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Analytical applications of X-Ray photoelectron spectroscopy Chan, Paul Ka-Hang 1987

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C • I . ANALYTICAL APPLICATIONS OF X-RAY PHOTOELECTRON SPECTROSCOPY by PAUL KA-HANG CHAN B.Sc.(Hons.), UNIVERSITY OF VICTORIA, 1985 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Chemistry) We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l 1987 ® Paul Ka-Hang Chan, 1987 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date fYfrRyV. \ . D E - 6 ( 3 / 8 1 ) ABSTRACT The s u r f a c e - s p e c i f i c a n a l y t i c a l technique of X-ray p h o t o e l e c t r o n spectroscopy (XPS) i s d e s c r i b e d , and was used to study v a r i o u s geochemical m a t e r i a l s and organic compounds. V a r i a t i o n of s u r f a c e p y r i t e d e n s i t y with c o a l p a r t i c l e s i z e (53-250/um) i n a t y p i c a l Canadian c o a l (Minto) provided some i n t e r e s t i n g data- i t i s very l i k e l y that as c o a l i s crushed, one e v e n t u a l l y reaches a p a r t i c l e s i z e where the su r f a c e p y r i t e / c a r b o n r a t i o maximizes. I t i s t h i s parameter that i s examined here, and c o r r e l a t i o n s were found between ( i ) s u r f a c e p y r i t e c o n c e n t r a t i o n , ( i i ) s u r f a c e p y r i t e / s u l f a t e r a t i o , and ( i i i ) o x i d i z e d and non-oxidized s u l f u r with p a r t i c l e s i z e . T h i s i s i n f o r m a t i o n which should f i n d u s e f u l a p p l i c a t i o n i n c o a l c l e a n i n g technology. For non-oxidized c o a l , we f i n d the area of exposed p y r i t e on the c o a l s u r f a c e i s approximately i n v e r s e l y p r o p o r t i o n a l to c o a l p a r t i c l e r a d i u s . However, f o r o x i d i z e d c o a l the appearance of curves depends on the o x i d a t i o n times, but there i s a p a r t i c l e s i z e which e x h i b i t s maximum s u r f a c e p y r i t e r e l a t i v e to 1/radius, corresponding to the i n t e r c e p t point of the two l i n e a r segments (low and higher values of 1/R) f o r the non-oxidized coa l ( f i g . 3 . 9 ) , and which i s e v i d e n t l y that we w i l l c a l l the " c h a r a c t e r i s t i c " s i z e of c o n s t i t u e n t p y r i t e . i i XPS 2p s u l f u r peaks from major s u l f u r c o n s t i t u e n t s 3/2 other than s u l f a t e i n sediment recovered from Mahoney Lake ( s o u t h - c e n t r a l B r i t i s h Columbia, Canada) show a s u r p r i s i n g l y p e r i o d i c v a r i a t i o n i n s u l f u r 2p binding energy (BE), and 3/2 hence molecular s t r u c t u r e , with sediment age. The p a t t e r n ceases at a core depth of about 3.2 meters, where a major dep o s i t of f i n e sandy sediment occurs j u s t below ash d e p o s i t i o n (2.6m) from a major e r u p t i o n of Mount Mazama, Oregon, approximately 6500 year ago, which l e d to the formation of C r a t e r Lake. Sedimental s u l f u r e x i s t s mainly as s u l f a t e ; however, there i s a pronounced i n c r e a s e i n amount of the lower BE s u l f u r s p e c i e s r e l a t i v e to s u l f a t e toward lower depths. The "reduced" s p e c i e s a l s o shows a trend towards s l i g h t l y higher o x i d a t i o n l e v e l at lower depths. We are able to suggest the probable chemical forms i n which the s u l f u r s p e c i e s e x i s t , which i s of i n t e r e s t "to b i o l o g i s t s working on s u l f u r t r a n s f o r m a t i o n s t u d i e s i n l a k e s . BE' s f o r n i t r o g e n Is and s u l f u r 2p i n the metal 3/2 c h e l a t e s of d i b e n z y l d i t h i o c a r b a m i c a c i d M(DBDTC) n f o r n=2, M=Cu(II) and Z n ( I I ) , and f o r n=3, M=Bi(III), have been measured. The nature of the s p e c t r a l peaks and core BE's i n d i c a t e s that the n i t r o g e n atom i n the DBDTC i s not i n t r a m o l e c u l a r l y bound with the metal as had been p r e v i o u s l y suggested. The r e l a t i v e l y high BE's f o r the n i t r o g e n Is o r b i t a l s i n d i c a t e planar geometry f o r the coordinated l i g a n d s , and the form / *s C^^-^NR 1to reasonably represent I 2 21 t h e i r s t r u c t u r e , which agrees with previous i n f r a r e d s t u d i e s . ACKNOWLEDGEMENTS I am s i n c e r e l y indebted to Pr o f e s s o r David C. F r o s t , f o r h i s w i l l i n g n e s s to d i s c u s s the many problems a s s o c i a t e d with the instrument and the a n a l y t i c a l a p p l i c a t i o n s of XPS. The most important are h i s i n v a l u a b l e words of encouragement and sense of humour during these two years. I would l i k e to thank P r o f e s s o r s A.P. Watkinson, J.S. Laskowski, and Dr. K.C. Teo; P r o f e s s o r s T.G. Northcote and K.J. H a l l ; P r o f e s s o r K.L. Cheng and Dr. S. Lawton f o r s t i m u l a t i n g d i s c u s s i o n s and h e l p f u l suggestions. I express my a p p r e c i a t i o n to Mr. C. McCafferty and Mr. J . Shim, of the UBC Machine and E l e c t r o n i c shops, f o r t h e i r s k i l l f u l t e c h n i c a l a s s i s t a n c e . I would e s p e c i a l l y l i k e to thank my parents and my f i a n c e e , Monica, f o r t h e i r constant support, encouragement and understanding. F i n a l l y , I would l i k e to thank the N a t i o n a l Research C o u n c i l of Canada and the U n i v e r s i t y of B r i t i s h Columbia f o r t h e i r f i n a n c i a l support. i x CONTENTS ABSTRACT ACKNOWLEDGEMENTS CONTENTS LIST OF FIGURES LIST OF TABLES CHAPTER ONE: X-RAY PHOTOELECTRON SPECTROSCOPY (XPS) 1.1 INTRODUCTION 1 a) H i s t o r i c a l 1 b) Basi c P r i n c i p l e s 4 1.2 THE SURFACE ANALYTICAL TECHNIQUE OF XPS 14 a) Sampling Depth 14 b) C a l i b r a t i o n of the Binding Energy Scale 16 c) Chemical S h i f t 19 d) I n t e r p r e t a t i o n of Data 23 REFERENCES 30 CHAPTER TWO: INSTRUMENTATION 2.1 INTRODUCTION 33 2.2 THE VARIAN IEE-15 SPECTROMETER 34 a) General C o n s i d e r a t i o n 34 b) The X-ray Source 41 c) The Sample 47 d) Data C o l l e c t i o n 48 REFERENCES . 50 PAGE i i i i i x v i v i i i V PAGE CHAPTER THREE: THE DEPENDENCY OF PYRITE OXIDATION IN COAL ON PARTICLE SIZE 3.1 INTRODUCTION 51 3.2 EXPERIMENTAL 55 3.3 RESULTS AND DISCUSSION 57 REFERENCES 74 CHAPTER FOUR: SULFUR SPECIATION IN LAKE SEDIMENT 4.1 INTRODUCTION 76 4.2 EXPERIMENTAL 78 4.3 RESULTS AND DISCUSSION 78 REFERENCES 85 CHAPTER FIVE: THE BONDING OF DIBENZYLDITHIOCARBAMATE METAL COMPLEXES 5.1 INTRODUCTION 87 5.2 EXPERIMENTAL 88 5.3 RESULTS AND DISCUSSION 89 REFERENCES 94 SUMMARY 95 v i LIST OF FIGURES FIGURE TITLE PAGE 1.1 The Energy L e v e l s Involved i n XPS 5 1.2 E l e c t r o n Binding Energies f o r Some N-, C-0- and S- Co n t a i n i n g Compounds 10 1.3 Auger and X-ray Fluorescence R e l a x a t i o n 11 1.4 P r o b a b i l i t y of Auger E l e c t r o n Emission and X-ray Fluorescence as a Function of Atomic Number 13 1.5 R e l a t i o n s h i p Between E l e c t r o n Escape Depth i n S o l i d s and K i n e t i c Energy 15 2.1 Block Diagram of the Varia n IEE-15 XPS Spectrometer 35 2.2 Schematic of the IEE-15 Spectrometer Analy-zer 37 2.3 XPS Spectrum of the 4f Gold Doublet 40 7/2,5/2 2.4 The X-ray Tube Assembly 45 3.1 A Proposed S t r u c t u r e f o r Coal 52 3.2 XPS Spectrum of Carbon I s / G r a p h i t e 58 3.3 Carbon Is and S u l f u r 2p XPS Spectra f o r 3/2 "3M" Scotch Tape 59 3.4 C o r r e l a t i o n Between XPS C Is Peak Height and the % by Weight of Carbon Present i n Sample 60 3.5 Carbon Is and S u l f u r 2p XPS Spectra of 3/2 Non-oxidized ( f r e s h ) Minto Coal Samples f o r D i f f e r e n t P a r t i c l e S i z e 63 V l l 3.6 Carbon Is and S u l f u r 2p XPS Spectra f o r 3/2 Minto Coal Samples of D i f f e r e n t P a r t i c l e S i z e s A f t e r 3 h Oxi d a t i o n 64 3.7 Carbon Is and S u l f u r 2p XPS Spectra f o r 3/2 Minto Coal Samples of D i f f e r e n t P a r t i c l e S i z e s A f t e r 15 h Oxi d a t i o n 65 3.8 Minto Coal P y r i t e / C a r b o n vs 1 / P a r t i c l e Radius 66 3.9 Minto Coal P y r i t e / C a r b o n vs P a r t i c l e Radius 67 3.10 Minto Coal P y r i t e / S 0 2 ~ ; S0 2~/C; P y r i t e / C vs 4 4 1 / P a r t i c l e Rad i u s ( A f t e r 3 h. O x i d a t i o n ) 69 3.11 Minto Coal P y r i t e / S 0 2 " ; S0,2~/C; P y r i t e / C vs 4 4 1 / P a r t i c l e Radius ( A f t e r 15 h Oxi d a t i o n ) 70 4.1 S u l f u r 2p XPS Spectra of Some Mahoney 3/2 Lake Sediments (at i n d i c a t e d core sample depths) 79 4.2 S u l f u r 2p Binding Energies of the Pre-3/2 dominant "Lower O x i d a t i o n S t a t e " S u l f u r Species i n Mahoney Lake Sediments 82 5.1 Nitrogen Is XPS Spectra of Some D i b e n z y l d i t h i o -carbamates, Cu ( II )h*2-EDTA and Bi ( I I I ) H-EDTA 92 5.2 S u l f u r 2p XPS Spectra of Some D i b e n z y l -3/2 dithiocarbamates 93 v i i i LIST OF TABLES TABLE TITLE PAGE 1.1 O r b i t a l I o n i z a t i o n P o t e n t i a l s f o r D i f f e r e n t Atoms 8 1.2 P e n e t r a t i o n Depths of P a r t i c l e s i n t o a T y p i c a l S o l i d Surface 15 1.3 D i f f i c u l t i e s Caused by Sample Charging 20 1.4 P h o t o e l e c t r i c C r o s s - s e c t i o n s R e l a t i v e to C Is L e v e l f o r Mg Ka(1253.60 eV) R a d i a t i o n 26 2.1 Some X-ray L i n e s Used i n XPS 43 3.1 Centigrade Table of R e l a t i v e Humidity 56 3.2 Ra t i o s of XPS Peak Heights at Various P a r t i c l e S i z e s f o r Fresh and Oxi d i z e d Samples of Minto Coal 72 5.1 XPS Data on Some D i b e n z y l d i t h i o c a r b a m a t e s , Cu(II)H 2-EDTA and Bi(III)H-EDTA 91 1 CHAPTER ONE X-RAY PHOTOELECTRON SPECTROSCOPY (XPS) 1.1: INTRODUCTION a) H i s t o r i c a l One of the e a r l i e s t r e f e r e n c e s to the importance of the r e l a t i o n s h i p between the s u r f a c e composition of s o l i d m a t e r i a l s and i t s e f f e c t on the m a t e r i a l s ' p r o p e r t i e s , appeared i n the i l l u m i n a t e d manuscript "De P r o p r i e t a t i b u s 1 Rerum" (The P r o p e r t i e s of Things) of 1250 AD: "When a p l a t e of gold s h a l l be bonded with a p l a t e of s i l v e r , or j o i n e d t h e r e t o , i t i s necessary to beware of three t h i n g s , of dust, of wind, and of moisture: f o r i f any come between the gold and s i l v e r they may not be jo i n e d together It remains a f a c t that the manner i n which any s o l i d s u r f a c e i n t e r a c t s with i t s environment, or with any other s o l i d s u r f a c e , i s determined by the p r e c i s e nature of the specimen s u r f a c e . Although bulk and s u r f a c e compositions may be r e l a t e d , to a grea t e r or l e s s e r degree, i t i s g e n e r a l l y i n f l u e n c e d accepted that more by sample 2 the nature of a s o l i d s u r f a c e environment than the bulk. i s C ompositional, chemical s t a t e and molecular bonding i n f o r m a t i o n about the topmost atomic l a y e r s of s o l i d specimens (metals, organic and i n o r g a n i c chemicals, polymers and powders, f i b r e s and f i l m s ) i s t h e r e f o r e c r u c i a l i f we are to understand how they behave when placed i n a p a r t i c u l a r environment. XPS i s an e x c e l l e n t technique f o r supplyi n g such i n f o r m a t i o n , and i t has many a p p l i c a t i o n s i n areas of sc i e n c e and technology where s o l i d s u r f a c e s or i n t e r f a c e s are i n v o l v e d . As an a n a l y t i c a l technique, XPS has i t s foundations i n 2 the p i o n e e r i n g work of Siegbahn and co-workers i n Sweden during the 1950's. However, the h i s t o r y of the f i e l d goes 3 back to the time of H e i n r i c h Hertz , who i n 1887 n o t i c e d that the maximum length of the spark of an i n d u c t i o n c o i l i s incr e a s e d by i l l u m i n a t i n g the gap with u l t r a v i o l e t l i g h t . He had, i n f a c t , produce p h o t o e l e c t r o n s between the gap A e l e c t r o d e s . In 1905, E i n s t e i n showed that the k i n e t i c energy of the p h o t o e l e c t r o n s i s equal to the quantum energy of the r a d i a t i o n minus the e l e c t r o n binding energy (BE) and the "work f u n c t i o n " of the s o l i d . This was c a l l e d the " p h o t o e l e c t r i c e f f e c t " . By reducing the X-ray wavelength i t was p o s s i b l e to i n v e s t i g a t e the i o n i z a t i o n of inner e l e c t r o n s h e l l s . T h i s was f i r s t c a l l e d X-ray absorption-edge s p e c t r o -5 scopy, and i n 1920, Bergengren r e l a t e d the p o s i t i o n s of X-3 ray absorption-edges to o x i d a t i o n s t a t e s of atoms i n v a r i o u s m a t e r i a l s . I t l a t e r became c l e a r that not only could one p r o f i t a b l y examine the a b s o r p t i o n of X-rays i n the d i r e c t i o n of the X-ray beam, but a l s o p h o t o e l e c t r o n s e j e c t e d at r i g h t angles to i t . Siegbahn found that BE's of atomic e l e c t r o n s could be s u f f i c i e n t l y a f f e c t e d by the chemical environment of that atom to produce a measureable s h i f t i n the ph o t o e l e c t r o n spectrum. T h i s was termed the "chemical s h i f t " . R e a l i s i n g that any t o o l capable of r e a d i l y d i f f e r e n t i a t i n g between chemical s t a t e s of the same element had enormous p o t e n t i a l i n chemical a n a l y s i s , Siegbahn designated the new spectroscopy as E l e c t r o n Spectroscopy f o r Chemical A n a l y s i s , or "ESCA", but i t i s more u s u a l l y r e f e r r e d to as "XPS", or X-ray P h o t o e l e c t r o n Spectroscopy. For many years, XPS remained p r i m a r i l y an academic research technique, simply because of the expensive i n s t r u m e n t a t i o n r e q u i r e d , and the n e c e s s i t y of taking extreme care i n i t s a p p l i c a t i o n and i n t e r p r e t a t i o n . The advent of modern commercial spectrometers, however, has enabled s p e c t r o s c o p i s t s to improve t h e i r understanding and use of the technique to the point where i t i s a major c o n t r i b u t o r i n i n d u s t r i a l a n a l y s i s and problem s o l v i n g , as w e l l as i n pure r e s e a r c h . 4 b) Basic P r i n c i p l e s The b a s i c p h o t o i o n i z a t i o n process promotes an e l e c t r o n to the f r e e e l e c t r o n l e v e l ( f i g . 1 . 1 ) . XPS, l i k e UPS ( u l t r a -v i o l e t P h o t o e l e c t r o n S p e c t r o s c o p y ) , i s based on the same p h o t o e l e c t r i c e f f e c t , but employs s o f t X-rays to remove core 6 r a t h e r than valence s h e l l e l e c t r o n s . In the b a s i c XPS 2 experiment, a small (< 2 cm ) sample s u r f a c e i s i l l u m i n a t e d with X-rays, g e n e r a l l y A l Ko(1486.60 eV) or Mg Ka(1253.60 eV), -7 -8 -11 under high vacuum (10 t o r r ) . U l t r a - h i g h vacuum (10 -10 t o r r ) i s d e s i r a b l e when gas a b s o r p t i o n on the s u r f a c e i s a f a c t o r , to ensure that s u r f a c e s once cleaned w i l l remain f r e e from contamination. High vacuum a l s o ensures that p h o t o e l e c t r o n s emitted w i l l have a s u f f i c i e n t l y long mean 7 f r e e path to s u r v i v e energy a n a l y s i s to the d e t e c t o r . In the XPS of s o l i d s , the sample i s placed on a metal probe. I f the sample i s m e t a l l i c , there are two conductors i n contact and t h e i r Fermi l e v e l s w i l l a d j ust to c o i n c i d e n c e . I f the sample i s n o n - m e t a l l i c , a s u f f i c i e n t number of f r e e charge c a r r i e r s are u s u a l l y present i n the sample f o r i t to a d j u s t to the thermodynamic e q u i l i b r i u m s t a t e and so again 8 the Fermi l e v e l s w i l l c o i n c i d e . F i g u r e 1.1 r e l a t e s the BE, work f u n c t i o n s , e t c . r e l a t i v e to the Fermi l e v e l of the m a t e r i a l s . I f there i s any d i f f e r e n c e i n the work f u n c t i o n s 5 F E L I CB V B t K E J _ C B V B F E L F E R M I L E V E L E x - r a y B E S A M P L E S P E C T R O M E T E R C O R E L E V E L E - Energy E r » Re c o i l Energy » Work Function of Spectrometer ^ • Work Function of Sample CB - Conduction Band VB - Valence Band FEL » Free Electron Level E « Energy of Exc i t i n g X-radiation x-ray " • KE ' - Kin e t i c Energy of Photoelectrons Before Entering the Monochrometor KE - Kinetic Energy of Photoelectrons After Entering the Monochromator BE - Binding Energy of Core Level Electron Ejected F i g u r e 1 . 1 : The Energy L e v e l s Involved X P S 6 of the spectrometer and samples, i . e . the energy r e q u i r e d to move an e l e c t r o n from the Fermi l e v e l to f r e e e l e c t r o n l e v e l , the r e s u l t of the formation of a common Fermi l e v e l i s to make the energy d i f f e r e n c e between the sample and the probe equal to the d i f f e r e n c e i n t h e i r work f u n c t i o n s . P h o t o i o n i z a t i o n takes place i n the sample s u r f a c e . In the absence of e l e c t r i c a l contact between the sample and the probe, the r e s u l t a n t p h o t o e l e c t r o n s have a k i n e t i c energy KE' r e l a t e d to the X-ray energy ho by the E i n s t e i n r e l a t i o n : KE' = ho - BE - </> - E s r where <p i s the sample work f u n c t i o n , and E i s the r e c o i l energy, r e s u l t i n g from c o n s e r v a t i o n of momentum i n the ej e c t e d e l e c t r o n p a r e n t - i o n system. T h i s equation can r e a d i l y be understood by . examining f i g u r e 1.1 again. In a r e a l system with e l e c t r i c a l c o n t a c t between the spectrometer and the sample we have an a d d i t i o n a l term, the contact p o t e n t i a l , V c: KE = ht) - BE - <t> - E - V s r c where KE i s s l i g h t l y d i f f e r e n t from KE'. As alre a d y d i s c u s s e d , V c i s the d i f f e r e n c e between the work f u n c t i o n s of the probe and the sample. Thus we have, on s u b s t i t u t -sp ing f o r V and <f> : c s KE = ho - BE - 0 - E sp r 7 In p r a c t i c e the r e c o i l energy i s extremely small (<< 1 eV) and can be n e g l e c t e d . For Mg Ka r a d i a t i o n , which i s used i n our l a b o r a t o r y , the maximum r e c o i l energy i s l e s s than 0.1 eV, and E v a r i e s d i r e c t l y with X-ray energy and i n v e r s e l y 2 with the mass of the parent atom . Moreover, the use of the Fermi l e v e l as a BE r e f e r e n c e zero allows the same work f u n c t i o n c o r r e c t i o n to be a p p l i e d to a l l measurements, i . e . i s constant f o r a given spectrometer. The above sp c o n s i d e r a t i o n s lead to the simple r e l a t i o n s h i p : KE = ho - BE- 4> sp KE i s w e l l d e f i n e d and any u n c e r t a i n t y i n the determination of BE i s due to the n a t u r a l width of the l e v e l from which the e l e c t r o n has been e j e c t e d and to the width of the 9 i n c i d e n t X-ray l i n e . In p r a c t i c e i n s t r u m e n t a l broadening w i l l add to t h i s (see below). In XPS emission of p h o t o e l e c t r o n s with w e l l - d e f i n e d KE's r e s u l t s i n p h o t o e l e c t r o n s p e c t r a - i n e f f e c t d i r e c t images of the atomic energy l e v e l s i f Koopmans' Theorem (KT) were true (see s e c t i o n 1.2c). Table 1.1 shows the v a r i a t i o n s of e l e c t r o n BE's with atomic number. The core l i n e s of the c o n s t i t u e n t elements of a compound are c h a r a c t e r i s t i c a l l y atomic and t h e i r o r d e r i n g does not depend on chemical environment,which makes l i n e assignment very 9 easy . Thus, i n f o r m a t i o n on e l e c t r o n s ' BE's w i t h i n a sample allows q u a l i t a t i v e elemental a n a l y s i s . E l e c t r o n BE's w i t h i n 8 ELECTKON BINDING ENEROES (eV) l j.il »1>|,l 3p,„ *J.;l 3J.il M| M|| Mm Mnr M» "I A n 14 23 55 111 IU> 3(4 399" 332' CI6> •47 1072' IMS' IJ60' 1839' 2149' 2472" 2823' 5203 34" It 49 63> •9" II 52 IIS* 74 7) I49» 100 99 II9> IX. 1)9 229' 165 164 270" 202 200 320' 247 245 S77« 297 294 43l» MO 547 500' 407 402 5«4» 461 493 628» 520 51) 695» 514 575 769' •92 641 «4«> 723 710 926' 794 779 I008> 172 •55 I09*» 951 931 1194 1044 1021 1298 114) 1116 1411* 1249 1217 1527 1)99 1)23 1654 1476 1436 1712' 1596' 1350' 1921 1727 1675 16 II 25 54 54 59 66 74 •4 95 101 112 120 1)7 II 26 22 14 II 43 56 60 66 74 S7 151 107 103 II III 129 122 29 204 147 141 41 232 161 162 57 257 119 112 70 69 27 2t»» 223 214 •9 24 322 241 2)9 112 111 X 391 210 269 135 I)) 31 20 593 313 301 160 198 46 26 431 145 331 113 IH 52 29 469 J79 563 208 309 SI 34 SOS 410 39) 230 227 62 23 544* 445 425 257 25) 68* 39 $85 413 461 284 279 73 43 627 521 496 312 307 •1 41 670 S39 3)1 340 333 •6 SI 717 602 571 373 367 99 62 ' 56 770 631 617 411 404 tot 67 •26 702 664 451 443 122 77 M4 757 715 494 483 137 19 944 • 13 766 537 528 152 99 32 1006 •70 • 19 582 572 168 110 40 1072 931 175 631 620 116 123 so 1145* 999 937 685* 672 20«* 147 63" 1217 1065 998 740 726 231 172 162 79 77 129) 1137 1063 796 711 253 192 110 93 90 1362 1205 1124 149 •32 271 206 192 99 1435 1273 1186 902 184 290 224 208 III 1511 1)38 124) 951 931 305 2)7 218 114 1576 140) 1298 1000 971 316 244 229 III 1690* 1472* I337« 1052" 1027* 331* 293* 237* 121* 1724 1942 1421 1107 1081 347 267 249 130 1100 1614 1481 1161 1131 360 214 237 134 • 111 1689 1944 1218 1186 376 219 271 141 1961 1761 1612 1276 1242 391 311 2*6 141 2047 1142 1676 13)2 1295 416 332 293 154 2128 1923 1741 1391 1331 436 343 306 161 2207 2006 1112 1453 1409 449 366 320 177 16* 2307 2090 1885 1519 1468 472 346 337 i n 2)97 2172 1949 1576 1927 4*7 396 343 197 114 2491 2264 2024 1640 1589 506 410 399 205 193 11" 2601 2363 2108 1716 1662 938 437 380 224 214 19" 2708 2469" 2194" 1793" 1735" 566 469 409 242 2)0 2 7 " 2 9 " 2820 2575'• 2281» 1872" 1810" 595 492 426 299 246 37" 34" 2932 2682" 2)67" 1949" 118)" 625 III 449 274 260 4 7 " 4 5 " 5049 2792" 2438" 2031" I960" 699 547 469 290 27) 52" SO" 5174 2909'" 2551" 2116" 2041" 690 577 499 312 293 63" 6 0 " 3298" 3021" 2646" 2202» 212l"> 724" 608" 644 9I9*> 331"° 3I4»° 74" TO" 3423" 3130" 274)" 2291" 2206" 799 546 332 334 • 7 " • 3 " 3962 3279 2647" 2)85" 2295" •00 677 571 379 . 360 10)" 9 9 " 3704 3416» 2957" 2485" 2390" 146 722 609 407 316 122" 111" 3151 35541' 3067" 2586" 2484" 194 764 645 435 413 143" 131" 199 9 " 3697" 3177" 2688" 2580" 939" • 0 6 " 679" 464" 440" 16)" 151" 4149 3154 4001 3302 2798 2683 993 •51 709 500 473 IS4* 4317* 1424 2909 2717 1042* 186 740 S33 507* 210* 1097' 929 761 567 541* 238* 1133* 980 110 603 577 268* 1208* 1058 •79 636 60) 299 1269" 1080 •90 675 639* 319* 333" 1330" 1168" 968" 714" 677" 144" 1387 1224 1007 743 708 371 360 1442" 1273" 1049" 710" 7 )1" 392 381 1901" 1328" 1087" • 1 7 " 77)" 415" 404" 1998 1377 1120 • 4 9 " • 0 1 " 422 1617 1412 1136* 179 121 440* able 1 .1 : O r b i t a l I o n i z a t i o n P o t e n t i a l s f o r D i f f e r e n t Atoms 10 any one element are f a r from f i x e d ( f i g . 1 . 2 ) , and v a r i a t i o n s (of up to 10 eV) may occur, depending on molecular environment. These so c a l l e d "chemical s h i f t s " may be used to provide v a l u a b l e i n f o r m a t i o n on chemical 11 bonding A f t e r p h o t o i o n i z a t i o n , the ion w i l l r e v e r t to the ground s t a t e e i t h e r by r e l e a s i n g a photon (X-ray f l u o r e s c e n c e ) or by e l e c t r o n (Auger) emission. In the former case the vacant o r b i t a l may be f i l l e d by an e l e c t r o n from an outer o r b i t a l and the energy emitted as an X-ray, while i n the l a t t e r an e l e c t r o n from an outer o r b i t a l drops i n t o the 10 primary h o l e , and other e l e c t r o n ( s ) are e j e c t e d ( f i g 1.3) As an example of Auger terminology, KL L i n d i c a t e s that I I I I the primary vacancy occurred i n the K s h e l l , the e l e c t r o n underwent a t r a n s i t i o n to f i l l the primary vacancy and a e l e c t r o n was e j e c t e d . The three processes are represented i n the f o l l o w i n g equations: P h o t o i o n i z a t i o n : +* M + h o ( l ) M + e (1) X-ray F l u o r e s c e n c e : + * + M M + ho(2) R e l a x a t i o n Auger Emission: [ Processes +* ++ M •M + e (2) where ht>(l) i s the e x c i t i n g r a d i a t i o n . 10 Compound N i t r o g e n CrN V N P . N , C.H .N BN C*H S CN K S C N N a N - ' N N * (OONHJ)J Na,(ON»NO,) C H . N (C,H,)4Na N a N j O , S . N 4 N H . O H d ( C H , ) , N B , H , ( C H , ) , N O NaO^N 'N) N a a ( O N N " 0 2 ) N a N O j C , H 5 N O , /(-•NOjCtH.CONH, fra«IC^en)j(NOa),P>,,0, N a N ' O , Carbon H f C T i C W'C C (graphite) O H . C H j N H , C ' H j C O O H C.H. K C N CH,C»H,NH, CH,OH Qxyqen S — S Me M e (CH,),*? N a , S O , FeSO« CtHtOiSOj* S O F , rfNO, soa, C H . O / S O , 396.6 0.0 397.2 0.6 S Compound 165 TTO tV 397.8 1.2 RSNo/K 0 i 398.0 1.4 NoSC(S)NR, D 398.2 1.6 S-C(SSR)NR, 1 398.4 1.8 ^sof 1 1 398.5 1.9 KSC(S)0R 1 399.3 2.7 RSH 1 IB 1 400.0 3.4 RSCSKR 1 400.9 4.3 RSR Annul 401.1 4.5 RSSR cissa nrnn 401.1 401.3 4.5 4.7 RSSC(S)NR, «S-C«5 'l 1 402.1 5.5 ®s 1 402.1 S.S RSSCTR 0 402.2 5.6 RSSOf 1 402.2 5.6 RSSOjR 0 403.7 7.1 RJNSNRJ 1 1 403.9 7.3 Rsa < 1 404.1 405.1 7.5 6.5 RSNKj Sthbphene 1 405.9 9.3 RS0R O 406.8 10.2 ROSSOR 1 407.4 10.8 F.CSSO.R RSOf 1 ipn 281.0 281.7 -4.0 -3.3 RS(0)R RSOSR not a m 282.9 -2.1 SO® 284.0 -0.7 eso'scf i 285.0 0.0 RS(0)0R a 285.0 0.0 RS0.R 285.0 0.0 « 5 o ; s e 1 285.0 0.0 •stQjflR, 1 285.7 287.2 0.7 2.2 RSOf ROS(0)OR RS0,SR RS0.NR./H, Bin a CD a so, i 530.3 0.0 SOCI, «*(0)R, i i RS0,0R RS0XI SQ? ^SOjSR ^OjSOf RS020S0,R ID 531.3 531.9 532.5 532.5 1.0 1.6 2.2 2.2 no i o i i 533.0 2.7 RSOjSCF, i 533.0 2.7 R0S0,0R i 533.6 3.3 RSOjF D 534.1 3.7 SOfj F i g u r e 1 . 2 : E l e c t r o n B i n d i n g Energies f o r Some N-, C-0- and S- C o n t a i n i n g Compounds 11 P R I M A R Y I O N I Z A T I O N L i n L II • • • K R E L A X A T I O N L. AUGER 1 • • • O— 9 • K in X - R A Y in L. t • • • • K -• 9-F i g u r e 1.3: Auger and X-ray Fluorescence R e l a x a t i o n 12 The r e l a t i v e p r o b a b i l i t i e s of Auger and X-ray emission are given i n f i g u r e 1.4. Auger processes are dominant i n l i g h t elements, which i s an i n d i c a t i o n of the d i f f i c u l t y of X-ray f l u o r e s c e n c e techniques i n t h i s area and the value of 8 the XPS method . Auger e l e c t r o n spectroscopy (AES) i s a su r f a c e a n a l y t i c a l technique i n i t s own r i g h t , and f u r t h e r i n f o r m a t i o n can be found i n r e f e r e n c e s 12-16. 13 1.0-1 t 0.84 m 0.6-§ 0.4-°- 02-0.0-AUGER X -RAY i 1 ' 1 1 1 1 1 5 10 15 20 25 30 35 40 ATOMIC NUMBER F i g u r e 1.4: P r o b a b i l i t y of Auger E l e c t r o n Emission and X-ray Fluor e s c e n c e as F u n c t i o n of Atomic Number 14 1.2: THE SURFACE ANALYTICAL TECHNIQUE OF XPS a) Sampling Depth The average sampling depth (depth from which photo-7 e l e c t r o n s may escape without i n e l a s t i c s c a t t e r i n g ) from a s o l i d i s c o n s i d e r a b l y l e s s than the p e n e t r a t i o n depth of the e x c i t i n g r e d i a t i o n . Table 1.2 gi v e s t y p i c a l p e n e t r a t i o n depths of photons, e l e c t r o n s , and ions i n the energy range commonly used f o r s u r f a c e a n a l y s i s . A photon-in/ photon-out technique w i l l c l e a r l y not normally be s u r f a c e s p e c i f i c and e i t h e r the beam-in or the beam-out must i n v o l v e e l e c t r o n s or ions . The escape depth of a p a r t i c l e such as an e l e c t r o n or ion depends on the energy of the p h o t o e l e c t r o n and the matrix environment through which i t t r a v e l s to the 17,18 su r f a c e ; the r e l a t i o n s h i p of the escape depth of 15 e l e c t r o n s versus KE i s given i n f i g u r e 1.5 . Notably, very low and high energy e l e c t r o n s have r e l a t i v e l y l a r g e escape depths, the former of l i t t l e use i n a n a l y t i c a l a p p l i c a t i o n s s i n c e they mostly r e s u l t from i n e l a s t i c c o l l i s i o n s . E l e c t r o n s having energies of about 100 eV have the lowest escape depths, and so spectroscopy i n v o l v i n g high energy or low energy e l e c t r o n s w i l l l i k e l y not be s u r f a c e s p e c i f i c . In XPS, however, the p h o t o e l e c t r o n s are u s u a l l y i n the 100-1000 eV range, from depths of <10-20 A, i . e . the f i r s t two or three molecular l a y e r s . For t h i s reason, XPS i s c l a s s i f i e d 15 F i g u r e 1.5: R e l a t i o n s h i p Between E l e c t r o n Escape Depth i n S o l i d s and K i n e t i c Energy INCIDENT vmnrw PENETRATION SPECIES ENERGY(eV) DEPTH (A) PHOTON lOOO > lOOOO E L E C T R O N lOOO 20 ION lOOO 10 TABLE 1.2 P e n e t r a t i o n Depths of P a r t i c l e s i n t o a T y p i c a l S o l i d Surface 16 as a su r f a c e spectroscopy. Comprehensive data on e x p e r i -mentally determined escape depths are contained i n re f e r e n c e 17. It i s p o s s i b l e to use the v a r i a t i o n i n escape depths with k i n e t i c energies to d i s t i n g u i s h between elements which 19 are present on the s u r f a c e . In s e c t i o n 1.2d, the e f f e c t of sampling depth on q u a n t i t a t i v e s u r f a c e a n a l y s i s w i l l be d i s c u s s e d . XPS s t u d i e s must take s u r f a c e i m p u r i t i e s and con-taminants i n t o account. Contamination l a y e r ( s ) u s u a l l y comprise s p e c i e s adsorbed from the atmosphere, through handli n g , or o r g i n a t i n g from the vacuum system. I t i s ob v i o u s l y h i g h l y d e s i r a b l e to have u l t r a - h i g h vacuum and a sample p r e p a r a t i o n chamber i n which samples may be cleaned p r i o r to a n a l y s i s . In our case, vacuum i s achieved using extremely low vapour-pressure o i l d i f f u s i o n pumps i n co n j u n c t i o n with the usual r o t a r y backing pumps. The d i f f u s i o n pump o i l s used i n the Varian IEE-15 system are Convalex-10 and Neovac-SY which are polyphenyl ether and a l k y l a t e d diphenyl e t h e r , r e s p e c t i v e l y . b) C a l i b r a t i o n of the Binding Energy Scale In order to r e a l i z e the p o t e n t i a l of XPS f u l l y , a ccurate c a l i b r a t i o n of the s p e c t r a l energy s c a l e i s e s s e n t i a l , s i n c e d i f f e r e n t chemical environments u s u a l l y 17 cause only small ( t y p i c a l l y 1/10 to a few eV) s h i f t s i n peak p o s i t i o n . G e n e r a l l y speaking, BE's are accurate to +0.2 eV in our measurements; however, having a s o l i d sample makes c a l i b r a t i o n d i f f i c u l t i n the f o l l o w i n g sense- the BE zero f o r a s o l i d i s a r b i t r a r i l y set at the Fermi l e v e l , the highest e l e c t r o n i c energy l e v e l that i s occupied by an e l e c t r o n i n conductors at room temperature, whereas f o r a fr e e molecule i t i s def i n e d with respect to the e j e c t e d 20 e l e c t r o n at r e s t at i n f i n i t y . T h e r e f o r e , i t i s a n a t u r a l choice f o r conductors to be used as standards, because i n p r i n c i p l e , any would d e f i n e the Fermi l e v e l . If one i s bombarding a su r f a c e with charged p a r t i c l e s or charged p a r t i c l e s are emitted from a s u r f a c e , sample charging may occur. T h i s i s a s i g n i f i c a n t problem i n a l l such s p e c t r o s c o p i c techniques i n v o l v i n g e l e c t r o n s or ions -f o r example, sample charging can d i s t o r t s p e c t r a and s h i f t the l o c a t i o n of peaks on the energy s c a l e , the extent depending on the nature of the sample. For i n s u l a t o r s , charging e f f e c t s can be qu i t e severe, while i n conductors they are r e l a t i v e l y s m a l l . F o r t u n a t e l y , the surface c o n d u c t i v i t y of most samples i s much greater than the bulk c o n d u c t i v i t y , and so an even charge d i s t r i b u t i o n b u i l d s up 7 on the su r f a c e to a steady s t a t e value . In XPS there are f r e q u e n t l y s t r a y e l e c t r o n s i n the v i c i n i t y of the sample which a l s o help to reduce sample charge, e s p e c i a l l y i n our spectrometer, i n which e l e c t r o n s are e j e c t e d from the aluminum f i l m between the X-ray source and sample. Because 18 of t h i s conducting cloud the sample w i l l assume a p o t e n t i a l 8 very c l o s e to that of the f i l m . For i n s u l a t o r s , there are two p o s s i b l e approaches to the c a l i b r a t i o n problem: one may attempt to prevent charge from b u i l d i n g up on the sample or one can measure the magnitude of the charge and apply a c o r r e c t i o n . The f i r s t approach almost always employs s t r a y e l e c t r o n s (as mentioned above) or by using an e l e c t r o n " f l o o d gun" - t h a t i s a source of low energy (thermal) e l e c t r o n s from a heated f i l a m e n t . The second approach i s to employ c a l i b r a t i o n m a t e r i a l ( i n t e r n a l or e x t e r n a l ) that e f f e c t i v e l y monitors s u r f a c e charge and changes the spectrometer work f u n c t i o n i n the computer program to compensate. Using i n t e r n a l c a l i b r a n t s , when a s e r i e s of r e l a t e d compounds i s being examined, one s i g n a l from a common element i n the compounds i s chosen and a l l other s i g n a l s are measured r e l a t i v e to i t , which works q u i t e s a t i s f a c t o r i l y . A l s o , an i n t i m a t e mixture of sample with v a r i o u s i n e r t m a t e r i a l s of known BE such as g r a p h i t e (our p r e f e r e n c e ) , metal oxides, and potassium 2 1 , 2 2 s a l t s can be used f o r c a l i b r a t i o n purposes. R a t i o n a l e of t h i s i n t e r n a l procedure i s that the s u r f a c e charge produces a f i e l d around the sample which a f f e c t s the KE of a l l p h o t o e l e c t r o n s , and hence a l l peaks experience the same s h i f t . Use of e x t e r n a l c a l i b r a n t s r e l i e s on the c a l i b r a n t and sample being charged to the same extent. The C Is peak of a g r a p h i t e sample and the adhesive " s c o t c h " 19 23 tape or the 4f peak of m e t a l l i c gold are e x c e l l e n t 7/2,5/2 e x t e r n a l c a l i b r a n t s . Table 1.3 summarizes some of the d i f f i c u l t i e s caused by sample cha r g i n g . c) Chemical S h i f t s When atoms are brought c l o s e together to form molecules, the o r b i t a l s of i n d i v i d u a l atoms are perturbed and may then be represented by molecular o r b i t a l s (MO's). Since valence-s h e l l e l e c t r o n s are d i r e c t l y i n v o l v e d i n bonding, they are very s e n s i t i v e to s u b s t i t u e n t and other s t r u c t u r a l e f f e c t s , but because the MO's they occupy are g e n e r a l l y m u l t i - c e n t r e d , i t i s not u s u a l l y p o s s i b l e to i d e n t i f y the i n d i v i d u a l component atoms of a molecule from i t s v a l e n c e - s h e l l photo-e l e c t r o n spectrum. Moreover, d i f f e r e n t types of valence-s h e l l o r b i t a l s have very s i m i l a r e n e r g i e s , r e s u l t i n g i n overlap of adjacent bands i n p h o t o e l e c t r o n s p e c t r a . However, inner core o r b i t a l s do r e t a i n t h e i r atomic i d e n t i t y to a gre a t e r extent, which allows XPS to provide a v i a b l e elemental a n a l y s i s . Notwithstanding the above comments, core e l e c t r o n s do s u f f e r small changes i n BE as t h e i r environment i s changed. This may be explained by reg a r d i n g i n d i v i d u a l atoms i n a molecule as spheres of d i f f e r i n g p o t e n t i a l s . Inside each charged sphere the atomic p o t e n t i a l , set up by charge s h i f t to or from i t s surface, to the neighbouring atom(s), i s 20 P R O B L E M S I N X P S : DISTORTION OF SPECTRA SHIFTS OF PEAK LOCATION MOVEMENT ON SURFACE E X T E N T O F P R O B L E M : INSULATORS » SEMICONDUCTORS > CONDUCTORS C O M P E N S A T I O N ? SURFACE CONDUCTION STRAY ELECTRONS CALIBRATION FLOOD-GUN 1.3: D i f f i c u l t i e s Caused by Sample Chargi 21 constant, according to c l a s s i c a l e l e c t r o s t a t i c s . The r e s u l t of t h i s change i n atomic p o t e n t i a l i s a s h i f t of the whole inner energy l e v e l system of any atom by a small 24,25 amount . These changes or "chemical s h i f t s " due to e l e c t r o s t a t i c i n t e r a c t i o n allow u s e f u l s t r u c t u r a l deductions to be made from XPS s p e c t r a . The a p p l i c a t i o n of XPS to the s o l u t i o n of s t r u c t u r a l problems can be s i m p l i f i e d by developing methods f o r p r e d i c t i n g reasonably accurate BE values f o r any p a r t i c u l a r s t r u c t u r a l formula one might propose f o r a given compound. These methods w i l l not be de s c r i b e d i n d e t a i l here as q u a n t i t a t i v e chemical s h i f t a n a l y s i s has not been attempted i n the work described i n t h i s t h e s i s . I t i s important to mention Koopmans' Theorem (KT), a f a c t o r u s u a l l y i n v o l v e d i n molecular o r b i t a l c a l c u l a t i o n s 26-28 based on s e m i - e m p i r i c a l and e m p i r i c a l methods . T h i s 29 theorem r e l a t e s o r b i t a l energies with the i o n i z a t i o n energy obtained from p h o t o e l e c t r o n s p e c t r a , and i s a u s e f u l approximation b r i d g i n g experimental data and t h e o r e t i c a l c a l c u l a t i o n s . The theorem assumes that the molecular o r b i t a l s are u n a l t e r e d by i o n i z a t i o n ( f r o z e n o r b i t a l a p r o x i m a t i o n ) , thus the BE's c a l c u l a t e d using KT u s u a l l y d i f f e r from the a c t u a l experimental BE's. The most important l i m i t a t i o n i s the ne g l e c t of c o r r e l a t i o n energy between the n e u t r a l molecule and the ion - t h i s should make the c a l c u l a t e d 22 i o n i z a t i o n energy too s m a l l . The f u r t h e r approximation that the e l e c t r o n i n t e r a c t i o n s are e x a c t l y the same i n the ion and molecule, which i s m a n i f e s t l y not t r u e , as the e l e c t r o n s i n the ions can always a t t a i n a more s t a b l e s t a t e than the one defined by t h e i r motions i n the molecule, should give a c a l c u l a t e d i o n i z a t i o n energy that i s too h i g h . In g e n e r a l , the c a n c e l l a t i o n of e r r o r s i n using KT due to n e g l e c t of both c o r r e l a t i o n energy and e l e c t r o n i n t e r a c t i o n s enables the theorem to e f f e c t a good approximation f o r many small molecules; however, there i s s u b s t a n t i a l evidence which shows that use of KT i s not j u s t i f i e d where r e l a t i v i s t i c e f f e c t s 30 ,31 ( e l e c t r o n c o r r e l a t i o n and r e l a x a t i o n ) are important The most accurate c a l c u l a t i o n i s the "exact" Hartree-Fock SCF ( s e l f c o n s i s t e n t f i e l d ) methods; these have a s e r i o u s disadvantage with regard to the s i z e of computations i n v o l v e d . In s p i t e of these problems, good c o r r e l a t i o n s between experimental chemical s h i f t s and t h e o r e t i c a l estimates of them, and t h e r e f o r e of the atomic p o t e n t i a l , have been 32 obtained . The s i m p l e s t model i s provided by the e l e c t r o -33,34 s t a t i c point charge p o t e n t i a l which r e l a t e s the chemical s h i f t , AE, of the atom to s e v e r a l f a c t o r s : A E ( i ) = k q ( i ) + V + 1 where q ( i ) i s the charge d i f f e r e n c e on an atom i between the molecule under c o n s i d e r a t i o n and a r e f e r e n c e molecule, and V 23 i s the i n t e r a t o m i c e f f e c t i v e p o t e n t i a l , or molecular potent-i a l found from: V = £ q ( i ) / R j * i J The parameter k and 1 of the equation are constants determined by a l e a s t squares p l o t to f i t the experimental data c h a r a c t e r i z i n g the type of chemical compound. The 35 atomic charge can be estimated by P a u l i n g ' s procedure , or 36 by CNDO (Complete Neglect of D i f f e r e n t i a l Overlap) methods I t i s a l s o p o s s i b l e to determine "group s h i f t " parameters i n which chemical s h i f t s are represented by the sum of para-meters c h a r a c t e r i s t i c of the atom(s) or s u b s t i t u e n t groups 35 attached to the atom under c o n s i d e r a t i o n . d) I n t e r p r e t a t i o n of Data XPS i s one of the most u s e f u l s u r f a c e techniques to emerge i n t o widespread use i n the l a s t decade. P r o v i d i n g the e x c i t i n g energy i s high enough, core l e v e l s p e c t r a can be obtained f o r a l l elements of the p e r i o d i c t a b l e except H and He, and the BE's of these core l e v e l s are s u f f i c i e n t l y unique f o r t h e i r unambiguous assignment ( c f . t a b l e 1.1). Since the BE of an atom w i t h i n the molecule r e f l e c t s i t s environment, i t should be p o s s i b l e to c o n s t r u c t c o r r e l a t i o n t a b l e s ( c f . f i g . 1 . 2 ) which would serve to i d e n t i f y the o x i d a t i o n c o n d i t i o n of an atom w i t h i n a molecule and thereby 24 help to i d e n t i f y i t s immediate environment. When a ph o t o e l e c t r o n spectrum from the sample of i n t e r e s t has been obtained, a n a l y s i s may be c a r r i e d out at one or more of s e v e r a l l e v e l s of i n t e r p r e t a t i o n , where the degree of complexity of the task i n c r e a s e s with the l e v e l of s o p h i s t i c a t i o n . At the lowest l e v e l , i d e n t i f i c a t i o n and assignment of the p h o t o e l e c t r o n peaks i n the spectrum allow a q u a l i t a t i v e a n a l y s i s to be c a r r i e d out. The a n a l y s i s may be made q u a n t i t a t i v e to a f a i r l y high degree of accuracy by measurement of r e l a t i v e p h o t o e l e c t r o n peak i n t e n s i t i e s or areas. Since the i n t e n s i t y of a core l e v e l peak from s p e c i e s X i n the spectrum i s p r o p o r t i o n a l to the s u r f a c e c o n c e n t r a t -ion of the s p e c i e s (where s u r f a c e , here, means the top 2 nm or l e s s of the s o l i d ) , i t i s p o s s i b l e to a r r i v e at a value f o r the s u r f a c e c o n c e n t r a t i o n of X, p r o v i d i n g the r e l e v a n t s e n s i t i v i t y f a c t o r s are known. D e t a i l s of t h i s procedure w i l l be discussed l a t e r . XPS s e n s i t i v i t y ( i n bulk terms) i s only about 0.1 atom % f o r elements with average c r o s s - s e c t i o n s ; however, i t s su r f a c e s p e c i f i c i t y puts i t i n a much higher c l a s s as a 37 a n a l y t i c a l t o o l . S o p h i s t i c a t e d spectrum de c o n v o l u t i o n i s 38 necessary to obt a i n high r e s o l u t i o n s p e c t r a i n some cases , and from these the ab s o l u t e BE's of the core l e v e l s can be measured (to w i t h i n 0.1 eV), and the presence of any f i n e s t r u c t u r e detected (such as m u l t i p l e t s p l i t t i n g , shake-up e t c ) . Knowledge of the core l e v e l BE and f i n e s t r u c t u r e 25 permits more r i g o r o u s assignments of the f e a t u r e s to be made, s i n c e i t i s p o s s i b l e to d i s t i n g u i s h more e a s i l y between d i f f e r e n t o x i d a t i o n s t a t e s of an element, or even to i n f e r the presence of p a r t i c u l a r chemical s p e c i e s such as the v a r i o u s form of p y r i t e ( f u r t h e r i n f o r m a t i o n may be found i n r e f e r e n c e s 10, 39 and 40). The i n t e n s i t y of a peak de r i v e d from a given o r b i t a l i s p r o p o r t i o n a l to photon f l u x , o r b i t a l occupancy and the photo-41 i o n i z a t i o n c r o s s - s e c t i o n of the o r b i t a l (see t a b l e 1.4) The c r o s s - s e c t i o n v a r i e s i n a complex way with a number of f a c t o r s such as the shape, s i z e and number of nodes of the 20 o r b i t a l and the energy of the i o n i z a t i o n r a d i a t i o n . Peak i n t e n s i t y may a l s o be a f f e c t e d by other processes which take 42 pl a c e , such as "shake-up" and " s h a k e - o f f " . In the former, an e l e c t r o n i s e x c i t e d to a higher l e v e l w i t h i n the molecules at an expenditure of E eV, r e s u l t i n g i n a peak E eV above the main i o n i z a t i o n peak. A "s h a k e - o f f " process i s one i n which a second e l e c t r o n i s e j e c t e d , g i v i n g r i s e to double i o n i z a t i o n . Notwithstanding these d i f f i c u l t i e s , Wend't et a l . and 43,44 Swingle have used a s i m p l i f i e d e x p r e s s i o n r e l a t i n g measured p h o t o e l e c t r o n i n t e n s i t y to fundamental parameters, f o r an i n f i n i t e l y t h i c k sample: Kj) = K 7 ( j ) N ( j ) / S ( j ) where I ( j ) i s the peak i n t e n s i t y corresponding to photo-« • ! « 3 M < J '"at '"3/a -Wjn " s f i « M / I *'\n *"3ii  M3n M S - I « ' » / » *«M-2 N M / J W J , I w 5 / i ^ H - J » ' T - J 6. , , , 8», r t S P J , 1 H 0.0002 1 Ht 0.0089 3 Li 0.059 0.0009 4 B* 0.200 0.007 5 B 0.492 0.022 • C 1.00 0.047 0.0006 0.0012 T N 1.78 0.084 0.0025 0.0049 • O 2.85 0.134 0.0073 0.0145 t r 4.28 0.199 0.0178 0.0352 10 N* 5.95 0.277 0.0381 0.0751 11 Nt 7.9* 0.390 0.0714 0.141 0.0059 12 Mf 0.525 0.121 0.239 0.0261 13 Al 0.681 0.193 0.380 0.0485 0.0012 0.0023 14 SI 0.855 0.292 0.573 0.0726 0.0050 0.0097 19 P 1.05 0.422 0.828 0.0998 0.0129 0.0253 1« S 1.25 0.590 1.155 0.130 0.0269 0.0527 IT ci 1.48 0.810 1.564 0.163 0.0493 0.0963 18 Ar 1.71 1.06 2.07 0.199 0.0503 0.161 19 K 1.96 1.37 2.67 0.249 0.122 0.238 M Ca 2.11 1.74 3.39 0.305 0.169 0.330 21 Sc 2.46 2.18 4.24 0.356 0.216 0.420 22 TI 2.72 2.68 6.22 0.408 0.268 0.521 23 V 2.98 3.28 6.33 0.462 0.326 0.633 24 Cr 3.23 3.92 7.60 0.511 0.382 0.740 25 Ma 3.48 4.63 8.99 0.575 0.460 0.891 M r# 3.70 5.43 10.5 0.634 0.535 1.04 27 Co 3.92 6.28 12.2 0.693 0.616 1.19 2« Nl 4.16 7.18 13.9 0.753 0.701 1.36 .29 Cu 4.38 8.18 15.9 0.605 0.779 1.50 30 Zn 4.55 9.29 19.0 0.873 0.882 1.70 31 C . 10.6 20.5 0.945 0.993 1.92 32 Gt 21.2 1.02 1.11 2.15 33 At 1.10 1.24 2.40 34 St 1.18 1.37 2.65 35 Br 1.26 1.50 2.92 36 Kr 1.35 1.64 3.20 3? Rb 1.43 1.79 3.48 38 Sr 1.52 1.93 3.78 39 Y 1.61 2.08. 4.09 40 Zr 1.70 2.24 4.40 41 Nb 1.79 2.39 4.71 42 Mo 1.89 2.54 5.03 43 Tc 1.98 269 5.36 44 Ru 2.07 2.64 5.68 45 Rh 2.15 3.98 6.00 0.006 0.023 0.002 0.003 0.027 0.006 0.009 0.031 0.014 0.021 0.034 0.030 0.044 0.014 0.048 0.071 0.040 0.079 0.116 0.043 0.122 0.179 0.045 0.181 0.26» 0.048 0.268 0.390 0.019 0.365 0.532 0.052 0.485 0.708 0.074 0.006 0.011 0.631 0.920 0.094 0.016 0.034 0.602 1.17 0.114 0.037 0.071 1.00 1.46 0.134 0.064 0.123 1.24 1.80 0.156 0.100 0.191 1.50 2.19 0.178 0.144 0.276 1.81 2.63 0.209 0.187 0.361 0.006 2.15 3.14 0.242 0.230 0.445 0.021 2.54 3.70 0.273 0.2SB 0.521 0.013 0.019 0025 2.97 4.33 0.305 0.307 0.596 0.035 0.051 0.029 3.45 5.01 0.333 0.340 0.661 0.082 0.118 0.013 3.97 6.77 0.364 0.379 0.739 0.130 0.187 0.014 4.54 6.60 0.397 0.419 0.818 0.192 0.278 0.015 6.17 7.51 0.429 0.460 0.899 0.269 0.387 0.016 5.84 8.48 0.463 0.501 0.981 0.365 0.524 0.016 Table 1 . 4 : P h o t o e l e c t r i c C r o s s - s e c t i o n s R e l a t i v e to C Is Level f o r Mg Ka (1253.60 eV) R a d i a t i o n I ' l l! * M « * M / 1 3 , t / l " i f l "3/1 *>3lt M W 4 . m * , m 4 » j „ 4. l 3 „ - w 4 r 5 , , « f T r t S . M B r , , , S » 3 , , W j , , M 5 / 1 V 8 „ B f T „ 6 . l r t « , , „ 9 , , , , 46 Pd 2.24 3.11 6.33 6.38 9.54 0.494 0.338 1.06 0.493 0.707 4T Al 2.33 3.25 6.64 7.36 10.7 0.331 0.366 1.13 0.616 0 184 0.018 48 Cd 2.40 3.39 6.96 8.11 11.9 0.571 0.636 1.26 0.747 1.07 0.046 49 In 2.48 3.51 7.27 9.11 13.1 0.611 0.669 1.37 0.893 1.29 0.063 0.006 SO Sn 2.54 3.61 7.58 10.1 14.6 0.653 0.743 1.48 1.03 1.31 0.076 0.017 SI Sb 1.60 3.71 7.86 11.1 18.1 0.696 0.799 1.60 1.11 1.76 0.090 0.033 52 T« 1.67 3.79 6.14 11.1 17.7 0.741 0.836 1.73 140 101 0.104 0.054 S3 1 1.75 3.87 B.3T 13.3 19.3 0.785 0.913 1.86 1.58 2.29 0.117 0.080 S4 X* 1.83 3.95 8.64 14.5 11.1 0.831 0.971 1.99 1.78 137 0.133 0.112 35 C» 1.84 4.04 8.94 13.9 11.8 0.877 1.03 2.11 1.99 188 0.131 0.140 36 Ba 4.10 9.26 17.0 14.8 0.914 1.09 1.16 1.11 3.20 0.174 0.166 3T La 4.06 9.51 16.1 16.3 0.971 1.15 3.40 2.44 3.33 0.193 0.189 36 Ce 9.67 19.7 28.6 1.00 1.18 1.49 1.58 3.74 0.069 0.088 0.189 0.180 5S Pr B.7B 11.1 30.7 1.04 1.23 1.39 1.76 4.00 0.126 0.161 0.196 0.186 60 Nd 11.6 31.9 1.07 1.26 1.70 1.93 4.28 0.200 0.157 0.203 0.191 •1 Pm » 14.3 33.1 1.10 1.30 1.91 3.14 4.35 0.296 0.378 0.108 0.196 82 Sm 16.1 37.B 1.14 1.14 1.91 3.33 4.82 0.416 0.531 0.113 0.201 63 Eu 18.1 40.9 1.17 1.37 3.01 3.31 5.09 0.362 0.716 0.119 0.105 64 Gd 14.3 43.4 1.10 1.41 3.13 3.73 341 0.693 0.887 0.233 0.123 65 Tb 10.6 1.11 1.43 3.11 3.86 3.61 0.949 1.31 0.228 0.111 66 Dy 1.13 1.43 1.30 4.03 387 1.10 1.51 0.231 0.114 61 He 1.17 1.47 3.39 4.11 6.13 1.49 1.89 0.237 0.116 6S Er 1.39 1.49 3.48 4.39 6.37 1.81 1.31 0.240 0.119 69 Tm 1.31 1.30 3.36 4.36 661 2.20 1,78 0.244 0.210 70 Yb 1.31 1.31 3.64 4.71 6.83 1.61 3.33 0.247 0.121 71 Ui 1.34 1.31 3.73 4.91 7.11 3.03 3.87 0.261 0.137 72 Hf 1.36 1.33 3.83 3.10 7.41 3.30 4.43 0.271 0.151 73 Ta 1.38 1.34 3.93 5.29 7.13 3.99 3.08 0.290 0.268 74 W 1.39 1.53 4.03 6.48 6.01 4.91 3.73 0.306 0.183 75 R» 1.41 1.53 4.13 3.67 8.30 3.08 6.46 0.321 0.299 76 Oa 1.43 1.33 4.34 3.86 8.60 3.67 7.11 0.337 0.314 77 tr 1.43 1.35 4.34 6.03 8.90 6.30 8.03 0.350 0.324 78 Pt 1.44 1.34 4.43 6.14 9.10 6.97 8.87 0.366 0.340 79 Au 1.43 1.53 4.35 6.41 9.30 7.68 9.79 0.380 0.353 80 H| 1.43 1.32 4.63 6.60 9.79 8.43 10.8 0.397 0.368 • 1 Tl 1.46 1.30 4.73 6.76 10.1 9.11 11.8 0.413 0.363 82 Pb 1.46 1.47 4.86 6.94 10.4 10.0 11.8 0.430 0.398 83 BI 1.44 1.43 4.96 7.11 10.6 10.9 14.0 0.446 0.411 0.063 0.104 0.156 0.218 0.278 0.334 0.392 0.363 0.378 0.390 0.400 0.412 0.422 0.463 0.440 0.449 0.437 0.464 0.471 0.478 0.319 0.562 0.606 0.651 0.697 0.743 0.774 0.825 0.877 0.933 0.996 1.06 1.12 0.019 0.017 0.011 0.031 0.021 0.054 0.098 0.153 0.117 0.193 0.431 0.308 0.619 0.707 0.029 0.074 0.136 0.212 0.303 0.410 0.393 0.709 0.863 0.997 0.804 1.14 0.900 1.29 0.997 1.44 0.003 0.017 0.011 0.016 0.018 0.018 0.018 0.019 0.019 0.022 0.019 0.019 0.019 0.019 0.019 0.019 0.023 0.026 0.029 0.031 0.033 0.033 0.017 0.017 0.040 0.031 0.004 0.008 0.039 0.011 0.023 0.066 0.021 0.046 ~4 Table 1.4 (cont.) 28 e l e c t r o n j , i s the e m i s s i v i t y of atom j (approximated by the atomic p h o t o e l e c t r i c c r o s s - s e c t i o n ) , N(j) i s the number of these atoms per cubic centimeter, K i s an instrument response f u n c t i o n dependent on the KE of the p h o t o e l e c t r o n and S ( j ) i s the t o t a l p h o t o e l e c t r o n s c a t t e r i n g c o e f f i c i e n t i n cm *. E l e c t r o n s c a t t e r i n g accounts f o r s u r f a c e s e n s i t i v i t y of XPS, and S ( j ) i s i n v e r s e l y r e l a t e d to the i n e l a s t i c mean o f r e e path or escape depth, A (as mention i n s e c t i o n 1.2a). Moreover A vs KE p l o t s show that i n the r e g i o n of i n t e r e s t 1/2 f o r XPS, A i s approximately a f u n c t i o n of E (see f i g 5), o which a l s o e x p l a i n s why S (or A) may not be the same f o r each component ( j ) of a s u r f a c e , because A f o r an e l e c t r o n i s determined by i t s KE. The KE of a p h o t o e l e c t r o n i s i n turn a f u n c t i o n of the BE which, of course, w i l l be d i f f e r e n t f o r each component. The K term v a r i e s approximately -1 as E f o r instruments employing f i x e d a n a l y z e r t r a n s m i s s i o n e nergies (as i n our s p ectrometer), or as E f o r those employing f i x e d r e t a r d a t i o n r a t i o s of emitted e l e c t r o n KE to a n a l y z e r pass e n e r g i e s . The former type of instrument tends to be more common and, s i n c e both K and S ( j ) are i n v e r s e l y p r o p o r t i o n a l to KE i n t h i s case, some degree of c a n c e l l a t i o n r e s u l t s i n the p revious equation, and I ( j ) w i l l c o r r e l a t e 41 ,45-47 reasonably w e l l with *y(j) and N(j) The c o n c e n t r a t i o n d i s t r i b u t i o n of a measured sp e c i e s as 29 a f u n c t i o n of depth can s e r i o u s l y a f f e c t q u a n t i t a t i v e s u r f a c e measurements, because the c o n c e n t r a t i o n of the species can vary from the outermost surface l a y e r i n t o the bulk, i . e . 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(1983) 33 CHAPTER TWO INSTRUMENTATION 2.1 INTRODUCTION In an XPS spectrometer an X-ray f l u x i r r a d i a t e s the t a r g e t sample and the e j e c t e d p h o t o e l e c t r o n s enter an e l e c t r o n analyzer to be subsequently d e t e c t e d . Since the 4 number of i o n i z a t i o n events i s so small ( t y p i c a l l y 10 / s e c ) , pulse counting techniques must be used. Pulses are counted and recorded i n a manner s u i t a b l e f o r data a n a l y s i s . An XPS spectrometer should have r e s o l u t i o n s u f f i c i e n t to r e v e a l the i n f l u e n c e of the chemical surroundings upon the energy l e v e l s , and s e n s i t i v i t y as high as p o s s i b l e ; however, as i n many forms of spectroscopy, a compromise must be reached s i n c e the two requirements c o n f l i c t . A Varian IEE-15 ("Induced E l e c t r o n Emission") s o l i d phase X-ray p h o t o e l e c t r o n spectrometer was used i n the work desc r i b e d i n t h i s t h e s i s . The spectrometer was e x t e n s i v e l y upgraded as part of t h i s work, and now f u n c t i o n s as w e l l i f not b e t t e r than when new. In t h i s chapter the spectrometer w i l l be d e s c r i b e d i n some d e t a i l . A b r i e f d e s c r i p t i o n of 1,2 the spectrometer has a l s o appeared elsewhere 34 2.2: THE VARIAN IEE-15 SPECTROMETER a) General C o n s i d e r a t i o n s A block diagram of the IEE spectrometer, i l l u s t r a t i n g the main components, i s shown i n f i g u r e 2.1. The s p e c t r o -meter system c o n t a i n s an X-ray source, a vacuum system, an e l e c t r o s t a t i c e l e c t r o n a n a l y z e r , a Varian 620/i computer, an IEE/computer i n t e r f a c e with ASR33 t e l e t y p e w r i t e r , an X-Y r e c o r d e r , and an o s c i l l o s c o p e f o r s p e c t r a l d i s p l a y . Most of the c o n s t i t u e n t parts of the spectrometer (X-ray anode, e l e c t r o n m u l t i p l i e r , i on and e l e c t r o n sources) w i l l operate only i n high vacuum. Moreover, as mentioned before, the high s u r f a c e s e n s i t i v i t y of XPS r e q u i r e s as good a vacuum as p o s s i b l e i n order to maintain a c l e a n sample -9 s u r f a c e . However even at 10 t o r r , exposure to a high 3 vacuum system f o r 1 h w i l l approximate monolayer coverage . F o r t u n a t e l y , i f the sample i s not s u s c e p t i b l e to degradation by small amounts of r e s i d u a l gases such as oxygen, water, -6 vacuum pump f l u i d vapours, a vacuum i n the range of 10 -8 10 t o r r may be t o l e r a t e d . The normal background pressure -7 i n the present IEE spectrometer i s about 1x10 t o r r , and i s maintained by employing two pumping systems and baking the chambers. Since a bakeable valve i s used to separate the sample p r e p a r a t i o n ( i n l e t ) chamber from the a n a l y z e r chamber, these two regions are pumped s e p a r a t e l y by o i l d i f f u s i o n pumps (285 L/sec f o r the former and 1500 L/sec f o r the 35 R E T A R D I N G V O L T A G E I N T E R F A C E C O M P U T E R AND INTERFACE TTY S C O P E RE1ARDING V O L T A G E P O W E R S U P P L Y P U L S E P R E - A M P C O U N T E R R E C O R D E R S A M P L E INLET A N A L Y Z E R C O L O TRAP MECHICAL PUMP DIFFUSION PUMP IONIZATION G A U G E H A S T I N G G A U G E VALVE ® VALVE F O C U S C O N T R O L C O TRJ LD OIFFI PU ISION MP G A U G E C O N T R O L M E C H A N I C A L P U M P F i g u r e 2.1: Block Diagram of the Va r i a n IEE-15 XPS Spectrometer 36 l a t t e r ) , f i t t e d with b a f f l e s and water c o l d t r a p s to minimize backstreaming of pump o i l i n t o the system. Each d i f f u s i o n pump i s backed by a r o t a r y pump, and i n a d d i t i o n , a l i q u i d n i t r o g e n - c o o l e d trap i s f i t t e d before each chamber to condense water and pump o i l vapors, the two major p o t e n t i a l contaminants. Moreover, the sample can be changed without a f f e c t i n g c o n d i t i o n s i n the a n a l y z e r . The vacuum lock through which sample t r a n s f e r r a l takes place must not be - 5 opened u n t i l the p r e p a r a t i o n chamber i s below 10 t o r r , otherwise rupture of the d e l i c a t e aluminum f o i l window (7A»«0 would occur. T h i s t h i n window i s used to separate the X-ray tube and sample; moreover, i t a l s o prevents low energy r a d i a t i o n from causing u n d e s i r a b l e e f f e c t s i n the sample, and s c a t t e r e d e l e c t r o n s from the X-ray gun reaching the 4 sample . As mentioned p r e v i o u s l y , low energy e l e c t r o n s from the i n s i d e of the window serve to n e u t r a l i z e sample c h a r g i n g . A f r a c t i o n of the p h o t o e l e c t r o n s produced from the sample pass through the annular entrance s l i t i n t o the s p h e r i c a l a n a l y z e r , e s s e n t i a l l y two c o n c e n t r i c spheres with a p o t e n t i a l d i f f e r e n c e between them. The e l e c t r o n image of the entrance s l i t i s at the annular e x i t s l i t , e l e c t r o n s passing through being focussed on the aperture of a 5 b e r i l l i u m - c o p p e r e l e c t r o n m u l t i p l i e r (see f i g . 2 . 2 ) . T y p i c a l l y , a p o s i t i v e v o l t a g e i s a p p l i e d to the sample so that i n i t i a l p h o t o e l e c t r o n s are retarded to 100 eV upon passing through the entrance s l i t . The geometry of the 37 F i g u r e 2.2: Schematic zer of the IEE-15 Spectrometer Analy-38 retarded region does not appear to be c r i t i c a l as long as a l l e l e c t r o n s e n t e r i n g the anal y z e r o r i g i n a t e from a s t r o n g l y i r r a d i a t e d p o r t i o n of sample s u r f a c e . However, the ph o t o e l e c t r o n s ' paths are c o n s t r a i n e d to prevent second order a b e r r a t i o n s from degrading the r e s o l u t i o n of the a n a l y z e r . T h e r e f o r e , at a given r e s o l u t i o n , the ph o t o e l e c t r o n s ' t r a n s m i s s i o n i s l i m i t e d by the s l i t - w i d t h and second order angular a b e r r a t i o n s . The e f f e c t of angular a b e r r a t i o n i s most e a s i l y v i s u a l i z e d as i n the case of throwing a b a l l i n a uniform g r a v i t a t i o n a l f i e l d . The b a l l w i l l t r a v e l the maximum h o r i z o n t a l d i s t a n c e when i t i s e launched at 45 . I f S i s the d i s t a n c e thrown, and S Q i s the maximum d i s t a n c e then: 2 S - S 2S ( 0 - TT/4 ) o o S Q i s a f i r s t order f o c a l point with second order a b e r r a t i o n s . T h i s i s a general e x p r e s s i o n v a l i d f o r f o c u s s i n g of charged 1,2 p a r t i c l e s i n e l e c t r i c and magnetic f i e l d s . An XPS spectrum i s generated by va r y i n g the voltage a p p l i e d to the sample, with respect to the an a l y z e r entrance s l i t . T h i s has the advantage that the anal y z e r d e f l e c t i o n v o l t a g e s remain f i x e d and the r e s o l u t i o n of the an a l y z e r remains constant throughout a scan. One can improve the ana l y z e r r e s o l u t i o n by reducing the energy at which the e l e c t r o n s are analyzed. Nominally, e l e c t r o n s are analyzed at an energy of 100 eV, and with the spectrometer designed 39 6 fo r 1% r e s o l u t i o n , we commonly obtain a t o t a l spectrometer r e s o l u t i o n f o r the Au 4f doublet of ~1 . 7 eV f u l l width 7/2,5/2 h a l f maximum ( i n c l u d i n g the Mg Ka r a d i a t i o n l i n e w i d t h of 7 about 0.7 eV , f i g u r e 2.3). With an X-ray power output of 280 watts, an e l e c t r o n count r a t e of >10,000 counts/sec can be obtained at t h i s peak maximum. In order to o b t a i n a p e r f e c t r e g i s t r y of the image of the annular entrance s l i t onto the annular e x i t s l i t , i t i s necessary to use an e l e c t r o s t a t i c octopole focus c o r r e c t o r to adjust the p o s i t i o n and shape of the f i n a l e l e c t r o n image (see f i g . 2.2). In summary, the spectrometer has the f o l l o w i n g f e a t u r e s : 1. S p h e r i c a l a n a l y z e r 2. E l e c t r o n r e t a r d a t i o n between sample and analyzer 3. E l e c t r o n i c a l l y v a r i a b l e r e s o l u t i o n 4. Annular source and d e t e c t o r s l i t s 5. Annular X-ray tube (to be discu s s e d i n the next s e c t i o n ) 6. Octopole focus adjustment 7. C y l i n d r i c a l e l e c t r o n f i l t e r between an a l y z e r and de t e c t o r Since e l e c t r o n paths are i n f l u e n c e d by magnetic f i e l d s , i t i s necessary that the e l e c t r o n t r a j e c t o r y be determined only by the f i e l d of the a n a l y z e r , and so the earth's magnetic f i e l d (~500 mG) i n the v i c i n i t y of an e l e c t r o n spectrometer must be n u l l i f i e d . T h i s may be done by using 8 9 Helmholtz c o i l s or ferromagnetic s h i e l d i n g m a t e r i a l s ; 40 F i g u r e 2.3: XPS Spectrum of the 4f Gold Doublet 7/2,5/2 41 however, a b a s i c Helmholtz system compensates only f o r s t a t i c and not f o r v a r y i n g magnetic f i e l d s . In p r a c t i c e , f l u c t u a t i o n s i n f i e l d can be observed i n the 2-10 mG region 3 depending upon time of day and day of the week. Therefore the e l e c t r o s t a t i c a n a l y z e r used i n the IEE spectrometer i s s h i e l d e d by a double l a y e r of mu-metal, about 1 foot i n diameter and A f e e t high, which reduces the earth's magnetic f i e l d t o ~ 0 . 3 mG at the spectrometer c e n t e r . b) The X-ray Source An i d e a l X-ray source should provides an i n t e n s e , constant l e v e l of sample r a d i a t i o n . The simplest type f o r use i n XPS i s one which u t i l i z e s c h a r a c t e r i s t i c emission l i n e s from an anode bombarded by high energy e l e c t r o n s . The energies of these l i n e s are dependent only on the anode m a t e r i a l . In a d d i t i o n to these c h a r a c t e r i s t i c l i n e s , a continuous spectrum dependent upon the primary e l e c t r o n energy (Bremsstrahlung) i s a l s o produced from the X-ray tube, which i n c r e a s e s the background l e v e l of the p h o t o e l e c t r o n spectrum. However, a more s e r i o u s problem concerns X-ray s a t e l l i t e l i n e s . The most commonly used X-ray l i n e s i n XPS are the A l and Mg Ka^ ^ emission l i n e s , which have l i n e w i d t h s of 0.85 eV and 0.7 eV and e n e r g i e s of 1486.60 eV and 1253.60 eV r e s p e c t i v e l y . The K« l i n e i s i »^ e s s e n t i a l l y an unresolved doublet r e s u l t i n g from 2p 3/2,1/2 42 to Is t r a n s i t i o n s ; however, l i n e s r e s u l t i n g from s i m i l a r 10 t r a n s i t i o n s w i t h i n m u l t i p l y i o n i z e d atoms (Ko_,Ko ) and 3 4 from valence band to Is t r a n s i t i o n (K/3) are a l s o present. In the case of A l (and Mg) there are two s a t e l l i t e s about 10 eV to the high energy s i d e of the main X-ray l i n e which are produced by t r a n s i t i o n s i n v o l v i n g the K hole of a doubly i o n i z e d atom. In XPS s p e c t r a these Ka and Ka s a t e l l i t e s 3 4 can i n t e r f e r e and obscure weak peaks. As mentioned before, one of the primary l i m i t i o n s on i n s t r u m e n t a l r e s o l u t i o n i s the X-ray bandwidth; the narrower t h i s i s the more accurate w i l l be the i n f o r m a t i o n o b t a i n e d . Hence, when c o n s i d e r i n g the choice of a s u i t a b l e X-ray l i n e , l i n e w i d t h i s always a major c o n s i d e r a t i o n , the other being the energy i t s e l f s i n c e t h i s determines the a c t u a l atomic o r b i t a l s which can be i o n i z e d . Recently, there has been c o n s i d e r a b l e i n t e r e s t i n the use of X-ray sources other than the t r a d i t i o n a l Mg and Al Ka, s i n c e a l t e r n a t i o n of source e n e r g i e s would i n c r e a s e the f l e x i b i l i t y of the technique. Obviously harder X-ray sources make deeper core l e v e l s a c c e s s i b l e ; however,although more ph o t o e l e c t r o n peaks would by obtained, the X-ray l i n e w i d t h i n c r e a s e s so there i s some r e d u c t i o n i n accuracy i n d e t e r -minating peak p o s i t i o n s . Some c h a r a c t e r i s t i c X-ray energ i e s and l i n e w i d t h s of the sources that have been used i n XPS 7,11 are given i n t a b l e 2.1. These represent a wide range of energies a v a i l a b l e to photoeject core e l e c t r o n s of most elements of i n t e r e s t . The comparative broadness of the X-43 Line Be Y Zr No Mo Ru Rh C Ti Ti O a K M{ M{ MC M ; MC M{ K U L a K L a Energy (eV) 108.9 132.3 151.4 171.4 192.3 236.9 260.1 278 395.3 452.2 524.9 572.8 Width (eV) 5.0 0.47 0.77 1.21 1.53 2.49 4.0 6 3 3 4 3 Line Ne Ni Cu Zn Na Mg A l Zr Ti Cr Cu K a La La L a K a K a K a La K a K a K a Energy (eV) 849 851.5 929.71011.71041.01253.61486.6 2042 4510 5417 8048 Width (eV) 0.3 2.5 3.8 2.0 0.42 0.7 0.85 1.7 2.0 2.1 2.6 Table 2 . 1 : Some X-ray L i n e s Used i n XPS 44 ray sources, together with t h e i r attendant s a t e l l i t e s (K«^ ^  and K0) i s u n d e s i r a b l e , because they overlap with other f e a t u r e s i n the spectrum. In modern commercial instruments, the source i s o f t e n monochromatized using a quartz c r y s t a l d i f f r a c t i o n g r a t i n g to y i e l d a narrow X-ray l i n e devoid of 6 s a t e l l i t e s . T y p i c a l l y the A l Ka l i n e may be reduced from 0.85 to 0.6 eV FWHM. The X-ray f l u x i s u s u a l l y somewhat reduced. As a r e s u l t , the monochromated source i s used only i n circumstances where s u f f i c i e n t b e n e f i t i s d e r i v e d from i t s narrow l i n e w i d t h to o f f s e t the r e d u c t i o n i n s e n s i t i v i t y . Synchrotron r a d i a t i o n i s a l s o a v a i l a b l e and g i v e s high count r a t e s . T h i s r a d i a t i o n i s produced by e l e c t r o n s a c c e l e r a t e d i n a r i n g under s t a t i c magnetic f i e l d c o n d i t i o n s , y i e l d i n g a 12 tunable continuous r a d i a t i o n of high i n t e n s i t y . Obviously a synchrotron f a c i l i t y i s l a r g e and expensive, but many s u c c e s s f u l XPS experiments have been done at S t a n f o r d , Madison, e t c . The X-ray source used i n t h i s study i s shown i n f i g . 2.4. E l e c t r o n s produced by thermionic emission from a hot annular tungsten f i l a m e n t (0.18mm diameter) s t r i k e the anode of the annular X-ray tube, c r e a t i n g Mg Ka X - r a d i a t i o n at a photon energy of 1253.60 eV. S e v e r a l supports are provided so the f i l a m e n t does not d i s t o r t upon h e a t i n g . The anode i s copper, vacuum-coated with approximately 0.3 cm of high--7 grade magnesium. The X-ray tube i s maintained at ~ 2 x l 0 t o r r under t y p i c a l o p e r a t i n g c o n d i t i o n s i n order to prevent vo l t a g e breakdown, and ..to i n c r e a s e f i l a m e n t l i f e t i m e . 45 F i g u r e 2.4: The X-ray Tube Assembly 46 Disadvantages i n the t r a d i t i o n a l X-ray design are that f a s t e l a s t i c a l l y s c a t t e r e d e l e c t r o n s from the anode can s t r i k e the aluminum window, and the l i n e - o f - s i g h t design r e s u l t s i n contamination of the anode by evaporation of the fi l a m e n t m a t e r i a l . The l a t t e r f a c t o r c l e a r l y r e s u l t s i n a t t e n t u a t i o n of X-ray f l u x and i n t r o d u c e s impurity l i n e s . The former can generate Al Ka X-rays and secondary e l e c t r o n s i n the window which may then i r r a d i a t e the sample. The presence of Al Ka i s a disadvantage i f an Mg anode i s i n use; a l s o secondary e l e c t r o n s can cause sample 6 degradation . F o r t u n a t e l y some of these e l e c t r o n s can be thermalized to reduce sample charging (mentioned above). To bypass such problems a p o s i t i v e p o t e n t i a l (7-10 kV) i s a p p l i e d to the anode while the f i l a m e n t , maintained at ground p o t e n t i a l , i s placed out of d i r e c t l i n e - o f - s i g h t of 13 the anode . The p o s i t i v e p o t e n t i a l ensures that s c a t t e r e d e l e c t r o n s are drawn back towards the anode and do not s t r i k e the window. Since the anode cannot "see" the cathode, a c y l i n d r i c a l s t a i n l e s s s t e e l s h i e l d between the anode tubing and the f i l a m e n t i s in t r o d u c e d to focus the e l e c t r o n beam onto the face of the anode. To prevent d e s t r u c t i o n of the aluminum window and the anode i t s e l f by o v e r h e a t i n g , the X-ray tube i s water-cooled using e l e c t r i c a l l y i n s u l a t e d p i p e s , and the anode i t s e l f i s i n s u l a t e d from ground by ceramic/metal s e a l s . Anode p o t e n t i a l s between 7 to 10 kV give e f f i c i e n t production of 47 c h a r a c t e r i s t i c r a d i a t i o n from Mg - o p e r a t i o n of the X-ray gun below 5 kV i s avoided s i n c e space charge around the f i l a m e n t would l i m i t the e l e c t r o n c u r r e n t . c) The Sample In p r i n c i p l e , any type of sample, i n c l u d i n g gases and 14 l i q u i d s , may be s t u d i e d by XPS. However, the i n s t r u m e n t a l design of the IEE-15 spectrometer r e s t r i c t s us to low vapour -6 pressure (<10 t o r r ) s o l i d s . The sample i s i n s e r t e d through a vacuum lock and i t s temperature can be cooled by l i q u i d n i t r o g e n f l o w i n g i n the i n t e r i o r of the probe. Use of such a. probe permits a wide range of m a t e r i a l s to be i n v e s t i g a t e d . The sample s i z e must be of diameter <1 cm; however, the spectrometer can accommodate three samples simultaneously on the sample h o l d e r . The sample must not, 15,16 of course, decompose under high vacuum 7 Methods of sample mounting are numerous , and are o f t e n d i c t a t e d by the nature of the sample i t s e l f . Bulk s o l i d samples may be held i n a c l i p . Powdered samples are f i n e l y ground ( p e s t l e / m o r t a r ) and spread on s i n g l e - s i d e d adhesive " s c o t c h " tape attached to the probe with s t i c k y s i d e out. T h i s method i s widely used but leads to a longer recovery time f o r the pressure i n the sample chamber because of outgassing of the adhesive tape. E i t h e r method can be used f o r f i b r e samples. 48 I t i s worth n o t i n g , with regard to the sample i n s e r t i o n mechanism, that i f a cooled probe i s being used a t e f l o n 'O'ring c o n t r a c t s and may cause u n d e s i r a b l e movement of the probe, p o s s i b l y causing l o s s of vacuum. d) Data C o l l e c t i o n An i n t r i n s i c problem i n t h i s type of spectrometer i s the low signa1/background (S/B) count r a t i o . The background i s mainly due to the continuum r a d i a t i o n produced by d e c e l e r a t i o n of the primary e l e c t r o n s on h i t t i n g the X-ray t a r g e t (Bremsstrahlung r a d i a t i o n ) . The r a t i o may be improved by X-ray monochromatization but t h i s o ption i s not a v a i l a b l e f o r the Varian instrument. The computer i s programmed to c o n t r o l most of the spectrometer f u n c t i o n s . The program w i l l time.average up to 2000 channels of data i n up to 10 d i s c r e t e r e g i o n s . D i f f e r e n t combinations of number of channels, scan time (1-1000 s e c ) , scan width (1-100 eV) and number of scans (1-999) allow optimum choice of parameters. Increase of source i n t e n s i t y w i l l r e s u l t i n improving data only to a p o i n t , even i f i n c r e a s e d source i n t e n s i t y does not induce sample decomposition, s i n c e high source i n t e n s i t y i s accompanied by an r e a l i n c r e a s e i n background. The S/B r a t i o can be improved by accumulation- of data over a period of time with 49 m u l t i p l e scanning. Random noise should c a n c e l , while r e a l s i g n a l s r e i n f o r c e . When the counting s t a t i s t i c s are s u f f i c i e n t l y good, accumulation of data may be terminated and the data stored f o r l a t e r use. T h i s type of s i g n a l enhancement i s p a r t i c u l a r l y important when one i s d e a l i n g with weak s i g n a l s , such as from f r a c t i o n a l monolayers on a s u r f a c e . Moreover, the data may be smoothed by l e a s t squares 6 f i t t i n g to polynomials before being p l o t t e d . 50 REFERENCES 1. J.C. Helmer and N.H. Weichert, App. Phys. L e t t s . , 13(8), 266 (1968) 2. J.C. Helmer and N.H. Weichert, "Advances i n X-ray A n a l y s i s " , Plenum, N.Y. (1970) 3. D.M. Hercules and S.H. Hercules, J . Chem. Edc., 61(5), 402 (1984) 4. D.M. Hercules, Anal Chem., 42(1), 20A (1970) 5. E.M. P u r c e l l , Phys. Rev., 54, 818 (1938) 6. IEE System Manual, P u b l i c a t i o n * 87-121-000, Varian Instrumental D i v i s i o n (1971) 7. D. B r i g g s , "Handbook of X-ray and U l t r a v i o l e t P h o t o e l e c t -ron Spectroscopy",Heyden, London (1977) 8. C.K. Jorgensen, Chimia, 25, 213 (1971) 9. W. Bermer and F. Linneman, Chem Ztg., 95, 1011 (1971) 10. Z. Horak, Proc. Phys. Soc. London, 77, 980 (1961) 11. K. Yated and R.H. West, Surf. I n t e r f a c e Anal., 5, 133 (1983) 12. M.T. Anthony, Chem. i n B r i t a i n , Jan., 33 (1986) 13. B.L. Henke, J . Adv. X-ray Anal., 4, 244 (1961) 14. K. Siegbahn e t . a l . , "ESCA-Atomic, Molecular, and S o l i d State S t r u c t u r e s Studied by Means of E l e c t r o n S pectro-scopy", Nova Acta Regiae Soc. S c i . Uppsala, 28 (1967) 15. S.C. Avanzino and W.L. J o l l y , J . Am. Chem. Soc., 100, 2228 (1978) 16. P.K. Chan, D.C. F r o s t and K. Venkateswaran, S p e c t r o s c . L e t t s . , 19(9), 1071 (1986) 51 CHAPTER THREE THE DEPENDENCY OF PYRITE OXIDATION IN COAL ON PARTICLE SIZE 3.1 : INTRODUCTION Coal i s a complex m a t e r i a l of v a r i a b l e composition on both the macroscopic and mi c r o s c o p i c l e v e l s , and of t e n c o n t a i n s s i g n i f i c a n t q u a n t i t i e s of s u l f u r i n the form of a s s o c i a t e d minerals such as p y r i t e s and /or s u l f u r i n 1 organic combination as part of the c o a l substance . The major elements present i n c o a l are carbon, hydrogen and oxygen, with minor q u a n t i t i e s of s u l f u r , s i l i c o n , n i t r o g e n , aluminum, i r o n , c alcium, magnesium, potassium, sodium and t i t a n i u m , e t c . Carbon i s by f a r the major element present, and combined with smaller amounts of hydrogen and oxygen provides the s t r u c t u r a l i n t e g r i t y and most of the heat content and reducing power of the f u e l . Despite the obvious s t r u c t u r a l c o m p l e x i t i e s and composition v a r i a b l e s , there has been c o n s i d e r a b l e s p e c u l a t i o n concerning the s t r u c t u r e of what might be c a l l e d a " t y p i c a l c o a l molecule". For 2 example, Sheer has proposed a complex s t r u c t u r e ( f i g . 3 . 1 ) i n which aromatic and a l i p h a t i c u n i t s are l i n k e d together i n the b a s i c c o a l s t r u c t u r e . 52 F i g u r e 3.1: A Proposed S t r u c t u r e f o r Coal 53 Coal i s a resource c e r t a i n to be a v a i l a b l e to meet f u t u r e energy demands f o r some c o n s i d e r a b l e time; however, the major cause of a i r p o l l u t i o n has been shown to a r i s e from combustion of f o s s i l f u e l to form s u l f u r oxides harmful to both i n d u s t r y and the ecosystem. Moreover, s u l f u r i s a p o t e n t i a l c o n t r i b u t o r to spontaneous combustion of c o a l s i n 3 mines and s t o c k p i l e s and the p o l l u t i o n of ground water 1 f l o w i n g from or through c o a l mines . G e n e r a l l y , the higher the s u l f u r content the worse the problems, which has l e d to government l e g i s l a t i o n imposing s t r i n g e n t c o n t r o l s on c o a l u t i l i z a t i o n . C e r t a i n l y , a b e t t e r understanding of the nature and d i s t r i b u t i o n of s u l f u r - b e a r i n g minerals and organic s u l f u r -c o n t a i n i n g f u n c t i o n a l groups and t h e i r concomitant chemistry w i l l have a d e s i r a b l e impact on c o a l p r e p a r a t i o n processes and the environmental a c c e p t a b i l i t y of c o a l - d e r i v e d energy. Current ASTM p r a c t i c e i s to analyze f o r v a r i o u s forms of 4,5 s u l f u r i n c o a l such as o r g a n i c , p y r i t i c , and s u l f a t i c Determination of organic s u l f u r i s c a r r i e d out by a s u b t r a c t i o n technique, which, being i n d i r e c t , tends to accumulate e r r o r s i n v o l v e d i n the i n o r g a n i c analyses 8 (themselves somewhat u n r e l i a b l e and t e d i o u s ) . C l e a r l y , an i n s t r u m e n t a l method g i v i n g a d i r e c t q u a n t i t a t i v e i n d i c a t i o n of c o a l - a s s o c i a t e d s u l f u r would be most d e s i r a b l e . Comparatively few XPS s t u d i e s have been reported r e l a t -8-18 12-18 ing to geochemical m a t e r i a l s and c o a l s c i e n c e , 54 which i s s u r p r i s i n g s i n c e the p r o p e r t i e s of c o a l s u r f a c e s play such an important r o l e i n a number of c o a l u t i l i z a t -ion t e c h n o l o g i e s . F l o t a t i o n , c o a l - l i q u i d mixing and coking p r o p e r t i e s a l l depend on the chemical nature of c o a l 18 s u r f a c e s . Since i n i t i a l i n t e r a c t i o n of a s o l i d with i t s environment occurs v i a i t s s u r f a c e , XPS has obvious advantages. Of the a v a i l a b l e methods f o r c o n t r o l l i n g s u l f u r oxide emissions, the p h y s i c a l removal of p y r i t i c s u l f u r p r i o r to combustion i s the most t e c h n o l o g i c a l l y developed and c o s t -19 e f f e c t i v e . XPS has been employed i n our l a b o r a t o r y 12 before , to study the s u l f u r atoms' molecular environment i n v a r i o u s c o a l s . Since XPS S 2p peaks from the probable 3/2 c o a l - o r g a n i c s u l f i d e - or t h i o l - (R-S-R' ,-SH), d i s u l f i d e -(R-S-S-R'), or thiophene- ( R — — R ' ) type s u l f u r m o i e t i e s s 12,13 can be expected to overlap with the p y r i t e S 2p peak 3/2 f u r t h e r study suggested that s u r f a c e p y r i t e could be separated by o x i d a t i o n to r e v e a l and d i r e c t l y measure the 17 organic s u l f u r present . C o a l - c l e a n i n g methods are normally capable of removing only p y r i t e , and not o r g a n i c a l l y bonded s u l f u r , s i n c e only p y r i t e may be o x i d i z e d to s o l u b l e 12 s u l f a t i c s a l t . T h e r e f o r e , one of the important f a c t o r s i n p y r i t e e l i m i n a t i o n would appear to i n v o l v e c r u s h i n g the c o a l to the p o i n t where su r f a c e p y r i t e maximizes, which our experiments i n d i c a t e i t to do. We c a l l t h i s the " c h a r a c t -e r i s t i c " s i z e , and r e f e r to i t l a t e r . 55 3.2: EXPERIMENTAL Experimental c o n d i t i o n s are as d e s c r i b e d i n Chapter 2. Bulk Minto c o a l samples as r e c e i v e d from Chemical En g i n e e r i n g UBC were crushed very gently before s i e v e -s c r e e n i n g , to produce p a r t i c l e s ranging from 53 to 250 ttm in diameter, which were immediately placed i n s i d e a vacuum d e s i c c a t o r u n t i l a n a l y s i s . Some samples were o x i d i z e d by * using the "Blue M" c o n t r o l l e d temperature and humidity o chamber, under 99+1 % r e l a t i v e humidity at 45 C f o r two d i f f e r e n t i n t e r v a l s (3 and 15 h) to see i f p y r i t e o x i d a t i o n was time dependent. Table 3.1 i n d i c a t e s the a p p r o p r i a t e s e t t i n g s of wet and dry bulb temperatures f o r a p a r t i c u l a r r e l a t i v e humidity. Samples were mounted on the XPS sample holder before o x i d a t i o n and subsequent analysed i n the IEE-15. The XPS S 2p and C Is peak he i g h t s were used to 3/2 c a l c u l a t e the r a t i o s of p y r i t e to s u l f a t e , p y r i t e to carbon, and s u l f a t e to carbon. BE's were accurate to +0.2 eV. R e l a t i v e l y low X-ray i n t e n s i t i e s (280 watts) i n ex-posures of about 1 h were used to minimize r a d i a t i o n damage, and i n f a c t no evidence f o r sample d e t e r i o r a t i o n was d e t e c t e d . For a l l c o a l samples, spectrometer s e t t i n g s were maintained constant i n order to remove spectrometer * Manufacturer: Blue M E l e c t r i c Co., Blue I s l a n d , I l l i n o i s , USA 56 Dry Dlffcri met B*tw*tn Kaadlt if* al Wat ai ai Dry Bolba to DtfTMa Caattjnta Dry •alb - -Mb 1MB. - Da f. C * O J IJO 1.3 1.0 1.3 3.0 13 44 4.5 5 6 7 8 9 10 11 12 11 14 13 16 19 20 22 24 26 21 M 12 14 M 18 4 0 «• 2 92 83 73 97 59 32 43 36 27 20 2 4 93 85 77 70 63 36 4 8 41 34 28 13 4 e 94 87 80 73 6A 60 54 47 41 33 23 11 • • 94 87 81 74 68 82 56 60 43 39 28 17 • 10 94 88 82 76 71 63 60 54 49 44 34 23 14 10 12 94 89 84 78 73 68 63 58 33 48 38 30 21 12 4 II 14 93 90 84 79 74 69 65 00 53 81 41 33 24 16 10 14 1« 95 90 83 SI 70 71 67 62 58 54 43 37 29 21 14 7 16 IS 85 90 66 82 78 73 68 63 61 57 48 42 35 27 20 13 6 19 20 96 91 87 82 78 74 70 66 62 68 61 44 36 30 23 17 11 20 21 96 92 87 83 79 73 72 68 64 60 S3 46 40 34 27 21 16 11 23 24 96 92 88 83 81 77 74 70 66 63 56 49 43 87 31 2 6 21 14 10 14 26 96 92 89 85 81 77 74 71 67 64 57 61 45 39 34 28 23 18 13 36 26 9fl 92 89 83 82 78 73 72 68 63 59 53 47 42 37 31 20 21 17 13 38 W 96 93 89 86 82 79 76 73 70 67 61 55 SO 4 4 39 33 30 24 30 16 12 80 12 96 93 90 86 83 80 77 74 71 68 62 56 51 48 41 38 32 27 23 19 IS 13 M 97 93 90 87 84 81 77 74 71 69 63 58 63 48 43 38 34 30 26 22 18 10 34 M 87 83 90 87 64 81 78 73 72 70 64 59 54 50 45 41 36 32 28 24 21 13 M 16 97 94 90 87 84 81 79 75 73 70 63 60 56 51 46 42 38 34 30 26 23 10 10 38 40 97 94 91 86 65 62 79 76 74 71 66 61 37 32 48 44 40 16 32 29 25 19 13 40 42 97 94 91 68 SS 82 80 77 74 72 67 62 58 53 48 45 41 38 34 31 27 21 15 42 44 97 94 91 88 B6 83 80 77 75 73 68 63 59 54 so 47 43 39 36 32 26 23 17 12 44 46 97 94 91 89 86 83 "81 78 76 73 68 64 60 65 32 48 44 4 1 37 34 3 1 25 19 14 46 46 87 94 92 89 86 84 81 78 76 74 69 63 61 56 S3 49 45 42 39 35 33 27 21 16 12 48 10 97 64 92 89 87 84 62 79 77 73 70 65 62 57 54 60 47 43 40 37 34 28 23 18 14 SO 12 97 94 92 89 87 84 82 79 77 73 70 66 62 59 S3 61 4B 44 41 38 35 30 25 30 16 11 81 •4 97 95 82 80 87 85 82 80 78 76 71 67 63 58 56 52 49 45 42 39 3 6 31 2 6 21 17 13 84 56 97 95 92 90 87 63 83 80 78 76 72 68 64 60 57 5.1 SO 40 43 40 3 8 32 27 23 19 15 11 36 IS 97 95 93 90 88 83 63 80 79 77 72 68 64 61 57 54 51 47 44 42 39 33 28 24 20 16 12 86 60 68 95 93 90 88 84 83 81 79 77 73 68 65 62 68 55 52 48 45 43 40 33 30 26 21 18 14 II 60 62 98 95 93 91 88 86 84 81 79 78 73 69 68 62 SO 66 53 49 4 6 43 41 30 31 27 23 19 15 12 •2 64 98 95 93 81 68 86 84 82 80 78 74 70 66 63 50 56 63 50 47 4« 42 37 32 28 24 20 17 11 64 66 98 95 93 91 89 86 84 82 80 78 74 70 67 64 60 57 SI 51 48 45 43 3S 33 29 23 21 18 15 12 66 46 98 95 93 91 89 87 85 82 81 79 75 71 67 64 61 5S 55 52 49 40 44 39 34 30 28 22 19 16 13 68 TO 98 96 93 91 88 87 83 83 81 78 75 71 68 63 61 58 55 52 50 47 4 4 40 35 31 27 23 30 17 14 II 70 T2 68 96 94 92 89 87 83 83 81 80 76 72 69 65 62 59 56 51 so 48 43 40 36 32 28 24 21 18 15 12 71 T4 98 96 94 92 90 87 83 83 82 80 78 72 69 66 63 60 57 54 51 4 S 4 6 41 37 33 29 25 22 19 16 13 11 74 T« 99 96 94 92 90 88 86 84 82 80 76 73 70 66 63 60 57 51 52 4 9 47 42 38 34 30 26 23 20 17 14 12 76 T» 98 9C 94 92 90 88 66 84 82 81 77 73 70 67 64 01 SS 53 32 50 47 43 38 34 30 27 24 21 18 15 13 10 78 60 98 96 94 92 90 68 86 84 83 81 77 74 71 67 64 61 SR 50 53 SO 48 4.1 19 35 31 28 24 22 19 16 14 11 80 62 98 96 94 92 90 88 86 84 63 81 77 74 71 SM 65 62 SO 50 54 51 40 4 4 40 36 32 29 25 22 20 17 15 12 0 82 64 98 96 94 92 90 88 66 83 83 81 78 74 71 68 6 5 62 59 s r 54 52 48 4 5 40 37 33 29 26 23 20 18 16 13 1 84 66 98 96 94 92 91 89 87 85 83 82 78 75 72 eu 60 63 60 3 7 55 52 JO 4 3 41 37 34 30 27 34 21 19 16 14 2 86 66 98 96 95 93 91 80 87 83 83 82 78 75 72 69 60 63 B D 5S 53 53 51 4 f i 42 38 34 31 2n 25 22 18 17 15 3 68 60 98 97 93 93 91 89 87 B5 84 82 78 76 73 69 67 64 6 1 68 56 53 51 47 42 39 35 32 28 26 23 20 18 16 4 90 62 98 97 95 93 91 89 87 86 84 82 79 76 73 70 67 64 61 50 50 54 52 47 43 38 36 32 28 26 24 21 19 16 4 92 94 99 97 95 83 91 89 88 86 84 83 79 76 73 70 6 7 65 62 59 57 54 62 48 4 4 40 36 33 30 27 24 22 19 17 5 94 •6 99 97 95 93 91 90 88 86 84 S3 80 76 74 70 61 6 5 62 60 57 55 5.1 48 4 4 41 37 34 31 28 23 22 20 18 16 96 96 99 97 93 93 92 90 88 86 83 83 80 77 74 71 6 8 65 63 60 5S 3 3 S3 49 45 41 38 34 31 28 26 23 21 IB 16 98 100 99 97 93 93 92 90 88 86 85 83 80 77 74 71 68 6G 63 60 58 56 54 45 42 38 35 32 29 26 24 22 19 17 109 Table 3.1: Centigrade Table of R e l a t i v e Humidity 57 v a r i a t i o n s from the analyses as much as p o s s i b l e . Moreover, a l l BE determinations were referenced to the C Is peak from g r a p h i t e at 284 eV ( f i g . 3.2). 3.3: RESULTS AND DISCUSSION T y p i c a l C Is and S 2p s p e c t r a from "3M" Scotch tape are shown i n f i g u r e 3.3. T h i s C Is BE i s s h i f t e d from that of the carbon i n c o a l or g r a p h i t e , r e f l e c t i n g the d i f f e r e n t chemical environments of carbon i n these m a t e r i a l s . Carbon i s d i f f i c u l t to study by XPS because of the high c o n c e n t r a t i o n of carbon present i n the adhesive tape and the p o s s i b i l i t y of contamination of sample s u r f a c e s by hydrocarbon present i n the pumping system. To minimize background carbon, the l i q u i d n i t r o g e n traps are kept f i l l e d at a l l times, the adhesive tape should be completely covered with sample, and a l l samples should be scanned f o r carbon at the same time i n t e r v a l a f t e r the i n t r o d u c t i o n i n t o the spectrometer. T h i s procedure does permit a r e l a t i v e c a l i b r a t i o n procedure as i l l u s t r a t e d i n f i g u r e 3.4. S u l f u r a n a l y s i s normally should not present the same problems as carbon, s i n c e i n s u f f i c i e n t s u l f u r - c o n t a i n i n g s p e c i e s are present and able to a f f e c t the samples during the course of t h i s work. Probably the most v a l u a b l e aspect of the XPS technique i s i t s a b i l i t y to provide i n f o r m a t i o n on the e l e c t r o n i c 58 F i g u r e 3.2: XPS Spectrum of Carbon Is/Graphite 59 B I N D I N G E N E R G Y (eV) F i g u r e 3.3: Carbon Is and S u l f u r 2p XPS Spectra f o r %3M» Scotch Tape 60 30 40 50 60 70 80 90 100 CARBON C7BY WEIGHT] F i g u r e 3.4: C o r r e l a t i o n Between XPS C Is Peak Height and the % by Weight of Carbon Present i n Sample (The e r r o r bar i s the estimated maximum e r r o r ) 61 environment of a sur f a c e atom (see ref e r e n c e 8 ) . This w i l l be v a l u a b l e i n s t u d i e s r e l a t i n g to f l o t a t i o n mechanisms s i n c e i n r e a c t i o n s between s o l i d s and s o l v a t e d s p e c i e s the outermost atomic l a y e r s of the s o l i d are p r i m a r i l y i n v o l v e d , and the c o a l f l o t a t i o n process i s based on d i f f e r e n c e s i n 20 s u r f a c e p r o p e r t i e s . Moreover, the S 2p peak provides a 3/2 good i n d i c a t i o n of the S o x i d a t i o n s t a t e , s i n c e the s u l f u r atomic e l e c t r o n s are p a r t i c u l a r l y s e n s i t i v e to t h e i r atomic environment. XPS i s s e n s i t i v e enough to be used to detect low l e v e l p y r i t e o x i d a t i o n i n c o a l at a very e a r l y stage, when oxygen i n c o r p o r a t i o n i s too low to be detected by con v e n t i o n a l a n a l y s i s . W a s h a b i l i t y t e s t s must be conducted to determine the proper method and equipment needed i n deep-cleaning; these i n v o l v e stage crus h i n g to r e l e a s e i m p u r i t i e s and s p e c i f i c g r a v i t y f r a c t i o n a t i o n to determine the q u a l i t y and qu a n t i t y 20,21 of the cleaned product . Even though these t e s t s are extremely v a l u a b l e i n p r e d i c t i n g w a s h a b i l i t y , they are c o s t l y . We demonstrate here how XPS i n f o r m a t i o n can be used to give maximum washing e f f i c i e n c y by i n d i c a t i n g one of the important c o n d i t i o n s f o r p y r i t e e l i m i n a t i o n , the best s i z e of c o a l p a r t i c l e to use. XPS s p e c t r a of c o a l samples u s u a l l y showed two S 2p 3/2 XPS peaks, with a s e p a r a t i o n averaging 5.5 eV. These are assigned to i r o n s u l f i d e s (163.5 eV) and i r o n s u l f a t e (169.0 62 12 eV) . Surface o x i d a t i o n o f t e n a r i s e s during sample p r e p a r a t i o n (e.g. g r i n d i n g ) ; i n f a c t s e v e r a l workers have r e l a t e d the e f f e c t s of e s t e r c r o s s - l i n k a g e development 22-24 during o x i d a t i o n to d e t e r i o r a t i o n of coking . A s l i g h t shoulder to the higher BE s i d e of the main hydrocarbon XPS peak i s c h a r a c t e r i s t i c of carbon s i n g l y bonded to 17,18 oxygen . R e f e r r i n g to our XPS measurements, i t appears that our f r e s h c o a l was low i n s u l f a t i c s u l f u r ( f i g . 3.5), but s i g n i f i c a n t amounts appear i n weathered or o x i d i z e d samples ( f i g s . 3.6 & 3.7). I t appears, then, that s u r f a c e o x i d a t i o n i s not s i g n i f i c a n t when samples are prepared by crushing at ambient c o n d i t i o n s , the S 2p s p e c t r a showing 3/2 l i t t l e or no s u l f a t e (and the C Is peak no s h o u l d e r ) . In the process of c r u s h i n g , there may w e l l be a degree of p r e f e r e n t i a l cleavage along the mineral planes which w i l l reduce the amount of heat generated and hence su r f a c e o x i d a t i o n . As shown i n f i g u r e s 3.8 and 3.9, the p y r i t e / c a r b o n r a t i o measured by XPS decreases as p a r t i c l e s i z e i n c r e a s e s , which i s c o n s i s t e n t with having r e l a t i v e l y more p y r i t e d i s p e r s e d over the c o a l s u r f a c e i n small c o a l p a r t i c l e s than f o r l a r g e ones. Furthermore, r e l a t i v e l y l e s s p h o t o e l e c t r o n s w i l l be e j e c t e d from S atoms i n l a r g e r p y r i t e p a r t i c l e s as compared to those escaping from small ones. T h e r e f o r e , the p y r i t e / c a r b o n i n t e n s i t y r a t i o i s approximately i n v e r s e l y p r o p o r t i o n a l to r a d i u s f o r non-oxidized samples. T h i s has a p o s s i b l e i m p l i c a t i o n that p y r i t e i s f i n e l y d i s p e r s e d or 63 .. r f\ ^^--/ 250 nm-208 *••»-150 125 V ' W 106 Mm-|^ 90M«n-75 Mm-61 nm— 53 Mm i 174 294 169 H 1 I I I t 1 164 159 154 1 289 BINDING ENERGY (eV) 284 279 274 F i g u r e 3.5: Carbon I s l a n d S u l f u r 2p XPS Spectra of 3/2 Non-oxidized ( f r e s h ) Minto Coal Samples f o r D i f f e r e n t P a r t i c l e S i z e 208 M » * 174 169 *—i—i—i i i—i—t—i—i—i i i s 2 164 159 154 1 BINDING ENERGY (eV) F i g u r e 3.6: Carbon Is and S u l f u r 2p XPS Spectra f o r 3/2 Minto Coal Samples of D i f f e r e n t P a r t i c l e S i z e s A f t e r 3 h Ox i d a t i o n F i g u r e 3 . 7 : Carbon Is and S u l f u r 2p XPS Spectra f o r 3/2 Minto Coal Samples of D i f f e r e n t P a r t i c l e S i z e s A f t e r 15 h O x i d a t i o n 66 0.3 0 8 i S 20 zZ iS To x lo-fym-': RADIUS F i g u r e 3.8: Minto Coal P y r i t e / C a r b o n vs 1 / P a r t i c l e Radius (The e r r o r bar i s the estimated maximum e r r o r ) 67 0.61 0.3 H 1 1 1 r——r——i 1 1 1 1 1 1 1 0 10 20 30 40 50 60 70 80 90 100 110 120 130 R A D I U S (/JUtl) F i g u r e 3.9: Minto Coal P y r i t e / C a r b o n vs P a r t i c l e Radius "The s i g n i f i c a n c e of the i n t e r c e p t p o i n t I i s explained i n the t e x t , page 71". (The e r r o r bar i s the estimated maximum e r r o r ) 68 homogeneously d i s t r i b u t e d i n Minto c o a l , s i n c e otherwise l i n e a r i t y i n f i g u r e 3.8 w i l l not be obtained. The above ob s e r v a t i o n may a l s o r e f l e c t the f r a c t u r e mechanism of the 18 c o a l p a r t i c l e s A f t e r the non-oxidized samples were exposed to humid a i r f o r some hours, the s u l f a t e peak i n c r e a s e d i n i n t e n s i t y , and the s u l f i d e peak decreased ( f i g s . 3.6 & 3.7). The o x i d a t i o n was performed under mild c o n d i t i o n s (99+1 % o o r e l a t i v e humidity at 45 C) s i n c e temperatures >75 C could r e s u l t i n l o s s of S - c o n t a i n i n g molecules from the sample 12 s u r f a c e , and d e t e r i o r a t i o n of f o r i n s t a n c e , coking 22-24 p r o p e r t i e s as mentioned above, during o x i d a t i o n F i g u r e s 3.6 & 3.7 show no evidence of such d e t e r i o r a t i o n . XPS r e s u l t s on p y r i t e o x i d a t i o n a f t e r 3 and 15 h are shown i n f i g u r e s 3.10 & 3.11 where we see p y r i t e maximized and s u l f a t e minimized at 5 3+ 3 M M i n r a d i u s . Note that prolonged exposure to humid a i r does not change t h i s " c h a r a c t e r i s t i c " p y r i t e s i z e f o r the c o a l we are using here. The same ex p l a n a t i o n can be a p p l i e d to the r i s i n g p o r t i o n of the curve as f o r non-oxidized c o a l . However, when one p r o g r e s s i v e l y approaches the s m a l l e r p a r t i c l e s i z e , s u r f a c e o x i d a t i o n seems to be dominant, and the p y r i t e o x i d i z e s r a p i d l y to s u l f a t e r e g a r d l e s s of the p a r t i c l e s i z e at which maximum p y r i t e s u r f a c e c o n c e n t r a t i o n o c c u r s . T h i s i s because at t h i s stage one i s c r u s h i n g p y r i t e i t s e l f and s u r f a c e o x i d a t i o n w i l l l i k e l y be approximately i n v e r s e l y dependent on r a d i u s , 1 / P a r t i c l e Radius ( A f t e r 3 h O x i d a t i o n ) (For e r r o r e s t i m a t i o n see t a b l e 3.2) R A D I U S 2- 2-Figure 3.11: Minto Coal P y r i t e / S 0 4 ; SO^ /C; P y r i t e / C vs 1 / P a r t i c l e Radius ( A f t e r 15 h O x i d a t i o n ) (For e r r o r e s t i m a t i o n see t a b l e 3.2) 71 i . e . the smaller the p y r i t e p a r t i c l e s i z e on the c o a l , the f a s t e r the r a t e of o x i d a t i o n . I n c i d e n t a l l y , t h i s " c h a r a c t e r i s t i c " s i z e a l s o corresponds to the i n t e r c e p t s of the curve e x t r e m i t i e s ( p o i n t I) f o r the non-oxidized c o a l p y r i t e / c a r b o n versus p a r t i c l e r a d i u s curve i n f i g u r e 3.9. The mathematical i m p l i c a t i o n s of t h i s are by no means c l e a r . The r e s u l t s obtained may be a p p l i e d as a p a r t i t i o n apparatus f o r producing c o a l p a r t i c l e s which are of optimum c l e a n i n g s i z e s i n c e they are c l o s e to the same s i z e as the p y r i t e c l u s t e r s . T h i s optimum s i z e i s almost c e r t a i n l y rank-dependent, owing to v a r i a t i o n s i n morphology, and mode of c o a l e v o l u t i o n a r i s i n g from the geochemical environments i n 5 which the minerals were o r i g i n a l l y formed . The determination of the i n t e n s i t y of XPS peaks i s important i f q u a n t i t a t i v e a n a l y s i s i s d e s i r e d . However, i t i s o f t e n d i f f i c u l t to determine a "base" r e f e r e n c e l e v e l i n a spectrum. In t h i s study, an a p p r o p r i a t e base l i n e i s formed by base l i n e e x t r a p o l a t i o n . The base l i n e s f o r a set of s p e c t r a from s i m i l a r samples are approximately p a r a l l e l , which improves the accuracy of our r a t i o values ( i . e . the i n e l a s t i c a l l y s c a t t e r e d e l e c t r o n spectrum and a l l the peak widths w i l l be very s i m i l a r ) . Table 3.2 summarizes the r e l a t i v e peak i n t e n s i t i e s of the f r e s h and o x i d i z e d c o a l samples. FRESH THREE HOURS OXIDATION FIFTEEN HOURS OXIDATION SIZE 0«m) RADIUS Oim) RADIUS"1 ( x i o ; 2 Fe-S/C Fe-S/C Fe-S/S0 4 max min max mm 2-max mm SO. 2 " /C Fe-S/C max mm max mm Fe-S/S0 4 2-max mm s o 4 2 " / c max mm 53.0 26.5 61.0 30.5 75.0 37.5 90.0 A5.0 106.0 53.0 125.0 62.5 150.0 75.0 180.0 90.0 208.0 104.0 250.0 125.0 3 3 2, 2, 77 28 67 22 1. 89 1 .60 1. 33 1 .11 0 . 96 0 . 8 0 .589 .573 .513 .484 .458 .439 .423 .414 .400 .401 .572 .544 .494 .460 .434 .424 .397 .392 .386 .376 .300 .386 .396 .410 .436 .390 .380 .355 .338 .292 .373 .387 .400 .428 .382 .372 .350 .329 .375 .973 1.11 1.38 1.55 1.35 1.24 1.13 .502 .363 .940 1.08 1.32 1.49 1.30 1.20 1.10 .487 .809 .404 .361 .304 .288 .295 .312 .319 .679 .796 .395 .353 .295 .280 .288 .303 .314 .670 .371 .365 .458 .455 .496 .486 .401 .395 .381 .344 .35* .358 .444 .445 .482 .475 .392 .383 .371 ,334 .328 .410 .515 .540 .910 .811 .674 .562 .541 .333 .314 .402 .500 .523 .872 .790 .656 .543 .526 .322 1.14 .897 .896 .849 .558 .608 .602 .710 .710 1.04 1.12 .884 .884 .838 .534 .594 .591 .699 .698 1.03 Table 3 . 2 : Ratios of XPS Peak Heights at Various P a r t i c l e S i z es f o r Fresh and Oxi d i z e d Samples of Minto Coal 73 It i s b e l i e v e d that c o a l s u r f a c e c h a r a c t e r i z a t i o n obtained by XPS a f t e r v a r i o u s c o n d i t i o n i n g steps, and during f l o t a t i o n , could w e l l allow both "chemical s h i f t " a n a l y s i s , and s e m i - q u a n t i t a t i v e a n a l y s i s based on r e l a t i v e i n t e n s i t y measurements. 74 REFERENCES D.R. 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Perry and A. G r i n t , F u e l , 62, 1024 (1983) 19. G. Blackmore, "Coal C l e a n i n g , Steam R a i s i n g and E n v i r o n -mental C o n t r o l , An Economic I n t e r p r e t a t i o n of Systems, Integrated E nvironmental C o n t r o l f o r C o a l — F i r e d Power Pl a n t s Symposium", Denver, February 22-25 (1981) 20. J.L. C e c i l e , " F l o t a t i o n of S u l f i d e M i n e r a l s " (ed. K.S.E. Forssberg) E l s e v i e r , Amsterdam, p.61-80 (1985) 21. J.A. C a v a l l a r o , M.T. Johnston and A.W. Deurbrouck, "U.S.B.M. R.I. No.8118", (1976) 22. H.M. Wachowska, B.N. Nandi and D.S. Montgomery, F u e l , 53, 212 (1974) 23. B.S. Ignasiak, A.J. Szladow and D.S. Montgomery, F u e l , .53, 229 (1974) 24. P.C. P a i n t e r , M.M. Coleman, R.W. Synder, 0. Mahajan, M. Komatsu and P.L. Walker J r . , Appl. S p e c t r o s c . , 35,106 (1981) 76 CHAPTER FOUR SULFUR SPECIATION IN LAKE SEDIMENT 4.1: INTRODUCTION Various s u l f u r - c o n t a i n i n g compounds have long been known to be major p o l l u t a n t s i n many i n d u s t r i a l i z e d parts of 1 the world . S u l f u r , an e s s e n t i a l b i o l o g i c a l n u t r i e n t , i s found i n both organic and i n o r g a n i c forms. In l i v i n g m a t e r i a l i t i s present p r i m a r i l y i n i t s most reduced s t a t e s , f o r example as -SH. When organic S-compounds are decomposed by b a c t e r i a , the i n i t i a l product i s g e n e r a l l y the reduced form hydrogen s u l p h i d e . Since s u l f a t e , as s u l f u r i c 2,3 a c i d , i s a major component of a c i d i c p r e c i p i t a t i o n , a c l e a r understanding of S dynamics and d i a g e n e s i s i n lakes i s most d e s i r a b l e s i n c e S t r a n s f o r m a t i o n s markedly a f f e c t t h i s 4,5 a c i d i f i c a t i o n process Mahoney Lake i s a small h y p e r s a l i n e meromictic lake (20 m deep) l o c a t e d i n southeast B r i t i s h Columbia, with the sediment being preserved much as i t was l a i d down o r i g i n a l l y . The seasonal changes i n i t s various 6,7 l i m n o l o g i c a l f e a t u r e s have been de s c r i b e d , and i t appears that s u l f u r i n i t s many o x i d a t i o n s t a t e s plays a dominant 77 r o l e i n the chemical and b i o l o g i c a l processes o c c u r r i n g i n t h i s l a k e . The elemental composition of the s u r f a c e l a y e r s can be reasonably r e l a t e d to the o r i g i n s of the sediments, because lake sediments and t h e i r i n t e r f a c e s with o v e r l y i n g water are 8,9 major s i t e s f o r s u l f u r accumulation and t r a n s f o r m a t i o n Moreover, geochemical r e a c t i o n s occur o f t e n only to a depth of a few atomic l a y e r s , thus an a n a l y t i c a l technique with high s u r f a c e s e n s i t i v i t y such as XPS i s i d e a l l y s u i t e d to analyse the elemental composition as w e l l as the chemical 10-12 s t a t e ( s ) of molecules present which may be of 13 geochemical and/or m i n e r a l o g i c a l i n t e r e s t Although g e o l o g i s t s have made ample use of Scanning E l e c t r o n Microscopy-Energy D i s p e r s i v e X-ray A n a l y s i s (SEM-EDX) and E l e c t r o n Microprobe A n a l y s i s (EMA), such techniques are seldom able to i d e n t i f y unambiguously the phase(s) present, and so they are of t e n only of value f o r p r e l i m i n a r y 10 surveys of s u r f a c e s . Moreover, c u r r e n t ASTM p r a c t i c e to analyze v a r i o u s forms of s u l f u r i s a l s o somewhat u n r e l i a b l e and t edious (see Chapter t h r e e ) . XPS has r e c e n t l y found a p p l i c a t i o n i n s e v e r a l e n v i r o n -14-17 mental s t u d i e s and here we have attempted to c h a r a c t e r i z e , i n a general way, the d i s t r i b u t i o n of the i n o r g a n i c and organic S c o n s t i t u e n t s i n a core of Mahoney Lake sediments. 7 8 4.2: EXPERIMENTAL Experimental c o n d i t i o n s are as d e s c r i b e d i n chapter 2. The sediment sample upon r e c e i p t e d from P r o f e s s o r T.Northcote and h i s a s s o c i a t e s (Zoology Department UBC) was subdivided i n t o many 10 cm s e c t i o n s to a depth of 455 cm, which were o stored i n a vacuum d e s i c c a t o r at about -15 C u n t i l a n a l y s i s . X-ray power of 320 watts was used, and no evidence of sample decomposition during a n a l y s i s was seen. The e l e c t r o n BE's were st a n d a r d i z e d a g a i n s t the potassium 2p (294.0 eV) l i n e 3/2 from a 3/7 mixture of potassium c h l o r i d e / s a m p l e . The measurements were repeated a f t e r exposure of the samples to a i r f o r two months, and no changes were seen. 4.3: RESULTS AND DISCUSSION S u l f u r can accumulate by decomposition of p a r t i c u l a t e matter or be d e r i v e d by i m m o b i l i z a t i o n of s o l u b l e forms, 18 predominantly s u l f a t e , from the water . A l l the XPS S 2p s p e c t r a show two peaks- one r e p r e s e n t i n g s u l f u r atoms 3/2 i n the sulphate ion and the second, s u l f u r i n more reduced forms. Some t y p i c a l s p e c t r a are shown i n f i g u r e 4.1. The second peak, s l i g h t l y broader, has a maximum value of BE 79 80 BINDING ENERGY (eV) F i g u r e 4.1 (cont.) 81 which v a r i e s with sediment depth i n a remarkably p e r i o d i c way (f i g . 4.2 ) . Many s u l f u r - c o n t a i n i n g molecules have been s t u d i e d by 19 XPS , and hence we are able to p r e d i c t the most l i k e l y S-c o n t a i n i n g s p e c i e s at the va r i o u s depths, as i n d i c a t e d i n f i g u r e 4.2. The S d i s t r i b u t i o n i n the sediment p r o f i l e r e f l e c t s the exchanges of s u l f a t e among other i n o r g a n i c and 20 organic S - c o n s t i t u e n t s . We f i n d a p r o g r e s s i v e d e c l i n e of t o t a l sulphate c o n c e n t r a t i o n with sediment depth, to the point where at 400 cm the lower BE S repr e s e n t s about 40% of the t o t a l s u l f u r , (Whereas f o r example, at 35 cm depth i t was only about 16 % ) . Although organic s u l f u r compounds of t e n lead to format-ion of hydrogen s u l f i d e i n water, i t i s of i n t e r e s t that 2-D e s u l f o v i b r i o b a c t e r i a can reduce S0^ to h^S by u t i l i z i n g 21 i t i n the o x i d a t i o n of organic matter : 2- + SO + 2(CH 0) + 2H •*» H S + 2C0 + 2H 0 4 2 2 2 2 B a c t e r i a l generation of h^S from s u l f a t e i o n s , with subsequent formation of i r o n s u l f i d e s , has been c l e a r l y e s t a b l i s h e d i n c o a s t a l lagoons and lakes/oceans i n which s a l i n i t i e s and sulphate contents are high and organic 22 p r o d u c t i v i t y i s not dominant . The r e f o r e , the major pathway f o r i n c o r p o r a t i o n of S i n sediment may w e l l be through s u l f a t e r e d u c t i o n and the formation of Fe-S compounds of low s o l u b i l i t i e s . For example, i n a lake 169 t\V so4s LU ai 164f-10 50 100 150 200 250 300 350 400 450 500 SEDIMENT DEPTH (cm) Figure 4.2 S u l f u r 2p Binding Energies of the Pre-3/2 dominant "Lower O x i d a t i o n S t a t e " S u l f u r Species i n Mahoney Lake Sediments (The e r r o r bar i s the estimated maximum e r r o r ) 83 e x p e r i m e n t a l l y a c i d i f i e d with s u l p h u r i c a c i d , Fe-S compounds 23 were found to be concentrated at the sediment s u r f a c e The process of p y r i t e formation o r i g i n a t e s as the r e s u l t of the r e a c t i o n of d i s s o l v e d hydrogen s u l f i d e with f i n e -18 grained i r o n m i n e r a l s . However, Berner noted that the substances which form by the r e a c t i o n of i r o n m i n e r als or d i s s o l v e d f e r r o u s ion with hydrogen s u l f i d e under simulated sedimentary c o n d i t i o n s i n the l a b o r a t o r y are mainly non-c r y s t a l l i n e Fe-S, and g r e g i t e ( c u b i c - F e S ). None of these 3 4 phases i s thermodynamically s t a b l e under sedimentary c o n d i t i o n s . T h e r e f o r e the f i n a l t r a n s f o r m a t i o n of these metastables ( i r o n s u l f i d e s to p y r i t e ) i s b e l i e v e d to i n v o l v e 18 the r e a c t i o n s with elemental s u l f u r . Moreover, 2-i n c o r p o r a t i o n of l a b e l l e d aqueous SO, by organic matter 24 A has been observed to be b a c t e r i a l l y m o b i l i z e d and then recovered as both e s t e r - and carbon-bonded S. The b i o d e g r a d a t i o n of the S - c o n t a i n i n g amino a c i d s c y s t e i n e , c y s t i n e and methionine can r e s u l t i n production of v o l a t i l e o r g a n i c S compounds such as methyl t h i o l and dimethyl d i s u l f i d e ; however, carbon-bonded s u l f i d e s and p o l y s u l f i d e s are r e l e a s e d as HS or p o s s i b l y as mercaptans, which i f f u r t h e r hydrolyzed may a l s o c o n t r i b u t e to p y r i t e 22 formation . The above processes would account f o r the pronounced i n c r e a s e of o r g a n i c / p y r i t i c s u l f u r and l o s s of s u l f a t e i n the basal zone. The f l u c t u a t i o n s evident i n f i g u r e 4.1 & 4.2 depend on 20 a complexity of a b i o t i c , b i o t i c and watershed f a c t o r s and 84 may w e l l prove to be a u s e f u l index i n a s s e s s i n g v a r i o u s l i m n o l o g i c a l processes, i n which t r a n s f o r m a t i o n and t r a n s l o c a t i o n of S i n i t s many o x i d a t i o n s t a t e s play c r i t i c a l r o l e s i n the chemical and b i o l o g i c a l e n e r g e t i c s of lakes With our present i n s t r u m e n t a l r e s o l u t i o n i t i s impossible to separate the c o n t r i b u t i o n s of the v a r i o u s organic S compounds to the 'reduced' peak at low BE. A l s o , s i g n a l s from s u l f u r i n c e r t a i n Fe-S compounds ( i . e . FeS, FeS , Fe S ), may be superposed, and so the average elemental 2 3 4 compositions of the s u r f a c e l a y e r s of the m a t e r i a l as i n d i c a t e d here may d e v i a t e from those given by t o t a l 17 a n a l y s i s . However, the s t r e n g t h of XPS a n a l y s i s i s i n p r o v i d i n g data which r e f l e c t s the o r i g i n and/or the h i s t o r y of the sediment samples, which have acted as ce n t r e s f o r s u l f u r accumulation and t r a n s f o r m a t i o n . The i n t e r p r e t a t i o n of these data i s an ongoing p r o j e c t i n c o n j u n c t i o n with other a n a l y t i c a l s t u d i e s on N, 0 and P e t c . i n the sample core, i n our l a b o r a t o r y and i n the Zoology Department UBC. 85 REFERENCES 1. W.W. K e l l o g g , R.D. Cadle, E.R. A l l e n , A.L. Lazrus and E.A. M a r t e l l , Science, 175, 587 (1972) 2. J.N. Galloway and D.M. Whelpdale, Atoms. E n v i r . , 14, 407 ( 1980) 3. G.E. Likens and T.J. B u t l e r , i b i d , 15, 1103 (1981) 4. M.J. M i t c h e l l D.H. Landers and D.F. Brodowski, Water A i r S o i l P o l l . , 16, 351 (1981) 5. C.A. K e l l y , J.W. Rudd, R.B. Cook and D.W. S c h i n d l e r , Limnol. Oceanog., 27, 868 (1982) 6. T.G. Northcote and T.G. Haksey, J . F i s h . Res. Bd. Can., 26, 1763 (1969) 7. T.G. Northcote and K.J. H a l l , H y d r o b i o l o g i a , 105, 179 (1983) 8. S.H. Zinder and D.T. Brock, Appl. E n v i r o n . M i c r o b i o l . , 35, 344 (1978) 9. W.R. Winfrey and J.G. Zeikus, i b i d . , 33, 275 (1977) 10. N.S. Mcl n t y r e , Canadian Chemical News, 13, May (1985) 11. L.A. Casper and C.J. Powell, " I n d u s t r i a l A p p l i c a t i o n of Surface A n a l y s i s " , ACS Symposium S e r i e s , 199 (1982) 12. H. Windawi and C D . Wagner, "Applied E l e c t r o n Spectro-scopy f o r Chemical A n a l y s i s " (H. Windawi & F.F. Ho eds), Wiley,N.Y. (1982) 13. G.M. Ba n c r o f t , J.R. Brown and W.S. Fyfe, Chem. Geol., 25, 227 (1979) 14. S.G. Chang, T. Novakov, L. Gundel and R.L. Dod, " A n a l y t -i c a l Aspects of Environmental Chemistry" (D.S. Natusch & P.K. Hopke), Wiley, N.Y. (1983) 15. R.W. L i n t o n , D.T. Harvey and G.E. Cabaniss, i b i d . 16. K. Okamoto, " P r e p a r a t i o n , A n a l y s i s and C e r t i f i c a t i o n of Pond Sediment C e r t i f i e d Reference M a t e r i a l s (Research Report from the N a t i o n a l I n s t i t u t e f o r Environmental S t u d i e s ) " , Japan, 38 (1982) 86 17. M. Soma, H. Seyama and K. Okamoto, T a l a n t a , 32, 177 (1985) 18. R.A. Berner, " P r i n c i p l e s of Chemical Sedimentology", McGraw H i l l , N.Y. (1971) 19. T.A. C a r l s o n , " P h o t o e l e c t r o n and Auger Spectroscopy", Plenum, N.Y. (1975) 20. M.J. M i t c h e l l , M.B. David and A.J. Uutala, H y d r o b i o l o g i a , 121, 121 (1985) 21. S.E. Manahan, "Environmental Chemistry", 4th ed., P.W.S., Boston (1984) 22. Z.S. A l s c h u l e r , M.M. Schnepfe, C C . S i l b e r , and F.O. Simon, Science, 221, 221 (1983) 23. R.B. Cook and D.W. S c h i n d l e r , Environmental Biochemistry E c o l . B u l l . , 35, 115 (1983) 24. J.R. Frenay e t . a l . , S o i l B i o l . Biochem., 3, 133 (1971) 87 CHAPTER FIVE THE BONDING OF DIBENZYLDITHIOCARBAMATE METAL COMPLEXES 5.1: INTRODUCTION Dithiocarbamate metal c h e l a t e s are used i n a wide v a r i e t y of chemical processes i n c l u d i n g rubber v u l c a n i z a t i o n and the pro d u c t i o n of a g r i c u l t u r a l f u n g i c i d e s and 1 p e s t i c i d e s . S o l u b l e dithiocarbamate s a l t s have found exten-s i v e a p p l i c a t i o n i n the e x t r a c t i o n of metal c h e l a t e s from 2 a c i d s o l u t i o n s , and the chemical and p h y s i c a l p r o p e r t i e s of these complexes are of much i n t e r e s t , p a r t i c u l a r l y the nature of the nitrogen-metal bonding. 3,4 5,6 Previous experimental data from NMR, IR and e l e c t -7 8-12 ron spectroscopy, as w e l l as XPS have i n d i c a t e d two p o s s i b i l i t i e s f o r the bonding i n these compounds: C< yM X N — c f . >l M TYPE I TYPE II 88 S t r u c t u r e I i l l u s t r a t e s the long-held view that the l i g a n d forms a s t a b l e four-membered c h e l a t e r i n g . E a r l y XPS s t u d i e s supported t h i s idea s o l e l y on the bas i s of the 8-11 e x i s t e n c e of only one broad sulphur 2p peak : however 3/2 recent XPS s t u d i e s on N Is BE's i n C u ( I I ) - , P b ( I I ) - and 12 S n ( I I ) - d i e t h y l d i t h i o c a r b a m a t e complexes by Cheng et a l . suggested that there may e x i s t an a d d i t i o n a l bond between the N atom and the metal M ( s t r u c t u r e II) to form a second four-membered r i n g , s i n c e they found the BE of the N Is peak to be approximately the same as that i n the complexes formed between the ions of the a l k a l i n e earths 13 and EDTA ( e t h y l e n e d i a m i n e t e t r a a c e t i c a c i d ) , where the N atoms are c e r t a i n l y c oordinated to the metal i o n . In order to c l a r i f y t h i s i s s u e , we extended the XPS s t u d i e s to i n c l u d e the more thermally s t a b l e C u ( I I ) , Zn(II) and B i ( I I I ) d i b e n z y l d i t h i o c a r b a m a t e (DBDTC) compounds. 5.2: EXPERIMENTAL a) P r e p a r a t i o n of M e t a l - D i b e n z y l d i t h i o c a r b a m a t e Chelate The metal (Cu(II) and B i ( I I I ) } c h e l a t e s were sy n t h e s i z e d by the general procedure o u t l i n e d f o r the 12 s y n t h e s i s of d i e t h y l d i t h i o c a r b a m i c a c i d s . Zinc-DBDTC was obtained from Eastman Kodak Chemical Company and was r e c r y s t a l l i z e d using a 1:1 mixture of chloroform and 89 ethanol as s o l v e n t . The sodium s a l t of DBDTC was prepared from Zn-DBDTC by ion exchange, and the other c h e l a t e s were 14 then s y n t h e s i z e d from the sodium s a l t . The c h e l a t e p r e c i p i t a t e s were f i l t e r e d , washed with water, d r i e d i n vacuum, and f i n a l l y r e c r y s t a l l i z e d from another c h l o r o f o r m -ethanol s o l v e n t . b) Instrumental The l i q u i d n i t r o g e n cooled probe was used i n t h i s study, to minimize sample decomposition. Since metal c h e l a t e d dithiocarbamates powders are e l e c t r i c a l i n s u l a t o r s , a l l BE's were c a l i b r a t e d with r e f e r e n c e to the potassium 2p peak (294.0 eV) from a 3/7 mixture of potassium 3/2 chloride/sample as an i n t e r n a l standard. 5.3: RESULTS AND DISCUSSION The study of the s t r u c t u r e of i n o r g a n i c or organic complexes by XPS i s o f t e n hampered by the decomposition of the compounds during measurement. We obtained N Is BE data on the d i b e n z y l d i t h i o c a r b a m a t e metal complexes c o n t a i n i n g C u ( I I ) , Z n ( I I ) , and B i ( I I I ) , choosing benzyl compounds i n preference to the l e s s s t a b l e e t h y l d e r i v a t i v e compounds used i n previous s t u d i e s , -even so, with the lowest p r a c t i c a l X-ray power and sample probe cooled by l i q u i d n i t r o g e n , the 90 hydrogen and sodium complexes decomposed i n the spectrometer The data obtained from the remaining complexes are given i n t a b l e 5.1, and the s p e c t r a are shown i n f i g u r e s 5.1 and 5.2. Our r e s u l t s show that from Zn(II) to B i ( I I I ) DBDTC the S 2p BE i n c r e a s e s , but the N Is BE remains e s s e n t i a l l y 3/2 unchanged, as opposed to the case of the s i m i l a r molecules CuH"2-EDTA and BiH-EDTA, where there i s N-M bonding, and where we see a d i f f e r e n c e i n N Is BE of approximately 2.0 eV. T h i s i s a strong i n d i c a t i o n that N i s not bonded to the metal i n the DBDTC complexes. Furthermore, the N Is e l e c t r o n s i n the DBDTC complexes have a BE of about 1.0 eV gre a t e r than " n e u t r a l " n i t r o g e n i n t r i p h e n y l a m i n e , which I * + 8~ f suggests that the N-C bond i n / R j j ^ i i i C S t i s p o l a r , l e a d i n g t 2 2> to l o c a l p l a n a r i t y , as has been p r e v i o u s l y observed i n the 15,16 X-ray c r y s t a l l o g r a p h y s t u d i e s of d i e t h y l complexes. 91 PEAK SAMPLE BINDING ENERGY (eV) PEAK WIDTH (eV) FWHM Zn(DBDTC) 400. .50 2. .10 Cu(DBDTC) 400. .55 2, .16 CuH (EDTA) 9 400. .20 2. .85 L. Bi(DBDTC) 400. .50 1. .95 BiH(EDTA) 402. .25 2. .47 Zn(DBDTC) 162. .20 2. .75 Cu(DBDTC) 162. ,20 2. .61 Bi(DBDTC) 162. .60 2. .67 Table 5.1: XPS Data on Some D i b e n z y l d i t h i o c a r b a m a t e s , Cu(II)H -EDTA and Bi(III)H-EDTA 92 F i g u r e 5.1: Nitrogen Is XPS Spectra of Some D i b e n z y l d i t h i o carbamates, Cu(II)H -EDTA and Bi(III)H-EDTA 93 Bi(ni) Cu(ii) A » —»\»-4—I—I—I—|—|—I—I—I—|—I—I—|—I—I—I—|—I—|—|—(-174 169 164 159 BINDING ENERGY (eV) 154 F i e u r e 5.2: S u l f u r 2p XPS Spectra of Some Dibenzyl-6 3/2 dithiocarbamates 94 REFERENCES 1. C.D. Thorn and R.A. Ludwig, "The Dithiocarbamates and Related Compounds", E l s e v i e r , Amsterdam (1962) 2. A. Hulanicke, T a l a n t a , 14, 137 (1967) 3. G.St. Ni k o l o v , Inorg. Nucl. Chem. L e t t . , 7, 120 (1971) 4. G.St. Nikolov, N. Jordanov and I. Havezov, J . Inorg. Nucl. Chem., 33, 1055 (1971) 5. F. Bonati and R. Ugo, J . Organomet. Chem., 10, 257 (1967) 6. C O . Connor, J.D. G i l b e r t and G. Wi l k i n s o n , J . Chem. Soc. A, 84 (1969) 7. G. St. Nikolov, N. Jordanov, and I. Havezov. J . Inorg. Nucl. Chem., 33, 1059 (1971) 8. P.M.Th.M. Van Attekum and J.M. T r o o s t e r , J . Chem. Soc., Dalton Trans., 1, 201 (1980) 9. D. Perry and R.A. Geanangel, Inorg. Chim. Acta, 13(2), 185 (1980) 10. D.M. Hercules, L.E. Cos, S. O n i s i c k , G.D. N i c h o l s and J.C. Carver, Anal. Cham., 45(11), 1973 (1973) 11. J . Willemse, J.A. Crass, J.G. Wijnhoven and P.T. Beur-skens, R e e l . Trav. Chim. Pays-Bas., 92(11), 1199 (1973) 12. H.P. Chang and K.L. Cheng, Sp e c t r o s . L e t t s . , 14(11&12), 795 (1981) 13. K.L. Cheng, J.C. Carver, and T.A. C a r l s o n , Inorg. Chem. 12 ( 7 ) , 1702 (1973) 14. L. Cambi and L. Szego, Chem. Ber., 64, 2591 (1931) 15. M. Bonamico, G. Dessy, A. Mugnoli, A. Vaciago and L. Zambonelli, Acta C r y s t . , 19, 886 (1965) 16. M. Bonamico, G. Mazzone, A. Vaciago and L. Zambonelli, Acta C r y s t . , 19, 898 (1965) 95 SUMMARY The minimal p e r t u r b i n g e f f e c t of the p h o t o e l e c t r o n makes X-ray p h o t o e l e c t r o n spectroscopy an e x c e l l e n t technique f o r s u r f a c e a n a l y s i s of almost any type of s o l i d sample. The sample can take any shape or form, and oft e n needs no s p e c i a l p r e p a r a t i o n p r i o r to a n a l y s i s . High r e s o l u t i o n XPS instruments are now commercially a v a i l a b l e , the use of which can lead to much improved i n f o r m a t i o n on chemical bonding i n a d d i t i o n to basic elemental a n a l y s i s . The study on coal demonstrates how the XPS technique may be used to study how the r e l a t i o n s h i p s between c o n c e n t r a t i o n r a t i o s i n v o l v i n g s u l f u r i n p y r i t e and s u l f a t e , and carbon, on c o a l s u r f a c e s are dependent on p a r t i c l e s i z e . I t a l s o gives i n f o r m a t i o n to improve s u l f u r e l i m i n a t i o n i n c o a l c l e a n i n g processes, p a r t i c u l a r l y f l o t a t i o n , that depend on d i f f e r e n c e s i n sur f a c e p r o p e r t i e s . T h i s should have s i g n i f i c a n t economic i m p l i c a t i o n s , f o r in s t a n c e r e d u c t i o n i n c a p i t a l investment r e q u i r e d f o r p r e p a r a t i o n p l a n t equipment and o p e r a t i o n a l costs r e l a t i n g to the flow v a r i a b l e s of mineral d r e s s i n g . Moreover, the r e s u l t s a l s o r e i n f o r c e the importance of XPS i n t e n s i t y r a t i o s as a measure of element d i s p e r s i o n . 96 There are few s t u d i e s which have considered the depth d i s t r i b u t i o n of s u l f u r i n sediments. The s u l f u r content i n sedimental d e p o s i t s from Mahoney Lake i s p r i m a r i l y of two types at any p a r t i c u l a r depth, - s u l f a t e , and a second, which i s probably a mixture of s e v e r a l s u l f u r compounds i n s i m i l a r o x i d a t i o n forms, which d i s p l a y s a remarkable c y c l i c v a r i a t -ion of o x i d a t i o n s t a t e (more p r o p e r l y , s u l f u r atom e l e c t -r o n i c environment) with depth beneath the lake bottom. The 2-change i n p r o p o r t i o n of s u l f u r i n SO, to S i n other forms, 4 with time ( l a k e sediment depth i n our c a s e ) , most probably r e s u l t s from S exchange r e a c t i o n s i n the lake i t s e l f . More-over, p o r t i o n s of t h i s s u l f u r pool are l a b i l e as r e f l e c t e d i n temporal f l u c t u a t i o n s . C l e a r l y , XPS has a great p o t e n t i a l f o r studying s u l f u r dynamics i n l a k e s , and hence to o b t a i n i n f o r m a t i o n on the r o l e of t h i s element i n l i m n e t i c n u t r i e n t c y c l e s , as w e l l as f o r i d e n t i f i c a t i o n of g e o l o g i c s u r f a c e s t r u c t u r e s . However to obtain accurate i d e n t i f i c a t i o n and q u a n t i f i c a t i o n of such complex heterogeneous m a t e r i a l s s p e c t r a of much higher r e s o l u t i o n and s e n s i t i v i t y such as could be provided by today's s t a t e - o f - a r t instruments would be needed, e s p e c i a l l y when studying lakes of lower s u l f u r content than Mahoney. In the DBDTC complexes s t u d i e d , S 2p b i n d i n g e n e r g i e s 3/2 are shown to vary with the o x i d a t i o n s t a t e of the metal. The same change i n o x i d a t i o n s t a t e has no e f f e c t on the N Is 97 peak, -strong evidence that there i s no metal-nitrogen i n t e r -a c t i o n and consequently no i n t r a m o l e c u l a r N-M bond, as had been p r e v i o u s l y thought to be the case. XPS i s now a w e l l e s t a b l i s h e d s u r f a c e a n a l y t i c a l t o o l , f i n d i n g widespread a p p l i c a t i o n i n both fundamental res e a r c h and i n d u s t r i a l p r o b l e m - s o l v i n g . I t i s l i k e l y to become i n c r e a s i n g l y v a l u a b l e as both our knowledge and under-standing of s u r f a c e chemistry i n c r e a s e s , and the power of the technique becomes more widely r e c o g n i z e d . 

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