UBC Theses and Dissertations
A muSR study of the interactions between muoniated radicals and metallic nanoparticles in mesoporous silica hosts Xiao, Jiyu
Transverse field (TF-muSR) and Avoided level crossing resonance (ALCR) techniques have been used in this very first investigation on the structure and dynamics of the Mu-cyclohexadienyl (MuC₆H₆) radical interacting with Au and Pt metal nanoparticles supported in mesoporous silica (SBA-15), providing novel information about the interactions of radicals with silica environments. Two different loadings of benzene were investigated by the TF-μSR and its companion Fast Fourier Transform (FFT) technique, revealing that the observed radicals are shielded from the metallic nanoparticle surfaces probably by a layer of benzene coating. At lower loading, the adsorption of benzene on nanoparticles increases the opportunity of an encounter between the muonium and benzene near the NP surface. The ALCR Δ₁ resonance arising from hyperfine anisotropy is observable in MNP-loaded samples up to 418 K but not in the unloaded SBA-15, while towards lower temperature it is the Δ₀ resonance mainly from isotropic hyperfine interactions that is observed in the small gold nanoparticle loaded SBA-15 sample. Such an difference is attributed to the stronger binding of MuC₆H₆ to the benzene coated MNPs than silica nanopore surfaces, so it is less perturbed by the surrounding atoms. Different dynamic behaviors have been seen in MCM-41 silica host compared to SBA-15, which is thought due to the agglomeration of benzene in the smaller pore sizes. In fact, the dynamics in this case are more like that seen for small Au NP-loaded SBA-15, giving an environment closer to that of liquid benzene due to this agglomeration. Disappointingly, the isotropic hyperfine coupling constants, determined from the positions of the Δ₀ and Δ₁ resonances, are almost the same in the MNP-loaded and unloaded samples, implying that the electronic structure of MuC₆H₆ is only weakly perturbed by the presence of the MNPs from an indirect interaction between MuC₆H₆ and the MNP. The only information on a direct interaction comes from the temperature-dependent longitudinal field T₁ relaxation rates at a monolayer benzene loading, giving also a clear direction for relevant future studies.
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