Figure 8.

Kar et al. [155] utilized UPS to characterize band energetics in their work on enhanced CH₄ yield via photocatalytic CO₂ reduction using TiO₂ nanotube arrays (TNAs) grafted with Au, Ru, and ZnPd nanoparticles (NP). In (a) the work functions of Au-TNA, Ru-TNA, and ZnPd-TNA are extracted to equal 4.50, 4.66, and 3.81 eV, respectively. In order to determine the positions of the valence band maxima for each structure, (b) UPS high binding energy cut-off spectra are utilized with cut-off energies at 3.20, 2.75, and 2.97 eV in Au-TNA, Ru-TNA, and ZnPd-TNA, respectively. The importance of these measurements is illustrated in (c), where the band structures of the noble metal–semiconductor composites are elucidated. Since a He laser of incident energy 21.21 eV was utilized, the work function can be calculated from the expression 21.21—E_cut-off, where E_cut-off is the cut-off energy. Given the earlier values found in (a,b), the band-bending at the NP-TNA interfaces is measured. Thus, the UPS spectra assist in the significant observation of the differing band bending dynamics that occur in TNAs in contact with Ru NPs (upward bending) and TNAs in contact with ZnPd NPs (downward bending). This is particularly helpful in facilitating hypotheses and discussions on the charge transfer dynamics that may occur in such composite systems involving metal NP co-catalysts on metal oxide semiconductor supports, and their subsequent use as potential photocatalysts for a variety of chemical reactions. Reprinted with permission from Ref [155] Copyright Springer Nature (2016).