Nano-TiO<sub>2</sub>/TiN Systems for Electrocatalysis:\nMapping the Changes in Energy Band Diagram across the Semiconductor|Current\nCollector Interface and the Study of Effects of TiO<sub>2</sub> Electrochemical\nReduction Us

TiO<sub>2</sub> is the most widely used material in photoelectrocatalytic\nsystems. A key parameter to understand its efficacy in such systems\nis the band bending in the semiconductor layer. In this regard, knowledge\non the band energetics at the semiconductor/current collector interface,\nespecially for a nanosemiconductor electrode, is extremely vital as\nit will directly impact any charge transfer processes at its interface\nwith the electrolyte. Since direct investigation of interfacial electronic\nfeatures without compromising its structure is difficult, only seldom\nare attempts made to study the semiconductor/current collector interface\nspecifically. This work utilizes ultraviolet photoelectron spectroscopy\n(UPS) to determine the valence band maximum (<i>E</i><sub>VBM</sub>) and Fermi level (<i>E</i><sub>F</sub>) at different\ndepths in a nano-TiO<sub>2</sub>/TiN thin-film system reached using\nan Ar gas-clustered ion beam (GCIB). By combining UPS with GCIB depth\nprofiling, we report an innovative approach for truly mapping the\nenergy band structure across a nanosemiconductor/current collector\ninterface. By coupling it with X-ray photoelectron spectroscopy (XPS),\ncorrelations among chemistry, chemical bonding, and electronic properties\nfor the nano-TiO<sub>2</sub>/TiN interface could also be studied.\nThe effects of TiO<sub>2</sub> in situ electrochemical reduction in\naqueous electrolytes are also investigated where UPS confirmed a decrease\nin the semiconductor work function (WF) and an associated increase\nin n-type Ti<sup>3+</sup> centers of nano-TiO<sub>2</sub> electrodes\npost use in a 0.2 M potassium chloride solution. We report the use\nof UPS to precisely determine the energy band diagrams for a nano-TiO<sub>2</sub>/TiN thin-film interface and confirm the increase in TiO<sub>2</sub> n-type dopant concentrations during electrocatalysis, promoting\na much more comprehensive and intuitive understanding of the TiO<sub>2</sub> activation mechanism by proton intercalation and therefore\nfurther optimizing the design process of efficient photocatalytic\nmaterials for solar conversion.