Photoreactivity of Deep VB Titania Attained Via Molecular Layer Deposition; Interplay of Metal Oxide Thin Film Built-in Strain and Molecular Effects

Calcination of hybrid organic–inorganic Titanium-Ethylene Glycol (Ti-EG) thin films yield non-stoichiometric titania with deep valence band maximum (VBM) with down-shift of up to 450 meV relative to stoichiometric TiO₂. We previously reported the enhanced photocatalytic (PC) reactivity of these materials by demonstrating direct photocatalytic production of H₂O₂ and high degradation rates of organic molecules (Ishchuk et al. in ACS Nano 6:7263–7269, 2012; Kaynan et al. in J Mater Chem A 2:13822–13826, 2014; Sarkar et al. in J Phys Chem C 120:3853–3862, 2016). Here we show that the down shift in VBM is related to strain effects at the metal oxide thin film originating from the calcination process. The deep VBM position, namely high oxidative potential of photogenerated holes allows the study of PC degradation reactions of challenging targets such as Benzoic Acids (BAs) in water and porphyrin monolayers. Systematic study of these systems as probe molecules provide insights regarding the electronic details, including the molecular details, such as ionization potentials, molecular dipoles, and charge densities at the molecular side, and molecule-oxide interactions in determining the overall PC process. The analyses provide insights regarding (i) the role of strain in attaining deep VB titania and, (ii) specific insights regarding PC degradation of relatively robust, high oxidation potential materials on metal oxide (MO) thin film catalysts. Deep-VB oxide materials may pave the way for attaining a promising strategy for extending the use of light energy for removal of organic pollutants and for photocatalysis in general. Specifically, deep VB titania is found to be effective in handling BAs, which is highly challenging due to their chemical robustness, high bio-recalcitrance, and high oxidation potentials making these compounds difficult to degrade by oxidative paths. This combination of properties leads to broad impact on water reservoirs in extensive areas. Therefore, there is an on-going drive to set strategies that utilize nontoxic materials with high degradation activities towards aromatic carboxylic acids.