Surface-charge-tuned Vanadia Nanowire Photolytic Hydrogen Generators
Abstract
The potential of hydrogen (H2) is greater than most other fuels in terms of energy density and clean emissions. H2 possesses an energy content per unit weight of 120 MJ/kg, three times that of gasoline. H2 generation by photocatalytic splitting of water is considered as the most efficient way of producing a fully renewable and sustainable fuel. However, development of an efficient photocatalyst generating H2 is challenged by a number of factors, one of which is efficient channeling of photogenerated electrons and holes to redox reactions. The present work investigates sol-gel synthesized vanadium oxyhydrate (V3O7�H2O) nanowires decorated with Au nanoparticles. Upon conjugation of vanadia nanowires with Au nanoparticles, the vanadia oxidizes to V2O5�H2O with optical band gap changing from 2.3 eV indirect to 2.7 eV direct. Previously in our lab, reproducible conversion and external quantum efficiencies of 5.3% and 11.3% were demonstrated, respectively, using V2O5�H2O/Au nanoconjugates, as measured by gas chromatography for the first hour of photolysis under 470 nm excitation (8 mW/cm2). Interestingly, a discrepancy was observed as the conduction band edge of V2O5�H2O is determined to be 0.6 eV below standard H+ reduction potential when measured by ultraviolet photoelectron spectroscopy (UPS), indicating H2 reduction cannot be possible under normal conditions. Therefore, to explain the observed hydrogen generation, a hypothesis that the vanadia electron energy levels are being raised by some negative surface charge is put forth. In order to validate this hypothesis, cyclic current-voltage measurements were performed on aqueous suspensions of V3O7�H2O nanowires and V2O5�H2O/Au nanoconjugates. The derived conduction and valence band edge energies exhibit exceptional consistency with optical band gaps, and also validate the hypothesized energy shift of the valence band by 1.9 and 1.6 eV, respectively. The negative surface charge is also corroborated by measured zeta potentials, determined to be ?57.57 and ?57.90 mV, respectively. Based on measured pH values of 2.5 and 2.4, respectively, the negative surface charge is attributed to Lewis acid nature of the nanowires, establishing coordinative bonding with OH? adsorbates. The present work establishes that surface charge can be instrumental and enabling in photolytic fuel production.
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- OSU Theses [15752]