Ternary metal oxides M2V2O7−δ (M = Zn and Cu) were synthesized by dissolving binary metal oxide precursors in an environmentally benign deep eutectic solvent (DES), which is a eutectic mixture of a hydrogen bond donor and acceptor, followed by annealing in an open crucible.

Metavanadates MV 2 O 6−δ (M = Zn and Cu) are synthesized by using a deep eutectic solvent (DES), a mixture of hydrogen bond donor and acceptor, as a reaction medium. Dissolution of stable binary metal oxides in a DES followed by a heat treatment yields phase-pure vanadates. According to in situ powder X-ray diffraction, ternary phases (α-Zn 2 V 2 O 7 and β-Cu 2−x V 2 O 7 , x ∼ 0.27) are intermediates in the reaction pathway taken. Identifying a polymorphic phase transformation temperature for CuV 2 O 6 as well as the narrow temperature range between formation and decomposition for both metavanadates allows for tackling the challenge of the synthesis of these materials. The oxygen vacancy introduced by the DES route is accompanied by the formation of reduced V 4+ and Cu + in the oxide matrix, based on X-ray photoelectron spectroscopy. These oxygen vacancies modify the vibrational modes in the corresponding Raman spectra and are also responsible for broad optical absorptions in the 1.8−1.1 eV range. The optical band gaps of the materials are found at 1.8 eV (CuV 2 O 6 ) and 2.2 eV (ZnV 2 O 6 ), approximately 0.1−0.3 eV below the values reported in the literature. The reduced band gaps and sub-band gap photon absorption are key features of the oxygendeficient metavanadates. Surface photovoltage spectroscopy reveals that all the synthesized vanadates are n-type materials with electrons as the majority charge carriers, and photoelectrochemical measurements confirm photoanodic currents for methanol oxidation. These results provide insight into the synthesis and structure−property relationship of the metavanadates for their potential use as photoanodes.

The space charge region (SCR) is a majority carrier-depleted region in a semiconductor. The width of this region and the size of the potential drop across it control photochemical charge separation under illumination, as relevant to solar energy conversion with photocatalyst particles, for example. For photocatalysts, the space charge region is often difficult to evaluate due to the small dimensions of the particles. Here, we show that surface photovoltage spectroscopy (SPS) can be used to observe the SCR in nano- and microparticles of strontium titanate (SrTiO3), aluminum-doped strontium titanate, and gallium arsenide (GaAs) on gold substrates. Depending on particle film thickness and thermal annealing conditions, we observe negative or positive photovoltages corresponding to majority or minority carrier diffusion toward the gold substrate. The direction of charge transport is controlled by the potential barrier across the depletion layer at the gold-particle interface. From the inverted photovoltage signal, potential barriers are estimated between 0.005 and 0.189 V and SCR widths between 1.3 and 2.8 μm. The observed barriers are 10–100 times smaller than the theory for an ideal Schottky junction, and the observed SCR widths are up to 100 times larger. The difference can be attributed to trapping of majority carriers in surface states arising from broken bonds and disorder, in the case of GaAs, or from chemisorbed oxygen, in the case of SrTiO3. Trapping of majority carriers in surface states (Fermi level pinning) also diminishes the effective carrier concentrations in the particles, explaining the larger SCR width in the films. These findings showcase the importance of surface states for charge separation in photocatalyst particles and their films.

Gallium nitride (GaN) nanowire arrays on silicon are able to drive the overall water-splitting reaction with up to 3.3% solar-to-hydrogen efficiency. Photochemical charge separation is key to the operation of these devices, but details are difficult to observe experimentally because of the number of components and interfaces. Here, we use surface photovoltage spectroscopy to study charge transfer in i-, n-, and p-GaN nanowire arrays on n+-Si wafers in the presence and absence of Rh/Cr2O3 co-catalysts. The effect of the space charge layer and sub-bandgap defects on majority and minority carrier transport can be clearly observed, and estimates of the built-in potential of the junctions can be made. Transient illumination of the p-GaN/n+-Si junction generates up to −1.4 V surface photovoltage by carrier separation along the GaN nanowire axis. This process is central to the overall water-splitting function of the n+-Si/p-GaN/Rh/Cr2O3 nanowire array. These results improve our understanding of photochemical charge transfer and separation in group III–V semiconductor nanostructures for the conversion of solar energy into fuels.

CuWO4 is a medium bandgap (2.3 eV) n-type semiconductor capable of photoelectrochemical water oxidation under applied electrical bias. Here, we show for the first time that suspended microcrystals CuWO4 evolve oxygen photocatalytically under visible illumination from solutions of 0.05 M AgNO3 (10.8 μmol/hour; AQE of 0.56% at 400 nm) and 0.0002 M FeCl3 (1.5 μmol/hour). No oxygen is detected with 0.002 M [Fe(CN)6]3− as sacrificial agent. The activity dependence on the redox potential of the acceptors is due to the presence of Cu2+ based electron trap states in CuWO4. According to surface photovoltage spectroscopy and electrochemistry, these states are located on the particle surface, 1.8 eV above the valence band edge of the material. Controlling the chemistry of these states will be key to uses of CuWO4 particles in tandem catalysts for overall water splitting.