Thermal annealing of metal oxides in oxygen‐deficient atmosphere, particularly reducing hydrogen gas, has been demonstrated to induce oxygen vacancy formation for enhanced photoactivity of the materials. Here, it is demonstrated that argon annealing (another prevalently used oxygen‐deficient gas) in the temperature range of 300–700 °C greatly affects the activity of dual‐faceted BiVO4 microcrystals for photocatalytic O2 generation and photocurrent generation. While treatment at 300 °C has little to no effect, higher temperatures of 500 and 700 °C significantly improve the crystallinity, alter the local structure distortion, and reduce the bandgap energy of the treated BiVO4. The higher temperature treatment also favors formation of new subgap states attributed to oxygen vacancies, as supported by surface photovoltage and electron paramagnetic resonance spectroscopies. Despite the most profound improvements in structural, optical, and electronic properties displayed by the 700 °C‐treated BiVO4, the sample annealed at 500 °C exhibits the highest photoactivity. The lower activity of the 700 °C‐treated BiVO4 is ascribed to the creation of bismuth vacancies and the loss of well‐defined crystal facets, contributing to impeded electron transport and poor charge separation.

Surface photovoltage spectroscopy (SPS) is used to measure the photopotential across a Ru-SrTiO:Rh/BiVO particle tandem overall water splitting photocatalyst. The tandem is synthesized from Ru-modified SrTiO:Rh nanocrystals and BiVO microcrystals by electrostatic assembly followed by thermal annealing. It splits water into H and O with an apparent quantum efficiency of 1.29% at 435 nm and a solar to hydrogen conversion efficiency of 0.028%. According to SPS, a photovoltage develops above 2.20 eV, the effective band gap of the tandem, and reaches its maximal value of -2.45 V at 435 nm (2.44 mW cm), which corresponds to 96% of the theoretical limit of the photocatalyst film on the fluorine-doped tin-oxide-coated glass (FTO) substrate. Charge separation is 82% reversible with 18% of charge carriers being trapped in defect states. The unusually strong light intensity dependence of the photovoltage (1.16 V per decade) is attributed to depletion layer changes inside of the BiVO microcrystals. These findings promote the understanding of solar energy conversion with inorganic particle photocatalysts.

No abstract

Any optimization of dye-sensitized solar cells (DSSCs) must consider the energetics and charge transfer kinetics of the dye, substrate, and redox couple. Here, we use surface photovoltage spectroscopy to probe the energetics and photochemical charge transfer efficiency in fluorenyl-thiophene dye (OD-8)-sensitized ZnO films. Discrete photochemical charge transfer events at the dye–ZnO interface and at the dye– I–/I3 – or [Co­(2,2′-bipyridyl)3]3+/2+ interfaces can be observed as negative photovoltage under dye excitation at 1.7 eV (460 nm). Without a redox couple, charge separation at the ZnO/dye interface is only 4% effective, likely due to the short electron hole separation distance. In the presence of the redox couples, charge separation approaches 26–54% of the theoretical limit, emphasizing the importance of the dye regeneration reaction via the redox couple. On the basis of the open circuit voltage, charge separation in fully assembled DSSCs is 100% efficient with iodide, but only 61% efficient with the cobalt redox couple. This suggests that device improvements are possible by optimizing the dye regeneration reaction with the cobalt redox couple.

Microstructure controlled ammonolysis allowed the synthesis of oxynitrides La_1−xY_xTa^IVO₂N (x ≤ 0.3) and YTa(O,N)₃ of which the first ones showed a remarkable up-built of photovoltage even in the presence of reduced tantalum (e.g. Ta⁴⁺).