Nanoparticulate TiN Loading to Promote Z-Scheme Water Splitting Using a Narrow-Bandgap Nonoxide-Based Photocatalyst Sheet.

Some oxide-based particulate photocatalyst sheets exhibit excellent activity during the water-splitting reaction. The replacement of oxide photocatalysts with narrow-bandgap photocatalysts based on nonoxides could provide the higher solar-to-hydrogen energy conversion efficiencies that are required for practical implementation. Unfortunately, the activity of nonoxide-based photocatalyst sheets is low in many cases, indicating the need for strategies to improve the quality of nonoxide photocatalysts and the charge transfer process. In this work, single-crystalline particulate SrTaO2N is studied as an oxygen evolution photocatalyst for photocatalyst sheets applied to Z-scheme water splitting, in combination with La5Ti2Cu0.9Ag0.1O7S5 and Au as the hydrogen evolution photocatalyst and conductive layer, respectively. The loading of SrTaO2N with CoOx provided increases activity during photocatalytic water oxidation, giving an apparent quantum yield of 15.7% at 420 nm. A photocatalyst sheet incorporating CoOx-loaded SrTaO2N is also found to promote Z-scheme water splitting under visible light. Notably, the additional loading of nanoparticulate TiN on the CoOx-loaded SrTaO2N improves the water splitting activity by six times because the TiN promotes electron transfer from the SrTaO2N particles to the Au layer. This work demonstrates key concepts related to the improvement of nonoxide-based photocatalyst sheets based on facilitating the charge transfer process through appropriate surface modifications.

Co-doping of a La5Ti2Cu0.9Ag0.1O7S5 photocatalyst (λ < 700 nm) with Ga and Al to enhance photocatalytic H2 evolution.

La5Ti2Cu0.9Ag0.1O7S5 (LTCA) (λ < 700 nm) can function as a photocatalyst for H2 evolution. Co-doping LTCA with Ga3+ and Al3+ at Ti4+ sites effectively enhanced the H2 evolution activity of LTCA, yielding an apparent quantum efficiency of 18% at 420 nm. The activity of this material was greater than that previously reported for Ga-doped LTCA by a factor of 1.6. Such activity enhancement is attributed to increasing the population of long-lived photogenerated electrons and facilitating the electron transfer to the cocatalyst. This work significantly improved the LTCA-based photocatalyst for H2 evolution, making it a promising material for future application in non-sacrificial Z-scheme water splitting.

Improved water oxidation with metal oxide catalysts via a regenerable and redox-inactive ZnOxHy overlayer.

We report a regenerable and redox-inactive ZnOxHy layer that was in situ deposited onto metal oxides MOz (M = Co, Fe, and Ni) in alkaline media containing [Zn(OH)4]2- species during water oxidation. An interface dipole was developed at the MOz/Zn interface, resulting in a decrease of the OER overpotential. Exemplified by the CoOz/ZnOxHy bilayer structure, it presented a 155 mV lower overpotential to deliver 10 mA cm-2 and long-term stability relative to the unmodified CoOz film.

Prospects and challenges in designing photocatalytic particle suspension reactors for solar fuel processing.

Photocatalytic approaches for the production of solar hydrogen or hydrocarbons are interesting as they provide a sustainable alternative to fossil fuels. Research has been focused on water splitting and on the synthesis of photocatalyst materials and compounds, and their characterization. The material-related challenges include the synthesis and design of photocatalysts that can absorb visible light at a high quantum efficiency, cocatalysts that are selective and can accelerate the reduction and/or oxidation reactions, and protection layers that facilitate migration of the minority carriers to the surface-active sites while reducing charge recombination and photo-corrosion. Less attention has been paid to the conceptual design of reactors, and how design and coupled transport can affect the material choice and requirements. This perspective discusses the various possible conceptual designs for particle suspension reactors and the related implications on the material requirements to achieve high energy conversion efficiencies. We establish a link between the thermodynamic limits, materials requirements, and conceptual reactor designs, quantify changes in material requirements when more realistic operation and losses are considered, and compare the theory-derived guidelines with the ongoing materials research activity.

Effects of Se Incorporation in La5Ti2CuS5O7 by Annealing on Physical Properties and Photocatalytic H2 Evolution Activity.

Oxysulfoselenide semiconductor photocatalysts absorb light at longer wavelengths than the corresponding oxysulfides. However, the synthesis of oxysulfoselenides is challenging due to excessive particle growth and the limited availability of metal selenide precursors. In this study, a La5Ti2CuS5O7 (LTCSO) oxysulfide was annealed with Se powder in sealed, evacuated quartz tubes to obtain LTCSO:Se photocatalysts, and the properties of these materials were investigated. Se was found to be incorporated into the LTCSO upon heating at 973 K or higher, and the Se/(S + Se) ratio was increased to a maximum of 0.3 upon repeating the heat treatment twice. The addition of Se extended the absorption edge of the LTCSO and thus increased its photocatalytic H2 evolution activity at longer wavelength. Even so, the apparent quantum yield at shorter wavelengths was reduced, which is similar to the results obtained for La5Ti2Cu(S1- xSe x)5O7 (LTCS1- xSe xO) solid solutions. Overall water splitting was achieved by constructing photocatalyst sheets using LTCSO:Se and LTCS1- xSe xO as hydrogen evolution photocatalysts and BiVO4 as an oxygen evolution photocatalyst. Heat treatment with Se is evidently an effective method for the transformation of oxysulfide photocatalysts to oxysulfoselenides that promote photocatalytic H2 evolution and have longer absorption edge wavelengths.