Cu2Sn x Ge1–x S3 (CTGS) particles were synthesized via a solid-state reaction and assessed, for the first time, as both photocatalysts and photocathode materials for hydrogen evolution from water. Variations in the crystal and electronic structure with the Sn/Ge ratio were examined experimentally and theoretically. The incorporation of Ge was found to negatively shift the conduction band minimum, such that the bandgap energy could be tuned over the range 0.77–1.49 eV, and also increased the driving force for the photoexcited electrons involved in hydrogen evolution. The effects of the Sn/Ge ratio and of Cu deficiency on the photoelectrochemical performance of Cu2Sn x Ge1–x S3 and Cu y Sn0.38Ge0.62S3 (1.86 < y < 2.1) based photocathodes were evaluated under simulated sunlight. Both variations in the band-edge position and the presence of a secondary impurity phase affected the performance, such that a particulate Cu1.9Sn0.38Ge0.62S3 photocathode was the highest performing specimen. This cathode gave a half-cell solar-to-hydrogen energy conversion efficiency of 0.56% at 0.18 V vs a reversible hydrogen electrode (RHE) and an incident-photon-to-current conversion efficiency of 18% in response to 550 nm monochromatic light at 0 VRHE. More importantly, these CTGS particles also demonstrated significant photocatalytic activity during hydrogen evolution and were responsive to radiation up to 1500 nm, representing infrared light. The chemical stability, lack of toxicity, and high activity during hydrogen evolution of the present CTGS particles suggest that they may be potential alternatives to visible/infrared light responsive Cu–chalcogenide photocatalysts and photocathode materials such as Cu­(In,Ga)­(S,Se)2 and Cu2ZnSnS4.

The photocatalyst titania film surface acquires a high hydrophilicity after it is exposed to UV light, which is induced by changes in the densities of the surface hydroxyl groups and charge. A xanthene dye, fluorescein, was deposited from a solution onto a titania film after UV irradiation in order to probe the titania surface change. The change in the surface acidity was confirmed by the ratio of dianion to monoanion of fluorescein by Raman spectroscopy. The ratio increased by UV irradiation, indicating that the surface became more basic. Transient absorption spectroscopy revealed the transformation from the monoanion to dianion via the excited states, that is, photoinduced proton transfer from fluorescein to the titania surface. The UV irradiation increases the basic hydroxyl groups on the titania surface, which accepts protons from the water molecules or proton donors on the surface and has a positive charge.

A novel “photovoltaics (PV) + electrolyzer” concept is presented using a simple, small, and completely stand-alone non-biased device for solar-driven overall water splitting. Three or four spherical-shaped p-n junction silicon balls were successfully connected in series, named “SPHELAR.” SPHELAR possessed small projected areas of 0.20 (3PVs) and 0.26 cm2 (4PVs) and exhibited working voltages sufficient for water electrolysis. Impacts of the configuration on the PV module performance were carefully analyzed, revealing that a drastic increase in the photocurrent (≈20%) was attained by the effective utilization of a reflective sheet. Separate investigations on the electrocatalyst performance showed that non-noble metal based materials with reasonably small sizes (<0.80 cm2) exhibited substantial currents at the PV working voltage. By combining the observations of the PV characteristics, light management and electrocatalyst performance, solar-driven overall water splitting was readily achieved, reaching solar-to-hydrogen efficiencies of 7.4% (3PVs) and 6.4% (4PVs).

This study investigated the properties of a photoanode fabricated by depositing a p-type CdTe thin film on a CdS-coated FTO substrate (CdTe/CdS/FTO) via close-space sublimation. This CdTe/CdS/FTO electrode was found to work as a photoanode with a long absorption edge wavelength of 830 nm. In a CdTe-based photoanode such as this, the p-n junction formed at the CdTe/CdS interface promotes charge separation of photoexcited carriers and forces photogenerated holes to move toward the photoanode surface to promote oxidation reactions on the electrode surface. A MoO buffer layer was also found to play a crucial role in facilitating the transfer of photogenerated holes to surface reaction sites through decreasing the energy barrier at the interface between the CdTe and a surface protective layer. A biphotoelectrode photoelectrochemical cell composed of a CdTe-based photoanode and a CdTe-based photocathode exhibited a solar-to-hydrogen conversion efficiency of 0.22% without an external voltage in response to illumination by AM 1.5G light.

A particulate (ZnSe) 0.85 (CuIn 0.7 Ga 0.3 Se 2 ) 0.15 (ZnSe:CIGS)-based photocathode modified with RuO 2 and Pt species exhibited improved photoelectrochemical activities and stability for hydrogen evolution as well as production of methylcyclohexane, the promising hydrogen carrier, from toluene using a highly alkaline aqueous solution as a hydrogen source under sunlight with almost 100% of faradaic efficiency. It was revealed that the co-loading of RuO 2 with Pt changed the Pt oxidation state, partly explaining the improved activity and stability, associated with an anchoring effect of Pt. It was also determined that such highly alkaline conditions promote selective MCH production, possibly because of the improved performance of the anion exchange membrane. The present study involving the construction of a surface protective/catalytic layer suggests a novel approach to artificial photosynthesis for solar energy harvesting in the form of organic hydrides.

(ZnSe)0.85(CuIn0.7Ga0.3Se2)0.15 (ZnSe:CIGS) photocatalytic particles are a promising candidate material for photocathodes in sunlight-driven photoelectrochemical (PEC) hydrogen evolution systems responsive up to 800-850 nm. However, the ZnSe:CIGS particles used in prior...

A particulate solid solution, (ZnSe)0.85(CuIn0.7Ga0.3Se2)0.15, was synthesized by the flux method using various amounts of a Cu precursor (to make Cu-deficient, stoichiometric, or Cu-excess specimens) and/or a Na2S additive, to assess the effects of synthesis conditions on photoelectrochemical (PEC) properties.

The development of semitransparent photoanodes is required for the construction of tandem photoelectrochemical (PEC) water splitting cells incorporating photocathodes. However, the poor stability of transparent conductive oxides at high temperatures hampers the growth of non-oxide photoanodes with intense visible light absorption. In this work, semitransparent Ta 3 N 5 thin film photoanodes were prepared on quartz glass substrates coated with carbon nanotubes (CNTs) by sputtering and thermal nitridation. This process makes use of the high thermal and chemical stability as well as the tunable conductivity and transmittance of CNT substrates. The photoanodic current produced by these Ta 3 N 5 photoanodes at negative potentials is also enhanced by surface modification with Mg species. Conductive semitransparent CNT substrates such as these will assist in the development of new tandem PEC cells for water splitting.Photoelectrochemical (PEC) water splitting has been intensively investigated as a promising means of harvesting solar energy in the form of hydrogen as a chemical energy carrier. [1] A tandem-type configuration, composed of a top semitransparent photoanode and a bottom narrow-gap photocathode, is a potential means of realizing high solar-to-hydrogen energy conversion efficiencies. [2] However, in prior research, the upper photoanode materials have been primarily limited to oxides such as semitransparent TiO 2 , [3] BiVO 4[4] and α-Fe 2 O 3 , [5] owing to the thermal and chemical instability of conventional transparent conductive oxides.Ta 3 N 5 is a promising nitride photoanode material for PEC oxygen evolution from water, as it has an absorption edge of 600 nm. [6] Ta 3 N 5 thin film photoanodes synthesized by the sputtering of Ta-based precursors and subsequent thermal nitridation under an NH 3 flow have shown high anodic photocurrents at positive potentials in the vicinity of the reversible oxygen evolution potential of 1.23 V (vs. a reversible hydrogen electrode (V RHE )). [7] Notably, semitransparent Ta 3 N 5 photoanodes can be obtained by employing thermally and chemically stable n-type GaN-coated sapphire as transparent and electrically conductive substrate. [8] The exploration of stable, transparent and conductive materials such as this is expected to result in new opportunities to produce semitransparent non-oxide photoelectrodes with intense visible light absorption.Carbon nanotubes (CNTs) can form highly conductive and transparent thin films on substrates via repetitive dispersion/ extraction and vacuum filtration processes. [9] CNT films are also highly stable during high temperature processing under an NH 3 flow and thus are ideal as substrates for the fabrication of Ta 3 N 5 thin film photoanodes. There is a trade-off relationship between the conductivity and transparency of CNT films, based on the amount of CNTs that is loaded. [9] Even so, it is likely that semitransparent Ta 3 N 5 photoanodes can be prepared by employing CNT thin films as current collecting electrodes.In the present stud...