RESUMO
Recombination of photoexcited carriers at interface states is generally believed to strongly govern the photoelectrochemical (PEC) performance of semiconductors in electrolytes. Sacrificial reagents (e.g., methanol or Na2SO3) are often used to assess the ideal PEC performance of photoanodes in cases of minimised interfacial recombination kinetics as well as accelerated surface reaction kinetics. However, varying the sacrificial reagents in the electrolyte means simultaneously changing the equilibrium potential and the number of electrons required to perform the sacrificial reaction, and thus the thermodynamic and kinetic aspects of the PEC reactions cannot be distinguished. In the present study, we propose an alternative methodology to experimentally evaluate the energy levels of interfacial recombination centres that can reduce PEC performance. We prepare nonaqueous electrolytes containing three different Ru complexes with different bipyridyl ligands; redox reactions of Ru complexes represent one-electron processes with similar charge transfer rates and diffusion coefficients. Therefore, the Ru complexes can serve as a probe to isolate and evaluate only the thermodynamic aspects of PEC reactions. Recombination centres at the interface between a nonaqueous electrolyte and a Zn0.25Cd0.75Se particulate photoanode are elucidated using this method as a model case. The energy level at which photocorrosion proceeds is also determined.
RESUMO
Ball milling has been shown empirically to produce fine photocatalytic particles from large bulky particles but to drastically reduce the photocatalytic activity of such material during water splitting due to mechanical damage to the photocatalyst surfaces. If the damaged photocatalyst surfaces could be removed or reconstructed, the reduced particle sizes resulting from milling would be expected to provide enhanced photocatalytic activity. In the present study, fine particles of crystalline Cu2Sn0.38Ge0.62S3 (CTGS), which is responsive to long wavelength light up to the near-infrared region, were synthesized by a flux method and subsequent ball milling. A photocathode made of such particles showed significantly enhanced photoelectrochemical (PEC) performance under simulated sunlight while the photocatalytic hydrogen evolution activity of a powder suspension system made from the same material exhibited a typical decrease. The CTGS crystalline particles synthesized using the flux method were found to be highly crystalline but to have relatively large micrometer-scale sizes. Ball milling reduced the particle size but produced an amorphous coating of oxidized species that lowered the photocatalytic activity of the powder suspension system. Typical surface modifications of a photocathode made from this material, consisting of wet chemical processes, also served as an etching treatment to successfully remove the minimally crystalline surface layer and provide greater PEC activity. These data suggest the benefits of combining flux crystal growth with ball milling and the appropriate chemical etching process to obtain high-crystallinity fine photocatalytic particles responsive to long wavelength light with improved PEC hydrogen evolution activity.
RESUMO
Cu2SnxGe1-xS3 (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 Cu2SnxGe1-xS3 and CuySn0.38Ge0.62S3 (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.
