On the structural evolution of nanoporous optically transparent CuO photocathodes upon calcination for photoelectrochemical applications

Copper oxides are promising photocathode materials for solar hydrogen production due to their narrow optical band gap energy allowing broad visible light absorption. However, they suffer from severe photocorrosion upon illumination, mainly due to copper reduction. Nanostructuring has been proven to enhance the photoresponse of CuO photocathodes; however, there is a lack of precise structural control on the nanoscale upon sol-gel synthesis and calcination for achieving optically transparent CuO thin film photoabsorbers. In this study, nanoporous and nanocrystalline CuO networks were prepared by a soft-templating and dip-coating method utilizing poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic® F-127) as a structure-directing agent, resulting for the first-time in uniformly structured, crack-free, and optically transparent CuO thin films. The photoelectrochemical properties of the nanoporous CuO frameworks were investigated as a function of the calcination temperature and film thickness, revealing important information about the photocurrent, photostability, and photovoltage. Based on surface photovoltage spectroscopy (SPV), the films are p-type and generate up to 60 mV photovoltage at 2.0 eV (0.050 mW cm-2) irradiation for the film annealed at 750 °C. For these high annealing temperatures, the nanocrystalline domains in the thin film structure are more developed, resulting in improved electronic quality. In aqueous electrolytes with or without methyl viologen (as a fast electron acceptor), CuO films show cathodic photocurrents of up to -2.4 mA cm-2 at 0.32 V vs. RHE (air mass (AM) 1.5). However, the photocurrents were found to be entirely due to photocorrosion of the films and decay to near zero over the course of 20 min under AM 1.5 illumination. These fundamental results on the structural and morphological development upon calcination provide a direction and show the necessity for further (surface) treatment of sol-gel derived CuO photocathodes for photoelectrochemical applications. The study demonstrates how to control the size of nanopores starting from mesopore formation at 400 °C to the evolution of macroporous frameworks at 750 °C.

Optical and Electrical Modulation Strategies of Photoelectrodes for Photoelectrochemical Water Splitting

When constructing efficient, cost-effective, and stable photoelectrodes for photoelectrochemical (PEC) systems, the solar-driven photo-to-chemical conversion efficiency of semiconductors is limited by several factors, including the surface catalytic activity, light absorption range, carrier separation, and transfer efficiency. Accordingly, various modulation strategies, such as modifying the light propagation behavior and regulating the absorption range of incident light based on optics and constructing and regulating the built-in electric field of semiconductors based on carrier behaviors in semiconductors, are implemented to improve the PEC performance. Herein, the mechanism and research advancements of optical and electrical modulation strategies for photoelectrodes are reviewed. First, parameters and methods for characterizing the performance and mechanism of photoelectrodes are introduced to reveal the principle and significance of modulation strategies. Then, plasmon and photonic crystal structures and mechanisms are summarized from the perspective of controlling the propagation behavior of incident light. Subsequently, the design of an electrical polarization material, polar surface, and heterojunction structure is elaborated to construct an internal electric field, which serves as the driving force to facilitate the separation and transfer of photogenerated electron-hole pairs. Finally, the challenges and opportunities for developing optical and electrical modulation strategies for photoelectrodes are discussed.

Emerging Chalcogenide Thin Films for Solar Energy Harvesting Devices

Chalcogenide semiconductors offer excellent optoelectronic properties for their use in solar cells, exemplified by the commercialization of Cu(In,Ga)Se₂- and CdTe-based photovoltaic technologies. Recently, several other chalcogenides have emerged as promising photoabsorbers for energy harvesting through the conversion of solar energy to electricity and fuels. The goal of this review is to summarize the development of emerging binary (Sb₂X₃, GeX, SnX), ternary (Cu₂SnX₃, Cu₂GeX₃, CuSbX₂, AgBiX₂), and quaternary (Cu₂ZnSnX₄, Ag₂ZnSnX₄, Cu₂CdSnX₄, Cu₂ZnGeX₄, Cu₂BaSnX₄) chalcogenides (X denotes S/Se), focusing especially on the comparative analysis of their optoelectronic performance metrics, electronic band structure, and point defect characteristics. The performance limiting factors of these photoabsorbers are discussed, together with suggestions for further improvement. Several relatively unexplored classes of chalcogenide compounds (such as chalcogenide perovskites, bichalcogenides, etc.) are highlighted, based on promising early reports on their optoelectronic properties. Finally, pathways for practical applications of emerging chalcogenides in solar energy harvesting are discussed against the backdrop of a market dominated by Si-based solar cells.

Synthesis and control strategies of nanomaterials for photoelectrochemical water splitting

Photoelectrochemical water splitting to produce hydrogen using solar energy can capture and directly convert solar energy into chemical energy, which is an effective way to deal with the current energy and environmental problems. The conversion efficiency of solar energy depends on the performance of semiconductor photoelectrodes in photoelectrochemical water splitting. This article presents our recent advances in the design and performance control of high-efficiency photoelectrocatalytic materials, followed by the discussion of the strategies employed for improving the performances of photoelectrodes in terms of photon absorption, charge separation and migration, as well as surface chemical reactions.

A ZnO@CuO core–shell heterojunction photoanode modified with ZnFe-LDH for efficient and stable photoelectrochemical performance

A highly efficient ZnO@CuO core–shell heterojunction photoanode modified with cocatalyst ZnFe-layered double hydroxides was designed and synthesized in this work.

A promising p-type Co-ZnFe2O4 nanorod film as a photocathode for photoelectrochemical water splitting

A p-type Co-ZnFe₂O₄ film with a one-dimensional (1D) rod-like morphology is fabricated for the first time on fluorine-doped tin oxide (FTO) through a hydrothermal reaction and sintering treatment. The p-type Co-ZnFe₂O₄ is obtained by doping Co ions into n-type ZnFe₂O₄, in which Zn sites are substituted by Co. Compared with the n-type ZnFe₂O₄, the light absorption edge of Co-ZnFe₂O₄ is clearly shifted from 589 to 624 nm, and the positions of the valence/conduction band of Co-ZnFe₂O₄ meet the thermodynamic requirements for water splitting. The photocurrent density of p-type Co-ZnFe₂O₄ is -0.22 mA cm^-2 at 0 V vs. the reversible hydrogen electrode (RHE), which is enhanced 7.33-times vs. that of n-type ZnFe₂O₄ (-0.03 mA cm^-2 at 0 V vs. RHE). This work provides useful insights into tuning the p-n character of semiconductors to realize efficient photoelectrochemical (PEC) water splitting.