Photoelectrochemical water splitting for hydrogen production provides a pollution-free and sustainable way to alleviate the over-reliance on fossil fuels. Cu 2 O is one of the photocathodes most concerned with superior performance, but it suffers from photocorrosion in the electrolyte, which is unfavorable to hydrogen production. Therefore, a reasonable and simple protection strategy to inhibit the photocorrosion of Cu 2 O is urgently needed in the long term hydrogen production device. In this work, we proposed a Cu 2 O-based photocathode protection strategy, which uses atomic layer deposition technology to deposit TiN thin film as the protective layer. The optimized photocathode shows remarkably improved stability: the photocurrent density maintains the initial value of −1.4 mA cm −2 for more than 35 min at 0 V vs RHE; after 1-h testing, it still remains at 71% of the initial value, with 16-fold enhancement in comparison with that of the bare photocathode. This work offers a way to solve the stability issue of unstable photoelectrodes.

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environmental problems. Hydrogen gas (H 2 ) is a clean and storable energy with a higher gravimetric energy density than gasoline (120 vs 44 MJ kg −1 ). Among various hydrogen production strategies, photoelectrochemical (PEC) water splitting has attracted much attention due to its high abundance of water, high purity of generated H 2 , and great reduction of CO 2 emission. [1][2][3][4][5] As the core of the PEC cell, the photoelectrode is composed of a photoanode and a photo cathode, which drives the oxygen evolution reaction (OER, +1.23 V vs reversible hydrogen electrode (RHE)) and hydrogen evolution reaction (HER, 0 V vs RHE), respectively. [1][2][3] At present, both photoanode and photo cathode are facing severe challenges. Among them, photocathode is mainly restricted by the few types of ptype semiconductors (SCs), which need to be improved. Cuprous oxide (Cu 2 O) is one of the most important photocathode materials and has many advantages, such as nontoxicity, element abundant, and high activity. [6,7] It has a narrow direct bandgap of ≈2.0 eV that favors visible light absorption. Its conduction band edge is more negative than −0.7 V versus RHE, providing a large driving force for HER. [6][7][8][9] The theoretical value of photocurrent density is as high as In the field of photoelectrochemical (PEC) water splitting, cuprous oxide (Cu 2 O) is one of the most promising photocathode materials, but its performance is restricted by poor carrier separation ability and low photovoltage. In order to overcome these limitations, a new kind of Cu 2 O-ZnO blended heterojunction photocathode is designed and prepared by novel one-step thermal oxidation method. ZnO granules are uniformly distributed in Cu 2 O film matrix, forming a granular structure, which enhances the band bending of Cu 2 O in the electrolyte and improves the photovoltage. In addition, the formed ladder type band alignment of Cu 2 O-ZnO facilitates the spatial separation of photoexcited carriers. Different from the conventional layered heterojunction, the granular structured heterojunction proposed in this work extends the built-in electric field region and further promotes the transmission of photoexcited carriers from the photoelectrode to the electrolyte. At 0 V vs reversible hydrogen electrode (RHE), the photocurrent density of Cu 2 O-ZnO film is as high as −8.7 mA cm −2 , which is over 6 times that of bare Cu 2 O (−1.3 mA cm −2 ). The onset potential positively shifts from 0.57 V vs RHE to 0.78 V vs RHE. This work provides an effective strategy for improving the PEC performance from the perspective of band alignment and material structure.

The Z-scheme overall solar water splitting is a mimic of natural photosynthesis to convert solar energy into chemical energy. Since the energy levels of most organic semiconductors match well with the hydrogen evolution potential, they have great application prospects as photocathodes in Z-scheme photoelectrochemical systems. However, due to the weak light absorption and difficult carrier separation, the photocurrent density and onset potential of organic photocathodes are still low. To solve these problems, we introduced a copper nanosheets array (Cu NSA) framework under organic layers to increase the surface reaction sites, improve the light absorption and enhance the distribution range of built-in electric field simultaneously. As a result, the photocurrent density and onset potential of poly(3-hexylthiophene) : [6,6]-phenyl-C 61 -butyric acid (P3HT : PCBM) photocathode were enhanced significantly. The onset potential increased by 50 mV to 0.65 V vs. RHE, and the photocurrent density reached À 1 mA cm À 2 at 0 V vs. RHE, which was 18 times that of the sample without Cu NSA. The optimized photocathode was connected with titanium dioxide nanorods array photoanode in a tandem manner to realize the spontaneous overall water splitting. Without bias and co-catalyst, the photocurrent density was maintained at 110 μA cm À 2 and the solar-to-fuel conversion efficiency was 0.14 % in neutral solution. These results provide a feasible method for optimizing the performance of organic photocathodes.