Design and In Situ Growth of Cu₂O‐Blended Heterojunction Directed by Energy‐Band Engineering: Toward High Photoelectrochemical Performance

Abstract

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.