Hydrothermal and solid-state reaction methods are commonly used to prepare the delafossite CuCrO 2 photocatalyst. It has been reported that the photocatalytic performances of CuCrO 2 samples prepared by these methods are quite different. In order to explore the possible influence of different preparation processes on the photocatalytic performance and the corresponding improvement strategies, this work compares the microstructure and physicochemical properties of the samples prepared by these two methods on the basis of optimizing the process conditions. A CuCrO 2 sample prepared by a hydrothermal method is characterized by high purity, low crystallinity, small grain size, and relatively higher photocatalytic activity. A CuCrO 2 sample prepared by a solid-state reaction method is characterized by low purity, high crystallinity, large grain size, and relatively lower photocatalytic activity. In combination with DFT calculations, it is confirmed that the CuCrO 2 sample prepared by a solid-state reaction method contains a certain amount of interstitial oxygens. Due to the presence of interstitial oxygens, CuCrO 2 has strong light absorption in the visible region, presents semimetallic ferromagnetism, and changes the carrier transport, reaction process, and rate on the electrode surface. These findings will contribute to the further development of efficient CuCrO 2 -based photocatalysts.

The built-in electric field generated by polar materials is one of the most effective strategies to promote the separation of photogenerated electron−hole pairs in the field of photocatalysis. However, because of the complexity and diversity of the built-in electric field in polar materials, it is not clear how to enhance the photocatalytic performance and how to control the polar electric field effectively. To this end, four-layered bismuth oxyhalides, BiOX, and BiOXO 3 (X = Br, I) were synthesized by a simple hydrothermal method. X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy analysis confirmed that they all have the structure characteristics of a sillenite phase. Scanning electron microscopy images show that they all have the morphology of nanosheets. Among them, BiOBrO 3 was successfully synthesized and characterized for the first time in the present work. The order of photocatalytic performance (including carrier's lifetime, photocurrent density, and H 2 evolution rate) of the four compounds is listed as follows: BiOBrO 3 > BiOI > BiOIO 3 > BiOBr. In the bulk of the BiOXO 3 photocatalyst, the spontaneous polar built-in electric field along the [001] direction is the crucial factor to inhibit the recombination of photogenerated electron−hole pairs, while the surface polar electric field in BiOI can outstandingly inhibit the recombination of photogenerated electron−hole pairs due to the breaking of the mirror symmetry. Therefore, regulating the microstructure and composition of the structure unit, which generates the built-in electric field, can indeed control the magnitude, direction, and effects of built-in electric fields. In practice, we should carefully adjust the strategy according to the actual situation so as to reasonably design and use the polar electric field, giving full play to its role and enhancing the photocatalytic performance.

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