Converting CO2 into chemical fuels with sunlight is a very attractive approach to solve the greenhouse effect and fossil fuel crisis. Metal halide perovskite nanocrystals (NCs) have been identified as ideal semiconductor photocatalysts for photocatalytic CO2 reduction due to their unique properties, such as strong light absorption, low exciton binding energy, tunable bandgaps, and low cost. However, the pristine perovskite NCs suffering from inevitable defects, which lower their charge transfer efficiency and are detrimental to photocatalytic performance toward CO2 reduction. Herein, a facile approach to modify the surface defects of CsPbBr3 NC is demonstrated using tetrafluoroborate salts as defects treatment agent and loading Co2+ as a cocatalyst. As a result, the optimized Co2+ on the surface of defect‐free CsPbBr3‐BF4 shows a remarkable photocatalytic CO2 activity of 83.8 μmol g−1 h−1, which indicates that the surface modification can effectively suppress the undesired charge recombination in CsPbBr3 NC and promote its charge separation efficiency. This work provides an effective method to modify the surface defects of the CsPbBr3 NCs for high efficient photocatalytic CO2 reduction and broadens the photocatalytic applications of halide perovskites.

Screening of stable visible-light-responsive water oxidation semiconductor photocatalysts is highly desired for the development of photocatalytic water splitting systems. Herein, a visible-light-absorbing Sr2NiWO6 double perovskite oxide photocatalyst was successfully prepared via a conventional solid-state reaction method. The intrinsic Sr2NiWO6 shows photocatalytic oxygen evaluation reaction (OER) activity of 60 μmol h–1 g–1, even without loading any cocatalysts. The DFT calculation indicates that the Ni species on the surface is the active site for the OER. The photocatalytic OER activity was further improved by loading Pt and RuO2 dual redox cocatalysts on the surface of Sr2NiWO6 to achieve a photocatalytic OER activity of 420 μmol h–1 g–1, which corresponds to a remarkable apparent quantum efficiency (AQE) of 8.6% (λ ≈ 420 nm). The result indicates that Sr2NiWO6 is one of the best double perovskite oxide-based photocatalysts for the photocatalytic OER, and the activity is even comparable to the benchmark BiVO4-based photocatalyst. The improvement of the photocatalytic OER activity is due to the provision of more active redox sites as well as the synergetic effect of the dual redox cocatalysts in facilitating charge separation and transfer. This work demonstrates that double perovskite oxides may serve as a novel class of efficient and stable oxide-based semiconductor photocatalysts for water splitting.

Development of photocatalytic oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) photocatalysts with a narrow bandgap is important for solar water splitting. Herein, narrow bandgap Sr2CoWO6 double perovskites with a light absorption edge of ≥700 nm are synthesized by a solid‐state reaction method varying the precursor ratios. The sample synthesized with a precursor Co/W ratio of 1:4 has a conduction band (CB) and valence band (VB) located at −0.82 and 0.95 V versus the normal hydrogen electrode (NHE) at pH = 7, respectively. As a result, both the photocatalytic OER and HER are observed even without loading any cocatalysts. After loading Pt and Rh cocatalysts, the average photocatalytic OER and HER rates are 188 μmol h−1 g−1 (apparent quantum efficiency of 3% at ≈420 nm) and 30 μmol h−1 g−1, respectively. Density functional theory calculations indicate that the OER active sites may shift from a high overpotential W‐site to a low overpotential Co‐site when the W content is increased, which renders high photocatalytic activity for W‐rich samples. Therefore, W‐rich Sr2CoWO6 double perovskite is identified as a novel narrow bandgap bifunctional semiconductor photocatalyst for photocatalytic OER and HER, which is rare for oxide semiconductor photocatalysts. This work opens up a new avenue for the development of oxide‐based double perovskite semiconductor photocatalysts for photocatalytic water splitting.

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Exploiting spontaneous polarization of ferroelectric materials to achieve high charge separation efficiency is an intriguing but challenging research topic in solar energy conversion. This work shows that loading high work function RuO2 cocatalyst on BiFeO3 (BFO) nanoparticles enhances the intrinsic ferroelectric polarization by efficient screening of charges to RuO2 via RuO2/BFO heterojunction. This leads to enhancement of the surface photovoltage of RuO2/BFO single nanoparticles nearly 3 times, the driving force for charge separation and transfer in photocatalytic reactions. Consequently, efficient photocatalytic water oxidation is achieved with quantum efficiency as high as 5.36 % at 560 nm, the highest activity reported so far for ferroelectric materials. This work demonstrates that, unlike low photocurrent density in film‐based ferroelectric devices, high photocatalytic activity could be achieved by regulating the ferroelectric spontaneous polarization using appropriate cocatalyst to enhance driving force for efficient separation and transfer of photogenerated charges in particulate ferroelectric semiconductor materials.

The limiting factor for low photocurrent density of polarization switchable ferroelectric BiFeO3 film is due to severe charge recombination at the interfaces of the domain walls rather than recombination inside the domains.

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