Converting sunlight to solar fuels by artificial photosynthesis is an innovative science and technology for renewable energy. Light harvesting, photogenerated charge separation and transfer (CST), and catalytic reactions are the three primary steps in the processes involved in the conversion of solar energy to chemical energy (SE‐CE). Among the processes, CST is the key “energy pump and delivery” step in determining the overall solar‐energy conversion efficiency. Efficient CST is always high priority in designing and assembling artificial photosynthesis systems for solar‐fuel production. This Review not only introduces the fundamental strategies for CST but also the combinatory application of these strategies to five types of the most‐investigated semiconductor‐based artificial photosynthesis systems: particulate, Z‐scheme, hybrid, photoelectrochemical, and photovoltaics‐assisted systems. We show that artificial photosynthesis systems with high SE‐CE efficiency can be rationally designed and constructed through combinatory application of these strategies, setting a promising blueprint for the future of solar fuels.

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.

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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The development of efficient and stable semiconductor photocatalysts for water splitting is regarded as a viable means to produce renewable hydrogen using abundant solar energy. Although some oxide semiconductors have already been demonstrated as efficient and stable photocatalysts for overall water splitting under UV light irradiation, no oxide semiconductor photocatalysts have been reported so far to split water efficiently under visible light irradiation. Hence, screening of visible-light-absorbing oxide semiconductors is highly demanded for photocatalytic water splitting. Herein, Sr2CoTaO6 double perovskite oxide has been identified as a visible-light-absorbing semiconductor photocatalyst with an electronic band structure suitable for overall water splitting by density of states (DOS) analysis. To demonstrate the capability of photocatalytic water splitting properties of Sr2CoTaO6, cubic-shaped Sr2CoTaO6-F and irregular-shaped Sr2CoTaO6-S were synthesized by the facile flux method (F) and the conventional high-temperature solid-state reaction method (S). The experimental analysis of the band structure indicates that the synthesized Sr2CoTaO6 is an n-type semiconductor with a bandgap of 2.7 eV, and the minimum of conduction band (CB) positions and the maximum of valence band (VB) positions are located at −0.87 and +1.83 V vs normal hydrogen electrode (NHE, pH = 7), respectively. Sr2CoTaO6-F showed much higher photocatalytic oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) activities than Sr2CoTaO6-S, indicating the advantage of the flux method in synthesizing double perovskite oxides in control of crystallinity and morphology. Without loading any cocatalysts, Sr2CoTaO6-F showed bifunctional photocatalytic oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) activities in the presence of sacrificial reagents under visible light irradiation, which is rarely reported for metal-oxide-based photocatalysts. The photocatalytic OER and HER activities could be further enhanced by loading RuO2 and Rh cocatalysts, respectively. This work further supports that visible-light-absorbing double perovskite oxide semiconductors are promising candidates worth exploring for photocatalytic water splitting.