Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

Solar-driven reduction of dinitrogen (N2 ) to ammonia (NH3 ) is severely hampered by the kinetically complex and energetically challenging multielectron reaction. Oxygen vacancies (OVs) with abundant localized electrons on the surface of bismuth oxybromide-based semiconductors are demonstrated to have the ability to capture and activate N2 , providing an alternative pathway to overcome such limitations. However, bismuth oxybromide materials are susceptible to photocorrosion, and the surface OVs are easily oxidized and therefore lose their activities. For realistic photocatalytic N2 fixation, fabricating and enhancing the stability of sustainable OVs on semiconductors is indispensable. This study shows the first synthesis of self-assembled 5 nm diameter Bi5 O7 Br nanotubes with strong nanotube structure, suitable absorption edge, and many exposed surface sites, which are favorable for furnishing sufficient visible light-induced OVs to realize excellent and stable photoreduction of atmospheric N2 into NH3 in pure water. The NH3 generation rate is as high as 1.38 mmol h-1 g-1 , accompanied by an apparent quantum efficiency over 2.3% at 420 nm. The results presented herein provide new insights into rational design and engineering for the creation of highly active catalysts with light-switchable OVs toward efficient, stable, and sustainable visible light N2 fixation in mild conditions.

Light-Switchable Oxygen Vacancies in Ultrafine Bi 5 O 7 Br Nanotubes for Boosting Solar-Driven Nitrogen Fixation in Pure Water

Precursor-reforming protocol to 3D mesoporous g-C3N4 established by ultrathin self-doped nanosheets for superior hydrogen evolution

The burgeoning development of ammonia (NH3) synthesis technology addresses the urgency of food intake required to sustain the population growth of the last 100 years. To date, NH3 has mostly been synthesized by the Haber–Bosch process in industry. Under the ever-increasing pressure of the fossil fuel depletion crisis and anthropogenic global climate change with continuous CO2 emission in the 21st century, research targeting the synthesis of NH3 under mild conditions in a sustainable and environment friendly manner is vigorous and thriving. Therefore, the focus of this review is the state-of-the-art engineering of efficient photocatalysts for dinitrogen (N2) fixation toward NH3 synthesis. Strenuous efforts have been devoted to modifying the intrinsic properties of semiconductors (i.e. poor electron transport, rapid electron–hole recombination and sluggish reaction kinetics), including nanoarchitecture design, crystal facet engineering, doping and heterostructuring. Herein, this review provides insights into the most recent advancements in understanding the charge carrier kinetics of photocatalysts with respect to charge transfer, migration and separation, which are of fundamental significance to photocatalytic N2 fixation. Subsequently, the challenges, outlooks and future prospects at the forefront of this research platform are presented. As such, it is anticipated that this review will shed new light on photocatalytic N2 fixation and NH3 synthesis and will also provide a blueprint for further investigations and momentous breakthroughs in next-generation catalyst design.

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Photocatalytic fixation of nitrogen to ammonia: state-of-the-art advancements and future prospects

The development of highly efficient photocatalysts with broad spectrum light response is a primary goal in the photocatalysis domain Here we report on a novel ternary photocatalyst comprised of single atom-dispersed silver and carbon quantum dots, co-loaded with ultrathin g-C 3 N 4 (SDAg-CQDs/UCN), which exhibited a highly enhanced photoresponse and broad-spectrum (UV, visible, and near-infrared light) photocatalytic activity The content of 10 wt% of CQDs and 30 wt% of Ag resulted in a 10-fold higher reaction rate than that of UCN under visible light irradiation This improved broad-spectrum photocatalytic performance may be attributed to the surface plasmon resonance effect of Ag, up-converted fluorescent properties of CQDs, narrowed energy gap, as well as the electron separation and transfer capacity of both the Ag and CQDs An electron spin resonance (ESR) technique, and reactive species (RS) scavenging experiments indicated that 1 O 2 and O 2 − were the dominant active species involved in the degradation of naproxen (NPX) Product identification and reaction site prediction revealed that the photocatalytic degradation of NPX included decarboxylation, hydroxylation, as well as the opening of the naphthalene ring Mineralization experiments indicated that NPX and its degradation products would be finally transformed into CO 2 and H 2 O Reactions in different water matrices indicated that SDAg-CQDs/UCN can be effectively employed for the degradation of NPX under ambient water conditions Therefore, SDAg-CQDs/UCN offers a new strategy for the broad-spectrum utilization of solar light and provides a promising method for the remediation of water contamination

Novel ternary photocatalyst of single atom-dispersed silver and carbon quantum dots co-loaded with ultrathin g-C3N4 for broad spectrum photocatalytic degradation of naproxen

Bismuth-rich bismuth oxyhalides (Bi–O–X; X = Cl, Br, I) display high photocatalytic reduction activity due to the promoting conduction band potential. In this work, two Bi5O7I nanosheets with different dominant facets were synthesized using either molecular precursor hydrolysis or calcination. Crystal structure characterizations, included X-ray diffraction patterns (XRD), field emission electron microscopy and fast Fourier transformation (FFT) images, showed that hydrolysis and calcination resulted in the dominant exposure of {100} and {001} facets, respectively. Photocatalytic data revealed that Bi5O7I–001 had a higher activity than Bi5O7I–100 for N2 fixation and dye degradation. Photoelectrochemical data revealed that Bi5O7I–001 had higher photoinduced carrier separation efficiency than Bi5O7I–100. The band structure analysis also used to explain the underlying photocatalytic mechanism based on the different conduction band position. This work presents the first report about the facet-dependent photocat...

Facet-Dependent Photocatalytic N2 Fixation of Bismuth-Rich Bi5O7I Nanosheets

Bismuth oxyhalides (BiOX, X = Br, I) photocatalysts are rarely applied for photocatalytic reduction reaction withthe photo-induced electron, as this is impeded by their low conduction band. As a widely used approach for enhancing the photocatalytic reduction activity, bismuth-rich strategy results the bismuth content of BiOX photocatalysts increasing. In this paper, a solid solutions of bismuth-rich Bi4O5BrxI2-x were prepared applying the molecular precursor method. Bi4O5BrxI2-x were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), element mapping, Brunauer–Emmett–Teller surface analysis (BET), UV–vis diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS). The obtained photocatalytic data showed that Bi4O5BrxI2-x solid solutions had higher photocatalytic activities than Bi4O5Br2 and Bi4O5I2. At an optimal ratio of x = 1, the Bi4O5BrI photocatalyst showed the highest photocatalytic reduction activity for CO2 conversion (22.85 μmol h−1 g−1 CO generation, AQE was 0.372 at 400 nm) and Cr(VI) removal (88%). CO2 adsorption data and CO2 temperature programmed desorption (CO2-TPD) revealed that Bi4O5BrI exhibited the highest chemical adsorption ability of CO2 molecules Photocurrent and electrochemical impedance (EIS) spectroscopy demonstrated the enhanced photo-induced carrier separation efficiency of Bi4O5BrI. These mechanistic studies suggest that Bi4O5BrxI2-x solid solutions are excellent photocatalysts for solar fuel generation and environmental remediation.

Synthesis of hierarchical bismuth-rich Bi4O5BrxI2-x solid solutions for enhanced photocatalytic activities of CO2 conversion and Cr(VI) reduction under visible light

Chen Ting

Papers

Facet-Dependent Photocatalytic N2 Fixation of Bismuth-Rich Bi5O7I Nanosheets

Synthesis of hierarchical bismuth-rich Bi4O5BrxI2-x solid solutions for enhanced photocatalytic activities of CO2 conversion and Cr(VI) reduction under visible light

Bismuth-rich Bi4O5X2 (X = Br, and I) nanosheets with dominant {1 0 1} facets exposure for photocatalytic H2 evolution

Size-dependent role of gold in g-C3N4/BiOBr/Au system for photocatalytic CO2 reduction and dye degradation

An innovative method to introduce magnetism into demulsifier