Advances in the Application of Bi-Based Compounds in Photocatalytic Reduction of CO2

Bi-based semiconductor materials have special layered structure and appropriate band gap, which endow them with excellent visible light response ability and stable photochemical characteristics. As a new type of environment-friendly photocatalyst, they have received extensive attention in the fields of environmental remediation and energy crisis resolution and have become a research hotspot in recent years. However, there are still some urgent issues that need to be addressed in the practical large-scale application of Bi-based photocatalysts, such as the high recombination rate of photogenerated carriers, limited response range to visible spectra, poor photocatalytic activity, and weak reduction ability. In this paper, the reaction conditions and mechanism of photocatalytic reduction of CO₂ and the typical characteristics of Bi-based semiconductor materials are introduced. On this basis, the research progress and application results of Bi-based photocatalysts in the field of reducing CO₂, including vacancy introduction, morphological control, heterojunction construction, and co-catalyst loading, are emphasized. Finally, the future prospects of Bi-based photocatalysts are prospected, and it is pointed out that future research directions should be focused on improving the selectivity and stability of catalysts, deeply exploring reaction mechanisms, and meeting industrial production requirements.

Research Progress of Co-Catalysts in Photocatalytic CO2 Reduction: A Review of Developments, Opportunities, and Directions

With the development of the global economy, large amounts of fossil fuels are being burned, causing a severe energy crisis and climate change. Photocatalytic CO2 reduction is a clean and environmentally friendly method to convert CO2 into hydrocarbon fuel, providing a feasible solution to the global energy crisis and climate problems. Photocatalytic CO2 reduction has three key steps: solar energy absorption, electron transfer, and CO2 catalytic reduction. The previous literature has obtained many significant results around the first two steps, while in the third step, there are few results due to the need to add a co-catalyst. In general, the co-catalysts have three essential roles: (1) promoting the separation of photoexcited electron–hole pairs, (2) inhibiting side reactions, and (3) improving the selectivity of target products. This paper summarizes different types of photocatalysts for photocatalytic CO2 reduction, the reaction mechanisms are illustrated, and the application prospects are prospected.

CO2 Reduction Using Water as an Electron Donor over Heterogeneous Photocatalysts Aiming at Artificial Photosynthesis

Photocatalytic and photoelectrochemical CO₂ reduction of artificial photosynthesis is a promising chemical process to solve resource, energy, and environmental problems. An advantage of artificial photosynthesis is that solar energy is converted to chemical products using abundant water as electron and proton sources. It can be operated under ambient temperature and pressure. Especially, photocatalytic CO₂ reduction employing a powdered material would be a low-cost and scalable system for practical use because of simplicity of the total system and simple mass-production of a photocatalyst material.In this Account, single particulate photocatalysts, Z-scheme photocatalysts, and photoelectrodes are introduced for artificial photosynthetic CO₂ reduction. It is indispensable to use water as an electron donor (i.e., reasonable O₂ evolution) but not to use a sacrificial reagent of a strong electron donor, for achievement of the artificial photosynthetic CO₂ reduction accompanied by ΔG > 0. Confirmations of O₂ evolution, a ratio of reacted e^- to h⁺ estimated from obtained products, a turnover number, and a carbon source of a CO₂ reduction product are discussed as the key points for evaluation of photocatalytic and photoelectrochemical CO₂ reduction.Various metal oxide photocatalysts with wide band gaps have been developed for water splitting under UV light irradiation. However, these bare metal oxide photocatalysts without a cocatalyst do not show high photocatalytic CO₂ reduction activity in an aqueous solution. The issue comes from lack of a reaction site for CO₂ reduction and competitive reaction between water and CO₂ reduction. This raises a key issue to find a cocatalyst and optimize reaction conditions defining this research field. Loading a Ag cocatalyst as a CO₂ reduction site and NaHCO₃ addition for a smooth supply of hydrated CO₂ molecules as reactant are beneficial for efficient photocatalytic CO₂ reduction. Ag/BaLa₄Ti₄O₁₅ and Ag/NaTaO₃:Ba reduce CO₂ to CO as a main reduction reaction using water as an electron donor even in just water and an aqueous NaHCO₃ solution. A Rh-Ru cocatalyst on NaTaO₃:Sr gives CH₄ with 10% selectivity (Faradaic efficiency) based on the number of reacted electrons in the photocatalytic CO₂ reduction accompanied by O₂ evolution by water oxidation.Visible-light-responsive photocatalyst systems are indispensable for efficient sunlight utilization. Z-scheme systems using CuGaS₂, (CuGa)_1-xZn_2xS₂, CuGa_1-xIn_xS₂, and SrTiO₃:Rh as CO₂-reducing photocatalyst, BiVO₄ as O₂-evolving photocatalyst, and reduced graphene oxide (RGO) and Co-complex as electron mediator or without an electron mediator are active for CO₂ reduction using water as an electron donor under visible light irradiation. These metal sulfide photocatalysts have the potential to take part in Z-scheme systems for artificial photosynthetic CO₂ reduction, even though their ability to extract electrons from water is insufficient.A photoelectrochemical system using a photocathode is also attractive for CO₂ reduction under visible light irradiation. For example, p-type CuGaS₂, (CuGa)_1-xZn_2xS₂, Cu_1-xAg_xGaS₂, and SrTiO₃:Rh function as photocathodes for CO₂ reduction under visible light irradiation. Moreover, introducing a conducting polymer as a hole transporter and surface modification with Ag and ZnS improve photoelectrochemical performance.

Bionic Optical Leaf for Photoreduction of CO2 from Noble Metal Atom Mediated Graphene Nanobubble Arrays

The reduction of CO₂ to useful chemicals by solar irradiation has been of great interest in recent years to tackle the greenhouse effect. Compared with inorganic metal oxide particles, carbonaceous materials, such as graphene, are excellent in light absorption; however, they lack in activity and selectivity because of the challenge to manipulate the band gap and optimize the electron-hole separation, which drives the photoreduction process. In this work, inspired by the delicate natural plant leaf structure, we fabricated orderly stacked graphene nanobubble arrays with nitrogen dopant for the coordination of noble metal atoms to mimic the natural photoreduction process in plant leaves. This graphene metamaterial not only mimics the optical structure of leaf cells, which scatter and absorb light efficiently, but also drives the CO₂ reduction via nitrogen coordinated metal atoms as the chlorophyll does in plants. Our characterizations show that the band gap of nitrogen-doped graphene could be precisely tailored via substitution with different noble metal atoms on the doped site. The noble atoms coordinated on the doped site of graphene metamaterial not only enlarge the light absorption volume but also maximize the utilization of noble metals. The bionic optical leaf metamaterial coordinated with Au atoms exhibits high CO productivity up to 11.14 mmol g_cat^-1 h^-1 and selectivity to 95%, standing as one of the best catalysts among the carbonaceous and metal-based catalysts reported to date. This catalyst also maintained a high performance at low temperatures, manifesting potential applications of this bionic catalyst at polar regions to reduce greenhouse gases.

Crystal-Structure-Dependent Photocatalytic Redox Activity and Reaction Pathways over Ga2O3 Polymorphs

Differentiated crystal structures generally affect the surface physicochemical properties of catalysts, causing variety in catalytic activity between polymorphs. However, the underlying mechanism has not been completely revealed, especially the influence of surface physicochemical properties on photocatalytic redox activity and the reaction mechanism. In this work, we reveal the mechanism of surface redox properties on different crystal forms of gallium oxide from a molecular level. α-Ga₂O₃ and β-Ga₂O₃ exhibit a slight difference in catalytic oxidation of organic pollutants due to comprehensive influencing factors, including their valence band position, reactive oxygen species, and pore structure properties related to the adsorption-reaction-desorption process. But the catalytic reduction ability of CO₂ is obviously different due to the large differences of interaction between the surface of crystal structures and CO₂ molecules, which are critical to determine the catalytic performance and reaction pathways. The enhanced adsorption and activation of CO₂ on the α-Ga₂O₃ surface could promote the reduction reaction efficiency. Moreover, the large energy barrier of CH₂* formation on β-Ga₂O₃ makes the formation of methane (CH₄) relatively difficult compared to that on α-Ga₂O₃. The yield rate of CH₄ (1.8 μmol·g^-1·h^-1) on α-Ga₂O₃ is three times better than that on β-Ga₂O₃ (CH₄: 0.6 μmol·g^-1·h^-1). The current findings can offer novel insights into the understanding of crystal-structure-dependent photocatalytic performances and the design of new catalysts applied in energy conversion and environmental purification by crystal structure-tuning.

Elegant Construction of ZnIn2S4/BiVO4 Hierarchical Heterostructures as Direct Z-Scheme Photocatalysts for Efficient CO2 Photoreduction

The ZnIn₂S₄/BiVO₄ heterostructures were elegantly designed through assembling ZnIn₂S₄ nanosheets onto the surface of BiVO₄ decahedrons. This composite photocatalyst exhibits efficient photocatalytic conversion of CO₂ into CO with a detectable amount of CH₄ in the presence of water vapor. An electron spin-resonance spectroscopy (ESR) technique and density function theory (DFT) calculation affirm the direct Z-scheme structure in ZnIn₂S₄/BiVO₄. The larger surface photovoltage (SPV) change and the longer liquid photoluminescence (PL) lifetime of the heterostructure, compared to the individual ZnIn₂S₄ and BiVO₄ components, demonstrate that the Z-scheme structure can effectively promote the recombination of the photogenerated holes in the valence band (VB) of the ZnIn₂S₄ nanosheet with the electrons in the conduction band (CB) of the decahedral BiVO₄ and lead to the abundant electrons surviving in the CB of ZnIn₂S₄ and holes in the VB of BiVO₄, thus enhancing photocatalytic CO₂ reduction performance. This study may make a potential contribution to the rational construction and deep understanding of the underlying mechanism of direct Z-schemes for advanced photocatalytic activity.

Imparting CO2 reduction selectivity to ZnGa2O4 photocatalysts by crystallization from hetero nano assembly of amorphous-like metal hydroxides

Imparting an enhanced CO₂ reduction selectivity to ZnGa₂O₄ photocatalysts has been demonstrated by controlled crystallization from interdispersed nanoparticles of zinc and gallium hydroxides. The hydroxide precursor in which Zn(ii) and Ga(iii) are homogeneously interdispersed was prepared through an epoxide-driven sol-gel reaction. ZnGa₂O₄ obtained by a heat-treatment exhibits a higher surface basicity and an enhanced affinity for CO₂ molecules than previously-reported standard ZnGa₂O₄. The enhanced affinity for CO₂ molecules of the resultant ZnGa₂O₄ leads to highly-selective CO evolution in CO₂ photo-reduction with H₂O reductants. The present scheme is promising to achieve desirable surface chemistry on metal oxide photocatalysts.

A silver-manganese dual co-catalyst for selective reduction of carbon dioxide into carbon monoxide over a potassium hexatitanate photocatalyst with water

A Ag-Mn dual co-catalyst deposited on a K2Ti6O13 photocatalyst significantly enhances the photocatalytic CO2 reduction into CO with an extremely high selectivity of 98% by using H2O as an electron donor, owing to the properties of Ag and MnOx species for promoting CO and O2 formation, respectively.

Important Role of Strontium Atom on the Surface of Sr2KTa5O15 with a Tetragonal Tungsten Bronze Structure to Improve Adsorption of CO2 for Photocatalytic Conversion of CO2 by H2O

Sr_1.6K_0.37Na_1.43Ta₅O₁₅, which belongs to the Na-substituted Sr₂KTa₅O₁₅ series of compounds with a tetragonal tungsten bronze structure, was fabricated using a flux mixture of KCl and NaCl (KCl/NaCl molar ratio = 55:45). It exhibited higher CO formation rate (94.6 μmol h^-1), better selectivity for CO evolution (85.5%), and better stability of the photocatalytic activity than those of bare Sr₂KTa₅O₁₅ and other Na-substituted Sr₂KTa₅O₁₅ samples synthesized from flux mixtures with different KCl/NaCl ratios. X-ray photoelectron spectroscopic studies revealed that the surface atomic Sr/Ta ratio of Sr_1.6K_0.37Na_1.43Ta₅O₁₅ was larger than that of Sr₂KTa₅O₁₅. To clarify the factor responsible for the improvement in the photocatalytic activity facilitated by Na substitution, as well as to elucidate the reaction mechanism, the surface species were characterized by in situ Fourier transform infrared spectroscopy. It was observed that the bicarbonate species (HCO₃^-) adsorbed on the active Sr sites of Sr_1.6K_0.37Na_1.43Ta₅O₁₅ was reduced to CO via the formate species during photoirradiation. The plot of the CO formation rate vs. the surface atomic Sr/Ta ratio for tetragonal tungsten bronze-type Sr-K-Ta-O complex oxides had the summit, indicating that Sr atoms on the surface enhance the photocatalytic activity, while an excessive amount of Sr on the surface leads to the decrease in the photocatalytic activity. Hence, it can be concluded that while the presence of Sr on the surface has a determining effect on the adsorption of CO₂ and eventually on the photocatalytic activity, excess Sr on the surface that exists as SrCO₃ or Sr₂Ta₂O₇ suppresses the photocatalytic activity. Thus, Sr_1.6K_0.37Na_1.43Ta₅O₁₅ showed higher CO formation rate than Sr₂KTa₅O₁₅ did.

Photocatalytic Activity of Ga2O3 Supported on Al2O3 for Water Splitting and CO2 Reduction

We have examined the photocatalytic activity of Ga₂O₃ supported on Al₂O₃ (Ga₂O₃/Al₂O₃ catalyst) without a noble metal cocatalyst for water splitting and reduction of CO₂ with water under UV light irradiation by changing the loading amount of Ga₂O₃. All prepared Ga₂O₃/Al₂O₃ catalysts show photocatalytic activities for both water splitting and CO₂ reduction, and their activities are significantly improved compared to those of nonsupported Ga₂O₃ and Al₂O₃. The water splitting is dominated for Ga₂O₃/Al₂O₃ with less than 1.0 vol % of Ga₂O₃ loaded, whereas the CO₂ reduction, for higher Ga₂O₃-loaded samples (2.6, 4.2 vol %). Crystalline structure characterizations of Ga₂O₃/Al₂O₃ catalysts indicate that active sites for both reactions are different. The water splitting proceeds on nanometer-sized Ga₂O₃ rods dispersed on an Al₂O₃ support consisting of a little distorted α-Ga₂O₃ phase. On the other hand, the CO₂ reduction proceeds on sub-micrometer-sized Ga₂O₃ particles consisting of mixed phases of α-Ga₂O₃ and γ-Ga₂O₃ or with appearance of boundaries between the α and γ phases, which plays a critical role. Al₂O₃ used as the support of the Ga₂O₃ particles does not seem to play an important role in the photocatalytic CO₂ reduction.

Robust Binding between Carbon Nitride Nanosheets and a Binuclear Ruthenium(II) Complex Enabling Durable, Selective CO2 Reduction under Visible Light in Aqueous Solution

Carbon nitride nanosheets (NS-C₃ N₄ ) were found to undergo robust binding with a binuclear ruthenium(II) complex (RuRu') even in basic aqueous solution. A hybrid material consisting of NS-C₃ N₄ (further modified with nanoparticulate Ag) and RuRu' promoted the photocatalytic reduction of CO₂ to formate in aqueous media, in conjunction with high selectivity (approximately 98 %) and a good turnover number (>2000 with respect to the loaded Ru complex). These represent the highest values yet reported for a powder-based photocatalytic system during CO₂ reduction under visible light in an aqueous environment. We also assessed the desorption of RuRu' from the Ag/C₃ N₄ surface, a factor that can contribute to a loss of activity. It was determined that desorption is not induced by salt additives, pH changes, or photoirradiation, which partly explains the high photocatalytic performance of this material.

Mechanistic View of the Main Current Issues in Photocatalytic CO2 Reduction

After 40 years of research on photocatalytic CO₂ reduction, there are still many unknowns about its mechanistic aspects even for the most common TiO₂-based photocatalytic systems. These uncertainties include the pathways inducing visible-light activity in wide-band gap semiconductors, the charge transfer between semiconductors and plasmonic metal nanoparticles, the unambiguous determination of the origin of C-bearing products, the very first step in the activation of the CO₂ molecule, the factors determining the selectivity, the reasons for photocatalyst deactivation, the closure of the catalytic cycle by the hole-scavenging reagent, and the detailed reaction pathways and the most suitable techniques for their determination. This Perspective discusses these controversial issues based on the most relevant investigations reported so far. For that purpose, we have tried to view the complex CO₂ reduction in a holistic manner, considering today's state-of-the-art approaches, strategies, and techniques for the study of one of the hottest topics in energy research.

Deep-Ultraviolet Photodetectors Based on Epitaxial ZnGa2O4 Thin Films

A single-crystalline ZnGa₂O₄ epilayer was successfully grown on c-plane (0001) sapphire substrate by metal-organic chemical vapor deposition. This epilayer was used as a ternary oxide semiconductor for application in high-performance metal–semiconductor–metal photoconductive deep-ultraviolet (DUV) photodetectors (PDs). At a bias of 5 V, the annealed ZnGa₂O₄ PDs showed better performance with a considerably low dark current of 1 pA, a responsivity of 86.3 A/W, cut-off wavelength of 280 nm, and a high DUV-to-visible discrimination ratio of approximately 10⁷ upon exposure to 230 nm DUV illumination than that of as-grown ZnGa₂O₄ PDs. The as-grown PDs presented a dark current of 0.5 mA, a responsivity of 2782 A/W at 230 nm, and a photo-to-dark current contrast ratio of approximately one order. The rise time of annealed PDs was 0.5 s, and the relatively quick decay time was 0.7 s. The present results demonstrate that annealing process can reduce the oxygen vacancy defects and be potentially applied in ZnGa₂O₄ film-based DUV PD devices, which have been rarely reported in previous studies.

Synergistic interplay of Zn and Rh-Cr promoters on Ga2O3 based photocatalysts for water splitting

The photocatalytic water splitting activity of a wide-bandgap material, Ga2O3, is greatly boosted with the addition of a Zn and Rh-Cr co-catalyst at optimum loadings. To date, however, the exact roles of the co-catalysts and particularly the origin of their synergistic functions have not been clarified. Herein, we present how the optimum Zn loading on Ga2O3 leads to creation of a ZnGa2O4/Ga2O3 heterojunction favorable for charge separation through information on the occupied and unoccupied electronic states of Zn and Ga elucidated by X-ray absorption and emission spectroscopic methods. The function of Rh-Cr as an electron sink and reduction site was proven by photocatalytic experiments using an electron scavenger (Ag+) and by learning where Ag deposits and its effects on photocatalytic activity. Finally, perturbation of the Zn electronic structure by photoactivation was evidenced by modulation excitation X-ray absorption spectroscopy. Importantly, Rh-Cr markedly enhanced the level of the perturbation, serving as proof of the direct communication and synergy between the electronic states of Zn, present in ZnGa2O4, and Rh-Cr deposited on Ga2O3.

Necessary and sufficient conditions for the successful three-phase photocatalytic reduction of CO2 by H2O over heterogeneous photocatalysts

Artificial photosynthesis has recently drawn an increasing amount of attention due to the fact that it allows for direct solar-to-chemical energy conversion. However, one of the basic steps of this process, namely the reduction of CO2 by H2O to afford O2 and CO2 reduction products (CO2RPs) such as HCOOH, CO, HCHO, CH3OH, and CH4, is very difficult to achieve. In contrast to the CO2 reduction in plants and homogenous systems, the reduction of CO2 to CO2RPs over heterogeneous photocatalysts was challenged by the competing reduction of H+ to H2. Unfortunately, most of the research performed so far has focused only on the reduction of CO2, rather than the characterization of the H2O oxidation and H2 production. Moreover, the fact that the heterogeneous photocatalytic reduction of CO2 into CO2RPs by H2O should satisfy several selectivity criteria has often been ignored. Herein, we propose three such evaluation criteria, namely (1) the origin of carbon in CO2RPs (determined using isotopically labeled CO2 (13CO2)), (2) the relative amount of H2 and CO2RPs produced, and (3) the amount of O2 produced by the oxidation of H2O. If all these criteria are satisfied, i.e., the carbons of CO2RPs originate from CO2, the amount of H2 produced is negligible, and a stoichiometric amount of O2 is produced by the oxidation of H2O, then CO2 introduced into the gas phase is believed to be reduced by H2O to CO2RPs in the aqueous phase.

Cocatalysts in Semiconductor-based Photocatalytic CO2 Reduction: Achievements, Challenges, and Opportunities

Ever-increasing fossil-fuel combustion along with massive CO₂ emissions has aroused a global energy crisis and climate change. Photocatalytic CO₂ reduction represents a promising strategy for clean, cost-effective, and environmentally friendly conversion of CO₂ into hydrocarbon fuels by utilizing solar energy. This strategy combines the reductive half-reaction of CO₂ conversion with an oxidative half reaction, e.g., H₂ O oxidation, to create a carbon-neutral cycle, presenting a viable solution to global energy and environmental problems. There are three pivotal processes in photocatalytic CO₂ conversion: (i) solar-light absorption, (ii) charge separation/migration, and (iii) catalytic CO₂ reduction and H₂ O oxidation. While significant progress is made in optimizing the first two processes, much less research is conducted toward enhancing the efficiency of the third step, which requires the presence of cocatalysts. In general, cocatalysts play four important roles: (i) boosting charge separation/transfer, (ii) improving the activity and selectivity of CO₂ reduction, (iii) enhancing the stability of photocatalysts, and (iv) suppressing side or back reactions. Herein, for the first time, all the developed CO₂ -reduction cocatalysts for semiconductor-based photocatalytic CO₂ conversion are summarized, and their functions and mechanisms are discussed. Finally, perspectives in this emerging area are provided.

Enhancement of CO Evolution by Modification of Ga2O3 with Rare-Earth Elements for the Photocatalytic Conversion of CO2 by H2O

Modification of the surface of Ga₂O₃ with rare-earth elements enhanced the evolution of CO as a reduction product in the photocatalytic conversion of CO₂ using H₂O as an electron donor under UV irradiation in aqueous NaHCO₃ as a pH buffer, with the rare-earth species functioning as a CO₂ capture and storage material. Isotope experiments using ¹³CO₂ as a substrate clearly revealed that CO was generated from the introduced gaseous CO₂. In the presence of the NaHCO₃ additive, the rare-earth (RE) species on the Ga₂O₃ surface are transformed into carbonate hydrates (RE₂(CO₃)₃·nH₂O) and/or hydroxycarbonates (RE₂(OH)_2(3-x)(CO₃)_x) which are decomposed upon photoirradiation. Consequently, Ag-loaded Yb-modified Ga₂O₃ exhibits much higher activity (209 μmol h^-1 of CO) than the pristine Ag-loaded Ga₂O₃. The further modification of the surface of the Yb-modified Ga₂O₃ with Zn afforded a selectivity toward CO evolution of 80%. Thus, we successfully achieved an efficient Ag-loaded Yb- and Zn-modified Ga₂O₃ photocatalyst with high activity and controllable selectivity, suitable for use in artificial photosynthesis.

Highly Active NaTaO3 -Based Photocatalysts for CO2 Reduction to Form CO Using Water as the Electron Donor

Doped NaTaO₃ (NaTaO₃ :A, where A=Mg, Ca, Sr, Ba, or La) has arisen as a highly active photocatalyst for CO₂ reduction to simultaneously form CO, H₂ , and O₂ using water as the electron donor when used with an Ag cocatalyst, under UV irradiation, and with 1 atm (0.1 MPa) of CO₂ . The ratio of the number of reacted electrons/holes was almost unity, indicating that water was consumed as the electron donor. A liquid-phase reduction method for loading of the Ag cocatalyst was superior to photodeposition and impregnation methods. The Ag cocatalyst-loaded NaTaO₃ :Ba was the most active photocatalyst in water with no required additives. The addition of bases, such as hydrogencarbonate, was effective to enhance the CO formation for Mg-, Ca-, Sr-, Ba-, and La-doped NaTaO₃ photocatalysts with an Ag cocatalyst. Ca- and Sr-doped NaTaO₃ photocatalysts showed especially high activity along with the Ba-doped photocatalyst in the aqueous NaHCO₃ solution. The selectivity for the CO formation [CO/(CO+H₂ )] on Ca-, Sr-, and Ba-doped NaTaO₃ photocatalysts with Ag cocatalyst reached around 90 %.

Photocatalytic CO2 reduction using water as an electron donor by a powdered Z-scheme system consisting of metal sulfide and an RGO–TiO2 composite

CuGaS₂, (AgInS₂)_x–(ZnS)_2−2x, Ag₂ZnGeS₄, Ni- or Pb-doped ZnS, (ZnS)_0.9–(CuCl)_0.1, and ZnGa_0.5In_1.5S₄ showed activities for CO₂ reduction to form CO and/or HCOOH in an aqueous solution containing K₂SO₃ and Na₂S as electron donors under visible light irradiation. Among them, CuGaS₂ and Ni-doped ZnS photocatalysts showed relatively high activities for CO and HCOOH formation, respectively. CuGaS₂ was applied in a powdered Z-scheme system combining with reduced graphene oxide (RGO)-incorporated TiO₂ as an O₂-evolving photocatalyst. The powdered Z-scheme system produced CO from CO₂ in addition to H₂ and O₂ due to water splitting. Oxygen evolution with an almost stoichiometric amount indicates that water was consumed as an electron donor in the Z-schematic CO₂ reduction. Thus, we successfully demonstrated CO₂ reduction of artificial photosynthesis using a simple Z-scheme system in which two kinds of photocatalyst powders (CuGaS₂ and an RGO–TiO₂ composite) were only dispersed in water under 1 atm of CO₂.

Phase-dependent enhancement for CO2 photocatalytic reduction over CeO2/TiO2 catalysts

The addition of CeO₂ can increase the activity of rutile for CO₂ photoreduction under simulated sunlight irradiation because of the presence of Ti defects at the CeO₂–rutile interfaces, and this is beneficial to the interfacial separation of photogenerated charge carriers.

A ZnTa2O6 photocatalyst synthesized via solid state reaction for conversion of CO2 into CO in water

The photocatalytic activity of ZnTa₂O₆ for the conversion of CO₂ using H₂O as a reductant was demonstrated. CO was produced as a reduction product of CO₂ in the presence of a Ag cocatalyst, accompanied by a stoichiometric amount of O₂ evolution.