Photocatalysts For CO2 Reduction Research Articles

MOFs materials as photocatalysts for CO2 reduction: Progress, challenges and perspectives

Photocatalytic reduction of carbon dioxide (CO2) presents a pivotal solution to address meteorological and ecological challenges. Currently, metal-organic frameworks (MOFs) with their crystalline porosity, adjustable structures, and diverse chemical functionalities have garnered significant attention in the realm of photocatalytic CO2 reduction. This review provides a brief introduction to CO2 reduction and MOF material and their applications in CO2 reduction. Then, it undertakes a comprehensive examination of MOFs, summarizing their key attributes, including porosity, large surface area, structural multifunctionalities, and responsiveness to visible light, along with an analysis of heterojunctions and their methods of preparation. Additionally, it elucidates the fundamental principle of photocatalysis and CO2 reduction, encompassing both half and overall reactions. Furthermore, the classification of MOF-based materials is explored, along with the proposed mechanism for CO2 reduction and an update on recent developments in this field. Finally, this review outlines the challenges and potential opportunities for utilizing MOFs in CO2 reduction, offering valuable insights to scholars seeking innovative approaches not only to enhance CO2 reduction but also to advance other photocatalytic processes.

Recent Advances in Graphene-Based Single-Atom Photocatalysts for CO2 Reduction and H2 Production

Recent Advances in Graphene-Based Single-Atom Photocatalysts for CO2 Reduction and H2 Production

Selenium doping to improve internal electric field for excellent photocatalytic efficiency of g-C3N4 nanosheets

Graphitic carbon nitride is a promising photocatalyst to address environmental issues and energy applications. Herein, we develop an efficient defect-containing functionalized system of 2D selenium doped g-C3N4 porous nanosheets via one pot with two-step temperature for the first time. During heat treatment, Se-infused g-C3N4 nanosheets loosen stacking and form thinner nanosheets, which could effectively shorten the electronic path of charge migration. The introduction of nitrogen vacancies creates a midgap energy state below the CB, which improves the light-harvesting efficiency. Moreover, the dual effect of N vacancy and Se doping provides a driving force for the dissociation of excitons and achieves an effective spatial charge separation. High specific surface area (104.1 m2/g), suitable bandgap (2.65 eV), and good photocurrent response (0.45 µA cm−2) help to enhance the photocatalytic efficiency of 1-SeCN remarkably. Henceforth, high photocatalytic H2 production rate of 5441 µmol g−1 h−1 and CO evolution rate of 31.5 µmol g−1 h−1 verifies the effective Se doping enhances the photocatalytic efficiency of g-C3N4. Furthermore, DFT calculations are consistent with experimental results and this facile engineering strategy provides a scalable way to develop a novel, efficient g-C3N4-based photocatalyst for H2 production and CO2 reduction.

Dual Role of a Novel heteroleptic Cu(I) Complex in Visible-Light-Driven CO2Reduction.

A novel mononuclear Cu(I) complex was synthesized via coordination with a benzoquinoxalin-2'-one-1,2,3-triazole chelating diimine and the bis[(2-diphenylphosphino)phenyl] ether (DPEPhos), to target a new and efficient photosensitizer for photocatalytic CO2 reduction. The Cu(I) complex absorbs in the blue-green region of the visible spectrum, with a broad band having a maximum at 475 nm (ε = 4500 M-1cm-1), which is assigned to the metal-to-ligand charge transfer (MLCT) transition from the Cu(I) to the benzoquinoxalin-2'-one moiety of the diimine. Surprisingly, photo-driven experiments for the CO2 reduction showed that this complex can undergo a photoinduced electron transfer with a sacrificial electron donor and accumulate electrons on the diimine backbone. Photo-driven experiments in a CO2 atmosphere revealed that this complex can not only act as a photosensitizer, when combined with an Fe(III)-porphyrin, but can also selectively produce CO from CO2. Thus, owing to its charge-accumulation properties, the non-innocent benzoquinoxalin-2-one based ligand enabled the development of the first copper(I)-based photocatalyst for CO2 reduction.

Ionic Liquid Modified Fe-Porphyrinic Metal-Organic Frameworks as Efficient and Selective Photocatalysts for CO2 Reduction.

Porphyrin-based metal-organic frameworks (MOFs) are ideal platforms for heterogeneous photocatalysts toward CO2 reduction. To further explore photocatalytic MOF systems, it is also necessary to consider their ability to fine-tune the microenvironments of the active sites, which affects their overall catalytic operation. Herein, a kind of ionic liquid (IL, here is 3-butyric acid-1-methyl imidazolium bromide, BAMeImBr) was anchored to iron-porphyrinic Zr-MOFs with different amounts to obtain ILx@MOF-526 (MOF-526 = Zr6O4(OH)4(FeTCBPP)3, FeTCBPP = iron 5,10,15,20-tetra[4-(4'-carboxyphenyl)phenyl]-porphyrin, x = 100, 200, and 400). ILx@MOF-526 series was designed to investigate the effects of the microenvironmental and electronic structural modification on the efficiency and selectivity of the photochemical reduction of CO2 after introducing IL fragments. Compared to parent MOF-526, the production and selectivity of CO were greatly improved in the absence of any photosensitizer under visible light by the ILx@MOF-526 series. Among them, the CO yield of IL200@MOF-526 was up to 14.0 mmol g-1 within 72 h with a remarkable CO selectivity of 97%, which is superior to that of MOF-526 without BAMeIm+ modification and other amounts of BAMeIm+ loaded. Furthermore, density functional theory calculations were performed to study the mechanism of the CO2 reduction.

A review of g‐C3N4‐based photocatalytic materials for photocatalytic CO2 reduction

AbstractCurrently, the concentration of carbon dioxide (CO2) has exceeded 400 ppm in the atmosphere. Thus, there is an urgent need to explore CO2 reduction and utilization technologies. Photocatalytic technology can convert CO2 to valuable hydrocarbons (CH4, CH3OH, and C2H5OH, etc.), realizing the conversion of solar energy to chemical energy as well as solving the problems of fossil fuel shortage and global warming. Graphitic carbon nitride (g‐C3N4), as a two‐dimensional nonmetallic semiconductor material, shows great potential in the field of CO2 photoreduction due to its moderate bandgap, easy synthesis method, low cost, and visible light response properties. This review elaborates the research progress of g‐C3N4‐based photocatalysts for photocatalytic CO2 reduction. The modification strategies (e.g., morphology engineering, elemental doping, crystallinity modulation, cocatalyst modification, and constructing heterojunction) of g‐C3N4‐based photocatalysts for CO2 reduction application have been discussed in detail. Finally, the challenges and development prospects of g‐C3N4‐based photocatalytic materials for CO2 reduction are presented.

Synergistic CdS@CeO2 nanocomposites: Harnessing Z-scheme electron transfer for enhanced photocatalytic CO2 conversion and aqueous Cr(VI) reduction

The present work describes the fabrication of CdS-D@CeO2 Z-scheme heterojunctions as highly stable, efficient and recyclable photocatalysts for the visible light assisted reduction of CO2 to methanol as well as Cr(VI) to Cr(III). The bare CeO2 produced methanol with the rate of 111 μmolh−1g−1 that is much lower in comparison to CdS-D@CeO2 (501 μmolh−1g−1). In addition, the photocatalytic efficiency remained nearly same for three successive runs, indicating the higher stability and recycling ability of the heterojunctions CdS-D@CeO2. It was also observed that under direct sunlight irradiation, CdS-D@CeO2 efficiently reduced 100% Cr(VI) to Cr(III) in an aqueous medium in just 40 minutes. Moreover, >95% removal was maintained even after 5 catalytic cycles. Additionally, for mimicking real contaminated water samples, the influence of the foreign ions on Cr(VI) reduction was evaluated by using lake water sample and no adverse effects on the result were observed. Thus, this work showed a way to develop cost-effective, easily synthesized, durable and recyclable composite photocatalysts for CO2 utilization and proficient removal of waste water pollutants by employing the clean, green, and renewable solar energy.

Hybrid photocatalyst constructed using polymerized metal complexes and semiconductor powders for photocatalytic carbon dioxide reduction

Abstract Hybrid photocatalysts prepared by the adsorption of metal complexes on semiconductors via anchoring groups are among notable photocatalysts for selective CO2 reduction using abundant electron donors. However, their photocatalytic activities are limited by their low adsorption amounts and strength. In this study, we demonstrate the reductive polymerization of vinyl groups in metal complexes triggered by excited electrons on semiconductors as a new immobilization method. This approach significantly enhanced the adsorption amount and durability, thus increasing the photocatalytic performance.

Construction of Bi2WO6/g-C3N4/Cu foam as 3D Z-scheme photocatalyst for photocatalytic CO2 reduction

Photocatalytic CO2 reduction to hydrocarbon fuels represents a promising solution to CO2 emission challenges. However, photocatalytic CO2 reduction systems face hurdles in practical utilization for the low efficiency of photocatalysts and challenge in catalyst immobilization. Therefore, a novel monolithic three-dimensional Z-scheme photocatalyst was synthesized by fabricating Bi2WO6/g-C3N4 heterojunction on Cu foam substrate (Bi2WO6/g-C3N4/Cu foam) via thermal polymerization and electrophoresis deposition. A series of characterization results, including X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and N2 adsorption–desorption, illustrate the crystalline properties, pore structure, and morphology of the composite photocatalyst. Consequently, Bi2WO6/g-C3N4/Cu foam achieved CO production yield of 33.84 μmol/g cat under visible light illumination for 6 h and retained 96.2 % of initial photocatalytic activity after 30 h of reaction. UV–Vis diffuse reflection spectra, photoluminescence spectra and photoelectrochemical analysis was involved to reveal the mechanism of the photocatalytic activity of composite photocatalyst.The synergistic effect of the Z-scheme mechanism and the hierarchical macroporous structure improves the light absorption, light-induced electron-hole pair separation, and charge transmission. This catalyst construction strategy provides a viable approach for facilitating practical photocatalytic CO2 reduction.

Interlayer anions modulated ZnAl-layered double hydroxides for enhanced photocatalytic CO2 reduction

Photocatalytic CO2 reduction has drawn widespread attention across all sectors of society. Layered double hydroxides (LDHs) have emerged as promising photocatalysts, renowned for their adjustable bandgaps and reaction sites. As a typical anionic layered material, LDHs exhibit intricate interactions between interlayer anions and brucite-like metal layers, greatly influencing their inherent physicochemical properties. In this work, we synthesized ZnAl-LDH samples intercalated with four distinct anions: carbonate (CO32–), chloridion (Cl–), nitrate (NO3–), and dodecyl sulfate (DS–), denoted as LDH-CO3, LDH-Cl, LDH-NO3, and LDH-DS, respectively. These LDHs samples have comparable crystallinity and morphology but differ in their interlayer spacing. XRD results reveal interlayer spacings of 0.77 nm for LDH-CO3, 0.78 nm for LDH-Cl, 0.88 nm for LDH-NO3, and 1.24 nm for LDH-DS, between adjacent brucite-like ZnAl hydroxide layers. Remarkably, LDH-NO3 demonstrates the highest photocatalytic CO2 reduction performance, with CO as the primary reduction product. Specifically, the average CO production rate of LDH-NO3 are 3.80 μmol g–1 h–1, surpassing that of LDH-CO3, LDH-Cl, and LDH-DS by 1.9, 2.1, and 2.5 times, respectively. This superior performance can be attributed to the intermediate interlayer spacing of LDH-NO3, which facilitates strong light absorption and a robust local built-in electric field. Theses properties enhance charge generation and separation abilities, leading to efficient photocatalytic reactions. The obtained results highlight the critical role of controlling small molecules within the interlayer, alongside defects, in the design and fabrication of layered-structure photocatalysts. This study provides valuable insights into optimizing the performance of LDH-based photocatalysts for CO2 reduction and potentially other photocatalytic applications.

Two 2D layered Co-modified Anderson-type polyoxometalate-based highly efficient photocatalysts for CO2 reduction

The design of efficient and low-price photocatalysts is an effective way to relieve excessive carbon dioxide emissions and energy dilemma. Polyoxometalates are ideal materials due to their adjustable structures and semiconductor-like properties. Limited by their light absorption capacity, it is a feasible way to construct inorganic-organic composite polyoxometalate-based photocatalysts. Here, two efficient Anderson-type polyoxometalate-based photocatalysts, (C10H9N2)·{[Co(H2O)2]·[H6CrMo6O24]}·4H2O (1), {[Co(H2O)3]·[Co(DAPSC)]2·[H6CrMo6O24]2}·11H2O (2) [2,6-diacetylpyridine bis(semicarbazone), abbr. DAPSC], were prepared by hydrothermal synthesis method. The structural analysis indicates that compounds 1 and 2 share similarities: {CrMo6} units were linked by cobalt ions to form two-dimensional (2D) frameworks. The addition of Co ions provided catalytic active center for the catalyst, compounds 1 and 2 exhibited promising yield into CO, as high as 3061.6 μmol g−1·h−1 and 9309.8 μmol g−1·h−1, respectively. Due to their excellent properties, Anderson-type POMs can be used as the basic units of photocatalysts. At the same time, four cycling tests prove the stability of compound 2 under a long reaction time.

Tuning Photoredox Catalysis of Ruthenium with Palladium: Synthesis of Heterobimetallic Ru-Pd Complexes That Enable Efficient Photochemical Reduction of CO2.

New Ru-Pd heterobimetallic complexes were synthesized and structurally characterized utilizing 6,6″-bis(phosphino)-2,2':6',2″-terpyridine as a scaffold for the metal-metal bond. The dicationic Ru-Pd complex was found to exhibit high catalytic activity as a photocatalyst for photochemical reduction of CO2 to CO under visible light irradiation. This study established a new design of transition metal catalysts that tune photoredox catalysis with metalloligands.

Surface Coordination Environment Engineering on PtxCu1-x Alloy Catalysts for the Efficient Photocatalytic Reduction of CO2 to CH4.

Alloy catalysts have been reported to be robust in catalyzing various heterogeneous reactions due to the synergistic effect between different metal atoms. In this work, aimed at understanding the effect of the coordination environment of surface atoms on the catalytic performance of alloy catalysts, a series of PtxCu1-x alloy model catalysts supported on anatase-phase TiO2 (PtxCu1-x/Ti, x = 0.4, 0.5, 0.6, 0.8) were developed and applied in the classic photocatalytic CO2 reduction reaction. According to the results of catalytic performance evaluation, it was found that the photocatalytic CO2 reduction activity on PtxCu1-x/Ti showed a volcanic change as a function of the Pt/Cu ratio, the highest CO2 conversion was achieved on Pt0.5Cu0.5/Ti, with CH4 as the main product. Further systematic characterizations and theoretical calculations revealed that the equimolar amounts of Pt and Cu in Pt0.5Cu0.5/Ti facilitated the generation of more Cu-Pt-paired sites (i.e., the higher coordination number of Pt-Cu), which would favor a bridge adsorption configuration of CO2 and facilitate the electron transfer, thus resulting in the highest photocatalytic CO2 reduction efficiency on Pt0.5Cu0.5/Ti. This work provided new insights into the design of excellent CO2 reduction photocatalysts with high CH4 selectivity from the perspective of surface coordination environment engineering on alloy catalysts.

Gas-Solid Phase Reaction Derived Silver Bismuth Iodide Rudorffite: Structural Insight and Exploring Photocatalytic Potential of CO2 Reduction.

Photocatalytic reduction of CO2 is a promising strategy to mitigate the effects of global warming by converting CO2 into valuable energy-dense products. Silver bismuth iodide (SBI) is an attractive material owing to its tunable bandgap and favorable band-edge positions for efficient CO2 photoreduction. In this study, SBI materials, including AgBi2I7, AgBiI4, Ag2BiI5, and Ag3BiI6 are first synthesized, through gas-solid reaction by controlling the stoichiometric ratio of reactants. The X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) results revealed that the distance between Ag-I is proportional to the degree of Ag ions delocalization, which occupies the vacant sites. That greatly retards the charge recombination at vacant sites. In addition, the surface potential via photo-assisted Kelvin probe force measurements of various SBI catalysts shows that Ag3BiI6 exhibits the highest surface potential change due to the rich delocalized Ag ions. This results in effective charge carrier transport and prevention of charge recombination at vacant sites. Taking the above advantages, the averaged CO and CH4 production rates for Ag3BiI6 achieved 0.23 and 0.10µmolg-1h-1, respectively. The findings suggest that Ag3BiI6 has a high potential as a novel photocatalyst for CO2 reduction and sheds light on the possibility of solving environmental contamination and sustainable energy crises.

Construction of an Electron Capture and Transfer Center for Highly Efficient and Selective Solar-Light-Driven CO2 Conversion.

Exploring high-efficiency photocatalysts for selective CO2 reduction is still challenging because of the limited charge separation and surface reactions. In this study, a noble-metal-free metallic VSe2 nanosheet was incorporated on g-C3N4 to serve as an electron capture and transfer center, activating surface active sites for highly efficient and selective CO2 photoreduction. Quasi in situ X-ray photoelectron spectroscopy (XPS), soft X-ray absorption spectroscopy (sXAS), and femtosecond transient absorption spectroscopy (fs-TAS) unveiled that VSe2 could capture electrons, which are further transferred to the surface for activating active sites. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations revealed a kinetically feasible process for the formation of a key intermediate and confirmed the favorable production of CO on the VSe2/PCN (protonated C3N4) photocatalyst. As an outcome, the optimized VSe2/PCN composite achieved 97% selectivity for solar-light-driven CO2 conversion to CO with a high rate of 16.3 μmol·g-1·h-1, without any sacrificial reagent or photosensitizer. This work offers new insights into the photocatalyst design toward highly efficient and selective CO2 conversion.

A covalent organic framework integrating photocatalytic center and electron reservoir for photocatalytic CO2 reduction to HCOOH

Direct and efficient photocatalytic conversion of CO2 into fuels and high value-added chemicals under sunlight is of great significance in alleviating the energy crisis and environmental issues. Covalent organic frameworks (COFs) have attracted much attention in the photocatalytic field as materials that can be pre-designed on demand. Taking advantage of the pre-designable properties of COFs materials, the integration of organic building blocks with photocatalytic activity and photoelectron-resident properties into frameworks can improve the electron-hole separation efficiency and photocatalytic performance. Here, we report a newly designed COF (COF-TVBT-N) with triazine and alkenyl groups that can be used as a photocatalyst to accomplish the photoreduction of CO2 to HCOOH under simulated sunlight without the assistance of metal sites and photosensitizers. Furthermore, we unambiguously reveal that the photocatalytic active center in COF-TVBT-N is TVBT through the combination of experiments and theoretical calculations. DFT calculations suggest that the adsorption of H near the active center is beneficial for the adsorption and photoreduction of CO2 in this photocatalytic system. The photoexcitation process and photocatalytic CO2 reduction mechanism of COF-TVBT-N are also reasonably deduced based on DFT calculations. This work provides ideas for the design of COFs as photocatalysts for CO2 reduction and gives insights into the mechanism of CO2 conversion to HCOOH.

Photocatalytic Enhancement for CO2 Reduction Using Au Nanoparticles Supported on Fe‐Doped SrTiO3−δ Perovskite

This study focuses on the use of Au nanoparticles supported on the SrTiO3−δ perovskite doped with Fe ions as a photocatalyst for CO2 reduction and methanol production using water as the hydrogen supplier. The photocatalytic evaluation of the synthesized materials shows that Au nanoparticles supported on the SrTi(1−x)FexO3−δ perovskites can enhance photocatalytic CO2 reduction. The sample with the doping level x = 0.125 has the highest methanol production, 9.5 times more than the undoped sample and 3.5 times more than its counterpart without Au nanoparticles. The results of the DFT + U calculations show that the contributions from the iron atoms states reduce the energy gap in the perovskite composite, SrTi(1−x)FexO3−δ. Additionally, Fe incorporation endows the system with the capacity to create electron–hole pairs via direct band transitions at the high‐symmetry point R with the doping levels of x = 0.125 and x = 0.250.

Photocatalytic Efficiency for CO2 Reduction of Co and Cluster Co2O2 Supported on g-C3N4: A Density Functional Theory and Machine Learning Study.

It is well known from experimental results that a single atom of cobalt supported on g-C3N4 is an efficient photocatalyst for the reduction of CO2 to CO, with a better photocatalytic activity than g-C3N4. In this work, we investigate the performance as catalysts for the CO2 reduction of single atoms of cobalt and Co2O2 clusters supported on graphitic carbon nitride (g-C3N4). Employing density functional theory plus Hubbard (DFT + U) calculations, we investigate in detail the reduction mechanisms to CO and HCOOH for the first time. We find that deposition of cobalt on g-C3N4 decreases the work function of g-C3N4 to 6.6 eV and provides a better candidate for the reduction reaction. In addition, we find that the preferred product of CO2 reduction on Co@g-C3N4 is CO, with a rate-determining barrier of 0.97 eV, while on Co2O2@g-C3N4, CO2 reduces to formate with a rate-determining barrier of 0.44 eV. We determine the creation of CO2 from COOH to only take place on Co2O2@g-C3N4, with a (relatively high) barrier of 2.27 eV. In order to obtain more easily the transition state energies of the reactions mentioned above, we applied machine learning methods to search for high-importance descriptors for these quantities, in the case of single transition metal atoms supported on C3N4. Interestingly, our results show that our quantities of interest are closely related to the adsorption energies of products and normalized valence electrons of the products of the elementary reactions as well as those of the metal atoms. The former of these two sets of features can be straightforwardly obtained via DFT, while the latter energies are extensively tabulated. Our results offer guidance for the design of catalysts and photocatalysts for CO2 reduction on single-metal atoms supported on C3N4.

G-C3N4 as ballistic electron transport “Tunnel” in CsPbBr3-based ternary photocatalyst for gas phase CO2 reduction

Perovskite CsPbBr3 quantum dot shows great potential in artificial photosynthesis, attributed to its outstanding optoelectronic properties. Nevertheless, its photocatalytic activity is hindered by insufficient catalytic active sites and severe charge recombination. In this work, a CsPbBr3@Ag-C3N4 ternary heterojunction photocatalyst is designed and synthesized for high-efficiency CO2 reduction. The CsPbBr3 quantum dots and Ag nanoparticles are chemically anchored on the surface of g-C3N4 sheets, forming an electron transfer tunnel from CsPbBr3 quantum dots to Ag nanoparticles via g-C3N4 sheets. The resulting CsPbBr3@Ag-C3N4 ternary photocatalyst, with spatial separation of photogenerated carriers, achieves a remarkable conversion rate of 19.49 μmol·g−1·h−1 with almost 100 % CO selectivity, a 3.13-fold enhancement in photocatalytic activity as compared to CsPbBr3 quantum dots. Density functional theory calculations reveal the rapid CO2 adsorption/activation and the decreased free energy (0.66 eV) of *COOH formation at the interface of Ag nanoparticles and g-C3N4 in contrast to the g-C3N4, leading to the excellent photocatalytic activity, while the thermodynamically favored CO desorption contributes to the high CO selectivity. This work presents an innovative strategy of constructing perovskite-based photocatalyst by modulating catalyst structure and offers profound insights for efficient CO2 conversion.

Defect-enriched BiOIO3/Ti3C2 MXene 2D/2D Schottky-type heterostructure for efficient and selective CH4 production via CO2 photoreduction: Unveiling the roles of defect inclusion and Ti3C2 MXene co-catalyst

The photoreduction of CO2 using solar energy to produce energy-efficient fuels is a sustainable technology that addresses energy needs while reducing carbon emissions. However, synthesizing efficient and robust photocatalysts for this process is challenging. This study introduces a viable approach for highly selective CO2 photoreduction to CH4 production by integrating defect-enriched BiOIO3 (DEBI) with a Ti3C2 (TC) MXene co-catalyst, forming an efficient 2D/2D Schottky-type heterostructure. The DEBI, enhanced with precise defect engineering, showed improved light absorption and charge separation efficiency. In tandem, the TC MXene co-catalyst facilitated rapid electron transfer and significantly minimized charge recombination. Consequently, the DEBI/TC-2 heterostructure, with an optimal 2 wt% TC MXene loading, achieved a CH4 yield of 52.8 μmol h−1 g−1, representing a remarkable 20.5- and 6.3-fold increase over pristine BiOIO3 and DEBI, respectively. The Schottky-type 2D/2D heterostructure also demonstrated an impressive apparent quantum yield of 0.72%, 99% CH4 selectivity over H2 generation, and remarkable stability across multiple cycles. This study underscores the synergistic advantages of defect engineering and MXene co-catalyst integration in a single system, proposing a novel direction for designing highly efficient photocatalysts for solar-driven CO2 reduction in energy-efficient fuel production.