Recent Advances in Heterogeneous Photocatalytic CO2 Conversion to Solar Fuels

Photoreduction of CO2 into CH4 Using Novel Composite of Triangular Silver Nanoplates on Graphene-BiVO4

Plasmonic photocatalysis, combing noble metal nanoparticles (NMNPs) with semiconductors, has been widely studied and proven to perform better than pure semiconductors. The plasmonic effects are mainly based on the localized surface plasmon resonance (LSPR) of NMNPs. The LSPR wavelength depends on several parameters, such as size, shape, the surrounding media, and the interdistance of the NMNPs. In this study, graphene-modified plate-like BiVO4 composites, combined with silver nanoplates (AgNPts), were successfully prepared and used as a photocatalyst for CO2 photoconversion. Triangular silver nanoplates (TAgNPts), icosahedral silver nanoparticles (I-AgNPs), and decahedra silver nanoparticles (D-AgNPs) were synthesized using photochemical methods and introduced to the nanocomposites to compare the shape-dependent plasmonic effect. Among them, T-AgNPts/graphene/BiVO4 exhibited the highest photoreduction efficiency of CO2 to CH4, at 18.1 μmolg−1h−1, which is 5.03 times higher than that of pure BiVO4 under the irradiation of a Hg lamp. A possible CO2 photoreduction mechanism was proposed to explain the synergetic effect of each component in TAgNPts/graphene/BiVO4. This high efficiency reveals the importance of considering the compositions of photocatalysts for converting CO2 to solar fuels.

Molecular-level insight into photocatalytic CO2 reduction with H2O over Au nanoparticles by interband transitions

Achieving CO₂ reduction with H₂O on metal photocatalysts and understanding the corresponding mechanisms at the molecular level are challenging. Herein, we report that quantum-sized Au nanoparticles can photocatalytically reduce CO₂ to CO with the help of H₂O by electron-hole pairs mainly originating from interband transitions. Notably, the Au photocatalyst shows a CO production rate of 4.73 mmol g^-1 h^-1 (~100% selectivity), ~2.5 times the rate during CO₂ reduction with H₂ under the same experimental conditions, under low-intensity irradiation at 420 nm. Theoretical and experimental studies reveal that the increased activity is induced by surface Au-O species formed from H₂O decomposition, which synchronously optimizes the rate-determining steps in the CO₂ reduction and H₂O oxidation reactions, lowers the energy barriers for the *CO desorption and *OOH formation, and facilitates CO and O₂ production. Our findings provide an in-depth mechanistic understanding for designing active metal photocatalysts for efficient CO₂ reduction with H₂O.

Photoreduction of carbon dioxide and photodegradation of organic pollutants using alkali cobalt oxides MCoO2 (M = Li or Na) as catalysts

The recycling of lithium batteries should be prioritized, and the use of discarded alkali metal battery electrode materials as photocatalysts merits research attention. This study synthesized alkali metal cobalt oxide (MCoO₂, M = Li or Na) as a photocatalyst for the photoreduction of CO₂ and degradation of toxic organic substances. The optimized NaCoO₂ and LiCoO₂ photocatalysts increased the photocatalytic CO₂-CH₄ conversion rate to 21.0 and 13.4 μmol g^-1 h^-1 under ultraviolet light irradiation and to 16.2 and 5.3 μmol g^-1 h^-1 under visible light irradiation, which is 17 times higher than that achieved by TiO₂ P25. The rate constants of the optimized reactions of crystal violet (CV) with LiCoO₂ and NaCoO₂ were 2.29 × 10^-2 and 4.35 × 10^-2 h^-1, respectively. The quenching effect of the scavengers and electron paramagnetic resonance in CV degradation indicated that active O₂^•-, ¹O₂, and h⁺ play the main role, whereas ^•OH plays a minor role for LiCoO₂. The hyperfine splitting of the DMPO-^•OH and DMPO-^•CH₃ adducts was a_N = 1.508 mT, a_Hβ = 1.478 mT and a_N = 1.558 mT, a_Hβ = 2.267 mT, respectively, whereas the hyperfine splitting of DMPO^+• was a_N = 1.475 mT. The quenching effect also indicated that active O₂^•- and h⁺ play the main role and that ^•OH and ¹O₂ play a minor role for NaCoO₂. The hyperfine splitting of the DMPO-^•OH and DMPO^+• adducts was a_N = 1.517 mT, a_Hβ = 1.489 mT and a_N = 1.496 mT, respectively. Discarded alkali metal battery electrode materials can be reused as photocatalysts to address environmental pollution.

Single-Atom Catalysts (SACs) for Photocatalytic CO2 Reduction with H2 O: Activity, Product Selectivity, Stability, and Surface Chemistry

In recent years, single-atom catalysts (SACs) have attracted the interest of researchers owing to their suitability for various catalytic applications. For instance, their optoelectronic features, site-specific activity, and cost-effectiveness make SACs ideal for photocatalytic CO₂ reduction. The activity, product selectivity, and photostability of SACs depend on various factors such as the nature of the metal/support material, the interaction between the metal atoms and support, light-harvesting ability, charge separation behavior, CO₂ adsorption ability, active sites, and defects. Consequently, it is necessary to investigate these factors in depth to elucidate the working principle(s) of SACs for catalytic applications. Herein, the recent progress in the development of SACs for photocatalytic CO₂ reduction with H₂ O is reviewed. First, a brief overview of CO₂ photoreduction and SACs for CO₂ conversion is provided. Several synthesis strategies and useful techniques for characterizing SACs employed in heterogeneous catalysis are then described. Next, the challenges of SACs for photocatalytic CO₂ reduction and related optimization strategies, in terms of activity, product selectivity, and stability, are explored. The progress in the development of noble metal- and transition metal-based SACs and dual-SACs for photocatalytic CO₂ reduction is discussed. Finally, the prospects of SACs for CO₂ reduction are considered.

Designing Nanoengineered Photocatalysts for Hydrogen Generation by Water Splitting and Conversion of Carbon Dioxide to Clean Fuels

Semiconductor photocatalysis has received tremendous attention in the past decade as it has shown great promise in the context of clean energy harvesting for environmental remediation. Sunlight is an inexhaustible source of energy available to us throughout the year, although it is rather dilutely dispersed. Semiconductor based photocatalysis presents one of the best ways to harness this source of energy to carry out chemical reactions of interest that require external energy input. Photocatalytic hydrogen generation by splitting of water, CO₂ mitigation, and CO₂ conversion to green fuel have therefore become the highly desirable clean and sustainable processes for a better tomorrow. Although numerous efforts have been made and continue to be expended to search and develop new classes of photocatalyst materials in recent years, several significant challenges still remain to be resolved before photocatalysis can reach its commercial potential. Therefore, major attention is required towards improving the efficiencies of the existing photocatalysts by further manipulating them and parallelly employing newer strategies for novel photocatalyst designs. This personal account aims to provide a broad overview of the field primarily invoking examples of our own research contributions in the field, which include photocatalytic hydrogen generation and CO₂ reduction to value added chemicals. This account reviews the state-of-the-art research activities and scientific possibilities which a functional material can offer if its properties are put to best use through goal-oriented design by combining with other compatible materials. We have addressed fundamental principles of photocatalysis, different kind of functional photocatalysts, critical issues associated with them and various strategies to overcome the related hurdles. It is our hope that this current personal account will provide a platform for young researchers to address the bottleneck issues in the field of photocatalysis and photocatalysts with a sense of clarity, and to find innovative solutions to resolve them by a prudent choice of materials, synthesis protocols, and approaches to boost the photocatalysis output. We emphasize that a targeted or goal-directed photocatalyst nanoengineering as perhaps the only way to realize an early success in this multiparametric domain.

Z-Scheme Heterojunction of SnS2/Bi2WO6 for Photoreduction of CO2 to 100% Alcohol Products by Promoting the Separation of Photogenerated Charges

Using sunlight to convert CO₂ into solar fuel is an ideal solution to both global warming and the energy crisis. The construction of direct Z-scheme heterojunctions is an effective method to overcome the shortcomings of single-component or conventional heterogeneous photocatalysts for photocatalytic CO₂ (carbon dioxide) reduction. In this work, a composite photocatalyst of narrow-gap SnS₂ and stable oxide Bi₂WO₆ were prepared by a simple hydrothermal method. The combination of Bi₂WO₆ and SnS₂ narrows the bandgap, thereby broadening the absorption edge and increasing the absorption intensity of visible light. Photoluminescence, transient photocurrent, and electrochemical impedance showed that the coupling of SnS₂ and Bi₂WO₆ enhanced the efficiency of photogenerated charge separation. The experimental results show that the electron transfer in the Z-scheme heterojunction of SnS₂/Bi₂WO₆ enables the CO₂ reduction reactions to take place. The photocatalytic reduction of CO₂ is carried out in pure water phase without electron donor, and the products are only methanol and ethanol. By constructing a Z-scheme heterojunction, the photocatalytic activity of the SnS₂/Bi₂WO₆ composite was improved to 3.3 times that of pure SnS₂.

Selective Photocatalytic CO2 Reduction to CH4 on Tri-s-triazine-Based Carbon Nitride via Defects and Crystal Regulation: Synergistic Effect of Thermodynamics and Kinetics

Realizing the high selectivity of CH₄ from the photocatalytic CO₂ reduction reaction (CO₂ RR) remains a great challenge owing to the lower efficiency of multi-electron transfer and the similar thermodynamic properties of CH₄ and CO. Herein, nitrogen-deficient carbon nitride two-dimensional (2D) nanosheets were prepared via the high-temperature crystalline phase transformation process. Optimizing crystallinity enhances the in-plane polarization along the a-axis. Owing to the increased electron density of the N defect, the kinetic possibilities of CH₄ production have increased. Furthermore, the potential energy of the mid-gap states introduced by the N defect favors the thermodynamics of CH₄ production. The selectivity values of CH₄ based on yield and electrons are 87.1 and 96.4%. This work unravels the mechanism to selectively produce CH₄ from CO₂ photoreduction through the crystalline phase and defect regulation and provides significant guidance for the rational design of CO₂ reduction photocatalysts for selective CH₄ production.

Light-Induced Structural Dynamic Evolution of Pt Single Atoms for Highly Efficient Photocatalytic CO2 Reduction

Revealing the structural evolution of the real active site during photocatalysis is very important for understanding the catalytic mechanism, but it remains a great challenge. By employing single atoms (SAs) as the mechanism research platform, we investigated the variation of the SA structure under light and the corresponding reaction pathway controlment mechanism. In particular, taking the defect anchoring strategy, Pt SAs are anchored on the metal ion vacancy-rich ZnNiTi layered double hydroxide-etched (ZnNiTi-LDHs-E) support. It is proved by CO-Fourier transform infrared and X-ray absorption fine structure characterization methods that the Pt SAs could gain photoelectrons to form cationic Pt(IV), electron-rich Pt(II), and near-neutral Ptδ+ species at different light intensities. By in situ inducing the above different Pt SAs in photocatalytic CO2 reduction, a dramatic product distribution is observed: (1) under weak light, Pt(IV) SAs cannot activate CO, so CO cannot be further transformed into hydrocarbons; (2) under the moderate light, electron-rich Pt(II) SAs could cooperate with adjacent LDH surface sites (Ni2+/Ti4+) to open up the C-C coupling route for C2H6 generation; and (3) Pt SAs in the state of near-neutral Ptδ+ could directly hydrogenate CO into CH4. This work reveals the structural evolution of Pt SAs in photocatalysis and the corresponding effect on catalytic performance, which provides a new idea for the construction of highly efficient photocatalysts.

Study of Intermolecular Interaction between Small Molecules and Carbon Nanobelt: Electrostatic, Exchange, Dispersive and Inductive Forces

The conjugated structure of carbon is used in chemical sensing and small molecule catalysis because of its high charge transfer ability, and the interaction between carbon materials and small molecules is the main factor determining the performance of sensing and catalytic reactions. In this work, Reduced Density Gradient (RDG) and Symmetry-Adapted Perturbation Theory (SAPT) energy decomposition methods were used in combination to investigate the heterogeneity of catalytic substrates commonly used in energy chemistry with [6, 6] the carbon nanobelt ([6, 6] CNB, the interaction properties and mechanisms inside and outside the system). The results show that most of the attractive forces between dimers are provided by dispersive interactions, but electrostatic interactions cannot be ignored either. The total energy of the internal adsorption of [6, 6] CNB was significantly smaller than that of external adsorption, which led to the small molecules being more inclined to adsorb in the inner region of [6, 6] CNB. The dispersive interactions of small molecules adsorbed on [6, 6] CNB were also found to be very high. Furthermore, the dispersive interactions of the same small molecules adsorbed inside [6, 6] CNB were significantly stronger than those adsorbed outside. In [6, 6] CNB dimers, dispersion played a major role in the mutual attraction of molecules, accounting for 70% of the total attraction.

Tailoring the Surface and Interface Structures of Copper-Based Catalysts for Electrochemical Reduction of CO2 to Ethylene and Ethanol

Electrochemical CO₂ reduction to valuable ethylene and ethanol offers a promising strategy to lower CO₂ emissions while storing renewable electricity. Cu-based catalysts have shown the potential for CO₂ -to-ethylene/ethanol conversion, but still suffer from low activity and selectivity. Herein, the effects of surface and interface structures in Cu-based catalysts for CO₂ -to-ethylene/ethanol production are systematically discussed. Both reactions involve three crucial steps: formation of CO intermediate, CC coupling, and hydrodeoxygenation of C₂ intermediates. For ethylene, the key step is CC coupling, which can be enhanced by tailoring the surface structures of catalyst such as step sites on facets, Cu⁰ /Cu^δ+ species and nanopores, as well as the optimized molecule-catalyst and electrolyte-catalyst interfaces further promoting the higher ethylene production. While the controllable hydrodeoxygenation of C₂ intermediate is important for ethanol, which can be achieved by tuning the stability of oxygenate intermediates through the metallic cluster induced special atomic configuration and bimetallic synergy induced the double active sites on catalyst surface. Additionally, constraining CO coverage by the complex-catalyst interface and stabilizing CO bond by N-doped carbon/Cu interface can also enhance the ethanol selectivity. The structure-performance relationships will provide the guidance for the design of Cu-based catalysts for highly efficient reduction of CO₂ .

Green synthesis of SrO bridged LaFeO3/g-C3N4 nanocomposites for CO2 conversion and bisphenol A degradation with new insights into mechanism

Very recently the green synthesis routes of nanomaterials have attracted massive attention as it overcome the sustainability concerns of conventional synthesis approaches. With this heed, in this novel research work we have synthesized the g-C₃N₄ nanosheets based nanocomposites by utilizing Eriobotrya japonica as mediator and stabilizer agent. Our designed bio-caped and green g-C₃N₄ nanosheets based nanocomposites have abundant organic functional groups, activated surface and strong adsorption capability which are very favorable for conversion CO₂ into useful products and bisphenol A degradation. Beneficial to further upgrade the performances of g-C₃N₄ nanosheets, the resulting pristine g-C₃N₄ nanosheets are coupled with LaFeO₃ nanosheets via SrO bridge. Based on our experimental results such as TEM, XRD, DRS, TPD, TGA, PL, PEC and FS spectra linked with OH amount it is confirmed that the biologically mediated green g-C₃N₄ nanosheets are eco-friendly, highly efficient and stable. Furthermore, the coupling of LaFeO₃ nanosheets enlarged the surface area, enhanced the charge separation, while the insertion of SrO bridge worked as facilitator for electron transportation and photo-electron modulation. In contrast to pristine green g-C₃N₄ nanosheets (GCN), the activities of final resulting sample 6LFOS-(4SrO)-GCN are improved by 8.0 times for CO₂ conversion (CH₄ = 4.2, CO = 9.2 μmol g^-1 h^-1) and 2.5-fold for bisphenol A degradation (88%) respectively. More specifically, our current research work will open a new gateway to design cost effective, eco-friendly and biological inspired green nanomaterials for CO₂ conversion and organic pollutants degradation which will further support the net zero carbon emission manifesto and the optimization of carbon neutrality level.

Visible light-driven CO2 reduction with a Ru polypyridyl complex bearing an N-heterocyclic carbene moiety

A novel Ru polypyridyl complex with an N-heterocyclic carbene ligand was successfully synthesised and characterised. The complex exhibited an intense absorption band in the visible-light region derived from the strong electron-donating character of the carbene ligand, and efficiently catalysed the visible light-driven CO₂ reduction with the reaction rate of 36.7 h^-1.

Non-Noble Plasmonic Metal-Based Photocatalysts

Solar-to-chemical energy conversion via heterogeneous photocatalysis is one of the sustainable approaches to tackle the growing environmental and energy challenges. Among various promising photocatalytic materials, plasmonic-driven photocatalysts feature prominent solar-driven surface plasmon resonance (SPR). Non-noble plasmonic metals (NNPMs)-based photocatalysts have been identified as a unique alternative to noble metal-based ones due to their advantages like earth-abundance, cost-effectiveness, and large-scale application capability. This review comprehensively summarizes the most recent advances in the synthesis, characterization, and properties of NNPMs-based photocatalysts. After introducing the fundamental principles of SPR, the attributes and functionalities of NNPMs in governing surface/interfacial photocatalytic processes are presented. Next, the utilization of NNPMs-based photocatalytic materials for the removal of pollutants, water splitting, CO₂ reduction, and organic transformations is discussed. The review concludes with current challenges and perspectives in advancing the NNPMs-based photocatalysts, which are timely and important to plasmon-based photocatalysis, a truly interdisciplinary field across materials science, chemistry, and physics.

A Critical Study of Cu2O: Synthesis and Its Application in CO2 Reduction by Photochemical and Electrochemical Approaches

Copper oxide (Cu2O) is a potential material as a catalyst for CO2 reduction. Cu2O nanostructures have many advantages, including interfacial charge separation and transportation, enhanced surface area, quantum efficiency, and feasibility of modification via composite development or integration of the favorable surface functional groups. We cover the current advancements in the synthesis of Cu2O nanomaterials in various morphological dimensions and their photochemical and electrochemical applications, which complies with the physical enrichment of their enhanced activity in every application they are employed in. The scope of fresh designs, namely composites or the hierarchy of copper oxide nanostructures, and various ways to improve CO2 reduction performance are also discussed in this review. Photochemical and electrochemical CO2 transformations have received tremendous attention in the last few years, thanks to the growing interest in renewable sources of energy and green facile chemistry. The current review provides an idea of current photochemical and electrochemical carbon dioxide fixing techniques by using Cu2O-based materials. Carboxylation and carboxylative cyclization, yield valuable chemicals such as carboxylic acids and heterocyclic compounds. Radical ions, which are induced by photo- and electrochemical reactions, as well as other high-energy organic molecules, are regarded as essential mid-products in photochemical and electrochemical reactions with CO2. It has also been claimed that CO2 can be activated to form radical anions.

Engineering Hierarchical Architecture of Metal-Organic Frameworks for Highly Efficient Overall CO2 Photoreduction

Previous studies on syntheses of metal-organic frameworks (MOFs) for photocatalytic CO₂ reduction are mainly focused on the exquisite control over the net topology and the functionality of metal clusters/organic building blocks. This contribution demonstrates that the rational design of MOF-based photocatalyst can be further extended to the hierarchical structure at micrometer scales well beyond the conventional MOF design at the molecular level. By taking advantage of the disparity of two selective MOFs in nucleation kinetics, a hierarchical core-shell MOF@MOF structure is successfully constructed through a simple one-pot synthesis. Besides inheriting the high porosity, crystallinity, and robustness of parent MOFs, the obtained heterojunction exhibits extended photoresponse, optimized band alignment with large overpotential, and greatly enhanced photogenerated charge separation, which would be hardly realized by the merely molecular-level assembly. As a result, the challenging overall CO₂ photoreduction is achieved, which generates a record high HCOOH production (146.0 µmol/g/h) without using any sacrificial reagents. Moreover, the core-shell structure exhibits a more effective use of photogenerated electrons than the individual MOFs. This work shows that harnessing the hierarchical architecture of MOFs present a new and effective alternative to tuning the photocatalytic performance at a mesoscopic level.

Research Progress in Semiconductor Materials with Application in the Photocatalytic Reduction of CO2

The large-scale burning of non-renewable fossil fuels leads to the gradual increase of the CO2 concentration in the atmosphere, which is associated with negative impacts on the environment. The consequent need to reduce the emission of CO2 resulting from fossil fuel combustion has led to a serious energy crisis. Research reports indicate that the photocatalytic reduction of CO2 is one of the most effective methods to control CO2 pollution. Therefore, the development of novel high-efficiency semiconductor materials has become an important research field. Semiconductor materials need to have a structure with abundant catalytic sites, among other conditions, which is of great significance for the practical application of highly active catalysts for CO2 reduction. This review systematically describes various types of semiconductor materials, as well as adjustments to the physical, chemical and electronic characteristics of semiconductor catalysts to improve the performance of photocatalytic reduction of CO2. The principle of photocatalytic CO2 reduction is also provided in this review. The reaction types and conditions of photocatalytic CO2 reduction are further discussed. We believe that this review will provide a good basis and reference point for future design and development in this field.

Photocatalytic CO2 Reduction Using TiO2-Based Photocatalysts and TiO2 Z-Scheme Heterojunction Composites: A Review

Photocatalytic CO₂ reduction is a most promising technique to capture CO₂ and reduce it to non-fossil fuel and other valuable compounds. Today, we are facing serious environmental issues due to the usage of excessive amounts of non-renewable energy resources. In this aspect, photocatalytic CO₂ reduction will provide us with energy-enriched compounds and help to keep our environment clean and healthy. For this purpose, various photocatalysts have been designed to obtain selective products and improve efficiency of the system. Semiconductor materials have received great attention and have showed good performances for CO₂ reduction. Titanium dioxide has been widely explored as a photocatalyst for CO₂ reduction among the semiconductors due to its suitable electronic/optical properties, availability at low cost, thermal stability, low toxicity, and high photoactivity. Inspired by natural photosynthesis, the artificial Z-scheme of photocatalyst is constructed to provide an easy method to enhance efficiency of CO₂ reduction. This review covers literature in this field, particularly the studies about the photocatalytic system, TiO₂ Z-scheme heterojunction composites, and use of transition metals for CO₂ photoreduction. Lastly, challenges and opportunities are described to open a new era in engineering and attain good performances with semiconductor materials for photocatalytic CO₂ reduction.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Emerging Stacked Photocatalyst Design Enables Spatially Separated Ni(OH)2 Redox Cocatalysts for Overall CO2 Reduction and H2 O Oxidation

Construction of photocatalytic systems with spatially separated dual cocatalysts is considered as a promising route to modulate charge separation/transfer, promote surface redox reactivities, and prevent unwanted reverse reactions. However, past efforts on the loading of spatially separated double-cocatalysts are limited to hollow structured semiconductors with inner/outer surface and monocrystalline semiconductors with different exposed facets. To overcome this limitation, herein, enabled by a unique stacked photocatalyst design, a facile and versatile strategy for spatial separation of redox cocatalysts on various semiconductors without structural and morphological restriction is demonstrated. The smart design begins with the deposition of light-harvesting semiconductors on reduced graphene oxide (rGO) nanosheets, followed with the coverage of Ni(OH)₂ outer layer. The ternary photocatalysts exhibit superior activities and stabilities of H₂ O oxidation and selective CO₂ -to-CO reduction, remarkably surpassing other counterparts. The origin of the enhanced performance is attributed to the synergistic interplay of rGO@Ni(OH)₂ reduction cocatalysts surrounding the semiconductors and Ni(OH)₂ oxidation cocatalysts directly supported by the semiconductors, which mitigates the charge recombination, supplies highly active and selective sites for overall reactions, and preserves the semiconductors from photocorrosion. This work presents a new approach to regulating the position of dual cocatalysts and ameliorating the net efficiency of photoredox catalysis.

Photocatalysis and perovskite oxide-based materials: a remedy for a clean and sustainable future

The massive use of non-renewable energy resources by humankind to fulfill their energy demands is causing severe environmental issues. Photocatalysis is considered one of the potential solutions for a clean and sustainable future because of its cleanliness, inexhaustibility, efficiency, and cost-effectiveness. Significant efforts have been made to design highly proficient photocatalyst materials for various applications such as water pollutant degradation, water splitting, CO2 reduction, and nitrogen fixation. Perovskite photocatalyst materials are gained special attention due to their exceptional properties because of their flexibility in chemical composition, structure, bandgap, oxidation states, and valence states. The current review is focused on perovskite materials and their applications in photocatalysis. Special attention has been given to the structural, stoichiometric, and compositional flexibility of perovskite photocatalyst materials. The photocatalytic activity of perovskite materials in different photocatalysis applications is also discussed. Various mechanisms involved in photocatalysis application from wastewater treatment to hydrogen production are also provided. The key objective of this review is to encapsulate the role of perovskite materials in photocatalysis along with their fundamental properties to provide valuable insight for addressing future environmental challenges.

Solving the structure of "single-atom" catalysts using machine learning - assisted XANES analysis

"Single-atom" catalysts (SACs) have demonstrated excellent activity and selectivity in challenging chemical transformations such as photocatalytic CO₂ reduction. For heterogeneous photocatalytic SAC systems, it is essential to obtain sufficient information of their structure at the atomic level in order to understand reaction mechanisms. In this work, a SAC was prepared by grafting a molecular cobalt catalyst on a light-absorbing carbon nitride surface. Due to the sensitivity of the X-ray absorption near edge structure (XANES) spectra to subtle variances in the Co SAC structure in reaction conditions, different machine learning (ML) methods, including principal component analysis, K-means clustering, and neural network (NN), were utilized for in situ Co XANES data analysis. As a result, we obtained quantitative structural information of the SAC nearest atomic environment, thereby extending the NN-XANES approach previously demonstrated for nanoparticles and size-selective clusters.

Atom manufacturing of photocatalyst towards solar CO2reduction

Photocatalytic CO₂reduction reaction (CO₂RR) is believed to be a promising remedy to simultaneously lessen CO₂emission and obtain high value-added products, but suffers from the thwarted activity of photocatalyst and poor selectivity of product. Over the past decade, aided by the significant advances in nanotechnology, the atom manufacturing of photocatalyst, including vacancies, dopants, single-atom catalysts, strains, have emerged as efficient approaches to precisely mediate the reaction intermediates and processes, which push forward in the rapid development of highly efficient and selective photocatalytic CO₂RR. In this review, we summarize the recent developments in highly efficient and/or selective photocatalysts toward CO₂RR with the special focus on various atom manufacturing. The mechanisms of these atom manufacturing from active sites creation, light absorbability, and electronic structure modulation are comprehensively and scientifically discussed. In addition, we attempt to establish the structure-activity relationship between active sites and photocatalytic CO₂RR capability by integrating theoretical simulations and experimental results, which will be helpful for insights into mechanism pathways of CO₂RR over defective photocatalysts. Finally, the remaining challenges and prospects in this field to improve the photocatalytic CO₂RR performances are proposed, which can shed some light on designing more potential photocatalysts through atomic regulations toward CO₂conversion.

Dynamic Interface with Enhanced Visible-Light Absorption and Electron Transfer for Direct Photoreduction of Flue Gas to Syngas

The direct usage of CO₂ in the flue gas to produce fuels or chemicals is of great significance from energy-saving and low-cost perspectives, yet it is still underexplored. Herein, we report the photoreduction of CO₂ from the simulated industrial exhaust by synergistic catalysis of TEOA and a metal-free composite (COF1-g-C₃N₄) fabricated via covalently grafting COF1 with g-C₃N₄. The hydrogen bond interaction between TEOA and hydrazine units on COF1 is detected in diluted CO₂, which leads to significantly enhanced light absorption in the whole visible-light region. Also, the photo-induced electrons undergo fast transfer from COF1 to g-C₃N₄. This kind of dynamic interface with enhanced light absorption and electron transfer effects promotes the photosynthetic yield of syngas to 165.6 μmol·g^-1·h^-1 with the use of simulated exhaust gas as a raw material directly. The photosynthetic yield of syngas ranks among the highest of known metal-free catalysts in diluted CO₂. This work provides a general rule for designing efficient catalysts via a controlled catalytic interface and new insights into the role of TEOA in photochemical CO₂ reduction.

Efficient Photoinduced Electron Transfer from Pyrene-o-Carborane Heterojunction to Selenoviologen for Enhanced Photocatalytic Hydrogen Evolution and Reduction of Alkynes

A series of pyrene or pyrene-o-carborane-appendant selenoviologens (Py-SeV²⁺ , Py-Cb-SeV²⁺ ) for enhanced photocatalytic hydrogen evolution reaction (HER) and reduction of alkynes is reported. The efficient photoinduced electron transfer (PET) from electron-rich pyrene-o-carborane heterojunction (Py-Cb) with intramolecular charge transfer (ICT) characteristic to electron-deficient selenoviologen (SeV²⁺ ) (k_ET = 1.2 × 10¹⁰ s^-1 ) endows the accelerating the generation of selenoviologen radical cation (SeV^+• ) compared with Py-SeV²⁺ and other derivatives. The electrochromic/electrofluorochromic devices' (ECD and EFCD) measurements and supramolecular assembly/disassembly processes of SeV²⁺ and cucurbit[8]uril (CB[8]) results show that the PET process can be finely tuned by electrochemical and host-guest chemistry methods. By combination with Pt-NPs catalyst, the Py-Cb-SeV²⁺ -based system shows high-efficiency visible-light-driven HER and highly selective phenylacetylene reduction due to the efficient PET process.

Theoretical insights into effective electron transfer and migration behavior for CO2 reduction on the BiOBr(001) surfaces

Carbon dioxide (CO₂) activation by effective electrons has been regarded as the rather necessary first-step for a CO₂ reduction reaction (CO₂RR). In addition, the electron migration and photoreaction selectivity are closely associated with the dominant crystal surface of a catalyst. Therefore, it is very interesting and important to elucidate the electron transfer and charge density effects on the catalyst surface for the CO₂RR. In this work, the dominant highly-active BiOBr(001) surfaces with Bi-, O- and Br-termination atoms are designed so that their electron distributions and CO₂RR behaviors can be observed. The electron-rich sites on the BiOBr(001) surfaces, where more effective electrons will migrate to achieve the activation of the adsorbed CO₂, are firstly confirmed by the electron density difference based on density functional theory calculations. Next, the CO₂RR pathways at the electron-rich sites are investigated to explore the migration mechanism of effective photo-induced electrons. The results obtained reveal that if a larger number of electrons transfer to CO₂, then less energy is needed to break the CO bond, and the formation of a *COOH intermediate corresponds to the ability of the surface to take part in protonation. Furthermore, the interface Bi atom can boost the transfer efficiency of effective electrons to CO₂, but the exposed Br atom with a longer electron transfer distance, because of the steric hindrance of the interface Br atoms, makes it difficult for the electrons to migrate, resulting in it being harder to fracture the CO bond to benefit the formation of the HCOOH product. These findings should give deep insight into the migration behaviors of effective electrons for CO₂ photoreduction on the BiOBr(001) surface and provide new perspectives for better understanding the structure-performance relationship at the molecular level.

Opto-Electronic Characterization of Photocatalysts Based on p,n-Junction Ternary and Quaternary Mixed Oxides Semiconductors (Cu2O-In2O3 and Cu2O-In2O3-TiO2)

Semiconductor materials are the basis of electronic devices employed in the communication and media industry. In the present work, we report the synthesis and characterization of mixed metal oxides (MOs) as p,n-junction photocatalysts, and demonstrate the correlation between the preparation technique and the properties of the materials. Solid-state UV-visible diffuse reflectance spectroscopy (UV-VIS DRS) allowed for the determination of the light absorption properties and the optical energy gap. X-ray photoelectron spectroscopy (XPS) allowed for the determination of the surface speciation and composition and for the determination of the valence band edge. The opto-electronic behavior was evaluated measuring the photocurrent generated after absorption of chopped visible light in a 3-electrode cell. Scanning electron microscopy (SEM) measurements allowed for auxiliary characterization of size and morphology, showing the formation of composites for the ternary Cu2O-In2O3 p,n-mixed oxide, and even more for the quaternary Cu2O-In2O3-TiO2 MO. Light absorption spectra and photocurrent-time curves mainly depend upon the composition of MOs, while the optical energy gap and defective absorption tail are closely related to the preparation methodology, time and thermal treatment. Qualitative electronic band structures of semiconductors are also presented.

Evaluation of novel ZnO–Ag cathode for CO2 electroreduction in solid oxide electrolyser

CO2 and steam/CO2 electroreduction to CO and methane in solid oxide electrolytic cells (SOEC) has gained major attention in the past few years. This work evaluates, for the very first time, the performance of two different ZnO–Ag cathodes: one where ZnO nanopowder was mixed with Ag powder for preparing the cathode ink (ZnOmix–Ag cathode) and the other one where Ag cathode was infiltrated with a zinc nitrate solution (ZnOinf –Ag cathode). ZnOmix–Ag cathode had a better distribution of ZnO particles throughout the cathode, resulting in almost double CO generation while electrolysing both dry CO2 and H2/CO2 (4:1 v/v). A maximum overall CO2 conversion of 48% (in H2/CO2) at 1.7 V and 700 °C clearly indicated that as low as 5 wt% zinc loading is capable of CO2 electroreduction. It was further revealed that for ZnOinf –Ag cathode, most of CO generation took place through RWGS reaction, but for ZnOmix–Ag cathode, it was the synergistic effect of both RWGS reaction and CO2 electrolysis. Although ZnOinf –Ag cathode produced trace amount of methane at higher voltages, with ZnOmix–Ag cathode, there was absolutely no methane. This seems to be due to strong electronic interaction between Zn and Ag that might have suppressed the catalytic activity of the cathode towards methanation.

Photothermal Chemistry Based on Solar Energy: From Synergistic Effects to Practical Applications

With the development of society, energy shortage and environmental problems have become more and more outstanding. Solar energy is a clean and sustainable energy resource, potentially driving energy conversion and environmental remediation reactions. Thus, solar-driven chemistry is an attractive way to solve the two problems. Photothermal chemistry (PTC) is developed to achieve full-spectral utilization of the solar radiation and drive chemical reactions more efficiently under relatively mild conditions. In this review, the mechanisms of PTC are summarized from the aspects of thermal and non-thermal effects, and then the interaction and synergy between these two effects are sorted out. In this paper, distinguishing and quantifying these two effects is discussed to understand PTC processes better and to design PTC catalysts more methodically. However, PTC is still a little far away from practical. Herein, several key points, which must be considered when pushing ahead with the engineering application of PTC, are proposed, along with some workable suggestions on the practical application. This review provides a unique perspective on PTC, focusing on the synergistic effects and pointing out a possible direction for practical application.

Grafting Hypercrosslinked Polymers on TiO2 Surface for Anchoring Ultrafine Pd Nanoparticles: Dramatically Enhanced Efficiency and Selectivity toward Photocatalytic Reduction of CO2 to CH4

Metal deposition with photocatalyst is a promising way to surmount the restriction of fast e^- /h⁺ recombination to improve the photocatalytic performance. However, the improvement remains limited by the existing strategies adopted for depositing metal particles due to the serious aggregation and large unconnected area on photocatalyst surface. Here, a strategy is proposed by directly grafting hypercrosslinked polymers (HCPs) on TiO₂ surface to construct Pd-HCPs-TiO₂ composite with uniform dispersion of ultrafine Pd nanoparticles on HCPs surface. This composite with surface area of 373 m² g^-1 exhibits improved photocatalytic CO₂ conversion efficiency to CH₄ with an evolution rate of 237.4 µmol g^-1 h^-1 and selectivity of more than 99.9%. The enhancement can be ascribed to the grafted porous HCPs with high surface area and N heteroatom on TiO₂ surface for the stabilization of Pd nanoparticles, favoring the electron transfer and CO₂ adsorption for selective CH₄ production. This strategy may hold the promise for design and construction of porous organic polymer with semiconductor for efficient photocatalytic conversion.

Metal center regulation of the porphyrin unit in covalent organic polymers for boosting the photocatalytic CO2 reduction activity

Regulating the porphyrin's metal center of metalloporphyrin(MPor)/Ru(ii)-pincer complex(RuN3) covalent organic polymers (COPs) effectively boosted the CO2 photoreduction by promoting charge separation and sacrificial electron donor oxidation.