Enhanced photoreduction of CO2 into methanol by facet-dependent Cu2O/reduce graphene oxide

The prospect of CuxO-based catalysts in photocatalysis: From pollutant degradation, CO2 reduction, and H2 production to N2 fixation

The topic of photocatalysis and CuxO-based materials has been intertwined for quite a long time. Its relatively high abundance in the earth's crust makes it an important target for researchers around the globe. One of the properties exploited by researchers is its ability to exist in different oxidation states (Cu0, Cu+, Cu2+, and Cu3+) and its implications on photocatalytic efficiency improvement. Recently, they have been extensively used as photocatalytic materials for dye and pollutant degradation. However, it has almost reached saturation levels, therefore, currently, they are being mostly utilized for CO2 reduction and H2 evolution. Hence, this review will discuss the evolution (in application) of CuxO-based photocatalysts, relating to their past, present, and future. Moreover, photocatalytic efficiency improvement strategies such as doping, heterojunction formation, and carbonaceous construction with other materials will also be touched upon. Finally, the prospect of Cu2O-based photocatalysts will be discussed in the field of photocatalytic N2 fixation to ammonia. The significance of N2 chemisorption on photocatalysts to maximize ammonia production will also be given importance.

A Review on Cu2O-Based Composites in Photocatalysis: Synthesis, Modification, and Applications

Photocatalysis technology has the advantages of being green, clean, and environmentally friendly, and has been widely used in CO₂ reduction, hydrolytic hydrogen production, and the degradation of pollutants in water. Cu₂O has the advantages of abundant reserves, a low cost, and environmental friendliness. Based on the narrow bandgap and strong visible light absorption ability of Cu₂O, Cu₂O-based composite materials show infinite development potential in photocatalysis. However, in practical large-scale applications, Cu₂O-based composites still pose some urgent problems that need to be solved, such as the high composite rate of photogenerated carriers, and poor photocatalytic activity. This paper introduces a series of Cu₂O-based composites, based on recent reports, including pure Cu₂O and Cu₂O hybrid materials. The modification strategies of photocatalysts, critical physical and chemical parameters of photocatalytic reactions, and the mechanism for the synergistic improvement of photocatalytic performance are investigated and explored. In addition, the application and photocatalytic performance of Cu₂O-based photocatalysts in CO₂ photoreduction, hydrogen production, and water pollution treatment are discussed and evaluated. Finally, the current challenges and development prospects are pointed out, to provide guidance in applying Cu₂O-based catalysts in renewable energy utilization and environmental protection.

Photocatalytic Reduction of Carbon Dioxide to Methanol: Carbonaceous Materials, Kinetics, Industrial Feasibility, and Future Directions

Photocatalytic carbon dioxide reduction (PCCR) for methanol synthesis (CH₃OH) targeting renewable energy resources is an attractive way to create a sustainable environment and also balance the carbon-neutral series. The application of PCCR to methanol enables the generation of solar energy while reducing CO₂, killing two birds with one stone in terms of energy and the environment. In recent years, research on CO₂ utilization has focused on hydrogenation of CO₂ to methanol due to global warming. This article mainly focuses on selective carbonaceous materials such as graphene, mesoporous carbon, and carbon nanotubes (CNTs) as catalysts for heterogeneous photocatalytic CO₂ reduction to methanol. In addition, special emphasis will be placed on the state of the art of PCCR catalysts as this type of research will be of great benefit for further development in this field. The main features of the reaction kinetics, techno-economic study, and current technological developments in PCCR are covered in detail.

Advances in CO2 utilization employing anisotropic nanomaterials as catalysts: a review

Anisotropic nanomaterials are materials with structures and properties that vary depending on the direction in which they are measured. Unlike isotropic materials, which exhibit uniform physical properties in all directions, anisotropic materials have different mechanical, electrical, thermal, and optical properties in different directions. Examples of anisotropic nanomaterials include nanocubes, nanowires, nanorods, nanoprisms, nanostars, and so on. These materials have unique properties that make them useful in a variety of applications, such as electronics, energy storage, catalysis, and biomedical engineering. One of the key advantages of anisotropic nanomaterials is their high aspect ratio, which refers to the ratio of their length to their width, which can enhance their mechanical and electrical properties, making them suitable for use in nanocomposites and other nanoscale applications. However, the anisotropic nature of these materials also presents challenges in their synthesis and processing. For example, it can be difficult to align the nanostructures in a specific direction to impart modulation of a specific property. Despite these challenges, research into anisotropic nanomaterials continues to grow, and scientists are working to develop new synthesis methods and processing techniques to unlock their full potential. Utilization of carbon dioxide (CO₂) as a renewable and sustainable source of carbon has been a topic of increasing interest due to its impact on reducing the level of greenhouse gas emissions. Anisotropic nanomaterials have been used to improve the efficiency of CO₂ conversion into useful chemicals and fuels using a variety of processes such as photocatalysis, electrocatalysis, and thermocatalysis. More study is required to improve the usage of anisotropic nanomaterials for CO₂ consumption and to scale up these technologies for industrial use. The unique properties of anisotropic nanomaterials, such as their high surface area, tunable morphology, and high activity, make them promising catalysts for CO₂ utilization. This review article discusses briefly about various approaches towards the synthesis of anisotropic nanomaterials and their applications in CO₂ utilization. The article also highlights the challenges and opportunities in this field and the future direction of research.

Converting CO2 into Value-Added Products by Cu2 O-Based Catalysts: From Photocatalysis, Electrocatalysis to Photoelectrocatalysis

Converting CO₂ into value-added products by photocatalysis, electrocatalysis, and photoelectrocatalysis is a promising method to alleviate the global environmental problems and energy crisis. Among the semiconductor materials applied in CO₂ catalytic reduction, Cu₂ O has the advantages of abundant reserves, low price and environmental friendliness. Moreover, Cu₂ O has unique adsorption and activation properties for CO₂ , which is conducive to the generation of C₂₊ products through CC coupling. This review introduces the basic principles of CO₂ reduction and summarizes the pathways for the generation of C₁ , C₂ , and C₂₊ products. The factors affecting CO₂ reduction performance are further discussed from the perspective of the reaction environment, medium, and novel reactor design. Then, the properties of Cu₂ O-based catalysts in CO₂ reduction are summarized and several optimization strategies to enhance their stability and redox capacity are discussed. Subsequently, the application of Cu₂ O-based catalysts in photocatalytic, electrocatalytic, and photoelectrocatalytic CO₂ reduction is described. Finally, the opportunities, challenges and several research directions of Cu₂ O-based catalysts in the field of CO₂ catalytic reduction are presented, which is guidance for its wide application in the energy and environmental fields is provided.

Recent Progress in Metal Oxide-Based Photocatalysts for CO2 Reduction to Solar Fuels: A Review

One of the challenges in developing practical CO₂ photoconversion catalysts is the design of materials with a low cost, high activity and good stability. In this paper, excellent photocatalysts based on TiO₂, WO₃, ZnO, Cu₂O and CeO₂ metal oxide materials, which are cost-effective, long-lasting, and easy to fabricate, are evaluated. The characteristics of the nanohybrid catalysts depend greatly on their architecture and design. Thus, we focus on outstanding materials that offer effective and practical solutions. Strategies to improve CO₂ conversion efficiency are summarized, including heterojunction, ion doping, defects, sensitization and morphology control, which can inspire the future improvement in photochemistry. The capacity of CO₂ adsorption is also pivotal, which varies with the morphological and electronic structures. Forms of 0D, 1D, 2D and 3DOM (zero/one/two-dimensional- and three-dimensional-ordered macroporous, respectively) are involved. Particularly, the several advantages of the 3DOM material make it an excellent candidate material for CO₂ conversion. Hence, we explain its preparation method. Based on the discussion, new insights and prospects for designing high-efficient metallic oxide photocatalysts to reduce CO₂ emissions are presented.

Mechanisms of biochar enhanced Cu2O photocatalysts in the visible-light photodegradation of sulfamethoxazole

Cu₂O nanoparticles are decorated with biochars derived from spent coffee grounds (denoted as Cu₂O/SCG) and applied as visible-light-active photocatalysts in the sulfamethoxazole (SMX) degradation. The physicochemical properties of Cu₂O/SCG are identified by various spectral analysis, electrochemical and photochemical techniques. As a result, the Cu₂O/SCG exhibits the higher removal efficiency of SMX than the pristine Cu₂O under visible light irradiation. We can observe that Cu₂O could be incorporated onto the SCG biochars with rich oxygen vacancies/adsorbed hydroxyl groups. In addition, the Cu₂O/SCG has the lower charge transfer resistance, faster interfacial electron transfer kinetics, decreased recombination of charge carriers and superior absorbance of visible light. The construction of band diagrams for Cu₂O/SCG and pristine Cu₂O via UV-vis spectra and Mott-Schottky plots suggest that the band energy shifts and higher carrier density of Cu₂O/SCG may be responsible for the photocatalytic activity enhancements. From the radical scavenger experiments and electron paramagnetic resonance spectra, the aforementioned energy shifts could decrease the energy requirement of transferring photoinduced electrons to the potential for the formation of active superoxide radicals (·O₂^-) via one and two-electron reduction routes in the photocatalytic reaction. A proposed degradation pathway shows that ·O₂^- and h⁺ are two main active species which can efficiently degrade SMX into reaction intermediates by oxidation, hydroxylation, and ring opening. This research demonstrates the alternative replacement of conventional carbon materials for the preparation of biochar-assisted Cu₂O photocatalysts which are applied in the environmental decontamination by using solar energy.

Crystal Facet Dependent Energy Band Structures of Polyhedral Cu2O Nanocrystals and Their Application in Solar Fuel Production

We demonstrated a facile hydrothermal method to synthesize the (100)-, (110)- and (111)-oriented Cu₂O nanocrystals (NCs) by controlling the concentration of the incorporated anions (CO₃^2- and SO₃^2-). The crystal facet dependent activity of the orientation controlled Cu₂O NCs in the rhodamine B (RhB) photodegradation and photocatalytic hydrogen (H₂) evolution was found to follow the trend: (111) > (110) > (100). The mechanism was investigated by characterizing the optical property, energy band structure, interfacial charge carrier dynamics and reducing ability. The results indicated that the (111)-oriented Cu₂O NCs exhibit the higher conduction band (CB) potential as compared with the (110)-oriented and (100)-oriented Cu₂O NCs, which resulted in the largest driving force of interfacial electron transfer for (111)-oriented Cu₂O NCs to carry out solar fuel generation. The current study offers an easy strategy for crystal facet engineering of semiconductors and provides important physical insights into their electronic properties for the desired solar energy conversions.

Cu/PCN Metal-Semiconductor Heterojunction by Thermal Reduction for Photoreaction of CO2-Aerated H2O to CH3OH and C2H5OH

g-C₃N₄-based materials show potential for photoreduction of CO₂ to oxygenates but are subjected to fast recombination of photogenerated charge carriers. Here, a novel Cu-dispersive protonated g-C₃N₄ (PCN) metal-semiconductor (m-s) heterojunction from thermal reduction of a Cu₂O/PCN precursor was prepared and characterized using in situ X-ray diffraction, scanning transmission electron microscopy, X-ray photoelectron spectroscopy, ultraviolet-visible (UV-vis) spectra, photoluminescence (PL) spectra, transient photocurrent response, and electrochemical impedance spectroscopy (EIS). The Cu amount in Cu/PCN and the reduction temperature affected the generation of CH₃OH and C₂H₅OH from the photoreaction of CO₂-aerated H₂O. During calcination of Cu₂O/PCN in N₂ at 550 °C, Cu₂O was completely reduced to Cu with even dispersion, and a m-s heterojunction was obtained. With thermal exfoliation, Cu/PCN showed a specific surface area and layer spacing larger than those of PCN. Cu/PCN-0.5 (12.8 wt % Cu) exhibited a total carbon yield of 25.0 μmol·g^-1 under UV-vis irradiation for 4 h, higher than that of Cu₂O/PCN (13.6 μmol·g^-1) and PCN (6.0 μmol·g^-1). The selectivity for CH₃OH and C₂H₅OH was 51.42 and 46.14%, respectively. The PL spectra, transient photocurrent response, and EIS characterizations indicated that Cu/PCN heterojunction promotes the separation of electrons and holes and suppresses their recombination. The calculated conduction band position was more negative, which is conducive to the multielectron reactions for CH₃OH and C₂H₅OH generation.

Cu/ZnV2O4 Heterojunction Interface Promoted Methanol and Ethanol Generation from CO2 and H2O under UV-Vis Light Irradiation

Adopting the concurrent reduction of Cu2O during hydrothermal preparation of ZnV2O4, metal-semiconductor heterojunction Cu/ZnV2O4 nanorods were synthesized and applied to the catalytic generation of methanol and ethanol from CO2 aerated water under UV-vis light irradiation. 10Cu/ZnV2O4 obtained from 10 wt % composite amount of Cu2O exhibited a total carbon yield of 6.49 μmol·g-1·h-1. The yield of CH3OH and C2H5OH reached 3.30 and 0.86 μmol·g-1·h-1, respectively. 2.5Cu/ZnV2O4 displayed the highest ethanol yield of 1.58 μmol·g-1·h-1 due to the strong absorption in the visible light. Cu/ZnV2O4 was characterized using X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible (UV-vis) spectra, photoluminescence (PL) spectra, transient photocurrent response, and electrochemical impedance spectroscopy (EIS). Results showed that composite Cu0-ZnV2O4 increased the surface area and tuned the energy band position, which matches the reaction potential toward methanol and ethanol. The photocatalytic activity toward CH3OH and C2H5OH on Cu/ZnV2O4 is attributed to faster transmission and a slow recombination rate of photogenerated carriers at the heterojunction interface. Multielectron reactions for the production of CH3OH and C2H5OH are promoted. Free radical capture experiments indicated that the active species boost the reaction in the order of •OH > e- > h+.

Recent Advances of Photocatalytic Hydrogenation of CO2 to Methanol

Constantly increasing hydrocarbon fuel combustion along with high levels of carbon dioxide emissions has given rise to a global energy crisis and environmental alterations. Photocatalysis is an effective technique for addressing this energy and environmental crisis. Clean and renewable solar energy is a very favourable path for photocatalytic CO2 reduction to value-added products to tackle problems of energy and the environment. The synthesis of various products such as CH4, CH3OH, CO, EtOH, etc., has been expanded through the photocatalytic reduction of CO2. Among these products, methanol is one of the most important and highly versatile chemicals widely used in industry and in day-to-day life. This review emphasizes the recent progress of photocatalytic CO2 hydrogenation to CH3OH. In particular, Metal organic frameworks (MOFs), mixed-metal oxide, carbon, TiO2 and plasmonic-based nanomaterials are discussed for the photocatalytic reduction of CO2 to methanol. Finally, a summary and perspectives on this emerging field are provided.

A Critical Review on Black Phosphorus-Based Photocatalytic CO2 Reduction Application

Energy shortages and greenhouse effects are two unavoidable problems that need to be solved. Photocatalytically converting CO₂ into a series of valuable chemicals is considered to be an effective means of solving the above dilemmas. Among these photocatalysts, the utilization of black phosphorus for CO₂ photocatalytic reduction deserves a lightspot not only for its excellent catalytic activity through different reaction routes, but also on account of the great preponderance of this relatively cheap catalyst. Herein, this review offers a summary of the recent advances in synthesis, structure, properties, and application for CO₂ photocatalytic reduction. In detail, the review starts from the basic principle of CO₂ photocatalytic reduction. In the following section, the synthesis, structure, and properties, as well as CO₂ photocatalytic reduction process of black phosphorus-based photocatalyst are discussed. In addition, some possible influencing factors and reaction mechanism are also summarized. Finally, a summary and the possible future perspectives of black phosphorus-based photocatalyst for CO₂ reduction are established.

Copper Oxide-Based Photocatalysts and Photocathodes: Fundamentals and Recent Advances

This work aims at reviewing the most impactful results obtained on the development of Cu-based photocathodes. The need of a sustainable exploitation of renewable energy sources and the parallel request of reducing pollutant emissions in airborne streams and in waters call for new technologies based on the use of efficient, abundant, low-toxicity and low-cost materials. Photoelectrochemical devices that adopts abundant element-based photoelectrodes might respond to these requests being an enabling technology for the direct use of sunlight to the production of energy fuels form water electrolysis (H2) and CO2 reduction (to alcohols, light hydrocarbons), as well as for the degradation of pollutants. This review analyses the physical chemical properties of Cu2O (and CuO) and the possible strategies to tune them (doping, lattice strain). Combining Cu with other elements in multinary oxides or in composite photoelectrodes is also discussed in detail. Finally, a short overview on the possible applications of these materials is presented.

Synthesis of Cu2O Nanostructures with Tunable Crystal Facets for Electrochemical CO2 Reduction to Alcohols

Electrochemical CO₂ reduction enables the conversion of intermittent renewable energy to value-added chemicals and fuel, presenting a promising strategy to relieve CO₂ emission and achieve clean energy storage. In this work, we developed nanosized Cu₂O catalysts using the hydrothermal method for electrochemical CO₂ reduction to alcohols. Cu₂O nanoparticles (NPs) of various morphologies that were enclosed with different crystal facets, named as Cu₂O-c (cubic structure with (100) facets), Cu₂O-o (octahedron structure with (111) facets), Cu₂O-t (truncated octahedron structure with both (100) and (111) facets), and Cu₂O-u (urchin-like structure with (100), (220), and (222) facets), were prepared by regulating the content of a polyvinyl pyrrolidone (PVP) template. The electrochemical CO₂ reduction performance of the different Cu₂O NPs was evaluated in the CO₂-saturated 0.5 M KHCO₃ electrolyte. The as-synthesized Cu₂O nanostructures were capable of reducing CO₂ to produce alcohols including methanol, ethanol, and isopropanol. The alcohol selectivity of the different Cu₂O NPs followed the order of Cu₂O-t < Cu₂O-u < Cu₂O-c < Cu₂O-o (with the total Faradaic efficiencies of alcohol products of 10.7, 25.0, 26.2, and 35.4%). The facet-dependent effects were associated with the varied concentrations of oxygen-vacancy defects, different energy barriers of CO₂ reduction, and distinct Cu-O bond lengths over the different crystal facets. The desired Cu₂O-o catalyst exhibited good reduction activity with the highest partial current density of 0.51 mA/cm² for alcohols. The Faradaic efficiencies of alcohol products were 4.9% for methanol, 17.9% for ethanol, and 12.6% for isopropanol. The good electrochemical CO₂ reduction performance was also associated with the surface reconstruction of Cu₂O, which endowed the catalyst with abundant Cu⁰ and Cu⁺ sites for promoted CO₂ activation and stabilized CO* adsorption for enhanced C-C coupling. This work will provide a new route for enhancing the alcohol selectivity of nanostructured Cu₂O catalysts by crystal facet engineering.

Recent advances in Cu2O-based composites for photocatalysis: a review

Cu₂O-based composites for photocatalysis have been extensively explored owing to their promising application in solving environmental and energy problems. At present, the research on photocatalysis is focused on improving the photocatalytic performance of materials. It has been reported that adjusting the morphology and size of Cu₂O can effectively improve its photocatalytic property. However, photocorrosion is still an inevitable problem, which hinders the application of Cu₂O in photocatalysis. The strategies of constructing heterogeneous nanostructures and ion doping can significantly improve the light stability, light absorption capacity and separation efficiency of electron-hole pairs. Cu₂O-based composites exhibit superior performances in degrading organic matter, producing hydrogen, reducing CO₂ and sterilization. Therefore, the construction of multi-materials will be one of the future directions in their photocatalytic application. This review summarizes the recent strategies for enhancing the photocatalytic activity of Cu₂O by analyzing different Cu₂O-based photocatalysts, and the charge transfer pathway is further discussed in detail. Finally, several opportunities and challenges in the field of photocatalysis are illustrated.

Tale of Two Layered Semiconductor Catalysts toward Artificial Photosynthesis

The ever-increasing reliance on nonrenewable fossil fuels due to massive urbanization and industrialization created problems such as depletion of the primary feedstock and raised the atmospheric CO₂ levels causing global warming. A smart and promising approach is artificial photosynthesis that photocatalytically valorizes CO₂ into high-value chemicals. The inexpensive layered semiconductors like g-C₃N₄ and rGO or GO have the potential to make the process practically feasible for real applications. The suitable band positions with respect to the reduction potentials coupled with the typical surface properties of these layered semiconductors play a beneficial role in photoreduction of CO₂. Additionally, the creation of heterojunction interfaces to achieve the Z-scheme by anchoring g-C₃N₄ and rGO with another semiconductor with proper band alignment and dispersing plasmonic nano metals to obtain Schottky barriers on the layered surfaces also help retarding the electron-hole recombination and boost up the catalytic efficacy. Extensive exploration happened in recent years toward artificial photosynthesis over these materials, which needs a critical compendium. Surprisingly, in spite of the recent explosion of studies on photocatalytic reduction of CO₂ over metal-free semiconductors, there is not a single review on comparing the mechanistic aspects of photoreduction of CO₂ over the layered semiconductors g-C₃N₄ and rGO. This review stands out as a unique documentation, where the mechanism of photocatalytic reduction of CO₂ over this set of materials is critically examined in the context of band and surface modifications. An overall conclusion and outlook at the end indicates the need to develop prototypes for artificial photosynthesis with these well-studied semiconducting layered materials to yield solar fuels.

Effect of bandgap alignment on the photoreduction of CO2 into methane based on Cu2O-decorated CuO microspheres

Semiconductors' band gap alignment is important for the photoreduction of CO2 to methane. In the paper, two kinds of Cu2O-decorated CuO microspheres composed with nanoflakes were prepared by using two different methods. Their electron behaviors were studied from the XPS spectra and photoelectrochemical measurements. Both samples are p-type CuO covered with an amount of Cu2O nanoparticles on their surface. Combined with their bandgaps and flat band potentials, CuO-Mic has a well-matched bandgap alignment between Cu2O and CuO, which is favorable for the separation of photogenerated electron-hole pairs. Those photogenerated carriers are beneficial for the conversion of CO2 to CH4, as an 8-electron process for the conversion of CO2 to CH4 will consume more photogenerated electrons for the chemical reactions than that of the 2-electron process for CO2 reduction to CO. Therefore, CuO-Mic has much better photocatalytic activity for CO2 reduction to CH4 with a CH4 yield ten times higher than that of CuO-Hyd under a visible light irradiation, the CO yields of the CO2 reduction are identical.

Aspect Ratio-Dependent Charge Carrier Dynamics in Matchstick-like Ag2S-ZnS Nanorods for Solar Hydrogen Generation

Matchstick-like Ag₂S-ZnS nanorods (NRs) with a tunable aspect ratio (AR) were synthesized using one-pot thermal decomposition. The ultraviolet photoelectron spectra and time-resolved photoluminescence spectra of the Ag₂S-ZnS NRs were collected to study their electronic band structures and charge carrier dynamics. The energy difference (ΔE) at the interface between the ZnS stem and Ag₂S tip was altered as the AR of Ag₂S-ZnS NRs increased from 11.9 to 18.4, resulting in an enlarged driving force for the delocalized electrons along the conduction band of ZnS being injected into that of Ag₂S. The interfacial electron transfer rate constant (k_et) from ZnS to Ag₂S could be enhanced by ∼2 orders of magnitude from 5.27 × 10⁶ to 3.24 × 10⁸ s^-1, leading to a significant improvement in the efficiency of solar hydrogen generation. This investigation provides new physical insights into the manipulation of charge carrier dynamics by means of AR adjustment in semiconductor nanoheterostructures for photoelectric conversions.

Methanol Synthesis from CO2: A Review of the Latest Developments in Heterogeneous Catalysis

Technological approaches which enable the effective utilization of CO2 for manufacturing value-added chemicals and fuels can help to solve environmental problems derived from large CO2 emissions associated with the use of fossil fuels. One of the most interesting products that can be synthesized from CO2 is methanol, since it is an industrial commodity used in several chemical products and also an efficient transportation fuel. In this review, we highlight the recent advances in the development of heterogeneous catalysts and processes for the direct hydrogenation of CO2 to methanol. The main efforts focused on the improvement of conventional Cu/ZnO based catalysts and the development of new catalytic systems targeting the specific needs for CO2 to methanol reactions (unfavourable thermodynamics, production of high amount of water and high methanol selectivity under high or full CO2 conversion). Major studies on the development of active and selective catalysts based on thermodynamics, mechanisms, nano-synthesis and catalyst design (active phase, promoters, supports, etc.) are highlighted in this review. Finally, a summary concerning future perspectives on the research and development of efficient heterogeneous catalysts for methanol synthesis from CO2 will be presented.