A review on modification of facet-engineered TiO2 for photocatalytic CO2 reduction

Single Atom Catalysts Based on Earth-Abundant Metals for Energy-Related Applications

Anthropogenic activities related to population growth, economic development, technological advances, and changes in lifestyle and climate patterns result in a continuous increase in energy consumption. At the same time, the rare metal elements frequently deployed as catalysts in energy related processes are not only costly in view of their low natural abundance, but their availability is often further limited due to geopolitical reasons. Thus, electrochemical energy storage and conversion with earth-abundant metals, mainly in the form of single-atom catalysts (SACs), are highly relevant and timely technologies. In this review the application of earth-abundant SACs in electrochemical energy storage and electrocatalytic conversion of chemicals to fuels or products with high energy content is discussed. The oxygen reduction reaction is also appraised, which is primarily harnessed in fuel cell technologies and metal-air batteries. The coordination, active sites, and mechanistic aspects of transition metal SACs are analyzed for two-electron and four-electron reaction pathways. Further, the electrochemical water splitting with SACs toward green hydrogen fuel is discussed in terms of not only hydrogen evolution reaction but also oxygen evolution reaction. Similarly, the production of ammonia as a clean fuel via electrocatalytic nitrogen reduction reaction is portrayed, highlighting the potential of earth-abundant single metal species.

Synergistic effect of atomically dispersed Cu species and Ti-defects for boosting photocatalytic CO2 reduction over hierarchical TiO2

The photocatalytic water-mediated CO2 reduction reaction, which holds great promise for the conversion of CO2 into valuable chemicals, is often hindered by inefficient separation of photogenerated charges and a lack of suitable catalytic sites. Herein, we have developed a glycerol coordination assembly approach to precisely control the distribution of atomically dispersed Cu species by occupying Ti-defects and adjusting the ratio between Cu species and Ti-defects in a hierarchical TiO2. The optimal sample demonstrates a ∼4-fold improvement in CO2-to-CO conversion compared to normal TiO2 nanoparticles. The high activity could be attributed to the Ti defects, which enhance the photogenerated charge separation and simultaneously facilitate the adsorption of water molecules, thereby promoting the water oxidation reaction. Moreover, by means of in situ EPR and FTIR spectra, we have demonstrated that Cu species can effectively capture photogenerated electrons and facilitate the adsorption of CO2, so as to catalyze the reduction of CO2. This work provides a strategy for the construction of atomic-level synergistic catalytic sites and the utilization of in situ techniques to reveal the underlying mechanism.

Atomic/molecular layer deposition strategies for enhanced CO2 capture, utilisation and storage materials

Elevated levels of carbon dioxide (CO2) in the atmosphere and the diminishing reserves of fossil fuels have raised profound concerns regarding the resulting consequences of global climate change and the future supply of energy. Hence, the reduction and transformation of CO2 not only mitigates environmental pollution but also generates value-added chemicals, providing a dual remedy to address both energy and environmental challenges. Despite notable advancements, the low conversion efficiency of CO2 remains a major obstacle, largely attributed to its inert chemical nature. It is imperative to engineer catalysts/materials that exhibit high conversion efficiency, selectivity, and stability for CO2 transformation. With unparalleled precision at the atomic level, atomic layer deposition (ALD) and molecular layer deposition (MLD) methods utilize various strategies, including ultrathin modification, overcoating, interlayer coating, area-selective deposition, template-assisted deposition, and sacrificial-layer-assisted deposition, to synthesize numerous novel metal-based materials with diverse structures. These materials, functioning as active materials, passive materials or modifiers, have contributed to the enhancement of catalytic activity, selectivity, and stability, effectively addressing the challenges linked to CO2 transformation. Herein, this review focuses on ALD and MLD's role in fabricating materials for electro-, photo-, photoelectro-, and thermal catalytic CO2 reduction, CO2 capture and separation, and electrochemical CO2 sensing. Significant emphasis is dedicated to the ALD and MLD designed materials, their crucial role in enhancing performance, and exploring the relationship between their structures and catalytic activities for CO2 transformation. Finally, this comprehensive review presents the summary, challenges and prospects for ALD and MLD-designed materials for CO2 transformation.

Flexible Antibacterial Respiratory Monitoring Sensor Based on Controllable Au-Modified Surface of Highly {001} Preferred Anatase Titanium Dioxide Thin Film

Respiratory signals are critical clinical diagnostic criteria for respiratory diseases and health conditions, and respiratory sensors play a crucial role in achieving the desired respiratory monitoring effect. High sensitivity to a single factor can improve the reliability of respiratory monitoring, and maintaining the hygiene of the sensors is also important for daily health monitoring. Herein, we propose a flexible Au-modified anatase titanium dioxide resistive respiratory sensor, which can be mechanically compliantly attached to curved surfaces for respiratory monitoring in different modalities (i.e., respiratory intensity, frequency, and rate). The uniform and preferentially oriented anatase titanium dioxide films gained by the polymer-assisted deposition technique can be fabricated on flexible substrates through a liquid-assisted transferring process. The Au modification can enhance surface plasmon resonance to facilitate the photocatalytic activity of titanium dioxide, and the optimized distribution of Au on the surface of titanium dioxide film made the sensor have an excellent antibacterial effect. The uniquely designed encapsulation can effectively control the contact between the surface of titanium dioxide films and electrodes, allowing the flexible sensor to exhibit fast response time (0.71 s) and recovery time (1.06 s) to respiratory as well as insensitivity or low sensitivity to other factors (i.e., gas composition, humidity, temperature, stress, and strain). This work provided an effective strategy for flexible wearable respiratory sensors and has great potential in daily respiratory monitoring for health management and pandemic control.

2D Atomic Layers for CO2 Photoreduction

Artificial photosynthesis can convert carbon dioxide into high value-added chemicals. However, due to the poor charge separation efficiency and CO2 activation ability, the conversion efficiency of photocatalytic CO2 reduction is greatly restricted. Ultrathin 2D photocatalyst emerges as an alternative to realize the higher CO2 reduction performance. In this review, the basic principle of CO2 photoreduction is introduced, and the types, advantages, and advances of 2D photocatalysts are reviewed in detail including metal oxides, metal chalcogenides, bismuth-based materials, MXene, metal-organic framework, and metal-free materials. Subsequently, the tactics for improving the performance of 2D photocatalysts are introduced in detail via the surface atomic configuration and electronic state tuning such as component tuning, crystal facet control, defect engineering, element doping, cocatalyst modification, polarization, and strain engineering. Finally, the concluding remarks and future development of 2D photocatalysts in CO2 reduction are prospected.

Recent Progress of Covalent Organic Frameworks-Based Materials in Photocatalytic Applications: A Review

Covalent organic frameworks (COFs) are one type of porous organic materials linked by covalent bonds. COFs materials exhibit many outstanding characteristics such as high porosity, high chemical and thermal stability, large specific surface area, efficient electron transfer efficiency, and the ability for predesigned structures. These exceptional advantages enable COFs materials to exhibit remarkable performance in photocatalysis. Additionally, the activity of COFs materials as photocatalysts can be significantly upgraded by ion doping and the formation of heterojunctions. This paper summarizes the latest research progress on COF-based materials applied in photocatalytic systems. Initially, typical structures and preparation methods of COFs are analyzed and compared. Moreover, the essential principles of photocatalytic reactions over COFs-based materials and the latest research developments in photocatalytic hydrogen production, CO2 reduction, pollutants elimination, organic transformation, and overall water splitting are indicated. At last, the outlook and challenges of COF-based materials in photocatalysis are discussed. This review is intended to permit instructive guidance for the efficient use of photocatalysis based on COFs in the future.

Elucidating the mechanism of photocatalytic reduction of bicarbonate (aqueous CO2) into formate and other organics

The photocatalytic reduction of CO2 under solar irradiation is an ideal approach to mitigating global warming, and reducing aqueous forms of CO2 that interact strongly with a catalyst (e.g., HCO3-) is a promising strategy to expedite such reductions. This study uses Pt-deposited graphene oxide dots as a model photocatalyst to elucidate the mechanism of HCO3- reduction. The photocatalyst steadily catalyzes the reduction of an HCO3- solution (at pH = 9) containing an electron donor under 1-sun illumination over a period of 60 h to produce H2 and organic compounds (formate, methanol, and acetate). H2 is derived from solution-contained H2O, which undergoes photocatalytic cleavage to produce •H atoms. Isotopic analysis reveals that all of the organics formed via interactions between HCO3- and •H. This study proposes mechanistic steps, which are governed by the reacting behavior of the •H, to correlate the electron transfer steps and product formation of this photocatalysis. This photocatalysis achieves overall apparent quantum efficiency of 27% in the formation of reaction products under monochromatic irradiation at 420 nm. This study demonstrates the effectiveness of aqueous-phase photocatalysis in converting aqueous CO2 into valuable chemicals and the importance of H2O-derived •H in governing the product selectivity and formation kinetics.

MoS2-Based Nanocomposites for Photocatalytic Hydrogen Evolution and Carbon Dioxide Reduction

Photocatalysis is a facile and sustainable approach for energy conversion and environmental remediation by generating solar fuels from water splitting. Due to their two-dimensional (2D) layered structure and excellent physicochemical properties, molybdenum disulfide (MoS₂) has been effectively utilized in photocatalytic H₂ evolution reaction (HER) and CO₂ reduction. The photocatalytic efficiency of MoS₂ greatly depends on the active edge sites present in their layered structure. Modifications like reducing the layer numbers, creating defective structures, and adopting different morphologies produce more unsaturated S atoms as active edge sites. Hence, MoS₂ acts as a cocatalyst in nanocomposites/heterojunctions to facilitate the photogenerated electron transfer. This review highlights the role of MoS₂ as a cocatalyst for nanocomposites in H₂ evolution reaction and CO₂ reduction. The H₂ evolution activity has been described comprehensively as binary (with metal oxide, carbonaceous materials, metal sulfides, and metal-organic frameworks) and ternary composites of MoS₂. Photocatalytic CO₂ reduction is a more complex and challenging process that demands an efficient light-responsive semiconductor catalyst to tackle the thermodynamic and kinetic factors. Photocatalytic reduction of CO₂ using MoS₂ is an emerging topic and would be a cost-effective substitute for noble catalysts. Herein, we also exclusively envisioned the possibility of layered MoS₂ and its composites in this area. This review is expected to furnish an understanding of the diverse roles of MoS₂ in solar fuel generation, thus endorsing an interest in utilizing this unique layered structure to create nanostructures for future energy applications.

Plasmon-Enhanced Photocatalytic CO2 Reduction for Higher-Order Hydrocarbon Generation Using Plasmonic Nano-Finger Arrays

The carbon dioxide reduction reaction (CO2RR) is a promising method to both reduce greenhouse gas carbon dioxide (CO₂) concentrations and provide an alternative to fossil fuel by converting water and CO₂ into high-energy-density chemicals. Nevertheless, the CO2RR suffers from high chemical reaction barriers and low selectivity. Here we demonstrate that 4 nm gap plasmonic nano-finger arrays provide a reliable and repeatable plasmon-resonant photocatalyst for multiple-electrons reactions: the CO2RR to generate higher-order hydrocarbons. Electromagnetics simulation shows that hot spots with 10,000 light intensity enhancement can be achieved using nano-gap fingers under a resonant wavelength of 638 nm. From cryogenic ¹H-NMR spectra, formic acid and acetic acid productions are observed with a nano-fingers array sample. After 1 h laser irradiation, we only observe the generation of formic acid in the liquid solution. While increasing the laser irradiation period, we observe both formic and acetic acid in the liquid solution. We also observe that laser irradiation at different wavelengths significantly affected the generation of formic acid and acetic acid. The ratio, 2.29, of the product concentration generated at the resonant wavelength 638 nm and the non-resonant wavelength 405 nm is close to the ratio, 4.93, of the generated hot electrons inside the TiO₂ layer at different wavelengths from the electromagnetics simulation. This shows that product generation is related to the strength of localized electric fields.

Tuning the physicochemical features of titanium oxide nanomaterials by ultrasound: Elevating photocatalytic selective partial oxidation of lignin-inspired aromatic alcohols

The research for "green" and economically feasible approaches such as (photo)catalysis especially for biomass valorization such as selective oxidation of biomass derived compounds like aromatic alcohols to corresponding aldehyde by avoiding the harsh reaction conditions and the addition of reagents concentrate the focus of attention the last years. Hence, design and development of novel photocatalyst for the partial selective oxidation is highly desirable. In this research work, ultrasonication of different frequencies (22, 40, 80 kHz) and different amplitudes was utilized as synthesis tool in order to obtain novel materials by precipitation method. The synthesized samples were characterized by using different techniques such as N₂ sorption, TEM, XPS, XRD, thermal analysis, and diffuse reflectance spectroscopy. The synthesized sample by using low ultrasound frequency (22 kHz) and amplitude showed a mixed morphological and structural nature consisting of asymmetric 1-dimensional (nanorods-like), layered nano-structures and not well-defined areas, leading to elevate for metal oxide specific surface areas up to 155 m²/g. The observed 1-D nanostructures have diamentions in the range of 20-60 nm. This sample revealed the highest photo-oxidation efficiency for the selective conversion of two biomass-derived, and more specifically lignin-inspired model compounds, benzyl alcohol and cinnamyl alcohol to benzaldehyde and cinnamyl aldehyde, respectively, and hence the highest yield towards the desired aldehydes. The selective photo-oxidation activity was retained even after 5 photocatalytic cycles, while no leaching of Ti was recorded.

Cu-doped SnS2 nanosheets with superior visible-light photocatalytic CO2 reduction performance

Photocatalytic CO₂ reduction utilizing solar energy is a clean, environment-friendly strategy converting CO₂ into hydrocarbon fuels to solve the energy crisis and climate issues. Herein, we report the synthesis of Cu-doped SnS₂ nanosheets via a simple hydrothermal method. The prepared Cu-doped SnS₂ composite displays superior photocatalytic CO₂ reduction activity. The optimized CH₃OH yield of the composite is two times higher than that of pure SnS₂. The enhanced photocatalytic performance is attributed to effective charge separation resulting from the thinner nanosheets and delocalization of electrons from SnS₂ to Cu, high visible light utilization efficiency, enlarged S_BET, negative shift of the flat-band potential and reduced charge transfer resistance with the introduction of Cu atoms. This work suggests the potential application of Cu-doped SnS₂ in photocatalytic CO₂ reduction.

N-doped rutile TiO2 nanorod@g-C3N4 core/shell S-scheme heterojunctions for boosting CO2 photoreduction activity

A novel core/shell structure composed of N-doped rutile TiO2@g-C3N4 (NT@CNx) with an S-scheme heterojunction is successfully synthesized. The S-scheme heterojunction optimizes the electrochemical property and redox ability of the NT@CNx composite.

Photocatalytic Oxidation of Carbon Monoxide Using Synergy of Redox-Separated Photocatalyst and Ozone

Separating the redox centers of photocatalysts is the most promising strategy to enhance photocatalytic oxidation efficiency. Herein, I investigate a site-selective loading of Pt on facet-engineered TiO₂ to achieve carbon monoxide (CO) oxidation at room temperature. Spatially loaded Pt on {101} facets of TiO₂ attracts photoinduced electrons efficiently. Thereby, oxygen dissociation is facilitated on the Pt surface, which is confirmed by enhanced oxidation of CO by 2.4 times compared to the benchmark of Pt/TiO₂. The remaining holes on TiO₂ can be utilized for the oxidation of various gaseous pollutants. Specifically, gaseous ozone, which is present in indoor and ambient air, is converted to a hydroxyl radical by reacting with the hole; thus, the poisoned Pt surface is continuously cleaned during the CO oxidation, as confirmed by in situ diffuse reflectance infrared transform spectroscopy. While randomly loaded Pt can act as recombination center, reducing photocatalytic activity, redox-separated photocatalyst enhances charge separation, boosting CO oxidation and catalyst regeneration via simultaneous ozone decomposition.

Recent review of BixMOy (M=V, Mo, W) for photocatalytic CO2 reduction into solar fuels

The utilization of solar energy for CO₂ conversion not only enables a green and low-carbon recycling of CO₂ with renewable energy, but also solves ecological problems. Bi_xMO_y (M = V, Mo, W) materials have typical layered structures and unique electronic properties that provide suitable band gaps and potential to meet the basic conditions for CO₂ reduction. However, pristine Bi_xMO_y faces with problems such as small specific surface area, insufficient active sites, low charge carriers' separation and utilization efficiency. This review comprehensively described the basic principles and reaction pathways of photocatalytic CO₂ reduction, and further presented the research progress of Bi_xMO_y catalysts in CO₂ conversion reactions. In this perspective, we further focus on the design concepts and modification strategies to improve the photocatalytic CO₂ reduction activity of Bi_xMO_y, such as morphology control, constructing surface vacancies and heterojunction fabrication. Finally, based on representative researches, the present review will be expected to provide updated information and insights for developing advanced Bi_xMO_y materials to further improve CO₂ reduction activity and selectivity.

One-Step Hydrothermal Synthesis of Anatase TiO2 Nanotubes for Efficient Photocatalytic CO2 Reduction

The hydrothermal dissolution-recrystallization process is a key step in the crystal structure of titania-based nanotubes and their composition. This work systematically studies the hydrothermal conditions for directly synthesizing anatase TiO₂ nanotubes (ATNTs), which have not been deeply discussed elsewhere. It has been well-known that ATNTs can be synthesized by the calcination of titanate nanotubes. Herein, we found the ATNTs can be directly synthesized by optimizing the reaction temperature and time rather than calcination of titanate nanotubes, where at each temperature, there is a range of reaction times in which ATNTs can be prepared. The effect of NaOH/TiO₂ ratio and starting materials was explored, and it was found that ATNTs can be prepared only if the precursor is anatase TiO₂, using rutile TiO₂ leads to forming titanate nanotubes. As a result, ATNTs produced directly without calcination have excellent photocatalytic CO₂ reduction than titanate nanotubes and ATNTs prepared by titanate calcination.

An Artificial Photosystem of Metal-Insulator-CTF Nanoarchitectures for Highly Efficient and Selective CO2 Conversion to CO

It is of pivotal significance to explore robust photocatalysts to promote the photoreduction of CO₂ into solar fuels. Herein, an intelligent metal-insulator-semiconductor (MIS) nano-architectural photosystem was constructed by electrostatic self-assembly between cetyltrimethylammonium bromide (CTAB) insulator-capped metal Ni nanoparticles (NPs) and covalent triazine-based frameworks (CTF-1). The metal-insulator-CTF composites unveiled a substantially higher CO evolution rate (1254.15 μmol g^-1  h^-1 ) compared with primitive CTF-1 (1.08 μmol g^-1  h^-1 ) and reached considerable selectivity (98.9 %) under visible-light irradiation. The superior photocatalytic CO₂ conversion activity over Ni-CTAB-CTF nanoarchitecture could be attributed to the larger surface area, reinforced visible-light response, and CO₂ capture capacity. More importantly, the Ni-CTAB-CTF nanoarchitecture endowed the photoexcited electrons on CTF-1 with the ability to tunnel across the thin CTAB insulating layer, directionally migrating to Ni NPs and thereby leading to the efficient separation of photogenerated electrons and holes in the photosystem. In addition, isotope-labeled (¹³ CO₂ ) tracer results verified that the reduction products come from CO₂ rather than the decomposition of the photocatalysts. This study opens a new avenue for establishing a highly efficient and selective artificial photosystem for CO₂ conversion.

A MXene-based multiple catalyst for highly efficient photocatalytic removal of nitrate

Photocatalytic removal of nitrate in wastewater has attracted wide attention because of its simple operation and environmental protection. However, the preparation of photocatalysts with high efficiency and high nitrogen selectivity is still a challenge. In this paper, TiO₂ is grown in situ on Ti₃C₂ MXene by a simple calcination method and modified with silver particles. The presence of Ti₃C₂ reduces the recombination rate of photogenerated electrons and generates more photogenerated electrons. At the same time, the silver particles also increase the photoelectron density and further improve the carrier separation of the catalyst. Due to its unique structure and optical properties, the prepared photocatalyst shows an excellent nitrate removal rate under a high-pressure mercury lamp. At 500 mg_N/L, the nitrate removal rate reaches 96.1%, and the nitrogen selectivity reaches 92.6%. Even after 5 cycles, the prepared photocatalyst still maintains a high nitrate photocatalytic removal efficiency (89%). The electron transfer path is verified by density functional theory calculations.

Fabrication of Electrospun Xylan-g-PMMA/TiO2 Nanofibers and Photocatalytic Degradation of Methylene Blue

Herein, xylan-g-PMMA was synthesized by grafting poly(methyl methacrylate) (PMMA) onto xylan and characterized by FT-IR and HSQC NMR spectroscopies, and the xylan-g-PMMA/TiO₂ solution was used to electrospun nanofibers at the voltage of 15 Kv, which was the first time employing xylan to electrospun nanofibers. Moreover, the electrospinning operating parameters were optimized by assessing the electrospinning process and the morphology of electrospun fibers, as follows: the mixed solvent of DMF and chloroform in a volume ratio of 5:1, an anhydroxylose unit (AXU)/MMA molar ratio lower than 1:2, the flow speed of 0.00565–0.02260 mL/min, and a receiving distance of 10–15 cm. Diameters of the electrospun fibers increased with increasing DMF content in the used solvent mixture, MMA dosage, and receiving distance. TiO₂ nanoparticles were successfully dispersed in electrospun xylan-g-PMMA nanofibers and characterized by scanning electron microscopy, energy dispersive X-ray diffraction spectrum, and X-ray photoelectron spectroscopy, and their application for methylene blue (MB) degradation presented above 80% photocatalytic efficiency, showing the good potential in water treatment.

Transition Metal Doping Induces Ti3+ to Promote the Performance of SrTiO3 @TiO2 Visible Light Photocatalytic Reduction of CO2 to Prepare C1 Product

Transition metal Fe, Co, Ni and Cu doped strontium titanate-rich SrTiO₃ @TiO₂ (STO@T) materials were prepared by hydrothermal method. The prepared doped materials exhibit better photocatalytic CO₂ reduction to CH₄ ability under visible light conditions. Among them, Fe-doped and undoped SrTiO₃ @TiO₂ under visible light conditions CO₂ reduction products only CO, while M-STO@T (M=Co, Ni, Cu) samples converted CO₂ to CH₄ . The average methane yield of Ni-doped STO@T samples are as high as 73.85 μmol g^-1  h^-1 . The production of methane is mainly due to the increase in the response of the doped samples to visible light. And the increase in the separation rate of photogenerated electrons and holes and the efficiency of electron transport caused by the generation of impurity levels. The impurity level caused by Ti³⁺ plays an important role in the production of methane by CO₂ visible light reduction. Ni doping effectively improves the photocatalytic performance of STO@T and CO₂ reduction mechanism were explained.

Metallic Copper-Containing Composite Photocatalysts: Fundamental, Materials Design, and Photoredox Applications

Semiconductor photocatalysis has long been regarded as a potential solution to tackle the energy and environmental challenges since the first discovery of water splitting by TiO₂ almost 50 years ago. The past few years have seen a tremendous flurry of research interest in the modification of semiconductors because of their shortcomings in the aspects of solar harvesting, electron-hole pairs separation, and utilization of photogenerated carriers. Among the various strategies, the introduction of metallic copper into the photocatalysis system can not only enhance the absorption of sunlight and the separation efficiency of photogenerated electrons and holes, but also increase the adsorption ability of substrate and the number of active sites, so as to realize the high solar to chemical energy conversion efficiency. This review focuses on the rational design of copper-based composites and their applications in photoredox catalysis. First, the preparation methods of metallic copper-containing composites are discussed. Then, the applications of different types of copper-based composites in the photocatalytic removal of pollutants, splitting of water to hydrogen production, reduction of carbon dioxide, and conversion of organic matter are introduced. Finally, the opportunities and challenges in the design and synthesis of copper-based composites and their applications in the photocatalysis are prospected.

Efficient Combination of G-C3 N4 and CDs for Enhanced Photocatalytic Performance: A Review of Synthesis, Strategies, and Applications

Recently, heterogeneous photocatalysts have achieved much interest on account of their great potential applications in resolving many tough energy and environmental troubles around the world through an ecologically sustainable way. Heterogeneous nanocomposites composed of graphitic carbon nitride (g-C₃ N₄ ) and carbon dots (CDs) possess broad spectrum absorption, appropriate electronic band structures, rapid carrier mobility, abundant reserves, excellent chemical stability, and facile synthesis methods, which make them promising composite photocatalysts for suitable applications such as photocatalytic solar fuels production and contaminant decomposition. With the rapid development in photocatalysis by hybridization of g-C₃ N₄ and CDs, a systematic summary and prospection of performance improvement are urgent and meaningful. This review first focuses on various kinds of effectively synthetic methods of composites. Following, the strategies available for enhanced performance, including morphology optimization, spectral absorption improvement, ternary or quaternary composition hybrid, lateral or vertical heterostructures construction, heteroatom doping, and so forth, are fully discussed. Then, the applications mainly in efficient photocatalytic hydrogen generation, photocatalytic carbon dioxide reduction, and organic pollutants degradation are systematically demonstrated. Finally, the remaining issues and prospect of further development are proposed as some kind of guidance for powerful combination of g-C₃ N₄ and CDs with high efficiency to photocatalysis.

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.

Photocatalysis and Li-Ion Battery Applications of {001} Faceted Anatase TiO2-Based Composites

Anatase TiO2 are the most widely used photocatalysts because of their unique electronic, optical and catalytic properties. Surface chemistry plays a very important role in the various applications of anatase TiO2 especially in the catalysis, photocatalysis, energy conversion and energy storage. Control of the surface structure by crystal facet engineering has become an important strategy for tuning and optimizing the physicochemical properties of TiO2. For anatase TiO2, the {001} crystal facets are the most reactive because they exhibit unique surface characteristics such as visible light responsiveness, dissociative adsorption, efficient charge separation capabilities and photocatalytic selectivity. In this review, a concise survey of the literature in the field of {001} dominated anatase TiO2 crystals and their composites is presented. To begin, the existing strategies for the synthesis of {001} dominated anatase TiO2 and their composites are discussed. These synthesis strategies include both fluorine-mediated and fluorine-free synthesis routes. Then, a detailed account of the effect of {001} facets on the physicochemical properties of TiO2 and their composites are reviewed, with a particular focus on photocatalysis and Li-ion batteries applications. Finally, an outlook is given on future strategies discussing the remaining challenges for the development of {001} dominated TiO2 nanomaterials and their potential applications.

Applications of electron spin resonance spectroscopy in photoinduced nanomaterial charge separation and reactive oxygen species generation

Nano-metals, nano-metal oxides, and carbon-based nanomaterials exhibit superior solar-to-chemical/photo-electron transfer properties and are potential candidates for environmental remediations and energy transfer. Recent research effort focuses on enhancing the efficiency of photoinduced electron-hole separation to improve energy transfer in catalytic reactions. Electron spin resonance (ESR) spectroscopy has been used to monitor the generation of electron/hole and reactive oxygen species (ROS) during nanomaterial-mediated photocatalysis. Using ESR coupled with spin trapping and spin labeling techniques, the underlying photocatalytic mechanism involved in the nanomaterial-mediated photocatalysis was investigated. In this review, we briefly introduced ESR principle and summarized recent advancements using ESR spectroscopy to characterize electron-hole separation and ROS production by different types of nanomaterials.

Recent Advances in TiO2-Based Heterojunctions for Photocatalytic CO2 Reduction With Water Oxidation: A Review

Photocatalytic conversion of CO₂ into solar fuels has gained increasing attention due to its great potential for alleviating the energy and environmental crisis at the same time. The low-cost TiO₂ with suitable band structure and high resistibility to light corrosion has proven to be very promising for photoreduction of CO₂ using water as the source of electrons and protons. However, the narrow spectral response range (ultraviolet region only) as well as the rapid recombination of photo-induced electron-hole pairs within pristine TiO₂ results in the low utilization of solar energy and limited photocatalytic efficiency. Besides, its low selectivity toward photoreduction products of CO₂ should also be improved. Combination of TiO₂ with other photoelectric active materials, such as metal oxide/sulfide semiconductors, metal nanoparticles and carbon-based nanostructures, for the construction of well-defined heterostructures can enhance the quantum efficiency significantly by promoting visible light adsorption, facilitating charge transfer and suppressing the recombination of charge carriers, resulting in the enhanced photocatalytic performance of the composite photocatalytic system. In addition, the adsorption and activation of CO₂ on these heterojunctions are also promoted, therefore enhancing the turnover frequency (TOF) of CO₂ molecules, so as to the improved selectivity of photoreduction products. This review focus on the recent advances of photocatalytic CO₂ reduction via TiO₂-based heterojunctions with water oxidation. The rational design, fabrication, photocatalytic performance and CO₂ photoreduction mechanisms of typical TiO₂-based heterojunctions, including semiconductor-semiconductor (S-S), semiconductor-metal (S-M), semiconductor-carbon group (S-C) and multicomponent heterojunction are reviewed and discussed. Moreover, the TiO₂-based phase heterojunction and facet heterojunction are also summarized and analyzed. In the end, the current challenges and future prospects of the TiO₂-based heterostructures for photoreduction of CO₂ with high efficiency, even for practical application are discussed.

Photoelectrochemical performance of facet-controlled TiO2 nanosheets grown hydrothermally on FTO

Single crystal anatase TiO₂ nanosheets (TiO₂-NSs) are grown hydrothermally on fluorine-doped tin oxide (FTO). By systematically changing the hydrothermal conditions such as reaction time, initial concentration of Ti precursor, F precursor, and HCl as an additive, a wide variety of TiO₂-NSs, with different morphologies and faceting have been synthesized. For the different morphologies and different facet ratios (anatase S ₀₀₁/S _001+101), the photoelectrochemical response is characterized and compared. We find that for photoanodes in neutral electrolytes, the magnitude of the photocurrent depends strongly on the growth parameters, that is, peak IPCEs can vary from 11.7% to 61%. For a wide range of parameters, the key parameter deciding on the photocurrent is the effective electrochemically active area of the electrode. Only for very high facet ratios >91%, the photoresponse can be strongly influenced by faceting - for samples with a S ₀₀₁/S _001+101 of 91%, IPCE value of ≈84% is obtained. This work defines not only optimized synthesis conditions for a most effective growth of these single crystalline electrode, but also represents fundamental data for further applications of such electrodes.

Photocatalytic reaction mechanisms at the gas–solid interface for environmental and energy applications

Five gas–solid photocatalytic reactions including the oxidation of NOx, VOCs and NH3, and reduction of CO2 and N2 are summarized. Besides, basic properties of gas molecules, their adsorption and activation, and various reaction pathways are analyzed.

Photocatalytic CO2 reduction to CH4 on iron porphyrin supported on atomically thin defective titanium dioxide

The synergistic effect of OVs and FeTPP on 2D TiO2 improves the efficiency and selectivity of CO2 photoreduction to CH4.

Photocatalytic treatment of natural waters. Reality or hype? The case of cyanotoxins remediation

This review compiles recent advances and challenges in the photocatalytic treatment of natural water by analyzing the remediation of cyanotoxins. The review frames the treatment need based on the occurrence, geographical distribution, and legislation of cyanotoxins in drinking water while highlighting the underestimated global risk of cyanotoxins. Next, the fundamental principles of photocatalytic treatment for remediating cyanotoxins and the complex degradation pathway for the most widespread cyanotoxins are presented. The state-of-the-art and recent advances on photocatalytic treatment processes are critically discussed, especially the modification strategies involving TiO₂ and the primary operational conditions that determine the scalability and integration of photocatalytic reactors. The relevance of light sources and light delivery strategies are shown, with emphasis on novel biomimicry materials design. Thereafter, the seldomly-addressed role of water-matrix components is thoroughly and critically explored by including natural organic matter and inorganic species to provide future directions in designing highly efficient strategies and scalable reactors.

Visible-light-driven photocatalytic CO2 reduction over ketoenamine-based covalent organic frameworks: role of the host functional groups

Compared to the electron-withdrawing groups, the electron-donating groups in TpBD can accelerate the photogenerated charge separation and transfer, thereby improving the photocatalytic performance for photocatalytic CO₂ reduction.

Photocatalytic Reactivity of Carbon–Nitrogen–Sulfur-Doped TiO2 Upconversion Phosphor Composites

Sol–gel synthesized N-doped and carbon–nitrogen–sulfur (CNS)-doped TiO2 solutions were deposited on upconversion phosphor using a dip coating method. Scanning electron microscopy (SEM) imaging showed that there was a change in the morphology of TiO2 coated on NaYF4:Yb,Er from spherical to nanorods caused by additional urea and thiourea doping reagents. Fourier transform infrared (FTIR) spectroscopy further verified the existence of nitrate–hyponitrite, carboxylate, and SO42− because of the doping effect. NaYF4:Yb,Er composites coated with N- and CNS-doped TiO2 exhibited a slight shift of UV-Vis spectra towards the visible light region. Photodecomposition of methylene blue (MB) was evaluated under 254 nm germicidal lamps and a 300 W Xe lamp with UV/Vis cut off filters. The photodegradation of toluene was evaluated on TiO2/NaYF4:Yb,Er and CNS-doped TiO2/NaYF4:Yb,Er samples under UV light illumination. The photocatalytic reactivity with CNS-doped TiO2/NaYF4:Yb,Er surpassed that of the undoped TiO2/NaYF4:Yb,Er for the MB solution and toluene. Photocatalytic activity is increased by CNS doping of TiO2, which improves light sensitization as a result of band gap narrowing due to impurity sites.

CuMoxW(1-x)O4 Solid Solution Display Visible Light Photoreduction of CO2 to CH3OH Coupling with Oxidation of Amine to Imine

The photoreduction of carbon dioxide (CO₂) to valuable fuels is a promising strategy for the prevention of rising atmospheric levels of CO₂ and the depletion of fossil fuel reserves. However, most reported photocatalysts are only active in the ultraviolet region, which necessitates co-catalysts and sacrificial agents in the reaction systems, leading to an unsatisfied economy of the process in energy and atoms. In this research, a CuMo_xW_(1-x)O₄ solid solution was synthesized, characterized, and tested for the photocatalytic reduction of CO₂ in the presence of amines. The results revealed that the yield of CH₃OH from CO₂ was 1017.7 μmol/g under 24 h visible light irradiation using CuW_0.7Mo_0.3O₄ (x = 0.7) as the catalyst. This was associated with the maximum conversion (82.1%) of benzylamine to N-benzylidene benzylamine with high selectivity (>99%). These results give new insight into the photocatalytic reduction of CO₂ for valuable chemical products in an economic way.

Visible light driven Ti3+ self-doped TiO2 for adsorption-photocatalysis of aqueous U(VI)

The photocatalytic reduction of U(VI) is recognized as an economical and effective way for U(VI) removal/recovery from solutions. To improve the photocatalytic activity of TiO₂ under visible light, TiO₂ was hydrogenated by NaBH₄ to generate Ti³⁺ self-doped black TiO₂ (BT_n). The self-doped Ti³⁺ alongside oxygen vacancies (Ov) could act as interband level to increase visible light capture and reduce the recombination of photogenerated carriers. The obtained BT_n samples showed high performance for U(VI) elimination under near neutral conditions, and held an outstanding anti-interference for U(VI) over competing metal cations and anions. Methanol and ethanol could act as sacrificial donors, being favorable for the photocatalytic reduction of U(VI), while the presence of EDTA inhibited the photoreduction of U(VI). The BT_n photocatalysts showed relatively high stability and reusability during the photocatalysis and elution processes. The XPS, TEM and XRD results revealed that U(VI) was photo-reduced to form UO₂ on the surface of BT_n. This work may serve as an important reference for improving the photocatalytic reactivity of TiO₂ as well as for the efficient removal/recovery of U(VI) from aqueous solutions.

Recent Advances in Solar-Driven Carbon Dioxide Conversion: Expectations versus Reality

Solar-driven carbon dioxide (CO₂) conversion to fuels and high-value chemicals can contribute to the better utilization of renewable energy sources. Photosynthetic (PS), photocatalytic (PC), photoelectrochemical (PEC), and photovoltaic plus electrochemical (PV+EC) approaches are intensively studied strategies. We aimed to compare the performance of these approaches using unified metrics and to highlight representative studies with outstanding performance in a given aspect. Most importantly, a statistical analysis was carried out to compare the differences in activity, selectivity, and durability of the various approaches, and the underlying causes are discussed in detail. Several interesting trends were found: (i) Only the minority of the studies present comprehensive metrics. (ii) The CO₂ reduction products and their relative amount vary across the different approaches. (iii) Only the PV+EC approach is likely to lead to industrial technologies in the midterm future. Last, a brief perspective on new directions is given to stimulate discussion and future research activity.