Cuprous ion (Cu+) doping induced surface/interface engineering for enhancing the CO2 photoreduction capability of W18O49 nanowires

Advances in Defect Engineering of Metal Oxides for Photocatalytic CO2 Reduction

Photocatalytic CO2 reduction technology, capable of converting low-density solar energy into high-density chemical energy, stands as a promising approach to alleviate the energy crisis and achieve carbon neutrality. Semiconductor metal oxides, characterized by their abundant reserves, good stability, and easily tunable structures, have found extensive applications in the field of photocatalysis. However, the wide bandgap inherent in metal oxides contributes to their poor efficiency in photocatalytic CO2 reduction. Defect engineering presents an effective strategy to address these challenges. This paper reviews the research progress in defect engineering to enhance the photocatalytic CO2 reduction performance of metal oxides, summarizing defect classifications, preparation methods, and characterization techniques. The focus is on defect engineering, represented by vacancies and doping, for improving the performance of metal oxide photocatalysts. This includes advancements in expanding the photoresponse range, enhancing photogenerated charge separation, and promoting CO2 molecule activation. Finally, the paper provides a summary of the current issues and challenges faced by defect engineering, along with a prospective outlook on the future development of photocatalytic CO2 reduction technology.

Fabrication of Ultrafine Cu2 O Nanoparticles on W18 O49 Ultra-Thin Nanowires by In-Situ Reduction for Highly Efficient Photocatalytic Nitrogen Fixation

Photocatalytic ammonia synthesis technology is one of the important methods to achieve green ammonia synthesis. Herein, two samples of Cu ion-doped W18 O49 with different morphologies, ultra-thin nanowires (Cu-W18 O49 -x UTNW) and sea urchin-like microspheres (Cu-W18 O49 -x SUMS), are synthesized by a simple solvothermal method. Subsequently, Cu2 O-W18 O49 -x UTNW/SUMS is synthesized by in situ reduction, where the NH3 production rate of Cu2 O-W18 O49 -30 UTNW is 252.4 µmol g-1  h-1 without sacrificial reagents, which is 11.8 times higher than that of the pristine W18 O49 UTNW. The Cu2 O-W18 O49 -30 UTNW sample is rich in oxygen vacancies, which promotes the chemisorption and activation of N2 molecules and makes the N≡N bond easier to dissociate by proton coupling. In addition, the in situ reduction-generated Cu2 O nanoparticles exhibit ideal S-scheme heterojunctions with W18 O49 UTNW, which enhances the internal electric field strength and improves the separation and transfer efficiency of the photogenerated carriers. Therefore, this study provides a new idea for the design of efficient nitrogen fixation photocatalysis.

A Full-Spectrum ZnS Photocatalyst with Gradient Distribution of Atomic Copper Dopants and Concomitant Sulfur Vacancies for Highly Efficient Hydrogen Evolution

A rarely discussed phenomenon in the realm of photocatalytic materials involves the presence of gradient distributed dopants and defects from the interior to the surface. This intriguing characteristic has been successfully achieved in the case of ZnS through the incorporation of atomic monovalent copper ions (Cu+) and concurrent sulfur vacancies (Vs), resulting in a photocatalyst denoted as G-CZS1-x. Through the cooperative action of these atomic Cu dopants and Vs, G-CZS1-x significantly extends its photoabsorption range to encompass the full spectrum (200-2100 nm), which improves the solar utilization ability. This alteration enhances the efficiency of charge separation and optimizes Δ(H*) (free energy of hydrogen adsorption) to approach 0 eV for the hydrogen evolution reaction (HER). It is noteworthy that both surface-exposed atomic Cu and Vs act as active sites for photocatalysis. G-CZS1-x exhibits a significant H2 evolution rate of 1.01 mmol h-1 in the absence of a cocatalyst. This performance exceeds the majority of previously reported photocatalysts, exhibiting approximately 25-fold as ZnS, and 5-fold as H-CZS1-x with homogeneous distribution of equal content Cu dopants and Vs. In contrast to G-CZS1-x, the H adsorption on Cu sites for H-CZS1-x (ΔG(H*) = -1.22 eV) is excessively strong to inhibit the H2 release, and the charge separation efficiency for H-CZS1-x is relatively sluggish, revealing the positive role of a gradient distribution model of dopants and defects on activity enhancement. This work highlights the synergy of atomic dopants and defects in advancing photoactivity, as well as the significant benefit of the controllable distribution model of dopants and defects for photocatalysis.

W18O49 Nanowhiskers Decorating SiO2 Nanofibers: Lessons from In Situ SEM/TEM Growth to Large Scale Synthesis and Fundamental Structural Understanding

Tungsten suboxide W18O49 nanowhiskers are a material of great interest due to their potential high-end applications in electronics, near-infrared light shielding, catalysis, and gas sensing. The present study introduces three main approaches for the fundamental understanding of W18O49 nanowhisker growth and structure. First, W18O49 nanowhiskers were grown from γ-WO3/a-SiO2 nanofibers in situ in a scanning electron microscope (SEM) utilizing a specially designed microreactor (μReactor). It was found that irradiation by the electron beam slows the growth kinetics of the W18O49 nanowhisker, markedly. Following this, an in situ TEM study led to some new fundamental understanding of the growth mode of the crystal shear planes in the W18O49 nanowhisker and the formation of a domain (bundle) structure. High-resolution scanning transmission electron microscopy analysis of a cross-sectioned W18O49 nanowhisker revealed the well-documented pentagonal Magnéli columns and hexagonal channel characteristics for this phase. Furthermore, a highly crystalline and oriented domain structure and previously unreported mixed structural arrangement of tungsten oxide polyhedrons were analyzed. The tungsten oxide phases found in the cross section of the W18O49 nanowhisker were analyzed by nanodiffraction and electron energy loss spectroscopy (EELS), which were discussed and compared in light of theoretical calculations based on the density functional theory method. Finally, the knowledge gained from the in situ SEM and TEM experiments was valorized in developing a multigram synthesis of W18O49/a-SiO2 urchin-like nanofibers in a flow reactor.

Coralloid W18O49@covalent organic frameworks S-scheme heterojunction for high efficiency photocatalytic aerobic oxidation

Reasonably designing and constructing efficient artificial S-mechanism photocatalysts, expanding their application in the field of photocatalytic organic synthesis, have become a hot and challenging topic in the photocatalysis. Herein, a series of coral-like W18O49@TpPa-H (TpPa-H represents COFs generated by the reaction of 1,3,5-triformylphloroglucinol (Tp) and p-phenylenediamine (Pa-H)) composites were successfully prepared by using a simple in-situ encapsulation strategy. Given the internal electric field at the S-scheme interface, W18O49 acts as an oxidative photocatalyst with sufficient positive valence band (VB) position and TpPa-H as a reductive one with enough negative conduction band (CB) position for the efficient amines oxidative coupling to imines. The resulting W18O49@TpPa-H-0.1 hybrid material shows both optimal benzylamine to imine conversion and selectivity exceeding 99 % within 4 h under 10 W 420 nm LED light irradiation, which is 9.9 and 2.8 fold greater than that of W18O49 and TpPa-H, respectively. The photocatalytic activity is even extended to 740 nm. Furthermore, the photocatalytic mechanism research confirmed that a high efficiency S-scheme heterojunction was formed between W18O49 and TpPa-H, and multiple active species, such as ·O2-, 1O2, and h+, synergistically participated in the reaction, imparting its excellent photocatalytic performance. This work may open new avenues for the development of high-efficiency COFs-based S-scheme heterojunction for organic photosynthesis.

Lead-Free Halide Perovskite Cs2AgBiBr6/Bismuthene Composites for Improved CH4 Production in Photocatalytic CO2 Reduction

CO2 photocatalytic conversion into value-added fuels through solar energy is a promising way of storing renewable energy while simultaneously reducing the concentration of CO2 in the atmosphere. Lead-based halide perovskites have recently shown great potential in various applications such as solar cells, optoelectronics, and photocatalysis. Even though they show high performance, the high toxicity of Pb2+ along with poor stability under ambient conditions restrains the application of these materials in photocatalysis. In this respect, we developed an in situ assembly strategy to fabricate the lead-free double perovskite Cs2AgBiBr6 on a 2D bismuthene nanosheet prepared by a ligand-assisted reprecipitation method for a liquid-phase CO2 photocatalytic reduction reaction. The composite improved the production and selectivity of the eight-electron CH4 pathway compared with the two-electron CO pathway, storing more of the light energy harvested by the photocatalyst. The Cs2AgBiBr6/bismuthene composite shows a photocatalytic activity of 1.49(±0.16) μmol g-1 h-1 CH4, 0.67(±0.14) μmol g-1 h-1 CO, and 0.75(±0.20) μmol g-1 h-1 H2, with a CH4 selectivity of 81(±1)% on an electron basis with 1 sun. The improved performance is attributed to the enhanced charge separation and suppressed electron-hole recombination due to good interfacial contact between the perovskite and bismuthene promoted by the synthesis method.

Tungsten and Copper (II) Oxide Mixtures as Gasless Time Delay Compositions for Mining Detonators

The widespread use of pyrotechnic compositions in time delay detonators is the reason for research aimed at expanding knowledge of the combustion properties of new pyrotechnic mixtures, whose components react with each other in the solid or liquid state. Such a method of combustion would make the rate of combustion independent of the pressure inside the detonator. This paper presents the effect of the parameters of W/CuO mixtures on their properties of combustion. As this composition has not been the subject of previous research and is not described in the literature, the basic parameters, such as the burning rate and the heat of combustion, were determined. In order to determine the reaction mechanism, a thermal analysis was performed, and the combustion products were determined using the XRD technique. Depending on the quantitative composition and density of the mixture, the burning rates were between 4.1-6.0 mm/s and the heat of combustion in the range of 475-835 J/g was measured. The gas-free combustion mode of the chosen mixture was proved using DTA and XRD. Determination of the qualitative composition of the combustion products and the heat of combustion allowed estimation of the adiabatic combustion temperature.

1D/1D W18O49/Cd0.9Zn0.1S S-scheme heterojunction with spatial charge separation for high-yield photocatalytic H2 evolution

Semiconductor photocatalytic water splitting is a green way to convert solar energy into chemical energy, but the recombination of electron and hole pairs and the low utilization of sunlight restrict the development of photocatalytic technology. By comparing the morphologies and hydrogen production properties of different proportions of solid solutions (Cd_xZn_1-xS), one-dimensional (1D) Cd_0.9Zn_0.1S nanorods (NRs) with the best photocatalytic properties are obtained. In addition, 1D W₁₈O₄₉ nanowires are assembled on the surface of 1D Cd_0.9Zn_0.1S NRs to construct a novel 1D/1D step-scheme (S-scheme) W₁₈O₄₉/Cd_0.9Zn_0.1S heterojunction photocatalyst. The W₁₈O₄₉/Cd_0.9Zn_0.1S heterojunction expands the optical absorption capacity of Cd_0.9Zn_0.1S NRs to provide more energy for the photoexcitation of electrons. The optimal hydrogen production rate of W₁₈O₄₉/Cd_0.9Zn_0.1S NRs with W₁₈O₄₉ content of 9 wt% is as high as 66.3 mmol·h^-1·g^-1, which is 5.7 times and 1.6 times higher than that of Cd_0.9Zn_0.1S NRs and 1 wt% Pt/Cd_0.9Zn_0.1S NRs. The apparent quantum efficiency (AQE) of 9 wt% W₁₈O₄₉/Cd_0.9Zn_0.1S reaches 56.0 % and 25.9 % under light wavelength irradiation at 370 and 456 nm, respectively. After the 20 h cycle stability test, the activity of photocatalytic hydrogen evolution does not decrease, due that the severe photo-corrosion of Cd_0.9Zn_0.1S NRs is efficiently inhibited. This work not only provides a simple and controllable synthesis method for the preparation of heterojunction structure, but also opens up a new way to improve the hydrogen evolution activity and stability of sulfur compounds.

Oxygen vacancies-rich Cu-W18O49 nanorods supported on reduced graphene oxide for electrochemical reduction ofN2to NH3

High-performance nitrogen fixation is severely limited by the efficiency and selectivity of a catalyst of electrochemical nitrogen reduction reaction (NRR) under ambient conditions. Here, the RGO/WOCu (reduced graphene oxide and Cu-doping W₁₈O₄₉) composite catalysts with abundant oxygen vacancies are prepared by the hydrothermal method. The obtained RGO/WOCu achieves an enhanced NRR performance (NH₃ yield rate:11.4 μg h^-1 mg_cat^-1, Faradaic efficiency: 4.4%) at -0.6 V (vs. RHE) in 0.1 mol L^-1 Na₂SO₄ solution. Furthermore, the NRR performance of the RGO/WOCu still keeps at 95% after four cycles, demonstrating its excellent stability. The Cu⁺-doping increases the concentration of oxygen vacancies, which is conducive to the adsorption and activation of N₂. Meanwhile, the introduction of RGO further improves the electrical conductivity and reaction kinetics of the RGO/WOCu due to the high specific surface area and conductivity. This work provides a simple and effective method for efficient electrochemical reduction ofN₂.

W18O49/MnWO4 heterojunction for highly efficient photocatalytic reduction of CO2 under full spectrum light

Solar-energy-driven CO₂ reduction for chemical reagents production, such as CH₃OH, CH₄ and CO, has tremendous potential for carbon neutrality in the energy industries. However, the low reduction efficiency limits its applicability. Herein, W₁₈O₄₉/MnWO₄ (WMn) heterojunctions were prepared via one-step in-situ solvothermal process. Through this method, W₁₈O₄₉ tightly combined with the surface of MnWO₄ nanofibers to form nanoflower heterojunction. It was found that under full spectrum light irradiation for 4 h, the yields of photoreduction of CO₂ to CO, CH₄ and CH₃OH by 3-1 WMn heterojunction were 61.74, 71.30 and 18.98 μmol/g, respectively, which were 2.4, 1.8 and 1.1 times that of pristine W₁₈O₄₉, and ca.20 times that of pristine MnWO₄ towards CO production. Furthermore, even in the air atmosphere, the WMn heterojunction still performed excellent photocatalytic performance. Systematic investigations demonstrated that the catalytic performance of WMn heterojunction was improved by superior light utilization and more efficient photo-generated carrier separation and migration as compared with W₁₈O₄₉ and MnWO₄. Meanwhile, the intermediate products of the photocatalytic CO₂ reduction process were also studied in detail by in-situ FTIR. Therefore, this study provides a new way for designing high efficiency of heterojunction for CO₂ reduction.

Chitosan Sponge/Cu-WO3-x Composite for Photodynamic Therapy of Wound Infection

One of the potential treatments for bacterial wound infections is photodynamic therapy. WO_3-x semiconductor materials can generate reactive oxygen species when exposed to light, which can inactivate bacteria. In this work, we improved their photocatalytic performance by doping WO_3-x with Cu. The wound dressing was prepared by loading Cu-WO_3-x into a highly biocompatible chitosan sponge. The composite sponge dressing showed significant inactivation of Escherichia coli and Staphylococcus aureus, and in vitro toxicity assays on L929 cells demonstrated the biosafety of the dressing. Through in vivo wound healing trials, composite sponge dressings have been shown to accelerate wound healing, and this composite chitosan sponge can be possibly used for photodynamic therapy of bacterial wound infections.

Functional Aerogels Composed of Regenerated Cellulose and Tungsten Oxide for UV Detection and Seawater Desalination

Functional aerogels composed of regenerated cellulose and tungsten oxide were fabricated by implanting tungsten-oxide nanodots into regenerated cellulose fiber. This superfast photochromic property benefitted from the small size and even distribution of tungsten oxide, which was caused by the confinement effect of the regenerated cellulose fiber. The composite was characterized using XRD and TEM to illustrate the successful loading of tungsten oxide. The composite turned from pale white to bright blue under ambient solar irradiation in five seconds. The evidence of solar absorption and electron paramagnetic resonance (EPR) demonstrated the fast photochromic nature of the composite and its mechanism. Furthermore, carbon fiber filled with preferential growth tungsten-oxide nanorods was obtained by annealing the photochromic composite in a N₂ atmosphere. This annealed product exhibited good absorption across the whole solar spectrum and revealed an excellent photothermal conversion performance. The water evaporation rate reached 1.75 kg m^-2 h^-1 under one sun illumination, which is 4.4 times higher than that of pure water. The photothermal conversion efficiency was 85%, which shows its potential application prospects in seawater desalination.

Progress and perspectives on 1D nanostructured catalysts applied in photo(electro)catalytic reduction of CO2

Reducing CO₂ into value-added chemicals and fuels by artificial photosynthesis (photocatalysis and photoelectrocatalysis) is one of the considerable solutions to global environmental and energy issues. One-dimensional (1D) nanostructured catalysts (nanowires, nanorods, nanotubes and so on.) have attracted extensive attention due to their superior light-harvesting ability, co-catalyst loading capacity, and high carrier separation rate. This review analyzed the basic principle of the photo(electro)catalytic CO₂ reduction reaction (CO₂ RR) briefly. The preparation methods and properties of 1D nanostructured catalysts are introduced. Next, the applications of 1D nanostructured catalysts in the field of photo(electro)catalytic CO₂ RR are introduced in detail. In particular, we introduced the design of composite catalysts with 1D nanostructures, for example loading 0D, 1D, 2D, and 3D materials on a 1D nanostructured semiconductor to construct a heterojunction to optimize the photo-response range, carrier separation and transport efficiency, CO₂ adsorption and activation capacity, and stability of the catalyst. Finally, the development prospects of 1D nanostructured catalysts are discussed and summarized. This review can provide guidance for the rational design of advanced catalysts for photo(electro)catalytic CO₂ RR.

Cu Nanoparticles Modified Step-Scheme Cu2O/WO3 Heterojunction Nanoflakes for Visible-Light-Driven Conversion of CO2 to CH4

In this study, Cu and Cu2O hybrid nanoparticles were synthesized onto the WO3 nanoflake film using a one-step electrodeposition method. The critical advance is the use of a heterojunction consisting of WO3 flakes and Cu2O as an innovative stack design, thereby achieving excellent performance for CO2 photoreduction with water vapor under visible light irradiation. Notably, with the modified Cu nanoparticles, the selectivity of CH4 increased from nearly 0% to 96.7%, while that of CO fell down from 94.5% to 0%. The yields of CH4, H2 and O2 reached 2.43, 0.32 and 3.45 mmol/gcat after 24 h of visible light irradiation, respectively. The boosted photocatalytic performance primarily originated from effective charge-transfer in the heterojunction and acceleration of electron-proton transfer in the presence of Cu nanoparticles. The S-scheme charge transfer mode was further proposed by the in situ-XPS measurement. In this regard, the heterojunction construction showed great significance in the design of efficient catalysts for CO2 photoreduction application.

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+.

Tailored Linker Defects in UiO-67 with High Ligand-to-Metal Charge Transfer toward Efficient Photoreduction of CO2

Defect engineering can be used as a potential tool to activate metal-organic frameworks by regulating the pore structure, electronic properties, and catalytic activity. Herein, linker defects were effectively controlled by adjusting the amount of formic acid, and UiO-67 with different CO₂ reduction capabilities was obtained. Among them, UiO-67-200 had the highest ability to selectively reduce CO₂ to CO (12.29 μmol g^-1 h^-1). On the one hand, the results based on time-resolved photoluminescence decay curves and photochemical experiments revealed that UiO-67-200 had the highest charge separation efficiency. On the other hand, the linker defects affected the band structure of UiO-67 by changing the lowest unoccupied molecular orbital (LUMO) based on the density functional theory and UV-vis spectra. Hence, the proper linker defects enhanced the ligand-to-metal charge transfer process by promoting the transfer of electrons between the highest occupied molecular orbital and LUMO. Additionally, in situ Fourier transform infrared spectra and ¹³CO₂ labeling experiments also indicated that COOH* was an important intermediate for CO formation and that CO originated from the photoreduction of CO₂.

The covalent Coordination-driven Bi2S3@NH2-MIL-125(Ti)-SH heterojunction with boosting photocatalytic CO2 reduction and dye degradation performance

Herein, the optional and controllable growth of Bi₂S₃ onto NH₂-MIL-125 via covalent conjunction strategy was reported. The experimental results demonstrate that the obtained heterojunction exhibits boosting photocatalytic reduction CO₂ and organic dye degradation. The 18-Bi₂S₃@NH₂-MIL-125-SH displays the highest yield of 12.46 μmol g^-1h^-1 of CO, >13 times that of pure NH₂-MIL-125. Meanwhile, the reaction kinetic of 18-Bi₂S₃@NH₂-MIL-125-SH in the degradation of methylene blue is uppermost, which is 160 times than that of the commercial P25. The enhancement of photocatalytic performance could be ascribed to the covalent coordination-driven intimate interfacial interaction in n-scheme heterojunction. Meanwhile, the plausible mechanism was also investigated by UV-vis diffuse reflectance (UV-vis), photoluminescence (PL), electrochemical photocurrent, electron spin resonance (ESR) and electrochemical impedance spectroscopy (EIS).

Bioinspired spike-like double yolk–shell structured TiO2@ZnIn2S4 for efficient photocatalytic CO2 reduction

A spike-like double yolk–shell structured TiO2@ZnIn2S4 (D-Y-TiO2@ZnIn2S4) photocatalyst was designed, which possesses superior photocatalytic CO2 reduction efficiency.

Ultrathin indium vanadate/cadmium selenide-amine step-scheme heterojunction with interfacial chemical bonding for promotion of visible-light-driven carbon dioxide reduction

The formation of interfacial chemical bonding in heterostructures plays an important role in the transport of carriers. Herein, we firstly prepared ultrathin InVO₄ nanosheet (Ns) with a thickness of 1.5 nm. Diethylenetriamine-modified CdSe (CdSe-DETA) nanobelts are in-situ deposited on the surface of ultrathin InVO₄ Ns to build a InVO₄/CdSe-DETA step-scheme (S-scheme) heterojunction photocatalysts. The protonated DETA acts as an amine-bridge to promote the formation of a tight chemical bond at the interface of InVO₄/CdSe-DETA, thereby promoting the transfer of carriers at the interface. For photocatalytic CO₂ reduction, the rationally designed InVO₄/CdSe-DETA S-scheme photocatalyst exhibits a remarkable CO generation rate of 27.9 µmol h^-1 g^-1 at 420 nm, which is 3.35 and 3.39 times higher than that of CdSe-DETA and InVO₄ Ns, respectively. The new method by using interfacial chemical bonding to facilitate interfacial charge transportation provide a promising strategy for improve photocatalysis.

Constructing electron transfer pathways and active centers over W18O49 nanowires by doping Fe3+ and incorporating g-C3N5 for enhanced photocatalytic nitrogen fixation

A compound constructed from fluffy and porous g-C₃N₅ with OV-rich Fe-W₁₈O₄₉ was employed in the photocatalytic nitrogen fixation. The formation rate of ammonia reached 131.6 μmol g⁻¹ h⁻¹ when Fe-W₁₈O₄₉/g-C₃N₅ was employed as the catalyst.