Metal–Organic Framework Coating Enhances the Performance of Cu2O in Photoelectrochemical CO2 Reduction

Dye-sensitized NiO photocathode sensor based on signal-sensitive change strategy for MC-LR detection

A novel photoelectrochemical (PEC) sensor for the detection of microcystic toxins (MC-LR) was developed on the basis of signal-sensitive change strategy. NiO nanoarray as a basic photoactive material was grown directly on the ITO glass electrode via calcination after hydrothermal reaction, while dye N719 was used to sensitize the electrode for enhancing visible light absorption, and the first signal-on stage was obtained. In the meantime, p-type Cu2O was applied as the signal probe attached to probe DNA (DNA2) to improve the sensitivity, and the second "signal-on" stage appeared because of its synergistic effect with NiO nanoarrays. The PEC signal decreases after the target analyte MC-LR is modified on the electrode due to the stronger affinity between MC-LR and its complementary aptamer DNA; part of the Cu2O-DNA2 will dissociate from the electrode. This sensitive signal change strategy allows the detection limit of the MC-LR sensor to be as low as 1.7 pM, which offers an optional method for the sensitive and selective detection of other target molecules, with potential applications in environmental monitoring and toxin determination.

Near-infrared light-driven lab-on-paper cathodic photoelectrochemical aptasensing for di(2-ethylhexyl)phthalate based on AgInS2/Cu2O/FeOOH photocathode

Di(2-ethylhexyl)phthalate (DEHP) is commonly released from plastics in aqueous environment, which can disrupt endocrine system and cause adverse effects on public health. There is a pressing need to highly sensitive detect DEHP. Herein, a near-infrared (NIR) light-driven lab-on-paper cathodic photoelectrochemical aptasensing platform integrated with AgInS2/Cu2O/FeOOH photocathode and "Y"-like ternary conjugated DNA nanostructure-mediated "ON-OFF" catalytic switching of hemin monomer-to-dimer was established for ultrasensitive DEHP detection. Profiting from the collaborative roles of the effective photosensitization of NIR-response AgInS2 and the fast hole extraction of FeOOH, the NIR light-activated AgInS2/Cu2O/FeOOH photocathode generated a markedly enhanced photocathodic signal. The dual hemin-labelled "Y"-like ternary conjugated DNA nanostructures made the hemin monomers separated in space and they maintained highly active to catalyze in situ generation of electron acceptors (O2). The hemin monomers were relocated in close proximity with the help of target-induced allosteric change of DNA nanostructures, which could spontaneously dimerize into catalytically inactive hemin dimers and fail to mediate electron acceptors generation, resulting in a decreased photocathodic signal. Therefore, the ultrasensitive DEHP detection was realized with a linear response range of 1 pM-500 nM and a detection limit of 0.39 pM. This work rendered a promising prototype to construct powerful paper-based photocathodic aptasensing system for sensitive and accurate screening of DEHP in aqueous environment.

Photoelectrothermocatalytic reduction of CO2 to glycol via Cu2S/MoS2-Vs octahedral heterostructure with synergistic mechanism

The defects and interface engineering are efficient approaches to adjust the physical and chemical properties of nanomaterials to enhance catalytic performance. In this study, we report a new MOFs-driven porous Cu2S/MoS2-Vs octahedral semiconductor with heterostructure and photothermal effect. The introduction of sulfur vacancies directly improves the adsorption performance of CO2, and the formation of heterostructure significantly increases the charge transfer rate. The C-penetrating material obtained from MOFs not only acts as an octahedral skeleton support, but also gives photothermal effects under photoelectric conditions. The formation rate of sole C2 products in photoelectrocatalytic CO2 reduction by using Cu2S/MoS2-Vs heterostructure is up to 52 μM·h-1·cm-2 equal to the total electron transfer rate of 541 μM·h-1·cm-2. The carbene mechanism and reaction pathways were proposed and verified by 13CO2 isotopic labelling and operando Fourier transform infrared (FT-IR) spectra. The important intermediates of *CO2-, *CO, *CHO and *CHO-CHO were identified by operando FT-IR spectra. In the comparative experiments, the photothermal electrons are beneficial to C2 products. DFT calculations indicate that the presence of S vacancies (Vs) reduces the energy barrier for product generation.

Influence of solution pH dependency on structure, optical with photoelectrochemical characteristics of SILAR deposited copper oxide thin films

Photoelectrochemical (PEC) technology is a promising approach for converting solar energy into chemical energy, offering significant potential for renewable energy applications. In this work, the CuO thin film was fabricated with different pH value in between 8.5 ± 0.1 and 10.5 ± 0.1 via Successive Ionic Layer Adsorption and Reaction (SILAR) method. The Effect of pH on thickness, structural, morphological, elemental composition and optical properties were investigated by using stylus profilometry, XRD, SEM, TEM, EDX, UV-vis and PL. The XRD results showed that as the pH increased, the crystallite size increased from 19.24 nm to 25.62 nm, with a monoclinic phase along the (111) direction. The CuO film deposited at pH value 10.5 ± 0.1 exhibit well defined identical particle with its size in the range between 200 and 300 nm was confirmed by SEM and TEM analysis. As the pH increased from 8.5 ± 0.1 to 10.5 ± 0.1, the CuO film bandgap (Eg) value reduced from 1.52 eV to 1.42 eV with indirect transition. The CuO photocathode deposited at pH 10.5 ± 0.1 shows maximum photocurrent density of 1.45 mA/cm2 at -0.1 V vs. RHE in 0.5 M Na2SO4 solution. Furthermore, the Electrochemical Impedance Spectroscopy (EIS) analysis shows, the CuO (pH 10.5 ± 0.1) electrode have higher conductivity value of 0.6862 S/cm compared CuO at pH 8.5 ± 0.1 (0.2779 S/cm) and CuO at pH 9.5 ± 0.1 (0.4646 S/cm) electrodes.

Linkage-Mediated Electronic Structure Modulation in Multicomponent Covalent Organic Frameworks for Dramatically Promoted Photocatalytic Hydrogen Evolution

Linkage chemistry is an essential aspect to covalent organic framework (COF) applications; it is highly desirable to precisely modulate electronic structure mediated directly by linkage for efficient COF-based photocatalytic hydrogen evolution, which however, remains substantially challenging. Herein, as a proof of concept, a collection of robust multicomponent pyrene-based COFs with abundant donor-acceptor (D-A) interactions has been judiciously designed and synthesized through molecularly engineering linkage for photogeneration of hydrogen. Controlled locking and conversion of linkage critically contribute to continuously regulating COFs' electronic structures further to optimize photocatalytic activities. Remarkably, the well-modulated optoelectronic properties turn on the average hydrogen evolution rate from zero to 15.67 mmol g-1 h-1 by the protonated quinoline-linked COF decorated with the trifluoromethyl group (TT-PQCOF-CF3). Using diversified spectroscopy and theoretical calculations, we show that multiple modifications toward linkage synergistically lead to the redistribution of charge on COFs with extended π-conjugation and reinforced D-A effect, making TT-PQCOF-CF3 a promising material with significantly boosted carrier separation and migration. This study provides important guidance for the design of high-performance COF photocatalysts based on the strategy of linkage-mediated electronic structure modulation in COFs.

Ultrathin zigzag-surface copper nanowire assembled hierarchical microspheres to enhance oxygen reduction catalysis

Ultrathin catalysts predominantly expose surface active atoms to deliver promising applications in oxygen reduction reactions (ORRs). However, they are commonly synthesized at high reaction temperatures, with tedious chemical routes involved. Herein, we report a low temperature (273 K) electric field driven route to synthesize zigzag-surface ultrathin copper nanowires. Interestingly, the ultrathin copper nanowires assemble into three-dimensional microspheres, which exhibit hydrophobic-aerophilic features, eventually resulting in good ORR activities. The aerophilicity and hydrophobicity of copper nanowires are related to their Cu2O active sites and hierarchical protuberances, respectively. Our findings open a new door to grow ultrathin catalysts for new energy storage systems.

Exploring the CO2 Electrocatalysis Potential of 2D Metal-Organic Transition Metal-Hexahydroxytriquinoline Frameworks: A DFT Investigation

Metal-organic frameworks have demonstrated great capacity in catalytic CO2 reduction due to their versatile pore structures, diverse active sites, and functionalization capabilities. In this study, a novel electrocatalytic framework for CO2 reduction was designed and implemented using 2D coordination network-type transition metal-hexahydroxytricyclic quinazoline (TM-HHTQ) materials. Density functional theory calculations were carried out to examine the binding energies between the HHTQ substrate and 10 single TM atoms, ranging from Sc to Zn, which revealed a stable distribution of metal atoms on the HHTQ substrate. The majority of the catalysts exhibited high selectivity for CO2 reduction, except for the Mn-HHTQ catalysts, which only exhibited selectivity at pH values above 4.183. Specifically, Ti and Cr primarily produced HCOOH, with corresponding 0.606 V and 0.236 V overpotentials. Vanadium produced CH4 as the main product with an overpotential of 0.675 V, while Fe formed HCHO with an overpotential of 0.342 V. Therefore, V, Cr, Fe, and Ti exhibit promising potential as electrocatalysts for carbon dioxide reduction due to their favorable product selectivity and low overpotential. Cu mainly produces CH3OH as the primary product, with an overpotential of 0.96 V. Zn primarily produces CO with a relatively high overpotential of 1.046 V. In contrast, catalysts such as Sc, Mn, Ni, and Co, among others, produce multiple products simultaneously at the same rate-limiting step and potential threshold.

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.

Density Functional Study of Electrocatalytic Carbon Dioxide Reduction in Fourth-Period Transition Metal-Tetrahydroxyquinone Organic Framework

This study investigates the utilisation of organometallic network frameworks composed of fourth-period transition metals and tetrahydroxyquinone (THQ) in electrocatalytic CO2 reduction. Density functional theory (DFT) calculations were employed in analysing binding energies, as well as the stabilities of metal atoms within the THQ frameworks, for transition metal TM-THQs ranging from Y to Cd. The findings demonstrate how metal atoms could be effectively dispersed and held within the THQ frameworks due to sufficiently high binding energies. Most TM-THQ frameworks exhibited favourable selectivity towards CO2 reduction, except for Tc and Ru, which experienced competition from hydrogen evolution reaction (HER) and required solution environments with pH values greater than 5.716 and 8.819, respectively, to exhibit CO2RR selectivity. Notably, the primary product of Y, Ag, and Cd was HCOOH; Mo produced HCHO; Pd yielded CO; and Zr, Nb, Tc, Ru, and Rh predominantly generated CH4. Among the studied frameworks, Zr-THQ displayed values of 1.212 V and 1.043 V, corresponding to the highest limiting potential and overpotential, respectively, while other metal-organic frameworks displayed relatively low ranges of overpotentials from 0.179 V to 0.949 V. Consequently, it is predicted that the TM-THQ framework constructed using a fourth-period transition metal and tetrahydroxyquinone exhibits robust electrocatalytic reduction of CO2 catalytic activity.

Oxygen-Activated Boron Nitride for Selective Photocatalytic Coupling of Methanol to Ethylene Glycol

The controllable photocatalytic C-C coupling of methanol to produce ethylene glycol (EG) is a highly desirable but challenging objective for replacing the current energy-intensive thermocatalytic process. Here, we develop a metal-free porous boron nitride catalyst that demonstrates exceptional selectivity in the photocatalytic production of EG from methanol under mild conditions. Comprehensive experiments and calculations are conducted to thoroughly investigate the reaction mechanism, revealing that the OB3 unit in the porous BN plays a critical role in the preferential activation of C-H bond in methanol to form ⋅CH2OH via a concerted proton-electron transfer mechanism. More prominent energy barriers are observed for the further dehydrogenation of the ⋅CH2OH intermediate on the OB3 unit, inhibiting the formation of some other by-products during the catalytic process. Additionally, a small downhill energy barrier for the coupling of ⋅CH2OH in the OB3 unit promotes the selective generation of EG. This study provides valuable insights into the underlying mechanisms and can serve as a guide for the design and optimization of photocatalysts for efficient and selective EG production under mild conditions.

High carrier mobility along the [111] orientation in Cu2O photoelectrodes

Solar fuels offer a promising approach to provide sustainable fuels by harnessing sunlight1,2. Following a decade of advancement, Cu2O photocathodes are capable of delivering a performance comparable to that of photoelectrodes with established photovoltaic materials3-5. However, considerable bulk charge carrier recombination that is poorly understood still limits further advances in performance6. Here we demonstrate performance of Cu2O photocathodes beyond the state-of-the-art by exploiting a new conceptual understanding of carrier recombination and transport in single-crystal Cu2O thin films. Using ambient liquid-phase epitaxy, we present a new method to grow single-crystal Cu2O samples with three crystal orientations. Broadband femtosecond transient reflection spectroscopy measurements were used to quantify anisotropic optoelectronic properties, through which the carrier mobility along the [111] direction was found to be an order of magnitude higher than those along other orientations. Driven by these findings, we developed a polycrystalline Cu2O photocathode with an extraordinarily pure (111) orientation and (111) terminating facets using a simple and low-cost method, which delivers 7 mA cm-2 current density (more than 70% improvement compared to that of state-of-the-art electrodeposited devices) at 0.5 V versus a reversible hydrogen electrode under air mass 1.5 G illumination, and stable operation over at least 120 h.

A high-performance photoelectrochemical sensor based on CdS-Au composite nanomaterials and localized surface plasmon resonance for ultrasensitive detection of ascorbic acid

Ascorbic acid (AA), which plays a vital role in the metabolism of the human body, is closely correlated with various diseases, including rheumatoid arthritis, scurvy, Parkinson's disease, urinary stones, and diarrhea. The detection of AA is of great significance for early prevention and diagnosis of related diseases. In this paper, a high-performance photoelectrochemical (PEC) sensor was constructed based on cadmium sulfide-gold (CdS-Au) composite nanomaterials for ultrasensitive ascorbic acid (AA) detection. Due to the localized surface plasmon resonance (LSPR) effect of gold nanoparticles (AuNPs), the PEC performance of CdS-Au composite nanomaterials was significantly improved compared to CdS semiconductor nanomaterials. Under the optimal conditions, the AA concentration was linearly related to the photocurrent signal in the range of 0.01 μM-200 μM, with the detection limit being 0.2 nM (S/N = 3) and the sensitivity being 642.9 μA mM-1 cm-2. In addition, the mechanism of the PEC sensor based on CdS-Au composite nanomaterials for ultrasensitive AA detection was discussed. Lastly, the self-constructed PEC sensors have been successfully applied in detecting AA in vitamin C tablets and actual blood samples, meeting the detection criteria required by the Chinese Pharmacopoeia (CP, 2020 edition). The self-fabricated PEC sensors in this paper are expected to be used for quality assessment of AA-related drugs and diagnosis of relevant diseases.

Manipulation of interfacial charge dynamics for metal-organic frameworks toward advanced photocatalytic applications

Compared to other known materials, metal-organic frameworks (MOFs) have the highest surface area and the lowest densities; as a result, MOFs are advantageous in numerous technological applications, especially in the area of photocatalysis. Photocatalysis shows tantalizing potential to fulfill global energy demands, reduce greenhouse effects, and resolve environmental contamination problems. To exploit highly active photocatalysts, it is important to determine the fate of photoexcited charge carriers and identify the most decisive charge transfer pathway. Methods to modulate charge dynamics and manipulate carrier behaviors may pave a new avenue for the intelligent design of MOF-based photocatalysts for widespread applications. By summarizing the recent developments in the modulation of interfacial charge dynamics for MOF-based photocatalysts, this minireview can deliver inspiring insights to help researchers harness the merits of MOFs and create versatile photocatalytic systems.

Metal-organic frameworks (MOFs) for photoelectrocatalytic (PEC) reducing carbon dioxide (CO2) to hydrocarbon fuels

MOF-based photoelectrocatalysis (PEC) using CO2 as an electron donor offers a green, clean, and extensible way to make hydrocarbon fuels under more tolerant conditions. Herein, basic principles of PEC reduction of CO2 and the preparation methods and characterization techniques of MOF-based materials are summarized. Furthermore, three applications of MOFs for improving the photoelectrocatalytic performance of CO2 reduction are described: (i) as photoelectrode alone; (ii) as a co-catalyst of semiconductor photoelectrode or as a substrate for loading dyes, quantum dots, and other co-catalysts; (iii) as one of the components of heterojunction structure. Challenges and future wave surrounding the development of robust PEC CO2 systems based on MOF materials are also discussed briefly.

Efficient Cu2O Photocathodes for Aqueous Photoelectrochemical CO2 Reduction to Formate and Syngas

Photoelectrochemical carbon dioxide reduction (PEC-CO2R) represents a promising approach for producing renewable fuels and chemicals using solar energy. However, attaining even modest solar-to-fuel (STF) conversion efficiency often necessitates the use of costly semiconductors and noble-metal catalysts. Herein, we present a Cu2O/Ga2O3/TiO2 photocathode modified with Sn/SnOx catalysts through a simple photoelectrodeposition method. It achieves a remarkable half-cell STF efficiency of ∼0.31% for the CO2R in aqueous KHCO3 electrolyte, under AM 1.5 G illumination. The system enables efficient production of syngas (FE: ∼62%, CO/H2 ≈ 1:2) and formate (FE: ∼38%) with a consistent selectivity over a wide potential range, from +0.34 to -0.16 V vs the reversible hydrogen electrode. We ascribe the observed performance to the favorable optoelectronic characteristics of our Cu2O heterostructure and the efficient Sn/SnOx catalysts incorporated in the PEC-CO2R reactions. Through comprehensive experimental investigations, we elucidate the indispensable role of Cu2O buried p-n junctions in generating a high photovoltage (∼1 V) and enabling efficient bulk charge separation (up to ∼70% efficiency). Meanwhile, we discover that the deposited Sn/SnOx catalysts have critical dual effects on the overall performance of the PEC devices, serving as active CO2R catalysts as well as the semiconductor front contact. It could facilitate interfacial electron transfer between the catalysts and the semiconductor device for CO2R by establishing a barrier-free ohmic contact.

Photo/electrocatalytic hydrogen evolution using Type-II Cu2O/g-C3N4 Heterostructure: Density functional theory addresses the improved charge transport efficiency

One of the most efficient ways for the photogenerated charge carriers is by the development of heterojunction between p-type and n-type semiconductors, which creates an interfacial charge transfer between two semiconductors. By enhancing the bifunctional characteristics for hydrogen generation via photocatalytic and electrocatalytic water splitting reaction, we report the type-II Cu2O/g-C3N4 heterostructure in this article. Due to significantly increased catalytically active sites for the hydrogen evolution reaction (HER) reaction during electrocatalysis and decreased charge transfer resistance, the as-prepared heterostructure exhibits a lower overpotential of 47 and 72 mVdec-1 for the HER and oxygen evolution reactions (OER), respectively, when compared to alone g-C3N4. In addition, Cu2O/g-C3N4 heterostructures have a higher photocatalytic hydrogen evolution of 3492 µmol gcat-1 in the presence of Triethanolamine as a sacrificial agent, which is nearly 2-fold times greater than g-C3N4 (1818 µmol gcat-1) after 5 h of continuous light-irradiation. Moreover, produced heterostructure exhibits 81% of Faradaic efficiency and 18% of apparent quantum yield. This work successfully explains how the rise in water splitting is induced by the transfer of photogenerated electrons in a cascade way from p-type Cu2O to the n-type g-C3N4 using density functional theory (DFT) calculations.

Theoretical Prediction of Electrocatalytic Reduction of CO2 Using a 2D Catalyst Composed of 3 d Transition Metal and Hexaamine Dipyrazino Quinoxaline

Transition metals and organic ligands combine to form metal-organic frameworks (MOFs), which possess distinct active sites, large specific surface areas and stable porous structures, giving them considerable promise for CO2 reduction electrocatalysis. In the present study, using spin polarisation density-functional theory, a series of 2D MOFs constructed from 3d transition metal and hexamethylene dipyrazoline quinoxaline(HADQ) were investigated. The calculated binding energies between HADQ and metal atoms for the ten TM-HADQ monolayers were strong sufficient to stably disperse the metal atoms in the HADQ monolayers. Of the ten catalysts tested, seven (Sc, Ni, Cu, Zn, Ti, V and Cr) exhibited high CO2 reduction selectivity, while Mn, Fe and Co required pH values above 2.350, 6.461 and 6.363, respectively, to exhibit CO2 reduction selectivity. HCOOH was the most important producer for Sc, Zn, Ni and Mn, while CH4 was the main producer for Ti, Cr, Fe and V. Cu and Co were less selective, producing HCHO, CH3 OH, and CH4 simultaneously at the same rate-determining step and limiting potential. The Cu-HADQ catalyst had a high overpotential for the HCHO product (1.022 V), while the other catalysts had lower overpotentials between 0.016 V and 0.792 V. Thus, these results predict TM-HADQ to show excellent activity in CO2 electrocatalytic reduction and to become a promising electrocatalyst for CO2 reduction.

Toward high-efficiency photovoltaics-assisted electrochemical and photoelectrochemical CO2 reduction: Strategy and challenge

The realization of a complete techno-economy through a significant carbon dioxide (CO2) reduction in the atmosphere has been explored to promote a low-carbon economy in various ways. CO2 reduction reactions (CO2RRs) can be induced using sustainable energy, including electric and solar energy, using systems such as electrochemical (EC) CO2RR and photoelectrochemical (PEC) systems. This study summarizes various fabrication strategies for non-noble metal, copper-based, and metal-organic framework-based catalysts with excellent Faradaic efficiency (FE) for target carbon compounds, and for noble metals with low overvoltage. Although EC and PEC systems achieve high energy conversion efficiency with excellent catalysts, they still require external power and lack complete bias-free operation. Therefore, photovoltaics, which can overcome the limitations of these systems, have been introduced. The utilization of silicon and perovskite-based solar cells for photovoltaics-assisted EC (PV-EC) and photovoltaics-assisted PEC (PV-PEC) CO2RR systems are cost-efficient, and the III-V semiconductor photoabsorbers achieved high solar-to-carbon efficiency. This work focuses on PV-EC and PV-PEC CO2RR systems and their components and then summarizes the special cell configurations, including the tandem and stacked structures. Additionally, the study discusses current issues, such as low energy conversion, expensive PV, theoretical limits, and industrial scale-up, along with proposed solutions.

A novel design of Cu(I) active site on the metal-organic framework for exploring the structural transformation of Fenton-like catalysts through in situ "capturing" OH. J Colloid Interface Sci 2023;648:778-786. [PMID: 37321097 DOI: 10.1016/j.jcis.2023.05.189] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The mutual transformation of reactive oxygen species may affect the structural transformation of catalysts during the Fenton-like processes. Its in-depth understanding is essential to achieve high catalytic activity and stability. In this study, a novel design of Cu(I) active sites based on the metal-organic framework (MOF) is proposed to "capture" OH^- produced via Fenton-like processes and re-coordinate the oxidized Cu sites. The Cu(I)-MOF presents an excellent removal efficiency for sulfamethoxazole (SMX), with a high removal kinetic constant of 7.146 min^-1. Combing DFT calculations with experimental observations, we have revealed that the Cu of Cu(I)-MOF exhibits a lower d-band center, enabling efficient activation of H₂O₂ and spontaneous "capturing" of OH^- to form Cu-MOF, which can be reorganized into the Cu(I)-MOF through molecular regulation for recycle. This research demonstrates a promising Fenton-like approach for solving the trade-off between catalytic activity and stability and provides new insights into the design and synthesis of efficient MOF-based catalysts for water treatment.

Recent Progress and Perspectives on Photocathode Materials for CO2 Catalytic Reduction

The continuous consumption of fossil energy and excessive emissions of carbon dioxide (CO₂) have caused a serious energy crisis and led to the greenhouse effect. Using natural resources to convert CO₂ into fuel or high-value chemicals is considered to be an effective solution. Photoelectrochemical (PEC) catalysis utilizes abundant solar energy resources, combined with the advantages of photocatalysis (PC) and electrocatalysis (EC), to achieve efficient CO₂ conversion. In this review, the basic principles and evaluation criteria, of PEC catalytic reduction to CO₂ (PEC CO₂RR), are introduced. Next, the recent research progress on typical kinds of photocathode materials for CO₂ reduction are reviewed, and the structure-function relationships between material composition/structure and activity/selectivity are discussed. Finally, the possible catalytic mechanisms and the challenges of using PEC to reduce CO₂ are proposed.

Stability of Photocathodes: A Review on Principles, Design, and Strategies

Photoelectrochemical devices based on semiconductor photoelectrode can directly convert and store solar energy into chemical fuels. Although the efficient photoelectrodes with commercially valuable solar-to-fuel energy conversion efficiency have been reported over past decades, one of the most enormous challenges is the stability of the photoelectrode due to corrosion during operation. Thus, it is of paramount importance for developing a stable photoelectrode to deploy solar-fuel production. This Review commences with a fundamental understanding of thermodynamics for photoelectrochemical reactions and the fundamentals of photocathodes. Then, the commercial application of photoelectrochemical technology is prospected. We specifically focus on recent strategies for designing photocathodes with long-term stability, including energy band alignment, hole transport/storage/blocking layer, spatial decoupling, grafting molecular catalysts, protective/passivation layer, surface element reconstruction, and solvent effects. Based on the insights gained from these effective strategies, we propose an outlook of key aspects that address the challenges for development of stable photoelectrodes in future work.

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.

Photoelectrochemical CO2 Reduction at a Direct CuInGaS2/Electrolyte Junction

Photoelectrochemical (PEC) CO2 reduction has received considerable attention given the inherent sustainability and simplicity of directly converting solar energy into carbon-based chemical fuels. However, complex photocathode architectures with protecting layers and cocatalysts are typically needed for selective and stable operation. We report herein that bare CuIn0.3Ga0.7S2 photocathodes can drive the PEC CO2 reduction with a benchmarking 1 Sun photocurrent density of over 2 mA/cm2 (at -2 V vs Fc+/Fc) and a product selectivity of up to 87% for CO (CO/all products) production while also displaying long-term stability for syngas production (over 44 h). Importantly, spectroelectrochemical analysis using PEC impedance spectroscopy (PEIS) and intensity-modulated photocurrent spectroscopy (IMPS) complements PEC data to reveal that tailoring the proton donor ability of the electrolyte is crucial for enhancing the performance, selectivity, and durability of the photocathode. When a moderate amount of protons is present, the density of photogenerated charges accumulated at the interface drops significantly, suggesting a faster charge transfer process. However, with a high concentration of proton donors, the H2 evolution reaction is preferred.

Embedding Plasmonic Metal into Heterointerface of MOFs-Encapsulated Semiconductor Hollow Architecture for Boosting CO2 Photoreduction

Coupling hollow semiconductor with metal-organic frameworks (MOFs) holds great promise for constructing high-efficient CO₂ photoreduction systems. However, energy band mismatch between them makes it difficult to exert their advantages to maximize the overall photocatalytic efficiency, since that the blockage of desirable interfacial charge transfer gives rise to the enrichment of photoelectrons and CO₂ molecules on the different locations. Herein, an interfacial engineering is presented to overcome this impediment, based on the insertion of plasmonic metal into the heterointerfaces between them, forming a stacked semiconductor/metal@MOF photocatalyst. Experimental observations and theoretical simulations validate the critical roles of embedded Au in maneuvering the charge separation/transfer and surface reaction: (i) bridges the photoelectron transfer from hollow CdS (H-CdS) to ZIF-8; (ii) produces hot electrons and shifts them to ZIF-8; (iii) induces the formation of ZIF-8 defects in promoting the CO₂ adsorption/activation and transformation to CO with low energy barriers. Consequently, the as-prepared H-CdS/Au@ZIF-8 with optimal ZIF-8 thickness exhibits distinctly boosted activity and superb selectivity in CO production as compared with H-CdS@ZIF-8 and other counterparts. This work provides protocols to take full advantages of components involved for enhanced solar-to-chemical energy conversion efficiency of hybrid artificial photosynthetic systems through rationally harnessing the charge transfer between them.

Enhancing the durability of Au clusters in CO2 photoreduction via encapsulation in Cu-based metal-organic frameworks

Here, we report the encapsulation of Au₂₅ nanoclusters in a Cu₃(BTC)₂ metal-organic framework (Au₂₅@Cu-BTC), which can achieve CO₂ photoreduction for selective CO production in a gas-solid reaction system at low-concentration CO₂ atmospheres (even to 0.1%), with remarkably enhanced durability up to at least 48 h.

Photocurrent-Polarity-Switching Photoelectrochemical Biosensor for Switching Spatial Distance Electroactive Tags

This work presents a photocurrent-polarity-switching-based photoelectrochemical (PEC) biosensing platform for ultrasensitive detection of microRNA-21 (miR-21) through target-triggered catalytic hairpin assembly (CHA) for modulation of methylene blue (MB) and ferrocene (Fc) positional configurations using double-shelled Cu-doped ZnS nanocages (NCs)-Au nanoparticles (NPs) as photoactive materials. In the presence of miR-21, the assembly of MB-labeled HP1 and Fc-labeled HP2 leads to the generation of a large amount of double-stranded DNA (HP1-HP2), which pushes MB away from the electrode surface and brings Fc close to the electrode surface, resulting in effectively quenching the enhanced PEC signal to activate the photocurrent-polarity-switching system. Benefiting from the distance-controllable strategy, the designed PEC bioanalysis can effectively eliminate false-positive and false-negative signals due to the change of different signal expression patterns (from traditional the "signal-on" mode to the photocurrent-polarity-switching mode), thereby significantly improving the sensing specificity and sensitivity. The proposed PEC sensing system exhibited satisfying photocurrent responses toward target miR-21 within the working range from 1.0 fM to 1 nM at a low limit of detection (LOD) of 0.58 fM. More importantly, we demonstrated the successful integration of the proposed PEC biosensor with a handheld wireless device for instant detection of miR-21 concentrations in practical samples.

Bias-Free Solar-Driven Syngas Production: A Fe2 O3 Photoanode Featuring Single-Atom Cobalt Integrated with a Silver-Palladium Cathode

Photoelectrochemical syngas production from aqueous CO₂ is a promising technique for carbon capture and utilization. Herein, we demonstrate the efficient and tunable syngas production by integrating a single-atom cobalt-catalyst-decorated α-Fe₂ O₃ photoanode with a bimetallic Ag/Pd alloy cathode. A record syngas production activity of 81.9 μmol cm^-2  h^-1 (CO/H₂ ratio: ≈1 : 1) was achieved under artificial sunlight (AM 1.5 G) with an excellent durability. Systematic studies reveal that the Co single atoms effectively extract the holes from Fe₂ O₃ photoanodes and serve as active sites for promoting oxygen evolution. Simultaneously, the Pd and Ag atoms in bimetallic cathodes selectively adsorb CO₂ and protons for facilitating CO production. Further incorporation with a photovoltaic, to allow solar light (>600 nm) to be utilized, yields a bias-free CO₂ reduction device with solar-to-CO and solar-to-H₂ conversion efficiencies up to 1.33 and 1.36 %, respectively.

Metal-Organic Framework-Based Photocatalysis for Solar Fuel Production

Metal-organic frameworks (MOFs) represent a novel class of crystalline inorganic-organic hybrid materials with tunable semiconducting behavior. MOFs have potential for application in photocatalysis to produce sustainable solar fuels, owing to their unique structural advantages (such as clarity and modifiability) that can facilitate a deeper understanding of the structure-activity relationship in photocatalysis. This review takes the photocatalytic active sites as a particular perspective, summarizing the progress of MOF-based photocatalysis for solar fuel production; mainly including three categories of solar-chemical conversions, photocatalytic water splitting to hydrogen fuel, photocatalytic carbon dioxide reduction to hydrocarbon fuels, and photocatalytic nitrogen fixation to high-energy fuel carriers such as ammonia. This review focuses on the types of active sites in MOF-based photocatalysts and discusses their enhanced activity based on the well-defined structure of MOFs, offering deep insights into MOF-based photocatalysis.

Two-Dimensional (2D) TM-Tetrahydroxyquinone Metal-Organic Framework for Selective CO2 Electrocatalysis: A DFT Investigation

The resource utilization of CO2 is one of the essential avenues to realize the goal of "double carbon". The metal-organic framework (MOF) has shown promising applications in CO2 catalytic reduction reactions due to its sufficient pore structure, abundant active sites and functionalizability. In this paper, we investigated the electrocatalytic carbon dioxide reduction reactions of single-atom catalysts created by MOF two-dimensional coordination network materials constructed from transition metal-tetrahydroxybenzoquinone using density function theory calculations. The results indicate that for 10 transition metals, TM-THQ single levels ranging from Sc to Zn, the metal atom binding energy to the THQ is large enough to allow the metal atoms to be stably dispersed in the THQ monolayer. The Ni-THQ catalyst does not compete with the HER reaction in an electrocatalytic CO2 reduction. The primary product of reduction for Sc-THQ is HCOOH, but the major product of Co-THQ is HCHO. The main product of Cu-THQ is CO, while the main product of six catalysts, Ti, V, Cr, Mn, Fe, and Zn, is CH4. The limit potential and overpotential of Ti-THQ are the highest, 1.043 V and 1.212 V, respectively. The overpotentials of the other monolayer catalysts ranged from 0.172 to 0.952 V, and they were all relatively low. Therefore, we forecast that the TM-HQ monolayer will show powerful activity in electrocatalytic carbon dioxide reduction, making it a prospective electrocatalyst for carbon dioxide reduction.

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.

A Potential Roadmap to Integrated Metal Organic Framework Artificial Photosynthetic Arrays

Metal organic frameworks (MOFs), a class of coordination polymers, gained popularity in the late 1990s with the efforts of Omar Yaghi, Richard Robson, Susumu Kitagawa, and others. The intrinsic porosity of MOFs made them a clear platform for gas storage and separation. Indeed, these applications have dominated the vast literature in MOF synthesis, characterization, and applications. However, even in those early years, there were hints to more advanced applications in light-MOF interactions and catalysis. This perspective focuses on the combination of both light-MOF interactions and catalysis: MOF artificial photosynthetic assemblies. Light absorption, charge transport, H2O oxidation, and CO2 reduction have all been previously observed in MOFs; however, work toward a fully MOF-based approach to artificial photosynthesis remains out of reach. Discussed here are the current limitations with MOF-based approaches: diffusion through the framework, selectivity toward high value products, lack of integrated studies, and stability. These topics provide a roadmap for the future development of fully integrated MOF-based assemblies for artificial photosynthesis.

Controllable States and Porosity of Cu-Carbon for CO2 Electroreduction to Hydrocarbons

The electrocatalytic carbon dioxide reduction reaction (CO₂ RR) to value-added chemical products is an effective strategy for both greenhouse effect mitigation and high-density energy storage. However, controllable manipulation of the oxidation state and porous structure of Cu-carbon based catalysts to achieve high selectivity and current density for a particular product remains very challenging. Herein, a strategy derived from Cu-based metal-organic frameworks (MOFs) for the synthesis of controllable oxidation states and porous structure of Cu-carbon (Cu-pC, Cu₂ O-pC, and Cu₂ O/Cu-pC) is demonstrated. By regulating oxygen partial pressure during the annealing process, the valence state of the Cu and mesoporous structures of surrounding carbon are changed, leads to the different selectivity of products. Cu₂ O/CuO-pC with the higher oxidation state exhibits FE_C2H4 of 65.12% and a partial current density of -578 mA cm^-2 , while the Cu₂ O-pC shows the FE_CH4 over 55% and a partial current density exceeding -438 mA cm^-2 . Experimental and theoretical studies indicate that porous carbon-coated Cu₂ O structures favor the CH₄ pathway and inhibit the hydrogen evolution reaction. This work provides an effective strategy for exploring the influence of the various valence states of Cu and mesoporous carbon structures on the selectivity of CH₄ and C₂ H₄ products in CO₂ RR.

Plasmonic Ag-decorated Cu2O nanowires for boosting photoelectrochemical CO2 reduction to multi-carbon products

The generation of multi-carbon products on the Cu₂O photocathode remains a great challenge. Herein, effective charge separation and surface catalytic reaction are achieved for photoelectrochemical CO₂ reduction through plasmon metal (Ag) decoration on Cu₂O nanowires. The Cu₂O/Ag composite photocathode achieves a 47.7% faradaic efficiency for CH₃COOH and the generation rate is 212.7 μmol cm^-2 h^-1 under illumination, which is about five times that in dark (44.4 μmol cm^-2 h^-1) at -0.7 V vs. RHE.

Emerging Trends in Sustainable CO2 -Management Materials

With the rising level of atmospheric CO₂ worsening climate change, a promising global movement toward carbon neutrality is forming. Sustainable CO₂ management based on carbon capture and utilization (CCU) has garnered considerable interest due to its critical role in resolving emission-control and energy-supply challenges. Here, a comprehensive review is presented that summarizes the state-of-the-art progress in developing promising materials for sustainable CO₂ management in terms of not only capture, catalytic conversion (thermochemistry, electrochemistry, photochemistry, and possible combinations), and direct utilization, but also emerging integrated capture and in situ conversion as well as artificial-intelligence-driven smart material study. In particular, insights that span multiple scopes of material research are offered, ranging from mechanistic comprehension of reactions, rational design and precise manipulation of key materials (e.g., carbon nanomaterials, metal-organic frameworks, covalent organic frameworks, zeolites, ionic liquids), to industrial implementation. This review concludes with a summary and new perspectives, especially from multiple aspects of society, which summarizes major difficulties and future potential for implementing advanced materials and technologies in sustainable CO₂ management. This work may serve as a guideline and road map for developing CCU material systems, benefiting both scientists and engineers working in this growing and potentially game-changing area.

Cupric porphyrin frameworks on multi-junction silicon photocathodes to expedite the kinetics of CO2 turnover

Photoelectrochemical CO₂ reduction utilizing silicon-based photocathodes offers a promising route to directly store solar energy in chemical bonds, provoking the development of heterogeneous molecular catalysts with high turnover rates. Herein, an in situ surface transformation strategy is adopted to grow metal-organic frameworks (MOFs) on Si-based photocathodes, serving as catalytic scaffolds for boosting both the kinetics and selectivity of CO₂ reduction. Benefitting from the multi-junctional configuration for enhanced charge separation and the porous MOF scaffold enriching redox-active metalloporphyrin sites, the Si photocathode demonstrates a high CO faradaic efficiency of 87% at a photocurrent density of 10.2 mA cm^-2, which is among the best seen for heterogeneous molecular catalysts. This study highlights the exploitation of reticular chemistry and macrocycle complexes as Earth-abundant alternatives for catalyzing artificial photosynthesis.

Metal-organic frameworks and their composites for fuel and chemical production via CO2 conversion and water splitting

Increase in the global energy demand has been leading to major energy crises in recent years. The use of excess fossil fuels for energy production is causing severe global warming, as well as energy shortage. To overcome the global energy crisis, the design of various chemical structures as efficient models for the generation of renewable energy fuels is very much crucial, and will limit the use of fossil fuels. Current challenges involve the design of Metal-Organic Framework (MOF) materials for this purpose to diminish the energy shortage. The large surface area, tunable pore environment, unique structural property and semiconducting nature of the highly porous MOF materials enhance their potential applications towards the production of enhanced energy fuels. This review is focused on the architecture of MOFs and their composites for fuels and essential chemicals production like hydrogen, methane, ethanol, methanol, acetic acid, and carbon monoxide, which can be used as renewable fuel energy sources to limit the use of fossil fuels, thereby reducing global warming.