Improvement of Catalytic Activity by Nanofibrous CuInS2 for Electrochemical CO2 Reduction

Co-sensitization of Copper Indium Gallium Disulfide and Indium Sulfide on Zinc Oxide Nanostructures: Effect of Morphology in Electrochemical Carbon Dioxide Reduction

Recent advances in nanoparticle materials can facilitate the electro-reduction of carbon dioxide (CO2) to form valuable products with high selectivity. Copper (Cu)-based electrodes are promising candidates to drive efficient and selective CO2 reduction. However, the application of Cu-based chalcopyrite semiconductors in the electrocatalytic reduction of CO2 is still limited. This study demonstrated that novel zinc oxide (ZnO)/copper indium gallium sulfide (CIGS)/indium sulfide (InS) heterojunction electrodes could be used in effective CO2 reduction for formic acid production. It has been determined that Faradaic efficiencies for formic acid production using ZnO nanowire (NW) and nanoflower (NF) structures vary due to structural and morphological differences. A ZnO NW/CIGS/InS heterojunction electrode resulted in the highest efficiency of 77.2% and 0.35 mA cm-2 of current density at a -0.24 V (vs. reversible hydrogen electrode) bias potential. Adding a ZTO intermediate layer by the spray pyrolysis method decreased the yield of formic acid and increased the yield of H2. Our work offers a new heterojunction electrode for efficient formic acid production via cost-effective and scalable CO2 reduction.

An insight of light-enhanced electrochemical kinetic behaviors and interfacial charge transfer of CuInS2/MoS2-based sensing nanoplatform for ultra-sensitive detection of chloramphenicol

Owing to the effective combination between MoS₂ sheets with CuInS₂ nanoparticles (NPs), a direct Z-scheme heterojunction was successfully constructed and proved as a promising structure to modify the working electrode surface with the aim of enhancing overall sensing performance towards CAP detection. Herein, MoS₂ was employed as a high mobility carrier transport channel with a strong photo-response, large specific surface area, and high in-plane electron mobility, while CuInS₂ acted as an efficient light absorber. This not only offered a stable nanocomposite structure but also created impressive synergistic effects of high electron conductivity, large surface area, highlight exposure interface, as well as favorable electron transfer process. Moreover, the possible mechanism and hypothesis of the transfer pathway of photo-induced electron-hole pairs on the CuInS₂-MoS₂/SPE as well as their impacts on the redox reaction of K₃/K₄ probes and CAP were proposed and investigated in detail via a series of calculated kinetic parameters, demonstrating the high practical applicability of light-assisted electrodes. Indeed, the detection concentration range of the proposed electrode was widened from 0.1 to 50 μM, compared with that of 1-50 μM without irradiation. Also, the LOD and sensitivity values were calculated to be approximately 0.06 μM and 0.4623 μA μM^-1, which is better than that of 0.3 μM and 0.095 μA μM^-1 without irradiation.

Copper indium sulfide quantum dots in photocatalysis

Since the advent of photocatalytic technology, scientists have been searching for semiconductor materials with high efficiency in solar energy utilization and conversion to chemical energy. Recently, the development of quantum dot (QD) photocatalysts has attracted much attention because of their unique characteristics: small size, quantum effects, strong surface activity, and wide photoresponse range. Among ternary chalcogenide semiconductors, CuInS₂ QDs are increasingly examined in the field of photocatalysis due to their high absorption coefficients, good matching of the absorption range with sunlight spectrum, long lifetimes of photogenerated electron-hole pairs and environmental sustainability. In this review paper, the structural and electronic properties, synthesis methods and various photocatalytic applications of CuInS₂ QDs are systematically expounded. The current research status on the photocatalytic properties of materials based on CuInS₂ QD is discussed combined with the existing modification approaches for the enhancement of their performances. Future challenges and new development opportunities of CuInS₂ QDs in the field of photocatalysis are then prospected.

Probing the Roles of Indium Oxides on Copper Catalysts for Enhanced Selectivity during CO2-to-CO Electrochemical Reduction

The electrochemical CO₂ reduction reaction (CO₂RR) provides an alternative protocol to producing industrial chemicals with renewable electricity sources, and the highly selective, durable, and economic catalysts should expedite CO₂RR applications. Here, we demonstrate a composite Cu-In₂O₃ catalyst in which a trace amount of In₂O₃ decorated on Cu surface greatly improves the selectivity and stability for CO₂-to-CO reduction as compared to the counterparts (Cu or In₂O₃), realizing a CO faradaic efficiency (FE_CO) of 95% at -0.7 V (vs RHE) and no obvious degradation within 7 h. In situ X-ray absorption spectroscopy reveals that In₂O₃ undergoes the redox reaction and preserves the metallic state of Cu during the CO₂RR process. Strong electronic interaction and coupling occur at the Cu/In₂O₃ interface which serves as the active site for selective CO₂RR. Theoretical calculation confirms the roles of In₂O₃ in preventing oxidation and altering the electronic structure of Cu to assist COOH* formation and demote CO* adsorption at the Cu/In₂O₃ interface.

Electrochemical organic reactions: A tutorial review

Although the core of electrochemistry involves simple oxidation and reduction reactions, it can be complicated in real electrochemical organic reactions. The principles used in electrochemical reactions have been derived using physical organic chemistry, which drives other organic/inorganic reactions. This review mainly comprises two themes: the first discusses the factors that help optimize an electrochemical reaction, including electrodes, supporting electrolytes, and electrochemical cell design, and the second outlines studies conducted in the field over a period of 10 years. Electrochemical reactions can be used as a versatile tool for synthetically important reactions by modifying the constant electrolysis current.

Facet-dependent CO2 reduction reactions on kesterite Cu2ZnSnS4 photo-electro-integrated electrodes

Photoelectrochemical CO₂ reduction by Cu₂ZnSnS₄ (CZTS) photocathodes is a potentially low-cost and high-efficiency CO₂ conversion approach. However, the current CZTS-based photocathodes for the CO₂ reduction reaction (CO₂RR) are challenged by the active side reaction of the hydrogen evolution reaction (HER) and the incompatibility with efficient electrocatalysts. In this work, by means of density functional theory (DFT), we predict that a (220)-facet-suppressed kesterite CZTS could be an efficient photo-electro-integrated photocathode for formic acid production in the CO₂RR. The results show that the competitive HER is mostly favored on the (220) facet. And the CO₂RR for formic acid production on the (112) and (312) facets exhibits a thermodynamic energy barrier lower than 0.26 eV. Different from the d-band theory in metal electrocatalysts, it is found that the density of low energy unoccupied states in the S 3p orbital plays a key role in determining the CO₂RR reaction path of the kesterite CZTS. Furthermore, two different trends of adsorption energy depending on the chemical characteristic of adsorbates are analyzed. Our study unveils the potential for selectively reducing CO₂ into formic acid with kesterite CZTS and provides a possible route for manipulating the electrocatalytic properties of metal sulfide catalysts.

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

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

Copper-alloy catalysts: structural characterization and catalytic synergies

Recent progress in the development of copper-alloy catalysts is highlighted, focusing on the structural and mechanistic characterizations of the catalysts in different catalytic reactions, and challenges and opportunities in future research.

Advanced electrospun nanomaterials for highly efficient electrocatalysis

We highlight the recent developments of electrospun nanomaterials with controlled morphology, composition and architecture for highly efficient electrocatalysis.

Challenges in TEM sample preparation of solvothermally grown CuInS2 films

Transmission electron microscopy (TEM) is a widely used tool to characterize materials. The required samples need to be electron transparent which should be achieved without changing the microstructure. This work describes different TEM sample preparation techniques of nanostructured CuInS₂ thin films on fluorine-doped tin oxide substrates, synthesized solvothermally using l-cysteine as sulfur source. Focused ion beam lamellae, conventional cross section samples and scratch samples have been prepared and investigated. It was possible to prepare appropriate samples with each technique, however, each technique brings with it certain advantages and disadvantages. FIB preparation of solvothermally synthesized CuInS₂ suffers from two main drawbacks. First, the whole CuInS₂ layer displays a strongly increased Cu content caused by Cu migration and preferential removal of In. Further, electron diffraction shows the formation of an additional CuS phase after Ga⁺ bombardment. Second, diffraction analysis is complicated by a strong contribution of crystalline Pt introduced during the FIB preparation and penetrating into the porous film surface. The conventional cross sectional CuInS₂ sample also shows a Cu signal enhancement which is caused by contribution of the brass tube material used for embedding. Additionally, Cu particles have been observed inside the CuInS₂ which have been sputtered on the film during preparation. Only the scratch samples allow an almost artefact-free and reliable elemental quantification using energy-dispersive X-ray spectroscopy. However, scratch samples suffer from the drawback that it is not possible to determine the layer thickness, which is possible for both cross sectional preparation techniques. Consequently, it is concluded that the type of sample preparation should be chosen dependent on the required information. A full characterization can only be achieved when the different techniques are combined.

Semiconductor-Based Photoelectrochemical Conversion of Carbon Dioxide: Stepping Towards Artificial Photosynthesis

The photoelectrochemical (PEC) carbon dioxide reduction process stands out as a promising avenue for the conversion of solar energy into chemical feedstocks, among various methods available for carbon dioxide mitigation. Semiconductors derived from cheap and abundant elements are interesting candidates for catalysis. Whether employed as intrinsic semiconductors or hybridized with metallic cocatalysts, biocatalysts, and metal molecular complexes, semiconductor photocathodes exhibit good performance and low overpotential during carbon dioxide reduction. Apart from focusing on carbon dioxide reduction materials and chemistry, PEC cells towards standalone devices that use photohybrid electrodes or solar cells have also been a hot topic in recent research. An overview of the state-of-the-art progress in PEC carbon dioxide reduction is presented and a deep understanding of the catalysts of carbon dioxide reduction is also given.

Biofunctionalized conductive polymers enable efficient CO2 electroreduction

Selective electrocatalysts are urgently needed for carbon dioxide (CO₂) reduction to replace fossil fuels with renewable fuels, thereby closing the carbon cycle. To date, noble metals have achieved the best performance in energy yield and faradaic efficiency and have recently reached impressive electrical-to-chemical power conversion efficiencies. However, the scarcity of precious metals makes the search for scalable, metal-free, CO₂ reduction reaction (CO₂RR) catalysts all the more important. We report an all-organic, that is, metal-free, electrocatalyst that achieves impressive performance comparable to that of best-in-class Ag electrocatalysts. We hypothesized that polydopamine-a conjugated polymer whose structure incorporates hydrogen-bonded motifs found in enzymes-could offer the combination of efficient electrical conduction, together with rendered active catalytic sites, and potentially thereby enable CO₂RR. Only by developing a vapor-phase polymerization of polydopamine were we able to combine the needed excellent conductivity with thin film-based processing. We achieve catalytic performance with geometric current densities of 18 mA cm^-2 at 0.21 V overpotential (-0.86 V versus normal hydrogen electrode) for the electrosynthesis of C₁ species (carbon monoxide and formate) with continuous 16-hour operation at >80% faradaic efficiency. Our catalyst exhibits lower overpotentials than state-of-the-art formate-selective metal electrocatalysts (for example, 0.5 V for Ag at 18 mA cm^-1). The results confirm the value of exploiting hydrogen-bonded sequences as effective catalytic centers for renewable and cost-efficient industrial CO₂RR applications.