Stable Quantum Dot Photoelectrolysis Cell for Unassisted Visible Light Solar Water Splitting

Sunlight is an ideal source of energy, and converting sunlight into chemical fuels, mimicking what nature does, has attracted significant attention in the past decade. In terms of solar energy conversion into chemical fuels, solar water splitting for hydrogen production is one of the most attractive renewable energy technologies, and this achievement would satisfy our increasing demand for carbon-neutral sustainable energy. Here, we report corrosion-resistant, nanocomposite photoelectrodes for spontaneous overall solar water splitting, consisting of a CdS quantum dot (QD) modified TiO₂ photoanode and a CdSe QD modified NiO photocathode, where cadmium chalcogenide QDs are protected by a ZnS passivation layer and gas evolution cocatalysts. The optimized device exhibited a maximum efficiency of 0.17%, comparable to that of natural photosynthesis with excellent photostability under visible light illumination. Our device shows spontaneous overall water splitting in a nonsacrificial environment under visible light illumination (λ > 400 nm) through mimicking nature’s “Z-scheme” process. The results here also provide a conceptual layout to improve the efficiency of solar-to-fuel conversion, which is solely based on facile, scalable solution-phase techniques

Light-Induced In Situ Transformation of Metal Clusters to Metal Nanocrystals for Photocatalysis

In situ transformation of glutathione-capped gold (Au_<i>x</i>) clusters to gold (Au) nanocrystals under simulated solar light irradiation was achieved and utilized as a facile synthetic approach to rationally fabricate Au_<i>x</i>/Au/TiO₂ ternary and Au/TiO₂ binary heterostructures. Synergistic interaction of Au_<i>x</i> clusters and Au nanocrystals contributes to enhanced visible-light-driven photocatalysis

High Spin State Promotes Water Oxidation Catalysis at Neutral pH in Spinel Cobalt Oxide

In this work, we present Co₃O₄ quantum dots (QDs) as a highly efficient and stable oxygen evolution reaction (OER) catalyst at neutral pH. The Co₃O₄ QDs with a mean size of 5 nm were synthesized by reacting cobalt acetate with benzyl alcohol in the presence of ammonia under reflux conditions. The as-synthesized Co₃O₄ QDs show extraordinary water oxidation activity with onset overpotential as low as 398 mV and mass activity as high as 567 A/g (at 1.75 V vs RHE) in a 0.2 M phosphate buffer electrolyte (pH ∼7), which are among the most efficient Earth-abundant OER catalysts at neutral pH reported in the literature, reaching a stable current density of 10 mA/cm² at an overpotential of ∼490 mV with a Tafel slope of 80 mV/decade. Through in-depth investigations by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, the high spin Co²⁺ and Co³⁺ cations on the surface of Co₃O₄ QDs were found to be important to promote the OER kinetics at neutral pH

High Spin State Promotes Water Oxidation Catalysis at Neutral pH in Spinel Cobalt Oxide

In this work, we present Co₃O₄ quantum dots (QDs) as a highly efficient and stable oxygen evolution reaction (OER) catalyst at neutral pH. The Co₃O₄ QDs with a mean size of 5 nm were synthesized by reacting cobalt acetate with benzyl alcohol in the presence of ammonia under reflux conditions. The as-synthesized Co₃O₄ QDs show extraordinary water oxidation activity with onset overpotential as low as 398 mV and mass activity as high as 567 A/g (at 1.75 V vs RHE) in a 0.2 M phosphate buffer electrolyte (pH ∼7), which are among the most efficient Earth-abundant OER catalysts at neutral pH reported in the literature, reaching a stable current density of 10 mA/cm² at an overpotential of ∼490 mV with a Tafel slope of 80 mV/decade. Through in-depth investigations by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, the high spin Co²⁺ and Co³⁺ cations on the surface of Co₃O₄ QDs were found to be important to promote the OER kinetics at neutral pH

In Situ Spectroscopic Identification of μ‑OO Bridging on Spinel Co₃O₄ Water Oxidation Electrocatalyst

The formation of μ-OO peroxide (Co–OO–Co) moieties on spinel Co₃O₄ electrocatalyst prior to the rise of the electrochemical oxygen evolution reaction (OER) current was identified by in situ spectroscopic methods. Through a combination of independent in situ X-ray absorption, grazing-angle X-ray diffraction, and Raman analysis, we observed a clear coincidence between the formation of μ-OO peroxide moieties and the rise of the anodic peak during OER. This finding implies that a chemical reaction step could be generally ignored before the onset of OER current. More importantly, the tetrahedral Co²⁺ ions in the spinel Co₃O₄ could be the vital species to initiate the formation of the μ-OO peroxide moieties

Bipyridine-Confined Silver Single-Atom Catalysts Facilitate In-Plane C–O Coupling for Propylene Electrooxidation

The electrooxidation of propylene presents a promising route for the production of 1,2-propylene glycol (PG) under ambient conditions. However, the C–O coupling process remains a challenge owing to the high energy barrier. In this work, we developed a highly efficient electrocatalyst of bipyridine-confined Ag single atoms on UiO-bpy substrates (Ag SAs/UiO-bpy), which exposed two in-plane coordination vacancies during reaction for the co-adsorption of key intermediates. Detailed structure and electronic property analyses demonstrate that CH3CHCH2OH* and *OH could stably co-adsorb in a square planar configuration, which then accelerates the charge transfer between them. The combination of stable co-adsorption and efficient charge transfer facilitates the C–O coupling process, thus significantly lowering its energy barrier. At 2.4 V versus a reversible hydrogen electrode, Ag SAs/UiO-bpy achieved a record-high activity of 61.9 gPG m–2 h–1. Our work not only presents a robust electrocatalyst but also advances a new perspective on catalyst design for propylene electrooxidation

Heterojunction of Zinc Blende/Wurtzite in Zn_1–<i>x</i>Cd_<i>x</i>S Solid Solution for Efficient Solar Hydrogen Generation: X‑ray Absorption/Diffraction Approaches

In the past decade, inorganic semiconductors have been successfully demonstrated as light absorbers in efficient solar water splitting to generate chemical fuels. Pseudobinary semiconductors Zn_1–<i>x</i>Cd_<i>x</i>S (0 ≤ <i>x</i> ≤ 1) have exhibited a superior photocatalytic reactivity of H₂ production from splitting of water by artificial solar irradiation without any metal catalysts. However, most studies had revealed that the extremely high efficiency with an optimal content of Zn_1–<i>x</i>Cd_<i>x</i>S solid solution was determined as a result of elevating the conduction band minimum (CBM) and the width of bandgap. In addition to corresponding band structure and bandgap, the local crystal structure should be taken into account as well to determine its photocatalytic performance. Herein, we demonstrated the correlations between the photocatalytic activity and structural properties that were first studied through synchrotron X-ray diffraction and X-ray absorption spectroscopy. The crystal structure transformed from zinc blende to coexisted phases of major zinc blende and minor wurtzite phases at a critical point. The heterojunction formed by coexistence of zinc blende and wurtzite phases in the Zn_1–<i>x</i>Cd_<i>x</i>S solid solution can significantly improve the separation and migration of photoinduced electron–hole pairs. Besides, X-ray absorption spectra and UV–vis spectra revealed that the bandgap of the Zn_0.45Cd_0.55S sample extended into the region of visible light because of the incorporation of Cd element in the sample. These results provided a significant progress toward the realization of the photoelectrochemical mechanism in heterojunction between zinc blende and wurtzite phases, which can effectively separate the charge-carriers and further suppress their recombination to enhance the photocatalytic reactivity