Ultrathin NiFeS Nanomeshes with Sulfur Vacancy for Electrocatalytic Hydrogen Evolution

Unveiling the Role of Sulfur Vacancies in Enhanced Photocatalytic Activity of Hybrids Photocatalysts

Photocatalysis represents a sustainable strategy for addressing energy shortages and global warming. The main challenges in the photocatalytic process include limited light absorption, rapid recombination of photo-induced carriers, and poor surface catalytic activity for reactant molecules. Defect engineering in photocatalysts has been proven to be an efficient approach for improving solar-to-chemical energy conversion. Sulfur vacancies can adjust the electron structure, act as electron reservoirs, and provide abundant adsorption and activate sites, leading to enhanced photocatalytic activity. In this work, we aim to elucidate the role of sulfur vacancies in photocatalytic reactions and provide valuable insights for engineering high-efficiency photocatalysts with abundant sulfur vacancies in the future. First, we delve into the fundamental understanding of photocatalysis. Subsequently, various strategies for fabricating sulfur vacancies in photocatalysts are summarized, along with the corresponding characterization techniques. More importantly, the enhanced photocatalytic mechanism, focusing on three key factors, including electron structure, charge transfer, and the surface catalytic reaction, is discussed in detail. Finally, the future opportunities and challenges in sulfur vacancy engineering for photocatalysis are identified.

Immersion-Driven Structural Evolution of NiFeS Nanosheets for Efficient Water Splitting

The development of low-cost, highly active, and stable electrocatalytic water-splitting catalysts is crucial to solving the current energy crisis and environmental pollution. Herein, a simple two-step conversion strategy is proposed to successfully prepare NiFeS nanosheet structure catalyst through the "immersion-sulfurization" strategy. The self-supported electrode can be prepared in large quantities due to its simple preparation process. As an active substance, NiFeS can grow directly on the NiFe foam substrate, avoiding the use of adhesives or conductive agents, and directly used as electrodes. The as-obtained NiFeS/NFF-300 displays efficient catalytic activity in electrocatalytic water splitting. The overpotential required for OER of the NiFeS/NFF-300 electrode at a current density of 10 mA cm-2 is 230 mV. The electrode underwent a stability test at 10 mA cm-2 for 24 h, and the overpotential remained essentially unchanged, demonstrating excellent stability. Moreover, NiFeS/NFF-300 exhibits considerable HER performances compared with NiFeC2O4/NFF and NiFe foam. The unique nanosheet structure and the presence of Niδ+ and Ni2+ formed by NiFe foam substrate on the NiFeS surface are responsible for its excellent electrocatalytic activity.

Bio-Inspired FeMo2S4 Microspheres as Bifunctional Electrocatalysts for Boosting Hydrogen Oxidation/Evolution Reactions in Alkaline Solution

Developing robust and effectual nonprecious electrocatalysts for the bifunctional hydrogen oxidation and evolution reactions (HOR and HER) in alkaline electrolyte is of critical significance for the realization of future hydrogen economy but challenging. Herein, this work demonstrates a new routine for the preparation of bio-inspired FeMo₂S₄ microspheres via the one-step sulfuration of Keplerate-type polyoxometalate {Mo₇₂Fe₃₀}. The bio-inspired FeMo₂S₄ microspheres feature potential-abundant structural defects and atomically precise iron doping and act as an effective bifunctional electrocatalyst for hydrogen oxidation/reduction reactions. The FeMo₂S₄ catalyst presents an impressive alkaline HOR activity compared to FeS₂ and MoS₂ with the high mass activity of 1.85 mA·mg^-1 and high specific activity as well as excellent tolerance to carbon monoxide poisoning. Meanwhile, FeMo₂S₄ electrocatalyst also displayed prominent alkaline HER activity with a low overpotential of 78 mV at a current density of 10 mA·cm^-2 and robust long-term durableness. Density functional theory (DFT) calculations indicate that the bio-inspired FeMo₂S₄ with a unique electron structure possesses the optimal hydrogen adsorption energy and enhanced adsorption of hydroxyl intermediates, which accelerates the potential-determining Volmer step, thus promoting the HOR and HER performance. This work provides a new pathway for designing efficient noble-metal-free electrocatalysts for the hydrogen economy.

Solvent-Type Passivation Strategy Controls Solid-State Self-Quenching-Resistant Behavior in Sulfur Dots

Treatment of sulfur dots with polyethylene glycol (PEG) has been an efficient way to achieve a high luminescence quantum yield, and such a PEG-related quantum dot (QD)-synthesis strategy has been well documented. However, the polymeric insulating capping layer acting as the "thick shell" will significantly slow down the electron-transfer efficiency and severely hamper its practical application in an optoelectric field. Especially, the employment of synthetic polymers with long alkyl chains or large molecular weights may lead to structural complexity or even unexpected changes of physical characteristics for QDs. Therefore, in sulfur dot preparation, it is a breakthrough to use short-chain molecular species to replace PEG for better control and reproducibility. In this article, a solvent-type passivation (STP) strategy has been reported, and no PEG or any other capping agent is required. The main role of the solvent, ethanol, is to directly react with NaOH, and the generated sodium ethoxide passivates the surface defects. The afforded STP-enhanced emission sulfur dots (STPEE-SDs) possess not only the self-quenching-resistant feature in the solid state but also the extension of fluorescence band toward the wavelength as long as 645 nm. The realization of sulfur dot emission in the deep-red region with a decent yield (8.7%) has never been reported. Moreover, a super large Stokes shift (300 nm, λ_ex = 345 nm, λ_em = 645 nm) and a much longer decay lifetime (109 μs) have been found, and such values can facilitate to suppress the negative influence from background signals. Density functional theory demonstrates that the surface passivation via sodium ethoxide is dynamically favorable, and the spectroscopic insights into emission behavior could be derived from the passivation effect of the sulfur vacancy as well as the charge-transfer process dominated by the highly electronegative ethoxide layer.