High-Efficiency Electrochemical Hydrogen Evolution Based on Surface Autocatalytic Effect of Ultrathin 3C-SiC Nanocrystals

Good understanding of the reaction mechanism in the electrochemical reduction of water to hydrogen is crucial to renewable energy technologies. Although previous studies have revealed that the surface properties of materials affect the catalytic reactivity, the effects of a catalytic surface on the hydrogen evolution reaction (HER) on the molecular level are still not well understood. Contrary to general belief, water molecules do not adsorb onto the surfaces of 3C-SiC nanocrystals (NCs), but rather spontaneously dissociate via a surface autocatalytic process forming a complex consisting of −H and −OH fragments. In this study, we show that ultrathin 3C-SiC NCs possess superior electrocatalytic activity in the HER. This arises from the large reduction in the activation barrier on the NC surface enabling efficient dissociation of H₂O molecules. Furthermore, the ultrathin 3C-SiC NCs show enhanced HER activity in photoelectrochemical cells and are very promising to the water splitting based on the synergistic electrocatalytic and photoelectrochemical actions. This study provides a molecular-level understanding of the HER mechanism and reveals that NCs with surface autocatalytic effects can be used to split water with high efficiency thereby enabling renewable and economical production of hydrogen

Facet Cutting and Hydrogenation of In₂O₃ Nanowires for Enhanced Photoelectrochemical Water Splitting

Semiconductor nanowires (NWs) are useful building blocks in optoelectronic, sensing, and energy devices and one-dimensional NWs have been used in photoelectrochemical (PEC) water splitting because of the enhanced light absorption and charge transport. It has been theoretically predicted that the {001} facets of body center cubic (bcc) In₂O₃ nanocrystals can effectively accumulate photogenerated holes under illumination, but it is unclear whether facet cutting of NWs can enhance the efficiency of PEC water splitting. In this work, the photocurrent of square In₂O₃ NWs with four {001} facets is observed to be an order of magnitude larger than that of cylindrical In₂O₃ NWs under the same conditions and subsequent hydrogen treatment further promotes the PEC water splitting performance of the NWs. The optimized hydrogenated In₂O₃ NWs yield a photocurrent density of 1.2 mA/cm² at 0.22 V versus Ag/AgCl with a Faradaic efficiency of about 84.4%. The enhanced PEC properties can be attributed to the reduced band gap due to merging of the disordered layer-induced band tail states with the valence band as well as improved separation of the photogenerated electrons/holes between the In₂O₃ crystal core and disordered layer interface. The results provide experimental evidence of the important role of facet cutting, which is promising in the design and fabrication of NW-based photoelectric devices

Optical Identification of Topological Defect Types in Monolayer Arsenene by First-Principles Calculation

Recent theoretical research has demonstrated that a new two-dimensional material, the monolayer of gray arsenic (arsenene), can respond to the blue and ultraviolet light leading to possible optoelectronic applications. However, some topological defects often affect various properties of arsenene. Here we theoretically investigate the arsenene with monovacancy (MV), divacancy (DV), and Stone–Wales (SW) defects. Three kinds of MVs are identified and the reconstructed structures of DV and SW defects are confirmed. The dynamical stability, rearrangement, and migration for these defects are investigated in detail. Optical spectral calculations indicate that the MVs enhance optical transitions in the forbidden bands of arsenene and two new characteristic peaks appear in the dielectric and absorption spectra. However, there is only one new peak in the spectrum induced by DV and SW defects. Calculations of band structures indicate that the MV induces two defect bands in the forbidden bands of pristine arsenene, which are responsible for the two new peaks in the dielectric and absorption spectra. Our findings suggest that the optical dielectric and absorption spectra can help identify the types of topological defects in arsenene

Tunable Silver Nanocap Superlattice Arrays for Surface-Enhanced Raman Scattering

We report on a convenient nanotechnique to fabricate large-area silver nanocap superlattice arrays templated by the base of porous anodic alumina membranes as robust and cost-efficient surface-enhanced Raman scattering substrate. The topography can be tuned to optimize the enhancement factor by adjusting anode voltages or the time of silver magnetron sputtering. Our technique is especially promising considering their easy fabrication and evenly distributed plasmonic fields to cm-dimensions featuring high average enhancement factor, thereby boding well for application in the sensing device

Enhanced Photodegradation of Methyl Orange Synergistically by Microcrystal Facet Cutting and Flexible Electrically-Conducting Channels

By performing precise facet cutting during hydrothermal synthesis, single-morphological and uniform-sized octahedral and cubic Cu₂O microcrystals respectively with {111} and {100} facets are synthesized and subsequently encapsulated with reduced graphene oxide (rGO). Electrochemical impedance spectroscopy shows that the rGO/Cu₂O polyhedral composite has excellent conductivity, indicating that rGO can serve as a flexible electrically conducting channel. On account of the accumulation of a large amount of photoexcited electrons on the {111} facets of the octahedrons and efficient electron transfer to the rGO sheet, photodegradation of methyl orange by the rGO/Cu₂O octahedral composite is enhanced by a factor of 4 compared to both bare Cu₂O octahedrons and rGO-encapsulated cubes with hole accumulation on the {100} facets, and the stability of the rGO/Cu₂O octahedrons is obviously improved due to no direct touch with water molecules in comparison with Cu₂O microcrystals without rGO wrap reported previously. This work shows that the combination of crystal facet cutting and conducting channels is an effective strategy to design new composites with enhanced photocatalytic properties

A General and Facile Approach to Heterostructured Core/Shell BiVO₄/BiOI <i>p–n</i> Junction: Room-Temperature <i>in Situ</i> Assembly and Highly Boosted Visible-Light Photocatalysis

Development of core/shell heterostructures and semiconductor <i>p–n</i> junctions is of great concern for environmental and energy applications. Herein, we develop a facile <i>in situ</i> deposition route for fabrication of a BiVO₄/BiOI composite integrating both the core/shell heterostructure and semiconductor <i>p–n</i> junction at room temperature. In the BiVO₄/BiOI core/shell heterostructure, the BiOI nanosheets are evenly assembled on the surface of the BiVO₄ cores. The photocatalytic performance is evaluated by monitoring the degradation of the dye model Rhodamine B (RhB), colorless contaminant phenol, and photocurrent generation under visible-light irradiation. The heterostructured BiVO₄/BiOI core/shell photocatalyst shows drastically enhanced photocatalysis properties compared to the pristine BiVO₄ and BiOI. This remarkable enhancement is attributed to the intimate interfacial interactions derived from the core/shell heterostructure and formation of the <i>p–n </i>junction between the <i>p</i>-type BiOI and <i>n</i>-type BiVO₄. Separation and transfer of photogenerated electron–hole pairs are hence greatly facilitated, thereby resulting in the improved photocatalytic performance as confirmed by electrochemical, photoelectrochemical, radicals trapping, and superoxide radical (•O₂^–) quantification results. Moreover, the core/shell BiVO₄/BiOI also displays high photochemical stability. This work sheds new light on the construction of high-performance photocatalysts with core/shell heterostructures and matchable band structures in a simple and efficient way

Photothermal Contribution to Enhanced Photocatalytic Performance of Graphene-Based Nanocomposites

Photocatalysts possessing high efficiency in degrading aquatic organic pollutants are highly desirable. Although graphene-based nanocomposites exhibit excellent photocatalytic properties, the role of graphene has been largely underestimated. Herein, the photothermal effect of graphene-based nanocomposites is demonstrated to play an important role in the enhanced photocatalytic performance, which has not been considered previously. In our study on degradation of organic pollutants (methylene blue), the contribution of the photothermal effect caused by a nanocomposite consisting of P25 and reduced graphene oxide can be as high as ∼38% in addition to trapping and shuttling photogenerated electrons and increasing both light absorption and pollutant adsorptivity. The result reveals that the photothermal characteristic of graphene-based nanocomposite is vital to photocatalysis. It implies that designing graphene-based nanocomposites with the improved photothermal performance is a promising strategy to acquire highly efficient photocatalytic activity

Oxygen Vacancy Enhanced Gas-Sensing Performance of CeO₂/Graphene Heterostructure at Room Temperature

Oxygen vacancies (O_v) as the active sites have significant influences on the gas sensing performance of metal oxides, and self-doping of Ce³⁺ in CeO₂ might promote the formation of oxygen vacancies. In this work, hydrothermal process is adopted to fabricate the composites of graphene and CeO₂ nanoparticles, and the influences of oxygen vacancies as well as Ce³⁺ ions on the sensing response to NO₂ are studied. It is found that the sensitivity of the composites to NO₂ increases gradually, as the proportion of Ce³⁺ relative to all of the cerium ions is increased from 14.6% to 50.7% but decreases after that value. First-principles calculations illustrate that CeO₂ becomes metallic at the Ce³⁺ proportion of <50.7%, the chemical potential of electrons on surface decreases, and the Fermi level shifts upward due to the existence of low-electronegativity Ce³⁺ ions, resulting in reduced Schottky barrier height (SBH) at the CeO₂/graphene interface, enhanced interfacial charge transfer, and high gas sensing performance. However, deep energy level will be induced at the Ce³⁺ proportion of >50.7%, and the Fermi level is pinned at the interface. As a result, the density of free electrons is reduced, leading to increased SBH and poor gas sensing response. It demonstrates that an appropriate concentration of oxygen vacancies in CeO₂ is needed to enhance the gas sensing performance to NO₂

Synergistic WO₃·2H₂O Nanoplates/WS₂ Hybrid Catalysts for High-Efficiency Hydrogen Evolution

Tungsten trioxide dihydrate (WO₃·2H₂O) nanoplates are prepared by <i>in situ</i> anodic oxidation of tungsten disulfide (WS₂) film on carbon fiber paper (CFP). The WO₃·2H₂O/WS₂ hybrid catalyst exhibits excellent synergistic effects which facilitate the kinetics of the hydrogen evolution reaction (HER). The electrochromatic effect takes place via hydrogen intercalation into WO₃·2H₂O. This process is accelerated by the desirable proton diffusion coefficient in the layered WO₃·2H₂O. Hydrogen spillover from WO₃·2H₂O to WS₂ occurs via atomic polarization caused by the electric field of the charges on the planar defect or edge active sites of WS₂. The optimized hybrid catalyst presents a geometrical current density of 100 mA cm^–2 at 152 mV overpotential with a Tafel slope of ∼54 mV per decade, making the materials one of the most active nonprecious metal HER catalysts

Fabrication of Multiple Heterojunctions with Tunable Visible-Light-Active Photocatalytic Reactivity in BiOBr–BiOI Full-Range Composites Based on Microstructure Modulation and Band Structures

The fabrication of multiple heterojunctions with tunable photocatalytic reactivity in full-range BiOBr–BiOI composites based on microstructure modulation and band structures is demonstrated. The multiple heterojunctions are constructed by precipitation at room temperature and characterized systematically. Photocatalytic experiments indicate that there are two types of heterostructures with distinct photocatalytic mechanisms, both of which can greatly enhance the visible-light photocatalytic performance for the decomposition of organic pollutants and generation of photocurrent. The large separation and inhibited recombination of electron–hole pairs rendered by the heterostructures are confirmed by electrochemical impedance spectra (EIS) and photoluminescence (PL). Reactive species trapping, nitroblue tetrazolium (NBT, detection agent of ^•O₂^–) transformation, and terephthalic acid photoluminescence (TA-PL) experiments verify the charge-transfer mechanism derived from the two types of heterostructures, as well as different enhancements of the photocatalytic activity. This article provides insights into heterostructure photocatalysis and describes a novel way to design and fabricate high-performance semiconductor composites