Nontypical Wulff-Shape Silicon Nanosheets with High Catalytic Activity

Nanostructured silicon with an equilibrium shape has exhibited hydrogen evolution reaction activity mainly owing to its high surface area, which is distinct from that of bulk silicon. Such a Wulff shape of silicon favors low-surface-energy planes, resulting in silicon being an anisotropic and predictably faceted solid in which certain planes are favored, but this limits further improvement of the catalytic activity. Here, we introduce nanoporous silicon nanosheets that possess high-surface-energy crystal planes, leading to an unconventional Wulff shape that bolsters the catalytic activity. The high-index plane, uncommonly seen in the Wulff shape of bulk Si, has a band structure optimally aligned with the redox potential necessary for hydrogen generation, resulting in an apparent quantum yield (AQY) of 12.1% at a 400 nm wavelength. The enhanced light absorption in nanoporous silicon nanosheets also contributes to the high photocatalytic activity. Collectively, the strategy of making crystals with nontypical Wulff shapes can provide a route toward various classes of photocatalysts for hydrogen production.

Solvent-assisted preparation of low-temperature SnO2 electron transport layers for efficient and stable perovskite solar cells made in ambient conditions

High-quality SnO2 electron transport layers were synthesized by a simple low-temperature solvothermal method.

Fabrication of large size individual octahedral tungsten oxide hydrate and Au NPs as SERS platforms for sensitive detection of cytochrome C

Surface-enhanced Raman scattering (SERS) has attracted much attention with its powerful trace detection and analysis capabilities, especially biological and environmental molecules. However, building a protein SERS detection platform based on semiconductor devices is a huge challenge. Herein, through the synergy of NH₃ and nickel foam, a large-sized semiconductor tungsten oxide hydrate platform (WOHP) was synthesized. The crystal plane of a single WOHP particle is larger than the excitation spot. As a SERS substrate, WOHP can make full use of the excitation light without destroying the structure during the protein molecules detection process. Through the synergy of WOHP and Au NPs, the enhancement factor is 1.5 × 10⁴. Raman peaks of WOHP can be used as references for the detection of typical protein cytochrome C (Cyt C). As the Cyt C concentration decreases, the I_{Cyt C}/I_WOHP ratio decreases, and the signal can still be obtained when the concentration is as low as 5 × 10^-9 mol L^-1. More importantly, the method does not affect the catalytic activity of Cyt C and can be applied to the detection of Cyt C concentration in serum.

Understanding the Structural and Electronic Properties of Photoactive Tungsten Oxide Nanoparticles from Density Functional Theory and GW Approaches

Tungsten trioxide (WO₃)-derived nanostructures have emerged recently as feasible semiconductors for photocatalytic purposes due to their visible-light harvesting that overcomes the drawbacks presented by TiO₂-derived nanoparticles (NPs). However, applications are still limited by the lack of fundamental knowledge at the nanoscale due to poor understanding of the physical processes that affect their photoactivity. To fill this gap, we report here a detailed computational study using a combined density functional theory (DFT)-GW scheme to investigate the electronic structure of realistic WO₃ NPs containing up to 1680 atoms. Different phases and morphologies are considered to provide reliable structure-property relationships. Upon proper benchmark of our DFT-GW methodology, we use this highly accurate approach to establish relevant rules for the design of photoactive WO₃ nanostructures by pointing out the most stable morphologies at the nanoscale and the appropriate size regime at which the photoactive efficiency is enhanced.

Titanium Oxide (TiO2) Nanoparticles for Treatment of Wound Infection

Wound infections is one of the major problems worldwide. Millions of people around the world require several medical treatments for wound infections. The extensive use of antibiotics to treat wound infection leads to emerging new microbial strains that are resistant to many antibiotics. There is a growing concern on the emergence and re-emergence of drug-resistant pathogens such as multi-resistant bacterial strains. Hence, the development of new antimicrobial compounds or the modification of those that already exist to improve antibacterial activity is a high research priority. Metallic nanoparticles (NPs) are considered as new alternative treatment for wound infection with superior antibacterial activity. In this study, new formulation of titanium oxide (TiO2) NPs with different sizes were synthesized and characterized. Genotoxicity, mutagenicity and antibacterial activities of TiO2 NPs against the causative agents of wound infection were investigated. Antibacterial activity of TiO2 NPs was conducted against three ATCC® bacterial strains: methicillin-resistant Staphylococcus aureus (MRSA), Escherichia coli and Pseudomonas aeruginosa. The results clearly illustrate a superior antibacterial activity of all newly formulated TiO2 NPs against the most causative agents of wound infection. Most of our TiO2 NPs showed non-genotoxic and non-mutagenic results at the maximum concentrations. Findings of this study will enhance the future of the therapeutic strategies against the resistant pathogenic strains that cause wound infections.

Polyethylenimine modified potassium tungsten oxide adsorbent for highly efficient Ag+ removal and valuable Ag0 recovery

Elemental Ag⁰ is well known for its remarkable catalytic and antibacterial properties, thus the regeneration of valuable Ag⁰ metal from Ag⁺ wastewater is of great significance. In this study, a novel polyethylenimine (PEI) modified potassium tungsten oxide (N-K₂W₄O₁₃) adsorbent was prepared for Ag⁺ removal and reduction to Ag⁰ using glutaraldehyde as crosslinking agent. XPS and FT-IR spectra verified PEI successfully anchored on the surface O and W atoms of K₂W₄O₁₃ through aldehyde bridges. The content of PEI in N-K₂W₄O₁₃ was calculated as 8.74wt% by TG curve. A heterogeneous PEI coating was observed in the SEM and TEM images. The N-K₂W₄O₁₃ exhibited larger Ag⁺ uptake (48.25mg/g) than the raw K₂W₄O₁₃ (42.50mg/g) though required a longer equilibrium time. This was due to the combined results of strong chelation and weak electrostatic repulsion that meanwhile occurring on the positive-charged surface of N-K₂W₄O₁₃. The maximum Ag⁺ uptake on N-K₂W₄O₁₃ was 72.5mg/g, which was larger than many of the reported adsorbents. Furthermore, the prepared N-K₂W₄O₁₃ displayed good anti-interference toward background ions (Na⁺, K⁺) and hold a stable Ag⁺ removal (>95%) after five runs of recycling tests. The mechanism studies elucidated that NH/N groups from the PEI modified N-K₂W₄O₁₃ mainly accounted for the Ag⁺ adsorption and Ag⁰ recovery in the adsorption-reduction process. Ion-exchange between Ag⁺ and K⁺ from the N-K₂W₄O₁₃ lattice also occurred. This work provided a facile method to synthesize a promising adsorbent for Ag⁺ wastewater remediation and valuable Ag⁰ recovery.

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies

Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.

Tuning the micromorphology and exposed facets of MnOx promotes methyl ethyl ketone low-temperature abatement: boosting oxygen activation and electron transmission

MnO_x nanowires with highly exposed {101} facets of Mn₃O₄ possess excellent low-temperature activity and stability for methyl ethyl ketone destruction.

A novel composite of W18O49 nanorods on reduced graphene oxide sheets based on in situ synthesis and catalytic performance for oxygen reduction reaction

A novel composite catalyst based on in situ synthesis of W₁₈O₄₉ nanorods on reduced graphene oxide sheets was successfully fabricated through a one-pot solvothermal route.

Tungsten Oxide Materials for Optoelectronic Applications

Tungsten oxide is a versatile transition-metal oxide with a vast number of polymorphs and sub-stoichiometric compositions, featuring innate tunnels and oxygen vacancies. The structure-determined nature, such as altered optical absorption and metal-like conductivity, makes tungsten oxide an attractive candidate for optoelectronic applications. A brief summary of the recent progress in tungsten oxide for optoelectronic applications is provided, including not only the traditional field of electrochromism/photochromism, but also new areas of application, such as visible-light-driven photocatalysis, photothermal therapy, and surface enhanced Raman spectroscopy (SERS). Also, the prospects for future applications of tungsten oxide are summarized and highlighted.

Monoclinic Tungsten Oxide with {100} Facet Orientation and Tuned Electronic Band Structure for Enhanced Photocatalytic Oxidations

Exploring surface-exposed highly active crystal facets for photocatalytic oxidations is promising in utilizing monoclinic WO3 semiconductor. However, the previously reported highly active facets for monoclinic WO3 were mainly toward enhancing photocatalytic reductions. Here we report that the WO3 with {100} facet orientation and tuned surface electronic band structure can effectively enhance photocatalytic oxidation properties. The {100} faceted WO3 single crystals are synthesized via a facile hydrothermal method. The UV-visible diffuse reflectance, X-ray photoelectron spectroscopy valence band spectra, and photoelectrochemical measurements suggest that the {100} faceted WO3 has a much higher energy level of valence band maximum compared with the normal WO3 crystals without preferred orientation of the crystal face. The density functional theory calculations reveal that the shift of O 2p and W 5d states in {100} face induce a unique band structure. In comparison with the normal WO3, the {100} faceted WO3 exhibits an O2 evolution rate about 5.1 times in water splitting, and also shows an acetone evolution rate of 4.2 times as well as CO2 evolution rate of 3.8 times in gaseous degradation of 2-propanol. This study demonstrates an efficient crystal face engineering route to tune the surface electronic band structure for enhanced photocatalytic oxidations.

Faceted metal and metal oxide nanoparticles: design, fabrication and catalysis

The review addresses new advances in metal, bimetallic, metal oxide, and composite particles in their nanoregime for facet-selective catalytic applications. The synthesis and growth mechanisms of the particles have been summarized in brief in this review with a view to develop critical examination of the faceted morphology of the particles for catalysis. The size, shape and composition of the particles have been found to be largely irrelevant in comparison to the nature of facets in catalysis. Thus selective high- and low-index facets have been found to selectively promote adsorption, which eventually leads to an effective catalytic reaction. As a consequence, a high density of atoms rest at the corners, steps, stages, kinks etc on the catalyst surface in order to host the adsorbate efficiently and catalyze the reaction. Again, surface atomic arrangement and bond length have been found to play a dominant role in adsorption, leading to effective catalysis.

Dopant-controlled morphology evolution of WO3 polyhedra synthesized by RF thermal plasma and their sensing properties

In this paper, a simple way is developed for the synthesis of Cr-doped WO3 polyhedra controlled by tailoring intrinsic thermodynamic properties in RF thermal plasma. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy are used to characterize the detail structures and surface/near-surface chemical compositions of the as-prepared products. Kinetic factors showed little effects on the equilibrium morphology of Cr-doped WO3 polyhedra, while equilibrium morphologies of WO3 polyhedra can be controlled by the thermodynamic factor (Cr doping). Set crystal growth habits of pure WO3 as an initial condition, coeffects of distortions introduced by Cr into the WO3 matrix, and a chromate layer on the crystal surface could reduce the growth rates along [001], [010], and [100] directions. The morphology evolution was turning out as the following order with increasing Cr dopants: octahedron-truncated octahedron-cuboid. 2.5 at. % Cr-doped WO3 polyhedra exhibit the highest sensing response due to coeffects of exposed crystal facets, activation energy, catalytic effects of Cr, and particle size on the surface reaction and electron transport units. By simply decorating Au on Cr-doped WO3 polyhedra, the sensing responses, detection limit, and response-recovery properties were significantly improved.

Tuning of the crystal engineering and photoelectrochemical properties of crystalline tungsten oxide for optoelectronic device applications

WO₃crystals with {002} or {111} facets primarily exposed, WO₃films with dominant orientations, doping and heterostructuring are highlighted.

A comprehensive review on synthesis methods for transition-metal oxide nanostructures

Recent developments of transition-metal oxide nanostructures with designed shape and dimensionality, including various synthesis methods and applications, are presented.

Exposed facet and crystal phase tuning of hierarchical tungsten oxide nanostructures and their enhanced visible-light-driven photocatalytic performance

Tungsten oxide hierarchical nanostructures controllably assembled with one dimensional nanostructures which exhibit different exposed facets and crystal phases were synthesized via a facile hydrothermal reaction assisted by urea.

Crystal facet tailoring arts in perovskite oxides

This review highlights various facet tailoring arts in perovskite structure oxides.

Morphological evolution and visible light driven degradation of tetracycline by Bi3.84W0.16O6.24 nanostructures

In this work, two morphologies of Bi_3.84W_0.16O_6.24 nanostructures, namely the nanobelt (T2) and nanooctahedral structure (T15), were synthesized via a simple, microwave-assisted method.

Photocatalytic O2 production using WO3 nanoparticles prepared by annealing ethylenediamine tungstate/CxNy gel

WO₃ nanoparticles have extraordinary potential applications in water oxidation and degradation of organic pollutes.

The Facile Hydrothermal Preparation of WO3 and its Photocatalytic Performance under Visible-Light Irradiation

The orthorhombic WO3 nanoplates were fabricated via a hydrothermal process, using HBF4 as the acid source. They were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV-vis diffused reflectance spectroscopy (UV-Vis DRS), respectively. The WO3 nanoplates exhibited nice UV-light driven photocatalytic performance in the degradation of Rhodamine B (RhB).

Observing the role of graphene in boosting the two-electron reduction of oxygen in graphene-WO₃ nanorod photocatalysts

The new role of graphene (GR) in boosting the two-electron reduction of O2 to H2O2 has been first identified in the GR-WO3 nanorod (NR) nanocomposite photocatalysts, which are fabricated by a facile, solid electrostatic self-assembly strategy to integrate the positively charged branched poly(ethylenimine) (BPEI)-GR (BGR) and negatively charged WO3 NRs at room temperature. Photoactivity test shows that, as compared to WO3 NRs, BGR-WO3 NRs with an appropriate addition ratio of GR exhibit remarkably enhanced and stable visible-light photoactivity toward the degradation of Rhodamine B. Besides the common roles of GR observed in the GR-based composite photocatalysts in the literature, including enhancing the visible-light absorption intensity, improving the lifetime and transfer of photogenerated charge carriers, and increasing the adsorption capacity for reactants, we have observed the new role of GR in boosting the two-electron reduction of O2 to H2O2 in this specific BGR-WO3 NR photocatalyst system. Importantly, this new role of GR does contribute to the overall photoactivity enhancement of BGR-WO3 NR nanocomposites. The synergistic contribution of GR on improving the photoactivity of WO3 NRs and the underlying reaction mechanism have been analyzed by the structure-photoactivity correlation analysis and controlled experiments using radicals scavengers.

Semiconductor photocatalysts for water oxidation: current status and challenges

Artificial photosynthesis is a highly-promising strategy to convert solar energy into hydrogen energy for the relief of the global energy crisis. Water oxidation is the bottleneck for its kinetic and energetic complexity in the further enhancement of the overall efficiency of the artificial photosystem. Developing efficient and cost-effective photocatalysts for water oxidation is a growing desire, and semiconductor photocatalysts have recently attracted more attention due to their stability and simplicity. This article reviews the recent advancement of semiconductor photocatalysts with a focus on the relationship between material optimization and water oxidation efficiency. A brief introduction to artificial photosynthesis and water oxidation is given first, followed by an explanation of the basic rules and mechanisms of semiconductor particulate photocatalysts for water oxidation as theoretical references for discussions of componential, surface structure, and crystal structure modification. O2-evolving photocatalysts in Z-scheme systems are also introduced to demonstrate practical applications of water oxidation photocatalysts in artificial photosystems. The final part proposes some challenges based on the dynamics and energetics of photoholes which are fundamental to the enhancement of water oxidation efficiency, as well as on the simulation of natural water oxidation that will be a trend in future research.

Advanced chemical compositions and nanoarchitectures of bismuth based complex oxides for solar photocatalytic application

Bismuth based complex oxide photocatalysts with high activity and stability could be obtained by engineering chemical compositions, morphologies, and microstructures.

Colloidal semiconductor nanocrystals: controlled synthesis and surface chemistry in organic media

State-of-the-art development of the controlled synthesis of colloidal semiconductor nanocrystals has been discussed in this review article.

Room-temperature synthesis of zinc oxide nanoparticles in different media and their application in cyanide photodegradation

Cyanide is an extreme hazard and extensively found in the wastes of refinery, coke plant, and metal plating industries. A simple, fast, cost-effective, room-temperature wet chemical route, based on cyclohexylamine, for synthesizing zinc oxide nanoparticles in aqueous and enthanolic media was established and tested for the photodegradation of cyanide ions. Particles of polyhedra morphology were obtained for zinc oxide, prepared in ethanol (ZnOE), while spherical and some chunky particles were observed for zinc oxide, prepared in water (ZnOW). The morphology was crucial in enhancing the cyanide ion photocatalytic degradation efficiency of ZnOE by a factor of 1.5 in comparison to the efficiency of ZnOW at an equivalent concentration of 0.02 wt.% ZnO. Increasing the concentration wt.% of ZnOE from 0.01 to 0.09 led to an increase in the photocatalytic degradation efficiency from 85% to almost 100% after 180 min and a doubling of the first-order rate constant (k).

Pseudocapacitive hausmannite nanoparticles with (101) facets: synthesis, characterization, and charge-transfer mechanism

Hausmannite Mn3 O4 octahedral nanoparticles of 18.3 ± 7.0 nm with (101) facets have been prepared by an oxygen-mediated growth. The electrochemical properties of the Mn3 O4 particles as pseudocapacitive cathode materials were characterized both in half-cells and in button-cells. The Mn3 O4 nanoparticles exhibited a high mass-specific capacitance of 261 F g(-1), which was calculated from cyclic voltammetry analyses, and a capacitive retention of 78% after 10,000 galvanostatic charge-discharge cycles. The charge-transfer mechanisms of the Mn3 O4 nanoparticles were further studied by using synchrotron-based in situ X-ray absorption near edge spectroscopy and XRD. Both measurements showed concurrently that throughout the potential window of 0-1.2 V (vs. Ag/AgCl), a stable spinel structure of Mn3 O4 remained, and a reversible electrochemical conversion between tetrahedral [Mn(II) O4 ] and octahedral [Mn(III) O6 ] units accounted for the redox activity. Density functional theory calculations further corroborated this mechanism by confirming the enhanced redox stability afforded by the abundant and exposed (101) facets of Mn3 O4 octahedra.

Well shaped Mn₃O₄ nano-octahedra with anomalous magnetic behavior and enhanced photodecomposition properties

Very uniform and well shaped Mn₃O₄ nano-octahedra are synthesized using a simple hydrothermal method under the help of polyethylene glycol (PEG200) as a reductant and shape-directing agent. The nano-octahedra formation mechanism is monitored. The shape and crystal orientation of the nanoparticles is reconstructed by scanning electron microscopy and electron tomography, which reveals that the nano-octahedra only selectively expose {101} facets at the external surfaces. The magnetic testing demonstrates that the Mn₃O₄ nano-octahedra exhibit anomalous magnetic properties: the Mn₃O₄ nano-octahedra around 150 nm show a similar Curie temperature and blocking temperature to Mn₃O₄ nanoparticles with 10 nm size because of the vertical axis of [001] plane and the exposed {101} facets. With these Mn₃O₄ nano-octahedra as a catalyst, the photodecomposition of rhodamine B is evaluated and it is found that the photodecomposition activity of Mn₃O₄ nano-octahedra is much superior to that of commercial Mn₃O₄ powders. The anomalous magnetic properties and high superior photodecomposition activity of well shaped Mn₃O₄ nano-octahedra should be related to the special shape of the nanoparticles and the abundantly exposed {101} facets at the external surfaces. Therefore, the shape preference can largely broaden the application of the Mn₃O₄ nano-octahedra.