Chromium Oxide-modified Mesoporous Zirconium Dioxide: Efficient Heterogeneous Catalyst for the Synthesis of 5-Hydroxymethylfurfural.

A highly efficient carbohydrate conversion to 5-hydroxymethylfurfural (5-HMF) is a promising method to achieve green and sustainable development. However, most currently reported strategies are energy consuming, and the 5-HMF yield is relatively lower in the aqueous phase. Herein, a facile method is reported to obtain effective Cr/ZrO2 catalysts with high acidity and their catalytic performances were investigated for catalyzing fructose to 5-HMF at different temperatures and times. With the catalysis of 15 % Cr/ZrO2 catalyst, the highest fructose conversion of 98 %, 5-HMF yield of 48.8 %, and 5-HMF selectivity of 49.8 % are achieved in green solvent with good recyclability. The possible reaction process of the improved catalysis performance is attributed to the highly crystalline and strong acidity of the Cr/ZrO2 catalyst. The Lewis acid sites could increase the overall rate of fructose conversion by promoting side reactions and might suppress fructose to glucose isomerization. In addition, Cr leakage during the reaction might act as the Bronsted acids to catalyze the fructose dehydration to 5-HMF. The reported method of introducing chromium oxides into ZrO2 catalyst will open a new avenue to promote the practical application of biomass and sustainable development in the future.

Two-Dimensional Transition Metal Oxide and Hydroxide-Based Hierarchical Architectures for Advanced Supercapacitor Materials.

The supercapacitor has been widely seen as one of the most promising emerging energy storage devices, by which electricity is converted from chemical energy and stored. Two-dimensional (2D) metal oxides/hydroxides (TMOs/TMHs) are revolutionizing the design of high-performance supercapacitors because of their high theoretical specific capacitance, abundance of electrochemically active sites, and feasibility for assembly in hierarchical structures by integrating with graphitic carbon, conductive polymers, and so on. The hierarchical structures achieved can not only overcome the limitations of using a single material but also bring new breakthroughs in performance. In this article, the research progress on 2D TMOs/TMHs and their use in hierarchical structures as supercapacitor materials are reviewed, including the evolution of supercapacitor materials, the configurations of hierarchical structures, the electrical properties regulated, and the existence of advantages and drawbacks. Finally, a perspective covering directions and challenges related to the development of supercapacitor materials is provided.

Manganese-Modulated Cobalt-Based Layered Double Hydroxide Grown on Nickel Foam with 1D-2D-3D Heterostructure for Highly Efficient Oxygen Evolution Reaction and Urea Oxidation Reaction.

Hydrogen production by energy-efficient water electrolysis is a green avenue for the development of contemporary society. However, the oxygen evolution reaction (OER) and the urea oxidation reaction (UOR) occurring at the anode are impeded by the sluggish reaction kinetics during the water-splitting process. Consequently, it is promising to develop bifunctional anodic electrocatalysts consisting of nonprecious metals. Herein, a bifunctional CoMn layered double hydroxide (LDH) was grown on nickel foam (NF) with a 1D-2D-3D hierarchical structure for efficient OER and UOR performance in alkaline solution. Owing to the significant synergistic effect of Mn doping and heterostructure engineering, the obtained Co1 Mn1 LDH/NF exhibits satisfactory OER activity with a low potential of 1.515 V to attain 10 mA cm-2 . Besides, the potential of the Co1 Mn1 LDH/NF catalyst for UOR at the same current density is only 1.326 V, which is much lower than those of its counterparts and most reported electrocatalysts. An urea electrolytic cell with a Co1 Mn1 LDH/NF anode and a Pt-C/NF cathode was established, and a low cell voltage of 1.354 V at 10 mA cm-2 was acquired. The optimized strategy may result in promising candidates for developing a new generation of bifunctional electrocatalysts for clean energy production.

Two-dimensional ultrathin BiOCl nanosheet/graphene heterojunction with enhanced photocatalytic activity.

2D semiconductors and their heterostructures promise great potential in solar-driven photocatalysis owing to their unique properties that result from structural planar confinement. Herein, we fabricate the BiOCl nanosheet/graphene hybrid with 2D heterojunction through a hydrothermal and physical mixing two-step process. Studies on the heterostructure reveal the close interfacial contact between the BiOCl nanosheet and graphene through van der Waals interaction rather than chemical bonding. Compared with BiOCl-bulk/graphene hybrid, the BiOCl nanosheet/graphene hybrid showed superior photocatalytic activity towards dye decolorization and decomposition, featuring a remarkable enhancement of a 15-fold higher reaction rate. A series of characterizations reveal that the co-effect of the surface defect of the BiOCl nanosheet and graphene planar structure contributes to an extended light absorption range, better electrical conductivity and larger dye adsorption capacity. Furthermore, the unique van der Waals close contact between graphene and BiOCl promoted the fast and continuous consumption of photogenerated electrons, thereby effectively facilitating the separation of electron-hole pairs. This work underscores the key role of face-to-face heterojunction for the interfacial charge transfer process.

A ternary cobalt-molybdenum-vanadium layered double hydroxide nanosheet array as an efficient bifunctional electrocatalyst for overall water splitting.

Active CoMoV LDH nanosheet arrays grown on nickel foam were obtained for the first time. The doping of high-valence Mo and V in the Co-based LDH could tune the electronic structure and keep Co in a high-valence state for the OER, and high-valence Mo could also act as the active site for the HER, thus obtaining superior bifunctional water splitting performance.

Nix Co3-x O4 Nanoneedle Arrays Grown on Ni Foam as an Efficient Bifunctional Electrocatalyst for Full Water Splitting.

Developing highly active, stable and robust electrocatalysts based on earth-abundant elements for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is important for many renewable energy conversion processes. Herein, Nix Co3-x O4 nanoneedle arrays grown on 3D porous nickel foam (NF) was synthesized as a bifunctional electrocatalyst with OER and HER activity for full water splitting. Benefiting from the advantageous structure, the composite exhibits superior OER activity with an overpotential of 320 mV achieving the current density of 10 mA cm-2 . An exceptional HER activity is also acquired with an overpotential of 170 mV at the current density of 10 mA cm-2 . Furthermore, the catalyst also shows the superior activity and stability for 20 h when used in the overall water splitting cell. Thus, the hierarchical 3D structure composed of the 1D nanoneedle structure in Nix Co3-x O4 /NF represents an avenue to design and develop highly active and bifunctional electrocatalysts for promising energy conversion.

Insights into a rutile/brookite homojunction of titanium dioxide: separated reactive sites and boosted photocatalytic activity.

Benefiting from studies into Degussa TiO2, forming junctions via combining different phases of a semiconductor may provide new insights into the design of efficient photocatalysts, which are a key element in current solar-driven fuel production and environmental remediation. In this work, we aimed at creating a highly efficient rutile/brookite homojunction through precise crystal phase control. Characterization of the morphology and structure revealed that the ultrafine brookite phase TiO2 particles were uniformly attached to the surfaces of the rod-like rutile phase, not only readily forming a homojunction but also stabilizing the brookite phase. Surprisingly, the rutile/brookite-TiO2 homojunction exhibited a synergetic effect, improving the photocatalytic activity for both hydrogen generation and organic dye degradation. This was attributed to the well-matched band alignment and separated reaction sites, effectively promoting the charge separation efficiency. These results highlight the potential for bifunctional photocatalyst design with separated reactive sites for simultaneous redox reactions.

Nickel-cobalt-layered double hydroxide nanosheet arrays on Ni foam as a bifunctional electrocatalyst for overall water splitting.

Bifunctional electrocatalysts, which are active both for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), can increase the efficiency of overall water splitting, which is considered an attractive technology for hydrogen production. In this study, we report NiCo-LDH ultrathin nanosheets grown on nickel foam as a bifunctional electrocatalyst for overall water splitting. When used as OER and HER electrodes, NiCo-LDH/NF demonstrated excellent water splitting performance, achieving a current density of 10 mA cm-2 at 1.66 V. Thus, the synthesis of hierarchical 3D porous structure composed of 2D ultrathin nanosheets in NiCo-LDH/NF demonstrated the significance of structural features for achieving a high electrocatalytic activity in overall water splitting.

The dominant {001} facet-dependent enhanced visible-light photoactivity of ultrathin BiOBr nanosheets.

The ability to suppress the recombination of the photoinduced charges is the key prerequisite for an excellent photocatalyst, which has attracted extensive and continuous interest in the field of photocatalysis. Herein, we presented a convenient strategy for the one-step selective synthesis of ultrathin BiOBr nanosheets with atomic thickness through a simple solvothermal method. These ultrathin BiOBr nanosheets not only show high exposure percentage of active (001) facets but also have an optimized band structure, which synergistically facilitates the electron-hole pair separation to realize significantly promoted visible-light photocatalytic activity. Our results provide a new avenue and direction for the design of photocatalysts with high visible-light photocatalytic performance.

Photoelectrodes based upon Mo:BiVO4 inverse opals for photoelectrochemical water splitting.

BiVO4 has been regarded as a promising material for photoelectrochemical water splitting, but it suffers from a major challenge on charge collection and utilization. In order to meet this challenge, we design a nanoengineered three-dimensional (3D) ordered macro-mesoporous architecture (a kind of inverse opal) of Mo:BiVO4 through a controllable colloidal crystal template method with the help of a sandwich solution infiltration method and adjustable post-heating time. Within expectation, a superior photocurrent density is achieved in return for this design. This enhancement originates primarily from effective charge collection and utilization according to the analysis of electrochemical impedance spectroscopy and so on. All the results highlight the great significance of the 3D ordered macro-mesoporous architecture as a promising photoelectrode model for the application in solar conversion. The cooperating amplification effects of nanoengineering from composition regulation and morphology innovation are helpful for creating more purpose-designed photoelectrodes with highly efficient performance.

High-performance flexible electrochromic device based on facile semiconductor-to-metal transition realized by WO3·2H2O ultrathin nanosheets.

Ultrathin nanosheets are considered as one kind of the most promising candidates for the fabrication of flexible electrochromic devices (ECDs) due to their permeable channels, high specific surface areas, and good contact with the substrate. Herein, we first report the synthesis of large-area nanosheets of tungsten oxide dihydrate (WO3·2H2O) with a thickness of only about 1.4 nm, showing much higher Li(+) diffusion coefficients than those of the bulk counterpart. The WO3·2H2O ultrathin nanosheets are successfully assembled into the electrode of flexible electrochromic device, which exhibits wide optical modulation, fast color-switching speed, high coloration efficiency, good cyclic stability and excellent flexibility. Moreover, the electrochromic mechanism of WO3·2H2O is further investigated by first-principle density functional theory (DFT) calculations, in which the relationship between structural features of ultrathin nanosheets and coloration/bleaching response speed is revealed.

Vacancy associates promoting solar-driven photocatalytic activity of ultrathin bismuth oxychloride nanosheets.

Crystal facet engineering of semiconductors is of growing interest and an important strategy for fine-tuning solar-driven photocatalytic activity. However, the primary factor in the exposed active facets that determines the photocatalytic property is still elusive. Herein, we have experimentally achieved high solar photocatalytic activity in ultrathin BiOCl nanosheets with almost fully exposed active {001} facets and provide some new and deep-seated insights into how the defects in the exposed active facets affect the solar-driven photocatalytic property. As the thickness of the nanosheets reduces to atomic scale, the predominant defects change from isolated defects V(Bi)‴ to triple vacancy associates V(Bi)‴V(O)••V(Bi)‴, which is unambiguously confirmed by the positron annihilation spectra. By virtue of the synergic advantages of enhanced adsorption capability, effective separation of electron­hole pairs and more reductive photoexcited electrons benefited from the V(Bi)‴V(O)••V(Bi)‴ vacancy associates, the ultrathin BiOCl nanosheets show significantly promoted solar-driven photocatalytic activity, even with extremely low photocatalyst loading. The finding of the existence of distinct defects (different from those in bulks) in ultrathin nanosheets undoubtedly leads to new possibilities for photocatalyst design using quasi-two-dimensional materials with high solar-driven photocatalytic activity.

Controlled synthesis of olive-shaped Bi2S3/BiVO4 microspheres through a limited chemical conversion route and enhanced visible-light-responding photocatalytic activity.

Well-defined olive-shaped Bi(2)S(3)/BiVO(4) microspheres were synthesized through a limited chemical conversion route (LCCR), where olive-shaped BiVO(4) microspheres and thioacetamide (TAA) were used as precursors and sulfur source, respectively. The as-synthesized products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high-resolution transmission microscope (HRTEM), X-ray photoelectron spectra (XPS), UV-visible diffuse-reflectance spectroscopy (UV-vis DRS), and photoluminescence (PL) spectra in detail. Compared with pure BiVO(4) microspheres and Bi(2)S(3) nanorods, the Bi(2)S(3)/BiVO(4) products showed obviously enhanced photocatalytic activity for the degradation of rhodamine B (Rh B) in aqueous solution under visible-light irradiation (λ > 400 nm). In addition, the Bi(2)S(3)/BiVO(4) composite microspheres showed good visible-light-driven photocatalytic activity for the degradation of refractory oxytetracycline (OTC) as well. On the basis of UV-vis DRS, the calculated energy band positions, and PL spectra, the mechanism of enhanced photocatalytic activity of Bi(2)S(3)/BiVO(4) was proposed. The present study provides a new strategy to design composite materials with enhanced photocatalytic performance.

From hollow olive-shaped BiVO4 to n-p core-shell BiVO4@Bi2O3 microspheres: controlled synthesis and enhanced visible-light-responsive photocatalytic properties.

In this study, hollow olive-shaped BiVO(4) and n-p core-shell BiVO(4)@Bi(2)O(3) microspheres were synthesized by a novel sodium bis(2-ethylhexyl)sulfosuccinate (AOT)-assisted mixed solvothermal route and a thermal solution of NaOH etching process under hydrothermal conditions for the first time, respectively. The as-obtained products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy, Brunauer-Emmett-Teller surface area, and UV-vis diffuse-reflectance spectroscopy in detail. The influence of AOT and solvent ratios on the final products was studied. On the basis of SEM observations and XRD analyses of the samples synthesized at different reaction stages, the formation mechanism of hollow olive-shaped BiVO(4) microspheres was proposed. The photocatalytic activities of hollow olive-shaped BiVO(4) and core-shell BiVO(4)@Bi(2)O(3) microspheres were evaluated on the degradation of rhodamine B under visible-light irradiation (λ > 400 nm). The results indicated that core-shell BiVO(4)@Bi(2)O(3) exhibited much higher photocatalytic activities than pure olive-shaped BiVO(4). The mechanism of enhanced photocatalytic activity of core-shell BiVO(4)@Bi(2)O(3) microspheres was discussed on the basis of the calculated energy band positions as well. The present study provides a new strategy to enhancing the photocatalytic activity of visible-light-responsive Bi-based photocatalysts by p-n heterojunction.