Zhengxiao Guo

Visible-light driven heterojunction photocatalysts for water splitting – a critical review

Solar driven catalysis on semiconductors to produce clean chemical fuels, such as hydrogen, is widely considered as a promising route to mitigate environmental issues caused by the combustion of fossil fuels and to meet increasing worldwide demands for energy. The major limiting factors affecting the efficiency of solar fuel synthesis include; (i) light absorption, (ii) charge separation and transport and (iii) surface chemical reaction; therefore substantial efforts have been put into solving these problems. In particular, the loading of co-catalysts or secondary semiconductors that can act as either electron or hole acceptors for improved charge separation is a promising strategy, leading to the adaptation of a junction architecture. Research related to semiconductor junction photocatalysts has developed very rapidly and there are a few comprehensive reviews in which the strategy is discussed (A. Kudo and Y. Miseki, Chemical Society Reviews, 2009, 38, 253–278, K. Li, D. Martin, and J. Tang, Chinese Journal of Catalysis, 2011, 32, 879–890, R. Marschall, Advanced Functional Materials, 2014, 24, 2421–2440). This critical review seeks to give an overview of the concept of heterojunction construction and more importantly, the current state-of-the art for the efficient, visible-light driven junction water splitting photo(electro)catalysts reported over the past ten years. For water splitting, these include BiVO4, Fe2O3, Cu2O and C3N4, which have attracted increasing attention. Experimental observations of the proposed charge transfer mechanism across the semiconductor/semiconductor/metal junctions and the resultant activity enhancement are discussed. In parallel, recent successes in the theoretical modelling of semiconductor electronic structures at interfaces and how these explain the functionality of the junction structures is highlighted.

Highly Efficient Photocatalytic H2 Evolution from Water using Visible Light and Structure‐Controlled Graphitic Carbon Nitride

The major challenge of photocatalytic water splitting, the prototypical reaction for the direct production of hydrogen by using solar energy, is to develop low-cost yet highly efficient and stable semiconductor photocatalysts. Herein, an effective strategy for synthesizing extremely active graphitic carbon nitride (g-C3N4) from a low-cost precursor, urea, is reported. The g-C3N4 exhibits an extraordinary hydrogen-evolution rate (ca. 20 000 μmol h−1 g−1 under full arc), which leads to a high turnover number (TON) of over 641 after 6 h. The reaction proceeds for more than 30 h without activity loss and results in an internal quantum yield of 26.5 % under visible light, which is nearly an order of magnitude higher than that observed for any other existing g-C3N4 photocatalysts. Furthermore, it was found by experimental analysis and DFT calculations that as the degree of polymerization increases and the proton concentration decreases, the hydrogen-evolution rate is significantly enhanced.

Mechanical alloying and electronic simulations of (MgH2+M) systems (M=Al, Ti, Fe, Ni, Cu and Nb) for hydrogen storage

Abstract Mg-based alloys are promising candidates for hydrogen storage applications. Here, mechanical alloying (MA) was used to process powder mixtures of MgH2 with 8 mol % M (M=Al, Ti, Fe, Ni, Cu and Nb) in order to modify hydrogen storage properties of the Mg hydride. Electronic simulations of the systems were carried out to clarify the mechanisms of the alloy effects. X-ray diffraction (XRD) of the milled samples revealed the formation of new phases: a bcc solid solution phase for the (MgH2+Nb) mixture; TiH2 phase for the (MgH2+Ti); and MgCu2 phase for the (MgH2+Cu). For all the mixtures, a high-pressure phase, γ-MgH2, was also identified after mechanical alloying. Further qualitative and quantitative phase analyses were carried out using the Rietveld method. Scanning electron microscopy (SEM) of the milled powder clearly showed substantial particle size reduction after milling. Dehydrogenation at 300°C under vacuum shows that the (MgH2+Ni) mixture gives the highest level of hydrogen desorption and the most rapid kinetics, followed by MgH2 with Al, Fe, Nb, Ti and Cu. Theoretical predictions show that the (MgH2+Cu) system is the most unstable, followed by (MgH2+Ni), (MgH2+Fe), (MgH2+Al), (MgH2+Nb), (MgH2+Ti). The predicted alloying effects on the stability of MgH2 generally agree with the experimentally observed change in the hydrogen desorption capacity. The differences were discussed in the text.

Coupled quantitative simulation of microstructural evolution and plastic flow during dynamic recrystallization

Abstract A new modelling approach that couples fundamental metallurgical principles of dynamical recrystallization (DRX) with the cellular automaton (CA) method has been developed to simulate the microstructural evolution and the plastic flow behaviour during thermomechanical processing with DRX. It provides an essential link for multiscale modelling to bridge mesostructural dislocation activities with microstructural grain boundary dynamics, allowing accurate predictions of microstructure, plastic flow behaviour, and property attributes. Variations of dislocation density and growth kinetics of each dynamically recrystallizing grain (R-grain) were determined by metallurgical relationships of DRX, and the flow stress was evaluated from the average dislocation density of the matrix and all the R-grains. The growth direction and the shape of each R-grain were simulated using the CA method. The predictions of microstructural evolution and the flow behaviour at various hot working conditions agree well with the experimental results for an oxygen free high conductivity (OFHC) copper. It is identified that the oscillation of the flow stress–strain curve not only depends on thermomechanical processing parameters (strain rate and temperature) but also the initial microstructure. The mean size of R-grains is only a function of the Zener–Hollomon parameter. However, the percentage of DRX is not only related with the Zener–Hollomon parameter, but also influenced by the nucleation rate and the initial microstructure.

Exceptional CO2 capture in a hierarchically porous carbon with simultaneous high surface area and pore volume

A new type of hierarchically porous carbon (HPC) structures of simultaneously high surface area and high pore volume has been synthesised from carefully controlled carbonization of in-house optimised metal–organic frameworks (MOFs). Changes in synthesis conditions lead to millimetre-sized MOF-5 crystals in a high yield. Subsequent carbonization of the MOFs yield HPCs with simultaneously high surface area, up to 2734 m2 g−1, and exceptionally high total pore volume, up to 5.53 cm3 g−1. In the HPCs, micropores are mostly retained and meso- and macro- pores are generated from defects in the individual crystals, which is made possible by structural inheritance from the MOF precursor. The resulting HPCs show a significant amount of CO2 adsorption, over 27 mmol g−1 (119 wt%) at 30 bar and 27 °C, which is one of the highest values reported in the literature for porous carbons. The findings are comparatively analysed with the literature. The results show great potential for the development of high capacity carbon-based sorbents for effective pre-combustion CO2 capture and other gas and energy storage applications.

Strain and Orientation Modulated Bandgaps and Effective Masses of Phosphorene Nanoribbons

Passivated phosphorene nanoribbons, armchair (a-PNR), diagonal (d-PNR), and zigzag (z-PNR), were investigated using density functional theory. Z-PNRs demonstrate the greatest quantum size effect, tuning the bandgap from 1.4 to 2.6 eV when the width is reduced from 26 to 6 Å. Strain effectively tunes charge carrier transport, leading to a sudden increase in electron effective mass at +8% strain for a-PNRs or hole effective mass at +3% strain for z-PNRs, differentiating the (mh*/me*) ratio by an order of magnitude in each case. Straining of d-PNRs results in a direct to indirect band gap transition at either -7% or +5% strain and therein creates degenerate energy valleys with potential applications for valleytronics and/or photocatalysis.

Reaction synthesis of TiB2–TiC composites with enhanced toughness

Abstract In situ toughened TiB 2 –TiC x composites were fabricated using reaction synthesis of B 4 C and Ti powders at high temperatures. The resulting materials possessed very high relative densities and well developed TiB 2 plate-like grains, leading to a rather high fracture toughness, up to 12.2 MPa⋅m 1/2 . The microstructure was examined by means of XRD, SEM, TEM and EDAX. The reaction products mainly consisted of TiB 2 and TiC x . No other phases, e.g. Ti 3 B 4 , TiB, Ti 2 B 5 and free Ti, were observed regardless of whether the starting composition was Ti:B 4 C=3:1 or 4.8:1, and whether the sintering temperature was 1700 or 1800°C. The microstructural morphology is characterised by TiB 2 plate-like grains distributed uniformly in the TiC x matrix. Some inclusions and defects were found in TiB 2 grains. The very high reaction temperature was believed to be responsible for the formation of plate-like grains, which, in turn, is responsible for the much improved mechanical properties. The main toughening mechanisms were likely to be crack deflection, platelet pull-out and the micro-fracture of TiB 2 grains.

High‐Performance All‐Carbon Yarn Micro‐Supercapacitor for an Integrated Energy System

Single-walled carbon nanotubes and chitosan composite yarn is prepared using a wet-spinning method. After thermal treatment, mesoporous all-carbon yarn is obtained. Based on this material, flexible all-solid-state yarn micro-supercapacitors are fabricated. Electrochemical results show high specific capacitance and energy density, good rate capability and stable cycling life. Results of this research offer prospect for application in portable and wearable electronics.

Co-enhanced SiO2-BN ceramics for high-temperature dielectric applications

Abstract Two typical high-temperature dielectric materials, fused silica and BN, have been used to form a composite with an attempt to overcome their own drawbacks. In the resultant BN–SiO 2 composites, BN platelike grains were preferentially orientated by hot pressing and homogeneously distributed in the fused silica matrix. An evident co-operative enhancement has been achieved by the combination of the constituents. The sinterability and the thermal shock resistance of the BN materials were increased and the ablation surface temperature was decreased by the involvement of the fused silica. On the other hand, the strength, fracture toughness, and flame ablation resistance of the fused silica were increased due to the addition of BN. Furthermore, an amorphous Si–B–O–N structure was identified in the surface layer of the ablated composites, to which attention should be further paid in the development of new elevated temperature dielectric materials.

Switching effective oxygen reduction and evolution performance by controlled graphitization of a cobalt–nitrogen–carbon framework system

We report a purposely designed route for the synthesis of a promising carbon-based electrocatalyst for both ORR (oxygen reduction reaction) and OER (oxygen evolution reaction) from zeolitic imidazolate frameworks (ZIFs). Firstly, precursor ZIFs are rationally designed with a blend of volatile zinc to induce porosity and stable cobalt to induce graphitic domains. Secondly, the self-modulated cobalt–nitrogen–carbon system (SCNCS) is shown to be an effective ORR catalyst after graphitization at mild temperatures. Finally, the best OER catalyst is developed by enhancing graphitization of the SCNCS. For the first time, solely by switching the graphitization conditions of SCNCS, excellent ORR or OER performance is realized. This approach not only opens up a simple protocol for simultaneous optimization of nitrogen doping and graphitization at controlled cobalt concentrations, but also provide a facile method of developing such active catalysts without the use of extensive synthesis procedures.