Mo2CF2/WS2: Two-Dimensional Van Der Waals Heterostructure for Overall Water Splitting Photocatalyst from Five-Step Screening.

With the increasing demand for renewable energy and clean energy, photocatalysis is considered an economical and feasible source of energy. In this work, we select two-dimensional (2D) materials of X2CT2 (X = Cr, Hf, Mo, Sc, Ti, Zr; T = Cl, F, O, OH), Mxene, and MS2 (M = Mo, W) to form 20 systems of 2D van der Waals (vdW) heterostructures. We establish five screening steps, and the 2D Mo2CF2/WS2 vdW heterostructures meet all the screening conditions. Mo2CF2/WS2 is a type II semiconductor with a band gap of 1.58 eV, proper band edge position and high solar-to-hydrogen efficiency (17.15%) and power conversion efficiency (23.4%). An excellent electron-hole recombination time of 21.2 ps and electron (hole) migration time of 149 (265) fs are obtained in the 2D Mo2CF2/WS2 vdW heterostructure. In addition, the calculation results of Gibbs free energy show that a hydrogen reduction reaction and water oxidation reaction can proceed smoothly under the driving of photogenerated holes.

In Situ Growth of Iron Sulfide on Fast Charge Transfer V2 C-MXene for Superior Sodium Storage Anodes.

Due to the upstream pressure of lithium resources, low-cost sodium-ion batteries (SIBs) have become the most potential candidates for energy storage systems in the new era. However, anode materials of SIBs have always been a major problem in their development. To address this, V2 C/Fe7 S8 @C composites with hierarchical structures prepared via an in situ synthesis method are proposed here. The 2D V2 C-MXene as the growth substrate for Fe7 S8  greatly improves the rate capability of SIBs, and the carbon layer on the surface provides a guarantee for charge-discharge stability. Unexpectedly, the V2 C/Fe7 S8 @C anode achieves satisfactory sodium storage capacity and exceptional rate performance (389.7 mAh g-1  at 5 A g-1 ). The sodium storage mechanism and origin of composites are thoroughly studied via ex situ characterization techniques and first-principles calculations. Furthermore, the constructed sodium-ion capacitor assembled with N-doped porous carbon delivers excellent energy density (135 Wh kg-1 ) and power density (11 kW kg-1 ), showing certain practical value. This work provides an advanced system of sodium storage anode materials and broadens the possibility of MXene-based materials in the energy storage.

Strong Metal-Support Interactions of Ni-CeO2 Effectively Improve the Performance of a Molten Hydroxide Direct Carbon Fuel Cell.

A strong metal-support interaction (SMSI) type catalyst has been synthesized and applied to a molten hydroxide direct carbon fuel cell (MHDCFC) to enhance the reaction activity of the anode carbon fuel through the interaction between the metal Ni and the support CeO2. Two catalysts have been prepared by a direct precipitation method (denoted NiO@CeO2) and a hydrothermal method (denoted NiO-CeO2), which are reduced by H2 to obtain Ni@CeO2 and Ni-CeO2, respectively. X-ray photoelectron spectroscopy (XPS), Raman, and temperature-programmed hydrogen reduction (H2-TPR) analysis results show that there are obvious oxygen vacancies and a Ni-O-Ce interface structure in NiO-CeO2 and Ni-CeO2, which is induced by the interaction between Ni and CeO2. The calculation results of current density and power density show that the performance of the MHDCFC is significantly improved in the presence of Ni-CeO2. The function fitting curves of the logarithm of the reaction rate constant (ln k) and the reciprocal of the temperature (1/T) show that the slope of the curve is decreased significantly after the addition of Ni-CeO2. In combination with density functional theory (DFT), the anode carbon reaction path is simulated in the MHDCFC, and the calculation results show that the reaction energy for the anodic carbon to generate carbon dioxide is decreased by 1.03 eV in the presence of Ni-CeO2.

Self-Powered Photoelectrochemical Biosensor Based on CdS/RGO/ZnO Nanowire Array Heterostructure.

A CdS/reduced graphene oxide (RGO)/ZnO nanowire array (NWAs) heterostructure is designed, which exhibits enhanced photoelectrochemical (PEC) activity compared to pure ZnO, RGO/ZnO, and CdS/ZnO. The enhancement can be attributed to the synergistic effect of the high electron mobility of ordered 1D ZnO NWAs, extended visible-light absorption of CdS nanocrystals, and the formed type II band alignment between them. Moreover, the incorporation of RGO further promotes the charge carrier separation and transfer process due to its excellent charge collection and shuttling characteristics. Subsequently, the CdS/RGO/ZnO heterostructure is successfully utilized for the PEC bioanalysis of glutathione at 0 V (vs Ag/AgCl). The self-powered device demonstrates satisfactory sensing performance with rapid response, a wide detection range from 0.05 mm to 1 mm, an acceptable detection limit of 10 µm, as well as certain selectivity, reproducibility, and stability. Therefore, the CdS/RGO/ZnO heterostructure has opened up a promising channel for the development of PEC biosensors.

Highly transparent triboelectric nanogenerator for harvesting water-related energy reinforced by antireflection coating.

Water-related energy is an inexhaustible and renewable energy resource in our environment, which has huge amount of energy and is not largely dictated by daytime and sunlight. The transparent characteristic plays a key role in practical applications for some devices designed for harvesting water-related energy. In this paper, a highly transparent triboelectric nanogenerator (T-TENG) was designed to harvest the electrostatic energy from flowing water. The instantaneous output power density of the T-TENG is 11.56 mW/m(2). Moreover, with the PTFE film acting as an antireflection coating, the maximum transmittance of the fabricated T-TENG is 87.4%, which is larger than that of individual glass substrate. The T-TENG can be integrated with silicon-based solar cell, building glass and car glass, which demonstrates its potential applications for harvesting waste water energy in our living environment and on smart home system and smart car system.

Influence of the carrier concentration on the piezotronic effect in a ZnO/Au Schottky junction.

The piezotronic effect, which utilizes the piezopotential to engineer the interface characteristics, has been widely exploited to design novel functional device or to optimize the device performance, which is intimately related to the carrier concentration. Here, by constructing a general Schottky diode, the piezotronic effect dependence on the carrier concentration was investigated systematically using ultraviolet (UV) illumination. Scanning Kelvin Probe Microscopy was employed to quantify the carrier concentration in ZnO nanorods under UV illumination. The results showed that the carrier concentration increases with increasing light intensity and an average value of up to 5.6 × 10(18) cm(-3) under 1.2 mW cm(-2) light illumination was obtained. Furthermore, with increasing UV light intensity, an increasingly imperceptible variation in the current-voltage characteristics under strain was observed, which finally disappeared under 1.2 mW cm(-2) light illumination. This phenomenon was attributed to the weakened modulation ability of the piezopotential due to the strengthened screening effect. In addition, the gradual disappearing in the barrier also contributed to the gradual disappearance of the piezotronic effect. This study provides an in-depth understanding of piezotronics, which could be extended to other piezoelectric devices and guide the design and optimization of piezotronic and even piezophototronic devices.

Gold nanoparticles coated zinc oxide nanorods as the matrix for enhanced L-lactate sensing.

In this study, an enzymatic electrochemical biosensor for L-lactate detection was proposed. The device was developed based on gold nanoparticles (Au NPs) modified zinc oxide nanorods (ZnO NRs). The sensing performance of the device was examined by cyclic voltammetry and amperometry. Compared with pristine ZnO based biosensor, Au/ZnO based sensor exhibited higher sensitivity of 24.56 µA cm(-2) mM(-1), smaller K(M)(app) of 1.58 mM, lower detection limit of 6 µM and wider linear range of 10 µM-0.6 mM for L-lactate detection. The introduction of Au NPs enhances electro-catalytic ability and electron migration, which contributes to the improvement of the sensing performance. Hence, the results confirm the essential character of Au NPs in such semiconductor based electrochemical biosensing system.

Enhanced photoelectrochemical property of ZnO nanorods array synthesized on reduced graphene oxide for self-powered biosensing application.

We have realized the direct synthesis of ZnO nanorods (ZnO NRs) array on reduced graphene layer (rGO), and demonstrated the enhanced photoelectrochemical (PEC) property of the rGO/ZnO based photoanode under UV irradiation compared with the pristine ZnO NRs array. The introduction of the rGO layer resulted in a favorable energy band structure for electron migration, which finally led to the efficient photoinduced charge separation. Such nanostructure was subsequently employed for self-powered PEC biosensing of glutathione in the condition of 0 V bias, with a linear range from 10 to 200 µM, a detection limit of 2.17 µM, as well as excellent selectivity, reproducibility and stability. The results indicated the rGO/ZnO nanostructure is a competitive candidate in the PEC biosensing field.

Investigation on the mechanism of nanodamage and nanofailure for single ZnO nanowires under an electric field.

The electrical service behavior of ZnO nanowires (NWs) with various diameters was investigated by a nanomanipulation technique. The nanodamage and nanofailure phenomena of the ZnO NWs were observed when external voltages were applied. The threshold voltages of the ZnO NWs increased linearly from 15 to 60 V with increasing diameter. The critical current densities were distributed from 19.50 × 10(6) to 56.90 × 10(6) A m(-2), and the reciprocal of the critical current density increased linearly with increasing diameter as well. The thermal core-shell model was proposed to explain the nanodamage and nanofailure mechanism of ZnO NWs under an electric field. It can be expected that the investigation on the nanodamage and nanofailure of nanomaterials would have a profound influence on practical applications of photoelectric, electromechanical, and piezoelectric nanodevices.

Asymmetric behavior in flexible piezoelectric strain sensors made of single ZnO nanowires.

Piezoelectric strain sensors were fabricated using single ZnO nanowires on flexible polystyrene substrates and the asymmetric behavior in this kind of piezoelectric strain sensor made of single nanowire was firstly introduced. The I~V curves of the sensors under a serious of tensile and compressive strains were measured and agreed with the thermionic emission-diffusion theory. The Schottky barrier heights were obtained. The change in Schottky barrier heights of the sensors under compressive strain is much slower than that under tensile strain and the asymmetric elastic response of the sensors is caused by the bending of the ZnO nanowires under compressive strains when the applied strain is larger than a critical strain εcr. And the conclusion that the critical strain εcr only relates to the geometry parameters of the nanowire is drawn. The reduction of the axial compressive strain in compression was calculated. This asymmetric behavior in strain sensors is firstly introduced and is important for the design and fabrication of this type of strain sensors.

First-principles studies on transport properties and contact effects of Cu(111)/ZnO-nanobelt(1010)/Cu(111) systems.

The transport properties of ZnO nanobelts along the (101¯0) non-polarized direction coupled with Cu electrodes were studied via non-equivalent Greens functions method and density functional theory formalism. The transport properties were greatly affected by interfacial spacing and nanobelt widths. The conductance decreased exponentially with the widths of the nanobelts. Ohmic behavior was found in narrow nanobelts, while rectifying characteristics were observed in wide nanobelts. In the case of narrow belts, the current-voltage characteristics were changed from ohmic type to rectifying characteristics as the interspace increased, corresponding to the contacts transforming from chemical to physical interactions. However, the conductance in the wider nanobelts declined exponentially as the interfacial distance increased. The change of metal induced gap states (MIGS) depends strongly on the interfacial distance but not significantly on the thickness of ZnO nanobelts. An n-type Schottky barrier between copper and ZnO nanobelts is induced by interfacial polarization effects. The Schottky barrier heights for the narrowest and widest nanobelts with equilibrium interfacial spacing were 0.37 eV and 0.44 eV, respectively, which is in good agreement with the experimental values. Additionally, the Schottky barrier heights increased almost linearly as the width of the nanobelts changed from 0.34 nm to 1.2 nm.

Facile synthesis of highly uniform Mn/Co-codoped ZnO nanowires: optical, electrical, and magnetic properties.

In this article, Co/Mn-codoped ZnO nanowires (NWs) were successfully synthesized on a silicon substrate by the thermal evaporation method with Au catalyst. The X-ray diffraction pattern indicated that the Co/Mn-codoped ZnO NWs are a hexagonal wurtzite structure without a second phase, and energy dispersive X-ray spectroscopy revealed that the Co and Mn ions were introduced into the ZnO NWs with the content of ∼0.8 at% and ∼1.2 at%, respectively. Photoluminescence spectra and Raman spectra showed that the Co/Mn were doped into the NWs and resulted in the shift of the near-band-edge emission. Moreover, the novel Raman peak at 519.3 cm(-1) has suggested that the two kinds of cations via doping could affect the local polarizability. Compared with the undoped ZnO NW, the electrical measurement showed that the Co/Mn-codoping enhanced the conductivity by an order of magnitude due to the presence of Co, Mn cations. The electron mobility and carrier concentration of a fabricated field effect transistor (FET) device is 679 cm2 V(-1) s(-1) and 2×10(18) cm(-3), respectively. Furthermore, the M-H curve demonstrated that the Co/Mn-codoped ZnO NWs have obvious ferromagnetic characteristics at room temperature. Our study enhances the understanding of the novel performances of transition-metal codoped ZnO NWs and also provides a potential way to fabricate optoelectronic devices.

Transverse piezoelectric field-effect transistor based on single ZnO nanobelts.

A transverse piezoelectric field-effect transistor (TP-FET) based on single ZnO nanobelts has been fabricated on a metallic graphite substrate in an atomic force microscope (AFM). The source-to-drain current of the TP-FET was found to decrease with increasing loading force under a positive bias due to the carrier-trapping effect and the creation of a charge-depletion zone. This TP-FET can be applied as a force/pressure sensor for measuring nanoNewton forces ranged from 0 to 700 nN.

Field-emission properties of individual ZnO nanowires studied in situ by transmission electron microscopy.

The field-emission properties of individual zinc oxide (ZnO) nanowires, grown by a solid-vapour phase thermal evaporation process, were studied in situ by transmission electron microscopy (TEM) using a home-made piezo-manipulator. The results indicate that ZnO nanowires present an outstanding field-emission property with low turn-on voltage and high emission current; the proper linearity of 1/V-ln(I/V(2)) curves basically accords with the Fowler-Nordheim model, and the dependence of the field-enhancement factor ß on the distance d between the nanowire tip and its counter anode fits a linear relationship. The investigations show that ZnO nanowires show promise for potential applications as field emitters.