MoSSe/Hf(Zr)S2 heterostructures used for efficient Z-scheme photocatalytic water-splitting.

Direct Z-scheme water-splitting is a promising route to enhancing the photocatalytic performance due to the effective separation of photogenerated carriers while simultaneously preserving the strong oxidation activity of holes and reduction activity of electrons. In this work, the MoSSe/XY2 (X = Hf, Zr; S, Se) heterostructures (HSs) with different contacts are proposed for Z-scheme photocatalytic water-spitting by first principles calculation. The separation of photogenerated carriers for HfSe2/SMoSe and ZrSe2/SMoSe HSs is limited by the type-I band alignment, while the hydrogen production ability of HfSe2/SeMoS and ZrSe2/SeMoS HSs is limited by the lower conduction band edge positions relative to the water reduction potential. The HfS2/SMoSe, HfS2/SeMoS, ZrS2/SMoSe, and ZrS2/SeMoS HSs are direct Z-scheme water-splitting photocatalysts with the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) occurring at the Hf(Zr)S2 layer and MoSSe layer, respectively. More excitingly, the S (or Se) vacancies effectively lower the HER overpotentials. Besides, the solar-to-hydrogen efficiencies are 6.1%, 5.9%, 6.4%, and 6.3% for HfS2/SMoSe, HfS2/SeMoS, ZrS2/SMoSe, and ZrS2/SeMoS HSs, respectively. This work paves the way for designing highly efficient overall water-splitting photocatalysts using 2D materials.

Rotational design of BP/XY2 (X = Mo, W; Y = S, Se) composites for overall photocatalytic water-splitting.

Photocatalytic water-splitting for hydrogen generation is a promising way to solve the energy crisis, yet the design of efficient photocatalysts is still a challenge. By utilization of first principles calculations, we predict the photocatalytic properties of monolayer boron phosphide (BP) based BP/XY2 (X = Mo, W; Y = S, Se) composites of different rotated configurations. Our results suggest that the BP/XY2 composites can be stably formed, and the narrowed bandgaps ensure these composites are suitable for absorbing visible light. The bandgaps and band edge positions are slightly affected by the rotation angles. The BP/MoS2, BP/MoSe2, and BP/WSe2 are type II heterostructures. Furthermore, the transferred charge from BP to XY2 layers leads to the formation of electric fields, which efficiently separate the photoinduced carriers. The band alignments of BP/MoS2, BP/MoS2, BP/MoSe2, and BP/WSe2 satisfy the requirements of overall water-splitting within the pH scope of 3.6-7.9, 6.8-7.9, 4.0-8.0, and 8.7-8.8. This work will provide valuable insight for designing efficient water-splitting photocatalysts.

Broadband Anisotropy in Terahertz Metamaterial With Single-Layer Gap Ring Array.

To convert the polarization of terahertz waves pumped by a femtosecond laser in a terahertz time domain system, a broadband anisotropic metamaterial is proposed. The metamaterial is constructed with a single-layer gapped metallic ring array, which supports different resonant modes in orthogonal directions. With the aid of simulations and measurements, the anisotropy of the terahertz transmission is demonstrated and discussed. The experimental results of THz transmission in the metamaterial indicate that the anisotropic band is as wide as 0.56 THz, which accords well with our theoretical prediction.

Electrical Properties of Double-Sided Polymer Surface Nanostructures.

In this study, double-sided polymer surface nanostructures are fabricated using twice nanoimprint lithography and metal deposition technique. We perform electrical property measurement on these double-sided surface nanostructures. Open-circuit voltage and short-circuit current of the as-prepared samples with double-sided surface nanostructures and conductive electrode are recorded using an oscilloscope with applying different external force. The measurements are carried out at room temperature. We find that the intensity of open-circuit voltage and short-circuit current for the double-sided surface nanostructures depends strongly on the sizes, shapes, and arrangements of nanostructures and pressure force. The strongest electrical property can be observed in the hexagon nanopillar arrays with the diameter of about 400 nm containing sub-50-nm resolution sharp structures at the force of about 40 N. We discuss the physical mechanisms responsible for these interesting research findings. The experimental results we study are relevant to the applications of double-sided surface nanostructures such as a nanogenerator, pressure sensors, and nano-optoelectronic devices.

Effect of orientation on polarization switching and fatigue of Bi3.15Nd0.85Ti2.99Mn0.01O12 thin films at both low and elevated temperatures.

Bi3.15Nd0.85Ti2.99Mn0.01O12 (BNTM) thin films with (200)-orientations, (117)-orientations, and mixed-orientations were prepared by sol-gel methods. The influence of orientations on polarization fatigue behaviors of BNTM thin films were systematically investigated at both low and elevated temperatures. It was found that the changed trends of the polarization fatigue of (200)-oriented and (117)-oriented BNTM thin films at elevated temperatures were opposite. The fatigue properties become exacerbated for the (200)-oriented ones and become improved for the (117)-oriented ones, while the reduction of remanent polarization first decreases and then increases for the mixed-oriented ones. It can be assumed that the different roles played by domain walls and interface layer with increasing T in these thin films have caused such differences, which was certified by the lower activation energies (0.12-0.13 eV) of (200)-oriented BNTM thin films compared to those of BNTM thin films (0.17-0.31 eV) with other orientations through the temperature-dependent impedance spectra analysis. With the aid of piezoresponse force microscopy (PFM), the non-neutral tail-to-tail or head-to-head polarization configurations with greater probabilities for (117)-oriented and mixed-oriented thin films were found, while a majority of the neutral head-to-tail polarization configurations can be observed for (200)-oriented ones.

Strain-Tunable Visible-Light-Responsive Photocatalytic Properties of Two-Dimensional CdS/g-C3N4: A Hybrid Density Functional Study.

By means of a hybrid density functional, we comprehensively investigate the energetic, electronic, optical properties, and band edge alignments of two-dimensional (2D) CdS/g-C 3 N 4 heterostructures by considering the effect of biaxial strain and pH value, so as to improve the photocatalytic activity. The results reveal that a CdS monolayer weakly contacts with g-C 3 N 4 , forming a type II van der Waals (vdW) heterostructure. The narrow bandgap makes CdS/g-C 3 N 4 suitable for absorbing visible light and the induced built-in electric field between the interface promotes the effective separation of photogenerated carriers. Through applying the biaxial strain, the interface adhesion energy, bandgap, and band edge positions, in contrast with water, redox levels of CdS/g-C 3 N 4 can be obviously adjusted. Especially, the pH of electrolyte also significantly influences the photocatalytic performance of CdS/g-C 3 N 4 . When pH is smaller than 6.5, the band edge alignments of CdS/g-C 3 N 4 are thermodynamically beneficial for oxygen and hydrogen generation. Our findings offer a theoretical basis to develop g-C 3 N 4 -based water-splitting photocatalysts.

Two dimensional ZnO/AlN composites used for photocatalytic water-splitting: a hybrid density functional study.

Using hybrid density functionals, we study the interfacial interactions and electronic properties of ZnO/AlN composites with the consideration of rotation angles and biaxial strains in order to enhance the photocatalytic performance for water-splitting. The different rotated composites, and -2% strained, original, and 2% strained ZnO/AlN composites can be easily prepared owing to the negative interface formation energies. The bandgaps and band alignments of ZnO/AlN composites can be significantly tuned by biaxial strains. Particularly, the appropriate bandgap for visible light absorption, proper band alignment for spontaneous water-splitting, and the formed electric field promoting photoinduced carrier separation make the 2% strained ZnO/AlN composite a potential candidate for photocatalytic water-splitting. This work shines some light on designing two dimensional heterostructured photocatalysts.

Hybrid Density Functional Study on the Photocatalytic Properties of Two-dimensional g-ZnO Based Heterostructures.

In this work, graphene-like ZnO (g-ZnO)-based two-dimensional (2D) heterostructures (ZnO/WS2 and ZnO/WSe2) were designed as water-splitting photocatalysts based on the hybrid density functional. The dependence of photocatalytic properties on the rotation angles and biaxial strains were investigated. The bandgaps of ZnO/WS2 and ZnO/WSe2 are not obviously affected by rotation angles but by strains. The ZnO/WS2 heterostructures with appropriate rotation angles and strains are promising visible water-splitting photocatalysts due to their appropriate bandgap for visible absorption, proper band edge alignment, and effective separation of carriers, while the water oxygen process of the ZnO/WSe2 heterostructures is limited by their band edge positions. The findings pave the way to efficient g-ZnO-based 2D visible water-splitting materials.

Improvement of light extraction efficiency of GaN-based light-emitting diodes using Ag nanostructure and indium tin oxide grating.

Based on the analysis of the evanescent wave from total internal reflection, a light-emitting diode (LED) structure with a plasmonic Ag nanostructure and indium tin oxide (ITO) grating was proposed to enhance the extraction efficiency. The two-dimensional finite-difference time-domain method was used to study the spectral properties of the hybrid structure and the effects of structure parameters on extraction enhancement. The results demonstrate that the plasmonic Ag nanostructure can couple the evanescent wave to a propagation wave around the GaN/ITO interface, and then the photons are scattered out of the LED chips by the ITO grating with high extraction efficiency. Under the optimal parameters, the light extraction efficiency can reach approximately three times the original value at a relatively longer wavelength.