Synthesizing graphdiyne with a well-defined structure is a great challenge. We reported herein a rational approach to synthesize graphdiyne nanowalls using a modified Glaser-Hay coupling reaction. Hexaethynylbenzene and copper plate were selected as monomer and substrate, respectively. By adjusting the ratio of added organic alkali along with the amount of monomer, the proper amount of copper ions was dissolved into the solution, thus forming catalytic reaction sites. With a rapid reaction rate of Glaser-Hay coupling, graphdiyne grew vertically at these sites first, and then with more copper ions dissolved, uniform graphdiyne nanowalls formed on the surface of copper substrate. Raman spectra, UV-vis spectra, and HRTEM results confirmed the features of graphdiyne. These graphdiyne nanowalls also exhibited excellent and stable field-emission properties.

In-plane anisotropic layered materials such as black phosphorus (BP) have emerged as an important class of two-dimensional (2D) materials that bring a new dimension to the properties of 2D materials, hence providing a wide range of opportunities for developing conceptually new device applications. However, all of recently reported anisotropic 2D materials are relatively narrow-bandgap semiconductors (<2 eV), and there has been no report about this type of materials with wide bandgap, restricting the relevant applications such as polarization-sensitive photodetection in short wave region. Here we present a new member of the family, germanium diselenide (GeSe) with a wide bandgap of 2.74 eV, and systematically investigate the in-plane anisotropic structural, vibrational, electrical, and optical properties from theory to experiment. Photodetectors based on GeSe exhibit a highly polarization-sensitive photoresponse in short wave region due to the optical absorption anisotropy induced by in-plane anisotropy in crystal structure. Furthermore, exfoliated GeSe flakes show an outstanding stability in ambient air which originates from the high activation energy of oxygen chemisorption on GeSe (2.12 eV) through our theoretical calculations, about three times higher than that of BP (0.71 eV). Such unique in-plane anisotropy and wide bandgap, together with high air stability, make GeSe a promising candidate for future 2D optoelectronic applications in short wave region.

This Article focuses on the fabrication of highly ordered nanotubes and some novel nanostructures of titania (TiO2) with a two-step anodization method. The first-step anodization was actually a pretreatment of the Ti foil surface and provided well-ordered imprints that served as a template for the further growth of nanotubes. As a result, the TiO2 nanotubes growing in the second-step anodization appreciably outperformed those fabricated with the conventional one-step Ti anodization in terms of size uniformity and arrangement orderliness. The parameters of the anodization were then modulated to obtain more complex structures. When the voltage in the second-step anodization was lower than that in the first-step anodization, a lotus root-shaped TiO2 nanostructure, in which each imprint contained several smaller nanopores, was achieved. When the second anodization was further divided into two stages, double-layered nanotube arrays were synthesized. They contained two distinctly separated parts, i.e., the bamboo-shaped upper one and the smooth-walled lower one. These results have demonstrated the effectiveness and controllability of the two-step anodization method in producing high-quality TiO2 nanotubes, which are believed to have potential applications in such fields as solar cells, photonic crystals, and hydrogen storage.

Polarization-sensitive photodetection in the UV region is highly indispensable in many military and civilian applications. UV-polarized photodetection usually relies on the use of wide bandgap semiconductors with 1D nanostructures requiring complicated nanofabrication processes. Although the emerging anisotropic 2D semiconductors shed light on the detection of polarization with a simple device architecture, bandgaps of such reported 2D semiconductors are too small to be applied for visible-blind UV-polarized photodetection. Here, germanium disulfide (GeS 2 ), the widest bandgap (>3 eV) in the family of in-plane anisotropic 2D semiconductors explored to date, is introduced as an ideal candidate for UV-polarized photodetection. The structural, vibrational, and optical anisotropies of GeS 2 are systematically investigated from theory to experiment. GeS 2 -based photodetectors show a strong polarization-dependent photoresponse in the UV region. GeS 2 with a wide bandgap and high in-plane anisotropy not only enriches the family of anisotropic 2D semiconductors but also expands the polarized photodetection from the current visible and near-infrared to the brand-new UV region.

In this communication, nickel diselenide (NiSe2) nanoparticles are synthesized by a facile and low-cost hydrothermal method. The synthesis method can be extended to other metal diselenides as well. The electrode made of NiSe2 exhibits superior electrocatalytic activity in the hydrogen evolution reaction (HER). A low Tafel slope of 31.1 mV per decade is achieved for NiSe2, which is comparable to that of platinum (∼30 mV per decade). Moreover, the catalytic activity of NiSe2 is very stable and no obvious degradation is found even after 1000 cyclic voltammetric sweeps.

As a new member of 2D materials, GeSe has attracted considerable attention recently due to its fascinating in-plane anisotropic vibrational, electrical, and optical properties originating from the low-symmetry crystal structure. Among these anisotropic properties, the anisotropic optical property, as a new degree of freedom to manipulate optoelectronic properties in 2D materials, is of great importance for practical applications. However, the fundamental understanding of the optical anisotropy of GeSe is still under exploration, severely restricting its utility in polarization-sensitive optical systems. Here, a systematic study about the in-plane optical anisotropy of GeSe is reported, including its anisotropic optical absorption, reflection, extinction, and refraction. The anisotropic band structure of GeSe is experimentally observed for the first time through angle-resolved photoemission spectroscopy, explaining the origin of the optical anisotropy. The anisotropic reflection and refraction of GeSe are further directly visualized through the angle-dependent optical contrast of GeSe flakes by azimuth-dependent reflectance difference microscopy and polarization-resolved optical microscopy, respectively. Finally, GeSe-based photodetectors exhibit a polarization-sensitive photoresponsivity due to the intrinsic linear dichroism. This study provides fundamental information for the optical anisotropy of GeSe, forcefully stimulating the exploration of novel GeSe-based optical and optoelectronic applications.

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