Photoluminescence of Graphene Oxide in Visible Range Arising from Excimer Formation

Graphene oxide (GO) has attracted considerable attention due to its interesting structure and properties. The photoluminescence (PL) of GO is much stronger than that of graphene owing to the opening of an energy band gap. However, the origin of the PL bands in the ultraviolet and visible ranges remains controversial. In this paper, we report the dependence of the PL spectrum of GO on the pH value and concentration of GO aqueous solutions. It was discovered that PL in the visible range becomes prominent when the pH value is low and/or the GO concentration is high. As revealed by the time-resolved photoluminescence, the lifetime of the PL in the visible range is longer than that in the UV range. These results evidence the formation of excimers and prove that the PL band at the long wavelength is caused by the GO excimers

Aqueous Solution Synthesis of Pt–M (M = Fe, Co, Ni) Bimetallic Nanoparticles and Their Catalysis for the Hydrolytic Dehydrogenation of Ammonia Borane

Platinum-based bimetallic nanocatalysts have attracted much attention due to their high-efficiency catalytic performance in energy-related applications such as fuel cell and hydrogen storage, for example, the hydrolytic dehydrogenation of ammonia borane (AB). In this work, a simple and green method has been demonstrated to successfully prepare Pt–M (M = Fe, Co, Ni) NPs with tunable composition (nominal Pt/M atomic ratios of 4:1, 1:1, and 1:4) in aqueous solution under mild conditions. All Pt–M NPs with a small size of 3–5 nm show a Pt <i>fcc</i> structure, suggesting the bimetallic formation (alloy and/or partial core–shell), examined by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray absorption fine structure (XAFS) analysis. The catalytic activities of Pt–M NPs in the hydrolytic dehydrogenation of AB reveal that Pt–Ni NPs with a ratio of 4:1 show the best catalytic activity and even better than that of pure Pt NPs when normalized to Pt molar amount. The Ni oxidation state in Pt–Ni NPs has been suggested to be responsible for the corresponding catalytic activity for hydrolytic dehydrogenation of AB by XAFS study. This strategy for the synthesis of Pt–M NPs is simple and environmentally benign in aqueous solution with the potential for scale-up preparation and the <i>in situ</i> catalytic reaction

Synthesis and Structure-Dependent Optical Properties of ZnO Nanocomb and ZnO Nanoflag

The structure-dependent optical properties of ZnO nanostructures have attracted considerable attention due to their fascinating optoelectronic properties and great structural diversity. Novel ZnO nanocomb and ZnO nanoflag have been successfully synthesized by chemical vapor deposition (CVD) method using Au nanoparticles (NPs) as the catalyst at the deposition temperatures of 900 and 950 °C, respectively. X-ray diffraction and high-resolution transmission electron microscopy results show that the ZnO nanocomb handle and its teeth grow in [01̅11] and [0001] orientations, respectively, while the ZnO nanoflag sheet and its pole grow along [0001] and [21̅1̅0] orientations, respectively. Au NPs as well as deposition temperature played an important role in the growth of the nanocomb handle and nanoflag pole. Synchrotron-based scanning transmission X-ray microscopy (STXM) reveals the thickness distribution and the crystallinity of ZnO nanocomb and ZnO nanoflag. For the near-surface emission, photoluminescence and cathode luminescence spectra of these two ZnO nanostructures show band gap emission from both nanocomb and nanoflag but green emission from only ZnO nanocomb. Synchrotron-based two-dimensional X-ray absorption near-edge structure–X-ray excited optical luminescence (2D XANES–XEOL) further reveals that the green (defect) emissions come from both the surface and bulk of nanostructures. In the ZnO nanocomb, the O excitation channel contributes more favorably to the band gap emission compared to the defect emission, while the Zn excitation channel contributes less favorably to the band gap emission than the defect emission. Meanwhile, ZnO nanoflag displays an excellent crystallinity with weak defect emission; the Zn and O excitation channels both contribute predominantly to the band gap emission