The peak fluorescence emission of conventional fluorophores such as organic dyes and inorganic quantum dots is independent of the excitation wavelength. In contrast, the position of the peak fluorescence of graphene oxide (GO) in a polar solvent is heavily dependent on the excitation wavelength. The present work has discovered that the strong excitation wavelength dependent fluorescence in GO is originated from the "giant red-edge effect", which breaks Kasha's rule. When GO sheets are present in a polar solvent, the solvation dynamics slow down to the same time scale as the fluorescence due to the local environment of the GO sheet. Consequently, the fluorescence peak of GO broadens and red-shifts up to 200 nm with an increase in the excitation wavelength. The giant red-edge effect of GO disappears in a nonpolar solvent, leading to a narrow fluorescence peak that is independent of the excitation wavelength. Discovery of the underlying strong excitation wavelength dependent fluorescence mechanism provides guidelines for the design of graphene oxide-based optical devices.
The possibility of superconductivity in tetragonal FeS has attracted considerable interest because of its similarities to the FeSe superconductor. However, all efforts made to pursue superconductivity in tetragonal FeS have failed so far, and it remains controversial whether tetragonal FeS is metallic or semiconducting. Here we report the observation of superconductivity at 5 K in tetragonal FeS that is synthesized by the hydrothermal reaction of iron powder with sulfide solution. The obtained samples are highly crystalline and less air-sensitive, in contrast to those reported in the literature, which are meta-stable and air-sensitive. Magnetic and electrical properties measurements show that the samples behave as a paramagnetic metal in the normal state and exhibit superconductivity below 5 K. The high crystallinity and the stoichiometry of the samples play important roles in the observation of superconductivity. The present results demonstrate that tetragonal FeS is a promising new platform to realize high-temperature superconductors.
Well‐crystallized Nb‐doped anatase TiO2 nanoparticles are prepared by a novel synthetic route and successfully used as the photoanode of dye‐sensitized solar cells (DSSCs). The homogenous distribution of Nb in the TiO2 lattice is confirmed by scanning transmission electron microscopy (STEM) elemental mapping and line‐scanning analyses. After Nb doping, the conductivity of the TiO2 powder increases, and its flat‐band potential (Vfb) has a positive shift. The energy‐conversion efficiency of a cell based on 5.0 mol% Nb‐doped TiO2 is significantly better, by about 18.2%, compared to that of a cell based on undoped TiO2. The as‐prepared Nb‐doped TiO2 material is proven in detail to be a better photoanode material than pure TiO2, and this new synthetic approach using a water‐soluble precursor provides a simple and versatile way to prepare excellent photoanode materials.
A p-n junction photoanode has been fabricated by depositing p-type NiO nanoparticles on the n-type hematite thin film. Such a photoanode is employed for a photoelectrochemical cell. NiO not only facilitates the extraction of accumulated holes from hematite via the p-n junction, but also lowers the barrier for oxygen evolution reaction.
Polyaniline (PANI)/graphene (GP) thermoelectric (TE) composite films were prepared by a combination of in situ polymerization and a solution process. It was found that there existed a large number of graphenepolyaniline nano-interfaces in the composite films with graphene nanoplates aligned in the PANI matrix in the direction parallel to the substrate. SEM, TEM, Raman, XPS and UV-Vis analyses indicated that polyaniline coated on the surface of graphene by the strong p-p conjugation interactions during in situ polymerization, and then the PANI molecular chains were expanded by the chemical interactions between polyaniline and solution. Both the in situ polymerization process and solution process contributed to the uniform dispersion of graphene in the PANI matrix, which not only increased the number of graphene-polyaniline nano-interfaces in the composite, but also strengthened the p-p conjugation interactions between graphene and polyaniline, resulting in more ordered regions forming in the composite films. Consequently, the Seebeck coefficient of the composite films was remarkably improved and higher than the values calculated based on the series-connected two-component mixture model. The optimal electrical conductivity and Seebeck coefficient of the composite with 48 wt% graphene reached 814 S cm À1 and 26 mV K À1 , respectively, resulting in a maximum power factor of 55 mW m À1 K À2 , which is the highest value among the reported polymer/graphene composite TE materials.
Supercapacitors suffer either from low capacitance for carbon or derivate electrodes or from poor electrical conductivity and electrochemical stability for metal oxide or conducting polymer electrodes. Transition metal nitrides possess fair electrical conductivity but superior chemical stability, which may be desirable candidates for supercapacitors. Herein, niobium nitride, Nb4N5, is explored to be an excellent capacitive material for the first time. An areal capacitance of 225.8 mF cm−2, with a reasonable rate capability (60.8% retention from 0.5 to 10 mA cm−2) and cycling stability (70.9% retention after 2000 cycles), is achieved in Nb4N5 nanochannels electrode with prominent electrical conductivity and electrochemical activity. Faradaic pseudocapacitance is confirmed by the mechanistic studies, deriving from the proton incorporation/chemisorption reaction owing to the copious +5 valence Nb ions in Nb4N5. Moreover, this Nb4N5 nanochannels electrode with an ultrathin carbon coating exhibits nearly 100% capacitance retention after 2000 CV cycles, which is an excellent cycling stability for metal nitride materials. Thus, the Nb4N5 nanochannels are qualified for a candidate for supercapacitors and other energy storage applications.
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