Jiawen Fang

Nitrogen self-doped graphitic carbon nitride as efficient visible light photocatalyst for hydrogen evolution

Nitrogen self-doped graphitic carbon nitride (C3N4+x) was successfully synthesized by the co-thermal condensation of the precursor with a nitrogen-rich additive. The resultant self-doped semiconductor was characterized by X-ray photoelectron spectroscopy (XPS), which indicated that the nitrogen atom substituted the sp2 carbon atom. The photocatalytic hydrogen evolution was systematically evaluated under visible light irradiation (λ > 400 nm). The average hydrogen evolution rate for C3N4+x was 1.8 times higher than that of pristine graphitic carbon nitride, and the superiority lay in greatly improved optical, emission and electronic properties of the nitrogen modified carbon nitride. This study filled the research gap of self-doping for 2D polymeric carbon nitride and will stimulate intensive investigations in the further improvement of photocatalytic hydrogen evolution.

A simple melamine-assisted exfoliation of polymeric graphitic carbon nitrides for highly efficient hydrogen production from water under visible light

Polymeric graphitic carbon nitride with a two-dimensional (2D) structure has intensive potential applications in hydrogen production from water splitting under visible light irradiation. Searching for an efficient technology is the key to synthesizing 2D materials from bulk powders. Here, a simple, highly-efficient, large-scale and low-cost melamine-assisted exfoliation route is reported to obtain quasi-2D carbon nitride using an oil bath. Quasi 2D carbon nitride possesses a high specific surface area (116.76 m2 g−1), a larger bandgap (by 0.13 eV), an enhanced electronic transport ability in the in-plane direction, a prolonged photo-excited charge carrier lifetime, and a lowered recombination of photo-induced electrons and holes resulting from the quantum confinement effect. These make enormous contributions to the photoactivity for hydrogen production under visible light. Therefore, the melamine-assisted liquid exfoliation route can be applied to large-scale polymeric carbon nitride photocatalyst production and is envisaged to have great promise for the exfoliation of other materials with layered structures.

Gas-sensing and electrical properties of perovskite structure p-type barium-substituted bismuth ferrite

Pure and Ba-substituted bismuth ferrite (BiFeO3, BFO) powders were successfully synthesized via a sol–gel method. The effects of the Ba-substitution on the morphology, gas-sensing and electrical properties of BFO were studied. The gas-sensing tests show that the sensor based on Bi0.9Ba0.1FeO2.95 (BBFO10) has high sensitivity, quick response, effective selectivity and excellent long-time stability. The conduction mechanism and gas-sensing mechanism of a BBFO10 sample were investigated by the impedance spectroscopy and it was found that the conduction is dominated by p-type hole conduction. The conductivity of the sensor is dependent on the oxygen partial pressures and the type of gas atmosphere. The enhanced gas-sensing performances of the BBFO10 sensor are attributed to the higher oxygen vacancy concentration which was induced by the substitution of Bi3+ ion by an aliovalent Ba2+ ion at the A-site of the perovskite structure.

Pt-decorated zinc oxide nanorod arrays with graphitic carbon nitride nanosheets for highly efficient dual-functional gas sensing.

In this work, well-aligned ZnO nanorods were grown on the substrate of exfoliated g-C3N4 nanosheets via a microwave-assisted hydrothermal synthesis, and then Pt/ZnO/g-C3N4 nanostructures were obtained after the deposition of Pt nanoparticles. The growth of vertically ordered ZnO nanorods was occurred on g-C3N4 nanosheets through the bonding interaction between Zn and N atoms, which was confirmed by XPS, FT-IR data and molecular orbital theory. The Pt/ZnO/g-C3N4 nanostructures sensor exhibited the remarkable sensitivity, selectivity, and fast response/recovery time for air pollutants of ethanol and NO2. The application of Pt/ZnO/g-C3N4 nanostructures could be used as a dual-functional gas sensor through the controlled working temperature. Besides, the Pt/ZnO/g-C3N4 nanostructures sensor could be applied to the repeating detection of ethanol and NO2 in the natural environment. The synergistic effect and improved the separation of electron-hole pairs in Pt/ZnO/g-C3N4 nanostructures had been verified for the gas sensing mechanism. Additionally, Pt/ZnO/g-C3N4 nanostructures revealed the excellent charge carriers transport properties in electrochemical impedance spectroscopy (EIS), such as the longer electron lifetime (τn), higher electron diffusion coefficient (Dn) and bigger effective diffusion length (Ln), which also played an important role for Pt/ZnO/g-C3N4 nanostructures with striking gas sensing activities.

Novel sintering and band gap engineering of ZnTiO3 ceramics with excellent microwave dielectric properties

Pure phase ZnTiO3 ceramics were successfully synthesized for the first time by a solid state reaction method. The synthesis temperature was higher than the phase transition temperature, wherein the ZnO nanoparticles acted as inhibitors to prevent the formation of the secondary phase, Zn2TiO4, which was inevitable by conventional preparation methods. As the small nano-ZnO regions dispersed in the ceramic grains, the bulk diffusion of Ti ions, formation of nucleation centers and migration of phase boundaries were largely suppressed, indicating that nano-ZnO was desirable for stabilizing the ZnTiO3 phase above the phase transition temperature. The R (no. 148) space groups of the single phase were determined by X-ray diffraction Rietveld analysis. X-ray photoelectron spectroscopy and photoluminescence emission spectroscopy were also carried out to investigate the electronic microstructure of the obtained ZnTiO3 phase. Finally, excellent microwave dielectric properties were achieved (er ∼ 31.5, Q × f ∼ 59 800 GHz and τf ∼ 1.2 ppm °C−1) with a high sintering temperature (900–950 °C). Moreover, given its good chemical compatibility with the Ag electrode and the merits of easy scale-up, high-efficiency, low-cost and environmentally benign synthesis, ZnTiO3 is a promising candidate for LTCC applications. This work paves a great way towards practical applications.

Hydrothermally Induced Oxygen Doping of Graphitic Carbon Nitride with a Highly Ordered Architecture and Enhanced Photocatalytic Activity

As an amorphous or semicrystalline material, graphitic carbon nitride (g-C3 N4 ) displays poor photocatalytic activity owing to rapid recombination of the photogenerated charge carriers, which is mainly caused by a high density of defects in the graphitic structure. In this work, a porous O-doped g-C3 N4 (P-CNO) nanosheet with a highly ordered architecture is fabricated by introducing a novel hydrothermal treatment to the precursor before the final thermal condensation. The photocatalytic hydrogen evolution rate (HER) and HER per surface area of P-CNO are 13.9 and 1.7 times higher than that of bulk g-C3 N4 . The improved photocatalytic activity is ascribed to a synergistic effect of O doping, a porous sheet-like morphology, and increased crystallinity. This work also provides a new approach for the synthesis of other polymer-based photocatalysts with high crystallinity and excellent performance.

Unusual devisable high-performance perovskite materials obtained by engineering in twins, domains, and antiphase boundaries

With the widespread application, engineering of microstructures, domains, twins, and antiphase boundaries (APBs) is attracting significant attention. However, the origin of the domains, especially in the paraelectric phase, as well as the mechanism of variation in twins or domains and their relationship are still not clear. Generally, these structures are recognized as one of the key origins of intrinsic loss. Our studies, however, reveal that the formation of twins is closely related to the asymmetry of the crystal structure. With the introduction of a lattice blockage-trigonal NdAlO3 phase, the increase in the symmetry of the CaTiO3 tetragonal phase results in a transformation from the (110)-oriented twins to the (111)-oriented twins. Then, it forms a new ordered structure. With the help of peak-differentiation and imitation of the Raman spectra, the A-site in the perovskite structure is found to be a dominant factor in lattice energy and performance. By designing an A-site displacement, i.e., Sr2+ or Ba2+ substitution into Ca2+, we created a controllable structure of twins by symmetry regulation and APBs by introducing ferroelectric spontaneous polarization. Via selected area electron diffraction patterns (SAED) and variable temperature electric field piezoresponse force microscopy (PFM) images, we found that 180° and 90° domains could coexist in the grains of xCaTiO3–(1 − x)NdAlO3 ceramics. Interestingly, the 90° domains and twin boundaries (TB) play more important roles in the anisotropic resistance to the electron/hole transfer. Our studies prove that the defect engineering can realize a controllable enhanced dielectric performance by defect regulation within the host lattice. These may pave a possible way for the design of the microstructure of the defects to achieve better predictable performances of the materials.