Novel WO3/g-C3N4 composite photocatalysts were prepared by a calcination process with different mass contents of WO3. The photocatalysts were characterized by thermogravimetric analysis (TG), powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS), high-resolution transmission electron microscopy (HRTEM), UV-vis diffuse reflection spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and electrochemical impedance spectroscopy (EIS). The photocatalytic activity of the photocatalysts was evaluated by degradation of methylene blue (MB) dye and 4-chlorophenol (4-CP) under visible light. The results indicated that the WO3/g-C3N4 composite photocatalysts showed higher photocatalytic activity than both the pure WO3 and pure g-C3N4. The optimum photocatalytic activity of WO3/g-C3N4 at a WO3 mass content of 9.7% under visible light irradiation was up to 4.2 times and 2.9 times as high as that of the pure WO3 and pure g-C3N4, respectively. The remarkably increased performance of WO3/g-C3N4 was mainly attributed to the synergistic effect between the interface of WO3 and g-C3N4, including enhanced optical absorption in the visible region, enlarged specific surface areas and the suitable band positions of WO3/g-C3N4 composites.

First-order solid–solid phase transition of crystalline solids at the nanoscale has attracted an increasing interest in solid-state physics and chemistry, which can be used to alter the properties of materials without changing chemical compositions. Herein, we report the results of our comparative studies on phase transitions between tetragonal (t), orthorhombic (β), and cubic (α) polymorphs in Ag2Se nanocrystals. A significant discrepancy in stability and phase transition behavior is determined for t-Ag2Se nanocrystals, which were prepared separately by two different methods. Differential scanning calorimetry (DSC) and variable-temperature XRD studies reveal that the t-Ag2Se nanocrystals prepared by the oleylamine (OLA)-mediated method show a highly temperature- and time-sensitive metastability and undergo a t → β → α → β phase transition during the thermal cycling, in which the t → β transition is exothermic and irreversible, whereas the β → α transition is reversible. Similarly, the reversible β → α structure transition is detected in the β-Ag2Se nanocrystals, which were also prepared using the OLA-mediated method with different post-treatment manners and stabilized conditions. In contrast, the t-Ag2Se nanocrystals prepared by the PVP-assisted solvothermal method are more stable and exhibit a direct, reversible t → α phase transition without undergoing the β phase; however, when heated to a high temperature, for example, ≥250 °C, the stability of the t phase and the reversibility of the t → α transition will be destroyed due to the sintering and size increase of the sample, which is confirmed by the determination of the t → α → β phase transition in the DSC cycle. The formation of the t phase is attributed to the α → t structure transformation with the temperature cooled from synthetic temperatures (160–220 °C) to room temperature. Moreover, the reasons for the difference in the stabilities and phase transitions of t-Ag2Se nanocrystals prepared in our two methods are discussed based on the influences of size, surface (or shape), and defects on the thermodynamics and kinetics of a solid–solid structure transformation.

A simple and low-energy-consuming approach to synthesize highly stable and dispersive silver nanoparticle-graphene (AgNP-GE) nanocomposites has been developed, in which the stability and dispersivity of the composites are varied greatly with the pH value and temperature of the reaction. The results demonstrate that the optimal reaction conditions are pH 11 at room temperature for 70 min. As-synthesized composites display excellent antimicrobial activity, and can completely inhibit the growth of Escherichia coli cells at a concentration of 20 mg L(-1) (20 ppm). After treatment with 10 ppm AgNP-GE composites, the cells are killed completely within 3 h. The unique structure imparts such good antimicrobial properties to the composites. Firstly, the sheetlike AgNP-GE tends to be adsorbed and accumulated onto the surface of cells, which can change the permeability and enhance the antimicrobial activity. Secondly, Ag(+) released from AgNPs can act on the cells effectively and fully, thereby resulting in cell death.

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