Sichao Xu

Synthesis of mesoporous carbon capsules encapsulated with magnetite nanoparticles and their application in wastewater treatment

Mesoporous carbon capsules encapsulated with Fe3O4 nanoparticles were prepared by the successive coating of a silica layer and a subsequent mesoporous silica/carbon layer on the surface of Fe3O4 nanoparticles followed by chemical etching with NaOH solution. TEM observations show that the as-obtained samples had a rattle-like structure: Fe3O4 nanoparticles were encapsulated in the interior of the mesoporous carbon capsules. The typical nitrogen adsorption/desorption results demonstrate that the specific surface area for the as-prepared samples is up to 1570 m2 g−1, and the total pore volume is about 3.02 cm3 g−1. The porous wall structure of the lateral carbon capsules provides the sufficient spaces that contribute to high adsorption capacities and faster adsorption rates of pollutants molecules in aqueous media. The nanocomposites are superparamagnetic at room temperature with a saturation magnetization of 5.5 emu g−1, which provides the prerequisite for the fast magnetic separation in wastewater treatment application. Water treatment experiments indicated that the as-prepared samples exhibited higher adsorption rates and more effective removal capacity of organic pollutants compared with commercial activated carbon (AC), and their maximum adsorption capabilities for methylene blue (MB), congo red (CR), and phenol reached 608.04, 1656.9 and 108.38 mg g−1, respectively. The multifunctional nanocomposites can be potentially used as absorbents for fast, convenient, and highly efficient removal of pollutants from the wastewater, which will play important roles in the purification or desalination of natural water and industrial effluents.

Efficient removal of Cr(VI) from wastewater under sunlight by Fe(II)-doped TiO2 spherical shell

Fe(II)-doped TiO2 spherical shell catalyst was synthesized by one-pot hydrothermal method. The photocatalytic removal of Cr(VI) from plating wastewater under sunlight of the catalyst was demonstrated. It was found that the removal effectiveness of about 99.99% for initial Cr(VI) concentration of 102.3 ppm and 99.01% for 153.4 ppm under 3h sunlight irradiation is realized. The Fe(II) ions serve not only as reducing agents for reducing the Cr(VI) to Cr(III) but also as an intermedium of a two-step reduction, in which the TiO2 photoreduces the Fe(II) ions to Fe atoms firstly, and then the Fe atoms reduce the Cr(VI) to Cr(III). The improved photocatalytic activity of the catalyst is considered due to the synergistic effect of a multi reducing process by Fe(II) doping. The extended optical response and effectively utilization of sunlight of the special spherical-shell-like morphology also contribute to the enhanced photocatalytic activity.

Highly efficient and recyclable triple-shelled Ag@Fe3O4@SiO2@TiO2 photocatalysts for degradation of organic pollutants and reduction of hexavalent chromium ions

Herein, we demonstrate the design and fabrication of the well-defined triple-shelled Ag@Fe3O4@SiO2@TiO2 nanospheres with burr-shaped hierarchical structures, in which the multiple distinct functional components are integrated wonderfully into a single nanostructure. In comparison with commercial TiO2 (P25), pure TiO2 microspheres, Fe3O4@SiO2@TiO2 and annealed Ag@Fe3O4@SiO2@TiO2 nanocomposites, the as-obtained amorphous triple-shelled Ag@Fe3O4@SiO2@TiO2 hierarchical nanospheres exhibit a markedly enhanced visible light or sunlight photocatalytic activity towards the photodegradation of methylene blue and photoreduction of hexavalent chromium ions in wastewater. The outstanding photocatalytic activities of the plasmonic photocatalyst are mainly due to the enhanced light harvesting, reduced transport paths for both mass and charge transport, reduced recombination probability of photogenerated electrons/holes, near field electromagnetic enhancement and efficient scattering from the plasmonic nanostructure, increased surface-to-volume ratio and active sites in three dimensional (3D) hierarchical porous nanostructures, and improved photo/chemical stability. More importantly, the hierarchical nanostructured Ag@Fe3O4@SiO2@TiO2 photocatalysts could be easily collected and separated by applying an external magnetic field and reused at least five times without any appreciable reduction in photocatalytic efficiency. The enhanced photocatalytic activity and excellent chemical stability, in combination with the magnetic recyclability, make these multifunctional nanostructures promising candidates to remediate aquatic contaminants and meet the demands of future environmental issues.

Self-assembly of noble metallic spherical aggregates from monodisperse nanoparticles: their synthesis and pronounced SERS and catalytic properties

Here, an oil-in-water emulsion method was used to assemble monodisperse noble metallic nanoparticles (NPs) in the oil phase into aggregates in the aqueous phase. The original size of the NPs is an essential factor in determining the final morphology: beyond the critical size, spherical aggregates with the original NPs can be formed; otherwise, complex aggregates with coarsing NPs come into being. The aggregation level of NPs and consequent properties of the as-constructed spherical assemblies can be optimized through tuning the evaporation rate of the organic solvent. The fabricated assemblies can serve as highly sensitive surface-enhanced Raman scattering (SERS) platforms for the detection of chemical or biological molecules, showing significantly high SERS activity toward rhodamine 6G (R6G) dye and 4-aminothiophenol (4-ATP) molecules. In addition, we further demonstrate the use of these highly sensitive SERS-active substrates to identify melamine at 1 × 10−7 M to insure food safety and biosecurity. Furthermore, the catalytic reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) in the presence of excess NaBH4 was also investigated, in which the as-prepared silver assemblies exhibited excellent catalytic activities due to their special 3D architecture and large number of active catalytic sites.

Optimization of microstructure and optical properties of VO2 thin film prepared by reactive sputtering

VO2 (M) thin films with good optical switching properties have been grown by reactive sputtering method. The influence of sputtering parameters on the structural and optical properties of the as-grown VO2 thin films was investigated, and the correlation between the microstructure and optical switching properties were studied. It was found that the phase transition temperature, hysteresis width, and the amplitude of the transition depend on the sputtering gas pressure, and the amplitude of the transition can reach as high as 70% with an approximately zero infrared transmission in metal state at a wavelength of 2.5 μm. The anomalous optical properties of the VO2 thin films were analyzed and discussed together with the studies of the refractive index and optical band gap.

Recyclable magnetic photocatalysts of Fe2+TiO2 hierarchical architecture with effective removal of Cr(VI) under UV light from water

We report the synthesis and photocatalytic removal of Cr(VI) from water of hierarchical micro/nanostructured Fe(2+)/TiO(2) tubes. The TiO(2) tubes fabricated by a facile solvothermal approach show a three-level hierarchical architecture assembled from dense nanosheets nearly vertically standing on the surface of TiO(2) microtube. The nanosheets with a thickness of about 20 nm are composed of numerous TiO(2) nanocrystals with size in the range of 15-20 nm. Ferrous ions are doped into the hierarchical architecture by a reduction route. The Fe(2+)/TiO(2) catalyst demonstrates an effective removal of Cr(VI) from water under UV light and the removal effectiveness reaches 99.3% at the initial Cr(VI) concentration of 10 mg L(-1). The ferrous ion in the catalyst serves not as the photo-electron trap but as an intermedium of a two-step reduction. The TiO(2) photoreduces the Fe(2+) ions to Fe atoms firstly, then the Fe atoms reduce the Cr(VI) to Cr(III), and the later is removed by adsorption. The hierarchical architecture of the catalyst serves as a reactor for the photocatalytic reaction of Cr(VI) ions and an effective absorbent for the removal of Cr(III) ions. The catalyst can be easily magnetically separated from the wastewater after photocatalytic reaction and recycled after acid treatment.

TiO2 Nanospheres: A Facile Size-Tunable Synthesis and Effective Light-Harvesting Layer for Dye-Sensitized Solar Cells

A facile route to synthesize amorphous TiO2 nanospheres by a controlled oxidation and hydrolysis process without any structure-directing agents or templates is presented. The size of the amorphous TiO2 nanospheres can be easily turned from 20 to 1500 nm by adjusting either the Ti species or ethanol content in the reaction solution. The phase structure of nanospheres can be controlled by hydrothermal treatment. The TiO2 nanospheres show excellent size-dependent light-scattering effects and can be structured into a light-harvesting layer for dye-sensitized solar cells with a quite high power conversion efficiency of 9.25 %.

Thermal conductivity of a single Bi0.5Sb1.5Te3 single-crystalline nanowire

Single-crystalline Bi₀.₅Sb₁.₅Te₃ nanowires were fabricated by a template-assisted pulsed electrodeposition technique; the thermal conductivity of a single Bi₀.₅Sb₁.₅Te₃ nanowire of different diameters was characterized through a self-heating 3 ω method. The temperature-dependent resistance measurements prove the semiconductor behavior of the nanowires. The extremely low thermal conductivity of the nanowires was found compared with the corresponding bulk, and the Umklapp peaks shift to a higher temperature as the decreasing nanowires diameter decreases, which qualitatively agrees with the theoretical calculations based on the Callaway model. The boundary scattering plays an important role in the reduction of the thermal conductivity and in the shift of the Umklapp peak of the Bi₀.₅Sb₁.₅Te₃ nanowires.

Strong localization induced anomalous temperature dependence exciton emission above 300 K from SnO2 quantum dots

SnO2 quantum dots (QDs) are potential materials for deep ultraviolet (DUV) light emitting devices. In this study, we report the temperature and excitation power-dependent exciton luminescence from SnO2 QDs. The exciton emission exhibits anomalous blue shift, accompanied with band width reduction with increasing temperature and excitation power above 300 K. The anomalous temperature dependences of the peak energy and band width are well interpreted by the strongly localized carrier thermal hopping process and Gaussian shape of band tails states, respectively. The localized wells and band tails at conduction minimum are considered to be induced by the surface oxygen defects and local potential fluctuation in SnO2 QDs.

TiO2 seed-assisted growth of VO2(M) films and thermochromic performance

This paper reports the TiO2 seed-assisted hydrothermal growth of VO2 films on a quartz substrate. The rutile TiO2 seed layer can promote the oriented growth of VO2(M) at the initial stage of the hydrothermal reaction, and then, the VO2(D) phase further grows on top of the VO2(M) layer. It was found that a pure VO2(M) thin film can grow on the TiO2 seed layer at a temperature as low as 200 °C. A subsequent annealing treatment can transform the VO2(D) phase to the VO2(M) phase. The VO2(M)/TiO2 composite films display thickness-dependent optical and electrical performance, and nearly 55% infrared modulation at 3 μm and 3 orders of magnitude resistance modulation can be realized. Our results demonstrate important progress in growth of VO2(M) thermochromic thin films, which will find potential application in constructing VO2-based switching devices.