Jian-Wen Shi

Preparation, characterization and photocatalytic activities of holmium-doped titanium dioxide nanoparticles

Holmium-doped TiO2 nanoparticles with high photocatalytic activities were prepared by sol-gel method and characterized by X-ray diffraction, transmission electron microscopy, ultraviolet-visible diffuse reflectance spectroscopy, and surface area measurement by nitrogen adsorption in this study. Experimental results indicated holmium doping could increase the surface area of TiO2 nanoparticles, and inhibit the growth of crystalline size and the anatase-to-rutile phase transformation. The results of photodegrading methyl orange showed holmium doping improved the photocatalytic activity of TiO2, and the reasons could be attributed to the synergetic effects of large surface areas, small crystallite size, lattice distortion and more charge imbalance of holmium-doped TiO2. In our experiment, the optimal doped amount was 0.3mol.% for the maximum photocatalytic degradation ratio when holmium-doped TiO2 was calcined at 500 degrees C, and the optimal calcined temperature was 600 degrees C when the doped amount was 0.5mol.%.

Combination treatment of ultrasound and ozone for improving solubilization and anaerobic biodegradability of waste activated sludge

The hydrolysis is known to be the rate-limiting step of biological sludge anaerobic degradation. The disruptions of sludge flocs and microbial cell walls by ultrasound combined with ozone treatment (US/O(3)) were investigated in laboratory-scale experiments. The results showed that temperature, O(3) dose, US energy density and pH had a positive effect on the disintegration of sludge. The organic substrates were released into the liquor, which induced the increases of soluble chemical oxygen demand (COD(S)) and turbidity in the aqueous phase. Accordingly, the biodegradability of sludge was improved. The COD(S) increased from 1821 to 2513 mg/l after reaction for 30 min when NaHCO(3) was added, which indicated that the ozone molecule played a major role in the disintegration of waste activated sludge. The COD(S) was 2483 mg/l after 60 min O(3) treatment followed by 60 min US treatment, and it changed into 3040 mg/l after 60 min US/O(3) treatment, which proved that US/O(3) induced a synergetic effect. The pH-drop of sludge from 6.8 to 5.21 might be attributed to the increase of volatile fatty acid from 61.35 to 111.96 mg/l during the US/O(3) treatment process.

Synthesis of porous magnetic ferrite nanowires containing Mn and their application in water treatment

Two kinds of porous magnetic ferrite nanowires containing manganese (MnFe2O4 and Mn doped Fe3O4) have been successfully synthesized by thermal decomposition of organometallic compounds, using nitrilotriacetic acid (NA) as a chelating agent to coordinate with various ratios of Fe(II) and Mn(II) ions. The resultant MnFe2O4 and Mn doped Fe3O4 nanostructures are superparamagnetic, and have magnetization saturation values of about 45.9 and 48.7 emu g−1 for MnFe2O4 and Mn doped Fe3O4, respectively. The Brunauer–Emmett–Teller specific surface areas of the MnFe2O4 and Mn doped Fe3O4 are 37.8 and 45.4 m2 g−1, respectively. The as-prepared porous MnFe2O4 and Mn doped Fe3O4 nanowires exhibit excellent ability to remove heavy metal ions and organic pollutant in waste water. In addition, these porous magnetic ferrites may be useful in other fields such as biomedicine and Li-ion batteries.

TiO2/activated carbon fibers photocatalyst: Effects of coating procedures on the microstructure, adhesion property, and photocatalytic ability

In order to more easily separate TiO(2) photocatalyst from the treated wastewater, TiO(2) film was immobilized on the surface of activated carbon fibers (ACFs) by employing two kinds of coating procedures, dip-coating, and hydrothermal treatment. The effects of coating procedures on microstructure of TiO(2)-coated ACFs (TiO(2)/ACFs), such as morphology, porous property, crystal structure, and light absorption characteristics were investigated in detail. The adhesion property between TiO(2) film and ACFs was evaluated by ultrasonic vibration, and the photocatalytic activity of TiO(2)/ACFs was tested by the photocatalytic decoloration of methylene blue solution. The results show that hydrothermal treatment presented many advantages to obtain high-performance TiO(2)/ACFs photocatalyst in comparison with dip-coating. Hydrothermal treatment could improve the binding property between TiO(2) films and ACFs, which endowed the as-obtained TiO(2)/ACFs photocatalyst with improved reusable performance, and TiO(2)/ACFs synthesized by hydrothermal treatment presented higher photocatalytic activity.

Carbon‐doped Titania Hollow Spheres with Tunable Hierarchical Macroporous Channels and Enhanced Visible Light‐induced Photocatalytic Activity

Titania, not just the wife of Oberon: Carbon-doped titania hollow spheres (THS) were prepared by using carbon spheres as both template and the source of carbon doping. The THS interconnect with each other through smaller pores to form hierarchical macroporous channel structure. Scale bar: 1 μm.

Recent progress in the preparation and application of semiconductor/graphene composite photocatalysts

Graphene is a new material with a single-layer laminar structure of carbon atoms that possesses favorable physical and chemical properties such as high electrical conductivity, high chemical stability, and large specific surface area. Combining graphene with semiconductors to form composite photocatalysts can extend its light absorption edge, improve the migration rate of charge carriers, and enhance the adsorption capacity of contaminants. The unique two-dimensional planar structure of graphene endows composite photocatalysts with many excellent properties. Herein, the properties of graphene, semiconductor, and composite photocatalysts are first introduced. The various preparation methods of semiconductor/graphene composite photocatalysts are then presented, and the mechanisms behind enhanced photocatalysis are summarized. Four typical applications of composite photocatalysts: elimination of organic pollutants, hydrogen production, organic fuel production via CO2 reduction, and photocatalytic sterilization, are described in detail. Finally, the direction of future research on semiconductor/graphene composite photocatalysts is explored.

Highly Efficient Photocatalyst Based on a CdS Quantum Dots/ZnO Nanosheets 0D/2D Heterojunction for Hydrogen Evolution from Water Splitting

A novel CdS/ZnO heterojunction constructed of zero-dimensional (0D) CdS quantum dots (QDs) and two-dimensional (2D) ZnO nanosheets (NSs) was rationally designed for the first time. The 2D ZnO NSs were assembled into ZnO microflowers (MFs) via an ultrasonic-assisted hydrothermal procedure (100 °C, 12 h) in the presence of a NaOH solution (0.06 M), and CdS QDs were deposited on both sides of every ZnO NS in situ by using the successive ionic-layer absorption and reaction method. It was found that the ultrasonic treatment played an important role in the generation of ZnO NSs, while NaOH was responsible to the assembly of a flower-like structure. The obtained CdS/ZnO 0D/2D heterostructures exhibited remarkably enhanced photocatalytic activity for hydrogen evolution from water splitting in comparison with other CdS/ZnO heterostructures with different dimensional combinations such as 2D/2D, 0D/three-dimensional (3D), and 3D/0D. Among them, CdS/ZnO-12 (12 deposition cycles of CdS QDs) exhibited the highest hydrogen evolution rate of 22.12 mmol/g/h, which was 13 and 138 times higher than those of single CdS (1.68 mmol/g/h) and ZnO (0.16 mmol/g/h), respectively. The enhanced photocatalytic activity can be attributed to several positive factors, such as the formation of a Z-scheme photocatalytic system, the tiny size effect of 0D CdS QDs and 2D ZnO NSs, and the intimate contact between CdS QDs and ZnO NSs. The formation of a Z-scheme photocatalytic system remarkably promoted the separation and migration of photogenerated electron-hole pairs. The tiny size effect effectively decreased the recombination probability of electrons and holes. The intimate contact between the two semiconductors efficiently reduced the migration resistance of photogenerated carriers. Furthermore, CdS/ZnO-12 also presented excellent stability for photocatalytic hydrogen evolution without any decay within five cycles in 25 h.

Phosphorus recovery from wastewater by struvite crystallization: Property of aggregates

Struvite crystallization is a promising method to remove and recover phosphorus from wastewater to ease both the scarcity of phosphorus rock resources and water eutrophication worldwide. To date, although various kinds of reactor systems have been developed, supporting methods are required to control the struvite fines flushing out of the reactors. As an intrinsic property, aggregation is normally disregarded in the struvite crystallization process, although it is the key factor in final particle size and therefore guarantees phosphorus recovery efficiency. The present study developed a method to analyze the characteristics of struvite aggregates using fractal geometry, and the influence of operational parameters on struvite aggregation was evaluated. Due to its typical orthorhombic molecular structure, struvite particles are prone to crystallize into needle or rod shapes, and aggregate at the corners or edges of crystals. The determined fractal dimension (Dpf) of struvite aggregates was 1.52-1.31, with the corresponding range of equivalent diameter (d0.5) at 295.9-85.4 μm. Aggregates formed in relatively low phosphorus concentrations (3.0-5.0 mmol/L) and mildly alkaline conditions (pH 9.0-9.5) displayed relatively compact structures, large aggregate sizes and high aggregation strength. Increasing pH values led to continuous decrease of aggregate sizes, while the variation of Dpf was insignificant. As to the aggregate evolution, fast growth in a short time followed by a long steady stage was observed.

Rationally Designed Porous MnOx–FeOx Nanoneedles for Low-Temperature Selective Catalytic Reduction of NOx by NH3

In this work, a novel porous nanoneedlelike MnOx-FeOx catalyst (MnOx-FeOx nanoneedles) was developed for the first time by rationally heat-treating metal-organic frameworks including MnFe precursor synthesized by hydrothermal method. A counterpart catalyst (MnOx-FeOx nanoparticles) without porous nanoneedle structure was also prepared by a similar procedure for comparison. The two catalysts were systematically characterized by scanning and transmission electron microscopy, X-ray diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy, hydrogen temperature-programmed reduction, ammonia temperature-programmed desorption, and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFT), and their catalytic activities were evaluated by selective catalytic reduction (SCR) of NOx by NH3. The results showed that the rationally designed MnOx-FeOx nanoneedles presented outstanding low-temperature NH3-SCR activity (100% NOx conversion in a wide temperature window from 120 to 240 °C), high selectivity for N2 (nearly 100% N2 selectivity from 60 to 240 °C), and excellent water resistance and stability in comparison with the counterpart MnOx-FeOx nanoparticles. The reasons can be attributed not only to the unique porous nanoneedle structure but also to the uniform distribution of MnOx and FeOx. More importantly, the desired Mn4+/Mnn+ and Oα/(Oα + Oβ) ratios, as well as rich redox sites and abundant strong acid sites on the surface of the porous MnOx-FeOx nanoneedles, also contribute to these excellent performances. In situ DRIFT suggested that the NH3-SCR of NO over MnOx-FeOx nanoneedles follows both Eley-Rideal and Langmuir-Hinshelwood mechanisms.

Fabrication of 3D porous Mn doped α-Fe2O3 nanostructures for the removal of heavy metals from wastewater

Three-dimensional porous Mn doped α-Fe2O3 nanostructures are successfully fabricated by calcined carbon spheres containing Fe(II) and Mn(II) ions, which were obtained by hydrothermal treatment of glucose, Fe(II), and Mn(II) mixed solutions. The obtained nanostructures exhibit excellent abilities for the removal of Pb(II), Cr(VI), and As(III) ions from wastewater with easy magnetic separation.