An-Wu Xu

Water-dispersible magnetite-graphene-LDH composites for efficient arsenate removal

A composite material, containing magnetite particles, graphene and layered double hydroxides (LDHs) was fabricated through a simple two-step reaction. Graphene was used as the matrix for supporting magnetite particles and LDH nanoplates. The synthesized magnetite-graphene-LDH (MGL) composites were characterized by field emission scanning electron microscopic (FE-SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transformed infrared (FTIR) spectroscopy, N2adsorption-desorption, and X-ray photoelectron spectroscopy (XPS). The MGL composites were applied to remove arsenate from aqueous solutions and could be easily separated by magnetic separation process. The results showed enhanced adsorption capacity of arsenate on the MGL as compared to that of pure Mg/Al LDHs. The surface area of MGL is greatly enhanced through the incorporation of magnetite particles and graphene, which provides more active sites for arsenate uptake. Moreover, LDHs were hybridized with mechanically and chemically stable graphene materials, providing an accessible diffusion pathway in the macropore domain, and therefore their adsorption capacity was enhanced. The fast and efficient adsorption of arsenate from solution to MGL suggests that the MGL composites are potential and suitable materials in the preconcentration of arsenate from large volumes of aqueous solutions in wastewater treatment.

Novel one-dimensional Bi2O3–Bi2WO6 p–n hierarchical heterojunction with enhanced photocatalytic activity

A novel one-dimensional (1D) Bi2O3 nanorod–Bi2WO6 nanosheet p–n junction photocatalyst was prepared by a three-step synthetic route. The obtained products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and N2-sorption–desorption and Brunauer–Emmett–Teller (BET) surface area. Bi2O3 rods with the diameter of about 200 nm were obtained by calcining a Bi(OHC2O4)·2H2O precursor. Bi2WO6 nanosheets vertically grew on the Bi2O3 rods along the long axial direction. The photocatalytic activity required to degrade Rhodamine B (RhB) and phenol under solar/visible light by p–n junction of Bi2O3–Bi2WO6 nanorods was investigated. The results demonstrate that the novel Bi2O3–Bi2WO6 p–n heterostructures display higher photocatalytic activity than single Bi2O3 nanorods or Bi2WO6 flowers. The enhancement of the photocatalytic activity of the Bi2O3–Bi2WO6 p–n junction structures can be ascribed to strong visible light absorption and the effective separation of photogenerated electrons and holes by the internal electrostatic field in the junction region. More importantly, 1D p–n heterostructures made of ordered nanosheets are beneficial for the transport of photogenerated carriers and for increasing the rate of the photocatalytic reaction. This work could offer a new insight into the design and fabrication of advanced materials with heterojunction structures for photocatalytic applications and optoelectronic devices.

Stable blue TiO2−x nanoparticles for efficient visible light photocatalysts

Tailored fabrication of non-stoichiometric semiconductors has attracted considerable interest since the oxygen vacancy is the fundamental and intrinsic defect in reduced semiconductors and has critical impacts on their physicochemical properties such as tuning optical absorption, increasing conductivity, etc. Therefore, it is highly important to have a comprehensive understanding of the methods and the techniques of Ti3+ generation as well as Ti3+ property exploration. Here we have developed an effective strategy for the large-scale synthesis of blue titania with Ti3+ localized in the core of TiO2−x nanoparticles (NPs) based on Le Chateliers principle. Electron paramagnetic resonance (EPR) spectra confirm the presence of Ti3+ in as-prepared samples, which is attributed to be the origin of its excellent visible-light photocatalytic activity. Further analysis based on X-ray photoelectron spectroscopy (XPS) spectra of Ti 3d indicates that only the rhombic Ti3+ is localized in the bulk, rather than on the surface of obtained TiO2−x NPs. It can be inferred that the electronic structure of blue titania NPs is determined by the unique defective and non-stoichiometric TiO2−x core and stoichiometric TiO2 shell structure. As a consequence, our samples show excellent stability and retain their blue color upon storage in ambient atmosphere for at least one year. The structure, crystallinity and morphology of the as-prepared samples were characterized systematically. The blue TiO2−x NPs exhibit higher photocatalytic activity for the photooxidation of methylene blue than the commercial P25 NPs under visible light irradiation (λ ≥ 400 nm). It is found that the molar ratio of Ti4+–Ti3+ in the reaction system plays a key role in the photocatalytic activity of Ti3+ self-doped TiO2−x under visible light since the electronic structures of the resulting TiO2−x NPs can be finely tuned by the molar ratios of Ti4+–Ti3+ in the precursors. The optimal molar ratio of Ti4+–Ti3+ is 1 : 40, at which the obtained titania NPs display a deep blue appearance and the highest catalytic activity. Therefore, our present work highlights the feasibility of simultaneous engineering of surface energetics and optical properties for designing novel TiO2-based nanomaterials.

Nano-structured manganese oxide as a cathodic catalyst for enhanced oxygen reduction in a microbial fuel cell fed with a synthetic wastewater

Microbial fuel cells (MFCs) provide new opportunities for the simultaneous wastewater treatment and electricity generation. Enhanced oxygen reduction capacity of cost-effective metal-based catalysts in an air cathode is essential for the scale-up and commercialization of MFCs in the field of wastewater treatment. We demonstrated that a nano-structured MnO(x) material, prepared by an electrochemically deposition method, could be an effective catalyst for oxygen reduction in an MFC to generate electricity with the maximum power density of 772.8 mW/m(3) and remove organics when the MFC was fed with an acetate-laden synthetic wastewater. The nano-structured MnO(x) with the controllable size and morphology could be readily obtained with the electrochemical deposition method. Both morphology and manganese oxidation state of the nano-scale catalyst were largely dependent on the electrochemical preparation process, and they governed its catalytic activity and the cathodic oxygen reduction performance of the MFC accordingly. Furthermore, cyclic voltammetry (CV) performed on each nano-structured material suggests that the MnO(x) nanorods had an electrochemical activity towards oxygen reduction reaction via a four-electron pathway in a neutral pH solution. This work provides useful information on the facile preparation of cost-effective cathodic catalysts in a controllable way for the single-chamber air-cathode MFC for wastewater treatment.

Enhanced visible-light photocatalytic activity of Z-scheme graphitic carbon nitride/oxygen vacancy-rich zinc oxide hybrid photocatalysts

With the objectives of enhancing the stability, optical properties and visible-light photocatalytic activity of photocatalysts, we modified oxygen vacancy-rich zinc oxide (Vo-ZnO) with graphitic carbon nitride (g-C3N4). The resulting g-C3N4/Vo-ZnO hybrid photocatalysts showed higher visible-light photocatalytic activity than pure Vo-ZnO and g-C3N4. The hybrid photocatalyst with a g-C3N4 content of 1 wt% exhibited the highest photocatalytic degradation activity under visible-light irradiation (λ ≥ 400 nm). In addition, the g-C3N4/Vo-ZnO photocatalyst was not deactivated after five cycles of methyl orange degradation, indicating that it is stable under light irradiation. Finally, a Z-scheme mechanism for the enhanced photocatalytic activity and stability of the g-C3N4/Vo-ZnO hybrid photocatalyst was proposed. The fast charge separation and transport within the g-C3N4/Vo-ZnO hybrid photocatalyst were attributed as the origins of its enhanced photocatalytic performance.

A core–shell structure of polyaniline coated protonic titanate nanobelt composites for both Cr(VI) and humic acid removal

The current methods for chromium and natural organic matter decontamination from wastewater present limitations, such as high cost, poor reproducibility, and detrimental environmental effects as well as by secondary waste. Herein, we synthesized a core–shell structure of polyaniline/hydrogen-titanate nanobelt (PANI/H-TNB) composites through chemical oxidation in the presence of phytic acid, which played an important role in the formation and regeneration of PANI. The adsorption performance of PANI/H-TNB composites as an adsorbent of Cr(VI) and humic acid (HA) from aqueous solutions was tested. A batch technique was adopted to investigate the removal efficiency toward Cr(VI) and HA under various environmental conditions. The PANI/H-TNB composites exhibited excellent adsorption capacity toward Cr(VI) (156.94 mg g−1) and HA (339.46 mg g−1), outperforming that of PANI nanowires and many other materials. Large Kd values (>104 mL g−1) demonstrated the high affinity of the composites for both of Cr(VI) and HA. The analysis of Fourier transformed infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) showed that the removal of Cr(VI) was a combined effect of reduction Cr(VI) to Cr(III) and chemical sorption, while HA adsorption was mainly via surface complexation between the disassociated HA macromolecules and the positively charged PANI. The PANI/H-TNB composites presented satisfactory regeneration performance and reusability, which greatly reduced the wastewater disposal expenses. For the sake of industrial application, the PANI/H-TNB composites with high adsorption capacities can be applied as a suitable adsorbent for simultaneous removal of Cr(VI) and HA in wastewater cleanup.

Novel CeO2 yolk–shell structures loaded with tiny Au nanoparticles for superior catalytic reduction of p-nitrophenol

Direct fabrication of core-shell or yolk-shell functional nanomaterials via a facile template-free method remains a challenge. In this work, we present a novel approach that involves straightforward chemical transformation and thermal treatment of the infinite coordination polymer particles to obtain composition-tunable CeO(2) yolk-shell structures. Uniform CeO(2) yolk-shell hollow spheres with a high surface area are promising support materials for tiny gold nanoparticles (ca. 4 nm), forming Au-CeO(2) nanocomposites which exhibit a remarkable catalytic activity and high stability for the reduction of p-nitrophenol. A possible mechanism for the formation of CeO(2) yolk-shell microspheres is also proposed.

Efficient adsorption/photodegradation of organic pollutants from aqueous systems using Cu2O nanocrystals as a novel integrated photocatalytic adsorbent

In this work, novel uniform Cu2O nanocrystals (NCs) with a size of 4 nm were synthesized by a remarkably simple and facile route via a synproportionation reaction of Cu2+ with metal copper powder at room temperature. The obtained Cu2O NCs as an integrated photocatalytic adsorbent (IPCA) exhibited high adsorption affinity combined with superior photocatalytic activity for the removal of various organic pollutants (humic acid (HA), Congo red (CR), methyl orange (MO) and tetracycline (TC)) in our experiments. The results show that the Langmuir isotherms were applicable to describe the adsorption processes and the adsorption kinetics followed the pseudo-second-order mode. The adsorption mechanism that is responsible for superior adsorption capacity occurs mainly via surface complexation as well as coagulation on the surface of Cu2O NCs. A remarkable maximum adsorption capacity toward HA (405.5 mg g−1) was achieved on Cu2O NCs, which is higher than any currently reported adsorbents. On the basis of the batch adsorption experiments, the as-prepared Cu2O NCs as IPCA were further applied to photodegradation experiments. More than 99.5% HA molecules could be degraded within 2 h, and the photocatalytic efficiency of Cu2O NCs did not decrease obviously after five cycles, indicating that our Cu2O NCs are stable IPCAs. Moreover, the Cu2O NCs also exhibit excellent degradation efficiency for other organic pollutants (99% for CR, 90% for MO, and 75% TC, respectively). In addition, more than 94% of natural organic matter (NOM) was eliminated by Cu2O NCs from real wastewater, which is served as drinking water in Togtoh County, Inner Mongolia, China. Therefore, obtained Cu2O NCs can be used as a novel IPCA material for the efficient purification of NOM of contaminated ground water.

Cobalt phosphate nanoparticles decorated with nitrogen-doped carbon layers as highly active and stable electrocatalysts for the oxygen evolution reaction

One promising approach to the production of clean hydrogen energy from electrochemical water splitting mainly relies on the successful development of earth-abundant, highly efficient and stable electrocatalysts for the oxygen evolution reaction (OER). Herein, we report the synthesis of robust cobalt phosphate nanoparticles (NPs) decorated with nitrogen-doped carbon layers (denoted as Co3(PO4)2@N-C) using O-phospho-DL-serine as both phosphate and carbon sources by hydrothermal treatment. The obtained Co3(PO4)2@N-C catalyst exhibits a remarkable electrocatalytic performance for the OER in alkaline media. A current density of 10 mA cm−2 is generated at a overpotential of only 317 mV with a small Tafel slope of 62 mV per decade in 1 M KOH electrolyte, which is even superior to those of state-of-the-art noble metal catalysts such as benchmark IrO2 catalysts. Notably, the Co3(PO4)2@N-C electrode shows excellent stability evaluated by 1000 potential cycles and operation with a high current density at a fixed potential for 8 h, which is highly desirable for a promising electrocatalyst. The excellent activity can be attributed to the unique network structure of materials, a large number of active sites and good conductivity under catalytic conditions. Our findings imply the possibility for the development of robust and cost-efficient cobalt phosphate as a promising candidate to replace high-cost and scarce noble metal catalysts for electrochemical water splitting.

Synthesis of one-dimensional WO3–Bi2WO6 heterojunctions with enhanced photocatalytic activity

WO3–Bi2WO6 heterostructures were synthesized by a facile hydrothermal method using WO3 nanorods and Bi(NO3)3 solution as raw materials. The Bi2WO6 nanosheets uniformly anchored onto the surface of the WO3 nanorods. The photocatalytic activity of the samples was assessed for degradation of rhodamine B (RhB) and phenol under solar light irradiation. The WO3–Bi2WO6 heterostructures showed higher photocatalytic activities than pure WO3 and Bi2WO6. As the content of Bi2WO6 increased, the photocatalytic activity of the WO3–Bi2WO6 heterojunction was enhanced and the optimal sample was WO3–Bi2WO6 with a nWO3 : nBi3+ mole ratio of 5 : 3. The efficient separation of electron–hole pairs because of the staggered band potentials between WO3 and Bi2WO6 may account for the higher photoactivity of WO3–Bi2WO6 hybrid structures. Radical scavenger experiments indicate that holes (h+) and superoxide radicals (˙O2−) were the main active species for RhB degradation during the photocatalytic process.