Changhua Wang

Electrospun Nanofibers of p-Type NiO/n-Type ZnO Heterojunctions with Enhanced Photocatalytic Activity

One-dimensional electrospun nanofibers of p-type NiO/n-type ZnO heterojunctions with different molar ratios of Ni to Zn were successfully synthesized using a facile electrospinning technique. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance (DR) spectroscopy, resonant Raman spectroscopy, photoluminescence (PL) spectroscopy, and surface photovoltage spectroscopy (SPS) were used to characterize the as-synthesized nanofibers. The results indicated that the p-n heterojunctions formed between the cubic structure NiO and hexangular structure ZnO in the NiO/ZnO nanofibers. Furthermore, the photocatalytic activity of the as-electrospun NiO/ZnO nanofibers for the degradation of rhodamine B (RB) was much higher than that of electrospun NiO and ZnO nanofibers, which could be ascribed to the formation of p-n heterojunctions in the NiO/ZnO nanofibers. In particular, the p-type NiO/n-type ZnO heterojunction nanofibers with the original Ni/Zn molar ratio of 1 exhibited the best catalytic activity, which might be attributed to their high separation efficiency of photogenerated electrons and holes. Notably, the electrospun nanofibers of p-type NiO/n-type ZnO heterojunctions could be easily recycled without a decrease of the photocatalytic activity due to their one-dimensional nanostructural property.

In situ assembly of well-dispersed gold nanoparticles on electrospun silica nanotubes for catalytic reduction of 4-nitrophenol

The tubular nanocomposite with well-dispersed distribution of small gold nanoparticles (AuNPs) assembled on the inside and outside surfaces of silica nanotubes (SNTs) was fabricated by combining the single capillary electrospinning technique and an in situ reduction approach. The AuNPs/SNTs nanocomposite exhibited a good catalytic activity for reduction of 4-nitrophenol (4-NP).

SnO2 Nanostructures-TiO2 Nanofibers Heterostructures: Controlled Fabrication and High Photocatalytic Properties

Combining the versatility of the electrospinning technique and hydrothermal growth of nanostructures enabled the fabrication of hierarchical SnO(2)/TiO(2) composite nanostructures. The results revealed that not only were secondary SnO(2) nanostructures successfully grown on primary TiO(2) nanofiber substrates but also the SnO(2) nanostructures were uniformly distributed without aggregation on TiO(2) nanofibers. By adjusting fabrication parameters, the morphology as well as coverage density of secondary SnO(2) nanostructures could be further controlled, and then SnO(2)/TiO(2) heterostructures with SnO(2) nanoparticles or nanorods were facilely fabricated. The photocatalytic studies suggested that the SnO(2)/TiO(2) heterostructures showed enhanced photocatalytic efficiency of photodegradation of Rhodamine B (RB) compared with the bare TiO(2) nanofibers under UV light irradiation.

A Facile in Situ Hydrothermal Method to SrTiO3/TiO2 Nanofiber Heterostructures with High Photocatalytic Activity

Heterostructured SrTiO3/TiO2 nanofibers were fabricated by in situ hydrothermal method using TiO2 nanofibers as both template and reactant. The as-fabricated heterostructures composite included SrTiO3 nanocubes or nanoparticles assembled uniformly on the surface of TiO2 nanofibers. Compared with the pure TiO2 nanofibers, SrTiO3/TiO2 nanofibers exhibited enhanced photocatalytic activity in the decomposition of Rhodamine B (RB) under ultraviolet light. The enhanced photocatalytic activity of SrTiO3/TiO2 nanofibers could be attributed to the improvement of charge separation derived from the coupling effect of TiO2 and SrTiO3 nanocomposite.

Tubular nanocomposite catalysts based on size-controlled and highly dispersed silver nanoparticles assembled on electrospun silica nanotubes for catalytic reduction of 4-nitrophenol

Tubular nanocomposites of silver nanoparticles (AgNPs)/silica nanotubes (SNTs) with the nearly uniform diameters of 250–350 nm were successfully fabricated by combining the single capillary electrospinning technique (for SNTs as the supports) and an in situreduction approach (for AgNPs). The highly dispersed AgNPs assembled on the inner and outer surface of SNTs through the in situreduction of Ag+ by Sn2+ ions were confirmed by transmission electron microscopy (TEM), UV-Vis absorption spectra and X-ray photoelectron spectroscopy (XPS). It was interesting to note that the size of AgNPs on the surface of SNTs could be controlled by appropriately adjusting the amount of ammonia solution during the above in situreduction reaction. The catalytic activities of the as-prepared tubular nanocomposites were evaluated by using a model reaction based on the reduction process of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) in the presence of NaBH4 as the reductant. The results indicated that all the tubular nanocomposites catalysts with high specific surface area (185–250 m2 g−1) exhibited excellent catalytic activities because the highly dispersed AgNPs were exposed on the inner and outer surface of electrospun SNTs, allowing effective contact with the reactants and catalysis of the reaction. In particular, the tubular nanocomposite catalysts containing small size AgNPs had higher catalytic activities than those containing the large size ones, which was attributed to the size-dependent Ag redox potential and surface-to-volume ratio influencing interfacial electron transfer from AgNPs surface to 4-NP in the presence of highly electron injecting BH4− ions. Those tubular catalysts based on AgNPs/SNTs nanocomposites could be easily recycled without a decrease of the catalytic activities due to their one-dimensional nanostructural property.

Water−Dichloromethane Interface Controlled Synthesis of Hierarchical Rutile TiO2 Superstructures and Their Photocatalytic Properties

A water-dichloromethane interface is used for synthesis and assembly of rutile TiO(2) nanorods. By hydrothermal treatment of a dichloromethane solution of TiCl(4) at the interface of water-dichloromethane, turning to no surfactant or template, hierarchical rutile TiO(2) superstructures are developed. By tuning the molar ratio of reactants r(w) (H(2)O/TiCl(4)), the size and shape of the samples significantly change. At a low value of r(w), highly extended, robust, porous, and thick titania film with ordered rutile nanorod bundles are deposited at the interface. At a high value of r(w), powders consisting of hierarchical rutile nanorod spheres together with disordered nanorods are obtained. A rational formation mechanism is proposed on the basis of a range of experiments. The main factors influencing the morphologies of the samples may be attributed to the acidity of the reaction system and the adsorption ability of the precursor nanoparticles to the water-dichloromethane interface. The as-obtained rutile TiO(2) hierarchical superstructures show higher photocatalytic property to decompose methylene blue (MB) dye compared with that of commercial P25, which can be ascribed to the contribution of high surface area and high crystallinity. Other applications, such as solar energy conversion, environmental remediation, and advanced optical/electric nanodevices may also benefit from the unique properties of the hierarchically rutile TiO(2) superstructures.

Bi4Ti3O12 nanosheets/TiO2 submicron fibers heterostructures: in situ fabrication and high visible light photocatalytic activity

In this paper, a facile two-step synthesis route combining an electrospinning method and hydrothermal technique has been accepted as a straightforward protocol for the exploitation of Bi4Ti3O12/TiO2 heterostructures which are composed of Bi4Ti3O12 nanosheets on the surface of TiO2 submicron fibers. The thickness of the as-grown nanosheets was about 20 nm and the size of the nanosheets increase with the increase of precursor concentration. Photocatalytic tests display that the Bi4Ti3O12/TiO2 heterostructures possess a much higher degradation rate of rhodamine B (RB) than the unmodified TiO2 submicron fibers under visible light. The enhanced photocatalytic activity can be attributed to the extended absorption in the visible light region resulting from the Bi4Ti3O12 nanosheets, and the effective separation of photogenerated carriers driven by the photoinduced potential difference generated at the Bi4Ti3O12/TiO2 junction interface.

Growth of single-crystalline rutile TiO2 nanowire array on titanate nanosheet film for dye-sensitized solar cells

A titanate nanosheet (TN) film is employed as a seed layer for the hydrothermal growth of a single-crystalline rutile TiO2 nanowire array on SnO2:F (FTO) conductive glass for dye-sensitized solar cell (DSSC) applications. TiO2 nanowires grown on a TN film appear to be thinner, more uniform, and well separated from each other, compared with those grown directly on FTO conductive glass. Besides FTO conductive glass, TiO2 nanowires can also be grown on silicon wafers and glass slides when a TN film is involved. Scanning electron microscope observations showed that the TN film underwent surface coarsening and thinning during the nucleation and growth of the TiO2 nanowires, suggesting that the film may act as a sacrificial seed layer. When applied in a DSSC, the nanowire photoanode with a TN film involved is significantly superior to that without the film in terms of all cell parameters, and gave an overall solar energy conversion efficiency of over 3% under AM 1.5G solar irradiation, about 3.4 times greater than that without the TN film. The combination of increased dye loading amount and reduced charge recombination at the FTO glass/electrolyte interface due to the involvement of the TN film should contribute to the significant improvement in cell performance.

Rutile TiO2 nanowires on anatase TiO2 nanofibers: A branched heterostructured photocatalysts via interface-assisted fabrication approach

A water-dichloromethane interface-assisted hydrothermal method was employed to grow rutile TiO(2) nanowires (NWs) on electrospun anatase TiO(2) nanofibers (NFs), using highly reactive TiCl(4) as precursor. The water-dichloromethane interface inhibited the formation of rutile NWs in water phase, but promoted the selective radial growth of densely packed rutile NWs on anatase NFs to form a branched heterojunction. The density and length of rutile NWs could be readily controlled by varying reaction parameters. A formation mechanism for the branched heterojunction was proposed which involved (1) the entrapment of rutile precursor nanoparticles at water-dichloromethane interface, (2) the growth of rutile NWs on anatase NFs via Ostwald ripening through the scavengering of interface-entrapped rutile nanoparticles. The heterojunction formed at anatase NF and rutile NW enhanced the charge separation of both under ultraviolet excitation, as evidenced by photoluminescence and surface photovoltage spectra. The branched TiO(2) heterostructures showed higher photocatalytic activity in degradation of rodamine B dye solution than anatase NFs, and the mixture of anatase NFs, and P25 powders, which was discussed in terms of the synergistic effect of enhanced charge separation by anatase-rutile heterojunction, high activity of rutile NWs, and increased specific area of branched heterostructures.

Rutile TiO2 nanowire array infiltrated with anatase nanoparticles as photoanode for dye-sensitized solar cells: enhanced cell performance via the rutile–anatase heterojunction

The structural and interfacial design of TiO2 photoanodes plays an important role in improving the solar energy conversion performance of dye-sensitized solar cells (DSSCs). Herein, we report that a rutile nanowire (NW) array infiltrated with anatase nanoparticles (NPs) can combine the advantages of the one dimensional electron transportation and light scattering of a NW array and the large dye-loading capacity of NPs, due to the presence of a rutile–anatase heterojunction. The dye-sensitized NW–NP composite film (1.4 μm thick) with a roughness factor of ∼114.7 displays a significantly improved light harvesting ability than a NW array (roughness factor ∼28.2), as manifested by diffuse transmittance and reflection spectra, and even higher light harvesting than a NP film (1.5 μm thick) with a roughness of ∼414.7. Moreover, the dye-sensitized NW–NP composite film shows slower charge recombination kinetics than both the NW array and NP film, as measured by open-circuit photovoltage decay and transient absorption spectroscopy. As a result, the dye-sensitized TiO2 NW–NP composite photoanode exhibits 2.2 times and 1.5 times higher overall efficiency than NW array and NP film photoanodes, respectively, under AM 1.5G simulated solar irradiation, demonstrating the synergistic effect of rutile NW and anatase NP for photoelectrochemical solar energy conversion.