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.

One-dimensional ZnO−SnO2 nanofibers with high photocatalytic activity have been successfully synthesized by a simple combination method of sol−gel process and electrospinning technique. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) spectroscopy, nitrogen adsorption−desorption isotherm analysis, UV−vis diffuse reflectance (DR), and photoluminescence (PL) spectroscopy were used to characterize the as-synthesized nanofibers. The results indicated that the ZnO−SnO2 nanofibers with diameters of 100−150 nm consisted of wurtzite ZnO and rutile SnO2. The photocatalytic activity of the ZnO−SnO2 nanofibers for the degradation of rhodamine B (RB) was much higher than that of electrospun ZnO and SnO2 nanofibers, which could be attributed to the formation of a ZnO−SnO2 heterojunction in the ZnO−SnO2 nanofibers and the high specific surface area of the ZnO−SnO2 nanofibers. Notably, the ZnO−SnO2 nanofibers could be easily recycled without the decrease of the photocatalytic activity due to their one-dimensional nanostructural property.

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.

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.

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).

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 situ reduction approach (for AgNPs). The highly dispersed AgNPs assembled on the inner and outer surface of SNTs through the in situ reduction of Ag + by Sn 2+ 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 situ reduction 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 NaBH 4 as the reductant. The results indicated that all the tubular nanocomposites catalysts with high specific surface area (185-250 m 2 g À1 ) 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 BH 4 À 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.

In this work, ZnO hollow nanofibers with diameters of 120-150 nm were successfully fabricated by electrospinning the precursor solution consisting of polyacrylonitrile (PAN), polyvinylpyrrolidone (PVP), and zinc acetate composite through a facile single capillary, followed by thermal decomposition for removal of the above polymers from the precursor fibers. The as-prepared nanofibers were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), resonant Raman spectra, thermal gravimetric and differential thermal analysis (TG-DTA), and Fourier transform infrared spectroscopy (FT-IR) spectra, respectively. The results indicated that, during the electrospinning process, there occurred phase separation between the electrospun composite materials, while the obtained precursor nanofibers of PAN, PVP, and zinc acetate composite might possess a core-shell structure (PAN as the core and PVP/zinc acetate composite as the shell). Furthermore, the composite nanofibers with core/shell structure could play a structural directing template role for preparing ZnO hollow nanofibers during the calcination process. The ZnO hollow nanofibers exhibited excellent sensing properties against ethanol due to their special one-dimensional nanostructural properties.

Doping TiO 2 photocatalysts with foreign ions has been deemed an effective method to enhance visible light absorption, and thus increase their photocatalytic performance. Herein, we report that Nb-doped TiO 2 porous microspheres prepared by ultrasonic spray pyrolysis of peroxide precursor solution are yellow colored, and the yellow coloration becomes increasingly conspicuous with increasing Nb dopant concentration.Comprehensive spectral analyses show that both surface peroxo species and bulk Ti 3+ are introduced into TiO 2 microsphere samples together by charge compensation with Nb 5+ dopant, and are responsible for the coloration of TiO 2 . The Nb-doped microspheres show higher photocatalytic rates than undoped TiO 2 for the degradation of gaseous acetaldehyde under visible irradiation, but slower rates under ultraviolet light. Moreover, the photocatalytic mineralization rates of acetaldehyde to CO 2 are lowered with Nb-doping under both visible and UV irradiation. Correlation between the results of surface photovoltage spectroscopic (SPS) characterizations and photocatalytic tests suggests that surface peroxo states are relevant to the visible light stimulated charge separation and photocatalytic reactions, albeit holes trapped in these states have lower reactivity than in valence band. On the other hand, the enhanced photoluminescence in the near-infrared region, reduced SPS response in the UV region as well as photochromic phenomena during photocatalytic process indicated that Ti 3+ defects serve as charge carrier recombination centers and display adverse effect to photocatalytic activity of Nb-doped TiO 2 , especially under UV irradiation.