Yaan Cao

Nanocrystalline Tungsten Oxide Thin Film: Preparation, Microstructure, and Photochromic Behavior

A nanocrystalline tungsten oxide photochromic thin film was prepared by colloid chemistry method. The microstructure, phase transition involved in the solution process, photochromic behavior, and mechanism of the film were investigated by means of transmission electron microscope, x-ray diffraction, ultraviolet-visible absorption spectra, and x-ray photoelectron spectra. It was found that the particle size and crystallinity of the thin film could be easily controlled by adjusting the concentration of oxalic acid in the colloid solution of tungsten oxide hydrate. With the increase of the oxalic acid concentration, the size of nanoparticles in the film decreased sharply, and meanwhile, a blue shift of the absorption peaks caused by the quantum size effect was observed accordingly. With the increase of the pH in the solution, tungsten oxide hydrate was gradually transformed into an oxided 12-tungstate with Keggin structure, which led to the change of photochromic property of the films. The photochromism of the film is believed to be due to the electron transfer between the different valence states of tungsten ions located in adjacent sites.

Photocatalytic activity of TiO2 films grown on different substrates.

Titanium dioxide films were prepared on glass, indium-tin oxide (ITO) glass and p-type monocrystalline silicon and studied for the photocatalytic degradation of rhodamine B in an aqueous medium. Raman, AFM, and XPS spectroscopic investigations of these films indicated that microstructure of titanium oxide films were greatly affected by the substrate materials. Rutile was confirmed to be easily formed on the surface of ITO glass, and TiO2 tended to grow as closely packed particles that were elongated strips with an average size of 20 nm, and had lovely contrast with the perfectly round particles grown on p-type monocrystalline silicon. Charge transfer between the film and silicon substrate was verified by surface photovoltage spectra. This may be the real reason why the films grown on ITO glass and silicon substrates exhibit higher photocatalytic reactivity than the film on glass substrate. Moreover, the different surface properties also seem to be responsible for the different activity.

Improved photochromism of WO3 thin films by addition of Au nanoparticles

A WO3/Au composite thin film is prepared by depositing gold nanoparticles onto the surface of a vacuum-evaporated WO3 thin film through a spin-coating technique. It is found that the photochromism of the WO3 thin films are enhanced greatly after the modification with Au nanoparticles. The enhancement effect is explained according to the Schottky barrier formed at the WO3/Au interface and the surface effect of the gold nanoparticles. The results may be helpful in technical applications and provide a more detailed understanding of the photochromic mechanism.

Photocatalytic activity of TiO 2 prepared at low temperature by a photo-assisted sol-gel method

TiO 2 nanocrystals were prepared by a photo-assisted sol-gel process in which tetrabutoxide titanate was hydrolyzed in acidic medium under ultraviolet irradiation. X-ray diffraction and Raman spectra showed that the as-prepared TiO 2 particles without further annealing were well-crystallized anatase. Such TiO 2 particles were easily immobilized on dacron cloth and showed very high photocatalytic activity. In contrast, TiO 2 particles were ill crystallized and showed lower activity when no light was introduced under otherwise equal conditions.

Enhanced visible-light coloration and its mechanism of MoO3 thin films by Au nanoparticles

A new kind of MoO3/Au film was prepared by depositing Au nanoparticles on the surface of MoO3 thin film through spin-on coating technique. After cathodic polarization, the MoO3/Au thin film was found to show enhanced visible-light coloration compared with MoO3 thin films. The formation of Schottky barrier at the MoO3/Au interface, and the visible-light coloration mechanism of the MoO3/Au film are elucidated by the energy band theory based on surface photovoltaic spectroscopy measurements.

Promoted phase transition of titania nanoparticles prepared by a photo-assisted sol-gel method

A simple and efficient methodology, in which ultraviolet irradiation was introduced into the preparation of the titania colloids, has been established for the synthesis of phase-pure anatase with a grain size of about 3.6 nm at calcination temperatures as low as 100 °C. The XRD and Raman results indicate that the phase transition of TiO2 from amorphous to anatase is promoted by UV light irradiation. It is also documented that the promoted formation of anatase upon calcination is due to the oxygen vacancy defects induced by photo-irradiation of the colloid during the sol-gel process.

Enhanced electrochromism of WO3 thin film by gold nanoparticles

Gold nanoparticles synthesized by the chemical reduction of hydrogen tetrachloroaurate were used to modify the surface of vacuum evaporated WO3 thin films through a spin coating technique. It was found that the gold nanoparticles could improve the electrochromic performance of the WO3 film greatly. The experimental results indicated that the conductivity of the thin film was increased after the surface modification. In addition, it was found that the composite thin film could show a better electrochromic performance in aqueous acid system than in organic solution.

Effects of zinc(II) and iron(III) doping of titania films on their photoreactivity to decompose rhodamine B

The heterogeneous photocatalytic oxidation of rhodamine B in aqueous solution containing pure or zinc (iron)-doped titania films has been studied. N-deethylation of rhodamine B was accelerated by iron(III) and zinc(II) doping as compared with pure titania film. It is shown that improvement of electron transfer from dye molecules to the film may be responsible for the high N-deethylation rate for iron-doped (0.5 mol%) film, while for zinc-doped (20 mol%) film, high surface roughness may be the main reason. In addition, both iron and zinc doping brought a new shallow trap to the intragap meaning that the surface defects had increased after doping; this is a possible reason doped films present relative low photoreactivity to catalyze the direct degradation of dye molecules.

Study on the photo-induced interfacial charge transfer in TiO2/Fe2O3 heterostructured composite film

Heterostructured TiO 2 /Fe 2 O 3 composite film was obtained by spin-coating α-Fe 2 O 3 hydrosol on the TiO 2 nanoparticulate film prepared by the sol-gel method. The composite film was characterized by surface photovoltaic spectroscopy (SPS) and electric-field-induced surface photovoltaic spectroscopy (EFSPS), and the results showed that the composite film had the extended photovoltaic response which can be modified by the external electric field. Based on these results, a photoinduced interfacial charge transfer mechanism was deduced and the energy band structure of the composite film was concluded.

Study on photoelectric properties of a TiO2 nanoparticle

Here we report a study of surface states in nanometer-sized TiO2 particles (P1) prepared in our lab with the aid of surface photovoltage spectroscopy, electric field-induced photovoltage spectroscopy (EFISPS), and fluorescence spectrum. The diameter determined by x-ray diffraction is 3.5 nm. The EFISPS of P1 shows several subband gap absorption bands at 350, 375, and 460 nm, respectively. The fluorescence peaks of dry powder and an aqueous suspension of P1 at 361, 391, and 469 nm were detected. The above results are concluded to be relative with the located surface states within the band gap. The surface states were deduced to be oxygen vacancies, i.e., Ti3+, Ti2+, Ti+, whose maximum densities lie at 0.32, 0.50, and 1.02 eV below the maximum density of states in the conduction band, respectively.