Preparation,Structure And Properties Of Multilayer Composite And Co-Doped TiO₂Thin Films

Owning to the excellent physicochemical properties, such as relatively high photocatalytic activity, cheapness and nontoxicity, nanocrystalline titanium dioxide (TiO2) materials is considered to be one of the most practical prospect of photocatalytic materials. However, due to the wide band gap of TiO2, the generated valence band holes and conduction electrons recombination in the bulk decreases the catalytic efficiency of TiO2, limiting its practical application. Meanwhile, the commonly used TiO2powder at present exist some drawbacks such as difficult recovery, easy reunion, secondary pollution and high cost. Therefore, the development of an efficient photocatalytic material with high photocatalytic activity has become an important task of the current materials research workers.In this paper, monolayer films(TiO2and CU2O), bilayer films composite film(Cu2O/TiO2and Cu2O/noble metal), three layer films composite films (Cu2O/Ag/TiO2> Cu2O/Pt/TiO2、Cu2O/TiO2/Pt、Cu2O/TiO2/Ag) and doped thin films were prepared, respectively. The crystal structures, morphologies and photocatalytic activities of the films were investigated by the scanning electric microscope (SEM), X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), PL, Uv-vis and Raman. Meanwhile, we have attempted to modulate the band gap of anatase-titania using hybrid density functional theory (DFT) calculations to examine the geometry structure and band structure.The TiO2thin films were prepared by magnetron sputtering by a high vacuum multifunctional magnetron sputtering equipment using TiO2targets on glass at room temperature. It was fond that with the increment of the annealing temperature, the phase of TiO2thin film transformed from anatase to rutile phase, and when the annealing temperature was500℃, the thin films showed the highest photocatalytic activity. Besides, the copper films prepared by magnetron sputtering annealed at different temperature(100℃to500℃), the Cu film Cu/TiO2hybrid films, pure Cu2O film, Cu2O/CuO mixed films and pure CuO films could be obtained respectively. And their morphology changed a lot with the increase of the annealing temperature; the photocatalytic experiment showed that the Cu/Cu2O film at150℃and Cu2O at200℃displayed the highest photocatalytic activity.Similar to the method of TiO2monolayer film, Cu2O/TiO2, Cu2O/Ag, Cu2O/Pt bilayer films were prepared by magnetron sputtering. It was found that when the TiO2layer in the bottom, the particle size of the Cu2O was significantly larger, and some of the Cu2O oxidated into CuO. The absorption range of Cu2O/TiO2was extended to the visible region. In the visible light irradiation, the electrons transfered between TiO2and Cu2O band, the photogenerated electrons and holes are separated, so the photocatalytic activity was improved. Meanwhile, for the Cu20/Ag, Cu2O/Pt two composite film, it was found that the underlying precious metals have some impacts on the morphology of the upper Cu2O. When the precious metal substrate is thin, the porosity of Cu2O was much larger; while at the same time, the thin precious metal substrate also makes Cu2O more stable, and not easily oxidized in air. The moderate amount of the underlying precious metals also increased the light absorption ability and photocatalytic activity.Cu2O/Ag/TiO2,, Cu2O/Pt/TiO2, Cu2O/TiO2/Ag and Cu2O/TiO2/Pt three layer thin films were prepared also by magnetron sputtering method. The presence of Ag and Pt precious metals play an important role in making the Cu2O layer composite film surface particle size decreases, with a unique morphology and rough surfaces and increases the surface active sites, leading to higher photocatalytic efficiency. It is also found that the presence of Ag and Pt layer reduces the photo-generated electron and hole recombination rate of the three-layer composite thin films, and promotes the separation of light and the light-generated charge electrons migrate. This is because the noble metal particles of Ag and Pt can as an electron well to promote the effective charge separation and electron transfer to the electron acceptor assist, thus preventing photo-generated electrons and holes recombine. Analysis showed that the presence of noble metal layer such as Pt and Ag in the TiO2film, the photocatalytic property can be improved a lot compared to the two layer composite film. Especially when the precious metals (Ag and Pt layer) in the middle of Cu2O/Ag/TiO2, Cu2O/Pt/TiO2three-layer composite film, the three-layer composite film has a relatively higher visible light catalytic ability. The above study has important guiding significance for the design of efficient composite film material.For the Cu-TiO2, N-TiO2and Cu-N-TiO2thin films prepared by the same method, Cu single doping and co-doping could facilitate the phase transitions, and when the films were calcined at500℃, the rutile phase appeared. Cu, N single doping or co-doped could inhibit the growth of TiO2particles, and the particle size of the doped TiO2films was much smaller, about10nm. In addition, for the N-doped and Cu, N co-doped TiO2films, N partial replace some of the O in the TiO2lattice to form Ti-N and O-Ti-N. It is also found that for Cu doping TiO2, Cu exist in the form of Cu2+, and the Ti4+of TiO2valence state was not changed. The light absorption experiment indicate that Cu, N single-doped and co-doped TiO2showed a redshift, where for the N-doped TiO2, some of O atoms were replaced by N atom, so that the N2p orbital and O2p orbital overlap to form a new valence band, leading to the narrowing of band gap, reducing the energy required for electronic transitions, and resulting redshift. Photocatalytic experiments showed that the photocatalytic activity order of the ultraviolet region sequence is Cu-N co-doped> N single-doped> Cu single doped> pure TiO2, while photocatalytic activity order in the visible region is Cu-N co-doped Miscellaneous> Cu doped single> single N-doped> pure TiO2.For doping experiments, density functional theory method was used to calculate the Cu, N single-doped and Cu, N co-doped geometry around anatase TiO2-It is fond that the quantifying stability gap of Cu doping system is higher than the replacement of doping system. Calculations show that for the Cu-N co-doped anatase TiO2, because of the hybridization Ti3d and N2p energy level, a new energy level appeared at about1.6eV. UV-visible absorption spectra calculations show that doping makes the absorption edge to the redshift. The photocatalytic activity order in the visible region of the sequence is Cu-N-doped> Cu doped> N-doped> pure anatase. The theoretical results confirm experimental studies and lay the foundation for further research and practical application.