Preparation And Properties Of TiO₂ Nanocrystalline Film By Atmospheric Plasma Chemical Vapor Deposition

Titanium dioxide (TiO2) has shown great applications in new energy, environmental and photocatalytic fields because of its excellent chemical stability, bio-compatability, photocatalytical property and low cost. The preparation, characterization and applications of various new structured TiO2 film have long been an interesting research subject, especially for some actual applications. Plasma chemical vapor deposition (PCVD) is an effective method to fabricate TiO2 film at low temperature. Atmospheric plasma chemical vapor deposition (APCVD) has attracted much attention in recent years owing to its low cost and easy operating at atmospheric pressure.In this paper, we use a homemade APCVD set up to synthesize a 3D anatase TiO2 nanocrystalline film on quartz substrate in one-step from Ar/O2/TiCl4 mixture.Firstly, we analyzed the discharge process of the Ar/O2/TiCl4 APCVD though optical emission spectrograph (OES) and oscillograph. The results indicated that the morphology of the discharge is uniform and there is no any filaments. Both the waveform of the applied voltage and the discharge current are sinusoidal and the phase of the discharge current leads ahead of the voltage. Besides, the peak value of applied voltage and discharge current increased as the discharge power increased, which indicates the capacitive coupling glow discharge characteristics of the plasma. By studying the optical emission spectra of the Ar/O2/TiCl4 plasma discharge, the electron energy was calculated. The electron temperature is about 1.9 eV.The average ion temperature (gas temperature) is about 600 K according to the thermal couple measurement. The optical emission spectra of Ar/O2/TiCl4 also indicated that active particles such as Ti+, O, Cl, ClO, Ar* existed in the plasma, most of which were important for synthesis of such structured titania films.Then, the surface morphology and crystal structure of the deposited titania films were extensively studied. Several parameters such as discharge power, deposition time, flow rate of the carrier gas and ratio of the reactive gas were considered. It is found that the discharge power and the flow rate of argon and oxygen play a key role in the morphologies and crystallization of the titania film when the content of precursor TiCl4 is about 0.2 sccm. When the discharge power is 100 W and below, the TiO2 film is anatase phase. When the discharge power increases to 120 W, the TiO2 film is the mixed phase of rutile and anatase. Deposition time is the main influence parameter on the TiO2 crystal growth. The longer the deposition time, the better the crystal growth. When the discharge power and monomer flow rate is set, the morphology of the TiO2 film was influenced by the flow rate and ratio of Ar and O2. As the increase of Ar flow rate and the decrease of O2 ratio, the size and thickness of the anatase crystal sheet increase and the concentration of {001} facets and the connection of the crystal sheets decrease. We also investigated the influence of the substrate position on the TiO2 film morphology. In the center of the plasma discharge zone, the film displays a closely-packed cauliflower-like morphology. In the afterglow zone, a 3D anatase TiO2 nanocrystalline film with large exposed (001) plane was observed. Between these two areas, a dendritic crystalline film with exposed (001) plane was demonstrated.At last, the luminescence and photocatalysis of such 3D anatase TiO2 crystalline films were studied. It was found that the film exhibits strong white visible light under the irradiation of 325 nm violet light, which is strong enough to be viewed with naked eyes. The photoluminescence (PL) ranges from 350 nm to 700 nm. It is suggested that the PL comes from the band-to-band emission, self-trapped excitations emission and defects emission. The large concentration of oxygen vacancies is mainly responsible for the strong observable visible PL through the X-ray photoelectron spectroscopy (XPS). Compared with the sol-gel synthesized TiO2 films, an improved photocatalytic degradation of the PECVD TiO2 film for methylthionine chloride was also observed. This improved photocatalysis is attributed to the large exposed active (001) planes of such structured 3D anatase TiO2 nanocrystalline film.The new structured 3D anatase TiO2 crystalline film with enhanced PL and photocatalysis could found great applications such as in bio-imaging or bio-probes or photocatalysis fields.