Synthesis Of Nanocrystalline TiO₂ Via Atmospheric-pressure Cold Plasma Generated By Dielectric Barrier Discharge

Anatase TiO₂ plays an important role in extensive applications such as photocatalysis, dye-sensitized solar cells and self-cleaning. Among these applications, TiO₂ films are usedinstead of TiO₂ powders. Low temperature film-fabrication processes are essential for allthermally sensitive or unstable substrate materials such as organic polymers, textiles. Coldplasma chemical vapor deposition (CVD) is highly competent for low temperature fabrication, but low pressure incorporated with sophisticated discharge and vacuum systems has mainlybeen adopted. Dielectric barrier discharge (DBD) is a simple and low-cost approach forgenerating cold plasma at atmospheric pressure. Thereby, DBD has been used to synthesizenanocrystalline anatase TiO₂ in this paper. The results are as follows:1. Using volume DBD, for the first time, nanocrystalline anatase TiO₂ was successfullysynthesized from TiCl₄ and O₂ precursors with one-step procedure at atmospheric pressureand low temperature. The effects of the mode of feeding O₂, the molar ratio of TiCl₄ to O₂(R_TiCl4/O2) and discharge power were investigated.At the same conditions, highly nanocrystalline anatase TiO₂ was achieved feeding O₂seperated from TiCl₄, whereas amorphous structure was mainly formed feeding O₂ mixedwith TiCl₄. In the optical emission spectra, the former gives very strong atomic Ar (4p→4stransition) and Ti emissions, while the latter gives weak atomic Ar (4p→4s transition)emissions. Intensity variation of the characteristic peak, corresponding to anatase A(101), with R_TiCl4/O2 tends to be a single peak. This is in a good agreement with intensity variationof atomic Ar and Ti emissions with R_TiCl4/O2. From XRD, HRTEM and SAED measurements, it was found that TiO₂ crystalline increases and the particle size decreases with the increase indischarge power. Also, it was found that the chlorine contamination dramatically decreases athigh discharge power from EDX measurements.MS in-situ measurements proved that the plasma processes for synthesizingnanocrystalline TiO₂ proceeded stably. TiCl₄ precursor was almost completely consumed andCl₂ was the main gaseous product.2. Using a cyclic "adsorption-discharge" approach, nanocrystalline TiO₂ photocatalystsupported on porous materials was prepared, i.e. in a support-filled DBD reactor, TiCl₄ wasfirst adsorbed ontoγ-Al₂O₃ pellets and then the adsorbed-state TiCl₄ was oxidized to TiO₂ inO₂/Ar plasma generated by DBD. Using this cyclic "adsorption-discharge" approach, the supported nanocrystallineTiO₂/γ-Al₂O₃ photocatalysts, showing high photocatalytic activity and stability, wereprepared successfully. At the experimental conditions, TiO₂/γ-Al₂O₃ photocatalysts preparedby using 6 cycles give the highest photocatalytic activity.According to MS in-situ measurements, no peak related to TiCl₄ appears during O₂/Ardischarge for oxidizing the adsorbed-state TiCl₄. MS signal of the gaseous product, Cl₂, roserapidly at the first several minutes of discharge, followed by a slow increase to a maximum, and then decreased gradually to the baseline at the complete consumption of theadsorbed-state TiCl₄. Compared with O₂/Ar discharge in the absence of adsorbed TiCl₄, thepresence of adsorbed TiCl₄ led to a dramatic decrease of the atomic O (777.0nm, 3⁵P→3⁵S)and Ar (772.4 nm, 4²P_1/24 ²S_1/2; 794.8nm, 4²P_3/2→4 ²S_3/2) emissions. When the adsorbedTiCl₄ was completely consumed, the atomic O and Ar emissions reached nearly the sameintensity as that of the case without TiCl₄ adsorption.3. Using coplanar DBD, an atmospheric-pressure, cold plasma CVD approach fornanocrystalline TiO₂ thin film fabrication was explored. This process allows TiO₂film-deposition on a wide range of materials especially on heat-sensitive materials likeorganic polymers.Nanocrystalline TiO₂ films were successfully fabricated, for the first time, through thiscoplanar DBD-induced plasma CVD route at atmospheric pressure and room temperature.SEM and AFM images indicate that the extremely flat surface of the as-deposited TiO₂ films, composed of uniform-size nanospheres (20～25 nm in diameter), with 0.42 nm of thestatistical root-mean-square (RMS) roughness. The fractured cross-section SEM image showsthat the TiO₂ films appear to have uniform and compact structure over the substrate.Additionally, scratch adhesion test result, giving 27 N of the critical load, proves that theas-deposited TiO₂ films have good adhesion onto the substrate. The UV-Vis spectra showmore than 92% visible light transparency and strong UV absorption for the deposited TiO₂layer with～70 nm thickness. Band gap energy for the as-deposited TiO₂ films was estimatedat 3.3 eV. The clear lattice configuration shown in HRTEM images and the bright diffractionspots shown in SAED patterns strongly support that the as-deposited TiO₂ films are quitecrystalline with anatase formation prevalent under co-presence of both very fine anatase andrutile nanocrystals.As photocatalysts for the destruction of stearic acid the as-deposited TiO₂ films weretested and the kinetics of stearic acid destruction are first order.