Preparation of TiO2 powder using titanium tetraisopropoxide decomposition in a plasma enhanced chemical vapor deposition (PECVD) reactor

Abstract

The preparation of titanium oxide powders has been the subject of many studies in recent years. Titania has many industrial applications as a white pigment, catalyst support, etc. Its use in semiconductor-sensitized photomineralization of organic compounds by oxygen is a common topic of research. Numerous studies have been devoted to investigate the synthesis of titania powders with improved photoactivity [1–3]. The TiO2-sensitized photodegradation of organic compounds has been proposed as an alternative advanced oxidation process (AOP) for the decontamination of water and air [4]. The process is initiated by the generation of hole/electron pairs upon absorption of UV light with energy equal to or higher than the band gap. In titania the threshold energy is ∼3.2 eV (anatase). Electrons and holes are photogenerated in the bulk of the semiconductor, and move within the particle. When the charges reach the particle surface, electrons are able to reduce an electron acceptor (i.e. oxygen) and positive holes can oxidize an electron donor (i.e. organic compounds). The electron/hole recombination is a competitive process that decreases the activity of the TiO2. Crystal defects or impurities may also act as photoactivity-quenching traps. The properties of titania powder, like crystal structure, surface area, morphology, particle size, surface chemistry, etc. are determined by the preparation conditions of the different synthetic routes used in their preparation and strongly influence their photoactivity. Materials with high surface area can adsorb a large amount of organic molecules. For this reason, the preparation of titania powders with high surface area has been extensively studied. Unfortunately, high surface area powders are usually associated with a large amount of crystalline defects that favors the recombination of electrons and positive holes leading to a poor performance in photoactivity. Recently, it has been reported that the photocatalytic activity of amorphous TiO2 is negligible or very poor indicating that crystallinity is an important requirement [5]. Then, finding a compromise between surface area and crystallinity must be attained. The most convenient TiO2 crystalline structure, either anatase or rutile, that could lead to a higher photoactivity is also a point of controversy. Some authors point out that the anatase structure is the more convenient, although a combination of anatase and rutile, or even pure rutile has proven to be favorable for the treatment of some contaminants [3, 6]. The preparation of titania powders was carried out in a cold-wall-type plasma enhanced chemical vapor deposition (PECVD) reactor with an external radiofrequency electrode [7]. The excitation frequency employed was 40 kHz. Titanium tetraisopropoxide (TTIP, supplied by Aldrich) was used as the precursor and oxygen as the carrier gas. The oxygen flow rate was controlled by a mass flowmeter and maintained at 50 sccm. The experiments were carried out under reduced pressure conditions at 30 Pa (measured by a capacitive gauge coupled to a throttle valve). The powder was deposited on the cold areas of the reactor at temperatures not higher than 90 ◦C. The titania powder was formed by vapor phase decomposition of the plasma-activated TTIP precursor in an oxygen atmosphere. The time of synthesis was 90 min. Structural characterization of powders was carried out by powder X-ray diffraction (XRD) in a Rigaku Rotaflex RU-200B. Measurements of powder specific surface area (SBET) were performed by using N2 adsorption/desorption isotherms following the Brunauer– Emmett–Teller (BET) method in a Micromeritics ASAP 2000 apparatus. Morphological characterization was performed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) at 300 kV. The as-grown powder presented a snow flake shape and was amorphous. Scanning electron microscopy (SEM) showed that the particles were spherical with a uniform size. The powder was hydrophobic, probably due to the presence of traces of organic species from the TTIP decomposition which still remain at the powder surface. This was confirmed by an IR spectrum.