Organometallic Titanium Oxides Obtained by Low-Pressure Plasmas of Water on Titanium Tetrapropoxide ()
1. Introduction
Titanium oxide nanoparticles have been produced by solgel, hydrolysis, precipitation or micro-emulsion processes [1-4]. The syntheses are usually carried out in two stages: nucleation from the first chemical reactions with the precursor and growth in which agglomerations of molecules appear. Both can occur in one phase, usually liquid, or combined with vapor or solid phases. The resulting compounds are particles of Ti oxides and residues of the chemical reactions, which depend on the initial precursors. The choice of a precursor, organic or inorganic, to obtain TiO depends on the synthesis route. The most popular precursors are: Titanium tetrachloride (TiCl4), Titanium butoxide (IV) (Ti(OBu)4), Titanium acetyl acetonate (AAT), Titanium tetraisopropoxide (TTIP) and Titanium tetrapropoxide (TTP). The route used in the synthesis of TiO using TTIP is sol-gel, however, it has the disadvantage of needing long synthesis time.
Techniques using accelerated particles in plasmas may be an option for obtaining TiO in less time and with similar morphologies to those obtained by the traditional routes. Low energy plasmas have proven to be very active in the degradation of organic materials and in the formation of polymer nanoparticles with particle size between 35 and 350 nm [5]. On its part, high energy plasmas have been used in the preparation of TiO in amorphous or crystalline phases, as anatase and rutile. The morphology of TiO obtained by plasma has been mainly as partially crystalline thin films with O/Ti atomic ratios between 1.92 and 2.7 [6]. TiO coexists in crystalline and amorphous phases with surfaces up to approximately 79 m2×g−1 for crystalline states and up to 385 m2·g−1 in the amorphous phases [7]. The thermal stability can reach up to 900˚C, depending on the combinations of phases and on the organic content.
The superficial properties of TiO and its derivatives are related to its, intense white color, thermal stability and its ability to absorb and reflect an incident light. When absorbing light, the incident energy can be transferred to other materials on its surface through a photochemical potential. The photo-activity of TiO can be applied to oxidize molecules or to create free radicals or OH groups. This activity can be combined with the large active area of nanospheres to degrade molecules, in the sorption of pollutants [8,9] and in small-scale semiconductor devices [10-12].
In this context, this work has the objective of studying the synthesis of TiO particles with a TTP precursor in plasma glow discharges of water. This precursor is formed with Oxygen atoms around the Ti atom and chains of 3-C hydrocarbon segments attached to each O atom, see Figure 1(a). The accelerated ions in the plasma have the role of removing the organic part of TTP molecules, the 3-C chains, to leave the metal oxide fraction, O-Ti-O, see Figure 1(b). To do this, it is important to balance the synthesis conditions between particles and films.
2. Synthesis of TiO
The syntheses of TiO were made in a vacuum tubular glass reactor of 1500 cm3 with stainless steel flanges and electrodes, 6.5 cm diameter separated 8 cm, see Figure 2. The electric discharges were produced at 13.56 MHz with resistive coupling. TTP was spread on sample holders, frozen with liquid nitrogen and placed inside the reactor, between the electrodes. The degradation of TTP to produce TiO was obtained with water vapor plasmas.
The syntheses were made in the following intervals: 0.3 - 0.9 mbar, 100 - 150 W and 60 - 240 min. Depending on the electric field applied to the reactor, the particles ionize and accelerate in the plasma to form a rain of ions and electrons, which in similar experimental arrangements, the electronic density and energy have been calculated in the order of 108 part/cm3 and 15 eV, respectively [13,14]. In these conditions, the atomic bonds of
(a)
(b)
Figure 1. Illustration of collisions of plasma particles on TTP molecules. (a) TTP. TTP molecule. Ti is the central atom; the red circles around it are oxygen atoms. The branches are composed of 3-C hydrocarbon segments; (b) TTP and particles. A TTP molecule interacting with plasma particles. The smaller dots represent accelerated charged particles.
(a)
(b)
Figure 2. Images of the reactor used in the synthesis of TiO with glow discharges in resistive coupling. (a) Lateral view of the reactor, electrodes and glow discharge; (b) Sample holders with the precursor on the surface; see the position between the electrodes.
TTP molecules, whose energies do not exceed 10 eV, break in multiple points. This effect produces the partial separation of organic and inorganic parts of TTP with the consequent formation of TiO.
The inorganic fraction remains on the sample holder. The smallest organic molecules evaporate and leave the surface of the holders. However, the fragments react among them to form large complex molecules that are difficult to evaporate. No attempts were made to separate the TiO fraction from the organic components in the final products. For this reason, there are traces of the organic content in the TiO powder. This combination is studied calculating the O/Ti and C/Ti atomic ratios in the final products.
3. Results
3.1. Elemental Analysis
The elemental analysis of the organometallic TiO compounds synthesized with two different conditions was done by X-ray energy dispersive spectroscopy (EDS) with an Oxford 7279 probe coupled with a scanning electron microscope and is shown in Table 1. The elements of the analysis were C, O and Ti, see Figure 3. This analysis does not consider the participation of H atoms.
Comparing these numbers with the elemental content of the precursor, it was found that the percentage of C reduced almost 5 times, since the participation in the precursor was 70.6% and decreased to 15.6% in the final compounds. A similar analysis can be applied to the Ti content which increased about 5 times. On its part, the O content increased twice, however this number has the additional contribution of the water added to the synthesis.
These data suggest that in the process some small organic molecules are released, but that the rest remain with the Ti oxides in the final compounds. The C/Ti atomic ratio was reduced by approximately 20 times from the precursor, 11.98, to 0.63 and 0.54 in the final compounds. This ratio suggests that the organic residues are half of the inorganic content. On the other hand, the O/Ti atomic ratio was reduced from 4 in the precursor to 1.96 and 1.89 in the final compounds. These relationships are very close to the typical Ti oxide, 2, (TiO2).
3.2. Crystallography
Figure 3 shows the X-ray diffraction of TiO synthesized by plasma at 100 W and 60 min, the other conditions used generated similar diffraction patterns. The spectra