Electrochemically Catalysed Sol-Gel Process Synthesis and Characterizations of TiO2 Anatase

Journal of Chemistry, Vol. 41, No. 1, P. 125 - 133, 2003 Electrochemically Catalysed Sol-Gel Process Synthesis and Characterizations of TiO2 Anatase Received 2-8-2002 Tran Trung Department of Electrochemistry, Faculty of Chemical Technology, HUT summary TiO2 nanostructured film and powder were prepared by electrochemically catalysed sol-gel process in absolutely ethyl alcohol at room temperature. The obtained TiO2 anatase was confirmed by XRD investigation. The morphology and chemically surface characterization were also investigated by using SEM photograph and FTIR measurements. The average particle size was ranged from 20 to 50 nm. The mechanism of the electrochemically catalysed sol-gel process and the presence and origin of a sub-oxide phase of Ti+3-O species incorporated in the obtained TiO2 were discussed. Key words: Electrochemically catalysed sol-gel, TiO2 anatase, XRD, SEM, FTIR. I - Introduction Sol-gel process, a most usefully and effectively process, has a lot of advantage for preparation of a variety of advanced materials in various structures and sizes, via polymerization of metal and semiconductor hydroxides or via hydrolysis and condensation of their alkoxides, since in nucleophilic substitution (SN) reaction and nucleophilic addition (AN) reaction, the substituent with the largest partial negative charge, -, is the nucleophile, and in SN reactions the substituent with the largest positive charge, +, is the leaving group or nucleofugal. Even though sol-gel products in any aspect, they are either in powder by sol dispersion or in coating by film preparation, the reactions taking place in sol-gel process, for example for metal alkoxides, popular precursors are quite complex, but can be simply represented as follows: nMe(OR)k + [(nk/2) + z] H2O MenOp(nk/2-(x+y)](H2O)z(OH)x(OR)y + [nk-y]ROH (1) In any reaction media, nucleophiles, which attack metal alkoxide molecules and can cause hydrolysis and condensation then are usually water molecules and/or hydroxyl groups. Thus the hydrolysis and condensation of metal alkoxides in a variety of mixed solvents can be controlled by adjustment of pH value. And initially the hydrolysis can be carried out at low pH values by nucleophilic substitution mechanism involving in nucleophilic addition and the formation of the transition states and removal of the protonated species, which is shown in the well-known reaction: H2O+MeOR H O Me OR H (2) OH Me O MeOH + ROH H 125 Followed the condensation of hydrolysed monomers is accomplished at higher pH values. In spite of the fact that the popularity of sol-gel technique used in many various areas of materials science and engineering, especially for sol-gel formation, the electrochemical approach also has been applying for sol-gel processing materials, which have been rapidly developing in recent years [1 - 3]. Under appropriately chosen conditions, some of electrolyte components can be electro-chemically adsorbed onto electrode surface and react together to form nucleophiles, such as usually water molecules and/or hydroxyl groups, which can then enjoy in nucleophilic substitution and addition reactions mentioned above. More recently the electrochemical method also has been used for surface modification of the films prepared by sol-gel process [4, 5]. A number of works, which showed the deposition of metal hydroxide or metal oxide from metallic inorganic salts by electrochemical means, have been reported [5 -10]. These electrodepositions were taken place in organic and aqueous solution employed as a source of hydroxide ions. But very a few works described the electro-deposition of metal oxide films from organic media [10]. In early work we reported the preparation of nanostructured TiO2 anatase films from very fine sols of TiO2 by using spin-coating technique [11]. Followed for the development of methologies for sol-gel film formatiom, we show here the electro-deposition of titanium oxide from tetra isopropyl ortho titanate in organic media, which based on the electrochemical generation of the condensation catalysts. The advantages, which electro-chemical approach can provide, are the low processing temperatures (usually at room temperature), the control of the film thickness by monitoring the delivered charge, and the coelectrodeposition to form the metal oxide/ metal composite films. Electrochemically deposited semiconductor films are strongly adherent to the substrate surface and are composed of aggregated nanocrystallites. Also the nanocrystal size distribution can be controlled by the electrodeposition current and temperature. This work will present our primary studies to develop electrochemical approach for 126 codeposition of very dissimilar materials, such as metal/metal oxide nanocomposites. II - Experimental All starting materials for synthesis involving absolute ethyl alcohol, glacial acetic acid (AcOH) and tetra isopropyl ortho titanate (Ti(O-Pri)4) were used as obtained from supplier, Aldrich. The preparation of electrolyte and all electrochemical experiment were taken place under nitrogen atmosphere and at room temperature. The solution which prepared by dissolving glacial acetic acid into absolutely ethyl alcohol in the ratio of 1 : 60 (v : v) was used as stock solution and maintained at 10oC prior to addition Ti(O-Pri)4. For deposition in a galvanostatic regime, a two-electrode cell composed of a platinum plate foil serving as a counter electrode and an Indium-Tin-Oxide electrode (ITO, S = 1 cm2) serving as a working electrode, cathode electrode. Just before electrodepositions were carried out, Ti(O-Pri)4 were added into stock solution in the AcOH : Ti = 4 : 1 ratio and under vigorous stirring. The products prepared by electrochemically catalysed sol-gel process were dried at 100o C for 24 hours, and heated then at 450oC for 3 hours. The obtained products were studied by using a Jeol scanning electron microscope, model of JSM–5410 LV for surface morphology. The phase content were determined by X-ray diffraction with diffractometer SIEMEN D5000 using mono- chromatized CuK radiation. For investigation of surface-chemical charac-terizations, FT-IR measurements were taken place at wave number ranging from 400 to 4000 cm-1 by using IMPACT 410-NICOLET. All measure-ments were carried out at room temperature (22 ¸ 25oC). III - Results and discussion 1. Electrode reactions, the source providing nucleophiles In order to get most useful information on the important role of electrode reactions, which catalysed hydrolysis and condensation reactions occurred then for the formation of TiO2 colloids in electrolyte, we set up chemically experiments of very fine sols in the same component of those for electrochemically catalysed sol-gel process. The amount of stock solution (60 mL) was dropwise Ti(O-Pri)4 as mentioned above and kept in a covered conical beaker under nitrogen atmosphere and at room temperature. This solution is transparent colorless for long time, at least 27 dates. No change has been observed in UV-Vis spectra. It means that hydrolysis reactions of tetra isopropyl ortho titanate were not occurred under the above conditions during that time. Whilst we also set up electrochemically experiments and observed the change in color of the electrolysis solution and the change of pH values increasing in time and in increasing current. Figure 1: The change of pH value in the electrolyzing duration of stock solution (60 mL) containing 1 mL tetra isopropyl ortho titanate in galvanostatic regime, I = 0.15 mA. Region A presents the relationship of time-pH values before electrolysis, and B corresponds to electrolyzing. The inset shows the change of pH value in the early stage of electrolysis. Figure 1 shows the change in pH values of electrolysis solution at zero current (before electrolyzing) and at a constant current of 0.15 mA. Just before addition of Ti(O-Pri)4, pH value of stock solution is of about 2.5 (measured by using HI8424 pHmeter, HANNA Instruments). This addition increases the measured pH value and kept constant of about 5.0. At this moment the current passed through electrode surface is still zero. However pH value increases rapidly and up to the value of about 8.7 just after a few minutes since the current applied (Fig. 1). This pH value was kept at the values approximately of 8.7 for ten hours before down to then the pH values of about 4.3 and 5.4. It indicates that there coexist two processes related to electrode reactions occurring during the current applied. The first is of the generation of nucleophiles such as water molecules and hydroxyl groups that will be participated in SN and AN reactions then. The other ones consume the performed nuleophiles or perform electrophiles such as protons. For further discussions, we listed here the reactions, which are available to act onto electrode surface and generating nucleophiles or 127 consuming them. In the vicinity of anode, organic molecules and water can be electrochemically adsorbed onto electrode surface via bridging oxygen atom and react together as follows: 2C2H5OH 2(C2H5OH)ad (C2H5)2O +H2O (Etherification) (3) CH3COOH (CH3COOH)ad +(C2H5OH)ad C2H5OOCCH3 + H2O (4) (Esterification) H2O ( H2O)ad - e- 2H++Oad (5) (C2H5OH)ad + 2Oad (CH3COOH)ad +H2O (6) At cathode: H2O ( H2O)ad + e- 2OH- +1/2H2 (7) The reactions (3) and (4) can also occur with organic molecules electrochemically adsorbed onto cathode surface via their bridging hydrogen atoms. Interestingly, there also exist the fact that,(as shown in figure 2), the increase in pH value of stock solution without tetra isopropyl ortho titanate, when constant current of 0.15 mA applied, also supports the existence of electrode reactions as the resource of generation of nucleophiles. But the pH maximum value of 5.5 is much less than the one (pH ~ 8.7) measured in the case of the Ti(O-Pri)4-containing solution. It supports again the coexistence of both mentioned above: generation and consumption of nucleophiles. And it seems that the present of tetra isopropyl ortho titanate molecules adsorbed onto the electrode surface, participate in electrode reactions and accelerate reactions of generating nucleophiles. Figure 2: The change of pH value in the electrolyzing duration of stock solution (60 mL, without tetra isopropyl ortho titanate) in galvanostatic regime of I = 0.15 mA and 0.25.mA The meaning of acceleration can be understood as being increase rapidly in concentration of hydroxyl ion in solution. So the increase can be resulted from either acceleration of occurring reactions or new arising reactions or from both. At present, mechanism of the acceleration is not known well. However we can consider following reactions: Ti(O-Pri)4 + x EtOH Ti(O-Pri)4-x(OEt)x + xPrOH (8) Ti(O-Pri)4 + y AcOH Ti(O-Pri)4-y(OAc)y + yPrOH (9) 128 which can occur simultaneously onto electrode surface and in bulk solution. Excepted for the SN reactions (8) and (9), in a mixture containing acetic acid and Ti(O-Pri)4 usually there exist a 2:2 complex composed of two bridging and four terminal propoxy groups and two bridging CH3COO- ligands [12 - 14]. Therefore, their intermediates denoted as Ti-AcOH-EtOH complexes may contain both AcOH and EtOH molecules. And under these states the reactions (3) and (4), etherification and esterification may occur speedily more than before. Our further studies have been still going on for elucidating mechanism. Also there usually exist the fact that meanwhile the change in color of the solution in sol-gel process is only from apparently transparent to opaque white, but during electrolyzing, peroxotitania complexes were performed in accompanied with the change in color of the solution from apparently transparent to yellow-reddish and finally to opaque white. The color of electrolysis solution has been kept in yellow-reddish for at least ten hours in our experiments. Especially when the ratio of AcOH : EtOH increasing up to 1 : 6 (v : v), the deposit on cathode is consisted of clear yellow-reddish colloids, but its color darker than that of electrolyte. It may be interpreted by the existence of complex colloids of Ti(O)2-(x+y)(OH)x(AcO)y(H2O)z. In the other words, nucleophiles, water molecules, attack atomic titanium of the Ti(O-Pir)4-x(AcO)x complexes and new intermediates of AcOH-Ti-water complex were performed. (a) (b) Figure 3: SEM photographs of TiO2 deposited film (a) and TiO2 powdery sample (b). Both were prepared by electrochemically catalysed sol-gel process. The mentioned above indicates that at the electrode surface tetra isopropyl ortho titanate molecules can be electrochemically adsorbed due to titanium atom (in the molecules) is partial positive charge and also unfilled-d-orbital atom, and electrons will be transferred into unoccupied orbitals in titanium atoms. According to electrochemically induced effect occurred then, it can lead to weakening bonds of titanium atom and isopropyl groups via bridging oxygen atom and the mentioned above complexes were performed easier. 2. SEM studies The SEM micrographs of both the TiO2 film deposited onto ITO electrode and the obtained TiO2 powdery sample, which were prepared in the same conditions were shown in Fig. 3. And both reveal the nanocrystallite structure. Most of the TiO2 nanocrystallites are of spherical-like particles sized in the range of 20 and 70 nm for TiO2 deposited film (Fig. 3a) and from 20 to 50 nm for powdery sample (Fig. 3b). The deposited film is homogeneous and smooth, neither cleavages nor cracks are observed. In the powdery sample, as seen in figure 3b, TiO2 nanocrystallites can be interconnected in two- 129 ... - tailieumienphi.vn