Synthesis of titanium oxide particles in supercritical CO2: Effect of operational variables in the characteristics of the final product

J. of Supercritical Fluids 39 (2007) 453–461

Synthesis of titanium oxide particles in supercritical CO2: Effect of operational variables in the characteristics of the final product E. Alonso ∗ , I. Montequi, S. Lucas, M.J. Cocero Dpto. Ingenier´ıa Qu´ımica y Tecnolog´ıa del Medio Ambiente, Universidad de Valladolid, Prado de la Magdalena s/n, Valladolid 47005, Spain Received 1 June 2005; received in revised form 1 February 2006; accepted 6 March 2006

Abstract A new chemical precursor is proposed for the synthesis of TiO2 anatase nanoparticles in supercritical CO2 : the organometallic diisopropoxititanium bis(acetylacetonate) (DIPBAT). DIPBAT thermohydrolysis in supercritical carbon dioxide (SC-CO2 ) has been studied in the range 10.0–20.0 MPa and 200–300 ◦ C, and compared with that of titanium tetraisopropoxide (TTIP). The proposed reaction mechanism is a thermohydrolysis, where the hydrolysis of acetylacetonate groups is the limiting step of the reaction rate. The addition of water directly to the reaction favours the growth of formed particles, whereas ethanol offers better results as hydrolysing reactant, leading to smaller particles. Experiments have been performed first in a batch process and secondly in a semi-continuous one, varying the residence time in the reactor from 30 s to 2 min. The effect of operational variables in the final product and their influence in the different steps of the process have been studied. Results have shown that product crystallinity is related with temperature, and temperatures higher than 250 ◦ C are necessary to obtain well-crystallized TiO2 anatase. In the same sense, area Brunauer–Emmett–Teller (BET) is connected with crystallinity, and amorphous product, Ti(OH)4 , shows the highest surface area. Particle size and particle size distribution (PSD) are controlled by instantaneously supersaturation degree, and precursor concentration together with pressure are the main responsible of particle size control. Operational conditions influence solubility of species, mass transfer, chemical reaction and nucleation and particle growth and they mark the final characteristics of the product and its application. In such a way, good crystallized TiO2 anatase particles of about 200 nm in diameter have been obtained working at 300 ◦ C, 20.0 MPa and residence time of 2 min, with a reaction medium composed by CO2 /ethanol (80/20, v/v). Such particles present good optical properties and specific surface area BET of around 150 m2 /g. At lower working temperatures the obtained particles present worse crystallinity; however, their specific surface area increases to 350 m2 /g and they are suitable as support of metal clusters in heterogeneous catalysis. © 2006 Elsevier B.V. All rights reserved. Keywords: TiO2 ; Particle synthesis; Supercritical synthesis

1. Introduction The properties of the supercritical fluids, mainly H2 O or CO2 , and their tunability with operational parameters are very attractive for particle generation and precipitation. They allow controlling particle properties such as morphology, structure or particle size and particle size distribution (PSD), very important for the final application of the product. Adschiri [1] has obtained different metal oxide nanoparticles in subcritical and supercritical water, showing that hydrothermal synthesis is a promising technique for particle formation. Supercritical CO2 has also been

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used for the synthesis of particles or films interesting at nano- or micro-size such as magnetic nanoparticles [2], metal films [3] or membrane modification [4]. Supercritical CO2 allows working at milder operational conditions than supercritical water, and products with different morphology and/or crystallinity can be obtained. Metal alkoxides are the main organic precursors for the precipitation reactions in supercritical CO2 [5], whereas inorganic salts such as nitrates or sulphates are the main reactants for hydrothermal synthesis [6]. Frequently the addition of a second reactant, or a stabilization agent [7], to the reaction medium is necessary in order to carry out the reaction and to obtain smaller particles. This is the case of microemulsions [8]. A review of the use of supercritical fluids for material synthesis can be found in references [9–11]. However, in the literature there is not a systematic study of operational variables effect in the process and

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their influence in the different stages, and this fact makes difficult an experimental planning for future experiments and products. In this work, titanium dioxide has been selected as desired product to carry out a study of the operational parameter effect, and this oxide can represent a model compound for such kind of processes. Titanium oxide is an important material that has been extensively used because of various interesting physical and chemical properties. Titanium dioxide is used in heterogeneous reactions, as photocatalyst, in solar cells, as white pigment, as a corrosion-protective coating, as an optical coating, in ceramics and in electrical devices, where the control of particle size, crystallinity and surface area is very important. Applications of TiO2 are extensively described in reference [12]. Titanium oxide has been prepared by liquid-phase methods such as the sol–gel synthesis and by gas-phase methods as chemical vapour deposition (CVD). In both cases the precursor reactant is an alkoxide. In the case of the sol–gel process a hydrolysis reaction takes place, otherwise when the CVD is carried out the main involved reaction is a thermolysis. At first, the use of supercritical fluids for this purpose was related to the drying stage of the sol–gel process [13]. Removing the solvent with supercritical fluids it is possible to obtain particles with higher specific area than doing it by heating. The synthesis of titanium oxide in a supercritical media has been studied, leading to lower temperatures compared to classical methods (CVD or sol–gel) and higher reaction rates. In that sense, several authors have proposed titanium tetraisopropoxide (TTIP) thermal decomposition in supercritical ethanol or isopropyl alcohol [14,15]. Supercritical water has also been used for hydrothermal synthesis of TiO2 nanoparticles; Adschiri et al. have prepared TiO2 nanoparticles from inorganic titanium salts such as TiCl4 and Ti(SO4 )2 [16]. In this case the water acts as a reactant in the formation of the metal oxide, and the pro-

posed mechanism is a two step mechanism: first, the hydrolysis reaction in which the metal hydroxide is produced, and then, the dehydration of the metal hydroxide takes place in order to obtain the metal oxide. Water is also a modifier that can be added to the CO2 medium to obtain TiO2 particles. Reverchon et al. [17] and Papet et al. [18] have used a CO2 /H2 O medium to carry out the Ti(OH)4 synthesis from TTIP, H2 O takes part in the reaction as a hydrolysis reactant. A reaction medium CO2 /alcohol has been selected for the present work, and the influence of pressure, temperature, addition of cosolvents, precursor concentration and residence time has been investigated for particle morphology, size distribution and PSD, specific surface area, crystallinity and chemical purity in the synthesis of TiO2 anatase from two different precursors: diisopropoxititanium bis(acetylacetonate) (DIPBAT) and titanium tetraisopropoxide. 2. Experimental 2.1. Chemicals Diisopropoxititanium bis(acetylacetonate) 75 wt% solution in isopropanol provided by Sigma–Aldrich, and titanium tetraisopropoxide pure by Fluka, were used as chemical precursors without further purification. Three different additional reactants were tested: absolute ethanol (analytical reagent) and isopropyl alcohol (analytical reagent) both provided by Panreac, and distilled water. The CO2 (99.9% purity) was used as received from the commercial supplier Carburos Met´alicos S.A. 2.2. Process A versatile pilot plant has been developed in the Chemical Engineering Department at the University of Valladolid, to oper-

Fig. 1. Flow diagram of the laboratory plant used for the synthesis of TiO2 particles in SC-CO2 (P-120: CO2 pump; P-130: DIPBAT pump; R-110: reactor; K-112: filter; V-1: backpressure valve; V-2: decompression valve; S-140: liquid–gas separator).

E. Alonso et al. / J. of Supercritical Fluids 39 (2007) 453–461

ate up to 35.0 MPa and 400 ◦ C. The facility is used for solubility measurements, using a dynamic way, and for solid precipitation reactions. The flow diagram is presented in Fig. 1. The plant has a stainless steel vessel (R-110) with 100 mL of internal volume and it is equipped with pressure and temperature sensors. The vessel (that acts as reactor in these experiments) is heated by means of two electrical resistances and it is located into a furnace in order to avoid heat losses. Two high-pressure pumps (P-120 and P-130) allow the entry of CO2 and precursor solution, respectively, into the reactor. Both of them are piston pumps supplied by Dosapro Milton Roy (type Milroyal D). A backpressure valve (V-1), from GO Regulator, Inc., is used to keep the pressure constant during the semi-continuous process. An in-line filter (K-112), 0.5 ␮m pore size, is placed before the backpressure valve to filter out larger particles in order to avoid clogging in the backpressure valve. The TiO2 obtained is collected at the end of each operation at the bottom of the reactor. The residual liquids (unreacted precursor, alcohol, byproducts of the reaction, . . .) were collected at the phase separator (S-140). Experiments have been performed in batch and in semicontinuous operation, and in both cases the facility presented in Fig. 1 has been used: - Batch process: Reactants are charged into the reactor and CO2 is pumped to the reactor until the work pressure is reached. At the same time the system is heated up until the desired reaction temperature. Once these values are reached time starts to count. In all batch experiments residence time was 2 h. After this time the system is decompressed and TiO2 is recovered inside the reactor (R-110). Heating time is less than 5 min. - Semi-continuous process: The precursor alcoholic solution and the CO2 are continuously pumped to the reactor. Particles are recovered inside the reactor at the end of the process. At first, the empty reactor is heated up until the desired reaction temperature. Once it is reached, first the CO2 pump and then the solution pump are connected in order to raise the work pressure. CO2 mass flowrate is instantaneously measured by a Coriolis flowmeter, and the solution flow is determined by level measurement in the feed tank. 2.3. Characterization techniques The crystal structure of the TiO2 powders was investigated by X-ray powder diffraction (XRD) using Cu K␣ radiation and ASTM D-476-84:1 for TiO2 anatase crystal structure identification. The morphology and average particle size of the samples were characterized by scanning electron microscopy (SEM) using JEOL JSM-T300. The particle size distribution was determined by using a laser scattering particle size distribution analyzer (Horiba LA900). The specific area of the powders was evaluated by means of nitrogen adsorption Brunauer–Emmett–Teller (BET) (Omnisorp 100 CX). The TiO2 purity was determined by analysis of the carbon (analyzer of C and S with Leco Cs225 determinator), since it is assumed that all the contamination of the samples comes from the organic part of the precursor molecule.

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2.4. Experimental planning In order to plan this kind of experimental work, it is necessary to consider what is happening inside the reactor and to analyze the different stages that take place. It is possible to distinguish four different steps: (a) solubilization, (b) mass transfer, (c) chemical reaction and (d) nucleation and crystal growth. Each of them is analyzed in the following sections. 2.4.1. Solubilization Fluid phase composition is important from two points of view: solubility of reactants and solubility of products. The solubility of chemical precursor marks reactant concentration in the supercritical phase and therefore, it has an important effect on reaction rate. The solubility of desired solid product initiates the nucleation step, with the precipitation of the product controlled by the supersaturation degree of the medium. The size of the particles obtained in the reaction is very sensitive to supersaturation, and the higher the supersaturation degree, the smaller the particles produced [19]. Moreover, solubility of byproducts in the supercritical phase is directly related to product pollution, since these compounds must be removed with the solvent, and therefore, they must be completely soluble in the solvent phase. DIPBAT solubility in supercritical carbon dioxide (SC-CO2 ) has been determined in the range 10.0–20.0 MPa and 80–200 ◦ C using a dynamic method [20]. According to these experiments, DIPBAT solubility increases with pressure up to 0.074 g DIPBAT/g CO2 at 20.0 MPa and 80 ◦ C, and it decreases with temperature, being minimum at 10.0 MPa and 200 ◦ C (solubility = 0.0005 g DIPBAT/g CO2 ). Thus, from the point of view of precursor solubility reaction must occur at high pressures, but at the lowest possible temperature. Moreover, the addition of 20 vol% ethanol increases up to 12 times these values, as it acts as cosolvent increasing the polarity of the medium. TTIP solubility in CO2 is reported by Reverchon et al. [17] and it varies from 0.015 g TTIP/g CO2 to 0.03 g TTIP/g CO2 in the range 40–60 ◦ C and for pressures between 10.0 MPa and 15.0 MPa. 2.4.2. Mass transfer Mixing of reactants and diffusion are favoured by turbulence. When reactants are in one single phase mass transfer limitations are eliminated. That is the case of CO2 /ethanol mixtures that are completely miscible. In these experiments an organometallic is added to this reaction medium and a ternary system is created. In order to guaranty one phase reaction organometallic precursor concentration must be controlled by equilibrium. On the other hand, particle growth is directly related to mass transfer. Collisions between nuclei or new material adhesion to form particles are controlled by mass transfer, but this aspect is exposed later. 2.4.3. Chemical reaction Chemical reaction mechanism is difficult to determine, but once reactants are in the same phase (the supercritical) reactions are very fast. Pressure and temperature speed up chemical reaction and therefore, precursor decomposition.

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TiO2 formation from DIPBAT can be described by a hydrolysis. Global DIPBAT hydrolysis reaction can be written: Ti(OC3 H7 )2 (C5 H7 O2 )2 + 2H2 O → TiO2 ↓ + 2C3 H7 OH + 2C5 H7 OOH In this work, H2 O necessary for the reaction has been added from alcohol decomposition, always in presence of CO2 . The solvent facilitates particle drying and recovery, since after decomposition at the end of the process CO2 is in gas phase, in opposition to the use of only alcohol. Two different alcohols have been tested: ethanol and isopropyl alcohol. Both alcohols are polar compounds and they, therefore, increase precursor solubility, acting as cosolvents. However, their difference in polarity influences specially byproduct solubility and therefore TiO2 pollution as is described by the following: Different ratios of alcohol/DIPBAT have been tested: 42 mol alcohol/mol DIPBAT, 28 mol alcohol/mol DIPBAT, 14 mol alcohol/mol DIPBAT and 0 mol alcohol/mol DIPBAT. H2 O has also been tested as reactant maintaining CO2 as solvent. Gourinchas et al. [15] have obtained kinetic data for the formation of TiO2 powder from TTIP in supercritical isopropyl alcohol in the temperature range of 258–295 ◦ C. They proposed a reaction mechanism based on a hydrolytic decomposition of TTIP by water produced in a catalytic dehydration of the isopropyl alcohol used as solvent and where the limiting steps are the decomposition reactions of the formed titanium hydroxides. The global TTIP hydrolysis reaction is: Ti(OC3 H7 )4 + 2H2 O → TiO2 ↓ + 4C3 H7 OH 2.4.4. Nucleation and crystal growth Relative rate of these two steps, nucleation and particle growth, affects the final size of the particles and PSD. These stages are controlled by the three former ones: phase equilibrium, mass transfer and chemical reaction. Supersaturation degree in supercritical fluids is extremely high and the number of generated nuclei in an instant is very elevated. Therefore, very small particles are produced, since they are stabilized at these sizes and coalescence and particle growth are minimized. Supersaturation controls particle size, and as is shown later, the parameters that have more influence in particle size are those that affect phase equilibrium: temperature and pressure, together with nuclei concentration in a certain moment [16,17,21]. Nucleation process in supercritical fluids can be described with the classical theory of particle growth by condensation and coagulation [21,22]. In these experiments, particles with a mean size ranging from 100 nm to 600 nm have been obtained. Probably condensation is more important than coagulation and particles smaller than those in GAS process can be obtained. The present work includes experiments for two different organometallic precursors, DIPBAT and TTIP, and three different sources of H2 O for the hydrolysis: ethanol, isopropyl alcohol and distilled water. Temperature is varied in the range

Fig. 2. SEM photography of TiO2 anatase formed at 20.0 MPa and 250 ◦ C, CO2 /EtOH.

200–300 ◦ C and pressure in the range 10.0–20.0 MPa. In all the experiments, initial concentration of the organometallic precursor is 0.05 g/g in order to guaranty precursor solubilization in the reaction medium. 3. Results and discussion 3.1. Influence of process variables Morphology of the formed TiO2 particles is spherical for the tested range of pressures and temperatures. As example, Fig. 2 shows a SEM photograph of TiO2 formed at 20.0 MPa, and 250 ◦ C, using ethanol as hydrolysis reactant. In previous literatures [23,24] it was demonstrated that morphology changes with the feed concentration, pH and T or P around the critical point, due to the change in the chemical species distribution and thus in crystal habits. However, in this experimental work, operational conditions are far from the critical point, and pH and concentration do not change substantially. 3.1.1. Chemical nature of the precursor TTIP has four isopropoxy ligands joined to the titanium (Fig. 3), and it is easily hydrolysable. Working with TTIP in a continuous synthesis process is difficult because of its easiness to form Ti(OH)4 at ambient temperature in presence of humidity. Because of this, it is necessary to implement a drying stage in the pumping line of the reactant, and alcohol must be added inside the reactor. DIPBAT has the titanium atom linked to two acetylacetonate (␤-diketone) and to other two isopropoxy (alkoxide) ligands (Fig. 3). The acetylacetonate is a bidentate chelate ligand; this kind of ligands is very stable thermically and kinetically; therefore, breaking the Ti–acetylacetonate bond is more difficult than

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Fig. 3. (A) DIPBAT chemical structure and (B) TTIP chemical structure.

breaking the Ti-isopropoxide one. This means that the hydrolysis reaction of the TTIP happens easily than the hydrolysis of the DIPBAT. Experiments using TTIP and DIPBAT have been performed in order to compare these two precursors in terms of operational manipulation and obtained TiO2 characteristics. Effect of operational variables, T (200–300 ◦ C) and P (10.0–20.0 MPa), has been tested. TiO2 properties are similar for the two reactants. The main difference is observed in particle size. Particles with a mean diameter of 270 nm with a standard deviation of 125 nm are obtained from TTIP decomposition at 20.0 MPa and 300 ◦ C. Under the same operational conditions, TiO2 particles are smaller (200 nm) and with a narrower PSD (standard deviation = 100 nm) when DIPBAT is used as precursor, as is shown in Fig. 4. When TTIP is used as precursor the apparition of Ti(OH)4 nuclei during the heating step favours the step of particle growth, because they act as seeds for material agglomeration. According to these results, the reaction mechanism of hydrolysis followed by thermal decomposition of the hydroxide to form TiO2 , proposed by Gourinchas et al. [15], for the TTIP reaction can also be applied to DIPBAT. The following mechanisms can then be proposed for DIPBAT transformation into TiO2 under our experimental conditions:

Ti(OR)(OR )2 (OH) + H2 O ↔ Ti(OR )2 (OH)2 + ROH Ti(OR )2 (OH)2 + H2 O ↔ Ti(OR )(OH)3 + R OH Ti(OR )(OH)3 + H2 O ↔ Ti(OH)4 + R OH Ti(OH)4 ↔ TiO2 + 2H2 O

Ti(OR)2 (OR )2 + H2 O ↔ Ti(OR)(OR )2 (OH) + ROH

where R: isopropoxy group (–C3 H7 ); R : acetylacetonate group (–C5 H7 O). The hydrolysis of the isopropoxy group is faster than that of acetylacetonate, and this hydrolysis step together with thermal decomposition of titanium hydroxides to form TiO2 could control the reaction rate. TTIP causes operational problems for continuous pumping due to its easiness for hydrolysis, even with ambient humidity, with precipitation of titanium hydroxides leading to pipe clogging. However, DIPBAT can be mixed with the ethanol before pumping, because the hydrolysis does not take place until the temperature is raised, whereas for TTIP a third pumping line is necessary to insert ethanol directly to the reactor. Taking into account these experimental results, DIPBAT offers better features as chemical precursor for the synthesis of TiO2 under supercritical conditions, since it facilitates the operation in continuous way and its manipulation is easier. Moreover, TiO2 particles are smaller when DIPBAT is used as precursor instead of TTIP.

Fig. 4. TiO2 particle size distribution for the synthesis using TTIP and DIPBAT (20.0 MPa, 300 ◦ C, CO2 /EtOH).

3.1.2. Effect of hydrolysis reactant Ethanol and isopropyl alcohol have been used as hydrolysis reactants, as has been mentioned in Section 2.4.3. Experimental results show that reaction stoichiometry requires ratios of alcohol/DIPBAT higher than 28 mol alcohol/mol DIPBAT, for TiO2 anatase to be formed with yields higher than 75%. When this amount of ethanol is used, a white and fine powder of TiO2 anatase is formed with a carbon content inferior of 1%. Experiments with lower quantities of ethanol produce small amounts of brown solids inside the reactor. The same ratio of isopropyl alcohol/organometallic precursor is also suitable to form TiO2 anatase. However, when isopropyl alcohol is used instead of ethanol, pollution of formed TiO2 is higher (7% C), since it is less polar and byproducts are worse eliminated because their solubility is lower. When smaller quantities of ethanol are used

EtOH ↔ ethene + H2 O

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Fig. 6. Influence of pressure on particle size distribution (300 ◦ C, CO2 /EtOH). Fig. 5. TiO2 particle size distribution for the synthesis using H2 O or ethanol as reactant (20.0 MPa, 300 ◦ C).

at the same pressure and temperature, a wet pasty residue is obtained inside the reactor. The X-ray diffraction analysis of these samples has shown that no TiO2 is formed, and that these residues show a high degree of carbon (>15%), showing that reaction is not completed. Experiments performed with distilled water have generated bigger particles and larger PSD than those obtained with alcohols at the same operational conditions. In order to obtain TiO2 anatase particles, without pollution, and with yields over 75%, molar ratios higher than 9 mol H2 O/mol DIPBAT are necessary. Fig. 5 shows PSD of TiO2 obtained using a molar ratio of 9 H2 O/DIPBAT compared with that obtained with ethanol in a molar ratio of 28, both of them operating at 20.0 MPa and 300 ◦ C. Particles obtained using water as reactant have a mean diameter of 450 nm, whereas those obtained using ethanol present a mean diameter of 200 nm. However, PSD is narrower when H2 O was used as reactant. This result can be explained by the limited solubility of H2 O in SC-CO2 compared with the total miscibility of this solvent with alcohols. This fact could generate a liquid phase reaction, leading to slower reaction rates, and thus, lower nucleation rates. Moreover, the presence of liquid water in the reactor favours particle agglomeration resulting in bigger particle diameters. 3.1.3. Effect of pressure Pressure effect has been studied in the range 10.0–20.0 MPa, and the same trend has been observed both in batch and in semicontinuous operations. TiO2 anatase is the only crystalline phase obtained in all cases; however, the degree of crystallinity is poorer when pressure is decreased. At 20.0 MPa, the reaction product is a white powder, fine and dry, whereas at 10.0 MPa, the obtained powder exhibits a yellow colour due to rest of reactant and/or byproducts, and therefore, pollution of the product is higher for lower pressures. The analysis of the carbon has confirmed this fact, and the content of carbon was 7% at 10.0 MPa against 1% at 20.0 MPa. This result can be explained from the point of view of byproducts solubility that decreases when pressure is decreased, and their elimination is worse. However, when pressure is increased particle size is increased. Higher pressures create higher medium densities, and

the degree of supersaturation decreases, diminishing nucleation rate, and particle growth is favoured. Fig. 6 shows particle size distribution at 10.0 MPa and 20.0 MPa where mean particle size is 200 nm and 450 nm, respectively. This same effect of pressure on particle size is also described by Reverchon et al. [17] for the precipitation of Ti(OH)4 . 3.1.4. Effect of temperature Temperature is an important operational parameter since it has effect on many steps of the process: solubilities, chemical reaction and nucleation and particle growth. Three different temperatures have been tested for DIPBAT decomposition: 200 ◦ C, 250 ◦ C and 300 ◦ C, with a constant pressure of 20.0 MPa. When correct amount of ethanol is added to the medium (molar ratio EtOH/DIPBAT > 28, as is detailed pre-

Fig. 7. X-ray diffractogram of TiO2 particles obtained at 200 ◦ C, 250 ◦ C and 300 ◦ C (20.0 MPa, and molar ratio EtOH/DIPBAT = 28).

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at 200 ◦ C to 150 m2 /g at 300 ◦ C. In terms of particle size, for bigger particles BET specific area is smaller. This result can be explained by the fact that specific area represents the external surface, but also the internal one; when particle crystallinity is not good, particle porosity is increased. This increase of particle porosity, when bigger particles are formed, has been observed by other authors in the precipitation of zinc acetate in supercritical medium by GAS process [17]. 3.1.5. Effect of residence time In order to study residence time, experiments have been performed in a semi-continuous way, and residence time has been varied from 20 s to 2 min, working at 20.0 MPa and 300 ◦ C. TiO2 anatase is the single crystalline phase that has been obtained. However, crystallinity is poor for residence times lower than 1 min. This means that reaction time is not a limitation and reaction is very quick, but crystallization is influenced by residence time. Carbon analyses have revealed that pollution of the samples is independent of residence time, and carbon content of TiO2 particles is inferior to 1%. Moreover, operational problems of piping plugs took place for very short residence times, lower than 1 min. From these results, it can be concluded that residence times greater than 1 min are required to obtain TiO2 anatase well crystallized, and to avoid operational problems. Particle size is smaller for shorter residence times.

Fig. 8. Specific surface area BET of TiO2 particles in terms of operational temperature and residence time.

viously), TiO2 anatase is obtained in all cases. However, the crystallinity degree increases with temperature, and temperatures higher than 250 ◦ C are needed in order to obtain a good crystallinity. Fig. 7 shows this tendency, and one analysis of this diffractogram looking for the relative height among peaks, the peak width and the noise of the base line, confirms a poor crystallinity for product obtained at 200 ◦ C. The carbon analyses have revealed that pollution of TiO2 has increased from 1.5% C to 12% C when temperature is decreased from 300 ◦ C to 200 ◦ C. Temperature has therefore a marked effect on byproduct solubility. Particle size is also dependent on operational temperature. Higher temperatures lead to bigger particles, obtaining particles with a mean size of 450 nm at 300 ◦ C, and 20.0 MPa. When temperature is increased, density is decreased increasing supersaturation degree. This fact favours nucleation and particle growth. Experimental results have shown that specific area (BET) of TiO2 particles is related to crystallinity. Particles well crystallized (high operational temperatures) present lower value of BET area, whereas amorphous particles show the biggest surface area. Fig. 8 shows values of BET area that varies from 350 m2 /g

3.2. Applications According to the previous results, that are summarized in Table 1, the selection of the optimal synthesis conditions will depend on the future application of the TiO2 . Before developing the synthesis process it is necessary to evaluate which are the required properties for the particles: for some purposes it is desired to obtain particles with a high specific area, for other ones a high crystallinity, a very high purity or a low average particle size is necessary. When the specific area is required to be high and crystallinity does not mind the process must be carried out at relatively low

Table 1 Summary of product characteristics at selected operational conditions P (MPa)

T (◦ C)

TiO2 anatase

Yield (%)

Average size (nm)

Product pollution (% C)

Crystallinity

BET area (m2 /g)

Batch processa 10.0 300 20.0 300 20.0 250 20.0 200

Yes Yes Yes ¿?

74 88 75 –

200 450 300 –

7.1 1.2 2.0 12.0

Very good Very good Good Poor

108 113 160 350

Semi-continuous processb 20.0 300 120 20.0 300 90 20.0 300 60 20.0 300 30 20.0 300 20

Yes Yes Yes ¿? ¿?

85 82 80 – –

250 125 100 – –

<1 <1 <1 – –

Very good Very good Very good Poor Poor

159 152 140 – –

a b

Residence time (s)

Initial concentration = 0.05 g DIPBAT/g reaction medium. Hydrolysis reactant: ethanol. Molar ratio EtOH/DIPBAT = 28. Hydrolysis reactant: ethanol. Molar ratio EtOH/DIPBAT = 28.

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temperatures (T = 200 ◦ C) and low residence times (tr = 1 min). Particles obtained working at these conditions are virtually amorphous, with a very high porosity, and a high value of area BET (350 m2 /g). In this sense, TiO2 is used as support of metal clusters in heterogeneous catalysis. In other cases, TiO2 particles are used because of their optical properties, which are directly related with crystallinity. Therefore, in these cases the reaction temperature and the residence time must be relatively high (T = 300 ◦ C, tr = 2 min) in order to obtain TiO2 anatase with a very high crystallinity. Some of these purposes are the following ones: TiO2 is used as a pigment in almost every kind of paints because of its high refractive index. It is also used in thin-film optical-interference coatings that are based on the interference effects between light reflected from both the upper and lower interfaces of a thin film [12]. The semiconducting nature of TiO2 makes it useful for several applications: TiO2 is employed as a gas sensor (overall as an oxygen gas sensor) since semiconducting metal oxides change their conductivity upon gas adsorption; this change in electrical signal is used for gas sensing. TiO2 is also employed as a photocatalyst to carry out the photo-assisted degradation of organic molecules in purification of wastewaters. In this case the charge carriers created upon irradiation with sunlight migrate to the surface where they react with adsorbed water [12]. According to this it is important that TiO2 have a large surface area when it works as a gas sensor or as a photocatalyst, but it is also necessary in both cases that it presents a relatively high crystallinity in order to have good semiconducting properties. For crystalline particles the smaller is the particle size the higher is the specific area; therefore, we must work at high temperature (300 ◦ C) and residence times greater than 1 min in order to obtain particles with a very high crystallinity and particle size as small as possible. In this case it is very important to avoid the agglomeration of the particles. 4. Conclusions DIPBAT has been proposed as a new organometallic precursor for the synthesis of TiO2 anatase. TiO2 anatase particles well crystallized, with a mean diameter of 200 nm, can be obtained at 20.0 MPa, and 300 ◦ C using a reaction medium composed by CO2 /EtOH. Precursor concentration in the medium was 0.05 g/g and PSD of formed TiO2 is narrow with a standard deviation of 100 nm. This organometallic precursor exhibits some advantages over TTIP, the precursor that has been used for other authors in the literature. DIPBAT can be pumped in continuous operation without humidity elimination, and it leads to smaller TiO2 particles than TTIP. A reaction mechanism of thermohydrolysis describes the chemical reaction that takes place in this precipitation reaction. Water for the hydrolysis is generated by decomposition of ethanol in the reaction medium where SC-CO2 is used as solvent. A molar ratio of at least 28 mol EtOH/mol DIPBAT is needed in order to complete the reaction and to obtain TiO2 anatase. Lower quantities of alcohol do not form TiO2 anatase. Isopropyl alcohol can also be used as water source for the hydrolysis but pollution

in final product is higher due to its lower polarity, that origins poorer solubility of byproducts. Pressure affects particle pollution and particle size distribution, since it acts on byproduct solubility and supersaturation degree, respectively. With a pressure of 10.0 MPa particles with a mean diameter of 200 nm are obtained, whereas an augment to 20.0 MPa increases particle size to 450 nm. Temperature is the most sensible parameter; crystallinity, particle size and pollution improve with temperature, leading to smaller particles and less contaminated for the highest temperature, 300 ◦ C. TiO2 pollution must be controlled by temperature and/or pressure, and therefore, by specie solubilities, and not with residence times. Specific area (BET) of TiO2 particles is related to crystallinity. Particles well crystallized (high operational temperatures) present lower value of BET area (150 m2 /g), whereas amorphous particles show the biggest surface area (350 m2 /g). Acknowledgements Authors thank Spanish Education and Science Ministry, Project Reference: PPQ2003-07209, FPI Grant of I. Montequi, and Regional Project of Junta de Castilla y Le´on Reference: VA025/04 for financial support. References [1] T. Adschiri, Applications of supercritical fluids in powder processing, KONA Powder Part. 16 (1998) 89. [2] A.S. Teja, L.J. Holm, Production of magnetic nanoparticles using supercritical fluids, in: Y.-P. Sun (Ed.), Supercritical Fluid Technology in Materials Science and Engineering, Marcel Dekker Inc., 2002, ISBN 0-82-470651-X, p. 311. [3] J.M. Blackburn, D.P. Long, A. Caba˜nas, J.J. Watkins, Deposition of conformal copper and nickel films from supercritical carbon dioxide, Science 294 (2001) 141. [4] J. Brasseur, C. Pommier, K. Chhor, Synthesis of supported TiO2 membranes using supercritical alcohols, Mater. Chem. Phys. 64 (2000) 156. [5] R. M’Hamdi, J.F. Bouquet, K. Chhor, C. Pommier, Solubility decomposition studies on metal chelates in supercritical fluids for ceramic precursors powders synthesis, J. Supercrit. Fluids 4 (1991) 55. [6] T. Adschiri, K. Arai, Hydrothermal synthesis of metal oxide nanoparticles under supercritical conditions, in: Y.-P. Sun (Ed.), Supercritical Fluid Technology in Materials Science and Engineering, Marcel Dekker Inc., 2002, ISBN 0-82-470651-X, p. 311. [7] P.S. Shah, J.D. Holmes, R.C. Doty, K.P. Johnston, B.A. Korgel, Steric stabilization of nanocrystals in supercritical CO2 using fluorinated ligands, J. Am. Chem. Soc. 122 (2000) 4245. [8] K.P. Johnston, et al., Microemulsions, emulsions and latexes, in: J. Jessu (Ed.), Chemical Synthesis Supercritical Fluids, John Wiley and Sons, New York, 1999, p. 127. [9] J. Jung, M. Perrut, Particle design using supercritical fluids: literature and patent survey, J. Supercrit. Fluids 20 (2001) 179. [10] Y.-P. Sun, Supercritical Fluid Technology in Materials Science and Engineering, Marcel Dekker Inc., 2001, ISBN 0-82-470651-X. [11] F. Cansell, C. Aymonier, A. Loppinet-Serani, Review on materials science and supercritical fluids, Curr. Opin. Solid State Mater. Sci. 7 (4–5) (2003) 331. [12] U. Diebold, The surface science of titanium dioxide, Surf. Sci. Rep. 48 (2003) 53.

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