... maintained at a selected value between 1 and 7 by addition of a NaOH solution (3 .... NaNO3 (4M) (rutile) or Na2SO4 (4M) (anatase), and (b) at ( ) 95°C, ...
Mat. Res. Soc. Symp. Proc. Vol. 822 © 2004 Materials Research Society
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Synthesis of nanometric TiO2 in aqueous solution by soft chemistry: obtaining of anatase, brookite and rutile with controlled shapes Magali Koelsch, Sophie Cassaignon and Jean-Pierre Jolivet Laboratoire de Chimie de la Matière Condensée, UMR-CNRS 7574 Université Pierre et Marie Curie 4 place Jussieu, 75252 Paris Cédex 05, France ABSTRACT Nanometric particles of titania, exhibiting anatase, brookite or rutile polymorphs, were synthesized by thermohydrolysis of TiCl4 in aqueous medium. The adjustement of physicochemical parameters (acidity, ionic strength, anions, temperature) allows to tune the crystalline structure, the size and the morphology of the particles. Brookite results from the precipitation of titanium in HCl, HBr or HNO3 whereas anatase is formed in H2SO4 medium. Adding salts in HCl medium leads to ionic strength or complexation effect. Varying the temperature of thermohydrolysis implies modification on yield, size and morphology of the particles. INTRODUCTION Titanium dioxide has attracted increasing attention because of its wide applications in many fields such as a main component of white pigments, paintings, inks and fillers [1]. The high absorption of UV-light allows its utilization in cosmetics. TiO2 has also received much attention for applications as a semiconductor in environmental photocatalysis processes such as removal of pollutants from air and water [2-4] and moreover it is also a common material for photovoltaic devices [5,6]. The performances for a given application are strongly influenced by the crystalline structure, the morphology and the size of the particles [7,8]. Particles with a nanometric size have also a particular interest because of their high surface/volume ratio inducing specific surface properties. Because of its three main crystallographic forms, each with their own physical properties, titanium dioxide is a very versatile system to study. This study reports on the role of acidity, anions and temperature on the synthesis of TiO2 polymorphs in aqueous medium by thermohydrolysis of TiCl4. We show that a precise adjustment of experimental conditions allows the control of the crystalline structure and the morphology of particles. EXPERIMENTAL DETAILS Synthesis A known volume of TiCl4 (Fluka 98 %) was cautiously added to mineral acid solutions whith concentrations ranging from 0.5 to 5 mol.L-1. All experiments were performed with [Ti] = 0.15 mol.L-1, the final concentration of Ti(IV) being measured by complexometry [9]. The so obtained solutions were aged at 60, 95 or 120°C for 24 hours or several weeks. Another procedure was to precipitate a solid by addition of a base. The pH of the mixture was automatically maintained at a selected value between 1 and 7 by addition of a NaOH solution (3
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mol.L-1) using a Combi Titreur 3D Metrohm apparatus. The particles were then centrifuged and washed with distilled water, the solid was dried under nitrogen atmosphere for X-ray diffraction characterization. Techniques X-Ray diffraction. XRD patterns were recorded using a powder diffractometer (Philips PW1830) operating in the reflection mode with CuKα radiation and equipped with a graphite back monochromator. The angular domain was between 20 and 80° (2θ). The proportions of the different titanium dioxide polymorphs were evaluated from the relative area of the 110, 121 and 101 diffraction lines of rutile, brookite and anatase phases respectively; the calibration method has been reported elsewhere [10]. The size of particles was calculated from the Scherrer formula [11]. Transmission electron microscopy (TEM). Transmission electron images were obtained using a JEOL 100 CX apparatus operating at 100kV and high resolution microscopy was performed on a Philips CM20/STEM operating at 200kV. Samples were prepared by evaporation of very dilute aqueous suspensions onto carbon-coated grids. RESULTS AND DISCUSSION Hydrolysis of TiCl4 in low acidity conditions When pH above 1, the precipitation of titanium is complete and instantaneous. The early obtained solid is very poorly cristallized. Aging the suspensions at 60°C for 24 h leads to the crystallization of anatase, figure 1a. During crystallization, there is no significative modification of the particle size, which suggests a mechanism for the formation of anatase that implies dehydration and in situ ordering within the amorphous phase. Rutile is obtained as a by-product (5%) when pH is lower than 2. Brookite is also formed and represents about 10 to 20% of the precipitated solid. The size of the anatase particles is strongly dependent on the acidity of the reaction medium, figure 1c. The lower the pH the smaller the particle size. The sizes evaluated from the 004, 200 and 101 XRD line width are very close, indicating that the anatase particles are likely spherical. A precise morphology is difficult to infer from the TEM micrographs. However, the particles seem to be isotropic, in agreement with XRD, figure 1b. Increasing the aging of the suspensions to one month at 60°C does not affect the particle size.
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The variation of particle size with the pH of synthesis is closely related to the variation of the electrostatic surface charge density. Analogous correlation has already been evidenced for the precipitation of magnetite, Fe3O4 [13]. The phenomenon can be qualitatively interpreted from a change in surface properties with pH [12]. The precipitation of pure anatase occurs when sulphate ions are added to the initial solution. The influence of the pH on the size and the morphology of the particles is analogous to that previously reported. Precipitation of TiO2 nanoparticles by thermohydrolysis in acidic medium -influence of the mineral acid The results of the thermohydrolysis of TiCl4 depend on the mineral acid used. With HClO4, the thermodynamical stable phase, rutile, is obtained because of the poor complexing ability of perchlorate anions. When H2SO4 is used, the well known precipitation of anatase takes place. For HCl [10], HBr and HNO3, brookite occurs in specific domains of acidity, figure 2. For HCl, HBr, HClO4 at 1 mol.L-1 and 95°C, rutile particles exhibit a rod shape (100 nm x 10 nm) with a specific crystalline orientation. The rods grow along the [001] (d = 0.29 nm) axis and their square cross section is delimitated by the [110] (d = 0.32 nm) and [1-10] planes. The tips of the rods exhibit a pyramidal shape with the [001] as base and [111] (d = 0.22 nm) planes as ridge. With HCl and HBr at 5 mol.L-1 and 95°C needles are obtained, figure 3c, with a different crystalline orientation. Now, the [001] plane is parallel to the needle lenght. Such changes in morphology are due to the large difference in titanium solubility and precipitation rate with acidity. In the most concentrated HCl solutions, the formation of titanium anionic hydroxochloro complexes drastically lowers nucleation and the growth of particles is enhanced. In HCl 3 mol.L-1, thermolysis leads mainly to the formation of brookite with little of rutile. The proportion of brookite is about 80% in the mixture. Peptization of the solid with nitric acid (pH 2) allows the separation of brookite from rutile. The particles of brookite are dispersed in solution whereas the large particles of rutile floculate. Pure dispersion of brookite is therefore obtained, figure 4b, and consist mainly of aggregates of 40 to 60 nm mean size. HRTEM micrographs show well-defined platelets diamond-shaped (9 nm), arranged in an ordered stacking, figure 4b. The lateral faces of the particles correspond to [1-1-1] and [11-1] planes with d-spacing of 0.34 nm, and the basal face is a [301] plane. 100
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acid concentration (mol.L ) b) a) Figure 2. Relative proportions of (○) brookite, (●) rutile and (x) anatase formed after thermohydrolysis of TiCl4 at 95°C for 48h, (a) in (--) HCl, () HBr, ( -) HClO4 and (b) in () HNO3, ( -) H2SO4
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a) b) c) Figure 3. TEM micrographs of rutile particles formed a) for HCl = 1 mol.L-1, b) for HCl = 5 mol.L-1 and NaNO3 = 4 mol.L-1 and c) for HBr = 5 mol.L-1 and aged in suspension for 48h at 95°C. The chloride ions (as Br- or NO3-) appear to be essential for the formation of brookite. The optimal range for the formation of brookite (proportion above 50%) corresponds to a Cl/Ti molar ratio ranging from 17 to 35. These values correspond to the domain of predominance of the zero charged complex [Ti(OH)2Cl2(OH2)2]0 in dilute titanium solution [14]. This strongly suggests that such a complex could be the precursor of brookite [12]. In HNO3, brookite is still stabilized for the highest acid concentration. This observation could be explained by a very low solubility of TiO2 in such medium. Indeed with more concentrated HCl or HBr the formation of other chloro complexes leads to the formation of rutile. -influence of anions and temperature in HCl solution Adding salts in an acidic solution has several effects, the first is to modify the ionic strength. Experiments were performed in hydrochloric medium, figure 5a. When NaCl (4 mol.L-1) was added, the brookite formation is shifted toward lower acidity and takes place in a reduced domain now centred on HCl = 1 mol.L-1. This phenomenon agrees with the need of a specific Cl/Ti molar ratio which would yield the formation of a specific complex with two chloride anions in its coordination sphere. Adding sulphate salts leads to the exclusive formation of spherical anatase (5 nm), the precipitation going through a titanium sulphate complex [Ti(OH)2SO4(OH2)2]0. Addition of sodium nitrate favors the precipitation of rutile with specific morphologies depending on the acid concentration. At the lowest acidities, rods of rutile are obtained with a size of 100 x 10 nm, whereas at the highest acidities, platelets of rutile are formed with a side size of 10 nm, figure 3b. Small rutile particles are likely formed because of high salts concentration which may limit the growth. The temperature has an influence on the size, the morphology and the yield of the various phases. When the thermohydrolysis is performed at 60°C, pure brookite and rutile are obtained. The particles are formed slowly following the same process than described above and dissolution crystallization process is less operative. At 120°C, brookite particles are very well defined and present a platelet morphology. If the aspect of brookite is compared at the different temperature, we can see that different morphology and size coul be obtained, spheres (9nm), platelets (9 and 25nm), figure 4.
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a) b) c) Figure 4. TEM micrographs of brookite particles formed for 48h in HCl = 3 mol.L-1 a) at 60°C b) at 95°C and c) at 120°C and aged in suspension for 48h at 95°C. 100
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HCl (mol.L ) b) a) Figure 5. Relative proportions of (○) brookite, (●) rutile and (x) anatase formed after thermohydrolysis of TiCl4 for 48h in HCl, (a) at 95°C with () HCl, (--) NaCl (4M), ( -) NaNO3 (4M) (rutile) or Na2SO4 (4M) (anatase), and (b) at () 95°C, ( -) 120°C, (--) 60°C.
CONCLUSIONS The nature of counter ions is a major and a determinant factor on the synthesis of titanium dioxide polymorphs. We have shown that the nitrate ions, which are known to be poor complexant, plays a role in the obtention of the brookite phase. The complexes of titanium in a nitrate medium are not known but at the weakest acidities (HNO3 < 3 mol.dm-3), it seems that it could be linked to the behavior of titanium in hydrochloric medium. The control of the synthesis parameters such as the acidity, the nature of the counter-ions, the titanium concentration or the thermohydrolysis temperature, allows to control the nature of titania particles. We can tune the morphology, the size and the structure of the TiO2 polymorphs. Preliminary results show that the different polymorphs of TiO2 exhibit distinct electrochemical behavior (spectroelectrochemistry, photosensibilisation). As expected, the properties depends on the phase, size and morphology of the particles. ACKNOWLEDGEMENTS We are grateful to Fabienne Warmont (CRMP, Université P. et M. Curie) for TEM measurements and to Dominique Jalabert (CME, Université d’Orléans) for the HRTEM images.
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