J. Phys. Chem. C 2007, 111, 4969-4976
4969
Anatase, Brookite, and Rutile Nanocrystals via Redox Reactions under Mild Hydrothermal Conditions: Phase-Selective Synthesis and Physicochemical Properties Ji-Guang Li,*,† Takamasa Ishigaki,† and Xudong Sun‡ National Institute for Materials Science, Nano Ceramics Center, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, and Department of Materials Science and Engineering, School of Materials and Metallurgy, Northeastern UniVersity, Shenyang 110004, P.R. China ReceiVed: NoVember 6, 2006; In Final Form: December 20, 2006
Anatase, rutile, and especially brookite nanocrystals have been selectively synthesized in this work via a redox route under mild hydrothermal conditions (180 °C, 3 h), employing trichloride as the titanium source and ammonium peroxodisulfate (APS), hydrogen peroxide, nitric acid, or perchloric acid as the oxidant. Characterizations of the three pure phases were achieved by XRD, Raman spectroscopy, FTIR, TG, HRTEM, UV-vis, and BET. The use of APS consistently yields anatase, but the particle morphology can be tuned from wormhole-structured agglomerates to more dispersed nanocrystallites. The use of other oxidants yields almost identical results, and phase selection can be attained in this case by controlling the reactant concentration and solution pH. The three phases show their distinctive crystal shapes: rounded nanocrystals for anatase, nanoplates for brookite, and nanorods for rutile. Both the optical band gap (3.11 eV) and the indirect band gap (2.85 eV) of brookite were found to lie in between those of anatase and rutile. Under the same surface area of loaded TiO2, the brookite nanoplates exhibit the highest efficiency in the beaching of methyl orange solution under UV irradiation. The mechanism of phase selection was discussed based upon a systematic investigation into the effects of synthetic parameters on phase constituents of the hydrothermal products.
1. Introduction Titanium dioxide (TiO2) has three most commonly encountered crystalline polymorphs: anatase, brookite, and rutile. All the three crystal structures are made up of distorted TiO6 octahedra, but in different ways. Rutile adopts a tetragonal structure (space group: D14 4h), in which two opposing edges of each octahedron are shared to form linear chains along the [001] direction and the TiO6 chains are then linked together via corner connection.1 Anatase (tetragonal, D19 4h) has no corner sharing but has four edges shared per octahedron. The crystal structure of anatase can be viewed as zigzag chains of the octahedra linked together through edge sharing.1 As for brookite (orthorhombic, D15 2h), the octahedra share three edges and also corners, and the dominant structural feature is a chain of edge sharing: the distorted TiO6 octahedra are arranged parallel to the c-axis and are cross-linked by shared edges.2 The crystal built-up (in terms of the number of shared edges) and some known physical properties of brookite seem to go between those of anatase and rutile. For example the refractive index of anatase, brookite, and rutile increases in the order 2.52, 2.63, and 2.72, while the theoretical density 3.84, 4.11, and 4.26 g/cm3.1,3 TiO2 currently finds wide technological applications including pigments, cosmetics ultrathin capacitors,4 photovoltaic cells,5 and catalysis.6 The applications for TiO2 strongly depend upon the crystal structure, morphology, and size of the particles. Each crystalline modification has different physiochemical properties, such as density, refractive index, and photochemical reactivity.1,3 * Corresponding author. Tel: +81-29-860-4394. Fax: +81-29-860-4701. E-mail:
[email protected]. † National Institute for Materials Science. ‡ Northeastern University.
Rutile has the highest density and refractive index among the three phases and therefore has been widely employed in pigments and cosmetics industries.3 Anatase generally shows better performances than its rutile counterpart in photocatalytic applications.6 The brookite phase is the least studied in many aspects of its properties, mainly owing to the difficulties encountered in obtaining its pure form, though it seems to have marked photocatalytic activity (comparable to anatase) in the dehydrogenation of propan-2-ol and in Ag desopition.7 Size, shape, and phase structure controlled synthesis of TiO2 nanocrystallites has long been one of the main themes in TiO2 research. Many synthetic techniques have been utilized in the preparation of TiO2 nanocrystals, among which hydrothermal treatment has been drawing much attention considering that it directly produces well-crystallized nanocrystallites of a wide range of compositions within a short period of reaction time.8 With alkoxides or tetrachloride as the titanium source, previous work showed that selective crystallization of anatase and rutile is readily achievable and the phase selection depends upon several factors including solution pH, reactant concentration, and the mineralizer used. It was widely observed that a highly acidic condition or mineralizers such as NaCl, NH4Cl, and SnCl4, etc., favors rutile formation,8c,d,f,g,i,l while the presence of some carboxylic acids promotes anatase crystallization.8e,h,i Although brookite is frequently encountered as a byproduct in the solgel or hydrothermal products,8c,d,i,k,l,9 there is no doubt to say that this phase is the most difficult to obtain as nanocrystallites in a phase-pure form. Pure brookite was classically obtained from aqueous or organic media as large crystals via hydrothermal treatment at high-temperature (such as 300 °C) and with proper amounts of alkaline salts as the mineralizer.10 Jolivet et
10.1021/jp0673258 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/09/2007
4970 J. Phys. Chem. C, Vol. 111, No. 13, 2007 al.9a,d obtained pure brookite nanocrystallites via fractionating anatase/brookite or rutile/brookite mixtures by peptization with nitric acid. We previously reported the direct formation of pure brookite under ambient pressure, but in that case the nanosized brookite crystallites are aggregated into spherical particles of submicron dimensions.11 Previous work on TiO2 synthesis largely started with titanium (IV) compounds, mainly TiCl4 and alkoxides, which are highly sensitive to atmospheric moisture and therefore requires special precautions. Titanium trichloride (TiCl3) solution, which is easily manipulatable, has been used in this work as a titanium source for TiO2 synthesis. Through controlling the synthetic parameters and therefore the reaction kinetics, pure anatase, brookite, and rutile nanocrystallites have all been selectively synthesized via one single redox route under mild hydrothermal conditions (180 °C, 3h) without the use of any mineralizer. The thus-made three types of TiO2 exhibit their distinctive morphologies: rounded primary nanocrystallites for anatase, nanoplates for brookite, and nanorods for rutile. The three phase pure powders were characterized in detail by combined means of XRD, HRTEM, Raman Spectroscopy, FTIR, TG, BET, and UV-vis spectroscopy, and their photocatalytic performances were evaluated via bleaching methyl orange solutions under UV illumination. The phase selection mechanism was also discussed based upon a systematic investigation into the effects of processing parameters on phase constituents of the hydrothermal products. 2. Experimental Section 2.1. Nanoparticle Synthesis. The titanium source for TiO2 synthesis is a 20% titanium trichloride (TiCl3) solution purchased from Wako Pure Chemical Industries, Osaka, Japan. The generation of TiO2 from a Ti3+ starting material requires an oxidation reaction, which is achieved by using one of the following common oxidants: 30% hydrogen peroxide solution (H2O2, Reagent grade, Junsei Chemical Co., Ltd., Tokyo, Japan), ammonium peroxodisulfate ((NH4)2S2O8, reagent grade, Kanto Chemical Co., Inc., Tokyo), 60% nitric acid (HNO3, reagent grade, Kanto Chemical Co., Inc.), and 60% perchloric acid (HClO4, analytical grade,Wako Pure Chemical Industries). Urea (CO(NH2)2, reagent-grade, Wako Pure Chemical Industries), which releases ammonia via forced hydrolysis at elevated temperatures, is used as an in situ pH regulator whenever necessary. Adjustment of solution pH was also made with 30% HCl solution (analytical grade, Wako Pure Chemical Industries). All the chemicals are used as received without further purification, and in all the cases the oxidant/Ti3+ molar ratio is kept constant at 1:1. For powder synthesis, the desired amounts of reactants are mixed together and diluted with distilled water to the intended concentration in terms of titanium. Eighty milliliters of the thus-made solution is then transferred to a Teflon-lined autoclave of 120 mL capacity and is autoclaved at 180 °C (the highest applicable temperature) in an air oven for 3 h. Preliminary experiments showed that lower hydrothermal temperatures yield low crystallinity or amorphous products. After cooling naturally to room temperature, the resultant solids are recovered via centrifuge under 12 557g for 30 min and then washed repeatedly with distilled water for 5 times via ultrasonic dispersion and centrifuge before drying at 100 °C for 12 h. 2.2. Characterization Techniques. Phase identification was performed via X-ray diffractometry (XRD) in conjunction with Raman spectroscopy. XRD analysis was conducted on a Philips PW1800 diffractometer (Philips Research Laboratories, Eindhoven, The Netherlands) operating at 40 kV/50 mA using nickelfiltered Cu-KR radiation and a scanning speed of 0.5° 2θ/min.
Li and Ishigaki Raman spectroscopy was made using Ar+ laser excitation (514.5 nm) with an incident power of 50 mW and a resolution of 1 cm-1 (Model NR-1800, JASCO, Tokyo). For a mixture of TiO2 polymorphs, phase constituents of the powder were analyzed by the method of Zhang and Banfield12 from the integrated intensities of the anatase (101), brookite (121), and rutile (110) XRD peaks. Fourier transform infrared (FTIR) spectroscopy (Model FTS-65, Bio-RAD Laboratories, Tokyo) of the powders was performed by the standard KBr method. Thermogravimetry (TG, Model TAS-200, Rigaku, Tokyo) of the powder is made in stagnant air with a heating rate of 10 °C/min. The specific surface area of the powder is determined by BrunauerEmmett-Teller (BET) analysis (Model Belsorp 18, Bell Japan Inc., Tokyo) via nitrogen adsorption at 77 K. Particle morphology was observed via transmission electron microscopy (TEM, Model JEOL FEM-3000F, Tokyo) operating at 300 kV. 2.3. Specular Reflection Measurements and Photocatalytic Evaluation. Optical properties of the TiO2 powders are studied via UV-vis absorption spectroscopy (Model V-570, JASCO Co., Tokyo). Photocatalytic activity is tested via bleaching 20 µM methyl orange (C14H14N3NaO3S, reagent-grade, Wako Pure Chemical Industries, Ltd.) solution under UV irradiation. The UV light, composed of five wavelengths of 300, 316, 365, 405, and 436 nm with an intensity ratio of 40:82:100:39:30, is generated using 200 W high-pressure mercury lamp by UVF203S Type A light source (San-Ei Electric Co., Ltd. Osaka). Photocatalysis is performed by shining the UV light (1 mW/ cm2) on the top surface of 10 mL dye solution (in brown-colored cylinder bottle of 20 mL capacity) with a certain amount of TiO2 nanocrystallites ultrasonically dispersed in it. All the photocatalytic tests are conducted under magnetic stirring of the suspension. After being illuminated for a certain period of time, the suspension is centrifuged under 12 557g for 30 min to achieve solid/liquid separation, and the relative concentration of methyl orange in the recovered solution is determined through UV-vis spectroscopy by comparing its intensity of the 465 nm absorption with that of the original methyl orange solution. 3. Results The results obtained in this work can be classified into two categories: (1) the use of (NH4)2S2O8 as the oxidant consistently yields anatase, irrespective of synthetic conditions (solution pH and the initial Ti3+ concentration); (2) under a fixed reaction condition, the use of HClO4, H2O2, or HNO3 produces almost identical results. In this case, phase selective synthesis of anatase, brookite, and rutile nanocrystallites can be attained by controlling the reactant (TiCl3) concentration and solution pH. 3.1. Anatase Powders Made with Ammonium Peroxodisulfate (APS). The powders synthesized with APS are exclusively anatase, but the particle morphology can be controlled either by changing the initial TiCl3 concentration or the solution pH. Figure 1 shows the anatase TiO2 nanopowders made under some typical reaction conditions. With high TiCl3 concentration (0.9 M) and low pH (
