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Facile synthesis of mesoporous titanium dioxide nanocomposites with controllable phase compositions by microwave-assisted esterification
Microporous and Mesoporous Materials 117 (2009) 444–449
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Facile synthesis of mesoporous titanium dioxide nanocomposites with controllable phase compositions by microwave-assisted esterification Yafeng Li a,b, Hongfang Li a, Taohai Li a, Guoliang Li a,b, Rong Cao a,* a b
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, The Chinese Academy of Sciences, Fuzhou 350002, China Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 1 November 2007 Received in revised form 5 May 2008 Accepted 23 June 2008 Available online 23 July 2008 Keywords: Mesoporous Titanium dioxide Phase composition Microwave Photocatalytic
a b s t r a c t Mesoporous titanium dioxide nanocomposites with controllable phase compositions and high surface areas were synthesized through convenient, fast, and one-step microwave-assisted esterification method. The introduction of microwave in the synthetic reactions not only accelerates the esterification reaction but also promotes the fast crystallization. By changing reaction temperature, microwave irradiation time, the amount of staring materials and the composition of solvents, pure anatase, pure rutile or mixed phase titanium dioxide nanocomposites were obtained in minutes. Meanwhile, the size of crystallite can be controlled by the reaction temperature. The as-synthesized materials display significative photocatalytic activities without any further disposal, among which the material with 3.6% rutile shows the best effect in degradation of methylene blue under UV-light irradiation. Moreover, the nitrogen adsorption–desorption results illustrate the obtained samples are mesophases. It is also interesting that the pure rutile has specific surface area as high as 210 m2 g1 calculated by BET equation. The method can control the hydrolysis of TiCl4 well and dramatically shorten the preparation time of titanium dioxide. Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction As one of the most important oxide semiconductor materials, nanocrystalline titanium dioxide has been extensively studied in recent years for its specific properties, such as photocatalysis, high chemical stability, sensitivity to humidity and gas, low toxicity and so on [1–4]. Natural titania exists in three crystalline forms: anatase, rutile and brookite. Anatase is a kinetic product and its photocatalytic activity is believed to be much higher than that of rutile phase [5]. However, some authors showed opposite results [6], and many experimental evidences indicated that the existence of anatase and rutile mixed phases enhanced the photocatalytic activity [7]. The commercially available P-25, which consists of 80% anatase and 20% rutile, has been proved to be very efficient in many catalytic reactions. It is considered that the spatial charge separation and hindered recombination of electrons and holes contribute to the enhancement of photocatalytic efficiency of mixed phases [8]. Gray and Matsumura discovered that the crystal structure and morphology on phase interfaces also played important roles for the synergistic effect between anatase and rutile [9,10]. Therefore, the control of the phase compositions can be utilized to optimize the photocatalytic properties of titanium dioxide. Several strategies have been employed to prepare titanium dioxide, such as vapor-phase method, combustion synthesis, hydrothermal and sol–gel methods * Corresponding author. Tel./fax: +86 591 83796710. E-mail address: [email protected] (R. Cao). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.06.042
[11–21]. Among them wet-chemical reaction is a traditional method for the preparation of TiO2, where the hydrolysis of titanium halides and alkoxides has been extensively studied. This method does not need complicated equipments and the cost is low. The crystal sizes, and structures, as well as the phase compositions of TiO2 nanoparticles obtained by this method are influenced by acidity, solvent, additive, temperature and aging time [22–24], and the control on phase compositions has been realized by fine tuning these factors [25,26]. However, wet-chemical method usually involves long reaction time and high temperature, which can reduce the surface area and cause sintering or aggregation. Sometimes, fussy operations are needed, making the preparation of TiO2 a little boring. Hence, to develop new methods for ready fabrication of TiO2 with controllable phase compositions is of great significance. In this work, a new method, namely microwave-assisted esterification, was developed for the convenient and rapid synthesis of TiO2, by which mesoporous nanocrystalline TiO2 can be obtained in minutes and the phase compositions can also be controlled easily. Esterification reactions have been utilized to generate ‘‘in situ” water molecules, providing excellent control of the hydrolysis and condensation rate [27–29]. However, the reported works by esterification reaction employed solvethermal method and did not show advantage in preparation time. In order to overcome the shortcoming, we introduced microwave irradiation into the esterification reaction instead of the traditional heating. To date, microwave irradiation has been used to prepare many composite materials successfully [30–36], for instance, many ceramic powders were
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synthesized by microwave irradiation [37–40]. Titanium dioxide was also prepared by microwave-hydrothermal method, but the control on the hydrolysis of TiCl4 and the phase compositions were not studied in detail. In current work, we tried to control both the release of H2O by esterification, instead of the direct introduction of water, and the hydrolysis process of TiCl4. The microwave irradiation in our reaction systems plays two important roles: (a) it accelerates the esterification reaction of ethanol and acetic acid; (b) when titanium tetrachloride encounters water molecules generated by the esterification reactions, it promotes the nucleation and the growth of titanium dioxide nanocomposites. On the other hand, the proposed method can lead to the formation of mesophases in the as-synthesized titania with high specific surface areas. The mesophases are formed without addition of any surfactant, and a long reaction time necessary for preparation of porous titanium dioxide in sol–gel process is also avoided. 2. Experimental 2.1. Materials Titanium tetrachloride (98%) and all solvents were of reagent grade and used without further purification. All experiments were carried out in a microwave oven (Initiator 8 EXP, 2450 MHz frequency, Biotage Corp.) equipped with magnetic stirring. The single-mode applicator with the proven ‘‘Dynamic Field Tuning” feature offers fast heating of a broad range of solvents. The capacity is from 0.2 mL to 20 mL. A maximum power of 400 W was selected for all the syntheses. The real power was automatically adjusted according to the temperature. In a typical synthesis, 0.6 mL TiCl4 was added into a mixed solution of 12 mL absolute ethanol and 6 mL acetic acid under magnetic stirring, a clear solution was quickly formed. Then the solution was transferred into a 20 mL vial and the synthesis was carried out in the microwave oven, the temperature and reaction time are 120 °C and 15 min, respectively. The experiment was performed under magnetic stirring with a rate of 600 rpm (the default value of the oven). The molar ratio was about TiCl4/EtOH/AcOH = 1/38.3/19.5. When the reaction was finished, the precipitates were separated by centrifugation and washed several times with absolute ethanol. Finally, the obtained powders were dried at 60 °C under vacuum.
The starting concentration of MB in the solution was 8 mg L1 and the photocatalyst loading amounted to 0.4 g L1. Vigorous stirring was employed to ensure the adsorption equilibrium and to eliminate any diffusion effect. After stirring for 2 h in dark, the equilibrium MB concentration was recorded as the initial concentration. The reaction solution was illuminated with a 300 W mercury lamp, emitting UV-light (kmax = 365 nm), positioned at 25 cm around the reactor. A portion of 2–3 mL solution was taken out in every 30 min, and the catalyst was separated by centrifugation. The final MB concentration in the solution was determined by UV absorbance at 664 nm from which the yield could be calculated. 3. Results and discussion 3.1. Influence of reaction temperature Factors influencing the specimens of titanium dioxide, such as reaction temperature, microwave irradiation time, the amount of starting materials and solvents, were investigated. Fig. 1 shows the XRD patterns of TiO2 nanocrystals synthesized at different temperatures, the microwave irradiation time is 15 min. As seen from the XRD patterns, when the temperature is 90 °C, only pure anatase phase (PDF#21-1272) is observed. When the temperature is increased to 100 °C and higher, mixed phases of anatase and rutile (PDF#21-1276) are obtained, and no photocatalytically inactive brookite phase is observed. Comparing the patterns it can be noticed that the crystallinity is improved with the increase of temperature. Interestingly, the bulk content of rutile phase doesn’t increase monotonically as the temperature rises. According to the formula reported in literature [41], the weight fraction of rutile has a maximum of 29% at 110 °C, other temperatures lead to the decrease of rutile content, for instances, 10%, 24% and 16% of rutile contents were observed for 100 °C, 120 °C and 150 °C, respectively. We suggest that this is the result of competition between anatase and rutile, and 110 °C may be the optimized temperature for the formation of latter. On the basis of well-known Scherrer’s equation, the grain sizes are estimated to be about 9 nm for the sample obtained at 150 °C and 11 nm for those obtained at other temperatures by applying peak broadening analysis to anatase (1 0 1) and rutile (1 1 0) diffractions. This is reasonable because
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2.2. Characterization of the composites
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To determine the phase compositions of the obtained TiO2 nanocomposites, X-ray diffraction spectra were taken using a Dmax 2500 diffractometer (Cu Ka radiation, k = 1.54 Å). Infrared spectra were measured with the SpectrumOne spectrometer, and the measurements were performed with pressed pellets which were made by using KBr powder as diluents. Surface areas were determined using an ASAP2020 analyzer and calculated by BET method. The pore-size distribution was calculated using the BJH (Barrett–Joyner–Halenda) equation from the adsorption branch of the isotherm. Samples were degassed for 7 h at 100 °C prior to the N2 adsorption analysis, and the analysis was carried out at liquid nitrogen temperature. Thermogravimetric analysis was performed using a STA449C thermal analyzer with air as carrier gas. HRTEM analysis for the sample was investigated on a JEM 2010 microscope. The UV absorbance of methylene blue was recorded by a Lambda35 UV/visible spectrophotometer at 664 nm.
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2θ (degree) 2.3. Catalytic activity measurement The catalytic performance of the obtained titania specimens was probed by the photodegradation of methylene blue (MB).
Fig. 1. XRD spectra of TiO2 nanocrystals obtained at different temperatures, the microwave irradiation time was kept at 15 min and the solution comprised 12 mL ethanol, 6 mL acetic acid and 0.6 mL TiCl4 (A = anatase, R = rutile): (a) 90 °C, (b) 100 °C, (c) 110 °C, (d) 120 °C and (e) 150 °C.
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j i h
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g f e d c Fig. 2. Typical HRTEM image of specimen obtained at 120 °C, the microwave irradiation time was 15 min and the solution contained 12 mL ethanol, 6 mL acetic acid and 0.6 mL TiCl4.
b a
the presence of more furious vibrations under higher temperature goes against the growth of crystal nucleus, thus the grain size can be controlled by the temperature. Fig. 2 is a typical TEM image of the sample obtained at 120 °C. The crystallites are estimated to be about 8–13 nm, corresponding to the XRD result.
As seen from Fig. 3, it is amazing that pure anatase can be obtained in 1 min. Factually, the microwave irradiation only affected the formation of nanocrystals of TiO2 in about 40 s because of the working mechanism of our microwave instrument. When the temperature reaches the setting value by the microwave irradiation, there is amortization and the temperature can be 3 °C higher. Before the temperature decreases to the right value, the real power of microwave irradiation is zero. The result indicates that the esterification along with the nucleation and growth of the nanocomposites are very fast. When TiCl4 is dissolved in the solutions of ethanol and acetic acid, Ti4+ coordinates with acetic acid, forming complex Ti(OOCCH3)4 [42]. At the same time the reaction gives rise to proton, which has a catalytic activity on esterification reactions. This process may partially accounts for the rapid esterification and the subsequent formation of TiO2 nanocrystals. The grain size of all products is estimated to be between 10 nm and 12 nm, and the contents of rutile do not change regularly. Prolongation of microwave irradiation time can improve the crystallinity and the yield of TiO2 powders, for example, when the microwave irradiation time is shorter than 5 min, the color of the resulted solution is light-cyan, indicating the incomplete transformation of TiCl4 to TiO2. Under typical synthetic conditions 15 min is sufficient for a satisfied yield (>98%). A prolonged time is needed at higher concentration of TiCl4 for the purpose of yield of TiO2 colloids. As a matter of fact, the particles of all specimens can form stable colloid solutions when washing them with distilled water. These particles are difficult to separate completely from the solutions by centrifugation, indicating strong interactions between the particles and water molecules. The speciality of the obtained samples is due to the surface coating of hydroxyl and carboxyl, which can be confirmed by infrared spectra. As seen from Fig. 4, the band at 3253 cm1 can be ascribed to the stretching vibrations of –OH, while the bands at 1568 cm1
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2θ (degree) Fig. 3. XRD patterns of TiO2 nanocrystals obtained at different microwave irradiation time, the temperature was kept at 120 °C and the solution contained 12 mL ethanol, 6 mL acetic acid and 0.6 mL TiCl4: (a) 1 min, (b) 2 min, (c) 3 min, (d) 4 min, (e) 5 min, (f) 10 min, (g) 15 min, (h) 20 min, (i) 25 min and (j) 30 min.
and 1415 cm1 correspond to the symmetric and asymmetric vibrations of COO [28], illuminating the presence of CH3COO bidentate chelating, which was illustrated in Weller’s scheme [43]. The bands at 2976 cm1 and 2928 cm1, although weakened by the screening effect of the KBr matrix, can be ascribed to the stretching vibrations of CH2 and CH3 groups, indicating the existence of adsorbed ethanol molecules on the surface of the sample. Besides the direct hydrolysis process of TiCl4 (Eq. (1)), as a known growth model of the Ti–O–Ti network [44], the following hydroly-
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3.2. Effect of microwave irradiation time
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Wave number (cm-1) Fig. 4. Infrared spectrum of particles obtained in 10 min, and the microwave temperature was 120 °C. The solution contained 12 mL ethanol, 6 mL acetic acid and 0.6 mL TiCl4.
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sis (Eq. (3)) and polycondensation (Eq. (4)) may occur in our experiments:
TiCl4 + 2H2 O ! TiO2 + 4HCl
ð1Þ
TiCl4 + 4ROH ! Ti(OR)4 + 4HCl
ð2Þ
Ti(OR)4 + xH2 O ! Ti(OH)x (OR)4x + xROH
ð3Þ
(OR)4x (OH)x1 Ti—OH + RO—Ti(OH)x (OR)3x
the elimination of water generated by oxolation, and the combustion of these functional groups. 3.3. Effect of initial amount of TiCl4 The amount of TiCl4 in the initial solutions is the most important factor in our experiments. This group of parallel experiments was carried out at 120 °C and the microwave irradiation time was set as 15 min. From the previous results we know that mixed phases of anatase and rutile were obtained when 0.6 mL TiCl4 was added to the solution of ethanol and acetic acid. If the amount of TiCl4 was increased to 0.8 mL, pure rutile could be obtained; when the amount was decreased to 0.3 mL, nearly pure anatase was produced. As illustrated in Fig. 6, the content of rutile synchronously increases when the initial amount of TiCl4 gradually increases from 0.3 mL to 0.8 mL. The phase compositions are very sensitive to the concentrations, for example, the rutile content increases from 23.5% to 84.0% when the amount of TiCl4 is changed from 0.6 mL to 0.7 mL. As is well-known, anatase phase TiO2 is composed of point-sharing octahedrons while rutile phase TiO2 is made up of edge-sharing ones. To explain this phenomenon, we suppose that the amount of water released by esterification may vary little in a given time, so when the quantity of TiCl4 is increased, the TiO2 octahedrons adopted the more compact packing mode, which was more efficient in using oxygen atoms obtained from the limited water molecules. Thus, by carefully adjusting the initial amount of TiCl4, mixed phases with different content of rutile can be obtained to optimize the photocatalytic activity. It is also reasonable to conclude that the hydrolysis rate is faster than the esterification one.
ð4Þ
! (OR)4x (OH)x1 Ti—O—Ti(OR)3x (OH)x + ROH
where R = CH3CO–. Acetic acid usually has a strong coordination capacity on metal centers, so an intermediate complex with TiCl4 may be formed (Eq. (2)). The process stood by Eqs. (3) and (4) accounts for the formation mechanism of hydroxyl and carboxyl of obtained powders. We hypothesize that organics are occluded in the product, meanwhile, there are abundant hydroxyl and adsorbed molecules on the surface of products. As seen from the TGA curve (Fig. 5), the dramatic mass loss at the initial segment may be caused by loss of the adsorbed molecules; the following mass loss are mainly caused by
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3.4. Effect of solvent
88 The choice of solvent is crucial. Firstly, the mixed solutions of different alcohols and acids have different viscosities. Li found that the viscosity not only affected the particle size, but also the morphologies and structures of TiO2 [28]. It was suggested that the higher viscosity inhibited the nucleation of the product and kept the particles from growing larger. We also investigated the influence of viscosity by substituting ethylene glycol and glycerol for ethanol. When the reaction temperature was kept at 120 °C, all powders, using ethylene glycol and glycerol as alcohols, were amorphous. To reduce the viscosity, a part of ethylene glycol and
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Temperature (°C) Fig. 5. TGA pattern of particles obtained in 20 min, and the reaction temperature was 120 °C. The solution contained 12 mL ethanol, 6 mL acetic acid and 0.6 mL TiCl4.
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Fig. 6. XRD patterns of TiO2 nanocrystals obtained at different concentrations, the reaction temperature was kept at 120 °C and the microwave irradiation time was set as a constant of 15 min. The solution was composed of 12 mL ethanol and 6 mL acetic acid. The starting amounts of TiCl4 were: (a) 0.3 mL, (b) 0.4 mL, (c) 0.5 mL, (d) 0.6 mL, (e) 0.65 mL, (f) 0.7 mL and (g) 0.8 mL. Right figure illustrates the relationship between starting TiCl4 amounts and the phase compositions.
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Volume adsorption ( cm3g-1 )
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glycerol was replaced by ethanol, but the anatase or rutile phase was still not obtained. Pure anatase phase was obtained when the temperature was increased to 170 °C with ethylene glycol acting as alcohol; however the samples obtained from the solution of glycerol and acetic acid still gave rise to amorphous products (Fig. 7). This clearly indicates the inhibition role of higher viscosity. To our interests, when ethylene glycol was used to replace ethanol, the products thus obtained were pure anatase phase, while the same amount of ethanol gave rise to the mixed phases of anatase and rutile. It seemed that the higher viscosity also led to the inhibited growth of rutile phase. The grain size of obtained particles was estimated to be 10 nm, which maybe a result of higher temperature. So it was not clear here that whether the smaller grain size was caused by the higher viscosity. Secondly, the ratio of ethanol to acetic acid can control the phase compositions too. Keeping the temperature at 120 °C, we decreased the amount of acetic acid from 6 mL to 4 mL, the phase of products changed from the mixed phase containing 23.5% rutile to pure rutile. Here the temperature once again showed its effect. When the temperature was kept at 100 °C, the product was pure anatase phase. Keeping on decreasing the amount of acetic acid to 2 mL, there were just a few aggregations; maybe this was the result of slowed esterification. In pure ethanol solutions, no particles were obtained; the results were the same in pure acetic acid solutions. The results illuminated that the quick production of TiO2 particles was due to the water generated in reaction systems. When the solvent was composed of 6 mL ethanol and 12 mL acetic acid, the yield was very low too; this should be an integrated result of slowed esterification and the enhanced chelation between acetic acid and Ti4+. 3.5. Nitrogen adsorption–desorption properties The present preparation methodology also led to the formation of mesophases in the titanium dioxide nanocomposites, and their porosity was investigated using nitrogen adsorption–desorption isotherms. To our knowledge, this is the first report that mesoporous titanium dioxide was synthesized under microwave irradiation without using any surfactants or templates. Fig. 8 gives the nitrogen adsorption–desorption isotherm of the specimen obtained at 120 °C, showing type IV-like isotherm with an inflection of nitro-
Fig. 8. Nitrogen adsorption–desorption isotherms for the synthesized titanium dioxide under the typical conditions.
gen adsorbed volume at around P/P0 = 0.50 (type H4 hysteresis loop). It indicates the presence of defined mesoporosity in the samples. According to the BJH model pore-size distribution is very broad using the adsorption branch of the isotherm. The specific surface area of the TiO2 particles obtained under a typical synthesis condition is about 75 m2/g using the BET method and its pore volume is about 0.062 cm3/g according to single point adsorption. Compared with the mixed phase product, pure rutile has much higher specific surface area (210 m2 g1), and the pore volume dramatically increases to 0.24 cm3/g, indicating another advantage of our method that the usage of surfactants for obtaining high specific surface area and porous TiO2 is unnecessary. These mesopores were formed by the stacking of the nanoparticles of titanium dioxide. 3.6. Photocatalytic activity Photocatalysis is one of the most important applications of titanium dioxide. We evaluated the photocatalytic activity of our samples by the degradation of methylene blue under the UV-light irradiation. It is shown from Fig. 9 that the sample with 3.6% rutile content has the highest catalytic activity, which was obtained by adding 0.4 mL TiCl4 to the mixed solutions of ethanol and acetic acid. Pure rutile phase has little photocatalytic activity, and its degradation curve is almost the same as that of the blank sample with-
0.0 3.6 23.5 60.4 84.0 100 blank
100%
Degradation (%)
Fig. 7. XRD spectrums of specimens obtained when the reaction time was 15 min. The solution contained 0.6 mL TiCl4 and other experiments parameters were: (a) 9 mL ethylene glycol and 9 mL acetic acid, 170 °C, (b) 4 mL ethanol and 6 mL ethylene glycol and 8 mL acetic acid, 170 °C, (c) 9 mL glycerol and 9 mL acetic acid, 170 °C, (d) 4 mL ethanol and 6 mL glycerol and 8 mL acetic acid, 170 °C, (e) 12 mL ethanol and 6 mL acetic acid, 120 °C, (f) 12 mL ethanol and 4 mL acetic acid, 120 °C and (g) 12 mL ethanol and 4 mL acetic acid, 100 °C.
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Time (h) Fig. 9. Changes of photodegradation of methylene blue with the increase of UVlight illuminating time. The legend denoted the rutile content of samples.
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out addition of titanium dioxide. Notably, the catalytic activity of pure anatase is lower even than the specimen with 60.4% rutile content, proving the synergistic effect of mixed phases. Because the catalytic activity is sensitive to the phase compositions, and the phase compositions are determined by the concentration of TiCl4, the catalytic activity can be optimized by changing the starting amount of TiCl4 very easily. However, under the same conditions the commercial product Degussa P-25 shows higher photocatalytic activity (it can degrade about 96.7% MB in 40 min) than our products, the reason is that our products have poorer crystallinity than that of P-25 and high photocatalytic activity involves good crystallinity. 4. Conclusions In summary, we have provided a new method for the convenient and rapid one-step preparation of mesoporous titanium dioxide nanocomposites with controllable phase compositions. Comparing to traditional wet-chemical methods, the microwaveassisted esterification method is simple and time-saving. By changing the microwave temperature, the initial amount of titanium tetrachloride and the ratio of ethanol to acetic acid, nanoscale anatase, rutile or mixed phases titanium dioxide with the grain size about 9–12 nm can be obtained. Because of the simple operation and economics, the method can simplify not only the fabrication of TiO2 in laboratory, but also the scaled-up industry production. Acknowledgments This work was financially supported by 973 Program (2007CB815303, 2006CB932903), NSFC (20521101, 20731005), NSF of Fujian Province (2005HZ01-1), Fujian Key Laboratory of Nanomaterials (2006L2005), and (CAS, 2007J0231). References [1] M. Yan, F. Chen, J. Zhang, M. Anpo, J. Phys. Chem. B 109 (2005) 8673. [2] A. Linsebigler, G. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735. [3] A.M. Tonejc, M. Goti, B. Greta, S. Music, S. Popovi, R. Trojko, A. Turkovi, I. Musevic, Mater. Sci. Eng. 40 (1996) 177.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Facile synthesis of mesoporous titanium dioxide nanocomposites with controllable phase compositions by microwave-assisted esterification Yafeng Li a,b, Hongfang Li a, Taohai Li a, Guoliang Li a,b, Rong Cao a,* a b
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, The Chinese Academy of Sciences, Fuzhou 350002, China Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 1 November 2007 Received in revised form 5 May 2008 Accepted 23 June 2008 Available online 23 July 2008 Keywords: Mesoporous Titanium dioxide Phase composition Microwave Photocatalytic
a b s t r a c t Mesoporous titanium dioxide nanocomposites with controllable phase compositions and high surface areas were synthesized through convenient, fast, and one-step microwave-assisted esterification method. The introduction of microwave in the synthetic reactions not only accelerates the esterification reaction but also promotes the fast crystallization. By changing reaction temperature, microwave irradiation time, the amount of staring materials and the composition of solvents, pure anatase, pure rutile or mixed phase titanium dioxide nanocomposites were obtained in minutes. Meanwhile, the size of crystallite can be controlled by the reaction temperature. The as-synthesized materials display significative photocatalytic activities without any further disposal, among which the material with 3.6% rutile shows the best effect in degradation of methylene blue under UV-light irradiation. Moreover, the nitrogen adsorption–desorption results illustrate the obtained samples are mesophases. It is also interesting that the pure rutile has specific surface area as high as 210 m2 g1 calculated by BET equation. The method can control the hydrolysis of TiCl4 well and dramatically shorten the preparation time of titanium dioxide. Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction As one of the most important oxide semiconductor materials, nanocrystalline titanium dioxide has been extensively studied in recent years for its specific properties, such as photocatalysis, high chemical stability, sensitivity to humidity and gas, low toxicity and so on [1–4]. Natural titania exists in three crystalline forms: anatase, rutile and brookite. Anatase is a kinetic product and its photocatalytic activity is believed to be much higher than that of rutile phase [5]. However, some authors showed opposite results [6], and many experimental evidences indicated that the existence of anatase and rutile mixed phases enhanced the photocatalytic activity [7]. The commercially available P-25, which consists of 80% anatase and 20% rutile, has been proved to be very efficient in many catalytic reactions. It is considered that the spatial charge separation and hindered recombination of electrons and holes contribute to the enhancement of photocatalytic efficiency of mixed phases [8]. Gray and Matsumura discovered that the crystal structure and morphology on phase interfaces also played important roles for the synergistic effect between anatase and rutile [9,10]. Therefore, the control of the phase compositions can be utilized to optimize the photocatalytic properties of titanium dioxide. Several strategies have been employed to prepare titanium dioxide, such as vapor-phase method, combustion synthesis, hydrothermal and sol–gel methods * Corresponding author. Tel./fax: +86 591 83796710. E-mail address: [email protected] (R. Cao). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.06.042
[11–21]. Among them wet-chemical reaction is a traditional method for the preparation of TiO2, where the hydrolysis of titanium halides and alkoxides has been extensively studied. This method does not need complicated equipments and the cost is low. The crystal sizes, and structures, as well as the phase compositions of TiO2 nanoparticles obtained by this method are influenced by acidity, solvent, additive, temperature and aging time [22–24], and the control on phase compositions has been realized by fine tuning these factors [25,26]. However, wet-chemical method usually involves long reaction time and high temperature, which can reduce the surface area and cause sintering or aggregation. Sometimes, fussy operations are needed, making the preparation of TiO2 a little boring. Hence, to develop new methods for ready fabrication of TiO2 with controllable phase compositions is of great significance. In this work, a new method, namely microwave-assisted esterification, was developed for the convenient and rapid synthesis of TiO2, by which mesoporous nanocrystalline TiO2 can be obtained in minutes and the phase compositions can also be controlled easily. Esterification reactions have been utilized to generate ‘‘in situ” water molecules, providing excellent control of the hydrolysis and condensation rate [27–29]. However, the reported works by esterification reaction employed solvethermal method and did not show advantage in preparation time. In order to overcome the shortcoming, we introduced microwave irradiation into the esterification reaction instead of the traditional heating. To date, microwave irradiation has been used to prepare many composite materials successfully [30–36], for instance, many ceramic powders were
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synthesized by microwave irradiation [37–40]. Titanium dioxide was also prepared by microwave-hydrothermal method, but the control on the hydrolysis of TiCl4 and the phase compositions were not studied in detail. In current work, we tried to control both the release of H2O by esterification, instead of the direct introduction of water, and the hydrolysis process of TiCl4. The microwave irradiation in our reaction systems plays two important roles: (a) it accelerates the esterification reaction of ethanol and acetic acid; (b) when titanium tetrachloride encounters water molecules generated by the esterification reactions, it promotes the nucleation and the growth of titanium dioxide nanocomposites. On the other hand, the proposed method can lead to the formation of mesophases in the as-synthesized titania with high specific surface areas. The mesophases are formed without addition of any surfactant, and a long reaction time necessary for preparation of porous titanium dioxide in sol–gel process is also avoided. 2. Experimental 2.1. Materials Titanium tetrachloride (98%) and all solvents were of reagent grade and used without further purification. All experiments were carried out in a microwave oven (Initiator 8 EXP, 2450 MHz frequency, Biotage Corp.) equipped with magnetic stirring. The single-mode applicator with the proven ‘‘Dynamic Field Tuning” feature offers fast heating of a broad range of solvents. The capacity is from 0.2 mL to 20 mL. A maximum power of 400 W was selected for all the syntheses. The real power was automatically adjusted according to the temperature. In a typical synthesis, 0.6 mL TiCl4 was added into a mixed solution of 12 mL absolute ethanol and 6 mL acetic acid under magnetic stirring, a clear solution was quickly formed. Then the solution was transferred into a 20 mL vial and the synthesis was carried out in the microwave oven, the temperature and reaction time are 120 °C and 15 min, respectively. The experiment was performed under magnetic stirring with a rate of 600 rpm (the default value of the oven). The molar ratio was about TiCl4/EtOH/AcOH = 1/38.3/19.5. When the reaction was finished, the precipitates were separated by centrifugation and washed several times with absolute ethanol. Finally, the obtained powders were dried at 60 °C under vacuum.
The starting concentration of MB in the solution was 8 mg L1 and the photocatalyst loading amounted to 0.4 g L1. Vigorous stirring was employed to ensure the adsorption equilibrium and to eliminate any diffusion effect. After stirring for 2 h in dark, the equilibrium MB concentration was recorded as the initial concentration. The reaction solution was illuminated with a 300 W mercury lamp, emitting UV-light (kmax = 365 nm), positioned at 25 cm around the reactor. A portion of 2–3 mL solution was taken out in every 30 min, and the catalyst was separated by centrifugation. The final MB concentration in the solution was determined by UV absorbance at 664 nm from which the yield could be calculated. 3. Results and discussion 3.1. Influence of reaction temperature Factors influencing the specimens of titanium dioxide, such as reaction temperature, microwave irradiation time, the amount of starting materials and solvents, were investigated. Fig. 1 shows the XRD patterns of TiO2 nanocrystals synthesized at different temperatures, the microwave irradiation time is 15 min. As seen from the XRD patterns, when the temperature is 90 °C, only pure anatase phase (PDF#21-1272) is observed. When the temperature is increased to 100 °C and higher, mixed phases of anatase and rutile (PDF#21-1276) are obtained, and no photocatalytically inactive brookite phase is observed. Comparing the patterns it can be noticed that the crystallinity is improved with the increase of temperature. Interestingly, the bulk content of rutile phase doesn’t increase monotonically as the temperature rises. According to the formula reported in literature [41], the weight fraction of rutile has a maximum of 29% at 110 °C, other temperatures lead to the decrease of rutile content, for instances, 10%, 24% and 16% of rutile contents were observed for 100 °C, 120 °C and 150 °C, respectively. We suggest that this is the result of competition between anatase and rutile, and 110 °C may be the optimized temperature for the formation of latter. On the basis of well-known Scherrer’s equation, the grain sizes are estimated to be about 9 nm for the sample obtained at 150 °C and 11 nm for those obtained at other temperatures by applying peak broadening analysis to anatase (1 0 1) and rutile (1 1 0) diffractions. This is reasonable because
A
2.2. Characterization of the composites
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To determine the phase compositions of the obtained TiO2 nanocomposites, X-ray diffraction spectra were taken using a Dmax 2500 diffractometer (Cu Ka radiation, k = 1.54 Å). Infrared spectra were measured with the SpectrumOne spectrometer, and the measurements were performed with pressed pellets which were made by using KBr powder as diluents. Surface areas were determined using an ASAP2020 analyzer and calculated by BET method. The pore-size distribution was calculated using the BJH (Barrett–Joyner–Halenda) equation from the adsorption branch of the isotherm. Samples were degassed for 7 h at 100 °C prior to the N2 adsorption analysis, and the analysis was carried out at liquid nitrogen temperature. Thermogravimetric analysis was performed using a STA449C thermal analyzer with air as carrier gas. HRTEM analysis for the sample was investigated on a JEM 2010 microscope. The UV absorbance of methylene blue was recorded by a Lambda35 UV/visible spectrophotometer at 664 nm.
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2θ (degree) 2.3. Catalytic activity measurement The catalytic performance of the obtained titania specimens was probed by the photodegradation of methylene blue (MB).
Fig. 1. XRD spectra of TiO2 nanocrystals obtained at different temperatures, the microwave irradiation time was kept at 15 min and the solution comprised 12 mL ethanol, 6 mL acetic acid and 0.6 mL TiCl4 (A = anatase, R = rutile): (a) 90 °C, (b) 100 °C, (c) 110 °C, (d) 120 °C and (e) 150 °C.
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j i h
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g f e d c Fig. 2. Typical HRTEM image of specimen obtained at 120 °C, the microwave irradiation time was 15 min and the solution contained 12 mL ethanol, 6 mL acetic acid and 0.6 mL TiCl4.
b a
the presence of more furious vibrations under higher temperature goes against the growth of crystal nucleus, thus the grain size can be controlled by the temperature. Fig. 2 is a typical TEM image of the sample obtained at 120 °C. The crystallites are estimated to be about 8–13 nm, corresponding to the XRD result.
As seen from Fig. 3, it is amazing that pure anatase can be obtained in 1 min. Factually, the microwave irradiation only affected the formation of nanocrystals of TiO2 in about 40 s because of the working mechanism of our microwave instrument. When the temperature reaches the setting value by the microwave irradiation, there is amortization and the temperature can be 3 °C higher. Before the temperature decreases to the right value, the real power of microwave irradiation is zero. The result indicates that the esterification along with the nucleation and growth of the nanocomposites are very fast. When TiCl4 is dissolved in the solutions of ethanol and acetic acid, Ti4+ coordinates with acetic acid, forming complex Ti(OOCCH3)4 [42]. At the same time the reaction gives rise to proton, which has a catalytic activity on esterification reactions. This process may partially accounts for the rapid esterification and the subsequent formation of TiO2 nanocrystals. The grain size of all products is estimated to be between 10 nm and 12 nm, and the contents of rutile do not change regularly. Prolongation of microwave irradiation time can improve the crystallinity and the yield of TiO2 powders, for example, when the microwave irradiation time is shorter than 5 min, the color of the resulted solution is light-cyan, indicating the incomplete transformation of TiCl4 to TiO2. Under typical synthetic conditions 15 min is sufficient for a satisfied yield (>98%). A prolonged time is needed at higher concentration of TiCl4 for the purpose of yield of TiO2 colloids. As a matter of fact, the particles of all specimens can form stable colloid solutions when washing them with distilled water. These particles are difficult to separate completely from the solutions by centrifugation, indicating strong interactions between the particles and water molecules. The speciality of the obtained samples is due to the surface coating of hydroxyl and carboxyl, which can be confirmed by infrared spectra. As seen from Fig. 4, the band at 3253 cm1 can be ascribed to the stretching vibrations of –OH, while the bands at 1568 cm1
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2θ (degree) Fig. 3. XRD patterns of TiO2 nanocrystals obtained at different microwave irradiation time, the temperature was kept at 120 °C and the solution contained 12 mL ethanol, 6 mL acetic acid and 0.6 mL TiCl4: (a) 1 min, (b) 2 min, (c) 3 min, (d) 4 min, (e) 5 min, (f) 10 min, (g) 15 min, (h) 20 min, (i) 25 min and (j) 30 min.
and 1415 cm1 correspond to the symmetric and asymmetric vibrations of COO [28], illuminating the presence of CH3COO bidentate chelating, which was illustrated in Weller’s scheme [43]. The bands at 2976 cm1 and 2928 cm1, although weakened by the screening effect of the KBr matrix, can be ascribed to the stretching vibrations of CH2 and CH3 groups, indicating the existence of adsorbed ethanol molecules on the surface of the sample. Besides the direct hydrolysis process of TiCl4 (Eq. (1)), as a known growth model of the Ti–O–Ti network [44], the following hydroly-
Transmittance
3.2. Effect of microwave irradiation time
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2976 3254 1568 4000
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Wave number (cm-1) Fig. 4. Infrared spectrum of particles obtained in 10 min, and the microwave temperature was 120 °C. The solution contained 12 mL ethanol, 6 mL acetic acid and 0.6 mL TiCl4.
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sis (Eq. (3)) and polycondensation (Eq. (4)) may occur in our experiments:
TiCl4 + 2H2 O ! TiO2 + 4HCl
ð1Þ
TiCl4 + 4ROH ! Ti(OR)4 + 4HCl
ð2Þ
Ti(OR)4 + xH2 O ! Ti(OH)x (OR)4x + xROH
ð3Þ
(OR)4x (OH)x1 Ti—OH + RO—Ti(OH)x (OR)3x
the elimination of water generated by oxolation, and the combustion of these functional groups. 3.3. Effect of initial amount of TiCl4 The amount of TiCl4 in the initial solutions is the most important factor in our experiments. This group of parallel experiments was carried out at 120 °C and the microwave irradiation time was set as 15 min. From the previous results we know that mixed phases of anatase and rutile were obtained when 0.6 mL TiCl4 was added to the solution of ethanol and acetic acid. If the amount of TiCl4 was increased to 0.8 mL, pure rutile could be obtained; when the amount was decreased to 0.3 mL, nearly pure anatase was produced. As illustrated in Fig. 6, the content of rutile synchronously increases when the initial amount of TiCl4 gradually increases from 0.3 mL to 0.8 mL. The phase compositions are very sensitive to the concentrations, for example, the rutile content increases from 23.5% to 84.0% when the amount of TiCl4 is changed from 0.6 mL to 0.7 mL. As is well-known, anatase phase TiO2 is composed of point-sharing octahedrons while rutile phase TiO2 is made up of edge-sharing ones. To explain this phenomenon, we suppose that the amount of water released by esterification may vary little in a given time, so when the quantity of TiCl4 is increased, the TiO2 octahedrons adopted the more compact packing mode, which was more efficient in using oxygen atoms obtained from the limited water molecules. Thus, by carefully adjusting the initial amount of TiCl4, mixed phases with different content of rutile can be obtained to optimize the photocatalytic activity. It is also reasonable to conclude that the hydrolysis rate is faster than the esterification one.
ð4Þ
! (OR)4x (OH)x1 Ti—O—Ti(OR)3x (OH)x + ROH
where R = CH3CO–. Acetic acid usually has a strong coordination capacity on metal centers, so an intermediate complex with TiCl4 may be formed (Eq. (2)). The process stood by Eqs. (3) and (4) accounts for the formation mechanism of hydroxyl and carboxyl of obtained powders. We hypothesize that organics are occluded in the product, meanwhile, there are abundant hydroxyl and adsorbed molecules on the surface of products. As seen from the TGA curve (Fig. 5), the dramatic mass loss at the initial segment may be caused by loss of the adsorbed molecules; the following mass loss are mainly caused by
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3.4. Effect of solvent
88 The choice of solvent is crucial. Firstly, the mixed solutions of different alcohols and acids have different viscosities. Li found that the viscosity not only affected the particle size, but also the morphologies and structures of TiO2 [28]. It was suggested that the higher viscosity inhibited the nucleation of the product and kept the particles from growing larger. We also investigated the influence of viscosity by substituting ethylene glycol and glycerol for ethanol. When the reaction temperature was kept at 120 °C, all powders, using ethylene glycol and glycerol as alcohols, were amorphous. To reduce the viscosity, a part of ethylene glycol and
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Temperature (°C) Fig. 5. TGA pattern of particles obtained in 20 min, and the reaction temperature was 120 °C. The solution contained 12 mL ethanol, 6 mL acetic acid and 0.6 mL TiCl4.
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Fig. 6. XRD patterns of TiO2 nanocrystals obtained at different concentrations, the reaction temperature was kept at 120 °C and the microwave irradiation time was set as a constant of 15 min. The solution was composed of 12 mL ethanol and 6 mL acetic acid. The starting amounts of TiCl4 were: (a) 0.3 mL, (b) 0.4 mL, (c) 0.5 mL, (d) 0.6 mL, (e) 0.65 mL, (f) 0.7 mL and (g) 0.8 mL. Right figure illustrates the relationship between starting TiCl4 amounts and the phase compositions.
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Volume adsorption ( cm3g-1 )
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glycerol was replaced by ethanol, but the anatase or rutile phase was still not obtained. Pure anatase phase was obtained when the temperature was increased to 170 °C with ethylene glycol acting as alcohol; however the samples obtained from the solution of glycerol and acetic acid still gave rise to amorphous products (Fig. 7). This clearly indicates the inhibition role of higher viscosity. To our interests, when ethylene glycol was used to replace ethanol, the products thus obtained were pure anatase phase, while the same amount of ethanol gave rise to the mixed phases of anatase and rutile. It seemed that the higher viscosity also led to the inhibited growth of rutile phase. The grain size of obtained particles was estimated to be 10 nm, which maybe a result of higher temperature. So it was not clear here that whether the smaller grain size was caused by the higher viscosity. Secondly, the ratio of ethanol to acetic acid can control the phase compositions too. Keeping the temperature at 120 °C, we decreased the amount of acetic acid from 6 mL to 4 mL, the phase of products changed from the mixed phase containing 23.5% rutile to pure rutile. Here the temperature once again showed its effect. When the temperature was kept at 100 °C, the product was pure anatase phase. Keeping on decreasing the amount of acetic acid to 2 mL, there were just a few aggregations; maybe this was the result of slowed esterification. In pure ethanol solutions, no particles were obtained; the results were the same in pure acetic acid solutions. The results illuminated that the quick production of TiO2 particles was due to the water generated in reaction systems. When the solvent was composed of 6 mL ethanol and 12 mL acetic acid, the yield was very low too; this should be an integrated result of slowed esterification and the enhanced chelation between acetic acid and Ti4+. 3.5. Nitrogen adsorption–desorption properties The present preparation methodology also led to the formation of mesophases in the titanium dioxide nanocomposites, and their porosity was investigated using nitrogen adsorption–desorption isotherms. To our knowledge, this is the first report that mesoporous titanium dioxide was synthesized under microwave irradiation without using any surfactants or templates. Fig. 8 gives the nitrogen adsorption–desorption isotherm of the specimen obtained at 120 °C, showing type IV-like isotherm with an inflection of nitro-
Fig. 8. Nitrogen adsorption–desorption isotherms for the synthesized titanium dioxide under the typical conditions.
gen adsorbed volume at around P/P0 = 0.50 (type H4 hysteresis loop). It indicates the presence of defined mesoporosity in the samples. According to the BJH model pore-size distribution is very broad using the adsorption branch of the isotherm. The specific surface area of the TiO2 particles obtained under a typical synthesis condition is about 75 m2/g using the BET method and its pore volume is about 0.062 cm3/g according to single point adsorption. Compared with the mixed phase product, pure rutile has much higher specific surface area (210 m2 g1), and the pore volume dramatically increases to 0.24 cm3/g, indicating another advantage of our method that the usage of surfactants for obtaining high specific surface area and porous TiO2 is unnecessary. These mesopores were formed by the stacking of the nanoparticles of titanium dioxide. 3.6. Photocatalytic activity Photocatalysis is one of the most important applications of titanium dioxide. We evaluated the photocatalytic activity of our samples by the degradation of methylene blue under the UV-light irradiation. It is shown from Fig. 9 that the sample with 3.6% rutile content has the highest catalytic activity, which was obtained by adding 0.4 mL TiCl4 to the mixed solutions of ethanol and acetic acid. Pure rutile phase has little photocatalytic activity, and its degradation curve is almost the same as that of the blank sample with-
0.0 3.6 23.5 60.4 84.0 100 blank
100%
Degradation (%)
Fig. 7. XRD spectrums of specimens obtained when the reaction time was 15 min. The solution contained 0.6 mL TiCl4 and other experiments parameters were: (a) 9 mL ethylene glycol and 9 mL acetic acid, 170 °C, (b) 4 mL ethanol and 6 mL ethylene glycol and 8 mL acetic acid, 170 °C, (c) 9 mL glycerol and 9 mL acetic acid, 170 °C, (d) 4 mL ethanol and 6 mL glycerol and 8 mL acetic acid, 170 °C, (e) 12 mL ethanol and 6 mL acetic acid, 120 °C, (f) 12 mL ethanol and 4 mL acetic acid, 120 °C and (g) 12 mL ethanol and 4 mL acetic acid, 100 °C.
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Time (h) Fig. 9. Changes of photodegradation of methylene blue with the increase of UVlight illuminating time. The legend denoted the rutile content of samples.
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out addition of titanium dioxide. Notably, the catalytic activity of pure anatase is lower even than the specimen with 60.4% rutile content, proving the synergistic effect of mixed phases. Because the catalytic activity is sensitive to the phase compositions, and the phase compositions are determined by the concentration of TiCl4, the catalytic activity can be optimized by changing the starting amount of TiCl4 very easily. However, under the same conditions the commercial product Degussa P-25 shows higher photocatalytic activity (it can degrade about 96.7% MB in 40 min) than our products, the reason is that our products have poorer crystallinity than that of P-25 and high photocatalytic activity involves good crystallinity. 4. Conclusions In summary, we have provided a new method for the convenient and rapid one-step preparation of mesoporous titanium dioxide nanocomposites with controllable phase compositions. Comparing to traditional wet-chemical methods, the microwaveassisted esterification method is simple and time-saving. By changing the microwave temperature, the initial amount of titanium tetrachloride and the ratio of ethanol to acetic acid, nanoscale anatase, rutile or mixed phases titanium dioxide with the grain size about 9–12 nm can be obtained. Because of the simple operation and economics, the method can simplify not only the fabrication of TiO2 in laboratory, but also the scaled-up industry production. Acknowledgments This work was financially supported by 973 Program (2007CB815303, 2006CB932903), NSFC (20521101, 20731005), NSF of Fujian Province (2005HZ01-1), Fujian Key Laboratory of Nanomaterials (2006L2005), and (CAS, 2007J0231). References [1] M. Yan, F. Chen, J. Zhang, M. Anpo, J. Phys. Chem. B 109 (2005) 8673. [2] A. Linsebigler, G. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735. [3] A.M. Tonejc, M. Goti, B. Greta, S. Music, S. Popovi, R. Trojko, A. Turkovi, I. Musevic, Mater. Sci. Eng. 40 (1996) 177.
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