Effects of alcohol content and calcination temperature on the textural properties of bimodally mesoporous titania

Applied Catalysis A: General 255 (2003) 309–320

Effects of alcohol content and calcination temperature on the textural properties of bimodally mesoporous titania Jiaguo Yu a,∗ , Jimmy C. Yu b , Wingkei Ho b , Mitch K.-P. Leung b , Bei Cheng a , Gaoke Zhang a , Xiujian Zhao a a

b

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Department of Chemistry, Materials Science & Technology Research Center, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China Received 4 May 2003; received in revised form 15 July 2003; accepted 15 July 2003

Abstract Bimodally mesoporous titania was prepared by hydrolysis of titanium tetraisopropoxide in pure water or the EtOH–H2 O mixed solution under ultrasonic irradiation. Effects of alcohol content and calcination temperature on the phase composition and porosity of bimodally mesoporous titania was investigated by thermogravimetric and differential thermal analysis (TG–DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET) surface areas. The results showed that for all TiO2 powders calcined from 400 to 600 ◦ C, the pore size distribution is bimodal with fine intra-particle pore diameter at maximum pore diameters of ca. 2–4 nm and larger inter-particle pore diameter at maximum pore diameters of ca. 18–50 nm. The EtOH/H2 O molar ratios obviously influenced the crystallization, crystallite size, BET surface areas, porosity and morphology of the prepared TiO2 powders. © 2003 Elsevier B.V. All rights reserved. Keywords: Bimodally mesoporous titania; Titanium tetraisopropoxide; Hydrolysis; Calcination temperature; Water content; Porosity; Phase composition

1. Introduction To solve the increasingly serious problems of environmental pollution, various catalytic techniques are being applied in the fields of environmental protection. Photocatalysis is one technique that has great potential to control aqueous organic contaminates or air pollutants. It is believed to have several advantages over conventional oxidation processes, such as: ∗ Corresponding author. Tel.: +86-27-87883610; fax: +86-27-87883610. E-mail addresses: [email protected] (J. Yu), [email protected] (J.C. Yu).

(1) complete mineralization of the pollutants; (2) use of the near-UV or solar light; (3) no addition of other chemicals; and (4) operation at near room temperature [1–6]. Although photocatalytic degradations of trace toxic organic compounds in water or air have been investigated intensively in the past decade, there still remain some problems in practical applications [5]. Fundamental research regarding the preparation of photocatalyst with highly photocatalytic activity, the immobilization of powder photocatalyst, and the improvement of photocatalyst performance are priorities to be considered [5–7]. Among various oxide semiconductor photocatalysts, titania is a very important photocatalyst for its

0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00570-2

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strong oxidizing power, non-toxicity and long-term photostability. Titania has three different crystalline phases: rutile, anatase and brookite, among which rutile is thermodynamic stable state while the latter two phases are metastable state [8]. Photocatalytic activity of titania may strongly depend on its phase structure, crystallite size, the specific surface areas and pore structure, and so on, for example, many studies have confirmed that the anatase phase of titania is the superior photocatalytic materials for air purification, water disinfection, hazardous waste remediation, and water purification [1,2]. Sol–gel processing can be used to make nanometersized photocatalysts by a chemical reaction in solution starting with metal alkoxides as a precursor at room temperature [9,10]. An advantage of the sol–gel processing is that it is possible to obtain large amount of powders with a high level of chemical purity [11]. In sol–gel synthesis of titania, hydrolysis and polycondensation reactions occur simultaneously when titanium tetraisopropoxide (TTIP) mixes with water [12,13]. The polycondensation reaction induces polymerization forming higher molecular weight products (nuclei and subsequently particles) [14,15]. These two reactions are sensitive to the reaction conditions, such as type and amount of solvent, water concentration, type and amount of catalysts, reaction temperature, mixing conditions, and so on. Thus, the photocatalytic activity and microstructures of the obtained titania powders depend strongly on the above reaction conditions. Especially, the composition of mixed solvents significantly influences textures and photocatalytic activity of titania powders [16–18]. In the previous research, Song and Pratsinis [16] synthesized the bimodally porous titania powders from hydrolysis of TTIP by controlling the water concentration during hydrolysis and calcination temperatures. They also reported the effects of hydrolysis temperatures, dopants, type and amount of catalyst and alcohol solvents on synthesis of bimodally nanostructured porous titania powders [19–22]. Anpo et al. [23] and others [24–26] investigated the effects of the calcinations temperature on the improvement of the photocatalytic activity, particle size, size quantization effects, and band structure of TiO2 . Ito et al. [10] found that the volume ratio of H2 O to EtOH was a key factor influencing crystallinity and photocatalytic activity of TiO2 particles prepared from the EtOH–H2 O mixed solution of TiOSO4 . Very

recently, we have also reported the enhancing effects of water content and ultrasonic irradiation on photocatalytic activity of nanometer-sized TiO2 powders prepared by the sol–gel method, and the photocatalytic activity of the obtained TiO2 powders exceeded that of Degussa P25 [17,18]. However, to the best of our knowledge, effects of alcohol content and calcination temperatures on the phase transformation and pore structures of titania powders prepared by hydrolysis of TTIP and ultrasonic irradiation have received little attention. Sonochemistry arises from acoustic cavitation, the formation, growth, and implosive collapse of bubbles in a liquid. The collapse of bubbles generates localized hot spots with transient temperatures of about 5000 K, pressures of about 5 × 107 Pa, and heating and cooling rates greater than 109 K s−1 [17,18]. Therefore, the use of ultrasound to enhance the rate of reaction has become a routine synthetic technique for many homogeneous and heterogeneous chemical reactions. The sonochemistry has been used to prepare various oxides and amorphous metal powders [17,18]. In this study, mesoporous nanometer-sized TiO2 photocatalyst with different phase compositions has been prepared by sol–gel method and ultrasonic treatment in pure water or the EtOH–H2 O mixed solution. The effects of initial alcohol concentration and calcination temperatures on the characteristics of mesoporous titania powders were investigated by thermogravimetric and differential thermal analysis (TG–DTA), X-ray diffraction (XRD), nitrogen adsorption, and scanning electron microscopy (SEM). 2. Experimental 2.1. Preparation All chemicals used in this study were reagent-grade supplied from Aldrich and were used as received. Millipore water was used in all experiments. Titanium tetraisopropoxide (Ti(OC3 H7 )4 , 97%) was used as a titanium source. Hydrolysis was carried out at room temperature by adding controlled amounts of TTIP slowly to EtOH–H2 O mixed solution under vigorous stirring for 30 min. Sol samples obtained by the hydrolysis process were irradiated with ultrasonic in an ultrasonic cleaning bath (Bransonic ultrasonic

J. Yu et al. / Applied Catalysis A: General 255 (2003) 309–320

cleaner, model 3210E DTH, 47 kHz, 120 W, USA) for 1 h, followed by aging in closed beaker at room temperature for 24 h in order to further hydrolyze the TTIP. After aging, these samples were evaporated at 100 ◦ C for about 8 h in air in order to remove water and alcohol in the gels and then ground to fine powders to obtain xerogel samples. The xerogel samples were calcined at 400, 500, 600 and 700 ◦ C in air for 1 h, respectively. Powders were prepared at EtOH/H2 O molar ratios (REtOH ) of 0, 1 and 10 and labeled as R0, R1 and R10, respectively. 2.2. Characterization The X-ray diffraction patterns obtained on a Philips MPD 18801 X-ray diffractometer using Cu K␣ radiation at a scan rate (2θ) of 0.05◦ s−1 were used to determine the identity of any phase present and their crystallite size. The accelerating voltage and the applied current are 35 kV and 20 mA, respectively. The phase content of a sample can be calculated from the integrated intensities of anatase (1 0 1), rutile (1 1 0) and brookite (1 2 1) peaks. If a sample contains only anatase and rutile, the mass fraction of rutile (WR ) can be calculated from [27]: WR =

AR 0.886AA + AR

(1)

where AA and AR represent the integrated intensity of the anatase (1 0 1) and rutile (1 1 0) peaks, respectively. If brookite is also present in sample, similar relations can be derived [27]: WA =

KA A A K A A A + A R + K B AB

(2a)

WR =

AR KA A A + A R + K B A B

(2b)

WB =

KB AB KA A A + A R + K B A B

(2c)

where WA and WB represent the mass fraction of anatase and brookite, respectively. AB is the integrated intensity of the brookite (1 2 1) peak, and KA and KB are two coefficients and their values are 0.886 and 2.721, respectively. With Eq. (2), the phase contents in any TiO2 samples can be calculated. The average crystallite sizes of anatase, rutile, and brookite were determined according to the Scherrer equation using

311

the full-width half-maximum data of each phase after correcting the instrumental broading [27]. The crystallization behavior is also monitored using a TG–DTA machine (model TG–DTA 92-16, Setaram, France). Particle sizes and shapes were observed using scanning electron microscopy (Japan). The Brunauer–Emmett–Teller (BET) surface area (SBET ) of the powders was analyzed by nitrogen adsorption in a Micromeritics ASAP 2000 nitrogen adsorption apparatus. For R0, R1 and R10 xerogel samples dried at 100 ◦ C, the samples were degassed at 100 ◦ C prior to actual measurements. However, for the calcined xerogel samples at high temperature (from 400 to 700 ◦ C), the degassed temperature is 180 ◦ C. The BET surface area was determined by the multipoint BET method using the adsorption data in the relative pressure (P/P0 ) range 0.05–0.25. The desorption isotherm was used to determine the pore size distribution using the Barret–Joyner–Halender (BJH) method with cylindrical pore size [28].

3. Results and discussion 3.1. Thermal analysis Fig. 1(a) and (b) show DTA curves of the R0 and R10 TiO2 xerogel powders dried at 100 ◦ C,

Fig. 1. DTA curves of the R0 (a) and R10 (b) xerogel TiO2 powders dried at 100 ◦ C for 8 h.

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respectively. It can be seen from Fig. 1(b) that a broad endothermic peak at below 150 ◦ C and a relatively small exothermic peak at 266 ◦ C are due to desorption of water and alcohol and thermal decomposition of organic substances contained in the xerogel, respectively. At 373 ◦ C, a very sharp exothermic peak is observed due to the phase transformation of amorphous to anatase phase [12,29]. Owing to the difference of the xerogel compositions, the positions and intensities of these peaks are obvious different in Fig. 1(a). Especially, the exothermic peak intensity of formation of anatase phase in Fig. 1(a) is much smaller than that in Fig. 1(b). This is assigned to the former mostly transformed into anatase phase at low temperature (as shown in Fig. 3(a)). Usually, the concentration of alkyl (OR) groups in the network structure decreases with increasing the amount of water in the hydrolysis medium. However, in the titania system, it was reported that a certain concentration of OR groups always remains in the structure, regardless of the amount of water present [13]. Therefore, the exothermic peak at 241 ◦ C in Fig. 1(a) is due to thermal decomposition of residual unhydrolyzed alkyls in TiO2 xerogel samples prepared from pure water, but the relative intensity of peaks decreased owing to the decrease in the amount of the unhydrolyzed residual alkyls. Fig. 2 shows the TG curves for the R0 and R10 xerogel powders dried at 100 ◦ C. Whether the xerogel

powders were R0 or R10, two main zones in weight loss can be roughly identified. The first zone from 50 to 150 ◦ C corresponds to the removal of physically adsorbed water and alcohol [29]. The second zone from 150 to 400 ◦ C corresponds to the oxidation of residual organic components. However, for the R10 xerogel powders, its weight loss is obviously larger than that of the R0 powders due to a larger amount of residual unhydrolyzed alkyls in the xerogel. 3.2. Phase composition Fig. 3(a)–(c) show the XRD patterns of the R0, R1 and R10 TiO2 powders calcined at different temperatures, respectively. The R0 xerogel powders dried at 100 ◦ C contain the anatase and brookite phases. The R1 xerogel powders dried at 100 ◦ C only contain the anatase phase. While the R10 xerogel powders dried at 100 ◦ C show an amorphous phase. It is apparent that the phase transformation temperature from amorphous to anatase depends on the initial EtOH/H2 O molar ratio. With increasing the initial EtOH/H2 O molar ratio, the phase transformation temperature of amorphous to anatase increases. This is probably due to the fact that when the water concentration during hydrolysis reaction was small, large amount of unhydrolyzed alkyls remained in the powders. These alkyls prevent crystallization to anatase so that powders were

2

Mass reduction (%)

0 R EtOH = 0 R EtOH = 10

-2 -4 -6 -8 -10 -12 -14 -16 -18 -20 0

100

200

300

400

500

600

700

o

Temperature ( C) Fig. 2. TG curves of the R0 and R10 xerogel powders dried at 100 ◦ C for 8 h.

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313

Fig. 3. XRD patterns of the R0 (a), R1 (b) and R10 (c) TiO2 powders calcined at various temperatures. The peaks marked A, R and B represent the anatase, rutile and brookite phase, respectively.

amorphous by adsorbing on the surfaces of TiO2 particles [10]. On the other hand, with increasing water concentration, a stronger nucleophilic substitute reaction between H2 O and alkoxide molecules will occur and more alkoxyl groups in the alkoxide will be substituted by hydroxyl groups of H2 O [30], the amount of residual alkyls preventing crystallization to anatase was small. Therefore, the molecular structure of the powder resembled that of the anatase phase, which is more stable than the amorphous phase [12]. It can also be seen from Fig. 3(a)–(c) that the initial EtOH/H2 O molar ratios obviously influence the phase composition and transformation temperatures of amorphous to anatase and anatase to rutile. For R0

sample, with increasing calcination temperature (from 100 to 600 ◦ C), the peak intensities of anatase increase and the width of the (1 0 1) plane diffraction peak of anatase (2θ = 25.4◦ ) becomes narrower, the crystallization of titania was enhanced. At 600 ◦ C, rutile phase appears, TiO2 powders meanwhile contain three different phase: anatase, brookite and rutile. At 700 ◦ C, rutile is a main phase and brookite disappears. For R1 sample, brookite phase only appears at 400 and 500 ◦ C, also, its content is obviously lower than that in R0 sample [18]. At 600 ◦ C, rutile phase appears and brookite phase disappears. For R10 sample, there is no trace of brookite phase and the phase transformation of amorphous to anatase and anatase to rutile

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Fig. 4. Schematic model of the phase transformations in the samples calcined at different temperatures. PA : anatase; PB : brookite; PR : rutile. The concentrations of different phases are roughly illustrated by the area size of the circles.

occurs at 400 and 700 ◦ C, respectively. For R0 and R1, the phase transformation temperature of anatase to rutile is 600 ◦ C. For R10, however, the phase transformation of anatase to rutile occurs at 700 ◦ C. This is due to the increase of steric hindrance caused by the residual unhydrolyzed alkyls [17,18]. These different mechanisms of phase transformation occurring in the three samples are illustrated in Fig. 4. Fig. 5 shows the average crystalline size of anatase phase in the R0, R1 and R10 TiO2 samples as a func-

tion of calcination temperature. For R0 and R1 samples, with increasing calcination temperature, the average crystallite size of anatase increases. From 500 to 700 ◦ C, there is a steep increase of anatase crystal size due to phase transformation of anatase to rutile phase. It can also be seen from Fig. 5 that the average crystallite size of anatase in the R10 powders is obviously larger than that in the R0 and R1 powders. This is ascribed to the phase transformation of amorphous to anatase occurred at about 373 ◦ C and combustion

50

Anatase crystallite size (nm)

45

R0 R1 R10

40 35 30 25 20 15 10 5 0 100

200

300

400

500

600

700

o

Temperature ( C) Fig. 5. Average crystallite size of anatase in the R0, R1 and R10 powders as a function of calcination temperature.

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of residual organic components in xerogel at about 266 ◦ C, which causing the growth of anatase crystallites by providing the heat of phase transformation and combustion. Also, the steep increase of anatase crystal size for R10 appeared at higher temperature (between 600 and 700 ◦ C) due to the phase transformation of anatase to rutile phase occurred at higher temperature (between 600 and 700 ◦ C). 3.3. BET surface areas and pore structure Fig. 6 shows the specific surface areas of the powders prepared at different temperature. All powders show a monotonic decrease of the specific surface area with increasing calcination temperature due to the phase transformation and crystallite growth. It is interesting to note that the R10 powder show higher surface areas than the R1 and R0 powders at 100 ◦ C, but exhibit smaller surface area at ≥400 ◦ C. This can be explained by the XRD and TG–DTA analyses as follows: at 100 ◦ C, the R0 or R1 powders had anatase and brookite or anatase phases, respectively, as confirmed by XRD analysis in Fig. 3(a) and (b), and thus the particle sizes of the powders are bigger than those of R10 powders which showed amorphous titania determined by XRD. Hence, the specific surface areas of the R0 and R1 powders are smaller than those of the R10 powder. However, the R10 powders

315

go through the phase transformation of amorphous to anatase and the thermal decomposition of residual organics occurred in between 300 and 400, and 150 and 300 ◦ C, respectively, as confirmed by DTA, TG and XRD analyses in Figs. 1, 2 and 3(c), which results in larger TiO2 crystallites due to the heat of phase transformation and thermal decomposition. Therefore, the specific surface areas of the R10 powder decrease rapidly. Moreover, at above 400 ◦ C, the specific areas of the R10 powder become smaller than those of the R0 and R1 powders due to its larger crystallite. Fig. 7 shows the isotherms of the xerogel powders dried at 100 ◦ C and prepared at various EtOH/H2 O molar ratios. The isotherm of the R10 powder is a combination of types I and IV (BDDT classification) with two very distinct regions: at low relative pressure, the isotherm exhibits high adsorption, indicating that the powder contains micropores (type I). However, at high relative pressures between 0.8 and 1.0, the curve exhibits a hysteresis loop indicating the presence of mesopores (type IV). The shape of the hysteresis loop is of a type H3, associate with plate-like particles giving rise to narrow slit-shaped pores [31,32]. On the other hand, the R0 and R1 powders have isotherms of type IV which exhibit hysteresis loops mostly of type H3. Thus, the powders are mesoporous and the pores have narrow slit-shaped shapes. Unlike the isotherm of the R10 powder with one hysteresis

R10 R1 R0

500

3

BET surface area (cm /g)

600

400 300 200 100 0 100

200

300

400

500

600

700

o

Temperature ( C) Fig. 6. The specific surface area of the R0, R1 and R10 powders as a function of calcination temperatures.

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3

Adsorbed volume (cm /g)

400 R10 R1 R0

350 300 250 200 150 100 50 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0) Fig. 7. Nitrogen adsorption and desorption isotherms obtained from the powders prepared at different initial EtOH/H2 O molar ratios and dried at 100 ◦ C.

loop, the isotherms of the R0 and R1 powders have two hysteresis loops, indicating bimodal pore size distributions in the mesoporous region. Fig. 8 shows the pore size distributions of the powders prepared at various initial EtOH/H2 O molar ratios and dried at 100 ◦ C. All powders show bimodal pore size distributions consisting of intra-aggregated pores with maximums pore diameters from micropores to 2.1 nm and inter-aggregated pores with maximum

pore diameters of ca. 16–24 nm. According to Kumar et al. [33], the bimodal pore size distribution arose from the hard aggregates in the powders. In addition, they reported that there are two types of pores present in the bimodal pore size distribution. One is a fine intra-aggregated pore (represented by the hysteresis loop in the lower P/P0 range), and the other is a larger inter-aggregated pore (hysteresis loop in the higher P/P0 range). In Fig. 8, the maximum pore volume of

3

Pore volume (cm /g)

0.14 R10 R1 R0

0.12 0.10 0.08 0.06 0.04 0.02 0.00 1

10

100

Pore size (nm) Fig. 8. Pore size distribution curves of the powders prepared at different initial EtOH/H2 O molar ratios and dried at 100 ◦ C.

J. Yu et al. / Applied Catalysis A: General 255 (2003) 309–320

the intra-aggregated pores of the R10 powders lies in the microporous region (pore diameter less than 2 nm). With decreasing the initial EtOH/H2 O molar ratio, the maximum pore size of the intra-aggregated pores shifts into the mesoporous region. However, the maximum pore sizes of the intra-aggregated pores in R0 and R1 powders do not exhibit significant difference compared to those of the R10 powders. Apparently, for the R10 powder, the particles within the aggregates tend to be packed more closely compared to those of the R0 and R1 powders and the interstitial voids of the packed primary particles within the aggregates increase with decreasing initial EtOH/H2 O ratio. On the contrary, it can also be seen from Fig. 8 that the maximum pore size of the inter-aggregated pores increases with increasing initial EtOH/H2 O ratio, which is ascribed to the fact that with increasing the EtOH/H2 O molar ratio, the zeta potentials and the dielectric constants decrease, the larger aggregate particles are obtained (as shown in Fig. 10), which result in the larger maximum pore size of the inter-aggregated pores. Fig. 9(a)–(c) show the pore size distributions of R10, R1 and R0 powders, respectively, calcined at different temperatures. For the R10 powder, the pores show bimodal distributions consisting of the intraand inter-aggregated pores from 400 to 600 ◦ C. At 700 ◦ C, the pores exhibit monomodal distribution of the inter-aggregated pores due to the collapse of intra-aggregated pores. The maximum pore volume of the intra-aggregated pores in the powder calcined at 400 ◦ C lies in 2 nm. However, when the powder is calcined 500 and 600 ◦ C, the maximum pore size of the intra-aggregated pores shifts into larger mesoporous region (ca. 2.2 and 2.4 nm, respectively) indicating the growth of pores. This is caused by the growth of anatase crystallites. The bimodal pore size distributions are also observed in the R1 and R0 powders calcined from 400 to 600 ◦ C (shown in Fig. 9(b) and (c)). At 700 ◦ C, the pores also exhibit monomodal distribution of the inter-aggregated pores for R1 and R0 powders. Therefore, it can be inferred that, at 700 ◦ C, the EtOH/H2 O molar ratio has no influence on the pore size distributions. 3.4. Effect of mixed solvents on particle morphology The composition of mixed solvents also affects the morphology of the resulting particles. Fig. 10 shows

317

SEM micrographs of R10 (a), R1 (b) and R0 (c) powders calcined at 500 ◦ C. For R0 (c) and R1 (b) powders, the precipitated particles are found to be very fine and highly agglomerated. In contrast, the R10 powders are spherical and discrete particles, and some are non-spherical particles due to grounding. These differences in morphologies indicate that the colloidal stability of the precipitated particles in the mixed solvent of EtOH and water is different from that of water solvent [34]. These results can be understood by considering the parameters determining the colloidal stability, such as, the dielectric constants and the zeta potentials [34,35]. Usually, the maximum repulsive force can be estimated from the equation of 2πε0 εr καψ2 for electrostatically stabilized particles, where ε0 is the permittivity in free space, εr the dielectric constant of the continuous phase, κ the Debye–Huckel parameter, α the particle diameter, and ψ the particle surface potential [34]. Under the constant ionic strength of the solvent, the maximum repulsive force depends on the particle surface potential, the dielectric constant, and the particle size. According to Derjaquin–Landau–Verwey–Overbeek (DLVO) theory [35], the energy barrier between two particles which inhibits agglomeration can be also expressed as: 1 Vb = − 12 Aκα + 2πε0 εr καψ2

(3)

where A is the effective Hamaker constant. The effective Hamaker constant depends on the dispersion medium. The Hamaker constants of water and several aliphatic alcohols in free space are reported to have similar values, on the order of 10−20 J [34,35]. Considering that the mixed solvent is composed of EtOH and water, the mixed solvent may not greatly influence the effective Hamaker constant [34,35]. Furthermore, there are no electrolytes in the mixed solvents in this work. Therefore, the ionic strength is supposed to be constant in the solvent. Under these conditions, the magnitude of the energy barrier and maximum repulsive force is determined mainly by the surface potential, the dielectric constant, and the particle size. As the EtOH/H2 O molar ratio increases, the zeta potentials and the dielectric constants rapidly decrease. For the solvent with EtOH/H2 O molar ratio of 0, the dielectric constant of the solvent and the zeta potential of precipitates are very high. According to Eq. (3), the potential energy barrier is relatively high. Under this condition, relatively small particles may be stable

318

0.06

0.008

0.004

0.04

0.02

0.00

0.000 1

10

1

100

Pore size (nm)

10

Pore size (nm)

(b)

0.10 o

R0

0.08 3

Pore volume (cm /g)

(a)

o

400 C o 500 C o 600 C o 700 C

3

3

0.012

400 C o 500 C o 600 C o 700 C

Pore volume (cm /g)

R10

400 C o 500 C o 600 C o 700 C

0.06

0.04

0.02

0.00 1

(c)

10

100

Pore size (nm)

Fig. 9. Pore size distribution curves of the R10 (a), R1 (b) and R0 (c) powders calcined at different temperature (from 400 to 700 ◦ C).

100

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Pore volume (cm /g)

R1

o

0.016

J. Yu et al. / Applied Catalysis A: General 255 (2003) 309–320

319

Fig. 10. SEM micrographs of R10 (a), R1 (b) and R0 (c) powders calcined at 500 ◦ C.

against the aggregation, and the resulting particles appear to fine as shown in Fig. 10(c). In the case of the solvent with EtOH/H2 O molar ratio of 10, the zeta potential and the dielectric constants are low. The precipitates have a low potential energy barrier and maximum repulsive force. Therefore, the colloidal stability of the precipitates increases with increase in the particle size, because the potential energy barrier and the maximum repulsive force increase with the particle size. Hence, the resulting precipitates, shown in Fig. 10(a), are composed of large and discrete sphere particles. These results show that the composition of the mixed solvent affects the particle size and morphology through the change of the zeta potential of precipitates and the dielectric constant of the mixed solvent.

4. Conclusions The content of alcohol during the hydrolysis of TTIP plays an important role on the phase and pore

structures of the final titania powders. When the initial alcohol concentration was high, the powders dried at 100 ◦ C was amorphous titania due to a large amount of residual organic components prevented the crystallization to anatase phase. When the initial alcohol concentration was small, the powders dried at 100 ◦ C were anatase. Furthermore, when the initial alcohol concentration was zero, or the hydrolysis of TTIP occurred in pure water, the powders dried at 100 ◦ C were anatase and brookite. Also, with increasing the initial alcohol concentration, the phase transformation temperatures of anatase to rutile increase. All powders dried at 100 ◦ C after being prepared with different EtOH/H2 O molar ratios resulted in bimodal pore size distributions consisting of fine intra-aggregated and larger inter-aggregated pores. The maximum size of the intra-aggregated pores in the R10 powder prepared with a high alcohol concentration (EtOH/H2 O = 10) lied in the microporous region. With decreasing the initial alcohol concentration, the maximum size of the intra-aggregated

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pores shifted into the mesoporous region. At 700 ◦ C, the pore size distributions of all samples exhibit monomodal distribution of the inter-aggregated pores due to the collapse of the intra-aggregated pores.

Acknowledgements This work was partially supported by the National Natural Science Foundation of China (50272049), The Excellent Young Teachers Program of MOE of P.R.C., and The Project-Sponsored by SRF for ROCS of SEM. This work was also financially supported by a grant from the National Natural Science Foundation of China and Research Grant Council of the Hong Kong Special Administrative Region, China (Project No. N CUHK433/00). References [1] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [2] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C: Photochem. Rev. 1 (2000) 1. [3] A.L. Linsebigler, G. Lu, J.T. Yates Jr., Chem. Rev. 95 (1995) 735. [4] H. Tada, M. Yamamoto, S. Ito, Langmuir 15 (1999) 3699. [5] J.A. Wang, R. Limas-Ballesteros, T. Lopez, A. Moreno, R. Gomez, O. Novaro, X. Bokhimi, J. Phys. Chem. B 105 (2001) 9692. [6] J.C. Yu, J.G. Yu, J.C. Zhao, Appl. Catal. B: Environ. 36 (2002) 31. [7] J.G. Yu, J.C. Yu, W.K. Ho, Z.T. Jiang, New J. Chem. 26 (2002) 607. [8] M. Gopal, W.J. Moberly Chan, L.C. De Jonghe, J. Mater. Sci. 32 (1997) 6001. [9] K.N.P. Kumer, K. Keizer, A.J. Burggraaf, T. Okubo, H. Nagamoto, S. Morooka, Nature 358 (1992) 48. [10] S. Ito, S. Inoue, H. Kawada, M. Hara, M. Iwasaki, H. Tada, J. Colloid Interface Sci. 216 (1999) 59.

[11] G.W. Koebrugge, L. Winnubst, A.J. Burggraaf, J. Mater. Chem. 3 (1993) 1095. [12] K. Terabe, K. Kato, H. Miyazaki, S. Yamaguchi, A. Imai, Y. Iguchi, J. Mater. Sci. 29 (1994) 1617. [13] B.E. Yoldas, J. Mater. Sci. 21 (1986) 1087. [14] H.K. Bowen, Mater. Sci. Eng. 65 (1986) 1574. [15] J.S. Chappell, L.J. Procopio, J.D. Birchall, J. Mater. Sci. Lett. 9 (1990) 1329. [16] K.C. Song, E. Pratsinis, J. Mater. Res. 15 (2000) 2322. [17] J.C. Yu, J.G. Yu, W.K. Ho, L.Z. Zhang, Chem. Commun. (2001) 1942. [18] J.C. Yu, J.G. Yu, W.K. Ho, J.C. Zhao, J. Photochem. Photobiol. A: Chem. 148 (2002) 263. [19] J. Kim, K.C. Song, E. Pratsinis, J. Nanopart. Res. 2 (2000) 419. [20] J. Kim, K.C. Song, S. Foncillas, S.E. Pratsinis, J. Eur. Ceram. Soc. 21 (2001) 2863. [21] K.C. Song, E. Pratsinis, J. Am. Ceram. Soc. 84 (2001) 92. [22] K.C. Song, E. Pratsinis, J. Colloid Interface Sci. 231 (2000) 289. [23] M. Anpo, T. Shima, S. Kodama, Y. Kubokawa, J. Phys. Chem. 91 (1987) 4305. [24] J.G. Yu, J.C. Yu, M.K.P. Leung, W.K. Ho, B. Cheng, X.J. Zhao, J.C. Zhao, J. Catal. 217 (2003) 69. [25] G. Colon, M.C. Hidalgo, J.A. Navio, Appl. Catal. A 231 (2002) 185. [26] Y. Tanaka, M. Suganuma, J. Sol Gel Sci. Technol. 22 (2001) 83. [27] H. Zhang, J.F. Banfield, J. Phys. Chem. B 104 (2000) 3481. [28] E.P. Barrett, L.G. Joyner, P.H. Halenda, J. Am. Chem. Soc. 73 (1951) 373. [29] J.G. Yu, X.J. Zhao, Q.N. Zhao, Thin Solid Films 379 (2000) 7. [30] H.F. Yu, S.M. Wang, J. Non-Cryst. Solids 261 (2000) 260. [31] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603. [32] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, UK, 1982. [33] K.N.P. Kumar, J. Kumar, K. Keizer, J. Am. Ceram. Soc. 77 (1994) 1396. [34] H.K. Park, D.K. Kim, C.H. Kim, J. Am. Ceram. Soc. 80 (1997) 743. [35] R.J. Hunter, Foundation of Colloid Science, vol. 1, Clarendon Press, Oxford, UK, 1987, p. 443.