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The synthesis of microporous and mesoporous titania with high specific surface area using sol–gel method and activated carbon templates
Materials Letters 87 (2012) 47–50
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
The synthesis of microporous and mesoporous titania with high specific surface area using sol–gel method and activated carbon templates Vorrada Loryuenyong a,b,n, Achanai Buasri a,b, Chonticha Srilachai a, Hathaichanok Srimuang a a b
Department of Materials Science and Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, Nakhon Pathom 73000, Thailand National Center of Excellence for Petroleum, Petrochemicals and Advanced Materials, Chulalongkorn University, Bangkok 10330, Thailand
a r t i c l e i n f o
abstract
Article history: Received 7 May 2012 Accepted 22 July 2012 Available online 31 July 2012
In this study, porous titania nanoparticles were synthesized by sol–gel template method, using titanium (IV) tetraisopropoxide as a starting precursor and isopropanol as a solvent. Different commercial activated carbons were used as templates at 10 wt% in titania sol. The templates were completely removed during the calcination in air at 500 1C for 3 h, forming porous titania. Nanocrystalline anatase titania with specific surface area as high as 700 m2/g was successfully obtained in this study. This specific surface area is one of the highest values reported in the literature under similar sol–gel conditions. & 2012 Elsevier B.V. All rights reserved.
1. Introdution Recently, nanocrystalline titania (TiO2) has been widely used in many promising applications including photocatalysis, absorbents, photonic crystals, solar cells and coatings for self-cleaning surfaces. This is due to its interesting properties such as high chemical stability, wide bandgap, high photocatalytic activity, non-toxicity, low cost and environmentally safety [1–4]. Titania occurs in nature as three main polymorphs: anatase, rutile and uncommon brookite phases. Anatase phase is generally the most photocatalytically active, owing to its wide bandgap, high specific surface area, and low recombination rates of electron–hole pairs [5,6]. On the other hand, rutile is a hightemperature stable phase and has high refractive index and weatherability [4–5]. Nevertheless, many researchers have reported that the mixed phase of anatase and rutile has higher photocatalytic activity and better optical properties than pure anatase phase [4–7]. Despite crystalline structure, other important properties include morphology, particle size and distribution, specific surface area and pore size distribution. Among many techniques used to synthesize titania nanoparticles, sol–gel method is widely used due to its low cost, high controllability, and low-temperature processing. In conjunction
with template method, highly-active porous titania photocatalysts could be synthesized [9–11]. Examples of commonly used templating materials are silica [9], calcium carbonate (CaCO3) [10] and activated carbon (AC) [11]. Activated carbon is well known as an efficient support due to its stability, mechanical resistance, high surface area and optimum porosity [11]. Moreover, carbon can act as an efficient doping agent of titania to induce high optical absorption at lower wavelength range with respect to other doping agents [12], resulting in higher photocatalytic activity under solar irradiation. The purpose of this work is to investigate a potential method for the preparation of crystalline anatase with high specific surface area and good crystallinity. Owing to their relatively low
100 96 Weight (%)
Keywords: Nanocrystalline TiO2 Sol–gel process Activated carbon Template Micropores
(a)
92 (c)
88
(b)
84
(d)
n
Corresponding author at: Silpakorn University, Department of Materials Science and Engineering, Faculty of Engineering and Industrial Technology, Faculty of Engineering and Indust, Nakornpathom 73000, Thailand. Tel./fax: þ66 034 219363. E-mail addresses: [email protected], [email protected] (V. Loryuenyong). 0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.07.090
100
200
300 400 500 600 Temperature (°C)
700
800
Fig. 1. TGA graphs of titania: (a) TiO2, (b) TiO2/AC1, (c) TiO2/AC2 and (d) TiO2/AC3.
48
V. Loryuenyong et al. / Materials Letters 87 (2012) 47–50
cost, different commercial activated carbons were used as templates in the amount of 10% by weight in relative to the amount of expected final titania products. The template effects on properties of obtained porous titania nanoparticles were studied in details.
Intensity (a.u.)
(e)
(d)
*
*
*
*
(c)
*
*
*
*
(b)
*
*
*
*
*
*
*
*
(a)
10
20
30
40
50
60
70
80
2θ θ (degree) Fig. 2. XRD patterns of titania: (a) P25, (b) TiO2, (c) TiO2/AC1, (d) TiO2/AC2 and (e) TiO2/AC3. (’ Anatase,%Rutile, Brookite).
2. Experimental procedure 2.1. Preparation of titania nanoparticles Titania nanoparticles were prepared by sol–gel template method, employing titanium (IV) tetraisopropoxide (TIP) as a precursor and three different commercial activated carbons as templates: AC1 (05120, Fluka, specific surface area¼788 m2/g, pore volume¼0.74 cm3/g [12]), AC2 (DarcosG60, Sigma-Aldrich, specific surface area¼762–923 m2/g, pore volume¼0.95 cm3/g [13]), and AC3 (CGO-200, C. Gigantic Carbon, specific surface area¼795 m2/g, pore volume¼0.47 cm3/g). It could be observed that all of activated carbons had similar specific surface area, but AC3 had the lowest pore volume. This implied that AC3 contained a large amount of smaller pores with an average pore size of 2.8 nm. In a preparation procedure, 5.5 ml of TIP was first dissolved in 71.8 ml isopropanol, and, if applicable, 0.145 g activated carbon (10% by weight) was added into the solution. In a separate flask, another solution was prepared by mixing 19 ml water and 1.68 ml HCl. The solution was added dropwise into titania-activated carbon suspensions under vigorous stirring at room temperature. The mixture was further stirred for 3 h, and
Table 1 Physical properties of titania. Sample
P25 TiO2 TiO2/AC1 TiO2/AC2 TiO2/AC3
XRD Crystallite size (nm)
% Phase
Anatase
Rutile
Anatase
Rutile
18 16 11 12 13
57 52 37 46 0
89 53 96 89 100
11 47 4 11 0
BET surface area (m2/g)
Pore size (nm)
Pore volume (cm3/g)
Pore size range (nm)
61 78 62 83 700
67.3 12.7 13.3 14.8 6.8
1.03 0.26 0.21 0.30 1.20
1.4–1945.0 5.6–17.2 6.2–18.7 6.2–18.4 1.5–485.8
900
3
600
2 1
300 2100
0 3
140
2 1 0 3
1500
Dv(log d) (cc/g)
Volume (cc/g)
70
100 50 1800
2 1 0 3 2
120
1
60 8000
0 3
600
2
400
1
200
0
0 0.0
0.2
0.4 0.6 0.8 Relative Pressure (P/P0)
1.0
10
100
1000
Pore diameter (Angstrom)
Fig. 3. N2 adsorption/desorption isotherms and corresponding BJH pore size distribution graphs of titania: (a) P25, (b) TiO2, (c) TiO2/AC1, (d) TiO2/AC2 and (e) TiO2/AC3.
V. Loryuenyong et al. / Materials Letters 87 (2012) 47–50
the obtained gel was centrifuged, washed to remove excess reactants and catalyst, and dried in the oven at 80 1C for 24 h. The dried samples were crushed and calcined at 500 1C for 3 h at a heating rate of 5 1C/min. The calcined titania nanoparticles were labeled as TiO2/AC1, TiO2/AC2, TiO2/AC3 for the synthesis with AC1, AC2, and AC3 templates, respectively. Commercial Aeroxides P25 titania from Evonik Industries (P25) and titania synthesized without templates (TiO2) were used for comparison purposes.
2.2. Characterization Thermal decomposition evolution of the samples was investigated by thermogravimetric analysis (TGA), conducted in air at a heating rate of 5 1C/min. X-ray diffraction (XRD) patterns for the phase analysis were obtained with a Rigaku, MiniflexII diffractometer, using Cu-Ka radiation. The morphology and average particle size were characterized by transmission electron microscopy (TEM), using JEOL, JEM-1230. N2 adsorption–desorption isotherms were acquired using Quantachrome, Autosorb-1 instrument. The specific surface area and pore size distribution were calculated by Brunauer–Emmett–Teller (BET) and Barret–Joyner– Halende (BJH) methods, respectively.
49
3. Results and discussion Fig. 1 illustrates TGA graphs of as-synthesized titania and carbon–titania composites. The initial weight loss starting at 60 1C relates to the elimination of water and alcohol. The second step at temperatures of 400–600 1C in the composites corresponds to the decomposition of activated carbon. The 1st derivative peak of TiO2/AC1, TiO2/AC2, and TiO2/AC3 are 526, 507, and 476 1C, respectively. It has been clearly observed that the decomposition temperature of AC3 is lower than that of AC1 and AC2. Fig. 2 shows XRD patterns of calcined titania nanoparticles. Based on the XRD results, the crystallite size and phase percentage of anatase and rutile can be determined [13–14]. As listed in Table 1, titania nanoparticles synthesized with activated carbon templates have smaller crystallite size and larger amount of anatase phase. This indicates that the presence of activated carbon templates could retard the growth and the anatase-torutile phase transformation of titania nanoparticles. The results are in agreement with the those reported previously [4–8]. The highest amount of anatase phase is obtained in TiO2/AC3 samples. However, it is found that the addition of activated carbons tends to favor the formation of brookite phase as well. Fig. 3 shows the N2 adsorption/desorption isotherms and the corresponding BJH pore size distribution of titania nanoparticles.
Fig. 4. TEM images of titania: (a) P25, (b) TiO2, (c) TiO2/AC1, (d) TiO2/AC2 and (e)–(f) TiO2/AC3. (scale bar ¼50 nm).
50
V. Loryuenyong et al. / Materials Letters 87 (2012) 47–50
From the figure, it is observed that the isotherm of P25 belongs to type II, which describes adsorption on macroporous adsorbents. The isotherms obtained for TiO2, TiO2/AC1, and TiO2/AC2 can be ascribed to the type IV with typical H2 hysteresis loops. The TiO2/AC3, on the other hand, displays a type IV isotherm with a H3 hysteresis loop, corresponding to bimodal and wide pore size distribution. A summary of BET results is illustrated in Table 1. As shown, activated carbon templates are likely to increase the specific surface areas of the synthesized titania nanoparticles. The lower specific surface area in TiO2/AC1 could be explained by the agglomeration of the particles, as shown in TEM images (Fig. 4c). TiO2/AC3 with the highest nitrogen adsorption shows a very large specific surface area of 700 m2/g, which is attributed to its small pore size (6.8 nm). This value is considerably one of the highest values reported for sol–gel derived crystalline titania [15]. The morphologies of calcined P25, TiO2, TiO2/AC1, TiO2/AC2 and TiO2/AC3 are shown in Fig. 4. It is found that titania nanoparticles synthesized from AC1 and AC2 as templates have small nanoparticles with average particle sizes of 22 and 15 nm, respectively. With AC3 as a template, the particle size is larger (average particle size ¼43 nm). The similarity between the particle size by TEM and crystallite size by XRD indicates a low degree of titania agglomeration. The high magnification TEM image of TiO2/AC3 (Fig. 4e–f) also shows very rough surfaces of porous titania nanoparticles, which corresponds to its high specific surface area.
4. Conclusion In this paper, titania nanoparticles were successfully synthesized by sol–gel method with activated carbon as templates. Anatase titania nanoparticles with specific surface area as high as 700 m2/g was obtained through the use of micro-mesoporous activated carbons. The results revealed that the presence of activated carbon during the synthesis and calcination processes retarded not only the anatase-to-rutile transformation but also the crystallite growth of titania.
Acknowledgement The authors wish to thank Department of Materials Science and Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, and National Center of Excellence for Petroleum, Petrochemicals and Advanced Materials for supporting and encouraging this investigation. References [1] Hafizah N, Sopyan I. Nanosized TiO2 photocatalyst powder via sol–gel method: effect of hydrolysis degree on powder properties. Int J Photoenergy 2009 Article ID 962783. [2] Benkacem T, Agoudjil N. Synthesis of mesoporous titania with surfactant and its characterization. Am J Appl Sci 2008;5:1437–41. [3] Lin X, Rong F, Ji X, Fu D. Carbon-doped mesoporous TiO2 film and its photocatalytic activity. Microporous Mesoporous Mater 2011;142:276–81. [4] Perego C, Revel R, Durupthy O, Cassaignon S, Jolivet JP. Thermal stability on TiO2-anatase: impact of nanoparticles morphology on kinetic phase transformation. Solid State Sci 2010;12:989–95. [5] Mahshid S, Askari M, Ghamsari MS, Afshar N, Lahuti S. Mixed-phase TiO2 nanoparticles preparation using sol–gel method. J Alloys Compd 2009;478: 586–9. [6] Fujishima A, Zhang X, Tryk DA. TiO2 photocatalysis and related surface phenomena. Surf Sci Rep 2008;63:515–82. [7] Slimen H, Houas A, Nogier JP. Elaboration of stable anatase TiO2 through activated carbon addition with high photocatalytic activity under visible light. J Photoch Photobio A 2011;221:13–21. [8] Song GB, Liang JK, Liu FS, Peng TJ, Rao GH. Preparation and phase transformation of anatase–rutile crystals in metal doped TiO2/muscovite nanocomposites. Thin Solid Films 2005;491:110–6. [9] Han S, Kim M, Hyeon T. Direct fabrication of mesoporous carbons using insitu polymerized silica gel. Carbon 2003;41:1525–32. [10] Loryuenyong V, Angamnuaysiri K, Sukcharoenpong J, Suwannasri A. Sol–gel derived mesoporous titania nanoparticles: effects of calcination temperature and alcoholic solvent on the photocatalytic behavior. Ceram Int. 2012;38:2233–7. [11] Huang D, Miyamoto Y, Ding J, Gu J, Zhu S, Liu Q, et al. A new method to prepare high-surface-area **N–TiO2/activated carbon. Mater Lett 2011;65:326–8. [12] Chen D, Jiang Z, Geng J, Wang Q, Yang D. Carbon and nitrogen co-doped TiO2 with enhanced visible-light photocatalytic activity. Ind Eng Chem Res 2007;46:2741–6. [13] Klug P, Alexander LE. X-ray diffraction procedures. New York: Wiley; 1974. [14] Chen J, Yao M, Wang X. Investigation of transition metal ion doping behaviors on TiO2 nanoparticles. J Nanopart Res 2008;10:163–71. [15] Joo JB, Zhang Q, Lee I, Dahl M, Zaera F, Yin Y. Mesoporous anatase titania hollow nanostructures though silica-protected calcination. Adv Funct Mater 2012;22:166–74.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
The synthesis of microporous and mesoporous titania with high specific surface area using sol–gel method and activated carbon templates Vorrada Loryuenyong a,b,n, Achanai Buasri a,b, Chonticha Srilachai a, Hathaichanok Srimuang a a b
Department of Materials Science and Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, Nakhon Pathom 73000, Thailand National Center of Excellence for Petroleum, Petrochemicals and Advanced Materials, Chulalongkorn University, Bangkok 10330, Thailand
a r t i c l e i n f o
abstract
Article history: Received 7 May 2012 Accepted 22 July 2012 Available online 31 July 2012
In this study, porous titania nanoparticles were synthesized by sol–gel template method, using titanium (IV) tetraisopropoxide as a starting precursor and isopropanol as a solvent. Different commercial activated carbons were used as templates at 10 wt% in titania sol. The templates were completely removed during the calcination in air at 500 1C for 3 h, forming porous titania. Nanocrystalline anatase titania with specific surface area as high as 700 m2/g was successfully obtained in this study. This specific surface area is one of the highest values reported in the literature under similar sol–gel conditions. & 2012 Elsevier B.V. All rights reserved.
1. Introdution Recently, nanocrystalline titania (TiO2) has been widely used in many promising applications including photocatalysis, absorbents, photonic crystals, solar cells and coatings for self-cleaning surfaces. This is due to its interesting properties such as high chemical stability, wide bandgap, high photocatalytic activity, non-toxicity, low cost and environmentally safety [1–4]. Titania occurs in nature as three main polymorphs: anatase, rutile and uncommon brookite phases. Anatase phase is generally the most photocatalytically active, owing to its wide bandgap, high specific surface area, and low recombination rates of electron–hole pairs [5,6]. On the other hand, rutile is a hightemperature stable phase and has high refractive index and weatherability [4–5]. Nevertheless, many researchers have reported that the mixed phase of anatase and rutile has higher photocatalytic activity and better optical properties than pure anatase phase [4–7]. Despite crystalline structure, other important properties include morphology, particle size and distribution, specific surface area and pore size distribution. Among many techniques used to synthesize titania nanoparticles, sol–gel method is widely used due to its low cost, high controllability, and low-temperature processing. In conjunction
with template method, highly-active porous titania photocatalysts could be synthesized [9–11]. Examples of commonly used templating materials are silica [9], calcium carbonate (CaCO3) [10] and activated carbon (AC) [11]. Activated carbon is well known as an efficient support due to its stability, mechanical resistance, high surface area and optimum porosity [11]. Moreover, carbon can act as an efficient doping agent of titania to induce high optical absorption at lower wavelength range with respect to other doping agents [12], resulting in higher photocatalytic activity under solar irradiation. The purpose of this work is to investigate a potential method for the preparation of crystalline anatase with high specific surface area and good crystallinity. Owing to their relatively low
100 96 Weight (%)
Keywords: Nanocrystalline TiO2 Sol–gel process Activated carbon Template Micropores
(a)
92 (c)
88
(b)
84
(d)
n
Corresponding author at: Silpakorn University, Department of Materials Science and Engineering, Faculty of Engineering and Industrial Technology, Faculty of Engineering and Indust, Nakornpathom 73000, Thailand. Tel./fax: þ66 034 219363. E-mail addresses: [email protected], [email protected] (V. Loryuenyong). 0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.07.090
100
200
300 400 500 600 Temperature (°C)
700
800
Fig. 1. TGA graphs of titania: (a) TiO2, (b) TiO2/AC1, (c) TiO2/AC2 and (d) TiO2/AC3.
48
V. Loryuenyong et al. / Materials Letters 87 (2012) 47–50
cost, different commercial activated carbons were used as templates in the amount of 10% by weight in relative to the amount of expected final titania products. The template effects on properties of obtained porous titania nanoparticles were studied in details.
Intensity (a.u.)
(e)
(d)
*
*
*
*
(c)
*
*
*
*
(b)
*
*
*
*
*
*
*
*
(a)
10
20
30
40
50
60
70
80
2θ θ (degree) Fig. 2. XRD patterns of titania: (a) P25, (b) TiO2, (c) TiO2/AC1, (d) TiO2/AC2 and (e) TiO2/AC3. (’ Anatase,%Rutile, Brookite).
2. Experimental procedure 2.1. Preparation of titania nanoparticles Titania nanoparticles were prepared by sol–gel template method, employing titanium (IV) tetraisopropoxide (TIP) as a precursor and three different commercial activated carbons as templates: AC1 (05120, Fluka, specific surface area¼788 m2/g, pore volume¼0.74 cm3/g [12]), AC2 (DarcosG60, Sigma-Aldrich, specific surface area¼762–923 m2/g, pore volume¼0.95 cm3/g [13]), and AC3 (CGO-200, C. Gigantic Carbon, specific surface area¼795 m2/g, pore volume¼0.47 cm3/g). It could be observed that all of activated carbons had similar specific surface area, but AC3 had the lowest pore volume. This implied that AC3 contained a large amount of smaller pores with an average pore size of 2.8 nm. In a preparation procedure, 5.5 ml of TIP was first dissolved in 71.8 ml isopropanol, and, if applicable, 0.145 g activated carbon (10% by weight) was added into the solution. In a separate flask, another solution was prepared by mixing 19 ml water and 1.68 ml HCl. The solution was added dropwise into titania-activated carbon suspensions under vigorous stirring at room temperature. The mixture was further stirred for 3 h, and
Table 1 Physical properties of titania. Sample
P25 TiO2 TiO2/AC1 TiO2/AC2 TiO2/AC3
XRD Crystallite size (nm)
% Phase
Anatase
Rutile
Anatase
Rutile
18 16 11 12 13
57 52 37 46 0
89 53 96 89 100
11 47 4 11 0
BET surface area (m2/g)
Pore size (nm)
Pore volume (cm3/g)
Pore size range (nm)
61 78 62 83 700
67.3 12.7 13.3 14.8 6.8
1.03 0.26 0.21 0.30 1.20
1.4–1945.0 5.6–17.2 6.2–18.7 6.2–18.4 1.5–485.8
900
3
600
2 1
300 2100
0 3
140
2 1 0 3
1500
Dv(log d) (cc/g)
Volume (cc/g)
70
100 50 1800
2 1 0 3 2
120
1
60 8000
0 3
600
2
400
1
200
0
0 0.0
0.2
0.4 0.6 0.8 Relative Pressure (P/P0)
1.0
10
100
1000
Pore diameter (Angstrom)
Fig. 3. N2 adsorption/desorption isotherms and corresponding BJH pore size distribution graphs of titania: (a) P25, (b) TiO2, (c) TiO2/AC1, (d) TiO2/AC2 and (e) TiO2/AC3.
V. Loryuenyong et al. / Materials Letters 87 (2012) 47–50
the obtained gel was centrifuged, washed to remove excess reactants and catalyst, and dried in the oven at 80 1C for 24 h. The dried samples were crushed and calcined at 500 1C for 3 h at a heating rate of 5 1C/min. The calcined titania nanoparticles were labeled as TiO2/AC1, TiO2/AC2, TiO2/AC3 for the synthesis with AC1, AC2, and AC3 templates, respectively. Commercial Aeroxides P25 titania from Evonik Industries (P25) and titania synthesized without templates (TiO2) were used for comparison purposes.
2.2. Characterization Thermal decomposition evolution of the samples was investigated by thermogravimetric analysis (TGA), conducted in air at a heating rate of 5 1C/min. X-ray diffraction (XRD) patterns for the phase analysis were obtained with a Rigaku, MiniflexII diffractometer, using Cu-Ka radiation. The morphology and average particle size were characterized by transmission electron microscopy (TEM), using JEOL, JEM-1230. N2 adsorption–desorption isotherms were acquired using Quantachrome, Autosorb-1 instrument. The specific surface area and pore size distribution were calculated by Brunauer–Emmett–Teller (BET) and Barret–Joyner– Halende (BJH) methods, respectively.
49
3. Results and discussion Fig. 1 illustrates TGA graphs of as-synthesized titania and carbon–titania composites. The initial weight loss starting at 60 1C relates to the elimination of water and alcohol. The second step at temperatures of 400–600 1C in the composites corresponds to the decomposition of activated carbon. The 1st derivative peak of TiO2/AC1, TiO2/AC2, and TiO2/AC3 are 526, 507, and 476 1C, respectively. It has been clearly observed that the decomposition temperature of AC3 is lower than that of AC1 and AC2. Fig. 2 shows XRD patterns of calcined titania nanoparticles. Based on the XRD results, the crystallite size and phase percentage of anatase and rutile can be determined [13–14]. As listed in Table 1, titania nanoparticles synthesized with activated carbon templates have smaller crystallite size and larger amount of anatase phase. This indicates that the presence of activated carbon templates could retard the growth and the anatase-torutile phase transformation of titania nanoparticles. The results are in agreement with the those reported previously [4–8]. The highest amount of anatase phase is obtained in TiO2/AC3 samples. However, it is found that the addition of activated carbons tends to favor the formation of brookite phase as well. Fig. 3 shows the N2 adsorption/desorption isotherms and the corresponding BJH pore size distribution of titania nanoparticles.
Fig. 4. TEM images of titania: (a) P25, (b) TiO2, (c) TiO2/AC1, (d) TiO2/AC2 and (e)–(f) TiO2/AC3. (scale bar ¼50 nm).
50
V. Loryuenyong et al. / Materials Letters 87 (2012) 47–50
From the figure, it is observed that the isotherm of P25 belongs to type II, which describes adsorption on macroporous adsorbents. The isotherms obtained for TiO2, TiO2/AC1, and TiO2/AC2 can be ascribed to the type IV with typical H2 hysteresis loops. The TiO2/AC3, on the other hand, displays a type IV isotherm with a H3 hysteresis loop, corresponding to bimodal and wide pore size distribution. A summary of BET results is illustrated in Table 1. As shown, activated carbon templates are likely to increase the specific surface areas of the synthesized titania nanoparticles. The lower specific surface area in TiO2/AC1 could be explained by the agglomeration of the particles, as shown in TEM images (Fig. 4c). TiO2/AC3 with the highest nitrogen adsorption shows a very large specific surface area of 700 m2/g, which is attributed to its small pore size (6.8 nm). This value is considerably one of the highest values reported for sol–gel derived crystalline titania [15]. The morphologies of calcined P25, TiO2, TiO2/AC1, TiO2/AC2 and TiO2/AC3 are shown in Fig. 4. It is found that titania nanoparticles synthesized from AC1 and AC2 as templates have small nanoparticles with average particle sizes of 22 and 15 nm, respectively. With AC3 as a template, the particle size is larger (average particle size ¼43 nm). The similarity between the particle size by TEM and crystallite size by XRD indicates a low degree of titania agglomeration. The high magnification TEM image of TiO2/AC3 (Fig. 4e–f) also shows very rough surfaces of porous titania nanoparticles, which corresponds to its high specific surface area.
4. Conclusion In this paper, titania nanoparticles were successfully synthesized by sol–gel method with activated carbon as templates. Anatase titania nanoparticles with specific surface area as high as 700 m2/g was obtained through the use of micro-mesoporous activated carbons. The results revealed that the presence of activated carbon during the synthesis and calcination processes retarded not only the anatase-to-rutile transformation but also the crystallite growth of titania.
Acknowledgement The authors wish to thank Department of Materials Science and Engineering, Faculty of Engineering and Industrial Technology, Silpakorn University, and National Center of Excellence for Petroleum, Petrochemicals and Advanced Materials for supporting and encouraging this investigation. References [1] Hafizah N, Sopyan I. Nanosized TiO2 photocatalyst powder via sol–gel method: effect of hydrolysis degree on powder properties. Int J Photoenergy 2009 Article ID 962783. [2] Benkacem T, Agoudjil N. Synthesis of mesoporous titania with surfactant and its characterization. Am J Appl Sci 2008;5:1437–41. [3] Lin X, Rong F, Ji X, Fu D. Carbon-doped mesoporous TiO2 film and its photocatalytic activity. Microporous Mesoporous Mater 2011;142:276–81. [4] Perego C, Revel R, Durupthy O, Cassaignon S, Jolivet JP. Thermal stability on TiO2-anatase: impact of nanoparticles morphology on kinetic phase transformation. Solid State Sci 2010;12:989–95. [5] Mahshid S, Askari M, Ghamsari MS, Afshar N, Lahuti S. Mixed-phase TiO2 nanoparticles preparation using sol–gel method. J Alloys Compd 2009;478: 586–9. [6] Fujishima A, Zhang X, Tryk DA. TiO2 photocatalysis and related surface phenomena. Surf Sci Rep 2008;63:515–82. [7] Slimen H, Houas A, Nogier JP. Elaboration of stable anatase TiO2 through activated carbon addition with high photocatalytic activity under visible light. J Photoch Photobio A 2011;221:13–21. [8] Song GB, Liang JK, Liu FS, Peng TJ, Rao GH. Preparation and phase transformation of anatase–rutile crystals in metal doped TiO2/muscovite nanocomposites. Thin Solid Films 2005;491:110–6. [9] Han S, Kim M, Hyeon T. Direct fabrication of mesoporous carbons using insitu polymerized silica gel. Carbon 2003;41:1525–32. [10] Loryuenyong V, Angamnuaysiri K, Sukcharoenpong J, Suwannasri A. Sol–gel derived mesoporous titania nanoparticles: effects of calcination temperature and alcoholic solvent on the photocatalytic behavior. Ceram Int. 2012;38:2233–7. [11] Huang D, Miyamoto Y, Ding J, Gu J, Zhu S, Liu Q, et al. A new method to prepare high-surface-area **N–TiO2/activated carbon. Mater Lett 2011;65:326–8. [12] Chen D, Jiang Z, Geng J, Wang Q, Yang D. Carbon and nitrogen co-doped TiO2 with enhanced visible-light photocatalytic activity. Ind Eng Chem Res 2007;46:2741–6. [13] Klug P, Alexander LE. X-ray diffraction procedures. New York: Wiley; 1974. [14] Chen J, Yao M, Wang X. Investigation of transition metal ion doping behaviors on TiO2 nanoparticles. J Nanopart Res 2008;10:163–71. [15] Joo JB, Zhang Q, Lee I, Dahl M, Zaera F, Yin Y. Mesoporous anatase titania hollow nanostructures though silica-protected calcination. Adv Funct Mater 2012;22:166–74.