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Materials Science and Engineering A 435–436 (2006) 158–162

Preparation of titania/silica mesoporous composites withactivated carbon template in supercritical carbon dioxide

Qun Xu ∗, Haijuan Fan, Yiqun Guo, Yanxia CaoCollege of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450052, China

Received 10 May 2006; received in revised form 8 June 2006; accepted 25 July 2006

bstract

Titania/silica mesoporous composites have been prepared with nanoscale casting process using activated carbon as template in supercriticalarbon dioxide (SC CO2). The composite precursor of tetrabutyl titanate and tetraethyl orthosilicate (TEOS) were dissolved in supercritical CO2,nd then coated on activated carbon in the desired supercritical condition. After removal of activated carbon template by calcinations in air at00 ◦C, TiO2/SiO2 mesoporous composite was obtained. The effects of CO2 pressure and temperature have been studied on the coating ratio duringhe supercritical condition. The products were characterized by XRD, nitrogen sorption isotherms, SEM and TEM. SEM result indicates that the

omposite product has pretty well porous structure and the product pore structure information had been calculated from the nitrogen sorptionsotherms. The titanium dioxide was in anatase phase and the largest BET surface area is up to 318.70 m2/g. TEM experimental result shows thatn the composite product, some titania nano-wire was formed.

2006 Elsevier B.V. All rights reserved.

eywords: Supercritical carbon dioxide; Titania; Silica; Activated carbon

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. Introduction

Titania particle has been considered as an important photo-atalyst because of its high activity, strong oxidation capabilitynd its chemical stability in the reaction conditions [1]. It isell known that the photocatalytic activity of titanium dioxide

trongly relies on its crystallinity and specific surface area, andhe anatase phase with large surface area is desired in photo-atalytic applications [2,3]. The addition of silica was foundo improve the thermal stability and photocatalytic activity ofitanium dioxide [4–8].

Traditionally, silica-modified titania is synthesized by sol-gelethods [9–11], chemical vapor deposition and hydrothermalethod [12], but many problems exist in the prepared products,

uch as serious particle agglomeration, small surface area andarge crystal grain, which results in low photocatalytic activ-

ty of titania. In order to avoid these limitations, nanocasting

ethod with the assistance of supercritical CO2 was proposed byakayama and Fukushima [13–15]. There are several specific

∗ Corresponding author. Tel.: +86 371 67767827; fax: +86 371 67763561.E-mail address: qunxu@zzu.edu.cn (Q. Xu).

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921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.07.107

easons to consider supercritical CO2 as alternative solvents forhe synthesis and processing of porous materials [16]. First, CO2s inexpensive, environmentally benign and nonflammable. Andts mild critical conditions (Pc = 73.8 bar; Tc = 31.1 ◦C) allowO2 to be used with safe laboratory and commercial operationonditions. Another advantage is that CO2 can be easily andompletely removed from products and porous structure can bebtained without collapse structure. In addition, the solvent cane easily recycled from gaseous CO2 after the pressure is dimin-shed.

In this article, the preparation of titania/silica mesoporousomposite with activated carbon template in SC CO2 haseen investigated and the effects of supercritical conditionspressure and temperature) on coating ratio, thus on BETurface area and pore structures of titania/silica compositeave also been studied. The product was characterized by-ray diffraction (XRD), nitrogen sorption isotherms, scan-ing electron microscope (SEM) and transmission electronicroscope (TEM). SEM result shows that the pores struc-

ure of activated template have been well replicated by com-osite product. From XRD, it indicates that the titaniumioxide in anatase phase with high BET surface area wasbtained.

Engineering A 435–436 (2006) 158–162 159

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Q. Xu et al. / Materials Science and

. Experimental

.1. Chemicals

Tetrabutyl titanate and tetraethyl orthosilicate (TEOS), bothf which were offered by China Medicine Group Shanghaihemical Reagent Co., were used as titania source and silica

ource. Activated carbon (AC), granules, were offered by Tian-in Kermel Chemical Reagent Development Center. CO2 withurity of 99.9% was provided by Zhengzhou Shuangyang Gaso. and used as received.

.2. Coating in SC CO2

In a typical experiment, 2 mL of tetrabutyl titanate and 2 mLf TEOS were placed into the bottom of a stainless steel auto-lave (Hai’an High Pressure Autoclave Factory, China) of 50-L capacity. A desirable amount of activated carbon was placed

n a stainless steel cage fixed at the upper part of the autoclave forhe purpose not to contact with the precursor mixture solution.he autoclave temperature was adjusted up to desired experi-ental value and CO2 was filled into the autoclave by a syringe

ump (DB-80, Beijing Satellite Manufacturing Factory), untilesired pressure was obtained. And the supercritical conditionas kept for 8 h. Then the pressure was released by venting and

he coated activated carbon was taken out.

.3. Thermal treatments

After cooling to room temperature, samples were heated at05 ◦C for 12 h in normal atmosphere. Then mass of coatingctivated carbon was weighed and symbolized as mc. The coatedctivated carbon was calcinated at 600 ◦C for 12 h in air flow toemove the template. In order to study the experimental effectsonveniently, coating ratio was described as follows:

oating ratio = mc − ma

ma× 100%

here ma is the mass of activated carbon before coating and mcs the mass of activated carbon after coating.

.4. Characterization

The calcinated products, coated at different supercritical con-itions, were characterized with nitrogen adsorption–desorptionxperiment and the isotherms at 77 K were collected on a Quan-achrome NOVA 1000e surface area and pore size analyzer.efore this measurement, all the samples were heated at 473 K

n 10−6 Torr for 1 h for degassing. Barret–Joyner–HallenderBJH) method was used to calculate pore size distribution. X-ray

iffraction data were recorded on a Rigaku D/MAX-3B usingu K� radiation at a scanning speed of 6◦ (2θ)/min in the rangef 10–70◦. The micromorphology of the products was observedith a JEOL JSM-5600LV scanning electron microscope (SEM)

t 15 kV acceleration voltage and with a FEI Tecnai transmissionlectron microscope (TEM).

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ig. 1. SC CO2 pressure effect on coating ratio (all the samples were coated inC CO2 at 40 ◦C).

. Results and discussion

.1. Pressure and temperature effect on coating ratio

To investigate the effect of different SC CO2 pressures onoating ratio, series of experiments were performed at 40 ◦Cnd the results were shown in Fig. 1. It can be seen that coatingatio firstly increases with the increasing pressure, reaches aaximum at 26 MPa, and then reduces continuously from 26

o 30 MPa. The existence of the maximum point is due to theomplex factors as: (1) solvent power increased with pressuren supercritical condition, and more precursors were dissolvednd carried by SC CO2 to coat on activated carbon, which is aositive factor; (2) solvent viscosity increased with pressure inupercritical condition, which is a negative factor for precursor tooat on the activated carbon; (3) the surface adsorption behavior

ig. 2. SC CO2 temperature effect on coating ratio (all the samples were coatedn SC CO2 at 26 MPa).

160 Q. Xu et al. / Materials Science and Engineering A 435–436 (2006) 158–162

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ig. 3. X-ray diffraction pattern of titania/silica composite (prepared in SC CO2

t 40 ◦C and 20 MPa, followed by calcinations).

Fig. 2 shows the effect of different temperatures on coatingatio. This series of experiments were carried out at 26 MPa.rom this figure it can be seen that coating ratio increased with

he increasing temperature continuously. Usually for supercrit-cal fluid, increase of temperature will induce the decrease ofolvent power [17] and it is not helpful for the solubility of pre-ursor. However, with the increase of temperature, the vaporressure of solute increase, which will increase the solubilityf solute in supercritical CO2. Therefore, increasing tempera-ure could increase the solubility. And the obtained experimentalesult depends on the competition between the solvent power ofO2 and the vapor pressure of solute.

.2. Characterization

Fig. 3 shows XRD pattern of titania/silica composite. It cane seen that the composite are composed of anatase and amor-hous silica. The diffraction peak is not too sharp, which indicatehat the incomplete crystallization of anatase and the existencef amorphous silica. No other peaks existing shows that acti-ated carbon template was completely removed from the product

hrough calcinations.

Table 1 lists the data of BET surface area, total pore volumend average pore diameter of 11 samples which were prepared inifferent experimental conditions. Samples 1–6 were prepared

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able 1ET surface area, total pore volume and pore diameter of calcinated products

ample Temperature (◦C) Pressure (MPa) BET surfa

1 40 20 212.352 40 22 223.213 40 24 258.584 40 26 318.705 40 28 295.196 40 30 263.037 50 26 269.448 60 26 232.939 70 26 227.850 80 26 255.601 90 26 262.26

ig. 4. Nitrogen sorption isotherm of titania/silica composite (coated in SC CO2

t 40 ◦C and 26 MPa, followed by calcinations).

n SC CO2 at 40 ◦C and at the pressure range from 20 to 30 MPaollowed by calcinations. It can be seen that BET surface areand total pore volume both increase in the pressure range from0 to 26 MPa and then decrease from 26 to 30 MPa. So there ismaximal BET surface area for the product in the experimen-

al supercritical CO2 coating condition. And this experimentalesult is totally in accordance with the result of pressure effectn the coating, which is shown in Fig. 1. So the higher amountf coating, the larger BET surface area for the product and theore accurate replication of template.Samples 4, 7–11 were all prepared in SC CO2 at 26 MPa

nd at the temperature range from 40 to 90 ◦C followed byalcinations. It can be seen that in temperature range from0 to 70 ◦C, BET surface area and pore volume decrease;rom 70 to 90 ◦C, BET surface area and pore volume increasegain. And the BET surface maximum of 318.70 m2/g andore volume maximum of 0.38 cm3/g for the product is fromhe experimental condition of 40 ◦C and 26 MPa. So it can beoncluded from our study that for the mixer precursor of tetra-utyl titanate and tetraethyl orthosilicate to prepare TiO2/SiO2

esoporous composite, the ideal experimental condition is

0 ◦C and 26 MPa, where for the same experimental pressure,he energy cost is the least and the replication degree is theighest.

ce area (m2/g) Pore volume (cm3/g) Pore diameter (nm)

0.28 5.190.30 5.030.32 4.900.38 4.710.34 4.550.30 4.540.33 4.920.31 5.250.30 5.200.31 4.930.32 4.88

Q. Xu et al. / Materials Science and Engin

Fig. 5. Pore size distribution of titania/silica composite (coated in SC CO2 at40 ◦C and 26 MPa, followed by calcinations).

Fig. 6. SEM photos of: (a) activated carbon template and (b) titania/silica com-posite (prepared in SC CO2 at 40 ◦C and 22 MPa, followed by calcinations).

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eering A 435–436 (2006) 158–162 161

Fig. 4 shows the typical nitrogen sorption isotherm of meso-orous titanium dioxide/silica. There exists a hysteresis loop,hich is accounted for the filling of mesopores. Pore size dis-

ribution of titania/silica composite calculated by BJH methodrom nitrogen sorption isotherm is shown in Fig. 5. It can beeen that most of pores are in the range of 2–10 nm in diameter.

SEM photos of original activated carbon and titania/silicaomposite are shown in Fig. 6(a) and (b), respectively. It can beeen that in the exterior surface of activated carbon in Fig. 6(a),here are interspaces between the crystalloid of irregular size.s seen in Fig. 6(b), the titania/silica composite has well crys-

allized, and there are holes between the wafers. So it is provedhat the composite product has well replicated the template andas more ideal porous structure.

Fig. 7 illustrated the TEM graphs of composite producthich was treated in SC CO2 at ideal experimental conditionf 40 ◦C and 26 MPa followed by calcinations at 600 ◦C. It cane observed nano-wire crystal structure for this composite at

ig. 7. TEM graphs of product (treated in SC CO2 at 40 ◦C and 26 MPa, pre-ursor ratio = 1:1, followed by calcinations at 600 ◦C).

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ifferent places. The experimental results reveal that the forma-ion of titania nanowire may attribute to the preferred orientatedggregation and growth of them maybe due to two reasons: ones the intrinsically inclined crystalline anisotropy and the otherne is the coordination effect of precursor TEOS. Further it iseeded to study the detail mechanism and try to prepare moreegular nano-wire. And referring SC CO2 assisted nanowire for-ation process, some study result was reported by others [18].

. Conclusion

TiO2/SiO2 mesoporous composites were prepared withanoscale casting process using activated carbon as template inC CO2. The effects of experimental conditions were studied onoating ratio. Nitrogen sorption experimental result shows thathe porous structure of activated carbon template had been welleplicated by composite product. BET surface area and productore structure information had been calculated from the nitro-en sorption isotherms and it was found that SC CO2-treatedondition has prominent effect on the pore microstructure ofhe composite product. Further from TEM graphs, nano-wiretructure of TiO2 had been observed.

cknowledgements

We are grateful for the financial support from the Prominentesearch Talents in University of Henan Province, the National

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eering A 435–436 (2006) 158–162

atural Science Foundation of China (No. 20404012) and therominent Youth Science Foundation of Henan Province (No.512001200).

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