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Synthesis and characterization of mesoporous TiO2 with wormhole-like framework structure
Applied Catalysis A: General 246 (2003) 161–170
Synthesis and characterization of mesoporous TiO2 with wormhole-like framework structure Wang Yu-de a,b , Ma Chun-lai a,∗ , Sun Xiao-dan a , Li Heng-de a a b
Department of Materials Science & Engineering, Tsinghua University, 100084 Beijing, PR China Department of Materials Science & Engineering, Yunnan University, 650091 Kunming, PR China
Received 12 June 2002; received in revised form 9 December 2002; accepted 21 December 2002
Abstract A neutral templating route for preparing mesoporous TiO2 with wormhole-like framework and high surface area is demonstrated based on self-assembly between a neutral amine surfactant (dodecylamine) and a neutral inorganic precursor (tetrabutyl titanate). The effects of the variation of surfactant-to-Ti alkoxide ratio and the aging temperature on mesoporous TiO2 are investigated. Powder X-ray diffraction (XRD) and the transmission electron microscopy (TEM) measurements have been used to characterize the mesostructure that forms at room temperature as well as the calcined materials. The XRD patterns and the TEM images confirm that these materials have the disordered hexagonal wormhole-like topology. The thermogravimetric analysis (TGA) showed that the ethanol extraction is an efficient approach to remove the surfactant template. The pore diameters and the surface areas of materials, evaluated from the N2 -sorption isotherms, indicate average pore diameters of about 3.0 and 5.0 nm, and surface areas about 246 and 124 m2 /g for calcination at 300 and 350 ◦ C, respectively. The mesoporous TiO2 is formed due to the presence of the hydrogen-bonding interactions between neutral template and inorganic precursors, which are supposed to self-assemble around the surfactant head groups. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Mesoporous TiO2 ; Neutral template; Surface area; Extraction; Aging temperature
1. Introduction Since the discovery of mesoporous aluminosilcate materials at Mobil Corporation in 1992 [1,2], much research has been reported on the synthesis of mesoporous molecular sieves. The mesoporous materials were derived with supramolecular assemblies of surfactants, which acted as templates of the inorganic components during synthesis [3]. Soon afterwards, this technique of using supramolecular templates to ∗ Corresponding author. Tel.: +86-10-62772977; fax: +86-10-62771160. E-mail address: [email protected] (M. Chun-lai).
produce materials was considered very promising for preparation of mesostructured/mesoporous metal oxides. The mesoporous materials with different compositions, new pore systems and novel properties have attracted considerable attentions because of their remarkably large surface areas and narrow pore size distributions, which make them ideal candidates for catalysts [4], molecular sieves, gas sensors and electrodes in solid-state ionic devices [5]. Design of mesostructured transition metal oxides based on the synthesis mechanism has been developed. Recently, oxides of transitional metals have attracted the interest of many scientists because transitional metal atoms are multi-valent so that they have some advantages
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over aluminosilicate materials for use in electromagnetics, photoelectronics, high surface area catalysis, especially in partial oxidation reactions [6–11]. However, it is quite difficult to synthesize transitional metal oxides with stable mesoporous structures because they can have a multitude of different coordination numbers and oxidation states [12]. The neutral templating route provides for the synthesis of transition metal oxide mesostructure that may be less readily accessible by electrostatic templating pathways [13]. Neutral templating route has important advantages over the electrostatic pathway because most metals form alkoxides or other neutral complexes suitable for hydrolysis and assembly as neutral inorganic precursors [14]. The diversity of compositions of neutral inorganic precursors allows for the synthesis of mesostructured oxides that are difficult or impossible to achieve by electrostatic assembly mechanisms. Mesoporous TiO2 with large surface area will provide a highly active photo-catalyst material [15] and is expected to be utilized as materials for high efficient solar cells [16–19]. To date, several preparative approaches utilizing a supramolecular templating mechanism have been reported for the preparation of mesoporous titanium dioxide. Mesoporous TiO2 was first prepared using a phosphate surfactant through a modified sol–gel process [9,20,21]. However, products were not pure titanium oxides because a significant amount of phosphorous still remained in these materials, and they underwent partial collapse of the mesostructure during template removal by calcination. Yang et al. prepared mesoporous TiO2 using amphiphilic poly(alkylene oxide) block copolymers as structure-directing agents and titanium inorganic salts as precursors in a non-aqueous solution [22–24]. The preparation of non-phosphated mesoporous TiO2 using dodecylamine as directing agent, titanium isopropoxide as inorganic precursor, and emptying the pore by extraction has been reported by Antonelli [25]. However, the porous structure has not been retained after heat treatment in dry air at 300 ◦ C. Peng et al. prepared mesoporous TiO2 stable up to 500 ◦ C with BET surface area 603 m2 /g using tri-block copolymers, (EO)n –(PO)m –(EO)n as directing agents and titanium butoxide as precursor in an aqueous solution [26]. Herein we describe the synthesis of titanium dioxide, a high surface area mesoporous transition metal oxide material. We investigated several factors to
prepare mesoporous structure titanium dioxide such as surfactant-to-Ti alkoxide ratio and aging temperature in order that such material can be synthesized in an easier and more secure way. The properties of the materials are characterized by X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray photoelectron spectrum (XPS) and N2 adsorption–desorption experiments. 2. Experimental 2.1. Synthesis All the chemical reagents used in the experiments were obtained from commercial sources as guaranteedgrade reagents and were used without further purification and treatment. The purities of dodecylamine (C12 H27 N) and tetrabutyl titanate (C16 H36 O4 Ti) are not less than 98%. The synthesized method was based on the use of neutral surfactant (dodecylamine), as structure-directing agent and tetrabutyl titanate as neutral inorganic precursor through self-assembly between the neutral surfactant and the neutral inorganic precursor. In a typical process, a different surfactant-to-Ti alkoxide ratio (0.5:1, 1.5:1, 2.0:1, or 2.5:1) mixture of dodecylamine and tetrabutyl titanate was stirred. After 10 min, ethanol (20 ml) was added and the solution was stirred until a homogenous solution was obtained. Some distilled deionized water (20 ml) was added and the mixture was stirred for 2 h, which caused the immediate precipitation of a white solid. The reaction mixture was aged at ambient temperature for 48 h, and then further aged at a different temperature (40, 60, 80, or 100 ◦ C) for 48 h in an autoclave. The resulting products were filtered, washed with the mixture solution of distilled water and ethanol, and dried at ambient temperature. For comparison, the blank reacts of titanium alkoxide under the same conditions without the dodecylamine were run, too. After air-drying, the as-synthesized product with a dodecylamine-to-C16 H36 O4 Ti ratio of 0.5:1 at 80 ◦ C aging temperature was heat-treated with 100 ml ethanol for 1 h twice, and then filtered and washed with ethanol to remove the surfactant. This product was air-dried for 72 h.
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2.2. Characterization Powder X-ray diffraction (XRD) data were obtained with a Rigaku D/max-RB diffractometer equipped with Cu K␣ radiation (λ = 1.5418 Å). The sample was scanned from 1.2 to 20◦ and 20 to 80◦ (2θ) in steps of 0.02◦ . The dh k l indexes of materials were calculated using Bragg’s diffraction equation: λ = 2dh k l sin θ. The transmission electron micrographs (TEM) were made on the Hitachi-800 transmission electron microscope operated at 200 kV. The samples for TEM were prepared by directly dispersing the fine powders of the products onto holey carbon–copper grids. Thermogravimetric analysis (TGA) curves were obtained in flowing nitrogen on TGA2050 with a temperature-increasing rate of 10 ◦ C/min. Differential scanning calorimetry (DSC) measurements were carried out on a DSC 2910 Modulated DSC with a scanning rate of the temperature of 10 ◦ C/min in a dry nitrogen atmosphere to avoid thermal noise due to oxidation reactions. X-ray photoelectron spectra (XPS) were measured in a Perkin–Elmer PHI 5300 ESCA system. During XPS analysis, an Al K␣ X-ray beam was adopted and the power was set to 250 W. N2 adsorption–desorption isotherms were recorded on a Micromeritics ASAP 2010 automated sorption analyzer using nitrogen as adsorbate at −196 K. The samples were outgassed for 4 h at 150 ◦ C. We have applied the Barrett–Joyner–Halenda (BJH) method to the determination of pore size. 3. Results and discussion For silica-based materials, variation of surfactantto-Si alkoxide ratios is important to form mesophases. The general trend is the derivation of hexagonal (MCM-41), cubic Ia 3d (MCM-48), and finally lamellar (MCM-50) phase with increasing surfactant-to-Si ratios. In general, MCM-41 is favored for below 0.6:1, while MCM-48 is observed at ratios between 1.0 and 1.2:1, and MCM-50 is favored at ratios higher than 1.2:1 [21]. However, it is found that the concentration of the surfactant does not affect the quality of mesoporous TiO2 . Fig. 1 shows the XRD patterns of the as-synthesized samples from dodecylamine-to-C16 H36 O4 Ti ratios of 0.5:1, 1.5:1, 2.0:1, and 2.5:1 at 100 ◦ C aging temperature. The
Fig. 1. The XRD patterns of as-synthesized mesoporous TiO2 with variation of dodecylamine-to-C16 H36 O4 Ti ratios at aging temperature of 100 ◦ C.
patterns contain a low single-angle diffraction peak characteristic of mesoporous materials corresponding to the (1 0 0). Higher order Bragg reflections of the hexagonal structure are not resolved. However, others [8,9,13,21,27–29] have demonstrated that similar “single-reflection” products still have short-range hexagonal symmetry. The intensity and sharpness of the patterns increases slightly with increasing proportions of surfactant. Further work needs to be done to get a definite understanding for the cubic and lamellar phases with variation of surfactant-to-Ti ratios. The material exhibits reflections in the region 2θ 20–80◦ that are characteristic of anatase. The influence of the aging temperature on mesoporous titanium oxide is investigated. The experimental results of the surfactant-to-Ti ratio of 0.5:1 at different aging temperatures are showed in Fig. 2. The characteristics of low-angle peak changed little when the aging temperature changed. However, with the increasing of aging temperature, d-spacing of the (1 0 0) layers slightly increases from 3.2 to 3.4 nm, indicating that d-spacing of mesoporous material increases during the aging. Such results indicated that the high aging temperature has an advantageous effect on the stability of materials after template removal because the thicker pore walls can improve the thermal and hydrothermal stability of the mesopore framework
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Fig. 2. The XRD patterns of mesoporous TiO2 at different aging temperatures with dodecylamine-to-C16 H36 O4 Ti ratio of 0.5:1.
[27,28]. However, the low-angle diffraction peak disappears and the high-angle XRD patterns indicate that the materials are crystalline rather than amorphous when the aging temperature is more than 140 ◦ C. We suggest that this should correspond to the crystallization of TiO2 and the decomposing of surfactant dodecylamine, which result in the collapse of mesostructure. In addition, for the sample without do-
decylamine, no low-angle peaks have been observed with the change of aging temperature. According to the above results, we selected the sample aged at 80 ◦ C and with surfactant-to-Ti ratio of 0.5:1 to carry out the following experiments. The XRD patterns of samples as-synthesized and of these treated with ethanol are shown in Fig. 3. The patterns are similar in intensity and position of peak. So the removal of the template by solvent extraction tends to preserve the mesostructure. The TGA of the sample treated by ethanol gave about 26% total weight loss with ∼7% corresponding to water desorption and surface dehydroxylation (Fig. 4). The surfactant can be removed from the framework by ethanol extraction. The templating displacement from the mesoporous framework by ethanol extraction indicates that the ethanol extraction is also an efficient approach to remove the surfactant template. In addition, templating removal without the need to exchange ions or ion pairs shows there is weak interaction between surfactant molecular and inorganic precursor. This is strong evidence in support of the neutral templating mechanism. Thermogravimetric analysis of the as-synthesized product under N2 showed the loss of the water below 153 ◦ C, surfactant loss started at 336 ◦ C and was completely removed at about 500 ◦ C (Fig. 4). The analysis of the as-synthesized sample revealed ∼46% total
Fig. 3. The XRD patterns of the mesoporous TiO2 of as-synthesized and treated by ethanol.
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Fig. 4. The TGA curve recorded for the as-synthesized and treated by ethanol samples.
weight loss on heating to 500 ◦ C. The first effect is attributed to the release of adsorbed water, the second to desorption and decomposition of the template, and the third to dehydroxylation of the surface. The observed mass loss suggests that the composition of the mesoporous amine adduct is (TiO2 )3.0 ·1.3H2 O·amine. The DSC pattern of the as-synthesized sample is shown in Fig. 5. The DSC curve of sample shows a shallow peak in the range of 50–220 ◦ C, corresponding to desorption of water and ethanol adsorbed on the surface of mesoporous TiO2 . Fig. 5 also displays the two broad
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peaks from 220 to 360 ◦ C and from 360 to 500 ◦ C, which are well known in the literature [30] as the decomposition of the organic template and the crystallization of amorphous TiO2 , respectively. The calcination of the samples in air has been performed at three temperatures; namely, at 300, 350 and 450 ◦ C. The samples heat-treated at 300, 350 and 450 ◦ C are incompletely calcined and still contained a small quantity of carbon. Fig. 6 shows X-ray diffraction (XRD) patterns of the as-synthesized sample and those calcined in air at 300, 350 and 450 ◦ C, respectively. The sample calcined at 450 ◦ C and the other samples have similar patterns and exhibit a single diffraction peak corresponding to d-spacing of 3.3, 3.1, and 2.5 nm, respectively. There is considerable reduction in the intensity of patterns of samples calcined at 300 and 350 ◦ C as compared to the as-synthesized sample. It revealed that there is partial loss of structure and reduction in crystalline domain size on calcination. The d-spacing reduction indicated that there is a small degree of pore shrinkage on thermal treatment. The calcination at 450 ◦ C seems to result in total collapse of structure because XRD patterns did not show the presence of any low-angle peaks. All four materials exhibit reflections of comparable integral intensity in the region 2θ 20–80◦ that are characteristic of anatase as can be observed in previous work [22–24]. These single XRD peaks are related to the uniform pore size rather than to the ordered arrangement of pores. TEM is a powerful tool to visualize
Fig. 5. The DSC profile for the as-synthesized sample.
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Fig. 6. The XRD patterns of the mesoporous TiO2 : as-synthesized, and calcined at different temperatures for 1 h.
different pore orderings [31]. Therefore, the pore topology was later confirmed by TEM images study. TEM micrographs showing the characters of among as-synthesized, treated by ethanol and calcined mesoporous TiO2 are presented in Fig. 7. There is no long-range order in the pore structure. However, the samples as-synthesized and treated by ethanol show mesostructure clearly and display the patterns characteristic of the disordered mesostructure (Fig. 7a and b). Moreover, the calcined mesoporous samples show the pore-packing motif that can be described as having wormhole-like topology (Fig. 7c and d). These results are similar to others previously reported [30,32–34]. XPS measurements are performed to characterize the surface compositions up to a depth of about 5 nm. The surface/near surface chemical composition of the samples analyzed by XPS is shown in Fig. 8a and b, respectively, these were measured within a range of binding energies of 0–1000 eV. In addition to the peaks for titanium, peaks corresponding to carbon and oxygen are observed for samples as-synthesized and calcined at 450 ◦ C. The disappearance of the nitrogen peak in the calcined sample shows that the surfactant template is completely removed from the sample. Spectra of the individual line of Ti 2p measured at high resolution show (Fig. 8c and d) narrow range
scans for the samples as-synthesized and calcined at 450 ◦ C, respectively. The XPS spectrum shows two peaks of 2p3/2 and 2p1/2 at 457.9 and 463.7 eV for as-synthesized sample (Fig. 8c) and at 459.8 and 465.5 eV for calcined sample (Fig. 8d) with a better symmetry, and they are assigned to the lattice titanium in titanium oxide. They have peak separations of 5.8 and 5.7 eV between these two peaks, respectively. The values correspond to a 2p3 binding energy of Ti(IV) ion (indexed Standard ESCA Spectra of the Elements and Line Energy Information, Co., USA). Binding energy and line shape of the Ti 2p3/2 peaks (Fig. 8c and d) reflect the contributions of anatase. The red shift in the 2p3/2 peak position from anatase to the mesoporous structure (from 459.8 to 457.9 eV) indicates a change of microenvironments for titanium. This shift (1.9 eV) is due to the interaction of dodecylamine with TiO2 [30]. The thermal stability of the mesoporous structure has also been studied by N2 adsorption and desorption analysis. N2 -sorption isotherms are respectively recorded for the sample treated by ethanol, and for the samples calcined at 300, 350 and 450 ◦ C for 1 h. Table 1 lists the surface area, the total volume of pore and the average pore size data measured from N2 adsorption–desorption isotherms for the mesoporous
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Fig. 7. TEM micrographs of the mesoporous TiO2 : (a) as-synthesized; (b) treated by ethanol, and after being calcined at (c) 300 ◦ C; and (d) 350 ◦ C for 1 h.
Table 1 Characterization of mesoporous structured titanium oxide Samples
d-Spacing (nm)
Total pore volume (cm3 /g)
BET surface area (m2 g−1 )
Average pore diameter (BJH) (nm)
As-synthesized Treated by ethanol Calcined at 300 ◦ C Calcined at 350 ◦ C Calcined at 450 ◦ C
3.3 3.0 3.1 2.5 –
– 0.064 0.126 0.082 0.047
– 82 246 124 48
– 10.9 3.0 5.0 12.6
N dash indicates that the variable was not measured.
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Fig. 8. XPS spectra for the as-synthesized samples (a) and (c) and the samples calcined at 450 ◦ C (b) and (d).
titanium dioxide samples treated by ethanol and calcined at 300, 350 and 450 ◦ C, respectively. The N2 adsorption–desorption isotherms are shown in Fig. 9. The sharp decline in desorption curve is indicative of mesoporosity. The shapes of the hysteresis loop in the adsorption–desorption isotherms of the treated by ethanol and calcined samples are the diffusion bottleneck that are possibly caused by pore damage. The pore size distributions for mesoporous TiO2 materials are determined using the BJH model and the adsorption branch isotherm. The mesoporous TiO2 produced after calcination at 300 and 350 ◦ C have Brunauer–Emmett–Teller (BET) surface areas of 246 and 124 m2 /g. At the same time, the average pore sizes are 3.0 and 5.0 nm. These results indicate that the num-
ber of pore decreased as a result of sintering while at the same time the pore size increased. The evaluation of the surface area of the calcined material allows us to compare it to previously reported mesoporous titanium oxide materials. Mesoporous materials are a consequence of a self-assembly process in an aqueous solution containing organic surfactant (anionic, cationic, or neutral) and inorganic cations, anions or neutral ions [26]. In this neutral templating route, the approach is based on self-assembly and hydrogen bonding between neutral surfactant (S0 ) and neutral inorganic precursors (I0 ). This approach seems feasible to prepare mesoporous TiO2 . The formation of the mesoporous materials occurs through the organization of the surfactant
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Fig. 9. Adsorption–desorption isotherm (a) and pore size distribution plot (b) of mesoporous structured TiO2 .
molecules into neutral rodlike micelles. The resultant Ti(OC4 H9 )4−x (OH)x species most likely participate in hydrogen-bonding interactions with the surfactant head groups because of the hydrolysis of tetrabutyl titanate. Further hydrolysis and condensation of the alkoxy titanium groups result in short-range hexagonal packing of the micelles and in framework wall formation.
4. Conclusion The mesoporous structured TiO2 was obtained through self-assembly between the neutral surfactant and the neutral inorganic precursor. Effects of synthetic factors, such as aging temperature and surfactant-to-Ti alkoxide ratios, on the formation and the structure of titanium dioxide mesophase are investigated. The different mesophase structures cannot be obtained through the variation of surfactant-to-Ti alkoxide ratios. The surfactant template can be removed from the mesoporous framework by ethanol extraction without any effect on the structure of materials. The efficient removal of the template confirmed that there is a weak interaction (hydrogen bonding) between surfactant molecular and inorganic precur-
sor. It also supported the neutral template mechanism. Based on these studies, we may conclude that the mesoporous structured TiO2 has high specific surface area (246 m2 /g) after calcination at 300 ◦ C. Future work will focus on improving the thermal stability of the mesoporous structured TiO2 while the surfactant is being removed, so that the applications for catalysts and electrochemistry can be improved.
Acknowledgements The authors acknowledge the financial support of the National Natural Science Foundation of China (no. 59832070) and thank X.-H. Chen and M.-J. Zhao for their help in the TEM experiments and X.Y. Ye for her help in the XPS experiments. Y.-L. Liu is acknowledged for her assistance in the TGA experiments. References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834.
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Synthesis and characterization of mesoporous TiO2 with wormhole-like framework structure Wang Yu-de a,b , Ma Chun-lai a,∗ , Sun Xiao-dan a , Li Heng-de a a b
Department of Materials Science & Engineering, Tsinghua University, 100084 Beijing, PR China Department of Materials Science & Engineering, Yunnan University, 650091 Kunming, PR China
Received 12 June 2002; received in revised form 9 December 2002; accepted 21 December 2002
Abstract A neutral templating route for preparing mesoporous TiO2 with wormhole-like framework and high surface area is demonstrated based on self-assembly between a neutral amine surfactant (dodecylamine) and a neutral inorganic precursor (tetrabutyl titanate). The effects of the variation of surfactant-to-Ti alkoxide ratio and the aging temperature on mesoporous TiO2 are investigated. Powder X-ray diffraction (XRD) and the transmission electron microscopy (TEM) measurements have been used to characterize the mesostructure that forms at room temperature as well as the calcined materials. The XRD patterns and the TEM images confirm that these materials have the disordered hexagonal wormhole-like topology. The thermogravimetric analysis (TGA) showed that the ethanol extraction is an efficient approach to remove the surfactant template. The pore diameters and the surface areas of materials, evaluated from the N2 -sorption isotherms, indicate average pore diameters of about 3.0 and 5.0 nm, and surface areas about 246 and 124 m2 /g for calcination at 300 and 350 ◦ C, respectively. The mesoporous TiO2 is formed due to the presence of the hydrogen-bonding interactions between neutral template and inorganic precursors, which are supposed to self-assemble around the surfactant head groups. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Mesoporous TiO2 ; Neutral template; Surface area; Extraction; Aging temperature
1. Introduction Since the discovery of mesoporous aluminosilcate materials at Mobil Corporation in 1992 [1,2], much research has been reported on the synthesis of mesoporous molecular sieves. The mesoporous materials were derived with supramolecular assemblies of surfactants, which acted as templates of the inorganic components during synthesis [3]. Soon afterwards, this technique of using supramolecular templates to ∗ Corresponding author. Tel.: +86-10-62772977; fax: +86-10-62771160. E-mail address: [email protected] (M. Chun-lai).
produce materials was considered very promising for preparation of mesostructured/mesoporous metal oxides. The mesoporous materials with different compositions, new pore systems and novel properties have attracted considerable attentions because of their remarkably large surface areas and narrow pore size distributions, which make them ideal candidates for catalysts [4], molecular sieves, gas sensors and electrodes in solid-state ionic devices [5]. Design of mesostructured transition metal oxides based on the synthesis mechanism has been developed. Recently, oxides of transitional metals have attracted the interest of many scientists because transitional metal atoms are multi-valent so that they have some advantages
0926-860X/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0926-860X(03)00055-3
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over aluminosilicate materials for use in electromagnetics, photoelectronics, high surface area catalysis, especially in partial oxidation reactions [6–11]. However, it is quite difficult to synthesize transitional metal oxides with stable mesoporous structures because they can have a multitude of different coordination numbers and oxidation states [12]. The neutral templating route provides for the synthesis of transition metal oxide mesostructure that may be less readily accessible by electrostatic templating pathways [13]. Neutral templating route has important advantages over the electrostatic pathway because most metals form alkoxides or other neutral complexes suitable for hydrolysis and assembly as neutral inorganic precursors [14]. The diversity of compositions of neutral inorganic precursors allows for the synthesis of mesostructured oxides that are difficult or impossible to achieve by electrostatic assembly mechanisms. Mesoporous TiO2 with large surface area will provide a highly active photo-catalyst material [15] and is expected to be utilized as materials for high efficient solar cells [16–19]. To date, several preparative approaches utilizing a supramolecular templating mechanism have been reported for the preparation of mesoporous titanium dioxide. Mesoporous TiO2 was first prepared using a phosphate surfactant through a modified sol–gel process [9,20,21]. However, products were not pure titanium oxides because a significant amount of phosphorous still remained in these materials, and they underwent partial collapse of the mesostructure during template removal by calcination. Yang et al. prepared mesoporous TiO2 using amphiphilic poly(alkylene oxide) block copolymers as structure-directing agents and titanium inorganic salts as precursors in a non-aqueous solution [22–24]. The preparation of non-phosphated mesoporous TiO2 using dodecylamine as directing agent, titanium isopropoxide as inorganic precursor, and emptying the pore by extraction has been reported by Antonelli [25]. However, the porous structure has not been retained after heat treatment in dry air at 300 ◦ C. Peng et al. prepared mesoporous TiO2 stable up to 500 ◦ C with BET surface area 603 m2 /g using tri-block copolymers, (EO)n –(PO)m –(EO)n as directing agents and titanium butoxide as precursor in an aqueous solution [26]. Herein we describe the synthesis of titanium dioxide, a high surface area mesoporous transition metal oxide material. We investigated several factors to
prepare mesoporous structure titanium dioxide such as surfactant-to-Ti alkoxide ratio and aging temperature in order that such material can be synthesized in an easier and more secure way. The properties of the materials are characterized by X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray photoelectron spectrum (XPS) and N2 adsorption–desorption experiments. 2. Experimental 2.1. Synthesis All the chemical reagents used in the experiments were obtained from commercial sources as guaranteedgrade reagents and were used without further purification and treatment. The purities of dodecylamine (C12 H27 N) and tetrabutyl titanate (C16 H36 O4 Ti) are not less than 98%. The synthesized method was based on the use of neutral surfactant (dodecylamine), as structure-directing agent and tetrabutyl titanate as neutral inorganic precursor through self-assembly between the neutral surfactant and the neutral inorganic precursor. In a typical process, a different surfactant-to-Ti alkoxide ratio (0.5:1, 1.5:1, 2.0:1, or 2.5:1) mixture of dodecylamine and tetrabutyl titanate was stirred. After 10 min, ethanol (20 ml) was added and the solution was stirred until a homogenous solution was obtained. Some distilled deionized water (20 ml) was added and the mixture was stirred for 2 h, which caused the immediate precipitation of a white solid. The reaction mixture was aged at ambient temperature for 48 h, and then further aged at a different temperature (40, 60, 80, or 100 ◦ C) for 48 h in an autoclave. The resulting products were filtered, washed with the mixture solution of distilled water and ethanol, and dried at ambient temperature. For comparison, the blank reacts of titanium alkoxide under the same conditions without the dodecylamine were run, too. After air-drying, the as-synthesized product with a dodecylamine-to-C16 H36 O4 Ti ratio of 0.5:1 at 80 ◦ C aging temperature was heat-treated with 100 ml ethanol for 1 h twice, and then filtered and washed with ethanol to remove the surfactant. This product was air-dried for 72 h.
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2.2. Characterization Powder X-ray diffraction (XRD) data were obtained with a Rigaku D/max-RB diffractometer equipped with Cu K␣ radiation (λ = 1.5418 Å). The sample was scanned from 1.2 to 20◦ and 20 to 80◦ (2θ) in steps of 0.02◦ . The dh k l indexes of materials were calculated using Bragg’s diffraction equation: λ = 2dh k l sin θ. The transmission electron micrographs (TEM) were made on the Hitachi-800 transmission electron microscope operated at 200 kV. The samples for TEM were prepared by directly dispersing the fine powders of the products onto holey carbon–copper grids. Thermogravimetric analysis (TGA) curves were obtained in flowing nitrogen on TGA2050 with a temperature-increasing rate of 10 ◦ C/min. Differential scanning calorimetry (DSC) measurements were carried out on a DSC 2910 Modulated DSC with a scanning rate of the temperature of 10 ◦ C/min in a dry nitrogen atmosphere to avoid thermal noise due to oxidation reactions. X-ray photoelectron spectra (XPS) were measured in a Perkin–Elmer PHI 5300 ESCA system. During XPS analysis, an Al K␣ X-ray beam was adopted and the power was set to 250 W. N2 adsorption–desorption isotherms were recorded on a Micromeritics ASAP 2010 automated sorption analyzer using nitrogen as adsorbate at −196 K. The samples were outgassed for 4 h at 150 ◦ C. We have applied the Barrett–Joyner–Halenda (BJH) method to the determination of pore size. 3. Results and discussion For silica-based materials, variation of surfactantto-Si alkoxide ratios is important to form mesophases. The general trend is the derivation of hexagonal (MCM-41), cubic Ia 3d (MCM-48), and finally lamellar (MCM-50) phase with increasing surfactant-to-Si ratios. In general, MCM-41 is favored for below 0.6:1, while MCM-48 is observed at ratios between 1.0 and 1.2:1, and MCM-50 is favored at ratios higher than 1.2:1 [21]. However, it is found that the concentration of the surfactant does not affect the quality of mesoporous TiO2 . Fig. 1 shows the XRD patterns of the as-synthesized samples from dodecylamine-to-C16 H36 O4 Ti ratios of 0.5:1, 1.5:1, 2.0:1, and 2.5:1 at 100 ◦ C aging temperature. The
Fig. 1. The XRD patterns of as-synthesized mesoporous TiO2 with variation of dodecylamine-to-C16 H36 O4 Ti ratios at aging temperature of 100 ◦ C.
patterns contain a low single-angle diffraction peak characteristic of mesoporous materials corresponding to the (1 0 0). Higher order Bragg reflections of the hexagonal structure are not resolved. However, others [8,9,13,21,27–29] have demonstrated that similar “single-reflection” products still have short-range hexagonal symmetry. The intensity and sharpness of the patterns increases slightly with increasing proportions of surfactant. Further work needs to be done to get a definite understanding for the cubic and lamellar phases with variation of surfactant-to-Ti ratios. The material exhibits reflections in the region 2θ 20–80◦ that are characteristic of anatase. The influence of the aging temperature on mesoporous titanium oxide is investigated. The experimental results of the surfactant-to-Ti ratio of 0.5:1 at different aging temperatures are showed in Fig. 2. The characteristics of low-angle peak changed little when the aging temperature changed. However, with the increasing of aging temperature, d-spacing of the (1 0 0) layers slightly increases from 3.2 to 3.4 nm, indicating that d-spacing of mesoporous material increases during the aging. Such results indicated that the high aging temperature has an advantageous effect on the stability of materials after template removal because the thicker pore walls can improve the thermal and hydrothermal stability of the mesopore framework
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Fig. 2. The XRD patterns of mesoporous TiO2 at different aging temperatures with dodecylamine-to-C16 H36 O4 Ti ratio of 0.5:1.
[27,28]. However, the low-angle diffraction peak disappears and the high-angle XRD patterns indicate that the materials are crystalline rather than amorphous when the aging temperature is more than 140 ◦ C. We suggest that this should correspond to the crystallization of TiO2 and the decomposing of surfactant dodecylamine, which result in the collapse of mesostructure. In addition, for the sample without do-
decylamine, no low-angle peaks have been observed with the change of aging temperature. According to the above results, we selected the sample aged at 80 ◦ C and with surfactant-to-Ti ratio of 0.5:1 to carry out the following experiments. The XRD patterns of samples as-synthesized and of these treated with ethanol are shown in Fig. 3. The patterns are similar in intensity and position of peak. So the removal of the template by solvent extraction tends to preserve the mesostructure. The TGA of the sample treated by ethanol gave about 26% total weight loss with ∼7% corresponding to water desorption and surface dehydroxylation (Fig. 4). The surfactant can be removed from the framework by ethanol extraction. The templating displacement from the mesoporous framework by ethanol extraction indicates that the ethanol extraction is also an efficient approach to remove the surfactant template. In addition, templating removal without the need to exchange ions or ion pairs shows there is weak interaction between surfactant molecular and inorganic precursor. This is strong evidence in support of the neutral templating mechanism. Thermogravimetric analysis of the as-synthesized product under N2 showed the loss of the water below 153 ◦ C, surfactant loss started at 336 ◦ C and was completely removed at about 500 ◦ C (Fig. 4). The analysis of the as-synthesized sample revealed ∼46% total
Fig. 3. The XRD patterns of the mesoporous TiO2 of as-synthesized and treated by ethanol.
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Fig. 4. The TGA curve recorded for the as-synthesized and treated by ethanol samples.
weight loss on heating to 500 ◦ C. The first effect is attributed to the release of adsorbed water, the second to desorption and decomposition of the template, and the third to dehydroxylation of the surface. The observed mass loss suggests that the composition of the mesoporous amine adduct is (TiO2 )3.0 ·1.3H2 O·amine. The DSC pattern of the as-synthesized sample is shown in Fig. 5. The DSC curve of sample shows a shallow peak in the range of 50–220 ◦ C, corresponding to desorption of water and ethanol adsorbed on the surface of mesoporous TiO2 . Fig. 5 also displays the two broad
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peaks from 220 to 360 ◦ C and from 360 to 500 ◦ C, which are well known in the literature [30] as the decomposition of the organic template and the crystallization of amorphous TiO2 , respectively. The calcination of the samples in air has been performed at three temperatures; namely, at 300, 350 and 450 ◦ C. The samples heat-treated at 300, 350 and 450 ◦ C are incompletely calcined and still contained a small quantity of carbon. Fig. 6 shows X-ray diffraction (XRD) patterns of the as-synthesized sample and those calcined in air at 300, 350 and 450 ◦ C, respectively. The sample calcined at 450 ◦ C and the other samples have similar patterns and exhibit a single diffraction peak corresponding to d-spacing of 3.3, 3.1, and 2.5 nm, respectively. There is considerable reduction in the intensity of patterns of samples calcined at 300 and 350 ◦ C as compared to the as-synthesized sample. It revealed that there is partial loss of structure and reduction in crystalline domain size on calcination. The d-spacing reduction indicated that there is a small degree of pore shrinkage on thermal treatment. The calcination at 450 ◦ C seems to result in total collapse of structure because XRD patterns did not show the presence of any low-angle peaks. All four materials exhibit reflections of comparable integral intensity in the region 2θ 20–80◦ that are characteristic of anatase as can be observed in previous work [22–24]. These single XRD peaks are related to the uniform pore size rather than to the ordered arrangement of pores. TEM is a powerful tool to visualize
Fig. 5. The DSC profile for the as-synthesized sample.
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Fig. 6. The XRD patterns of the mesoporous TiO2 : as-synthesized, and calcined at different temperatures for 1 h.
different pore orderings [31]. Therefore, the pore topology was later confirmed by TEM images study. TEM micrographs showing the characters of among as-synthesized, treated by ethanol and calcined mesoporous TiO2 are presented in Fig. 7. There is no long-range order in the pore structure. However, the samples as-synthesized and treated by ethanol show mesostructure clearly and display the patterns characteristic of the disordered mesostructure (Fig. 7a and b). Moreover, the calcined mesoporous samples show the pore-packing motif that can be described as having wormhole-like topology (Fig. 7c and d). These results are similar to others previously reported [30,32–34]. XPS measurements are performed to characterize the surface compositions up to a depth of about 5 nm. The surface/near surface chemical composition of the samples analyzed by XPS is shown in Fig. 8a and b, respectively, these were measured within a range of binding energies of 0–1000 eV. In addition to the peaks for titanium, peaks corresponding to carbon and oxygen are observed for samples as-synthesized and calcined at 450 ◦ C. The disappearance of the nitrogen peak in the calcined sample shows that the surfactant template is completely removed from the sample. Spectra of the individual line of Ti 2p measured at high resolution show (Fig. 8c and d) narrow range
scans for the samples as-synthesized and calcined at 450 ◦ C, respectively. The XPS spectrum shows two peaks of 2p3/2 and 2p1/2 at 457.9 and 463.7 eV for as-synthesized sample (Fig. 8c) and at 459.8 and 465.5 eV for calcined sample (Fig. 8d) with a better symmetry, and they are assigned to the lattice titanium in titanium oxide. They have peak separations of 5.8 and 5.7 eV between these two peaks, respectively. The values correspond to a 2p3 binding energy of Ti(IV) ion (indexed Standard ESCA Spectra of the Elements and Line Energy Information, Co., USA). Binding energy and line shape of the Ti 2p3/2 peaks (Fig. 8c and d) reflect the contributions of anatase. The red shift in the 2p3/2 peak position from anatase to the mesoporous structure (from 459.8 to 457.9 eV) indicates a change of microenvironments for titanium. This shift (1.9 eV) is due to the interaction of dodecylamine with TiO2 [30]. The thermal stability of the mesoporous structure has also been studied by N2 adsorption and desorption analysis. N2 -sorption isotherms are respectively recorded for the sample treated by ethanol, and for the samples calcined at 300, 350 and 450 ◦ C for 1 h. Table 1 lists the surface area, the total volume of pore and the average pore size data measured from N2 adsorption–desorption isotherms for the mesoporous
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Fig. 7. TEM micrographs of the mesoporous TiO2 : (a) as-synthesized; (b) treated by ethanol, and after being calcined at (c) 300 ◦ C; and (d) 350 ◦ C for 1 h.
Table 1 Characterization of mesoporous structured titanium oxide Samples
d-Spacing (nm)
Total pore volume (cm3 /g)
BET surface area (m2 g−1 )
Average pore diameter (BJH) (nm)
As-synthesized Treated by ethanol Calcined at 300 ◦ C Calcined at 350 ◦ C Calcined at 450 ◦ C
3.3 3.0 3.1 2.5 –
– 0.064 0.126 0.082 0.047
– 82 246 124 48
– 10.9 3.0 5.0 12.6
N dash indicates that the variable was not measured.
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Fig. 8. XPS spectra for the as-synthesized samples (a) and (c) and the samples calcined at 450 ◦ C (b) and (d).
titanium dioxide samples treated by ethanol and calcined at 300, 350 and 450 ◦ C, respectively. The N2 adsorption–desorption isotherms are shown in Fig. 9. The sharp decline in desorption curve is indicative of mesoporosity. The shapes of the hysteresis loop in the adsorption–desorption isotherms of the treated by ethanol and calcined samples are the diffusion bottleneck that are possibly caused by pore damage. The pore size distributions for mesoporous TiO2 materials are determined using the BJH model and the adsorption branch isotherm. The mesoporous TiO2 produced after calcination at 300 and 350 ◦ C have Brunauer–Emmett–Teller (BET) surface areas of 246 and 124 m2 /g. At the same time, the average pore sizes are 3.0 and 5.0 nm. These results indicate that the num-
ber of pore decreased as a result of sintering while at the same time the pore size increased. The evaluation of the surface area of the calcined material allows us to compare it to previously reported mesoporous titanium oxide materials. Mesoporous materials are a consequence of a self-assembly process in an aqueous solution containing organic surfactant (anionic, cationic, or neutral) and inorganic cations, anions or neutral ions [26]. In this neutral templating route, the approach is based on self-assembly and hydrogen bonding between neutral surfactant (S0 ) and neutral inorganic precursors (I0 ). This approach seems feasible to prepare mesoporous TiO2 . The formation of the mesoporous materials occurs through the organization of the surfactant
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Fig. 9. Adsorption–desorption isotherm (a) and pore size distribution plot (b) of mesoporous structured TiO2 .
molecules into neutral rodlike micelles. The resultant Ti(OC4 H9 )4−x (OH)x species most likely participate in hydrogen-bonding interactions with the surfactant head groups because of the hydrolysis of tetrabutyl titanate. Further hydrolysis and condensation of the alkoxy titanium groups result in short-range hexagonal packing of the micelles and in framework wall formation.
4. Conclusion The mesoporous structured TiO2 was obtained through self-assembly between the neutral surfactant and the neutral inorganic precursor. Effects of synthetic factors, such as aging temperature and surfactant-to-Ti alkoxide ratios, on the formation and the structure of titanium dioxide mesophase are investigated. The different mesophase structures cannot be obtained through the variation of surfactant-to-Ti alkoxide ratios. The surfactant template can be removed from the mesoporous framework by ethanol extraction without any effect on the structure of materials. The efficient removal of the template confirmed that there is a weak interaction (hydrogen bonding) between surfactant molecular and inorganic precur-
sor. It also supported the neutral template mechanism. Based on these studies, we may conclude that the mesoporous structured TiO2 has high specific surface area (246 m2 /g) after calcination at 300 ◦ C. Future work will focus on improving the thermal stability of the mesoporous structured TiO2 while the surfactant is being removed, so that the applications for catalysts and electrochemistry can be improved.
Acknowledgements The authors acknowledge the financial support of the National Natural Science Foundation of China (no. 59832070) and thank X.-H. Chen and M.-J. Zhao for their help in the TEM experiments and X.Y. Ye for her help in the XPS experiments. Y.-L. Liu is acknowledged for her assistance in the TGA experiments. References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834.
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