Synthesis and characterization of amorphous TiO2 with wormhole-like framework mesostructure

Journal of Non-Crystalline Solids 319 (2003) 109–116 www.elsevier.com/locate/jnoncrysol

Synthesis and characterization of amorphous TiO2 with wormhole-like framework mesostructure Yu-de Wang, Chun-lai Ma *, Xiao-dan Sun, Heng-de Li Department of Materials Science and Engineering, Tsinghua University, 100084 Beijing, PeopleÕs Republic of China Received 18 April 2002; received in revised form 4 October 2002

Abstract Using neutral amine surfactant (dodecylamine) as an organic template and neutral inorganic material (tetrabutyl titanate) as a precursor, amorphous TiO2 with wormhole-like framework mesostructure was synthesized with the variation of surfactant-to-Ti alkoxide ratios. Powder X-ray diffraction (XRD), thermogravimetric analysis, Fourier transformed infrared spectra, transmission electron microscopy (TEM), nitrogen adsorption–desorption, and X-ray photoelectron spectroscopy (XPS) have been used to characterize the TiO2 mesostructure. The interaction between surfactant and titanium dioxide was displayed by XPS. The samples exhibit a wormhole-like framework from XRD patterns and TEM images, and high surface area (221 m2 /g) for the sample calcined at 450 °C for 2 h. The formation of the titanium oxide mesostructure is proposed to be due to the presence of the interactions between surfactant head group and inorganic precursors prior to hydrolysis, and the condensation under condition favorable for liquid crystal formation. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 81.07.Pr; 81.16.Dn; 82.30.Rs; 82.80.Pv

1. Introduction Since the discovery of mesoporous aluminosilicate 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 supra*

Corresponding author. Tel.: +86-10 6277 2977; fax: +86-10 6277 1160. E-mail address: [email protected] (C.-l. Ma).

molecular templates to produce materials was considered very promising for preparation of mesophase or mesoporous metal oxides. Titania, especially anatase is attractive for its potential applications, such as photocatalysts [4], electrodes for wet solar cells [5], gas sensor [6] and electrochromic devices [7]. Mesoporous TiO2 with large surface area offers a prospect to provide a highly active photocatalyst material and is expected to be utilized as materials for high efficient solar cells [8–11]. To date, several preparative approaches utilizing a supramolecular templating mechanism had been reported for the preparation of mesoporous titanium dioxide. Mesoporous TiO2

0022-3093/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-3093(02)01956-7

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was first prepared using a phosphate surfactant through a modified sol–gel process [12–14]. However, they were not pure titanium oxides because a significant amount of phosphorous still remained in these materials, and underwent partial collapse of the mesostructure during template removal by calcination. Yang and co-workers prepared mesoporous TiO2 using amphiphilic poly(alkylene oxide) block copolymers as structure-directing agents and titanium inorganic salts as precursors in a nonaqueous solution [15–17]. The preparation of nonphosphated mesoporous TiO2 using dodecylamine as directing agent, titanium isopropoxide as inorganic precursor, and emptying the pore by extraction has been reported by Antonelli [18]. However, the porous structure had not been retained after heat treatment in dry air at 300 °C, and the synthesis of mesoporous titanium oxide deeds a long time (up to 7 days). Peng and co-workers 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 [19]. However, the mesoporous TiO2 was easily deliquesced at ambient temperature. Herein we synthesized the amorphous TiO2 material with wormhole-like framework mesostructure and high surface area using the neutral amine surfactant (dodecylamine) as the structure director and tetrabutyl titanate as the inorganic precursor. A study of the variation of surfactant-to-Ti ratios was undertaken, and wanted to see if variation of surfactant-to-Ti ratios could lead to mesostructured hexagonal, layered and cubic phase. The properties of the materials were characterized by X-ray diffraction analysis (XRD), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). Fourier transformed infrared (FTIR) spectra, nitrogen adsorption–desorption, and X-ray photoelectron spectroscopy (XPS).

2. Experimental 2.1. TiO2 mesostructure preparation All the chemical reagents used in the experiments were obtained from commercial sources as

guaranteed-grade reagents and used without further purification. The purities of dodecylamine (C12 H27 N) and tetrabutyl titanate (C16 H36 O4 Ti) were 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. Reactions were performed at room temperature. The starting surfactant-to-Ti alkoxide ratio (molar) were 0.5:1, 1.0:1, 1.5:1, 2.0:1, and 2.5:1, respectively. According to the surfactant-to-Ti alkoxide ratio, the resulting products were labeled with from sample 1 to sample 5. In a typical process (sample 1), the synthetic procedures were as follows: a 0.5:1 mixture of dodecylamine (0.272 g, 1.47 mmol) and tetrabutyl titanate (1.0 g, 2.94 mmol) was stirred and warmed until a homogeneous colorless solution was obtained (about 1 min, 60 °C). To this solution, distilled deionized water (60 ml) was added and the mixture was stirred for 30 min, which caused the immediate precipitation of a yellow solid. The reaction mixture was aged at ambient temperature for 24 h, and then further aged at 100 °C for 48 h in an autoclave. The assynthesized products were filtered and washed with the mixture solution of ethanol and distilled deionized water to remove the surfactant, and dried at ambient temperature. 2.2. Characterization of TiO2 mesostructure Powder XRD data were carried out with CuKa
). The sample was scanned radiation (k ¼ 1:5418 A from 1.2° to 20° and 20° to 80° (2h) in steps of 0.01° and 0.02°, respectively. The dhkl indexes of materials were calculated using BraggÕs diffraction equation: k ¼ 2dhkl sin h. TGA curves were obtained in flowing air with a temperature increasing rate of 10 °C/min. FTIR spectra were recorded in the range of 4000–400 cm1 . The samples for FTIR were prepared using the KBr technology, which were calibrated by polystyrene. During XPS analysis, Al Ka X-ray beam was adopted and power was set to 250 W. Vacuum pressure of the instrument chamber was 1  107 Pa as read on the panel. Measured spectra were decomposed into Gaussian components by a least-square fitting

Y. Wang et al. / Journal of Non-Crystalline Solids 319 (2003) 109–116

method. Bonding energy was calibrated with reference to C 1s peak (285.0 eV). The transmission electron micrographs (TEM) were made operated at 200 KV. The samples for TEM were prepared by directly dispersing the fine powders of the products onto holey carbon copper grids. N2 adsorption–desorption isotherms were recorded with automated sorption analyzer using nitrogen as adsorbate at 77 K. The samples were outgassed 4 h at 150 °C. We have applied the Barrett–Joyner– Halenda (BJH) method to the determination of pore size.

3. Results Fig. 1 shows XRD patterns of the as-synthesized products from dodecylamine-to-C16 H36 O4 Ti ratios of 0.5:1, 1:1, 1.5:1, 2.0:1, and 2.5:1. All patterns are similar and contain a low single-angle diffraction peak characteristic of mesostructured materials corresponding to the (1 0 0). The patterns exhibit a single diffraction peak corresponding to
, d-spacing of 34.5, 35.9, 35.2, 36.5, and 36.7 A respectively. Higher order Bragg reflections of the hexagonal structure are not resolved. The low single-angle diffraction peak shows the existence of the disordered wormhole-like framework mesostructure, which had been demonstrated by others

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[12,20–24]. The wide-angle (in the region 2h 20– 80°) XRD of all five materials shows that the TiO2 are amorphous (Fig. 1, inset). According to the above results, the sample 3 (surfactant-to-Ti alkoxide ratio is 1.5:1) was selected to carry out the following experiments. TGA of the as-synthesized product under air shows the loss of the water below 153 °C and surfactant loss starts at about 200 °C and is completely removed at about 336 °C (Fig. 2). The analysis of the as-synthesized sample reveals 46% total weight loss on heating to 500 °C. Little further weight loss in the TGA curve is observed at temperature above 500 °C, indicating the completion of any reaction involving a weight change. The FTIR spectra in the range 4000–400 cm1 of as-synthesized and 450 °C calcined TiO2 mesostrucrure in air for 2 h are shown in Fig. 3. The single XRD peaks are related to the uniform pore size rather than the ordered arrangement of pores. TEM is a powerful tool to visualize different pore orderings [25]. Therefore, the pore topology is later confirmed by TEM images study. TEM micrographs showing the characters of among assynthesized and calcined TiO2 mesostructure are presented in Fig. 4. XPS measurements are performed to characterize the surface compositions up to a depth of about 5 nm. The XPS electron spectrum is measured within a range of binding energies of 0–1000 eV and shown in Fig. 5. The thermal stability of the mesoporous structure has also been studied by N2 adsorption and desorption analysis. N2 -sorption isotherm is recorded for

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Fig. 1. XRD patterns of amorphous TiO2 mesostructure at dodecylamine-to-Ti alkoxide ratios of (a) 0.5:1, (b) 1.0:1, (c) 1.5:1, (d) 2.0:1, and (e) 2.5:1.

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b

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Fig. 3. FTIR spectra of TiO2 mesostructure (a) as-synthesized; (b) after calcination at 450 °C for 2 h.

TiO2 mesostructure calcined at 450 °C in air for 2 h and shown in Fig. 6.

4. Discussion The surfactant-to-metal ratios are very important to certain mesostructured materials because

the liquid crystal-type phases of the mesoporous materials are sensitive to the relative ratios [14,26]. For silica-based materials, the hexagonal (MCM41) phase is favored for ratios below 0.6:1, while Cubic Ia3d (MCM-48) is observed at ratios between 1.0 and 1.2:1, and finally the lamellar (MCM-50) structure is favored at ratios higher than 1.2:1. For the metal-based materials, for example, Ying and co-workers prepared mesoporous niobium oxide molecular sieves using ligandassisted liquid crystal templating [14]. The samples prepared at ratios less than 1.25:1 surfactantto-alkoxide are hexagonal phases (Nb-TMS1). At a 1.5:1 ratio, the sample is a hexagonal phase (NbTMS2), which can be indexed to a p63=mmc unit cell. At a 2.0:1, the materials obtained are layered MCM-50 analogues (Nb-TMS4). However, only the intensity and sharpness of patterns increases slightly with increasing proportions of surfactant from Fig. 1. The hexagonal, cubic, and lamellar phases are not observed with variation of surfactant-to-Ti alkoxide ratios. It is found that Ti is not different with Si and Nb, and the concentration of the surfactant does not affect the quality of TiO2 mesostructure. Further work is to be done to get a definite understanding.

Fig. 4. TEM micrographs of the TiO2 mesostructure: (a) as-synthesized, (b) calcined at 450 °C for 2 h.

Y. Wang et al. / Journal of Non-Crystalline Solids 319 (2003) 109–116 300000

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Fig. 5. XPS electron spectrum measured with TiO2 mesostructure as-synthesized (a) and calcined at 450 °C (c); (b) and (d) XPS spectra of individual lines of Ti 2p measured at high resolution, respectively.

According to the TGA curve, the first effect (below 153 °C) is attributed to the release of adsorbed water, the second (200–336 °C) to desorption and decomposition of the template, and the third (336–500 °C) to dehydroxylation of the surface. The observed mass loss suggests the composition of the mesoporous amine adduct to be ðTiO2 Þ3:0  1:3H2 O  amine. From Fig. 3, some bands are observed in the region 3300–3500 cm1 are due to N–H stretches of dodecylamine and O–H stretches of adsorbed water for the as-synthesized material. Two sharp bands at 2853 and 2913 cm1 are due to C–H stretches of the hydrocarbone chain of dodecylamine. The sharp bands in the region of 1370–1600 cm1 are attributed to the deformation of –CH2 –

and –CH3 of the incorporated dodecylamine. The CH2 stretching vibrations are considered to be related with the physical state (monomer, micelle or solid) of dodecylamine. These band vibrations indicate that the dodecylamine is present in the assynthesized titanium oxide mesostructure. They provide evidence for the incorporation of dodecylamine into the hydrous oxide [27]. The 886 cm1 band can be assigned as free or complexed – O–O– vibration. After calcination at 450 °C in air for 1 h, the bands due to dodecylamine disappeared, indicating the complete removal of dodecylamine from the titanium oxide mesostructure. The broad band between 3200–3600 cm1 and the band centered at 1622 cm1 found on sample are assigned to O–H stretching and deformation

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(b) Fig. 6. Nitrogen adsorption–desorption isotherm (a) and BJH PSD plot (b) for TiO2 mesostructure calcined at 450 °C for 2 h.

vibrations of weak-bound water. In fact, as already found in the FTIR spectra, the amount of OH groups in the titanium oxides mesostructure is quite high, even in the material calcined at 450 °C. It is because the measured sample adsorbed the water for the KBr. A little C–H band is observed from Fig. 3, we suggest that the surfactant is not completely removed at this temperature. The broad bands between 800–1400 cm1 are attributed to the lattice vibrations of titanium oxide. For the pore topology, there is no long-range order in the pore structure from Fig. 4. However, the samples as-synthesized and calcined at 450 °C in air are clearly mesostructure and display the pattern characteristic of the pore-packing motif that can be described as having wormhole-like topology. Similar patterns can be observed in

previous work [28–31]. The wormhole-like channel motif is a potentially important structural feature for catalytic reactivity, in part, because channel branching within the framework can facilitate access to reactive sites on the framework wall [31]. For the as-synthesized product, the data of XPS contain the expected elements: titanium, oxygen, nitrogen and carbon (Fig. 5(a)). Spectrum of individual lines of Ti 2p measured at high resolution shows (Fig. 5(b)) two peaks of 2p3=2 and 2p1=2 at 457.6 and 463.3 eV with a better symmetry. They are assigned to the lattice titanium in titanium oxide. The spectrum measured within binding energies 0–1000 eV of TiO2 mesostructure calcined at 450 °C in air is shown in Fig. 5(c), and contain only the elements: titanium, oxygen and carbon. The nitrogen peak is not observed in the calcined sample. The high resolution individual lines of Ti 2p spectrum is displayed in Fig. 5(d). The 2p3=2 and 2p1=2 peaks are at 459.0 and 464.7 eV, respectively. The as-synthesized and calcined samples all have a peak separation of 5.7 eV between the 2p3=2 and 2p1=2 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, U Co., USA). Disappearance of nitrogen peak in the calcined sample shows the surfactant template is completely removed from the sample. Binding energy and line shape of the Ti 2p3=2 peaks (Fig. 5(b) and (d)) reflect the contributions of both anatase and rutile. Values of binding energies of individual components and their possible interpretations are obtained on comparison of these values with published data for various chemical states. According to the experimental results, the values correspond to a 2p3 binding energy of Ti (IV) ion. XPS also points out that values of the binding energies (for the Ti 2p line, the measured value of the binding energy of 457.6 eV was obtained) and the Ti 2p spectrum structure are identical to the anatase. It could be assumed that the –O–Ti–O– Ti– network is organized into the anatase-identical structure [32]. The red shift in the 2p3=2 peak position from anatase to the mesostructure (from 459.0 to 457.6 eV) indicates a change of microenvironments for titanium. This shift (1.4 eV) is due to the interaction of dodecylamine with TiO2 [27].

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From N2 adsorption–desorption isotherms, the surface area, the total volume of pore, and the average pore size data are 221 m2 /g, 0.250 cm3 /g, and 3.89 nm, respectively. The N2 adsorption–desorption isotherm is shown in Fig. 6(a). The sharp decline in desorption curve is indicative of mesoporosity. The shape of the hysteresis loop in the adsorption–desorption isotherm of the calcined samples is the diffusion bottleneck that is possibly caused by pore damage. The isotherm is essentially of type IV being diagnostic of mesoporous materials [33]. The isotherm shows also a little rising up portion at P =P0 > 0:95, which reveals some properties, related to type II isotherm. The pore size distribution (PSD) for TiO2 mesostructure materials is determined using the BJH model and the adsorption branch isotherms. Fig. 6(b) is the PSD of material calcined at 450 °C in air for 2 h. The PSD curve shows the main porosity in the pore width range 2–5 nm, sharply peaking at 3.4 nm. The narrow PSD curve implies that the material has very regular pore channel in the mesoporous region. It shows that this TiO2 is a mesostructured material after the removal of dodecylamine through calcination. In this organic templating synthesis, the approach is similar to that employed by Ying and coworkers [14], and is thought that this approach is feasible to prepare TiO2 mesophase. They synthesized mesoporous niobium oxide based on a novel ligand-assisted liquid crystal templating mechanism in which a discrete covalent bond is used to direct the templating interaction between the neutral primary amine micelles and neutral inorganic precursors. They proposed that the formation of the mesoporous materials occurs through a mechanism involving self-assembly with concomitant condensation between the head group of surfactant and the inorganic moiety. Further work is to be done to get a definite understanding.

5. Conclusion The amorphous TiO2 with wormhole-like framework mesostructure was obtained using neutral surfactant (dodecylamine) as organic template, and tetrabutyl titanate as neutral inor-

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ganic precursor. The different mesostructures cannot be obtained through the variation of surfactant-to-Ti alkoxide ratios that can form the hexagonal, cubic, and lamellar mesophases for silica-based materials. The TiO2 mesostructure was shown to have high specific surface area (221 m2 /g) and narrow PSD after calcination at 450 °C for 2 h. The TiO2 with wormhole-like framework mesostructure will have a potential application in catalysts. The formations of the mesoporous materials depend on an interaction between the inorganic precursor and the surfactant, and condensation under condition favorable for liquid crystal formation. Future work will focus on improving thermal stability of the TiO2 mesostructure 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 S.-Q. Sun for her help in the FRIT and X.Y. Ye for her help in the XPS experiments. Y.-L. Liu is acknowledged for her assistance in the TGA experiments.

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