Hydrothermal Synthesis and Morphological Evolution of Mesoporous

J. Phys. Chem. C 2009, 113, 20335–20348

20335

Hydrothermal Synthesis and Morphological Evolution of Mesoporous Titania-Silica Zheng Ying Wu,† Yu Fei Tao,‡ Zhi Lin,*,† Long Liu,‡ Xiao Xing Fan,‡ and Ying Wang‡ Department of Chemistry, CICECO, UniVersity of AVeiro, 3810-193 AVeiro, Portugal, and Eco-materials and Renewable Energy Research Center (ERERC), School of Chemistry and Chemical Engineering, Nanjing UniVersity, China ReceiVed: April 24, 2009; ReVised Manuscript ReceiVed: October 12, 2009

Mesoporous titania-silica (TiO2-SiO2) composites have been directly synthesized through a hydrothermal method in weak acidic conditions by using TiCl3 solution as a titanium source and triblock copolymer P123 (EO20PO70EO20) as a template with the assistance of zinc acetate. Characterizations by XRD, TEM, and N2 sorption measurements show that highly ordered mesostructured TiO2-SiO2 with the BET specific area of 882 m2/g was obtained with the Ti/Si molar ratio of 0.1. Morphology of the composites can be controlled by the incorporation of different amounts of titanium species. Highly curved shapes such as winding, wormlike, and wheat grain-like particles were obtained by this direct synthetic method. Acidity and zinc acetate concentration can also affect the morphology of the composites. UV-vis, FTIR and 29Si MAS NMR results display that titanium species exist both in the framework and extra-framework of the mesoporous SiO2. Wide angle XRD, Raman, and HRSEM results show that the outer surface of the SiO2 (probably Ti-doped) is decorated by round anatase nanocrystallites (probably Si-doped), whereas X-ray mapping results reveal that titanium species are highly dispersed on the final mesoporous composites. The formation mechanism of the mesophase was discussed based on characterization results. Photocatalytic results show that the TiO2-SiO2 composites prepared in this way have high catalytic activity in the photodegradation of isopropanol due to both framework and surface titanium species. 1. Introduction 1,2

Since the discovery of mesoporous SiO2 in the early 1990s, mesoporous materials have attracted more and more interest due to their potential use in a wide range of technologically advanced as well as conventional application fields.3,4 With high specific area, uniform and tunable pore size (2-50 nm), and high pore volume, mesoporous materials are widely used in heterogeneous catalysis, separation processes, biomedical systems, and host-guest chemistry.5,6 Among those materials, the block copolymer templated mesoporous SiO2 is especially interesting because they have larger pores and thicker walls than other organic templated materials.7 One of the typical and prominent copolymer templated mesoporous SiO2 is SBA-15, which was synthesized under strong acidic conditions in 1998.8 SBA-15 has a highly ordered hexagonal mesostructure, large pores, and remarkable hydrothermal stability. Therefore, it can be potentially used in many applications such as catalysis, sorption, and advanced material design.9 However, the neutral framework and the lack of any acidic or redox properties in the pure mesoporous SiO2 materials limit their use as catalysts for many reactions.10 Hence, much effort has been made to incorporate heteroatoms into the framework or graft metal oxides in the channels or on the surface of mesoporous SBA-15 in order to generate active sites for catalytic applications.11 Nanocrystalline titanium dioxide (TiO2) is widely used in photocatalysis, solar energy conversion, sensors, optoelectronics and so on.12,13 Among various metal oxide semiconductors, TiO2, especially anatase, has been proven to be the most efficient * Corresponding author. Telephone: +351-234370368. Fax: +351234370084. E-mail: [email protected]. † University of Aveiro. ‡ Nanjing University.

photocatalyst due to its superior photoreactivity, nontoxicity, long-term stability and relatively low price.14 However, the low specific surface area of TiO2 hinders its applications.15 As a result, much work has been done to prepare Ti-substituted mesoporous SBA-15 and related materials through postsynthetic grafting procedures16–20 or direct synthesis.21–27 Postsynthesis is a useful method to introduce high loading titanium species on the surface or in channels of the mesostructure, but it often results in the blockage of mesopores and is relatively complex.18 Comparatively, the direct synthesis can avoid the pore blockage and provide well distribution of titanium species in the framework and surface without any decrease of mesopore size. Moreover, direct synthesis is simpler and energy saving. The first direct synthesis of Ti-substituted mesoporous SBA-15 was reported by Newalkar et al.22 Under the microwave-assistedhydrothermal conditions, ordered mesostructured Ti-SBA-15 can be obtained up to a bulk Ti/Si ratio of about 0.05 in the synthesis gel.22 Since then several modified methods have been tried to incorporate titanium into SBA-15 and related mesoporous materials by one-pot synthesis. Those methods include adding fluoride ions into the synthesis solution to accelerate the hydrolysis of the silicon precursor,9 prehydrolyzing the silicon precursor,23 or adjusting the pH value of the reaction system to increase the amount of incorporated titanium.24–26 Decreasing the acid concentration seems to be an effective way to introduce metal species into the mesostructures.26–29 Similar and more comprehensive work revealed that titanium loading is influenced by synthesis temperature, hydrothermal treatment time, silicon precursor and acid concentrations.27 Obviously, weak acidic condition is more favorable for titanium incorporation than strong acidic conditions because of the easy dissociation of metal-O-Si bonds under strong acidic hydrothermal conditions.9 Recently, highly ordered hexagonal and cubic mesopo-

10.1021/jp9037842 CCC: $40.75  2009 American Chemical Society Published on Web 11/03/2009

20336

J. Phys. Chem. C, Vol. 113, No. 47, 2009

Wu et al.

TABLE 1: Physical Property of the Mesoporous TiO2-SiO2 Samples sample

a0 (nm)

Ti/Si* (mol/mol)

SBET (m2/g)

Vp (cm3/g)

DKJS (nm)

Wd (nm)

Smic (m2/g)

Vmic (cm3/g)

(Sp × DKJS) /Vmeso

ZT0 ZT003 ZT005 ZT007 ZT01 ZT02 ZT03 CT01 CT02 CT03

11.2 12.1 12.1 11.2 12.3 12.0 12.3 13.6 14.0

0.03 0.06 0.07 0.10 0.21 0.30 0.10 0.24 0.35

678 617 675 785 882 514 474 621 505 432

0.82 0.65 1.02 1.37 1.34 0.62 0.71 1.34 1.33 1.23

7.3 8.2 8.5 8.5 8.5 7.3 7.8 9.4 11.4 11.4

3.0 3.6 3.6 2.7 5.0 4.2 2.9 2.2 2.6

86 94 32 0 29 0 0 32 15 0

0.03 0.04 0 0 0 0 0 0.01 0 0

5.5 7.0 5.4 4.9 5.4 4.2

a0: lattice parameter calculated from a0 ) d100 × 2/31/2. *: the Ti/Si molar ratio was detected by EDS. SBET: BET specific surface area. Vp: total pore volume. DKJS: pore diameter calculated by improved Kruk-Jaroniec-Sayari (KJS) method. Wd: wall thickness calculated from Wd ) a0 - DKJS. Sp) SBET - Smic. Vmeso ) Vp - Vmic.

rous materials have been synthesized in very weak acidic conditions (pH > 2) with the addition of different kinds of acetates.30 Such weak acidic synthesis conditions have great prospect to be used for the introduction of titanium species into the mesostructure. The titanium precursors used in post grafting or direct synthesis of Ti-substituted mesoporous material are expensive titanium alkoxides (titanium ethoxide, isopropoxide, and butoxide) or moisture sensitive titanium chloride (TiCl4), the hydrolysis of which is virtually instantaneous, and sometimes anhydrous organic solvents need be used to dissolve such titanium sources.20,31 On the other hand, the hydrolysis rate of a SiO2 precursor such as tetraethylorthosilicate (TEOS) is much slower, which prevents its coprecipitation with titanium sources. Thus, the decrease of the hydrolysis rate of titanium precursors and/or the acceleration of the hydrolysis rate of silicon precursors are very important for the synthesis of titanium incorporated mesoporous material.9 From this point of view, utilization of the inexpensive and acidic-stable titanium trichloride (TiCl3) may slow down the hydrolysis rate of titanium species. However, no titanium can be incorporated into the mesostructure if TiCl3 was directly introduced into the precursor solution containing surfactant P123 and prehydrolyzed TEOS under the strong acidic conditions (the concentration of H+ higher than 1 mol/L).25 When the hydrogen chloride (HCl) concentration of the reaction solution is lower than 1 mol/L, titanium can be incorporated though the Ti/Si molar ratio is much lower in the final products (1/300) than that in the initial gel (1/20).25 At the same time, H2O2 need be added to the titanium solution; otherwise, the incorporation of titanium will prevent the formation of the ordered mesostructure of SBA-15.25 Even if various papers reported on the synthesis of titanium contained SBA-15 materials, there are still great challenges in the synthesis of Ti-SBA-15 with ordered mesostructure and high titanium content by using TiCl3 as Ti precursors. In this paper, we wish to report a novel direct synthetic method for the preparation of mesoporous titanium substituted SiO2 with the Ti/Si molar ratio up to 0.3, by using TiCl3 and TEOS as precursors, with the presence of Pluronic P123, hydrochloric acid and zinc acetate in aqueous solution. With the assistance of zinc acetate, the reaction solution was weak acidic (pH near 3), which is beneficial for controlling both the hydrolysis and condensation of the silicon precursors, and the hydrolysis, oxidation, co-condensation of the TiCl3 species. Under such weak acidic condition, the titanium species in the initial gel can be completely incorporated into the final composites without using H2O2. The effects of stirring time, aging temperature, and zinc acetate concentration have been

studied. The formation mechanism of the mesoporous TiO2-SiO2 was also proposed. Further more, the obtained mesoporous TiO2-SiO2 products show special twisted, winding, worm-like, and well-defined wheat grain like morphologies. Morphology control is also a very important subject for material scientists due to its potential application in nanodevice and medical areas.32–40 Methods such as introducing inorganic salts27,33–35 or organic additives36,37 into the reaction system, adopting supercritical carbon dioxide38 or mixed template,39 and adjusting the composition, stirring rate, and the reaction temperature33,36,40 all can induce a morphology alternation of the mesoporous materials. In present work, morphology of the mesoporous SiO2 is changed and highly curved morphology is obtained after the incorporation of different amount of titanium species. Apart from the special morphology, the directly synthesized mesoporous TiO2-SiO2 composites also show good performance in the photocatalytic decomposition of isoproponal. 2. Experimental Section 2.1. Synthesis. The mesoporous TiO2-SiO2 composites with different Ti/Si molar ratios were hydrothermally synthesized by using TiCl3 solution [15 wt % TiCl3, 10 wt % HCl, Merck] and TEOS [98 wt %, Aldrich] as precursors and triblock copolymer P123 (Aldrich) as a template in the aqueous solution containing hydrochloric acid [HCl 37 wt %, Sigma-Aldrich] and zinc acetate [Zn(CH3COO)2 · 2H2O, Reidel-de Haen]. In a typical synthesis, 1.0 g of Pluronic P123 and 8.97 g of Zn(CH3COO)2 · 2H2O were dissolved in 7.5 g of H2O with a measurable amount of 2 M HCl, and the mixture was heated to 35 °C. Then, 2.13 g of TEOS and a calculated amount of TiCl3 solution were added. The initial molar composition of the mixture, TEOS/TiCl3/Zn(CH3COO)2 · 2H2O/P123/HCl/H2O, was 1/x/4/0.017/6/192, where x varied from 0 to 1.0. Thereafter, the mixture was stirred at 35 °C for 24 h, aged in autoclave at 100 °C for another 24 h. The product was filtered off, thoroughly washed, dried at 50 °C, and then calcined at 550 °C for 5 h to remove the template. The resultant mesoporous TiO2-SiO2 samples are denoted as ZTx, where x relates to the Ti/Si molar ratio in the initial gel (for example, ZT005 represents Ti/Si ratio of 0.05). For detecting the influence of stirring time and hydrothermal treatment temperature, a set of samples was synthesized using a similar method except with a stirring time of 48 h and hydrothermal treatment temperature of 120 °C. With the Ti/Si molar ratio from 0.05 to 0.2, those samples are designated as CTx. EDS analysis indicates that all Ti was incorporated into final products (table 1). No Zn was found in all samples, which is the same as the synthesis of pure SiO2

Synthesis and Evolution of Mesoporous Titania-Silica with zinc acetate in the precursor.30 In order to test the effect of zinc acetate and its concentration on the textural property and morphology of the mesoporous TiO2-SiO2, two sets of samples were prepared by the same reaction method as that for ZTx samples but with different molar compositions. One series of samples was synthesized with Ti/Si/Zn/H molar compositions of 0.1/1/x/6 (x ) 1-4) and denoted as ZHx (x ) 1, 2, 3, 4). Another series of samples was prepared by adjusting the Zn and HCl contents at the same time for modulating the pH value of the reaction solution in the range of 2-3. Detailed molar compositions of those samples are listed in Table 3, and the obtained composites are labeled as ZXx (x ) 1 - 4, which is the Zn/Si ratio). 2.2. Characterization. Low-angle and wide-angle X-ray diffraction (XRD) patterns were recorded on a Philips X’pert MPD diffractometer using Cu KR radiation. Scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), and X-ray mapping were performed on Hitachi S-4100 and Hitachi FE-SEM Su-70 microscopes with Rontec EDS system. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were performed on a Hitachi H9000-NA microscope. Nitrogen adsorption-desorption measurements were performed at 77 K with a Micromeritics Gemini 2375 system. Samples were evacuated at 200 °C for more than 4 h in the degas port. The Brunauer-Emmett-Teller (BET) specific surface areas were calculated using adsorption data in the relative pressure of 0.04 to 0.2, and the total pore volumes were determined from the amount adsorbed at a relative pressure of about 0.99. The pore size distribution (PSD) curves were calculated using the improved Kruk-Jaroniec-Sayari (KJS) method.41 The primary mesopore size of all samples was estimated on the basis of the relative pressure of the condensation step in the adsorption branches of the isotherms. Micropore surface area was calculated by t-plot method with the reference thickness equation of t ) [13.99/(0.034 - log(p/p0))]0.5. Fourier transform infrared (FTIR) spectra of powdered samples suspended in KBr pallets were acquired between 400 and 4000 cm-1 on a Mattson 7000 spectrometer, with resolution 2 cm-1. Fourier transform Raman spectra were obtained at room temperature on a Bruker RFS 100/S spectrometer in the range of 50-4000 cm-1 using a Nd: YAG laser (1064 nm, 500 mW). UV-vis diffuse reflectance (DR) spectra were recorded on a UV-2401 (Shimadzu) spectrometer adapted with a praying mantis accessory using BaSO4 as the reference material. 29Si magic-angle spinning (MAS) NMR spectra were recorded at 79.49 MHz on a Bruker Avance 400 (9.4 T, wide-bore) spectrometer with 40° radio frequency (rf) pulses, a spinning rate of 5.0 kHz, and 60 s recycle delays. 1 H-29Si cross-polarization (CP) MAS spectra were recoded with 4.0 µs 1H 90° pulses, a spinning rate of 5.0 kHz, and 5 s recycle delays. Chemical shifts are quoted in ppm from tetramethylsilane. 2.3. Photodegradation of Isopropanol. The photocatalytic activities of the selected samples for the oxidation of isopropanol (IPA) in air were examined at room temperature.42 In a typical process, the powder sample (0.1 g) was put on a 6-cm2 glass groove. The glass with powder photocatalyst was then placed into a 500-mL gastight reactor with a quartz window, filled with air to one atmospheric pressure. Then, 10 mL of gaseous IPA (with a concentration of ca. 108.75 g/m3) was introduced into the reactor. The initial carbon dioxide (CO2) concentration was detected after IPA gas was injected and adsorbed by the catalyst for 30 min. The light source for the catalytic reaction was a 300-W Xe arc lamp. The evolved CO2 was detected by a

J. Phys. Chem. C, Vol. 113, No. 47, 2009 20337

Figure 1. Low angle XRD patterns of the calcined TiO2-SiO2 composites synthesized with different Ti/Si molar ratios.

Shimadzu GC-14B gas chromatograph equipped with a methanizer and a FID detector. 3. Results and Discussion 3.1. Mesostructure and Morphology of the TiO2-SiO2 Composites. The low angle X-ray diffraction (LXRD) patterns of the samples prepared with Ti/Si molar ratios from 0.01 to 1.0 are shown in Figure 1. Highly ordered mesoporous TiO2-SiO2 can be easily synthesized via this hydrothermal method. The LXRD pattern (Figure 1A) of the calcined mesoporous TiO2-SiO2 with a Ti/Si molar ratio of 0.1 shows three well-resolved diffraction peaks with d-spacing ratios of 1:3:2 at 2θ of 0.5-2.0°, which can be indexed as the 100, 110, and 200 reflections of typical 2-D hexagonal mesostructure (space group P6mm), respectively.43 All samples with Ti/Si molar ratio below 0.1 give these three well-resolved diffraction peaks at low angle, which indicates long-range highly ordered hexagonal mesostructures (Figure 1A). However, mesostructures of the composites are partly damaged when the Ti/Si molar ratio is higher than 0.1. As shown in Figure 1B, one peak is retained in the LXRD patterns of the samples synthesized with Ti/Si molar ratios of 0.2 or 0.3, indicating less-ordered mesostructures or structures possessing short-range hexagonal symmetry with uniform pore diameter.44 Mesostructures of the samples become much less-ordered with higher titanium content. Diffraction of the sample with a Ti/Si molar ratio of 0.5 becomes very weak while the peak vanishes for the samples with Ti/Si molar ratios of 0.7 and 1.0, indicating complete damage of the mesostructure. The variation of mesostructures is mainly due to two reasons: (a) The introduced titanium species affect the phase formation and (b) the titanium oxides are occluded in the mesopores so that diffraction peaks become weaker due to the loss of diffraction contrast between wall and pore.45 In our case, the former reason induces the variation of LXRD patterns, which will be discussed by combining the N2 physisorption results. Nitrogen adsorption-desorption isotherms and pore size distribution curves of the TiO2-SiO2 composites are shown in Figures 2 and 3. The pore structure parameters, such as the specific area (SBET), cumulative pore volume (Vp), pore diameter, and wall thickness (Wd) of the samples are listed in Table 1. N2 physisorption isotherms of mesoporous samples with Ti/Si molar ratios from 0 to 0.1 show typical type IV curves with sharp capillary condensation and evaporation steps in the p/p0 range of about 0.62-0.82 (Figure 2), suggesting very narrow pore size distributions. Those samples also have hysteresis loops of H1 type, implying uniform pore geometries.46 The pore sizes

20338


Figure 2. N2 adsorption-desorption isotherms of the mesoporous TiO2-SiO2 composites with different Ti/Si molar ratios. (The isotherms for the samples synthesized with Ti/Si molar ratios of 0.03, 0.05, 0.07, 0.1, and 0.2 are offset vertically by 15, 24, 36, 7, and 6 mmol/g, respectively.)

Figure 3. PSD curves for the ZTx samples calculated by the improved KJS method.

of those well-ordered mesoporous TiO2-SiO2 calculated from the adsorption data using the improved KJS model are 8.2-8.5 nm (Figure 3, Table 1). It should be pointed out that the pore sizes of the samples with Ti/Si ratios of 0.05, 0.07, and 0.1 are very similar, which implies that most of the titaneous species are not in the mesopores of the obtained composites. Microporosity of the samples was evaluated by the wS/V ratio where w, S, and V are mesopore diameter, mesopore surface area, and mesopore volume, respectively.47 The wS/V ratio for the mesoporous TiO2-SiO2 remains higher than 4.4 (Table 1), indicating the existence of some micropore in the Ti-substituted mesoporous SiO2.47,48 The pore wall thickness calculated from the pore size and unit cell varies from 2.7 to 3.6 nm with Ti/Si molar ratios between 0.03 and 0.1. The N2 physisorption isotherm of the sample changes significantly when the Ti/Si molar ratios are 0.2 and 0.3. Hysteresis loops of these two samples like H2 type, which is indicative of blocked and/or less-open pore channels.49 Considering the less-ordered structures detected by LXRD, it is reasonable to conclude that large

Wu et al. amount of titaneous species in the reaction solution (Ti/Si ) 0.2 and 0.3) disturbs the phase formation of the mesostructure. The calculated BET specific surface area slightly decreases from 678 (Ti/Si ) 0) to 617 m2/g when a small amount of titaneous species (Ti/Si ) 0.03) was introduced into the mesoporous SiO2, which may be due to the different density of TiO2 and SiO2. In fact, the specific surface area of the sample increases remarkably with the incorporated titanium content. At Ti/Si ratios of 0.07 and 0.1, the specific areas are 785 and 882 m2/g, respectively. At the same time, pore volume of the mesoporous composite also increases after the introduction of titaneous species into the reaction solution. The mesoporous composite with a Ti/Si molar ratio of 0.05 has a pore volume of 1.02 cm3/g, which is much higher than that of pure SiO2 sample (0.82 cm3/g). Pore volumes of the samples synthesized with Ti/Si molar ratios of 0.07 and 0.1 are 1.37 and 1.34 cm3/ g, as shown in Table 1. The specific surface areas of the samples depend on titanium content and with a threshold at the Ti/Si molar ratio of 0.1 (Table 1). At the Ti/Si molar ratio lower or equal to 0.1, the specific surface areas of the samples increase with titanium content. However, when Ti/Si molar ratios are 0.2 and 0.3, the surface areas of these two samples decrease to 514 and 474 m2/g, respectively. Simultaneously, pore volume of these two samples reduces from 1.34 (Ti/Si ) 0.1) to 0.62 and 0.71 cm3/g (Ti/Si ) 0.2 and 0.3), indicating that the titaneous species occlude the mesopores. Pore volume is similar when the Ti/Si ratios are 0.2 and 0.3, suggesting a similar pore structure of the two samples. TEM images in Figure 4 show that composites with Ti/Si ratios from 0.03 to 0.1 have high 2-D hexagonal regularity. The sample synthesized with a Ti/Si molar ratio of 0.2 has a lessordered mesostructure. Straight channels disappear while wormlike pores widely exist in this sample (Figure 4H), which is consistent with the LXRD and N2 physisorption results. A lot of dark particles with a size about 30-60 nm dispersed on the TiO2-SiO2 composites (Figure 4A, C, E, F and H). High resolution TEM results (figure not shown) reveal that those particles are randomly oriented and overlap the pore channels of the composites. The lattice fringe of the particles can be observed during the HRTEM detection process but can not be recorded by photos due to the instability of those nanocrystallites. The number of dark particles increases with the titanium content. At a Ti/Si molar ratio of 0.2, almost all of the outer surface of the samples is covered by dark crystalline nanoparticles (Figure 4H). Those dark particles are expected to be a TiO2 nanocrystallite concentrated region (It will be discussed later). Those particles disperse as multilayers and make the mesopores of the sample seem illegible. TEM results also show various morphologies of the TiO2-SiO2 composites. Some circle like and curved wormlike particles can be observed from the TEM images (Figure 5). A detailed morphology transition of ZTx samples was observed by SEM (Figure 6). ZT0 sample has a lot of agglomerated and irregular particles with rough surfaces (Figure 6A). After the introduction of titaneous species into the reaction solution (Ti/Si ) 0.01 and 0.02), the morphology of the resultant TiO2-SiO2 composites became more regular and the surface is smoother than that of the ZT0 sample (Figure 6, panels B and C). At a Ti/Si molar ratio of 0.03, particles with large domains of uniform curved wormlike morphology appear (Figure 6D). The length of one “worm” is about 2-3 µm. The surface of the winding wormlike particle is not smooth but decorated with round nanoparticles of 30-60 nm (also see Figure 4A,B). At Ti/Si ratios between

Synthesis and Evolution of Mesoporous Titania-Silica

Figure 4. TEM images of the TiO2-SiO2 synthesized with Ti/Si molar ratios of 0.03 (A, B), 0.05 (C, D), 0.07 (E), 0.1 (F, G), and 0.2 (H).

0.03 and 0.1, the macroscopic morphology maintains the twisted wormlike morphology as that of the ZT003 (Figure 6E-H). Curvature of the samples becomes a bit higher for samples with more titanium species (Figure 6H). Simultaneously, the outer surfaces of those samples are also covered by a considerable amount of round nanoparticles. Moreover, the particle size of nanospheres on the surface of ZT01 is about 50 nm, which is more uniform than that of ZT003 (Figure 6, panels D and H). The sample of ZT02 has a special morphology. The small curved “worm” is grown larger and more complex. The length of the “worm” increased to 8-10 µm or even longer (Figure 6I-K). The breadth of the “worm” becomes wider and it seems that several small “worms” are grown together and form a larger one (Figure 6I,J). Particles of this sample are very regular and like wheat-grains (Figure 6K). The particles of ZT03 are much longer and more irregular than that of ZT02. The morphology of ZT03 is rope like with a length in the range of 30-100 µm (Figure 6L). The samples with Ti/Si molar ratios higher than 0.3 are macro brick like, of which mesostructures are disordered; therefore, we will not discuss the morphology alteration. Recent reports showed that the incorporation of heteroatoms into the framework of mesoporous SiO2 can lead to a morphology control of the sample.10,27 Regular fine spheres can be obtained by introducing an appropriate amount of Ti into mesoporous SBA-1,10 and highly curved shapes were found in the niobium incorporated SBA-15.27 The key factor to control

J. Phys. Chem. C, Vol. 113, No. 47, 2009 20339 the morphology of the mesoporous sample is not the acidity of the reaction solution but the Si/metal ratio.10,27 Therefore, doping heteroatoms into the mesostructure has the potential for preparing functional mesoporous SiO2 with various morphologies, which is confirmed by our research. In our study, the pH of the solution is fixed between 2.85 and 2.95 by adding zinc acetate into the reaction system. However, the morphology of the mesoporous composite apparently depends on the titanium content in the reaction system. Smoother and curved shapes are found when the Ti/Si ratio is 0.03. At a Ti/Si ratio of 0.1, the morphology becomes very fine, and winding-shaped particles can be seen everywhere. The sample with a Ti/Si ratio of 0.2 also possesses a well-defined curved morphology, but the particles become larger and more regular in a macroscopic scale. At a Ti/Si ratio of 0.3, the morphology changes to long ropelike, and the regularity and the shape curvature decrease (Figure 6L), which is due to the excessive incorporation of titanium into the sample. 3.2. Oxidation and Dispersion States of Titanium Species in the SiO2 Matrix. The wide angle XRD (WXRD) patterns of the calcined mesoporous samples in Figure 7 show clearly broad diffraction peaks of anatase phase,50 indicating a formation of TiO2 nanocrystallites. Diffraction peaks of anatase can be clearly observed for ZT002. With the increase in titanium content, the diffraction peaks become stronger and more visible (Figure 7B). The formation of anatase in the mesoporous composite with very low titanium content suggests that part of the titanium species is accumulated in the mesoporous SiO2 matrix or on the outer surface of the particles. The growth of the anatase nanocrystalline can occur in the hydrothermal treatment process at 100 °C. Diffraction peaks of the anatase phase appear when the Ti/Si molar ratio is 0.03 (Figure 7A) and those peaks can be clearly seen in ZT005 and ZT007. At Ti/Si ratios of 0.1, 0.2, and 0.3, the diffraction peaks of the as-synthesized samples are as strong as those of the calcined samples, confirming that anatase nanocrystallines formed during the hydrothermal process. According to Scherrer’s equation, the size of anatase nanocrystallites (calculated from (101) peak) in the calcined samples with Ti/Si molar ratios of 0.05, 0.1, and 0.2 is about 13, 12, and 11 nm, respectively. The size of anatase nanocrystallites does not increase with titanium content in the initial gel, which implies that the generation of TiO2 nanocrystallites is not due to the increase in the titanium concentration but the intrinsic character of Ti in this weak acidic condition. The increase of the titanium concentration in the reaction solution can not result in a larger size of anatase nanocrystallites. On the other hand, the calculated size of the as-synthesized ZT02 is about 12 nm, which is very similar to the calcined sample, suggesting that no further growth of TiO2 nanocrystallites occurs during the calcination process. However, the intensity of the XRD peaks of anatase nanocrystallites for the calcined samples is slightly stronger than that of the as-synthesized samples, indicating the improved crystallinity of anatase after calcinations. The Raman spectra (Figure 8) support the WXRD results that anatase nanocrystallites are formed in the composites. According to factor group analysis, anatase has six Raman active models (A1g + 2B1g +3Eg) and those allowed models appear at 144 cm-1 (Eg), 197 cm-1 (Eg), 399 cm-1 (B1g), 513 cm-1 (A1g), 519 cm-1 (B1g), and 639 cm-1 (Eg) for an anatase single crystal.51–54 In ZT001 sample, there is only one faint peak at about 148 cm-1, which is assigned to the strongest Eg(1) peak. The intensity of the Eg(1) peak increases with the TiO2 content while no other Raman peaks can be observed in the samples until the Ti/Si

20340


Wu et al.

Figure 5. TEM images of the TiO2-SiO2 synthesized with Ti/Si molar ratios of 0.05 (A) and 0.1 (B).

Figure 6. SEM images of the TiO2-SiO2 composites. ZT0 (A), ZT001 (B), ZT002 (C), ZT003 (D), ZT005 (E), ZT007 (F), ZT01 (G abd H), ZT02 (I-K), and ZT03 (L).

ratio reaches 0.1. At a Ti/Si ratio of 0.1 (ZT01), three other unconspicuous peaks at 396, 516, and 639 cm-1 appear in the Raman spectra, which is due to the relative high titanium content. The intensity of the Raman peaks of ZT02 and ZT03 is very similar, and the five Raman peaks of anatase become obvious, indicating that a large amount of anatase nanocrystallites exists in the TiO2-SiO2 composites. The frequency of the main Raman Eg(1) shift depends on the size of the anatase nanocrystallites and red-shifts occur for the relatively small anatase nanocrystallites.53 There is no distinctive red-shift for the Eg(1) Raman peak in our samples, indicating

that the size of the anatase nanocrystallites in the samples synthesized with different Ti/Si ratios is very similar. This result is in accordance with the particle size calculation from the WXRD patterns. On the other hand, the Eg(2) model of anatase can not be observed in Raman spectra for the samples with a Ti/Si ratio up to 0.1, which indicates that the anatase nanocrystallites in these samples are not very well-ordered. At Ti/Si ratios of 0.2 and 0.3, the Eg(1) Raman peak is clearly unsymmetric, and a shoulder at ca. 200 cm-1 can be seen. The FTIR spectra (Figure 9) for the pure siliceous and Ticontaining samples show characteristic absorption bands at ca.



Figure 9. FTIR spectra of the TiO2-SiO2 composites synthesized with different Ti/Si molar ratios.

Figure 7. Wide angle XRD patterns of the TiO2-SiO2 samples synthesized with different Ti/Si molar ratios. Figure 10. UV-vis DR spectra of calcined TiO2-SiO2 composites synthesized with different Ti/Si ratios.

Figure 8. Raman spectra of calcined TiO2-SiO2 composites synthesized with Ti/Si molar ratios of 0 (a), 0.01 (b), 0.03 (c), 0.05 (d), 0.07 (e), 0.1 (f), 0.2 (g), and 0.3 (h).

465, 802, and 960 cm-1. In general, the infrared spectrum of zeolite can be classified into two groups of vibration. One is internal vibrations of framework TO4, which are insensitive to structural variation. The other is vibrations related to the external linkage of the TO4 units in the structure, which are sensitive to structural variation.55 The band at ca. 960 cm-1 may be the evidence of the existence of framework titanium νas(Si-O-Ti).22,55,56 However, silanol groups ν(Si-OH) of mesoporous SiO257 can also contribute to this band because it exists in mesoporous SiO2 without titanium. Therefore, we cannot estimate the interactions between titanium and siliceous species from this band. The intensity of the bands at ca. 465 and 802 cm-1 decreases with the increase in titanium content (Figure 9). These two bands belong to the deformation models of Si-O-Si [δb(Si-O-Si)] and symmetrical stretching of Si-O-Si [νs(Si-O-Si)]. The reduction of those bands implies some interactions between titanium and

siliceous species. The WXRD and Raman results clearly show the existence of anatase nanoparticles. Here, FTIR results suggest that those anatase nanoparticles are not physically deposited in the SiO2 matrix, and connection between titanium species and SiO2 occurs. UV-vis diffuse reflectance and NMR spectroscopies are used to further study the interactions between the titanium species and SiO2 matrix. UV-vis diffuse reflectance spectra (DRS) can be used to obtain information of the nature and coordination of Ti atoms in the titanium-substituted composites because the position of adsorption bands depends on the chemical environment of Ti atoms in SiO2 matrix.10,11 The existence of titanium(IV) in the framework is characterized by a band at ca. 210-230 nm, which is assigned to a ligand-to-metal chargetransfer (LMCT) transition in isolated TiO4 or HOTiO3 units. This band is the direct evidence that titanium is incorporated into SiO2 matrix.10 The band at 240-250 nm is a clue to the existence of isolated titanium(IV) in an octahedral environment.11 The band at ca. 330 nm indicates the formation of TiO2 clusters. The band between these adsorptions, such as a band centered at ca. 270 or 280 nm, is attributed to penta- or hexacoordinated Ti species, which are not isolated but polymeric Ti species most likely generated through the hydration of the tetrahedrally coordinated sites.10,19,20 The UV-vis DRS of the mesoporous TiO2-SiO2 composites display three bands at 215, 265, and 305 nm (Figure 10). According to the discussion above, it is possible to propose that the three bands in the spectra of the composites can be attributed, respectively, to isolated Ti in a nearly tetrahedral coordination [Ti-(O-Si)4] (band ca. 215 nm), Ti in a nearly octahedral coordination, probably with Ti-O-Si-O-Ti structures (band at 265 nm) and small TiO2 clusters.20 For the ZT001 sample, two UV-vis absorption bands at 215 and 260 nm can be

20342


Wu et al. TABLE 2: 29Si MAS NMR Chemical Shifts and Intensity Ratios (Q2 + Q3)/Q4

Figure 11. Solid-state 29Si MAS and 1H-29Si CP/MAS NMR spectra of the mesoporous TiO2-SiO2 composites synthesized with different Ti/Si molar ratio.

observed, indicating that no obvious extra-framework titanium oxides formed in the sample. When the Ti/Si ratios are between 0.03 and 0.1, the framework titanium species seem to enhance monotonically with the Ti/Si ratio (Figure 10). At Ti/Si ratios of 0.2 and 0.3, the intensity of the band at 215 nm is very similar, implying that no more titanium is incorporated into the SiO2 framework. At the same time, the band at 300-305 nm (TiO2 nancoclusters) becomes more and more intense with the increase of titanium content. For the samples with the Ti/Si molar ratio up to 0.1, the band at ca. 305 nm appears as a shoulder. However, when the Ti/Si ratio reaches 0.2 and 0.3, it becomes the main peak in the UV-vis spectra, indicating that large amount of TiO2 nanoclusters existed in those samples. 29 Si MAS NMR spectrum of mesoporous SiO2 (Figure 11A) exhibits three broad signals centered at ca. -92, -101, and -110 ppm for different chemical environments of Si: corresponding to terminal silanols (SiO)2Si(OH)2, Q2; isolated silanol groups (SiO)3SiOH, Q3; and silicon in the siloxane without hydroxyl groups (SiO)4Si, Q4, respectively.9,21,58,59 In accordance with this assignment, signals at ca. -92 and -101 ppm in 1 H-29Si CP/MAS NMR spectrum of mesoporous SiO2 (Figure 11B) show a marked increase in the relative intensity in comparison with the 29Si MAS spectrum, confirming that these silicon atoms are attached to hydroxyl groups. 29Si MAS NMR spectra of mesoporous TiO2-SiO2 composites with different Ti/Si molar ratios also exhibit three broad signals centered at ca. -92, -101, and -110 ppm. However, the relative intensities of three resonances are clearly different from those of the pure SiO2. At the same time, signals at ca. -92 and -101 ppm for mesoporous TiO2-SiO2 composites are not as sensitive as that of pure siliceous sample to the 1H-29Si cross-polarization (Figure 11B). The MAS spectra have been deconvoluted and the relative intensity ratios of the three silicon environments (Q2, Q3, and Q4) have been calculated from the integrated intensities and presented in Table 2. Compared with pure mesoporous SiO2, the intensities of Q2 and Q3 substantially increase for Tisubstituted samples. [(Q2 + Q3)/Q4] ratio increases from 0.7 to ca. 1.0 after the incorporation of titanium. Concerning the chemical environment surrounding a silicon atom in Tisubstituted samples, the following configurations need be considered: Si(OSi)4, Si(OSi)3(OTi), Si(OSi)3(OH), Si(OSi)2(OH)2, Si(OSi)2(OH)(OTi), and Si(OSi)2(OTi)2. Each configuration gives resonance at different place. For example, crystalline microporous titanosilicates may give Si(OSi)2(OTi)2 resonances between -86 and -96 ppm, Si(OSi)3(OTi) reso-

sample

ZT0

ZT005

ZT01

ZT02

29Si MAS NMR results chemical shift Q2 (ppm) % area chemical shift Q3 (ppm) % area chemical shift Q4 (ppm) % area relative area ratio ((Q2 + Q3)/Q4)

-92.1 3.5 -101.1 38.5 -109.6 58.0 0.72

-91.5 3.8 -101.3 46.6 -110.0 49.6 1.02

-90.6 7.4 -100.3 44.8 -108.8 47.8 1.09

-90.0 6.0 -100.7 45.4 -109.3 48.6 1.06

nances between -94 and -100 ppm.60 These resonances will overlap with Q2 and Q3 from pure SiO2 phase, precluding a reliable quantitative analysis. Therefore, for Ti-substituted mesoporous SiO2, the resonances at ca. -91 and -101 ppm also have the contributions from (SiO)2Si(OH)x(OTi)2-x (x ) 0, 1) and (SiO)3Si(OTi), respectively. In fact, a close inspection of 29Si MAS spectra of mesoporous TiO2-SiO2 composites displays that the Q2 and Q3 resonances in spectra are not totally symmetric and can not be well deconvoluted by a single peak, which implies the other faint contribution at near chemical shifts. The significant increase of the [(Q2 + Q3)/Q4] ratio for the Tisubstituted samples strongly suggests that titanium species are incorporated into the SiO2 matrix of mesoporous composite.21 On the other hand, the [(Q2 + Q3)/Q4] ratio for the Ti-substituted samples is very similar, suggesting that limited amount of titanium species are able to be incorporated into the SiO2 matrix. The remaining titanium species become clusters. Since WXRD, Raman, 29Si NMR, and UV-vis results all suggest that considerable anatase nanocrystallites exist in the mesoporous TiO2-SiO2 composites, HRSEM was used to observe the local morphologies of the samples (Figure 12). Morphology alteration of the particles is not only from the macroscopical view but also from the micro level. The surfaces of the samples are decorated by smooth roundish nanoparticles with an average size of ca. 50-60 nm (Figure 12). The amount of the spherical nanoparticles increases monotonically with the titanium content. No particles were found on the surface of the ZT0 sample while several visible spherical particles appear on the surface of the ZT003 sample. The amount of the nanoparticles on the surface of the sample further increases while the regularity of the spherical nanoparticles becomes better with a Ti/Si ratio up to 0.2 (Figure 12B-E). At a Ti/Si ratio of 0.2, all of the outer surface of the composites is decorated by regular spherical nanoparticles (Figure 12E). At a Ti/Si ratio of 0.3, the nanoparticles on the surface become compact and irregular (Figure 12F). Combined with WXRD, TEM, Raman, and UV-vis results, it is concluded that these nanoparticles are anatase nanocrystallites. Furthermore, the ZT02 has a topology like a core-shelled structure (Figure 13). Mesoporous TiO2SiO2 acts as a core inside while anatase nanoparticles (probably Si doped) form a shell surrounding the mesoporous matrix. Figure 13 gives the schematic diagram of the described particle growth and morphology evolution of the TiO2-SiO2 composites. When the Ti/Si molar ratio increases from 0.05 to 0.2, the mesostructure of the TiO2-SiO2 composite changed from wellordered hexagonal pores to less-ordered wormhole like pores while the number of the anatase nanoparticles which decorated on the outer surface of the sample increased. Figure 14 shows the X-ray mapping images of the mesoporous TiO2-SiO2 composites with Ti/Si molar ratios of 0.03, 0.1, and 0.2. In those images, green and red colors represent Ti and Si, respectively. It shows that titanium species are homogeneously distributed in TiO2-SiO2 composites. In the samples with Ti/Si molar ratios of 0.03 and 0.1, titanium species are



Figure 12. HRSEM images of the TiO2-SiO2 composites. ZT0 (A), ZT003 (B), ZT005 (C), ZT01 (D), ZT02 (E), and ZT03 (F).

Figure 13. Schematic diagram of the described particle growth and morphology evolution of the TiO2-SiO2 composites with different Ti/ Si ratios (the gray parts: mesoporous SiO2, probably Ti-doped; the dark particles: TiO2 nanoparticles, probably Si-doped).

well-proportionally dispersed in the silicon matrix, whereas in the sample with a Ti/Si ratio of 0.2, titanium species are highly concentrated on the surface although they are still well distributed in the silicon matrix (Figure 14). Although titanium atoms are homogeneously doped in the TiO2-SiO2 composites, we are not able to identify, from the view of the X-ray mapping, whether these titanium species are on the outer surface or incorporated inside the SiO2 matrix, because the depth that the

beam can penetrate in the sample is about 0.2-1 µm, whereas the size of the anatase on the outer surface is only about 50 nm. 3.3. Effect of Stirring Time, Hydrothermal Treatment Temperature, and Zinc Acetate. Influence of stirring time and hydrothermal treatment temperature during the synthesis was examined by using CTx samples to get further information for understanding the formation of mesoporous TiO2-SiO2. The LXRD pattern of the sample CT01 gives three well-reserved peaks, indicating a hexagonal structure, which is similar to that of sample ZT01 (Figure 15A). At Ti/Si ratios of 0.2 and 0.3, less-ordered mesostructures were gained (Figure 15B). It is interesting to note that the unit cell expands for CTx samples (Table 1 and Figure 15A), the reason of which is still unclear. However, it may be because more titanium species are incorporated into the framework of the CTx samples than that of the ZTx samples. Introducing Ti species into the framework of crystalline silicates to replace Si can expand unit cell due to the fact that the atomic radius of Ti4+ (0.56 Å) is larger than that of Si4+ (0.40 Å).10 Unit cell expansion of the mesoporous SiO2 after introducing titanium into the framework was also observed and cell increases with the Ti content.10 Hence, the unit cell expansion of the CTx sample implies that more Ti species are introduced into the framework due to longer stirring time and higher hydrothermal treatment temperature. This assumption is in accordance with the UV-vis results.

20344


Wu et al.

Figure 16. UV-vis DR spectra of calcined TiO2-SiO2 composites synthesized with different Ti/Si molar ratios and method: ZT01 (a); CT01 (a′); ZT02 (b); CT02 (b′); ZT03 (c); CT03 (c′).

Figure 14. X-ray mapping images of the mesoporous TiO2-SiO2 composites synthesized with a Ti/Si molar ratio of 0.03 (top), 0.1 (middle), and 0.2 (bottom). Green and red represent Ti and Si, respectively.

Figure 15. Low angle XRD of the mesoporous TiO2-SiO2 composites: (A) samples synthesized with the same Ti/Si molar ratio (0.1) but different stirring time and hydrothermal treatment temperature (ZT01: 24 h, 100 °C; CT01: 48 h, 120 °C); (B) CTx samples with different Ti/Si molar ratios which were synthesized at the stirring time of 48 h and under hydrothermal treatment temperature of 120 °C.

Figure 16 shows the UV-vis results of the ZTx and CTx samples. At a Ti/Si ratio of 0.1, the three bands at 215, 265, and 305 nm in the spectra are similar for both samples. However, when the Ti/Si ratios are 0.2 and 0.3, the absorption bands at

215 and 265 nm of the CTx samples become more intense than those of the ZTx samples, whereas intensities of the band at 305 nm are very similar for both CTx and ZTx samples, indicating that longer stirring time and higher hydrothermal treatment temperature can result in a remarkable increase of framework titanium species but not the extra-framework titanium species. Furthermore, the resultant Ti/Si molar ratios of the calcined CT02 and CT03 samples are 0.24 and 0.35, which are slightly higher than those in the initial gel (0.20 and 0.30), indicating that the total amount of introduced titanium species in the CT02 and CT03 samples is also increased. With longer stirring time, more titanium species can react with siliceous species and higher hydrothermal treatment temperature also increases the interactions between Ti and Si species, which therefore causes the increased framework titanium content. Textural characteristics of the calcined TiO2-SiO2 composites CTx are detected by N2 adsorption-desorption. The pore structure parameters are listed in Table 1. The N2 physisorption isotherm of CT01 sample is typical type IV with an H1 type hysteresis loop, implying uniform pore geometries. The hysteresis loops of the CT02 and CT03 samples deviate from H1 type and have tails at p/p0 of about 0.65, indicating less-ordered mesostructures. Pore size distribution of CT01 is narrower than those of the CT02 and CT03 samples, which is similar to that of ZTx samples. However, the BET specific surface area of CT01 (621 m2/g) is much lower than that of ZT01 (882 m2/g). Also, the microporosity of the CT01 sample was dramatically reduced and the calculated wS/V ratio decreases to 4.2. On the other hand, the surface areas of CT02 and CT03 are similar to those of ZT02 and ZT03 while their pore volumes are different (Table 1). Pore volumes of CT02 and CT03 are 1.33 and 1.23 cm3/g, respectively, which are similar to that of the CT01 sample (1.34 cm3/g) but much higher than those of ZT02 and ZT03 samples (0.62 and 0.71 cm3/g). At the same time, pore sizes of CT02 and CT03 samples are also larger than those of ZT02 and ZT03 samples (Table 1). Wide angle XRD peaks display that anatase nanocrystallites existed in the CT01 sample, but no distinctive differences were found between the ZT01 and CT01 samples (Figure 17), indicating a similar size of anatese nanocrystallites after longer stirring time and higher hydrothermal treatment temperature. However, HRSEM images show that the round particles on the outer surface of the CT01 sample are more regular than that of the ZT01 sample (Figure 18). Effect of zinc acetate on textural properties and morphology evolution was also detected by N2 adsorption and SEM methods


Figure 17. Wide angle XRD patterns of the calcined ZT01 and CT01 TiO2-SiO2 composites.

using samples with Ti/Si/Zn/H molar ratios of 0.1/1/x/6, (x ) 1-4). Table 3 illustrates the N2 adsorption results of the samples prepared under different concentration of zinc acetate. All four samples have high BET specific area around 900 m2/g and large micropore area around 100 m2/g with the exception of ZH4. The ZH4 sample has large BET surface area but relatively low micropore area, which may be due to the low acidity of the reaction solution. The pH value of the reaction solution was enhanced and the acidity of the synthetic system was decreased when increasing the zinc acetate amount at a constant Ti/Si/H molar ratio (When Zn/Si was increased to 4, the pH value of the reaction solution is about 2.9, as shown in Table 3). The decrease of the micropore area was also observed from the sample ZX3, which was synthesized under similar modulated acidic conditions of pH 2.89 (Table 3). ZXx samples were synthesized under a restrictive acidic condition (pH value is in the range of 2-3) by modulating the Zn/H molar ratio with fixed Ti/Si ratio of 0.1; detailed molar compositions for synthesis of ZXx composites are listed in Table 3. BET surface area of the all ZXx samples (x ) 1-4) is almost same. As mentioned above, the micropore area of the ZX3 sample (27 m2/g) is much lower than those of ZX1 and ZX2 samples (72 and 120 m2/g), which is mainly due to the acidity of the reaction solution but not the zinc acetate concentration. With the same Zn/Si ratio of 3, ZH3 sample has a micropore area of 110 m2/g, which is much higher than that of ZX3. ZX3 and ZH3 were synthesized under the same reaction conditions but with different H/Zn molar ratio. Consequently, the pH values of the reaction solution for ZX3 and ZH3 are different (for ZX3: H/Zn ) 5/3, pH ) 2.89; for ZH3: H/Zn ) 6/3; pH ) 2.08). As a result, samples with relatively low micropore area are obtained at relatively low reaction acidity. Hence, the primary factor which affects the micropore area of the TiO2-SiO2 composites may be the acidity of the reaction solution. Morphology of the ZHx (Zn/Ti/Si/H ) x/0.1/1/6) samples is greatly changed according to the zinc acetate content. ZH1 composite has a large domain of curved shapes and a small quantity of irregular puny particles (Figure 19A). Morphology of the ZH2 sample became more irregular. Some unformed brick like and few small particles are mixed together with the large “bricks” (Figure 19B) in the ZH2 sample. The shape of the ZH3 sample changes to be regular but the surface of the particle is not smooth (Figure 19C). Morphology of ZH4 becomes very regular (Figure 19, panels D and H). Large domains of uniform curved wormlike particles exist in the ZH4 sample. The concentration of zinc acetate obviously affects the morphology of the mesoporous TiO2-SiO2 composites. However, acidity

J. Phys. Chem. C, Vol. 113, No. 47, 2009 20345 of the reaction solution for preparing ZHx was simultaneously changed according to the zinc acetate content. Hence, ZXx were prepared by adjusting the H+ concentration when alternating the zinc acetate content for modulating the pH value of the reaction solution in the range of 2-3. All four ZXx samples have uniform curved worm-like shapes though they are synthesized at different zinc acetate concentration. The morphology of the ZXx sample becomes more uniform and the surface of the particles become smoother when increasing the zinc acetate amount in the reaction solution (Figure 19 E-H). When the Zn/ Si ratio is enhanced from 1 to 4, the obtained “worms” become larger, longer, fatter, and more regular, as shown in Figure 19. EDS results show that titanium content in these ZXx composites is similar (Ti/Si ratio around 0.1), which means morphology of the mesoporous TiO2-SiO2 composites can also be controlled by the zinc acetate concentration in the initial reaction solution. 3.4. Formation Mechanism and Photocatalytic Property of the TiO2-SiO2. The hydrolysis and oxidation of TiCl3 are very slow in strong acidic conditions, and oxidants or organic additives such as H2O225 or amine17 need be added into the reaction solutions in order to incorporate Ti into the mesoporous SiO2 matrix. However, high TiO2 contained mesoporous SiO2 with well-ordered mesostructure is still hard to prepare using TiCl3 as Ti source. As a result, few works have been reported on the synthesis of TiO2 contained mesoporous SiO2 by using TiCl3. In this work, TiCl3 was used and Ti3+ species were easily hydrolyzed and oxidized under the weak acidic conditions. At the same time, hydrolysis and condensation of the siliceous precursors are also slowed to match the hydrolysis rate of the titanium species. TiCl3 solution contains Ti3+ aqueous complexes (violet).61 Up to pH ) 1, most of the Ti3+ is present as the Ti(OH2)63+, whereas between pH ) 1 and 3, Ti(OH)(OH2)52+ becomes the main species. Above pH ) 3, the precipitation (dark blue) of the partially oxidized compound Ti(OH)3+x occurs, where x corresponding to the oxidation rate. Ti(OH)3+x is not stable under air and is easy to react with the dissolved oxygen and then forms a white participate.61 In this work, zinc acetate was introduced into the reaction system to adjust the reaction solution to pH ) 2.85-2.95. Under this condition, Ti(OH)(OH2)52+ would be the main species at the very beginning after TiCl3 was added into the solution. Along with the reaction process, the oxidation of the Ti(OH)(OH2)52+ species occurred simultaneously together with the hydrolysis and condensation of alkoxysilane species. Proton will instantaneously be consumed during the period when alkoxysilane species are hydrolyzed,8 which can decrease the local acidity of the solution and accelerate the oxidation of Ti3+ species. At the same time, the introduction of TiCl3 can also facilitate the hydrolysis and condensation of the siliceous species. Precipitation appears at about 90 min after the introduction of TEOS into the reaction solution without TiCl3. With the addition of TiCl3, precipitation takes place within 90 min and becomes faster and faster when the amount of the introduced TiCl3 was gradually increased. A color of dark green is observed after the introduction of TiCl3 into the reaction mixture. The dark color becomes light after the precipitation happens, and the color changes from dark green to gray and white along with the reaction process. After stirring for 24 h at 35 °C, the reaction mixture (precipitate and solution) is white for the low Ticontained samples (ZT001-ZT003) and light pink (ZT005-ZT01) or yellowish-gray (ZT02-ZT03) for the high Ti-contained samples, which implies that small amounts of Ti3+ species have not been completely oxidized in the high Ti-contained samples

20346


Wu et al.

Figure 18. SEM images of the calcined CT01 (A) and ZT01 (B) TiO2-SiO2 composites.

TABLE 3: BET Surface Area and t-Plot Micropore Area of the Mesoporous TiO2-SiO2 Samples sample

Zn/H/Si/Tia (molar ratio)

Ti/Sib (mol/mol)

pHc

SBET (m2/g)

Smic (m2/g)

ZH1 ZH2 ZH3 ZH4 ) ZT01 ZX1 ZX2 ZX3 ZX4 ) ZT01

1/6/1/0.1 2/6/1/0.1 3/6/1/0.1 4/6/1/0.1 1/2/1/0.1 2/4/1/0.1 3/5/1/0.1 4/6/1/0.1

Hydrothermal Synthesis and Morphological Evolution of Mesoporous

Recommend Documents