Journal of Colloid and Interface Science

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1 Journal of Colloid and Interface Science 323 (2008) Contents lists available at ScienceDirect Journal of Colloid and Interface Science Layer-by-layer assembly of TiO 2 colloids onto diatomite to build hierarchical porous materials Yuxin Jia a,b,weihan a,b, Guoxing Xiong a,, Weishen Yang a a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, Liaoning , People s Republic of China b Graduate School of the Chinese Academy of Sciences, Beijing , People s Republic of China article info abstract Article history: Received 31 December 2007 Accepted 8 April 2008 Availableonline11April2008 Keywords: TiO 2 colloids Nanoparticles Diatomite Layer-by-layer assembly Phytic acid Hierarchical porous materials TiO 2 colloids with the most probably particle size of 10 nm were deposited on the surface of macroporous diatomite by a layer-by-layer (LBL) assembly method with using phytic acid as molecular binder. For preparation of colloidal TiO 2, titanium(iv) isopropoxide (Ti(C 3 H 7 O) 4 ) was used as titanium precursor, nitric acid (HNO 3 ) as peptizing agent and deionized water and isopropanol (C 3 H 7 OH) as solvent. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), N 2 adsorption desorption, and UV vis spectra are used to assess the morphology and physical chemistry properties of the resulting TiO 2 coated diatomite. It was shown that the mesoporosity has been introduced into macroporous diatomite by LBL deposition. The mesoporosity was originated from closepacking of the uniform TiO 2 nanoparticles. More TiO 2 could be coated on the surface of diatomite by increasing the deposition cycles. This hierarchical porous material has potential for applications in catalytic reactions involved diffusion limit, especially in photocatalytic reactions Elsevier Inc. All rights reserved. 1. Introduction Nanometer-scaled TiO 2 is extensively used as catalysts and supports in a wide variety of reactions for its humidity- and gassensitive behavior and excellent dielectric properties. Its unique photocatalytic properties make it suitable for the oxidation of organic pollutants and other contaminants from wastewater or for drinking water supplies. In particular, it was found that nanosized semiconducting TiO 2 crystals of less than 10 nm shown significant enhancement in photocatalytic reactivity, which can be attributed to the quantum size effect [1]. In order to optimize the performance of TiO 2 for a specific application, it is desirable to combine different levels of porosity into one hierarchical porous material [2 8]. In applications such as catalysis, where the diffusion of molecules through the pore structure is vital for optimum performance, a highly ramified network of macro- and mesopores is desired. However, TiO 2 when present as a high surface area powder is not thermally stable and loses surface area readily. Therefore, effort has been devoted in recent years to coating titanium oxides on high surface area supports such as silica or alumina [9]. Recently, the diatomite was examined in perfect support in preparation of hierarchical porous materials for its inherent hierarchical porosity, inexpensiveness and easy availability. For instance, Anderson * Corresponding author. Fax: address: [email protected] (G. Xiong). et al. first build hierarchical porous materials through diatom zeolitization [10]. Tang et al. prepared and characterized zeolitized diatomite by treating seeded diatom with ethylenediamine, triethylamine and H 2 O mixed steam [11]. Diatomite is tiny single celled plants that live inside a hard shell. They occur in great quantities both in salt and fresh water. When the plant dies, the shell sinks to the bottom. Over time, large quantities of these shells accumulate, eventually forming the materials called diatomaceous earth or the more lightweight rock called diatomite [12]. Diatomite has been widely used in sound and heat insulation, as filters, abrasives, and in the manufacture of explosives. Besides, the diatomite has been tested as a support in the reactions such as hydrogenation [13,14], oxidation[15], Fischer Tropsch synthesis [16] and CO 2 reduction to alkanes [17]. In this report, it is demonstrated that well-defined TiO 2 coating may be formed by employing a layer-by-layer (LBL) deposition procedure based on diatomite. The basis of LBL assembly procedure is primarily the electrostatic attraction between oppositely charged species deposited from solution onto colloidal spheres, lends itself well to the task of producing colloidally stable, homogeneously coated particles [18 21]. This flexible and facile procedure permits the coating of colloids of various shapes and sizes with uniform layers of diverse composition. In order to continue the deposition process and to form a three-dimensional coating of TiO 2 nanoparticles, a binder molecule can be employed. Here, phytic acid has been selected as the ideal binder molecule for TiO 2 [22 24]. Phytic acid is a well-known naturally occurring acid with six phosphate functional groups attached symmetrically to /$ see front matter 2008 Elsevier Inc. All rights reserved. doi: /j.jcis

2 Y. Jia et al. / Journal of Colloid and Interface Science 323 (2008) Fig. 1. (a) Particle diameter distribution of TiO 2 sol measured by a laser light scattering meter; (b) TEM image of synthesized TiO 2 sol. a cyclohexanehexol ring [25]. The prepared samples have been characterized by dynamic laser scattering (DLS), XRD, SEM, TEM, UV vis DRS, and nitrogen adsorption desorption method. 2. Experimental The diatom used in the study was purchased from Sigma Aldrich (Celatom FW 80). For preparation of colloidal TiO 2, titanium(iv) isopropoxide (TIP, Ti(C 3 H 7 O) 4 ) was used as titanium precursor, nitric acid (HNO 3 )as peptizing agent and deionized water and isopropanol (C 3 H 7 OH) as solvent [26]. The 1 M TIP/ isopropanol solution was added to 30 C water under vigorous stirring to obtain a white TiO 2 suspension. By adding a correct amount 1.6 M HNO 3 ([H + ]/[Ti] = 0.4) under vigorous stirring, the precipitate was peptized to form a highly dispersed and stable TiO 2 sol. Finally, the sol was heated at 70 Cfor 3 h to evaporate the isopropanol. The synthesized TiO 2 colloids were coated on the surface of diatomite through a layer-by-layer assembly procedure. Initial diatomite (i-diatom) was calcinated at 550 C for 6 h to remove organic components before used. The procedure for the formation of TiO 2 coated diatomite consisted of: (i) immersion of diatomite into colloidal TiO 2 solution for 1 min, then filter and rinsing with distilled water, (ii) immersion into aqueous 40 mm phytic acid for 1 min and filter and rinsing with distilled water. By repetition of this sequence, a TiO 2 phytate coating is formed layer-by-layer. The samples were denoted TCD ( denotes the cycles of deposition). To remove phytic acid, the TiO 2 coated diatomite samples were treated for 6 h at 400 Cinair. The radius of the TiO 2 colloids and the particle-size distribution in the TiO 2 prior to the deposition were determined with dynamic light scattering (Coulter N4 plus) at a 90 angle to the light beam. A He Ne laser operating at 10 mw was used as the light source. Drops of the sol were diluted in the sample cuvette with water to give the appropriate intensity for the measurement. TEM was performed on a TECNAI G2 SPIRIT microscope with a field emission and an excitation voltage of 200 kv. The samples were dispersed in ethanol and placed on a 400-mesh copper grid. SEM was performed on a QUANTA 200 FEG microscope with a field emission and an excitation voltage of 30 kv. N 2 adsorption desorption isotherms of the samples were obtained at 77 K using the static volumetric method with an Omnisorp-100CX apparatus from Coulter. The solid samples were first degassed at 623 K under vacuum Fig. 2. UV vis absorption spectra of TiO 2 sol. (10 6 Torr) for at least 3 h before recording their isotherms. The sample s specific surface area was calculated based on the BET theory. The mesopore size distribution was calculated using the BJH method from the adsorption branch of the isotherms. Powder XRD was performed on a Rigaku D/max-2500PC powder diffractometer using Cu Kα radiation. Diffraction patterns were collected over a range 5 50 using a step scan mode with a step size of 0.02 and a step rate of 5 /min at 40 kv and 100 ma. UV vis diffuse reflectance spectra were recorded on a Jasco V550 spectrophotometer. 3. Results and discussion 3.1. Characterization of colloidal TiO 2 Fig. 1a shows the particle size distribution of the synthesized TiO 2 sol. The size distribution of the particles was 5 20 nm. TEM image of the synthesized TiO 2 sol is shown in Fig. 1b. The spherical nanocrystals were observed from TEM images with size about 5 10 nm. The discrepancy between the particles sizes determined

3 328 Y. Jia et al. / Journal of Colloid and Interface Science 323 (2008) Fig. 3. (a) SEM image and (b) XRD pattern of initial diatomite. Fig. 4. SEM images of TiO 2 coated diatomite. (a) and (b) TCD1, (c) and (d) TCD5. by DLS analysis and TEM could be due to the fact that the synthesized TiO 2 sol is composed of a large quantity of agglomerated nanoparticles. The UV vis absorption spectra of TiO 2 sol was shown in Fig. 2. The UV vis absorption band edge is a strong function of the crystallite size of nanosize TiO 2 catalyst. Usually the band gap between the valance band and the conduction band of semiconductor increases with decreasing particle size [27]. Arep- resentative UV vis absorption spectrum of the TiO 2 colloids is

4 Y. Jia et al. / Journal of Colloid and Interface Science 323 (2008) Fig. 6. (a) UV vis diffuse reflectance spectra of initial diatomite and TiO 2 coated diatomites; (b) UV vis diffuse reflectance spectra of TiO 2 coated diatomites minus the data of initial diatomite. sponding to λ = nm. The E g = 0.14 ev of this blue shift compared to bulk anatase TiO 2 particles indicated a size of TiO 2 crystallites smaller than 10 nm due to so called quantum size effect [27]. A particle size of 6 nm was estimated by the effective mass approximation based on the observed band gap shift [29], which is in accord with the results observed from TEM and DLS Characterization of initial diatomite Fig. 3 shows the SEM image and XRD pattern of initial diatomite. The SEM image of initial diatomite shows a cylindrical structure with a length of ca μm and internal pore diameter of about 4 μm. There is a nearly regular array of submicron pores in a diameter of 500 nm in the wall. The initial diatomite has an XRD pattern consistent with the crystalline silica form Characterization of TiO 2 coated diatomite Fig. 5. TEM images of (a) initial diatomite, (b) TCD1, and (c) TCD5. shown in Fig. 2. The onset of absorption of wavelength (λ) and the corresponding band gap energy (E g )oftio 2 material are well known to be λ = 385 nm and E g = 3.23 ev for anatase phase, respectively [28]. Extrapolating the spectral curve, the band gap energy of the TiO 2 sol was measured to be E g = 3.37 ev corre- The morphology changes during deposition are clearly revealed by SEM as shown in Fig. 4. The cylindrical shape and macropore structure of the diatomite are both well preserved (Figs. 4a and 4c). Compared to the clean surface of initial diatomite (Fig. 3a, insert), the TCD1 and TCD5 samples (Figs. 4b and 4d) were apparently rougher than the uncoated diatomite. After coating one layer TiO 2, uniform nanoparticles were observed on the surfaces of the TCD1. And increasing cycles of TiO 2 deposition to five, TCD5 with smoother surface than TCD1 was obtained. TEM images of initial diatomite and TiO 2 coated diatomite are shown in Fig. 5. TiO 2 nanocrystals with size of about 5 10 nm were observed on the wall of all coated diatomite. However, TEM results gave no evidence that more TiO 2 was coated on the diatomite with more coating cycles. This is due to the visualization of the nanoparticles with conventional TEM gives no information about the shape and three-dimensional orientation of the particles. UV vis DRS spectrums of initial diatomite and TiO 2 coated diatomite are shown in Fig. 6. A broad band between 200 and 350 nm centered at 260 nm is observed in initial diatomite sample. This band has been assigned to the Fe impurity in initial diatomite [30]. For TiO 2 coated diatomite, the onset of absorption shifted to about 370 nm, which is ascribed to the deposition of small size of TiO 2 crystallite on the surface of diatomite as observed by TEM. After subtraction of the absorbance for initial diatomite (Fig. 6b), an increase of the absorbance centered at

5 330 Y. Jia et al. / Journal of Colloid and Interface Science 323 (2008) Fig. 7. patters of initial diatomite and TiO 2 coated diatomite. (a) Full range, (b) enlarged view. about 320 nm was also observed with increasing the deposition cycles, which implied more TiO 2 was coated on diatomite by repeating deposition. Fig. 7 provides XRD patterns of TiO 2 coated diatomite samples. No wide Bragg peaks corresponding to the anatase phase appear in the region 5 50 (Fig. 7a). However, careful analyzing the XRD patterns of TiO 2 coated diatomite and initial diatomite, an increase between 22 and 27 centered at about 25 corresponding to anatase form of TiO 2 was observed with increasing the deposition cycles (Fig. 7b). The absence of intense Bragg peaks corresponding to the anatase phase could be contributed to the broadening of peaks originated from the small sizes of the TiO 2 nanocrystals and the interference from the intense reflection corresponding to the α-cristobalite form of silica in initial diatomite. The mesoporosity was introduced into macroporous initial diatomite by LBL deposition as characterized by N 2 adsorption desorption experiments. The isotherms for initial diatomite, shown in Fig. 8a, indicated few N 2 was adsorbed on initial diatomite due to its macroporosity. For TiO 2 coated diatomite samples, the capillary condensation step at a partial pressure >0.9 indicates the filling of textural mesopores (Figs. 8b and 8c), which was originated from close-packing of the uniform TiO 2 nanoparticles. The corresponding mesopore distribution of TCD1 and TCD5 is shown in Fig. 9. The texture properties of the initial diatomite and different zeolitized diatomites from nitrogen adsorption analysis were listed in Table 1. Compare with the surface area of m 2 /g initial diatomite had, the BET surface area and mesopore volume obtained for the TiO 2 coated diatomite increased with increasing Fig. 8. N 2 adsorption desorption isotherms of (a) initial diatomite, (b) TCD1, and (c) TCD5. Fig. 9. Mesopore size distribution of TiO 2 coated diatomite determined by the BJH method from the adsorption branch. the amounts of TiO 2 deposited, which is in accord with the UV vis and XRD results. Higher surface area composites can be obtained by more cycles TiO 2 deposition.

6 Y. Jia et al. / Journal of Colloid and Interface Science 323 (2008) Table 1 Texture properties from nitrogen adsorption analysis Sample S a BET (m 2 /g) D b meso (nm) V c meso (cc/g) i-diatom TCD TCD a S BET : BET surface area; b D meso : mean mesopore size determined by the BJH c method from adsorption branch; V meso : mesopore volume. 4. Conclusion In conclusion, we have demonstrated a simple method to fabricate hierarchical porous TiO 2 /SiO 2 materials by layer-by-layer coating TiO 2 nanoparticles on the surface of diatomite with using phytic acid as molecular binder. As revealed by SEM, TEM, XRD, UV vis spectrum, the TiO 2 nanocrystals with size of ca nm were uniformly coated on the surface of diatomite and the macroporous structure of the initial diatomite is well preserved under synthesis condition. N 2 adsorption characterization indicated that the mesoporosity was introduced by the close-packing of TiO 2 nanoparticles. Higher surface area composites can be obtained by more cycles of TiO 2 deposition. The current works suggest new possibilities for the creation of advanced hierarchical structured catalysts, especially photocatalysts. Acknowledgments This work was supported by SINOPEC (No. X503008) and NSFC (No ). References [1] M. Anpo, M. Takeuchi, J. Catal. 216 (2003) [2] M. Iwasaki, S.A. Davis, S. Mann, J. Sol Gel Sci. Technol. 32 (2004) [3] J.C. Lytle, H.W. Yan, R.T. Turgeon, A. Stein, Chem. Mater. 16 (2004) [4] S. Nagamine, T. Ueda, I. Masuda, T. Mori, E. Sasaoka, I. Joko, Ind. Eng. Chem. Res. 42 (2003) [5] X.C. Wang, J.C. Yu, C.M. Ho, Y.D. Hou, X.Z. Fu, Langmuir 21 (2005) [6] D. Yang, L.M. Qi, J.M. Ma, Adv. Mater. 14 (2002) [7] H. Zhang, G.C. Hardy, Y.Z. Khimyak, M.J. Rosseinsky, A.I. Cooper, Chem. Mater. 16 (2004) [8] L.Z. Zhang, J.C. Yu, Chem. Commun. (2003) [9] R. Castillo, B. Koch, P. Ruiz, B. Delmon, J. Mater. Chem. 4 (1994) [10] M.W. Anderson, S.M. Holmes, N. Hanif, C.S. Cundy, Angew. Chem. Int. Ed. 39 (2000) [11] Y.J. Wang, Y. Tang, A.G. Dong, X.D. Wang, N. Ren, Z. Gao, J. Mater. Chem. 12 (2002) [12] P.J. Lopez, J. Descles, A.E. Allen, C. Bowler, Curr. Opin. Biotech. 16 (2005) [13] S.M. Echeverria, V.M. Andres, Appl. Catal. 66 (1990) [14] D.J. Zhou, D.Q. Zhou, X.H. Cui, F.M. Wang, M.Y. Huang, Y.Y. Jiang, Polym. Adv. Tech. 15 (2004) [15] C.S. Yao, H.S. Weng, Chem. Eng. Sci. 47 (1992) [16] S. Bessell, Appl. Catal. A Gen. 96 (1993) [17] K. Ogura, M. Kawano, D. Adachi, J. Mol. Catal. 72 (1992) [18] L. Wang, T. Sasaki, Y. Ebina, K. Kurashima, M. Watanabe, Chem. Mater. 14 (2002) [19] K.S. Mayya, D.I. Gittins, A.M. Dibaj, F. Caruso, Nano Lett. 1 (2001) [20] F. Caruso, X.Y. Shi, R.A. Caruso, A. Susha, Adv. Mater. 13 (2001) [21] X.C. Guo, P. Dong, Langmuir 15 (1999) [22] K.J. McKenzie, P.M. King, F. Marken, C.E. Gardner, J.V. Macpherson, J. Electroanal. Chem. 579 (2005) [23] K.J. McKenzie, F. Marken, M. Oyama, C.E. Gardner, J.V. Macpherson, Electroanalysis 16 (2004) [24] K.J. McKenzie, F. Marken, M. Opallo, Bioelectrochemistry 66 (2005) [25] K.J. McKenzie, F. Marken, M. Hydeb, R.G. Compton, New J. Chem. 26 (2002) [26] Y. Zhang, Thesis: The Synthesis, Characterization and Application of NiO/Al 2 O 3 and M x O y -TiO 2 Catalysts Prepared by the Sol Gel Method, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, [27] A. Henglein, Ber. Bunsen-Ges. Phys. Chem. Chem. Phys. 86 (1982) [28] C. Kormann, D.W. Bahnemann, M.R. Hoffmann, J. Phys. Chem. 92 (1988) [29] L.E. Brus, J. Chem. Phys. 80 (1984) [30] Y.X. Jia, W. Han, G.X. Xiong, W.S. Yang, Sci. Technol. Adv. Mater. 8 (2007)