Preparation of Oxide Nanocrystals with Tunable Morphologies by the

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Langmuir 2003, 19, 967-971

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Notes Preparation of Oxide Nanocrystals with Tunable Morphologies by the Moderate Hydrothermal Method: Insights from Rutile TiO2 Qinghong Zhang and Lian Gao* The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China Received April 1, 2002. In Final Form: October 18, 2002

Introduction The ability to systematically manipulate the shapes of inorganic nanocrystals remains an important goal of modern materials chemistry. The physical and photophysical properties of inorganic nanocrystals are influenced by the shape and the size of the nanoparticles.1-4 One means of achieving shape control is by using a static template to enhance the growth rate of one crystallographic face over another. For example, rod-, arrow-, teardrop-, and tetrapod-shaped CdSe nanocrystals were obtained by growth of the nanoparticles in a mixture of hexylphosphonic acid and trioctylphosphine oxide.4 The organic surfactants may be undesired for many applications, and a relatively high temperature is needed to decompose and combust them. Unfortunately, such a high temperature generally induces dramatic growth of nanoparticles such that ultrafine nanoparticles free of templating and stabilizing agents could not be obtained by this route. The development of bulk solution synthetic methods that offer shape control is of paramount importance if the full potential of materials is to be realized.5-7 In recent years, there has been a great deal of research on the preparation of nanostructured titania due to its extensive applications. There is much literature on the synthesis of anatase TiO2 particles with the size range from 5 nm to several microns and with a variety of shapes, for various applications.8 However, unlike anatase, it is found that * Corresponding author. Tel: +0086-21-52412718. Fax: +008621-52413122. E-mail: [email protected]. (1) Lieber, C. M. Solid State Commun. 1998, 107, 607. (2) Smalley, R. E.; Yakobson, B. I. Solid State Commun. 1998, 107, 597. (3) Alivisatos, A. P. Science 1996, 271, 933. (4) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700. (5) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (6) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (7) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 617. (8) Matsuda, A.; Kotani, Y.; Kogure, T.; Tatsumisago, M.; Minami, T. J. Am. Ceram. Soc. 2000, 83, 229. Parala, H.; Devi, A.; Bhakta, R.; Fischer, A. J. Mater. Chem. 2002, 12, 1625. Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 1613. Zaban, A.; Aruna, S. T.; Tirosh, S.; Gregg, B. A.; Mastai, Y. J. Phys. Chem. B 2000, 104, 4130. Bersani, D.; Lottici, P. P.; Ding, X.-Z. Appl. Phys. Lett. 1998, 72, 73.

the synthesis of ultrafine rutile particles is much more difficult.9 When anatase nanoparticles are calcined at a temperature higher than 873 K, anatase transforms into rutile, and only rutile on the micrometer scale can be obtained due to the dynamical growth during the heat treatment.10 Rutile nanocrystals were prepared successfully by hydrothermal treatment or peptization of titanium alkoxide derived amorphous precipitates at a temperature in the region from 433 to 573 K.11-13 Bacsa et al.14 reported an improvement of the anatase-to-rutile phase transformation by peptizing the hydrolyzed precipitates with nitric acid before the hydrothermal treatment; however, this phase transformation was not completed after a hydrothermal treatment at 473-523 K. Wang et al.15 prepared pure rutile titania with an average grain size of 49 nm by peptizing for 7 days in 1 M HNO3. Generally, a mixture of spherical, broomlike, and rod-shaped rutile titania was obtained by these routes,11,13,16 but the separation of nanocrystals in one morphology from others is very difficult technically and how to control the morphology of rutile nanocrystals is less known. Here, we report a novel and simple method to prepare ultrafine rutile titania crystallites with controllable morphologies by simply varying the hydrolytic conditions. Using a combination of these parameters, we demonstrate the controlled formation of rutile titania nanocrystals with rod, sphere, and peanut shapes. Experimental Section Preparation. Titanium tetrachloride (98% TiCl4) was used as a main starting material without any further purification. A desired amount of TiCl4 was dissolved in distilled water in an ice-water bath to obtain 3 M TiCl4 as a stock solution. The aqueous solution was diluted with distilled water at room temperature, and then the acid or the salt solution was added to adjust the acidity and the concentration of chloride ion. The mixture was stirred at high speed while the amount of TiCl4 solution necessary for the desired [H2O]/[Ti] molar ratio was added dropwise. In a typical process, 35 mL of TiCl4 aqueous solution was put into a Teflon vessel. The vessel was then placed in a stainless steel vessel, which was closed tightly, and held at 333 to 423 K for 12 h, respectively. The precipitates after the hydrothermal treatment were washed well with distilled water and then were separated from the washing solution by centrifugation. The hydrous oxide was dried at 383 K and then was ground to a fine powder. Characterization. Powder X-ray diffraction (XRD) was used for the crystal phase identification and the estimation of the anatase-to-rutile ratio and the crystallite size of each phase present. The XRD intensities of the anatase (101) peak at 2θ ) (9) Aruna, S. T.; Tirosh, S.; Zaban, A. J. Mater. Chem. 2000, 10, 2388. (10) Ovenstone, J.; Yanagisawa, K. Chem. Mater. 1999, 11, 2770. (11) Zheng, Y.; Shi, E.; Cui, S.; Li, W.; Hu, X. J. Am. Ceram. Soc. 2000, 83, 2634. (12) Yang, J.; Mei, S.; Ferreira, J. M. F. J. Am. Ceram. Soc. 2000, 83, 1361. (13) Cheng, H.; Ma, J.; Zhou, Z.; Qi, L. Chem. Mater. 1995, 7, 663. (14) Bacsa, R. R.; Gra¨tzel, M. J. Am. Ceram. Soc. 1996, 79, 2185. (15) Wang, C.-C.; Ying, J. Y. Chem. Mater. 1999, 11, 3113. (16) Sasamoto, T.; Enomoto, S.; Shimoda, Z.; Saeki, Y. J. Ceram. Soc. Jpn. 1993, 101, 230. Park, N.-G.; Schlichtho¨rl, G.; Lagemaat, J. Van de; Cheong, H. M.; Mascarenhas, A.; Frank, A. J. J. Phys. Chem. B 1999, 103, 3308.

10.1021/la020310q CCC: $25.00 © 2003 American Chemical Society Published on Web 12/31/2002

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Notes

Table 1. Preparation Parameters and Properties of Productsa properties conditions

Sa/ sample CT T/K CH/CC/CS/CA m2g-1 A B C D E

0.3 0.3 0.3 0.3 0.3

333 363 393 393 393

1.2/1.2/0/0 1.2/1.2/0/0 1.2/1.2/0/0 2.2/2.2/0/0 4.2/4.2/0/0

202.2 154.5 92.4 82.5 49.3

F G H

0.3 393 1.2/4.2/3.0/0 0.3 393 1.2/4.2/0/3.0 0.3 423 4.2/4.2/0/0

I J

0.3 423 1.2/4.2/3.0/0 56.5 0.1 423 0.4/0.4/0/0 133.3

K L

0.3 423 1.2/1.2/0/0 0.5 423 2.0/2.0/0/0

69.1 61.3

M

1.0 423 4.0/4.0/0/0

46.8

62.9 72.8 46.7

phase

dp/nm

R R R R R+B χ ) 0.832 R R R+B χ ) 0.777 R R+A χ ) 0.461 R R

7.3 8.4 10.1 9.8 9.3 (R) 10.8 (B) 8.1(a) × 95.6(c) 9.1(a) × 103.5(c) 12.9 (R) 13.2 (B) 9.5(a) × 100.2(c) 8.2 (A) 17.4 (R) 12.9 17.1 27.2(a) × 109.1(c) R+B 18.4 (R) χ ) 0.827 9.9 (B)

a C , C , C , C , and C represent the concentration of Ti4+, H+, T H C S A Cl-, Na+, and NH4+ in mol L-1 (M), respectively. R, A, and B denote rutile, anatase, and brookite, respectively. Sa is specific surface area; dp is particle size calculated by the XRD broadening of rutile {110}, brookite (121), and anatase (101) diffraction, respectively. χ is the weight fraction of rutile in powders. The letters a and c in brackets represent the crystallite size in diameter (along the a- or b-axis of rutile) and in length (along the c-axis of rutile), respectively.

25.3°, the brookite (121) peak at 2θ ) 30.8°, and the rutile (110) peak at 27.4° were analyzed. The fraction of rutile in the samples can be estimated from the following equation:17

χ ) (1 + 0.8IA/IR)-1

(1)

where χ is the weight fraction of rutile in the powder, and IA and IR are the respective XRD peak intensities of the anatase and the rutile peaks, respectively. For the mixtures consisting of rutile and brookite, the weight fraction of brookite was calculated by using the equation presented by Zhang and Banfield.18 XRD patterns were obtained at room temperature with a diffractometer D/max 2550V using Cu KR radiation. Transmission electron microscopy (TEM) observations were carried out using a JEOL-200CX electron microscope. The BrunauerEmmett-Teller (BET) surface area was determined using a Micromeritics ASAP 2010 nitrogen adsorption apparatus.

Results The effects of hydrothermal conditions on the properties of TiO2 nanocrystals are summarized in Table 1. The specific surface area of the products decreases with the raising of the hydrothermal temperature, and phase-pure rutile nanoparticles with specific surface areas in the range of 56.5-202.2 m2/g are obtained by the hydrothermal treatment of the TiCl4 solution. The highest specific surface area of rutile is 202.2 m2/g (sample A), which is much higher than the value of 96 m2/g reported for rutile titania prepared by a hydrothermal treatment of HNO3-peptized precipitate.12 In our previous works, we demonstrated that the rutile titania powder with a huge surface area also showed much higher photocatalytic activity in the photooxidation of phenol.19,20 It is also found in Table 1 that the addition of HCl, NH4Cl, and NaCl into the TiCl4 solution has a similar effect on lowering the specific surface area of rutile crystallites. Therefore, a series of ultrafine rutile (17) Spurr, R. A.; Myers, H. Anal. Chem. 1957, 29, 760. (18) Zhang, H.; Banfield, J. F. J. Phys. Chem. B 2000, 104, 3481. (19) Zhang, Q.; Gao, L.; Guo, J. Appl. Catal. B 2000, 26, 207. (20) Zhang, Q.; Gao, L.; Zheng, S. Acta Chim. Sin. 2001, 59, 1909.

Figure 1. XRD patterns of titania powders prepared by the hydrothermal treatment of TiCl4 solutions: (a) sample A, (b) sample C, (c) sample J, (d) sample K, (e) sample L, and (f) sample M. R, A, and B denote rutile, anatase, and brookite, respectively.

titania particles with tunable surface areas is obtained simply by varying the hydrothermal temperature, the concentration of TiCl4, and the addition of HCl, NH4Cl, or NaCl into the TiCl4 solution. Besides the effect of the additives on the surface area of rutile nanoparticles, the effects of additives on other aspects such as morphologies and phase composition of the resulting materials are also attractive topics. As shown in Figure 1, the obtained powders are confirmed to be rutile TiO2 by XRD in most cases, and the analysis of diffraction line widths indicates the crystalline domains of samples A and B are under 10 nm in diameter. Assuming all particles are spherical, the BET surface area of samples A and B listed in Table 1 corresponds to a crystallite size of 7.0 nm for sample A and 9.1 nm for sample B. The formation of brookite TiO2 in the highly acidic media (samples E, H, and M) is also worthy to be noted. The formation conditions for brookite are similar to those reported by Pottier et al.21 However, no shuttleshaped particles attributed to brookite11 can be found by TEM observation in the present work. Figure 2 shows TEM micrographs of typical samples. Sample A is aggregates of micropores, mesopores, and nanoparticles. The TEM-observed particle size of sample A shown in Figure 2a is consistent with the XRD determination though there is considerable distribution in size and morphology. The particle sizes of samples C (21) Pottier, A.; Chane´ac, C.; Trone, E.; Mazerolles, L.; Jolivet, J.-P. J. Mater. Chem. 2001, 11, 1116.

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Figure 2. TEM micrographs of titania powders prepared by the hydrothermal treatment of TiCl4 solutions: (a) sample A, (b) sample C, (c) sample J, (d) sample K, (e) sample L, and (f) sample M.

and J that were calculated from the broadening of the rutile XRD (110) peak at 27.4° are comparable with the diameters observed in parts b and c of Figure 2, respectively, but much finer than the length of the rodlike rutile. As shown in Figure 2, the particle size of powders derived from a fixed concentration of TiCl4 solution increases with the raising of the hydrothermal temperature. Accordingly, the specific surface area and the area-to-volume ratio decline with increasing temperature. The trend that nonagglomerated particles formed at higher temperatures (such as 393 and 423 K) is also seen in the TEM micrographs. In addition, samples L and M shown in parts e and f of Figure 2, respectively, consisted of dominantly spherical particles and a portion of nanorods. Fewer nanorods were formed in sample M compared to the samples prepared by the hydrolysis of TiCl4 solutions at lower concentrations. From the hydrolytic equations of the TiCl4 solution, we knew that the higher the concentration of TiCl4, the stronger the acidity of the solution.22 The excess of H+ ions may suppress the hydrolytic process of the TiCl4 solution, and thus fewer nuclei are formed in the highly concentrated acidic solution. The trend that more spherical (22) Zhang, Q.; Gao, L.; Guo, J. Nanostruct. Mater. 1999, 11, 1293.

particles and fewer nanorods were formed in sample M suggests that rutile nanoparticles with tunable morphologies may be prepared by simply adjusting the acidity or the concentration of chloride. Figure 3 shows XRD patterns of some samples prepared by hydrothermal treatment of TiCl4 and SnCl4 solutions containing an excess of acidity or chloride ions. Both the diffraction (101) planes of TiO2 and SnO2 are sharper and less broadened than the diffraction (110) plane. Moreover, for example, the relative intensity of I101/I110 for SnO2 nanocrystals is 96/100 in the present work, which is significantly different from 100/75 reported as JCPDS 411445. Figure 4a shows the TEM micrograph of sample H, which was prepared by the hydrothermal treatment of 0.3 M TiCl4 containing an excess of H+ and Cl- ions. Sphere-, peanut-, and a few rod-shaped rutile nanoparticles occur in this sample; however, these rods (elongated particles) are much shorter than the nanorods in samples C, J, and K and their aspect ratio is only about 2. In addition, sample H has the lowest specific surface area among all samples listed in Table 1. Figure 4b shows a TEM micrograph of rutile nanoparticles prepared by hydrothermal treatment of 0.3 M TiCl4 containing an excess of Na+ and Cl- ions. It is interesting that rutile nanorods with the aspect ratio close to 10 were obtained

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Figure 3. XRD patterns of titania powders prepared by the hydrothermal treatment of different solutions: (a) sample L, (b) sample H, (c) sample I, and (d) 0.3 M SnCl4 + 3 M HCl at 423 K. R and B denote rutile and brookite, respectively.

in the excess of chloride ions. When NH4Cl was added instead of NaCl, anisotropic rutile nanoparticles were prepared successfully. High-resolution transmission electron microscopy (HRTEM) shows that the long direction of all rods is parallel to the {110} planes. The oriented growth of rutile nanorods is also supported by XRD analysis, where the (110) reflection shows a smaller intensity and a larger width than the (101) reflection does. The effects of other parameters such as the concentration of salts and the hydrothermal treatment time on the morphologies were also investigated. When the concentration of NH4Cl or NaCl was 1.5 or 0.5 M, respectively, the powder was composed of nanorods and spheroids with a morphology similar to that shown in the TEM micrograph in Figure 2e, except that these nanorods exhibited a relatively lower aspect ratio. Since the concentration of 3 M is close to the concentration of the saturated solution of NaCl, a solution containing more than 3 M NaCl was not used to prepare rutile nanorods. In the highly acidic solutions, it is found that the average grain size of the products increases with prolonging reaction time. Their morphology is essentially the same as in Figure 4a. Up to now, rutile nanorods parallel to the c-axis with the aspect ratio of about 10 as well as the spheroids and peanut-shaped particles have been prepared by us using this method; however, nanorods along the vertical plane of {110} could not be obtained. Rutile nanorods with different aspect ratios may be achieved by adjusting the relative contents of NaCl and HCl in the TiCl4 solution. This work combined with the investigation of spectral properties of rutile titania crystallites with different morphologies and the dependence of properties on morphologies is in progress. Discussion Table 1 shows the properties of samples E, F, and G, which were prepared at equal [Cl-]. The rutile nanoparticles of sample E prepared at [H+] ) 4.2 mol L-1 had the lowest surface area. On the other hand, for samples

Notes

C, F, and G, their acidity is equal, while the concentration of chloride ion is varied. Higher concentrated chloride ions derived rutile titania with a lower surface area (the surface areas of samples C and F are 92.4 and 62.9 m2 g-1, respectively). These results imply that both acidity and the concentration of chloride ion influence the properties of rutile titania nanoparticles significantly. Moreover, in highly acidic solution the equilibrium between dissolution and precipitation of nanoparticles is significantly different from that in weakly acidic solution for the following considerations. First, the nucleation in concentrated acidic media is very difficult in this H+-releasing reaction, and the growth just takes place outside a limited number of nuclei by a layer-by-layer growth mechanism. Second, nuclei finer than the critical size are dissolved rapidly in concentrated acidic media under hydrothermal conditions, and only relatively large and stable nanoparticles remain in the concentrated acidic solution. The latter is also supported by the fact that the higher the hydrothermal temperature, the larger the particle size, because more and larger particles are dissolved at relatively high temperatures. When some NH4Cl or NaCl was added into TiCl4 solution, the resultant rutile nanocrystals were characteristically elongated particles (nanorods). Particularly, as the concentration of chloride was increased, the particles turned into rodlike particles. The axis of revolution of these nanorods and peanut-type particles was found to agree with the c-axis of the tetragonal crystal system of rutile. Meanwhile, the addition of HCl generally favors the formation of rutile crystallites with a more isotropic morphology compared to those nanorods. Recently, rutile TiO2 nanowhiskers23 and SnO2 nanorods24 have been prepared by annealing Ti(OH)4 and SnO2 powder in NaCl flux in the presence of surfactant NP9 at temperatures ranging from 750 to 800 °C, respectively. The effect of NaCl on the formation of nanorods was explained simply by taking into account the decreasing of the viscosity of the flux.24 Our experimental conditions are different from those in the works mentioned above; thus this mechanism is not applicable to the present case. We tentatively interpret the formation mechanism of nanorods according to the following aspects. A selective adsorption of impurities leads to a decrease in the vertical growth rate of the adsorbed crystal plane, which should be beneficial to the formation of nanorods. Oliver and co-workers calculated the surface and attachment energies based on atomistic simulation, and they claimed the surface energies of {011}, {110}, {100}, and {221} were 1.85, 1.78, 2.08, and 2.02 J m-2, respectively.25 The most stable surface of rutile, the {110}, contains five- and six-coordinate species, which in part may be the source of the selective adsorption (chemical, physical, or both) of the chloride or anionic complex of titanium(IV) ions in the acidic medium. However, for the consideration of lowering the surface energy, tips composed of the surface of {101}, which has a higher surface energy than that of {110}, can adsorb more anionic complex of titanium(IV) ions than tips composed of {110} do. These complexes of titanium(IV) ions dehydrate on the tips of nanorods and form units of [TiO6] continuously, consequently growing anisotropically along the c-axis. Now, the question is why could one not form nanorods by the addition of equimolar [Cl-] in the form of HCl instead of NaCl or NH4Cl into TiCl4 solution? (23) Li, G. L.; Wang, G. H.; Hong, J. M. J. Mater. Res. 1999, 14, 3346. (24) Wang, W.; Xu, C.; Wang, X.; Liu, Y.; Zhan, Y.; Zheng, C.; Song, F.; Wang, G. J. Mater. Chem. 2002, 12, 1922. (25) Oliver, P. M.; Watson, G. W.; Kelsey, E. T.; Parker, S. C. J. Mater. Chem. 1997, 7, 563.

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Figure 4. TEM micrographs of titania powders prepared by the hydrothermal treatment of (a) sample H and (b) sample I.

Before answering this question, we should examine the coordination of Ti(IV) ions in different acidity and [Cl-]. The ligand field strength of the OH group is larger than that of the Cl- ion. Titanium(IV) exists as the [Ti(OH)nClm(H2O)6-n-m](n+m-4)- complex species in the TiCl4 solution,21 where n and m are concerned with the acidity and [Cl-] in feedstock, respectively, that is, the higher either the acidity or [Cl-], the bigger the value of m. In a highly acidic solution of HCl, the formed neutral molecule Ti(OH)2Cl2(H2O)2 seemed to be the precursor of brookite.21 This mechanism is also favored to explain the occurrence of brookite in samples E, H, and M listed in Table 1. The linking between [TiO6] units is carried out by the dehydration reaction between OH ligands in [Ti(OH)nClm(H2O)6-n-m](n+m-4)- complex ions.26 When the acidity in the feedstock is lower, the number of OH ligands in [Ti(OH)nClm(H2O)6-n-m](n+m-4)- is more. The values of n, m, and 6-n-m also influence the symmetry of titanium(IV) complex ions, and a more symmetrical structure is favored to form rutile. These different complexes likely coexist in equilibrium, and it is difficult to precisely distinguish the respective roles of acidity and [Cl-] on the evolution of the soluble complexes during the hydrothermal process. In general, the equilibrium concentration and exact nature of individual complex ions of titanium(IV) are depending directly on the acidity and [Cl-], and (26) Jolivet, J.-P. Metal Oxide Chemistry and Synthesis: from Solution to Solid State; John Wiley and Sons: Chichester, 2000.

both affect the selective adsorption on the side faces or tips of nanorods; thus sphere-, peanut-, and rod-shaped rutile nanocrystals can be synthesized by adjusting the relative concentration of acid and chloride ions. The formation of rutile spheroids at an extremely high pH is explained by a layer-by-layer growth mechanism. It seems that hydrogen ions most strongly suppress the growth of rutile nanoparticles in all directions, and thus spheroids and peanutlike particles are formed. We now describe a minimum set of requirements to achieve size and shape control of rutile titania nanocrystals by adding corresponding inorganic counterions. Chloride ions are found to tune the morphology very effectively, and TiO2 and SnO2 nanorods in the rutile phase are prepared successfully from metal chlorides. This strategy can also easily be adapted to synthesize oxides of other metals in the rutile phase (such as SnO2). The ease, reproducibility, and versatility of this synthetic approach will facilitate the development of new materials and the examination of their size- and shape-dependent properties. The simplicity of the method suggests that it is amenable to commercial scale-up. Acknowledgment. We thank Professor Meiling Ruan for the help in TEM. This work is supported by the National Key Project of Fundamental Research for Nanomaterials and Nanostructures (Grant No. 19990645-06). LA020310Q