Materials Chemistry and Physics 124 (2010) 809–815
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Synthesis and structural properties of ultra-small oxide (TiO2 , ZrO2 , SnO2 ) nanoparticles prepared by decomposition of metal alkoxides Mauro Epifani a,∗ , Jordi Arbiol b , Eva Pellicer c,1 , Nicolas Sergent d , Thierry Pagnier d , Joan R. Morante c,e a
Istituto per la Microelettronica e i Microsistemi, IMM-CNR, Via Monteroni, 73100 Lecce, Italy Institució Catalana de Recerca i Estudis Avanc¸ats (ICREA) and Institut de Ciència de Materials de Barcelona, CSIC, Campus de la UAB, 08193 Bellaterra, CAT, Spain M2E-XaRMAE, Departament d’Electrònica, Universitat de Barcelona, C. Martí i Franquès 1, 08028 Barcelona, CAT, Spain d Laboratoire d’Electrochimie et de Physicochimie des Matériaux et Interfaces, GIT-CNRS-UJF BP 75, 38402 Saint Martin d’Hères, France e Institut de Recerca en Energia de Catalunya (IREC), C/Josep Pla 2, 08019 Barcelona, Spain b c
a r t i c l e
i n f o
Article history: Received 30 April 2010 Received in revised form 21 July 2010 Accepted 26 July 2010 Keywords: Nanocrystals SnO2 polymorphs Transmission electron microscopy Raman spectroscopy
a b s t r a c t The decomposition of metal (Ti, Zr, Sn) alkoxides at 250 ◦ C in a solution of tetradecene and dodecylamine resulted in the formation of ultra-small (1–2 nm) oxide nanoparticles. The nanoparticles showed unusual structural properties. The high-pressure orthorhombic phase was found for SnO2 . The as-synthesized ZrO2 nanoparticles were only partially crystallized. It was possible to observe their in situ crystallization under the TEM beam. The TiO2 nanoparticles appeared amorphous, with only a few nanocrystals dispersed in the sample. The temperature evolution of the samples was investigated in situ by Raman spectroscopy. All SnO2 was converted to stable cassiterite at 350 ◦ C. TiO2 and ZrO2 samples displayed phase stability reversal over a broad range of temperatures. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The peculiar structural transformations of nanosized systems are well known. Haase and Alivisatos [1] first reported on the zinc blende-rock salt phase transition in 4 nm CdS nanocrystals. Other peculiar structural transformation properties have been reported by several research groups: (a) pressure- and temperature-induced transformations in CdSe [2], GaAs [3], ZnO [4], etc.; (b) ambientpersistence of metastable phases in CoO [5]; (d) surface-induced structural modifications in nanocrystalline zirconia [6] and nanosized titania [7]. Other examples include the phase transformation kinetics [8] and the surface energy [9] of nanomaterials. In this paper we present further examples of the intriguing structural properties of nanoparticle systems, focusing on SnO2 , TiO2 and ZrO2 nanoparticles. We explored alternative uses of metal alkoxides [10] as precursors for the synthesis of metal oxide nanoparticles. The alkoxide hydrolytic processing has been used for synthesizing TiO2 [11], ZrO2 [12], In2 O3 [13] and BaTiO3 [14]. The non-hydrolytic sol–gel approach has been used for preparing ferroelectrics [16], In2 O3 and SnO2 [16], Y2 O3 [17], Ta2 O5 and HfO2 [18]. We instead tried to process metal alkoxides as a particular case of oxygen-rich
∗ Corresponding author. Tel.: +39 0831507219; fax: +39 0831507659. E-mail address:
[email protected] (M. Epifani). 1 Current address: Departament de Física, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Spain. 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.07.066
precursors. In particular, we studied the decomposition of Sn, Ti and Zr alkoxides. In this way very small (about 1–1.5 nm) nanoparticles were prepared, displaying peculiar structural properties: unusual crystalline phases were observed for SnO2 ; highly distorted, partially amorphous ZrO2 nanoparticles were obtained. The latter were crystallized in situ under the TEM beam. Raman spectroscopy, could evidence a phase stability reversal for TiO2 and ZrO2 over a broad range of temperatures. 2. Experimental All the chemicals were provided by Sigma–Aldrich, apart for Sn isopropoxide, provided by Gelest. A solution of 10 ml of tetradecene (technical grade, 92%), 3.1 mmol of a metal (Ti, Zr, Sn) alkoxide and a variable amount of n-dodecylamine (DA, 98% purity) or n-hexylamine (HA, 99% purity) was poured in a degassed flask. The amine:metal molar ratio (in the following denoted by RDA or RHA ) was varied from 0.35 to 1.4. The investigated metal alkoxides were: Ti n-butoxide (Ti(OC4 H9 )4 ), Ti isopropoxide (Ti(2-OC3 H7 )4 ), Zr n-butoxide (Zr(OC4 H9 )4 , 80 wt% solution in nbutanol), Zr propoxide (Zr(OC3 H7 )4 , 70 wt% solution in n-propanol), Sn isopropoxide (Sn(2-OC3 H7 )4 ). All the solvents used for purifications were of analytical grade. The solutions were heated up to 250 ◦ C (heating rate: 10 ◦ C/min), where they were kept for 5 min (TiO2 ) or 1 h (ZrO2 ), while in the case of SnO2 the heating was stopped as soon as the temperature reached 250 ◦ C. The heating temperature and times were empirically adjusted for avoiding the sudden formation of carbonaceous, insoluble residuals. When the temperature reached the final value, the solutions suddenly became greenish-yellow (TiO2 ) or light-yellow (ZrO2 ). In the case of SnO2 a yellow, gelatinous precipitate suddenly formed. After cooling the flask, the synthesis product was recovered by the addition of methanol followed by centrifugation. After further washing with methanol, the precipitate was dried at 80 ◦ C. We observed that the powders could be redispersed in hexane by addition of trioctylphosphine, forming clear and stable suspensions. White (TiO2 , ZrO2 ) or yellow (SnO2 ) powders
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were obtained after drying. The typical yields were about 15 mg for TiO2 , 25 mg for SnO2 and 120 mg for ZrO2 . The crystal structure of the powders was characterized by X-ray diffraction (XRD) with a Panalytical Alfa diffractometer with the Cu-K␣1 radiation ˚ The structural and morphological characterization of the nanocrystals ( = 1.5406 A). was also carried out by transmission electron microscopy (TEM). In order to obtain the high-resolution TEM (HRTEM) results, we used a field emission gun microscope, JEOL 2010F, operated at 200 kV and with a point-to point resolution of 0.19 nm. Raman spectroscopy measurements were carried out with a Renishaw InVia Spectrometer. Spectra were obtained with the green line of an Ar-ion laser (514.53 nm) in micro-Raman configuration (objective 50×). The dried powders were introduced into a laboratory-made cell allowing heat-treatment in flowing gas [19].
3. Results and discussion 3.1. Synthetic considerations and structural features of the as-prepared nanoparticles Our synthesis approach was inspired by metal acetylacetonate aminolysis [15] and non-hydrolytic reactions of metal alkoxides [16]. We did not use amine excess, and avoided solvothermal conditions. Long-chain amines were supposed to originate nucleophilic attack on the metal centers in the alkoxide molecules. Such centers are indeed known to be susceptible to nucleophilic attack (sudden precipitation of hydrated oxides upon alkoxide reaction with water is a known example). On the other hand, amine bonding to the metal oxide species was also expected, resulting in capping and size control. Another plausible reaction mechanism is the alkoxide condensation by ether elimination: M–OR + M–OR → M–O–M + R–O–R This mechanism was investigated and proposed by Niederberger and coworkers for explaining the nanoparticle formation starting from alkoxides [18]. It is plausible to suppose that our synthesis could involve both the above-described mechanisms. Thus any mechanism investigation would be very complex and it was not carried out more in detail. Only amines were tested since with other ligands such as phosphines no product was obtained. An early evaluation of the synthesis products was carried out by XRD. The results are reported in Fig. 1. The syntheses were carried out with RDA = 0.35, and using Ti(OC4 H9 )4 , Zr(OC4 H9 )4 and Sn(2-OC3 H7 )4 as metal precursors. We will consistently refer to these samples throughout the work. The XRD patterns show that very small or poorly crystallized nanoparticles were prepared for all the systems. Very broad peaks for SnO2 and ZrO2 correspond to the most intense reflections of the tetragonal and monoclinic phases, respectively, which are the most stable for the corresponding bulk systems. The TiO2 pattern only shows very weak and broad reflections. Similar results, reported in the Supplementary Material (Figure S1), were obtained by changing the amine, the alkoxy ligand and the solvent. The extremely small size was also shown by the optical absorption spectra measured on colloidal suspensions of the nanoparticles in hexane (Supplementary Material, Figure S1). The spectra displayed a large blue shift of the absorption bands with respect to the bulk values. The XRD data were ambiguous about the crystalline nature of the obtained products. Hence, a systematic TEM investigation of the nanoparticles was carried out. The same samples of Fig. 1 were investigated. In Fig. 2 the results of the observation of the TiO2 nanoparticles are reported. The crystallographic parameters of the nanoparticles were similar to anatase TiO2 . The nanoparticles showed about 10% distortions with respect to the bulk cell parameters. Some nanoparticles of 3.5 nm were found, but typically they were smaller than 1.5 nm. In Figure S2 (Supplementary Material) images of a less distorted nanocrystal are shown. In Fig. 3 the results are reported for the ZrO2 nanoparticles. Few crystallites with size lower than 1.5 nm were found. SAED patterns show a wide ring
Fig. 1. XRD patterns measured on the indicated nanoparticles. The synthesis parameters are described in the text. Adsorbed solvent may be contributing to the XRD background.
denoting a large distortion of cell parameters. However the ring is centered on 0.297 nm. The latter is the interplanar distance for the (1 0 1) atomic planes of tetragonal ZrO2 . Other images are shown in Figure S3 of the Supplementary Material. Finally, in Fig. 4 the results obtained on the SnO2 sample are shown. The HRTEM analysis showed the presence of small SnO2 nanoparticles, with sizes between 1 and 2.5 nm. It was difficult to determine their structure, as they were widely distorted. However the usual normal cassiterite SnO2 phase or, in some cases, the SnO2 orthorhombic (Pbcn space group) phase were evidenced. Distortions with respect to the bulk cell parameters were about 10% in both cases. This distortion is substantially larger than commonly reported for larger nanocrystals. We attribute the high distortion observed to surface effects. In nanoparticles with very small size, the percentage of surface
Fig. 2. General view, high-resolution image of the marked area, and related FFT spectrum for the TiO2 nanoparticles of Fig. 1.
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Fig. 3. General view, high-resolution image of the marked area, and SAED pattern for the ZrO2 nanoparticles of Fig. 1.
Fig. 4. General view, high-resolution image of the marked area, and related FFT spectrum for the SnO2 nanoparticles of Fig. 1.
atoms is high compared to the “bulk” atoms. Surface atoms have free bonds, not keeping the perfect cell structure, and thus allowing high distortions in their lattice parameters. If we consider a bigger nanoparticle, there is a much larger bulk contribution to be considered, and the lattice parameters of the nanoparticle will be in good agreement with those of the bulk. High distortions observed in our 1 and 1.5 nm nanoparticles are then attributed directly to the remarkable surface contribution, due to the precarious bonding state of the mean part of the atoms composing such ultra-small nanoparticles. Fig. 4 refers to an orthorhombic nanocrystal. In Figure S4 of the Supplementary Material cassiterite nanocrystals are also shown. It is
well known that SnO2 crystallizes as a bulk material in cassiterite phase (SnO2 -I) [20]. Even SnO2 nanostructures display the cassiterite phase, in general. However, under high pressure, the stacking of the lattice basic cells is modified and the crystallographic phase becomes different. So, it has been reported by Suito et al. [21] and later by Mueller [22] that bulk SnO2 , obtained at high pressure, can be found in the orthorhombic Pbcn phase (SnO2 -II). Later studies have evidenced a complex system of phase transitions at very high pressures [23]. Pressures beyond 150 kbar are required to form the orthorhombic phase. On the other hand, nonconventional crystal phases in nanostructures synthesized in normal pressure condi-
Fig. 5. General TEM view and power spectra of ZrO2 nanoparticles observed after the indicated times of exposure under the TEM beam.
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tions have been reported for other materials [24] and even SnO2 [25]. Our result was interpreted as a consequence of the metastable configuration of nanostructures comprising a very small number of atoms, and characterized by the remarkable strain evidenced by TEM studies. A more striking result was observed in the case of ZrO2 nanoparticles. Upon prolonged exposure under the TEM beam, the structural transformation of the ZrO2 nanoparticles was directly observed. A representative sequence of TEM images and the related power spectra during the under-beam crystallization is shown in Fig. 5. A complete movie of the crystallization process, built up of images taken every 4 s, is available in the Supplementary Material. The power spectra indexation demonstrates that tetragonal ZrO2 [26] nanocrystals were formed. The spots shown in Fig. 5 after 72 s of exposure belong to the (1 0 1) tetragonal ZrO2 crystallographic plane. The synthesis of zirconia nanocrystals requires high temperatures and/or pressures. Hence the observation of the structural transformation directly under the TEM beam was surprising. These phenomena are well documented for other systems [27]. 3.2. Raman spectroscopy investigation of the structural stability of the nanoparticles 3.2.1. SnO2 nanoparticles The presence of metastable features or of unusual phases stimulated the question about their stability. This topic was investigated by Raman spectroscopy measurements with in situ heating. Fig. 6 shows the Raman spectra of the SnO2 sample recorded at increasing temperature from 25 ◦ C up to 500 ◦ C in flowing air, in the range 150–3800 cm−1 . The Raman spectrum recorded at 25 ◦ C is dominated by strong bands associated with the amine used in the synthesis, together with broad bands in the 400–700 cm−1 range attributed to surface modes of nanocrystalline SnO2 [28]. From 100 ◦ C, the organic bands disappear progressively until complete removal at 200 ◦ C. From 150 ◦ C, two broad bands can be
Fig. 6. Raman spectra of the SnO2 sample recorded in situ at the corresponding temperature of treatment in air. All the spectra have been compensated for the strong background by baseline subtraction.
observed at 1385 and 1585 cm−1 . Their intensity reaches a maximum at 200 ◦ C and then decreases. These bands can be attributed to the presence of more or less crystallized graphite at the surface of the sample [29]. Finally, from 250 ◦ C, these carbon species are progressively oxidized until complete removal at 400 ◦ C. This interpretation was closely confirmed by the results of the thermal analyses, shown in the Supplementary Material (Figure S5). The intensity of the surface SnO2 bands in the 400–700 cm−1 decreases with increasing temperature, while a new band appears at 621 cm−1 from 150 ◦ C. This band is characteristic of the A1g mode of cassiterite SnO2 . Its intensity increases with increasing temperature. At 25 ◦ C after cooling, the spectrum exhibits the
Fig. 7. Raman spectra of the TiO2 sample recorded in situ at the corresponding treatment temperature in air. (A) From 25 to 500 ◦ C and at 25 ◦ C after cooling; (B) from 100 to 300 ◦ C; (C) expanded view from 250 to 350 ◦ C.
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three modes of cassiterite tin oxide at 476 (Eg ), 631 (A1g ) and 772 cm−1 (B2g ) together with the surface modes in the 200–400 and 400–700 cm−1 ranges [28]. It was not possible to discern features attributable to the orthorhombic phase, in the region of the surface modes. The persistence of such modes showed that the sample size remained in the nanometric range. A sample was indeed heat-treated for 1 h at 350 ◦ C. The related XRD pattern, shown in the Supplementary Material (Figure S6), confirmed the nanometric size of the sample. It was also concluded that any orthorhombic nanocrystal was converted to the more stable tetragonal phase. 3.2.2. TiO2 nanoparticles Fig. 7A shows the Raman spectra of the TiO2 sample recorded at increasing temperatures from 25 ◦ C up to 500 ◦ C in flowing air, in the range 150–3800 cm−1 . The Raman spectrum recorded at 25 ◦ C exhibits a strong fluorescence background. However, some bands associated with amine groups can be observed around 2900 cm−1 . These bands together with the fluorescence background disappear progressively with increasing temperature until complete removal at 300 ◦ C (see Fig. 7B). It was concluded that the organics responsible for the fluorescence were progressively decomposed when heating in air. From 250 ◦ C, additional bands can be observed (clearly shown in Fig. 7C): (i) four bands at 243, 425, 607 and 823 cm−1 can be attributed to rutile TiO2 [30]. These bands are already present at 25 ◦ C, but the strong fluorescence prevents their clear observation; (ii) the band at 149 cm−1 can be attributed to anatase TiO2 . From 450 ◦ C, the spectrum of anatase TiO2 is clearly evidenced by the bands at 149, 399, 520 and 641 cm−1 [30]; (iii) at 25 ◦ C after cooling, the spectrum is characteristic of anatase TiO2 , but the presence of rutile TiO2 cannot be excluded. While rutile is the most stable TiO2 phase, the observation of anatase in asprepared samples is commonly reported in the literature. Anatase is then converted to rutile after heat-treatment at high temperatures. Instead, we still observed anatase after heat-treatment at 450 ◦ C, despite rutile had begun appearing at lower temperatures. The influence of small sizes (below 3 nm) on the stability of titania polymorphs has been recently reported by Hummer et al. [31]. Interestingly, the authors explain the stability of anatase with respect to rutile, below a size of 3 nm, to the surface energy differences in nanosized polymorphs. They prepared the nanoparticles by a surfactant-free aqueous route. In our case, we prepared our nanoparticles with an organic capping. We then attributed the phase composition of our samples to an additional surface energy term. Such contribution is to be attributed to the amine ligands. 3.2.3. ZrO2 nanoparticles For the ZrO2 sample, no Raman spectrum could be recorded at temperature higher than 100 ◦ C due to the condensation of products on the cell window. The heat-treatment procedure had to be changed as follows: the sample was heated in flowing air from 25 ◦ C up to 200 ◦ C then cooled down to 25 ◦ C. After cleaning the cell window, the sample was heated from 25 ◦ C up to 800 ◦ C in flowing air. Fig. 8A shows the Raman spectra obtained in the range 150–3800 cm−1 . The Raman spectrum recorded at 25 ◦ C exhibits a fluorescence background together with bands associated with organic residuals. From 25 ◦ C to 100 ◦ C, these bands together with the fluorescence background disappear progressively: the organics are evaporated (condensation on the cell window) and/or decomposed. From 200 ◦ C to 500 ◦ C, the spectra are only dominated by a strong fluorescence background. From 550 ◦ C, the fluorescence background decreases while new bands appear in the 150–800 cm−1 range (clearly shown in Fig. 8B). Between 650 ◦ C and 800 ◦ C, no more change was observed in the Raman spectra. It was concluded that the organics were progressively transformed into
Fig. 8. Raman spectra of the ZrO2 sample recorded in situ at the corresponding temperature of treatment in air. All the spectra have been normalized over the whole wavenumber range. (A) From 25 to 800 ◦ C and at 25 ◦ C after cooling, (B) expanded view from 550 to 800 ◦ C.
species responsible for the fluorescence, then these species were decomposed when heating in air. The bands in the 150–800 cm−1 range can be attributed to tetragonal ZrO2 [32]. Finally, at 25 ◦ C after cooling, the Raman spectrum (Fig. 9) is characteristic of both tetragonal and monoclinic ZrO2 [32,33] in agreement with the nanocrystalline nature of the powder [34]. Once again, the small nanoparticle size stabilizes the phase that should be found only at high temperatures. The most stable monoclinic phase began to appear after high-temperature heat-treatment and the subsequent cooling.
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Fig. 9. Raman spectra of the ZrO2 sample recorded at 25 ◦ C after in situ treatment up to 800 ◦ C in air. T = Tetragonal ZrO2 .
in the present work. In another work [36], we have showed the ligand effect on the size dependent crystallization threshold of CdSe nanoparticles. It seems reasonable to conclude that in the present work the structural disorder is due to peculiar kinetic factors originated from both synthesis (alkoxide decomposition rate, monomers condensation, etc.) and the ligand bonding to the surface. Finally, we explicitly consider the possibility of the defects influence on phase stability. This topic is well known in the case of TiO2 . For instance, the defects induced by dopants are known to stabilize the rutile phase. We observed this phenomenon in TiO2 thin films doped with Pt [37]. Above a threshold Pt concentration, the rutile phase was directly formed instead of anatase. In our samples there is large surface carbon concentration due to the capping amine. Hence, the resulting surface defects may play an essential role in further influencing the structure of the nanoparticles. What emerges is an essential role of the nanoparticles surface in the evolution of the structural properties. The surface must be necessarily considered as the combination of the outer atoms and the ligand molecules (amine) bonded to them.
3.3. Structural issues related to the ultra-small size
4. Conclusions
The Gibbs free energy for a nanoparticle system is composed of bulk and a surface term, GB and GS :
The processing of metal alkoxides in amine solution allowed the preparation of SnO2 , TiO2 and ZrO2 nanoparticles in a size range below 2.5 nm. The size range resulted in the observation of peculiar structural features, in particular: (i) the presence of orthorhombic crystallographic phase for SnO2 ; (ii) the metastability of zirconia nanoparticles, which converted to the final crystalline phase under the TEM beam; (iii) the phase metastability in the case of the TiO2 and ZrO2 samples. These results were attributed to the nanoparticle surface composition and in particular to the species bonded to the surface metal atoms.
G = GB + GS With decreasing the system size, the surface term is increasingly important and can result in dramatic changes in the relative polymorphs stability. This phenomenon occurs if for each polymorph there is a specific surface contribution. The structure of the surface term further evidences these differences, since we have: GS = A where is the interfacial tension and A the surface area (and excluding, for simplicity, any electrostatic term). It has been shown [9] that, by itself, has a size dependence in the nanosized regime. Moreover, the presence of a capping layer can be seen as an effective surface with its own interfacial tension contribution. These considerations help to understand the reason for the unusual structural properties of our nanoparticles. An exact consideration of the surface contribution for each of the investigated systems is outside the aims of the present work. In this section we further discuss peculiar factors that can be responsible for the observed phenomena. First of all, it is necessary to consider that for small nanoparticles the energy provided by the post-synthesis treatment can substantially perturb the systems. In particular, we cannot exclude that the nanoparticles observed for SnO2 and TiO2 systems were directly crystallized by the TEM beam, but on much faster time scale than for ZrO2 . We carried out prolonged exposure of a TiO2 sample under the TEM beam, but the sample remained amorphous, as shown in the additional figures in the Supplementary Material (Figure S7). Nevertheless, we cannot consider this observation as conclusive as concerns the lack of effect of the TEM beam. On the other hand, it is reasonable to conclude that the post-synthesis drying process is not capable of inducing crystallization. One more question concerns the effect of the capping ligands on the structural transformations. The ligands presence is clearly shown by the FTIR spectra measured, as an example, on SnO2 nanoparticles, and reported in the Supplementary Material (Figure S8). In recent works [35], we have investigated SnO2 nanocrystals prepared by a different technique, showing the importance of amine ligands in the final chemical composition of the materials. Anyway the resulting nanocrystals, with a size of about 2 nm, did not display structural disorder comparable to that observed
Acknowledgements The authors thank Serveis Cientificotècnics of Universitat de Barcelona for the use of their TEM equipment. This work was partially supported by the Spanish Government projects Consolider Ingenio 2010 CSD2009 00013 IMAGINE and CSD2009 00050 MULTICAT. Jordi Arbiol acknowledges the funding from the Spanish CSIC project NEAMAN. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.matchemphys.2010.07.066. References [1] M. Haase, A.P. Alivisatos, J. Phys. Chem. 96 (1992) 6756. [2] (a) C.C. Chen, A.B. Herhold, C.S. Johnson, A.P. Alivisatos, Science 276 (1997) 398; (b) J. Wickham, A.B. Herhold, A.P. Alivisatos, Phys. Rev. Lett. 84 (2000) 923; (c) K. Jacobs, D. Zazinski, E.C. Scher, A.B. Herhold, A.P. Alivisatos, Science 293 (2001) 1803. [3] S. Kodiyalam, R.K. Kalia, A. Nakano, P. Vashishta, Phys. Rev. Lett. 93 (2004) 203401. [4] F. Decremps, J. Pellicer-Porres, F. Datchi, J.P. Itié, A. Polian, F. Baudelet, J.Z. Jiang, Appl. Phys. Lett. 81 (2002) 4820. [5] J.F. Liu, Y. He, W. Chen, G.Q. Zhang, Y.W. Zeng, T. Kikegawa, J.Z. Jiang, J. Phys. Chem. C 111 (2007) 2. [6] B.L. Kirsch, A.E. Riley, A.F. Gross, S.H. Tolbert, Langmuir 20 (2004) 11247. [7] (a) B. Chen, H. Zhang, B. Gilbert, J.F. Banfield, Phys. Rev. Lett. 98 (2007) 106103; (b) M.P. Finnegan, H. Zhang, J.F. Banfield, J. Phys. Chem. C 111 (2007) 1962. [8] (a) F. Huang, J.F. Banfield, J. Am. Chem. Soc. 127 (2005) 4523; (b) H. Zhang, J.F. Banfield, Chem. Mater. 17 (2005) 3421. [9] H.M. Lu, Q.J. Jiang, J. Phys. Chem. B 108 (2004) 5617. [10] (a) D.C. Bradley, R.C. Mehrothra, D.P. Gaur, Metal Alkoxides, Academic Press, London, 1978;
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