Interface Energy Impact on Phase Transitions: The Case of TiO2

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Interface Energy Impact on Phase Transitions: The Case of TiO2 Nanoparticles Denis Machon,*,† Marlene Daniel,‡ Pierre Bouvier,§,|| Stephane Daniele,‡ Sylvie Le Floch,† Patrice Melinon,† and Vittoria Pischedda† †

Universite Lyon 1, LPMCN, CNRS-UMR 5586, 69622 Villeurbanne Cedex, France Universite Lyon 1, IRCELYON, CNRS-UMR 5256, 2 Avenue A. Einstein, 69626 Villeurbanne Cedex, France § Laboratoire des Materiaux et du Genie Physique, CNRS, Grenoble Institute of Technology, MINATEC, 38016 Grenoble, France European Synchrotron Radiation Facility ESRF, BP 220, 6 Rue Jules Horowitz, 38043 Grenoble Cedex, France

)

‡

bS Supporting Information ABSTRACT: We investigate the interface energy impact on phase stability using the shining example of TiO2 nanoparticles under pressure. We revisit the previously reported phase diagram of this system and propose a new mechanism allowing the control of pressureinduced amorphization of TiO2 ultrafine particles. We demonstrate that the size effect is necessary for stabilizing the amorphous state but is not sufficient in the sense that surface chemical functionalization of nanoparticles is determinant. This discovery opens the possibility to select the high-pressure phase in nanomaterials and, consequently, the recovered structure under ambient conditions.

’ INTRODUCTION Nanosized materials show extensive new physical and chemical properties compared with bulk samples. It is now wellestablished that nanoscience will be a base for a new technological revolution, and, in fact, some nanomaterials are already used for industrial applications.1 Therefore, they have been the focus of an extensive research interest in the attempt to understand, control, and tune the stability and the properties of functional nanomaterials. Phase stability in finite materials can be modified by surface energy control. However, if the concept of surface energy exists to describe free nanoparticles properties, this model can hardly be generalized to describe phase transitions, in particular, in highpressure experiments. As a matter of fact, nanoparticles are embedded in a medium, a situation that creates an interface energy. The nature of this interface (chemical interaction, defects, etc.) may determine the phase equilibria. The use of pressure as a control parameter is fundamental because it allows us to explore the energy landscape of nanoparticles emphasizing the contribution of surface energy in phase stability. It has been shown that high-pressure polymorphism can differ in nanoparticles and bulk samples. Previous works established a new type of phase diagram using pressure and particle size as parameters. In this approach, TiO2 has gained special attention because of its outstanding properties (TiO2 is currently used in photocatalysis, optical filters, electronic devices, ceramic industries, as well as a chemical sensor or pigment) and for its extremely rich polymorphic behavior with varying size, temperature, and pressure conditions. Below a particle diameter of 50 nm, the rutile (space group: P42/mnm) structure is no longer the most stable phase and the anatase phase (space group: I4/amd) is r 2011 American Chemical Society

stabilized by surface energy effect.2 This size-induced structural change is partially responsible for the enhancement of the UVB absorption properties. Applying large stresses to TiO2 nanoparticles led to interesting observations. First, it was shown that an intermediate high-pressure phase (α-PbO2 columbite space group:Pbcn) usually observed around 3 to 4 GPa in bulk TiO2 was suppressed when dealing with nanocrystals.3 Second, the transition pressure to the next high-pressure phase (monoclinic baddeleyite space group: P21/c) is dependent on the particle size.2 Recently, it has been proposed that below 10 nm, TiO2 nanoparticles undergo a pressure-induced amorphization instead of a transition to the usual baddeleyite phase.2 5 The mechanism proposed for an isolated 4 nm size particle suggests that the disorder initiates in the surface crust of the particles and propagates in the core with increasing pressure.4,6 In addition, an extremely rich pressure-induced polyamorphism in nanoanatase has been evidenced, emphasizing further the complexity of the energy landscape at the nanoscale in this compound.7 Here to better understand the role of surface-dependent stability of nanoparticles, we compare the high-pressure behavior of ultrafine bare TiO2 with citrate-surface functionalized TiO2 nanoparticles. Pressurizing the samples in a diamond anvil cell and using synchrotron X-ray diffraction and Raman spectroscopy, we investigate the sensitivity of TiO2 phase stability in relation to the surface chemistry. We demonstrate that the presence of a small proportion of citrate onto the surface leads Received: August 25, 2011 Revised: October 5, 2011 Published: October 05, 2011 22286

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The second and third samples of nanohybrid titania, that is, TiO2 nanoparticles with citrate molecules at the surface, have been produced according to ref 9. In the present study, we used (Cit)1(TiO2)100 and (Cit)5(TiO2)100 giving the initial titanium to citrate molar ratio used in the synthesis. In the following, the two samples will be referred to as TiO2@cit1 and TiO2@cit5, respectively. These syntheses led to well-crystallized nanoparticles with a mean diameter of 6 nm and a narrow size distribution as demonstrated by HRTEM investigations.8,9 In all experiments, high-pressure was generated using a membrane diamond-anvil cell with low-fluorescence diamonds. Nanoparticles were placed into a 125 μm chamber drilled in an indented stainless steel gasket. No pressure-transmitting medium (PTM) was intentionally used during these experiments. Because the aim of the present study is to understand the effect of interface energy on the thermodynamics of TiO2 nanoparticles, the effect of a PTM would interfere in introducing an additional interface energy term. Raman spectra of our three samples of TiO2-based nanoparticles were obtained using a customized high-throughput optical system based on Kaiser optical filters and an Acton 300i spectrograph (gratings 1800) with sensitive CCD detection. Samples were excited using 514.5 nm radiation from an air-cooled Ar+ laser. The beam was focused on the sample using a Mitutoyo 50 objective, with beam diameter ∼2 μm at the sample. The scattered light was collected in backscattering geometry using the same objective. Synchrotron radiation measurements were performed at beamline ID27 at ESRF via angle-dispersive diffraction techniques. We used a monochromatic beam with a 0.3738 Å wavelength selected by an iodine K-edge filter and focused to a beam size of ∼3 μm. The signal was collected on a MarCCD (345) area detector. The sample-to-detector distance and the image plate inclination angles were calibrated using a LaB6 powder standard. The 2D diffraction images were analyzed using the FIT2D software,10 yielding 1D intensity versus diffraction angle 2θ patterns. The fluorescence of ruby was used as a pressure gauge.11

Figure 1. Raman spectra obtained with increasing pressure for (a) 6 nm bare TiO2 particles (a transformation above 19.1 GPa to a crystalline phase is observed) and (b) 6 nm TiO2@Cit1 particles (TiO2 coated with citrate molecules). A transformation above 21.2 GPa to an amorphous state is observed. Arrows show the peak at ∼510 cm 1 reminiscent of the baddeleyite structure.

to a switch between a crystal-to-crystal and a crystal-to-amorphous pressure-induced transition.

’ EXPERIMENTAL SECTION Three samples of TiO2-based nanoparticles have been studied in the present work. The first one is bare TiO2 synthesized by sol gel process using ammonium bromide salts as catalysts.8

’ RESULTS AND DISCUSSION Figures 1a,b show the Raman spectra of 6 nm bare TiO2 and TiO2@cit1 under pressure, respectively. In both compounds, the starting phase is anatase. For bare TiO2 nanoparticles, no particular change is observed up to 18 GPa. Above this pressure, a new peak around 510 cm 1 appears. This is concomitant with a broadening of the low-energy peak. These spectral evolutions are the signature of a sluggish pressure-induced transformation that is completed at pressures >25 GPa. The peaks from the highpressure crystalline phase are rather broad and are associated with an emerging background resulting from the increasing disorder in the structure. This disorder is expected because this transition is reconstructive. The high-pressure spectra strongly resemble the ones obtained for larger TiO2 particle sizes12 and are assigned to the baddeleyite structure. The high-pressure phase has been identified using synchrotron X-ray diffraction (Figure 2a). The structural transition toward a baddeleyite crystalline phase above 18 GPa is confirmed. The diffraction patterns at 30.9 GPa are indexed using a monoclinic space group P21/c with cell parameters a = 4.54(1) Å, b = 4.91(1) Å, c = 4.71(1) Å, and β = 95.8(1)° in good agreement with literature values for bulk counterparts.13 In both experiments, an emerging background may indicate that part of the sample became highly 22287

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Figure 2. (a) Synchrotron X-ray diffraction of 6 nm TiO2 bare particles with increasing pressure. (b) Synchrotron X-ray diffraction patterns of 6 nm TiO2 particles at ambient pressure (anatase), at high pressure (baddeleyite) and recovered from the high-pressure treatment (columbite). X-ray pattern at 30.9 GPa is indexed in a monoclinic phase (space group: P21/c) with cell parameters a = 4.54(1) Å, b = 4.91(1) Å, c = 4.71(1) Å, and β = 95.8(1)°. X-ray pattern of the recovered sample is indexed in a orthorhombic phase (space group: Pbcn) with cell parameters a = 4.53(1) Å, b = 5.50(1) Å, and c = 4.95(1) Å.

disordered (possibly amorphous). However, our main observation is to point out that a part of the sample remains crystalline. On decompression, we observe the usual downshift of the Raman peaks (Figure 3a). However, below 10 GPa, new features appear such as a new peak at 150 cm 1 and a change in relative intensity of several peaks. These new spectral features indicate that a phase transition has occurred and is reported here for the first time. Unfortunately, it was not possible to study this phase by X-ray diffraction as during the decompression, we missed the narrow range of pressure where this phase is stabilized. Therefore, the structure of this phase remains to be established. Below 2.5 GPa, this intermediate phase disappears, and a Raman spectrum typical of the columbite structure is observed. The recovered phase is confirmed to be the α-PbO2 columbite structure by X-ray diffraction (Figure 2b). The surprising result is that this high-pressure behavior has never been observed before for anatase particles of diameter below 10 nm.14 In

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Figure 3. (a) Raman spectra obtained with decreasing pressure for 6 nm bare TiO2 particles. An intermediate phase is observed with the appearance of a new Raman peak at ∼150 cm 1 (arrow). This phase transforms to the columbite structure below 2.5 GPa. (b) Raman spectra obtained with decreasing pressure for 6 nm TiO2@Cit1 particles (TiO2 coated with citrate molecules). These spectra are in agreement with those of ref 5. The low-pressure spectra can be assigned to a mixture between an amorphous state and partially recrystallized columbite.

nanoparticles of these sizes and obtained through different chemical synthesis leading to a pollution of the particles surfaces, a pressure-induced amorphization of the anatase structure and a polyamorphic transformation has been observed.4,5,7 In a second experiment, we studied pressure-induced evolution of 6 nm TiO2@cit1 particles (Figure 1b). No clear sign of phase transition could be observed up to 21.2 GPa. However, at higher pressure, we observed a broadening of the spectra, typical of an amorphous state. A very weak peak centered at ∼510 cm 1 may indicate the presence of a small amount of baddeleyite. This transition is completed at 28.5 GPa. On decompression, rather sharp peaks are observed below 3.3 GPa. These may be due to a partial recrystallization in the columbite crystalline structure (Figure 3b). 22288

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Figure 5. Phase diagram including size, pressure, and surface functionalization as control parameters. In the absence of functionalization, the anatase structure in nanoparticles will transform to the baddeleyite phase (at least for d > 6 nm). In the case of surface functionalization, the defects density (see the text) will orientate the transformation to an amorphous state for d < 10 nm. This critical value is taken from the literature, but the degree of functionalization was not known in these studies.2 5 The higher the density of defects, the lower the transition pressure.

Figure 4. Raman spectra of 6 nm TiO2 @Cit5 particles (TiO 2 coated with citrate molecules) obtained (a) with increasing pressure (a transformation above 16.8 GPa to an amorphous state is observed) and (b) with decreasing pressure. The amorphous state is observed down to 1.6 GPa. Below this pressure, a recrystallization mainly to an anatase phase can be obtained because the laser used in Raman spectroscopy experiments heated the sample.

A third high-pressure experiment has been conducted using 6 nm diameter TiO2@cit5 particles. Figure 4a shows the Raman spectra recorded during the pressurization of these 6 nm TiO2@Cit5 particles. The starting phase is anatase. We can notice some broadening of the Raman peaks that is expected because of some disordering induced by the functionalization. Above 16.8 GPa, the sample almost transforms totally to an amorphous structure. During decompression (Figure 4b), this amorphous state is observed down to 1.6 GPa. At lower pressures, the amorphous state has the tendency to recrystallize mainly into anatase structure under the laser irradiation, which probably induce some heating of the particles. In Figure 5, we summarize these results with a phase diagram including three control parameters: pressure, size, and surface functionalization rate (at 0, 1, and 5% molar of citrate with respect to TiO2 content). We can first point out that the presence of a very low quantity of citrate at the surface of particles prevents the anatase-baddeleyite

transition and favors a pressure-induced amorphization instead. Second, the amorphization pressure is lowered as the quantity of citrate increases. On the basis of the results from literature,5 amorphization occurs below a critical diameter size of ∼10 nm for functionalized nanoparticles. We here demonstrate that contrary to previous propositions, the surface energy is not the only critical parameter inducing the stabilization of the amorphous state for particle diameters smaller than 10 nm. This seems to be in good agreement with predictions based on energetic considerations on the nucleation and growth of a spherical nucleus in a matrix which estimate that the minimum diameter size to obtain a metastable baddeleyite (or any other crystalline) phase should be ∼4 nm.12 A concomitant major result linked to the absence of amorphization in pure anatase is that we stabilized the columbite structure in a particle of 6 nm. This opens the possibility to obtain materials with modified or amplified properties by nanoconfinement as columbite presents an electronic structure different from those of rutile and anatase.15 To the best of our knowledge, this is the first observation of a crystalline columbite phase in ultrafine (d < 10 nm) TiO2. The apparent disagreement between our results and the literature results is lifted when one considers the 6 nm sample with 1% of citrate added during the synthesis process and located at the surface of the particle. In this case, the pressure-induced amorphization is indeed observed in the same pressure range as the anatase-baddeleyite transition in bare TiO2 nanoparticle, and these results are in agreement with the literature. In particular, our spectra at high pressure of nanohybrid anatase (TiO2@cit1) strongly resemble the ones obtained by Swamy et al.5 for TiO2 nanoparticles with diameter size below 10 nm. In our Raman spectrum at 33.3 GPa (Figure 1b), after amorphization, we 22289

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The Journal of Physical Chemistry C observe a peak at 500 cm 1 in agreement with ref 5. This peak is absent in the spectra of the high-density amorphous state obtained by the pressurization of nanometric amorphous TiO2.7 This additional peak is centered at the position of the most intense Raman peak of the baddeleyite phase (Figure 1a) suggesting a coexistence of a high-density amorphous state and a disordered (or short-range ordered) baddeleyite structure. By X-ray absorption, it has been shown that ultrafine TiO2 nanoanatase undergoes a transition to an intermediate-range crystalline phase prior to amorphization and that the same batch of nanoanatase can follow different structural pathways.16 The frustrated attempt of a transition to a crystalline phase would also explain the presence of additional peaks during decompression (observed in our spectra (Figure 3b) and in ref 5) that may correspond to a disordered (or poorly crystalline) columbite phase. Therefore, we suggest that to observe a transition to an amorphous state under pressure two conditions are required: a critical particle diameter and a sufficient amount of surface functionalization. Below a critical size (likely ∼10 nm) and for a sufficient amount of functionalization the crystal crystal phase transition is inhibited and an amorphous state is observed. The question to address now is: what is the mechanism leading to this change of regime under pressure? From recent results concerning the effect of irradiation on TiO2 structures,17,18 it has emerged that the anatase phase shows the most propensity for metamictization (transformation from a crystalline phase to an amorphous phase under the effect of irradiation). Above some degree of defects, this structure undergoes an amorphization because of its structural configuration.17 In our particular case, surface functionalization induces surface defects through charge transfer as TiO2 has a marked ionic character.19 The citrate molecules are capable of making up to three bonds with the titanium atoms. One can calculate that for TiO2@cit1 almost 7% of the Ti atoms at the surface are bonded to citrate molecules. For TiO2@cit5, this proportion goes up to 33%. From the broadening of the peaks in the X-ray diffraction patterns of these two nanohybrids (see X-ray diffraction patterns in ref 9), we can propose that a disordering effect has been induced in the structure by the bonding of the citrate molecule at the surface of the TiO2 nanoparticles. Using numerical simulations, it has been shown that the pressure-induced amorphization in TiO2 nanoparticles is initiated at the surface4,6 through bonding distortion and defects and propagates in the core of the particle. Therefore, we propose that the citrate molecules adsorbed at the nanoparticle surface induce an initial disorder that propagates inside the particles under pressure. The increasing disorder will orientate the transition path to an amorphous structure instead of a crystalline structure, as would happen in the absence of citrate. This mechanism is particularly adapted to the anatase structure, being the most sensitive structure (existing under ambient conditions) to transform to an amorphous state in the presence of defects.17,18 The link between pressure-induced amorphization and metamictization has already been described theoretically by Toledano et al. as resulting from a similar mechanism based on a spatial distribution of the primary order parameter.20,21 It is then natural to suggest that the high-pressure evolution of ultrafine anatase TiO2 nanoparticles is dependent on the degree of disorder at the surface that may be enhanced by surface functionalization. This treatment may modify the transition path, lower the transition pressure, or both, as it has been observed in our work. This scenario is coherent with the present data and the

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existing literature. However, it remains hypothetical and requires further experimental or numerical evidence.

’ CONCLUSIONS In summary, our observations deeply modify the picture of the size-effect on pressure evolution of TiO2 nanoparticles. Results relating pressure-induced amorphization driven by size-effect that neglect chemical contribution should be re-examined. Functionalization allows tuning from a surface energy contribution to an interface energy contribution that modifies the phase stability. Other works have previously underlined the importance of surface chemistry to structural stability even under ambient conditions.1,22 This is amplified on the nanoscale as the surface is extremely important. Here we showed that by pressurizing 6 nm diameter bare TiO2 nanoparticles we did not encounter amorphization. However, adding a small amount of organic molecules (citrate) at the particle surface allows for switching from a crystalto-crystal transition to an amorphization. Therefore, the size effect is necessary for stabilizing the amorphous state but is not sufficient. This effect has to be combined with a chemical effect. We relate the chemical impact to the introduction of defects at the surface of particles through charge transfer that will drive the transition toward a disordered phase. We also showed that it is possible to obtain new metastable structures of TiO2 with diameters below 10 nm (columbite in our case) that could possibly have new interesting physical and chemical properties. From an applications point of view, the possibility to tune high-pressure transformations through chemical control may prefigure synthesizing new metastable phases under more industrially accessible conditions.23 Finally, it is worth noting that our results underline the necessity that highpressure experiments should be done after a careful characterization of the nanoparticles. Moreover, the interface energy between the PTM and the nanoparticles may modify the phase stability and should be considered in the interpretation of highpressure works. ’ ASSOCIATED CONTENT

bS

Supporting Information. Williamson-Hall plots for the starting anatase phase and the recovered columbite phase after a pressure cycle. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ REFERENCES (1) Chen, X.; Mao, S. S. Chem. Rev. 2007, 107, 2891. (2) Swamy, V.; Kuznetsov, A.; Dubrovinsky, L. S.; Caruso, R. A.; Shchukin, D. G.; Muddle, B. C. Phys. Rev. B 2005, 71, 184302. (3) Wang, Z. W.; Saxena, S. K. Solid State Commun. 2001, 118, 75. (4) Pischedda, V.; Hearne, G. R.; Dawe, A. M.; Lowther, J. E. Phys. Rev. Lett. 2006, 96, 035509. (5) Swamy, V.; Kuznetsov, A.; Dubrovinsky, L. S.; McMillan, P. F.; Prakapenka, V. B.; Shen, G.; Muddle, B. C. Phys. Rev. Lett. 2006, 96, 135702. (6) Lowther, J. E. High Pressure Res. 2006, 26, 131. (7) Machon, D; Daniel, M.; Pischedda, V.; Daniele, S.; Bouvier, P.; LeFloch, S. Phys. Rev. B 2010, 82, 140102(R). 22290

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(8) Goutailler, G.; Guillard, C.; Daniele, S.; Hubert-Pfalzgraf, L. G. J. Mater. Chem. 2003, 13, 342. (9) Mendez, V.; Caps, V.; Daniele, S. Chem. Commun. 2009, 21, 3116. (10) Hammersley, A. P.; Svensson, S. O.; Hanfland, M.; Fitch, A. N.; Hausermann, D. High Pressure Res. 1996, 14, 235. (11) Mao, H. K.; Bell, J.; Shaner, W.; Steinberg, D. J. J. Appl. Phys. 1978, 49, 3276. (12) Hearne, G. R.; Zhao, J.; Dawe, A. M.; Pischedda, V.; Maaza, M.; Nieuwoudt, M. K.; Kibasomba, P.; Nemraoui, O.; Comins, J. D. Phys Rev. B 2004, 70, 134102. (13) Haines, J.; Leger, J. M. Physica B 1993, 192, 233. (14) Nanoparticles have a tendency to form aggregates, especially if the high-pressure treatment is combined with a temperature cycle. (See Wang, Y.; Zhao, Y.; Zhang, J.; Xu, H.; Wang, L.; Luo, S.-N.; Daemen, L. L. J. Phys.: Condens. Matter 2008, 20, 125224.) To verify if aggregation had occurred, increasing the size of the particles before and after the transition pressure, we used the Williamson plot analysis. (See the Supporting Information.) The average size of the recovered columbite structure was still centred to 6 nm, discarding any sintering effect. In addition, this effect on the nanoscale requires some compatibility between particle facets, which is not the case here as the nanoparticles show a pronounced spherical symmetry.8 (15) Kuo, M. Y.; Chen, C. L.; Hua, C. Y; Yang, H. C.; Shen, P. J. Phys. Chem. B 2005, 109, 8693. (16) Flank, A.-M.; Lagarde, P.; Itie, J.-P.; Polian, A.; Hearne, G. R. Phys. Rev. B 2008, 77, 224112. (17) Lumpkin, G. R.; Smith, K. L.; Blackford, M. G.; Thomas, B. S.; Whittle, K. R.; Marks, N. A.; Zaluzec, N. J. Phys. Rev. B 2008, 77, 214201. (18) Lumpkin, G. R.; Blackford, M. G.; Smith, K. L.; Whittle, K. R.; Zaluzec, N. J.; Ryan, E. A.; Baldo, P. Am. Mineral. 2010, 95, 192. (19) Guo, Y. Y.; Kuo, C. K.; Nicholson, P. S. Solid State Ionics 1999, 123, 225. (20) Toledano, P.; Machon, D. Phys. Rev. B 2005, 71, 024210. (21) Toledano, P.; Bismayer, U. J. Phys.: Condens. Matter 2005, 17, 6627. (22) Barnard, A. S.; Curtiss, L. A. Nano Lett. 2005, 5, 1261. (23) Grey, I. E.; Li, C.; Madsen, I. C.; Braunshausen, G. Mater. Res. Bull. 1988, 23, 743.

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