Pressure-Induced Disordering in SnO2 ... - ACS Publications

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Pressure-induced Disordering in SnO Nanoparticles Helainne T Girao, Thibaut Cornier, Stephane Daniele, Regis Debord, Maria A Caravaca, Ricardo Antonio Casali, Patrice Melinon, and Denis Machon J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 22 Jun 2017 Downloaded from http://pubs.acs.org on June 23, 2017

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The Journal of Physical Chemistry

Pressure-induced disordering in SnO2 nanoparticles Helainne T. Girão1, Thibaut Cornier2, Stéphane Daniele2, Régis Debord1, Maria A. Caravaca3, Ricardo A. Casali4,∞, Patrice Mélinon1, Denis Machon1,* 1 - Université de Lyon, F-69000 Lyon, France and Institut Lumière Matière, CNRS, UMR 5306, Université Lyon 1, F-69622 Villeurbanne, France 2 - Université́ Lyon 1, IRCELYON, CNRS-UMR 5256, 2 Avenue A. Einstein, 69626 Villeurbanne Cedex, France 3 - Departamento de Fisico-Quimica, Facultad de Ingenieria, UNNE, Av. Las Heras 727, C.P. 3500 Resistencia, Argentina 4 - Departamento de Fisica, Facultad de Ciencias Exactas y Nat. y Agr., UNNE, Av. Libertad 5600, C.P. 3400 Corrientes, Argentina

* [email protected] ∞ in memory of our respected collaborator Ricardo.

Abstract: The high-pressure behavior of SnO2 nanoparticles (~2.8 nm) is studied up to approximately 20 GPa using Raman spectroscopy in a diamond anvil cell and ab initio simulations. Above ~7 GPa, the disordering, initially located at the surface, propagates to the core of nanoparticles leading ultimately to amorphous-like spectra. This observation is interpreted as a disordering of the oxygen sub-lattice specially probed by Raman spectroscopy in contrast to powder X-ray diffraction techniques. The low frequency mode is related to the nanoparticle vibration as an elastic isotropic sphere motion. The pressure-induced shift of this mode allows for constraining mechanical properties data reported in the literature.

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I. INTRODUCTION Pressure-induced phase transition studies in nanomaterials are a bench test for thermodynamics at the nanoscale. An expected 1/R dependence of the transition pressure (R: particle radius) has been found experimentally1,2 and demonstrated theoretically using different models going from the Gibbs approach to the Landau theory of phase transition.3,4 However, such a situation seemingly straightforward covers more delicate aspects. First, experimental discrepancies are abnormally abundant. The transition pressure and high-pressure phases may vary depending on the sample source3 and/or the storage conditions (chemi- and physisorption).5 As underlined recently, when dealing with nanoparticles, the particle size is not the only pertinent parameter. When the diameter decreases, several size-dependent effects should be taken into account (quantum confinement, surface state, defects, etc). The phase stability should then be represented in a multidimensional plot.6 When studying the pressure-induced transitions in nanomaterials, one has access only to the projection of this multi-dimensional representation on the (P, R) plane. For instance, defects strongly influence the phase stability of nanomaterials, and they can promote size-dependent pressure-induced amorphization (PIA).7 SnO2 is a good candidate to explore such aspects as defects strongly influence its properties. Tin dioxide (SnO2) is an n-type semiconductor with a large band gap (Eg = 3.6 eV).8 SnO2 and SnO2-based nanodevices are applied as gas sensors,9,10 transistors, catalysts,11 dye-base solar cells, electrode materials (transparent and conductive12 or Li-ion battery13), electrochromic devices.10,14 Its properties such as particle size, crystallinity, electrical, morphological, structural are dependent on the synthesis route15 and on the presence of impurities and its stoichiometry is related to oxygen.16 SnO2 crystals have been studied under high-pressure as a bulk or as nanopowder samples mainly using X-ray diffraction techniques. As a bulk sample, SnO2 adopts a rutile-type structure (SG N°136 - P42/mnm) at ambient conditions. Under pressure, it transforms to a CaCl2-type structure (SG N°58 - Pnnm) at around 12 GPa through a ferroelastic transition driven by the softening of the B1g mode.17–19 Depending on the degree of hydrostaticity, a small amount of PbO2-type (SG N°60 – Pbcn) phase may appear.

17

At

pressure above 21 GPa, a cubic phase has been reported, and two structures have been proposed to


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describe this phase: (i) a pyrite-type (SG N°205 - Pa3 ) or (ii) a fluorite-type (SG N°225 - Fm3m) .17,20 In nanopowder, the initial structure is also a rutile-type. However, it is not clear whether the CaCl2-type phase is present under pressure as the broadening of the diffraction peaks due to the size prevents from observing subtle changes in the diffraction patterns associated with this second-order phase transition.21 However, a recent study with a micron-size X-ray beam spot on 30 nm-diameter particles confirmed the existence of such a second-order phase transition.22 Meanwhile, the transformation to the cubic phase can be observed in all studies. In a work on nanoparticles, it has been found that the transition pressure increased with decreasing particle size.21 For 3 nm particle diameter, no transformation to the cubic phase could be observed at pressure as high as 39 GPa.21 In the same study, for 8 nm and 14 nm, the transition pressures are 29 ± 2 GPa and 30 ± 2 GPa, respectively. It is worth noting that in addition to the shift of the transition pressures, the width of the transition (coexistence of phases) is extremely large as, in all cases, the low-pressure phase persists up to ~40 GPa. Such effect has been explained theoretically by Machon et al.3 using the Ginzburg-Landau approach. In a more recent study, 5 nm diameter SnO2 nanocrystals prepared by chemical precipitation method have been investigated under pressure. The transition from rutile-tocubic (eventually CaCl2-type to cubic) occurred at 18 GPa. Here again, the width of the transition is large as the phases still coexist at pressures ~33GPa. On decompression, the high-pressure phase may be retained,23 but sometimes the α-PbO2 is stabilized during the decompression path.24 One of these previous studies reported Raman spectra under pressure but the weak signal and a fluorescence background make it difficult to complete a proper analysis.25 To the best of our knowledge, no complete high-pressure study using Raman spectroscopy on SnO2 nanoparticles has been reported so far despite the complementary structural information provided by optical spectroscopies that have different chemical sensitivity and coherence length than X-ray diffraction techniques.26 In this work, we aim at exploring the pressure-induced phase transformation in SnO2 nanoparticles by Raman spectroscopy as it is a particularly powerful technique to study SnO2 crystal and nanoparticles.27 The paper is organized as follows. After introducing the experimental and numerical techniques, the sample characterization will be detailed with emphasis on the low-


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frequency region recorded in the Raman spectra. Pressure-induced evolution of the low-frequency mode will be shown and modeled using mechanical data from the literature. In addition, X-ray diffraction will be shown to support the use of these data. In the final part, the high-frequency region of the Raman spectra of SnO2 nanoparticles under pressure will be presented and discussed in the light of ab initio simulations.

II. METHODS A. Experimental 1. Synthesis and characterization at ambient pressure 7mL of Sn(OiPr)4(HOiPr)2 in 10% weight per volume isopropanol/toluene (1.97 mmol) were added to a refluxing aqueous solution (50mL) under vigorous stirring. After an immediate precipitation, the reflux was maintained for 2hrs. The solid was separated by centrifugation, was washed twice with deionized water and once with ethanol, and was then dried at 70°C for 20 h to give a powder. The morphology of the particles was examined by transmission electron microscopy (TEM, JEOL JEM-2010) and quasi-spheroidal nanocrystallites of about 2-3 nm were revealed (Figure 1).

Figure 1. Transmission Electron Micrographs of the as-prepared SnO2 sample. Excellent crystallinity may be inferred from these images.


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SnO2 adopts a rutile-type structure (Space group N°136 - D144h) with a unit cell formed by six atoms: two Sn atoms and four O atoms. Each tin atom is surrounded by six oxygen atoms, which are located at the corners of a regular octahedron.28 X-ray diffraction data shows that the sample is crystalline and may be indexed in its cassiterite (rutile) structure with cell parameters a ~ 4.74Å and c ~ 3.19Å, in agreement with the literature data (Figure 2).17 A Williamson-Hall plot29 gives an average size of ~2.7 nm with no residual stress. X-ray diffraction and electron microscopy techniques show a crystalline phase with a wellordered atomic network. However, these techniques are mainly sensitive to high-Z elements and, in a routine use, they give only little information of defects in materials such as in oxides where defects are mainly oxygen vacancies. Complementary techniques such as optical spectroscopy, for

40

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(321)

(310) (112) (301)

(211)

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instance, are required to get access to the low-Z sublattice.

Intensity (arbitr. units)

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80

[email protected]Å

Figure 2. X-ray diffraction pattern of the as-prepared SnO2 sample. The peak positions and relative intensities of the rutile structure are plotted. The diffraction pattern may be indexed in this structure with usual cell parameters a ~ 4.74 Å and c ~ 3.19 Å.


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Raman spectra were collected using the Labram HR Evolution Spectrometer, which allows the acquisition of spectra down to 5 cm-1 (Fig. 3a). The Raman spectrum is in good agreement with previous publications.30 The vibrational representation of the normal vibration modes at the center of the Brillouin zone is given by Γ = A1g +A2g + B1g + B2g + Eg + A2u + 2 B1u + 3 Eu

(1)

Four of those modes are infrared-active (A2u and Eu), four modes are Raman-active (A1g, B1g, B2g, Eg) and two are silent (A2g and B1u). In the Raman-active modes, the oxygen atoms vibrate while Sn atoms are at rest, making Raman spectroscopy a technique particularly sensitive to changes in the oxygen sublattice. The modes A1g, B1g, and B2g vibrate in the plane perpendicular to the c axis while the doubly degenerated Eg mode vibrates in the direction of the c axis. B1g mode is a libration mode related to oxygen atoms rotating around the c axis31,32. Raman spectra for single crystals have been reported and the positions of three peaks are well established with ω(Eg) ~ 475 cm-1, ω(A1g) ~ 634 cm-1, and ω(B2g) ~ 776 cm-1

31

(Fig. 3(a)). However, the B1g mode shows

surprising features. As it is a libration mode, it is difficult to observe and is absent in several spectra reported in the literature. Its position has also been a matter of debate and has been reported as lying from 87 to 184 cm-1 (18,33,34 and references therein). When considering nanocrystals, the Raman spectrum of SnO2 is strongly modified (Fig.3a). Xie et al.35 and Abello et al.30 reported the drastic changes that appear when reducing the particle size that are interpreted based on surface modes and defect-related effects.35–37 The understanding of the features associated to a nanosize effect have been studied35,37–39 and in particular by Diéguez et al.27,37 Along with a broadening and a shift of the classical modes of SnO2, apparently in agreement with a spatial correlation model,27 two main bands were found to grow with the proportion of surface atoms in the high-frequency region of the spectrum.30 Many reasons have been invoked to explain the appearance of new peaks and broadening of the existing peaks: phonon confinement models, surface modes, disorder-induced breaking of selection rules.27 Besides the usual Raman modes, additional low-frequency modes (below 100 cm-1) may be measured in nanoparticles. These bands are due to confined acoustic vibrations and are attributed to spheroidal vibration modes of a spherical elastic body.40 These low-frequency vibrations are usually


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considered from a classical point of view by considering the particle acoustic vibration as a whole. A theoretical explanation for the origin of such low-frequency vibrations was first introduced by Lamb41 and recently developed by Saviot et al.42 The Lamb solution is for a homogeneous, elastically isotropic continuum sphere and the frequency of the vibration is given by

ω = S1, 0

vl L.c

(2)

where vl is the longitudinal sound velocity ( vl = 6.53 × 105 cm.s-1),31 L is the particle size, c is the light velocity, and S1, 0 is a coefficient ( S1, 0 = 0.887).27 Low-frequency Raman scattering proves to be very effective in providing the size of very small nanoparticles

27

and, using this relation, the average diameter can be determined and has been

estimated to be centered at around 2.6 nm, in good agreement with the XRD pattern and TEM images. In addition, the particle size distribution can be extracted, as shown by Diéguez et al.,27 by comparing size distributions obtained by Raman spectroscopy and TEM measurements. Here, Figure 3b shows the extracted particle size distribution of our sample. The particle size is mainly comprised between 2 and 4 nm.

(a)

Nano

Bulk B1g 100

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A1g

Eg 300

(b)

Normalized size distribution

Relative intensity (Arbitr. units)

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Figure 3. (a) Raman spectrum of the SnO2 nanoparticles used in this study compared to the one of a bulk sample. Ticks show the position of the Raman peaks in a bulk sample. (b) Size distribution of the particles deduced from the low-frequency peak.


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2. X-ray diffraction at high-pressure A High-pressure X-ray diffraction experiment was conducted with a microfocus Oxford Xcalibur Mova diffractometer operated with a molybdenum source compatible with our DAC setup. High pressure was produced using a screw-driven diamond-anvil cell (DAC). SnO2 nanoparticles were placed inside a 250 µm chamber drilled in an indented stainless steel gasket, along with a ruby acting as a pressure gauge.43 No pressure-transmitting medium has been used. The highest pressure achieved was 13.0 GPa.

3. Raman spectroscopy at high pressure Raman spectra were obtained using the LabRAM HR Evolution Raman Spectrometer (Horiba Scientific) using an excitation energy of 532 nm and a power set at 5 mW at the entrance of the DAC to avoid heating. The beam was focused on the sample using a 50x objective, with beam diameter ~2 µm at the sample. The scattered light was collected in backscattering geometry using the same objective. High pressure was generated using a membrane DAC 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 to prevent from further interactions with the PTM. The pressure was probed by the shift of the R1 fluorescence line of a small ruby chip.

B. Numerical simulations Density Functional Theory (DFT) and its implementation in several electronic structure codes have proven to be reliable tools to study the solid-state bulk and small aggregate of atoms (clusters) with defects, surfaces and interfaces. Recent developments of localized ab initio implementation of DFT by means of pseudo-potentials and localized atomic wave functions were introduced in SIESTA code. 44 This flexible implementation allows the study of systems in bulk45 and in aperiodic systems with a larger number of atoms than in other methods. An example of this is a recent study of stability and thermodynamic properties of SnO2 nanoparticles which include up to 1309 atoms.46 In that study, surface formation energies and thermodynamic stability of


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nanoparticles were estimated as a function of O2 pressure and temperature. In addition, electronic properties of the surfaces, complex formations, and details about relaxations of atoms inside, close, and at the surface of the nanoparticles were investigated. These electronic, structural, and physicochemical properties were correlated with the changes in the nanoparticle stoichiometry. In that study for the search of minimum energy structure of each particle, an annealing method with variable finite temperature technique improved the conventional conjugate gradient method. The present study is an extension of that work, but using target pressure and temperature of the Parrinello-Rahman technique47,48 together with a Nose thermostat. The time step was fixed between 1 and 3 femtoseconds, and the number of steps was in the order of at least 2000, making a total relaxation time of at least 3-to-6 picoseconds. The system investigated in these simulations is a nanoparticle of approximately the same size as the particles studied experimentally. Such sophisticated simulation on such a large system is state-of-the-art to understand the interface impact on the system evolution under pressure.

III. RESULTS AND DISCUSSION The low-frequency region was studied under high pressure (Figure 4). The main peak is related to confined acoustic vibrations and was fitted using a log-normal function. The Raman peak position shifts to higher frequencies as pressure increases. The low-pressure peak may be correctly fitted up to 12 GPa. At pressures between 7 and 12 GPa, the low-frequency region is modified. At higher pressure, a new peak increases in intensity while the initial low-frequency peak decreases in intensity. At pressure P ~ 20 GPa, a component centered around 200 cm-1 is of similar intensity of the initial low-frequency peak. These changes are concomitant with a broadening of the highfrequency peaks and the loss of the structuration of the Raman spectra that will be detailed hereafter. The pressure-induced shift of the peak is related to the changes in both R, the radius of the particle, and vL. The pressure dependence of the frequency related to the variation of vL and R with pressure is: vL = ((B +(4/3)G)/ρ)1/2 ,

(3)

with


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R = ((3M)/(4πρ))1/3,

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(4)

B and G being bulk and shear moduli, respectively, ρ being the mass density, and M the mass of a spherical nanoparticle. The pressure-dependent factor in the position of the Raman peaks is therefore (B + (4/3)G)1/2ρ−1/6 for the spheroidal vibration peak.49 Calculating the vibration frequency at all pressures is not straightforward. It requires the knowledge of bulk and shear modulii values at ambient pressure but also their pressure dependencies, and these values show some dispersion in literature. Haines et al.17 reported a bulk modulus of 205 ± 7 GPa and its pressure-derivative was found to be 7.2 ± 2.0. In another study, He et al.21 found B0 = 252 ± 10 GPa for a fixed value of B0’ = 4. Concerning G and its pressuredependencies, data are obtained using the Brillouin measurements obtained by Hellwig et al.18 We tested the different sets of B0 and B0’ values to calculate the frequencies as a function of pressure and found an excellent agreement with the data by He et al. (Figure 4b). Other data gave a highly overestimated pressure dependency. The values of He et al. were obtained on a similar system (particles of 3 nm) whereas data by Haines were obtained on a micrometric powder. Our result is in favor of a possible increase of the mechanical properties of nanoscale materials (see for instance50 even though the effect of non-hydrostaticity usually cannot be estimated for nanoparticles because of aggregation and inter-grain contacts.


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(a)

P(GPa)

(b)

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Intensity (Arbitr. Units)

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Figure 4. (a) Pressure-induced evolution of the low-frequency range. (b) Peak position as a function of pressure along with the calculated variation (black line) using B0 and B0’ values reported by He et al.21


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Experimental data BM EOS

1.00

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10.7

Relative Intensity

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0.99 0.98 0.97 0.96

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Figure 5. (a) XRD diffraction (λ = 0.7093 Å) patterns with increasing pressure. The asterisk shows the diffraction signal coming from the stainless-steel gasket. (b) Relative volume as a function of pressure (points) along with a second order Birch-Murnaghan equation of state (line) using B0 = 252 GPa. X-ray diffraction data are shown in Figure 5. The maximum pressure obtained during this experiment is 13.0 GPa. Because of the size-induced broadening of the peaks, no subtle change can be analyzed during compression. For instance, the second-order phase transition to the CaCl2-type is not detected. It requires larger particles and/or smaller beam size to avoid the pressure gradient leading to an apparent coexistence of phases.22 This result is in agreement with previous works on nanoparticles of the same size.21 It is worth noting at this point that the oxygen sublattice is hardly accessible through X-ray diffraction because of the Z-contrast. Volume as a function of pressure has been extracted (Fig. 5(b)) and the relative volume has been fitted using a second-order Birch-Murnaghan equation of state with B0 = 252 GPa, the value used to fit the pressure-induced variation of the low-frequency Raman mode. The agreement


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between experimental data and this model equation of state is satisfactory up to 12 GPa. At higher pressure, structural changes occur as discussed in the following. Figure 6a shows the pressure-induced evolution of the Raman spectra in the range 100 cm-1 to 850 cm-1. The high-frequency region of the Raman spectra of SnO2 nanoparticles under high pressure was fitted using a minimal number of functions to reproduce the spectra. Five Gaussian components were sufficient to model the spectra and to analyze the pressure-induced evolution.

(a)

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19.8

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Figure 6. (a) Raman spectra of SnO2 nanoparticles with increasing pressure (b) Pressure-induced evolution of the centers of the Gaussian functions. Lines are the results of the fits of these positions (see Table I). Dashed line at ~13.8 indicates the observed structural change leading to broad features in the spectra. Their pressure-dependencies are plotted on figure 6b and the slopes ∂ω / ∂P are indicated in Table 1. Two peaks are related to the ones observed in bulk samples. The peak at ~ 633 cm-1 corresponds to the A1g mode ( ∂ω / ∂P =4.9 cm-1. GPa-1 in bulk) and the one at 778 cm-1 corresponds


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to the B2g mode ( ∂ω / ∂P =5.4 cm-1. GPa-1 in bulk). Additional modes have been proposed to result from the surface and disorder states. A change in the general shape of the broad features, ranged between 400 and 800 cm-1, is observed above 7.2 GPa. At pressures higher than 13.8 GPa, new features between 200 and 300 cm1

start to emerge in the spectra. It is difficult to relate this change to the transition to the CaCl2-type

structure even though this transition should occur in this pressure range. At 19.8 GPa, two broad features (~200 cm-1 and ~600 cm-1) are observed. The structuration initially present in the spectra has disappeared and amorphous-like spectra.

Table 1. Peak position at ambient pressure and their pressure dependencies Initial Peak Position (cm-1)

Slope (cm-1. GPa-1)

778(1)

4.0(1)

633(1)

4.6(1)

575(1)

2.1(2)

515(1)

1.1(2)

434(1)

2.5(3)

To obtain more structural information about the pressure-induced changes, ab initio simulations have been carried out. Figure 7 shows the results of ab initio simulations on a similar system (a particle with a diameter of 2.8 nm containing 984 atoms). The internal order of the particle can be studied by means of the Pair Distribution Function (g (r)) for the two atomic species Sn and O. At P = 0 GPa, the nanoparticles are characterized by a slight disorder at the surface but with a wellordered core. This situation changes under high pressures and the disorder, initially located at the surface, increases significantly above 6 GPa deduced from the spreading of the Sn-O distances. Under pressure, the O-O pair distances diminished appreciably and the two peaks, located at 2.58 and 2.92 Å, change their distribution towards only one broad peak located near 2.60 Å at 19 GPa. Similar behavior is found for Sn-Sn pairs, with peaks in the gSnSn(r) distribution located at 3.26 and 3.73 Å merging into one broad peak located near to 3.40 Å at 19 GPa. This could be due to a


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distorted lattice including a possible phase transition to a high-density phase like a distorted fluorite-structure, e.g. the Pa3 (pyrite) phase, or α-PbO217 in some part inside the nanoparticle. Interestingly, the inner core of the nanoparticle remains in the rutile structure (being not so much distorted) at all range of pressures.

Figure 7. Snapshots obtained by ab initio simulations of a particle with a diameter of 2.8 nm (984 atoms) at different P = 0, 9 and 19 GPa. The internal order of the particle can be studied by means of the Pair Distribution Function (g(r)), for the two atomic species Sn and O. Therefore, the broadening of the Raman spectra for pressures between 7 and 12 GPa may be related to the increasing of disorder of the shells of nanoparticles. At higher pressures, new features are observed. The combination of phase transitions (to the CaCl2-type structure at P~12 GPa and to the pyrite-type structure at P~19 GPa), of disordering, and of nanosize effects leads to a Raman spectrum resembling vibrational density of states and is similar to an amorphous-like spectrum.51 However, alternative hypotheses may be considered and discussed. First, this may be due to a fragmentation of the nanoparticles in smaller domains due to the first-order nature of the transition to the cubic phase. The new peak located at around 200 cm-1 would then be of the same nature as the peaks at ~ 75 cm-1 (acoustic vibrations of the particles considered as an elastic medium) but for


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smaller particles. However, the Raman signal almost reverses when pressure is brought back down, in particular, the position of the low-frequency spheroidal mode is inversely proportional to the particle size (Figure 8a). This observation rules out the hypothesis of the fragmentation of the nanoparticles in smaller domains as such phenomenon would be irreversible. Secondly, the peak centered around 200 cm-1 in the amorphous-like spectra has been assigned to vibrations of sub-stoichiometric SnO2-y. Such decomposition effect is indeed invoked to describe the amorphous state as an intermediate, kinetically hindered state as in the case of SnI452. However, here again, reversibility rules out this possibility. The reversibility upon the pressure release may be due either to a relaxation of mechanical stresses or a structural reversibility from cubic to rutile initiated by the core of the nanoparticles remaining in the rutile-type structure, according to the simulations and acting as a nucleus for the back transformation. It is worth noting that the low-frequency peak position after pressure release is slightly shifted to the low-frequency side (Figure 8b). It may look like the average particle size increased during the compression (Figure 8c). However, potential source of frequency variation (and then of this apparent size variation) could be the change in the shape of the nanoparticles under pressure associated with a plastic deformation, for instance, and/or residual mechanical stresses affecting the mechanical properties.


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(a)


P(GPa)

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After cycle Before cycle

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0.8 8.4 10.9 13.5 19.8 0

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After cycle

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-1

Raman shift (cm )

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2

3

4

5

6

7

8

Average grain size (nm)

Figure 8. (a) Raman spectra upon decompression showing the reversibility of the pressureinduced transformations. (b) Raman spectra before and after the pressure cycle (c) Distribution of the particle size using the low-frequency region of the Raman spectra before and after the pressure cycle.

IV. CONCLUSIONS SnO2 nanoparticles of ~2-3 nm have been investigated under high-pressure both experimentally (Raman spectroscopy and X-ray diffraction) and numerically (ab initio simulations). If such nanoparticles have already been studied using powder X-ray diffraction (PXRD), this is the first report of a complete study using Raman spectroscopy. PXRD on particles of this size concluded that no important structural changes occur at pressure as high as 30 GPa, indicating that the transition pressure to the high-pressure cubic phase is drastically shifted due to size effect. Raman spectroscopy combined with ab initio simulations provides complementary information. A first stage, from ambient pressure to 6-7 GPa, does not show noticeable changes. At higher pressure, the disorder, initially located at the surface of the particles, starts to propagate to the core


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of the nanoparticles. Spectra at high pressures are very similar to the ones of an amorphous state indicating a possible pressure-induced amorphization (PIA). This seems to be in contradiction with X-ray diffraction data on SnO2 that has never reported PIA whatever the nanoparticles size and the synthesis routes. However, the seeming paradox can be explained by the chemical selectivity associated with both experimental techniques. X-ray diffraction is affected by the heaviest atoms (Sn in this case) whereas Raman spectroscopy is mainly sensitive to the oxygen sublattice, particularly in the case of rutile-type structure where the Raman-active modes only involve oxygen displacements. Such sublattice disordering has already been reported,53 in particular in Eu2(MoO4)3.26 In compounds with a high contrast on the Z of the constituent atoms (such as some oxides), the combination of different characterization techniques is required to obtain complementary information on the structure and its evolution. In this work, no pressure-transmitting medium has been used. The compression is nonhydrostatic even though, in nanoparticles, because of multiple contact points, the transmission of pressure is rather uniform

54

. Using a pressure-transmitting medium does not necessarily ensure

hydrostaticity because nanoparticles have tendency to aggregate and the PTM may not surround each nanoparticle. This point was discussed in ref. [3] on ZnO nanoparticles where we compared the pressure-induced phase transitions with methanol-ethanol as the PTM and without any PTM. No difference on the transition features could be observed. However, no hydrostaticity is known to favor amorphization under pressure53. This effect has been discussed in ref [3] where a GinzburgLandau approach is used to detail the size-dependent pressure-induced amorphization. This transformation is kinetically favored and is controlled by the so-called Ginzburg term. This latter one gathers different effects together, non-hydrostaticity being one of them. Even tough, up to now, non-hydrostaticity has not been found to be a preponderant term in amorphization in nanoparticles, this cannot be discarded with the available data. Further works using helium has the PTM is required to determine whether this sub-lattice disordering may be related to the lack of hydrostaticity.


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ACKNOWLEDGMENTS We would like to thank the National Council of Technological and Scientific Development (CNPQ), Brazil) for H.T.G. funding and Ms Laurence Burel and Ms Marlène Daniel from IRCELYON for TEM studies.

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