Amorphization in Nanoparticles - The Journal of Physical Chemistry C

Article pubs.acs.org/JPCC

Amorphization in Nanoparticles Lucas Piot,† Sylvie Le Floch,† Thibaut Cornier,‡ Stéphane Daniele,‡ and Denis Machon*,† †

Institut Lumière Matière, UMR5306 Université Lyon 1-CNRS, Université de Lyon, 69622 Villeurbanne Cedex, France Université Lyon 1, IRCELYON, CNRS-UMR 5256, 2 Avenue A. Einstein, 69626 Villeurbanne Cedex, France

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ABSTRACT: Nanoparticles have more propensities for amorphization than their bulk counterparts, opening the opportunity to achieve the amorphous states in well-known poor glass-former compounds. Classical size effects are often invoked to explain such phenomenon. However, this argument is not sufficient to cover all experimental results such as functionalization effect. In this work, Y2O3 nanoparticles of 7 nm diameter are investigated under pressure. Special care is taken on the surface state of the particles by comparing the pressure-induced transformation of nanoparticles stored under different conditions (atmospheric vs argon environment). A clear difference is reported as one batch shows a transformation to an amorphous state, whereas the second undergoes a crystal-to-crystal transition. These results are discussed in terms of interface energy taking into account not only the usual surface energy but also the surface state contribution. The link between different types of amorphization (pressure-, mechanical-, and radiation-induced amorphization) in nanoparticles is underlined as a critical defect density is required to achieve the crystal-to-amorphous transformation. Defect creation may arise from multiple sources: irradiation, functionalization, and reconstructive transition.

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INTRODUCTION

This mechanism must be exploited to obtain amorphization in materials that are recognized as being difficult to amorphize. In bulk materials, some compounds are poor-glass formers, and the synthesis of the amorphous state of such materials may be highly challenging (e.g., ZrO2, TiO2, Y2O3, etc.). However, amorphization may be induced in such materials in nanoparticles with higher propensity to form a glass by creating a sufficiently high defect density (by irradiation or ball-milling, for instance). Recently, a new process for amorphizing nanoparticles has been reported: size-dependent pressure-induced amorphization. Under a critical diameter, an amorphous state is observed under pressure instead of a high-pressure polymorph. For instance, it has been shown that, under a diameter of 10 nm, TiO2 nanoparticles undergo pressure-induced amorphization (PIA), whereas nanoanatase transforms to a baddeleyite structure when the diameter exceeds 10 nm.10,11 Such size dependent pressure-induced amorphization has also been reported in Y2O3 nanoparticles.12 When the particles diameter is above 21 nm, the initial cubic structure exhibits a transition to a hexagonal structure phase at 14 GPa. A different scenario is observed for particles with a diameter of 16 nm, for which the cubic phase transforms to an amorphous state at 25 GPa. Apparently, nanoparticles of TiO2 and Y2O3 undergo pressure-induced amorphization when a critical diameter of the particles is reached. However, it has been recently shown that a reduced particle size is not sufficient in order to explain

At the nanoscale, materials do exhibit physical and chemical behaviors different from the bulk situation. On the one hand, properties vary in a nonlinear fashion as the size decreases, leading to potential applications. The key point is that for a given physical property with its characteristic length, nanostructures with lower sizes obey classically to a scaling factor with possibly additional quantum effects.1 On the other hand, the contribution of the surface energy in the energetic balance can lead to the stabilization of new polymorphs at ambient conditions that may exhibit original properties.2 For instance, bulk TiO2 adopts a rutile structure, whereas TiO2 nanoparticles crystallize in anatase structure;3 these nanoparticles are widely used for their photocatalytic properties.4 Another striking example of phase equilibria modified by surface effects is the melting process.5 In small particles, the effect on the melting temperature has been known since 1871 when Thomson described that the melting point decreases with the inverse of the particle radius according to the Gibbs−Thomson equation. This has been experimentally evidenced in nanoparticles.6−8 On a microscopic point of view, melting has also been related to the chemical disordering that may be induced by point defects using the generalized Lindemann melting criterion.5,7,9 In nanoparticles, the surface corresponds to an extended defect, and it should therefore be associated with a positive excess entropy. Extra configurational entropy can also be introduced into the surface by the formation of surface defects. Thus, the formation of point defects in nanoparticles leads to a favorized stabilization of the liquid phase. If this happens at a sufficiently low temperature, an amorphous state may be observed. © XXXX American Chemical Society

Received: January 31, 2013 Revised: April 20, 2013

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dx.doi.org/10.1021/jp401121c | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. (Top) (a) Experimental protocol of the present study. A batch of nanoparticles is divided into two samples. Sample A is stored under air atmosphere, whereas Sample B is kept under argon atmosphere. After storage, both samples were subjected to high pressure conditions in a highpressure diamond-anvil cell. (Bottom) (b) Transmission electron microscopy image, (c) X-ray diffraction pattern, and (d) Raman spectrum of starting Y2O3 nanoparticles. The size ranges between 7 and 10 nm. The particles adopt a cubic C-type structure.

pressure, ball-milling, etc.) and the role devoted to defect density. In the first part, we show the effect of the surface state on pressure-induced transformations in Y2O3 by carefully controlling the environment prior to pressure exposure. The drastic and unexpected effect of air on the nanoparticles is key to understand previously reported size-dependent pressureinduced amorphization. On the basis of these results, we propose a unifying scheme relating particle size, defect density, and amorphization. This phenomenological approach displays the relationship between different amorphization processes in nanoparticles by applying thermodynamic considerations and the concept of energy landscape.

the amorphization process. In TiO2, the possible high-pressure transformations (crystalline baddeleyite vs amorphous) in nanoparticles with d < 10 nm depend also on the degree of functionalization.13 Adsorbed molecules at the surface induce disorder leading the transition pathway to an amorphous state at high-pressure. Therefore, the notion of surface energy is not sufficient and interface energy should be privileged by taking into account the surface state (defects, chemistry, etc.). The understanding of the key parameters necessary to tune the high-pressure transformation may have useful practical applications. This opens the opportunity to adjust the properties related to the degree of crystallinity of the recovered nanomaterials. As is found in amorphous bulk materials, nanosized amorphous materials exhibit unique physical and chemical properties as compared with their crystalline counterparts with potential applications in science and technology.14−21 For instance, it has been found that amorphous nanoparticles can have advanced catalytic properties compared to the traditional (nano)crystalline catalysts.22,23 In addition, they may display specific photoluminescence24 or enhanced bioactivity.25 Thus, understanding the critical parameters that control amorphization at the nanoscale is crucial in nanomaterials design. The question that arises is the possible relationship among different amorphization processes in nanoparticles (radiations,

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EXPERIMENTAL METHODS Y2O3 nanoparticles were synthesized following the procedure described in ref 26 through hydrolysis of Y5O(OtBu)13 and calcination of the as-prepared powder at 900 °C for 4 h in air. The same batch of Y2O3 nanoparticles was divided into two samples: Sample A was cooled down and stored at ambient conditions, while Sample B was kept within an argon atmosphere following the same experimental protocol (Figure 1a). The powder X-ray diffraction (PXRD, Bruker D8 Advance A25) patterns of both Y2O3 samples were similar consisting only of crystalline cubic Y2O3 (JCSPD no. 00-043-1036). The average particle size, as estimated from line-broadening B



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Figure 2. Raman spectra of cubic Y2O3 nanoparticles stored under atmospheric conditions (sample A) (a) with increasing pressure (up to ∼26.0 GPa) and (b) during decompression. Pressure-induced amorphization is observed around 22.4 GPa.

Figure 3. Raman spectra of cubic Y2O3 nanoparticles stored under argon atmosphere (sample B) (a) with increasing pressure and (b) during decompression. No pressure-induced amorphization is observed, and a phase transition to a hexagonal phase occurs around 20.0 GPa.

transformation in Y2O3 nanoparticles, a PTM would interfere by introducing an additional interface energy term. It has to be noted that, in nanoparticles, because of multiple contact points, the transmission of pressure is rather uniform.28 Raman spectra of our samples of Y2O3 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. The casual heating of the sample by the laser was checked by recording spectra at different incident powers. An incident power of 10 mW was found to be convenient for the present study. Infrared spectra were recorded between 5000 and 370 cm−1 using a Perkin-Elmer 2000 FTIR spectrometer. The

according to the Scherrer equation, was about 7−8 nm for both samples. The morphology of the particles was examined by transmission electron microscopy (TEM, JEOL JEM-2010) and dense, homogeneous, quasi-spheroidal nanocrystallites of about 7−10 nm were revealed (Figures 1b,c). Both sample batches were studied under high pressure using Raman spectroscopy. It has to be noted that it has been recently demonstrated that no coalescence occurred with nanoparticles (more specifically in the case of ZrO2) by monitoring the vibrations of the nanoparticules under high stresses.27 In all experiments, high-pressure was generated using a membrane diamond-anvil cell (DAC) with low-fluorescence diamonds. Nanoparticles were placed into a 125 μm chamber drilled in an indented stainless steel gasket. No pressuretransmitting medium (PTM) was intentionally used during these experiments. Since the aim of the present study is to understand the effect of interface energy on the phase C



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phase transition. This means that the storage conditions prior to pressure exposure play an important role in the phase transformation mechanisms of Y2O3 nanoparticles. It has been underlined in previous studies that the Y2O3 highpressure polymorphism is sensitive to the presence of defects. Thus, Wang et al.31 compared the high-pressure polymorphism in Y2O3 and Y2O3:Eu3+ (molar ratio Y3+:Eu3+ = 99:1). In pure yttria, the C-type structure transforms to a hexagonal phase (Atype in sesquioxides classification) above 12.1 GPa, which persists up to a pressure of 34.5 GPa.31 On decompression, a Btype (monoclinic symmetry) is recovered. In doped Y2O3:Eu3+, the phase sequence upon increasing pressure is as follows: Ctype → B-Type → A-type with transition pressures at 7.9 and 25.8 GPa, respectively. On decompression, the A-type phase back-transforms to the B-type phase that can be recovered to ambient conditions. May this high sensitivity to doping/impurity be invoked to understand the results shown in the present studies? In the previous work on high-pressure Y2O3 nanoparticles, the pressure-induced amorphization has been attributed to an increase of the free energy associated with grain boundaries.12 It has been proposed that with increasing grain boundary fraction, i.e., with decreasing size, a critical diameter must be reached, below which the increase of energy is so large that the amorphous state is stabilized. This interesting model requires a more detailed analysis. If amorphization is purely due to an increase of surface energy, a smaller particle size would lead to a lower transition pressure. However, in our experiments, amorphization is observed at approximately the same pressure as in ref 12 despite the fact that the present particles are half the size. Actually, the set of experimental evidence is similar to the ones obtained in TiO2.13 In this work, it has been shown that the pressure-induced amorphization in nanoparticles was not only dependent on the particle size but also on the surface state. Therefore, the same mechanism should apply here to explain the results in Y2O3. The pressure-induced amorphization is related to nanoparticles with a diameter below a critical value. However, this requirement may not be sufficient, and a definite degree of defects should be attained to alter the crystalto-crystal transition to an amorphization process. In the case of Y2O3, it seems that the storage under atmospheric conditions has a direct impact on the surface state. FTIR spectra obtained for samples A and B are shown in Figure 4. The main difference is the intensity of the peaks located at ∼1404 and ∼1521 cm−1 for sample A. These peaks correspond to the ν3 asymmetric stretching modes of CO32− ions due to Lux−Flood acid−base reaction between atmospheric CO2 and the Y2O3 surface, which results additionally in surface disordering. As a matter of fact, it has been reported previously that a large surface area of ultrafine yttria is responsible for undesired adsorption of atmospheric gases, such as CO2 on the particle surfaces.32 Therefore, two sources of defects may be identified in Y2O3 nanoparticles: (i) defects related to the surface state modified by the presence of CO32− ions and (ii) intrinsic defects induced by the first-order, reconstructive phase transition under pressure. The structural reorganization at ambient temperature leads to a defective structure due to the slow kinetics, which cannot be avoided. In the case of Y2O3 nanoparticles exposed to atmospheric conditions and subsequently pressurized, the two different types of defect (i and ii) add up and create a

standard KBr pellet method was used. Y2O3 powders were diluted to 1 wt % in KBr.

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RESULTS AND DISCUSSION The previous group-theoretical analysis indicates that 22 Raman-active modes are expected for cubic Y2O3. Among them, only 14 can be observed by polarized-Raman study on single-crystals due to overlaps of modes. The main intense peak is centered at ∼380 cm−129 (Figure 1d). Let us first describe the results obtained on sample A nanoparticles stored under atmospheric conditions. In Figure 2 is shown the pressure-induced change of the Raman spectra. At very low-pressure (0.4 GPa), the Raman spectrum displays an intense peak at 380 cm−1 and several broad and weak peaks at 137, 160, 328, 437, 470, and 594 cm−1. All peaks are related to different vibration modes of cubic Y2O329 in agreement with the X-ray diffraction results. The main peak at ∼380 cm−1 shifts toward high wavenumber by ∼3.1 cm−1·GPa−1 up to a pressure of 18.6 GPa. Above 17.1 GPa, a strong background develops, and at pressure higher than 20.5 GPa, broad features are present in the spectra along with a weak remaining peak indicating a small amount of the crystalline phase. Above 22.4 GPa, this peak completely disappears and one may conclude that the sample underwent a PIA in agreement with the previous results.12 On decompression (Figure 2b), the broad feature centered at ∼430 cm−1 at 25.9 GPa shifts back to lower wavenumbers. A broad band at low frequency, weak at high pressure, increases in intensity below 1.6 GPa but without any other apparent modification in the Raman spectra. It is worth noting that, in some spectra (for instance, the one at 3.1 GPa in Figure 2b), weak peaks can be observed at ∼143 and 160 cm−1. These features can be attributed to the presence of a small amount of a monoclinic phase as detailed in the following section. Nanoparticles in sample B come from the same synthesis. However, the sample was kept under argon atmosphere, and the high-pressure cell loading was performed in a glovebox. Surprisingly, the evolution of Raman spectra under compression is different from sample A as shown in Figure 3. The lowpressure spectrum contains several resolved peaks, which can be attributed to active Raman modes of cubic Y2O3. In addition, no background is observed at the lowest pressure. The quality of the spectra for sample B is due to a lower defect density as discussed hereafter. The high-pressure behavior is rather different compared to sample A. Even though some background becomes detectable above 21.1 GPa, followed by the complete disappearance of the main peak characteristic of the cubic phase at 25.5 GPa, the nanoparticles remain crystalline throughout the pressure experiment. The strong background shows some structure, and a relatively sharp low-frequency peak centered at ∼180 cm−1 is clearly a signature of a crystalline phase. Moreover, the positions of the observed peaks allow identifying the high-pressure phase as being hexagonal following results by Husson et al.30 Thus, the same high-pressure polymorphism as in bulk Y2O3 is observed here.31 The main difference is the transition pressure: 12 GPa in bulk yttria and 17−25 GPa in nanoparticles. On decompression, Y2O3 transforms to a monoclinic phase, a conclusion based on the splitting of the low-frequency mode (Figure 3b).30 In summary, both samples show clear differences in their high-pressure behavior. The sample stored in air exhibits a pressure-induced amorphization, whereas an identical sample kept under an argon atmosphere undergoes a crystal-to-crystal D



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(cubic) phase. At a critical pressure, the Gibbs energy of this phase is equal to that of the hexagonal phase, which has a smaller molar volume. This thermodynamic transition pressure is about 12 GPa in Y2O3 for the cubic phase.31 When the sample consists of nanoparticles, it can be expected that the transition pressure increases as a result of the surface energy.34−36 This is demonstrated with our experiment using sample B where the cubic-to-hexagonal transformation occurs above 20 GPa. For a fixed chemical composition and particle size, the transition pressure in different samples should be rather similar and toward the same high-pressure phase as the surface energy increase is identical. However, it is usual to observe discrepancies depending on the synthesis method.13 In this case, the surface state is a major parameter and an interface energy should be invoked. The results of experiment on sample A are fully demonstrative of this effect. In this case, an additional energy term related to the defective surface should be taken into account. If this contribution is high enough, the corresponding pressure increase reaches the energy field of the amorphous state. This description is based on classical thermodynamics taking into account surface/interface energy to the Gibbs potential. It allows to describe the effect of grain size on pressure-induced transformations, specifically in the present study, size-dependent pressure-induced amorphization. However, it is important to put this type of crystal-toamorphous transformation in a broader picture and to relate it to other amorphization processes. The competition between the transformation to a crystalline phase or an amorphous state is governed by a delicate balance between thermodynamic and kinetic factors. Amorphization may occur if the nucleation and growth of an equilibrium crystalline phase is kinetically hindered. To better represent this aspect, the use of the configurational energy landscape concept may be useful. This landscape can be depicted as a multidimensional plot of the potential energy (E) corresponding to different local and long-

Figure 4. Fourier-transform infrared spectra of samples A and B. Strong absorption bands at ∼1404 and ∼1521 cm−1 for sample A indicate the presence of CO32− ions.

sufficiently high amount of defects to change the crystal-tocrystal transformation to a crystal-to-amorphous. On the basis of the similarity of melting and amorphization, it has been proposed that the crystal-to-amorphous transformation can be interpreted as defect-induced melting of metastable crystals driven beyond the critical state of disorder at which the melting temperature falls below the glass transition temperature. Solid-state amorphization occurs when the increase of the free energy of the original crystalline phase is equal or higher than the free energy of the amorphous state, which is believed to be an undercooled liquid without transformation to other crystalline phases.33 For the interpretation of our results, Figure 5 shows a thermodynamic picture of different Y2O3 morphologies at high pressure. The G(P) representation is useful to understand the effect of reduced size and defects on transformation pathways at high pressure. For bulk samples, pressure induces an energy increase proportional to the molar volume of the low-pressure

Figure 5. Gibbs energy as a function of pressure for three different morphologies of the same compound: bulk material, nanoparticles and defective nanoparticles. Surface energy considerations are responsible for the increase of the transition pressure (nanoparticles) or pressure-induced amorphization (defective nanoparticles). E



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Figure 6. Energy landscapes of two nanocrystalline Y2O3 samples at high pressure. Sample A contains a high defect density due to formation of CO32− ions at the surface. Under pressure, a transformation to the amorphous state is kinetically favorable for this sample. Crystal-to-crystal transformation occurs in sample B with a low defect density.

energy barrier between two crystalline phases, which undergo reconstructive phase transformations, is higher due to bond breaking and atomic movements, amorphization may be easier to achieve in such materials. In contrast, displacive crystalline phase transformations with a relative low energy barrier may be favorable over crystal−amorphous transformations. Our experimental data fully agree with this model. In the case of bare nanoparticles, the increase of energy minima and of the kinetics barrier is not enough to switch toward the amorphous state. In the case of defective nanoparticles, the combination of surface energy and of the effect of defects is sufficient to obtain a position of the potential barrier above the one governing the crystal-to-amorphous path. The cubic-to-hexagonal transition is of reconstructive nature and leads to defect creation that facilitates the transformation to the amorphous state. This scheme applies also to other amorphization processes, such as the radiation-induced amorphization, which has been shown to occur only when a critical defect concentration is reached.38 The influence of defects on the amorphization of nanoparticles has been reported in previous irradiation experiments.39 As a function of irradiation dose, point defects, complex defects, and lattice strain are being accumulated in the structure and increase the free energy of the crystalline compound, which results in the formation of a high-energy metastable phase. In the case of bulk ZrO2, ion irradiation induces a transformation from the initial monoclinic phase to a tetragonal or cubic phase.39 In our interpretation scheme, this means that the monoclinic phase is increasingly destabilized as compared with the tetragonal or cubic phase due to the accumulation of defects. However, the energy increase is not sufficient to surpass the energy barrier governing the transformation path to the amorphous state. This situation is different for nanocrystalline ZrO2 (3 nm), where radiationinduced amorphization is observed.39 Thus, the combination of size and defect density plays a crucial role in the amorphization of nanozirconia, one of the most radiation-resistant ceramics. In this situation, both surface and defect energies bring the kinetic barrier sufficiently high to promote the amorphous state. Ball milling is a sample preparation method to reduce the grain size and induce a certain level of defects. This technique is

range arrangements of atoms in the system and to the type of bonding.37 Each arrangement constitutes a configuration that is often shown schematically in two or three dimensions as a plot of E vs the generalized configurational coordinate (Figure 6). Each crystalline polymorph in the system corresponds to a deep energy minimum. For either temperature or pressure increase, a different crystalline polymorph becomes more stable and a polymorphic transition takes place. The ease of this transition is dictated by the height of the energy barriers separating the two phases and the transformation pathway between them, involving breaking and reforming of bonds and displacement of relative atomic coordinates. Additionally to the deep and sharp energy minima representing ordered, crystalline phases, a landscape of the amorphous states is formed by a number of shallow minima or basins of similar energies, separating the regions of stability by relatively small energy barriers. Figure 6 shows a possible energy landscape at the transition pressure between the cubic and hexagonal phase for Y2O3 nanoparticles with a high (sample A) and low (sample B) defect density, respectively. In the latter case, the energy minima of both crystalline phases are equally deep but separated by a potential energy barrier that governs the kinetics of this transition process. In the case of a high defect density (sample A), the energy minimum of each crystalline phase is significantly increased, whereas the landscape of amorphous states is only slightly affected, as amorphous states are a defective and flexible structure for which the relaxation processes are facilitated. At a critical defect density, the relative height of the energy barrier between cubic and hexagonal phases may exceed the critical value for the cubic-to-amorphous transformation, resulting in a kinetically preferred crystal−amorphous transformation instead of a cubic-to-hexagonal transformation. In summary, two critical factors govern the amorphization process: (i) the depth of the energy minima representing different crystalline states and (ii) the height of the energy barrier with respect to the amorphous megabasin, which can be increased by the number of defects (doping, irradiation, etc.), surface/interface energy, strain, etc. Whenever this increase is sufficiently high, the initial crystal structure may be destabilized in favor of a new, noncrystalline atomic configuration. Since the F



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well-known for producing amorphous nanoparticles. When the defect concentration is large enough, a defect-induced solidstate amorphization may take place.40 The requirement of a critical grain size for amorphization has been clearly demonstrated for silicon, where nanocrystalline and amorphous sample forms coexist.41 Above a critical grain size, solid-state amorphization is inhibited. Interestingly, Y2O3 and ZrO2 amorphize during ball-milling,40,42 which is also related to an energy increase due to both surface energy and defects. Thus, independently of the type of the amorphization process (pressure-, radiation-, or mechanically-induced), the underlying thermodynamic explanation may be identical. It has to be noted that, in some cases, defect recovery is more effective in nanosized materials compared with the bulk condition due to the proximity of free surfaces.43 In these situations, a critical defect density may be difficult to obtain, and no amorphous state may be observed.

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AUTHOR INFORMATION

Corresponding Author

*(D.M.) E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully thank one of the referees for his/her advices that improved our article.

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REFERENCES

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CONCLUSIONS The high-pressure polymorphism of Y2O3 nanoparticles was investigated in the present study. Two different samples have been used, which were obtained through the same synthesis but stored within different environments. The first sample was kept at ambient atmosphere before loading in a high-pressure cell. Raman spectra confirmed a pressure-induced amorphization starting at ∼18 GPa. The second sample was kept under an argon atmosphere prior to high-pressure experiments, which revealed a clear crystal-to-crystal phase transformation at ∼20 GPa (completed at ∼25 GPa). A monoclinic structure was recovered to ambient conditions. The difference in the highpressure polymorphism was explained by surface chemistry. Under atmospheric conditions, yttria nanoparticles have the tendency of carbonation, whereas this reaction cannot proceed under an argon atmosphere as demonstrated by FTIR. Pressure-induced amorphization requires a critical particles size at the nanoscale (bulk Y2O3 does not amorphize under pressure) but also a certain degree of defects. In the present case, this critical degree of defects is obtained when the system undergoes a reconstructive phase transition creating an amount of intrinsic supplementary defects. Pressure-induced amorphization with its dependence on particle size and defect density can be linked to other solid-state amorphization processes by applying the energy landscapes concept. The elastic energy induced by high pressure coupled with the interface (surface and defects) energy allow to explore the different structural configurations at ambient temperature, including a quenching to a megabasin dominated by an amorphous state. Similarly, nanoparticles subject to radiation or ball-milling have similar energy landscapes, which depend on grain size (surface energy) and defect contributions. If both requirements are fulfilled, a crystal-to-amorphous transformation can be induced by these treatments. This concept can be applied to poor-glass forming compounds to obtain an amorphous state, which can be further studied. For instance, amorphous TiO2 nanoparticles have been studied under high-pressure and revealed surprising behavior. Depending on the starting amorphous states, different sequences of amorphous−amorphous transformations have been obtained.44 Therefore, the exploration of different amorphous states becomes possible, which provide fundamental insights and new opportunities for the design of unique nanomaterials. G



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Amorphization in Nanoparticles - The Journal of Physical Chemistry C

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