Article pubs.acs.org/Langmuir
Ag Nanoparticles from the Mechanochemical Decomposition of Ag Oxalate Francesco Delogu* Dipartimento di Ingegneria Meccanica, Chimica e dei Materiali, Università degli Studi di Cagliari, via Marengo 2, I-09123 Cagliari, Italy ABSTRACT: The present work focuses on the chemical reactivity of Ag oxalate powders under mechanical processing conditions. The powders were submitted to mechanical loads in the presence of an aqueous solution containing a polymeric surfactant. A gradual decrease of the total mass of powders was observed, ascribable to the occurrence of a decomposition process. X-ray diffraction and UV−vis spectrophotometric analyses indicated that the Ag oxalate decomposes into metallic Ag and gaseous carbon dioxide. Transmission electron microscopy showed that metallic Ag exists in the form of particles with average size of about 5 nm. The formation of nanometer-sized Ag particles can be related to the plastic deformation and attrition processes taking place at the points of contacts between neighboring particles during the mechanical loading at collision.
1. INTRODUCTION Mechanochemistry can be defined as the branch of chemistry that investigates the effects of nonhydrostatic mechanical stresses and plastic strain on the chemical processes inducing a change of the energy and entropy as well as of the structure and chemical composition of molecules, crystals, and other aggregates of matter.1−6 In the case of molecular systems, mechanochemical effects directly stem from the directional character of mechanical stresses.6−9 Purely mechanical forces are able to selectively break and reform covalent bonds in individual molecules, thus promoting or depressing the chemical reactivity of covalent systems.6−9 The scenario changes in the case of solids. Mechanical forces can be no longer selectively applied to individual chemical species located at the sites of the crystalline lattice.10−14 Rather, even the most localized mechanical stresses can affect a relatively large volume as a consequence of the interaction forces operating across the lattice.15 Therefore, atomic, ionic, or molecular solids exhibit a cooperative response to mechanical deformation, mediated by the nucleation and migration of point, line, and plane lattice defects.16 Due to the different intensity of interactions between the elementary units forming the crystal, atomic, ionic, and molecular solids must be expected in general to exhibit a different response to mechanical deformation.15,16 For example, the absence of direct chemical bonds between individual molecules, as well as the relative weakness of intermolecular forces, permit the plastic deformation of molecular crystals at low mechanical stresses.16,17 Molecules can be displaced from their original position, and the crystalline lattice considerably disordered, without any noticeable chemical behavior due to the activation of intramolecular bonds.17 Conversely, the chemical behavior of metallic phases is intimately connected with their plastic deformation.18−25 In the regions of solid in which mechanical stresses are more intense, © 2012 American Chemical Society
unusual mass transport processes take place, which induce in turn the intimate mixing of atomic species.18−25 Coordination shells are severely affected, finally resulting in new crystalline, or amorphous, chemical systems.18−25 Compared with the above-mentioned molecular and metallic solids, ionic crystals can exhibit in principle a broader spectrum of behavior.26 Cations and anions strongly interact with each other through very intense electrostatic forces, which give rise to high cohesive energies.26 At the same time, the intensity of electrostatic forces also results in the brittle character of ionic crystals.26 Even relatively small deformations can produce a brittle fracture as a consequence of the intense repulsive forces arising when ions of the same charge face each other.26 On this basis, it can be expected that ionic crystals could be significantly reduced in size during the mechanical processing, which has been experimentally observed.27 Whereas the behavior described above is common to all of the ionic crystals, it must be noted that broader responses to deformation can be obtained when either cations, or anions, or both, exhibit relatively complicated structures. In this case, the intense electrostatic forces arising in connection with deformation processes can significantly destabilize the molecular ions. More specifically, deformation can give rise to intense intramolecular stresses originating from electrostatic repulsion and attraction. In turn, these can potentially activate the rupture of the covalent chemical bonds, and then induce a strongly localized chemical reaction.28 In the past, mechanically induced chemical reactions have been profitably used to fabricate various nanometer-sized systems, including Ag nanoparticles.29−31 For example, mechanochemical methods have been employed to synthesize active catalysts by depositing small Ag clusters at the surface of Received: April 30, 2012 Revised: June 27, 2012 Published: July 1, 2012 10898
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a support material.31 Also, it has been shown that carbon nanotubes can be decorated with Ag nanoparticles smaller than 4 nm by mechanically processing a solid mixture of carbon nanotubes and thermally decomposable Ag salts, such as acetates.30 Therefore, the mechanochemical methods can be legitimately included in the list of methodologies allowing the synthesis of Ag nanoparticles.32 With the aim of providing an original contribution along such line, the present study focuses on a specific chemical reaction of Ag oxalate (Ag2C2O4) crystals activated by the mechanical processing. The anhydrous Ag2C2O4 crystal is a white solid with a monoclinic P21/m crystalline structure, the different chemical species forming approximately planar chains of Ag+ cations and C2O42‑ anions.33 It has been known for long time that Ag2C2O4 starts decomposing around 373 K, with formation of Ag crystals and gaseous carbon dioxide (CO2).34,35 In the past, the thermal decomposition process has been thoroughly investigated as a paradigmatic case study for solid state chemical kinetics.36−39 It is characterized by an initial induction period, followed by the rise of the decomposition rate to a maximum value, and its subsequent decrease.39 The process exhibits a marked sensitivity to the details of preparation methods, to additives, and to external perturbations related to electric fields, irradiation, and mechanical deformation.39 In this latter case, it has been shown that the mechanical processing of Ag2C2O4 powders facilitates the thermal decomposition process, possibly inducing an explosive-like regime.39 Starting from these premises, the present study shows that, under suitable conditions, mechanical stresses operating at collisions can induce the decomposition of anhydrous Ag2C2O4 powders, with formation of nanometer-sized Ag particles and gaseous CO2.
designed mechanochemical reactor. It consists of a stainless steel cylindrical reactor, with a rounded bottom base, roughly 20 cm in height, and 2 cm in diameter. A stainless steel ball of 20 g was placed inside the reactor together with a total mass mp of powder equal to 2 g. Afterward, 20 mL of the aqueous solution containing PVP was added to the powder. The solution roughly occupies 24% of the reactor volume. Then, the reactor chamber was sealed under Ar atmosphere and the reactor fixed on a mechanical arm that undergoes a harmonic oscillation along the vertical direction. In the present case, the device was operated at amplitude and frequency values so as to determine a collision every 5 s. Collisions occurred with an average energy of about 0.08 J. Due to the characteristics of the reactor displacement, collisions occur exclusively on the bottom base of the reactor. Here, the ball traps part of the Ag2C2O4 powders, and dissipates its energy, which is completely transferred to the powders. The top basis of the reactor is equipped with a gas-tight septum connector, which allows the sampling of the gas and liquid phases. A schematic description of the reaction chamber is shown in Figure 1.
2. EXPERIMENTAL OUTLINE Anhydrous Ag2C2O4 crystalline powders were dispersed in distilled water containing 5 g L−1 of polyvinylpyrrolidone (PVP), and mechanically processed under Ar atmosphere. The reactivity of this chemical system was investigated by using gravimetric, diffractometric, and spectrophotometric methods as well as electron microscopy. The degree of decomposition was related to the number of mechanical deformation cycles carried out by ball drop experiments. 2.1. Materials. Anhydrous Ag2C2O4 crystals were synthesized starting from sodium oxalate (Na2C2O4) powders with a nominal purity higher than 99%, purchased from Aldrich. An aqueous solution of 0.5 M Na2C2O4 was prepared. A 60 mL portion of this solution was mixed with 100 mL of an aqueous solution of 0.5 M Ag nitrate (AgNO3) (Aldrich, 99.9%) under magnetic stirring at room temperature. The Ag+ cations and the C2O42‑ anions formed by the dissociation of Na2C2O4 rapidly form a white precipitate consisting of Ag2C2O4 hydrated crystals. The precipitate was filtered, washed three times with distilled water, and finally dried in an oven at 110 °C for 10 h. Afterward, it was placed into a dark glass bottle, and sealed under Ar atmosphere. The procedure was repeated every time new amounts of anhydrous Ag2C2O4 powders were needed. Mechanical activation experiments were performed on such powders. Regarding PVP, it is worth remembering that it is a homopolymer formed by a backbone of polyvinyl structural units including chemically bonded amide groups.40 The amide groups allow the PVP to exhibit good stabilizing properties for the surface of transition metal particles, especially in the case of noble metals. In the present work, it was employed as a surfactant due its widely recognized capability of avoiding the aggregation of Ag nanoparticles dispersed in a liquid medium.40 2.2. Mechanical Processing. The mechanical treatment of anhydrous Ag2C2O4 powders was carried out by employing a suitably
Figure 1. Mechanochemical reactor at rest with the Ag2C2O4 powders at the bottom of the reactor chamber, the ball, and the aqueous solution containing PVP. The remaining volume is occupied by Ar gas. Here, it is worth noting that it can be reasonably expected that the use of reactors and/or balls of different size could affect the results of the mechanical processing, at least from the quantitative point of view. Also, it can be expected that the purity of initial reactants and the concentration of solutions could influence the amount, and the properties, of the final product of the mechanochemical reaction. Conversely, no change in the mechanochemical behavior should be induced, from a qualitative point of view, by slightly modified experimental conditions. 2.3. Gravimetric Analyses. The decomposition of the Ag2C2O4 compound determines the formation of metallic Ag and gaseous CO2. Correspondingly, the powders undergo a mass loss that can be detected provided that a suitably sensitive precision balance is used. Aimed at evaluating the total mass lost as a consequence of the mechanically activated decomposition process, the reactor was 10899
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emptied after selected numbers of collisions. The removal of the material from the reactor was performed by gently washing the reactor walls with distilled water. The material removed was placed in a beaker, and put into an oven under Ar atmosphere, where it was dried at the constant temperature of 70 °C. The remaining material was weighted in a laboratory precision balance able to read four decimal places to the right of the decimal point, and the mass of powder including both Ag2C2O4 and Ag contributions estimated by subtracting the mass of PVP left as a residual from the drying process. 2.4. X-ray Diffraction (XRD). The formation of anhydrous Ag2C2O4 crystals, and subsequently the formation of nanometersized Ag particles, was confirmed by wide-angle XRD analyses. The analyses were performed on disk-shaped specimens of dry powder compacted at about 0.1 MPa. Powders coming from the wet mechanical treatment were dried under Ar gas flux at 50 °C for 5 h. XRD patterns were collected with a Miniflex II Rigaku diffractometer equipped with Cu Kα radiation tube over a scattering angle 2θ range approximately from 20° to 110°. Discrete angular displacements of 0.05° and acquisition times up to 100 s long per point were used. The collected patterns were analyzed by the so-called Rietveld method, which allows reliably estimating the average size of coherent diffraction domains, and the average content of lattice disorder, by a suitable mathematical reconstruction of the XRD peak profiles.41 2.5. UV−Vis Spectrophotometry. Similar to other systems, nanometer-sized Ag particles exhibit size-dependent optical properties.42 In particular, their absorption spectra exhibit broad peaks in the UV−vis range, due to the excitation of surface plasma resonances or to interband transitions.42 The relatively strong absorption of nanometersized Ag particles can be used to point out their formation, or their presence in a colloidal solution, by UV−vis spectrophotometry. In the present work, UV−vis spectrophotometric analyses were performed by using a Varian Cary 50 Scan apparatus. The aqueous solution containing PVP was separated from the residual Ag2C2O4 powders by centrifugation. Suitable amounts of the obtained aqueous solutions were introduced into a suitable PMMA cuvette with optical path 1 cm long. The absorption spectrum was recorded in the wavelength range between 300 and 900 nm. 2.6. Scanning Electron Microscopy (SEM). SEM observation was employed to visualize the particles of the starting anhydrous Ag2C2O4 powders, and to estimate their average size. The observations were performed with a Zeiss EVO LS15 scanning electron microscope. 2.7. Tunneling Electron Microscopy (TEM). TEM observation was used to visualize the nanometer-sized Ag particles formed by decomposition of the Ag2C2O4 powders, and to estimate their size. A FEI Tecnai G12 TEM microscope was employed.
Figure 3. Total mass mp of powder inside the reactor as a function of the number n of collisions. The best-fitted line is also shown.
to 2.42 × 10−4 g. It follows that each collision induces a mass loss equal, on the average, to about 2.42 × 10−4 g. The observed reduction in the total mass of powders is only apparently small. In fact, the conversion degree per collision measured is about 2 or 3 orders of magnitude higher than the ones typically detected in the case of metals and minerals.13 In addition, on the basis of previous works,13 it is expected that the amount of powder transformed is not related to the PVP concentration in solution, or to the total amount of PVP solution introduced in the reactor. Rather, it should be connected with the mechanical energy deposited into powders at each collision.13 With the aim of identifying the origin of the observed mass loss, XRD analyses were performed. A typical XRD pattern of mechanically processed powders is shown in Figure 4 together with one of the starting powders of anhydrous Ag2C2O4. The intensity I of the scattered X-rays is plotted as a function of the scattering angle 2θ.
Figure 4. Intensity I of the scattered X-rays as a function of the scattering angle 2θ for the starting anhydrous Ag2C2O4 powders (lower pattern), and for powders that have undergone 100 collisions (upper pattern). The XRD peaks of the metallic Ag phase are indicated by the symbol ○.
3. RESULTS An SEM micrograph of anhydrous Ag2C2O4 powders is shown in Figure 2. Powder particles exhibit an average size of about 2
The comparison between the two XRD patterns shown in Figure 4 indicates that the mechanical processing induces a general decrease of the intensity of the reflections of the Ag2C2O4 phase. Broadening and texture are also observed. Moreover, two additional peaks appear close to 38° and 45°. These can be assigned to the families of (111) and (200) atomic planes of metallic Ag with the usual face-centered cubic crystalline structure.43 The width of the peaks, and the fact that the ideal relative intensities are not respected, suggests that the metallic Ag phase is characterized by a nanometer-sized structure. In particular, the numerical analysis of the XRD peak profile according to the well-known Rietveld method41 indicates that the characteristic length of the nanostructured Ag phase exhibits an average value equal to 9.2 ± 3.4 nm. Instead, the average strain content, which measures the amount of disorder
Figure 2. SEM micrograph of anhydrous Ag2C2O4 powders.
μm. Their mechanical processing induces a detectable decrease of the mass of powders inside the reactor even after the very first collisions. The values of the total mass mp of powder inside the reactor is shown in Figure 3 as a function of the number n of collisions. It can be seen that mp decreases linearly as n increases. The best-fitted line has a slope approximately equal 10900
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of the crystalline lattice, is roughly equal to 2 × 10−4 ± 4 × 10−5. It follows that the Ag lattice can be considered as substantially free from structural defects. Under these circumstances, the relatively large uncertainty associated with the average size of the coherent diffraction domains is more likely ascribable to the breadth of the size distribution rather than to an intrinsic difficulty in estimating the average size of the coherent diffraction domains.44 A further support to the hypothesis that nanometer-sized metallic Ag forms comes from spectrophotometric analyses. The UV−vis absorption curves of the aqueous solution containing PVP are shown in Figure 5, where the absorbance
Now, it is worth noting that the decomposition process takes place at an average temperature of about 293 K. Therefore, Ag2C2O4 powders are relatively far from the temperatures at which decomposition is thermally activated.39 Of course, it cannot be excluded that frictional forces arising between powder particles during the collision can induce local temperature rises. Actually, phenomenological models as well as numerical simulations suggest that temperature rises in the range between 50 and 80 K are plausible.46 However, it must be noted that such temperature rises are not due to a generalized heating of powders as in the case of thermal activation. Rather, they are the result of strongly localized deformation processes mediated by the nucleation and migration of lattice defects under the effect of the mechanical stresses operating at collision. It follows that the rise of temperature induced by the mechanical deformation, and the deformation process itself, takes place under far-from-equilibrium conditions.47 This means that although a thermal component cannot be ruled out, it must be expected that the decomposition process is intimately related to the severe deformation conditions imposed by the mechanical loading.6,47 As a consequence, the mechanically activated decomposition of anhydrous Ag2C2O4 can be regarded as a truly mechanochemical process. Further insight into the details of the mechanochemical decomposition of Ag2C2O4 can be gained by estimating the average size of the nanometer-sized Ag particles. This was done by using TEM observations. A typical TEM micrograph is shown in Figure 6. The Ag particles exhibit characteristic
Figure 5. Absorbance A of the aqueous solutions containing PVP as a function of the UV−vis radiation wavelength λ. Data refer to experiments in which Ag2C2O4 powders have undergone, from bottom to top, 50 (black), 70 (red), 100 (green), and 120 (blue) collisions. The upper curve (magenta) refers to a colloidal aqueous solution of nanometer-sized Ag particles.
A is plotted as a function of the radiation wavelength λ. The various spectra have been collected after a different number of collisions. The curves point out a gradual increase of absorbance as the number n of collisions undergone by Ag2C2O4 powders increases. The absorption curves exhibited by the aqueous solution containing PVP can be compared with the absorption curve exhibited by a colloidal aqueous solution of nanometer-sized Ag particles, also shown in Figure 5. The colloidal Ag was prepared by a classical reduction method, based on the mixing of an aqueous solution of 1 mM AgNO3 with an aqueous solution of sodium citrate in the presence of PVP as surfactant as described in the literature.45 The shapes of the absorption curves, and the absorption range, of the aqueous solutions containing PVP are similar to the ones of the colloidal Ag. This suggests that the aqueous solutions containing PVP also contain nanometer-sized Ag particles. The increase in absorbance A taking place as the number n of collisions undergone by Ag2C2O4 powders increases can be reasonably related to the increase in the concentration of nanometer-sized Ag particles in solution, in agreement with the observed decrease of the total mass mp of powders in the reactor. On the basis of this evidence, the loss of mass can be ascribed to the occurrence of the decomposition process described by the following chemical equation: Ag 2C2O4 → 2Ag + 2CO2
Figure 6. TEM micrograph of nanometer-sized Ag particles isolated from the aqueous solution containing PVP. The micrograph refers to a sample obtained after the Ag2C2O4 powders have undergone 50 collisions.
lengths in the range between 2 and 12 nm. The distribution, not shown for brevity, is quite narrow, and indicates that the nanometer-sized Ag particles have average size around 5 nm. It is worth noting here that the breadth of the size distribution of Ag nanoparticles suggests that the size of the region involved in local Ag2C2O4 decomposition processes can change significantly from site to site. More specifically, obtaining particles with size in the range between 2 and 12 nm indicates that the size of Ag nanoparticles could depend on the intensity of the mechanical stresses operating, on a local basis, within the compressed granular bed formed by the powder particles trapped at collision. In particular, it can be expected that the intensity of the mechanical stresses could proportionally affect the size of the region in which the deformation conditions for the decomposition of Ag2C2O4 are satisfied. It follows that the size of Ag nanoparticles can be changed by changing the intensity of the mechanical stresses at
(1)
The data shown in Figure 3 indicate that the mass loss per collision is roughly equal to 2.42 × 10−4 g. According to eq 1, such quantity corresponds to the decomposition of about 2.75 × 10−6 mol of Ag2C2O4, and the consequent formation of 5.5 × 10−6 mol of Ag and of 5.5 × 10−6 mol of CO2. 10901
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The results obtained suggest that suitably designed mechanochemical processes can be profitably used to carry out controlled chemical reactions. Governed by the effects of localized mechanical stresses, the far-from-equilibrium chemical reactivity of the anhydrous Ag2C2O4 powders allows the synthesis of nanometer-sized Ag particles. This demonstrates that the mechanical activation of powders should be no longer regarded as a rough processing method based on mere deformation and fracture processes, but rather as a truly chemical method, with almost unexplored potential.
collision, which is a function of the mechanical energy deposited by the ball at collision. The evaluation of the average size of Ag particles allows us to relate the mass loss observed during the mechanical processing to the formation of metallic Ag at individual collisions. Under the assumption that only 5 nm Ag particles are formed, the decomposition of 2.75 × 10−6 mol of Ag2C2O4 on the average at each collision results in the formation of about 8.64 × 1014 Ag particles. Unfortunately, neither XRD analysis, nor TEM observations, can be used to give direct experimental support to this latter inference. The nanometer-sized Ag particles form as a consequence of the mechanochemical decomposition process. Their size provides an indirect estimate of the size of the regions of Ag2C2O4 crystals separately involved in the decomposition process at collision. It can be hypothesized that such regions correspond to the points of contact between Ag2C2O4 particles. As shown by mesoscopic models,48 it is there that the mechanical stresses operating at collision are more intense. On the basis of the number of Ag particles formed at each collision, the total number of points of contact between Ag2C2O4 particles can be put equal to 8.64 × 1014. Now, it must be taken into account that the total number of points of contact is a dynamic quantity. At collision, the disordered packing of particles exhibits a dynamics dependent on the distribution of mechanical stresses in its interior. The powder particles move with respect to each other, undergoing at the same time a modification of their shape due to fracture, plastic deformation, and decomposition processes. Under such conditions, it must be expected that the total number of contacts is significantly larger than the one observed in a static disordered packing of spherical particles, where each particle is surrounded, on the average, by 6 neighboring particles.48 Along this line, the 8.64 × 1014 points of contact can be referred to all of the Ag2C2O4 particles somehow involved in the collision process. An estimate of this latter quantity can be obtained by considering the mass of Ag2C2O4 decomposed at each collision, equal to about 8.35 × 10−4 g on the average. This includes about 3.94 × 107 particles. Correspondingly, each Ag2C2O4 particle experiences about 2.19 × 107 contacts. Alternatively, 2.19 × 107 can be roughly regarded as the number of events per particle in which localized mechanical stresses induce the decomposition of a small region of the Ag2C2O4 crystal, and the formation of a nanometer-sized Ag particle. On the basis of these considerations, it can be expected that the total number of the points of contact between the powder particles, and the intensity of the mechanical stresses operating at each point of contact, can be changed by changing the amount of kinetic energy deposited in the anhydrous Ag2C2O4 powders at collision. In turn, this can be increased or decreased by increasing or decreasing the energy with which the ball collides with the reactor base. More specifically, it can be expected that the higher the collision energy, the higher the mechanical stresses at the points of contact between the powder particles. As a consequence, it can be reasonably expected that the number of the points of contact between particles, and the volume of anhydrous Ag2C2O4 crystals involved in mechanical deformation processes, increases as the collision energy increases. It follows that higher collision energies could induce the formation of larger Ag nanoparticles. The experimental validation of these inferences is left to future work.
5. CONCLUSIONS Anhydrous Ag2C2O4 powders have been submitted to mechanical processing in the presence of an aqueous solution containing PVP as a polymeric surfactant. The mechanical treatment induced a decrease of the total mass of powders placed inside the reactor. This indicated the occurrence of some sort of decomposition process changing their chemical nature. The XRD patterns of the processed powders pointed out the formation of metallic Ag with nanometer-sized structure. The spectrophotometric analyses of the aqueous solution containing PVP confirmed such evidence, indicating that nanometer-sized Ag particles were dissolved in the liquid phase. TEM observations supported the conclusions drawn from other experimental findings, allowing the observation of Ag particles with average size around 5 nm. The results obtained indicate that the mechanical stresses operating at the points of contact between the particles involved in individual collisions are able to promote the decomposition of the Ag2C2O4. Similar to the thermally activated process, the mechanically activated decomposition results in the formation of metallic Ag and gaseous CO2. However, at least two differences between thermal and mechanical activation emerge. First, the mechanically activated decomposition process takes place on a local scale, involving only very small amounts of Ag2C2O4 powders. Second, the decomposition leads to the formation of nanometer-sized Ag particles, and not of massive Ag. This can be ascribed to the fact that the most intense mechanical stresses develop at the points of contact between Ag2C2O4 particles. The experimental findings clearly indicate that, under suitable conditions and for specific chemical systems, the mechanical processing of powders can be used as a refined method to carry out selective chemical reactions. In general, it must be expected that these mechanochemical reactions have a chemical path different from thermally activated ones. For its potential in science and engineering, it is highly desirable that this almost unexplored area of research could be suitably investigated in the future.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Dr. Paola Meloni, Department of Mechanical, Chemical, and Materials Engineering, University of Cagliari, and Dr. Gianfranco Carcangiu, Institute of Environmental Geology and Geoengineering, CNR, are gratefully acknowledged for the XRD and SEM analyses carried out at the Laboratory of 10902
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Materials Science “Colle di Bonaria”. Dr. Annalisa Vacca and Dr. Anna Da Pozzo, Department of Mechanical, Chemical, and Materials Engineering, University of Cagliari, are also gratefully acknowledged for assistance in UV−vis spectrophotometry. The author is indebted to Dr. Giovanna Mura, Department of Electrical and Electronic Engineering, University of Cagliari, and Dr. Elodia Musu, Industrial Telemicroscopy Laboratory, Sardegna Ricerche, for TEM observations. Financial support has been given by the University of Cagliari.
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