Article pubs.acs.org/JPCC
Role of Interfacial Energy and Crystallographic Orientation on the Mechanism of the ZnO + Al2O3 → ZnAl2O4 Solid-State Reaction: II. Reactivity of Films Deposited onto the Sapphire (001) Face Sonia Pin,†,‡ Marco Suardelli,‡ Francesco D’Acapito,∥ Giorgio Spinolo,‡ Michele Zema,§ Serena C. Tarantino,§ Luisa Barba,⊥ and Paolo Ghigna*,‡ †
General Energy Research (ENE), Laboratory for Bionenergy and Catalysis, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland INSTM, Department of Chemistry and §Department of Earth and Environment Sciences, University of Pavia, I27100 Pavia, Italy ∥ CNR-IOM-OGG c/o ESRF, GILDA-CRG, BP 220, F38043 Grenoble Cedex, France ⊥ CNR - Institute of Crystallography, UOS Trieste, Strada Statale 14 - km 163,5 Area Science Park I34149 Basovizza, Trieste Italy ‡
ABSTRACT: The initial steps of the reaction between ZnO and (001)-oriented Al2O3 single crystals have been investigated with X-ray diffraction, atomic force microscopy, and Xray absorption spectroscopy at the Zn−K edge starting from 45 nm thick zincite films. The reaction eventually yields the ZnAl2O4 spinel on this interface but via a complex mechanism involving side and intermediate nonequilibrium compounds, the spinel initially forming with a distribution of lattice parameters. Evidence is given of the fact that one of the side compounds has a crystal structure close to that of the Zn3In2O6 compound. The results are discussed in the general context of the same reaction on different single-crystal substrates and different film thicknesses.
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and (012) faces, while the present article deals with the films deposited onto the (001) face. Part I also accounts for previous literature concerning (a) studies on the mechanistic aspects of solid-state reactions before the onset of the diffusion-driven regime,8−24 (b) the interest toward a better understanding of the reaction of the simple oxides to form the ZnAl-spinel (ZnAl2O4) from the point of view of basic research, and (c) its relation to the synthesis and properties of oxide nanostructures.25−33 Part I also includes sections that are pertinent to both papers, but are not rewritten here. These regard: (a) the details of the experimental parts concerning the deposition of 45 ± 5 nm thick films and the common characterization procedures (X-ray absorption spectroscopy (XAS), X-ray powder diffraction (XRPD), and atomic force microscopy (AFM)) and (b) a discussion of the underlying thermodynamic aspects as well as (c) some general conclusions already emerging from an extremely rich and complex set of experimental situations characterized, in particular, by several intermediate phases. On the other side, in addition to the discussion of the results for the films deposited onto the (001) face, the present Part II (a) reports another experimental section concerning the glancing incidence X-ray diffraction (GIXRD) technique, (b) compares the results obtained with 45 ± 5 nm thick films on all substrates, and (c) discusses the
INTRODUCTION This article reports part of a series of investigations aiming at improving the understanding of the initial steps of solid−solid heterogeneous reactions,1−7 that is, the processes occurring when a single interface between two reactants turns into a couple of interfaces with the product in between. The experimental protocol is based on a multitechnique approach and essentially monitors the time evolution of a reactive system made of a thin layer of one reagent deposited onto a singlecrystal slab of the other reagent and compares the results obtained with films deposited onto different crystal orientations and with different thicknesses. The underlying idea of the work is that considering various film thicknesses and various crystal orientations allow us to tune the contribution of interfacial free enthalpy to the overall driving force of the reaction and to explore various reaction regimes. Experiments have indeed shown that this approach is effective in enhancing or depressing different mechanistic steps, stabilizing intermediate compounds, and so detecting intermediate phases that are otherwise impossible to study or do not occur at all. The experimental model here investigated is the reaction of Al2O3 (sapphire structure) and ZnO (zincite, wurtzite-type structure) in the form of films deposited onto sapphire single crystals. In detail, previous works3,6 discuss the reactivity of 10−15 nm thick ZnO films. Thicker (45 ± 5 nm) films are considered in the present article and in its accompanying Part I,7 with Part I essentially dealing with the films deposited onto the (110) © 2013 American Chemical Society
Received: December 19, 2012 Revised: March 5, 2013 Published: March 11, 2013 6113
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whole set of results together with those of our previous studies on thinner (10−15 nm) films.3,6
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EXPERIMENTAL SECTION ZnO films have been obtained by RF-magnetron sputtering of ZnO at room temperature. For each substrate orientation, a batch of equal films has been prepared and treated for different times in air at different temperatures to start the spinel forming reaction and to monitor its advancement by withdrawing a film from the batch after each treatment. Fluorescence XAFS (X-ray absorption fine structure) data were collected at GILDA beamline (European Synchrotron Radiation Facility, ESRF, Grenoble) at the Zn−K edge (at ca. 9660 eV). The measurements were performed at room temperature. XRPD patterns have been collected at room temperature with a laboratory (D8 Bruker AXS) diffractometer using a Cu anticathode. AFM images have been acquired using a Molecular Imaging PicoSPM scanning probe microscope operating in contact mode. More details on starting products and deposition procedure as well as on the apparatuses and data treatments of the above characterization techniques are reported in Part I.7 GIXRD measurements were performed at the X-ray diffraction beamline 5.2 of the Synchrotron Radiation Facility Elettra in Trieste (Italy). The X-ray beam emitted by the wiggler source was monochromatized at 0.7 Å by a Si(111) double-crystal monochromator, focused on the sample and collimated by a double set of slits giving a spot size of 0.2 × 0.2 mm. The samples were oriented by means of a four-circle diffractometer in the horizontal plane containing the X-ray beam employing a laser light. Bidimensional diffraction patterns were recorded with a 2 M Pilatus silicon pixel X-ray detector (DECTRIS, Baden, Switzerland) perpendicular to the incident beam at a distance of 200 mm from the sample. The sample inclination to the beam was kept at ω = 0.2°. Patterns were calibrated by means of a LaB6 standard and integrated using the software Fit2d34 obtaining powder-like patterns, corrected for geometry, Lorentz, and beam polarization effects. The experiments were performed at room temperature. The 2θ range observed spanned up to 44.88°, with the resolution reaching 0.92 Å. Peaks positions were extracted by means of the program Winplotr;35 cell parameters were inferred with the help of the program Checkcell.36
Figure 1. XAS dichroic spectra at Zn−K edge for the (001)zincite ∥ (001)sapphire sample, treated at 900 °C for 1 h.
orientations, the EXAFS of the thinner films was found to agree with a structure based on an O first-coordination shell around Zn similar to the tetrahedral coordination of the wurtzite-type structure but showing in addition a considerable amount of static disorder in the next (metal−metal) shell. Approximately one-third of the Zn positions are filled by Al ions3,6 in the most reliable local structural model accounting for this disorder. The similarity of the EXAFS analyses, however, does not imply that the intermediate phases found for the different orientations belong to the same crystal structure. On the contrary, differences in some minor features suggest a somewhat different structure, and there is evidence, in particular, that a smaller substitution of Zn with Al occurs for the present (001) orientation than for the (012) case.3 EXAFS also singled out the presence of another local structure, which can be equally well-assigned to a new structure or to regions having the previous structure but modified composition. In summary, there is no doubt about the presence of at least one unknown reaction product instead of the regular spinel. This product also was detected at long reaction times without signs of spinel formation. Morphologically, the reacting thinner film was found to collapse into islands (Figure 2a−d). The present (thicker) films are morphologically stable and do not collapse into islands. When the temperature is increased to 1000 °C, XANES and EXAFS show (Figure 3) that the regular reaction ZnO + Al2O3 → ZnAl2O4 takes place. As for the case described in the previous paper7 (thicker film on (110)sapphire), an advancement degree can be retrieved by fitting the spectra as a combination of the phase specific spectra. The plot (see Figure 4) now shows, however, a large induction period followed by a linear growth with a kinetic constant k = 4.8(1) × 10−9 cm s−1. Once started, the spinel growth is then more than four times faster than that on (110)sapphire and appears to be almost complete after only 1 h at 1000 °C. For completeness, we note the appearance of a very small (511) spinel peak in the diffraction pattern taken at 900 °C (see Figure 5): the feature shows that the reaction also runs at 900 °C but at a much lower rate that cannot be well-determined by XAS. X-ray diffraction with a laboratory powder diffractometer shows, in good analogy now to the results for the films deposited onto the (110)sapphire face,7 that at 1000 °C the spinel initially grows with a preferred orientation with respect to the substrate, as clearly assessed by the large intensity of the (511) peak with respect to the other diffraction effects. Also, the original orientation of the ZnO film is seemingly maintained during almost the entire course of the reaction. Only at the very end does some loss of orientation occurs, as revealed by the
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RESULTS For easier presentation, the results are systematically compared with those obtained3,6 on 10−15 nm thick ZnO films deposited onto the same (001) face of the sapphire single crystal: those films are hereafter referred to as “thinner films” for short. The linear dichroism at the Zn−K edge (Figure 1) shows that the c axis of the ZnO film is perpendicular to the sample surface: (001)zincite ∥ (001)sapphire. This is in agreement with the literature37,38 and with our previous work on thinner films deposited onto the same sapphire face3 and on films of the same thickness deposited onto the (110) face. When reacting the thinner ZnO films deposited onto this face, (001), of the sapphire single crystal, we found3,6 that even at long reaction times the regular spinel fingerprint does not appear in the EXAFS, a feature shown also by the equally thin films but deposited onto the (012) sapphire face. For both 6114
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Figure 4. Kinetics for the (001)zincite ∥ (001)sapphire interfacial reaction.
Figure 2. Evolution of morphology of the films during the (001)zincite ∥ (001)sapphire interfacial reaction, as seen by AFM. (a) 15 nm film heated to 800 °C for 20 min; (b) 15 nm film heated to 800 °C for 40 min; (c) 15 nm film heated to 800 °C for 60 min; (d) 15 nm film heated to 800 °C for 120 min; (e) 45 nm film heated to 800 °C for 60 min; and (f) 45 nm film heated to 1000 °C for 120 min.
Figure 5. Evolution of the XRPD patterns at 900 and 1000 °C for the (001)zincite ∥ (001)sapphire interface. The patterns are shifted along the y axis for the sake of clarity. The indexes reported are as follows: A: alumina; Z: ZnO; S: spinel. Unindexed peaks are marked with asterisks. The peaks marked Fe are due to the sample holder.
of the reaction, with a very small fwhm and with an intensity that increases as the reaction advances. Then, the phase responsible for this diffraction effect is more reasonably assigned here the role of a product of a reaction that runs concurrently with the reaction leading to the regular spinel. Other diffraction effects are more probably pertinent to intermediate product(s) of the main reaction. In our opinion, the experiments do not give a clear explanation of the induction period in the time evolution of the spinel, as obtained from the EXAFS response on the reaction at 1000 °C (Figure 5): it can be either the result of the competition between concurrent processes with different time laws or the direct effect of a preliminary step on the kinetics of the overall process eventually leading to the spinel. A more detailed understanding of the reactivity can be provided by the grazing incidence diffraction frames of the films reacted 1 h (Figure 6A) and 2 h (Figure 6B) at 1000 °C. Let us start from panel B: this is clearly made of powder rings, overlapping with spotty effects. All of these diffraction effects (rings and spots) correspond to d spacings of the ZnAl2O4 spinel, as evidenced by the pattern of panel C (red trace). So, at the end of the reaction, the spinel is the only phase present and appears as essentially polycrystalline with only some amount of preferred orientation. Looking now at panel A, we note that at the shorter heating time (1 h) at 1000 °C the sample already shows all of the diffraction effects of the ZnAl2O4 spinel. With respect to panel B, well apparent differences are that along the rings the intensity now appears more focused into a smaller number of larger spots and that rings and spots are generally much more broadened. In particular, around the mean positions of the diffraction effects the intensity shows a large distribution, which
Figure 3. Evolution of the XANES and EXAFS spectra at the Zn−K edge at 1000 °C for the (001)zincite ∥ (001)sapphire interface. For better reference, the spectra of bulk ZnO and ZnAl2O4 are also reported. The spectra have been shifted along the y axis for better clarity.
onset of other diffraction lines of the spinel, with a weak intensity. We remark the presence of a particular unindexed line at d = 4.12 Å, already found with films deposited on other substrate faces. Here this line appears during the entire course 6115
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concern various issues. We hereafter discuss these issues following the time evolution of the chemical reaction. Concerning the starting points of the reaction, the ZnO films deposited onto (110) and (001) sapphire faces are always highly oriented, with their c axis perpendicular to the film surface, independently of film thickness. For the (012) face, the as-deposited film is polycrystalline and the onset of epitaxy occurs after a thermal treatment: the c axis then becomes parallel to the film surface. For both thin and thick films, the starting orientations are (001)zincite ∥ (110)sapphire, (110)zincite ∥ (012)sapphire, and (001)zincite ∥ (001)sapphire and are all in agreement with previous literature.37−40 The important role of orientation and film thickness in driving the reaction is directly made apparent by considering the morphology and its time evolution. Generally speaking, the thick films are morphologically stable and show only small changes in time, such as some grain size increase, whereas the thin films typically collapse into islands. The morphology of the reaction of thin zincite film on (012)sapphire, for example, shows a complex time evolution that seemingly involves two different phases and suggests a 2D nucleation and growth process. The (001)zincite ∥ (110)sapphire reaction appears peculiar in this regard because the thin films closely parallel the thick films in showing no significant morphology change during reaction and no collapse into islands. For comparison, remember that a smooth surface represented by atomically flat terraces and half unit cell high steps has been observed41 using in situ reflection high energy electron diffraction (RHEED) and ex situ AFM during ZnO growth by plasma-assisted molecular beam epitaxy at 750 °C on the same sapphire face. Another remarkable difference between various orientations and film thicknesses concerns the actual formation of the regular spinel product at the end of the reaction. Using thick films, the spinel product always appears among the reaction products. On the contrary, for the (110)zincite ∥ (012)sapphire and (001)zincite ∥ (001)sapphire interfaces, the spinel features never appear in the EXAFS when reacting thin films. Also, in this regard the (001)zincite ∥ (110)sapphire interface behaves differently because formation and growth of the spinel phase is detected for both thick and thin films. Incidentally, the last result indicates that the absence of the spinel in the reactions at other interfaces is not due to stoichiometric constraints. The chemical reaction generally starts with a cation counterdiffusion that changes the compositions at the two sides of the original interface and possibly also its position in space. The diffusional step has been best characterized for the thin films deposited onto the sapphire (110) face and the finer details of the results so found are in agreement with the thermodynamic analysis reported at the end of Part I.7 In the overall reactive process, the role of the preliminary diffusional step is to control compositions and strain at the original interface, to build composition and strain gradients in the reactant phases, and possibly to change the size and shape of the small-sized reactant. All of these tune the related contributions to free enthalpy and finally trigger the successive phase formation steps. The experiments show that several intermediate phases are formed after the preliminary diffusional process and before the onset of the spinel product. In some cases, there are experimental insights into a growth in parallel to the growth of spinel. The crystal structure determinations of these phases are still in progress. What can be said now is that they do not appear in the equilibrium phase diagram of the pseudobinary
Figure 6. Grazing incidence X-ray diffraction pattern of the reacting films after 1 (A) and 2 h (B). Panel A′ shows in better detail the low θ part of panel A. In panel B, the upper half is due to direct scattering, whereas the scattered beam is partially absorbed through the sapphire substrate in the lower half. Panel C shows a comparison of the integrated diffraction patterns corresponding to panels A (red trace) and B (blue trace). Panel D shows the 2θ region between 3 and 10° of the integrated diffraction pattern for the sample heated for 1 h (panel A). Concerning panel C, discontinuities in the integrated trace are due to the blind stripes of the Pilatus detector.
in most cases appears remarkably bimodal. This can be better appreciated by the blue trace of panel C, corresponding to the integration of panel B. Several peaks of the spinel here appear as followed at higher θ by an additional diffraction effect, with intensity well related to that of the major peak. Instead of a distinct phase, the general features of the frame suggest a single spinel phase with a wide (and essentially bimodal) distribution of composition and lattice constant. Then, the experimental evidence is that the spinel initially forms with a few preferred orientations with respect to the sapphire substrate and with large disorder. The first aspect is in agreement with the other results previously reported.1−7 As the reaction proceeds, microstructural rearrangements take place, resulting both in some loss of preferred orientation and in more structurally ordered crystallites, where only one component of the earlier bimodal distribution of composition and lattice dimension survives and grows at the expense of the other one. We must also remark on a number of diffraction effects that are not related to the spinel structure and occur at much lower diffraction angle (much higher d spacing, see panels A′ and D). It is particularly appealing to infer from these the presence of a phase at an earlier time (or possibly a set of related phases) having a very large unit cell and oriented parallel to the base plane (001) of alumina.
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DISCUSSION AND CONCLUSIONS We here discuss the results concerning the ZnO films deposited onto the (001) sapphire face together with those obtained on the films with the same thickness (40 ± 5 nm) deposited onto the (110) and (012) sapphire faces7 as well as those previously obtained with the “thinner” (i.e., 10 - 15 nm thick) films deposited onto the same sapphire faces.3,6 The reactivity of ZnO films deposited onto sapphire single crystal shows widely different features that strongly depend on film thickness and substrate orientation, and the differences 6116
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system,42,43 and their number, composition, structure, and kinetic role strongly depend on substrate orientation and film thickness. A general trend is that for a given substrate orientation a lower number of intermediate phases is seen when reacting thicker films. The enhanced stability of intermediate phases when reacting thinner films can be easily explained7 using simple thermodynamic arguments and considering the effect of the additional contributions to the free enthalpy of the reactive system. Within the scheme of a thermodynamic analysis, it is therefore possible to gain a rational approach to some general features of the reactivity. Some other features, however, cannot be satisfactorily explained with this approach: this regards in particular a kind of chemical oscillation found on the thicker films deposited onto the sapphire (012) face. A common and extremely distinctive feature of the thick film cases is that the pertinent starting orientations between zincite and sapphire are maintained during the whole reactive process, except for the very last time. The experimental evidence comes from various techniques and is essentially due to the fact that a preferred orientation is found also at later reaction times. So, the orientation is preserved during the preliminary diffusional process, when an intermediate phase is formed, and when this later disappears. Some information on the structure of the intermediate phases is given by EXAFS. For the films deposited onto the (001) and (012) sapphire faces, EXAFS data agree on the presence of (Zn,Al)O solutions essentially based on an hcp arrangement of oxide anions and a Zn-rich composition. The composition is intermediate between zincite and spinel; in particular, it is closer to pure zincite in the former case and more displaced toward spinel composition in the latter case. When combined with the observations concerning the initial diffusional step, the EXAFS results strongly suggest for the intermediate steps crystal structures essentially based on those of the reactant phases and showing for the cations some ordering scheme that possibly becomes more effective (and simpler) with increasing time. A well -elated insight is given by the long spacing structures found in the thicker films deposited on the (001) sapphire face, with the warning that Al-rich solutions based on the sapphire structure are most probably involved here. It is interesting to compare this result with a combined Al−K edge XANES/ computational study on supersaturated ZnO-rich (Zn, Al)/O solutions by Yoshioka et al.44 These authors found clearly different structures when exploring solutions with various Al compositions prepared (a) by conventional solid state reaction of the component oxides in air at 1300 °C or (b) by pulsed laser deposition using the reacted ceramic powders as target. In the ceramic samples, the fingerprint of the spinel phase starts to appear in the XANES above the Al solubility limit in the zincite phase, which is lower than 2% at the pertinent conditions. (See also ref 45.) In the film samples, on the contrary, a supersaturated solution structurally close to wurtzite was found even at much higher Al compositions. XANES; however, it is not consistent with Al in four-fold coordination (as is the regular Zn site of wurtzite structure). The authors therefore suggest a structure analogous to the rhombohedral structure of Zn3In2O6.46 So, despite the quite different reactive system, there are nice points of agreement with the present results, such as the unquestionable difference between bulk samples and film samples and the onset of an intermediate phase stabilized by small size and characterized by a structure in some way related
to the hcp packing of oxide anions of zincite. The agreement is strongly supported by the fact that the diffraction effects at 1.20 (2θ = 80.01°), 2.34 (38.55°), 2.39 (37.53°), 2.75 (32.50°), 4.10 (21.63°), and 8.25 Å (10.21°) in the diffraction pattern for 1000 °C, 1 h in Figure 5 can be indexed as (0 2 23), (0 0 21), (1 0 11), (0 0 18), (0 0 12) and (0 0 6), respectively, using hexagonal axes a = 3.34(1) Å, and c = 49.3(3) Å: these cell dimensions are impressively close to those of the Zn3In2O6 compound. We also remark, incidentally, that in these metastable solutions Al doping of the reference zincite structure follows a quasichemical defect reaction (formation of Al substitutional defects coupled to cation vacancies on Zn sites) that is quite different from that occurring in equilibrium solutions formed at lower Al content, where the formation of Al substitutional defect is balanced by electrons. Concerning the process going from the intermediate phases to the spinel, the most sensible insights come from the glancing incidence diffraction data previously discussed for the thicker film deposited on the (001)sapphire face. The data show that the regular spinel product is associated with regions where the same basic structure appears with different lattice dimensions, reasonably as a result of a different composition. The transition between these regions and spinel appears as discontinuous in the sense that we have not found phases with intermediate compositions or a smooth change in time of the lattice spacings. For an example of the relation between the control of phase formation, film orientation, and stress due to molar volume change, we make reference to a work on the MgO + GeO2 reaction by Blum and Hesse.47 In the cases where the spinel eventually forms, near the end of the reaction it appears as polycrystalline. This does not derive, however, from spinel nucleation occurring at random orientations but from loss of orientation of a spinel phase that is initially formed with a few symmetry-related preferred orientations with respect to the reactants. Some previous investigations on related topics are worth recalling here. During the atomic layer deposition of alumina/zincite/alumina thin layers, Jang et al.48 commonly observed the ZnO (001)// ZnAl2O4 (111) and ZnO [100]//ZnAl2O4 [110] orientation between zincite and the newly formed spinel and in only a few cases were mismatches with respect to this orientation observed. According to the authors, spinel formation is essentially due to diffusion of ZnO into alumina, and the mismatches occur where alumina is present as an amorphous phase, that is, where the movement of oxygen is less restrictive than in the crystal phase. As said, Kotula and Carter investigated11 the nucleation of the solid-state reaction between nickel oxide and aluminum oxide using an experimental setup similar to that of the present work and made of an NiO thin film deposited onto sapphire (001). They found that NiO is deposited with its (111) plane parallel to the substrate and in the form of three twins, which are rotated to each other around the normal to that plane [111]. The spinel product forms where the twin boundaries in the film meet the substrate and contains the same variants as the NiO film: in the crystal structure of NiO, the (111) plane is the close correspondent of the (001) plane of ZnO. Our data, combined with all of these previous findings, suggest that the spinel orientation with respect to (001) zincite is probably the same for thick films deposited onto (110) and (001). The loss of orientation occurs when most of the spinel is formed. The important insight here is that a concerted rearrangement takes place: this breaks the layer into a polycrystalline material and involves the whole 6117
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“Laurea” degree in Chemistry at Università di Pavia, and of a Ph.D. degree in Chemical Sciences at Università di Pavia and at the Universitée Franco-Italienne (thèse en cotutelle).
thickness of the product layer and a length (parallel to the substrate) much larger than the layer thickness. We recall that Kotula and Carter49 have already reported a somehow similar process where a whole product layer rotates away from its initial alignment, carrying a reactant overlayer along with it. We finally note that, once formed, the spinel growths four times faster on (001)sapphire than on (110)sapphire. A possible explanation is as follows. The growth can be controlled by the mobility of the zincite/spinel interface or the spinel/sapphire interface. The same zincite face (001) is involved with both substrates, according to the starting orientations that are maintained during most of the reaction. The understanding therefore is that nucleation of spinel is much slower at the (001)zincite/(001)sapphire interface, but the interface mobility at (111)spinel/(001)sapphire is four times faster than that at the (111)spinel/(110)sapphire, whereas the (001)zincite/(111)spinel interface shows the fastest mobility and therefore never appears as rate-determining. Going now to more general conclusions, first of all we want to stress that the experimental protocol based on a reactive system made of a thin layer of one reagent deposited onto a single crystal slab of the other reagent has been proven to be very effective in giving sound insights into the mechanism and kinetics of the interfacial processes that represent the initial stages of the reactions in the solid state, in agreement with previous insights.11 A strong argument in favor of this protocol is, in particular, its capability of controlling the (meta) stability of intermediate compounds and therefore detecting and investigating the kinetic role of phases that are otherwise impossible to access experimentally. A very general common feature of the reactivity at the various interfaces is represented by the occurrence of a preliminary diffusional step that is potentially able to produce supersaturation in the reagent phases, an effect that has been experimentally verified in at least one case and seemingly plays a key role in triggering the successive reaction steps. For thick films, the starting orientations between zincite and sapphire are impressively maintained during the whole reactive process, that is, during the preliminary diffusional process, when an intermediate phase is formed and also (if that applies) when this later disappears and the regular spinel product is formed. Within this common landscape with a clear epitactic character, each substrate orientation represents a different case by itself, with a different reactive path, with specific intermediate phase(s), and with different kinetic regimes and rates. Close to the end, it is again possible to find a common behavior because most cases show a concerted rearrangement that breaks the epitactic nature of the whole process and produces a polycrystalline material.
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REFERENCES
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Corresponding Author
*Phone: +39-0382-987574. Fax: +39-00382-987575. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS ESRF and ELETTRA are acknowledged for provision of beam time (experiments CH-3151 and 20115047, respectively). This paper is essentially based on the thesis submitted by two of the authors (M.S. and S.P.) in fulfillment, respectively, of a 6118
dx.doi.org/10.1021/jp312517w | J. Phys. Chem. C 2013, 117, 6113−6119
The Journal of Physical Chemistry C
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