Sulfidation Mechanism of Pure and Cu-Doped ZnO Nanoparticles at

Article pubs.acs.org/JPCC

Sulfidation Mechanism of Pure and Cu-Doped ZnO Nanoparticles at Moderate Temperature: TEM and In Situ XRD Studies Igor Bezverkhyy,*,† Jonathan Skrzypski,† Olga Safonova,‡ and Jean-Pierre Bellat† †

Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR6303 CNRS − Université de Bourgogne, 9 av. A. Savary, BP47870, 21078 Dijon Cedex, France ‡ Paul Scherrer Institut, 5232 Villigen, Switzerland ABSTRACT: Sulfidation mechanism of pure and Cu-doped ZnO nanoparticles (Cu0.03Zn0.97O and Cu0.06Zn0.94O) at 250 and 350 °C was studied by transmission electron microscopy (TEM) and in situ synchrotron XRD. For nondoped ZnO, we observed by TEM that partial reaction with H 2 S is accompanied by the formation of voids at the ZnO/ZnS interface. This phenomenon (known as the Kirkendall effect) confirms that sulfidation of nanosized ZnO by gaseous H2S proceeds via the outward growth of ZnS: Zn2+ and O2− are transferred to the external (ZnS/gas) surface, where zinc is combined with sulfur and oxygen reacts with protons yielding H2O. During sulfidation of Cu-doped ZnO, the cavities do not form, showing that the sulfidation proceeds by another mechanism, the inward growth, which implies that S2− anions diffuse from the external surface to the internal ZnO/ZnS interface, where they exchange with O2− anions. The change of the transformation mechanism is attributed to a significant acceleration of sulfur transport (lattice or grain boundary) through the Cu-containing ZnS layer due to the presence of sulfur vacancies formed after the charge compensation of Cu1+ replacing Zn2+. The conclusion about the enhanced sulfur diffusion in Cu-containing ZnS is further supported by the time-resolved in situ XRD measurements. It is found that in the case of nondoped ZnO the size of formed ZnS crystallites remains constant during reaction. In contrast, a pronounced crystalline growth takes place in Cu-doped samples during sulfidation under rather mild conditions (250 °C for Cu0.06Zn0.94O) pointing out a high mobility of sulfur anions in Cu-containing ZnS particles.

1. INTRODUCTION ZnO is an important industrial sorbent for the removal of H2S from the gaseous streams of various nature. In the chemical industry, ZnO-based materials have been successfully used for many years to purify the feedstocks at 350−500 °C, allowing to reduce the concentration of H2S to 1 ppm level.1 In some emerging technologies, for example, low-temperature polymer electrolyte fuel cells (PEMFCs) or Fischer−Tropsch synthesis, the desulfurization conditions are more stringent. The concentration of H2S in syngas or reformate gas cannot exceed 0.1 ppm, and in addition, the desulfurization should be done at moderate temperatures (200−300 °C) to simplify the process scheme.2,3 The reactivity of existing sorbents in this temperature range is insufficient, and rational development of more efficient materials is hardly possible without a clear understanding of the mechanism of the reaction between ZnO and H2S. It is a common knowledge that the drop of the reactivity of ZnO at lower temperatures is related to the fact that after sulfidation of the surface a protecting layer of ZnS is formed and further transformation of sorbent necessitates the matter to be transported through this layer. Whereas the first stage (surface reaction) is rapid even at room temperature,4 the second one (bulk reaction) is much slower due to a strong © 2012 American Chemical Society

diffusional resistance. Given the existence of these two stages, the ZnO-based sorbents should absorb more H2S at low temperature if they contain more atoms exposed on the surface, that is, if they are used in a finely divided form. A significant enhancement of the sulfur capacity has indeed been observed in many studies, describing the reactivity of ZnO nanoparticles in both pure and supported form.5−13 This approach is, however, of a limited utility, as even in nanoparticles an important fraction of ZnO remains unsulfided, and the capacity of such sorbents is still lower than the theoretical value. To increase sulfur uptake by ZnO-based sorbents at low temperatures, one has to achieve a rapid and complete bulk sulfidation of ZnO. To attain this goal, the second stage of the transformation, diffusion through the ZnS product layer, must be accelerated. The detailed information about the atomic scale mechanism of diffusion during ZnO sulfidation would be very helpful in accomplishing this task. Such information is, however, very scarce despite a large number of studies dealing with modeling of ZnO sulfidation kinetics. It has been for a long time implicitly admitted that ZnS layer is formed by the Received: April 3, 2012 Revised: May 3, 2012 Published: June 14, 2012 14423

dx.doi.org/10.1021/jp303181d | J. Phys. Chem. C 2012, 116, 14423−14430

The Journal of Physical Chemistry C

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Figure 1. Two alternative mechanisms of ZnS growth during ZnO sulfidation: inward growth (a) and outward growth (b).

inward growth:14,15 sulfur diffuses from the external (ZnS/gas) interface through the ZnS layer toward ZnS/ZnO interface, and then oxygen atoms are transferred in the inverse direction to combine with protons and yield water molecules (Figure 1a). Recently, new data have become available that allow a better understanding of the reaction mechanism. Thus, it was shown that sulfidation of ZnO nanorods with H2S in liquid16,17 or gas phase 18 results in the formation of nanotubes. This phenomenon pointed out the Kirkendall effect during ZnO sulfidation and therefore permitted to conclude about the outward growth of the ZnS layer.18 It means that Zn2+ and O2− diffuse through the ZnS layer to the external ZnS/gas interface where Zn2+ cations combine with sulfur and O2− anions react with protons yielding H2O molecules (Figure 1b). Despite this valuable information, some important questions remain unanswered. Thus, it is well known that stability of porous structures formed through the Kirkendall effect depends strongly on their size and shape.19,20 It is not therefore clear if the cavities observed at ZnO/ZnS interface for well-separated rods of 100−200 nm in diameter will also be present in tightly agglomerated ZnO nanoparticles composing the industrially relevant sorbents. Another important question is how the reaction mechanism is influenced by the dopant atoms present in ZnO. In particular, it would be instructive to get such information for Cu-doped ZnO sorbent, which was previously shown to react with H2S much more rapidly than pure ZnO due to a strong increase in diffusion rate through the ZnS layer.21 To clarify these points, we studied sulfidation of pure and Cu-doped ZnO nanoparticles at 250 and 350 °C by H2S. TEM and in situ synchrotron XRD were employed to characterize in detail the microstructure of the ZnS phase formed after partial and complete sulfidation.

recording the sulfidation profiles. The degree of sulfidation of the samples was controlled by their weight change. After attaining the desired sulfidation degree, the flow of the reaction mixture was stopped, and the samples were cooled under N2 flow. For TEM characterization, the samples were grounded and dispersed in ethanol, and after sonication the obtained suspension was deposited on carbon-covered Cu grid. TEM micrographs were obtained on a JEOL JEM 2100 instrument equipped with LaB6 cathode operating at 200 kV. The in situ XRD measurements were realized at the BM01B beamline at ESRF (Grenoble, France) using the high-resolution powder diffractometer (wavelength of 0.5 Å). The sample powder (ca. 5 mg) was placed inside the capillary plug flow reactor (1 mm quartz capillary with 20 μm walls) and connected to the gas source and the exhaust. The experiments were done at the atmospheric pressure. A gas blower oven was used to control the sample temperature inside the capillary. All samples before reaction were heated to 350 °C in the flow of N2 for 1 h. After this step, the sample was brought to the chosen temperature; then, the reaction mixture containing 500 ppm of H2S in H2/N2 (1/1) flow was introduced in the reactor. The acquisition time per one XRD spectrum was 12 min. The XRD patterns were treated using the FullProf software package.22

3. RESULTS AND DISCUSSION 3.1. TEM. The properties of the solids used in the work are summarized in Table 1 and their TEM images are given in Table 1. Properties of the Samples Used in the Work

2. EXPERIMENTAL The samples of ZnO and Cu-doped ZnO (Cu0.03Zn0.97O and Cu0.06Zn0.94O) used in the present work are the same solids that we previously studied; the details of the preparation procedure and their characterization can be found in ref 21. In summary, the samples were synthesized by (co)precipitation with Na2CO3 from aqueous nitrate solution at room temperature. After drying at 100 °C and calcination at 400 °C, the nanostructured solids were prepared. To obtain the sulfided samples for transmission electron microscopy (TEM), we sulfided the oxides in the H2/N2 (1/1) mixture containing 0.2 vol % H2S using the homemade thermogravimetric setup that was used in the previous work for

sample

composition

ZnO crystallite size, nma

BET surface area, m2/g

ZnO Cu3-ZnO Cu6-ZnO

ZnO Cu0.03Zn0.97O Cu0.06Zn0.94O

21 14 14

29 44 46

a

Calculated from XRD peak broadening.

Figure 2. One can conclude that all samples consist of aggregated nanosized crystallites of isotropic shape. In accordance with XRD data (Table 1), the particle size decreases slightly when ZnO is doped with Cu. Sulfidation of ZnO. The representative TEM images of ZnO after sulfidation at 250 °C are given in Figure 3a,b. After a partial sulfidation, one can distinguish on ZnO particles a compact shell of ZnS, which is separated from the oxide core by voids at the ZnO/ZnS interface (Figure 3a). In the partially 14424



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fraction of the sample consists of tightly agglomerated crystallites without any voids (arrow 2). After sulfidation of ZnO at 350 °C, the voids are also observed in the partially transformed solid (Figure 3c, arrow 1). However, the shape and size of the voids is obviously less regular than after reaction at 250 °C. Also, the regions without any voids are present already in a partially sulfided sample (arrow 2). After a complete transformation of ZnO at 350 °C, the obtained ZnS consists of strongly agglomerated particles of 30−50 nm without any voids (Figure 3d). Appearance of voids at the ZnO/ZnS interface after a partial sulfidation of ZnO nanoparticles suggests the Kirkendall effect during reaction. This allows us to conclude that their sulfidation also occurs according to the outward mechanism of ZnS growth as it was previously proposed for ZnO nanorods.18 Despite this similarity, the behavior of nanorods and nanoparticles bears an important difference. The obtained TEM images reveal that the formed voids are less stable during sulfidation of ZnO nanoparticles. Indeed, it was shown in refs 17 and 18 that a complete sulfidation of nanorods results in the formation of ZnS nanotubes; that is, the integrity of the voids is preserved until the end of the reaction. In contrast, we observe that in ZnS formed from nanosized ZnO particles the voids become less numerous after a complete sulfidation at 250 °C. Their stability is even lower at 350 °C: in the fully sulfided sample, the voids are absent, and even after a partial reaction a significant fraction of them is collapsed (Figure 3c,d). This phenomenon of disappearance of voids can be explained by a low stability of such hollow structures. It was shown in fact that the structures formed through the Kirkendall effect are thermodynamically instable because the vacancies accumulated in the voids tend to diffuse to the external surface resulting in the collapse of the hollow structure.19 It was shown that the time needed for a void to collapse depends linearly on the temperature and cubically on the particle size (eq 9 in ref 19). The strong dependence of the collapse time on the size allows us to account for the higher stability of the hollow ZnS structures formed from the ZnO nanorods (100−200 nm) than from nanoparticles (20 nm). The influence of the temperature explains why the voids are absent in ZnS formed after a complete sulfidation at 350 °C, whereas they persist until the end of reaction, when it is done at 250 °C. A partial collapse of the voids at ZnO/ZnS interface allows us to explain a striking difference between ZnO sulfidation profiles at 250 and 350 °C observed in our previous study.21 We found that sulfidation kinetics of nanostructured ZnO at 350 °C obeys the equations derived from the shrinking core model. In contrast, after lowering the temperature to 250 °C, the reactivity of the solid drops dramatically, and the same model is no longer applicable. Using the present TEM data, we suggest that the difference between the shapes of the sulfidation profiles at 250 and 350 °C is due to higher stability of the voids at lower temperature. Indeed, the presence of voids at 250 °C leads to a pronounced separation of the ZnS layer from ZnO core, which considerably decreases the matter flow through the ZnO/ZnS interface. Such an additional strong resistance for mass transfer is not taken into account by the shrinking core model, which becomes, therefore, inapplicable. At 350 °C, the microstructure of ZnS/ZnO interface is different according to our TEM images. The voids are less stable than at 250 °C, and their collapse allows to restore a close contact between the ZnS layer and ZnO core. The disappearance of the barrier related to the presence of voids at the ZnO/ZnS interface makes the

Figure 2. TEM images of ZnO (a), Cu3-ZnO (b), and Cu6-ZnO (c). Scale bar is 50 nm for panel a and 20 nm for panels b and c.

sulfided sample, the voids are systematically present in all observed particles. After the complete sulfidation at 250 °C, the microstructure is different (Figure 3b). Whereas the voids can still be distinguished (zone pointed by arrow 1), the important 14425



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Figure 3. TEM images of ZnO sample after sulfidation at 250 °C to 50% (a) and 100% (b) and at 350 °C to 50% (c) and 100% (d). Scale bar is 20 nm.

the presence of Cu the reaction proceeds via the previously described inward diffusion of sulfur (Figure 1): S2− anions migrate to the ZnO/ZnS interface, where they exchange with O2−, which are then transferred to the external (ZnS/gas) interface. In this case, the voids are not formed because the unbalanced flow of the species out of the spherical particles does not exist contrarily to the outward growth mechanism. Analysis of the scheme given in Figure 1 permits us to suppose that the observed transition from the outward growth in non-doped ZnO to the inward growth in Cu-doped ZnO reflects a considerable increase in mobility of sulfur anions within the sulfide layer in the presence of Cu. In defect-free ZnS formed during sulfidation of a nondoped ZnO, the transport of both oxygen and sulfur should be extremely slow. The outward growth of ZnS layer observed in this case implies that oxygen diffusion is faster than that of sulfur, the difference being consistent with a smaller size of O2− in comparison with S2−. The inward growth of ZnS layer observed during sulfidation of Cu-doped ZnO points out that in this case the ratio of transport rates is inverse: sulfur diffusion becomes faster than that of oxygen. We suggest that the acceleration of sulfur diffusion in Cucontaining ZnS is due to the presence of sulfur vacancies in high concentration. Indeed, as we have shown in our previous work,21 after sulfidation the dopant sulfide phase Cu2S is not separated but forms a solid solution with ZnS. The charge compensation of 2Cu+ replacing 2Zn2+ results in the formation of one sulfur vacancy.23 It is generally accepted that if the concentration of point defects exceeds 0.1 atomic %, then they

diffusion through the ZnS product layer the unique diffusional resistance, allowing thus the application of the shrinking core model for fitting the conversion profiles. Sulfidation of Cu3-ZnO and Cu6-ZnO. The microstructure of the sulfide layer in Cu3-ZnO sample transformed to 50% at 250 °C is very different from that observed for pure ZnO (Figure 4a). The sulfide shell in this case consists of loosely aggregated small crystallites of ca. 3 to 4 nm without any observable voids at the ZnO/ZnS interface. A similar microstructure is observed after a complete transformation of Cu3-ZnO sample with crystallites growing to ca. 10 nm (Figure 4b). During reaction of Cu3-ZnO with H2S at 350 °C, the structure of ZnS phase changes much more strongly with sulfidation degree (Figure 4c,d). After transformation to 50%, the sample resembles that obtained at 250 °C, whereas the complete sulfidation of Cu3-ZnO provokes a significant increase in particle size up to 70 nm (Figure 4d). The TEM images of Cu6-ZnO after a partial reaction with H2S at 250 °C (Figure 5a) show the microstructure of ZnS/ ZnO interface without any visible cavities similarly to the case of Cu3-ZnO. After a complete sulfidation of Cu6-ZnO at 250 °C, the texture of the sample is somewhat different from that observed for Cu3-ZnO. In particular, we note that the tendency to sintering observed for Cu3-ZnO only at 350 °C (Figure 4d) can be distinguished in Cu6-ZnO solid already at 250 °C, giving ZnS crystallites of 30−40 nm (Figure 5b). Absence of voids in partially sulfided Cu-doped ZnO indicates that in this case the sulfidation mechanism is different from that observed for the nondoped ZnO. We suppose that in 14426



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Figure 4. TEM images of Cu3-ZnO sample after sulfidation at 250 °C to 50% (a) and 100% (b) and at 350 °C to 50% (c) and 100% (d). Scale bar is 20 nm for panels a−c and 50 nm for panel d.

cannot exist in isolated form and must associate in some way, with the grain boundaries being the most favorable vacancy “sink”.24 Given a high Cu concentration in our samples, the grain boundaries in the Cu-doped ZnS particles should be enriched with sulfur vacancies as compared with the nondoped ZnS. Even if a detailed atomic structure of these highly defective grain boundaries is not clear, it can be supposed that diffusion of S2− through the ZnS layer along such boundaries must be considerably accelerated. 3.2. In situ XRD. Sulfidation of ZnO. The series of XRD patterns representing transformation of pure ZnO (bottom) into ZnS (top) at 250 and 350 °C show that the characteristic peaks of both sphalerite and wurtzite modifications of ZnS appear simultaneously when the reaction progresses (Figure 6). There are, however, some particular features in the patterns that have already been observed in our previous work:21 wurtzite peak (102) (2θ = 12.5°) is absent, and peak (103) (2θ = 16.2°) forms a broad hump. These particularities, characteristic of a random stacking of cubic and hexagonal layers inside ZnS crystallites,25 prevent us from using a full profile refinement of these patterns. That is why to estimate the crystallite size of ZnO and ZnS during reaction we fitted the region 2θ = 8−12° with a superposition of single peaks having a pseudo-Voight profile. (An example of such fitting is depicted in Figure 7.) For ZnO, all maxima in the fitted domain give the same value of crystallite size, reflecting their isotropic shape. This is clearly not the case for ZnS: the w(101) peak is much broader than others due to a stacking disorder, and it therefore does not reflect the size of crystallites. The crystallite size of

ZnS was estimated therefore from the broadening of the Bragg peak at 2θ = 9.2° corresponding to the superposition of w(002) and s(111) (Figure 7). Analysis of the evolution of crystallite size of ZnO and ZnS with time (Figure 8) revealed that this parameter for both phases remains constant from the beginning until the end of the reaction. This effect is rather unexpected because transformation of a crystallite of ZnO into ZnS should naturally proceed by shrinkage of the oxide and growth of the sulfide counterpart. We suggest that the absence of noticeable variation of the crystallite size is due to a small width of the reaction front, containing partially transformed crystallites, in comparison with that of the X-ray beam used for the measurements. Under this condition, the fraction of the partially sulfided crystallites is rather small in comparison with nonsulfided or completely transformed ones. Accordingly, the diffraction pattern at different moments is a superposition of the patterns corresponding to pristine ZnO and to formed ZnS particles in a varying proportion, and the size of sulfide crystallites calculated from the XRD patterns corresponds to the completely transformed particles. The size of the formed ZnS crystallites varies with temperature increasing from 12 nm at 250 °C to 20 nm at 350 °C. Comparison with the corresponding TEM images (Figure 3) shows that the crystallites of similar size can be distinguished after reaction at 250 °C, even if they are tightly agglomerated. In contrast, in the sulfide formed at 350 °C, the observed particles are much larger than 20 nm. This difference points out that the ZnS particles formed at 350 °C are 14427



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Figure 6. XRD patterns recorded during sulfidation of ZnO at 250 °C (a) and 350 °C (b). The time interval between the patterns is 24 min. Figure 5. TEM images of Cu6-ZnO sample after sulfidation at 250 °C to 50% (a) and 100% (b). Scale bar is 20 nm.

polycrystalline, which is consistent with the strong variation of the contrast observed throughout the particles in the corresponding TEM images (Figure 3d). Sulfidation of Cu3-ZnO and Cu6-ZnO. The XRD patterns of Cu3-ZnO recorded during sulfidation at 250 °C closely resemble those of pure ZnO (Figure 9a). In both cases, a mixture of sphalerite and wurtzite is formed, and the size of crystallites for oxide and sulfide counterparts remains constant during sulfidation. The results are, however, very different for Cu3-ZnO reacting with H2S at 350 °C (Figure 9b) and for Cu6-ZnO reacting at 250 °C (Figure 9c). When compared with the nondoped ZnO, two striking differences can be noticed. First, nearly pure sphalerite phase is formed in these two cases as it follows from the disappearance of a broad shoulder at 2θ = 9.8°. Second, the width of the Bragg peaks of ZnS decreases sharply with time, indicating that after formation the ZnS crystallites grow. Analysis of the peak broadening shows that for Cu3-ZnO reacting at 350 °C the crystallite size rises from 25 nm after 12 min of reaction to ca. 70 nm after 90 min (Figure 10a). Even if less pronounced, the same trend is observed for

Figure 7. Fitting of the XRD pattern of ZnO partially sulfided at 350 °C. The peaks of wurtzite are denoted by w, and those of sphalerite are denoted by s.

Cu6-ZnO reacting at 250 °C: the crystallites of 35 nm grow out in 90 min from the crystallites of 10 nm present at the beginning of the sulfidation (Figure 10b). The final size of the 14428



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Figure 8. Evolution of crystallite size for ZnO (a) and ZnS (b) during sulfidation of nondoped ZnO at 250 °C.

crystallites is consistent with the data obtained from our TEM images (see Figures 4d and 5b). The striking contrast between the stability of pure ZnS crystallites and a rapid growth of Cu-containing ZnS crystallites shows that mobility of atoms in these two cases is rather different. Indeed, any crystallite growth in a binary compound like ZnS necessitates the transport of both zinc and sulfur. Because due to larger size (r(Zn2+) = 0.74 Å and r(S2−) = 1.84 Å26) the anion diffusion is much more slower than the cationic one, the growth is limited by the transport of anions.24 The rapid crystallite growth under mild conditions revealed in our study by in situ XRD suggests therefore that in Cu-doped ZnS the mobility of sulfide anions is much higher than in pure ZnS. We attribute the acceleration of sulfur diffusion to the presence of anionic vacancies in Cu-doped ZnS formed through Cu1+ charge compensation as previously mentioned (Section 3.1). The phenomenon of sintering revealed by in situ XRD gives an additional support to this interpretation. The drop of the sintering temperature from 350 to 250 °C when passing from Cu3-ZnO to Cu6-ZnO shows that sulfur mobility increases with Cu content. The rapid crystallite growth observed in Cu-doped ZnS under mild conditions therefore supports our conclusion about the presence of the anionic vacancies as the main reason of strong acceleration of sulfur diffusion observed during sulfidation of the Cu-doped ZnO.

4. CONCLUSIONS The microstructure of ZnS shell formed during sulfidation of pure and Cu-doped ZnO nanoparticles by gaseous H2S at moderate temperatures (250 and 350 °C) was studied using TEM and in situ XRD. For pure ZnO nanoparticles reacting at 250 °C, formation of voids at the ZnO/ZnS interface was observed. This finding is in line with previous studies dealing with ZnO nanorods.18 It is interpreted as a consequence of the Kirkendall effect, suggesting the outward growth of the ZnS layer. When the reaction between ZnO and H2S is done at 350 °C, the voids are still observed after a partial transformation, but they are absent in the completely transformed solid. The collapse of the voids attests their limited stability in nanosized ZnO sulfided at high temperature. In contrast with pure ZnO, sulfidation of Cu-doped ZnO is not accompanied by the formation of voids. This fact implies the inward growth of ZnS in the presence of Cu. This radical change in mechanism is attributed to the presence of sulfur vacancies in the ZnS layer due to replacement of Zn2+ by Cu1+.

Figure 9. XRD patterns recorded during sulfidation of Cu-doped ZnO: Cu3-ZnO at 250 °C (a) and 350 °C (b) and Cu6-ZnO at 250 °C (c). The time interval between the patterns is 12 min.

We suppose that the presence of anionic vacancies, probably located at the grain boundaries, enables a strong acceleration of sulfur diffusion, making the inward growth of ZnS more rapid than the outward one observed for nondoped ZnO. The assumption about a highly defective nature of Cu-doped ZnS is in line with the results of in situ XRD of the samples during sulfidation. It was found that the non-doped ZnO transforms in 14429



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(15) Lew, S.; Sarofim, A. F.; Flytzani-Stephanopoulos, M. AIChE J. 1992, 38, 1161. (16) Yi, R.; Qiu, G.; Liu, X. J. Solid State Chem. 2009, 182, 2791. (17) Wang, X.; Gao, P.; Li, J.; Summers, C. J.; Wang, Z. L. Adv. Mater. 2002, 14, 1732. (18) Neveux, L.; Chiche, D.; Bazer-Bachi, D.; Favergeon, L.; Pijolat, M. Chem. Eng. J. 2012, 181−182, 508. (19) Tu, K. N.; Gösele, U. Appl. Phys. Let. 2005, 86, 093111. (20) Fan, H. J.; Gösele, U.; Zacharias, M. Small 2007, 3, 1660. (21) Skrzypski, J.; Bezverkhyy, I.; Heintz, O.; Bellat, J.-P. Ind. Eng. Chem. Res. 2011, 50, 5714. (22) Rodriguez-Carvajal, J. Reference Guide for the Computer Program FullProf; Laboratoire Leon Brillouin CEA-CNRS: Saclay, France, 2001. (23) Kröger, F. A. The Chemistry of Imperfect Crystals; NorthHolland: Amsterdam, 1964. (24) Mrowec, S. Defects and Diffusion in Solids; Elsevier: New York, 1980. (25) Zhang, H.; Banfield, J. F. J. Phys. Chem. C 2009, 113, 9681. (26) Greenwood, N. N.; Earnshow, A. Chemistry of the Elements, 2nd ed.; Butterworth-Heinemann: Boston, 1997.

Figure 10. Evolution of ZnS crystallite size during sulfidation of Cu3ZnO sample at 350 °C (a) and Cu6-ZnO at 250 °C (b).

ZnS, whose crystallite size remains constant during reaction, whereas in Cu-doped ZnO a rapid growth of ZnS crystallites was observed at temperature as low as 250 °C. This phenomenon reflects a high mobility of sulfur anions, thus confirming our conclusion about the presence of anionic vacancies.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +33 380 39 60 38. Fax: +33 380 39 61 32. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS We thank the SNBL at the ESRF for the provision of beamtime. REFERENCES

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Sulfidation Mechanism of Pure and Cu-Doped ZnO Nanoparticles at

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