Sulfidation Mechanism of Pure and Cu-Doped ZnO Nanoparticles at Moderate Temperature: TEM and In Situ XRD Studies

Sulfidation mechanism of pure and Cu-doped ZnO nanoparticles (Cu0.03Zn0.97O and Cu0.06Zn0.94O) at 250 and 350 degrees C was studied by transmission electron microscopy (TEM) and in situ synchrotron XRD. For nondoped ZnO, we observed by TEM that partial reaction with H2S 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 S' 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 degrees C for Cu0.06Zn0.94O) pointing out a high mobility of sulfur anions in Cu containing ZnS particles.