The role of substrate materials on stabilization of CdO, 2CdO⋅CdSO4 and 2CdS⋅2CdO⋅CdSO4 from CdS powder film annealed in air

The relative stability of CdS and CdO with respect of substrates has been widely studied up to 500 °C, the upper temperature in many applications. The physical-chemical properties of the deposited films are often revealed to be influenced by deposition substrates. In this paper, using XRD, SEM and EDX methods, we have studied the oxidation/desulphurization of CdS to CdO and the intermediate stabilized materials therein (i.e. 2CdO·CdSO4 and 2CdS·2CdO·CdSO4), by using CdS films deposited on different substrates (i.e. nickel, Si-polycrystalline, glaze porcelain and α-Al2O3) which were annealed in air up to 800 °C. At the annealing temperatures used in this study, different phases were found to be stabilized on different substrates. The relative stability of CdO and 2CdO·CdSO4 phases is greater on α-Al2O3 than on glazed porcelain substrate. A mixed phase of CdO and 2CdO·CdSO4 was obtained at the annealing temperature of 800 °C if films are deposited on Ni and polycrystalline silica. The amount of the oxidized CdS (i.e. CdO and 2CdO·CdSO4) obtained by mixing CdS and Al2O3 powders at different Al2O3/CdS ratios and annealed up to 800 °C, were also investigated. Decreasing the Al2O3/CdS ratio, the interactions between Al2O3 and CdS particles are reduced, consequently the CdO stabilization is strongly retarded in favour of 2CdO·CdSO4, while reverse results was proven for high values of Al2O3/CdS ratio. The results show the role of the interface area between Al2O3 and Cd-materials for stabilization of CdO. A low interface area was also obtained by filling an α-Al2O3 crucible with CdS powder. After annealing, the CdO stabilization was strongly reduced, in line with the low Al2O3/CdS ratio sample. Spreading the CdS powder on the α-Al2O3 substrate, instead of drop coating deposition method, showed a remarkable CdO stabilization in line with the results obtained with drop coating method. To account for the role of the interface area a model is proposed (Stabilization model), based on surface energy, as responsible to phase stabilization to explain the nucleation – stabilization of the CdO and 2CdS·2CdO·CdSO4. The morphologies of the CdO and 2CdS·2CdO·CdSO4 resemble hexagonal-prism shapes and compact chips, respectively, while 2CdO·CdSO4 phases has a sponge-like appearance.