Identification of a deleterious phase in photocatalyst based on Cd_1 - xZn_xS/Zn(OH)₂ by simulated XRD patterns

The X-ray diffraction (XRD) pattern of a deleterious phase in the photocatalyst based on Cd_1 - xZn_xS/Zn(OH)₂ contains two relatively intense asymmetric peaks with d-spacings of 2.72 and 1.56 Å. Very small diffraction peaks with interplanar distances of (d) ≃ 8.01, 5.40, 4.09, 3.15, 2.49 and 1.35 Å are characteristic of this phase but not always observed. To identify this phase, the XRD patterns for sheet-like hydroxide β-Zn(OH)₂ and sheet-like hydrozincite Zn₅(CO₃)₂(OH)₆ as well as for turbostratic hydrozincite were simulated. It is shown that the XRD pattern calculated on the basis of the last model gives the best correspondence with experimental data. Distances between layers in the turbostratically disordered hydrozincite fluctuate around d ≃ 8.01 Å. This average layer-to-layer distance is significantly higher than the interlayer distance 6.77 Å in the ordered Zn₅(CO₃)₂(OH)₆ probably due to a deficiency of CO₃^2- anions, excess OH^- and the presence of water molecules in the interlayers. It is shown by variable-temperature XRD and thermogravimetric analysis (TGA) that the nanocrystalline turbostratic nonstoichiometric hydrozincite-like phase is quite thermostable. It decomposes into ZnO in air above 473 K.