(Cd,Mn)S in the Composite Photocatalyst: Zinc Blende and Wurtzite Particles or Integrowth of These Two Modifications?

In this study, the crystalline structure and particle shape of Cd1-xMnxS (x similar to 0.3) in the composite photocatalysts prepared by hydrothermal synthesis at different temperatures (T = 80, 100, 120, and 140 degrees C) were analyzed. Along with mixed Cd-Mn sulfide, the catalysts contain a small amount of beta-Mn3O4. XRD patterns of (Cd,Mn)S have features inherent to both cubic zinc blende and hexagonal wurtzite structure. Moreover, XRD peaks are anisotropically broadened. First, the heterogeneous (or two-phased) model was considered by the commonly used Rietveld method. Phase ratio, average crystallite sizes, and strains for both phases were formally determined. However, it was shown that this model is not correct because relatively narrow and broad peaks cannot be fitted simultaneously. Then, the homogeneous model was tested by Debye Function Analysis. This model assumes that particles are statistically homogeneous, but each particle contains lamellar intergrowth of zinc blende and wurtzite modifications. The probability of stacking faults, as well as the average radii of spherical and ellipsoidal particles, were varied. It was shown that nanocrystalline Cd0.7Mn0.3S particles have an ellipsoidal shape. Ellipsoids are elongated along the direction normal to the plane of defects. An increase in the hydrothermal synthesis temperature from 80 degrees C to 140 degrees C leads to an enlargement of particles and a gradual decrease in the probability of stacking faults in the wurtzite structure from 0.47 to 0.36. Therefore, with increasing temperature, the structure of (Cd,Mn)S nanoparticles transforms from almost random polytype cubic/hexagonal (ZB:WZ = 47:53) to a preferably hexagonal structure (ZB:WZ = 36:64). Mn2+ ions facilitate CdS phase transformation from zinc blende to wurtzite structure. There is no direct correlation between the structure and photocatalytic activity.

Automated summary

This study analyzed the crystalline structure and particle shape of Cd1-xMnxS (x similar to 0.3) in composite photocatalysts prepared by hydrothermal synthesis at different temperatures (T = 80, 100, 120, and 140 degrees C). The catalysts contained mixed Cd-Mn sulfide and a small amount of beta-Mn3O4. The XRD patterns showed features of both cubic zinc blende and hexagonal wurtzite structures, with anisotropically broadened peaks. A heterogeneous model was initially considered but proved incorrect, and a homogeneous model with lamellar intergrowth of zinc blende and wurtzite modifications was found to be more accurate. The synthesis temperature affected particle size, probability of stacking faults, and the transformation of the nanoparticle structure.