Synthesis of high surface area mesoporous ZnAl₂O₄ with excellent photocatalytic activity for the photodegradation of Congo Red dye

The large-scale release by the textile and other industries of organic dyes into water courses degrades the aquatic environment and results in serious health problems for wildlife, humans, and domestic animals. The removal of such dyes from wastewater is therefore a pressing and important challenge. The efficient photocatalytic destruction of these dyes using sunlight is an elegant and attractive potential solution. In this work, mesoporous ZnAl₂O₄ nanomaterials were prepared with high specific surface area, high pore volume and uniform pore size distribution using a modified sol–gel synthetic protocol. BET analysis showed that the highest surface area achieved for ZnAl2O4 reached 344 m²/g. The texture, structure and chemical composition of these materials were further studied using XRD, FTIR, TEM, EDS, CHN and XPS analysis and zeta potential measurements. Band gap energies were obtained from UV/vis spectra and showed a strong decrease as particle size decreased, shifting absorption into the visible range for the finest materials and indicating the potential to improve the efficiency of photocatalysis under solar irradiation. Photocatalytic activity was evaluated by studying the photodegradation of Congo Red dye and, for the best materials, showed that 99 % of the dye was degraded in only 30 mins. These results compared very favourably with those of previously reported studies. The degradation products of the Congo Red were further investigated using mass spectrometry and this detailed study confirmed that true photoinduced fragmentation of the dye molecule occurred rather than simple ‘photobleaching’.