Facile synthesis of novel microporous CdSe/SiO₂ nanocomposites selective for removal of methylene blue dye by tandem adsorption and photocatalytic process

CdSe is considered a promising photocatalyst that exhibits a lower band gap energy (1.7 eV). However, the high cost of its metal precursors, low surface area and the rapid electron–hole recombination are the main problems that limit its industrialization. In this research work, incorporation of various proportions of CdSe (0–10 wt%) on the surface of microporous silica (432 m²/g) were carried out through ultrasonic route to enhance the chemical interaction between the synthesized nanoparticles and the support. X-ray diffraction (XRD), N₂-adsorption–desorption isotherm, diffuse reflectance spectra, photoluminescence, field emission scanning electron microscope and high resolution transmission electron microscope (HRTEM) were carried out to investigate the physicochemical properties of the novel nanocomposites. HRTEM and XRD analysis indicate the homogeneous dispersion of CdSe nanoparticles of crystallite size (8–19) nm on silica surface. The influence of varying the concentration of CdSe on its degree of dispersion on silica via the hydroxyl group that dispersed on silica negatively charged surface was monitored. The experimental results have pointed out that the nominal value required for spreading CdSe as monolayer on silica surface is 3.25 molecules of CdSe per 100 Å² of silica that is considered the most reactive photocatalyst in removal of MB dye. Below this nominal value, the amount of CdSe dispersed on silica is not enough to produce the required number of reactive radicals responsible for dye degradation. However, above the nominal value, the aggregation of CdSe layers on silica surface prevent the light penetration and induce electron–hole centers that reduce the lifetime of the photocatalyst. The removal of methylene blue dye via tandem adsorption and photocatalytic route increases with increasing CdSe concentration up to 10 wt% that is responsible for removal 90% of MB dye followed by reducing the reactivity for the sample containing 15 wt% CdSe. A direct relationship between adsorption and the photocatalytic reactivity is purposed assuming that the photocatalytic process require adsorption on high surface area nanoparticles as prime key step in increasing the contact probability between the photocatalyst and the organic dye that facilitate the production of large number of reactive radicals available for dye mineralization. TOC and COD analysis confirmed the complete mineralization of MB dye. The scavenger results indicate that both positive hole and hydroxyl radicals are the predominant reactive species for degradation of MB dye over CdSe and CdSe10. The achievement of controlling the dispersion of metal selenide on a support using ultrasonic route would certainly open a new approach for catalysis preparation.