Focus on the photocatalytic pathway of the CdS-AgBr nano-catalyst by using the scavenging agents

Abstract

The CdS-AgBr nano-composite (NC) was prepared in-situ and characterized by XRD, SEM-EDX, UV–Vis DRS, and photoluminescence (PL) techniques. Crystallite size of about 53 nm was estimated for the NC sample by the Williamson-Hall equation. Then, the samples were used in the photodegradation of methylene blue (MB) in aqueous solution. In initial experiments, the photocatalytic activity of NC was 1.16 and 1.42 times greater than those of individual AgBr and CdS NPs in a 2 ppm MB solution without any optimization, respectively. The kinetic effects of some scavenging agents were studied to elucidate the photodegradation pathway and the change in k-values was compared statistically. For example, the average k-values of 9.13 × 10−2 and 5.02 × 10−2 for a 0.5 mM MB at pH 6 in the absence and presence of 0.5 mM nitrate were compared and an experimental t-value of 37.5 was obtained which is greater than the critical value of t0.05,4 = 2.78. This means that the two compared averages are significantly different at a 0.95% confidence interval. The scavenging effects depended on the solution pH and concentration of MB and the type of the scavenger. For example, a lower scavenging effect for carbonate species was observed at pH 2.5 concerning pH 4.5 and 9. The following trend was achieved for the investigated scavengers in a decrease in the rate of MB photodegradation process: Chloride > Carbonate > AA ≈ Nitrate, confirming the important roles of the photogenerated holes and hydroxyl radicals for the MB degradation. The results confirmed that the MB photodegradation pathway by the proposed composite proceeds via a direct Z-Scheme mechanism.