Sunlight-assisted degradation of methylene blue using surface-decorated AuNP@ZIF-8 and AuNS@ZIF-8 nanocomposites synthesised via a time-gated growth strategy

Abstract

Methylene blue is a toxic and persistent dye that requires effective removal from aquatic environments. This study reports sunlight-assisted degradation of methylene blue using surface-decorated AuNP@ZIF-8 and AuNS@ZIF-8 nanocomposites prepared by a time-gated growth strategy, in which preformed gold colloids were added 1 h after the onset of ZIF-8 crystallisation to favour surface localisation. Under indirect indoor sunlight, 1 mg/L methylene blue showed 7% and 14% degradation after 150 minutes with AuNP@ZIF-8 and AuNS@ZIF-8, respectively, while methylene blue alone showed negligible photolysis. UV–Vis spectral changes indicated oxidative degradation rather than simple chromophore bleaching. Under direct outdoor sunlight at 10 mg/L methylene blue and 10 mg/L catalyst, all catalyst systems showed similar degradation of about 57-58% over 240 minutes, whereas methylene blue alone reached 47%, indicating that high photon flux can mask plasmon-specific effects. SEM, PXRD, Raman and UV–Vis confirmed successful composite formation, preserved ZIF-8 crystallinity, and stronger light-matter interaction in AuNS@ZIF-8. These stable, visible-light-responsive nanocomposites show promise for low-resource environmental remediation.