Surface-associated Cu2S nanodomains on FAU NaX zeolite for sulfidation-dependent sunlight-assisted methylene blue decolorization

Abstract

Cu₂S nanoparticles supported on commercial F9 NaX zeolite (FAU) were prepared and evaluated for sunlight-assisted decolorization of methylene blue in water. Cu²⁺ was introduced by aqueous ion exchange (0.025-0.10 M CuSO₄, 50 °C, 25 min; natural pH) and converted to sulfide by in situ sulfidation with Na₂S under matched conditions. XRD and FTIR support retention of the FAU framework after both steps. Diffuse reflectance shows broad visible absorption (350–500 nm) with a maximum near 438 nm that increases with nominal sulfidation concentration. HRTEM analysis of ZF9 + Cu₂S-0.1 reveals dispersed crystalline nanodomains (18 ± 2 nm, n = 100) associated with the host; lattice fringes (d = 1.99 Å) are consistent with Cu₂S (110). Decolorization tests were performed under sunlight using methylene blue (15 ppm, 25 mL) and 0.10 g solid after 20 min dark equilibration; A0 was defined after the dark step and absorbance was monitored at 664 nm. Dark adsorption and photolysis blanks were included, and outcomes are reported as methylene blue decolorization from the 664 nm absorbance response. ZF9 + Cu₂S-0.1 reached ~ 95% overall decolorization at 100 min, whereas lower nominal sulfidation concentrations were largely adsorption-driven. Overall, the sustained decrease in the MB band during sunlight exposure occurs only for ZF9 + Cu₂S-0.1, which exhibits the strongest Cu-S-related optical response, with NaX acting as a growth and stabilization matrix rather than a strict micropore-confinement medium.