TiO2@ZIF-8 hybrid as a type II heterojunction photocatalyst:adsorption/photocatalytic properties, kinetics, and effect of humidity

TiO 2 @ZIF-8 hybrid and its humidified forms are synthesized as type 2 heterojunction systems using a facile sol–gel method at different relative humidity (RH) conditions. The hybrids' surface characteristics and optical properties are thoroughly examined, followed by investigations into adsorption capacities and photocatalytic activities using anionic-methyl orange (MO) and cationic-methylene blue (MB) dyes. TiO 2 @ZIF-8 hybrid generates a mesoporous structure with a higher surface area (135.6 m 2 g −1 ) than bare TiO 2 (40.1 m 2 g −1 ). Band gap energies of TiO 2 @ZIF-8 (3.00 eV) and humidified hybrids (2.95–2.91 eV) decrease compared to the individual phases of TiO 2 (3.08 eV) and ZIF-8 (4.9 eV). XPS analysis verifies the electron donation from ZIF-8 to TiO 2 . Adsorptions of MO and MB differ depending on the affinities of MO or MB’s functional groups on the TiO 2 @ZIF-8 and humidified hybrids. TiO 2 @ZIF-8 demonstrates 98.7% MO and 89.5% MB degradations within 100 min irradiation. Dark adsorption studies follow Langmuir model with Langmuir constants of K L = 0.445 L mg −1 for MO in the presence of TiO 2 @ZIF-8 and K L = 0.409 L mg −1 for MB in the presence of TiO 2 @ZIF-8 (84% RH). Meanwhile, pseudo-second-order kinetic model is applicable for dark experiments with rate constants of k 2 = 0.918 g mg −1 min −1 for MO in the existence of TiO 2 @ZIF-8 and k 2 = 0.917 g mg −1 min −1 for MB in the existence of TiO 2 @ZIF-8 (84% RH). Under irradiation, TiO 2 @ZIF-8 (84% RH) has higher first-order rate constant ‘ k ’ values for both MO (0.0134 min −1 ) and MB (0.0146 min −1 ) relative to that of TiO 2 @ZIF-8 (0.0129 min −1 for MO and 0.0112 min −1 for MB). Based on Langmuir–Hinshelwood model, TiO 2 @ZIF-8 (84% RH)) shows the highest adsorption coefficients ( K ) of 0.164 L mg −1 for MO and 0.192 L mg −1 for MB and rate constants ( k ) of 0.197 mg L −1 min −1 for MO and 0.182 mg L −1 min −1 for MB. Notably, the hybrids maintain high stability and photocatalytic efficiency after four consecutive degradation cycles, highlighting their potential for sustainable environmental remediation.