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

TiO₂@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. TiO₂@ZIF-8 hybrid generates a mesoporous structure with a higher surface area (135.6 m² g^-1) than bare TiO₂ (40.1 m² g^-1). Band gap energies of TiO₂@ZIF-8 (3.00 eV) and humidified hybrids (2.95-2.91 eV) decrease compared to the individual phases of TiO₂ (3.08 eV) and ZIF-8 (4.9 eV). XPS analysis verifies the electron donation from ZIF-8 to TiO₂. Adsorptions of MO and MB differ depending on the affinities of MO or MB's functional groups on the TiO₂@ZIF-8 and humidified hybrids. TiO₂@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 TiO₂@ZIF-8 and K_L = 0.409 L mg^-1 for MB in the presence of TiO₂@ZIF-8 (84% RH). Meanwhile, pseudo-second-order kinetic model is applicable for dark experiments with rate constants of k₂ = 0.918 g mg^-1 min^-1 for MO in the existence of TiO₂@ZIF-8 and k₂ = 0.917 g mg^-1 min^-1 for MB in the existence of TiO₂@ZIF-8 (84% RH). Under irradiation, TiO₂@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 TiO₂@ZIF-8 (0.0129 min^-1 for MO and 0.0112 min^-1 for MB). Based on Langmuir-Hinshelwood model, TiO₂@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.