Photodegradation of 4-nitrophenol over B-doped TiO2 nanostructure: effect of dopant concentration, kinetics, and mechanism.

The 4-nitrophenol (4-NP) is one of the carcinogenic pollutants listed by US EPA and has been detected in industrial wastewater. This study investigates the photocatalytic degradation of 4-NP with TiO2 and boron (B)-doped TiO2 nanostructures. The degradation on undoped and B-doped TiO2 with various boron loadings (1-7%) was studied to establish a relationship between structure, interface, and photo-catalytic properties. The results of XRD, micro Raman, FTIR, and HRTEM show that the B doping has improved the crystallinity and induces rutile phase along with anatase (major phase). The N2 adsorption-desorption, SEM-EDX, and XPS indicated that the B induced the formation of mesoporous nanostructures in TiO2 and occupies interstitial sites by forming Ti-O-B type linkage. The surface area of pure TiO2 was decreased from 235.4 to 63.3 m2/g in B-TiO2. The photo-physical properties were characterized by UV-Vis DRS, which showed decrease in the optical band-gap of pure TiO2 (2.98 eV) to B-TiO2 (2.95 eV). The degradation results demonstrated that the B doping improved the photocatalytic activity of TiO2; however, this improvement depends on the B concentration in doped TiO2. B-doped TiO2 (> 5% B) showed 90 % degradation of 4-NP, whereas the undoped TiO2 can degrade only 79 % of 4-NP. The degradation followed pseudo-first-order kinetics with rate constant values of 0.006 min-1 and 0.0322 min-1 for pure TiO2 and B-TiO2 respectively. The existence of a reduced form of Ti3+ on the surface of TiO2 (as evidence from XPS) was found responsible for enhancement in photocatalytic activity.