Synthesis and characterization of TiO2-based nanostructures via fluorine-free solvothermal method for enhancing visible light photocatalytic activity: Experimental and theoretical approach

Surface-fluorinated anatase TiO2 nanosheets have been widely prepared using hydrofluoric acid (HF) as morphology controlling agent with several titanium alkoxides. However, hazardous and other scientific problems associated with HF are major obstacles towards its global commercialization. Therefore, this study introduces a facile, green and fluorine-free solvothermal synthetic approach to prepare anatase TiO2 nanosheets (TNSs), involving commercial TiO2 nanoparticles as titanium source and N, N dimethylformamide (DMF) as morphology controlling agent under alkaline environment. A Box-Behnken-Design 3-level experimental design was applied to optimize the major key synthesis parameters for tuneable TNSs performance by manipulating the reaction temperature (120-180 degrees C), reaction time (3-48 hr) and dose of DMF (5-40 mL). The proposed model was verified through the analysis of variance and performing experiments at predicated optimal conditions (tem-perature =165.35 degrees C, time = 25.50 h and DMF dose =22.50 mL). The quadratic model was found to be best-fitted with the maximum correlation coefficients (R-2 = 0.989 and adjusted R-2 = 0.975) between predicted and actual results. The crystal growth mechanism of TNSs was also proposed. The formation of tetragonal TNSs (10 nm) containing the highest surface area with least bandgap upto 2.62 eV, confirmed through XRD, TEM, BET, DRS, FTIR and XPS measurements. The excellent photocatalytic performance was exhibited by as-developed TNSs for phenol degradation upto 95.33 % under visible-light. The photoactivity of TNSs was found to be strongly associated with synthesis parameters including hydrothermal temperature, reaction time and DMF dose. The phenol degradation data was well explained via pseudo-first-order kinetics and Langmuir isotherm models. Thermodynamic parameters were suggested the nature of phenol degradation process. This enhanced degradation activity of TNSs was attributed to the well-defined morphology, larger surface area, improved energy harvesting aptitude and abated recombination rate of photogenerated electron hole-pairs resulting an enhancement in the photogenerated charge carrier.