Preparation Of Three-Dimensional TiO₂ Microspheres By Solvothermal Method And The Photocatalytic Activities

Recently,threedimensional(3D)hierarchical TiO2 microspheres structure selfassembled from onedimensional(1D)nanostructures was reported to be an effectivemethod to improve the quantum efficiency and recyclability of TiO2.And the solvothermal method is considered as the most appropriate way to synthesize thus 3D hierarchical structure.However,the non-polar solvents with higher toxicity were mainly used as the reaction medium for solvothermal method,the successful preparation based on polar solvents has rarely been reported,and the mechanisms remained unclear.Moreover,carbon doping can widen the light response region of TiO2 to visible light region.Nevertheless,most of the carbon doping methods rely on external carbon sources and high temperature calcination,which not only increase the synthesis cost but also might damage structure of the obtained samples,and result in poor photocatalytic activity.Therefore,this study aims to fabricate a carbon-doped 3D hierarchical TiO2 microspheres structure using polar solvent as reaction medium and carbon precursor via a facile solvothermal method under mild reaction conditions.The main contents and results are as follows:In the first phase of this study,several typical solvents,including alkane,aromatic hydrocarbons,halohydrocarbon,ketone,organic acid,mono-and dihydric alcohols,were selected tosynthesize 3D hierarchical TiO2 microspheres via solvothermal method.Combining the morphologies,crystal structures,photocatalytic activities of the finalproducts with solvent properties,the dielectricconstant of solvents was the decisive factor for the formation of quasi-3D urchin-like TiO2 microspheres selfassembled from 1D nanostructures,and the low pH of reaction medium was also helpful.In addition,it is worth noting that the sample prepared from the polar solvent acetone is a unique 3D hierarchical structure,unlike the samples synthesized by other polar solvents in this study.Based on the results of the first phase,the golden wattle-like 3D hierarchical TiO2 microspheres was successfully synthesized by solvothermal method using acetone and hydrochloricacid(HCl)as solvent and morphology control agent,respectively.The effect of HCl addition on the final product was investigated as well.According to the experimental results,the amount of HCl added during thesolvothermal reaction was found to play an important role in adjustingthe morphology and properties of the final products,and an optimum HClamount at 2ml was discovered.In addition,the growth mechanism of the goldenwattle-like TiO2 microspheres was supposed to be a three-stepprocess:(?)fast nucleation and nuclei growth;(?)nanoparticledissolution and recrystallization;(?)nanorod growth and selfassembly.In order to farther improve the visible induced photocatalytic activity of TiO2 and based on the optimal operationconditions obtained in the first and second phase of this study.The carbon doped golden wattle-like 3D hierarchical TiO2 microspheres was obtained by solvothermal method.During the synthesis,growth and self-assembly of TiO2 single-crystal nanorods were successfully achieved simultaneously with in-situ carbon doping via employing only acetone as reaction medium and carbon source.The obtained sample exhibited remarkable photodegradation capability for ciprofloxacin(CIP)under visible light irradiation since the carbon doping and the unique 3D hierarchical structure.Calcination temperature during the synthesis process was proved to be essential for the properties of the final products,and theoptimum calcination temperature was 200?.In addition,both ·O2-and valence band holes were proved to be the main active species for CIP degradation,and CIP followed piperazine side chain degradation pathwayduring the photocatalytic reaction over the synthesized product.