Nanostructure Regulation Of TiO₂ On Flexible Substrates And The Characteristics Towards Photocatalytic Degradations Of Typical Organic Pollutants

Because of its merits of environmental benignity,low cost,and chemical stability,titania(TiO2)is widely recognized as a photocatalyst most likely finding practical applications in wastewater treatments.Several photocatalytic apparatus utilizing TiO2 nanoparticles in slurry to decompose organics pollutants in water have been commercialized.Nanosized TiO2 in a slurry system exhibits an efficiency several times that of immobilized one;yet the recovery of nanoparticles from the effluents remains a nuisance.It is thus of great interest to develop TiO2 thin films with high photocatalytic activity.To utilize immobilized TiO2,the choice of substrates deserves a serious consideration.An appropriate substrate should be of high flexibility so that the design and fabrication of photocatalytic reactors could be facilitated.Titanium mesh is an appropriate flexible substrate.In this study,a low temperature synthetic route was adopted on flexible Ti mesh substrates to deposit nanostructured TiO2 arrays with high photocatalytic activity,which was further improved by various branching tactics.Moreover,we combined the bulk compositing and surface functionalization routes to achieve a polymeric photocatalytic ultrafiltration membrane via surface functionalization with TiO2 nanowires.Both the photocatalytic activity and ultrafiltration performances of the organic-inorganic hybrid membrane were further enhanced by using a subsequent TiCl4 treatment to.achieve branches on TiO2 nanowires.Few-layer titanate nanobelts were firstly deposited on metallic Ti mesh using an aqueous H2O2 solution containing hexamethylenetetramine(HMT)and HNO3,which were then subjected to a subsequent hot water treatment to achieve crystallized TiO2 nanorod arrays.The Ti mesh substrate allows for overlapping of TiO2 thin films to make full use of the limited spaces in a photoreactor.The TiO2 nanorod arrays on Ti mesh(three layers overlapped)exhibited an exceptional high efficiency when utilized to assist photodegradation of rhodamine B in water under UV light illumination,which is 25 times that of P25 film,and even slightly higher than that of P25 nanoparticles in a slurry system.For photodegradation of phenol in water,the three-layer Ti mesh with surface TiO2 nanorods also exhibited a reaction rate constant 6.9 times that of P25 film.The reduced band gap,abundant surface hydroxyl groups,and mixed phase effects arising from the low temperature approach contribute to the high photocatalytic activity.The titanate arrays on Ti mesh were subjected to calcination in air to achieve anatase TiO2 nanowires,which were further immersed in three kinds of precursor solutions to obtain hierarchical nanostructures.The branching tactic is effective to increase the photocatalytic activity.The remained solution of the Ti-H2O2 approach to grow titanate films on Ti mesh(named as HMT precursor solution for simplicity)contains H202,HNO3 and Ti(IV)ions.The TiO2 nano wire arrays were immersed in the HMT precursor solution for 6-16 h to grow rutile branches.The branch length increased with increasing immersing duration.Thanks to the high specific surface area and mixed crystal effects,the photo-response and photocatalytic performance improved significantly.Under the UV light illumination to assist photodegradations of rhodamine B and phenol in water and toluene in air,the reaction rate constant of the branched TiO2 nanowires is 2.9,3.5 and 1.6 times that of the pristine TiO2 nano wires,respectively.A solution combustion combined with wet chemistry route was adopted to prepare a precursor solution containing H2O2 and Ti(IV)ions.Branches of anatase TiO2 nanosheets were grown on the anatase TiO2 nanowires after immersing them in the precursor solution for 30-75 min.The branching tactic increased the effective TiO2 loading and specific surface area.Under the UV light illumination to assist photodegradations of rhodamine B and phenol in water and toluene in air,the reaction rate constant of the branched TiO2 nanowires is 3.5,2.5 and 1.7 times that of the pristine TiO2 nanowires,respectively.After immersing the anatase TiO2 nanowires in the TiCl4 solution for 15-60 min,rutile TiO2 nanoparticles arising from the TiCl4 hydrolysis decorated on the anatase TiO2 nanowire trunks.Under the UV light illumination to assist photodegradations of rhodamine B and phenol in water and toluene in air,the reaction rate constant of the branched TiO2 nanowires is 2.05 3.3 and 1.8 times that of the pristine TiO2 nanowires,respectively.Based on the low temperature precipitation and crystallization techniques,an organic-inorganic hybrid filtration membrane was developed for photocatalytic and ultrafiltration applications.A flexible hybrid membrane of polyetherimide(PEI)and commercial P25 TiO2 nanoparticles was firstly synthesized via a solvent-induced phase separation technique,which was then surface functionalized by quasi-aligned TiO2 nanowires adhering tightly to the membrane,through multi-steps of surface roughening,titanate nanowires deposition and finally hot water treatment.The mercury porosimetry measurement shows that the mesopore size ranges from 2 to 100 nm with the peak maxima at 26 nm,and the porosity is ca.48.7%.Such TiO2 functionalized PEI/P25 membranes exhibited high and stable efficiency to assist photodegradations of rhodamine B in water under UV light illumination.The membrane possessed also high hydrophilicity,permeability and resistance to fouling with bovine serum albumin(BSA)solution.The surface roughened PEI/P25 membrane exhibited a pure water flux of 2165 L m-2 h-1,with a BSA retention rate of only 3.2%.After precipitated with TiO2 nanowires,the BSA retention rate increased to 93.3%while a high water flux of 595 L m-2 h-1 was still attained.After water rinsing followed by the UV light illumination,the membrane exhibited a water flux recovery ratio(FRR)of 88%,which suggests an enhanced antifouling performance.In order to enhance the photocatalytic and ultrafiltration performance of the TiO2/PEI/P25 membrane,the surface TiO2 nanowires were immersed the TiCl4 solution to deposit branches.Because of the high specific surface area and mixed crystal effects,the photocatalytic performance improved.The branches filled certain gaps between neighbouring nanowires,which in turn improved the ultrafiltration performances.The BSA retention rate increased to 96%whilst the water flux decreased to 430 L m-2 h-1,and the FRR increased to 94%.