Molten Salt Assisted Doping Of Transition Metal Into TiO₂ Nanowires And Their Photocatalytic Properties

Semiconductor photocatalysis displays important application prospect in combating environmental pollution and energy shortage.Therein,TiO₂ becomes benchmark photocatalyst due to advantages such as non-toxic,high activity,acid and alkali resistance and anti-photo corrosion.Recent study found that doping by part of the transition metal can broaden absorption of TiO₂ to visible light region,overcoming the shortcoming of low activity of TiO₂ under visible light irradiation.However,whether transition metal doping is benefical to improve the efficiency of TiO₂ photocatalysis,especially for the enhancement of UV activity remains controversial.In fact,many factors such as TiO₂ crystal phase and quality,valence state of doping ion,doping concentration,doping method do play important role in the evaluation of effect of doping on activity enhancement.Among transition metal ions,Nb⁵⁺ is favored because of its similar electronegativity and ionic radius to that of Ti⁴⁺.However,charge mismatch between Nb⁵⁺and Ti⁴⁺inevitably brings additional defects such as Ti³⁺ and oxygen vacancy into TiO₂,which in turn acts as recombination centre of electrons and holes and hence results in decreased activity under UV light.In comparison with anatase,rutile could introduce less defects at the same doping concentration.Meanwhile,the higher stability of rutile phase than that of antase provides more choice of doping ways for doping of Nb into highly crystalline TiO₂.In this thesis,we firstly attempt to employ a molten salt assisted crystallization route to access to highly crystalline rutile TiO₂ nanostructures.By selection of NaCl/ Na₂HPO₄ as molten salt media and P25 TiO₂ as precursor,calcinations at 825 ? could produce single crystalline rutile TiO₂ nanowires with diameters of 50-200 nm and length of 0.5-10 ?m.For Nb doping into TiO₂,it is found that direct use of Nb₂O₅ precursor surprisingly work well in uniform distribution of Nb dopant into TiO₂,which can reduce the cost of doping.Structural characterization results reveal following issues:?1?At a 2 at% Nb dopant concentration,the growth of rutile nanowirs is restricted,the diameters are narrowed to 20-100 nm and lengths are shorten to 50-100 nm.However,the high crystallinity can be well maintained.?2?Phase transition of anatase to rutile can be inhibited by Nb doping,the residue proportion of anatase is 22.61%.?3?Different from previous Nb⁵⁺doped TiO₂,molten salt assisted doing make Nb dopant exist as Nb⁴⁺,which fundamentally avoid the introduction of unfavored defects induced by charge compensation.By monitoring the degradation of acetaldehyde and evolution of CO₂ over catalyst under solar light irradiation,the Nb doped TiO₂ nanowires display significantally higher activity than that of pristine TiO₂.In other words,the reaction rate constant increases from 4.51×10-3min-1 to 37.3×10-3min-1,suggesting that molten saltassisted doping successfully boost the activity of TiO₂ under solar light.Further photoelectochemistry measurement indicates that the enhancement of activity is beneficial from upshift of Fermi level induced by Nb doping,fewer defects and high crystallinity of nanowires.To investigate the applicability of transition metal doping by molten salt flux,we employ Ta₂O₅?MoO₃?WO₃ as doping source,respectively,to prepare transition metal TiO₂ nanowires.As expected,other transition metal doped TiO₂ nanowires with high crystallinty and even dopant distribution are againly produced.Photocatalytic tests via acetaldehyde degradation show that Ta doping can also improve the activity of TiO₂,and whereas Mo and W doping give rise to little contribution on activity enhancement.Finally,the activity of transition metal doped TiO₂ can be further improved by loading of CuOx cluster,which confirms their potential value of application on environment purification and exploring of alternative energy.