One-step Synthesis Of Nano-crystal TiO₂Photocatalyst From Oxidation Of Titanium Tetraisopropoxide In Gliding Arc Plasma

Titanium dioxide (TiO2) as one of the most promising photocatalysts has been widely used for the purification of various toxic organic compounds in water and air. Crystallinity, phase composition and specific surface area of TiO2are the key factors to affect its photocatalytic activity, which is relevant to its synthesis technique. In this paper, using tetraisopropyltitanate (TTIP) as titanium source, gliding arc plasma was explored for one-step synthesis of nano-crystal TiO2UV photocatalyst and carbon-doped TiO2visible-light photocatalyst for the first time. The results are as follows:1. Using air as swirl gas, one-step synthesis of nano-crystal TiO2photocatalyst in gliding arc plasma was studied. Specific energy input (SEI) of gliding arc plasma was changed via varying discharge power and air flow rate, respectively. The effects of SEI on crystal composition, crystallinity, specific surface area, particle size, morphology and photocatalytic activity of as-synthesized TiO2nanopowders were investigated.Gliding arc plasma can directly synthesize TiO2nanopowder which was almost fully crystalline and consisted of anatase and rutile. At SEI=38.2kJ/mol, the anatase weight fraction (fA) of as-synthesized TiO2nanopowders was64.6%, BET specific surface area (SBET) was58.2m2/g, the morphology of most particles was non-spherical, the the distribution of particle size ranged from10to60nm, the average size was about24nm. With increasing SEI, the anatase weight fraction (fA) increased linearly,SBET decreased first rapidly and then slowly, the morphology of particle changed from non-spherical to spherical, while the distribution of particle size widened. At SEI=76.7kJ/mol,fA rapidly increased to96.6%, SBET decreased to24.4m2/g, the morphology of almost all particles was spherical, the the distribution of particle size ranged from10to150nm, the average size was about63nm. The O/Ti atomic ratio of as-synthesized nanopowders was about2.1and no N-doped TiO2was formed.SEI of gliding arc plasma had significant effect on photocatalytic activity of as-synthesized samples. UV-irradiated apparent rate constant (kUV) increased slowly with SEI when SEI was below47kJ/mol.kUV remained constant when SEI ranged from47to61kJ/mol. kUV rapidly increased when SEI increased from61to69kJ/mol. kUV decreased slowly with SEI when SEI was beyond69kJ/mol.According to the characterization of samples and ICCD imaging of gliding arc discharge, the mechanism of TiO2synthesized in gliding arc plasma was inferred:the spherical particles of TiO2were formed via vapor-liquid-solid mechanism in the arc channel; the non-spherical particles of TiO2were formed via vapor-solid mechanism in the arc channel diffusion region.2. TiO2nanopowders with different morphology and fA were synthesized by gliding arc plasma and calcined at different temperatures. The effects of thermal treatment on crystal phase, particle size and SBET of TiO2nanopowders were investigated and compared with the amorphous TiO2nanopowders. The effect of fA on the photocatalytic activity of TiO2nanopowders was investigated.The anatase-to-rutile transformation temperature of spherical nano-crystal TiO2was around650℃, which was higher than that of non-spherical nano-crystal TiO2. When particle size and the initiaL fA were similar, the anatase-to-rutile transformation rate of spherical particle was higher than that of non-spherical particle at the same calcination temperature.After thermal treatment at different temperatures fA of samples varied in large range, but crystal size and SBET were similar. With the increase of fA, kUV of photocatalytic reaction presented three profiles:kUV slowly increased when fA was below70%; kuv rapidly increased when fA incrased from70%to85%; kUV rapidly decreased when fA was beyond85%. The result confirmed that there was a synergetic effect between anatase and rutile and small amount of rutile would improve the photocatalytic activity of nano-crystal TiO2.3. Using nitrogen as swirl gas and TTIP as the precursor, one-step synthesis of C-doped TiO2visible-light photocatalyst by tuning the molar ratio of O2to TTIP (O2/TTIP) in gliding arc plasma was studied.With the increase of O2/TTIP,fA of as-synthesized samples showed non-monotonic change:fA was90%when O2/TTIP was0; fA was kept at-65%when O2/TTIP ranged from18to36;fA decreased to57%when O2/TTIP increased to54;fA turned to increase to90%when O2/TTIP increased to147. The relative crystallinity of samples dramatically rose from0.53to1.1when O2/TTIP increased from0to18. Subsequently, the crystallinity increased slowly with O2/TTIP. The characterization of BET showed that, SBET of sample was56.9m2/g when O2/TTIP was0; SBET was kept at～40-50m2/g when O2/TTIP ranged from18to54; SBET rapidly decreased to25.7m2/g when O2/TTIP increased to147. XPS indicated that carbon atoms were incorporated into the interstitial positions of the TiO2lattice and strong covalent bonds with two adjacent O atoms were formed. Interstitial carbon dopant resulted in the formation of Ti3+and oxygen vacancy (□). With the decrease of O2/TTIP ratio, the content of carbon dopant gradually increased, the Ti3+/□/Ti ratio of the C-doped titania samples firstly increased then decreased. The Ti3+/□/Ti ratios of S18. S27. S36and S147were calculated at0.6/0.3/1,0.7/0.35/1,0.3/0.15/1and0/0/1, respectively. With the decrease in O2/TTIP ratio from147to18, the band gap of samples dropped slowly from2.97to2.71eV, the band gap of SO further drastically dropped to2.31eV when O2/TTIP ratio decreased to0. When the O2/TTIP ratio ranged from18to36, the as-synthesized samples showed high visible-light photocatalytic activity. For the reaction of MB degradation under visible-light irradiation (λ>420nm), the visible-light-irradiated apparent rate constant (kVlS) of S18, S27and S36is 0.054,0.079and0.063h-1, respectively. S0, S54and S147did not exibit any visible-light photocatalytic activity.