Synthesis of titanate nanotubes via microwave hydrothermal treatment: Study on the characterization and photocatalytic potential

Despite TiO2-based photocatalysis has been extensively investigated and examined over the past decades, it is still a highly engrossing technology owing to the stability and low cost. Recently, TiO2-induced titanate nanotubes (TNTs) have received much attention as a result of high specific surface area. Traditional method in fabricating TNTs, however, needs at least 20 hr reaction time to achieve a perfect tube structure. Therefore, this research aimed to the rapid formation kinetics of TNTs with the aid of microwave irradiation and attempted to investigate the effect of microwave irradiation on the characterization of titanate nanotubes (microwave-induced TNTs). Photocatalytic behavior of microwave-induced TNTs towards the degradation of gaseous trichloroethylene (TCE) and aqueous ammonia (NH3/NH4+) were also examined to survey the photocatalytic potential of microwave-induced TNTs.ased on the performance of BET surface area (SBET), TNTs synthesized at 130℃ for 1.5 hr with and without 400W irradiation presented the SBET of 256 and 76 m2g-1, respectively. The result indicates that the formation kinetics of TNTs is significantly enhanced via microwave hydrothermal treatment. The microwave-induced TNTs are preferentially assigned for NaxH2-xTi3O7 whose Na/H ratio is dominated by the applied lever of microwave irradiation during fabrication process. This phenomenon can be evidenced by various determinations including powder X-ray diffraction, NH3-temperature programmable desorption, X-ray photoelectron spectroscopic, and ionic coupled plasma-atomic emission spectrometry. Regarding the behavior of TNTs after thermal treatment, TNTs with abounding H atoms presented anatase phase at 500℃ through rearrangement and restacking of [TiO6]. The sintered TNTs synthesized under high irradiation power presented the rod shape at 700℃ which mainly comprise of Na2Ti6O13. The (Ti6O13)2- unit within Na2Ti6O13 is constructed by two (Ti3O7)2- layers within TNTs via the topotactical connection along the [110] direction during thermal process.s for the photocatalytic potential of TNTs, a pure TNTs phase presents no powerful ability towards photocatalytic NH3/NH4+ while the photocatalytic efficiency can be enhanced with the presence of rutile phase within TNTs. Regarding the effect of acid-washing treatment on TNTs, the acid-treated TNTs with enhanced ion exchangeability considerably improve the NH3/NH4+ degradation and NO2-/NO3- yields. This result is likely ascribed to the easy intercalation of NH3/NH4+ into the structure of acid-washing TNTs so that the photocatalytic oxidation of intercalated NH3/NH4+ is not limited to the shielding effect resulting from the overload of TNTs. In the case of photocatalytic TCE over TNTs, the efficiency of TCE degradation enhances with increasing sintering temperature until 700℃。This phenomenon is attributed to the recrystallization of anatase phase and the construction of inter-particle electron transfer effect. Photocatalytic TCE over Pt/Pd doped TNTs-induced TiO2 was also investigated in terms of the effect of Pt and Pd on the TCE degradation and on the yields of dichloroacetyl chloride (DCAC) and phosgene. In the presence of Pt and Pd, the degradation of TCE was retarded; especially Pd had a significantly negative effect on TCE degradation, which was ascribed to the intercalation of Pd into the lattice of TiO2. Moreover, Pt had no influence on the selectivity toward DCAC and phosgene while the selectivity toward phosgene in the presence of Pd was enhanced. As for the behavior of Pt and Pd in TCE degradation, Pt doped TiO2 exhibited the same photocatalytic behavior as P25 TiO2 whereas Pd doped TiO2 led to a different photocatalytic mechanism. Although microwave-induced TNTs have no powerful ability in photocatalysis and ion exchange, they can still be considered as a potential material in some applications owing to the corresponding bi-functions.