Determination of X-Ray Diffraction on the Phase Transformation of Microwave-Assisted Titanate Nanotubes during Thermal Treatment

Based on the determination of X-ray powder diffraction, this study aims to investigate the thermal effect on the phase transformation of microwave-assisted titanate nanotubes (MTNTs). The phase transformation is highly dependent on the intercalating amount of Na(I) within MTNTs and on the heating atmosphere. In other words, the presence of Na(I) favors the transformation of TNTs phase into Na2Ti6O13 whereas anatase phase selectively formed in the case of MTNTs with less Na(I) amount. Furthermore, H2 versus O2 is able to form anatase phase and establish a newly transformation pathway. The photocatalytic ability of the calcined MTNTs was also evaluated based on the observed rate constant of trichloroethylene degradation. In addition to anatase phase, the newly phase including Na2Ti6O13 and Ti2O3 with calcined MTNTs is able to photocatalyze trichloroethylene. MTNTs calcined with the presence of H2 also exhibit a superior photocatalytic performance to P25 TiO2.


Introduction
Titanate nanotubes (TNTs) have received engrossing interests owing to the vast applications including solar cells, photocatalysis, and electroluminescent hybrid device [1][2][3][4][5].An overview regarding the fabrication, characterization, and application of TNTs was also demonstrated in [6].In spite of the debate on the formation mechanism and chemical structure of TNTs, the application of TNT or even its derivates (nanoparticles and nanorods) on the photocatalytic field has been emerging [3][4][5].Several studies have reported that TNTs without further treatment have no powerful photocatalytic ability [3,4,7].Tsai and Teng [3] indicated that the thermal-treated TNTs versus P25 TiO 2 exhibited a better performance on NO conversion.Similar result was indicated by Yu et al. [4] in terms of the microstructure of calcined TNTs.Furthermore, Štengl et al. [7] investigated the preparation, microstructure characterization, and photocatalytic activity of sodium titanate nanorods which were fabricated by using TNTs as materials.The single-crystalline TiO 2 nanorods with highly activity and long-term stability have also been demonstrated in [8].
Our previous reports have characterized microwaveassisted TNTs (MTNTs) whose chemical structure was preferentially assigned for Na x H 2−x Ti 3 O 7 [5,9].The intercalation amount of Na(I) within MTNTs increases with enhancing applied level of irradiation powers during fabrication process [9].The photocatalytic oxidation of aqueous ammonia over microwave-induced MTNTs was also examined recently in [5].A specific degradation mechanism of photocatalytic ammonia over MTNTs was also proposed that the reaction of photocatalytic ammonia was not suffered from shielding effect.In the present study, the thermal effects including calcination temperature and atmosphere on the phase transformation of MTNTs were examined.The photocatalytic ability of calcined MTNTs towards gaseous trichloroethylene (TCE) degradation was also investigated.

Preparation and Thermal Treatment of MTNTs.
The fabrication of MTNTs is described in detail elsewhere in our previous studies in [5,9].In a typical process, the microwave digestion system (Ethos Touch Control, MILESTONE Corporation) with a double-walled consists of an inner Teflon liner and outer shell of high strength Ultem polyetherimide, was used.The applied temperature, power level , as well as treatment time, are programmable with an integrated computer.The slurry containing 0.6 g P25 TiO 2 and 70 mL 10 N NaOH was treated at 130 • C for 3 hours under different microwave irradiation powers of 70, 400, and 700 W which are abbreviated as MTNTs-70 W, MTNTs-400 W, and MTNTs-700 W, respectively.After microwave thermal treatment, the resulting powders were washed by 0.5 N HCl until the pH of slurry achieved 7. The resulting powders were subsequently washed by deionized water repeatedly to remove the excess of HCl adhered on the surface of MTNTs.MTNTs were subsequently obtained after the filtration and dryied under the vacuum freeze dried for 20 hours (−56 • C; 100∼200 Mtorr).Regarding the thermal treatment of TNTs, the experiments were conducted in the tubular oven under the atmospheres of 20% O 2 /80% N 2 and 20% H 2 /80% N 2 at the progressively elevated temperatures for 3 hours (100, 300, 500, and 700 • C).The ramping rate was controlled at 2 • C/min.The following powders subsequently cooled down to room temperature and were characterized by phase determinations.

XRPD Determinations of MTNTs.
The X-ray powder diffractometer (XRPD) was employed to determine the crystalline phases of MTNTs after thermal treatment.The patterns were recorded from 5 to 70 • with a scan rate of 2 • /min by Philips X' Pert Pro MPD which was equipped with a CuKα X-ray source (λ = 1.5405Å) operating under a voltage of 40 kV and a current of 30 mA, respectively.

Photocatalytic Oxidation of TCE over MTNTs.
Batch experiments of photocatalytic TCE over MTNTs were conducted in a flat-plate photoreactor, which has been described in detail in our previous publication [10].MTNTs sample was coated on a silica board with the normalized concentration of 0.83 mg/cm 2 .The as-prepared photocatalyst was deposited on the bottom of the reactor without further treatment.The initial concentration of TCE was prepared at 70 ± 2.5 μM, and the initial relative humidity and oxygen concentration were controlled at 25 ± 2.5% and 3.53 mM, respectively.Illumination provided by three 8W black-light fluorescent lamps principally emits at 365 nm with the light intensity of 2.34 ± 0.23 mW/cm 2 .The concentration of TCE, under a series of time sequence during photocatalytic experiments, was quantified via the manual injection into the gas chromatograph (5890II) equipped with the flame ionization detector.The photocatalytic ability of TNTs was quantitatively evaluated by the corresponding observed rate constants (k obs ) of TCE degradation.

XRD Determinations on the MTNTs after Thermal
Treatment.XRPD determination was used to investigate the phase of MTNTs after calcination at increasing temperatures (100∼700 • C) under the atmosphere of either O 2 or H 2 .In   the following materials, samples indexed as MTNTs X−Y • C represent MTNTs calcined at Y • C under X atmosphere.Figures 1-3 show the dependence of phase change of MTNTs on the heating temperature and atmosphere.Either in the presence of O 2 and H 2 , no appreciable change of MTNTs phase was observed as the heating temperature was lower than 300 • C. In addition, an interesting phenomenon was observed that the phase transformation of MTNTs-70W begins at 500 • C whereas no phase change was observed for MTNTs-700W calcined at the same temperature.According to our previous report [5,9], a high level of microwave irradiation during fabrication process favors the intercalation of Na(I) into MTNTs.In other words, the intercalated Na(I) is advantageous to the stability of MTNTs during thermal treatment.
As for MTNTs-70W, anatase phase dominates for either MTNTs-70W O2-500 • C (Figure 1(a)) or MTNTs-70W H2-500 • C (Figure 1(b)), and an increasing temperature results in the transformation of anatase phase into rutile phase (TNTs-70W H2-700 • C and MTNTs-70W O2-700 • C ). TNTs-70W H2-700 • C versus MTNTs-70W O2-700 • C exhibits a stronger intensity of the anatase phase, suggesting that H 2 is beneficial to maintain anatase phase.In the case of MTNTs-400W (Figure 2), anatase phase along with a trace amount of rutile phase exists in MTNTs-400W O2-500 • C and an increase in temperature to 700 • C results in the formation of Na 2 Ti 6 O 13 at the expense of anatase phase (Figure 2(a)).Comparatively, TNTs phase still dominates at MTNTs-400W H-500 • C whereas TNTs-400W H2-700 • C presents the relatively significant anatase, rutile, and Na 2 Ti 6 O 13 phase (Figure 2(b)).This result not only corresponds to the aforementioned conclusion in that H 2 helps to establish anatase phase within MTNTs but also indicates that O 2 provides the prevailing ability to transform TNTs phase.Regarding the phase transformation of TNTs-700W, Na 2 Ti 6 O 13 is the predominant phase in MTNTs-700W O2-700 • C (Figure 3(a)) whereas Ti 2 O 3 and Na 2 Ti 6 O 13 with trace anatase phase appear in the TNTs-700W H2-700 • C (Figure 3(b)).These results reveal that the presence of H 2 leads to a selective transformation pathway as MTNTs was intercalated with abounding amount of Na(I).
Usually, for pure anatase TiO 2 samples, the rutile phase starts to appear at ca. 700 • C [11].Tsai and Teng [3] have also indicated that the rutile phase arose at 900 • C as far as the traditional TNTs are concerned.In our study, rutile phase can be observed as MTNTs were treated at 500 • C under O 2 atmosphere (Figure 1(a)), which suggests that [TiO 6 ] octahedra are easy to be transformed.This result can be ascribed to the application of microwave irradiation during fabrication process of MTNTs.Furthermore, the appearance of Na 2 Ti 6 O 13 can be derived from the dehydration of Na x Ti 2-x Ti 2 O 5 (H 2 O) and is found to be more abundant for TNTs washed and suspended under more basic conditions, that is, more Na(I) was intercalated into TNTs [12,13].The demonstration in our result is consistent to the results of above researches and again confirms that the applied microwave irradiation favors the intercalation of Na(I) into MTNTs structure.

Photocatalytic Oxidation of Gaseous TCE over MTNTs.
Figure 4 demonstrates the dependence of k obs on the heating temperature and atmosphere.No TCE degraded over MTNTs without thermal treatment, suggesting that MTNTs have no powerful ability to TCE degradation.This result corresponds to the XRPD determination that anatase phase, a major crystalline phase responsible for organic degradation, arose at the heating temperature higher than 300 • C. As for MTNTs-70W O2 and MTNTs-400W O2 (Figure 4(a)), k obs increase with increasing calcination temperature until 500 • C. A further increase in heating temperature to 700 • C results in a decrease in k obs .The phenomenon can be ascribed to the absence or low crystallization of anatase phase as MTNTs calcined at 700 • C. Na 2 Ti 6 O 13 phase appearing in MTNTs-700W O2-700 • C is also effective to the degradation of TCE despite the k obs value (0.74 ± 0.15 min −1 ) is not excellent.The photocatalytic ability of Na 2 Ti 6 O 13 can be attributed to the corresponding tunnel structure, which provides a number of active sites and has a significant effect on the efficiency of separation of the irradiated generated photos [14,15].Regarding the MTNTs calcined under H 2 atmosphere (Figure 4(b)), k obs increase with an increase in calcination temperature no matter which type of MTNTs was investigated.An interesting result was also noticed that TNTs-700W H2-700 • C without the presence of anatase phase has k obs of 3.29 ± 0. In addition to the contribution of anatase phase in the photocatalytic reaction, the photocatalytic ability can be attributed to the well-known interparticle electron process (IPET process) [16,17].The band gaps between two coupled semiconductors/crystal phases establish a selective pathway for irradiated electrons to migrate.This candidate pathway is able to result in an efficient separation between irradiated photoholes and photoelectrons.Therefore, more hydroxyl radicals (OH•) are expected to produce with IPET process.This effect can be evidenced in terms of the comparison of k obs between MTNTs-70W H2-500 • C and MTNTs-70W H2-700 • C .MTNTs-70W H2-500 • C and MTNTs-70W H2-700 • C exhibit a comparative intensity of anatase phase (Figure 1(b)) but the degradation of TCE is better for TNTs-70W H2-700 • C versus TNTs-70W H2-500 • C .This is owing to the coexistence of anatase and rutile phases in TNTs-70W H2-700 • C whereas only anatase phase was observed in TNTs-70W H2-500 • C .The same result was found between the cases of TNTs-70W H2-700 • C and TNTs-400W H2-700 • C that TNTs-400W H2-700 • C exhibits diversified phases including

Conclusions
MTNTs with less amount of Na(I) prefer to form anatase phase after thermal treatment.In contrast, Na 2 Ti 6 O 13 is a predominant phase as MTNTs with abounding amount of Na(I) was thermally treated.The presence of H 2 during thermal treatment establishes a selective crystalline pathway in which Ti 2 O 3 phase forms for MTNTs with abounding amount of Na(I).Regarding the TCE degradation over MTNTs, both the presence of anatase phase and IPET process contribute to the TCE degradation.Despite not all calcined MTNTs are superior to commercial P25 TiO 2 in TCE degradation, MTNTs fabricated via microwave hydrothermal process exhibit a well-defined behavior during thermal treatment.We are confident that some insight into the nature and behavior of MTNTs during thermal treatment can be useful for designing some novel catalysts.