Photocatalytic Properties of Nanotubular-Shaped TiO 2 Powders with Anatase Phase Obtained from Titanate Nanotube Powder through Various Thermal Treatments

1 INAME, Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 143-747, Republic of Korea 2 Department of Environment Engineering, Sunchon National University, Sunchon, Jeonnam 540-742, Republic of Korea 3 Nanohybrid & Energy Materials Research Center, Korea Electrotechnology Research Institute, Changwon, Gyeongnam 641-120, Republic of Korea 4 Wire Rod Research Group, Technical Research Laboratory, POSCO, Pohang, Gyeongbuk 790-785, Republic of Korea


Introduction
Nanostructured TiO 2 powder with the particle sizes under 100 nm has been studied as one of the most important matters in materials industry over the last decades due to its great advantages as unique electrical, chemical, optical, and photochemical properties [1][2][3][4].Recently, for getting around severe agglomeration between nanoparticles in the powder with high quality in their properties, the synthesis researches are mainly focused on the maximization of specific surface area of obtained particles by rearrangement of bulk TiO 2 materials like changing their particle shapes into nanotube [5], nanowire [6], or nanorod [7] with one-dimensional structure.Among them, TiO 2 -based nanotube powder which had been firstly reported by Kasuga et al. in 1998 [5] is structurally very similar to carbon nanotube and has been actively used in researches in application fields for solar cell electrode [8], photoelectrochemical water splitting catalyst [9], secondary battery electrode [10], hydrogen storage [11], photocatalyst [12], and so on [13][14][15] because it has high chemical stability as well as specific physical properties such as high aspect ratio, high surface area, and fast ionic path.
The TiO 2 -based nanotube material synthesized by the hydrothermal process is generally referred to as titanate phase, which has TiO 6 octahedra consisting of a layered structure for H x Ti y O z obtained normally with its rearrangement in a strong basic NaOH solution and also has a particular crystallinity that differs from anatase, rutile, and brookite phases [3].However, large amount of Na + ions existed on the surface of and inside the tubular particles gone through water washing and drying steps after synthesis and it has been already well known that Na + ions significantly decrease photocatalytic activity of TiO 2 [16,17].Also, it was reported by Qamar et al. [18,19] that the photocatalytic activity of the titanate powder increased due to its transformation into anatase phase with calcination, only when completely removing Na + ions on surface of and inside the powder by ion-exchange process compared to that of the powder without ion-exchange process.According to their research, the most excellent photocatalytic property of hydrothermally prepared titanate powder was obtained at the calcination of 400 • C for 2 hours after completely removing the Na + ions, where their photocatalytic ability were still lower than that of commercial P25 powder.Also, their titanate nanotube powder after calcination had relatively very smaller value in the specific surface area, compared to original ion-exchanged titanate nanotube powder.In general, there is a large amount of H 2 O content or moisture in the finally obtained powder with high surface area through the solution-based hydrothermal process.The H 2 O content or moisture inside and outside the particles with increasing calcination temperatures causes severe problem that the specific surface area of powders is greatly decreased due to the rapid mass transportation.Therefore, in this study, for high efficient photocatalytic TiO 2 powder from the titanate powder, we quantitatively evaluate the effects on photocatalytic activities of titanate nanotube with variations of particle shape, specific surface area, and crystal structure by controlling the heating time, temperatures, and atmospheres with removing H 2 O content or moisture in the powder.

Experimental Procedure
For preparing titanate nanotube powder, 0.1 M P25 (Degussa Co.) powder was at first put into 10 M aqueous NaOH (Junsei Chem.Co.) solution, and then the solution was vigorously stirred for 2 hours at room temperature using magnetic stirrer.The solution after complete mixing was poured into nickel-lined stainless steel autoclave, and it was hydrothermally heated at 150 • C for 48 hours.After hydrothermal process, white-colored precipitate was obtained and the precipitate was repetitively washed with deionized water until the solution pH became 7.The removal of Na + ions on the surface of and inside the obtained precipitated particles was carried out by ion-exchanging process in 0.1 N aqueous HCl solution for 24 hours to have less than 0.1 at % Na + .After ion-exchanging process, the precipitate was washed again with deionized water, followed by freeze drying (ALPHA 1-2 LD, Christ) at −57 • C for 24 hours.Then, the dried precipitates were calcined with variables in temperature and time using tube furnace-controlled various atmospheres such as oxygen, air and hydrogen (10 vol% H 2 with balanced N 2 ).
Particle shapes of as-prepared and calcined powders were observed using scanning electron microscope (S-4700, Hitachi), and specific surface area of the powder was measured using BET (KICT-SPA3000, MiraeSI Co.).Crystal structure of the powder was analyzed using X-ray diffractometer (D/MAX-2500/PC, Rigaku) and ambient Raman Spectroscopy (InVia Raman Macroscope, Renishow).Finally, to evaluate photocatalytic properties of the calcined powders, 0.1 g of the obtained powders were mixed together with 0.1 mM aqueous 4-chlorophenol (4-CP) solution.Then, UV-A light source (PowerArc UV-100, UV Process Co.) with ∼400 nm of wavelength and 100 W of power was irradiated on the mixed solution for 3 hours for the photocatalytic reaction.After the reaction, the remaining organic carbon in the mixed solution was measured to check the extent of photocatalytic decomposition of 4-CP using TOC analyzer (TOC-VCPH, Shimadzu Co.)

Result and Discussion
Figure 1(a) shows XRD results for the ion-exchanged titanate nanotube (H-TiNT) powders calcined at temperatures from 400 • C to 500 • C for 30 min in O 2 atmosphere.It can be seen that the diffraction intensities for characteristic peaks of anatase TiO 2 phase at 2θ = 25.35,37.84, 48.14, 53.97, and 55.18 • rise gradually with increasing temperature by transformation of the titanate into anatase phases, when there coexist a peak of the titanate phase at 2θ = 24.5 • and other weak anatase peaks at 400 • C.Those are well matched with mixed Raman spectra at 400 • C of Figure 1(b), which includes the titanate characteristic peaks of 197, 280, and 450 cm −1 and the anatase characteristic peaks of 145, 390, 506, and 638 cm −1 .It can be also seen that the titanate phase transforms almost perfectly into anatase phase at 450 • C, although very small traces of the titanate phase at 280 cm −1 undetectable in normal scale exist.On the other hand, Figure 2 shows SEM images of the calcined powders corresponding to the XRD patterns of Figure 1(a), respectively.Well-developed nanotubular shape is observed at 400∼450 • C but the agglomerations between tubular particles above 450 • C become more severe.This observation about tubular shape was also confirmed using TEM as previously reported [20].It was also observed that they were shortened like nanorod and then became nanoparticles when the heating temperature approached 500 • C or higher for 2 hours of calcination [18].From these results, therefore, it can be said that the nanotubular-shaped TiO 2 powder with perfect anatase phase from the H-TiNT powder was obtained by calcination at 475 • C for 30 min in O 2 atmosphere.It was also observed that the calcined powders in air atmosphere showed almost the same results as Figures 1 and  2. However, it was observed in H 2 atmosphere that the fast transformation into anatase phase was occurred with some reduction into TiO 2-x on the surface of H-TiNT particle.
Figure 3 shows photocatalytic decomposition abilities in aqueous-mixed solutions of 4-CP and calcined H-TiNT powders of Figure 1.For reference, well-known commercial P25 powder shows 52.50% in the 4-CP decomposition with the same photocatalytic reaction under UV-A irradiation for 3 hours, together with not showing decomposition at all when applying without photocatalyst ("no powder" marked).The photocatalytic decomposition of 4-CP in the mixed solution by the calcined H-TiNT powder increases gradually with increasing calcination temperature and then shows the maximum decomposition of 67.21% at 475 • C. Here, it was confirmed that the H-TiNT powder before calcination shows only 2.43% in the 4-CP decomposition.It means that the H-TiNT powder with the titanate phase has no photocatalytic ability under UV-A irradiation, and its photocatalytic ability is enhanced as the titanate phase transforms into the anatase phase by the calcination.However, the powder with perfect anatase phase by the calcination at 500 • C shows a little higher decomposition ability of 4-CP than P25 powder, although the specific surface area of the powder (162 m 2 /g) is about 3 times larger than that of P25 powder (∼50 m 2 /g).Especially, the 4-CP decomposition ability of the calcined powder at 475 • C only is as high as 15% compared to that of P25, regardless of having high surface area (170 m 2 /g).When simply grinding the powder calcined at 500 • C, its decomposition was measured having almost the same value as that of the powder at 475 • C.
From these results, it can be said that the effects on the photocatalytic properties of the calcined H-TiNT powders with anatase phase do not depend on the specific surface area alone.Like these, the decrease in the photocatalytic ability at higher calcination temperature results from that the calcined H-TiNT powders might not be dispersed well in the 4-CP solution by severe agglomerations or sintering between the calcined particles as shown in Figure 2. Therefore, the photocatalytic decomposition would be hindered due to the screening or shadow effect by the growth of the particles although their specific surface area for surficial sites was large.Anyway, in order to apply the titanate nanotube particle with anatase phase by simple heating for a photocatalyst with high specific surface area, it can be suggested that the optimized temperature for the calcination for 30 min in O 2 atmosphere should be in the ranges of 450 • C∼ 500 • C from particle observations and crystallinity analysis of Figures 1 and 2.
Yu et al. [21] and Qamar et al. [19] had already reported that the particles with well-developed anatase phase obtained by calcining ion-exchanged titanate nanotube particles at 200 • C in long time or higher than 400 • C were changed into nanorod or nanospherical typed.From our previous works [22][23][24] and other researcher's reports [25][26][27], it can be said that the structural changes and particle growth of the H-TiNT powder might be controlled by appropriately controlling the calcination time and temperature.
It can be observed in Figure 4 that the shape of the H-TiNT particle apparently remains unchanged when calcining at 475 • C for 10 min but the particles in longitudinal direction are greatly shortened at the same temperature for 60 min when the calcination time increases.Here, the calcination temperature of 475 • C was chosen from the condition of the highest photocatalytic activity, as shown in Figure 3. Furthermore, with increasing calcination time to 120 minutes, the particles are more shortened and gradually become larger spherically due to sintering between the particles.With the same photocatalytic reaction of Figure 3, when the effect on the calcination time in the 4-CP decomposition by the photocatalytic reaction was checked, the highest value at the powder obtained at 475 • C for 30 min was measured as shown in Figure 5. Compared with the powders calcined at 475 • C with the calcination times of 10 and 30 min, it is likely that there is no difference in the particle shape as that of the original H-TiNT but the powder obtained for 10 min might show a little lower decomposition ability due to relatively weak crystallinity.Also, the particles calcined at longer times became severely agglomerated showing decreased photocatalytic ability.
On the other hand, Figure 6 shows the photocatalytic decomposition abilities of the calcined H-TiNT powders  with various heating times under air and H 2 atmospheres.It can be seen that the powders calcined under H 2 atmosphere show relatively low photocatalytic abilities and the maximum ability (45.50%) at 500 • C becomes lower at higher temperature compared to other atmospheres.But the powders under air atmosphere have almost the same abilities compared to that under O 2 atmosphere.It was observed that these particles, obtained under H 2 and air atmospheres, had the same crystallinity with the same calcination temperature and time under O 2 atmosphere, as shown in Figures 1 and 2, through XRD and Raman analysis, and also showed apparently almost the same nanotubular shape in these temperature ranges regardless of different calcination atmospheres.However, as shown in TEM photos in Figure 7 for further investigation on particle shapes with the calcinations, the H 2 atmosphere for the calcination at 500 • C made the fast shape change of nanotube into nanorod caused comparatively small suface area of the powder.It could be additionally confirmed that, at temperatures higher than 450 • C in H 2 atmosphere, the sintering phenomenon between the nanorod typed particles was locally observed (Figure 7(b)) and was enhanced when phase transformation of anatase into rutile phase occurred at higher temperatures, whereas in the air atmosphere the nanotubular shape of the titanate particles was almost perfectly maintained.Thus, it seems that the low photocatalytic ability of the powder calcined under H 2 atmosphere would be ascribed to the defect formations of TiO 2-x with grayed color on the surface of the calcined particles and to the destruction of nanotubular shape by reducing atmosphere.Consequently, for the application of the H-TiNT powder as a high efficient photocatalyst, it can be thought that the powder should be calcined to have fully anatase phase at the condition of an appropriate time and temperature like 30 min at 475 • C, under comparatively no severe agglomeration by the calcination of O 2 or air atmosphere.In addition, from the BET data, it could be thought that to obtain a high efficient anatase powder from the H-TiNT, powder should be calcined   for a short time at the temperature as high as possible because the long time of calcination at lower temperature also makes severe agglomerations between the particles.

Conclusions
Photocatalytic activities of H-TiNT powders were characterized by controlling the calcination conditions as the function of times and atmospheres in the ranges of 400∼ 500 • C. With increasing calcination temperature and time,  the titanate phase in the ion-exchanged H-TiNT was changed into anatase phase and at the same time nanotube-shaped particles were naturally turned into nanoparticles at higher temperature and longer time.The calcined powders at 475 • C for 30 min in O 2 or air atmosphere to maintain nanotubular shape with high surface area showed the highest decomposition ability in the photocatalytic reaction, whereas  the powders calcined at 500 • C or higher showed low photoactivity due to their severe agglomeration.Therefore, it can be thought that to obtain a high efficient anatase powder from the ion-exchanged H-TiNT powder should be calcined for a short time at a temperature high as much as possible because the long time calcination at lower temperature makes severe agglomerations between the particles.

Figure 1 :
Figure 1: X-ray diffraction patterns (a) and Raman spectra (b) of the calcined powders with different temperatures in O 2 atmosphere for 30 min.

Figure 5 :
Figure 5: Photocatalytic 4-CP decomposition results of the powders calcined at 475 • C for various times in O 2 atmosphere and variation of specific surface area shown in inset.

Figure 6 :
Figure 6: Photocatalytic 4-CP decomposition abilities of the powders with various calcination conditions.

Figure 7 :
Figure 7: TEM photos for some samples in Figure 6; (a) 400 • C for 30 min in H 2 , (b) 500 • C for 10 min in H 2 , (c) 475 • C for 30 min in air and (d) 500 • C for 10 min in air atmosphere.