Synthesis and characterization of anatase TiO2 nanotubes with controllable crystal size by a simple MWCNT template method

Journal of Solid State Chemistry 196 (2012) 435–440

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Synthesis and characterization of anatase TiO2 nanotubes with controllable crystal size by a simple MWCNT template method Ju Hyung Kim a, Xiao Hui Zhang a, Jae Deuk Kim a, Hoon Mo Park b, Sang Bok Lee c, Jin Woo Yi c, Seung Il Jung a,n a

Future Industry R&D Center, DH Holdings Co., LTD, 705-32 Yeoksam-Dong, Kangnam-Gu, Seoul 135-080, Republic of Korea Research & Development Division, Hyundai Motors, 460-30 Sam-dong, Uiwang-si, Gyeongg do 437-815, Republic of Korea c Composite Materials Research Group, Korea Institute of Materials Science, 797 Changwondaero, Seongsna-gu, Changwon 641-831, Republic of Korea b

a r t i c l e i n f o

abstract

Article history: Received 30 April 2012 Received in revised form 29 June 2012 Accepted 30 June 2012 Available online 9 July 2012

Highly crystalline phase-pure anatase TiO2 nanotubes were produced by performing a simple calcinations process in air of the TiO2 coated MWCNTs composite material prepared by an in situ sol–gel method. A homogeneous thin coating layer of TiO2 on the surface of MWCNTs is firstly obtained. Calcination in ambient air at appropriate temperatures is performed to transform phase of TiO2 into anatase and remove MWCNTs simultaneously, resulting in pure anatase TiO2 nanotubes. Both end tips of the nanotubes are observed to be closed, probably due to covering up MWCNTs with TiO2 particles. The crystallization of anatase phase was formed upon 350 1C, and MWCNTs are completely oxidized between 500 and 650 1C, leaving anatase TiO2 nanotubes with an average crystal size increasing from about 8 to 24 nm as the temperature rises. Moreover, the tubular structure was found to collapse after calcinations at 700 1C. & 2012 Elsevier Inc. All rights reserved.

Keywords: MWCNT TiO2 Anatase Crystalline Tubular

1. Introduction Titanium oxide (TiO2), one of the most important transition metal oxide, has two main phases: the kinetically favored anatase phase and rutile phase which is thermodynamically stable phase [1]. Fine TiO2 semiconductor materials have attracted a great deal of interest due to their chemical stability, nontoxicity, and high photocatalytic reactivity. A broad of applications include photocatalyst [2,3], dye-sensitized solar cells [4,5], and sensor materials [6,7]. TiO2 materials may also be used as electrode materials due to their high electronic conductivity and corrosion stability [8,9]. Usually rutile phase need to be transformed from anatase phase at high temperatures. As a result of that, the crystal size of rutile is much larger than that of anatase, resulting in smaller surface area than anatase. Generally larger surface area leads to higher catalytic activity. Therefore anatase TiO2 is more favored in catalysis application compared with rutile. Also anatase is preferred over rutile, as anatase exhibits a higher electron mobility, lower dielectric constant, lower density, and lower deposition temperature [10]. Recently nanostructured tubular materials have attracted much attention due to their exceptional properties and potential

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Corresponding author. Fax: þ82 2 2083 1336. E-mail addresses: [email protected], [email protected] (S.I. Jung).

0022-4596/$ - see front matter & 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2012.06.045

applications. Morphology and structure of TiO2 nanotubes, including large specific surface area, high pore volume, thin tube wall, and optimal tube length, are important factors to achieve a good performance. And nanotubes and nanofibrils have the advantage that they enable three-dimensional mechanically coherent architectures, providing ready gas access to a high surface area [11]. TiO2 nanostructured materials have been mainly prepared by three approaches: the hydrothermal synthesis [12–14], the anodic oxidation [15–17] and the template synthesis [18–22]. Among those, the advantage of the template-based synthesis route is the straightforward control over the morphology of the resulting TiO2 nanostructured materials [22]. Recently CNTs have been used as a template to synthesis TiO2 nanotubes since they can support the tubular morphology. In addition, the diameter and length of TiO2 nanotubes could be determined by the initial dimensions of CNTs/ CNFs, and the morphology can be controlled efficiently by the initial thickness of the coating. Apart from TiO2, similarly, H. Ogihara et al. have reported a simple CNF-template method to synthesis transition metal oxide nanotubes, such as SiO2, Al2O3, Fe2O3, and NiO, that a sintering process at an appropriate temperature is performed after coating of metal oxide on CNFs [23–25]. According to the combination manner, the synthesis techniques can be categorized as ex situ and in situ routes. The ex situ (building block) approach modifies and attaches the inorganic component, which is prepared in desired dimensions and morphology, to the

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surface of CNTs via covalent [26], noncovalent [27], and so on. While in the in situ approach, typically such as electrodeposition [28,29], the sol–gel method [30,31], and hydrothermal technique [32], the inorganic component grows as particles, nanowires, or thin films onto the surface of CNTs. Among these methods, the sol– gel process followed by a heat treatment at elevated temperatures, is the most commonly adopted method to introduce the inorganic phase onto the CNTs substrate [31]. It provides particularly good control from the molecular precursor to the final product, as well as giving high purity and homogeneity [21]. In this respect, we tried to synthesize anatase TiO2 nanotubes by performing a simply calcinations process on TiO2 coated MWCNTs composite materials. First of all, to ensure a better quality of anatase TiO2 nannotubes, we tried to coat a thin homogeneous layer of TiO2 on the surface of MWCNTs via sol–gel method. Benzyl alcohol (BA) was added to act as surfactant, which could enhance the coating of pristine CNTs with TiO2 and provide an excellent tool to control the size of the deposited TiO2 particles upon crystallization and phase transformation [21]. Then the obtained TiO2 coated MWCNT composite material was heat treated in air at an appropriate temperatures to remove MWCNTs and crystallize TiO2 simultaneously, resulting in anatase nanotubes, different with some studies that crystallize TiO2 first and then remove MWCNTs. Changes of morphology and structure after calcinations at different temperatures are characterized by using following equipments: transmission and scanning electron microscopy (TEM and SEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA).

to 7 using ammonia solution. In our experiment, the molar ratio of the catalyst was 1:0.1:1:0.5 (Fe/Mo/Mg/citric acid). After mixing it at room temperature, the catalyst solution was carefully poured into an alumina boat. The combustion and synthesis process were serially conducted in a quartz tube reactor (100 mm i.d., and 1000 mm long) mounted in a furnace. The boat containing the catalyst solution was placed at the center of the reactor tube. The quartz tube was rapidly heated to 550 1C in an xAr atmosphere. After the temperature reached 550 1C, it was maintained at this temperature for 5 min in an Ar atmosphere. Without any intermission, the quartz tube was then heated to

2. Experimental 2.1. Materials preparation (Fe–Mo/Mg)O catalyst was prepared according to the following procedure. A mixture of Fe(NO3)3  9H2O (99%, Aldrich) and molybdenum solution (Aldrich, ICP/DCP standard, 9.8 mg/mL in H2O) was dissolved in DI water by stirring for 30 min. Also, Mg(NO3)2  6H2O (99%, Aldrich) and citric acid were dissolved in DI water by stirring for 30 min. The mixed Fe–Mo nitrate solution was then introduced into the Mg nitrate–citric acid solution while stirring for 30 min and the pH of the final catalyst solution was set

Fig. 1. SEM images of (a) as-synthesized MWCNTs coated with amorphous TiO2, and after calcinations in air at various temperatures: (b) 350 1C, (c) 500 1C, and (d) 650 1C, respectively.

Fig. 2. TEM images of (a) low, (b) high magnification of as-synthesized MWCNTs coated with amorphous TiO2; and after calcinations in air at various temperatures: (c) low and (d) high magnification at 350 1C; (e) low and (f) high magnification at 500 1C; (g) low and (h) high magnification at 650 1C. The insets show the diffraction pattern.

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700 1C in an O2 atmosphere and kept at this temperature for 2 min. And then, a mixture of Ar (200 sccm) and C2H2 (100 sccm) was introduced into the reactor for 30 min. After reaction, the reactor was rapidly cooled to room temperature in an Ar atmosphere. Finally, we obtained pristine MWCNTs with the diameter of about 30 nm [33].

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were explored at a scan rate of 61 min  1 with step of 0.021. The thermal behavior of the composite material was analyzed with a Q500 (TA Instrument, USA), by being heated to 900 1C at 10 1C/ min heating rate under air flow (60 ml/min).

3. Results and discussions 2.2. MWCNTs coating with TiO2 For preparation of the composite materials, 2 g of the prepared MWCNTs are dispersed in 0.4 kg of ethanol and followed by sonication for 30 min at room temperature. Subsequently a solution obtained by dissolving 43 g of Benzyl Alcohol in 11.91 g of water is added. The mixture is sonicated for another 30 min and then transferred to the reactor, which is kept at a temperature 0 1C with a stirring speed of 350 rpm in N2 atmosphere. In the next step titanium butoxide (TNBT, Sigma Aldrich) and ethanol, each with a total amount of 60 ml, are introduced into the solution slowly at the same rate of 30 ml/h through a feeder machine. The final molar ratio of the mixture of TNBT:BA:EtOH:H2O is 1:3:76:5. Finally after the reaction is completed the suspension is filtered using a membrane with 1 mm pore size and washed by ethanol, followed by vacuum drying at 60 1C for 12 h. 2.3. Characterization of materials The morphology of the catalyst was observed by SEM (JSM-6701F, JEOL, Japan) and TEM (Tecnai G2 F20, FEI, USA) both for low and high resolution imagines. The elemental composition of the composite material was investigated by energy-dispersive X-ray attached to the TEM equipment. X-ray diffraction (XRD) analysis was performed using the Rigaku X-ray diffractometer with CuKa-source. The 2y angular regions between 201 and 901

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Fig. 1(a), (b), (c), and (d) show the SEM images of the samples prepared at different calcinations temperatures, respectively: (a) no calcinations, (b) 350 1C, (c) 500 1C, and (d) 650 1C. The homogeneous coating of TiO2 on the surface of MWCNTs without apparent aggregates of TiO2 nanoparticles could be verified from Fig. 1(a). The tubular morphology is not destroyed during the crystallization and phase transformation in the heat treatment. However, the surface became rougher as the heat treatment temperature rise, indicating increase of crystal size. As can be seen from these figures, since both head and tail of MWCNTs are closed, which is probably due to covering up MWCNTs with a TiO2, the inner hollow structure is hard to be confirmed. Hence, TEM analyses are performed. Fig. 2 show the TEM images of those samples shown in Fig. 1. The initial prepared TiO2 coated MWCNT composite material is estimated to have an outer diameter of around 60 nm. As the diameter of the prepared MWCNTs is around 30 nm, the thickness of the coating layer of TiO2 is estimated to be 15 nm. The formation of TiO2 nanotubes after complete oxidization of MWCNT could be obviously observed after calcinations at 500 and 650 1C. EDX spectrum will be performed to further support this. To further analyze effect of calcinations temperature on the morphology, the initial TiO2 coated MWCNT composite material is also calcinated at 700 1C. However, the TEM image (Supplementary material: Fig. 1) showed that hollow tubular structure

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Fig. 3. TEM EDX spectrum of (a) as-synthesized MWCNTs coated with amorphous TiO2, and after calcinations in air at various temperatures: (b) 350 1C, (c) 500 1C, and (d) 650 1C, respectively.

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collapsed and a fibrous structure was formed. The diffraction patterns, which are shown in the inset of Fig. 2, show that the initial coating of TiO2 was amorphous and then was transformed into anatase phase at 350 1C and 500 1C, and possibly a mixture of anatase and rutile phase at 650 1C. It could be clearly noticed as the calcinations temperature increases the crystal size increases significantly. As shown in Fig. 3, C (Carbon) still exists after calcinations at 350 1C, while C does not exist anymore after calcinations at 500 and 650 1C, which indicate the MWCNTs have been completely oxidized. This also explain that the color of the samples changed from black to white as the temperature increased from 350 to 500 1C and 650 1C. Therefore, TiO2 nanotubes with an approximate outer diameter of 60 nm, could be confirmed to have been successfully produced. To further analyze the element composition and structure of the materials, TEM line scan has been performed on these samples, the results of which are shown in Fig. 4(a), (b), (c), and (d), respectively. Before calcinations and after calcinations at 350 1C, the convex carbon intensity images indicate that carbon exists in the inner center, while the concave Ti and O intensity images indicate that Ti and O exist in the surface of the tube,

which are consistent with the result of TiO2 coating on the surface of MWCNTs observed in the TEM images. However, after calcinations at 500 and 650 1C, existence of carbon is not detected and the concave Ti and O intensity images indicate absence of Ti and O in the internal part, suggesting formation of hollow TiO2 nanotubes which is consistent with the previous observations. The kinetically favored anatase phase could be transformed to rutile via heat treatment at high temperatures (e.g. 600–900 1C). However, grain growth usually happened during these high temperatures and thus the specific surface area is considerably reduced. XRD analysis is carried out to further investigate the crystallization and phase transformation of TiO2 in the heat treatment, the results of which are shown in Fig. 5. No diffraction peak of TiO2 is seen before calcinations, indicating that the initial coating of TiO2 is amorphous. After calcinated at 350, 500, and 650 1C, the crystallization of TiO2 is formed as the anatase diffraction peak of TiO2 is observed. Moreover, two minor rutile diffraction peaks are observed at 650 1C, indicating that the TiO2 nanotube obtained at 650 1C was a mixture of anatase and rutile phase. It is worth to notice that the intensity of anatase diffraction peaks increases considerably and the width at half height of the peaks decreases as the temperature increases from 350 to 650 1C,

Fig. 4. TEM line scan analysis of (a) as-synthesized MWCNTs coated with amorphous TiO2, and after calcinations in air at various temperatures: (b) 350 1C, (c) 500 1C, and (d) 650 1C, respectively.

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Fig. 5. XRD data of (a) as-synthesized MWCNTs coated with amorphous TiO2, and after calcinations in air at various temperatures: (b) 350 1C, (c) 500 1C, and (d) 650 1C, respectively.

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Fig. 6. Crystal size of TiO2 for the samples after calcinations at various temperatures.

indicating a significant grain growth. To further investigate the influence of temperature on crystal size, the initial TiO2 coated MWCNT composite material is heat treated in air at other temperatures (XRD data are shown in Supplementary material: Fig. 2). The TiO2 crystal sizes are estimated from the line broadening by using Scherrer’s equation: d ¼0.89l/(b cos(y)), where l ¼1.5406 A˚ and b is the full width at half maximum. The results are summarized in Fig. 6. The crystal size of anatase TiO2 after calcinations at 350, 500, and 650 1C are estimated to be 8.4, 11.6, and 23.3 nm, respectively, exhibiting an approximate linear growth relation between the crystal size and the temperature. Hence, it could be concluded that higher temperature favors crystallization and agglomeration of TiO2 particles. Dominik Eder et al. the studied the effect of temperature on crystallization and phase transformation and reported that thin coatings ( o50 nm) generally resulted in a smooth coating with very small and uniform crystals (d o10 nm), while thicker coatings commonly developed a rough coating with large crystals (diameters up to 50 nm) [18,21], the result of which is consistent with ours. Finally, the thermal behavior and stability of the samples are investigated by thermo-gravimetric analysis (Fig. 7). For the initial TiO2 coated MWCNTs composite material, three steps were

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Fig. 7. Thermo-gravimetric analysis of (a) as-synthesized MWCNTs coated with amorphous TiO2, and after calcinations in air at various temperatures: (b) 350 1C, (c) 500 1C, and (d) 650 1C, respectively.

observed during the weight loss. The first one, accompanied by an endothermic effect around 60 1C and an approximate weight loss of 10%, is assigned to the removal of absorbed water and/or solvent. The second one, with an approximate weight loss of 5%, corresponds to the removal of structural water and residual BA surfactant occurs between 180 and 460 1C. The last one, with an approximate weight loss of 15% due to the MWCNTs combustion, occurs between 460 and 660 1C and is featured by an exothermal effect with a maximum at 570 1C. For the sample prepared at calcinations temperature of 350 1C, the TGA curves exhibited a similar feature, except that the weight loss is much less. A similar weight loss due to the MWCNTs combustion occurs, while the weight loss corresponding to the removal of water and structural water is much less. This could contributed to that the temperature of 350 1C is high enough to remove water and structural water, however, not high enough to oxidize MWCNTs. For both pure anatase TiO2 nanotubes obtained after calcinations at 500 and 650 1C, there is almost no weight loss since carbon does not exist and the structural water and surfactant has been almost completely removed during the heat treatment. Up to this point we have discussed the characterization and fabrication of pure anatase TiO2 nanotubes prepared by the CNTtemplated synthesis method. Hollow inside and convex closed tip structure of the TiO2 nanotubes are confirmed. Desired crystal size could be controlled by the thickness of coating layer and the heat treatment temperature. The application of the produced pure anatase TiO2 will be discussed in the near future.

4. Conclusion In summary, pure anatase TiO2 nanotubes are successfully produced by a CNT template method. First of all, an amorphous homogeneous coating of TiO2 on the surface of MWCNTs was obtained by sol–gel method having a noncovalently chemical reaction at a molar ratio of TNBT: benzyl alcohol (BA): ethanol: D.I.water¼1:3:76:5. The initial coating thickness of TiO2 is about 15 nm, indicating an outstanding achievement on the coating of a very thin and even TiO2 layer on the surface of MWCNTs. Through calcinations in ambient air at various temperatures between 350 and 650 1C, the amorphous phase of TiO2 is transformed into pure anatase phase up to 600 1C, and a mixture of anatase and rutile phase at 650 1C with anatase being dominant phase. The crystal size of anatase TiO2 increases significantly from around 8.4 to 23.3 nm. When the temperature is above 500 1C, MWCNTs

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are completely oxidized, leaving pure anatase TiO2 nanotubes between 500 and 600 1C, and anatase-rutile mixed TiO2 nanotubes at 650 1C.

Acknowledgments This research was supported by a grant from the Fundamental R&D Program for Technology of World Premier Materials funded by the Ministry of Knowledge Economy, Republic of Korea. Also, this work was supported by the development program of local science park funded by the ULSAN Metropolitan City and the MEST (Ministry of Education, Science and Technology).

Appendix A. Supplementary information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2012.06. 045.

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