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TiO2 rutile–anatase core–shell nanorod and nanotube arrays for photocatalytic applications Downloaded by Nanyang Technological University on 07 February 2013 Published on 09 January 2013 on http://pubs.rsc.org | doi:10.1039/C3RA22842H

Cite this: DOI: 10.1039/c3ra22842h

Lei Pan,a Hui Huang,*b Chiew Keat Lim,a Qing Yao Hong,a Man Siu Tsea and Ooi Kiang Tan*a Rutile and anatase mixed TiO2 is a promising material for photocatalytic applications. In this work, rutile– anatase core–shell nanotube (NT) and nanorod (NR) structures were fabricated directly on a fluorine doped SnO2 (FTO) glass substrate using a hydrothermal growing/etching and TiCl4 post-treatment method. A pure rutile phase NT/NR core was coated with a roughened anatase shell. The nanostructures Received 10th November 2012, Accepted 6th January 2013

were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, UV-vis reflectance and transmittance spectra. The photocatalytic capability was measured by

DOI: 10.1039/c3ra22842h

investigating the speed of methyl blue degradation UV. The core–shell NT structure showed superior photocatalytic properties due to larger light absorbing areas, higher light trapping capability and

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enhanced charge separation efficiency.

1. Introduction TiO2 is one of the most prominent oxide materials used as a photocatalyst for pollutant degradation (e.g., volatile organic compounds1) and elimination, and nanostructured TiO2 materials are believed to be future materials that can ease the energy shortage due to their capabilities in solar power utilization applications.2 In recent years, various TiO2 nanostructures including nanotubes (NT), nanorods (NR), and nanowires3–5 have been prepared via titanium foil anode oxidization,6 hydrothermal growth,7 sol–gel synthesis,8 and chemical vapor deposition9 for applications in photovoltaic devices,10,11 photocatalysis12 and gas sensing.13 For example, Liu et al.14 developed a facile hydrothermal method to grow oriented single-crystalline rutile TiO2 NR films on FTO substrates, and dye-sensitized solar cells (DSCs) were fabricated with the TiO2 NR arrays. However, the main limitations of rutile TiO2 nanorods with smooth surfaces are the relatively small surface area and lower photocatalytic activity compared to anatase phase nanostructures. Compared to NRs, TiO2 NTs have also received a lot of attention, which can be attributed to their larger scattering area and direct electron pathway.6,15 To date, TiO2 NTs are mainly prepared using an anode oxidization method and ZnO template growth.15,16 DSCs have been demonstrated based on a polycrystalline TiO2 NT structure prepared by anode oxidization. Enhanced charge collection efficiency and stronger light scattering properties were both observed.17 The increasing of the light path length of photons a

School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore. E-mail: eoktan@ntu.edu.sg b Surface Technology Group, Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, 638075, Singapore. E-mail: hhuang@SIMTech.a-star.edu.sg

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within the NT structure increased the chance of light absorption.18 Apart from the geometric structure, which has a large effect on device performance, the different crystal forms of TiO2 involved in the reaction system are also critical for the best light energy utilization. There are two main crystal structures for TiO2 photocatalytic applications: rutile and anatase. As previously reported, the anatase phase of TiO2 shows superior photocatalytic performance because of the lower recombination rate of electron–hole pairs.19 For example, the activity of the commercialized product P25 which consists of both rutile and anatase phase (1/4 w/w) exceeds that of pure anatase phase and pure rutile phase,20 and this is attributed to the effective charge separation at the surface-phase junction.21 In this paper, a hydrothermal etching method was applied to convert rutile TiO2 NR arrays into NT arrays grown on FTO glass. In order to enhance the surface area and create large amount of anatase to rutile phase junctions, the rutile–anatase core–shell structures were further grown by TiCl4 posttreatment in order to coat another anatase phase shell layer onto the core rutile NT. The photocatalytic abilities of different nanostructure arrays were evaluated by the speed of degradation of 30 ppm methyl blue aqueous solution under UV irradiation.

2. Experimental 2.1 Growth of TiO2 NR and NT arrays The TiO2 NTs were grown in two steps. First, the TiO2 NR arrays were grown on FTO glass using a hydrothermal method similar to Liu et al.,14 and then the as-grown TiO2 NRs were

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Paper converted into NTs by hydrothermal etching in HCl. The FTO substrate (TEC-7, 7V, Hartford Glass, USA) was cut into 3 6 6 cm pieces and was ultrasonically cleaned using deionized (DI) water, acetone and ethanol, and dried under a stream of N2. A 20 nm anatase TiO2 seed layer was grown using atmospheric pressure chemical vapor deposition (APCVD). 0.6 ml titanium isopropoxide (TTIP, 99.999%, Sigma Aldrich) was added into 30 ml 5 M hydrochloric acid (37%, Merck) and stirred for 5 min, and then the precursor was transferred into a 50 ml Teflon-lined stainless steel autoclave. The FTO substrate was immersed in the precursor solution and the autoclave was kept in an oven at 150 uC for 14 h. The as-grown TiO2 NR arrays were rinsed with deionized water and were dried under a stream of nitrogen. Then, the TiO2 NR arrays were immersed in 30 ml 5–6.67 M HCl again without any TTIP and posttreated at 150 uC for 6–8 h. The samples were rinsed with deionized water and dried under a stream of nitrogen. For surface modification, the TiO2 NR/NT arrays were immersed in a TiCl4 aqueous solution at 70 uC for 1.5 h followed by annealing at 450 uC for 30 min. 2.2 Characterization The crystal structure of the TiO2 nanostructures was recorded using a Siemens D5005 X-ray diffractometer (XRD) with Cu-Ka radiation. The morphology of the TiO2 nanostructures was observed using a Leo 1550 field emission scanning electron microscope (SEM). The microstructure of the TiO2 nanostructures was studied with a JEOL TEM-2010F transmission electron microscope (TEM). The UV-vis transmittance/reflectance spectra were measured using a PerkinElmer Lambda 950 UV/Vis/NIR spectrophotometer system. The samples were cut into 3 6 3 cm sections and immersed in 8 ml 30 ppm methyl blue aqueous solution in a W40 mm glass Petri dish. The light source used was 8 W 6 6 pcs black light tube (360 nm, Phillips, made in Holland), and the light intensity was 260 lx at the sample level. At 30 min intervals during light illumination, the methyl blue solution was sampled and the absorbance change at 532 nm was monitored using UV-vis spectroscopy (Shimadzu 2450).

3. Results and discussion 3.1 Growth of TiO2 NR arrays TiO2 NR arrays were grown hydrothermally at 150 uC for 5–41 h. During the first 5 h growth, the TiO2 started to nucleate on the FTO substrate and some short nanorods were observed at the boundaries of FTO crystals (Fig. 1a). After growth for 14 h, the NRs grew longer and were rectangular (Fig. 1b). The size of the NRs was around 40–100 nm. With a further increase in the growth time to 41 h (as shown in Fig. 1d), the NRs grew larger and longer (length from 1.8 mm to 2.8 mm). The XRD results (Fig. 4d) revealed that the TiO2 film could be classified as tetragonal rutile (JCPDS, card no. 21-1276).22 The peak in the (002) direction reflects the uniform growing direction of the nanorods, which is perpendicular to the FTO glass. The predeposition of seed layer could enhance the NR growing orientation towards the [001] direction, which has also been

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Fig. 1 SEM images of the TiO2 nanorod arrays grown on FTO glass at 150 uC for 5–41 h. (a) 5 h, (b) 14 h, planar view, (c) 14 h, and (d) 41 h, cross-sectional view.

reported before.23 This effect is confirmed by the much stronger (002) peak of the NRs grown on the seed layer than the NRs grown on FTO glass in the XRD analysis shown in Fig. 4d. The TEM image in Fig. 2a shows the smooth surface of a TiO2 NR. The NR is 100 nm with a cone-shape growing front. The interplanar spacing is 3.3 Å, which corresponds to the rutile (110) plane. The TiO2 NRs grow along [001] direction determined by HRTEM image and selected area electron diffraction (SAED) as shown in Fig. 2b and 2c, which is consistent with XRD results shown in Fig. 4d. 3.2 Hydrothermal etching of NRs A second hydrothermal treatment was conducted in the same Teflon-lined autoclave to convert the TiO2 NRs to hollow NT structures. The effects of hydrothermal etching time and concentration of the HCl on the morphology transformation were studied. It was found that the solid TiO2 NRs could be fully transformed into hollow NT structures (as shown in Fig. 3a by hydrothermal etching at 150 uC in 6.67 M HCl solution for 7 h. With lower concentrations of the HCl, the TiO2 NRs were insufficiently etched and only at the tip position (Fig. 3b). A longer etching time than 7 h or with a higher concentration of HCl caused over-etching of the NRs, and led to the TiO2 NR arrays peeling off the glass substrate (Fig. 3c). A TEM image of the NTs is shown in Fig. 4a. A wedge-shaped hollow was created through the original NR. At the top end of NT shown in Fig. 4b, the wall of the NT exhibits a fence structure which could hardly be observed from the NR structure. The alignment of the surface detail shows that the NT is a polycrystalline structure divided by clear grain boundaries. The nanotube is made up of a 20 nm wide single-crystal ‘‘fence’’. It is apparent that the fence part of the nanotube is more resistant to HCl etching than the center part, which results in the hollow tube structure. The preferential etching of the nanorod could be explained by surface energy theory. The surface energy of rutile structure

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Fig. 2 TEM and HRTEM images of a TiO2 NR grown for 14 h. (a) as-grown TiO2 NR. (b) SAED pattern of a NR. (c) HRTEM image of the wall of a NR.

follows the sequence of (110) , (100) , (101) , (001)24,25 which shows that the (110) has the highest stability. Thus the (001) core part undergoes faster etching than the (110) nanorod sidewalls. Comparing the XRD results in Fig. 4d for NR and NT, the phase remains as rutile after the etching with almost unchanged characteristic peaks, except for the huge drop of the (002) peak. This in fact agrees well with the hypothesis that the etching of (001) face dominates.

Fig. 4 TEM images of (a) hollow TiO2 NTs; (b) the fence structure of one TiO2 NT; (c) the wall of one single-crystal NT. (d) XRD patterns of bare FTO glass, NT, NR and NR grown on an APCVD seed layer.

3.3 Core–shell structure formation The anatase shell layer was deposited onto the TiO2 NR and NT arrays by TiCl4 hydrolysis post-treatment. The post-treatment was conducted by soaking the NR/NT arrays in 0.2 M TiCl4 aqueous solution at 70 uC for 1.5 h followed by annealing at 450 uC for 30 min. The surface was roughened compared to the original smooth NR and NT surface as shown in the SEM images (Fig. 5a and 5b). Both an increase and a decrease in the

surface areas of the TiO2 nanostructures were reported when TiCl4 treatment was done on nanocrystalline (NC) structures.26,27 This undetermined surface area effect of posttreatment on NC structure could be attributed to the clogging of pores during TiCl4 treatment.

Fig. 3 SEM images of TiO2 NR arrays after different etching processes (a) complete etching of TiO2 NTs in 6.67 M HCl at 150 uC for 7 h; (b) partially etched TiO2 NRs in 5 M HCl at 150 uC for 7 h; (c) Over-etched TiO2 NRs (FTO glass exposed) in 6.67 M HCl at 150 uC for 8 h.

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3.4 Optical and photocatalytic properties

Fig. 5 SEM images of (a) TiO2 core–shell NR arrays after TiCl4 post-treatment; (b) TiO2 core–shell NT arrays after TiCl4 post-treatment; (c) TEM image of a single TiO2 core–shell NT after post-treatment; (d) HRTEM image of TiO2 rutile–anatase core–shell NT.

However, for TiO2 NT and NR core–shell structure, the shell thickness was around 8 to 10 nm and the core diameter was around 150 nm (Fig. 5c). The thickness ratio of shell to core was only 1/20 which made the roughening factor more significant relative to the small size of the NC. This is because for small structures, the roughened shell layer could either increase the surface area by roughening the original smooth surface or decrease the surface area by filling limited inner gaps. We expected an increase in surface area after the posttreatment of NRs and NTs. Fig. 5c shows a TEM image of a core–shell NT. Both the inner and outer surfaces were covered by an anatase TiO2 shell. The HRTEM image in Fig. 5d clearly shows that the core structure is rutile phase growing along the [001] direction while the nanocrystalline shell is anatase phase.

The transmittance and reflectance properties of the TiO2 NR and NT arrays before and after TiCl4 post-treatment were measured. As shown in Fig. 6a, there is a slight downshift of the transmittance curve after TiO2 NR conversion into NT, which indicates the better light trapping capability of the NT structure. After TiCl4 post-treatment, the absorption edge of TiO2 rutile–anatase core–shell NT arrays shifts to the left a little, which proves the existence of the anatase phase in the system, because anatase phase TiO2 has a 0.2 eV wider band gap than rutile phase.28–31 The reflectance spectra of TiO2 NR and NT arrays are shown in Fig. 6b. In the visible light range where excitons are not involved in the measurement, TiO2 rutile–anatase core–shell NR/NT arrays show lower reflectance after the post-treatment process. As the band gaps of both rutile and anatase phases are large enough not to absorb any visible light, the reflecting property within the visible range shows pure photon scattering and reflecting effects within the nanostructure. The photocatalytic properties were measured by the degradation speed of methyl blue under UV light (as shown in Fig. 7a). The bare TiO2 NT arrays showed better photocatalytic performance than the bare TiO2 NR arrays. Such an improvement can be attributed to the better scattering properties of the NT structure than NR structure and the larger surface area brought by the intrinsic geometric characteristics of NTs over NRs.17,18 After TiCl4 post-treatment, both the TiO2 rutile– anatase core–shell NR and NT arrays showed a significant improvement in degradation rate. This performance leap of the TiO2 rutile–anatase core–shell NR and NT arrays can be explained by the synergistic effects of following reasons. The first is the superior photocatalytic properties of anatase phase TiO2 over rutile phase TiO2, which has been reported previously.32,33 Anatase phase TiO2 exhibits a higher absorptive affinity for organic compounds34 and lower rates of recombination.35 The post-treatment introduces anatase phase TiO2 into the photocatalytic system which would increase the degradation speed. Secondly, as mentioned in the analysis of the SEM images of the TiO2 rutile–anatase core–

Fig. 6 (a) UV-vis transmittance spectra and (b) UV-vis reflectance spectra of TiO2 NR and NT arrays before and after TiCl4 post-treatment.

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Paper phase TiO2 and the formation of a large number of rutile– anatase surface-phase junctions in the system.

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Acknowledgements

Fig. 7 (a) Photodegradation of 30 ppm methyl blue under UV light by NT, NR, core–shell NT, core–shell NR and FTO substrate; (b) Schematic drawing of the photocatalytic degradation mechanism of a rutile–anatase core–shell NT with enhanced charge-separation efficiency.

shell NR and NT arrays (Fig. 5), the surface area of the surfacemodified nanostructures increases. More contact area between the TiO2 NR/NTs and the pollutants and light absorption area could also promote the photocatalytic activity. The intrinsic optical properties of dense 1-D nanostructure arrays and higher volume filling factor for arrays after post-treatment are both responsible for the performance improvement.23 Thirdly, the TiO2 rutile–anatase core–shell NR and NT arrays formed favor the photodegradation activity because of the surfacephase junctions created in between the core and shell.21 There are two transfer mechanisms reported for this. The first one suggests quick electron transfer from the conduction band of anatase to rutile (Fig. 7b) because of the 0.2 eV36 energy level difference.37 The second one suggests that the electron transfer is from the rutile conduction band to an anatase lattice trapping site.38 Both of the charge transferring mechanism could enhance the charge-separation efficiency if there is a surface-phase junction. Large numbers of ‘‘hotspots’’ are created at the rutile–anatase interface, serving as oxidation sites for organic pollutants.

Conclusions In summary, TiO2 rutile–anatase core–shell NT arrays were successfully prepared on FTO glass using a three-step growing/ etching/coating method. The NT core structure grown via a hydrothermal growing/etching method was of pure rutile phase and showed an ordered polycrystalline structure along the [001] direction. The shell structure coating was prepared by TiCl4 hydrolysis. The anatase-phase shell shifted the transmission curve towards shorter wavelength as a result of its wider band gap. Photocatalytic performance was evaluated by examining the degradation rate of a methyl blue solution under UV. By comparison, the NT structure showed a minor improvement over the NR structure attributed to the larger surface area of the tube structure and the stronger scattering properties. The core–shell NT structure achieved better decomposition performance because of the larger surface area of the roughened shell structure, the existence of anatase

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L. Pan wishes to acknowledge the funding support for this project from Nanyang Technological University under the Undergraduate Research Experience on CAmpus (URECA) Programme. H. Huang gratefully acknowledges the LKY PDF Start-up Grant (Grant No. LKY 3/08) for financial support.

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