Materials Letters 75 (2012) 175–178
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Facile microwave-assisted hydrothermal synthesis of TiO2 nanotubes L. Cui a, K.N. Hui b,⁎, K.S. Hui a,⁎⁎, S.K. Lee c, W. Zhou d, Z.P. Wan e, Chi-Nhan Ha Thuc f a
Department of Systems Engineering & Engineering Management, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong Department of Materials Science and Engineering, Pusan National University, 30 Jangjeon-Dong, Kumjeong-Gu, Busan 609-735, Republic of Korea c Energy Policy Research Center, Korea Institute of Energy Research, 71-2, Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea d School of Engineering, Sun Yat-sen University, No.132, East Waihuan Road, Guangzhou Higher Education Mega Center, Guangzhou, China e School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou, 510640, China f Faculty of Materials Science, University of Science, HoChiMinh City, Vietnam b
a r t i c l e
i n f o
Article history: Received 19 December 2011 Accepted 2 February 2012 Available online 9 February 2012 Keywords: Anatase TiO2 nanotubes Microwave-assisted hydrothermal synthesis
a b s t r a c t Anatase TiO2 nanotubes grown on Ni foam substrates were successfully synthesized by treating TiO2 nanocrystals coated on an Ni foam in alkaline solution using a facile microwave-assisted hydrothermal method. The as-synthesized TiO2 nanotubes were characterized by SEM, TEM, and XRD. The effects of NaOH concentration, reaction time, hydrothermal temperature, and calcination temperature on the morphology, as well as crystal structure, of the nanotubes were investigated. TiO2 nanotubes synthesized under the optimal conditions were polycrystalline anatases with 4 to 5 multilayers, approximately 10 nm wide, and several tens of nanometers long. The possible formation mechanism for TiO2 nanotubes was also discussed in the current paper. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Titanium dioxide (TiO2) nanostructured materials have attracted considerable attention because of their applications in various fields such as in dye-sensitized solar cells [1], photocatalysis [2,3], photoelectrochemical applications [4], and water splitting [5]. TiO2 nanotubes possess remarkably superior properties in these applications compared with other forms of nanocrystalline TiO2 because of their high surface area and charge transport property. Therefore, the synthesis of TiO2 nanotubes has emerged as an important research field. Various methods for the synthesis of TiO2 nanotubes have been developed in recent years, including anodic oxidation [6,7], template synthesis [8], and hydrothermal synthesis [9]. The hydrothermal synthesis has become one of the most promising techniques because of its capability for large-area growth of nanotubes and simplicity [10,11]. This process is typically based on the reaction of TiO2 nanoparticles with concentrated NaOH aqueous solution, followed by water or acid treatment and calcination. However, the conventional hydrothermal synthesis method has several limitations such as long reaction time (24 h to 72 h) and large energy consumption. Although TiO2 nanotubes grown on Ti substrates have been previously reported [12–14], very little information on whether TiO2 nanotubes can directly grow on other conductive substrates, such as Ni foam, is available. Ni foam can be a potential catalyst support because of its good mechanical property, thermo stability, and high conductivity. A modified approach ⁎ Corresponding author. Tel.: + 82 051 510 2467; fax: +82 051 514 4457. ⁎⁎ Corresponding author. Tel.: +852 3442 4759; fax: +852 3442 0172. E-mail addresses:
[email protected] (K.N. Hui),
[email protected] (K.S. Hui). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.02.004
to synthesize TiO2 nanotubes directly grown on Ni foam without consuming much time and energy should be explored. Microwave-assisted heating offers more rapid heating, faster kinetics, higher yield, and better reproducibility of products, compared with conventional heating. A facile and effective method to prepare anatase TiO2 nanotubes on Ni foam substrate using microwaveassisted hydrothermal method is reported in the present study. The proposed method includes the pre-deposition of TiO2 powder onto an Ni foam and the subsequent growth of TiO2 nanotubes based on the TiO2 layer under microwave irradiation, followed by calcinations in air. The effects of the different experimental conditions, such as NaOH concentration, reaction time, hydrothermal temperature, and calcination temperature on the morphology, as well as crystal structure, of the nanotubes were also investigated. The corresponding samples were characterized through XRD, SEM, and TEM. 2. Experimental TiO2 nanotubes were synthesized directly onto the surface of an Ni foam substrate through a microwave-assisted hydrothermal method. The Ni foam (1 cm×1 cm×0.17 cm [L×W×T], 110 pores/inch, 320 g/m2; Artenano Company limited, Hong Kong) was degreased by acetone and rinsed with distilled water to remove surface impurities. Approximately 0.2 g TiO2 powders (Artenano Company limited, Hong Kong) and 2 mL polyvinyl alcohol (PVA) aqueous solution (15%) were added to 18 mL de-ionized (DI) water and stirred for 10 min. Then, the Ni foam was immersed in the solution, sonicated for 5 min, and dried to coat the TiO2 powders onto the foam. The TiO2-coated Ni foam was placed in an NaOH solution
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Table 1 Synthesis conditions and morphology of the as-synthesized products. No. NaOH concentration (M)
Time (h)
Temperature (°C)
Morphology of the products
1 2
3 5
3 3
150 150
3
8
3
150
4 5
10 10
3 1
150 150
6 7 8
10 10 10
5 3 3
150 100 200
Small platelets Nanotubes + large platelets Nanotubes + large platelets Nanotubes Aggregation of nanoparticles Nanotubes Nanotubes Nanotubes
and microwave-irradiated in a commercial instrument (MDS-6, Sineo Microwave Chemical Technology Co. Ltd). The temperature during irradiation was measured by a thermocouple that was inserted into the reaction vessel. Different reaction conditions were conducted to investigate the effect of NaOH concentration, time, and temperature on the formation of the nanotubes. Table 1 lists the morphology of the as-synthesized TiO2 materials prepared under different experimental conditions. The intermediate products that were reacted for 1.5 h were removed for measurement to investigate the formation process of the nanotubes. After the microwave-assisted hydrothermal process, the Ni foam was washed and aged in 100 mL DI water for 2 h. Finally, the as-synthesized Ni foam was calcined at 350, 550, and 650 °C for 2 h. The morphology of the TiO2 nanotubes was observed by scanning electron microscopy (SEM, JEOL JSM-5600) at 20 kV and transmission electron microscopy (TEM, FEI Teccai G2 S-Twin, Philips). XRD patterns were obtained using Siemens D500
diffractometer with a step of 0.02° with Cu Kα (λ = 0.1542 nm) radiation at 40 kV and 30 mA.
3. Results and discussion The SEM images of the titanate nanostructures that were synthesized under different NaOH concentrations (samples 1 to 4) at 150 °C for 3 h are shown in Fig. 1. Remarkable differences in the morphologies of the products were observed. Many aggregations of unreacted powders were noted, and no tubular structure was formed under a low NaOH concentration (Fig. 1a, sample 1). A combination of high aspect ratio nanotubes and large or small platelet structures were produced under the 5 and 8 M NaOH conditions (Fig. 1b to c, samples 2 and 3). Pure nanotubes were fully grown under 10 M NaOH (sample 4). These nanotubes were randomly oriented and entwined, forming an intertexture-like hierarchical structured film. NaOH concentration is therefore a very important factor in the synthesis of TiO2 nanotubes through the microwave-assisted hydrothermal method. The effects of reaction time and temperature on the morphology of the as-synthesized products are presented in the section of Supplementary Information. The XRD patterns of the nanotubes (sample 4) calcined at different temperatures for 2 h in ambient atmosphere are shown in Fig. 2. All of the samples were scraped from the Ni foam prior to analysis. The calcination temperature proved to be a key factor that affects the crystalline phase of the nanotubes. The broadened diffraction peaks of the samples uncalcined and calcined at 350 °C can be indexed as H2Ti3O7 [15]. The broadening of the peaks was attributed to the nanoscale size of the nanotubes. The anatase phase of TiO2 started to observe at calcinations temperature of 350 °C. The characteristic diffraction peaks became stronger and sharper as the calcination temperature increased to 550 °C, indicating an improvement in
Fig. 1. SEM images of products synthesized under different NaOH concentrations: (a) 3 M (sample 1), (b) 5 M (sample 2), (c) 8 M (sample 3), and (d) 10 M (sample 4).
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Fig. 2. XRD patterns of sample 4 under different calcination temperatures.
crystallinity. The transformation from titanate to TiO2 has been completed. The TEM results show that the nanotubes synthesized under this condition have approximate outer diameters of 10 nm, inner diameters of approximately 3 nm to 5 nm, and several tens of nanometers long (Fig. 3a). A single nanotube has 4 to 5 multi-layers, and the layer spacing is approximately 0.35 nm. Each nanotube tends to have a constant diameter along its length. The corresponding SAED image (insert in Fig. 3a) also confirms the anatase phase of the nanotubes, and the continuous rings indicate that the TiO2 nanotubes are polycrystalline in structure. A rutile phase was observed, aside from the anatase phase, for the nanotubes calcined at 650 °C, suggesting that the occurrence of the anatase-to-rutile phase transition. The strong and sharp diffraction peaks indicate the high crystallinity of the nanotubes. The following formation process of TiO2 nanotubes is proposed according to Fig. 2. The Ti–O–Ti bonds of the TiO2 powder were broken during the early stage of reaction. Ti–O–Na and Ti–OH nanosheets were formed under the action of NaOH (Fig. 3b, TEM image of the intermediate product). The high surface energy of the Ti–O–Na and Ti–OH nanosheets resulted in the curling of the nanosheets and subsequent formation of the nanotubes. The curling of the nanosheets may occur by the scrolling of one single nanosheet from one end to the other, which can be corroborated by the asymmetric number of the multilayers of the nanotubes (Fig. 3a). New Ti–O–Ti bonds of TiO2 were produced after the water treatment and calcination. The bond distance from one Ti to the next Ti on the surface decreased, which can also explain the right shift of the diffraction peaks in the XRD pattern (Fig. 2c).
Fig. 3. TEM and SAED (insert) of TiO2 nanotubes. (a) TiO2 nanotubes calcined at 550 °C (sample 4) and (b) intermediate products reacted for 1.5 h.
Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.matlet.2012.02.004. References
4. Conclusion TiO2 anatase nanotubes were synthesized successfully through a simple microwave hydrothermal method. NaOH concentration, reaction time, hydrothermal temperature, and calcination temperature have remarkable effects on the morphology, as well as crystal structure, of the nanotubes. The nanotubes calcined at 550 °C temperature are of polycrystalline anatase phase, and each nanotube has 4 to 5 shell layers. The nanotubes are approximately 10 nm wide and several tens of nanometers long. Microwave heating can significantly reduce the reaction time. Acknowledgement The work described in this paper was supported by grants from the City University of Hong Kong (project nos. 7008056 and 9667037).
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