Effect of Anodization Parameters on Morphology and Photocatalysis Properties of TiO2 Nanotube Arrays

Accepted Manuscript Title: Effect of Anodization Parameters on Morphology and Photocatalysis Properties of TiO2 Nanotube Arrays Author: Lianjie Qin, Qijing Chen, Ruijun Lan, Runqian Jiang, Xiao Quan, Bin Xu, Feng Zhang, Yongmin Jia PII: DOI: Reference:

S1005-0302(15)00117-6 http://dx.doi.org/doi:10.1016/j.jmst.2015.07.012 JMST 531

To appear in:

Journal of Materials Science & Technology

Received date: Revised date: Accepted date:

21-1-2015 16-2-2015 15-3-2015

Please cite this article as: Lianjie Qin, Qijing Chen, Ruijun Lan, Runqian Jiang, Xiao Quan, Bin Xu, Feng Zhang, Yongmin Jia, Effect of Anodization Parameters on Morphology and Photocatalysis Properties of TiO2 Nanotube Arrays, Journal of Materials Science & Technology (2015), http://dx.doi.org/doi:10.1016/j.jmst.2015.07.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of Anodization Parameters on Morphology and Photocatalysis Properties of TiO2 Nanotube Arrays Lianjie Qin1*, Qijing Chen1, Ruijun Lan2, Runqian Jiang1, Xiao Quan1, Bin Xu1, Feng Zhang1, Yongmin Jia1 1

School of Environment and Material Engineering, Yantai University, Yantai 264005, China

2

School of Opto-electrical Information Science and Technology, Yantai University, Yantai

264005, China

* Corresponding author. Prof., Ph.D.; Tel.: +86 535 6901817; Fax: +86 535 6902191. E-mail address: [email protected] (Lianjie Qin). [Manuscript received 21 January 2015; received in revised form 16 February 2015; accepted 15 March 2015] Highly ordered TiO2 nanotube arrays were fabricated via electrochemical anodization of high purity Ti foil in fluoride-containing electrolyte. The effects of applied anodization potential, anodization time on the formation of TiO2 nanotube arrays and the photocatalytic degradation of methylene blue (MB) were discussed. The TiO2 nanotube arrays calcined at 500 ºC for 2 h show pure anatase phase. The pore diameters of TiO2 nanotube arrays can be adjusted from 30 to 90 nm using different anodization voltage. Anodization time mainly influenced TiO2 tube length, and with increasing the anodization time, the nanotube length became longer gradually. When the anodization potential was 40 V, the average growth rate of TiO2 nanotube was about 4.17 μm/h. Both anodization potential and time had important effects on the photocatalytic efficiency.The TiO2 nanotube arrays obtained at anodization potential of 40 V for 1 h, showed the best photocatalytic degradation ratio of MB. Key words: TiO2 nanotubes; Electrochemical anodization; Photocatalysis

1. Introduction Photocatalysis has been intensively investigated in recent years. In a photocatalytic process, electron-hole pairs are generated under illumination, followed by oxidation and/or reduction reactions at the surface of the photocatalyst[1]. In the presence of a photocatalyst, organic contaminants can be oxidized directly by photogenerated holes or indirectly by reactive oxygen species (ROS), such as hydroxyl radical OH·produced in the solution [2–4]. TiO2 is a widely used photocatalyst material. It exhibits photocatalytic activity under UV

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illumination[5–8]. However, immobilized TiO 2 film usually presents a lower photocatalytic activity due to the diminution of surface area and the limitation of mass diffusion[9]. Therefore, it is necessary to improve the photocatalytic activity of TiO2 film by optimizing several factors, such as thickness, porosity, and crystal structure[10]. Recently, highly ordered TiO 2 nanotube arrays prepared by electrochemical anodization[11–14] have attracted much attention due to their unique architecture. TiO2 nanotube array can be formed on a titanium substrate, which provides a large internal surface area. Such an infrastructure is especially favorable for mass diffusion and capturing incident illumination in combination with minimal radial dimensions providing facile separation of photogenerated charges[15]. Although the electrochemical technique is simple and effective to obtain nano-tubular structures, some problems could arise due to the reproducibility of the prepared surfaces. The morphology of a semiconductor directly influences its performance in the photocatalytic oxidation of organic compounds [16]. It is well-known that the morphology of nanostructure depends on the operative conditions: applied potential, electrolyte composition and anodization time [17]. Herein we report the study on the effects of applied anodization potential and anodization time on the formation of TiO 2 nanotube arrays and the photocatalytic degradation efficiency of methylene blue (MB). The structure and morphology of the prepared films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and the degradation kinetics of methylene blue were monitored by UV-Vis spectroscopy. 2. Experimental 2.1. Preparation of TiO2 nanotube arrays Titanium (Ti) foils (250 µm in thickness, 99.7% purity) were cut into pieces of 15 mm × 20 mm. Briefly, they were degreased by ultrasonication in acetone and then ethanol, respectively, for about 15 min, followed by a thorough rinse with deionized (DI) water, and finally dried in the air before use. Highly ordered TiO 2 nanotube arrays were prepared by a potentiostatic anodization with a platinum (Pt) sheet as the counter electrode. A DYY-7 SourceMeter was used as the power supply and applied anodization potential was 30, 40, 50, and 60 V. The Ti foils

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were anodized in a 0.25 wt% NH 4F (purity >96 %, AR) and 75 ml ethylene glycol (purity >99.7% anhydrous) solution with 2 vol% water. All the experiments were conducted at 20 ºC without magnetic stirring. In order to discuss the effect of the anodization time on the morphology, the anodization process separately lasted for 0.5, 1, and 1.5 h when the anodization potential was 40 V. Afterwards, the samples were rinsed with DI water and dried in air. A subsequent heating at 500 ºC for 2 h with a temperature ramp rate of 2 ºC/min in air was applied to achieve the crystallization of TiO2 nanotube arrays[18].

2.2. Structure and morphology characterization The structure of the films was determined by a SHIMADZU XRD-7000 powder X-ray diffractmeter with Cu Kα radiation (λ = 0.154065 nm). The surface morphology of the films was observed by field emission-scanning electron microscopy (Hitachi S-4800 FE-SEM). 2.3. Evaluation of photocatalytic activity Photocatalytic properties of TiO 2 nanotube arrays under UV light illumination (365 nm, 1.08 mW/cm 2) were evaluated by the photodegradation of MB (oxidation) using UV-Vis spectroscopy (TU-1810). Each 8 mm × 8 mm thin film was immersed in 3 ml solution of MB (concentration 10 ppm) in dark at room temperature. After irradiating the solution for a defined time with UV light (365 nm, 1.08 mW/cm2), the concentration of MB was measured by the absorbance of MB at 665 nm with UV-Vis spectroscopy.

3. Results and Discussion 3.1. X-ray diffraction analysis XRD measurements confirmed that all of the TiO 2 nanotube films prepared under different conditions had polycrystalline anatase structures as shown in Fig. 1. All the etched films had their own preferred orientation (002) or (101). The average crystallite size of anatase in the samples can be calculated with the Scherrer formula[19] using the anatase (002) diffraction peaks: D  k  /(  cos  )

where D is the average crystallite size, k is a constant (0.89 here),  is the

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wavelength of the X-ray radiation (0.154 nm), β is the band broadening (full width at half-maximum) and  is the diffraction angle. The grain sizes were calculated to be 55.9, 59.6, 65.5, and 64.1 nm when the anodization potential was 30, 40, 50, and 60 V. The concrete analysis can be also seen in Table 1. 3.2. SEM analysis As shown in Fig. 2 and Fig. 3, the morphologies of the TiO2 nanotube arrays were examined by FESEM. The electrochemical etching of titanium foil in a fluoride medium produced an ordered array of hollow TiO 2 tubes. The details on the formation mechanism [20] of the TiO2 tubular array structure on a titanium substrate were described as follows. As the anodization process begins, the initial oxide layer formed due to interaction of the surface Ti 4+ ions with oxygen ions (O 2–) in the electrolyte, is seen uniformly across the surface. The overall reactions for anodic oxidation of titanium can be represented as: 2H 2O  O 2  4e  4H



(1)

Ti  O 2  T i O2

(2)

At the initial stages of the anodization process, field-assisted dissolution dominates chemical dissolution due to the relatively large electric field across the thin oxide layer. Small pits formed due to the localized dissolution of the oxide, act as pore forming centers, as shown in the following reaction: TiO



2



 6 F  4 H  TiF 6

2

 2H 2O

(3)

Then, these pits convert into bigger pores and the pore density increases. After that, the pores spread uniformly over the surface. The pore growth occurs due to the inward movement of the oxide layer at the pore bottom (barrier layer) due to processes (1)–(3). The Ti4+ ions migrating from the metal to the oxide/electrolyte interface dissolve in the NH 4F electrolyte. The rate of oxide growth at the metal/oxide

interface

and

the

rate

of

oxide

dissolution

at

the

pore-bottom/electrolyte interface ultimately become equal; thereafter the thickness of the barrier layer remains unchanged although it moves further into the metal, making the pore deeper. Fig. 2 shows FESEM images of TiO2 nanotube arrays on Ti foil fabricated by anodization at 30, 40, 50, and 60 V, respectively, revealing a regularly arranged

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pore structure of the film. The diameter of these pores gradually increased with increasing applied anodization potential. Depending on the anodization voltage, pore diameters of the resulting nanotube arrays ranged from 30 to 90 nm. Bauer et al.[21] reported that nanotubes with diameters ranging between 15 and 120 nm can be produced by anodization in an electrolyte composing of 1 mol/L H3PO4 + 0.3 wt% HF at voltages between 1 and 25 V. It was shown by Gong et al. [22] that anodization in an aqueous electrolyte containing 0.5 wt% hydrofluoric acid (HF), at voltages increased from 3 to 20 V, resulted in the formation of tubular structures with pore diameters increasing from 15 to 30 nm. Also, Macak et al.[23] showed that an increased TiO2 nanotube diameter can be obtained from 20 nm for anodization at 2 V to 300 nm for anodization at 40 V when Ti foils were anodized in water/glycerol mixtures with 0.27 mol/L NH4F. These studies showed that the nanotube diameter, d, is related to the applied voltage, V by d [ nm ]  k  V , where k equals 2  f g ; f g is the growth factor for anodic oxides and typically between 2 and 2.5 nm/V for TiO 2 films[24]. It is different from our experiment as shown in Fig. 4. Fitting the experimental values obtained for the nanotube diameter with a straight-line equation showed that the diameter increased at a growth factor, f g  0 . 64 nm/V. The reason was that the pore size not only was related to the applied anodization potential but also affected by electrolyte category and/or water content[25]. In addition, during anodization the color of the titanium oxide layer normally changed from purple to blue, light green, and then finally light red. Fig. 3 shows that with increasing anodization time, the nanotube length grew longer which was in accordance with previous reports[26]. As the anodization time increased, the tube lengths became from 2.3, 3.9, to 6.0 μm. It can be calculated that the growth rate was 4.17 μm/h at the applied voltage of 40 V. 3.3. Photocatalytic degradation of MB The photocatalytic degradation of organic pollutants in water generally follows a Langmuir-Hinshelwood mechanism [27], with the rate being proportional to the coverage  : r   d C / d t  k   k ( KC /(1  KC ))

(4)

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where k is the true rate constant which is dependent upon various parameters such as the mass of catalyst, the flux of efficient photons, the coverage in oxygen, etc., K is the adsorption constant, t is the time, and C is the concentration of organic pollutant (in this case, MB). For the low initial concentrations of pollutants, the term KC in the denominator can be neglected with respect to unity and the photocatalytic oxidation rate approaches first order: R   d C / d t  kKt  K ' C

(5)

where k is the apparent rate constant of the pseudo-first order kinetics. The integral form, C  f (t ) of the rate equation is: ln( C / C 0 )   k ' t

(6)

where C 0 is the initial concentration of MB. Two sets of MB degradation experiments were carried out in the solution with an initial concentration of 10 ppm under UV illumination using TiO 2 nanotube arrays fabricated under different conditions [28]. The experimental results are shown in Fig. 5 and Table 2. It can be seen that when the applied anodization potential was 40 V for 1 h, the photocatalytic efficiency was the best (k' = 0.4266 h–1). Under this anodization conditions, TiO 2 nanotube arrays characterized by pore diameter of 40–60 nm and tube length of 3.9 μm. Such structure had a larger surface area and made the mass diffusion more easily. With a suitable length of 3.9 μm, the incident illumination in combination with minimal radial dimensions can be effectively captured, providing facile separation of photogenerated charge.

4. Conclusion

Highly ordered TiO2 nanotube arrays with anatase structures were fabricated via electrochemical anodization of high purity Ti foil in a 0.25 wt % NH4F and 75 ml ethylene glycol solution with 2 vol % water. The applied potential affected the pore diameter and tube crystallinity. Depending on the anodization voltage, pore diameters of the resulting nanotube arrays ranged from 30 to 90 nm. The grain sizes were calculated to be 55.9, 59.6, 65.5, and 64.1 nm under different anodization potential of 30, 40, 50, and 60 V. The tube lengthsincrease with the

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increasing anodization time.. It can be calculated that the growth rate was 4.17 μm/h under the applied voltage of 40 V. When the applied anodization potential was 40 Vfor 1 h, the obtained TiO2 nanotube arrays showed the best photocatalytic degradation ratio of MB (k' = 0.4266 h–1). Acknowledgements This work was partly supported by the National Natural Science Foundation of China (Grant No. 61405171), the Shandong Province Natural Science Foundation (No. ZR2012FQ014), and the Shandong Province Higher Educational Science and Technology Program (No. G12LA08, No. J13LJ05). REFERENCES [1] Z.H. Zhang, Y. Yuan, G.Y. Shi, Y.J. Fang, L.H. Liang, H.C. Ding, L.T. Jin, Environ. Sci. Technol. 41 (2007) 6259–6263. [2] S.G. Yang, Y.Z. Liu, C. Sun, Appl. Catal. A: General 301 (2006) 284–291. [3] M.Y. Guo, M.K. Fung, F. Fang, X.Y. Chen, A.M.C. Ng, A.B. Djurišić, W.K. Chan, J. Alloy. Compd. 509 (2011) 1328–1332. [4] S. Sreekantan, R. Hazan, Z. Lockman, Thin Solid Films 518 (2009) 16–21. [5] Y.L. Su, X.W. Zhang, M.H. Zhou, S. Han, L.C. Lei, J. Photochem. Photobiol. A: Chemistry 194 (2008) 152–160. [6] Y. Xie, G. Ali, S.H. Yoo, S.O. Cho, ACS Appl. Mater. Interface. 2 (2010) 2910–2914. [7] A.Y. Zhang, M.H. Zhou, L. Han, Q.X. Zhou, J. Hazard. Mater. 186 (2011) 1374–1383. [8] Q.X. Zhou, Z. Fang, J. Li, M.Y. Wang, Microporous Mesoporous Mater. 202 (2015) 22–35. [9] M. Bockmeyer, B. Herbig, P. Löbmann, Thin Solid Films 517 (2009) 1596–1600. [10] C.H. Jiang, J.S. Zhang, J. Mater. Sci. Technol. 29 (2013) 97–122. [11] S.S. Park, S.M. Eom, D.H. Seo, Y.G. Shul, Res. Chem. Intermed. 36 (2010) 77–82. [12] Z.C. Xu, Q. Li, S. Gao, J.K. Shang, J. Mater. Sci. Technol. 28 (2012) 865–870. [13] M.H. Yaaco, A.Z. Sadek, K. Latham, K. Kalantar-zadeh, W. Wlodarski, Procedia Chem. 1 (2009) 951–954. [14] G.G. Bessegato, J.C. Cardoso, M.V. B. Zanoni, Catal. Today 240 (2015) 100–106. [15] S. Palmas, A.D. Pozzo, M. Mascia, A. Vacca, A. Ardu, R. Matarrese, I. Nova, Int. J. Hydrogen Energy 36 (2011) 8894–8901.

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Table and Figure Captions: Fig. 1 XRD patterns of the TiO 2 nanotube arrays fabricated at different anodization potentials for 1 h (a) and at 40 V for different anodization time (b) annealed at 500 °C for 2 h in ambient atmosphere, respectively (A=anatase). Fig. 2 Top-view FESEM images of TiO2 nanotube arrays on Ti foil fabricated at different anodization potentials for 1 h: (a) 30 V, (b) 40 V, (c) 50 V, (d) 60 V. Fig. 3 FESEM cross-section images of TiO2 nanotube arrays at different anodization time: (a) 0.5 h, (b) 1 h, (c) 1.5 h at 40 V and (a'), (b'), (c') correspond to the magnified images. Fig. 4 Plot of the TiO2 nanotube diameter vs. anodization voltage at different voltages for 1 h. Fig. 5 UV light-induced photodegradation of MB by TiO 2 nanotube arrays fabricated at different anodization potential for 1 h (a) and (b); at 40 V for different anodization time (c) and (d). C0 is original [MB] concentration and C is the concentration after a specified UV light irradiation time (h).

Table list Table 1 Pore diameter and grain size of the TiO 2 nanotube arrays fabricated at different anodization potential for 1 h Anodization voltage Average tube diameter Average grain size (V) (nm) (nm) 30 30 55.9 40 50 59.6 50 70 65.5 60 90 64.9

Table 2 k' values of TiO2 nanotube arrays prepared under different conditions. Different anodization potentials for 1 h 30 V k'

0.3357

Different anodization time at 40 V

40 V

50 V

60 V

0.5 h

1h

1.5 h

0.4266

0.3578

0.3466

0.1719

0.4266

0.1892

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