The effect of anodization parameters on the formation of nanoporous TiO2 layers and their photocatalytic activities

Journal of Alloys and Compounds 604 (2014) 66–72

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The effect of anodization parameters on the formation of nanoporous TiO2 layers and their photocatalytic activities Mustafa Erol a,b,c,⇑, Tuncay Dikici a,b,d, Mustafa Toparli a,e,f, Erdal Celik a,e,f a

Dokuz Eylul University, Department of Metallurgical and Materials Engineering, Buca 35160, Izmir, Turkey Dokuz Eylul University, The Graduate School of Natural and Applied Sciences, Buca 35160, Izmir, Turkey c Hitit University, Department of Metallurgical and Materials Engineering, 19000 Çorum, Turkey d Izmir Katip Celebi University, Department of Materials Science and Engineering, Cigli 35620, Izmir, Turkey e Dokuz Eylul University, Center for Fabrication and Applications of Electronic Materials (EMUM), Buca 35160, Izmir, Turkey f Dokuz Eylul University, Department of Nanoscience and Nanoengineering, Buca 35160, Izmir, Turkey b

a r t i c l e

i n f o

Article history: Received 23 January 2014 Received in revised form 17 March 2014 Accepted 21 March 2014 Available online 28 March 2014 Keywords: Nanoporous TiO2 Anodization parameters Photocatalytic activity Surface morphology

a b s t r a c t In this work, nanoporous titanium dioxide (TiO2) layers were successfully formed by electrochemical anodization method on titanium (Ti) surface in fluorine containing electrolytes with different processing parameters. The effects of anodization voltages, electrolyte temperature and anodization time on the microstructure and photocatalytic performance of nanoporous TiO2 layers were investigated and compared in details. Nanoporous structures were annealed at 480 °C for 2 h in air in order to obtain anatase transformation and increase crystallinity. The phase structure and surface morphology of the samples characterized by means of X-ray diffraction (XRD) and scanning electron microscope (SEM) respectively. The photocatalytic activity tests of the samples were evaluated by the degradation of aqueous methylene blue (MB) solutions under UV light illumination for different periods of time. The results showed that the processing parameters on production of nanoporous TiO2 layers played important roles in the degradation of aqueous MB solutions. To sum up, the highest photocatalytical activity was obtained at the sample anodized under 30 V for 30 min at 20 °C among the samples. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction In recent years, nanostructured titanium dioxide (TiO2) has been widely studied in several scientific and commercial application fields such as photocatalysis [1], solar cells [2], sensors [3], self-cleaning surfaces [4] and biomedical industry [5] owing to its unique physical and chemical properties [6–8]. Subsequent to novel study of Fujishima and Honda on photocatalytic water splitting using TiO2 by in 1972, the studies performed on TiO2 as a photocatalyst for purification of groundwater and wastewater have increased significantly [9–14]. In order to perform these studies, structure and properties of TiO2 must be illuminated in details. There are three different crystalline phases of TiO2 namely rutile (tetragonal structure), anatase (tetragonal structure) and brookite (orthorhombic structure). Among the three main crystal phases of TiO2, anatase phase is metastable and is referred to be the most

⇑ Corresponding author at: Dokuz Eylul University, Department of Metallurgical and Materials Engineering, Buca 35160, Izmir, Turkey. E-mail address: [email protected] (M. Erol). http://dx.doi.org/10.1016/j.jallcom.2014.03.105 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

photoactive structure although rutile has a high chemical stability [15,16]. TiO2 films and/or surface modifications can be fabricated by vacuum evaporation, magnetron sputtering, sol–gel, chemical vapor deposition (CVD), liquid-phase deposition (LPD) methods and electrochemical anodization, etc. Of these techniques, electrochemical anodization can be noted as more efficient and cost effective method of preparing nanostructured TiO2 layers on titanium substrates [17–19]. Recently, TiO2 layers with a nanotubular and porous structure have been also prepared via anodic oxidation in an aqueous HF-based electrolyte by some researchers [18,20]. In contrast to our previous study where we have addressed the photocatalytic activities of anodized titanium surfaces [21], here we aimed to investigate anodization parameters and their effects on the photocatalytic reactions to determine optimal processing parameters. Anodized TiO2 film synthesis is directly affected from the production parameters such as electrolyte type, anodizing voltage, bath temperature, and anodization time. Any optimization issues afforded on these parameters will directly change some properties such as the crystal structure, surface morphology, and phase stability. It must be noted that the mentioned properties

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are very important and must be controlled for the desired photocatalytical properties [22]. In this study, nanoporous TiO2 layers have been prepared on titanium surfaces using an electrochemical anodizing technique under different processing parameters. The effects of anodization parameters such as anodization voltages, electrolyte temperature and anodization time on the phase structures, surface morphologies, photocatalytic activities of nanostructured TiO2 layers were scrutinized in details.

2. Experimental details Samples with a 25 mm diameter and a 5 mm thickness were cut from commercially pure titanium (ASTM grade 2) cylindrical bar, and then mechanically polished with 80-grit to 2000-grit emery paper. After polishing process, the titanium samples were washed with acetone, ethanol and distilled water in an ultrasonic cleaner at room temperature for 15 min in air. Prior to the anodization experiments, the samples were cleaned in a mixture of nitric acid (HNO3) and hydrofluoric acid (HF) solutions for 10 s to remove the air-formed oxide layer. Applied anodization parameters were listed in Table 1. The anodization treatment was performed in 1% hydrofluoric acid (HF) solution through a DC power supply and the distance between the anode–cathode was kept about 3 cm for all the experiments. All the anodized samples were annealed at 480 °C for 2 h in air to improve the oxide crystallinity and anatase transformation as reported in some similar studies [21,23]. X-ray diffraction (XRD, Rigaku D/MAX-2200/PC) patterns of the specimens were determined to identify phase structure with the help of a diffractometer with a Cu Ka irradiation. The surface morphology and microstructure of the specimens were characterized by a scanning electron microscope (SEM, JEOL 6060).

Table 1 Anodization parameters of nanoporous TiO2 layers. Group

Voltages (V)

Time (min)

Temperature (°C)

Group 1

20 30 40 50

30 30 30 30

20 20 20 20

Group 2

20 20 20 20

30 60 120 240

20 20 20 20

Group 3

20 20 20 20

20 20 20 20

20 30 40 50

20-30-20

1

Photocatalytic degradation experiments of the specimens were carried out using a setup including aqueous methylene blue (MB) solutions for all catalysts and a UV light source (Osram, UltraVitalux E27, 300 W). In order to record catalyst-free degradation of MB under radiation an arbitration sample of the solution was also prepared. MB precursor powder used for the solution preparation was purchased from Merck in the laboratory grade and used without further purification. The MB solutions of 30 mL volume were poured into beakers and catalysts were placed across to light source with a distance approximately 200 mm. Lambert Beer Law denotes that, the molarity of a solution is directly proportional to its absorbance [21,24]. Therefore the absorbance measurement with a UV–visible spectrophotometer (V-530 JASCO UV/VIS) at 664 nm (which is maximum absorption wavelength of MB) was performed. Additionally, absorbance versus molarity calibration curve of MB was plotted to determine corresponding molarities. Times versus morality values of all degraded solutions were recorded for different durations. Rate of a photocatalytic reaction is an important issue for determination of any catalyst to be transferred into device or industrial applications. Because of this reality, as the last step of characterization, photocatalytic reaction kinetics on MB degradation were calculated for all samples using the catalysts.

3. Result and discussion 3.1. XRD studies It is successfully reported in the literature that films growth by anodization process provides a highly amorphous TiO2 structure [25,26]. XRD patterns of the nanoporous TiO2 films annealed with the aim of anatase transformation were given in Figs. 1–3 for sample groups 1, 2 and 3, respectively. According to all of these patterns, it is possible to express that all the anodized and annealed samples were constructed from anatase and metallic titanium phases as marked with 1 and 2 for titanium (JCPDS 044-1294) and anatase (JCPDS 021-1272) respectively. The peaks (1) for metallic titanium substrates were also obtained in all patterns because of X-ray penetration into the substrate through the thin nanoporous TiO2 layers. Beside the similar character of all patterns, it can be inferred that the highest intensity of anatase peak (2) was obtained with the sample anodized with 20 V at 20 °C for 240 min in group 2. It is worth noting that, an increase in the time of anodization provides the increasing thickness of the TiO2 layer regarding to the increasing amount of relative anatase content that is important for photocatalytical issues. The obtained results can be supported by referring to study of Sreekantan et al. As they reported, in the early stage of anodization random small pits formed. Subsequent to pits formation, it is reported that tube formation started and kept growing from 20th minute to 360th minutes which means film thickness (nanotube length) increased [27].

1: Titanium 2: Anatase

1 1

1

2

1 1 1

Intensity (CPS)

30-30-20

40-30-20

50-30-20

10

20

30

40

50

60

70

800 600 400 200 0 800 600 400 200 0 600 400 200 0 600 400 200 0

80

2θ (Degree) Fig. 1. XRD patterns of the samples anodized at 20 °C for 30 min at different voltages (a) 20 V, (b) 30 V, (c) 40 V and (d) 50 V.

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20-30-20

1

1: Titanium 2: Anatase

1 1

1

2

1 1 1

Intensity (CPS)

20-60-20

20-120-20

20-240-20

10

20

30

40

50

60

70

800 600 400 200 0 800 600 400 200 0 600 400 200 0 600 400 200 0

80

2θ (Degree) Fig. 2. XRD patterns of the samples anodized at 20 V and 20 °C for different times (a) 30 min, (b) 60 min, (c) 120 min and (d) 240 min.

20-30-20

1

1: Titanium 2: Anatase

1 1

1

2

1 1 1

Intensity (CPS)

20-30-30

20-30-40

-20-30-50 -

10

20

30

40

50

60

70

800 600 400 200 0 800 600 400 200 0 600 400 200 0 600 400 200 0

80

2θ (Degree) Fig. 3. XRD patterns of the samples anodized at 20 V for 30 min at different electrolyte temperatures (a) 20 °C, (b) 30 °C, (c) 40 °C and (d) 50 °C.

The obtained structural data for group 1 samples were found to be in a good agreement with the studies of Ref. [28] where they have investigated the effect of anodization voltage. It is reported that the anatase to rutile transformation is occurred over 30 V. It must be noted that the dominancy of the Ti peaks weakens the rutile peaks. As Su et al. reported, the anodized films grow at higher electric field strength when applied with a higher anodization voltage. The higher field strength across the oxide films and the localize temperature rise induced by the resulting enhanced joule heating will both accelerate the crystallization of the oxides grown [29]. Increasing electrolyte temperature is thought to be primarily active since it changes the conditions of the anodization reactions in thermodynamically and chemically manners. It can be inferred that increasing anodization temperature tends to decrease electrolyte–specimen interaction where the anodization come through. Effect of anodization temperature on phase formation requires the assistance of discussions for photocatalytical results. With this manner it will be further clarified. 3.2. Surface morphology The anodizing potential is one of the most important parameters pertaining to the surface morphology of TiO2 nanostructures

[30]. The surface morphologies of group 1 samples anodized at different voltages are depicted in Fig. 4. It was observed that uniform and ordered TiO2 nanopores formed on titanium substrate at 20 V (see Fig. 4a for details). Note that as the voltage increased, micrometer scale pores formed on the samples. Generally speaking, voltages in the 30–40 V range resulted in similar nanoporous TiO2 structures as can be seen from Fig. 4b and c. This is what once the anodization voltage increases to 50 V, micro pores gradually began to disappear. On the other hand, the shape of nanopores exhibited a noticeable change with increasing applied voltage. The observed morphologies for varying applied anodization voltages were found to be similar to the study of Ref. [27]. The occurring reactions during anodization are carefully explained by Alivov et al. Growth of TiO2 nanotubes is governed by a competition between formation of TiO2 and formation/dissolution of Ti–F complexes during anodization. If anodization voltages are thought to be as the driving force for nanotube formation regarding to local electrical fields, disappearance of the nanotubes and changes in their shapes with higher anodization voltages can be advocated [31]. Fig. 5 shows the effects of anodization time on the surface morphology of nanoporous TiO2 films. Our studies indicate that uniform and highly ordered TiO2 nanopores formed on the

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Fig. 4. SEM micrographs of the samples anodized at 20 °C for 30 min at different voltages (a) 20 V, (b) 30 V, (c) 40 V and (d) 50 V.

Fig. 5. SEM micrographs of the samples anodized at 20 V and 20 °C for different times (a) 30 min, (b) 60 min, (c) 120 min and (d) 240 min.

titanium substrates with increasing anodization duration (Fig. 5a–d). It is known that the thickness of the TiO2 film increases with increasing anodizing time [32]. The results showed that the morphologies of the TiO2 nanopores can be controlled by alteration of the anodization parameters. It should be kept in mind that, generally in group 1 samples, an increase in the

number of micro pores was determined when nanopore formation become irregular at a 30 V. The most striking feature of TiO2 layers is that, congruently to the obtained XRD results, the highly ordered and homogenous nanoporous structure was obtained with the sample anodized 20 V at 20 °C for 240 min as depicted in Fig. 5d which can be

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Fig. 6. SEM micrographs of the samples anodized at 20 V for 30 min at different electrolyte temperatures (a) 20 °C, (b) 30 °C, (c) 40 °C and (d) 50 °C.

evaluated as a result of the increasing thickness [32]. Nonetheless, it must be considered that the increase in the thickness of the film might cause strain formation due to crystalline structure mismatch with the substrate [33]. Fig. 6 denotes the surface morphology of nanostructured TiO2 films produced at various electrolyte temperatures. An important point here is that no significant changes were observed regarding to the diameter of the nanopores produced in ranging from 20 °C to 40 °C (Fig. 6a–c) in a good agreement with similar study applied for aluminum anodization [33]. On the other hand, nanoporous structures were not observed for 50 °C as represented in Fig. 6d. It can be expressed that the rate of chemical etching at this temperature is found to be faster than the anodic oxide formation rate which are important reactions in the anodization process [27].

As mentioned before, rate of a photochemical reaction is important for any industrial reactor design and applications. Photocatalytical oxidation kinetics of many organic compounds such as MB, referred to Langmuir–Hinshelwood kinetics model (L–H) since the compound was adsorbed on the catalyst surface and the reaction is surface controlled. With this manner; some important units C, C0, m and t must be defined as the instant molarity, starting molarity, rate constant and time respectively [34]. The results depicted in Fig. 8a–c indicate that the kinetics of photocatalytic reaction fit the Langmuir–Hinshelwood kinetics model for sample groups 1, 2 and 3 respectively. Regarding to the kinetic data plotted in Fig. 8a, it can be inferred that the best photocatalytical performance was obtained with sample anodized using anodization voltages of 30 V and 40 V with linear fitted functions according to the Langmuir–Hinshelwood

3.3. Photocatalytic activity of nanoporous TiO2 layers

y Experimental Data Lineer Fit

0,6 0,5

Absorbance

Photocatalytical performances of the catalyst were tested by using artifactual dyes as impurities. Within the framework of the present study, MB was chosen as a dye in our study. Prior to derivation of photocatalytic degradation efficiencies of the films on MB, Lambert–Beer correlation curve was plotted to determine linear relation between absorbance and concentration as shown in Fig. 7. According to Fig. 7, a linear function as y = 68254.54x was obtained with R2 = 0.9977 where x, y, and R denote molarity, absorbance and mean square errors respectively. Regarding function will help us on molarity determination of the degraded MB solutions. Subsequent to determination of the correlation function, absorbance measurements of the samples were employed for different UV irradiation duration. It is important to note that measured maximum absorbance values at 664 nm wavelength time 0–300 min of irradiation were converted to molarities using the function.

0,7

y=68254.54x 2 R =0.9977

0,4 0,3 0,2 0,1 0,0

x

-0,1 0,0

-6

2,0x10

-6

4,0x10

-6

6,0x10

-6

8,0x10

Molarity (mol/L) Fig. 7. Absorbance–molarity correlation plot.

-5

1,0x10

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(a) 2,0 1,6 1,2 0,8

1,2 0,8

0,4

0,4

0,0

0,0 0

60 min. 30 min. 120 min. 240 min.

1,6

Ln (C0 /C)

Ln (C0 /C)

(b) 2,0

20V 30V 40V 50V

50

100

150

200

250

0

300

50

100

(c)

200

250

300

1,6 o

20 C o 30 C o 40 C o 50 C

1,2

Ln (C0 /C)

150

Time (mins.)

Time (mins.)

0,8

0,4

0,0 0

50

100

150

200

250

300

Time (mins.) Fig. 8. Photocatalytical kinetics of the samples (a) group 1, (b) group 2, and (c) group 3.

Table 2 Photocatalytical test results of nanoporous TiO2 layers anodized at different anodization parameters. Group 1 Anodization parameters Kinetic constant (m) R2 Degradation efficiency (%)

20 V 0.0047 0.996 76

Group 2 30 V 0.0061 0.997 83

40 V 0.0058 0.997 82

50 V 0.0046 0.993 74

30 min 0.0047 0.996 76

kinetics model. In addition to the obtained results, it can be stated out that 20 V anodization voltage is not enough for nanopore formation while 50 V tends to film spillage as supported with the SEM results. Kinetic data obtained for group 2 samples were given in Fig. 8b. Based on the calculations mentioned above, it is easy to express that the best photocatalytical kinetics were obtained with the sample anodized for 240 min. The obtained results were found to be in a good agreement with the SEM micrographs point out the uniform and thicker layers. In the scope of group 3 samples the best photocatalytical performance was obtained with the sample anodized at 20 °C as seen in Fig. 8c which means the increase in the electrolyte temperature decreases the film formation, as observed in morphological results. Dependent on these results, film growth at different temperature might be attributed to; the increase of ion migration, and increases of the chemical etching as electrolyte temperature increases. Also the obtained results for group 3 were found to be in a good agreement with the study employed with aluminum [33]. In order to perform a good comparison of phocatalytical performances, a list of calculated Langmuir–Hinshelwood kinetics

Group 3 60 min 0.0031 0.938 64

120 min 0.0042 0.978 72

240 min 0.0051 0.994 81

20 °C 0.0047 0.996 76

30 °C 0.0031 0.968 68

40 °C 0.0021 0.965 55

50 °C 0.0012 0.787 34

function slopes (m, reaction rates) and degradation percentages and for all samples was also listed in Table 2 with mean square errors. The degradation percentages in the table were calculated using the equation; [100 (C300  100)/C0)] where C300 and C0 denote the molarities of starting time and 300th minute respectively. Samples of group 1 anodized with 30 V, 40 V and group 2 anodized for 240 min were found to be the best photocatalysts regarding to their m values listed in Table 2. Owing to this reality, it can be inferred that the optimum temperature for anodization can be defined as 20 °C and voltages between 30 V and 40 V. In addition to the generalization on temperature and voltage, time for anodization can be defined by the operations requirements since the increasing time results thicker film thickness and photocatalytical activity. 4. Conclusion In summary, nanoporous TiO2 layers were successfully formed on titanium substrates using electrochemical anodization in HF containing electrolytes at different anodizing parameters. Subsequent to production all anodized layers transformed from amorphous to anatase structure thanks to heat treatment process.

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In addition to structural optimization, different microstructures and morphologies were obtained with varying processing parameters. Owing to photocatalytic activity of TiO2, photodegradation tests were performed. According to the tests results, the best activity was recorded between 30 V and 40 V with slight differences for varying voltage samples (group 1). The best performance for samples classified depending on the increasing time of anodization (group 2) was determined as 240 min. No significant change was obtained with the samples of varying temperatures (group 3). Our investigations showed that anodizing parameters effect on the microstructure and photocatalytic activity of nanoporous TiO2 layers. Among all the obtained data the optimal anodization parameters for the formation of nanoporous TiO2 film having the highest photocatalytic performance were determined as 20 °C, 30–40 V and 240 min for further studies if the results above were considered. Acknowledgements The authors are indebted to State Planning Foundation (DPT) and Dokuz Eylul University financial and infrastructural support for establishment of Dokuz Eylul University, Center for Production and Applications of Electronic Materials (EMUM) where this research was carried out. References [1] H.D. Jang, S.K. Kim, S.J. Kim, Effect of particle size and phase composition of titanium dioxide nanoparticles on the photocatalytic properties, J. Nanopart. Res. 3 (2001) 141–147. [2] M. Grätzel, Photoelectrochemical cells, Nature 414 (2001) 338–344. [3] O.K. Varghese, D. Gong, M. Paulose, K.G. Ong, C.A. Grimes, Hydrogen sensing using titania nanotubes, Sens. Actuators, B 93 (2003) 338–344. [4] I.P. Parkin, R.G. Palgrave, Self-cleaning coatings, J. Mater. Chem. 15 (2005) 1689–1695. [5] R. Rodriguez, K. Kim, J.L. Ong, In vitro osteoblast response to anodized titanium and anodized titanium followed by hydrothermal treatment, J. Biomed. Mater. Res. A 65 (2003) 352–358. [6] S. Hrapovic, B.L. Luan, M. D’amours, G. Atankhah, G. Jerkiewicz, Morphology, chemical composition, and electrochemical characteristics of colored titanium passive layers, Langmuir 17 (2001) 3051–3060. [7] O.K. Varghese, D. Gong, M. Paulose, K.G. Ong, E.C. Dickey, C.A. Grimes, Extreme changes in the electrical resistance of titania nanotubes with hydrogen exposure, Adv. Mater. 15 (2003) 624–627. [8] J.W. Schultze, M.M. Lohrengel, Stability, reactivity and breakdown of passive films. Problems of recent and future research, Electrochim. Acta 45 (2000) 2499–2513. [9] A. Fujishima, Kenichi Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [10] O. Prieto, J. Fermoso, Y. Nuez, J.L. Del Valle, R. Irusta, Decolouration of textile dyes in wastewaters by photocatalysis with TiO2, Sol. Energy 79 (2005) 376– 383. [11] A.L. Linsebigler, G. Lu, J.T. Yates, Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758.

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