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ARTICLE IN PRESSG ModelPSUSC-27234; No. of Pages 5

Applied Surface Science xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h om ep age: www.elsev ier .com/ locate /apsusc

hotocatalytic degradation of textile dyestuffs using TiO2 nanotubesrepared by sonoelectrochemical method

erya Tekin ∗

aculty of Engineering, Department of Chemical Engineering, Atatürk University, 25240 Erzurum, Turkey

r t i c l e i n f o

rticle history:eceived 23 October 2013eceived in revised form 7 January 2014ccepted 6 February 2014vailable online xxx

a b s t r a c t

TiO2 nanotubes were prepared by anodization of Ti plates by conventional electrochemical technique aswell as by an emerging sonoelectrochemical technique. Scanning electron miscroscope (SEM) analysisshowed that ultrasound assisted anodization yielded more ordered and controllable TiO2 tube banks withhigher tube diameter. The photocatalytical activities of TiO2 nanotubes were tested in the photocatalyt-ical degradation of Orange G dye. The results showed that sonoelectrochemically prepared TiO2 tubes

eywords:iO2 nanotubeshotocatalysisltrasoundnodization

exhibited 10% higher photocatalytic performance than the electrochemical prepared ones, and more than18% higher activity than the other TiO2 samples.

© 2014 Published by Elsevier B.V.

range G dye

ntroduction

TiO2 is inexpensive, stable, readily, available and the mostxtensively studied semiconductor photocatalyst for the purifi-ation of water and air. For these reasons TiO2 has been usedommonly for photocatalysis processes [1–3]. TiO2 nanotubes ofubular structure possess unique physical and chemical proper-ies including large specific surface area, good electron/protononductivity and high aspect ratio [4]. In particular, they possessutstanding charge transport and long carrier lifetime propertiesince the unique structure of TiO2 nanotubes provides electron per-olation pathways for effective vectorial charge transfer [5]. Theharge carriers easily move along the longitudinal direction of theubular nanostructure, which is favorable to improve the separa-ion ability of photoinduced electron/hole pairs. All these excellentroperties make TiO2 nanotubes more suitable for use as photocat-lysts or catalyst supports for photocatalytic reactions [6].

Methods of fabricating TiO2 nanotubes currently developedomprise the assisted-template method [1], the sol–gel process2], electrochemical anodic oxidation [7,8] and hydrothermalreatment [9]. The anodization of titanium using phosphoric acid

Please cite this article in press as: D. Tekin, Photocatalytic degradationtrochemical method, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016

nd sodium fluoride or hydrofluoric acid has also recently beeneported [10]. The anodization approach builds self-organized

∗ Tel.: +90 442 2314583; fax: +90 442 2314544.E-mail address: deryatekin@atauni.edu.tr

ttp://dx.doi.org/10.1016/j.apsusc.2014.02.018169-4332/© 2014 Published by Elsevier B.V.

titanium nanotubular arrays with controllable tube diameter, gooduniformity, and conformability over large areas.

TiO2 nanotubes prepared by anodization have been recentlyused as photocatalyst in various studies [11,12]. Xu et al. [13] inves-tigated the photocatalytic degradation of methyl orange using aTiO2 nanotube which was modified by adsorbed Zn2+ ions. Con-versely, Qamar et al. [14] prepared a TiO2 nanotube photocatalystusing the sol–gel method; TiOCl2 solution was hydrothermallytreated to induce the sol, which was conditioned using NaOHsolution and finally thermally treated in an autoclave to generatenanotubes in an autoclave.

The aim of this study was to prepare TiO2 nanotube by andozingtitanium using electrochemical and sonoelectrochemical methodas well, and to test its photocatalytic activity using the degradationof a commercial Orange G dye.

Materials and methods

The preparation of TiO2 nanotube

TiO2 nanotube array was grown by anodization of titaniumsheets in a hydrofluoric acid solution stirred magnetically, using atwo electrode electrochemical cell with a large platinum coil as acathode. Anodization was carried out by varying the applied poten-

of textile dyestuffs using TiO2 nanotubes prepared by sonoelec-/j.apsusc.2014.02.018

tial from 20 V using a DC power supplies (Cole-Parmer, 1627A)and the anodization time was 40 min. The anodized titaniumsheets were annealed in dry oxygen environment at 500–650 ◦Cfor 1 h, heating and cooling rates were kept at 2.5 ◦C min−1. In

ARTICLE IN PRESSG ModelAPSUSC-27234; No. of Pages 5

2 D. Tekin / Applied Surface Science xxx (2014) xxx–xxx

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before calcinations. Crystals were formed during annealing [16].TiO2 films treated at a temperature 500 ◦C and below 550 ◦C werepolycrystalline and did not have a rutile phase. However, the rutilephases were incrementally observed with temperature increases

Fig. 1. The experimental setup for ph

onoelectrochemical method, ultrasound was applied using anorn type ultrasonic homogenizer (Cole-Parmer, 750 W).

xperimental system

Fig. 1 shows the experimental setup used in photocatalyticegradation of Orange G dye. The experiments were performed in

jacketed reactor isolated from the outer surface against the light.he reaction temperature was kept constant with a programmableater circulator. For investigating the effect of the intensity of light,iffering numbers of Pen-Ray UV lambs emitting at 254 nm with

light intensity of 44 W/m2 (Cole-Parmer) were immersed in theeactor. Air was supplied at constant flow rate to ensure saturated2 concentration in the reaction medium.

esults and discussion

orphology of TiO2 nanotubes

Characterization of TiO2 nanotubes prepared by electrochemi-al and sonoelectrochemical methods was performed by scanninglectron microscopy (SEM). SEM images of both TiO2 nanotubeamples were shown in Figs. 2 and 3, respectively. The averageiameter of these nanotubes was around 85 and 100 nm, respec-ively.

The formation mechanism of the TiO2 nanotubes can bexplained as follows [10]. In aqueous acidic medium, titanium oxi-izes to form TiO2:

i + 2H2O → TiO2+ 4H+ (1)

The pit initiation on the oxide surface is a complex process.lthough TiO2 is stable thermodynamically at a pH range 2–12, aomplexing ligand (F−) leads to substantial dissolution. The mech-nism of pit formation due to F− ions is given by

iO2+6F−+4H+→ [TiF6]−2+2H2O (2)

This complex formation leads to breakage in the passive oxideayer, with pit formation continuing until repassivation occurs [10].anotube formation goes through the diffusion of F− ions and

Please cite this article in press as: D. Tekin, Photocatalytic degradationtrochemical method, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016

imultaneous effusion of the [TiF6]−2 ions. The faster rate of for-ation of TiO2 nanotubes using ultrasonic waves can be explained

y the faster mobility of the F− ions into the nanotubular reactionhannel and effusion of the [TiF6]−2 ions from the channel.

talytic degradation of Orange G dye.

As it is well known, the thermal–mechanical–chemical effects ofultrasonic energy are due to cavitation bubbles. Negative pressureis applied to the product of micro-cavitation bubbles. Reductionin the tensile strength of the weak points because of the presenceof fluid cavitation result relatively from low acoustic pressure [15].Collapse of transient bubbles causes a jet of liquid to impinge on thesurface [15]. On a microscopic scale, impingement of a liquid jet onthe surface could increase the dissolution reaction rate. Ultrasoni-fication helps break the double layer and thus hastens the diffusionof F− ions into the nanotubes and effusion of [TiF6]−2 ions from thenanotubes.

XRD analysis

Fig. 4 shows the XRD patterns of nano-TiO2 oxidized at a temper-ature range of 500–650 ◦C for 3 h. Nanotube arrays were amorphous

of textile dyestuffs using TiO2 nanotubes prepared by sonoelec-/j.apsusc.2014.02.018

Fig. 2. SEM analysis of TiO2 nanotube prepared using the electrochemical method.

Please cite this article in press as: D. Tekin, Photocatalytic degradationtrochemical method, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016

ARTICLE ING ModelAPSUSC-27234; No. of Pages 5

D. Tekin / Applied Surface Scie

Fig. 3. SEM analysis of TiO2 nanotube prepared using the sonoelectrochemicalmethod.

Fig. 4. XRD patterns of sample nano-

PRESSnce xxx (2014) xxx–xxx 3

from 500 to 650 ◦C. When the temperature reached 650 ◦C, the char-acteristic peaks of rutile phase were monitored other anatase peaksdecreased in intensity [17].

Photocatalytic activity of TiO2 nanotubes

In order to test the photocatalytic activity of TiO2 nanotubesthe photocatalytic decomposition of Orange G dye was used. Theresults obtained using different TiO2 photocatalysts (Degussa P25TiO2 and Riedel-de Haen TiO2) were compared to those of the pho-tocatalytic decomposition. All test results are shown in Fig. 5.

As shown in Fig. 5, degradation of Orange G dye using TiO2 nano-tubes prepared by sonoelectrochemical method was much fasterthan that of the other catalyst used for degradation of Orange Gdye. For example, in the experiments of degradation of Orange Gdye, at the end of 20 min, degradation was 79% in photocatalyticaldegradation using TiO2 nanotubes prepared by sonoelectrochemi-cal method; 68% in the experiment using TiO2 nanotubes preparedby electrochemical method; 60% in the photocatalytic experiment

of textile dyestuffs using TiO2 nanotubes prepared by sonoelec-/j.apsusc.2014.02.018

using Degussa P-25 TiO2 and 32% in the photocatalytic experimentusing Riedel-de Haen TiO2. At the end of 40 min, the degradationwas, respectively, 92%, 83%, 76%, >and 51%. At the end of 60 min,the values were 98%, 89%, 84%, and 60%.

TiO2 at different temperatures.

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0

5

10

15

20

25

30

706050403020100

Dye

Con.

(mg/

L)

With nanotube TiO2(Elec trochem. Metod)With nanotube TiO2(Sonoe lectroche m. Meto d)With Degussa P-25 TiO2)

With Ri edel-d e Ha en TiO2)

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t/(C

o-C)

ln(Co/C)/(Co-C)

Time (min)

Fig. 5. Degradation of Orange G dye in different photocatalyst.

inetic model

The reaction mechanism of photocatalytic dye degradation isroposed by considering the reaction steps reported in the litera-ure [18] is as follows:

iO2 + h� (� < 390 nm)k1←→TiO2(e−) + TiO2(h+) (3)

iO2(h+) + H2Ok2−→TiO2(OH•) + H+ (4)

iO2(e−) + O2k3−→TiO2(O−•2 ) + H+ (5)

iO2(OH•) + Cdyek4−→Products + TiO2 (6)

iO2(O−•2 ) + Cdyek5−→Products + TiO2 (7)

The first step is the adsorption of dye over the catalyst surfacend the attainment of adsorption desorption equilibrium. An elec-ron hole pair is generated when UV radiation having energy equalo or greater than the band gap of a catalyst strikes it. This is becausef the transfer of an electron from the valence band to conduc-ion band. These charge carriers, i.e., e- and h+ can recombine, ifhey do not transfer their charge to other species. This recombina-ion eventually turns up into decreased efficiency of photocatalyticrocess.

This proposed mechanism starts with the photocatalyst irradi-tion by UV light, followed by the formation of hydroxyl radicalnd superoxide radical. The hydroxyl radical and superoxide rad-cal then attack the dye compound as shown in reactions (6) and7), respectively.

The kinetic expression for this mechanism is shown in the fol-owing form [19,20]:

dCdye

dt= kaCdye

(1 + kbCdye)(8)

When Eq. (8) is integrated and linearized, following equation (9)re obtained:

t

(C0 − Cdye)= kb

ka+ 1

ka

ln(C0/Cdye)(C0 − Cdye)

(9)

The values of ka and kb model constants for photocatalyticxperiment were obtained from (ln(C0/Cdye) / (C0− Cdye)) vs. (t/C0− Cdye)) drawing as presented in Fig. 6. The linearity of this curve,

Please cite this article in press as: D. Tekin, Photocatalytic degradationtrochemical method, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016

hich is the result of Eq. (9), indicates that the photocatalytic degra-ation kinetics of Orange G dye obeys the model. The values of ka

nd kb from the intercepts and slopes of the lines shown in Fig. 6ere calculated as 27.47 and 0.71, respectively.

[

Fig. 6. Plot of ln(C0/Cdye) / (C0 − Cdye)) vs. (t/(C0 − Cdye)).

Conclusions

The rate of formation of the TiO2 nanotubes by the sonoelectro-chemical method is found to be almost twice as fast, larger and moreregular as the electrochemical method. To test their photocatalyticactivities, TiO2 nanotube arrays prepared by sonoelectrochemicalmethod were tested with various TiO2 photocatalysts used in thedegradation of Orange G dye. It was discovered that using TiO2nanotubes prepared by sonoelectrochemical method, photoelec-trocatalytic degradation was much faster.

The photocatalytic degradation of Orange G dye using TiO2nanotubes prepared by sonoelectrochemical method was nearly10% more rapid on average than the photocatalytic degradationusing TiO2 nanotubes prepared by electrochemical method; nearly18% more rapid than the reaction of photocatalytic degradationrealized by Degussa P-25 TiO2 photocatalyst, a very good catalystand 48% quicker than the photocatalytic degradation using Riedel-de Haen TiO2.

The photocatalytic degradation rate of Orange G dye was foundto be −rdye = kaCdye/(1 + kbCdye). The kinetics model has shown goodagreement with the experimental findings.

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