research article wetting and photocatalytic properties of...

Research ArticleWetting and Photocatalytic Properties of TiO2Nanotube Arrays Prepared via Anodic Oxidation ofE-Beam Evaporated Ti Thin Films

Soon Wook Kim1 Hong Ki Kim1 Jong Won Yun1 Eui Jung Kim2 and Sung Hong Hahn1

1Department of Physics University of Ulsan Ulsan 680-749 Republic of Korea2Department of Chemical Engineering University of Ulsan Ulsan 680-749 Republic of Korea

Correspondence should be addressed to Eui Jung Kim ejkimulsanackr and Sung Hong Hahn shhahnulsanackr

Received 28 July 2015 Accepted 30 September 2015

Academic Editor Xuxu Wang

Copyright copy 2015 Soon Wook Kim et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

TiO2nanotube arrays (TNAs) are fabricated on quartz substrate by anodizing E-beam evaporated Ti films E-beam evaporated

Ti films are directly anodized at various anodizing voltages ranging from 20 to 45V and their morphological wetting andphotocatalytic properties are examined The photocatalytic activity of the prepared TNAs is evaluated by the photodecompositionof methylene blue under UV illumination The TNAs prepared at an anodizing voltage of 30V have a high roughness of 301 nmand a low water contact angle of 75∘ resulting in a high photocatalytic performance The surface roughness of the TNAs is foundto correlate inversely with the water contact angle High roughness (ie high surface area) which leads to high hydrophilicity isdesirable for effective photocatalytic activity

1 Introduction

TiO2has been actively studied as photocatalytic material due

to its large band-gap (anatase 32 eV) stable and nontoxicnature [1ndash4] To improve the photocatalytic property of TiO

2

several methods have been considered such as controllingband-gap energy through doping nonmetal materials [5ndash8]and enlarging the surface area by producing TiO

2in the form

of wire rod and particle [9 10] TiO2nanotube (TNA) is

known to have a high photo collection efficiency and largesurface areavolume ratio and it can be easily filled withliquid thus enabling intimate contact with dye and electrolyte[11ndash14]

Commonly TNAs can be formed via anodic oxidationof Ti foil by applying a potential to the anode connected tothe Ti foil in the electrolyte The diameter of the TNAs canbe controlled by changing the anodizing voltage [15] Thismethod of using the Ti foil requires a subsequent process likepeeling off the TNAs from the Ti foil However it is difficultto separate large-area TNAs without cracking Furthermorethis method requires an additional process to attach theseparated TNAs to substrate in order to fabricate the device

In this work we prepared Ti films using an E-beamevaporation method and investigated relationship betweenroughness hydrophilicity and photocatalytic performanceE-beam evaporated Ti thin films were anodized at variousapplied voltages and annealed at different temperatures Theprepared TNAs were characterized using various techniquessuch as field emission scanning electron microscopy (FE-SEM) water contact angle goniometry X-ray diffraction(XRD) spectroscopy and photocatalysisThe TNAs preparedat an applied voltage of 30V showed the best photocatalyticproperties for the photodegradation of methylene blue underblack-light irradiation

2 Material and Methods

The TNAs were prepared by anodizing Ti thin films on thequartz glass substrate using an E-beam evaporator (TemescalFC-2000 USA) with a Ti tablet (9999 pure) Prior todeposition the E-beam evaporator was evacuated to a basepressure of 4 times 10minus7 Torr The electron gun voltage and thecurrentwere 996 kV and 115mA respectivelyThedeposition

Hindawi Publishing CorporationInternational Journal of PhotoenergyVolume 2015 Article ID 320625 6 pageshttpdxdoiorg1011552015320625

2 International Journal of Photoenergy

E-beam evaporation

Substrate

Ti

Anodization of TiO2 O2 O2 O2

Substrate Substrate

TiO2

(a) (b) (c)

Figure 1 A schematic diagram for the preparation of TNAs (a) deposition of Ti thin film on quartz substrate using E-beam evaporationmethod (b) anodic oxidation of Ti thin film and (c) formation of TNAs

RutileAnatase

2120579 (deg)

Inte

nsity

(au

)

600 (∘C)

500 (∘C)

400 (∘C)

50 60403020

(a)

200nm

(b) 400∘C

200nm

(c) 500∘C

200nm

(d) 600∘C

Figure 2 XRD patterns and surface SEM images of TNAs annealed at 400∘C 500∘C and 600∘C

rate was 05 nms and the substrate was rotated at 20 rpm toobtain uniform Ti films The Ti thin films were anodized ina mixture of glycerol NH

4F (02 wt) and H

2O (25 vol)

as an electrolyte using a carbon counter electrode for 2 hto form the TNAs and then cooled from room temperatureto 10∘C using a WiseCircu fuzzy control system to enhancethe binding force of the TNAs The samples prepared at anapplied anodizing voltage of 20V 25V 30V 35V 40V and

45V are referred to as T20 T25 T30 T35 T40 and T45respectively To compare with the TNAs TiO

2thin film was

prepared by using an E-beam evaporation method After theE-beam evaporator was evacuated to a base pressure of 6times 10minus6 Torr TiO

2thin films were deposited at a working

oxygen gas pressure of 5 times 10minus5 Torr and a temperature of200∘C The electron gun voltage was 70 kV and the currentwas 200mA for the TiO

2film deposition The substrate was

International Journal of Photoenergy 3

200nm

(a) T20

200nm

(b) T25

200nm

(c) T30

200nm

(d) T35

200nm

(e) T40

200nm

(f) T45

Figure 3 Top-view SEM images of TNAs prepared at different anodizing voltages

rotated at 15 rpm to obtain uniform TiO2films The samples

were annealed at 500∘C for 2 hThe surfacemorphology of the TNAswas examined using

FESEM (JSM-6500F JEOL) The crystalline structure wasdetermined by XRD spectroscopy (PW3710 Philips) with K120572radiation (120582 = 15406 A) The surface hydrophilicity wasevaluated using contact angle goniometry To test their pho-tocatalytic activities the TNA thin films (20 times 20 cm2) wereimmersed in 10mL of 10minus5M methylene blue (C

16H18N3S-

Cl-3H2O) solution and irradiated with 4 surrounding 20 W

black-light (UVA) lamps (wavelength range 315ndash400 nm)The photocatalytic performance of the samples was evaluatedby measuring the absorbance of the methylene blue solutionat 665 nm using a UV-Vis spectrophotometer (HP8453)

3 Results and Discussion

Figure 1 shows the growth mechanism of TNAs preparedby anodic oxidation of E-beam evaporated Ti thin filmFirstly the Ti thin film was deposited by E-beam evaporation(Figure 1(a)) The titanium was anodized to TiO

2by reacting

withH2Oat the interfaceTheTiO

2thin film as a barrier layer

is formed on the surface of the Ti film Then it was etchedby Fminus ions developing holes in the TiO

2film (Figure 1(b))

Chemical reactions involved in the etching of the Ti film areas follows

Ti4+ + 2H2O 997888rarr TiO

2+ 4H+ + 4eminus

TiO2+ 6Fminus + 4H+ 997888rarr [TiF

6]2minus

+ 2H2O

(1)


20 120583m

20120583m

57nm

minus53nm

(a) T20

20 120583m20

120583m

010 120583m

minus015 120583m

(b) T25

20120583m

20 120583m

016 120583m

minus012 120583m

(c) T30

20 120583m20

120583m

031120583m

minus011 120583m

(d) T35

20 120583m

20120583m

013 120583m

minus009 120583m

(e) T40

20 120583m20

120583m

007 120583m

minus006 120583m

(f) T45

Figure 4 AFM images of TNAs prepared at different anodizing voltages

As the reaction proceeds the holes grow deeper into theTiO2film and the metallic regions at the pore leading to the

formation of the nanotube structure (Figure 1(c))Figure 2 shows the XRD patterns and SEM images of

TNAs anodized at 20V and annealed at 400 500 and 600∘CThe anatase and rutile phases of TiO

2were identified from

XRD patterns in Figure 2(a) After annealing at 400∘C onlyanatase phase peaks appeared at 253∘ 387∘ and 541∘ whichcorrespond to (101) (004) and (105) planes respectively(JCPDS number 01-089-4921) The intensity of the majoranatase peak at 253∘ was increased with increasing anneal-ing temperature from 400∘C to 500∘C indicating improved

crystallinity at 500∘C As the annealing temperature wasfurther increased to 600∘C rutile phase peaks at 275∘ and362∘ which are consistent with JCPDS number 01-078-2485appeared in the XRD patternThe SEM images of the samplesin Figures 2(b)ndash2(d) show the morphological properties ofTNAs with different annealing temperatures In Figures 2(b)-2(c) the TNAs were fairly uniform in shape and size At600∘C (Figure 2(d)) the TNAs were nonuniform in sizeand shape (Figures 2(c) and 2(d)) Due to the appearanceof rutile phase the surface coarsening took place and thegrain size increased These results may cause a decreasedphotocatalytic activity of the sample [16] In this study


T45T40T35T30T25T20TiO2 thin filmBare

Irradiation time (min)

CC

0

10

08

06

041801501209060300

(a)

Con

tact

angl

e (de

g)

20

15

10

5

Anodizing voltage (V)

120578(

)

Photocatalytic efficiencyContact angle

50

40

30

20

454035302520

(b)

Figure 5 (a) Variations of methylene blue concentration with UV irradiation time for the samples (b) Photocatalytic efficiency and watercontact angle of TNA film as a function of anodizing voltage

we employed the 500∘C-annealed TNAs with anatase phaseand good crystallinity to investigate their structural wettingand photocatalytic properties by changing the anodizingvoltage in synthesizing the TNAs

Figure 3 illustrates the SEM images of TNAs preparedat different anodizing voltages from 20 to 45V The TNAsamples prepared at an anodizing voltage of 20V 25V30V 35V 40V and 45V are referred to as T20 T25 T30T35 T40 and T45 respectively The average inner diameterof the T20 T25 T30 T35 T40 and T45 samples was231 nm 288 nm 424 nm 578 nm 684 nm and 622 nmrespectively The inner diameter increased with increasinganodizing voltage and the average wall thickness of the tubesalso increased from 90 to 218 nm as the anodizing voltagewas increased from 20V to 45V From the AFM images(Figure 4) the roughness of the samples was determinedas 154 nm 247 nm 301 nm 207 nm 205 nm and 191 nmfor the T20 T25 T30 T35 T40 and T45 respectivelyAs can be seen in Table 1 the 30V-anodized sample hadthe lowest contact angle of 75∘ and the 20V- and 45V-anodized ones had high values of 183∘ and 180∘ respectivelyThe water contact angle was found to correlate inverselywith the roughness According to the Wenzel theory a lowerwater contact angle indicates a higher hydrophilicity [17]Thismeans that a rough surface can bemore hydrophilic thana smooth surface

The photocatalytic performance of the samples wasevaluated by the photodecomposition of methylene blueunder black-light irradiation Figure 5(a) shows variations ofmethylene blue concentration with UV irradiation time forTiO2film and TNA films prepared at different anodizing

voltages The concentration of methylene blue was deter-mined from the UV-Vis absorbance at 644 nm The TNA

Table 1 Roughness wetting energy and water contact angle ofTNAs prepared at different anodizing voltages

20V 25V 30V 35V 40V 45VRoughness (nm) 154 247 301 207 205 191Wetting energy (mNm) 691 703 722 713 715 694Water contact angle (∘) 183 151 75 117 121 180

films displayed better photodecomposition ability than theTiO2film The 30V-anodized TNAs exhibited the best pho-

tocatalytic performance Figure 5(b) shows the photocatalyticefficiency andwater contact angle of TNAfilmas a function ofanodizing voltage The photocatalytic efficiency 120578 is definedas (1198620minus119862)119862

0times100() where119862

0is the initial concentration

of methylene blue and 119862 is its concentration after 180min ofUV illumination The 30V-anodized TNAs had the highestphotocatalytic decomposition efficiency of 439 while the45V-anodized TNAs had the lowest efficiency of 188Rough surface (large surface area) and high hydrophilicitymake water easily get into the inside of the tube Accordinglya high surface area and hydrophilicity would be desirablefor achieving an excellent photocatalytic activity towardorganic pollutant degradation (Figure 5(b)) The results ofFigure 5 demonstrate that a hydrophilic surface is favorableto enhanced photocatalytic performance

4 Conclusion

We have developed an effective method for preparing TNAsby anodizing E-beamevaporatedTi thin film and investigatedthe structural hydrophilic and photocatalytic properties ofthe TNAs prepared at different anodizing voltages ranging


from 20V to 45VThe TNAs had anatase phase after anneal-ing at 500∘C and rutile phase at 600∘C The inner tubediameter of the TNAs increased from 231 nm to 622 nmwith increasing anodizing voltage from 20V to 45V The30V-anodized TNAs had the highest roughness of 301 nmand the lowest contact angle of 75∘ resulting in the highestphotocatalytic activity The TNAs with a rough surfacewere more hydrophilic than those with a smooth surfacedemonstrating that high surface area and hydrophilicity arecrucial to photocatalytic activity

Conflict of Interests

All authors declare that there is no conflict of interestsregarding the publication of this paper

Authorsrsquo Contribution

SoonWookKim andHongKiKim contributed equally to thiswork

Acknowledgments

This research was financially supported by the Ministryof Education (MOE) and National Research Foundation ofKorea (NRF) through the Human Resource Training Projectfor Regional Innovation (2012H1B8A2026179) and the BasicScience Research Program (2012R1A1A4A01015466)

References

[1] X Pang W Chang C Chen H Ji W Ma and J Zhao ldquoDeter-mining the TiO

2-photocatalytic aryl-ring-opening mechanism

in aqueous solution using oxygen-18 labeled O2and H

2Ordquo

Journal of the American Chemical Society vol 136 no 24 pp8714ndash8721 2014

[2] P Wang J Wang T Ming et al ldquoDye-sensitization-inducedvisible-light reduction of graphene oxide for the enhancedTiO2photocatalytic performancerdquo ACS Applied Materials and

Interfaces vol 5 no 8 pp 2924ndash2929 2013[3] S Suzuki T Tsuneda and K Hirao ldquoA theoretical investigation

on photocatalytic oxidation on the TiO2surfacerdquo Journal of

Chemical Physics vol 136 no 2 Article ID 024706 2012[4] M J Munoz-Batista M N Gomez-Cerezo A Kubacka D

Tudela and M Fernandez-Garcıa ldquoRole of interface contact inCeO2-TiO2photocatalytic composite materialsrdquo ACS Catalysis

vol 4 no 1 pp 63ndash72 2014[5] S Yang and L Gao ldquoNew method to prepare nitrogen-doped

titanium dioxide and its photocatalytic activities irradiated byvisible lightrdquo Journal of the American Ceramic Society vol 87no 9 pp 1803ndash1805 2004

[6] X-X Xue W Ji Z Mao et al ldquoSERS study of co-doped TiO2

nanoparticlesrdquo Chemical Research in Chinese Universities vol29 no 4 pp 751ndash754 2013

[7] A W Tan B Pingguan-Murphy R Ahmad and S A AkbarldquoAdvances in fabrication of TiO

2nanofibernanowire arrays

toward the cellular response in biomedical implantations areviewrdquo Journal of Materials Science vol 48 no 24 pp 8337ndash8353 2013

[8] A Bumajdad M Madkour Y Abdel-Moneam and M El-Kemary ldquoNanostructured mesoporous AuTiO

2for photocat-

alytic degradation of a textile dye the effect of size similarity ofthe deposited Au with that of TiO

2poresrdquo Journal of Materials

Science vol 49 no 4 pp 1743ndash1754 2014[9] S S Mali C S Shim H K Park J Heo P S Patil and C

K Hong ldquoUltrathin atomic layer deposited TiO2for surface

passivation of hydrothermally grown 1D TiO2nanorod arrays

for efficient solid-state perovskite solar cellsrdquo Chemistry ofMaterials vol 27 no 5 pp 1541ndash1551 2015

[10] D V Bavykin L Passoni and F C Walsh ldquoHierarchical tube-in-tube structures prepared by electrophoretic deposition ofnanostructured titanates into a TiO

2nanotube arrayrdquo Chemical

Communications vol 49 no 62 pp 7007ndash7009 2013[11] J Y Kim K Zhu N R Neale and A J Frank ldquoTransparent

TiO2nanotube array photoelectrodes prepared via two-step

anodizationrdquo Nano Convergence vol 1 article 9 2014[12] K Lee RKirchgeorg andP Schmuki ldquoRole of transparent elec-

trodes for high efficiency TiO2nanotube based dye-sensitized

solar cellsrdquoThe Journal of Physical Chemistry C vol 118 no 30pp 16562ndash16566 2014

[13] H Cui W Zhao C Yang et al ldquoBlack TiO2nanotube arrays for

high-efficiency photoelectrochemical water-splittingrdquo Journalof Materials Chemistry A vol 2 no 23 pp 8612ndash8616 2014

[14] T H T Vu H T Au L T Tran et al ldquoSynthesis of titaniumdioxide nanotubes via one-step dynamic hydrothermal pro-cessrdquo Journal of Materials Science vol 49 no 16 pp 5617ndash56252014

[15] A Mohammadpour and K Shankar ldquoMagnetic field-assistedelectroless anodization TiO

2nanotube growth on discontinu-

ous patterned Ti filmsrdquo Journal of Materials Chemistry A vol2 no 34 pp 13810ndash13816 2014

[16] D A H Hanaor and C C Sorrell ldquoReview of the anatase torutile phase transformationrdquo Journal of Materials Science vol46 no 4 pp 855ndash874 2011

[17] D Tahk T-I Kim H Yoon M Choi K Shin and K Y SuhldquoFabrication of antireflection and antifogging polymer sheet bypartial photopolymerization and dry etchingrdquo Langmuir vol26 no 4 pp 2240ndash2243 2010

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Inorganic ChemistryInternational Journal of

Hindawi Publishing Corporation httpwwwhindawicom Volume 2014

International Journal ofPhotoenergy


Carbohydrate Chemistry

International Journal of


Journal of

Chemistry


Advances in

Physical Chemistry

Hindawi Publishing Corporationhttpwwwhindawicom

Analytical Methods in Chemistry

Journal of

Volume 2014

Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

SpectroscopyInternational Journal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Medicinal ChemistryInternational Journal of


Chromatography Research International


Applied ChemistryJournal of



Theoretical ChemistryJournal of


Journal of

Spectroscopy

Analytical ChemistryInternational Journal of


Journal of


Quantum Chemistry


Organic Chemistry International

ElectrochemistryInternational Journal of



CatalystsJournal of


E-beam evaporation

Substrate

Ti

Anodization of TiO2 O2 O2 O2

Substrate Substrate

TiO2

(a) (b) (c)

Figure 1 A schematic diagram for the preparation of TNAs (a) deposition of Ti thin film on quartz substrate using E-beam evaporationmethod (b) anodic oxidation of Ti thin film and (c) formation of TNAs

RutileAnatase

2120579 (deg)

Inte

nsity

(au

)

600 (∘C)

500 (∘C)

400 (∘C)

50 60403020

(a)

200nm

(b) 400∘C

200nm

(c) 500∘C

200nm

(d) 600∘C

Figure 2 XRD patterns and surface SEM images of TNAs annealed at 400∘C 500∘C and 600∘C

rate was 05 nms and the substrate was rotated at 20 rpm toobtain uniform Ti films The Ti thin films were anodized ina mixture of glycerol NH

4F (02 wt) and H

2O (25 vol)

as an electrolyte using a carbon counter electrode for 2 hto form the TNAs and then cooled from room temperatureto 10∘C using a WiseCircu fuzzy control system to enhancethe binding force of the TNAs The samples prepared at anapplied anodizing voltage of 20V 25V 30V 35V 40V and

45V are referred to as T20 T25 T30 T35 T40 and T45respectively To compare with the TNAs TiO

2thin film was

prepared by using an E-beam evaporation method After theE-beam evaporator was evacuated to a base pressure of 6times 10minus6 Torr TiO

2thin films were deposited at a working

oxygen gas pressure of 5 times 10minus5 Torr and a temperature of200∘C The electron gun voltage was 70 kV and the currentwas 200mA for the TiO

2film deposition The substrate was


200nm

(a) T20

200nm

(b) T25

200nm

(c) T30

200nm

(d) T35

200nm

(e) T40

200nm

(f) T45





16H18N3S-





2by reacting




2film (Figure 1(b))



2+ 4H+ + 4eminus


6]2minus

+ 2H2O

(1)


20 120583m

20120583m

57nm

minus53nm

(a) T20

20 120583m20

120583m

010 120583m

minus015 120583m

(b) T25

20120583m

20 120583m

016 120583m

minus012 120583m

(c) T30

20 120583m20

120583m

031120583m

minus011 120583m

(d) T35

20 120583m

20120583m

013 120583m

minus009 120583m

(e) T40

20 120583m20

120583m

007 120583m

minus006 120583m

(f) T45











CC

0

10

08

06

041801501209060300

(a)

Con

tact

angl

e (de

g)

20

15

10

5


120578(

)


50

40

30

20

454035302520

(b)













4 Conclusion








Acknowledgments


References




2Ordquo















2for photocat-































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


200nm

(a) T20

200nm

(b) T25

200nm

(c) T30

200nm

(d) T35

200nm

(e) T40

200nm

(f) T45





16H18N3S-





2by reacting




2film (Figure 1(b))



2+ 4H+ + 4eminus


6]2minus

+ 2H2O

(1)


20 120583m

20120583m

57nm

minus53nm

(a) T20

20 120583m20

120583m

010 120583m

minus015 120583m

(b) T25

20120583m

20 120583m

016 120583m

minus012 120583m

(c) T30

20 120583m20

120583m

031120583m

minus011 120583m

(d) T35

20 120583m

20120583m

013 120583m

minus009 120583m

(e) T40

20 120583m20

120583m

007 120583m

minus006 120583m

(f) T45











CC

0

10

08

06

041801501209060300

(a)

Con

tact

angl

e (de

g)

20

15

10

5


120578(

)


50

40

30

20

454035302520

(b)













4 Conclusion








Acknowledgments


References




2Ordquo















2for photocat-































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


20 120583m

20120583m

57nm

minus53nm

(a) T20

20 120583m20

120583m

010 120583m

minus015 120583m

(b) T25

20120583m

20 120583m

016 120583m

minus012 120583m

(c) T30

20 120583m20

120583m

031120583m

minus011 120583m

(d) T35

20 120583m

20120583m

013 120583m

minus009 120583m

(e) T40

20 120583m20

120583m

007 120583m

minus006 120583m

(f) T45











CC

0

10

08

06

041801501209060300

(a)

Con

tact

angl

e (de

g)

20

15

10

5


120578(

)


50

40

30

20

454035302520

(b)













4 Conclusion








Acknowledgments


References




2Ordquo















2for photocat-































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of




CC

0

10

08

06

041801501209060300

(a)

Con

tact

angl

e (de

g)

20

15

10

5


120578(

)


50

40

30

20

454035302520

(b)













4 Conclusion








Acknowledgments


References




2Ordquo















2for photocat-































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of







Acknowledgments


References




2Ordquo















2for photocat-































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of










Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

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