Photocatalytic Properties of TiO2 Nano Structures

Journal of The Electrochemical Society, 156 �9� K160-K165 �2009�K160

Photocatalytic Properties of TiO2 Nanostructures Fabricatedby Means of Glancing Angle Deposition and AnodizationYuriy Pihosh,a,*,z Ivan Turkevych,b,* Jinhua Ye,c Masahiro Goto,a

Akira Kasahara,a Michio Kondo,b and Masahiro Tosaa

aMaterials Reliability Center and cPhotocatalytic Materials Center, National Institute for MaterialsScience, Ibaraki 305-0047, JapanbResearch Center of Photovoltaics, National Institute of Advanced Industrial Science and Technology,Ibaraki 305-8568, Japan

Structural, optical, and photocatalytic properties of various TiO2 nanostructures prepared by glancing angle deposition �GLAD�and by electrochemical anodic oxidation of Ti have been studied. The TiO2 nanorods were prepared on unheated glass substratesby using reactive sputtering of Ti in the GLAD regime. TiO2 nanotubes and brush-type nanostructures were fabricated by anodicoxidation of flat Ti films and Ti nanorods prepared by GLAD, respectively. The optical studies revealed that the nanotubes andbrush-type nanostructures possess antireflection properties. The photocatalytic activity of TiO2 nanostructures was characterizedby following decomposition of isopropanol under visible and UV light irradiation and found to be significantly higher in nano-structured samples than in their flat counterparts. Also, TiO2 nanotubes and brush-type nanostructures showed superior photocata-lytic activity in comparison with nanorods due to a significantly higher specific surface area.© 2009 The Electrochemical Society. �DOI: 10.1149/1.3160622� All rights reserved.

Manuscript submitted April 18, 2009; revised manuscript received June 3, 2009. Published July 22, 2009. This was Paper 342presented at the Honolulu, Hawaii Meeting of the Society, October 12–17, 2008.

0013-4651/2009/156�9�/K160/6/$25.00 © The Electrochemical Society

In recent years, the serious increase in global environmentalpollution1,2 and sequestration of toxic substances into environmentalwaters and wetlands within the global carbon cycle3,4 created a greatdemand for stable and environmentally friendly materials, whichcan perform efficient photocatalytic decomposition of hazardoussubstances before their emission to the environment. The mostpromising and suitable material for this purpose is titanium oxide�TiO2�, which possesses a unique combination of optical and pho-tochemical properties. TiO2 has a high refractive index,5 excellenttransparency6 in visible �VIS� and near-IR region, and shows a highphotocatalytic performance under UV light irradiation, i.e., degrada-tion of organic pollutants in water7 and in gas phase,8,9 and disin-fection of environment contaminated with pathogenicmicro-organisms.10 In addition, TiO2 has been used as a white pig-ment for centuries, which is one of the main arguments that confirmits safety for the environment and human beings.

Many techniques have been developed for the preparation oftitanium dioxide films, such as pulsed laser deposition,11 reactiveevaporation,12 and chemical vapor deposition.13 Although the TiO2films usually have a low specific surface area and therefore weakerphotocatalytic activity, they are not subjected to serious practicalproblems typical for TiO2 powders, which are associated with syn-thesis, annealing, and immobilization. Thus, besides the optimiza-tion of the photocatalytic performance of TiO2- 14-16 and TiO2-basedcomposite materials,17 much attention has been paid recently to thefabrication of nanostructured films of TiO2 with enhanced specificsurface area.18-21

In this work we studied TiO2 nanostructures fabricated by usingtwo different techniques: The glancing angle deposition �GLAD�and the electrochemical anodic oxidation. In the GLAD regime, theangles measured between the substrate normal and the direction ofincident flux are typically bigger than 80°. Therefore atoms thathave already been deposited on the substrate create shadows behindthem and shield that area from other incident atoms. The shadowingeffect and limited adatom diffusion eventually produce a microstruc-ture of small isolated columns slanting toward the incident beam.

Another technique in focus is electrochemical anodic oxidationthat converts Ti to TiO2 with the formation of long oxide nanotubes�NTs�.22,23 We performed anodization of Ti films and nanorods

* Electrochemical Society Active Member.z E-mail: [email protected]

Downloaded 22 Jul 2009 to 144.213.253.16. Redistribution subject to E

�NRs� sputtered on glass substrate in normal and GLAD regimesand, respectively, obtained simple TiO2 NTs as well as more com-plex brush-type TiO2 nanostructures.24

The prepared TiO2 films and nanostructures were characterizedby X-ray diffraction �XRD�, scanning electron microscopy �SEM�,and UV spectroscopy. The photocatalytic properties of nanostruc-tured TiO2 films were evaluated by decomposition of gaseous iso-propanol �IPA� under VIS and UV light. The influence of the shapeof the films on their photocatalytic activity was investigated.

Experimental

The TiO2 films of various morphologies, such as flat film, NRs,NTs, and brush-type nanostructures �BTNs�, were prepared on whitecrown/potassium glass substrates �Matsunami Glass Industries Ltd.�.The samples with TiO2 flat film and NRs were prepared directly byreactive radio-frequency �rf� magnetron sputtering of Ti in the mix-ture of O2 and Ar on the substrates positioned in normal and GLADregimes �Fig. 1a�, respectively. The NTs and BTNs were preparedby anodization of Ti films �Fig. 2a� and Ti NRs �Fig. 2b� sputtered inpure Ar on the substrates positioned in normal and GLAD regime,respectively.

The sputtering of Ti in the Ar/O2 mixture and in pure Ar wasperformed at low pressure �0.1 Pa� and at long throw �85 mm� toreduce the scattered components of flux from magnetron. A shutterplaced between the magnetron and the special rotated substrateholder allowed presputtering of the materials for at least 5 min be-fore deposition. The purity of the Ti target was 99.9% �FuruchiChemical Corporation, diameter of 50 mm and thickness of 5 mm�.The distance measured between the substrate and the target centerswas 95 mm. The angle � between the substrate normal and theincident flux was 85° for all depositions in the GLAD regime andwas held constant in each experiment. The rotation of the substrateand the GLAD process was computer controlled. By moving thesubstrate in a controlled way during the deposition, we were able toaccurately control the shape of the columns. The influence of sub-strate rotation regimes during GLAD on the morphology of depos-ited columnar structures is described in detail elsewhere.25,26 Figure1b-d shows TiO2 NRs prepared by reactive magnetron sputtering inthe GLAD configuration by using our experimental setup. For ex-ample, three basic microstructures were obtained by changing �throughout the deposition: Slanted �Fig. 1b�, vertical �Fig. 1c�, andhelical �Fig. 1d� NRs. The increase in the � angle increased indiameter of the NRs.

The TiO NTs and BTNs were prepared by anodization of Ti
2
CS license or copyright; see http://www.ecsdl.org/terms_use.jsp

NTs and �b� BTNs.

2

K161Journal of The Electrochemical Society, 156 �9� K160-K165 �2009� K161


films and Ti NRs, respectively. The anodization was performed in0.3 wt % solution of NH4F in ethylene glycol in a two-electrodeconfiguration with Ti layer, as the anode, and platinum mesh, as thecounter electrode. The Ti films and NRs were anodized at roomtemperature at constant potential of 60 V for the flat films and 15 Vfor the NRs.

The flat film and NRs of Ti were fabricated by rf sputtering in Aratmosphere. The thickness of the flat Ti film was 1.5 �m. The TiNRs with an average diameter of 200 nm and a length of 1.5 �mwere fabricated in the GLAD regime at 85°. Before switching to theGLAD regime, the flat Ti film with the thickness of 300 nm waspresputtered onto the glass substrate, as shown in Fig. 2b. The an-odic oxidation is a self-organized nanoscale level process, whichconverts Ti into vertically aligned TiO2 NTs. During anodization theNTs grow perpendicular to the Ti surface exposed to the electrolyte.For Ti NRs, the conformal growth of small TiO2 NTs results in theformation of highly porous BTNs, which replace former Ti NRs.The as-grown amorphous TiO2 NTs and BTNs were converted tothe single-phase anatase TiO2 by annealing at 450°C in air.22

XRD �Rigaku RINT-2500� measurements were performed forstructural characterization. The parameters of a diffractometer �U= 40 kV, I = 300 mA� were the same for all samples. The grainsize DC was calculated from anatase �101� reflection by using theScherrer equation. The surface morphology of the nanostructuredfilms was observed by SEM �Hitachi S-4300�. Optical transmissionand reflection of TiO2 films were measured by a spectrophotometer�Hitachi U-4100, Japan� in the wavelength range from 250 to 800nm. The optical bandgap Eg was determined from the data on ab-sorption coefficient by using the Tauc expression for indirect inter-band transition.

The photocatalytic activity of prepared TiO2 films was studied byfollowing the decomposition of gaseous IPA under VIS �380–780nm� and UV �310–370 nm� light irradiations in a quartz reactor withthe working volume of 500 mL. Each sample was placed in thecenter of the reactor, and then the atmosphere inside of the reactorwas replaced by the IPA gas. The IPA gas with concentration of ca.200 ppm �for experiments with VIS light� and ca. 60 ppm �for ex-periments with UV light� was introduced into the reactor stored inthe dark until the concentration of IPA became balanced, i.e., thesystem reached an equilibrium state of adsorption. Then the reactorwas transferred to the special black box space, where it was irradi-ated by VIS and UV lights, respectively. The light intensity was31 �W/cm2 for VIS and 14.5 �W/cm2 for UV light, which wasdetermined by a spectroradiometer �USR-40D, Ushio Co., Japan�.The concentrations of IPA, acetone, and CO2 were measured byusing a gas chromatograph �GC-14B, Shimadzu Co., Japan�equipped with a flame-ionized detector and a methanizer. The gasreactor system and the measurement of photocatalytic activity aredescribed in detail elsewhere.27

Results and Discussion

Several nanostructured TiO2 films with different shapes andthicknesses were prepared on glass substrates by rf sputtering,GLAD, and electrochemical anodic oxidation �Table I�. It is wellknown that the substrate temperature and partial oxygen pressuredetermine parameters for the formation of TiO2 films with differentcrystal structures. We found optimized experimental conditions forthe preparation of crystalline anatase TiO2 by rf reactive sputtering

Thickness�nm� Phase

DC�Å�

Rrms�nm�

Eg�eV�

r mixture 500 Anatase 272 7.5 3.14in GLAD regime 1500 Anatase 388 10.7 3.1

anodization 1500 Anatase 366 33.7 2.95ime and anodization 1500 Anatase 363 24.1 2.86

Glass Glass

Ti Ti

Glass GlassTi Ti

GLADα= 85°α= 0°

Ti

Anodization

TiO2

TiO2

TiGlass

Ti TiO2

Sputtering

Glass

Ti

Glass

AnodizationSputtering

Brush-type nanostructures

Nanotubes(a)

(b)

TiO2

Glass

TiO2

Figure 2. Schematic representation of the fabrication process for TiO2 �a�

500nm

α = 65-85 °

α

φSubstrate

α

Incident beam

Adatomdiffusion

Shadow

Slanted rods

(b) (c) (d)

Vertical rods

(a)

φ rotationcontinuous 90o stepsfixed

Helices

500nm500nm

Figure 1. �Color online� �a� Schematic representation of the GLAD tech-nique and the shadowing effect. SEM images of �b� slanted, �c� vertical, and�d� helical TiO2 NRs prepared by reactive rf sputtering of Ti in the GLADregime by controlled � and � rotations of the substrate.

Table I. Fabrication methods and main properties of TiO2 samples.

Sample Fabrication methods

Flat TiO2 RF reactive sputtering in O2/ATiO2 nanorods RF reactive sputtering in O2/Ar mixtureTiO2 nanotubes RF sputtering in Ar �0.1 Pa� andTiO brush-type nanostructures RF sputtering in Ar �0.1 Pa� in GLAD reg


K162 Journal of The Electrochemical Society, 156 �9� K160-K165 �2009�K162

without heating the substrate.28 The optimal conditions for thepreparation of anatase TiO2 flat films and NRs are listed in Table II.

Figure 3 presents SEM images obtained from the studied TiO2samples. Although the flat film of TiO2 deposited onto a glass sub-strate by rf reactive magnetron sputtering exhibits a columnar struc-ture, there is no space between the columns. Figure 3a shows thedense-packed arrays of polygons on the surface of the film withboundaries between them. The TiO2 films prepared by GLAD �Fig.3b� consist of individual NRs with diameters of around 120–140 nmwhich are well separated from each other.

Titanium oxide NTs and BTNs are shown in Fig. 3c and d, re-spectively. It can be seen that NTs are monodisperse with the outerdiameter of around 150 nm, which correspond to the applied anod-ization potential of 60 V. The important feature of the Ti anodizationprocess is that TiO2 NTs grow perpendicular to the surface of Tiexposed to the electrolyte. Therefore, during anodization of Ti NRs,the TiO2 NTs grow on the walls of Ti NRs until all Ti is converted

Table II. The optimal conditions for the deposition of anatase TiO2.

Sample name Phase RF power �W�

TiO2 flat Anatase 350TiO2 nanorods Anatase 350

500nm

a

c

d

500nm

500nm500nm

500nm500nm

b

500nm500nm

Figure 3. Surface morphologies and cross sections of a TiO2 �a� flat filmprepared by rf reactive sputtering, �b� NRs prepared by rf reactive sputteringin the GLAD regime, �c� NTs prepared by anodization of a flat Ti film, and�d� BTNs prepared by anodization of Ti NRs.


to the oxide. The smaller anodization potential of 15 V used in thiscase promoted formation of small NTs with the diameter of around20 nm. As a result, the highly porous material that consists of brush-type TiO2 nanostructures was formed.

The diffraction patterns of the titanium oxide films are shown inFig. 4. All peaks �2� = 25.16, 37.82, 47.9, 53.9, and 54.9� corre-spond to the known diffraction maxima of the anatase phase. Theaverage crystallite size DC of TiO2 was calculated by Scherrer’sequation using the full width at half-maximum of the XRD peaks ofA�101�. The size of the anatase grains in the flat TiO2 film is smallerif compared with the nanostructured films prepared by GLAD orcombination of GLAD and anodization with average sizes of 27, 39,and 37 nm, respectively.

Different shapes of nanostructures in the studied samples lead todifferent optical transmittances and reflectances, as shown in Fig. 5aand b, respectively. In the VIS range the transmittance of the samplewith the flat film and with NRs is around 90%. An abrupt decreasein optical transmission in the region of 320–350 nm observed in allsamples was caused by interband electron transitions in TiO2. Themultiple reflection in the samples with TiO2 flat film and NRs re-sulted in the interference fringes in both transmission and reflectionspectra. For NTs and BTNs no interference fringes and almost noreflectance were observed. The transparency of the NT and BTNfilms exhibits a sharp decrease in the UV region as well, but theabsorption edge is a bit shifted to the longer wavelengths.

Calculations of optical absorption coefficient from the transmit-tance T and reflectance R helped to eliminate the effects of theinterference, though we used an approximate formula

��h�� = d−1 ln�1 − R

T� �1�

where d is the thickness of the film and ��h�� is the absorptioncoefficient at a photon energy h�. The optical bandgap Eg of the

Ptot �Pa� Ar �sccm� O2 �sccm�

2 8.4 2.70.1 6.0 8.0

20 30 40 50 60

A(114)

A(103)

A(211)

A(105)

A(200)

A(004)

A(101)

(d)

(c)

Intensity[a.u]

brush-type nanostructures

nanotubes

nanorods

flat film

(b)

2Θ [deg]

(a)

Figure 4. �Color online� XRD spectra for a TiO2 �a� flat film, �b� NRs, �c�NTs, and �d� BTNs.



TiO2 films can be derived from the data on the absorption coefficient��h�� by utilizing the Tauc expression for indirect allowed inter-band transitions

const � �h� − Eg�2 = ��h�� h� �2�The optical bandgaps of the films derived by extrapolation of

theoretical curves are listed in Table I. The reason for the redshift ofthe absorption edge for the NTs and BTNs in comparison with NRsand flat films is a little smaller bandgap observed in TiO2 nanostruc-tures prepared by anodization. The NTs and BTNs can contain im-purities such as F, C, and N, which comes from the electrolyteduring anodization and from air during annealing and are known todecrease the bandgap in TiO2.29

The photocatalytic activity of the TiO2 flat and nanostructuredfilms was characterized by decomposition of IPA under VIS and UVlights. Although the thickness of the flat film is smaller, it absorbs allthe UV light at the wavelengths below the absorption edge. Becausethe TiO2 film is compact, i.e., there are no pores, the increase in thefilm thickness results neither in better absorption nor in larger spe-

0

20

40

60

80

100300 400 500 600 700 800

Wavelength [nm]

Transmittance[%]

(a)

300 400 500 600 700 800

0

5

10

15

20(b)

Reflectance[%]

Wavelength [nm]

Flat filmNanorodsNanotubesBrush-type nanostructures

250 300 350 400

0

2

4

6

8

10

Figure 5. �Color online� �a� Transmittance and �b� reflectance spectra ofTiO2 flat film, NRs, NTs, and BTNs prepared by different techniques onglass substrates. The transmittance spectrum for the glass substrate is alsoshown.


cific surface. Also, the photocatalytic activity of compact TiO2 filmssaturates, when the film thickness is larger than a threshold value,typically around 150–300 nm.30

The mechanism for the photocatalytic oxidation of IPA by UV-illuminated TiO2 has already been described.31,32 The photocatalyticactivity of titanium dioxide is based on the formation of electron-hole pairs via photoexcitation

TiO2 + h� → TiO2�h+ + e−� �3�

The electrons and holes generated in TiO2 are highly active and ableto oxidize most organic compounds completely, i.e., all hydrocarbonintermediate products can be oxidized to carbon dioxide and water.The holes on the TiO2 interface can be captured by water moleculeswith the formation of highly active radicals

H2O + h+ → OH· + H+ �4�

At the same time free electrons generated in the conduction band byillumination can be trapped by adsorbed oxygen forming superoxideions O2

− and O• radicals

O2 + e−• → O2− �5�

O2− + h+ → 2O• �6�

The superoxide ions and oxide radicals have been suggested as be-ing capable of reacting directly with organic molecules adsorbed onthe TiO2 surface in the gaseous phase33,34 as well as in the aqueousphase.35 The OH· and O· radicals react with IPA abstracting a hy-drogen atom

�CH3�2CHOH + •OH → �CH3�2C•OH + H2O �7�

2�CH3�2CHOH + O• → 2�CH3�2C·OH + H2O �8�

The �CH3�2C•OH radicals are transformed to acetone

�CH3�2C•OH → �CH3�2CO + H+ + e− �9�

The acetone is oxidized further to CO2.36

Figures 6 and 7 show the time course of IPA, acetone, and CO2concentrations vs irradiation time in the presence of samples withdifferent TiO2 structures under VIS and UV light irradiations. Ac-etone and CO2 evolved from photocatalytic decomposition of IPAduring irradiation with both VIS and UV lights. We found that TiO2NTs and BTNs possess the best photocatalytic properties. The TiO2NTs are able to completely decompose IPA gas under UV irradia-tion. As can be seen from Fig. 6c, the concentration of acetone,which is the intermediate product of the IPA decomposition, reacheda maximum and then decreased with time, indicating that acetonewas further decomposed to CO2. The higher photocatalytic activityof TiO2 NTs and BTNs is attributed to their significantly higherspecific surface area and antireflection properties in comparison withTiO2 NRs and flat-film samples.

Conclusions

We studied various TiO2 nanostructures prepared by GLAD di-rectly and by anodic oxidation of Ti and characterized their struc-tural and optical properties by XRD and UV spectroscopy. The TiO2NRs were prepared on unheated glass substrates by using reactivesputtering of Ti in the GLAD regime. TiO2 NTs and BTNs werefabricated by anodic oxidation of flat Ti films and Ti NRs preparedby GLAD, respectively. The optical studies show that the nanostruc-tured films possess antireflection properties. The photocatalytic ac-tivity of TiO2 nanostructures was characterized by following decom-position of IPA under VIS and UV light irradiations. Thephotocatalytic activity of all nanostructured samples was signifi-cantly higher than that in their flat counterparts. Also, TiO2 NTs andBTNs showed higher photocatalytic activity in comparison withNRs deposited by GLAD due to a significantly higher specific sur-face area.


K164 Journal of The Electrochemical Society, 156 �9� K160-K165 �2009�K164

Acknowledgments

This study was supported by the Japan Society for the Promotionof Science �JSPS� and by the New Energy Development Organiza-tion �NEDO� of Japan.

National Institute for Materials Science assisted in meeting the publica-tion costs of this article.

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0 100 200 300 400 500 600 700

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Photocatalytic Properties of TiO2 Nano Structures