research article effect of low cobalt loading on tio ...research article effect of low cobalt...

Research ArticleEffect of Low Cobalt Loading on TiO2 Nanotube Arrays forWater-Splitting

Alfonso Pozio

ENEA CR Casaccia Via Anguillarese 301 S Maria di Galeria 00123 Rome Italy

Correspondence should be addressed to Alfonso Pozio alfonsopozioeneait

Received 28 July 2014 Revised 14 October 2014 Accepted 14 October 2014 Published 9 November 2014

Academic Editor Emmanuel Maisonhaute

Copyright copy 2014 Alfonso PozioThis is an open access article distributed under the Creative CommonsAttribution License whichpermits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

This work is intended to define a new possiblemethodology for the TiO2doping through the use of an electrochemical deposition of

cobalt directly on the titaniumnanotubes obtained by a previous galvanostatic anodization treatment in an ethylene glycol solutionThis method does not seem to cause any influence on the nanotube structure showing final products with news and interestingfeatures with respect to the unmodified sample Together with an unmodified photoconversion efficiency underUV light the cobaltdoped specimen reports an increase of the electrocatalytic efficiency for the oxygen evolution reaction (OER)

1 Introduction

The publication of the fundamental work of Gong et al [1] inwhich the authors created the basis for the development of anew synthesis model for the titania nanotubes based on theanodic oxidation of a titanium foil in fluoride based solutionsopened the way to a new methodology able to combine asimplicity of preparation of the material with a completecontrol of physical characteristics of the nanosystem [2ndash5] Besides its particular geometric shape is particularlyappropriate for application as photo-anode in the photo-electrolysis of water [6] For this reason many studies havebeen directed towards this field obtaining elevated values ofUV photoconversion efficiency for these nanosystems [7 8]In the meanwhile a large range of different applicationsfor this material has been discovered In fact for exampleit is reported that the electrical resistance of the titaniananotubes was highly sensitive to the chemisorbed hydrogenmolecules hydrogen sensing [9 10] creating a new route inthe hydrogen sensing research field [11 12] But many othersimilar examples of the wide versatility of the TiO

2nanotube

arrays are available in literature as the dye-sensitized solarcells [13ndash17] lithium batteries [18] and also in differentbiological and medical researches like the osteoblast growth[19ndash22] or drug elution [23ndash25] As regards the applicationof the TiO

2nanotube arrays as photo-electrodes for water

photoelectrolysis it is important to emphasize that althoughmany important results have been reached in this field thecommercialization of such nanosystem is still far because ofthe high band gap of titania which limits the light adsorptiononly to limited UV region [6] In addition also the titaniaelectrocatalytic activity for the OER is very low if comparedwith that obtained on conventional metallic electrodes (PtNi etc) These problems limit the use of this material due tothe low current density produced both in conventional modeor in photo-assistedmode For this reason a different strategybased on the use of cocatalyst able to enhance the activity forthe O2evolutionmaintaining the photo-activity constant can

be usefulThis type of electrode could be used indifferently ina photo-assisted or in a conventional electrolyser in absenceor in the presence of a UV light source In the past mostresearchers focused their attention on the doping of thesenanostructures mainly in order to shift the light absorptionto lower energy region [26ndash39] As alternative the use ofcocatalyst to enhance reactivity was first observed for thephotoconversion of H

2O to H

2and O

2using the Pt-TiO

2

system [40] The addition of metals to a semiconductor canchange the photocatalytic process by changing the semicon-ductor surface propertiesThemetal can enhance the yield ofa particular product or the rate of the photocatalytic reactionOn the other side the metal can be important also becauseof its own electrocatalytic activity In our case the functions

Hindawi Publishing CorporationInternational Journal of ElectrochemistryVolume 2014 Article ID 904128 7 pageshttpdxdoiorg1011552014904128

2 International Journal of Electrochemistry

of oxidation cocatalyst are applied to O2evolution from the

electrocatalytic water oxidation Representative water oxida-tion inorganic cocatalysts include ruthenium oxide cobaltoxide and iridium oxide [41] Deposition of IrO

119909 CoO

119909 and

RuO119909cocatalysts on n-semiconductors seems all to enhance

the activity for O2evolution and CoO

119909was found to be

the best one As example Surendranath et al described theself-assembly of a highly active cobalt-based oxygen evolvingcatalyst that forms as a thin film on inert electrodes whenaqueous solutions of Co2+ salts are electrolyzed in presenceof phosphate or borate [42] These authors evidenced thatthis catalyst can be interfaced with light absorbing andcharge separating materials to affect photoelectrochemicalwater-splitting In addition it can be formed in situ undermild conditions on a variety of conductive substrates and itexhibits high activity at high pH and room temperature

In the past Shrestha et al loaded self-organized TiO2

nanotubes grown by anodization of Ti-substrate in glycerol-water electrolyte containing fluoride with Ni oxide nanopar-ticles by a simple chemical bath precipitation techniqueUnfortunately the photocurrent inUV light region decreasedsignificantly which was possibly due to the blockage of UVlight by Ni oxide particles [43]

In this work we electrodeposited very small amountof cobalt on TiO

2nanotubes grown by anodization of Ti-

substrate in ethylene glycol electrolyte containing fluorideusing an in situ method already tested in the past on nickelanodes [44 45] We have found that cobalt-titania nanotubearrays own different features with respect to the startingstructure The photoconversion efficiency did not changethe energy gap and the levels of conduction and valenceband were equal In contrast the catalytic activity for theOER increased greatly at the same level of a conventionalmetallic electrodeWe retain the fact that thismethodology ifoptimized and deepened could open a new route to the besteffective doping of the titania nanotube arrays able to workboth in photo-assisted mode and conventional mode

2 Experiment

21 Materials and Photo-Electrode Preparation A small diskof commercially pure grade-3 titanium (Titania Italy) hasbeen used as substrate for the nanotube growth The cir-cular sample had a diameter of 15mm with a thickness of05mm and was arranged to show an active circular areaof 1 cm2 The unmodified sample (TiO

2Ti) was prepared

with the methodology previously developed in differentarticles [8 44] Briefly after 3min pickling in an HF (CarloErba)HNO

3(Carlo Erba) solution made by a volumetric

ratio of 1 3 and diluted in deionised water until 100mLthe titanium disks have been set in a three-electrode cellcontaining a 1M KOH solution (Carlo Erba) and subjectedto a prefixed and optimized density current (1mAcm2)generated by a potentiostatgalvanostat (Solartron 1286) for3min The counter-electrode was a platinum sheet whilethe reference was a standard calomel electrode (SCE) Theanodic growth of the nanotube arrays has been obtainedin a two-electrode cell with a platinum counter-electrode

Generator

Anode Cathode Multimeter

CellResistance

PCDigital recorder

Figure 1 Scheme of anodization system

using a glycol ethylene (Ashland) solution with 1wt H2O

and 02wt NH4F and applying 60V for 3 h by means of a

potentiostatgalvanostat PS251-2 (Aldrich) The current hasbeen measured with a Keithley 2000 multimeter and hasbeen acquired with a MadgeTech Volt101 digital recorderplaced in series with a calibrated resistance (300Ω) (Leedsand Northrup) (Figure 1)

After the anodization the sample was washed in glycolethylene and left overnight in a dry room Then after a heattreatment at 90∘C in vacuum for 3 hours the sample has beenplaced in a tubular furnace (Lenton) for 1 h at 580∘C with aslope of 1∘C minminus1 in air This step transforms amorphousTiO2nanotubes into a crystalline anatase phase which shows

a higher photo-sensibilityAfter the physical and electrochemical characterizations

the same TiO2Ti sample was loaded with cobalt in order to

obtain modified nanotube arrays Co-TiO2Ti Using Figure 1

system cobaltrsquos atoms have been electrodeposited on theTiO2Ti specimenThe electrodes were placed at a distance of

1 cm in a KOH 25M solution containing Co (NO3)2times 6H2O

(Carlo Erba) corresponding to 20 ppm of cobalt and TiO2Ti

electrode was anodized at 02mA for 18 hAfter the deposition the sample was washed in distilled

water and left overnight in a dry room Then the dopedsample has underwent again the physical and electrochemicalcharacterizations in order to evaluate the difference

22 Surface Analysis Thematerial structure was investigatedwith a Rigaku Miniflex diffractometer The patterns wereobtained using a Cu K120572 radiation from a rotating anodesource operating at 30 kV and 15mA The specimens werescanned at 002∘ sminus1 in the continuous scan mode over the 2120579range 20ndash120∘

The morphology of samples was investigated with ascanning electron microscopy JEOL JSM5510LV

23 Electrochemical Measurements The electrochemicalmeasurements were performed using a system similar tothe one described by Shankar et al [7] Briefly it is made ofa Pyrex cell with a 15 cm diameter quartz window wherethe light emitted by UV (Ultravitalux Osram) lamp is

International Journal of Electrochemistry 3

Counter-Pt Reference-SCE PTFE support

Lens

Electrolyte Working Quartz window

Lightsource

Figure 2 Experimental setup

placed at distance of 45 cm The source has a spectrumwith peak intensity in the UVA region at 360 400 nm andthe UV intensity which is measured on the sample by aphoto-radiometer HD23020 (Delta OHM) over the spectralrange 220ndash400 nm is 24Wmminus2 Along the optical path to thecell a lens was interposed in order to collimate the radiationof the light source The active surface of the sample (1 cm2)was immersed in a KOH 1M solution and placed at 05 cmfrom the quartz window (Figure 2) Photocurrents andldquoopen-circuit potentialrdquo (OCP) measurements were made inthe cell of Figure 2 via a potentiostat 1287 (Solartron) Themeasurements of photo-current were performed with a scanrate of 20mV sminus1 in the range of potential minus123 divide 170Vversus NHE in conditions of presence and absence of UVThe OCP measures were recorded in the presence andabsence of UV

3 Results and Discussion

31 Co Electrodeposited TiO2Nanotube Arrays The loading

of the TiO2Ti specimen is based on the evidence that in situ

activation of Ni anodes with cobalt ions in KOH electrolyteduring water electrolysis results in a marked increase ofelectrocatalytic activity [45] In a basic environment thecobalt is precipitated as hydroxide (pKps = 148) howeverat high pH as in the case in question there is the formation ofthe complex Co (OH)3minus where the constant of complexationis 120573 = 63 times 10minus5

Co2+ + 2OHminus larrrarr Co (OH)2

(1)

Co (OH)2+OHminus larrrarr Co (OH)minus

3(2)

From equation of the equilibrium constant of complexation(2) we obtain the concentration of the cobalt ion in theform of complex which is equal to about 15 ppm In previousworks bymeans of atomic absorption a cobalt concentrationof about 2 ppm was measured [45 46]

So we haveCo(OH)minus3ion release in theKOHsolution that

can be used as doping agent for the TiO2nanotubes cathode

Obviously during the long-time electrolysis the main anodereaction is the oxygen evolution (OER) but cobalt atoms canbe electrodeposited into the nanotubes surface in the form ofcobalt oxide according to the reaction

3Co (OH)minus3larrrarr Co

3O4+OHminus + 4H

2O + 2eminus (3)

0200400600800

100012001400

20 25 30 35 40 45 50 55 60 65 70 75 80

Sign

al in

tens

ity (a

u) A

R

A

RA

A

TiO2Ti

Co-TiO2Ti

2120579 (∘)

Figure 3 XRD pattern of nanotube TiO2Ti and Co-TiO

2Ti

obtained through electrosynthesis route (A) anatase (R) rutile and(998787) titanium The dashed lines indicate the Co

3O4and the fcc-Co

position [49]

Mag = 25000 kX

Figure 4 SEM top view of sample Co-TiO2Ti

The XRD pattern of nanotube TiO2Ti obtained through

electrosynthesis route is depicted in Figure 3 From com-parison with the Co-TiO

2Ti-substrate pattern (black trace)

no difference can be noted and all features typical of thetitanium metal and TiO

2are observed Particularly a few

peaks ascribable to titanium oxidematerial anatase and rutilecan be recognized [47 48] The results indicate that Codoping does not introduce changes in the TiO

2structure

This result is confirmed by the SEM analysis for theCo-TiO

2Ti reported in Figure 4 The structure of titanium

oxide nanotubes is clearly highlighted by the top view imageNo difference can be noted with typical TiO

2Ti nanotube

structure already evidenced in previous works [8 44]

32 Analysis of the Photoelectrochemical Performance Fig-ure 5 shows the open-circuit potential (119864ocv) for TiO2Ti andCo-TiO

2Ti with UV on and off When applying the UV

119864ocv decreases sharply and it reaches a steady state at a lowerpotential After switching off the light source 119864ocv increasesrapidly in the first seconds and then slowly until a new steadystate level In UV on condition the doped electrode shows an119864ocv of minus049V versus NHE while the undoped one minus054VThe low decrease of potential indicates a minimal effect onthe degree of band bending for the doped 119899-type metal oxidesemiconductor

Figure 6 shows the curves of photocurrent and photo-electrochemical performance of TiO

2Ti and Co-TiO

2Ti


Table 1 Photo-electrochemical characteristic of TiO2Ti and Co-TiO2Ti anodes

Sample 119864ocv 120578 119864fb asymp 119864CB 119864119861

11989415VV vs NHE V vs NHE (eV) V vs NHE mAcmminus2

TiO2Ti minus056 798 minus061 (minus405) minus101 040Co-TiO2Ti minus049 822 minus055 (minus399) minus095 313

UV on

UV off

0 05 10 15 20 25minus075

minus050

minus025

0

025

050

Time (h)

Eoc

v(V

) ver

sus S

HE

TiO2TiCo-TiO2Ti

Figure 5 Open-circuit potential at UV on and off for TiO2Ti (red)

and Co-TiO2Ti (blue)

0005101520253035404550

02468101214

minus10 minus05 00 05 10 15 20

Electrode potential (V) versus SHE

E998400998400 E998400

TiO2Ti Phot

ocon

vers

ion

effici

ency

()

Phot

ocur

rent

den

sity

(mA

cmminus2) 120578(TiO2Ti)

120578(

Co-TiO2Ti

Co-TiO2Ti)

Figure 6 UV photocurrent density and efficiency versus voltage forTiO2Ti (∘) and Co-TiO

2Ti in UV (times) and in dark (mdash)

The corresponding dark current for Co-TiO2Ti is also

represented for comparisonThe photoconversion efficiency 120578 which is the light

energy to chemical energy conversion efficiency is calculatedas [7]

120578 () = 119894ph[1198640

rev minus10038161003816100381610038161003816119864app10038161003816100381610038161003816]

1198690

times 100 (4)

where 119894ph is the photocurrent density (mA cmminus2) 119894ph1198640

revis the total power output 119894ph|119864app| is the electrical powerinput and 119869

0is the power density of incident light which

is 24mWcmminus2 1198640rev is the standard reversible potential of123VNHE The applied potential can be calculated

119864app = 119864meas minus 119864ocv (5)

where 119864meas is the electrode potential (versus NHE) of theworking electrode at which photocurrent was measured

00E + 00

25E minus 08

50E minus 08

75E minus 08

10E minus 07

minus1 minus075 minus05 minus025 0 025 05


TiO2Ti

i2 ph(A

2)

Co-TiO2Ti

Figure 7 Square photocurrent versus electrode potential forTiO2Ti (times) and Co-TiO

2Ti (∘)

under illumination and119864ocv is the electrode potential (versusNHE) of the same working electrode at open-circuit condi-tions under the same illumination and in the same electrolyte

After doping the efficiency of photoelectrochemicalremains virtually unchanged (sim8) as well as the photo-potential onset of 11986410158401015840 for the discharge of the water aroundminus075V versus NHE and the potential onset of 1198641015840 077V ver-susNHEHowever at the same time there is a complementaryincrease in the electrocatalytic activity highlighted by theincrease in current for higher potential to 1198641015840 that reaches680 at 15 V

The flat band potential 119864fb has been determined for bothelectrodes from the photocurrentmeasurements applying theequation [50 51]

1198942

ph = (212057611990312057601198901205721198690

119873119863

) (119864meas minus 119864fb) (6)

where119873119863denotes the donor density 120576

119903the relative dielectric

constant of the TiO2anodic film (55) [52] 120576

0the vacuum

permittivity (886 times 10minus14 F cmminus1) 119890 the charge of an electron(1602 times 10minus19 C) and 120572 the light absorption coefficient forthe material

The flat band potential (119864fb) calculated in Figure 7 as theintercept with the abscissa remains constant showing thatthe cobalt doping does not change the energy levels of theconduction band and valence from the sample nanotube

All the photoelectrochemical characteristics of TiO2Ti

and Co-TiO2Ti are summarized in Table 1

In considering light receptor electrodes for photoelectro-chemical cells an important criterion is the absolute positionof the valence (VB) and conduction bands (CB) In factfast electron transfer across the electrolyte semiconductorinterface occurs when the appropriate band overlaps theredox level of the electrolyte This is important since recom-bination of the photo-generated electron-hole pair competes


with the electron transfer and thus these relative ratesinfluence quantum efficiency The absolute position of thebands effectively controls the degree of band bending 119864

119861

since the surface Fermi level 119864119865 for the heavily doped 119899-type

metal oxide semiconductor essentially coincides with the CBlevel The value of 119864

119861is generally taken as the difference

between 119864119865and the ldquoFermi levelrdquo of the electrolyte which is

taken as the redox potential 119864redox for the reaction occurringat the photo-electrode [53]

119864119861= 119864119865minus 119864119865-Redox (7)

For water oxidation in alkaline media (6) and for TiO2Ti 119899-

type semiconductor photo-electrode 119864redox is the O2 | OHminus

potential

2h+ + 2OHminus 997888rarr 12O2(g) +H2O(l) (8)

119864∘= 0401V versus NHE (9)

The maximum open-circuit photo-potential value is 119864119861and

if it exceeds 123V then it is possible to run water-splittingwithout any other energy source other than the light Tothe extent that 119864

119861falls short of 123V (plus any required

overvoltage) one requires an additional energy input assistingthe effect an applied potential 119881app must be supplied toprovide the difference Since 119864

119865= 119864CB for the materials

of interest here we can conclude from this discussion thatthe CB position must be 123V above the O

2| OHminus

This analysis evidences that Co doping does not change theabsolute position of the valence and conduction band thusleaving the photo-catalytic properties unchanged

4 Conclusion

In this work we have reported a preliminary study for a newpossible doping route for TiO

2nanotubesThismethodology

yet to be improved and optimized shows as a first positivefeature the fact that the doping with Co into the TiO

2nan-

otubes does not produce relevant changes in the electronicstructure thusmaintaining the photochemical properties butit improves greatly the electrocatalytic properties of the elec-trode for the OERThe in situ activation of titania nanotubesin alkaline environment with cobalt ions is a very cheapmethod to produce electrodes able to work indifferently inphoto-assisted or conventional mode

Conflict of Interests

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

References

[1] D Gong C A Grimes O K Varghese et al ldquoTitaniumoxide nanotube arrays prepared by anodic oxidationrdquo Journalof Materials Research vol 16 no 12 pp 3331ndash3334 2001

[2] G K Mor O K Varghese M Paulose N Mukherjee andC A Grimes ldquoFabrication of tapered conical-shaped titania

nanotubesrdquo Journal of Materials Research vol 18 no 11 pp2588ndash2593 2003

[3] QCaiM PauloseOKVarghese andCAGrimes ldquoThe effectof electrolyte composition on the fabrication of self-organizedtitanium oxide nanotube arrays by anodic oxidationrdquo Journal ofMaterials Research vol 20 no 1 pp 230ndash236 2005

[4] A G Kontos A I Kontos D S Tsoukleris et al ldquoPhoto-induced effects on self-organized TiO

2nanotube arrays The

influence of surface morphologyrdquo Nanotechnology vol 20 no4 Article ID 045603 2009

[5] G K Mor K Shankar M Paulose O K Varghese and CA Grimes ldquoEnhanced photocleavage of water using titaniananotube arraysrdquo Nano Letters vol 5 no 1 pp 191ndash195 2005

[6] C AGrimes O K Varghese and S RanjanTheSolarHydrogenGeneration by Water Photoelectrolysis Springer New York NYUSA 2008

[7] K Shankar G K Mor H E Prakasam et al ldquoHighly-orderedTiO2nanotube arrays up to 220 120583m in length use in water

photoelectrolysis and dye-sensitized solar cellsrdquo Nanotechnol-ogy vol 18 no 6 Article ID 065707 2007

[8] F Mura A Masci M Pasquali and A Pozio ldquoStable TiO2

nanotube arrays with high UV photoconversion efficiencyrdquoElectrochimica Acta vol 55 no 7 pp 2246ndash2251 2010

[9] O K Varghese D Gong M Paulose K G Ong E C Dickeyand C A Grimes ldquoExtreme changes in the electrical resistanceof titania nanotubes with hydrogen exposurerdquo Advanced Mate-rials vol 15 no 7-8 pp 624ndash627 2003

[10] O K Varghese D Gong M Paulose K G Ong and C AGrimes ldquoHydrogen sensing using titania nanotubesrdquo Sensorsand Actuators B Chemical vol 93 no 1ndash3 pp 338ndash344 2003

[11] Q Chen D Xu Z Wu and Z Liu ldquoFree-standing TiO2

nanotube arrays made by anodic oxidation and ultrasonicsplittingrdquo Nanotechnology vol 19 no 36 Article ID 3657082008

[12] E Sennik Z Colak N Kilinc and Z Z Ozturk ldquoSynthesisof highly-ordered TiO

2nanotubes for a hydrogen sensorrdquo

International Journal of Hydrogen Energy vol 35 no 9 pp4420ndash4427 2010

[13] G K Mor K Shankar M Paulose O K Varghese and CA Grimes ldquoHigh efficiency double heterojunction polymerphotovoltaic cells using highly ordered TiO

2nanotube arraysrdquo

Applied Physics Letters vol 91 Article ID 152111 2007[14] G K Mor J Basham M Paulose et al ldquoHigh-efficiency forster

resonance energy transfer in solid-state dye sensitized solarcellsrdquo Nano Letters vol 10 no 7 pp 2387ndash2394 2010

[15] Y Wang H Yang Y Liu et al ldquoThe use of Ti meshes withself-organized TiO

2nanotubes as photoanodes of all-Ti dye-

sensitized solar cellsrdquo Progress in Photovoltaics Research andApplications vol 18 no 4 pp 285ndash290 2010

[16] Y Alivov and Z Y Fan ldquoDye-sensitized solar cells using TiO2

nanoparticles transformed from nanotube arraysrdquo Journal ofMaterials Science vol 45 no 11 pp 2902ndash2906 2010

[17] Z Liu and M Misra ldquoBifacial dye-sensitized solar cells basedon vertically oriented TiO

2nanotube arraysrdquo Nanotechnology

vol 21 no 12 Article ID 125703 2010[18] D Fang S-Q Liu R-Y Chen et al ldquoFabrication and charac-

terization of highly ordered porous anodic titania on titaniumsubstraterdquo Journal of Inorganic Materials vol 23 no 4 pp 647ndash651 2008

[19] S-H Oh R R Finones C Daraio L-H Chen and S JinldquoGrowth of nano-scale hydroxyapatite using chemically treated


titanium oxide nanotubesrdquo Biomaterials vol 26 no 24 pp4938ndash4943 2005

[20] S Oh and S Jin ldquoTitanium oxide nanotubes with controlledmorphology for enhanced bone growthrdquoMaterials Science andEngineering C vol 26 no 8 pp 1301ndash1306 2006

[21] H J Oh J H Lee Y J Kim S J Suh and C S Chi ldquoSurfacecharacteristics of porous anodic TiO

2layer for biomedical

applicationsrdquo Materials Chemistry and Physics vol 109 no 1pp 10ndash14 2008

[22] K Das A Bandyopadhyay and S Bose ldquoBiocompatibilityand in situ growth of TiO

2nanotubes on Ti using different

electrolyte chemistryrdquo Journal of the American Ceramic Societyvol 91 no 9 pp 2808ndash2814 2008

[23] K C Popat M Eltgroth T J LaTempa C A Grimes and TA Desai ldquoDecreased Staphylococcus epidermis adhesion andincreased osteoblast functionality on antibiotic-loaded titaniananotubesrdquo Biomaterials vol 28 no 32 pp 4880ndash4888 2007

[24] K C Popat M Eltgroth T J LaTempa C A Grimes and TA Desai ldquoTitania nanotubes a novel platform for drug-elutingcoatings for medical implantsrdquo Small vol 3 no 11 pp 1878ndash1881 2007

[25] L Peng A D Mendelsohn T J LaTempa S Yoriya C AGrimes and T A Desai ldquoLong-term small molecule andprotein elution from TiO

2nanotubesrdquo Nano Letters vol 9 no

5 pp 1932ndash1936 2009[26] YWang C Feng Z Jin J Zhang J Yang and S Zhang ldquoAnovel

N-doped TiO2with high visible light photocatalytic activityrdquo

Journal of Molecular Catalysis A Chemical vol 260 no 1-2 pp1ndash3 2006

[27] AGhicov JMMacakH Tsuchiya et al ldquoIon implantation andannealing for an efficient N-doping of TiO

2nanotubesrdquo Nano

Letters vol 6 no 5 pp 1080ndash1082 2006[28] A Ghicov J M Macak H Tsuchiya et al ldquoTiO

2nanotube

layers dose effects during nitrogen doping by ion implantationrdquoChemical Physics Letters vol 419 pp 426ndash429 2006

[29] K Shankar K C Tep G K Mor and C A Grimes ldquoAn elec-trochemical strategy to incorporate nitrogen in nanostructuredTiO2thin films modification of bandgap and photoelectro-

chemical propertiesrdquo Journal of Physics D Applied Physics vol39 no 11 pp 2361ndash2366 2006

[30] Q Li and J K Shang ldquoSelf-organized nitrogen and fluorine co-doped titanium oxide nanotube arrays with enhanced visiblelight photocatalytic performancerdquo Environmental Science andTechnology vol 43 no 23 pp 8923ndash8929 2009

[31] L Dong Y Ma Y Wang et al ldquoPreparation and characteriza-tion of nitrogen-doped titania nanotubesrdquoMaterials Letters vol63 no 18-19 pp 1598ndash1600 2009

[32] J Xu Y Ao M Chen and D Fu ldquoPhotoelectrochemicalproperty and photocatalytic activity of N-doped TiO

2nanotube

arraysrdquo Applied Surface Science vol 256 no 13 pp 4397ndash44012010

[33] J H Park S Kim and A J Bard ldquoNovel carbon-doped TiO2

nanotube arrays with high aspect ratios for efficient solar watersplittingrdquo Nano Letters vol 6 no 1 pp 24ndash28 2006

[34] K S RajaMMisra V KMahajan T Gandhi P Pillai and S KMohapatra ldquoPhoto-electrochemical hydrogen generation usingband-gap modified nanotubular titanium oxide in solar lightrdquoJournal of Power Sources vol 161 no 2 pp 1450ndash1457 2006

[35] G Wu T Nishikawa B Ohtani and A Chen ldquoSynthesis andcharacterization of carbon-doped TiO

2nanostructures with

enhanced visible light responserdquo Chemistry of Materials vol 19no 18 pp 4530ndash4537 2007

[36] S K Mohapatra M Misra V K Mahajan and K S RajaldquoDesign of a highly efficient photoelectrolytic cell for hydrogengeneration by water splitting Application of TiO

2minus119909C119909nan-

otubes as a photoanode and PtTiO2nanotubes as a cathoderdquo

The Journal of Physical Chemistry C vol 111 no 24 pp 8677ndash8685 2007

[37] R Hahn A Ghicov J Salonen V-P Lehto and P SchmukildquoCarbon doping of self-organized TiO

2nanotube layers by

thermal acetylene treatmentrdquo Nanotechnology vol 18 no 10Article ID 105604 2007

[38] N Lu H Zhao J Li X Quan and S Chen ldquoCharacterizationof boron-doped TiO

2nanotube arrays prepared by electro-

chemical method and its visible light activityrdquo Separation andPurification Technology vol 62 no 3 pp 668ndash673 2008

[39] Y Su S Han X Zhang X Chen and L Lei ldquoPreparation andvisible-light-driven photoelectrocatalytic properties of boron-doped TiO

2nanotubesrdquo Materials Chemistry and Physics vol

110 no 2-3 pp 239ndash246 2008[40] A L Linsebigler G Lu and J T Yates ldquoPhotocatalysis on

TiO2surfaces principles mechanisms and selected resultsrdquo

Chemical Reviews vol 95 no 3 pp 735ndash758 1995[41] J Yang D Wang H Han and C Li ldquoRoles of cocatalysts in

photocatalysis and photoelectrocatalysisrdquo Accounts of ChemicalResearch vol 46 no 8 pp 1900ndash1909 2013

[42] Y Surendranath MW Kanan and D G Nocera ldquoMechanisticstudies of the oxygen evolution reaction by a cobalt-phosphatecatalyst at neutral pHrdquo Journal of the American ChemicalSociety vol 132 no 46 pp 16501ndash16509 2010

[43] N K Shrestha M Yang Y-C Nah I Paramasivam and PSchmuki ldquoSelf-organized TiO

2nanotubes visible light acti-

vation by Ni oxide nanoparticle decorationrdquo ElectrochemistryCommunications vol 12 no 2 pp 254ndash257 2010

[44] F Mura A Masci M Pasquali and A Pozio ldquoEffect of agalvanostatic treatment on the preparation of highly orderedTiO2nanotubesrdquo Electrochimica Acta vol 54 no 14 pp 3794ndash

3798 2009[45] L Giorgi and A Pozio ldquoNickel anodes in situ activation with

cobalt in alkaline electrolyzersrdquo in Proceedings of the 1st WorldConference on Environmental Catalysis for a better World andLife vol 1 pp 647ndash650 Italian Chemical Society 1995

[46] S Scaccia ldquoDetermination of traces of Ni Co and Fe inLi2CO3K2CO3melts by flame atomic absorption spectrome-

tryrdquo Talanta vol 49 no 2 pp 467ndash472 1999[47] K Thamaphat P Limsuwan and B Ngotawornchai ldquoPhase

characterization of TiO2powder by XRD and TEMrdquo Natural

Science vol 42 pp 357ndash361 2008[48] R Nashed P Szymanski M A El-Sayed and N K Allam

ldquoSelf-Assembled nanostructured photoanodes with staggeredbandgap for efficient solar energy conversionrdquo ACS Nano vol8 no 5 pp 4915ndash4923 2014

[49] N Fischer E van Steen and M Claeys ldquoPreparation ofsupported nano-sized cobalt oxide and fcc cobalt crystallitesrdquoCatalysis Today vol 171 no 1 pp 174ndash179 2011

[50] H Gerischer ldquoThe role of semiconductor structure and sur-face properties in photoelectrochemical processesrdquo Journal ofElectroanalytical Chemistry and Interfacial Electrochemistry vol150 no 1 pp 553ndash569 1982

[51] M Radecka M Rekas A Trenczek-Zajac and K ZakrzewskaldquoImportance of the band gap energy and flat band potential forapplication of modified TiO

2photoanodes in water photolysisrdquo

Journal of Power Sources vol 181 no 1 pp 46ndash55 2008


[52] D Scharnweber R Beutner S Roszligler and H Worch ldquoElec-trochemical behavior of titanium-based materialsmdashare thererelations to biocompatibilityrdquo Journal of Materials ScienceMaterials in Medicine vol 13 no 12 pp 1215ndash1220 2002

[53] J M Bolts and M S Wrighton ldquoCorrelation of photocurrent-voltage curves with flat-band potential for stable photoelec-trodes for the photoelectrolysis of waterrdquoThe Journal of PhysicalChemistry vol 80 no 24 pp 2641ndash2645 1976

Submit your manuscripts athttpwwwhindawicom

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Inorganic ChemistryInternational Journal of

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Physical Chemistry

Hindawi Publishing Corporationhttpwwwhindawicom

Analytical Methods in Chemistry

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Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

SpectroscopyInternational Journal of


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Applied ChemistryJournal of



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Analytical ChemistryInternational Journal of


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Organic Chemistry International

ElectrochemistryInternational Journal of



CatalystsJournal of


of oxidation cocatalyst are applied to O2evolution from the

electrocatalytic water oxidation Representative water oxida-tion inorganic cocatalysts include ruthenium oxide cobaltoxide and iridium oxide [41] Deposition of IrO

119909 CoO

119909 and

RuO119909cocatalysts on n-semiconductors seems all to enhance

the activity for O2evolution and CoO

119909was found to be

the best one As example Surendranath et al described theself-assembly of a highly active cobalt-based oxygen evolvingcatalyst that forms as a thin film on inert electrodes whenaqueous solutions of Co2+ salts are electrolyzed in presenceof phosphate or borate [42] These authors evidenced thatthis catalyst can be interfaced with light absorbing andcharge separating materials to affect photoelectrochemicalwater-splitting In addition it can be formed in situ undermild conditions on a variety of conductive substrates and itexhibits high activity at high pH and room temperature

In the past Shrestha et al loaded self-organized TiO2

nanotubes grown by anodization of Ti-substrate in glycerol-water electrolyte containing fluoride with Ni oxide nanopar-ticles by a simple chemical bath precipitation techniqueUnfortunately the photocurrent inUV light region decreasedsignificantly which was possibly due to the blockage of UVlight by Ni oxide particles [43]

In this work we electrodeposited very small amountof cobalt on TiO

2nanotubes grown by anodization of Ti-

substrate in ethylene glycol electrolyte containing fluorideusing an in situ method already tested in the past on nickelanodes [44 45] We have found that cobalt-titania nanotubearrays own different features with respect to the startingstructure The photoconversion efficiency did not changethe energy gap and the levels of conduction and valenceband were equal In contrast the catalytic activity for theOER increased greatly at the same level of a conventionalmetallic electrodeWe retain the fact that thismethodology ifoptimized and deepened could open a new route to the besteffective doping of the titania nanotube arrays able to workboth in photo-assisted mode and conventional mode

2 Experiment

21 Materials and Photo-Electrode Preparation A small diskof commercially pure grade-3 titanium (Titania Italy) hasbeen used as substrate for the nanotube growth The cir-cular sample had a diameter of 15mm with a thickness of05mm and was arranged to show an active circular areaof 1 cm2 The unmodified sample (TiO

2Ti) was prepared

with the methodology previously developed in differentarticles [8 44] Briefly after 3min pickling in an HF (CarloErba)HNO

3(Carlo Erba) solution made by a volumetric

ratio of 1 3 and diluted in deionised water until 100mLthe titanium disks have been set in a three-electrode cellcontaining a 1M KOH solution (Carlo Erba) and subjectedto a prefixed and optimized density current (1mAcm2)generated by a potentiostatgalvanostat (Solartron 1286) for3min The counter-electrode was a platinum sheet whilethe reference was a standard calomel electrode (SCE) Theanodic growth of the nanotube arrays has been obtainedin a two-electrode cell with a platinum counter-electrode

Generator

Anode Cathode Multimeter

CellResistance

PCDigital recorder

Figure 1 Scheme of anodization system

using a glycol ethylene (Ashland) solution with 1wt H2O

and 02wt NH4F and applying 60V for 3 h by means of a

potentiostatgalvanostat PS251-2 (Aldrich) The current hasbeen measured with a Keithley 2000 multimeter and hasbeen acquired with a MadgeTech Volt101 digital recorderplaced in series with a calibrated resistance (300Ω) (Leedsand Northrup) (Figure 1)

After the anodization the sample was washed in glycolethylene and left overnight in a dry room Then after a heattreatment at 90∘C in vacuum for 3 hours the sample has beenplaced in a tubular furnace (Lenton) for 1 h at 580∘C with aslope of 1∘C minminus1 in air This step transforms amorphousTiO2nanotubes into a crystalline anatase phase which shows

a higher photo-sensibilityAfter the physical and electrochemical characterizations

the same TiO2Ti sample was loaded with cobalt in order to

obtain modified nanotube arrays Co-TiO2Ti Using Figure 1

system cobaltrsquos atoms have been electrodeposited on theTiO2Ti specimenThe electrodes were placed at a distance of

1 cm in a KOH 25M solution containing Co (NO3)2times 6H2O

(Carlo Erba) corresponding to 20 ppm of cobalt and TiO2Ti

electrode was anodized at 02mA for 18 hAfter the deposition the sample was washed in distilled

water and left overnight in a dry room Then the dopedsample has underwent again the physical and electrochemicalcharacterizations in order to evaluate the difference

22 Surface Analysis Thematerial structure was investigatedwith a Rigaku Miniflex diffractometer The patterns wereobtained using a Cu K120572 radiation from a rotating anodesource operating at 30 kV and 15mA The specimens werescanned at 002∘ sminus1 in the continuous scan mode over the 2120579range 20ndash120∘

The morphology of samples was investigated with ascanning electron microscopy JEOL JSM5510LV

23 Electrochemical Measurements The electrochemicalmeasurements were performed using a system similar tothe one described by Shankar et al [7] Briefly it is made ofa Pyrex cell with a 15 cm diameter quartz window wherethe light emitted by UV (Ultravitalux Osram) lamp is



Lens


Lightsource








(1)


3(2)






3O4+OHminus + 4H

2O + 2eminus (3)

0200400600800

100012001400

20 25 30 35 40 45 50 55 60 65 70 75 80

Sign

al in

tens

ity (a

u) A

R

A

RA

A

TiO2Ti

Co-TiO2Ti

2120579 (∘)


2Ti


3O4and the fcc-Co

position [49]

Mag = 25000 kX








2structure




2Ti nanotube






2Ti and Co-TiO

2Ti






UV on

UV off

0 05 10 15 20 25minus075

minus050

minus025

0

025

050

Time (h)

Eoc

v(V

) ver

sus S

HE

TiO2TiCo-TiO2Ti



0005101520253035404550

02468101214

minus10 minus05 00 05 10 15 20


E998400998400 E998400

TiO2Ti Phot

ocon

vers

ion

effici

ency

()

Phot

ocur

rent

den

sity

(mA


120578(

Co-TiO2Ti

Co-TiO2Ti)






120578 () = 119894ph[1198640

rev minus10038161003816100381610038161003816119864app10038161003816100381610038161003816]

1198690

times 100 (4)







00E + 00

25E minus 08

50E minus 08

75E minus 08

10E minus 07



TiO2Ti

i2 ph(A

2)

Co-TiO2Ti


2Ti (∘)




1198942

ph = (212057611990312057601198901205721198690

119873119863

) (119864meas minus 119864fb) (6)




0the vacuum








119861






119864119861= 119864119865minus 119864119865-Redox (7)



potential


119864∘= 0401V versus NHE (9)







2| OHminus


4 Conclusion




2nan-




References






















































2nanotube







2nanotube









2minus119909C119909nan-




2nanotube layers by








































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of



Lens


Lightsource








(1)


3(2)






3O4+OHminus + 4H

2O + 2eminus (3)

0200400600800

100012001400

20 25 30 35 40 45 50 55 60 65 70 75 80

Sign

al in

tens

ity (a

u) A

R

A

RA

A

TiO2Ti

Co-TiO2Ti

2120579 (∘)


2Ti


3O4and the fcc-Co

position [49]

Mag = 25000 kX








2structure




2Ti nanotube






2Ti and Co-TiO

2Ti






UV on

UV off

0 05 10 15 20 25minus075

minus050

minus025

0

025

050

Time (h)

Eoc

v(V

) ver

sus S

HE

TiO2TiCo-TiO2Ti



0005101520253035404550

02468101214

minus10 minus05 00 05 10 15 20


E998400998400 E998400

TiO2Ti Phot

ocon

vers

ion

effici

ency

()

Phot

ocur

rent

den

sity

(mA


120578(

Co-TiO2Ti

Co-TiO2Ti)






120578 () = 119894ph[1198640

rev minus10038161003816100381610038161003816119864app10038161003816100381610038161003816]

1198690

times 100 (4)







00E + 00

25E minus 08

50E minus 08

75E minus 08

10E minus 07



TiO2Ti

i2 ph(A

2)

Co-TiO2Ti


2Ti (∘)




1198942

ph = (212057611990312057601198901205721198690

119873119863

) (119864meas minus 119864fb) (6)




0the vacuum








119861






119864119861= 119864119865minus 119864119865-Redox (7)



potential


119864∘= 0401V versus NHE (9)







2| OHminus


4 Conclusion




2nan-




References






















































2nanotube







2nanotube









2minus119909C119909nan-




2nanotube layers by








































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of






UV on

UV off

0 05 10 15 20 25minus075

minus050

minus025

0

025

050

Time (h)

Eoc

v(V

) ver

sus S

HE

TiO2TiCo-TiO2Ti



0005101520253035404550

02468101214

minus10 minus05 00 05 10 15 20


E998400998400 E998400

TiO2Ti Phot

ocon

vers

ion

effici

ency

()

Phot

ocur

rent

den

sity

(mA


120578(

Co-TiO2Ti

Co-TiO2Ti)






120578 () = 119894ph[1198640

rev minus10038161003816100381610038161003816119864app10038161003816100381610038161003816]

1198690

times 100 (4)







00E + 00

25E minus 08

50E minus 08

75E minus 08

10E minus 07



TiO2Ti

i2 ph(A

2)

Co-TiO2Ti


2Ti (∘)




1198942

ph = (212057611990312057601198901205721198690

119873119863

) (119864meas minus 119864fb) (6)




0the vacuum








119861






119864119861= 119864119865minus 119864119865-Redox (7)



potential


119864∘= 0401V versus NHE (9)







2| OHminus


4 Conclusion




2nan-




References






















































2nanotube







2nanotube









2minus119909C119909nan-




2nanotube layers by








































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of



119861






119864119861= 119864119865minus 119864119865-Redox (7)



potential


119864∘= 0401V versus NHE (9)







2| OHminus


4 Conclusion




2nan-




References






















































2nanotube







2nanotube









2minus119909C119909nan-




2nanotube layers by








































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of




















2nanotube







2nanotube









2minus119909C119909nan-




2nanotube layers by








































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

research article effect of low cobalt loading on tio ...research article effect of low cobalt...

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