DOI: 10.1002/cphc.201100450

Transition from Anodic Titania Nanotubes to Nanowires:Arising from Nanotube Growth to Application in Dye-Sensitized Solar CellsLidong Sun,[a] Sam Zhang,*[a] Xiu Wang,[b] Xiao Wei Sun,[c] Duen Yang Ong,[b]

Xiaoyan Wang,[a] and Dongliang Zhao[d]

1. Introduction

The emergence of anodic aluminum oxide has triggered thedevelopment of a spectrum of nanostructured materials withhighly ordered porous structure as a template. CdS nanowirearrays,[1] carbon nanotubes,[2] and titanium dioxide (titania)nanotube arrays[3] are a few examples. In the past decade, vari-ous metal oxide nanoporous or nanotube structures were di-rectly produced using the same electrochemical anodizingmethod, in the absence of the templates. Examples include ti-tanium,[4] zirconium,[5] hafnium,[6] niobium,[7] tantalum,[8] andtungsten,[9] to name but a few. In particular, anodic titaniananotube arrays with highly oriented alignment have generat-ed worldwide interest, owing to their intriguing performancewhen applied in dye-sensitized solar cells (DSSCs),[10–12] gas sen-sors,[13, 14] water splitting,[15, 16] and so forth.

Recently, a few researchers reported anodic formation of ti-tania nanowires on top of the nanotubes in an organic electro-lyte.[17–20] Several methods have been employed to suppressformation of the nanowires, such as growing a compact rutilelayer,[21] producing a photoresist coating,[22] or polishing titani-um foils prior to anodization,[23] and to eliminate the nanowireswith an ultrasonication process after anodization.[24] The forma-tion of the nanowires has been interpreted using a bamboo-splitting model[17] based on alumina nanowires and nanotubesproduced by etching alumina templates.[25] Although there area number of common features in anodizing titanium and alu-minum,[26] the difference in key features is obvious: anodiza-tion of titanium gives rise to individual nanotubes while anodi-zation of aluminum results in an integrated nanoporous struc-ture. Thus, the growth mechanism may not be the same. Inthis study, a reasonable model is proposed based on theoreti-cal analyses and experimental justifications. Two types of be-havior are considered in this model : stretching under an elec-

tric field and splitting under tensile stress, which explain wellthe mechanism to remove the nanowires with ultrasonication.Interestingly, the nanotube–nanowire transition also takesplace when the tubes are applied in DSSCs. This is detrimentalin view of the long-term stability of the solar cells. Accordingto the transition model proposed, an effective strategy to sup-press nanowire formation is used to improve the cell perfor-mance.

2. Results and Discussion

2.1. Transition during Nanotube Growth

Figure 1 a shows the nanowires produced by electrochemicalanodization, beneath which the nanotubes are clearly visible.

Anodic formation of titania nanowires has been interpretedusing a bamboo-splitting model; however, a number of phe-nomena are difficult to explain with this model. Herein, transi-tion from nanotubes to nanowires is investigated by varyingthe anodizing conditions. The results indicate that the transi-tion requires a large number of hydrogen ions to reduce thepassivated area of tube walls, and therefore can be observedonly in an intermediate chemical dissolution environment. Ac-cordingly, a model in terms of stretching and splitting is pro-

posed to interpret the transition process. The model providesa basis to suppress the nanowires with surface treatmentsbefore anodization and to clear the nanowires with an ultra-sonication process after anodization. The nanotube–nanowiretransition also arises when the tubes are directly used in dye-sensitized solar cells. Treatment with titanium tetrachloride so-lution for about 10 h is found to be effective in suppressingthe nanowires, and thus improving the photovoltaic propertiesof the solar cells.

[a] L. Sun, Prof. Dr. S. Zhang, X. WangSchool of Mechanical and Aerospace EngineeringNanyang Technological University50 Nanyang Avenue, Singapore 639798 (Singapore)Fax: (+ 65) 67924062E-mail : MSYZhang@ntu.edu.sg

[b] X. Wang, D. Y. OngSchool of Materials Science and EngineeringNanyang Technological University50 Nanyang Avenue, Singapore 639798 (Singapore)

[c] Prof. Dr. X. W. SunSchool of Electrical and Electronic EngineeringNanyang Technological University50 Nanyang Avenue, Singapore 639798 (Singapore)

[d] Prof. Dr. D. ZhaoCentral Iron and Steel Research InstituteNo. 76 Xueyuan Nanlu, Beijing 100081 (P.R. China)

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cphc.201100450.

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Similar structures were previously reported elsewhere[17–21] andexplained using a bamboo-splitting model.[17] However, anumber of phenomena are difficult to explain with this model.In Figure 1 b, each single tube presents a springlike structurethat bends and elongates. Gen-erally only a single nanowire ini-tiates from each nanotube, as re-vealed in Figure 1 c. The tubesare already transformed intoquasi-nanosprings before forma-tion of the nanowires, as shownin Figure 1 d. To better under-stand the transition from nano-tubes to nanowires and hencecontrol the morphology of thenanostructures, the chemicalbonding is first examined withX-ray photoelectron spectrosco-py (XPS) measurements.

In Figure 2, the XPS spectra ofas-grown nanowires and nano-tubes are compared. The oxida-tion states of Ti element (Ti 2p3/2,binding energy 458.9 eV; Ti 2p1/2,binding energy 464.7 eV) in bothnanostructures are identical toTi4+ ,[27, 28] together with themajor peak of O 1s (bindingenergy 530.3 eV), thereby indi-cating that the pertinent bond-ing structure is TiO2. The bindingenergy of F 1s is 684.6 eV, in linewith the F� ions that physicallyadsorbed on the TiO2 surface.[29]

Obviously, both the O 1s peaksare asymmetric and can be de-convoluted into two peaks, asshown in Figure 2 d and e. Thesmaller peaks at the shoulders(binding energy 532.0 eV) are as-signed to surface species (i.e. Ti�OH and Ti�O�O�) as reportedelsewhere.[27, 30, 31] The corre-sponding content of these spe-

cies in the O 1s peak is �20 and�45 % for nanowires and nano-tubes, respectively. The atomicratio of Ti :O:F is about1:2.25:0.27 in nanowires and1:2.89:0.37 in nanotubes basedon the relevant peak areas dis-played in Figure 2 a–c. Accord-ingly, the number of F� ions persurface Ti atom is 2.7:1.8 fornanowires with respect to nano-tubes (i.e. 0.27/[1–2.25�(1–20 %) � (1/2)] = 2.7 for nanowires

and 0.37/[1–2.89�(1–45 %) � (1/2)] = 1.8 for nanotubes; here,the terms in the square brackets are the percentage of Ti ele-ment bonding with surface species only).

Figure 1. Top-view field-emission scanning electron microscopy (FESEM) images of as-grown anodic titania nano-tubes prepared at 25 V and 40 8C for 2 h: a) nanowires on top of nanotubes; b, c) local magnifications in (a);d) nanotubes underneath the nanowires.

Figure 2. Comparison of XPS spectra of as-grown nanowires and nanotubes prepared at 60 V for 8 h: a) Ti 2p corelevel, b) O 1s core level, c) F 1s core level ; d, e) respective curve fitting results of the O 1s peaks.

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Transition from Anodic Titania Nanotubes to Nanowires

As such, the following scenario is figured out. Initially a largenumber of anions (mainly F� and OH� ions) are adsorbed onthe nanotube surface under the electric field. Considering thechemical dissolution process, that is, TiO2 + 4H+ + 6F�![TiF6]2�+ 2H2O,[32, 33] only the regions with F� ions are activatedand able to be etched, while the regions with OH� ions arepassivated. According to the calculation above, the activatedregion is only 45 % (1.8/4 = 45 %) for the nanotubes whereas itis �68 % (2.7/4�68 %) for the nanowires. To trigger the transi-tion, more H+ ions are required to react with OH� ions andpave the way for F� adsorption. Therefore, the nanotube–nanowire transition can be observed only under a specialchemical dissolution environment, as will be revealed below. Inaddition, this result also elucidates that high-aspect-ratio nano-tubes are obtainable in an organic electrolyte because of thepassivated tube walls.

Herein, the environment for nanotube growth is presentedwith three regions of high, low, and intermediate chemical dis-solution rate. At a high chemical dissolution rate, for example,in an aqueous[34, 35] or acidic electrolyte,[36, 37] no nanowires formdue to fast dissolution. Under this condition, the springlikestructure can still be observed near the tube mouths (seeimages in refs. [34, 37] and Figure S1 of the Supporting Infor-mation). At a low chemical dissolution rate, for instance, in anorganic electrolyte at room temperature for short anodizingdurations,[38] the tube arrays are covered by a thin layer ofporous oxide, as shown in Figure 3 a; hence no nanowires

form either. On the contrary, at an intermediate chemical disso-lution rate, the nanowires initiate and evolve easily. To producethe special chemical dissolution environment, a series of ex-periments was carried out under the same conditions as in Fig-ure 3 a but at an elevated temperature. Figure 3 b shows thatthe pore size of the oxide layer is larger at 40 8C than that atroom temperature owing to increased dissolution, and thenanotubes beneath the oxide layer are exposed in regions. Thespringlike structure also appears at the tube mouths, as illus-trated in the inset in Figure 3 b. Prolonged anodization at 40 8Cresults in the formation of the wires as displayed in Figure 1. Incontrast, no wires are fashioned at room temperature for thesame duration (see Figure S2 of the Supporting Information).This indicates that chemical dissolution plays an essential rolein the transition from tubes to wires, thus interpreting theeffect of water content[17, 18, 20] on formation of the nanowires.

As the water content increases in an organic electrolyte, thechemical dissolution rate also increases and hence facilitatesthe development of the nanowires. Therefore, under the sameconditions but with a relatively higher water content, nano-wires can initiate at a lower potential,[17] in a shorter time,[18] orwith a lower threshold content when HF is used instead ofwater.[20] All these results demonstrate that the transition fromtubes to wires is visible only under an intermediate chemicaldissolution environment, consistent with the XPS analysisabove.

In the growth of anodic titania nanotubes, the chemical dis-solution rate increases with anodizing duration (Figure 3 a andFigure S2 of the Supporting Information) due to localizedacidification.[33] Therefore, on the basis of the above discussion,it is believed that the nanotubes will evolve into nanowireseventually for a long enough duration even in an organic elec-trolyte at room temperature. This has been revealed in Fig-ure 4 a, where the nanowires grown for 14 h are displayed. Asimilar transition can also be obtained at a higher potential fora relatively shorter duration (e.g. at 60 V and room temperaturefor 8 h, as shown in Figure S3 of the Supporting Information).

The existence of a layer of nanowires on top of a nanotubearray blocks the way the nanotubes are meant to function, forexample, in DSSCs to facilitate dye loading and electrolyte infil-tration;[39] thus, this layer of wires is undesirable and should beremoved in some applications. However, once formed, thenanowires cannot self-dissolve completely during anodization.Ultrasonic cleaning is effective in clearing away the wires, asshown in Figure 4 b. It is noteworthy that the nanowires grownfor “short” durations (e.g. at 25 V and 40 8C for 2, 4, and 6 h)are unable to be eliminated even under the ultrasonic clean-ing, as presented in Figure 5 a. In the first 6 h, the length ofthe nanotubes together with the nanowires increases linearly.The corresponding growth rate is �1.39 mm h�1, which dropsafter 6 h since the lengthened nanowires keep curving and col-lapse on the tube surface (see Figures 5 b and S4 in the Sup-porting Information). In Figure 5 a, two values of tube lengthare given at 8 h, one for nanotubes and nanowires together(Figure 5 b) and one for nanotubes only (Figure 5 c). Thereafter,clear nanotubes are obtainable via ultrasonic cleaning. The re-sultant net tube growth rate is �0.37 mm h�1. The reason why

Figure 3. Top-view FESEM images of as-grown anodic titania nanotubes pre-pared at 25 V for 30 min: a) at room temperature (�20 8C) and b) at 40 8C.The inset in (b) shows a local region.

Figure 4. Surface morphology of as-grown anodic titania nanotubes pre-pared at 25 V and room temperature (�20 8C) for 14 h: a) without ultrasoniccleaning; b) with ultrasonic cleaning for 10 min after anodization. The insetin (b) is a local magnification showing the quasi-nanosprings at the tubemouths.

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a clear tube surface can be obtained by ultrasonication onlyafter a long anodizing duration will be discussed later.

2.2. Nanospring Model

Based on the above results, a model to illustrate the transitionprocess is proposed in Figure 6. Initially the nanotubes growsteadily underneath a compact oxide layer formed at the verybeginning of anodization,[40] as illustrated in Figure 6 a. At thesame time, periodic rings form on the tube walls (see Figure S5in the Supporting Information),[35, 41] which play an importantpart in the subsequent transition process. As the anodization isprolonged, thinning of the tube walls takes place especiallynear the tube mouths. This thinning process is unsymmetricalover the tube walls due to the uneven thickness, and proceedsslowly at relatively thicker parts (e.g. at the rings), as depictedin Figure 6 b. At the tube mouth, a springlike structure (see theinset of Figure 3 b) arises under significant etching after the ini-tial oxide layer is etched away (or detached from the nano-tubes). The above course is widely recognized as a pure chemi-cal dissolution process at the tube mouths. However, it cannotbe achieved without the assistance of an electric field, as theTi�O bonds are polarized and weakened along the tube axialdirection under the field.[42] This in turn makes the bonds easily

broken in the presence of H+ and F� ions. Continuous dissolu-tion results in progressive thinning of the tube walls and even-tually the formation of quasi-nanosprings for long durations,as shown in Figure 1 d and the inset of Figure 4 b. This couldbe the reason why nanowires grown for long durations can becleared away with ultrasonic cleaning, since the cracks on thequasi-nanosprings propagate easily along the rings under theultrasonication process.

After formation of the quasi-nanosprings in Figure 6 b, nano-wires start to be fashioned in two ways: stretching and split-ting. In the stretching mode, cracks at the tube mouths (seethe inset of Figure 3 b) propagate in a direction perpendicularto the electric field or with an angle, because of the polarizedand weakened Ti�O bonds along the field, as illustrated in Fig-ure 6 c. Meanwhile, the broken tubes will stretch out with themotion of ionic species in the electrolyte, thereby forming theprimal nanowires, as shown in Figure 6 d. To prove this mode,a series of experiments was designed. First of all, the essentialrole of the electric field was studied with reference to nano-wire evolution. After formation of the quasi-nanosprings andinitiation of the cracks at tube mouths (Figure 3 b), the nano-tubes were etched under the same conditions in the absenceof the field (i.e. leaving the samples in the electrolyte afterswitching off the power supply) for 1.5 h. The results revealthat no nanowires are formed without the assistance of thefield, as shown in Figure S6a of the Supporting Information, incontrast to the results under the field (Figure 1). The same re-sults are obtained when etching the tubes for even 7.5 h (seeFigures S4 and S6b in the Supporting Information). These dem-onstrate that an electric field is required to trigger the transi-

Figure 5. a) Length of nanotubes formed at 25 V and 40 8C for differentanodizing durations. b, c) FESEM cross-sectional images at 8 h: b) without ul-trasonic cleaning and c) with ultrasonic cleaning for 10 min after anodiza-tion. NT and NW denote nanotubes and nanowires, respectively.

Figure 6. Schematic diagram of the transition from nanotubes to nanowires:a) steady growth of a nanotube underneath an initial oxide layer ; b) thin-ning of the tube mouth and formation of the quasi-nanosprings under anintermediate chemical dissolution environment. Stretching mode: c) propa-gation of a crack along the quasi-nanosprings under an electric field;d) stretching of a nanowire under the field. Splitting mode: e) bending ofthe quasi-nanosprings; the sketch on the right-hand side is a correspondingside view of the rectangular region and the arrow pairs on the left and rightdenote compressive and tensile stresses, respectively; f) initiation of a singlenanowire due to splitting.

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Transition from Anodic Titania Nanotubes to Nanowires

tion from the tubes to wires. Secondly, after formation of thestructure in Figure 3 b, the anodization was continued in ethyl-ene glycol with 2 vol % H2O only (i.e. removing the ionic spe-cies of NH4F) at 25 V and 40 8C for 1.5 or 7.5 h. The resultsshow that no nanowires are fashioned in the absence of NH4Feither, as displayed in Figure S7 of the Supporting Information.

The above experiments disclose two prerequisites for thetransition: an electric field to polarize the Ti�O bonds and anionic species to attack the weakened bonds. Accordingly, thenanowires stretching out of the tube surface can be observedwhen adopting a relatively weaker electrolyte. As such, afterformation of the initial structure in Figure 3 b, the anodizationwas continued further in an electrolyte of ethylene glycol con-taining 0.09 m NaCl and 2 vol % H2O for 1.5 h, considering thepossibility of growing nanotubes using a chlorine-based solu-tion.[43] Figure 7 a indicates that a number of short nanowires

appear on the tube surface after anodization. Under the cur-rent anodizing conditions, no substance can be deposited onthe surface. Therefore, these wires originate from the tubes un-derneath the porous oxide layer, as can be confirmed fromone end of each wire anchoring to the surface (Figure 7 b–d).Obviously, the wires protruding from the surface can only beachieved via an approach such as that proposed in Figure 6 cand d. The stretching mode can be further consolidated withthe zigzag edges of the wires, which emanate from crack prop-agation across the rings. Also, the wires keep curving whenlong enough (see Figure S4 in the Supporting Information).

In the other manner of splitting, the resultant quasi-nano-springs (as in Figure 6 b) can bend, elongate, and swing in theelectrolyte, especially for the ones with enough room. Whentwo neighboring nanotubes bend toward opposite directions,cracks initiate, coalesce, and propagate, as shown in Figure 1 b.In principle, the bending gives rise to a tensile stress for one

side of a quasi-nanospring but a compressive stress for theother side, as illustrated in Figure 6 e. This will accelerate thechemical dissolution of the regions under tensile stresses, es-pecially with the assistance of an electric field. As a conse-quence, the regions finally break up and thus form the primalnanowire structure, as illustrated in Figure 6 f and revealed inFigure 1 c. The splitting mode requires a certain amount ofroom for bending and hence takes place at the cracking re-gions only.

The model proposed herein reveals that formation of thequasi-nanosprings plays an essential role in nanotube–nano-wire transition. Accordingly, it provides a basis for the methodspresented in refs. [21–23] to suppress the transition with sur-face treatments before anodization (i.e. deterring the formationof the quasi-nanosprings) and that in ref. [24] to clear thenanowires with ultrasonication after anodization (i.e. enablingthe cracks on the quasi-nanosprings to propagate). In addition,it also suggests an approach to increase the tube stabilityduring application in DSSCs, as will be shown below.

2.3. Transition during Application in Dye-Sensitized SolarCells

The formation of nanowires also arises during application ofnanotubes in DSSCs. In Figure 8 a, the original nanotubes (i.e.without TiCl4 treatment or at 0 h) exhibit a regular tube surfacewith some cracks resulting from the formation of initial nano-wires (Figures 1 and 4). After assembly into DSSCs, the nano-tubes bundle together especially along the cracks, as shown inFigure 8 b at 0 h. Detailed observation reveals that nanowiresappear once again at the bundling regions, as displayed in Fig-ure 8 c, where the quasi-nanosprings can also be seen. Appa-rently this transition process is detrimental in view of the long-term stability of the solar cells. Considering the similar transi-tion that occurs during nanotube growth, thinning of tubewalls is a crucial factor. Accordingly, a counter strategy to thick-en the tube walls is proposed to suppress the formation ofnanowires. One effective method is TiCl4 treatment, which isfrequently used to promote the photovoltaic performance ofDSSCs by forming an additional highly pure TiO2 layer on theoriginal nanoparticles,[44] thereby improving electron percola-tion[45] and increasing the surface area.[46] A TiO2 layer 1 nm inthickness on the surface of nanoparticles was obtained byGr�tzel’s group using 40 mm aqueous TiCl4 solution at 70 8Cfor 30 min.[47] Under the same conditions but adopting a 0.2 m

solution, TiO2 layers of thickness �13 nm were formed onboth the inner and outer walls of the anodic nanotubes bySchmuki’s group.[48, 49] As such, the concentration of 40 mm

was selected in light of the closely packed configuration of thenanotubes used in this study, otherwise a higher concentrationmay result in clogging of the intertube spacing and even thetube tops as in ref. [48] .

Figure 8 a shows that the surface morphology of the nano-tubes changes little with the duration of the TiCl4 treatment,whereas the pertinent tube stability has been improved signifi-cantly as displayed in Figure 8 b. In comparison to the pristine(Figure 8 b at 0 h) and the original (Figure 8 a) nanotubes, a

Figure 7. Surface morphology of the nanotubes anodized at 25 V and 40 8Cin ethylene glycol with 0.3 wt % NH4F (or 0.09 m) and 2 vol % H2O for 30 min,and then in ethylene glycol with 0.09 m NaCl and 2 vol % H2O for 1.5 h. Thefour images were obtained at different locations and with different magnifi-cations.

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treatment duration of 10 h and above gives rise to a relativelymore stable tube structure. The photovoltaic performance ofthe relevant solar cells is summarized in Figure 9 a (see Fig-ure S8 in the Supporting Information for typical J–V curves).The open-circuit voltage (Voc) of cells with nanotubes underTiCl4 treatment is higher than that without the treatment. Thisis attributed to the improved electron percolation in the nano-tubes, especially in the quasi-nanosprings where the connec-tions between rings may act as the electron-trapping sites. Noobvious changes in Voc can be observed as the duration is pro-longed.

The short-circuit current density (Jsc) is drastically enhancedupon treatment (cf. 0 and 0.5 h) and gradually increases there-after (i.e. from 0.5 to 10 h), whereas it drops dramatically be-tween 10 and 15 h but increases steadily after 15 h. This trendis in good agreement with the dye desorption measurementspresented in Figure 9 b. Clearly there is also an abrupt decreasein dye loading amount that is adsorbed on the nanotubes inthe range of 10–15 h. In general, the dye loading amount isclosely related to the surface area of a photoanode. According-ly, the surface area of the nanotubes increases with the treat-ment duration, as more tiny particles form on both the innerand outer tube walls.[46, 48] However, the surface area will dimin-ish considerably when the intertube spacing (or at least thatnear the tube tops, considering the diffusion process in the so-lution for the hydrolysis) is fully filled by the nanoparticles,

since the outer surface of thenanotubes is hardly used anymore. This is corroborated bythe morphology changes shownin Figure 9 c–f. After 10 h, the in-tertube spacing is too small forthe dye molecules to access theouter tube walls, particularlywhen taking into account thedye loading process which nor-mally occurs by diffusion fromthe tube tops to the bottoms. Itis believed that the progressiveincrement in Jsc after 15 h origi-nates from the increased innertube surface area.

Figure 9 a also reveals that thefill factor (FF) of the solar cells isimproved by the TiCl4 treatmentbut degrades gradually after10 h. This is in line with the var-iation in series resistance (Fig-ure 9 b). Eventually the most effi-cient cell is obtained with thenanotubes under TiCl4 treatmentfor 10 h. The above results dem-onstrate that a TiCl4 treatmentduration of over 5 h is necessaryand effective in suppressing theformation of nanowires, thus in-creasing the stability of the

nanotubes. On the other hand, longer treatment duration(over 10 h) will result in degradation of photovoltaic perfor-mance, due to failure in using the outer tube surface. There-fore, the treatment duration of �10 h is optimal for the cur-rent study.

3. Conclusions

Transition from anodic titania nanotubes to nanowires can beobserved only under intermediate chemical dissolution condi-tions (e.g. at elevated temperature or prolonged duration in anorganic electrolyte), as it requires a large number of hydrogenions to reduce the passivated area of tube walls. The nano-wires grown for long durations (e.g. �8 h at 25 V and 40 8C)can be cleared away with ultrasonic cleaning due to the forma-tion of quasi-nanosprings. A model in terms of stretching andsplitting is proposed to interpret the transition process, andprovides a basis to suppress or eliminate the nanowires. Thetransition also arises during application of the nanotubes inDSSCs. To increase the tube stability, a 10 h treatment with40 mm TiCl4 solution at 70 8C is proven effective in this study.

Experimental Section

Prior to electrochemical anodization, titanium foils (0.25 mm,99.7 % purity, Sigma–Aldrich) were cleaned in an ultrasonic bath

Figure 8. Surface morphology of the nanotubes used in DSSCs: a) before and b) after assembly into devices;c) typical nanowire and quasi-nanospring structures present in (b) at 0 h. The times on the images indicate thedurations of TiCl4 treatment of the relevant nanotubes.

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Transition from Anodic Titania Nanotubes to Nanowires

using acetone, ethanol, and deionized water for 10 min each, anddried with an air stream. The anodization was carried out using atypical two-electrode configuration, with titanium foil as the work-ing electrode (size: �11 mm � 20 mm) and platinum gauze as thecounter electrode (size: 15 mm � 25 mm). The distance betweenthe electrodes was kept at 13 mm. The anodizing electrolyte solu-tion used was ethylene glycol (anhydrous, 99.8 %, Sigma–Aldrich)containing 0.3 wt % ammonium fluoride (98 + %, ACS reagent,

Sigma–Aldrich) and 2 vol % deionized water, unless otherwise indi-cated. A Keithley 2400 SourceMeter was adopted as a powersupply. The temperature of 40 8C was controlled by a water bath.For use in DSSCs, the as-grown nanotubes (anodized at 25 V and40 8C for 14 h, tube length: �11 mm) were cleaned in deionizedwater ultrasonically for 20 min immediately after anodization, driedin air, and thereafter annealed in air at 450 8C for 3 h with a heatingand cooling rate of 5 8C min�1. The annealed samples were treatedin 40 mm aqueous solutions of TiCl4 at 70 8C for different durations.The TiCl4 solution prepared under ice-cooled conditions wasloaded into a closed vessel with one sample in partial vacuumeach time. Again the obtained samples were annealed under thesame conditions as described above. The resultant samples weresensitized with N719 dye (0.3 mm, cis-diisothiocyanato-bis(2,2’-bi-pyridyl-4,4’-dicarboxylato)ruthenium(II) bis(tetrabutylammonium),Solaronix) in acetonitrile/tert-butanol (1:1, v/v) solvent for 24 h. Thedye loading process was also performed in a partial vacuum. Trans-parent platinized counter electrodes were prepared by dropspreading H2PtCl6/isopropanol solution (0.6 m) on a fluorine-dopedtin oxide (FTO, 15 W sq�1) glass substrate, followed by thermal de-composition at 400 8C for 15 min. For cell assembly, a hot-moltenspacer (SX1170, Solaronix, 60 mm) was sandwiched between thesample and the counter electrode. Electrolyte solution (EL HPE,Dyesol) containing iodide/triiodide redox couple was introducedinto the cells from one hole on the counter electrode under partialvacuum.The photocurrent–voltage characteristics were measured using anAM 1.5 solar simulator (Oriel, 300 W xenon lamp) with an activearea of 0.26 cm2. Dye desorption measurements were carried outby immersing the dye-sensitized samples in NaOH solution (0.1 m,3 mL) in a mixed solvent (water/ethanol = 1:1, v/v) for 1 h; there-after, the UV/Vis absorption spectrum of the resultant solution wasmeasured to estimate the dye loading amount. FESEM (JEOL, JSM-6340F) was used to characterize the morphology of the nanotubesand nanowires. XPS (KRATOS, AXIS ULTRA) measurements were car-ried out using a monochromated AlKa (1486.7 eV) X-ray source at apower of 150 W (15 kV � 10 mA). A charge neutralizer was em-ployed to neutralize charge accumulation during analysis. All thespectra were referenced to the binding energy of the C 1s peak(285.0 eV) arising from adventitious carbon. Prior to peak deconvo-lution, the inelastic background (Shirley-type) was subtracted. Themeasurements were applied to the surface of bulk nanowires andthe cross section of the same lift-off nanotube membrane.

Acknowledgements

We thank Chow Shiau Kee of the Materials Lab of the School ofMechanical and Aerospace Engineering, Nanyang TechnologicalUniversity, for her assistance in XPS measurements.

Keywords: electrochemistry · nanostructures · nanotubes ·photovoltaic effect · solar cells

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Received: June 12, 2011

Revised: October 3, 2011

Published online on November 7, 2011

ChemPhysChem 2011, 12, 3634 – 3641 � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemphyschem.org 3641

Transition from Anodic Titania Nanotubes to Nanowires