Journal of Industrial and Engineering Chemistry 29 (2015) 227–237

Fabrication of hierarchical porous anodized titania nano-network withenhanced active surface area: Ruthenium-based dye adsorptionstudies for dye-sensitized solar cell (DSSC) application

V.C. Anitha a, Arghya Narayan Banerjee a,*, Sang Woo Joo a,*, Bong Ki Min b

a School of Mechanical Engineering, Yeungnam University, Gyeongsan 712-749, South Koreab Center for Research Facilities, Yeungnam University, Gyeongsan 712-749, South Korea

A R T I C L E I N F O

Article history:

Received 25 September 2014

Received in revised form 18 March 2015

Accepted 29 March 2015

Available online 18 April 2015

Keywords:

Two-step anodization

Hierarchical TiO2

Conductivity

Surface area

Dye absorption

A B S T R A C T

Hierarchical (HC) porous titania (TiO2) nano-networks are developed via a two-step anodization method

in a mixed viscous electrolyte of hydrofluoric acid (HF) and glycerol. An ordered imprint was generated

on metallic titanium (Ti) substrate and subsequently anodized this imprint in aged HF/glycerol at

different conditions to create HC nanostructures. The optimal HF/glycerol volume ratio, conductivity,

and initial water/glycerol content of the electrolyte are 1:5, 10 to 12 mS/cm, and 17/83 vol%, respectively,

for obtaining HC structures. Also, ruthenium-based dye adsorption analysis demonstrates better dye

adsorption on HC–TiO2 as compared to conventional TiO2 nanotubes (TNAs), due to the enhanced active

surface area of the HC nano-network.

� 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

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jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

Introduction

Titanium dioxide with one-dimensional (1-D) nanostructures,especially the ordered nanotube materials, has attracted a greatdeal of attention in various fields including dye-sensitized solarcells [1–3], electrochemical sensors [4–6], photo catalysts [7,8],and implants [9,10]. In all of these applications, geometricalparameters such as diameter, length, and wall thickness, as wellas the surface morphology of the nanotubes, have profound effectson performance. Gong et al. [11] established the synthesis of thefirst generation of vertically-oriented titania nanotube arraysextending up to 500 nm in length by electrochemical oxidationof titanium in aqueous HF electrolyte. Great progress in thefabrication of TiO2 nanotubular structures was achieved afterMacak et al. [12,13] reported the formation of very smooth, regular,and long (1000 mm) nanotubes in organic viscous electrolytes. Thisopened the possibility of fabricating ultra-long multilayer TiO2

nanotube arrays with novel properties. For example, by adopting asimilar synthesis protocol to that used by Macak et al. [12,13],

* Corresponding authors. Tel.: +82 53 810 2453/+82 53 810;

fax: +82 53 810 2062/+82 53 810 2062.

E-mail addresses: arghya@ynu.ac.kr, banerjee_arghya@hotmail.com

(A.N. Banerjee), swjoo@yu.ac.kr (S.W. Joo).

http://dx.doi.org/10.1016/j.jiec.2015.03.032

1226-086X/� 2015 The Korean Society of Industrial and Engineering Chemistry. Publis

Paulose et al. [14] reported the possibility of increasing thenanotube length up to 1 mm after the complete anodization of a1 mm-thick titanium foil for bio-filtration applications. Similarly,Macak et al. [15] and John et al. [16] synthesized double-layeredand double-walled TiO2 nanotube arrays, respectively, for waterphoto-electrolysis. However, very little attention was paid to thevariations in surface morphologies, especially the smoothness ofthe top layer, and the uniformity of TiO2 nanotubes. It is wellknown that surface morphology plays a pivotal role in improvingthe photo-electrochemical performance of electro-optical devices[17]. More interestingly, the demand for a strict periodicity of thephotonic crystal entails very good uniformity of TiO2 nanotubes inan array. Therefore, proper tuning of the surface properties and theperiodicity of the TiO2 nanotube arrays may initiate new andinteresting applications in diverse fields. Nevertheless, all of thephotocatalytic applications have in common the fact that a higheroverall reaction rate is achieved using high surface area geome-tries. In addition to the standard conditions of anodization such aselectrolyte composition, HF concentration, anodization duration,and voltage, we have observed that the surface morphology and thequality of the resulting nanotubes are dependent upon several otherfactors including the aging of the electrolyte system, HF concentra-tion, and volume ratios of the components in the electrolyte system[18–20]. Because of the remarkable morphology and the potentialapplications, the formation of titania nanostructures in non-aqueous

hed by Elsevier B.V. All rights reserved.

Table 1Conductivity of HF/glycerol mixture at different anodization voltages.

Aging

voltage (V)

Ratio of HF/glycerol Conductivity (mS/cm)

20 V 1:1 1083

1:5 10.7

1:10 2.2

1:15 1.3

40 V 1:1 1447

1:5 11.3

1:10 2.4

1:15 1.4

80 V 1:1 1543

1:5 11.7

1:10 2.7

1:15 1.7

V.C. Anitha et al. / Journal of Industrial and Engineering Chemistry 29 (2015) 227–237228

organic electrolyte could be an interesting subject for furtherresearch [21]. The water content and the HF concentration arecritical in regulating the surface morphology of nanotubes [22–26]. Additionally, multi-step anodizations with varying anodizationconditions at different steps show the formation of some interestingTiO2 nanostructures, such as multi-layer and graded TiO2 nanotubearrays with different nanotubular morphologies along the growthdirection [27,28].

Based on the results of the present study, we report theformation of a top-porous and bottom-tubular hierarchical (HC)titania nanostructure via a two-step anodization process by usingpatterned Ti substrates and aged HF/glycerol electrolytes. Depend-ing upon the characteristics of the aged electrolytes (conductivity,water content), different surface morphologies of titania nanos-tructures are obtained. Furthermore, for the first time (to best of ourknowledge), the percentage of the initial content of water, HF, andglycerol in the HF/glycerol mixture are correlated with the chemicaldissolution process to understand the variation of surfacemorphological characteristics of the nanostructures. The optimumvalues of water and HF content within the electrolyte mixturewere obtained to fabricate a highly-ordered porous network ofhierarchical titania nanostructures, which could lead to interestingand diverse interfacial applications. Also in this study, for the firsttime, we have correlated the profound effect of the aging condition(in terms of conductivity) and fluoride ion concentration (in termsof the HF-to-glycerol volume ratio) of the electrolyte on theformation of hierarchical titania nanostructures. These hierarchicalnanotube layers have showed better performance in water splittingand in dye-sensitized solar cells than the conventional TiO2

nanotubes (TNAs) [29–31]. It is imperative that the photoanodeused in dye-sensitized solar cells (DSSCs) has a high surface area tofacilitate dye adsorption which, in turn, enhances the lightabsorption characteristics and performance of the cell [32–34]. The overall efficiency-limiting factor in TiO2 nanotube-basedsolar cells is specific dye loading. Anodically self-organized TiO2

hierarchical porous structures have a significantly a higher specificsurface area than conventional tubular layers. Thus, they are capableof improving the performance of TNA-based solar cells [35]. Hence,in this study, the active surface areas of conventional andhierarchical TiO2 surfaces were analyzed by using a dye adsorptiontest. The results show that HC nanostructures have a larger surfacearea compared to conventional TNAs. Thus, HC TNAs can be apromising candidate for the improved performance of DSSCs.

Experimental: Materials and methods

Commercially available titanium plates (14 mm2 area, 99.9%purity, Sigma-Aldrich) were polished using silicon carbide emerypapers with grit sizes of 800 and 2000. These titanium sampleswere further polished using 1 micron diamond paste followed by300 nm alumina suspensions. Furthermore, the samples wereultrasonically cleaned in acetone (Sigma-Aldrich) followed by de-ionized water, and then air-dried. The lab-built anodization set-upconsists of a metallic titanium sample (anode) and a platinumelectrode (cathode) connected to a regulated direct current (DC)power supply (EP-5001, PNCYS, Korea). In the first anodization stepof the two-step anodization process, polished Ti samples werepotentiostatically anodized at 40 V in a hydrofluoric acid (HF) (48%,Sigma-Aldrich)/ethylene glycol (EG) (99.8%, Sigma-Aldrich) mix-ture (1:1 volume ratio) for 15 h, resulting in conventional TiO2

nanotubes [36]. This particular anodization condition helps toremove the nanotubular layer easily. Then, using adhesive tape,this titania nanotubular (TNA) layer was peeled off, and thesubstrate was ultrasonically cleaned using acetone, resulting in aglossy Ti imprint surface that was used for the second step of theanodization.

Hierarchical porous TNA formation

In the second step of anodization, patterned Ti substrate,obtained in Step 1, and aged HF/glycerol electrolyte, obtained fromthe aging process, are used. Prior to the second step of anodization,aged electrolytes were prepared by anodizing dummy Ti plates in aHF/glycerol (99.5%, Fisher Chemical, Korea) mixture at threedifferent aging voltages (20, 40, and 80 V) and four different HF-to-glycerol volume ratios (1:1, 1:5, 1:10, and 1:15) for 10 h. Theconcentration of HF was fixed to 0.25 M and the total volume of theelectrolyte was fixed to 32 ml. This process produces ‘‘aged’’electrolytes with different properties (such as the difference inconcentrations of Ti–F complex ions, water content, etc., whichbasically leads to variations in the conductivity [37,38] for differentanodization conditions (cf. Table 1)). In the second step ofanodization, a patterned Ti substrate was used as the anode andPt plate as the counter-electrode with an aged HF/glycerol mixtureas the active electrolyte under a potentiostatic anodizationcondition of 40 V for 10 h. This step is repeated by using differentaged electrolytes produced via different aging conditions (e.g., threedifferent aging voltages of 20, 40, and 80 V, and four different HF-to-glycerol ratios of 1:1, 1:5, 1:10, and 1:15), leading to the formationof various porous hierarchical titania nanostructure networks as afunction of the aging conditions. A detailed layout and pictorialrepresentation of the process is shown in Fig. 1(a) and (b).

Characterizations

Surface morphology and dimensional characteristics of nano-tubes were carried out using field emission scanning electronmicroscopy (FE-SEM) (S-4800, Hitachi, Japan) and high resolutiontransmission electron microscopy (HR-TEM) (G2 F20, Tecnai,Japan). For the HRTEM analysis, the nanotubes are lifted fromthe substrate and dispersed in alcohol, followed by stirring anddrop-casting onto the carbon-coated Cu grid. The crystallinity ofthe anodized samples was analyzed using X-ray diffraction (XRD)(PAnalytical Xpert Pro, MPD, Japan). To improve the crystallinity,the samples were air-annealed at 500 8C for 3 h in a conventionalbox furnace (Thermo Scientific, Lind Berg Blue M 10 kA, 120/24 V,Korea). Conductivity measurements of the aged electrolytes werecarried out using a conductivity meter (Cond 3110, Incli.Teta Con@325, 2CA101, Germany). The surface area of nanopolished Ti wasdeduced using atomic force microscopy (JSPM-5200 scanningprobe microscope, JEOL, Japan).

Surface area analysis via dye adsorption test

The active surface area of conventional and hierarchical TiO2

nanotubes was analyzed by performing a dye adsorption test

Fig. 1. (a) Layout of the entire anodization process. (b) Schematic and pictorial representation of the different steps involved in two-step anodization process.

V.C. Anitha et al. / Journal of Industrial and Engineering Chemistry 29 (2015) 227–237 229

[39]. Titanium plates containing conventional titania nanotubes(prepared via first step anodization, as previously described),hierarchical porous nano-networks (prepared via second stepanodization in different aged electrolytes), and a control polishedtitanium (Ti) having equal dimensions were ultrasonically cleanedusing distilled water and acetone, and dried before the experiment.Triplicate experiments with three samples each were used for thestudy. The samples were immersed in a 0.3 mM solution of di-tetrabutylammonium cis-bis(isothiocyanato) bis(2,20-bipyridyl-4,40-dicarboxylato) ruthenium(II) (N719, Solaronix, Switzerland)in a mixture containing acetonitrile and tert-butanol in a 1:1volume ratio for 48 h. After incubation with dye, the sampleswere cleaned with ethanol to remove any un-adsorbed dye fromthe surface of the samples, and to remove the dye adsorbed on thelower unmodified surfaces of the titanium plates as well as the fouredges. The dyes adsorbed onto the nanostructured surfaces wereextracted using 0.1 M potassium hydroxide (KOH) throughrepeated washing (three times), and the absorbance of the dyewas recorded at 380 nm. The amount of dye adsorbed on eachsurface was determined from a standard curve of absorbance

versus known concentrations of dye run in parallel with theexperimental samples. The quantity of dye adsorbed per cm2 of thesamples was deduced from the surface area of the nanopolishedcontrol titanium obtained from AFM (see Supporting informationin Fig. S1). Thereafter, considering this as the reference, the surfacearea of the nanostructured titanium plates was calculated usingthe amount of dye adsorbed/cm2.

Results and discussion

Conventional TNAs

Fig. 2(a)–(c) shows SEM images of the top, bottom, and lateralviews of the titania nanotubes, respectively, obtained via the firstanodization step. In Fig. 2(a), the average inner pore diameter ofthe TNAs is seen to be approximately 160 nm with an average wallthickness of around 50 nm. Fig. 2(b) shows that the TNAs have aclosed-bottom and open-top structure, similar to a ‘‘nano-testtube.’’ The lateral view of the TNAs (Fig. 2(c)) reveals an averagetube length of 2 mm. The imprint patterns, created on the Ti

Fig. 2. Representative SEM images of (a) top surface of titania nanotubes, (b) bottom view of nanotubes formed in HF/EG electrolyte in Step I of anodization, (c) tilted view of

the samples showing the lateral dimension of the nanotubes, (d) imprints on Ti surface generated by peeling off the nanotube layer after Step I anodization, (e) TEM

micrographs of TNAs top-view, and (f) lateral view. Insets show the corresponding magnified images.

V.C. Anitha et al. / Journal of Industrial and Engineering Chemistry 29 (2015) 227–237230

substrate by peeling off the titania nanotubes (TNAs) usingadhesion tape, is shown in Fig. 2(d). Fig. 2(e) and (f) shows TEMimages of titania nanotube arrays created by the first anodizationstep. The top view of the nanotubes (Fig. 2(e) and inset) depicts anaverage inner pore diameter of around 100 to 150 nm with a porewall thickness of around 40 to 50 nm, as observed in SEM images(cf. Fig. 2(a)). Fig. 2(f) and inset represent the corresponding cross-sectional view of the titania nanotube arrays, revealing the closedbottom of a single nanotube (marked by dotted circles).

Hierarchical TNAs

The imprinted titanium substrate (Fig. 2(d)) was found tocontain dimples with hexagonal and pentagonal shapes with anaverage size of around 200 nm. When these polygonal dimplesare used as a substrate for the second anodization step, the pits

of the dimpled structure provide energetically favored sites forfurther pore formation, leading to the creation of a hierarchicalporous network with several inner pores within an outer cell. Aschematic diagram of the hierarchical nanotube formation isshown in Fig. S2. Figs. 3–5 show SEM images of titaniananostructures formed by the second anodization step under0.25 M HF/glycerol electrolyte, aged at different anodizationvoltages and volume ratios. Conventional TNAs were obtainedin non-aged electrolytes. Fig. 3 represents titania nanostruc-tures formed in aged (at 20 V) HF/glycerol electrolyte at fourdifferent HF-to-glycerol volume ratios (1:1, 1:5, 1:10, and1:15). In Fig. 3(a), conventional nanotubes are formed with a1:1 HF/glycerol volume ratio. It is well known that the contentof HF plays a key role in controlling the surface morphology oftitania nanotubes, through its role as a pore opening reagent[23]. The overall oxidation and chemical dissolution reactions

Fig. 3. SEM images of porous titania nano-structures formed in 0.25 M HF/glycerol aged at 20 V. (a) 1:1, (b) 1:5, (c) 1:10, and (d) 1:15 HF/glycerol volume ratios. Inset of (a)

shows cross-sectional view of conventional TNAs.

V.C. Anitha et al. / Journal of Industrial and Engineering Chemistry 29 (2015) 227–237 231

for the anodization of titanium are represented as follows[40,41]:

Ti þ 2H2O ! TiO2 þ 4Hþ þ 4e� (1)

TiO2 þ 4Hþ þ 6F�! TiF6½ �2� þ 2H2O (2)

Ti4þ þ 6F�! ½TiF6�2� (3)

Due to the higher content of HF in this case (cf. Fig. 3(a)), thedissolution reaction is very fast, leading to the development ofconventional TNAs. The inset of Fig. 3(a) depicts the formation offree-standing TNAs with an inner diameter and length of around�200 nm and �1.0 mm, respectively. Clearly, the diameters ofthese TNAs are found to be similar to the dimension of theimprinted Ti structures (shown in Fig. 2(d)) because theseimprinted dimples acted as pore formation sites in the secondanodization step. However, for increased glycerol content (1:5,1:10, and 1:15), the structure appears quite different fromconventional TNA structures (cf. Fig. 3(b)–(d)). A hierarchicalporous network, having a top porous layer with small pores withinthe top layer, is clearly visible at the 1:5 HF/glycerol volume ratio(Fig. 3(b)). Due to the lower relative HF content in this casecompared to the previous case (Fig. 3(a)), the dissolution reactionbecomes more controlled, and thus multiple pore formation hasoccurred within the pits of each dimpled pattern, as shownschematically in Fig. S2. On the other hand, at a higher HF content(Fig. 3(a)), due to highly elevated dissolution rate, etching ofthe entire bottom surface of each dimpled pattern occurs,yielding conventional TNAs. With further increases in the glycerolcontent to 1:10 and 1:15, nano-patterns closely resembling theimprint morphology (Fig. 2(d)) are obtained along with some oxidedeposits on the surface (cf. Fig. 3(c) and (d)). Additionally,the influence of a viscous component such as glycerol in theelectrolyte mixture is also found to play a pivotal role in thepore structure (detailed discussions are given in Section 3.5). The

presence of a large amount of glycerol in the electrolyte mixture atratios of 1:10 and 1:15 inhibits the fluoride ions from participating inthe dissolution reaction. Thus, the imprint morphology is retained toa great extent on the surface (cf. Fig. 3(c) and (d)). Therefore, abalance should be maintained between the dissolution and theinhibition process to obtain an ordered porous network of titaniananostructures. When the second anodization step is repeated with40 V-aged and 80 V-aged electrolytes, similar trends of over-etchingof the top porous layer at a higher HF content (1:1 HF/glycerolvolume ratio) and under-etching (or no-etching) at a lower HFcontent (1:10, 1:15 HF/glycerol volume ratio) are observed (cf.Figs. 4 and 5). The reason for higher etching in this case is due to thehigher conductivity of the 80 V-aged electrolyte compared to that ofthe 40 V-aged electrolyte (cf. Table 1), which is discussed in furtherdetail in Sections 3.3–3.5. On the other hand, Fig. 44(c),(d) andFig. 55(c),(d) show very little pore formation under 40 V-aged and80 V-aged electrolytes for 1:10 and 1:15 HF/glycerol volume ratios,respectively. On the contrary, similar to the samples treated under20 V-aged electrolytes for a 1:5 HF/glycerol ratio (cf. Fig. 3(b)),samples treated under 40 V-aged and 80 V-aged electrolytes with a1:5 HF/glycerol ratio reveal a double-layer hierarchical nanotubegeometry, with a top porous first layer of cells having typical sizes of200 to 250 nm and a second layer with smaller pore sizes (�40 nm)inside the bigger cells (cf. Figs. 4(b) and 5(b)). The multiple pores thatform simultaneously within the cells of one imprint create ahoneycomb-like structure embedding several nanotubes inside oneimprint. Therefore, under the current experimental conditions, a 1:5HF/glycerol volume ratio is found to be optimum for the formation ofhierarchical titania nanotube arrays having smaller nanotubeswithin the bigger cells of a honeycomb-like network.

Effect of aging conditions on electrolyte conductivity

To explain the effect of the aged electrolytes on the surfacemorphology and formation of titania nanostructures, we investigated

Fig. 4. SEM images of titania nano-structures formed in 0.25 M HF/glycerol aged at 40 V. (a) 1:1, (b) 1:5, (c) 1:10, and (d) 1:15 HF/glycerol volume ratios. Inset of (b) shows

corresponding cross-sectional view of hierarchical porous titania.

V.C. Anitha et al. / Journal of Industrial and Engineering Chemistry 29 (2015) 227–237232

the effect of aging conditions on electrolyte conductivity. Table 1shows the measured values of the conductivities of the agedelectrolytes prepared at different anodization voltages (20, 40, and80 V) and HF:glycerol volume ratios (1:1, 1:5, 1:10, and 1:15). Wenote that the variations in electrolyte conductivity at 20 decreasedwith an increasing glycerol volume ratio (or decreased with a relativedecrease in HF content) from 1:1 to 1:15. The same trend is alsoobserved in 40 V-aged and 80 V-aged samples. On the other hand, ifwe consider one particular HF/glycerol volume ratio (for example,1:1) for 20, 40, and 80 V aging conditions, the conductivity is found toincrease with increasing aging voltages (e.g., 1083, 1447, and1543 mS/cm for 20, 40, and 80 V aging voltages, respectively). Thesame trend follows in 1:5, 1:10, 1:15 ratios. Therefore, theconductivity is found to increase with increasing aging voltage aswell as with higher HF content, which is apparently due to the higherfield-assisted dissolution of Ti ions and hence higher Ti–F complexformation at larger aging voltage/HF content within the agedelectrolytes [42]. Additionally, the use of aged electrolytes greatlyimproved the adhesion between nanotubes and the underlying Timetal substrate, which further improved the photoconversionefficiency [42]. This trend shows that the conductivity of theelectrolyte increases with anodization duration and smaller inter-electrode spacing [41–44]. In the present study, the conductivity ofaged HF/glycerol mixtures increased with anodization voltage and HFcontent due to the faster reaction rate, and decreased with higherglycerol content because of the inhibition of the reaction via anincrease in the internal resistance of the electrolyte [43].

Effect of electrolyte conductivity on the surface morphology of TiO2

nanostructures

The increase in the conductivity of the electrolyte at higheraging voltages (40 and 80 V) and lower glycerol content (1:1 HF/glycerol) provides a larger fraction of the applied potential

available for the second anodization process. This occurs becausethe potential drop across the electrolyte decreases with time,leading to faster oxide growth and higher dissolution of the toplayer of titania to form some aggregates on the surface via a near-complete destruction of the porous network (cf. Figs. 4(a) and 5(a)).For the same reason, lower conductivity of the electrolytes at ahigher glycerol content (1:10, 1:15 HF/glycerol) leads to negligibledissolution, resulting in insignificant or disordered pore formation,as observed in Figs. 3(c),(d), 4(c),(d) and 5(c),(d). On the other hand,the optimum conductivity of the aged electrolytes required toobtain a hierarchical, ordered titania nanotube network (cf. Figs.3(b), 4(b), and 5(b)) is found to be 10 to 11 mS/cm for a 1:5 HF/glycerol volume ratio.

In this study, to the best of our knowledge, we are reporting forthe first time the profound effect of the aging condition (in terms ofconductivity) and fluoride ion concentration (in terms of the HF-to-glycerol volume ratio) of the electrolyte on the surfacemorphology of a titania nanotube network. The nature of theelectrolyte, HF concentration, and composition of electrolytes, for agiven applied potential, must be appropriately tuned to obtainbetter control of the nanotubular morphologies [44,37,45]. Gener-ally, the dynamics of the titania pore formation process consists offour simultaneous chemical (field-induced) and electrochemical(current-assisted) mechanisms [38,46–48]: (1) the oxidation of theTi–surface at the Ti–electrolyte interface to form titania, (2) thedissolution of oxides at the titania–electrolyte interface viainteraction with H+ and F� ions, (3) field-induced migration ofoxygen/fluoride/titanium ions between titania–electrolyte andtitania–Ti interfaces, and (4) oxide growth at the titania–Tiinterface (cf. Fig. 6(a)). The rates of oxide dissolution at thetitania–electrolyte interface, oxide growth at the titania–Tiinterface, and the migration of ions between the interfaces dependon the local ion concentrations as well as on the local electric fieldand current density imposed on the Ti–titania–electrolyte ternary

Fig. 5. SEM images of porous nano-structures formed in 0.25 M HF/glycerol aged at 80 V. (a) 1:1, (b) 1:5, (c) 1:10, and (d) 1:15 HF/glycerol volume ratios. Inset of (b) depicts

the magnified image of one single outer cell consisting of several inner pores.

Fig. 6. Schematic illustration of (a) titania nanotube growth in presence of fluorides40, (b) dissolution of the bottom of the outer pores in excess water content of the electrolyte

to obtain conventional TNAs.

V.C. Anitha et al. / Journal of Industrial and Engineering Chemistry 29 (2015) 227–237 233

V.C. Anitha et al. / Journal of Industrial and Engineering Chemistry 29 (2015) 227–237234

system through the applied anodization conditions. If the fluoridecontent is very low, a stable compact oxide layer is formed (cf. Figs.3(d), 4(d) and 5(d)). If the fluoride concentration is very high, nooxide formation can be observed and all of the formed Ti4+

immediately reacts with the abundant fluoride to form watersoluble TiF6½ �2�. The reaction is then controlled by the diffusion ofTiF6½ �2� (cf. Figs. 4(a) and 5(a)). In intermediate fluoride

concentrations, a competition between oxide formation and thecomplex formation of Ti4+ takes place, and porous oxide ornanotube formation can be observed [38,45] (cf. Figs. 3(b),(c),4(b),(c) and 5(b),(c)).

Effect of glycerol content

From Eq. (1), it is apparent that lower water content in theelectrolyte reduces the oxidation rate at the electrolyte–Tiinterface, which further reduces the concentration of H+ ionsand thus reduces the dissolution rate of the pore-propagation frontvia a lower formation rate of water soluble [TiF6]2� complex(Eq. (2)). Therefore, the diffusion control (of the [TiF6]2� complex)on the dissolution rate can effectively be controlled by tuning thewater content of the electrolyte [38], which is found to be one ofthe decisive factors for hierarchical pore formation. At a higherwater content, the selective dissolution of the entire bottom of thebigger cells leads to the formation of conventional titaniananotubes (Fig. 6(b)). Therefore, an optimum content of waterand HF helps to balance these two competing processes, whichfurther results in an ordered porous network of hierarchical titaniananostructures. In the present case, the amount of glycerol withinthe electrolyte was observed to be crucial in order to control thewater content, and thus was found to be the effective way ofcontrolling the diffusion and dissolution reactions needed toobtain tailored hierarchical nanostructures. The calculated values(based on the density, assay, and the volume taken in each ratio) ofinitial water content, HF, and glycerol in the HF/glycerol mixtureare shown in Table 2. From the table, we note that by adjusting theglycerol content, the water content within the electrolyte iscontrolled effectively. In a condition of lower glycerol content (i.e.,a higher water content), due to lower viscosity of the electrolyte,the chemical dissolution rate becomes relatively faster, thusaccelerating the dissolution of TiO2 at the nanotube walls as well asat the bottom of the nanotubes. This results in the formation ofconventional TNAs with larger diameters and thinner walls[49]. Also, as previously stated, a higher water content (i.e., alower glycerol content) leads to a relatively faster oxidation rate,thus producing oxide deposits on the surface to form destroyednanotubular structures (Figs. 3(a), 4(a) and 5(a)). Likewise, a higherglycerol content (i.e., lower water content) at the 1:10 and 1:15ratios resulted in a higher viscosity of the electrolyte, yielding amuch lower chemical dissolution rate, which resulted in theformation of a small number of pores (Figs. 3(c),(d), 4(c),(d) and5(d)). We note that in the case shown in Fig. 5(c), a combination ofhigher conductivity (cf. Table 1) and higher viscosity (due to higherglycerol content, cf. Table 2) leads to the formation of disorderedpores within destroyed outer cells of imprinted structures.However, a 1:5 HF/glycerol volume ratio is found to have the

Table 2Calculated values for initial content of HF, water, and glycerol in HF/glycerol

mixture.

Medium (vol%) Ratio of HF/glycerol

1:1 1:5 1:10 1:15

HF 0.1893 0.0630 0.0344 0.0236

H2O 50.062 17.02 9.51 6.69

Glycerol 49.75 82.92 90.46 93.29

optimum conductivity and glycerol/HF/water content needed toproduce controlled oxidation and dissolution reactions to obtainan ordered porous network of hierarchical titania. We repeated thesecond anodization step in 1:2 and 1:3 HF/glycerol volume ratios(aged at 40 V), and observed that in these cases the top layersdissolve out and form conventional TNAs with the destroyed toplayer (Fig. S3).

To observe the effect of anodization time on the TiO2

nanostructures, we performed the second anodization step for a1:1 HF/glycerol ratio (aged at 40 V) for a shorter length of time(4 h), and still observed conventional nanotube formation. Thisindicates that the dissolution rate higher and uncontrolled toobtain ordered hierarchical titania nanotubes (Fig. S4a). Similarly,for a 1:15 HF/glycerol ratio, an increase in the second anodizationtime to 24 h led to the transformation of the top porous layer into athick oxide layer (Fig. S4b), indicating that the viscosity of theelectrolyte is too high to yield effective pore formation. When theHF/glycerol ratio is 1:5 and the second step anodization time isincreased from 10 to 15 h, a hierarchical nanostructure formationwas observed, having smaller pores within the honeycomb-likestructure (Fig. S4c). Interestingly, these honeycomb-like hexagonaland pentagonal structures are separated by some voids due to theextended dissolution of the outer walls of the honeycombstructures, attributed to a longer anodization time. An electrolytehaving high viscosity will produce nanopores/nanotubes withsmall diameters and vice versa. At a 1:5 HF/glycerol ratio, theviscosity of glycerol is found to be 945 cP at 25 8C [50], which isenough to control the diffusion of H+ and F� ions effectively. Thus,the nanotubes with smaller pores are formed inside each biggerimprint. Consequently, the growth of nanopores starts within thetop porous pre-patterned imprints during the second anodizationstep in the glycerol-based electrolyte, and the diameter of thesmaller pores becomes circular in shape with a honeycomb-likemorphology of the outer pores. The thickness of honeycomb-likepatterned hexagonal rings is also greater after the secondanodization in glycerol compared to the thickness of originalhoneycomb-like hexagonal pre-patterns before the second anodi-zation step (Fig. 2(d)). This is due to the diffusion-controlleddissolution process in viscous electrolyte.

XRD measurements

To evaluate the effect of two-step anodization on the crystallinityof the nanotubes, X-ray diffraction analyses were performed on as-synthesized and air-annealed (@ 500 8C for 3 h) conventional andhierarchical TNAs. Fig. 7 curve (a) shows the XRD spectrum ofconventional TNAs formed after the first anodization step. The curveshows an almost amorphous morphology of the titania nanotubes.The Ti peaks originate from the titanium substrates (1999 JCPDS-ICDD File Card #12-0754). A possible TiO2 peak (indicated by ‘‘*’’) isalso observed (1999 JCPDS-ICDD File Card #44-1294), which wasremoved after air annealing. After air annealing these TNAs at 500 8Cfor 3 h, the crystallinity increased considerably with a dominantanatase phase, as shown in Fig. 7 curve b (1999 JCPDS-ICDD File Card#83-2243). On the other hand, the as-synthesized hierarchical TNAs,prepared after the second anodization (with a 1:5 HF/glycerolvolume ratio for 40 V-aged electrolyte), show a similar amorphousnature (Fig. 7 curve c). After air annealing at 500 8C for 3 h, thecrystallinity increased considerably (Fig. 7 curve d). A relativecomparison of the positions (2u) and full-width at half-maxima (b)of the (1 0 1), (2 0 0), (2 1 1), etc. diffraction peaks of anatase TiO2 forconventional and hierarchical TNAs reveal that these peaks areslightly left-shifted (toward smaller 2u) and broadened (higher b)for hierarchical TNAs compared to the conventional values (cf. Fig. 7curves b and d). For example, the 2u and b values of (1 0 1) peaksof annealed conventional TNAs and hierarchical (HC) TNAs

80706050403020

Ti

*

TiTiTiTiTi

Ti

2θ (deg. )

Con ventional TNAs

A(1

07)

Ti

(b)

(a)

A (0

04)

TiTi

Ti

TiA(2

11)

A(1

05)

A(2

00)

Ti

A (1

01) Annea led Con vent ional TNAs

Ti

(c)TiTi

TiTiTi

Ti

Ti

Ti

Hierarchi cal TNAs

Ti(d) A (0

04)

Ti

TiTi

A(1

07)

A(2

11)

A(1

05)

Ti

TiA(2

00)

Ti

Ti

TiA

(101

) Anneal ed hierarchica l TNAs

Inte

nsity

(a.u

.)

Fig. 7. XRD of as-prepared and annealed at 500 8C (3 h) nanotubes for conventional and hierarchical structures. ‘‘A’’ represents the anatase phase of TiO2.

V.C. Anitha et al. / Journal of Industrial and Engineering Chemistry 29 (2015) 227–237 235

are obtained as follows: (2uTNAð101Þ ¼ 25:4�; 2uHC

ð101Þ ¼ 25:3�;bTNA

101 ¼ 0:21�; bHC101 ¼ 0:40�). Similar trends are observed for

other peaks as well. Generally, nanocrystalline materials arereported to be always subjected to a strain, which is reflectedeither by the broadening of the diffraction peaks (i.e., an increase inthe b-values, which is due to nonuniform strain, called ‘‘strainbroadening’’) and/or a shift toward a high 2u value (for uniformcompressive strain) or a low 2u value (for uniform tensile strain)[51]. Therefore, the observed increment in the b values and a low 2ushift of the diffraction peaks of the hierarchical TNAs with respect toconventional TNAs indicate the generation of both uniform and non-uniform tensile strain within the hierarchical TNAs against theconventional titania nanotubes. Because of the formation of severalsmaller nanotubes within the bigger pores with hierarchicalhoneycomb-like structures, a tensile stress is generated withinthe system, and is reflected by an increase in the strain within thehierarchical TNAs. For a freely growing system like ours, generationof both uniform and nonuniform strain is thermodynamically moreprobable, and is reflected by the changes in the b and 2u values.Physically, the oxide growth at the Ti–TiO2 interface generates atensile stress at the metal–oxide interface, as shown in Fig. S5.Microscopically, within the free-standing hierarchical TNAs, thesestresses generate both uniform and nonuniform tensile strains alongthe pore walls and at the pore bottoms, respectively, as revealedfrom the XRD analysis.

Dye adsorption studies

Specific dye loading and corresponding surface area measure-ments were carried out on selected hierarchical TNAs (Figs. 3(b)(HC-1), 4(b) (HC-2), and 5(b) (HC-3)) with respect to conventionalTNA and nanopolished Ti substrate (NP) using dye adsorptionanalysis. The length of all the nanotubular structures was �1 mm.The amounts of dye adsorbed in these nanostructures are shown inFig. 8(a), and corresponding active surface areas are given in

Table 3. These results demonstrate high dye adsorption for all HCnanostructures as compared to conventional TNA and nanopol-ished Ti substrates (NP). This is due to the higher active surfaceareas of hierarchical nanostructures against TNA and NP samples,as depicted in Table 3, and clearly implies that the electrolyteconductivity-modulated two-step anodization process was able togenerate hierarchical structures that showed a nearly two-foldincrease in the active surface area, in comparison to conventionalTNAs. Generally, the dye adsorption reaction is represented as[52,53] follows:

N719 þ TiO2! TiO2 : N719 (4)

where TiO2:N719 represents adsorbed N719 dye on the TiO2

nanotube surface. The molecular structure of the N719 dye isshown in Fig. 8(b). The corresponding schematic bondingmechanism with the TNA surface is shown in Fig. 8(c). Generally,the carboxylic groups of the dye interact with the TiO2 surface indifferent binding modes involving two or three of the –COOHgroups anchoring with the metal oxide surface [54]. Additionally,the interaction of the NCS groups with the titania surface, as well ashydrogen bonding with surface hydroxides of titania, can alsooccur [55–57], as described in Fig. 8(c). All three anchoringmechanisms are shown together in Fig. 8(c), but all of these maynot necessarily occur simultaneously during dye adsorption on atitania surface. Depending upon the surface properties andadsorption dynamics, one (or some) of these would have a higherprobability of occurrence than other(s) [54]. Other possible –COOHbinding modes are described in detail in [54]. Regarding theaforementioned bonding mechanisms, the majority of studies havereported that N719 dye adsorption occurs via two carboxylicgroups with the surface oxygen atoms of anatase titania, therebyproducing the strongest binding state [57,58]. In our case ofhierarchically porous anatase titania nanotubes with higher activesurface areas (i.e., higher than the conventional TNAs), the number

Fig. 8. (a) Amount of dye adsorption on different nanostructured surfaces, (b) molecular structure of N719 dye, (c) schematic representation of N719 dye anchoring

mechanism with TNA surfaces: (from left to right) interaction of S atom of -NCS group with the TNA surface, interaction of one oxygen atom of the carboxylic group with the

TiO2 surface, and hydrogen bonding between the H-atom of the –COOH group and the titania surface.

Table 3Comparison of dye quantification analysis and surface area measurement of

conventional nanotubes and hierarchical TNAs.

Sample

identity

Quantity of

ruthenium dye

adsorped (mg/ml)

Surface area

of samples

(cm2)

NP 1.12 � 0.01 2.77 � 0.015

TNA 1.27 � 0.03 3.14 � 0.004

HC-1 1.52 � 0.02 3.76 � 0.005

HC-2 1.96 � 0.005 4.85 � 0.05

HC-3 1.77 � 0.007 4.38 � 0.06

V.C. Anitha et al. / Journal of Industrial and Engineering Chemistry 29 (2015) 227–237236

of bonding states are also much higher. Hence, the dye adsorptionefficiency is also higher, as clearly shown in Fig. 8(a). Among thedifferent HC nanostructures, HC-2 shows the maximum activesurface area (4.85 cm2). Thus, it also has highest specific dyeadsorption (1.96 mg/ml) versus the other hierarchical nanostruc-tures. This is mainly because of its distinct surface morphologyagainst other HC nanostructures. A relative comparison of various

Table 4Comparison of outer/inner pore diameters and pore density of the different hierarchica

glycerol ratio.

0.25M HF:

glycerol

vol. ratio

Aging

voltage

(V)

Sample identity Avg. outer

cell diameter

(nm)

1:5 20 HC-1: Fig. 3b 200 � 10

40 HC-2: Fig. 4b 240 � 5

80 HC-3: Fig. 5b 250 � 7

physical parameters of the nanopores of the three different (HC)nanostructures (shown in Figs. 3(b), 4(b) and 5(b)) is given inTable 4, which reveals that the inner pore density for HC-2 is thehighest with respect to other HC nanostructures (HC-1 and HC-3).For example, the average number of inner pores per outer cell forHC-2 is highest compared to HC-1 and HC-3, although the averageouter cell diameter (�250 nm) of HC-3 is higher than that of HC-2(�240 nm). This results in the average inner pore density of HC-2(�3 � 1010 cm�2) being the highest with respect to HC-1 and HC-3.Hence, these data indicate that HC-2 has the highest effectivesurface area compared to other hierarchical nanostructures (HC-1and HC-3) as described in Table 3. Flores et al. [59] reported that ahigh surface area of TiO2 nanotubes allows more sensitizer dyes tobe chemically attached to the semiconductor surface. As aconsequence, the device’s performance at a light intensity of100 mW cm�2 (Xe(Hg) lamp) is significantly improved, reaching4.03%, in comparison to the same device using TiO2 nanoparticles.Therefore, hierarchical TiO2 nanotubes with various inner andouter diameters can be a promising nanostructure material for useas a photoanode in a DSSC for enhanced performance.

l nanostructures obtained at three different electrolyte aging voltages in a 1:5 HF/

Avg. Inner

pore diameter

(nm)

Avg. no. of

inner pores

per outer cell

Avg. no. of inner

pores per unit area

of the sample (cm�2)

35 � 7 7 � 2 1.5 � 1010

40 � 5 14 � 3 3 � 1010

40 � 6 12 � 2 2 � 1010

V.C. Anitha et al. / Journal of Industrial and Engineering Chemistry 29 (2015) 227–237 237

Conclusion

We fabricated a porous network of hierarchical titaniananotubular morphologies by modulating the electrolyte compo-sition and titanium surface substrate via a two-step anodizationprocess. The use of aged electrolytes and imprinted Ti substratewas integrated with the two-step anodization process for thegeneration of hierarchical titania nanostructures having a meso-porous membrane within a hierarchical honeycomb-like network.During electrolyte aging, two parameters (the aging voltage andHF-to-glycerol volume ratio) were tuned, which led to thefollowing variations in the properties of the aged electrolyte thatmanifest different conductivities of the electrolyte:

(1) A variation in the aging voltage leads to a change in the Ti4+ ionconcentration within the electrolyte.

(2) A variation in the HF/glycerol content leads to a change in theF� ion concentration, and thus a variation in the Ti–F complexion formation and a change in the viscous component of theelectrolyte.

(3) Additionally, the change in the glycerol content also controlsthe optimum water content within the electrolytes, which(along with the electrolyte conductivity) controls the overalloxidation and dissolution of the nanopore propagation frontsduring the second anodization step to obtain hierarchical TNAswith unique surface morphologies. The overall sequentialvariation of the morphological structure of hierarchical porousanodized titania, as a function of the HF-to-glycerol volumeratio and electrolyte conductivity in terms of aging voltage, issummarized in Fig. S6 of the supplementary data. This pictureshows that a judicious tuning of these parameters leads to theformation of hierarchical porous nanotube networks withnovel morphologies. The present work shows that an initialcontent of 17/83 water/glycerol volume % and 0.063% HF in theaged electrolyte mixture results in the growth of orderedporous hierarchical titania nanostructure. Additionally, wenote that subtle variations in the surface morphology enhancedthe active surface area of the nanostructures. The key toimproved efficiency in DSSCs is an increase in the dye loading ofthe material, which is achieved because of the improvedsurface area of the hierarchical nanostructures having ahoneycomb-like nanotube morphology. Hence, we suggestthat improvements in DSSC performance can be aided by subtlealterations in the surface morphology of titania nanotubes.

Acknowledgement

This work was supported by a World Class University grant(no.R32-2008-000-20082-0) of the Ministry of Education, Scienceand Technology of the Korean government. The authors gratefullyacknowledge this support.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.jiec.2015.03.032.

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