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Electrochimica Acta 56 (2011) 7696– 7702

Contents lists available at ScienceDirect

Electrochimica Acta

j ourna l ho me pag e: www.elsev ier .com/ locate /e lec tac ta

abrication of morphology controllable rutile TiO2 nanowire arrays byolvothermal route for dye-sensitized solar cells

hen Wei, Ruoshi Li, Tao Huang, Aishui Yu ∗

epartment of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Fudan University, Shanghai 200438, China

r t i c l e i n f o

rticle history:eceived 16 May 2011eceived in revised form 17 May 2011ccepted 13 June 2011vailable online 21 June 2011

a b s t r a c t

Highly ordered, vertically oriented TiO2 nanowire arrays (TNAs) are synthesized directly on transpar-ent conducting substrate by solvothermal procedure without any template. The X-ray diffraction (XRD)pattern shows that TiO2 array is in rutile phase growing along the (0 0 2) direction. The field-emissionscanning electron microscopy (FE-SEM) images of the samples indicate that the TiO2 array surface mor-

eywords:itania nanowire arrayolvothermal crystal growthye-sensitized solar cellshotoanode

phology and orientation are highly dependent on the synthesis conditions. In a typical condition ofsolvothermal at 180 ◦C for 2 h, the TNAs are composed of nanowires 10 ± 2 nm in width, and severalnanowires bunch together to form a larger secondary structure of 60 ± 10 nm wide. Dye-sensitized solarcell (DSSC) assembled with the TNAs grown on the FTO glass as photoanode under illumination of simu-lated AM 1.5G solar light (100 mW cm−2) achieves an overall photoelectric conversion efficiency of 1.64%.

. Introduction

Up to date DSSCs have attracted much attention due to theirow manufacturing costs and relatively high energy conversion effi-iencies [1,2]. The electron-collecting layer in a DSSC is typically a0 �m thick nanocrystalline film comprised of a three dimensionaletwork of interconnected 15–20 nm sized nanoparticles [3]. Com-ared with TiO2 nanoparticles, TiO2 nanorod/wire/tube arrays offerirect pathways for electrons and can increase the electron trans-ort rate, which improves the performance of the device [4–6]. Dueo their ability to constrain the movement of electrons and pho-ons in one direction, TiO2-based materials with one-dimensionaltructures have been intensively investigated [7].

Various synthesis techniques have been used to form ori-nted TiO2 nanorod/wire/tube arrays including template-assistedethod, electrochemical anodic oxidation method, chemical vapor

eposition (CVD) and hydrothermal method [8–11]. Among theseethods, hydrothermal/solvothermal synthesis of TiO2 nanowire

rrays is a promising approach due to its simple process, fasteaction velocity and low cost. Therefore, it has the potential forarge-scale preparation of TNAs [12]. Recent reports by Grimes ando-workers [5] and Zhou and co-workers [13] present a straight-

orward solution growth method to prepare oriented rutile TiO2anowire arrays from transparent conducting oxide (TCO) sub-trates without any template, and it shows a promising potential

∗ Corresponding author. Tel.: +86 21 51630320; fax: +86 21 51630320.E-mail address: asyu@fudan.edu.cn (A. Yu).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.06.038

© 2011 Elsevier Ltd. All rights reserved.

application in the photovoltaic field. But the detailed microscopicmorphology and structure of the TNA film have not been systemat-ically studied and proficiently controlled, which severely restrictsits applications.

We provide here a systematic evaluation of how reaction condi-tions affect the nucleation and growth of the TNAs, and how theseparameters affect the surface morphology and also photoelectricconversion efficiency of TNA film. The TNA film was prepared via anon-polar solvent/hydrophilic substrate interfacial reaction undermild solvothermal conditions. Conditions for crystal growth, suchas conducting surface of the substrate facing up and down, reactiontemperature (140–220 ◦C) and reaction duration (0.5–6 h) werepreliminarily examined and optimized. The as prepared TNAs usedas photoanodes to assemble DSSCs exhibit a maximum efficiencyof 1.64%.

2. Experimental procedure

2.1. Preparation of TiO2 nanowire arrays

Fluorine-doped tin dioxide (FTO) (14 � cm−2) conducting glasswas purchased from Wuhan Geao Instruments Science & Tech-nology Co., Ltd.; and the other chemicals were purchased fromShanghai Chemical Co., Ltd. All chemicals were used as receivedwithout any further purification.

In a typical synthesis, FTO conducting glass was initially cleanedby being immersed in isopropanol saturated with potassiumhydroxide for 24 h, subsequently cleaned by ultrasonication in ace-tone, ethanol and deionised water for 30 min, respectively.

Z. Wei et al. / Electrochimica Acta 56 (2011) 7696– 7702 7697

2 h, with conducting side facing up and down: (a and b) up, (c and d) down.

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(f)

(002)

100806040200

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nsity

(a.

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(a)

(b)

(c)

(002)

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Fig. 1. FE-SEM images of TiO2 nanowire arrays fabricated at 180 ◦C for

Before the solvothermal experiment, a compact TiO2 layer waseposited onto the well-cleaned FTO glass substrate by immersion

n a 0.05 M TiCl4 aqueous solution at 70 ◦C for 30 min, then heatedn air at 500 ◦C for 30 min [5]. We note that the TNA films without

compact layer tend to peel off from the substrate due to poordhesion.

10 mL of toluene, 1 mL of tetrabutyl titanate, 1 mL of tita-ium tetrachloride (1 M in toluene) and 1 mL of hydrochloric acid37 wt%) were added to a sealed Teflon autoclave (50 mL) as theolvothermal solution. FTO glass substrate was placed into the auto-lave with conducting surface facing up or facing down. To evaluateow reaction duration affects the TNAs’ microscopic morphology,he durations are selected as 0.5 h, 1 h, 2 h, 4 h and 6 h, with theeaction temperature fixed at 180 ◦C. Correspondingly, to evalu-te reaction temperature, the temperatures are selected as 140 ◦C,60 ◦C, 180 ◦C, 200 ◦C and 220 ◦C, with the reaction duration fixed at

h. After the solvothermal experiment, the resulting samples wereinsed with ethanol, and then dried in air.

Dye-sensitized solar cells were assembled using the TiO2anowires grown on FTO glass as the photoanode. Prior to dyedsorption, the as-prepared sample was placed in 0.05 M TiCl4queous solution at 70 ◦C for 0.5 h. Then the TiCl4-treated sampleas annealed in air at 450 ◦C for 30 min inside a furnace. The sen-

itizer used in this work was cis-bis(isothiocyanato)bis(2,2′-ipyrridyl-4-4′-dicarboxylato)-ruthenium(II) bis-etrabutylammonium dye (N-719 as received from Solaronix).he dye was adsorbed to titania film by soaking for 24 h in a.3 mM ethanol solution of N-719. A platinum-coated FTO sub-trate spaced from the TiO2 nanorod photoanode with 25-�mhick Teflon strips (Pike Technologies) was used as the counterlectrode. A thin layer of electrolyte was introduced into the

nterelectrode space. The electrolyte contains 0.1 M LiI, 0.05 M2, 0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide (Tomiyamaure Chemical Industries Ltd.) and 0.5 M 4-tertbutylpyridine (TBP)n dehydrated acetonitrile.

2-Theta

Fig. 2. XRD patterns of TiO2 nanowire arrays fabricated at 180 ◦C for different dura-tion: (a) FTO glass, (b) 0.5 h, (c) 1 h, (d) 2 h, (e) 4 h, (f) 6 h.

7698 Z. Wei et al. / Electrochimica Acta 56 (2011) 7696– 7702

rent d

2

dtuasatS(sd

Fig. 3. FE-SEM images of TiO2 nanowire arrays fabricated at 180 ◦C for diffe

.2. Characterization

The crystal structure of the sample was identified by X-rayiffraction (XRD) analysis on a Bruker D8 Advance X-ray diffrac-ometer using Cu K� radiation. The morphology was characterizedsing field emission scanning electron microscopy (FE-SEM) with

Hitachi S-4800 microscope and transmission electron micro-cope (TEM JEM-2100F). The TNA film thickness was measured by

Veeco Dektak 150 Surface Profiler. The UV–vis adsorption spec-ra of dye N719 molecule adsorbed on TNAs was measured by

himadz UV-3150. The electrochemical impedance spectroscopyEIS) measurements were performed on an electrochemical work-tation (CHI 660A, Chenhua Instrument Company). The currentensity–voltage curve was measured with a calibrated solar sim-

uration: (a1 and a2) 0.5 h, (b1–b3) 1 h, (c1–c3) 2 h, (d1–d3) 4 h, (e1–e3) 6 h.

ulator, using air mass 1.5 global (AM 1.5G) illumination with anintensity of 100 mW cm−2..

3. Results and discussion

3.1. Effect of conducting surface

Fig. 1(a and b) illustrates the FE-SEM images of the titaniumdioxide deposited on FTO glass at 180 ◦C for 2 h, with conducting

surface facing up when the solvothermal process is proceeding,and Fig. 1(c and d) for conducting surface facing down. High-magnification cross-sectional image is shown as inset of Fig. 1(d).The images show that TiO2 film fabricated with conducting surface

ica Acta 56 (2011) 7696– 7702 7699

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Fig. 4. TEM images of TiO2 nanowire arrays fabricated at 180 ◦C for 2 h.

Table 1The relationship between a single wire width, the diameter of a bunch of wires, theTNA thickness and the reaction duration.

A single wirewidth (nm)

Diameter of a bunchof wires (nm)

The TNAthickness (�m)

0.5 h Less than 10 120 ± 20 Less than 11 h Less than 10 100 ± 20 1.92 h 10 ± 2 60 ± 10 6.8

thick, and it grows thicker at longer time. At reaction period of 6 h,the film reaches 18.0 �m thick but it is likely to peel off from thesubstrate.

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(00 2)

(a)

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Z. Wei et al. / Electrochim

acing up contains two parts: nanospheres composed of nanowiresn the top and nanowire array at the bottom. The width of theanowires of the nanosphere is in the range of 200–300 nm, and theiameter of a whole nanosphere is about 9 �m. In Fig. 1(c and d), noanosphere layer can be observed, which can be explained by thathe nanosphere layer is formed by precipitation of TiO2 formed inolution via homogeneous nucleation procedure, and the nanowirerray is produced by TiO2 formed in solution/FTO glass interface viaeterogeneous nucleation procedure. So by placing the FTO glassith conducting surface down, TiO2 particles formed by homo-

eneous nucleation will not aggregate on the conducting surfacend TNAs can be achieved. Fig. 1(d) is cross-sectional image of theNAs and shows that the as prepared TNAs are highly ordered, wellriented and grows perpendicular to the substrate. Since conduct-ng surface plays an important role in the fabrication of TNAs, weelected conducting surface facing down in the following studies.

.2. Effect of growth time

Fig. 2(a) shows the XRD peaks of FTO substrate and Fig. 2(b–f)re XRD patterns of TiO2 nanowire arrays fabricated at 180 ◦C for.5 h, 1 h, 2 h, 4 h and 6 h, respectively. Fig. 2(b) shows the same XRDeaks as Fig. 2(a), which illustrates that both the TNA film growingor 0.5 h and the TiO2 compact layer fabricated before solvothermaleaction are so thin that no other peaks appear. For TNA growingor 1 h, diffraction peak at 2� = 62.7◦ is observed, which correspondso the (0 0 2) crystal plane of rutile TiO2 (JCPDS No. 21-1276). Inonger times of 2 h, 4 h, 6 h, the intensity of (0 0 2) peak becomesxtremely strong, which indicates the TNA is well crystallized androws perpendicular to the substrate along the (0 0 2) direction. It isnown that the (1 1 0) plane of rutile has the lowest surface energy14]. In crystal growth, the plane with the lowest surface energywns the fastest growing velocity along the plane, but the slowestrowing velocity perpendicular to the plane. Therefore the (0 0 2)rystal plane survives after the growth.

Fig. 3 indicates the FE-SEM images of TiO2 nanowire arrays fab-icated at 180 ◦C for different duration, and Fig. 1(b3, c3, d3, e3) areross-sectional images of these samples. The TNA synthesized at80 ◦C for 0.5 h is too thin to get a clear cross-sectional image. Athort duration of 0.5 h in Fig. 1(a1 and a2), the stone-like bulks areTO substrate, and small one-dimensional structures tend to growp on FTO substrate. The sample synthesized for 1 h is consistedf big nanorods about 100 ± 20 nm wide with not very uniformhape, and each nanorod is composed of many small strings, whichs too small to identify its width. For reaction times of 2 h, 4 h and

h, highly ordered, well-oriented TiO2 nanowires are fabricated.nd the surface shape grows from needle like for 2 h to rectan-le for 6 h. From the high magnification images, we can tell thessential structure of the sample is a single nanowire, and severalanowires bunch together to form a larger structure that is a bunchf nanowires.

Fig. 4 shows TEM images of TiO2 nanowire arrays fabricated at80 ◦C for 2 h. A single nanowire is around 12 nm wide, and severalanowire bunch together to form a secondary structure. This resultgrees with the FESEM images.

The relationship between a single wire width, the diameter of bunch of wires, the TNA thickness and the reaction duration ishown in Table 1. The nanowire is formed by different growthelocities along axial and lateral directions, and comparison of theingle wire width and film thickness reveals intuitively competi-ion growth in this two directions. In short reaction times of 0.5 hnd 1 h, axial growth is faster than lateral growth, so nanowires

ecome to appear, but not in a uniform and ordered structure. In

onger times of 2 h and 4 h, highly orderly, vertically oriented TiO2rrays are synthesized, with the size of nanowires very uniform.ith the longer reaction period, the average width of a single wire

4 h 18 ± 2 90 ± 10 13.46 h 30 ± 3 No apparent bunch 18.0

increases and the uniformity of nanowire width becomes better.Several nanowires aggregate together to form a bunch, which hasnot been mentioned in literatures before. In even longer time of 6 h,the single nanowire width grows up to 30 ± 3 nm, and the bunchdisappears, as shown in Fig. 3(e2 and e3), comparing with apparentbunches in Fig. 3(c2, c3, d2, d3). This may be attributed to that TiO2grows in a circumstance of severe vibration during solvothermalprocedure, so the neighboring nanowires are easily to touch andconnect with each other and will not separate apart. In prolongedtime of 6 h, nanowires become so thick that they cannot adherewith each other any more, so a single nanowire stand and grow byitself. In short reaction time of 0.5 h, the film is just less than 1 �m

2-Theta

Fig. 5. XRD patterns of TiO2 nanowire arrays fabricated for 2 h at different temper-ature: (a) 140 ◦C, (b) 160 ◦C, (c) 180 ◦C, (d) 200 ◦C, (e) 220 ◦C.

7700 Z. Wei et al. / Electrochimica Acta 56 (2011) 7696– 7702

F rature

3

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ig. 6. FE-SEM images of TiO2 nanowire arrays fabricated for 2 h at different tempe

.3. Effect of growth temperature

Fig. 5 shows the XRD patterns of TiO2 nanowire arrays fabri-ated for 2 h at different temperatures. All the diffraction peaks arescribed to tetragonal rutile phase (JCPDS No. 21-1276). By increas-ng the reaction temperature, peaks for (0 0 2) crystal plane become

: (a and b) 140 ◦C, (c and d) 160 ◦C, (e and f) 180 ◦C, (g and h) 200 ◦C, (i and j) 220 ◦C.

stronger, which indicates that the orientation along (0 0 2) directionis better.

Fig. 6 illustrates the typical FE-SEM images of TiO2 nanowirearrays fabricated for 2 h at different reaction temperatures. Thesamples prepared at different temperatures are also constructed bybunches of nanowires. At low temperatures of 140 ◦C and 160 ◦C,

Z. Wei et al. / Electrochimica Acta 56 (2011) 7696– 7702 7701

Table 2The relationship between a single wire width, the diameter of a bunch of wires, theTNA thickness and the reaction temperature.

A single wirewidth (nm)

Diameter of a bunchof wires (nm)

The TNAthickness (�m)

140 ◦C Less than 10 160 ± 40 5.9160 ◦C Less than 10 100 ± 20 6.2180 ◦C 10 ± 2 60 ± 10 6.8

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nly bulks of nanorods not uniform in shape are found. Whenhe temperature is raised up to 180–220 ◦C, highly ordered, well-riented TiO2 nanowire arrays are achieved.

The relationship between a single wire width, the diameter of aunch of wires, the TNA thickness and the reaction temperature ishown in Table 2. With the higher reaction temperature, the aver-ge width of a single wire increases and the nanowires grow moreniformly. For the temperatures from 140 ◦C to 180 ◦C, the TNAlm thickness grows from 5.9 �m to 6.8 �m that is because therowth velocity increases and also the orientation becomes better.t the temperatures higher than 180 ◦C, the film thickness remainsearly the same, which may because the growth velocity at 180 ◦Cas nearly reached the maximum and increases not so apparentlyt higher temperatures. At the same time the lateral growth veloc-ty also increases at higher temperatures, which hinders the axialrowth too.

.4. Current density–voltage characteristics

Fig. 7 shows the UV–vis adsorption spectra of dye N719 moleculedsorbed on TNAs fabricated at 180 ◦C for 2 h, 4 h and 6 h. Allhe three adsorption peaks appear at around 515 nm, which areypical characteristics of dye N719. The absorption peak intensityecreases with longer reaction time, which illustrates that effectiveurface area to absorb dye molecules becomes smaller at prolongedeaction duration.

Fig. 8 shows the typical current density versus voltage curves ofhe TiO2 nanowire arrays fabricated at 180 ◦C for 2 h, 4 h and 6 h.he photovoltaic behavior of 0.5 h and 1 h is very poor. The short-ircuit current density (J ) of the cell of 2 h, 4 h and 6 h reduces

sc

rom 3.16 to 2.49 to 2.07 mA cm−2. Although longer reaction timeeads to longer nanowire length, the width also increases, and theurface area is much more sensitive to the width than the length, so

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ig. 7. Shows the UV–vis adsorption spectra of dye N719 molecule adsorbed onNAs fabricated at 180 ◦C for 2 h, 4 h and 6 h.

Fig. 8. Current density–voltage curves of TiO2 nanowire arrays fabricated at 180 ◦Cfor different duration: (a) 2 h, (b) 4 h, (c) 6 h.

surface area reduces with longer length. Besides, in longer length,it is more difficult for dye molecules to infiltrate to the bottom ofthe TNAs, because the interspace between nanowires is not roomyenough. The open circuit voltage (Voc) of the cell of 2 h, 4 h and 6 halso reduces from 780 mV to 750 mV to 730 mV. Voc can be derivedfrom Ref. [15]

Voc = mc ln(Q ) (1)

where mc is a constant in the same system. Q is the photoinducedcharge density, and is proportional to Jsc. So higher Voc is achieved atlarger Jsc. In our calculation Voc and ln(Jsc) complies with linear rela-tionship, which is in good agreement with former reports [16,17].Fill factor (FF) of 2 h, 4 h and 6 h are determined by the followingequations

FF = VmJmVocJsc

(2)

where Vm and Jm are the voltage and current density at maximumpower output. The calculation using the J–V curves yield FF at 0.67,0.64 and 0.62, respectively, not having much difference. The overallphotoelectric conversion efficiency of the cells of 2 h, 4 h and 6 h are1.64%, 1.19% and 0.93%.

The efficiency presented here is lower than recent reports(3% and 5%), respectively [5,6]. It is likely that the small inter-space between nanowires results in reduced effective surface areaand thus weak dye adsorption. Further improvement is eagerlyexpected by fabricating titania arrays comprised of thin nanowiresseparating apart from each other.

4. Conclusions

Highly oriented TiO2 nanowire arrays were synthesized directlyon the transparent conducting substrate by solvothermal pro-cedure without any template. The as prepared TNAs have beensystematically studied at various reaction conditions, the essentialstructure of the synthesized TNAs is single nanowires, and severalnanowires bunch together to form a larger secondary structure,namely a bunch of nanowires. FTO substrate should be placed withconducting surface facing down to eliminate the interference ofhomogeneous nucleation of TiO2 in solution. Well-ordered, verti-cally oriented TiO2 nanowire arrays can be achieved at appropriate

conditions (2 h, 4 h, 6 h; 180 ◦C, 200 ◦C, 220 ◦C), and the nanowiresgrow along the (0 0 2) direction. The nanowire width, the width of abunch of nanowires and the nanowire length increases with longerreaction time and higher temperature. In reaction period too short

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r temperature too low, only bulks of nanorods in not regular shapean be obtained. The overall photoelectric conversion efficiency ofhe cells comprised of TNAs prepared at 180 ◦C for 2 h reaches asigh as 1.64%, and the value reduces to 0.93% for longer nanowiresrepared for 6 h because of diminishing of effectual surface area.urther improvement is expected by fabricating titania arrays com-rised of thin nanowires separating apart from each other. Ourtudy has opened the great possibility for morphology controllableutile TiO2 nanowire arrays to apply in many fields including dyeensitized solar cell [6], photocatalysis [18], gas sensors [19] andydrogen generation by water photoelectrolysis [20].

cknowledgements

This work is financially supported by the National High Tech-ology Research and Development Program of (“863” program, No.009AA033701) and Science & Technology Commission of Shang-ai Municipality (08DZ2270500), China.

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