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Electrochimica Acta 89 (2013) 469– 478

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

ontrolling the microstructure and properties of titania nanopowdersor high efficiency dye sensitized solar cells

.E. Shalana,b,∗, M.M. Rashada, Youhai Yub, Mónica Lira-Cantúb, M.S.A. Abdel-Mottalebc

Central Metallurgical Research & Development Institute (CMRDI), Electronic and Magnetic Materials Division, Advanced Materials Department, P.O. Box 87, Helwan, Cairo, EgyptCentre de Investigacio en Nanociencia I Nanotecnologia (Cin2, CSIC), ETSE, Campus UAB, Edifici Q, 2nd Floor, Bellaterra (Barcelona) E-08193, SpainNano-Photochemistry and Solar Chemistry Lab, Department of Chemistry, Faculty of Science, Ain Shams University, 11566 Abbassia, Cairo, Egypt

r t i c l e i n f o

rticle history:eceived 11 September 2012eceived in revised form1 November 2012ccepted 23 November 2012vailable online 29 November 2012

eywords:itania nanoroditania nanoparticleemiconducting metal oxide electrode

a b s t r a c t

A low temperature hydrothermal process have been developed to synthesize titania nanorods (NRs)and nanoparticles (NPs) with controlled size for dye sensitized solar cells (DSSCs). Effect of calcinationtemperature on the performance of TiO2 nanoparticles for solar cells was investigated and discussed.The crystallite size and the relative crystallinity of the anatase phase were increased with increasing thecalcination temperature. The structures and morphologies of both (TiO2 nanorods and nanoparticles)were characterized using XRD, SEM, TEM/HRTEM, UV–vis Spectroscopy, FTIR and BET specific surfacearea (SBET) as well as pore-size distribution by BJH. The size of the titania nanorods was 6.7 nm widthand 22 nm length while it was 13 nm for nanoparticles. Efficiency of dye-sensitized solar cells (DSSCs)fabricated with oriented TiO2 nanorods was reported to be more superior compared to DSSC based onmesoporous TiO2 nanoparticles due to their high surface area, hierarchically mesoporous structures, low

emperature effectye sensitized solar cell

charge recombination and fast electron-transfer rate. With increasing calcination temperature of theprepared nanopowders, the light-electricity conversion efficiency (�) decreased. The efficiency of theassembly solar cells was decreased due to the agglomeration of the particles and difficulty of electronmovement. The power efficiency was enhanced from 1.7% for TiO2 nanoparticles cells at hydrothermallytemperature 500 ◦C and 5.2% for TiO2 nanoparticles cells at hydrothermally temperature 100 ◦C to 7.2%for TiO2 nanorods cells under AM1.5 illumination (100 mW cm−2).

. Introduction

Dye-sensitized solar cells (DSSCs) have recently attracted con-iderable attention due to their promising potential for highfficiency and low production cost [1,2]. Efficiency of solar cellsepends on its light harvest efficiency, the quantum yield for charge

njection, and the charge collection efficiency at the electrodes3]. Since Grätzel reported the efficient TiO2-based DSSCs, manyttempts have been made to enhance the photon–electron con-ersion efficiency of this low-cost solar cell [4–8]. It has beenemonstrated that to improve the conversion efficiency, it is nec-

ssary to increase the TiO2 layer thickness [9]. However, due tohe lack of a depletion layer on the TiO2 nanocrystallite surface, aurther increase in TiO2 layer thickness results in higher electron

∗ Corresponding author at: Central Metallurgical Research & Development Insti-ute (CMRDI), Electronic and Magnetic Materials Division, Advanced Materialsepartment, P.O. Box 87, Helwan, Cairo, Egypt. Tel.: +20 2 25010640x43;

ax: +20 2 25010639.E-mail addresses: a.shalan133@gmail.com, genral zero@yahoo.com

A.E. Shalan).

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2012.11.091

© 2012 Elsevier Ltd. All rights reserved.

recombination and consequently a further increase in conversionefficiency has been limited by electron recombination during thecharge transport process [10]. In this respect, to enhance the elec-tron transport properties in the photoanode, a highly ordered,vertically oriented TiO2 nanorods array of different aspect ratiosand surface qualities provides better replacement to substitutethe sintered TiO2 nanoparticle films (as shown in Fig. 1a) usedin DSSCs [11–15]. Generally, nanostructured TiO2 materials likenanorods and nanoparticles can be prepared using dry and wetprocesses. The effect of reaction conditions, such as reactants, reac-tion medium, temperature, and pH of solution, can be chosen inthe wet processes. Therefore, the crystallite size, crystal shape,and surface structure of the nanocrystals can be controlled moreeasily than that in the dry processes [16,17]. Sol–gel and hydro-thermal processes are the most popular methods for synthesis ofTiO2 nanocrystals, in which titanium salts hydrolyzed in solutionand then crystallized [18–22]. In these processes, the crystallite sizecan be controlled by crystal growth rate, while the crystal shape is

strongly depended on the crystal growth direction that is not easyto control in normal cases [23]. Up to now, most studies on the pho-tocatalytic properties of TiO2 nanocrystals have carried out on theeffects of surface area and crystallinity, and the results suggest that

470 A.E. Shalan et al. / Electrochimica Acta 89 (2013) 469– 478

iO2 na

algpnpTpafWb2spmtTmbdetn1Fpstocmpnsp

2

2

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Fig. 1. (a) A highly ordered, vertically oriented TiO2 nanorods compared with T

large surface area and a high crystallinity of TiO2 nanocrystalsead to a high photocatalytic activity [24]. Among renewable ener-ies, DSSCs created by Grätzel in 1991 have a recognized and simplerocess [25,26]. DSSCs are of great interest as a cost-effective alter-ative to conventional silicon photovoltaics [27]; they consider arominent member of the larger group of thin film photovoltaics.he improvement in efficiency is due to the large surface area of theorous film, more efficient light absorption of the dye molecules,nd faste electron transport and/or transfer between the inter-aces of film material/electrolyte/counter electrode within DSSCs.

e report successful preparation of nanocrystalline TiO2 powdersy hydrothermal process at hydrothermal temperature 100 ◦C for4 h (as made sample), and compared such sample by the formedamples calcined at different temperatures from 200 to 500 ◦C,rocessing, device fabrication and assembly, solar cell performanceeasurements, and device modelling. In our study we made a new

rend by calcination the titania nanopowders before assembly theiO2 electrode. Although in the most other studies the authorsake calcination for the electrode after assembly [28]. From the

est of our knowledge, the study of the calcination of TiO2 pow-ers synthesized via hydrothermal pathway before assembly TiO2lectrode and its effect on the photovoltaic efficiency is not men-ioned before. In this article, we report on synthesis of titaniaanorods, nanoparticles at low temperature hydrothermal process00 ◦C. The formed titania nanorods were characterized using SEM,TIR, PL, UV-Spectroscopy, XRD, HR-TEM, and SBET. Moreover, theroduced titania nanorods were utilized to fabricate DSSCs. Wetudy the effect of calcination temperature for the powders syn-hesized by hydrothermal route before assembly the TiO2 electroden the photovoltaic efficiency of the dye sensitized solar cells. Thehange in crystal structure, crystallite size, specific surface area,icrostructure and optical properties was investigated. We com-

ared the solar cell performance of both TiO2 nanoparticles andanorods and observed higher solar cell efficiency with nanorodtructures than nanoparticles due to the physical and electronicroperties of nanorods and nanoparticles.

. Experimental

.1. Chemicals and materials

Titanium isopropoxide [Ti (OCH (CH3)2)4] (99.99%), (Sigmaldrich) was used to synthesize anatase TiO2 nanorods andanoparticles via hydrothermal method. Pure ammonia solution

33%), (Fluka) was used as a base. Fluorinated tin oxide (FTO)Asahi Glass Co. Ltd.) and indium tin oxide (ITO) glass sub-trates were bought from SOLEMS, 10 and 70 � cm−2, cleanedith soap water, mili-Q water, acetone and ethanol (99.5%) for

nopaticles and (b) Dye sensitized solar cell fabricated using sealing technique.

10 min before use. The substrates were dried under an N2 flux andfinally cleaned for 20 min in a UV-surface decontamination system(Novascan, PSD-UV) connected to an O2 gas source. O2 (BIP qual-ity) and N2 (BIP quality, <0.02% O2) were purchased from CarburosMetalicos (Air Products) and used at <0.5 bar pressure. The dyesolution was prepared by dissolving 0.17 mM cis-di (thiocyanato)bis (2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium-(II) (also calledN719, Solaronix SA, Switzerland) in dry ethanol (from SigmaAldrich). An electrolyte was made with 0.1 M lithium iodide (LiI)(Aldrich), 0.1 M iodine (I2) (Aldrich), 0.6 M tetrabutylammoniumiodide (Fluka), and 0.5 M tert-butylpyridine (Aldrich) in dry ace-tonitrile (Fluka).

2.2. Procedure

2.2.1. Preparation of TiO2 nanorods, nanoparticles10 ml titanium isopropoxide [Ti (OCH (CH3)2)4] was added to

100 ml distilled water under vigorous stirring for 10 min. Then,ammonia solution was dropped until stable pH 7 and pH 4. Afterstirring for another 10 min to achieve more homogeneity, themixed solution was transferred into a 150 ml Teflon-lined stainlesssteel autoclave. Then, the produced solutions were hydrothermallytreated at 100 ◦C for 24 h. After cooling down to room temperature,samples was washed with deionized water several times and driedat 80 ◦C overnight. Then, the as made samples were calcined at dif-ferent temperatures from 200 to 500 ◦C for 2 h to study the effectof temperature on the efficiency of DSSCs.

2.2.2. Preparation of photoanodes1.0 g portion of TiO2 (nanorods) powder was mixed with 1.0 ml

distillated water, 5 ml absolute ethanol, and stirred using hot platemagnetic stirrer for 10 h. Fluorinated tin oxide (FTO) glass sub-strates were cleaned with soap water, mili-Q water, acetone andethanol (99.5%) for 10 min before use. The substrates were driedunder an N2 and finally cleaned for 20 min in a UV-surface decon-tamination system. Deposit a TiO2 blocking layer using titaniumtetrachloride (TiCl4). For the reason for processing TiCl4 at eachelectrode, the initial TiCl4 treatment influences positively the TiO2working electrode in two manners, firstly through enhancing thebonding strength between the FTO substrate and the porous-TiO2layer, and secondly, by blocking the charge recombination betweenelectrons emanating from the FTO and the I3− ions present inthe I−/I3− redox couple. Cover the FTO glass with tape as spacerand then coated with TiO2 paste by doctor blade method. The

thickness of titania film can be controlled by changing the con-centration of the paste and the layer numbers of the adhesive tape(Scotch, 50 �m). For comparison, electrode using (nanoparticles)was also prepared by the doctor blade technique. The films were

himica Acta 89 (2013) 469– 478 471

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ried under ambient conditions and then annealed at 450 ◦C for0 min to remove the binders in the paste and to increase the crys-allinity of the nanorods. Then, the films were cooled down to 80 ◦Cor dye sensitization. Dye sensitization was achieved by immers-ng the TiO2 nanorods electrodes in a 0.3 mM N719 dye (Solaronix)n ethanol solution overnight, followed by rinsing in ethanol andrying in air.

.2.3. Fabrication of DSSCs based on the nanorods TiO2 andanoparticles TiO2

The sensitized TiO2 film (nanorods or nanoparticles) was rinsedith ethanol and assembled with a platinum covered FTO elec-

rode (TEC 15, 15 � cm−2) containing a hole into a sandwich-typeonfiguration using sealing technique (as shown in Fig. 1b). Theounter electrode was prepared by adding 50 nm layer of plat-num (Pt) on the FTO surface using spin coating pyrolysis technique.he two electrodes sealed with a 25 �m thick polymer spacerSurlyn, DuPont). The void between the electrodes then filled withn iodide/tri-iodide based electrolyte, containing 0.1 M lithiumodide, 0.1 M iodine, 0.6 M tetrabutylammonium iodide, and 0.5 Mert-butylpyridine in dry acetonitrile in 1:1 acetonitrile propylenearbonate by firm press, via air pump vacuum backfilling through aole pierced through the Surlyn sheet. The hole then sealed with andhesive sheet and a thin glass to avoid leakage of the electrolyte.he resulting cell had an active area of ∼0.25 cm2.

.3. Physical characterization

The crystallite phases of different samples were identifiedy X-ray diffraction (XRD) using a step size of 0.02◦ and at

scanning rate of 0.16◦/min on a Brucker axis D8 diffrac-ometer with crystallographic data software Topas 2 using Cu� radiation operating at accelerating voltage and applied cur-ent were 40 kV and 80 mA, respectively. The diffraction dataas recorded for 2� values between 20◦ and 80◦. The crys-

allite size of anatase titania nanorods was calculated applyingcherrer formula for the most intense peak of (1 0 1) plane.he particles morphologies were performed using the scan-ing electron microscope (JEOL, SEM, JSM-5400). Transmissionlectron microscopy (TEM) and high resolution transmissionlectron microscopy (HRTEM) were recorded with a JEOL-JEM-230 microscope. Nitrogen adsorption–desorption isotherms wasbtained on an ASAP 2020 (Micromeritics Instruments, USA) nitro-en adsorption apparatus. All the samples degassed at 180 ◦Crior to Brunauer–Emmett–Teller (BET) measurements. The spe-ific surface area (SBET) was determined by a multipoint BETethod using the adsorption data in the relative pressure P/P0

ange of 0.05–0.25. The desorption isotherm used to deter-ine the pore-size distribution using the Barret–Joyner–Halender

BJH) method. The UV–vis absorption spectrum was recordedy a UV–vis–NIR scanning spectrophotometer (Jasco-V-570 Spec-rophotometer, Japan) using a 1 cm path length quartz cell. Fourierransformer infrared absorption spectrum (FTIR) was performedy JASCO 3600 spectrophotometer in the wave number range200–4000 cm−1). Photocurrent–voltage J–V characteristic curves

easurements were investigated using the solar simulation whicharried out with a Steuernagel Solarkonstant KHS1200. Light inten-ity was adjusted at 1000 W/m2 with a bolometric Zipp & KonenM-4 pyranometer. Calibration of the sun simulator was made byeveral means: with a calibrated S1227-1010BQ photo diode fromamamatsu and a mini spectrophotometer from Ava-Spec 4200.

he AM1.5 simulated sunlight reference spectrum was accordingo an ASTM G173 standard. Solar decay and IV-curves were mea-ured using a Keithley2601 multimeter connected to a computernd software.

Fig. 2. XRD patterns of (a) TiO2 nanorods (NRs), (b) TiO2 nanoparticles (NPs), (c)TiO2 nanoparticles annealed at 200 ◦C, (d) TiO2 nanoparticles annealed at 300 ◦C, (e)TiO2 nanoparticles annealed at 400 ◦C and (e) TiO2 nanoparticles annealed at 500 ◦C.

The photoelectric conversion efficiency (�) was calculatedaccording to the following equation:

�% = Jsc · Voc · FFPin

· 100 (1)

where the fill factor (FF) is the ratio between the maximum outputpower density available (Jm·Vm) and the maximum power com-bining short-circuit and open-circuit situations (Eq. (2)) and itdescribes the “squareness” of the J–V curve.

FF% = Jm · Vm

Jsc · Voc· 100 (2)

The incident monochromatic photoelectric conversion effi-ciency (IPCE) analyses was carried out using a QE/IPCE mea-surement system from Oriel at 10 nm intervals between 300and 700 nm, where a monochromator was used to obtain themonochromatic light from a 300 W Xe lamp. The IPCE is definedas mention in the following equation:

IPCE(%) = 12, 400 × Jsc(�A cm−2)

�(nm) × Pin(�W cm−2)(3)

A calibrated photodiode (S1227-1010BQ from Hamamatsu) wasused before each IPCE analyses. In the above two formulas, � isthe global efficiency, Voc, Jsc, and FF are open circuit voltage, shortcircuit current density, and fill factor, respectively. Pin and � are thelight energy and wavelength of the incident monochromatic light,respectively.

3. Results and discussion

3.1. Crystal structure

The effect of pH on the conversion, of anatase phase was deter-mined at constant temperature of 100 ◦C and after time of 24 h.XRD patterns of as-prepared TiO2 nanopowders precipitated at pH7 and pH 4 hydrothermally treated at 100 ◦C for 24 h compared

with XRD patterns of with as made sample calcined at differenttemperatures from 200 to 500 ◦C for 2 h are given in Fig. 2. It isclear that the crystal phase of the final product derived from thehydrothermal method related to anatase TiO2 (JCPDS# 04-0477).

472 A.E. Shalan et al. / Electrochimica

Table 1Effects of calcination temperature on crystalline size, relative crystallinity and bandgap energy of anatase TiO2 nanoparticles.

Temp. (◦C) Crystallite size(nm)

Relativecrystallinity

Band gapenergy, (eV)

100 12.3 1.00 3.30200 13.3 1.02 3.25300 14.4 1.16 3.20400 15.2 1.50 3.15

Ttm1tcTupip

cles calcined at 300 ◦C and 500 ◦C is smooth (as shown in Fig. 3c

Fn

500 16.3 1.78 3.10

he crystallite size of the formed titania nanorods and nanopar-icle powders given by applying Debye-Scherrer formula for the

ost intense peak plane (1 0 1) of the anatase TiO2. It was 3 and3 nm for the sample precipitated at pH 7 and 4. It was found thathe surface area of the obtained nanorods more than nanoparti-les which indicate that the efficiency of DSSCs fabricated usingiO2 nanorods will be more than the efficiency of DSSCs fabricatedsing TiO2 nanoparticles. With further increase in calcination tem-

eratures from 100 to 500 ◦C, the peak intensities of anatase phase

ncrease, indicating the enhancement of crystallization of anatasehase and the growth of crystallites.

ig. 3. SEM micrographs of the produced powders (a) Titania spheres particles, (b) asanoparticles annealed at 500 ◦C.

Acta 89 (2013) 469– 478

Table 1 lists the average crystalline sizes and relative anatasecrystallinity of TiO2 samples prepared at different calcination tem-peratures. It is clear that the average crystalline sizes and relativeanatase crystallinity was increased by increasing the calcinationtemperature.

3.2. Microstructure

Fig. 3 shows the SEM images of titania nanorods and titaniananoparticles in comparison with such calcined at 300 and 500 ◦C.In Fig. 3a, it is observed that the titania nanoparticles were homo-geneous and good interconnection formed between particles. Inaddition to a small amount of aggregated nanoparticles, proba-bly coming from precursors, most titania nanoparticles spheresstill keep with original spherical morphology, confirming the pre-pared titania nanoparticles spheres powders with high mechanicalstrength. From the SEM analysis of as-prepared TiO2 nanorods sam-ple, (as shown in Fig. 3b), it is seen that the particles appearedas nanorods like structure. The surface of the TiO2 nanoparti-

and d). After calcination, the film surfaces become rough withthe appearance of some pores. Many small nanoparticles was sin-tered together as illustrated by the enlarged area. The connection

-prepared TiO2 nanorods, (c) TiO2 nanoparticles annealed at 300 ◦C and (d) TiO2

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f the small nanoparticles at high calcination temperatures (i.e.00 and 500 ◦C) are affecting on the electron transport withinhe TiO2 nanoparticles. Moreover, the particles were fine andegularly agglomerated. Two possible mechanisms are worthy ofonsideration during the crystallization of the surface amorphoustructures. The first one is that the surface amorphous structuresrystallized in situ and the small nanoparticles merged and/or con-ected into larger ones. The second possible mechanism is thathe surface amorphous structure of some particles diffused quicklyo the surface of the other particles and then crystallized. Fur-hermore, the larger particle size in the porous structure may bescribed to the physical accumulation of the small nanoparticles.

The mesopouros characteristics of the TiO2 nanorods parti-les were confirmed by TEM micrographs as shown in Fig. 4a.t is clear that the produced TiO2 nanorods particles exhibited

regular morphology consisting in the rods-like structure. Theimensions of the obtained powders was 6.7 nm width and 22 nm

ength and the average particles size found between 3 and 5 nm,hich was a good agreement with the crystallite size given based

n Debye–Scherrer formula obtained from XRD patterns data. Theigh-resolution transmission micrograph (HRTEM) image of a TiO2anorods (Fig. 4b) shows a lattice fringes with an interplanar spac-

ng of 0.352 nm, which corresponds to the (1 0 1) plane of anatasehase. Furthermore, (Fig. 4c–e) displays TEM micrographs of the asade TiO2 nanoparticles sample hydrothermally at 100 ◦C for 24 h

ompared with as made sample calcined at 300 and 500 ◦C. Thenatase TiO2 nanoparticles consists of spherical particles that growith increasing calcination temperature. The mesoporous charac-

eristics within nanoparticles indicating that TiO2 nanoparticles hasomparatively good stability during the hydrothermal process. Fur-hermore, The amorphous structure was completely crystallizedfter calcination at 500 ◦C for 2 h. EDX curve of the calcined prod-ct (Fig. 4f, is supporting the information) reveals the presence ofi and O as the only elementary components and the atomic ratiof Ti and O was about 1:2. Traces amount of Cu was found due tohe grid of the sample.

.3. Optical properties

Figs. 5 and 6 indicate the transmittance spectra of titaniaanorods and titania nanoparticles (hydrothermally treated at00 ◦C for 24 h) in comparison with such calcined samples at dif-erent calcination temperatures. The transmittance T% spectrumecorded for as prepared TiO2 nanoparticles and nanorods com-ared with such calcined samples in the range of 200–600 nm isiven in Fig. 5. The TiO2 nanorods showed an absorption thresholdt less than 400 nm (3.2 eV). It can be observed that the reflectivityf the TiO2 nanorods reaches its minimum value at the wavelengthf 320 nm. In the current measurement, the onset of absorptionas at around 300 nm which is in agreement with those reported

y others for anatase-TiO2 nanorods [29,30]. The mechanism of UVbsorption in such materials involves the use of photon energy toxcite electrons from the valence band to conduction band. It is seenhat with increasing calcination temperature, the transmittancedge was shifted to the lower wavelength direction. The opticalandgap energy Eg was determined from the absorption coefficient, and by plotting (˛h�)1/2 vs. h� (for indirect allowed transitions)nd (˛h�)2 vs. h� (for direct allowed transitions) where h is thelanck constant and � is the frequency. The absorption coefficientor TiO2 nanocrystals was determined applying the following equa-ion:

m

h�) = h� = Eg (4)

here is the absorption coefficient, h� is the photon energy, Eg ishe band gap energy, m = 1/2 or 3/2 for indirect allowed and indirectorbidden transitions, and m = 2 or 3 for direct allowed and direct

Acta 89 (2013) 469– 478 473

forbidden transitions. The results illustrate that the ideal straightline plot extended over most data points was (˛h�)2 vs. h� to get theband gap energy, as shown in Fig. 6. However, the band energy gapswere in the range from 3.3 to 3.1 eV with increase the calcinationtemperature from 100 to 500 ◦C as shown in Table 1.

The blue shift in the absorption band edge with small valuehas been claimed as a consequence of exciton confinement withdecrease particle size (the so-called quantum-size effect) in TiO2. Itwas reported that the blue shift of optical energy gap is attributedto the change of the energy gap of the disorder crystal in TiO2nanopowders [31]. The particle size of TiO2 in anatase phase wasincreased from 12.5 to 16.5 nm with the increasing the calcinationtemperature from 100 to 500 ◦C, while the corresponding opticalenergy gaps was varied about 0.19 eV. As temperature increases,the band gap energy was decreased because the crystal latticeexpands and the interatomic bonds were weakened. Weaker bondsmeans less energy is needed to break a bond and get an electron inthe conduction band [32].

The FT-IR spectrum of obtained samples is shown in Fig. 7. PureTiO2 has a strong and broad band in the range of 400–1000 cm−1,due to Ti O stretching vibration modes, as the result of TiO2 anatasephase [33]. The absorption band of Ti O stretching vibration modeswas observed to shift towards lower wave number for titaniananoparticles. In addition, weak features observed at 1633 and3436 cm−1, due to OH binding and stretching modes of adsorbedwater, respectively [34]. However, the peaks related to hydroxylgroup were disappeared by increasing the calcination temperature.

3.4. Brunauer–Emmett–Teller (BET) surface area (SBET)measurement

Fig. 8 shows nitrogen adsorption–desorption isotherms andthe corresponding pore-size distribution curve (inset) of titaniananoparticles and as prepared TiO2 nanorods in comparison withsuch calcined samples at different calcination temperatures. Allthe samples show the type IV isotherms with type H2 hysteresisloops according to Brunauer–Deming–Deming–Teller (BDDT) clas-sification [35,36], indicating the presence of mesoporous structure(2–50 nm). Moreover, with as prepared TiO2 nanorods, the hys-teresis loops shifted to the region of higher relative pressure andthe position of the hysteresis loops was gradually decreased, sug-gesting that the pore size decrease and the specific surface areasincrease. Furthermore, the results are attributed to the increase incrystallite size in TiO2 nanopowders with high temperature than inTiO2 nanopowders with low temperature. However, with increas-ing calcination temperature, the crystallinity, average crystallitesize and pore size increase. The high surface area can provide moresites for dye adsorption, while fast photoelectron-transfer chan-nel can enhance the photogenerated electron transfer to completethe circuit. The specific surface area SBET was 77.14 m2 g−1 for TiO2nanorods while it was 35.5 m2 g−1 for titania nanoparticles. Table 2lists the physical properties of both titania nanoparticles and TiO2nanorods compared with (Nps) annealed at 500 ◦C which can beevaluated from the SBET measurements, as it found the differencein specific surface areas, porosity, relative anatase crystallinity inboth materials.

3.5. Photovoltaic performance

In this paper the titania nanorods synthesized have a differ-ent shape compared with titania spheres and this affect on themovement of the electrons. Hence, by facilitating the movement

of the electrons within the materials, the efficiency is improvedeven though the surface area is not large.

Fig. 9 shows the J–V curves of DSSCs fabricated with the titaniananoparticles based electrodes and of the obtained TiO2 nanorods.

474 A.E. Shalan et al. / Electrochimica Acta 89 (2013) 469– 478

Fig. 4. TEM micrographs of the produced powders (a) TiO2 nanorods, (b) HRTEM image of TiO2 nanorods shows diffraction patterns and lattice, (c) TiO2 nanoparticles at100 ◦C, (d) TiO2 nanoparticles annealed at 300 ◦C, (e) TiO2 nanoparticles annealed at 500 ◦C and (f) EDX spectrum of the TiO2 nanopowders product.

Table 2Physical properties of TiO2 nanorods and nanoparticles compared with (NPs) annealed at 500 ◦C.

Sample SBET (m2 g−1) Crystallite size(nm)

Pore volume(cm3 g−1)

Average poresize (nm)

Porosity (%) BJH adsorption(nm)

BJH desorption(nm)

TiO2 (NPs) 50.10 30 0.41 5.45 65 4.76 3.66TiO2 (NRs) 77.14 3 0.16 8.81 40 6.90 5.26(NPs) at 500 ◦C 35.50 16.3 0.11 7.3 35 4.55 5.95

A.E. Shalan et al. / Electrochimica Acta 89 (2013) 469– 478 475

Table 3Comparison of the I–V characteristics of DSSCs made from TiO2 nanorods and nanoparticles compared with (NPs) at different temperatures.

Sample Voc Jsc (mA/cm2) FF � (%) Active area (cm2)

TiO2 (NRs) 0.734 13.23 73.68 7.2 0.25TiO2 (NPs) 0.743 9.73 71.35 5.2 0.188(NPs) at 200 0.696 7.01 71.16 3.4 0.191(NPs) at 300 0.718 5.35 71.07 2.7 0.205(NPs) at 400 0.690 4.61 65.55 2.1 0.215(NPs) at 500 0.672 3.42 70.32 1.7 0.212

200 30 0 40 0 50 0 60 0

0

10

20

30

40

50

60

70

80

90

Inte

nsity (

T%

)

Wavelen gth ( nm)

(a)

(c)

(b)

(d)(e)

(f)

Fig. 5. UV–vis transmittance spectrum T% of (a) as-prepared of TiO2 nanorods, (b)T(

TteodtDt

4000 350 0 300 0 250 0 200 0 150 0 100 0

80

85

90

95

100

105

waven umber (cm-1)

transm

issio

n (

T%

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(d)(e)

(f)

(c)(b)

(a)

(a) (NRs), (b) (NPs), (c) 200oC

(d)300oC, (e)4 00

oC,( f)500

oC

Fig. 7. FT-IR spectra of (a) TiO2 (NRs), (b) TiO2 (NPs), (c) TiO2 nanoparticles annealed

F1

iO2 nanoparticles, (c) TiO2 nanoparticles annealed at 200 ◦C, (d) annealed at 300 ◦C,e) annealed at 400 ◦C and (f) annealed at 500 ◦C.

he values of Voc, Jsc, FF, and conversion efficiencies (�) of the elec-rodes are shown in Table 3. All DSSCs fabricated titania nanorodslectrodes display higher Voc as compared to the solar cell basedn titania nanoparticles electrode. Generally, Voc of a solar cell is

etermined by the difference between the Fermi level for elec-rons in the TiO2 electrode and the redox potential of I3−/I−.ue to the one-dimensional structure of the present nanorods,

he charge recombination could decrease and charge transfer rate

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2

(b) 3.15 ev(a) 3.1 ev

(αh

υ )

2

hυ ( eV)

ig. 6. Band gap spectrum of (a) the prepared TiO2 nanoparticles annealed at 500 ◦C, (b00 ◦C.

at 200 ◦C, (d) TiO2 nanoparticles annealed at 300 ◦C, (e) TiO2 nanoparticles annealedat 400 ◦C and (f) TiO2 nanoparticles annealed at 500 ◦C.

could increase because of its straight charge transfer path dividedby the nanorods, which result in an increase of electron densityin TiO2 electrode, and thus the positive shift of Fermi level. Fur-thermore, it is reasonable to think the relatively larger size of thepresent nanorods than titania nanoparticles could enlarge the poresize of the film to improve the diffusion of liquid electrolyte, which

is beneficial for electron transfer in I3−/I− electrolyte [37]. Theabove-mentioned low electron recombination and fast electrolytediffusion are beneficial for lowering the concentration of I3− and

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2

(d) 3.25 ev

(e) 3.3 ev(c) 3.2 ev

hυ (e V)

) annealed at 400 ◦C, (c) annealed at 300 ◦C, (d) annealed at 200 ◦C and (e) NPs at

476 A.E. Shalan et al. / Electrochimica Acta 89 (2013) 469– 478

0 20 40 60 80 100

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40 TiO

2 (NPs)

TiO2 (NRs)

(NPs) at 500 oC

(b)

Po

re v

olu

me

(cc/g

)

Pore di ameter (nm)

0.0 0.2 0.4 0.6 0.8 1.0

0

20

40

60

80

100

120

(a)

TiO2 (NPs)

TiO2 (NRs)

(NPs) at 500oC

Vo

lum

e a

dso

rbe

d (

cm

3/g

)

Relative Pr ess ure P /PO

F g porh

tIfindJwrDf

vtvTos

F(24

increasing of Isc observed in Fig. 9 when the cell is illuminatedat (non monochromatic) 1-sun light intensity. Although the effi-

ig. 8. (a) N2 adsorption–desorption isotherms surface area and (b) correspondinydrothermally at low temperature compared to nanoparticles annealed at 500 ◦C.

he Voc depends logarithmically on the inverse concentration of3

−[38]. As can see in Table 3, Jsc shows an increasing behaviourrst from 11.9 to 13.12 mA cm−2 when the cell fabricated using tita-ia nanorods in compared with titania nanoparticles sample. Theependence of � on the nanorods content is consistent with that of

sc, which achieves the maximum energy conversion efficiency 7.2%ith the cell of TiO2 nanorods and then decreases when the paste

eplaced with titania nanoparticles. Apparently the performance ofSSCs in our case is more dependent on Jsc behaviour than other

actors.At calcination temperature 500 ◦C, the lowest photoelectric con-

ersion efficiency (�) was observed (� = 1.72%). There are at leastwo factors indicating the reasons for the lowest photoelectric con-ersion efficiency (�) at 500 ◦C. The first reason is that the calcined

iO2 nanopowders were aggolomerated together and then affectn the distribution of the particles in the surface. The second rea-on was the amount of light absorbed was decreased which affect

0.0 0. 2 0. 4 0. 6 0. 8

0

2

4

6

8

10

12

14(a) TiO

2 (NRs)

(f) 50 0 oC ( η=1.72 %)

(e) 40 0 oC ( η=2.1%)

(d) 30 0 oC ( η=2.7%)

(c) 20 0 oC (η=3.4%)

(b) 10 0 oC ( η=5.1%)

Vol tag e ( V)

Photo

curr

ent (m

A c

m-2)

ig. 9. Comparison of the I–V characteristics of DSSCs made from (a) TiO2 nanorods,b) the prepared TiO2 nanoparticles at 100 ◦C, (c) TiO2 nanoparticles annealed at00 ◦C, (d) TiO2 nanoparticles annealed at 300 ◦C, (e) TiO2 nanoparticles annealed at00 ◦C and (f) TiO2 nanoparticles annealed at 500 ◦C.

e size distribution of the produced TiO2 nanorods and nanoparticles synthesized

on efficiency of the cells. However, the overall conversion efficien-cies of DSSCs were decreased from 5.2 to 1.72% with increasing thecalcination temperature from 100 to 500 ◦C as shown in Fig. 9 andTable 3. The overall conversion efficiency of TiO2 as made samplewith active area of 0.2 cm2 was 5.2%, which was higher than that ofTiO2 nanopowders calcined at 500 ◦C (� = 1.72%).

Fig. 10 depicts the IPCE of DSSCs made from titania nanopar-ticles and TiO2 nanorods films in comparison with such calcinedsamples at different calcination temperatures as a function of wave-length. At the maximum value of the IPCE spectra at 535 nm, theIPCE of titania nanoparticles is slightly higher than that of theTiO2 nanorods film. This increase is in good agreement with the

ciency of TiO2 nanorods film is higher than the efficiency of titaniananoparticles due to the crystallinity and orientation of nanorods

300 40 0 50 0 60 0 70 0 80 0

0

20

40

60

80

100

(f)

(e)

(b)

(d)

(c)

(a)

QE

%

Wav ele ngth (nm)

Fig. 10. IPCE of DSSCs made from (a) the prepared TiO2 nanoparticles at 100 ◦C,(b) TiO2 nanorods, (c) TiO2 nanoparticles annealed at 200 ◦C, (d) TiO2 nanopar-ticles annealed at 300 ◦C, (e) TiO2 nanoparticles annealed at 400 ◦C and (f) TiO2

nanoparticles annealed at 500 ◦C.

himica

itiTbthnetiHtTtFccrrlpctsedvwamt

4

s

1

2

3

4

5

6

[

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A.E. Shalan et al. / Electroc

n the titania film which provide a faster electron transfer rate dueo a decreased number of contact barriers between particles. Its found that IPCE of titania nanoparticles is higher than that ofiO2 nanorods film DSSC. This attributed to TiO2 that nanorodsased electrode not only provide a straight path for the electronransport but also function as scattering particles to increase lightarvesting efficiency of the solar cell. Light scattering from TiO2anorods, extends the distance that light travels within the photolectrode film and provides the photons with more opportunitieso be absorbed by the dye molecules. This leads to a significantncrease in the light-harvesting capability of the photo electrode.owever, the results were due to the scattering only occur when

he particle size is comparable to the wavelength of incident light.his should be attributed to the reduced charge recombination andhe sufficient scattering effect of the nanorods in the film electrode.urthermore, the present of nanorods scattering layer lowers theell IPCE performance. It is can be inferred that the TiO2 nanorodsan also reflect the incident light as scattering particles due to theirelatively small particles, most of the penetrating light has beeneflected inner the film before reaching the nanorods scatteringayer [39]. All the results indicate the present TiO2 nanorods areromising in enhancing the performance of dye sensitized solarells. The IPCE was decreased significantly with the calcinationemperature of the TiO2 nanopowders. The low IPCE values of theample calcined at 500 ◦C annealed sample at the high- and low-nergy side of the peak maximum is attributed to the relatively lowye concentration (poor light absorption by the cell). The low IPCEalues of the samples with increasing the calcination temperatureere as the result of agglomeration of the powders at high temper-

ture which make the amount of light absorbed decrease and theovement of electrons within the cell decrease then decreasing

he efficiency and IPCE values of the produced cells.

. Conclusion

Based on our experimental results, the most remarkable conclu-ions were summarized as the following:

. Titania nanorods and nanoparticles with high surface area andnarrow pore size distribution have been synthesized using lowtemperature hydrothermal method.

. The effect of calcination temperature of the hydrothermally syn-thesized TiO2 nanopowders on crystal structure, crystallite size,surface area, microstructure, optical properties and photovoltaicefficiency of the formed solar cell was investigated and discussedin details.

. The size of the formed titania nanorods was 6.7 nm width and22 nm length, the specific surface area SBET was 77.14 m2 g−1.Furthermore, SBET of the formed powders was decreased from80.9 to 50.1 m2 g−1 whereas BJH adsorbtion was decreased from6.17 to 4.55 nm with increasing the calcination temperaturefrom 100 to 500 ◦C.

. With increasing calcinations temperature, the crystallinity, aver-age crystallite size and pore size increase.

. The oriented nanorods with appropriate lengths are beneficialto improve the electron transport property and thus lead to theincrease of photocurrent, together enhancing the power conver-sion efficiency due to higher surface area and fastest interfacialcharge transfer.

. Under 100 mW cm−2-simulated sunlight, the titania nanorodsDSSC showed solar energy conversion efficiency of 7.2%,

open circuit voltage Voc = 0.73 V, short current densityJsc = 13.23 mA cm−2, and fill factor = 0.73. In addition to, theefficiency of TiO2 nanostructures DSSCs rapidly was decreasedwith increasing the calcinations temperature from 100 to 500 ◦C.

[

Acta 89 (2013) 469– 478 477

Acknowledgements

This work was financially supported by the Academy of Sci-entific Research and Technology (ASRT) and Ministry of ScientificResearch of Egypt. Ahmed Shalan acknowledges Prof Monica Liraand her lab in Centre de Investigacio en Nanociencia I Nanotecnolo-gia (Cin2, CSIC), ETSE, Campus UAB, Bellaterra (Barcelona), Spainfor their support and helping in pursue part of the experimentalsection.

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