Improved Electron Diffusion Coefficient in Electrospun TiO2 Nanowires

P. S. Archana,† R. Jose,*,† C. Vijila,‡ and S. Ramakrishna†

National UniVersity of Singapore, 2 Engineering DriVe 1, 117576, Singapore, and Institute of MaterialsScience and Engineering, A-STAR, 3 Research Link, 117602, Singapore

ReceiVed: August 26, 2009; ReVised Manuscript ReceiVed: October 15, 2009

TiO2 nanowires with a diameter of ∼150 nm, length of ∼3-4 µm, and aspect ratio of 10:1, were preparedby ultrasonically dispersing electrospun continuous nanofibers in monocarboxylic acids. The resulting pasteswere used for making nanowire films on conducting glass substrates with thicknesses in the range of 500 nmto 100 µm, good adhesion, and high nanowire packing. These films were used to fabricate dye-sensitizedsolar cells using the D131 dye and the iodide/triiodide electrolyte. Transient photocurrent measurements showeda high electron diffusion coefficient in those nanowire films. The measured diffusion coefficient in thoseTiO2 nanowires was orders of magnitude higher than that observed in nanoparticles under similar experimentalconditions. The charge-transport mechanism in the nanowire sample is discussed in support with the measuredopen-circuit voltage decay curves.

1. Introduction

One-dimensional (1-D) metal oxide nanostructures, typicallynanowires (NWs), are attracting much attention recently due totheir unique properties exploitable for device applications.1-4

Moreover, these structures are ideal to study the dependence ofphysical properties on directionality. Most commonly, NWs areproduced by the bottom-up approach, that is, by self-assemblyof the target material from vapor, liquid, or solid phases throughnucleation and growth under optimized conditions of temper-ature, pressure, or concentration.1 One of the emerging bottom-up techniques for fabrication of NWs or even continuous 1-Dnanostructures is electrospinning that works under the principleof asymmetric bending of a charged liquid jet when it isaccelerated by a longitudinal electric field.5-8 The electrospin-ning technique is characterized by its simplicity, versatility, andcost effectiveness.5 The chemical processing of the solution forelectrospinning provides controllable crystallinity and morphol-ogy, and physical processes involved, such as asymmetricbending, make the technique suitable for large-scale productionof nanostructures economically. The diameter and alignmentof the nanofibers could be controlled by the liquid injection rate,intensity of the electric field,8,9 and geometry of the collectorsurface, respectively.8,10 If the polymeric solution contains metalions for making an inorganic solid, then appropriate postelec-trospinning annealing produces continuous nanofibers of thetarget material.6,7,11,12

The continuous electrospun inorganic nanofibers are to bedeposited on suitable substrates for fabrication of solid-statedevices. Morphology, and, therefore, charge-transport propertiesof the final films are affected by the film fabrication technique.For example, electrospun TiO2 nanofiber based dye-sensitizedsolar cells (DSCs), in which photogenerated electrons arecollected through diffusion, were developed on conducting glasssubstrates (e.g., fluorine-doped tin oxide coated glass, FTO) byseveral groups.13-16 TiO2 films were fabricated by electrospin-ning the composite fibers directly onto preheated FTO substrates,

followed by hot pressing and/or solvent treatments.13,14 However,the fibrous morphology was distorted and/or the continuousnanofibers were shortened to nanorods of several hundrednanometers in length. In another method,15,16 the electrospunnanofibers were mechanically ground to nanorods, therebycompromising the initial high aspect ratio, and were sprayedonto FTO to fabricate DSCs. In both of the above methods,shortening of the nanofibers down to several hundred nanome-ters increased the grain boundary density and resulted in anenhanced scattering, thereby leading to poor diffusivity. Mukher-jee et al.17 recently fabricated DSCs using continuous TiO2

nanofibers by directly electrospinning onto FTO substrates andhot pressed at a lower temperature and pressure than that in theprevious reports.13,14 The electron diffusion coefficient (Dn) ofthe resulting film was lower than that of the TiO2 nanoparticles,despite the lower transport resistance observed for the nanofi-bers. High transit time due to the continuous fibers that wereparallel to the substrate and their poor crystallinity additionallycontributed to the observed lower Dn.17

The purpose of the present study is to optimize morphologyby making use of the lower transport resistance in electrospunnanofibers such that higher diffusion coefficients could beachieved. Such optimized structures are inevitable in view ofthe increased importance of electrospinning for its ability toproduce nanofibers in commercial scale as well as suitabilityof the 1D TiO2 nanostructures for energy harvesting andphotocatalysis. Continuous nanofibers and short nanorods couldbe ruled out based on the previous experiences.13-16 Weobserved that nanowires with a length of ∼3-4 µm have anorder of magnitude higher Dn compared with the continuousnanofibers in the presence of iodide/triiodide electrolyte. It wasalso noted that annealing the fibers for a longer time (∼24 h)increased the crystallinity without compromising much the highsurface area of the fibers and resulted in an improved Dn.

2. Experimental Details

The TiO2 nanofibers were prepared by reported methods withmodifications.9 The sol for electrospinning was prepared frompolyvinyl acetate (PVAc, Mw ) 500,000), dimethyl formamide(DMF), titanium(IV) isopropoxide, and acetic acid. The poly-

* To whom correspondence should be addressed. E-mail: nnijr@nus.edu.sg(R.J.), Seeram@nus.edu.sg (S.R.).

† National University of Singapore.‡ Institute of Materials Science and Engineering, A-STAR.

J. Phys. Chem. C 2009, 113, 21538–2154221538

10.1021/jp908238q 2009 American Chemical SocietyPublished on Web 10/30/2009

meric solution was prepared by dissolving PVAc in DMF (11.5wt. %). Titanium(IV) isopropoxide (2 g) was added to the PVAcsolution (4.5 g) together with acetic acid (0.5 g). The resultingsol was contained in an airtight bottle and stirred for 12 h beforeelectrospinning. Electrospinning was carried out on a com-mercial machine (NANON, MECC, Japan) at a 25 kV ac-celerating voltage and at a 1 mL/h flow rate. The polymericfibers containing Ti4+ ions were collected on a grounded rotatingdrum placed ∼10 cm below the spinneret. The samples weresintered in air at 500 °C for 1-24 h to remove PVAc and allownucleation and growth of TiO2 particles in the fiber structure.The annealed fibers were characterized by examining theirmorphology, surface, and crystal structure. Scanning electronmicroscopy (SEM, Quanta 200 FEG System, FEI Company,U.S.A.) and transmission electron microscopy (TEM, JEOL2010Fas) of the annealed products were carried out to examinethe morphology, surface, and crystallinity. The crystal structureof the nanofibers was examined by X-ray (XRD) and electrondiffraction techniques. The XRD patterns were recorded by aSiemens D5005 X-ray diffractometer employing Ni-filtered CuKR radiation. The electron diffraction was carried out duringthe TEM measurements.

The annealed fibers were ultrasonically dispersed in variousmonocarboxylic and mercapto acids. Typically 0.05 g ofnanofibers was ultrasonically dispersed in 0.6 mL of acetic acidfor durations of 30 min to 15 h and developed into a paste. Thepaste was then added with ethylene glycol and ethyl celluloseand coated on FTO glass substrates (1.5 cm × 1 cm; 25 Ω/0,Asahi Glass Co. Ltd., Japan) by the doctor blade technique.The thickness of the NW films could be varied between 500nm to 100 µm. A thin (∼100 nm) layer of TiO2 was developedon the FTO substrate by spin-coating a sol prepared fromtitanium isopropoxide in hydrogen peroxide before fabricationof the NW films. The films were then annealed at 500 °C for1-24 h.

The DSCs were prepared by soaking a 0.28 cm2 TiO2 NWelectrode in a 1:1 volume mixture of acetonitrile and tert-butanolof D131 dye for 24 h at room temperature. Selection of D131dye as the light harvester was due to its similarity in the LUMOsurface with many of the conventional Ru-based dyes.18 Thedye-sensitized samples were then washed in ethanol to removeunanchored dye and dried in air. Samples were sealed using a50 µm spacer. Aetonitrile containing 0.1 M lithium iodide, 0.03M iodine, 0.5 M 4-tert-butylpyridine, and 0.6 M 1-propyl-2,3-dimethyl imidazolium iodide was used as the electrolyte. A Pt-sputtered FTO glass was used as the counter electrode.Photocurrent measurements of the assembled DSC were per-formed using a solar simulator (San Ei, Japan) at AM 1.5Gcondition. I-V curves were obtained using a potentiostat(Autolab PGSTAT30, Eco Chemie B.V., The Netherlands). Theinstrument has a current sensitivity down to several nanoamperes.

The electron transport in TiO2 NW films was studied usinga transient photocurrent technique. Samples for the photocurrentmeasurements were DSCs fabricated using the TiO2 NW filmswith a thickness of ∼13 µm. Samples from two typical batches,viz. annealed for 1 h (S1) and 24 h (S2), were selected forphotocurrent measurements. In the transient photocurrent ex-periments, the cells were excited with a low intensity laser pulse(532 nm Nd:YAG laser, pulse width < 5 ns) superimposed ona large background white light illumination. The intensity ofthe white bias light was varied in order to study the effect ofphotocarrier density on the Dn. The intensity of the laser lightwas controlled with neutral density filters to keep the magnitudeof photocurrent transients less than the dc level due to the white

bias light. The cells were illuminated through the substrate side,and the photocurrent transients were recorded using a digitaloscilloscope (Agilent, 1 GHz) under short-circuit conditions.The RC time constant of the setup was < 20 µs.

3. Results and Discussion

Figure 1 shows the morphological and structural details ofthe TiO2 nanofibers obtained after heating the as-spun compositefibers for 1 and 24 h, respectively. The nanofibers maintainedcross-sectional uniformity throughout the length, indicating asmooth injection of the fine TiO2 sol dispersed in the polymermatrix during electrospinning using the commercial setup. Thecomposite polymeric and the sintered metal oxide fibersmaintained similar continuous fibrous morphology; however,the diameter of the sintered fibers was less by a fraction of nearlythree than that of the composite polymer fibers. No appreciablechange in the fiber diameter was noted when heated for differentdurations; the fiber diameter remained at ∼150 nm when theywere heated for 1-24 h. However, crystallinity of the fibersincreased considerably due to grain growth from ∼10-15 nm(1 h) to 25-50 nm (24 h), which was well-reflected in the SAEDpatterns (insets of Figure 1C,D), HREM images (Figure 1E,F),and XRD (Supporting Information) patterns. The SAED pattern

Figure 1. SEM images of the TiO2 nanofibers sintered at 500 °C for(A) 1 h and (B) 24 h. The second panels (C, D) display a bright-fieldTEM image recorded using the samples heated for 1 and 24 h annealing,respectively. The insets of (C) and (D) are their corresponding SAEDpatterns. The third panels (E, F) display a lattice image recorded at thehigh-resolution mode. An enhanced grain boundary volume clearlyrevealed in the HREM of the sample heated for 1 h d with the other.

Electrospun TiO2 Nanowires J. Phys. Chem. C, Vol. 113, No. 52, 2009 21539

of S1 displayed continuous rings, which are characteristics ofthe polycrystalline materials. As expected, a spotty pattern wasobtained for S2, indicating the improvement in its crystallinityafter annealing for 24 h. The lattice images shown in Figure1E,F clearly show the difference in crystallinity; the particlesgrew two to four times upon prolonged heating. The SAEDand XRD patterns were indexed for anatase phase. The latticeparameters calculated from the XRD pattern were a ) 3.789(4)Å and c ) 9.497(1) Å, which are consistent with the reportedvalues and also that calculated from the SAED patterns.

The TiO2 nanofibers were ultrasonically dispersed in mono-carboxylic acids (formic acid, acetic acid, dichloroacetic acid,triflouroacetic acid, oleic acid, and stearic acid) and mercaptoacids (mercaptosuccinic acid and mercaptopropionic acid) fortime intervals ranging from 30 min to 15 h. The rationales ofthis selection are (i) these acids bind the TiO2 surface and couldprevent the breaking down of continuous nanofibers intonanorods through molecular level interaction and (ii) acetic acidcould be used for chemical sintering and to prepare high qualityTiO2 paste.19 The SEM images of the nanofibers ultrasonicallydispersed in acetic acid for 1-15 h well prove the firsthypothesis (Figure 2). The continuous fibers were cut into NWswith an average length of 4 µm upon ultrasonic dispersion. Theaverage aspect ratio of the NWs was 10:1. Prolonged ultrasoni-cation of the TiO2 nanofiber suspension in the acids used here

did not appreciably reduce the length of the wires. High stabilityof the fibers against mechanical degradation is thought to arisefrom the molecular level binding of the monocarboxylic andmercapto acids with TiO2. The current method of breaking thefibers into large aspect ratio NWs also solved another crucialissue: fabrication of thick films (10-15 µm) of electrospunnanostructures with a high aspect ratio. Previous attemptscompromised either the thickness and/or the aspect ratio of theelectrospun TiO2 films.13-17

The efficiency of the cell was calculated from the short-circuitcurrent density (JSC), open-circuit voltage (VOC), and fill factor(FF) determined from the J-V curves. The photovoltaicparameters of S1 (S2) are JSC ∼ 7.79 (8.46) mA/cm2, VOC ∼815 (815) mV, FF ∼ 59.8 (60), and η ∼ 4.12 (4.29). Both S1and S2 showed similar dye-loading (7 × 10-8 mol/cm2),indicating the similar quantity of active surfaces and concentra-tion of injected electrons. A small difference observed in theJSC of S2 is attributed to its improved charge transport.

The effective diffusion coefficient of the electrons movingthrough the nanofiber film was measured by transient photo-current measurements. Figure 3 shows the photocurrent tran-sients observed for two devices fabricated using S1 and S2.The photocurrent was observed immediately after laser excita-tion in the device S1 and reaches the peak at around 0.7 ms,followed by a single exponential decay at long times. It was

Figure 2. SEM images of the NWs obtained by ultrasonically dispersing the TiO2 nanofibers in acetic acid for 30 min and thick film developedusing the NW paste. The bottom panels display magnified images of the corresponding samples.

21540 J. Phys. Chem. C, Vol. 113, No. 52, 2009 Archana et al.

also observed that rise time of the photocurrent transient in S2is much faster than that in S1, followed by a single exponentialdecay. The photocurrent collection times (τc) were extractedby fitting the photocurrent transient using a single exponentialdecay. The photocharge collection time (τc) for S1 was ∼4.8ms, which increases with the decrease of bias light intensity.The τc for S2 was ∼1.5 ms, which is almost 3 times shorterthan the τc estimated for S1 under similar measurementconditions. The Dn was estimated using the relation τc) d2/2.35Dn, where d is the thickness of the samples (∼13 µm). TheDn of the NW samples was ∼10-4 cm2/s, which showed an orderof magnitude increase with the continuous nanofibers (∼4 ×10-5 cm2/s).17 The observed enhancement in the diffusioncoefficient of NWs compared with continuous nanofibers, bothobtained by electrospinning, is attributed to the lower photo-current collection time due to the NW morphology.

The photoexcitation density for various white light illuminationintensities was calculated by numerically integrating the photo-current transients. The inset of Figure 3 shows variation of Dn withthe density of photoexcitation. An increase in Dn with photoexci-tation density was observed in both the samples, similar to thatobserved for nanoparticles20,21 and indicative of the trap-limited

diffusion process. Traps in nanocrystalline semiconductors resultfrom their increased defects, such as unsaturated bonds, deviationfrom bulk bond lengths, etc. and are distributed over a broad energyrange.22,23 The rate of increase of Dn with photoexcitation densitychanged appreciably for samples S1 and S2. The Dn of S2 (∼4.6× 10-4 cm2/s) was nearly 3 times that of S1 (∼1.5 × 10-4 cm2/s)for a photoexcitation density of ∼2 × 1016cm-3. Note thatKopidakis et al. measured the Dn of TiO2 nanoparticles to be 10-7

cm2/s for a photoexcitation density of ∼1016/cm3.20,21,24 Recently,Jemmings et al.25 measured the Dn of 1D nanotubes using intensitymodulated photoelectron spectroscopy. They observed a Dn of 10-6

cm2/s for an incident photon flux of 1016 /cm3. The Dn measuredfor electrospun TiO2 nanowires for a photoexcitation density of∼1016/cm3 was ∼10-4cm2/s, which is about to 2-3 orders ofmagnitude higher than that measured for the nanoparticles.

Many efforts have been devoted to understanding the chargetransport through mesoporous TiO2 nanoparticles26 using a numberof techniques, including transient photocurrent measurements.24,27

It has been generally accepted that photoinjected electrons in themesoporous network diffuse through a well-defined conductionband minimum, EC, and is interrupted by a series of trapping anddetrapping events. On the other hand, not much is known aboutcharge transport through random 1D nanostructures except onordered nanotubes.28,29 Figure 4 shows a schematic that explainsthe source of difference in Dn observed for mesoporous particlesand NW samples. The smaller particle size and higher space chargeregion thereby in the mesoporous network lead to completedepletion of macroscopic electric fields, and therefore, the conduc-tion band remains flat throughout the particle. On the other hand,the NWs in the present study are characterized by a dense packingof grains along their length and diameter, in contrast to the randomparticle network (Figure 4), which can support a small electric fielddue to a partially depleted space charge region within its volume.This space charge free region is thought to further accelerate theelectrons. The spatial extend of the electric field increases withincrease in the size of the grains composing the NWs (Figure 4).We believe that the difference in the magnitude of the depletedspace region between the mesoporous network and random 1D

Figure 4. Schematics showing the diffusion process in nanoparticle and nanowire systems. The bottom and top panel display the morphologiesand energy levels, respectively. The red arrow indicates the diffusion process, and the blue curves indicate the recombination processes. (A) Mesoporousnanoparticle system in which the conduction band is flat throughout the particle. Typically, 25 nm particles have an effective surface area of ∼100m2/g. The mesoporosity of the films has the advantage of large dye-anchoring, consequently, with increased recombination. (B) A nanowire withan average diameter of 150 nm composed of particles of ∼12 nm; in the present experiment, similar structures were obtained by annealing theas-spun composite polymeric fibers for 1 h. These fibers had an effective BET surface area of ∼60 m2/g. The dyes could be anchored only on theirsurface, consequently, with a reduced recombination rate. Close packing of nanoparticles in the nanowires leads to a depleted space charge regionin the volume of the nanowires and, therefore, with an improved diffusion process. (C) A nanowire with an average diameter of 150 nm composedof particles of ∼50 nm; in the present experiment, similar structures were obtained by annealing the as-spun composite polymeric fibers for 1 h.These fibers had an effective BET surface area of ∼50 m2/g. Enhanced particle size further reduced the space charge region, thereby leading to animproved diffusion process.

Figure 3. Transient photocurrent observed for NW films with annealedTiO2 nanofibers for 1 h (S1) and 24 h (S2). Inset: diffusion coefficientsof S1 and S2 as a function of photoexcitation density.

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NWs is the source of the improved effective Dn in the latterstructure. Annealing the samples for a long time increases theparticle size, thereby removing the surface traps. Therefore, thehigh Dn observed for S2 is assigned to its enhanced crystallinitycompared with that of S1.

Removal of surface traps upon increasing the particle sizewas further studied by measuring the open-circuit voltage decay(OCVD)30,31 of the two samples, S1 and S2. The OCVDmeasures the lifetime of the electrons as a function of the open-circuit voltage. The OCVD curves were recorded by turningoff the illumination in a steady state and monitoring thesubsequent photovoltage decay. As the measurement is per-formed in the dark, recombination with the oxidized dyemolecule does not take place, but this is acceptable becausethe electrolyte accounts for the majority of the recombinationeven under illumination. Thus, assuming a first-order recom-bination reaction, the electron lifetime is given by τn )-(kT/e)(dVOC/dt)-1,30,31 where kT is the thermal energy, e is thepositive elementary charge, and dVoc/dt is the first-order timederivative of the VOC. Figure 5 shows the electron lifetime as afunction of VOC for S1 and S2. The shape of the OCVD curvefor S1 shows a dependence on the quasi-fermi level, confirminga trap-assisted conduction mechanism similar to those ofnanoparticles. Similar OCVD was reported for electrospuncontinuous nanofibers directly spun on FTO, followed byannealing for 1 h.17 The depression seen in the curve at around0.3 V indicates the presence of surface trap states that couldresult in recombination of electrons with the electrolyte throughtunneling. Furthermore, the strength of the deviation from linearsuggests a high rate constant for such recombination. Interest-ingly, the dependence of τn on VOC was found to be linear forthe sample S2, indicating the removal of the surface traps,thereby accounting for its larger Dn compared with that of S1.

4. Conclusions

In conclusion, smooth, porous, and thicker TiO2 NW films ofhigh aspect ratio were developed on conductive glass substratesby combining coordination chemistry and electrospinning. Elec-trospun metal oxide nanofibers were coordinated using organicacids, which prevented breaking of nanofibers into tiny submi-crometer nanorods during the film fabrication process. The Dn ofthe resulting NW shows an order of magnitude enhancement dwith the random continuous nanofibers that are parallel to thesubstrate. The observed enhancement in Dn is attributed to a small

electric field in dense NWs due to a partially depleted space chargeregion within its volume. The width of this space charge free regionincreases with increase in the particle size, which further enhancedthe Dn. Thus, electrospinning offers a possibility to enhance thecharge mobility without compromising much to the specific surfacearea of the nanostructures thereby produced. The high electrondiffusion coefficient observed in annealed electrospun metal oxideNWs in the present study provides opportunities to fabricateelectronic devices with better performance.

Acknowledgment. This project is financially supported bythe Clean Energy Program Office, National Research Founda-tion, Singapore.

Supporting Information Available: X-ray diffraction pat-terns of the NW samples under different annealing conditions.This material is available free of charge via the Internet at http://pubs.acs.org.

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JP908238Q

Figure 5. OCVD curve measured for the samples S1 (black dots) andS2 (red dots).

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