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Applied Surface Science 256 (2010) 7217–7221

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

study on photo-generated charges property in highly ordered TiO2

anotube arrays

u Zhanga, Dejun Wanga, Shan Pangb, Yanhong Lina, Tengfei Jianga, Tengfeng Xiea,∗

College of Chemistry, Jilin University, Changchun 130023, PR ChinaKey Laboratory for Special Functional Materials, Henan University, Kaifeng 475001, PR China

r t i c l e i n f o

rticle history:eceived 2 March 2010eceived in revised form 4 May 2010ccepted 13 May 2010

a b s t r a c t

In this study TiO2 nanotube arrays were fabricated by potentiostatic anodization of titanium sheet. The X-ray diffraction (XRD) pattern and field emission scanning electron microscopy (FE-SEM) image indicatedthe TiO2 nanotube arrays were of pure anatase form and highly ordered. The properties of the photo-

vailable online 21 May 2010

eywords:itania nanotube arrayhoto-generated chargeurface photovoltage

generated charges in the nanotube arrays were investigated by transient photovoltage (TPV) techniqueand surface photovoltage (SPV) technique based on lock-in amplifier with dc bias, in comparison with thecommercial powder derived film. The separation processes of the photo-induced charges in the system ofTiO2 nanotubes on Ti have been demonstrated to be correlated with the incident light intensity, surfacetrapping states, and the interfacial electric field between Ti and TiO2. The results also show that the highlyordered nanotube film could generate much stronger SPV responses under external electric field than

erive

the commercial powder d

. Introduction

Over the past decades, the TiO2 nanomaterials have attractedide attention because of their superior photoactive properties.

iO2 nanostructures with different patterns such as particles,ubules, wires, and rods have been already reported [1–8]. Com-ared with all these structures, the highly ordered TiO2 nanotuberrays are fabricated by relatively simple process. As photovoltaicaterial they represent larger surface area, convenient for trans-

ort in the 1-dimensional scale and dramatically improved chargeransport properties. The first work on nano tubular TiO2 formed bynodization in fluoride containing media was done by Zwilling et al.9]. After a period of ten years, the TiO2 nanotube arrays have beenpplied in many areas including dye-sensitized solar cells [10–12],hoto-catalysis [13,14], gas sensor [15,16]. For any of these applica-ions it is very important to know the kinetic of photo-generationf excess carriers in this nanotube structure, which is helpful forxploring the mechanism and making efficient photovoltaic device.

Transient photovoltage (TPV), also known as time-resolved pho-ovoltage (PV), monitors the changes of surface potential as a

unction of time in the materials upon illumination. We can directlybtain information about charge dynamics, including generation,eparation and recombination of photo-generated charges by thisechnique. TPV technique can be used for characterizing porous

∗ Corresponding author. Tel.: +86 431 85168093.E-mail address: xietf@jlu.edu.cn (T. Xie).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.05.054

d film.© 2010 Elsevier B.V. All rights reserved.

semiconductors [17], heterojunction structures [18], and somepolymer materials [19]. Dittrich and coworkers have conducted anin-depth study on the diffusion process in nanocrystalline porousTiO2 using the TPV method [17,20]. Recently, Anta and cowork-ers investigated the dynamics of charge separation and transportin metal-oxide nanostructures employing both TPV technique anda random walk numerical simulation (RWNS) method [21]. TheRWNS simulation reproduced well the main experimental TPV fea-tures and gave strong evidence that electron trapping is governingthe dynamics of the injected charge. The surface photovoltage (SPV)spectra, known as spectral photovoltage, are well-established tech-nique for the characterization of semiconductor of bulk materialsor nanomaterials [22]. Since the photovoltaic effect is based on thephotogeneration of excess carriers followed by their spatial separa-tion, the SPV results contain both optical properties and transportproperties of different regions in the material. Therefore, both TPVand SPV measurements are suitable for studying the photovoltaicproperties of highly ordered arrays of TiO2 nanotubes.

In this work we prepared TiO2 nanotube arrays grown on Timetal substrate by potentiostatic anodization of titanium sheetin fluoride containing electrolyte. TPV measurements and SPVtechnique were both employed to characterize the behavior ofphoto-generated charge carriers in the TiO2 nanotube arrays.

2. Experiment

In our experiments, we used the method of anodization to growthe nanotube arrays from titanium thin sheets [10]. For all exper-

7218 Y. Zhang et al. / Applied Surface Science 256 (2010) 7217–7221

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ig. 1. FE-SEM images of TiO2 nanotube arrays on Ti substrate anodized in DMSOontaining 1% HF at +30 V (vs Pt) for 72 h at room temperature and then annealedt 450 ◦C for 5 hours. The scale bar in the image is 1 �m.

ments titanium sheet with a thickness of 250 �m (99.6% purity)as cleaned with deionized (DI) water, followed by sonicating in

cetone and ethanol successively, after that, the titanium sheetas dried in a nitrogen stream. An organic electrolyte includingdimethyl sulfoxide (DMSO; 99.6%) and 1% hydrofluoric acid (HF;0% aqueous solution) mixture was prepared. The anodization waserformed in a two-electrode configuration with titanium sheet ashe working electrode and platinum foil as the counter electrode.itanium sheet has been anodized at 30 V for 72 hours achievinganotube arrays with length of 6-8 �m. After anodization, the filmf titania nanotube arrays grown on titanium sheet was washedith ethanol and dried in air. All the above experiments were

arried out at room temperature. Then the highly ordered titaniaanotube arrays were crystallized by annealing in air at 450 ◦C forh. The morphologies of the titania nanotube arrays (see Fig. 1)ere observed by field emission scanning electron microscope (FE-

EM; JEOL JSM-6700F). The XRD pattern of the sample was obtainedn a Siemens D5005 diffractometer. The work function of samplesas measured by a commercial Kelvin probe system (SKP5050, KP

echnology, Ltd) at room temperature and under ambient pressure.or comparison the nanocrystalline TiO2 film (P25 Degussa Co.) wasabricated on FTO glass by doctor-blading.

TPV measurements were carried out on a device which wasescribed in our previous paper [18]. A sample chamber like aarallel-plate capacitor consisted of the film of TiO2 nanotuberrays on the substrate with Ti sheet, a 10 �m thick mica and alatinum wire gauze top electrode (with the transparency of ca.0%). A laser radiation pulse (wavelength of 355 nm and pulseidth of 5 ns) from a third harmonic Nd:YAG laser (Polaris II, Newave Research, Inc.) was used to excite the TPV. The signals were

egistered by a 500 MHz digital phosphor oscilloscope (TDS 5054,ektronix) with a preamplifier. The formation of a TPV signal isetermined by the factors of light absorption, transport of excessarriers, structural and electric characteristics in a semiconductingaterial.The SPV measurement system is made up of a source of

onochromatic light, a lock-in amplifier (SR830-DSP) with a lighthopper (SR540), a sample cell and a computer. A 500 W Xenon

amp, and a double-prism monochromator providing monochro-

atic light [23]. In the photovoltaic cell the Ti sheet with TiO2anotube arrays acted as a bottom electrode. A glass substrate cov-red with ITO was used for top electrode and a spacer of micaas inserted between the two electrodes. With lock-in amplifier

Fig. 2. X-ray diffractogram of the sample annealed at 450 ◦C for 5 hours. The diffrac-tion peaks of TiO2 (anatase) and Ti are found in the pattern.

we can obtain SPV amplitude and phase spectra. The SPV spectrawith dc bias, also called field-induced surface photovoltage spectra(FISPS), are developed by our group [23,24]. It can be a supple-ment to the SPV spectroscopy method. In FISPS, the external electricfields are applied between the two electrodes. Some obscure pro-cesses will be enhanced under external electric fields. The TPV andSPV measurements were performed in air atmosphere, and at roomtemperature.

3. Result and Discussion

Fig. 1 shows the top view FE-SEM image of the TiO2 nanotubearrays. We know the pore size diameter of ∼60 nm and a wall thick-ness of ∼50 nm at the top. The growth mechanism for the nanotubearrays is mainly due to two effects of the fluoride ion [25]: the abilityto form water-soluble TiF6

2- complexes, and the small ionic radiusthat makes them suitable to enter the growing TiO2 lattice and tobe transported through the oxide by the applied field. Steady stateis established when the thickness reducing dissolution rate of theoxide film at the outer interface is identical to the pore growth rateat the metal oxide interface. That means the thickness of the bot-tom TiO2 layer would be fixed. For the surface stress during theformation of films the nanotubes are piled together in this kindof electrolyte [26]. Almost all the nanotubes are perpendicular tothe Ti substrate, and there are a high proportion of the nanotubeswith top opening. The light may irradiate the surface or penetratethrough the tube to the substrate, which would generate very dif-ferent photovoltage signals. In addition, since the TiO2 nanotubesprepared by etching in DMSO at room temperature are amorphous,the wall of each nanotube should be constructed of TiO2 crystallitesafter annealing. Fig. 2 shows XRD patterns of the Ti-based nanotubearrays after calcination at 450 ◦C. The diffraction peaks of anataseand titanium appear, indicating the TiO2 nanotube arrays annealedare of pure anatase form.

3.1. Transient photovoltage

The transient photovoltage measurements were first carried outto explore the photovoltaic properties. The TPV responses of TiO2

nanotube arrays are shown in Fig. 3a. The excess carriers are excitedwith photons of hv > EG. Since the optical band gap (EG) of anataseTiO2 is 3.2 eV, excitation under 355 nm (photon energy hv = 3.5 eV)leads to a band-to-band transition. We find that under differentlaser intensities the signals are strongly varied. When we use a low-

Y. Zhang et al. / Applied Surface Science 256 (2010) 7217–7221 7219

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ig. 3. (a) The transient photovoltage responses of the nanotube arrays. (b) The trased in this measurement are 30, 50, 100, and 200 �J/pulse. The wavelength of lase

ower laser, it appears a negative sign at about 10−7∼10−6 s (theast component). Then the response moves upward with a positivealue at 10−6∼10−2 s (the slow component). While under a highaser intensity the situation is much more complex. The fast com-onent of TPV response becomes positive and rises rapidly. Then itisplays positive TPV sign at 10−6∼10−2 s just like the previous case.ith the laser power enhanced from 30 �J/pulse to 200 �J/pulse

here is a trend of signal inversion at fast component. However, theeature of polarity changes for TiO2 nanotube arrays is not found in25 film (Fig. 3b) for which only positive signals appear under allaser intensities. For the TPV responses, positive sign implies thathe electric potential of the top electrode is positive with respecto the bottom electrode. Some mechanisms may exist such as thatositive charges transfer towards the top electrode and accumulatet the area nearby. On the contrary, a negative sign is produced dueo negative charges accumulation at the surface area nearby the toplectrode.

For the above phenomena, we consider three main factors thatffect the movement of photo-generated charges in TiO2 nan-tube arrays: 1. the trapping of photo-generated electron on TiO2urface, 2. the diffusion of photo-generated charge carriers alonghe nanotube under concentration gradient, 3. the drift of photo-enerated charge carriers at the TiO2/Ti interface. When the laserower is low, it can produce a small quantity of free carriers in theiO2 nanotube arrays. As the anodized TiO2 nanotubes are amor-hous and have rough surface [15], many surface defects shoulde reserved in the crystallization process by a high-temperaturenneal. Some of the surface defects will play the role of electronrapping states. Therefore, some electrons excited from the valenceand to the conduction band will be trapped by intra-particle sur-ace states immediately, generating a negative TPV response athorter timescale (see Fig. 3a). Then, the non-homogeneous absorp-ion of light from surface to bulk with photons of hv > EG caused theradient of excess electron and hole concentrations, and photo-enerated charge carriers can diffuse through the particles underhis concentration gradient. This diffusion belongs to the inter-article process which is slower than the intra-particle separation27], and electrons usually have a higher diffusion coefficient thanxcess holes [17]. So the surface is charged by holes resulting inositive sign at long timescale.

When irradiating the sample with high laser intensity, relativelyore free charge carriers are generated. But not all the electronsould be trapped because surface trapping states may get satu-

ated. The curve has two peaks at about 3 × 10−7 s and 4 × 10−5 s

as shown in red line in Fig. 3a), which presents two different kindsf charge carrier transport ways: drift and diffusion, respectively28]. Generally, the drift related TPV response (caused by surfacer interface built-in field) always appears immediately within aew hundreds nanoseconds after the exciting laser pulse, while the

t photovoltage responses of the P25 film based on FTO glass. The excitation levelse is 355 nm.

emergence of diffusion related TPV is relatively late compared withthe former [28,29]. So, there should be a weak charge drift process,which corresponds to the peak at 3 × 10−7. Because the light trans-mission in the tube could induce charge carriers along the wholenanostructure, the photo-generated charges at the bottom will beaffected by the interface built-in field [28,29]. If the direction ofbuilt-in field is from Ti to TiO2, the injecting of photo-generatedelectrons from TiO2 to Ti will happen. If the light is strong enough,the impact of the drift of photo-generated electrons at TiO2/Ti inter-face will exceed the effect of electron trapping and generate weakpositive signal as shown in Fig. 3(a) (red line), which correspondsto the first peak (10−7 s). Wei et al. have studied the interfaceeffect between the TiO2 nanoparticles and FTO substrate [29]. Theyproved that there is built-in electric field at TiO2/FTO interface, theseparation and transport of photo-generated charges at TiO2/FTOinterface is primarily controlled by the built-in electric field. Inorder to obtain the direction about interface (TiO2/Ti) built-in field,Kelvin Probe technique, an effective tool to characterize inorganicand organic functional materials [18], was used to get work func-tion of TiO2 nanotube arrays and Ti substrate. The contact potentialdifferences (CPD) vs Au of TiO2 nanotube arrays and Ti substrate areshown in Fig. 4a. As the work function of the gold reference probeis 5.1 eV, we can calculate the work function of Ti and TiO2 by theequation: Wx-WAu = CPD, where Wx is the work fuction of the sam-ple. So the work function of Ti and TiO2 are Wtitanium = 4.29 eV andWtitania = 5.13 eV respectively. When they contact, the work func-tion difference can form a negative space charge region in TiO2.The orientation of built-in electric fields at the Ti/TiO2 interface isfrom Ti substrate towards TiO2 (see Fig. 4b and c). Thus, the photo-generated electrons and holes will separate under the driving forceof the built-in field rapidly [29].

The TPV responses at long timescale (10−6∼10−2s) should beattributed to the diffusion of photo-generated charge carriers alongthe nanotube under concentration gradient. The positive sign ofthe TPV implies that the photoexcited electrons move faster thanholes. It can be found that the diffusion processes made a majorcontribution to the TPV responses under strong light intensity irra-diation for both TiO2 nanotube array and TiO2 nanoparticle film.However, by comparing their diffusion photovoltage, we find thattheir signal strength and peak time are different. When irradiatedwith the same light intensity, the TiO2 nanoparticulate film pro-duces stronger TPV signal than TiO2 nanotube array and a differenceof peak time about several tens of microseconds can be observed.Dittrich and coworkers have proposed that the diffusion photovolt-

age can be split up into two contributions: total amount of charge,and distance between the centers of charge of the positive and neg-ative carrier distributions [20]. As the electron diffusion length inTiO2 nanotube is slightly higher than that in TiO2 nanoparticlutefilm [30,31], we consider that the relatively weak TPV for TiO2

7220 Y. Zhang et al. / Applied Surface Science 256 (2010) 7217–7221

Fig. 4. (a) The contact potential differences (CPD) of TiO2 nanotube arrays and Ti foil.(b) The schematic band diagrams of the Ti/TiO2 interface: Before the Ti substrate andTiO2 contact (left), the work function values of Ti and TiO2 are Wtitanium = 4.29 eV andWtitania = 5.13 eV, respectively. After contact (right), EF’ is the Fermi level at equilib-rium conditions. E0: local vacuum level. EV: valence band. EC: conduction band. Thearrows indicate the orientation of the built-in electric field. (c) Sketch of the distri-bution of surface trapping states and built-in electric field in a TiO nanotube. Thesom

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than the changes of P25 film (see Fig. 6). The higher externalfield is applied, the stronger response is obtained. Furthermore,the SPV signal of TiO2 nanotube is about one order of magni-tude larger than the P25 film under biases. All these featuresshow that the highly ordered nanotube film has more excellent

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urface trapping states may exist in the top and wall of the nanotube. At the bottomf the nanotube there is the build-in electric field between the TiO2 layer and Tietal.

anotube array may be caused by the above-mentioned processestrapping and drift) which could reduce the total amount of chargenvolved in diffusion process. According to ref. 21, the retardationf tpeak may be relevant to some parameters, for example the val-es of tpeak increase with decreasing lattice constant or increasingcreening length. Another possible reason for the retardation ofpeak may be the higher grain boundaries between TiO2 particles inhe nanoparticulate TiO2 film. This may result in the formation of a

arrier for electron transport and inhibit the carrier recombination,ccordingly extending the time for carrier transport.

Our results indicate that the drift and diffusion of electrons havehe same direction (from surface to substrate) in the TiO2 nanotube

Fig. 5. FISPS of TiO2 nanotube arrays under different dc biases.

arrays, which is helpful for the electronic collection of photoelectricdevice based on the structure.

3.2. Field-induced surface photovoltage spectroscopy

The FISPS results for the TiO2 nanotube arrays are shown inFig. 5. The black line represents the SPV signal of the TiO2 nan-otube arrays under 0 V bias. Here, a positive signal implies thatthe positive charges accumulate at the surface area and a nega-tive signal is produced due to negative charges accumulation atthe surface area nearby the top electrode. The threshold is about390 nm (photon energy hv =3.2 eV), showing a character of anataseTiO2. For comparison the P25 film has also been studied by FISPS.The new sub-band-gap (>390 nm) SPV of TiO2 nanotube underexternal electric fields may be due to the effect of the high den-sity of surface states and we didn’t find sub-band-gap responses inP25 film by FISPS measurement. Evidently, the changes in photo-voltaic responses of the TiO2 nanotubes are much more obvious

Fig. 6. Photovoltage as a function of the applied bias for the two samples, the exci-tation wavelength is 350 nm.

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eparation properties of the photo-generated charges than theommercial powder derived film under external electric fields.his is mainly because the separating processes above-mentionedould contribute to the changing of surface charges under differ-nt biases. As the 500 W Xenon lamp produces a much weakeray than the laser radiation pulse (the light intensity ranged from00 nm to 600 nm is below 0.03 W/cm2, and the laser intensity is.3 × 104 W/cm2 for 20 �J/pulse), the processes of electron trap-ing on TiO2 surface and diffusion under concentration gradientshould be dominant in poor light irradiation that we have demon-trated by TPV measurements. When positive dc bias is applied,t will induce the injecting of photo-generated electrons to sur-ace trapping states under the electric fields, resulting in enhancedegative response. But the surface trapping states tend to be filledith photo-generated electrons and get more saturated underigher bias, so the relation of photovoltage and bias is non-linearnder positive biases (see Fig. 6). When negative dc bias is applied,owever, the diffusion of photo-generated electrons to bulk wille promoted to meet the direction of external electric field. The

inear change in negative bias could reflect the diffusion-relatedrocess.

All the interesting photovoltaic properties of highly orderediO2 nanotubes arrays will be helpful for us to fabricate effi-ient photoelectric device. The excellent photoelectric responses ofISPS indicate its potential application, for example, an ultraviolethoto-detector based on this structure. Furthermore, the changes

n polarity of the TPV with different light intensity can be vieweds features of a light-controlled detector which might find use inpplication in the area of fast optoelectronic switches.

. Conclusion

We have investigated the photovoltaic properties of the TiO2anotube arrays linked with Ti substrate by TPV and SPV technol-gy. Under different laser intensities there is a polarity change in thePV responses. The unique TPV responses depend on the incidentight intensity, surface trapping states, and the interfacial electriceld between Ti and TiO2. The SPV measurements show a verybvious signal changes under external electric field in UV region.he formation of tubal structure significantly affects the separation,ransport and recombination of the photo-generated charges, andisplays the unique property when comparing with commercialowder derived film.

cknowledgement

For financial support, we are grateful to the National Basicesearch Program of China (973 Program) (No.2007CB613303).his work is also supported by the National Natural Science Foun-ation of China (No. 20673049, 20703020, 20873053).

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.apsusc.2010.05.054.

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