uv sensor based on tio2 nanorod arrays on fto thin film

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Sensors and Actuators B 156 (2011) 114–119 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb UV sensor based on TiO 2 nanorod arrays on FTO thin film Chunlan Cao a,b,, Chenguo Hu a,∗∗ , Xue Wang a , Shuxia Wang a , Yongshu Tian a,b , Hulin Zhang a a Department of Applied Physics, Chongqing University, Chongqing 400044, PR China b Department of Power Engineer, Chongqing Communication College, Chongqing 400035, PR China article info Article history: Received 30 January 2011 Received in revised form 26 March 2011 Accepted 31 March 2011 Available online 8 April 2011 Keywords: TiO2 nanorod arrays UV sensor Photocurrent response abstract Highly oriented TiO 2 nanorod arrays were fabricated directly on fluorine-doped tin oxide-coated glass (FTO) substrate by the hydrothermal method. The diameter, length, and density of the nanorods could be varied by changing the growth parameters, such as time, temperature, and initial reactant concentra- tion. The fabricated samples were characterized with X-ray diffraction, field-emission-scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. The TiO 2 nanorod array was applied to construct photoelectric devices, by which highly sensitive and steady photocurrent responses were obtained. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Nanostructural TiO 2 has attracted considerable attention due to its unique and excellent properties in optics, electronics, photochemistry and biology, as well as its applications in photo- voltaic devices [1–3], lithium ion batteries [4,5], dye-sensitized solar cells [6,7], photocatalysts [8,9]. At present, one-dimensional TiO 2 nanoarrays can be prepared by many methods, including hydrothermal [10–14], template synthesis [15,16], electrochemical etching [17,18], chemical vapor deposition [19–21] and sol–gel method [22,23]. Among these methods, the hydrothermal synthe- sis of TiO 2 nanoarrays is a promising approach due to its simple process and low cost. To date, preparing TiO 2 arrays growing on transparent sub- strates by hydrothermal method is rarely reported. Feng et al. [24] reported the TiO 2 papillae assembled by nanorod arrays in a random pattern, and Varghese et al. [25] reported dense nanorod arrays which are connected together and only separated at top. Very recently, Liu et al. [26] systematically investigated the rutile TiO 2 nanorod arrays growing directly on FTO substrate and investigated its photovoltage properties. Wang et al. [27] fabricated oriented, single-crystalline rutile TiO 2 nanorods on a large diversity of substrates including Si, Si/SiO 2 , sapphire, Si pillars, and FTO-covered glass. Up to now, the photocurrent generated directly by TiO 2 nan- otubes in the exposure to different lights has been extensively Corresponding author at: Department of Applied Physics, Chongqing University, Chongqing 400044, PR China. Tel.: +86 2365105890; fax: +86 2365111245. ∗∗ Corresponding author. Tel.: +86 2365105890; fax: +86 2365111245. E-mail addresses: [email protected] (C. Cao), [email protected] (C. Hu). investigated, such as their possible use in UV sensors, solar cells, and biosensors [21,28,29]. However, the photoelectrochemistric responses based on the TiO 2 nanorod arrays growing on a trans- parent substrate via hydrothermal method is rarely reported. Low cost and convenient preparation of large-area highly oriented TiO 2 nanorod arrays still remains a challenge. In this article, we report the controllable preparation of highly oriented TiO 2 nanorod arrays growing on FTO by the hydrothermal method, in which the growth time and temperature as well as reac- tant concentration were systemically examined. The morphology of TiO 2 nanorod arrays was characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and diffuse reflectance spectroscopy (DRS). In addition, the effect of nanorod size on the conductivity was studied via electrochemical impedance spectroscopy (EIS). The photocurrent responses and stability of the TiO 2 nanorod arrays on FTO under a simulated sunlight and UV illumination were investigated by the amperometric measurement in 0.5 M sodium sulfate aqueous solu- tion. The results indicate that TiO 2 nanorod arrays could be a well- defined structure for highly sensitive and optoelectronic sensors. 2. Experimental 2.1. Chemical FTO glass substrate with a thickness of 2.2 mm (F:SnO 2 , Tec 15, 15 /sq) was purchased from Japan. Ethanol, acetone, Na 2 SO 4 , and HCl (36%) were from Chongqing Chemical Reagent Co. All chemicals were of analytical grade and were used as received. Deionized water was used to prepare all the solutions. 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.03.080

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Page 1: UV sensor based on TiO2 nanorod arrays on FTO thin film

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Sensors and Actuators B 156 (2011) 114–119

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

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

V sensor based on TiO2 nanorod arrays on FTO thin film

hunlan Caoa,b,∗, Chenguo Hua,∗∗, Xue Wanga, Shuxia Wanga, Yongshu Tiana,b, Hulin Zhanga

Department of Applied Physics, Chongqing University, Chongqing 400044, PR ChinaDepartment of Power Engineer, Chongqing Communication College, Chongqing 400035, PR China

r t i c l e i n f o

rticle history:eceived 30 January 2011

a b s t r a c t

Highly oriented TiO2 nanorod arrays were fabricated directly on fluorine-doped tin oxide-coated glass(FTO) substrate by the hydrothermal method. The diameter, length, and density of the nanorods could

eceived in revised form 26 March 2011ccepted 31 March 2011vailable online 8 April 2011

eywords:iO nanorod arrays

be varied by changing the growth parameters, such as time, temperature, and initial reactant concentra-tion. The fabricated samples were characterized with X-ray diffraction, field-emission-scanning electronmicroscopy, transmission electron microscopy, high resolution transmission electron microscopy andenergy dispersive X-ray spectroscopy. The TiO2 nanorod array was applied to construct photoelectricdevices, by which highly sensitive and steady photocurrent responses were obtained.

2

V sensorhotocurrent response

. Introduction

Nanostructural TiO2 has attracted considerable attentionue to its unique and excellent properties in optics, electronics,hotochemistry and biology, as well as its applications in photo-oltaic devices [1–3], lithium ion batteries [4,5], dye-sensitizedolar cells [6,7], photocatalysts [8,9]. At present, one-dimensionaliO2 nanoarrays can be prepared by many methods, includingydrothermal [10–14], template synthesis [15,16], electrochemicaltching [17,18], chemical vapor deposition [19–21] and sol–gelethod [22,23]. Among these methods, the hydrothermal synthe-

is of TiO2 nanoarrays is a promising approach due to its simplerocess and low cost.

To date, preparing TiO2 arrays growing on transparent sub-trates by hydrothermal method is rarely reported. Feng et al.24] reported the TiO2 papillae assembled by nanorod arraysn a random pattern, and Varghese et al. [25] reported denseanorod arrays which are connected together and only separatedt top. Very recently, Liu et al. [26] systematically investigatedhe rutile TiO2 nanorod arrays growing directly on FTO substratend investigated its photovoltage properties. Wang et al. [27]abricated oriented, single-crystalline rutile TiO2 nanorods on

large diversity of substrates including Si, Si/SiO2, sapphire, Siillars, and FTO-covered glass.

Up to now, the photocurrent generated directly by TiO2 nan-tubes in the exposure to different lights has been extensively

∗ Corresponding author at: Department of Applied Physics, Chongqing University,hongqing 400044, PR China. Tel.: +86 2365105890; fax: +86 2365111245.∗∗ Corresponding author. Tel.: +86 2365105890; fax: +86 2365111245.

E-mail addresses: [email protected] (C. Cao), [email protected] (C. Hu).

925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2011.03.080

© 2011 Elsevier B.V. All rights reserved.

investigated, such as their possible use in UV sensors, solar cells,and biosensors [21,28,29]. However, the photoelectrochemistricresponses based on the TiO2 nanorod arrays growing on a trans-parent substrate via hydrothermal method is rarely reported. Lowcost and convenient preparation of large-area highly oriented TiO2nanorod arrays still remains a challenge.

In this article, we report the controllable preparation of highlyoriented TiO2 nanorod arrays growing on FTO by the hydrothermalmethod, in which the growth time and temperature as well as reac-tant concentration were systemically examined. The morphology ofTiO2 nanorod arrays was characterized by X-ray diffraction (XRD),field emission scanning electron microscopy (FESEM), transmissionelectron microscopy (TEM), energy-dispersive X-ray spectroscopy(EDS), and diffuse reflectance spectroscopy (DRS). In addition,the effect of nanorod size on the conductivity was studied viaelectrochemical impedance spectroscopy (EIS). The photocurrentresponses and stability of the TiO2 nanorod arrays on FTO under asimulated sunlight and UV illumination were investigated by theamperometric measurement in 0.5 M sodium sulfate aqueous solu-tion. The results indicate that TiO2 nanorod arrays could be a well-defined structure for highly sensitive and optoelectronic sensors.

2. Experimental

2.1. Chemical

FTO glass substrate with a thickness of 2.2 mm (F:SnO2, Tec 15,15 �/sq) was purchased from Japan. Ethanol, acetone, Na2SO4, andHCl (36%) were from Chongqing Chemical Reagent Co. All chemicalswere of analytical grade and were used as received. Deionized waterwas used to prepare all the solutions.

Page 2: UV sensor based on TiO2 nanorod arrays on FTO thin film

ctuators B 156 (2011) 114–119 115

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.2. Synthesis of TiO2 nanorod arrays

In a typical reaction: (1) FTO glass substrate was cut into aimension of 10 mm × 40 mm and was degreased by sonicating incetone, ethanol and deionized water in sequence for about 30 min,nd finally dried in air. (2) 0–15 mL deionized water was mixed with–15 mL concentrated hydrochloric acid (36% by weight) to reachtotal volume of 20 mL in a Teflon-lined stainless steel autoclave

25 mL volume). The mixture was stirred at ambient conditions for0 min. (3) 0.4–0.8 mL titanium butoxide was added into the aboveentioned solution under magnetic stirring for 30 min. (4) One

iece of FTO substrate was placed at an angle against the wall of theeflon-vessel with the conducting side facing down. The hydrother-al synthesis was conducted at 120–180 ◦C for 4–24 h in an electric

ven, as shown in Table 1. (5) After a specified time, the vessel wasaken out and let cool to room temperature naturally, which tookpproximately 60 min. (6) The FTO substrate was taken out, rinsedith deionized water several times and allowed to dry in ambient

ir.

.3. Characterization

An energy dispersive X-ray spectroscopy (EDS) and X-rayiffraction measurement (XRD) with Cu K� radiation (� = 1.5418 A)t a 2◦/min scanning speed in the 2� rang from 20◦ to 80◦ were usedo investigate the crystal phase and chemical composition. The sizend morphology of the prepared samples were measured by FESEMFEI Nova 400, at 10 kV) and transmission electron microscopyTEM, HD-2000, at 200 kV, Hitachi). The selected-area electroniffraction (SEAD) pattern was taken on the TEM. An UV–Vis-IR Spectrophotometer (Hitachi U-4100) was used to measure

he optical properties of the samples. A CHI 660C electrochemi-al workstation (Shanghai Chenhua Instruments) was employedor the EIS and photocurrent measurements, which were carriedut with a conventional three-electrode electrochemical cell. Ptoil and Ag/AgCl (saturated KCl) were used respectively as theounter and reference electrodes. For the electrochemical exper-ments, light intensity was measured by a radiant power energy

eter (Photoelectric Instrument Factory of Beijing Normal Uni-ersity). The photoelectrochemical measurements were performedn a 0.5 M Na2SO4 solution. The working electrodes were illumi-ated using a simulated sunlight source (CHF-XM-500 W) under

rradiation of 100 mW cm−2. A UV light illumination at 365 nm wasbtained using an optical filter under irradiation of 3 mW cm−2. Allhe experiments were performed at room temperature (20 ◦C).

. Results and discussion

.1. Morphological characterization of the as-prepared TiO2

anorod arrays

Typical XRD patterns of the samples synthesized in differentonditions are shown in Fig. 1. XRD shows that the films on FTOubstrates are rutile TiO2. All the diffraction peaks agree well with

able 1he samples synthesized by hydrothermal method under different conditions.

Sample 36% HCl (mL) Deionized water (mL)

S-a 10 10S-b 10 10S-c 10 10S-d 10 10S-e 10 10S-f 10 10S-g 10 10S-h 10 10

Fig. 1. XRD patterns of the samples, FTO, (a) S-a, (b) S-b, (c) S-c, (d) S-d, (e) S-e, (f)S-f, (g) S-g and (h) S-h.

those of the rutile TiO2 (JCPDS 89-4920). The XRD intensity is var-ied for the different samples prepared by changing the titaniumbutoxide concentration in the reaction solution, growth time andtemperature. Some rutile diffraction peaks including (1 1 0), (1 0 1),(2 0 0) and (2 1 1) are very weak and are finally disappeared fromcurve a–h, and only (0 0 2) diffraction peak is observed in curve hin Fig. 1, which suggests that the TiO2 nanorods grow in the [0 0 1]direction with the growth axis vertical to the FTO substrate. Theseresults were confirmed by FESEM and TEM.

Fig. 2 shows typical FESEM images of the morphology of TiO2film growing at 150 ◦C for 8 h. The images reveal that the TiO2nanorod arrays grow uniformly on the FTO substrate. Top (Fig. 2A)and side (Fig. 2B) views show that the nanorods are nearly perpen-dicular to the FTO substrate. The nanorods are tetragonal in shapewith square top facets, the expected growth habit for the tetragonalcrystal structure. The average diameter and length are 100–200 nmand 3.0 �m, respectively.

Fig. 3 shows typical FESEM images of the morphology of nanorodarrays growing in different conditions. In Fig. 3A, the surface of sam-ple S-a consists of the highly oriented TiO2 nanorods of 40–60 nmin diameter, 6.0 �m in length and a tetrahedron tip. However, therods are congregated in parallel axis direction and only separated attips. As the temperature increases from 150 ◦C to 180 ◦C, the formedTiO2 nanorods are congregated with a diameter of 50–100 nm anda length of 2.0 �m (Fig. 3E). When the temperature decreases from150 ◦C to 120 ◦C, the nanorods grow slightly smaller and shorterwith 20–40 nm in diameter and 0.5 �m in length (Fig. 3D). Toinvestigate the influence of solution concentration on the morphol-ogy, experiments with different concentrations are carried out, asshown in Fig. 2A and F. There appear some bigger nanorods when

the concentration of the Titanium butoxide is increased. It appearsthat when the concentration of the Titanium butoxide is low, thenanorods are not as aligned as those obtained using higher concen-tration solution. To find out the influence of the growth time, the

Titanium butoxide (mL) Temperature (◦C) Time (h)

0.4 150 40.4 150 80.4 150 240.4 120 40.4 180 40.8 150 40.8 150 160.8 150 24

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116 C. Cao et al. / Sensors and Actuators B 156 (2011) 114–119

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ig. 2. FESEM images of oriented rutile TiO2 nanorod film grown on FTO substratet 150 ◦C for 4 h (S-a). Top view of (a) low and (b) high magnified SEM image. The in

amples were prepared in different time, from 4 to 24 h for sam-

les S-a, S-b, and S-c, as shown in Fig. 3A–C, respectively. Clearly,he TiO2 nanorods grow bigger as the synthesis time increases.here is a strange phenomenon that some nanorods collide withneighboring nanorod and aggregate to a big nanorod and eventu-

ig. 3. FESEM images of the samples, (a) S-a, (b) S-b, (c) S-c, (d) S-d, (e) S-e, (f) S-f, (g) S-g aniews of each sample.

L of deionized water, 10 mL of hydrochloric acid, and 0.4 mL of titanium butoxidethe corresponding cross-sectional view of S-a.

ally stop growing when the synthesis time increases (Fig. 3B, C, and

G, F). The insets are the corresponding side view of the sample. Thenanorod arrays were also analysed using energy-dispersive X-rayspectroscopy (EDS), and the atomic ratio of Ti and O is close to 1:2(Fig. 3I).

d (h) S-h. (i) The EDS spectra of S-h. The insets are the corresponding cross-sectional

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C. Cao et al. / Sensors and Actuators B 156 (2011) 114–119 117

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The Growth of TiO2 nanorod arrays on FTO is stronglyelated to rutile’s inherent growth habit and hydrothermal prepa-ation conditions. The formation of rutile crystal nucleus ini(OCH2CH2CH2CH3)4 strongly acidic precursor solution can beescribed by the following process [30]. First, single polymerTiO(OH2)5]2+ forms by the hydrolysis of Ti(OCH2CH2CH2CH3)4.hen the single polymer [TiO(OH2)5]2+ combine through dehy-rating each other to form straight chain polymer by the edgeonnection. Finally, the straight chain polymers connect through

oints to form rutile crystal nucleus. Basing on the heterogeneousucleation of crystalline phase in solution, TiO2 nanocrystal par-icles coated onto FTO followed by heat treatment can be serveds the seeds of heterogeneous nucleation. The nanorods can be

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Fig. 5. Transmittance spectra of the (a) S-a and (b) S-b.

Fig. 6. Nyquist plots of the (a) S-b and (b) S-g electrodes in (A) dark and (B) under100 mW cm−2 simulated sunlight.

formed via the deposition and reorganization of TiO2 seeds dur-ing the dynamic chemical dissolution and deposition processes. Inaddition, a strong hydrochloric acid plays a significant role whenTiO2 growth units deposit on TiO2 crystal seeds, and it can promoteTiO2 crystal to grow into rods instead of particles.

The TEM image and the selected area electron diffraction (SAED)pattern of the as-prepared S-h are shown respectively in Fig. 4.The TEM image (Fig. 4A) shows the typical rod about 400 nm inwidth and 3.2 �m in length. The SAED pattern (Fig. 4B) reveals thesingle-crystalline structure of the rod growing along [0 0 1] direc-tion, which is in agreement with the XRD results.

Fig. 5 shows the transmittance spectra of TiO2 rod arrays onFTO from 240 to 850 nm, in which curves (a) and (b) refer to thetransmittance spectra of the S-a and S-b, respectively. The opticalband gap Eg can be obtained from the formula ˛h� = C(h� − Eg)2,where h is Planck constant, C is a constant, and ˛ is the opticalabsorption coefficient. The absorption edge for the TiO2 nanorodarrays is found to be 380 nm. The results show that the Eg of therutile TiO2 nanorod arrays is 3.26 eV, similar to the band gap ofTiO2 films [31–33]. Beside, it reveals that the TiO2 nanorod arrayscould only absorb the UV light less than 380 nm. The transparencyof sample S-b is smaller than that of sample S-a, indicating strongerdiffuse reflection from the separated nanorods of S-b, consistentwith the SEM image (Fig. 2B).

3.2. EIS characteristics

To reveal the difference in the interfacial characteristics of

the photoelectrodes made from the TiO2 nanorod arrays on FTOsubstrates, we measured EIS spectra of the samples in dark andillumination with the light intensity of 100 mW cm−2 at the elec-trode potential 0.1 V in 0.5 M NaSO4 at frequency ranging from0.5 Hz to 100 kHz. Fig. 6 shows EIS spectra of the sample S-a
Page 5: UV sensor based on TiO2 nanorod arrays on FTO thin film

118 C. Cao et al. / Sensors and Actuat

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Fig. 7. Photocurrent responses in 0.5 M NaSO4 solution for the (a) S-a and (b)S-b electrodes under radiation of (A) 100 mW cm−2 simulated sunlight and (B)3 mW cm−2 UV light illumination. The stability test (C) in exposure to different wave-lv

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reveal that the band gap of the rutile TiO2 nanorod arrays is 3.26 eV,which is similar to that of TiO2 films reported previously. The pho-

ength lights: 365, 405, 436, 546, 577 nm for the S-b electrodes. The potential is 0 Vs. Ag/AgCl. The magnified range of 300–500 nm is shown in the inset.

A) and S-b (B) electrodes, respectively. The Nyquist plots exhibithat the overall charge-transfer resistance of the S-a electrode ismaller than that of the S-b electrode not only in the dark butlso under light illumination. It is widely accepted that the diffu-ion of an electron within the photoanode film is in competitionith the recombination process at the photoanode film/electrolyte

nterface. Such aligned thinner nanorod channels have more inter-

aces between the nanorods and the solution to enhance chargeransfer as well as photosensitivity in the photoelectrochemicalrocess.

ors B 156 (2011) 114–119

3.3. Photocurrent effects

The photocurrent performance of TiO2 nanorod arrays in a 0.5 MNa2SO4 solution under different light sources are displayed in Fig. 7,which shows the typical real time photocurrent response of theTiO2 nanorod arrays when different light sources are switched onand off, exhibiting rapid photocurrent rise and decay. The responsetime of photocurrent is clearly less than 0.1 s. The dark currentdensity was found to be negligible for the S-a and S-b electrodes;however, once light is turned on, a photocurrent is instantly gen-erated and their intensity jump up to 47.69 and 14.93 �A cm−2,respectively, under irradiation of 100 mW cm−2 simulated sunlight.This is due to their different morphology and crystallinity. In termsof semiconductor physics, when an irradiation provides energyhigher than the band gap of TiO2, the energy can excite the elec-trons from valence band to conducting band and leave a hole invalence band. Thus, the photocurrent is mainly determined by theefficiency of photogenerated hole transfer at the TiO2/electrolyteand the electron diffusion to the back contact. The electron–holecould contribute to the photocurrent, leading to a low resistance inthe illumination state. When light is turned off, this photocurrentinstantly decreases to initial value in the dark. Clearly, larger pho-tocurrent response is observed for the S-b electrode under the samelight irradiation, which is almost 3 times as much as that of the S-aelectrode under irradiation of both 100 mW cm−2 simulated sun-light and 3 mW cm−2 UV light. These results imply that the higherspecific surface area from the separated nanorods on the S-b elec-trode plays an important role in the charge transfer and results ina better performance.

It is known that the stability of a photocurrent sensor is a keyfactor for its applications. Fig. 7C shows the stability of the S-belectrode, which is evaluated respectively by measuring the pho-tocurrents for 600 s in the exposure to the different wavelengthlight of 365, 405, 436, 546 and 577 nm. The plot reveals excellentstability of the photocurrent for the S-b electrode, which provesthat the influence from the gaseous atmosphere, humidity, andthermal effect from the IR in simulated sunlight could be effectivelyavoided. The photocurrent generated by the radiation of 405, 436,546 and 577 nm lights are too small to be considered when com-pared with that (12.87 �A cm−2) from the radiation of 365 nm light.It indicates that the S-b electrode is only sensitive to the 365 nmlight, as it provides energy higher than that of the bandgap of TiO2and can excite electrons from valence band to conduction band togenerate a photocurrent. The small photocurrent from the radia-tion of these visible lights might come from the imperfect blockingoff UV light by the filters as shown in Fig. 7C. The good stabilityof the sample demonstrates that the sensors can be used as a UVdetector in sunlight and other light sources. It is the original prop-erties of the TiO2 nanorods: rod shape and small dimension thatmight be considered as the most important parameters becausethey have a tremendous effect on transport properties and lightharvesting.

4. Conclusions

A facile hydrothermal method was used to grow highly orientedrutile TiO2 nanorod arrays on FTO substrates. The growth parame-ters like the growth time, the growth temperature, initial reactantconcentration could be controlled to fabricate the TiO2 nanorodarrays with desired lengths and densities. The UV–visible spectra

toelectric effect of the device has been systematically investigatedunder simulated sunlight and UV illumination via electrochemistrytechniques. The highly sensitive responses and excellent stability

Page 6: UV sensor based on TiO2 nanorod arrays on FTO thin film

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the synthesis of functional crystal, nanomaterials and its properties investigation.

Yongshu Tian is a Ph.D. student of the Department of Applied Physics at theChongqing University.

C. Cao et al. / Sensors and A

o the UV light reveal that the TiO2 nanorod arrays can be used toabricate the UV sensors/switches.

cknowledgements

This work is financially supported by the NSFC (60976055),nd Postgraduates’ Innovative Training Project (S-09109) of therd-211 Project, and the large-scale equipment sharing fund ofhongqing University.

eferences

[1] M. Gratzel, Sol–gel processed TiO2 films for photovoltaic applications, J. Sol–GelSci. Technol. 22 (2001) 7–13.

[2] Z.X. Ge, A.X. Wei, J. Liu, Synthesis photovoltaic devices performance of singlecrystalline TiO2 nanowire bundle arrays, J. Inorg. Mater. 25 (2010) 1105–1109.

[3] H. Chen, W.Y. Fu, H.B. Yang, Photosensitization of TiO2 nanorods with CdSquantum dots for photovoltaic devices, Electrochem. Acta 56 (2010) 919–924.

[4] M. Inaba, Y. Oba, F. Niina, Y. Murota, Y. Ogino, A. Tasaka, K. Hirota, TiO2(B) as apromising high potential negative electrode for large-size lithium-ion batteries,J. Power Sources 189 (2009) 580–584.

[5] Y.K. Zhou, L. Cao, F.B. Zhang, B.L. He, H.L. Li, Lithium insertion into TiO2 nan-otube prepared by the hydrothermal process, J. Electrochem. Soc. 150 (2003)A1246–A1249.

[6] P. Charoensirithavorn, Y. Ogomi, T. Sagawa, S. Hayase, S. Yoshikawa, A facileroute to TiO2 nanotube arrays for dye-sensitized solar cells, J. Cryst. Growth311 (2009) 757–759.

[7] M. Anpo, Preparation, characterization, and reactivities of highly functionaltitanium oxide-based photocatalysts able to operate under UV–visible lightirradiation: approaches in realizing high efficiency in the use of visible light,Bull. Chem. Soc. Jpn. 77 (2004) 1427–1442.

[8] B.Q. Su, Y.J. Ma, Y.L. Du, C. Yang, C.M. Wang, In situ photoelectrocatalyticdegradation behavior of methylene blue on nano-TiO2 modified electrode, J.Electrochem. Soc. 155 (2008) F213–F217.

[9] Y.B. Xie, Photoelectrochemical application of nanotubular titania photoanode,Electrochim. Acta 51 (2006) 3399–3406.

10] Y.P. Guoa, N.H. Leea, Hyo-Jin Oha, Cho-Rong Yoon, Preparation of titanatenanotube thin film using hydrothermal method, Thin Solid Films 516 (2008)8363–8371.

11] M. Paulose, K. Shankar, S. Yoriya, H.E. Prakasam, O.K. Varghese, G.K. Mor, T.A.Latempa, A. Fitzgerald, C.A. Grimes, Anodic growth of highly ordered TiO2 nan-otube arrays to 134 �m in length, J. Phys. Chem. B 110 (2006) 16179–16184.

12] S.K. Pradhan, P.J. Reucroft, F. Yang, Dozier Growth of TiO2 nanorods by met-alorganic chemical vapor deposition, J. Cryst. Growth 256 (2003) 83–88.

13] A. Zaban, S.T. Aruna, S. Tirosh, B.A. Gregg, Y. Mastal, The effect of the preparationcondition of TiO2 colloids on their surface structures, J. Phys. Chem. B 104 (2000)4130–4133.

14] S. Srimala, C.W. Lai, Study on the formation and photocatalytic activity oftitanate nanotubes synthesized via hydrothermal method, J. Alloys Compd. 490(2010) 436–442.

15] C. Suwanchawalit, S. Wongnawa, Triblock copolymer-templated synthesis ofporous TiO2 and its photocatalytic activity, J. Nano Res. 12 (2010) 2895–2906.

16] A. Michailowski, D. AlMawlawi, G.S. Cheng, M. Moskovits, Highly regularanatase nanotubule arrays fabricated in porous anodic templates, Chem. Phys.Lett. 349 (2001) 1–5.

17] Y.S. Tian, C.G. Hu, X.S. He, C.L. Cao, G.S. Huang, K.Y. Zhang, Titania nanotubearrays for light sensor and UV photometer, Sens. Actuators B 144 (2010)

203–207.

18] P. Xiao, D.W. Liu, B.B. Garcia, S. Sepehri, Electrochemical and photoelectricalproperties of titania nanotube arrays annealed in different gases, Sens. Actua-tors B 134 (2008) 367–372.

19] W. Li, S.I. Shah, C.-P. Haung, C. Ni, Mater, Metallorganic chemical vapor deposi-tion and characterization of TiO2 nanoparticles, Sci. Eng. B 96 (2002) 247–253.

ors B 156 (2011) 114–119 119

20] T. Kuykendall, P. Ulrich, S. Aloni, P.D. Yang, Complete composition tunabil-ity of InGaN nanowires using a combinatorial approach, Nat. Mater. 6 (2007)951–956.

21] D.A. Boyd, L. Greengard, M. Brongersma, M.Y. El-Naggar, D.G. Goodwin,Plasmon-assisted chemical vapor deposition, Nano Lett. 6 (2006) 2592–2597.

22] B. Mukherjee, C. Karthik, N. Ravishankar, Hybrid Sol–Gel combustion synthesisof nanoporous anatase, J. Phys. Chem. C 113 (2009) 18204–18211.

23] K. Kanie, T. Sugimoto, Shape control of anatase TiO2 nanoparticles by aminoacids in a gel–sol system, Chem. Commun. (Cambridge) (2004) 1584–1585.

24] X.J. Feng, J. Zhai, L. Jiang, The fabrication and switchable superhydrophobicityof TiO2 nanorod films, Angew. Chem. Int. Ed. 44 (2005) 5115–5118.

25] X. Feng, K. Shankar, O.K. Varghese, M. Paulose, T.J. Latempa, C.A. Grimes, Verti-cally aligned single crystal TiO2 nanowire arrays grown directly on transparentconducting oxide coated glass: synthesis details and applications, Nano Lett. 8(2008) 3781–3786.

26] B. Liu, E.S. Aydil, Growth of oriented single-crystalline rutile TiO2 nanorods ontransparent conducting substrates for dye-sensitized solar cells, J. Am. Chem.Soc. 131 (2009) 3985–3990.

27] H.E. Wang, Z.H. Chen, Y.H. Leung, C.Y. Luan, C.P. Liu, Y.B. Tang, C. Yan, W.J.Zhang, J.A. Zapien, I. Bello, S.T. Lee, Hydrothermal synthesis of ordered single-crystalline rutile TiO2 nanorod arrays on different substrates, Appl. Phys. Lett.96 (2010) 263104.

28] G.K. Mor, O.K. Varghese, M. Paulose, C.A. Grimes, Transparent highly orderedTiO2 nanotube arrays via anodization of titanium thin films, Adv. Funct. Mater.15 (2005) 1291–1296.

29] X.S. He, C.G. Hu, B. Feng, B.Y. Wan, Y.S. Tian, Vertically aligned TiO2 nanorodarrays as a steady light sensor, J. Electrochem. Soc. 157 (11) (2010) J381–385.

30] E.W. Shi, Z.Z. Chen, R.L. Yuan, Y.Q. Zheng, Hydrothermal Crystallography, Sci-ence Press, Beijing, 2004, p. 150.

31] J.R. Simpson, H.D. Drew, S.R. Shinde, R.J. Choudhary, S.B. Ogale, T. Venkatesan,Optical band-edge shift of anatase Ti1−xCoxO2-delta, Phys. Rev. B 69 (2004)193205.

32] D. Mardare, M. Tasca, M. Delibas, G.I. Rusu, On the structural properties andoptical transmittance of TiO2 r.f. sputtered thin films, Appl. Surf. Sci. 156 (2000)200–206.

33] J.G. Yu, G.P. Dai, B.B. Huan, Fabrication characterization of visible-light-drivenplasmonic photocatalyst Ag/AgCl/TiO2 nanotube arrays, J. Phys. Chem. C 113(2009) 16394–16401.

Biographies

Chunlan Cao received her MS degree from Chongqing University in 2006. At presentshe is a Ph.D. student under the supervision of Prof. Chenguo Hu of the Departmentof Applied Physics at Chongqing University.

Chenguo Hu is a professor of Physics in Chongqing University. She received herPh.D. in materials from Chongqing University in 2003. Her research interestsinclude methodology of synthesising nanomaterials, investigation of morphology-dependent properties of nanomaterials and gas- or bio-sensors.

Xue Wang is a Ph.D. student of the Department of Applied Physics at the ChongqingUniversity.

Shuxia Wang is a professor of Physics in Chongqing University. She received herPh.D. in materials from Chongqing University in 2002. Her main fields of interest are

Hulin Zhang is a Ph.D. student of the Department of Applied Physics at ChongqingUniversity.