CdS and PbS quantum dots co-sensitized TiO2 nanorod arrays with improved performance for solar cells application

Materials Science in Semiconductor Processing 16 (2013) 435–440

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CdS and PbS quantum dots co-sensitized TiO2 nanorod arrays with improved performance for solar cells application Jie Jiao, Zheng-Ji Zhou, Wen-Hui Zhou, Si-Xin Wu n The Key Laboratory for Special Functional Materials of MOE, Henan University, Kaifeng 475004, PR China.

a r t i c l e i n f o

abstract

Available online 26 September 2012

In this paper, we report a novel CdS and PbS quantum dots (QDs) co-sensitized TiO2 nanorod arrays photoelectrode for quantum dots sensitized solar cells (QDSSCs). TiO2 film consisting of free-standing single crystal nanorods with several microns high and 90–100 nm in diameter were deposited on a conducting glass (SnO2:FTO) substrate by hydrothermal method. Then CdS/PbS QDs were deposited in turn on TiO2 nanorods by facile SILAR technique. The FTO/TiO2/CdS/PbS, used as photoelectrode in QDSSCs, produced a light to electric power conversion efficiency (Eff) of 2.0% under AM 1.5 illumination (100% sun), which shows the best power conversion efficiency compared with single CdS or PbS sensitized QDSSCs. One dimension TiO2 nanorod provides continuous charge carrier transport pathways without dead ends. The stepwise structure of the band edges favored the electron injection and the hole-recovery for both CdS and PbS layers in photoelectrode, which may gave a high electric power conversion efficiency. The facile preparation and low cost nature of the proposed method and structure make it has a bright application prospects in photovoltaic areas in the future. & 2012 Elsevier Ltd. All rights reserved.

Keywords: Quantum dots sensitized solar cells Co-sensitized TiO2 nanorod arrays

1. Introduction Quantum dot-sensitized solar cells (QDSSCs) have become one of the most popular subjects for research into the next generation of solar cells, because quantum dots (QDs) offer the impressive ability to harvest sunlight, advantageous features of photostability, high molar extinction coefficients, size-dependent optical properties, ease of fabrication and low costs [1–6]. Theoretically, the maximum thermodynamic conversion efficiency of a QDSSC can reach 44% [7]. Although QDSSCs have been studied for many years, at present the highest values of efficiency that are around 5.0% [8–10].

n Corresponding author. Tel.: þ86 378 388 1358; fax: þ 86 378 388 1358. E-mail address: [email protected] (S.-X. Wu).

1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.08.009

In order to improve the performance of solar cell devices, strategies that focused on using nanostructures to improve the electron transport rate [11,12], light harvesting efficiency [13,14], and decrease the degree of charge recombination [15–17] have been investigated. Using one-dimensional (1D) single crystal TiO2 nanorods to take the place of spherical TiO2 nanoparticles can improve the rate of electron transport between electrode and QDs, thereby increasing the incident photon-to-current conversion efficiency [18,19]. Recently, Mallouk has demonstrated the diffusion coefficient in the single-crystal rutile nanorod-based solar cell is over 200 times higher than that of the rutile nanoparticles-based cell [20]. In addition, co-sensitized QDSSC incorporating at least two different types and/or sizes QDs could improve the light harvesting efficiency [21,22]. Moreover, one of the sensitizer may act as a buffer layer [23,24] to control the band alignments of semiconductor at the interface, accordingly modifying the surface states and reducing recombination. So co-sensitized is a promising way to combine the

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advantages of photo capture and charge separation, thus improving the efficiency of final QDSSCs devices. At present, more and more researchers have focused their attention on co-sensitized strategy in QDSSCs, such as CdS/CdSe [22], CdS/CdTeS [25] co-sensitizers have been used in solar cell devices and showed higher conversion efficiency than single nanocrystals sensitized QDSSCs. PbS with a bulk band gap of 0.41 eV can absorb sunlight in the infrared region [26], thereby using PbS as a semiconductor sensitizer in cosensitized QDSSCs can broaden the optical absorption area and further improve the performance [27,28]. In this work, a novel, simple and low cost CdS and PbS quantum dots (QDs) co-sensitized TiO2 nanorod arrays were used as photoelectrode for QDSSCs. Firstly, one-dimensional (1D) rutile TiO2 nanorod arrays were synthesized by a hydrothermal method on the FTO substrate. Then the PbS and CdS QDs were attached to TiO2 nanorod by a sequential successive ionic adsorption and reaction (SILAR) technique, respectively. Compared with TiO2 nanorods photoelectrode sensitized with single QDs (CdS or PbS), the CdS/PbS QDs cosensitized TiO2 nanorods photoelectrode showed broadened absorption spectrum and remarkable photovoltaic performance. A short circuit photocurrent density of 11.5 mA/cm2 and a conversion efficiency of 2.0% under AM 1.5 illumination were achieved in our work. 2. Experimental section 2.1. Materials Titanium butoxide, chromic nitrate [Cd(NO3)2] and lead acetate trihydrate (PbAc) were purchased from SigmaAldrich China. Concentrated hydrochloric acid (36.5–38% by weight), Na2S  9H2O were purchased from Tianjin Chemical Reagents Co. All the reagents were used without further purification. Triply-deionized water (resistivity of 18.2 MO  cm  1) was obtained from MilliQ ultra-pure water system (Millipore, USA). FTO coated glass slides (F:SnO2, 14 O/square, Nippon Sheet Glass Group, Japan) were thoroughly washed with a mixed solution of deionized water, acetone, and 2-propanol (volume rations of 1:1:1) under sonication for 60 min. 2.2. Preparation of TiO2 nanorod arrays The rutile TiO2 nanorod arrays were synthesized by a hydrothermal method, details for the fabrication of TiO2 nanorod arrays similarly to that described by Liu and Aydil [29]. Typically, 30 mL deionized water was mixed with 30 mL concentrated hydrochloric acid, after the mixture was stirred at ambient conditions for 5 min, 1 mL of titanium butoxide was added to the mixture and stirred for another 5 min. Then the mixture was transferred to a sealed Teflon reactor (100 mL volume), and two pieces of cleaned FTO substrates were placed within the reactor. The hydrothermal synthesis was conducted at 150 1C in an electric oven for 12 h. After synthesis, the Teflon reactor was cooled to room temperature under flowing water and the FTO substrates were taken out and rinsed extensively with deionized water and ethanol.

2.3. TiO2 nanorod arrays Decorated with PbS and CdS QDs PbS and CdS QDs were attached to TiO2 by a sequential successive ionic adsorption and reaction technique. Typically, the process involved dipping the TiO2 nanorod arrays film into an mixed solution of ethanol and aqueous (which leads to better penetrating in the nanorod array than aqueous or ethanol solution) containing 0.1 M Cd(NO3)2 for 2 min and rinsing with ethanol. Then the TiO2 nanorod arrays film was dipped into an ethanol and aqueous solution containing 0.1 M Na2S for another 2 min, rinsed with ethanol and dried at 200 1C for 10 min. The entire procedure is termed as cycle 1 and the incorporated amount of CdS can be increased by repeating the assembly of cycles 1. Then the CdS decorated TiO2 nanorod arrays film was dipped into an ethanol and aqueous solution containing 0.1 M PbAc for 2 min and rinsed with ethanol. After that the CdS decorated TiO2 nanorod arrays film was dipped into an ethanol and aqueous solution containing 0.1 M Na2S for another 2 min, rinsed with ethanol and dried at 200 1C for 10 min. This entire procedure is termed as cycle 2 and the incorporated amount of PbS can be increased by repeating the assembly of cycles 2. 2.4. Characterization A model JEOL JSM-6700 F field-emission scanning electron microscopy fitted with an energy dispersive X-ray spectrometer (EDX) was used to characterize the morphologies and elemental analysis of the samples. Transmission electron microscopy (TEM) images were obtained with a Philips CM-10 transmission electron microscope with a digital camera. Diffuse reflectance absorption spectra of bare TiO2 nanorod arrays and TiO2 nanorod arrays decorated with PbS and CdS QDs were recorded in the range from 200 to 1100 nm using a CARY5000 spectroscopy with BaSO4 as a reference. 2.5. Solar cell fabrication and photoelectricity measurements An open sandwich-type cell was fabricated in the air by clamping the QDs sensitized TiO2 nanorod arrays, a drop of electrolyte, and a counter electrode with two clips. The counter electrode was fabricated by painting another conductive glass plate with highly conductive Pt paint. The higher efficiency in polysulfide electrolyte is due to its ability to inhibit photocorrosion of CdS and PbS QDs [30], so polysulfide electrolyte was used in the test. The Oriel solar simulator served 100 mW/cm2 (100% sun) as a light source. The illuminated area of the cell was 0.1 cm2. Current–potential (I–V) measurements of QDs–TiO2 films were investigated using a Keithley 2420 programmable Source Meter and an Oriel solar simulator. 3. Results and discussion 3.1. Morphology analysis Morphologies of FTO/TiO2 and FTO/TiO2/PbS/CdS photoelectrodes are shown in Fig. 1. It is obvious from Fig. 1(a)

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contact between the TiO2 nanorod and the CdS/PbS QDs decorated TiO2 nanorods. 3.2. XRD and element analysis Fig. 2(A) shows XRD patterns of the FTO, FTO/TiO2 and FTO/TiO2/CdS/PbS photoelectrodes. Compared to XRD pattern of FTO glass substrates (JCPDS no. 41-1445), XRD pattern of FTO/TiO2 photoelectrode exists tetragonal rutile TiO2 diffraction peaks Fig. 2A (b). After decorated by CdS/ PbS QDs, the height of diffraction peaks ascribe to TiO2 reduced which indicates that the PbS/CdS QDs had been attached to the TiO2 nanorods. Additional diffraction peaks appeared in Fig. 2A (c) can be attributed to cubic CdS (JCPDS no. 75-1546) and/or PbS (JCPDS no. 78-1054). Due to the similarity of XRD diffraction peaks of CdS and PbS QDs, EDX was applied to determine the elements composition of the FTO/TiO2/CdS/PbS photoelectrode. As shown in Fig. 2(B), Ti, O, Cd, Pb and S elements present which suggested the TiO2 nanorods were successfully decorated by CdS/PbS QDs (each for 3 cycles). The atomic ratio of Ti:O:Cd:Pb:S is determined to be 17.61: 80.22: 0.50: 0.54: 1.14. It is found that the atomic ratio of Cd/Pb and (CdþPb)/S are almost one. These results provide powerful evidence for successful coating of PbS and CdS QDs on the surface of TiO2 nanorods. 3.3. UV–vis absorption spectroscopy For the comparison of light absorption properties of different QDs sensitized TiO2 nanorod array films, the spectra of the photoelectrodes prepared by 6 cycles for QDs are Fig. 1. Morphologies of FTO/TiO2 and FTO/TiO2/CdS/PbS photoelectrodes: typical top view SEM images of FTO/TiO2 photoelectrode (a) and FTO/TiO2/CdS/PbS photoelectrode (b); typical cross-sectional view of the well-aligned TiO2 nanorod array (c)and FTO/TiO2/CdS/PbS photoelectrode(d); typical TEM image of the single bare TiO2 nanorod (e) and single TiO2 nanorod deposited with CdS QDs (3 cycles) and PbS QDs (3 cycles) (f).

that the entire surface of the FTO substrate is covered uniformly and densely with TiO2 nanorods. The nanorods are tetragonal in shape with square top facets, which is the expected growth habit for the tetragonal crystal structure. From the cross-sectional view of the sample, we can see that the well aligned nanorods are nearly vertically on the FTO substrate Fig. 1(c). The length of the nanorods is about 3mm and the average diameter is about 100 nm. From Fig. 1(b) and (d) which show TiO2 nanorods decorated with CdS/PbS QDs, we can see the surface of the nanorod arrays became rough and the diameters increased. It is obvious that PbS and CdS QDs were deposited evenly on the top and lateral of the TiO2 nanorods. The morphology of the TiO2 nanorods and PbS/CdS QDs decorated TiO2 nanorods were further characterized by TEM, the results are showed in Fig. 1(e) and (f). Fig. 1(e) shows the TEM image of a bare single TiO2 nanorod which is very smooth. Fig. 1(f) is the typical TEM image of a TiO2 nanorod array deposited with CdS/PdS QDs for 3 cycles. After deposited with CdS/PdS QDs, the surface of the nanorod is rough, and large amounts of CdS/PbS QDs had been deposited on the TiO2 nanorod. The diameter of CdS/PbS QDs was about 10–20 nm. These figures show good

Fig. 2. (A) XRD pattern of FTO (a) FTO/TiO2 (b) and FTO/TiO2/CdS/PbS (c).The peaks correspond to cubic CdS/PbS and rutile TiO2. Fig. 2(B) EDX spectrum of FTO/TiO2/CdS/PbS photoelectrode (CdS/PbS QDs each for 3 cycles).

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12

Absorbance(a.u.)

d

TiO2

b

TiO2/PbS

c

TiO2/CdS

d

TiO2/PbS/CdS

e

c b

a

d

TiO2/CdS/PbS

Current Density (mA/cm2)

e

a

10

TiO2/PbS/CdS

b

TiO2/PbS

c

TiO2/CdS

d

TiO2/CdS/PbS

8 c

6 b

4 2

a

a 0

200

300

400

500

600

700

800

900

0.0

1000 1100

0.1

0.2

Wavelength(nm) Fig. 3. Diffuse reflectance absorption spectra of FTO/TiO2, FTO/TiO2/PbS (PbS QDs for 6 cycles), FTO/TiO2/CdS (CdS QDs for 6 cycles), FTO/TiO2/ CdS/PbS (CdS and PbS QDs each for 3 cycles) and FTO/TiO2/PbS/CdS (PbS and CdS QDs each for 3 cycles).

shown in Fig. 3. The maximum peak of the TiO2 nanorod arrays film occurs at around 380 nm and has no significant absorption in visible-light region due to its large energy gap (3.2 eV). The absorption intensity of the QDs sensitized TiO2 nanorod arrays films is increased after deposition of CdS and/ or PbS QDs in visible-light region. We can see that the absorption of the FTO/TiO2/CdS/PbS photoelectrode decayed more slowly than that of FTO/TiO2, FTO/TiO2/CdS, FTO/TiO2/ PbS and FTO/TiO2/PbS/CdS photoelectrode. Compared to other photoelectrodes, the FTO/TiO2/CdS/PbS photoelectrode showed broadened and enhanced photo absorption. The band gap of PbS QDs were determined to be 1.4 eV from the absorption spectrum, which is larger than that of bulk PbS (0.41eV), indicating that the sizes of PbS on the TiO2 nanorod are within the nanoscale (like QDs). Using the empirical equations proposed by Yu et al. [31], the diameter of PbS was determined to be about 3 nm. 3.4. Photovoltaic performance of the photoelectrodes All the photoelectrodes were measured versus Pt under simulated sunlight with an illumination intensity of 100 mW/cm2 (100% sun). The I–V measurements show that single PbS and CdS QDs sensitized TiO2 nanorod arrays yielded a Jsc of 4.80 mA/cm2 and 8.84 mA/cm2, respectively (Fig. 4). The Jsc of TiO2 nanorod arrays co-sensitized by CdS and PbS QDs is larger than that of any single sensitized samples (PbS/TiO2 or CdS/TiO2). The characteristic parameters of these solar cells, short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF) and overall photoconversion efficiency (Eff) were summarized in Table 1. It can be seen that the Eff of the device use FTO/TiO2/PbS and FTO/ TiO2/CdS as the photoelectrode only achieve 0.61% and 1.12%. Under 100% sun illumination, the co-sensitized FTO/ TiO2/CdS/PdS photoelectrode exhibited the best performance, the Voc, Jsc, FF and Eff of TiO2/CdS (3 cycles)/PbS (3 cycles) photoelectrode increased to 581 mV, 11.5 mA/cm2, 0.30% and 2.0%, respectively. The efficiency of the device use FTO/ TiO2/CdS/PbS as the photoelectrode is larger than the sum

0.3

0.4

0.5

0.6

Voltage(V) Fig. 4. I–V curves of FTO/TiO2/PbS/CdS photoelectrode (PbS/CdS QDs deposited each for 3 cycles, (a)); FTO/TiO2/PbS photoelectrode (PbS QDs deposition for 6 cycles, (b)); FTO/TiO2/CdS photoelectrode (CdS QDs deposition for 6 cycles, (c)) and FTO/TiO2/CdS/PbS photoelectrode (CdS/ PbS QDs deposited each for 3 cycles, (d)).

Table 1 Parameters obtained from the photocurrent–voltage measurements of the solar cells using various photoelectrodes. Sensitizer

Jsc (/msA cm  2)

Voc (/mV)

FF

Eff (%)

PbS(6 CdS(6 PbS(3 CdS(3

4.80 8.84 1.21 11.5

474 490 347 581

0.27 0.26 0.35 0.30

0.61 1.12 0.15 2.02

cycles) cycles) cycles)/CdS(3 cycles) cycles)/PbS(3 cycles)

efficiency of the devices use FTO/TiO2/PbS and FTO/TiO2/CdS as photoelectrodes separately. As contrast, the position of the two kind QDs was also reversed to study the structure effect of the co-sensitizers on the device performance. The I–V curve and the related parameters for the device using FTO/ TiO2/PbS (3 cycles)/CdS (3 cycles) photoelectrode are also shown in Fig. 4 and Table 1. Compared with the FTO/TiO2/ CdS/PbS device, the FTO/TiO2/PbS/CdS device causes a significant decrease of Jsc and Voc, and leading to significant decrease of efficiency value (Eff¼ 0.15%). These results indicate that a cascade structure of QDs sensitizers may greatly enhance the performance of a QDSSC. However, the position of the two kinds of QDs should be placed in proper order on TiO2 nanorod arrays. The band edges of TiO2, CdS, and PbS are 3.2 eV, 2.4 eV and 1.4 eV, respectively. And the possible explanation for increased photovoltaic performance of FTO/TiO2/CdS/PbS photoelectrode and decreased photovoltaic performance of the FTO/TiO2/PbS/CdS photoelectrode are as below. When CdS and PbS were brought in the cascade structure, the energy levels difference (Fermi level difference if they follow Fermi–Dirac statistic) between CdS and PbS causes the electrons flow from CdS (higher level) to PbS (lower level). Such electron transfer is known as the Fermi level alignment if the material can be described by Fermi–Dirac distribution. The redistribution of the electrons between CdS and PbS is supposed to trigger a downward and upward shift of the band edges for CdS and PbS, respectively. Therefore,

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ee-

e-

ePbS 1.4

PbS 1.4

CdS 2.4 TiO2 3.2

439

CdS 2.4 h+

TiO2 3.2 h+

h+

h+

Scheme 1. A proposed band edges structure for TiO2/CdS/PbS electrode after re-distribution of electrons between CdS and PbS interface (a, termed as Fermi level alignment for bulk materials) and a proposed band edges structure for TiO2/PbS/CdS electrode (b).

the resulting band edges for the TiO2/CdS/PbS device are inferred to have a stepwise structure as shown in Scheme 1a. That is, the insertion of a CdS layer between TiO2 and PbS layer elevates the conduction band edge of PbS, giving a higher driving force for the injection of excited electrons from PbS layer to CdS layer. This is why a higher Eff was obtained for the TiO2/CdS/PbS device compared to CdS or PbS QDs single-sensitized device. When both CdS and PbS are photoexcited under white light illumination, the stepwise structure of the band edges is advantageous not only to the electron injection but also to the hole-recovery for both CdS and PbS layers. On the contrary, in the TiO2/PbS/CdS device, the conduction and valence bands edges of intermediate PbS will be higher to those of CdS (Scheme 1b). In such an energy structure, energy barriers exist for injecting an excited electron from CdS to PbS layer and transferring a hole from PbS to Cds layer, which causes a low Eff of the TiO2/PbS/CdS device [22].

4. Conclusion In summary, we have successfully used CdS and PbS QDs as co-sensitizers to improve the performance of QDSSCs. In comparison to single QDs sensitized QDSSCs, the CdS/PbS co-sensitized photoelectrode showed substantially improved overall photoconversion efficiency. The efficiency of the CdS/PbS co-sensitized solar cell is as high as 2.0% under one sun illumination (AM1.5, 100 mW/cm2). The high electric power conversion efficiency may contribute to one dimension TiO2 nanorod and cascade band gap structure of FTO/TiO2/CdS/PbS. This work also demonstrated the potential applications of CdS/ PbS co-sensitized TiO2 nanorod arrays for QDSSCs. More importantly, the proposed co-sensitized CdS/PbS QDSSC fabrication approach offers low-cost and easy preparation nature, and this device can reach better photovoltaic performance. We believe that this proposed method and structure have a bright application prospects in photovoltaic areas in the future.

Acknowledgment This project is supported by the National Natural Science Foundation of China (20871041 and 20903033) and the New Century Excellent Talents in University (NCET-08–0659). References [1] S.M. Yang, C.H. Huang, J. Zhai, Z.S. Wang, L. Jiang, Journal of Materials Chemistry 12 (2002) 1459–1464. [2] R.D. Schaller, V.I. Klimov, Physics Review Letters 92 (2004) 186601–186604. [3] P.V. Kamat, Journal of Phyics Chemistry C 112 (2008) 18737–18753. [4] H. Lee, M. Wang, P. Chen, D.R. Gamelin, S.M. Zakeeruddin, ¨ M. Gratzel, M.K. Nazeeruddin, Nano Letters 9 (2009) 4221–4227. [5] A. Braga, S. Gime!nez, I. Concina, A. Vomiero, !I. Mora-Sero, Journal of Phyics Chemistry Letters 2 (2011) 454–460. [6] F.Y. Zhao, G.S. Tang, J.B. Zhang, Y. Lin, Electrochimica Acta 62 (2012) 396–401. [7] M.C. Hanna, A.J. Nozik, Journal of Applied Physics 100 (2006) 074510–074518. [8] Q. Zhang, X. Guo, X. Huang, S. Huang, D. Li, Y. Luo, Q. Shen, T. Toyoda, Q. Meng, Physical Chemistry Chemical Physics 13 (2011) 4659–4667. [9] J.A. Chang, J.H. Rhee, S.H. Im, Y.H. Lee, H.J. Kim, S.I. Seok, M.K. Nazeeruddin, M. Gratzel, Nano Letters 10 (2010) 2609–2612. [10] P.K. Santra, P.V. Kamat, Journal of the American Chemical Society 134 (2012) 2508–2511. [11] H.B. Wu, H.H. Hng, X.W. Lou, Advanced Materials 24 (2012) 2567–2571. [12] F. Xu, L.T. Sun, Energy and Environmental Science 4 (2011) 818–841. ¨ [13] O. Niitsoo, S.K. Sarkar, C. Pejoux, S. Ruhle, D. Cahen, G. Hodes, Journal of Photochemistry and Photobiology A 181 (2006) 306–313. [14] G.Y. Lan, Z. Yang, Y.W. Lin, Z.H. Lin, H.Y. Liao, H.T. Chang, Journal of Materials Chemistry 19 (2009) 2349–2355. [15] Q. Shen, J. Kobayashi, L.J. Diguna, T. Toyoda, Journal of Applied Physics 103 (2008) 084304–084309. [16] I. Mora-Sero´, S. Gime´nez, F. Fabregat-San´tiago, R. Go´mez, Q. Shen, T. Toyoda, J. Bisquert, Accounts of Chemical Research 42 (2009) 1848–1857. [17] S. Liu, B. Liu, K. Nakata, T. Ochiai, T. Murakami, A. Fujishima, Journal of Nanomaterials, http://dx.doi.org/10.1155/2012/491927 2012 (2012), article ID 491927, 5 pp. [18] A. Kongkanand, K. Tvrdy, K. Takechi, M. Kuno, P.V. Kamat, Journal of the American Chemical Society 130 (2008) 4007–4015. [19] X.Y. Liu, W.J. Zhou, Z.M. Yin, X.P. Hao, Y.Z. Wu, X.G. Xu, Journal of Materials Chemistry 22 (2012) 3916–3921. [20] X.J. Feng, K. Zhu, A.J. Frank, C.A. Grimes, T.E. Mallouk, Angewandte Chemie International Edition 51 (2012) 1–5.

440

J. Jiao et al. / Materials Science in Semiconductor Processing 16 (2013) 435–440

[21] W. Lee, S.H. Kang, S.K. Min, Y.E. Sung, S.H. Han, Electrochemistry Communications 10 (2008) 1579–1582. [22] Y.L. Lee, C.F. Chi, S.Y. Liau, Chemistry of Materials 22 (2010) 922–927. [23] X. Hu, Q.X. Zhang, X.M. Huang, D.M. Li, Y.H. Luo, Q.B. Meng, Journal of Materials Chemistry 21 (2011) 15903–15905. [24] T.B. Yang, W.Z. Cai, D.H. Qin, E.G. Wang, L.F. Lan, X. Gong, J.B. Peng, Y. Cao, Journal of Physical Chemistry C 114 (2010) 6849–6853. [25] Y. Medina-Gonzalez, W.Z. Xu, B. Chen, N. Farhanghi, P.A. Charpentier, Nanotechnology 22 (2011) 065603–065611. [26] T.R. Heera, L. Cindrella, Materials Science in Semiconductor Processing 14 (2011) 151–156.

[27] H.J. Lee, H.C. Leventis, S.J. Moon, P. Chen, S. Ito, S.A. Haque, T. Torres, ¨ F. Nuesch, T. Geiger, S.M. Zakeeruddin, M. Gr¨atzel, M.K. Nazeeruddin, Advanced Functional Materials 19 (2009) 2735–2742. ¨ ¨ [28] L. Etgar, T. Moehl, S. Gabriel, S.G. Hickey, A. Eychmuller, M. Gratzel, ACS Nano 6 (2012) 3092–3099. [29] B. Liu, E.S. Aydil, Journal of the American Chemical Society 131 (2009) 3985–3990. [30] Y. Tachibana, H.Y. Akiyama, Y. Ohtsuka, T. Torimoto, S. Kuwabata, Chemistry Letters 36 (2007) 88–89. [31] W.W. Yu, X. Peng, Angewandte Chemie International Edition 41 (2002) 2368–2371.