Materials Letters 65 (2011) 548–551
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Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Low temperature aqueous chemical synthesis of CdS sensitized ZnO nanorods S.A. Vanalakar a, R.C. Pawar a, M.P. Suryawanshi a, S.S. Mali a, D.S. Dalavi a, A.V. Moholkar a,b, K.U. Sim b, Y.B. Kown b, J.H. Kim b, P.S. Patil a,⁎ a b
Thin film Materials Laboratory, Dept. of Physics, Shivaji University, Kolhapur-416 004, India Dept. of Materials Science and Engineering, Chonnam National University, Gwangju-500 757, South Korea
a r t i c l e
i n f o
Article history: Received 18 September 2010 Accepted 21 October 2010 Available online 29 October 2010 Keywords: CdS nanoparticles sensitized ZnO nanorods
a b s t r a c t Cadmium sulfide nanoparticles (CNPs) sensitized zinc oxide nanorod arrays (ZNRs) were synthesized in the two step deposition process at relatively low temperature. The vertically aligned ZNRs were grown on the conducting glass substrates (FTO) using aqueous chemical method, followed by the deposition of CNPs at 70 °C using chemical bath deposition (CBD) technique. The samples were characterized by optical absorption, X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and photoluminescence spectroscopy (PL). Further, the photoelectrochemical (PEC) performance of ZNRs with and without CNPs sensitization was tested in Na2S–NaOH–S and Na2SO4 electrolyte, respectively. When the CNPs are coated on the ZNRs, the optical absorption is enhanced and band edge is shifted towards visible region (525 nm) as compared with ZNRs (375 nm). The sample sensitized with CNPs shows higher photoelectrochemical (PEC) performance with maximum short circuit current of (Isc) 2.60 mA/cm2. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The one-dimensional (1D) nanostructures exhibit unique optoelectronic properties and provide efficient photon absorption, fast electron transport and collection. The 1D structure of metal oxides such as ZnO and TiO2 constitutes a new class of photoelectrode materials in solar cells, and number of other novel devices [1]. However, their wide band gap energy requires ultraviolet (UV) light irradiation, and therefore only a small fraction of solar spectrum is harnessed for power conversion. Many researchers attempted to extend their solar response more efficiently from UV to visible region. For this, considerable efforts like dye and narrow band gap semiconductor nanoparticle sensitization have been made and studied. The dye sensitized solar cells (DSSC) is exhaustively investigated (efficiency of up to 11%) [2]. But, it is very difficult to grow TiO2 anisotropically to obtain hierarchical and ordered structures. However, ZnO favors formation of anisotropic structures, exhibit much higher electron mobility than TiO2 (155 cm2 V−1 s−1 vs. 10−5 cm2 V−1 s−1)5, exciton binding energy (60 meV), breakdown strength and exciton stability [3]. These properties make ZnO more desirable in DSSC application. However, the carboxylic group of dye molecules reacts with Zn+ ions and form aggregates, which reduces the electron injection efficiency and stability of the solar cells.
⁎ Corresponding author. Fax: +91 231 2691533. E-mail address:
[email protected] (P.S. Patil). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.10.067
Fig. 1. XRD pattern of ZnO and CdS–ZnO samples.
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multiple electron-hole pair generation per incident photon to achieve higher conversion efficiency. The numbers of researchers have proposed visible band gap semiconductors for effective sensitization, such as CdSe, InP, CdTe, PbS and CdS [4–8]. Among these sensitizers, CdS has shown much promise as an effective sensitizer due to its reasonable band gap (2.4 eV) and offer new opportunities for light harvesting [9]. Among the various chemical and physical routes chemical bath deposition (CBD) has been employed to grow ZNRs and CNPs including layer-by-layer assembly [10–12]. In CBD, the growth rate is controllable by solution, pH, temperature and relative concentrations of the reactants in the bath solution. In the present work, we have synthesized ZNRs using aqueous chemical method, and the grown arrays were sensitized with CNPs by CBD method. The CNPs sensitized ZNRs were successfully employed in PEC cells. The structural and optical properties have been investigated in detail.
2. Experimental details Fig. 2. UV–Vis absorption spectra of ZnO and CdS–ZnO samples. Inset shows the room temperature photoluminescence spectra of CdS and CdS–ZnO samples.
The visible band gap semiconductor sensitized solar cell (SSSC) is the best option to improve efficiency and stability in which semiconductor materials replace the organic dyes. The SSSC provides additional opportunities that are not available in DSSC. The shape and size of semiconductor can be adjusted to cover the solar spectrum from UV to Vis range, and are easy to process. The SSSC can potentially utilize
The seed solution was prepared in an absolute ethanol with 0.05 M zinc acetate and 0.05 M diethanolamine. The cleaned glass substrate was dip coated for 10 s in a seed solution and then kept at room temperature for drying. The dried film was annealed at 400 °C for 5 min in air atmosphere to yield ZnO seed layer. The seeded substrate was placed vertically in the 200 ml solution of zinc acetate and hexamethylenetetramine (HMTA) having equimolar concentration of 0.05 M and refluxed at 95 ± 3 °C for 5 h to grow ZNRs with 22 aspect ratio. The grown film was removed and then rinsed in distilled water and then used for CNPs coating.
Fig. 3. (a–e) XPS spectrum of CdS/ZnO sample.
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The CNPs were deposited on ZNRs from a chemical bath containing 1 mM cadmium sulfate, 1 mM thiourea and 25% ammonium hydroxide. The substrate containing ZNRs was vertically immersed into the chemical bath solution. The substrate was dipped into the bath solution for 100 min at 70 °C. 3. Results and discussion The XRD patterns of ZnO and CdS/ZnO were shown in Fig. 1. The comparison of observed XRD patterns with the standard JCPDS data (80-0074) confirms the formation of ZnO having hexagonal wurtzite crystal structure with diffraction peaks (100), (002), (101) and (102). The characteristic peaks along (111), (200), (220), (311), (220) and (400) planes corresponding to CdS are observed with the cubic structure (JCPDS card no. 80-0019). XRD shows the good crystalline character for ZnO and CdS/ZnO samples. From the optical absorption spectra (Fig. 2), it is evident that the ZnO sample exhibits intrinsic absorption at about 375 nm. A significant shift in the spectral photoresponse (525 nm) is observed for CdS/ZnO sample. It clearly illustrates effective photon capturing in the visible region [13]. The general characteristics of the emitted PL from the CdS/ZnO are similar to those from CdS NPs. Both the samples show a broad band over 350– 500 nm (inset, Fig. 2). The CNPs exhibit a strong PL peak centered at 403 nm, due to the exciton-related emission near the band-edge. Besides UV emission, it can be seen that a broad green emission around 525 nm, generally referred to surface states or traps is observed for CdS/ZnO. A quenching of the PL peak for CdS/ZnO sample, at least five-folds of magnitude, due to electron injection from CdS conduction band to ZnO conduction band [14,15]. The five fold
quenching in the present study suggests that more than one CdS particles are capable of interacting with the single ZnO NR and taking active part in the charge injection process [16], thereby minimizing recombination of photogeneration electron-hole pair. The XPS of the CdS/ZnO sample (Fig. 3) indicates the presence of cadmium (Cd), sulfur (S), Zinc (Zn) with the carbon (C) and oxygen (O). The C peaks stem from the atmospheric contamination. The binding energies obtained in the XPS analysis were corrected taking into account the specimen charging and by referring to C1s at 285.02 eV. Fig. 3(c and d) shows a narrow range scans for the Cd and S peaks of the same samples. The two-peak structure in Cd3d core level arises from the spin–orbit interaction with the Cd3d5/2 peak position at 404.28 eV and the 3d3/2 at 410.48 eV. The XPS binding energies of Cd3d at 404.28 eV and the S2p at 160.89 eV are indicative of the CdS chemistry. Fig. 3(b and e) shows a narrow range scans for the Zn and O peak region of the similar samples. From the spectral graph it is clear that, Cd and Zn exhibit narrow, well defined feature for doublet structure. The two-peak structure in Zn core level arises from the spin–orbit interaction with the Cd 3d5/2 peak position at 1021.4 eV and the 3d3/2 at 1044.52 eV. The peak for O is at 530.82 eV. Fig. 4(a) shows FE-SEM image of the ZNRs without CNPs and its corresponding cross-sectional image. The well aligned ZNRs are observed, with ~90 nm diameter and ~ 2 μm length (Fig. 4b). The vertical alignment of ZNRs is beneficial for the improvement in the charge transfer mechanism of the solar cells. Fig. 4(c) shows the top view FE-SEM image of ZNRs coated with the CNPs. The faceted hexagonal ZnO rods are seen to be covered with CNPs. A cross-sectional image (Fig. 4d) shows that the CNPs are attached uniformly to the ZNRs covering entire ZNR length. Contact angle (CA) of a water drop is influenced
Fig. 4. FE-SEM images of (a) Top view of ZnO NRs, (b) cross-sectional view of ZnO NRs, (c) top view of CdS-ZnO, and (d) cross-sectional view of CdS–ZnO samples. Inset shows the contact angle of corresponding samples.
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Fig. 5. I–V curves for ZnO and CdS–ZnO samples.
by the roughness of the surface. Inset shows water CA measurement of ZnO and CdS/ZnO and are found to be 145° and 90° (±1°) respectively. ZNR film shows hydrophobic nature since the water CA is greater than 90°, while CdS/ZnO film shows hydrophilic nature as compared to ZNRs film because the sulfate ions from the CdS precursor strongly influence the transformation from hydrophobic to hydrophilic nature. This is beneficial to the better access of electrolyte into the film structure which enhances the PEC performance. Fig. 5 shows the I–V characteristics of ZNRs and CdS/ZnO respectively. ZnO cell exhibits the power conversion efficiency (η) of 0.025% with (JSC) = 139 μA/cm2, open-circuit voltage (VOC) = 310 mV and fill factor (FF) = 0.24. However, a huge change in PEC performance was observed for CdS/ZnO cell with JSC = 2.60 mA/cm2, VOC = 378 mV and FF = 0.29 which is higher than reported values [17,18]. The η of the cell is about 0.72% under the illumination of light intensity 40 mWcm−2. These results demonstrate successful sensitization of ZnO with CNPs grown by a facile and cost effective method. 4. Conclusions The CNPs were chemically deposited on the ZNRs at relatively low temperature. The XRD and XPS analyses gave the formation of CdS/ ZnO. The absorption spectrum shows the red shift of absorption edge, which is crucible for solar cell. The SEM images clearly show the vertically aligned ZnO nanorods, which are helpful for the charge transfer in the PEC cell. The CNPs sensitized ZNRs have shown a promising candidate for an efficient solar energy conversion devices.
Acknowledgement Authors are thankful to University Grants Commission (UGC), New Delhi, India for financial support under the DRS-SAP-II and UGC-ASIST programme.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Gao Y, Masayuki N. Langmuir 2006;22:3936–40. Dhas V, Subas S, Lee W, Han SH, Ogale S. Appl Phys Lett 2008;93:243108–11. O'Regan B, Grätzel M. Nature 1991;335:737–40. Leschkies K, Divakar R, Basu J. Nano Lett 2007;7:1793–8. Zaban A, Micic O, Gregg B, Nozik A. Langmuir 1998;14:3153–6. Aga R, Gunther D, Ueda A, Pan Z, Collins W, Mu R, Singer K. Nanotechnology 2009;20:4652041–8. Vogel R, Hoyer P, Weller H. J Phys Chem B 1994;98:3183–8. Chang C, Lee Y. Appl Phys Lett 2007;91:053503–5. Robel I, Kuno M, Kamat P. J Am Chem Soc 2007;129:4136–7. Wu P, Zhang H, Du N, Ruan L, Yang D. J Phys Chem C 2009;113:8147–51. Banerjee S, Mohapatra S, Das P, Misra M. Chem Mater 2008;20:6784–91. Song X, Fu Y, Xie Y, Song J. Semicond Sci Technol 2010;25:045031–5. Zhang Q, Tammy P . Angew Chem Int Ed 2008;47:2402–6. Kamat P, Dimitrijevic N. Sol Energy 1990;44:83–98. Spanhel L, Weller H, Henglein A. J Am Chem Soc 1987;109(22):6632–5. Hotchandani S, Kamat P. J Phys Chem 1992;96:6834–9. Chen J, Li C, Song JL, Sun X, Lei W, Deng W. Appl Surf Sci 2009;255:7508–11. Zhang Y, Xie T, Jiang T, Wei X, Pang S, Wang X, Wang D. Nanotechnology 2009;20: 155707–12.
