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CdS-Sensitized ZnO Nanorod Photoelectrodes: Photoelectrochemistry and Photoinduced Absorption Spectroscopy

CdS-Sensitized ZnO Nanorod Photoelectrodes: Photoelectrochemistry and Photoinduced Absorption... Hindawi Publishing Corporation Advances in OptoElectronics Volume 2011, Article ID 915123, 5 pages doi:10.1155/2011/915123 Research Article CdS-Sensitized ZnO Nanorod Photoelectrodes: Photoelectrochemistry and Photoinduced Absorption Spectroscopy Idriss Bedja CRC, Department of Optometry, College of Applied Medical Sciences, King Saud University, Riyadh 11433, Saudi Arabia Correspondence should be addressed to Idriss Bedja, bedja@ksu.edu.sa Received 15 June 2011; Accepted 25 July 2011 Academic Editor: Surya Prakash Singh Copyright © 2011 Idriss Bedja. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Thin films of ZnO semiconductor nanorods (ZnO-nr) of 6 µm length and thin ZnO nanoparticulate films (ZnO-np) have been prepared and modified with Q-dots CdS for comparison study. PIA (photoinduced absorption spectroscopy), a multipurpose tool in the study of dye-sensitized solar cells, is used to study a quantum-dot-modified metal-oxide nanostrucutred electrode. Q- dot CdS-sensitized ZnO-nr (1D network) sensitized photoelectrode has demonstrated best performances in both photoelectrical response (IPCE max = 92%) and broadening response into far visible comparing to ZnO-np-based CdS solar cell. Preadsorbing ZnO-nr with ZnO-np does not bring further improvement. Time constant for electron injection into ZnO-nr conduction band was relatively fast decay of 6.5 ms, similar to TiO -coated CdS, and proves at least a well pore filling of ZnO-nr film by ultrafine CdS particles. Unidirectional electron transfer mechanistic in ZnO-nr has played a major role in these performances. 1. Introduction generated in the semiconductor remains there and cannot migrate to MO. Thus, the two charges will be separated When used as electrodes in regenerative photoelectron- effectively. chemical cells, wide bandgap nanostructured metal oxide Dye-sensitized solar cells (DSCs) based on one-dimen- (MO) semiconductor materials can serve as carriers of solar sional (1D) ZnO nanostructures, which exhibit significantly absorbers such as organometallic dyes [1–5] or inorganic higher electron mobility than that of both TiO and ZnO-np narrow bandgap semiconductors (quantum dots: Q-dots) films [10], have recently been attracting increasing attention [6–9]. Power conversion efficiencies in the range of 8–12% [10, 11]. in diffuse daylight have been obtained in the sensitization In contrast with the dye-sensitized solar cells, fundamen- of highly porous TiO film with only a submonolayer tal understanding for the factors controlling the interfacial required ruthenium complex [1, 2]. On the other hand, electron transfer reactions in Q-dots-modified metal-oxide- wide bandgap semiconductors have been sensitized by based solar cells is limited. Photoinduced absorption spec- short bandgap (Q-dots) semiconductor materials CdSe/TiO troscopy is a suitable method to obtain spectral and kinetic [6], CdS/TiO -SnO [9] as alternative to dye sensitization. 2 2 information of the Q-dot-sensitized MO electrodes. Vogeland coworkers[7] have investigated the sensitization In this paper, we report QD-CdS-sensitized ZnO-np, of nanoporous TiO , ZnO by Q-sized CdS. Photocurrent ZnO-nr, and ZnO-nr preadsorbed with ZnO-np photoelec- quantum yields of up to nearly 80% and opencircuit voltages trodes. We described photoelectrochemical properties and up to 1 V range were obtained. Under visiblelight irradiation, only the sensitizer is excited, and electrons transferred to for the first time photoinduced absorption spectroscopy for mechanistic study. We compare our results to those obtained their conduction band are injected to the inactivated MO semiconductor conduction band. If the valence band of the recently on CdS-adsorbed TiO using PIA measurements, in sensitizer is more cathodic than the valence band of MO, hole a separate paper submitted for publication. 2 Advances in OptoElectronics Potentiostat Lens + Lamp Sample Monochromator Working electrode Optional filters Light LED driver + lense 4mm Counter electrode Signal analysis Si detector Reference electrode Figure 2: Photoinduced absorption (PIA) setup. Laser pulse/ To detector a blue LED (Luxeon Star 1 W, Royal Blue, 470 nm), which or LED pulse was square-wave modulated (on/off) by electronical means Figure 1: Photoelectrochemical cell (Quartz) [13]. using an HP 33120 A waveform generator and a home- built LED driver system. The beam, with an intensity in the range of 0.5–30 mW/cm ,excited asampleareaofabout 2. Experimental Section 2 1cm . White probe light was provided by a 20 W tungsten- halogen lamp. A cutoff filter (Schott RG715) was used to 2.1. Preparation of ZnO Nanorod (1D) Films. Firstly, 300 nm minimize excitation of the sample by the probe light where ZnO seed layer was prepared on the optically conducting indicated. The transmitted probe light was focused onto a glass, indium-doped SnO (ITO) substrate. Two drops of monochromator (Acton Research Corporation SP-150) and 5 mM solution of zinc acetate-dihydrate in ethanol absolute, detected using a UV-enhanced Si photodiode, connected to a rinsed in ethanol, and blow dried with nitrogen gas. This lock-in amplifier via a current amplifier (Stanford Research is repeated 4 times before sintering at 350 Cinair for Systems models 830 and 570, resp.). For the time-resolved 30 min and cooled down to room temperature. This process studies the output of the current amplifier was connected to is repeated twice [12]. a data acquisition board (National Instruments PCI-6052E). Secondly, the deposited ZnO seed substrate was All PIA measurements were done at room temperature. immersed into an aqueous solution of 25 mM zinc nitrate hexahydrate, 25 mM hexamethylenetetramine and 5 mM polyethyleneimine at 90 C for a hydrothermal reaction for a 3. Results and Discussion total of 12 hours. The solution was replaced by a fresh one 3.1. Microstructure. SEM image in Figure 3 shows a 1- every 4 hours. The obtained ZnO nanorods were rinsed with dimensional network of ZnO nanorods about 6 µm length. deionized water and dried in air at room temperature. The diameter of these nanorods was estimated to be less than 300 nm. Also in Figure 4 are SEM pictures of ZnO-nr films 2.2. Surface Modification of ZnO by Quantum Dots of after modification with Q-dots CdS. Compared to ZnO-nr CdS. ZnO metal oxide nanostructured and ZnO nanorod SEM alone, we clearly can observe a good adsorption of CdS electrodes were successively dipped into an aqueous solution particles around ZnO nanorods (see both Figures 4(a) and of saturated Cd(ClO ) and 0.1 M Na S for 1 and 2 min, 4 2 2 4(b)). respectively. After each CdS layer deposition, the electrodes were heated at 125 C for 5 min. 3.2. UV-VIS Absorption Spectra. Absorption spectra have been recorded for both ZnO-nr films before and after CdS 2.3. Characterization Methods. UV-Vis spectra were record- modification (Figure 5). Coating ZnO-nr film with CdS ed using a Hewlett-Packard 8453 diode array spectrometer. particles extends further its adsorption into the visible up to The photoelectrochemical measurements were carried out in 700 nm. a layer cell consisting of a 1 cm path-length quartz cuvette [13]. Two electrodes were inserted consisting of a reference (Ag/AgCl) and counter- (Pt wire) electrodes similar to that 3.3. Incident Photon-to-Current Conversion Efficiency (IPCE) shown in Figure 1. A Princeton Applied Research (PAR) Spectra. In Figure 6 is shown a comparison of photore- Model 173 and 175 universal potentiostats were used in sponses in the visible spectrum of the three-based ZnO electrochemical measurements. The setups for recording network—CdS photoelectrodes, namely. High IPCE mea- incident photon to current efficiency (IPCE) spectra and I-V surements have been found with all based ZnO network films curves have been described elsewhere [12]. using 1D ZnO network. 1D network of ZnO broadens the For PIA spectroscopy (see described setup in Figure 2), response to larger wavelengths compared to ZnO-np alone excitation of the sample was provided by light from or ZnO-nr preadsorbed with ZnO-np. The broadening of LED Advances in OptoElectronics 3 3.6 3.1 2.6 2.1 1.6 1.1 2 µm EHT = 5.00 kV Signal A = ln lens Date: 14 Sep 2008 Mag = 10.36 kX 0.6 WD = 4mm Photo no. = 1089 Time: 15:32 380 480 580 680 780 Wavelength (nm) Figure 3: SEM cross-section picture of ZnO nanorod film. Figure 5: Absorption spectra of (a) ZnO-nr particulate film and (b), (c), and (d) different CdS coatings modified ZnO-nr particulate film. 2 µm 3 µm (a) (b) Figure 4: (a) SEM top picture of ZnO nanorod film modified with CdS. (b) SEM cross-section of ZnO nanorod film modified with CdS. the response with ZnO-nr-based films may originate from 400 450 500 550 600 the possible aggregation of some CdS particles between the Wavelength (nm) nanorods, thus red-shifting in the absorption. By CdS aggre- Figure 6: Incident photon-to-current conversion efficiencies gation we should observe lowering in IPCE response, but (IPCE) of ZnO-nr photoelectrode before modification with CdS the fact that 1D ZnO network will enhance unidirectional (dark blue square) and after modification with CdS (violet square); e-transfer to the substrate will thus compensate the loss ZnO-np/CdS (cross) and ZnO-nr/ZnO-np/CdS (yellow triangle). due to aggregation and keep the visible broadening higher. Electrolyte was Na S0.1 M, Na SO 0.01 M. 2 2 4 Regarding ZnO-nr preadsorbed with ZnO-np, this network design has not brought an enhancement to the simple ZnO- nr based cell, but on the contrary it lowers the response, and also the broadening into visible spectrum. 1E−05 3.4. PIA Spectroscopy. Figure 7 shows a PIA spectrum of 0E+00 500 550 600 650 700 750 800 850 CdS-modified ZnO-nr and ZnO-np without electrolyte for comparison. The PIA spectrum clearly reflects the −1E−05 differential spectrum of CdS upon formation injection of electrons into ZnO conduction band, with a bleach of the −2E−05 main absorption around 470 nm. The remaining hole in CdS absorbs light, and because valence band electrons are −3E−05 missing, an apparent increase in bandgap is seen (bleach, Moss-Burstein shift). Almost negligible PIA signal is detected −4E−05 Wavelength (nm) with ZnO-np preadsorbed with ZnO-np-based CdS solar cell. Both ZnO-nr- and ZnO-np-based CdS films depict Figure 7: Photoinduced absorption (PIA) spectra of quantum CdS- different onset bleaching. Although they show similar PIA modified ZnO-np (triangle) and ZnO-nr (square) electrodes in air. intensity above 670 nm, this difference in onsets is probably The spectra were recorded using blue light (460 nm) excitation −2 due to the broadening of visible absorption of CdS particles (42 mW cm ) with a modulation frequency of 9 Hz. IPCE (%) Absorbance ΔA 4 Advances in OptoElectronics −5 species, and their kinetics can be explored using time- ×10 resolved techniques. PIA can monitor slow processes and is much cheaper compared to laser flash photolysis. ZnO-nr- (1D network) based CdS photoelectrode has demonstrated best performances in both photoelectrical response and broadening response into far visible comparing to ZnO-np- based CdS solar cell. Preadsorbing ZnO-nr with ZnO-np does not bring further performances, but on the contrary it lowers photoresponse and broadening to almost the same level as with ZnO-np-based cell. Unidirectional electron transfer mechanistic observed in ZnO-nr has played a major role in these performances. Acknowledgments −0.02 0 0.02 0.04 0.06 0.08 0.1 The research project is funded by the National Plan for Time (s) Science and Technology Program, Grant no. 09-NAN859-02, King Saud University, Riyadh, Saudi Arabia. I. Bedja would Figure 8: PIA decay transient absorption of quantum dots like to thank Professor Anders Hagfeldt for his authorization CdS-modified ZnO-nr electrode after excitation with blue light to use his laboratory facilities at Uppsala University in 2 −1 (11 mW/cm ) recorded at 750 nm, using a sampling rate of 103 s Sweden. and averaged 100 times. References after aggregation between ZnO nanorods, thus red-shifting [1] B. O’Regan and M. Gratz ¨ el, “A low-cost, high-efficiency solar the PIA onset in the visible spectrum. cell based on dye-sensitized colloidal TiO films,” Nature, vol. 353, no. 6346, pp. 737–740, 1991. 3.5. PIA Kinetics. Study of the kinetics in semiconductor sen- [2] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, and H. Pettersson, sitizing solar cells is not only feasible by laser flash photolysis “Dye-sensitized solar cells,” Chemical Reviews, vol. 110, no. 11, but also possible using time-resolved PIA measurements. pp. 6595–6663, 2010. Figure 8 shows an example of such PIA transient, here [3] I. Bedja, S. Hotchandani, and P. V. Kamat, “Preparation and photoelectrochemical characterization of thin SnO2 decay recorded at 750 nm for CdS-modified ZnO-nr. It nanocrystalline semiconductor films and their sensitization is clear that the recombination yield between generated with bis(2,2-bipyridine)(2,2-bipyridine-4,4-dicarboxylic electrons and holes does not follow simple first- or even acid)ruthenium(II) complex,” Journal of Physical Chemistry, second-order kinetics but is characterized by a range of vol. 98, no. 15, pp. 4133–4140, 1994. recombination times. Transport of electrons through ZnO- [4] I. Bedja, P. V. Kamat, and S. Hotchandani, “Fluorescence and nr nanocrystals could also be one relation. For solar cell photoelectrochemical behavior of chlorophyll a adsorbed on a performance the pseudo-first-order rate constant under nanocrystalline SnO2 film,” Journal of Applied Physics, vol. 80, steady-state conditions is a relevant parameter as it can no. 8, pp. 4637–4643, 1996. give direct information on possible recombination losses [5] T. A. Heimer, E. J. Heilweil, C. A. Bignozzi, and G. J. Meyer, due to the reaction of electrons with holes. Analysis of the “Electron injection, recombination, and halide oxidation decay in Figure 8 during the first 1 ms (using a sampling dynamics at dye-sensitized metal oxide interfaces,” Journal of rate of 1 MHz) gives a recombination lifetime of 6.5 ms Physical Chemistry A, vol. 104, no. 18, pp. 4256–4262, 2000. approximately (similar to that found with CdS-coated TiO - 2 [6] D. Liu and P. V. Kamat, “Electrochemically active nanocrys- np of 8.5 ms, in a separate article submitted for publication talline SnO2 films: surface modification with thiazine and in Hindawi journals, 2011), which does not follow simple oxazine dye aggregates,” Journal of the Electrochemical Society, first-order kinetic but is characterized by a range of injection vol. 142, no. 3, pp. 835–839, 1995. [7] R.Vogel,P.Hoyer,and H. Weller,“Quantum-sizedPbS, times. This relatively fast decay proves at least a well pore CdS, Ag2S, Sb2S3, and Bi2S3 particles as sensitizers for filling of ZnO-nr film by ultrafine CdS particles. Almost no various nanoporous wide-bandgap semiconductors,” Journal transient decay has been observed for ZnO-nr preadsorbed of Physical Chemistry, vol. 98, no. 12, pp. 3183–3188, 1994. with ZnO-np-based CdS films. [8] J. Rabani, “Sandwich colloids of ZnO and ZnS in aqueous solutions,” JournalofPhysicalChemistry, vol. 93, no. 22, pp. 4. Conclusion 7707–7713, 1989. [9] I.Bedja,S.Holchandani, andP.V.Kamat,“Photosensitization Photoinduced absorption spectroscopy where the excitation of composite metal oxide semiconductor films,” General & is provided by an on/off monochromatic light source can Introductory Chemistry, vol. 101, no. 11, pp. 1651–1653, 1997. give direct information on electron-injection and hole- [10] M. Law, L. E. Greene, A. Radenovic, T. Kuykendall, J. Liphardt, electron recombination rates using spectra of transient and P. Yang, “ZnO-Al O and ZnO-TiO core-shell nanowire 2 3 2 ΔA Advances in OptoElectronics 5 dye-sensitized solar cells,” Journal of Physical Chemistry B, vol. 110, no. 45, pp. 22652–22663, 2006. [11] M. Guo, P. Diao, X. Wang, and S. Cai, “The effect of hydrothermal growth temperature on preparation and pho- toelectrochemical performance of ZnO nanorod array films,” Journal of Solid State Chemistry, vol. 178, no. 10, pp. 3210– 3215, 2005. [12] C. Bauer, G. Boschloo, E. Mukhtar, and A. Hagfeldt, “Electron injection and recombination in Ru(dcbpy)2(NCS)2 sensitized nanostructured Zno,” Journal of Physical Chemistry B, vol. 105, no. 24, pp. 5585–5588, 2001. [13] I. Bedja, Photophysics and photoelectrochemistry studies on nanocrystalline semiconductor systems. Mechanistic studies of photosensitization and modu-lation of electron transfer kinetics, Ph.D. thesis, University of Quebec at Trois-Rivieres, Quebec, Canada, 1996. 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CdS-Sensitized ZnO Nanorod Photoelectrodes: Photoelectrochemistry and Photoinduced Absorption Spectroscopy

Bedja, Idriss
Advances in OptoElectronics , Volume 2011 – Oct 3, 2011
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Hindawi Publishing Corporation
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Copyright © 2011 Idriss Bedja. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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10.1155/2011/915123
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Hindawi Publishing Corporation Advances in OptoElectronics Volume 2011, Article ID 915123, 5 pages doi:10.1155/2011/915123 Research Article CdS-Sensitized ZnO Nanorod Photoelectrodes: Photoelectrochemistry and Photoinduced Absorption Spectroscopy Idriss Bedja CRC, Department of Optometry, College of Applied Medical Sciences, King Saud University, Riyadh 11433, Saudi Arabia Correspondence should be addressed to Idriss Bedja, bedja@ksu.edu.sa Received 15 June 2011; Accepted 25 July 2011 Academic Editor: Surya Prakash Singh Copyright © 2011 Idriss Bedja. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Thin films of ZnO semiconductor nanorods (ZnO-nr) of 6 µm length and thin ZnO nanoparticulate films (ZnO-np) have been prepared and modified with Q-dots CdS for comparison study. PIA (photoinduced absorption spectroscopy), a multipurpose tool in the study of dye-sensitized solar cells, is used to study a quantum-dot-modified metal-oxide nanostrucutred electrode. Q- dot CdS-sensitized ZnO-nr (1D network) sensitized photoelectrode has demonstrated best performances in both photoelectrical response (IPCE max = 92%) and broadening response into far visible comparing to ZnO-np-based CdS solar cell. Preadsorbing ZnO-nr with ZnO-np does not bring further improvement. Time constant for electron injection into ZnO-nr conduction band was relatively fast decay of 6.5 ms, similar to TiO -coated CdS, and proves at least a well pore filling of ZnO-nr film by ultrafine CdS particles. Unidirectional electron transfer mechanistic in ZnO-nr has played a major role in these performances. 1. Introduction generated in the semiconductor remains there and cannot migrate to MO. Thus, the two charges will be separated When used as electrodes in regenerative photoelectron- effectively. chemical cells, wide bandgap nanostructured metal oxide Dye-sensitized solar cells (DSCs) based on one-dimen- (MO) semiconductor materials can serve as carriers of solar sional (1D) ZnO nanostructures, which exhibit significantly absorbers such as organometallic dyes [1–5] or inorganic higher electron mobility than that of both TiO and ZnO-np narrow bandgap semiconductors (quantum dots: Q-dots) films [10], have recently been attracting increasing attention [6–9]. Power conversion efficiencies in the range of 8–12% [10, 11]. in diffuse daylight have been obtained in the sensitization In contrast with the dye-sensitized solar cells, fundamen- of highly porous TiO film with only a submonolayer tal understanding for the factors controlling the interfacial required ruthenium complex [1, 2]. On the other hand, electron transfer reactions in Q-dots-modified metal-oxide- wide bandgap semiconductors have been sensitized by based solar cells is limited. Photoinduced absorption spec- short bandgap (Q-dots) semiconductor materials CdSe/TiO troscopy is a suitable method to obtain spectral and kinetic [6], CdS/TiO -SnO [9] as alternative to dye sensitization. 2 2 information of the Q-dot-sensitized MO electrodes. Vogeland coworkers[7] have investigated the sensitization In this paper, we report QD-CdS-sensitized ZnO-np, of nanoporous TiO , ZnO by Q-sized CdS. Photocurrent ZnO-nr, and ZnO-nr preadsorbed with ZnO-np photoelec- quantum yields of up to nearly 80% and opencircuit voltages trodes. We described photoelectrochemical properties and up to 1 V range were obtained. Under visiblelight irradiation, only the sensitizer is excited, and electrons transferred to for the first time photoinduced absorption spectroscopy for mechanistic study. We compare our results to those obtained their conduction band are injected to the inactivated MO semiconductor conduction band. If the valence band of the recently on CdS-adsorbed TiO using PIA measurements, in sensitizer is more cathodic than the valence band of MO, hole a separate paper submitted for publication. 2 Advances in OptoElectronics Potentiostat Lens + Lamp Sample Monochromator Working electrode Optional filters Light LED driver + lense 4mm Counter electrode Signal analysis Si detector Reference electrode Figure 2: Photoinduced absorption (PIA) setup. Laser pulse/ To detector a blue LED (Luxeon Star 1 W, Royal Blue, 470 nm), which or LED pulse was square-wave modulated (on/off) by electronical means Figure 1: Photoelectrochemical cell (Quartz) [13]. using an HP 33120 A waveform generator and a home- built LED driver system. The beam, with an intensity in the range of 0.5–30 mW/cm ,excited asampleareaofabout 2. Experimental Section 2 1cm . White probe light was provided by a 20 W tungsten- halogen lamp. A cutoff filter (Schott RG715) was used to 2.1. Preparation of ZnO Nanorod (1D) Films. Firstly, 300 nm minimize excitation of the sample by the probe light where ZnO seed layer was prepared on the optically conducting indicated. The transmitted probe light was focused onto a glass, indium-doped SnO (ITO) substrate. Two drops of monochromator (Acton Research Corporation SP-150) and 5 mM solution of zinc acetate-dihydrate in ethanol absolute, detected using a UV-enhanced Si photodiode, connected to a rinsed in ethanol, and blow dried with nitrogen gas. This lock-in amplifier via a current amplifier (Stanford Research is repeated 4 times before sintering at 350 Cinair for Systems models 830 and 570, resp.). For the time-resolved 30 min and cooled down to room temperature. This process studies the output of the current amplifier was connected to is repeated twice [12]. a data acquisition board (National Instruments PCI-6052E). Secondly, the deposited ZnO seed substrate was All PIA measurements were done at room temperature. immersed into an aqueous solution of 25 mM zinc nitrate hexahydrate, 25 mM hexamethylenetetramine and 5 mM polyethyleneimine at 90 C for a hydrothermal reaction for a 3. Results and Discussion total of 12 hours. The solution was replaced by a fresh one 3.1. Microstructure. SEM image in Figure 3 shows a 1- every 4 hours. The obtained ZnO nanorods were rinsed with dimensional network of ZnO nanorods about 6 µm length. deionized water and dried in air at room temperature. The diameter of these nanorods was estimated to be less than 300 nm. Also in Figure 4 are SEM pictures of ZnO-nr films 2.2. Surface Modification of ZnO by Quantum Dots of after modification with Q-dots CdS. Compared to ZnO-nr CdS. ZnO metal oxide nanostructured and ZnO nanorod SEM alone, we clearly can observe a good adsorption of CdS electrodes were successively dipped into an aqueous solution particles around ZnO nanorods (see both Figures 4(a) and of saturated Cd(ClO ) and 0.1 M Na S for 1 and 2 min, 4 2 2 4(b)). respectively. After each CdS layer deposition, the electrodes were heated at 125 C for 5 min. 3.2. UV-VIS Absorption Spectra. Absorption spectra have been recorded for both ZnO-nr films before and after CdS 2.3. Characterization Methods. UV-Vis spectra were record- modification (Figure 5). Coating ZnO-nr film with CdS ed using a Hewlett-Packard 8453 diode array spectrometer. particles extends further its adsorption into the visible up to The photoelectrochemical measurements were carried out in 700 nm. a layer cell consisting of a 1 cm path-length quartz cuvette [13]. Two electrodes were inserted consisting of a reference (Ag/AgCl) and counter- (Pt wire) electrodes similar to that 3.3. Incident Photon-to-Current Conversion Efficiency (IPCE) shown in Figure 1. A Princeton Applied Research (PAR) Spectra. In Figure 6 is shown a comparison of photore- Model 173 and 175 universal potentiostats were used in sponses in the visible spectrum of the three-based ZnO electrochemical measurements. The setups for recording network—CdS photoelectrodes, namely. High IPCE mea- incident photon to current efficiency (IPCE) spectra and I-V surements have been found with all based ZnO network films curves have been described elsewhere [12]. using 1D ZnO network. 1D network of ZnO broadens the For PIA spectroscopy (see described setup in Figure 2), response to larger wavelengths compared to ZnO-np alone excitation of the sample was provided by light from or ZnO-nr preadsorbed with ZnO-np. The broadening of LED Advances in OptoElectronics 3 3.6 3.1 2.6 2.1 1.6 1.1 2 µm EHT = 5.00 kV Signal A = ln lens Date: 14 Sep 2008 Mag = 10.36 kX 0.6 WD = 4mm Photo no. = 1089 Time: 15:32 380 480 580 680 780 Wavelength (nm) Figure 3: SEM cross-section picture of ZnO nanorod film. Figure 5: Absorption spectra of (a) ZnO-nr particulate film and (b), (c), and (d) different CdS coatings modified ZnO-nr particulate film. 2 µm 3 µm (a) (b) Figure 4: (a) SEM top picture of ZnO nanorod film modified with CdS. (b) SEM cross-section of ZnO nanorod film modified with CdS. the response with ZnO-nr-based films may originate from 400 450 500 550 600 the possible aggregation of some CdS particles between the Wavelength (nm) nanorods, thus red-shifting in the absorption. By CdS aggre- Figure 6: Incident photon-to-current conversion efficiencies gation we should observe lowering in IPCE response, but (IPCE) of ZnO-nr photoelectrode before modification with CdS the fact that 1D ZnO network will enhance unidirectional (dark blue square) and after modification with CdS (violet square); e-transfer to the substrate will thus compensate the loss ZnO-np/CdS (cross) and ZnO-nr/ZnO-np/CdS (yellow triangle). due to aggregation and keep the visible broadening higher. Electrolyte was Na S0.1 M, Na SO 0.01 M. 2 2 4 Regarding ZnO-nr preadsorbed with ZnO-np, this network design has not brought an enhancement to the simple ZnO- nr based cell, but on the contrary it lowers the response, and also the broadening into visible spectrum. 1E−05 3.4. PIA Spectroscopy. Figure 7 shows a PIA spectrum of 0E+00 500 550 600 650 700 750 800 850 CdS-modified ZnO-nr and ZnO-np without electrolyte for comparison. The PIA spectrum clearly reflects the −1E−05 differential spectrum of CdS upon formation injection of electrons into ZnO conduction band, with a bleach of the −2E−05 main absorption around 470 nm. The remaining hole in CdS absorbs light, and because valence band electrons are −3E−05 missing, an apparent increase in bandgap is seen (bleach, Moss-Burstein shift). Almost negligible PIA signal is detected −4E−05 Wavelength (nm) with ZnO-np preadsorbed with ZnO-np-based CdS solar cell. Both ZnO-nr- and ZnO-np-based CdS films depict Figure 7: Photoinduced absorption (PIA) spectra of quantum CdS- different onset bleaching. Although they show similar PIA modified ZnO-np (triangle) and ZnO-nr (square) electrodes in air. intensity above 670 nm, this difference in onsets is probably The spectra were recorded using blue light (460 nm) excitation −2 due to the broadening of visible absorption of CdS particles (42 mW cm ) with a modulation frequency of 9 Hz. IPCE (%) Absorbance ΔA 4 Advances in OptoElectronics −5 species, and their kinetics can be explored using time- ×10 resolved techniques. PIA can monitor slow processes and is much cheaper compared to laser flash photolysis. ZnO-nr- (1D network) based CdS photoelectrode has demonstrated best performances in both photoelectrical response and broadening response into far visible comparing to ZnO-np- based CdS solar cell. Preadsorbing ZnO-nr with ZnO-np does not bring further performances, but on the contrary it lowers photoresponse and broadening to almost the same level as with ZnO-np-based cell. Unidirectional electron transfer mechanistic observed in ZnO-nr has played a major role in these performances. Acknowledgments −0.02 0 0.02 0.04 0.06 0.08 0.1 The research project is funded by the National Plan for Time (s) Science and Technology Program, Grant no. 09-NAN859-02, King Saud University, Riyadh, Saudi Arabia. I. Bedja would Figure 8: PIA decay transient absorption of quantum dots like to thank Professor Anders Hagfeldt for his authorization CdS-modified ZnO-nr electrode after excitation with blue light to use his laboratory facilities at Uppsala University in 2 −1 (11 mW/cm ) recorded at 750 nm, using a sampling rate of 103 s Sweden. and averaged 100 times. References after aggregation between ZnO nanorods, thus red-shifting [1] B. O’Regan and M. Gratz ¨ el, “A low-cost, high-efficiency solar the PIA onset in the visible spectrum. cell based on dye-sensitized colloidal TiO films,” Nature, vol. 353, no. 6346, pp. 737–740, 1991. 3.5. PIA Kinetics. Study of the kinetics in semiconductor sen- [2] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, and H. Pettersson, sitizing solar cells is not only feasible by laser flash photolysis “Dye-sensitized solar cells,” Chemical Reviews, vol. 110, no. 11, but also possible using time-resolved PIA measurements. pp. 6595–6663, 2010. Figure 8 shows an example of such PIA transient, here [3] I. Bedja, S. Hotchandani, and P. V. Kamat, “Preparation and photoelectrochemical characterization of thin SnO2 decay recorded at 750 nm for CdS-modified ZnO-nr. It nanocrystalline semiconductor films and their sensitization is clear that the recombination yield between generated with bis(2,2-bipyridine)(2,2-bipyridine-4,4-dicarboxylic electrons and holes does not follow simple first- or even acid)ruthenium(II) complex,” Journal of Physical Chemistry, second-order kinetics but is characterized by a range of vol. 98, no. 15, pp. 4133–4140, 1994. recombination times. Transport of electrons through ZnO- [4] I. Bedja, P. V. Kamat, and S. Hotchandani, “Fluorescence and nr nanocrystals could also be one relation. For solar cell photoelectrochemical behavior of chlorophyll a adsorbed on a performance the pseudo-first-order rate constant under nanocrystalline SnO2 film,” Journal of Applied Physics, vol. 80, steady-state conditions is a relevant parameter as it can no. 8, pp. 4637–4643, 1996. give direct information on possible recombination losses [5] T. A. Heimer, E. J. Heilweil, C. A. Bignozzi, and G. J. Meyer, due to the reaction of electrons with holes. Analysis of the “Electron injection, recombination, and halide oxidation decay in Figure 8 during the first 1 ms (using a sampling dynamics at dye-sensitized metal oxide interfaces,” Journal of rate of 1 MHz) gives a recombination lifetime of 6.5 ms Physical Chemistry A, vol. 104, no. 18, pp. 4256–4262, 2000. approximately (similar to that found with CdS-coated TiO - 2 [6] D. Liu and P. V. 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[9] I.Bedja,S.Holchandani, andP.V.Kamat,“Photosensitization Photoinduced absorption spectroscopy where the excitation of composite metal oxide semiconductor films,” General & is provided by an on/off monochromatic light source can Introductory Chemistry, vol. 101, no. 11, pp. 1651–1653, 1997. give direct information on electron-injection and hole- [10] M. Law, L. E. Greene, A. Radenovic, T. Kuykendall, J. Liphardt, electron recombination rates using spectra of transient and P. Yang, “ZnO-Al O and ZnO-TiO core-shell nanowire 2 3 2 ΔA Advances in OptoElectronics 5 dye-sensitized solar cells,” Journal of Physical Chemistry B, vol. 110, no. 45, pp. 22652–22663, 2006. [11] M. Guo, P. Diao, X. Wang, and S. Cai, “The effect of hydrothermal growth temperature on preparation and pho- toelectrochemical performance of ZnO nanorod array films,” Journal of Solid State Chemistry, vol. 178, no. 10, pp. 3210– 3215, 2005. [12] C. Bauer, G. Boschloo, E. Mukhtar, and A. Hagfeldt, “Electron injection and recombination in Ru(dcbpy)2(NCS)2 sensitized nanostructured Zno,” Journal of Physical Chemistry B, vol. 105, no. 24, pp. 5585–5588, 2001. [13] I. Bedja, Photophysics and photoelectrochemistry studies on nanocrystalline semiconductor systems. Mechanistic studies of photosensitization and modu-lation of electron transfer kinetics, Ph.D. thesis, University of Quebec at Trois-Rivieres, Quebec, Canada, 1996. 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