Study of modular PEC solar cells for

International Journal of Hydrogen Energy 32 (2007) 1680 – 1685 www.elsevier.com/locate/ijhydene

Study of modular PEC solar cells for photoelectrochemical splitting of water employing nanostructured TiO2 photoelectrodes P.R. Mishra a , P.K. Shukla b , O.N. Srivastava a,∗ a Department of Physics, Banaras Hindu University, Varanasi 221005, India b Vindhya Institute of Technology and Science, Satna 485002, India

Received 2 August 2006; received in revised form 1 October 2006; accepted 2 October 2006 Available online 17 November 2006

Abstract This report is aimed at the development of modular PEC solar cells for hydrogen production. For fabrication of cell, photoelectrodes in the from of nanostructured TiO2 film over Ti-metal base have been synthesized by alkoxide sol–gel method using Ti[OCH(CH3 )2 ]4 as precursor followed by deposition using spin on technique. Crystalline phase and microstructure of the deposited film have investigated with the help of X-ray diffraction and TEM techniques. The photoelectrochemical response of the individual film electrodes of different area as well as parallel combination of two and four electrodes have been recorded and compared using conventional three electrode configuration and 1 M-NaOH as electrolyte. The photoconversion efficiencies have also been determined and compared for both the configurations. The rates of hydrogen production have been determined by measuring volume of the gas evolved over cathode. It has been found that the optimum area of the photoelectrode for modular PEC solar cell design in the present study corresponds to ∼ 0.40 cm2 . 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Solar hydrogen production; Modular PEC solar cell; Nanostructured (ns) TiO2

1. Introduction Today, energy crisis triggered by depletion and pollution aspect of fossil fuels, particularly of petroleum, is one of the perennial problems. Therefore, there is a constant search of clean and renewable energy, which can effectively substitute petroleum. Decades of R&D efforts have shown that hydrogen is the best substitute. Hydrogen can be produced through various routes, particularly most attractive routes are photoelectrochemical and photocatalytic decomposition of water [1–8]. In 1972, Fujishima and Honda [1] have demonstrated photoelectrolysis of water on n-type TiO2 single crystal electrode and proposed this reaction to be used for solar energy conversion and storage in the form of hydrogen. Hydrogen production

∗ Corresponding author. Tel.: +91 542 2368468; fax: +91 542 2369889.

E-mail address: [email protected] (O.N. Srivastava).

through photoelectrolysis of water with the use of semiconductor photoelectrodes is a nonpolluting, wasteless and renewable method of H2 production. Impressive progresses have been made in the search of stable and efficient photoelectrodes during last quarter of century. It has been found that metal oxide semiconductors (particularly TiO2 ) are the most promising candidates for light-driven photoelectrolysis. Recent developments in the preparation of nanostructured photoelectrodes and surface decoration with suitable light harvesting systems have raised the hope of realizing commercial, high efficiency photoelectrochemical solar cells [6–10]. However, unlike dry photovoltaic cells, there are only few works available on the design considerations of the modular PEC solar cells [11–13]. One of the important aspects in this regard is determination of effective photoelectrode area for the development of modular PEC solar cells. In the present investigation, we have addressed this issue in order to explore the possibilities of developing a feasible water photoelectrolysis system. For this, we have fabricated and studied the photoelectrochemical response of photoelectrodes of different area as well as modules with parallel-connected photoelectrodes.

0360-3199/$ - see front matter 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2006.10.002

P.R. Mishra et al. / International Journal of Hydrogen Energy 32 (2007) 1680 – 1685

2. Experimental details

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Ti-sheet

2.1. Preparation of nanostructured TiO2 films Nanocrystalline TiO2 (ns-TiO2 ) film electrodes have been prepared through sol–gel process involving hydrothermal treatment step [14,15]. The precipitation process involves controlled hydrolysis of Titanium-tetra-isopropoxide, Ti[OCH(CH3 )2 ]4 , which results in formation of mainly amorphous TiO2 sol. Autoclaving of this sol at 200 ◦ C for 12 h allows the controlled growth of primary particles and improves the crystallinity. During hydrothermal growth step, smaller particles dissolve and fuse leading to relatively large particles. The obtained sol was then dispersed in the solvent and stirred for 4 h before mixing the binder (Carbowax M-20000). Post stirring of this sol for 4 h resulted in final viscous sol, ready for deposition on the substrate. The Ti-metal sheets (∼ 0.3 mm thickness, Good fellow metals, UK) were used as conducting substrate for the preparation of photoelectrodes. The TiO2 sol was deposited by spin-on technique using a photoresist spinner. Nearly uniform films of TiO2 have been deposited by placing ultrasonically cleaned Ti-sheet over vacuum inter-locked spinner holder followed by spreading one drop of sol over the substrate rotating at 3000 rpm. The thickness of the film thus obtained was ∼ 1 m. The deposition process was repeated five times to achieve optimum film thickness of ∼ 5 m. After air drying for 20 min. at 60 ◦ C and for 30 min at 100 ◦ C, the electrodes were fired for 30 min at 450 ◦ C in air to decompose the binder and other organic components present [15,16]. Microstructural characterization employing transmission electron microscope (Philips EM-CM (12)) revealed that these TiO2 films were nanostructured. The average grain size was found to be ∼ 2–10 nm. The contact between the particles was produced by sintering at 500 ◦ C for1 h in Argon atmosphere. A mesoporous structure of nanocrystalline TiO2 film (∼ 5 m thick) over the substrate with a very high effective surface area is thereby formed. 2.2. Structural characterization The gross structure and crystalline behavior of the films were analyzed by X-ray diffractrometry (Phillips; PW 1710). The microstructural feature of the film was investigated under transmission electron microscope (Phillips; EM-CM-12). These studies showed porous nanocrystalline TiO2 films. 2.3. Fabrication of PEC solar cell The photoelectrochemical solar cells were fabricated using perspex having a quartz window for illumination. The photoelectrodes prepared by spin coating technique were fixed over a perspex sheet having a hole of predefined area using a chemically inert epoxy resin. The ohmic contact was made on the back of Ti-sheet using Cu-wire with the help of silver glue and sealed with epoxy resin. For modular cell, the parallel combination of two and four photoelectrodes of specified area (e.g. 1.85 cm2 and 0.40 cm2 each, respectively) were obtained from

Insulating Mask

ns-TiO2 Film (a) POTENTIOSTAT CE RE WE A V

4

O2

H2 3

hν

Eg

5 2

6

7

(b) Fig. 1. (a) Parallel-connected photoelectrodes in the modular PEC cell and (b) schematic representation of a photoelectrolysis cell showing conventional three-electrode controlled potential measurement system along with various components. CE: counter electrode terminal, RE: Reference electrode terminal, WE: working electrode terminal, 1: TiO2 working electrode, 2: PtCE fused in Pyrex glass, 3: saturated calomel electrode, 4: inverted burettes for H2 & O2 collection, 5: quartz window, 6: electrolyte, 7: perspex cell.

the master plate by stripping off part of the TiO2 layer (1 mm finger width) and filling it with the help of epoxy resin. This may be taken to be like finger mask. Thus, the individual photoelectrodes in the module are internally connected through the base Ti-metal sheet (as shown in Fig. 1(a)). The part of the cell between quartz window and photoelectrode was filled with electrolyte (viz. 1 M-NaOH). A platinum sheet with 5-cm2 area was used as counter electrode and a saturated calomel electrode (SCE) with KCl bridge as the reference electrode. The overall cell configuration can be represented as Pt//1 MNaOH//(ns)TiO2 /Ti. All the chemicals used were of analytical grade (Qualigens, ExelR unless otherwise indicated) and the electrolyte used was prepared by using deionized distilled water. The conventional three-electrode configuration along with other components used for controlled potential measurements in the present investigation are schematically shown in Fig. 1b.

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2.4. Photocurrent measurement The photoelectrochemical (I –V ) measurements were carried out by employing standard three-electrode configuration. The photocurrent as a function of applied potential versus SCE was measured using a computer controlled scanning potentiostat/galvanostat (EG & G Princeton Applied Research: Model 263A). A 400 W Hg–Xe lamp (Oriel Corporation, USA) was used as light source. 2.5. Hydrogen production measurements The hydrogen production measurement has been performed under bias in the conventional three-electrode configuration, i.e., Pt-wire//1 M-NaOH//ns-TiO2 /Ti. The H2 gas was collected over Pt-wire in an inverted burette by displacing electrolyte in the burette column. The volume of hydrogen produced was measured for the single cell as well as for parallel combination of the electrodes (i.e., module) for specific bias potentials in the two sets of experiments. In one of them, the module contains two cells with each of photoelectrode area 1.85 cm2 and in the other, four cells with the area of each photoelectrode as 0.40 cm2 . 3. Results and discussion 3.1. Structural characterization A typical X-ray diffraction pattern of the as-deposited nsTiO2 film is shown in Fig. 2. All the X-ray peaks observed in X-ray diffraction pattern have been indexed on the basis of standard ASTM values. The XRD pattern revealed that the dominant TiO2 phase was anatase (tetragonal) type. The lattice parameters were calculated as a = 3.785 Å and c = 9.514 Å, which were closely corresponded to the standard values. Apart from the peaks corresponding to anatase phase, some peaks corresponding to underlying rutile phase of TiO2 (tetragonal:

(011)

- Anatase - Rutile - Substrate

180

120 100

(101)

(Counts)

140

(002)

(110)

160

(012)

200

80

(105) (215) (220)

(200)

(210)

20

(111)

40

(200)

(010) (101)

60

0 20

30

40 (2θ)

50

60

Fig. 2. A typical X-ray diffraction pattern for ns-TiO2 film deposited over Ti substrate after initial firing and annealing process.

Fig. 3. (a) Transmission electron micrograph and (b) corresponding selected area electron diffraction pattern for spin deposited ns-TiO2 film. The oxide film layer scrapped from the substrate was used for the TEM analysis.

a = 4.593Å and c = 2.959 Å) and substrate Ti (hexagonal: a = 2.950 Å, and c = 4.686 Å) are also present. The dominance of anatase phase was confirmed through electron diffraction as will be outlined in the next section. Transmission electron microscopy (TEM) technique was employed for microstructural characterization of the TiO2 film. Fig. 3a shows a typical TEM micrograph of the as-synthesized and processed TiO2 film bringing out the microstructure of the film. From the TEM micrograph it can be seen that mesoporous network of nanocrystalline film is composed of TiO2 grains of average grain size ∼ 2–10 nm. The electron diffraction patterns were also taken for the phase identification of the material. Fig. 3b depicts the selected area (from the region shown in Fig. 3a) electron diffraction pattern, which confirms the formation of anatase crystallites. The nanostructured form of the deposited films leads to increase in effective surface area due to the attainment of higher roughness factor, which in some cases, can be a few hundred times the geometric surface area.


Fig. 4. Current–voltage (I –V ) characteristics under illumination for (a) single cell (area 1.85 cm2 ), larger area cell (area 3.7 cm2 ) and modular cell (2×1.85 cm2 ), and (b) single cell (area 0.40 cm2 ), larger cell (area 1.60 cm2 ) and modular cells (4 × 0.4 cm2 ).

3.2. Photoelectrochemical response The variation of the photocurrent density (Jp ) as a function of measured potential (Emeas ) versus SCE for the PEC solar cells with the electrode area 1.85 cm2 and 3.70 cm2 have been shown by the representative I –V characteristics as in Fig. 4a. The photocurrent density for module of two parallel-connected electrodes, with each of area 1.85 cm2 (hereafter 2 × 1.85 cm2 module) have also been recorded under identical conditions and incorporated in this figure for comparison. It is observed that the photocurrent density for PEC cell with photoelectrode area 1.85 cm2 is 1.50 mA cm−2 at −0.40 V versus SCE (maximum power point of the I –V response) that decreases to 1.08 mA cm−2 on doubling the photoelectrode area (i.e., 3.70 cm2 ). This corresponds to a decrease in Jp by ∼ 28% on doubling the photoelectrode area. On the other hand, the value of photocurrent density for module (2 × 1.85 cm2 ) is found to

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be 1.38 mA cm−2 at the same measured potential. Interestingly, for modular cell with the same effective area of photoelectrode and at the same measured potential, the photocurrent density is found to decrease only by 8%. Similar set of experiment has been performed by combining the photoelectrodes with smaller area (viz. 0.40 cm2 ). The observed dependence of Jp as a function of measured potential is shown in Fig. 4b. The value of photocurrent density for the PEC cells with photoelectrode area 0.40 cm2 and 1.60 cm2 at −0.52 V versus SCE (maximum power point of theI –V response) correspond to 2.77 mA cm−2 and 1.75 mA cm−2 , respectively, whereas the photocurrent for modular cell with four parallel-connected photoelectrodes, each of area 0.40 cm2 (hereafter 4 × 0.40 cm2 module) correspond to 2.69 mA cm−2 at the same measured potential. From this figure, it is evident that the value of photocurrent density for PEC cell with photoelectrode area 0.40 cm2 at maximum power point decreases by 36% on taking four times larger photoelectrode area (i.e., 1.60 cm2 ). On the other hand, the decrease in the photocurrent density is only 3% for the same effective area of photoelectrode at the same measured potential in the case of modular cell (4 × 0.40 cm2 ). From the above observations, it is evident that the photocurrent density decreases rapidly on increasing the photoelectrode area. Such a behavior of photocurrent density is known to be due to increase in the defect states originating mostly from grain boundaries/surface defects acting as the recombination center. In contrast to this, the slight decrease in the photocurrent density in the case of modular cells may be related to shadow effect at the edges of the finger mask or to the decrease in photon density in the light beam used for the illumination on moving outwards from the center of photoelectrode. However, it should be mentioned here that on increasing the number of photoelectrodes in modular PEC cells, the noneffective area (i.e., the areas underlying the finger mask) will also increase, which will add to the cost of modules. 3.3. Photoconversion efficiency The photoconversion efficiency (%) of light to chemical energy in the presence of applied potential Eappl was calculated using the following relation [12,17]: 0 (%) = [Jp (Erev − |Eapp |)/I0 ] × 100,

(1)

0 is the standard where Jp is the photocurrent density, Erev state reversible potential (which is 1.23 eV for water splitting reaction), I0 is the power density of incident light (I0 = 85 mW cm−2 ) falling over the photoelectrode and |Eapp | is the absolute value of applied potential Eapp which is obtained as,

Eapp = Emeas − Eaoc ,

(2)

where Emeas is the photoelectrode potential with respect to reference electrode at which Jp was measured and Eaoc is the photoelectrode potential with respect to the same reference electrode at open circuit condition in the same electrolyte solution and under the same illumination condition.

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is mainly related to decrease in the fill factor on upscaling the photoelectrode area [11], which is quite visible from the observed photocurrent response (Fig. 4). From the above description, it is clear that in order to improve the photoconversion efficiency while upscaling the photoelectrode area, modular PEC solar cell with parallel-connected photoelectrodes of suitably chosen size is must. How to optimize the performance of PEC cell modules for water splitting is still under investigation? The efficiency amongst other things crucially depends on the photocurrent density. As it has already been described in the earlier section (Section 3.2), the small area electrode cell module consisting of several electrodes connected in parallel via Ti-base has higher current density and hence this would have higher efficiency. One of the important ways in which efficiency can be enhanced further is to carry out suitable doping of TiO2 so as the spectral response of TiO2 covers besides ultraviolet visible parts of the solar spectrum. There are several such studies of this type [17]. However, it appears that this area of investigation needs further exploration lowering the band gap of TiO2 from ∼ 3.0 eV to ∼ 2 eV so that the spectral response covers almost whole of the visible region.

3.4. Hydrogen production measurements

Fig. 5. Photoconversion efficiency (%) as a function of applied bias Eapp for (a) single cell (area 1.85 cm2 ), larger area cell (area 3.7 cm2 ) and modular cells (2 × 1.85 cm2 ). Maximum in conversion efficiency corresponds to Eapp = 0.45 V and (b) single cell (area 0.40 cm2 ), larger cell (area 1.60 cm2 ) and modular cell (4 × 0.4 cm2 ). Maximum in conversion efficiency corresponds to Eapp = 0.57 V.

Fig. 5(a,b) shows the dependence of percent photoconversion efficiency on the applied potential Eapp for the individual photoelectrodes along with the modules for both the cases. The maximum photoconversion efficiency was found to be 2.52% (Fig. 5(b)) for the PEC cell with photoelectrode area 0.40 cm2 and it was found to decrease on increasing the photoelectrode area. This is quite similar to the variation in the photocurrent density as discussed in Section 3.2. However, a closer look of Fig. 5 reveals that the maximum photoconversion efficiency, for PEC cell with relatively small photoelectrode area as well as for modular cell made of smaller photoelectrodes, corresponds to a lower applied potential (viz. Eapp = 0.45 V) in comparison to the PEC cells and modular cell made of relatively large photoelectrodes (where max correspond to Eapp = 0.57 V). The shift of applied potential Eapp towards higher value on increasing photoelectrode area (for both, single cells as well as modules)

The photocurrent response of the individual cells and modules was also evaluated by measuring the rate of water splitting reaction to produce hydrogen and oxygen. The hydrogen production measurements have been performed under identical conditions on both types of PEC cells described above. The only difference is that an inverted burette was used to collect hydrogen gas evolving over platinum wire electrode that was used as cathode for this set of measurement. The volume of gas evolved was determined from displacement of the electrolyte in the burette column. The hydrogen production rate was determined for an applied bias Eapp corresponding to maximum in photoconversion efficiency (Fig. 5) for both type of cells, i.e., for single cells with photoelectrode area 1.6 cm2 and 3.70 cm2 , as well as, for modular cells (2 × 1.85 cm2 ) and (4 × 0.4 cm2 ). The rates of hydrogen production for single cell with photoelectrode area 3.70 cm2 and for module (2 × 1.85 cm2 ) have been found to be 4.15 l h−1 m−2 and 5.31 l h−1 m−2 , respectively. Similarly, for the case of PEC cell with photoanode area 1.6 cm2 and modular cell (4 × 0.4 cm2 ), the measured values of hydrogen production rate correspond to 6.72 l h−1 m−2 and 10.35 l h−1 m−2 , respectively. Thus, in the former case, the hydrogen production rate increases only by 27% on using modular cell of the same effective area. On the other hand, for the module with individual electrodes of relatively smaller area (i.e., for 4 × 0.4 cm2 module) hydrogen production rate increases by 54%. Therefore, the hydrogen production rate can be improved by employing modular PEC cells in the form of parallel-connected photoelectrodes of smaller area. The rate of hydrogen production mainly depends on the charge carrier generation and their combination with hydrogen ions at the counter electrode and hence on the photocurrent density in the photoelectrochemical solar cell. The PEC current,


in the present case, depends strongly on the microstructural defect induced recombination centers and electron diffusion coefficient in the TiO2 photoelectrodes [18]. In the present case, we are comparing the advantages of performance of several small area photoelectrodes connected in parallel to an equivalent large area photoelectrode in regard to photocurrent density and hence hydrogen production rate. In the light of this, the important parameter here will be defect based recombination centers. For small area (∼ 0.40 cm2 ) module cell, due to lower defect density, the photogenerated charge carriers would posses lower recombination rate as compared to the large area (∼ 1.85 cm2 ) photoelectrode. This will lead to improved PEC current and hence higher hydrogen production rate (see Fig. 4). When such small area cells are connected in parallel via the Ti-sheet base, the effect is to have not only large effective area but also high photocurrent density. This would not have been possible with a single large area cell due to high defect density; which will lead to high charge recombination rate and hence lower photocurrent density. 4. Conclusions The enhancement in photocurrent density and hydrogen production rate has been obtained by parallel combination of suitable area nanostructured TiO2 photoelectrodes. The observed values of photocurrent density at maximum power point in the I –V response for the cells with electrode area 1.85 cm2 , 3.70 cm2 and for modular cell (2×1.85 cm2 ) has been found to be 1.50 mA cm−2 , 1.08 mA cm−2 and 1.38 mA cm−2 , respectively. On the other hand, the recorded values of photocurrent density at maximum power point in the I –V response correspond to 2.77 mA cm−2 , 1.75 mA cm−2 and 2.69 mA cm−2 for the cells with photoelectrode area 0.4 cm2 , 1.6 cm2 and for module (4 × 0.4 cm2 ). The photocurrent density was found to increase by 28% in case of modular cell (2 ×1.85 cm2 ) in comparison to the PEC cell with single photoelectrode of same effective area, whereas the increase in photocurrent density was found to be 36% in the case of modular cell (4 × 0.4 cm2 ) in comparison to cell with single photoelectrode of same effective area. The maximum value of photoconversion efficiency for water-splitting reaction in the case of modular cell (4×0.4 cm2 ) was found to be 2.45% for an applied bias of 0. 45 V. The hydrogen production rate was found to increase by a factor of 54% on using modular cell (4×0.4 cm2 ) in comparison to PEC cell with single photoelectrode of same effective area. Acknowledgments The authors are grateful to Prof. A.R. Verma, Prof. C.N.R. Rao, Dr. G.V. Subba Rao, Prof. S. Lele and Dr. P.N. Dixit

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for helpful discussions. The present work was financially supported by the Ministry of Nonconventional Energy Sources, Government of India. One of the authors (P.R. Mishra, Senior Research Fellow) is grateful to Ministry of Nonconventional Energy Sources, Government of India, for awarding the prestigious National Renewable Energy (NRE) Fellowship. References [1] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972;238:37–8. [2] Tributsch H. Photovoltaic strategies for hydrogen generation. Chem Ind 1999;53:410–20. [3] Bard AJ, Fox MA. Artificial Photosynthesis: Solar splitting of water to hydrogen and oxygen. Acc Chem Res 1995;28:141–5. [4] Nozik AJ, Memming R. Physical chemistry of semiconductor-liquid interface. J Phys Chem 1996;100:13061–78. [5] Bak T, Nowotny J, Rekas M, Sorrell CC. Photoelectrochemical hydrogen generation from water using solar energy. Materials-related aspects. Int J Hydrogen Energy 2002;27:991–1022. [6] Gratzel M. Photoelectrochemical cells. Nature 2001;414:338–44. [7] Kocha SS, Montgomery D, Peterson MW, Turner JA. Photoelectrochemical decomposition of water utilizing monolithic tandem cells. Sol Energy Mater Sol Cells 1998;52:389–97. [8] Aroutiounian VM, Arakelyan VM, Shahnazaryan GE. Metal oxide photoelectrodes for hydrogen generation using solar radiation-driven water splitting. Solar Energy 2005;78:581–92. [9] Robert F. Tricks for beating the heat help pannels see the light. Science 2003;300:1219. [10] Kubo W, Kambe S, Nakade S, Kitamura T, Hanabusa K, Wada Y. et al. Photocurrent determining process in quasi-solid dye sensitized solar cells using ionic gel electrolytes. J Phys Chem B 2003;107:4374–81. [11] Dai S, Weng J, Sui Y, Shi C, Huang Y, Chen S. et al. Dye sensitized solar cells from cell to module. Sol Energy Mater Sol Cells 2004;84:125–33. [12] Mishra PR, Shukla PK, Singh AK, Srivastava ON. Investigation and optimization of nanostructured TiO2 photoelectrode in regard to hydrogen production through photoelectrochemical process. Int J Hydrogen Energy 2003;28:1089–94. [13] Sorensen B, Noisen A, Sorensen F, Lund T, West K, Hansen EB. et al. Industrial development of photoelectrochemical modules. http://mmf.ruc.dk/energy/PE montrial01.pdf. [14] Shukla PK, Karn RK, Mishra PR, Singh AK, Srivastava ON. On the hydrogen production through PV assisted PEC Solar cells. Proceedings IAHE, WHEC; 2002, B2.4. [15] Gratzel M. The artificial leaf, bio-mimetic photocatalysis. Catal Tech 1999;3(1):4–17. [16] Karn RK, Srivastava ON. On the synthesis of nanostructured TiO2 anatase phase and the development of the photoelectrochemical solar cells. Int J Hydrogen Energy 1999;24:27–34. [17] Khan SUM, Al-Shahry M, Ingler Jr WB. Efficient Photochemical water splitting by a chemically modified n-TiO2 . Science 2002;297:2243–5. [18] Shen Q, Arae D, Toyoda T. Photosensitization of nanostructured TiO2 with CdSe quantum dots: effects of microstructure and electron transport in TiO2 substrates. J Photochem Photobiol A: Chem 2004;164:78–80.