Antireflective Photoanode Made of TiO2 Nanobelts and a ZnO

Jun 17, 2010 - The network fabricated with anatase TiO2 NBs demonstrated a lower resistance than that of the TiO2 NP film. In the network, the TiO2 NB...
0 downloads 0 Views 3MB Size
Download PDF
J. Phys. Chem. C 2010, 114, 11375–11380

11375

Antireflective Photoanode Made of TiO2 Nanobelts and a ZnO Nanowire Array Haiyan Li,† Bin Jiang,‡ Rebecca Schaller,† Jianfeng Wu,† and Jun Jiao*,† Department of Physics and Department of Mathematics and Statistics, Portland State UniVersity, Portland, Oregon 97207 ReceiVed: February 17, 2010; ReVised Manuscript ReceiVed: April 30, 2010

We fabricated an antireflective hybrid nanostructure using anatase TiO2 nanobelts synthesized by an alkaline hydrothermal route and a ZnO nanowire array grown via a low-temperature solution-phase process. The replacement of TiO2 nanoparticles with TiO2 nanobelts improved the electron transport in the TiO2 porous film. Importantly, rigorous coupled-wave analysis and reflectance measurements indicate that the well-designed composite of TiO2 nanobelt-ZnO nanowire array acted as an efficient antireflection coating. The photoanode made of this hybrid nanostructure may enhance the performance of dye-sensitized solar cells by minimizing the electron-hole recombination-related and reflection-induced energy loss. Introduction Photosensitization of TiO2 by dye molecules is a promising and low-cost way to prepare sunlight-absorbing materials for dye-sensitized solar cells (DSSCs).1 In a DSSC, the anchored dye molecules generate electron-hole pairs after harvesting photons. The excited electrons are thermodynamically injected into the TiO2 conduction band and transported through the TiO2 layer to the transparent conducting glass. The holes diffuse through the electrolyte (usually I-/I3-) to the opposing electrode.2-5 In general, the photoanode of the DSSC is highly porous and made of stacking crystalline TiO2 nanoparticles (NPs), allowing the photoanode to load adequate dye molecules for sufficient light absorption.6 To reach the transparent conducting glass, the injected electrons need to encounter on average more than 106 TiO2 NPs along random and complicated pathways in the porous TiO2 NP film.7 The excessive electron traps related to the surface defects, which exist at the weak contact between the neighboring TiO2 NPs and on the TiO2 NP surface, cause the trap-limited electron diffusion rate to be several orders of magnitude slower than that in TiO2 single crystals.7-10 This leads to the exacerbation of the energy depletion connected to the electron-hole recombination across the oxide-electrolyte interface. Adachi et al. aligned and attached TiO2 NPs to nanowires (NWs), resulting in a high rate of electron transfer through the TiO2 single-crystal-like anatase NW network.11 They further reported the use of TiO2 nanorods (NRs) instead of TiO2 NPs to decrease the Ohmic loss during transport of the injected electrons.12 This indicates that onedimensional TiO2 nanostructures, such as TiO2 NRs, NWs, or nanobelts (NBs), are ideal materials for DSSC fabrication due to their electron diffusion length over tens of micrometers in comparison to that of TiO2 NPs.13-16 Similarly, ZnO NWs were utilized for DSSCs owing to their substantially larger electron diffusion lengths related to the high electron mobility of 1-5 cm2 s-1 V-1 along the ZnO crystal c axis and the radial electric field in ZnO NW’s crystalline core.17,18 The vertically aligned ZnO NWs can form continuous electron paths between the dye molecule and the transparent conducting glass. The electrontransport rate in the ZnO NW array-based DSSC measured by * To whom correspondence should be addressed. E-mail: [email protected]. † Department of Physics. ‡ Department of Mathematics and Statistics.

Law et al. is 1 or 2 orders of magnitude greater than that in the TiO2 NP-based DSSC.19,20 However, the replacement of the tiny NPs with thick NRs, NWs, or NBs leads to a great loss of surface area, which is proportional to the amount of absorbed sensitizers. To overcome the dilemma of having to compromise the DSSC anode’s light harvesting for enhanced electron transport or vice versa, we integrated TiO2 NBs and a ZnO NW array into a hybrid architecture. The network fabricated with anatase TiO2 NBs demonstrated a lower resistance than that of the TiO2 NP film. In the network, the TiO2 NBs may serve as fast electron paths and as efficient dye absorbers due to their thin diameters. The rigorous coupled-wave analysis (RCWA) model and reflectance measurements reveal that, in the hybrid nanostructure of TiO2 NBs and a ZnO NW array, the TiO2 NB filled ZnO NW array is an efficient antireflection coating (ARC) between the woven TiO2 NB network and the transparent conducting glass. Experimental Section Synthesis and Integration of TiO2 NBs and a ZnO NW Array into a Hybrid Nanostructure. The TiO2 NBs were synthesized by treating 0.2 g of anatase TiO2 NPs (diameter ) 100-200 nm, 99.8%, Aldrich) with a KOH aqueous solution (15 M), maintaining a 75% fraction in a Teflon-lined autoclave (0.05 L) at 185 °C for 12-48 h. The products were washed with 0.1 M nitric acid and deionized water until the pH value of the solution reached 7. The ZnO array was synthesized by a two-step chemical method. First, the ZnO seed layer was prepared by spin-coating (3000 rpm) a mixture of zinc acetate, ethanolamine, and 2-methoxy-ethanol and then annealing at 400 °C for 1 h. Second, the ZnO NW arrays were grown on the ZnO seed-covered fluorine-doped SnO2 (FTO) substrates (Tec 8, sheet resistance ) 9 Ω/sq, Pilkington) in an aqueous solution of 20 mM zinc nitrate hydrate (99.999%, Aldrich), 20 mM hexamethylenetetramine (97%, Aldrich), and 100 mM 1,3diaminopropane (99%, Aldrich) at 90 °C for 6 h. The products were washed with deionized water and annealed at 400 °C for 0.5 h to remove any residual chemicals and improve their nanostructures. To fabricate the hybrid nanostructure, the suspension solution consisting of (10 mg/mL) as-synthesized TiO2 NBs was poured into a cube, which had been constructed

10.1021/jp101478t  2010 American Chemical Society Published on Web 06/17/2010

11376

J. Phys. Chem. C, Vol. 114, No. 26, 2010

Figure 1. (a, b) SEM and TEM images of the TiO2 NB/NP mixture synthesized with a reaction time of 12 h. (c, d) SEM and TEM images of the TiO2 NBs synthesized with a reaction time of 48 h. The inset of (d) is the corresponding selected area electron diffraction pattern.

by pasting the top of the ZnO NW array-covered FTO substrate with 70 µm thick planar adhesive tapes. After evaporation of the water and annealing at 300 °C for 2 h, the TiO2 NBs filled the ZnO NW array and had partially woven into a network. Morphological and Structural Characterization and Electrical and Reflectance Measurements. The morphology and the internal structure of the as-synthesized materials were analyzed using an FEI Sirion field emission scanning electron microscope (SEM) equipped with an energy-dispersive X-ray (EDX) spectrometer, an FEI Strata 237 dual-beam focused ion beam (FIB) microsocpe, and an FEI Tecnai F-20 transmission electron microscope (TEM). Raman measurements were carried out at room temperature on a HORIBA Jobin Yvon LabRAM HR Raman microscope using a laser with a wavelength of 532 nm as the excitation source. The electrical measurements were performed using an Agilent 4156C precision semiconductor parameter analyzer and a Cascade Microchamber probe station operated at room temperature and in the dark. Reflectance was measured using a UV-vis-NIR spectrophotometer (UV 3600, Shimadzu) equipped with a UV-vis-NIR integrating sphere attachment (ISR 3100, Shimadzu). Results and Discussion The TiO2 NBs were synthesized through treating the TiO2 NPs with KOH aqueous solution at 185 °C. During the reaction, the erosion of the TiO2 NPs led to the formation of [Ti(OH)6]2monomers. In a saturated state, the adjacent [Ti(OH)6]2monomers were arranged by oxolation or olation,21 resulting in the growth of TiO2 NBs. Figure 1a shows the mixture of the TiO2 NBs and the residual TiO2 NPs after treating TiO2 NPs with KOH aqueous solution at 185 °C for 12 h. It is interesting to note that some TiO2 NBs grew in an organized fashion on the TiO2 NPs. The intrinsic connection between the TiO2 NB and the TiO2 NP is shown in Figure 1b. We found that, in some cases, the TiO2 NBs were primarily oriented in two directions on the same TiO2 NP. In each given direction, the TiO2 NBs grew parallel to one another. This characteristic growth pattern of the TiO2 NBs is probably attributed to the oxolation or olation between [Ti(OH)6]2- monomers and the Ti-OH bonds on the TiO2 NP surface, where there are monatomic-height step defects aligned along two directions.22,23 Continuous erosion of the TiO2

Li et al. NPs and oxolation or olation of adjacent [Ti(OH)6]2- monomers elongated these TiO2 NBs. When the reaction time reached 48 h, most of the TiO2 NPs were converted to the TiO2 NBs, as shown in Figure 1c. The EDX analysis confirms that these NBs are pure TiO2. A high-resolution TEM (HRTEM) image indicates that these TiO2 NBs are single-crystalline, with a growth direction along the [100] orientation (anatase TiO2), as shown in Figure 1d. This was confirmed by the corresponding selected area electron diffraction pattern, as shown in the inset of Figure 1d. The products were further characterized using a Raman microscope. Figure 2 shows that all Raman peaks of the TiO2 NB/NP mixture were related to the typical six Raman-active modes (3Eg + 2B1g + A1g) of octahedral anatase TiO2.24,25 Compared to the TiO2 NPs, the TiO2 NBs demonstrated a peak at 235 cm-1 related to the multiphoton process of rutile TiO2, indicating a fraction of rutile in the TiO2 NBs. Furthermore, the peaks of TiO2 NBs were broadened, and the A1g/E1g(2) and Eg(3) peaks show a significant red shift. This is related to the effects of decreasing particle size on the force constants and vibration amplitudes of the nearest-neighbor bonds, as the large size TiO2 NPs were converted to the thin TiO2 NBs.26 The thin diameter of TiO2 NBs also resulted in the formation of the additional peak at 255 cm-1, which has been observed from small-sized (4 nm) TiO2 NPs.27 In the TiO2 NP film, the slow electron diffusion is dominated by the localized surface electron traps of Ti (III)-OH related to the oxygen vacancies after calcinations. These are located at the TiO2 NP surface and occur especially at the boundaries between the weak contacted TiO2 NPs.28-30 To test whether the placement of the TiO2 NPs with the TiO2 NBs increased the conductivity of the porous TiO2 film, we performed a comparative electrical measurement on the TiO2 NP film (Figure 3a), the TiO2 NB/NP film (Figure 3b), and the TiO2 NB networks (Figure 3c), which were fabricated by the deposition of the TiO2 NPs, the as-synthesized TiO2 NB/NP mixture, and the assynthesized TiO2 NBs on the insulating glass substrates. The linear behavior of the current-voltage (I-V) curves indicates barrier-free contacts between the measured film and the probes. The measured electrical resistivities of the TiO2 NP film, the TiO2 NB/NP film, and the TiO2 NB networks were 1.5 × 107, 2.14 × 103, and 5.85 × 102 Ω · cm (Figure 3d), respectively. We also measured the resistance of the three samples at 120 °C. The resistance of the TiO2 NP film increased an average of 5 times when the temperature was increased from room temperature to 120 °C. However, the resistances of the TiO2 NB/NP film and the TiO2 NB networks were several hundred times greater than those of the room-temperature networks. Water adsorption on the TiO2 crystal surfaces partially contributes to the conductivity of the TiO2 film at room temperature by hopping protons through a percolation cluster formed through the bonded water molecules on the TiO2 crystal surfaces.31 These results suggest that the conductivities of the TiO2 NB/NP film and the TiO2 NB networks were related to the absorption of water on the TiO2 NB surface, which correlated with the specific surface area (porosity) of TiO2 and ambient temperature. After minimizing the effects of the humidity under 120 °C, the resistance of the TiO2 NP films was still more than 1 order of magnitude higher than those of both the TiO2 NB/NP film and the TiO2 NB networks. This shows that the TiO2 NBs could be the continuous paths for transport of the injected electrons in DSSCs. The optimization of the photoanode architecture for maximizing light-harvesting efficiency will improve the efficiency of

Photoanode Made of TiO2 NBs and a ZnO NW Array

J. Phys. Chem. C, Vol. 114, No. 26, 2010 11377

Figure 2. Ambient pressure Raman spectra of the TiO2 NB/NP mixture and the TiO2 NBs recorded by the HR800 Raman system with a laser wavelength of 532 nm.

Figure 3. SEM images of (a) the TiO2 NP film, (b) the TiO2 NB/NP film, and (c) the TiO2 NB network. (d) I-V characteristics of the films shown in (a-c).

DSSCs.32,33 In particular, the application of well-designed ARC materials is a promising approach to reducing reflection-induced energy loss.34 In the porous TiO2 film-based DSSC, the notable difference of the refractive index between the TiO2 film (n ) 2.49) and the FTO layer (n ) 1.7) leads to the significant reflection of sunlight. The gradual change of the refractive index between two media can reduce the light reflection. For example, Lee et al. employed the ZnO (refractive index, n ) 2.0) NW array as an efficient ARC layer between Si (n ) 3.6) and air (n ) 1), showing a broad-band reflection suppression of visible light.35 To fabricate an antireflective photoanode, we built an ARC between the FTO substrate and the TiO2 film using the ZnO NW array filled with TiO2 NBs, which caused a smooth transition of the effective refractive index from n ) 1.7 of the FTO layer to n ) 2.49 of the TiO2 layer. A schematic diagram

of the cross section of the photoanode is shown in Figure 4a. In the ZnO NW array, the roots of the ZnO NW joined firmly together and the tips of the ZnO NWs were tapered. To approximate the structure of the ZnO NW for model simplification, the ARC between the FTO layer and the TiO2 NB network is divided into three layers: (I) fused base layer of ZnO, (II) ZnO NW stems mixed with TiO2 NBs, and (III) the mixture of ZnO NW tips with TiO2 NBs. A theoretical prediction, using RCWA,36,37 was performed in order to optimize the parameters of the photoanode’s architecture. Because the photoanodes both without and with an ARC have the same air/glass, glass/FTO, and TiO2/electrolyte interfaces, the reflectance difference between these two photoanodes was solely from the introduced ARC and the interfaces of TiO2/ZnO and ZnO/FTO. Therefore, our model calculated

11378

J. Phys. Chem. C, Vol. 114, No. 26, 2010

Li et al.

Figure 4. (a) Schematic diagram of the cross section of the photoanode consisting of five layers defined as the model for calculation. The ARC between the top layer of the TiO2 NB network and the FTO layer consisted of three layers: (I) fused base layer of ZnO, (II) hybrid structure of ZnO NW stems and TiO2 NBs, and (III) hybrid structure of ZnO NW tips with TiO2 NBs. (b-d) The simulated reflectance of the photoanode depended on the thicknesses of layers I, II, and III, respectively.

the reflectance from the TiO2/FTO interface for the photoanode without ARC and the reflectance from the three-layer-structured ARC and the interfaces of TiO2/ZnO and ZnO/FTO for the photoanode with an ARC. In addition, the scattering in the substrate parallel direction is negligible due to the vertically aligned ZnO NW with a thin diameter of 40-80 nm. The RCWA model is set up as follows: First, we need to calculate the effective refractive indices n1, n2, and n3 for layers I, II, and III of the ARC structure, respectively. It is obvious that n1 is the refractive index of ZnO for layer I, which is composed of ZnO only. As we observed from SEM, the volume ratios of ZnO to TiO2 were estimated to be 0.7:0.3 for layer II and 0.3: 0.7 for layer III, respectively. On the basis of the effective medium theory,38 the effective refractive index n2 can be approximated by n2 ) [0.7 × nZnOq + 0.3 × nTiO2q]1/q, where q ) 2/3. Similarly, n3 ) [0.3 × nZnOq + 0.7 × nTiO2q]1/q holds in layer III. Second, we set up Maxwell’s equation to express the electromagnetic field in all the layers, including layers I, II, and III of the ARC, the bottom FTO layer, and the top TiO2 NB network. By matching the tangential components of the electrical and magnetic fields between all neighboring layers, we can then calculate the reflectance and transmittance of the whole system at normal incidence. This model shows that the calculated weighted reflectance (Rw) related to the TiO2/FTO interface (3.54%) was extremely suppressed to less than 0.50% by using the inserted ARC. Using

this model, thickening layer I led to a reduction of the reflection at wavelengths ranging from 520 to 900 nm, but it also enhanced the reflection at wavelengths from 350 to 520 nm (Figure 4b). By fixing the thicknesses of layers II and III as 3000 and 50 nm, the Rw of the photoanode was 0.60, 0.52, and 0.54% when the thicknesses of layer I were set to be 50, 60, and 70 nm. Next, by fixing the thicknesses of layers I and III as 60 and 50 nm, we found that the thickness of layer II has less effect on the reflectance than the thicknesses of layers I and III (Figure 4c). Its optimal value can be chosen from the wide range of 1000-5000 nm. Finally, we fixed the thicknesses of layers I and II as 60 and 3000 nm and increased the thickness of layer III, causing amplitude magnification of the periodical reflectance variation at wavelengths ranging from 350 to 520 nm and amplitude suppression at wavelengths from 520 to 900 nm (Figure 4d). The Rw was 0.68, 0.52, and 0.57% when layer III had a thickness of 30, 50, and 70 nm. The above calculations indicate that the optimized thicknesses of layers I, II, and III were around 60, 1000-5000, and 50 nm. Because both the thickness of the fused ZnO layer (layer I) and the height of the ZnO NWs (layer II) were proportional to the growth time, the reaction time of 3 h was adopted for the growth of the ZnO NWs. The tip shape of the ZnO NWs was controlled by adjusting the concentration of hexamethylenetetramine. Figure 5a demonstrates a cross-sectional view of the as-synthesized ZnO NW array with similar geometrical param-

Photoanode Made of TiO2 NBs and a ZnO NW Array

J. Phys. Chem. C, Vol. 114, No. 26, 2010 11379

Figure 5. (a) Cross section of the as-synthesized ZnO NW array. (b) Top view of the photoanode made of a TiO2 NB-ZnO NW array hybrid nanostructure. The inset of (b) is the 30° side view of a ZnO NW array filled with TiO2 NBs. (c) Measured reflectance spectra of the photoanodes without or with the ARC of a TiO2 NB-ZnO NW array. (d) Experimental and simulated data of ARC-induced reflectance suppression.

eters to the optimal simulation values (layers I, II, and III were around 60, 1000-5000, and 50 nm, respectively). The aqueous suspension of the as-synthesized TiO2 NBs was coated on the ZnO NW array. Because of the small size (5-40 nm wide, 1-5 nm thick) of the as-synthesized TiO2 NBs, these TiO2 NBs filled partially in the ZnO NW array (inset of Figure 5b). A top view of the TiO2 NB-ZnO NW array photoanode is shown in Figure 5b. Compared with the photoanode without the ARC, the reflectance of the photoanode with the ARC of the TiO2 NB-ZnO NW hybrid nanostructure was noticeably suppressed (Figure 5c). Figure 5d shows the simulated and the measured ARC-induced reflectance suppressions calculated by subtracting the reflectance of the photoanode without the ARC from that of the photoanode with the ARC. Note that the measured ARCinduced reflectance suppression was higher than the simulated value, especially in the UV region. A plausible explanation for this difference is that, in the simulated results, we did not consider the effect of ZnO NW absorption and reflection. The fact is that the ZnO NW array has a strong absorption for light with wavelengths less than 375 nm, whereas it shows a weak absorption in the 450-550 nm range due to the recombination of photogenerated holes with the electrons occupying the oxygen vacancies.39 If taking into consideration of these effects, the simulated ARC-induced reflectance suppression matches with our experimental data. In conclusion, we fabricated a hybrid nanostructure composed of TiO2 NBs and a ZnO NW array, which was capable of improving electron transport and antireflection. Because the band gaps and band-edge energies of ZnO and TiO2 are similar,20 the large diffusion length enables the ZnO NWs of the array to effectively collect the injected electrons from the TiO2 NB network. Therefore, the photoanode made of this hybrid nanostructure is able to reduce the electron-hole recombination-

related and reflection-induced energy loss of DSSCs. Moreover, our SEM and FIB results suggest that, in the current hybrid nanostructure, the TiO2 NBs did not completely fill in the ZnO NW arrays yet. It is expected that the antireflective photoanode could be further improved by optimizing the density of both TiO2 NBs and ZnO arrays. Acknowledgment. This work was supported, in part, by the National Science Foundation under Grant Nos. ECCS0348277 and DMR-0649280 and a grant from the ONAMI/ DOD’s program. References and Notes (1) O’Regan, B.; Gra¨tzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737– 740. (2) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gra¨tzel, M. Combined Experimental and DFT-TDDFT Computational Study of Photoelectrochemical Cell Ruthenium Sensitizers. J. Am. Chem. Soc. 2004, 127, 16835–16847. (3) Robertson, N. Optimizing Dyes for Dye-Sensitized Solar Cells. Angew. Chem., Int. Ed. 2006, 45, 2338–2345. (4) Kuang, D. B.; Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M. Stable Mesoscopic Dye-Sensitized Solar Cells Based on Tetracyanoborate Ionic Liquid Electrolyte. J. Am. Chem. Soc. 2006, 128, 7732–7733. (5) Horiuchi, T.; Miura, H.; Sumioka, K.; Uchida, S. High Efficiency of Dye-Sensitized Solar Cells Based on Metal-Free Indoline Dyes. J. Am. Chem. Soc. 2004, 126, 12218–12219. (6) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphrybaker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. Conversion of Light to Electricity by cis-X2Bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium(II) Charge-Transfer Sensitizers (X ) C1-, Br-, I-, CN-, and SCN-) on Nanocrystalline TiO2 Electrodes. J. Am. Chem. Soc. 1993, 115, 6382–6390. (7) Benkstein, K. D.; Kopidakis, N.; van de Lagemaat, J.; Frank, A. J. Influence of the Percolation Network Geometry on Electron Transport in Dye-Sensitized Titanium Dioxide Solar Cells. J. Phys. Chem. B 2003, 107, 7759–7767.

11380

J. Phys. Chem. C, Vol. 114, No. 26, 2010

(8) Kavan, L.; Gra¨tzel, M.; Gilbert, S. E.; Klemenz, C.; Schell, H. J. Electrochemical and Photoelectrochemical Investigation of Single-Crystal Anatase. J. Am. Chem. Soc. 1996, 118, 6716–6723. (9) Kopidakis, N.; Benkstein, K. D.; van de Lagemaat, J.; Frank, A. J. Transport-Limited Recombination of Photocarriers in Dye-Sensitized Nanocrystalline TiO2 Solar Cells. J. Phys. Chem. B 2003, 107, 11307– 11315. (10) Chen, X.; Mao, S. S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem. ReV. 2007, 107, 2891– 2959. (11) Adachi, M.; Murata, Y.; Takao, J.; Jiu, J.; Sakamoto, M.; Wang, F. Highly Efficient Dye-Sensitized Solar Cells with a Titania Thin-Film Electrode Composed of a Network Structure of Single-Crystal-like TiO2 Nanowires Made by the “Oriented Attachment” Mechanism. J. Am. Chem. Soc. 2004, 126, 14943–14949. (12) Jiu, J.; Isoda, S.; Wang, F.; Adachi, M. Dye-Sensitized Solar Cells Based on a Single-Crystalline TiO2 Nanorod Film. J. Phys. Chem. B 2006, 110, 2087–2092. (13) Tan, B.; Wu, Y. Dye-Sensitized Solar Cells Based on Anatase TiO2 Nanoparticle/Nanowire Composites. J. Phys. Chem. B 2006, 110, 15932– 15938. (14) Ohsaki, Y.; Masaki, N.; Kitamura, T.; Wada, Y.; Okamoto, T.; Sekino, T.; Niihara, K.; Yanagida, S. Dye-Sensitized TiO2 Nanotube Solar Cells: Fabrication and Electronic Characterization. Phys. Chem. Chem. Phys. 2005, 7, 4157–4163. (15) Kuang, D.; Brillet, J.; Chen, P.; Takata, M.; Uchida, S.; Miura, H.; Sumioka, K.; Zakeeruddin, S. M.; Gra¨tzel, M. Application of Highly Ordered TiO2 Nanotube Arrays in Flexible Dye-Sensitized Solar Cells. ACS Nano 2008, 2, 1113–1116. (16) Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J. Enhanced ChargeCollection Efficiencies and Light Scattering in Dye-Sensitized Solar Cells Using Oriented TiO2 Nanotubes Arrays. Nano Lett. 2007, 7, 69–74. (17) Tornow, J.; Schwarzburg, K. Transient Electrical Response of DyeSensitized ZnO Nanorod Solar Cells. J. Phys. Chem. C 2007, 111, 8692– 8698. (18) Martinson, A. B. F.; Elam, J. W.; Hupp, J. T.; Pellin, M. J. ZnO Nanotube Based Dye-Sensitized Solar Cells. Nano Lett. 2007, 7, 2183– 2187. (19) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nanowire Dye-Sensitized Solar Cells. Nat. Mater. 2005, 4, 455–459. (20) Law, M.; Greene, L. E.; Radenovic, A.; Kuykendall, T.; Liphardt, J.; Yang, P. ZnO-Al2O3 and ZnO-TiO2 Core-Shell Nanowire DyeSensitized Solar Cells. J. Phys. Chem. B 2006, 110, 22652–22663. (21) Wang, Y.; Du, G.; Liu, H.; Liu, D.; Qin, S.; Wang, N.; Hu, C.; Tao, X.; Jiao, J.; Wang, J.; Wang, Z. L. Nanostructured Sheets of Ti-O Nanobelts for Gas Sensing and Antibacterial Applications. AdV. Funct. Mater. 2008, 18, 1131–1137. (22) Gong, X.; Selloni, A.; Batzill, M.; Diebold, U. Steps on Anatase TiO2(101). Nat. Mater. 2006, 5, 665–670.

Li et al. (23) Batzill, M.; Morales, E.; Diebold, U. Influence of Nitrogen Doping on the Defect Formation and Surface Properties of TiO2 Rutile and Anatase. Phys. ReV. Lett. 2006, 96, 026103. (24) Beattie, I. R.; Gilson, T. R. Single Crystal Laser Raman Spectroscopy. Proc. R. Soc. London, Ser. A 1968, 307, 407–429. (25) Ohsaka, T.; Izumi, F.; Fujiki, Y. J. Raman Spectrum of Anatase, TiO2. Raman Spectrosc. 1978, 7, 321–324. (26) Choi, H. C.; Jung, Y. M.; Kim, S. B. Size Effects in the Raman Spectra of TiO2 Nanoparticles. Vib. Spectrosc. 2005, 37, 33–38. (27) Swamy, V.; Kuznetsov, A.; Dubrovinsky, L. S.; Caruso, R. A.; Shchukin, D. G.; Muddle, B. C. Finite-Size and Pressure Effects on the Raman Spectrum of Nanocrystalline Anatase TiO2. Phys. ReV. B 2005, 71, 184302. (28) Nelson, J. Continuous-Time Random-Walk Model of Electron Transport in Nanocrystalline TiO2 Electrodes. Phys. ReV. B 1999, 59, 15374– 15380. (29) Qian, X.; Qin, D.; Song, Q.; Bai, Y.; Ji, T.; Tang, X.; Wang, E.; Dong, S. Surface Photovoltage Spectra and Photoelectrochemical Properties of Semiconductor-Sensitized Nanostructured TiO2 Electrodes. Thin Solid Films 2001, 385, 152. (30) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. ReV. 1995, 95, 69–96. (31) Garcia-Belmonte, G.; Kytin, V.; Dittrich, T.; Bisquert, J. Effect of Humidity on the ac Conductivity of Nanoporous TiO2. J. Appl. Phys. 2003, 94, 5261. (32) Ferry, V. E.; Sweatlock, L. A.; Pacifici, D.; Atwater, H. A. Plasmonic Nanostructure Design for Efficient Light Coupling into Solar Cells. Nano Lett. 2008, 8, 4391–4397. (33) Zhu, J.; Yu, Z.; Burkhard, G. F.; Hsu, C.; Connor, S. T.; Xu, Y.; Wang, Q.; McGehee, M.; Fan, S.; Cui, Y. Optical Absorption Enhancement in Amorphous Silicon Nanowire and Nanocone Arrays. Nano Lett. 2009, 9, 279–282. (34) Kuo, M.; Poxson, D. J.; Kim, Y. S.; Mont, F. W.; Kim, J. K.; Schubert, E. F.; Lin, S. Realization of a Near-Perfect Antireflection Coating for Silicon Solar Energy Utilization. Optic. Lett. 2008, 33, 2527–2529. (35) Lee, Y.; Ruby, D. S.; Peters, D. W.; McKenzie, B. B.; Hsu, J. W. P. ZnO Nanostructures as Efficient Antireflection Layers in Solar Cells. Nano Lett. 2008, 8, 1501–1505. (36) Sun, C.; Jiang, P.; Jiang, B. Broadband Moth-Eye Antireflection Coatings on Silicon. Appl. Phys. Lett. 2008, 92, 061112. (37) Min, W.; Jiang, B.; Jiang, P. Bio-inspired Self-Cleaning Antireflection Coatings. AdV. Mater. 2008, 20, 1–5. (38) Stavenga, D. G.; Foletti, S.; Palasantzas, G.; Arikawa, K. Light on the Moth-Eye Corneal Nipple Array of Butterflies. Proc. R. Soc. B 2006, 273, 661–667. (39) Wang, Y. D.; Zang, K. Y.; Chua, S. J.; Fonstad, C. G. CatalystFree Growth of Uniform ZnO Nanowire Arrays on Prepatterned Substrate. Appl. Phys. Lett. 2006, 89, 263116.

JP101478T