www.advmat.de www.MaterialsViews.com
Siva Krishna Karuturi, Jingshan Luo, Chuanwei Cheng, Lijun Liu, Liap Tat Su, Alfred Iing Yoong Tok,* and Hong Jin Fan* In recent years, photoelectrochemical (PEC) cells have attracted worldwide attention as cheap alternatives to conventional devices for solar energy conversion. Crucial to the light harvesting and conversion efficiency of a PEC cell is a nanostructured photoanode, in which the incident photons are captured, electron–hole pairs are generated, and the subsequent electron transfer takes place.[1,2] To realize highly efficient PEC cells, a nanostructured photoanode should possess several favorable intrinsic characteristics, such as adequate specific surface area to permit high photosensitizer loading (in the case of TiO2), direct electron transport pathways for long electron diffusion length, and strong light scattering to promote the light harvesting ability by confining the light within the cell.[3–6] It is thus highly desirable to develop a photoanode that meets all the above requirements. Towards this goal, immense efforts have been concentrated on tailoring the nanometer-scale features of photoanode materials.[7] Nanoparticle films provide very high surface areas to increase the amount of sensitizer loading, but they lack direct electrical contacts and light-scattering ability.[8,9] On the other hand, one-dimensional (1D) nanostructures such as nanowires and nanotubes offer superior electron transport pathways and improved light scattering, but they suffer from very low surface area (roughly an order of magnitude lower than nanoparticle films).[10,11] In conjunction with these efforts, 3D inverse opal (IO) nanostructures possessing highly ordered interconnected shells, high porosity (∼75%), and photonic crystal light localization have shown to be promising in photovoltaics and PEC cells.[12–20] While the fabrication scalability of these photoanodes has been established using a combination S. K. Karuturi,[+] Dr. L. J. Liu, Dr. L. T. Su, Prof. A. I. Y. Tok School of Materials Science and Engineering, Nanyang Technological University 639798 Singapore E-mail:
[email protected] J. S. Luo,[+] Prof. C. W. Cheng, [‡] Prof. H. J. Fan Division of Physics and Applied Physics School of Physical and Mathematical Sciences Nanyang Technological University 637371 Singapore E-mail:
[email protected] [‡] Present Address: Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, Department of Physics, Tongji University, Shanghai 200092, P. R. of China [+] These authors contributed equally to this work.
DOI: 10.1002/adma.201104428
Adv. Mater. 2012, 24, 4157–4162
COMMUNICATION
A Novel Photoanode with Three-Dimensionally, Hierarchically Ordered Nanobushes for Highly Efficient Photoelectrochemical Cells
of a simple doctor blading method and atomic layer deposition (ALD),[19] the highest efficiencies reported so far still fall behind those of mesoporous nanoparticle films, mainly because of the surface area disparity. Highest PEC efficiencies of 3.47% and 2.7% have been reported for dye-sensitized and quantum dot (QD)-sensitized unmodified TiO2 IO photoanodes, respectively.[12,16] It is envisaged that the promise of these 3D photoanodes can be further achieved when a substantial proportion of the pore volume is carefully exploited to resolve the issue of surface area shortage. In this Communication, we demonstrate a novel nanoarchitecture consisting of 3D ordered hierarchical nanobushes, using TiO2 IOs as the host template for the facile solution growth of ZnO nanowire networks. The TiO2 IO/ZnO nanowire hybrid nanostructure is sensitized with CdS QDs and investigated as a PEC photoanode. The key idea here is to couple the ZnO nanowires with the TiO2 IO to achieve higher sensitizer loading and larger contact interface areas with the electrolyte, and enhance light scattering. PEC performance measurements of the nanobush photoanode do indeed show an increased level of photocurrent density. The fabrication procedure for the nanobush photoanode is outlined in Scheme 1. Self-assembled opal templates with facecentered cubic (fcc) lattice structure and long-range ordering were developed using monodisperse polystyrene particles of 500 nm diameter that were infiltrated with TiO2 by ALD to close to theoretical filling fractions, as demonstrated in previous publications.[21–25] After the as-infiltrated opal templates were heattreated at 450 °C, TiO2 IOs with highly ordered, interconnected spherical pores in hexagonal arrangements were obtained, as shown in Figures 1a,b. The thickness of the TiO2 shells can be estimated to be ∼40 nm based on the maximum possible coating thickness, which is around 7.75% of the diameter of polystyrene particles. The obtained TiO2 IOs of ∼6 μm thickness were coated with a ∼10 nm thick conformal ZnO film, which serves as the seed layer for subsequent solution growth of ZnO nanowires, forming a 3D ordered nanobush structure. Growth conditions were optimized to control the nanowire diameter and length as detailed in the Experimental Section. Figure 1c shows a top-view field emission scanning electron microscopy (FE-SEM) image of the obtained nanobush structure after 9 h of solution growth. The higher magnification image (inset of Figure 1c) shows a single nanobush within an IO pore composed of multiple ZnO nanowires randomly aligned and covering the entire pore uniformly. The cross-sectional view of the nanobush structure observed at a crack area shown in Figure 1d
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
4157
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Scheme 1. Fabrication process for the nanobush photoanode.
depicts the hierarchical organization of the nanobushes. This kind of 3D-ordered growth of nanobushes is observed over the entire high-aspect-ratio template of 1 cm2. Note that although 9 h growth was used to achieve the longest nanowires within the pore, the wire length can be conveniently controlled by changing the reaction time. In addition, the size of the nanobushes can also be tuned by changing the pore size (Figure S1, Supporting Information). The nanometer-scale features of the as-fabricated nanobushes were further examined using transmission electron microscopy (TEM). Figure 1e reveals the densely grown nanobushes within each opal pore. The diameter of the nanowires is ∼30 nm as estimated from Figure 1f. The higher magnification image, in Figure 1g, shows the interface between the nanowires and ZnO seed film. The nanowires are highly crystalline, exhibiting an interplane spacing (0.24 nm) that closely matches the d-spacing of the (101) plane in hexagonal ZnO. One notable feature of the hierarchical nanobushes compared to arrays of vertically aligned ZnO nanowires grown directly on transparent conducting substrates is the complete elimination of the fusion of wires at their roots that commonly occurs during growth of nanowires that are several micrometers long.[26] In addition, the dense networks of nanowires with small diameters can be hierarchically extended to tens of micrometers by simply adjusting the thickness of the opal templates, fulfilling the high interfacial area requirement for PEC cells. Owing to the highly periodic arrangement of spherical pores, TiO2 IOs are 3D photonic crystals exhibiting photonic bandgaps, where the propagation of light is prohibited. TiO2 IOs possess photonic stop bands centered at 934 nm (Figure 2a, spectrum i) with higher order photonic bands in the visible part (∼450 nm). After the nanobushes had been grown, the stop band red-shifted to 1086 nm (Figure 2a, spectrum iii) with comparable bandwidth and higher order photonic bands, 4158
wileyonlinelibrary.com
demonstrating that the long-range structural order is preserved overall. Since the position and width of the bandgap are dictated by the lattice size and refractive indices,[27] the red shift of the bandgap occurred because of the increased dielectric index after the deposition of ZnO seed film and the growth of nanowires within the IO pores. Given the general interest in the optical properties of ZnO nanowires and the ability of photonic crystals to confine, manipulate, and guide photons, this result augurs well for interesting optoelectronic properties of such a coupled structure.[28,29] To examine comparatively the PEC performance of the nanobush photoanode, photosensitization with CdS QDs was carried out for the three nanostructured photoanodes, namely, TiO2 IOs, nanobushes, and the ZnO nanowire array. The thickness of each of the nanostructured photoanodes was kept at ∼6 μm (Figure S2, Supporting Information). Hexagonal CdS QDs were confirmed from X-ray diffraction (XRD) patterns (Figure S3, Supporting Information). Curves ii and iv in Figure 2a show the photonic stop bands after the CdS sensitization. Photonic bandgap shifts of 37 nm and 162 nm can be observed for the TiO2 IOs and nanobushes, respectively. The larger shift for the nanobushes means that the amount of CdS QDs loading is increased several times by the higher specific surface area provided by the networks of nanowires within each pore. Quantitative analysis of the XRD patterns also confirms that the QD loading in the nanobushes is three times that in the TiO2 IO. Of major concern here is whether there exist photonic bands for the photoanodes within the absorption wavelength range of CdS (below ∼530 nm). As seen from Figure 2a, no major specular reflectance features below 530 nm were found for QD-sensitized nanobushes, whereas a high-order peak centered at ∼470 nm exists for the sensitized TiO2 IO. The absence of specular reflection features in the nanobushes is beneficial for light harvesting. The FE-SEM image in Figure 2b shows a cross-sectional view of the QD-sensitized nanobush photoanode. Distinct CdS/ZnO core/shell nanowires without any aggregation can be observed. Interconnection of the TiO2 shells that provide direct electron transport pathways can also be seen. A TEM image of a nanowire detached from the nanobush structure verifies that there is a CdS shell of ∼7 nm around the entire ZnO nanowire. In order to investigate the light-scattering ability of different photoanodes, the UV-vis diffuse reflectance spectra before and after CdS QD sensitization were measured (see Figure 3). In the absence of CdS coating, the highest scattering was found in the nanobush structure, followed by TiO2 IO and the ZnO nanowire array. Given the fact that effective Mie scattering originates from particles with size comparable to the wavelength of the incident light, the void size of the IO used here (∼500 nm) apparently contributed to the strong visible light scattering.[30] This clearly highlights the benefits of the 3D structured photoanodes with specially engineered scattering centers compared to the 1D ZnO nanowire array. Previous reports also suggest that disorder in photonic crystals induces broadband diffuse scattering.[31–33] The higher disorder present in the nanobushes as a result of the growth of ZnO nanowires is also expected to contribute additionally
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2012, 24, 4157–4162
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 1. a,b) Top-viewFE-SEM images of TiO2 IOs at different magnifications. c,d) FE-SEM images of the nanobush structure: c) top view and d) cross-sectional view. The insets of (c) and (d) show the magnified top view and cross-sectional view of a single nanobush structure, respectively. e,f,g) TEM images of the nanobush structure taken at different magnifications.
to the improved diffuse scattering. As expected, all the photoanodes after sensitization showed decreased diffuse reflectance in the wavelength range up to 550 nm, owing to light absorption by CdS. However, the nanobush CdS photoanode shows very low diffuse reflectance in comparison to the TiO2 IO CdS photoanode. This is attributed to the much higher amount of CdS anchored to the ZnO nanowire networks, which promotes stronger absorption of the scattered light in comparison to the TiO2 IO CdS photoanode, thereby effectively capturing the benefits of strong light scattering with minimum reflection losses. Moreover, the presence of higher order photonic bands for the TiO2 IO CdS photoanode (Figure 2a, spectrum ii) may have contributed to specular reflections. The PEC performance of different photoanodes was investigated by conducting the current density versus potential (J–V) measurements in the dark and under simulated sunlight illumination (AM 1.5) in a three-electrode cell configuration. As shown in Figure 4a, all the photoanodes displayed pronounced photocurrent density upon illumination, implying efficient light harvesting and charge separation. Typically, all the photoanodes showed a similar onset potential with continuously
Adv. Mater. 2012, 24, 4157–4162
increasing photoresponse with increasing bias voltage. The photocurrent density for the nanobush CdS photoanode was observed to be 6.2 mA cm−2 at 0 V vs Ag/AgCl, compared to 3.6 and 2.9 mA cm−2 for the photoanodes based on TiO2 IO CdS and the ZnO nanowire array, respectively, measured under the same conditions. To evaluate the wavelength-dependent light harvesting efficiency of different photoanode structures, incident-photon-tocurrent conversion efficiency (IPCE) tests for all three photoanodes were performed from 300 to 600 nm wavelength under 0.5 V bias. Strong photoactivity for all the photoanodes is observed in the visible light region from 375 to 525 nm. In contrast to the ZnO nanowire array CdS photoanode, both the 3D structured photoanodes show broader photoresponse, indicating the indispensable role of light scattering to improve the light harvesting efficiency. The highest efficiencies for nanobush and ZnO nanowire array photoanodes can be determined to be 60% and 30%, respectively, at a wavelength of ∼460 nm. Considering the transmission losses of ∼20% through the fluorine-doped tin oxide (FTO) and the cell, the 60% efficiency emphasizes the immense potential of the hierarchical 3D nanobushes for PEC cell applications. For the TiO2 IO photoanode,
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
4159
www.advmat.de
(a)
(a) i TiO2 IO
i
80
971
Diffuse Reflectance (%)
iii Nanobushes iv Nanobushes-CdS
80
100
934
ii TiO2 IO-CdS
100
Reflectance (%)
COMMUNICATION
www.MaterialsViews.com
ii 1086
60
iii 1248
iv
40 20
ZnO nanowire array TiO2 IO Nanobushes
60
40
20
0 400
600
800 1000 1200 Wavelength (nm)
0 350
1400
(b)
400
450 500 Wavelength (nm)
550
600
550
600
(b) 100
Diffuse Reflectance (%)
80
Nanobushes-CdS
60
40
20
0 350
4160
ZnO nanowire array-CdS TiO2 IO-CdS
400
450 500 Wavelength (nm)
Figure 2. a) UV-vis-NIR specular reflectance spectra showing the photonic stop bands of i) TiO2 IO, ii) CdS QD–sensitized TiO2 IO, iii) nanobushes, and iv) CdS QD–sensitized nanobushes. b) Cross-sectional FE-SEM images of CdS QD–sensitized nanobush photoanode. Inset: HRTEM image of a single nanowire sensitized with CdS.
Figure 3. a,b) UV-vis diffuse reflectance spectra of different photoanodes before (a) and after (b) CdS QD sensitization. Strong light scattering exhibited by the 3D photoanodes is expected to increase the path length traveled by the light within the photoanode, and thus the PEC light harvesting.[6,30]
the maximum efficiency of 36% can be discerned at ∼405 nm with relatively flat conversions in the active region. This contrasting behavior could be attributed to the reflection losses at wavelengths >420 nm with the presence of higher order photonic stop bands (see Figure 2a, spectrum ii, and Figure 3b). IPCE results for all the photoanodes are also consistent with their corresponding J–V characteristics. Fast photoresponse and good photostability were observed for all the photoanodes from amperometric J–t curves (see Figure 4c). The improved performance of the nanobush photoanode may also be partly attributed to the fact that the ZnO seed film coated on the electron transporting TiO2 shells may function as an energy barrier to suppress the recombination of photoinjected electrons with redox ions of the electrolyte.[34–36] In summary, we have achieved 3D-ordered nanobushes consisting of dense networks of ZnO nanowires embedded within
TiO2 IOs through a facile fabrication strategy. Justified by several favorable attributes, such as high specific surface area, direct electron transport networks, and strong light scattering, the promise of the novel hierarchical photoanode is unambiguously demonstrated for PEC hydrogen generation. With CdS QD sensitization, a photocurrent density of 6.2 mA cm−2 at 0 V vs Ag/AgCl under AM1.5 light illumination is achieved. This nanobush photoanode is expected to be advantageous also for other PEC applications, including quantum dot solar cells and dye-sensitized solar cells.
wileyonlinelibrary.com
Experimental Section Fabrication of TiO2 IOs: Carboxylate-modified, monodisperse polystyrene particles of 500 nm diameter were bought from Duke Scientific Corporation (Palo Alto, CA) and were assembled onto the
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2012, 24, 4157–4162
www.advmat.de www.MaterialsViews.com
Current density (mA/cm2)
12 ZnO nanowire array TiO2 IO
10
Nanobushes 8 6 4 2
Dark currents 0 -0.9
-0.6
-0.3
0.0
0.3
0.6
Potential (V vs Ag/AgCl)
(b) 80 70
ZnO nanowire array TiO2 IO Nanobushes
IPCE (%)
60 50 40 30 20 10 0 300
350
400
450
500
550
600
Wavelength (nm)
(c)
ZnO nanowire array TiO2 IO
Current density (mA/cm2)
8
Nanobushes 6
4
2
0 0
100
200
300
400
Time (s) Figure 4. a) Linear sweep voltammetry measurements of the CdS QD– sensitized photoanodes; dotted lines show the measurements collected in the dark and the solid lines show the measurements collected under 100 mW cm−2 simulated sunlight illumination. b) IPCE spectra of different photoanodes. c) Amperometric J–t curves for CdS QD–sensitized photoanodes at an applied potential of 0 V vs AgCl/Ag at 100 mW cm−2 illumination with 50 s light on/off cycles.
Adv. Mater. 2012, 24, 4157–4162
FTO-coated glass substrates via a vertical deposition process at 90 °C.[37] Subsequently, the self-assembled PS opals were infiltrated with TiO2 using a self-made stopped-flow reactor type of ALD system at 70 °C.[21] Titanium tetrachloride (99.99%, Sigma-Aldrich) and H2O were used as the Ti and O precursors for TiO2 deposition. Finally, IO structures were developed by burning original polystyrene spheres in air at 450 °C for 2 h. Reactive ion etching (RIE, NSC ES371) was used to cut the top surface and open up the pores. Fabrication of nanobushes: A conformal seed layer of ∼10 nm ZnO (50 cycles of ALD) was deposited on the internal surface of TiO2 IOs using the stopped-flow ALD method. Subsequently, ZnO nanowires were grown based on the ZnO seed layer within the IO structure using a standard solution growth method.[38] The TiO2 IO substrates were immersed into a 35 mL aqueous solution of equimolar zinc nitrate, Zn(NO3)2·6H2O (0.025 M), and hexamethylenetetramine (C6H12N4) in an autoclave. The reaction was conducted at 95 °C for controlled growth times. Fabrication of the ZnO nanowire array: A ZnO nanowire array on FTO glass was developed using the standard solution growth method based on an ALD ZnO seed layer of ∼15 nm (70 cycles of ALD) as described above. The growth reaction was conducted for 6 h and repeated again with freshly added chemicals for 6 h to achieve the desired thickness of ∼6 μm. Preparation of photoanodes: The three different types of nanostructures on the FTO glass substrate, namely, TiO2 IOs, nanobushes, and a ZnO nanowire array, were sensitized with CdS QDs under the same conditions using a modification of a previously reported “successive ionic layer adsorption and reaction” (SILAR) route.[39] In a typical procedure, the nanostructures on FTO glass substrates were immersed in a solution containing 50 mM cadmium acetate tetrahydrate, Cd(Ac)2·2H2O (Alfa Aesar, 98%), in ethanol for 1 min, to allow Cd2+ to adsorb onto the TiO2. They were dried in a stream of N2. The dried substrates were then dipped into a solution containing 50 mM sodium sulfide nonahydrate (98% Na2S, Alfa Aesar) in methanol for 1 min, where the pre-adsorbed Cd2+ reacted with S2− to form the desired CdS. The substrates were then rinsed in water for 1 min to remove the excessive ions and dried again with N2. This typical procedure was repeated ten times to obtain the desired thickness of CdS. The as-sensitized substrates were annealed at 400 °C for 30 min in an Ar protective atmosphere to improve the crystallinity. Materials characterization: The morphology and microstructures of the nanostructured films were examined using a JEOL JSM-7600F field emission scanning electron microscope and a JEM 2100F transmission electron microscope. The XRD patterns were recorded using Shimadzu thin film XRD equipment using Cu Kα radiation. The specular reflectance spectra were collected at 20° with respect to the normal incidence of light using a UV-vis-NIR spectrophotometer (Cary 5000, Varian). The diffused reflection spectra were taken using a Zolix Solar Cell quantum efficiency (QE)/IPCE measurement system equipped with an integrating sphere and a silicon diode detector. Photoelectrochemical characterizations: The PEC performance measurements were conducted in three-electrode configuration with as-prepared nanostructured photoanodes as working electrodes, Ag/ AgCl in saturated KCl as a reference electrode, and a Pt foil as the counter electrode. Na2S (0.25 M) and Na2SO3 (0.35 M) mixed solution was used as the electrolyte. The photoresponse was measured under chopped illumination from a 150 W Xe lamp (Sciencetech SS150) equipped with an AM1.5 G filter, calibrated with a standard Si solar cell to simulate AM1.5 illumination (100 mW cm−2). Photocurrent stability tests were carried out by measuring the photocurrents under AM1.5 sunlight irradiation (light/dark cycles of 50 s) at a fixed bias of 0 V vs Ag/AgCl. The IPCE measurements were taken as a function of wavelength from 300 to 650 nm using a specially designed IPCE system for solar cells (Zolix Solar Cell Scan100), with two-electrode configuration under 0.5 V bias (the photocurrent level at this potential is comparable to 0 V vs Ag/ AgCl). A 150 W Xe lamp equipped with gratings was used to generate a monochromatic beam. The incident light intensity was calibrated by a standard silicon photodiode.
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
COMMUNICATION
(a)
4161
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements A.I.Y.T. acknowledges the financial support from the Ministry of Education, Singapore by Tier 2 Academic Research Fund (grant no. T208A1225, ARC 5/08). Received: November 21, 2011 Revised: February 29, 2012 Published online: May 29, 2012
[1] P. V. Kamat, K. Tvrdy, D. R. Baker, J. G. Radich, Chem. Rev. 2010, 110, 6664. [2] M. Grätzel, Nature 2001, 414, 338. [3] O. K. Varghese, M. Paulose, C. A. Grimes, Nat. Nanotechnol. 2009, 4, 592. [4] D. Chen, F. Huang, Y. B. Cheng, R. A. Caruso, Adv. Mater. 2009, 21, 2206. [5] Y. Luo, D. Li, Q. Meng, Adv. Mater. 2009, 21, 4647. [6] Q. Zhang, T. P. Chou, B. Russo, S. A. Jenekhe, G. Cao, Angew. Chem. Int. Ed. 2008, 47, 2402. [7] Q. Zhang, G. Cao, Nano Today 2011, 6, 91. [8] Y. L. Lee, C. F. Chi, S. Y. Liau, Chem. Mater. 2010, 22, 922. [9] K. Tvrdy, P. A. Frantsuzov, P. V. Kamat, Proc. Natl. Acad. Sci. USA 2011, 108, 29. [10] K. Zhu, N. R. Neale, A. Miedaner, A. J. Frank, Nano. Lett. 2007, 7, 69. [11] F. Zhuge, J. Qiu, X. Li, X. Gao, X. Gan, W. Yu, Adv. Mater. 2011, 23, 1330. [12] E. S. Kwak, W. Lee, N. C. Park, J. Kim, H. Lee, Adv. Funct. Mater. 2009, 19, 1093. [13] L. Liu, S. K. Karuturi, L. T. Su, A. I. Y. Tok, Energy Environ. Sci. 2011, 4, 209. [14] S. Nishimura, N. Abrams, B. A. Lewis, L. I. Halaoui, T. E. Mallouk, K. D. Benkstein, J. van de Lagemaat, A. J. Frank, J. Am. Chem. Soc. 2003, 125, 6306. [15] S. Guldin, S. Hüttner, M. Kolle, M. E. Welland, P. Müller-Buschbaum, R. H. Friend, U. Steiner, N. Tétreault, Nano Lett. 2010, 10, 2303.
4162
wileyonlinelibrary.com
[16] L. J. Diguna, Q. Shen, J. Kobayashi, T. Toyoda, Appl. Phys.Lett. 2007, 91, 023116. [17] C. Cheng, S. K. Karuturi, L. Liu, J. Liu, H. Li, L. T. Su, A. I. Y. Tok, H. J. Fan, Small 2012, 8, 37. [18] L. Zhang, C. Baumanis, L. Robben, T. Kandiel, D. Bahnemann, Small 2011, 7, 2714. [19] N. Tétreault, E. Arsenault, L.-P. Heiniger, N. Soheilnia, J. Brillet, T. Moehl, S. Zakeeruddin, G. A. Ozin, M. Grätzel, Nano Lett. 2011, 11, 4579. [20] X. Chen, J. Ye, S. Ouyang, T. Kako, Z. Li, Z. Zou, ACS Nano 2011, 5, 4310. [21] S. K. Karuturi, L. Liu, L. T. Su, Y. Zhao, H. J. Fan, X. Ge, S. He, A. T. I. Yoong, J. Phys. Chem. C 2010, 114, 14843. [22] L. Liu, S. K. Karuturi, L. T. Su, Q. Wang, A. I. Y. Tok, Electrochem. Commun. 2011, 1163. [23] B. H. Juárez, P. D. García, D. Golmayo, A. Blanco, C. López, Adv. Mater. 2005, 17, 2761. [24] J. S. King, E. Graugnard, C. J. Summers, Adv. Mater. 2005, 17, 1010. [25] M. Scharrer, X. Wu, A. Yamilov, H. Cao, R. P. H. Chang, Appl. Phys. Lett. 2005, 86, 1. [26] C. Xu, P. Shin, L. Cao, D. Gao, J. Phys. Chem. C 2010, 114, 125. [27] K. Busch, S. John, Phys. Rev. E 1998, 58, 3896. [28] P. Lodahl, A. F. van Driel, I. S. Nikolaev, A. Irman, K. Overgaag, D. Vanmaekelbergh, W. L. Vos, Nature 2004, 430, 654. [29] D. Vanmaekelbergh, L. K. van Vugt, Nanoscale 2011, 3, 2783. [30] Q. Zhang, T. P. Chou, B. Russo, S. A. Jenekhe, G. Cao, Adv. Funct. Mater. 2008, 18, 1654. [31] J. I. L. Chen, G. von Freymann, V. Kitaev, G. A. Ozin, J. Am. Chem. Soc. 2007, 129, 1196. [32] N. R. Neale, B. G. Lee, S. H. Kang, A. J. Frank, J. Phys. Chem. C 2011, 115, 14341. [33] J. F. Galisteo-López, E. Palacios-Lidón, E. Castillo-Martínez, C. López, Phys. Rev. B 2003, 68, 115109. [34] E. Palomares, J. N. Clifford, S. A. Haque, T. Lutz, J. R. Durrant, J. Am. Chem. Soc. 2003, 125, 475. [35] C. Chen, Y. Xie, G. Ali, S. H. Yoo, S. O. Cho, Nanotechnology 2011, 22, 015202. [36] W. Lee, S. H. Kang, J. Y. Kim, G. B. Kolekar, Y. E. Sung, S. H. Han, Nanotechnology 2009, 20. [37] F. Marlow, Muldarisnur, P. Sharifi, R. Brinkmann, C. Mendive, Angew. Chem. Int. Ed. 2009, 48, 6212. [38] C. W. Cheng, B. Yan, S. M. Wong, X. L. Li, W. W. Zhou, T. Yu, Z. X. Shen, H. Y. Yu, H. J. Fan, ACS Appl. Mater. Interfaces 2010, 2, 1824. [39] D. R. Baker, P. V. Kamat, Adv. Funct. Mater. 2009, 19, 805.
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2012, 24, 4157–4162
