此研究是利用水熱合成法及高溫熱還原合成二硫化鉬/還原態參氮氧化石墨烯奈米複合材料(molybdenum disulfide/ nitrogen reduced graphene oxide)，覆蓋在透明導電玻璃之對電極，替代貴金屬鉑作為染料敏化太陽能電池。利用Hummers法將天然石墨製作出剝離呈片狀的氧化石墨烯。利用水熱合成法將聯氨中之氮原子置換氧化石墨烯中之氧原子，形成還原石墨烯，從中及可修補在石墨烯上大量的孔洞缺陷以及提升導電性。再利用高溫熱還原，將附著在還原態氧化石墨烯四硫代鉬酸銨還原成二硫化鉬，此可借用二硫化鉬的良好催化能力，提升在對電極上催化碘三根離子還原的能力。從穿透式電子顯微鏡(TEM)中可發現有層層堆疊之片狀結構，除此之外還可發現兩種不同層間距之二硫化鉬及還原態參氮氧化石墨烯。從X射線光電子能譜儀(XPS)中可發現，還原石墨烯具有N1s之峰值，且還原石墨烯之O1s峰值較氧化石墨烯之O1s來的低，還原態氧化石墨烯中組成C1s峰值之含氧官能基也隨著還原後降低，由此可推斷還原態氧化石墨烯上之含氧官能基被N原子所取代而形成具有吡啶分子結構之石墨烯，在Mo3d和S2p之峰值中可發現主要是由四價之Mo及二價之S所組成的，由此更可判定石墨烯上確實有成功還原之二硫化鉬。此種奈米複合材料，可結合還原態參氮氧化石墨烯與二硫化鉬之優點，具有良好催化活性及大反應面積之二硫化鉬，可取代白金作為對電極來催化碘三根離子還原，再加上還原態氧化石墨烯的良好導電性，在二硫化鉬做催化時更能有有效的將電子從導電玻璃上傳遞到二硫化鉬中，能提高電荷交換頻率，具有良好之催化活性。其光電轉換效率達5.95%，對照以傳統鉑作為標準對電極之光電轉換效率6.43%，還原石墨烯對電極之效率為鉑對電極之效率的93.4%。因此具有低成本之還原石墨烯可替代昂貴之鉑，以作為高效率染料敏化太陽能電池之對電極。

In this study, the molybdenum disulfide/ nitrogen doped graphene oxide nanocomposite (MoS2/nGO) was synthesized with nitrogen doped reduced graphene oxide (nGO) by hydrothermal synthesis method and molybdenum disulfide (MoS2) by thermal reduce method. This hybrid nanocomposite coated on fluorine doped tin oxide (FTO) glass as a platinum-free counter electrode (CE). The characteristics of MoS2/nGO were investigated by high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. The electrochemical property of MoS2/nGO was characterized by cyclic voltammetry (CV), electrochemical impedance spectra (EIS) and Tafel-polarization measurement. This MoS2/nGO CE exhibited well photovoltaic conversion efficiency (PCE = 5.95 %). Compared with the conventional Platinum, the MoS2/nGO CE was up to 93.4% of using conventional Pt CE (PCE = 6.43 %). As a result of the MoS2/nGO nanocomposite could provide an alternative selection to replace the noble platinum as CE for DSSCs.

Contents
摘要 i
Abstract ii
致謝 iii
Contents v
Figure list viii
Table list xi
Chapter 1 Introduction 1
1-1 Introduction to solar cells 2
1-1-1 Classification and principle of solar cells 2
1-1-2 Dye-sensitized solar cells 4
1-2 Motivation 6
Chapter 2 Literature review 7
2-1 The working principle of DSSCs 7
2-2 Composition of DSSCs 11
2-2-1 Working electrode 11
2-2-2 Photosensitizer dye 15
2-2-3 Electrolytes 18
2-2-4 Counter electrode 20
2-3 Graphene oxide 21
2-4 Reduced graphene oxide 23
2-5 Molybdenum disulfide 25
2-6 Photocurrent-voltage characteristic 27
Chapter 3 Experimental 28
3-1 Process flow 28
3-2 Experimental steps 30
3-2-1 Preparation of graphene oxide 30
3-2-2 Nitrogen doped reduced graphene oxide 33
3-2-3 Molybdenum disulfide process 35
3-2-4 Conductive glass cleaning 37
3-2-5 Preparation of working electrode 38
3-2-6 Preparation of counter electrode 41
3-2-7 Composition of DSSCs 42
3-3 Characterizations 43
Chapter 4 Results and discussion 45
4-1 Structure analysis 45
4-2 Chemical analysis 47
4-2-1 X-ray photoelectron spectra 47
4-2-2 Raman spectrum 51
4-3 Electrocatalytic analysis 52
4-3-1 Voltammetry analysis 52
4-3-2 Electrochemical impedance spectra 54
4-3-3 Tafel-polarization measurements 56
4-4 Photovoltaic performance 59
Chapter 5 Conclusion 62
Chapter 6 Reference 63

[1] V. Sivakov, G. Andra, A. Gawlik, A. Berger, J. Plentz, F. Falk, and S. H. Christiansen, "Silicon Nanowire-Based Solar Cells on Glass: Synthesis, Optical Properties, and Cell Parameters," Nano Letters, vol. 9, pp. 1549-1554, Apr 2009.
[2] M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphrybaker, E. Muller, P. Liska, N. Vlachopoulos, and M. Gratzel, "Conversion of Light to Electricity by Cis-X2bis(2,2'-Bipyridyl-4,4'-Dicarboxylate)Ruthenium(Ii) Charge-Transfer Sensitizers (X = Cl-, Br-, I-, Cn-, and Scn-) on Nanocrystalline Tio2 Electrodes," Journal of the American Chemical Society, vol. 115, pp. 6382-6390, Jul 14 1993.
[3] G. P. Smestad, "Conversion of heat and light simultaneously using a vacuum photodiode and the thermionic and photoelectric effects," Solar Energy Materials and Solar Cells, vol. 82, pp. 227-240, May 1 2004.
[4] T. Surek, "Crystal growth and materials research in photovoltaics: progress and challenges," Journal of Crystal Growth, vol. 275, pp. 292-304, Feb 15 2005.
[5] H. Spanggaard and F. C. Krebs, "A brief history of the development of organic and polymeric photovoltaics," Solar Energy Materials and Solar Cells, vol. 83, pp. 125-146, Jun 15 2004.
[6] D. Wohrle and D. Meissner, "Organic Solar-Cells," Advanced Materials, vol. 3, pp. 129-138, Mar 1991.
[7] B. Oregan and M. Gratzel, "A Low-Cost, High-Efficiency Solar-Cell Based on Dye-Sensitized Colloidal Tio2 Films," Nature, vol. 353, pp. 737-740, Oct 24 1991.
[8] M. Gratzel, "Solar energy conversion by dye-sensitized photovoltaic cells," Inorganic Chemistry, vol. 44, pp. 6841-6851, Oct 3 2005.
[9] W. G. Yang, F. R. Wan, Q. W. Chen, J. J. Li, and D. S. Xu, "Controlling synthesis of well-crystallized mesoporous TiO2 microspheres with ultrahigh surface area for high-performance dye-sensitized solar cells," Journal of Materials Chemistry, vol. 20, pp. 2870-2876, 2010.
[10] H. C. Weerasinghe, F. Z. Huang, and Y. B. Cheng, "Fabrication of flexible dye sensitized solar cells on plastic substrates," Nano Energy, vol. 2, pp. 174-189, Mar 2013.
[11] A. Yella, H. W. Lee, H. N. Tsao, C. Y. Yi, and A. K. Chandiran, "Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte exceed 12 percent efficiency (vol 334, pg 629, 2011)," Science, vol. 334, pp. 1203-1203, Dec 2 2011.
[12] J. H. Heo, S. H. Im, J. H. Noh, T. N. Mandal, C. S. Lim, J. A. Chang, Y. H. Lee, H. J. Kim, A. Sarkar, M. K. Nazeeruddin, M. Gratzel, and S. I. Seok, "Efficient inorganic-organic hybrid heterojunction solar cells containing perovskite compound and polymeric hole conductors," Nature Photonics, vol. 7, pp. 487-492, Jun 2013.
[13] M. Gratzel, "Photoelectrochemical cells," Nature, vol. 414, pp. 338-344, Nov 15 2001.
[14] M. Gratzel, "Mesoporous oxide junctions and nanostructured solar cells," Current Opinion in Colloid & Interface Science, vol. 4, pp. 314-321, Aug 1999.
[15] D. Cahen, G. Hodes, M. Gratzel, J. F. Guillemoles, and I. Riess, "Nature of photovoltaic action in dye-sensitized solar cells," Journal of Physical Chemistry B, vol. 104, pp. 2053-2059, Mar 9 2000.
[16] K. Kalyanasundaram and M. Gratzel, "Applications of functionalized transition metal complexes in photonic and optoelectronic devices," Coordination Chemistry Reviews, vol. 177, pp. 347-414, Oct 1998.
[17] A. Hagfeldt and M. Gratzel, "Light-Induced Redox Reactions in Nanocrystalline Systems," Chemical Reviews, vol. 95, pp. 49-68, Jan-Feb 1995.
[18] S. Nakade, T. Kanzaki, W. Kubo, T. Kitamura, Y. Wada, and S. Yanagida, "Role of electrolytes on charge recombination in dye-sensitized TiO2 solar cell (1): The case of solar cells using the I-/I3(-) redox couple," Journal of Physical Chemistry B, vol. 109, pp. 3480-3487, Mar 3 2005.
[19] R. Jose, V. Thavasi, and S. Ramakrishna, "Metal Oxides for Dye-Sensitized Solar Cells," Journal of the American Ceramic Society, vol. 92, pp. 289-301, Feb 2009.
[20] V. Thavasi, V. Renugopalakrishnan, R. Jose, and S. Ramakrishna, "Controlled electron injection and transport at materials interfaces in dye sensitized solar cells," Materials Science & Engineering R-Reports, vol. 63, pp. 81-99, Jan 29 2009.
[21] K. M. Lee, V. Suryanarayanan, and K. C. Ho, "A study on the electron transport properties of TiO2 electrodes in dye-sensitized solar cells," Solar Energy Materials and Solar Cells, vol. 91, pp. 1416-1420, Sep 22 2007.
[22] M. K. Nazeeruddin and M. Gratzel, "Transition metal complexes for photovoltaic and light emitting applications," Photofunctional Transition Metals Complexes, vol. 123, pp. 113-175, 2007.
[23] V. Shklover, M. K. Nazeeruddin, S. M. Zakeeruddin, C. Barbe, A. Kay, T. Haibach, W. Steurer, R. Hermann, H. U. Nissen, and M. Gratzel, "Structure of nanocrystalline TiO2 powders and precursor to their highly efficient photosensitizer," Chemistry of Materials, vol. 9, pp. 430-439, Feb 1997.
[24] V. Shklover, Y. E. Ovchinnikov, L. S. Braginsky, S. M. Zakeeruddin, and M. Gratzel, "Structure of organic/inorganic interface in assembled materials comprising molecular components. Crystal structure of the sensitizer bis[(4,4 '-carboxy-2,2 '-bipyridine)(thiocyanato)]ruthenium(II)," Chemistry of Materials, vol. 10, pp. 2533-2541, Sep 1998.
[25] V. Aranyos, H. Grennberg, S. Tingry, S. E. Lindquist, and A. Hagfeldt, "Electrochemical and photoelectrochemical investigation of new carboxylatobipyridine (bis-bipyridine)ruthenium(II) complexes for dye-sensitized TiO2 electrodes," Solar Energy Materials and Solar Cells, vol. 64, pp. 97-114, Sep 30 2000.
[26] S. Ito, S. M. Zakeeruddin, R. Humphry-Baker, P. Liska, R. Charvet, P. Comte, M. K. Nazeeruddin, P. Pechy, M. Takata, H. Miura, S. Uchida, and M. Gratzel, "High-efficiency organic-dye-sensitized solar cells controlled by nanocrystalline-TiO2 electrode thickness," Advanced Materials, vol. 18, pp. 1202-+, May 2 2006.
[27] J. H. Wu, Z. Lan, S. C. Hao, P. J. Li, J. M. Lin, M. L. Huang, L. Q. Fang, and Y. F. Huang, "Progress on the electrolytes for dye-sensitized solar cells," Pure and Applied Chemistry, vol. 80, pp. 2241-2258, Nov 2008.
[28] S. Y. Huang, G. Schlichthorl, A. J. Nozik, M. Gratzel, and A. J. Frank, "Charge recombination in dye-sensitized nanocrystalline TiO2 solar cells," Journal of Physical Chemistry B, vol. 101, pp. 2576-2582, Apr 3 1997.
[29] K. Tennakone, G. R. R. A. Kumara, I. R. M. Kottegoda, K. G. U. Wijayantha, and V. P. S. Perera, "A solid-state photovoltaic cell sensitized with a ruthenium bipyridyl complex," Journal of Physics D-Applied Physics, vol. 31, pp. 1492-1496, Jun 21 1998.
[30] B. Oregan and D. T. Schwartz, "Efficient Photo-Hole Injection from Adsorbed Cyanine Dyes into Electrodeposited Copper(I) Thiocyanate Thin-Films," Chemistry of Materials, vol. 7, pp. 1349-1354, Jul 1995.
[31] J. H. Park, K. J. Choi, S. W. Kang, Y. Seo, Y. S. Kang, J. Kim, and S. S. Lee, "Ionic diffusion in various electrolytes and the implications for dye-sensitized solar cells," Journal of Photochemistry and Photobiology a-Chemistry, vol. 213, pp. 1-6, Jun 10 2010.
[32] P. Balraju, P. Suresh, M. Kumar, M. S. Roy, and G. D. Sharma, "Effect of counter electrode, thickness and sintering temperature of TiO2 electrode and TBP addition in electrolyte on photovoltaic performance of dye sensitized solar cell using pyronine G (PYR) dye," Journal of Photochemistry and Photobiology a-Chemistry, vol. 206, pp. 53-63, Jul 5 2009.
[33] A. Hauch and A. Georg, "Diffusion in the electrolyte and charge-transfer reaction at the platinum electrode in dye-sensitized solar cells," Electrochimica Acta, vol. 46, pp. 3457-3466, Aug 1 2001.
[34] V. A. Macagno, M. C. Giordano, and A. J. Arvia, "Kinetics and Mechanisms of Electrochemical Reactions on Platinum with Solutions of Iodine-Sodium Iodide in Acetonitrile," Electrochimica Acta, vol. 14, pp. 335-&, 1969.
[35] L. M. Dane, L. J. J. Janssen, and J. G. Hoogland, "Iodine/Iodide Redox Couple at a Platinum Electrode," Electrochimica Acta, vol. 13, pp. 507-&, 1968.
[36] H. C. Schniepp, J. L. Li, M. J. McAllister, H. Sai, M. Herrera-Alonso, D. H. Adamson, R. K. Prud'homme, R. Car, D. A. Saville, and I. A. Aksay, "Functionalized single graphene sheets derived from splitting graphite oxide," Journal of Physical Chemistry B, vol. 110, pp. 8535-8539, May 4 2006.
[37] Y. Yamada, H. Yasuda, K. Murota, M. Nakamura, T. Sodesawa, and S. Sato, "Analysis of heat-treated graphite oxide by X-ray photoelectron spectroscopy," Journal of Materials Science, vol. 48, pp. 8171-8198, Dec 2013.
[38] D. Pandey, R. Reifenberger, and R. Piner, "Scanning probe microscopy study of exfoliated oxidized graphene sheets," Surface Science, vol. 602, pp. 1607-1613, May 1 2008.
[39] A. V. Talyzin, V. L. Solozhenko, O. O. Kurakevych, T. Szabo, I. Dekany, A. Kurnosov, and V. Dmitriev, "Colossal Pressure-Induced Lattice Expansion of Graphite Oxide in the Presence of Water," Angewandte Chemie-International Edition, vol. 47, pp. 8268-8271, 2008.
[40] A. V. Talyzin, T. Szabo, I. Dekany, F. Langenhorst, P. S. Sokolov, and V. L. Solozhenko, "Nanocarbons by High-Temperature Decomposition of Graphite Oxide at Various Pressures," Journal of Physical Chemistry C, vol. 113, pp. 11279-11284, Jul 2 2009.
[41] S. Stankovich, R. D. Piner, X. Q. Chen, N. Q. Wu, S. T. Nguyen, and R. S. Ruoff, "Stable aqueous dispersions of graphitic nanoplatelets via the reduction of exfoliated graphite oxide in the presence of poly(sodium 4-styrenesulfonate)," Journal of Materials Chemistry, vol. 16, pp. 155-158, 2006.
[42] C. Gomez-Navarro, R. T. Weitz, A. M. Bittner, M. Scolari, A. Mews, M. Burghard, and K. Kern, "Electronic transport properties of individual chemically reduced graphene oxide sheets," Nano Letters, vol. 7, pp. 3499-3503, Nov 2007.
[43] L. J. Cote, R. Cruz-Silva, and J. X. Huang, "Flash Reduction and Patterning of Graphite Oxide and Its Polymer Composite," Journal of the American Chemical Society, vol. 131, pp. 11027-11032, Aug 12 2009.
[44] C. Gomez-Navarro, R. T. Weitz, A. M. Bittner, M. Scolari, A. Mews, M. Burghard, and K. Kern, "Electronic Transport Properties of Individual Chemically Reduced Graphene Oxide Sheets. (vol 7, pg 3499, 2007)," Nano Letters, vol. 9, May 2009.
[45] S. Eigler, M. Enzelberger-Heim, S. Grimm, P. Hofmann, W. Kroener, A. Geworski, C. Dotzer, M. Rockert, J. Xiao, C. Papp, O. Lytken, H. P. Steinruck, P. Muller, and A. Hirsch, "Wet Chemical Synthesis of Graphene," Advanced Materials, vol. 25, pp. 3583-3587, Jul 12 2013.
[46] Z. G. Mou, X. Y. Chen, Y. K. Du, X. M. Wang, P. Yang, and S. D. Wang, "Forming mechanism of nitrogen doped graphene prepared by thermal solid-state reaction of graphite oxide and urea," Applied Surface Science, vol. 258, pp. 1704-1710, Dec 15 2011.
[47] H. Terrones, R. T. Lv, M. Terrones, and M. S. Dresselhaus, "The role of defects and doping in 2D graphene sheets and 1D nanoribbons," Reports on Progress in Physics, vol. 75, Jun 2012.
[48] B. Lei, G. R. Li, and X. P. Gao, "Morphology dependence of molybdenum disulfide transparent counter electrode in dye-sensitized solar cells," Journal of Materials Chemistry A, vol. 2, pp. 3919-3925, 2014.
[49] R. Tenne, L. Margulis, M. Genut, and G. Hodes, "Polyhedral and Cylindrical Structures of Tungsten Disulfide," Nature, vol. 360, pp. 444-446, Dec 3 1992.
[50] K. Kobayashi and J. Yamauchi, "Electronic-Structure and Scanning-Tunneling-Microscopy Image of Molybdenum Dichalcogenide Surfaces," Physical Review B, vol. 51, pp. 17085-17095, Jun 15 1995.
[51] Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman, and M. S. Strano, "Electronics and optoelectronics of two-dimensional transition metal dichalcogenides," Nature Nanotechnology, vol. 7, pp. 699-712, Nov 2012.
[52] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis, "Single-layer MoS2 transistors," Nature Nanotechnology, vol. 6, pp. 147-150, Mar 2011.
[53] O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic, and A. Kis, "Ultrasensitive photodetectors based on monolayer MoS2," Nature Nanotechnology, vol. 8, pp. 497-501, Jul 2013.
[54] G. C. Stevens and T. Edmonds, "Catalytic Activity of Basal and Edge Planes of Molybdenum-Disulfide," Journal of the Less-Common Metals, vol. 54, pp. 321-330, 1977.
[55] C. B. Roxlo, M. Daage, D. P. Leta, K. S. Liang, S. Rice, A. F. Ruppert, and R. R. Chianelli, "Catalytic Defects at Molybdenum-Disulfide Edge Planes," Solid State Ionics, vol. 22, pp. 97-104, Dec 1986.
[56] D. C. Marcano, D. V. Kosynkin, J. M. Berlin, A. Sinitskii, Z. Z. Sun, A. Slesarev, L. B. Alemany, W. Lu, and J. M. Tour, "Improved Synthesis of Graphene Oxide," Acs Nano, vol. 4, pp. 4806-4814, Aug 2010.
[57] D. H. Long, W. Li, L. C. Ling, J. Miyawaki, I. Mochida, and S. H. Yoon, "Preparation of Nitrogen-Doped Graphene Sheets by a Combined Chemical and Hydrothermal Reduction of Graphene Oxide," Langmuir, vol. 26, pp. 16096-16102, Oct 19 2010.
[58] S. Ito, P. Chen, P. Comte, M. K. Nazeeruddin, P. Liska, P. Pechy, and M. Gratzel, "Fabrication of screen-printing pastes from TiO2 powders for dye-sensitised solar cells," Progress in Photovoltaics, vol. 15, pp. 603-612, Nov 2007.
[59] N. Papageorgiou, W. F. Maier, and M. Gratzel, "An iodine/triiodide reduction electrocatalyst for aqueous and organic media," Journal of the Electrochemical Society, vol. 144, pp. 876-884, Mar 1997.
[60] G. G. Tang, J. R. Sun, C. Wei, K. Q. Wu, X. R. Ji, S. S. Liu, H. Tang, and C. S. Li, "Synthesis and characterization of flowerlike MoS2 nanostructures through CTAB-assisted hydrothermal process," Materials Letters, vol. 86, pp. 9-12, Nov 1 2012.
[61] Y. Hou, B. Zhang, Z. H. Wen, S. M. Cui, X. R. Guo, Z. He, and J. H. Chen, "A 3D hybrid of layered MoS2/nitrogen-doped graphene nanosheet aerogels: an effective catalyst for hydrogen evolution in microbial electrolysis cells," Journal of Materials Chemistry A, vol. 2, pp. 13795-13800, 2014.
[62] T. V. Khai, H. G. Na, D. S. Kwak, Y. J. Kwon, H. Ham, K. B. Shim, and H. W. Kim, "Influence of N-doping on the structural and photoluminescence properties of graphene oxide films," Carbon, vol. 50, pp. 3799-3806, Aug 2012.
[63] J. L. Brito, M. Ilija, and P. Hernandez, "Thermal and Reductive Decomposition of Ammonium Thiomolybdates," Thermochimica Acta, vol. 256, pp. 325-338, Jun 1 1995.
[64] H. L. Guo, P. Su, X. F. Kang, and S. K. Ning, "Synthesis and characterization of nitrogen-doped graphene hydrogels by hydrothermal route with urea as reducing-doping agents," Journal of Materials Chemistry A, vol. 1, pp. 2248-2255, 2013.
[65] B. C. Windom, W. G. Sawyer, and D. W. Hahn, "A Raman Spectroscopic Study of MoS2 and MoO3: Applications to Tribological Systems," Tribology Letters, vol. 42, pp. 301-310, Jun 2011.
[66] C. J. Bender and H. T. Tien, "Electrical-Properties of Bilayer Lipid-Membranes Containing Iodine and Iodide, Investigated by Cyclic Voltammetry," Analytica Chimica Acta, vol. 201, pp. 51-58, Oct 15 1987.
[67] G. Tsekouras, A. J. Mozer, and G. G. Wallace, "Enhanced performance of dye sensitized solar cells utilizing platinum electrodeposit counter electrodes," Journal of the Electrochemical Society, vol. 155, pp. K124-K128, 2008.
[68] D. J. Driscoll, W. Martir, J. X. Wang, and J. H. Lunsford, "Formation of Gas-Phase Methyl Radicals over Mgo," Journal of the American Chemical Society, vol. 107, pp. 58-63, 1985.
[69] T. B. Hunter, P. S. Tyler, W. H. Smyrl, and H. S. White, "Impedance Analysis of Poly(Vinylferrocene) Films - the Dependence of Diffusional Charge Transport and Exchange Current-Density on Polymer Oxidation-State," Journal of the Electrochemical Society, vol. 134, pp. 2198-2204, Sep 1987.
[70] G. T. Yue, J. H. Wu, Y. M. Xiao, M. L. Huang, J. M. Lin, and J. Y. Lin, "High performance platinum-free counter electrode of molybdenum sulfide-carbon used in dye-sensitized solar cells (vol 1, pg 1495, 2013)," Journal of Materials Chemistry A, vol. 1, pp. 15553-15553, 2013.