- Email: [email protected]
Stabilizing molecular sensitizers in aqueous environs
Author's Accepted Manuscript
Stabilizing Molecular Sensitizers in Aqueous Environs Mark D. Losego, Kenneth Hanson
www.elsevier.com/nanoenergy
PII: DOI: Reference:
S2211-2855(13)00133-X http://dx.doi.org/10.1016/j.nanoen.2013.07.007 NANOEN257
To appear in:
Nano Energy
Received date: 1 July 2013 Accepted date: 17 July 2013 Cite this article as: Mark D. Losego, Kenneth Hanson, Stabilizing Molecular Sensitizers in Aqueous Environs, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2013.07.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Stabilizing Molecular Sensitizers in Aqueous Environs Mark D. Losego1 and Kenneth Hanson2 1Department of Chemical and Biomolecular Engineering, North Carolina State University,
Raleigh, NC USA 2Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL USA The recent transition of dye‐sensitized solar cells (DSCs) from the lab to small‐scale production validates the commercial viability of energy technologies that combine molecular functionality with nanomaterials. Commercialization of DSC technology is the result of advances in both device performance and lifetime.1 While academic researchers often focus on the former (performance), the latter (lifetime) is essential for commercial success. Increasing the lifetime of molecularly sensitized devices in either humid or aqueous environments will reduce device costs and enable a range of new device opportunities. Dye‐sensitized devices are typically composed of molecular sensitizers attached to conductive, nanostructured inorganic scaffolds as depicted in Fig. 1a. Light absorption by the molecular sensitizer results in an excited state of sufficient energy to transfer an electron to the inorganic scaffold. In DSCs, this electron transfer process generates electricity. However, similar molecularly sensitized schemes are being developed for other applications, including hydrogen fuel generation (i.e., solar water splitting),2 photocatalytic water purification,3 and CO2 reduction to hydrocarbon fuels.4 While numerous researchers actively investigate the electron transfer processes and catalytic mechanisms of these molecular sensitizers, little attention is being given to the critical challenge of extending device lifetime by maintaining attachment of the molecular sensitizer to the inorganic
scaffold. Without a robust organic‐inorganic linkage, these devices are useless from a practical standpoint. In the presence of water, the attachment groups used for most molecular sensitizers (e.g., ‐COOH, ‐PO3H ) are susceptible to hydrolytic attack, resulting in bond breakage between molecule and inorganic scaffold (Fig. 1b). Long‐term stability in DSCs has been possible because they operate with non‐aqueous electrolytes and module encapsulation has been extensively engineered to prevent water permeation. For emerging energy applications where water is a necessary reactant (e.g., hydrogen fuel production) or ambient conditions are preferred (e.g., CO2 reduction), this challenge of molecular attachment in the presence of water can no longer be avoided. External stimuli including pH, illumination, and electrical current can further destabilize molecular attachment. Recently, we published a standard protocol to evaluate the role of environmental effects on the stability of surface bound molecules under continuous illumination.5 We show that photodesorption of the phosphonate bound molecule is strongly pH dependent with desorption rates increasing by a factor 5 when the pH is increased from 1 to 5. Several approaches have been explored to improve aqueous stability including stronger surface binding groups,6 electropolymerizized overlayers,7 and amphiphilic structuring.8 Unfortunately, these strategies have had limited success and/or require complex organic syntheses that hurt economic viability.9 Within the last year, we have demonstrated that atomic layer deposition (ALD) could be used to coat surface bound molecules and prevent their detachment from the surface.10 ALD is a vapor phase deposition process that proceeds through a binary sequence of self‐limiting surface
reactions and permits conformal, subnanometer precision of inorganic material deposition over three‐dimensional nano‐scale architectures. The ALD layers, which are < 1 nm thick, presumably protect the bond that links the molecule to the inorganic scaffold from hydrolytic attack as depicted in Fig. 1c. The observed improvements in stability are even maintained when the systems are stressed with high pH, illumination, and/or electrical current flow. From these preliminary experiments we have demonstrated a 1 to 2 order of magnitude increase in molecular attachment lifetimes under simulated operating conditions. We believe that these ALD treatments represent a new technological platform for stabilizing molecular sensitizer intended for aqueous environs. An order of magnitude increase in DSC device lifetime could have major implications for economic viability. It has been estimated that the energy cost of an organic photovoltaic could be reduced by ~30% if the device lifetime is doubled from 10 years to 20 years (assuming 15 % efficiency).11 Our preliminary investigations suggest that this technique can be applied to a wide variety of molecular dyes and catalysts. The ALD process is sufficiently mature that nearly any elemental oxide layer could be used for protection, necessitating a much broader examination of the potential materials sets for optimal device performance. In particular, considerable fundamental research is still needed to fully understand how these ALD treatments affect electron transfer events and electronic band structure at the organic‐ inorganic interface. Ultimately, extensive interdisciplinary collaboration will be required to transition this technology from the lab bench to the commercial market. Our new ALD protection platform directly emerged from a multidisciplinary collaborative research effort supported by the Research Triangle Solar Fuels Institute in North Carolina (www.solarfuels.org).
Interactions between synthetic chemists, spectroscopists, materials scientists, physicists, and engineers will be necessary to ultimately achieve the level of fundamental understanding to rationally design ALD protecting layers that optimize the stability and performance of molecularly sensitized devices for use in aqueous solutions or humid ambient conditions.
Fig. 1: (a) Scheme for a dye‐sensitized device architecture showing the photoexcitation and electron transfer events. (b) Current molecular sensitizer technology exposed to aqueous conditions and other external stimuli cause detachment of the molecules from the surface. (c) New work in ALD deposition shows promise in preventing molecular detachment under similar aqueous conditions. Email addresses: [email protected] (M. Losego); [email protected] (K. Hanson)
Mark D. Losego is an n Assistant Research Professor of Chemical an nd Biomolecular h.D. in materials science Engineerring at Nortth Carolina SState University. He eaarned his Ph and engiineering at N North Carollina State Un niversity in 2008 and cconducted p postdoctoral research h in nano‐sccale heat traansport at th he Universitty of Illinoiss until 2011. Losego’s research h focuses on n understanding transp port phenom mena at orgaanic / inorgganic interfaaces and with hin three‐dimensionally y nanostrucctured systeems used in energy and d environmeental applicatiions.
Kenneth h Hanson is an assiistant Proffessor in the Department of Chemistry and Biochem mistry at Flo orida State U University. H He earned aa B.S. in cheemistry from m St. Cloud State Universiity in 2005 5 and, undeer the direection of Mark M E. Tho ompson, a Ph.D. from m the
University of Southern California in 2010. While a post‐doctoral researcher in the lab of Thomas J. Meyer at the University of North Carolina at Chapel Hill (2010‐2013), he investigated the photophysical properties of molecules on metal oxide surfaces. His current research focuses on understanding the structure‐property relationship of molecular light absorbers/emitters in dye‐sensitized solar cells and other applications.
References
1. Hagfeldt, A.; Boschloo, G.; Sun, L. C.; Kloo, L.; Pettersson, H., "Dye‐Sensitized Solar Cells." Chem. Rev. 110 6595‐6663 (2010). 2. Alibabaei, L.; Luo, H. L.; House, R. L.; Hoertz, P. G.; Lopez, R.; Meyer, T. J., "Applications of metal oxide materials in dye sensitized photoelectrosynthesis cells for making solar fuels: let the molecules do the work." J Mater Chem A 1 4133‐4145 (2013). 3. Zhang, F. L.; Zhao, J. C.; Zang, L.; Shen, T.; Hidaka, H.; Pelizzetti, E.; Serpone, N., "Photoassisted degradation of dye pollutants in aqueous TiO2 dispersions under irradiation by visible light." J Mol Catal aChem 120 173‐178 (1997). 4. Chen, Z. F.; Chen, C. C.; Weinberg, D. R.; Kang, P.; Concepcion, J. J.; Harrison, D. P.; Brookhart, M. S.; Meyer, T. J., "Electrocatalytic reduction of CO2 to CO by polypyridyl ruthenium complexes." Chem. Commun. 47 12607‐12609 (2011). 5. Hanson, K.; Brennaman, M. K.; Luo, H. L.; Glasson, C. R. K.; Concepcion, J. J.; Song, W. J.; Meyer, T. J., "Photostability of Phosphonate‐Derivatized, Ru‐II Polypyridyl Complexes on Metal Oxide Surfaces." Acs Applied Materials & Interfaces 4 1462‐1469 (2012). 6. Szpakolski, K.; Latham, K.; Rix, C.; Rani, R. A.; Kalantar‐zadeh, K., "Silane: A new linker for chromophores in dye‐sensitised solar cells." Polyhedron 52 719‐732 (2013). 7. Moss, J. A.; Stipkala, J. M.; Yang, J. C.; Bignozzi, C. A.; Meyer, G. J.; Meyer, T. J.; Wen, X. G.; Linton, R. W., "Sensitization of nanocrystalline TiO2 by electropolymerized thin films." Chem. Mater. 10 1748‐+ (1998). 8. Zakeeruddin, S. M.; Nazeeruddin, M. K.; Humphry‐Baker, R.; Péchy, P.; Quagliotto, P.; Barolo, C.; Viscardi, G.; Grätzel, M., "Design, Synthesis, and Application of Amphiphilic Ruthenium Polypyridyl Photosensitizers in Solar Cells Based on Nanocrystalline TiO2 Films." Langmuir 18 952‐954 (2002). 9. Osedach, T. P.; Andrew, T. L.; Bulovic, V., "Effect of synthetic accessibility on the commercial viability of organic photovoltaics." Energy & Environmental Science 6 711‐718 (2013). 10. Hanson, K.; Losego, M. D.; Kalanyan, B.; Ashford, D. L.; Parsons, G. N.; Meyer, T. J., "Stabilization of [Ru(bpy)(2)(4,4 '‐(PO3H2)bpy)](2+) on Mesoporous TiO2 with Atomic Layer Deposition of Al2O3." Chem. Mater. 25 3‐5 (2013). 11. Kalowekamo, J.; Baker, E., "Estimating the manufacturing cost of purely organic solar cells." Sol Energy 83 1224‐1231 (2009).
Graphical Abstract
Highlights • Research into molecularly sensitized energy systems is disproportionately weighted towards optimizing device performance compared to lifetime. • Extending the lifetime of these systems in aqueous or humid environments could reduce costs and enable new devices. • A new atomic layer deposition treatment shows great promise for extending the lifetime of molecular attachment to inorganic surfaces under aqueous conditions.
Stabilizing Molecular Sensitizers in Aqueous Environs Mark D. Losego, Kenneth Hanson
www.elsevier.com/nanoenergy
PII: DOI: Reference:
S2211-2855(13)00133-X http://dx.doi.org/10.1016/j.nanoen.2013.07.007 NANOEN257
To appear in:
Nano Energy
Received date: 1 July 2013 Accepted date: 17 July 2013 Cite this article as: Mark D. Losego, Kenneth Hanson, Stabilizing Molecular Sensitizers in Aqueous Environs, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2013.07.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Stabilizing Molecular Sensitizers in Aqueous Environs Mark D. Losego1 and Kenneth Hanson2 1Department of Chemical and Biomolecular Engineering, North Carolina State University,
Raleigh, NC USA 2Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL USA The recent transition of dye‐sensitized solar cells (DSCs) from the lab to small‐scale production validates the commercial viability of energy technologies that combine molecular functionality with nanomaterials. Commercialization of DSC technology is the result of advances in both device performance and lifetime.1 While academic researchers often focus on the former (performance), the latter (lifetime) is essential for commercial success. Increasing the lifetime of molecularly sensitized devices in either humid or aqueous environments will reduce device costs and enable a range of new device opportunities. Dye‐sensitized devices are typically composed of molecular sensitizers attached to conductive, nanostructured inorganic scaffolds as depicted in Fig. 1a. Light absorption by the molecular sensitizer results in an excited state of sufficient energy to transfer an electron to the inorganic scaffold. In DSCs, this electron transfer process generates electricity. However, similar molecularly sensitized schemes are being developed for other applications, including hydrogen fuel generation (i.e., solar water splitting),2 photocatalytic water purification,3 and CO2 reduction to hydrocarbon fuels.4 While numerous researchers actively investigate the electron transfer processes and catalytic mechanisms of these molecular sensitizers, little attention is being given to the critical challenge of extending device lifetime by maintaining attachment of the molecular sensitizer to the inorganic
scaffold. Without a robust organic‐inorganic linkage, these devices are useless from a practical standpoint. In the presence of water, the attachment groups used for most molecular sensitizers (e.g., ‐COOH, ‐PO3H ) are susceptible to hydrolytic attack, resulting in bond breakage between molecule and inorganic scaffold (Fig. 1b). Long‐term stability in DSCs has been possible because they operate with non‐aqueous electrolytes and module encapsulation has been extensively engineered to prevent water permeation. For emerging energy applications where water is a necessary reactant (e.g., hydrogen fuel production) or ambient conditions are preferred (e.g., CO2 reduction), this challenge of molecular attachment in the presence of water can no longer be avoided. External stimuli including pH, illumination, and electrical current can further destabilize molecular attachment. Recently, we published a standard protocol to evaluate the role of environmental effects on the stability of surface bound molecules under continuous illumination.5 We show that photodesorption of the phosphonate bound molecule is strongly pH dependent with desorption rates increasing by a factor 5 when the pH is increased from 1 to 5. Several approaches have been explored to improve aqueous stability including stronger surface binding groups,6 electropolymerizized overlayers,7 and amphiphilic structuring.8 Unfortunately, these strategies have had limited success and/or require complex organic syntheses that hurt economic viability.9 Within the last year, we have demonstrated that atomic layer deposition (ALD) could be used to coat surface bound molecules and prevent their detachment from the surface.10 ALD is a vapor phase deposition process that proceeds through a binary sequence of self‐limiting surface
reactions and permits conformal, subnanometer precision of inorganic material deposition over three‐dimensional nano‐scale architectures. The ALD layers, which are < 1 nm thick, presumably protect the bond that links the molecule to the inorganic scaffold from hydrolytic attack as depicted in Fig. 1c. The observed improvements in stability are even maintained when the systems are stressed with high pH, illumination, and/or electrical current flow. From these preliminary experiments we have demonstrated a 1 to 2 order of magnitude increase in molecular attachment lifetimes under simulated operating conditions. We believe that these ALD treatments represent a new technological platform for stabilizing molecular sensitizer intended for aqueous environs. An order of magnitude increase in DSC device lifetime could have major implications for economic viability. It has been estimated that the energy cost of an organic photovoltaic could be reduced by ~30% if the device lifetime is doubled from 10 years to 20 years (assuming 15 % efficiency).11 Our preliminary investigations suggest that this technique can be applied to a wide variety of molecular dyes and catalysts. The ALD process is sufficiently mature that nearly any elemental oxide layer could be used for protection, necessitating a much broader examination of the potential materials sets for optimal device performance. In particular, considerable fundamental research is still needed to fully understand how these ALD treatments affect electron transfer events and electronic band structure at the organic‐ inorganic interface. Ultimately, extensive interdisciplinary collaboration will be required to transition this technology from the lab bench to the commercial market. Our new ALD protection platform directly emerged from a multidisciplinary collaborative research effort supported by the Research Triangle Solar Fuels Institute in North Carolina (www.solarfuels.org).
Interactions between synthetic chemists, spectroscopists, materials scientists, physicists, and engineers will be necessary to ultimately achieve the level of fundamental understanding to rationally design ALD protecting layers that optimize the stability and performance of molecularly sensitized devices for use in aqueous solutions or humid ambient conditions.
Fig. 1: (a) Scheme for a dye‐sensitized device architecture showing the photoexcitation and electron transfer events. (b) Current molecular sensitizer technology exposed to aqueous conditions and other external stimuli cause detachment of the molecules from the surface. (c) New work in ALD deposition shows promise in preventing molecular detachment under similar aqueous conditions. Email addresses: [email protected] (M. Losego); [email protected] (K. Hanson)
Mark D. Losego is an n Assistant Research Professor of Chemical an nd Biomolecular h.D. in materials science Engineerring at Nortth Carolina SState University. He eaarned his Ph and engiineering at N North Carollina State Un niversity in 2008 and cconducted p postdoctoral research h in nano‐sccale heat traansport at th he Universitty of Illinoiss until 2011. Losego’s research h focuses on n understanding transp port phenom mena at orgaanic / inorgganic interfaaces and with hin three‐dimensionally y nanostrucctured systeems used in energy and d environmeental applicatiions.
Kenneth h Hanson is an assiistant Proffessor in the Department of Chemistry and Biochem mistry at Flo orida State U University. H He earned aa B.S. in cheemistry from m St. Cloud State Universiity in 2005 5 and, undeer the direection of Mark M E. Tho ompson, a Ph.D. from m the
University of Southern California in 2010. While a post‐doctoral researcher in the lab of Thomas J. Meyer at the University of North Carolina at Chapel Hill (2010‐2013), he investigated the photophysical properties of molecules on metal oxide surfaces. His current research focuses on understanding the structure‐property relationship of molecular light absorbers/emitters in dye‐sensitized solar cells and other applications.
References
1. Hagfeldt, A.; Boschloo, G.; Sun, L. C.; Kloo, L.; Pettersson, H., "Dye‐Sensitized Solar Cells." Chem. Rev. 110 6595‐6663 (2010). 2. Alibabaei, L.; Luo, H. L.; House, R. L.; Hoertz, P. G.; Lopez, R.; Meyer, T. J., "Applications of metal oxide materials in dye sensitized photoelectrosynthesis cells for making solar fuels: let the molecules do the work." J Mater Chem A 1 4133‐4145 (2013). 3. Zhang, F. L.; Zhao, J. C.; Zang, L.; Shen, T.; Hidaka, H.; Pelizzetti, E.; Serpone, N., "Photoassisted degradation of dye pollutants in aqueous TiO2 dispersions under irradiation by visible light." J Mol Catal aChem 120 173‐178 (1997). 4. Chen, Z. F.; Chen, C. C.; Weinberg, D. R.; Kang, P.; Concepcion, J. J.; Harrison, D. P.; Brookhart, M. S.; Meyer, T. J., "Electrocatalytic reduction of CO2 to CO by polypyridyl ruthenium complexes." Chem. Commun. 47 12607‐12609 (2011). 5. Hanson, K.; Brennaman, M. K.; Luo, H. L.; Glasson, C. R. K.; Concepcion, J. J.; Song, W. J.; Meyer, T. J., "Photostability of Phosphonate‐Derivatized, Ru‐II Polypyridyl Complexes on Metal Oxide Surfaces." Acs Applied Materials & Interfaces 4 1462‐1469 (2012). 6. Szpakolski, K.; Latham, K.; Rix, C.; Rani, R. A.; Kalantar‐zadeh, K., "Silane: A new linker for chromophores in dye‐sensitised solar cells." Polyhedron 52 719‐732 (2013). 7. Moss, J. A.; Stipkala, J. M.; Yang, J. C.; Bignozzi, C. A.; Meyer, G. J.; Meyer, T. J.; Wen, X. G.; Linton, R. W., "Sensitization of nanocrystalline TiO2 by electropolymerized thin films." Chem. Mater. 10 1748‐+ (1998). 8. Zakeeruddin, S. M.; Nazeeruddin, M. K.; Humphry‐Baker, R.; Péchy, P.; Quagliotto, P.; Barolo, C.; Viscardi, G.; Grätzel, M., "Design, Synthesis, and Application of Amphiphilic Ruthenium Polypyridyl Photosensitizers in Solar Cells Based on Nanocrystalline TiO2 Films." Langmuir 18 952‐954 (2002). 9. Osedach, T. P.; Andrew, T. L.; Bulovic, V., "Effect of synthetic accessibility on the commercial viability of organic photovoltaics." Energy & Environmental Science 6 711‐718 (2013). 10. Hanson, K.; Losego, M. D.; Kalanyan, B.; Ashford, D. L.; Parsons, G. N.; Meyer, T. J., "Stabilization of [Ru(bpy)(2)(4,4 '‐(PO3H2)bpy)](2+) on Mesoporous TiO2 with Atomic Layer Deposition of Al2O3." Chem. Mater. 25 3‐5 (2013). 11. Kalowekamo, J.; Baker, E., "Estimating the manufacturing cost of purely organic solar cells." Sol Energy 83 1224‐1231 (2009).
Graphical Abstract
Highlights • Research into molecularly sensitized energy systems is disproportionately weighted towards optimizing device performance compared to lifetime. • Extending the lifetime of these systems in aqueous or humid environments could reduce costs and enable new devices. • A new atomic layer deposition treatment shows great promise for extending the lifetime of molecular attachment to inorganic surfaces under aqueous conditions.