pubs.acs.org/Langmuir This article not subject to U.S. Copyright. Published 2009 by the American Chemical Society
Metalloporphyrin Assemblies on Pyridine-Functionalized Titanium Dioxide Michael T. Brumbach,*,† Andrew K. Boal,‡ and David R. Wheeler† †
Sandia National Laboratories, Albuquerque, New Mexico 87123, and ‡MIOX Corporation, Albuquerque, New Mexico 87113 Received March 31, 2009. Revised Manuscript Received July 22, 2009
Porphyrin adsorption on TiO2 nanoparticles has been achieved for multiple porphyrins, and in mixed porphyrin assemblies, via axial ligation to surface-bound pyridine anchored by either para carboxylic or phosphonic functionalizations. Homogenous assemblies were prepared and characterized, while mixed metalloporphyrin assemblies were demonstrated by controlling the concentration ratios of respective porphyrins in the modifying solution. Evaluation of the assemblies using spectroscopic techniques and electrochemistry confirms high porphyrin retention, while exhibiting their surface bound optical and electrochemical properties. A thorough study is discussed where several metalloporphyrins have been evaluated (Ru(CO)OEP, Ru(CO)TPP, and ZnTPP) for relative comparisons and relationships to pyridyl axial binding strengths. The systematic study evaluates multiple background cases using either H2TPP, TiO2 modification with benzoic acid, or unmodified TiO2 to confirm the high affinity of Ru and Zn porphyrins for surfaceanchored pyridyl sites. The simple method of step-by-step coordinative anchoring of porphyrins to TiO2 using small, commercially available molecules is highly adaptable for use in dye-sensitized solar cells (DSSC) where intimate contact between the absorbing dye and the semiconductor is required. DSSC devices with novel mixed porphyrin assemblies were shown to give higher power performance than DSSCs utilizing sensitization with only one type of porphyrin.
Introduction The use of high-absorptivity molecules as surface sensitizers on nanocrystalline semiconductor materials provides the light absorbing and charge generating component of the dye-sensitized solar cell (DSSC).1,2 Many factors affect the overall DSSC energy conversion efficiency including the physical and electronic properties of the semiconductor film, the redox species in the electrolyte solution, the charge transfer rates at the front and back electrodes, and the nature of the absorbing dye. Research on the dye often focuses on increasing the overall light harvesting efficiency to subsequently increase the overall photovoltaic efficiency.1-12 Other molecular features of the dye which affect efficiency include the nature and orientation of the functional group used to anchor the dye to the semiconductor surface,1,4-8 *Corresponding author. E-mail:
[email protected]. (1) Kalyanasundaram, K.; Gr€atzel, M. Coord. Chem. Rev. 1998, 177, 347–414. (2) O’Regan, B.; Gr€atzel, M. Nature 1991, 353, 737–40. (3) Snaith, H. J.; Karthikeyan, C. S.; Petrozza, A.; Teuscher, J.; Moser, J. E.; Nazeeruddin, M. K.; Thelakkat, M.; Gr€atzel, M. J. Phys. Chem. C 2008, 112, 7562–7566. (4) He, J.; Benko, G.; Korodi, F.; Polvka, T.; Lomoth, R.; Kermark, B.; Sun, L.; Hagfeldt, A.; Sundstrum, V. J. Am. Chem. Soc. 2002, 123, 4922–4932. (5) He, J.; Hagfeldt, A.; Lindquist, S.-E.; Grennberg, H.; Korodi, F.; Sun, L.; A˚kermark, B. Langmuir 2001, 17, 2743–2747. (6) Duffy, N. W.; Dobson, K. D.; Gordon, K. C.; Robinson, B. H.; McQuillan, A. J. Chem. Phys. Lett. 1997, 266, 451–455. (7) Bramblett, A. L.; Boeckl, M. S.; Hauch, K. D.; Ratner, B. D.; Sasaki, T.; Rogers, J. W., Jr. Surf. Interface Anal. 2002, 33, 506–515. (8) Rochford, J.; Galoppini, E. Langmuir 2008, 24, 5366–5374. (9) Campbell, W. M.; Burrell, A. K.; Officer, D. L.; Jolley, K. W. Coord. Chem. Rev. 2004, 248, 1363–1379. (10) Campbell, W. M.; Jolley, K. W.; Wagner, P.; Wagner, K.; Walsh, P. J.; Gordon, K. C.; Schmidt-Mende, L.; Nazeeruddin, M. K.; Wang, Q.; Gr€atzel, M.; Officer, D. L. J. Phys. Chem. C 2007, 111, 11760–11762. (11) Jasieniak, J.; Johnston, M.; Waclawik, E. R. J. Phys. Chem. B 2004, 108, 12962–12971. (12) Nazeeruddin, Md. K.; Humphry-Baker, R.; Gr€atzel, M.; Murrer, B. A. Chem. Commun. 1998, 719–720. (13) Van Ryswyk, H.; Gabor, R. S.; Awasthi, S.; Bodzin, D. J.; Orosz, K. S. Langmuir 2004, 20, 11815–11817. (14) Eberspacher, T. A.; Collman, J. P.; Chidsey, C. E. D.; Donohue, D. L.; Ryswyk, H. V. Langmuir 2003, 19, 3814–3821.
Langmuir 2009, 25(18), 10685–10690
the physical and chemical nature of the spacing group between the anchoring functionality and the dye,1,8-17 and the dye molecule itself.1-12 Porphyrins have generated a substantial amount of interest for use as the dye component of a DSSC, often inspired by the light harvesting networks found in plants.8-17 DSSC dyes are typically dyes where an appropriate oxide binding functionality has been tethered to the dye, such as a carboxylic or phosphonic functional group.1-6,8 These dyes can then be adsorbed onto a semiconductor surface for subsequent photophysical studies and demonstration of charge transfer in a photovoltaic device. An alternative approach, as is taken in this work, relies on the favorable properties of self-assembly, where simple components of the overall system are separately selfassembled into larger functional units, typically via noncovalent processes. Other attempts to bind porphyrins to TiO2 have been attempted in a similar manner to this work.12-17 Functional DSSC devices exploiting axial ligation of porphyrins have been prepared, resulting in good photovoltaic behavior for porphyrin DSSC devices.12,17 Several studies have focused on porphyrin tethering through modification of gold with alkanethiols with long, nonconductive alkyl chains terminated by a pyridine functionalization.13-15 4-Pyridinethiol has also been used to modify gold with subsequent axial ligation of porphyrins.16 Alternatively, axial ligation has been performed on porphyrins in solution prior to surface adsorption.12,17 Here, short aromatic amines were tethered to TiO2 nanoparticles and were used to scavenge porphyrin from solution and anchor the porphyrin, via axial ligation, in close proximity to the electrode surface. Attachment of porphyrins via axial coordination provides a number of advantages including the ability to control chromophore orientation (15) Offord, D. A.; Sachs, S. B.; Ennis, M. S.; Eberspacher, T. A.; Griffin, J. H.; Chidsey, C. E. D.; Collman, J. P. J. Am. Chem. Soc. 1998, 120, 4478–4487. (16) Zhang, Z.; Imae, T.; Sato, H.; Watanabe, A.; Ozaki, Y. Langmuir 2001, 17, 4564–4568. (17) Yanagisawa, M.; Korodi, F.; He, J.; Sun, L.; Sundstr€om, V.; A˚kermark, B. J. Porphyrins Phthalocyanines 2002, 6, 217–224.
Published on Web 08/13/2009
DOI: 10.1021/la901130a
10685
Article
Brumbach et al.
Figure 1. Scheme for modification of TiO2 nanoparticles with small molecule acids is shown in step 1. Pyridine phosphonic acid ((a) pyphos) and pyridine carboxylic acid ((b) pycarbox) were used to anchor pyridyl groups to titania nanoparticles. Benzene carboxylic acid ((c) bz) was used as a control. Subsequent exposure of modified TiO2 NPs to metalloporphyrin solutions gave adsorbed porphyrin via an axial linkage as illustrated for the isonicotinic acid modified TiO2 and a Ru(CO) porphyrin representation.
relative to the surface, simplified syntheses using commercially available materials for modular assembly of porphyrin sensitization, reduction of surface aggregation, and a controllable mechanism for stepwise construction of multiporphyrin arrays.
Experimental Methods Porphyrins (Aldrich) used in binding studies included 5,10,15,20-tetraphenylporphyrin (H2TPP), 5,10,15,20-tetraphenylporphyrin zinc (ZnTPP), 5,10,15,20-tetraphenylporphyrin ruthenium(II) carbonyl (Ru(CO)TPP), and 2,3,7,8,12,13,17,18octaethylporphyrin ruthenium(II) carbonyl (Ru(CO)OEP). Three surface modifiers were used to generate the functionalized nanoparticles: pyridine phosphonic acid (pyphos), isonicotinic acid (pycarbox), and benzoic acid (bz) as a control (see step 1 in Figure 1). Pycarbox and bz were obtained from Aldrich. 2-(pyridin4-yl)ethylphosphonic acid, or pyridine phosphonic acid (pyphos), was synthesized by literature procedures.18,19 All other chemicals and reagents were purchased from commercial sources and used as received as described further in the Supporting Information. Anatase TiO2 nanoparticles (
