Técnicas e instrumentos para la recolección de datos

The results presented here suggest possibilities for exploration of electron transfer rate control of the electrocatalytic oxidation of water by the IrOX NPs in higher valent states. Preliminary results (Figure 3.6) are highly promising. A strong increase in oxidation currents

is seen at positive potentials, which are known20,22,23 to reflect high-valent NP

electrocatalysis of water oxidation. In a rotated ring-disk electrode experiment20 _{in which the} disk was IrOX NP-SAM-coated, O2 was detected at the ring.42 The significant aspect of Figure 3.6 is that the water oxidation process becomes shifted to more positive potentials as

Figure 3.6 CV of NPs immobilized on C16, C12, and C8 SAMs. The large currents at positive potentials correspond to electrocatalyzed water oxidation, which occurs at increasing over-potential, !, with increasing SAM chain length. CVs were performed at 500 mV/s in 0.1 M NaOH (pH ~13) electrolyte with 0.106 cm2 electrode area. Surface coverage, "IrO2, was ~1.2 # 10-9, 1.1 # 10-9, and 1.0 # 10-9 mol Ir/cm2 for C16, C12, and C8 SAMs,

respectively. The dotted line at 0.5 mA/cm2 marks C16, C12, and C8 potentials for catalysis of water oxidation at 0.68, 0.55, 0.546 V, respectively. With repeated scans, the NP-SAM becomes degrade by Au oxide formation.

the NP-SAM chain length is increased, signaling a SAM-imposed kinetic control of the reaction. The potentials for water oxidation at 0.5 mA/cm2 are 0.546, 0.55 and 0.68 V for C8, C12, and C16 SAMs, respectively. For comparison, the potential for water oxidation by electroflocculated43 IrOX NP films is 0.50 V.20 The unmistakable retarding effect of NP- SAM chain length in Figure 3.6 opens a potential avenue to studying the water oxidation reaction mechanism at manipulable reaction rates.

3.4 Conclusions

The electron transfer kinetics study of IrOx NPs attached to SAMs on a flat electrode surface encourages further study with regard to control, manipulation, and mechanistic unraveling of water oxidation catalysis. Varying the chain length of NP-SAM assemblies provides a model for understanding the role of electron transfer rates in other water oxidation immobilized-catalyst schemes found in the literature.44 Such an investigation is necessary for ultimately optimizing water oxidation catalysis. More broadly, the measurement of ET rates for SAMs-attached IrOX NPs demonstrates that electroactive nanoparticles can be studied and controlled in a similar manner to small redox species.

3.5 Acknowledgement

This work was supported in part (theory and consultation by Steve W. Feldberg) by an award from NSF (CHE-0950320) and in part by the UNC EFRC: Center for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001011. I also

acknowledge a graduate research fellowship from the Eastman Chemical Company (Kingsport, TN) for summer of 2011.

3.6 References

(1) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103.

(2) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G.

J. Am. Chem. Soc. 1989, 111, 321.

(3) Eckermann, A. L.; Feld, D. J.; Shaw, J. A.; Meade, T. J. Coord. Chem. Rev. 2010,

254, 1769.

(4) Vericat, C.; Vela, M. E.; Benitez, G.; Carro, P.; Salvarezza, R. C. Chem. Soc. Rev.

2010, 39, 1805.

(5) Ulman, A. Chem. Rev. 1996, 96, 1533.

(6) Kasmi, A. E.; Wallace, J. M.; Bowden, E. F.; Binet, S. M.; Linderman, R. J. J. Am. Chem. Soc. 1998, 120, 225.

(7) Liu, B.; Bard, A. J.; Mirkin, M. V.; Creager, S. E. J. Am. Chem. Soc., 2004, 126, 1485.

(8) Smalley, J. F.; Sachs, S. B.; Chidsey, C. E. D.; Dudek, S. P.; Sikes, H. D.; Creager, S. E.; Yu, C. J.; Feldberg, S. W.; Newton, M. D. J. Am. Chem. Soc. 2004, 126, 14623. (9) Smalley, J. F.; Finklea, H. O.; Chidsey, C. E. D.; Linford, M. R.; Creager, S. E.;

Ferraris, J. P.; Chalfant, K.; Zawodzinsk, T.; Feldberg, S. W.; Newton, M. D. J. Am. Chem. Soc. 2003, 125, 2004.

(10) Smalley, J. F.; Feldberg, S. W.; Chidsey, C. E. D.; Linford, M. R.; Newton, M. D.; Liu, Y.-P. J. Phys. Chem. 1995, 99, 13141.

(11) Chidsey, C. E. D. Science. 1991, 251, 919.

(12) Collinson, M.; Bowden, E. F; Tarlov, M. J. Langmuir. 1992, 8, 1247. (13) Chen, S.; Deng, F. Proc. of SPIE, 2002, 4807, 93.

(14) Chen, S. J. Phys. Chem. B. 2000, 104, 663. (15) Peng, Z.; Qu, Z.; Dong, S. Langmuir, 2004, 20, 5. (16) Chan, E. W. L.; Yu, L. Langmuir, 2002, 18, 311.

(17) Malinsky, M. D.; Kelly, K. L. Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc.

(18) Sheibley, D.; Tognarelli, D. J.; Szymanik, R.; Leopold, M. C. J. Mater. Chem. 2005,

15, 491.

(19) Sardar, R.; Beasley, C. A.; Murray, R. W. Anal. Chem. 2009, 81, 6960.

(20) Nakagawa, T.; Beasley, C. A.; Murray, R. W. J. Phys. Chem. C. 2009, 113, 12958. (21) Gambardella, A. A.; Bjorge, N. S.; Alspaugh, V. K.; Murray, R. W. J. Phys. Chem. B.

2011, 115, 21659.

(22) Nakagawa, T.; Bjorge, N. S.; Murray, R. W. J. Am. Chem. Soc. 2009, 131, 15578. (23) Yagi, M.; Tomita, E.; Sakita, S.; Kuwabara, T.; Nagai, K. J. Phys. Chem. B. 2005,

109, 21489.

(24) Kondo, T.; Sumi, T.; Uosaki, K. J. Electroanal. Chem. 2002, 538, 59. (25) Zhong, C-J.; Porter, M. D. J. Electroanal. Chem. 1997, 425, 147.

(26) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860. (27) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (28) Bard, A. J.;, Faulkner, L. R. Electrochemical Methods: Fundamentals and

Applications, 2nd ed.; John Wiley & Sons, Inc.: New York, 2001, pp 96 and 591.

(29) Feldberg, S. W.; Rubinstein, I. J. Electroanal. Chem. 1988, 240, 1. (30) Laviron, E. J. Electroanal. Chem. 1979, 101, 19.

(31) Madhiri, N.; Finklea, H. O. Langmuir. 2006, 22, 10643.

(32) Haddox, R. M.; Finklea, H. O. J. Phys. Chem. B. 2004, 108, 1694.

(33) Haddox, R. M.; Finklea, H. O. J. Electroanal. Chem. 2003, 550-551, 351. (34) Murgida, D. H.; Hildebrandt, P. J. Am. Chem. Soc. 2001, 123, 4062.

(35) Zhang, W.; Rosendahl, S. M.; Burgess, I. J. J. Phys. Chem. C. 2010, 114, 2738. (36) Zhang, W.; Burgess, I. J. Phys. Chem. Chem. Phys. 2010, 13, 2151.

(38) Lemmer, C.; Bouvet, M.; Meunier-Prest, R. Phys. Chem. Chem. Phys. 2011, 13, 13327.

(39) Ludlow, M. K.; Soudackov, A. V.; Hammes-Schiffer, S. J. Am. Chem. Soc. 2010,

132, 1234.

(40) Abhayawardhana, A. D.; Sutherland, T. C. J. Phys. Chem. C. 2009, 113, 4915. (41) Clark, R. A.; Bowden, E. F. Langmuir. 1997, 13, 559.

(42) Gambardella, A. A.; Murray, R. W. unpublished results, UNC, Fall 2011.

(43) To allow comparison, the values reported are at the current density used in previous reports. (Ref. 21)

(44) Youngblood, W. J.; Lee, S.-H. A.; Kobayashi, Y.; Hernandez-Pagan, E. A.; Hoertz, P. G.; Moore, T. A.; Moore, A. L.; Gust, D.; Mallouk, T. E. J. Am. Chem. Soc. 2009,

131, 926.

(45) Youngblood, W. J.; Lee, S.-H. A.; Maeda, K.; Mallouk, T. E. Acc. Chem. Res. 2009,