pubs.acs.org/Langmuir © 2011 American Chemical Society
Frequency Effects on the Surface Coverage of Nitrophenyl Films Ultrasonically Grafted onto Indium Tin Oxide Fakhradin Mirkhalaf,*,† Timothy J. Mason,† David J. Morgan,‡ and Veronica Saez† †
Sonochemistry Centre, Faculty of Health and Life Sciences, Coventry University, Coventry CV1 5FB, U.K., and ‡Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Cardiff CF10 3AT, U.K. Received November 4, 2010. Revised Manuscript Received December 13, 2010
The covalent attachment of various organic molecules on conductive supports including indium tin oxide (ITO) using diazonium chemistry has been known for many years. A commonly used method to achieve this is the electrochemical reduction of diazonium compounds leading to radicals, followed by binding of the radicals to the support. In the present report, an alternative method using ultrasound at different frequencies (20, 582, 863, and 1142 kHz) to induce the surface grafting of nitrobenzene diazonium onto an ITO surface is described. It is shown that the grafting on ITO is more efficient in the lower ultrasonic frequency range that is ascribed to changes in the balance between the physical and chemical effects of cavitation with frequency. Both the physical and chemical effects of cavitation play important roles at all frequencies, but at high ultrasound frequencies, the physical effects are relatively small. At 20 kHz, the physical component, including mass transport, is larger than at higher frequencies, and mechanisms based on these observations have been proposed. Ultrasonic grafting has an advantage over electrografting in that it may provide more control over surface coverage, thus it is suggested that the ultrasonic method is used where the surface concentration of the organic layer must be controlled.
1. Introduction The surface modification of carbon and metals by the electrochemical reduction of diazonium compounds has been a topic of considerable research interest over the last two decades.1-3 Diazonium-mediated surface modification relies on the generation of a phenyl radical species through the reduction of the diazonium group followed by the elimination of N2.The reduction of diazonium salts allows the covalent bonding of organic molecules to solid supports through the formation of aryl radicals. This method has been used to modify carbon materials,4-10 metals,11,12 silicon,13,14 gallium arsenide,15 and organic materials such as PTFE and polyaniline.16 There are several factors that influence the structure of the film that is formed, including the type and crystalline structure of the support *Corresponding author. E-mail:
[email protected]. Phone: þ44-24 7688 8624. Fax: þ44-24 7688 8173.
(1) Delmar, M.; Hitmi, R.; Pinson, J.; Saveant, J.-M. J. Am. Chem. Soc. 1992, 114, 5883–5884. (2) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J.-M. J. Am. Chem. Soc. 1997, 119, 201–207. (3) Liu, Y.-C.; McCreery, R. L. J. Am. Chem. Soc. 1995, 117, 11254–11259. (4) Solak, A. O.; Eichorst, L. R.; Clark, W. J.; McCreery, R. L. Anal. Chem. 2003, 75, 296–305. (5) Downard, A. J.; Prince, M. J. Langmuir 2001, 17, 5581–5586. (6) Kariuki, J. K.; McDermott, M. T. Langmuir 2001, 17, 5947–5951. (7) Marwan, J.; Addou, T.; Belanger, D. Chem. Mater. 2005, 17, 2395–2403. (8) Liu, S.; Tang, Z.; Shi, Z.; Niu, L.; Wang, E.; Dong, S. Langmuir 1999, 15, 7268–7275. (9) Bahr, J. L.; Tour, J. M. Chem. Mater. 2001, 13, 3823–3824. (10) Kuo, T.; McCreery, R. L.; Swain, G. M. Electrochem. Solid-State Lett. 1999, 2, 288–290. (11) Bernard, M.-C.; Chausse, A.; Cabet-Deliry, E.; Chehimi, M. M.; Pinson, J.; Podvorica, F.; Vautrin-Ul, C. Chem. Mater. 2003, 15, 3450–3462. (12) Hurley, B. L.; McCreery, R. L. J. Electrochem. Soc. 2004, 151, B252–B259. (13) Hartig, P.; Dittrich, T.; Rappich, J. J. Electroanal. Chem. 2002, 524-525, 120-126. (14) Ahlberg, E.; Helgee, B.; Parker, V. D. Acta Chem. Scand. 1980, B34, 181–186. (15) Stewart, M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.; Stapleton, J. J.; McGuiness, C. L.; Allara, D. L.; Tour, J. M. J. Am. Chem. Soc. 2004, 126, 370–378. (16) Combellas, C.; Kanoufi, F.; Mazouzi, D.; Thiebault, A.; Bertrand, P.; Medard, N. Polymer 2002, 44, 19–24.
Langmuir 2011, 27(5), 1853–1858
material, diazonium salt concentration, media (aprotic, aqueous, electrolytes used, solution pH), modification potential, reaction time, and nature of the diazonium derivative employed.17 A change in any of these factors directly affects the surface concentration obtained and hence the compactness of the film. The nature of the bond formed when the radical species interacts with a surface site is very much dependent on the surface material. In particular, when the surface in question has free electrons and if electron transfer is thermodynamically favorable, then a spontaneous reduction of the diazonium salt can take place, resulting in a self-sustaining surface-modification reaction.18 The resulting molecular layers are generally robust, and the modified surfaces can be used in many different studies. However, it should be noted that the aggressive nature of the radical-mediated bonding pathway readily leads to multilayer films. The thickness of these films can be controlled by adjusting the conditions and measured using atomic force microscopy.18 The attachment of organic molecules to solid surfaces including semiconductors, metals, and metal oxide surfaces is of great importance in the development of electronics and sensors. The importance of modified ITO surfaces has grown rapidly during the past several years because of the development of electroanalytical sensors, organic light-emitting diodes (OLED), and new thin film photovoltaics (PV) where rates of electron transfer to adsorbed or covalently attached molecules are critical. For this reason, currently there is considerable effort being put into developing efficient deposition procedures. Ideally, these should be robust, sustainable, and green. Sonochemistry is an emerging technology capable of enhancing radical formation, improving mass transport, and affecting surface activity.19 The technology is based upon the effects resulting from the collapse of acoustic cavitation bubbles that generate regions of extremely high local temperature and pressure.19 (17) Pinson, J.; Podvorica, F. Chem. Soc. Rev. 2005, 34, 429–439. (18) Mahmoud, A. M.; Bergren, A. J.; McCreery, R. L. Anal. Chem. 2009, 81, 6972–6980.
Published on Web 01/10/2011
DOI: 10.1021/la104402z
1853
Article
Mirkhalaf et al.
This can cause homogeneous ruptures of covalent bonds and result in the formation of radicals that can enter into a great variety of reactions.19 It is expected that sonochemical effects will be frequency-dependent because of the influence of frequency on the collapse time and hence the size of the bubbles.20 Chehimi et al. have recently reported the ultrasonically assisted electroless grafting of 4-nitrophenyl groups onto diamond nanoparticles.21 This was achieved by simple insonation of diamond nanoparticles dispersed in an aqueous solution in the presence of the corresponding diazonium compounds in a 37 kHz ultrasonic cleaning bath.21 To our knowledge, this is the only example reported to date of the use of ultrasound for the modification of solid surfaces by the decomposition of diazonium derivatives. However, no report was found in the literature on the effects of changes in the frequency of ultrasound on the progress of the decomposition of diazonium derivatives. In the present work, 4-nitrophenyl groups (NP) were attached to indium tin oxide (ITO) surfaces (designated as ITO/NP) by the decomposition of the 4-nitrobenzene diazonium (NBD) salt achieved by both electrochemical and sonochemical induction. A comprehensive investigation has been carried out using the XPS technique in order to study the effects of changes in the ultrasound frequency on the surface coverage of nitrophenyl groups grafted onto ITO surfaces.
2. Experimental Section Indium tin oxide-coated glass slides were purchased from Optics Balzers GmbH (Germany) with a resistance of 50 Ω cm-2 and were cut to 2.5 0.9 cm2. ITO surfaces were cleaned by dipping in piranha solution (H2SO4/H2O2 3:1) for 30 s followed by extensive rinsing with pure water. (Caution! Piranha solution is a strong oxidizing agent.) The unmodified and cleaned ITO surfaces were then dried in air and dipped in 10 mM nitrobenzene diazonium (NBD) in acetonitrile (AN). The slides were then exposed to ultrasound (US) using either a probe system (20 kHz with a tip diameter of 1.27 cm; Sonics, Sonics & Materials; model VCX600) or a multifrequency bath (582, 863, 1142 kHz; Meinhardt E805TM) for 15 min. In each case, a constant distance (30 mm) was maintained between the surface and the emitting face. All experiments were performed with a constant ultrasonic power of 20 W/dm2 as measured by calorimetry. During the irradiation, a constant temperature of 21 ( 2 °C was maintained in the solution by circulating water via a water jacket around the cell. The slides were then washed with acetonitrile (AN) and water several times in order to remove physically adsorbed compounds and then dried in air. XPS spectra were obtained with a Kratos Axis Ultra-DLD system (Kratos Analytical, vacuum of
