Ozone- and Thermally Activated Films of Palladium Monolayer

Penner and co-workers demonstrated this type of behavior for an array of .... a film of C6 Pd MPCs and forming electrical contacts to the Au electrode...
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Langmuir 2006, 22, 9789-9796

9789

Ozone- and Thermally Activated Films of Palladium Monolayer-Protected Clusters for Chemiresistive Hydrogen Sensing Francisco J. Iban˜ez and Francis P. Zamborini* Department of Chemistry, UniVersity of LouisVille, LouisVille, Kentucky 40292 ReceiVed June 15, 2006. In Final Form: September 6, 2006

Here we describe the chemiresistive H2-sensing properties of drop-cast films comprised of 3.0 nm average diameter hexanethiolate-coated Pd monolayer-protected clusters (C6 Pd MPCs) bridging a pair of electrodes separated by a 23 µm gap. The gas-sensing properties were measured for 9.6-0.11% H2 in a H2/N2 mixture. The sensing mechanism is based on changes in the resistance of the film upon reaction of Pd with H2 to form PdHx, which is known to be larger in volume and more resistive than pure Pd. As-prepared Pd MPC films are highly insensitive to H2, requiring O3 and thermal treatment to enhance changes in film resistance in the presence of H2. Exposure to O3 for 15 min followed by activation in 100% H2 leads to an increase in film conductivity in the presence of H2, with a detection limit of 0.11% H2. When exposed to temperatures of 180-200 °C, the conductivity of the film increases and a decrease in conductivity occurs in the presence of H2 with a detection limit of 0.21%. The sensing behavior reverses after further heating to 260 °C, exhibiting an increase in conductivity in the presence of H2 as in O3-treated films and a detection limit of 0.11%. The sensitivity of the variously treated films follows the order O3 > high temp > low temp, and the response times at 1.0% H2 range from 10 to 50s, depending on the treatment. FTIR spectroscopy, Raman spectroscopy, and atomic force microscopy provide information about the C6 monolayer, Pd metal, and film morphology, respectively, as a function of O3 and heat treatment to aid in understanding the observed sensing behavior. This work demonstrates a simple chemical approach toward fabricating a fast, reversible sensor capable of detecting low concentrations of H2.

Introduction H2 is a feasible source of energy, which may someday replace or serve as an important alternative to the current fossil-based transportation fuels. It is also an important reagent in chemical industry. However, H2 has a very low flash point (-253 °C) making it highly explosive above 4 vol %.1 Accordingly, applications for H2 require sensors to detect H2 flow and leakage at early stages. Solid-state H2 sensors1 based on pure Pd or Pd-containing alloys2-8 have been thoroughly explored due to the interaction of Pd with H2 to form PdHx. This well-known, highly selective reaction has been exploited for H2 sensing because PdHx has a larger volume, different optical properties, lower work function, and higher resistance compared to pure Pd. Accordingly, optical,2-4,9-13 mass,1,14 and electronic6-8,15-32 transduction * To whom correspondence should be addressed. Email: f.zamborini@ louisville.edu. Fax: 502-852-8149. (1) Christofides, C.; Mandelis, A. J. Appl. Phys. 1990, 68, 1-30. (2) Zhao, Z.; Sevryugina, Y.; Carpenter, M. A.; Welch, D.; Xia, H. Anal. Chem. 2004, 76, 6321-6326. (3) Zhao, Z.; Carpenter, M. A.; Xia, H.; Welch, D. Sens. Actuators B 2006, 113, 532-538. (4) Zhao, Z.; Carpenter, M. A. J. Appl. Phys. 2005, 97, 124301. (5) Sun, Y.; Tao, Z.; Chen, J.; Herricks, T.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 5940-5941. (6) Hughes, R. C.; Schubert, W. K.; Zipperian, T. E.; Rodriguez, J. L.; Plut, T. A. J. Appl. Phys. 1987, 62, 1074-1083. (7) Hughes, R. C.; Schubert, W. K. J. Appl. Phys. 1992, 71, 542-544. (8) Huang, L.; Gong, H.; Peng, D.; Meng, G. Thin Solid Films 1999, 345, 217-221. (9) Garcia, J. A.; Mandelis, A. ReV. Sci. Instrum. 1996, 67, 3981-3983. (10) Be´venot, X.; Trouillet, A.; Veillas, C.; Gagnaire, H.; Cle´ment, M. Sens. Actuators B 2000, 67, 57-67. (11) Lin, H.; Gao, T.; Fantini, J.; Sailor, M. J. Langmuir 2004, 20, 51045108. (12) Kalli, K.; Othonos, A.; Christofides, C. ReV. Sci. Instrum. 1998, 69, 33313338. (13) Kalli, K.; Othonos, A.; Christofides, C. J. Appl. Phys. 2002, 91, 38293840. (14) Smith, A. L.; Shirazi, H. M. Thermochim. Acta 2005, 432, 202-211.

methods are most commonly used to detect H2 gas. The advantage of optical sensors is that they are highly sensitive at room temperature, operable in harsh or explosive environments, and useful for remote sensing.11 The drawback is the need for a light source, optics, and a photodetector, making them less portable. Electronic sensors are easily miniaturized using microfabrication technology, which makes them highly portable, but the operation environment is more limited. Electronic-based hydrogen sensors involving Pd include pyroelectronic,1 electrochemical,1,32 semiconductor/field-effect,26-31 and chemiresistive sensors.15-25 (15) Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 293, 2227-2231. (16) Walter, E. C.; Favier, F.; Penner, R. M. Anal. Chem. 2002, 74, 15461553. (17) Kaltenpoth, G.; Schnabel, P.; Menke, E.; Walter, E. C.; Grunze, M.; Penner, R. M. Anal. Chem. 2003, 75, 4756-4765. (18) Yun, M.; Myung, N. V.; Vasquez, R. P.; Lee, C.; Menke, E.; Penner, R. M. Nano Lett. 2004, 4, 419-422. (19) Yu, S.; Welp, U.; Hua, L. Z.; Rydh, A.; Kwok, W. K.; Wang, H. H. Chem. Mater. 2005, 17, 3445-3450. (20) Xu, T.; Zach, M. P.; Xiao, Z. L.; Rosenmann, D.; Welp, U.; Kwok, W. K.; Crabtree, G. W. Appl. Phys. Lett. 2005, 86, 203104. (21) Dankert, O.; Pundt, A. Appl. Phys. Lett. 2002, 81, 1618-1620. (22) Luongo, K.; Sine, A.; Bhansali, S. Sens. Actuators B 2005, 111-112, 125-129. (23) Morris, J. E.; Kiesow, A.; Hong, M.; Wu, F. Int. J. Electron. 1996, 81, 441-447. (24) Wolfe, D. B.; Love, J. C.; Paul, K. E.; Chabinyc, M. L.; Whitesides, G. M. Appl. Phys. Lett. 2002, 80, 2222-2224. (25) Sakamoto, Y.; Takai, K.; Takashima, I.; Imada, M. J. Phys.: Condens. Matter 1996, 8, 3399-3411. (26) Kong, J.; Chapline, M. G.; Dai, H. AdV. Mater. 2001, 13, 1384-1386. (27) Sayago, I.; Terrado, E.; Lafuente, E.; Horrillo, M. C.; Maser, W. K.; Benito, A. M.; Navarro, R.; Urriolabeitia, E. P.; Martinez, M. T.; Gutierrez, J. Synth. Met. 2005, 148, 15-19. (28) Mizsei, J.; Voutilainen, J.; Saukko, S.; Lantto, V. Thin Solid Films 2001, 391, 209-215. (29) Kolmakov, A.; Klenov, D. O.; Lilach, Y.; Stemmer, S.; Moskovits, M. Nano Lett. 2005, 5, 667-673. (30) Kang, W. P.; Gu¨rbu¨z, Y. J. Appl. Phys. 1994, 75, 8175-8181. (31) Dwivedi, D.; Dwivedi, R.; Srivastava, S. K. Sens. Actuators B 2000, 71, 161-168. (32) Lutz, B. J.; Fan, Z. H. Anal. Chem. 2005, 77, 4969-4975.

10.1021/la0617309 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/17/2006

9790 Langmuir, Vol. 22, No. 23, 2006 Scheme 1. Illustration of the Two Main Types of H2 Sensing for Pd-Based Chemiresistors.

Nanostructures have become increasingly important in electronic-based H2 sensing technology. For example, titania nanotubes were used to sense H2 by changes in resistance of the nanotubes due to electron injection into the titania upon H adsorption.33,34 Semiconducting p-type carbon nanotubes decorated with Pd nanoparticles sensed H2 on the basis of changes in the hole carrier mobility in the nanotubes when converting Pd into PdHx.26 Pd wires,15-18 nanotubes,19 and nanoparticles20 have been utilized in chemiresistive H2 sensors. In addition to lower costs and great miniaturization potential, a major benefit of nanoscale sensors is their fast response times due to fast diffusion of H into these high surface-to-volume ratio materials. Nanoscale sensors also often provide a lower limit of detection due to a larger change in their electronic properties upon surface adsorption and overall greater adsorptive properties. Chemiresistive sensors are the simplest type of electronic-based H2 sensor and the main focus of this study. Two main types of sensing mechanisms have been demonstrated previously with Pd-based chemiresistors, as shown in Scheme 1. Type I involves a well-connected Pd film whose resistance increases in the presence of H2 due to the formation of the more resistive PdHx material.25,35 The higher resistance is due to H2 adsorbing and dissociating into atomic H on the Pd surface then diffusing into the bulk of the Pd, forming PdHx with increased volume and a larger lattice constant.11 Evaporated,7 sputtered,19 and microcontact printed24 films of Pd have been shown to behave in this manner. These are considered traditional chemiresistive sensors for H2, and they are often plagued by moderate sensitivity and slow response times (often several minutes), depending on the film dimensions. Recently, Pd nanotube chemiresistors exhibited lower detection limits (500 ppm) and faster response times (a few to tens of seconds) when directly compared to thin sputtered Pd films due to more available adsorption sites and faster H diffusion in the nanotubes.19 Type II (A and B) in Scheme 1 represents the response to H2 observed for nanostructures containing high resistance break (33) Varghese, O. K.; Gong, D.; Paulose, M.; Ong, K. G.; Dickey, E. C.; Grimes, C. A. AdV. Mater. 2003, 15, 624-627. (34) Varghese, O. K.; Gong, D.; Paulose, M.; Ong, K. G.; Grimes, C. A. Sens. Actuators B 2003, 93, 338-344. (35) Lewis, F. A. The Palladium/Hydrogen System; Academic Press, Inc.: London, 1967.

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junctions or discontinuous Pd thin films. In this case, the Pd structures initially exhibit a very large resistance due to a break in the circuit. In the presence of H2, the formation of the PdHx lowers the resistance as expansion in volume creates a more connected structure with fewer high resistance junctions. Despite the fact that PdHx is more resistive than Pd, the formation of PdHx at the open junctions improves the conductivity by forming connections. Penner and co-workers demonstrated this type of behavior for an array of electrochemically synthesized Pd mesowires, which displayed good sensitivity to H2 and remarkably fast response times (20 ms to 5 s).15,16 Atomic force microscopy (AFM) images definitively showed that the increase in current in the presence of H2 was due to the mending of break junctions in the Pd wires (Type II-A). Others successfully prepared thin discontinuous deposited Pd films just below the percolation limit,20-23 which is analogous to disconnected wires. In the presence of H2, expansion of the Pd leads to films above the percolation threshold and the resistance of the film decreases dramatically (Type II-B). For example, Pd films evaporated onto a siloxane-based self-assembled monolayer (SAM) controllably below the percolation threshold sensed H2 in this manner down to 25 ppm with 70 ms response times.20 A benefit of the Type II sensing scheme is the extremely low background currents and fast response times due to the nanoscale dimensions of the structures. The goal in this work was to prepare films of chemically synthesized Pd monolayer-protected clusters (MPCs)36 for Type II-B H2 sensing. While the catalytic properties37 and H2 reactivity38 of synthetic Pd nanoparticles have been explored, there are surprisingly no reports on H2-sensing applications. MPCs are roughly spherical particles (diam ) 1-5 nm) comprised of a metal core surrounded by an organic alkanethiolate shell.39 We and others previously reported on the use of Au MPCs for chemiresistive sensing of volatile organic compounds (VOCs).40-55 With the exception of Pt,45 the use of other metal MPCs for (36) Zamborini, F. P.; Gross, S. M.; Murray, R. W. Langmuir 2001, 17, 481487. (37) Niu, Y.; Yeung, L. K.; Crooks, R. M. J. Am. Chem. Soc. 2001, 123, 6840-6846. (38) Pundt, A.; Suleiman, M.; Ba¨htz, C.; Reetz, M. T.; Kirchheim, R.; Jisrawi, N. M. Mater. Sci. Eng. 2004, B108, 19-23. (39) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (40) Ahn, H.; Chandekar, A.; Kang, B.; Sung, C.; Whitten, J. E. Chem. Mater. 2004, 16, 3274-3278. (41) Evans, S. D.; Johnson, S. R.; Cheng, Y. L.; Shen, T. J. Mater. Chem. 2000, 10, 183-188. (42) Han, L.; Daniel, D. R.; Maye, M. M.; Zhong, C.-J. Anal. Chem. 2001, 73, 4441-4449. (43) Han, L.; Shi, X.; Wu, W.; Kirk, F. L.; Luo, J.; Wang, L.; Mott, D.; Cousineau, L.; Lim, S. I.-I.; Lu, S.; Zhong, C.-J. Sens. Actuators B 2005, 106, 431-441. (44) Joseph, Y.; Besnard, I.; Rosenberger, M.; Guse, B.; Nothofer, H.-G.; Wessels, J. M.; Wild, U.; Knop-Gericke, A.; Su, D.; Schlogl, R.; Yasuda, A.; Vossmeyer, T. J. Phys. Chem. B 2003, 107, 7406-7413. (45) Joseph, Y.; Guse, B.; Yasuda, A.; Vossmeyer, T. Sens. Actuators B 2004, 98, 188-195. (46) Joseph, Y.; Krasteva, N.; Besnard, I.; Guse, B.; Rosenberger, M.; Wild, U.; Knop-Gericke, A.; Schlo¨gl, R.; Krustev, R.; Yasuda, A.; Vossmeyer, T. Faraday Discuss. 2004, 125, 77-97. (47) Krasteva, N.; Besnard, I.; Guse, B.; Bauer, R. E.; Mullen, K.; Yasuda, A.; Vossmeyer, T. Nano Lett. 2002, 2, 551-555. (48) Krasteva, N.; Guse, B.; Besnard, I.; Yasuda, A.; Vossmeyer, T. Sens. Actuators B 2003, 92, 137-143. (49) Krasteva, N.; Krustev, R.; Yasuda, A.; Vossmeyer, T. Langmuir 2003, 19, 7754-7760. (50) Leopold, M. C.; Donkers, R. L.; Georganopoulou, D.; Fisher, M.; Zamborini, F. P.; Murray, R. W. Faraday Discuss. 2004, 125, 63-76. (51) Vossmeyer, T.; Guse, B.; Besnard, I.; Bauer, R. E.; Mullen, K.; Yasuda, A. AdV. Mater. 2002, 14, 238-242. (52) Wohltjen, H.; Snow, A. W. Anal. Chem. 1998, 70, 2856-2859. (53) Zhang, H.-L.; Evans, S. D.; Henderson, J. R.; Miles, R. E.; Shen, T.-H. Nanotechnol. 2002, 13, 439-444. (54) Iban˜ez, F. J.; Growrishetty, U.; Crain, M. M.; Walsh, K. M.; Zamborini, F. P. Anal. Chem. 2006, 78, 753-761.

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vapor or gas sensing has been unexplored. There are two main advantages of Pd MPCs over Pd films and other nanostructures for H2 sensing. First, they are easily synthesized chemically on a benchtop and drop-cast deposited from solution across electrodes. This is much simpler compared to high-vacuum thermal or electron-beam deposition20-22 or electrochemical methods.15 Second, the alkanethiolate monolayer surrounding the Pd core serves as a well-controlled spacer that allows tuning of the electronic properties of the film by controlling the interparticle distance. This is important because the size of the Pd MPCs and length of the alkanethiolate spacer may be chemically controlled in order to create films just below the percolation limit so that the response to H2 by the Type II-B mechanism is maximized. These two advantages are important in terms of simplicity, reproducibility, and the success rate of the sensing devices. Experimental Section Chemicals. Hexanethiol (96%), sodium borohydride (99%), tetraoctylamonium bromide (99%), toluene (99.9%), and 2-propanol (99.9%) were purchased from VWR Scientific Products and used as received. Barnstead Nanopure water (17.8 MΩ‚cm) was employed for all aqueous solutions. Synthesis of Pd MPCs. Hexanethiolate-coated Pd MPCs (C6 Pd MPCs) were synthesized according to a modified Brust reaction56 as reported previously.36 Briefly, 0.40 g of K2PdCl4 was dissolved in 25 mL of water and 1.00 g of tetraoctylammonium bromide (TOABr) was dissolved in 150 mL of toluene. The two solutions were combined and stirred until all of the PdCl42- transferred into the toluene phase. The toluene phase was separated, and 90 µL of hexanethiol, corresponding to a 1:2 thiol/Pd ratio, was added to the toluene and stirred. The solution was cooled to ∼0 °C using an ice bath and a 10-fold excess of NaBH4 (0.46 g in 10 mL of water) with respect to Pd was added to the toluene solution with stirring. The solution turned black within a few seconds, indicating the formation of metallic Pd MPCs. Additional water (10 mL) was added, and the solution was stirred overnight. The toluene layer was separated and removed by rotary evaporation. The remaining black solid was suspended in 200 mL of acetonitrile and collected by filtration on a glass-fritted Bu¨chner funnel. The black solid product was washed with an additional 250 mL of acetonitrile and thoroughly dried before collecting. The average diameter of Pd MPCs prepared this way is 3.0 nm according to the literature.36 Sensor Device. Two Au electrodes separated by 23 µm were fabricated in a clean room by photolithography on a Si/SiOx substrate. Wire leads were attached to the Au contact pads with Ag epoxy (cured 12 h, 80 °C), which was further insulated with an overlayer of Torr-seal epoxy (cured 12 h, 80 °C). The electrode was cleaned by rinsing in acetonitrile, dichloromethane, acetone, ethanol, and 2-propanol before drying under N2. The device was then placed in a UVO ozone cleaner (Jelight Company Inc., Irvine, CA) for 10 min before drop-casting a film of C6 Pd MPCs from a 40 mg/mL toluene solution (1-3 drops). Gas-Sensing Experiments. Gas-sensing experiments were performed with a CH Instruments 660A (Austin, TX) electrochemical workstation operating in chronoamperometry mode. The current was monitored with time while a -0.3 V potential was applied between the two electrodes and the sample was exposed to alternating flows of pure N2 and different concentrations of H2/N2. Varied concentrations of hydrogen were obtained using a set of flow meters (Cole Palmer, 2% error at full scale) located between the sample and gas cylinders. The different concentrations of H2 and the total flow rates (H2 + N2) used were as follows. 9.6 ( 0.3% (3.1 ( 0.1 L‚min-1), 6.2 ( 0.2% (4.9 ( 0.1 L‚min-1), 3.2 ( 0.1% (4.7 ( 0.1 L‚min-1), (55) Zamborini, F. P.; Leopold, M. C.; Hicks, J. F.; Kulesza, P. J.; Malik, M. A.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 8958-8964. (56) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Chem. Commun. 1994, 7, 801-802.

Figure 1. Digital pictures of the electrode device before (A) and after (B) drop-casting a film of C6 Pd MPCs and forming electrical contacts to the Au electrodes. Optical microscope images of the gap between the Au electrodes before (C) and after (D) drop-casting C6 Pd MPCs. Scheme of electron hopping conductivity through the film of Pd MPCs is shown at the bottom. 1.0 ( 0.1% (4.7 ( 0.1 L‚min-1), 0.50 ( 0.02% (4.6 ( 0.1 L‚min-1), 0.21 ( 0.02% (4.6 ( 0.1 L‚min-1), and 0.11 ( 0.02% (4.6 ( 0.1 L‚min-1). Ozone Treatment. Samples containing drop-cast films of C6 Pd MPCs were placed in a UVO ozone cleaner (Jelight Company Inc., Irvine, CA) from 0 to 60 min at 15 min intervals. Sensing, FTIR, Raman, and AFM measurements were performed on samples either (1) directly after O3 treatment or (2) after O3 treatment followed by dipping in 2-propanol and drying under N2 (as indicated). Thermal Treatment. Samples containing drop-cast films were heated in air in a Model FB1315M Type 1300 muffle furnace (Barnstead International, Dubuque, IA) for 5 min at temperatures from 180 to 220 °C ((2 °C) at intervals of 20 °C, for 2 min at 240 °C, and then for 2 min at 260 °C for up to three times. Sensing, FTIR, Raman, and AFM measurements were obtained following each heat treatment. Characterization. Pd MPC films were drop-cast deposited onto Si(100)/TiW (50 Å)/Au(2000 Å), Si(100) with a native oxide layer, and Si/SiOx samples with interdigitated electrodes for FTIR spectroscopy, Raman spectroscopy, and AFM experiments, respectively. FTIR data were acquired using a Digilab FTS 7000 spectrometer (Varian, Cambridge, MA) in reflectance mode with a liquid N2-cooled MCT detector. Raman spectra were collected at 632.8 nm excitation using a Renishaw micro Raman system. AFM images were acquired with a Veeco Digital Instruments Nanoscope 3A multimode scanning probe microscope (Santa Barbara, CA) using a Si tip operating in tapping mode.

Results and Discussion Sensor Device. A digital picture of the full sensor device and an enlarged optical microscope image of the 23 µm electrode gap area (indicated by the box) are shown in panels A and C of Figure 1, respectively. Panels B and D of Figure 1 show a digital picture of the device and an enlarged optical microscope image of the electrode gap, respectively, after attachment of the wire leads and drop-casting of the Pd MPCs across the electrode gap. Upon applying a potential across the two electrodes, electronic

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current flows through the Pd MPCs by an electron hopping mechanism, as shown in the cartoon at the bottom of Figure 1. This well-documented conduction method described for Au MPCs depends on the cluster edge-to-edge distance and the dielectric of the material between the MPCs.55,57-59 The hexanethiolates therefore act as a spacer or tunneling barrier between the Pd MPCs. The conductivity of MPC films has been shown to vary exponentially with alkanethiolate chainlength, as expected for an electron-hopping process.55,57-59 Figure 2A (bottom trace, dashed box) shows a chronoamperometry (CA) plot of current versus time for a Pd MPC film as it is exposed to H2/N2 mixtures from 100% down to 0.11% H2. H2 “off” correlates with exposures to 100% N2, and H2 “on” correlates with exposure to H2 at the percent indicated in a H2/N2 mixture. The plot in Figure 2A is flat, showing no response to a flow of 100% H2 at the displayed current scale. However, the expanded plot in Figure 2B of the dashed box area from Figure 2A shows that the as-prepared film exhibits a small decrease in current in the presence of H2, although it is insensitive below 1.0% and there is a small drift in the baseline current over time. The low sensitivity could be due to one or more of four possibilities. First, the C6 monolayer may passivate the Pd surface, preventing complete formation of PdHx. Second, the H2 may not produce a large enough expansion of the Pd MPCs to cause a dramatic change in current. On the basis of the known expansion of ∼11 vol %,11,20 the diameter of the MPCs would increase from 3.0 to only 3.1 nm when saturated with H2. A 1 Å decrease in cluster-to-cluster distance could increase the conductivity significantly on the basis of previous studies of electron hopping through alkane chains;58,59 however, the formation of the more resistive PdHx could counteract it. Third, a decrease in current could be caused by a slight increase in cluster-to-cluster distance due to H2 partitioning into the alkane chains between the particles (instead of reacting with Pd), which is similar to what is observed for organic vapors.54,55 Finally, it is possible that a small amount of H may adsorb to the surface of Pd MPCs and increase the barrier for electron hopping between particles as described previously.21,23 In any case, the data show that as-prepared Pd MPCs do not react significantly with H2 to provide a Type II sensor response. While the highly controlled spatial properties of the hexanethiolate chains are desirable, the small size of the MPCs and barrier properties of the monolayer are detrimental. For this reason, we exposed C6 Pd MPC films to O3 and thermal treatment in order to promote the reaction between Pd and H2 by partial desorption of the C6 monolayer and film restructuring, which leads to greater H2 sensitivity as described in the following paragraphs. Effect of Ozone on H2 Response. Figure 2A (top trace) shows the chemiresistive response to H2 for a selected C6 Pd MPC film that was exposed to O3 for 15 min followed by a gentle 2-propanol rinse. The CA plot interestingly shows that the current increased by ∼2 orders of magnitude (4.26 × 10-9 to 1.82 × 10-7 A) once exposed to the flow of 100% H2. This is dramatically different when compared to the very small decrease in current observed with the as-prepared film (Figure 2A, bottom plot). Upon reexposure to N2, the current does not return to the original baseline, showing that this initially large response is an irreversible process. However, smaller, fast, and reversible increases in current occur (57) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.-h.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson Jr., C. S.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537-12548. (58) Wuelfing, W. P.; Green, S. J.; Pietron, J. J.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 11465-11472. (59) Zamborini, F. P.; Smart, L. E.; Leopold, M. C.; Murray, R. W. Anal. Chim. Acta 2003, 496, 3-16.

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Figure 2. Chronoamperometric plots (current vs time) for (A) asprepared Pd MPC film (bottom trace) and Pd MPC film exposed to O3 for 15 min and dipped in 2-propanol (top trace). (B) Zoom-in region of as-prepared Pd MPC film shown by the dashed rectangle in (A). (C) Zoom-in region of O3 treated film shown by the solid rectangle in (A). Samples were exposed to H2/N2 mixtures from 100% to 0.11% H2 by volume as indicated.

for subsequent H2 exposures in the concentration range of 9.60.11%, as shown in the expanded CA plot in Figure 2C. Although the sample shown in Figure 2 was rinsed with 2-propanol, we found that this step is not necessary, as discussed later. Effect of Thermal Treatment on H2 Response. Thermal treatment to the films also leads to improved responses to H2, as shown in Figure 3, where both Type I and Type II responses occur depending on temperature. The response before treatment is similar to that shown in Figure 2A (bottom) and B as already described. Figure 3A shows a CA plot for a selected sample that was heated at 180 °C for 5 min and 200 °C for 5 min. The current increased from 10-9 to the 10-6 A range following thermal treatment and the sensing behavior exhibits a very sharp, reversible decrease in current in the presence of H2 from 9.6% to 1.0%. The response from 0.50% to 0.11% H2 (Figure 3B) is observable but less pronounced and largely washed out by the sloping baseline. The response exhibits Type I behavior reminiscent of traditional Pd thin films. Upon further heating of the same film to 240 °C for 2 min and 260 °C three times for 2 min, the current decreased 1 order of magnitude to the 10-7 A range and the sensing behavior of the film reversed, where the current increased (resistance decreased) in the presence of H2 as in a Type II sensor. The first exposure to 9.6% H2 is slightly irreversible, while the response from 6.2% to 1.0% is well-pronounced and reversible (Figure 3C). Responses from 0.50% to 0.11% (Figure 3D) are clearly observed on top of the sloping baseline. Comparison Between Ozone and Thermal Treatment. The analytical signal used for chemiresistive sensors is % response

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Figure 3. Chronoamperometric plots showing the Type I response to (A) high and (B) low H2 concentrations for a Pd MPC film thermally treated at 180 and 200 °C for 5 min. Type II response to (C) high and (D) low H2 concentrations for the same Pd MPC film after further treatment at 240 °C for 5 min and 260 °C (3 times) for 2 min each.

Figure 4. Calibration curves showing % response versus (A) 0-1.0% and (B) 0-9.6% H2 concentration for all of the Pd MPC sensors described in this study. The curves represent the average of three devices, and the as-prepared sensor was omitted since it did not respond appreciably below 3.2% H2.

as described in the following equation,

% response ) (ir - ib)/ib × 100 ) ∆i/ib × 100 where ib is the initial baseline current in 100% N2, ir is the current in the presence of H2, and ∆i ) (ir - ib). A negative value is equal to a decrease in the current upon exposure to H2. Figure 4 shows the average calibration curves plotting the % response (y axis) versus the H2 concentration (x axis) for the O3- and

thermally treated samples (not forced through the origin). The points and curves represent the average of three samples. The standard deviations are omitted for clarity (full details in Table S-1 and Figure S-1 of the Supporting Information). The relative standard deviation in % response generally ranged from 20% to 50% with a few outliers; however, all samples undergoing the same treatment showed the same trend in sensing behavior. Frames A and B in Figure 4 show calibration curves for the low % H2 region (0 to 1.0%) and entire region (0 to 9.6%), respectively. PdHx formation involves a phase transition from the R phase for pure Pd and low H2 concentrations to the β phase at higher concentrations.1,35 Intermediate percentages contain a mixture of R and β phase PdHx. This phase transition and eventual H saturation leads to a nonlinear calibration curve over the entire H2 concentration as in Figure 4B. However, the low-concentration region is often quite linear as in Figure 4A.19 The phase transition has been reported to occur anywhere from 0.3%11 to 2.0%1,35 depending on the temperature, film thickness, and type of measurement. The calibration curves in Figure 4 are generally consistent with those reported in the literature and the phase dependence on H2 concentration. The slopes of the linear plots in Figure 4A reflect the following sensitivity order: O3 with 2-propanol rinse (1.7) > O3 no rinse (1.5) > high-temp Type II (1.0) > low-temp Type I (-0.06). Although all treatments lead to sensors responding below the explosive limit for H2, those exhibiting Type II behavior were most sensitive and had the lowest detection limit. The response time measured at 1.0% H2 for the various samples were 20-50, 50-60, and ∼10 s for O3-treated, high-temperature, and low-temperature treated samples, respectively. Table 1 summarizes the sensing characteristics of the three treatments tested. Spectroscopic and Microscopic Characterization. FTIR spectroscopy, Raman spectroscopy, and AFM provide information about the C6 monolayer, Pd metal, and film morphology, respectively, to better understand the effect of O3 and thermal treatment on the Pd MPC films and develop a model for the observed sensing behavior. We obtained surface reflectance FTIR

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Table 1. H2 Sensing Characteristics of Differently Treated C6 Pd MPC Films

treatment type O3 + rinse O3 no rinse high temp low temp

response direction current increase current increase current increase current decrease

limit of detection (% H2)

sensitivity

response time at 1.0% H2 (s)

0.11

1.7

20-50

0.21

1.5

20-50

0.11

1.0

50-60

0.21

-0.06

8-10

data on a C6 Pd MPC film drop-cast deposited onto a reflective Au surface as a function of time in O3 to determine its effect on the monolayer of hexanethiolates (C6) surrounding the Pd clusters in the film. Figure 5A shows two plots of the % loss of hexanethiolates in the Pd MPC film versus time the film was exposed to O3 from 0 to 60 min. The % loss was determined by the change in peak height of the asymmetric CH2 stretch at ∼2920 cm-1 of the C6 monolayer measured from the FTIR spectrum. As indicated, the difference between the two plots is that one film was dipped in 2-propanol and dried under N2 after O3 treatment while the other one was not treated further. O3 is known to oxidize thiolates to sulfonates60-64 and readily remove organics from surfaces. On the basis of previous O3 studies on selfassembled monolayers (SAMs), the 2-propanol was expected to improve the removal of hexanethiolates that were oxidized to sulfonates but not desorbed from the Pd MPCs.63 Consistent with these studies, there is a larger average decrease in the asymmetric CH2 stretch for the sample rinsed with 2-propanol, but the difference is not dramatic. The data show a 60-70% decrease in the monolayer after 1 h in O3, indicating that some of the alkane chains remain on the surface (likely as alkyl sulfonates). After 15 min in O3 (the treatment used for sensing), the loss of hexanethiolates is 40-60%. AFM images (Figure S-2, Supporting Information) of nontreated and ozone-treated Pd MPC films before and after H2 show morphological changes only in ozone-treated films. For example, the same area of a nontreated Pd MPC film is indistinguishable before and after exposure to 100% H2. In contrast, the same area of the same film shows small changes in film morphology after 15 min in O3 and much larger changes, including the appearance of numerous islands on the surface, after subsequent exposure to 100% H2. FTIR spectroscopy, Raman spectroscopy, and AFM provide evidence about the C6 monolayer, the Pd metal, and film morphology, respectively, during thermal treatment. Figure 5B shows a plot of % loss of hexanethiolates (assessed from the peak height of the asymmetric CH2 stretch) as a function of temperature from 180 to 260 °C. The two points where the sensing behavior changes significantly (Figure 3) are 200 and 260 °C, where the loss of hexanethiolates is ∼40% and >90%, respectively. AFM images over this temperature range (Figure S-3, Supporting Information) show that the film collapses slightly and undergoes small morphological changes from 180 to 220 °C and more significant changes occur at higher temperatures; several islands appear on the surface at 240 °C and grow larger after (60) Norrod, K. L.; Rowlen, K. L. J. Am. Chem. Soc. 1998, 120, 2656-2657. (61) Poirier, G. E.; Herne, T. M.; Miller, C. C.; Tarlov, M. J. J. Am. Chem. Soc. 1999, 121, 9703-9711. (62) Schoenfisch, M. H.; Pemberton, J. E. J. Am. Chem. Soc. 1998, 120, 45024513. (63) Zhang, Y.; Terril, R. H.; Tanzer, T. A.; Bohn, P. W. J. Am. Chem. Soc. 1998, 120, 2654-2655. (64) Zhang, Y.; Terrill, R. H.; Bohn, P. W. Chem. Mater. 1999, 11, 21912198.

Figure 5. (A) % loss of hexanethiolates as a function of time in O3 for C6 Pd MPC films rinsed (triangles) and not rinsed (squares) with 2-propanol after O3 treatment. (B) % loss of hexanethiolates as a function of temperature treatment (5 min at 180-220 °C and 2 min at 240-260 °C). % loss of hexanethiolates is determined by the change in the peak height of the asymmetric CH2 stretch at ∼2920 cm-1 from the surface reflectance FTIR spectra. The average and standard deviations are from three samples measured.

Figure 6. (A) Intensity of Pd-O stretch in the Raman spectrum as a function of temperature for a Pd MPC film treated with temperature only (red line) and for a Pd MPC film treated with temperature and exposed to 100% H2 for 5 min after treatment at 260 °C (blue, dashed line). Values are the average of three samples and offset for comparison purposes. (B) Actual Raman spectra for one of the samples treated with temperature and then exposed to 100% H2 for 5 min.

heating to 260 °C. In addition, a small crack in the film appears and grows wider over the temperature range. We used Raman spectroscopy to determine if PdO formation occurs during thermal treatment. Figure 6A shows the Raman intensity of the Pd-O stretch measured at 651 cm-1 of a Pd MPC film thermally treated from 180 to 260 °C (same conditions used for the sensing

Films of Palladium Monolayer-Protected Clusters

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Scheme 2. Proposed Sensing Mechanism for (A) O3 Treatment and (B) Low-Temperature Treatment.

experiments). The top line shows the Pd-O intensity of a film treated with the indicated temperatures only (never exposed to H2), while the bottom line is that for a film treated with the indicated temperature and exposed to 100% H2 for 5 min after the 260 °C treatment. Figure 6A represents the average of three samples, while Figure 6B shows the Raman spectra for one representative sample. Interestingly, the only evidence of Pd oxide formation in the Raman occurs after 260 °C, which is the same temperature where the sensing response reversed its behavior in Figure 3. The samples heated to 260 °C and then exposed to H2 had a smaller amount of PdO on average (Figure 6A) and complete loss of PdO for the sample in Figure 6B, which is likely due to reduction of the oxide by H2.65 Incomplete reduction of PdO by H2 may occur during this process due to the formation of a Pd shell on top of PdO.65 Sensing Mechanisms. Scheme 2A and B illustrates the effect of O3 and low temperature on the Pd MPC films and provides a sensing mechanism for these two treatments, respectively. For the case of Pd MPC films treated for 15 min with O3, the sensing data in Figure 2A show that the current displays a large irreversible increase by ∼2 orders of magnitude upon exposure to 100% H2. Although a recent study showed that an alkanethiolate SAM on a microcontact-printed Pd film was transparent to H2, we believe that in our system the strongly bound C6 monolayer initially acts as a poison to the Pd catalyst, preventing reactivity with H2. The O3-induced loss of 40-60% of the C6 monolayer, as shown in the FTIR data, removes the poisoning effect and promotes the reactivity of Pd with H2 to form PdHx, resulting in a large increase in film conductivity. Since PdHx formation is a reversible process, the irreversibility of the current increase shows that some other process also occurs. One possibility was that O3 treatment led to partial PdO formation, which was reduced back to Pd upon exposure to H2.65 Raman spectroscopy, however, showed no evidence of a Pd-O stretch at 651 cm-1 after O3 exposures up to 60 min, and the conductivity of the film did not change (before H2 exposure) as might be expected for oxide formation, ruling this possibility out. Another potentially irreversible process is the reduction of sulfonates back to thiolates in the presence of H2. This may occur, but it is not likely to result in a 2 order of magnitude increase in current. We believe that restructuring of the film upon the first exposure to 100% H2 accounts for the large irreversible increase in current. The large morphology change observed in the AFM image after O3 and 100% H2 (65) Su, C. S.; Carstens, J. N.; Bell, A. J. Catal. 1998, 176, 125-135.

exposure supports this conclusion, and structural changes within Pd are well-known upon exposure to and removal from 100% H2.35 Importantly, fast reversible responses to H2 occur following the irreversible process, as shown in Figure 2C. Taken together, the data are consistent with the proposed mechanism illustrated in Scheme 2A. O3 removes part of the monolayer and allows Pd clusters to react, restructure, and expand in the presence of H2. This leads to an initially large irreversible increase in current. Subsequent exposures to smaller H2 concentrations lead to a reversible decrease in film resistance due to lowering of the electron hopping barrier between particles upon expansion of Pd during PdHx formation in a manner analogous to discontinuous thin films (Scheme 1, Type II-B). The FTIR and AFM data support the model by showing that O3-induced loss of the C6 monolayer leads to enhanced sensing response to H2 and significant morphological changes to the film in the presence of H2. Scheme 2B illustrates a model for the sensing behavior of the film following thermal treatment at 200 °C based on the FTIR, AFM, Raman, and sensing data. The FTIR indicates that ∼40% of the C6 monolayer desorbs from the surface at 200 °C (Figure 5B), which is similar to the 15 min O3 treatment (Figure 5A). However, the conductivity of Pd films after 200 °C increased by 3 orders of magnitude (Figure 3), whereas the conductivity of O3-treated films (before exposure to 100% H2) remained about the same (Figure 2). This is due to thermal annealing of the Pd metal, which accompanies thermal desorption of the C6 monolayer and leads to a significant decrease in film resistance, as evidenced by the small morphology changes of the Pd MPC film in the AFM image at 200 °C (Figure S-3, frame C). As illustrated in Scheme 2B, thermal desorption of C6 and annealing of the film leads to a more continuous, well-connected film exhibiting microamp currents and sensing behavior reminiscent of conventionally prepared thin Pd films with Type I behavior (Scheme 1), where the current decreases in the presence of H2 (Figure 3A and B). Figure 3C and D shows that after treatment at 260 °C the conductivity of the film decreases and the sensing reversed its behavior by displaying an increase in current in the presence of H2 similar to Type II films. Unfortunately, we do not have a model to explain this interesting behavior, which coincides with nearly full removal of the C6 monolayer from Pd as evidenced by FTIR and the formation of PdO as evidenced by Raman spectroscopy. In addition, the AFM shows the development of

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large islands on the surface at high temperatures, whose composition is unknown (pure Pd or PdO), and crack formation throughout the temperature range. It is not certain which of these observations is responsible for the reversed sensing behavior. The removal of thiolates does not likely play a dominant role since we usually have to heat the sample two to three times (2 min each time) at 260 °C. Since thiolate desorption occurs fairly rapidly and is complete after one heating at 260 °C, multiple heatings would not be necessary if it was responsible for the sensing change. The drop in conductivity after heating at 260 °C suggests that increased heating and then cooling may lead to the formation of break junctions or discontinuous films, which would explain the decreased resistance in the presence of H2 (Type II sensors). This seems plausible, except that on one occasion the current through the film did not change after several treatments at 260 °C, but the sensing response still reversed its behavior. Other than the appearance of a large crack in the AFM image, we have no evidence to support the formation of small nanometerscale discontinuities in the film with high thermal treatment. More work is clearly needed to better understand the behavior at high temperature, but we believe that PdO formation plays a key role and that morphology changes and thiolate desorption may also contribute to some degree.

Conclusions We prepared drop-cast films of chemically synthesized C6 Pd MPCs between two Au electrodes separated by a 23 µm gap and measured the chemiresistive sensing response to H2 at concentrations of 9.6-0.11% in a H2/N2 mixture. The as-prepared MPC films are initially highly insensitive to H2. O3 treatment and activation in 100% H2 leads to a fast, reversible, Type II sensor that detects H2 down to 0.11% by a decrease in the film resistance. The improved response is associated with partial removal of the C6 monolayer surrounding the Pd MPCs and activation/ restructuring of the film in 100% H2. Thermal treatment leads to two different responses, depending on the temperature. At 200 °C, partial removal of hexanethiolates and thermal annealing creates a Type I H2 sensor whose resistance increases in the presence of H2. At 260 °C, the hexanethiolates completely desorb

Iban˜ ez and Zamborini

from the Pd MPCs and Pd partially oxidizes, leading to a Type II sensor with a resistance decrease in H2. All treatment methods detect H2 well below the explosive level with response times high temperature > low temperature. The sensors are durable, although the response decreases over a 4 month period (Figure S-4, Supporting Information), and are operable when fabricated with microscale dimensions using Ag paint as contacts (Figure S-5, Supporting Information). A major benefit of these sensors is that Pd MPCs can be chemically synthesized in large quantities, enabling easy mass production of devices by simply drop-casting Pd MPCs from solution across electrodes. Further details about the sensing mechanism are still needed. Future experiments will explore H2 sensing with different pure Pd and Pd alloy MPCs as a function of cluster size and type of thiolate surrounding the particles to determine the optimal conditions in terms of sensitivity, detection limit, response time, and cost. Acknowledgment. We gratefully acknowledge the National Science Foundation (CHE-0518561) and the Kentucky Science and Engineering Foundation for partial support of this research. We also acknowledge Guillermo Cohen from HDC Maquinola LLC for providing instrumentation and valuable input regarding the gas sensing design. We thank Kevin M. Walsh and Mark M. Crain from the Department of Electrical and Computer Engineering at the University of Louisville for use of the clean room to microfabricate the electrode devices. We also thank Dr. Gamini Sumanasekera and Romaneh Jalilian from the Department of Physics at the University of Louisville for assistance in acquiring the Raman spectroscopy data. Supporting Information Available: Table and bar graph showing the % response to H2 for each sample with averages and standard deviations, AFM images of nontreated and O3-treated Pd MPC films before and after exposure to 100% H2, AFM images of thermally treated Pd MPC films, chronoamperometry plots showing response to H2 over time (device durability), and the response to H2 using microscale films of Pd MPCs connected with Ag paint. This material is available free of charge via the Internet at http://pubs.acs.org. LA0617309