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A Porous Silicon-Palladium Composite Film for Optical Interferometric Sensing of Hydrogen Haohao Lin, Ting Gao, Joshua Fantini, and Michael J. Sailor* Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093-0358 Received January 29, 2004. In Final Form: April 5, 2004 Porous Si Fabry-Pe´rot films are coated with Pd via immersion plating. The materials are characterized by electron microscopy and infrared spectroscopy. The Pd-coated porous Si samples exhibit distinct FabryPe´rot fringes in the optical reflection spectrum due to thin film optical interference in the porous Si layer, though the reflectivity spectrum loses fidelity upon Pd coating. The effect of H2 exposure on the interference spectrum is studied. Absorption of hydrogen into Pd induces a lattice expansion, which results in a shift of the optical fringes and a decrease in the reflected intensity. The detection limit measured at room temperature is ∼0.2% (by volume) in an N2 carrier gas, with a response time of a few seconds.
Introduction Large quantities of hydrogen are used commercially in the Haber ammonia process, the hydrogenation of fats and oils, hydrodealkylation, hydrocracking, and hydrodesulfurization. Hydrogen is also attracting increased attention as a clean energy source with a high heat content.1 The explosive limits of hydrogen in air range between 4.65 and 93.9 vol % H2.2 Therefore, it is important to develop a safe and reliable sensor to monitor hydrogen in and below this range.3,4 Recently, palladium mesowire arrays,5 Pd-coated nanotubes,6 and Pd-coated porous Si7 have been used to detect hydrogen. Most H2 sensors are based on electrical or optical phenomena. In general, optical sensors are preferable over electrical sensors when used in an explosive atmosphere, and they are amenable to remote sensing applications.4 Herein, we report an optical hydrogen sensor based on Pd-coated porous Si. When prepared appropriately, thin porous Si films can exhibit Fabry-Pe´rot fringes in their reflection spectra.8 The spectral position of the fringes depends on the refractive index of the film, and an analyte can be detected by monitoring the shift in these fringes when it enters the porous matrix.8 On the basis of this principle, sensors for biomolecules,9-13 HF,14 chemical * Corresponding author. E-mail:
[email protected]. (1) Yu¨ru¨m, Y., Ed. Hydrogen energy system: production and utilization of hydrogen and future aspects; Kluwer Academic Publishers: Boston, MA, 1995; Vol. 295. (2) Weast, R. C., Ed. Handbook of Chemistry and Physics, 56th ed.; CRC Press, Inc.: Boca Raton, FL, 1976. (3) Lundstrom, I.; Armgarth, M.; Petersson, L.-G. CRC Crit. Rev. Solid State Mater. Sci. 1989, 15, 201-278. (4) Christofides, C.; Mandelis, A. J. Appl. Phys. 1990, 68, R1-R30. (5) Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 293, 2227-2231. (6) Kong, J.; Chapline, M. G.; Dai, H. Adv. Mater. 2001, 12, 13841386. (7) Tsamis, C.; Tsoura, L.; Nassiopoulou, A. G.; Travlos, A.; Salmas, C. E.; Hatzilyberis, K. S.; Androutsopoulos, G. P. IEEE Sens. J. 2002, 2, 89-95. (8) Curtis, C. L.; Doan, V. V.; Credo, G. M.; Sailor, M. J. J. Electrochem. Soc. 1993, 140, 3492-3494. (9) Lin, V. S.; Motesharei, K.; Dancil, K. S.; Sailor, M. J.; Ghadiri, M. R. Science 1997, 278, 840-843. (10) van Noort, D.; Welin-Klintstrom, S.; Arwin, H.; Zangooie, S.; Lundstrom, I.; Mandenius, C.-F. Biosens. Bioelectron. 1998, 13, 439449. (11) Dancil, K.-P. S.; Greiner, D. P.; Sailor, M. J. J. Am. Chem. Soc. 1999, 121, 7925-7930.
warfare agents,15 and condensable vapors16-19 have been developed. Using this methodology, porous Si by itself is not expected to detect gases, such as hydrogen, because most noncondensable gases have very similar refractive indices and will not induce a detectable shift of the FabryPe´rot fringes. Pd can interstitially dissolve H2 up to ∼1300 relative volumes (H/Pd ) 1.0, 50 atom % H) and even more under certain conditions.20,21 In the present study, Pd was plated on porous Si for use as a hydrogen trap, after a method first demonstrated by Tsamis et al.7 Because the absorption of hydrogen results in a significant expansion of the Pd lattice and a concomitant change in the refractive index and the optical extinction coefficient, hydrogen can be detected by the composite Pd/porous Si structures using optical interferometric methods similar to those previously reported.9,11,14 For most optical-based hydrogen sensors, hydrogen is detected by the measurement of either a change in intensity or frequency of light reflected from or transmitted through an active medium such as palladium.22-31 These (12) Chan, S.; Fauchet, P. M.; Li, Y.; Rothberg, L. J.; Miller, B. L. Phys. Status Solidi A 2000, 182, 541-546. (13) Chan, S.; Horner, S. R.; Miller, B. L.; Fauchet, P. M. J. Am. Chem. Soc. 2001, 123, 11797-11798. (14) Le´tant, S.; Sailor, M. J. Adv. Mater. 2000, 12, 355-359. (15) Sohn, H.; Le´tant, S.; Sailor, M. J.; Trogler, W. C. J. Am. Chem. Soc. 2000, 122, 5399-5400. (16) Gao, J.; Gao, T.; Sailor, M. J. Appl. Phys. Lett. 2000, 77, 901903. (17) Gao, T.; Gao, J.; Sailor, M. J. Langmuir 2002, 18, 9953-9957. (18) Allcock, P.; Snow, P. A. J. Appl. Phys. 2001, 90, 5052-5057. (19) Snow, P. A.; Squire, E. K.; Russell, P. S. J.; Canham, L. T. J. Appl. Phys. 1999, 86, 1781-1784. (20) Moore, G. A. Trans. Electrochem. Soc. 1939, 75, 237-267. (21) Hansen, M. Constitution of Binary Alloys; McGraw-Hill Book Company: New York, 1958. (22) Butler, M. A. Appl. Phys. Lett. 1984, 45, 1007-1009. (23) Bevenot, X.; Trouillet, A.; Veillas, C.; Gagnaire, H.; Clement, M. Sens. Actuators, B 2000, 67, 57-67. (24) Tabib-Azar, M.; Sutapun, B.; Petrick, R.; Kazemi, A. Sens. Actuators, B 1999, 56, 158-163. (25) Tobiska, P.; Hugon, O.; Trouillet, A.; Gagnaire, H. Sens. Actuators, B 2001, 74, 168-172. (26) Bevenot, X.; Trouillet, A.; Veillas, C.; Gagnaire, H.; Clement, M. Meas. Sci. Technol. 2002, 13, 118-124. (27) Sutapun, B.; Tabib-Azar, M.; Kazemi, A. Sens. Actuators, B 1999, 60, 27-34. (28) Villatoro, J.; Diez, A.; Cruz, J. L.; Andres, M. V. Electron. Lett. 2001, 37, 1011-1012. (29) Wang, C.; Mandelis, A.; Au-Ieong, K. P. Sens. Actuators, B 2001, 73, 100-105.
10.1021/la049741u CCC: $27.50 © 2004 American Chemical Society Published on Web 05/14/2004
Porous Si Sensor for H2
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measurements are susceptible to a variety of errors. Instead of measuring the absolute value of hydrogen concentration, some techniques measure only a relative change in hydrogen concentration. A few advanced designs, such as a Mach-Zender interferometer, use a reference arm together with a sensing arm in order to reduce measurement error.32-34 In the present study, sensing of hydrogen is achieved by simultaneously monitoring the wavelength and intensity of the Fabry-Pe´rot fringes, obtained from the interferometric reflectance spectrum of a Pd-coated porous Si film. The intensity of the fringes depends on the reflectivity of the Pd/porous Si composite, and their wavelength is dependent in part on the refractive index of the Pd film. Since this latter property is an intrinsic bulk property of the Pd/porous Si composite, the interference measurement is independent of factors such as the incident light intensity or the distance from the light source to the detector. The relatively simple design based on Pd-coated porous Si provides a reliable means of sensing hydrogen well below the explosive limit. In addition, the sensor design is amenable to remote sensing.35 The response time of the sensor is also quantified. It is found to be shorter than that of the Pd-coated porous Si conductivity-based sensors previously reported, although the sensitivity in the present system is not as high.36 Due to the specificity of the Pd/H system, the sensor is highly selective to H2.
temperature (nominally 21 °C). Interferometric reflectance spectra of the porous Si samples were obtained using an Ocean Optics S2000 fiber optic spectrometer. A tungsten light source was focused to a spot size of ∼1 mm2 on the Pd-coated porous Si surface, and the reflected light was collected with a microscope objective lens and focused on the fiber optic input of the spectrometer, as previously described.37 Illumination and detection of the reflected light was performed at an incident angle of 0° to the surface normal. Values of optical thickness were obtained from a Fourier transform of the reflectance spectra. Reflected intensity was measured at the wavelength of the most intense Fabry-Pe´rot fringe observed in the reflectance spectra.
Experimental Section
Results and Discussion
Materials. Degenerately boron doped p-type Si wafers with resistivities in the range 0.6-1.0 mΩ‚cm, with 〈100〉 orientation, and of a 500-550 µm thickness were obtained from International Wafer Service. PdCl2 (5 wt % in an aqueous solution containing 10 wt % HCl) was purchased from Aldrich Chemicals, Inc. Ethanol (200 proof) was purchased from AAPER Alcohol. All gases were ultrahigh purity grade. Porous Si Sample Preparation. Polished pieces of Si of an ∼1.3 cm2 area were placed in a Teflon etching cell, pressed against a piece of aluminum foil as a back contact. In the etching process, the cell was filled with a 3:1 (v/v) mixture of 48% aqueous HF (Fisher Chemicals) and absolute ethanol. A platinum mesh was immersed in the solution as the counter electrode. An anodic current of 70 mA/cm2 was passed for 30 s in the absence of light. The cell and the sample were then rinsed thoroughly with ethanol and dried under a stream of nitrogen. Immersion Plating of Pd on Porous Si. A few drops of a solution 0.014 M in PdCl2 and 0.14 M in HCl in a 1:1 (v/v) mixture of ethanol and water were added to the surface of the freshly etched porous Si sample. After 1 min, the Pd solution was washed off with deionized water and ethanol, and the sample was then dried in a nitrogen stream. Scanning Electron Microscopy. Scanning electron microscope (SEM) images were obtained with a Cambridge (LEO) S360 electron microscope with an Oxford energy dispersive X-ray (EDS) analyzer. Gas Dosing Experiments. The Pd-coated porous Si samples were placed in a gas-flow cell equipped with a transparent window, whose inlet was connected to a gas-mixer/mass-flow controller (Cole-Parmer). Pure nitrogen was used as the carrier gas, and H2 was passed through a moisture trap before introduction to the mixer. All measurements were performed at room
Immersion Plating of Porous Si with Pd. There are many techniques that can be used to deposit a thin metal film, such as chemical vapor deposition (CVD), sputtering deposition, electrochemical deposition, and laser ablation. One advantage of immersion plating over these is its simplicity. The hydride-terminated porous Si substrate acts as a reducing agent for dissolved noble metal ions, providing a metallized material in ∼1 min. The mechanism of this deposition procedure has been proposed to follow a localized cell model by Ogata and co-workers,38 who have studied the metallization reaction in great detail. The morphology of Pd-coated porous Si is revealed by scanning electron microscopy (Figure 1). A high density of Pd particles with diameters typically between 50 and 100 nm is randomly distributed on the porous Si surface, in accordance with the Volmer-Weber mechanism.38,39 The identity of elemental Pd is confirmed by energy dispersive spectroscopy (EDS). Diffuse reflectance Fourier transform infrared (FT-IR) measurements indicate that silicon oxide appears in the porous Si film concomitant with the growth of the Pd particles. A reaction sequence consistent with these observations is provided in eqs 1.11.3.
(30) Bodzenta, J.; Burak, B.; Gacek, Z.; Jakubik, W. P.; Kochowski, S.; Urbanczyk, M. Sens. Actuators, B 2002, 87, 82-87. (31) Okazaki, S.; Nakagawa, H.; Asakura, S.; Tomiuchi, Y.; Tsuji, N.; Murayama, H.; Washiya, M. Sens. Actuators, B 2003, 93, 142-147. (32) Butler, M. A.; Ginley, D. S. J. Appl. Phys. 1988, 64, 3706-3712. (33) Bearzotti, A.; Caliendo, C.; Verona, E.; D’Amico, A. Sens. Actuators, B 1992, 7, 685-688. (34) Chtanov, A.; Gal, M. Sens. Actuators, B 2001, 79, 196-199. (35) Schmedake, T. A.; Cunin, F.; Link, J. R.; Sailor, M. J. Adv. Mater. 2002, 14, 1270-1272. (36) Polishchuk, V.; Souteyrand, E.; Martin, J. R.; Strikha, V. I.; Skryshevsky, V. A. Anal. Chim. Acta 1998, 375, 205-210.
Figure 1. Plan-view scanning electron micrograph (secondary electron) image of Pd-plated porous Si typical of the samples used in this study.
2Si(surface) + H2O f Si-O-Si(surface) + 2H+(aq) + 2e- (1.1) 2Si-H(surface) + H2O f Si-O-Si(surface) + 4H+(aq) + 4e- (1.2) Ag+(aq) + e- f Ag(s)
(1.3)
The amount of Pd that deposits on the porous Si film can be controlled by changing the Pd solution concentra(37) Gao, J.; Gao, T.; Li, Y.; Sailor, M. J. Langmuir 2002, 18, 22292233. (38) Harraz, F. A.; Tsuboi, T.; Sasano, J.; Sakka, T.; Ogata, Y. H. J. Electrochem. Soc. 2002, 149, C456-C463. (39) Oskam, G.; Long, J. G.; Natarajan, A.; Searson, P. C. J. Phys. D: Appl. Phys. 1998, 31, 1927-1949.
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intensity reflected from the top of the porous Si layer (at the air-porous Si interface, Itop) and the intensity reflected from the bottom of the layer (at the silicon-porous Si interface, Ibottom), eq 3.
Imax ) Itop + Ibottom
(3)
Generally, the incident light on a surface is partly reflected and partly transmitted in a ratio that relates to the refractive indices of the media, eqs 4 and 5,42
Figure 2. Reflectance spectra of a porous Si sample before (dashed) and after (solid) Pd immersion plating. The inset shows a schematic diagram of the Pd-coated porous Si film represented by the solid trace.
tion and the deposition time. An appropriate amount of palladium is critical for the optical interferometric sensors to operate effectively. It is found that a small amount of deposited Pd does not provide a detectable response to hydrogen, while too much Pd does not allow penetration of light into the porous Si interferometer element. It is also found that addition of ethanol to the aqueous Pd deposition solution improves the sensor response. Presumably, the ethanol assists in wetting of the pores for better penetration of the deposition solution, providing durable, adherent films. Chan and co-workers have found that metals prepared by immersion plating on porous Si deposit mostly near the entrance of the pores rather than deep inside the pores.40 This observation is confirmed in the present work by cross-sectional EDS measurements of the Pd-coated porous Si samples. Optical Properties of Pd-Coated Porous Si Films. The thin porous Si films prepared in this study are transparent and of uniform thickness, and their pore dimensions are much smaller than the wavelength of visible light. Thus, they display Fabry-Pe´rot interference fringes in their optical reflectivity spectrum (Figure 2).8 For light of normal incidence, maximal reflection is obtained when the wavelengths of incident light satisfy the Fabry-Pe´rot equation, eq 2,41
mλ ) 2nd
(2)
where m is an integer (the spectral order of a particular fringe), λ is the wavelength, d is the film thickness, and n is the refractive index of the porous layer. A typical optical reflectivity spectrum displaying several optical fringes is presented in Figure 2. The value of the optical thickness (the product of the thickness (d) and the refractive index (n)) can be obtained by curve fitting the spectrum or from the Fourier transform of the intensity versus frequency spectrum. The Fourier transform method is used in this work. The optical reflectivity spectrum of the same porous Si sample after deposition of Pd is also presented in Figure 2 (solid line). The presence of a thin Pd overlayer slightly red-shifts and significantly reduces the fidelity of the Fabry-Pe´rot interference fringes. The total reflectivity of the film also decreases upon Pd deposition. At a wavelength where constructive interference occurs, the intensity of the reflected light (Imax) is the sum of the (40) Chan, S.; Kwon, S.; Koo, T.-W.; Lee, L. P.; Berlin, A. A. Adv. Mater. 2003, 15, 1595-1598. (41) Hecht, E. Optics, 3rd ed.; Addison-Wesley: Reading, MA, 1998.
R ) Ir/I0 ) (n2 - n1)/(n1 + n2)
(4)
T ) It/I0 ) 2n1/(n1 + n2)
(5)
where R is the reflection coefficient, T is the transmission coefficient, and I0, Ir, and It are the incident light, reflected light, and transmitted light intensities, respectively. n1 is the refractive index of the medium the incident light comes from, and n2 is the refractive index of the medium the light is transmitted into. When the incident light goes through a film, the light intensity decreases according to eq 6.43
Id ) I exp(-2ωkd/c)
(6)
Here, I is the incident light intensity, ω is its frequency, and c is its velocity. Id is the intensity of light after it goes through a film with extinction coefficient k and thickness d. As a metal, Pd has a large extinction coefficient (k) (4.42 at 659.5 nm),44 while the extinction coefficient of Si is much smaller (0.0141 at 652.6 nm).45 The strong attenuation by Pd can significantly decrease the intensity of light reflected from the bottom of the porous Si layer (Ibottom), which is the interface between the porous Si film and the bulk Si substrate. Presumably, the Pd particles plated on the surface also contribute to the decrease in the intensity of light reflected from the top of the porous Si film because of the scattering of light by those particles. The approximate feature size in these films is on the order of 50-100 nm (Figure 1), so the scattering efficiency at visible wavelengths is not large enough to completely obscure the Fabry-Pe´rot interference. Sensing of Hydrogen with Pd-Coated Porous Si Films. Hydrogen is expected to change the specific extinction coefficient, the refractive index, and the thickness upon absorption into Pd. All three of these parameters are expected to modify the Fabry-Pe´rot interference spectrum. Figure 3 presents the optical thickness (the value of nd) and the intensity of reflected light (Imax) as a function of time during which the sample is dosed with hydrogen. Hydrogen concentrations ranging from 0.17 to 1.70% in a N2 carrier gas are presented. An increase in optical thickness and a decrease in total reflected light intensity is observed with increasing H2(g) concentration. The increase in optical thickness observed is attributed primarily to expansion of the Pd lattice upon hydrogen absorption. The refractive index of Pd is also known to change somewhat upon hydrogen absorption.46 At wave(42) Smith, F. G.; Thomson, J. H. Optics; Wiley: New York, 1988. (43) Jiles, D. Introduction to the Electronic Properties of Materials; Chapman & Hall: New York, 1994. (44) Borghesi, A.; Piaggi, A. In Handbook of Optical Constants of Solids II; Palik, E. D., Ed.; Academic Press: San Diego, CA, 1985; p 475. (45) Geist, J. In Handbook of Optical Constants of Solids III; Palik, E. D., Ed.; Academic Press: San Diego, CA, 1998; p 529. (46) von Rottkay, K.; Rubin, M.; Duine, P. A. J. Appl. Phys. 1999, 85, 408-413.
Porous Si Sensor for H2
Figure 3. Room-temperature response of Pd-coated porous Si as a function of hydrogen gas at concentrations ranging from 0.17 to 1.71% (by volume) in N2, as indicated: (A) optical thickness measured by Fourier transform of the reflectance vs frequency spectrum; (B) intensity of the largest Fabry-Pe´rot peak in the reflectance spectrum.
lengths less than ∼600 nm, the index of PdHx (Pd saturated with H2) is lower than that of Pd, and at wavelengths larger than ∼600 nm, the index of PdHx is higher than that of Pd.46 The situation is further complicated by the existence of two separate PdHx phases.47 Pure palladium and palladium hydride with low hydrogen content are in the R phase, and they transform to the β phase at high hydrogen content. At intermediate hydrogen concentrations, the R and β phases coexist. Both phases possess a face-centered cubic (fcc) arrangement of the Pd atoms with H atoms in interstitial positions, while there is a 3% difference of the lattice parameter corresponding to a change of ∼10% in volume. As the value of the H/Pd ratio increases from 0 to ∼0.015 (the upper limit of the R phase), the lattice constant gradually increases from 3.889 to 3.890 Å to 3.893-3.895 Å.48,49 The lattice constant of the β phase increases from 4.025 to ∼4.040 Å when the value of the H/Pd ratio reaches ∼0.7.50 A H/Pd ratio of 0.7 corresponds to Pd in equilibrium with 1 atm of H2. The optical thickness changes observed in the present study are in agreement with the known relationship between the lattice constant and hydrogen loading in the PdHx system. As shown in Figure 3A, the increase in optical thickness of the film is small at low H2(g) concentrations, when PdHx is mostly in the R phase. As the H2(g) concentration in the gas stream increases above (47) Lewis, F. A. The Palladium Hydrogen System; Academic Press Inc.: New York and London, 1967. (48) Simons, J. W.; Flanagan, T. B. J. Phys. Chem. 1965, 69, 35813587. (49) Simons, J. W.; Flanagan, T. B. J. Phys. Chem. 1965, 69, 37733781. (50) Aben, P. C.; Burgers, W. G. Trans. Faraday Soc. 1962, 58, 19891992.
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∼0.3% (v/v), there is a sudden increase in the optical thickness signal, in accordance with the R to β phase transformation. At H2(g) concentrations above ∼0.8%, when the PdHx film is completely in the β phase, the change of the signal with increasing H2(g) concentration becomes small again. This highly nonlinear response due to the R to β phase transition suggests that this system may be more appropriate as a threshold detector for hydrogen concentrations above 0.5% (v/v), rather than as a sensor. The response of the optical thickness to hydrogen exposure is mirrored in the reflectivity of the film, displayed in Figure 3B. Hydrogen absorption decreases the intensity of the Fabry-Pe´rot fringes in the reflectance spectra, shown in Figure 3B for the most intense peak at ∼650 nm. As with the measurement of optical thickness, the reflectivity response is consistent with the Pd/H phase diagram. Reflectivity is related to index contrast via eqs 4 and 5 and to optical extinction via eq 6. In addition, increases in the Pd particle sizes are expected to increase incoherent losses due to light scattering effects. Thus, a quantitative interpretation of the reflectivity data is somewhat complicated. In the present system, the effect of hydrogen absorption is a net decrease in reflectivity of the sample at all wavelengths measured (400-1000 nm). Parts A and B of Figure 3 are obtained from the same set of reflectance spectra, and the results are interpreted as arising primarily from Pd lattice expansion upon hydrogen absorption. The expansion of Pd particles increases the average refractive index of the porous Si layer by taking up a larger volume fraction inside the pores. The change in volume attributed to the Pd/H R to β phase transition is ∼10%. It is reasonable to observe a 1% change of optical thickness (as shown in Figure 3) based on a 10% volume change in Pd. At the same time, the transition also changes the refractive index and thus the reflectivity of the PdHx layer. As shown in Figure 3, the reflected intensity measurement provides a larger signal-to-noise ratio than that of the optical thickness measurement, and it might be expected to represent a superior method for the detection of hydrogen. However, the optical thickness measurement is more reliable. Arising from a bulk property of the porous Si film (refractive index) instead of just a surface reflectivity phenomenon, shifts in the optical thickness are relatively independent of fluctuations in incident light intensity or of changes in surface scattering efficiency. For example, a more significant drift of the baseline is observed in the reflected intensity measurement (Figure 3B) than in the optical thickness measurement (Figure 3A). The drift in the reflectivity measurement is attributed to the deformation and fatigue of the Pd overlayer that occurs upon repeated hydrogen absorption and desorption cycles.51 The reflected intensity measurement is susceptible to changes in surface morphology and roughness because these phenomena increase the efficiency of incoherent scattering of light. Response Time of Pd-Coated Porous Si Films. The temporal response of the Pd-coated porous Si films displays an unusual dependence on H2(g) concentration. As shown in Figure 4, the response time at high or low H2(g) concentrations is on the order of the mixing time of our apparatus (30 s), while at intermediate H2(g) concentrations (0.5%) it is significantly longer. However, the time required for recovery to 0% H2(g) is quite constant for all concentration ranges, ∼1-2 min. This phenomenon is observed in both the reflected intensity measurement (51) Krause, W.; Kahlenberg, L. Trans. Electrochem. Soc. 1935, 68, 449.
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Figure 4. Relative change in intensity of the maximum FabryPe´rot peak in the reflectance spectrum upon exposure of the Pd-coated porous Si film to hydrogen gas at the indicated concentrations as a function of time. The y-axis is presented as the relative change in reflected intensity, defined as I/I0 - 1, where I is the intensity at time t and I0 is the intensity at time t ) 0. The value of (I/I0 - 1) for the sample exposed to hydrogen gas at the lowest concentration (0.17%) was multiplied by a factor of 12 for clarity of presentation. In all traces, the relevant concentration of H2(g) was introduced at time t ) 110 s.
(Figure 4) and the optical thickness measurement (not shown). Similar time-dependent behavior has been reported on the frequency response of Pd-coated surface acoustic wave resonators52 and in the conductivity response of Pd nanowires,53,54 and such hysteresis in apparent violation of the phase rule has been long recognized in the Pd/H2 system.55 The hysteresis has been attributed to plastic deformation of the R phase induced by the large lattice expansion that occurs as it transforms to the β phase. The average rate of advance of the β phase regions in Pd has been reported to range from 0.126 to 0.20 mm/hr,56 so the phase transformation time for a 100 nm thick Pd film corresponds to ∼2-3 s, which is close to the response time of the present sensor at high H2(g) concentrations. Under intermediate H2(g) concentrations of ∼0.5%, the response time is found to be surprisingly long. In this concentration range, both R and β palladium hydride phases are expected to coexist, and thus, the strain-induced hysteresis effect should be most pro(52) Jakubik, W. P.; Urbanczyk, M. W.; Kochowski, S.; Bodzenta, J. Sens. Actuators, B 2002, 82, 265-271. (53) Walter, E. C.; Favier, F.; Penner, R. M. Anal. Chem. 2002, 74, 1546-1553. (54) Walter, E. C.; Penner, R. M.; Liu, H.; Ng, K. H.; Zach, M. P.; Favier, F. Surf. Interface Anal. 2002, 34, 409-412. (55) Scholtus, N. A.; Hall, W. K. J. Chem. Phys. 1963, 39, 868-870. (56) Smith, D. P.; Barrett, C. S. J. Am. Chem. Soc. 1940, 62, 25652566.
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nounced. The observed difference between absorption and desorption times is interpreted to be a reflection of different states of strain in the lattice during expansion and contraction, and the long response time at intermediate H2(g) partial pressures can be thought of as a straininduced pinning of the phase boundaries. Poisoning and Specificity Tests. Practical hydrogen sensors must operate in the air instead of pure nitrogen. Most electrical hydrogen sensors, including the wellknown metal oxide semiconductor sensors, are affected by the presence of oxygen.3,57 It has been reported that hydrogen can be catalytically oxidized by oxygen on Pdcoated porous Si.7 The porous-Si-based sensor used in the current study is found to respond equally to H2(g) in an air or in a nitrogen carrier gas. In addition, no significant difference is observed using dry hydrogen or watersaturated hydrogen in N2(g). Due to the specificity of the Pd/H system, only hydrogen gas dissolves into the palladium lattice to cause the expansion, and sensors based on nanoscale Pd have been found to be remarkably immune to poisoning.53,54 However, the optical sensor reported in the present work is poisoned by the competitive surface adsorbate carbon monoxide. Exposure of the sensor to CO impedes both absorption and desorption of hydrogen and results in very long (>1 h) response and recovery times, presumably by blocking activation of hydrogen at the Pd surface.58,59 Conclusions Porous Si can be coated with a thin layer of Pd by immersion plating. H2 gas at a concentration as low as 0.17 vol % can be detected in a few seconds by monitoring either the optical thickness change or the change in reflected light intensity obtained from the interferometric reflectance spectrum. The combination of those two complementary transduction methods, optical thickness measurement and reflected light measurement, yields a reliable and sensitive sensing technique of H2. The optical H2 sensor is safe, sensitive, selective, and reproducible, and it displays a short response time and operates at room temperature. It is highly susceptible to poisoning by CO. Acknowledgment. The authors thank the U.S. EPA STAR program (Grant No. R829619) and the Air Force Office of Scientific Research (Grant No. F49620-02-1-0288) for funding. J.F. thanks the NSF Undergraduate Research Program in Solid State Chemistry, administered by the Division of Materials Research, for support. This paper is fondly dedicated to Nathan S. Lewis, who instructed one of the authors (M.J.S.) early in his career on the wonders of the Pd/H electrochemical system. LA049741U (57) Johansson, M.; Lundstrom, I.; Ekedahl, L.-G. J. Appl. Phys. 1998, 84, 44-51. (58) Wilzen, L.; Petersson, L.-G. Vacuum 1995, 46, 1237-1240. (59) Kok, G. A.; Noordemeer, A.; Nieuwenhuys, B. E. Surf. Sci. 1983, 135, 65-80.
