Langmuir 1993,9, 3441-3445
3441
Reductive Deposition of Pd on Porous Silicon from Aqueous Solutions of PdCl2: An X-ray Absorption Fine Structure Study I. Coulthard, D.-T. Jiang, J. W. Lorimer,* and T.K. Sham' Department of Chemistry, University of Western Ontario, London, Ontario N6A 5B7,Canada
X.-H. Feng Canadian Synchrotron Radiation Facility, Synchrotron Radiation Center, University of Wisconsin-Madison, Stoughton, Wisconsin 53589 Received April 5,1993. I n Final Form: September 30,199P A method for the deposition of palladium on the vast surface of porous silicon from aqueous solutions of PdCl2 is described. The deposited Pd and the porous silicon substrate have been characterized using X-ray absorption fine structure (XAFS)spectroscopy. It is found that deposition can be carried out in a controlled manner,that the deposited Pd is metallic, and that the oxidation-reduction reaction responsible for the reductive deposition of Pd from PdClz(aq) takes place at specific surface sites.
Introduction One of the most intriguing properties of porous silicon is that it exhibits intense luminescence in the visible at room temperature,' a phenomenon which has already stimulated much work in the fabrication of optoelectronic devicesq2Another intriguing property, yet to be exploited, is the combined effect of ita enormous internal surface area and ita ability to chemically reduce many substances including water and metal ions in s ~ l u t i o n .We ~ have been able to use the strong reducing power of porous silicon to disperse noble metals reductively onto the vast internal surface of porous silicon from appropriate aqueous solutions. We shall demonstrate, through characterization of the deposition of Pd on porous silicon from an aqueous solution of PdCl2 by Pd L-edge, Pd K-edge, and Si K-edge X-ray absorption fine structure (XAFS), that this can indeed be carried out under controlled conditions. Thus it appears that porous silicon may have interesting implications to the modification of industrial catalysts for processes such as petroleum reformingP6 and catalytic conversion of automobile exhaust.' XAFS refers to the oscillations of the X-ray absorption coefficient of a core level of an element in a particular environment and is sensitive to chemical changes experienced by the absorbing atom. The origin of XAFS is now reasonably understood in terms of a multiple scattering formalism.8 Traditionally, the absorption coefficient at the first -50 eV above the edge jump is called the near edge structure and the coefficient beyond that and 0
Abstract published in Advance ACS Abstracts, November 15,
1993. (1) Canham, L. T. Appl. Phys. Lett. 1990,57,1046. ( 2 ) Sailor, M. J.; Kavanagh, K. L. Adu. Mater. 1992,4, 432. (3) McCord,P.;Yau,S.-L.;Bard, A. J. Science 1992,267,68. Coulthard, I.; Lorimer, J. W.; Sham,T. K. Paper presented at the 118th Meeting, T h e Electrochemical Society, Toronto, O N 15, October, 1992. (4) Gatas, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979. (6) Sielt, J. H. Bimetallic Catalysts: Discoveries, Concepts, and Applications; John Wiley & Sone: New York, 1983. (6) S i e l t , J. H. Reu. Mod Phys. 1979,51, 569. (7) Root, T.W.; Schmidt, L. D.; Fisher,G. B. Surf. Sci. 1985,150,173. (8)Proceed@ of the VIIth International Conference on XAFS,Kobe, Japan, Auguet, 1992; to be published in Jpn. J. Appl. Phys. (9) Lytle,F.W.; Wei, P. S. P.; Greegor, R. B.; Via, G. H.; Sinfelt, J. H. J. Chem. Phys. 1979,70,4649. Sinfelt, J. H.; Via, G. H.; Lytle,F.W. J. Chem. Phys. 1980, 72,4832.
up to as much as 10oO eV above the edge is called the EXAFS (extended X-ray absorption fine structure). XAFS has been shown to be a unique and powerful tool for the structural characterization of highly dispersed metallic catalysta.Sl2 In this paper we use the near edge structure, which is dominated by dipole transitions from atomic to bound and quasi-bound states and is also sensitive to the local environment, to probe the deposited Pd as well as the porous silicon substrate. In general, the first peak a t the absorption edge is characteristic of dipole transition from the core to the unoccupied densities of state of p (K,L1 edge) and d (L3,2edge) holes just above the Fermi level in metallic and semiconductor systems. These transitions often result in a sharp peak at the edge jump called a whiteline. Features beyond the whiteline are transitions to the upper bands and are characteristic of the crystal structure of which the extended local structure (up to three shells about the absorbing atom) makes the most significant contribution. A comparison of the Pd K-edge EXAFS between Pd/PS and Pd metal is also presented to confirm the Pd L-edge observation.
Experimental Section Porous silicon specimens were prepared electrochemicallyin a 1:l ethanol-48% aqueous HF solution. A boron-doped ptype Si(100)substrate with a resistivity of -3 Q cm was used as the anode while Pt was used as the counter electrode. A current density of 20 mA/cma was applied for 20 min in a typical preparation. Under these conditions,a thin f i i of porous silicon with an area (nominal)of 1.3 cma and a thickness of -10 pm was produced on the Si(100)substrate. Freshly prepared porous silicon samples were either blown dry of electrolytes with dry nitrogen or rinsed quickly with distilled water and blown dry. Theae samples were then characterizedusing several spectrascopic methods (UV-visible, ESR,X-ray diffraction, SEM,infrared, Raman);s the W excited luminescence maximum appears at -680 nm. Infrared spectra of the porous silicon clearly exhibit both Si-H and Si-OH stretching vibrations indicating the presence of these bonds, most likely on the surface.
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(10) Sham,T. K.;Ohta,T.; Yokoyama, T.; Takata, Y.; Kitajima, Y.; Funabashi, M.; Kuroda, H. J. Chem. Phys. 1991,95,8725. (11)Meitzner, G.; Fischer, D. A.; Sinfelt, J. H. Catal. Lett. 1992,16, 219. (12) Coulthard, I.; Lorimer, J. W.;
Sham,T. K. Unpublished.
0743-7463/93/2409-3441$04.00/00 1993 American Chemical Society
3442 Langmuir, Vol. 9,No. 12, 1993 Reductive deposition of Pd on the porous silicon surface was carried out by immersing the porous silicon specimen into a vial containing -4 mL of an aqueous solution of PdC12. Two nominal concentrations, 0.001 and O.OOO1 M, were used and the time of immersion was 15 min in both cases. Evolution of hydrogen gas was noted upon immersion of the specimen in the solution.12The resultant sample, especially that from the more concentrated solution, appears to show a metallic coating under the optical microscopeand the scanning electron microscope;the UV excited luminescence is quenched somewhat for the 0.O001 M sample and significantly for the 0.001 M sample. X-ray absorption measurements of the Pd L s , ~ edge and the Si K-edge were carried out on the double crystal monochromator beamline at the Canadian Synchrotron Radiation Facility at Synchrotron Radiation Center (Aladdin), University of Wisconsin-Madison, Stoughton, WI. Aladdin operates at 800 MeV11 GeV with a start-up current of 200 mAl80 mA. InSb(111) crystals were used as the monochromator. This monochromator is extremely stable under these conditions and delivers a typical photon flux of -1O"Jat the specimen with a resolution of -0.9 eV at the Si K-edge (1839 eV).I3 The monochromatic photon beam was at a fixed exit position and was monitored with a calibrated nitrogen gas ionization chamber at 1Torr pressure so that absolute photon flux could be obtained. XAF'S measurements were made in a vacuum chamber using total electron yield (TEY). In all the XAFS spectra reported here, the total electron yield (normalized to photon flux) is displayed as a function of photon energy so that their relative intensities are directly comparable. To confirm the metallic nature of PdIPS, Pd K-edge extended X-ray absorption fine structures (EXAFS) have also been obtained for the 0.001 M Pd/PS and Pd metal at the C2 station of Cornell High Energy Synchrotron Source (CHESS) using a Si(220) double crystal monochromator.
Results and Discussion Reflectance infrared spectra of the PS samples which had been in contact only with the electrolyte showed Si-H stretching frequencies but no Si-OH, while sampleswhich had been in contact with pure water showed Si-OH frequencies as well. All the porous silicon samples used in the reaction had been rinsed with water. Figure 1 shows the Pd L3,2edge near edge structure of PdClz and Pd metal.ls These are used for the comparative analysis of the Pd L-edge spectra of Pd on porous silicon (henceforth denoted Pd/PS) prepared from the 0.001and 0.0001 M solutions. Figure 2 shows the Pd L3,2 edge structure of Pd/PS prepared from these solutions. A more detailed comparison of the Pd L3-edge of the two Pd/PS spectra is shown in Figure 3 where the intensity of edge jump of the 0.0001 M sample has been multiplied by 10. From comparison of Figures 1-3,it is apparent that the Pd/PS spectra resemble that of the Pd metal. In fact a close inspection of Figure 1 and Figure 3 shows that the sample prepared from the 0.001 M solution exhibits a spectrum identical to that of the Pd metal in every detail. Figure 4 shows the Si K-edge near edge structure of the porous silicon sample before and after different Pd depositions from the two solutions. The spectrum in Figure 4a is typical of as-prepared porous silicon16J7and exhibits a three-peak pattern just above the threshold with the whiteline (first peak just above the edge threshold) at (13)Yang, B.-X.; Middleton, F.; Olsson, B. G.; Bancroft, G. M.; Chen, J. M.; Sham,T. K.; Tan, K. H.; Wallace, D. J. Nucl. Instrum. Methods Phys. Res., Sect. A 1992,A316,422. (14)Holroyd, R. A.;Sham,T. K.; Yang, B.-X.; Feng, X.-H. J. Phys. Chem. 1992,96,1438. (15) Sham,T. K. Phys. Reu. B 1986,13,1903. (16)Sham,T.K.; Feng, X.-H.; Jiang,D.-T.; Yang, B.-X.;Xiong, J. Z.; Bzowslu, A.; Houghton, D. C.; Bryskiewicz, B.; Wang, E. Can. J. Phys. 1992,70, 813. (17)Sham,T. K.; Jiang, D. T.; Coulthard, I.; Lorimer, J. W.; Feng, X.-H.; Tan, K. H.; Frigo, S. P.; Rosenberg, R. A,; Houghton, D. C.; Bryskiewicz, B. Mater. Res. SOC.Symp., R o c . 1993,281,525.
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Figure 1. Pd L3.2 near-edge spectra of (a) PdCl2 and (b) Pd metal. 1.0
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Figure 2. Pd La3 near-edge spectra of Pd/PS from (a) a 0.001 M PdCldaq) solution and (b) a O.OOO1 M PdCl2(aq) solution.
-1840 eV, followed by two peaks at 1844 and 1847 eV. The origin of this three-peak pattern has been attributed to Si-Si, Si-H/Si-OH, and S i 4 bonding, respectively.lG18 p band The assignment of the whiteline to a Si 1s (unoccupied densities of states at the conduction band with p character) transition in PS and the third peak to a Si-0 bond signature in silicon oxide is well established, and there is good evidence (such as infrared, XAF'S of hydrogenated amorphous silicon and siloxene, and what is presented below) to show that the second peak most probably involves Si-H and Si-OH bonding. The Si-H species are formed as the result of H-passivation of the Si surface in HF, while the Si-OH species result from the hydrolysis of silicon oxide on the surface when exposed to water or moisture in the ambient atmosphere. This three-peak pattern has been seen in all the as-prepared porous silicon samples we have studied and is independent of the current density (20-200 mA/
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(18)Sham,T. K.;Jiang, D. T.; Coulthard, I.; Lorimer, J. W.; Feng, X.-H.; Tan,K. H.; Frigo, S. P.;Rosenberg, R. A.; Houghton, D. C.; Bryskiewicz, B. Nature 1993,363,331.
Langmuir, Vol. 9, No. 12, 1993 3443
Reductive Dispersion of Pd on Porous Silicon 0.9
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samples. These spectra also do not match those of Pd silicide of which the whiteline intensity is significantly greater.19 The detailed analysis of the Pd metal L3,2 edge has been reported previously.16 The near edge features can be interpreted in terms of a band modelm or a cluster model.21fn In either case the whiteline intensity corresponds to the number of d holes at the Fermi level (p d dipole transition) and the structures beyond that are due to transitions to upper bands (band model) or multiple scattering with the extended local structures (clustermodel including the first several shells of neighboring atoms) contributing the most to the most important pathways responsible for the near edge structure.21 Cluster calculations show that generally a cluster size including at least three shells of neighboring atoms surrounding the absorbing atom is required to reproduce nearly all the essential features of the near edge structure of the bulk metal, and that a smaller cluster would produce a spectrum with less-pronounced near-edge features.21 From the above considerations we can interpret the spectrum of Pd/PS from the O.OOO1 M PdC12 solution as that of a very thin Pd film of small Pd clusters (-several monolayers) on the porous silicon surface since the lesspronounced near edge structures at 3190 eV indicate that the Pd cluster has not acquired a full bulk character.21It is also possible that the Pd atoms at the Pd/Si interface may contribute somewhatto the overall appearance. The peaks at 3202 and 3228 eV, however, are characteristic of contributions from the nearest Pd neighbors (first shell) in the Pd metal. A simulation of the first shell contribution to XAFS using the theoretical phase and amplitude21p22 reproduces the relative intensity and separation of these two peaks, although the absolute position is several electronvolta higher than the experiment. From the edge-jump intensity in Figure 2, we deduce that the Pd film on 0.001 M Pd/PS is about 20 times thicker than the monolayer-thick 0.0001 M sample. These estimates can be compared with the upper limits of the Pd ions available (4 mL) for reduction ( 2.4 X lOl8 and 2.4 X lo1' ions for the 0.001 and 0.0001 M solutions, respectively). If the active internal surface area of the porous silicon is that of the lower bound of typical PS (200 m2/ cm3)23and our PS has a nominal surface area of 1.3 cm2 and a thickness of -10 pm, we would expect an upperlimit coverage of 1 X 1016 and 1 X 1014 Pd atom/cm2 (-0.67 and -0.067 monolayer) for total conversion at the two concentrations, compared to the surface density of 1.5 X 10l6atom/cm2for a closest-packed monolayer. This estimate, though very rough given the large uncertainty in the percent of conversion and the percent of the active surface sites involved in the reduction process, is within the order of magnitude suggested by the experimental results: submonolayer for the 0.0001 M sample and at least 1order of magnitude higher for the 0.001 M sample. In both cases Pd atoms aggregated to form islands on the surface.
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Figure 3. Detailed Pd La near-edge spectra of Pd/PS as shown in Figure 2. The spectrum in (b) has been multiplied by a factor of 10.
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Figure 4. Si K-edge near edge spectra of the porous silicon substrate: (a) as-depositedporous silicon before Pd deposition; (b) Pd/PS from the 0.001M PdCl, solution; (c) Pd/PS from the O.OOO1 M PdClz solution.
cm2) and duration (20min typical) used in the preparation and the period of aging (a few days to 10 months). However, the relative intensities of the three peaks change considerably. In general the Si oxide feature (1847 eV) increases at the expense of the whiteline as the current density increases or as the sample ages in the ambient atmosphere. The second and the third peaks are greatly reduced or disappear on treatment with a HF solution and this is accompanied by the reappearance of the porous silicon whiteline. Parts b and c of Figure 4 show the Si K-edge spectra of PS after the reductive deposition of Pd from 0.001 and O.OOO1 M PdCl2 solutions, respectively. It can be seen from these figures that the intensities of the first two peaks of the three-peak pattern of the porous silicon have been quenched significantly, particularly in the spectrum corresponding to the more concentrated solution, while the Si-oxide peak (1847 eV) becomes more intense. Returning to Figures 1-3, it is evident that the Pd deposited on porous silicon from the 0.001 M PdCl2 solution is Pd metal. The O.OOO1 M sample also displays a Pd metallike spectrum albeit with much weaker overall intensity and less-pronounced XAFS beyond the whiteline. The Pd L3 edge of the less concentrated sample shifts slightly to higher energy (0.5 eV) while the width and area under the whiteline are essentially the same for both
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(19) Rossi, G.; Jaeger, R.; Stohr, J.; Kendelwin, T.; Lmdau, I. Phys. Reu. E 1983,27,5154. (20) Muller, J. E.; Jepsen, 0.; Wilkina, J. W. Solid State Commun. 1982, 42, 365.
(21) Bianconi, A.; Garcia, J.; Benfatto, M. In Synchrotron Radiation in Chemistry and Biology I, Topics in Current Chemistry 145; Springer-Verlag: Berlin, 1986. 422) Rehr,J. J.; de Leon, Muatre; Zabinsky, S. I.; Albers, R.C . J. Am. Chem. SOC.1991,113,5135. (23) Ito, T.; Yamama, A.; Hiraki, A.; Satou, M. Appl. Surf. Sci. 1989, 41/42,301. Herino, R.; Billat, S.; Basiesy, F.; Gaspard, M.; Michalcesca, I.; Ramestain, R.; Vial, J. C. Phys. Scr. 1992, T45,300.
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3444 Langmuir, Vol. 9,No. 12, 1993 CI
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