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Langmuir 1998, 14, 2736-2743
Infrared Spectroscopic and Temperature-Programmed Desorption Studies of Dimethylphenylphosphine Adsorbed on the Coinage Metals H. Kariis,† G. Westermark,‡ I. Persson,‡ and B. Liedberg*,† Laboratory of Applied Physics, Department of Physics and Measurement Technology, Linko¨ ping University, S-581 83 Linko¨ ping Sweden, and Department of Chemistry, Swedish University of Agricultural Sciences, P.O. Box 7015, S-750 07 Uppsala Sweden Received November 13, 1997. In Final Form: February 19, 1998 The adsorption of dimethylphenylphosphine (dmpp) on gold, silver, and copper has been studied with infrared reflection absorption spectroscopy and temperature programmed desorption mass spectroscopy. This study focuses on phosphine layers that have been prepared on the coinage metals by deposition from the gas phase in ultrahigh vacuum (UHV). Significant shifts in several infrared bands as well as high stability when heated indicate that a thin chemically bound layer of dmpp has been formed on the coinage metals. The activation energy of desorption as revealed by temperature programmed desorption mass spectroscopy shows that the interaction with gold is significantly stronger than with copper and silver. The properties of the UHV-deposited layers are also compared with analogous layers prepared by spontaneous adsorption from dilute toluene solutions. Infrared spectroscopy suggests that the two methods of preparation lead to chemically bound layers with dissimilar orientation and coordination geometries. Moreover, preliminary X-ray photoelectron spectroscopy data suggest that the solution-prepared layer contains a large fraction of oxidized dmpp. It is also found that the chemical interaction between the phosphine ligands and the metal surfaces is stronger for the solution than for the UHV-deposited layers. The solutiondeposited layers on all metals and the gas-phase-deposited layers on gold were found to decompose upon desorption, while the gas-phase-deposited dmpp molecules on copper and silver desorbed as intact entities.
Introduction Formation of monolayers of organic molecules on metal surfaces attracts much attention in diverse areas of fundamental surface physics and chemistry,1 as well as in areas of technological importance. Several molecules containing anchoring groups, like thiols,2,3 disulfides,4,5 and xanthates6 are known to bind strongly to clean gold surfaces, a behavior that has been extensively studied. Tertiary phosphines are also known to form a strongly bound layer on several metal surfaces including the coinage metals7-9 as well as the platinum group metals.10,11 In this paper the interest is focused on the chemisorption of a tertiary phosphine on gold, silver, and copper. Trimethylphosphine would be the smallest tertiary phosphine to choose for this type of investigation. However, dimethylphenylphosphine (dmpp), Figure 1, was selected because of the unique infrared signature of the phenyl † ‡
Linkoping University. Swedish University of Agricultural Sciences.
(1) Ullman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly: Academic Press Inc.: New York, 1991. (2) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733-740. (3) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (4) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 44814483. (5) Liedberg, B.; Carlsson, C.; Lundstro¨m, I. J. Colloid Interface Sci. 1987, 120, 64-75. (6) Ihs, A.; Uvdal, K.; Liedberg, B. Langmuir 1993, 9, 733-9. (7) Uvdal, K.; Persson, I.; Liedberg, B. Langmuir 1995, 11, 12521256. (8) Steiner, U. B.; P., N.; Caseri, W. R.; Suter, U. W.; Stucki, F. Langmuir 1992, 8, 90. (9) Westermark, G.; Kariis, H.; Liedberg, B.; Persson, I. Colloids Surf., in press. (10) Edwards, H. G. M.; Lewis, I. R.; Turner, P. H. Inorg. Chim. Acta 1994, 216, 191-199. (11) Westermark, G.; Persson, I. Colloids Surf., in press.
Figure 1. Dimethylphenylphosphine.
group that makes it easy to detect the presence of dmpp on a surface with infrared reflection absorption spectroscopy (IRAS). In this study we focus the attention on the coinage metals gold, silver, and copper, but results from similar experiments on platinum, iridium, and rhodium surfaces will be published in a subsequent paper.12 Infrared reflection absorption spectroscopy and temperature-programmed desorption (TPD) are employed to study the presence, orientation, and coordination of dmpp on the coinage metal surfaces. The infrared assignments rely on a recent assignment work based on comparisons to similar molecules and normal coordinate analysis of dmpp.13 Most of the experiments are undertaken in ultrahigh vacuum (UHV) to maintain surfaces free from interfering oxides and organic contaminations. This is important for silver and, to an even higher degree, for copper, on which a layer of copper(I) oxide is rapidly formed upon exposure to air.14 No similar oxidation problem occurs on gold, a metal that, on the other hand, quickly (12) Kariis, H.; Westermark, G.; Persson, I.; Liedberg, B. Submitted to Appl. Surf. Sci. (13) Westermark, G. Chemisorption of Phosphines on Coinage and Platinum Group Metal Surfaces. Thesis, Swedish University of Agricultural Sciences, Uppsala, 1987. (14) Allen, J. A. Trans. Faraday Soc. 1952, 48, 273.
S0743-7463(97)01232-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/21/1998
Dimethylphenylphosphine Adsorbed on the Coinage Metals
becomes contaminated by various organic compounds when exposed to air. Comparisons are also made between solution-prepared structures and multi- and monolayer structures prepared by gas-phase deposition in UHV. During these experiments special considerations were taken to reduce the time a metal surface was exposed to air. Experimental Section Materials. Dimethylphenylphosphine (97%) was obtained from Aldrich and used without further purification. The quality was checked regularly with gas chromatography to ensure a low content of oxidation products of the material used in the experiments. Ethanol (99.5%, Kemetyl, Stockholm) and toluene (99.5%, Merck) were used as purchased. The silicon wafers (100) were obtained from OKMETIC, Espoo, Finland. Silver (99.95%) and gold (99.95%) for the evaporation were obtained from ANA A ¨ delmetall, Helsingborg, Sweden, and copper (>99%) was from Balzers. Substrate Preparation. The metal surfaces were prepared by electron beam evaporation to a thickness of 2000 Å of the coinage metal on 40 × 20 or 20 × 20 mm slides of a standard (100)-silicon wafer. The silicon slides were cleaned in a 5:1:1 mixture of MilliQ water, ammonia (25% aqueous solution), and hydrogen peroxide (30% aqueous solution) called TL1, heated to 80 °C for 5 min, rinsed in MilliQ water, and blown dry in nitrogen. The silicon slides were coated with a 25 Å thick titanium adhesion layer prior to the deposition of the coinage metals. The coinage metal films were evaporated with the rate 5 Å/s. The base pressure in the chamber was 2 × 10-9 Torr, and the pressure during evaporation was held below 2 × 10-7 Torr. Metal evaporations took place at a temperature only slightly elevated from room temperature due to radiation heat from the liquid metal. Identically prepared surfaces have previously been studied with X-ray diffraction, transmission electron microscopy, and atomic force microscopy. These measurements show that for gold (111) texture dominates strongly (95%) and the typical grain size is 200-500 Å.15 Also for copper16 and silver17 the (111) face is strongly preferred. Deposition from Solution. Two different methods were used to deposit dmpp on the metal surfaces. Solution deposition, which is described in detail elsewhere,9 was made from a 2 mmol dm-3 solution of dmpp in toluene in which the sample was immersed for at least 20 h. The gold surfaces had been cleaned in TL1 and blown dry in nitrogen prior to immersion in the dmpp solution. Silver surfaces, which might be damaged by TL1, were either taken directly from the evaporator into the toluene solution or ultrasonicated in ethanol and blown dry in nitrogen. Copper surfaces were taken from the evaporation chamber directly to the dmpp solution to avoid oxidation. The total exposure time to air was held below 2 min. After the dmpp treatment the metals were rinsed in toluene, ultrasonicated in toluene (2 min), rinsed in ethanol, ultrasonicated in ethanol (2 min), blown dry in clean nitrogen gas, and immediately mounted in the IR spectrometer or the UHV system. Infrared Spectroscopy. Infrared reflection absorption spectroscopy measurements on the solution-deposited samples were made on a Bruker IFS113v (vacuum ∼10 Torr) Fourier transform spectrometer equipped with a grazing angle of incidence reflection accessory aligned at 83°. The infrared radiation was polarized parallel to the plane of incidence. Interferograms were apodized with a three-term BlackmannHarris function before transformation. The spectra were recorded by averaging 2000 interferograms at 4 cm-1 resolution using a deuterated triglycine sulfate (DTGS) detector. The transmission infrared spectroscopic measurements on dmpp in the liquid phase were made on a Bruker IFS 48 Fourier transform infrared spectrometer continuously purged with dry air. One drop of dmpp was placed between two KBr windows at (15) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141-149. (16) Ihs, A.; Liedberg, B.; Uvdal, K.; To¨rnkvist, C.; Bodo¨, P.; Lundstro¨m, I. J. Colloid Interface Sci. 1990, 140, 192. (17) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167.
Langmuir, Vol. 14, No. 10, 1998 2737 the sample position. Each spectrum was obtained by averaging 200 interferograms at 4 cm-1 resolution using a DTGS detector. Combined IRAS and TPD Experiments in UHV. The ultrahigh vacuum experiments were conducted in a threechamber UHV system of our own design built at the Linko¨ping laboratory. The system is designed for simultaneous IRAS and TPD measurements on thin organic layers on metal surfaces and has been described in detail elsewhere.18 The pressure in the preparation chamber, in which dmpp was deposited, was in the range of 10-9 Torr, while the IRAS and TPD measurement were conducted in a chamber holding a pressure in the range of 10-10 Torr. The sample could quickly be transferred between these chambers in UHV. Gas-Phase Deposition in UHV. The 20 × 20 mm gold samples were cleaned with TL1, rinsed in MilliQ water, and blown dry in nitrogen before being mounted in the UHV system. The silver and copper samples were quickly taken out from the metal evaporation UHV chamber and immediately put into the analysis UHV system. The exposure time to air was below 10 min. The samples were sputter cleaned with neon gas at a pressure of 2 × 10-4 Torr with 4 kV Ne ions until no traces of contaminating hydrocarbon bands were detected in the 30002800 cm-1 region with IRAS. Dmpp was then deposited on the cleaned metal surface in the preparation chamber of the vacuum system,18 holding a base pressure below 5 × 10-9 Torr. A few drops of dmpp, which is a liquid at room temperature, was placed in a test tube connected to the UHV system through a leak valve. The test tube was pumped with a turbomolecular pump until the gas in the test tube (at a pressure close to 1 × 10-4 Torr) held a high content of dmpp vapor. The leak valve was opened and closed a number of times to pump away residual gases trapped in cavities inside the valve. A Hiden HAL 2/301 quadrupole mass spectrometer was employed to ensure that pure dmpp gas was introduced into the chamber. The dmpp entered the chamber from the valve through a narrow pipe positioned 3 cm in front of the sample. Deposition experiments were made with sample temperatures ranging from 153 to 326 K. A typical deposition was made with the partial pressure of dmpp at 5 × 10-7 Torr for 30 s, giving a dose of ca. 15 Langmuir. The entire deposition process was monitored with mass spectroscopy. After dmpp deposition the sample was transferred to the IRAS measurement position in the analysis chamber with a base pressure lower than 2 × 10-10 Torr. IRAS Measurements in UHV. IRAS spectra were recorded using IR radiation from another port of the Bruker 113v spectrometer incident on the sample at an angle of 82°. The reflected radiation was focused on a liquid nitrogen cooled mercury cadmium telluride (MCT) detector. Due to the higher sensitivity level in the MCT detector, only 500 scans were needed to get an acceptable signal-to-noise ratio in these measurements. Also, here, a three-term Blackmann-Harris function was used for apodization. After thermal desorption the samples were sputtered clean with Ne ions and another IRAS spectrum was recorded. This spectrum was used as a reference in the further evaluation. Temperature-Programmed Desorption Measurements. The sample temperature, measured with a Pt100 element, was held constant during the entire deposition, transfer, and measurement procedure. After verification of a successful deposition with IRAS the sample was resistively heated and the desorbing fragments analyzed with an ion counting quadrupole gas analyzer (Hiden HAL 3F/301 PIC). Time evolutions of the partial pressures of all masses from 2 to 139 atomic mass units were recorded together with the corresponding measured sample temperature. The heating process could be interrupted and another IR spectrum recorded at any temperature without moving the sample. From the temperature at which desorption occurred, an approximate value for the activation energy of desorption can be calculated using the method proposed by Redhead.19 TPD experiments were also performed on solution-deposited dmpp films to investigate the differences in adhesion strength between solution and gas-phase deposited dmpp. These samples were (18) Engquist, I.; Lundstro¨m, I.; Liedberg, B. J. Phys. Chem. 1995, 99, 12257. (19) Redhead, P. A. Vacuum 1962, 12, 203-211.
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treated in the same way as the solution-deposited samples used for IRAS measurements before being mounted in the UHV system. X-ray Photoelectron Spectroscopy. The XPS measurements were done on a VG instrument with a CLAM2 analyzer and a twin Mg/Al anode. The measurements were carried out with unmonochromized Mg KR photons (1253.6 eV). The resolution was determined from the full width at half-maximum (fwhm) of the Au (4f7/2) line that was 1.3 eV with a pass energy of 50 eV. The power of the X-ray gun was kept constant at 300 W. The spectra were lined up through the C(1s) peak (284.5 eV). The pressure in the analysis chamber was approximately 5 × 10-10 Torr, and the temperature of each sample was about 30 °C during measurement.
Results and Discussion Solution-Deposited dmpp. A recent infrared study revealed that solution-deposited dmpp forms a thin firmly bound layer on the coinage metals after removal of a thick physisorbed layer by ultrasonication. A detailed evaluation and discussion of these spectra can be found elsewhere.9 Therefore, only a brief description will be given here. Several of the more prominent bands in the bulk phase spectrum, e.g., those at 1433, 895, and 740 cm-1, displayed relatively large frequency shifts in the adsorbed state. These shifts were attributed to a soft donor-acceptor interaction between the phosphorus lone pair and vacant orbitals of the metal. Other bands, such as 895 and 1426 cm-1, completely disappeared, indicating that the adsorbed molecules had a preferential orientation. Moreover, broad bands in the 1040-1150 cm-1 region were believed to originate from partly oxidized phosphinecontaining P-O-C units. Tertiary phosphines are known to be sensitive to oxygen7 and in these experiments no special precautions were taken to protect the samples from air exposure. The thin layers were also found to be stable, and no phosphine desorbed from the surface at room temperature either in air or in the mild vacuum (∼10 Torr) inside the infrared spectrometer. Striking similarities were seen in IRAS spectra of adsorbed layers when compared to IR spectra from the coordination complexes Au(dmpp)2BF4, Ag(dmpp)4ClO4, and Cu(dmpp)4ClO4, indicating that the two different types of complexes exhibit similar coordination. Thus, the coordination complexes served as a better model for the surface complexes than the pure phosphine. Gas-Phase-Deposited dmpp Layers on Gold. The results from the UHV-deposition experiments are somewhat different from those obtained for the solutiondeposited dmpp.9 A multilayer was formed on the surface of a gold sample cooled to 173 K prior to exposure of 30 Langmuir of dmpp; see Figure 2a. The spectrum of this thick layer resembles the bulk phase spectrum and does not show the same shifts as the solution-deposited phosphine, indicating that no strong specific interaction with the metal is present. The small shifts (less than 2 wavenumbers) seen for several bands in the reflectionabsorption spectrum compared to those in the transmission spectrum of the pure phosphine (see Figure 3a,b) are most likely caused by optical20 and/or solid state effects. One difference, however, is that a strong previously unassigned band at 912 cm-1 is observed, along with the methyl rocking bands at 895 and 945 cm-1. The 912 cm-1 band will be discussed later in the text. The peak frequencies, assignments and directions of transition dipole moments of bulk dmpp13 are summarized in Table 1. (20) Allara, D. L.; Baca, A.; Pryde, C. A. Macromolecules 1978, 11, 1215.
Figure 2. Thick and thin dimethylphenylphosphine layers on gold: (a) IRAS spectrum of thick layer deposited at 173 K. Inset: TPD trace of mass 69 during desorption. (b) IRAS spectrum taken at 313 K, after desorption of the multilayer.
Figure 3. IRAS spectra of dmpp on gold as a function of deposition temperature: (a) dmpp bulk material; (b) dmpp deposited on Au at 173 K, thick layer; (c) dmpp deposited on Au at 263 K; (d) dmpp deposited on Au at 326 K.
The molecular mass of dmpp is 138 amu. However, the mass spectrum obtained upon introduction of dmpp in the UHV chamber also revealed several characteristic masses at m/e ) 55, 57, 60, 64, 69, 77, 91, 121, and 123, due to cracking of the molecule in the mass spectrometer; see Table 2. Mass 69 was chosen as an indicator of dmpp in the subsequent analyses and desorption traces plotted. This peak is supposed to be proportional to the partial pressure of desorbed dmpp since the strength of this peak was proportional to the pressure when phosphine was introduced into the preparation chamber of the UHV system. When the metal with the thick dmpp layer is slowly (0.3 K/s) heated, most of the dmpp layer is leaving the surface starting at around 170 K, Figure 2 (inset). The shape of the peak resembles a normal first-order desorption TPD trace with a maximum desorption rate at 194 K. The Redhead equation,19 assuming a preexponential factor of 1013 s-1, gives an activation energy of
Dimethylphenylphosphine Adsorbed on the Coinage Metals Table 1. Infrared Frequencies, Assignments, and Dipole Moments of dmpp (Bulk Phase) wavenumber (cm-1)
assignment
direction vs phenyl ringa
740b 865 895 941 1000 1028 1070 1106 1416 1425 1433 1485
phenyl C-H out of plane methyl rocking methyl rocking methyl rocking phenyl in plane symm phenyl in plane symm phenyl in plane asymm phenyl in plane symm methyl asymm bending methyl asymm bending phenyl asymm CC stretch phenyl asymm CC stretch
x x-y-zc x-y-zc x-y-zc z z y z x-y-zc x-y-zc y z
a With the z-direction understood to be the direction along the P-Ph bond in the plane of the phenyl ring. The y-direction is in the phenyl ring plane perpendicular to the z-direction and x is perpendicular to the plane of the phenyl ring; see Figure 1. b Out of the spectral range of the UHV setup. c We assume a low barrier of rotation around the P-CH3 bond.
desorption of 0.56 eV/molecule (54 kJ/mol), much less than the bond energy for a chemisorbed layer. For example, the strength of the S-Au bond in a chemisorbed thiol layer is estimated to be 1.7 eV/molecule (167 kJ/mol).21 Exposure of the cold metal to a higher dose of dmpp vapor prior to desorption gives an increased size of the desorption peak but does not change the desorption temperature, confirming the assumption of a first-order desorption, i.e., a desorption rate proportional to the number of physisorbed molecules on the surface, which follows when no lateral interaction between the molecules occurs. The cracking pattern is quite similar to that seen upon introducing dmpp gas into the chamber, the only significant difference is observed for m/e ) 55, a mass for which a lower intensity is detected. This fraction is supposed to originate either from a short hydrocarbon chain or from an oxygen-containing species according to our tentative assignments, Table 2. The IRAS spectrum recorded at 313 K, a temperature at which the mass spectrometer no longer detects any desorption, shows that a thin layer of dmpp is still left on the surface, Figure 2b. Several bands have shifted or disappeared compared to the spectrum of the thick layer. Some of the shifts are interpreted as being due to specific chemical interaction between the dmpp molecule and the gold surface. The most pronounced changes can be observed in the 850-950 cm-1 region. The methyl rocking modes seen near 865, 895, and 945 cm-1 in the multilayer and bulk spectra, Figures 2a and 3a, are all strongly perturbed and can hardly be seen at all in the 313 K spectrum, Figure 2b. Instead, a series of new bands appears in the region 920-960 cm-1, and the 912 cm-1 band becomes the most prominent one. It is believed that the 912 cm-1 band is the methyl rocking at 895 cm-1 that is shifted upward in frequency due to electronic interaction with the gold substrate. Shifts in the same direction have been observed for thin layers prepared by solution deposition.9 Thus, it seems plausible that the 912 cm-1 band seen as a high-frequency shoulder on the 895 cm-1 band in the multilayer spectrum, Figure 2a, originates from a very first layer of strongly bound (chemisorbed) dmpp on the gold surface, whereas the physisorbed molecules on top of this inner layer do not exhibit this shift. At 313 K, Figure 2b, the 912 cm-1 band still has the same strength whereas most other bands appear with (21) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569.
Langmuir, Vol. 14, No. 10, 1998 2739
decreased intensity due to the desorption of the physisorbed dmpp molecules. The large magnitude of the shift of the 895 cm-1 band could indicate that a substantial reorganization of the electronic structure around the phosphorus atom occurs when the free electron pair is donated to the metal or that a lateral organization (packing) different from that in the bulk phase is established during the adsorption process. However, since the 912 cm-1 peak is observed also in the spectrum of the coordination compound Au(dmpp)2BF4,11 for which lateral organization phenomena are expected to be of less importance, it seems reasonable to assume that the shifts in the methyl rocking modes originate from a reorganization of the electronic structure around the phosphorus atom in dmpp. XPS data confirm the assumption that the molecules are intact on the surface excluding the possibility that the shifts are caused by some sort of decomposition reaction of dmpp molecules on the surface. The bulk phase bands at 865, 1070, 1416, and 1425 cm-1 appear with very low intensity or disappear completely and the band at 895 cm-1 is shifted. These bands correspond to vibrations with a significant component of the transition dipole moment in the y-direction;13 see Figure 1. On the other hand, the z-bands at 1028 and 1485 cm-1 (phenyl modes) are still clearly visible at their original wavenumbers, indicating that these vibrations are not affected by the adsorption of the molecule to the surface. Two reasons for the disappearance of four of the five methyl bending bands (i.e., 1425, 1416, 945, and 865 cm-1) can be conceivable: (i) their transition dipole moments are aligned parallel to the surface and thus are invisible in IRAS according to the surface selection rule; (ii) the frequencies are shifted to coincide with another band. However, to conclusively decide which of these suggestions that apply, ab initio quantum chemical calculations of the mode frequencies of dmpp connected to a metal cluster are necessary. Various in-plane interaction phenomena may also hinder the methyl hydrogen vibrations geometrically, leading to a reduction in the corresponding mode intensities and/or to frequency shifts. The methyl bending vibration at 895 cm-1 is shifted to 912 cm-1, with only a small trace of the band at its original frequency, Figure 2b, revealing that almost all the molecules that are left on the surface at 313 K are directly attached to the gold substrate. The strong phenyl C-C stretching band at 1433 cm-1, Figure 2a, which also is assigned to have its transition dipole moment in the y-direction, appears with reduced relative intensity in Figure 2b. This reduction appears consistent with the overall behavior of the y-polarized modes; see discussion above. Another interesting observation is that this band appears at the same frequency in the thin and thick film spectra, Figure 2. This is a very interesting observation in gross contrast to the findings in the study of the coordination complexes and solutionprepared layers in which this band was found to consistently shift upward by 5-6 cm-1.9 The absence of this shift is somewhat surprising since it was taken as one of the characteristic markers for a strongly bound (chemisorbed) phosphine molecule in our previous solution deposition study.9 The two methods of preparing thin layers display also large variations in relative band intensities between the phenyl (in-plane) modes at 1433 cm-1 (y-polarized) and 1485 cm-1 (z-polarized). The I1433/ I1485 intensity ratio is about 1.5 for the gas phase and 15 for the solution-deposited layers.9 Thus, the binding to the metal and the molecular orientation are both very different for the solution and gas-phase-prepared layers.
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Table 2. Tentative Assignments and Relative Intensities for Selected m/e Values upon Desorption When Deposited from Toluene Solution (sol) and Gas Phase (gas) on Gold, Silver and Copper m/e
tentative assignment
introa
Au thick
Ag thick
Cu thick
Au gas
Ag gas
Cu gas
Au sol
Ag sol
Cu sol
55 57 60 64 65 69 77 79 91 107 109 121 123 138
O-C3H3, C4H7 P-C2H2 P-C2H5, P-O-CH P-O2-H C5H5 138/2, C4H5O Ph Ph-H2 Ph-CH2 Ph-P-(H) Ph-P-H Ph-P-CH Ph-P-CH3 dmpp
40 77 9 8 45 60 75 71 100 15 7 14 14 6
6 64 6 3 37 45 76 83 100 11 8 36 25 8
4 80 8 5 44 51 87 94 100 10 7 26 21 9
4 73 7 4 37 47 82 88 100 9 7 30 23 9
81 100 10 9 9 37 15 8 6 1 1 1 1 0
15 126 16 13 44 93 87 91 100 4 6 18 21 9
14 112 11 7 67 81 104 85 100 9 4 16 11 5
100 70 21 75 9 26 10 2 1 0 0 0 0 0
100 88 25 38 2 29 9 3 1 0 1 0 0 0
100 92 63 77 5 21 2 1 0 0 0 0 0 0
a Recordings from inlet of pure dmpp into the UHV chamber and desorption of a thick frozen film at 200 K from the metals are also shown.
Figure 4. IRAS spectra of dmpp deposited on different metals at low temperature and heated: (a) dmpp bulk material; (b) dmpp layer on copper at 223 K; (c) dmpp layer on silver at 223 K; (d) dmpp layer on gold at 313 K.
Continuation of the TPD measurement above room temperature gives information about the strength of the chemical bond between the phosphine and the gold surface. In Figure 5 (thick line) it is seen that the desorption peaks above 450 K, the highest sample temperature at which the mass spectrometer could be used due to a too high background pressure of water and carbon oxide desorbing from the sample holder. This temperature, according to the Redhead equation,19 corresponds to an activation energy of desorption of 1.3 eV/molecule (120 kJ/mol). The increase of the partial pressure due to the desorption of the remaining very thin dmpp layer is close to the noise level of the mass spectrometer, but IRAS confirms that all, within the detection limit of the spectrometer, dmpp has desorbed at 470 K. In the cracking pattern, a significant difference is seen, compared to the desorption of the physisorbed thick layer at 200 K. The lower m/e values, such as 55, 57, and 69, now strongly dominate, while very low signals are recorded for m/e values corresponding to more or less intact molecules, such as 138, 121, and 107; see Table 2. As the same experimental setup is used, this must be due to cracking of the molecule on the surface prior to or at desorption. All the masses observed show similar desorption traces, with no desorption at all seen between 220 and 330 K. This
Figure 5. Temperature-programmed desorption traces of thin UHV-deposited dmpp layers on Au, Ag, and Cu.
observation suggests that the decomposition occurs at desorption and that it is the decomposition process itself that leads to desorption. If the molecule decomposes on the surface prior to desorption, it would be unlikely that all fragments leave at exactly the same temperature. Deposition at Different Temperatures. Exposure of 30 Langmuir of dmpp on a gold surface at room temperature caused low but still detectable adsorption with IRAS. Deposition of dmpp at different temperatures resulted in minor but reproducible spectral differences, as shown in Figure 3. At 263 K the 903 and 912 cm-1 bands as well as the 940 and 950 cm-1 bands are clearly visible, indicating a state in which a loosely bound physisorbed layer and a strongly bound inner layer coexist. At 326 K (Figure 3d) a band at 909 cm-1 dominates the spectrum and a single band is observed near 955 cm-1. It should be emphasized that at this substrate temperature the amount of adsorbed dmpp remains constant regardless of the exposure. The medium intensity band at 930 cm-1 seen in Figure 2b is, however, missing in the IRAS spectra, Figure 3c,d. Comparison of Different Deposition Methods. Three methods to form a thin layer of dmpp on a gold surface have been mentioned; deposition from toluene solution (I), condensing a thick layer on a cold sample in UHV followed by desorption of the physisorbed molecules upon heating to room temperature (II), and deposition in
Dimethylphenylphosphine Adsorbed on the Coinage Metals
UHV on a sample held at or slightly above room temperature (III). In case I the dmpp layer is allowed to equilibrate with a solution containing the dmpp molecules for at least 20 h. The solution assembly process can therefore be regarded to approach equilibrium-like conditions where the continuous exchange between surfaceadsorbed and solution molecules will ensure that an inner layer of strongly bound dmpp molecules, with a packing density as high as the geometry permits, is formed. The situation is different in case II where almost all dmpp molecules hitting the cold surface are believed to stick to it during formation of a multilayer of randomly distributed (oriented) molecules, Figure 3b. The majority of molecules in this multilayer, including those in the inner layer that are oriented with their lone-pair orbital away from the metal, will desorb near 200 K, Figure 2 (inset), and leave a thin layer of dmpp molecules on the surface. As estimated from the IRAS intensities the coverage (density) of such a gas-phase-deposited layer is lower by a factor of 2 than of those layers prepared from solution. This agrees well with the above assumption since it is only the properly oriented molecules within the very first layer that are able to develop sufficiently strong bonds with the metal. However, these bonds, as well as those formed after deposition at elevated temperatures (III), are according to our infrared results not necessarily identical to those formed in (I) (cf. the TPD data below). The layers prepared from solution, I, seem to induce a larger reorganization of the electronic structure around the phosphorus atom in dmpp, which manifests itself as significant shifts in both the phenyl and the methyl modes and large changes in relative band intensities. For example, the shift from 1433 to 1438 cm-1 for the phenyl CC band, I, is most likely a consequence of such a chargetransfer interaction with the metal. The absence of the shift in the 1433 cm-1 band for the UHV layers may be due to a change in the Au-P bond length. A longer Au-P bond will inevitably lead to a smaller overlap and to a reduced reorganization of the electronic structure around the phosphorus atom. In this context it should be remembered that the solution-deposited layer, which has been exposed to oxygen for a substantial time, also contains a higher fraction of oxidized phosphine than the sample that has been handled in UHV. This effect may also lead to large changes in electronic structure and thereby to a large frequency shift of the phenyl C-C mode. One may argue that the phenyl group equally well can interact directly with the metal through its p-orbitals and thereby cause approximately the same shift of the phenyl C-C stretching band. We consider this explanation less likely since the same shift is observed for the coordination compound Au(dmpp)2BF4,11 in which metal-ligand bonding is established exclusively via phosphorus lone-pair donation into vacant orbitals of the metal followed by backdonation into a phosphorus 3d orbital.22 The conclusion from this comparison is that the two methods used to deposit dmpp on gold lead to overlayer structures with different lateral organization, packing, orientation, and coordination geometry. Gas-Phase-Deposited dmpp on Silver. Deposition of a thick (30 Langmuir) dmpp layer frozen onto silver at 173 K gives an IRAS spectrum identical to that obtained for gold; Figure 2. When heated to room temperature most of the dmpp desorb below 200 K, just as for gold; see Figure 2 (inset). Also the cracking pattern in the mass spectrometer is the same as for gold; see Table 2. These (22) Carty, A. J.; Taylor, N. J.; Coleman, A. W.; Lappert, M. F. J. Chem. Soc., Chem. Commun. 1979, 639.
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observations confirm our assumption that the thick layer is physisorbed and that the deposition process at low temperature is independent of the substrate material. After desorption of the frozen thick overlayer of dmpp, some differences are observed in the spectrum recorded at 223 K (see Figure 4c). The shifts in peak positions for silver are much smaller than for gold, which might indicate a weaker chemical interaction between dmpp and the metal. For example, the 895 cm-1 band has shifted upward by 8 cm-1, compared to 15 cm-1 for gold, and the shift in the 941 cm-1 band is only a few wavenumbers for silver. The total coverage is slightly lower than for gold. The assumption of a weaker chemisorption for silver than for gold is confirmed by TPD measurements on the UHVdeposited films. As seen from Figure 5, dmpp desorb from the silver already at 305 K, corresponding to an activation energy for desorption at 0.88 eV/molecule (85 kJ/mol), i.e., about 0.4 eV lower than that for gold. Thus, the magnitude of the frequency shift for the 895 cm-1 band seems to correlate with the activation energy of desorption. In the case of silver, the TPD cracking patterns for the physisorbed and chemisorbed layers are much more similar than in the case of gold; see Table 2. Only a weak tendency toward lower masses is seen. Note also that intact molecules, m/e ) 138, are detectable with the expected relative intensity. These observations agree well with the assumption of a weaker chemisorption and suggest that decomposition upon desorption from silver does not occur. The desorption of the chemisorbed layer on gold, occurring at higher temperatures, could, however, be due to decomposition of the chemisorbed layer, a thermally activated process. Thus, no intact dmpp molecules are seen desorbing from gold. Another interesting correlation is that between the shift in the 895 cm-1 band and the metal-phosphorus bond distance. In phosphine complexes of monovalent coinage metal ions with the same configuration the P-metal bond length is for gold 2.45 Å23 and for silver 2.67 Å,24 as determined by X-ray diffraction. Such metal complexes have been shown to be good microscopic model systems for phosphines adsorbed on a metal surface.11 Thus, the longer bond distance for silver seems to result in less charge transfer (overlap) and thereby a smaller shift in the 895 cm-1 band and a weaker surface chemical bond than for gold, consistent with the TPD data in Figure 5. This is in accordance with the assumption that this shift is caused by a chemical interaction between the phosphine lone pair orbitals and the metal surface. Gas-Phase-Deposited dmpp on Copper. The IRAS spectra from a thick dmpp layer deposited at 173 K on copper do not differ from those obtained on gold and silver. Also, here, a thick physisorbed layer desorbs at below 200 K, with the same cracking pattern, Table 2. After heating to 223 K, the thin layer of dmpp on copper does not give such a well-defined spectrum as for gold and silver. Note that the copper surface was neon sputtered immediately before dmpp deposition in order to reduce the influence of the native copper(I) oxide formed on copper metal when exposed to oxygen. A strong band is seen at 920 cm-1 that is believed to be this methyl rocking vibration strongly shifted from its original position at 895 cm-1. For copper(I) ions the bond length to a tertiary phosphine in a complex is even shorter (23) Andersson, L.; Hulte´n, F. In Thermodynamic and Structural Studies of Silver(I) Complexes with Phosphine, Arsine and Stibine in Pyridine. Thesis, Go¨teborg University, Go¨teborg, 1986. (24) Edler, R. C.; Zeiher, M.; Onady, M.; Whittle, R. R. J. Chem. Soc., Chem. Commun. 1981, 900.
2742 Langmuir, Vol. 14, No. 10, 1998
Figure 6. Temperature-programmed desorption traces of thin solution-deposited dmpp layers on Au, Ag, and Cu.
than for gold, 2.26 Å.25 Thus, following the same way of reasoning as for silver the shift in the 895 cm-1 band is expected to be even greater than for gold, which is observed, Figure 4b. However, the TPD data in Figure 5 show that the desorption peak occurs at the same temperature for copper as for silver, indicating a similar activation energy for desorption (0.88 eV/molecule, 85 kJ/ mol). Thus, the magnitude of the shift in the 895 cm-1 band seems, in this particular case, not to be correlated with a strong chemisorption. On the other hand, the shape of the 920 cm-1 band is very complex, making a detailed analysis difficult. In the TPD cracking pattern, Table 2, a slightly stronger tendency toward smaller masses than for silver is seen for copper, but not close to the large shifts toward lower m/e values seen for gold. It seems that, for both silver and copper, most molecules desorb as intact entities while they for gold, where the chemisorption is stronger, decompose before reaching the mass spectrometer. TPD of Solution-Deposited Layers. The solutiondeposited dimethylphenylphosphine thin films generally remained attached to the metal at temperatures higher than for those deposited from gas phase in UHV, Figure 6. For copper the difference is significant; the desorption peaks at 305 K for gas-phase-deposited and at a temperature well above 450 K (1.3 eV/molecule, 120 kJ/mol) for solution-deposited dmpp. It should be noted that the copper surface used for solution-deposition had been exposed to air up to 2 min, which might be a possible explanation for the differences seen for this metal. It is well-known that copper(I) ions form strong complexes with phosphines26 and that a layer of copper(I) oxide is quickly formed on a copper surface in air.14 Thus, the oxide layer may act as source of copper(I) ions that could form strong surface-bound complexes with dmpp. As for the gas-phase-deposited layers, the phosphine sticks to the metal at higher temperatures on gold than on silver. The desorption peak maximum for gold appears well above 450 K, the peak temperature for gas-phasedeposited dmpp. The desorption peak from the silver surface occurs at around 450 K (1.3 eV/molecule, 120 kJ/ mol) to be compared with the temperature observed for the gas-phase-deposited layers, 305 K. These results (25) Dempsey, D. F.; Girolami, G. S. Organometallics 1988, 7, 1208. (26) Ahrland, S.; Berg, T.; Trinderup, P. Acta Chem. Scand., Ser. A 1977, 31, 775.
Kariis et al.
support the assumption that the binding mechanism in the layers deposited from solution is quite different from that in the gas-phase-deposited layers and that the chemisorption is stronger for the solution-prepared layers. Also the cracking patterns, Table 2, confirm that there is a significant difference between the deposition methods. For all three metals a substantial decomposition of the dmpp molecules occurs upon desorption. The tendency of forming low-mass fragments at desorption is even more pronounced than for the gas-phase-deposited thin layer on gold. It is also interesting to note that the relative intensities of several low-mass fragments containing oxygen, Table 2, increase for the solution-prepared dmpp layers, a phenomenon in full agreement with the high sensitivity to oxidation for dmpp. XPS Analysis. To further study the electronic interaction between dmpp and the metal surfaces, X-ray photoelectron spectroscopy was employed. These studies will be published separately.27 In brief, the XPS results confirm the IRAS and TPD results reported here. The relative C:P intensity is 8.0 for solution-deposited dmpp on gold and 10.8 for gas-phase-deposited dmpp, strongly supporting that the molecules form stable and intact monolayers at room temperature. The degree of oxidation is also confirmed to be considerably higher for the solutiondeposited layers than for the layers prepared in UHV. The P(2p) spectrum for the solution-prepared layer on gold exhibits two peaks at 130.9 and 132.5 eV, respectively. The 130.9 eV peak is due to pure phosphine chemisorbed to gold via its lone pair orbital, and the 132.5 eV peak originates from partly oxidized molecules. Only the 130.9 eV peak is seen for the UHV-deposited layers. The O(1s) spectrum is complex for the solution-prepared layer with two peaks at 530 and 532.5 eV, where the lower one is assigned to a P-O-C species. This peak disappears and the 532.5 eV peak (contaminants and water) is not observed in the O(1s) spectrum of the UHV-deposited layer. For more discussion and spectra see the forthcoming paper.27 Conclusions It has earlier been shown that solution deposition of dmpp results in a strongly bound layer on all the three coinage metals. Significant differences are observed between the IRAS spectra of solution-deposited phosphine and those of phosphine deposited in UHV. XPS also reveals that the oxidation state of dmpp is different for the two sets of layers. Therefore, the structure and binding of dmpp deposited from the gas phase in UHV are different from those obtained in toluene solution. Thermal desorption experiments furthermore suggest that the bond between dmpp and the metal is stronger for solutiondeposited layers, which is consistent with the larger shifts of some of the bands in the infrared spectra. The dmpp ligand is more strongly chemisorbed on gold than on copper and silver when the deposition is performed from the gas phase. This correlation does not hold for the solution prepared layer where no significant difference was seen between the metals. Detailed analysis of the desorbing fragments shows that the thick physisorbed dmpp layers desorb as intact molecules. The same is true for the desorption of thin layers of dmpp left on silver and copper after removal of the physisorbed layer deposited in UHV. A considerable decomposition of the molecules occurred upon desorption for the UHV-deposited thin layer on gold as well as for solution-deposited films on all metals. The (27) Kariis, H.; Uvdal, K.; Wirde, M.; Westermark G.; Gelius, U.; Persson, I.; Liedberg, B. Manuscript in preparation.
Dimethylphenylphosphine Adsorbed on the Coinage Metals
Langmuir, Vol. 14, No. 10, 1998 2743
strong correlation between the desorption temperature and the degree of fragmentation suggests that the fragmentation is a thermally activated process, which causes desorption of chemisorbed dmpp.
University for Agricultural Sciences in Uppsala, was supported by the Swedish Research Council for Engineering Sciences (TFR). We also thank Dr. Kajsa Uvdal for help with the XPS measurements.
Acknowledgment. This work, which is part of a joint project between Linko¨ping University and the Swedish
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