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Development of Boron Carbide as a Suitable Substrate for Physisorption Studies K. Tejasen,† A. Diama,† D. Wittmer,‡ and A. D. Migone*,† Departments of Physics and Mechanical Engineering, Southern Illinois University, Carbondale, Illinois 62901 Received July 31, 1997. In Final Form: November 24, 1997 Adsorption isotherms of Ar and CH4 were used to study the suitability of boron carbide as substrate for physisorption. Samples from three different groups of boron carbide were used. The samples were subjected to different cleaning treatments. The best results were obtained on powders and on whiskers heated to 1770 °C in a N2 atmosphere. There are four resolvable steps in multilayer adsorption isotherm data measured on these high-temperature-treated substrates. Detailed monolayer adsorption data measured at 77.3 K for CH4 on these samples reveal the presence of a substep near monolayer completion. This feature is probably related to a solidification transition in the monolayer as the coverage is increased. The specific surface areas of the boron carbide samples used range between 0.3 and 0.15 m2/g. Our results indicate that boron carbide can, when cleaned at sufficiently high temperatures, result in a suitable substrate for physisorption studies.
I. Introduction Interest in physisorbed films stems in part from the fact that they provide one of the few experimentally realizable approximations to two-dimensional (2D) matter.1 Adsorbed films, however, are not just 2D entities; their characteristics and behavior are the result of the combined properties of the adsorbates and those of the substrates.2 The nature of the phases present on a film,2,3 the structures of the adsorbed solid phases,2 their symmetries,4 the temperatures at which phase transitions take place in films,3 etc., are all the product of the interplay of the characteristics of the substrate and those of the adsorbate species. Experimentally the only way to explore the dependence of physisorbed systems on the adsorbent species is to study films on different high-quality substrates. For this reason, the search for suitable new substrates for use in adsorption is a topic of continuing interest for researchers in field. A relatively large number of high-quality substrates have been studied to date. Among them: graphite,1 BN,3 MgO,4 and about a dozen of the lamellar halides.5 The extent of the utilization of these different materials as substrates in adsorption studies varies greatly. Graphite is, by far, the most commonly used substrate in adsorption, followed by BN and MgO.3 The level of utilization of a substance as substrate in physisorption studies is deter† ‡
Department of Physics. Department of Mechanical Engineering.
(1) A large number of examples can be found in Phase Transitions in Surface Films 2, Taub, H., Torzo, G., Lauter, H. J., Fain, S. C., Jr., Eds.; NATO ASI Series B; Plenum: New York, 1991; Vol. 267. (2) Lakhlifi, A.; Girardet, C. Surf. Sci. 1991, 241, 400. Hoang, P. N. M.; Girardet, C.; Sidoumu, M.; Suzanne, J. Phys. Rev. B 1993, 48, 12183. (3) Li, W.; Shrestha, P.; Migone, A. D.; Marmier, A.; Girardet, C. Phys. Rev. B 1996, 54, 8833. Meldrim, J. M.; Migone, A. D. Phys. Rev. B 1995, 51, 4435. Alkhafaji, M. T.; Shrestha, P.; Migone, A. D. Phys. Rev. B 1995, 50, 11088. (4) Coulomb, J. P.; Vilches, O. E. J. de Phys. (Paris) 1984, 48, 1381. Coulomb, J. P.; Sullivan, T. S.; Vilches, O. E. Phys. Rev. B 1984, 30, 4753. Ma, J.; Kingsbury, D. L.; Liu, F. C.; Vilches, O. E. Phys. Rev. Lett. 1988, 61, 2348. Vilches, O. E. J. Low Temp. Phys. 1992, 89, 267. Maruyama, M.; Bienfait, M.; Liu, F. C.; Liu, Y. M.; Vilches, O. E.; Rietvord, F. Surf. Sci. 1993, 283, 333. (5) Larher, Y. J. Colloid Interface Sci. 1971, 37, 836. Millot, F.; Larher, Y.; Tessier, C. J. Chem. Phys. 1982, 76, 3327.
mined by a combination of the properties material and its adsorption characteristics. From a practical point of view, one of the appealing features shared by graphite, BN, and MgO is that all three substances can be cleaned by high-temperature heating. In addition, graphite and BN can withstand to be exposed to air without significant quality deterioration. By contrast, the lamellar halides cannot be heated to the high temperatures used to clean BN or graphite because they decompose; these materials are also very hygroscopic, so in order to get best adsorption results the samples have to be prepapred and kept under vacuum until the time in which they are used.5 The greater difficulties in prepapration and handling presented by the lamellar halides account, to a great extent, for their much more infrequent use. In what follows, we discuss results of adsorption isotherm measurements and scanning electron microscopy studies used to explore the suitability of boron carbide (B4C) as a substrate for physisorption. Boron carbide is commercially available in different grades, some of which have sufficiently large specific areas to permit the performance of adsorption isotherm measurements on them. Because of its high melting point (in excess of 2400 °C),6 boron carbide can be cleaned by subjecting it to heating at sufficiently high temperatures. This material had never before been investigated as a substrate in adsorption studies. Boron-rich solids have attracted much attention during this past decade.7,8 Boron carbides exist over a wide range of compositions as single-phase materials.7,9 Boron carbides with compositions between 10 and 20 atom % of C have icosahedral structures which are based on the structure of R-rhombohedral boron. The C atoms can be located at different positions within this icosahedral structure; the variability in the location of the C atoms accounts for the wide range of compositions with the same structure.7 (6) Spohn, M. T. Am. Ceram. Soc. Bull. 1995, 74, 113. (7) Emin, D. Phys. Today 1987, 40 (1), 55. (8) See, for example: Boron-Rich Solids; AIP Conf. Proc. 140; Emin, D. et al., eds.; AIP: New York, 1986. Boron-Rich Solids; AIP Conf. Proc. 231; Emin, D., et al., Eds.; AIP: New York, 1991. (9) Spohn, M. T. Am. Ceram. Soc. Bull. 1993, 72, 88.
S0743-7463(97)00850-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/25/1998
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There has been no determination of the structure of the planes which constitute the exposed surfaces of the boron carbides. From the perspective of physisorption this is quite unfortunate, because it is on these surfaces where the adsorption takes place. On the other hand, some of the characteristics of boron carbide (in particular, the possibility of cleaning this substrate by heating it to high temperature and its resistance to exposure to air) are highly desirable qualities in a substrate. II. Experimental Section Substrates. Three different groups of boron carbide samples, all of the same nominal composition B4C, were used in these measurements. We studied boron carbide powders manufactured by the Advanced Ceramics Corp., powders from Johnson Matthey, and whiskers also from Johnson Matthey. The specific surface areas of the boron carbide substrates varied between 0.3 (for the powders) and 0.15 m2/g (for the whiskers). The surface areas of the substrates were calculated from the monolayer capacity of methane isotherms; we assumed a specific area of 15.7 A2/ molecule for methane. The monolayer capacity of a sample of boron carbide was determined using the point B method (i.e., by determining the intercept in the coverage axis of the linear portion of the isotherm data between the first and second layers). We used different cleaning treatments for the boron carbide substrates: Some samples from all three groups were used as substrates in as-received condition. A portion of the B4C powders manufactured by Advanced Ceramics was subjected to heat treatment in a vacuum at 900 °C prior to the performance of adsorption measurements. This treatment was performed in a tube furnace, with the powder placed inside a quartz tube which had been evacuated below 1 × 10-5 Torr using a liquid-nitrogentrapped diffusion pump. Finally, samples from all three groups of B4C were heated to 1770 °C in a N2 atmosphere, in a hightemperature oven. For this high-temperature treatment the samples were placed in either BN or graphite boats, and the boats were placed on the oven’s conveyor belt. The transit time of the boat through the high-temperature section of the oven was on the order of 3 h. The oven was thoroughly purged with N2 before the beginning of the run, and a N2 gas overpressure was maintained in it throughout the heating process. In all three cases the boron carbide samples were transferred, in air, to the sample cell in which the isotherm measurements were performed. Apparatus. Two, essentially identical, computer-controlled, isotherm setups were used for the performance of our adsorption measurements. In one of the setups, low temperatures were achieved using a helium closed-cycled refrigerator. In the other setup low temperatures were attained by means of a liquid-N2 bath. All pressures were measured with MKS capacitance gauges located at room temperature. The amounts of gas dosed into the sample cell were controlled by a computer via relays and electropneumatic valves. A more detailed description of the apparatus has been provided elsewhere.10 The majority of the adsorption isotherms were performed using CH4 gas as the adsorbate species. A smaller number of isotherms were conducted using Ar. Both Ar and CH4 were Research grade Matheson products. Electron micrographs of selected boron carbide samples were obtained on a Hitachi S 570 scanning electron microscope. The micrographs were taken at the Center for Electron Microscopy of Southern Illinois University.
III. Results Isotherms are sensitive tools with which to probe the adsorption quality of surfaces. On highly homogeneous, uniform substrates, at sufficiently low temperatures (generally below 0.85Tt, with Tt the adsorbate’s triple point) adsorption can take place in stepwise fashion.11 At low temperatures, each step in the adsorption isotherm corresponds to the formation of a new layer of film on the (10) Shrestha, P.; Alkhafaji, M. T.; Lukowitz, M. M.; Yang, G.; Migone, A. D. Langmuir 1994, 10, 3244. (11) Hess, G. B. ref 1, p 357.
Figure 1. Isothermal compressibility data for Ar at 77.3 K on a sample of boron carbide powder in as-received condition. The two-dimensional isothermal compressibility is plotted against the pressures scaled in terms of the saturated vapor pressure (P/P0). There are two peaks in the compressibility, which correspond to two layer-steps in the isotherm (the lower pressure peak, due to the first layer formation, is not shown in the figure).
substrate.11 The number, size, and sharpness of the adsorption isotherm steps provide a measure of the degree of binding energy uniformity of the substrate’s surface.4,5,10 If a substrate were ideal, steps in an isotherm would take place at a fixed value of the pressure; this would correspond to adsorption taking place at a constant value of the chemical potential and, consequently, at a fixed value of the adsorption potential.12 In real substrates, where imperfections and finite size are always present, isotherm steps are never vertical; they can, however, be very steep. The pressure difference between the top and bottom of a quasi-vertical step in an adsorption isotherm provides a quantitative measure of the degree of binding energy heterogeneity of the substrate.13 Because of their sensitivity to surface quality, adsorption isotherms can be used to monitor the effectiveness of surface-cleaning treatments.4,5,10 Comparison of the sharpness of isotherm steps obtained on substrates before and after a given treatment provides a good indication of the cleaning method’s efficacy. A quantitative measure of the sharpness of the steps of an isotherm can be obtained by calculating the isothermal compressibility of the adsorbed film from the adsorption isotherm data. The film’s isothermal compressibility is given by:3
KT2D ) (1/n2v)[dn/dP]T In this expression n is the coverage on the surface, v is the specific volume of the gas in the three-dimensional vapor, and P is its pressure. This relation has the virtue of expressing a purely 2D property, KT2D, in terms of 3D measurable quantities. Peaks in the isothermal compressibility correspond to steps in the isotherm; the sharper the adsorption isotherm step, the larger the isothermal compressibility peak which corresponds to it. In Figure 1 we present isothermal compressibility data for Ar measured at liquid-nitrogen temperature on an Advanced Ceramics boron carbide powder which was utilized in as-received condition. In addition to the peak present at low pressures, corresponding to first layer formation, there is a weak peak present at a scaled pressure, P/P0 (P0 the saturated vapor pressure) of approximately 0.5. At 77.3 K Ar is at about 0.91 of its triple point, Tt. In all likelihood the Ar layers forming on (12) See, for example: Dash, J. G. Films on Solid Surfaces; Academic Press: New York, 1973. Steele, W. A. The Interaction of gases with solid surfaces; Pergamon: New York, 1974. (13) Ecke, R. E.; Ma, J.; Migone, A. D.; Sullivan, T. S. Phys. Rev. B 1986, 33, 1746.
Boron Carbide as Substrate for Physisorption Studies
Figure 2. Isothermal compressibility corresponding to an Ar adsorption isotherm measured at 77.3 K on a sample of boron carbide heated in a vacuum to 900 °C. The 2D-isothermal compressibility, in m/N, is presented as a function of the scaled pressures. Four peaks in the isothermal compressibility are present; the first one, corresponding to the formation of the first layer on the substrate occurs below a scaled pressure of 0.05, and the others occur at 0.45, 0.7, and 0.85. Comparing Figures 1 and 2, we can see the improvement in substrate quality as a result of heating.
Figure 3. CH4 adsorption isotherm at 77.3 K measured on the same substrate as that used for the Ar data of Figure 2. The steps in the isotherm are sharper for CH4 than for Ar.
the substrate at this temperature are fluid (a fact which partially accounts for the lack of sharpness of the steps). After the boron carbide powder was subjected to the vacuum heat treatment at 900 °C, the isotherm steps obtained increased in sharpness. This can be readily seen in Figure 2, which displays the isothermal compressibility of the film of Ar adsorbed on the heat-treated substrate. In Figure 2 we can resolve four isothermal compressibility peaks: one at low pressure, below P/P0 of 0.05; another near P/P0 of 0.45; and, two peaks at scaled pressures of approximately 0.7 and 0.85. These last two peaks correspond to steps in the isotherm which are not present in the data measured on the as-received powders, shown in Figure 1. Figures 1 and 2 two provide evidence of substrate improvement upon heating. Since the same adsorbate was used in both measurements, the additional isothermal compressibility peaks present for the data measured on the boron carbide powder heated to 900 °C are due to improvement of the substrate’s surface. In Figure 3 we present an adsorption isotherm of CH4 on boron carbide powder heated to 900 °C under vacuum. We display data up to relative pressures close to 1.0. At least three steps are resolvable. The isothermal compressibility data corresponding to Figure 3 are displayed in Figure 4. The peaks are sharper in Figure 4 than in
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Figure 4. Isothermal compressibility data corresponding to the CH4 isotherm shown in Figure 3. Four peaks are present in these data. The isothermal compressibility peaks for CH4 are sharper than those for Ar, presented in Figure 2, even though the same substrate was used in both sets of measurements.
Figure 5. CH4 adsorption data on a boron carbide powder sample heated to 1770 °C in a N2 atmosphere. The steps are sharper than those shown in Figure 3, which were obtained for the same adsorbate and temperature but on a substrate heated only to 900 °C under vacuum. This shows that the substrate clearly improves upon high-temperature heating. The total mass of the sample used in these measurements was 7.4 g.
Figure 2. The boron carbide sample for Figures 3 and 4 is the same as that used in Figure 2. Consequently, the increase in the sharpness of the isothermal compressibility peaks is entirely due to the difference in adsorbates: at 77.3 K, CH4 is further below from its triple point than Ar. The CH4 steps probably correspond to the growth of a solid film. Isotherm steps corresponding to the formation of solid layers on the substrate are sharper than those corresponding to fluid layers. The quality of the boron carbide substrate exhibits substantial improvement after heat treatment at 1770 °C. In Figure 5 we display results of CH4 adsorption on a sample of boron carbide powder heated to this temperature in a N2-saturated atmosphere. The location of the first three isotherm steps is the same as those present for the isotherm shown in Figure 3; however, the steps are now much sharper. In addition, a fourth step is clearly resolvable in the isotherm. The corresponding isothermal compressibility peaks are much sharper than those in Figure 4. It is clear from Figures 1-5 that the best quality substrates were obtained with the heat treatment to 1770 °C. The sharpness and quality of the isotherm steps measured on boron carbides treated to 1770 °C were substantially similar for all three groups, regardless of the origin or specific surface area of the boron carbide.
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Figure 7. Detailed monolayer adsorption isotherm at 77 K for CH4 adsorbed on boron carbide whiskers heat treated to 1770 °C. There is little, if any, concavity in the data measured at the lowest coverages (and pressures), indicating the absence of highenergy binding sites. A substep is present in the adsorption data near a scaled pressure of 0.09. It very likely corresponds to the solidification transition of the monolayer. The amount of CH4 adsorbed on the substrate is given in cm3 Torr at 273 K (1 cm3 Torr at 273 K corresponds to 3.54 × 1016 molecules). The pressures are scaled by the saturated vapor pressure.
Figure 6. Adsorption isotherms measured at different temperatures for CH4 on boron carbide whiskers heated to 1770 °C in a N2 atmosphere. The temperatures shown, from top to bottom, are 67, 74, 75, and 77 K. Four steps are resolvable in the data. The total mass of the sample used in these data sets was 8.5 g.
For this reason we limited our adsorption isotherm studies at different temperatures only to one set of boron carbide substrates: Johnson-Matthey boron carbide whiskers, heated in a N2 atmosphere at 1770 °C. In Figure 6 we present the lower coverage portions of four of the six multilayer CH4 adsorption isotherms which we measured for temperatures between 67 and 82 K. The coverage in layers is plotted against the scaled pressures, P/P0. Four isotherm steps are resolvable in these data. While we have not attempted to conduct here a detailed study of the multilayer phase diagram of CH4 on boron carbide, it is apparent that the quality of the substrate is sufficiently high to permit the performance of such a study. The results displayed in Figure 6 show that only a finitethickness solid CH4 film forms on the boron carbide when the saturated vapor pressure is reached. That is, our data indicate that solid CH4 incompletely wets the boron carbide substrate. We have only performed two detailed adsorption isotherm measurements at monolayer coverages, both at liquid-nitrogen temperature. The isotherms were measured on samples of Johnson-Matthey boron carbide whiskers and on boron carbide powders, both heat treated
to 1770 °C in an oversaturated N2 atmosphere. The results of the measurements for the whisker sample are displayed in Figure 7. There are two features of interest in the data. First, at the lowest pressures studied there is very little concave curving of the adsorption data toward the coverage axis. That is, little adsorption occurs at the lowest values of the pressure. A concave curving of the isotherm corresponds to the presence of high-energy binding sites, i.e., those which will fill up before the rest of the surface.14 The lack of concavity in the data provides a clear indication that the high-temperature treatment is an effective method for cleaning the substrate. The absence of highenergy binding sites is a desirable substrate characteristic. Second, there is a small substep present in the monolayer data (near a reduced pressure of 0.09). This substep is due to a phase transition taking place in the monolayer film. The height of the substep is proportional to the 2D density difference between the phases undergoing the transition. In this case, because the substep occurs near monolayer completion and because there is only a small coverage difference between the top and bottom of the substep, we can conclude that the transition is taking place between two dense phases. The most likely scenario is that the isotherm feature corresponds to a solidification transition from a liquid to a solid monolayer which takes place as the coverage is increased. The ability of this physisorbed system to display a phase transition occurring at monolayer coverages is additional proof of the high quality of the substrate. From the electron micrographs we determined what appeared to be flat regions on the order of 10 µm in diameter for the powders. Some improvement on the flatness of the surfaces was observed on the boron carbide powders upon heating. Electron micrographs of the boron carbide whiskers showed the presence of much longer particles, some of the order of 200-300 µm. In the whiskers sample these larger particles were present together with smaller particles, of only tens of micrometers in size. (14) Lowell, S.; Shields, J. E. Powder Surface Area and Porosity, 3rd ed.; Chapman and Hall: London, 1991.
Boron Carbide as Substrate for Physisorption Studies
IV. Conclusions We have explored the use of boron carbide as substrate for physisorption. High-temperature heating of the boron carbide in a N2 atmosphere results in physisorption substrates of high quality. Adsorption isotherms for methane reveal the formation of four steps on boron carbide substrates subjected to the high-temperature treatment. The number and sharpness of the steps obtained on these substrates are comparable to those which can be obtained on other high-quality substrates. The surface areas of the boron carbide substrates used were between 0.3 and 0.15 m2/g. While these are not overly large values, they obviously allow the performance of adsorption measurements. The boron carbide substrates obtained after hightemperature cleaning are of sufficiently high quality to allow the study of the evolution of multilayer films as a
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function of temperature. They also permit to detect features corresponding to phase transitions occurring at monolayer coverages. Our results show that very high quality boron carbide substrates can be produced. We hope that these measurements will stimulate further research on this substrate. For the purpose of adsorption, it would be of particularly high interest to have an experimental determination of the structure of the exposed surfaces of the boron carbide substrates. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund of the American Chemical Society for support of this research. We thank Ms. Ruth Ann Wolfson for taking the electron micrographs of the boron carbide samples and Ms. Shyamali Saha for assistance provided in producing some of the figures. LA9708501
