2868
Langmuir 1996, 12, 2868-2871
Comparative Study of the Substrate Quality of BN Powders Ruth Ann Wolfson, Liv M. Arnold, Praful Shrestha, and Aldo D. Migone* Department of Physics, Southern Illinois University, Carbondale, Illinois 62901 Received November 20, 1995. In Final Form: February 23, 1996
I. Introduction Physisorbed films are regarded as providing good physical realizations of matter in two dimensions (2D).1 Until recently, the vast majority of experimental studies of these films were conducted on graphite substrates. Graphite is, in many respects, close to an ideal substrate. It is, nevertheless, important to have high-quality substrate alternatives on which to study adsorbed films. The refinement attained in computer simulations and in experimental studies has made it clear that the behavior of a film is the result of the subtle interplay between the characteristics of the adsorbate and those of the substrate;2 hence, it is desirable to have a variety of substrates available. Excessive reliance on a single substrate makes it difficult to discern which behavior of a film is intrinsically 2D and which is substrate-induced. Interest in adsorption on BN substrates has been increasing in recent years.3-7 BN, an insulator, is isostructural with graphite, but has 2% larger lattice parameters.8 Commercially available BN powders come in a variety of grades. They differ from each other in their impurity concentrations and in their specific surface areas. We have conducted a comparative study of the substrate quality of different BN powders. We employed two techniques: adsorption isotherms and scanning electron microscopy. Prior to this work, there was no study available which systematically compared different grades of BN. By contrast, the different types of graphite have been very well characterized.9,10 Information on the quality of the different BN powders is useful for researchers trying to select the grade of BN best suited for use in a given experiment and for anyone wanting to compare the results of the different published adsorption studies on BN. (1) Monolayer and multilayer adsorptions are reviewed: Phase Transitions in Surface Films 2; Taub, H., Torzo, G., Lauter, H. J., Fain, S. C., Jr., Eds.; NATO ASI Series B; Plenum Press: New York, 1991; Vol. 267. (2) Experiments and simulations on rare gas films on graphite are reviewed extensively by Shrimpton, N. D.; Cole, M. W.; Steele, W. A.; Chan, M. H. W. In Surface Properties of Layered Structures; Benedek, G., Ed.; Kluwer Academic Publishers: Dordrecht, 1992; p 219. (3) Evans, M. D.; Patel, N.; Sullivan, N. S. J. Low Temp. Phys. 1992, 89, 653. (4) Wiechert, H. Bull. Am. Phys. Soc. 1995, 40, 525. Wiechert, H. Private communication. (5) Migone, A. D.; Alkhafaji, M. T.; Vidali, G.; Karimi, M. Phys. Rev. B 1993, 47, 6685. Alkhafaji, M. T.; Migone, A. D. Phys. Rev. 1992, 45, 8767. (6) Wiechert, H.; Maus, E.; Knorr, K. Jpn. J. Appl. Phys. 1987, 26, Suppl. 26-3, 889. (7) Shrestha, P.; Alkhafaji, M. T.; Lukowitz, M. M.; Yang, G.; Migone, A. D. Langmuir 1994, 10, 3244. (8) Wyckoff, R. W. G. Crystal Structures, 2nd ed.; Interscience Publishers: New York, 1963; Vol. I. (9) Birgeneau, R. J.; Heiney, P. A.; Pelz, J. P. Physica 1982, 109 & 110B, 1785. (10) Morishige, K.; Kawamura, K.; Yamamoto, M.; Ohfuji, I. Langmuir 1990, 6, 1417.
S0743-7463(95)01052-3 CCC: $12.00
II. Experimental Section We used four different grades of BN: Advanced Ceramics grades HCM (2.7 m2/g), HCPL (7.7 m2/g), and HCP (19.3 m2/g) and Johnson-Matthey grade Alfa (2.9 m2/g). Prior to performing the isotherms, all the powders underwent the same cleaning treatment: wash in methanol, filtration, and heating under vacuum at 900 °C (heating was continued until the pressure over the BN powder was below 3 × 10-6 Torr). This treatment, on which we have recently reported,7 has been proven effective in removing soluble borate impurities from the BN. Most of the isotherms measured in this study used Ar as the adsorbate and were performed at 77.3 K. We also investigated CH4 films at high coverages for one of the powders. Our measurements were conducted on a fully automated setup. A computer controls five electropneumatic valves which regulate the gas flow into (and out from) the sample cell. Pressures were measured using four MKS Baratron capacitance gauges (1, 10, 100, and 1000 Torr). Details of the apparatus have been provided elsewhere.7 Typically, data were taken at less than 0.01 of a layer coverage increments for the monolayer runs and at larger intervals for the multilayer isotherms. The fine spacing between data points allowed us to reliably obtain the two-dimensional isothermal compressibility (KT2D) of the film, which was calculated from the isotherm data. Scanning electron micrographs were taken from the four grades of BN studied. Because BN is an insulator, it had to be coated with an electron-emitting substance before the micrographs could be taken. We tried two coatings: Au films and C films. The C films produced a more uniform covering of the BN and better micrographs.
III. Results (A) Adsorption Isotherms. Our measurements focused on three portions of the isotherms: the low coverage region (below 0.2 of a layer); the region near monolayer completion (where Ar films undergo a solidification transition); the multilayer region (where we performed adsorption-desorption cycles with the films). (i) Low Coverage. The low coverage data provide information on the presence of high-energy binding sites on the substrate. High-energy binding sites become occupied first (i.e., at low pressures), before the rest the substrate gets covered.11 When high-energy binding sites are present, the isotherm will curve toward the coverage axis; the fraction of the layer over which this curving occurs provides a measure of the degree of binding energy inhomogeneity of the substrate. In Figure 1 we present a plot of the low-pressure region (pressures below 0.3 Torr) for Ar isotherms. Thermal transpiration corrections were applied to the pressures in this regime.12 The fraction of the monolayer which is convex toward the coverage axis ranges from a low of about 0.03 of a layer for the Alfa and HCM powders to a high of approximately 0.08 of a layer for the HCP grade powder. Higher specific surface area powders have a larger inhomogeneous fraction present in them. (ii) Monolayer Completion. Argon films adsorbed on BN undergo a solidification transition near monolayer completion.5 In an isotherm at 77.3 K this transition appears as a small substep (on the order of 0.025 of a monolayer) at pressures near 36 Torr. On a perfectly homogeneous, infinite substrate the transition will occur at a single value of the chemical potential (and, correspondingly, at a single value of the pressure). If the (11) See, for example: Adamson, A. W. Physical Chemistry at Surfaces, 4th ed.; John Wiley and Sons: New York, 1982. Lowell, S.; Shields, J. R. Powder Surface Area and Porosity, 2nd ed.; Chapman and Hall: London, 1984. (12) Takaishi, T.; Sensui, Y. Trans. Faraday Soc. 1962, 59, 2503.
© 1996 American Chemical Society
Notes
Langmuir, Vol. 12, No. 11, 1996 2869
Figure 1. Low-coverage portions of Ar adsorption isotherms on the different substrates (from top to bottom): Alfa (2.9 m2/ g), HCM (2.7 m2/g), HCPL (7.7 m2/g), and HCP (19.3 m2/g). The arrow corresponds to a coverage interval of 1/10 of a layer. The isotherms have been displaced along the Y axis for the sake of clarity; the pressure (X axis) is in Torr.
substrate is either heterogeneous or finite-sized (or both), there will be a spread of pressures over which the transition takes place.13 On a heterogeneous substrate the transition on different portions of the substrate will take place at different values of the chemical potential. For a finite-sized substrate there will be rounding of any phase transition taking place on it. The sharper the isotherm substep corresponding to a monolayer phase transition, the more the substrate approaches the ideal, infinite substrate. In Figure 2 we display our isotherm and isothermal compressibility (KT2D) results in the vicinity of the Ar solidification transition. The isothermal compressibility peaks range in height from approximately 47 m/N on the Alfa grade BN (specific surface area 2.9 m2/g) to about 11 m/N on the HCP grade (specific surface 19.3 m2/g). The full width at half maximum (fwhm) of the compressibility peaks (in units of µ/kB) is about 2 K for the three smaller specific surface area powders and goes up to 3.5 K for the HCP powder, the largest specific area substrate used. The KT2D peak heights and fwhm provide a quantitative measure of the sharpness of the isotherm substep. The two lower specific area substrates have the sharpest features, while grade HCP (the largest specific surface area substrate studied) has the broadest. (iii) Multilayer. Cycles of adsorption to high multilayer coverages followed by desorption measurements were used to investigate capillary condensation and the total pore volume for the different BN substrates. The equivalent of a minimum of 20 layers were adsorbed on the substrate before beginning the desorption measurements. This was done in order to ensure that all pores present were filled. Multilayer adsorption isotherms for Ar on BN are stepwise:5 each step corresponds to the formation of a layer on the substrate. In the adsorption branch, capillary condensation appears as a coverage increase occurring in the pressure interval between the end of one layer step and the beginning of the next layer step.14 These coverage increases between layers become rapidly larger as the pressure approaches the saturated vapor pressure.
A clearer sign of capillary condensation is the presence of a hysteresis loop in an adsorption-desorption cycle.11 For the same total coverage, the chemical potential on the desorption branch is lower than that on the adsorption branch.11 In Figure 3 we display the adsorption-desorption cycles for one of the powders studied (the Alfa powder, which has the second smallest pore volume). A hysteresis loop is clearly present. The closing of the hysteresis loop along the desorption branch gives rise to an additional identifiable feature indicative of capillary condensation: there is an extra step in the desorption isotherms, not present in the adsorption branch.11,15 Correspondingly, there is an extra compressibility peak at the closing of the loop. In Figure 4 we display the KT2D peaks associated with a multilayer adsorption-desorption cycle. The arrow marks the position of the additional step due to the closing of the loop. We have used the Kelvin equation to determine the pore size distribution and the total pore volume of our substrates from the desorption data.11 We have found a general trend correlating greater values of the total pore volume per unit mass with greater specific areas. Our results for this quantity are summarized in Table 1.
(13) Dash, J. G.; Puff, R. D. Phys. Rev. B 1981, 24, 295. Ecke, R. E.; Dash, J. G.; Puff, R. D. Phys. Rev. B 1982, 26, 1288. (14) Ser, F.; Larher, Y.; Gilquin, B. Mol. Phys. 1989, 67, 1077.
(15) Lysek, M. J.; La Madrid, M.; Day, P.; Goodstein, D. L. Langmuir 1992, 8, 898. Lysek, M. J.; La Madrid, M.; Day, P.; Goodstein, D. L. Langmuir 1993, 9, 1040.
Figure 2. Shown from top to bottom are the adsorption isotherm and isothermal compressibilities in the vicinity of the monolayer solidification transition on the four powders studied: Alfa, HCM, HCPL, and HCP. The pressure is plotted in scaled units (P/Pmax) with Pmax the pressure at the steepest point in the isotherm. The coverage is presented in fractions of a layer and the isothermal compressibilities in m/N. The arrow corresponds to a coverage interval of 0.25 layers and to an isothermal compressibility interval of 50 m/N. The melting substep is on the order of 0.025 of a layer. The compressibility provides a quantitative measure of the sharpness and steepness of the adsorption isotherm substep.
2870
Langmuir, Vol. 12, No. 11, 1996
Notes
Figure 3. Adsorption-desorption cycle for Ar on the Alfa BN substrate. Along the desorption branch the values of the pressure are lower than those on the adsorption branch for the same coverage. The arrow corresponds to a coverage interval of 5000 cm3 Torr.
Figure 5. Electron micrographs of the Alfa powder (top) and the HCP powder (bottom). There is a higher degree of uniformity, regularity in shape, and larger platelets for the Alfa powder than for the HCP powder. Figure 4. Isothermal compressibility corresponding to the adsorption (upper graph) and desorption (lower graph) cycles on the HCPL substrate. Peaks in the isothermal compressibility along the adsorption branch correspond to layers forming on the substrate. There is an extra peak in the isothermal compressibility along the desorption branch (marked by an arrow). The double-headed arrow along the Y axis corresponds to an isothermal compressibility interval of 400 m/N. Table 1. Resultsa powder specific surface area (m2/g) height of KT (m/N) KT fwhm (K) isotherm substep height (layer) specific pore volume (cm3/g) fraction of high-energy sites (layer)
ALFA HCM HCPL HCP 2.90 47.2 2.26 0.027 0.018 0.03
2.7 31.1 1.84 0.026 0.010 0.03
7.7 29 2.08 0.028 0.027 0.06
19.3 11.1 3.52 0.024 0.046 0.08
a The specific surface area is calculated from the coverage at the solidification substep. The peak height of the isothermal compressibility is in m/N. The units of the fwhm for the isothermal compressibility peaks are obtained by plotting the compressibility as a function of the chemical potential relative to the bulk chemical potential. The substep height is in layers, as is the fraction of high-energy binding sites. The specific pore volume is in cm3/g.
(B) Electron Micrographs. Scanning electron micrographs in the secondary electron mode were taken with a Hitachi S 570 scanning electron microscope. The insulating BN powder was placed on a sample holder with a small amount of adhesive. The powder was coated with
Au and with C films; both types of coatings were deposited by evaporation under high vacuum. The Au coating was not completely uniform and gave the appearance of cracks. The C coating was more uniform, resulting in smooth covering of the BN (probably due to the great deal of similarity between the structures of BN and graphite, which should allow for the smooth growth of a C film on top of the substrate). Two representative micrographs are shown in Figure 5. On the one taken of the Alfa powder, we discern platelets which mostly are in the range from 10 to 11 µm in linear dimensions; there are very few as small as 3 µm. The particles of this powder have more or less the shape of pancakes. By contrast, on the micrograph taken from the HCP powder there are no platelets greater than 7 µm, and there are a few as small as 0.8 µm. There is also a much greater variety of shapes present than in the Alfa powder. The semiquantitative features of the micrographs agree well with what we would expect from the thermodynamic results: The more uniform, larger-platelet, BN powders are those which have the best substrate quality, sharper isotherm steps, and smaller porosity. Conversely, BN samples which appear in the electron micrographs as less uniform, and being formed by smaller size platelets, also have lower quality in the thermodynamic measurements.
Notes
Langmuir, Vol. 12, No. 11, 1996 2871
IV. Conclusion We have used adsorption isotherms and electron microscopy to perform a comparative study of the substrate quality of four commercially available BN powders. We found a roughly inverse correlation between the quality of the BN substrate and its specific surface area. The adsorption isotherms indicate that BN substrates with low specific surface areas (HCM and Alfa) have fewer highenergy binding sites, sharper solidification adsorption isotherm substeps, and, less specific total pore volume than those grades of BN with larger specific areas (HCPL and HCP). Our main thermodynamic results are summarized in Table 1. The qualitative information obtained in the electron microscopy study is in good agreement
with the thermodynamic results: the better quality substrates have a more uniform shape distribution of particles and larger platelet sizes. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. We thank Dr. Morgan Evans and Professor Neil S. Sullivan for suggesting the use of Johnson-Matthey Alfa grade BN, and for making their results available to us prior to publication. We are thankful to the staff of the Center for Electron Microscopy of Southern Illinois University for assistance with the electron micrograph portion of this study. LA951052I
