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J. Phys. Chem. B 2001, 105, 3776-3782
Low-Temperature Adsorption Microcalorimetry: Pb on MgO(100)† D. E. Starr and C. T. Campbell* Department of Chemistry, UniVersity of Washington, Seattle, WA 98195-1700 ReceiVed: September 21, 2000; In Final Form: January 18, 2001
The heat of adsorption of Pb on an MgO(100) thin film at 190 K is measured calorimetrically as a function of coverage using a removable pyroelectric polymer ribbon as the heat-detecting element. This is the first demonstration that this versatile method of heat detection can be used for single-crystal adsorption calorimetry at cryogenic conditions. The results are compared to earlier measurements at room temperature. Similar to room temperature, the initial heat of adsorption at 190 K is ∼100 kJ/mol. However, the heat of adsorption grows much more slowly with coverage toward the bulk heat of sublimation, consistent with a larger Pb island density (and smaller, flatter islands) at low temperature. Measurement of the sticking probability for Pb onto the MgO(100) thin film at temperatures down to 167 K are also consistent with this.
I. Introduction A crucial role has been played by calorimetric measurements in advancing organic and inorganic chemistry to their current state. The whole basis for fundamentally understanding reactivity trends lies in their thermodynamic underpinnings revealed by calorimetry. These fields are so highly advanced that the researchers in these areas routinely develop valuable new products using a cognitive, as opposed to an Edisonian, approach. The extent of advancement of the field allows one to simply use Benson’s bond or group additivity tables1 to get reasonably accurate estimates of the enthalpy changes, entropy changes, and equilibrium constants for a broad range of organic reactions. In this sense, surface chemistry lags far behind, although some very important systematics of bond energies are beginning to be revealed.2-7 With respect to this, the singlecrystal adsorption calorimetry results from King’s group are making a big impact.8-14 These accurate calorimetric measurements not only provide the basic data for such bond additivity calculations in surface chemistry, but they also provide benchmarks against which to compare various quantum mechanical approximations. Although admittedly crude, bond additivity estimates at this level often prove to be valuable in estimating reaction heats, predicting reaction pathways, and distinguishing between alternative mechanisms.1,2,4,15,16 Accurate knowledge of enthalpy changes for reactions can also be very useful in interpreting and predicting kinetic data. For example, the fact that the reaction energy equals the difference between the activation energies for the forward and reverse reactions allows the calculation of one from the reaction energy if the other is known. As a result, calorimetric measurements of adsorption energies could help clarify the energetics, kinetics, and mechanisms of many important processes in thin film growth, molecular beam epitaxy (MBE), and chemical vapor deposition (CVD). Furthermore, we have shown recently 17,18 that they can provide also the adhesion energies and interfacial energies of solid/solid interfaces, which are important in many materials applications. †
Part of the special issue “John T. Yates, Jr. Festschrift”. * Corresponding author. E-mail:
[email protected]. Fax: (206)-685-8665.
While temperature-programmed desorption (TPD) provides a desorption activation energy that is equal to the heat of adsorption, in many cases where adsorption is nonactivated, frequently the adsorbate does not desorb intact but predominantly dissociates during TPD. Furthermore, an adsorbate may diffuse into the bulk or form another high-temperature phase before reaching the temperature at which it can desorb. An example of this is late transition metal adsorbates on oxide surfaces, which, generally, cluster into large three-dimensional particles before desorbing. Finally, the substrate may decompose or restructure during heating before the adsorbate desorbs. Thus, TPD can only provide heats of adsorption for the species of interest in a relatively few limited cases. Adsorption isotherms are also of limited value in this respect for similar reasons. Thus, there is a clear need for direct calorimetric measurements of adsorption energies on single-crystal surfaces. Calorimetric measurements of heats of adsorption on higharea metal films were common in the early days of surface science.19 However, it was often not clear what the adsorbates being formed on such films really were, or which of the many different sites on its heterogeneous surface were being populated. Adsorption calorimetry is most powerful when the heats being measured are associated with the production of a single, welldefined surface structure. Since this can only be realized on single-crystalline surfaces in ultrahigh vacuum (UHV), the most interesting adsorption calorimetric measurements would be on single crystals, i.e., single-crystal adsorption calorimetry (SCAC). With the development of the first apparatus for adsorption microcalorimetry that could work routinely on single-crystal surfaces by David King’s group in England,8-14 these measurements can now be made as a detailed function of coverage under conditions where the nature and site for the adsorbate are well defined. Their method for SCAC relies on ultrathin metal single crystals whose transient temperature rise after adsorption of ∼1% monolayer pulses of gas is measurable with infrared optical pyrometry. The ability to measure at cryogenic conditions is very important in adsorption calorimetry. Certain forms of molecular adsorbates and certain molecular fragments or reaction intermediates can only be captured if the surface is held at low temperature, since adsorbates often dissociate or desorb even below room
10.1021/jp003411a CCC: $20.00 © 2001 American Chemical Society Published on Web 03/09/2001
Pb on MgO(100) temperature. In thin film growth, many interesting structures can only be stabilized below room temperature. In metal adsorption on oxide surfaces, which is the subject of the present study, the metal atoms are very mobile at room temperature and often agglomerate into thick three-dimensional (3D) islands upon adsorption.17,20 Therefore, the heat of adsorption of metal atoms which one measures at room temperature is really the energy associated with making these 3D particles and, thus, contains only weak information about the strength of the metaloxide bonding. Since true 2D structures (islands and/or isolated adatoms) should be able to be produced at low temperatures where surface mobility is less rapid, one has a better chance at low temperature of making measurements that reflect more directly the strength of the metal binding to the oxide surface. The list of adsorption systems wherein a very interesting structure can only be measured at temperatures well below 300 K is long. Thus, there is a strong need for extending singlecrystal adsorption microcalorimetry to cryogenic conditions. King’s group recognized this and, indeed, reported the first example of cryogenic SCAC.21 Their method, which was shown to work at 100 K, was based on a completely different heat detection scheme than was used in all their other SCAC studies. It required that the thin single-crystal sample be permanently mounted directly onto a thick LiTaO3 pyroelectric detector, which was in a holder that could be cooled with liquid nitrogen.21 They do not currently use this method for their SCAC experiments because of the difficulties it presented in sample preparation. (For detection based on infrared optical pyrometry, currently used so successfully and almost exclusively by King’s group, the sensitivity theoretically decreases by a factor of 27 between 100 and 300 K,21,22 which would make measurements at 100 K with this standard room-temperature method impossible.) The cryogenic method for SCAC reported by King’s group has a very important limitation; the sample has to be permanently mounted to the pyroelectric crystal, allowing annealing temperatures (often necessary for surface pretreatment) only up to ∼940 K (the Curie temperature of the LiTaO3 pyroelectric used in their method23). Higher temperatures damage the pyroelectric crystal. Since higher annealing temperatures on many types of single crystals are often required to get a clean and well-ordered surface, this limits the range of surfaces that can be studied. For example, the Mo(100) crystal used as the substrate for the cryogenic SCAC measurements reported here requires annealing to 1900 K for preparation, which would clearly be impossible with King’s method for cryogenic SCAC. Recently, we have developed a new method for SCAC, based in many ways on King’s methods but using a different method of heat detection that has similar sensitivity.18,24 Our method for detecting the heat of adsorption uses a thin pyroelectric polymer ribbon which is gently pressed against the back of the single-crystal sample (during calorimetric measurements only). Our heat detection method for SCAC has several distinct advantages over King’s methods, including the abilities to more easily prepare samples, use thicker samples, and thus use a wider variety of substrates.18,24-29 Note that the pyroelectric detector is removed from the single crystal before and after heat measurements so that the single crystal can be pretreated to any temperature desired without damaging the detector. For this type of detector, it is expected that the magnitude of the calorimeter voltage signal for a given heat input should be almost the same when the sample is cooled by liquid nitrogen as when it is at 300 K.30 Indeed, this was one of the reasons we chose this type of detector in designing this new calorimeter.
J. Phys. Chem. B, Vol. 105, No. 18, 2001 3777 In this paper, we describe apparatus modifications allowing SCAC with our calorimeter type at cryogenic conditions and report the first low-temperature calorimetric measurements using it. The data presented are for a system previously studied at room temperature,31 Pb adsorption onto a thin MgO(100) film grown on Mo(100). The previous room-temperature results31 indicated an initial heat of adsorption of 103 kJ/mol. The heat of adsorption then increased steeply with coverage, reaching the heat of sublimation of bulk Pb by ∼1.5 monolayers (ML) of Pb(111). Auger electron spectroscopy (AES) results indicated growth of three-dimensional Pb islands at least above 0.1 ML. In the current study, the metal’s heat of adsorption and sticking probabilities as a function of coverage have been measured at temperatures ranging from room temperature down to 164 K. The combination of these measurements provides insight into the effect of film morphology on adsorption energetics. To achieve these measurements, we mounted the sample onto a moveable platten which is mounted directly on the calorimeter’s heat detector housing during measurements. The platten/ detector assembly can be cooled with liquid-nitrogen (or, in principle, helium) for cryogenic measurements. The use of a sample platten should also increase the rate of data acquisition in general, since it could be coupled with a sample holding stage for multiple moveable plattens on a simple linear manipulator, thus allowing several samples to be studied on the same day. II. Experimental Section The single-crystal adsorption microcalorimetry apparatus and procedures have been described in detail elsewhere18,24 and will be briefly summarized here. During the experiment, a pulse of gaseous metal atoms from a chopped atomic beam impinge onto a thin single-crystal sample. When they adsorb onto the surface of the sample, energy is released into the crystal, causing a small transient temperature rise. The temperature rise is detected with a 9 µm thick ribbon of pyroelectric polymer (β-PVDF) pressed in contact with the backside of the sample. The temperature rise of the crystal induces a temperature change in the pyroelectric detector, causing it to develop a transient face-to-face voltage, which produces the measured signal. (The pyroelectric ribbon is coated on both the front- and backsides with 20-60 nm thick NiAl film providing electrical contact to measure the induced voltage.) The sensitivity, in voltage output per joule of absorbed heat (V/Jabs), varies somewhat from contact to contact due to changes in the thermal contact between the sample and the pyroelectric ribbon. The detector voltage output is calibrated directly before (and after) each heat measurement with pulses of He-Ne laser light of known energy. One difficulty associated with these experiments is that the effusive source of the metal atom beam produces radiation which contributes to the measured signal. To quantify the portion of the signal due to the radiation, we placed a BaF2 window in front of the metal source. This allows the passage of a known fraction of the radiation (given by the measured transmittance of the BaF2, typically 0.92) while blocking all of the metal atoms, thus providing a measurement of the amount of signal due to the radiation alone. To report heats of adsorption per mole adsorbed, we must also measure the metal’s sticking probability as a function of coverage. To achieve this, we detected the amount of metal that is reflected off the sample when the metal pulse impinges onto the sample with a line-of-sight quadrupole mass spectrometer (QMS). The QMS signal is calibrated using a hot Ta foil (T ∼1300 K), which reflects (or rapidly desorbs) the entire pulse of atoms. A T1/2 scaling of the signal is applied to correct for
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Figure 2. Schematic of the movable sample platten and calorimeter head. Left: front view. Right: side view: (a) Calorimeter head approaching contact and (b) the head inside the platten in contact with the sample. The dashed line is a cutout to show the beveling of the inside of the sample platten used to guide the conical portion of the calorimeter head into the platten and also tighten the fit to reduce mechanical vibrations. The pyroelectric ribbon, as shown, compresses against the backside of the sample, allowing thermal contact and, thus, pyroelectric detection. The left-hand side of the figure shows a front view of the sample platten and the sample mounting.
Figure 1. New calorimeter design for cryogenic measurements. In panel a, the sample platten is in the manipulator forks and being lowered into the reservoir forks. In panel b, the sample has been placed into the reservoir forks and the manipulator arm removed. In panel c, the calorimeter head has been driven into the back of the sample platten.
the different thermal velocities of the molecules passing through the ion source during the calibration measurements and the sticking probability measurements. The QMS is placed at the “magic angle” such that variations in the signal due to differing distributions of atoms coming off of the surface are minimized.37 (The incident beam is directed almost normal to the surface to help in this respect.) The 1 µm thick, 1.0 cm diameter Mo(100) single-crystal samples used in these experiments were purchased from Jacques Chevallier at Aarhus University. The sample is spot-welded between the stainless steel sample platten (see Figures 1 and 2) and a 0.25 mm Ta sheet, both containing an ∼7 mm diameter hole to allow access to both sides of the sample. Once mounted, the Mo(100) sample was cleaned by cycles of thermal annealing (with electron beam heating) to ∼1500 K in a 10-7 mbar background pressure of oxygen followed by annealing to ∼1900 K in UHV. This sample gave a very sharp p(1 × 1) LEED pattern and was free of impurities, as checked by Auger electron spectroscopy (AES). MgO(100) thin films were prepared on the Mo(100) single crystal by the thermal evaporation of Mg metal at a rate of ∼5 × 1014 atoms/(cm2 min) in a background of ∼3 × 10-7 mbar of oxygen. AES analysis of the deposition rate of Mg indicated that the films as grown were ∼4.0 nm thick. This method of producing MgO(100) thin films has been described previously.32,33 Following growth, the thin film was briefly annealed at ∼850 K in UHV to order the film. This procedure produced
LEED patterns of the MgO(100) thin film similar in quality to those reported by Wu et al.32,33 The films produced in this manner have defect densities corresponding to several percent of the surface area, which may affect our measured heats of adsorption, since metals tend to adsorb first at defects on oxides.17 II.1. Technical Requirements for Low-Temperature Microcalorimetry Experiments. If the microcalorimetry experiment is to be performed at temperatures other than room temperature, the drift in temperature cannot affect the peak-topeak voltage response of the pyroelectric detector. A measure of the required temperature stability can be found as follows. Assuming a 1 µm thick Mo sample with heat capacity of 2.55 J/(cm3 K) and that the heated volume of the sample is localized in the area of the incident atomic beam, 0.138 cm2, the temperature rise for a 0.02 monolayer pulse with a heat of adsorption of 200 kJ/mol is ∼0.03 K. Typical pulse-to-pulse standard deviations in the peak-to-peak voltage response are ∼2%. Given the pulse rise time of ∼0.2 s, the temperature stability required is
