Pyroelectric heat detector for measuring adsorption energies on thicker single crystals

The direct measurement of adsorption heats on the well-defined and highly ordered surfaces of single crystals (i.e., single crystal adsorption calorimetry, SCAC) has until now been performed only on samples with thickness of 0.2 or 1-8 μm, depending on the heat detection method. This paper reports a modification to a β-polyvinylidene fluoride (PVDF) pyroelectric heat detector, which allows measurements on much thicker samples (∼80 μm) with similar signal-to-noise. The improvement is achieved by increasing the compression between the single crystal and the 9 μm thick PVDF detector ribbon by pressing it with a thicker, insulating polymer, and by using a similarly-mounted reference detector to subtract away some of the voltage noise due to mechanical vibrations (also detected by the piezoelectric PVDF ribbons). For a 75 μm NiAl(1 1 0) single crystal, a sensitivity of 5 V per adsorbed joule and a pulse-to-pulse standard deviation of 4% in the measured heats are seen for adsorbed laser pulses of 3.7 μJ over a 0.138 cm2 area. Application of this method to measure the heats of adsorption of Pb onto NiAl(1 1 0) is demonstrated. The pulse-to-pulse standard deviation of the heat of adsorption at high coverages, where the heat is independent of coverage, is about ±5.2%, or 10.2 kJ/mol. This broadens the range of surfaces that can be studied by calorimetry, since this thickness can be achieved with nearly any single crystal material by simple mechanical thinning, whereas that is not possible with samples thinner than ∼50 μm over the required surface area of ∼1 cm2.