Pyroelectric heat detector for measuring adsorption energies on thicker
single crystals
Sensors And Actuators B-Chemical, 107 (1):
454--460 (May 2005)DOI: 10.1016/j.snb.2004.11.037
Abstract
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 mu m, depending on the heat
detection method. This paper reports a modification to a P-polyvinylidene
fluoride (PVDF) pyroelectric heat detector, which allows measurements
on much thicker samples (similar to 80 mu m) with similar signal-to-noise.
The improvement is achieved by increasing the compression between
the single crystal and the 9 mu m 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 mu m NiAl(110) 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
mu J over a 0.138 cm(2) area. Application of this method to measure
the heats of adsorption of Pb onto NiAl(110) 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 similar to
50 mu m over the required surface area of similar to 1 cm(2). (c)
2004 Elsevier B.V. All rights reserved.