Introduction to Nanotechnology

8.2. INFRARED FREQUENCY RANGE 199
dioxide on the surface. Note that the OH absorption signal is in the negative
(downward) direction. This means that the initial activated titania surface, whose
spectrum had been subtracted, had many more OH groups on it than the same
surface after CO adsorption. Apparently OH groups originally present on the surface
have been replaced by C02 groups. Also the spectrum exhibits structure in the range
from 2100 to 2400 cm-' due to the vibrational-rotational modes of the CO and C02.
In addition, the gradually increasing absorption for decreasing wavenumber shown
at the right side of the figure corresponds to a broad spectral band that arises from
electron transfer between the valence and conduction bands of the n-type titania
semiconductor.
To learn more about an infrared spectrum, the technique of isotopic substitution
can be employed. We know from elementary physics that the frequency of a simple
harmonic oscillator o of mass m and spring constant C is proportional to (C/m)''2,
which means that the frequency o, and the energy E given by E= Fim, both
decrease with an increase in the mass m. As a result, isotopic substitution, which
involves nuclei of different masses, changes the IR absorption frequencies of
chemical groups. Thus the replacements of ordinary hydrogen 'H by the heavier
isotope deuterium 2D (0.015% abundant), ordinary carbon I2C by 13C (1.1 1%
abundant), ordinary 14N by 15N (0.37% abundant), or ordinary I6O by "0 (0.047%
abundant) all increase the mass, and hence decrease the infrared absorption
frequency. The decrease is especially pronounced when deuterium is substituted
for ordinary hydrogen since the mass ratio mD/mH = 2, so the absorption frequency
is expected to decrease by the factor fi 1.414. The FTIR spectrum of boron
nitride (BN) nanopowder after deuteration (H/D exchange), presented in Fig. 8.5,
exhibits this fi shift. The figure shows the initial spectrum (tracing a) of the BN
nanopowder after activation at 875 K, (tracing b) of the nanopowder after subsequent
deuteration, and (tracing c) after subtraction of the two spectra. It is clear that the
deuteration converted the initial B-OH, B-NH2, and B2-NH groups on the surface
to B-OD, B-ND2, and B2-ND, respectively, and that in each case the shift in
wavenumber (Le., frequency) is close to the expected a. The overtone bands that
vanish in the subtraction of the spectra are due to harmonics of the fundamental BN
lattice vibrations, which are not affected by the H/D exchange at the surface. Boron
nitride powder is used commercially for lubrication. Its hexagonal lattice, with
planar B3N3 hexagons, resembles that of graphite.
A close comparison of the FTIR spectra from gallium nitride GaN nanoparticles
illustrated in Fig. 8.6 with the boron nitride nanoparticle spectra of Fig. 8.5 show
how the various chemical groups -OH, -NH2, and -NH and their deuterated
analogues have similar vibrational frequencies, but these frequencies are not pre-
cisely the same. For example, the frequency of the B-ND2 spectral line of Fig. 8.5 is
somewhat lower than that of the Ga-ND2 line of Fig. 8.6, a small shift that results
from their somewhat different chemical environments. The H/D exchange results of
Fig. 8.6 show that all of the Ga-OH and Ga-NH2 are on the surface, while only
some of the NH groups were exchanged. Notice that the strong GaH absorption band
near 21,000 cm-' was not appreciably disturbed by the H/D exchange, suggesting
that it arises from hydrogen atoms bound to gallium inside the bulk.