JChimPhys (1991)88,1339-1352
©Elsevier, Paris
Electroreflectance of well-defined Pt surfaces
FVMolina1, RParsons2*
i Departmento de Química Inorganica, Analitica y Química Fisica, FCEN, Universidad de Buenos Aires,
Pabellón 2, Ciudad Universitaria, (1428) Buenos Aires, Argentina;
zDepartment of Chemistry, University of Southampton, Southampton, S09 5NH, UK
ABSTRACT
A method has been devised to carry out UV/vis electroreflectance studies on
electrodes making a meniscus contact with the electrolyte. In this way the
electrochemical state of the surface can be monitored at the same time as the
electroreflectance is studied. Experiments have been carried out on Pt single
crystals oriented to expose the low index plans (111), (100) and (110). The region
from the adsorption of hydrogen to the early stages of the adsorption of oxygen
species has been studied in the wavelength range 365 nm to 630 nm.
The
characteristics of the reflectance change in the hydrogen region differ from those
in the oxygen region and confirm that the so-called 'anomalous' region may be
attributed to hydrogen adsorption.
RÉSUMÉ
On présente un dispositif expérimental dans lequel on peut mesurer
l'electroreflectivité, UV/visible, d'une surface métallique en contact capillaire
avec 1'electrolyte. L’état électrochimique de l'échantillon peut être contrôlé
pendant les mesures optiques par voltammétrie cyclique. Les résultats sont obtenus
pour le platine polycristallin et les surfaces orientées (111) (100) et (110). On
a utilisé le domaine des longueurs d'onde entre 365 nm et 630 nm.
Les
caractéristiques de l'électroréflectivité dans le domaine d'adsorption d'hydrogène
diffère de celles dans le domaine d'adsorption d’oxygène. On vérifie que le
domaine dit "anormal" est vraiment lié à l'adsorption d'hydrogène.
INTRODUCTION
The electroreflectance of polycrystalline platinum was used about 20 years ago
to investigate the nature of hydrogen adsorbed on these surfaces [1-4], Two
distinct types of behaviour were observed corresponding to the two peaks in the
linear sweep voltammograms of the Pt electrode in acid solution, i.e. to the
strongly bound and weakly bound hydrogen. Bewick and Tuxford [3, 4) argued that
the strongly bound hydrogen, which corresponded to an increase in reflectivity,
Correspondenceandreprints
— 1340 —
could be identified with a state in which protons lie within the electronic surface
of the metal and the electrons are in the conduction band.
On the other hand the
weakly adsorbed hydrogen has more conventional optical behaviour characteristic
of a chemisorbed species on the surface of the metal.
Since those measurements were done, a great deal of work has been done on
hydrogen adsorption on Pt electrode using electrodes with well-defined surfaces and
it has become clear that the nature of hydrogen adsorbed on Pt is more complicated
than the above model would suggest, as many states can be observed on low index
crystal planes (see e.g. 5, 6). While it seems clear that the two main states on
a polycrystalline or poly-oriented electrode may be attributed to (110) like sites
with no long range order (weakly adsorbing hydrogen) and (100) like sites (strongly
adsorbing hydrogen), some of the other attributions remain the subject of argument.
This is particularly true of the 'unusual states' observed first on (111) surfaces
having a high degree of long-range order [7] . It might be expected that the nature
of the adsorbed hydrogen could be investigated by IR spectroscopy but a recent
study using a high performance SNIFTIRS technique has confirmed that no direct
measurements of the vibrational properties of submonolayer hydrogen can be made
[8],
It therefore seemed useful to return to the UV-visible electroreflectance
technique with the aim of relating the older observations on polycrystalline
surfaces to the more detailed electrochemical studies on well defined surfaces.
To carry out such measurements it is important that the state of the surface
is monitored closely by voltammetry.
Preliminary experiments [9] were done witli
completely immersed electrodes where this was not possible.
In the work presented
here, the cell and the optical path were designed so that the dipping configuration
[8] could be used in the same experiment as the reflectivity was studied.
EXPERIMENTAL
Platinum single crystal electrodes prepared using the technique developed by
Clavilier [7] have been used.
In order to have well controlled experimental
conditions, the cell was designed so that the electrode could be kept face down
during the reflectivity measurements (see figure 1); in that way, after recording
the voltammogram using the dipping technique (position a in figure 1) to check the
surface conditions, the electrode was immersed in the solution (position bj for the
optical experiments; it was necessary to immerse the electrode to avoid the
interference arising from internal reflection on the solution surface.
All the
experiments were conducted at room temperature under computer control, and the
waveform was generated digitally. The reflectivity changes were measured directly
— 1341 —
Figure 1; Experimental setup.
a: electrode position for voltammograms ;
b. position for reflectance measurements; L: xenon lamp; M: monochromator;
L1, L2 : lenses; M1, M2; mirrors; PM: photomultiplier; CA: current amplifier;
E. DC voltage source; AD; analog/digital interface; C; computer; (- - -) light
path.
using cyclic voltammetry, usually at 3 V/s, averaging enough cycles to diminish
the noise to acceptable levels; between 100 and 500 cycles were usually required,
depending on the conditions.
It was always checked that throughout the time that
the electrode was immersed the surface conditions were good; usually the recording
of a curve involved several stages of a number of cycles each, separated by
electrode annealing in order to recover the initial surface conditions.
The
experiments were conducted in both 0.1 M perchloric acid and 0.5 M sulfuric acid,
at incidence angles of 45 and 70 degrees and evaluating the response for both s and
P polarized light in the visible range (365-630 nm). The solutions were prepared
using Aristar grade sulfuric acid and perchloric acid (Suprapur); the water was
— 1342 —
obtained from a Millipore system, and sometimes was redistilled in addition.
RESULTS AND DISCUSSION
(a) Polvcrvstalline. (llO) and (100) surfaces
Figure 2 shows some typical results obtained with polycrystalline Pt.
Figure 2: Reflectivity variation for polycrystalline platinum in 0.5 M H2S04 with
the corresponding voltammograms on the same potential scale. Angle of incidence,
φ - 45°; sweep rate v = 1 Vs-1.
Comparison of the two upper reflectivity curves with curves i and j of figure 2 in
reference 4 shows satisfactory agreement in the general shape of the curve as well
— 1343 —
as Che magnitude of the effect.
The increase in reflectivity with adsorption of
the strongly bound hydrogen and the decrease with adsorption of weakly bound
hydrogen is confirmed.
The latter region according to the analysis of Armand and Clavilier [6] would
be expected to be similar to the corresponding potential region of the (110)
surface and figure 3 shows that this is indeed so, the amplitude of the
Figure 3 •_ Reflectivity variations for Pt(110) in 0.5 M H2S04; v - 3 V/s; ^ - 45°.
— 1344 —
reflectivity change being somewhat greater in accord with the larger number of
(110) sites on the single crystal surface.
On the other hand the former region
*should be compared with a (100) surface and figure 4 shows clearly that the
increase in reflectivity with adsorption of hydrogen occurs over the main hydrogen
Figure 4; Reflectivity
φ = 45°; v = 3 Vs-1.
variations
for
Pt(100)
in 0.5 M H2So4, at 546 nm and
region of this surface.
It should be noted that the voltammogram of figure 4 is
characteristic of a (100) without a substantial amount of long range order [11, 12]
and so will be comparable with the (100)-like sites on a polycrystalline surface.
These comparisons show clearly that the behaviour of polycrystalline Pt is
— 1345 —
determined primarily by the two types of site as already indicated by the
voltammetry [6], It may also relate to the explanation given by Bewick and Tuxford
[4] in that the deposited H atom can pass into the sub-surface and increase the
surface electron concentration when it is adsorbed on the more open four-fold site
of the (100) while on the closer packed three-fold site on the (110) surface, the
H atom must remain in the usual position for a chemisorbed species.
(b) (1111 surfaces
Figure 5 shows some typical results obtained for Pt(lll) in perchloric acid
Figure 5: Reflectivity variations for Pt(lll) in 0.1 M HC104; v - 3 V/s; φ - 45°;
potential scan range: 0.05 to 0.90 V.
— 1346 —
when the potential scan range is limited at the positive end, to 0.95 V (all
potentials are expressed on the reversible hydrogen scale) . At the more
potentials where,
as
it is generally accepted,
negative
a weakly bound layer of
electrosorbed hydrogen is formed, there is a decrease in reflectivity at low
wavelengths upon hydrogen adsorption, but at approximately 530 run there is a change
in the sign of the slope so that in the red end of the spectrum the reflectivity
increases with the increase in hydrogen coverage.
Apart from this last behaviour this is consistent with the optical properties
*of weakly bound hydrogen on polycrystalline Pt or Pt(110) as might be expected from
the presence of somewhat similar three-fold sites.
At the more positive potentials there is a sharp change in reflectivity
associated with the peak of the "anomalous feature" at 0.75 to 0.80 V.
If this
feature is due to hydrogen adsorption, the reflectivity change is an increase with
amount of adsorption like that of the strongly bound region on polycrystalline or
(100) Pt. However, if it were due to the adsorption of an oxygen species it would
have the more normal type of behaviour for a chemisorbed species. To investigate
these possibilities further, the upper limit was extended to 1.15 V where oxygen
electrosorption is known to occur [10], (figure 6), a further decrease with
significant hysteresis develops, specially in the violet end of the spectrum,
which appears clearly related to the oxygen adsorption-desorption peaks; this part
of the reflectance curve seems to have different characteristics from that at 0.77
V (see later) .
Comparison of the results for perchloric acid with those for
sulfuric acid (figure 7) shows that the optical behaviour in the so called
"anomalous hydrogen" region in both media is similar, that is, the sharp variation
in reflectivity appears in sulfuric acid displaced to about 0.35-0.40 V, in accord
with the displacement of the anomalous feature. When the spectral behaviour of the
differential reflectivity (obtained by differentiation of the A R/R curves with
respect to potential is studied (figure 8) , it is seen that this similarity holds
over the visible region, in that the spectrum for H2S04, solution at 0.35 V (A) is
closely similar to that for HC104 at 0.75 V (□). In contrast the spectrum obtained
for the oxygen electrosorption peak at 1.08 V (0) shows a different behaviour with
much more negative values at the blue end.
This suggests that the current in the
anomalous region in perchloric acid has the same origin as that on sulfuric acid,
and that it is not due to early OH adsorption.
While it is not possible to rule out completely the adsorption of sulphate
species as the source of the steep change in reflectivity around 0.4 V (figure 7b),
the radiotracer measurements of sulphate adsorption on Pt(lll) [14] indicate that
— 1347 —
Reflectivity variations for Pt(lll) in 0.1 M HC104; v = 3 V/s; φ = 43°;
:
6
e
r
u
g
i
F
Potential scan range: 0.05 Co 1.15 V.
1348 —
Figure 7: Reflectivity variations at 440 nm for Pt(lll) in: (a) 0.1 M HC104,
(b) 0.5 M H2SO4; v = 3 V/s; φ = 45°; potential scan range: 0.05 to 0.90 V.
this increases over a broad potential range 0.1 to 0.6 V which is inconsistent with
the much sharper change in reflectivity in the range 0.3 to 0.45 V.
In the potential region corresponding to the weakly bound hydrogen, a very
similar behaviour is found in the two solutions, both in the shape of the curves
(figure 7) and in the spectral characteristics (figure 9) as would be expected
because of the absence of adsorbed anions in this region. A distinctive feature,
as pointed out earlier, is the sign inversion in the reflectivity variations around
530 run.
— 1349 —
Figure 8: Differencial reflectivicy vs. wavelength for Pt(lll): (A) 0.5 M H2S04
at 0.35 V; (□) o.l M HC104 at 0.75 V; (◊) 0.1 M HC104 at 1.07 V. φ = 45°; (a) s
P° arization; (b) p polarization.
—
1350 —
Figure 9: Differential reflectivity vs. wavelength for weakly bound hydrogen on
Pt(lll): (Δ) H.5 M H2SO4, s polarization; (0)0.5 M H2S04, p polarization;
(□) 0.1 M HC104, s polarization; (◊ ) 0.1 M HC104, p polarization φ = 45°
It may therefore be concluded that the more probable explanation is that the
"anomalous region" corresponds to strongly bound hydrogen in agreement with the
original hypothesis of Clavilier [7, 10] and with the indications from the
experiments on the reduction of N20 [13].
The introduction of random steps in Pt(lll) (figure 10) causes, as expected,
the vanishing of the features associated with the anomalous hydrogen wave, and the
shape of the reflectance curve becomes somewhat similar to that of the polycrystal;
specially, in the weakly bound region, a sharp decrease analogous to the (110) case
develops as the current peak at ca. 0.1 V grows.
—
1351 —
Figure 10: Effect of the introduction of random steps on Pt(lll) in 0.5 M H2S04 at
365 nm and 45° , p polarization. Left: voltainmogram and reflectivity variations of
the electrode after annealing; right: the same after 6 cycles scanning up to
1.4 V.
ACKNOWLEDGEMENTS
One of
the authors
(FVM)
is
indebted
to
the Consejo Nacional
de
Investigaciones Cientificas y Tecnicas de la Republica Argentina for a fellowship.
We are grateful to Dr. J. Clavilier for supplying us with oriented single crystals
of Ft, and to Dr. L. Berlouis for the use of his electroreflectance equipment.
—
1352 —
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The papers published in this Special Issue on "Electrochemistry at well-defined metal surfaces" have
been received on 05 january 1991 to 20 april 1991 and accepted for publication on 15 may 1991.