Phases of underpotentially deposited Hg on Au(111): An in situ surface X-ray diffraction study

Article abstract of DOI:10.1021/jp963861w

We report on an in situ surface X-ray diffraction study of the underpotential deposition (UPD) of mercury on Au(111). We have observed three UPD phases present at potentials prior to bulk mercury deposition. These phases consist of two well-ordered intermediate states and what appears to be either a fully discharged two-dimensional liquid Hg layer or a monolayer of an amorphous Hg-Au alloy. Both ordered intermediate phases have hexagonal structures with lattice vectors that are rotated 30° from those of the Au(111) substrate. The first phase (phase I), present at a potential of +0.68 V, was only observed on fresh flame-annealed Au(111) electrodes and appears to be an open incommensurate structure with a lattice constant of 3.86 ± 0.03 A?. This phase appears to be metastable since it changes to a second ordered phase (phase II) after a certain time at +0.68 V or after the potential is moved to more negative values (+0.63 V). The second phase has a more compact lattice with a = 3.34 ± 0.01 A? and appears to be a commensurate 2×2 structure with 2/3 of the Hg atoms at threefold hollow sites and 1/3 on atop sites. Similar to the first one, this phase is also metastable and can be transformed to a final, fully discharged, state of a two-dimensional liquid Hg layer or an amorphous Hg-Au alloy. The entire Hg UPD process, from Hg²⁺ to the fully discharged metallic Hg layer, agrees well with a multistep mechanism based on previous electrochemical kinetic studies on polycrystalline Au electrodes. Our results also show that the UPD of Hg on Au(111) electrodes is quite different from that of other metals such as Cu, Ag, Tl, and Pb.

Full text of DOI:10.1021/jp963861w

J. Phys. Chem. B 1997, 101, 2907-2916
2907
Phases of Underpotentially Deposited Hg on Au(111): An in Situ Surface X-ray Diffraction
Study
Jun Li and He´ctor D. Abrun˜a*
Department of Chemistry, Baker Laboratory, Cornell UniVersity, Ithaca, New York 14853-1301
ReceiVed: NoVember 19, 1996; In Final Form: February 3, 1997X
We report on an in situ surface X-ray diffraction study of the underpotential deposition (UPD) of mercury on
Au(111). We have observed three UPD phases present at potentials prior to bulk mercury deposition. These
phases consist of two well-ordered intermediate states and what appears to be either a fully discharged two-
dimensional liquid Hg layer or a monolayer of an amorphous Hg-Au alloy. Both ordered intermediate
phases have hexagonal structures with lattice vectors that are rotated 30° from those of the Au(111) substrate.
The first phase (phase I), present at a potential of +0.68 V, was only observed on fresh flame-annealed
Au(111) electrodes and appears to be an open incommensurate structure with a lattice constant of 3.86 (
0.03 Å. This phase appears to be metastable since it changes to a second ordered phase (phase II) after a
certain time at +0.68 V or after the potential is moved to more negative values (+0.63 V). The second
phase has a more compact lattice with a ) 3.34 ( 0.01 Å and appears to be a commensurate 2×2 structure
with 2/3 of the Hg atoms at threefold hollow sites and 1/3 on atop sites. Similar to the first one, this phase is
also metastable and can be transformed to a final, fully discharged, state of a two-dimensional liquid Hg
layer or an amorphous Hg-Au alloy. The entire Hg UPD process, from Hg2+ to the fully discharged metallic
Hg layer, agrees well with a multistep mechanism based on previous electrochemical kinetic studies on
polycrystalline Au electrodes. Our results also show that the UPD of Hg on Au(111) electrodes is quite
different from that of other metals such as Cu, Ag, Tl, and Pb.
1. Introduction
electrochemical methods.8-12 It appears that mercury elec-
trodeposition on polycrystalline gold electrodes involves some
intermediate states and this was interpreted as being due to the
presence of a mixture of Hg(0) and Hg(I) species (likely Hg22+).
These intermediate states are subsequently reduced to a full layer
of metallic mercury. It has also been found that the mercury
overlayer is stable in the UPD regime (i.e., at submonolayer
coverage). Once the coverage exceeds one monolayer, the Hg
atoms appear to diffuse into the bulk Au crystal to form an
alloy (amalgam). However, this process is not well understood
because of the lack of detailed microscopic structural informa-
tion.
Salie´ and Bartels13-16 found that in perchloric acid solutions
containing Hg(I) or Hg(II), the UPD of mercury involves a
nonintegral partial charge transfer. Two different processes were
detected during the deposition of Hg(I) at polycrystalline Au
electrodes in the underpotential range by potentiostatic measure-
ments at a rotating ring-disk electrode13-15 and by impedance
spectroscopy.16 The first process was proposed to be the
adsorption of metal ions onto the gold substrate surface to form
an intermediate species with a partial charge of about 0.53e-
per Hg atom. The second process, whose kinetics appeared to
be substantially slower, was one in which the partially charged
Hg ions were completely discharged to form a full overlayer of
metallic Hg. The first process can be thought of as chemisorp-
tion where the chemical bonds of adsorbed ions with the
substrate metal lattice and surrounding electrolyte species
determine the partial charge of the adsorbed ions. The second
process would correspond to the completion of the full UPD
layer and the onset of alloy formation. Within the UPD regime,
measurements taken in Hg(II) solutions were found to be
essentially the same as in Hg(I) solutions. Vicente-Beckett17
also found, by potentiostatic measurements at a rotating gold
The underpotential deposition (UPD) of monolayer or sub-
monolayer amounts of metals onto foreign metal substrates is
a fundamental electrochemical process which has attracted
longstanding interest. The processes involved are also closely
related to adsorption/desorption, charge transfer, nucleation and
growth, and electrocatalysis. Within the UPD regime, the
overlayer coverage and structure can be modulated and/or
controlled by the electrode potential and the presence of
coadsorbates, especially anions. The structures of UPD over-
layers have been found to depend strongly on specific crystal-
lographic planes and specific interactions. Much effort has been
placed on correlating microscopic structures with electrochemi-
cal reactivities in UPD systems. In recent years, significant
progress has been made in understanding UPD processes, in
part due to the advent of in situ structural characterization
techniques including scanning tunneling microscopy (STM),1,2
atomic force microscopy (AFM),3 and synchrotron X-ray
scattering.4-6
In this paper, we report on three distinct phases of Hg
monolayers underpotentially deposited on Au(111) observed by
in situ surface X-ray scattering measurements. This study
2+
follows our recent report on a coadsorbed structure of Hg2
2-
and SO4 at the initial stages of Hg UPD.7 These studies
present a very detailed picture of the microscopic processes
involved in Hg UPD and agree well with previous electrochemi-
cal kinetic studies.
The Hg UPD process is of particular importance due to the
special electrochemical properties of Hg surfaces and the
formation of amalgams by Hg with a variety of metal substrates.
There have been extensive studies of the electrodeposition of
Hg on Au, especially polycrystalline surfaces, by conventional
X Abstract published in AdVance ACS Abstracts, March 1, 1997.
S1089-5647(96)03861-8 CCC: $14.00 © 1997 American Chemical Society

2908 J. Phys. Chem. B, Vol. 101, No. 15, 1997
Li and Abrun˜a
ring-disk electrode, that the UPD of Hg in sulfuric acid solution
is similar to that in perchloric acid media.
by Gewirth et al.18 found a close-packed hexagonal overlayer
(with a ) 2.9 to 3.1 ( 0.3 Å) in all four electrolytes. This
suggests that an ordered metallic Hg monolayer is indeed formed
on Au(111) surfaces in all cases.
We report here direct in situ surface X-ray diffraction
evidence of two intermediate phases of UPD Hg layers during
the electrodeposition process and propose a mechanism for the
entire process which is consistent with previous electrochemical
studies.
In this paper, we report on three additional phases present
during Hg UPD on Au(111) in sulfuric acid solutions. Two of
these phases are ordered Hg adlayer structures which are present
as intermediate states. The third and final state of Hg UPD
just prior to bulk deposition represents a monolayer of two-
dimensional liquid Hg or an amorphous Hg-Au alloy. These
three phases were revealed by synchrotron-based surface X-ray
scattering studies. The paper is organized as follows. The
experimental details (section 2) will be briefly discussed first
and then the electrochemical behavior of Hg UPD on Au(111)
electrodes is presented in section 3.1. The grazing incidence
X-ray diffraction (GIXD) measurements of the two ordered Hg
UPD phases and corresponding in-plane real space structures
will be presented next (section 3.2). Specular crystal truncation
rod (CTR) measurements of both phases are presented in section
3.3 to reveal the structure along the surface normal. In section
4, we discuss the time dependence of these two Hg UPD phases
and correlate the structures with the deposition mechanism.
Conclusions are presented in section 5.
There have been previous AFM and STM studies intended
to derive the atomic structures of Hg UPD overlayers on
Au(111). Using AFM, Gewirth and co-worker18 investigated
the structure of UPD Hg overlayers in four different supporting
electrolytes and revealed a strong dependence of the Hg
overlayer structure on the nature of the anions. They found
that, in sulfate, nitrate, and perchlorate electrolytes, the overlayer
presented the same open hexagonal lattice with a lattice constant
a ) 5.8 ( 0.2 Å at potentials just prior to bulk deposition and
a close-packed hexagonal lattice with an atom-atom spacing
of a ) 2.9 ( 0.3 Å just after bulk deposition. On the other
hand, in acetate solutions, the Hg overlayer presented a close-
packed hexagonal lattice with a ) 3.1 ( 0.2 Å just prior to
bulk deposition. At more positive potentials, they found two
additional open structures.
However, in recent in situ STM studies, Itaya and co-
workers19 found that the structure of the Hg adlayer at the first
stage of UPD in sulfuric acid is different from that in perchloric
acid. In both cases, the Hg overlayers present commensurate
open rectangular lattices which are different from the hexagonal
lattice reported by Gewirth.18 Although there is a clear
discrepancy in these results, they both indicate a strong effect
of supporting electrolyte anions on the resulting Hg UPD
structures. It is also clear that mercury appears to form ordered
adlayers when the deposition is within the monolayer regime.
The ambiguity in the specific Hg adlayer structure may actually
be due to anions adsorbed on top of the Hg adlayer since both
AFM and STM are most sensitive to the topmost surface. The
strong tip-adsorbate interaction makes it difficult for STM and
AFM techniques to probe the real undisturbed overlayer
structure.
On the other hand, synchrotron radiation based surface X-ray
diffraction has the advantage over STM and AFM techniques
in that X-ray photons penetrate down to the bulk substrate
without any disturbance to the overlayers. Thus X-ray scattering
measurements can probe the precise undisturbed multilayer
structure along the surface normal as well as the atomic
arrangement within each layer. This is extremely valuable
particularly for UPD processes which often involve the coad-
sorption of other electrolyte species.
In a recent paper,7 we reported that there appears to be a
highly ordered coadsorbed structure in the first stage of Hg UPD
on Au(111) (at +0.80 V e E e +0.88 V vs Ag/AgCl(3 M
KCl)). This structure consists of a distorted honeycomb lattice
of mercurous (Hg22+) ions (0.375 monolayer of Hg22+) with
sulfate anions adsorbed in the hollow sites (0.375 monolayer)
and above the plane of Hg(I). This study agreed well with
electrochemical data20 and explained the very sharp peaks in
the cyclic voltammogram at +0.93 V. We believe that the large
open lattices observed by STM and AFM likely arise from the
disturbed sulfate anions instead of Hg adlayers, as has been
previously found for Cu UPD on Au(111).1-3
2. Experimental Section
The experimental setup and measurement conditions were
described in detail in our previous report.7 A Au(111) single-
crystal disk (∼9 mm in diameter and ∼2 mm thick) was used
as the working electrode for both electrochemical and X-ray
scattering measurements. The crystal was chemically etched
and flame-annealed before being placed in the X-ray scattering
cell. A Ag/AgCl (3 M KCl) reference electrode was used
without regard for the liquid junction. A coil of platinum wire
was employed as the counter electrode. High-purity H2SO4 (J.
T. Baker) and ultrapure water (Milli-Q) were used to prepare
solutions. Solutions containing Hg2+ were prepared by dis-
solving HgO (Alfa, 99.998%) into appropriate solutions.
Electrochemical experiments were carried out with a BAS CV-
27 potentiostat and recorded with a BAS X-Y recorder. In some
cases, a PAR 283 potentiostat interfaced with Corrware software
was employed.
X-ray diffraction experiments were performed at the Exxon
X10B beamline at the National Synchrotron Light Source using
a four-circle diffractometer. X-ray photons of 1.1287 Å
wavelength were employed. The sample cell has a reflection
geometry with the crystal at the center. X-ray photons penetrate
through a 2.5 µm Mylar film (Chemplex) as well as a thin film
of electrolyte (estimated to be ∼30 µm thick) covering the
Au(111) crystal. During X-ray scattering measurements, the
electrolyte was withdrawn to achieve a thin layer configuration
in which the background and absorption were reduced. Each
time the potential was changed to a new value, the Mylar film
was inflated by adding more electrolyte and held in this
condition for about 5 min to ensure equilibration. Typically, a
complete specular CTR measurement takes about 6-8 h.
However, the freshly deposited intermediate Hg phases were
found to be stable only for 3-6 h. To ensure that we were
measuring the same structure, we stripped and redeposited Hg
adlayers every 2 h.
However, there is still a lack of detailed information about
the additional stages of Hg UPD on Au(111) in sulfuric acid
solutions where a full monolayer of metallic Hg is formed just
prior to bulk deposition. STM images are very difficult to
acquire at these potentials due to the electrochemical oxidation
of mercury that takes place at the tip electrode.19 AFM studies
In this paper, the X-ray reflections are referred to the
reciprocal lattice units of the hexagonal coordinates of the
Au(111) substrate, with as and bs along the nearest-neighbor
direction in the surface plane (as ) bs ) 2.885 Å) and cs (cs )
2.356 Å) normal to the Au(111) plane. The Q vectors are
described by two components with Q| ) has* + kbs* in the

Phases of Underpotentially Deposited Hg on Au(111)
J. Phys. Chem. B, Vol. 101, No. 15, 1997 2909
Figure 2. Reciprocal space map of the in-plane diffraction from Hg
UPD phase I (solid circles) and Hg UPD phase II (solid squares) relative
to that of Au(111) surface (open circles). The reflections from Au(111)
surface are indexed as (h,k)s and those from Hg UPD phase I and phase
II are indexed as (h,k)I and (h,k)II, respectively.
Figure 1. Cyclic voltammogram of a Au(111) electrode in 0.10 M
sulfuric acid containing 1.0 mM Hg2+ at a scan rate of 2 mV/s. Inset:
voltammetric profile over the range of +1.05 to +0.70 V vs Ag/AgCl
(3 M KCl).
+0.60 V appears to correspond to the deposition of the first
Hg monolayer and the sharp spike at +0.52 V likely indicates
the completion of the Hg monolayer and the onset of amalgam
formation. The sharp feature at +0.93 V is consistent with our
previously reported coadsorption structure which is reversible
and has rapid kinetics. On the other hand, the broad diffusional-
like feature between +0.50 and +0.70 V suggests a slow
process. This might be accounted by the existence of interme-
diate states during the reduction of Hg(I) to Hg(0), as indicated
in previous electrochemical studies on polycrystalline Au.13-17
In this paper, we will focus on the structure and kinetics of Hg
UPD layers at potentials within this broad wave.
surface plane and Qz along the surface normal. GIXD measure-
ments were carried out in azimuth fixed mode where the incident
and outgoing angles were kept small (R ) â ≈ 3.5°, at L ) 0.1
reciprocal lattice unit (rlu)) so as to reduce the background and
absorption by the Mylar film and electrolyte solution.
A
complete CTR measurement consists of a series of rocking
curves at Qz’s from 0 to 2 rlu. Each of these rocking curves
was fitted to a Lorentzian line shape to derive the integrated
intensity which was subsequently used for fitting.
3. Results
3.2. Grazing Incidence X-ray Diffraction Results. GIXD
measurements were taken at a series of potentials from +1.05
to +0.50 V at 50 mV intervals. As we described in our previous
paper, an ordered coadsorbed structure of Hg2SO4 was found
at +0.80 V e E e + 0.88 V. When the potential was changed
to approximately +0.68 V (the onset of the broad reduction
wave), we found an ordered hexagonal structure corresponding
to a Hg UPD adlayer (referred to as UPD phase I). This
structure was transformed to the second ordered hexagonal
structure (referred to as UPD phase II) when the potential was
moved to values below +0.63 V but above that of bulk
deposition (around +0.45 V).
Figure 2 shows the experimentally observed in-plane dif-
fraction spots of Hg UPD phase I and phase II relative to the
Au(111) substrate. The open circles represent the reciprocal
lattice points of the Au(111) surface which are indexed as (h,k)s.
The solid circles are the measured diffraction spots from the
Hg UPD phase I (indexed as (h,k)I) and the solid squares are
from the Hg UPD phase II (indexed as (h,k)II). Both of these
Hg UPD phases have hexagonal lattices rotated by 30° relative
to the Au(111) substrate orientation. The diffraction spots from
Hg UPD phase II fall on, within our experimental error (<0.5%),
the spots indexed with the substrate coordinates (h/2, k/2)s (with
h, k as integers), indicating that the overlayer is likely com-
mensurate with the Au(111) substrate. On the other hand, Hg
UPD phase I is clearly incommensurate with the substrate
surface. For both structures, higher order reflections at
(1,-1)I, (2,1)I, and (1,-1)II, (2,1)II were also observed, which
unambiguously defined the hexagonal lattice of the Hg UPD
adlayers.
3.1. Cyclic Voltammetry. Figure 1 shows the cyclic
voltammogram of a Au(111) electrode in 0.10 M H2SO4
containing 1.0 mM Hg2+ at a scan rate of 2 mV/s, which is
similar to that reported by Itaya and co-workers.19 The inset
shows the voltammogram when the potential is cycled between
+0.70 and +1.05 V, where one can observe two sets of very
sharp peaks around +0.93 V. In addition, there are two much
smaller and broader peaks at +0.74 and +0.81 V, respectively.
As discussed in our previous paper,7 the two sets of sharp peaks
around +0.93 V correspond to the desorption of preadsorbed
sulfate anions and the deposition of Hg2SO4. An ordered
coadsorbed structure of Hg2SO4 was observed at potentials
+0.80 e E e +0.88 V. The sharpness of these features would
suggest that these processes are quite rapid. The nature of the
small waves at +0.81 and +0.74 V is not clear at this time.
Upon further scanning the potential to more negative values, a
broad and diffusional-like reduction peak appears at +0.60 V,
as shown in the main panel of Figure 1. There is also a very
sharp spike at +0.52 V superimposed on the broad reduction
wave. However, no corresponding oxidation peak is observed
if the scan direction is reversed after it passes this spike. Instead,
only a broad oxidation peak, corresponding to the broad
reduction peak, appears. Once the electrode potential is scanned
beyond the sharp spike at +0.52 V, both the sharp double peaks
at +0.93 V and the single peak at +0.52 V are dramatically
broadened and diminished in amplitude in successive scans. This
change is clearly evident when comparing the main panel with
the inset to Figure 1. However, if the scan direction is reversed
right on the sharp spike at +0.52 V, a sharp oxidation spike
also appears on the anodic sweep and the change in the double
peaks at +0.93 V is greatly minimized. The broad peak at
From the size and geometry of the reciprocal lattice, we can
derive the hexagonal real space lattice of the Hg adlayers as

2910 J. Phys. Chem. B, Vol. 101, No. 15, 1997
Li and Abrun˜a
be superimposed on a commensurate c(2×2) structure, as shown
in Figure 4. In Figure 4a, one-third of the Hg atoms (dots filled
circles) sit on atop sites while the other two-thirds of the Hg
atoms (hatched circles) sit at threefold hollow sites. The nearest
Hg-Hg distance in this structure is 3.3313 Å which is within
the error of our experimental results. The lattice expansion in
this case is probably due to the Hg-substrate interactions for
forming a commensurate structure rather than to Coulombic
repulsion as in Hg UPD phase I. We can correlate this structure
with an intermediate state with nearly zero charge as has been
previously proposed based on electrochemical studies.13-17
However, experimentally, we did not observe any diffraction
corresponding to the 2×2 superlattice. This might be because
the small surface corrugation of the Au(111) gives rise to little
modulation to the heights of overlayer atoms to which grazing
incidence X-ray diffraction is not very sensitive. This has also
been the case for other UPD systems on Au(111) and Ag(111)
electrodes.21,22
Figure 3. Schematic of the real space model of Hg UPD phase I.
With the in-plane diffraction data alone, we cannot unam-
biguously establish the exact registry of the Hg atoms on the
Au(111) surface. Figure 4b shows an alternate model which is
also consistent with the in-plane diffraction data. This one is
also a c(2×2) commensurate structure with one-third of the Hg
atoms (dots filled circles) located at bridge sites and the other
two-thirds of the Hg atoms (hatched circles) located at quasi
threefold hollow sites. In principle, the two models shown in
Figure 4 could be distinguished by fitting the intensity profile
along L at nonspecular spots, i.e., nonspecular rod scans.
However, a detailed structural determination by this method
requires a large amount of data taken with a highly ordered
and stable system and has only been successfully demonstrated
in the model system of Cu UPD on Au(111).23 In the present
case of Hg UPD, the metastable nature of the Hg UPD layers
prevented us from carrying out a reliable measurement of
nonspecular rods.
The in-plane radial scans across the first-order diffraction
spots (1,0)s from the Au(111) surface, (1,0)I from Hg UPD phase
I, and (1,0)II from Hg UPD phase II are shown in Figure 5, a,
b, and c, respectively. The diffraction from the Au(111) surface
is better fitted with a Lorentzian rather than a Gaussian
distribution. The fitted full width at half-maximum is 0.005 49
( 0.000 36 Å-1 which is limited by the detector slit. However,
it establishes that the coherence length of the Au(111) surface
is at least 1144 ( 40 Å.
Figure 4. Schematics of two possible commensurate real space
structural models of Hg UPD phase II.
In-plane radial scans of both Hg UPD phases are better fitted
with Gaussians than Lorentzians. The scan of Hg UPD phase
I in Figure 5b is slightly asymmetric due to the tail from a nearby
mosaic (within 0.1° in azimuth) picked up by the relaxed
detector resolution. Therefore, it was fitted with an additional
Gaussian to account for the small tail. The major peak was
fitted with a width of 0.0165 ( 0.0016 Å-1 which corresponds
to a coherence length of about 380 ( 37 Å. The scan of Hg
UPD phase II in Figure 5c can be fitted with one Gaussian.
The width is 0.0139 ( 0.000 36 Å-1, smaller than that of Hg
UPD Phase I. This gives rise to a larger coherence length of
about 450 ( 12 Å.
Both Hg UPD phases have coherence lengths much smaller
than the Au(111) substrate, indicating that they are not limited
by the substrate quality. It is likely that the Hg adlayers form
two-dimensional islands on the substrate surface rather than
being homogeneously distributed. Since liquid Hg has a very
large surface tension, it would not be surprising to observe the
formation of 2-D islands. On the other hand, azimuthal scans
(not shown) of both Hg UPD phases can be fitted with
Lorentzians with a fwhm of 0.10° ( 0.015°, which is identical
shown in Figures 3 and 4, respectively. Figure 3 shows the
incommensurate structure of Hg UPD phase I with lattice
constants a ) b ) 3.86 ( 0.03 Å and R ) 60°. The error bar
is calculated from the standard deviation of measurements at
different spots and with different samples. The open circles
represent substrate atoms of the Au(111) surface, and the
hatched circles are the Hg atoms (with a diameter of 3.01 Å).
If the first Hg atom is placed in a threefold hollow site, the
other Hg atoms clearly do not fall on any specific sites,
indicating that the overlayer is incommensurate with the
substrate. The lattice constant of this structure is 28.2% larger
than the nearest neighbor distance in bulk Hg (3.01 Å).
Considering that this structure might correspond to one of the
partially charged states,13-17 the expanded lattice can be readily
rationalized by the Coulombic repulsion between partially
charged Hg atoms.
Hg UPD phase II also has a hexagonal lattice with lattice
parameters a ) b ) 3.34 ( 0.01 Å and R ) 60°. The lattice
constant of this structure is still 11% larger than the nearest
Hg-Hg distance in frozen bulk Hg. However, this lattice can

Phases of Underpotentially Deposited Hg on Au(111)
J. Phys. Chem. B, Vol. 101, No. 15, 1997 2911
Figure 6. Potential dependence of the lattice parameter of Hg UPD
phase II.
present uniaxially commensurate rectangular structures which
were distorted from the bulk hexagonal structure. The incom-
mensurate lattice was also compressed with decreasing electrode
potential. Therefore, it appears that the adatom-adatom
interactions in these systems are likely the most important while
the adatom/substrate, the anion/substrate, and the anion/adatom
interactions have little effect on the structure of the complete
UPD monolayers.21
In contrast, the two ordered Hg UPD phases that we observed
have expanded structures compared to frozen bulk Hg. Phase
I was only observed in a rather narrow potential range (<50
mV). Within this range, we did not observe any clear potential
dependence of the lattice parameters. However, the statistical
error calculated from several samples deposited under different
conditions is rather large. This is likely due to the metastable
nature of this structure, which prevented us from carrying out
a careful potential dependence study of this structure. On the
other hand, Hg UPD phase II is more stable and has much
smaller statistical errors. Figure 6 presents the potential
dependence of the lattice parameter of Hg UPD phase II. In
general, there is only a very small change, barely larger than
the error bars. If fitted with a linear function, the slope of the
line is about 0.028 Å/V, which is at least 1 order of magnitude
smaller than the corresponding values of Tl, Pb, and Bi UPD
on Au(111) and Au(100) surfaces.21,22,24-26 This provides an
additional and strong evidence that Hg UPD phase II is a
commensurate phase as we proposed above.
Figure 5. In-plane radial scans of (a) Au(111) substrate, (b) Hg UPD
phase I, and (c) Hg UPD phase II. The Au(111) surface peak is fitted
by a Lorentzian while the Hg UPD peaks are fitted with Gaussians.
3.3. Specular CTR Results. It is important in our studies
to relate the in-plane surface structure to the surface normal
structure. This can be done with the complementary information
derived from X-ray reflectivity studies, i.e., the measurement
of the intensity profiles of specular crystal truncation rods. In
these studies, the weak scattering between (0,0,L) Bragg
reflections is very sensitive to the average density and arrange-
ment of adsorbate layers.
to the Au(111) substrate. Therefore, no additional orientational
disordering is introduced into the Hg adlayers.
The UPD of Hg on Au(111) is quite different from that of
the neighboring metal elements in the same row of the periodical
table, such as Tl, Pb, and Bi.21,22 The nearest-neighbor distances
in bulk solids of Tl, Pb, and Bi are 3.46, 3.50, and 3.07 Å,
respectively. All of these values are much larger than the 2.885
Å nearest Au-Au distance in bulk gold. The UPD layers of
Tl and Pb just prior to bulk deposition on Au(111) and Ag(111)
have been found to have essentially identical structures to their
vapor-deposited counterparts in vacuum environments. In both
cases, the UPD layers present incommensurate hexagonal lattices
rotated by 2°-5° relative to the substrate orientation. The lattice
parameters were found to be compressed by only abut 3% from
the bulk values and to decrease with decreasing electrode
potential. On the other hand, the UPD layers of Bi on
Au(111),24 Tl on Au(100),25 and Ag(100)26 were found to
In the case of in situ electrochemical studies, the specular
CTR curve of a Au(111) crystal covered with an overlayer can
be calculated as27
2
256π2r0
4
2
abs/Qz
R(0,0,L) ) |T(Qz)|
|s(0,0,L)| e-Q
(1)
4
2
[
]
3as Qz
where the sum over atomic layers is

2912 J. Phys. Chem. B, Vol. 101, No. 15, 1997
Li and Abrun˜a
∞
2
2σn
z
z
s(0,0,L) ) F (Q )
Fn e-Q /2 eiQ nd
+
∑
Au
z
n)1
2
2σ0
FAu(Qz) F0e-Q /2eiQ (d -d)
+
/2 eiQ (d
z
z
0
-Qz2σads
ads-d0+d)
2
z
Fads(Qz) F
e
(2)
ads
In eq 1, T(Qz) is a factor that accounts for the enhancement at
the critical angle and exp(-Qabs/Qz) accounts for the absorption
by the polymer and electrolyte film with a constant thickness.
The first term in eq 2 corresponds to the sum of the scattering
amplitude over the bulk Au(111) layers. Each atomic layer is
described by three parameters: the coverage or density Fm
(defined as the atomic ratio of the species in the nth layer to
that of Au atoms in a bulk Au(111) layer), the atomic root mean
square (rms) displacement σn, and the interlayer distance d. We
fixed these values at Fm ) 1, σm ) 0.085 Å, and d ) 2.3556 Å
as reported for bulk Au(111) layers.28 The second term
corresponds to the scattering from the top Au(111) layer and
the third term corresponds to that from the Hg UPD layer. The
atomic scattering factors FAu and Fads were numerically
calculated from the empirical equations in the International
Tables for Crystallography.29 The coverage of the top Au(111)
layer was fixed at F0 ) 1 and the other parameters σ0, d0, Fads
ads, and dads were left free to vary during fitting.
Figure 7 shows the CTR data of the Au(111) electrode in
,
σ
Figure 7. CTR of the Au(111) electrode in 0.10 M H2SO4 containing
0.10 mM Hg2+ measured at E ) +1.00 V (a, solid circles), +0.68 V
(b, open squares), and +0.63 V (c, open circles). The data at +0.68
and +0.63 V correspond to Hg UPD phases I and II, respectively. Each
set of data is shifted down by 1 order of magnitude relative to the
previous one to give a better view. The dashed lines are the calculated
CTR curves for an ideally truncated Au(111) crystal. The continuous
lines are the fits to the experimental data with a multilayer model.
0.10 M H2SO4 containing 0.10 mM Hg2+ at (a) E ) +1.00 V
(solid circles), (b) +0.68 V (open squares), and (c) +0.63 V
(open circles), respectively. The CTR data at +0.68 and +0.63
V correspond to Hg UPD phase I and II, respectively. Since
one complete set of CTR measurements can take more than 8
h, during which time the Hg UPD structure might be changed,
in-plane surface diffraction measurements were carried out
periodically to ensure that the Hg UPD structure was the same.
Data set at +0.68 and +0.63 V were shifted down by 1 and 2
orders of magnitude, respectively, in order to give a clearer view.
The dashed lines are the CTR curves for an ideally truncated
Au(111) surface and the continuous lines are the fits to the
experimental data. The spikes at L ) 1 and 2 correspond to
the first- and second-order diffraction from the bulk Au(111)
planes, respectively. Around the valleys (L ) 0.5 and 1.5), the
scattering from the substrate layers cancels out and thus the
specular CTR intensity is most sensitive to the surface structure.
Specifically, it provides the average electron density profile in
a direction normal to the surface. It is clear that, in all cases,
the data show deeper valleys than the ideal Au(111) surface.
This cannot be accounted for with just an enhanced rms
displacement of Au(111) surface atoms. Instead, adsorbed
overlayers have to be incorporated onto the surface in order to
fit the data. The scattering from adsorbates interferes with that
from the Au(111) substrate and thus gives rise to deeper valleys
in the CTR profiles.
enhanced from 0.085 Å (bulk value) to 0.184 ( 0.012 Å due
to the strong chemical interaction of the adsorbed sulfate/
bisulfate anions. The top layer to second layer distance was
only slightly expanded from 2.356 Å to 2.406 ( 0.004 Å.
As mentioned earlier, at E ) +0.68 V, we observed the in-
plane diffraction of Hg UPD phase I. The CTR curve (Figure
7b) at this potential is quite different from curve 7a with a deeper
1
valley around (0, 0, /2) which is asymmetrically shifted to a
lower value of L. In contrast to the two-layer coadsorbed
structure at +0.80 V e E e +0.88 V in our previous paper,7
this set of CTR data can be fitted with a single Hg overlayer
on the relaxed Au(111) surface. The Au(111) surface rms
displacement and the top to second layer distance are only
slightly smaller than those at E ) +1.00 V (Table 1). The Hg
overlayer is fitted with a density of Fads ) 0.332 ( 0.014 and
an atomic rms displacement σads ) 0.44 ( 0.06 Å. However,
this density is much lower than the value of 0.56 calculated
from the hexagonal in-plane structure (with lattice constants a
) b ) 3.86 ( 0.03 Å). This suggests that the Hg UPD
overlayer forms two-dimensional islands on the Au(111) surface.
At this potential, the bare Au surface is likely covered with a
layer of water and some sulfate/bisulfate anions. The fitted Hg
overlayer to Au(111) surface layer distance is 2.558 ( 0.022
Å which is larger than the interlayer distance in bulk Au(111).
Assuming that Hg and Au atoms are hard balls with diameters
of 3.01 and 2.885 Å, respectively, the Hg to Au(111) distance
can be calculated as 2.570 Å at bridge sites, 2.432 Å at three-
fold hollow sites, and 2.9475 Å on atop sites. The fitted value
is close to that at bridge sites and in between the other two
extreme values, which is consistent with the averaged value over
the incommensurate structure shown in Figure 3.
The fitted parameters at the three potentials are listed in Table
1. At E ) +1.00 V, the CTR data are identical to those
measured in pure 0.10 M H2SO4 solution at the same potential.
The data can be fitted with a sulfate/bisulfate overlayer of
density Fads ) 0.371 ( 0.02, as shown in Figure 7a. As
discussed in our previous paper, this density is in agreement
with both a 0.40 monolayer of bisulfate and a 0.20 monolayer
of sulfate incorporating some water or hydronium ions (g0.4
monolayer). The fitting was relatively insensitive to the
overlayer rms displacement σads. Thus, it was fixed at 0.20 Å
during fitting. The distance between the overlayer and the first
substrate layer, dads, was found to be 2.082 ( 0.038 Å which is
also in agreement with the value obtained in pure 0.10 M H2SO4
solution. The rms displacement of the Au(111) surface was
At E ) +0.63 V, the in-plane surface diffraction measurement
shows the formation of Hg UPD phase II. However, the CTR
profile (Figure 7c) has very similar features to that corresponding

Phases of Underpotentially Deposited Hg on Au(111)
J. Phys. Chem. B, Vol. 101, No. 15, 1997 2913
TABLE 1: Specular CTR Fitting Parameters at Different Potentials
E (V)
adsorbate species
F0
σ0 (Å)
d0 (Å)
Fads
σ
ads (Å)
dads (Å)
2-
+1.00 V
+0.68 V
+0.63 V
SO4
Hg
Hg
1
1
1
0.184 ( 0.012
0.165 ( 0.007
0.178 ( 0.0079
2.406 ( 0.004
2.338 ( 0.0066
2.349 ( 0.0068
0.371 ( 0.02
0.332 ( 0.014
0.38 ( 0.11
0.2
0.44 ( 0.06
0.75 ( 0.26
2.082 ( 0.038
2.558 ( 0.022
2.46 ( 0.14
to Hg UPD phase I (Figure 7b). This set of data can also be
fitted with a single Hg overlayer on the relaxed Au(111) surface.
The fitted parameters are F0 ) 1, σ0 ) 0.178 ( 0.0079 Å, and
d0 ) 2.349 ( 0.0068 Å for the top Au(111) layer, and Fads
)
0.38 ( 0.11, σads ) 0.75 ( 0.26 Å, and dads ) 2.46 ( 0.14 Å
for the Hg UPD layer. The density of the Hg layer is slightly
higher than that corresponding to Hg UPD phase I, but much
lower than 0.751 as calculated from the in-plane structure.
Therefore, this Hg UPD overlayer also appears to be present as
two-dimensional islands on the Au(111) surface. The distance
from the Hg overlayer to the Au(111) surface is 2.46 ( 0.14
Å, which is smaller than the average values of 2.604 and 2.657
Å calculated from the two c(2×2) commensurate models shown
in Figure 4, a and b, respectively. This might be due to a strong
chemical bond between Hg and Au atoms. A similar phenom-
enon has been previously observed in Tl UPD on Au(111).22
On the other hand, the rms displacement of Hg atoms of Hg
UPD phase II (0.75 ( 0.26 Å) is much larger than that of Hg
UPD phase I (0.44 ( 0.06 Å). Of the two models shown in
Figure 4, model a would give an rms displacement much larger
than that of model b, suggesting that model a is likely the correct
one.
4. Discussion: The Kinetics of Hg UPD Phases
Figure 8. CTR corresponding to Hg UPD phase II (open circles) on
the Au(111) electrode and that measured after holding potential at +0.63
V for 12 h (solid squares). The latter is shifted down by 2 orders of
magnitude for a better view. Dashed lines are the calculated CTR
curves for an ideal Au(111) electrode and continuous lines are fits to
the data.
In the previous sections, we have presented the structures of
two ordered Hg UPD phases at potentials of +0.68 and +0.63
V, respectively. Both of these phases are within the potential
region of the broad reduction wave in Figure 1. The specular
CTR data suggested that both phases are present as two-
dimensional islands. We found that these two phases exhibited
strong kinetic effects which might account for the broad
reduction wave in the cyclic voltammetric response.
Figure 8 shows the specular CTR data of a freshly deposited
Hg UPD phase II (open circles) along with that of the same
sample measured after the Hg overlayer had been deposited and
held at +0.63 V for more than 12 h (filled squares; displaced
by 2 orders of magnitude to give a clearer view). The data for
the fresh Hg UPD phase II was taken by stripping the Hg layer
at +1.00 V and redepositing it at +0.63 V every 2 h. The other
set of data was taken after 12 h without any other disturbance
once the Hg had been deposited. In this case, there is a dramatic
change in intensity around the anti-Bragg points and the CTR
profile is close to that of an ideal Au(111) surface (dashed line).
As mentioned before, the CTR data of Hg UPD phase II could
be fitted with a single layer of Hg on the Au(111) surface. On
the other hand, the CTR data after 12 h could be fitted, with
the same quality, by either a single Hg overlayer or a single
water layer on the relaxed Au(111) surface. The fitted
parameters are listed in Table 2. When fitted with a Hg
overlayer, the parameters for the Au(111) surface are F0 ) 1,
σ0 ) 0.1004 ( 0.0069 Å, and d0 ) 2.354 ( 0.002 Å. Fitting
parameters for the Hg overlayer are Fads ) 0.8916 ( 0.0089,
σads ) 0.0906 ( 0.0093 Å, and dads ) 2.344 ( 0.004 Å. The
fitted coverage is consistent with a fully covered close-packed
Hg layer (with a calculated coverage of 0.92). Both the rms
displacement and the overlayer to substrate distance are close
to the values of bulk Au(111) (0.085 and 2.3556 Å, respec-
tively).
Hg UPD phase I was observed only when Hg was deposited
on a freshly flame-annealed Au(111) electrode and was stable
for about 3-4 h after being deposited. During CTR measure-
ments of this phase, the Hg layer had to be stripped at +1.00
V and redeposited at +0.68 V every two hours to ensure that
we were studying the same structure. However, after about 7-8
h past the initial deposition, phase I disappeared and we were
not able to recover it without reannealing the crystal. Instead,
the redeposited Hg UPD overlayer showed the structure of Hg
UPD phase II. This phase was more stable and could be
repeatedly stripped at E > +0.90 V and redeposited at +0.50
V e E e +0.70 V as long as the deposition was within the
monolayer regime. Hg UPD phase I could also be converted
to Hg UPD phase II by changing the potential from +0.68 V
(where Hg UPD phase I was observed) to +0.63 V. Clearly,
Hg UPD phase I is a metastable structure whose instability is
likely due, at least in part, to its partially charged chemical state.
Hg UPD phase II also shows some kinetic effects. This was
found during specular CTR measurements. Typically, a set of
specular CTR measurements consists of rocking curves at a
series of Qz’s, which takes more than 8 h to complete. The
same rocking curve around (0, 0, 0.5) repeated after 6 h was
found to have an intensity that was several times higher than
before, suggesting that the structure of the Hg UPD changed
during that time period. The intensity becomes constant again
after the Hg UPD phase II is deposited for more than 12 h, so
we believe that it forms yet another structure.
If fitted with a monolayer of water, the parameters for the
Au(111) surface layer are about the same as before, with F0 )
1, σ0 ) 0.121 ( 0.013 Å, and d0 ) 2.354 ( 0.006 Å. The
parameters for the water overlayer are fitted as Fads ) 0.768 (

2914 J. Phys. Chem. B, Vol. 101, No. 15, 1997
Li and Abrun˜a
TABLE 2: Specular CTR Fitting Parameters for a Sample Held at E ) +0.63 V for More Than 12 h
adsorbate species
F0
σ0 (Å)
d0 (Å)
Fads
σ
ads (Å)
dads (Å)
Hg
H2O
1
1
0.1004 ( 0.0069
0.121 ( 0.013
2.354 ( 0.002
2.354 ( 0.006
0.892 ( 0.0089
0.768 ( 0.082
0.091 ( 0.0093
0.2
2.3442 ( 0.0041
2.12 ( 0.16
0.082, σads ) 0.2 Å, and dads ) 2.12 ( 0.16 Å. The rms
displacement σads was fixed at 0.20 Å to prevent it from going
to zero. As mentioned before, the value of this parameter has
little, if any, effect on the other fitting parameters. The fitted
coverage of the water overlayer is a bit smaller than that for a
Au(111) electrode in 0.10 M H2SO4 at +0.70 V. This is
consistent with our previous result that the adsorbed water
molecules are desorbed from the Au(111) surface when the
potential is moved from +0.70 to +0.40 V.7
Thus, the specular CTR results suggest two possible models.
In the first model, Hg deposition on the Au(111) surface
continues when the potential is held at +0.63 V and a full
monolayer of close-packed Hg is formed after about 12 h. This
is based on the assumption that the Hg atoms are stable on the
Au(111) surface without diffusing into the bulk if the deposition
is within one monolayer. However, Hg is known to have a
relatively high diffusion coefficient in bulk Au crystals. If Hg
atoms on the Au surface were to diffuse into the bulk crystal,
the Hg coverage would decrease with time rather than increase.
After about 12 h, all the deposited Hg atoms would disappear
from the surface assuming there is no more deposition from
bulk solution. The disturbance to the bulk Au crystal might
not be particularly dramatic since the total amount of Hg
deposited would be less than one monolayer. Since the diameter
of Hg atoms is 0.125 Å larger than that of Au atoms, this will
induce only a small disturbance to the Au crystal. Thus, the
second model of a weakly adsorbed water layer on the Au(111)
surface cannot be rigorously excluded.
Based on the specular CTR data alone, it is difficult to
unambiguously determine which is the correct model. However,
previous electrochemical studies have found that the Hg
overlayer might form an intermetallic compound with the Au
substrate.8-12 Since alloy formation would likely change the
electrochemical properties of the Au electrode, one could, based
on the electrochemical response prior to and after the formation
of this structure, distinguish between these two models. Figure
9 shows the cyclic voltammograms of a Au(111) electrode in
0.10 M H2SO4 solution containing 0.10 mM Hg2+ at a scan
rate of 20 mV/s. Curve A (thin line) was measured with a fresh
flame-annealed Au(111) crystal and curve B (thick line) was
measured after depositing Hg at +0.63 V for about 12 h. The
fresh Au(111) sample shows a pair of sharp peaks around +0.90
V similar to the features shown in Figure 1. The cathodic wave
was broadened due to the relatively low Hg2+ concentration
(0.10 mM). After holding the potential at +0.63 V for 12 h,
the pair of sharp peaks around +0.90 V is replaced by a pair of
broad peaks around +0.76 V. The peak position is shifted
negatively by 140 mV, and the separation in peak potentials
increased relative to a fresh Au(111) crystal. Clearly, after 12
h, the Hg-surface interaction is weaker and more negative
potentials are required for the Hg deposition. This change could
be due to the formation of a layer of an intermetallic compound
or a full Hg monolayer. On the other hand, it cannot be readily
explained by the second model which should give rise to a clean
Au(111) surface.
Figure 9. Cyclic voltammograms of a Au(111) electrode in 0.10 M
sulfuric acid containing 0.10 mM Hg2+ at a scan rate of 20 mV/s. Curve
A (thin line) was measured with a fresh flame-annealed Au(111) crystal
and curve B (thick line) was measured after depositing Hg at +0.63 V
for about 12 h.
to the fully discharged metallic mercury on the Au(111) surface.
The amount of mercury in this state is very small on a fresh
Au(111) surface but builds up slowly. We found that this
process is strongly dependent on the potential at which the
electrode is held, the Hg2+ concentration, and the types of anions
present in solution. Detailed electrochemical studies are
underway and will be published elsewhere.20
To further understand this kinetic process, we carried out both
rocking scans across (0, 0, 0.5) and in-plane radial scans
following the deposition of Hg over a 12 h period. Figure 10a
shows the integrated intensity under the rocking curve vs time
which reflects the change in the surface normal structure during
the kinetic process. Clearly, there is a dramatic change after
about 6 h. Between 0 to 5 h and 7 to 12 h, there appear two
plateaus. The two sets of data shown in Figure 8 were taken
in these two regions, respectively. The peak width of the in-
plane radial scan first decreased slightly and then increased after
6-8 h (see Figure 10b). This indicates that the domain size of
the Hg UPD layer first increases and then decreases. However,
the change is small (about 20%). On the other hand, the
integrated intensity of the in-plane radial scan first increases
and then decreases after about 6 h (see Figure 10c). Since this
quantity is proportional to the total amount of Hg atoms in the
ordered surface layer, one can conclude that the ordered Hg
UPD islands likely grow at first. After about 6 h, some of them
appear to change to a third phase which gives no in-plane surface
diffraction. This phase could be either a liquid Hg overlayer
or a layer of an amorphous Hg-Au intermetallic compound or
alloy. However, at this time, we do not have any direct
experimental evidence to derive the details of this structure.
We can speculate that the formation of the third phase requires
a certain critical size or coverage of the Hg overlayer. Once
the islands grow over this size (after about 6 h), they may
coalesce to form a 2-D liquid or an amorphous structure. This
was indicated by the specular CTR data in Figure 10a. The
intensity at (0, 0, 0.5) shows a transition at about 6 h consistent
Careful examination of Figure 9 also shows that a very small
wave at about +0.76 V is also present in the CV taken with a
fresh Au(111) electrode (curve A). This wave (to which we
alluded to earlier, section 3.1) is also present in the inset of
Figure 1. It is consistent with the assumption that it corresponds

Phases of Underpotentially Deposited Hg on Au(111)
J. Phys. Chem. B, Vol. 101, No. 15, 1997 2915
structures involved in Hg UPD. Further studies are in progress
to elucidate the effects of anions on the kinetics of Hg UPD on
Au surfaces.31
Our results can be compared with previous electrochemical
kinetic studies of Hg UPD on polycrystalline Au electrodes.
As mentioned before, Salie´ and Bartels13-16 proposed that
initially Hg(II) is rapidly reduced to Hg(I) and then slowly
reduced to a fully discharged metallic layer through two
intermediate states. In our previous paper,7 we reported on the
formation of a Hg2SO4 layer (with mercury present as mercurous
dimers) at the initial stages of Hg UPD. Starting from this
potential (+0.80 V e E e +0.88 V) and scanning to more
negative values, we observed two ordered Hg UPD structures
which are in excellent qualitative agreement with the mechanism
proposed by Salie´ and Bartels. They describe the underpotential
deposition of a metal according to a general theory of chemi-
sorption reactions with multiple steps.13-16
In the present case, the deposition process can be described
by eqs 3-6.
Hg2+(soln) + e- a ((Hg2)SO4)(ads)
(3)
(4)
((Hg2)SO4)(ads) + 0.47e- a Hg/UPD-I
Hg/UPD-I + 0.53e- a Hg/UPD-II
Hg/UPD-II a Hg0/UPD
(5)
(6)
where Hg2+(soln), ((Hg2)SO4)(ads), Hg/UPD-I, Hg/UPD-II, and
Hg0/UPD represent, respectively, mercuric ions in solution,
adsorbed mercurous sulfate, UPD phases I and II, and the
metallic mercury overlayer. Potentiostatic measurements with
a polycrystalline Au ring-disk electrode and impedance spec-
troscopy on a polycrystalline Au electrode have been employed
to determine the partial charge on the electrodeposited mercury
species.13-16 Assuming similar values for single-crystal Au(111)
electrodes, we expect that the charge of each Hg atom is about
0.53e- in Hg UPD phase I and close to zero in Hg UPD phase
II. These values are consistent with the open structure of Hg
UPD phase I as being due to the Coulombic repulsion forces
between the partially charged Hg atoms. In Hg UPD phase II,
the commensurate structure may be at a local free energy
minimum and thus would be more stable than phase I. Finally,
the transition from Hg UPD phase II to the fully discharged
2-D liquid Hg or amorphous alloy structure is also consistent
with the last alloy formation step according to the studies by
Salie´ and Bartels.
Figure 10. X-ray scattering intensity changes during Hg UPD at +0.63
V: (a) integrated intensity of the rocking curve across specular CTR
at (0,0,0.5); (b) full width at half-maximum of the in-plane radial scan
across (1,0)II of Hg UPD phase II; (c) integrated intensity of the in-
plane radial scan across (1,0)II.
with the in-plane data. A 2-D island growth model would
predict a continuous increase in the overlayer density before
the sudden jump at about 6 h to form a 2-D liquid or an
amorphous structure. However, a detailed understanding would
require a series of time-resolved specular CTR data. Unfortu-
nately, it is not feasible in this case since it takes about 8 h to
complete the measurement of one set of specular CTR data.
Future experiments using time-resolved X-ray scattering tech-
niques would be most valuable in exploring the transition
processes30 and we are currently exploring such possibilities.
At this stage, it is not well understood why the kinetics of
reducing mercury ions to the fully discharged metallic state is
so slow. It could be due to anion-coadsorption effects, a
disprotionation reaction involving the adsorbed intermediate-
(s), or slow nucleation processes. We have carried out both
GIXD and specular CTR measurements in solutions containing
different anions such as SO42-, ClO4-, Cl-, and CH3COO- and
the results clearly show a strong anion dependence of the
5. Conclusions
In summary, we report on two ordered intermediate Hg UPD
phases and a fully discharged 2-D liquid Hg or an amorphous
Hg-Au alloy structure during the UPD of Hg on Au(111)
electrodes in 0.10 M H2SO4 solution. These results, combined
with our previous report on the first stage of Hg UPD on
Au(111) electrodes, present a detailed understanding of the UPD
of Hg from Hg(II) to a fully discharged metallic state. Both
ordered intermediate phases have hexagonal structures with
lattice vectors rotated 30° relative to those of the Au(111)
substrate. The first phase was observed only on a fresh flame-
annealed Au(111) electrode at an applied potential of +0.68
V. It is incommensurate with the Au(111) surface with lattice
constants a ) b ) 3.84 ( 0.03 Å. The open, incommensurate,
structure is likely due to the Coulombic repulsion forces between
the partially charged Hg atoms. We found that this phase is a
metastable state which changes with time (on a time scale of

2916 J. Phys. Chem. B, Vol. 101, No. 15, 1997
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(29) International Tables for X-ray Crystallography; Ibers, J. A.,
Hamilton, W. C., Eds.; Kynoch: Birmingham, U.K., 1974; Vol. IV.
(30) Finnefrock, A. C.; Buller, L. J.; Ringland, K. L.; Brock, J. D.;
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Acknowledgment. This work was supported by the Office
of Naval Research and the National Science Foundation
(CHE9407009). Most synchrotron X-ray experiments were
performed at Exxon beamline ×10B at the National Synchrotron
Light Source, Brookhaven National Laboratory, which is
supported by the U.S. Department of Energy, Division of
Materials Science and Division of Chemical Sciences (DOE
contract No. DE-AC02-76CH0016). Preliminary X-ray mea-
surements were also performed at the Cornell High Energy
Synchrotron Source, which is supported by National Science
Foundation. We thank Dr. K. Liang for his technical help and
scientific discussions. We also thank Sean Smith, Shaowei
Chen, and Dr. Enrique Herrero for participating in parts of these
experiments and for their helpful suggestions and discussions.
References and Notes
(1) Magnussen, O. M.; Hotlos, J.; Nichols, R. J.; Kolb, D. M.; Behm,
R. J. Phys. ReV. Lett. 1990, 64, 2929.
(2) Hachiya, T.; Honbo, H.; Itaya, K. J. Electroanal. Chem. 1991, 315,
275.