The structure of gold and platinum deposits grown on gold and platinum wire cathodes, respectively, by electroreduction of the corresponding oxide layers has been analyzed in terms of surface fractals by measuring the perimeter L and area A of intergranular voids. Perimeter and area were determined from scanning tunneling microscopy (STM) topographic images of the deposit surfaces. Fractal behavior, L ∞ AD'/2 with D'= 1.5 ± 0.1, is found for both platinum and gold deposits. From these measurements, the fractal dimension of the surfaces was 2.5. The D' value of the gold deposits decreases by aging in the electrolyte at 325 K as a consequence of surface diffusion processes.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA)Facultad de Ciencias Exacta

Arvia, Alejandro Jorge

Baró, A. M.

Gómez-Rodríguez, J. M.

Salvarezza, Roberto Carlos

Vara, J. M.

Vázquez, L.

The Journal of Physical Chemistry A

English

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J. Phys. Chem. 1992, 96, 347-350 347
(multilayer state) and 280 K (monolayer state, desorption acti-
vation energy of 16.6 kcal/mol). Hydrazine decomposed exten-
sively into NH3, N2, H, and N. B adatoms (electron-acceptor
species) enhanced the bonding energy of hydrazine molecules
(electron-donor species) on Ru(0001).
(2) Thermal desorption spectra from N/Ru(0001) surfaces
showed N2 evolution in a sharp peak at ~510 K and in a broad
feature between 600 and 800 K.
(3) Boron-nitrogen adlayers with stoichiometry close to 1:1 were
formed: (a) after coadsorption of B2H6 and N2H4 at 90 K, and
subsequent annealing to temperatures above 450 K, and (b) by
simultaneous dosing of B2H6 and N2H4 to Ru(0001) at 450 K.
(4) For films with 0N < 1 ML, adsorbed atomic oxygen reduced
the stability of the boron-nitrogen adlayers by forming strong B-0
bonds.
Acknowledgment. This work was supported by the Texas
Advanced Research Program under Grant No. 160769.
Registry No. B2H6, 19287-45-7; N2H4, 302-01-2; BN, 10043-11-5;
Ru, 7440-18-8.
Fractal Surfaces of Gold and Platinum Electrodeposits. Dimensionality Determination by
Scanning Tunneling Microscopy
J. M. Gomez-Rodriguez, A. M. Bard,*
Departamento de Fisica de la Materia Condensada, C-III, Universidad Autonoma de Madrid, 28049 Madrid,
Spain
L. Vazquez,
Instituto de Ciencias de Materiales, Sede B, CSIC, Departamento de Fisica Aplicada, C-XII, Universidad
Autonoma de Madrid, 28049 Madrid, Spain
R. C. Salvarezza/ J. M. Vara,
Departamento de Quimica Fisica Aplicada, C-II, Universidad Autonoma de Madrid, 28049 Madrid, Spain
and A. J. Arvia
INIFTA, Universidad Nacional de La Plata, Sucursal 4, Casilla de Correo 16, 1900 La Plata, Argentina
(Received: April 30, 1991; In Final Form: August 20, 1991)
The structure of gold and platinum deposits grown on gold and platinum wire cathodes, respectively, by electroreduction
of the corresponding oxide layers has been analyzed in terms of surface fractals by measuring the perimeter L and area A
of intergranular voids. Perimeter and area were determined from scanning tunneling microscopy (STM) topographic images
of the deposit surfaces. Fractal behavior, L a Aal1 with D'= 1.5 ± 0.1, is found for both platinum and gold deposits. From
these measurements, the fractal dimension of the surfaces was 2.5. The D' value of the gold deposits decreases by aging
in the electrolyte at 325 K as a consequence of surface diffusion processes.
Introduction
The kinetics of surface reactions on metals is extremely de-
pendent on the topography of the reacting surfaces. Real systems
exhibit complicated shapes with a high degree of irregularity or
disorder. In these cases fractal geometry has been useful to
characterize rough surfaces in very general terms. Metal films
grown under nonequilibrium conditions exhibit complicated
structures and rough surfaces.1-4 An important case is the process
of metal electrodeposition either from aqueous solutions or from
hydrous metal oxide layers in aqeous acid electrolytes. The first
process can generate dendritic, dense radial and DLA patterns
according to the growth conditions.2,3 The second process, al-
though less studied, can generate thin metal films with colum-
nar-like structures similar to those described for metal deposits
grown from the vapor phase1 2345 or thick metal film showing self-
similar patterns.6 Fractal characterization of these structures
is usually made in the micrometer to centimeter range so that little
information is available on the fractal nature of the deposit surfaces
at the nanometer level. In this paper we present an island type
analysis7 8based on determination of the perimeter L and area A
of intergranular voids of metal electrodeposits measured by
scanning tunneling microscopy (STM) as a method to investigate
+ Permanent address: INIFTA, La Plata, Argentina.
the fractal nature of the electrodeposit surface at the nanometer
level and to estimate the corresponding fractal dimension D.
Experimental Section
Gold and platinum films (thickness sl0-5-10-4 cm) were
prepared in 0.5 M H2S04 at 298 K electroreducing the corre-
sponding oxide layers grown on polycrystalline gold and platinum
wires (length 0.4 cm; radius 0.05 cm). The electrochemical
procedure for the preparation of these metal deposits has been
reported elsewhere.5 8 The electrodeposition of the gold and
platinum oxide layers was made by applying potential sweep rates
of 0.02 and 0.005 V/s for gold and platinum substrates, respec-
tively. Under these conditions the growth rate of the electrodeposit
(1) Matsushita, M.; Sano, M.; Hayakagawa, Y.; Honyo, H.; Sawada, Y.
Phys. Rev. Lett. 1984, 53, 286.
(2) Grier, D. G.; Kessler, D. A.; Sander, L. M. Phys. Rev. Lett. 1987, 59,
2315.
(3) Brady, R. M.; Ball, R. C. Nature 1984, 309, 225.
(4) Witten, T. A.; Sander, L. M. Phys. Rev. Lett. 1981, 47, 1400.
(5) Vazquez, L.; Bartolome, A.; Baro, A. M.; Alonso, C.; Salvarezza, R.
C.; Arvia, A. J. Surf. Sci. 1989, 215, 171.
(6) Pajkossy, T. J. Electroanal. Chem. 1991, 300, 1.
(7) Feder, J. Fractals', Plenum Press: New York, 1988.
(8) Chialvo, A. C.; Triaca, W. E.; Arvia, A. J. J. Electroanal. Chem. 1984,
71, 303.
0022-3654/92/2096-347S03.00/0 &copy; 1992 American Chemical Society
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348 The Journal of Physical Chemistry. Vol. 96. No. I. 1992
Figure 1. (a, top) 1400 X 1000 nm2 3D STM image of a nonaged gold
electrodeposit with voids filled as lakes, (b, bottom) log L vs log A plot
for the voids shown in (a). The yardstick is 4 = 5.6 nm. The inset in
this figure corresponds to the analysis of a smaller area (460 X 460 nm2)
of the same sample. The yardstick is 4 — 1.8 nm.
is in the 10-6—10-5 cm/s range, which is consistent with the de-
velopment of columnar structures with large surfaces.9 The
concept of surface area for these samples is a difficult one due
to the various accidents like cracks, pores, and indentations that
a surface can have. We measured the area S of the deposits
through the O-electroadsorption charge density (0.4 mC cm'2).5-10
The interesting point about the structure of these samples is the
great value of S obtained. For typical deposits with thickness close
to 10~4 cm we obtain S = 8 cm2, 80 times greater than that of
the starting metal wires.
Other types of deposits were prepared by aging the gold and
platinum deposits during different times fg at open circuit in the
0.5 H2SO4 solution at 325 K. Under these experimental conditions
there is a marked decrease in S for gold (i.e., from 8 to 1 cm2
for fg = 2 X 104 s) whereas for platinum only a slight decrease
in 5 is observed (i.e., from 8 to 6 cm2 for fg = 6 X 104 s).
Characterization of the surface of the electrodes before elec-
trodeposition was also made by measuring the time dependence
of the current I of the Fe(CN)64_/Fe(CN)63' reaction6 under
diffusion control. In this case we observed / <* r" with n = 0.5
in the 5 X 10 3 to 10 s time range. As D = 2n + 1, the surface
of the starting electrodes wires has D = 2.
STM measurements were all made in the constant-current mode
with bias voltages between 0.25 and 0.80 V and tunneling currents
between 1 and 2 nA. The data were acquired with a fully au-
tomated workstation and stored as digitized images. A detailed
description of the STM measurements are reported in ref 5.
(9) Srolowitz, D.; Mazor, A.; Bukiet, B. J. Vac. Sci. Technol. A 1988, 6,
2371.
(10) Trassati, S.; Petrii, O. A. Real Surface Area Measurements in
Electrochemistry; Comission on Electrochemistry, IUPAC: Oxford, in press.
Gomez-Rodriguez et al.
LOG,0 [AREA (nm2)]
Figure 2. (a, top) 1100 X 1100 nm2 3D STM image of an aged gold
electrodeposit (l, = 2 X 10* s) with voids filled as lakes, (b, bottom) log
L vs log A plot for the voids shown in (a). The yardstick is 4 = 4.5 nm.
The inset in this figure corresponds to the analysis of a smaller area (830
X 830 nm2) of the same sample. The yardstick is 4 = 3.2 nm.
Results and Discussion
The fractal nature of the surfaces of thin metal films has been
studied in recent years.1 '~13 Computer simulations reveal that
the columnar units'2 and the deposit surface13 are not self-similar
but self-affine fractals, although certain controversy exists on this
point.6,11 Thus, we propose a simple method to obtain the fractal
dimension of surfaces in the nanometer range which applies to
both self-similar and self-affine fractals. It is based on the fact
that the intersection of a plane with a self-similar or a self-affine
fractal surface generates self-similar lakes or islands.14 We
analyze the fractal nature of the surface by ex situ STM imaging.
Figure la is a STM 3D image of a nonaged gold electrodeposit.
The time elapsed between formation and imaging was around 1
h. On the other hand. Figure 2 shows the 3D image of an aged
gold deposit (fg = 2 X 104 s). In a way similar to that reported
for the Earth coastlines'4 we simulate the “filling with water* of
the surface topography up to a given level. By using threshold
detection and optimal connectivity checking algorithms, individual
voids “filled with water" were isolated. Then, their perimeter L,
defined as the number of water pixels on the digitized grid having
non-water neighbors, and their area A, defined as the number of
water pixels for a given void, were measured. The yardstick value
4 of the digitized grid is given by the total area scanned and the
(11) Yehoda, J. E.; Messier, R. Applic. Surface Sci. 1986, 22/23, 590.
(12) Meakin, P. J. Phys. A 1987, 20, LI 113.
(13) Family, A.; Vicsek, T. J. Phys 1985. 18, L75.
(14) Mandelbrot, B. B. The Fractal Geometry of Mature; Freeman: San
Francisco, 1977.
Fractal Surfaces of Au and Pt Electrodeposits
Figure 3. (a, top) 930 X 890 nmJ 3D STM image of a nonaged platinum
electrodeposit with voids filled as lakes, (b, bottom) log L vs log A plot
for the voids shown in (a). The yardstick is 3 = 3.6 nm.
number of pixels in the image (typically 256 X 256). An initial
point of concern is the minimum lake size that one should consider,
since the measurement of the length and area of a lake in a
digitized grid requires £ to be small enough so as to account for
the smaller features. Kaye15 suggests that a rugged perimeter
has to be measured with a yardstick smaller than 0.3 times the
projected total length. We have been analyzing several images
in order to determine empirically the more suitable choice. As
our yardstick is fixed for each image (i.e., the yardstick is equal
to one pixel length), we have considered only lakes with an area
greater than 30£:, which is a more restricted condition than Kaye’s.
L and A for similar shaped objects are related by7
L — aD'Ac//1 (1)
where a is a constant and D'is the fractal dimension of the void
perimeter. From the rule of thumb7-14 it follows
D' = D - 1 (2)
where D is the fractal dimension of the deposit surface.
The log L vs log A plot, covering two decades in area scale,
yields a straight line with D' = 1.5 ± 0.06 and D' = 1.3 ± 0.06
for nonaged and aged gold electrodeposits, respectively (Figures
lb and 2b). The yardstick value in Figure lb was 5.6 nm, while
in Figure 2b it was 4.5 nm. We show as insets in Figures lb and
2b the plots obtained from different STM measurements on
smaller areas of the electrodeposits. Though the yardstick were
(15) Kaye. B. H. In The Fractal Approach to Heterogeneous Chemistry;
Avnir, D., Ed.; Wiley; New York, 1989; p 58.
The Journal of Physical Chemistry, Vol. 96, No. I, 1992 349
Figure 4. (a, top) 510 x 490 nm2 3D STM image of an aged platinum
electrodeposit (f, = 4 X 10* s) with voids filled as lakes, (b, bottom) log
L vs log A plot for the voids shown in (a). The yardstick is 3 - 2.0 nm.
smaller, the D'values remained the same: D' = 1.5 ± 0.06 and
D' = 1.3 ± 0.06 for the nonaged and aged gold electrodeposits,
respectively.
The same “filling with water” procedure was performed on
nonaged and aged (I, = 4X 104 s) platinum electrodeposits. Their
3D STM images (Figures 3a and 4a) looked similar to those of
the gold surfaces (Figures la and 2a). From the generated void
patterns the log L vs log A plots were obtained (Figures 3b and
4b). However, in this case, one obtains D'= 1.5 ± 0.06 for both
types of deposits.
A limitation of the STM method is related to the fact that the
tip itself limits the roughness resolution as the STM images are
the convolution of the surface with the tip shape. A quantitative
estimation of the tip shape effect requires the knowledge of the
real tip geometry,16 which is rather difficult in practice. Thus,
we used an experimental approach to solve this problem analyzing
several sample sets (2 X 10~5 cm < film thickness < 2 X 10 4 cm)
using different STM probing tips. We were not able to detect
any marked tip influence on the D values, which remained
practically constant within ±0.1 error. A similar observation was
already reported for metal fracture surfaces.16 The fact that the
tip geometry has not influenced our results could be explained
considering that the smallest lakes which should be mostly affected
by the tip shape are discarded due to the more restrictive condition
imposed by the perimeter/area measurements. This limitation
was also analyzed by using several water-filling heights in a ±5-nm
range, yielding again a D' nearly independent (within ±0.1) on
(16) Dcnley, D. R. Ultramicroscopy 1989. 33, 83.
350 J. Phys. Chem. 1992, 96, 350-357
the depth at which the surface was studied. Thus, in our ex-
perimental system, we cannot determine D with an error less than
0.1.
From eq 2, D values of 2.5 ± 0.1 and 2.3 ± 0.1 result for the
nonaged and aged gold surfaces, respectively, in agreement with
the smoothing of the deposit surface as tg increases (Figures la
and 2a). These values indicate a marked reconstruction of the
gold surface in order to decrease the surface area energy of the
system. On the other hand, the D value for nonaged and aged
platinum surfaces is 2.5 ±0.1, showing that platinum deposits
are more stable than gold deposits under similar experimental
conditions. This conclusion is consistent with the slower mobility
of the platinum surface (surface diffusion coefficient, A.Pt - 1
X 10~18 cm2/s)17 in relation to the surface mobility of gold (A,au
s 1 X 10“13 cm2/s)17 in this electrolyte and temperature. Similar
results were recently reported on the surface diffusion of gold and
platinum in air at room temperature.18 The effect of the aging
process on the D values was verified by measuring the time de-
pendence of the diffusional current of the Fe(CN)64~/Fe(CN)63“
reaction.6 We obtained D = 2.5 ± 0.1 and D = 2.3 ± 0.1 for
recently prepared and aged gold electrodeposits in the 5 X 10'3
and 0.1 s time range,19 respectively, in good agreement with the
STM results. On the other hand, the value of D = 2.5 ± 0.1
obtained for our platinum electrodeposits by the STM method
is also in good agreement with D = 2.46 reported for a thicker
platinum deposit exhibiting self-similar patterns obtained by the
same electrochemical procedure.6
Finally, it should be noted that information concerning the
growth mechanism of the deposits should be derived from the
analysis of the D value of nonaged samples (D s 2.5). For
diffusion-limited deposition with DLA patterns one expects D =
2.53 whereas for a simple ballistic deposition D' gives 1.67,20 both
(17) Alonso, C.; Salvarezza, R. C.; Vara, J. M.; Arvia, A. J.; Vazquez, L.;
Bartolome, A.; Baro, A. M. J. Eleclrochem. Soc. 1990, 137, 2161.
(18) Sommerfeld, D. A.; Cambron, R.; Beebe, T., Jr. J. Phys. Chem. 1990,
94, 8926.
(19) Ocon, P.; Herrasti, P.; Vazquez, L.; Salvarezza, R. C.; Vara, J. M.;
Arvia, A. J. J. Electroanal. Chem., in press.
in three dimensions. Our experimental values of D are closer to
that expected for DLA model. Computer simulations and ex-
perimental work are in progress in our laboratory to verify this
point.
Conclusions
(1) We have used STM images to measure the fractal di-
mension of the surface of thin gold and platinum electrodeposits
by using perimeter/area relationships. This island type analysis,
which applies to both self-similar and self-affine fractals, is based
on the fact that STM is able to give real-space three-dimensional
images of the surface. Also the good lateral resolution of STM
allows us to reach the nanometer range, and the data show that
the fractal nature of the surface reaches this small unit.
(2) For freshly prepared gold and platinum electrodeposits, a
D value equal to 2.5 ± 0.1 was obtained. The aging of the gold
films in the electrolyte at 325 K produced a decrease in the D
value from 2.5 ±0.1 to 2.3 ±0.1, whereas no change in D was
detected for aged platinum films. The correlation of this with
surface diffusivities suggests that diffusion of atoms on surfaces
can cause changes over time of fractal dimension,21 and different
values of this dimension should be obtained by changing the metal,
electrolyte composition, and temperature.
Acknowledgment. A fellowship from the Comision Intermin-
isterial de Ciencia y Tecnologia (Spain) and Consejo Nacional
de Investigaciones Cientificas y Tecnicas (Argentina) to R.C.S.
is gratefully acknowledged. Financial support was obtained from
the CICYT through Contract No. MAT89-0204. We ac-
knowledge the critical reading of the manuscript by F. Guinea.
We also thank J. Gomez-Herrero for valuable discussions.
Registry No. Au, 7440-57-5; Pt, 7440-06-4; H2S04, 7664-93-9; Fe-
(CN)63-, 13408-62-3; Fe(CN)64~, 13408-63-4; gold oxide, 39403-39-9;
platinum oxide, 11129-89-8.
(20) Vicsek, T. Fractal Growth Phenomena', World Scientific: Singapore,
New Jersey, London, 1989.
(21) Hepel, T. J. Eleclrochem. Soc. 1987, 134, 2687.
Catalytic and Spectroscopic Studies of the Water Gas Shift Reaction over RuNa-X and
RuH-X Zeolites
Guan-dao Lei and Larry Kevan*
Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: May 7, 1991)
The low-temperature water gas shift reaction has been studied over Ru(NH3)6C13 exchanged into the H-X and Na-X zeolites.
The RuNa-X and RuH-X zeolite catalysts are investigated under water gas shift reaction conditions by electron spin resonance,
electron spin echo modulation, and infrared and diffuse reflectance spectroscopies. A Ru(II)-dicarbonyl complex is found
to be the catalytically active species for the reaction, and the evidence suggests that it is located at a four-ring site in the
a-cage of the X zeolite. In the RuH-X zeolite, an inactive Ru species is formed inside the /3-cage, which is responsible for
its lower activity compared to the RuNa-X zeolite.
Introduction
The conversion of carbon monoxide and water to produce
carbon dioxide and hydrogen as in eq 1 is known as the water gas
CO + h2o -* C02 + H2 (1)
shift (WGS) reaction. Its industrial importance derives from its
role in the production of ammonia gas,1 which requires high-purity
hydrogen, and from its ability to increase the H2:CO ratio in
(1) Newsome, D. S. Catal. Rev.—Sci. Eng. 1980, 21, 275.
0022-3654/92/2096-350S03.00/0
gaseous feedstock for methanation and Fisher-Tropsch synthesis.2
Commercially, the WGS reaction is carried out over iron-based
and copper-based catalysts, which operate at temperatures of
300-400 and 200-250 °C and are called high-temperature shift
catalysts and low-temperature shift catalysts, respectively. At-
tempts have been made to use homogeneous catalysts for the WGS
reaction such as rhodium carbonyl iodide and ruthenium carbonyl
in alkaline solution.3 4 Ruthenium supported on faujasite-type
(2) Storch, H. H.; Golumbic, N.; Anderson, R. B. The Fisher-Tropsch and
Related System; Wiley: New York, 1951.
&copy; 1992 American Chemical Society


Fractal surfaces of gold and platinum electrodeposits. Dimensionality determination by scanning tunneling microscopy

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Fractal surfaces of gold and platinum electrodeposits. Dimensionality determination by scanning tunneling microscopy

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