Techniques for anion adsorption investigation Vladimir D. Jovic Center for Multidisciplinary Studies, Belgrade University, 11030 Belgrade, P.O.Box 33, Serbia

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Techniquesfor anion adsorption
investigationVladimir D. JovicCenter for
Multidisciplinary Studies, Belgrade
University,11030 Belgrade, P.O.Box 33, Serbia
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Double layer structure and corresponding
potential distribution
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Double layer structure and corresponding
potential distributionin the presence of
specifically adsorbed anions
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Differential capacity (EIS) measurementsfor
determination of the properties of the double
layer
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Differential capacity measurementsDetermination
of the potential of zero charge,
Epzc(non-adsorbing electrolytes)G. Quincke,
Ann. Phys., 113 (1861) 513.
G.Valette, A.Hamelin, J.Electroanal.Chem.,
45(1973)301.
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Differential capacity measurementsNon-adsorbing
electrolyte with addition of adsorbing Cl- ions
G.Valette, R.Parsons, J.Electroanal. Chem., 204
(1986) 291.
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In situ Scanning Tunneling Microscopy (STM)
determination of ordered structures during anion
adsorption
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In situ STM image of ordered sulfate structures
adsorbed onto Ag(111) M.Schweizer, D.M.Kolb,
Surf. Sci., 544 (2003) 93-102
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Structure model of the c(3xv3) sulfate structure
adsorbed onto Ag(111) M.Schweizer, D.M.Kolb,
Surf. Sci., 544 (2003) 93-102
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In situ STM image of ordered sulfate structures
adsorbed onto Ag(100) M.Schweizer, D.M.Kolb,
Surf. Sci., 544 (2003) 93-102
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Structure model of the (1.3 x 3.0) sulfate
structure adsorbed onto Ag(100) M.Schweizer,
D.M.Kolb, Surf. Sci., 544 (2003) 93-102
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Bromide adlayer observed in the potential region
III (0.15 V) and underlying Au(111)-(1x1)
substrate (-0.05 V) observed in the potential
region II.A.Cuesta, D.M.Kolb, Surf. Sci., 465
(2000) 311-316
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Adsorption of sulfate anions onto Cu(111)Series
of STM images showing the Moire formation
processduration of the series 12 min.
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In situ STM image of Pd(111) surface obtained at
0.3 V, just before hydrogen adsorption (sharp
peak). Li-Jun Wan et al., J.Electroanal.Chem.,
484 (2000) 189-193
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In situ STM image of ordered sulfate structure
adsorbed onto Pd(111) Li-Jun Wan et al.,
J.Electroanal.Chem., 484 (2000) 189-193
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In situ x-ray determination of ordered structures
during anion adsorption(it requires high energy
electrons obtained from the National Synchrotron
Light Source at Brookhaven National Laboratory,
New York, USA)
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In situ x-ray technique (it can provide
information about distribution of species
parallel and normal to the surface)
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EQMC and in situ stress measurements during anion
and cation adsorption.UPD of Cu onto Au(111) and
sulfate adsorption/desorptionO.E. Kongstein, U.
Bertocci, G.R. Stafford, J. Electrochem. Soc.,
152 (2005) C111-C123
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EQCM and in situ stress measurementsAu(111)
textured substrate, 0.1M H2SO4 0.01M CuSO4
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Stress measurements during sulfate
adsorption/desorption
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IMPORTANT REMARKS

1. There are some other in situ techniques used
   for determining the presence of anions in the
   double layer, such as FTIR and Raman spectroscopy
   and some ex situ techniques such as LEED etc.
2. For the application of each of these techniques
   it is necessary to obtain CV first in order to
   define the system, for easier interpretation of
   ordered adsorbed structures
3. None of the techniques is able to provide
   information about randomly distributed adsorbed
   structures except CV to some extent (qualitative
   interpretation broad peaks).

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New approach to the interpretation of the process
of anion adsorption onto real single crystal
surfaces
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EQUATIONS FOR THE DOUBLE LAYER CAPACITY in the
presence of adsorbing anions
Determination of the double layer capacities is
based on either, differential capacity
measurements (Cdiff vs. E) performed at a single
frequency, or on impedance measurements performed
in a broad range of frequencies and the analysis
of impedance diagrams using the adsorption
impedance theory. According to this theory, the
capacitance spectrum, C(?), calculated from the
measured impedance spectrum, Z(?), can be
expressed by the equation
where Rs represents resistance of the solution,
Cdl the double layer capacity, while Cad, Rad and
?ad correspond to the capacity, resistance and
Warburg coefficient of the adsorbate,
respectively. From this equation it can be
concluded that at high frequencies and low
concentrations of adsorbate, the contribution of
the second term becomes insignificant and the
C(?) spectrum corresponds to the double layer
capacity only. The Cdiff for such a case is given
by the equation
where cad and Dad represent the concentration and
diffusion coefficient of the adsorbing anions,
respectively. All the above mentioned
consideration is valid for systems where the
double layer capacity behaves as an ideal double
layer, without frequency dispersion in the
range of low frequencies, i.e. assuming
homogeneous electrode surfaces. If this is not
the case, constant phase element (CPE) must be
introduced (ZCPE Y0(j?)?, Y0 ?-1cm-2s?).
For parallel connection of CPE and R can be
expressed by two different equations
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EQUIVALENT CIRCUITS FOR DOUBLE LAYER
REPRESENTATION in the presence of adsorbing
anions
Cdl
Rs
CPE
Rs
(c)
Zads
Cad
Rad
CPE
Zads
(a)
(d)
Zads
(b)
Rs
Rad
Cad
Zw
Rad
Cad
Double layer capacity is represented by the
parallel plate condenser (homogeneous charge
distribution)
Double layer capacity is represented by the
Constant Phase Element (nonhomogeneous charge
distribution)
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Simulation of the differential capacity vs.
frequency curves(homogeneous charge distribution
parallel plate condenser)
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Simulation of the differential capacity vs.
frequency curves(homogeneous charge distribution
parallel plate condenser)
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Simulation of the differential capacity vs.
frequency curves(non-homogeneous charge
distribution constant phase element)
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In situ STM results on real single crystal
surfaces
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Model and equivalent circuit for anion adsorption
onto real single crystals Hence, considering all
above mentioned it could be concluded that the
equivalent circuit for anion adsorption onto real
single crystal surfaces should be represented by
two impedances, one corresponding to the process
of anion adsorption onto heterogeneous part of
the surface (monoatomic steps), Zadhe, and
another one corresponding to the process of anion
adsorption (formation of ordered structures) onto
homogeneous part of the surface (flat terraces),
Zadho. Such equivalent circuit is presented here
with Radhe and CPEdlhe
corresponding to the charge transfer resistance
and constant phase element on the heterogeneous
part of the surface respectively and Radho and
Cad corresponding to the charge transfer
resistance and capacity on the homogeneous part
of the surface respectively.
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Equations for the real and imaginary component of
capacitance
Commonly accepted procedure, particularly in the
case of diffusion controlled anion adsorption, is
based on the complex-plane CIm vs. CRe
capacitance presentation and its analysis. Using
the values for Cdl 60 ?F, Cad 200 ?F, Radho
50 ? and Radhe 5000 ? and varying the value of
? from 1.00 to 0.85 a complex-plane CIm vs. CRe
capacity diagram presented in a following figure
are obtained by simulation process.
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Cyclic voltammetry and differential capacity
measurementsof anion adsorption
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Adsorption of chloride anions onto Ag(111)
surfaceV.D. Jovic and B.M. Jovic, J.
Electroanal. Chem., 541 (2003) 1 11.
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Impedance measurements
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Differential capacity vs. potential curves
recorded for different frequencies
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Differential capacity vs. frequency curves
obtained from Cdiff vs. E curves
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Results obtained by fitting procedure
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Adsorption of bromide anions onto Ag(100)
surfaceV.D. Jovic and B.M. Jovic, 57th ISE
Meeting, Edinburgh, 2006.
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Impedance measurementsAg(100), 0.01M KBr

• ? E -1.1 V
• ? E -0.5 V
• ? E -0.3 V

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CRe vs. E and CIm ? Cdiff vs. E dependences
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Differential capacity vs. frequency curves
obtained from Cdiff vs. E curves

• ? E - 1.2 V
• ? E - 1.1 V
• E - 1.0 V
• E -0.8 V
• E - 0.75 V
• ? E - 0.6 V
• ? E - 0.1 V

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Results obtained by fitting procedure
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CONCLUDING REMARKS

• From the presented results it is obvious that
  the most sensitive dependence for anion
  adsorption investigation is Cdiff vs. f(?)
  function
• Considering charges under Cad vs. E curves for
  the system Ag(111)/0.01M NaCl (29 ?C cm-2) and
  Ag(100)/0.01M KBr (31 ?Ccm-2) and assuming
  that the electrosorption valence corresponds to
  the formation of ordered adsorbed structures, it
  appears that ? - 0.4 and ? - 0.3
  respectively, i.e. both adsorbed anions are
  partially discharged. Hence, this analysis
  clearly indicates that neither the charge under
  the CV, nor that under Cdiff vs. E curve recorded
  at a single frequency, can be considered as
  relevant for determining either the structure of
  adsorbed anions or the value of ?
• Finally, it should be stated that the
  combination of cyclic voltammetry, in situ STM
  technique and Cdiff vs. E (f) curve analysis
  could be the best way for qualitative and
  quantitative interpretation of anion adsorption
  processes.