TechniquesTechniquesfor anion adsorption for anion adsorption
investigationinvestigation
Vladimir D. JoviVladimir D. Jovićć
Center for MultidisciplinarCenter for Multidisciplinaryy Studies Studies, Belgrade University,, Belgrade University,11030 Belgrade, P.O.Box 33, Serbia11030 Belgrade, P.O.Box 33, Serbia
Double layer structure and corresponding potential distributionDouble layer structure and corresponding potential distribution
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Double layer structure and corresponding potential distributionDouble layer structure and corresponding potential distributionin the presence of specifically adsorbed anionsin the presence of specifically adsorbed anions
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Differential capacity Differential capacity (EIS) measurements(EIS) measurements
for determination of the for determination of the properties of the double layerproperties of the double layer
Differential capacity measurementsDifferential capacity measurementsDetermination of the potential of zero charge, Determination of the potential of zero charge, EEpzcpzc
(non-adsorbing electrolytes)(non-adsorbing electrolytes)[G. Quincke, Ann. Phys., [G. Quincke, Ann. Phys., 113113 (1861) 513.] [G.Valette, A.Hamelin, J.Electroanal.Chem., (1861) 513.] [G.Valette, A.Hamelin, J.Electroanal.Chem.,
4545(1973)301.](1973)301.]
Differential capacity measurementsDifferential capacity measurementsNon-adsorbing electrolyte with addition of adsorbing ClNon-adsorbing electrolyte with addition of adsorbing Cl-- ions ions
[G.Valette, R.Parsons, J.Electroanal. Chem., [G.Valette, R.Parsons, J.Electroanal. Chem., 204204 (1986) 291.] (1986) 291.]
In situ Scanning Tunneling In situ Scanning Tunneling Microscopy (STM) determination of Microscopy (STM) determination of
ordered structures during anion ordered structures during anion adsorptionadsorption
In situ STM image of ordered sulfate structures adsorbed onto Ag(111) In situ STM image of ordered sulfate structures adsorbed onto Ag(111) [M.Schweizer, D.M.Kolb, Surf. Sci., [M.Schweizer, D.M.Kolb, Surf. Sci., 544544 (2003) 93-102] (2003) 93-102]
Structure model of the c(3xStructure model of the c(3x√3√3) sulfate structure adsorbed onto Ag(111) ) sulfate structure adsorbed onto Ag(111) [M.Schweizer, D.M.Kolb, Surf. Sci., [M.Schweizer, D.M.Kolb, Surf. Sci., 544544 (2003) 93-102] (2003) 93-102]
In situ STM image of ordered sulfate structures adsorbed onto Ag(100) In situ STM image of ordered sulfate structures adsorbed onto Ag(100) [M.Schweizer, D.M.Kolb, Surf. Sci., [M.Schweizer, D.M.Kolb, Surf. Sci., 544544 (2003) 93-102] (2003) 93-102]
Structure model of the (1.3 x 3.0) sulfate structure adsorbed onto Ag(100) Structure model of the (1.3 x 3.0) sulfate structure adsorbed onto Ag(100) [M.Schweizer, D.M.Kolb, Surf. Sci., [M.Schweizer, D.M.Kolb, Surf. Sci., 544544 (2003) 93-102] (2003) 93-102]
Bromide adlayer observed in the potential region III (0.15 V) and underlying 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.Au(111)-(1x1) substrate (-0.05 V) observed in the potential region II.
[A.Cuesta, D.M.Kolb, Surf. Sci., [A.Cuesta, D.M.Kolb, Surf. Sci., 465465 (2000) 311-316] (2000) 311-316]
Adsorption of sulfate anions onto Cu(111)Adsorption of sulfate anions onto Cu(111)Series of STM images showing the Moire formation process:Series of STM images showing the Moire formation process:
duration of the series 12 min.duration of the series 12 min.
In situ STM image of Pd(111) surface obtained at 0.3 V, In situ STM image of Pd(111) surface obtained at 0.3 V, just before hydrogen adsorption (sharp peak).just before hydrogen adsorption (sharp peak).
[Li-Jun Wan et al., J.Electroanal.Chem., [Li-Jun Wan et al., J.Electroanal.Chem., 484484 (2000) 189-193] (2000) 189-193]
In situ STM image of ordered sulfate structure adsorbed onto Pd(111) In situ STM image of ordered sulfate structure adsorbed onto Pd(111) [Li-Jun Wan et al., J.Electroanal.Chem., [Li-Jun Wan et al., J.Electroanal.Chem., 484484 (2000) 189-193] (2000) 189-193]
In situ x-ray determination of In situ x-ray determination of ordered structures during anion ordered structures during anion
adsorptionadsorption
(it requires high energy electrons (it requires high energy electrons obtained from the National Synchrotron Light obtained from the National Synchrotron Light Source at Brookhaven National Laboratory, Source at Brookhaven National Laboratory,
New York, USA)New York, USA)
In situ x-ray technique In situ x-ray technique (it can provide information about distribution of species parallel (it can provide information about distribution of species parallel
and normal to the surface)and normal to the surface)
EQMC and in situ stress EQMC and in situ stress measurements during anion and measurements during anion and
cation adsorption.cation adsorption.UPD of Cu onto Au(111) and sulfate
adsorption/desorption[O.E. Kongstein, U. Bertocci, G.R. Stafford, [O.E. Kongstein, U. Bertocci, G.R. Stafford,
J. Electrochem. Soc., J. Electrochem. Soc., 152152 (2005) C111-C123] (2005) C111-C123]
EQCM and in situ stress measurementsEQCM and in situ stress measurementsAu(111) textured substrate, 0.1M HAu(111) textured substrate, 0.1M H22SOSO44 + 0.01M CuSO + 0.01M CuSO44
Stress measurements during sulfate adsorption/desorptionStress measurements during sulfate adsorption/desorption
-0.5
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0.5
1
1.5
2
-0.06 -0.05 -0.04 -0.03 -0.02 -0.01 0 0.01 0.02
SO42- Adsorption/Desorption on (111)-Textured Au
Surf
ace
Stre
ss [N
/m]
Charge [mC cm -2 ]
SO 42- Desorption SO 4
2- Adsorption
ds/dq Weak Adsorbates (ClO
4-)
ds/dq Strong Adsorbates (Br -)
Figure 1: Surface stress associated with the adsorption/desorption of SO4
2- on (111)-textured Au.
IMPORTANT REMARKSIMPORTANT REMARKS
1.1. There are some other “in situ” techniques used There are some other “in situ” techniques used for determining the presence of anions in the for determining the presence of anions in the double layer, such as FTIR and Raman double layer, such as FTIR and Raman spectroscopy and some “ex situ” techniques spectroscopy and some “ex situ” techniques such as LEED etc.;such as LEED etc.;
2.2. For the application of each of these techniques For the application of each of these techniques it is necessary to obtain CV first in order to it is necessary to obtain CV first in order to define the system, for easier interpretation of define the system, for easier interpretation of ordered adsorbed structures;ordered adsorbed structures;
3.3. None of the techniques is able to provide None of the techniques is able to provide information about randomly distributed information about randomly distributed adsorbed structures except CV to some extent adsorbed structures except CV to some extent (qualitative interpretation – broad peaks).(qualitative interpretation – broad peaks).
New approach to the New approach to the interpretation of the interpretation of the
process of anion process of anion adsorption onto real adsorption onto real
single crystal surfacessingle crystal surfaces
EQUATIONS FOR THE DOUBLE LAYER CAPACITY EQUATIONS FOR THE DOUBLE LAYER CAPACITY in the presence of adsorbing anionsin 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
jCRjCCC
RZjC
a adad2/1
dad
addl
s )(1)(1)(
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
22/1adad
2ad
222/1adad
2/1adadad
dlcorr
diff )()1()1(''
RCC
CCCYC 2/1adad
22ad )2( DcFzRT
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
)(11)(CPE CRj
Z
CRj
Z
)(11)(CPE
EQUIVALENT CIRCUITS FOR DOUBLE LAYER REPRESENTATION EQUIVALENT CIRCUITS FOR DOUBLE LAYER REPRESENTATION in the presence of adsorbing anions in the presence of adsorbing anions
Rs
Cdl
Zads
Zads
Zads
Rs
Rad
Rs
CadRad
Rad Cad ZwCad
CPE
CPE
(a)
(b)(d)
(c)
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)
CRjZ
)(1
1)(CPE
Simulation of the differential capacity vs. frequency curvesSimulation of the differential capacity vs. frequency curves(homogeneous charge distribution – parallel plate condenser)(homogeneous charge distribution – parallel plate condenser)
Simulation of the differential capacity vs. frequency curvesSimulation of the differential capacity vs. frequency curves(homogeneous charge distribution – parallel plate condenser)(homogeneous charge distribution – parallel plate condenser)
Simulation of the differential capacity vs. frequency curvesSimulation of the differential capacity vs. frequency curves(non-homogeneous charge distribution – constant phase element) (non-homogeneous charge distribution – constant phase element)
CRjZ
)(1
1)(CPE
2ad
2ad
2ad1
addldiff 1)
2sin()()(
RCCRCC
In situ STM results on real single crystal surfacesIn situ STM results on real single crystal surfaces
Model and equivalent circuit for anion adsorption onto real single crystalsHence, 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), Zad
he, and another one corresponding to the process of anion adsorption (formation of ordered structures) onto homogeneous part of the surface (flat terraces), Zad
ho. Such equivalent circuit is presented here
with Radhe and CPEdl
he corresponding to the charge transfer resistance and constant phase element on the heterogeneous part of the surface respectively and Rad
ho and Cad corresponding to the charge transfer resistance and capacity on the homogeneous part of the surface respectively.
Equations for the real and imaginary component of capacitanceEquations for the real and imaginary component of capacitance
2hoad
2ad
2
hoad
2ad1he
addlhead
Re)()(1
)2
cos()()(1'RC
RCRCR
YC
2hoad
2ad
2ad1he
addldiffIm)()(1
)2
sin()()(''RC
CRCYCC
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, Rad
ho = 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.
)(11)(CPE CRj
Z
Cyclic voltammetry and differential Cyclic voltammetry and differential capacity measurementscapacity measurements
of anion adsorptionof anion adsorption
Adsorption of chloride anions onto Ag(111) surfaceAdsorption of chloride anions onto Ag(111) surface[V.D. Jovi[V.D. Jovićć and B.M. Jovi and B.M. Jovićć, J. Electroanal. Chem., , J. Electroanal. Chem., 541541 (2003) 1 – 11.] (2003) 1 – 11.]
Impedance measurementsImpedance measurements
Differential capacity vs. potential curves recorded for different frequenciesDifferential capacity vs. potential curves recorded for different frequencies
22corr
Re)''()'(
'' ZRZ
RZYC s
s
22scorr
diffIm)''()'(
'''' ZRZ
ZYCC
Differential capacity vs. frequency curves obtained from Differential capacity vs. frequency curves obtained from CCdiffdiff vs. vs. EE curves curves
Results obtained by fitting procedureResults obtained by fitting procedure
Adsorption of bromide anions onto Ag(100) surfaceAdsorption of bromide anions onto Ag(100) surface[V.D. Jovi[V.D. Jovićć and B.M. Jovi and B.M. Jovićć, 57, 57thth ISE Meeting, Edinburgh, 2006.] ISE Meeting, Edinburgh, 2006.]
Impedance measurementsImpedance measurementsAg(100), 0.01M KBrAg(100), 0.01M KBr
E = -1.1 V E = -0.5 V E = -0.3 V
CCReRe vs. vs. EE and and CCImIm CCdiffdiff vs. vs. EE dependences dependences
Differential capacity vs. frequency curves obtained from Differential capacity vs. frequency curves obtained from CCdiffdiff vs. vs. EE curves 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
Results obtained by fitting procedureResults obtained by fitting procedure
CONCLUDING REMARKSCONCLUDING REMARKSFrom 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.