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

0

0.5

1

1.5

2

-0.06 -0.05 -0.04 -0.03 -0.02 -0.01 0 0.01 0.02

SO4

2- Adsorption/Desorption on (111)-Textured Au

Su

rfac

e S

tres

s [N

/m]

Charge [mC cm -2 ]

SO4

2- Desorption SO4

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 There are some other “in situ” techniques used for determining the presence of anions in used for determining the presence of anions in the double layer, such as FTIR and Raman the 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 For the application of each of these techniques it is necessary to obtain CV first in techniques it is necessary to obtain CV first in order to define the system, for easier order to define the system, for easier interpretation of ordered adsorbed structures;interpretation of 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

jCRjC

CC

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

CCC

YC 2/1

adad22ad )2( DcFz

RT

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

)(1

1)(CPE CRj

Z

CRj

Z

)(1

1)(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

addldiff1

)2

sin()()(RC

CRCC

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

RCRC

R

YC

2hoad

2ad

2ad1he

addldiffIm)()(1

)2

sin()()(''

RC

CRC

YCC

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.

)(1

1)(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

RZ

YC s

s

22scorr

diffIm)''()'(

''

'' ZRZ

Z

YCC

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.