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The evidence for a stoichiometric silver oxide-cyanide phase in cyanide SERS from silver electrodes
Volume 117. numbx
CHEMICAL
1
PHYSICS
C SILVER THE EVIDENCE FOR A STOICHIOMEIRI IN CYANIDE SERS FROM SILVER ELECI-RODES Merrick
R. MAHONEY
31 May 1985
LETlTXS
OXIDE-CYANIDE
PHASE
and Ralph P. COONEY
Chemisrry Deparrmenr. Unruersrryof Newcasrle. Ne-wcade, NS
W., Aurrraha 2308
Received 23 Oclober 19E4
Nine candidates for the intensely scattering phase in tilver/cyanide SERS are compared on the basis of eight exps;‘_menral criteria.New crireriainclude complex dissnciarion studies. ksts for radical involvement and the cathodic dsplacement of SERS with increaung ektrolyte
alkahn~~y. The favoured phase is a sliver oxide-cyanide
1.Introduction The present series of studies [l] of surface-enhanced Raman scattering (SEBS) effects has two objectives: to examine the correlation between laser surface damage and SERS, and to identify the molecular species responsible for the intense spectrum_ The evidence for the involvement of laser-assisted (i.e. photochemical) corrosion and dissolution processes wrthin the sampling microzone in cyanide SERS from silver eIectrodes is based on the following experimental tests. (1) Scanning Auger microscopy [2] and scanning electron microscopy [2,3] have identified laser damage zones (-0.13 mm in diameter). (2) Easer-interruption studies [3] reveal that the characteristic intensity decay pattern in silver/cyanide SERS is laser-assisted. (3) Uniform laser flux perturbations [3] of voltammograms and steady-state currents demonstrate that the electrochemical processes are modified by laser flux densities 51% of those employed in typical SEBS studies. The preliminary evidence for the involvement of a hydrolyzed (i.e. oxide or hydroxide) phase in cyanide SEBS from silver electrodes emerges from three experimental results: (1) An irreversible loss of intensity in the Y(CN) line (21 I1 cm-l) was observed when the surface was strongly reduced [4] _ This indicates that a reducible 0 009-2614/85/$03.30 @EIsevier Science Publishers B.V. (North-HolIand Physics publishing Division)
compound.
(Ag(I)) species formed as a result of the ORC was the origin of the intense spectrum. (2) The pH sensitivity of the spectrum (which goes to extinction at pH =r 7 [4]) suggests that the laser microzone surface phase incorporates silver oxide, which is thermodynamically stable only at pH 2 7. This aspect is further investigated in the present study. (3) The results of Auger surface analysis 123 reveal that the laser damage zone is oxygen-enriched, and silver- and carbon-depleted relative to the non-illuminated surface_ The question remaining unresolved is the precise form of chemical association between silver oxide/ hydroxide and cyanide, i.e. the identity of the intensely scattering phase. In table 1 nine possible species are analyzed according to their ability to explain seven key experimental results. For completeness the range of cyanide species on both silver metal and silver oxide are subjected to the experimental analysis. In addition the possibility of chemical compound formation (i.e. a stoichiometdc phase) is considered.
2. Experimental The lasers, Raman spectrometer, electrochemical equipment and spectroelectrochemical ceils have been described previously [6,7]. The materials including high-purity silver have also been reported previously [6]. The electrolyte used in the SERS studies was 0.1 71
x
J x
insenslff vity of SEKS to r&al scavmgat ~~fareor afterORC) (thiswork)
slmbihyofC!N”SERS fiorn &andAufl2j
4
~O~~U~R af Y(CN) m2113e& I41 poslthnhtcnslty,andmass scnsltivity of the 227cm’1 line,is, cvidenoe fat lattice node@j
x
lytcalkalhity(tl$swork)
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e) Not&n: (\h wnalstontwithexperiment; (I()inconshtentwithexpwlmcnt, end(7)uncertainresult,
0)
0
61
(8
(4)
bendUdlyofpotentialof mrtimumSERS to electto,
@~~Uon (31 ~fevc~ibie of SERSby calhodizetlon [4/
(NJ work}
(2) lntenaltyextiction et pH5 7
oxygen&handrrllverdeploted[2]
(11 Augerdatailasermlcro~ano
x
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x
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9
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Volume 117, number 1
CHEMICAL
PFWSICS
31 May 1985
LETITERS
M Na,SO,, 0.01 M KCN @Hz 10.5). The exciting line employed ti recording spectra was 514.5 run Ar+ (100 mw). Potentials are measured relative to a saturated calomel reference. The oxidation-reduction cycle (ORC) pretreatment was a pulse step (-0.8 V * +0.5 V) with a 5 s pause ar the anodic limit. Solutions of cyanosilver(I) complexes were prepared by the addition of various quantities of AgCN to 1 M KCN.
tensity was a result of the elimination of a surface (oxide) phase, and not a result of pHdependent changes in CN-/HCN solution equilibria. This assump tion appears to be supported by calculations of free cyanide ion concentration (assuming a pK, value of 9.31 [lo]), which indicate that atpH 6.5 the free cyanide ion concentration is -2 X 10e5 M. Measurements of cyanide surface coverage at 1.1 X 10e5 M revealed that significant cyanide adsorption had occurred [l l] _ While these results were recorded for an
3. Results and discussion
to an ORC nor exposed to laser flux, they confirm that conventional cyanide electrosorptlon would be reasonably insensitive to cyanide concentration changes resulting from adjustment of PH. This is further supported by recent data which reveal a very different pH dependence of cyanide SERS from copper surfaces in an identical electrolyte medium [12] _ Cyanide SERS from copper electrodes exlublts similar intensities atpH 10.3 and 3.5. These results together with the Auger analysis and cathodization data [24] support the proposition (see table 1) that the intensely scattering surface phase involves silver(I) oxide or some related partly hydrolyzed cyanide compound A further indication of the involvement of an oxide/hydroxide phase is found in the displacement of the potential of maximum intensity from - 0.95 to -1.30 V(see table 3) when the electrolyte is changed from 0.1 M Na,SO,, 0.01 M KCN to O-1 M KOH, 0.1 M KCN. The latter electrolyte has been employed because it facilitates comparison of silver electrode data with previous gold dissolution studies [9] in which oxide/hydroxide passivating films are known to form. The cathodic displacement of the potential of maximum mtensity for silver in a more alkaline electrolyte is consistent with the stabilization on an oxide/ hydroxide scattering phase. In order to test for the presence of cyanosilver(l) complex adsorption on the oxide phase (above), the pH dependence of v(CN) intensities of a series of aqueous solutions of such complexes were studied. Raman studies of the acid dissociation of [Ago (identified 1131 by a line maximum at 2141 cm-l) in 1 M aqueous solutions, reveal that on lowering the pff to 56 (i.e. the point of SERS extinction) no clear evidence of dissociation into lower complexes was ob served, i.e. there was little change in the position or intensity of the coordination-sensitive v(CN). A simi-
electrode
Three types of cyanidecontaining adsorbate (cyan0 complex, cyanide ion and radical) are considered for both silver metal and silver oxide surface (see table 1). In ad&tion the possibility that cyanide is incorporated in a stoichiometric silver(I) oxo- or hydroxo-compound (ie_ a non-adsorptive association) is considered (see table 1). This last possibility is considered because of the tendency [g] of isoelectronic (alo) metal Ions to form oxocyanide compounds (e.g. [O(HgCN),] 2) and aIso because of the known involvement of oxide or hydroxide passivation films in the closely related corrosion system: Au /cyanide in aqueous alkali [9]. A diagnostic test of considerable importance to the present species analysis (table 1) IS the pH dependence of the intensity of the principal SERS line at 2111 cm-l (see table 2). A decrease and virtual elimination of silver/cyanide SERS, as the pH is lowered from the natural electrolyte pH of 10-l 1 to ~7 was observed (table 2) It was assumed [4] that this decrease in inTable 2 SequentialpH
dependencz of Ag/cyanide SERS at -0.8
V
WE)
first ORC
secondORC
pIf=)
SEFCS intensity b,
10.1 7.1 6.5 58
100 8.5 3.0 10
5-B
10.1
1.0 _
33.0
al pH adjusted by ad&tion of aqueous HzS04 or KOH
point takeit IJI sequence from top to bottom. IJJII~S (typically: 100 units= 5 X 10’ counts s-‘1.
b) hblhary
surface
which
has been neither
subjected
73
Table 3
Variation of
v(CNl
SERS with changingpotential
Eleckode potential #
Relative intensity
Frequency ~GNJ G=rn-‘1
(a) Ag/O 1 hi NalS04. O-01 M KCN -1.20 2104*1 -1.00 2108 -0.95 2111 -0.90 2113 -0.80 2114 -0.70 2114 -0.60 2114 -0.40 2110 (b) Ag/O.l M KOH, 0 1 M KCN -1.40 -1 30 -1.20 -1.10 -1 00 -0.90
;;;;
70 90 100 78 63 42 37 28
47 100 41 36 9 -0
:;
2103 2104 2104 -
a) Some evidence for a weak band at -2005
cm-‘.
lar study of 1 M [Ag(CN),]2(2108 cm-1 [13,14]) revealed that at pW 2: 6, there was little change in iutensity of the Y(CN) prome_ However v(CN) was shifted to 2135 and 2091 cm-l, indicating [13,14] the formation of mainly [Ag(CN)2]and a small proportion of [Ag(CN)4]3-. (Associated with these pJf adjustments was significant dilution of the sample which was considered in the intensity calculations.)
Therefore the extinction of cyanide SERS at pH =;: 7 cannot be attributed to acid dissociation of cyanosilver(I) complexes within the sampling zone_ This conclusion is supported by the similar v(CN)
observed for the Au/CN- and Ag/CN- SERS systems [ 121. Cyanoaetal complexes of the two metals exhibit distinctly different v(CN) frequencies. Therefore the intensely scattering phase is likely to mvolve only weak or non-existent metal-cyanide interactions. To test for the presence of a cyamde radical species (e.g. the cyanogen radical anion [15]) a radical scavenger, hydroquinone, was added to the electrolyte in a typical electrochemical experiment exhibiting intense cyanide SERS at 2111 cm-l. Hydroquinone addition (at f3 r&l) to the electrolyte medium resulted in no observable loss of intensity in the u(CN) line. In another experiment hydroquinone (6 mM) was added to 74
31 May 1985
CHEMICAL PHYSICS LITrERs
Volume 117. number 1
the electrolyte (0.1 M Na2S04, 0.01 M KCN) before the ORC. After an ORC the intense SERS line was observed as usual at 2111 cm-l. It is therefore unlikely that a radical species is the origin of SERS from this system (see table 1). Therefore the intensely scattering phase appears to be either CN- adsorbed on an oxide phase or a stoichiometric
silver oxide cyanide corn-
pound. The mass sensitivity [5] of the dominant low-frequency line at 227 cm-’ provides a means of distinguishing between these final two possrbilities (table 1). Substitution of 13CN- for l*CN- results [S] in the displacement of the 227 cm-l band to lower frequencies by an increment of 8-10 cm-l. This result indicates that the 227 cm-’ line arises from a mass-sensitive mode involving CN-. This 227 c-m-l feature and the u(CN) SERS Lineat 2111 cm-l exhibit a constant ratio of peak heights (0.33 + 0.02) for almost all spectra recorded in this study and therefore are considered to arise from different modes of a common scattering species. It was concluded on the basis of acid-dissociation studies (above) that the common species responsible for the 2111 cm -l line was not a cyanosilver(I) complex. This conclusion is reinforced by the observation that [Ag(03] 2-_. which has a line at a2108 cm-l (i-e_ adjacent to the SERS dCN)),
has no detectable line in the region around 227 cm-l_ Also [Ag(CN)2]has a Y(CN) at 2141 cm-l (i e. well removed from the
SERS y(CN)), and neither its v(Ag-C) (360 cm-l) nor its G(AgCN) (250 cm-l) [13] coincide with the 227 cm-l SERS line. In general v(J4-C) for metal dicyarudes would be expected at higher frequencies (e-g. 412 cm-l for Hg(Cl+Q [ 141). Therefore it is proposed that the 227 cur-l line arises from a mass-sensitive lattice mode probably involving cyanide translations. This assignment is made by comparison with intense Raman-active lattice modes of ionic KCN 1161
(observed in the 100-300 cm-1 region). The 227 cm-1 feature is also adjacent to the mass-sensitive longitudinal optical (L.0) mode of silver halides (AgCl, 184 cm-l ;AgBr, 128 cm-l [17]). The assignment of the 227 cm-l line to a lattice mode involving cyanide ions suggests that extended (crystallographic) order exists among the cyamde ions in the SERS phase. Adsorption of cyanide on an
oxide surface (which may itself be disordered) ls unlikely to generate a highly ordered adsorbed phase
Volume 117, number 1
CHEMICAL PHYSICS LETTERS
especially at low coverage. Therefore the appearance of a cyanide lattice mode (227 cm-l) indicates that the SERS phase is highly ordered (i.e. polycrystalline).
Hence the SERS phase would appear to be a stoichiometric silver oxide/hydroxide cyanide compound (see table 1). This compound may form III the interfacial region of the laser damage microzone as a result of cyanide depletion followmg locahzed laser-assisted corrosion in the basic electrolyte_ The position of the 2111 cm-1
line can now be rationalized_ At least two possible explanations may be considered_ First, the cyanide ions in the lattice may associate with [Ag(OH Brgnsted acid groupings and would then exhibit a v(CN) which IS similar to HCN on oxide surfaces (=2100 cm-l) [IS]. Alternatively, the cyanide ions may be associated with defect (colour) centres created either by the excess of metal ions in the interfacial region or by intense laser flux. Cyanide ions associated with colour defects in ionic lattices exhibit split v(CN) features and also resonance enhancement [ 191. Such an enhancement may contribute to SERS intensities from this system. However, the exlstmg body of data does not permit these two possibilities to be resolved, The close similarity of cyanide SERS from silver and gold electrodes can now be understood. If the proposed silver oxide/hydroxide cyanide compound is a cationic 0x0 framework polymer wth cyanide counterions (as in [Hg302]n(X)2n), it is probable that
direct metal-cyanide interactions are relatively unimportant (see Breasted acid group discussIon, above). In such a case, analogous silver and gold compounds would exlubit v&N) which are senative to the metal present- The similarity of cyamde SERS from silver and gold is of particular significance because corresponding dicyano- and tricyanometal complexes of the two metals have different Y(W) [20] The apparent absence of a v(Ag0) of the oxide/ hydroxide compound in the SERS spectrum, is not unexpected. The weakness of Rarnan spectra of metal oxide substrates has been expIoited in studies of surface species 1203. Also, Rarnan spectra of a withdrawn electrode on which a thick layer of Ag20 had been formed, and of bulk AgzO, did not show any Raman lines in the 150-900 cm-l region [21].
31 May 1985
4 Conclusion On the basis of the analysis in table 1, it appears that a silver(I) oxide/hydroxide cyanide compound is the origin of cyanide SERS from silver electrodes. The distinction between cyanide ion adsorption on silver(l) oxide/hydroxide and such a stoichoimetric compound (see table 1) is relatively subtle It depends on the evidence for the assignment of the 227 cm-l line as a lattice (external) mode rather than an internal mode. It also depends on the similarity of the low-frequency mode (218-228 cm-l) for both silver and gold electrode systems exhibiting cyanide SERS [12]. Finally, the potential-sensitive dispiacement of the v(CN) SERS line is presumed to atise from a succession of closely related oxo-cyanide compounds differing in Ag/CN‘- ratio.
Acknowledgement The authors are grateful to the Australian Research Grants Scheme for supporting this project.
References [l] R-P Cooney and T.P. Mernagh. in: Electrochemistry
- the interfadng science. eds. D.A.J. Rand and A.M. Bond (Elsevier. titerclam, 1984); J. Electroanal. Chem.
168 (1984) 67. [2] R-P. Cooney, T.P Mcmagh, M R Mahoney and J.A. Spink, J. Phys. Chem 87 (1983) 5314.
[3] M-R. Mahoney and R.P. Cooney, J. Phys. Chem. 87 (1983) 4589 [4] M-R Mahoney and R.P Cooney, J. Raman Spcctry. (1981)
11
141_
M. Flciwhmann, 1-R HIU and NE. Pcmblc, J Elecuoanal. Chem. 136 (1982) 361 (61 M.R Mahoney, M-W. Howard and R.P. Cooney, Chem. Phys. Letters 71 (1980) 59. [7] M W. Howard, R.P. Cooney and A.J. McQuUan. J. [S]
Raman Spectry. 9 (1980) 273. [R] S. S~vnirar. 2. Krist. 118 (1963) 248 [9] D-W. Kirk, F.R. Foulkes and W.F. Graydon. J. Electro&em. Sot. 127 (1980)
[lo] R-C [ 111
Weast.
1962. ed., CRC handbook
of chemistry
and
physics, 60th Ed. (CRC Press. Cleveland, 1979). N A. Rogozhnikov and R.Yu. Bek. Elektrokhimiya 16
(1980) 662. R. Mahoney and R-P. Cooney, unpubhshed datx.
[ 121 M
75
Volume
117. number 1
CHEMICAL
[ 131 D.M. Adams, Metal-Ii-d and related vibrations (Arnold. London, 1967) pp_ 164-168. (141 K. Nakamoto, Infrared spectra of inorganic and coordination compounds wey-Interscience, New York, 1970) pp. 180.181. [15] A_ Regis, 1. Corset_ N. Jaffrezic-Renault and G. Blond-u, in Proceedings of the Wth International Raman Conferemx. ed. W.F. Murphy (North-Holland, Amsterdam, 1980) p_ 418.
76
PHYSICS
LETTERS
[16] [17]
31 May 1985
W. Dultz, Solid State Commun. 15 (1974) 595. S D. Ross, Inorganic infrared and Raman spectra (McGraw-Hill, New York, 1972) p. 97. [IS] M-J D. Law, N. Ra masubramaian, P. Ramamurthy and AV. Dee, J. Phys. Chem. 72 (1968) 2371. [ 191 Y Yang and F. Luty, Phys. Rev. Letters 51 (1983) 419. [20] RP Cooney, G.C. Curthoys and T-T. Nguyen, Advan_ Catal. 24 (1975) 293. [21] R. Kbtz and E. Yeager. J_ EIectroanaL Chem. 111 (1980) 105.
CHEMICAL
1
PHYSICS
C SILVER THE EVIDENCE FOR A STOICHIOMEIRI IN CYANIDE SERS FROM SILVER ELECI-RODES Merrick
R. MAHONEY
31 May 1985
LETlTXS
OXIDE-CYANIDE
PHASE
and Ralph P. COONEY
Chemisrry Deparrmenr. Unruersrryof Newcasrle. Ne-wcade, NS
W., Aurrraha 2308
Received 23 Oclober 19E4
Nine candidates for the intensely scattering phase in tilver/cyanide SERS are compared on the basis of eight exps;‘_menral criteria.New crireriainclude complex dissnciarion studies. ksts for radical involvement and the cathodic dsplacement of SERS with increaung ektrolyte
alkahn~~y. The favoured phase is a sliver oxide-cyanide
1.Introduction The present series of studies [l] of surface-enhanced Raman scattering (SEBS) effects has two objectives: to examine the correlation between laser surface damage and SERS, and to identify the molecular species responsible for the intense spectrum_ The evidence for the involvement of laser-assisted (i.e. photochemical) corrosion and dissolution processes wrthin the sampling microzone in cyanide SERS from silver eIectrodes is based on the following experimental tests. (1) Scanning Auger microscopy [2] and scanning electron microscopy [2,3] have identified laser damage zones (-0.13 mm in diameter). (2) Easer-interruption studies [3] reveal that the characteristic intensity decay pattern in silver/cyanide SERS is laser-assisted. (3) Uniform laser flux perturbations [3] of voltammograms and steady-state currents demonstrate that the electrochemical processes are modified by laser flux densities 51% of those employed in typical SEBS studies. The preliminary evidence for the involvement of a hydrolyzed (i.e. oxide or hydroxide) phase in cyanide SEBS from silver electrodes emerges from three experimental results: (1) An irreversible loss of intensity in the Y(CN) line (21 I1 cm-l) was observed when the surface was strongly reduced [4] _ This indicates that a reducible 0 009-2614/85/$03.30 @EIsevier Science Publishers B.V. (North-HolIand Physics publishing Division)
compound.
(Ag(I)) species formed as a result of the ORC was the origin of the intense spectrum. (2) The pH sensitivity of the spectrum (which goes to extinction at pH =r 7 [4]) suggests that the laser microzone surface phase incorporates silver oxide, which is thermodynamically stable only at pH 2 7. This aspect is further investigated in the present study. (3) The results of Auger surface analysis 123 reveal that the laser damage zone is oxygen-enriched, and silver- and carbon-depleted relative to the non-illuminated surface_ The question remaining unresolved is the precise form of chemical association between silver oxide/ hydroxide and cyanide, i.e. the identity of the intensely scattering phase. In table 1 nine possible species are analyzed according to their ability to explain seven key experimental results. For completeness the range of cyanide species on both silver metal and silver oxide are subjected to the experimental analysis. In addition the possibility of chemical compound formation (i.e. a stoichiometdc phase) is considered.
2. Experimental The lasers, Raman spectrometer, electrochemical equipment and spectroelectrochemical ceils have been described previously [6,7]. The materials including high-purity silver have also been reported previously [6]. The electrolyte used in the SERS studies was 0.1 71
x
J x
insenslff vity of SEKS to r&al scavmgat ~~fareor afterORC) (thiswork)
slmbihyofC!N”SERS fiorn &andAufl2j
4
~O~~U~R af Y(CN) m2113e& I41 poslthnhtcnslty,andmass scnsltivity of the 227cm’1 line,is, cvidenoe fat lattice node@j
x
lytcalkalhity(tl$swork)
x/
n
n
n
4
x
J
x
J
x
J
x
n
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li
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x
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e) Not&n: (\h wnalstontwithexperiment; (I()inconshtentwithexpwlmcnt, end(7)uncertainresult,
0)
0
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bendUdlyofpotentialof mrtimumSERS to electto,
@~~Uon (31 ~fevc~ibie of SERSby calhodizetlon [4/
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(2) lntenaltyextiction et pH5 7
oxygen&handrrllverdeploted[2]
(11 Augerdatailasermlcro~ano
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Volume 117, number 1
CHEMICAL
PFWSICS
31 May 1985
LETITERS
M Na,SO,, 0.01 M KCN @Hz 10.5). The exciting line employed ti recording spectra was 514.5 run Ar+ (100 mw). Potentials are measured relative to a saturated calomel reference. The oxidation-reduction cycle (ORC) pretreatment was a pulse step (-0.8 V * +0.5 V) with a 5 s pause ar the anodic limit. Solutions of cyanosilver(I) complexes were prepared by the addition of various quantities of AgCN to 1 M KCN.
tensity was a result of the elimination of a surface (oxide) phase, and not a result of pHdependent changes in CN-/HCN solution equilibria. This assump tion appears to be supported by calculations of free cyanide ion concentration (assuming a pK, value of 9.31 [lo]), which indicate that atpH 6.5 the free cyanide ion concentration is -2 X 10e5 M. Measurements of cyanide surface coverage at 1.1 X 10e5 M revealed that significant cyanide adsorption had occurred [l l] _ While these results were recorded for an
3. Results and discussion
to an ORC nor exposed to laser flux, they confirm that conventional cyanide electrosorptlon would be reasonably insensitive to cyanide concentration changes resulting from adjustment of PH. This is further supported by recent data which reveal a very different pH dependence of cyanide SERS from copper surfaces in an identical electrolyte medium [12] _ Cyanide SERS from copper electrodes exlublts similar intensities atpH 10.3 and 3.5. These results together with the Auger analysis and cathodization data [24] support the proposition (see table 1) that the intensely scattering surface phase involves silver(I) oxide or some related partly hydrolyzed cyanide compound A further indication of the involvement of an oxide/hydroxide phase is found in the displacement of the potential of maximum intensity from - 0.95 to -1.30 V(see table 3) when the electrolyte is changed from 0.1 M Na,SO,, 0.01 M KCN to O-1 M KOH, 0.1 M KCN. The latter electrolyte has been employed because it facilitates comparison of silver electrode data with previous gold dissolution studies [9] in which oxide/hydroxide passivating films are known to form. The cathodic displacement of the potential of maximum mtensity for silver in a more alkaline electrolyte is consistent with the stabilization on an oxide/ hydroxide scattering phase. In order to test for the presence of cyanosilver(l) complex adsorption on the oxide phase (above), the pH dependence of v(CN) intensities of a series of aqueous solutions of such complexes were studied. Raman studies of the acid dissociation of [Ago (identified 1131 by a line maximum at 2141 cm-l) in 1 M aqueous solutions, reveal that on lowering the pff to 56 (i.e. the point of SERS extinction) no clear evidence of dissociation into lower complexes was ob served, i.e. there was little change in the position or intensity of the coordination-sensitive v(CN). A simi-
electrode
Three types of cyanidecontaining adsorbate (cyan0 complex, cyanide ion and radical) are considered for both silver metal and silver oxide surface (see table 1). In ad&tion the possibility that cyanide is incorporated in a stoichiometric silver(I) oxo- or hydroxo-compound (ie_ a non-adsorptive association) is considered (see table 1). This last possibility is considered because of the tendency [g] of isoelectronic (alo) metal Ions to form oxocyanide compounds (e.g. [O(HgCN),] 2) and aIso because of the known involvement of oxide or hydroxide passivation films in the closely related corrosion system: Au /cyanide in aqueous alkali [9]. A diagnostic test of considerable importance to the present species analysis (table 1) IS the pH dependence of the intensity of the principal SERS line at 2111 cm-l (see table 2). A decrease and virtual elimination of silver/cyanide SERS, as the pH is lowered from the natural electrolyte pH of 10-l 1 to ~7 was observed (table 2) It was assumed [4] that this decrease in inTable 2 SequentialpH
dependencz of Ag/cyanide SERS at -0.8
V
WE)
first ORC
secondORC
pIf=)
SEFCS intensity b,
10.1 7.1 6.5 58
100 8.5 3.0 10
5-B
10.1
1.0 _
33.0
al pH adjusted by ad&tion of aqueous HzS04 or KOH
point takeit IJI sequence from top to bottom. IJJII~S (typically: 100 units= 5 X 10’ counts s-‘1.
b) hblhary
surface
which
has been neither
subjected
73
Table 3
Variation of
v(CNl
SERS with changingpotential
Eleckode potential #
Relative intensity
Frequency ~GNJ G=rn-‘1
(a) Ag/O 1 hi NalS04. O-01 M KCN -1.20 2104*1 -1.00 2108 -0.95 2111 -0.90 2113 -0.80 2114 -0.70 2114 -0.60 2114 -0.40 2110 (b) Ag/O.l M KOH, 0 1 M KCN -1.40 -1 30 -1.20 -1.10 -1 00 -0.90
;;;;
70 90 100 78 63 42 37 28
47 100 41 36 9 -0
:;
2103 2104 2104 -
a) Some evidence for a weak band at -2005
cm-‘.
lar study of 1 M [Ag(CN),]2(2108 cm-1 [13,14]) revealed that at pW 2: 6, there was little change in iutensity of the Y(CN) prome_ However v(CN) was shifted to 2135 and 2091 cm-l, indicating [13,14] the formation of mainly [Ag(CN)2]and a small proportion of [Ag(CN)4]3-. (Associated with these pJf adjustments was significant dilution of the sample which was considered in the intensity calculations.)
Therefore the extinction of cyanide SERS at pH =;: 7 cannot be attributed to acid dissociation of cyanosilver(I) complexes within the sampling zone_ This conclusion is supported by the similar v(CN)
observed for the Au/CN- and Ag/CN- SERS systems [ 121. Cyanoaetal complexes of the two metals exhibit distinctly different v(CN) frequencies. Therefore the intensely scattering phase is likely to mvolve only weak or non-existent metal-cyanide interactions. To test for the presence of a cyamde radical species (e.g. the cyanogen radical anion [15]) a radical scavenger, hydroquinone, was added to the electrolyte in a typical electrochemical experiment exhibiting intense cyanide SERS at 2111 cm-l. Hydroquinone addition (at f3 r&l) to the electrolyte medium resulted in no observable loss of intensity in the u(CN) line. In another experiment hydroquinone (6 mM) was added to 74
31 May 1985
CHEMICAL PHYSICS LITrERs
Volume 117. number 1
the electrolyte (0.1 M Na2S04, 0.01 M KCN) before the ORC. After an ORC the intense SERS line was observed as usual at 2111 cm-l. It is therefore unlikely that a radical species is the origin of SERS from this system (see table 1). Therefore the intensely scattering phase appears to be either CN- adsorbed on an oxide phase or a stoichiometric
silver oxide cyanide corn-
pound. The mass sensitivity [5] of the dominant low-frequency line at 227 cm-’ provides a means of distinguishing between these final two possrbilities (table 1). Substitution of 13CN- for l*CN- results [S] in the displacement of the 227 cm-l band to lower frequencies by an increment of 8-10 cm-l. This result indicates that the 227 cm-’ line arises from a mass-sensitive mode involving CN-. This 227 c-m-l feature and the u(CN) SERS Lineat 2111 cm-l exhibit a constant ratio of peak heights (0.33 + 0.02) for almost all spectra recorded in this study and therefore are considered to arise from different modes of a common scattering species. It was concluded on the basis of acid-dissociation studies (above) that the common species responsible for the 2111 cm -l line was not a cyanosilver(I) complex. This conclusion is reinforced by the observation that [Ag(03] 2-_. which has a line at a2108 cm-l (i-e_ adjacent to the SERS dCN)),
has no detectable line in the region around 227 cm-l_ Also [Ag(CN)2]has a Y(CN) at 2141 cm-l (i e. well removed from the
SERS y(CN)), and neither its v(Ag-C) (360 cm-l) nor its G(AgCN) (250 cm-l) [13] coincide with the 227 cm-l SERS line. In general v(J4-C) for metal dicyarudes would be expected at higher frequencies (e-g. 412 cm-l for Hg(Cl+Q [ 141). Therefore it is proposed that the 227 cur-l line arises from a mass-sensitive lattice mode probably involving cyanide translations. This assignment is made by comparison with intense Raman-active lattice modes of ionic KCN 1161
(observed in the 100-300 cm-1 region). The 227 cm-1 feature is also adjacent to the mass-sensitive longitudinal optical (L.0) mode of silver halides (AgCl, 184 cm-l ;AgBr, 128 cm-l [17]). The assignment of the 227 cm-l line to a lattice mode involving cyanide ions suggests that extended (crystallographic) order exists among the cyamde ions in the SERS phase. Adsorption of cyanide on an
oxide surface (which may itself be disordered) ls unlikely to generate a highly ordered adsorbed phase
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CHEMICAL PHYSICS LETTERS
especially at low coverage. Therefore the appearance of a cyanide lattice mode (227 cm-l) indicates that the SERS phase is highly ordered (i.e. polycrystalline).
Hence the SERS phase would appear to be a stoichiometric silver oxide/hydroxide cyanide compound (see table 1). This compound may form III the interfacial region of the laser damage microzone as a result of cyanide depletion followmg locahzed laser-assisted corrosion in the basic electrolyte_ The position of the 2111 cm-1
line can now be rationalized_ At least two possible explanations may be considered_ First, the cyanide ions in the lattice may associate with [Ag(OH Brgnsted acid groupings and would then exhibit a v(CN) which IS similar to HCN on oxide surfaces (=2100 cm-l) [IS]. Alternatively, the cyanide ions may be associated with defect (colour) centres created either by the excess of metal ions in the interfacial region or by intense laser flux. Cyanide ions associated with colour defects in ionic lattices exhibit split v(CN) features and also resonance enhancement [ 191. Such an enhancement may contribute to SERS intensities from this system. However, the exlstmg body of data does not permit these two possibilities to be resolved, The close similarity of cyanide SERS from silver and gold electrodes can now be understood. If the proposed silver oxide/hydroxide cyanide compound is a cationic 0x0 framework polymer wth cyanide counterions (as in [Hg302]n(X)2n), it is probable that
direct metal-cyanide interactions are relatively unimportant (see Breasted acid group discussIon, above). In such a case, analogous silver and gold compounds would exlubit v&N) which are senative to the metal present- The similarity of cyamde SERS from silver and gold is of particular significance because corresponding dicyano- and tricyanometal complexes of the two metals have different Y(W) [20] The apparent absence of a v(Ag0) of the oxide/ hydroxide compound in the SERS spectrum, is not unexpected. The weakness of Rarnan spectra of metal oxide substrates has been expIoited in studies of surface species 1203. Also, Rarnan spectra of a withdrawn electrode on which a thick layer of Ag20 had been formed, and of bulk AgzO, did not show any Raman lines in the 150-900 cm-l region [21].
31 May 1985
4 Conclusion On the basis of the analysis in table 1, it appears that a silver(I) oxide/hydroxide cyanide compound is the origin of cyanide SERS from silver electrodes. The distinction between cyanide ion adsorption on silver(l) oxide/hydroxide and such a stoichoimetric compound (see table 1) is relatively subtle It depends on the evidence for the assignment of the 227 cm-l line as a lattice (external) mode rather than an internal mode. It also depends on the similarity of the low-frequency mode (218-228 cm-l) for both silver and gold electrode systems exhibiting cyanide SERS [12]. Finally, the potential-sensitive dispiacement of the v(CN) SERS line is presumed to atise from a succession of closely related oxo-cyanide compounds differing in Ag/CN‘- ratio.
Acknowledgement The authors are grateful to the Australian Research Grants Scheme for supporting this project.
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