COLLOIDS AND A

Colloids and Surfaces SURFACES ELS EV I ER A: Physicochemical and Engineering Aspects 137 ( 1998 ) 339-344

Spectroelectrochemical investigations of the interaction of ethyl xanthate with copper, silver and gold:

III. SERS of xanthate adsorbed on gold surfaces

Ronald Woods *, Gregory A. Hope, Glen M. Brown Faculty of Science and Technology, Griffith University, Nathan Campus, Queensland4111, Australia

Received 4 September 1997; accepted 19 December 1997

Abstract

Raman spectroscopy has been applied to investigate the interaction of ethyl xanthate with gold surfaces. SERS spectra showed that xanthate was adsorbed on the gold surface at potentials below the reversible values for diethyl dixanthogen and gold ethyl xanthate formation. As found previously for copper and silver, the SERS spectra confirmed that ethyl xanthate retains its molecular integrity in the layer deposited at underpotentials. The stretching vibrations of the hydrocarbon groups in the xanthate molecule chemisorbed on gold were suppressed when SERS spectra were recorded in situ, analogous to the behaviour at silver and copper surfaces. Differences between the Raman spectra for diethyl dixanthogen and gold ethyl xanthate were utilised to record SERS spectra from the chemisorbed layer in the presence of a significant coverage of dixanthogen. © 1998 Elsevier Science B.V.

Keywords: Chemisorption; Surface enhanced Raman spectroscopy; Ethyl xanthate; Gold; Diethyl dixanthogen; Flotation

1. Introduction xanthate coverage could then occur at potentials prior to dixanthogen formation. The oxidation of

The anodic oxidation of ethyl xanthate on gold ethyl xanthate on gold does not exhibit a prewave differs from that on silver and copper in that the owing to chemisorption as with silver and copper dithiolate forms before the metal xanthate. This [4], because chemisorption occurs in the same results from the E ° for the ethyl xanthate-ethyl potential region as dixanthogen formation. Indeed, dixanthogen couple ( -0 .057 V [1]) being more the kinetics of oxidation of xanthate to dixantho- negative than the corresponding value for the gen indicate [5] that the process proceeds through formation of gold xanthate ( -0 .035 V, calculated a chemisorbed xanthate intermediate. from the solubility of gold (I) xanthate reported FTIR spectroscopy provides a means of distin- by Kakovsky [2]). This does not exclude the guishingmetalxanthatesfromdixanthogen. Metal possibility that xanthate chemisorbs on gold. xanthates display an absorption at approximately Chemisorption is expected to occur at underpoten- 1200 cm-1, whereas the equivalent feature for dix- tials on this metal as it does on silver and copper anthogen is approximately 50 cm-1 greater. (see Part II of this series [3]) and a significant Woods et al. [6] determined the ethyl xanthate

coverage on gold from the intensity of the * Corresponding author. 1204 cm- 1 absorbance and found xanthate adsorp-

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340 R. Woods et al. / Colloids Surfaces A." Physicochem. Eng. Aspects 137 (1998) 339-344

tion to commence at about 0.15V below the Mielczarski [7] has disputed the explanation potential at which diethyl dixanthogen deposits given by Ihs et al. [9] for the FTIR spectrum for and to reach a fractional surface coverage of about xanthate on gold, contending that there is no 0.2 at the reversible value of the ethyl xanthate-di- possibility that another band could be produced ethyl dixanthogen couple. The coverage was found as the result of a specific molecular orientation as to increase in parallel with the anodic current those authors had proposed. In view of the contro- arising from dixanthogen deposition and reach a versy surrounding the interpretation of the FTIR monolayer at approximately 0.4 V above the spectra, we have applied SERS to investigate the reversible xanthate-dixanthogen potential, interaction of ethyl xanthate with gold surfaces,

As pointed out in Part II of this series [3], the since Raman spectroscopy provides a technique identification of the FTIR spectrum observed on complementary to FTIR in obtaining information gold at potentials below where dixanthogen forms on the nature of adsorbed species. In Part III of as chemisorbed xanthate, has been challenged [7]. this series the results of this study are presented The FTIR spectrum in this potential region has a and discussed. dominant absorption at 1204 cm -1 and the other bands expected for xanthate have low intensity. Leppinen et al. [8], in a study of xanthate adsorp- 2. Experimental tion on silver, gold and silver-gold alloys consid- ered that this spectrum was due to an unknown The equipment used in these investigations, and surface species which was most likely a decomposi- the experimental procedure, were as presented in tionproductofxanthate. Ihset al. [9] subsequently Part II of this series [3]. Preparation of ethyl carried out an FTIR and X-ray photoelectron xanthate compounds was as described in Part spectroscopy (XPS) study of the adsorption of I [13]. ethyl xanthate on gold surfaces and concluded that A gold electrode of 99.99% purity was electro- the "unknown" species is, in fact, xanthate itself, chemically roughened by oxidation-reduction They interpreted the difference in relative inten- cycling (ORC); 4-5 cycles were applied between sities of the major FTIR absorptions in terms of 0.1 V and 0.9 V in 1 M KC1 acidified with HC1 orientation of the adsorbed xanthate on the gold with a polarisation period of approximately surface and pointed out that similar intensity 20 s before reversing the polarity. SERS spectra behaviour was observed [10] for the adsorption of were recorded in situ with the gold electrode p-methylbenzyl xanthate and p-(trifluoromethyl)- under potential control in a solution of benzyl xanthate on gold. Ihs et al. [9] also investi- 5 × 10 -4 mol dm -3 potassium ethyl xanthate in gated the adsorption of octyl xanthate on gold 0.05 mol dm -3 sodium tetraborate that was and found the ratio of the FTIR absorbances in de-oxygenated with high purity nitrogen. Spectra the 1000-1300cm -x region similar to those for were also recorded on emersed electrodes. This ethyl xanthate. With the long-chain xanthate, they involved removal of the solution from the cell were able to obtain additional information from under a constant flow of nitrogen to avoid ingress the high frequency bands just below 3000 cm -1 of oxygen from the atmosphere. Potentials are that correspond to C H 2 and CH 3 stretching modes, presented on the SHE scale. The characteristics of these spectra substantiated the identification of the spectrum with adsorbed xanthate and provided evidence regarding the ori- 3. Results and discussion entation and conformation of the adsorbed mole- cule. The FTIR and XPS data were also interpreted An in situ SERS spectrum from a gold electrode in terms of the coordination of both sulfur atoms polarised for 2 min at 0.10 V in 0.5 mol in the adsorbate to the metal surface as previously cm- a sodium tetraborate solution containing proposed by Mielczarski and Suoninen [11] and 5 × 10 -4 mol dm -3 potassium ethyl xanthate is Mielczarski and Yoon [12]. shown in Fig. l(b). A spectrum recorded after

R_ Woods et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 339-344 341

surface under conditions analogous to those in Fig. 1.

As found previously [3] for silver and copper, the hydrocarbon stretching vibrations give rise to strong bands for an emersed electrode, but are suppressed when the SERS spectrum is recorded in situ. However, the CH 3 antisymmetric deforma-

"~ tion and the CH3 and CHz rocking vibrations are similar for the two environments. The reason for the suppression of the hydrocarbon stretching vibrations in the aqueous environment was dis- cussed in Part II of this series [3]. The probable explanations is absorption of the Raman scattering intensity by the overlaying water in the wave- number regions of C-H stretches. The hydro-

~ ~ ~ ~ ~ g °° g °° ~ ~ r ~ g °° ~ § ~ ~ ~ ° ° °° °° ~ § ~g ~g phobic effect in which the organic species occupies era" a cavity surrounded by highly structured water

molecules [15] may also contribute. Fig. 1. SERS spectra of gold electrode at 0.10V in 0.5 mol cm 1 sodium tetraborate solution containing The spectra in Figs. 1 (b) and (c) exhibit bands 5 x 10 -4 mol dm 3 potassium ethyl xanthate recorded (b) in at 597 cm-1 and 491 cm-1 (Table 1) that are not situ and (c) after emersion. Curve (a) is FT-Raman spectrum present in the spectrum for gold ethyl xanthate. for solid gold xanthate (AuEX). Analogous bands were observed in SERS spectra

for ethyl xanthate chemisorbed on silver and copper [3] and can be confidently assigned to the

subsequently immersing the electrode is shown in presence of the gauche conformers of the ethyl Fig. 1 (c). An FT-Raman spectrum for gold ethyl xanthate molecule. The bulk compound is present xanthate [13] (curve a) is included for comparison in only the trans configuration as a result of the of the band positions. Assignment of the different organisation of the gold ethyl molecules in the bands, based on the work of Colthup and Powell crystal structure. The wavenumbers at which the [14], is presented in Table 1. The additional sharp other SERS bands occur are close to those for features in the spectra that are not included in the the FT-Raman spectrum of bulk gold ethyl xan- assignment are plasma spikes caused by the laser, thate (Table 1), except for the COC and OCC

It can be seen from Fig. 1 and Table 1 that a deformation vibrations which are 15 and SERS spectrum was obtained from the gold 10cm -1 lower, respectively. It is interesting to electrode treated under these conditions and that note that the bands from these vibrations for gold the band energies are similar to those of the bulk ethyl xanthate are approximately 20 cm- 1 greater compound. The applied potential was below the than those from the corresponding vibrations for reversible potentials of both the gold-gold ethyl the silver and copper compounds [3]. Thus, the xanthate and ethyl xanthate-diethyl dixanthogen shifts for the chemisorbed layer on gold make the couples for this solution, 0.160 V and 0.138 V, band positions closer to those of the analogous respectively [1], and hence must correspond to layer on silver and copper. underpotential deposition. No SERS spectrum was Notwithstanding these small shifts in band posi- observed when the gold electrode was polarised at tions, the SERS spectra clearly confirm that ethyl -0 .1 V in a solution of the same composition, xanthate retains its molecular integrity when it is These findings are in agreement with FTIR studies chemisorbed on gold at underpotentials. Bands of the gold ethyl xanthate system [6]. In that are observed corresponding to vibrations of each work, it was found that a surface xanthate frac- part of the xanthate molecule. tional coverage of 0.2 was present on the gold Raman spectra were also recorded for a gold

342 R. Woods et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 339-344

Table 1 SERS vibrations recorded for gold surfaces in the presence of ethyl xanthate compared with those for FT-Raman spectra of gold ethyl xanthate

Vibration Wavenumber (cm- 1)

AuEX (s) 100 mV 500 mV

in situ emersed in situ emersed emersed (2 h)

CH3 antisymmetric stretch 2987 2979 2979 - - 2985 2985 CH2 symmetric stretch 2941 2933 2933 2935 2935 2935 CH 3 symmetric stretch 2880 - - 2874 - - 2865 2865 CH a antisymmetric deformation 1453 1443 1443 1445 1445 1445 COCS2 antisymmetric stretch 1186 1200 1200 - - 1200 1200 CH 3 rock; CH 2 rock 1112 1109 1109 1110 1111 1111 CS2 antisymmetric stretch 1021 1000 1000 997 1000 1000 CCOC stretch 858 856 856 853 860 860 CS 2 symmetric stretch trans 644 641 641 641 641 643 CS2 symmetric stretch gauche - - 597 597 598 598 598 OCS2 out of plane wag 544 540 537 - - 543 539 SS stretch n.a. - - - - 499 497 - - COC deformation gauche - - 491 491 - - - - 491 COC deformation trans 467 452 452 450 455 455 OCC deformation 430 420 420 415 427 411

elect rode po la r i sed at 0 . 5 V in 0 . 5 m o l c m -1 sod ium te t r abora t e solut ion conta in ing 5 x 10-4 mol d m - 3 po tas s ium ethyl xanthate ;

record ing o f the spec t rum commenced af ter 2 min at the set potent ia l . These condi t ions co r re spond to an overpo ten t ia l for d ixan thogen fo rma t ion o f 0.362 V and a mul t i layer o f this species would have been depos i ted on the gold surface pr io r to and dur ing recording o f the spect ra [5, 6]. Previous F T I R studies [6] have shown tha t a b o u t a m o n o - layer o f chemisorbed xan tha te is also fo rmed on ~ ( the e lect rode under the prevai l ing condi t ions . The SERS spec t rum recorded in situ is shown in Fig. 2(c); the F T - R a m a n spect ra for d ixan thogen and gold ethyl xan tha te [ 13] are included in the figure as curves (a) and (b ) , respectively, for compar i son . SERS spect ra for an emersed e lec t rode recorded immedia te ly af ter r emova l o f ~ ~ ~, ~ ~ ~ ~ ~, ~ ~ ~ ~ ~ § ~ § ~ ~

Clltl "1

the solut ion, and af ter a pe r iod o f 2 h under a cons tan t flow o f d ry ni t rogen, are shown as Fig. 2. SERS spectra of gold electrode at 0.50V in Fig. 2 (d ) and (e), respectively. 0.5 mol cm-1 sodium tetraborate solution containing

5 x 10 -4 mol dm -3 potassium ethyl xanthate recorded (c) in The spect ra in Fig. 2 exhibi t the same suppres- situ, (d) immediately after emersion, (e) 2 h after emersion with

sion o f the h y d r o c a r b o n s t re tching v ibra t ions dry nitrogen flowing over electrode surface. Curves (a) and (b) (F ig . 1) as was observed for the unde rpo ten t i a l are FT-Raman spectra of liquid diethyl dixanthogen (EX2) and xan tha te layers on silver and copper surfaces [3]. solid gold xanthate (AuEX), respectively.

R. Woods et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 339-344 343

It can also be seen that curves (c) and (d) display in the wavenumber regions of C - H stretches, and a band at 498 cm-1 in addition to those character- the hydrophobic effect in which the hydrocarbon- istic of adsorbed xanthate on gold. The band arises xanthate end of the adsorbed xanthate molecule from the S-S stretching vibration of dixanthogen occupies a cavity surrounded by highly structured and demonstrates the presence of this compound water molecules. on the gold surface. Dixanthogen is volatile at In the potential region where diethyl dixantho- ambient temperature and evaporates under a gen formation occurs on the gold electrode, the stream of nitrogen [6]. Hence, the feature at chemisorbed layer was distinguished from the bulk 498 cm -1 is not present in Fig. 2(e). compound from SERS spectra. The intensity of

Under the conditions of Fig. 2(c) and (d), there SERS bands from the chemisorbed layer was sim- must be very much more dixanthogen on the gold ilar to that from a multilayer of dixanthogen. This surface than chemisorbed xanthate, but the inten- is considered to be due to the different surface sity of the 498 c m- 1 band from the former species enhancements of the two species resulting from is no greater than that from the CS2 stretching different attachments to the surface. The chemi- vibrations of the chemisorbed xanthate. This is sorbed monolayer is covalently bonded to gold considered to reflect the different nature of attach- atoms while dixanthogen is sited further away ment of the two species to the gold surface, from the surface and is retained at the interface Chemisorbed xanthate is covalently bonded to through hydrophobic interactions between hydro- gold sites in the metal surface and hence is condu- carbon chains. cive to a significant enhancement of Raman scat- tering and will give a strong SERS signal. Dixanthogen, however, will be sited further away Acknowledgment from the gold surface and be bound to the under- lying chemisorbed xanthate and to adjacent dixan- The authors are grateful to Dr P.M. Fredericks thogen molecules only by hydrophobic interactions of the Centre for Instrumental Development between the hydrocarbon parts of the molecule. Chemistry for access to Raman instrumentation. This could explain the fact that there is much lower, if any, surface enhancement.

References

4. Concluding remarks [1] R. Woods, in: P. Somasundaran, B.J. Moudgil (Eds.), Reagents in Mineral Processing, Surfactant Science Series,

SERS spectroscopy has confirmed previous vol. 27, Marcel Dekker, New York, NY, 1988, pp. 39-78. FTIR findings that ethyl xanthate is chemisorbed [2] I.A. Kakovsky, in: J.H. Schulman (Ed.) Proceedings of the on gold surfaces at potentials below the reversible 2nd International Congress on Surface Activity,

Butterworths, London, vol. 4, 1957, pp. 225-237. xanthate-dixanthogen and metal-metal ethyl xan- [3] R. Woods, G.A. Hope, G.M. Brown, Spectro- thate values and established that it retains its electrochemical investigations of the interaction of ethyl molecular integrity in the adsorbed layer, xanthate with copper, silver and gold after II SERS of

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pressed when the SERS spectra are recorded in [41 R. Woods, in: J.O'M. Bockris, B.E. Conway, R.E. White (Eds.), Modern Aspects of Electrochemistry No. 29,

situ but the associated rocking and deformation Plenum, New York, 1996, pp. 401-453. vibrations are unaffected. The stretching vibrations [5] R. Woods, J. Phys. Chem. 75 (1971) 354. are strong in the spectra for emersed electrodes. [6] R. Woods, D.S. Kim, C.I. Basilio, R.-H. Yoon, Colloids Possible explanations for the suppression of the Surfaces A 94 (1995) 67-74. hydrocarbon stretching vibrations when spectra [7] J.A. Mielczarski, Langmuir 13 (1997) 878-880.

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344 R. Woods et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 339-344

[10] N.-O. Persson, K. Uvdal, B. Liedberg, M. Hellsten, Prog. and gold after I FT-Raman and NMR spectra of the xan- Colloid Polym. Sci. 88 (1992) 100. thate compounds, Colloids Surfaces A.

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[12] J.A. Mielczarski, R.-H. Yoon, J. Phys. Chem. 93 (1989) [15] C. Tanford, The Hydrophobic Effect: Formation of 2034. Micelles and Biological Membranes, Wiley-Interscience,

[13] R. Woods, G.A. Hope, Spectroelectrochemical investiga- New York, 1980. tions of the interaction of ethyl xanthate with copper, silver