1
Formation and characterization of aromatic selenol and thiol monolayers on gold: in-situ IR studies and electrochemical measurements

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

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Adsorption processes and electrochemical properties of benzeneselenol and benzenethiol on gold electrodes were investigated by surface-enhanced IR absorption spectroscopy (SEIRAS) and electrochemical measurements.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

3
From the IR spectroscopic measurements, it was confirmed that benzeneselenol molecules adsorbed on the gold surface as benzeneselenolate–Au.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

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SEIRAS measurements revealed that benzeneselenol took a much longer time to adsorb on a gold surface than benzenethiol.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

5
Electrochemical reductive desorption of these monolayers in alkaline solution proved that benzeneselenoate chemisorbed on gold surfaces were more stable compared to benzenethiolate, since the reduction peak potential of benzeneselenolate shifted to more than 200 mV negative potential than that of benzenethiolate.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

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The total amounts of adsorbed benzeneselenol molecules were smaller than the case of benzenethiol.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

Introduction

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It has been well known that organosulfur compounds can chemisorb to metal surfaces and form ordered monolayers.1,2

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

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Modification of gold surfaces by self-assembled monolayers of organosulfur compounds such as alkanethiols and dialkyl disulfides have been widely studied and applied for improvement of surface properties and constructing functional films on metals.2

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

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Adsorbed thiolates on gold are stable enough to form nanometer scale fabrications by selective electrochemical desorption.3–6

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

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To construct desirable functional monolayers on the nano-order scale, not only selection of functional (tail) groups but also investigation of head (anchor) groups will be important.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

11
Recently, monolayer formation using chemical bonds between metals and heavy chalcogens have been reported.7–14

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

12
Adsorptivity and stability of organoselenol and diselenide on gold and silver surfaces were studied by surface-enhanced Raman spectroscopy (SERS), X-ray photoelectron spectroscopy (XPS), and other methods,7–14 however, the most important fundamental features of organoselenoate monolayers such as adsorption kinetics, number of adsorbates on the surface, and detailed electrochemical properties, have not been investigated sufficiently yet.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

13
In this study, we chose benzeneselenol as a model compound as an alternative of thiols to construct stable monolayers on gold.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met2

14
We investigated adsorption properties, adsorbed molecular orientation, stabilities, and other features of benzeneselenol on gold compared with benzenethiol by in situ surface-enhanced IR absorption spectroscopy (SEIRAS) and electrochemical measurements.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

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SEIRAS is one of the powerful tools to evaluate monolayer structures and orientations on the metal surface because its sensitivity is higher than the ordinary IR reflection–absorption spectroscopy method and shows negligible interference from the bulk solution.15,16

Type: Method | Advantage: Yes | Novelty: New | ConceptID: Met2

16
From the SEIRAS measurement, it became clear that benzeneselenol took a much longer time to adsorb on a gold surface than benzenethiol.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

17
Reductive desorptive measurements of monolayers in alkaline solution3,4,6,17–19 disclosed the difference of affinities between thiolate/selenolate species with gold electrodes and the total amounts of benzenethiolate/benzeneselenolate adsorbed on gold.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

Experimental

18
Benzeneselenol (Fluka, 97%) and benzenethiol (Sigma-Aldrich, 99%) were used without further purification.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

19
The reagents for the electrolyte solution were extra-pure grade.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

20
Millipore water (Milli-Q SP, Millipore Co.) was used to prepare electrolyte solutions.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

21
A gold thin film, which was evaporated on a hemi-cylindrical silicon prism, was used as the substrate for IR measurements.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

22
The thickness of the gold film was 20 nm.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

23
Spectra were measured by using an FTIR spectrometer (DIGILAB FTS-6000e) with a liquid-nitrogen cooled MCT detector and was operated at a resolution of 4 cm−1.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

24
The acquisition time of each spectrum was ca. 10 s.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

25
The incident angle of the IR beam was 80° with respect to the surface normal.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

26
Further details of experimental procedures are referred to in the literature.15

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

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An Au(111) plane mechanically polished with successively finer grades of alumina pastes was used for electrochemical measurements.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

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The Au(111) electrode was annealed in a hydrogen–oxygen flame and quenched quickly into ultra-pure water saturated with hydrogen.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

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Electrochemical measurements were conducted in a three-electrode configuration.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

30
Ag/AgCl (3 M NaCl) and Pt wire were used as the reference and auxiliary electrodes, respectively.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

31
A potentiostat–galvanostat (Hokuto Denko: HA-502) was used to control the potential of the working electrode.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

32
A function generator (Hokuto Denko: HA-105) was used to provide an external potential.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

33
Cyclic voltammograms (CVs) of the gold electrode for the reductive desorption of the adsorbed monolayers were recorded in 0.5 M KOH solution.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

34
Electrochemical measurements were carried out at room temperature after the solution was deaerated by flowing Ar gas.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

Results and discussion

Adsorption kinetics of benzeneselenol and benzenethiol

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Fig. 1 shows a series of IR spectra of the gold surface measured in pure water after injecting benzeneselenol solution (total concentration 0.1 mM).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

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The reference was the spectrum of a gold electrode measured in pure water without benzeneselenol.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met3

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When the gold surface is modified by organosulfur analogues, organic solvents such as ethanol and methanol are commonly used.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met4

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We adopted an aqueous solution for the adsorption experiments to avoid multilayer depositions of aromatic sulfur/selenide compounds.20

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met5

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IR spectra changed gradually after adding benzeneselenol.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

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Three positive going bands appeared at 1572, 1470 and 1439 cm−1 in the spectral region between 1300 and 1700 cm−1.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

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These bands are assigned to phenyl ring modes of benzeneselenol adsorbed on gold.11,21

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

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This result indicates that benzeneselenol molecule was gradually adsorbed on the gold electrode surface and the phenyl ring part was oriented to near the surface normal.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

43
The adsorption of benzeneselenol was also supported by the rise of the baseline level of the spectrum.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

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Molecule–substrate and molecule–molecule interactions were constant during the adsorption process because no peak shifts were observed in this adsorption period.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

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Fig. 2 shows the adsorption behavior of benzenethiol molecules on gold by IR measurement.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

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At the beginning of adsorption, the spectrum around 1650 cm−1, which was assigned as water molecules on the electrode surface22,23 and a small shoulder around 1450 cm−1 were observed, however, adsorption of benzenethiol on gold was complete within a few minutes.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

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Finally, three positive going bands appeared at 1574, 1470 and 1439 cm−1.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

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These bands did not shift in this adsorption process.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

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These were assigned to phenyl ring modes of benzenethiolate on gold.11,21,24

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

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The rise of the baseline level of the spectrum was also evidenced by the adsorption of benzenethiol.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

51
Benzenethiol molecules adsorbed on the gold surface with the phenyl ring part oriented near perpendicular to the surface, similarly as benzeneselenol.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

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Fig. 3(a) shows the relation between the adsorption time and the intensity of the 1470 cm−1 band of benzeneselenol on gold.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

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To compare the adsorption properties of benzenselenol with benzenethiol, the band intensities of the 1470 cm−1 of benzenethiol on gold are also shown (Fig. 3(b)).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

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In the case of benzenethiolate, the band intensity saturated within a few minutes and then increased gradually.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

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This result was quite similar to that in previous literature.24

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

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In contrast, the band intensity of benzeneselenoate increased gradually and approached a saturated value after around 40 min.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs10

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Benzeneselenol molecules needed a much longer time to reach the saturated adsorption value than that of benzenethiol.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

Adsorption structure of benzeneselenol molecules on gold

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Fig. 4(a) shows the spectrum extracted from Fig. 1, which was collected 25 min after benzeneselenol injection.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs11

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For comparison, the transmission spectrum of benzeneselenol (KBr) is also shown in Fig. 4(b).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs12

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In the spectrum of the modified electrode (Fig. 4(a)), the 2305 cm−1 band, corresponding to the Se–H stretching mode,25 disappeared completely.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs13

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Frequencies of phenyl-ring modes changed slightly after benzeneselenol adsorption.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs14

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A few bands, such as at 3070 and 1580 cm−1 in the bulk spectrum, disappeared and the 1477 cm−1 band shifted to lower frequency.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs15

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Adsorption of diphenyl diselenide on to a gold surface was reported by Huang et al.11

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

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In their SERS study, the ν(Se–Se) vibration was absent after adsorption and the Se–Se bond cleaved on the gold surface.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

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On the other hand, Bandyopadhyay et al. reported that the Se–Se bond was retained even after formation of a monolayer in their Raman study.13

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

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In our investigation, benzeneselenol was attached on the gold surface as benzeneselenoate–Au since the Se–H stretching mode disappeared completely.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

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This is similar to the adsorption of thiolates on gold.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

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In SEIRAS measurement, bands which have perpendicular dipole moments to the gold surface are observed strongly.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

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The twist angle between the benzeneselenol molecular plane and the surface plane was quite small on the grounds that band intensities depend upon tilt angle (bands of 3070, 1580 and 1439 cm−1 which were assigned as symmetries of the phenyl ring vibrations in refs. 24 and 26) became relatively weak after benzeneselenol molecular adsorption.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met7

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It was suggested that the phenyl ring of benzeneselenol was oriented to near the upright direction on the gold surface as bands originating from the phenyl ring were observed clearly.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met7

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In the case of benzenethiol, it has been reported that the phenyl ring plane was tilted about 30° from the surface normal.24

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met7

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The tilt angle of the benzeneselenol molecule from the surface normal should be small, similarly to benzenethiol, however, it can not be determined accurately because IR measurements were insufficient to analyze much lower frequency regions in which molecular orientations are strongly reflected.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

Electrochemical reductive desorption of benzeneselenoate and benzenethiolate monolayers

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Fig. 5 shows cyclic voltammograms (CVs) for reductive desorption of benzeneselenolate and benzenethiolate monolayers on Au(111) in 0.5 M KOH.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs16

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Porter and co-workers suggested that alkanethiolates on gold and other metal electrodes were desorbed in alkaline solution through a one-electron reductive path (RS–Au + e− → RS− + Au).17,18

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met8

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The electrochemical desorption of monolayers from gold and other metal surfaces has been widely used for the characterization of thiolate monolayers.3,4,6,17–19,27–31

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

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In the case of selenolate layers, a similar result was also considered for reductive desorption (RSe–Au + e− → RSe− + Au).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met8

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Benzeneselenolate and benzenethiolate monolayers showed clear desorption peaks at −0.82 and −0.61 V vs.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

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Ag/AgCl, respectively.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

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The charge and concentration of benzeneselenolates were 37.6 μC cm−2 and 3.9 × 10−10 mol cm−2.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

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In the case of benzenethiolates, the charge and concentration were 51.2 μC cm−2 and 5.3 × 10−10 mol cm−2.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

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The result for benzenethiolate in this study is very close to previous results reported by Zhong et al.(52 μC cm−2)31 and by Wan et al. (51 μC cm−2) for the same molecule.24

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

82
Bandyopadhyay et al. showed the surface coverage of diphenyl diselenide on polycrystalline gold was 99% by impedance, electrochemical, and XPS measurements, however, total numbers of adsorbed molecules were not mentioned.13

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met9

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It was reported that thiolate chemisorbed more strongly than selenolate, on the basis of the bond strengths (Au–Se < Au–S).10,32,33

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met10

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It was also observed that the adsorption of selenolates on gold or silver was more favorable than that of thiolates from SERS results.11,12

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met11

85
In our experimental results, the reductive desorption peak of benzeneselenolate was 0.21 V more negative than that of benzenethiolate.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs18

86
Differences in desorption potentials reflect differences in the binding strength of the compounds.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

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There are two possibilities for this: differences in the mode of binding of selenol/thiol to gold or differences in packing densities of adsorbed molecules.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

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The latter case reflects small contributions to differences of binding strength.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

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If thiolate/selenoate which have long alkyl chains are used for modification, lateral interactions between alkyl chains became larger, so that the packing density will be important when considering the difference of binding strength.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

90
The reductive desorptive potential of dodecaneselenol on gold was −1.2 V, which was quite similar to that of dodecanethiol.34

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

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It is feasible that the difference of reductive desorption potential is caused by a difference in binding of (benzene)thiolate and (benzene)selenoate to gold.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

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Our experimental result demonstrated clearly that the benzeneselenoate monolayer was more stable than benzenethiolate monolayer on gold.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

Conclusion

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We investigated the formation process and electrochemical properties of benzeneselenolate and benzenethiolate monolayers on gold by SEIRAS and electrochemical measurements.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

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SEIRAS measurements revealed that benzeneselenol took a much longer time to adsorb on the gold surface than benzenethiol.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

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Benzeneselenol molecules adsorbed on the gold surface as benzeneselenolate–Au.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res20

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Electrochemical reductive desorption of these monolayers proved that benzeneselenoate chemisorbed on a gold surface more strongly compared with benzenethiolate.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

97
Total amounts of adsorbed benzeneselenol molecules were smaller than in the case of benzenethiols.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

98
This phenomenon will be applicable to construct nano-scale domains.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

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STM studies on the morphologies of both monolayers are under way.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con5