Solution Effect on Diazonium-Modified Au(111): Reactions and

Electrografting via Diazonium Chemistry: From Multilayer to Monolayer Using Radical Scavenger. Thibaud Menanteau , Eric Levillain , and Tony Breton...
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Solution Effect on Diazonium-Modified Au(111): Reactions and Structures Bo Cui,†,‡ Jing-Ying Gu,†,‡ Ting Chen,† Hui-Juan Yan,† Dong Wang,*,† and Li-Jun Wan*,† †

CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ABSTRACT: Surface modifications of a Au(111) electrode with 4bromobenzenediazonium tetrafluoroborate (BBD) in acetonitrile (ACN) and 0.1 M HClO4 have been characterized by scanning tunneling microscopy (STM). In ACN, STM results reveal the formation of disordered thin organic films. The involvement of the radical as an intermediate is evidenced by the negative effect of radical scavengers on organic thin film formation. In contrast, the 4,4′-dibromobiphenyl monolayer is observed when the aqueous solution is used as a medium to carry out the grafting experiment. The biphenyl compound is considered to be generated by a radical−radical coupling reaction.

structures of aryl films on epitaxial graphene on SiC(0001) and highly oriented pyrolytic graphite (HOPG) have been characterized by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy.19,20 The adlayer was observed to be composed of aryl oligomers or diazonium derivatives, and multilayers could be effectively suppressed by long alkyl chains. The diazonium-based chemistry has been extended to the synthesis and functionalization of metal nanoparticles, and the formation of the gold−carbon bond has been evidenced by surface-enhanced Raman spectroscopy and theoretical simulations.17,21 Although it is well established that the spontaneous reduction of diazonium salts on metal surfaces represents a facile way to tailor the surface chemistry of metal surfaces, there exists some discrepancy in terms of the reaction conditions and the mechanisms. Vautrin-Ul et al. employed ACN as a solvent for diazonium salts to modify metal surfaces and proposed the aryl radical as an intermediate.15 Mesnage et al. investigated the diazonium-based surface chemistry on gold surface using an aqueous solution and found no evidence of a radical mechanism or the diazonium cation or aryl cation as an intermediate.22 Downard et al. investigated the diazonium chemistry on glassy carbon substrates in both aqueous acid and acetonitrile solutions and proposed the involvement of the rapid and the slow processes in the spontaneous grafting reactions.23 Herein, we compare the 4-bromobenzenediazonium tetrafluoroborate (BBD)-derived thin films on Au(111) in ACN and an aqueous acidic solution by employing high-resolution STM.

1. INTRODUCTION The covalent modification of surfaces with organic films, which is driven by various applications via tailoring surface properties such as sensors, molecular electronics, corrosion protection, and so on, has attracted much attention.1−6 The formation of self-assembled monolayers (SAMs) of thiol on gold surfaces via Au−S bonding is one of the most powerful approaches to modifying gold surfaces.7−9 However, the instability of the Au− S interaction under certain conditions, such as UV photooxidation, limits its application in electronics, sensors, and data storage.10 In recent years, a diazonium salt-based surface-modification technique has attracted a great deal of attention. In a typical process, the reduction of diazonium salts, which can be formed electrochemically or by electron donation from surfaces, results in the formation of highly reactive aryl radicals that can bond to virtually any surface and results in the formation of organic thin films. Generally, the metal−carbon bond formed by diazoniumbased radical grafting is stable relative to the metal−sulfur bond produced by self-assembly chemistry.11,12 The spontaneous reduction of diazonium salts on metal and semiconductor surfaces has been demonstrated on gold, copper, nickel, iron, zinc, and silicon and has resulted in organic thin films that have been investigated by a variety of surface-characterization techniques.13−17 Vautrin-UI and co-workers have studied the spontaneous grafting of the nitrophenyl group in ACN on glassy carbon, copper, nickel, iron, and zinc substrates by cyclic voltammetry, X-ray photoelectron spectroscopy, Fourier transform infrared reflection adsorption spectroscopy, and atomic force microscopy.18 All of these methods demonstrate that the thickness and morphology of organic layers depend on the reaction time and the diazonium salt concentration. The © 2013 American Chemical Society

Received: January 15, 2013 Revised: February 11, 2013 Published: February 15, 2013 2955

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Figure 1. STM images of a Au(111) surface modified with BBD for different times in ACN before and after sonication recorded in 0.1 M HClO4 at E = 550 mV. No obvious structure change is observed in the double-layer potential range (0−900 mV vs RHE). (a) 2 min before sonication with tunneling conditions of Vbias = −356 mV and It = 966.3 pA. (b) 2 min after sonication with tunneling conditions of Vbias = −170 mV and It = 2 nA. (c) 10 min before sonication with tunneling conditions of Vbias = −208 mV and It = 1.031 nA. (d) 10 min after sonication with tunneling conditions of Vbias = −223 mV and It = 994 pA. (e) 1.2 h before sonication with tunneling conditions of Vbias = −463 mV and It = 662.5 pA. (f) 1.2 h after sonication with tunneling conditions of Vbias = −281 mV and It = 1.033 nA. (g) Cross-sectional profile along the S−S′ line in panel e. (h) Crosssectional profile along the S−S′ line in panel f.

2. EXPERIMENTAL SECTION

dipping the Au(111) surface into the diazonium salt solution degassed with N2 for 1 min or longer. The results established that the dipping time and light have no detectable effect on the adlayer structure under aqueous grafting conditions. 2.3. Scanning Tunneling Microscopy (STM). STM imaging was carried out in 0.1 M HClO4 by using a Nanoscope E microscope (Bruker Inc.) with a tungsten tip etched in 0.6 M KOH. The tungsten tips were further sealed with transparent nail polish to minimize residual faradic currents. All STM images shown here were collected in constant-current mode. The imaging potentials were quoted at 550 mV versus the reversible hydrogen electrode (RHE).

2.1. Chemicals and Substrate. Acetonitrile (ACN, anhydrous, Alfa Aesar), 4-bromobenzenediazonium tetrafluoroborate (BBD, Acros Organics, 96%), flavone (Acros Organics, 99%), and 2,6-ditert-butyl-4methylphenol (BHT, J&K Chemical, 99.8%) were all used as received. An electrolyte solution was prepared with ultrapure HClO4 (Kanto Chemical Co.) and Milli-Q water (18.2 MΩ·cm, TOC ≤ 4 ppb). A Au(111) single-crystal electrode was prepared by melting an Au wire (99.999%) in a hydrogen−oxygen flame.24 2.2. Surface Modification. Surface modifications in ACN were carried out by submersing the cleaned Au(111) electrode into a 1.35 mM diazonium salt solution for 2 min, 10 min, or 1.2 h in the dark under an inert atmosphere at room temperature. In the control experiment, the Au(111) surface was immersed in the mixed solution of BHT (19 mM) or flavone (8.8 mM) and BBD (1 to 2 mM) for 2 to 3 h. The radical scavengers were present in stoichiometric excess compared to the amount of BBD in the mixed solutions to make sure that radical scavengers could react sufficiently with the diazonium salt.25 After modification, the substrates were thoroughly rinsed with pure ACN and water before introduction into an STM cell. To investigate the influence of sonication, the same electrode modified in ACN was studied by STM before and after sonication in pure ACN for 30 min. Surface modification in aqueous media was performed by

3. RESULTS AND DISCUSSION 3.1. Surface Modification with BBD in ACN. Figure 1 shows a series of STM images of BBD-modified Au(111) facets with different immersion times. After being in contact with the BBD solution for 2 min, a disordered adlayer forms on the surface as shown in Figure 1a. A large proportion of the adlayer can be removed after the electrode is ultrasonicated in ACN for 30 min (Figure 1b), which illustrates that the adsorbates are mainly weakly physisorbed species at short immersion times because a strongly attached layer should be able to resist sustained ultrasonic rinsing.11,26 The quantity of adsorbates increases with a prolonged reaction time of up to 10 min, and the thickness of the thin film is not uniform (Figure 1c). There are some bright aggregates about 1 nm in size dispersed on the surface. These clusters may be ascribed to multilayer formation or the aggregation of dimers, oligomers, or other compounds formed by the attack of aryl radicals.16 The surface morphology is not significantly different after ultrasonic treatment (Figure

When ACN is used as the solvent, the formation of disordered thin organic films is confirmed by STM. The effect of the reaction time on the composition of the adlayer structure is evaluated. The involvement of the radical as an intermediate is evidenced by the negative effect of radical scavengers on organic thin film formation. In contrast, high-resolution STM reveals the formation of the biphenyl compound when using an aqueous acidic solution to modify the gold surfaces.

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composed of physisorbed and chemisorbed species. To a certain extent, the result demonstrates that BHT could not entirely prevent the reaction from occurring. A similar conclusion has been drawn from a previous study on the spontaneous grafting of diazonium on Si.25 In contrast, a Au(111) surface dipped in a BBD and flavone mixture in ACN reveals a uniformly dispersed dotlike feature with only a few clusters (Figure 2e). The dotlike structure is ca. 0.5 nm in size and 0.1 nm in height (Figure 2f). The surface character is appreciably different from that in single BBD solution. This result illustrates that flavone could effectively react with an aryl radical intermediate of BBD and hinder molecular grafting or multilayer formation. 3.3. Surface Modification with BBD Aqueous Solution. An ordered monolayer appears on the surface when the Au(111) electrode is modified with BBD in 0.1 M HClO4. Figure 3a shows the high-resolution STM image of this structure. The striped adlayer consists of bright rods and dots that arrange alternately along direction a (⟨110⟩ direction of the substrate). The length and width of each bright rod and dot agree well with the shapes and dimensions of 4,4′dibromobiphenyl (ca. 1.1 nm in length and 0.4 nm in width) in a flat-lying or perpendicular orientation, respectively. The unit cell is outlined in Figure 3a,b with a = 2.0 ± 0.1 nm, b = 1.1 ± 0.1 nm, and α = 30 ± 2°, with the b direction along the ⟨121⟩ direction of the substrate. All of the STM results illustrate that the adlayer has the same molecular arrangement as the 4,4′-dibromobiphenyl monolayer observed in 0.1 M HClO4 at E = 550 mV.32 The same self-assembled monolayer has also been observed by STM for 4,4′-dibromobiphenyl adsorption on Au(111) in acetonitrile. The possible formation mechanism of 4,4′-dibromobiphenyl is depicted in Scheme 2. It is considered to be a two-step process: aryl diazonium is reduced to aryl radicals, which generate 4,4′-dibromobiphenyl via radical−radical coupling. The dimer is believed to exist in a diazonium grafting system by many groups,11,16,26,33 but few people observe it on the surface directly. 4-Bromobenzenediazonium tetrafluoroborate forms a covalently bonded disordered adlayer in the ACN medium, but an ordered biphenyl adsorption structure appears on Au(111) in aqueous solution. The results indicate that the solvent has a great effect on the diazonium modification behavior. The possible mechanism is proposed as follows. In ACN, the gold substrate acts as an electron donor that transfers electrons to the diazonium salt. Aryl radicals are formed by a dediazoniation reaction at the surface of the electrode and further bind with substrate. Because the aryl radicals are formed close to the surface, the surface grafting process is greatly facilitated. In contrast, aryl radicals formed in aqueous solution interact with each other, and the produced compounds form a self-assembled monolayer on the surface. The difference may be ascribed to how aryl radicals are produced. In ACN, aryl radicals are produced mainly from the reduction of diazonium salt at the electrode. However, in aqueous solution, the ions in aqueous solution may induce dediazoniation and produce radicals in solution that generate biphenyl compound by a radical−radical coupling reaction.34,35 The process proceeds quickly without the contribution from the substrate. The produced dimers then adsorb on the surface and arrange into an ordered adlayer. The compact, ordered physisorbed adlayer may potentially hinder electron donation from the electrode to the diazonium salts in solution and/or minimize the extent of covalent grafting simply by steric restriction.

1d), which is in sharp contrast to the sample shown in Figure 1a,b with a dipping time of 2 min. The thin film may consist of covalently bonded aromatic groups via the reduction of diazonium salt. When the immersion time changes to 1.2 h, there are more aggregates dispersed on the surface and the adlayer thickness (ca. 0.4 nm, Figure 1g) is greater than that for 2 or 10 min (ca. 0.2 nm). Although a more compact layer could be obtained by increasing the dipping time, the holes in the layer indicate that the adsorbates could not form a uniform, compact adlayer (Figure 1e). This result is consistent with the literature.19 It is worth noting that the bottom of the holes may also be occupied by physisorbed molecules. Therefore, the measured corrugation height from the holes in the STM image may be a little shorter than the real height of the grafted adlayer. Figure 1f,h shows that the thickness of the adlayer decreases noticeably to about 0.2 nm after sonication, which indicates that the adlayer is composed of physically and chemically bonded molecules by grafting for a long time. The molecules physically adsorbed on the surface could be removed by extended sonication, whereas the molecules covalently attached by Au−C bonding are relatively stable under extreme conditions. From the STM results, when the reaction time is short, physical adsorption is the main adsorption form. If the immersion time is long enough, then spontaneous grafting can occur via the reduction of BBD by a gold electrode, presumably producing reactive aryl radicals as the intermediate that subsequently react with the surface. At the mean time, according to previous studies,17,18,22,27 a multilayer film comprising covalently bound aryl groups and dimers produced by radical−radical coupling may be formed. The grafted aryl groups may include bromobenzene and a polyaryl film that result from the growth of the layer through the attack of aryl radicals obtained from the reduction of the diazonium salt on the grafted phenyl groups or the azo coupling with the attached rings. The film structure is proposed in Scheme 1. The adsorbates on Au(111) formed by Au−C bonding are more stable than the physisorbed molecules. Scheme 1. Scheme of the BBD-Grafted Film Structure in ACN

3.2. Surface Modification with BBD and Radical Scavengers in ACN. To investigate the nature of active intermediate during the diazonium grafting process, we have carried out the control experiments to investigate the effect of free radical scavengers on the grafting process. BHT and flavone are chosen as typical free radical scavengers.28−31 The chemical structures of BHT and flavone are shown in Figure 2a,d. Figure 2b shows that the adlayer structure modified in the mixed solution of BBD and BHT is similar to that in single BBD. The molecules gather into clusters dispersed on the surface with medium coverage. The average height of aggregates is ca. 0.4 nm (Figure 2c), similar to the layer thickness of BBD in ACN after 1.2 h. The adsorbates may be 2957

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Figure 2. (a) Molecular structure of BHT. (b) STM image of BHT-and-BBD-modified Au(111) at E = 550 mV under tunneling conditions of Vbias = −571 mV and It = 1.944 nA. (c) Cross-sectional profile along the S−S′ line in panel b. (d) Molecular structure of flavone. (e) STM image of flavoneand-BBD-modified Au(111) at E = 550 mV under tunneling conditions of Vbias = −101 mV and It = 1.379 nA. (f) Cross-sectional profile along the S−S′ line in panel e.

4. CONCLUSIONS The thin film structure of diazonium grafted onto the Au(111) substrate in ACN and the effect of the reaction time have been studied by STM on the molecular level. With a prolonged reaction time, the proportion of covalent modification in the adlayer increases, although the arrangement is disordered. As a radical scavenger, flavone decrease the molecular grafting more effectively than does BHT. When the modification process is carried out in 0.1 M HClO4, 4,4′-dibromobiphenyl is produced by radical−radical coupling. The compound assembles into an ordered monolayer on Au(111). STM results provide direct evidence for the aryl-radical-based grafting mechanism in ACN. In addition, the present work hints at the importance of reaction media in the diazonium salt grafting reaction that modifies surfaces.

Figure 3. (a) STM image of 4,4′-dibromobiphenyl adsorbed on Au(111) at E = 550 mV. Tunneling conditions: Vbias = −133 mV and It = 1.027 nA. No obvious structure change is observed from 350 to 700 mV (vs RHE). (b) Tentatively suggested structural model of the 4,4′dibromobiphenyl monolayer.

Scheme 2. Speculated Formation Mechanism of 4,4′Dibromobiphenyl

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest. 2958

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ACKNOWLEDGMENTS This work was supported by the National Key Project on Basic Research (grant nos. 2011CB808700 and 2011CB932900), the National Natural Science Foundation of China (grant nos. 21073204, 21121063, 91023013, and 21127901), and the Chinese Academy of Sciences.

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