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AFM2 2017

Direct observation of the initial adsorption stage of stearic acid on HOPG/aqueous interface using in-situ atomic force microscopy

Yanyan Liua*, Yang Hub, Shaoxian Songa

a School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan, 430070, Chinab Instituto de Metalurgia, Universidad Autonoma de San Luis Potosi, Av. Sierra Leona 550, San Luis Potosi, C.P. 78210, Mexico

Abstract

Direct observation of the initial adsorption stage of stearic acid on graphite surfaces by AFM imaging has been investigated. The adsorbed aggregates of the stearic acid (C17H35COOH) at the graphite/aqueous interface were observed with PeakForce Tapping® mode in air and in liquid. Initial adsorption stage of the stearic acid on HOPG has been identified. At a low stearic acid concentration, the adsorption initially occurred only along the cleavage edge and few on the basal plane of the graphite in a straight line or in a random individual distribution, which might be due to the higher surface energy at the edge and the oxidation or defects on the plane. At a high concentration, wormlike aggregates increased on the basal plane, which might be attributed to the semi-micelle adsorption of stearic anion on the plane. The initial adsorption at very low concentration may play a role to identify the area of oxidation or defects on graphite and graphene surfaces.

© 2018 Elsevier Ltd. All rights reserved.Selection and/or Peer-review under responsibility of 2017 International Workshop on Atomic Force Microscopy for Advanced Functional Materials.

Keywords: Initial adsorption; Adsorption aggregates; Defects; Atomic force microscopy; Highly Oriented Pyrolytic Graphite (HOPG).

1. Introduction

* * Corresponding author. Yanyan Liu. Tel.: +86-137-9706-4762.E-mail address: wulengheiyin@whut.edu.cn

2214-7853 © 2018 Elsevier Ltd. All rights reserved.Selection and/or Peer-review under responsibility of 2017 International Workshop on Atomic Force Microscopy for Advanced Functional Materials.

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Graphite, with numerous layers composed by packing its sp2-hybridized carbon atoms into the hexagonal lattice, has energetically homogeneous surfaces and edges that contain unpaired electrons [1, 2]. It is known that graphite is a good electrically conductive and hydrophobic material. However, these unique properties will be weakened if graphite is oxidized or the structure of it is partially broken. On the other hand, these oxidized sites on graphite can provide the active location for polymers, surfactants and nanoparticles to adsorb or assemble, which is helpful to make modification or composites of graphite based materials [3]. Hence, the study on the in-situ determination of oxidation areas on graphite surfaces is very important no matter in the theoretical acknowledge or practical applications. Also, it can be applied on other carbon based materials, such as carbon nanotubes, graphene and graphene oxide, etc.

Techniques such as X-ray diffraction (XRD), X-ray fluorescence (XRF) and Raman scattering are widely used to provide general information about the oxidation behaviors of materials, although they are not in-situ methods for the characterization of the oxidation area on material surfaces. Scanning electron microscopy (SEM) has advantages on direct observation of the microstructure of materials with oxidation reactivity or defects. It was found that small particles on the graphite as the catalytic impurities accumulated along the edges, which indicated that such a direct observation of the microstructure features may offer a possible way to assess the oxidation behaviors of materials [4]. Atomic force microscopy (AFM) also displays a great potential in direct measurement of the oxidation locations on materials. Kudernatsch et al. reported a direct visualization of the catalytically active sites at the FeOPt (111) interface using scanning tunneling microscopy (STM) [5]. Quintana et al. observed the deposition of Au nanoparticles on carbon nanotubes using AFM imaging [6]. Surface morphology showed that the Au nanoparticles accumulated only on the walls and at the end of the nanotubes, suggesting that the defects on the material surface played an essential role on Au nucleation. Such results demonstrated that AFM imaging technique may be employed as a useful tool for direct measurement of the oxidation locations or defects on the material surface. On other hand, the defects or cleavage planes on the mineral surfaces might affect the surfactant adsorption [7, 8]. Intensive research about the adsorption of surfactants on minerals has been widely reported [9-12]. Direct visualization of the adsorption aggregates using AFM has been also seen in much literature [13-15]. It is suggested that the crystalline cleavage basal planes on the graphite surface has a templating impact on the adsorption structure that displays differently between the edge and basal planes [14]. Therefore, the objective of current work is to directly observe the initial adsorption stage of stearic acid (SA) on HOPG, which may further offer information of the identification of the oxidation locations or defects on the graphite through AFM imaging.

2. Experimental

Highly-Oriented-Pyrolytic-Graphite (HOPG) and mica were purchased from Boyue Company, China. Stearic acid (SA) and octadearyl dimethyl ammonium chloride (OTAC), used here are analytically pure reagents (Sinopharm, China). The solutions were prepared using deionized and purified water (Mili-Q water) that was produced by Milli-Q system (Merck Millipore, Germany) with a value of 18 MΩ·cm, and was used for preparing all the aqueous solutions. The newly uncovered HOPG or mica surface was firstly mounted on the substrate of AFM and covered with the O-ring cell to which was later injected with the SA or OTAC solutions. The in-situ observation was conducted using PeakForce® Tapping AFM (Multimode 8, Bruker). Scanning three different locations was conducted to ensure the consistence and reproducibility of the results.

3. Results and discussion

Figure 1 showed the AFM images of the HOPG surfaces before and after being immersed in aqueous SA solutions. Before being immersed in the solution, the HOPG had very clear basal planes and edges, as shown in Fig 1a. After being immersed in 1 mol/L aqueous SA solution for 1 hour, the HOPG surface displayed aggregates formation that occurred mainly on the edge sites, whereas a small part of adsorption was observed on the basal planes with a globular or chain distribution randomly, as shown in Fig. 1b. Details of the edge-adsorption were displayed in Fig. 1c with a scanning size of 830×830 nm. Besides, several random particles were also seen on the

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basal plane. After being immersed in aqueous SA solution with the higher concentration of 5 mol/L, the HOPG on the basal planes was covered randomly by large amounts of adsorbed layers with aggregates curvatures in a wormlike, as shown in Fig. 1d. Such an observation of the surface morphology was also named as branched stripes (mesh) [15].

Fig. 1. AFM images of HOPG surfaces before being immersed in SA solutions (a); HOPG surface after being immersed in 1 mol/L aqueous SA solution, 5.5×5.5 µm size (b); HOPG surface after being immersed in 1 mol/L aqueous SA solution, 830×830nm size (c); HOPG surface after being immersed in 5 mol/L aqueous SA solution, 2.5×2.5 µm size (d).

(a) (b)

(c)(d)

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At the low SA concentration, the distribution of the adsorption aggregates on the edge site may be attributed to the high bond-energy at the edge. The random and chain accumulation of the aggregates on the basal planes might be related to the oxidation or the defects at these locations [16]. Figure 2 schematically illustrates the graphite structure with edges that can be easily oxidized with groups (-COOH, -OH, etc.) and the possible defects (scratch, hole, etc.) on the basal plane. Graphite has edges that consist of two different configurations named as “armchair” and “zigzag”. Generally, the edge may contain both the armchair and zigzag structure [1]. Each carbon atom in the zigzag edge structure has an unpaired electron, which provides it more chemical activity. Comparatively, the armchair carbon atom is more stable as the two open edge carbon atoms are triple covalently bonded [2]. Due to this unique properties, the graphite edge is commonly terminated by hydrogen atoms, and can be easily oxidized to form hydroxyl, carboxyl and edge ether [3]. Similar structure also appears in the vacancies and scratches on the surface of graphite, shown in Fig. 2b. Those species would interact with the polar groups in surfactants or polymers, leading to the formation of a cluster along the oxidized zigzag edges or the vacancies and scratches [17]. The cluster in Fig. 1b and 1c could be attributed to the semi-micelle adsorption of stearic anion on the edges and basal planes, in which the polar group oriented towards to the hydrophilic sites on the HOPG surface and the nonpolar tail oriented towards water. Fig. 3 schematically represents the adsorption of stearic anion on the oxidation sites of graphite (edge, scratch and hole) in aqueous SA solutions at low concentration (a) and high concentration (b), which may reflect that there are two-step adsorption process [9,18]: initial fast adsorption stage (Ⅰ), and slower-time-scale adsorption stage (Ⅱ). At the initial adsorption stage which was observed more obviously at very low concentration in Fig.1b, hydrogen-bond interaction domains due the existence of the oxidized step and defect sites. During this stage, the surfactants adsorbed quickly onto the edge sites. The head groups were oriented to the cleavage or defected sites, whereas the tail groups were towards to the solution. Once such adsorption sites were fully occupied, it reaches to the slow stage in which the hydrophobic interactions play a more significant role to attract the tail groups attaching to the HOPG surface, leading the formation of the wormlike admicelle on the HOPG surfaces until the adsorption was kept at a dynamic equilibrium stage with the effective adsorption sites fully occupied.

In the case of higher SA concentrations, the kinetics of the SA adsorption onto HOPG surface was faster since there are more SA close to the solid/aqueous interface for adsorption. In this situation, not only the edges and defects areas, but also the basal planes have been occupied quickly. Compared to the edge adsorption, basal adsorption was attributed that the SA nonpolar groups (hydrocarbon chain) interacted with the hydrophobic surfaces of the HOPG because of hydrophobic attraction, leading the SA anion to adsorb on the surfaces in the reverse way (the nonpolar group oriented toward the surfaces), as schematically represented in Fig. 3b. This adsorption would lead the great coverage of SA on the HOPG surfaces, as shown in Fig. 2d. Therefore, the in situ observation of the initial adsorption stage at very low concentration may shed light on the identification of the oxidation or defects on the basal plane. This observation agreed well with the reference [19] that also confirmed the LSHB polymer adsorption at very low concentration exclusively located along the polishing scratches with defects due to higher energy at these locations.

Fig. 2. Schematic drawing of graphite structure with oxygen groups (a) and scratch and hole (b)

(b)(a)

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Fig. 3. Adsorption of SA on oxidized areas of graphite in aqueous SA solution at low concentration (a); Adsorption of SA on graphite surfaces in aqueous SA solution at high concentration (b).

4. Conclusions

In summary, it concludes that the initial adsorption of the stearic acid onto graphite surface occurred firstly at the edge site, oxidized area, and locations with defects where the high bond-energy domains. Once such adsorption sites are fully occupied, it reaches to the slower-time-scale adsorption stage in which the hydrophobic interactions play a more significant role to attract the nonpolar groups attaching to the basal planes, leading the formation of the wormlike semi-micelle on the surfaces. In particular, this in-situ visualization of the initial adsorption aggregates that formed at the edge and basal locations at lower concentration is significant interesting. It may shed light on the identification of the oxidation areas and defects on graphite-based materials using the atomic force microscopy imaging technique. Further study will be continued.

Acknowledgements

Financial support from the National Natural Science Foundation of China (project No. 51504174) is gratefully acknowledged. Support from Hubei Provincial Collaborative Innovation Center for High Efficient Utilization of Vanadium Resources and Hubei Key Laboratory of Mineral Resources Processing and Environment is also appreciated.

References

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