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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers
Determining the surface tension of two-dimensional nanosheets by a low-rate advancing contact angle measurement Xinru Zhang, Xinzhi Cai, Kairu Jin, Zeyi Jiang, Hao Yuan, Yan Jia, Yang Wang, Limei Cao, and Xinxin Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04104 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 17, 2019
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Determining the surface tension of two-dimensional nanosheets by a low-rate advancing contact angle measurement Xinru Zhang,a, b# Xinzhi Cai,a# Kairu Jin,a Zeyi Jiang,a,
c*
Hao Yuan,a Yan Jia,a Yang
Wang,d Limei Cao,e Xinxin Zhang a, c
a
School of Energy and Environmental Engineering, University of Science and
Technology Beijing, Beijing, 100083, China b Beijing Engineering Research Center of Energy Saving and Environmental Protection,
University of Science and Technology Beijing, Beijing, 100083, China c Beijing
Key Laboratory for Energy Saving and Emission Reduction of Metallurgical
Industry, University of Science and Technology Beijing, Beijing, 100083, China d
School of Advanced Engineering, University of Science and Technology Beijing,
Beijing, 100083, China e
School of Mathematics and Physics, University of Science and Technology Beijing,
Beijing, 100083, China
Corresponding Author *Zeyi Jiang Prof. Mailing Address: School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing, 100083, China Phone: 86-10-62334971; E-mail:
[email protected] 1 / 23
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Abstract Because of their atomic thinness, two-dimensional (2D) nanosheets need be bound to a substrate or be dispersed in material in various applications. The surface tension (ST) of a 2D nanosheet is critical for analyzing the physicochemical interactions between 2D nanosheets and other materials. To date, the determination of the ST of 2D nanosheets has relied mainly on the contact angle (CA) method. However, because of the difficulty in measuring the thermodynamically significant Young’s CA, which is the only meaningful CA that can be used to determine the ST, significant differences exist in reported STs of 2D nanosheets. In this study, we obtained such unique Young’s CAs on graphene, boron-nitride, molybdenum-disulfide and tungsten-disulfide nanosheets by a low-rate advancing contact angle measurement using a rigorously designed experimental setup. By interpreting the CA with Neumann’s equation of state, the STs of these four nanosheets were determined to be 29.7 ± 0.6, 30.9 ± 0.7, 27.8 ± 0.7 and 29.1 ± 0.8 mJ/m2, respectively. The surface energies of these 2D nanosheets were estimated to be in the range of 95–120 mJ/m2 by considering the contribution of ST and surface entropy. The accuracy of these determined STs were validated by the exfoliation and dispersion of 2D nanosheets in liquids with a series of STs. The study may have important implications for understanding the physicochemical interactions between 2D nanosheets and other materials, and the development of 2D nanosheetbased devices.
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Introduction Two-dimensional (2D) nanosheets, such as graphene, boron-nitride (BN), and transition-metal dichalcogenides nanosheets, are an emerging class of layered nanomaterials that possess mono- to few-layer sheet-like structures.1-2 Owing to their fascinating physical and chemical properties, 2D nanosheets promise many potential applications in electronic devices, optoelectronics, nanocomposites, biological sensors and energy storage.3-5 In most applications, 2D nanosheets need be bound to a substrate or be dispersed in the material because of their atomic sheet-like structures.1 Based on the theory of surface thermodynamics, the surface tension (ST) of 2D nanosheets plays an important role in determining the physicochemical interactions between 2D nanosheets and other materials6-7, such as, adhesion of nanosheets onto substrate,8 dispersing nanosheets into polymer matrix,9 choosing suitable solvents in exfoliation of bulk material into nanosheets,10 estimating the binding interaction of adsorbent,11 and so forth. Therefore, an understanding and characterization of the ST of 2D nanosheets has significant implications for academia and industry. According to the Gibbs convention, the ST of 2D nanosheets (𝛾𝑁𝑉) has an energetic and an entropic component (i.e., 𝛾𝑁𝑉 = 𝐸𝑁𝑉 ―𝑇𝑆𝑁𝑉, where 𝐸𝑁𝑉 and 𝑆𝑁𝑉 are the surface energy and surface entropy of the 2D nanosheets, respectively, and T is the absolute temperature).6, 12 Owing to the difficulty in determining the 𝐸𝑁𝑉 and 𝑆𝑁𝑉,13 the current quantitative determination of the ST of 2D nanosheets relies mainly on the contact angle (CA) method, in which the CA of a sessile drop is measured on nanosheet films, followed by a theoretical interpretation that determines the ST. Using the CA 3 / 23
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method, Wang et al.14 determined the ST of graphene nanosheets (GNs) to be 46.7 mJ/m2, by measuring the CAs of 10-μL static droplets on a GNs film, followed by a theoretical interpretation based on Neumann’s equation of state. Kozbial et al.15 measured the CAs of 2-μL static droplets on graphene films, and determined the STs of fresh graphene and graphene exposed to ambient air for 24 h to be 63.8 and 57.4 mJ/m2, respectively, in conjunction with equation of state. In addition, based on the CA measurement, Gaur et al. reported ST of few layer MoS2 as 44.5 mJ/m2, while, Kozbial reported fresh MoS2 nanosheet has a ST of 54.5 mJ/m2.16-17 Evidently, these reported STs of 2D nanosheets are significantly different. According to the theory of interpreting CA in terms of ST, the difference in the reported STs of the 2D nanosheets may be because the measured static CA could not be regarded as Young’s CA, which is the only meaningful CA that can be used to determine the ST, and therefore a significant variation in the determined STs results.1820
Generally, in the CA measurement, due to the influence of the surface roughness,
heterogeneity, volume of sessile drops, and creeping of liquid into surface, 18-21 it is not a trivial task to determine the Young’s CA on 2D nanosheets. Based on the extensive research in the area of CA, Neumann and coworkers found that the thermodynamically significant Young’s CA can be obtained by a low-rate dynamic contact angle (LDCA) method.6, 22-23 In this LDCA method, an initial small drop was formed on the surface, then more liquid was injected continuously into the droplet from below the surface using a motorized syringe, and the final equilibrium low-rate advancing CA (LACA) could be approximated to Young’s CA.6 It is noted that supplying liquid from below 4 / 23
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the surface, rather than from above the droplet, was used to ensure that the shape of droplet was not affected by the needle, and the measured LACA was a proper and precise Young’s CA. Using this method, some researchers have obtained the CAs, which can be regarded as Young’s CA, on a wide variety of biological materials, including bacteria and microalgae cells. Their STs were then characterized by a theoretical interpretation of the CAs.6, 24-25 However, to date, little research has been conducted to determine the STs of 2D nanosheets with LDCA method. In this study, we determined the STs of 2D nanosheets, including graphene, BN, molybdenum-disulfide, and tungsten-disulfide nanosheets (i.e., GNs, BNNs, MoS2Ns and WS2Ns) by measuring the thermodynamically significant LACA on the nanosheet film using a rigorously designed experimental setup, in conjunction with Neumann’s equation of state. To verify the accuracy of the determined ST, we studied the exfoliation and dispersion of these four 2D nanosheets in acetone–water and ethanol– water mixtures with a series of STs. The results demonstrated that the highest dispersion was obtained in the liquid mixture with an ST that was similar to the determined ST of 2D nanosheets and confirmed the accuracy of the determined STs of the 2D nanosheets. The study may have important implications for fundamental understanding of the physicochemical interactions between 2D nanosheets and other materials. Experimental section Preparation and characterization of 2D nanosheets Commercially-available flaked graphite (Chengdu Organic Chemicals Co. Ltd., Chengdu, China), BN, MoS2 and WS2 (Nanjing XFNANO Materials Tech Co. Ltd, 5 / 23
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China) were used to prepare 2D nanosheets by liquid phase exfoliation via tip sonication. The morphology, thickness, and crystal structure of the exfoliated 2D nanosheet were characterized using a scanning-electron-microscopy (SEM) (Nova NanoSEM 430, FEI, Hillsboro, OR, USA) by pipetting the nanosheet dispersions onto a Si substrate, an atomic force microscope (AFM) (Dimension FastScan, Brucker Ltd., USA) by pipetting the nanosheet dispersions onto the mica, a Raman spectroscopy (LabRAM HR800, Horiba Jobin-Yvon, France) by using a 514-nm laser and depositing nanosheet films onto glass slides, and a transmission-electron microscopy (TEM) (JEM-2200FS, JEOL, Japan) by pipetting the 2D nanosheet dispersion onto carboncoated copper grid. Measurement of the thermodynamically significant LACA on 2D nanosheet The thermodynamically significant LACA on the filtered nanosheet film was measured by a LDCA method based on a rigorous experimental design (as shown in Figure 1) and a reliable CA analysis tool (axisymmetric drop-shape analysis).6 Before the measurement, the nanosheet films were prepared by filtering a 2D nanosheet dispersion using a polyvinylidene fluoride membrane (pore size: 0.45 μm, Whatman, England). It should be noted that, based on the previous study on the LDCA method, Neumann indicated that when the surface roughness was below approximately 0.1 μm, the CA hysteresis arising from roughness was absent.6, 26 Therefore, the microstructure of the prepared nanosheet film was observed by SEM, and the average roughness (Ra) of the film was measured by using a laser scanning confocal microscope (LSCM) (LEXT OLS4000, Olympus, Japan) and an AFM (Dimension FastScan, Brucker Ltd., 6 / 23
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USA). During the measurement, firstly, the nanosheet film-membrane was mounted onto two adjacent glass slides by using double-sided adhesive tape. Then, a microhole with a diameter of ~500 μm was pierced through the nanosheet film by using a needle. As shown in Figure 1, a deionized-water sessile drop with a radius of ~2 mm was deposited carefully on the nanosheet film from above to cover the microhole. Afterward, more deionized water was injected continuously into the initial sessile drop from below the nanosheet film by using a precisely controlled motorized syringe. In the experiments, the three-phase contact line of the sessile drop was advanced steadily at a low rate. The images of the droplet were taken successively with a high-speed camera. The CA, contact radius, and volume of sessile drop were calculated from the shape of the sessile drop by using axisymmetric drop-shape analysis.6 At least three samples were measured for each nanosheet film. The measured thermodynamically significant LACAs were shown in mean ± standard deviation. It should be noted that, using the LDCA method, the advancing CA (ACA) and receding CA (RCA) can both be measured. In this regard, previous study demonstrated that, for a heterogeneous surface consisting of low- and high-energy surface patches (roughness
