Article pubs.acs.org/Langmuir
Probing Anisotropic Surface Properties of Molybdenite by Direct Force Measurements Zhenzhen Lu, Qingxia Liu,* Zhenghe Xu, and Hongbo Zeng Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 2V4, Canada S Supporting Information *
ABSTRACT: Probing anisotropic surface properties of layer-type mineral is fundamentally important in understanding its surface charge and wettability for a variety of applications. In this study, the surface properties of the face and the edge surfaces of natural molybdenite (MoS2) were investigated by direct surface force measurements using atomic force microscope (AFM). The interaction forces between the AFM tip (Si3N4) and face or edge surface of molybdenite were measured in 10 mM NaCl solutions at various pHs. The force profiles were well-fitted with classical DLVO (Derjaguin−Landau−Verwey−Overbeek) theory to determine the surface potentials of the face and the edge surfaces of molybdenite. The surface potentials of both the face and edge surfaces become more negative with increasing pH. At neutral and alkaline conditions, the edge surface exhibits more negative surface potential than the face surface, which is possibly due to molybdate and hydromolybdate ions on the edge surface. The point of zero charge (PZC) of the edge surface was determined around pH 3 while PZC of the face surface was not observed in the range of pH 3−11. The interaction forces between octadecyltrichlorosilane-treated AFM tip (OTS-tip) and face or edge surface of molybdenite were also measured at various pHs to study the wettability of molybdenite surfaces. An attractive force between the OTS-tip and the face surface was detected. The force profiles were well-fitted by considering DLVO forces and additional hydrophobic force. Our results suggest the hydrophobic feature of the face surface of molybdenite. In contrast, no attractive force between the OTS-tip and the edge surface was detected. This is the first study in directly measuring surface charge and wettability of the pristine face and edge surfaces of molybdenite through surface force measurements.
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INTRODUCTION Molybdenite (MoS2), an important sulfide mineral, has found a wide range of applications in dry lubrication,1,2 catalysis,3−5 photovoltaics,6−8 batteries,9,10 and recently nanoelectronics and optoelectronics.11 Naturally occurred molybdenite is normally recovered with chalcopyrite through bulk flotation. The molybdenite is then separated by differential flotation through the depression of chalcopyrite. Extensive research has been conducted to illustrate the importance of anisotropic properties of molybdenite in flotation process.12−14 It was postulated that the variation in flotation recovery with the change of particle size might be attributed to the anisotropic surface properties of molybdenite.15 Molybdenite exhibits a laminar crystalline structure as shown in Figure 1a. The rupture of covalent bonds between S−Mo generates a high-energy surface that is known as the edge. The breakage of weak van der Waals interactions between S−Mo−S layers generates a low-energy surface that is known as the face or basal plane. It has been reported in the literature that the face surface is naturally hydrophobic.16 As the face to edge ratio may decrease with the reduction of particle size, the floatability of molybdenite may decrease with decreasing particle size. Probing anisotropic surface properties of molybdenite is fundamentally important in understanding of surface wettability and its relation to floatability.13,17 © 2015 American Chemical Society
Several surface techniques have been employed to study the surface properties of the face and edge of natural molybdenite. Zanin et al.17 conducted contact angle measurements on the face and scalpel-cut edge of molybdenite. The face surface was found to be hydrophobic with a contact angle of about 100° while the edge surface was found to be hydrophilic with a contact angle of about 18° and is reactive to Ca2+ or Mg2+ at a pH of 11. It is worthwhile to note that by scalpel cutting, the edge surface of molybdenite could be bent, distorted, or roughened, which may result in a large error in experimentally measured contact angle. Molecular dynamic simulation has shown the contact angle of 83° for face, 54° for armchair edge, and 24.3° for zigzag edge.18 Moreover, the face and edge electrodes were prepared by slicing molybdenite crystal along or through the face surface for electrochemistry measurements.14 The face electrode exhibited a higher resting potential than the edge electrode. The critical wetting surface tension of the face surface of molybdenite was determined to be 29 mN/ m, whereas the edge surface was estimated to be larger than 72.5 mN/m.19 The zeta potential of molybdenite has also been studied by electrophoretic measurement.13,20,21 Received: July 21, 2015 Revised: September 29, 2015 Published: October 4, 2015 11409
DOI: 10.1021/acs.langmuir.5b02678 Langmuir 2015, 31, 11409−11418
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Figure 1. (a) Crystal structure of molybdenite. (b) X-ray diffraction analysis of molybdenite mineral.
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Atomic force microscopy (AFM) has been widely applied in the characterization of physical and chemical properties of interfaces and systems, such as surface morphology, mechanical properties, and surface forces, etc.22−29 AFM imaging has been applied to map the cleavage face surface of molybdenite at atomic resolution29 and for the analysis of the adsorption of chemicals onto the face surface of molybdenite.30,31 AFM has also been employed for measuring the colloidal forces between mica and silica,27,32 between air bubble and silica,33 and between silica and gold,34 and the adhesion forces between illite particles and cleaved illite surfaces.23,35 Nevertheless, there is limited information on the force measurements conducted on the face and edge surfaces of molybdenite. One challenge associated with force measurements is the ability to prepare molecularly smooth face and edge surfaces of molybdenite. Recently, a microtome cutting technique was applied to obtain smooth edge surface of muscovite for force measurements by AFM.36 A molecularly smooth edge surface was achieved with platy minerals such as talc and muscovite37 and rodlike chrysotile38 by an ultramicrotome cutting technique for force measurements by AFM. In this study, ultramicrotome-cutting technique was employed to obtain molecularly smooth surface of molybdenite edge. Therefore, the surface charge property of the face and the edge surfaces of molybdenite can be evaluated by direct force measurement. In this paper, we investigated the anisotropic surface properties of molybdenite mineral by direct force measurements using AFM in 10 mM NaCl solutions over the range of pH 3−11. To probe surface potential of the face and edge surfaces, the interaction forces between a silicon nitride tip and the face and edge surfaces of molybdenite were measured. The obtained force profiles were fitted with DLVO (Derjaguin− Landau−Verwey−Overbeek) theory to determine the surface potential of both the face and edge surfaces of molybdenite. The interactions between a hydrophobic tip and the face and edge surfaces of molybdenite were also measured. An extended DLVO theory, incorporating additional hydrophobic force, was applied to best fit the force profiles. The hydrophobicity of the face and the edge surfaces of molybdenite was elucidated. The anisotropic surface properties of natural molybdenite mineral were, for the first time, determined by direct surface force measurements using AFM.
MATERIALS AND METHODS
Molybdenite mineral was bought from eBay and used without further purification. X-ray diffraction (XRD) analysis of the raw sample revealed that the bulk mineral was pure molybdenite (Figure 1b). In addition, 10 mM sodium chloride (NaCl) solution (SigmaAldrich) was used as the background electrolyte during AFM force measurements. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) (Sigma-Aldrich) were used to adjust the pH of the solutions. Milli-Q water with a resistivity of 18.2 MΩ·cm was used for preparing the solutions. All other reagents were from Sigma-Aldrich. The AFM tip was rinsed with ethanol and Milli-Q water and dried with ultrapure nitrogen gas. The cleaned tip was hydrophobized by immersing it in freshly prepared 1 mM octadecyltrichlorosilane (OTS) in toluene solution for 1 h. Special care was taken to remove water from the solvent. The excess OTS on tip surface was removed by gentle washing in chloroform, acetone, and pure water. The OTS-tip was then blow-dried with pure nitrogen gas and stored prior to use. The water contact angle of the OTS-tip is about 96°. Contact Angle Measurement. For contact angle measurements, freshly cleaved basal plane of molybdenite was used as the face surface. The water contact angles of sample surfaces were measured by a goniometer (KRUSS DSA 10, Germany) using a sessile drop method. In the experiment, 2 μL of 10 mM NaCl solution was dropped on the surface. Images were captured and analyzed with the goniometer at room temperature. The contact angle was measured by the goniometer based on the shape of the water droplet, and the measurement was repeated at least three times for each sample. Surface Preparation and Ultramicrotome Cutting. Molybdenite has perfect cleavage along the basal plane. The basal plane was, therefore, freshly cleaved using a sticky tape prior to the AFM measurement. To generate a fresh edge surface, an ultramicrotome cutting technique36 was employed. In the literature, the ultramicrotome technique has been applied for the preparation of smooth surfaces of mineral,37,38 polymer,39 semiconductive nanomaterial,40 and biological materials41 for TEM, SEM, and AFM analysis. The preparation process of edge surface of molybdenite is described as follows. First, a small and thin piece of molybdenite was laid flat and embedded in an epoxy resin and then cured as a block overnight. Second, the epoxy block was trimmed by a razor blade under an optical microscope, and the cutting direction was kept perpendicular to the molybdenite sheet to expose the edge surface of the molybdenite piece. After trimming, the block was mounted on an ultramicrotome (EM UC 7, Leica Microsystems Inc.) for cutting. A rough cut was performed with a glass knife to remove most of the resin around the sample and to generate a small and relatively smooth top surface. To better support the sample during cutting, the top surface of the block was cut to a rectangular shape and the edge surface of the 11410
DOI: 10.1021/acs.langmuir.5b02678 Langmuir 2015, 31, 11409−11418
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Figure 2. (a) Geometry of a model conical AFM tip with spherical apex used for DLVO calculations. α and β are the geometric angles for the spherical cap at the tip end and the conical tip, D is the distance between the tip end and substrate, L is the distance between a differential surface section of the tip and the substrate, r is the radius of the circle of the tip at a given vertical position, and R is the radius of the spherical cap at the tip end. (b) FE-SEM image of one of the AFM cantilever tips used in this study. The angle of the cone 2β was obtained. Inlet image is the zoom-in image of the tip end with two circles, which represent minimum and maximum curvature of the tip apex. molybdenite piece was sandwiched at the center. Then, a final delicate cutting was performed with a 45° diamond knife (Diatome AG, Biel, Switzerland) using the procedures for thin sectioning. The cutting speed of last thin sectioning was set at 1.00 mm/s, and in total around 500−700 nm thick of sample surface was cut off from the specimen to achieve the fresh and smooth edge surface. The exposed edge surface used in surface force measurement was only contacted with the cutting edge of inert diamond knife at very low speed. Thus, the cut sample for the colloidal force measurements is fresh and pristine edge surface of molybdenite. The finished surface of the block was shiny after sectioning. The edge surface was subjected to high-pressured nitrogen gas blow to remove any fine debris. The prepared edge surface was rinsed with Milli-Q water and dried with ultrapure nitrogen gas (Praxair) prior to use. To fix the sample, the sample block was glued onto the AFM liquid cell with the edge surface of molybdenite facing upward to AFM tip for force measurement. AFM Measurement. Imaging and surface force measurements on silica wafer and molybdenite mineral surfaces were carried out in 10 mM NaCl aqueous solutions at various pHs using an Asylum MFP-3D atomic force microscope (Oxford Instrument Company, UK). Silicon nitride tips (NP, Veeco Inc., U.S.) were used for all AFM images and force measurements. Prior to force measurement at each pH, a piece of silica wafer was cleaned with piranha solution, rinsed with Milli-Q water, and dried with nitrogen gas. The molybdenite face sample was cleaved along the basal plane to expose a fresh face surface. For edge sample, the ultramicrotome-cut molybdenite edge surface was rinsed thoroughly with Milli-Q water and dried with nitrogen gas before the force measurement at a given pH. The AFM tip was also rinsed with Milli-Q water and dried with nitrogen gas prior to force measurement. All substrates (silica wafer, molybdenite face, and edge surfaces) and AFM tip were immersed in the solution of given pH in a home-built liquid cell for at least 30 min to equilibrate the tip/solution/substrate system before any force measurement was conducted. For each pH value, the interaction forces between the tip and sample surface were measured at five different locations on each sample surface. The representative force profiles at each location were fitted to the classical or extended DLVO theory. For each pH of solution, the average value over five locations was calculated and reported in this study. All of the experiments were conducted at room temperature (22 ± 1 °C). The raw force data were analyzed with Asylum software (Igor Pro 6.34A) which converted the deflection distance data to force−separation curves. The spring constant of each cantilever was measured using the thermal tune method, which was a built-in option in the Asylum software. The spring constants of the cantilevers were determined to be 0.07−0.15 N/m. Field Emission Scanning Electron Microscope (FE-SEM). The dimensional analysis on the AFM silicon nitride tip was carried out
using a JAMP-9500F Auger scanning electron microprobe (JEOL). The instrument is equipped with a Schottky field emitter that produces an electron probe diameter of about 3−8 nm on the sample. Shown in Figure 2b, the radius of the tip apex curvature was determined from the FE-SEM images of the tips and varied between 22 and 76 nm. The angle of the cone 2β ranged between 43.8° and 47.3°, and the angle 2α was between 132.7° and 136.2°. For the following calculations, an average value of 67.3° was applied for angle α.
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MODEL FOR THEORETICAL CALCULATION
For a Si3N4 AFM tip, the force between the tip and sample surface was calculated using DLVO theory, which includes contributions from van der Waals (VDW) force and electric double-layer (EDL) force. Based on the pyramidal shape as shown in FE-SEM image (Figure 2b), the tip geometry can be reasonably approximated as conical with a spherical cap at its apex, as shown in Figure 2a. The Derjaguin approximation was applied to the derivation of the DLVO theory for the conical tip−flat substrate system.42 Although the use of the Derjaguin approximation may reduce the accuracy of the surface charge density/surface potential analysis, it is noted that previous studies43,44 indicated that such approximations do not necessarily invalidate analyses of systems with the pyramidalshaped AFM tips. The final equations used for force calculation were the following:43 van der Waals force: F vdw =
A ⎛ (R + D) − 2L1 R − D⎞ A ⎜ ⎟− − 2 6⎝ L1 D2 ⎠ 3 tan 2 α
⎛1 R sin α tan α − D − R(1 − cos α) ⎞ ⎟ ×⎜ + 2L12 ⎝ L1 ⎠
(1)
electrostatic double-layer force under constant potential condition: F ψ − ψ = 4πεε0ψTψS(a0e−κD − a1e−κL1) − 2πεε0(ψT 2 + ψS 2) × (a 2e−2κD − a3e−2κL1) +
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⎤ ⎛ψ 2 + ψ 2⎞ 4πεε0κ ⎡⎢ S ⎟ −2κL1⎥ b1ψTψS e−κL1 − b2⎜⎜ T e ⎟ ⎥⎦ tan α ⎢⎣ 2 ⎝ ⎠
(2)
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⎛ ∂W ψ − σ ⎞ ⎟ F ψ − σ = −⎜ ⎝ ∂D ⎠
electrostatic double-layer force under constant charge density condition: 4π σTσS(a0e−κD − a1e−κL1) εε0κ 2 2π + (σT 2 + σS 2)(a 2e−2κD − a3e−2κL1) εε0κ 2 ⎡ ⎤ ⎛σ 2 + σ 2⎞ 4π S ⎢b1σTσS e−κL1 + b2⎜ T ⎟e−2κL1⎥ + ⎥⎦ 2 εε0κ tan α ⎢⎣ ⎝ ⎠
F σ−σ =
where L1 = D + R(1 − cos α), a1 = κR cos α − 1,
b1 = R sin α +
1 κ tan α
b2 = R sin α +
1 2κ tan α
(3)
The theoretical electrostatic double-layer force can be calculated under either constant charge density or constant potential boundary condition. However, in certain systems, particularly at a short distance, the electric double layer force may fall in between those two conditions. In a mixed model, one surface is under constant potential condition and the other surface is under constant charge density.45 In our study, it was found that at a short distance less than 5−10 nm the mixed potential model appears to fit our force data better. Therefore, a mixed electrostatic double-layer force model was used to better fit the measured force curves. Based on the equation for the electrostatic potential energy per unit area for two plane parallel double layers under the mixed model,45 the electrostatic potential energy for flat substrate and the pyramid tip can be integrated. The tip geometry can be approximated as conical with a spherical cap as its apex (Figure 2a), and the final equations are given: The electrostatic potential energy for flat substrate and the spherical part of the tip is ⎡ W Sedl = πR ⎢2ψSσT ⎢⎣ ×
∫D
L1
∫D
L1
πb3 ⎡ ⎢2ψ σT tan α ⎢⎣ S
∫L
(4)
+
π ⎡ ⎢2ψ σT tan α ⎢⎣ S
σ=
∞
sec h(κL) dL
∫L
∫L
∞
1
⎤ (tanh(κL) − 1) dL ⎥ ⎥⎦
∞
sec h(κL)L dL
1
⎞ ⎛σ 2 + ⎜ T − εε0κψS 2⎟ ⎠ ⎝ εε0κ
∫L
∞
1
⎤ (tanh(κL) − 1)L dL ⎥ ⎥⎦
(5)
⎛ eψ ⎞ 8c0εε0kBT sinh⎜ ⎟ ⎝ 2kBT ⎠
⎛ D⎞ F H = −RC0 exp⎜ − ⎟ ⎝ D0 ⎠
The total electrostatic potential energy for flat substrate and the whole tip is W ψ − σ = W Sedl + WCedl
a3 = a1 + 0.5
(8)
where e is the elementary charge, c0 is the ion-number concentration in bulk solution, kB is the Boltzmann constant, and T is temperature. With an OTS-coated tip, the surface forces between the tip and the face surface were fitted by an extended DLVO theory. Apart from van der Waals force and electric double-layer force, the contribution of the additional hydrophobic force was also included for the extended DLVO theory. The hydrophobic force is represented by the following empirical relationship47 hydrophobic force:
1
⎞ ⎛σ 2 + ⎜ T − εε0κψS 2⎟ ⎠ ⎝ εε0κ
a 2 = a0 + 0.5,
Here, D is the distance between the tip end and the substrate, and R is the radius of the tip apex. The Greek symbols α and β are the geometrical angles for the spherical cap at the tip apex and the conical tip, where α + β = 90° (see Figure 2a). ε0 is the permittivity of the vacuum, ε is the dielectric constant of the medium separating surfaces, ψ is the surface potential, σ is the surface charge density, and κ−1 is the Debye length. A is the combined Hamaker constant of the tip−solution−substrate system. The Hamaker constant A for silicon nitride/water/silica system was calculated to be 2.43 × 10−20 J (see eq S1 in the Supporting Information). For the silicon nitride/water/ molybdenite system, the value of the Hamaker constant A was calculated to be 3.3 × 10−20 J (see eq S2). W is electrostatic potential energy for two surfaces. The subscripts S and T refer to the substrate and the tip, respectively. The superscripts ψ−ψ, σ−σ, and ψ−σ refer to the constant potential, constant charge density, and the mixed model, respectively. In the mixed model, the substrate surface (MoS2 here) is considered as constant potential and the tip surface of AFM is treated as constant charge density. The surface potential (ψ) and the surface charge density (σ) are linked to each other by the Grahame equation:46
The electrostatic potential energy for flat substrate and the conical part of the tip is WCedl =
a0 = κR − 1,
b3 = R sin α tan α − L1
⎛σ 2 ⎞ sec h(κL) dL + ⎜ T − εε0κψS 2⎟ ⎝ εε0κ ⎠
⎤ (tanh(κL) − 1) dL ⎥ ⎥⎦
(7)
(9)
where C0 and D0 are fitting constants. C0 is related to interfacial tension at the solid/liquid interface, while D0 is referred to as decay length.48 According to the above equations, a MATLAB program was developed to fit force profiles that were measured by AFM.
(6)
Then, the electrostatic double-layer force between flat substrate and the tip in mixed model is given by differentiating the total electrostatic potential energy. 11412
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Figure 3. (a) Force measurement between a silicon nitride tip (tip) and silica wafer (sa) in 10 mM NaCl solutions at various pH values fitted by DLVO theory under constant charge density. The symbols represent experimental data. The solid lines represent the theoretical fittings. (b) Comparison of surface potential values of silicon nitride determined by force fitting in this study (black solid sphere) with surface potential reported in the literature (open symbols). Black open square and red open circle represent zeta potentials by electroacoustic and electrophoretic measurements in 10 mM KCl.58 Blue open triangle represents the data determined by AFM fitting in 1 mM KCl solution.59
Figure 4. AFM images of molybdenite surfaces. (a) Molybdenite face surface with a mean roughness of 0.053 nm (RMS) over 4 μm2. (b) Molybdenite edge surface prepared by ultramicrotome cutting technique with a mean roughness of 1.579 nm (RMS) over 4 μm2.
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RESULTS AND DISCUSSION
was detected. This suggests that the surface charge of the Si3N4 tip is negative at pH 6 or higher, which is similar to silica. The measured force profiles were analyzed by the DLVO theory in order to evaluate the surface potentials of the Si3N4 tip at various pH values. As reported in the literature, constant charge density was found to be most appropriate for a ceramic surface such as silica.53,54 Therefore, DLVO fitting under constant charge density was applied in our system for silicon nitride surface. For each fitting, the value of the Debye length was calculated based on the ionic strength of the solution46 as listed in Figure 3b. The experimental force−separation profiles were found to be in reasonable agreement with theoretical predictions of DLVO under constant charge density, as shown by the solid lines in Figure 3a. At the separation distance less than 5 nm, a discrepancy between the experimental data and DLVO theory was observed for each pH value, which could be attributed to the presence of hydration forces between the silica surface and the tip.28,55−57 The surface potentials of silica and the silicon nitride tip at various pHs were determined by fitting AFM force curves. The fitted potential values of silica agreed well with the measured values in the literature as seen in Figure S1b. The fitted values for surface potentials of Si3N4 tip at pH 3.0, 6.0, 9.0, and 11.1 are +24, −45, −69, and −80 mV, respectively. As shown in Figure 3b, the surface potential values
Interaction between AFM Tip and Silica Wafer. In order to determine the surface charge properties of molybdenite surfaces by fitting AFM force profiles, it is essential to measure the surface potential of the tip at each testing condition. According to DLVO theory, interaction force is determined by surface potentials of both the tip and the substrate. If interaction force is measured and surface potential of either the tip or substrate is known, the other can be obtained via the fitting of the force profiles based on the DLVO model. The surface potential of silica at various pH conditions has been well studied; therefore, a clean silica wafer can be used to calibrate the surface potential of the AFM tip by measuring the interaction forces between them. As shown in Figure S1a, the AFM image of a silica wafer shows that the surface roughness (RMS) of the silica wafer is about 0.077 nm over a 4 μm2 area, which is quite smooth for AFM force measurements. It is well established that the PZC value of silica is around pH 2−3.49−52 The silica wafer is all negatively charged over the range of pH 3−11. The attractive interaction at pH 3.0 indicates that the surface charge of the tip could be positive. As pH value increased to 6.0 or higher, only repulsive interaction 11413
DOI: 10.1021/acs.langmuir.5b02678 Langmuir 2015, 31, 11409−11418
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Figure 5. Force profiles between a silicon nitride tip and molybdenite face (a) or edge surface (b) in 10 mM NaCl solutions at pH 3.0, 6.0, 9.0, and 11.1 and the DLVO fitting. The square symbol corresponds to experimental data. Solid and dashed curves represent fitting results of DLVO forces under constant charge density (CC, red), constant potential (CP, blue), and mixed (Mix, black) boundary conditions. Hamaker constant A of 3.3 × 10−20 for silicon nitride/aqueous solution/molybdenite system was used in the fitting. The face (basal plane) surface is simplified as fa, and the edge surface is represented as eg.
The interaction forces between the tip and face (or edge) surface of molybdenite were measured on at least five different spots on the surfaces in 10 mM NaCl solutions of various pH values. The force profile at each spot was fitted, and an average value was calculated for each pH solution as shown in Figure 6. All the force profiles and the fitting at each pH are shown in Figure 5. The AFM force profiles between the silicon nitride tip and the face or edge surface of molybdenite were fitted by DLVO theory to evaluate the surface potentials of molybdenite at various pHs. In the fitting, the surface potential of silicon nitride derived above was applied, and the Debye lengths were the same as shown in Figure 3b. Therefore, the surface potential of the face or edge surface of molybdenite was the only unknown parameter. The force profiles between the tip and face surface of molybdenite in 10 mM NaCl solutions at various pHs are displayed in Figure 5a. The interaction between the silicon nitride tip and the face surface is attractive at pH 3.0 and repulsive at pH 6.0 or higher. Since van der Waals force is attractive and independent of surface potential, the repulsive interaction is attributed to the electrical double layer force. The electrostatic repulsive force is greater under alkaline condition, indicating higher surface potential of the face surface. The theoretical electric double-layer force was calculated using both constant charge density and constant potential boundary
of the Si3N4 tip display a similar trend to the potential values obtained using other methods.58,59 However, the isoelectric point (IEP) of silicon nitride from electrophoretic mobility measurements was about 6,58 which is higher than the values of pH 3−4 as determined by this study. The lowered IEP is due to partial oxidation on the silicon nitride surface and a possible formation of silica layer.37 In addition, AFM tips were kept in air-based atmosphere, and they were usually aged for several weeks or longer. Thus, the surface condition of AFM tip is different from that of freshly exposed silicon nitride applied in electrophoretic mobility measurements. Therefore, the surface potential values obtained from force fitting are more representative for silicon nitride tip used in this study. Interactions between AFM Tip and Both the Face and Edge Surfaces of Molybdenite. Before performing force measurements on the molybdenite surfaces, the surface roughness of the face and edge surfaces were studied by AFM imaging as shown in Figure 4. The face surface of molybdenite is quite smooth with a root-mean-square (RMS) roughness of 0.053 nm over 4 μm2. The AFM image of molybdenite edge surface prepared by ultramicrotome cutting technique shows a RMS of 1.579 nm over 4 μm2 (Figure 4b). The large smooth areas of the face and edge surfaces of molybdenite are suitable for AFM force measurement. 11414
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This also agrees well with the data reported by Chander et al.15,60 Also, the surface potential curves of the face and edge surfaces as a function of pH from this study are compared with zeta potentials from the literature13,20,21 in Figure 6. As surface potential of the mineral is important in understanding its flotation behavior, the zeta potential of molybdenite was obtained by electrophoretic measurements.13,21 The value of the zeta potential for molybdenite varied in different literatures and especially with reduction of particle size. It is noted that the zeta potentials of molybdenite determined by electrophoretic measurements were averaged values over all surface area, which includes both face and edge surfaces. The pristine surface potentials of the face and edge of molybdenite are critically important for many applications. However, prior to this study, there is no report in the literature that the surface potentials of pristine face and edge surfaces of molybdenite have been determined, respectively. In this study, surface potentials on both pristine face and edge surface of molybdenite were determined by AFM force measurement. Our results suggest that both surfaces are negatively charged at pH 4 or higher. As the solution becomes acidic, both surfaces are less negative. When solution becomes alkaline, both surfaces become more negative, and the edge surface bears even more negative charge than the face surface at neutral or alkaline condition. To be noted that the face surface is relatively inactive as compared with the edge surface due to the lower surface energy of the face surface. The MoS2 edge surface can be partially oxidized by oxygen or water vapor, resulting in oxygenated sites on the surface.60,61 These sites are assumed to be either HMoO4− or MoO42−.60 Thus, the following reaction could occur on the edge surface of molybdenite:
Figure 6. Comparison between surface potentials of face (black solid square) and edge (red solid sphere) surfaces of molybdenite in 10 mM NaCl solutions in this study by force measurement and zeta potentials from literatures. Zeta potential in the literature* was in 2 mM KCl.21 Zeta potential in the literature** was in 5 mM KNO3.13
conditions. At pH 3.0, the measured force profiles fit well with both the constant charge and constant potential model (Figure 5a). As pH increases, theoretical forces under the constant charge model begin to separate with those under the constant potential condition. At pH of 9.0 and 11.1, the force profiles are closer to the constant charge condition, especially at a distance less than 5 nm, suggesting that the constant charge condition fits better for the face surface of molybdenite. Through this fitting, the measured force profiles agree with DLVO theory for all pH values. The surface potentials of the face surface of molybdenite were fitted at −27, −34, −44, and −52 mV for pH 3.0, 6.0, 9.0, and 11.1, respectively. The surface potentials of the face surface (basal plane) of molybdenite are negative and gradually become more negative with increasing pH value. The surface potential of the face surface versus pH curve is shown in Figure 6. The point of zero charge (PZC) of the face surface was not detected in the range of pH 3.0−11.1 in this study. PZC of molybdenite face surface is expected to be lower than pH 3.0. The force profiles between the tip and edge surface of molybdenite in 10 mM NaCl solutions at various pHs are displayed in Figure 5b. The interaction is weakly attractive at pH 3.0. The interaction force becomes purely repulsive at pH greater than or equal to 6.0, and the repulsive interaction force increases with increasing pH value. Since the surface potentials of the tip in the range of pH 6.0−11.1 are negative, this indicates that the surface potential of the edge surface in the pH range is also negative and the value decreases as pH becomes more alkaline. The force profiles were fitted well using DLVO under mixed model (Figure 5b). Comparing the force profiles of pH 6.0, 9.0, and 11.1, the magnitude of the repulsive forces on the edge surface are greater than those on the face surface. It is demonstrated that molybdenite edge surface carries more negative charge at neutral or alkaline condition. In Figure 6, the surface potential versus pH curves show the surface potentials of both the face and edge become more negative as pH value increases, while surface potential of the edge surface decreases more greatly than that of the face surface. Figure 6 also indicates that the PZC of edge surface of molybdenite is about pH 3. Combining PZC of the face (
