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. 2022 Mar 17;38(12):3666–3675. doi: 10.1021/acs.langmuir.1c03061

Tuning the Chemical and Mechanical Properties of Conductive MoS2 Thin Films by Surface Modification with Aryl Diazonium Salts

Dipankar Saha †, Shayan Angizi †, Maryam Darestani-Farahani †, Johnson Dalmieda ‡, Ponnambalam Ravi Selvaganapathy ‡,§, Peter Kruse †,*
PMCID: PMC8969871  PMID: 35298176

Abstract

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Molybdenum disulfide (MoS2) is a promising material for applications in sensors, energy storage, energy conversion devices, solar cells, and fuel cells. Because many of those applications require conductive materials, we recently developed a method for preparing a conductive form of MoS2 (c-MoS2) using dilute aqueous hydrogen peroxide in a simple and safe way. Here, we investigate modulating the chemical and mechanical surface properties of c-MoS2 thin films using diazonium chemistry. In addition to a direct passivation strategy of c-MoS2 with diazonium salts for electron-withdrawing groups, we also propose a novel in situ synthetic pathway for modification with electron-donating groups. The obtained results are examined by Raman spectroscopy and X-ray photoelectron spectroscopy. The degree of surface passivation of pristine and functionalized c-MoS2 films was tested by exposing them to aqueous solutions of different metal cations (Fe2+, Zn2+, Cu2+, and Co2+) and detecting the chemiresistive response. While pristine films were found to interact with several of the cations, modified films did not. We propose that a surface charge transfer mechanism is responsible for the chemiresistive response of the pristine films, while both modification routes succeeded at complete surface passivation. Functionalization was also found to lower the coefficient of friction for semiconducting 2H-MoS2, while all conductive materials (modified or not) also had lower coefficients of friction. This opens up a pathway to a palette of dry lubricant materials with improved chemical stability and tunable conductivity. Thus, both in situ and direct diazonium chemistries are powerful tools for tuning chemical and mechanical properties of conductive MoS2 for new devices and lubricants based on conductive MoS2.

1. Introduction

Over the past years, molybdenum disulfide (MoS2) has been extensively studied because of its unique physical and chemical properties.1−4 Most common among the known three phases of MoS2 is the semiconducting hexagonal (2H) phase, which occurs naturally and is stable at room temperature. It has a band gap of 1.8 eV as a monolayer and 1.23 eV in bulk.3 The metallic 1T phase can be obtained through various methods, like lithium intercalation or hydrothermal processes.5,6 Recently, we reported a safe and facile way of preparing conductive MoS2 using dilute hydrogen peroxide.7,8 There has been interest in MoS2 for many practical applications, such as water quality sensors,7,9 lubricants,10 field effect transistors,11 gas sensors,12,13 batteries,14 supercapacitors,15 solar cells,16 and electrocatalysts.17 Some applications require a conductive form of MoS2 (e.g., the metallic 1T phase, conductive composite materials, or doped conductive 2H-MoS2), so that a small applied voltage can achieve a reasonably high current density.7 Recent studies have shown that chemical functionalization of the MoS2 surface with selective groups can enhance its suitability for certain applications as a result of an increase in the surface area, selective detection of analytes, or acceleration of electrocatalytic hydrogen evolution.18,19

While there has been a lot of focus on the quality and scalability of preparing MoS2, much work remains to be done on its chemical functionalization. Chemical functionalization is a process to modify the reactivity, electronic property, and other surface properties of MoS2 with its many applications in mind.3 The use of thiol molecules has been demonstrated for the covalent functionalization of MoS2 on sulfur vacancy sites, which may be created during the exfoliation process or during post-processing (e.g., by ion beam irradiation).20 The limitation of thiol functionalization is its dependence upon the presence of unreacted sulfur vacancy sites, which limits the practically achievable surface coverage. Hence, it is important to develop functionalization processes that can achieve high functionalization densities without the need for a high density of unreacted sulfur vacancies.

The most direct way to attach new functional groups to MoS2 basal planes is to form a covalent bond between the functional groups and the surface sulfur atoms. MoS2 is known to have a negative surface charge (1T more so than 2H);19 hence, the use of electrophilic diazonium salts is beneficial to interact with the negative charges and achieve covalent functionalization. Upon the approach, the surface charge distribution of MoS2 causes the diazonium group to leave, forming an aryl radical, which then forms a covalent bond with sulfur to form a monolayer of functionalities at the surface (Figure 1).18,21 Knirsch et al.19 first demonstrated chemical functionalization of 1T-MoS2 using a methoxy-substituted phenyl diazonium salt. Similarly, Benson et al.22 demonstrated the surface functionalization of 1T-MoS2 with different electron-donating and -withdrawing substituent groups present in the aryl ring in the diazonium salts for the purpose of increasing the production of hydrogen gas. This opens opportunities to explore functionalization of other phases of MoS2 and with a wider range of functionalities following similar pathways.

Figure 1.

Figure 1

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Schematic representation of the modification process with aryl diazonium salts. “R” represents the −NO2 and tert-butyl groups.

MoS2 has long been known to be a good solid lubricant.10,23 It only has weak van der Waals forces between layers, which facilitates tangential shearing between the planes, resulting in good lubrication properties.10 Even though liquid lubricants, such as oil or grease, are very popular, they have limitations for use in sealing lubrication at extreme temperatures, in corrosive environments, at high pressures, or in vacuum.10 Solid lubricants also may reduce contamination, eliminate service requirements, and allow for weight reduction compared to oil and grease. MoS2 is widely used as a dry lubricant or as a coating on composite materials. The coefficient of friction (COF) for MoS2 is found to be between 0.15 and 0.30 in air24 or sometimes less depending upon the environmental conditions.24−26 Although many efforts have gone into lowering the COF of MoS2, research is still going on to improve the lubricant properties of MoS2.

Here, we demonstrate the use of both electron-donating (4-tertbutyl) and electron-withdrawing (4-nitro) substituted aryl diazonium chemistry to modify oxide-doped conductive MoS2, as verified by Raman spectroscopy and X-ray photoelectron spectroscopy. Because 4-terbutylbenzenediazonium salts are not stable or readily available, an in situ reaction pathway was introduced for passivation. Further, to elucidate the impact on chemical surface passivation, chemiresistive responses of both pristine and functionalized conductive MoS2 films were investigated. By passivating the surface with 4-nitroaryl and 4-tertbutylaryl groups, conductive MoS2 can be completely protected from participation in chemical reactions with metal ions. Lubrication properties were further studied using scratch testing to quantify changes in the COF of conductive MoS2. We found an ultralow friction coefficient of 0.028 for conductive MoS2 compared to semiconducting and bulk MoS2. The COF further decreased after passivating with 4-nitrobenzene and 4-tertbutylbenzene groups. Gaining control over the chemical and mechanical properties of conductive MoS2 using diazonium chemistry enables their use in a wide range of device and lubricant applications.

2. Materials and Methods

2.1. Materials

All organic solvents were high-performance liquid chromatography (HPLC)-grade and used without further purification. Bulk molybdenum disulfide powder (from ∼6 μm to a maximum of 40 μm, 98%, product number 69860, batch number WXBD2352V) was purchased from Sigma-Aldrich and used without further purification. Iron(II) chloride tetrahydrate, cobalt(II) chloride, zinc(II) chloride, and copper(II) chloride were purchased from Sigma-Aldrich. 4-Nitrobenzenediazonium tetrafluoroborate (97%), sodium tetrafluoroborate (98%), sodium nitrite (reagent plus, ≥99%), nitrobenzene (≥99%), 1,2-dichlorobenzene (anhydrous, 99%), isopentyl nitrite (96%), acetonitrile (HPLC gradient grade, ≥99.9%), N,N-dimethylformamide (≥99.8%), diethyl ether (HPLC, ≥99.9%, inhibitor-free), Whatman poly(tetrafluoroethylene) (PTFE) membrane filters (PTFE membrane circles, TE 36, 0.45 μm, 47 mm), and 4-tert-butylaniline (99%) were purchased from Sigma-Aldrich. Water used for experiments was ultrapure type I water (18.2 MΩ cm) from a Millipore Simplicity water purification system.

2.2. Exfoliation of Semiconducting MoS2 (2H-MoS2)

2H-MoS2 was exfoliated using 45% (v/v) ethanol in water via sonication (80 kHz frequency, 100% power, and sweep mode) for 12 h.27 The temperature (30 °C) was controlled during sonication by cooling the bath. The centrifugation process was then optimized for conditions to consist of a first step at 3500 rpm (820g) for 15 min. The supernatant underwent a second step of centrifugation at 4500 rpm (1700g) for 3 min. Thus, 2H-MoS2 was collected in the form of a gray precipitate. Collected 2H-MoS2 was washed with water, and the supernatant was discarded. A bath sonicator (Elmasonic P60H ultrasonic cleaner) was used for sonication, and an Eppendorf MiniSpin Plus microcentrifuge was used for centrifugation.

2.3. Exfoliation of Conductive MoS2 (c-MoS2)

Peroxide-induced c-MoS2 was exfoliated using our previously reported method.7 Briefly, semiconducting MoS2 (2H-MoS2) was exfoliated, and the precipitate was then washed with water. Aqueous H2O2 (0.06%, by volume) was added to the precipitate of 2H-MoS2 and sonicated (37 kHz frequency, 100% power, and sweep mode) for 20 min at 30 °C. The temperature during sonication was controlled using the built-in thermostat and heater of the sonicator. A cooling coil running with tap water was immersed into the sonicator bath for cooling. After sonication, the suspension was centrifuged at 3500 rpm (820g) for 8 min. The supernatant from this step was centrifuged at 10 000 rpm (6708g) for 15 min. The supernatant was discarded using a glass pipet, and the precipitate was collected for further use.

2.4. In Situ Functionalization of Conductive MoS2

A dispersion was prepared by sonicating 10 mg of conductive MoS2 in 10 mL of 1,2-dichlorobenzene (ODCB) for 10 min.28,29 Then, the mixture was transferred to a two-neck round-bottom flask, and 5 mL of acetonitrile and 4-tert-butyl aniline (2.6 mmol) were added to the mixture according to the reported procedure. The mixture was purged by bubbling nitrogen gas through it for at least 10 min, and then the flask was sealed with two septa. Isoamyl nitrite was added externally using a syringe, and the reaction mixture was heated to 65 °C using an oil bath for 18 h. A syringe was inserted into a septum of the round-bottom flask for the first 3 h to release excess pressure from the reaction. After 18 h, the reaction mixture was cooled to 45 °C and dimethylformamide (DMF) was added. Functionalized conductive MoS2 was filtered under vacuum using 0.45 μm PTFE membrane filter paper, washed with DMF several times, and dried. A blackish gray color sample was obtained and used for further characterization.

2.5. Fabrication of Chemiresistive Devices from Functionalized Conductive MoS2

Chemiresistive devices were fabricated using our previously reported procedure.7,30 Briefly, two terminal devices were fabricated on glass slides, which were cleaned by sonication in acetone, followed by methanol and water for at least 15 min during each step. Two parallel pads were drawn on the frosted end of the glass slide using a 9B pencil with a 1 cm gap. Kapton tape was used as a mask in the area for drop casting. About 150 μL of conductive MoS2 suspension was drop-casted in the masked area and dried at 100 °C to obtain a solid film. Afterward, the Kapton tape was removed, and two strips of conductive copper tape were pasted as contacts onto the parallel pencil line pads. Hot glue was used as a dielectric to cover the copper metal contacts using a hot glue gun.

2.6. Direct Functionalization of Conductive MoS2 Using Diazonium Salts

The deposited conductive MoS2 film on a glass slide was immersed in 10 mM aqueous 4-nitrobenezenediazonium tetrafluoroborate solution in a parafilm-sealed container under constant stirring on a hot plate at 35 °C.18 After the functionalization, the sample was rinsed with water and dried using nitrogen for further uses.

2.7. Raman Spectroscopy

A Renishaw inVia Raman spectrometer was used over a range of 100–3000 cm–1, with a spectral resolution of 2 cm–1, using a 20× objective in backscattering configuration. Spectra were obtained on two different spots of each sample using a fully focused 633 nm laser on a spot size of about 50 μm limited to 1% of laser power to avoid sample damage.

2.8. X-ray Diffraction (XRD)

XRD was performed using a Bruker D8 Discover instrument with Cu Kα radiation at a wavelength of 0.154 nm.

2.9. X-ray Photoelectron Spectroscopy (XPS)

The XPS analyses were carried out with a Kratos AXIS Supra X-ray photoelectron spectrometer using a monochromatic Al Kα source (15 mA and 15 kV). XPS can detect all elements, except hydrogen and helium, probes the surface of the sample to a depth of 7–10 nm, and has detection limits ranging from 0.1 to 0.5 atomic percent depending upon the element. The instrument work function was calibrated to give a binding energy (BE) of 83.96 eV for the Au 4f7/2 line for metallic gold, and the spectrometer dispersion was adjusted to give a BE of 932.62 eV for the Cu 2p3/2 line of metallic copper. The Kratos charge neutralizer system was used on all specimens. Survey scan analyses were carried out with an analysis area of 300 × 700 μm2 and a pass energy of 160 eV. High-resolution analyses were carried out with an analysis area of 300 × 700 μm2 and a pass energy of 20 eV. Spectra have been charge-corrected to the S 2p3/2 line of MoS2 set to 162.4 eV. Spectra were analyzed using CasaXPS software (version 2.3.14).

2.10. Scanning Electron Microscopy (SEM)

High-resolution images were obtained on a JEOL JSM-7000F scanning electron microscope at 3 kV.

2.11. Chemiresistive Device Measurements

Measurements were carried out using a four-channel eDAQ EPU452 Quad Multifunction isoPod with USB (purchased from eDAQ, Inc.). The chemiresistive devices were immersed in a bowl of 200 ppm of NaCl solution, and responses were obtained continuously in a two-probe configuration in biosensor mode (30 mV applied bias, 20 μA current range, 6 decimal places, and 15 points/min scan rate) at room temperature. The aqueous solution bowl was stirred at ∼200 revolutions per minute (rpm). Metal cation solutions at appropriate concentrations were added to the 3.42 mM (200 ppm) NaCl solution at about 40 min intervals. Two devices of each type were run in parallel, with multiple runs per experiment to ensure reproducibility.

2.12. Scratch Testing

Pristine and functionalized conductive MoS2 suspensions were drop-casted onto glass slides inside a 7 × 7 mm2 area masked with Kapton tape. The mask was removed after the film dried. In the case of the samples functionalized with 4-NBD, the pristine film was prepared first, followed by 4-NBD functionalization. 4-TBD-functionalized MoS2 was collected on filter paper, rinsed off with pure methanol, and sonicated for 1 min, and then the resultant suspension was drop-cast into the masked area.

An Anton Paar-RST3 Revetest scratch tester (Buchs, Switzerland) was used to perform microscratch tests on all samples in air at room temperature. A Rockwell diamond indenter with a 200 μm end radius was used for the tests using progressive mode. The tests were conducted with progressive loading from 500 to 530 mN over a scratch length of 0.6 mm at a scratching speed of 0.6 mm/min and loading rate of 30 mN/min.

3. Results and Discussion

3.1. Direct and In Situ Process for Chemical Surface Modification

Peroxide-exfoliated conductive MoS2 was functionalized with 4-NBD because it is commonly used to functionalize carbon nanotubes,29 graphene,31 and 2H-MoS2.18 However, before attempting to functionalize c-MoS2, 4-NBD functionalization was first demonstrated with exfoliated 2H-MoS2. Peaks at 1350 and 1595 cm–1 in the Raman spectra of functionalized 2H-MoS2 (Figure S1 of the Supporting Information) along with peaks at 385 and 410 cm–1 confirm functionalization of 2H-MoS2 with 4-NB.32 The peak at 1595 cm–1 is due to the aryl ring stretching of 4-NB on MoS2, and the peak at 1350 cm–1 is characteristic of the nitro group (Figure S1 of the Supporting Information).32 The same methodology can be applied to functionalize conductive MoS2. The characteristic Raman peaks for MoS2 (A1g and E2g)7 remain unchanged after functionalization, whereas several other peaks appear, which can be assigned to 4-NB (Figure 2). Importantly, the absence of any feature at 2300 cm–1 shows that the diazonium functionalities have been removed.32 The role of the diazonium group in the surface functionalization reaction was verified by a control experiment with nitrobenzene (instead of 4-NBD) and conductive MoS2 following the same reaction procedure. No nitro group and aryl ring stretching peaks appear in the Raman spectrum. This supports the proposed mechanism of the diazonium group departing to yield a phenyl radical that then attacks the sulfur atoms at the surface (Figure 1).

Figure 2.

Figure 2

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Raman spectra of aryl-functionalized and pristine conductive MoS2. Nitrobenzene with c-MoS2 was used as a control experiment. A 633 nm laser wavelength is used with 1% power, and all spectra are normalized to the ∼466 cm–1 peak.

Aryl diazonium salts for covalent functionalization of surfaces can also be formed in situ, which has been successfully demonstrated in the case of carbon nanotube (CNT) functionalization.28,29 This alternative route avoids the handling and storage of unstable aryl diazonium salts. In particular, aryl diazonium salts containing electron-donating moieties at the phenyl ring are not stable and not available for purchase as prepared salts, unlike some diazonium salts with strongly electron-withdrawing groups. Some diazonium salts that contain weakly electron-withdrawing groups are also unstable and cannot be stored for more than 1 or 2 days after their initial preparation. Here, the same in situ diazonium reaction conditions were applied to functionalize both conductive MoS2 and semiconducting exfoliated 2H-MoS2 with 4-tertbutylbenzene diazonium salt. In the Raman spectra of the products (both conductive and semiconducting), the characteristic feature peaks for tert-butylbenzene were observed at 997 and 1038 cm–1 in addition to the expected features at 383 and 410 cm–1 for MoS2 (Figure 2 and Figure S2 of the Supporting Information).33 The lack of a Raman feature around 2300 cm–1 indicates the absence of diazonium groups. A thermally induced reaction of MoS2 with diazonium compounds, which are generated by the in situ reaction of isopentyl nitrite and aniline derivative, has therefore been established as another pathway to MoS2 functionalization.29

Even though Raman spectroscopy can verify the presence of functional groups on the c-MoS2 surface, it does not provide definite proof of successful covalent bonding between c-MoS2 and the aryl groups. Hence, to better understand the formation of covalent chemical bonds during both direct and in situ chemical functionalization of c-MoS2 by diazonium salts, high-resolution XPS of C 1s (Figure S3 of the Supporting Information), Mo 3d (Figure S3 of the Supporting Information), S 2p (left panels of Figure 3), and N 1s (right panels of Figure 3) was performed on pristine and functionalized samples. For direct 4-NB functionalization, XPS spectra of N 1s at 405.9 eV in Figure 3b clearly indicate the presence of nitro groups at the pristine c-MoS2 surface. Nevertheless, this leaves open the possibility of dimer or oligomer formation as side products during the functionalization because the ratio of N from nitro groups and S(II) is around 1 (Table 1). Both functionalized samples show a significant increase in total carbon (XPS probes the top few nanometers of the sample only; therefore, this is surface carbon) to almost 60%, up from 36% for the pristine sample (Table 1). All samples will have accumulated some adventitious carbon during preparation, but this consistent increase is due to the presence of nitrobenzyl or tert-butyl benzyl groups on the c-MoS2 surface. However, there is little direct evidence in the S 2p XPS spectra of covalent bonding between carbon and sulfur atoms on the c-MoS2 surface, except for a low-intensity shoulder (3 at %) at 163.7 eV (panels b and c of Figure 3) that may be due to C–S bonding. This implies that only a smaller portion of the surface sulfur atoms has participated in the radical reaction with diazonium salts. We therefore propose that the functionalization occurs primarily at surface-defect sites (there are plenty of defect sites because the S/Mo ratio is less than 2 for all samples). Because buried defects are not affected, the electronic properties of pristine and functionalized c-MoS2 samples are nevertheless preserved, even after functionalization. The attachment of functional groups to the c-MoS2 surface is also reflected in the surface morphology of both pristine and modified conductive MoS2 films. High-resolution SEM images of a non-functionalized conductive MoS2 film deposited on a silicon dioxide substrate (panels a and b of Figure 4) show homogeneously distributed multilayered flakes. The functionalized c-MoS2 films (panels c–f of Figure 4) show a similar morphology of still intact multilayer flakes, although with some irregular features that may be due to chemical surface modification.

Figure 3.

Figure 3

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XPS high-resolution spectra of S 2p and Mo 3p + N 1s for (a) pristine c-MoS2, (b) 4-NB-modified c-MoS2, and (c) 4-TB-modified c-MoS2.

Table 1. Compositional Changes in the Pristine and Modified Samples from High-Resolution XPSa.

sample name total C total Mo total S total O total N N (−NO2)/S2– total N/total S
pristine c-MoS2 36.35 12.37 18.78 32.48      
4-NB-modified c-MoS2 58.35 4.49 3.81 27.94 5.35 0.96 1.38
4-TB-modified c-MoS2 59.40 0.80 0.94 38.84      
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a

Total atomic percentages of C, Mo, S, O, and N are obtained from the high-resolution XPS spectra. The atomic ratio of nitrogen in nitro groups and sulfur as sulfide is calculated from the total atomic percentages of S and N in the high-resolution S 2p and N 1s XPS spectra.

Figure 4.

Figure 4

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Surface morphology of pristine and modified c-MoS2, with SEM images of c-MoS2 samples. The left- and right-side SEM images are from the same sample. Samples are (a) pristine c-MoS2, scale bar of 1 μm; (b) pristine c-MoS2, scale bar of 100 nm; (c) nitrobenzene-functionalized c-MoS2, scale bar of 1 μm; (d) nitrobenzene-functionalized c-MoS2, scale bar of 100 nm; (e) tert-butyl-functionalized c-MoS2, scale bar of 1 μm; and (f) tert-butyl-functionalized c-MoS2, scale bar of 100 nm.

3.2. Modulation of the Chemical Surface Properties

When the surface of a pristine or functionalized two-dimensional (2D) material interacts with an aqueous solution containing metal cations, there will be an impact on the electronic structure of the material that can be observed as a result of surface doping or field effects as a result of its interactions with its chemical environment.30,34 These chemical changes can be probed in a chemiresistive thin-film format. Two different mechanisms are commonly invoked to explain chemiresistive responses: (a) an electrostatic gating mechanism as a result of changes in the electrochemical double layer (EDL) and (b) a surface charge transfer mechanism that modulates the dopant concentration of the conductive layer, e.g., as a result of complexation.35 Because the formation of an EDL upon exposure of the film to an aqueous electrolyte is unavoidable, we start by considering its impact. Increasing concentrations of NaCl were added to deionized water in contact with a bare c-MoS2 chemiresistive film (Figure 5a). While lower concentrations can lead to changes in the chemiresistive response as a function of the ionic strength of the solution, the chemiresistive response saturates at higher ionic strengths. The magnitude of the chemiresistive response is significantly lower for the addition of large concentrations of Na+ ions (Figure 5a) than it is for the addition of a smaller concentration of Fe2+ (Figure 5b and Figure S4a of the Supporting Information) or other ions. Therefore, we do not expect the chemiresistive response to Fe2+ to be primarily due to the electrostatic gating mechanism. In all following experiments (including Figure 5b and Figure S4a of the Supporting Information), a 3.42 mM NaCl solution was used instead of deionized (DI) water as the background electrolyte to minimize changes in the compactness of the EDL during the gradual addition of metal salt solutions. As a result of its equilibrium with CO2 from the ambient air, the pH of the unbuffered 3.42 mM NaCl solution was around 5.6 and remained stable upon the addition of different metal ions (Figure S6 of the Supporting Information). Although conductive MoS2 films respond to pH,7 the chemiresistive response in this case is not due to pH changes.

Figure 5.

Figure 5

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Ionic strength and chemiresistive behavior of conductive MoS2 films: (a) pristine conductive MoS2 thin-film response in different concentrations of NaCl along with their conductance values and (b) pristine, (c) 4-NB-functionalized, and (d) 4-TB-functionalized conductive MoS2 thin-film responses to different concentrations of Fe2+.

Four different metal ions (Fe2+, Zn2+, Co2+, and Cu2+) with concentration ranges of 0–0.29 mM (for Fe2+), 0–0.24 mM (for Zn2+), 0–0.27 mM (for Co2+), and 0–0.25 mM (for Cu2+) were chosen to understand the interactions with conductive MoS2 films. Current changes versus time were recorded, starting with immersion of the devices into a 3.42 mM NaCl solution. Increases in the chemiresistive response to Fe2+ ions were observed for exposure to different concentrations, from 3.17% at 2.87 μM over 56% at 0.05 mM to almost 237% at 0.29 mM relative to the baseline current (Figure 5b). Pristine films also responded reproducibly and strongly to the addition of Cu2+ ions but to a lesser extent to Zn2+ and Co2+ (Figure S4 of the Supporting Information). In the case of Fe2+, one might speculate about surface charge transfer as a result of a redox mechanism resulting in Fe3+ and a decrease in hole density in the p-doped MoS2 film. However, not only would that result in a current decrease (i.e., opposite chemiresistive response), but it would also not explain the (albeit much smaller) response to Zn2+ (Figure S4b of the Supporting Information), which is not redox-active under these conditions, and Co2+ (Figure S4d of the Supporting Information), which should respond with a decrease in the current if oxidized to Co3+ as the concentration is increased. A reduction of Cu2+ to Cu+ at the pristine MoS2 surface (Figure S4c of the Supporting Information) would be a possibility, but it is not a satisfactory explanation in the face of the data for the other cations. A Lewis acid base interaction of the divalent cations with the lone pairs of the sulfur atoms at the surface provides a more likely mechanism. Na+ as a hard Lewis acid would not interact as readily with the soft Lewis base sulfur lone pairs,36−38 which is why the additions of large amounts of NaCl background do not have any effect on the chemiresistive response. A close interaction with the double-positive charge at the borderline Lewis acids Fe2+, Zn2+, Cu2+, or Co2+ would remove some electron density from the sulfur atoms and increase the hole density in the p-doped MoS2 film, resulting in an increased current flow in all four cases, which is exactly what was observed (Figure 5 and Figure S4 of the Supporting Information).

When conductive MoS2 is chemically modified with either 4-NB or 4-TB, the scenario is quite different (panels c and d of Figure 5), because the surface modification prevents the formation of a complex between the surface sulfur sites and the divalent cations.38 No chemiresistive responses were observed upon exposure of 4-NB-modified devices to Fe2+, Zn2+, Cu2+, or Co2+ metal ions (Figure 3c for Fe2+ and Figure S5 of the Supporting Information for Zn2+, Cu2+, or Co2+). In solution phase, the nitroso group (as it is present in 4-NB) has been reported to form a complex with iron selectively compared to the other three metal ions.39 The surface geometry, however, restricts the formation of multiligand complexes,30 reducing the thermodynamic favorability of an iron complex with 4-NB groups anchored to c-MoS2. If a complex of the nitro group with any of the cations was formed at all, the resulting change in the electronic structure is not transmitted to the MoS2 surface (Figure 5c and Figure S5 of the Supporting Information). We also fabricated devices with 4-TB-modified conductive MoS2 to study the chemical reactivity of conductive MoS2 films, but no response was observed upon exposing different metal ions to the solution (Figure 5d for Fe2+ metal ion and Figure S7 of the Supporting Information). This is expected because the tert-butyl group is expected to be inert and not form any complexes with these metal ions. Both 4-NB and 4-TB can therefore be used as protective layers for MoS2 films.

There is another interesting consequence to the inertness of the 4-NB- and 4-TB-functionalized films to interactions with the divalent cations. While Raman spectroscopy is able to qualitatively confirm the success of the functionalization reactions (Figure 2), XPS data imply a rather modest degree of direct functionalization. It has been reported that the functionalization of 2H-MoS2 with diazonium salts is initiated at and aided by defects and then propagates along the surface without a guarantee of it going to completion.18 We can state here that the complete absence of a chemiresistive response in our functionalized samples (panels c and d of Figure 5 and Figures S5 and S7 of the Supporting Information) attests to a very high degree of coverage at our surfaces because any remaining exposed patches would have mirrored the responses of pristine samples. A polymeric surface film formed during the diazonium reaction and anchored to surface defect sites is therefore the most likely scenario.

3.3. Modulation of the Mechanical Properties

The lubricant properties of any material can be evaluated on the basis of its COF, which is defined as the ratio of friction force and applied normal load of that material during sliding. A lower COF signifies better lubrication properties. Hence, to evaluate the lubricant properties, COFs were measured in both pristine and functionalized conductive MoS2 thin films (Figure 6). In agreement with previous reports,10,24 the COF for bulk MoS2 was found to be approximately 0.10. This value is lowered to 0.07 upon exfoliation to a few layer 2H-MoS2 material as a result of an increase in the number of interflake sliding planes (bulk MoS2 has weak interactions between planes but is higher compared to exfoliated 2H-MoS2). Remarkably, however, the COF for peroxide-exfoliated conductive MoS2 was found to be even lower at 0.028 (Figure 6a). The low friction of MoS2 may arise from a scenario where the MoS2 flakes are rotated relative to each other in the surface-normal direction. This would lead to a mismatch in the basal plane pairs of MoS2 at the interfaces that come into contact during sliding, resulting in the systematic cancellation of lateral forces.10 It has been reported that edge oxidation by water can increase the COF in MoS2 as a result of the blocking of crystallite alignment and shearing of MoS2 planes.24 However, this is not what we observed for c-MoS2, because it showed a lower COF compared to other MoS2 materials. In our case, conductive MoS2 is in the +4 oxidation state in the bulk, whereas at the edges, MoS2 first formed +6 oxidation states followed by reduction of the +5 oxidation state upon exposure of hydrogen peroxide.8 Hence, the edge reduction by hydrogen peroxide in conductive MoS2 may contribute to lowering the COF of c-MoS2.24 XRD data of all pristine samples show a broad (002) peak at 2θ of ∼14°, but no new peaks were detected,7 confirming that all samples are in the 2H phase without changes in the interlayer spacings or layer interactions within the individual flakes. The changes in COF are thus a result of changes in flake thickness or surface chemistry. Not only is the lubricity of c-MoS2 higher compared to other forms of MoS2, but the material is conductive in nature as well, which is important for applications like wiring harness, contacts, connectors, sensors, etc.40

Figure 6.

Figure 6

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Scratch test experiments on different MoS2 samples. COF values and corresponding scratch test images of (a) pristine bulk, 2H, and conductive MoS2 and (b) functionalized c-MoS2 along with pristine c-MoS2 samples. The scale bar of all of the images is 50 μm.

No significant changes in the COF values were observed for 4-NB- and 4-TB-functionalized conductive MoS2 (Figure 6b). We further carried out XRD of conductive MoS2-functionalized samples, but no changes in the peak position or additional peaks were detected in those samples (Figure S8 of the Supporting Information). This indicates that functionalization only occurred at the surface of the flakes and not between layers inside the flakes. While MoS2 itself has been known to be a good lubricant, the hydrogen peroxide exfoliation process and diazonium-based surface modifications can be used to further improve its lubrication properties.

4. Conclusion

In summary, we have demonstrated chemical modification of oxide-doped conductive MoS2 with 4-NB using diazonium chemistry. Surface modification proceeds through a radical mechanism that results in the loss of the diazonium group and yields a functionalized surface of c-MoS2. An in situ functionalization strategy is also proposed for the first time to functionalize conductive MoS2 with an electron-donating 4-TB functional group. In situ functionalization helps to avoid stability issues in the handling of electron-donating aryl diazonium salts. Further, the extent of chemical interaction of four different metal ions (Fe2+, Zn2+, Cu2+, and Co2+) with pristine and functionalized conductive MoS2 films has been studied using a chemiresistive geometry. The chemiresistive response indicates that pristine films are prone to interact with certain metal ions, while the surfaces of the functionalized films were found to be completely passivated. Coefficients of friction for pristine and functionalized MoS2 surfaces were also measured to understand the impact of functionalization on the lubricant properties of MoS2. While further work is still required to optimize different diazonium functionalization strategies both in situ and direct, this work advances our ability to tune the chemical and mechanical properties of conductive MoS2 for a wide range of practical applications.

Acknowledgments

The authors are grateful to Dr. Mark Biesinger (Surface Science Western, Canada) for help with XPS, Prof. Alex Adronov (Chemistry and Chemical Biology, McMaster University) for use of the Raman spectrometer, Mohammad Shariful Islam Chowdhury (McMaster Manufacturing Research Institute, McMaster University) for help with the scratch test measurement, Chris Butcher [Canadian Centre for Electron Microscopy (CCEM), McMaster University] for help with high-resolution SEM, and Md. Ali Akbar, Peter Ho, Vinay Patel, Sudarshan Sharma, and Dr. Jayasree Biswas (all McMaster University) for fruitful discussions. Electron microscopy was performed at the CCEM (also supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and other government agencies). The work was financially supported by the NSERC through the Discovery Grant Program as well as the Canada First Research Excellence Fund Project “Global Water Futures”.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.1c03061.

  • Additional Raman and XPS spectra, chemiresistive measurements, and XRD data (PDF)

The authors declare no competing financial interest.

Supplementary Material

la1c03061_si_001.pdf (2.3MB, pdf)

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Supplementary Materials

la1c03061_si_001.pdf (2.3MB, pdf)

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