Soluble, Exfoliated Two-Dimensional Nanosheets as Excellent

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Soluble, Exfoliated Two-Dimensional Nanosheets as Excellent Aqueous Lubricants Wenling Zhang,*,† Yanlin Cao,† Pengyi Tian,‡ Fei Guo,‡ Yu Tian,*,‡ Wen Zheng,† Xuqiang Ji,† and Jingquan Liu*,† †

College of Materials Science and Engineering, Laboratory of Fiber Materials and Modern Textile, The Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, China ‡ State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Dispersion in water of two-dimensional (2D) nanosheets is conducive to their practical applications in fundamental science communities due to their abundance, low cost, and ecofriendliness. However, it is difficult to achieve stable aqueous 2D material suspensions because of the intrinsic hydrophobic properties of the layered materials. Here, we report an effective and economic way of producing various 2D nanosheets (h-BN, MoS2, MoSe2, WS2, and graphene) as aqueous dispersions using carbon quantum dots (CQDs) as exfoliation agents and stabilizers. The dispersion was prepared through a liquid phase exfoliation. The as-synthesized stable 2D nanosheets based dispersions were characterized by UV−vis, HRTEM, AFM, Raman, XPS, and XRD. The solutions based on CQD decorated 2D nanosheets were utilized as aqueous lubricants, which realized a friction coefficient as low as 0.02 and even achieved a superlubricity under certain working conditions. The excellent lubricating properties were attributed to the synergetic effects of the 2D nanosheets and CQDs, such as good dispersion stability and easy-sliding interlayer structure. This work thus proposes a novel strategy for the design and preparation of high-performance water based green lubricants. KEYWORDS: 2D nanosheets, dispersion stability, carbon quantum dots, synergistic effect, friction coefficient

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INTRODUCTION Recent developments of two-dimensional (2D) materials such as single- or few-layer sheets of graphene, layered double hydroxide (LDH), hexagonal boron nitride (h-BN), transition metal dichalcogenides (TMDs, e.g., MoS2, WS2, MoSe2, or WSe2), and phosphorene have far-reaching applications, such as hydrogen evolution, field effect transistors, supercapacitors, phototransistors, water desalination, lithium-ion batteries, and catalysis, etc.1−6 Moreover, these lamellar materials have proven potential applications as solid lubricant additives due to their easy interlayer sliding attributed to the relatively weak van der Waals interactions.7−11 Recently, the investigation of lubricants with excellent tribological properties has become essential for saving energy and increasing the service life of components.12−14 Compared with solid lubricants, liquid lubricant additives have the following advantages: (1) lower friction coefficient; (2) higher cooling capacity; (3) faster entrance and replenishment of the sliding interface; (4) easier removal of wear debris from the contact zone. Aqueous lubricants have been widely used since water is a low-cost lubricating liquid with superb thermal conductivity, easy cleaned, cost-effective, and environmentally friendly.15,16 If the limitations of water based lubricant additives, such as low viscosity, a low boiling point, and high corrosive properties, can be overcome, water © XXXX American Chemical Society

based lubricants could replace many oil based lubricants to satisfy the demands in various industrial applications.17,18 Therefore, developing high-performance aqueous lubricant additives is imperative. Owing to the weak interaction between the interlayers of 2D materials, these layered 2D materials can be dispersed in appropriate media and used as good liquid lubricant additives. Mechanical cleavage, liquid exfoliation, chemical intercalation, and chemical vapor deposition are the most widely used methods to obtain monolayer or few-layer sheets with great advantages, such as being defect-free, easily scalable in production, and low cost.19−25 Recently, sonicationassisted exfoliation has also been used to produce various nanosheet dispersions,26−28 such as graphene quantum dots (GQDs) stabilized graphene aqueous dispersion29 and graphite, h-BN, MoS2, WS2, and MoSe2 aqueous dispersions.20,28 Carbon quantum dots (CQDs) have been reported as stabilizers and exfoliation agents in the exfoliation of graphite to graphene in aqueous media.27 Due to their chemical and dispersion stability, small particle size, and low cytotoxicity,30−32 CQDs have been used in many applications, including Received: August 4, 2016 Accepted: November 7, 2016 Published: November 7, 2016 A

DOI: 10.1021/acsami.6b09752 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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were investigated. X-ray diffraction (XRD) patterns were recorded using a LabX XRD-6100 system. Lubricating Measurements. Standard four-ball tests and ball-ondisk tests were conducted on a Plint TE92 tribometer (Phoenix Tribology, Newbury, U.K.) and a UMT-3 microtribometer (CETR, Campbell, CA, USA), respectively. The former tests could evaluate the load carrying capacity of the lubricant, while the latter tests were used to study the comprehensive friction and lubrication properties of materials. In a four-ball test, four AISI 52100 bearing steels with a diameter of 12.7 mm were used as friction pairs. The CQDs decorated 2D nanosheets aqueous dispersions were filled over the lower three balls to act as lubricant. For the loading test, three different normal loads (100, 200, and 300 N) were applied to the upper steel ball which rotated at 1,000 rpm against the three lower stationary balls for 300 s. For the speed test, the rotation speed of the upper ball was controlled sequentially to be 50, 100, 250, 500, 1000, 1500, and 2000 rpm, respectively under a load of 100 N. The test was run for 60 s at each speed. All the friction tests were conducted under ambient conditions. In a ball-on-disk test, both the ball and disk were also made of AISI 52100 bearing steels. The schematic of the microtribometer is shown in the inset of Figure 8a. The upper stationary ball was attached to the upper holder fixed to a 2D force sensor which could measure the rotating friction force and the normal force. The lower rotating disk was attached to a horizontal plate with 3M double-sided tape. The lubricant was supplied continuously using a pipet. The steel ball with a diameter of 10 mm was pressed onto the substrate under a load of 5 N, to generate a maximum contact pressure of 800 MPa based on Hertz contact theory. Rotation speeds of 500 and 700 rpm were selected to generate different linear speeds for the track radii of 5 and 6 mm. Before testing, 30 min of accelerated presliding under a force of 20 N at 300 rpm in bare CQDs aqueous dispersion for the track radius of 3 mm was needed to generate a small platform at the top of the ball.

cell imaging, photocatalysis, chemical sensing, light-emitting diodes (LEDs), and storage devices.33−35 While the lubrication properties of CQDs based nanocomposites have already been investigated,36 CQDs based aqueous lubricant additives and their tribological performances are still waiting to be explored. The good lubrication and friction properties of 2D nanosheets, such as graphene oxide (GO) and h-BN, as lubrication additives in water have been revealed and ascribed to the formation of a thin tribofilm on worn surfaces by the repeated exfoliation and deposition of 2D nanosheets to friction surfaces during sliding.37,38 In addition, nanoparticles as additives in water, can either act as micro rolling bearings to reduce friction in the interface, or enter the friction interface to fill and/or repair the microdimples and worn zones on the substrates.39 Furthermore, experiments demonstrated that the combination of nanosheets and nanoparticles in water showed a better lubrication performance. Herein, we demonstrate a scalable method of preparing aqueous dispersions containing 2D nanosheets (h-BN, MoS2, MoSe2, WS2, and graphene) decorated by CQDs as liquid lubrication additives. The stable aqueous dispersions were obtained via physical peeling (π−π stacking between 2D sheets and CQDs) as well as mechanical exfoliation (by using ultrasonic waves). Excellent lubricating properties were observed with the aqueous dispersions of these CQDs decorated 2D nanosheets (h-BN, MoS2, MoSe2, WS2, and graphene).

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EXPERIMENTAL SECTION

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Synthesis of Aqueous Dispersions with 2D Nanosheets Decorated by CQDs. CQDs were prepared following the method in a published work.27 As shown in Supporting Information Scheme S1a, the CQDs were prepared using the following procedure: citric acid (10 g) and urea (10 g) were dissolved in ultrapure (UP) water (35 mL) with ultrasonication to afford a transparent solution. A microwave oven was preheated at 800 W for 5 min; the solution was then irradiated by microwave for 6 min. The transparent solution was changed into a black solid. The obtained solid was quickly re-dissolved in UP water using a sonifier (c. 600 W, 30 min). The homogeneous black suspension was filtered through a cellulose filtration membrane (0.22 μm pores) and further purified with UP water in a dialysis bag (1000 Da) for 5 days. Finally, the resulting black aqueous suspension was lyophilized to obtain CQDs powders. As for the preparation of 2D nanosheets decorated by CQDs (Scheme S1b), the resulting CQDs were first re-dispersed in 200 mL of UP water. A 0.05 g amount of bulk powder (h-BN, MoS2, MoSe2, WS2, or graphite) was then added to the CQDs aqueous dispersion, respectively. The resulting mixture was ultrasonically processed at 60 °C for 60 h. The operating power was 80 W, and the frequency was set at 40 kHz. The layered materials exfoliated by CQDs based dispersions were precipitated by centrifugation at 600 relative centrifugal force; the precipitates were weighed (Table S2). Finally, the upper dispersions were stored at 60 °C and naturally cooled to room temperature before lubricating measurements. For comparison, bare 2D nanosheets (h-BN, MoS2, MoSe2, WS2, and graphene) based aqueous dispersions were prepared with the same procedure but without using of CQDs. The production yields of these precipitates were recorded (Table S3). Structural and Morphological Characterization. The morphology of the samples was investigated using a scanning electron microscope (SEM, JEOL JSM-6700F) and high-resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F20 S-TWIN). Atomic force microscopy (AFM, Dimension Icon, Bruker) images were characterized to determine the thickness of the exfoliated nanosheets. Fourier transform infrared spectroscopic (FT-IR) signals were performed on a Nicolet Nexus FT-IR spectrometer. The Raman spectra (using a 488 nm laser, RENISHAW RM2000 Raman System) and X-ray photoelectron spectroscopy (XPS, Escalab 250XI) analysis

RESULTS AND DISCUSSION The generation of 2D materials decorated by CQDs is illustrated in Scheme 1. First, the CQDs were synthesized Scheme 1. Schematic of the Generation of 2D Nanosheets Decorated by CQDs

from urea and citric acid under microwave heating, and then, the layered materials were exfoliated and dispersed in UP water by CQDs under ultrasonication. The experimental material feeds are shown in Table S1. The as-prepared aqueous dispersions composed of CQDs and different 2D nanosheets of h-BN, MoS2, MoSe2, WS2, and graphene were designated as follows: h-BN/CQDs, MoS2/CQDs, MoSe2/CQDs, WS2/ CQDs, and graphene/CQDs, respectively. The characterization of the synthesized CQDs is illustrated in Figure S1. It can be seen that the synthesized CQDs were uniformly spherical with an average diameter of 3 nm, the lattice spacing was approximately 0.271 nm (Figure S1a), which is consistent with other reported values.27 FT-IR (Figure S1b) revealed the functional groups on CQDs.36 The shear viscosity of CQDs aqueous dispersion (0.1498 wt % CQDs) was 0.9789 B


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AFM was used to analyze the thicknesses of the 2D nanosheets decorated by CQDs. As shown in Figure 2a,f, the hBN/CQDs exhibited a thickness of approximately 0.68 nm. Similarly, the thicknesses of the MoS2/CQDs (Figure 2b,g), MoSe2/CQDs (Figure 2c,h), WS2/CQDs (Figure 2d,i), and graphene/CQDs (Figure 2e,j) were found to be approximately 1.05, 2.6, 1.0, and 1.1 nm, respectively. The obtained 2D nanosheets/CQDs were very thin, indicating the successful exfoliation of layered materials by CQDs. The thickness distribution of MoSe2/CQDs was counted from the AFM images. As shown in Figure S5 the average thickness for MoSe2/CQDs was approximately 1−3 nm, which corresponds to one to five layers.22 The 2D nanosheets/CQDs possess curled edges, rendering it possible to reveal their sheet thickness using HRTEM as shown in Figure S6. The lattice distance of h-BN/CQDs (Figure S6a) was about 0.33 nm, close to the (002) crystal planes of h-BN. As shown in Figure S6b, the interplanar spacing of 0.62 nm could be attributed to the (002) plane of MoS2 nanosheets. The clear lattice fringes of MoSe2/CQDs were separated by 0.64 nm, corresponding to the (002) plane of the hexagonal MoSe2 phase (Figure S6c). The lattice spacing with d-spacing of 0.34 nm in graphene/CQDs could be assigned to the atomically flat thickness of graphene. The HRTEM images revealed the thicknesses of these 2D nanosheets/CQDs, which agreed well with the results from AFM. The hexagonal atomic structures of 2D nanosheets could be simulated from the inverse fast Fourier transform (FFT). As shown in Figure S6, the clear FFT of these 2D nanosheets/CQDs indicated that the ordered structures of the few-layer nanosheets were wellreserved after exfoliation. Raman spectroscopy was used to investigate the structural changes of the as-obtained samples. As shown in Figure 3, the Raman spectra of bare CQDs and 2D nanosheets decorated by CQDs exhibited two characteristic peaks of a D-band centered at 1357 cm−1 and a G-band at 1574 cm−1, revealing the successful attachment of CQDs.27 It is worth mentioning that there was no 2D band at 2700−2900 cm−1, indicating that the obtained CQDs were nanosized spheres with multilayered structures, leading to good lubrication properties.27,36 The h-BN/CQDs (Figure 3b) exhibited a peak at around 1360 cm−1, which could be attributed to the typical E2g mode vibration of h-BN, differing from the value for cubic BN (1305 cm−1). The B−N vibration mode is analogous to the G-band of graphene.24 For theMoS2/CQDs (Figure 3c), two characteristic

mPa·s, which was higher than that of water (0.8949 mPa·s) at 25 °C (Figure S1c). In the ultraviolet−visible (UV−vis) adsorption spectrum of CQDs, two peaks at 338 and 372 nm appeared, representing the aromatic π system (Figure S1d).41 As shown in Figure S2a, the CQDs exhibited a relatively narrow size distribution and most of the CQDs have an average diameter of about 3 nm. The more detailed information for the CQDs can be found in the Supporting Information. The morphologies of the samples were characterized by SEM, TEM, and HRTEM. Typical SEM and TEM images of the as-received powders (h-BN, MoS2, WS2, MoSe2, and graphite) are shown in Figures S3 and S4, respectively. All of these powders had a lamellar structure with the thickness of several layers stacked together. When these powders were exfoliated in the CQD aqueous solutions, the CQDs were uniformly interspersed on these exfoliated monolayered or few-layered 2D nanosheets (Figure 1), evincing the successful exfoliation of these lamellar bulk

Figure 1. HRTEM images of (a) h-BN/CQDs, (b) MoS2/CQDs, (c) MoSe2/CQDs, (d) WS2/CQDs, and (e) graphene/CQDs.

materials. From the HRTEM images of h-BN/CQDs shown in Figure 1a, we could barely identify the single layer of h-BN due to its poor optical contrast against the flakes.28 It was worth noting that the CQDs were more densely distributed on graphene nanosheets as shown in Figure 1e, indicating that CQDs with an sp2 carbon structure can bind strongly to the basal plane of graphene sheets via π−π stacking interactions.

Figure 2. AFM images and the corresponding height profile of (a, f) h-BN/CQDs, (b, g) MoS2/CQDs, (c, h) MoSe2/CQDs, (d, i) WS2/CQDs, and (e, j) graphene/CQDs. C


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Figure 3. Raman spectra of (a) CQDs, (b) h-BN/CQDs, (c) MoS2/CQDs, (d) MoSe2/CQDs, (e) WS2/CQDs, and, (f) grapheme/CQDs. Insets: magnified spectra at the corresponding wavenumbers.

Figure 4. XPS survey spectra of (a) h-BN/CQDs, (b) MoS2/CQDs, (c) MoSe2/CQDs, and (d) WS2/CQDs. Insets: narrow XPS spectra with fitted curves.

peaks at 377 and 402 cm−1 were observed, corresponding to the E12g and A1g modes of vibration, respectively, indicating the presence of exfoliated hexagonal MoS2 nanostructuress.23,42,43 In the Raman spectrum of MoSe2/CQDs (Figure 3d), a characteristic A1g active mode of MoSe2 was found at 239 cm−1,44 confirming the successful dispersion of MoSe2. As shown in Figure 3e, the vibration modes A1g and E12g located at 417 and 350 cm−1 were assignable to the out-of-plane and inplane of W−S phonon modes, respectively.2 In general, the

blue shift of A1g mode represented the increase in the number of layers. Compared with the value of the A1g mode at 420 cm−1 recorded in the literature, this WS2/CQDs showed a red-shifted A1g peak at 417 cm−1, which was consistent with the value reported for WS2 monolayers.45 For the Raman spectrum of graphene/CQDs (Figure 3f), the D-band and G-band were slightly blue-shifted to 1372 and 1596 cm−1, respectively. The reduced ID:IG ratio proved the decrease of defection after the introduction of graphene nanosheets. D


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Figure 5. XRD patterns of (a) CQDs, (b) h-BN/CQDs, (c) MoS2/CQDs, (d) MoSe2/CQDs, (e) WS2/CQDs, and (f) graphene/CQDs.

bulk MoS2 (Figure S7b) matched the characteristic peaks of hexagonal 2H-MoS2 (JCPDS No. 65-1951), which showed a strongest peak at a 2θ value of 14.5° with an interlayer distance of 0.61 nm.50 The XRD pattern of MoSe2/CQDs exhibited a peak located near 13.67°, corresponding to the (002) plane of the hexagonal structure of MoSe2.44 Furthermore, the XRD pattern of as-received MoSe2 powder (Figure S7c) confirmed the identity of the standard values of MoSe2 materials (JCPDS No. 65-3481). The diffraction peaks of WS2/CQDs and WS2 were indexed perfectly to the reflection of the hexagonal 2HWS2 phase with no discernible impurity according to JCPDS No. 08-0237.51 A significant difference was that the presence of a strong, sharp diffraction peak located at 14.95° of bulk WS2 (Figure S7d) along with the (002) reflection of WS2 layers was much reduced and shifted to a lower angle (approximately 14.31°) in WS2/CQDs (Figure 5e). The sharp peak of bulk graphite (JCPDS No. 41-1487) at 26.76° shown in Figure S7e became broad, and the value of the peak shifted to a lower angle of about 23.98° in graphene/CQDs (Figure 5f). The XRD pattern of the bare silicon substrate is shown in Figure S7f as a reference. In general, the thicknesses of the nanosheets can be calculated by the broadness of the diffraction peak (002) based on the Scherrer equation, τ = Kλ/β/(cos θ), where τ is the nanoscale thickness, the parameter K is a constant, λ is the wavelength of the X-ray, β is the full width at half-maximum (fwhm), and θ is the Bragg angle.52 Compared to the pristine bulk powders, the broader fwhm of peak (002) and reduced 2θ values for the 2D nanosheets decorated by CQDs indicated the presence of much thinner nanosheets and the increase of interlayer distance after exfoliation under ultrasonication. After sonication, the 2D nanosheets/CQDs dispersions were precipitated via centrifugation. The concentrations and production yields of the exfoliated 2D nanosheets were also summarized in Table S2. It can be observed that the production yields are 20, 62, ∼100, 76, and 85 wt % for graphene nanosheets, MoS2 nanosheets,MoSe2 nanosheets, h-BN nanosheets, and WS2 nanosheets, respectively. It is worth noting that almost all the MoSe2 was exfoliated by CQDs to form MoSe2/ CQDs. However, the exfoliation of MoS2, WS2 and graphite were not complete, where the exfoliation of graphite left the most sediment. This could be ascribed to the following two

The XPS analyses were carried out to investigate the elemental composition and chemical bonds of the as-prepared samples. The XPS survey spectra of these samples revealed the existence of carbon (C), oxygen (O), and typical elements of 2D nanosheets, consistent with the EDS results shown in Table S4. As shown in Figure 4, a predominant peak of C 1s (281.6 eV), and an O 1s peak (528.1 eV) were recognized, confirming the existence of CQDs in these as-prepared samples.40 The N 1s peak (399.6 eV) might originate from the N-doped CQDs. The peak at 187.2 eV in the h-BN/CQDs was attributed to B 1s (Figure 4a). As shown in Figure 4b, the XPS spectrum of MoS2/CQDs presented two obvious peaks at 228.9 and 232.4 eV, which could be assigned to Mo4+ 3d5/2 and Mo4+ 3d3/2 binding energies of MoS2, respectively.46 The peaks at 165.1 eV can be attributed to the S 2p band. In the MoSe2/CQDs (Figure 4c), the peaks located at about 225.2 (Mo4+ 3d5/2) and 228.9 eV (Mo4+ 3d3/2) of Mo 3d were observed. The binding energy of Se 3d was located at 51.4 eV.44 As shown in Figure 4d, the peaks at 32.5 and 34.7 eV were typical values for W 4f7/2 and W 4f5/2, respectively. The peaks with binding energies of 162.1 and 163.4 eV can be assigned to S 2p1/2 and S 2p3/2, respectively.47 The crystal structure of these samples was investigated by using XRD patterns. The XRD patterns of the 2D nanosheets decorated by CQDs are shown in Figure 5, while the XRD peaks of as-received bulk powders and bare silicon substrate are shown in Figure S7. As shown in Figure 5a, the CQDs showed a broad peak centered at approximately 24.85°, corresponding to the highly amorphous carbon.42 All of these 2D materials decorated by CQDs showed a peak at about 23° to 24°, confirming the presence of CQDs. For the orange trace of hBN/CQDs, the diffraction peaks at 26.62° and 42.25° were observed, arising from the few-layered h-BN.48 Compared with the sharp peak (27.06°) of starting h-BN (JCPDS No. 34-0421) in Figure S7a, the peak of h-BN/CQDs showed a remarkably broadened width with dramatically reduced intensity, indicating the much less extended/ordered stacking of h-BN sheets.48,49 MoS2/CQDs exhibited distinct diffraction peaks at about 14.27°, and 32.94°, which were associated with the (002), (100) facets of MoS2, respectively, revealing the presence of MoS2 in the as-prepared MoS2/CQDs. The XRD pattern of E


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Figure 6. Lubricating properties of (a) CQDs, (b) h-BN/CQDs, (c) MoS2/CQDs, (d) MoSe2/CQDs, (e) WS2/CQDs, and (f) graphene/CQDs based aqueous dispersions under loads of 100, 200, and 300 N and a rate of rotation of 1,000 rpm based on the standard four-ball test.

the preparation of h-BN/CQDs, MoS2/CQDs, and WS2/ CQDs were investigated. As shown in Figure S12a−c, two obvious characteristic peaks observed at 399.6 and 187.2 eV can be assigned to N 1s and B 1s from BN, respectively, revealing the dominant presence of BN in the precipitate of h-BN/ CQDs. Similar to the XPS result for the precipitate of h-BN/ CQDs, strong peaks of MoS2 and WS2 were observed in the precipitate of MoS2/CQDs and WS2/CQDs (Figure S12d−i), evidencing the dominant composition of the precipitates. To discuss the role of CQDs in the exfoliation of layered materials, a set of control experiments with the same experimental procedures in the absence of CQDs were carried out. The precipitates were collected, and their weights were summarized in Table S3. Compared with the results in Table S2, the production yield of 2D nanosheets produced without the addition of CQDs was significantly decreased. The TEM images of these 2D nanosheets were collected as shown in Figure S13. It can be seen that these 2D nanosheets have been cut into small pieces, whereas the nanosheets were still very thick. As confirmed from the systematic experimental results, we can conclude that a long time of ultrasonication can only cut large sheets into small pieces. However, CQDs play a critical role to exfoliate layered materials into thin layers and prevent their restacking in water, generating homogeneous aqueous solutions. The exfoliation mechanism is described as follows: the hydrophilic CQDs attached on the 2D nanosheets enhance the affinity of 2D nanosheets with water as well as reduce the interactions between the adjacent layers of 2D nanosheets, thus readily pulling the 2D nanosheets into water.27 The XRD patterns of the precipitates after ultrasonication were measured. As shown in Figure S14, sharp characteristic peaks were observed, confirming that these precipitates were layered bulk materials that were not completely exfoliated. To assess the stability of the dispersions, we measured the ζ potential values of these aqueous dispersions as shown in Figure S15. In general, the value of ζ otential in the range from −40 to +40 mV is a typical feature of stable suspensions or colloids.28,57 As shown in Figure S15, all of these samples exhibited ζ potential values near −40 mV, confirming their good dispersion stability. The coefficient of friction (COF) of these aqueous dispersions was investigated under different loads based on a

reasons: (i) Graphene is a highly conjugated material and readily undergoes a restacking process to aggregate via π−π stacking interaction; (ii) the interlayer spacing of MoSe2 is 3.300 Å which is larger than that of MoS2 (3.165 Å) and WS2 (3.163 Å)53, while the band gap for MoSe2 (1.56 eV) is much lower than that for MoS2 (1.81 eV) and WS2 (1.97 eV).54,55 The total density of state (DOS) can be carried out to study the band structures. Compared with the value of MoS2, the narrower band gap of MoSe2 calculated from DOS indicated the better chemical activity56 and the valency charge density distribution on the surfaces of the 2D nanosheets might affect the interlayer energy. Therefore, the exfoliation of MoSe2 nanosheets is much easier than those for graphite, MoS2, WS2, etc. The SEM and TEM images of the precipitates collected from the preparation of 2D naonosheets/CQDs were obtained (Figure S8 and Figure S9). Similar to the parent layered materials (Figure S3 and Figure S4), the precipitates exhibited aggregated layered structure. FT-IR spectroscopy was used to identify the functional groups of the samples. The pristine hBN powder (Figure S10a) displayed two sharp absorption peaks at 1384 and 801 cm−1, which were derived from the B−N stretching vibration and B−N−B bending vibration, respectively. Both the h-BN/CQDs samples from the precipitate (Figure S10b) and from the suspension (Figure S10c) showed two absorption peaks at 1384 and 801 cm−1, confirming the presence of h-BN.19,20 Compared with the pristine h-BN, the relative peak intensity of h-BN/CQDs was decreased, which could be due to the smaller thickness of h-BN/CQDs. The typical peaks of CQDs at 1626 and 1706 cm−1 assigning to δ(CC) and ν(CO) were also found in h-BN/CQDs, revealing the existence of CQDs. In addition, a weak peak at 1706 cm−1 was also observed with precipitate of h-BN/CQDs, evidencing the less attachment of CQDs. As shown in Figure S11, the Raman spectrum of precipitate collected from the preparation of MoS2/CQDs was investigated. Differing from the the Raman spectrum shown in Figure 3c, the peaks of MoS2 (378 and 405 cm−1) with high intensity were clearly observed while peaks of CQDs (1358 and 1578 cm−1) were extremely weak, confirming that the precipitate collected from the preparation of MoS2/CQDs dispersion was an MoS2 bulk precursor. XPS measurements of the precipitates collected from F


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ACS Applied Materials & Interfaces standard four-ball test. As shown in Figure 6, all the samples exhibited a low COF under a load of 100 N. It was obvious that the COF decreased quickly and remained stable after the addition of 2D nanosheets even under a high load of 200 or 300 N. The smooth curves of h-BN/CQDs and graphene/ CQDs based aqueous dispersions were observed, demonstrating their excellent and stable lubricating behavior. As a contrast, the COF of UP water exhibited a rising trend even under a low load of 50 N (Figure S16). It was difficult to further increase the load due to the deteriorated friction conditions. Figure S17 shows the friction coefficient as a function of load and speed of 2D nanosheets decorated by CQD in aqueous dispersion. As described in Figure S17a, as the load increased, the COF of CQDs in aqueous dispersion increased, which agrees with the results shown in Figure 6. The addition of 2D nanosheets improved the lubricating properties, especially at high load. The WS 2/CQDs and h-BN/CQD aqueous dispersions exhibited a low COF of about 0.1 under a load of 200 N. Compared with bare CQDs aqueous dispersion, these 2D nanosheets decorated by CQD aqueous dispersions exhibited similar COF under low speeds as shown in Figure S17b. When the speed reached 1500 rpm, the COF of the CQDs increased sharply, in agreement with the results shown in Figure S17a. Under the same operating conditions, a lower COF and a larger load carrying capacity mean a better lubrication property of the lubricant. Moreover, since the viscosities of all the aqueous dispersions were almost identical and very close to the viscosity of ultrapure water, the substantial increased load carrying capacity of the 2D nanosheets decorated by CQD aqueous dispersions should be attributed to the addition of the 2D nanosheets. However, the bare 2D nanosheets aqueous dispersions possess poor lubrication properties that remain to be discussed later. If the components at the contact zone could be online detected by certain optical spectroscopy, the mechanism could be possibly verified. But due to the great difficulty to carry out a real online measurement, we could not achieve it at the present stage. However, in general, the lubricating mechanism for nanomaterials has been proposed by many teams. When the two opposite peaks contact and separate, the nanoparticles could be assembled on the rubbing surface to prevent their direct contact. For example, Liu et al. have demonstrated that the interfacial physisorption and hydration lubrication leads to the good lubricating performances of microgels.58 Wang et al. have reported that the nanoadditives can form a lubricating film on the sliding surfaces when asperity peaks meet and separate.59 When applied as lubricant oil additives, graphene sheets can form a protective layer on the surface of each steel ball, introducing the enhanced antiwear performance.60 Based on this mechanism, we proposed a model as illustrated in Figure 7. The CQDs tend to attach on the rubbing surfaces when asperities meet and separate, forming a lubricating film (Figure 7a). As load increased, CQDs could be extruded out of the contact area, resulting in significantly reduced lubricating property. While for the 2D nanosheets decorated by CQDs, the nanosheets assembled on the rubbing surface are difficult to be fully squeezed out of the contact area due to their relatively large surface area. In addition, the CQDs might also play a critical role as a wheel to help the easy sliding under external load, resulting in synergetic lubricating performances as shown in Figure 7b. However, an experimental online verification of the proposed mechanism is still a challenge at the present stage, which will be pursued in our future work.

Figure 7. Proposed lubrication model for (a) CQDs and (b) 2D nanosheets decorated by CQD based aqueous dispersions.

Figure 8 shows the friction coefficient curves of self-mated AISI 52100 bearing steels in MoS2/CQDs and MoSe2/CQD based aqueous dispersions at different speeds under a load of 5 N based on a ball-on-disk apparatus. At a speed of 0.183 m/s for MoS2/CQDs and 0.157 m/s for MoSe2/CQDs, the friction coefficients could be less than 0.02, especially for the former lubricant where it was less than 0.01, achieving superlubricity after a short running-in process of about 20 s. In contrast, the friction coefficients in bare h-BN and MoS2 nanosheets aqueous dispersions under 5 N and 0.183 m/s conditions were 0.325 and 0.125, respectively, as shown in Figure S18, which were much larger than that in their respective 2D nanosheets decorated by CQDs based aqueous dispersions. These results demonstrate that the synergistic effect of the CQDs and 2D nanosheets led to excellent lubrication properties (Figures 6, 8, and S17). Unlike the good properties reported elsewhere,61 the aqueous dispersions of the bare 2D nanosheets, especially h-BN, did not exhibit such good lubrication performance. To evaluate the antiwear properties, the wear scar diameter and optical microscopy images of the wear scars after each friction test were measured (Figures S19 and S20, respectively). The size of wear scars lubricated by 2D nanosheets decorated by CQDs was significantly reduced, the surface became smoother, and fewer defects were observed.

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CONCLUSION In summary, 2D nanosheets (h-BN, MoS2, MoSe2, WS2, and graphene) decorated by CQD based aqueous homogeneous dispersions were achieved from a liquid exfoliation procedure. The hydrophilic CQDs can enhance the affinity of 2D nanosheets with water and prevent the restacking of 2D nanosheets, playing a critical role as exfoliation agents and stabilizers in the liquid exfoliation process. Their excellent lubricating performances could be attributed to their dispersion stability and the synergistic effect of CQDs and 2D nanosheets owing to their easy-sliding interlayer properties. Moreover, the small size of these lubricating additives can enter the surface of the contact area and form a lubricating film, resulting in favorable wear resistance. The liquid-phase exfoliation method provides a facile and economic way to achieve large-scale, water based lubricant additives. Future work will focus on the G


Research Article


Figure 8. Friction coefficient curves of self-mated AISI 52100 bearing steels in (a) MoS2/CQDs and (b) MoSe2/CQDs based aqueous dispersions under a load of 5 N based on a ball-on-disk apparatus (ball diameter, 10 mm).

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synthesis of QDs from h-BN, MoS2, MoSe2, WS2, and graphene, etc., to decorate their respective precursors for potential lubricating applications, especially for precision instrument manufacture.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b09752. Chemicals, experimental recipe, tables of production yields of 2D nanosheets/CQDs and pure 2D nanosheets after ultrasonication, schematic for the preparation of CQDs and 2D nanosheets decorated by CQDs, characterizations of CQDs (HRTEM, FT-IR, viscosity, UV−vis, and fluorescence microscopic images), characterizations of the 2D nanosheets/CQDs (EDS, HRTEM, and statistical analysis of the thickness), characterizations of the as-received powders (SEM, TEM, and XRD), characterizations of the precipitates collected from 2D nanosheets/CQDs and pure 2D nanosheets (SEM, TEM, FT-IR, XPS, Raman spectrum, and XRD), ζ potentials, wear scar diameter, and optical microscopy images of the wear scars for the 2D nanosheets decorated by CQDs, COF of bare h-BN, and bare MoS2 aqueous dispersions as well as ultrapure (UP) water (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.L.Z.). *E-mail: [email protected] (Y.T.). *E-mail: [email protected] (J.L.). Notes

The authors declare no competing financial interest.

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REFERENCES

ACKNOWLEDGMENTS

J.L. received funding from Qingdao Basic & Applied Research project (15-9-1-100-jch). Y.T. received funding from the National Natural Science Foundation of China (Grant Nos. 51425502 and 51323006). W.L.Z. received funding from the National Natural Science Foundation of China (Grant No. 21603115), the China Postdoctoral Science Foundation (Grant No. 2016M592136), and the Tribology Science Fund of the State Key Laboratory of Tribology (Grant No. SKLTKF15A10). H


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Soluble, Exfoliated Two-Dimensional Nanosheets as Excellent

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