*Article* **Improvement of the Bias Stress Stability in 2D MoS2 and WS2 Transistors with a TiO2 Interfacial Layer**

**Woojin Park 1,**†**, Yusin Pak 2,**† **, Hye Yeon Jang 1, Jae Hyeon Nam 1, Tae Hyeon Kim 1, Seyoung Oh 1, Sung Mook Choi 3, Yonghun Kim <sup>3</sup> and Byungjin Cho 1,\***


Received: 11 July 2019; Accepted: 8 August 2019; Published: 12 August 2019

**Abstract:** The fermi-level pinning phenomenon, which occurs at the metal–semiconductor interface, not only obstructs the achievement of high-performance field effect transistors (FETs) but also results in poor long-term stability. This paper reports on the improvement in gate-bias stress stability in two-dimensional (2D) transition metal dichalcogenide (TMD) FETs with a titanium dioxide (TiO2) interfacial layer inserted between the 2D TMDs (MoS2 or WS2) and metal electrodes. Compared to the control MoS2, the device without the TiO2 layer, the TiO2 interfacial layer deposited on 2D TMDs could lead to more effective carrier modulation by simply changing the contact metal, thereby improving the performance of the Schottky-barrier-modulated FET device. The TiO2 layer could also suppress the Fermi-level pinning phenomenon usually fixed to the metal–semiconductor interface, resulting in an improvement in transistor performance. Especially, the introduction of the TiO2 layer contributed to achieving stable device performance. Threshold voltage variation of MoS2 and WS2 FETs with the TiO2 interfacial layer was ~2 V and ~3.6 V, respectively. The theoretical result of the density function theory validated that mid-gap energy states created within the bandgap of 2D MoS2 can cause a doping effect. The simple approach of introducing a thin interfacial oxide layer offers a promising way toward the implementation of high-performance 2D TMD-based logic circuits.

**Keywords:** MoS2; WS2; interfacial layer; contact resistance; bias stress stability

#### **1. Introduction**

The process of extreme scaling-down to reach a physical channel length limit of sub-100 nm has caused critical problems, such as a short channel effect and increased leakage current. To address these limitations, efforts have recently been made to scrutinize promising semiconducting materials. In particular, atomically thin layered transition metal dichalcogenides (TMDs) have attracted great attention due to their extraordinary electrical, optical, and mechanical properties [1–9]. One of their most attractive properties is the existence of a band-gap and its facile engineering. For instance, single-layer molybdenum disulfide (MoS2) has a direct band-gap of ~1.8 eV, and multilayer MoS2 has an indirect band-gap of ~1.2 eV [1]. The physical properties of 2D TMDs have led to their applications in various electronic devices such as transistors, memory devices, and opto-electronic devices [10–18]. Among them, the most promising device is the field caused effect transistor (FET), which functions as an essential switching component of display back-plane circuits [12].

However, a few challenging issues around employing 2D TMD-based FETs for practical applications have to be resolved. Fabricating large-scale, high-quality continuous 2D TMD films and the direct deposition of the gate dielectric layer on a 2D surface with a low surface energy are important issues in terms of the utilization of conventional Si fabrication infrastructures and the realization of high-mobility FETs. Furthermore, the unreliable performance of 2D TMD FETs has been a critical concern that must be preferentially addressed. Chemically and mechanically disordered surface and interface states are the origin of the performance instability of semiconductor devices, causing a large hysteresis window and a significant threshold voltage (VTH) shift.

The passivation of the polymer layer on the 2D TMDs is an efficient countermeasure against the instability of 2D semiconductor-based FET performance [19,20]. Using a similar method, Zheng et al. reported that the hysteresis window of the 2D layered materials capped with an Al2O3 was considerably reduced [20]. Meanwhile, the contact engineering strategy for modifying the interface states between a metal and a 2D semiconductor has been actively studied [21–28]. Because the operation of the 2D TMD FET is based on a modulation of the Schottky-barrier, the interface quality at the metal/TMD contact becomes more critical. Several approaches to reduce the contact resistance, including a doping technique and selection of proper work function metal, have been proposed [26,28]. Meanwhile, Fermi-level pinning usually occurs at a metal/semiconductor contact region, causing high contact resistance due to a fixed high band offset regardless of the work function value of the metal [25,26,29]. Because the interface states usually serve as carrier trapping sites, it is hard to realize the high performance of 2D TMD FETs. Thus, a reliable and simple approach for Fermi-level depinning is necessary. The corresponding result was reported for an MoS2 device with an interfacial oxide layer [29].

Herein, the effect of the interfacial buffer layer at the metal/2D TMD (MoS2 and WS2) contact on transistor performance was experimentally and theoretically investigated. Titanium dioxide (TiO2) was used as a buffer layer because its band offset with MoS2 and WS2 is relatively small and tunnel resistance can be minimized with the thin TiO2 layer. By employing a TiO2 interlayer, interface states were successfully reduced, achieving an increased drive current and the enhancement of long term bias stress stability. In addition, the role of the TiO2 layer on MoS2 was theoretically elucidated using a density function theory (DFT) simulation. It can be highlighted that we suggested a facile approach to achieve both higher transistor performance and stability at the same time.

#### **2. Materials and Methods**

A mechanical exfoliation method using scotch tape to obtain high-quality 2D TMD flakes was adopted, and then the exfoliated 2D TMD flakes (MoS2 and WS2) were transferred onto a SiO2 (300 nm)/heavily doped Si substrate. To identify the existence of the 2D TMDs, MoS2 was mechanically exfoliated from the bulk mineral, and the multilayer MoS2 was characterized using Raman spectroscopy (Figure 1a). LabRAM ARAMIS (laser wavelength: 473 nm, 50 mW) was used for Raman measurements. Two prominent peaks feature the in-plane E<sup>1</sup> 2g mode (~384 cm−1) and the out-of-plane A1g mode (~409 cm<sup>−</sup>1) of the MoS2. A frequency difference of ~25 cm−<sup>1</sup> between two vibrational modes indicates a multilayer MoS2. To determine the thickness of the exfoliated MoS2, we performed an atomic force microscopy (AFM) analysis. As shown in Figure 1b, the 92 nm-thick MoS2 was transferred onto the SiO2/Si substrate using a typical scotch-tape exfoliation method.

To investigate the effect of the TiO2 interlayer on the device's contact properties, 2D FET devices with back gate electrodes were fabricated: a control device without TiO2 and a testing device with TiO2. Figure 1c shows the 3D schematic image of the FET device with the 2D TMD-TiO2-Ti/Au structure. The TiO2 interfacial layer on the 2D TMDs was deposited using an atomic layer deposition (ALD) technique based on a tetrakis-dimethyl-amido-titanium (TDMAT) precursor at 200 ◦C. The pulse and purging times were 0.2 s and 20 s, respectively. The number of cycles were 15, resulting in a 2~3 nm thickness. The thickness of the TiO2 layer was also optimized to avoid high tunnel resistance. The 2D TMD transistor devices were made by a conventional photolithography process. Photolithography was

conducted after spin-coating of the photoresist (AZ 5214, MicroChemicals, Germany), and the metal was deposited by a physical vapor evaporator. Electron beam evaporation was selected to minimize the physical damage on the surface of the TMDs. Lift-off processes were sequentially performed to make the source and drain electrodes. The channel distance between source and drain was ~3 μm. After device fabrication, the post-annealing process was conducted in a vacuum environment at 300 ◦C. The process of the vacuum annealing step included a 30 min ramping time to 300 ◦C, for a 1 h duration, and a cool down at room temperature. The electrical characterization (transfer, output, and stress measurement) was performed with a Keithley 4200-SCS (Keithley, Cleveland, OH, US). Stress measurement followed the conventional stress-measure-stress sequence for 10,000 s, which is summarized in Figure S6 of the Supplementary Materials information.

**Figure 1.** (**a**) Raman spectrum and (**b**) atomic force microscopy (AFM) analysis of a multilayer MoS2; (**c**) 3D schematic image of a transition metal dichalcogenide field effect transistor (TMD FET) device; (**d**) a high-resolution transmission electron microscopy (HRTEM) image of the MoS2-TiO2-Ti stacked structure.

Figure 1d shows a cross-sectional high-resolution transmission electron microscopy (HRTEM) image of the MoS2-TiO2-Ti stacked structure. The lattice constant of the MoS2 was measured to be ~0.65 nm along the c-plane [0001] direction in a hexagonal close-packed crystal structure. A thin (~3 nm-thick) TiO2 layer, deposited using the atomic layer deposition process, was inserted between the Ti metal and MoS2. Interestingly, the discontinuous layers of the MoS2 layers exhibited a step-like crystal structure. Thus, it is reasonably expected that randomness in the defect density for the exposed edge planes and basal planes can cause considerable deviation from the physical interface states, thereby inducing a large difference in the electrical properties of MoS2. The structural disorder of the MoS2 surface is also a strong source for Fermi-level pinning, which caused some points of the band gap to be locked (pinned) to the Fermi-level. This made the Schottky-barrier height considerably insensitive to the metal's work function. The Fermi-level pinning phenomenon, with respect to various metals (for instance, Ti, Cr, Au, and Pd), is illustrated in Figure S1 in the Supplementary Materials information. Even in the corresponding literature studies, the existence of dangling bonds in TMD has

been proven via in-depth analyses, such as scanning tunneling microscopy and inductively coupled plasma-mass spectroscopy [30–33].

#### **3. Results and Discussion**

To investigate the influence of a TiO2 interfacial layer on the MoS2 and WS2 device performance, electrical measurements were performed. Basic electrical characterizations were carried out with a Keithley 4200-SCS (Keithley, Cleveland, OH, US) analyzer. Figure 2a shows a comparison between the transfer characteristics (IDS-VBG) of the MoS2-Ti and MoS2-TiO2-Ti devices. The gate-bias sweeping ranged from −50 to 20 V at a fixed drain voltage of 0.1 V. A typical unipolar n-type behavior and a depletion mode of MoS2 transistor devices were observed. The MoS2-TiO2-Ti device with a TiO2 interfacial layer showed more enhanced performance with a higher drive on current (ION). ION values for devices without and with the TiO2 layer are 0.36 and 1.22 μA, respectively. The field effect mobility (μFE) values for MoS2-Ti and MoS2-TiO2-Ti devices were estimated to be 1.38 and 6.08 cm<sup>2</sup>/V·s, respectively. The transfer curves at variable drain voltages and output characteristic also confirmed the better performance of the testing devices with the TiO2 layer (Figure S2 in the Supplementary Materials information). The μFE values of the MoS2-TiO2-Ti device as a function of gate voltage were higher than those of the MoS2-Ti device (Figure S3 in the Supplementary Materials information).

**Figure 2.** Transfer curves (IDS-VBG) for (**a**) MoS2-Ti and MoS2-TiO2-Ti, (**b**) WS2-Ti and WS2-TiO2-Ti, (**c**) and WS2-Pd and WS2-TiO2-Pd.

A more interesting result was observed on the WS2 FETs. Figure 2b shows a comparison of the IDS-VBG transfer characteristics of the WS2-Ti and WS2-TiO2-Ti structured devices. The bi-polar behavior of the WS2-Ti structured devices was observed, which is consistent with the previous results [34]. It is highly likely that the Fermi-level of the Ti metal exists within the mid-gap of WS2. The transfer curve of the WS2-TiO2-Ti structured device showed stronger n-type unipolar behavior with a higher ION current than that of the WS2-Ti device. As shown in Figure 2c, we also characterized the WS2 devices using Pd metal electrodes with a relatively high work function of ~5.1 eV to understand the mid-gap pinning and the effects of an interfacial layer. The addition of the TiO2 layer on the WS2 caused a change from a weak bipolar to a p-type unipolar behavior. This result indicates that a high Schottky-barrier can be effectively reduced by a contact engineering approach utilizing a very thin TiO2 interfacial layer. The IDS-VBG curves of the WS2 FETs at various drain voltages are also shown in Figure S4 of the Supplementary Materials information. The performance enhancement of the 2D FET devices with the interfacial TiO2 layer is attributed to the considerable reduction in the density of the diverse interface states, resulting from the direct contact between the metal and the 2D semiconductor channel. Comparison of the proposed band diagrams between the 2D TMD-Ti and 2D TMD-TiO2-Ti devices highlights the change in the Schottky-barrier height as shown in Figure S5 in the Supplementary Materials information. In principle, the theoretical Fermi-level alignment between the metal and semiconductor, called Fermi-level depinning, also creates a more effective carrier modulation of the 2D TMD FET device.

For practical transistor applications, the electrical stability of the MoS2 based FET devices was examined under a long-term positive gate-bias stress condition, as shown in Figure 3a–d. Figure 3a,b shows the shift of the IDS-VBG curves during the long-term gate-bias stress test. The transfer I-V curve properties were monitored every logarithmic time interval (1, 10, 100, 1000, and 10,000 s) while continuously applying +10 V to the gate electrode. Schemes to illustrate the stress measurement set up environment and the data checking points are shown in Figure S6 of the Supplementary Materials information. Even if the IDS-VBG curves in all of the MoS2-Ti and MoS2-TiO2-Ti devices were slightly shifted to the positive direction, the device with the TiO2 layer showed less of a shift than that without TiO2, indicating more stable electrical properties compared to the control device without TiO2. Interestingly, in Figure 3b, the variation of IOFF values for the MoS2-TiO2-Ti stack seems more severe than that of the control MoS2-Ti device. The actual differences of the minimum and maximum IOFF values are 4.20 <sup>×</sup> 10−<sup>12</sup> A and 3.68 <sup>×</sup> 10−<sup>10</sup> A for MoS2-Ti and MoS2-TiO2-Ti, respectively. The IOFF fluctuation of all the devices was less than 1 nA, and this fluctuation was negligible in operation. Figure 3c shows a summary of the threshold voltage (VTH) change for MoS2-Ti and MoS2-TiO2-Ti stacked devices as a function of stress time, which was extracted from the raw data from Figure 3a,b. The MoS2 FET without a TiO2 layer showed a more positive VTH shift than that of the MoS2 FET with a TiO2 layer. The VTH shift for the MoS2 FET without and with a TiO2 interfacial layer was 3.1 and 1.1 V, respectively. The TiO2 layer could serve as a buffer layer to mitigate the interfacial damage from electrical stress. As shown in Figure 3d, we also compared the field-effect mobility (μFE) values for the devices without and with a TiO2 layer. The μFE was estimated by following equation:

$$\mu\_{FE} = g\_{\rm w} \frac{L}{\mathcal{W}} \frac{1}{V\_{DS}} \frac{1}{C\_{\rm ox}} \text{ and } g\_{\rm w} = \frac{\partial I\_D}{\partial V\_G}$$

where *gm* is the maximum transconductance that can be achieved from IDS-VBG, *L* is the channel length, *W* is the channel width, *VDS* is the applied drain bias, and *Cox* is the gate oxide capacitance.

**Figure 3.** Transfer curves (IDS-VBG) of MoS2 FETs (**a**) without TiO2 and (**b**) with a TiO2 layer during a 10,000 s gate-bias stress measurement at room temperature. The summary of (**c**) the ΔVTH shift and (**d**) the μFE change as function of stress time for MoS2-Ti and MoS2-TiO2-Ti.

Overall, the μFE of MoS2-TiO2-Ti device was higher than that of the MoS2-Ti device. After 10,000 s stress time, the μFE was reduced from 0.22 to 0.17 cm2/Vs for the device without a TiO2 layer and from 8.05 to 6.14 cm2/Vs for the device with a TiO2 layer. Approximately, 25% of the μFE reduction was observed for both cases.

Additionally, the stability of the contact region for the WS2-based FET devices was also determined for the effect of the interfacial TiO2 layer on bias stress stability, as shown in Figure 4a,b. As can be seen, the transfer curves of the WS2-Ti contact FET device showed bipolar behavior where both electron and hole carriers contribute to the current flow of the semiconductor channel. Overall, a lower VTH shift was observed for the FET with a TiO2 layer compared to the FET without a TiO2 layer, indicating that the introduction of the TiO2 interfacial layer on the WS2 layered film is also an effective approach for improving the contact reliability of the WS2 device, as well as the case of MoS2 device. Specifically, the VTH shifts for the WS2 FET without and with a TiO2 interfacial layer were 8 and 4.3 V, respectively (Figure 4c). As shown in Figure 4d, the change of μFE as a function of stress time was also fitted: the mobility value was almost unchanged for the control device without a TiO2 layer and from 0.41 to 0.18 cm2/Vs for the testing device with a TiO2 layer.

**Figure 4.** Transfer curves (IDS-VBG) of WS2 FETs (**a**) without TiO2 and (**b**) with a TiO2 layer during a 10,000 s gate-bias stress measurement at room temperature. The summary of (**c**) the ΔVTH shift and (**d**) the μFE change as a function of the stress time for WS2-Ti and WS2-TiO2-Ti devices.

Indeed, the WS2 FET device was more vulnerable to electrical stress than MoS2, which might be due to greater number of interface states at the metal/semiconductor contact. The metal-induced gap states indispensably exist on the metal/semiconductor interface, which induces the instability of transistor performance. Additionally, there is a quantum mechanically long distance of 2–3 Å between the metal and 2D TMD, which increases the tunneling probability of charge carriers [35]. The more stable performance of the 2D TMD devices with an insulating TiO2 layer might be understood by a mitigation of those gap states and a reduction in physical distance.

To unveil how the TiO2 layer electronically influences the MoS2 semiconductor, we explored a theoretical simulation of electronic states for free-standing MoS2 and MoS2/TiO2 materials via a density functional theory (DFT) calculation (Figure 5). The density of states (DOS) calculation result of the free standing MoS2 showed the existence of a forbidden gap (Figure 5a). Meanwhile, the TiO2/MoS2 hybrid combination featured a spin-polarized metallic behavior. The calculated DOS clearly validates that the addition of the TiO2 layer leads to the modification of the electronical band structure of the junction region, offering the benefit of a doping effect on MoS2.

**Figure 5.** Density function theory (DFT)-calculated density of states (DOS) of (**a**) MoS2 and (**b**) TiO2/MoS2.

#### **4. Conclusions**

The effect of a TiO2 interfacial layer on metal/TMD (MoS2 and WS2) contact was experimentally and theoretically studied. The advantages of a Schottky-type FET device, possibly implemented according to the value of a metal work function, were achieved in the 2D TMD devices with a TiO2 layer. Furthermore, a more enhanced and stable electrical performance for the 2D TMD FET devices with the TiO2 interfacial layer could be obtained under a gate-bias stress condition. The TiO2 interfacial layer could serve as a Fermi-level de-pinning layer, reducing the density of the interface states. Additionally, the DFT calculation validates the doping effect of the TiO2 interfacial layer on the 2D MoS2. The strategy of inserting a very thin insulating layer into the contact region will be also applied to diverse 2D TMD-based FET devices.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/9/8/1155/s1, Figure S1: Fabrication process and band diagram for Fermi-level pinning with various metals; Figure S2: Transfer and output curves for MoS2 FETs without and with a TiO2 layer; Figure S3: Field-effect mobility (μFE) for MoS2 FETs without and with a TiO2 layer; Figure S4: Transfer curves for WS2-Ti, WS2-TiO2-Ti, WS2-Pd, and WS2-TiO2-Pd structured FET devices; Figure S5: Energy band diagrams for TMDC-Ti and TMDC-TiO2-Ti stacks; Figure S6: Bias stress measurement sequence.

**Author Contributions:** W.P. designed and conducted the experiments, and H.Y.J., J.H.N., and S.O. supported the electrical measurement and analysis. T.H.K. set up the experiment system. Y.P., Y.K. and S.M.C. supported the process of experiments and the analysis of data. B.C. supported and guided the experiment and the results. B.C. conceived and advised the publication of paper.

**Funding:** This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT; Ministry of Science and ICT) (No. 2017R1C1B1005076) and Fundamental Research Program (No. PNK6300) of the Korea Institute of Materials Science (KIMS). This research was also financially supported by the Ministry of Trade, Industry and Energy (MOTIE) and Korea Institute for Advancement of Technology (KIAT) through the National Innovation Cluster R & D program (P0006704\_Development of energy saving advanced parts).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Article* **Preparation and Tribological Properties of WS2 Hexagonal Nanoplates and Nanoflowers**

#### **Xianghua Zhang 1,\*, Jiangtao Wang 2, Hongxiang Xu 1, Heng Tan <sup>1</sup> and Xia Ye <sup>1</sup>**


Received: 27 April 2019; Accepted: 27 May 2019; Published: 1 June 2019

**Abstract:** This paper presents the facile synthesis of two different morphologies ofWS2 nanomaterials— WS2 hexagonal nanoplates and nanoflowers—by a sulfurization reaction. The phases and morphology of the samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The tribological performance of the two kinds of WS2 nanomaterials as additives in paraffin oil were measured using a UMT (Universal Mechanical Tester)-2 tribotester. The results demonstrated that the friction and wear performance of paraffin oil can be greatly improved with the addition of WS2 nanomaterials, and that the morphology and content of WS2 nanomaterials have a significant effect on the tribological properties of paraffin oil. The tribological performance of lubricating oil was best when the concentration of the WS2 nanomaterial additive was 0.5 wt %. Moreover, the paraffin oil with added WS2 nanoflowers exhibited better tribological properties than paraffin oil with added WS2 hexagonal nanoplates. The superior tribological properties of WS2 nanoflowers can be attributed to their special morphology, which contributes to the formation of a uniform tribo-film during the sliding process.

**Keywords:** WS2; lubricant additives; tribological properties

#### **1. Introduction**

In recent years, the global energy crisis and environmental pollution have been serious problems. Now, the regulatory requirements for reducing energy consumption and avoiding energy losses are becoming more stringent. Because of this, reducing energy consumption and greenhouse gas emissions has become an important focus for researchers. According to recent research by Holmberg et al., the friction of engines, gearboxes, tires, auxiliary equipment, and brakes in heavy vehicles consumes 33% of fuel energy [1], friction in cars consumes 28% of fuel energy [2], and the energy consumed by internal friction in the entire paper mill accounts for 15–25% [3]. Therefore, many attempts have been made to introduce various methods to overcome friction. Lubrication is known to be one of the most effective ways to reduce friction and wear, and the antifriction effect of lubricating oil is mainly affected by the lubricant additive. Recent studies have found that some nanomaterials have good antifriction performance due to their special structure. Therefore, increasing attention is now being paid to the use of nanomaterials as lubricant additives to improve the tribological properties of lubricating oil.

In the past few years, a variety of nanomaterials have been used as lubricant additives, and their tribological properties have been extensively studied. These materials can be classified into the following categories. The first type is metallic nanoparticles, including Cu, Fe, Ni, etc. [4–6]. The second includes carbon materials such as carbon nanotubes and graphene [7–10]. The third is composed of the transition metal chalcogenides, containing MoS2, WS2, MoSe2, WSe2, etc. [11–15]. The last category comprises other nanomaterials such as oxides, fluorides, and borides [16–20]. Among these different

types of materials, transition metal chalcogenides have received great attention due to their special layered structure.

WS2, as an important member of the transition metal chalcogenide material family, has attracted great attention for its intriguing electronic, electrochemical, and electrocatalytic properties, and for its extensive applications in field-effect transistors, energy storage, catalysis, and hydrogen storage media [21–28]. In addition, WS2 is an excellent solid lubricant due to its special layered structure, which is composed of strong S–W–S covalent bonds inside the layers, and the weak van der Waals force between the layers. The easy sliding between WS2 layers under small shear forces is often regarded as an important feature of its excellent lubricity [29]. Recently, WS2 nanomaterials with different morphologies have been synthesized, and their tribological properties and antifriction mechanisms have been studied. For example, Tenne et al. [11] investigated the tribological properties of fullerene-like WS2 nanoparticles as additives in a lubricating oil under harsh conditions, and the results showed that WS2 nanoparticles play a major role in alleviating friction and wear. Wu et al. [30] synthesized hollow WS2 spheres by a solvothermal process and compared their tribological properties with commercial colloidal MoS2 as an additive in liquid paraffin. Zhang et al. [12] prepared WS2 nanorods by using a self-transformation process and investigated the tribological performance of WS2 nanorods as an additive in lubricating oil. It was found that the antiwear ability of the base oil was improved by the addition of WS2 nanorods. Hu et al. [31] studied the tribological properties of WS2 and WS2/TiO2 nanoparticles dispersed in diisooctyl sebacate and found that the two nano-additives slightly affected the friction reduction effect, but WS2/TiO2 nanoparticles were found to remarkably improve the wear resistance of diisooctyl sebacate.

All of the above studies have shown that WS2 nanomaterials with different morphologies help to improve the tribological properties of lubricating oils. However, these studies have only investigated the tribological properties of WS2 nanomaterials with a single morphology, and did not explore the antifriction properties and mechanisms of WS2 nanomaterials with a different morphology under the same working conditions. Previous studies investigating MoS2 nanomaterials have demonstrated the complex relationship between the morphology size and tribological properties of MoS2. For example, Xu et al. reported that the lubricity of the sheet-like nano-MoS2 is inferior to that of the micro-scale MoS2 in rapeseed oil [32]. However, Raboso et al. reported that the size and morphology of MoS2 did not have a significant effect on the friction and wear of the polyalphaolefin oil [33]. Therefore, it is valuable to study the tribological properties and friction reduction mechanism of WS2 nanomaterials with different morphologies.

In this study, two different morphologies of WS2 nanomaterials—WS2 hexagonal nanoplates and nanoflowers—were synthesized by a different high-temperature solid-phase reaction process. The tribological properties of the two kinds of WS2 nanomaterials as additives in the paraffin oil were also investigated.

#### **2. Materials and Methods**

#### *2.1. Reagents and Materials*

Tungsten and sulfur powders were purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). Tungsten trioxide and thiourea were obtained from the Aladdin Chemical Reagent Company (Shanghai, China). All chemical reagents were used directly without further purification.

#### *2.2. Synthesis of WS2 Hexagonal Nanoplates*

In a typical method, high-purity tungsten and sulfur powder (W:S molar ratio of 1:3, S powder excess 50%) were poured into a steel kettle, and then the powders were mechanically ground with a speed of 300 rpm (rotations per minute) in a planetary ball mill for 12 h. Then, the ball-milled mixture was transferred into a stainless-steel reactor. The reactor was tightly closed and pushed into the middle of a tube furnace. The temperature of the tube furnace was raised to 650 ◦C at a rate of 5 ◦C min−<sup>1</sup> in an atmosphere of N2 and the temperature was maintained at 650 ◦C for 2 h. Subsequently, the tube was gradually cooled to room temperature and the prepared powders were then obtained.

#### *2.3. Synthesis of WS2 Nanoflowers*

The WS2 nanoflowers were synthesized according to the previous method [34] reported by us with minor modifications. This included 10 mmol of WO3, 60 mmol of sulfur powder and 140 mmol of thiourea were ground in a mortar for 30 min. Then 3 g of the ground mixture was loaded in an alumina boat. This boat was pushed into the hot zone of the tube furnace. The furnace temperature was maintained at 850 ◦C for 1 h in N2 atmosphere and then gradually cooled to room temperature.

#### *2.4. Materials Characterization*

The X-ray diffraction (XRD) pattern was recorded by a Shimadzu LabX XRD-6000 X-ray diffractometer using a Cu Ka X-ray source operating at 40 kV and 30 mA with a scanning range of 10◦ to 80◦. A JSM-7001F field-emission scanning electron microscope (FESEM) and a JEM-2100 transmission electron microscope (TEM) were used to record the sample morphology.

#### *2.5. Tribological Properties Test*

A UMT-2 tribotester (CETR, San Jose, CA, USA) was used to measure the tribological properties of the two WS2 samples. The prepared WS2 powders were dispersed into the paraffin oil by ultrasonic dispersion for 60 min which then resulted in the required lubricating oil with different WS2 contents. The tribological properties tests were performed in ball-and-disk mode with a load of 10–60 N and a rotational speed of 100–400 rpm for 30 min. The friction pair consisted of a ball with a diameter of 10 mm and a disc with size of Φ40 mm × 3 mm. The fixed upper sample (ball) is made of GCr15 bearing steel (AISI 52100) with a hardness of 62 HRC (Rockwellhardness) and the rotating lower sample (disk) is made of 45# steel. The surface of the steel disc was polished and cleaned with acetone before the test. The friction coefficient was automatically recorded during the contact friction, and the widths of the wear scars were measured by an optical microscope. The morphologies and elements of the wear scars on the surface of the lower disc were investigated by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS).

#### **3. Results and Discussion**

#### *3.1. Structure and Morphology Characterization*

The crystal structure and phase purity of the synthesized samples were verified by the XRD patterns, as presented in Figure 1. From the image, it can be seen that the diffraction patterns of the two samples were significantly different. The diffraction peaks of the nanoplates located at 14.30◦, 28.84◦, 32.74◦, 33.50◦, 39.50◦, 43.90◦, 49.70◦, 58.40◦, 59.80◦, 60.48◦, and 66.50◦ were assigned to the (002), (004), (100), (101), (103), (006), (105), (110), (008), (112), and (114) planes of WS2, respectively. Furthermore, a high and sharp (002) peak was observed from the XRD pattern, indicating that the WS2 nanoplates were stacked together with a highly ordered packing [35]. In contrast, only (002), (100), (101), and (110) peaks could be detected in the diffraction pattern of the nanoflower sample. Besides, the intensity of the (002) peak located at 13.76◦ was significantly weakened, and its position was shifted to the left by 0.56◦ from the standard card. This indicates that the number of stacks in the (002) layer of the WS2 nanoflowers was reduced, and the layer interval became larger [36]. All the diffraction peaks of the two patterns could be indexed to the hexagonal phase (p63/mmc space group) of WS2 (JSPDS No. 08-0237). No evidence of any other phases was detected, indicating that the samples were of high purity.

**Figure 1.** X-ray diffraction (XRD) pattern of the as-synthesizedWS2 hexagonal nanoplates and nanoflowers.

The morphology and size of the two fabricated WS2 samples were identified by SEM and TEM. The SEM images of the WS2 hexagonal nanoplates are presented in Figure 2a,b. From the low-magnification SEM image (Figure 2a), it can be seen that the sample was composed of a large number of regular nanoplates with the diameter of about 0.5–1 μm. The SEM image with higher magnification in Figure 2b presents a clear view of the surface morphology of the nanoplates. These nanoplates exhibited hexagonal morphology with a thickness of 50–100 nm. Figure 2c,d displays the SEM images of the WS2 nanoflowers. Some agglomerated WS2 flower-like structures are presented in Figure 2c. It can be seen from the enlarged image (Figure 2d) that these nanoflowers were composed of some ultrathin nanosheets, and the edges of these nanosheets were obviously curled. That is because these nanosheets are unstable and tend to form a closed structure by rolling up, thereby reducing the number of dangling bonds and the total energy of the system [37].

To further reveal the morphology and microstructure of these WS2 nanomaterials, TEM measurements were performed on the samples. As shown in Figure 2e, perfectly hexagonal WS2 nanoplates with diameters of 3.5 μm were observed. The TEM image of the WS2 nanoflower is shown in Figure 2f, from which it can be seen that the WS2 nanoflowers were dispersed into ultrathin nanosheets after sonication, but the nanosheets were still connected together. In addition, the edges of the nanosheets were significantly curled, which is consistent with the SEM photographs.

**Figure 2.** *Cont*.

**Figure 2.** Scanning electron microscopy (SEM) images of WS2 hexagonal nanoplates (**a**,**b**) and nanoflowers (**c**,**d**); transmission electron microscopy (TEM) images of WS2 hexagonal nanoplates (**e**) and nanoflowers (**f**).

#### *3.2. Analysis of Tribological Properties*

The tribological properties of the two different WS2 nanomaterials as lubricant additives in paraffin oil were investigated by a UMT-2 tribotester. Figure 3a shows the effect of the nanomaterial additive concentrations on the tribological properties with a working load of 20 N at 200 rpm for 30 min. From this, it could be found that the average friction coefficient of paraffin oil with the addition of the two different nanomaterials was smaller than that of pure paraffin oil. When the content of the additives was 0.5 wt %, the friction coefficient of the paraffin oil with WS2 nanoflowers reached the lowest value, a reduction of 29.1% in comparison with pure paraffin oil, while that of paraffin oil containing WS2 nanoplates was only reduced by 24.5%. Additionally, when the concentration of the nanomaterials was higher than 0.5 wt %, the friction reducing performance was gradually weakened with the increase of the additive concentration. It can be concluded that the friction coefficient will increase when the additive content is too low or too high. The reason is that when the concentration is too low, a continuous lubricating film cannot be formed on the surface of the friction pair, and when the concentration is too high, the additive will agglomerate, which affects the friction-reducing effect [38]. Figure 3b exhibits the real-time friction coefficient curve of pure paraffin oil and the two lubricating oils with 0.5 wt % nanomaterial added. In the beginning, the three curves had the same trend, and the friction coefficient changed from large to small, which is attributable to the lack of lubricant between the friction pairs. With the embedding of the lubricant, the friction coefficient was drastically reduced. However, after 10 min, the friction coefficient of the pure paraffin oil began to increase, and the coefficient fluctuated greatly. However, the friction coefficient of the paraffin oil with added nanomaterials was very stable, and the friction coefficient of the WS2 nanoflowers was always lower than that of the nanoplates. The above experimental results indicate that the paraffin oil containing WS2 nanoflowers possessed better lubricating properties than both the pure paraffin oil and the paraffin oil containing WS2 nanoplates.

**Figure 3.** (**a**) The changes of the average friction coefficient of the WS2 nanoplates and nanoflowers with different concentration, and (**b**) the real-time friction coefficient as a function of sliding time when lubricated by three different oil samples.

In order to further compare the tribological properties of the two WS2 nanomaterials, comparative experiments were carried out with different loads and different rotating speeds. Figure 4a shows the average friction coefficient as a function of applied load when the additive concentration was 0.5 wt % and the tribotester was operated with a rotating speed of 200 rpm for 30 min. Obviously, when the applied load was increased from 10 to 40 N, the average friction coefficient had a downward trend. However, when the load was increased to 60 N, the coefficient increased slightly. The variation of the average friction coefficient of the two kinds of WS2 nanomaterials with the change of rotating speed is presented in Figure 4b. The trend of the average friction coefficient in Figure 4b is similar to that in Figure 4a, which indicates that the friction coefficient first decreased with increasing speed, and then increased. In addition, under the same conditions, the average friction coefficient of the WS2 nanoflowers was always lower than that of the WS2 nanoplates.

From the above results, it was found that WS2 nanoflowers exhibited better tribological properties, and it was found that the friction coefficient could be remarkably decreased by adding two kinds of WS2 nanomaterials into the paraffin oil.

**Figure 4.** The changes of average coefficient of friction of the WS2 nanoflowers and nanoplates with different load (**a**) and different rotating speed (**b**).

In order to compare and analyze the antiwear properties of the two kinds of WS2 nanomaterials, the wear surface of the steel disc was examined by optical microscopy and SEM. The optical micrograph and SEM images of the wear surface lubricated with pure paraffin oil, and paraffin oil with 0.5 wt % added WS2 nanoplates and nanoflowers are shown in Figure 5. The test time was 30 min and the test load was 20 N. It can be seen from Figure 5a that when the friction pair was lubricated with the pure paraffin oil, the width of the wear scar was about 453 μm. When WS2 nanoplates and nanoflowers were added into the paraffin oil, the width of the wear scar was significantly reduced to 373 μm (Figure 5c) and 340 μm (Figure 5e). Furthermore, some deep and wide furrows were observed on the wear surface shown in Figure 5a, which clearly indicates that the surface was subjected to a large contact stress during sliding. The same result was also found in the SEM image. As shown in Figure 5b, many grooves and pits were discovered on the surface of the wear scar, and some abrasive grains with different sizes were attached to it. When 0.5 wt.% of WS2 nanoplates were added into the paraffin oil, the surface topography of the wear scars was significantly improved. A dark-colored tribo-film can be observed in the optical picture (Figure 5c), but the tribo-film was unevenly distributed. In the TEM image (Figure 5d), only some very shallow grooves and some very small grinding debris can be found. Compared to the above two lubricants, the friction surface added with the WS2 nanoflower lubricant was the least damaged. A large number of dark areas (tribo-film) were observed on the track in Figure 5e and only a few very shallow grooves were seen in Figure 5f and no wear debris was found. These results verify that WS2 nanosheets and nanoflowers can improve the antiwear performance of paraffin oil, but the antiwear ability of WS2 nanoflowers is better than that of nanoplates.

**Figure 5.** Optical images and SEM micrographs of wear scars lubricated with pure paraffin oil (**a**,**b**), paraffin oil + 0.5 wt % WS2 nanoplates (**c**,**d**), and paraffin oil + WS2 nanoflowers (**e**,**f**).

In order to investigate the lubrication mechanism of the WS2 nanoplates and nanoflowers, EDS was used to investigate the worn surface. The EDS spectra obtained from the worn scar of Figure 5d,f are presented in Figure 6. The elements of W and S were present on the worn surface. This could prove that there was WS2 deposited on the worn surface during the process of friction.

**Figure 6.** Energy-dispersive X-ray spectroscopy (EDS) of the worn scar of a steel disc lubricated with 0.5 wt % WS2 nanoplates (**a**) and nanoflowers (**b**).

There is a great deal of literature regarding the antifriction and antiwear mechanisms of nanomaterials as lubricant additives, and they can be summarized into the following three reasons. The first is that the nanomaterial produces a rolling effect on the surface of the friction pair [12]. The second reason is that the nanomaterial adsorbed on the surface of the friction pair forms a lubricating film [13]. The last reason is that nanomaterials have a repair effect on the surface of the friction pair [4,6].

According to the above experimental results, we could infer the reasons for the friction reduction and antiwear properties of the WS2 nanoplates and nanoflowers. The main reason can be attributed to the formation of a tribo-film on the rubbing surface, but there was still a difference in the mechanism of the antifriction and antiwear between the WS2 nanoplate and the nanoflower at the beginning. When the WS2 nanoplates were used as lubricant additive, the WS2 nanoplates would penetrate into the interface of the friction surface. However, due to the large thickness of the nanoplates, they could not be firmly adsorbed on the surface of the friction pair. Due to the layered structure of the nanoplate, some thin nanosheets would be peeled off from the nanoplates during the continuous extrusion process by the friction pair. These stripped nanosheets would be adsorbed on the surface of the friction pair and then form a lubricating film. However, due to the different thickness of the stripped nanosheets, the resulting lubricating film was uneven. In contrast, after ultrasonic dispersion, WS2 nanoflowers were decomposed into some ultrathin nanosheets, as demonstrated by the TEM image in Figure 2f. We have researched the antifriction and antiwear mechanism of the ultrathin WS2 nanosheets as additives in 500 SN base oil [38]. The antifriction mechanism of the WS2 nanoflowers and the WS2 ultrathin nanosheets is the same. When the nanoflowers were dispersed into nanosheets, the dispersed nanosheets quickly adhered to the surface of the friction pair and formed a lubricating film, further reducing the wear on the surface of the friction pair. Since the thickness of the ultrathin nanosheets forming the nanoflowers is substantially the same, when the nanoflowers are decomposed, a tribo-film with uniform thickness is formed. The uniform tribo-film can improve tribological performance. Therefore, WS2 nanoflowers as a lubricant additive have better antifriction and antiwear properties than WS2 nanoplates.

#### **4. Conclusions**

In this study, WS2 hexagonal nanoplates and nanoflowers were successfully synthesized by a solid-phase reaction. Tribological tests demonstrated that the tribological properties of paraffin oil could be greatly improved with the addition of the two kinds of WS2 nanomaterials, and the morphology and content of the WS2 nanomaterials had a significant effect on the tribological properties of paraffin oil. The optimum nanomaterial concentration was 0.5 wt %. The paraffin oil with added WS2 nanoflowers exhibited better friction reducing and antiwear properties than the WS2 hexagonal nanoplates. With the addition of the WS2 nanoflowers, the friction coefficient was stably maintained at a low value and the wear surface appeared to be smoother. The superior tribological performance of WS2 nanoflowers can be attributed to their special structure. Since the nanoflowers are decomposed into a number of ultrathin nanosheets, and these nanosheets are adsorbed on the surface of the friction pair which forms a uniform tribo-film, this can reduce friction and wear.

**Author Contributions:** X.Z. and X.Y. designed the experiments. X.Z., J.W., and H.X. performed the experiments. X.Z. and H.T. analyzed the data. X.Z. and X.Y. wrote the manuscript. All authors read and approved the final manuscript.

**Funding:** This research was supported by the Jiangsu Province Industry-University-Research Cooperation Project (BY2018314), the Scientific Research Foundation of Jiangsu University of Technology (KYY18030) and Jiangsu Overseas Visiting Scholar Program for University Prominent Young & Middle-aged Teachers and Presidents.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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