Full Length ArticleControllable synthesis of MoS2/graphene low-dimensional nanocomposites and their electrical properties
Graphical abstract
Introduction
Atomically thin two-dimensional (2D) materials have recently gained extensive attention because of their unique structures and intriguing physicochemical properties with potential applications [1], [2], [3], [4], [5], [6], [7]. Graphene, anatomic layer of sp2 bonded carbon atoms in a hexagonal lattice, is one of the most studied 2D materials. Many fascinating properties of graphenesuch as high electron mobility (~200 000 cm2 V−1 s−1), large specific surface area (~2600 m2 g−1) and excellent thermal conductivity (~5000 W−1 K−1) [8], [9], all of which make it a promising candidate for various applications including gas sensors [10], batteries [11], [12], supercapacitors [13], [14], fuel cells [15], photovoltaic devices [16], [17], [18], [19], [20], solar cells [21], [22], [23] and biosensors [24]. Beyond graphene, molybdenum disulphide (MoS2) is emerging as one of the most attractive 2D materials amongst the transition metal dichalcogenides (TMDs). Similar to graphene, MoS2 has a layered structure in which Mo and S atoms are covalently bonded to form 2D S–Mo–S tri-layers that stacked together by weak van der Waals interactions along the c-axisto form the bulk MoS2 crystal [4], [25]. MoS2 exhibits a thickness-dependent band gap [2] that changes from indirect band gap of ~1.3 eV for bulk MoS2 to direct band gap of ~1.9 eV in monolayer form [3], [26], exhibiting strong photoluminescence (PL) [2], high in-plane carrier mobility (~200–500 cm2 V−1 s−1) [27] and robust mechanical properties [28]. Interestingly, such indirect-to-direct gap transition due to quantum confinement results in giant enhancement in PL quantum yield [3], together with its strong interaction with light [29], [30], this has opened up the possibility of using few-layer MoS2-based materials in various practical applications such as field effect transistors, photodetectors [31], [32], light-emitting diodes [33], solar cells [34], spintronic [35] and environmental applications [36], [37].
Recently, the combination of MoS2 and graphene to fabricate MoS2/graphene hybrid structure has attracted significant interest due to their potential in combining properties of both individual components for specific applications. Importantly, these hybrid nanostructures have exhibited better performances in comparison to their single counterparts for various applications including photocatalysts [38], [39], batteries [40], [41], sensing [42], energy-harvesting [30] and memory devices [43], [44]. Such improved performance is primarily attributed to the robust hybrid structure and the synergetic effects between few-layer MoS2 and graphene sheets.
There are several routes for the synthesis of MoS2/graphene composites including ex-situ and in-situ strategies [45]. In the ex-situ synthetic strategy, each component (MoS2, graphene or GO) are prepared separately in advance, then the composites are fabricated by layer-by-layer assembly [46], liquid phase exfoliation [47], [48], [49], [50], and chemical exfoliation [51] methods. Despite many advantages include low cost and scalable production, the ex-situ synthetic strategy requires multiple complex and time-consuming steps to prepare raw component materials. Moreover, since the component material has completely formed, it is difficult to control the preparation of the composites due to its randomly dispersed and weak interactions. For the in-situ strategy, the synthetic process involves ionic reactions such as sol–gel [52], hydrothermal [53], [54] or solvothermal [55], [56] methods which hold capability of synthesis nanoscale materials with uniform dispersion and architectures compared to the ex-situ strategies. Therefore, the in-situ synthetic strategy is receiving large amount of attention, such as the preparation of GO and g-C3N4, to synthesize MoS2/GO [57] and MoS2/g-C3N4 [58] composite materials.
The crystalline structure, morphology and distribution of the nanostructures in a composite are strongly affecting the properties of the final materials [59], [60]. Therefore, the control of shape, size and morphology through the proper setting of the growth conditions plays a crucial role in controlling the functional properties. Previous studies have demonstrated that the morphology and crystallization behaviour of pure MoS2 could dramatically change with different reaction conditions [61], [62]. In hydrothermal process, MoS2/graphene composite is directly precipitated from the solution and the unique temperature-pressure interaction of the hydrothermal solution allows the preparation of different phases of MoS2 on graphene surfaces and edges. It is noted that the crystalline structure, size, and densityof MoS2 in the composites can be easily adjusted by controlling the reaction conditions. However, the morphologies of the reported composites mostly depend on their corresponding graphene substrates. The large specific surface areas of GO or graphene are beneficial to improve the dispersity of MoS2 along with other unique properties. In addition, the lattice mismatch is significant effective to the composite architectures. Graphene and MoS2 have the same hexagonal crystal structure and both are well-known quasi 2D materials, but the lattice constants of graphene (2.461 Å) [63] and MoS2 (2H-MoS2, 3.160 Å) [64] are quite different, causing a large lattice mismatch between them. For this reason, MoS2 can interact with graphene at atomic level to form different types of nanocomposite structures such as 2D-2D and 2D-3D architectures.
Despite the recent progress in hydrothermal synthesis with different combinations of synthetic parameters such as reaction temperature, time and precursor’s molar ratio, the required reaction time reported in literatures so far, is still significant long reaction time from several hours to several days [52], [65], [66] that remaining a huge challenge to develop fast simple, reliable and economical strategies for synthesis of MoS2/graphene nanocomposites.
In this report, we developed a novel hydrothermal synthesis strategy for the growth of MoS2 nanostructures on GO to fabricate MoS2/graphene nanocomposite by pre-introducing Mo4+ ions on graphene oxide surfaces through the sonication before in situ hydrothermal growth of MoS2 and simultaneously restoration of graphene from GO by adding sulphur source as a reducing agents. And following by the post-adding sulphur source (S2–) as reducing agents into hydrothermal medium, simultaneous with samples extraction technique for structural analysis that benefit for determination of suitable reaction time. To do this, we use a high pressure autoclave reactor with a specific sampling valve which enables to withdraw samples at a specific reaction time for instant analysis during the growth of MoS2/graphene composite. This strategy is not only capable of rapid evaluation of crystallization of MoS2/graphene microstructures and morphologies with acceptable quality and precise determination of suitable reaction time but also provides instant monitoring of the crystallization, microstructure, morphology of MoS2 and interlayer coupling in MoS2/graphene nanocomposite. Four MoS2/graphene composite samples with different Mo4+-to-C molar ratios, denoted as MoS2/C (1:2), MoS2/C (3:2), MoS2/C (2.5:1) and MoS2/C (3:1) were prepared, respectively. Our results show that MoS2 crystalline phase with nanopetal-like shapes can be grown on graphene surface at 230 °C within ~2 h. And we have successfully incorporated MoS2nanostructures into graphene surfaces that resulting three observed types of nanocompositearchitectures including “sandwich-like”, “layer-by-layer” and “anchored”by controlling the Mo4+-to-C precursor molar ratios. Interestingly, the as-prepared MoS2/C (3:2) sample depicts its memristive behavior through the Volt-Ampere characteristics that opened up a potential application in memory devices.
Section snippets
Preparation of GO
A detailed for the synthesis of the GO has been published elsewhere [67], [68]. In brief, a volume 180 mL of H2SO4 (98%) was added to a mixture of 5 g graphite flakes (~ 1 – 5 μm, 99.8%, Sigma Aldrich) and ~ 60 mL of H3PO4 (85%) within an ice bath media at ~ 0–5 °C. Then, 60 g of KMnO4 (98%) was added slowly to the mixture with stirring it for ~3 h at a temperature of 35 ± 5 °C. After that, ~450 mL of DI water was slowly added and stirred at 90 ± 5 °C for 5 h. Finally, ~ 800 mL of distilled
Characterizations of MoS2/graphene nanocomposites
The crystalline structure and phase components of the as-synthesized MoS2/graphene nanocomposites samples were examined by XRD. Fig. 1(a) shows XRD patterns of the as-prepared GO, pristine MoS2 and MoS2/graphene under different hydrothermal reaction time.The XRD pattern of GO shows a strong (0 0 1) peak at 2θ = ~ 10.8°, corresponding to interlayer distance (d-spacing) of ~8.18 Å(estimatedfrom Bragg equation) [74]. Such d-spacing is considerably larger than that of graphite (3.35 Å), indicating
Conclusions
In this report, we introduced a new synthesis strategy using the hydrothermal method to synthesize MoS2/graphene nanocomposites including two new points: (1) the synthetic strategy was modified by first introducing Mo4+ into the aqueous dispersion of GO before adding thioacetamide as a sulphur source, also a reducing agent (H2S gas forming under the hydrothermal condition); (2) The formation and growth of nanopetal-like MoS2 on graphene surface were analysed along the reaction process: sample
Acknowledgments
This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 104.03-2019.42; Vietnam National University Ho Chi Minh City (VNU-HCM) under grant number C2018-20-16; the Center for Innovative Materials and Architectures (INOMAR), Vietnam National University Ho Chi Minh City (VNU-HCM) under grant number NCM2019-50-01; and the HXPES measurements were performed under the approval of NIMS Synchrotron X-ray Station (Proposal No. 2017B4907).
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Le Ngoc Long and Tran Van Khai contributed equally to this work.