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Understanding the intrinsic water wettability of molybdenum disulfide (MoS2) Andrew Kozbiala, Xiao Gonga, Haitao Liub and Lei Lia,c* a Department of Chemical & Petroleum Engineering, Swanson School of Engineering,
University of Pittsburgh, Pittsburgh, PA 15261, USA b Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA c Department of Mechanical Engineering & Materials Science, Swanson School of
Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA Abstract 2D semiconductors allow for unique and ultrasensitive devices to be fabricated for applications ranging from clinical diagnosis instruments to low-‐energy light emitting diodes (LEDs). Graphene has championed research in this field since it was first fabricated; however, its zero bandgap creates many challenges. Transition metal dichalcogenides (TMDCs), e.g., MoS2, have a direct bandgap which alleviates the challenge of creating a bandgap in graphene-‐based devices. Water wettability of MoS2 is critical to device fabrication/performance and MoS2 has been believed to be hydrophobic. Herein, we report that water contact angle (WCA) of freshly exfoliated MoS2 shows temporal evolution with an intrinsic WCA of 69.0±3.8° that increases to 89.0±3.1° after 1 day exposure to ambient air. ATR-‐FTIR and ellipsometry show that the fresh, intrinsically mildly hydrophilic MoS2 surface adsorbs hydrocarbons from ambient air and thus becomes hydrophobic.
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Introduction Molybdenum disulfide (MoS2) has generated significant interest in the past several years as limitations of graphene become apparent due to its zero bandgap.1-‐8 Weak interlayer van der Waals forces allow MoS2 to be easily exfoliated to form atomic layers which, like graphene, can be used in electronic and optoelectronic devices for environmental, biological, and clinical applications.1,
2, 3
MoS2 and other 2D
transition metal dichalcogenides (TMDCs) such as MoSe2, WS2, and WSe2 are semiconductors that have an intrinsic bandgap which enhances device sensitivity and allows for fabrication of unique field-‐effect transistors (FETs), biosensors, solar cells, and light-‐emitting diodes (LEDs).3, 4, 5, 6 Additionally, the atomic thinness of TMDCs allow for flexible devices not possible with traditional organic semiconductors.5, 6, 7, 8 Sarkar et al. demonstrated that MoS2-‐based FET biosensors are over 74 times more sensitive than a graphene-‐based device and can be utilized for ultrasensitive protein sensing at extremely low concentrations of 100 femtomolar.9 Moreover, Lee et al. demonstrated efficacy of a MoS2 biosensor for detection of prostate antigens in order to diagnose prostate cancer. The minimum antigen concentration detected by their MoS2-‐based biosensor was 1 pg/mL which is 4000 times more sensitive than the current clinical cut-‐off level.3 Jiang et al. created a MoS2-‐based FET with a Hg+2 detection limit of 30 pM useful for monitoring anthropogenic mercury in drinking water.10 These studies show proof-‐of-‐concept that incredible, ultrasensitive devices, which are not possible with atomically thin graphene, can be realized using TMDCs.
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Understanding wettability of surfaces is critical for fabricating ultrasensitive devices because small changes in wettability can significantly influence adhesion in heterostructures and impact overall device performance. In 1988, Kelebek reported molybdenite to be hydrophobic with a critical surface tension of 29 mJ/m2.11 Meanwhile, Zhang et al. reported the water contact angle (WCA) of sputtered MoS2 as 85°.12 More recent work has corroborated the hydrophobicity of MoS2: the WCA of bulk MoS2 was reported as 88.37° and 75.8°.3, 13 Gaur et al. showed that increasing synthesis temperature of MoS2 thin films from 550°C to 900°C allowed for controlled diffusion of sulfur atoms through the Mo film to create a well ordered surface with a high degree of crystallinity, resulting in a WCA change from 23.8° (550°C) to 91.6° (900°C) for 2D MoS2 films. The low-‐WCA surface was attributed to high-‐energy vertically aligned edge sites due to low synthesis temperature. Moreover, they reported that WCA decreases with number of MoS2 monolayers to approach that of the bulk (88.37°)13, a phenomenon which was also reported on graphene.14 Strano et al. reported MoS2 and other TMDC’s to have surface energy of 65-‐75 mJ/m2 while Gaur et al. reported surface energy of few layer MoS2 as 44.5 mJ/m2 (Neumann method) and 40.47 mJ/m2 (Fowkes method).6, 13 The discrepancy between surface energy values could be due to different methods used to calculate surface energy along with spontaneous contamination by ambient hydrocarbons.15, 16 However, MoS
2 has been believed to be hydrophobic in all the aforementioned
articles with WCA of 76-‐92°.3, 6, 11, 12, 13 Interestingly, Chow et al. recently reported on the wetting behavior of monolayer and few-‐layer MoS2 and WS2 supported on
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silica. They showed that WCA of fresh monolayer WS2 increases from 70° to 83° upon exposure to ambient air.16 Surface contamination is a serious concern for any solid surface, including atomically thin material, since contaminants can affect water wettability. A classic example is the adsorption of airborne contaminants onto gold rendering the hydrophilic surface to appear to be hydrophobic. It took the surface science community more than forty years to conclude that the observed hydrophobicity of the gold is due to airborne hydrocarbon contaminants and the gold is intrinsically hydrophilic.17, 18, 19 Graphite has been traditionally believed to be hydrophobic with WCA of ca. 90°; however, recent studies indicated that graphite is intrinsically mildly hydrophilic with a WCA of ca. 53-‐65° and it adsorbs airborne hydrocarbons in the ambient air to minimize surface energy, i.e., appear to be more hydrophobic.20, 21, 22
Similar results were also reported for monolayer graphene on copper and
multilayer graphene on nickel15, 23 and the surface energy and wettability of freshly synthesized graphene on copper was found to be dependent upon exposure time to the ambient air.23 Lai et al. attributed the change in graphene wettability to adsorption of both water molecules and hydrocarbons24 and Nioradze et al. demonstrated that HOPG electroactivity is significantly affected by organic impurities in water and air.25 Boinovich et al. also showed that hydrocarbon contaminants spontaneously adsorb onto boron nitride nanotubes (BNNTs) and render the BNNTs to be hydrophobic, suggesting that other 2D materials are susceptible to spontaneous hydrocarbon contamination.26 In light of these
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observations, there is great importance in investigating how airborne contamination affects the wettability of MoS2 due to its salience as a 2D material beyond graphene. Herein, we investigate bulk MoS2 to elucidate its intrinsic water wettability, which is the foundation of that of the mono (few)-‐layer MoS2. WCA shows temporal evolution with an “intrinsic” value of 69.0±3.8° that increases to 89.0±3.1° after 1 day exposure to ambient air. Surface energy of fresh and aged MoS2 was calculated from contact angle measurements with data indicating that surface energy is a strong function of exposure time to ambient air. Attenuated total reflectance-‐ Fourier transform infrared spectroscopy (ATR-‐FTIR) and ellipsometry indicate that hydrocarbon contaminants adsorb onto freshly exfoliated MoS2, rendering the intrinsically mildly hydrophilic surface hydrophobic. Investigating wettability of bulk MoS2, as opposed to mono (few) -‐ layer MoS2, provides valuable insight to the true material properties without interfering effects from sample synthesis, processing, and substrate interactions. Experimental MoS2 preparation Bulk MoS2 (2D Semiconductors; ~10x5x2 mm) was exfoliated with Scotch brand 1-‐ inch tape to expose a fresh surface. The tape was applied to the upper sample surface and gently pressed to remove air bubbles and ensure contact between the tape and MoS2. The tape was then gently pulled away causing the upper MoS2 layer
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to be removed, thereby revealing a fresh surface on the bulk sample. The fresh surface was used for experiments only when exfoliation was clean with no flakes and the tape had a uniform coverage of removed material. This ensured that (1) the sample was actually exfoliated exposing a fresh surface and (2) tape residue did not remain on the bulk sample. The fresh MoS2 was tested within 10 seconds to obtain results on the pristine surface. Contact angle Contact angle measurements were taken in ambient air at 22-‐25°C and 20-‐30% relative humidity. Deionized (DI) water was provided from a Millipore Academic A10 with
