Growth and Tunable Surface Wettability of Vertical MoS2 Layers for

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Growth and Tunable Surface Wettability of Vertical MoS2 layers for Improved Hydrogen Evolution Reactions Ganesh R. Bhimanapati, Trevor Hankins, Yu Lei, Rafael A. Vila, Ian Fuller, Mauricio Terrones, and Joshua A. Robinson ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05848 • Publication Date (Web): 08 Aug 2016 Downloaded from http://pubs.acs.org on August 13, 2016

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ACS Applied Materials & Interfaces

Growth and Tunable Surface Wettability of Vertical MoS2 layers for Improved Hydrogen Evolution Reactions Ganesh R Bhimanapati1, Trevor Hankins1, Yu Lei1, Rafael A. Vilá1, Ian Fuller3, Mauricio Terrones1,2, Joshua A Robinson1* 1

The Department of Material Science and Engineering, Center for 2-Dimensional Layered Materials and NSF I/UCRC Center for Atomically Thin Multifunctional Coatings, Pennsylvania State University, University Park, PA-16802;

2

Department of Physics and Chemistry, Pennsylvania State University, University Park, PA-16802;

3

Angstron Materials inc.,1240 McCook Ave., Dayton, OH, 45404.

*[email protected]

ABSTRACT: Layered materials, especially the transition metal dichalcogenides (TMDs), are of interest for a broad range of applications. Among the class of TMDs, molybdenum disulfide (MoS2) is perhaps the most studied because of its natural abundance and use in optoelectronics, energy storage and energy conversion applications. Understanding the fundamental structure-property relations is key for tailoring the enhancement in the above mentioned applications. Here, we report a controlled powder vaporization synthesis of MoS2 flower-like structures consisting of vertically grown layers of MoS2 exhibiting exposed edges. This growth is readily achievable on multiple substrates such as graphite, silicon and silicon dioxide. The resulting MoS2 flowers are highly crystalline and stoichiometric. Further observations using contact angle indicate that MoS2 flowers exhibit the highest reported contact angle of ~160± 10o, making the material super hydrophobic. This surface wettability was further tuned by changing the edge chemistry of the MoS2 flowers using an ozone etching treatment. Hydrogen evolution reaction (HER) measurements indicate that the surface treated with UV-Ozone showed a reduction in the Tafel slope from 185 mV/dec to 54 mV/dec, suggesting an increase in the amount of reactive surface to generate hydrogen.

Keywords: Transition metal dichalcogenides, MoS2, contact angle, nano flowers, HERs.

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Introduction: After the successful isolation of graphene1 in 2004 by Geim et al., there has been a rapid increase in the interest in two-dimensional (2D) materials2. These novel materials range from highly conducting graphene to insulating hexagonal boron nitride (hBN)3. Transition metal dichalcogenides (TMDs) usually abbreviated as MX2 where M is the transition metal and X is the chalcogen atom (typically S, Se, or Te), are the most commonly studied systems due to their applications in various fields4. Within an individual TMD layer, the M and X atoms are covalently bonded whereas TMD layers are held together by van der Waals forces to form bulk solids. These weak van der Waals forces are characteristic of 2D materials and makes it possible to obtain individual layers via mechanical or liquid phase exfoliation techniques4–6. The most extensively studied TMD is MoS2 because of its optoelectronic properties and natural abundance7. The band gap for MoS2 ranges from 1.2 eV in bulk to 1.8 eV for the monolayer8. Recent advances have focused more in understanding the optical, electronic and spintronic properties of planar MoS2, and less effort is devoted in understanding the structure-property relationship of MoS2. For example, the wetting behavior of MoS2 is important as properties can deteriorate in oxidative environments such as water vapor or at high temperatures9,10. The wettability is an important topic for coatings, hydrogen evolution reactions (HER)11–13, lithium ion batteries (LiB’s)14–17 and nano electronic devices. Most of these properties depend on the degree of crystallinity, chemical composition and surface topology of the material used. For catalytic applications, especially photo induced H2 evolution, the active sites and wettability are dependent on the surface energy. By reducing the surface energy it is possible to stabilize a water droplet on the surface, thus enhancing the hydrophobic nature of the surface, and vice versa is true for a hydrophilic surface. Hence, understanding the surface wettability behavior is important and necessary. Previous studies18,19 report that MoS2 is hydrophobic in nature with a maximum contact angle of ~98o for chemical vapor deposited (CVD) grown samples. However, no reports on the control of the wetting behavior of MoS2 layers have been reported hitherto. In order to completely understand the properties for these applications, properties such as adhesion, hydrophilic versus hydrophobic and its compatibility with various working environments need to be studied. In this context, contact angle measurements using water droplets would help us to understand the surface wettability and adhesion properties of MoS2 layers as a function of the edges orientation and their surface functionalization. It has been reported that MoS2 nanoflowers can also grow via hydrothermal synthesis20–24 or when using molybdenum chloride12 as a precursor material. Furthermore, heterostructure growth of these flower like structures have also been reported using graphene oxide24 as template in a hydrothermal assisted synthesis. These techniques could involve reaction times of over 24 hours (in the case of hydrothermal synthesis), in addition to the use of dangerous precursors such as molybdenum chloride, which could potentially form harmful chloride vapors. Similar to the study by Ling. L et.al.,25 we also report a safe process for growing MoS2 flowers using safe precursors such as molybdenum oxide and sulfur powders, which are less harmful while giving us an ultra large area growth along with high


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density of these MoS2 flowers. We also describe a way to tune up the hydrophobicity of the MoS2 substrates by controlling the chemical reactivity of the MoS2 exposed edges using ozone plasma.

Figure 1: (a) Schematic showing the furnace and the position of the samples inside the furnace, (b) Image showing crucible before and after growth of vertical nanoflowers, (c) SEM image showing the growth of high density flowers on graphite paper and SiO2, Inset image showing the close-up view of the structure of the vertically oriented flowers and (d) Raman spectra showing the high quality MoS2 growth.

Experimentation The growth of MoS2 flowers on high quality graphite paper and silicon substrates was performed using powder vaporization of 0.1 g of molybdenum oxide (MoO3, Sigma Aldrich, 99.95%) and 1 mg of sulfur (Sigma Aldrich, 99.5%). A schematic of the CVD system is shown in Figure 1(a). A specially designed alumina crucible was used for this process which could hold 5 samples of 1 x 1 cm2 (Figure 1(b)). The alumina crucible was placed at the center of the furnace and sulfur boat was placed outside the furnace where a separate heater was placed to control the temperature of sulfur. Several optimization experiments were performed to identify optimal conditions. The growth performed at 750-800oC and < 500 torr gave the highest flower density structures. Ideal conditions for growth were found to be at a pressure of 250 torr, a temperature of 800oC and an Argon flow of 0.15 L/min through a 2-inch tube. In addition, maintaining a high MoO3:S ratio is necessary for this type of structure growth. In this case, ~1:10 ratio proved to give the best result. This



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provided the initial seeding of the MoO2 which further converted into MoS2 upon introduction of sulfur into the system. After the growth, the substrates had a purple color, indicating the formation of these flower-like structures. Further, it can be observed in Figure 1(b) that the color change is almost uniform across the entire substrate indicating the homogeneity of the flower growth. Previous reports on the growth of nanoflowers of MoS2 were performed using MoCl5 and sulfur in a hydrogen environment. The surface coverage of the MoS2 nanoflowers were observed using FESEM. Figure 1(c) and (d) shows the growth of MoS2 nanoflowers grown on graphite paper and silicon. It can be also observed that the growth was uniform irrespective of the substrate used, thus indicating the diversity of this technique. Further confirmation of the presence of MoS2 was obtained by Raman spectroscopy. The Raman signatures for MoS2 are observed at 380 cm-1 and 406 cm-1, corresponding to the E2g and A1g vibrational modes of MoS2, respectively. Further chemical information was observed using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) (Fig. 2). Elemental maps in AES were used to identify the uniformity of the chemical composition across a large area. The AES elemental maps presented in Figures 2(a) and (b) demonstrate that the sample consists of MoS2. Further chemical analysis was obtained using x-ray photoelectron spectroscopy (XPS). As it can be observed from the survey spectrum, very little oxygen and carbon were present on the sample and the molybdenum to sulfur ratio was ~ 1:2. Further deconvolution of the high-resolution peaks of Mo 3d showed a doublet at 229.5 and 232.6 eV which corresponds to Mo 3d5/2 and Mo 3d3/2. The peak width between the peaks was ~3.135 eV which corresponds to the MoS2 formation. The peak located at 226.5 eV corresponds to S 2s in MoS2, which is further confirmed by the S 2p doublet (Figure 2d) at 162.2 and 163.4 eV. The difference between the sulfur doublets 2p3/2 and 2p1/2 is 1.2 eV, and correspond to a divalent sulfide ion (S2-). A linear background fit was used for all the XPS measurements. This peak shape and shift is comparable to CVD-grown crystalline MoS2 26.


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Figure 2: (a), (b) Auger electron spectroscopy (AES) elemental map corresponding to Mo and S elements on the sample, (c) High resolution Mo 3d spectra showing the high quality of MoS2 where the Δ between the 3d5/2 and 3d3/2 is 3.135 eV, corresponding to Mo4+ state (d) High resolution S 2p spectra with a Δ~1.2 eV corresponding to S2- in the MoS2. (e) XPS survey spectrum showing the presence of very little carbon and oxygen. In order to understand the surface wettability, static contact angle measurements were performed on the synthesized samples. Surface characteristics typically cause an impact on the free energy of any surface when it interacts with a liquid or a vapor. As reported in the literature, the contact angle varied with the growth temperature19 (550-900oC) from 23o to 98o indicating an improvement of minimization of surface free energy of MoS2 and exposure of the highly crystalline (001) planes (edges). It also varied as a function of the number of layers. Although, surface roughness did not play a major role since it was an atomically smooth and layered sample27,28. In this case, these were only a few layers thick (2-10 nm), the surface roughness over a large region for the water contact droplet is high, thus minimizing the surface free energy required to stabilize the water droplet and hence we see an improvement in the contact angle. Furthermore, this surface energy was also minimized by increasing the extent of edge sites exposed also increased the total active sites in a given surface area when compared with horizontally aligned flakes. As water is a



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polar solvent with the highest surface tension at room temperature (~72 mJ/m2), its reactivity towards the edge sites is higher. Hence, when having so many reactive sites on the surface, templating the surface with a higher surface area improved the contact angle, thus indicating the importance of the flower shaped architecture on the surface.

Figure 3: Contact angle measurement for (a) Bare graphite paper, (b) As grown MoS2 flowers on GP, (c) As grown MoS2 flowers after 2 minutes of gentle UV-Ozone treatment, (d) 10 minute UV-Ozone treatment showing a significant reduction in contact angle showing an increase in the surface free energy, (e) As grown partially covered MoS2 flowers have a contact angle less than the UV-Ozone treated one and (f) complete wettability using a lower surface tension liquid (ethanol). The typical contact angle for high purity graphite (HOPG)29 is ~90o, which is considered hydrophobic. In this case, the graphite paper used as our substrate exhibits a contact angle of ~35o (Figure 3(a)), showing the effect of impurities from the assembly process that was used. Typically, graphite paper had much higher surface energy30 and this is highly dependent on the sheet assembly process and the surface impurities on graphite. After the growth of MoS2 flowers, the contact angle varied from 35 to 156.16 ± 5o (Figure 3(b)), indicating that the surface is now super-hydrophobic after the MoS2 flower growth. This can be controlled by treating the surface with UV-Ozone treatment. A simple 2 minute treatment reduced the contact angle to 1200 whereas 10 minute treatment reduced it to 910 (Figure 3(c)&(d)). Further, the super-hydrophobic MoS2 followed a perfect case of Wensel state where the liquid droplet spreads into the porous structure beyond the drop (Figure 4(a) and Figure 4(b)), and the transition from Cassie-Baxter state to Wensel state was only a fraction of second31 which was evident from the slight wetting that observed after the water drop was dried following the measurement. As the surface energy minimization is


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directly linked to the contact angle, this higher contact angle indicates that the surface energy for the MoS2 flower system is much lower than that was reported i.e., 46 mJ/m2.

Figure 4: Wetting transition from (a) Cassie-Baxter state to (b) Wensel state for water contacting on MoS2 flowers and (c) image showing the reduction of contact angle with the addition of the S-O functional groups on the surface.

HER measurements were carried out using a 3-electrode system with a graphite rod as counter electrode, and a saturated calomel electrode (SCE) as reference electrode. The reference electrode was calibrated with respect to reversible hydrogen electrode (RHE) by using high purity H2 saturated 0.5 M H2SO4 electrolyte which is the electrolyte for HER measurement. In order to perform the calibration, a platinum wire was used as the working electrode to run the cyclic voltammetry (CV) at a scan rate of 1 mV/s, and the average of the two potentials at which the current is zero was considered as the thermodynamic potential. Herein, E (RHE) = E (SCE) + 0.274 V. For HER measurement, the linear sweep voltammetry was conducted at a scan rate of 1 mV/s using a Versa STAT 4 potentiostat. The working electrodes are the self-standing films on the graphite paper. To evaluate the inherent activity of the catalysts, the Tafel plots were linearly fitted, yielding 185, 92, 78 and 54 mV/dec for MoS2, MoS2-1 min, MoS2-5 min, and MoS2-10 min, which can be observed in figure 5(b). By increasing the ozone plasma treatment time, more surface area is exposed to the electrolyte (more defects on edges), so that proton can be efficiently absorbed onto the surface and form the H2. Moreover, the potential for 10 mA/cm2 is another common figure of merit to evaluate efficiency of the HER catalyst32. Herein, to achieve 10 mA/cm2, 364 mV is needed for MoS2-10min, which is comparable to the reported vertically aligned MoS2 layers13. However, the Tafel slope of MoS2-10 min (54 mV/dec) is much smaller than that reported for vertically aligned MoS2 layers (ca. 105 mV/dec)11–13, which indicates higher intrinsic electro-activity after the ozone treatment. Electrochemical impedance spectroscopy (EIS) was used to understand the behavior of the MoS2 and uv-ozone treated MoS2 samples. It can be observed from figure 5 (c), the electron transfer resistance of the treated MoS2 sample is much larger than that of the untreated MoS2 indicating that the electron transfer is easier in the treated sample. This 4x reduction in the resistance value greatly promotes the conductivity leading to a higher HERs performance. Further, cycling performance was performed on the 10 minute uv-



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ozone treated sample and it can be observed from figure 5(d), the performance remained unchanged after 24 hours of immersion and 1000 cycles.

Figure 5: (a) Polarization curves of MoS2 with different ozone treatment time (1 min, 5 min, and 10 min); and (b) the corresponding Tafel slopes, the inset table are the fitted Tafel slopes,(c) EIS curves for untreated and 10 minute UV-ozone treated showing the reduction in resistance from 2000 to 500 ohms and (d) Cycling performance for UV-ozone treated sample showing no reduction in performance after 24 hour immersion in the solution.

In order to understand the oxidation effects, the samples were subjected to UV-Ozone treatment and the contact angle was measured before and after the treatment. It can be clearly observed in Figure 3(c) and Figure 3(d), that the contact angle changes drastically when there is a slight presence of oxygen. Treatment of UV-Ozone on these samples introduced an S-O33 bonding (Figure 4(c)) which reduced the surface energy of the system, hence reducing the contact angle of water. This presence of S-O species makes the surface more reactive, hence allowing the water to strongly interact with the surface, thus lowering


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the Tafel slope for MoS2 down to 54 mV/decade. A two-minute ozone treatment of the sample reduced the contact angle by ~30o (Figure 3(c)) and a 10-minute treatment almost reduced the contact angle by ~50% (Figure 3(d)), indicating the presence of more reactive sites on the surface (S-O species). This increase in the reactive sites on the surface had significant impact for HERs as the Tafel slope reduced drastically to 54 mV/dec. As reported earlier34, this oxygen incorporation creates a moderate degree of disorder in the MoS2, which reduces the band gap of MoS2 leading to enhancement in the intrinsic conductivity of MoS2. This further increases the presence of unsaturated sulfur atoms available as active sites for the HER reaction, hence improving the performance of the MoS2 catalyst even further. Further information on the surface chemistry can be found in the supplementary S3. This was further extended to understand the interaction with a lower surface tension liquid (ethanol @ 22.39 mJ/m2). As it can be observed in Figure 3 (e), ethanol completely wets the surface which further confirms the presence of high surface energy material. Hence these properties help us in understanding the adhesion and the stability of the material under oxidation conditions.

Conclusions: In the current work, large area MoS2 nanoflowers were synthesized using non-toxic precursors i.e., MoO3 and sulfur in an inert flow. These nanoflowers were grown on multiple substrates such as flexible graphite sheets and silicon with a uniform coverage and very high density across the entire substrate. Characterization techniques such as Raman, Auger electron spectroscopy and XPS were used to determine the physical structure and the chemical composition of the MoS2 nanoflowers. From XPS, it was clear that the Mo:S was in the ratio of ~1:2. Static contact angle measurements were performed to determine the water contact angle on these surfaces. The contact angle values ranged from 30-160o depending on the coverage of MoS2 nanoflowers. It is expected that the textured surfaces have higher contact angle, in this case, MoS2 has a contact angle of ~160 which is termed as super hydrophobic and it follows a classic example of Wensel state of surface wetness. This paper reports one of the highest contact angle reported for MoS2 surfaces in the literature. Along with controlling the contact angle of the surface using UV-Ozone, the defects that were created enhanced the HER performance and lowered the Tafel slope from 185 mV/dec to 54 mV/dec, which is reportedly lower than the reported values for textured MoS2 surfaces. ASSOCIATED CONTENT Supporting Information More information about the growth is shown in supporting information. SEM images showing the uniformity of the growth, graphite paper used as substrate characterization and XPS analysis of the surface when exposed to UV-Ozone treatment is shown. Contact angle study of the MoS2 on SiO2 is also shown. Further information about the HERs performance for the uv-ozone treatment beyond 10 minute samples is also compared. This material is available free of charge via the Internet at http://pubs.acs.org.



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Corresponding Author All the correspondence should be addressed to J.A. Robinson at [email protected]. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We would like to thank Angstron Materials Inc. for providing us the flexible graphite paper and Honda Inc. for their funding support. We would also like to thank Birgitt Boschitsch from Tak Sing-Wong’s group for helping us with the contact angle measurements. REFERENCES ((1)

(2)

(3)

(4)

(5) (6)

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(24) Centre, S.; Kumar, C. High Performance Supercapacitor Electrode Material Based on in Situ Reduced Graphene Oxide Wrapped Manganese Carbonate. Int. J. Latest Res. Sci. Technol. 2014, 3, 65–69. (25) Ling, L.; Wang, C.; Zhang, K.; Li, T.; Tang, L.; Li, C.; Wang, L.; Xu, Y.; Song, Q.; Yao, Y. Controlled Growth of MoS2 Nanopetals and Their Hydrogen Evolution Performance. RSC Adv. 2016, 6, 18483–18489. (26) Liu, K. K.; Zhang, W.; Lee, Y. H.; Lin, Y. C.; Chang, M. T.; Su, C. Y.; Chang, C. S.; Li, H.; Shi, Y.; Zhang, H.; Lai, C. S.; Li, L. J. Growth of Large-Area and Highly Crystalline MoS 2 Thin Layers on Insulating Substrates. Nano Lett. 2012, 12, 1538– 1544. (27) Latthe, S. S.; Terashima, C.; Nakata, K.; Fujishima, A. Superhydrophobic Surfaces Developed by Mimicking Hierarchical Surface Morphology of Lotus Leaf. Molecules 2014, 19, 4256–4283. (28) Patankar, N. A. On the Modeling of Hydrophobic Contact Angles on Rough Surfaces. Langmuir 2003, 19, 1249–1253. (29) Shin, Y. J.; Wang, Y.; Huang, H.; Kalon, G.; Wee, A. T. S.; Shen, Z.; Bhatia, C. S.; Yang, H. Surface-Energy Engineering of Graphene. Langmuir 2010, 26, 3798–3802. (30) Bico, J.; Thiele, U.; Quéré, D. Wetting of Textured Surfaces. Colloids Surfaces A Physicochem. Eng. Asp. 2002, 206, 41–46. (31) Murakami, D.; Jinnai, H.; Takahara, A. Wetting Transition from the Cassie-Baxter State to the Wenzel State on Textured Polymer Surfaces. Langmuir 2014, 30, 2061– 2067. (32) Benck, J. D.; Hellstern, T. R.; Kibsgaard, J.; Chakthranont, P.; Jaramillo, T. F. Catalyzing the Hydrogen Evolution Reaction (HER) with Molybdenum Sulfide Nanomaterials. ACS Catal. 2014, 4, 3957–3971. (33) Addou, R.; McDonnell, S.; Barrera, D.; Guo, Z.; Azcatl, A.; Wang, J.; Zhu, H.; Hinkle, C. L.; Quevedo-Lopez, M.; Alshareef, H. N.; Colombo, L.; Hsu, J. W. P.; Wallace, R. M. Impurities and Electronic Property Variations of Natural MoS 2 Crystal Surfaces. ACS Nano 2015, 9, 9124–9133. (34) Xie, J.; Zhang, J.; Li, S.; Grote, F.; Zhang, X.; Zhang, H.; Wang, R.; Lei, Y.; Pan, B.; Xie, Y. Controllable Disorder Engineering in Oxygen-Incorporated MoS2 Ultrathin Nanosheets for Efficient Hydrogen Evolution. J. Am. Chem. Soc. 2013, 135, 17881– 17888.


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ToC Figure:



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Figures:

Figure 1: (a) Schematic showing the furnace and the position of the samples inside the furnace, (b) Image showing crucible before and after growth of vertical nanoflowers, (c) SEM image showing the growth of high density flowers on graphite paper and SiO2, Inset image showing the close-up view of the structure of the vertically oriented flowers and (d) Raman spectra showing the high quality MoS2 growth.


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Figure 2: (a) Auger electron spectroscopy (AES) elemental map corresponding to Mo and S elements on the sample, (b) High resolution Mo 3d spectra showing the high quality of MoS2 where the Δ between the 3d5/2 and 3d3/2 is 3.135 eV, corresponding to Mo4+ state (c) High resolution S 2p spectra with a Δ~1.2 eV corresponding to S2- in the MoS2. (d) XPS survey spectrum showing the presence of very little carbon and oxygen.



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Figure 3: Contact angle measurement for (a) Bare graphite paper, (b) As grown MoS2 flowers on GP, (c) As grown MoS2 flowers after 2 minutes of gentle UV-Ozone treatment, (d) 10 minute UV-Ozone treatment showing a significant reduction in contact angle showing an increase in the surface free energy, (e) As grown partially covered MoS2 flowers have a contact angle less than the UV-Ozone treated one and (f) complete wettability using a lower surface tension liquid (ethanol).


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Figure 4: Wetting transition from (a) Cassie-Baxter state to (b) Wensel state for water contacting on MoS2 flowers and (c) image showing the reduction of contact angle with the addition of the S-O functional groups on the surface.



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Figure 5: (a) Polarization curves of MoS2 with different ozone treatment time (1 min, 5 min, and 10 min); and (b) the corresponding Tafel slopes, the inset table are the fitted Tafel slopes.


Growth and Tunable Surface Wettability of Vertical MoS2 Layers for

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