Research Article www.acsami.org
Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
MoSe2 Dispersed in Stabilizing Surfactant Media: Effect of the Surfactant Type and Concentration on Electron Transfer and Catalytic Properties C. Lorena Manzanares Palenzuela, Jan Luxa, Zdeněk Sofer, and Martin Pumera* Center for Advanced Functional Nanorobots, Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic S Supporting Information *
ABSTRACT: Layered transition metal dichalcogenides (TMDs) have gained attention from the scientific community because of their extended range of applications. Molybdenum diselenide (MoSe2) has been proven to be an efficient catalyst for the hydrogen evolution reaction (HER), having implications in the research of new catalysts for clean energy production. One way to produce large quantities of these materials involves the use of surfactants for liquid exfoliation. Herein, we investigate the effects of cationic, anionic, and nonionic surfactants within a concentration range on the heterogeneous electron transfer rates, electrocatalytic efficiency toward the HER of MoSe2, and on the stability of the dispersions. We found that surfactants can have a detrimental effect on the electrocatalytic properties of the material when used above a concentration threshold. In some cases, high surfactant levels also had a negative effect on the stability of the material. This report serves to gain an understanding on how the way TMDs are prepared, processed, and stabilized can have dramatic effects on their efficiency toward HER, one of their most popular applications, and how choosing the appropriate surfactant type and concentration is crucial to gain in stability without compromising the intrinsic properties of the material. KEYWORDS: layered transition metal dichalcogenides, molybdenum diselenide, surfactants, liquid exfoliation, catalyst, hydrogen evolution reaction, heterogeneous electron transfer, cyclic voltammetry
■
INTRODUCTION The interest in two-dimensional (2D) materials aside from graphene has increased dramatically in recent years. Transition metal dichalcogenides (TMDs) are layered materials almost as thin and flexible as graphene, with semiconducting properties and a great degree of tunability.1 Depending on their composition, structure, and dimensionality, they find applications in nanoelectronics, photonics, sensing, energy storage, optoelectronics, and electrocatalysis.1,2 Aside from their tunable band gap, another important feature is their high surface area carrying high density of edges which are potential active sites for the hydrogen evolution reaction (HER).2,3 This is important in the field of alternative energy: TMDs have been identified as inexpensive, electrochemically stable, and environmentally friendly catalysts to replace the currently used costly platinum for the generation of hydrogen gas through HER.2 Although the most studied HER catalyst within the TMD family has been MoS2, MoSe2 has recently shown analogue or more potential in the same range of applications.3−5 The higher electrical conductivity of MoSe2 because of the intrinsic metallike properties of Se is relevant for electrochemical applications in which a faster rate of heterogeneous electron transfer is favorable.4,6 Different methods for the fabrication of TMDs as well as different solvents used as dispersion media or for liquid© XXXX American Chemical Society
phase exfoliation can be found across the literature. Our group recently demonstrated that the dispersion media can have an important effect on both the charge-transfer and electrocatalytic (HER efficiency) properties of MoS2.7 With the aim of producing large quantities of TMD crystals in a commercially viable way, some works have proposed exfoliation of bulk materials in aqueous media containing stabilizing surfactant agents.8−10 According to these studies, surfactants not only favor the exfoliation process and increase the stability of the dispersed material by preventing the sheets from reaggregation but also play a role in tailoring the surface charge. Of particular interest is to find out whether these interactions can have a negative impact on the material’s electrocatalytic properties seeming that the blockage of active sites can lead to a significant reduction in the catalytic activity. Previous studies have found that surfactants inherent to graphene, for example, detrimentally alter the electrochemical properties of the material.11,12 Herein, we investigate the effect of cationic, anionic, and nonionic surfactants at different concentration levels used as Received: January 3, 2018 Accepted: April 23, 2018
A
DOI: 10.1021/acsami.7b19744 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
STEM measurements were carried out using a 30 kV electron beam. The water-dispersed samples were also observed by a transmission electron microscope JEOL JEM-1010 at an accelerating voltage of 80 kV. The pictures were taken by an SIS MegaView III digital camera (Soft Imaging Systems) and analyzed by Analysis v. 2.0 software. Highresolution TEM (HR-TEM) was also performed using an EFTEM JEOL 2200 FS microscope (JEOL, Japan). A 200 keV acceleration voltage was used for the measurements. Dynamic light scattering (DLS) was performed using a Zetasizer Nano ZS (Malvern, England). The measurements were done at room temperature (20 °C) with diluted MoSe2 dispersions using polystyrene cuvettes. The zeta-potential measurements were performed on a Malvern Zetasizer Nano ZS with the supernatant collected after the natural sedimentation of the MoSe2 dispersions. For electrochemical measurements, aliquots of 5 μL of the dispersions were then drop-casted on the GC electrodes immediately after sonication and dried. The GC electrode surfaces were renewed prior to new measurements by polishing with a 1.0 μm alumina particle slurry on a polishing pad and thoroughly washed with deionized water, followed by 15 s sonication in water and then in ethanol. The HER was investigated by linear sweep voltammetry in 0.5 M H2SO4 at a scan rate of 5 mV s−1. The measurements of the ΔV values, directly correlated with the heterogeneous electron transfer rates at the MoSe2 surfaces, were done using 0.1 M KCl as the supporting electrolyte with 1 mM of the ferro/ferricyanide redox probe. For this set of experiments, cyclic voltammetry was performed at a scan rate of 100 mV s−1. The solutions were purged with nitrogen gas before the measurements.
dispersion media for exfoliated MoSe2 on the heterogeneous electron transfer rates and HER performance of the material.
■
EXPERIMENTAL SECTION
Chemicals. Molybdenum selenide (99.9%) was obtained from Alfa Aesar, Germany. n-Butyllithium (1.6 M in hexane), potassium chloride, and Tween 20 were obtained from Sigma-Aldrich, Czech Republic. Hexane was obtained from Lachner, Czech Republic. Absolute ethanol and concentrated sulfuric acid were purchased from Penta, Czech Republic. Potassium hexacyanoferrate(II) and potassium hexacyanoferrate(III) were purchased from Lachner, Czech Republic. Hexadecyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) were acquired from Fluka, Czech Republic. Glassy carbon (GC) electrodes of 3 mm diameter, platinum wire, and Ag/ AgCl (1 M KCl) electrodes were purchased from CH Instruments (Texas, USA). Deionized water was used in all experiments. Procedure. The exfoliation of MoSe2 was carried out by dispersing 15 mmol of bulk MoSe2 powder in 20 mL of 1.6 M n-butyllithium in hexane. The solution was then stirred for 72 h at 25 °C under an argon atmosphere. The Li-intercalated material was separated by suction filtration under an argon atmosphere, and the intercalation compound was washed several times with hexane (dried over Na). The separated MoSe2 with intercalated Li was subsequently placed in water (100 mL) and repeatedly centrifuged (18 000g). A portion of this intercalated material was used for elemental analysis with inductively coupled plasma optical emission spectroscopy (ICP−OES). The final obtained material was dried in a vacuum oven at 50 °C for 48 h prior to further use. An aqueous dispersion of MoSe2 was first prepared in deionized water at a 1.0 mg mL−1 concentration with ultrasonication for 1 h to obtain a well-dispersed suspension. MoSe2 (1.0 mg mL−1) dispersions in CTAB, SDS, Tween-20 of 0.01, 0.1, 1, and 10 mM each (prepared in deionized water) were also prepared. The chemical composition of the exfoliated material was characterized by X-ray photoelectron (XPS), Raman, and energydispersive (EDS) spectroscopies. Wide-scan XPS surveys of all elements were performed, with subsequent high-resolution scans of the C 1s, O 1s, Se 3d, and Mo 3d regions. High-resolution XPS was performed using ESCA Probe P (Omicron Nanotechnology, Taunusstein, Germany) with a monochromatic aluminum X-ray radiation source (1486.7 eV). An electron gun was used to eliminate sample charging during measurement (1−5 V). Raman spectroscopy was performed with an inVia Raman microscope (Renishaw, England) in backscattering geometry with a charge-coupled device detector. A diode-pumped solid-state laser (532 nm, 50 mW) with an applied power of 0.5 mW and a 50× magnification objective was used for the measurement. The water-dispersed material was drop-casted onto a gold film/silicon substrate for both XPS and Raman measurements. The elemental composition and mapping were obtained using an EDS analyzer (X-Max Extreme) with a 20 mm2 silicon drift detector (Oxford instruments) and AZtecEnergy software. To conduct the measurements, the water-dispersed sample was casted on a Formvar carbon film on a 200-mesh copper grid (Tedpella Inc., California). The EDS measurements were carried out using a 10 kV electron beam. The ICP−OES measurements were performed using a SPECTRO ARCOS optical emission spectrometer (SPECTRO Analytical Instruments, Kleve, Germany) with radial plasma observation. The sample was prepared via acidic digestion using concentrated HNO3. For calibration, commercially available multi-element standard solutions (Analytika) were used. The powder X-ray diffraction data were collected at room temperature on a Bruker D8 DISCOVER (Bruker, Germany) powder diffractometer with a parafocusing Bragg−Brentano geometry using Cu Kα radiation (λ = 0.15418 nm, U = 40 kV, I = 40 mA) with a step size of 0.019° (2θ). The morphology was investigated using scanning transmission electron microscopy (STEM) with a Tescan Lyra dual beam microscope equipped with an FEG electron source. To conduct the measurements, the water- and surfactant-based dispersions were dropcasted on a Formvar carbon film on a 200-mesh copper grid. The
■
RESULTS AND DISCUSSION To gain information about the characteristics of the exfoliated MoSe2, different surface and bulk characterizations at different scales from micro to atomic levels were carried out. Figure 1 shows the HR-TEM images at different magnifications of individual MoSe2 sheets with sizes below 1 μm. The EDS analysis coupled with TEM shows a homogeneous distribution of Mo and Se and a low concentration of oxygen (Figure S1). The SEM/EDS of MoSe2 together with the elemental mapping
Figure 1. The TEM images of the exfoliated MoSe2 with the corresponding SAED and HR-TEM images of MoSe2. Scale bar corresponds to 5 nm. B
DOI: 10.1021/acsami.7b19744 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 2. Effect of surfactants on the heterogeneous electron transfer rates of MoSe2: (A) chemical structures of each surfactant and the graphical representation of the few-layered MoSe2 material; (B) cyclic voltammograms of MoSe2 dispersed in CTAB, SDS, and Tween-20 at different concentration levels, including that in deionized water (0 mM of the surfactant), measured in the presence of [Fe(CN)6]3−/4− in 0.1 M KCl as the supporting electrolyte (scan rate: 100 mV s−1); (C) bar chart showing the average ΔV values (n = 3) for each surfactant, at each concentration level. The dashed lines indicate where the CMC value is approximately located.
are characterized by the peaks found at lower binding energies (ca. 228 and 231 eV) (Figure S4B). The partial conversion of MoSe2 from the 2H to the metallic 1T form was enabled by Li intercalation through n-butyllithium. The signal recorded at a higher binding energy (∼235 eV) corresponds to Mo6+, which can be assigned to MoO3. The exfoliation process led to partial oxidation/reduction processes which promoted the formation of Mo3Se4 due to Mo reduction and elemental selenium because of selenide oxidation.15 The ICP−OES analysis was performed on the Li-intercalated compound to gain insight into the stoichiometry of MoSe2 after reaction with butyllithium. The obtained results were: 6.1 wt % of Li, 34.79 wt % of Mo, and 59.39 wt % of Se. The atomic ratios revealed a stoichiometry of 1:2 of the MoSe2 material. Three types of surfactants, cationic (CTAB), anionic (SDS), and nonionic (Tween-20), were used as model hydrophilic surfactants (Figure 2A) to address the effect of these stabilizing agents on the electrochemical properties of MoSe2. The typical surfactant concentration values found in the literature vary from 3.5 to 14 mM in the case of sodium cholate,8,9 whereas 27.4
of the exfoliated material carried out at lower magnifications can be found in Figure S2. The selected area electron diffraction (SAED) shown in Figure 1 indicates the hexagonal symmetry of MoSe2, in which the sharp diffraction pattern reveals the highly crystalline nature of the individual MoSe2 flakes. The high-magnification HR-TEM image shows clearly the crystal structure. Figure S3 shows the X-ray diffractograms of the material after exfoliation, in which a phase of MoSe2 together with a fraction of Mo3Se4 phase with clear diffraction peaks for hexagonal cell structures was observed. The presence of these phases is later discussed with the XPS measurements. The crystallographic parameters for MoSe2 are a = b = 3.287 Å, c = 12.925 Å, α = β = 90°; γ = 120° (PDF#29-0914). The measured Raman and XPS spectra are depicted in Figure S4A,B. Figure S4A shows the characteristic out-of-plane 1 vibrational mode A1g at ∼237 cm−1 and the in-plane E2g −1 mode of MoSe2 located at ∼284 cm . These agree well with the literature values for the few-layered MoSe2.13,14 The XPS spectrum shows contribution from the 1T phase as well as the Mo3Se4 phase (confirmed by X-ray diffraction). These phases C
DOI: 10.1021/acsami.7b19744 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 3. Cyclic voltammograms of MoSe2−surfactants (), MoSe2 without surfactants (- - - -), and surfactants without MoSe2 (········) in the presence of [Fe(CN)6]3−/4− in 0.1 M KCl as the supporting electrolyte (scan rate: 100 mV s−1). The surfactant concentration is 10 mM.
as molecular electronics, sensors and biosensors, and solar cells (i.e. photoinduced interfacial electron transfer). A surfacesensitive probe such as a ferro/ferricyanide redox pair is ideal to evaluate the electron transfer properties of a material, given that it requires interaction with the electrode to engage in an electrochemical reaction. Anything affecting the material’s intrinsic electron transfer properties would be observed in a cyclic voltammetric experiment performed with this redox pair. The heterogeneous electron transfer rate can be directly assessed from the peak separation (ΔV values) in cyclic voltammetric experiments. Figure 2B shows the cyclic voltammograms of MoSe2 dispersed in the three surfactants at different concentration levels and the average ΔV values (Figure 2C). The ΔV value of the surfactant-free dispersion (107.5 ± 27.6 mV) was taken as the reference. An apparent decrease was obtained in the CTAB dispersions down to an average of 74.9 (±3.7) mV for 10 mM, although, considering the standard deviation values, it cannot be stated that there is an enhanced electron transfer of MoSe2 with this surfactant. For the SDS dispersions of 1 and 10 mM, a sharp increase of ∼3 and ∼6.5 times ΔV from the water value, that is, a 3- and 6.5-fold decrease in the electron transfer rates, was obtained. For smaller concentration values, the effect was not as marked, with ΔV of 132.4 (±0.9) and 174.7 (±11.9) mV for 0.01 and 0.1 mM, respectively. The results obtained with both surfactants can be explained by the difference in their charge. Gupta et al.10 showed that the surfactant chains bind to the surface of the TMD material, changing its surface charge. CTAB−MoSe2 might be exerting an electrostatic attraction on the redox pair that results in lower ΔV values than those obtained with SDS, especially at higher concentrations (≥1 mM). On the other hand, the presence of the nonionic Tween20 produced a ∼3−4-fold decrease in the electron transfer rate without a concentration-dependent behavior. The current “evolution” seen at the positive potentials in nearly all the voltammograms is most likely to be the beginning of an anodic process because of the oxidation of Mo4+ to Mo6+, at ca. 1.0 V.7 This inherent electrochemistry becomes less apparent with the highest surfactant concentrations. Possibly, the material is coated by the surfactant chains, consequently decreasing the availability of the redox active moieties within the layered structure. We wanted to investigate whether these effects on the electron transfer rates were owed to the material−surfactant combinations, to the material itself, or to the surfactant (Figure 3). For the GC electrodes that were drop-casted with MoSe2 dispersed in 10 mM of each surfactant, the ΔV values for CTAB, SDS, and Tween-20 were 1.5, 63.8, and 70.5% higher than the values corresponding to the GC electrodes drop-
mM of CTAB and 36.7 mM of SDS were used in another work.10 Tween-20 has been used at ∼0.816 and ∼0.08 mM17 concentration levels for graphene exfoliation. Clearly, the concentration values found across the literature are diverse, and it is difficult to correlate different reports in terms of the surfactant types and concentration values, as well as the nature of the exfoliated material and the preparation method. Also, the final surfactant concentration in the dispersion after centrifugation (post exfoliation) will be inferior to the abovementioned values, and there is no information in the literature regarding the real amount of surfactant in the final dispersion, which is why we decided to cover three orders of magnitude of concentration, from i.e., 0.01 to 10 mM for each model surfactant. The critical micellar concentration (CMC) values of these detergents are ∼1, ∼8, and ∼0.06 mM for CTAB, SDS, and Tween-20, respectively. Backes et al.9 have stated that stable dispersions of layered materials can be produced when the surfactant concentration is kept below its CMC, that is, when using sodium cholate. However, the authors acknowledged that a comprehensive understanding of the role of the surfactant concentration is lacking. Gupta et al.10 carried out a comprehensive characterization of the interaction between SDS and CTAB with MoS2. They did not find a correlation between the stability of the MoS2 dispersions and the CMC values of these surfactants. They did, however, investigate the nature of the surfactant−TMD nanosheet interaction and characterize the organization and arrangement of the surfactant chains by means of different spectroscopic techniques. The experimental evidence provided in this work concluded that the surfactant chains interact with the 2D material by a random arrangement on the basal plane of the nanosheet. The surfactant chains lie flat on the surface of the nanosheets via dispersive interactions between the alkyl chains of the surfactant and the MoS2 sheets, leaving the charged headgroups “free” toward the dispersion. The bound chains undergo a rapid exchange with the free surfactant chains. Although this evidence clearly suggests that these hydrophilic surfactants can interact strongly with TMDs, a comprehensive characterization of the nature of such interactions with MoSe2 is beyond the scope of our work, and the model proposed by Gupta et al.10 should merely taken as orientative. Moreover, the way Tween-20 interacts with TMDs has not been explored to date, to the best of our knowledge. Herein, we aim at exploring how the presence of these surfactants in different concentration levels affects a material’s properties: heterogeneous electron transfer, electrocatalytical performance toward HER, and stability. The studies on the heterogeneous electron transfer of a material have implications in technologically relevant fields such D
DOI: 10.1021/acsami.7b19744 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 4. Effect of surfactants on the HER efficiency of MoSe2: (A) linear sweep voltammograms and (B) Tafel plots obtained for MoSe2 dispersed in different surfactant concentrations (colored, full lines), MoSe2 dispersed in deionized water (full black lines), the bare GC electrode (discontinuous lines), and the Pt foil (dashed lines), measured in 0.5 M H2SO4 at 5 mV s−1; (C) bar chart displaying the average overpotential values; and (D) the Tafel slopes (n = 3) for each surfactant, at each concentration level. The dashed lines indicate where the CMC value is approximately located.
electrode materials are judged by the overpotential, which is determined by both the Tafel slope and the onset potential (i.e., the potential at which the initial catalytic current is observed).19 Figure 4A,B shows the linear sweep voltammograms and the Tafel plots for each surfactant medium in which the material was dispersed. The overpotential values and Tafel slopes are shown in Figure 4C,D. The lowest overpotential was achieved by MoSe2 dispersed in 0.01 mM Tween-20 (353 ± 1.4 mV) and 0.01 mM SDS (354 ± 5.7 mV), ∼25 mV below the values of the surfactant-free dispersion (379 ± 1.4 mV), followed by 0.01 mM CTAB (362.5 ± 3.5 mV). However, the Tafel slope values obtained with the low surfactant concentrations were similar to the one obtained with the surfactantfree dispersion (83.2 ± 3.7 mV dec−1). The dispersions with 0.01 and 0.1 mM SDS and 0.01 mM Tween-20 displayed the lowest slope values (87.5 ± 5.7, 81.2 ± 0.9, and 83.7 ± 1.4 mV dec−1), whereas CTAB dispersions exhibited slightly higher values (92.9 ± 7.3 mV dec−1 for 0.01 mM). All of the surfactant dispersions resulted in a systematic increase of both overpotential and Tafel slope values with increasing concentration, especially above 0.1 mM, with SDS being the least detrimental
casted with the surfactants alone, respectively. While CTAB showed no significant difference in terms of ΔV, whether the material is present or not, suggesting that the surfactant itself is responsible for the low ΔV, the electron transfer rates of the SDS- and Tween-20-dispersed materials were considerably lower than those of the surfactants alone. The behavior resembles the one of a “blocking” surface adsorbate, known to negatively affect the electrochemistry of the ferro/ferricyanide redox pair.18 The surfactant-coated MoSe2 sheets impair the electron transfer between the probe and the electrode surface. Further studies were conducted to assess the effect of surfactants on the HER efficiency of MoSe2. Two metrics were withdrawn from the linear sweep voltammograms recorded in acidic media: the Tafel slope and the overpotential. The Tafel slope, an inherent property of the reaction kinetics determined by the electrode potential versus the logarithm of the current density, is characteristically used to designate the potential increase required to increase the resulting current by one order of magnitude. The overpotential is the potential required to reach an operating current density, often defined at −10 mA/ cm2. Therefore, low Tafel slopes are desirable, but ultimately all E
DOI: 10.1021/acsami.7b19744 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
by the observed sedimentation rates and zeta potential values. The pH values of most dispersions (including that without surfactant) were in the range 4.6−4.9, except for CTAB, in which the pH increased up to ∼5.5 (1 mM). Overall, the stability of the dispersions seemed to decrease considerably with increasing levels of the surfactant concentration, especially above the CMC value, with the exception of CTAB. The morphologies of the dispersed particles in different surfactants (1 mM) were further observed by STEM (Figure S9). A tendency to form aggregates or large clustered flakes in the CTAB media can be seen from these images. This effect was also partially observed for Tween-20, whereas the SDSmodified MoSe2 particles seemed to be more uniformly distributed forming smaller clusters. DLS measurements also showed smaller average sizes of the SDS-modified material up to 1 mM, with size distributions similar to that of the waterdispersed particles (Figures S10 and S11). An overall detrimental effect on polydispersity and size distribution can be seen with increasing surfactant concentration.
to catalysis. In general, we observe a negative effect on HER catalysis when working above the CMC value of the surfactant. As control experiments, we included the performances of a bare GC electrode (overpotential = 783.9 mV; Tafel slope = 119.5 mV dec−1) and a Pt foil (overpotential = 58.8 mV; Tafel slope = 35.2 mV dec−1). Our findings suggest that the surfactant chains, when in excess, bind to the material and can interfere with the production of hydrogen gas, which requires the adsorption of a H intermediate followed by the desorption of H2 for gas formation. As a result, there is a negative effect for HER catalysis generally above 0.1 mM of the surfactant in the final dispersion. However, when working in a low concentration range (0.01 mM), the catalytic efficiency can remain unaffected or slightly improved. A lower degree of reaggregation of the MoSe2 sheets in a surfactant-stabilized medium can explain this slight catalytic enhancement given the expected increase of the exposed active sites in the nonaggregated material. To assess whether the inclusion of surfactants in the dispersion media is favorable for stabilizing the nanosheets, we evaluated the sedimentation rates, zeta potential, and size distribution of the dispersed MoSe2 particles. The stability of the dispersions was evaluated after sonication (0 min) by leaving them to naturally sediment at different times (10, 30, 60, 120 min, and 48 h) and visually monitoring the extent of the sedimentation (Figures S5−S8). The zeta plots are presented in Figures S5−S8, and the average zeta values are plotted in Figure 5. The zeta potential values reveal that CTAB
■
CONCLUSIONS This paper highlights that the presence of surfactants can severely affect the electrochemical properties/applications of MoSe2 depending on their concentration and type. For sensing and biosensing applications, where usually a fast electron transfer is preferred, the selection of a surfactant-stabilized material should be evaluated carefully: CTAB-MoSe2 did not affect the heterogeneous electron transfer rates, whereas SDSand Tween-20-MoSe2 were found to worsen the rates. The catalytic efficiency of MoSe2 toward HER was also dramatically affected by the surfactant concentration. Overall, a detrimental effect with increasing surfactant concentration was observed. This paper shows that while incorporating a surfactant in the exfoliation medium can be useful to avoid reaggregation of the sheets and to produce large quantities of layered TMDs and other 2D materials, the selection and optimization of the surfactant type and concentration should be thoroughly assessed in terms of the effect on the intended application.
■
Figure 5. Effect of surfactants on the zeta potential of MoSe2 dispersions. The horizontal dashed lines indicate the stability threshold generally considered to be above +30 mV and below −30 mV. The dashed lines indicate where the CMC value is approximately located.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b19744. TEM image and corresponding EDS elemental distribution map of water-dispersed MoSe2; SEM image and corresponding elemental distribution mapping of waterdispersed MoSe2; X-ray diffractogram of the exfoliated material showing two combined phases, MoSe2 and Mo3Se4; the Raman spectra of exfoliated MoSe2 and the high-resolution XPS spectrum of Mo 3d peak area; photographs of the CTAB-dispersed MoSe2 taken after different times post sonication and the zeta potential plot displaying three measurements; photographs of the SDSdispersed MoSe2 taken after different times post sonication and the zeta potential plot displaying three measurements; photographs of the Tween-20-dispersed MoSe2 taken after different times post sonication and the zeta potential plot displaying three measurements; photographs of the water-dispersed MoSe2 taken after different times post sonication and the zeta potential plot displaying three measurements; typical STEM images of
generated a shift in the surface charge of the material when used above 1 mM, changing from −43 (±7.48) mV in the water-dispersed MoSe2 to +32.3 (±6.37) mV and +56.5 (±7.43) mV in the CTAB dispersions of 1 and 10 mM, respectively. These findings agree with Gupta et al.’s remarks on surface charge being tailored by surfactant binding.10 Figure S5 shows that the CTAB dispersions (above 0.1 mM) were highly stable, being easy to resuspend after 48 h of sedimentation. SDS did not induce substantial changes in the average zeta values, remaining generally below −30 mV, but the dispersions of 1 and 10 mM were found to be unstable (Figure S6), the latter having a high deviation in the zeta potential measurement. The Tween-20 (0.01 and 0.1 mM) MoSe2 dispersions were highly stable (Figure S7), that is, the sedimentation was seemingly slower after 2 h of sitting time than with most of the other surfactants or the water-based dispersion (Figure S8). The addition of increasing levels of Tween-20 to MoSe2 resulted in a decreased stability, as shown F
DOI: 10.1021/acsami.7b19744 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
■
(11) Bonanni, A.; Pumera, M. Surfactants Used for Dispersion of Graphenes Exhibit Strong Influence on Electrochemical Impedance Spectroscopic Response. Electrochem. Commun. 2012, 16, 19−21. (12) Wong, C. H. A.; Pumera, M. Surfactants Show Both Large Positive and Negative Effects on Observed Electron Transfer Rates at Thermally Reduced Graphenes. Electrochem. Commun. 2012, 22, 105− 108. (13) Ambrosi, A.; Sofer, Z.; Pumera, M. 2H → 1T phase transition and hydrogen evolution activity of MoS2, MoSe2, WS2 and WSe2 strongly depends on the MX2 composition. Chem. Commun. 2015, 51, 8450−8453. (14) Tonndorf, P.; Schmidt, R.; Böttger, P.; Zhang, X.; Börner, J.; Liebig, A.; Albrecht, M.; Kloc, C.; Gordan, O.; Zahn, D. R. T.; de Vasconcellos, S. M.; Bratschitsch, R. Photoluminescence emission and Raman response of monolayer MoS2, MoSe2, and WSe2. Opt. Express 2013, 21, 4908−4916. (15) Lim, C. S.; Tan, S. M.; Sofer, Z.; Pumera, M. Impact Electrochemistry of Layered Transition Metal Dichalcogenides. ACS Nano 2015, 9, 8474−8483. (16) Wang, T.; Quinn, M. D. J.; Notley, S. M. A benzoxazine surfactant exchange for atomic force microscopy characterization of two dimensional materials exfoliated in aqueous surfactant solutions. RSC Adv. 2017, 7, 3222−3228. (17) Smith, R. J.; Lotya, M.; Coleman, J. N. The importance of repulsive potential barriers for the dispersion of graphene using surfactants. New J. Phys. 2010, 12, 125008. (18) Chen, P.; McCreery, R. L. Control of Electron Transfer Kinetics at Glassy Carbon Electrodes by Specific Surface Modification. Anal. Chem. 1996, 68, 3958−3965. (19) 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.
MoSe2 dispersed in 1 mM solution of surfactant; DLS measurements of the MoSe2 dispersion in water; DLS measurements of the MoSe2 dispersions in CTAB, SDS, and Tween-20 at different surfactant concentration levels (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zdeněk Sofer: 0000-0002-1391-4448 Martin Pumera: 0000-0001-5846-2951 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the project Advanced Functional Nanorobots (reg. no. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR). Z.S. and J.L. were supported by Czech Science Foundation (GACR no. 17-11456S) and by specific university research (MSMT no. 20-SVV/2018). This work was created with the financial support of the Neuron Foundation for science support.
■
REFERENCES
(1) Choi, W.; Choudhary, N.; Han, G. H.; Park, J.; Akinwande, D.; Lee, Y. H. Recent Development of Two-Dimensional Transition Metal Dichalcogenides and Their Applications. Mater. Today 2017, 20, 116− 130. (2) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L.-J.; Loh, K. P.; Zhang, H. The Chemistry of Two-Dimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263−275. (3) Chia, X.; Ambrosi, A.; Sofer, Z.; Luxa, J.; Pumera, M. Catalytic and Charge Transfer Properties of Transition Metal Dichalcogenides Arising from Electrochemical Pretreatment. ACS Nano 2015, 9, 5164− 5179. (4) Eftekhari, A. Molybdenum Diselenide (MoSe2) for Energy Storage, Catalysis, And Optoelectronics. Appl. Mater. Today 2017, 8, 1−17. (5) Gholamvand, Z.; McAteer, D.; Backes, C.; McEvoy, N.; Harvey, A.; Berner, N. C.; Hanlon, D.; Bradley, C.; Godwin, I.; Rovetta, A.; Lyons, M. E. G.; Duesberg, G. S.; Coleman, J. N. Comparison of liquid exfoliated transition metal dichalcogenides reveals MoSe2 to be the most effective hydrogen evolution catalyst. Nanoscale 2016, 8, 5737− 5749. (6) Loo, A. H.; Bonanni, A.; Sofer, Z.; Pumera, M. Exfoliated Transition Metal Dichalcogenides (MoS2, MoSe2, WS2, WSe2): An Electrochemical Impedance Spectroscopic Investigation. Electrochem. Commun. 2015, 50, 39−42. (7) Chua, X. J.; Pumera, M. The Effect of Varying Solvents for MoS2 Treatment on its Catalytic Efficiencies for HER And ORR. Phys. Chem. Chem. Phys. 2017, 19, 6610−6619. (8) Smith, R. J.; King, P. J.; Lotya, M.; Wirtz, C.; Khan, U.; De, S.; O’Neill, A.; Duesberg, G. S.; Grunlan, J. C.; Moriarty, G.; Chen, J.; Wang, J.; Minett, A. I.; Nicolosi, V.; Coleman, J. N. Large-Scale Exfoliation of Inorganic Layered Compounds in Aqueous Surfactant Solutions. Adv. Mater. 2011, 23, 3944−3948. (9) Backes, C.; Higgins, T. M.; Kelly, A.; Boland, C.; Harvey, A.; Hanlon, D.; Coleman, J. N. Guidelines for Exfoliation, Characterization and Processing of Layered Materials Produced by Liquid Exfoliation. Chem. Mater. 2017, 29, 243−255. (10) Gupta, A.; Arunachalam, V.; Vasudevan, S. Water Dispersible, Positively and Negatively Charged MoS2 Nanosheets: Surface Chemistry and the Role of Surfactant Binding. J. Phys. Chem. Lett. 2015, 6, 739−744. G
DOI: 10.1021/acsami.7b19744 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
