Nanoflakes Heterostructure

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 8374−8382

pubs.acs.org/journal/ascecg

Vertically Aligned MoS2 Quantum Dots/Nanoflakes Heterostructure: Facile Deposition with Excellent Performance toward Hydrogen Evolution Reaction Amir Bayat,† Mohammad Zirak,‡ and Esmaiel Saievar-Iranizad*,† †

Department of Physics, Faculty of Basic Science, Tarbiat Modares University, P.O. Box 14115-111, Tehran, Islamic Republic of Iran Department of Physics, Hakim Sabzevari University, P.O. Box 961797648, Sabzevar, Islamic Republic of Iran

‡

ACS Sustainable Chem. Eng. 2018.6:8374-8382. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/04/18. For personal use only.

S Supporting Information *

ABSTRACT: A facile, low-cost, and straightforward procedure was developed for vertical deposition of MoS2 quantum dots (QDs)/nanoflakes (NFs) on fluorine-doped tin oxide (FTO) substrate without any binder. Bulk MoS2 powder was exfoliated in a bath sonicator followed by tip sonicator inside a water−ethanol (0.5/0.5 volume ratio) solution. The obtained MoS2 QDs/NFs are deposited on FTO substrate via a simple electrophoretic technique which resulted in vertical deposition of MoS2 nanoflakes. Field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) confirmed the formation of MoS2 nanoflakes with thickness of ∼20 nm and lateral dimension of ∼400 nm; transmission electron microscopy (TEM) and AFM analysis revealed that many single- or two-layer MoS2 quantum dots exist with lateral size of ∼5 nm on average on the basal plains of MoS2 nanoflakes. Electrochemical measurements indicated that the vertical MoS2 QDs/NFs/FTO electrode has a Tafel slope of 74 mV/decade and charge transport resistance (Rct) of 16 Ω which is ∼1.9 and 2.7 times smaller than the Tafel slope and Rct of the nonvertical MoS2 NFs/FTO electrode, respectively. This unique morphology exhibited excellent stability for the electrocatalytic hydrogen evolution reaction (HER) after a repeating linear sweep voltammetry (LSV) test for 1000 cycles. KEYWORDS: MoS2 quantum dots, Vertical alignment, High catalytic activity, Stability, Hydrogen evolution

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INTRODUCTION Up to now, platinum has been known to be the material with the best catalytic performance for the hydrogen evolution reaction (HER) because of its excellent stability and very low hydrogen evolution overpotential.1 However, its scarcity and high price limit the industrial application of platinum. Hence, considerable efforts have been undertaken to eliminate the expensive Pt and find an alternative counter electrode, which is highly active, stable, cost-effective, easy to handle, and composed of earth-abundant materials to advance the production of H2 fuel through water splitting via the HER. Several compounds and materials such as carbides,2 nitrides,3 graphite and graphene composites,4 and two-dimensional (2D) transition metal dichalcogenides (TMDs)5 have been introduced as promising Pt alternatives. Among these materials, 2D TMD nanoflakes have recently attracted renewed and tremendous interests for HER applications because of their metal-free composition, high activity, excellent stability, and economic cost-effective aspects. Molybdenum disulfide (MoS2) is the most studied member of the TMD family, specially for HER applications.6,7 The results of many studies showed that MoS2 can exhibit a catalytic activity even higher than the Pt catalyst. Zong et al. experimentally exhibited that the photocatalytic activity of © 2018 American Chemical Society

MoS2-loaded CdS was even higher than that of Pt/CdS under the same reaction conditions.8 The H2 evolution rate up to 1315 μmol/h was obtained using the MoS2/CdS photocatalyst which was much higher than 488 μmol/h reported for Pt/CdS photocatalyst.9 Because of these fascinating results, MoS2 has been rapidly gaining momentum in the economic HER area and has given a fresh impetus to achieve this long-conceived goal of Pt replacement. Molybdenum disulfide (MoS 2) is a transition metal dichalcogenide (TMD) compound with a layered structure similar to graphite with a trigonal prismatic structure. It is verified that the basal plane of MoS2 is inert for hydrogen reduction, while the edge sites of MoS2 are catalytically active.10 The unsaturated sulfur atoms, bonded at the terminal sites of MoS2 planes, exhibit superior catalytic activity toward H+ reduction. Therefore, it is of particular interest to increase the active edge sites exposed to the electrolyte to enhance HER efficiency. To achieve excellent catalytic activity of MoS2, various strategies have been widely employed to synthesize MoS2 micro-/nanomaterials with novel and superior morpholReceived: January 28, 2018 Revised: April 22, 2018 Published: May 25, 2018 8374

DOI: 10.1021/acssuschemeng.8b00441 ACS Sustainable Chem. Eng. 2018, 6, 8374−8382

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ACS Sustainable Chemistry & Engineering

Figure 1. Preparation procedure of the vertical MoS2 QDs/NFs heterostructure.

conductive substrate to investigate the electrochemical catalytic activity of MoS2 flakes toward hydrogen production. Although there are many report using MSS to produce a fewlayer MoS2 nanoflake suspension, preparing a MoS2 electrode with desired and controllable morphology using the obtained suspension is rarely reported. Herein, we have used MSS with some modifications to prepare MoS2 quantum dots (QDs) on few-layer MoS2 nanoflakes (NFs). Our modification resulted in the production of a novel MoS2 QDs/NFs morphology using MSS. Thereafter, the MoS2 QDs/NFs were deposited vertically on fluorine-doped tin oxide (FTO) substrates via the electrophoretic process for the first time without using any additional binders. The obtained vertically aligned MoS2 QDs/ NFs electrode exhibited high electrochemical catalytic H2 production activity with excellent stability. The introduced combined strategy can be easily conducted to prepare the MoS2 QDs/NFs/FTO electrode with vertical morphology and highly exposed active edge sites without using any complicated and expensive material or apparatus. This method provides a significant breakthrough toward the economic HER, because it allows us to synthesize cost-effective large-scale vertically deposited MoS2 QDs/NFs.

ogies to optimize the active edge sites of MoS2 over the inert basal plane sites in a controllable manner. For example, using amorphous defective MoS2 nanosheets,11 using mesoporous MoS2 nanostructures,12 lowering the thickness of MoS2 nanosheets toward a single layer,13 construction of threedimensional (3D) architectures,14 and synthesis of vertically aligned MoS2 nanosheets15 are some of these strategies. However, developing a straightforward and feasible strategy to produce large-scale and cost-effective MoS2-based electrodes with desired stability and activity is still a significant challenge. A facile, well-known method to prepare MoS2 few-layer nanoflakes is liquid exfoliation of layered bulk MoS2 (and other TMDs such as WS2) under ultrasonic wave irradiation.16,17 This method has great potential for the scalable production of two-dimensional (2D) nanosheet-based materials by adjusting its important parameters. During ultrasonic exfoliation, the selected solvent plays an important role, since physical properties such as boiling point, surface tension, and energy along with solubility parameters affect the process and hence the resulting 2D flakes. A popular high-yield solvent, N-methyl2-pyrrolidone (NMP), which has been recommended in several studies,18 has been employed as a high-yield reference. Even though high yields are obtained when using NMP, removal of solvent residues is difficult because of its relatively high boiling point (202 °C) and surface tension (40.79 mJ/m2 at 20 °C).19 Another important solvent for exfoliation of transition metal dichalcogenides is acetonitrile (ACN). Moreover, the boiling point of ACN is below 100 °C, but ACN can be metabolized to produce hydrogen cyanide, which is the source of the observed toxic effects.20 Furthermore, NMP and ACN are not appropriate solutions for the electrophoretic process. The mixed-solvent strategy (MSS) has been developed to overcome these drawbacks.21 This strategy uses the mixture of abundant and easily accessible solvents such as water and ethanol which are ecofriendly with low boiling temperatures and can be readily removed from the systems. Neither water nor ethanol can properly disperse MoS2 bulk powder with a high concentration, but their mixture can sufficiently disperse MoS2 powder to achieve a few-layer MoS 2 nanosheet dispersion.21 The few-layer MoS2 nanosheet dispersion in mixed water−ethanol solvent should be then deposited on a

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EXPERIMENTAL SECTION

Materials. All materials and chemical reagents were used without any further purification. Bulk MoS2 powder was provided from Aladdin Reagent Inc., and absolute ethanol (Merck) was provided. Preparation of MoS2 QDs/NFs Heterostructure Solution. A highly dispersed suspension of the MoS2 QDs/NFs heterostructure was prepared using a very simple liquid exfoliation-breaking technique involving a bath−tip sonication (cavitation process) followed by centrifugation in a water−ethanol solution. Typically, 200 mg of bulk MoS2 powder was dispersed in a 100 mL flask containing a mixed water−ethanol solution (with a 0.5/0.5 volume ratio). The flask was capped tightly and stirred for 1 h, and then was put in a bath sonicator and sonicated for 7 h at room temperature to form a black suspension. After this process, the mixture was centrifuged at 1000 rpm for 10 min. Then, the precipitate and the supernatant were collected separately. After 24 h of rest under ambient condition, 2/3 of the supernatant was selected and was sonicated for 1 h using a tip sonicator (the probe diameter was 3 mm) operating on pulsed mode with power of 200 W. The ultrasound taps were ON for 5 s and the turned OFF for 2 s to avoid tip damaging and to reduce solvent heating, and thus solution 8375


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Figure 2. (a) FESEM image of the exfoliated MoS2 deposited on FTO substrate in a nonvertical alignment. (b, c) FESEM images of the vertically aligned MoS2 QDs/NFs heterostructure deposited on FTO substrate. (d) TEM image of MoS2 NFs and (e) MoS2 QDs/NFs; the inset of part e is an HRTEM image that clearly shows the MoS2 QD on the MoS2 flake. For comparison, the TEM image of bare MoS2 QDs (without MoS2 NFs, obtained via prolonged tip sonication and high-rate centrifugation) is shown in part f. (g−i) AFM images of MoS2 NFs, QDs/NFs, and QDs, respectively. (j−l) Height profiles of MoS2 NFs, QDs/NFs, and QDs cast on mica substrate. was sealed and then placed at an oven at temperature of 180 °C for 1 h. The preparation procedure of the thin films is illustrated schematically in Figure 1. Electrochemical Measurements. Electrochemical impedance spectroscopy (EIS) measurements and linear sweep voltammetry (LSV) tests to obtain current density (J) versus applied voltage (V) curves (J−V curves) were conducted via a three-electrode electrochemical cell connected to a VSP-300 multichannel potentiostat/ galvanostat (Bio-Logic Science Instruments). In these setups, the MoS2 QDs/NFs (or MoS2 NF) electrode was used as working electrode; a saturated Ag/AgCl and a Pt plate were used as the reference and counter electrode, respectively. All the electrodes were put into 0.5 M H2SO4 aqueous solution as electrolyte. For EIS measurements, an ac voltage with amplitude of 5 mV and the voltage frequency ranging from 100 kHz to 1 Hz was applied. Characterization Techniques. The structural morphology of the samples was studied using field emission scanning electron microscopy (FESEM, HITACHI S-4160) and transmission electron microscopy (TEM, CM30 300 kV). The surface topography of the samples on mica substrates was studied by atomic force microscopy (AFM) using a commercial microscope (Veeco Autoprobe CP-research) in air with a 10 nm radius of tip curvature. Optical absorbance measurements were carried out using a spectrophotometer (Unico 4802). X-ray

degradation. The beaker was put into an ice/water system to keep the surroundings of the beaker cool (5 °C) during probe ultrasonication. After that, the product was obtained by centrifugation at 5000 rpm. Finally the MoS2 QDs/NFs heterostructure solution was kept for further use. The MoS2 NF solution was obtained in the same situation without the tip-sonication process. For the acquisition of the MoS2 QD solution, the tip-sonication process was performed for 5 h under the same situation, and supernatant containing MoS2 QDs was obtain after centrifugation of the solution at 7000 rpm. Preparation of the Vertical MoS2 QDs/NFs/FTO Electrode. The electrophoretic deposition (EPD) process was used for vertical deposition of MoS2 QDs/NFs on the FTO substrates. At first, FTO substrates were washed in a solution of deionized (DI) water and ethanol and dried in a nitrogen flow. Then, MoS2 nanostructures were deposited on the precleaned FTO substrates via the EPD process. In the EPD process, two parallel FTO substrates were immersed in the MoS2 QDs/NFs solution (or MoS2 NF solution) with a distance of 1 cm at an optimized applied dc voltage of 20 V (or 8 V) for 4 min. A high applied voltage (20 V) resulted in vertical deposition of MoS2 nanoflakes. Nonvertical MoS2 NFs were obtained via low applied voltage (8 V). Finally, for the acquisition of more stable MoS2 QDs/ NFs thin films, the samples were put in an autoclave including 50 mL of DI water, Na2MoO4 (0.05 g), and thiourea (0.05 g). The autoclave 8376


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ACS Sustainable Chemistry & Engineering diffraction patterns (XRD, Co Kα radiation source, Philips, X’Pert MPD) were studied to investigate the crystal structures, and Raman analysis (Takram P50C0R10) was used to investigate the composition of the samples.

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RESULTS AND DISCUSSION FESEM, TEM, and AFM Analysis. Figure 2a presents the FESEM image of exfoliated MoS2 NFs deposited on the FTO sheet using low voltage during EPD. As can be seen, MoS2 NFs have been deposited randomly on each other and have covered the entire surface of the FTO. Figure 2b,c presents FESEM images of the as-grown MoS2 QDs/NFs heterostructure deposited on the FTO sheets in two different scales. This figure reveals that the MoS2 QDs/NFs heterostructure has been deposited perpendicular to the FTO substrate when a high voltage was used during EPD. Figure 2b,c also shows that the entire surface of the FTO substrate is covered uniformly. MoS2 NFs (without QDs) can also be deposited vertically under high applied voltage. The SEM image of this electrode is shown in Figure S1 (Supporting Information). Transmission electron microscopy (TEM) was used to observe the MoS2 NFs, QDs, and QDs/NFs heterostructure and to determine the size of MoS2 QDs on the basal plane of MoS2 nanoflakes. Figure 2d,e shows TEM images of MoS2 NFs (which are obtained from bath sonication without tip sonication) and QDs/NFs, respectively. Clearly, the MoS2 QDs can be seen on MoS2 NFs. The Figure 2e inset shows a high-resolution TEM (HRTEM) image of a MoS2 QD placed on the basal plane of the MoS2 NF. The standing waves produced during tip-ultrasonic treatment are believed to vibrate the lamellar particles, and prolonged vibration results in the formation of quantum dots along with MoS2 nanoflakes. For comparison, Figure 2f shows a TEM image of the MoS2 QDs (without NFs) that are obtained from the prolonged tipsonication process and high centrifugation rate. Clearly MoS2 QDs have an average diameter of ∼5 nm. For further investigation of the morphology and thickness of the as-formed MoS2 NFs, quantum dots, and the underlying nanoflakes, AFM topography images were analyzed. AFM images of MoS2 NFs, QDs/NFs, and QDs on mica substrates are shown in Figure 2g−i, respectively. For the acquisition of the AFM image, a drop of MoS2 solution was cast on a mica substrate. Then, AFM images were taken from various parts of the sample surface. The surface of mica is very smooth, and the measurement error in the height profile measurements of the AFM analysis is 0.1 nm. Thus, single-layer (or two-layer) MoS2 QDs with a thickness of about ∼0.65 nm (∼1.23 nm)22−24 can be determined on the mica substrate. Height profiles of the samples along line 1, 2, and 3 related to MoS2 NFs, QDs/NFs, and QDs are shown in Figure 2j−l, respectively. As can be seen, the topographic height of the MoS2 QDs is less than 1.5 nm, suggesting that the MoS2 QDs are single-layer (or two-layer). According to the obtained FESEM, TEM, and AFM images, the MoS2 NFs have the average thickness of 20 nm and lateral size of 400 nm; the MoS2 QDs are single-layer (or two-layer) and have the average lateral size of 5 nm, and many MoS2 QDs exist on the basal plains of MoS2 NFs. Using Figure 2h, the surface coverage ratio (QD area/MoS2 NF area) was calculated to be around 20% in the MoS2 QDs/NFs nanostructure. Optical Analysis, XRD Patterns, and Raman Characterization. The absorption spectra of the dispersed bulk MoS2, QDs, and QDs/NFs solutions (in water−ethanol) are presented in Figure 3. General features of the absorption

Figure 3. UV−vis absorption spectra of the bulk, QDs, and QDs/NFs of MoS2.

spectrum of the dispersed MoS2 powder are in good agreement with other reports.25 As shown in Figure 3, the spectrum of bulk MoS2 displays four characteristic peaks at 680, 620, 450, and 400 nm (named as A, B, C, and D peaks, respectively). A and B, located at 680 and 620 nm, are excitonic transitions arising from spin orbit splitting of the K point of the Brillouin zone at the top of the valence band (VB).26,27 C and D excitonic peaks located at 450 and 400 nm are related to interband transitions from the occupied dz2 orbital to unoccupied dxy,x2−y2 and dxz,yz orbitals.27 Exfoliation of bulk MoS2 to few-layer nanoflakes leads to a blue-shift in A and B peaks. On the other hand, when microcrystals of MoS2 are changed completely into their quantum dots, the absorption spectra change drastically. The excitonic peaks disappear, and the band edge shifts to a shorter wavelength (λ < 350 nm).28 Therefore, simultaneous coexistence of QDs and NFs of MoS2 (in the QDs/NFs heterostructure form) resulted in blue-shift of A and B peaks and enhancement of absorption at the UV region. Figure 4 presents the XRD patterns of the raw bulk MoS2 powder and prepared MoS2 QDs/NFs. All the recorded diffraction peaks of the MoS2 bulk powder can be indexed to the 2H-MoS2 phase consistent with other reports.29,30 Periodicity in the c-axis is evident for the bulk material, with a strong peak (002) observed at 2θ = 14.3°. Previous studies

Figure 4. XRD patterns of bulk MoS2 and MoS2 QDs/NFs heterostructure. 8377


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vertical structure offers the efficient charge transport from the substrate to its active edge sites through individual layers, leading to minimizing ohmic losses.41 The electrochemical H2 evolution activities of vertical MoS2 QDs/NFs/FTO, vertical MoS2 NFs/FTO, and exfoliated (nonvertical) MoS2/FTO electrodes were investigated via LSV technique. The results are shown and compared in Figure 6a. As can be seen, the overpotential of vertical MoS2 QDs/ NFs/FTO electrodes is clearly lower than those of vertical MoS2 NFs/FTO and exfoliated (nonvertical) MoS2/FTO. For example, at a given current of −10 mA/cm2, the overpotentials of nonvertical MoS2/FTO and vertical MoS2 QDs/NFs/FTO are −0.49 and −0.25 V (versus reversible hydrogen electrode, RHE), respectively. Corresponding Tafel plots of the electrodes with calculated slopes can be seen in Figure 6b. The Tafel plot slope of the vertical MoS2 QDs/NFs/FTO structure is 74 mV/ decade which is significantly smaller than the slope obtained for the exfoliated (nonvertical) MoS2 electrode (143 mV/decade). The smaller Tafel slope means that, to reach a given current, lower applied voltage is needed.42 In other words, higher current can be obtained with lower power consumption. The obtained Tafel slope for our easily synthesized unique structure of vertical MoS2 QDs/NFs/FTO is much smaller than41,43−45 or comparable46−49 to those measured for many MoS2 structures obtained via high-cost and complicated procedures. Table S1 (Supporting Information) presents more comparison of various MoS2 nanostructures’ catalytic activities toward the HER reported in the literature recently. The Tafel slope of 143 mV/decade reveals that the Volmer reaction (H+ + e− → Hads) is a rate-determining step (RDS),50 and electron transfer from the conductive substrate to the top of the electrode is slower than other reaction steps toward H+ to H2 conversion. On the other hand, the Tafel slope of 74 mV/decade (lower than 120 mV/decade) of vertical MoS2 QDs/NFs/FTO indicates that the charge transfer reactions proceed much more quickly for this electrode.51 Therefore, the lower overpotential of the MoS2 QDs/NFs/FTO heterostructure can be attributed to the improved electrical conduction pathway in this electrode because of its vertical morphology.52 For verification of this concept, the exchange current densities (J0) of the electrodes were determined through Tafel plots. J0 is the current at zero overpotential in the absence of net electrolysis. It can be determined by extrapolating the Tafel plot slope to V = 0. When the J0 is higher, the reaction rate is faster.53 The obtained J0 values for nonvertical MoS2 NFs/FTO, vertical MoS2 NFs/FTO, vertical MoS2 QDs/NFs/FTO, and Pt electrodes are 0.09, 0.12, 0.15, and 0.2 mA/cm2, respectively. The higher J0 value of vertically aligned MoS2 NFs/FTO as compared to that of nonvertical MoS2 NFs/FTO verifies that charge carrier transport is more efficient in the vertical structure. In addition, the electrode containing vertical MoS2 QD/NFs/FTO presented a higher exchange current density in comparison with vertical MoS2 NFs without QDs, which could be attributed to the improved numbers of active edges in MoS2 QDs.54 Therefore, both vertical alignment (to facilitate charge carrier transport) and presence of MoS2 QDs (to increase the active edge sites) are key factors to enhance the catalytic activity of MoS2 nanosheets toward the HER. The electron transport conditions of these two electrodes were further investigated through Nyquist plots obtained via EIS, and the results are shown in Figure 6c. Clearly, the semicircle obtained for vertical MoS2 QDs/NFs/FTO has an

have demonstrated that the (002) peak weakens (or disappears) when transition metal dichalcogenides are restricted in their multilayer (single-layer and quantum dots) forms,31,32 and the other peaks disappear approximately. Ren et al.33 have proven that, for most samples, when the size of a particle is very small, there is no visible signature seen in XRD patterns. Wu et al.34 have found that for WS2/MoS2 there is no signal or peak in the XRD pattern because of quantum dot formation. Shaijumon et al.18 have shown that for MoS2 multilayer the (002) peak weakened, and other peaks disappeared. As shown in Figure 4, the weakening of the (002) reflection in the XRD pattern confirms that the bulk MoS2 has successfully exfoliated to few-layer nanoflakes. The Raman spectra of bulk, exfoliated (nonvertical), and vertically grown QDs/NFs MoS2 are shown in Figure 5.

Figure 5. Raman spectra of bulk, novertical, and vertically deposited QDs/NFs MoS2.

Typically for a crystalline MoS2 film, two prominent peaks at ∼407 and ∼382 cm−1 would appear,35,36 which are generated by the out-of-plane vibration mode of sulfur atoms (often referred as A1g) and in-plane vibration mode of molybdenum atoms and sulfur atoms (referred as E12g), respectively.37 For all of the samples, the expected characteristic peaks of MoS2 are clearly observed. Apart from positions of these two characteristic peaks, E12g and A1g, the intensity ratio A1g/E12g also provides information on the texture of as-grown MoS2 thin films.38 E12g, which corresponds to the in-plane Mo−S phonon modes, is preferentially excited for terrace-oriented thin films, whereas A1g, corresponding to the out-of-plane Mo−S phonon modes, is preferential for edge-oriented thin films.38 As can be seen from Figure 5, the intensity ratio of A1g/E12g increased significantly after vertical deposition of the MoS2 heterostructure on the FTO substrate, clearly indicating increased vertical orientation, matching well with the FESEM observations (Figure 2). Electrochemical Activity toward the HER. It is wellknown that the HER activity of the MoS2 plane is driven by its edge sites, and the basal plane is catalytically inert.39 The edgeterminated (vertically aligned) MoS2 thin films possess the maximal active edge sites over a given geometric surface. Moreover, the intralayer conductivity of MoS2 is ∼2 orders of magnitude higher than interlayer conductivity.40 Therefore, this 8378


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Figure 6. (a) LSV curves, (b) Tafel plots, and (c) Nyquist plots of nonvertical and vertical MoS2 NFs/FTO and vertical MoS2 QDs/NFs/FTO electrodes in comparison with the Pt electrode. (d) Stability test for catalytic activity of the vertical MoS2 QDs/NFs/FTO electrode.

Table 1. Electrochemical Properties of Various MoS2 Electrodes sample name

overpotential (V) at −10 mA/cm2

Tafel slope (mV/decade)

J0 (mA/cm2)

Rct (Ω)

nonvertical MoS2 NFs/FTO vertical MoS2/FTO vertical MoS2 QDs/NFs/FTO Pt

−0.49 −0.37 −0.25 −0.08

143 102 74 30

0.09 0.12 0.15 0.2

43 28 16 3

was examined by a repeating LSV test, and the result is presented in Figure 6d. Clearly, the LSV curve has been nearly left unchanged after repeating the test for 1000 cycles. It is worth noting that this excellent stability accompanied with high catalytic activity has been achieved via a facile, binder-free, and cost-effective procedure. Therefore, the introduced approach has great potential for reliable and large-scale catalytic applications. MoS2 QDs/NFs Formation Mechanism. The layered structure of MoS2 is constructed from the unit of S−Mo−S atomic trilayers. The van der Waals interactions between the neighboring units are weak compared to the strong of Mo−S covalent binding within the layer, and therefore, the sliding of planes is feasible.55 The cavitation process during sonication results in a strong acoustic streaming (liquid circulation) and the high shear stress near the bubble wall, leading to the formation of microjets near the solid surface (due to the asymmetric collapse of bubbles).56 Propagation of high-

obviously smaller diameter, indicating that this electrode has lower charge transfer resistance (Rct) in comparison with vertical MoS2 NFs/FTO and exfoliated (nonvertical) MoS2/ FTO. Rct values were calculated by fitting the EIS data with a simple Randles circuit, and the results are listed in Table 1. Accordingly, the R ct of the vertical MoS 2 QDs/NFs heterostructure is about 2.7 and 1.7 times smaller than those of nonvertical MoS2 NFs and vertical MoS2 NFs, respectively. Both LSV and EIS data are in good agreement with each other and strongly verify that vertical alignment of the MoS2 QDs/ NFs heterostructure with highly exposed active edge sites facilitates HER kinetics significantly. After the high catalytic activity of the vertical MoS2 QDs/ NFs/FTO electrode for the HER is verified, the stability of the synthesized samples is a concept of paramount importance for reliable and applicable utilization toward abiding H2 production through the electrocatalytic HER. Therefore, the stability of catalytic activity of the vertical MoS2 QDs/NFs/FTO electrode 8379


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Figure 7. (a) Schematic illustration to present the effects of bath and tip sonication on the formation of MoS2 nanostructures. Effect of (b) high and (c) low applied electrical voltage on deposition alignment of MoS2 NFs under parallel electrical field using the electrophoretic method. (d) Catalytic activity and charge transport condition of a (left) nonvertical and (right) vertically aligned MoS2 QDs/NFs/FTO electrode.

other hand, a low applied potential creates weak E, low induced charges, and therefore weak electrical force. As a result, the MoS2 NFs will be aligned randomly with no preferential direction, leading to random deposition of MoS2 NFs on the FTO substrate (Figure 7c). As was discussed, the most important parameter affecting deposition alignment is applied voltage rather than deposition time. Low applied voltage results in weak electrical field (E) (typically lower than 15 V/cm), and the electrophoretic process leads to nonvertical alignment of the MoS2 QDs/NFs. On the other hand, strong applied E (typically more than 15 V/cm such as 20 V/cm) leads to vertical alignment of the MoS2 QDs/ NFs. If the applied E is lower than critical value (Ecr = 15 V/ cm), long or short electrophoretic deposition time cannot result in vertical alignment. Above Ecr, the optimum time for vertical deposition in the electrophoretic process was found to be 4 min. After this time, excess nanoflakes are deposited above vertical nanoflakes leading to collapse of all nanoflakes on the FTO substrate. Therefore, vertical deposition changes to a random and nonvertical morphology. HER Mechanism on the Vertically Grown MoS2 QDs/ NFs Heterostructure. The catalytic activity and charge electron condition of vertically aligned MoS2 QDs/NFs are schematically illustrated in Figure 7d. When MoS2 QDs/NFs/ FTO is used as a cathode, the electrons are injected to the conductive FTO substrate, and then should be transferred to MoS2 flakes and QDs. When the MoS2 are deposited with a nonvertical alignment (Figure 7d, left), many nanoflakes cannot make direct contact with the FTO to receive electrons. Thus, the electrons are enforced to transfer from one MoS2 nanoflake to others. This direction is not feasible for electrons, and as a result, the charge transport resistance increases significantly (Figure 6c). In addition, many active edge sites of MoS2 QDs/ NFs are covered by MoS2 nanoflakes overlapping. Therefore, the HER efficiency toward H2 production decreases under these conditions (low active edge sites and high Rct). On the other hand, when MoS2 QDs/NFs are deposited in an ordered vertical alignment (Figure 7d, right), the nanoflakes’ over-

amplitude ultrasonic waves leads to the creation of voids and rapid formation of cavities (bubbles). These bubbles grow in the zone of negative pressure of the acoustic field, while they shrink in the zone of positive pressure. Continuous interaction between the bubbles and the acoustic field causes the deposition and violent collapse of the bubbles. The implosion of bubbles can create high-speed jets and intense shock waves on the surface of bulk materials and leads to the creation of nanometric materials. These processes can effectively fragmentize the top layers of MoS2 nanoflakes and minify them into QD dimensions. The mentioned processes are highly intense and localized when a tip sonicator is used in comparison with a bath sonicator. Ultrasound waves created inside a bath sonicator can only induce enough shear force to overcome a weak van der Waals bond across the MoS2 layers. Therefore, bath-sonication step results in thinning the bulk MoS2 and does not affect the lateral size significantly. It is necessary to use a more intense and localized sonication effect to decrease the lateral size of few-layer MoS2 flakes, which can be provide via a tip sonicator. In conclusion, for the preparation of a unique morphology of MoS2 QDs on few-layer MoS2 nanoflakes, use of both tip and bath sonicators is necessary (Figure 7a). Vertical Deposition Mechanism. It is well-known that the MoS2 flakes obtained via solvent-exfoliation methods are electrically charged.21 Applied external potential (ΔV) generates an even electrical field (E) perpendicular to the FTO surface during the electrophoretic process. This electrical field induces additional electrical charges on the surface of MoS2 QDs and MoS2 NFs. These electrical charges transport within the surface layers of MoS2 flakes and also result in MoS2 NF movement in the E direction. More charges (q) and strong E are created when high ΔV is applied during electrophoretic process, and as a result, an intense electrical force (FE) is applied to MoS2 NFs. This strong FE enforces the electrical charges to accumulate at the layer edges (Figure 7b). Therefore, MoS2 NFs are unwrapped and aligned parallel to E (normal to the FTO electrode) and then are moved toward the substrate electrode, resulting in vertical deposition. On the 8380


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(3) Tan, X.; Tahini, H. A.; Smith, S. C. p-Doped Graphene/Graphitic Carbon Nitride Hybrid Electrocatalysts: Unraveling Charge Transfer Mechanisms for Enhanced Hydrogen Evolution Reaction Performance. ACS Catal. 2016, 6 (10), 7071. (4) Shi, Y.; Gao, W.; Lu, H.; Huang, Y.; Zuo, L.; Fan, W.; Liu, T. Carbon-Nanotube-Incorporated Graphene Scroll-Sheet Conjoined Aerogels for Efficient Hydrogen Evolution Reaction. ACS Sustainable Chem. Eng. 2017, 5 (8), 6994. (5) Zhang, Y.; Zuo, L.; Huang, Y.; Zhang, L.; Lai, F.; Fan, W.; Liu, T. In-situ growth of few-layered MoS2 nanosheets on highly porous carbon aerogel as advanced electrocatalysts for hydrogen evolution reaction. ACS Sustainable Chem. Eng. 2015, 3 (12), 3140. (6) Tong, J.; Li, Q.; Li, W.; Wang, W.; Ma, W.; Su, B.; Bo, L. MoS2 Thin Sheet Growing on Nitrogen Self-Doped Mesoporous Graphic Carbon Derived from ZIF-8 with Highly Electrocatalytic Performance on Hydrogen Evolution Reaction. ACS Sustainable Chem. Eng. 2017, 5 (11), 10240. (7) Samadi, M.; Sarikhani, N.; Zirak, M.; Zhang, H.; Zhang, H.-L.; Moshfegh, A. Z. Group 6 transition metal dichalcogenide nanomaterials: synthesis, applications and future perspectives. Nanoscale Horizons 2018, 3 (115), 90. (8) Zong, X.; Yan, H.; Wu, G.; Ma, G.; Wen, F.; Wang, L.; Li, C. Enhancement of photocatalytic H2 evolution on CdS by loading MoS2 as cocatalyst under visible light irradiation. J. Am. Chem. Soc. 2008, 130 (23), 7176. (9) Chen, G.; Li, D.; Li, F.; Fan, Y.; Zhao, H.; Luo, Y.; Yu, R.; Meng, Q. Ball-milling combined calcination synthesis of MoS2/CdS photocatalysts for high photocatalytic H2 evolution activity under visible light irradiation. Appl. Catal., A 2012, 443, 138. (10) Jaramillo, T. F.; Jørgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Identification of active edge sites for electrochemical H2 evolution from MoS2 nanocatalysts. Science 2007, 317 (5834), 100. (11) Kim, J.; Byun, S.; Smith, A. J.; Yu, J.; Huang, J. Enhanced electrocatalytic properties of transition-metal dichalcogenides sheets by spontaneous gold nanoparticle decoration. J. Phys. Chem. Lett. 2013, 4 (8), 1227. (12) Kibsgaard, J.; Chen, Z.; Reinecke, B. N.; Jaramillo, T. F. Engineering the surface structure of MoS2 to preferentially expose active edge sites for electrocatalysis. Nat. Mater. 2012, 11 (11), 963. (13) Ye, G.; Gong, Y.; Lin, J.; Li, B.; He, Y.; Pantelides, S. T.; Zhou, W.; Vajtai, R.; Ajayan, P. M. Defects engineered monolayer MoS2 for improved hydrogen evolution reaction. Nano Lett. 2016, 16 (2), 1097. (14) Lu, X.; Lin, Y.; Dong, H.; Dai, W.; Chen, X.; Qu, X.; Zhang, X. One-Step Hydrothermal Fabrication of Three-dimensional MoS2 Nanoflower using Polypyrrole as Template for Efficient Hydrogen Evolution Reaction. Sci. Rep. 2017, 7, 42309. (15) Liu, C.; Kong, D.; Hsu, P.-C.; Yuan, H.; Lee, H.-W.; Liu, Y.; Wang, H.; Wang, S.; Yan, K.; Lin, D. Rapid water disinfection using vertically aligned MoS2 nanofilms and visible light. Nat. Nanotechnol. 2016, 11 (12), 1098. (16) Zirak, M.; Zhao, M.; Moradlou, O.; Samadi, M.; Sarikhani, N.; Wang, Q.; Zhang, H.-L.; Moshfegh, A. Controlled engineering of WS2 nanosheets−CdS nanoparticle heterojunction with enhanced photoelectrochemical activity. Sol. Energy Mater. Sol. Cells 2015, 141, 260. (17) Zirak, M.; Ebrahimi, M.; Zhao, M.; Moradlou, O.; Samadi, M.; Bayat, A.; Zhang, H.-L.; Moshfegh, A. Fabrication and surface stochastic analysis of enhanced photoelectrochemical activity of a tuneable MoS2−CdS thin film heterojunction. RSC Adv. 2016, 6 (20), 16711. (18) Gopalakrishnan, D.; Damien, D.; Shaijumon, M. M. MoS2 quantum dot-interspersed exfoliated MoS2 nanosheets. ACS Nano 2014, 8 (5), 5297. (19) Carey, B. J.; Daeneke, T.; Nguyen, E. P.; Wang, Y.; Ou, J. Z.; Zhuiykov, S.; Kalantar-Zadeh, K. Two solvent grinding sonication method for the synthesis of two-dimensional tungsten disulphide flakes. Chem. Commun. 2015, 51 (18), 3770. (20) Nguyen, E. P.; Carey, B. J.; Daeneke, T.; Ou, J. Z.; Latham, K.; Zhuiykov, S.; Kalantar-zadeh, K. Investigation of two-solvent grinding-

lapping is minimized, and maximal active edge sites are available for catalytic reactions. Moreover, the vertical morphology provides a low resistance and straightforward direction for electron transport with minimal ohmic losses (Figure 6c). Highly exposed active sites and a facile electron transport direction result in high HER performance.

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CONCLUSIONS The MoS2 quantum dots (QDs)/MoS2 nanoflakes (NFs) heterostructure was prepared via combining bath and tip sonication in mixed water−ethanol solution. Following a simple electrophoretic and then a hydrothermal method, the MoS2 QDs/NFs were deposited on FTO substrate in a vertically aligned morphology. This facile synthesized unique structure provides highly exposed active edge sites for the HER. In addition, vertical MoS2 alignment of nanoflakes facilitates charge carrier transport and decreases ohmic losses. Therefore, the vertical MoS2 QDs/NFs/FTO electrode exhibited high catalytic activity for the electrochemical HER with excellent stability. The low-cost, facile, binder-free, and straightforward preparation procedure, high catalytic activity, and more importantly excellent stability of the vertical MoS2 QDs/ NFs/FTO electrode make it a promising candidate for applicable cost-effective catalytic purposes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b00441. SEM image of vertically deposited MoS2 NFs (without QDs) and comparison of various MoS2 nanostructures’ catalytic activities toward the HER recently reported in the literature (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Esmaiel Saievar-Iranizad: 0000-0002-8545-2107 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Research and Technology Council of the Tarbiat Modares University and the Hakim Sabzevari University for financial support.

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REFERENCES

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