Nanoflakes Heterostructure

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Vertically Aligned MoS2 Quantum dots/Nanoflakes Heterostructure: Facile Deposition with Excellent Performance toward Hydrogen Evolution Reaction Amir Bayat, Mohammad Zirak, and Esmaiel Saievar Iranizad ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00441 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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ACS Sustainable Chemistry & Engineering

Vertically aligned MoS2 quantum dots/nanoflakes heterostructure: facile deposition with excellent performance toward hydrogen evolution reaction Amir Bayat†, Mohammad Zirak§, Esmaiel. Saievar-Iranizad†* †

Department of physics, Faculty of Basic Science, Tarbiat Modares University, Tehran, P.O. Box: 14115-111, Islamic Republic of Iran §

Department of Physics, Hakim Sabzevari University, Sabzevar, P. O. Box 961797648, Islamic Republic of Iran

KEYWORDS: MoS2 quantum dots, vertical alignment, high catalytic activity, stability, hydrogen evolution

*

Corresponding author: [email protected]

1 ACS Paragon Plus Environment

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ABSTRACT

A facile, low-cost and straightforward procedure was developed for vertically 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 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 there exist many single or two layer MoS2 quantum dots with lateral size of ~ 5 nm in average on the basal plains of MoS2 nanoflakes. Electrochemical measurements indicated vertical MoS2 QDs/NFs/FTO electrode has Tafel slope of 74 mV/decade and charge transport resistance (Rct) of 16 Ω which is ~ 1.9 and 2.7 times smaller than of Tafel slope and Rct of non-vertical MoS2 NFs/FTO electrode, respectively. This unique morphology exhibited excellent stability for electrocatalytic HER after repeating linear sweep voltammetry (LSV) test for 1000 cycles.

INTRODUCTION Up to now, platinum is known to be the material with the best catalytic performance for hydrogen evolution reaction (HER) because of its excellent stability and very low hydrogen evolution overpotential1. But its scarcity and high price limit industrial application of


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platinum. Hence, considerable efforts have been undertaken to eliminate the expensive Pt and find an alternative counter electrode, that is highly active, stable, cost-effective, easy to handle and composed of earth-abundant materials in order to advance the production of H2 fuel through water splitting via HER. Several compounds and materials such as carbides2, nitrides3, graphite and graphene composites4 and two-dimensional (2D) transition metal dichalcogenides (TMDs)5 have been introduced as promising Pt alternatives. Among these materials, 2D TMDs nanoflakes have recently attracted renewed and tremendous interests for HER application because of their metal-free composition, high activity, excellent stability, and economic cost-effective aspects. Molybdenum disulfide (MoS2) is the most studied member of TMDs family, specially for HER application6-7. The results of many studies represented that MoS2 can exhibit the catalytic activity even higher than Pt catalyst. Zong et al. experimentally exhibited that photocatalytic activity of MoS2-loaded CdS was even higher than that of Pt/CdS under the same reaction conditions8. The H2 evolution rate up to 1315 µmol.h−1 was obtained using the MoS2/CdS photocatalyst which was much higher than 488 µmolh−1 reported for Pt/CdS photocatalyst9. Because of these fascinating results, MoS2

have been rapidly gaining

momentum in the economic HER area and have given a fresh impetus to achieve this longconceived goal of Pt replacement. Molybdenum disulfide (MoS2) is a transition metal dichalcogenide (TMD) compound with a layered structure similar to graphite with a trigonal prismatic structure. It is verified that basal plane of MoS2 is inert for hydrogen reduction, while the edge sites of MoS2 are catalytically active10. 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



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employed to synthesis of MoS2 micro-nano-materials with novel and superior morphologies in order to optimize the active edge sites of MoS2 over the inert basal plane sites in a controllable manner. For example, using amorphous defective MoS2 nanosheets11, mesoporous MoS2 nanostructures12, lowering the thickness of MoS2 nanosheets toward single layer13, Construction of three-dimensional (3D) architectures14 and synthesis of vertically aligned MoS2 nanosheets15 are some of these strategies. But, 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 waves irradiation16-17. 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-methyl-2-pyrrolidone (NMP), which has been recommended in several studies18, has been employed as a high yield reference. Even though high yields are obtained when using NMP, removal of solvent residues is difficult due to its relatively high boiling point (202 °C) and surface tension (40.79 m Jm-2 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 metabolised to produce hydrogen cyanide, which is the source of the observed toxic effects20. Furthermore, NMP and ACN aren’t appropriate solutions for electrophoretic process. The mixed-solvent strategy (MSS) has been developed to overcome these drawbacks21. This strategy uses the mixture of abundant and easy accessible solvents such as water and ethanol which are eco-friendly with low boiling temperatures and can be


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readily removed from the systems. Neither water nor ethanol can properly disperse MoS2 bulk powder with a high concentration, but a mixture of them can sufficiently disperse MoS2 powder to achieve a few-layer MoS2 nanosheets dispersion21. The few-layer MoS2 nanosheets dispersion in mixed water-ethanol solvent should be then deposited on a conductive substrate to investigate the electrochemical catalytic activity of MoS2 flakes toward hydrogen production. Although there are many report using MSS to produce few-layer MoS2 nanosflakes suspension, preparing 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 production of a novel MoS2 QDs/NFs morphology using MSS. Thereafter, the MoS2 QDs/NFs were deposited vertically on FTO substrates via 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 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 economic HER, because it allows us to synthesize cost-effective large-scale vertically deposited MoS2 QDs/NFs.

EXPERIMENTAL Materials: All materials and chemical reagents were used without any further purification. Bulk MoS2 powder was provided from Aladdin Reagent Inc, China and absolute ethanol (Merck) was provided.



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Preparation of MoS2 QDs/NFs heterostructure solution: A highly dispersed suspension of MoS2 QDs/NFs heterostructure was prepared using a very simple liquid exfoliationbreaking technique involving a bath-tip sonication (cavitation process) followed by centrifugation in 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 rest under ambient condition, the 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 degradation. The beaker was put into an ice/water system to maintain the surrounding of 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. MoS2 NFs solution was obtained in a same situation without tip sonication process. To obtain MoS2 QDs solution, tip-sonication process was done for 5 h under same situation and supernatant containing MoS2 QDs was obtain after centrifugation of the solution at 7000 rpm. Preparation of vertical MoS2 QDs/NFs/FTO electrode: Electrophoretic deposition process (EPD) was used for vertically deposition of MoS2 QDs/NFs on the FTO substrates. At first, FTO substrates were washed in a solution of DI water and ethanol and dried in a nitrogen flow. Then, MoS2 nanostructures were deposited on the pre-cleaned FTO substrates via EPD process. In the EPD process, two parallel FTO substrates were immersed in the MoS2 QDs/NFs solution (or MoS2 NFs solution) with a distance of 1 cm at an optimized


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applied DC voltage of 20 V (or 8 V) for 4 min. High applied voltage (20 V) resulted in vertical deposition of MoS2 nanoflakes. Non-vertical MoS2 NFs were obtained via low applied voltage (8 V). Finally, to obtain more stable MoS2 QDs/NFs thin films, the samples were put in an autoclave including 50 ml DI water, Na2MoO4 (0.05 g) and thiourea (0.05 g). The autoclave was sealed and then placed at an oven at temperature of 180 ˚C for 1 h. The preparation procedure of the thin films has been 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 set up, MoS2 QDs/NFs (or MoS2 NFs) 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 to1 Hz was applied. Characterization techniques: Structural morphology of the samples was studied using a field emission scanning electron microscope (FESEM, HITACHI S-4160) and transmission electron microscope (TEM, CM30 300kV). 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 diffraction patterns (XRD, Co-Kα radiation source, Philips, X'Pert MPD) were studied in order to investigate the crystal structures and Raman analysis (Takram P50C0R10) was used to investigate the composition of the samples.



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

RESULTS AND DISCUSSION FESEM, TEM and AFM analysis: Figure 2(a) presents FESEM image of exfoliated MoS2 NFs deposited on the FTO sheet using low voltage during EPD. As it can be seen, MoS2 NFs have been deposited randomly on each other and have covered entire surface of the FTO. Figure 2(b and c) presents FESEM images of as-grown MoS2 QDs/NFs heterostructure deposited on the FTO sheets in two different scales. This figure reveals that the MoS2 QDs/NFs heterostructure have been deposited perpendicular to the FTO substrate when a high voltage was used during EPD. Figure 2(b and c) also show that the entire surface of the FTO substrate is covered uniformly. MoS2 NFs (without QDs) can also been deposited vertically under high applied voltage. The SEM image of this electrode has been shown in Figure S1 (supporting information).


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Transmission electron microscopy (TEM) was used to observe MoS2 NFs, QDs and QDs/NFs heterostructure and to determine the size of MoS2 QDs on the basal plane of MoS2 nanoflakes. Fig 2(d) and (e) show TEM images of MoS2 NFs (that obtained from bath sonication without tip sonication) and QDs/NFs, respectively. Clearly, the MoS2 QDs can be seen on MoS2 NFs. The Figure 2(e) inset shows a high resolution TEM (HRTEM) image of a MoS2 QD placed on basal plane of MoS2 NF. The standing waves produced during tipultrasonic 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 2(f) shows TEM image of the MoS2 QDs (without NFs) that obtained from prolonged tip sonication process and high centrifugation rate. Clearly MoS2 QDs have 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 have been shown in Figure 2(g), (h) and (i), respectively. To obtain AFM image, a drop of MoS2 solution was casted 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 2(j), (k) and (l), respectively. As it can be seen, topographic height of the MoS2 QDs is less than 1.5 nm, suggesting that the MoS2 QDs are single (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 (or two) layer and have the average lateral size of 5 nm and there exist many MoS2 QDs on the basal plains of MoS2 NFs. Using



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Figure 2(h), the surface coverage ratio (QDs area/MoS2 NFs area) was calculated to be around 20 % in MoS2 QDs/NFs nanostructure.


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Figure 2. (a) FESEM image of the exfoliated MoS2 deposited on FTO substrate in a non-vertical alignment, (b) and (c) are FESEM images of vertically aligned MoS2 QDs/NFs heterostructure deposited on FTO substrate. (d) TEM image of MoS2 NFs and (e) is the TEM image of MoS2 QDs/NFs and the inset of (e) is HRTEM image that clearly shows the MoS2 QD on MoS2 flake. For comparison the TEM image of bare MoS2 QDs (without MoS2 NFs, obtained via prolonged tip-sonication and high rate centrifugation) has been shown in (f). AFM images of MoS2 NFs,

QDs/NFs and QDs are shown in (g), (h) and (i), respectively. (j), (k) and (l) are height profiles of MoS2 NFs, QDs/NFs and QDs casted on mica substrate.



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Optical analysis, XRD patterns and Raman characterization: The absorption spectra of the dispersed bulk MoS2, QDs and QDs/NFs solutions (in water/ethanol) have been presented in Figure 3. General features of absorption spectrum of the dispersed MoS2 powder are in good agreement with other reports25. 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 K point of the Brillouin zone at the top of 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,

2

2

x -y

and dxz,yz orbitals27. Exfoliation of bulk MoS2

to few-layer nanoflakes leads to a blue-shift in A and B peaks. In 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 co-existence 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 UV region.

Figure 3. UV-Vis absorption spectra of the bulk, QDs and QDs/NFs of MoS2


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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 reports29-30. Periodicity in c-axis is evident for the bulk material, with a strong peak (002) observed at 2θ=14.3˚. Previous studies have demonstrated that the (002) peak weakens (or disappears) when transition metal dichalcogenides are restricted in their multilayer (single layer and quantum dots) forms31-32 and the other peaks disappear approximately. Ren et al.33 have proved that for most samples, when the size of 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 XRD pattern because of quantum dots 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 (002) reflection in the XRD pattern confirms that the bulk MoS2 has successfully exfoliated to few-layer nanoflakes.

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



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The Raman spectra of bulk, exfoliated (non-vertical) and vertically grown QDs/NFs MoS2 have been shown in Figure 5. Typically for a crystalline MoS2 film, two prominent peaks at ~407 and ~382 cm−1 would appear35-36, which are generated by 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 ), respectively37. 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 films38. E12g, which is corresponding 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 films38. As it can be seen from Figure 5, the intensity ratio of A1g/E12g increased significantly after vertical deposition of MoS2 heterostructure on the FTO substrate, clearly indicating increased vertical orientation, matching well with the FESEM observations (Figure 2).


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

Electrochemical activity toward HER: It is well known that the HER activity of MoS2 plane is driven by its edge sites and basal plane is catalytically inert39. The edge terminated (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 conductivity40. Therefore, this vertical structure offers the efficient charge transport from the substrate to its active edge sites through individual layers, leading to minimizing ohmic losses41. The electrochemical H2 evolution activities of vertical MoS2 QDs/NFs/FTO, vertical MoS2 NFs/FTO and exfoliated (non-vertical) MoS2/FTO electrodes were investigated via LSV technique. The results have been shown and compared in Figure 6(a). As it can be seen, the overpotential of vertical MoS2 QDs/NFs/FTO electrodes is clearly lower than vertical MoS2 NFs/FTO and exfoliated (non-vertical) MoS2/FTO. For example, at a given current of -10



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mA/cm2, the overpotentials of non-vertical MoS2/FTO and vertical MoS2 QDs/NFs/FTO are 0.49 and -0.25 V (vs RHE), respectively. Corresponding Tafel plots of the electrodes with calculated slopes can be seen in Figure 6(b). The Tafel plot slope of vertical MoS2 QDs/NFs/FTO structure is 74 mV/decade which is significantly smaller than that slope obtained for exfoliated (non-vertical) MoS2 electrode (143 mV/decades). Smaller Tafel slope means that to reach a given current, lower applied voltage is needed42. 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, 4345

or comparable46-49 to that measured for many MoS2 structures obtained via high-cost and

complicate procedures. Table S1 (supporting information) presents more comparison of various MoS2 nanostructures catalytic activity towards HER reported in literature recently. Tafel slope of 143 mV/decade reveals that Volmer reaction (H+ + e- → H

ads)

is 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. In other hand, Tafel slope of 74 mV/decade (lower than 120 mV/decade) of vertical MoS2 QDs/NFs/FTO indicate that the charge transfer

reactions proceeds much faster for this electrode51.

Therefore, the lower overpotential of MoS2 QDs/NFs/FTO heterostructure can be attributed to the improved electrical conduction pathway in this electrode because of its vertical morphology52. To verify 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. The higher the J0 the faster is the reaction rate53. The obtained J0 values for non-vertical 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. Higher J0 value of vertically aligned MoS2 NFs/FTO than nonvertical MoS2 NFs/FTO verifies that charge carriers transport is more efficient in vertical


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structure. in addition, the electrode containing vertical MoS2 QD/NFs/FTO presented 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 towards HER. The electron transport conditions of these two electrodes were more investigated through Nyquist plots obtained via EIS, and the results have been shown in Figure 6(c). Clearly, the semi-circle obtained for vertical MoS2 QDs/NFs/FTO has obviously smaller diameter, indicating that this electrode has lower charge transfer resistant (Rct) in comparison with vertical MoS2 NFs/FTO and exfoliated (non-vertical) MoS2/FTO. Rct values were calculated by fitting the EIS data with a simple Randles circuit, and the results have been listed in Table 1. Accordingly, the Rct of vertical MoS2 QDs/NFs heterostructure is about 2.7 and 1.7 times smaller than that of non-vertical 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 MoS2 QDs/NFs heterostructure with highly exposed active edge sites facilitate HER kinetics significantly.



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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 Pt electrode. (d) Stability test for catalytic activity of vertical MoS2 QDs/NFs/FTO electrode.

After verifying the high catalytic activity of the vertical MoS2 QDs/NFs/FTO electrode for HER, the stability of the synthesized samples is concept of paramount importance for reliable and applicable utilization toward abiding H2 production through electrocatalytic HER. Therefore, the stability of catalytic activity of the vertical MoS2 QDs/NFs/FTO electrode were examined by repeating LSV test, and the result has been presented in Figure 6(d). Clearly, the LSV curve has been nearly left unchanged after repeating the test for 1000 cycles. It is worth to note that this excellent stability accompanying 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. Table 1: electrochemical properties of various MoS2 electrodes


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Sample name

Overpotential (V) at -10 mA/cm2 -0.49

Non-vertical MoS2 NFs/FTO Vertical MoS2/FTO -0.37 Vertical MoS2 QDs/NFs/FTO -0.25 -0.08 Pt

Tafel slope J0 (mV/decade) (mA/cm2)

Rct (Ω)

143 102 74 30

43 28 16 3

0.09 0.12 0.15 0.2

MoS2 QDs/NFs formation mechanism: The layered structure of MoS2 is constructed from the unit of S-Mo-S atomic trilayers. The Vander 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 feasable55. 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 micro-jets near the solid surface (due to asymmetric collapse of bubbles)56. Propagation of high 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 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 weak Vander Waals bond across the MoS2 layers. Therefore, bath sonication step results in thinning bulk MoS2 and does not affect the lateral size significantly. It is necessary to use 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



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conclusion, to prepare unique morphology of MoS2 QDs on few-layer MoS2 nanoflakes, using both tip and bath sonicators are necessary (Figure 7-a). Vertical deposition mechanism: It is well kwon that the MoS2 flakes obtained via solventexfoliation methods are electrically charged21. Applied external potential (∆V) generates an even electrical field (E) perpendicular to FTO surface during 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 NFs movement in E direction. More charges (q) and strong E are created when high ∆V 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 7-b). Therefore, MoS2 NFs are unwrapped and aligned parallel to E (normal to FTO electrode) and then are moved toward substrate electrode, resulting in vertical deposition. On the other hand, low applied potential, create 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 7-c). As it was discussed, the most important parameter affecting on 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 non-vertical 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 electrophoretic process was found to be 4 min. After this time, excess nanoflakes are


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deposited above vertical nanoflakes leading to collapse of all nanoflakes on the FTO substrate. Therefore, vertical deposition changes to a random and non-vertical morphology. HER mechanism on vertically grown MoS2 QDs/NFs heterostructure: The catalytic activity and charge electron condition of vertically aligned MoS2 QDs/NFs has been schematically illustrated in Figure 7(d). When MoS2 QDs/NFs/FTO is used as a cathode, the electrons are injected to the conductive FTO substrate, and then should be transfer to MoS2 flakes and QDs. When the MoS2 are deposited with a non-vertical alignment (Figure 7d-left), many nanoflakes cannot contact directly to FTO to receive electrons. So, the electrons are enforced to transfer from one MoS2 nanoflakes to others. This direction is not feasible for electrons and as a result, the charge transport resistant increases significantly (Figure 6-c). In addition, many active edge sites of MoS2 QDs/NFs are covered by MoS2 nanoflakes overlapping. Therefore, 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 overlapping is minimized and maximal active edge sites are available for catalytic reactions. Moreover, vertical morphology provides a low resistant and straightforward direction for electron transport with minimal ohmic losses (Figure 6-c). Highly exposed active sites and facile electron transport direction result in high HER performance.



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

CONCLUSIONS MoS2 quantum dots (QDs)/MoS2 nanoflakes (NFs) heterostructure were 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 HER. In addition, vertical MoS2 alignment of nanoflakes facilitates charge carrier transport and decrease ohmic losses. Therefore, vertical 22 ACS Paragon Plus Environment

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MoS2 QDs/NFs/FTO electrode exhibited high catalytic activity for electrochemical HER with excellent stability. Low-cost, facile, binder-free and straightforward preparation procedure, high catalytic activity and more importantly, excellent stability of vertical MoS2 QDs/NFs/FTO electrode, make it a promising candidate for applicable cost-effective catalytic purposes.

ASSOCIATED CONTENT Supporting Information SEM image of vertically deposited MoS2 NFs (without QDs) and comparison of various MoS2 nanostructures catalytic activity towards HER recently reported in literature have been provided in supporting information file which can be found at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Esmaiel Saievar-Iranizad: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors would like to thank Research and Technology Council of the Tarbiat Modares University and the Hakim Sabzevari University for financial supports.

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TOC Vertically grown MoS2 quantum dots/MoS2 nanoflakes provide active edges sites and facilitate charge carriers transport, which is highly beneficial for stable HER. 27 ACS Paragon Plus Environment


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H+

Proton

H2 Hydrogen e-

Electron Electron pathway Mo atom S atom

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Nanoflakes Heterostructure

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