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Graphene as high performance electrocatalysts
international journal of hydrogen energy xxx (xxxx) xxx
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Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts Sunil P. Lonkar*, Vishnu V. Pillai, Saeed M. Alhassan** Department of Chemical Engineering, Khalifa University, P.O box 127788, Abu Dhabi, United Arab Emirates
article info
abstract
Article history:
A promising electrocatalyst material composed of 2D layered MoS2-WS2 heterostructure
Received 13 November 2018
hierarchically assembled into a 3D highly interconnected macroporous network of
Received in revised form
graphene was facilely fabricated. This in-situ synthesis method involves hydrothermal
17 March 2019
reaction followed by moderate thermal annealing which guarantees the uniform distri-
Accepted 20 March 2019
bution of the MoS2-WS2 heterojunctions within graphene matrix. The presence of 3D
Available online xxx
conductive and porous graphene network and the combined merits of MoS2 and WS2 endow the resulting 3D MoS2-WS2/graphene nanohybrids with unique conductivity path-
Keywords:
ways and channels for electrons and with outstanding electrocatalytic performance
3D assembly
towards enhanced hydrogen evolution reaction (HER). This 3D nanohybrid delivered the
Nanostructured hybrids
small overpotential of 110 mV, and the small Tafel slope of 41 mV per dec, demonstrating
Electrocatalysts
high HER activity. Furthermore, the resulting nanohybrids exhibit excellent stability as
Hydrogen evolution reaction
very trivial drop in the current density was noticed even after 2000 cycles. The superior
MoS2-WS2
electrocatalytic performance of 3D MoS2-WS2/graphene over other non-precious metal
Graphene
electrocatalysts is accredited to the robust synergism of 2D MoS2-WS2 with 3D graphene that offer ample active sites and improved conductivity for HER. The proposed approach can be further extended to modify other layered transition metal dichalcogenides with hierarchical 3D porous structure as a competent electrocatalysts for HER. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In order to address the aggravating energy and environment problems; a scalable, renewable yet environmentally clean alternative energy carrier systems are urgently required. On this front, hydrogen (H2) has emerged as a promising substitute to fossil fuel, primarily owing to its remarkable energy density and ecofriendly perspectives [1e6]. Especially, the production
of H2 using electrochemical water-splitting method is considered as highly appropriate, due to its efficiency, natural abundance and eco-friendly nature [2,7e12]. Nevertheless, to achieve broader practical attention of such electrocatalytic H2 and O2 production process, the advancement electrocatalysts those are of highly active and particularly sourced from Earthabundant elements is highly demanded [13e22]. Hence, the quest for exploration of suitable active materials other than
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S.P. Lonkar), [email protected] (S.M. Alhassan). https://doi.org/10.1016/j.ijhydene.2019.03.195 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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precious metal system and their hybridization by a facile preparation process is still hold significant importance. It is well-known that noble metals, platinum, iridium, ruthenium etc. and its alloys, exhibiting extraordinary electrocatalytic properties and considered as best electrocatalysts for HER [23e28]. However, their high cost and inadequate reserves hinder the sustainable hydrogen economy. Hence, the development of highly active HER catalysts that are economical and earth abundant is of prime importance. In this context, a myriad of nonprecious-metal materials including transition metal chalcogenides (TMDs), metal alloys and carbides, polymeric carbon nitride and have been pursued to enhance the electrocatalytic performance [13,29e32]. Amongst, the layered two-dimensional (2D) atomically-thick materials, i. e transition metal dichalcogenides (TMDs) especially molybdenum and tungsten sulfides (MoS2 and WS2), have been demonstrated as potential electrocatalysts owing to their high hydrogen binding activity, electrochemical stability, and abundance [33e36]. In further advancement, the heterostructured nanohybrids of these TMDs showed enhanced HER performance attributed due to the combined effect of these materials [36e38]. However, endowed with these fascinating properties, the electrocatalytic performance of these TMDs and their heterostructured hybrids is significantly restricted due to its low inherent electric conductivity and possible re-stacking under long cycles using high current density. Also, the limited surface area ensuing lower stability is responsible for the ordinary performance these electrocatalysts for HER applications [39,40]. In order overcome these issues, the use of TMD electrocatalysts composed with hierarchical structures in threedimensional (3D) form could offer an excellent structure integrity to achieve maximally exposing active edge sites, consequently, that guarantee the enhanced electrochemical durability without conceding their intrinsic electrocatalytic ability [41,42]. Recently, several 3D support have been used to incorporate the individual TMDs nanosheets that include carbonaceous materials such as graphene, CNTs, carbon cloth etc. and other metallic supports such as tungsten foil and nickel foams [43e47]. However, the resulting electrocatalyst efficiency for overall HER performances are still unsatisfactory considering the commercial hydrogen production. On other hand, bestowed with several fascinating properties, graphene based materials hold high potential as a 3D support that can accommodate TMD nanosheets and offer excellent surface area and electronic conductivity. So far, there exist only few reports on the nanohybrids that leverage advantage of graphene with TMDs for increasing their overall electrocatalytic performance [48,49]. Moreover, implementing the right synthesis strategy is highly critical in developing these 3D nanohybrid electrocatalyst because the common solvothermal methods derived materials displayed lower HER performance, ascribed to the poor TMD dispersion which could limit the active sites exposure and eventually lower their catalytic activity for the HER [50,51]. Therefore, it is imperative and of significant attention to develop facile procedure to fabricate heterostructured TMD/graphene nanohybrids which could ensure higher and well distributed active sites that could potentially enhance the overall electrocatalytic activity.
So far, numerous studies that report the fabrication of TMD nanoparticleegraphene composites are available, but there is no report using the heterostructured MoS2-WS2 nanosheets supported on 3D graphene as potential electrocatalysts. Recently, Zhou et al. [51] reported a very good HER activity for the electrocatalyst that is composed of WS2/graphene on to Ni foam in three-dimensional (3D) assembly. However, the considering the limitation of using Ni foam as a support in acidic electrolyte, these hybrids are not suitable from poor cyclic durability perspective. Moreover, the used synthesis protocol does not offer controllability of the active material deposition. Similarly, Chen et al. demonstrated the hydrothermal method to fabricate 3D MoS2egraphene hybrid using an isopropyl alcohol (IPA)/water system [52]. However, due to incompatible surface energies of TMDs precursors and IPA/ water mixture, it was difficult to achieve desired exfoliation of TMDs and required active sites density for the HER. Few reports also mention strong exfoliation ability toward TMDs in solvothermal process using organic solvents having high boiling points, such as N,N dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP). Although, few-layered TMDs can be achieved under such solvothermal treatment but the intricate processability is a major challenge [53]. Hence, if 3D nanohybrids of these heterostructured nanosheets with graphene is formed using facile hydrothermal process that could warrant higher active sites and better conductivity to achieve the remarkable HER activities. In this study, a facile integration of 2D MoS2-WS2 heterostructures within 3D graphene macrostructure was demonstrated while preserving their inherent properties that can be directly fabricated using simple hydrothermal processing that is sustainable and economical. Further, in this synergistic hybridization, 3D macroporous graphene network is expected to offer high specific area and minimize the phenomenon of aggregation in the in-situ formed TMDs nanosheets which inturn exposes more catalytic sites. Further, the strong interaction between these defect rich few layered 2D TMDs and conductive graphene substrate will increase electron and ion conductivity and subsequently formed conductive channels will offer effective electrolyte accessibility which is beneficial to the catalytic activity toward HER. Moreover, the interconnected porous network and high surface area boost the overall electrocatalytic performance with lower overpotential and long-term stability toward the HER. The structure and properties of the resulting nanohybrids have also been thoroughly investigated. The abundant catalytically active edges sites of MoS2-WS2 heterostructures and high surface areas are expected to endow resulting hierarchical 3D nanohybrids with excellent electrocatalytic HER performance governed by lower overpotential and higher current densities.
Materials and methods Graphite oxide (GO) synthesis GO was synthesized using improved Tours method with slight modification [54]. Briefly, 5 g of graphite flakes having 10 mesh particle sizes was thoroughly dispersed in the ice-cold mixture of 270 mL of sulfuric acid and 33 mL of phosphoric
Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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acid under constant stirring for 60 min. Successively, 2.8 g of potassium permanganate (KMnO4) was gradually added over 30 min under stirring, and the as-formed reaction mix was further mixed for 72 h at ambient conditions to achieve complete oxidation of graphite flakes. Further, the color of the reaction mixture was gradually changed from purple green to dark brown. After oxidation, the resulting oxidized graphite dispersion was cooled to ambient temperature and then reaction was completed by addition of 35% hydrogen peroxide (H2O2) solution. The color of final reaction mix was appeared to be golden yellow, affirming the highly oxidized graphite. The resulting GO dispersion was filtered and washed with 1 M HCl, and further dialyzed to eliminate impurities of residual salts and metal ions.
Preparation of 3D MoS2-WS2/graphene nanohybrids aerogels The TMD nanohybrid aerogels were prepared by using facile hydrothermal reaction followed by moderate thermal treatment. In a typical procedure, the ultra-sonicated GO aqueous dispersion (5 mg mL1) was mixed with aqueous solutions respective TMD precursor of MoS2 (ammonium tetrathiomolybdate; (NH4)2MoS4) and WS2 (ammonium tetrathiotungstate; (NH4)2WS4) under constant stirring. The resulting homogeneous mixture was hydrothermally processed in a seal-tight autoclave at 200 C for 4 h. Then the autoclave was cooled down and a blackish cylindrical hydrogel monolith was recovered which was carefully separated and thoroughly washed under the continuous de-ionized water stream to eliminate residual metallic ions and impurities. Further, the as-obtained 3D hydrogels composed of amorphous MoS2-WS2/ graphene nanohybrids were converted into 3D aerogels by using freeze drying method at liquid nitrogen temperature for 24 h. Subsequently, the resulting nanohybrid aerogels were thermal annealed at 600 C in quartz tube furnace under argon flow to ensure high crystallization of in-situ formed MoS2 and WoS2 phases and complete transformation RGO into graphene. The synthesis details are schematically presented in Scheme 1. The amount of graphene weight percent was varied from 0, 10, 20 and 30 wt-% and the as-prepared samples were marked as GMW-0, GMW-10, GMW-20 and GMW-30, respectively.
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Characterization The resulting 3D aerogels were thoroughly characterized by means of several characterization techniques. The morphologies were investigated by using scanning electron microscope (SEM) coupled with energy-dispersive X-ray analysis (EDX) was used (Zeiss (1540 XB). For the high resolution transmission electron microscope (HRTEM) imaging, the FEI Tecnai (G20) TEM operating at 200 kV was used. For the X-ray diffraction (XRD) measurements, the Philips X'Pert Pro X-ray diffractometer was used which was equipped with a scintillation counter and Cu Ka (l ¼ 1.5418 A) radiation reflection mode in the 2q range from 10 to 80 . The specific surface area and porosity of the resulting 3D NMG nanohybrids were obtained using ASPS 2010 (Micromeritics) BrunauerEmmettTeller (BET) nitrogen adsorptiondesorption at liquid N2 temperature. The samples were pretreated under high vacuum at 100 C for 24 h before N2 adsorption was performed using a Quantachrome Autosorb gas-sorption system. The Raman analysis was carried out on the powdered samples and studied using LabRAM HR Raman spectrometer with a laser excitation wavelength of 633 nm excitation line. X-ray photoelectron spectroscopy (XPS) analysis was performed using a SSX-100 system (Surface Science Laboratories, Inc.) equipped with a monochromated Al Ka Xray source, a hemispherical sector analyzer (HSA), and a resistive anode detector.
Electrocatalytic measurements for the HER The electrocatalytic performance of the resulting 3D nanohybrid aerogels were tested using a Biologic VP300 electrochemical workstation. The standard electrochemical cell (three-electrode) configured with reference electrode [saturated calomel electrode (SCE)] and platinum wire (counter electrode), respectively was employed. The working electrodes were fabricated from as synthesized 3D MoS2-WS2/ graphene nanohybrids by mixing it with conducting carbon black (CB), and polyvinylidene fluoride (PVDF) binder in the ratio of 80:10:10 wt-% and dispersing in to N-methyl-2-pyrrolidone (NMP). The resultant homogeneous mixture was
Scheme 1 e Schematic representation of the synthesis procedure to prepare MoS2/WS2egraphene hybrid aerogels. Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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uniformly casted on a graphite sheet and vacuum dried at 100 C to remove the solvent. Linear sweep voltammetry (LSV) tests were performed by using the current potential from 0.1 V to 0.4 V vs RHE at a sweeping rate of 3 mV s1 in 0.5 M H2SO4 (purged with argon) at ambient conditions. The measured potentials were calibrated in terms of reversible hydrogen electrode (RHE). The durability of the working electrode was assessed by continuous cyclic voltammetry (CV) tests between 0.4 V and 0.1 V vs RHE at a scan rate of 50 mV s1 for 2000 cycles. The electrochemical impedance spectroscopy (EIS) was performed between the frequencies ranging from 102e100 Hz using amplitude of 5 mV. The longevity of the resulting electrocatalysts was carried out by means of chronopotentiometry (CP) at the fixed current density of 10 mA/cm2 while monitoring the variation of applied potential during 30 h.
Result and discussion The 3D hybrid aerogels composed of 2D heterostructured MoS2-WS2 and graphene were hydrothermally fabricated followed by moderate thermal treatment. During hydrothermal process, graphene oxide (GO) sheets were expected to interact with molybdate and tungstate anions and this synergistic reduction process resulted in to 3D porous architectures having amorphous or poorly crystallized in-situ formed metal sulfides uniformly distributed into partially reduced GO (RGO) was achieved [55]. In second step, the resulting aerogels were thermally treated to obtain 3D assembly of highly crystallized heterostructured assembly of MoS2-WS2 uniformly decorated on thermally reduced graphite oxide (graphene) sheets, as demonstrated in Scheme 1. The surface morphologies of the MoS2/WS2egraphene nanohybrid aerogels were characterized by SEM as shown in Fig. 1. It is apparent that the surface morphology of the resulting nanohybrids is highly corrugated with interconnected pores and primarily consist the homogeneously dispersed MoS2/WS2 heterostructure nanosheets within ultrathin macroporous graphene network (Fig. 1a). Moreover, a lower-magnification SEM image (Fig. 1b) further confirms the surface robustness and homogeneity with dense confinement of the particles of resulting GMW nanohybrids verses the pristine graphene aerogel (Fig. S1). Hence, this unique 3D architecture effectively exposes the catalytic active sites and provide an interconnected conductivity path, which could greatly facilitate the electrocatalytic activity [56]. The EDX analysis of 3D nanohybrid aerogels (Fig. 1c) showed presence Mo, W, S and C elements peaks, which confirm the integrated structure of the resulting nanohybrids without any impurity. The energy dispersive X-ray spectroscopy (EDS) maps of GMW nanohybrids (Fig. S2) also confirms the uniform distribution of in-situ formed heterostructured assembly of MoS2-WS2 within graphene nanolayers. Further, the CeMoeW mixed element mapping (Fig. 2d) clearly indicates the close proximity of MoS2 and WS2 nanoparticles with carbon indicating uniform distribution of these nanolayers in the resulting 3D nanostructured aerogels. The contents of MoS2 and WS2 nanosheets are also calculated and tabulated in Table 1.
To further shed light on the morphological aspects of the resulting 3D nanohybrids, the TEM analysis was also performed. The TEM images shown in Fig. 2a clearly indicated that the intercrossed heterostructured MoS2-WS2 nanosheets are uniformly distributed throughout the crumpled graphene sheets to form heterostructured nanohybrid as evidenced from SEM analysis as well. Owing to their identical layered structure it's difficult to distinguish individual features of MoS2 and WS2. Further, the high-magnification TEM image of GMW-10 shows the existence of well-defined few layered MoS2-WS2 heterostructures with interconnected network. Furthermore, the few-layered structural arrangement of in-situ formed MoS2WS2 heterostructure was clearly evidenced from the HRTEM image (Fig. 2b). A confined layered crystal lattice structure with interlayer spacing of about 0.67 nm confirmed the correspondence with (002) lattice planes of both hexagonal MoS2 and WS2 as both of these TMDS holds similar crystal lattice structure [57,58]. The overall layer number of MoS2-WS2 was observed to be approximately 3e8. Furthermore, the crystalline structure of layered MoS2-WS2/graphene deduced from a selected area electron diffraction (SEAD) pattern (Fig. 2c) shows characteristic diffraction rings, corresponding to the (002), (100), (103) and (110) planes, respectively [59]. For high graphene content hybrids i. e GMW-30, the formation of oxide phases were clearly observed in TEM (Fig. S3). The crystal structural features and phase composition of the resulting 3D nanohybrids aerogels were investigated by using XRD analysis. The XRD diffractograms (Fig. 3) clearly shows that characteristic JCPDS patterns for MoS2 (JCPDS: 872416) and WS2 (JCPDS: 872417) showing peaks at 14.38, 28.81, 33.35, 39.68, 49.64, 58.79, and 69.33 corresponding to the (002), (004) (100), (103), (105), (110) and (200) peaks which reveal the presence of MoS2-WS2 heterostructures as the XRD patterns of both MoS2 and WS2 are very identical to each other [60]. Moreover, compared to pristine MoS2 (Fig. S3b), all the XRD peaks for nanohybrids was appeared to be broadened, indicating the formation of nanostructured MoS2-WS2 within graphene layers. Similarly, 3D MoS2-WS2/graphene samples (Fig. 3bed) show a broad peak around 24.2 (002) planes was ascribed due to the formation of graphene nanolayers after the in-situ reduction of graphite oxide (Fig. S4b). However, with high concentration graphite oxide samples (Fig. 3d), the graphene peaks have been overlapped with some of the oxides (Mo-WO3) that are formed due to the possible interactions with graphite oxide [61,62]. Fig. 4aed shows the chemical composition and elemental states of the as-synthesized 3D MoS2-WS2/graphene determined from XPS measurements. Fig. 4a shows the XPS survey spectrum of GMW-10 nanohybrid which indicates the existence of the C, Mo, W, O and S elements which confirm the coexistence of these elements within surface elemental composition of the resulting nanohybrid. Similarly, intensity decreases of O 1s peak with simultaneous peak intensity increase for C 1s versus pristine GO (Fig. S5) confirms that the GO was successfully reduced into graphene under thermal treatment. Similarly, the deconvoluted high resolution XPS spectra of Mo 3d peak (Fig. 4b) indicates two major peaks at 232.1 and 228.9 eV, affirming the presence of Mo 3d3/2 and Mo 3d5/2 orbitals of Mo in the Mo4þ oxidation state. Similarly, the XPS spectra of Wf (Fig. 4c) designates two main peaks at
Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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Fig. 1 e SEM images (a and b), EDAX spectra (c), and mixed CeMoeW EDS mapping d) of the GMW-10 nanohybrid.
binding energies of 32.3 and 34.5 eV assigned for W4þ oxidation state in WS2, which is also indicate the semiconducting prismatic 2H form. In addition, a small peak was observed at 37.4 eV, which are due to the presence of W6þ state originated from slightly oxidized WS2 phase. Further, the sulfur exhibits two peaks at 161.9 eV and 163.0 eV, which is attributed to the S 2p3/2 and S 2p1/2 orbitals. Overall XPS study indicates the formation of MoS2-WS2 heterostructure within macroporous graphene aerogel. The quantitative elemental analyses by XPS also reveal that the oxygen percentages in the nanohybrids increases with increase in GO content, which probably due to the trivial oxidation of in-situ formed MoS2-WS2 heterostructures by oxygenated functions of GO. The Raman spectrum of as prepared 3D nanohybrids showed in Fig. 5a displayed the characteristic Raman peaks corresponding to the in-plane E12g and out-of plane A1g vibrational modes derived from the relative motions of the Mo/W and S atoms in the MoS2-WS2 crystal, respectively. A slight redshift in these peaks of MoS2-WS2 in 3D nanohybrids, in compared to pristine MoS2-WS2 (Fig. S6) indicates the existence of few layered MoS2-WS2. Similarly, two unique signature peaks appeared at ~1587 cm1 and ~1341 cm1 was ascribed to the G-band and D-band peaks of graphene layers. Typically, the D band corresponds to the carbon in disordered form arising the defects and edges of graphene sheet, whereas the G band is assigned to the orderly carbon in sp2
configuration. Moreover, the weakened D-band intensity clearly indicated the high quality graphene [63]. Thermogravimetric analysis (TGA) was also performed to investigate the thermal stability and estimation of the weight composition of MoS2-WS2 in the resulting nanohybrids. The TGA curves showed in Fig. S7, indicates that the all GMW nanohybrid samples exhibit similar TGA profile. The major weight loss appears below 400 C can be ascribed due to the oxidation of MoS2 and WS2 to MoO3 and WO3, respectively [64]. The weight loss takes place below 550 C, which could be mainly contributed to the oxidation of graphitic carbon. It was assumed that the remaining residue after 550 C is the mixture of pure MoO3 and WO3, which has a weight percentage of approximately 86.1%, 79.5% and 70.9% of MoS2-WS2/ graphene nanohybrids, respectively. From this the weight percentage of MoS2-WS2 in the final composites were calculated to be 88.6%, 80.2% and 69.8% for the GMW-10, GMW-2- and GMW-30, respectively. The as-obtained weight percentage values are in alignment with SEM-EDS values tabulated in Table. 1. The specific surface area and porous structure of the resulting 3D MoS2-WS2/graphene heterostructured nanohybrids were also investigated by performing N2 absorption/ desorption (Fig. 5b). The specific surface area of the resulting 3D aerogel hybrids and pristine nano-MoS2-WS2 was obtained from Brunauer-Emmett-Teller (BET) isotherm and deduced to
Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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Fig. 2 e TEM images of (a and b), High-resolution TEM image b) and the SAED pattern of MoS2-WS2/graphene (GMW-10) nanohybrid. be 57.5, 76.9, 85.3, and 4.6 m2/g for GMW-0, GMW-10, GMW-20 and GMW-30, respectively. The results indicated that the 3D aerogels have higher specific surface area in compared to the pristine MoS2-WS2. Hence, it is evident that the surface features of GMW nanohybrids were greatly influenced by presence of the layered graphene. Similarly, the aerogels having higher graphene content showed improved surface area was also observed. Moreover, a very small H3 hysteresis loop that features of type IV IUPAC isotherms for 3D MoS2-WS2/graphene hybrid (GMW-10) indicates the pores are mainly mesopores and could be ascribed due to the improved mesoporous
Table 1 e Concentration of the elements estimated from SEM-EDS analysis. Sample
Mo
W
S
C
O
GMW-0 GMW-10 GMW-20 GMW-30
22.81 22.32 18.43 15.76
46.56 41.54 36.85 32.32
30.03 26.82 24.22 21.02
e 8.52 16.94 25.27
0.6 0.8 3.56 5.63
Fig. 3 e XRD patterns of 3D MoS2-WS2/graphene aerogels a) GMW-0, b) GMW-10, c) GMW-20, and GMW-30, respectively.
Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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Fig. 4 e XPS survey spectrum of the 3D MoS2-WS2/graphene (GMW-10) nanohybrids (a), high resolution XPS spectrum of Mo 3d (b), W 3d (c), and S 2p (d), respectively.
channels in the graphitic planes of the resulting 3D architecture (Fig. 5b). A Barrett-Joyner-Halenda (BJH) model derived pore-size distribution pattern of GMW-10 nanohybrid (inset of Fig. 5b) indicates a sharp peak around 18 nm and another
distribution with peak broadening in the range 20e45 nm, these pore distribution reveals mesoporous structure of the resulting nanohybrids. Therefore, the enhanced specific surface area for MoS2-WS2/graphene implies the evolution of
Fig. 5 e Raman spectra of bulk MoS2-WS2/graphene (GMW-10) nanohybrids obtained with 633 nm laser excitation (inset is schematics showing atomic displacement of Raman active modes in MoS2 and WS2, respectively) a) and N2 adsorptiondesorption plots of GMW-0 and GMW-10 nanohybrids (Inset- Pore size distribution curve) b). Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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hierarchical porous channels that are created during the in-situ synthesis of such 3D architecture. Overall, the enhanced surface properties could facilitate the better contact between the electrode material and electrolyte, thus ensuring the remarkable electrocatalytic activities of the 3D MoS2WS2/graphene [64]. The electrocatalytic activities of the as-prepared 3D nanohybrid aerogels were investigated using a threeelectrode electrochemical cell configuration in 0.5 M H2SO4 aqueous electrolyte (N2 purged) at the scan rate of 50 mV s1. The characteristic polarization curves derived from linear sweep voltammetry are depicted in Fig. 6. It was observed that the GMW-10 electrocatalyst exhibit a lowest onset overpotential of 0.11 V versus RHE when compared to the other GMW nanohybrids and significant HER activity (J ¼ 10 mA cm2) was recorded at 0.19 V, which is considerably lesser than GMW-20 (0.27 V), GMW-30 (0.32 V), and GMW0 (0.34 V), respectively. No HER activity was observed for pristine carbon electrode. In further investigations of the electrocatalytic efficiency of these nanohybrids, the Tafel plots derived linear segments were fitted with the as-obtained polarization curves by employing a Tafel equation i.e. h ¼ b log j þ a, where b corresponds to the resulting Tafel slope at applied current density ‘J’. As widely accepted, the smaller Tafel slope corresponds to a faster surge of HER rate
with the increasing potential [33]. In Fig. 6b, we noted that the GMW-10 electrocatalyst gives the lowest Tafel slope of 41 mV dec1, in compared to GMW-20 (70 mV dec1), GMW-30 (96 mV dec1), and GMW-0 (115 mV dec1), respectively. The resulting Tafel slope finding was remarkable and in-line with or even smaller when compared to literature reports of non-noble metal based HER electrocatalysts (Table S1). Generally, in any HER, conversion of Hþ in to H2 was primarily elucidated based on the nature of the resultant Tafel slopes. Generally, the HER is explained by different classical theories governed by Volmer, Heyrovsky, and Tafel reactions [65,66]. The Tafel slopes measured for these reactions were approximately around 120 mV dec1, 40 mV dec1, and 30 mV dec1, respectively (Eqs. (1)e(3)). In the first step (1) is a discharge step, in which protons (H3Oþ) are adsorbed onto the active sites on the catalyst's surface and combined with electrons (e þ EC) to form adsorbed hydrogen atoms (ECHads). Subsequently, the Tafel slope derived for the GMW-10 nanohybrid catalyst was around to 41 mV dec1, which indicated that the involved HER follows both Volmer reaction and the Heyrovsky reaction through the conversion of protons into absorbed hydrogen atoms on the catalyst surface, and finally the formation of surface hydrogen molecules, respectively. Overall, it can be concluded that in an acidic medium the H atoms were adsorbed on the electrocatalyst surface and
Fig. 6 e (a) Linear sweep voltammetry (LSV) curves with 10 mV s¡1 sweep rate, (b) Tafel plots obtained from the polarization curves, and (c) electrochemical impedance spectra d) MoS2-WS2 heterostructure electrocatalytic stability: polarization curves before and after 2000 CV cycles. Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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subsequently converted into hydrogen (H2) by reactive contribution of hydrated proton from the electrolyte and electron from the surface-coated carbon to produce the final H2. H3Oþ þ e þ EC / EC-Hads þ H2O H3Oþ þ e þ EC-Hads / EC þ H2 EC-Hads þ EC-Hads / 2 EC þ H2
(Volmer reaction)
(1)
(Heyrovsky reaction) (2) (Tafel reaction) (3)
Moreover, in order to further understand the charge transport in the resulting 3D GMW nanohybrids, the impedance analyses by EIS were performed as shown in Fig. 6c and fitted to an equivalent circuit (Fig. S8), where a constant phase element (CPE) is employed. As expected, the graphene loaded nanohybrids displayed lesser charge transfer impedance (Rct) in comparison to the pristine MoS2-WS2 electrocatalyst. Such impedance behavior is accredited to the greater electrical conductivity due to the presence of graphene. Among the graphene nanohybrids GMW-30 shows lower impedance ~31 U which is smaller than the GMW-10 (~43 U) and GMW-20 (~49 U). Moreover, the GMW-10 nanohybrid also display a smaller Rs (4 U) compared to GMW-0 Rs (4.6 U), indicating that the presence of graphene could also improve the electrical contact at the electrode interface (Fig. S9). A long term durability of an electrocatalyst is also an important characteristic for high performance applications. Hence, the stability of the synthesized 3D MoS2-WS2/graphene (GMW-10) catalyst was evaluated by subjecting the catalysts under galvanostatic sweeps at cathodic current of 10 mA cm2. After 24 h galvanostatic test (Fig. 6d), there is no noticeable degradation was observed affirming superior electrochemical HER stability. Moreover, around 2000 continuous The CV cycles were also performed which exhibited trivial losses of the cathodic current (Fig. S10). Moreover, in order to know the generation of stable currents during the HER process, the chronopotentiometry test was performed (Fig. S11). It was observed that the applied overpotentials were found to increase initially, and stabilize thereafter showing only a slight decrease over time. These results reveal that, after the initial stage of the testing, the electrode exhibits a reasonably good long-term stability towards HER. Notably, the electrode is able to produce stable anodic currents for more than one day during CP testing. Furthermore, in order to monitor the possible structural, microstructural, and compositional changes of the electrocatalyst, the XRD and SEM studies were performed after 30 h of continues HER process. The XRD confirms that the characteristic structural features of the electrocatalysts were preserved with slight peak shifts, which could be attributed to the layered dislocation in MoS2/WS2 heterostructure upon constant applied overpotential (Fig. S12). The SEM morphological study reveals that the after HER tests the surface of the electrocatalyst is became rough accompanied by the moderated scrolling of the nanolayers (Fig. S13). Overall, the structural and morphological integrity of the electrocatalyst was maintained without undergoing any poisoning in acidic electrolyte.
9
The superior electro-catalytic activity of GMW-10 in compared to the other nanohybrids can be attributed to the unique 3D porous structure with high surface area and strong electronic coupling between the 2D heterostructured MoS2WS2 which is uniformly distributed within highly porous yet conductive 3D graphene support. Moreover, the in-situ synthesis process warrants the abundant active MoS2-WS2 phase. This synergistic effect of the nanohybrids exhibit a remarkable HER activity, primarily owing to smaller overpotential and Tafel slope signifying that the resulting nanohybrids are potential contenders to replace platinum based commercial electrocatalysts for effective HER. The relatively poor HER performance for GMW-20 and GMW-30 could be due to the formation of oxides species which could possibly alter the electron transfer mechanism and block the electrocatalytically active sites of MoS2-WS2 phase. In case of GMW-0, the absence of conductive network of graphene could be the probable reason for deprived catalytic activity. A realtime picture of hydrogen evolution during HER catalyzed by 3D MoS2-WS2/graphene nanohybrid (GMW-10) is presented in Fig. S14.
Conclusions In summary, a 3D nanostructured HER electrocatalyst composed of heterostructured MoS2-WS2 homogeneously anchored on graphene was fabricated using a simplistic hydrothermal method followed by moderate thermal treatment. The presented in-situ process of assembling the hierarchical 3D catalyst structure not only exposes the ample active sites but also the increase of their density was greatly facilitated. Further thermal treatment ensured the improved conductivity which underlined the effective electron transfer between the electrode and the catalysts. The resulting 3D nanohybrids exhibited remarkable HER performance and displayed small onset potential and excellent stability. In compare to pristine MoS2-WS2 the nanohybrid electrocatalyst MoS2-WS2/graphene exhibit outstanding HER performance mainly owe to the plethora of widely exposed electrochemically active sites. Among the MoS2-WS2/graphene samples, the GMW-10 showed greater catalytic performance and long-term durability with very small onset potential of 110 mV and a Tafel slope of 41 mV per dec which is competing with Pt-based HER catalysts. Furthermore, owing to its simple and economical preparation approach, this work holds high potential to fabricate advanced yet promising Pt-free HER catalysts for high performance commercial applications.
Acknowledgements This work was supported by Abu Dhabi Department of Education and Knowledge (ADEK) Award for Research Excellence (AARE17). Authors also thank Samuel Stephen for TEM images, Ms. Abeer Al Yafeai for SEM imaging, Ms. Anjana Tharalekshmy for Raman analysis and Dr. Vishwanath Kalyani, IIT-Mumbai for XPS analysis.
Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.03.195.
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Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts Sunil P. Lonkar*, Vishnu V. Pillai, Saeed M. Alhassan** Department of Chemical Engineering, Khalifa University, P.O box 127788, Abu Dhabi, United Arab Emirates
article info
abstract
Article history:
A promising electrocatalyst material composed of 2D layered MoS2-WS2 heterostructure
Received 13 November 2018
hierarchically assembled into a 3D highly interconnected macroporous network of
Received in revised form
graphene was facilely fabricated. This in-situ synthesis method involves hydrothermal
17 March 2019
reaction followed by moderate thermal annealing which guarantees the uniform distri-
Accepted 20 March 2019
bution of the MoS2-WS2 heterojunctions within graphene matrix. The presence of 3D
Available online xxx
conductive and porous graphene network and the combined merits of MoS2 and WS2 endow the resulting 3D MoS2-WS2/graphene nanohybrids with unique conductivity path-
Keywords:
ways and channels for electrons and with outstanding electrocatalytic performance
3D assembly
towards enhanced hydrogen evolution reaction (HER). This 3D nanohybrid delivered the
Nanostructured hybrids
small overpotential of 110 mV, and the small Tafel slope of 41 mV per dec, demonstrating
Electrocatalysts
high HER activity. Furthermore, the resulting nanohybrids exhibit excellent stability as
Hydrogen evolution reaction
very trivial drop in the current density was noticed even after 2000 cycles. The superior
MoS2-WS2
electrocatalytic performance of 3D MoS2-WS2/graphene over other non-precious metal
Graphene
electrocatalysts is accredited to the robust synergism of 2D MoS2-WS2 with 3D graphene that offer ample active sites and improved conductivity for HER. The proposed approach can be further extended to modify other layered transition metal dichalcogenides with hierarchical 3D porous structure as a competent electrocatalysts for HER. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In order to address the aggravating energy and environment problems; a scalable, renewable yet environmentally clean alternative energy carrier systems are urgently required. On this front, hydrogen (H2) has emerged as a promising substitute to fossil fuel, primarily owing to its remarkable energy density and ecofriendly perspectives [1e6]. Especially, the production
of H2 using electrochemical water-splitting method is considered as highly appropriate, due to its efficiency, natural abundance and eco-friendly nature [2,7e12]. Nevertheless, to achieve broader practical attention of such electrocatalytic H2 and O2 production process, the advancement electrocatalysts those are of highly active and particularly sourced from Earthabundant elements is highly demanded [13e22]. Hence, the quest for exploration of suitable active materials other than
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S.P. Lonkar), [email protected] (S.M. Alhassan). https://doi.org/10.1016/j.ijhydene.2019.03.195 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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precious metal system and their hybridization by a facile preparation process is still hold significant importance. It is well-known that noble metals, platinum, iridium, ruthenium etc. and its alloys, exhibiting extraordinary electrocatalytic properties and considered as best electrocatalysts for HER [23e28]. However, their high cost and inadequate reserves hinder the sustainable hydrogen economy. Hence, the development of highly active HER catalysts that are economical and earth abundant is of prime importance. In this context, a myriad of nonprecious-metal materials including transition metal chalcogenides (TMDs), metal alloys and carbides, polymeric carbon nitride and have been pursued to enhance the electrocatalytic performance [13,29e32]. Amongst, the layered two-dimensional (2D) atomically-thick materials, i. e transition metal dichalcogenides (TMDs) especially molybdenum and tungsten sulfides (MoS2 and WS2), have been demonstrated as potential electrocatalysts owing to their high hydrogen binding activity, electrochemical stability, and abundance [33e36]. In further advancement, the heterostructured nanohybrids of these TMDs showed enhanced HER performance attributed due to the combined effect of these materials [36e38]. However, endowed with these fascinating properties, the electrocatalytic performance of these TMDs and their heterostructured hybrids is significantly restricted due to its low inherent electric conductivity and possible re-stacking under long cycles using high current density. Also, the limited surface area ensuing lower stability is responsible for the ordinary performance these electrocatalysts for HER applications [39,40]. In order overcome these issues, the use of TMD electrocatalysts composed with hierarchical structures in threedimensional (3D) form could offer an excellent structure integrity to achieve maximally exposing active edge sites, consequently, that guarantee the enhanced electrochemical durability without conceding their intrinsic electrocatalytic ability [41,42]. Recently, several 3D support have been used to incorporate the individual TMDs nanosheets that include carbonaceous materials such as graphene, CNTs, carbon cloth etc. and other metallic supports such as tungsten foil and nickel foams [43e47]. However, the resulting electrocatalyst efficiency for overall HER performances are still unsatisfactory considering the commercial hydrogen production. On other hand, bestowed with several fascinating properties, graphene based materials hold high potential as a 3D support that can accommodate TMD nanosheets and offer excellent surface area and electronic conductivity. So far, there exist only few reports on the nanohybrids that leverage advantage of graphene with TMDs for increasing their overall electrocatalytic performance [48,49]. Moreover, implementing the right synthesis strategy is highly critical in developing these 3D nanohybrid electrocatalyst because the common solvothermal methods derived materials displayed lower HER performance, ascribed to the poor TMD dispersion which could limit the active sites exposure and eventually lower their catalytic activity for the HER [50,51]. Therefore, it is imperative and of significant attention to develop facile procedure to fabricate heterostructured TMD/graphene nanohybrids which could ensure higher and well distributed active sites that could potentially enhance the overall electrocatalytic activity.
So far, numerous studies that report the fabrication of TMD nanoparticleegraphene composites are available, but there is no report using the heterostructured MoS2-WS2 nanosheets supported on 3D graphene as potential electrocatalysts. Recently, Zhou et al. [51] reported a very good HER activity for the electrocatalyst that is composed of WS2/graphene on to Ni foam in three-dimensional (3D) assembly. However, the considering the limitation of using Ni foam as a support in acidic electrolyte, these hybrids are not suitable from poor cyclic durability perspective. Moreover, the used synthesis protocol does not offer controllability of the active material deposition. Similarly, Chen et al. demonstrated the hydrothermal method to fabricate 3D MoS2egraphene hybrid using an isopropyl alcohol (IPA)/water system [52]. However, due to incompatible surface energies of TMDs precursors and IPA/ water mixture, it was difficult to achieve desired exfoliation of TMDs and required active sites density for the HER. Few reports also mention strong exfoliation ability toward TMDs in solvothermal process using organic solvents having high boiling points, such as N,N dimethylformamide (DMF) and N-methyl-2-pyrrolidone (NMP). Although, few-layered TMDs can be achieved under such solvothermal treatment but the intricate processability is a major challenge [53]. Hence, if 3D nanohybrids of these heterostructured nanosheets with graphene is formed using facile hydrothermal process that could warrant higher active sites and better conductivity to achieve the remarkable HER activities. In this study, a facile integration of 2D MoS2-WS2 heterostructures within 3D graphene macrostructure was demonstrated while preserving their inherent properties that can be directly fabricated using simple hydrothermal processing that is sustainable and economical. Further, in this synergistic hybridization, 3D macroporous graphene network is expected to offer high specific area and minimize the phenomenon of aggregation in the in-situ formed TMDs nanosheets which inturn exposes more catalytic sites. Further, the strong interaction between these defect rich few layered 2D TMDs and conductive graphene substrate will increase electron and ion conductivity and subsequently formed conductive channels will offer effective electrolyte accessibility which is beneficial to the catalytic activity toward HER. Moreover, the interconnected porous network and high surface area boost the overall electrocatalytic performance with lower overpotential and long-term stability toward the HER. The structure and properties of the resulting nanohybrids have also been thoroughly investigated. The abundant catalytically active edges sites of MoS2-WS2 heterostructures and high surface areas are expected to endow resulting hierarchical 3D nanohybrids with excellent electrocatalytic HER performance governed by lower overpotential and higher current densities.
Materials and methods Graphite oxide (GO) synthesis GO was synthesized using improved Tours method with slight modification [54]. Briefly, 5 g of graphite flakes having 10 mesh particle sizes was thoroughly dispersed in the ice-cold mixture of 270 mL of sulfuric acid and 33 mL of phosphoric
Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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acid under constant stirring for 60 min. Successively, 2.8 g of potassium permanganate (KMnO4) was gradually added over 30 min under stirring, and the as-formed reaction mix was further mixed for 72 h at ambient conditions to achieve complete oxidation of graphite flakes. Further, the color of the reaction mixture was gradually changed from purple green to dark brown. After oxidation, the resulting oxidized graphite dispersion was cooled to ambient temperature and then reaction was completed by addition of 35% hydrogen peroxide (H2O2) solution. The color of final reaction mix was appeared to be golden yellow, affirming the highly oxidized graphite. The resulting GO dispersion was filtered and washed with 1 M HCl, and further dialyzed to eliminate impurities of residual salts and metal ions.
Preparation of 3D MoS2-WS2/graphene nanohybrids aerogels The TMD nanohybrid aerogels were prepared by using facile hydrothermal reaction followed by moderate thermal treatment. In a typical procedure, the ultra-sonicated GO aqueous dispersion (5 mg mL1) was mixed with aqueous solutions respective TMD precursor of MoS2 (ammonium tetrathiomolybdate; (NH4)2MoS4) and WS2 (ammonium tetrathiotungstate; (NH4)2WS4) under constant stirring. The resulting homogeneous mixture was hydrothermally processed in a seal-tight autoclave at 200 C for 4 h. Then the autoclave was cooled down and a blackish cylindrical hydrogel monolith was recovered which was carefully separated and thoroughly washed under the continuous de-ionized water stream to eliminate residual metallic ions and impurities. Further, the as-obtained 3D hydrogels composed of amorphous MoS2-WS2/ graphene nanohybrids were converted into 3D aerogels by using freeze drying method at liquid nitrogen temperature for 24 h. Subsequently, the resulting nanohybrid aerogels were thermal annealed at 600 C in quartz tube furnace under argon flow to ensure high crystallization of in-situ formed MoS2 and WoS2 phases and complete transformation RGO into graphene. The synthesis details are schematically presented in Scheme 1. The amount of graphene weight percent was varied from 0, 10, 20 and 30 wt-% and the as-prepared samples were marked as GMW-0, GMW-10, GMW-20 and GMW-30, respectively.
3
Characterization The resulting 3D aerogels were thoroughly characterized by means of several characterization techniques. The morphologies were investigated by using scanning electron microscope (SEM) coupled with energy-dispersive X-ray analysis (EDX) was used (Zeiss (1540 XB). For the high resolution transmission electron microscope (HRTEM) imaging, the FEI Tecnai (G20) TEM operating at 200 kV was used. For the X-ray diffraction (XRD) measurements, the Philips X'Pert Pro X-ray diffractometer was used which was equipped with a scintillation counter and Cu Ka (l ¼ 1.5418 A) radiation reflection mode in the 2q range from 10 to 80 . The specific surface area and porosity of the resulting 3D NMG nanohybrids were obtained using ASPS 2010 (Micromeritics) BrunauerEmmettTeller (BET) nitrogen adsorptiondesorption at liquid N2 temperature. The samples were pretreated under high vacuum at 100 C for 24 h before N2 adsorption was performed using a Quantachrome Autosorb gas-sorption system. The Raman analysis was carried out on the powdered samples and studied using LabRAM HR Raman spectrometer with a laser excitation wavelength of 633 nm excitation line. X-ray photoelectron spectroscopy (XPS) analysis was performed using a SSX-100 system (Surface Science Laboratories, Inc.) equipped with a monochromated Al Ka Xray source, a hemispherical sector analyzer (HSA), and a resistive anode detector.
Electrocatalytic measurements for the HER The electrocatalytic performance of the resulting 3D nanohybrid aerogels were tested using a Biologic VP300 electrochemical workstation. The standard electrochemical cell (three-electrode) configured with reference electrode [saturated calomel electrode (SCE)] and platinum wire (counter electrode), respectively was employed. The working electrodes were fabricated from as synthesized 3D MoS2-WS2/ graphene nanohybrids by mixing it with conducting carbon black (CB), and polyvinylidene fluoride (PVDF) binder in the ratio of 80:10:10 wt-% and dispersing in to N-methyl-2-pyrrolidone (NMP). The resultant homogeneous mixture was
Scheme 1 e Schematic representation of the synthesis procedure to prepare MoS2/WS2egraphene hybrid aerogels. Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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uniformly casted on a graphite sheet and vacuum dried at 100 C to remove the solvent. Linear sweep voltammetry (LSV) tests were performed by using the current potential from 0.1 V to 0.4 V vs RHE at a sweeping rate of 3 mV s1 in 0.5 M H2SO4 (purged with argon) at ambient conditions. The measured potentials were calibrated in terms of reversible hydrogen electrode (RHE). The durability of the working electrode was assessed by continuous cyclic voltammetry (CV) tests between 0.4 V and 0.1 V vs RHE at a scan rate of 50 mV s1 for 2000 cycles. The electrochemical impedance spectroscopy (EIS) was performed between the frequencies ranging from 102e100 Hz using amplitude of 5 mV. The longevity of the resulting electrocatalysts was carried out by means of chronopotentiometry (CP) at the fixed current density of 10 mA/cm2 while monitoring the variation of applied potential during 30 h.
Result and discussion The 3D hybrid aerogels composed of 2D heterostructured MoS2-WS2 and graphene were hydrothermally fabricated followed by moderate thermal treatment. During hydrothermal process, graphene oxide (GO) sheets were expected to interact with molybdate and tungstate anions and this synergistic reduction process resulted in to 3D porous architectures having amorphous or poorly crystallized in-situ formed metal sulfides uniformly distributed into partially reduced GO (RGO) was achieved [55]. In second step, the resulting aerogels were thermally treated to obtain 3D assembly of highly crystallized heterostructured assembly of MoS2-WS2 uniformly decorated on thermally reduced graphite oxide (graphene) sheets, as demonstrated in Scheme 1. The surface morphologies of the MoS2/WS2egraphene nanohybrid aerogels were characterized by SEM as shown in Fig. 1. It is apparent that the surface morphology of the resulting nanohybrids is highly corrugated with interconnected pores and primarily consist the homogeneously dispersed MoS2/WS2 heterostructure nanosheets within ultrathin macroporous graphene network (Fig. 1a). Moreover, a lower-magnification SEM image (Fig. 1b) further confirms the surface robustness and homogeneity with dense confinement of the particles of resulting GMW nanohybrids verses the pristine graphene aerogel (Fig. S1). Hence, this unique 3D architecture effectively exposes the catalytic active sites and provide an interconnected conductivity path, which could greatly facilitate the electrocatalytic activity [56]. The EDX analysis of 3D nanohybrid aerogels (Fig. 1c) showed presence Mo, W, S and C elements peaks, which confirm the integrated structure of the resulting nanohybrids without any impurity. The energy dispersive X-ray spectroscopy (EDS) maps of GMW nanohybrids (Fig. S2) also confirms the uniform distribution of in-situ formed heterostructured assembly of MoS2-WS2 within graphene nanolayers. Further, the CeMoeW mixed element mapping (Fig. 2d) clearly indicates the close proximity of MoS2 and WS2 nanoparticles with carbon indicating uniform distribution of these nanolayers in the resulting 3D nanostructured aerogels. The contents of MoS2 and WS2 nanosheets are also calculated and tabulated in Table 1.
To further shed light on the morphological aspects of the resulting 3D nanohybrids, the TEM analysis was also performed. The TEM images shown in Fig. 2a clearly indicated that the intercrossed heterostructured MoS2-WS2 nanosheets are uniformly distributed throughout the crumpled graphene sheets to form heterostructured nanohybrid as evidenced from SEM analysis as well. Owing to their identical layered structure it's difficult to distinguish individual features of MoS2 and WS2. Further, the high-magnification TEM image of GMW-10 shows the existence of well-defined few layered MoS2-WS2 heterostructures with interconnected network. Furthermore, the few-layered structural arrangement of in-situ formed MoS2WS2 heterostructure was clearly evidenced from the HRTEM image (Fig. 2b). A confined layered crystal lattice structure with interlayer spacing of about 0.67 nm confirmed the correspondence with (002) lattice planes of both hexagonal MoS2 and WS2 as both of these TMDS holds similar crystal lattice structure [57,58]. The overall layer number of MoS2-WS2 was observed to be approximately 3e8. Furthermore, the crystalline structure of layered MoS2-WS2/graphene deduced from a selected area electron diffraction (SEAD) pattern (Fig. 2c) shows characteristic diffraction rings, corresponding to the (002), (100), (103) and (110) planes, respectively [59]. For high graphene content hybrids i. e GMW-30, the formation of oxide phases were clearly observed in TEM (Fig. S3). The crystal structural features and phase composition of the resulting 3D nanohybrids aerogels were investigated by using XRD analysis. The XRD diffractograms (Fig. 3) clearly shows that characteristic JCPDS patterns for MoS2 (JCPDS: 872416) and WS2 (JCPDS: 872417) showing peaks at 14.38, 28.81, 33.35, 39.68, 49.64, 58.79, and 69.33 corresponding to the (002), (004) (100), (103), (105), (110) and (200) peaks which reveal the presence of MoS2-WS2 heterostructures as the XRD patterns of both MoS2 and WS2 are very identical to each other [60]. Moreover, compared to pristine MoS2 (Fig. S3b), all the XRD peaks for nanohybrids was appeared to be broadened, indicating the formation of nanostructured MoS2-WS2 within graphene layers. Similarly, 3D MoS2-WS2/graphene samples (Fig. 3bed) show a broad peak around 24.2 (002) planes was ascribed due to the formation of graphene nanolayers after the in-situ reduction of graphite oxide (Fig. S4b). However, with high concentration graphite oxide samples (Fig. 3d), the graphene peaks have been overlapped with some of the oxides (Mo-WO3) that are formed due to the possible interactions with graphite oxide [61,62]. Fig. 4aed shows the chemical composition and elemental states of the as-synthesized 3D MoS2-WS2/graphene determined from XPS measurements. Fig. 4a shows the XPS survey spectrum of GMW-10 nanohybrid which indicates the existence of the C, Mo, W, O and S elements which confirm the coexistence of these elements within surface elemental composition of the resulting nanohybrid. Similarly, intensity decreases of O 1s peak with simultaneous peak intensity increase for C 1s versus pristine GO (Fig. S5) confirms that the GO was successfully reduced into graphene under thermal treatment. Similarly, the deconvoluted high resolution XPS spectra of Mo 3d peak (Fig. 4b) indicates two major peaks at 232.1 and 228.9 eV, affirming the presence of Mo 3d3/2 and Mo 3d5/2 orbitals of Mo in the Mo4þ oxidation state. Similarly, the XPS spectra of Wf (Fig. 4c) designates two main peaks at
Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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Fig. 1 e SEM images (a and b), EDAX spectra (c), and mixed CeMoeW EDS mapping d) of the GMW-10 nanohybrid.
binding energies of 32.3 and 34.5 eV assigned for W4þ oxidation state in WS2, which is also indicate the semiconducting prismatic 2H form. In addition, a small peak was observed at 37.4 eV, which are due to the presence of W6þ state originated from slightly oxidized WS2 phase. Further, the sulfur exhibits two peaks at 161.9 eV and 163.0 eV, which is attributed to the S 2p3/2 and S 2p1/2 orbitals. Overall XPS study indicates the formation of MoS2-WS2 heterostructure within macroporous graphene aerogel. The quantitative elemental analyses by XPS also reveal that the oxygen percentages in the nanohybrids increases with increase in GO content, which probably due to the trivial oxidation of in-situ formed MoS2-WS2 heterostructures by oxygenated functions of GO. The Raman spectrum of as prepared 3D nanohybrids showed in Fig. 5a displayed the characteristic Raman peaks corresponding to the in-plane E12g and out-of plane A1g vibrational modes derived from the relative motions of the Mo/W and S atoms in the MoS2-WS2 crystal, respectively. A slight redshift in these peaks of MoS2-WS2 in 3D nanohybrids, in compared to pristine MoS2-WS2 (Fig. S6) indicates the existence of few layered MoS2-WS2. Similarly, two unique signature peaks appeared at ~1587 cm1 and ~1341 cm1 was ascribed to the G-band and D-band peaks of graphene layers. Typically, the D band corresponds to the carbon in disordered form arising the defects and edges of graphene sheet, whereas the G band is assigned to the orderly carbon in sp2
configuration. Moreover, the weakened D-band intensity clearly indicated the high quality graphene [63]. Thermogravimetric analysis (TGA) was also performed to investigate the thermal stability and estimation of the weight composition of MoS2-WS2 in the resulting nanohybrids. The TGA curves showed in Fig. S7, indicates that the all GMW nanohybrid samples exhibit similar TGA profile. The major weight loss appears below 400 C can be ascribed due to the oxidation of MoS2 and WS2 to MoO3 and WO3, respectively [64]. The weight loss takes place below 550 C, which could be mainly contributed to the oxidation of graphitic carbon. It was assumed that the remaining residue after 550 C is the mixture of pure MoO3 and WO3, which has a weight percentage of approximately 86.1%, 79.5% and 70.9% of MoS2-WS2/ graphene nanohybrids, respectively. From this the weight percentage of MoS2-WS2 in the final composites were calculated to be 88.6%, 80.2% and 69.8% for the GMW-10, GMW-2- and GMW-30, respectively. The as-obtained weight percentage values are in alignment with SEM-EDS values tabulated in Table. 1. The specific surface area and porous structure of the resulting 3D MoS2-WS2/graphene heterostructured nanohybrids were also investigated by performing N2 absorption/ desorption (Fig. 5b). The specific surface area of the resulting 3D aerogel hybrids and pristine nano-MoS2-WS2 was obtained from Brunauer-Emmett-Teller (BET) isotherm and deduced to
Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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Fig. 2 e TEM images of (a and b), High-resolution TEM image b) and the SAED pattern of MoS2-WS2/graphene (GMW-10) nanohybrid. be 57.5, 76.9, 85.3, and 4.6 m2/g for GMW-0, GMW-10, GMW-20 and GMW-30, respectively. The results indicated that the 3D aerogels have higher specific surface area in compared to the pristine MoS2-WS2. Hence, it is evident that the surface features of GMW nanohybrids were greatly influenced by presence of the layered graphene. Similarly, the aerogels having higher graphene content showed improved surface area was also observed. Moreover, a very small H3 hysteresis loop that features of type IV IUPAC isotherms for 3D MoS2-WS2/graphene hybrid (GMW-10) indicates the pores are mainly mesopores and could be ascribed due to the improved mesoporous
Table 1 e Concentration of the elements estimated from SEM-EDS analysis. Sample
Mo
W
S
C
O
GMW-0 GMW-10 GMW-20 GMW-30
22.81 22.32 18.43 15.76
46.56 41.54 36.85 32.32
30.03 26.82 24.22 21.02
e 8.52 16.94 25.27
0.6 0.8 3.56 5.63
Fig. 3 e XRD patterns of 3D MoS2-WS2/graphene aerogels a) GMW-0, b) GMW-10, c) GMW-20, and GMW-30, respectively.
Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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Fig. 4 e XPS survey spectrum of the 3D MoS2-WS2/graphene (GMW-10) nanohybrids (a), high resolution XPS spectrum of Mo 3d (b), W 3d (c), and S 2p (d), respectively.
channels in the graphitic planes of the resulting 3D architecture (Fig. 5b). A Barrett-Joyner-Halenda (BJH) model derived pore-size distribution pattern of GMW-10 nanohybrid (inset of Fig. 5b) indicates a sharp peak around 18 nm and another
distribution with peak broadening in the range 20e45 nm, these pore distribution reveals mesoporous structure of the resulting nanohybrids. Therefore, the enhanced specific surface area for MoS2-WS2/graphene implies the evolution of
Fig. 5 e Raman spectra of bulk MoS2-WS2/graphene (GMW-10) nanohybrids obtained with 633 nm laser excitation (inset is schematics showing atomic displacement of Raman active modes in MoS2 and WS2, respectively) a) and N2 adsorptiondesorption plots of GMW-0 and GMW-10 nanohybrids (Inset- Pore size distribution curve) b). Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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hierarchical porous channels that are created during the in-situ synthesis of such 3D architecture. Overall, the enhanced surface properties could facilitate the better contact between the electrode material and electrolyte, thus ensuring the remarkable electrocatalytic activities of the 3D MoS2WS2/graphene [64]. The electrocatalytic activities of the as-prepared 3D nanohybrid aerogels were investigated using a threeelectrode electrochemical cell configuration in 0.5 M H2SO4 aqueous electrolyte (N2 purged) at the scan rate of 50 mV s1. The characteristic polarization curves derived from linear sweep voltammetry are depicted in Fig. 6. It was observed that the GMW-10 electrocatalyst exhibit a lowest onset overpotential of 0.11 V versus RHE when compared to the other GMW nanohybrids and significant HER activity (J ¼ 10 mA cm2) was recorded at 0.19 V, which is considerably lesser than GMW-20 (0.27 V), GMW-30 (0.32 V), and GMW0 (0.34 V), respectively. No HER activity was observed for pristine carbon electrode. In further investigations of the electrocatalytic efficiency of these nanohybrids, the Tafel plots derived linear segments were fitted with the as-obtained polarization curves by employing a Tafel equation i.e. h ¼ b log j þ a, where b corresponds to the resulting Tafel slope at applied current density ‘J’. As widely accepted, the smaller Tafel slope corresponds to a faster surge of HER rate
with the increasing potential [33]. In Fig. 6b, we noted that the GMW-10 electrocatalyst gives the lowest Tafel slope of 41 mV dec1, in compared to GMW-20 (70 mV dec1), GMW-30 (96 mV dec1), and GMW-0 (115 mV dec1), respectively. The resulting Tafel slope finding was remarkable and in-line with or even smaller when compared to literature reports of non-noble metal based HER electrocatalysts (Table S1). Generally, in any HER, conversion of Hþ in to H2 was primarily elucidated based on the nature of the resultant Tafel slopes. Generally, the HER is explained by different classical theories governed by Volmer, Heyrovsky, and Tafel reactions [65,66]. The Tafel slopes measured for these reactions were approximately around 120 mV dec1, 40 mV dec1, and 30 mV dec1, respectively (Eqs. (1)e(3)). In the first step (1) is a discharge step, in which protons (H3Oþ) are adsorbed onto the active sites on the catalyst's surface and combined with electrons (e þ EC) to form adsorbed hydrogen atoms (ECHads). Subsequently, the Tafel slope derived for the GMW-10 nanohybrid catalyst was around to 41 mV dec1, which indicated that the involved HER follows both Volmer reaction and the Heyrovsky reaction through the conversion of protons into absorbed hydrogen atoms on the catalyst surface, and finally the formation of surface hydrogen molecules, respectively. Overall, it can be concluded that in an acidic medium the H atoms were adsorbed on the electrocatalyst surface and
Fig. 6 e (a) Linear sweep voltammetry (LSV) curves with 10 mV s¡1 sweep rate, (b) Tafel plots obtained from the polarization curves, and (c) electrochemical impedance spectra d) MoS2-WS2 heterostructure electrocatalytic stability: polarization curves before and after 2000 CV cycles. Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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subsequently converted into hydrogen (H2) by reactive contribution of hydrated proton from the electrolyte and electron from the surface-coated carbon to produce the final H2. H3Oþ þ e þ EC / EC-Hads þ H2O H3Oþ þ e þ EC-Hads / EC þ H2 EC-Hads þ EC-Hads / 2 EC þ H2
(Volmer reaction)
(1)
(Heyrovsky reaction) (2) (Tafel reaction) (3)
Moreover, in order to further understand the charge transport in the resulting 3D GMW nanohybrids, the impedance analyses by EIS were performed as shown in Fig. 6c and fitted to an equivalent circuit (Fig. S8), where a constant phase element (CPE) is employed. As expected, the graphene loaded nanohybrids displayed lesser charge transfer impedance (Rct) in comparison to the pristine MoS2-WS2 electrocatalyst. Such impedance behavior is accredited to the greater electrical conductivity due to the presence of graphene. Among the graphene nanohybrids GMW-30 shows lower impedance ~31 U which is smaller than the GMW-10 (~43 U) and GMW-20 (~49 U). Moreover, the GMW-10 nanohybrid also display a smaller Rs (4 U) compared to GMW-0 Rs (4.6 U), indicating that the presence of graphene could also improve the electrical contact at the electrode interface (Fig. S9). A long term durability of an electrocatalyst is also an important characteristic for high performance applications. Hence, the stability of the synthesized 3D MoS2-WS2/graphene (GMW-10) catalyst was evaluated by subjecting the catalysts under galvanostatic sweeps at cathodic current of 10 mA cm2. After 24 h galvanostatic test (Fig. 6d), there is no noticeable degradation was observed affirming superior electrochemical HER stability. Moreover, around 2000 continuous The CV cycles were also performed which exhibited trivial losses of the cathodic current (Fig. S10). Moreover, in order to know the generation of stable currents during the HER process, the chronopotentiometry test was performed (Fig. S11). It was observed that the applied overpotentials were found to increase initially, and stabilize thereafter showing only a slight decrease over time. These results reveal that, after the initial stage of the testing, the electrode exhibits a reasonably good long-term stability towards HER. Notably, the electrode is able to produce stable anodic currents for more than one day during CP testing. Furthermore, in order to monitor the possible structural, microstructural, and compositional changes of the electrocatalyst, the XRD and SEM studies were performed after 30 h of continues HER process. The XRD confirms that the characteristic structural features of the electrocatalysts were preserved with slight peak shifts, which could be attributed to the layered dislocation in MoS2/WS2 heterostructure upon constant applied overpotential (Fig. S12). The SEM morphological study reveals that the after HER tests the surface of the electrocatalyst is became rough accompanied by the moderated scrolling of the nanolayers (Fig. S13). Overall, the structural and morphological integrity of the electrocatalyst was maintained without undergoing any poisoning in acidic electrolyte.
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The superior electro-catalytic activity of GMW-10 in compared to the other nanohybrids can be attributed to the unique 3D porous structure with high surface area and strong electronic coupling between the 2D heterostructured MoS2WS2 which is uniformly distributed within highly porous yet conductive 3D graphene support. Moreover, the in-situ synthesis process warrants the abundant active MoS2-WS2 phase. This synergistic effect of the nanohybrids exhibit a remarkable HER activity, primarily owing to smaller overpotential and Tafel slope signifying that the resulting nanohybrids are potential contenders to replace platinum based commercial electrocatalysts for effective HER. The relatively poor HER performance for GMW-20 and GMW-30 could be due to the formation of oxides species which could possibly alter the electron transfer mechanism and block the electrocatalytically active sites of MoS2-WS2 phase. In case of GMW-0, the absence of conductive network of graphene could be the probable reason for deprived catalytic activity. A realtime picture of hydrogen evolution during HER catalyzed by 3D MoS2-WS2/graphene nanohybrid (GMW-10) is presented in Fig. S14.
Conclusions In summary, a 3D nanostructured HER electrocatalyst composed of heterostructured MoS2-WS2 homogeneously anchored on graphene was fabricated using a simplistic hydrothermal method followed by moderate thermal treatment. The presented in-situ process of assembling the hierarchical 3D catalyst structure not only exposes the ample active sites but also the increase of their density was greatly facilitated. Further thermal treatment ensured the improved conductivity which underlined the effective electron transfer between the electrode and the catalysts. The resulting 3D nanohybrids exhibited remarkable HER performance and displayed small onset potential and excellent stability. In compare to pristine MoS2-WS2 the nanohybrid electrocatalyst MoS2-WS2/graphene exhibit outstanding HER performance mainly owe to the plethora of widely exposed electrochemically active sites. Among the MoS2-WS2/graphene samples, the GMW-10 showed greater catalytic performance and long-term durability with very small onset potential of 110 mV and a Tafel slope of 41 mV per dec which is competing with Pt-based HER catalysts. Furthermore, owing to its simple and economical preparation approach, this work holds high potential to fabricate advanced yet promising Pt-free HER catalysts for high performance commercial applications.
Acknowledgements This work was supported by Abu Dhabi Department of Education and Knowledge (ADEK) Award for Research Excellence (AARE17). Authors also thank Samuel Stephen for TEM images, Ms. Abeer Al Yafeai for SEM imaging, Ms. Anjana Tharalekshmy for Raman analysis and Dr. Vishwanath Kalyani, IIT-Mumbai for XPS analysis.
Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.03.195.
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Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195
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Please cite this article as: Lonkar SP et al., Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.03.195