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A facile route for the synthesis of nickel foam modified WSe2:CdS nanohybrid as an efficient catalyst for hydrogen evolution reaction (HER)

Nickel foam was synthesized via a easy and time consuming method with the help of microwave, the nickel foam was then modified with very impressive rod shaped nanoparticle WSe2-CdS. The pores of the nickel foam are very clear and uniform in size with the nanoparticle attached in the surface of the foam. The prepared nanohybrid foam (Nifoam@WSe2-CdS) shows good performance towards hydrogen evolution reaction with small onset potential of -0.30 V and very low tafel slope that is 44 mV/dec. The electrocatalyst was prepared with earth abundant material with low cost of synthesis. Prepared electrocatlyst shows high durability over acidic medium for several time periods with unchanged electrochemical behavior.


Introduction
In recent times two dimensional nanosheet of transition metal dichalcogenides (TMDs) has attracted the researchers, due to their highly efficient electrocatalytic properties. In case of TMDs, X-M-X like layered sandwich structure was observed where M is the metal ions of group IV, V and VI. Here, X is the chalcogen atom (X = S, Se and Te). Diverse properties (chemical, physical, optical, catalytic) of the TMDCs depend mainly on their atomic arrangement. Like that of graphene weak van der Waals interaction works between two adjacent layers and each atom in the same layer are covalently bonded in a hexagonal network. A satisfying stability in strong acids is obtained from these chalcogen atoms decorated on the edges of the transition metal with a beneficial band gap, results in their high catalytic properties [1]. Therefore, emphasize was given on enough edge sites designing and fabrication of the TMDs, which could be possible by the nano scientist via reducing their size to the nanoscale dimensions [2]. Earth abundant material like nickel have been extensively used as an electrocatalyst for HER because of their crystal structure, electrical conductivity, chemical composition, size distribution, low cost etc. However nickel based catalyst also have some limitations like dissolution in acidic medium and many more, but nickel foam have long term stability in acidic or basic medium, very high porosity thus can be used as an good electrocatalyst. Some of the articles reported in nickel foam for HER [3]. Guo [4]. Wang et al reported synthesis hierarchical Ni 3 S 2 -NiOOH grown on nickel foam for HER [5]. The author reported HER in 1.0 M NaOH with very low overpotential value. Cao et al electrodeposited Ni X P nanospheres on nickel foam as an active catalyst for HER as well as on supercapacitor [6]. Sustainable and renewable energy is the main way out for the great energy crisis and global warming and energy safety issues which can be a substitute of replacing fossil fuels energy [7]. Water splitting proved to be one of the efficient sustainable energy sources at cheap rate [8]. A best electrocatalyst is needed with minimal overpotential (η), high current density to overcome the internal slothful kinetics of hydrogen evolution reaction (HER). Primary pace towards evolution of hydrogen energy is the use of earth abundant and Pt-free electrocatalysts with high stability in nice potential range [9]. In this work we have made an easy approach for the synthesis of nickel foam and prepared a hybrid with scalable nanometer sized rod shaped WSe 2 -CdS with high quality and uniformity and explored its activity towards HER. The prepared nanohybrid (Ni foam @ WSe 2 :CdS) showed good HER performances, low TAFEL slope (44 mV/dec) and low overpotential value (230 mV).

Instrumentation
Powder X-ray diffraction (XRD) analysis of the samples were taken in Xpert Pro MPD diffractometer with Cu-Kα radiation with 40 KV/mA power. Zeiss SUPRA 55 was used for field emission scanning electron microscopy (FE-SEM) and Energy Dispersive X-ray spectroscopy (EDAX) analysis. All the electrochemical analysis was done on the CH instrument (USA, model number 440D). Every experiments were performed at room temperature.

Synthesis of rod shaped WSe 2 :CdS
To fabricate rod shaped WSe 2 :CdS nanohybrid, 5.0 mmol of Se powder was dissolved in 10 mL of hydrazine hydrate solution, then 0.41g (2.0mmol) of sodium tungstate and 2.0 mmol of cadmium sulphate were added to the above mixture. Again, 1.0 mmol of l-cysteine (dissolved in 0.3ml of oleylamine) was injected to the above solution maintaining pH of the solution neutral. After that, the solution was kept for constant stirring for 6 hour and poured into a 50.0 mL Teflon lined stainless steel autoclave and placed in the oven keeping the temperature at 200 °C for 36 hours. The product solution was centrifuged at 15,000 rpm, with distilled water and ethanol. Then the product was dried at 60 °C in vacuum for 12 h. 2.4. Synthesis of nickel foam using microwave Two milimole water-based solution of Ni(NO 3 ) 2 . 6H 2 O was prepared by taking 0.291g Nickel nitrate hexahydrate in 1 mL water. 0.450g of glycine was added to that mixture. Then this mixture was irradiated in microwave oven at 750W for 60s.

Synthesis of hybrid nickel foam with WSe 2 -CdS (Ni foam @ WSe 2 -CdS)
Nanoparticle of 0.1 g WSe 2 -CdS was dispersed in 10 ml DMF with the prepared nickel foam cut into small pieces with area 1cm 2 for 24 hours under vigorous stirring. The mixture was then added to 25ml Teflon autoclave for 24 hours. After the reaction the modified foam was washed with water ethanol mixture then dried in vaccum oven for next 48 hours.

Compositional and morphological characterization
The XRD of the nanohybrid (Ni foam @WSe 2 :CdS) was successfully done and the XRD spectra of the nanocomposite Ni foam @WSe 2 :CdS shows all the peaks of the nanoparticle and nickel foam which is shown in Figure 1 24.48°, 28.3°, 50.9° is due to that of the CdS. Extra peak at 44° and 50° concludes the successful synthesis of nickel foam with JCPDS no 04 The morphological study of the synthesized From the Figure 2(A) we can see the nm arranged in a homogeneous manner. In figure 2(B) and 2 (C) having uniform pores of size less than 10 µm at having no nanoparticles. After mod beside the pores in haphazard manner can be seen analysis we can tell the homogeneous distribution of W, Se, nanohybrid shown in figure 2(F) successfully synthesized.
is due to that of the CdS. Extra peak at 44° and 50° concludes the successful synthesis of nickel foam with JCPDS no 04-0850.
XRD of nanohybrid with nickel foam (Ni foam @WSe 2 :CdS synthesized nanohybrid has been done by means of FE we can see the clear rod shaped image of the nanoparticle WSe homogeneous manner. In figure 2(B) and 2 (C) we can see that less than 10 µm at lower and higher magnification with clear surface . After modification the nanoparticle are attached on the surface of the foam manner can be seen from figure 2(D) and 2 (E) the homogeneous distribution of W, Se, Cd, S, Ni atoms over the whole shown in figure 2(F). From the EDAX spectra we can say that the nanohybrid is is due to that of the CdS. Extra peak at 44° and 50° concludes the successful :CdS).  Electrocatalytic activity and Hydrogen evolution study of the prepared nanocomposite electrochemical measurements as well as HER analysis, nanomaterial modified pencil grap electrode (PGE) was used. Firstly, the PGE was washed with 6.0 M nitric acid followed by ed as an electrode was inserted in a micropipette tip a . Required amount of nanohybrid material and 5.0 µL of Nafion solution (5 wt %) were dispersed in 1.0 mL of water-ethanol (1:1) solution and was coated Electrochemical measurements like linear sweep voltammetry (LSV) an airtight cell (N 2 -atmosphere) using a three-electrode modified PGE as the working electrode, Ag/AgCl (in 3.0M KCl electrolyte) as reference The electrochemical investigation was done in 1N Ag/AgCl electrode and then converted to reversible hydrogen electrode the equation: E RHE = E Ag/AgCl + 0.059×pH + E Ag/AgCl . Here, E Ag/AgCl the experimental potential measured against the Ag/AgCl reference electrode and the standard respectively. The electroactive surface area was als 5.0 mM Fe[(CN)6] 4in 1.0 M KCl. The electroactive surface area was estimated according to the Randles-Sevcik equation [5]: where Ip is peak current, A is electroactive surface area, n is number of electrons involved diffusion coefficient of the analyte, v is scan rate and C 0 is concentration lytic activity of the electrodes was studied and observed in the Figure 3 nanocomposite modified pencil graphite nitric acid followed by water. The in a micropipette tip and further .0 µL of Nafion solution oated dropwise on the linear sweep voltammetry (LSV), Cyclic electrode system with a in 3.0M KCl electrolyte) as reference done in 1N H 2 SO 4 (pH = 0). All reversible hydrogen electrode Ag/AgCl and E Ag/AgCl are the experimental potential measured against the Ag/AgCl reference electrode and the standard was also analysed using The electroactive surface area was number of electrons involved, D is of the analyte. The in the Figure 3(A) and Table 1 nanohybrid modified PGE in comparison to the pristine foam modified PGE thus having high electrocatalytic surface area and high roughness factor for the nanohybrid m PGE. Optimization through the LSV run was done and it is observed that with 10 mg of the nanohybrid modified PGE electrode and at 5.0 mVs noticeable HER activity that is smaller onset potential and h figure 3(B), 3(C), 3(D).  Tafel slope is an important parameter of electrocatalysts determined by the mechanism critical insights into the HER reaction kinetics. between linear portion of the polarization curve (LSV) nanohybrid modified PGE in comparison to the pristine foam modified PGE observed in figure 3(A) thus having high electrocatalytic surface area and high roughness factor for the nanohybrid m the LSV run was done and it is observed that with 10 mg of the nanohybrid modified PGE electrode and at 5.0 mVs -1 scan rates, 1N H 2 SO 4 as supporting electrolyte noticeable HER activity that is smaller onset potential and high current density is obtained shown in where η is the overpotential, j is the current density and the other symbols have their usual meanings. From the value of tafel slope, there are TAFEL slope (120 mVdec -1 ), Heyrovsky reaction step (40 mVdec A Tafel plot with small slope value increase in overpotential with increasing current density From the figure 4(A) the LSV plot of the unmodified nickel foam and the nano foam is seen that Ni foam @WSe 2 -CdS result of -0.30 V than that of the pristine nickel foam with onset potential for Ni foam @WSe 2 -CdS is 230 mV and that of the overpotential value means higher is CdS is ~ 80 mA and that of the pristine foam is with Cyclic voltammetery (CV) for th potential range of 0 to -1.3 V for the nanocomposites under the same analytical condition figure 4(B). All the nanocomposites shows a cathodic sweep toward lower potential with a high current density was observed in (Ni foam @WSe 2 :CdS) was found to be 44 mV/dec and that unmodified TAFEL slope we can conclude that both the electrodes follow nanohybrid is much smaller than that of the unmodified 3.3. Stability study of the prepared nanocomposite Stability is one of the important parameter to justify good HER performan was performed to monitor the stability of the electrocatalyst. in the cyclic voltammetry data durin recorded in the potential window of 0 to 72 , 70 and 72 mA cm -2 shown in figure 5 (D) the onset potential is same and also the current also be attributed with the unchanged behavior of nanocomposite, after the whole electrochemical study shown in figure 5 (C). T unaltered for both the cases of fresh and used electrochemical studies). η is the overpotential, j is the current density and the other symbols have their usual meanings.
there are three possible reaction steps, Volmer reaction step having ), Heyrovsky reaction step (40 mVdec -1 ), Tafel reaction step (30 mVdec Tafel plot with small slope value leads to good reaction kinetics for HER th increasing current density. the LSV plot of the unmodified nickel foam and the nanoparticle CdS results the best electrocatalytic activity with 0.30 V than that of the pristine nickel foam with onset potential -0.44 V. The overpotential value CdS is 230 mV and that of the pristine nickel foam is 250mV. Smaller the higher is its electrocatalytic activity. The current density of Ni pristine foam is with 60 mA current density. for the nanohybrid was performed at a scan rate of 5 mV s 1.3 V for the nanocomposites under the same analytical condition . All the nanocomposites shows a cathodic sweep toward lower potential with a high current density was observed in the nanohybrid modified nickel foam. The Tafel slope of nanohybrid ) was found to be 44 mV/dec and that unmodified nickel is 65 mV/dec. TAFEL slope we can conclude that both the electrodes follow Heyrovsky pathway. nanohybrid is much smaller than that of the unmodified nickel foam shown in figure 5 (A) Stability study of the prepared nanocomposite Stability is one of the important parameter to justify good HER performances. Cyclic voltammetry tability of the electrocatalyst. No such significant change in the cyclic voltammetry data during repetition after 1st, 50th, 100 th and 500th cycles he potential window of 0 to -1.2 V with HER current densities recorded and shown in figure 5 (D). From the LSV study we can see that for all the cycles potential is same and also the current density shown in figure 5(B). The higher stability can unchanged behavior of nanocomposite, after the whole electrochemical The current density and onset potential value was fresh and used nanohybrid electrocatalyst (i.e. before and after (2) η is the overpotential, j is the current density and the other symbols have their usual meanings. reaction steps, Volmer reaction step having ), Tafel reaction step (30 mVdec -1 ). and means a slower particle modified nickel with lower onset potential 0.44 V. The overpotential value pristine nickel foam is 250mV. Smaller the The current density of Ni foam @WSe 2 -Cyclic votammetry runs for both the electrodes.
at a scan rate of 5 mV s -1 in the 1.3 V for the nanocomposites under the same analytical condition shown in . All the nanocomposites shows a cathodic sweep toward lower potential with a high The Tafel slope of nanohybrid 65 mV/dec. From the pathway.TAFEL slope of shown in figure 5 (A).
ces. Cyclic voltammetry significant change was observed and 500th cycles respectively HER current densities recorded and found as 74, From the LSV study we can see that for all the cycles The higher stability can unchanged behavior of nanocomposite, after the whole electrochemical tial value was remained almost (i.e. before and after all the

Conclusion
The prepared nanoparticle WSe synthesized and were characterized by different characterization synthesis of the materials. Among all the electrocatalytic activity with much lower (HER). So also, electroactive surface area of nickel foam making it best electro shows a great stability for several months which is very much important parameter catalyst.