Facile fabrication of SnSe nanorods embedded in GO nanosheet for robust oxygen evolution reaction

Excessive use of fossil fuels has caused their rapid depletion, resulting in a growing energy crisis as well as many environmental concerns, and prompting the development of the sustainable energy conversion system. In this study, we report GO based SnSe nanomaterial incorporated via the hydrothermal route engaging OER activity. The morphological, structural and textual effects of the material have been measured with scanning electron microscopy (SEM), X-ray diffraction (XRD) and Brunauer Emmett Teller (BET). Furthermore, the kinetic process, active sites, conductivity and stability of the electrode medium of GO, SnSe and GO/SnSe were estimated with electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), electrochemical active surface area (ECSA) and chronoamperometry under 1.0 M KOH with Ni foam as a conductive substrate. Its electrochemical results depicted the following: the GO based SnSe nanocomposite displayed lesser overpotential around 336 mV to reach a current density around 10 mA cm−2 and Tafel slope of 36 mV dec−1. Cyclic stability and chronoamperometry analysis of GO/SnSe nanocomposite show stability over 35 h. The exceptional electrochemical properties of the GO/SnSe nanocomposite differentiate it as a good material for use in electrical and in many other fields in future.


Introduction
Nonrenewable energy resources like natural gas, oil and coal have been utilized as the principal source of energy to sustain human activities since the eighteenth century [1].Our civilization has been pushed to establish a green and sustainable energy system for long-term growth as a result of rapidly rising global population and expanding industrialization, as well as the attendant environmental issues [2].Solar, wind and tidal energy are the examples of renewable and carbon-free energy sources that are used all over the world [3,4].Unfortunately, these energy sources are only available during specific seasons and weather conditions, resulting in an inefficient energy distribution and utilization [5][6][7][8][9].
However, the researchers have successfully replaced the polluted and season-dependent energy sources with a sustainable, clean, economical and efficient hydrogen source of energy [10].Therefore, several techniques such as steam reforming [11], gasification [12], chemical oxidation [13] and water splitting [14] are some of the ways for creating hydrogen energy.Among all, water spitting is the best source to produce renewable energy (H 2 and O 2 ) a suitable alternative for reducing dependency on fossil fuels, satisfying the growing energy demand, and fulfilling the safe environmental criteria due to the clean nature of H 2 as a fuel [15][16][17].OER (oxygen evolution reaction) at the anode (2H 2 O(l) O 2 (g) + 4H + (aq) + 4e − ) and HER (hydrogen evolution reaction) at the cathode (2H + (aq) 2e − H 2 (g)) are two half-reactions intricate in electrochemical water splitting [18].Furthermore, due to thermodynamic and kinetic restrictions, reaching 100% Faradaic efficiency is extremely difficult [19][20][21].
Modern OER electrocatalysts are often constructed by using valuable metals like Pt, Ru and Os which have limited availability and high cost, which restrict their application [22].Owing to their affordability, abundant availability, ecological sensitivity and considerable electrocatalytic performance, transition metal based materials have recently been widely explored for OER [23].The remarkable physical, chemical and electrical features of transition metal selenides (TMS) composites have piqued interest in their potential use for energy storage [24,25].Metal selenides exhibit more electrochem-ical activity in reactions than metal oxides/hydroxides because selenium has a lower electronegativity than oxides and sulphur, resulting in a weaker chemical link between the atom and the binding electrons.Several transition metal selenides such as including MoSe 2 [26], CoSe [27], Co 0.85 Se [27], SnSe 2 [28], Ni 3 Se 2 [29] and Cu 2 Se [30] have been reported.To improve specific capacitance and durability, SnSe is integrated into the carbon-based compound particularly graphene, which increases charge transfer capacity at the electrode/electrolyte interface and hence electrical conductivity [31].Excellent thermal conductivity, strong mechanical properties, outstanding electrical conductivity, significant chemical stability, high specific surface area and fast electromobility are some of the characteristics of graphene [32][33][34][35][36]. On the other hand, the strong Vander Walls contact between graphene layers may promote the aggregation of two-dimensional (2D) graphene sheets, reducing the active surface area and changing other features.Due to its unique structure, three-dimensional (3D) graphene may avoid interaction [37][38][39][40][41][42].It has been demonstrated that 3D graphene has a greater response sensitivity than 2D graphene.Because of its excellent conductivity and efficient electron transport channels, 3D graphene is employed as a communication platform to improve electron transmission and transit in HER and OER processes [43][44][45][46] In this research, we report the manufacturing of the GO/SnSe nanocomposite via the facile hydrothermal route employed towards the OER activity.The structural, morphological and textual analyses of the material were confirmed with scanning electron microscopy (SEM), X-ray diffraction (XRD) and Brunauer Emmett Teller (BET) analysis, correspondingly.Furthermore, the electrochemical production of electrocatalyst had been evaluated with various electrochemical parameters.Electrochemical findings imply GO based SnSe nanocomposite exhibited less overpotential around 336 mV in attaining current density of 10 mA cm −2 and Tafel slope of 36 mV dec −1 .This finding not only explains the importance of OER using the synthesized material as electrocatalyst but also showed by research to improve water splitting through different approaches and structural modification.

Fabrication of graphene oxide (GO)
The graphene oxide (GO) nanosheet was prepared via simple Hummer's process.For the fabrication of GO, 5 g graphite powder, 2.5 g NaNO 3 , 12 mL H 3 PO 4 and100 mL of H 2 SO 4 were added into a beaker and mixed under vigorous stirring, then comes the gradual accumulation of 15 g of KMnO 4 in aforementioned combination.However, the temperature of the reaction vessel was decreased by placing it in an ice bath.After half an hour, the reaction mixture became thick, so 300 mL of deionized water was added and agitated for 20 min.After that around 500 mL water and 50 mL of H 2 O 2 were added, then the mixture was centrifuged followed by washing finally with deionized water and 5% HCl to eliminate any residue from the suspension and subsequently the prepared material was dehydrated for one day at 60 °C in an electric oven [51].

Fabrication of GO/SnSe via the chemical reduction technique
For the synthesis of SnSe, 0.1 mM SnCl 2 and 0.1 mM Se powders were dissolved in 10 mL of 1 M KOH aqueous solution.After that 2 mL of N 2 H 4 was added in the aforementioned mixture involving vigorous stirring for 1 h at ambient reaction conditions.The resultant mixture was transformed into the Teflon-lined autoclave for 4 h at 180 °C.The resulting product was removed by centrifugation and then washed multiple times with deionized water along with ethanol.This obtained product was then dried around 60 °C and stored for further use.Furthermore, for the fabrication of the GO/SnSe composite, the same procedure was adopted except 0.5 g of the already fabricated GO was added into the prepared mixture which was used for the synthesis of SnSe.The synthesized SnSe and GO/SnSe nanocomposite were saved for further chemical and electrochemical characterization.

Electrode preparation
To produce the working electrode (WE), firstly, nickel foam (NF) was fragmented into tiny bits (1 * 1 cm) and activated by sonicating with ethanol, acetone and ultrapure deionized water 15 min for each step.After that NF was dried in an electric oven for 15 min at 50 °C.Additionally, the catalytic ink was fabricated with 10 mg of the electrocatalytic powder in 80 mL of deionized water and then sonicated for 20 min.The resulting catalytic ink was pasted on the Ni foam by a simple drop casting approach and this was dryed at room temperature for further process.

Physical characterization
A Bruker 2 D X-ray diffractometer equipped with Ni filtered Cu Kα (1.54178 Å) radiation was employed for the determination of the structure and crystalline phases for the fabricated material.The Quanta 200-FEG SEM combined with energy-dispersive x-ray (EDX) at an accelerating voltage of 100 kV was used; its morphology, microstructure, and elemental content of a synthesized sample were determined using a scanning electron microscope (SEM-EDX).Furthermore, the Nova2200e Quantachrome Brunauer Emmett Teller was investigated for textual properties of the synthesized material.

Electrochemical characterization
Electrochemical activity of the fabricated electrocatalyst was investigated with Metrohm Autolab (PGSTAT-204) armed via the 3 electrode system such as the Ag/AgCl function reference electrode, Pt wire function as counter and fabricated material dumped on the NF function as WE under 1.0 M alkaline KOH solution.OER performance was calculated via LSV, CV, EIS, ECSA and chronoamperometry by 2.1 NOVA software.However, recorded potentials were transformed to RHE with the following equation [52]: The CV polarization curve was used to measure the electrocatalytic activity and kinetics via the Tafel slope considering linear region of the steady state curve with equation [53].
The electrochemical surface area was computed by polarization curves at different sweep speeds in capacitive areas within the potential window that goes from 0.90 to 1.20 V vs RHE.The electrochemical surface area of the fabricated material was calculated with the following equation [54]: Electrochemical impedance spectroscopy was used to measure electrical conductivity of the material having a frequency region of 0.1-100,000 Hz.Moreover, the Nyquist plot was employed for the determination of reluctance to solution Rs and charge transfer resistance Rct of the manufactured electrode material by fitting along the Randles circuit.Also, the firmness of the fabricated electrode material was investigated via the chronoamperometry method at a fixed potential of 1.75 V vs. RHE.

Structural, morphology and textual properties.
Crystal structure, purity, crystallinity and size of the prepared substance were measured by XRD in 2-theta ranging from 0 o to 80 o .Figure 1(b) shows the peak at 25.19 °, 30.47 °, 31.04 °, 37.61 °, 47.96°correspond to 012, 111, 004, 113 and 115 indexed well to JCPDS card no.00-032-1382 and exhibited the orthorhombic crystal system having a = 11.420,b = 4.190, c = 4.460 Å cell constant with the Pnma space group.As shown in Figure 1, the distinctive broad peak appearing at 7.24°is attributed to 001 hkl value, confirming the synthesis of GO.Furthermore, the existence of both phases GO and SnSe peak observed in the diffractogram of GO/SnSe with diffraction peaks at 2θ = 6.45, 24.44, 30.47, 37.34, 49.65°indicates the successful synthesis of the GO/SnSe nanocomposite.The slight shifting in the XRD pattern of the composite occurred due to the synergetic effect of the GO and SnSe than their pristine material.The crystallinity and purity of the material were determined by the appearance of sharp peaks and the presence of the number of other peak depicts its crystalline nature and purity of the synthesized material [55].Crystallite size of the material was measured with Debye Scherrer equation ( 4) and is in range of 86, 72 and 59 nm for GO, SnSe and nanocomposite, respectively [56][57][58][59][60][61][62][63].
Raman spectra of SnSe, GO, and the GO/SnSe composite were acquired to confirm the presence of GO in the composite and gain an insight into the phases and orientation of the band in the fabricated materials (Figure 1(e)).The two spectral peaks at 159.99 and 185.32 cm −1 are associated with the B-plane vibrations related to SnSe that are typical of the Ag phonon modes.The peak at 114.25 cm −1 in SnSe may be explained by the B 3 g phonon mode, which has been produced by stiff shear modes in c directions.GO demonstrates a spectrum that includes a significant carbon D (1356 cm −1 ) and G (1610 cm −1 ) bands, which have been correlated with vibrations of sp 3 type carbon atoms (typically activated carbon along with disordered) and sp 2 -like carbon atoms (typical of graphitic layers), while the formation of the GO/SnSe nanosized composite leads to higher intensity of D as well as G band and other two bands such as D + G and 2D appeared at 2560.10 and 2753.11cm −1 , respectively, are related to the carbon-based materials.With the exception of a very little but detectable red shift, which may reflect an interaction between SnSe and GO, all peaks are likewise seen in the GO/SnSe spectra confirming the successful synthesis of the nanocomposite.
The microstructure and morphology of material were measured with SEM. Figure 2(a) depicts thick dendrite layers of GO, and these dendrites promoted electrochemical performance because of the enhanced surface area.However, the SnSe exhibited a smooth surface with randomly distributed nanorod morphology, which provides an extensive surface area as well as active sites for enhanced electrochemical performance as shown in Figure 2(b).Figure 2(c) denotes the GO/SnSe nanocomposite morphology showing the nanorod anchoring on the dendrite layers of graphene oxide because the rod-shaped SnSe was uniformly engaged between the stacked graphene layers which are accountable for the creation of restriction between the restacking of graphene layers during the electrochemical performance of the GO/SnSe nanocomposite, and also the synergetic effect provides a large surface area which is evidence of good electrochemical surface area (ECSA) analysis.The elemental configuration of the designed material was analysed using the EDX study.As depicted in Figure 2 The surface area of GO, SnSe and GO/SnSe powders was determined using the BET method, which established an adsorption-desorption isotherm.Figure 2(g) shows type III isotherms, indicating that the adsorbate and adsorbent have limited interaction.The absence of an adsorption-desorption hysteresis loop in the isotherm rules out the GO and SnSe powders are mesoporous [64].The isotherm, on the other hand, demonstrated multilayer gas adsorption, with GO, SnSe and GO/SnSe having specific BET surface areas of 66, 160, 249 m 2 g −1 , respectively.The pore size distribution of GO, SnSe as well as GO/SnSe was measured with desorption data from the Barret−Joyner−Halenda (BJH) model, as seen in Figure 2(g) inset.Calculated pore volumes for GO, SnSe and GO/SnSe are 0.1, 0.9, as well as 1.1 cm 3 g −1 , accordingly.Thus, enhanced pore volume and greater surface area of the GO/SnSe are attributed to the existence of nanorods supported on the graphene oxide sheet increase surface area and the number of active site accountable to robust OER activity.

Electrochemical study
Cyclic voltammetry was used to assess the electrochemical performance of the produced material coated on NF under 1.0 M alkaline KOH at a sweep speed of 5 mV s −1 .The cyclic voltammogram profile shows the peroxidative characteristics at 1.6 V due to the oxidation of Sn + 2 to Sn + 3 , followed by the steady decline at 1.4 V vs. RHE suggesting start of the OER process with the evolution of oxygen bubble at the surface of the working electrode as shown in Figure 3(a).Furthermore, the intrinsic electrocatalytic property of fabricated substances was investigated along with LSV at a   3(c,d) and comparison between the reported literature and fabricated materials are given in Table 1.Among all GO/SnSe had the least overpotential of 336 mV dec −1 to attain a current density around 10 mA cm −2 , as well as the lower value of the Tafel slope (36 mV dec −1 ) of the nanocomposite in contrast to the benchmark of RuO 2 and other fabricated substances suggest the rate determining step, easy electron transfer process and the kinetic mechanism of the fabricated substance which are given below During the OER process, OH − was adsorbed on the surface of a free metal ion, resulting in the production of MOH, as well as deprotonation transform of M-OH to M-O.There are two ways to create O 2 molecules, and then, M-O interacts with OH − to produce M-OOH, which deprotonates with active site renewal to form O 2 [65].
Electrochemical surface area of the material was determined with double layer capacitance in potential window ranging from 0.80-1.10V vs. RHE at sweep speed of 5, 10 and 15 mV s −1 in 1.0 M alkaline KOH as shown in Figure 4(a-c), and difference between anodic as well as cathodic current density as a function of scan rate is mentioned in Figure 4(d-f), and the calculated Cdl values of GO, SnSe, and GO/SnSe are 11.38, 4.105 and 17.3 mF, respectively, using Equation (3) and Figure 4(d-f).A larger Cdl value of the GO/SnSe suggests that the material exhibited high rate of charge accumulation and larger surface area with enhanced electrochemical active sites [75].
The solution resistance (Rs) and charge transfer resistance (Rct) of the fabricated electrode substance were calculated with EIS under 1.0 M alkaline KOH at 0.50 V vs Ag/AgCl electrode.The Nyquist plots displayed a smaller semicircle resultant via the circuit fitted by employing Nova 2.1 software.The electrochemical result revealed that GO, SnSe and GO/SnSe exhibited the Rct value of 0.2, 0.8, and 2.6 Ω, respectively.The smaller Rct value of GO/SnSe is attributed to their crystalline structure, good morphology, larger surface area, greater amounts of active sites and increased electrical conductivity in the alkaline medium [76].
Long-term stability of synthesized materials was confirmed via the chronoamperometry technique and reported as an outcome of potential vs time for 35 h (Figure 5(b)).Moreover, the cyclic stability of the substance was also measured with multiple CV cycles as depicted in Figure 5(c).Electrochemical stability results suggested that the material shows stable behaviour over 2000 cycle with negligible change, corroborative with the high stability of the resultant substance.Hence, the increased electrochemical performance of the fabricated GO/SnSe electrode material was because of the inclusion of rGO in the composite, which not only gave the physical support, avoiding volume expansion and electrode material aggregation, but it also allowed for rapid electron flow during the electrochemical process, high surface area, good morphology and high crystallinity of the resultant material.To confirm the structure, phase retaining the XRD was also performed after the stability test and is shown in Figure 5(d).The resultant XRD peaks' position approves that there is no prominent change in the phase, but smaller change in the intensity of the peaks occurs.

Conclusion
In this summary, we report the fabrication of the GO/SnSe nanocomposite via the hydrothermal route employed for the water splitting process.The various analytical characterizations are often utilized to examine the morphology, structure and textual characteristics of the fabricated material.Further, the electrochemical performance of the substance was measured in 1.0 M alkaline KOH solution, and electrochemical result suggests that the GO based SnSe exhibited lesser overpotential of 336 mV to attain a current density around 10 mA cm −2 as well as smaller Tafel slope of 36 mV dec −1 .However, the electrochemical efficiency of the GO/SnSe nanocomposite was further confirmed with EIS and chronoamperometry analysis, indicating good conductivity due to enhanced larger surface area supported by the rod-like structure and stable for 35 h.Hence, our electrochemical results indicate that GO-based SnSe combinations are highly efficient for OER activity, and also opens a new era for different applications.

Figure 1 .
Figure 1.(a) XRD pattern of SnSe, GO and GO/SnSe, and (b) Raman shift of all fabricated materials.
(d), the occurrence of C, O in GO, and the SnSe EDX spectrum contains Sn and Se elements in the sample as depicted in Figure 2(e), while on the other hand, in Figure 2(f) the GO/SnSe nanocomposite contains C, O, Sn, Se, in the relative EDX spectra.However, the existence of no extra peak shows the purity of the material and shows excellent consistency between the XRD results

Figure 3 .
Figure 3. (a) CV curves, (b) LSV curves at a sweep speed of 5 mV s −1 , (c) comparison of overpotential to reach current density of 10 mA cm −2 of all the fabricated catalysts, and (d) Tafel slope of the GO, SnSe, RuO 2 and GO/SnSe nanocomposite

Table 1 .
Comparison of OER performance with reported literatures and fabricated materials.