Enhanced Overall Water-Splitting Performance: Oleylamine-Functionalized GO/Cu2ZnSnS4 Composite as a Nobel Metal-Free and NonPrecious Electrocatalyst.

Using emergent highly proficient and inexpensive non-noble metal-based bifunctional electrocatalysts to overall water splitting reaction is a pleasingly optional approach to resolve greenhouse gases and energy anxiety. In this work, oleylamine-functionalized graphene oxide/Cu2ZnSnS4 composite (OAm-GO/CZTS) is prepared and investigated as a higher bifunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The OAm-GO/CZTS shows brilliant electrocatalytic performance and durability toward H2 and O2 in both acidic and basic media, with overpotentials of 47 mV for HER and 1.36 V for OER at a current density of 10 mA cm-2 and Tafel slopes of 64 and 91 mV dec-1, respectively, which are extremely higher to those of transition metal chalcogenide and as good as of commercial precious-metal catalysts.

Introduction

Intensification requirement of fossil fuels, the exhaustion of nonrenewable energy sources and ecological pollution have led to immediate requirement for economical, environmentally friendly, and highly developed energy production and storage systems.[1] Electrochemical water splitting into hydrogen and oxygen is broadly considered to be a critical step for remarkable nonconventional energy production, storage, and convention. To date, the best catalysts are still mainly from Noble metals Ir or Ru for the oxygen evolution reaction (OER)[2] and Pt for the hydrogen evolution reaction (HER).[3] Nevertheless, expensive metals rarely fulfill the requirements of large-scale consumption because of their shortage and costly. Intensive study is accordingly conducted to develop inexpensive and high-efficiency electrocatalysts that have comparable activity to precious electrocatalysts. In the times of yore, metal complexes,[4] heteroatom-doped,[5] metal alloys,[6] double-layered hydroxide,[7] metal phosphides,[8] hybrid nano–bio electrocatalysts,[9] polymer-embedded catalyst,[10] transition metals,[11] metal-free electrocatalyst,[12] metal–carbon heterostructures,[13] amine-functionalized electrocatalysts,[12c] and many more potential electrode materials have been investigated for H2- and O2-evolution reactions.[14]

Consequently, different approaches have been scrutinized to further fine-tune HER and OER activities of electrocatalysts: first, manufacturing of higher surface areas and earth-abundant nanostructures.[15] Second, doping is one of the scalable strategies that further improve the electrocatalytic activity of the electrocatalyst because of their abundant defects and enhanced number of the catalytic active sites. For example, pure MoS2 consists of inferior electrocatalytic activity than that of Ni-doped MoS2 electrocatalyst toward water splitting reaction.[16] Fe incorporates in MoP give an electrocatalytic performance than bare MoP,[17] and Co-doped CZTS enhanced their electrocatalytic activity than pure CZTS.[18] Third, heteroatom-doped materials are admired in the research areas of water-splitting reaction because of its abundant active sites and superb electrical properties. N/Co-doped PCP/NRGO demonstrated superior electrocatalytic performance when N/Co-doped PCP merges with NRGO sheets to form a hybrid, which is a valuable strategy to integrate their relevant virtues and improve the overall catalytic performance;[19] the Ni2P/NRGO hybrid displayed an improved catalytic activity with a small Tafel slope (59 mV dec–1) and minute overpotential 37 mV than the Ni2P/RGO hybrid catalyst;[20] and the amorphous Co–Ni–B electrocatalyst shows higher electrocatalytic performance toward HER in a wide range of pH.[21] Fourth, layered double hydroxides (LDHs), as a class of ionic lamellar compounds made up of positively charged brucite-like layers with an interlayer area including charge-balancing anions and solvation molecules. Co intake LDH ultrathin nanosheets show superior OER activity with small overpotentials and lesser Tafel slopes.[22] Fifth, extremely high electrically conductive materials, such as graphene oxide (GO),[23] can further increase the electrocatalytic activities of catalysts for water-splitting reaction,[24] thereby exposing more active sites for facilitating fast electron transport. For instance, MoS2/rGO and MoSe2/rGO composites exhibit a large number of catalytic edge sites plus their outstanding HER activity.[25] Three-dimensional CoS2 and CoSe2/graphene hybrid heterostructures are excellent HER catalysts.[26] So far, this strategy has actually improved the catalytic performances of the electrocatalyst, but the majority of them still have no comparison to the noble-metal Pt electrocatalyst.[27] Consequently, it is still well enviable and crucial to the formation of the electrocatalyst as compared to the Pt-like electrocatalyst. In our earlier work, we testify that the GO/CZTS composite further enhances the electrocatalytic activity of CZTS; here, functionalization of graphene with amine (electron-donating groups) might be a promising approach to get better electrical conductivity of graphene.[28]

Herein, we developed an oleylamine-functionalized GO/CZTS composite (OAm-GO/CZTS) which further enhances the electrocatalytic performance toward water-splitting reaction (mutual aid H2 and O2) compared to GO/CZTS. The results undoubtedly suggest that the introduction of electron-donating functional groups could significantly improve the catalytic activities and prospect a further avenue toward scheming very capable catalysts for water splitting with enormous potentials to substitute the precious Pt-based catalysts. The key advantage of oleylamine (OAm) over other amines is its ability to act as many different agents, including high boiling point, viscous solvent, reducing agent, and coordinating ligand via the terminal amine.[29] The OAm-GO/CZTS electrocatalyst exhibits excellent electrocatalytic activity with a minor Tafel slope of 64 and 91 mV dec–1 and a small overpotential of 47 mV and 1.11 V at 10 mA cm–2 obtained toward HER and OER, respectively. This effort not only presents a low cost, earth-abundant, and well-active electrocatalyst but also unlock a fresh research pathway toward the development of HER and OER catalytic activity in general.

Instruments

To identify the organic and inorganic components in the sample, Fourier transform infrared (FTIR) spectrometric (400–4000 cm–1) studies on a Bruker TENSOR 27 FT-IR spectrometer were carried out. X-ray diffraction (XRD) spectroscopy for the purpose used phase and structure analysis by a Siemens D-5005 diffractometer equipped with an X-ray tube (Cu Kα; λ = 1.5418 nm, 40 kV, 30 mA, with a step size of 0.01°). An equivalent Raman spectrum was obtained by Raman optics with a microscope, Seki Technotron Corp., Tokyo, with 532 nm laser. The Brunauer–Emmett–Teller (BET) surface area calculation of OAm-GO/CZTS and all necessary sample analysis by N2 adsorption at 77 K isotherms at 77.350 K was performed using a Quantachrome NovaWin 1994-2012, Quantachrome Instruments v11.02. Field emission scanning electron microscopy (FE-SEM) study was conducted using the Nova NanoSEM NPEP303 and transmission electron microscopy (TEM) instrument for examining the morphologies and sizes of the products.

Electrochemical Measurements

Electrocatalytic potential investigation studies were carried out using CHI Instrument 660E (USA) electrochemical workstation under room temperature. A typical three-electrode system consist of a reference electrode as a saturated calomel electrode (SCE), a working electrode as a glassy carbon electrode (GCE is 3.0 mm in dia.), and a counter electrode as a platinum wire for overall water splitting reaction. Before the GCE is used for an experiment, it makes mirror polished, by using alumina powders, in the order of 1, 0.3, and 0.05 μm and cleaned at the same time by using water and methanol to remove inorganic and organic impurities concurrently. For the manufacture of the working electrode, the active area of the electrode was coated with a calculated amount of a thin layer of the catalyst, which was prepared by ultrasonic mixing of 5.0 mg of the as-synthesized catalyst with 300 μL of isopropanol and 10 μL of Nafion solution for 1/2 h in order to form a uniform slurry. Then, 10 μL of the prepared slurry was loaded onto the surface of a GCE using a micropipette and dried at room temperature. All the potentials were carried out with respect to the reversible hydrogen electrode (RHE) in 0.5 M H2SO4E(RHE) = E(SCE) + 0.244 V, in 1.0 M KOH, E(RHE) = E(SCE) + 1.051 V, electrochemical impedance spectroscopy (EIS) analysis carried out from 1 000 000 to 0.002 Hz at a direct-current bias potential of 47 mV for HER and 1.11 V for OER, respectively, at room temperature.

Result and Discussion

Figure a demonstrated FT-IR spectra of GO, OAm, pure CZTS, OAm-GO, GO/CZTS composite, OAm-CZTS, and OAm-GO/CZTS over the range of 400–4000 cm–1. The characteristic bands of GO are observed at 1025 cm–1 (C–O–C stretching vibrations of epoxy groups), 1406 cm–1 (C–OH stretching), 1615 cm–1 (C=C skeleton vibrations of graphitic domains), 1725 cm–1 (C=O stretching vibrations of COOH groups), and 3442 cm–1 (−OH bending vibration.[30] The FTIR spectra of OAm shows absorption peaks at 720, 967, 1070, 1466, 1592, 1650, 2850, and 3003 cm–1, respectively. The FTIR absorption peak of OAm at 720 cm–1 corresponds to the (C–C) bond. The carbon-nitrogen (C–N) stretching vibrations (1070 cm–1) and nitrogen hydrogen bending vibrations (967 cm–1) of amine (NH2) group in OAm were unchanged. The absorption peak at 1465 and 1593 cm–1 corresponds to the bending of the CH3 and NH2 group in OAm. The band at 2850 cm–1 is for symmetric C–H stretching and 2920 cm–1 for asymmetric C–H stretching and the main band 3312 cm–1 is corresponding to the primary amine NH2 group of OAm, which is in good agreement with the reported literature.[31] The distinctive functional groups of CZTS were located in 1040, 1420, 1622, and 2351 cm–1 corresponding to the C−S stretching, coupled vibrations of C−N stretching, and N–H bending, S–H thiol functionalities, respectively, which is in concurrence with the literature ideals.[32,33] In the OAm-GO FTIR spectra, vanishing of the peak of the carboxyl group on the GO surface at 1725 cm–1 peak of the amine group on OAm at 3312 cm–1 stretching vibration can be observed, and the appearance of the peak of the N–H stretching vibration at 1552 cm–1 in the FTIR spectrum of OAm confirms that the successful functionalization with GO (OAm-GO) were attributed to the covalent bonding between amine groups of OAm and carboxyl groups of GO. On the other hand, the above-assigned peaks of CZTS were also observed in the GO/CZTS composites, with reduced intensity, which indicates that the oxygenated functional groups in GO were reduced partially. For the comparative studies, characteristic peaks obtained at 3311 cm–1 amines (NH2), 2854, and 2924 cm–1 (symmetric and asymmetric stretching C–H) are observed from plane OAm and the peak at 1040, 1420, and 1622 cm–1 equivalent to the C−S stretching, coupled vibrations of C−N stretching, and N–H bending of CZTS. In the case of OAm-GO/CZTS, the broad FTIR peak at 3438 cm–1 is attributed to the stretching of adsorbed water molecules and structural O–H groups (such as alcohol and carboxylic acid) from GO and peaks at 1565 cm–1 equivalent to amine bond (NH2). The intensity of every peak attributed to oxygen functional groups is remarkably reduced, representing that most of the GO oxygen group is reduced or substituted with amino groups during the functionalization. Hence, FTIR analysis confirms the amine functionalization of the GO/CZTS composite.[34]

Figure 1

(a) Superimposed FTIR spectra using dry KBr more than the range of 400–4000 cm–1 (i) GO (black), (ii) CZTS (dark green), and (iii) OAm (orange). (b) FTIR spectra using dry KBr more than the range of 400–4000 cm–1 of (i) OAm-GO (pink), (ii) OAm-CZTS (faint green), (iii) GO/CZTS (blue), and (iv) OAm-GO/CZTS (red). (c) Superimposed XRD patterns in the range of 2θ° (20°–80°) (i) GO (black), (ii) OAm-GO (pink), (iii) CZTS (dark green), (iv) OAm-CZTS (faint green), (v) GO/CZTS (blue), and (vi) OAm-GO/CZTS (red). (d) Superimposed Raman spectrum of (i) GO (black), (ii) pure CZTS (dark green), (iii) GO/CZTS (blue), and (iv) OAm-GO/CZTS (red). (e) Superimposed BET surface area measurement using N2 adsorption–desorption isotherms of (i) CZTS (dark green), (ii) GO/CZTS (blue), (iii) GO (black), and (iv) OAm-GO/CZTS (red).

X-ray Diffraction

Figure b shows the superimposed XRD patterns of GO, OAm-f-GO, pure CZTS, OAm-CZTS, GO/CZTS composite, and OAm-GO/CZTS. The XRD pattern of GO represents a distinct small peak at (002) corresponding to few-layer graphene.[5] Accordingly, the pattern for OAm-f-GO illustrates the interplanar spacing is slightly increased after surface functionalization of GO by OAm performing as a spacer and increases interplanar distances. It is apparent that the pure CZTS XRD patterns shown are the characteristic peaks at (112), (200), (220), and (312) crystal planes of the kesterite structure of CZTS (JCPDS card no: 26-0575).[35] Then, OAm-CZTS showed similar XRD patterns of pure CZTS, and except for their peak broadening, there is no any additional peaks observed. GO/CZTS composites possess the XRD pattern, the peaks observed at (002), (112), (200), (220), and (312) correspond to the diffraction planes of mixed GO and CZTS. However, OAm-GO/CZTS shows four pronounced diffraction planes at (112), (200), (220), and (312) of CZTS and (002) plane of GO, respectively, which shows that the functionalization of amine does not much affect the crystal structure of the GO/CZTS composite and OAm species is not observed. The broadened peaks indicate that the crystallite size of the OAm-GO/CZTS composite is relatively small.

Raman Spectroscopy

Raman spectroscopy was executed on GO, CZTS, composite GO/CZTS, and OAm-GO/CZTS, and the outcome is shown in Figure c. In the Raman profile of GO, the typical D and G bands are allocated to the k-point phonons of the A1g symmetry and E2g phonon of the sp2 carbon at 1360 and 1594 cm–1, respectively.[36] The characteristic peaks at 333 cm–1 were assigned to the A1 mode of CZTS NPs, which is the toughest mode observed from kesterite CZTS.[37] Intended for the GO/CZTS composite, all the Raman bands for CZTS and GO can be found which indicates that GO/CZTS consists of GO and CZTS. The intensity of the G band increases relatively to the D band when CZTS NPs are deposited on graphene and both bands shift to higher wavenumbers.[36] In the present study, the D and G bands for the OAm-GO/CZTS composite appeared at 1360 and 1594 cm–1 and the CZTS peak was observed at 333 cm–1, respectively. Therefore, the intensity ratio of ID/IG gives straight confirmation of the degree of functionalization and the ID/IG ratio of pure GO is 0.83, then ID/IG ratio of GO/CZTS and OAm-GO/CZTS was increased from 0.38 to 0.435, following OAm functionalization. This consequence also visibly signifies that the amine functional group was covalently bonded against the surface of GO after surface functionalization. The XRD patterns accompanied by Raman spectra all demonstrated the fact that the OAm-GO/CZTS composite material was obtained without any other extraneous species.

Brunauer–Emmett–Teller

The BET surface area for pure CZTS, GO/CZTS, GO, and OAm-GO/CZTS was surveyed using typical N2 adsorption–desorption isotherms. As shown in Figure d the special BET surface area of GO, CZTS, and GO/CZTS electrocatalytic systems is 41.7, 2.016, and 24.016 m2 g–1, which is significantly lower than OAm-GO/CZTS (50.716 m2 g–1), and this could be due to the presence of amine groups and conductive graphene with a huge surface area. The enhanced specific surface area supplies efficient transport pathways for charged ions and increases the electrode–electrolyte interfacial area, which is favorable for the improvement of electrochemical performance of the composite.

Field Emission Scanning Electron Microscopy

Figure a–c illustrate the surface morphologies of pure CZTS NPs, GO sheets, and OAm-GO/CZTS examined by FE-SEM. In Figure a, the layered structure of the stacked GO sheets can be seen, and there are many wrinkles through all the surfaces of GO sheets.[38] The solid GO sample is severely agglomerated because of its high specific surface area. Figure b shows an FE-SEM image of CZTS nanoparticles that were mainly of the spherical shape, with an average size diameter of 100–200 nm. The diameter was determined by averaged measurements of around 50 particles in the FE-SEM images. Uniform CZTS nanospheres are obtained by small aggregation.[39]Figure c after amine functionalization and careful inspection of the FE-SEM images of OAm-GO/CZTS revealed an average size diameter of 200–250 nm, and the size increases compared to CZTS because of the higher size of GO. When adding together, it can be evidently examined that CZTS was well dispersed in the graphene structure with no obvious aggregation, and the majority of these CZTS nanospheres were wrapped with graphene nanosheets.[36] Additionally, the high-resolution TEM image of OAm-GO/CZTS is shown in Figure S1, and it clearly shows that after amine functionalization, CZTS particles were enfolded in graphene sheets, which is reliable with the examination from FE-SEM images of OAm-GO/CZTS.

Figure 2

Morphology of OAm-GO/CZTS (a) FE-SEM image of GO, (b) FE-SEM of pure CZTS NPs size of 100–200 nm, and (c) FE-SEM image of OAm-GO/CZTS composite size of 200–250 nm (yellow arrows denoted graphene sheets and red arrows denote CZTS NPs).

Electrocatalytic Activities toward HER

The performance of the electrocatalyst of all the samples toward the HER was estimated on three set up systems, using a SCE as the reference electrode, platinum wire as the counter electrode, and GCE as the working electrode, in 0.5 M H2SO4 solution at a scan rate of 50 mV s–1. Figure a shows the linear sweep voltammetry (LSV) polarization curves of Pt, pristine GO, OAm-GO, pure CZTS, OAm-CZTS, GO/CZTS, and OAm-GO/CZTS at the scan rate of 50 mV s–1. As control experiments, the electrochemical performances of the commercial Pt electrode and pure GO were also investigated for comparison. Obviously, the Pt electrode demonstrates the lowest overpotential, representing the highest electrocatalytic activity for HER,[40] while the pure GO electrode exhibits insignificant activity toward the HER. Figure a shows significantly that OAm-GO/CZTS exhibits the onset overpotential of 47 and 96.6 mV to afford current densities of 10 and 20 mA cm–2, respectively, lower than the earlier-reported HER catalytic systems under given conditions,[41] demonstrating that OAm-GO (546 mV), pure CZTS (435 mV), OAm-CZTS (389 mV), and GO/CZTS composite (208 mV) at 10 mA cm–2 exhibit inferior HER activities as compared to the OAm-GO/CZTS electrocatalyst. However, improved electrochemical performance on OAm-GO/CZTS composites can be attributed to the chemical and electronic coupling between OAm and CZTS/GO support, and this result makes us consider that the presence of the amine group plays a noteworthy task in the superior electrocatalytic activity and also graphene can perform as an ideal conductive additive because of its distinctive electrical properties.[42] Besides, the Tafel slope is playing a vital role for analyzing HER activity, and a lesser Tafel slope leads to a faster augmentation of HER rate with increasing overpotential. The linear portions of the Tafel plots were fitted by the Tafel equation (η = b log j + a, where, j is the current density and b is the Tafel slope), in Figure b. The Pt Tafel value is superior, that is, 36 mV dec–1 which means it is smaller than the exhibited OAm-GO/CZTS Tafel slope value of 64 mV dec–1.[43] while it is much lower than that of OAm-GO (115 mV dec–1), pure CZTS (85 mV dec–1), OAm-CZTS (83 mV dec–1), and for GO/CZTS (81 mV dec–1) which is in good agreement with previous literature values for the nonprecious transition-metal HER electrocatalyst, such as MoS2,[13c] MoS2CFs,[44] and bare CoS2.[45] More comparison of the OAm-GO/CZTS composite with other electrocatalyst is listed in (Table S1). OAm-GO/CZTS is obtained with the exchange current density (j0) being 988 mA cm–2 for the HER, which is superior to GO/CZTS composites (884 mA cm–2) and also outperforming that of many reported non-noble metal HER catalysts.[46] These results suggest that the OAm-GO/CZTS could be used as an efficient electrocatalyst for the HER and are shown in Figure S2, which shows scan rate-dependant LSV polarization curve from 10 to 100 mV s–1. It can be seen that the current density increases with the increasing scan rate; it is informative that the catalytic activity of OAm-GO/CZTS toward HER is a slighter pretentious by scan rates authenticate electron transfer reaction (HER) is diffusion-controlled.[27d] In addition, we use the LSV using graphite as a counter electrode for a comparative study for the Pt counter electrode, but there are no significant results obtained (dissolution and reductive deposition of Pt on the cathode) see in Figure S3.

Figure 3

Superimposed (a) HER polarization curves for (i) Pt (cyan), (ii) OAm-GO/CZTS (red), (iii) GO/CZTS (blue), (iv) OAm-CZTS (green), (v) pure CZTS (dark green), (vi) OAm-GO (pink), (vii) GO (black) and showing the highest catalytic activity of OAm-GO/CZTS. (b) Corresponding to the Tafel plot (i) Pt (cyan), (ii) OAm-GO/CZTS (red), (iii) GO/CZTS (blue), (iv) OAm-CZTS (green), (v) pure CZTS (dark green), (vi) OAm-GO (pink). (c) Nyquist plot of (i) OAm-GO/CZTS (red), (ii) GO/CZTS (blue), (iii) OAm-CZTS (green), (iv) OAm-GO (pink) (v) GO (black). (d) Durability test of OAm-GO/CZTS (inset of d) shows the i–t chronoamperometry test for 1000 min in 0.5 M H2SO4.

To understand the role of electrode kinetics and interface reaction of OAm-GO/CZTS and its GO/CZTS composites on HER to perform the EIS measurement, Nyquist plots presented in Figure c is observed to be semicircle which is due to the charge transfer resistance (Rct) at the electrode and electrolyte interface. From the EIS, it has been observed that the GO/CZTS (23 Ω), OAm-CZTS (37 Ω), OAm-GO (50 Ω), and GO (78 Ω) show higher Rct values compared to OAm-GO/CZTS; herein, the charge transfer resistance (Rct) of OAm-GO/CZTS is 5 Ω which results in better enhancement in the electron transfer process after amine functionalization, which leads to the enhancement of HER activity.[25b,47] We further ensure the catalytic strength of OAm-GO/CZTS in the 0.5 M H2SO4 solution to exhibit the possibility of its practical application. The polarization curves of OAm-GO/CZTS in Figure d show an insignificant loss of it’s HER catalytic activity after 1000 CV cycles. In addition, the chronopotentiometric curve in the inset of Figure d shows that the OAm-GO/CZTS could deliver a stable current density at the overpotential of 47 mV up to 1000 min with negligible degradation. These results undoubtedly display the excellent durability of the OAm-GO/CZTS under the HER condition.[27d,46b,48] The turnover frequency (TOF) of H2 molecules evolved per second (symbolize as units of s–1) for each active site was deliberate. The TOF can be intended by the Jaramillo’s method.[27d,48,49] For this direct site-to-site evaluation, a contrast to other catalysts except for platinum (Pt), the OAm-GO/CZTS catalyst shows the uppermost TOF value of 3.673 s–1 at η = 250 mV, indicating a tremendous intrinsic HER activity which should be credited to the conductive GO that imparts CZTS with rapid electron transfer in the HER. Moreover, the GO sheets with a high specific surface area act as a superior supporter for the electrocatalyst, which is also essential for enhancing the active sites. TOF is the number of hydrogen molecules generated in each active site per second (see the Supporting Information for details of calculations). More detailed comparison of other electrocatalysts is shown in Table S1.

Electrocatalytic Activities toward OER

Electrocatalytic studies also show that the synthesized catalysts also possess prominent OER performance measured by LSV in basic electrolyte (1.0 M KOH) solution at a scan rate of 50 mV s–1 at room temperature and is shown in Figure a. Surprisingly, OAm-GO/CZTS exhibits a low overpotential of η = 1.36 V at 10 mA cm–2, and the observed overpotential for OER is one of the lowest that has been witnessed till date, also lesser than GO/CZTS (1.61 V), OAm-CZTS (1.80 V), pure CZTS (1.85 V), OAm-GO (2.30 V), and GO (2.46 V) appraisal listed in (Table S2) and the resulting Tafel slope of OAm-GO/CZTS is 91 mV dec–1 in Figure b. This is the lowest among those of electrocatalysts OAm-GO (160 mV dec–1), CZTS (144 mV dec–1) OAm-CZTS (142 mV dec–1), and GO/CZTS for (140 mV dec–1). These results reveal that OAm-GO/CZTS shows excellent electrocatalytic properties over the other catalysts and even newly reported OER-based catalysts.[16,50] Furthermore, the semicircular diameter is shown in Figure c. The OAm-GO/CZTS (Rct = 76 Ω) is much inferior to GO/CZTS (Rct = 125 Ω), OAm-CZTS (Rct = 155 Ω), CZTS (Rct = 200 Ω), OAm-GO (Rct = 150 Ω), and GO (Rct = 398 Ω) because of lower electron transfer resistance in alkaline solution. The OAm-GO/CZTS possesses superb durability in the alkaline electrolyte as shown in Figure d. Even after continuous 1000 cycling, the OAm-GO/CZTS catalyst presents a similar polarization curve and original appearance. This result is fabulous than other OER electrocatalysts.[51] The chronoamperometric (CA) test for 1000 min is shown in the inset of Figure d, and it confirms the superb stability performance of OAm-GO/CZTS with negligible loss of anodic current in the alkaline electrolyte.

Figure 4

Superimposed (a) OER polarization curves for pure (i) OAm-GO/CZTS (red), (ii) GO/CZTS (blue), (iii) OAm-CZTS (green), (iv) pure CZTS (olive), (v) OAm-GO (pink), and (vi) GO (black), showing the highest catalytic activity of OAm-GO/CZTS (b) corresponding to the Tafel plot (i) OAm-GO/CZTS (red), (ii) GO/CZTS (blue), (iii) OAm-CZTS (green), (iv) pure CZTS (olive), and (v) OAm-GO (pink) (c) Nyquist plot of (i) OAm-GO/CZTS (red), (ii) GO/CZTS (blue), (iii) OAm-CZTS (green), (iv) pure CZTS (olive), (v) OAm-GO (pink), and (vi) GO (black). (d) Durability test of OAm-GO/CZTS (inset of d) shows the i–t CA test for 1000 min in 1.0 M KOH.

Figure S4 shows that scan rate-dependent LSV performance of OAm-GO/CZTS toward OER in the resultant current density increases with increasing scan rate from 10 to 100 mV s–1, revealing that the electrocatalytic activity of the catalyst which is small is affected by scan rates. That is, superior HER and OER performance of the proposed OAm-GO/CZTS electrocatalyst is essentially ascribed to the following reasons:First, the amine functionalization plays an important role in the enhanced catalytic activity where the lower |ΔGH*| value of the amine group increases the electron transferability of GO, which is beneficial for water-splitting reaction. Second, the OAm-GO/CZTS electrocatalyst has a huge specific surface area and supports the mass transfer of ions in the electrolyte from GO that enhances HER catalytic performance. Third, the outstanding OER performance is initiated after oxidation and OAm functionalization of graphene; the graphene surface contains many amino groups and oxygen atoms, which present a greater catalysis activity for OER performance. Fourth, electronic coupling to the fundamental GO consistent conducting system provided speedy electron transport from the highly resistive CZTS NPs to the electrodes. To analyze this result, we verified electrochemical impedance spectroscopic (EIS) measurements. Thus, the fast charge transfer during the electrocatalytic reaction, unchanging with a large exposed active surface area, could have donated to the higher electrocatalytic activity of the OAm-GO/CZTS. Fifth, synergetic interaction between transition-metal chalcogenide (CZTS) and conducting GO composite hasten charge transfer and benefit fast dispersion and reaction at the electrolyte–electrode interface. Above rationalization indicates that OAm-GO/CZTS is a capable and stable bifunctional overall water-splitting electrocatalyst. In short, the electrochemical performance of OAm-GO/CZTS is given from the OAm-GO co-catalyst supported by the surface of CZTS. On the one hand, the amine group raises the electron transferability of GO and on the other hand, electrically conductive GO sheets as a skeleton facilitate the electron transfer to the CZTS catalyst; this work paves a new path for exploring efficient bifunctional water-splitting catalysts; see Scheme .

Scheme 1

Schematic Illustration of the OAm-GO/CZTS Electrocatalyst on the GCE for Overall Water-Splitting Reaction (HER and OER)

Conclusions

In summary, we prepared an OAm-GO/CZTS composite that exhibits high electrocatalytic activity toward overall water-splitting (HER and OER) reactions; the composite was obtained by the wrapping of GO on CZTS by an electrostatic reaction and the grafting of OAm via interaction with carboxylic groups on GO. Amine-functionalized graphene played a noteworthy role in enhancing H2 and O2 performance. The intimate contact of amine-functionalized graphene with CZTS enhanced charge transfer, resulting in a small Tafel slope of 64 mV dec–1 for HER and 91 mV dec–1 for OER with small overpotential of just about 47 mV in 0.5 M H2SO4 for the HER and 1.36 V in 1 M KOH for the OER, respectively, as well as superb stability of 1000 min for overall water splitting. We expect that amine-functionalized graphene does not only report a cost-effective catalyst for the water-splitting in both HER and OER. A similar approach is also applicable to propose the other catalysts with high efficiency.

Experimental Section

Chemicals and Materials

Graphite fine powder (extra pure), sulphuric acid (H2SO4, 98%), nitric acid (HNO3, 78%), hydrochloric acid (HCl, 98%), thionyl chloride (SOCl2), copper chloride (CuCl2·2H2O, 98%), zinc chloride (ZnCl2·2H2O, 96%), tin chloride (SnCl2·2H2O, 98%), thioacetamide, 2-methoxyethanol, OAm, monoethanolamine, and absolute ethanol of the AR grade were used for sonication. All the chemicals were procured from Sigma-Aldrich and were used without any further purification.

Preparation of GO

GO produced by a modified Hummers’ method is reported in the previous article.[52] In detail, 1 g of graphite powder as the graphene source was added in H2SO4/HNO3 typically in a 3:1 ratio of under continuous stirring in an ice bath for 30 min and sonicated at room temperature further for 6 h. The suspension was refluxed in an oil bath for the next 24 h. The combination was then frequently centrifuged and washed successively with water to remove surplus nitric acid and sulphuric acid. Finally, it was washed with 30% HCl to keep surface acid functionalities, followed by water and acetone. The filtered residue was dried in an oven at 200 °C for next 3–4 h.

Synthesis of CZTS Nanoparticles

The CZTS nanoparticles were synthesized by the sonochemical method described in our previous report.[53]

Synthesis of Amine Functionalization OAm-GO/CZTS Composite

Further, for the synthesis of OAm-GO/CZTS, 200 mg of as-synthesized GO was taken in a round-bottom flask and 20 mL of SOCl2 followed by 1 mL of dimethylformamide (DMF) was mixed in the cold condition, and this mixture was stirred for 1 h followed by reflux for the next 24 h. For additional functionalization by OAm, 20 mg of above G-COCl and 10 mL of OAm were mixed in 20 mL of DMF and this solution was sonicated for the next 4 h. In this OAm-GO solution, 200 mg of CZTS NPs was added and sonicated further for 2 h at room temperature. The obtained product was washed two times with ethanol to remove other impurities. Collected OAm-GO/CZTS annealed at 170 °C for 1 h. In this step, the amine group of OAm reacts with the carboxylic group of GO. The resulting amine-functionalized GO CZTS (OAm-GO/CZTS) consisted of CZTS nanoparticles coated with amine-functionalized GO. The overall procedure for fabricating OAm-GO/CZTS is schematically shown in Scheme .

Scheme 2

Schematic Illustration OAm-GO/CZTS Composites Synthetic Process (a) GO (b) G-COCl (c) OAm-GO (d) OAm-GO/CZTS (e) FE-SEM OAm-GO/CZTS (f) Water-Splitting Reaction