Architecture of the CdIn₂S₄/graphene nano-heterostructure for solar hydrogen production and anode for lithium ion battery

Abstract

The facile single step template free synthesis of hierarchical CdIn₂S₄/graphene nano-heterostructures with multi-functionality as a photocatalyst for solar hydrogen production and as an anode for lithium ion battery has been demonstrated. The nanopetals of CdIn₂S₄ are decorated on the graphene which shows extended visible light absorption. Hence, the photocatalytic hydrogen evolution study is performed under solar light. The nano-heterostructure showed excellent photocatalytic activity (4495 μmol h⁻¹ per 0.2 g) for hydrogen production. The enhanced photocatalytic activity is attributed to the inhibition of charge carrier recombination due to graphene which acts as an excellent electron collector and transporter. Furthermore, the use of nano-heterostructures as an anode for Li-ion batteries demonstrated very high reversible capacity i.e. 678 mA h g⁻¹ which on cycling, kept at 608 mA h g⁻¹ at an applied current of 150 mA g⁻¹ for 225 cycles and exhibited good rate capability. The excellent Li-storage properties of the nano-heterostructures is associated with the hierarchical flower like structure, high porosity of CdIn₂S₄ and the fast electron kinetics offered by the graphene support.

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1. Introduction

Research on clean energy has been a global focus due to the depletion of fossil fuels and the environmental issues associated with them. ‘Hydrogen’ is the key word for renewable energy as it is a clean fuel. Considering the alternative energy sources with increasing environmental concerns, energy storage devices such as lithium ion batteries and supercapacitors have attracted a lot of attention. Hence, global eco-friendly energy production and storage is becoming important and indispensable. Recently, multifunctional nanostructured materials or nanocomposites are fetching significance in energy generation and storage.¹ In this context, we have demonstrated the synthesis of nanostructured CdIn₂S₄/graphene nano-heterostructures with multifunctionality as a solar light active photocatalyst for hydrogen production and an anode for lithium ion battery. The energy rich, hydrogen can be produced from water and H₂S by utilizing the natural sun light which is abundantly available.² Since, past few decades, the photodecomposition of water using semiconductor photocatalyst has engaged much attention, among the various routes for hydrogen production. Along with water, hydrogen sulphide (H₂S) could become an alternative potential source of hydrogen. Emission of huge amounts of H₂S from oil refineries, other chemical industries and naturally occurring is a global threat to the environment. Every year, millions of tons of H₂S is produced around the world from oil-refinery plants or natural-gas extraction, which is predicted to increase in the future. Hence, decomposition of H₂S to a green product is in consideration to diminish its detrimental effects on the human life. Currently, the conventional Claus process is the most popular technique used for H₂S decomposition which produces sulphur and water.^3,4 However, this is not an economically feasible process, and it also creates acute environmental damages. Hence, large amounts of untreated H₂S are released into the atmosphere every year.⁴ Thus; it is highly desirable to develop a sustainable and economical process which can recover hydrogen and sulphur simultaneously from H₂S. In this context, the process of photo-cleavage of H₂S to H₂ by a semiconductor photocatalyst is a boon to mankind as it uses solar energy to generate hydrogen which is a green fuel by a cost-effective process. A number of oxide semiconductors⁵ have a wide band gap and show activity under UV light (λ < 389 nm), which is only 5% of the solar spectrum on the Earth's surface.⁶ Hence, the development of highly active visible light photocatalyst to utilize large amounts of energy from the solar spectrum has great significance and economically viable.

Our group has already demonstrated the photo-production of hydrogen by H₂S splitting under visible light irradiation using different hierarchical nanostructured photocatalysts such as CdS,⁷ In₂S₃ ⁸ etc. To overcome the photocorrosion problem of these catalysts, intercalation of metal sulphides into interlayers have been attempted.⁹ However, such stable catalysts showed very poor photocatalytic activity. Recently, the new class of multi-component sulphides have also been reported with good photocatalytic efficiency.¹⁰ Our group has introduced AB₂X₄ family of semiconductors i.e. nanostructured CdIn₂S₄, ZnIn₂S₄ as a photocatalyst for hydrogen generation and organic dye degradation.^3,11,12 CdIn₂S₄ has attracted wide interests because of its potential applications in different fields such as solar cells, optoelectronics, photochemical devices, light-emitting diodes, nonlinear optics, photoconductor optical sensing, biological labelling etc.¹³ Carbonaceous materials such as carbon nanotubes, activated carbon and graphene¹⁴ are centre of attraction due to their unique electronic/electrical properties. Graphene is a phenomenal material due to its unique two-dimensional nanostructure with noteworthy properties such as greater mechanical strength, superb mobility of charge carriers, elevated thermal conductivity and huge specific surface area.¹⁵ These exceptional features make graphene a potential candidate to be utilized as a promising support material for CdIn₂S₄ nanoparticles for better photocatalytic performance. To the best of our knowledge, there are reports on use of CdIn₂S₄ as photocatalytic applications^3,12,16 but there is no reports on synthesis of CdIn₂S₄/graphene composites and their use for photocatalytic and other applications. Considering the structure, porosity of the CdIn₂S₄ and its deposition on graphene, we were very curious to know its electrochemical properties. Hence, we also used this nano-heterostructures as an anode material in the lithium ion battery.

Moreover, due to the low reversible capacity of commercial graphitic anodes (372 mA h g⁻¹), the development of new electrode materials having high specific capacities and cyclabilities for lithium-ion batteries (LIB), which is required due to their wide applications such as in electric vehicles and large scale power storage systems¹⁷ is of high demand. Moreover, the elements from IV group undergo volume expansion, causing the pulverization, resulting into poor electrochemical performance.¹⁸ Thus, exploration of new electrode materials with higher capacity is one of the most important research fields for LIBs. More recently, binary metal sulfides/oxides such as MoS₂, MoO₂,¹⁹ CoS₂,²⁰ In₂S₃, In₂O₃ ²¹ etc., have been reported as possible alternatives to carbonaceous anode materials because of their higher capacities. However, ternary metal chalcogenides have been scarcely reported as an anode for Li-ion batteries. In particular, there is no report on the application of CdIn₂S₄ or its nano-heterostructures as an anode material for Li-ion batteries.

In the present investigation, we have demonstrated single step synthesis of CdIn₂S₄/graphene nano-heterostructures for the first time as well as characterized thoroughly and discussed structural, optical, electronic, electrochemical properties with new comprehensions. We have demonstrated hydrogen evolution from H₂S and H₂O splitting using CdIn₂S₄/graphene as a photocatalyst under solar light. Its noteworthy use as an anode material for lithium ion battery is hitherto unattempted as well as, has opened new avenues in energy storage.

2. Experimental section

2.1 Preparation of graphene oxide (GO)

Graphene oxide was prepared from graphite powder (99.99% S D fine Chemicals) by a Hummers and Offeman method.²²

2.2 Preparation of cadmium indium sulphide/graphene (CdIn₂S₄/graphene) nano-heterostructures

CdIn₂S₄/graphene nano-heterostructures were prepared by using a hydrothermal method. Analytical grade cadmium nitrate (Cd(NO₃)₂·4H₂O), indium nitrate (In(NO₃)₃·3H₂O) used during reaction. In a typical synthesis, the as prepared GO, (1%) was dispersed in distilled water and ultrasonicated for half an hour [Solution A]. The solutions of precursors Cd(NO₃)₂·4H₂O, In(NO₃)₃·3H₂O and thiourea was prepared in double distilled water and mixed in the proportion of 1 : 2 : 4 [Solution B]. Then, Solution A is slowly added into Solution B followed by stirring for 30 min using magnetic stirrer. The reaction mixture was then transferred into Teflon-lined stainless-steel autoclave of 200 ml capacity and reaction was performed at 150 °C for 30 h. After reaction, yellowish precipitate (C2) was obtained, which then washed with distilled water followed by ethanol for several times and dried at 70 °C. The same procedure was followed using various amount of GO (2%, 3%, 4%, 5% and 10%) to obtain respective CdIn₂S₄/graphene nano-heterostructures (samples named as C3, C4, C5, C6 and C7) and without addition of GO to obtain bare CdIn₂S₄ (C1). The final dried products were then subjected to XRD, UV-DRS, FTIR, Raman, XPS and TEM analysis for their characterization prior to hydrogen generation.

2.3 Photocatalytic hydrogen evolution from H₂S splitting

The cylindrical quartz photochemical reactor was filled with 200 ml 0.25 M aqueous KOH and purged with argon for 1 h. Hydrogen sulphide (H₂S) was bubbled through the solution for about 1 h at the rate of 2.5 ml min⁻¹ at room temperature. Bare CdIn₂S₄ and CdIn₂S₄/graphene nano-heterostructures (0.2 g) was introduced into the reactor and irradiated with a visible light source Xe-lamp (300 W, LOT ORIEL GRUPPE, EUROPA, LSH302, cut-off filter λ ≥ 420 nm) with constant stirring. The escaped hydrogen sulfide was trapped in NaOH solution. The amount of hydrogen gas evolved was collected and measured by an inverted cylinder method. The evolved hydrogen was then analyzed for its purity using a gas chromatograph (Model Shimadzu GC-14B, MS-5 Å column, TCD, Ar carrier). All the samples (C1–C7) were tested for their photocatalytic activity under identical conditions.

2.4 Photocatalytic hydrogen evolution from H₂O splitting

The photocatalytic H₂ production experiments were performed in a 100 ml Pyrex flask at ambient temperature and atmospheric pressure and two openings of the flask were sealed with a silicone rubber septum. In a typical photocatalytic experiment, 0.2 g of photocatalyst (bare CdIn₂S₄ and CdIn₂S₄/graphene nano-heterostructures) powder was suspended in aqueous solution containing 0.35 M Na₂SO₃ and 0.25 M Na₂S as sacrificial reagents. Before irradiation to 300 W Xe arc lamp, suspensions of the catalyst were dispersed in an ultrasonic bath and argon was bubbled through the reaction mixture for 20 min to remove the dissolved oxygen and to ensure the anaerobic conditions in the reaction system. The photocatalysts were irradiated with visible light (λ ≥ 420 nm) using a cut-off filter from a 300 W Xe arc lamp. The temperature of the reactant solution was maintained at room temperature by providing a flow of cooling water during the photocatalytic reaction. The amount of hydrogen evolved was determined with gas chromatography (Model Shimadzu GC-14B, MS-5 Å column, TCD, Ar carrier).

2.5 Electrochemical study

The electrochemical characteristics of the CdIn₂S₄ (C1) and its composite with graphene (C4) were investigated by fabricating the electrodes using a slurry casting process. The slurry was prepared by mixing active material (CdIn₂S₄ or CdIn₂S₄/graphene nano-heterostructure), carbon black, and binder at a weight ratio of 8 : 1 : 1. The resulting slurry was coated onto a copper foil (current collector) and dried overnight under vacuum at 80 °C. Further, 2032 coin type half-cells were assembled in Ar filled glove-box. The Li metal was used as counter and reference electrode while prepared electrode was a working electrode. A solution of 1 M LiPF₆ dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volume ratio of 1 : 1 was used as the electrolyte. The electrochemical properties of the obtained electrodes were investigated using cyclic voltammetry (CV) in the potential range of 0.0 to 3.0 V vs. Li/Li⁺ at a scan rate of 0.1 mV s⁻¹ with a potentiostat (Gamry-PC750). In addition, the cells were galvanostatically charged and discharged from 0.01 to 3 V vs. Li/Li⁺ using an automatic battery cycler (WonATech-WBCS 3000). All experiments and tests were conducted at 25 °C.

2.6 Materials characterization

The crystalline phases and the crystallite size of the nano-heterostructures were identified using an X-ray powder diffraction (XRD) technique (XRD-D8, Advance, Bruker-AXS) with Cu Kα radiation. The optical properties of the powder samples were studied using an UV-visible-near infrared spectrometer (UV-VIS-NIR, Perkin Elmer Lambda-950). Room temperature micro Raman scattering (RS) was performed using a HR 800-Raman Spectroscopy, Horiba Jobin Yvon, France, with an excitation at 632.81 nm by a coherent He–Ne ion laser and a liquid nitrogen cooled CCD camera to collect and process the scattered data. The nature of chemical bonds formed in CdIn₂S₄ and reduction of GO to graphene were examined using X-ray photoelectron spectroscopy (XPS, ESCA-3000, VG Scientific Ltd, England) with a base pressure greater than 1.0 × 10⁻⁹ Pa and Mg Kα radiation (1253.6 eV) was used as an X-ray source operated at 150 W. FTIR spectra were recorded with a Nicolet Magna 550 spectrometer. Photoluminescence (PL) emission spectra were recorded using HORIBA-JY, F-3 Fluorescence spectrophotometer, excited at 400 nm. The morphologies of the CdIn₂S₄/graphene nano-heterostructures were characterized by field emission scanning electron microscopy (FESEM, Hitachi, S-4800) and high resolution transmission electron microscopy (HRTEM, JEOL, 2010F).

3. Results and discussion

The facile template free hydrothermal process was designated for the in situ preparation of hierarchical CdIn₂S₄/graphene nano-heterostructures. The samples with various percentage of graphene have been prepared and further characterized for their structural, optical, morphological property investigation.

The XRD pattern of CdIn₂S₄ and CdIn₂S₄/graphene nano-heterostructures are shown in Fig. 1a–f. The XRD patterns indicates sharp peaks at 2θ value 27° (311) and 47.5° (440) which are the characteristic peaks of the cubic spinel phase of CdIn₂S₄ (JCPDS card no. 27-0060).¹² The absence of peaks pertaining to binary sulphides or oxides (from the reactants) is an indicative of formation of pure phase of CdIn₂S₄. Diffraction peaks for GO or graphene are not observed in any of the XRD patterns (Fig. 1b–f) of CdIn₂S₄/graphene nano-heterostructures as content of GO is low (1% to 5%). Further, when the amount of GO is increased up to 10% (C7) still there is no appearance of any characteristic peak related to any of the carbon species (Fig. S1†) due to its relatively low diffraction intensity and high dispersion.²³

Fig. 1 XRD patterns of (a) bare CdIn₂S₄ (C1), CdIn₂S₄/graphene nano-heterostructures prepared using different amount of GO (b) 1% (C2), (c) 2% (C3), (d) 3% (C4), (e) 4% (C5) and (f) 5% (C6).

Fig. 2 shows the UV-visible diffuse reflectance spectra of bare CdIn₂S₄ and CdIn₂S₄/graphene nano-heterostructures. The absorption edge of bare CdIn₂S₄ can be seen at 523 nm and corresponding band gap at 2.37 eV which makes it a profound candidate as visible light active photocatalyst.¹² However, there is no significant change observed in the band gap of CdIn₂S₄/graphene nano-heterostructures (i.e. 2.32 eV, 2.34 eV, 2.29 eV, 2.32 eV, 2.36 eV) due to presence of variable quantity of graphene in it. The increased absorption in the visible light region is definitely attributed to increase in amount of graphene which is quite well known.²⁴ The phenomenal increase in area under the curve also emphasizes the enhanced absorption in visible light region. For sample C1, bare CdIn₂S₄, the area under the curve is 282.39 which increase gradually with increase in amount of GO and for C6, CdIn₂S₄/graphene nano-heterostructures with 5% GO, it is observed as 826.25, which signifies 192% increase. This remarkable increase in visible light absorption assures the better exploitation of solar energy.

Fig. 2 UV-visible spectra of (a) bare CdIn₂S₄ (C1), CdIn₂S₄/graphene nano-heterostructures prepared using different amount of GO (b) 1% (C2), (c) 2% (C3), (d) 3% (C4), (e) 4% (C5) and (f) 5% (C6).

Ordered/disordered crystalline structures of carbonaceous materials like graphene oxide and graphene can be critically analysed by Raman spectroscopy. Fig. 3a–f shows the Raman spectrum of GO, bare CdIn₂S₄ (C1) and CdIn₂S₄/graphene (C2–C6) nano-heterostructures exclusively in the range of 1100–2000 cm⁻¹. It can be clearly seen that two prominent peaks centered at about 1325 cm⁻¹ (D band) and 1595 cm⁻¹ (G band), that are absent in Raman spectrum of bare CdIn₂S₄ (Fig. 3a) suggest that the structure of graphene is maintained in the nano-heterostructures. In case of GO (Fig. 3b), there are two typical Raman bands located at 1325 and 1591 cm⁻¹, which corresponds to disordered sp² carbon (D-band) and well-ordered graphite (G-band), respectively.²⁵

Fig. 3 Raman spectra of (a) bare CdIn₂S₄ (C1), (b) GO, CdIn₂S₄/graphene nano-heterostructures prepared using different amount of GO (c) 1% (C2), (d) 2% (C3), (e) 3% (C4), (f) 4% (C5) and (g) 5% (C6).

Further, the samples C2–C6 nano-heterostructures shows an increased I_D/I_G intensity ratio (1.14, 1.25, 1.31, 1.08, 1.10) in comparison to that of GO (0.91). This change is observed due to an increase in the average size of the sp² domains when graphene is produced typically by reduction of GO.²⁶ This also strengthens the argument of in situ formation of CdIn₂S₄/graphene nano-heterostructures by one step hydrothermal synthesis process.²⁷ The G band for samples C2–C6 is broadened and slightly red shifted to 1595 cm⁻¹ after the reduction of graphene oxide (1591 cm⁻¹), implying a close interaction between CdIn₂S₄ and graphene nanosheets with a firm interface.²⁸

Fig. 4 shows, the high resolution XPS spectra for C1s, Cd3d, In3d and S2p core levels of the bare CdIn₂S₄, GO, CdIn₂S₄/graphene nano-heterostructures prepared using 3% GO (C4). In CdIn₂S₄/graphene nano-heterostructure sample, the characteristic binding energy observed for Cd²⁺ (Fig. 4b) are 405.3 eV (Cd3p_5/2) and 412.2 eV (Cd3p_3/2). Similarly, the binding energies for In³⁺ (Fig. 4c) are 445.2 eV (In3p_5/2), 452.7 eV (In3p_3/2) and for S²⁻ (Fig. 4d) 161.8 eV (S2p). When compared to those observed in bare CdIn₂S₄, the peaks of In3d and S2p in CdIn₂S₄/graphene nano-heterostructure shift towards lower binding energy and the peak of Cd3d shift towards higher binding energy.¹² This trivial shift in binding energy for CdIn₂S₄/graphene nano-heterostructure clearly constitutes a strong interaction between CdIn₂S₄ and multilayer graphene sheets in the composite.²⁹

Fig. 4 XPS patterns of bare CdIn₂S₄, GO, CdIn₂S₄/graphene nano-heterostructure prepared using 3% GO (C4) (a) C1s, (b) Cd3d, (c) In3d, (d) S2p.

In Fig. 4a, the XPS spectrum of C1s from GO (solid black line) can be deconvoluted into three smaller peaks (dotted lines) which are ascribed to sp² bonded carbon (C–C, 284.6 eV), epoxy/hydroxyls (C–O, 286.6 eV) and carbonyls (C=O, 287.8 eV).³⁰ When compared to the XPS spectrum of C1s from CdIn₂S₄/graphene nano-heterostructure (Fig. 4a), the intensity of the peak indicating the presence of 2D carbon structure (C–C) is same but the peak intensities for oxygen containing functional groups (C–O and C=O) substantially decreased, which is a prominent sign of conversion of GO to graphene. In addition, based on the XPS data, the oxygen content decreased from 29.1% in GO to 9.3% in CdIn₂S₄/graphene nano-heterostructure. These results indicate that about 70% of the oxygen-containing functional groups were removed during hydrothermal reduction process.²⁹ The presence of small amount of hydrophilic groups on the surface of graphene, such as hydroxyl and carboxyl groups, can enhance the photocatalytic activity.³¹

Further, the presence of graphene in the CdIn₂S₄/graphene nano-heterostructures can be authenticated by FT-IR spectra in Fig. 5. FT-IR spectra of GO (Fig. 5a) shows peaks at 1054, 1420, 1622, 1741 cm⁻¹ corresponding to C–O stretching vibration, O–H deformation vibration of COOH group or bending vibration of epoxide, C=C i.e. skeletal vibration of graphene and C=O stretching vibration.³² The two peaks at 1399 and 1610 cm⁻¹ corresponding to the hydroxyl groups and surface absorbed water molecules are observed in bare CdIn₂S₄ (Fig. 5g).³³ The peaks of oxygen containing functional groups are very weak or completely vanished in CdIn₂S₄/graphene nano-heterostructures (Fig. 5b–f) but the peak corresponding to C=C skeletal vibration of graphene is present which clearly signifies the presence of graphene in CdIn₂S₄/graphene nano-heterostructures due to reduction of GO during the hydrothermal reaction.^34a The above XPS, RAMAN and IR study as well as our recent report^34b firmly shows the presence of graphene.

Fig. 5 FTIR spectra of (a) GO, CdIn₂S₄/graphene nano-heterostructures prepared using different amount of GO (b) 1% (C2), (c) 2% (C3), (d) 3% (C4), (e) 4% (C5), (f) 5% (C6) and (g) bare CdIn₂S₄ (C1).

The electronic transfer between CdIn₂S₄ and graphene can be studied using photoluminescence (PL) spectroscopy. Fig. 6a–e, shows the photoluminescence spectra of CdIn₂S₄/graphene nano-heterostructure and bare CdIn₂S₄ (Fig. 6f) recorded using excitation wavelength 400 nm. The bare CdIn₂S₄ shows a strong peak at about 550 nm. The decrease in PL intensities of CdIn₂S₄/graphene nano-heterostructure is credited to highly efficient electron transfer between CdIn₂S₄ nanopetals and graphene sheets. This highlights the fact of decrease in rate of direct recombination of holes and electrons due to their spatial separation. This remarkable decrease is sign of enhanced electronic coupling of CdIn₂S₄ nanopetals grown on multilayer graphene sheets in the nano-heterostructures.

Fig. 6 Photoluminescence spectra of CdIn₂S₄/graphene nano-heterostructures prepared using different amount of GO (a) 3% (C4), (b) 4% (C5), (c) 2% (C3), (d) 5% (C6), (e) 1% (C2) and (f) bare CdIn₂S₄ (C1).

Fig. 7a shows FESEM image of bare CdIn₂S₄. Beautiful marigold flower with thin, transparent petals of approximately 10–15 nm thickness can be seen. Two dimensional GO sheets with wrinkles can be seen in Fig. 7b which is the result of exfoliation of graphite during exhaustive oxidation and sonication. Fig. 7c shows FESEM image of surface of graphene sheet decorated with CdIn₂S₄ nanopetals (C4). Growth of CdIn₂S₄ nanopetals on graphene sheets provides close interfacial contact which is further confirmed by TEM analysis. Fig. 7d and e shows the TEM images of CdIn₂S₄/graphene nano-heterostructure, C4. From these images it is clear that the thin and translucent nanopetals of CdIn₂S₄ grown on two-dimensional sheets of graphene, which also shows that close interfacial contact of CdIn₂S₄ nanopetals and graphene sheet. The HRTEM image (Fig. 7e) of CdIn₂S₄ nanopetals showing clear well defined lattice fringes with spacing of 0.22 nm, which corresponds to (422) lattice planes of the cubic spinel CdIn₂S₄ (inset of Fig. 7e). The EDS pattern (Fig. 7f) also show presence of (220), (311) and (400) planes of cubic spinel CdIn₂S₄ which is in accordance with the XRD results (Fig. 1). The EDS also discloses the co-existence of C, Cd, In and S that authenticates the formation of CdIn₂S₄/graphene nano-heterostructure (Fig. S2†).

Fig. 7 Morphological study from FESEM images of (a) bare CdIn₂S₄, (b) GO, (c) CdIn₂S₄/graphene nano-heterostructure prepared using 3% GO (C4) and TEM images (d and e) of (C4) and (f) SEAD pattern of the same.

The formation mechanism of CdIn₂S₄/graphene nano-heterostructure is illustrated in Fig. 8. GO is prepared using the Hummer's method from pristine graphite (Fig. 8b). The strong van der Waals interactions among the GO layers get conquered after ultrasonic pre-treatment, leads to the separation of GO sheets (Fig. 8c). It is well known that GO has many hydrophilic functional groups (e.g., –OH, –COOH) on its surface, which gets attached to Cd²⁺ and In³⁺ cations (Fig. 8d) through hydrogen bonding or via chemisorptions.³⁵ The formation of cubic spinel CdIn₂S₄ using cadmium and Indium precursors in presence of thiourea under hydrothermal conditions is explained by following reaction.¹²

Cd(NO₃)₂ + 2In(NO₃)₃ + 8(NH₂)₂CS + 16H₂O → CdIn₂S₄ + 8HNO₃ + 8CO₂ + 16NH₃ + 4H₂S

Fig. 8 Schematic representation of formation mechanism of CdIn₂S₄/graphene nano-heterostructure.

In polar solvent like water, the degree of ionization of cadmium nitrate and indium nitrate is quite high. Initially, the tiny nuclei are generated in the supersaturated solution and further growth of nanoparticles takes place with time (Fig. 8e). These newly formed nanoparticles spontaneously aggregated to minimise their surface energy. These nanoparticles further grow anisotropically along the 2D directions, resulting in the formation of nanopetals. Finally, the CdIn₂S₄ nanopetals grow larger and denser (Fig. 8f) along with the reduction of GO to graphene simultaneously³⁶ by ammonia from decomposition of thiourea.³⁷ These CdIn₂S₄ nanopetals grown on the surface of graphene sheets forms nano-heterostructure which exhibit a good dispersion behavior, larger surface area and hence, can offer more active adsorption sites for reaction.

The photocatalytic activity of freshly prepared bare CdIn₂S₄, CdIn₂S₄/graphene nano-heterostructures for hydrogen generation via photodecomposition of H₂S and H₂O was performed. Fig. 9 presents a comparison of the photocatalytic activity of samples C1 to C6 for H₂ production from H₂S under solar light. The linearity of the graphs clearly shows that the rate of H₂ evolution is stable for all samples throughout the course of photocatalytic reaction (C1–C6). Table 1 shows the hydrogen evolution rate from the photodecomposition of H₂S using bare CdIn₂S₄ (C1) and CdIn₂S₄/graphene nano-heterostructures (C2–C6). Control experiments indicated that no appreciable hydrogen production was detected in the absence of irradiation or photocatalyst, suggesting that hydrogen was produced via photocatalytic reactions on the photocatalyst surface. The bare CdIn₂S₄ (2375 μmol h⁻¹) shows lowest photocatalytic activity when compared to the activity shown by other samples because of the rapid recombination between conduction band (CB) electrons and valance band (VB) holes resulting in small number of “active” electrons/holes for the necessary reaction in CdIn₂S₄. Addition of graphene increases the activity of the photocatalyst significantly. The photocatalytic activity of samples is found to be in close relation with the amount of graphene in nano-heterostructure. The H₂ production increases with the increase in amount of graphene (for 1, 2, and 3%) and then with further increase in amount of graphene (4 and 5%) it decreases, dramatically. The maximum H₂ production is achieved for C4 sample (4495 μmol h⁻¹), which contains 3.0 wt% graphene (Fig. 9d). This shows 100% increase in H₂ production with respect to H₂ production recorded for bare CdIn₂S₄. Hence, we can say, the graphene content is a major manipulating factor in the enhanced photocatalytic activity of CdIn₂S₄. The bare CdIn₂S₄ (C1) and CdIn₂S₄/graphene nano-heterostructure (C4) containing 3.0 wt% of graphene confers quantum yield of 3.34% and 6.33% respectively and the rate of H₂ evolution is also high when compared with the available literature (Table S1†).

Fig. 9 Time verses volume of H₂ (μmol) evolution of samples (a) bare CdIn₂S₄ (C1), CdIn₂S₄/graphene nano-heterostructure with different amount of GO (b) 1% (C2), (c) 2% (C3), (d) 3% (C4), (e) 4% (C5) and (f) 5% (C6).

Table 1 Hydrogen evolution rate of the samples

Sample code	Graphene content (%)	H2 evolution^a (μmol h^−1)
a 0.2 g sample used for each run, 300 W xenon lamp, cut-off filter (λ ≥ 420 nm), 200 ml 0.25 M aqueous KOH.
C1	0	2375
C2	1	3134
C3	2	3677
C4	3	4495
C5	4	3996
C6	5	3357

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The stability of CdIn₂S₄/graphene nano-heterostructure (C4) was evaluated by repeating the experiments under similar conditions using the same photocatalyst after recycling. After five recycles, H₂ evolution did not decrease (Fig. S3†) which indicates there is no corrosion of catalyst during the process. The XRD and Raman spectra of reused catalyst C4 (Fig. S4 and S5†) did not show any change in the phase purity or graphene content, which implies excellent stability of the photocatalyst for hydrogen generation. The major reaction steps in this mechanism under visible-light irradiation are described in following equations.

H₂S + OH⁻ ↔ HS⁻ + H₂O (1)

Oxidation reaction: 2HS⁻ + CdIn₂S₄ (2h⁺)_VB → 2S + 2H⁺ (3)

Reduction reaction: 2H⁺ + graphene (2e⁻)_CB → H₂ (4)

We also performed hydrogen evolution from water using the all as synthesised CdIn₂S₄/graphene nano-heterostructure under solar light. Looking at Fig. 10, we can conclude that the trend of change in rate of H₂ generation with increase in amount of graphene remains unaltered in water splitting, also. The bare CdIn₂S₄ (C1) shows minimum H₂ production (101 μmol h⁻¹ g⁻¹) which increases with increase in amount of graphene i.e. maximum H₂ production is observed in sample C4 (713 μmol h⁻¹ g⁻¹) and it further decreases with increase in amount of graphene. The difference in the energy required for both H₂S and water splitting is responsible for the different catalytic efficiencies of CdIn₂S₄/graphene nano-heterostructure towards hydrogen production from H₂S and H₂O. The energy required for H₂S splitting is ΔG° = 0.17 eV or 33.44 kJ mol⁻¹ whereas for water splitting ΔG° = 1.23 eV or 237.19 kJ mol⁻¹.

Fig. 10 Comparison of photocatalytic H₂ production activity by water splitting of bare CdIn₂S₄ (C1), CdIn₂S₄/graphene nano-heterostructures prepared using 1% (C2), 2% (C3), 3% (C4), 4% (C5) and 5% (C6) amount of GO.

Hence, there is significant difference in the hydrogen production from H₂S and water splitting and this difference in amount of generated hydrogen is observed. Fig. S6† shows the actual graphs of volume of H₂ production from water by fresh sample C4 (a) and after recycling the same catalyst for 5 times (Fig. S6†) under same experimental conditions. The linearity in the graphs shows the stable H₂ production in all the samples. Metal sulfides are usually subjected to photocorrosion during the photocatalytic reaction of water splitting.³⁸ The unaffected rate of H₂ production even after 5 recycles and the XRD, Raman spectra of reused catalyst (Fig. S7 and S8†) clearly emphasises the stability of photocatalyst for water splitting. The high stability of the photocatalyst is attributed to the close interaction between graphene and CdIn₂S₄ (ref. 39) which favours the vectorial transfer of the photogenerated electrons from the CB of CdIn₂S₄ to graphene. This space separation of the photogenerated electrons and holes is beneficial for preventing the reduction of Cd²⁺ and In²⁺. Furthermore, high Na₂S concentration in the solution can suppress the oxidation of S²⁻ on CdIn₂S₄. The sacrificial reagent system of Na₂S and Na₂SO₃, plays an extremely important role in evolution of hydrogen. Due to the strong reduction capacity of S²⁻ ions, the photogenerated holes irreversibly oxidize the S²⁻ ions instead of water. The various reactions occurred are shown below.⁴⁰

Graphene (e⁻)_CB + 2H₂O → H₂ + 2OH⁻ (6)

SO₃²⁻ + 2H₂O + CdIn₂S₄ (2h⁺)_VB → SO₄²⁻ + 2H⁺ (7)

2S²⁻ + CdIn₂S₄ (2h⁺)_VB → S₂²⁻ (8)

S₂²⁻ + SO₃²⁻ → S₂O₃²⁻ + S²⁻ (9)

The high hydrogen production activity of sample C4 under solar irradiation can be explained from the Fig. 11. Under solar irradiation, electrons are excited from the VB populated by S2p to the formed CB by hybridizing Cd3d with In3d which creates holes in VB. Normally; these charge carriers recombine rapidly resulting in a low photocatalytic hydrogen production rate of bare CdIn₂S₄. However, when CdIn₂S₄ nanopetals are grown on the surface of graphene, those photogenerated electrons in CB of CdIn₂S₄ tend to transfer to graphene, leading to the hole–electron separation. Graphene can function as an electron collector and transporter to lengthen the lifetime of the charge carriers, consequently improving the charge separation and photocatalytic activity. The photoluminescence study also clearly showed the effect of graphene on inhibition of charge carrier recombination. The strong interaction between the multilayer graphene sheets and CdIn₂S₄ nano-petals observed in XPS spectra leads to enhanced photocatalytic activity i.e. enhanced hydrogen generation.

Fig. 11 Schematic representation of photocatalytic mechanism of CdIn₂S₄/graphene nano-heterostructures.

In addition, the unique features of graphene facilitate the photocatalytic reaction to take place not only on the surface of semiconductor catalysts, but also on the graphene sheet, greatly enlarging the reaction space.⁴¹ Furthermore, since the standard reduction potential of graphene is about −0.08 V (vs. SHE, pH = 0) it can act as a co-catalyst to promote the separation and transfer of photo-excited electrons from CB of CdIn₂S₄ to the graphene, where H⁺ is reduced to hydrogen molecule ( vs. SHE, pH = 0); meanwhile, the holes in VB of CdIn₂S₄ can be consumed by the sacrificial agents (S²⁻, SO₃²⁻).

A further increase in the content of graphene leads to a reduction in the photocatalytic activity. It is clearly visible in the case of C5 and C6, the activity for H₂ production decreases, dramatically. Introduction of a large percentage of graphene leads to shielding the incident light, thus preventing the generation of electrons from the inside of the CdIn₂S₄ nanopetals.⁴² Excessive amount of graphene may cover the active sites on the surface of CdIn₂S₄ and also could hinder the contact of the sacrificial agents with CdIn₂S₄. Therefore, a suitable content of graphene is crucial for optimizing the photocatalytic activity of composite photocatalysts, which can also be proved by the previous studies.^41,43

Furthermore, the bare CdIn₂S₄ and CdIn₂S₄/graphene nano-heterostructure prepared using 3% GO (C4) was studied for Li storage application. They have been used as an anode in lithium ion battery and studied their performance.

Fig. 12 shows; the charge–discharge plots of the LIB half-cells containing C1 (bare CdIn₂S₄) and C4 (CdIn₂S₄/graphene nano-heterostructure) electrode at a specific current of 75 mA g⁻¹. For C4 electrode, it is clearly seen that there is a potential plateau in the discharge process at about 1.4 V (vs. Li/Li⁺), which might be due to the reduction reactions of Cd²⁺ to Cd⁰, In³⁺ to In²⁺ and In²⁺ to In⁰.^21a Similar potential plateau can also be seen in the charge–discharge profile of C1 electrode. The C1 electrode shows initial discharge and charge capacities of 842 and 602 mA h g⁻¹, while those of C4 are 1091 and 678 mA h g⁻¹. The initial loss in the capacity of both the electrode is observed, which is due to the irreversible loss of lithium ions in the formation of SEI film.⁴⁴ For appraisal, the charge–discharge curve for baseline graphene is given in Fig. S9.† The reversible capacity for baseline graphene is observed to be 300 mA h g⁻¹. Considering the amount of graphene (3%) in the CdIn₂S₄/graphene nano-heterostructure, its contribution in the reversible capacity of nano-heterostructure is quite small and can be neglected.

Fig. 12 First discharge–charge profile of Li-ion battery containing bare CdIn₂S₄ and CdIn₂S₄/graphene nano-heterostructure prepared with 3% GO (C4) loading.

In addition, the cyclic voltammetry (CV) measurement of the C1 and C4 electrode are shown in Fig. 13. In the CV curves for C1 electrodes (black line), the small peak at 1.6 and broad peak in between 1.3 and 1 V vs. Li/Li⁺ in the cathodic sweep correspond to the formation of metallic indium and cadmium and Li₂S (reaction (10)). Another two cathodic peaks at 0.35 V and 0.65 V were observed similar to the steps seen in the first discharge process (Fig. 12), which are due to the Li_xIn alloy or additional sites for lithium intercalation, suggesting a multistep process. In addition, the peak at 0.01 V is corresponding to the lithium storage by carbonous materials. In the anodic scan, the corresponding oxidation peaks were observed at 0.65, 1.9 and 2.35 V. On the other hand, the similar peaks with increased peak current were observed for C4 electrode, suggesting the increased kinetics of Li⁺ diffusion and electron transport due to presence of graphene in the composite. From next cycles, there is very small decrease in individual peaks of C1, corroborating the losses in the reversible capacity. In the subsequent discharge–charge processes for C4, the cathodic and anodic peaks are stable, which indicates that the Li_xIn alloying and dealloying is reversible (reaction (11)).

CdIn₂S₄ + 8Li⁺ + 8e⁻ → Cd + 2In + Li₂S (10)

In + xLi⁺ + xe⁻ ↔ Li_xIn (11)

Fig. 13 Cyclic-voltammetry curves recorded after first discharge–charge profile of Li-ion battery containing bare CdIn₂S₄ (C1) and CdIn₂S₄/graphene nano-heterostructure with 3% GO (C4).

Fig. 14a shows the cycling performance of CdIn₂S₄ and CdIn₂S₄/graphene nano-heterostructure at specific currents of 75 and 150 mA g⁻¹. It can be obviously observed that the composite samples show significantly improved capacity and cycling stability than pure CdIn₂S₄.

Fig. 14 (a) Specific capacity and (b) coulombic efficiency of bare CdIn₂S₄ and CdIn₂S₄/graphene nano-heterostructure (C4) with 3% GO loading electrode with cycle.

In Fig. 14a, the capacities of CdIn₂S₄ with 3% graphene is larger than that of bare CdIn₂S₄, which might be attributed to the two reasons: (1) firstly, in the CdIn₂S₄/graphene nano-heterostructure, GO precursors can provide more nucleation to support growth of CdIn₂S₄ nanoparticles along with the formation of graphene, leading to more uniform structures. (2) Secondly, higher electrical conductivity of graphene can offer easy electronic transport. After 25 cycles at applied current of 75 mA g⁻¹, the discharge and charge capacities are 610 and 607 mA h g⁻¹ for C4, whereas 508 and 504 mA h g⁻¹ for C1 electrodes. This decline in capacity during the first few cycles might be due to an activation of porous materials, which is caused by trapping of lithium ions in the CdIn₂S₄ frameworks in the initial cycles and then released gradually upon cycling.⁴⁵ Interestingly, it is noted that both the electrodes, when cycled at higher current of 150 mA g⁻¹, show an increase in capacity after 35 cycles. This process may be assigned to the gradual access of more electrolyte molecules into the meso- and micro-pores of the active material. The charge capacity was recovered to 532 mA h g⁻¹ for C1 upto 125 cycles compared to its initial reversible capacities of 602 mA h g⁻¹ and then declined to 481 mA h g⁻¹, after completion of 250 cycles. On the other hand, the reversible capacity achieved by C4 electrode after 250 cycle is 608 mA h g⁻¹, which is almost 90% of its initial reversible capacities of 678 mA h g⁻¹, indicating excellent cyclability over 250 cycles. The 20% decline in capacity for C1 during cycling might be owing to poor conductivity of bare CdIn₂S₄ materials, while the graphene support in C4 has improved the conductivity of composite and hence the life-cycle of the composite. These result suggest that the addition of graphene had not only increased the reversible charge capacity of composite but also improved the cycling stability of the same.

It is also seen that the capacity retained by CdIn₂S₄/graphene nano-heterostructure (90%) is higher than the bare CdIn₂S₄ (80%) after 250 cycles, corroborating the stability of composite at higher applied currents. The values of specific capacity and capacity retention over number of cycles obtained in the present work for the bare CdIn₂S₄ and the CdIn₂S₄/graphene nano-heterostructure has been compared to the literature values of metal sulphides and their carbonous composites as given in Table S2.† From those values, it is noted that the CdIn₂S₄/graphene nano-heterostructure has potential as anode material in Li-ion batteries. Furthermore, to investigate the reversibility of the material, the coulombic efficiency was calculated as shown in Fig. 14b. The coulombic efficiencies of bare CdIn₂S₄ and CdIn₂S₄/graphene nano-heterostructure in the first cycle were 71.4 and 62.2%, respectively. The coulombic efficiency of C4 in the first cycle was lower than that of C1. This might be due to the porous structure of C4 as compared to C1. The less coulombic efficiency is also due to irreversible trapping of Li-ions in the SEI layer due to higher surface area of the composite. After the second cycle, the coulombic efficiency of C4 becomes higher than that of C1, and after the 15^th cycle, the coulombic efficiency of C4 reached a stable value of up to ∼99%. The high coulombic efficiency of C4 is attributed to the nano-heterostructure formed by wrapping of graphene on CdIn₂S₄.

To understand superior electrochemical performance of C1 and C4 electrodes over 250 cycles, morphologies of both the electrodes were analyzed. To perform ex situ FE-SEM analysis, the cycled cells were initially opened in the glove-box. The obtained electrodes were then washed thoroughly with the solvent, dimethyl carbonate, to remove the electrolyte salt. The electrodes were then dried overnight in a vacuum oven. The FE-SEM images for both C1 and C4 electrodes before and after testing is presented in Fig. 15.

Fig. 15 FE-SEM images of CdIn₂S₄ (a and c) and CdIn₂S₄/graphene (b and d) nano-heterostructure before and after 250 charge–discharge cycles, respectively.

After comparing the image, it is observed that the microstructure of both the electrodes after test becomes smoother, due to formation of SEI layer during charge–discharge cycling, than the pristine one. The smooth microstructure of C4 observed in FE-SEM images (Fig. 15d) clearly demonstrate that the electrode remain intact and unchanged during electrochemical cycling except for a slight increase in particle size. This might be the reason for retaining 90% reversible charge capacity over 250 cycles. On the contrary, for C1 electrode, small voids on SEI layer indicating the loose connection between the particles (Fig. 15c), which could be attributed to the capacity fading.

The CdIn₂S₄ (C1) and CdIn₂S₄/graphene nano-heterostructure (C4) electrodes are tested at various applied current from 75–4800 mA g⁻¹ and are shown in Fig. 16. Further to avoid the induced effect due to the activation of the electrode, the cells cycled at 75 mA g⁻¹ for 10 cycles were used for rate capability test. The CdIn₂S₄/graphene nano-heterostructure (C4) electrode exhibited good rate capability with the average charge capacities of 644, 600, 569, 502, 442, 393, and 347 mA h g⁻¹ at specific current of 75, 150, 300, 600, 1200, 2400 and 4800 mA g⁻¹, respectively. Interestingly, the small decrease in charge capacity was found at high specific currents of 2400 and 4800 mA g⁻¹, and the electrode maintained 62% (at 2400 mA g⁻¹) of the initial charge capacity. These values are still higher than those observed for the graphite electrodes. On the other hand, the average charge capacities obtained for bare CdIn₂S₄ are 542, 481, 411, 368, 322, 279 and 237 mA h g⁻¹ at specific current of 75, 150, 300, 600, 1200, 2400 and 4800 mA g⁻¹, respectively. Moreover, the C4 electrode nearly recovered its original charge capacity, when the current returned to the initial value (75 mA g⁻¹).

Fig. 16 Rate capability of bare CdIn₂S₄ and CdIn₂S₄/graphene nano-heterostructure with 3% GO electrode at applied specific current 75–4800 mA g⁻¹.

The excellent discharge capacity of C4 at high specific current is attributed to the porous nature of the CdIn₂S₄/graphene nano-heterostructure, which provides the pathway for the diffusion of electrolyte and the high conductivity of graphene allows fast electronic kinetics at high applied current. More significantly, the bare CdIn₂S₄ has showed good capacity due to its hierarchical porous flower like nanostructure which accelerates the lithium ion intercalation. The further enhancement with graphene is quite understood due to high surface area and electrical properties of graphene as discussed above. The further study on CdIn₂S₄ nanocomposite with highly porous carbon may have good potential in lithium ion battery. Overall, the multifunctionality of CdIn₂S₄ and its graphene composites for energy generation (hydrogen production) and storage (Li-ion battery) has been demonstrated for the first time.

4. Conclusion

Novel nano-heterostructures were prepared for the first time by decorating CdIn₂S₄ nanopetals on graphene sheets by in situ hydrothermal method. A high efficiency of the photocatalytic H₂ production from hazardous H₂S waste and H₂O, with the help of abundantly available solar light was demonstrated using CdIn₂S₄/graphene nano-heterostructure as photocatalyst. The as prepared CdIn₂S₄/graphene nano-heterostructure sample showed excellent photocatalytic activity (4495 μmol h⁻¹) for the hydrogen production under visible light irradiation. The enhanced photocatalytic activity is because graphene can act as an excellent electron collector and transporter, thus reducing the recombination of charge carriers and enhancing the photocatalytic activity. Further, the usage of CdIn₂S₄/graphene nano-heterostructure as anode electrode for Li-ion batteries has demonstrated very high reversible charge capacities of 678 mA h g⁻¹. On cycling, CdIn₂S₄/graphene nano-heterostructure electrode maintained capacity of 608 mA h g⁻¹ at applied current of 150 mA g⁻¹ for 225 cycles and exhibited good rate capability. The excellent Li-storage properties of CdIn₂S₄/graphene nano-heterostructure is associated with the high porosity of CdIn₂S₄ flower structures and the fast electron kinetics offered by graphene support. This study opens a new possibility in the investigation of CdIn₂S₄/graphene nano-heterostructures and promotes their practical applications in environmental issues.

Acknowledgements

The authors would like to thank Executive Director, C-MET for encouragement and nanocrystalline materials group C-MET, Pune for support. Authors also thankful to Mr Rami Reddy & Dr M.V. Shelke, NCL, Pune for their experimental help. BBK would like to acknowledge DeitY for financial support. BBK and CJP is also thankful for the support by MSIP (Ministry of Science, ICT and future planning), Korea through Brain Pool Program. RSK would like to thank University Grants Commission, New Delhi for awarding D.S. Kothari Post-Doctoral Fellowship (F.4-2/2006 (BSR)/PH/14-15/0132). The authors MAM, SBK and RSK have contributed equally in the synthesis and testing.

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Footnotes

† Electronic supplementary information (ESI) available: XRD spectrum of sample C7, EDAX of sample C4, photocatalytic study of reuse and characterization of recycled sample C4. Comparison of electrochemical performance of metal sulphides in the literature and the materials prepared in the current work. See DOI: 10.1039/c6ra02002j

‡ These authors have contributed equally in the synthesis and testing.

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