Unique hierarchical SiO₂@ZnIn₂S₄ marigold flower like nanoheterostructure for solar hydrogen production

Abstract

The novel marigold flower like SiO₂@ZnIn₂S₄ nano-heterostructure was fabricated using an in situ hydrothermal method. The nanoheterostructure exhibits hexagonal structure with marigold flower like morphology. The porous marigold flower assembly was constructed using ultrathin nanosheets. Interestingly, the thickness of the nanopetal was observed to be 5–10 nm and tiny SiO₂ nanoparticles (5–7 nm) are decorated on the surface of the nanopetals. As the concentration of SiO₂ increases the deposition of SiO₂ nanoparticles on ZnIn₂S₄ nanopetals increases in the form of clusters. The optical study revealed that the band gap lies in the visible range of the solar spectrum. Using X-ray photoelectron spectroscopy (XPS), the chemical structure and valence states of the as-synthesized SiO₂@ZnIn₂S₄ nano-heterostructure were confirmed. The photocatalytic activities of the hierarchical SiO₂@ZnIn₂S₄ nano-heterostructure for hydrogen evolution from H₂S under natural sunlight have been investigated with regard to the band structure in the visible region. The 0.75% SiO₂@ZnIn₂S₄ showed a higher photocatalytic activity (6730 μmol⁻¹ h⁻¹ g⁻¹) for hydrogen production which is almost double that of pristine ZnIn₂S₄. Similarly, the hydrogen production from water splitting was observed to be 730 μmol⁻¹ h⁻¹ g⁻¹. The enhanced photocatalytic activity is attributed to the inhibition of charge carrier separation owing to the hierarchical morphology, heterojunction and crystallinity of the SiO₂@ZnIn₂S₄.

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1 Introduction

In recent years, the combined extreme energy shortages and environmental degradation have been triggered by technological development that demands a huge amount of fossil oil. Therefore, intensive studies focus on investigating alternative energy resources with renewable and environmentally-friendly advantages.^1,2 Among them, hydrogen will be considered a possible candidate for future use in conventional fuels.³ Photocatalytic H₂S & water splitting is one of the most promising approaches for hydrogen production, as it can perform a conversion that converts solar light into chemical energy.^4–6 Various materials with important photocatalytic hydrogen evolution properties, such as MoS₂, BiVO₄, CuO, Ti₃C₂ etc., have been prepared to date.^7–10 Ternary ZnIn₂S₄ is a semiconductor chalcogenide with great visible-light-responsive photocatalytic activity, considerable chemical stability and ample flat band potential which attracted considerable interest in lithium ion batteries and gas sensor applications.^11–15

As one of the most promising n-type semiconductors, ZnIn₂S₄ has captured much interest in the field of photocatalytic hydrogen generation, which can be attributed to its non-toxic properties and suitable band structure (2.2–2.5 eV)^16,17 Similar to other photocatalysts, ZnIn₂S₄ suffers electron–hole recombination, slow oxygen evolution reaction kinetics and low visible light utilization.¹⁸ Combining ZnIn₂S₄ with other semiconductors is an effective method to inhibit the recombination of photogenerated charge carriers and improve the photocatalytic activity. Until now, many ZnIn₂S₄ nanoheterostructures and doped ZnIn₂S₄ with various morphologies such as ZnIn₂S₄/Ni₁₂P₅,¹⁹ g-C₃N₄/Ag:ZnIn₂S₄,²⁰ SnS@ZnIn₂S₄,²¹ Ag-doped ZnIn₂S₄,²² CdS/QDs/ZnIn₂S₄,²³ Ti₃C₂ MXene@TiO₂/ZnIn₂S₄ (ref. 24) have been reported to exhibit admirable visible-light photocatalytic activity. These literatures emphasized that while some prodigious work has been done in the field of photocatalysis, catalytic efficiency is still insufficient. It also needs more studies to explore new ways of further improving it.

SiO₂ is an insulator mesoporous material. SiO₂ has been widely used in the fields of catalysis, separation and adsorption due to its large specific surface area, abundant surface hydroxyl groups, environmental friendliness, stable chemical properties, uniform pore size and relatively high mechanical strength.^25–27 SiO₂ is mainly used in many heterostructures to enhance the photocatalytic performance of other semiconductor by increasing the surface area of the catalyst and it affects on the charge carrier recombination.^28,29 In past, various attempts have been made to enhance the photocatalytic activity of individual semiconductors by coupling with SiO₂ such as g-C₃N₄/SiO₂,³⁰ SiO₂@TiO₂core@Shell,³¹ SiO₂/CNOs/TiO₂,³² gC3N₄/TiO₂/Fe₃O₄@SiO₂,³³ γ-Fe₂O₃@SiO₂@TiO₂,³⁴ TiO₂–SiO₂Ag,³⁵ NiO/SiO₂,³⁶ ZnFe₂O₄@SiO₂@TiO₂.³⁷ So, to overcome the problem of fast recombination of photoinduced electron–hole pair and enhance the photocatalytic performance of pure ZnIn₂S₄, we synthesized a ZnIn₂S₄ and decorated with SiO₂ nanoparticles to enhance the separation and transfer efficiency of electrons. With this motivation, we have prepared a new SiO₂@ZnIn₂S₄ hybrid semiconductor by coupling SiO₂ and ZnIn₂S₄. To the best of our knowledge, this is the first report on SiO₂@ZnIn₂S₄ heterostructure for H₂ production from H₂S & Water splitting.

In the view of above findings, we report the hierarchical SiO₂@ZnIn₂S₄ nano-heterostructure for efficient generation of hydrogen, which was synthesized using facile hydrothermal method. These fabricated heterostructures were thoroughly characterized using XRD, FESEM, UV-Vis spectroscopy, PL and TEM. The fabrication of SiO₂@ZnIn₂S₄ nano-heterostructure increases the charge separation and also enhances the solar light response. Thus, under solar light irradiation, the high output rate of hydrogen is observed. In addition, the component effect on photocatalytic efficiency of SiO₂@ZnIn₂S₄ nano-heterostructure and the proposed photocatalytic mechanism have been further investigated.

2 Experimental sections

Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), Indium nitrate (In(NO₃)₃·4H₂O), Thiourea (CSNH₂CS),Silicon Dioxide (SiO₂) used for the preparation of catalysts are of analytical grade (SD-fine-chem limited, India) and used without any further purification.

2.1 Synthesis of ZnIn₂S₄ and SiO₂@ZnIn₂S₄

All the chemicals used for SiO₂@ZnIn₂S₄ synthesis were analytical grade and used without further purification. The hydrothermal method was used to synthesise SiO₂@ZnIn₂S₄ photocatalysts and the procedure is briefly defined as follows: first, 1 mole of hexahydrate zinc nitrate (Zn(NO₃)₂·6H₂O) (99.99%) and 2 moles of indium nitrate (In(NO₃)₃·4H₂O) (99.99%) and 8 moles of thiourea were mixed in a beaker (a) in 70 mL distilled water and stirred at room temperature for 15 minutes. After stirring, 0.25 wt% of silicon dioxide was applied to the above solution in beaker (a). The clear solution was then moved to a Teflon autoclave with a capacity of 150 mL and held in an electric oven at 150 °C for 30 hours. As the synthetic catalyst was washed several times with absolute ethanol and deionized water, the yellow powder was obtained. For pure ZnIn₂S₄, 0.25%SiO₂@ZnIn₂S₄, 0.50%SiO₂@ZnIn₂S₄, 0.75%SiO₂@ZnIn₂S₄, 1% SiO₂@ZnIn₂S₄ were labelled as (ZS-1), (ZS-2), (ZS-3), (ZS-4) and as (ZS-5) respectively and were subsequently used for further analysis and comparison.

2.2 Photocatalytic study

2.2.1 Photocatalytic hydrogen generation from water. The photochemical reaction was carried in a 70 mL total volume air-tight cylindrical quartz reactor with a cooling jacket for water circulation. All the reactions were carried out at ambient conditions under natural sunlight on sunny days (March to May months) between 10 am to 3 pm at Pune, located in the Maharashtra state of India. The intensity of solar light was measured by using a digital Lux meter. The measured average intensity of solar light reaching the surface of the earth was 145 000 Lux. In a typical photocatalytic experiment, 15 mg of the photocatalyst was dispersed in 25 mL total volume containing 20% methanol (v/v) in aqueous solution. The 45 mL free space of photoreactor made airtight with rubber septum followed by ultrasonication for 5 min for the uniform dispersion of catalyst. The solution mixture was then purged with Ultra High Purity nitrogen gas (UHP-99.999%) to remove all the gases in the headspace of the reactor and dissolved oxygen from the reaction mixture. Before and after irradiation with solar light, the gas in the free space of the reactor was analyzed using gas chromatography (GC). The generated gas was analyzed immediately using GC with a specific time interval.

2.2.2 Photocatalytic hydrogen generation from water. The hydrogen generation is performed in a cylindrical quartz photochemical thermostatic reactor introducing 0.5 g SiO₂@ZnIn₂S₄. The reactor was filled with 700 mL 0.5 M aqueous KOH and purged with Argon for 30 min. Hydrogen sulfide is bubbled through the solution for 1 h at the rate of 2.5 mL min⁻¹ at 299 K. SiO₂@ZnIn₂S₄ photocatalyst is introduced as a suspension into a cylindrical quartz reactor and irradiated with the sunlight at a constant stirring with a continuous flow of H₂S (2.5 mL min⁻¹). The excess of H₂S was trapped in 10% NaOH solution. The amount of H₂ gas evolved was measured using a graduated gas burette and analyzed using gas chromatography technique (GC) equipped with thermal conductivity detector (TCD) and Porapak-Q packed column with N₂ as a carrier gas.

2.3 Samples characterization

The phase formation and crystallite size of all synthesized samples were estimated via X-ray Diffractometry (XRD-D8, Advance, Bruker-AXS) with Ni-filtered Cu-Kα radiation (λ = 1.5418 Å). Optical properties of the bare and Au loaded samples were studied by UV-Vis-DRS spectrophotometer (UV 2600 spectrometer, Lambda-950, Perkin-Elmer) in the spectral range of 200–800 nm. The surface morphology was characterized using Field Emission Scanning Electron Microscopy (FESEM; Hitachi, S-4800 II) and Field Emission Transmission Electron Microscopy (FETEM; JEM-2000 FS). Image processing and interplanar distance (d) evaluation were performed with the help of micrograph Gatan software. Surface characterization of all SiO₂@ZnIn₂S₄ samples was carried out using X-ray Photoelectron Spectroscopy (XPS, ESCA3000, VG Scientific Ltd.) at a pressure of >1 × 10–9 Torr. The general scan C1s, S2p, In3d, Sn3d, and Zn2p core level spectra were recorded with non-mono chromatized Mg-Kα radiation (photon energy-1253.6 eV). Baseline correction and peak fitting for all the samples were done using software package XPS peak-41. The core level binding energies (BEs) were aligned with respect to the C1s binding energy of 285 eV. The collected gas sample was analyzed using a Gas chromatography system (Shimadzu GC-2014) with portapak-Q packed column coupled with TCD detector and nitrogen (N2-UHP) as a carrier gas.

3 Results and discussions

3.1 Structural study

The phase purity and crystallinity of the as-synthesized pristine ZnIn₂S₄ and SiO₂@ZnIn₂S₄ nano-heterostructure were investigated by using X-ray diffraction analysis. Fig. 1 depicts the XRD diffractogram of the as-synthesized samples (ZS1–ZS5). The pristine ZnIn₂S₄ (ZS1) shows diffraction peaks at 27.93°, 47.57°, 33.62°, 44.44°, 29.14° and 56.89° are indexed to crystalline planes (311), (440), (400), (511), (222), and (533), respectively. The diffraction peaks reveal presence of cubic phase of ZnIn₂S₄ (JCPDS 00-023-0295).³⁸ For the SiO₂@ZnIn₂S₄ nano-heterostructures, it has the similar XRD patterns like pristine ZnIn₂S₄ and the diffraction peaks corresponding to SiO₂ are not observed because very small amount of SiO₂ (i.e. less than equal to 1%).This also could be because of nano tiny SiO₂ surface is covered with dense nano-petals of ZnIn₂S₄, which may prevent XRD from detecting the inner SiO₂. Additionally, the peak intensity of ZnIn₂S₄ slightly decreased with the increase of SiO₂ content. The crystalite size is obtained for nanoheterostructure is around 15 nm against bare ZnIn₂S₄ i.e. 13.65 nm. The lattice parameter of SiO₂@ZnIn₂S₄ is a = b = c 10.78 Å having cubic system (space group Fd3m & space group number 227). The XRD findings may suggest the coexistence of SiO₂ and ZnIn₂S₄ in the nano-heterostructures of SiO₂@ZnIn₂S₄, which will be further illustrated by the following observations from XPS, FESEM and TEM. No diffraction peaks were obtained for any impurities, which indicate the pure nature of the SiO₂@ZnIn₂S₄ samples as-synthesised.

Fig. 1 X-ray diffraction patterns of (ZS1) pure ZnIn₂S₄ (ZS2) 0.25% SiO₂@ZnIn₂S₄ (ZS3) 0.50% SiO₂@ZnIn2S₄ (ZS4) 0.75% SiO₂@ZnIn₂S4 and (ZS5) 1% SiO₂@ZnIn₂S₄ sample Photocatalyst.

3.2 Surface and morphological studies

The morphological investigation of as-synthesized SiO₂@ZnIn₂S₄ nano-heterostructures was observed by using field emission scanning electron microscopy (FESEM). Fig. 2 shows FESEM images of as-synthesized pristine ZnIn₂S₄ (ZS1) and SiO₂@ZnIn₂S₄ (ZS2–ZS5) nano-heterostructures. The low magnification image (Fig. 2a) of pristine ZnIn₂S₄ demonstrates the ultrathin curled nanosheets which self-assembled in the form of marigold flower. The average diameter of flower was found to be 7–12 um. The high magnification image (Fig. 2b) indicate that these flower are assembled by ultrathin interlocked nanosheets (nanopetal) with the thickness about 5–10 nm. The FESEM images of SiO₂@ZnIn₂S₄ nano-heterostructures (ZS-2 to ZS-5) as shown in Figure (2c to j). From figures, the effect of addition of SiO₂ on surface morphologies of nano-heterostructures was investigated.

Fig. 2 Representative FE-SEM images of (a and b) pure ZnIn₂S₄ (c and d) 0.25% SiO₂@ZnIn₂S₄ (e and f) 0.50% SiO₂@ZnIn₂S₄ (g and h) 0.75% SiO₂@ZnIn₂S₄ and (i and j) 1% SiO₂@ZnIn₂S₄ sample photocatalyst.

Fig. 2c–j depicts FESEM imagos of as-synthesized nano-heterostructures (ZS-2 to ZS-5), where the flowers like morphology was retained. Most significantly, extremely crystalline ultrathin nanosheets are seen in case of ZS-4 (Fig. 2g and h). This shows the fully grown ultrathin nanosheets curled and form nanoflowers. Furthermore, as percentage of SiO₂ increases (ZS-5), the small SiO₂ nanoparticles deposited on nanopetals of flower & ultrathin nanosheets (nanopetals) get distorted (Fig. 2i and j). As the concentration of SiO₂ increases there is a deposition of the SiO₂ particles on the nanopetals of ZnIn₂S₄. Further, increasing concentration, there is a formation of SiO₂ clusters on nanopetal of the flowers. We observed uniform deposition/coating for sample ZS-4 (Deposition of 0.75% SiO₂).

The microstructural aspects of as-synthesized SiO₂@ZnIn₂S₄ (ZS-4) nano-heterostructure were confirmed by field emission transmittance electron microscopy (FETEM). Fig. 3 depicts the TEM images of SiO₂@ZnIn₂S₄. The TEM images of SiO₂@ZnIn₂S₄ clearly shows flower-like structure formed by ultrathin nanosheets (nanopetals). These results are well in accordance with FESEM (Fig. 2). More specific details about the nano-heterostructure of SiO₂@ZnIn₂S₄ was further examined by HRTEM, as shown in Fig. 3c. The lattice fringes are distinctly identified with interplanar distances of 0.312 nm and 0.4 nm, which correlates to the d spacing of (311) and (101) planes of ZnIn₂S₄ and SiO₂, respectively.^39,40 Fig. 3d demonstrate the selected area diffraction pattern of as-synthesized sample, which clearly shows polycrystalline nature of SiO₂@ZnIn₂S₄ nano-heterostructure. From Fig. 3e, the observed diffraction dots can be assigned to (311) and (101) planes of ZnIn₂S₄ and SiO₂, respectively. These results are consistent with XRD results. To investigate the distribution of SiO₂ in SiO₂@ZnIn₂S₄ nano-heterostructure the EDS analysis was performed (Fig. 3e and f). These results clearly indicate the uniform distribution of Si, Zn, In, S, & O in SiO₂@ZnIn₂S₄ nano-heterostructure. More significantly, the TEM results confirm the successful formation of SiO₂@ZnIn₂S₄ nano-heterostructure and there exists intimate contact between SiO₂ and ZnIn₂S₄.

Fig. 3 (a) FE-TEM images of pure ZnIn₂S₄, (b) FE-TEM images of SiO₂@ZnIn₂S₄, (c) HRTEM (d) SAED pattern (e) EDAX spectra and (f) elemental mapping of 2% SiO₂ loaded ZnIn₂S₄.

We can easily predict the mechanism of the flower-like structure of SiO₂@ZnIn₂S₄ based on morphological studies. Initially, the nucleation begins to form the Zn²⁺, In³⁺ and S²⁻ ions with the dissociation of the sulphur source, zinc and indium nitrate precursor at a low temperature. The degree of dissociation of zinc and indium salts is greater in the water-like polar solvent. In the supersaturated solution, the petite nuclei were formed and there was further growth of nanoparticles over a time (Scheme 1). In order to minimize their surface energy that further evolves anisotropically along with 2D directions, these new nanoparticles impulsively agglomerated, aligned and confer the formation of ultra-thin nanosheets (Nanopetals). And then after prolonged hydrothermal reaction the curled nanopetals are formed because of surface tension created on the petals. Further, this curving nature petals self-assembled and form marigold like flower structure. During reaction the disperse SiO₂ nanoparticles gets deposited on the surface of the nanopetals.

Scheme 1 Schematic representation of the formation mechanism of SiO₂@ZnIn₂S₄ nanoheterostructure.

3.3 Optical properties

The optical absorption study of the pristine ZnIn₂S₄ and SiO₂@ZnIn₂S₄ nano-heterostructures is important for its photocatalytic application. The UV-Vis diffuse reflectance spectra of as-synthesized nano-heterostructures (ZS-1 to ZS-5) are shown in the Fig. 4. The ZnIn₂S₄ shows an absorption edge of about 570 nm and its band gap is estimated to be 2.16 eV.⁴¹ Compared to the pristine ZnIn₂S₄, the all SiO₂@ZnIn₂S₄ nano-heterostructures exhibited a small red shift in UV-vis diffuse reflectance absorption spectrum (DRS) and the visible light absorption intensity was also increased due to SiO₂ nanoparticles decoration. The addition of SiO₂ will greatly improve light harvesting, separation and transfer of photoinduced charge carriers (h⁺/e⁻) and can be shown by the following spectrum of photoluminescence. Thus, SiO₂@ZnIn₂S₄ nano-heterostructures demonstrated promising usage as a photocatalyst under visible light irradiation relative to pristine ZnIn₂S₄.

Fig. 4 UV-DRS spectra of (ZS1) pure ZnIn₂S₄ (ZS2) 0.25% SiO₂@ZnIn₂S₄ (ZS3) 0.50% SiO₂@ZnIn₂S₄ (ZS4) 0.75% SiO₂@ZnIn₂S₄ and (ZS5) 1% SiO₂@ZnIn₂S₄ sample photocatalyst.

Photoluminescence (PL) emission spectrum measurement was performed for pristine ZnIn₂S₄ (ZS1) and SiO₂@ZnIn₂S₄ (ZS2–ZS5) nano-heterostructures (Fig. 5). This spectrum can provide useful information on capture and recombination charge carriers in semiconductor. The ZS-1 PL spectrum displays a broad peak ∼572 nm, attributing to the band gap transfer emission with the light energy nearly equivalent to the ZnIn₂S₄ band gap energy.⁴² For SiO₂@ZnIn₂S₄ Samples, their PL spectra are similar to that of pristine ZnIn₂S₄, but after SiO₂ modification a significant decrease in PL intensity was observed. Such obvious reduction in PL intensity strength revealed the suppression in recombination rate of photogenerated e⁻ & h⁺ in the SiO₂@ZnIn₂S₄. Interestingly, ZS-4 has shown lowest PL Intensity which indicates optimum concentration of SiO₂ can inhibit electron hole recombination strongly. At very high concentration of SiO₂ i.e. 1% the transport of electrons get hindered which ultimately increased the electron hole recombination. However, at moderate SiO₂ concentration i.e. 0.75% to electron transport to the surface is quite high. Therefore, we conclude that of the ZS-4 sample has more efficient charge carrier separation compared to other samples.⁴³

Fig. 5 Photoluminescence spectra of (ZS1) pure ZnIn₂S₄ (ZS2) 0.25% SiO₂@ZnIn₂S₄ (ZS3) 0.50% SiO₂@ZnIn₂S₄ (ZS4) 0.75% SiO₂@ZnIn₂S₄ and (ZS5) 1% SiO₂@ZnIn₂S₄ sample photocatalyst.

3.4 XPS study

The SiO₂@ZnIn₂S₄ XPS spectra are analyzed in a contrastive manner in order to further confirm the formation of the SiO₂@ZnIn₂S₄ nano-heterostructure and determine its elemental composition and valence states. The Zn 2p, In 3d, S 2p, Si 2P, and O 1 s high-resolution XPS spectra provides more detailed information on the chemical state of these elements in Fig. 6a–e respectively. Fig. 6a shows the Zn 2p region in SiO₂@ZnIn₂S₄ nano-heterostructure with high resolution. Based on figure, 2p_1/2 and 2p_3/2 splitting respectively defined the doublet of Zn 2p. The binding energies at 1046.02 eV associated with Zn²⁺ p_1/2, whereas those of 1021.24 eV were correlated with Zn²⁺ p_3/2.⁴⁴ Fig. 6b shows the In 3d high-resolution spectrum and the SiO₂@ZnIn₂S₄ nano-heterostructure peak at 453.54 eV and 445.18 eV corresponds to In³⁺ 3d_3/2 and In³⁺ 3d_5/2 respectively.²¹ Furthermore, the XPS signal (Fig. 6c) observed at a 162.89 eV binding energy is attributed to S²⁻ 2p_3/2 which is inconsistent with ZnIn₂S₄ results.⁴⁵ Similarly, Fig. 6d and e shows the high resolution spectrum of the energy levels of Si 2P and O 1 s. The binding energies located at 102.1 eV and 532.3 eV verified the existence of Si & O element in SiO₂@ZnIn₂S₄ nano-heterostructure.⁴⁶ On the basis of the above results, it indicates that SiO₂ nanoparticle are distributed uniformly on ZnIn₂S₄. Together with the TEM and optical results, these results suggest that ZnIn₂S₄ was hybridized with SiO₂ by the successful formation of nano-heterostructure, which provides synergistic interaction between SiO₂ and ZnIn₂S₄ matrices for the transfer of charge carriers.

Fig. 6 XPS spectra of the as-prepared SiO₂@ZnIn₂S₄ (a) Zn2p, (b) In3d, (c) S2p, (d) Si3d.

3.5 Photocatalytic study

3.5.1 Photocatalytic activity measurement. As conferred earlier, SiO₂@ZnIn₂S₄ is a good semiconductor having a narrow band gap falling in the visible region at room temperature. Considering the good response towards solar light, photo-catalytic activities of SiO₂@ZnIn₂S₄ have been investigated. Herein, we report the photocatalytic H₂ evolution performance from water and H₂S under natural sun light.

3.5.2 Photocatalytic H₂ evolution from H₂O. The effect of SiO₂ loading on ZnIn₂S₄ and their photo-catalytic activity of hydrogen generation were studied under natural sunlight irradiation using Na₂S/Na₂S₂O₃ as a scavenger electron donor. A series of experiments were performed using different concentrations of SiO₂ on the surface of ZnIn₂S₄, and their results are given in Table 1. The investigation results indicated that the maximum hydrogen generation, i.e. 796 μmol h⁻¹ g⁻¹, was obtained for the sample ZS-4, whereas pure ZnIn₂S₄, shows hydrogen evolution rates of 138 μmol h⁻¹ g⁻¹. Furthermore, 0.25% SiO₂@ZnIn₂S₄, 0.50% SiO₂@ZnIn₂S₄ & 1% SiO₂@ZnIn₂S₄ give 237, 480 & 643 μmol h⁻¹ g⁻¹ respectively from water splitting. Fig. 7 shows the time-dependent hydrogen generation using the as-synthesized ZnIn₂S₄ and SiO₂@ZnIn₂S₄ nanoheterostructures. The linearity of the graph shows a continuous and stable hydrogen generation rate. Which reveals that most exciting electrons are available for proton reduction. For this experiment, we used 70 mL DI water and Na₂S/Na₂S₂O₃ mixture as a sacrificial agent. In the present case, 0.25 M Na₂S and 0.35 M Na₂S₂O₃ mixture were used to resist the photo-corrosion of the semiconductor. Moreover, the Na₂S solution acts as a hole scavenger, and it oxidized S²⁻ into S²⁻, which is beneficial for hydrogen generation from SiO₂@ZnIn₂S₄ rather than alcohol. The photocatalytic hydrogen generation mechanism from water is well reported and discussed in the previous literature.⁴⁷ The Semiconductor photocatalyst after interaction with solar light with energy greater than or equal to band gap energy generates electrons in CB and holes in VB. It causes redox reactions of adsorbed species on a semiconducting material. The photogenerated holes from VB irreversibly oxidize S²⁻, which was reduced back to S²⁻ by Na₂S₂O₃ and radially got adsorbed on the surface of semiconductor producing protons (H⁺) and free radicals, while electrons from CB reduce H⁺ ions into molecular hydrogen.

Table 1 The H₂ generation rates for as-synthesized ZnIn₂S₄, SiO₂ & SiO₂@ZnIn₂S₄

Sr. no.	Sample code	H2 evolution rate from water (μmol h^−1 g^−1)	H2 evolution rate from H2S (μmol h^−1 g^−1)
1	ZS1	138	3350
2	ZS2	237	4320
3	ZS3	480	5650
4	ZS4	796	6730
5	ZS5	643	6050

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Fig. 7 Time versus volume of H₂ (μmole) evolution from H₂O of the all samples synthesized at (ZS-1) pure ZnIn₂S₄ and SiO₂@ZnIn₂S₄ nanoheterostructure prepared with a different percent of SiO₂ loading (ZS-2) 0.25%, (ZS-3) 0.50%, (ZS-4) 0.75%, and (ZS-5) 1%.

Oxidation:

SO₃²⁻+ 2OH⁻ + 2h⁺ → SO₄²⁻ + 2H⁺ (2)

2S²⁻+ 2h⁺ → S₂²⁻ (3)

S₂²⁻ + SO₃²⁻ → S₂O₃²⁻ + S²⁻ (4)

SO₃²⁻ + S²⁻ + 2h⁺ → S₂O₃²⁻ (5)

Reduction:

H⁺ +2e⁻ → H₂ (6)

3.7.1 Photocatalytic H2 evolution from H₂S splitting. The photocatalytic activity of the as-synthesized pristine ZnIn₂S₄ and SiO₂@ZnIn₂S₄ nano-heterostructure is evaluated for hydrogen production via. H₂S splitting under natural sunlight. Fig. 8 and Table 1 shows amount of hydrogen evolved via. H₂S splitting using ZS-1 to ZS-5 samples. The graph linearity clearly shows stable evolution rate of hydrogen. Control experiments indicated that no evolution of hydrogen was detected in the absence of photocatalyst.

Fig. 8 Time versus volume of H₂ (μmole) evolution from H₂S of the all samples (ZS-1) pure ZnIn₂S₄ and SiO₂@ZnIn₂S₄ nanoheterostructure prepared with a different percent of SiO₂ loading (ZS-2) 0.25%, (ZS-3) 0.50%, (ZS-4) 0.75%, and (ZS-5) 1%.

From Fig. 8, it is observed that pristine ZnIn₂S₄ exhibits hydrogen production rate of 3350 μmol h⁻¹ g⁻¹, The enhanced photocatalytic H₂ generation can be attributed to the hydrothermal assisted growth of flower-like ZnIn₂S₄ assembled from ultrathin nanosheets (nanopetals) which facilitates more catalyst surface for reaction and light harvesting. The photocatalytic hydrogen production activity of SiO₂@ZnIn₂S₄ nano-heterostructure increases with increase in SiO₂ content. The sample ZS-2 exhibits significant H₂ production rate 4320 μmol h⁻¹ g⁻¹, which is 30% higher compared to the ZS1 sample. Amongst all of the photocatalysts (ZS1–ZS5), 0.75 wt% loaded SiO₂@ZnIn₂S₄ nano-heterostructure (ZS-4) showed the highest rate for H₂ production (6730 μmol h⁻¹ g⁻¹) under natural sunlight. The optimized 0.75 wt% loaded SiO₂@ZnIn₂S₄ shows twice hydrogen production rate as compared to pristine ZnIn₂S₄. The highest H₂ evolution was achieved for ZS-4 (0.75 wt% loaded SiO₂@ZnIn₂S₄) and is due to the high crystallinity, flower like morphology and band structure of SiO₂@ZnIn₂S₄. More specifically, in contrast to earlier studies, the H₂ evolution obtained for sample ZS-4 is much greater. The sample ZS-5 shows substantially less evolution of H₂ due to the increase in band gap relative to sample ZS-4. It is mainly due to the fact that the relatively high weight ratio of SiO₂ in SiO₂@ZnIn₂S₄ (ZS-5) can increase shielding in the light absorption and opacity, which ultimately reduce penetration of light through the depth of the reaction solution. The efficient charge separation and possible hydrogen production mechanism in SiO₂@ZnIn₂S₄ nano-heterostructure explained by considering the SiO₂@ZnIn₂S₄ band gap energy diagram as shown in Scheme 2. Initially, the nano-heterostructure SiO₂@ZnIn₂S₄ is irradiated under sunlight during photocatalysis. The photo-excitation of the charge carriers occurs in ZnIn₂S₄, which induces h⁺ in the valence band (VB) and e⁻ in the ZnIn₂S₄ conduction band (CB). The photo-induced holes in the VB oxidizes S₂²⁻/SO₃²⁻ to oxidation products (SO₄²⁻/S₂O₃²⁻).⁴⁸ Meanwhile, photo-induced electrons in the CB will move to the deposited SiO₂ nanoparticles in order to reduce H⁺ to H₂.

H₂S + OH⁻ ↔ HS⁻ + H₂O (7)

H₂S + OH⁻ ↔ S²⁻ + H₂O (8)

Photo catalyst → e_CB⁻ + h_VB⁺ (9)

2S²⁻+ 2 h_VB⁺ → S₂²⁻ (10)

2HS⁻ + 2h_VB⁺ → S₂²⁻ + 2H⁺ (11)

2H⁺ + 2e_CB⁻ → H₂ (12)

Scheme 2 Schematic representation of the photocatalytic mechanism of SiO₂@ZnIn₂S₄ heterostructure.

The existence of SiO₂ nanoparticles in SiO₂@ZnIn₂S₄ nano-heterostructure will thus prevent direct charge recombination and promote the separation of electrons and holes. Additionally, provide more active sites for surface reactions, resulting in the enhanced photocatalytic H₂ production using SiO₂@ZnIn₂S₄. The enhancement of hydrogen evolution rate is also supported by photoluminance (PL) study. PL Study shows that there is a decrease in PL intensity in case of ZS-4 Sample which indicates many vacancy defects in the composites which ultimately inhibits charge carrier recombination. In nutshell, the nanoheterostructure, flower like morphology is responsible for good hydrogen evolution in case of ZS-4 Sample.

4 Conclusions

In nutshell, SiO₂@ZnIn₂S₄ nanoheterostructure with marigold flower like structure was synthesized successfully. The photocatalytic efficiency of the SiO₂@ZnIn₂S₄ nanoheterostructure for the evolution of hydrogen under natural sunlight was investigated. The highest H₂ evolution i.e. 796 μmol h⁻¹ g⁻¹ for water splitting & 6730 μmol h⁻¹ g⁻¹ for H₂S Splitting using 0.75 wt percent SiO₂ loading on ZnIn₂S₄ was achieved which is almost twice of pristine ZnIn₂S₄. It is due to high crystallinity, flower-like morphology and band structure of SiO₂@ZnIn₂S₄. This study highlights the nanoheterostructure with marigold flower like morphology which shows efficient charge separations due to intimate contact of the SiO₂ with nanopetals of flowers. Such kind of other nanoheterostructures may have potential as a photocatalyst for higher hydrogen evolution.

Conflicts of interest

There is no any conflict of intrest.

Acknowledgements

ARG like to thanks Dr R. J. Barnabas (Principal Ahmednagar College Ahmednagar) for useful suggestions and discussion and JRD Tata, SPPU for financial support. The authors would like to thank Nanocrystalline materials group. BBK would like to thanks the Ministry of Electronics and Information Technology (MeitY), Government of India and C-MET Pune for providing research facilities.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra01140e

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