Fabrication of Hierarchical Two-Dimensional MoS2 Nanoflowers

Aug 3, 2018 - Synopsis. The current study based on decoration of MoS2 nanoflowers on microflower-like CaIn2S4 highlights the photocatalytic H2 evoluti...
2 downloads 0 Views 5MB Size
Download PDF
Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

Fabrication of Hierarchical Two-Dimensional MoS2 Nanoflowers Decorated upon Cubic CaIn2S4 Microflowers: Facile Approach To Construct Novel Metal-Free p−n Heterojunction Semiconductors with Superior Charge Separation Efficiency Gayatri Swain,† Sabiha Sultana,† John Moma,‡ and Kulamani Parida*,†

Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on August 3, 2018 at 18:32:07 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Centre for Nano Science and Nanotechnology, Siksha O Anusnadhan (Deemed to be University), Bhubaneswar 751030, Odisha, India ‡ School of Chemistry, University of the Witwatersrand, Jorissen Street, Braamfontein, Private Bag 3, PO WITS 2050, Johannesburg, South Africa S Supporting Information *

ABSTRACT: Due to the enormous demand for effective conversion of solar energy and large-scale hydrogen production, cost-effective and long-lasting photocatalysts are believed to be necessary for global production of sustainable and clean hydrogen fuel. Robust and highly efficient p−n heterojunction photocatalysts have a striking ability to enhance light-harvesting capacity and retard the recombination of photoexcitons. A series of p-MoS2/n-CaIn2S4 heterojunction composites with different MoS2 contents have been synthesized via a facile two-step hydrothermal technique in which rose-like p-MoS2 nanoflowers are decorated upon n-type cubic CIS microflowers. In the synthesis protocol highly dispersed MoS2 nanoflowers provided more active edge sites for the growth of c-CIS nuclei, leading to a hierarchical architecture with intimate interfacial contact. The formation of a hierarchical flower-like morphology of the photocatalyst has been established by an HRTEM and FESEM study. Electrochemical characterization, especially the slope of the curve from Mott−Schottky analysis and nature of the current from LSV, reveals the p−n heterojunction nature of the composite photocatalyst. The fabricated heterojunction photocatalysts were further examined for visible light photocatalytic H2 evolution. Far exceeding those for the neat c-CIS and MoS2, it is seen that the p-MoS2/n-CIS heterojunction photocatalyst with an optimum content of MoS2 exhibited enhanced H2 evolution using a 0.025 M Na2S/ Na2SO3 solution as hole quenching agent under visible light illumination. The 0.5 wt % p-MoS2/n-CIS photocatalyst presents a higher H2 production rate of 602.35 μmol h−1 with 0.743 mA cm−2 photocurrent density, 19 times and 8 times higher than those of neat c-CIS, respectively. This superior photocatalyic activity is due to the efficient separation of electron−hole charge carriers at the interface, as supported by a photoluminescence study and EIS measurements.

1. INTRODUCTION The design of visible-light-driven (VLD) heterojunction materials toward photocatalytic water splitting is a current trend in research.1 In recent years much effort has been delivered to develop different n−n, p−p, and p−n heterojunction photocatalysts with optimal charge separation efficiency. Importantly the p−n heterojunction has been highlighted as one of the most fascinating strategies in the development of antirecombination properties of electron−hole pairs in semiconducting materials in comparison to p−p and n−n heterojunctions.2−4 In addition, in comparison to singlecomponent systems, p−n heterojunctions can also be considered as unit components for nanodevices mainly due to their unique optoelectronics properties.5 A p−n junction formed from the coupling of p-type and n-type materials with an appropriate band gap position within one system has proven to be a promising candidate with various performances in photocatalytic application.6−9 The main aspect of a p−n junction is that the separation of photogenerated charge carriers are carried out through a space charge region which is © XXXX American Chemical Society

formed by the depletion of electrons and holes in n-type and ptype semiconductors, respectively. This difference in the surface potential creates an electrostatic field between them which readily separates the reduction and oxidation sites. Driven by the formed electric field, electrons and holes are readily separated and easily transported to the conduction band (CB) of an n-type semiconductor and the valence band (VB) of a p-type semiconductor, respectively.1−3,10 Furthermore, a p−n heterojunction could also increase the life span of photogenerated electrons and holes, which will endow photocatalytic activity.11 Regarding p−n heterojunction materials, there have been very limited studies reported in the field of photocatalysis such as Sr2TiO4/SrTiO3 (La,Cr),12 Ag2O-Bi2O2CO3,4 BiOCl-BiVO4,13 (AgIn)xS2-Ag2S,14 CuO/ PbTiO311 etc. Still more development needs to be made in order to construct highly efficient visible-light-responsive p−n heterojunction based composites. Received: May 4, 2018

A

DOI: 10.1021/acs.inorgchem.8b01221 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry The most promising “multimetal chalcogenide” AB2X4 (A = Ca, Cd, Cu, Zn, etc.; B = In, Ga, Al; X = chalcogenides such as S, Se, Te) based semiconductor has drawn significant attention in extensive scientific research areas.15−17 It serves as a VLD photcatalyst mainly owing to its special photochemical stabilities and also its unique electronic and optical properties.18,19 CaIn2S4 (CIS) is an alkali-earth-metal-based n-type photocatalyst which has been widely used in the field of hydrogen fuel generation like ZnIn2S4,20 CdIn2S4,21 etc. due to its visible light active nature and suitable negative band edge potential level for water reduction.19,22 In 2013 Ding et al. designed a novel CaIn2S4 photocatalyst that showed hydrogen evolution of about 30.92 μmol g−1 h−1 without any sacrificial agent.19 Later Ding et al. fabricated mesoporous monoclinic CaIn2S4, which exhibited a H2 generation rate of 30.2 μmol h−1 in Na2S/Na2SO3 aqueous solution.22 However, isolated CaIn2S4 has limited large-scale applications toward photocatalysis, which is especially due to the high rate of recombination of photogenerated electron−hole charge carriers as well as low separation efficiency.23 Thus, it was later combined with different metal oxides (CaIn2S4/TiO2 and CaIn2S4/Ag3PO4),16,24 novel metals (Au-Pt alloy/CaIn2S4, Au/ CaIn 2 S 4 , and Au-Cu/CaIn 2 S 4 ), 25−27 layered materials (CaIn2S4/g-C3N4 and CaIn2S4/RGO),28,29 and carbon materials (CNTs/CaIn2S4),23 but its hydrogen evolution rate is not comparable with those of the other known efficient metal sulfides. Thus, it is most challenging for ternary metal chalcogenides to achieve better improvement in photocatalytic H2 production, but this can be mitigated by coupling CIS with other semiconductors, such as two-dimensional (2D) layered metal dichalcogenides. More importantly, 2D layered materials are more efficient toward photocatalytic activity, thus shortening the charge transport time and distance. In addition, constructing composites with 2D materials enhances the interfacial contact owing to their excellent properties of restriction of particles toward agglomeration.30 Significant progress has been made with 2D layered molybdenum disulfide (MoS2), a chalcogenide derivative of molybdenum.31,32 It is an exceptionally cost effective substitute for novel metal catalysts33 and has received a fair share of attention owing to its unique optical, chemical, and mechanical properties34 as well as outstanding electronic properties with controlled band gap that ranges from indirect to direct band gap energy (1.2−1.9 eV).35 The building block of MoS2 is composed of Mo and S atoms in a covalently bonded S−Mo− S fashion in a stack of planes which are closely filled in a hexagonal arrangement and has a van der Waals interaction between two adjacent planes.33 Because of its structure it exhibits high chemical and thermal endurance. The lowtoxicity quasi-two-dimensional 2D crystal MoS2 has been explored in applications in areas ranging from photocatalysis31 and photovoltaics36 to biomedicine.37 Many studies have shown that MoS2 is not effective as a photocatalyst alone due to its low rate of charge separation, but when it is coupled with other cocatalysts and materials it can exhibit better photocatalytic activity.38 However, it is believed that the formation of hierarchical composites with 2D-layered MoS2 provides enormous active sites and immense surface area which are responsible for maintaining sufficient contact area and greatly enhancing the effectiveness of the reaction. Many recent studies have shown that the p-type semiconducting characteristics of MoS2 materials have been explored in various applications in photocatalysis9 as well as photoelectrochemical

applications.39 Zhao et al. designed an n-BiVO4/p-MoS2 core− shell heterojunction photocatalyst which exhibited excellent performance toward pollutant degradation: i.e., Cr6+ reduction and oxidation of crystal violet (CV).7 Ye et al. fabricated an MoS2/S-doped g-C3N4 heterojunction film which exhibited enhanced photoelectrocatalytic activities toward hydrogen generation.40 The novel BiVO4/Bi2S3/MoS2 n−p heterojunction constructed by Wang and co-workers were systematically examined for rhodamine B (RhB), methylene blue (MB), and malachite green (MG) photocatalytic degradation in visible light irradiation.41 Recent studies have shown that MoS2 is the best active component for a combination of different binary and ternary metal sulfides.39−42 Although many works have been reported with n-type metal sulfides such as CdS as well as ZnIn2S4 and other metal sulfides like Bi2S3,41 Ni3S2,43 WS2,44 etc., to the best of our knowledge a photocatalytic hydrogen production reaction involving MoS2/CIS has not been yet studied. In a review of the literature it was found that sensitizing n-type CIS with p-type MoS2 could create a new type II heterojunction (depending on the suitable band gap of the neat materials) system that could yield a better enhancement in hydrogen production rate under solar light irradiation. The present study details the fabrication of a strong coupling interface between p-MoS2 and n-CIS with varying percentages of MoS2 through a facile two-step hydrothermal technique. The main aspect of the current system is the design and coupling of low-cost metal sulfide semiconducting materials in order to construct two-dimensional (2D) p−n heterojunction photocatalysts. Initially MoS2 has a low charge carrier density, but after the formation of heterojunctions with n-type c-CIS, its charge carrier density increases, which is a favorable condition for the enhancement of the photocatalytic H2 evolution rate. The photocatalytic mechanism depends upon the presence of a built-in potential gradient between the two metal sulfide interfaces through an s−s linkage, which channelizes the photogenerated charge carriers (two major units of a photocatalytic mechanism). In the present case construction of a flowerlike morphology could be the main reason for the enhancement of photocatalytic H2 evolution. The aforementioned designed heterojunction between p-MoS2 and n-CIS could exhibit good chemical and excellent photocatalytic stability and also enhance interfacial contact area as well as suppress charge recombination. A possible electron−hole charge transfer as well as band-bending model and construction of p−n heterojunctions for an H2 production mechanism have been schematically proposed.

2. EXPERIMENTAL SECTION All precursor chemicals such as calcium nitrate (Ca(NO3)2·4H2O), indium nitrate In(NO3)3)·xH2O (M.W. = 318.83 g/mol, thioacetamide (CH3CSNH2), molybdenum trioxide (MoO3), potassium thiocyanate (KSCN), and ethanol were commercially supplied by MERCK India. All reagents are of analytical grade and were used without purification. Throughout all the experiments double-distilled water was used. 2.1. Synthetic Procedure for the Photocatalyst. The overall synthetic procedure of MoS2/CIS heterojunction photocatalyst was focused on a facile two-step hydrothermal technique described as follows. 2.1.1. Synthesis of Rose-Like MoS2 Nanoflowers. The starting material sources for the preparation of MoS2 nanoflowers are MoO3 and KSCN as Mo and sulfur sources, respectively. First an aqueous solution was prepared containing MoO3 and KSCN in 1:3 millimole B

DOI: 10.1021/acs.inorgchem.8b01221 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry ratio with 60 mL of distilled water with vigorous stirring at room temperature. The obtained suspension was transferred into a Teflonlined stainless steel autoclave with the temperature maintained at 180 °C for 24 h. After the reaction was over, the hydrothermal unit was cooled naturally to ambient temperature and the black precipitates that formed were collected and washed three times by centrifugation at 7000 rpm with distilled water and ethanol. The resulting product was then dried in an oven at 80 °C for 14 h to obtain the black roselike MoS2 nanoflowers.45 2.1.2. Fabrication of MoS2/c-CIS Heterojunction Photocatalyst. In a typical procedure a calculated amount of powdered MoS2 nanoflowers was first dispersed in 20 mL of ethanol and ultrasonicated until it became a homogeneous suspension. In a separate 100 mL beaker a transparent aqueous solution of Ca(NO3)2·4H2O, In(NO3)3·H2O, and CH3CSNH2 in a molar ratio of 1:2:8 was prepared by using 60 mL of distilled water with vigorous magnetic stirring for about 30 min. The dispersed MoS2 suspension was introduced slowly to the transparent solution with continuous stirring. After complete addition, the mixture was stirred for another 1 h, after which the resulting mixed solution was placed into a 100 mL Teflonlined stainless steel autoclave and the temperature maintained at 120 °C for 24 h. The autoclave was then cooled naturally to ambient temperature, and the product was collected after repeated centrifugation. The product was then further washed several times with distilled water and ethanol. The final product was obtained by drying the centrifuged suspension for under vacuum at 60 °C. Various MoS2/CIS heterojunction photocatalysts were prepared by varying the weight percentages of powder MoS2 nanoflowers: i.e., 0.25%, 0.5%, 1%, and 2%. Pure CIS was also synthesized for comparison by adopting the same synthetic procedure as described above in the absence of MoS2.16 2.2. Catalyst Characterization. 2.2.1. Physical Characterization. In order to characterize the crystal phase purity of the assynthesized photocatalyst, PXRD measurements were performed on a Rigaku Ultima IV instrument at 40 kV and 40 mA using Cu Kα radiation (λ = 0.154 nm). Ultraviolet−visible spectral measurements were recorded on a JASCO V-750 spectrophotometer in the range of 200−800 nm using BaSO4 as a reflectance standard to determine the absorbance capacity of the neat and composite materials. In order to better understand the phase composition, X-ray photoelectron spectroscopy measurements were done on an Omicron ESCA+ instrument. Photoluminescence (PL) analysis was carried out by using a JASCO FP-8300 spectrofluorometer with an excitation wavelength of around 440 nm. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy analyses were performed on a 200 kV JEOL JEM 2100 instrument to characterize the morphological characteristics of the materials. Field emission scanning electron microscopy (FESEM) analyses were carried out on a FEI Quanta 400 FEG_SEM to further confirm the morphology of the materials. 2.2.2. Photolectrochemical Characterization. The photoelectrochemical photocurrents of the as-prepared CIS and all MoS2/CIS heterojunction photocatalysts with various contents of MoS2 were measured on an IVIUMnSTAT multichannel electrochemical analyzer. The experimental setup is a three-electrode system consisting of an as-synthesized sample as a working electrode, an Ag/AgCl electrode as a reference electrode, and Pt foil as a counter electrode. A mixture of 0.5 M Na2S and 0.5 M Na2SO3 electrolyte solution having pH 13.01 was chosen for LSV measurements. The LSV curve was taken by sweeping the potential range from −1.4 to +0.1 V at a scan rate of 5 mV. In addition the electrochemical impedance spectroscopic (EIS) measurement of each material was carried out by using 0.5 M Na2S and Na2SO3 electrolyte solution with a frequency range from 100000 to 100 Hz. Again a Mott−Schottky analysis for c-CIS was carried out in 1 M KCl and that for neat MoS2 was carried out in 0.5 M H2SO4 at 1000 Hz frequency with a fixed ac potential. 2.3. Photocatalytic Hydrogen Evolution Experimental Setup. The overall experimental setup for the visible-light-responsive photocatalytic hydrogen evolution reaction has been carried out in a

100 mL sealed Pyrex glass photoreactor cell attached to a 150 W xenon arc lamp (>400 nm) as the irradiating source followed by 1 M NaNO2 solution as the UV cutoff filter. In a typical hydrogen production experiment, 20 mg of as-synthesized photocatalyst was suspended in a 20 mL mixed aqueous solution containing 0.025 M Na2S and 0.025 M Na2SO3 as sacrificial agents with continuous magnetic stirring in order to keep the photocatalyst in suspension throughout the experiment. Before the experiment was started, the solution in the photoreactor was thoroughly degassed by passing N2 gas through the reactor for about 30 min in order to completely remove all dissolved oxygen. The hydrogen gas produced during the reaction was collected by using the water displacement technique and was analyzed by gas chromatography. The detailed calculation process for apparent conversion efficiency of the current photocatalytic system (H2 evolution yield 602.35 μmol/h with a 150 W xenon lamp as the light source positioned 9 cm away from the photocatalyst reactor) has been estimated and is given below. The equation by which apparent conversion efficiency can be calculated is

conversion efficiency = H 2O → H 2 +

1 O2 2

stored chemical energy incident light energy

ΔHc = 285.8 kJ/mol

where ΔHc = heat of combustion of hydrogen in kJ/mol and stored chemical energy = (number of moles of hydrogen produced per second after the reaction) × ΔHc = 0.1673 mmol s−1 × 285.8 kJ/ mol−1 = 0.0480 J s−1 or W. The calculated illumination intensity on the flask is found to be around 70 mW cm−2. Now, incident light energy = 70 mW cm−2 × (area of the spherical surface on which light falls off) = 70 mW cm−2 × π × r2 (r is the radius of the circle and the value is 1.5 cm) = 70 mW cm−2 × 3.141 × (1.5 cm)2 = 0.4947 W and conversion efficiency = 0.0480 W/0.4947 W = 0.0971 = 9.71%.

3. RESULTS AND DISCUSSION 3.1. Characterization. The crystallographic phase purity of the as-synthesized c-CIS, MoS 2 , and MoS 2 /c-CIS heterojunction photoctalysts have been elucidated by XRD measurements and are shown in Figure 1. Pure MoS2 exhibits

Figure 1. XRD patterns of neat MoS2, neat c-CIS, and MoS2/c-CIS composites with different contents of MoS2.

characteristic peaks at 2θ = 14.1, 33.3, 58.9° in XRD spectra ascribed to the 002, 100, and 110 planes of hexagonal MoS2 in accordance with the JCPDS file 37-1492.39 The main diffraction peaks for neat c-CIS observed at 22.8, 27.5, 28.45, 33.5, 43.6, 47.8, 56.4, 59.7, 67.0, and 70.1° mainly correspond to the 220, 311, 222, 400, 511, 440, 533, 622, 731, and 800 planes, respectively, indexed to the plane of the cubic phase of C

DOI: 10.1021/acs.inorgchem.8b01221 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry CIS (JCPDS 16-0341).28 No other impurity peaks in the spectrum attributed to metal oxides, binary sulfides, or unreacted reactants are observed in XRD spectra of neat cCIS as well as of MoS2/c-CIS heterojunction photoctalysts, which imply a high phase purity along with good crystallinity. The intensity of the XRD diffraction peaks of c-CIS in the series of all MoS2/c-CIS heterojunction photocatalysts gradually decreases with the introduction of MoS2 which confirms the successful synthesis, good interaction and coexistence of both c-CIS and MoS2 in the MoS2/c-CIS hybrid. After the loading of MoS2, no obvious change in the peak position of c-CIS and the absence of an XRD peak for MoS2 are observed. The absence of XRD peaks for MoS2 can be attributed to the low loading and good dispersion of MoS2 in the composites. The presence of MoS2 in the composites was confirmed by XPS and EDAX measurements. Additionally, the higher the loading of MoS2, i.e. from 0.25% to 2 wt %, the more prominent is 220 plane of c-CIS in the MoS2/c-CIS hybrids, with a similar observation being reported by Xia et al. for the CNTs/CaIn2S4 system.23 The average crystallite size of the neat c-CIS and composite samples has been calculated considering the (440) planes by using the Scherrer equation,46 and the values are depicted in Table 1. From calculated data it is apparent that the crystallite size decreases with an increase in the loading amount of MoS2. Table 1. Average Crystallite Sizes of Neat c-CIS and Composite MoS2/c-CIS Samples As Determined by the Scherrer Equation sample

crystallite size (nm)

neat c-CIS 0.25% MoS2/c-CIS 0.5% MoS2/c-CIS 1% MoS2/c-CIS 2% MoS2/c-CIS

12.3 15.1 14.3 12.4 11.9

Furthermore, to determine the surface chemical composition status as well as the existence of various oxidation state of the as-synthesized heterojunction composite, X-ray photoelectron spectroscopy (XPS) measurements were carried out. The adventitious C 1s peak recognized from the XPS spectra having a binding energy value around 284.8 eV was taken as a reference component. The appearance of this carbon peak is a result of the use of carbon tape in the sample preparation and may also be attributed to the adsorption of CO2 from the atmosphere by the surface of the sample.16 The full scan XPS survey spectra corresponding to neat c-CIS, neat MoS2, and 0.5% MoS2/c-CIS as shown in Figure 2a illustrate the presence of Ca, In, Mo, and S with a trace amount of C and O and no additional peaks observed; these results corroborate the high purity of all the samples which supports both XRD and EDAX data. To precisely ensure the electron transfer from p-type MoS2 to n-type CIS in a p−n heterojunction system, an XPS analysis of the parent materials was also carried out. For pure c-CIS, Ca exhibits two XPS peaks (Figure 2b) at 351.20 and 347.52 eV corresponding to 2p1/2 and 2p3/2, respectively, with a peak splitting of 3.68 eV suggesting the +2 oxidation state of Ca. After the heterojunction formation with MoS2, the corresponding Ca 2p peaks for 0.5% MoS2/c-CIS are shifted toward a lower binding energy (shifting energy ∼0.16 eV), exhibiting XPS peaks at 351.04 eV (2p1/2) and 347.36 eV (2p3/2).24 Figure 2c represents the high-resolution

Figure 2. XPS spectra of neat c-CIS, neat MoS2, and 0.5%MoS2/cCIS composite: (a) full spectrum scan; (b) Ca 2p; (c) In 3d; (d) Mo 3d; (e) S 2p.

XPS peak of In in neat c-CIS, which indicated that the In 3d core peak can be fitted into two peaks around 452.50 and 444.93 eV, assigned to 3d3/2 and 3d5/2 having a peak splitting of around 7.57 eV, confirming the +3 oxidation state for In. However, for 0.5% MoS2/c-CIS, the In 3d peaks show a red shift of ∼0.16 eV resolved into two peaks at around 452.07 eV (InIII 3d3/2) and 444.51 eV (InIII 3d5/2).28 The deconvulted spectrum of Mo 3d for neat MoS2 is compared with that for Mo of 0.5% MoS2/c-CIS. As shown in Figure 2d, the XPS peaks of Mo can be resolved into four core level XPS peaks and the main two strongest peaks located at around 231.76 and 228.38 eV (for MoS2) assigned to MoIV 3d3/2 and MoIV 3d5/2, respectively, suggest the +4 oxidation state of the Mo. The peak at the binding energy 235.10 eV indicates the presence of Mo in a +6 oxidation state, attributed D

DOI: 10.1021/acs.inorgchem.8b01221 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 3. SEM images of neat c-CIS (a), neat MoS2 (b, c) and MoS2/c-CIS composite (d).

Figure 4. TEM images (a, b) and HRTEM image (c) of neat c-CIS, TEM image (d) and HRTEM image (e) of 0.5% MoS2/c-CIS composite photocatalyst, and SAED pattern of MoS2/c-CIS composite (f).

Mo in the composite photocatalyst are shifted slightly toward higher binding energy.47,48 The high-resolution S 2p spectrum (Figure 2e) can be deconvoluted into two main peaks for neat MoS2 and c-CIS and split into three peaks for 0.5% MoS2/c-CIS. Peaks at around 162.88 eV (2p1/2) and 161.70 eV (2p3/2) are assigned to the −2 oxidation state of the c-CIS. From the MoS2 deconvoluted spectra the peaks positioned at 162.47 and

to the exposure of materials to atmospheric oxygen. In addition to Mo another satellite peak at around 225.50 eV corresponds to the 2s peak of sulfur. Similarly the corresponding peaks for 0.5% MoS2/c-CIS has been observed at binding energies of around 231.99 eV (MoIV 3d3/2), 228.41 eV (MoIV 3d5/2), 235.87 eV (MoVI), and 226.08 eV (S 2s). The above results concluded that after a heterojunction with c-CIS the peaks of E

DOI: 10.1021/acs.inorgchem.8b01221 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

microflower-like c-CIS species (shown by yellow circles) are surrounded by tiny rose-like MoS2 nanoflowers (in accordance with the SEM image) shown by red circles with intimate contact. In the synthesis process, at first highly dispersed MoS2 nanoflowers provided a more active edge sites for the growth of CaIn2S4 nuclei, leading to the hierarchical architecture. Formation of an edge-anchored flower morphology of MoS2 with c-CIS as demonstrated in Figure 4d offers improved properties toward the photocatalytic activity mainly due to the presence of more edge sites, containing a large number of unsaturated sulfur ions which easily capture the H+ ions at the contact area of petals of both flowers, resulting in good hydrogen evolution performance.47 The corresponding Figure 4e shows the interplanner d spacing value of around d = 0.334 and 0.269 nm indexed to the 311 and 400 planes of cubic CIS microflowers, respectively,16 and the plane measured to be about 0.62 nm is attributed to the 002 plane for rose-like hexagonal MoS2.9 Moreover, the selected area electron diffraction (SAED) pattern in Figure 4f of MoS2/c-CIS composite clearly demonstrates the Debye−Scherrer ring patterns of (311), (400), and (440) which are in good agreement with XRD spectra.16 In order to confirm accurately the content of different elements present in the MoS2/CIS heterojunction photocatalyst, an energy dispersive X-ray spectroscopy mapping image has been analyzed and represented in the Figure S1, which further confirms the presence of MoS2. The optical absorption and electronic property information of the MoS2/c-CIS heterojunction photocatalyst has been confirmed from UV−vis diffuse reflectance spectroscopy measurement. Figure 5 displays the UV−vis DRS spectra of

161.22 eV corresponding to the S 2p1/2 and S 2p3/2 peaks suggest the presence of S2− species. In the XPS spectra of the MoS2/c-CIS heterojunction composite the deconvoluted peak of S has been shifted toward higher binding energy and lower binding energy in comparison to neat MoS2 and c-CIS, respectively. In addition to this, another S peak for the composite sample located at 163.5 eV is attributed to the presence of a bridging disulfide S22− ligand.47,49 From the above XPS results it has been affirmed that, after the formation of a p−n heterojunction, neat c-CIS undergoes a red shift in the binding energy while MoS2 exhibits a blue shift. These shifts in binding energy may be attributed to partial electron transfer from the electron-rich system MoS2 to the electron-poor system c-CIS. This electron transfer mechanism would increase the electron density of c-CIS in the composite material, resulting in reduction in the BEs of Ca, In, and S. Similarly the electron density of MoS2 in XPS spectra of 0.5% MoS2/c-CIS decreases and that leads to an increase in BEs of Mo and S. These types of variations in chemical environment in XPS result indicates the good electronic interaction between the p-type MoS2 and n-type c-CIS and also supports the formation of a heterojunction in which photogenerated electron−hole pairs are easily separated and migrated at the electric interface. Field-emission scanning electron microscopy (FESEM) was used to examine the morphology and crystal structure of the cCIS, MoS2, and 0.5% MoS2/c-CIS heterojunction photocatalyst, as shown in Figure 3. Figure 3a clearly demonstrates the microflower-like morphology of bare c-CIS with a wide size distribution having a diameter approximately within the range of 2−2.5 μm. Figure 3b,c show that MoS2 has a rose-like nanoflower morphology with diameter 500−800 nm.7 During the course of synthesis through hydrothermal treatment, partially monodispersed isolated spheres of MoS2 are generated, and after the completion of the hydrothermal process, the flower morphologies are formed by the growth of a number of petal-like nanosheets on the surface of the sphere of MoS2 in a random order. As shown in Figure 3a, the FESEM image of c-CIS demonstrates that the c-CIS microflowers are composed of curled and densely packed thin nanosheets on the microspheres. However, in the case of rose-like MoS 2 nanoflowers, the hierarchical monodispersed nanospheres are covered by a number of crumpled type petal nanosheets. In addition, c-CIS flowers in the composite marked in Figure 3d are not clearly observed due to exfoliation during the hydrothermal method of synthesis in the presence of MoS2 nanoflowers. The typical SEM image (Figure 3d) of the MoS2/ c-CIS heterojunction photocatalyst reveals that a large number of nanosheet-like petals of c-CIS microflowers are framed by MoS2 flowers in a loosely stacked order owing to the improved channelization of photogenerated excitons. To gain further insight into the coexistence of different elements as well as morphological information about the MoS2/c-CaIn2S4 heterojunction photocatalyst, TEM and HRTEM analyses were carried out. Figure 4 provides the TEM and HRTEM images of c-CIS and 0.5% MoS2/c-CIS heterojunction photocatalyst. The microflower-like morphology of c-CIS is clearly observed in Figure 4a, which is matched with the FESEM images as shown in Figure 3a. The lattice fringes of about d = 0.334 nm in Figure 4c are assigned to the 311 plane of cubic CIS and also explain the formation of the microflower-like morphology (Figure 4b) from the highdensity sheet of c-CIS.29 In Figure 4d it is seen that large

Figure 5. UV−visible diffuse reflectance spectra of the neat and composite photocatalyst.

c-CIS, neat MoS2, and MoS2/c-CIS composite photocatalyst with different amounts of MoS2. c-CIS shows broad absorption spectra in the visible region with an absorption edge end at around 621 nm corresponding to the band gap energy nearly equal to 2.11 eV attributed to the intrinsic band gap transition.28 As comparison to neat c-CIS, composites show longer wavelength absorption due to the narrow band gap and deep color of MoS2. As the loading level of MoS2 increases, the absorption range gradually increases, which is supported by the color change from burnt orange to dark yellow.39 Interestingly MoS2 has a wide absorption in the region of visible light owing to the presence of a humped peak located at about 450−750 nm, which enhances the light-harvesting efficiency that easily generates electron−hole charge carriers of the respective F

DOI: 10.1021/acs.inorgchem.8b01221 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry composite by improving the photocatalytic behavior;47 however, with a high content of MoS2 (1 and 2 wt %), the synthesized composite is not efficient for photocatalytic activity because the generation of local heating effects at the interface lowers the electron−hole charge carrier excitations.23 In addition the band gaps of pure c-CIS, MoS2, and MoS2/cCIS composite have been calculated from Schuster−Kubelka− Munk function transformed reflectance spectra (given below) by using Tauc plots (Figure S2 in the Supporting Information): αhv = α(hv − Eg )n

or

Figure 6. PL spectra of c-CIS and different MoS2/c-CIS composites.

(αhv)1/ n = α(hv − Eg )

composite has been quenched significantly after loading of various concentrations of MoS2. A notable change occurs in the composite, attributed to the decoration of flowerlike MoS2, which favors an efficient interfacial interaction between two flowers through S−S edge bonds and helps in prolonging the lifetime of photoinduced charge carriers. In contrast, the intensity of the PL peak of the composite photocatalyst effectively diminished to an optimal loading of MoS2, i.e. 0.5% MoS2/c-CIS, in compared to neat c-CIS. The most quenched PL intensity explores a good separation of photogenerated charge carriers, which thus results in a high hydrogen evolution rate. After the optimal level, the luminescence intensity increases as the MoS2 content loading increases, and this may be due to the blocking of active sites in hierarchical MoS2/c-CIS composite due to excess black MoS2 nanoflowers which block the transition of photons, thus decreasing the hydrogen evolution activity. The charge transfer mechanism was further studied by EIS. Electrochemical impedance spectroscopy is a very useful technique to evaluate the catalytic kinetics and interfacial reaction process.50 Figure 7 displays Nyquist plots which are

where α = absorption coefficient, h = Planck’s constant = 6.626 × 10−34 J sec, ν = frequency of light, A = proportionality constant, and Eg = band gap energy. By plotting graphs between (αhν)1/n vs hν and extrapolating the plot to the x-axis intercept, we can calculate the band gap energy of all the samples. Again here n is 2 or 1/2 depending upon the transition in the semiconductor. For a direct transition it is 2, and for an indirect transition its value is 1/ 2. Taking direct band gap transition in the present case, the band gap energies are calculated to be 2.11, 1.86, and 2.13 eV for c-CIS, MoS2, and 0.5% MoS2/c-CIS photocatalyst, respectively.46 To be a photocatalyst, a material should have conduction and valence bands, and they can be calculated by using the equations E VB = X − E e + 0.5Eg ECB = E VB − Eg

where EVB and ECB are the energies of valence band and conduction band edge potentials, respectively, X is the absolute electronegativity of the semiconductor and is 4.394 for c-CIS and 5.323 for MoS2, Ee is the energy of free electrons based on the hydrogen scale (∼4.5 eV), and Eg is the band gap energy.9,16 On the basis of the above equations the calculated VB position of the c-CIS and MoS2 is 0.94 and 1.75 eV respectively while the conduction band minimum value as calculated for both c-CIS and MoS2 is −1.17 eV and −0.11 eV respectively. According to the VB and CB positions of c-CIS and MoS2 resulting from the above equation, it has been concluded that a clear appropriate type II staggered band gap alignment is formed between neat c-CIS and neat MoS2. It is well-known that the photocatalytic activity of a photocatalyst closely depends on the separation efficiency of photoinduced excitons. Photoluminescence (PL) spectroscopy, a generic measurement, has been used to investigate the details about the rate of transfer and recombination of photogenerated charge carriers of photocatalytic materials.47 Figure 6 gives the comparison of the decreasing order of PL curves of the c-CIS and MoS 2 /c-CIS heterojunction composites all showing a profile similar to that of pure cCIS. All samples were excited at a wavelength of 440 eV at room temperature. In the PL spectra, c-CIS exhibits a luminescence emission peak in a broad range centered at around 554.7 nm, whereas the MoS2/c-CIS heterojunction photocatalyst shows a peak at around 553.4 nm. More importantly the integrated PL intensity of the MoS2/c-CIS

Figure 7. Nyquist plots of c-CIS and MoS2/c-CIS composites.

EIS representations of c-CIS and different wt % MoS2 loaded CIS. In general, the charge transfer resistance or the conductance can be analyzed through the radius of the semicircle area in the Nyquist plots occurring at the interface of the photoelectrode.51 A smaller semicircle diameter for the MoS2/c-CIS heterojunction was found in comparison to neat c-CIS, which implies that the incorporation of such a low amount of MoS2 can shorten the ion diffusion pathways, hence facilitating the charge transfer process across the electrode− electrolyte junction. As the diameter of the Nyquist plot is G

DOI: 10.1021/acs.inorgchem.8b01221 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 8. Mott−Schottky plots of neat c-CIS and 0.5%MoS2/c-CIS (a) and MoS2 (b).

inversely proportional to the rate of charge transfer in hydrogen evolution reactions, 0.5%MoS2/c-CIS exhibited the highest photocatalytic hydrogen evolution rate in comparison to other amounts of catalyst. Qualitative insights into the formation of p−n junctions can easily be evaluated by Mott−Schottky (MS) measurements, as shown in Figure 8. From the positive slope (Figure 8a) it was confirmed that c-CIS is an n-type material and MoS2 is a ptype material, which was confirmed from the negative slope of the MS plot of MoS2 (Figure 8b). By extrapolating the Mott− Schottky plot to 1/C2 = 0, a flat band potential was obtained, which was −1.07 V for CIS, −0.53 V (Efn) and 0.98 V (Efb) for 0.5%MoS2/c-CIS ,and 1.68 V for MoS2 (V vs NHE). In comparison with the c-CIS, the composite exhibited a positive shift in flat band potential, which indicated that the Fermi levels of p-type MoS2 shift upward while those of the n-type cCIS shift downward along with the energy bands in the composite.51,52 This positive shift in the composite in comparison to c-CIS, in addition to its n-type and p-type features which were seen in the Mott−Schottky plot of MoS2/ c-CIS composite photocatalyst, further confirmed the effective p−n junction formation.51 The H2 production activity of MoS2/c-CIS was remarkably enhanced due to the efficient charge separation occurring through the built-in electric field at the p−n junction. Further, from the Mott−Schottky plot, the carrier density can be calculated from the slope of the plot.50 Mathematically, the carrier density of the electrode material is inversely proportional to the slope of the plot53 and measurement can be done through the equation stated as follows:

Figure 9. I−V curves of neat c-CIS and various MoS2/c-CIS composites.

exhibited a very small photocurrent, i.e. 0.09 mA, whereas all the p−n junction materials exhibited high photocurrent and the nature of the plot is asymmetric in forward and reverse biasing.11 As is expected, the photocurrent density of MoS2/cCIS composite increased with an increase in loading percentage of MoS2 up to 0.5 wt % upon visible light illumination. Further increase in the MoS2 content caused a decrease in photocurrent density. The proper amount of MoS2 is crucial for the photocatalytic activity, since an excessive amount of MoS2 may result in blocking of the light absorption of c-CIS.48 In addition to this, it was seen that all of the composite materials exhibited much higher photocurrent density in comparison to the neat c-CIS, which indirectly confirms that there must be efficient separation of charge carriers across the junction.39 However, with an applied potential, at first band bending was carried out and then an electric field is formed at the interface of c-CIS and MoS2, which may be the reason behind the efficient exciton separation.11 Thus the p−n junction has promising merit to reduce the electron−hole recombination by injecting and transporting of electrons and holes from one semiconductor to the other in opposite directions through electric fields built between them, so that a greater number of charge carriers could participate in the chemical reactions instead of recombining. 3.2. Optimization of Photocatalytic Hydrogen Gas Production Mechanism of MoS2/c-CIS Heterojunction Photocatalyst. The photocatalytic performance of MoS2, cCIS, and MoS2/c-CIS composites has been tested for hydrogen production under visible light irradiation with a mixed solution containing 0.025 M Na2S and 0.025 M Na2SO3 serving as the hole scavenger under ambient conditions. A

N = (2/ϵϵ0e)[d(1/C 2)/dV ]−1

where ε denotes the dielectric constant of the material, ε0 is the permittivity of the vacuum (8.854 × 10−12 F m−1), the factor e is the electronic charge unit, i.e. 1.602 × 10−19 C, and V is the potential applied at the electrode. The slope of 0.5%MoS2/cCIS was much smaller in comparison to that of c-CIS, which confirmed that the donor density capacity for MoS2-modified c-CIS was much higher, which is the main reason for its high photocatalytic activity. The photocatalytic activity was further evaluated by the I vs V polarization curve as shown in Figure 9. A higher photocurrent corresponds to a high photocatalytic H 2 evolution. In this system Na2S/Na2SO3 mixed electrolyte was used as a hole scavenger, and as a result it releases more electrons and hence results in more electronegative shifting of onset potential.48 With the applied potential neat c-CIS H

DOI: 10.1021/acs.inorgchem.8b01221 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 10. Hydrogen evolution rate of neat c-CIS and MoS2/c-CIS composite (a) and reusability test of 0.5%MoS2/c-CIS composite (b).

Table 2. Hydrogen Generation Data of Neat CIS and CIS-Based Composite photocatalysis novel CaIn2S4

synthetic procedure

CaIn2S4/g-C3N4

facile hydrothermal method followed by a postcalcination process hydrothermal

CNTs/CaIn2S4

one-step microwave hydrothermal method

metalloid Ni2P/ CaIn2S4 mesoporous monoclinic CaIn2S4 Au-Pt alloy/CaIn2S4

hydrothermal followed by thermal reaction

MoS2/CaIn2S4

high-temp sulfurization approach high-temp sulfurization approach followed by photoreduction method two-step hydrothermal method

irradiation light source 300 W Xe arc lamp 300 W xenon lamp xenon arc lamp (350 W) 300 W xenon lamp 300 W Xe arc lamp 300-W Xe arc lamp 150W Xe lamp

sacrificial agent

H2 production rate (μmol g−1 h−1)

ref

nil

30.92

19

0.5 M Na2S and 0.5 M Na2SO3 0.5 M Na2S and 0.5 M Na2SO3 lactic acid

102

28

23.43

23

486

54

0.025 M Na2S/Na2SO3

3020 (30.2 in 10 mg)

22

Na2S/Na2SO3 (0.025 M)

10760 (107.6 in 10 mg) 26

0.025 M Na2SO3 and 0.025 M Na2S

30117.5 (602.35 in 20 mg)

this work

H2: i.e., 90.2 μmol h−1. The above order is in agreement with the PL, EIS, and photocurrent study. From the foregoing discussion it has been shown that the photocatalytic activity of the photocatalyst is increased for an optimal content of MoS2. We have calculated the apparent conversion efficiency for the 0.5% MoS2/c-CIS composite photocatalyst53 and obtained a value of 9.71%. A few studies on H2 generation have been carried out over cubic and monoclinic CaIn2S4-based composite materials, as shown in Table2. In addition to the photocatalytic efficiency of the photocatalyst, it is also necessary to know the lifetime of the photocatalyst, which is another decisive factor throughout the hydrogen evolution experiment. To investigate the photocatalytic stability for H2 production application of the assynthesized MoS2/c-CIS heterojunction photocatalyst four consecutive cyclic tests has been performed for durations of 4 h at a time interval of 1 h under the same reaction conditions as shown in Figure 10b. After the completion of four cycling experiments it has been obvious that there was no appreciable change in the rate of H2 generation, and that result suggested that the fabricated MoS2/c-CIS composite photocatalyst shows good photocatalytic stability against photocorrosion as well as high photocatalytic performance. In order to further confirm the photostability of the heterojunction photocatalyst, we have carried out an XRD and EDAX characterization after a fourcycle H2 evolution experiment. As shown in Figure S3a, no change in the XRD pattern has been observed, which confirms the photostability of the heterostructure. In addition to this, an EDAX (Figure S3b) analysis also denies the photocorrosive

comparison of the activities of the materials is represented in Figure 10a. The hydrogen production process is a photocatalytic reaction, because with the absence of light and photocatalyst there was no hydrogen evolution. From the comparative results of the rate of hydrogen evolution graph with different weight percentages of MoS2 in MoS2/c-CIS composite it can be seen that H2 evolution is negligible for neat MoS2, and this is in accordance with our previously reported paper,9 but pure c-CIS shows a certain amount of H2 evolution, i.e. about 32.8 μmol h−1, due to the rapid recombination of electron−hole pairs. However, after formation of the p−n heterojunction between MoS2 and CIS, the H2 evolution rate of the MoS2/CIS composite was remarkably enhanced with the loading content of MoS2. This may be due to the delayed recombination of photoexcitons. The optimized experiment of the aforementioned samples reveals that the composite with 0.5 wt % content of MoS2 results in an optimum hydrogen evolution rate of 602.35 μmol/h, i.e. 19 times higher than that of pristine c-CIS, suggesting the major role of MoS2. A notably high loading of MoS2 to the composite decreases the average rate of H2 production owing to the black color of MoS2 which hinders the transition of photons. Hence 2% MoS2/c-CIS results in a lower hydrogen evolution rate: i.e., about 240.3 μmol/h. The rate of H2 production of the composite photocatalyst for various contents of MoS2 is in the order 0.5 > 1 > 0.25 > 2 > neat c-CIS and the H2 generation amounts are 602.35, 359.03, 278.75, 245.3, and 32.8 μmol/h, respectively. Again we have performed the water splitting reaction of 0.5% MoS2/c-CIS photocatalyst in the absence of a sacrificial agent (pure water) and found a very low amount of I

DOI: 10.1021/acs.inorgchem.8b01221 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Possible Charge Transfer Route for H2 Evolution through p−n Junction Mechanism

the junction contact area. The accumulated electrons over MoS2 may raise the Efp value and further decrease the position of Efn in the composite. The Mott−Schottky plot of 0.5% MoS2/c-CIS heterojunction photocatalyst has an inverted V shape, confirming the positive slope for the n-type features (for c-CIS) and negative slope for the the p-type characteristics (for MoS2), which proves electron and hole migration. Further, extrapolating the tangent to 1/C2 = 0 results in the two Fermi level values, one for c-CIS (−0.53 V) and the other one for MoS2 (0.98 V) (V vs NHE). The two observed Fermi level values within the heterostructure confirm that the Efn value of c-CIS and Efp value of MoS2 shifted downward and upward, respectively, along with their VB and CB in order to attain the equilibration. For the Fermi level equilibration a band bending must occur, which is formed due to the difference in work functions between p-MoS2 and n-CIS. Here a huge band bending has been observed after the contact formation between two parent components. According to Anderson’s model39

nature of the combined metal sulfide in the presence of a sacrificial agent. The photostability of the prepared heterojunction composite is due to the incorporation of flowerlike pMoS2 to the n-type c-CIS, which simultaneously promotes the hydrogen evolution by inhibiting the photocorrosive properties of c-CIS.

4. POSSIBLE CHARGE TRANSFER MECHANISM In order to understand the effect of the p−n heterojunction on photocatalytic H2 production through a possible interface charge transfer mechanism, the relative band edge position of the composite photocatalyst has been taken into consideration, which gives an idea about the migration of photoinduced charge carriers. In order to make a p−n heterojunction system effective, the band edge potential of the two materials should be suitable so that the Fermi level can easily be equilibrated. In the present system the suitable band edge potential of p-type MoS2 and n-type c-CIS helps to construct a type II heterojunction photocatalyst. The band gap energies and respective band edge potentials of the photocatalytic samples were determined from both Mulliken electronegativity and electrochemical measurements. As we have calculated from a UV−vis DRS plot, the band edge potential of neat c-CIS was found to be −1.17 eV for the CB and 0.94 eV for the VB and that for the MoS2 CB is −0.11 eV and for the VB is 1.75 eV. Again from the Mott−Schottky measurement the Fermi level and the corresponding conduction band of c-CIS were −1.07 and −1.17 V, respectively. Similarly for MoS2 the Fermi level and corresponding valence band were calculated to be 1.68 and 1.78 V, respectively, which are nearly equal to the value obtained in accordance with the Mulliken electronegativity data. The Fermi level and the conduction band of p-MoS2 were somewhat lower than those for neat c-CIS before contact. However, electron diffusion occurs by the directional transfer of electrons from the higher Fermi level of n-type c-CIS to the lower Fermi level p-type MoS2 owing to a more negative conduction band of c-CIS, thus forming an intimate contact between the two individual systems. As a result negative charges were accumulated in MoS2 and positive charges in cCIS when hole transfer takes place from MoS2 to c-CIS near

band bending ∝

1 charge carrier density

it has been shown that a high band bending is mainly attributed to the introduction of MoS2, which has a poor charge carrier density. The Mott−Schottky plot also supported the band bending and effective formation of a p−n junction, as the flat band (E fb ) edge position of the composite heterojunction photocatalyst shifted toward positive potential in comparison to the neat c-CIS. Scheme 1 depicts the energy band schematic diagram for p-MoS2 and n-type c-CIS before and after contact. The photocatalytic process mainly depends on the excellent charge separation efficiency and easier transfer of photoexcitons within the semiconductor, which facilitates the better enhancement of photocatalytic activity. From the above discussion, it is apparent that the effective charge separation was carried out through the p−n junction. Individually the band edge potentials of both c-CIS and MoS2 have a suitable conduction edge potential values for the reduction of water to generate hydrogen. MoS2 has poor photocatalytic activity J

DOI: 10.1021/acs.inorgchem.8b01221 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

hydrothermal synthesis technique has been successfully demonstrated. As reported here under visible light illumination a high photocatalytic H2 evolution rate has been achieved by pMoS 2 /n-CIS using Na 2 S/Na 2 SO 3 aqueous solution in comparison to that in the neat photocatalyst owing to the excellent intimate interfacial contact through a large number of active sites of MoS2. The fabricated staggered type II heterojunction photocatalyst is expected to effectively improve the charge separation efficiency by reducing the recombination of photoexcitons at the interface and permit the charge injection from one semiconductor to another depending on the favorable band edge potential. The current work can provide a new experimental insight for the utilization of visible light via the synthesis of p-MoS2/n-CIS heterojunction photocatalysts which not only enhance the photocatalytic activity of the neat c-CIS and neat MoS2 but also be potential components of a promising platform in environmental remediation.

toward H2 generation in its bulk state owing to low charge mobility, since the S−Mo−S adjacent layers have poor conductivity. However, under hydrothermal treatment flowerlike MoS2 is formed in which a large number of MoS2 sheets randomly arrange in a flower shape, which provides a large number of active sites to facilitate photocatalytic reactions, but its low charge carrier density still limits its activity. The rapid recombination property of pristine c-CIS also made it impotent for the photocatalytic H2 evolution catalyst. This problem was mitigated by forming a heterojunction with pMoS2, where n-CIS microflowers are surrounded by a number of rose-like p-MoS2 nanoflowers, providing a large number of contact interfacial sites through which the photoinduced charge carriers easily migrate from one part to other through the junction. In this way the antirecombination properties of the photoinduced electrons and holes will be efficiently enhanced due to the presence of MoS2. In this system according to the foregoing discussion, a plausible mechanistic pathway has been adopted for the enhanced hydrogen production over the MoS2/CIS heterojunction upon visible light illumination in which the separation efficiency of the photoexciton charge carriers through a p−n heterojunction is enhanced. As the Fermi level becomes equilibrated, the photogenerated electrons and holes reside at the CB and VB of MoS2 and CIS, respectively. As soon as possible the photoexcited electrons on the conduction band of MoS2 migrate toward the conduction band of CIS since MoS2 has a more negative potential conduction band and CIS has a lower band and simultaneously the holes are moved up from higher valence band potential to lower potential of valence band: i.e., from n-CIS to p-MoS2 after contact formation. In particular the internal electric field region is formed by the band bending, which is beneficial for the transfer of whole electron−hole charge pairs which promotes the easier separation of photoexcitons that restrained the recombination probability of photogenerated charge carriers. The volume of electrons thus transferred from the CB of MoS2 to the CB of CIS were responsible for the reduction of H+ ions by absorbing them at the surface of CIS reactive sites, producing H2 (E°H+/H2 = −0.42 vs NHE at pH 7), and simultaneously the holes are captured by the sacrificial reagent mainly by S2− at the surface of the VB of MoS2. In addition to the p−n heterojunction sacrificial agent, i.e. Na2S and Na2SO3, the aqueous solution also plays a vital role in the transportation of photoinduced charge carriers. The schematic representation for energy band and the synergistic effect between p-type MoS2 and n-type CIS regarding the p−n heterojunction and the H2 production mechanism through a sacrificial agent is depicted in Scheme 1. It is very clear that Fermi level tunability is one of the efficient strategies of the 2D material in order to decrease the junction barrier height and that can be carried out by suppressing the static charge transfer between two 2D semiconducting materials which have different work function values. Thus, in the present photocatalytic system the static charge transfer could be restrained by the introduction of MoS2 to the neat CIS during the formation of the p-MoS2/n-CIS p−n heterojunction photocatalyst that can bring about a great improvement in photocatalytic activity.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01221. EDAX pattern for MoS2/CIS heterojunction composite, individual band edge potentials of various photocatalysts, and XRD and EDAX of heterostructure after the experiment (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail for K.P.: [email protected]. ORCID

Kulamani Parida: 0000-0001-7807-5561 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are very thankful to SOA management for their cooperation and encouragement. They are also obliged to the SERB (for funding the project EMR/2016/000606) for their financial support.

■

REFERENCES

(1) Moniz, S. J.; Shevlin, S. A.; Martin, D. J.; Guo, Z. X.; Tang, J. Visible-light Driven Heterojunction Photocatalysts for Water Splitting−A Critical Review. Energy Environ. Sci. 2015, 8, 731−759. (2) Ida, S.; Takashiba, A.; Koga, S.; Hagiwara, H.; Ishihara, T. Potential Gradient and Photocatalytic Activity of an Ultrathin p−n Junction Surface Prepared with Two-Dimensional Semiconducting Nanocrystals. J. Am. Chem. Soc. 2014, 136, 1872−1878. (3) Wang, H.; Zhang, L.; Chen, Z.; Hu, J.; Li, S.; Wang, Z.; Liu, J.; Wang, X. Semiconductor Heterojunction Photocatalysts: Design, Construction and Photocatalytic Performances. Chem. Soc. Rev. 2014, 43, 5234−5244. (4) Liang, N.; Wang, M.; Jin, L.; Huang, S.; Chen, W.; Xu, M.; He, Q.; Zai, J.; Fang, N.; Qian, X. Highly Efficient Ag2O/Bi2O2CO3 pn Heterojunction Photocatalysts with Improved Visible-light Responsive Activity. ACS Appl. Mater. Interfaces 2014, 6, 11698−11705. (5) Jeon, P. J.; Lee, Y. T.; Lim, J. Y.; Kim, J. S.; Hwang, D. K.; Im, S. Black Phosphorus−Zinc Oxide Nanomaterial Heterojunction for p−n Diode and Junction Field-effect Transistor. Nano Lett. 2016, 16, 1293−1298.

5. CONCLUSION In summary, for the first time a robust and effective hydrogen evolution p-MoS2/n-CIS composite system based on the K

DOI: 10.1021/acs.inorgchem.8b01221 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (6) Guo, F.; Shi, W.; Wang, H.; Huang, H.; Liu, Y.; Kang, Z. Fabrication of a CuBi2O4/g-C3N4 p−n Heterojunction with Enhanced Visible Light Photocatalytic Efficiency toward Tetracycline Degradation. Inorg. Chem. Front. 2017, 4, 1714−1720. (7) Zhao, W.; Liu, Y.; Wei, Z.; Yang, S.; He, H.; Sun, C. Fabrication of a Novel p−n Heterojunction Photocatalyst n-BiVO4@ p-MoS2 with Core−Shell Structure and Its Excellent Visible-Light Photocatalytic Reduction and Oxidation Activities. Appl. Catal., B 2016, 185, 242−252. (8) Nashim, A.; Martha, S.; Parida, K. M. Heterojunction Conception of n-La2Ti2O7/p-CuO in The Limelight of Photocatalytic Formation of Hydrogen under Visible Light. RSC Adv. 2014, 4, 14633−14643. (9) Swain, G.; Sultana, S.; Naik, B.; Parida, K. M. Coupling of Crumpled-Type Novel MoS2 with CeO2 Nanoparticles: A NobleMetal-Free p−n Heterojunction Composite for Visible Light Photocatalytic H2 Production. ACS Omega 2017, 2, 3745−3753. (10) Rawool, S. A.; Pai, M. R.; Banerjee, A. M.; Arya, A.; Ningthoujam, R. S.; Tewari, R.; Rao, R.; Chalke, B.; Ayyub, P.; Tripathi, A. K.; Bharadwaj, S. R. pn Heterojunctions in NiO: TiO2 Composites with Type-II Band Alignment Assisting Sunlight Driven Photocatalytic H2 Generation. Appl. Catal., B 2018, 221, 443−458. (11) Reddy, K. H.; Parida, K. M.; Satapathy, P. K. CuO/PbTiO3: A New-Fangled p−n Junction Designed for The Efficient Absorption of Visible Light with Augmented Interfacial Charge Transfer, Photoelectrochemical and Photocatalytic Activities. J. Mater. Chem. A 2017, 5, 20359. (12) Jia, Y.; Shen, S.; Wang, D.; Wang, X.; Shi, J.; Zhang, F.; Han, H.; Li, C. Composite Sr2TiO4/SrTiO3 (La, Cr) Heterojunction based Photocatalyst for Hydrogen Production under Visible Light Irradiation. J. Mater. Chem. A 2013, 1, 7905−7912. (13) He, Z.; Shi, Y.; Gao, C.; Wen, L.; Chen, J.; Song, S. BiOCl/ BiVO4 p−n Heterojunction with Enhanced Photocatalytic Activity under Visible-Light Irradiation. J. Phys. Chem. C 2014, 118, 389−398. (14) Abdullah, H.; Kuo, D. H. Facile Synthesis of n-type (AgIn)xZn2(1−x)S2/p-type Ag2S Nanocomposite for Visible Light Photocatalytic Reduction to Detoxify Hexavalent Chromium. ACS Appl. Mater. Interfaces 2015, 7, 26941−26951. (15) Wang, W.; Ng, T. W.; Ho, W. K.; Huang, J.; Liang, S.; An, T.; Li, G.; Jimmy, C. Y.; Wong, P. K. CdIn2S4 Microsphere as an Efficient Visible-Light-Driven Photocatalyst for Bacterial Inactivation: Synthesis, Characterizations and Photocatalytic Inactivation Mechanisms. Appl. Catal., B 2013, 129, 482−490. (16) Jo, W. K.; Natarajan, T. S. Facile Synthesis of Novel RedoxMediator-Free Direct Z-Scheme CaIn2S4 Marigold-Flower-Like/TiO2 Photocatalysts with Superior Photocatalytic Efficiency. ACS Appl. Mater. Interfaces 2015, 7, 17138−17154. (17) Chaudhari, N. S.; Bhirud, A. P.; Sonawane, R. S.; Nikam, L. K.; Warule, S. S.; Rane, V. H.; Kale, B. B. Ecofriendly Hydrogen Production from Abundant Hydrogen Sulfide using Solar LightDriven Hierarchical Nanostructured ZnIn2S4 Photocatalyst. Green Chem. 2011, 13, 2500−2506. (18) Yuan, W.; Yang, S.; Li, L. Synthesis of g-C3N4/CaIn2S4 Composites with Enhanced Photocatalytic Activity under Visible Light Irradiation. Dalton Trans. 2015, 44, 16091−16098. (19) Ding, J.; Sun, S.; Yan, W.; Bao, J.; Gao, C. Photocatalytic H2 Evolution on a Novel CaIn2S4 Photocatalyst under Visible Light Irradiation. Int. J. Hydrogen Energy 2013, 38, 13153−13158. (20) Chen, Y.; Tian, G.; Ren, Z.; Pan, K.; Shi, Y.; Wang, J.; Fu, H. Hierarchical Core−Shell Carbon Nanofiber@ZnIn2S4 Composites for Enhanced Hydrogen Evolution Performance. ACS Appl. Mater. Interfaces 2014, 6, 13841−13849. (21) Kale, B. B.; Baeg, J. O.; Lee, S. M.; Chang, H.; Moon, S. J.; Lee, C. W. CdIn2S4 Nanotubes and “Marigold” Nanostructures: A VisibleLight Photocatalyst. Adv. Funct. Mater. 2006, 16, 1349−1354. (22) Ding, J.; Hong, B.; Luo, Z.; Sun, S.; Bao, J.; Gao, C. Mesoporous Monoclinic CaIn2S4 with Surface Nanostructure: An Efficient Photocatalyst for Hydrogen Production under Visible Light. J. Phys. Chem. C 2014, 118, 27690−27697.

(23) Xia, Y.; Li, Q.; Wu, X.; Lv, K.; Tang, D.; Li, M. Facile Synthesis of CNTs/CaIn2S4 Composites with Enhanced Visible-Light Photocatalytic Performance. Appl. Surf. Sci. 2017, 391, 565−571. (24) Wan, S.; Ou, M.; Zhong, Q.; Zhang, S. Z-scheme CaIn2S4/ Ag3PO4 Nanocomposite with Superior Photocatalytic NO Removal Performance: Fabrication, Characterization and Mechanistic Study. New J. Chem. 2018, 42, 318−326. (25) Li, J.; Meng, S.; Wang, T.; Xu, Q.; Shao, L.; Jiang, D.; Chen, M. Novel Au/CaIn2S4 Nanocomposites with Plasmon-Enhanced Photocatalytic Performance under Visible Light Irradiation. Appl. Surf. Sci. 2017, 396, 430−437. (26) Ding, J.; Li, X.; Chen, L.; Zhang, X.; Sun, S.; Bao, J.; Gao, C.; Tian, X. Au−Pt Alloy Nanoparticles Site-Selectively Deposited on CaIn2S4 Nanosteps as Efficient Photocatalysts for Hydrogen Production. J. Mater. Chem. A 2016, 4, 12630−12637. (27) Ding, J.; Li, X.; Chen, L.; Zhang, X.; Tian, X. Photocatalytic Hydrogen Production Over Plasmonic AuCu/CaIn2S4 Composites with Different AuCu Atomic Arrangements. Appl. Catal., B 2018, 224, 322−329. (28) Jiang, D.; Li, J.; Xing, C.; Zhang, Z.; Meng, S.; Chen, M. TwoDimensional CaIn2S4/g-C3N4 Heterojunction Nanocomposite with Enhanced Visible-Light Photocatalytic Activities: Interfacial Engineering and Mechanism Insight. ACS Appl. Mater. Interfaces 2015, 7, 19234−19242. (29) Ding, J.; Yan, W.; Sun, S.; Bao, J.; Gao, C. Hydrothermal Synthesis of CaIn2S4-Reduced Graphene Oxide Nanocomposites with Increased Photocatalytic Performance. ACS Appl. Mater. Interfaces 2014, 6, 12877−12884. (30) Wang, F.; Shifa, T. A.; Zhan, X.; Huang, Y.; Liu, K.; Cheng, Z.; Jiang, C.; He, J. Recent Advances in Transition-Metal Dichalcogenide based Nanomaterials for Water Splitting. Nanoscale 2015, 7, 19764− 19788. (31) Lv, R.; Robinson, J. A.; Schaak, R. E.; Sun, D.; Sun, Y.; Mallouk, T. E.; Terrones, M. Transition Metal Dichalcogenides and Beyond: Synthesis, Properties, and Applications of Single-and Few-Layer Nanosheets. Acc. Chem. Res. 2015, 48, 56−64. (32) Kibsgaard, J.; Chen, Z.; Reinecke, B. N.; Jaramillo, T. F. Engineering The Surface Structure of MoS2 to Preferentially Expose Active Edge Sites for Electrocatalysis. Nat. Mater. 2012, 11, 963. (33) Kong, Z.; Yuan, Y. J.; Chen, D.; Fang, G.; Yang, Y.; Yang, S.; Cao, D. Noble-Metal-Free MoS 2 Nanosheet Modified-InVO 4 Heterostructures for Enhanced Visible-Light-Driven Photocatalytic H2 Production. Dalton Trans. 2017, 46, 2072−2076. (34) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699. (35) Gong, L.; Wang, L.; Lu, J.; Han, C.; Chen, W.; Sow, C. H. Photocurrent Response in Multiwalled Carbon Nanotube Core− Molybdenum Disulfide Shell Heterostructures. J. Phys. Chem. C 2015, 119, 24588−24596. (36) Domask, A. C.; Gurunathan, R. L.; Mohney, S. E. Transition Metal−MoS2 Reactions: Review and Thermodynamic Predictions. J. Electron. Mater. 2015, 44, 4065−4079. (37) Liu, T.; Shi, S.; Liang, C.; Shen, S.; Cheng, L.; Wang, C.; Song, X.; Goel, S.; Barnhart, T. E.; Cai, W.; Liu, Z. Iron Oxide Decorated MoS2 Nanosheets with Double PEGylation for Chelator-Free Radio Labeling and Multimodal Imaging Guided Photothermal Therapy. ACS Nano 2015, 9, 950−960. (38) Meng, F.; Li, J.; Cushing, S. K.; Zhi, M.; Wu, N. Solar Hydrogen Generation by Nanoscale p−n Junction of p-type Molybdenum Disulfide/n-type Nitrogen-Doped Reduced Graphene Oxide. J. Am. Chem. Soc. 2013, 135, 10286−10289. (39) Liu, Y.; Yu, Y. X.; Zhang, W. D. MoS2/CdS Heterojunction with High Photoelectrochemical Activity for H2 Evolution under Visible Light: The Role of MoS2. J. Phys. Chem. C 2013, 117, 12949− 12957. (40) Ye, L.; Wang, D.; Chen, S. Fabrication and Enhanced Photoelectrochemical Performance of MoS2/S-doped g-C3N4 Heterojunction Film. ACS Appl. Mater. Interfaces 2016, 8, 5280−5289. L

DOI: 10.1021/acs.inorgchem.8b01221 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (41) Wang, J.; Jin, J.; Wang, X.; Yang, S.; Zhao, Y.; Wu, Y.; Dong, S.; Sun, J.; Sun, J. Facile Fabrication of Novel BiVO4/Bi2S3/MoS2 np Heterojunction with Enhanced Photocatalytic Activities Towards Pollutant Degradation Under Natural Sunlight. J. Colloid Interface Sci. 2017, 505, 805−815. (42) Wei, L.; Chen, Y.; Lin, Y.; Wu, H.; Yuan, R.; Li, Z. MoS2 as Non-Noble-Metal Co-Catalyst for Photocatalytic Hydrogen Evolution Over Hexagonal ZnIn2S4 Under Visible Light Irradiations. Appl. Catal., B 2014, 144, 521−527. (43) Cao, J.; Zhou, J.; Zhang, Y.; Wang, Y.; Liu, X. Dominating Role of Aligned MoS2/Ni3S2 Nanoarrays Supported on Three-Dimensional Ni Foam with Hydrophilic Interface for Highly Enhanced Hydrogen Evolution Reaction. ACS Appl. Mater. Interfaces 2018, 10, 1752−1760. (44) Xu, S.; Li, D.; Wu, P. One-Pot, Facile and Versatile Synthesis of Monolayer MoS2/WS2 Quantum Dots as Bioimaging Probes and Efficient Electrocatalysts for Hydrogen Evolution Reaction. Adv. Funct. Mater. 2015, 25, 1127−1136. (45) Zhu, T.; Huang, L.; Song, Y.; Chen, Z.; Ji, H.; Li, Y.; Xu, Y.; Zhang, Q.; Xu, H.; Li, H. Modification of Ag3VO4 with GrapheneLike MoS2 for Enhanced Visible-Light Photocatalytic Property and Stability. New J. Chem. 2016, 40, 2168−2177. (46) Sahoo, D. P.; Nayak, S.; Reddy, K. H.; Martha, S.; Parida, K. M. Fabrication of A Co(OH)2/ZnCr LDH “p−n” Heterojunction Photocatalyst with Enhanced Separation of Charge Carriers for Efficient Visible-Light-Driven H2 and O2 Evolution. Inorg. Chem. 2018, 57, 3840−3854. (47) Zhang, S.; Yang, H.; Gao, H.; Cao, R.; Huang, J.; Xu, X. Onepot Synthesis of CdS Irregular Nanospheres Hybridized with OxygenIncorporated Defect-Rich MoS2 Ultrathin Nanosheets for Efficient Photocatalytic Hydrogen Evolution. ACS Appl. Mater. Interfaces. ACS Appl. Mater. Interfaces 2017, 9, 23635−23646. (48) Ji, J.; Guo, L.; Li, Q.; Wang, F.; Li, Z.; Liu, J.; Jia, Y. A Bifunctional Catalyst for Hydrogen Evolution Reaction: The Interactive Influences Between CdS and MoS2 on Photoelectrochemical Activity. Int. J. Hydrogen Energy 2015, 40, 3813−3821. (49) Zhu, X.; Wang, P.; Zhang, Q.; Wang, Z.; Liu, Y.; Qin, X.; Zhang, X.; Dai, Y.; Huang, B. CdS−MoS2 Heterostructures on Mo Substrates via in situ Sulfurization for Efficient Photoelectrochemical Hydrogen Generation. RSC Adv. 2017, 7, 44626−44631. (50) Lang, D.; Cheng, F.; Xiang, Q. Enhancement of Photocatalytic H2 Production Activity of CdS Nanorods by Cobalt-Based Cocatalyst Modification. Catal. Sci. Technol. 2016, 6, 6207−6216. (51) Zhang, Y.; Li, W.; Sun, Z.; Zhang, Q.; Wang, L.; Chen, Z. Insitu Synthesis of Heterostructured BiVO4/BiOBr Core-Shell Hierarchical Mesoporous Spindles with Highly Enhanced Visible-Light Photocatalytic Performance. J. Alloys Compd. 2017, 713, 78−86. (52) Ning, F.; Shao, M.; Xu, S.; Fu, Y.; Zhang, R.; Wei, M.; Evans, D. G.; Duan, X. TiO2/Graphene/NiFe-Layered Double Hydroxide Nanorod Array Photoanodes for Efficient Photoelectrochemical Water Splitting. Energy Environ. Sci. 2016, 9, 2633−2643. (53) Sultana, S.; Mansingh, S.; Scurrell, M.; Parida, K. M. Controlled Synthesis of CeO2NS-Au-CdSQDs Ternary Nanoheterostructure: A Promising Visible Light Responsive Photocatalyst for H2 Evolution. Inorg. Chem. 2017, 56, 12297−12307. (54) Wang, X. J.; Li, X. L.; Liu, C.; Li, F. T.; Li, Y. P.; Zhao, J.; Liu, R. H.; Li, G. D. Metalloid Ni2P and Its Behavior for Boosting the Photocatalytic Hydrogen Evolution of CaIn2S4. Int. J. Hydrogen Energy 2018, 43, 219−228.

M

DOI: 10.1021/acs.inorgchem.8b01221 Inorg. Chem. XXXX, XXX, XXX−XXX