Hybrid of g-C3N4 and MoS2 Integrated onto Cd0.5Zn0.5S: Rational

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A novel hybrid of g-C3N4 and MoS2 integrated onto Cd0.5Zn0.5S: Rational design with efficient charge transfer for enhanced photocatalytic activity Gaurangi Gogoi, Sam Keene, Anindya Sundar Patra, Tushar Kanta Sahu, Shane Ardo, and Mohammad Qureshi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00512 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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ACS Sustainable Chemistry & Engineering

A novel hybrid of g-C3N4 and MoS2 integrated onto Cd0.5Zn0.5S: Rational design with efficient charge transfer for enhanced photocatalytic activity Gaurangi Gogoi,a Sam Keene,b Anindya S. Patra,a Tushar K. Sahu,a Shane Ardoc,d and Mohammad Qureshi* a,c a

b

Department of Chemistry, Indian Institute Technology, Guwahati – 781039, Assam, India.

Department of Physics, cDepartment of Chemistry, dDepartment of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697, USA *E-mail: [email protected]

KEYWORDS: noble metal free composite, band alignment, charge transfer, charge separation, hydrogen production.

One of the authors, Dr. Mohammad Qureshi has performed some of the spectroscopic studies during his stay of five months at University of California, Irvine in collaboration with Dr. Shane Ardo.

ACS Paragon Plus Environment

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ABSTRACT: Rational design of hierarchical nano-composites is a promising approach for efficient energy harvesting and conversion. A noble-metal-free ternary hierarchical composite, Cd0.5Zn0.5S-g-C3N4-MoS2, has been developed. Materials were chosen based on their relative band-edge alignments and they were studied as a composite for photocatalytic properties. The photocatalytic activity was evaluated by measuring the rate of photo-driven H2 evolution with concomitant degradation of organic pollutants, such as Rhodamine B. Optimization of the loading of g-C3N4 and MoS2 onto Cd0.5Zn0.5S results in an enhanced yield of hydrogen evolution by ~120% (Cd0.5Zn0.5S-g-C3N4) and ~197% (Cd0.5Zn0.5S-g-C3N4-MoS2) compared to bare Cd0.5Zn0.5S. The ternary hybrid, Cd0.5Zn0.5S-g-C3N4-MoS2 resulted in an apparent quantum yield (AQY) of 38 % at 420 nm. The significant improvement in photocatalytic performance in the composite can be attributed to enhanced interfacial charge transfer of electrons from g-C3N4 to Cd0.5Zn0.5S and MoS2. We surmise that the close proximity of the energies of conduction band edge for each component in the ternary composite promotes better charge separation.

INTRODUCTION Combustion of fossil fuels to power our society results in CO2 emissions and global warming. This motivates the search for a renewable energy resource to diminish the effects of global warming as well as meet increasing energy demand. Hydrogen has been considered as a promising future energy storage medium and energy carrier due to atomic hydrogen’s abundance in both water and biomass, non-toxicity, high energy density (122 kJ/g), etc.1 Since the first report on using the TiO2 photocatalyst to generate hydrogen by splitting water,2 there has been extensive research to develop efficient photocatalyst materials for hydrogen evolution from


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water. Several inorganic materials capable of photocatalytically splitting water have been discovered.3-5 Among them, CdS is promising, because of its strongly visible-light-absorbing direct bandgap (Ebg ≈ 2.3 eV) and conduction band capable of reducing protons to H2.6-10 Yet, it is limited due to rapid recombination of photogenerated charge carriers and photocorrosion.11 ZnS is more stable than CdS but suffers from poor visible light absorption (Ebg ≈ 3.6 eV).12 Consequently, there has been an attempt to combine both CdS and ZnS in order to realize a stable and effective photocatalyst for H2 evolution. In this regard, ternary chalcogenide CdxZn1xS

is a promising alloy of CdS and ZnS with tunable optical properties by varying the x value.13

Cd0.5Zn0.5S has been shown to be an efficient photocatalyst for H2 evolution and has a suitable bandgap for visible light absorption (Ebg ≈ 2.4 eV) and conduction band edge for H2 evolution,1416

even though recombination of photogenerated electron–hole pairs is fast.13 Another very promising visible-light active photocatalyst is graphitic carbon nitride (g-

C3N4), a metal free, two-dimensional conjugated layered polymer. It is a good photocatalyst material due to its excellent stability, nontoxic nature, visible-light-absorbing band gap (Ebg ≈ 2.7 eV), band-edge positions suitable for water splitting, and facile synthesis from amply available precursors like urea and melamine.17-20 However, g-C3N4 suffers from rapid recombination of photoinduced electron–hole pair, poor visible-light absorption, and limited surface area.21-23 Efforts to decrease rates of recombination have included the development of heterogeneous systems by combining g-C3N4 with other visible-light active photocatalysts. These structures are designed to facilitate charge separation and slow charge recombination.24-26 In a heterostructure with different components absorbing in different regions can result in an enhanced light harvesting. Thus light harvesting ability can be promoted in a suitable hierarchical heterostructure in which the semiconductor has an appropriate band gap and band alignment.27-29


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Several works have reported synthesis and characterisation

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of the binary composite, g-

C3N4/CdxZn1-xS, where CdxZn1-xS is the electron acceptor to facilitate charge separation and slow charge recombination.30-32 The efficiency of a photocatalyst can be enhanced by integration of a cocatalyst to facilitate charge separation and catalysis.33-37 The photocatalytic efficiency of CdxZn1-xS has been shown to increase with loading of cocatalyts like Pt,38 Au,39 bimetallic AuPd,40 Pt-RuS2,41 and CoPt342. Molybdenum disulfide (MoS2) is a two-dimensional layered material widely used as a hydrogen evolution cocatalyst,35,43,44 including being integrated with CdxZn1-xS.45-47 The present work integrates Cd0.5Zn0.5S-g-C3N4 with MoS2 to form a ternary composite that exhibits better catalytic activity compared to the binary composite Cd0.5Zn0.5S-g-C3N4 and single component, Cd0.5Zn0.5S. We report the synthesis, characterisation, photocatalytic hydrogen evolution efficiency and dye degradation ability of Cd0.5Zn0.5S, its composite with gC3N4 (Cd0.5Zn0.5S-g-C3N4), and a ternary composite with MoS2 (Cd0.5Zn0.5S-g-C3N4-MoS2). The composites were prepared by ultrasonic mixing of specific weight percentage of each component. During the ultrasonic treatment, the sheets of MoS2 and g-C3N4 are expected to exfoliate and integrate onto Cd0.5Zn0.5S nanorods. As prepared, Cd0.5Zn0.5S-g-C3N4-MoS2 shows an enhanced photocatalytic H2 activity compared to Cd0.5Zn0.5S-g-C3N4 and Cd0.5Zn0.5S. The enhanced efficiency is attributed to the superior interfacial contact between the components and synergistic transportation and separation of the photogenerated charge carriers, thereby slowing recombination, as well as increased rate of catalysis due to the MoS2 cocatalyst.

EXPERIMENTAL SECTION Materials


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Analytical grade reagents were used without further purification. Zinc acetate dihydrate (Merck), cadmium acetate dihydrate (Merck), thioacetamide (Spectrochem), ethylene diamine (Merck) ammonium heptamolybdate (Sigma Aldrich), hexamine (Merck), L-cysteine (Sigma Aldrich), melamine (Sigma Aldrich), ethylenediaminetetraacetic acid disodium salt dehydrate (Himedia), p-Benzoquinone (Sigma Aldrich), Isopropanol (Merck), Sodium Sulfate anhydrous (Merck), Nafion perfluorinated resin solution (5 wt% in lower aliphatic alcohols and water) (Sigma Aldrich), FTO coated glass substrates (Sigma Aldrich)and ethanol were used as received. Preparation of Cd0.5Zn0.5S, MoS2 and g-C3N4 Cd0.5Zn0.5S and MoS2 were synthesised by solvothermal and hydrothermal methods following previous protocols.48,

49

g-C3N4 was prepared by self-condensation of melamine at high

temperature.50 The details of the experimental procedure are included in the supporting information. Preparation of Cd0.5Zn0.5S -g-C3N4 and Cd0.5Zn0.5S -g-C3N4-MoS2 composites Cd0.5Zn0.5S-x%g-C3N4 (x=10, 20, 30, 40, 50) and Cd0.5Zn0.5S-g-C3N4-x%MoS2 (x= 1, 3, 5, 7) composites were prepared by ultrasonication of the components in specific weight percentage for 1 h in ethanol (5 mL) to form a dispersion followed by drying at 60 °C overnight. Use of ultrasonication is expected to facilitate formation of intimate contacts between the individual components. Characterisation The crystal structure of as-prepared samples were characterised by powder X-ray diffraction measurements using a Rigaku X-ray diffractometer (model TTRAX III) with (Cu-Kα) X-ray


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generator (λ=1.54 Å) at an operating voltage of 50 kV and current of 100 mA. The scan range was 10°-80° with a step size of 0.03°/sec. X-ray photoelectron spectroscopy was carried out on a Kratos AXIS Supra X-ray photoelectron spectrometer instrument with a monochromatized AlKα X ray source (hν = 1486.6 eV) at a residual gas pressure of 10−6 Pa. Survey and highresolution core-level spectra were collected at 160 eV and 20 eV pass energies, respectively. Morphological and compositional analysis of the photocatalysts were carried out at an operating voltage of 1 − 5 kV in a Zeiss Sigma field-emission scanning electron microscope and the operating voltage for energy dispersive x-ray spectroscopy analysis was 20 kV. The transmission electron microscopy measurements were carried out in a JEOL JEM 2100 microscope at an operating voltage of 200 kV. Ultraviolet–visible diffuse reflectance spectra were recorded on a Jasco V-650 spectrophotometer. Spectra of Rhodamine B solutions were measured on a Perkin Elmer Lamda 25 spectrophotometer. Photoluminescence spectra were recorded in Horiba Scientific Fluoromax-4 spectrophotometer. Time-resolved photoluminescence measurements were performed on a LifeSpec II Edinburgh instrument. Fourier-transform infrared spectra were recorded using a Perkin-Elmer instrument with KBr pellet. BET surface area analysis was carried out in a Beckman-Coulter SA 3100 nitrogen adsorption apparatus at liquid N2 temperature with prior degassing at 150 °C for 3h. Photocurrent-time response measurements were carried out on a CH Instruments Electrochemical Workstation (CHI 1120B). Photocatalytic hydrogen evolution The photocatalytic hydrogen evolution reaction from water was carried out at room temperature and atmospheric pressure in a two neck double walled borosilicate round-bottomed flask (100 mL) with a water circulation through the outer jacket. The necks of the reactor were sealed with rubber septa to prevent leakage of the gas produced. The photocatalytic reaction was initiated by


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irradiation from a 500 W tungsten–halogen lamp (Halonix, India) placed 15 cm away from the reactor. In order to carry out the experiment, photocatalyst (20 mg) was dispersed in water (50 mL) with Na2SO3 (0.1 mol L-1) and Na2S (0.1 mol L-1) as sacrificial reagents. The reactor was purged with N2 for 10 min at a flow rate of 0.1 L min-1 monitored by a KIT rotameter and subsequently the system was evacuated in order to liberate the dissolved oxygen and any other gases. This process was repeated twice before irradiating the system by the light source. During irradiation, the suspension was continuously stirred so that the catalyst is uniformly exposed to the light source. Aliquots of the gaseous headspace were collected every 15 min for a total of 1 h using a 1 mL gastight syringe, which was then analysed by gas chromatography (Agilent 7890A), equipped with a thermal conductivity detector (TCD) and molesieve column with N2 as the carrier gas. Blank reactions in the absence of photocatalyst or light were also carried out and in which case no H2 was detected, suggesting the role of the photocatalysts in hydrogen evolution. The apparent quantum yield (AQY) was measured under irradiation at 420 nm by using a band pass filter (Thorlabs) and was calculated using the following equation:27 AQY =

=

Number of reacted electrons x 100% Number of incident photons

2 x Number of hydrogen molecules evolved in 1 hour x 100% (1) Number of incident photons in 1 hour

Photocatalytic dye degradation experiment Rhodamine B dye degradation experiments were performed separately in a round-bottom flask (100 mL) by irradiating with a 500 W tungsten–halogen lamp (Halonix, India) light source from a distance of 15 cm from the reactor. During the photocatalytic degradation experiment, photocatalyst (50 mg) was dispersed in aqueous Rhodamine B solution (50 mL, 10-5 M). Before


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irradiation, the mixture was stirred in the dark for ~1 h to achieve adsorption–desorption equilibrium among the dye, photo-catalyst particles, dissolved oxygen, and atmospheric oxygen. Solution aliquots (2mL) were collected from the photo-reactor every 10 min and up to 1 h followed by recording their electronic absorption spectra in the range of 200 – 800 nm. The degradation of Rhodamine B was determined by monitoring the decrease in the absorbance at 552 nm. The photocatalytic degradation efficiency was calculated as follows, Efficiency(%) =

(C − C) × 100 (2) C

where C0 and C are the values of initial and final concentrations of rhodamine B respectively.51 To detect the active species in the photo-oxidation of rhodamine B, ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) (1mM), benzoquinone (BQ) (1mM), and isopropanol (IPA) (1mM) were introduced in the rhodamine B dye degradation experiments to quench holes (h+), superoxide radicals (•O2-), and hydroxyl radicals (•OH), respectively. The experiments were further carried out as the photocatalytic dye degradation experiment.52

RESULTS AND DISCUSSIONS Powder X-ray diffraction The powder X-ray diffraction (PXRD) patterns of g-C3N4, MoS2, CdS, ZnS, Cd0.5Zn0.5S, Cd0.5Zn0.5S-30%g-C3N4 and Cd0.5Zn0.5S-30%g-C3N4-5%MoS2 (represented as Cd0.5Zn0.5S-gC3N4 and Cd0.5Zn0.5S-g-C3N4-MoS2 hereafter) are shown in Fig. 1. PXRD of g-C3N4 exhibited a characteristic major peak at 27.3° corresponding to the (002) crystal plane due to the stacking of conjugated aromatic system, while the low intensity peak at 13.2° corresponded to the (001) plane, indicative of a periodic arrangement of condensed tri-s-triazine units in the g-C3N4


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sheets.17 In the PXRD pattern of MoS2, diffraction peaks appearing at 2θ = 16.7°, 33.4°, 43.5° and 57.4° correspond to (002), (100), (103), and (110) lattice planes of a hexagonal phase (JCPDS file No. 37-1592). The as-prepared MoS2 is weakly crystalline, as proved by the broad (002) diffraction peak which indicates poor stacking and highly disordered packing of MoS2 layers.53 Bare ZnS shows diffraction at 2θ = 28.5°, 33.08°, 47.5°, 56.2°, 69.5°, 76.8° corresponding to (111), (200), (220), (311), (400), (420) planes of cubic phase ZnS (JCPDS file No. 05-0566). PXRD of CdS shows diffraction peaks at 2θ = 24.8°, 26.5°, 28.1°, 36.6°, 43.6°, 47.8°, 50.8°, 51.8°, 52.7°, 66.7°, 70.8°, 72.3°, 75.4° which are indexable to (100), (002), (101), (102), (110), (103), (200), (112), (201), (203), (211), (114), (105) planes of hexagonal phase CdS (JCPDS file No. 41-1049). The PXRD pattern of Cd0.5Zn0.5S synthesised by a solvothermal method shows a shift in position of diffraction peaks compared to bare CdS. It can be inferred that the as-prepared sample is not just a physical mixture of CdS and ZnS but a solid solution of CdxZn1-xS.54 In the alloyed CdxZn1-xS, Zn2+ (ionic radius (0.74 Å)) gets incorporated into the CdS lattice or may sit at interstitial sites leading to disorder in the crystal structure.48 Moreover, the PXRD pattern of Cd0.5Zn0.5S and its composites exhibit lower crystallinity than bare CdS or ZnS which can be attributed to the disorder generated in the crystal structure as a result of incorporation of Zn2+ into the CdS lattice.55,56 In the composites Cd0.5Zn0.5S-g-C3N4 and Cd0.5Zn0.5S-g-C3N4-MoS2 there is no apparent peaks of g-C3N4 and MoS2, due to the low percentage of loading of the components. The single-layer structure of MoS2 might also be attributed for the absence of MoS2 diffraction peak in Cd0.5Zn0.5S-g-C3N4-MoS2.45 Moreover, the significant peaks of g-C3N4 and MoS2 overlap with that of Cd0.5Zn0.5S which supresses its occurrence in the PXRD measurements of these composites.


9


Cd0.5Zn0.5S-g-C3N4-MoS2

Cd0.5Zn0.5S-g-C3N4

(001)

10

20

(105)

(211) (114)

ZnS (420)

(203) (400) (110)

(103)

(311)

(220)

(200) (112) (201)

(103)

(110)

(102) (200)

(100)

CdS

MoS2

(002)

(002)

(111)

(100) (002)

(101)

Cd0.5Zn0.5S

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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g-C3N4

30

40

50

60

70

80

2θ (degree) Fig. 1 Powder X-ray diffraction (PXRD) pattern of Cd0.5Zn0.5S-g-C3N4-MoS2, Cd0.5Zn0.5S-gC3N4, Cd0.5Zn0.5S, bare CdS, ZnS, MoS2 and g-C3N4. X-Ray Photoelectron Spectroscopy X-ray photoelectron spectroscopy (XPS) analyses were carried out in order to probe the local electronic environment of the prepared samples. All the peaks were calibrated with respect to C 1s at 284.7 eV. From the high-resolution core-level spectra of Cd 3d, Zn 2p and S 2p of Cd0.5Zn0.5S (Fig. 2(a), 2(b) and 2(c) respectively), the peaks corresponding to Cd2+ 3d3/2, Cd2+ 3d5/2, Zn2+ 2p3/2, Zn2+ 2p1/2, S2- 2p3/2 and S2- 2p1/2 are observed at binding energies of 404.6 eV, 411.32 eV, 1020.98 eV, 1044 eV, 161.23 and 162.4 respectively.32,57 The asymmetric N 1s spectra of g-C3N4 has been fitted using the deconvolution method with three major Gaussian


10

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component peaks at 398.51 eV, 399.26 eV and 400.65 eV respectively (Fig. 2(e)). This provides strong evidence for the existence of the chemical bonding modes of nitrogen species corresponding to sp2-bonded N (C-N=C), tertiary nitrogen N-(C)3 groups, and amino groups (CN-H) respectively in the g-C3N4 sheets.58 The peaks at 162.1 eV and 163.6 eV in S 2p of MoS2, correspond to the S 2p3/2 and S 2p1/2 of divalent sulfide (S2-) of MoS2 (Fig. 2(d)).59 The Mo 3d spectra of MoS2 can be deconvoluted into four components corresponding to the binding energy of S 2s (226.4 eV), 3d5/2 and 3d3/2 of Mo (229.26 and 233.09 respectively) and Mo6+ (236.5 eV) (Fig. S2 (a)). The observance of Mo6+ may be due to atmospheric oxidation of MoS2 during storage in laboratory conditions.60 In Fig. 2(a), 2(b) and 2(c), it is seen that in the composite Cd0.5Zn0.5S-g-C3N4-MoS2 the Cd 3d, Zn 2p and S 2p are slightly shifted to higher binding energy (405.14 eV and 411.89 eV for Cd2+ 3d3/2, Cd2+ 3d5/2; 1021.86 eV and 1044.93 eV for Zn2+ 2p3/2, Zn2+ 2p1/2; 161.7 eV and 162.86 eV for S2- 2p3/2 and S2- 2p1/2 respectively) compared to that of Cd0.5Zn0.5S. Due to the favorable band alignments at the interfaces, there is a possibility of electronic interactions among g-C3N4, Cd0.5Zn0.5S and MoS2 leading to a change in the electron density on all the components in the composite. Based on the interfacial positions, electrons will transfer from Cd0.5Zn0.5S (donor) to MoS2 (acceptor) leading to a decreased electron density around the S 2p of Cd0.5Zn0.5S and enhanced density distribution around S 2p of MoS2 (Pathway II in Fig. 2(f)). Moreover, there is another possible charge-transfer pathway based on the band alignments where Cd0.5Zn0.5S can act as an acceptor of an electron from photo-excited g-C3N4 (Pathway I in Fig. 2(f)). On comparing the S 2p core level spectra of Cd0.5Zn0.5S-g-C3N4-MoS2 with that of bare Cd0.5Zn0.5S, it is observed that S 2p of the composite is shifted towards higher binding energy (Fig. 2(c)) due to the reduction of S 2p electron density in Cd0.5Zn0.5S-g-C3N4MoS2 as compared to Cd0.5Zn0.5S.61 This change in the binding energy of S 2p reflects that


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Pathway II is more competitive than Pathway I. Analysis of N 1s peak of g-C3N4 in Cd0.5Zn0.5Sg-C3N4-MoS2 also shows a shift toward higher binding energy (398.56 eV, 399.31 eV and 400.7 eV respectively) compared to that of bare g-C3N4 (Fig. 2(e)). The shift is based on the favourable charge transfer from g-C3N4 to the other components of the composite, i.e. Cd0.5Zn0.5S and MoS2 (Pathway I and Pathway III in Fig. 2(f) respectively), which leads to a reduction of the electron density of N 1s in the composite. The peaks of S 2s, Mo 3d5/2 and Mo 3d3/2 in Cd0.5Zn0.5S-gC3N4-MoS2 are shifted toward lower binding energy (223.05 eV, 224.65 eV and 227.65 eV respectively) in comparison to that of MoS2 (Fig. S2 (a) and (b) (SI)). Comparing the individual MoS2 component to that of the composite, the S 2p of bare MoS2 has a higher binding energy than the S 2p of Cd0.5Zn0.5S-g-C3N4-MoS2 (Fig. 2(d)). In the composite there is an increase in the electron density on the conduction band of MoS2 in the composite than that of bare MoS2 leading to the shift of S 2p to a lower binding energy in the composite than MoS2 (Fig. 2(d)). The shift in binding energy between the composite and bare components is likely due to charge transfer from Cd0.5Zn0.5S and g-C3N4 to MoS2 (Pathway II and Pathway III in Fig. 2(f) respectively). The survey spectrum of Cd0.5Zn0.5S-g-C3N4-MoS2 is shown in Fig. S3.


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Fig. 2 X-Ray photoelectron spectroscopy (XPS) spectra of (a) Cd 3d, (b) Zn 2p, (c, d) S 2p, (e) N 1s, (f) Schematic representation of the different possible charge transfer pathways in the composite Cd0.5Zn0.5S-g-C3N4-MoS2.


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UV-Visible diffuse reflectance spectroscopy Fig. 3(a) represents the diffuse-reflectance ultraviolet–visible absorption spectra of Cd0.5Zn0.5S, g-C3N4, MoS2, ZnS, CdS and the composites Cd0.5Zn0.5S-g-C3N4 and Cd0.5Zn0.5S-g-C3N4-MoS2. Bare ZnS absorbs in the ultraviolet spectral region (absorbance onset at ~350 nm) and CdS has an absorbance onset in the visible region at ~590 nm as observed from Fig. 3(a). As expected, the absorption onset of Cd0.5Zn0.5S occurs between its constituents at ~520 nm. Fig. 3(b) depicts the Tauc plots of Cd0.5Zn0.5S-g-C3N4-MoS2, Cd0.5Zn0.5S-g-C3N4, Cd0.5Zn0.5S, ZnS, CdS, g-C3N4 and MoS2 (inset of Fig. 3(b)), which enables determination of the bandgap energy. Tauc plots show (αhν)2 versus the photon energy (hν), where α is the absorption. The bandgap values of Cd0.5Zn0.5S-g-C3N4-MoS2, Cd0.5Zn0.5S-g-C3N4 and Cd0.5Zn0.5S are estimated to be 2.41 eV, 2.38 eV and 2.35 eV respectively which is intermediate of ZnS (3.6 eV) and CdS (2.25 eV). The band gap of g-C3N4 and MoS2 are estimated to be 2.7 eV and 1.46 eV respectively. The estimated band gaps are tabulated in Table 1. Electronic absorption spectra of all compositions Cd0.5Zn0.5Sx%g-C3N4 (x=10, 20, 30, 40, 50) and Cd0.5Zn0.5S-g-C3N4-x%MoS2 (x=1, 3, 5, 7) are shown in Fig. S4 and S5 respectively. From Fig. S4 and S5, it is observed that the absorption onset of the compounds are in the visible spectral region.


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1.0

Absorbance (a.u.)

(a)

g-C3N4 MoS2 Cd0.5Zn0.5S-g-C3N4 Cd0.5Zn0.5S

0.8

Cd0.5Zn0.5S-g-C3N4-MoS2 CdS ZnS

0.6

0.4

0.2

0.0 300

400

500

600

800

700

Wavelength (nm)

200

(b)

Cd0.5Zn0.5S-g-C3N4-MoS2 Cd0.5Zn0.5S-g-C3N4 Cd0.5Zn0.5S

150

g-C3N4 CdS

(α hν)^2

ZnS

100

160

MoS2

140

50

120

(α hν )^2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 80 60 40 20

0

0

2.0

2.5

3.0

3.5

4.0

1.6

2

hν (eV)

4.5

2.4

2.8

5.0

hν (eV) Fig. 3 (a) Diffuse reflectance electronic absorption spectra of Cd0.5Zn0.5S-g-C3N4-MoS2, Cd0.5Zn0.5S-g-C3N4, Cd0.5Zn0.5S, g-C3N4, MoS2, CdS and ZnS (b) Tauc plot to estimate optical band gap values.


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Table 1 Estimated band gap values Compound

Band gap (eV)

CdS

2.25

ZnS

3.6

g-C3N4

2.7

MoS2

1.46

Cd0.5Zn0.5S

2.35

Cd0.5Zn0.5S-g-C3N4

2.38


2.41

Field Emission Scanning Electron Microscopy Fig. 4 and Fig. 5 show Field-Emission Scanning Electron Microscopy (FESEM) images of the as-prepared materials. Cd0.5Zn0.5S exhibits nanorod morphology with an average length of around 100 nm (Fig. 4(a) and 4(b)). The ethylenediamine used as solvent in the synthesis of Cd0.5Zn0.5S, is also a good bidentate coordinating agent with Cd2+ and Zn2+ to form [Cd(EDA)n]2+ and [Zn(EDA)n]2+ complexes.62,

63

As the temperature increases, thioacetamide,

used as sulphur source in Cd0.5Zn0.5S, decomposes gradually to release S2-. Finally, [Cd(EDA)n]2+ and [Zn(EDA)n]2+ complexes react with S2- and yield a CdxZn1-xS solid solution. Zn2+ strongly coordinates with EDA and therefore, with only Zn2+ (x = 0), ZnS-(EDA)0.5 forms instead of ZnS.51 The use of ethylene diamine as a solvent promotes the unidirectional growth of CdS along c-axis while it promotes the growth of ZnS in the a-b plane.48 It is observed that for x=0.5 in CdxZn1-xS, the morphology is predominantly rod shaped though with a lower aspect ratio. FESEM images of g-C3N4 reveal its flake-like and porous nature (Fig. 4(c) and 4(d)). MoS2


16

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consists of nanosheets as porous bunched structures that are ~500 nm in diameter (Fig. 4(e) and 4(f)). MoS2 has a large exposed surface area that can provide greater active area for H2 evolution electrocatalysis. This includes a larger fraction of edge sites which are indicated as active sites for H2 evolution.64


17


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Fig. 4 Field-Emission Scanning Electron Microscopy (FESEM) images of (a, b) Cd0.5Zn0.5S, (c, d) g-C3N4, (e, f) MoS2 at different magnifications.

The FESEM images of Cd0.5Zn0.5S-g-C3N4 composites are shown in Fig. 5. The images show the close contact among the components Cd0.5Zn0.5S and g-C3N4 (Fig. 5(a) and 5(b)) and those among components as well as MoS2 (Fig. 5(c) and 5(d)). It can be seen that the three components are present in a very close proximity, blending with each other thus this close contact should better assist charge migration. Energy dispersive X-ray analysis of the distribution of the components in the composite Cd0.5Zn0.5S-g-C3N4-MoS2, indicate that all the elements are distributed homogeneously throughout the material in the scan area (Fig. 6).


18

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Fig. 5 Field Emission Scanning Electron Microscopy (FESEM) images of (a, b) Cd0.5Zn0.5S-gC3N4, (c, d) Cd0.5Zn0.5S-g-C3N4-MoS2 at different magnifications.


19


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Fig. 6 Energy-dispersive X-ray (EDX) mapping of Cd0.5Zn0.5S-g-C3N4-MoS2 (g). (a), (b), (c), (d), (e) and (f) shows the homogenous elemental distribution of Cd, S, Zn, N, C and Mo respectively in the area (g). Transmission Electron Microscopy Particle size and microstructural analysis of Cd0.5Zn0.5S-g-C3N4-MoS2 was performed using Transmission Electron Microscopy (TEM) and High Resolution Transmission Electron Microscopy (HRTEM). Consistent with FESEM images, Cd0.5Zn0.5S was observed to be nanorod shaped with a particle size of about 100 nm (Fig. 7(a)). g-C3N4 is visible as a two-dimensional nanosheet structure around the clusters of Cd0.5Zn0.5S nanorods and MoS2 clusters (Fig. 7(a)). All components are well dispersed (Fig. 7(b)). Fig. 7(c) is the HRTEM image showing the lattice


20

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fringes of Cd0.5Zn0.5S with a d-spacing of ~0.32 nm which is assignable to the (101) lattice plane of hexagonal Cd0.5Zn0.5S.65 The lattice fringes with a d-spacing of ~0.62 nm corresponds to the (002) plane of hexagonal MoS2.33 Thus, Fig. 7(c) reveals the intimate contact between Cd0.5Zn0.5S and MoS2, which is expected to facilitate improved charge separation, thus resulting in enhanced photocatalytic activity. The EDX spectrum (Fig. 7(d)) of the composite proves the existence of Cd, Zn, S, Mo, C and N elements (The Cu peak originates from the substrate).


21


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Fig. 7 (a) Transmission electron microscopy (TEM) image of Cd0.5Zn0.5S-g-C3N4-MoS2, (b) Transmission electron microscopy (TEM) image of a part of Cd0.5Zn0.5S-g-C3N4-MoS2 with magnified image of the highlighted portion in the inset of (b), (c) High Resolution Transmission electron microscopy (HRTEM) image showing the lattice fringes of Cd0.5Zn0.5S and MoS2 (d) Energy-dispersive X-ray (EDX) spectrum.


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Steady State Photoluminescence study To gain insight into the excited-state electronic interaction among the components in the photocatalyst system, steady-state photoluminescence (PL) measurement was carried out. From the PL spectra in Fig. 8, it is observed that, on excitation at 380 nm, g-C3N4, Cd0.5Zn0.5S-g-C3N4, Cd0.5Zn0.5S-g-C3N4-MoS2 and Cd0.5Zn0.5S emits at ~470 nm, ~510 nm, ~504 nm, and ~515 nm respectively. Pure g-C3N4 has a very intense PL emission band centred at ~470 nm which is credited to the transition from the lone pair on the N atoms in g-C3N4 to its π* conduction band.58 On introduction of Cd0.5Zn0.5S as Cd0.5Zn0.5S-g-C3N4, the PL emission intensity at 470 nm decreases likely due to charge transfer from g-C3N4 to Cd0.5Zn0.5S. PL from the composite is also observed at ~515 nm, and is consistent with emission from Cd0.5Zn0.5S. On addition of MoS2 (Cd0.5Zn0.5S-g-C3N4-MoS2), there is further quenching in the PL intensity compared to Cd0.5Zn0.5S-g-C3N4. Electrons from CB of Cd0.5Zn0.5S and g-C3N4 might exhibit a faster migration to the MoS2 sheets due to the suitable CB positions. It leads to a reduction of electron density at both Cd0.5Zn0.5S and g-C3N4 sites which minimizes the chances of recombination. A reasonable mechanism for photogenerated charge transfer processes that take place in Cd0.5Zn0.5S-g-C3N4-MoS2 is schematically shown in the inset of Fig. 8.


23


Page 24 of 49

Fig. 8 Steady-state photoluminescence (PL) spectra of Cd0.5Zn0.5S, Cd0.5Zn0.5S-g-C3N4–MoS2, Cd0.5Zn0.5S-g-C3N4 and g-C3N4 at an excitation wavelength of 380 nm. Time Resolved Photoluminescence study To further show the synergistic effects of electron transfer between the materials in the composite, time-resolved photoluminescence (TRPL) spectroscopy was performed at an excitation wavelength of 375 nm and are shown in Fig. 9. The TRPL decay patterns monitored at 470 nm are fitted with a tri-exponential function, which implies that multiple processes are responsible for the emission. The average lifetime, ⟨τ⟩, was calculated using equation (3).66


24

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+ + (3) < >= + + Table 2 summarizes the detailed spectroscopic results for g-C3N4, Cd0.5Zn0.5S-g-C3N4, and Cd0.5Zn0.5S-g-C3N4-MoS2. From Table 2, it is observed that all the decay curves are well fitted with three components as assessed by the near unity value of χ2. The shorter decay time is associated with radiative recombination of electron hole pairs while the higher value of decay time attributes to trap state emission. In all the three compounds, the population with short decay time (less than 4 ns) is dominant (~85%), which implies rapid electron hole pair recombination.67 However, it is observed that the lifetime of all the components (τ1, τ2, τ3) decrease gradually from g-C3N4 to Cd0.5Zn0.5S-g-C3N4 and Cd0.5Zn0.5S-g-C3N4-MoS2. It is well known that a decrease in the lifetime (τ) value indicates more efficient separation of electrons and holes.68-71In order to be an efficient reduction catalyst, the lifetime of the photocatalyst is expected to be shorter in order to facilitate efficient charge transfer from the photocatalyst to cocatalyst surface.72 As materials are added to the composite, from the average lifetime value ⟨τ⟩, calculated using equation (3), it is observed that the average lifetime is much shorter in Cd0.5Zn0.5S-g-C3N4-MoS2 (9.17 ns) in comparison to Cd0.5Zn0.5S-g-C3N4 (10.54 ns) and g-C3N4 (12.35 ns) which indicates an excited state electronic interaction and efficient charge separation or charge delocalisation in the composite based on the bandgap energies and band alignments as shown in the inset to Fig. 8.


25


IRF g-C3N4 Fitted g-C3N4 Cd0.5Zn0.5S-g-C3N4 Fitted Cd0.5Zn0.5S-g-C3N4

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 49

Cd0.5Zn0.5S-g-C3N4-MoS2 Fitted Cd0.5Zn0.5S-g-C3N4-MoS2

4

5

6

7

8

9

10

11

12

Time (ns) Fig. 9 Time-resolved photoluminescence decay curves for Cd0.5Zn0.5S-g-C3N4-MoS2, Cd0.5Zn0.5S-g-C3N4 and g-C3N4 (excitation wavelength = 375 nm and emission wavelength = 470 nm). Table 2. Relative Percentage (α1, α2, α3), Excited-State Lifetime (τ1, τ2, τ3), Average Exciton Lifetime ⟨τ⟩ and Fitting Parameter (χ2), for Cd0.5Zn0.5S-g-C3N4-MoS2, Cd0.5Zn0.5S-g-C3N4 and gC3N4.


26

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α1

α2

α3

τ 1 (ns)

τ 2 (ns)

τ 3(ns)

g-C3N4

44.940

38.722

16.338

1.155

3.936

18.514

12.35

1.219

Cd0.5Zn0.5S-

47.351

37.645

15.003

0.995

3.499

16.215

10.54

1.264

57.355

29.004

13.641

0.929

3.217

14.288

9.17

1.238

⟨τ⟩ (ns)

χ2

g-C3N4 Cd0.5Zn0.5Sg-C3N4MoS2

BET surface area analysis BET surface area analysis of Cd0.5Zn0.5S-g-C3N4-MoS2 and Cd0.5Zn0.5S are shown in Fig. 10. Cd0.5Zn0.5S-g-C3N4-MoS2 and Cd0.5Zn0.5S show H3 hysteresis loops and exhibit type IV isotherms.

Although Type IV isotherms are typical for mesoporous materials, however, a

distinct increase in adsorbate volume in the low P/P0 region in type IV isotherms indicates the presence of micropores associated with mesopores.73 As an inflection is observed in the isotherm of Cd0.5Zn0.5S-g-C3N4-MoS2 and Cd0.5Zn0.5S in logarithmic scale as shown in Figure S7, it might infer the presence of both meso and micropores in the materials. Barrett–Joyner–Halenda (BJH) pore size distribution indicate the presence of micro to meso pores, with a higher distribution at ~0.6 nm and ~1.6 nm for Cd0.5Zn0.5S-g-C3N4-MoS2 and Cd0.5Zn0.5S respectively (inset to Fig. 10). The observed BET surface area of Cd0.5Zn0.5S-g-C3N4-MoS2 is ~16.7 m²/g which is lower than bare Cd0.5Zn0.5S (~19.1 m²/g). The decrease of the surface area after incorporation of gC3N4 and MoS2 is consistent with loss of the small pores in Cd0.5Zn0.5S as observed in the pore size distribution curve.


27


Volume adsorbed (cm3/g) at STP

100

100

Cd Zn S-g-C N -MoS 0.5 0.5 3 4 2 80

80

Cd Zn S 0.5 0.5 60

60 40

40

20

20 0

Pore Volume (cm3g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 49

0 2

4

6

8

10

Pore Diameter (nm) 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (Ps/P0)

Fig. 10 Nitrogen adsorption–desorption isotherms and Barrett–Joyner–Halenda (BJH) pore size distribution plots (inset) for Cd0.5Zn0.5S-g-C3N4-MoS2 and Cd0.5Zn0.5S. Hydrogen production and dye degradation analysis In order to evaluate the photocatalytic activity of the composite system, hydrogen evolution was estimated in an aqueous solution containing sodium sulphite and sodium sulphide as sacrificial reagent for Cd0.5Zn0.5S-g-C3N4-MoS2, Cd0.5Zn0.5S-g-C3N4, and Cd0.5Zn0.5S (Fig. 11). Photocatalytic H2 evolution by a metal sulphide semiconductor in the presence of sulphide and sulphite anions can be represented by equations 4-9.74


28

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Photocatalyst + light → e-CB +h+VB

(4)

H2O + 2 e-CB → H2 + 2OH-

(5)

SO32- + H2O + 2h+VB → SO42- + 2H+

(6)

2S2- + 2h+VB → S22-

(7)

S22- + SO32- → S2O32- + S2-

(8)

SO32- + S2- + 2h+VB → S2O32-

(9)

The photoexcited electrons in the conduction band carries out the reduction of water to H2. While the photogenerated holes in the valence band are consumed by SO32- and S2- and get converted into SO42- and S22- respectively. The production of excess S22- is supressed by its reaction with SO32- to form S2O32-. Although an additional component, S2O32-, is formed in the reaction chamber, it does not compete for light absorption with the photocatalyst because it is colourless. The risk of formation of sulphur defects in Cd0.5Zn0.5S is suppressed due to the presence of excess S2- in the solution, which stabilizes the photocatalyst.52 To evaluate the H2 production efficacy of the binary composite Cd0.5Zn0.5S-g-C3N4 in comparison to other binary combinations, we performed control experiment with Cd0.5Zn0.5S-5%MoS2 and 30%g-C3N45%MoS2 (both prepared by ultrasonication of the components in definite weight percentage and represented as Cd0.5Zn0.5S-MoS2, and g-C3N4-MoS2 respectively), and compared with Cd0.5Zn0.5S, Cd0.5Zn0.5S-g-C3N4 and Cd0.5Zn0.5S-g-C3N4-MoS2 (shown in Fig. S8). It is seen that the binary composites Cd0.5Zn0.5S-MoS2 and g-C3N4-MoS2 produce hydrogen at a rate of ~49 µmol/h/0.02g and ~20 µmol/h/0.02g respectively which is lesser than Cd0.5Zn0.5S-g-C3N4 (~68 µmol/h/0.02g). The ternary composite Cd0.5Zn0.5S-g-C3N4-MoS2 resulted in hydrogen production from water at a rate of ~91 µmol/h/0.02g with an AQY of 38% at 420 nm. The observed rate of hydrogen production of Cd0.5Zn0.5S-g-C3N4 and Cd0.5Zn0.5S are ~68 µmol/h/0.02g and ~30


29


Page 30 of 49

µmol/h/0.02g respectively (Fig. 11). Thus, the ternary system (Cd0.5Zn0.5S-g-C3N4-MoS2) exhibited a significantly improved photocatalytic hydrogen production activity compared to Cd0.5Zn0.5S-g-C3N4 and Cd0.5Zn0.5S. The observed trend in the rate of hydrogen production can be understood by the enhanced charge separation in Cd0.5Zn0.5S-g-C3N4-MoS2 owing to the electron transfers from g-C3N4 and Cd0.5Zn0.5S to the H2 evolution reaction sites of MoS2 by virtue of its favorable band alignments.

Fig. 11 Rate of photocatalytic hydrogen evolution from Cd0.5Zn0.5S, Cd0.5Zn0.5S-g-C3N4, and Cd0.5Zn0.5S-g-C3N4-MoS2. The durability of the best performing catalyst, Cd0.5Zn0.5S-g-C3N4-MoS2, is tested for three consecutive cycles, (details in supporting information, Fig S9). The amount of hydrogen generated gradually decreased with consecutive cycle which might be due to the photocorrosion of the sulphides present in our system. The stability of Cd0.5Zn0.5S, Cd0.5Zn0.5S-g-C3N4 and


30

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Cd0.5Zn0.5S-g-C3N4-MoS2 after 2 hr of irradiation is shown by PXRD (Fig. S10). The PXRD of the catalysts recovered after one and two photocatalytic cycles (1 and 2 hours of irradiation respectively) were found to be similar to the freshly synthesised catalysts which proved their stability. The suitable band positions and possible charge transfer pathways between the components in the Cd0.5Zn0.5S-g-C3N4-MoS2 composite is schematically shown in Scheme 1. The calculation of conduction band of the individual components by cyclic voltammetry is shown in Fig. S11. An overview of the present scenario of some CdxZn1-xS based systems or their binary composite with g-C3N4 or different cocatalysts in terms of the rate of hydrogen production is presented in Table S1. In order to further confirm the separation efficiency, photocurrent-time measurements were carried out and are shown in Figure S12. From the photoresponse curves it is seen that Cd0.5Zn0.5S-g-C3N4-MoS2 exhibits a higher transient photocurrent density than Cd0.5Zn0.5S-g-C3N4 and Cd0.5Zn0.5S, which indicates that the separation efficiency of photogenerated charge carriers is significantly improved at the semiconductor electrolyte interface, thus extending the lifetime of the electron–hole pairs effectively in Cd0.5Zn0.5S-g-C3N4-MoS2.


31


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Scheme 1 Band alignment and probable charge-transfer pathways among the components in Cd0.5Zn0.5S-g-C3N4-MoS2 composite. The as-synthesised composite also proved effective for degradation of organic pollutants frequently found in industrial waste water. We chose Rhodamine B as the model system to study degradation kinetics. Normalized concentration (C/C0) over time is shown in Fig. 12 (a) for Cd0.5Zn0.5S-g-C3N4-MoS2, Cd0.5Zn0.5S-g-C3N4, and Cd0.5Zn0.5S, where it is clear that Rhodamine B is degraded much faster by Cd0.5Zn0.5S-g-C3N4-MoS2 compared to Cd0.5Zn0.5S-g-C3N4 and Cd0.5Zn0.5S. By 30 min, the percent degradation is ~33%, ~65%, and ~86% for Cd0.5Zn0.5S, Cd0.5Zn0.5S-g-C3N4, and Cd0.5Zn0.5S-g-C3N4-MoS2 respectively. Active species trapping experiments were conducted in order to determine the primary active species involved in the degradation process. As seen in Figure 12 (b), a significant decrease in the rate was observed after the addition of BQ (scavenger of •O2-), demonstrating that •O2- is the main active species involved in the RhB photooxidation process. However, IPA (scavenger of •OH) and EDTA-2Na


32

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(scavenger of holes) had almost no effect on RhB degradation as the rate of degradation is not much varied with the introduction of these scavengers which proves that •OH and h+ do not have a significant contribution in the photodegradation process. First, control experiments for Rhodamine B degradation were carried out using each binary composition, Cd0.5Zn0.5S-g-C3N4, Cd0.5Zn0.5S-MoS2, and g-C3N4-MoS2 (Fig. S13), where Cd0.5Zn0.5S-g-C3N4 was found to be most effective. A degradation study for all composites with different weight percentage of g-C3N4 and MoS2, Cd0.5Zn0.5S-x%g-C3N4 (x=10, 20, 30, 40, 50) and Cd0.5Zn0.5S-30%g-C3N4-x%MoS2 (x=1, 3, 5, 7) are shown in the Supporting Information (Fig. S14 and S15). From Fig. S14, it is observed that Cd0.5Zn0.5S-30%g-C3N4 exhibits the maximum degradation rate among the Cd0.5Zn0.5S-x%g-C3N4 (x= 10, 20, 30, 40, 50) composites. Thus, Cd0.5Zn0.5S-30%g-C3N4-x%MoS2 composites were further studied. The degradation efficiencies of Cd0.5Zn0.5S-g-C3N4-x%MoS2 (x=1, 3, 5, 7) are shown in Fig. S15, where Cd0.5Zn0.5S-g-C3N4-5%MoS2 shows the best performance in degradation of Rhodamine B. In order to minimize the possibility of manual and experimental error at various steps, the dye degradation efficiency (%) of all the compositions along with the bare counterpart are calculated by carrying out duplicate set and considered the average efficiency (%). The results are tabulated in Table S2. A study of dye degradation ability of different CdxZn1-xS-based systems along with our materials, are presented in Table S3.


33


(a)

1.0

(b) 1.0

0.8

0.8

0.6

Cd0.5Zn0.5S-g-C3N4-MoS2 Isopropanol Benzoquinone EDTA-2Na

0.6

0.4

C/C0

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 49

Cd0.5Zn0.5S

0.4

Cd0.5Zn0.5S-g-C3N4 0.2


0.2

0.0 -60

-40

-20

0

20

40

60

0.0 0

Time (min)

10

20

30

40

50

60

Time (min)

Fig. 12 (a) Plot of C/C0 as a function of time (min) for Cd0.5Zn0.5S, Cd0.5Zn0.5S-g-C3N4, and Cd0.5Zn0.5S-g-C3N4-MoS2., (b) Rate of degradation of Rhodamine B in the presence of different reactive species scavengers

CONCLUSION We synthesized Cd0.5Zn0.5S, g-C3N4, and MoS2 by straightforward synthetic procedures. By ultrasonicating these three components, we developed a noble-metal-free ternary composite Cd0.5Zn0.5S-g-C3N4-MoS2. The photocatalytic efficiency of the ternary composite was studied by monitoring H2 and dye degradation during illumination. Addition of g-C3N4 and both g-C3N4 and MoS2 increase the rate of H2 production (~68 µmol/h/0.02g and ~91 µmol/h/0.02g respectively) compared to bare Cd0.5Zn0.5S (~31 µmol/h/0.02g). The significant improvement of photocatalytic properties in the ternary composite can mainly be credited to the enhanced interfacial charge transfer of electrons in the composite system leading to a decrease in the photoinduced charge recombination and hence an increased number of electrons available for reaction at the active


34

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sites. This conclusion was supported by extensive experimental techniques like electron microscopy images, photoluminescence data, XPS analysis etc.

ACKNOWLEDGMENT The authors acknowledge Department of Science and Technology for project no. SERB/EMR/2016/005123. Indian Institute of Technology (IIT) Guwahati, Central Instruments Facility, IIT Guwahati are acknowledged for the instrumentation facility. SAIF, NEHU is acknowledged for the TEM facility. XPS studies were performed at the UC Irvine Materials Research Institute (IMRI) using instrumentation funded in part by the National Science Foundation Major Research Instrumentation Program under grant no. CHE-1338173. MQ acknowledges his Fulbright fellowship grant: 2205/F-NAPE/2016 to visit University of California Irvine with Professor Shane Ardo. The authors declare no conflict of interest. Supporting Information. Preparation of Cd0.5Zn0.5S, MoS2 and g-C3N4, Powder X-ray diffraction (PXRD) pattern of ZnS.(EDA)0.5, XPS Survey Spectra of Cd0.5Zn0.5S-g-C3N4- MoS2, XPS of Mo 3d,UV-Visible absorption profile of the samples Cd0.5Zn0.5S- x%g-C3N4 (x=10, 20, 30, 40, 50) and Cd0.5Zn0.5S-g-C3N4-x% MoS2 (x=1, 3, 5, 7), Raman spectral analysis, isotherm in logarithmic scale, rate of photocatalytic hydrogen evolution of Cd0.5Zn0.5S-MoS2 and g-C3N4MoS2, reusability experiment, PXRD of (A) Cd0.5Zn0.5S, (B) Cd0.5Zn0.5S-g-C3N4 and (C) Cd0.5Zn0.5S-g-C3N4-MoS2 before irradiation and recorded after 1 and 2 hours of irradiation, Determination of CB positions of the components by Cyclic voltammetry scan, photocurrenttime measurement, C/C0 versus time (min) plots, average efficiency (%) of photocatalytic dye degradation of all the composites a study of hydrogen evolution and dye degradation ability of


35


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different CdxZn1-xS based systems along with our system, are given in the ESI. “This material is available free of charge via the Internet at http://pubs.acs.org.”

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Table of Content


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A ternary composite, Cd0.5Zn0.5S-g-C3N4-MoS2 shows enhanced photocatalytic activity owing to efficient charge separation in the composite.


49

Hybrid of g-C3N4 and MoS2 Integrated onto Cd0.5Zn0.5S: Rational

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