Enhanced Photocatalytic Activities of RhB Degradation and H2

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Enhanced Photocatalytic Activities of RhB Degradation and H2 Evolution from in Situ Formation of the Electrostatic Heterostructure MoS2/NiFe LDH Nanocomposite through the Z‑Scheme Mechanism via p−n Heterojunctions Susanginee Nayak, Gayatri Swain, and Kulamani Parida* Centre for Nano Science and Nano Technology, Siksha ‘O’ Anusandhan Deemed to be University, Bhubaneswar 751030, Odisha, India

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ABSTRACT: Designing of an efficient heterostructure photocatalyst for photocatalytic organic pollutant removal and H2 production has been a subject of rigorous research intended to solve the related environmental aggravation and enormous energy crises. Z-scheme-based charge-transfer dynamics in a p−n heterostructure could significantly replicate the inherent power of natural photosynthesis, which is the key point to affect the transportation of photoinduced exciton pairs. In this finding, a series of p-type MoS2 loaded with n-type NiFe-layered double hydroxide (LDH) forming a heterostructure MoS2/NiFe LDH were designed by electrostatic selfassembled chemistry and an in situ hydrothermal strategy for photocatalytic rhodamine B (RhB) dye degradation and H2 production. The creation of p−n heterojunctions of type-II and Z-scheme mode of charge transfer modified the optical and electronic property of the as-synthesized MSLDH3, thereafter promoting the generation, separation, and migration of photoinduced electron−hole pairs. The as-synthesized MSLDH3 showed superior photocatalytic activities in degradation of RhB with H2 evolution, which was enhanced by 3- and 4.5-fold and 10.9 and 19.2 times higher than that of NiFe LDH and MoS2, respectively. Last but not the least, heterostructure MSLDH3 possesses practical stability for its resultant enhanced photocatalytic activity with recyclability for everyday life. KEYWORDS: H2 evolution, water splitting, layered double hydroxide nanosheets, metal sulfide nanosheets, nanocomposite, p−n heterojunctions, Z-scheme charge separations

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INTRODUCTION Amid the numerous heterojunction photocatalysts, p−n heterostructures with a band alignment of staggered type-II have been portrayed as the hot topic of research interest because of their effective charge carrier separation at their interfacial region.1,2 There are primarily two kinds of charge carrier separation path around the contact area of type-II p−n heterostructures. The first type is a double-charge-transfer path in which the photogenerated electrons of higher −ve conduction band (CB) potential of one semiconductor transfer to more positive CB potential of another semiconductor, while hole transfer takes place in an opposite direction from more positive valence band (VB) potential to more negative VB potential. Regretfully, this kind of charge separation has some disadvantages which are adverse to many photocatalytic redox processes because reaction sites are specifically at lower oxidation and reduction potential. In contrast to the Zscheme-type charge separation mode, the charge transfer and separation path smoothen the reductive electrons and weaker oxidative holes in a new preferential direction.3 Previous study reported many Z-scheme photocatalysts for different types of © XXXX American Chemical Society

photocatalytic reactions such as water-splitting reactions and organic pollutant degradation.4−7 However, Z-scheme photocatalytic systems consisting of a layered-to-layered structure assembly have been scarcely developed. In addition, the visiblelight-responsive constituents in Z-scheme systems are limited to the metal complex that could be easily oxidized, which possibly leads to a decrease in material efficacy and stability. Therefore, it is urged to design highly efficient type-II electrostatic p−n heterostructures via a Z-scheme chargetransfer mechanism for stable materials toward photocatalytic efficiency.8−11 Bimetallic layered double hydroxides (LDHs) appeared to be stable and effective catalysts for photocatalytic or photoelectrochemical reaction studies such as in NiCo2S4/NiFe LDH,12 NiCo2O4/NiFe LDH,13 N-deficient porous g-C3N4 nanosheets/N-doped graphene/NiFe LDH,14 ZnCr LDH/ GO,15 NiTi LDH/reduced graphene oxide (RGO),16 CeO2/ Received: April 14, 2019 Accepted: April 29, 2019

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DOI: 10.1021/acsami.9b06511 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces MgAl LDH,17 and MgCr LDH to MgO/MgCr2O4.18 In particular, NiFe LDH is a visible-light-driven n-type semiconductor (2.2 eV) that could absorb light and cover up to 800 nm. These NiFe LDH materials have fascinated much attention around the scientific community owing to nontoxic nature, resource abundance, cost-effectiveness, effectual redox reaction action, and environment-friendliness, followed by a reasonable narrow band gap energy for significant harvestation of visible light out of the solar spectrum.19−23 The narrow band gap coupling with its VB and CB edge (VB = +2.19 and CB = −0.01 eV) makes it an excellent system for photocatalytic redox reactions.19 As mentioned above, the various works reported using NiFe LDH as the photocatalyst including our previous reports include g-C3N4/NiFe LDH,19 Ag@Ag3PO4/gC3N4/NiFe LDH,20 AgCl and BiOCl/NiFe LDH,21 sulfurated NiFe LDH to NiFeS NP composites with CdTe/CdS,22 and RGO/La2Ti2O7/NiFe-LDH23 for various photocatalytic reactions. Though there are many positive aspects of NiFe LDH being a photocatalyst, at this point, the major drawback lies within the low conductance through the electrical chargetransfer process and agglomeration of their delaminated nanosheets. These electrical charge efficiency issues certainly leave question marks on the stability and catalytic performances of NiFe LDHs. In contrast, molybdenum disulfide (MoS2) is an intrinsically p-type semiconductor containing a sandwich-like stacked structure of S−Mo−S atomic layers bonded by van der Waals force of attraction. The indirect and direct band gaps of MoS2 lie within 1.2−1.9 eV, which have been used for photocatalysis.24−26 This MoS2 is also regarded as a significant member of transition-metal dichalcogenide family, which is similar to the layered-structured material graphene whose Mo layer is sandwiched between two sulfur layers through the covalent force of attraction. However, it is conceived from the literature report that the development of 2D composite materials with layered MoS2 generates huge amount of active sites, which are accountable for preserving adequate interfacial area and increasing the efficacy of photocatalytic reaction.27−29 Recently, progress has been made on modified MoS2 for the superior photocatalytic activities in various ways and the examples include MoS2/CeO2,24 MoS2/CaIn2S4,25 MoS2/ CdS,26 RP/MoS2/RGO,30 and g-C3N4/Ag/MoS2,31 Recently, Zhao et al. reported n-BiVO4/p-MoS2 core−shell heterojunctions. 32 Ye et al. designed MoS 2 /S-doped g-C 3 N 4 heterojunctions.33 Wang and co-workers reported BiVO4/ Bi2S3/MoS2 n−p heterojunctions.34 Although previous works on MoS2/MgAl LDH have been reported for the reduction of Cr(VI),35 to the best of our knowledge, construction of a sufficient interface association to obtain type-II band structures with Z-scheme charge transport between p-type MoS2 and ntype NiFe LDH for the photocatalytic redox reaction has not been reported until now. Wang et al. reported that besides the charge transport behavior, the inherent physicochemical behavior of a semiconductor, for example, band gap engineering, band edge potentials, crystallinity, and internal morphological structure, greatly affects their photocatalytic performances. Among them, the positions of the electronic band edges play a significant role to determine the purpose of a semiconductor material to work properly in a solar light influencing the photocatalytic system.36 In the present study, the effective Z-scheme photocatalyst consisting of p-type MoS2 and n-type NiFe LDH as components for type-II MoS2/NiFe LDH electrostatic

heterostructures was constructed via electrostatic self-assembly and hydrothermal steps. Initially, LDH has low charge carrier conductivity, but heterojunction formation with p-type MoS2 increases their conductivity via charge carrier transportancy which is constructive for the augmented photocatalytic redox reactions. The continuous covering of MoS2 nanosheets on NiFe LDH nanosheets reveals the effectual path of hole transfer from NiFe LDH to the scavenger for the oxidation reaction and electrons on MoS2 for the reduction reaction proposed by the Z-scheme charge separation mechanism. Detailed investigations on the photocatalytic rhodamine B (RhB) degradation and H2 production activity of MoS2/NiFe LDH nanocomposites named as MSLDHx (x = 1, 3, 5, and 10 wt % of MoS2 loaded over NiFe LDH) were performed under the exposure of solar energy and visible light, respectively. Compared to solitary NiFe LDH and MoS2, the as-prepared MSLDHx nanocomposites were demonstrated to exhibit excellent photocatalytic activities and the selectivity of which could be changed by varying the wt % of MoS2 over NiFe LDH. At an optimal 3 wt % of MoS2, MSLDH3 exhibited an enhanced catalytic activity of 90% RhB dye degradation in 2 h and a H2 production rate of 550.9 μmol/h, which is mainly possible due to the effective charge carrier separation derived from the type-II p−n band alignment with the Z-scheme charge transportation mechanism in the electrostatic heterostructure MSLDH3.

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EXPERIMENTAL SECTION

Chemicals. Fe (NO3)3·9H2O (Aldrich Chemicals USA 99.0%), Ni (NO3)2·6H2O (Aldrich Chemicals USA, 99.0%), Na2MoO4 (Aldrich Chemicals USA, 99.0%), NaOH (SD Fine Chemicals India, 99.5%), CH3CH2OH (Merck India, 99%), and NH2CSNH2 (Merck India, 99%) were received and used for further reactions. The remaining solvents and chemicals purchased were of analytical grade with moisture-free quality and directly used for reactions. Fabrication of NiFe LDH. NiFe LDH was prepared by a minor modification of our work reported earlier.14 In this method, Ni (NO3)2·6H2O (0.02 M, 5.8162 g) and Fe (NO3)3·9H2O (0.004 M, 1.616 g) of the metal salt (Ni2+/Fe3+ = 5:1) were dissolved in deionized water (20 mL) to get clear solution. To this metal salt solution, NaOH solution was added dropwise in distilled water (5.0727 g in 10 mL) until pH 9, and a gel-like product found was stirred continuously for 12 h. The final product was recovered by filtration and washed several times with deionized water and finally with ethanol, followed by drying in vacuum at 60 °C for 24 h. NiFe LDH nanosheets were prepared according to our recently reported work.37 In this method, the NiFe LDH gel was exfoliated in the liquid-phase process (brown color, ∼2 g), recovered after NaOH addition, and dispersed in an ethanol (75 mL) and water (25 mL) mixed solvent (total volume = 100 mL) for about 30 min. Afterward, the obtained gel of NiFe LDH was treated hydrothermally at 160 °C for 8 h. Consequently, the targeted product was recovered from the unexfoliated portion by centrifugation at 5000 rpm for about 15 min. The supernatant of NiFe LDH nanosheets possessed a concentration of approximately 20 mg mL−1. Fabrication of MoS2. The precursors used for the synthesis of MoS2 were Na2MoO4 and thiourea as the source for Mo and sulfur, respectively. At first, Na2MoO4 and thiourea in 1:5 millimolar ratios in 20 mL of ethanol and 60 mL of distilled water were sonicated for 30 min, followed by constant stirring for 30 min at room temperature. Then the recovered suspension was poured into a Teflon-lined stainless steel autoclave and hydrothermally treated at 210 °C for 24 h. Afterward, the autoclave was allowable to cool down at room temperature, and a black colored precipitate was formed by centrifuging at 6500 rpm, followed by washing three times with distilled water and ethanol. The obtained hexagonal sheetlike MoS2 material was oven-dried at 80 °C for 12 h to get the final product. B


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ACS Applied Materials & Interfaces Preparation of the Electrostatic Heterostructure MoS2/NiFe LDH Nanocomposite. In an usual methodology, the calculated amount of the metal salt solution of [Ni]2+/[Fe]3+ of the 5:1 molar ratio was dissolved in double distilled water (20 mL), and further sonication (30 min) resulted in a clear solution of metal nitrates. Then the aqueous solution of NaOH (5.0727 g, 10 mL) was added dropwise to this metal salt solution until the pH of the solution remained constant at 9. The as-obtained NiFe LDH gel (yield ≈ 2 g) was dispersed in an ethanol (75 mL) and water (25 mL) mixed solvent (total volume = 100 mL) for 30 min. Then in another beaker, a calculated amount of Na2MoO4 and thiourea in 1:5 millimolar ratios of ethanol (20 mL) and distilled water (60 mL) was sonicated for 30 min and then vigorously stirred at room temperature for another 30 min. Afterward, the precursor of MoS2 suspension was dispersed and added dropwise to the transparent gel dispersion of NiFe LDH with constant stirring. Then, the suspension was stirred for another 1 h to maintain the electrostatic equilibrium. At this stage, the MoS2 precursor would deposit over the NiFe LDH gel surface to form a mixture, which was poured to a 100 mL Teflon container and hydrothermally treated at 210 °C for 24 h. After cooling of the autoclave, the product was obtained through centrifugation, and then a final product was obtained by washing with ethanol and deionized water, followed by vacuum drying at 60 °C. A set of electrostatic p−n heterostructure MoS2/NiFe LDH nanocomposites were synthesized by the weight percentage variation of the loading of the MoS2 precursor: that is, 1, 3, 5, and 10% against NiFe LDH and named as MSLDHx, x = 1, 3, 5, and 10 wt % against NiFe LDH, respectively. Materials Characterization. By using a Rigaku MiniFlex powder diffraction meter along with Cu Kα (λ = 1.54 Å, 30 kV, 50 mA) as the source, the powder X-ray diffraction (XRD) was performed. JASCO Fourier transform infrared (FTIR)-4600 and KBr reference matrix were used for the measurement of stretching and bending frequency mode of the materials, respectively. The ultraviolet−visible diffuse reflectance (UV−vis DR) spectra were recorded by using a JASCO-V750 UV−vis spectrophotometer in the range of 200−800 nm. Boric acid pellets were taken as the reference sample. The photoluminescence (PL) spectra were recorded in room temperature at an excitation wavelength of 380 nm. To determine the internal microstructure of the sample, high-resolution transmission electron microscopy (HR-TEM, JEM-2100F) was used and run at a 200 kV accelerating voltage. X-ray photoelectron spectroscopy (XPS) measurement was taken on a VG Microtech Multilab ESCA 3000 spectrometer. Mg Kα X-ray was used as the nonmonochromatized source. The C 1s spin state of the reference carbon peak was used for the binding energy (BE) correction. All photoelectrochemical measurements were done by the potentiostat−galvanostat (IviumStat) electrochemical workstation, equipped with 300 W xenon lamps as the visible-light source. The whole three-electrode arrangement system consisted of Pt, Ag/AgCl, and prepared fluorine-doped tin oxide (FTO), which were used as the counter, reference, and working electrodes, respectively. FTO was synthesized by an electrophoresis deposition method (catalyst = 30 mg and iodine powder = 20 mg were dispersed in 30 mL of acetone, and then two FTO were positioned within the solution in an opposite direction facing parallel to each other). Na2SO4 solution (0.1 M) was used as the electrolyte solution. The Nyquist plot was carried out at 105 Hz to 100 Hz in an open-circuit potential of 0.1 V. The linear sweep voltammetry (LSV) plots were measured by applying the potential range of −1.2 to +1.2 V at a scan rate of 5 mV s−1 under visible-light irradiation. The Mott−Schottky (MS) plot was observed at the frequency of 500 Hz. The chronoamperometric response of the time-dependent cathodic photocurrent of MSLDH3 electrodes was measured at −1.0 V applied potential. Experiment of the Photocatalytic Reaction. Photodegradation Experimental Study of the RhB Dye. The photodegradation experiment of the RhB dye was performed by the use of 20 ppm aqueous solution of RhB (20 mL) plus catalyst (0.02 g), which was then subjected to an average intensity of solar light ∼102 000 Lux at the time period of 120 min. The remaining RhB concentration (λmax = 554 nm) was spectrophotometrically measured by using a UV−vis

spectrophotometer. The dynamic agents associated in the photodegradation reactions were tested by using different scavengers such as dimethyl sulfoxide (DMSO), ethylenediaminetetraacetic acid (EDTA), para-benzoquinone (p-BQ), and isopropanol (IPA) for scavenging electron (e−), hole (h+), superoxide (•O2−) radicals, and hydroxyl radical (•OH), respectively. In an experimental procedure of the scavenger test, additional 5 mM scavenger was added to the RhB dye solution during the degradation process. Moreover, the reusability study of RhB dye degradation reactions was repeatedly carried out for five cycles to check the effectiveness of the catalyst. The photocatalyst was washed with ethanol and deionized water after every cycle consecutively and oven-dried at 80 °C. The samples recovered after the first cycle were washed properly with deionized water and ethanol so that they can be used for the next cycle. Photocatalytic H2 Production Reactions. H2 gas was formed through photocatalytic water splitting in a reactor connected with 125 W medium pressure Hg lamps as the visible light and a UV cutoff filter as 1 M NaNO2 to illuminate the light at a wavelength ≥400 nm. The reactor chamber was illuminated with the visible-light source of power density 100 mW cm−2. To carry out the water-splitting reactions for H2 production, 0.03 g of the catalyst (NiFe LDH, MoS2, and MSLDHx, x = 1, 3, 5, and 10 wt % of MoS2 over NiFe LDH) was dispersed in aqueous CH3OH solution (10 vol %, 30 mL) as the sacrificial reagent. Prior to the reaction, purging of the reactor with N2 gas was performed to maintain the inert atmosphere, and then the suspension of the catalyst was constantly stirred till the end of the reactions. The H2 formation reaction was identified by connecting to GC-17A containing column packing with 5 Å molecular sieve and attached to the thermal conductivity detector (TCD) (Scheme 1).

Scheme 1. Synthetic Steps of the Heterostructure MoS2/ NiFe LDH Nanocomposite

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RESULTS AND DISCUSSION UV−vis DR spectroscopy was thoroughly accustomed to evaluate the optical response of deliberate materials (Figure 1). Solitary NiFe LDH exhibits optical response in both the UV and visible zones (200−800 nm). The primary optical response in the UV zone, that is, band around 200−300 nm, could arise because of the ligand-to-metal charge-transfer transition (O 2p → Ni 3dt2g), whereas the crystal field splitting energy of the octahedral Ni2+ ion state and d−d transition bands appears in 300−800 nm.20,38 The metal-to-metal chargetransfer (MMCT) transitions in bimetallic Ni2+−O−Fe3+ to Ni+−O−Fe4+ metallocenters connected through the oxobridged linkage were characterized by a sharp absorption band at 520 nm. Generally, the MMCT band in the bimetallic LDH system is linked to the crystal field splitting energy of oxo-bridged bimetallocenter present in LDH.20,39 The spinallowed transitions of 3A2g(F) → 3T1g(P) and 3A2g(F) → 3 T1g(F) of the d8 geometry of Ni2+ ions in an octahedral crystal field splitting energy were linked with absorption bands at 380 and 740 nm, respectively. Similarly, the absorption bands at 420 and 645 nm are related to the spin-forbidden transitions of C


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the similar type of systems has been reported earlier. This insight is frequent with materials composed of MoS2, which has two distinct absorptions in the range of 250−750 nm and which are known to arise from the direct band gap transition at the K-point of the Brillouin zone between the maxima of the VB splits and the minima of the CB.41,42 The absorption intensity of the few-layered heterostructure MSLDH3 increases significantly with a decrease in the number of layers because of the indirect-to-direct band gap crossover in fewlayered MoS2. The blue-shifted absorption band position of MoS2 from multilayers to few layers (from 720 to 695 nm) indicates that the band gap increases as the layer number of MoS2 decreases. The band gap energies of NiFe LDH, MoS2, and MSLDH3 calculated from the corresponding Kubelka−Munk function and Tauc plot were as follows17−20

Figure 1. UV−vis DR spectral graph of (a) NiFe LDH, (b) MoS2, (c) MSLDH1, (d) MSLDH3, (e) MSLDH5, and (f) MSLDH10.

(αhν)1/ n = A(hν − Eg )

A2g(F) → 1T2g(D) to 3A2g(F) → 1Eg(D), respectively. Single MoS2 displays weak absorbance at around 578 nm, and their absorption edge is extended toward 900 nm.41 Similar type of absorbance properties was also found in the previously reported nano-sized MoS2.24,25 The weak absorbance in MoS2 was due to the contributions of the direct transition at the Kpoint or excitonic transitions.41,42 Similarly, the light absorption properties of the heterostructure MSLDH3 were measured to identify the key factors responsible for the excellent photocatalytic activities. The absorption intensity in the 600−800 nm region strengthens with the increasing amount of MoS2 which of reliable with the color of MSLDH3 composites that changes from brownish yellow to gray.24,25,34 This suggests the probability of an interfacial charge transfer between NiFe LDH and MoS2, and such a transition among 3

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(1)

As shown in Figure 2, the estimated optical band gap energy, Eg, of 2.2 eV for NiFe LDH was similar to our previous results.19,20 The optical band energy of MoS2 was calculated to be about 1.86 eV for the direct band gap model. This result implies that in the heterostructure MSLDH3, MoS2 grown on NiFe LDH is more like MoS2 thin layers containing a direct band gap energy value of 2.0 eV, consistent with the TEM observation of the as-synthesized materials. XRD analysis was performed to elucidate the structural configuration of NiFe LDH, MoS2, and MSLDHx samples, and the results are shown in Figure 3. The diffraction peaks of pure NiFe LDH could be perfectly assigned into the rhombohedral symmetry of 3R polytypic with space group r3̅m and basal reflection planes of (003), (006), (009), (012), (018), (1010),

Figure 2. Band gap energy values estimated through the Tauc plot of (a) NiFe LDH, (b) MoS2, and (c) MSLDH3. D


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Figure 4. FTIR spectra of (a) NiFe LDH, (b) MSLDH1, (c) MSLDH3, (d) MSLDH5, and (e) MSLDH10. Figure 3. XRD patterns of (a) NiFe LDH, (b) MSLDH1, (c) MSLDH3, (d) MSLDH5, (e) MSLDH10, and (f) MoS2.

3700−3500 cm−1 corresponds to overlapping stretching vibrations of the O−H group in the brucite layer of NiFe LDH.17,19,20 The brucite-like layers of NiFe LDH composed with lattice vibration bands of metal−oxygen and metal− oxygen−metal were detected at around 500−900 cm−1.19,20 In MSLDHx, the presence of MoS2 adsorbed on the LDH is confirmed by the characteristics of the peak at 1421 and 735 cm−1. Also for MoS2, water bonding and O−H stretching vibration band center were observed at 1632 and 3441 cm−1, respectively.48,49 The decrease in peak intensities attributed to NO3− anions in MSLDHx composites than pure NiFe LDH is due to the gradual decrease in the concentration of NO3− anions during the electrostatic self-assembly of exfoliated NiFe LDH with MoS2. The structure of heterojunction interface may restrain the exciton pair recombination factor and speed up the surface chemical kinetics. The alignment of energy bands and energy states at the interface evaluates the properties of junctions, and for this, we examined the energy states of constituent elements in MSLDH3 heterojunction interfaces using high-resolution XPS. The signals of C, Ni, Fe, Mo, O, and S were visualized in wide XPS plot, which confirm the creation of heterostructure nanocomposites. The transfer of electron toward p-type MoS2 to n-type NiFe LDH in the MSLDH3 system through the p−n heterojunction via Z-scheme charge transfer was further proved by XPS analysis of the pure MoS2 and NiFe LDH. The survey spectra of NiFe LDH, MoS2, and MSLDH3 are represented in Figure S1. In pristine NiFe LDH, for the Ni 2p3/2 spectrum (Figure 5a), the peaks at 855.48 and 873.09 eV correspond to the main Ni2+, followed by two prominent shake-up satellite peaks at 861.37 and 879.07 eV, respectively.50 Furthermore, the peak-fitted Ni 2p spectrum of MSLDH3 was analyzed for the elucidation of the nature of Ni phase as well as its interaction with MoS2 (Figure 5a). After the heterostructure formation of NiFe LDH with MoS2, the corresponding Ni 2p peaks of MSLDH3 were shifted toward a higher BE (blue shift, shifting energy ≈ 0.14 eV). The deconvoluted Ni 2p XPS spectrum of MSLDH3 also confirms the presence of Ni2+ in the system. In Figure 5a, two spin− orbit doublet peaks situated at the BEs of 855.60 and 879.07 eV corresponded to Ni 2p3/2 and Ni 2p1/2, respectively, with two satellite peaks at 860.95 and 879.07 eV.51 In neat NiFe LDH (Figure 5b), the peaks located at 712.77 and 724.95 eV are typical characteristic peaks of the Fe3+ species.50 However, for MSLDH3 (Figure 5b), the Fe 2p peaks show a blue shift of ∼0.06 eV and two major peaks at 713.79 and 725.01 eV, linked to Fe 2p3/2 and Fe 2p1/2, respectively.50 This indicated that Ni

(110), and (113) in reference with JCPDS card no. 380715.26,43 Solitary MoS2 exhibits diffraction peaks at 2θ = 14.1, 33.3, and 58.9, which are ascribed to the (002), (100), and (110) planes of hexagonal MoS2 in reference to the JCPDS card no. 37-1492.27,44 It could be found that the peak intensity at 2θ = 14.1° corresponding to the (002) plane of MoS2 was completely absent in MSLDHx, which indicates that the fewlayered structure of MoS2 hexagonal nanosheets was formed because of exfoliation of the multilayered MoS2 nanocrystals in MSLDHx.26 There were no other impurity phases detected in the XRD pattern of NiFe LDH as well as in the heterostructure MSLDHx nanocomposite related to the phases of metal oxides and metal sulfides, which indicate the yield of highly phase pure electrostatic heterostructure nanomaterials. The gradual increase in intensities of the diffraction peaks of NiFe LDH with the increase in wt % of MoS2 in the entire MSLDHx confirms the excellent coupling interaction between NiFe LDH and MoS2 in the resulting heterostructure for superior photocatalytic activities. After the loading of MoS2 in NiFe LDH, that is, in MSLDHx, intermediates of fully breathing peaks of NiFe LDH, that is, (1010) and (0111), were detected, and the growth of these planes exposed all catalytic active sites of NiFe LDH, which were further beneficial for the superior photocatalytic activities. The new peak identified at about 30° and 43° in MSLDHx is due to the (101) and (015) reflection planes of LDH.45 The absence of (003) and upward shifting of (009) diffraction planes of NiFe LDH indicate the formation of NiFe LDH nanosheets in MSLDHx.46 The contribution of MoS2 in the heterostructure MSLDHx nanocomposites was further proved by XPS and energy-dispersive X-ray (EDX) measurement studies. In addition, with the higher wt % content of MoS2, that is, from 3, 5, and 10 wt % in MSLDHx, the intensity of (006) and (012) planes of NiFe LDH became more prominent and sharp, which indicates that MoS2 boosts well the crystal growth of NiFe LDH and such type of diffraction pattern is also reported by Ma et al. in NiFe LDH nanosheets supported on a carbon cloth.47 The FTIR spectra of NiFe LDH and heterostructure MSLDHx samples within 4000−400 cm−1 are shown in Figure 4. Solitary NiFe LDH displays a sharp peak at 1384 cm−1, which could be assigned to the stretching mode of vibration of NO3 anions.19,20 The low-frequency bands in 500−900 cm−1 are associated with Fe−O, Ni−O, and Ni−O− Fe, that is, metal−oxygen and metal−oxygen−metal lattice vibrations of LDH.19,20 A broad absorption band at around E


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Figure 5. Comparative deconvoluted XPS spectral data of NiFe LDH and MoS2 with MSLDH3 for (a) Ni 2p spectrum, (b) Fe 2p spectrum, (c) O 1s spectrum, (d) Mo 3d spectrum, and (e) S 2p spectrum.

and Fe are present in the form of Ni2+ and Fe3+ oxidation state in the MSLDH3 nanocomposites, respectively.19,20 Furthermore, the presence of a satellite peak as well as higher BE shift of the Ni 2p main line confirms the presence of an octahedral geometry, that is, the [NiO6] moieties in MSLDH3.51 The deconvoluted O 1s spectrum of pure NiFe LDH is compared with O 1s of MSLDH3. For neat NiFe LDH (Figure 5c), the high-resolution O 1s spectrum could be resolved to two XPS peaks at 530.31 eV (OI) and 531.09 eV (OII) related to the lattice oxygen and surface hydroxyl group bonded to metal centers.52 The spectra consisting of O 1s of MSLDH3 (Figure 5c) disclose four different peaks ascribed to the lattice oxygen of metal center-OI (530.63 eV),53 surface hydroxyl groups of metal center-OII (531.65 eV),54 oxygen vacancies in lattice OIII (531.30 eV),55 and adsorbed water−OIV (532.90).56 These results reveal the p−n heterojunction impact of NiFe LDH with MoS2, and the effect was observed by the shifting of O 1s peaks of the MSLDH3 composite photocatalyst toward higher BE. A comparison study of the deconvoluted Mo 3d spectrum of pure MoS2 with Mo for MSLDH3 is shown in Figure 5d. In this Figure 5d, Mo XPS peaks in neat MoS2 could be split into

four peaks, of which the two main peaks are found at 233.11 eV (3d3/2) and 229.98 eV (3d5/2) for Mo in +4 oxidation number. The peaks at 234.09 and 227.17 eV could be noted for the Mo6+ oxidation state and S 2s state as well.33,57 In MSLDH3 (Figure 5d), Mo 3d XPS spectra exhibit two distinct doublet peaks at 232.34 (3d3/2) and 229.70 eV (3d5/2), which are in accordance with the XPS peaks of Mo4+. The peaks at 226.84 and 234.25 eV corresponded to the S 2s peak and Mo+6 oxidation state in MoS2, respectively.33,57 The high-resolution S 2p spectra of neat MoS2 (Figure 5e) could be deconvoluted into two doublet peaks at 162.09 and 163.29 eV, which were attributed to the 2p3/2 and 2p1/2 spin state of S 2p validating the −2 oxidation state of sulfur.57 Similarly, the high-resolution S 2p spectra of MSLDH3 could be deconvoluted into two doublet peaks at 161.97 and 163.19 eV, which were attributed to the 2p3/2 and 2p1/2 spin state of S 2p validating the −2 oxidation state of sulfur (Figure 5e).25,26,31,57 The above XPS spectral results reveal that with the creation of p−n heterojunctions via the Z-scheme mechanism of charge transfer in MSLDH3, neat MoS2 exhibits a red shift in the position of BE, and simultaneously, a blue shift was detected in the BE position of NiFe LDH. The cause of shifting of these F


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vacancies (Scheme 2c). Afterward, the oxygen vacancies in the LDH nanosheets form good coordination with Mo4+ ions and S2− ions. The nucleation, growth, and assembly of MoS2 layers over the positively charged surface of exfoliated LDH nanosheets preceded during the hydrothermal reaction (Scheme 2d−f), and the existence of MoS2 prevents the reconstruction of delaminated LDH nanosheets and progress the dispersion to each other through oppositely charged surface interactions forming an electrostatic MSLDH3 heterostructure (Scheme 2g). Moreover, delamination of the LDH nanosheet during the hydrothermal process and swelling properties of the surface hydroxyl group offers a cost-effective way to expand a high-performance heterostructure photocatalyst. The zeta potential values of NiFe LDH and MoS2 were examined to be +26 ± 0.3 and −28.5 ± 0.4 mV, respectively. By taking the advantages of oppositely charged surfaces of exfoliated LDH nanosheets and MoS2 sheets, the formation of MSLDH3 heterostructure nanocomposites could be achieved by self-assembled chemistry via electrostatic force of interactions. The microstructure features, elemental purity, and the crystalline nature of the as-synthesized NiFe LDH, MoS2, and MSLDH3 materials were further revealed through TEM and HR-TEM analyses as shown in Figure 6. In Figure 6a,

BE positions may be due to the transfer of electron from MoS2 to the NiFe LDH and then recombination of the accumulated electrons of LDH with the holes of MoS2. This phenomenon enriched the electron density of MoS2 in the heterostructure nanocomposite and causes lower BEs of Mo and S, respectively. Likewise, the decreased electron density of LDH in MSLDH3 causes an increment in the BE position of Ni, Fe, and O. This kind of shuffling in chemical environment provides evidence of the existence of charge-transfer-induced electric field and strong electronic interaction between the ptype MoS2 and n-type LDH through the formation of a p−n heterojunction via the Z-scheme mode of charge pair separation. In this way, separation of the photoinduced exciton pairs is easily restored at higher reduction and oxidation potential for excellent photocatalytic activities.8 Moreover, after hybridizing MoS2 with NiFe LDH, the percentage of sulfur active sites became prime (Figure 5e), which is supported with the loading of MoS2 on NiFe LDH nanosheets. Electrostatic self-assembly is an effectual strategy for designing distinctive nanocomposite materials. There are examples of reported works such as ZnCr LDH/graphene oxide nanosheets and NiTi LDH/RGO, which are bonded by strong electrostatic force of attraction.15,16 A similar phenomenon is also found in our previous reported works in g-C3N4/ NiFe LDH and Ag@Ag3PO4/g-C3N4/NiFe LDH for superior water-splitting reactions and pollutant degradation.19,20 Scheme 2 represents the formation mechanism of MSLDH3. Scheme 2. Schematic Illustration of the Formation Mechanism of Electrostatic MSLDH3: (a) Precursors of LDH, (b) Positively Charged Surface of LDH with Precursors of MoS2, (c) Adsorption Process of Mo4+ and S2− Ions over LDH, (d) Nucleation Process of MoS2 over LDH To Form an Interfacial Contact (Dotted Lines), (e) Growth of MoS2 Nanosheets over LDH Nanosheets, (f) Electrostatic Self-Assembly of Negatively Charged MoS2 Nanosheets with Positively Charged LDH Nanosheets and (g) Stacking Structure of the Electrostatic MoS2/LDH Heterostructure Nanocomposite

Figure 6. TEM and HR-TEM images of (a) NiFe LDH. TEM image of (b) MoS2 and (c,d) MSLDH3 and (e) HR-TEM images of MSLDH3.

LDH nanosheets have a very unique brucite-like structure consisting of an edge-shared MO6 octahedron, which gives ample site for the nucleation and growth of MoS2 nanosheets through electrostatic self-assembled chemistry. At first, when the LDH gel is mixed with the precursor of Mo and S (Scheme 2a,b), adsorption onto the positively charged surface of LDH is caused by the electrostatic force of interaction. Interestingly, layers of LDH separated during the hydrothermal reaction because of structural changes and separation of strongly bonded positive layers with creation of surface oxygen

NiFe LDH was formed as a hexagonal nanosheet aggregate, with an average diameter in the range of 20−30 nm. The lattice fringes detected in the HR-TEM image of NiFe LDH (inset in Figure 6a) suggest a defined crystal structure and a lattice spacing of 0.25 nm, corresponding to the (012) plane of hexagonal NiFe LDH nanosheets.12 The synthesized MoS2 nanosheets are interconnected with each other to form a selfG


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Figure 7. SEM images of (a) NiFe LDH, (b) MoS2, and (c) MSLDH3. (d−h) EDX elemental mapping images of MSLDH3.

the heterogeneous nanojunctions were verified by the selected area electron diffraction (SAED) pattern and EDX analysis as shown in Figure S2a,b. The compositional distribution of each constituent metal element of the electrostatic heterostructure MSLDH3 nanocomposite was calculated by EDX spectroscopy measurements (Figure S2b). All elements such as Ni, Fe, Mo, S, and O were detected with high intensity, indicating that each element was distributed evenly throughout the few-layered MSLDH3 nanocomposite. In the growth process, highly dispersed NiFe LDH nanosheets provide active planes for the growth of MoS2 nuclei, which is the primary factor responsible for the hierarchical structural design. Formation of heterostructures with edge-anchored few-layered MoS2 and NiFe LDH nanosheets results in enhanced physical and chemical properties toward the photocatalytic activity. The due occurrence of more basal edge sites containing unsaturated sulfur ions confines the H+ ions at the intimate contact area of both MoS2 and NiFe LDH nanosheets intended for excellent redox reaction activities.26 Figure 7a−c depicts a typical scanning electron microscopy (SEM) image of NiFe LDH, MoS2, and MSLDH3. The bare NiFe LDHs (Figure 7a) possessed a hexagonal nanoplatelet morphology, which are of typical characteristic of the exfoliated LDH material.18 Figure 7b shows that MoS2 has a flaky sheetlike morphology with an approximate diameter ranging within 50−100 nm. The morphology of MSLDH3 as shown in Figure 7c reveals that MoS2 hexagonal flaky nanosheets were successfully grown on the NiFe LDH sheets with a size of 5−20 nm. These results show that NiFe LDH efficiently hinders the agglomeration of MoS2 nanoplatelets in

assembled mass of exfoliated few-layered nanosheets as displayed in Figure 6b. The interlayer lattice fringes in the HR-TEM image of the lattice spacing of 0.227 nm matching to (103) crystallographic orientations of MoS2 are shown in Figure 6e.25 As compared to LDHs, the drastic changes in the morphological aspect of MSLDH3 could be visualized in Figure 6c,d. It is important to mention that LDHs were effectively covered by MoS2. Figure 6c,d depicts the TEM image of the heterostructure MSLDH3 nanocomposite, which shows the uniform morphology throughout the heterostructure nanocomposites. The TEM image in Figure 6d shows that the MoS2 nanosheets and NiFe LDH nanosheets are selfassembled to form a stacked nanosheet-like structure with proper alignment in the opposite growth direction of NiFe LDH and MoS2 forming heterointerfaces. Furthermore, the HR-TEM image in Figure 6e corresponding to the interplanar lattice spacing values of 0.227 and 0.25 nm was indexed to the (103) and (012) interplanar planes of few-layer MoS2 and NiFe LDH, respectively. The existence of heterogeneous nanojunctions of the MSLDH3 nanocomposite could be clearly found in the HR-TEM image as depicted in Figure 6e. The image clearly revealed the formation of lattice fringes in opposite direction and separate barriers of NiFe LDH and MoS2 lamellas. This implies that few-layer MoS2 with an interlayer spacing of 0.227 nm forms an intimate contact with NiFe LDH (typical region marked in Figure 6c,d), which favors the emergence of junctions with dimensions ranging in the nanometric scale for the transfer and separation of photoexcited exciton pair for augmented catalytic performances. The highly crystalline nature and the ultrahigh purity of H


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Figure 8. (a) Plotted graph of C/C0 vs time and (b) kinetic plot of NiFe LDH, MoS2, MSLDH1, MSLDH3, MSLDH5, and MSLDH10 during RhB degradation and (c) concentration of RhB absorbance vs time for the MSLDH3 catalyst (monitored through a UV−vis spectrophotometer).

scheme mechanism route that makes the reduction potential of MoS2 and oxidation potentials of LDH rich in electrons and holes, respectively. In this process, recombination of additional exciton pairs occurred over the Schottky barrier formed at the interfacial region of the heterojunction.8 This enhances the degradation rate of 75% (MSLDH1) to 90% (MSLDH3) and diminishes to 85% (MSLDH5) and 70% (MSLDH10). These results clearly show that the heterostructure MSLDH3 at the loading of MoS2 (3 wt %) displays superior degradation percentage than the other as-prepared materials of the series. In addition, an overload of MoS2 causes reduction in photocatalytic activity owing to shielding of the active sites and hinders the light from coming in contact with the NiFe LDH and an increase in the opacity due to excessive presence of the black material MoS2 prevents penetration of light to the reaction flask and causes decrease in the photocatalytic activities of MSLDH10. In these circumstances, a constant quantity of the catalyst is considerable for optimization of the photocatalytic performance of MSLDH3.24,25,35 These outcomes revealed that the cooperative role of MoS2 and NiFe LDH plays a major part in increasing the photocatalytic performances directly under the sunlight exposure. C/C0 versus time plot signifies the photodegradation rate as shown in Figure 8a using the following eq 2.

MSLDH3 and vice versa. In addition, electrostatic interaction between LDH and MoS2 retains their original hexagonal morphology in MSLDH3. The EDX elemental mapping images of MSLDH3 (Figure 7d−h) discloses the clear dispersion and uniform elemental distribution of Ni, Fe, Mo, O, and S. Photocatalytic Activity Evaluation. RhB Dye Degradation Study. Visible-Light-Triggered Photocatalytic Activities. In the process of evaluating the photocatalytic performance of NiFe LDH, MoS2, and MSLDHx, the RhB degradation study was conducted under natural sunlight irradiation. The photocatalytic degradation of the RhB dye was performed by dispersing 0.02 g of the catalyst in RhB solution (20 ppm, 20 mL) and by exposing it to natural sunlight for 2 h. The measured concentration of the RhB dye solution was observed to be constant in the absence of a catalyst or sunlight exposure, which shows the inefficiency in the self-degradation activity of RhB, but the existence of both catalyst and light energy source plays a tremendous role in the photodegradation of any kind of pollutants. The phenomenon of adsorption over the surface of a catalyst is considered to be prime factor in photodegradation reactions. Before proceeding to the degradation study, the RhB dye adsorption test over NiFe LDH, MoS2, and MSLDHx in dark was measured for 30 min to study the equilibrium reached in the adsorption−desorption process (Figure 8). By using NiFe LDH, the degradation of RhB dyes was achieved almost 30% under the irradiation of natural sunlight for 30 min, and no significant degradation of RhB was observed with pristine MoS2 catalysts. By comparing with pure NiFe LDH, MSLDHx composites with different wt % of MoS2 showed enhanced photocatalytic activities for the photodegradation of RhB. In MSLDHx, loading of MoS2 over NiFe LDH generates p−n heterojunctions with charge separation, followed by the Z-

% of photo degradation rate = (C0 − C /C0) × 100

(2)

where C0 represents the concentration of RhB dye (mg/L) when time“t” = 0 min and C represents the concentration of RhB dye (mg/L) when time“t” = t min. In this case, pseudo-first-order kinetics using the Langmuir− Hinshelwood model could be adopted using the following eq 3 for understanding the reaction kinetics. I


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Figure 9. (a) Role of different types of scavengers on the photodegradation of RhB with MSLDH3, (b) NBT spectral behavior curve of NiFe LDH, MoS2, and MSLDH3, and (c) TAOH fluorescence spectral emission behavior of NiFe LDH, MoS2, and MSLDH3.

ln(C0/C) = kappt

toward photodegradation of RhB as the model pollutant, the scavenger test was conducted by taking reagents such as p-BQ, IPA, DMSO, and EDTA for scavenging •O2− radicals, •OH radicals, e−, and h+, respectively. In the experimental procedure, each trapping reagent (5 mM) was added to 20 mL of the RhB dye solution (20 ppm) and the same was subjected to the photocatalytic reaction. The obtained results as depicted in Figure 9a reveal that the degradation activities for RhB drop to 18, 28, 41, and 86% with the addition of p-BQ, EDTA, IPA, and DMSO scavenging reagents, respectively. As a result, •O2−, h+, and •OH were the primary reactive species responsible for RhB degradation using MSLDH3. When DMSO was used as the scavenger, the degradation percentage is in close proximity to MSLDH3 without a scavenger, that is, 86%, so the minor role of electrons was identified in RhB degradation for the MSLDH3 catalyst. The •O2− radical detection was performed by the nitroblue tetrazolium (NBT) test. By this method, NBT was selected to measure the concentration of •O2− and reveal the photocatalytic efficiency of NiFe LDH, MoS2, and MSLDH3.58 Figure 9b shows the deviation in the concentration of NBT in the dispersion of NiFe LDH, MoS2, and MSLDH3 at unlike time period under sunlight irradiation. The decrease in highest absorbances in the UV−vis spectral data of NBT indicates the • O2− production in MSLDH3. However, there is no such difference in the spectral absorbance of NBT in NiFe LDH. Hence, no such •O2− radicals were formed on the potential edge of CB in NiFe LDH. These outcomes express the photocatalytic performance of MSLDH3, and •O2− production was influenced by the CB potential edge of MoS2 under the exposure of visible light. Similarly, terephthalic acid PL (TA-PL) probe technique was used for the measurement of •OH radical concentration in

(3)

Here, kapp represents the rate constant (apparent). The linear plot of ln(C0/C) versus exposure time represents the kinetics of the rate constant as shown in Figure 8b. The slopes of the ln(C0/C) versus exposure time plot give the value of k or kapp. The measured kapp values indicate that RhB degradation for NiFe LDH and MSLDHx obeys pseudo-first-order kinetics in the Langmuir−Hinshelwood model. Regression coefficient (R2) values of the fitted lines in Figure 8b of the as-synthesized materials were listed as 0.9666, 0.9735, 0.9859, 0.9728, and 0.9621 for NiFe LDH, MSLDH1, MSLDH3, MSLDH5, and MSLDH10, respectively. The corresponding reaction rate constants were determined to be −0.00414, −0.00617, −0.00746, −0.00688, and −0.00526 min−1 for NiFe LDH, MSLDH1, MSLDH3, MSLDH5, and MSLDH10, respectively. The improved reaction rate constant value of MSLDH3, that is, −0.00746, verifies its significance as the heterostructure photocatalyst toward mitigation of pollutants. The photocatalytic augmentation of MSLDH3 was attributed to the light harvestation and reduction in charge carrier’s recombination because of direct Z-scheme with Schottky barriers generated at the interfacial area of the p−n junction with high photoelectrical conversion efficiency and low resistance. Figure 8c denotes the variation in the UV−vis absorption spectra of the RhB dye after the MSLDH3 catalyst was added into the dye stuff solution, which corresponds to the decrease in RhB dye concentration in the solution without a shift in the absorption band from the exposure time of 0−120 min. The color of the RhB dye solution catalyzed by MSLDH3 changed from magenta to transparency, indicating high degradation efficiency of MSLDH3. In order to trace the reactive species participation (h+, •OH, • O2−, and e−) and to portray the right mechanistic pathway J


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Figure 10. Sequential change of the (a) H2 evolution (μmol/h) with NiFe LDH, MoS2, and MSLDHx (x = 1, 3, 5, and 10 wt % of MoS2 over NiFe LDH), (b) recyclability study of RhB in each 120 min, and (c) volume of H2 produced in four successive cycles in each 60 min of time interval with MSLDH3. Source of the superior performance.

the photocatalytic reactions.8 In the TA-PL reaction process, TA reacts with the •OH radical, generating a 2-hydroxyterephthalic acid (TAOH) fluorescent active molecule, and the corresponding emission band appeared at 426 nm when excited at 315 nm. In this technique, ∼0.02 g of the selected catalyst was added to 4 × 10−3 M NaOH solution containing TA (20 mL), and then the suspension was exposed to sunlight for 2 h. Then, the measured intensity of the fluorescence spectrum of the TAOH solution gives the quantitative proportionate of the •OH radical generated during photocatalytic reactions. The highest intense peak of MSLDH3 (Figure 9c) denotes the maximum percentage of formation of • OH radicals. In MSLDH3, equal contribution of NiFe LDH and MoS2 resulted in the formation of h+, •O2−, and •OH active radical species for excellent photocatalytic activities. Photocatalytic Activities of H2 Formation. The performances of photocatalytic H2 formation reactions in NiFe LDH, MoS2, and MSLDHx were accomplished by using aqueous CH3OH solution (30 mL) and then directly exposed under visible light (λ ≥ 400 nm) as shown in Figure 10a.17−19 No H2 evolution was found in the absence of a catalyst or light and hence explains the dependency of photocatalytic H2 formation reactions on the cooperative effect of the catalyst and light. NiFe LDH and MoS2 demonstrate the rate of H2 formation of 50.3 and 28.6 μmol/h, respectively. Interestingly, MSLDHx following the Z-scheme charge-carrier-transfer mechanism with p−n heterojunctions between NiFe LDH and MoS2 demonstrates superior H2 formation performance than bare NiFe LDH and solitary MoS2. When wt % variation of MoS2 started from 1 to 3 in MSLDHx, the H2 formation rate enhances from 414 μmol/h (MSLDH1) to 550.9 μmol/h (MSLDH3) and then decreases from 463 μmol/h (MSLDH5)

to 280μmol/h (MSLDH10). Single MoS2 exhibits a negligible amount of photocatalytic H2 production either because of inefficient charge separation or because of the miser transportation of the charge carriers to the hydrogen evolutionactive edges.59 The incremental photocatalytic performances are due to the increase in exposed active edges, quantum confinement, or light scattering effect of nanolayered MoS2 in MSLDH3, which is thermodynamically feasible for the hydrogen evolution reaction.26,30−32 However, extra loading of MoS2 resulted in the reduction of photocatalytic performance, which is owing to the shielding cause of MoS2 in MSLDH3 that creates hindrance for photon absorption. The augmentation in photocatalytic activities is attributed to the existence of p−n heterojunctions with the Z-scheme charge flow obtained due to incorporation of MoS2 on NiFe LDH as in the MSLDH3 heterostructure. Recyclable study of the material is very much essential for a common phenomenon in actual world as the catalytic effectiveness may reduce after repeated use in an aqueous medium by directly exposing to natural sunlight or visible light. In order to corroborate the photocatalytic stability and recyclability of a photocatalyst, the heterostructure MSLDH3 was recycled in four different cycles of the catalytic reaction in RhB degradation and H2 production activities. As shown in Figure 10b, MSLDH3 could maintain superior photocatalytic RhB dye degradation activities after four cycles of repeated use, which reveals their stable catalytic effectiveness in treating RhB-contaminated water bodies as an exemplary in real-time applications. Similarly, Figure 10c shows the catalytic recyclability test for H2 evolution experiments being repeatedly used for four different cycles. Also, Figure 10c depicts the constant rate of the H2 recyclable test reaction without deactivating steps in the repeated fourth cycle with the K


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Figure 11. PL plotted data of (a) NiFe LDH and MSLDH10, (b) MSLDH1, MSLDH3, MSLDH5, and MSLDH10.

Figure 12. MS graph of (a) NiFe LDH, (b) MoS2, and (c) MSLDH3.

To explore the source of the superior photocatalytic RhB dye degradation and H2 evolution over the MSLDH3 heterostructure photocatalyst, PL and electrochemical studies were taken into account for the detailed clarity of charge separation. The photophysical properties related to optical emission and interfacial charge separation efficiency of NiFe LDH and heterostructure MSLDHx were studied by PL spectroscopy with an excited energy of 380 nm (Figure 11a,b). The emission peak of NiFe LDH (Figure 11a) exhibits three distinct bands located at 442, 460, and 575 nm. The peak appeared at 442 and 460 nm was due to the radiative recombination of carrier pairs associated with the ligand field transition energy of the Ni2+ octahedron.60 The surface defective site of NiFe LDH nanosheets was reflected in terms of a steady emission at 575 nm. Because oxygen vacancy states found in XPS analysis (Figure 5c) lie below the CB of the n-type semiconductor as discussed below in electrochemical sections and the oxygen vacancy states stand as the electron-capturing centers were accountable for the large Stokes shift.18,20 The electrostatic self-assembled heterostruc-

exposure time, which established high stability of the MSLDH3 material toward photocatalytic H2 evolution activity too. In direct to perfect the RhB degradation and H2 formation reaction of MSLDH3, the physical mixture of MoS2 and NiFe LDH was used for RhB degradation with H2 evolution activities, and subsequently, the recyclability test was continued for two cycles (Figure S3a,b). Furthermore, Figure S3a,b shows the significant decrease in RhB degradation and H2 evolution rate in the physical mixture. Therefore, the interaction between MoS2 and NiFe LDH was very ineffective in the physical mixture by simply grinding both the bare materials using a mortar and pestle. In contrast, an effective interaction was developed during electrostatic self-assembly and in situ hydrothermal treatment between MoS2 and LDH in the physical mixture. This illustrates the high chemical stability of the electrostatic MSLDH3 heterostructure for clean reusability during photocatalytic RhB degradation and the H2 evolution process. A comparison table representing the catalytic performances of MSLDH3 with literature-reviewed materials is depicted in Table S1. L


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Figure 13. LSV plotted data of (a) NiFe LDH, (b) MoS2 with (c) MSLDH1, MSLDH3, and MSLDH5 with direct exposure of light.

ties. The following equation is used to accurate the flat band potential (Vfb)20,24,25,59

ture MSLDH3 (Figure 11b) dramatically showed the reduced intensity in the visible region with the red-shifted peak position, which was ascribed to the spectral overlapping of emission bands. MSLDH3 certainly showed two emission bands within the visible range, and the higher energy band at 567 nm agreed with the defect emission of NiFe LDH and the lower energy band below 650 nm (1.86 eV) matched with the band-to-band emission energy of MoS2.26 A previous study reported that the interlayer coupling has a significant impact on the charge transport and band alignment of the electrostatically self-assembled heterostructure as compared to their individual constituents.26 As revealed here from the intensities of peak position of materials as shown in Figure 11a,b, the electron−hole recombination becomes negligible for MSLDH3 as compared to MSLDH1, MSLDH5, MSLDH7, and MSLDH10, suggesting strong interlayer bending at the interface of few-layered MoS2 and NiFe LDH nanosheets, and formation of heterostructures drives the photogenerated charge separation. Furthermore, the induced electrical field generated in between the NiFe LDH and few-layered MoS2 owing to the work function difference further contributed efficient electron transfer at their interfaces for enhanced performances in photocatalytic reactions. Electrochemical Properties. To understand the MS plot, the impedance study was carried out at the frequency range of 100 000−100 Hz under dark condition. The analysis of MS plot reveals the effect of MoS2 layers on NiFe LDH that was reflected in the flat band edge position of MSLDH3. As shown in Figure 12, NiFe LDH showed a positive slope in the dark, which shows its n-type semiconductor conductivity.20 Solitary MoS2 showed negative slopes, which indicated their p-type conductivity behavior.24,25,59 At the same time, MSLDH3 showed both kinds of positive and negative slopes, which reflects n-type and p-type semiconductor conductivity proper-

ij 2 yzij y 1 zzjjV − V − kT zzz = jjj app fb z j 2 j qε0εNd zj q zz{ c k {k

(4)

The flat band potential value (Vfb) of a semiconductor could be calculated from the linear plot of intercept 1/C2 equal to 0. The approximate Vfb value lies in the potential edge of CB (ECB) of n-type semiconductors and potential edge of VB (EVB) of p-type semiconductors. In accordance with the MS plot intercept (Figure 12a−c), Vfb values of NiFe LDH, MoS2, and MSLDH3 were estimated to be −0.6, +1.56, and +0.85 V versus Ag/AgCl. Consequently, ECB and EVB of NiFe LDH, MoS2, and MSLDH3 were calculated to be (−0.01, +2.19), (−0.08, +1.78), and (−0.93, +1.07) versus NHE, respectively. Notably, the potentials obtained by the Ag/AgCl reference electrode changed to the NHE scale using the equation below.11 0 E(NHE) = E(Ag / AgCl) + EAg/AgCl + 0.059pH

(5)

Here, E(Ag/AgCl) is the potential measured using the Ag/AgCl reference electrode. E0Ag/AgCl = 0.197 at 25 °C. The electrolyte is 0.1 M Na2SO4 solution of pH value approximately at 6.5.20 For the calculation of the potential of MoS2 and MSLDH3, the pH value of the 0.5 M H2SO4 electrolyte was measured to be approximately 5.5.24,25 p-Type and n-type semiconductor properties of MoS2 and NiFe LDH make them compatible for a p−n heterostructure.20,24,25 Therefore, when MoS2 and NiFe LDH coupled to form a p−n heterostructure, an inverted “V-shape” curve in the MS plot was formed, as shown in Figure 12c. Mathematically, the carrier density of a semiconductor is inversely proportional to the slope of their MS graph, and the related theoretical equation could be stated as follows20,24,25,59 M


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ACS Applied Materials & Interfaces N = (2/εε 0 ·e)[d(1/C 2)/dV ]−1

ance of photoinduced charge pairs assimilating on the surface of the electrode. Figure 14 shows the EIS curve of different

(6)

In comparison with the NiFe LDH, a little sharp slope in the MS plot of MSLDH3 indicates poor level of donor density and revealed the existence of p-type MoS2. The negative shifting in the VB position of MSLDH3 (+1.07 V vs NHE) as compared to p-type MoS2 (+1.78 V vs NHE) shows the upward shifting of Fermi edge of p-type MoS2 while downward shifting of ntype NiFe LDH with the formation of a space-charge region for effective separation of exciton pairs.20,24,25,59 When Fermi edge equilibrates in the semiconductor component of MSLDH3, the space-charge region exerts motive force for the capturing of carrier charges inside the depletion layer through an in-built electric field near the interfacial region of the p−n junction, which consequently enhances the photocatalytic activities of MSLDH3. The superior photocatalytic performance of MSLDH3 was owing to the effective separation of carrier charge by the creation of an in-built electric field near the interfacial contact area of the p−n heterojunction. The photoelectrochemical performance of NiFe LDH, MoS2, and MLDH3 was calculated by plotting current density (I) versus potential (V) as displayed in Figure 13. For either independent NiFe LDH or MoS2, very low photocurrents were observed even at the potential of −0.5 V versus Ag/AgCl. Under light exposure, the onset potential of LDH was −0.10 V versus Ag/AgCl and the onset potential of MSLDH3 shifted to +0.05 V versus Ag/AgCl. The onset potential of the semiconductor photoelectrode for the generation of the photocurrent explores their catalytic efficiency.59 The shift in onset potential of the heterostructure MSLDH3 reveals the creation of a p−n junction with enhanced photocatalytic redox reaction activity. At high −ve potential, the photocurrent density denotes the rate of reduction reaction and is stabilized by the flow of electrons to the interface of the electrode/ electrolyte. When MoS2 coupled with NiFe LDH, the increased density of the photocurrent in the p−n-type MSLDH3 junction is shown in Figure 13c. This reveals that p−n heterojunctions make easier the separation of charge pairs and suppressed the recombination factor.20,24,25 The spacecharge layer of a p−n heterojunction in the nanometric region generates an in-built electric field and move apart the charge pairs under visible-light exposure. Therefore, p−n heterojunctions minimize the lacunas of NiFe LDH and become more active in enhancing the charge separation.14 The rate of catalytic redox reaction requires charge pair separation and recombination factor. The chronoamperometric curve was achieved for 9000 s to explore the charge recombination behaviors (Figure S3). Figure S4 shows the corresponding chronoamperometric response of the timedependent cathodic photocurrent of MSLDH3 electrodes in Na2SO4 solution (0.1 M, pH = 6.5) at −1.0 V applied potential versus Ag/AgCl. When the MSLDH3 semiconductor electrode was irradiated with light, the cathodic photocurrent density at the primary stage increases and then becomes stable with an increase in time period. The current density of MSLDH3 indicates a very slow reduction rate and a relative current of 85% is still preserved even after 9000 s, which reveals the inhibition of charge recombination rate owing to the establishment of a space-charge layer in nanometric p−n heterojunctions. In electrochemical impedance spectroscopy (EIS) spectra, the semicircular arc discloses the most appropriate sign of charge separation, transportation, and charge-transfer resist-

Figure 14. EIS curve of NiFe LDH, MoS2, MSLDH1, MSLDH3, MSLDH5, and MSLDH10 exposed under dark and light irradiation.

working electrodes under the dark as well as light condition to achieve the close neat of the charge-transfer process and photoinduced charge recombination on NiFe LDH, MoS2, and MSLDH3 electrode surface. The reduction in the diameter of semicircular arc in the EIS plot displays reduction in resistance of the charge-transfer process at the interfacial area of the electrode and electrolyte.18,20,25 On the basis of the above basic principle, the results show that the MSLDH3 electrode possesses low charge-transfer resistance (charge transfer/ interfacial) than NiFe LDH, which reveals the separation of the effective photogenerated carrier charge pair across the surface of MSLDH3 owing to the introduction of MoS2. These results follow the photocatalytic activity trend. The equivalent model circuit fitted for MSLDH3 is displayed in the outset of Figure 14. R1 is denoted as a series resistance of the circuit and associated with the resistance in the charge-transfer process throughout the interfacial contact area of the electrode/ electrolyte in the Pt counter electrode and represents the first semicircle in the range of 25−40 Ω (high frequency). The semicircle in the center frequency range of 15−25 Ω is denoted as the charge-transfer resistance (R2) across the interface of the electrode MSLDH3 and electrolytic solution. The R3 denoted as the resistance in the charge-carrier-transfer process in the Helmholtz double layer with CP1 with CP2 corresponds to the chemical capacitance. Finally, EIS spectra show the reduction in the diameter of the semicircle in the MSLDH3 electrode as compared to the other as-prepared electrodes. The drastic reduction in Rct value at the loading rate of MoS2 to NiFe LDH (Figure 14) indicates that the electrostatic heterostructure material MSLDH3 deeply enhances the charge transfer, which results in enhanced photocurrent density, and consequently enhanced the photocatalytic activities of MSLDH3. These results clearly reveal the quicker interfacial charge-transfer dynamics and charge separation efficiency of MSLDH3. As shown in Figure 14, the electrostatic MSLDH3 heterostructure system displays the reduced chargeN


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applied to Kubelka−Munk transformation (2.2 and 1.86 eV, Tauc plot as shown in Figure 2) and further verified with visible-light absorption capability at the wavelength covering 800 nm. In order to demonstrate the flow of electron in MSLDH3, the CB and VB positions of NiFe LDH and MoS2 semiconductors were calculated by combining the Tauc plot with MS analysis and further compared with the Mullikan method by using the following equations23,25

transfer resistance in both dark and under light exposure, owing to good electrical conductivity arising from the Mo nanosheets. The significant change in the charge-transfer resistance of MSLDH3 under light irradiation than in the dark confirms the stability and most efficient photocatalytic system among the series of the as-synthesized materials. The orders of the calculated value of charge-transfer resistance throughout the working electrode under dark are as follows: MSLDH3 (64.5 Ω) < MSLDH5 (69.1 Ω) < MSLDH1 (78.8 Ω) < MSLDH10 (123.9 Ω) < NiFe LDH (154.2 Ω). Analysis of the Possible Photocatalytic Mechanism of ZScheme Charge Pair Transfer and Separation Route in a p−nType Electrostatic Heterostructure MSLDH3 Nanocomposite. On the basis of the Experimental Section and Results and Discussion section, the band alignment structure and mechanism related to photogenerated carrier charge separation and transfer process in MSLDH3 are demonstrated in Figure 15 and Scheme 3, respectively. Determining proper band

ECB = X − Ee − 0.5Eg

(7)

E VB = ECB + Eg

(8)

By using eqs 7 and 8 and MS plot analysis, the VB and CB positions of NiFe LDH were found to be +2.19 and −0.01 eV, respectively. However, the VB and CB edge potentials of MoS2 were found to be +1.78 and −0.08 eV, respectively. For the ntype semiconductor, the Fermi edge is located at 0.1 eV below the CB and 0.1 eV higher than the VB potential edge of the ptype semiconductor.20 Consequently, LSV and MS plot reveal that NiFe LDH and MoS2 possess n- and p-type semiconductor properties.19 Hence, the Fermi edge positions of MoS2 and NiFe LDH were predicted to be +1.18 and +0.11 V versus NHE. The difference in energy in between the VB maxima of MoS2 and the CB minima of NiFe LDH could be estimated as +1.78 and −0.01 V, respectively. These indicate that on direct exposure to visible light, transportation of electrons takes place from the CB of NiFe LDH to the VB of MoS2 (direct Z-scheme) owing to enlarged overlapping energy levels associated with the p−n heterojunctions. Under the exposure of visible light, excitation of both p-type MoS2 and n-type NiFe LDH could produce photoinduced charge pairs in their relevant CB and VB edge potentials, respectively. The charge transfer in MSLDH3 was due to the Fermi level difference between MoS2 and NiFe LDH (Figure 15a), which leads to local band bending and establishment of an internal electric field at the interfacial contact area of p−n junctions as shown in Figure 15b. In a heterostructure of p−n junction type, the junction generates a “space-charge region” close to which depletion of exciton pairs takes place in the ntype semiconductor and p-type semiconductor. The spacecharge region builds an inner electric field and drives bulk carrier charge to flow in reverse direction proceeding redox reactions. Actually, the p−n junction interface is the perfect surface for the transfer of carrier charge in photocatalysis.61,62 Here, the potential gradient created at the interface area of sheet-to-sheet junction acts as the main vector force of the carrier charge transfer to the site of reaction.10 However, the role of the in-built electric field near the interfacial contact area of p−n junctions in the Z-scheme charge-transfer mechanism is to reduce the probability of bulk recombination. In the photocatalytic system consisting of p−n heterojunction (Figure 15b and Scheme 3a), the photoinduced electrons in the CB of photosystem I (PS I) and photoinduced holes in the VB of photosystem II (PS II) migrated toward the CB of PS II and VB of PS I, respectively.10 Consequently, exciton pairs are dimensionally separated, which significantly encourage the antirecombination process. In a different way, Zscheme reaction dynamics (Scheme 3b) give much emphasis on the reduction of bulk recombination factor. The photoinduced electrons aggregated at the CB edge of PS I turn it to be enriched of electrons and prevent PS I from the photooxidation reaction. Similarly, the photoinduced holes at the VB edge of PS II enrich PS II in holes and further prevent

Figure 15. Potential edge alignment for (a) n-type NiFe LDH and ptype MoS2 before contact and (b) p−n heterostructure MSLDH3 nanocomposite after contact.

Scheme 3. Proposed Schematic Representation of the Charge-Transfer Mechanism over MSLDH3 under Exposure of Visible Light for (a) Double Charge and (b) p− n Heterojunctions with the Direct Z-Scheme Mechanism

alignment or energy level of the constituent semiconductor is very much crucial to define the reaction mechanisms in the heterostructure MSLDH3. The band gap of NiFe LDH and MoS2 was measured by means of UV−vis DR spectral analysis O


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in highest absorbances in the UV−vis spectral data of NBT indicates the formation of •O2− in MSLDH3. Scheme 3a reveals that if charge carrier transfer is preceded by type-II band alignment between MoS2 and NiFe LDH in MSLDH3, then the CB potential edge of NiFe LDH could not have the capacity to produce •O2− and •HO2 because of higher positive potential of the CB edge in NiFe LDH (−0.01 V vs NHE) than the universal standard redox potential of •O2− (EΘ(O2/•O2−) = −0.33 eV vs NHE) and EΘ(O2/•HO2, −0.05 eV vs NHE), respectively.20 Similarly, the higher negative VB potential of MoS2 (+1.78 eV vs NHE) than the redox potential of •OH radicals, that is, EΘ(•OH/OH−) = +1.99 eV vs NHE, shows its inefficiency in producing •OH radicals.63 Hence, the holes present on the VB potential edge of MoS2 could not have sufficient potential to generate •OH radicals on reaction with H2O. On the contrary, the type-II p−n heterojunction chargetransfer mechanism could not deduce the photocatalytic RhB dye degradation results as shown by MSLDH3. Nevertheless, it would not follow Z-scheme charge separation path and participation of h+, •O2−, and •OH as the active species for enhanced RhB degradation. On the basis of band potentials and existence of powerful electrostatic force of attraction between MoS2 and NiFe LDH, the Z-scheme mode of chargetransfer path was proposed for carrier charge separation. MoS2 possesses a very narrow band gap and is unable to perform as the mediator in carrier charge transfer. Hence, MSLDH3 behaves as the direct Z-scheme photocatalyst.55,56 Then, MoS2 electrostatically bonded with the NiFe LDH nanosheets in MSLDH3 generates a fine Schottky barrier by the contact of defective surface of NiFe LDH. Afterward, photoinduced electrons on the CB potential of NiFe LDH rapidly recombined with photoinduced holes at the VB potential edge of MoS2, which effectively extend the lifetime of exciton pairs. In MSLDH3 (Scheme 3b), electrons accumulated on the CB of MoS2 possess enough potential to acquire dissolved oxygen and generate superoxide radicals EΘ(O2/•O2− = −0.33 eV vs NHE) or •O2− and react with H+ to form hydroperoxy radical EΘ(O2/•HO2 = −0.05 eV vs NHE), which disintegrated into •OH radicals and played a key role in the degradation of RhB to small organic molecules. The higher positive VB of NiFe LDH in MSLDH3 than EΘ(•OH/OH− = +1.99 eV vs NHE) was quite capable to generate •OH. As a result, reaction of holes with H2O at the VB potential of NiFe LDH generates • OH radicals, which further degraded RhB to nontoxic products. Moving toward the reduction reaction activities, Figure 10a,b shows the meager capability of solitary MoS2 in reduction of H+ to H2, and this might be owing to fast electron−hole recombination factor. Alternatively, enhanced photocatalytic H2 evolution could be explained appropriately if MSLDH3 follows a Z-scheme-based p−n heterojunction charge-transfer mechanism (Scheme 3b) and photoinduced electrons on the CB of NiFe LDH recombined with the holes on the VB edge of MoS2. This kind of separation of photoinduced charge pairs could indicate that electrons remain in the more −ve CB edge potential of MoS2 and holes in the more +ve VB edge potential of NiFe LDH, respectively. The electrons of CB edge potential of MoS2 in MSLDH3 exhibit strong reducibility owing to be in a higher −ve potential and reduce H+ to H2, and the results well agreed with H2 evolution experiments. Hence, the anticipated Z-scheme mechanism (Scheme 3b) route of photoinduced carrier charge separation is thermodynamically more positive in the heterostructure

PS II from photoreduction reactions. Nevertheless, photooxidation and photoreduction processes can easily occur in PS II and PS I, respectively. Therefore, the development of p−n type heterojunctions along with the Z-scheme charge-transfer path would contribute more toward charge separation and easily protect PS I and PS II from photocorrosion. The establishment of a Schottky junction in a p−n heterojunction would act as a barrier of charge recombination, thereby protecting PS I and PS II from self-corrosion. Hence, the formation of a p−n heterojunction with Z-scheme charge separation would greatly contribute toward faster charge separation in MSLDH3 for superior photocatalytic activities. Accordingly, two types of electron-transfer processes compete with each other as shown in Scheme 3. The first one is type-II p−n heterojunction band alignment (Scheme 3a), and the second one is the Z-scheme charge-transfer path (Scheme 3b). In type-II band alignment (Scheme 3a), photogenerated electrons could strike the Schottky barrier at the CB of MoS2 and transfer toward the CB of NiFe LDH. Consequently, transfer of holes occurs from the VB of NiFe LDH to the VB of MoS2, respectively. Nevertheless, these kinds of charge-transfer paths might decrease the redox capability of the heterostructure material and thereby decrease the photocatalytic activity of MSLDH3. In a charge-transfer route through the Z-scheme mechanism, the holes on the VB edge of MoS2 and electrons on the CB of NiFe LDH could be recombined next to the interfacial area parting electrons and hole pairs with higher potential at the CB edge of MoS2 and VB edge of NiFe LDH to proceed photocatalytic redox reactions as illustrated in Scheme 3b. The excited electrons of NiFe LDH have immense chance to recombine with the hole of MoS2 and generate a Z-scheme-type photocatalyst. At this point, photoinduced holes on the VB of NiFe LDH or photoinduced electrons on the CB of MoS2 have enough oxidizability and reducibility for the degradation of RhB. Alternatively, electrons with higher potential in MoS2 migrated to the surface and reduce H+ ions in order to produce H2, whereas holes at the VB edge of NiFe LDH are quenched by hole scavengers. Such p−n heterojunctions with Z-scheme mechanistic charge separation could clearly clarify the photodegradation activities of RhB degradation as well as H2 evolution reactions. Coming into the RhB degradation reaction mechanism, the active oxidative species of MSLDH3 in the RhB degradation were revealed through the scavenger experimental method. Generally, p-BQ (5 mM), IPA (5 mM), DMSO (5 mM), and EDTA (5 mM) were used as scavenging reagents for trapping • O2− radicals, •OH radicals, e−, and h+, respectively. Figure 9a shows the photocatalytic performances of MSLDH3 after adding scavengers, which indicate a fall in degradation performances of 28, 18, 86, and 41% by using EDTA, p-BQ, DMSO, and IPA as scavengers, respectively. Also, Figure 9a reveals the change in the catalytic activity of MSLDH3 after addition of DMSO, indicating the minor role of e− in RhB degradation. Thus, the major reactive species accountable for the RhB degradation comprises •O2−, •OH, and h+ by using the MSLDH3 catalyst. Additionally, to corroborate the capability of •O2− and •OH radical production by MSLDH3, NBT and TA tests were carried out and depicted in Figure 9.8,58 Moreover, the TA-PL spectral data of MSLDH3 (Figure 9c) show the highest fluorescence intensity, indicating the larger quantity of formation of •OH radicals, and the decrease P


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ACS Applied Materials & Interfaces MSLDH3 as compared to direct type-II band alignment among MoS2 and NiFe LDH (Scheme 3a). Therefore, the aligned band edge potential at the CB and VB of MSLDH3 in Z-scheme heterojunctions was thermodynamically more positive for RhB degradation and H2 evolution. The marked augmentation of photocatalytic RhB degradation and H2 evolution reaction in MSLDH3 is owing to the Zscheme mode of exciton pair separation. Further, the Z-scheme mode of exciton pair separation was confirmed by examining the band edge potential of the photogenerated hole by PL experimental results using TA.8,20,64 When a photocatalyst produces •OH radicals by the photooxidation of the OH− anion, then TA could be transformed into TAOH. The change in PL spectra of TA solution with MSLDH3 shows a significant intense PL peak under the exposure of visible light (Figure 9c). There were no intense peaks detected in MoS2 owing to their higher −ve VB potential (+1.78 V vs NHE) as compared to EΘ(•OH/OH− = +1.99 eV vs NHE), which is inadequate to generate •OH radical. NiFe LDH emits PL emission band owing to the +ve VB potential (+2.19 V vs NHE). Captivatingly, MSLDH3 resulted in more intense PL peak than NiFe LDH. Alternatively, the holes generated in the MSLDH3 photocatalyst tend to accumulate faster on the VB of NiFe LDH. However, the photoinduced hole on the VB potential of NiFe LDH in a conventional type-II heterojunction was migrated toward higher −ve VB of MoS2. Therefore, PL emission spectral data acquired from the TA probe test indicate that MSLDH3 follows a Z-scheme chargetransfer route of exciton pair separation. Accordingly, exciton pairs preserved in higher reduction and oxidation potentials were found in the Z-scheme MSLDH3 p−n-type heterojunction nanocomposite for the H2 evolution reaction and RhB dye degradation products. This MSLDH3 nanocomposite was regarded as an electrostatic heterostructure based on their synthetic approach.65−74

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Phone: +91-674-2351777. Fax: +91-674-2350642. ORCID

Kulamani Parida: 0000-0001-7807-5561 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their profound gratitude toward Siksha ‘O’ Anusandhan Deemed to be University for giving all necessary facilities and financial support to carry out this immense research work. The funding from MNRE project (grant no. 103/233/2014-NT) is greatly respected.

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REFERENCES

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CONCLUSIONS In brief, effort to fabricate the heterostructure MoS2/NiFe LDH nanocomposite was possible by the combination of hydrothermal and electrostatic self-assembly methods. The excellent catalytic activities of MSLDH3 were mostly connected with the existence of electrostatic force of interactions among NiFe LDH and MoS2 with effective separation of carrier charge by p−n heterojunction-based Zscheme mechanistic routes. The morphology of MSLDH3 as revealed from HR-TEM images shows the interconnectivity and tumbling of layers between NiFe LDH and MoS2 in the MSLDH3 hybrid. The •O2− radicals generated on the MoS2 surface and direct oxidation of h+ to •OH on the NiFe LDH surface after equilibration of the Fermi level strongly proved the Z-scheme mechanism route in MSLDH3 and played important roles in the RhB degradation process. MSLDH3 exhibited a H2 production rate of 550.9 μmol/h, which is enhanced by 10.9- and 19.2-fold times higher than that of NiFe LDH and MoS2, respectively. The status of MSLDH3 upraised as a gifted sunlight-driven photocatalytic material appropriate for remediation of environmental pollutant as well as suitable for clean energy production.

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XPS survey scan spectrum of NiFe LDH, MSLDH3, and MoS2; SAED pattern and EDX spectra of MSLDH3; photocatalytic rate constant and stability test for RhB degradation using the physical mixture of MoS2 and NiFe LDH under sunlight irradiation, rate of photocatalytic H2 evolution, and stability test between the physical mixture of MoS2 and NiFe LDH under visiblelight irradiation; chronoamperometric plot of MSLDH3 in 0.1 M Na2SO4 solution (pH = 6.5) at −1.0 V applied potential versus Ag/AgCl; and comparison of RhB degradation and H2 evolution on MSLDH3 with materials outlined in the literature (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06511. Q


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