Defect-Rich MoS2 Ultrathin Nanosheets-Coated Nitrogen-Doped ZnO

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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Defect-Rich MoS2 Ultrathin Nanosheets-Coated Nitrogen-Doped ZnO Nanorod Heterostructures: An Insight into in-Situ-Generated ZnS for Enhanced Photocatalytic Hydrogen Evolution Suneel Kumar,†,# Ajay Kumar,† Vempuluru Navakoteswara Rao,‡ Ashish Kumar,† Muthukonda Venkatakrishnan Shankar,‡ and Venkata Krishnan*,†

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School of Basic Sciences and Advanced Materials Research Center, Indian Institute of Technology Mandi, Mandi 175005, Himachal Pradesh, India ‡ Nanocatalysis and Solar Fuels Research Laboratory, Department of Materials Science & Nanotechnology, Yogi Vemana University, Kadapa 516005, Andhra Pradesh, India S Supporting Information *

ABSTRACT: Molybdenum disulfide has emerged as one of the promising materials, particularly as a co-catalyst for photocatalytic hydrogen evolution over the conventional and more-expensive platinum. Herein, we report novel onedimensional/two-dimensional (1D-2D) heterostructures consisting of nitrogen-doped ZnO nanorods coated with defect-rich MoS2 nanosheets having abundant edge sulfur atoms. The optimized heterostructure consists of 15 wt % of defect-rich MoS2 nanosheets-coated on N-ZnO showed the highest H2 evolution of 17.3 mmol h−1 gcat−1 under solar light irradiation. The improved photocatalytic H2 evolution can be attributed to (i) the in-situ-generated ZnS during the process, which increased the number of interfaces, (ii) the presence of abundant exposed sulfur edge atoms in defect-rich MoS2 nanosheets, which has strong affinity for H+ ions, and (iii) the intimate heterojunction formed between N-ZnO and MoS2, which facilitates charge transfer efficiency. Hence, this work offers a promising strategy for the design and development of defect engineered heterostructure photocatalysts for greatly enhanced solar-to-fuel conversion. KEYWORDS: Semiconductor heterostructures, defect engineering, synergistic effects, photocatalytic hydrogen evolution, solar-to-fuel conversion

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photocatalytic materials to generate H2 by water splitting.5,8−12 For photocatalytic H2 evolution reaction, the conduction band minima of a semiconductor must have more negative potential than the reduction potential of H+/H2 (0 V vs NHE at pH = 0) and valence band maxima must be more positive than the oxidation potential of H2O/O2 (1.23 V vs NHE at pH = 0).13 Zinc oxide (ZnO) is considered to be one of the promising materials, because of favorable band positions, high photosensitivity, nontoxicity, low cost, high exciton binding energy (60 meV), and excellent chemical and physical stability.14 ZnO

INTRODUCTION Artificial photosynthesis for photocatalytic hydrogen (H2) evolution by water splitting has gained considerable attention, because of its unique potential to solve global energy crisis and environmental pollution.1−3 Therefore, enormous research efforts have been dedicated in the field of photocatalysis for H2 evolution over semiconductor oxides by harvesting solar energy.4−6 Hydrogen is regarded as a zero-emission fuel, and a green energy source (because of its high combustion heat and clean combustion product); hence, it is one of the best alternatives to the fossil fuels.7 Since 1972,8 when the first report came on photoelectrochemical H2 evolution by using TiO2, various other semiconductors, such as CdS, MoS2, Fe2O3, ZnO, WO3, Cu2O, etc. have been utilized as © XXXX American Chemical Society

Received: April 22, 2019 Accepted: July 8, 2019

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DOI: 10.1021/acsaem.9b00790 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

rich nanosheets with abundant edge sites. It has been proved theoretically that MoS2 exists in a 2H semiconducting phase and a 1T metallic phase, with respect to the configuration of S atoms, and exhibit different electronic properties.41 The 1T metallic phase of MoS2 has both the edges and basal planes catalytically active, whereas, in the case of the 2H semiconducting phase, the basal plane remain inert and edges are active. Although there are some reports with regard to the use of defect-rich MoS2 for photocatalytic hydrogen generation, its composite with other materials has not been fully explored. In this work, our objective was to design and develop heterostructures of defect-rich ultrathin MoS2 nanosheets coated over N-doped ZnO nanorods by using hydrothermal synthesis method. The prepared catalysts have been utilized for photocatalytic H2 generation under solar light irradiation, in the presence of Na2S and Na2SO4 as sacrificial reagents. These nanocomposite heterostructures exhibit core−shell type of morphology, wherein N-ZnO nanorods are coated with defectrich MoS2 nanosheets, which provide large interfacial intimate contact between two components. Such a unique structure can exhibit abundant reaction sites on the edges of MoS 2 nanosheets and allow fast charge transfer across heterojunction for remarkably enhanced photocatalytic H2 evolution. Furthermore, the crystal structure and morphological studies of one of the recycled catalysts reveals the in situ formation of ZnS during the photocatalytic reaction, which significantly promotes the photoinduced charge transfer process by introducing an additional ZnS−ZnO interface. Therefore, this work not only provides an easy method to fabricate the NZnO−MoS2 nanosheet heterojunctions but also provides insight into the scope of exploitation of such materials for hydrogen fuel generation using water and sunlight.

is also considered as a superior photocatalyst, in comparison to TiO 2 , because of fast charge carrier transport, easy crystallization, and anisotropic crystal growth.15 ZnO is an ntype, group II−IV semiconductor with a band gap energy of 3.37 eV and a Hall mobility on the order of 200 cm2 V−1 s−1 at room temperature.15 It crystallizes into hexagonal wurtzite structure (a = 3.25 Å, c = 5.20 Å). Despite various advantages, the photocatalytic efficiency of ZnO is restricted because of various factors, such as the absorption of restrictive light photons (only in the ultraviolet (UV) region, which constitutes ∼5% of the solar spectrum), short lifetime of photoinduced charge carriers, and photocorrosion. 14 Hence, it is of considerable significance to fabricate the ZnO-based photocatalysts with excellent photoactivity, high stability, and wide applicability. In past years, the coupling of ZnO with other semiconductors, such as ZnO−TiO2,16 ZnO−CdS,17 ZnO− ZnS,18 ZnO−CuO,19 ZnO−BiOI,20 ZnO−Cu2O,21 etc. have been investigated mainly as photoelectrodes for H2 evolution and there are only limited reports on photocatalytic H2 evolution with ZnO-based heterostructures.22−25 The narrow-band-gap semiconductors can utilize visiblelight photons, but they also suffer from the fast recombination of photoinduced charge carriers, which drastically decreases their photocatalytic efficiency.26,27 Therefore, doping, inducing defects, such as oxygen vacancies in semiconductors suppress the photoinduced charge carriers recombination by introducing defect levels in the crystal structure.26,28,29 Furthermore, the combination of semiconductors to form heterojunction nanostructures is one more interesting strategy to prolong the recombination of the charge carriers by increasing their lifetime.30 The heterojunction formation between two semiconductors induces the electric field at the interface, which promotes the charge transfer to the catalyst surface, wherein transferred species undergo redox reaction and, hence, boosts photocatalytic activity.11,31 In addition, a semiconductor heterojunction improves the charge dynamics, such as charge transportation direction and separation distance. Among the visible light absorbing materials, molybdenum disulfide (MoS2) is an exciting material, which corresponds to a large family of two-dimensional (2D) layered transition-metal dichalcogenides (LTMDs) and is well-known for its diverse applications.4,32−34 The MoS2 exhibit an analogous structure with graphene, having a hexagonal crystal lattice in which S− Mo−S layers are have strong in-plane bonding and vertically stacked by weak van der Waals forces and exhibit several fascinating mechanical, electronic, electrochemical, optical, and thermal properties.35 It is well reported that the edges of 2D MoS2 layers have exposed unsaturated S atoms that have a strong affinity for H+ ions in solution, while its basal plane is catalytically inert.36,37 Thus, the unsaturated sulfur atoms on the edges of MoS2 layers acts as H2 evolution centers by reduction of H+ ions and, hence, plays an important role in improving the photocatalytic efficiency. In some recent reports, it has been shown that engineering of defects density on the edges of 2D MoS2 nanosheets can improve the photocatalytic H2 evolution performance.38−40 Therefore, enormous efforts have been made on enhancing the active sites by fabricating defect-rich materials with abundant edges and conductivity.39,40 For instance, Xie et al.40 have reported the fabrication of defect-rich ultrathin MoS2 nanosheets for enhanced hydrogen evolution. The defects were engineered by controlling the amount of thiourea precursor, which hinders the oriented crystal growth, resulting in defect-

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EXPERIMENTAL SECTION

Chemicals. Hexaammonium heptamolybdate tetrahydrate, (NH4)6Mo7O24·4H2O and thiourea (NH2CSNH2) were obtained from Merck, India. Zinc chloride (ZnCl2), aqueous ammonia solution (NH3), and sodium hydroxide (NaOH) were also supplied by Merck, India. Deionized (DI) water (18.2 MΩ cm) was obtained from ELGA PURELAB Option-R7 and was used for all experimental works. Materials Preparation. Synthesis of ZnO and N-Doped ZnO Nanorods. ZnO nanorods were synthesized by adopting the hydrothermal synthesis method as previously reported in the literature.42 In brief, 0.2 M ZnCl2 solution was prepared in 5 mL of ethanol. Furthermore, 35 mL of 0.5 M aqueous solution of NaOH was added into ZnCl2 solution with continuous stirring, which resulted in the formation of a white precipitate. This suspension was finally transferred into a 50 mL Teflon-lined stainless steel autoclave, sealed tightly, and treated at 180 °C for 12 h. After the completion of reaction time, autoclave was allowed to cool naturally and the obtained precipitates were washed with DI water and ethanol mixture by centrifugation. The final product was dried at 60 °C in a heating oven. The amount of the catalyst obtained was ∼150−200 mg in each batch. Nitrogen-doped ZnO nanorods (N-ZnO) were prepared by adopting the same method as reported earlier in one of our previous works.42 First of all, 100 mg of ZnO nanorods were dispersed in 40 mL of DI water. This was followed by the addition of 8 mL of aqueous ammonia solution under constant stirring to form a clear solution. Furthermore, the mixture was transferred into a Teflon-lined stainless steel autoclave of 50 mL and heated at 150 °C for 24 h. After the autoclave was cooled naturally, ∼70−80 mg of the final product was collected by centrifugation and washed several times with DI water and ethanol, dried at 60 °C. Fabrication of N-ZnO Nanorods Coated with Defect-Rich MoS2 Nanosheets (MNZ Heterostructures). Defect-rich MoS2 nanosheets B


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Scheme 1. Illustration of the Procedure for the Fabrication of N-ZnO Nanorods Coated with Defect-Rich MoS2 Nanosheets

Figure 1. XRD patterns of (a) MoS2 nanosheets and (b) ZnO, N-ZnO, MNZ5, MNZ8, MNZ11, MNZ15, and MNZ20 heterostructures. Evaluation of Photocatalytic Activity. The laboratory-scale batch-type reactor (Kjeldahl flask with a capacity of 185 mL) made of quartz is used for photocatalytic H2 generation experiments under natural sunlight irradiation. The detailed procedure and analysis conditions were elaborated in our recent publication.43 The chosen photocatalyst (10 mg) was dispersed in 50 mL of aqueous solution containing 0.3 M Na2S and Na2SO4 as a sacrificial agent. Since the photocatalytic experiments require an oxygen-free environment for H2 generation, the single port reactor top was sealed with a specially made silicon rubber septum, followed by evacuation using a vacuum pump and purged with nitrogen gas for 30 min in each stage. This reactor setup mounted with clamp and stirred magnetically and exposed to solar light from 10 am to 2 pm. At an interval of every hour, the generated H2 was analyzed and quantified using a gas chromatograph (Shimadzu, Model GC-2014) fitted with a molecular sieve/5 Å column and a thermal conductivity detector. The equipment was precalibrated with high-purity H2 and O2 gases. Photoelectrochemical Studies. A scanning potentiostat (Metrohom, Autolab) was employed to perform photoelectrochemical

were synthesized by following a previously reported hydrothermal synthesis route.40 Typically, 1 mM of hexaammonium heptamolybdate tetrahydrate, (NH4)6Mo7O24·4H2O was dissolved in 60 mL of DI water, followed by the addition of 30 mM of thiourea. The solution was vigorously stirred for 2 h and then transferred to a 50 mL Teflon-lined stainless steel autoclave maintained at 200 °C for 24 h. Finally, the product was washed with a mixture of DI water and ethanol and dried at 60 °C. In order to prepare the defect-rich MoS2 nanosheets-coated NZnO heterostructures, ∼200 mg of as-prepared N-ZnO nanorods was dispersed in DI water and subsequently defect-rich MoS2 nanosheets were added in appropriate amount. The solution was transferred to a 50 mL Teflon-lined stainless steel autoclave and heated at 200 °C for 24 h. The final product was washed with a mixture of DI water and ethanol and dried at 80 °C to yield ∼180−210 mg of nanocomposite. The MoS2 loading was varied by as 5%, 8%, 11%, 15%, and 20% over ZnO nanorods to form MNZ5, MNZ8, MNZ11, MNZ15, and MNZ20 nanocomposites, respectively. C


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Figure 2. Raman spectra of (a) MoS2 nanosheets and (b) ZnO, N-ZnO, MNZ5, MNZ8, MNZ11, MNZ15, and MNZ20 heterostructures.

Figure 3. SEM micrographs of (a) N-ZnO nanorods, (b) MoS2 nanosheets, and (c) MNZ15 heterostructure.

36-1451).25 Note that the diffraction peaks of N-ZnO are slightly broader than that of bare ZnO nanorods, which signify the slight decrease in the crystallinity due to the expansion in crystallite size. It is also clear that diffraction peaks of N-ZnO appear at slightly higher 2θ angles due to the tensile strain in the ZnO lattice that is caused by a slightly larger nitrogen dopant, compared to O (Figure S1 in the Supporting Information).44 The main diffraction peak of MoS2, at 2θ ≈ 14.5°, was not observed in the XRD pattern of N-ZnO-MoS2 heterostructures, because of the small amount of MoS2 present in the sample, and its reduced stacking is due to heterojunction formation with the N-ZnO nanorods. Raman Spectroscopic Analysis. Raman spectroscopic analysis has been performed on pristine, doped catalyst and heterostructures to further confirm their structure, and the corresponding data have been presented in Figure 2. The Raman spectra of MoS2 nanosheets displays peaks at 282, 378, 402, and 451 cm−1, which could be assigned to the E1g, E12g, A1g, and 2LA(M) vibration modes, respectively.45 The E1g and E12g modes correspond to the associated vibrations of two S atoms, with respect to the Mo atom, while the A1g vibration mode signify the out-of-plane vibration of S atoms in MoS2. For bare ZnO, Raman peaks are observed at 330 and 436 cm−1 which could be assigned to A1 and E2 (high) vibration modes, which are the characteristic bands for hexagonal wurtzite ZnO.45 The N-doping in ZnO crystal introduces the disorder activated Raman scattering, which give rise to additional bands assigned as silent B1 (low) mode at 271 cm−1 and B1 (high) mode at 578 cm−1, as previously reported in the literature.42 It is noteworthy to mention here that the A1 and E2 (high) vibration modes can be observed in N-ZnO, which indicates that the hexagonal structure remains intact after N-doping. The thin MoS2 nanosheets coated N-ZnO nanorod heterostruc-

studies using a three electrode setup. Typically, 4 mg of the photocatalyst and 40 μL Nafion were dispersed in a mixture of water and ethanol (2:1 volume ratio). The prepared mixture was than sonicated for at least half an hour and then drop-casted onto an indium tin oxide (ITO) substrate (working electrode). The transient photocurrent measurements have been performed at a potential of 0 V, wherein Ag/AgCl electrode was used as the reference, a platinum wire was used as the counter electrode, and 0.1 M Na2SO4 was used as the aqueous electrolyte. Subsequently, the Nyquist plots were recorded with frequencies ranging from 0.1 Hz to 105 Hz with an amplitude of 0.01 V.

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RESULTS AND DISCUSSION Heterostructure Fabrication and Crystal Phase Analysis. The procedure for the fabrication of N-ZnO nanorods coated with defect-rich MoS2 nanosheets is schematically illustrated in Scheme 1. The crystal structure and phase purity of as-prepared bare catalysts and N-ZnO-MoS2 heterostructures with different loading of MoS2 nanosheets were analyzed by X-ray diffraction (XRD) studies and the corresponding plots are presented in Figure 1. The defected MoS2 nanosheets exhibit diffraction peaks at 2θ = 14.5°, 32.8°, and 57.2°, which can be indexed to the (002), (100), and (110) reflection planes, respectively of hexagonal phase MoS 2 (Joint Committee on Powder Diffraction Standards (JCPDS) No. 37-1492), as presented in Figure 1a.25 The XRD pattern of ZnO, N-ZnO nanorods and defected MoS2-coated N-ZnO heterostructures (MNZ5, MNZ8, MNZ11, MNZ15, and MNZ20) can be seen in Figure 1b. These catalysts show diffraction peaks at 2θ ≈ 31.7°, 34.2°, 36.1°, 47.5°, 56.6°, 62.8°, 66.1°, 68.0°, and 69.2°, which could be respectively assigned to the (100), (002), (101), (102), (110), (103), (200), (112), and (201) lattice planes in the polycrystalline hexagonal wurtzite structure of ZnO nanorods (JCPDS No. D


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Figure 4. TEM images of (a) N-ZnO nanorods, (b) MoS2 nanosheets, (c) MNZ15 heterostructure; HRTEM images of (d) MoS2 nanosheets, (e) MNZ15 heterostructure, and (f) EDAX spectra of MNZ15 heterostructure.

heterostructure has been confirmed by EDAX spectra (Figure 4f). Furthermore, the scanning tunneling electron microscope (STEM) image and corresponding elemental mapping images of Zn, O, N, Mo, and S are presented in Figure S3 in the Supporting Information, wherein the dispersion of MoS2 can be observed over N-ZnO nanorods. The surface chemical composition and oxidation state of all the constituent elements in the MoS2, N-ZnO and MNZ15 (representative heterostructure) were investigated by performing X-ray photoelectron spectroscopy (XPS) measurements, and corresponding binding energy plots have been presented in Figure 5. Figure 5a shows the survey spectrum of MoS2, NZnO, and MNZ15 in the region of 0−1350 eV. The presence of all peaks in the survey spectra confirms the existence of all constituent elements. The survey scan of MNZ15 shows the presence of Zn 2p, O 1s, Mo 3d, S 2p, and N 1s peaks; also, the successful incorporation of nitrogen in ZnO is confirmed by the presence of the N 1s binding energy peak (see Figures 5a and 5b). The atomic percentage of nitrogen doping in NZnO nanorods and MNZ heterostructures was determined to be 3% from the XPS analysis. The N 1s peak has been deconvoluted into two peaks with binding energies at 398.4 and 399.6 eV, wherein the former binding energy peak indicated the presence of O−Zn−N moiety in the ZnO lattice and later peak represents the presence of the Zn−N bond.42 Therefore, the two binding energies for N 1s suggests the presence of doped N in two different chemical environments. The Zn 2p high-resolution scan spectrum in Figure 5d shows the presence of two characteristic binding energy peaks at 1021.6 and 1044.7 eV corresponding to the Zn 2p3/2 and Zn 2p1/2 electrons, respectively, which signify the +2 oxidation state of Zn in the MNZ heterostuctures. Compared to bare ZnO, there is a peak shift of 0.5 eV in MNZ15 heterostructure, which signifies the strong interactions between the components (Figure 5(c)). The O 1s binding energy peak has been deconvoluted into three peaks at 530.2, 531.4, and 532.3 eV, which suggests the presence of three different types of oxygen species in the catalyst, as depicted in Figure 5e. The binding energy peak at 530.2 eV could be assigned to lattice O contribution of NZnO, while the peak at 531.4 eV correspond to the presence of O (−OH) molecule on the surface of photocatalyst. The third

tures, MNZ5, MNZ8, MNZ11, MNZ15, and MNZ20 with different loadings of MoS2 exhibit the Raman bands corresponding to the crystal of N-ZnO and MoS2, which confirms the successful heterojunction formation. Morphology and Composition Analyses. Morphological features of as-prepared bare and heterostructure catalysts have been analyzed by field-emission scanning electron microscopy (FESEM), and corresponding micrographs has been presented in the Figure 3. The N-ZnO nanorods are 50− 300 nm wide, 20 nm thick, and several micrometers in length (Figure 3a). Figure 3b clearly shows that the bare MoS2 exhibits the self-assembled nanostructures to form a flowerlike morphology. The SEM micrograph of MNZ15 (representative heterostructure) presented in Figure 3c shows the coating of MoS2 nanosheets over N-ZnO nanorods to form a heterostructure with intimate contact between N-ZnO and defect-rich MoS2 nanosheets. To further investigate the nanosized heterostructures, the transmission electron microscopy (TEM) analysis has been performed and is presented in Figure 4. TEM results for NZnO nanorods also complements the SEM studies and confirms its nanorod-like morphology (Figure 4a). The TEM image of bare MoS2 nanosheets indicate its thin sheetlike structures giving a flowerlike morphology, as depicted in Figure 4b. The dislocations and distortions marked as white dashed circles in Figure 4c indicate the defect-rich nanosheets. In addition, Figure S2 in the Supporting Information presents another HRTEM image of defected MoS2 nanosheets depicting defects in the form of dislocation in lattice fringes, and these observations have been found to be consistent with the literature.38 Moreover, the dispersion of thin MoS2 nanosheets over N-ZnO nanorods in MNZ15 (a representative heterostructure) to form heterojunctions can be seen clearly evidenced in Figure 4d. The presence of lattice fringes corresponding to the N-ZnO nanorods and MoS2 nanosheets confirm the formation of N-ZnO-MoS2 heterojunction (MNZ) in Figure 4e. An interlayer spacing of 0.62 nm was observed in the heterostructure, which corresponds to the (002) crystal plane of MoS2, while interlayer d-spacing of 0.24 nm corresponds to the characteristic diffraction plane (101) of N-ZnO nanorods. The presence of all of the constituent elements (N, Zn, O, Mo, and S) in the MNZ15 E


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Figure 5. XPS spectra of (a) survey spectra of MoS2, N-ZnO, MNZ15; (b) N 1s (MNZ15); (c) Zn 2p (N-ZnO); (d) Zn 2p (MNZ15); (e) O 1s (N-ZnO); (f) O 1s (MNZ15); (g) Mo 3d (MoS2); (h) Mo 3d (MNZ15); (i) S 2p (MoS2); and (j) S 2p (MNZ15).

binding energy peak at 532.3 eV presents the characteristic peak of the O-vacancies in N-ZnO lattice due to N-doping.

The binding energy peaks of O 1s (MNZ15) appear at a slightly higher binding energy, compared to O 1s (N-ZnO) in F


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to extended absorption.42 The bare MoS2 nanosheets show absorption in the entire visible region of 200−800 nm. The plots obtained via the transformation based on Kubelka−Munk function versus the energy of light for MNZ5, MNZ8, MNZ11, MNZ15, and MNZ20 heterostructures are shown in Figure S5 in the Supporting Information, which presents the obtained band-gap values for all heterostructures and bare catalysts.47 The decrease in the band gap with increasing MoS2 content was observed in these heterostructures, which can be attributed to the intimate interfacial contact between N-ZnO and MoS2 to form N-ZnO-MoS2 heterojunction.42 The specific surface area of photocatalytic materials is highly significant to determine their photoactivity.48 The BET measurements were performed to investigate the surface area and pore size of bare MoS 2 , N-ZnO, and MNZ15 heterostructure, and the corresponding results are shown in Figures S6a−S6c in the Supporting Information and summarized in Table S1 in the Supporting Information). For MoS2 nanosheets, the determined specific surface area and pore volume are 16.32 m2 g−1 and 0.011 cm3 g−1, respectively. The BET specific surface areas and the pore volume of N-ZnO nanorods are 13.07 m2 g−1 and 0.015 cm3 g−1, respectively. The higher specific surface area (25.01 m2 g−1) and pore volume (0.016 cm3 g−1) of the MNZ15 heterostructure, compared to bare catalysts, could be ascribed to the uniform distribution of MoS2 nanosheets over N-ZnO nanorods, which can significantly decrease the agglomeration of N-ZnO and results in the increased surface area, which is beneficial for hydrogen evolution. From the Brunauer−Emmett−Teller classification, it can be concluded that the given isotherms (see Figures S6a−S6c) exhibit a typical H3 hysteresis loop, which suggests the presence of slitlike pores. The pore size distribution curves for MoS2, N-ZnO and MNZ15 heterostructure suggests the presence of mesopores 2−28 nm in size (see Figures S6d−S6f). Furthermore, the surface area plots are also obtained by linear fitting of N2 adsorption−desorption values, as shown in Figures S6g−S6i. Hence, the increased surface area, good pore size, and good pore volume aids to yield the high activity during photocatalytic H2 evolution. Photocatalytic Hydrogen Generation, Recyclability and Mechanism. The photocatalytic activity of all prepared photocatalysts have been demonstrated by studying the hydrogen generation from water splitting under natural solar light and aqueous 0.3 M Na2S and Na2SO4 as sacrificial agents. Control experiments were also done without photocatalyst and light illumination, wherein no H2 evolution were observed, which ascertains the role of catalyst and light in photocatalytic reaction. As depicted in Figure 7a, the bare catalysts (ZnO, defected MoS2) shows almost negligible H2 evolution, indicating their poor photocatalytic activity. The N-doped ZnO nanorods exhibit substantially improved H2 evolution rate of 4.661 mmol h−1 gcat−1. This superior photocatalytic activity could be attributed to the N-doping, which introduces the acceptor energy level near to the VB of ZnO and also leads to oxygen vacancies in the ZnO crystal. This facilitates the photoexcitation process by band gap narrowing and increases the lifetime of photoinduced charge carriers by suppressing their recombination. It is noteworthy to mention here that photocatalytic activity increases with the loading of defect-rich MoS2 nanosheets in heterostructures to reach a maximum and then decreases as the MoS2 content increases. For the 4 h of light irradiation, the rate of H2 evolution achieved for MNZ5, MNZ8, MNZ11, MNZ15, and MNZ20 heterostructures was

Figure 5f, indicating the electronic interactions between NZnO and MoS2 in the heterostructure, as previously reported.25 The high-resolution spectrum of Mo 3d in Figure 5g displays the three binding energy peaks, in which 232.1 and 235.6 eV could be assigned to the Mo 3d5/2 and Mo 3d3/2, respectively, which confirms the Mo(IV) state in the heterostructure, while the third peak at 226.4 eV corresponds to the S 2s. On the other hand, the S 2p spectrum shows two binding energy peaks, at 161.7 eV (S 2p3/2) and 162.9 eV (S 2p1/2), which signify the −2 oxidation state of S in the MNZ15 heterostructure. The binding energy shift in the Mo 3d and S 2p of MNZ15 heterostructure, compared to the characteristic peaks of Mo and S indicates the electron coupling in N-ZnO and MoS2, which eventually promotes the charge transfer across heterojunction during photocatalytic reaction. Oxygen incorporation is found in the MoS2 as can be seen from the survey spectrum of bare MoS2 (see Figure S4 in the Supporting Information). The O 1s peak can be deconvoluted into three peaks: at 530.4, 531.5, and 532.0 eV. The peak at the lower binding energy (530.4 eV) is due to the oxygen that is incorporated into MoS2 (the Mo−O bond) during sample preparation.46 Because of surface oxidation, the formation of oxysulfide can also be observed from the deconvoluted peak of oxygen at 531.5 eV. The peak at higher binding energy (532.0 eV) corresponds to surface adsorbed oxygen (−OH groups). Zhang et al.38 have also observed and reported the same oxygen incorporation phenomenon in their nanocomposites comprising of defect-rich MoS2 nanosheets. It has been proposed that oxygen incorporation in defect-rich MoS2 nanosheets decreases the energy barrier for H2 evolution reaction. UV-vis Diffuse Reflectance Spectroscopy and BET Surface Area Analysis. The optical properties of all prepared catalysts were analyzed by measuring the UV-vis diffuse reflectance spectroscopy, and the corresponding results are presented in Figure 6. As depicted in Figure 6, UV-vis diffuse

Figure 6. UV-vis diffuse reflectance spectra (DRS) of photocatalysts.

spectra of bare ZnO nanorods absorb in the UV region only and show an absorption edge at ∼384 nm, which belongs to the intrinsic band-gap absorption and band-gap energy of 3.23 eV.25 After nitrogen doping, N-ZnO shows enhanced absorption in visible light with a decrease in band-gap energy to 3.20 eV. This decrease in band gap occurs due to introduction of acceptor energy level near the valence band (VB) of ZnO and results in the reduced band gap energy due G


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Figure 7. (a) Volume of H2 generated over all the photocatalysts. Experimental conditions: catalyst (10 mg) dispersed in 0.3 M Na2S and Na2SO4 solution under natural solar light for 4 h. (b) Stability of MNZ15 photocatalyst efficient H2 evolution under natural solar light for 12 h of irradiation.

6.358, 6.511, 6.905, 17.363, and 1.987 mmol h−1 gcat−1, respectively. Notably, the photocatalyst with optimum concentration having 15% defect-rich MoS2 in N-ZnO (MNZ15) shows the highest H2 evolution (17.363 mmol h−1 gcat−1), compared to other heterostructures and bare catalysts. Further increasing the amount of MoS2 nanosheets to more than 15% results in the decrease of photocatalytic activity due to shielding of N-ZnO caused by this dark-colored material (MoS2), which is responsible for reduced exposure area and less light absorption by the active heterostructure.49 This explains the decreased photocatalytic activity of MNZ20 heterostructure for H2 evolution. A comparison of our photocatalyst with other reported MoS2-based photocatalysts for hydrogen evolution is provided in Table 1, which validates the superiority of our photocatalyst. The remarkably improved photocatalytic activity of MNZ15 has been ascribed to the following factors: (i) good wrapping (coating) of defect-rich MoS2 nanosheets over N-ZnO nanorods and its rough surface, which results in the defect-induced effective binding at the

interfacial contact region; (ii) abundant catalytic edge sites in defect-rich MoS2 nanosheets with plenty of exposed unsaturated S atoms at the edges with strong affinity for H+ ions in solution, which get reduced by photoinduced electrons to evolve H2 at catalyst surface; (iii) the in situ generation of ZnS during photocatalytic reaction over the surface of MNZ heterostructures, which impart the additional ZnO/ZnS interface for facile charge transfer; and (iv) unique structure of MNZ heterostructures, which causes suppressed recombination and expedient flow of photoinduced charge carriers across the heterojunction, which leads to reduction of H+ ions to hydrogen gas. To examine the stability and reusability of the MNZ15 heterostructure, the photocatalytic H2 evolution experiments were performed for three recycles under sunlight irradiation. As depicted in Figure 7b, after three cycles of photocatalytic reaction, only a minor loss in the photocatalytic activity was observed for the H2 evolution reaction. This minor loss in photocatalytic activity could be ascribed to (i) the oxidation of sacrificial agents, resulting in the decrease in initial concentration, and (ii) the formation of reaction intermediates, which leads to poor adsorption of reactants on the catalyst surface, which influences the generation of protons and, in turn, H2 gas evolution.43,56 In addition, we have characterized the recycled catalyst (MNZ15-R) by XRD and HRTEM analysis, and corresponding results have been presented in Figure 8. The XRD pattern of MNZ15-R has been presented along with that of fresh MNZ15 heterostructure, in Figure 8a. The emergence of a new diffraction peak in MNZ15-R at 2θ ≈ 28.2° could be indexed to the (111) diffraction plane of ZnS (JCPDS No. 05-0566).22 The notable decreases in the diffraction intensity of ZnO patterns clearly indicate the ZnS formation over the catalyst surface during photocatalytic reaction, which is consistent with some earlier reports.22,57 The HRTEM images of MNZ15-R in Figure 8b reveals the existence of lattice fringes having d-spacing of 0.32 nm, which corresponds to the (111) diffraction plane of ZnS and complements the XRD analysis.22 In addition, the HRTEM image also depicts the ZnO/ZnS interfaces in the MNZ15-R catalyst. These results suggest that the MNZ15 heterostructure photocatalyst is quite stable and is not deactivated during the

Table 1. Comparison of the Activity of MoS2-Based Photocatalyst for Hydrogen Evolution No.

photocatalyst system

1

TiO2−MoS2

2

TiO2−MoS2

3 4

TiO2−MoS2− RGO MoS2/CdS

5

CdS@MoS2

6

CdS/MoS2/ graphene MoS2−RGO/ ZnO Au@MoS2− ZnO ZnO−MoS2− RGO ZnO−MoS2

7 8 9 10

light source simulated solar light 300 W xenon arc lamp 300 W xenon arc lamp 300 W xenon arc lamp 300 W xenon lamp 300 W xenon lamp 300 W xenon lamp 300 W xenon arc lamp sunlight sunlight

H2 evolved (μmol h−1 g−1cat)

ref

4300

49

1600

32

206.6

50

685.0

51

2465.5

52

1913

53

288.4

54

813.6

55

28616

25

17363

this work H


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Figure 8. (a) XRD patterns of the MNZ15 (fresh catalyst) and MNZ15-R (recycled catalyst); (b) HRTEM image of the MNZ15-R catalyst.

Scheme 2. Schematic Illustration of Photocatalytic H2 Evolution over N-ZnO-MoS2 Heterostructure under Natural Solar Light Using Na2S and Na2SO4 as a Sacrificial Agent

reaction, despite the in situ formation of ZnS, which plays a favorable role in photocatalytic hydrogen generation. Based on one of the recent works by our group and a few literature reports, the ZnS formation occur over ZnO-based catalyst during photocatalytic reaction in the presence of Na2S and Na2SO4 as sacrificial agents. Upon light irradiation from solar simulator, the electron−hole pairs generation occurs in the conduction band (CB) and valence band (VB) of N-ZnO and MoS2 by absorbing UV and visible-light energy photons. The ZnO-based catalyst undergoes dissolution in alkaline sulfide solution and ZnS formation occurs on the catalyst surface. The entire process of ZnS generation can be summarized with the help of the following equations:25 (i) (ii)

2H 2O + e−CB → H 2 + 2OH−

(iii)

(vi)

S2 O4 2 − + H+ → HSO4 − + S

(vii)

S + 2e−CB → S2 −

(viii)

Zn 2 + + S2 − → ZnS

(ix)

The mechanism of enhanced photocatalytic H2 evolution over the N-ZnO-MoS2 heterostructure has been depicted in Scheme 2. The in-situ generation of ZnS during photocatalytic reaction over the catalyst surface plays an important role in facilitating the charge transfer process. To understand the mechanism, the relative band positions of N-ZnO and MoS2 were investigated because the band-edge potentials determine the photoinduced charge carriers transport across the heterojunction. The CB and VB edge potentials can be calculated by applying the following formula:58

N‐ZnO−MoS2 + hν → N‐ZnO−MoS2 (e−CB + h+ VB) SO4 2 − + H 2O + 2h+ VB → SO52 − + 2H+

SO4 2 − + S2 − + h+ VB → S2 O4 2 −

ECB = X − 0.5Eg + E0 ECB = E VB − Eg

2S2 − + 2h+ VB → S2 2 −

(iv)

S2 2 − + SO52 − → S2 O52 − + S2 −

(v)

Here, Eg correspond to the band-gap energy of semiconductors, E0 is a scale factor relating the reference electrode redox level to the absolute vacuum scale (E0 = −4.5 eV for the I


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Figure 9. (a) Photoluminescence spectra; (b) transient photocurrent response for ZnO, N-ZnO, MoS2, and MNZ15 photocatalysts; (c) Nyquist plots for ZnO, N-ZnO, MoS2, and MNZ15 under visible light irradiation; and (d) Nyquist plots for MNZ15 under dark conditions and visible light irradiation. Inset shows a Bode plot indicating the width of the frequency range.

normal hydrogen electrode), and X is the electronegativity of the semiconductors (ZnO is 5.79 eV, and MoS2 is 5.32 eV). Based on the equation given above, the CB bottom potential of N-ZnO and MoS2 is calculated to be −0.31 eV and −0.11 eV, respectively, with respect to NHE. Consequently, the top of VB potential is calculated to be 2.90 and 1.75 eV for N-ZnO and MoS2, respectively. Thus, the CB level of N-ZnO is more negative, compared to MoS2, while VB is more positive. Therefore, the calculated band-gap positions of both semiconductors indicate that the MNZ heterostructure is advantageous for the efficient separation of photoinduced charge carriers, thereby prolonging their lifetime by restricting their recombination. Importantly, the good wrapping of defect-rich MoS2 nanosheets over N-ZnO nanorods improves the visiblelight absorption by the entire heterostructure, because of sensitization. Under solar light irradiation, the electrons are promptly excited from the CB of the semiconductors (N-ZnO, ZnS), leaving holes in the VB by absorbing light energy. It has been well-reported in the literature that the CB potential of ZnS is more negative, compared to CB potential of ZnO, while VB of ZnO possess more positive potential, compared to the VB of ZnS.57 The intimate interfacial contact between N-ZnOZnS, as confirmed by the HRTEM images (Figure 9b), provides additional interfaces for charge transfer and predominantly enhances the photocatalytic activity. Meanwhile, the photoinduced electrons from the CB of ZnS are transferred to the CB of N-ZnO due to favorable band alignment in the heterostructure, which enables the fast electron mobility, shortens the diffusion path by reducing the electron−hole recombination probability across the intimate

interfacial contact region. The photoinduced electrons from the CB of N-ZnO gets transferred further to MoS2, which act as an excellent co-catalyst, because of its less negative potential. The transferred photoelectrons participate in reduction of adsorbed H+ ions on the edges of MoS2 nanosheets to evolve H2 due to more negative redox potential (−0.11 eV vs NHE) than H+/H2.13,59 The photoinduced holes from VB of N-ZnO and ZnS are scavenged by the hole scavengers. Accordingly, the heterojunction formation by coating N-ZnO nanorods with defect-rich MoS2 nanosheets and in-situ ZnS generation contribute significantly to the photocatalytic activity enhancement by effective charge separation and their facilitated transfer to achieve high H2 evolution rates. Photoluminescence and Photoelectrochemical Studies. The photoluminescence (PL) spectroscopy studies provide the insightful measure of the photoinduced charge separation and transfer process, as the emission intensity originates from their recombination and hence suggests the dynamics of photogenerated charge carriers.60 The prolonged lifetime of photoinduced charge carriers plays an important role in improving the photocatalytic activity by better separation and transfer to the catalyst surface, where the redox reaction occurs. Figure 9a shows the PL spectra of ZnO, N-ZnO, MoS2, and MNZ15 photocatalysts recorded at room temperature and excitation wavelength used was 372 nm. It can be clearly observed from PL spectra that bare ZnO shows three emission bands in the visible region corresponding to the 438, 488, and 508 nm.61 The violet emission at 438 nm arises due to zinc interstitial defects in the crystal lattice, and this peak results due to the recombination of photoinduced J


Article

ACS Applied Energy Materials electrons in these sites with holes.62 The emission band at 488 nm is also referenced as the blue band and indicates the surface state defects in ZnO, while the green emission band at 508 nm corresponds to the oxygen vacancies.63 Overall, the high emission intensity of bands in ZnO suggests the fast recombination rate of photoinduced charge carriers. The role of N-doping in ZnO crystal in suppressing the photoinduced charge carriers by introducing the additional acceptor energy level has also been observed in PL spectra of N-ZnO, which shows the decreased emission intensity. Notably, the MNZ15 heterostructure exhibit substantial decrease in emission intensity indicating the decreased recombination of photoinduced charge carriers due to their efficient transfer across the interface from N-ZnO to MoS2 to boost the reduction of H+ ions to evolve H2. The photocurrent measurements under visible light irradiation were performed for ZnO, N-ZnO, defect-rich MoS2, and MNZ15 heterostructure, and the corresponding photocurrent−time (I−t) curves are depicted in Figure 9b. The photocurrent density suggests the formation of photoinduced charge carriers and, hence, determines the efficiency of the photocatalysts. The enhanced photocurrent density indicates the effective separation of photoinduced charge carriers due to their hindered recombination and improved photocatalytic activity. Upon irradiating the visible light, the photocurrent increases rapidly to a maximum value, decays to the steady state, and quickly decreases when the light is turned off. The photocurrent density for bare catalysts, ZnO, N-ZnO, and MoS2 observed were 0.07, 0.33, and 0.14 μA cm−2. The low photocurrent response for bare catalysts reveals the high recombination rate of photoinduced charge carriers during photocatalysis, which restrict their transfer to the catalyst surface. The coating of defect-rich MoS2 nanosheets over NZnO nanorods leads to notable increase in photocurrent density in MNZ15 heterostructure and an enhanced photocurrent of 5.23 μA cm−2 was observed under visible-light irradiation. The photocurrent of MNZ15 heterostructure is many times greater than that of bare ZnO, which could be ascribed to the favorable band alignment of ZnO and MoS2 to form an intimate heterojunction to reduce the photoexcitation threshold energy. The electrochemical impedance spectroscopy (EIS) measurements were done to study the transfer of interfacial charge carriers, particularly in the MNZ15 heterostructure electrode, along with bare catalysts, ZnO, N-ZnO, and MoS 2 ; corresponding Nyquist plots are presented in Figures 9c and 9d. The first and second semicircle regions correspond to the charge transfer resistance at counter electrode and working electrode (photocatalyst), respectively. EIS analysis reveals the presence of large semicircle in the middle frequency region for ZnO, N-ZnO, and MoS2 under visible light (Figure 9c). Notably, for MNZ15 heterostructure electrode, the size of semicircle decreases remarkably under visible-light irradiation, which signifies the decreased resistance and high conductivity across the heterojunction, as depicted in Figure 9d. The high conductivity in the MNZ15 electrode is due to the coating of MoS2 nanosheets on N-ZnO nanorods and the formation of an intimate heterojunction, which facilitates the charge transfer by decreasing the recombination of photoinduced charge carriers. Overall, the MoS2 act as a co-catalyst by collecting the photoinduced electrons from CB of N-ZnO and ZnS (in-situgenerated), which boosts the H+ reduction to generate H2 and, hence, significantly enhances the photocatalytic activity.

Therefore, the EIS results support the photocatalytic H2 evolution results, wherein MNZ15 exhibited the highest activity, which is several fold higher than that of bare photocatalysts.

■

CONCLUSIONS In summary, one-dimensional/two-dimensional (1D−2D) heterostructures consisting of N-doped ZnO nanorods coated with defect-rich MoS2 nanosheets have been fabricated using facile hydrothermal route. The amount of defect-rich MoS2 nanosheets has been varied as 5%, 8%, 11%, 15%, and 20%, with respect to N-ZnO nanorods to form heterostructures having different compositions. In comparison to the bare catalysts, these binary heterostructures show remarkable increase in the photocatalytic H2 evolution rate by enhancing the light harvesting efficiency (both in UV and visible region), facilitated charge transfer across heterointerfaces and restricted recombination of charge carriers. The optimized heterostructure with 15% of MoS2 nanosheets (MNZ15) exhibit the highest H2 evolution of 17.3 mmol h−1 gcat−1 under solar light in the presence of Na2S and Na2SO4 as sacrificial agents. The plausible mechanism suggests that the photoexcitation process was facilitated by the acceptor energy level near to the valence band (VB) of ZnO introduced by N-dopant, followed by the electrons transfer from the conduction band (CB) of ZnS (in-situ-generated) to the CB of N-ZnO, because of favorable band potentials in the heterostructure. Moreover, the defect-rich MoS2 nanosheets possesses abundant edge sites with plenty of exposed unsaturated S atoms, which have strong affinity to H+ ions in solution and get reduced to generate H2 during photocatalytic process. Therefore, the synergetic effect of N-doping of ZnO, the in-situ generation of ZnS, and the formation of heterojunctions with defect-rich MoS2 nanosheets result in the effective utilization of wide spectrum of solar light, leading to high hydrogen evolution rates. Hence, this work provides the facile strategy for the rational design of efficient and recyclable photocatalysts for solar-to-hydrogen fuel conversion.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00790.

■

Materials characterization; enlarged XRD data showing Bragg diffraction angle shifts between ZnO and N-ZnO nanorods; HRTEM image of defect-rich MoS2 nanosheets; STEM and elemental mapping image of MNZ15 heterostructure; XPS spectra of O 1s (MoS2 nanosheets); plot of transformed Kubelka−Munk function versus the energy of light for ZnO, N-ZnO, MNZ5, MNZ8, MNZ11, MNZ15, the MNZ20 heterostructure, and MoS2 nanosheets; and summary of specific surface area and pore volume distribution of MoS2, N-ZnO, and MNZ15 heterostructure (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Suneel Kumar: 0000-0002-5259-1792 Ajay Kumar: 0000-0001-8775-4486 K


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Ashish Kumar: 0000-0003-3527-4952 Muthukonda Venkatakrishnan Shankar: 0000-0002-5284-1480 Venkata Krishnan: 0000-0002-4453-0914 Present Address #

Rabindranath Tagore Government College, Sarkaghat, Mandi 175024, Himachal Pradesh, India. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to Advanced Materials Research Centre (AMRC), IIT Mandi for laboratory and the characterization facilities. S.K. acknowledges a senior research fellowship from University Grants Commission (UGC), India, and A.K. acknowledges a doctoral scholarship from the Ministry of Human Resource Development (MHRD), India.

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Defect-Rich MoS2 Ultrathin Nanosheets-Coated Nitrogen-Doped ZnO

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