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Jul 17, 2017 - Optimization of Active Sites of MoS2 Nanosheets Using Nonmetal. Doping and Exfoliation into Few Layers on CdS Nanorods for. Enhanced ...
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Optimization of Active Sites of MoS2 Nanosheets Using Nonmetal Doping and Exfoliation into Few Layers on CdS Nanorods for Enhanced Photocatalytic Hydrogen Production D. Praveen Kumar,† Myeong In Song,† Sangyeob Hong, Eun Hwa Kim, Madhusudana Gopannagari, D. Amaranatha Reddy, and Tae Kyu Kim* Department of Chemistry and Chemical Institute for Functional Materials, Pusan National University, Busan 46241, Republic of Korea S Supporting Information *

ABSTRACT: Transition metal dichalcogenides (TMDs) have emerged as promising nonprecious noble-metal-free catalysts for photocatalytic applications. Among TMDs, MoS2 has been extensively studied as a cocatalyst due to its exceptional activity for photocatalytic hydrogen evolution. However, the catalytic activity of MoS2 is triggered only by the active S atoms on its exposed edges, whereas the majority of S atoms present on the basal plane are catalytically inactive. Doping of foreign nonmetals into the MoS2 system is an appealing approach for activation of the basal plane surface as an alternative for increasing the concentration of catalytically active sites. Herein, we report the development of earth-abundant, few-layered, boron-doped MoS2 nanosheets decorated on CdS nanorods (FBMC) employing simple methods and their use for photocatalytic hydrogen evolution under solar irradiation, with lactic acid as a hole scavenger, under optimal conditions. The FBMC material exhibited a high rate of H2 production (196 mmol·h−1·g−1). The presence of few-layered boron-doped MoS2 (FBM) nanosheets on the surface of CdS nanorods effectively separated the photogenerated charge carriers and improved the surface shuttling properties for efficient H2 production due to their extraordinary number of active edge sites with superior electrical conductivity. In addition, the observed H2 evolution rate of FBMC was much higher than that for the individual few-layered MoS2-assisted CdS (FMC) and bulk boron-doped MoS2/CdS (BBMC) photocatalysts. To the best of our knowledge, this is the highest H2 production rate achieved with MoS2-based CdS photocatalysts for water splitting under solar irradiation. Considering its low cost and high efficiency, this system has great potential as a photocatalyst for use in various fields. KEYWORDS: Photocatalyst, Hydrogen production, Nonmetal doping, Exfoliation, MoS2

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INTRODUCTION Rapid advancements in science and technology and massive population growth have led to serious environmental pollution and escalation of the energy crunch.1 Semiconductor photocatalysts have fascinated scientists in the search for solutions to these problems due to the extensive applicability of such systems.2 Photocatalytic H2 evolution from water on semiconductors is one attractive way to simultaneously combat environmental pollution and the energy crunch as the process is pollution free and economically viable and the materials are easily stored.3 Numerous photocatalysts have been described for the splitting of water in the presence or absence of sacrificial agents, generating H2 as a clean energy source.4−10 Nevertheless, the majority of these catalysts require ultraviolet (UV) light, and their effectiveness is unsatisfactory for practical application. The development of efficient visible-light-accessible photocatalysts is an extremely desirable challenge to achieve effective consumption of solar energy, given that visible light comprises a greater fraction of the solar spectrum (43%) than © 2017 American Chemical Society

UV light (4%). Among the various semiconductor photocatalysts, CdS, with a band gap of ∼2.4 eV and an appropriate conduction band potential, has been extensively investigated as a visible-light-driven photocatalyst for hydrogen generation.11 Nevertheless, investigations on bare CdS have proven it to be been inadequate due to its low photocatalytic efficiency, attributed to rapid recombination of the photogenerated electrons and holes.12 Since the invention of graphene, there has been much interest in auxiliary 2D layered materials due to their distinctive electronic properties and high surface-to-volume ratios, making them prospectively superior platforms for photocatalytic applications.13−16 Among the known 2D layered materials, transition metal dichalcogenides (TMDs)having extraordinary propertiesare of particular interest; for instance, Received: March 31, 2017 Revised: June 16, 2017 Published: July 17, 2017 7651

DOI: 10.1021/acssuschemeng.7b00978 ACS Sustainable Chem. Eng. 2017, 5, 7651−7658

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ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Representation of Preparation Procedure of FBMC Nanocomposites

doping (with B, N, P, and O) is an effective strategy for altering the physical properties of TMDs. Extensive research has been undertaken to increase the number of active S atoms on the exposed edges of MoS2 by conversion of many-layered MoS2 to mono- and few-layered species and conversion from the 2H phase to the 1T phase. However, the separated mono- and few-layer species and 1T phases are more unstable than the 2H phase. The MoS2 monolayer is known to have two phases: trigonal prismatic (labeled as 2H, D3h) and octahedral (labeled as 1T, Oh). The 2H phase is relatively stable but semiconducting and of poor conductivity. The 1T phase is metastable at room temperature but metallic and of better conductivity.35−38 Herein, we concentrate on increasing the catalytically active sites by activating S atoms on the basal plane; these S species are not generally as catalytically active as the surface edge sites. We report the synthesis of earth-abundant, few-layered, borondoped, MoS2 nanosheets decorated on CdS nanorods (FBMC) by simple methods. The synthesized materials are evaluated for photocatalytic hydrogen evolution under solar irradiation with lactic acid as a hole scavenger under the optimal conditions. The FBMC material exhibits a high rate of H2 production (196 mmol·h−1·g−1). The presence of few-layered boron-doped MoS2 (FBM) nanosheets on the surface of the CdS nanorods effectively separates the photogenerated charge carriers and improves the surface shuttling properties for efficient H2 production due to the extraordinary number of active edge sites with superior electrical conductivity.

photocatalysts can be combined with TMDs as cocatalysts to generate low-cost materials with an edge-terminated structure.17−21 TMDs are a class of materials with the general formula MX2, where M refers to the transition metal (such as Mo, W, Co, etc.) and X indicates the chalcogen (such as S, Se, Te, etc.).22 Among the TMDs, MoS2 is considered an excellent cocatalyst for photocatalytic H2 production from water because it possesses excellent H2 activation ability.23 MoS2 has emerged as the most attractive and promising TMD due to its peculiar properties. MoS2 generally adopts a sandwich structure comprising three stacked atomic layers (S−Mo−S) linked by van der Waals forces and has been used in photocatalysis.24−29 Moreover, density functional calculation of the free energy has shown that the S atoms on the exposed edges of MoS2 bond strongly to H+ in solution, and the bound protons are easily reduced to H2 by electrons.30 Theoretically, the catalytic activity of MoS2 is derived only from the active S atoms on its exposed edges, but most S atoms on the basal plane show no activity.31 Therefore, conversion of MoS2 nanosheets into fewlayered nanosheets with a large number of exposed active sites increases their activity, thereby making MoS2 a promising cocatalyst as a low-cost alternative to platinum. Despite the large number of exposed active sites in few-layered MoS2 nanosheets, the cocatalytic activity is restricted to some extent due to its inefficient electrical conductivity. The modification of TMDs by doping with transition metals/ nonmetals can expand their utility in such applications, thereby expanding their prospective for technological applications.32 The comparative quantity of active sites can be increased by morphological control, and in the case of TMDs, this can be accomplished by doping.33 Although the basal plane is not generally as catalytically active as the edge sites, doping offers a strategy for the activation of this surface as an alternative approach for increasing the density of catalytically active sites.34 Atomic doping of a material is a technique that can alter the structure and/or properties depending on the dopant and its concentration. Supersession can typically occur as direct supersession of atoms in the lattice (if the dopant is well matched in terms of size, valence, and coordination) or into interstitial sites between subsisting atoms in the lattice. In the context of layered materials, dopant atoms may also be intercalated between layers or alter the structure to engender formation of an incipient phase. In optoelectronic applications, doping can be utilized to control p- or n-type semiconducting behavior by modulating the Fermi level and/or band gap of the semiconductor.34 Judicious selection of the dopant can yield magnetic semiconductors: a potential technique that can be applied for the development of spintronics, which may form the substructure of solid-state storage devices and computing components.34 Metal-doped TMDs have been extensively studied relative to nonmetal doped TMDs, though nonmetal

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EXPERIMENTAL SECTION

Materials. Cadmium acetate dihydrate (Cd(CH3COO)2·2H2O), sodium molybdate dihydrate (Na2MoO4·2H2O), boric acid, and ethanol were purchased from Daejung Chemicals & Metals Co. Ltd., Korea. Thiourea (NH2CSNH2) and thioacetamide (C2H5NS) were obtained from Alfa Aesar. All chemicals were used without further purification. Synthesis of Few-Layered, Boron-Doped, MoS2/CdS Nanorods (FBMC). First, one-dimensional CdS nanorods and bulk borondoped MoS2 (BBM) nanosheets were synthesized separately using hydrothermal methods; the detailed experimental procedures are provided in the Supporting Information (SI).23,39 Finally the FBMC composites were synthesized by simple ultrasonication.23 Different weight percentages (1−7%) of BBM were separately dispersed in 10 mL of dimethylformamide (DMF) and ultrasonicated for 3 h at room temperature. The BBM suspension was exfoliated and homogeneously separated into a few layers by the sonication process. The as-prepared CdS nanorods (100 mg) were then added, and the mixture was again ultrasonicated for 1 h, followed by 12 h of stirring to improve the interaction between the exfoliated boron-doped MoS2 layers and the CdS nanorods. The black boron-doped MoS2 became greenish; this greenish solid product was washed thoroughly with deionized water and ethanol many times to eliminate DMF and then dried at 60 °C for 12 h. A schematic representation of the materials preparation is shown in Scheme 1. 7652

DOI: 10.1021/acssuschemeng.7b00978 ACS Sustainable Chem. Eng. 2017, 5, 7651−7658

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Figure 1. (a) XRD patterns and (b) UV/vis diffuse reflectance spectra of CdS and related composites (BBMC and FBMC).

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RESULTS AND DISCUSSION The solvothermally synthesized CdS nanorods (NRs) were structurally characterized by X-ray diffraction (XRD), which is shown in Figure 1(a). The diffraction pattern of the bare CdS NRs was clearly indexed to the pure hexagonal phase of CdS, which is in good agreement with the literature values (JCPDS card #: 41-1049). Although the boron-doped bulk MoS2/CdS (BBMC) and FBMC nanocomposites contained 6% borondoped MoS2 on the CdS substrate as a cocatalyst, no diffraction peaks of boron-doped MoS2 were observed, which may be ascribed to the relatively low amount and low diffraction intensity of boron-doped MoS2.23 Further, the formation of FBM by exfoliation was confirmed by the differences in the respective diffraction patterns of FBM and BBM, such as the shift in the diffraction peak at 2θ = 14.5° to a value of 10.0° upon exfoliation, providing a clear evidence of an increase in the space in between the boron-doped MoS2 layers (Figure S1, Supporting Information).11 Optical absorption of as-synthesized CdS NRs and composite materials were studied by UV/vis diffuse reflectance spectroscopy (DRS), as shown in Figure 1(b). The bare CdS NRs had a prominent absorption with an absorption edge at about 520 nm, which corresponds to the intrinsic band gap absorption of CdS NRs.11 The optical band gap of the CdS semiconductor was calculated to be 2.38 eV by utilizing the Kubelka−Munk method, which is consistent with the reported value.13 Decoration of boron-doped MoS2 on the CdS NRs gave rise to the same absorption edge as that of the bare CdS NRs, indicating that the band gap of CdS was unchanged. The deposition of boron-doped MoS2 on the surface of the CdS NRs did not alter the band structure of CdS. However, the broad background absorption of the composite was extended in the range of 530−750 nm because black boron-doped MoS2 can lead to a large increase in the opacity. The morphology of the FBMC composite was analyzed by transmission electron microscopy (TEM) and energy dispersive X-ray spectral (EDS) mapping analysis, as displayed in Figure 2. Figure 2(a−d) shows FBM dispersed on the CdS nanorods, with CdS exhibiting a rod-like 1-D morphology decorated with FBM nanosheets, as indicated by the red circles; the highresolution TEM (HRTEM) images of the composite (Figure 2(c) and (d)) also clearly confirm the presence of CdS lattice fringes with a spacing of 0.33 nm and FBM, respectively. The elemental composition was determined by EDS mapping

Figure 2. (a, b) TEM and HRTEM images of FBMC, respectively. (c, d) HRTEM images of CdS and boron-doped MoS2, respectively. (e−h) Elemental analysis of FBMC (Cd, S, and Mo elements).

analysis, providing good evidence of the presence of FBM on the CdS nanorods, as shown in Figure 2(e−h). Figure 2(e) confirms the presence of three elements, Cd, S, and Mo. Figure 2(f−h) shows the individual elemental mapping results. From EDS, it was confirmed that Mo was well dispersed but at a much lower percentage than Cd and S; this was due to the much lower weight percentage of FBM used in the synthesis of FBMC. We were unable to detect boron in the EDS mapping due to its smaller atomic size and significantly lower percentage compared to FBMC.38 FESEM and TEM images of CdS NRs and TEM images of bulk MoS2/CdS and few layered MoS2/ CdS are shown in Figure S2, confirming high agglomeration of MoS2 layers on CdS in bulk MoS2/CdS and few separated layers on CdS in few layered MoS2/CdS. The chemical states and elemental compositions at the surface of the FBMC nanocomposite were analyzed by X-ray photoelectron spectroscopy (XPS), as shown in Figure 3(a−e). According to XPS survey spectrum, we found the existence of B, Cd, Mo, and S and trace amounts of C and O elements in the FBMC. The B element peak in XPS clearly evidenced boron doping in FBM. Furthermore, the chemical states of the elements were confirmed from the high-resolution B 1s, Cd 3d, Mo 3d, and S 2p XPS spectra, showing peaks of B 1s at binding energies of 190.6 eV and of Cd 3d at 404.6 and 411.7 eV, which correspond to Cd 3d3/2 and Cd 3d5/2, respectively. The binding energy difference indicated that the chemical state of Cd in the nanocomposite was +2. The binding energy of Mo 3d3/2 (at 231.8 eV) and those of S 2p3/2 (at 161.5 eV) clearly confirm the 7653

DOI: 10.1021/acssuschemeng.7b00978 ACS Sustainable Chem. Eng. 2017, 5, 7651−7658

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Figure 3. (a−e) XPS spectra of FBMC composite showing expanded Cd, S, Mo, and B regions. (f) Photoluminescence (PL) spectra of CdS, BBMC, and FBMC.

presence of boron-doped MoS2 in the FBMC composite.11,13,39 To confirm the effect of boron doping in FBMC, we compared XPS results of FMC (Figure S6). Figure S6(a) shows the Cd 3d binding energies at 404.9 (3d5/2) and 411.5 eV (3d3/2), confirming the +2 oxidation state of Cd in CdS.29 In addition, the S 2p peaks are present at binding energies of 161.2 and 162.5 eV. The binding energies of the Mo 3d spectrum appear at 228.3 (Mo 3d5/2) and 232.0 eV (Mo 3d3/2), which indicates that Mo4+ is the dominant oxidation state. The binding energies of Mo and S are slightly different in FBMC and FMC, which could be ascribed to the presence of boron in FBMC. Photoluminescence (PL) experiments were performed to evaluate the efficiency of trapping and recombination of the photogenerated charge carriers. The PL spectra of CdS, BBMC, and FBMC were measured after 380 nm excitation (Figure 3(f)). A near-band-edge emission around 550 nm was observed for all samples, which is due to the exciton emission of CdS.13 In contrast, the decreased PL intensity of BBMC and FBMC indicated facilitated transport and separation of photogenerated electrons and holes. Notably, the BBMC material also exhibited an obvious decrease in the PL intensity, which can be ascribed to the lower probability of recombination of the photogenerated carriers. Moreover, the lower intensity of the emission bands indicates that most of the surface states were passivated, thereby making the electrons available for photocatalytic H2 evolution. The emission bands of FBMC were much less intense than those of BBMC, which implies that

FBM is favorable for the charge carrier transfer processes (on a faster time scale) because of the increased number of surface edge sites and separated layers. Furthermore, the obtained PL results were compared with few-layered MoS2/CdS (FMC) and bulk MoS2/CdS (BMC), as in Figure S5. Figure S5 clearly explains that PL intensity of FBMC and BBMC is much lower than other nanocomposites (FMC and BMC), thus indicating that effective charge carrier separations in FBMC are due to boron doping in the nanocomposites. To further evaluate the separation efficiency and charge carrier transport, photoelectrochemical (PEC) analysis was carried out by using indium tin oxide (ITO) electrodes coated with CdS NRs and BBMC and FBMC nanocomposites under simulated sunlight irradiation by employing an electrochemical potentiostat. Figure 4(a) shows the time-dependent photocurrent responses of CdS and the BBMC and FBMC composites with 30 s simulated sunlight on/off cycles. As expected, the FBMC nanocomposites showed the highest photocurrent intensity relative to the CdS NRs and BBMC, suggesting a higher separation efficiency of the photoexcited electron−hole pairs in the former. Consequently, more electrons could be captured by protons to form H2. Moreover, the photocurrent responses were highly reproducible for several on−off cycles and remained stable. This indicates that FBM on CdS can effectively prevent photocorrosion. The photocurrent data are consistent with PL results. On the basis of these results, 7654

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Figure 4. (a) Photoelectrochemical characterization of CdS and related composites and photocatalytic H2 production studies. (b) Effects of FBM loading ratio on CdS NRs. (c) Recyclability test for FBMC. (d) Long-term stability test of FBMC with 1 mg of catalyst dispersed in 15 mL of 20 vol % aqueous lactic acid solution under simulated solar light irradiation. (e) Effect of lactic acid concentration on performance of FBMC in aqueous solution with 1 mg of catalyst. (f) Effect of catalyst loading in 15 mL of 20 vol % aqueous lactic acid solution on H2 production.

FBM greatly influences the H2 production rate of CdS NRs under solar light irradiation. Photocatalytic H2 generation experiments employing CdS NRs and FBMC were performed under solar simulator irradiation using a lactic acid aqueous solution. It has been proven that lactic acid is a good hole scavenger for suppressing the recombination of photogenerated charge carriers. The data show that the amount of H2 gas generated increased linearly with respect to the irradiation time; the loading of FBM as a cocatalyst on the CdS NRs remarkably influenced the rate of H2 production (Figure 4(b)). The undecorated CdS NRs exhibited significantly lower activity (5 mmol·h−1·g−1). The loading of FBM was varied from 0 to 7.0 wt %. The composite catalyst showed a significantly higher rate of H2 generation than the CdS NRs; at a loading of 6% FBM, the rate of H2 production reached 196 mmol·h−1·g−1, which is 39.2-fold higher than that of the bare CdS NRs, indicating that FBM is an excellent cocatalyst. Above and below this optimum loading of FBM on the CdS nanorods the rate of H2 production was lower. This is ascribed to the lower number of catalytically

it is evident that the FBM nanostructures enhance the photocatalytic activity of the CdS nanostructures. The presence of FBM on CdS nanorods plays a crucial role on enhanced photocatalytic H2 production rate. Structural measurements indicate effective separations of few-layered MoS2 nanosheets, which is dissimilar from those of BBM. The effective layer separation into few layers increases the number of active sites with higher surface area. In addition, morphological properties clearly show that the better deposition of FBM on CdS NRs, which leads to better interactions between three components, benefits effective charge transportations. The optical properties of the FBMC provide evidence for a more effective solar light utilization compared to BBMC and CdS based on its higher absorption coefficient in the visible region. Charge carrier transfer studies (PL analyses) also confirm the fast migration of carriers in FBMC reducing charge carriers recombination. PEC studies support the fact that the separation and transportation of charge carriers in FBMC occur at a higher current density than those in BBMC and CdS. All these findings strongly indicate 7655

DOI: 10.1021/acssuschemeng.7b00978 ACS Sustainable Chem. Eng. 2017, 5, 7651−7658

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comprehensive description of the experimental procedure for the QE measurement and calculations is provided in the SI. A plausible mechanism for the efficient production of H2 under solar irradiation using lactic acid as a hole scavenger and FBMC as a photocatalyst is depicted in Figure 5. Excitation of

active sites at low FBM loading, whereas higher loading leads to obstruction of the incident light and inhibits the generation of electrons from the CdS NRs. In addition, higher loading leads to a greater number of S atoms on the basal planes; these S atoms are inactive, resulting in reduced photocatalytic activity as the cocatalytic activity of FBM depends strongly on S atoms on the surface exposed edges. To assess the stability of the catalyst, we evaluated the time course of photocatalytic hydrogen generation using the optimized FBMC catalyst with lactic acid as a scavenger as shown in Figure 4(c). Five reaction cycles, each lasting 5 h were performed; the rate of hydrogen production remained almost unchanged over the five cycles. However, a marginal decrease was observed after three reaction cycles, at which point an additional 3 mL of lactic acid was added, followed by two more reaction cycles. The performance was similar to that in the first cycle, which clearly indicates that the composite material is stable. To evaluate the stability and durability of the optimized FBMC photocatalysts, we also performed photocatalytic H2 generation experiments for an extended period of 60 h (Figure 4(d)). The rate of H2 evolution improved with respect to time for the first 50 h and then decreased slightly. We confirmed that the decrease in the production rate was due to the photoreactor being filled with H2 gas, resulting in a sluggish rate of H2 evolution. Thus, the generated gas was removed from the reactor, and the same experiment was performed for a further 10 h. The rate of H2 production remained similar to that in the first 10 h. The effect of the scavenger concentration on the rate of H2 production using the optimized FBMC catalyst was also evaluated (Figure 4(e)). Initially, the rate of H2 production (210 mmol·h−1·g−1) was proportional to the concentration of lactic acid up to 4 mL in 15 mL of reaction solution and thereafter was indirectly proportional to the increase in the scavenger concentration. The decreased rate of H2 production is due to the high formation of intermediates at a high concentration of lactic acid. Photoexperiments were conducted with different amounts (1 to 20 mg) of the optimized FBMC catalyst in the reaction solution (in 15 mL), demonstrating that the rate of H2 production differed significantly with respect to the amount catalyst in the reaction solution. At loadings of 1−5 mg, the rate of H2 production increased up to 277 μmol·h−1 with respect to the amount of catalyst. The H2 production rate became saturated at loadings exceeding 5 mg in 15 mL of reaction solution due to the large amount of catalyst, causing a shielding effect for the suspended catalyst particles (Figure 4(f)). In the comparative assessments of the H2 production rate for CdS, boron-doped MoS2, pristine few bulk MoS2/CdS (BMC), few-layered MoS2/CdS (FMC), BBMC, and FBMC, the H2 evolution rate of the optimized 6 wt % FBMC (196 mmol·h−1· g−1) is 39.2- and 2.7-fold greater than that of CdS (5 mmol·h−1· g−1) and BBMC (72 mmol·h−1·g−1), respectively (Figure S3(b)). We could not observe hydrogen production from boron-doped MoS2. The rate of H2 production of FMC and BMC are 125 and 60 mmol·h−1·g−1, respectively. We also studied effects of scavengers for 6 wt % FBMC catalyst (Figure S3(c)). The apparent quantum efficiency (QE) of the optimized FBMC nanocomposite under visible-light irradiation using a 150 W Xe lamp with a 425 nm band-pass filter was determined. The quantum yield was estimated to be around 31.5% (the rate of H2 production was 84 mmol·h−1·g−1); a

Figure 5. Schematic representation of the proposed mechanism of reaction of the FBMC nanocomposite.

the semiconducting CdS in FBMC by solar light generates electron−hole pairs at the conduction band (CB) and valence band (VB), respectively. The FBM nanosheets effectively separate the photogenerated charge carriers and improve the surface shuttling properties; the transfer is followed by reduction of protons to produce molecular hydrogen. The sacrificial agent (i.e., lactic acid) is consumed by the photogenerated holes in the VB of CdS to generate pyruvic acid. However, FBMC exhibits superior activity relative to that of BBMC and FMC. As supported by all experimental data, this improvement is due to the few-layered nature of boron-doped MoS2 and nonmetal doping into the MoS2 system, which strongly hinder the recombination of the photogenerated charge carriers and enhance the surface shuttling properties by increasing the surface active sites and thus the electrical conductivity. Mott−Schottky (Figure S4, SI) and photocurrent measurements are good supports for the proposed reaction mechanism. In addition, the conduction band potential of FBM is very close to the CdS conduction band potential, which leads to very fast migration of electrons from FBM to CdS.

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CONCLUSIONS The study demonstrates that earth-abundant, noble-metal-free, few-layered, boron-doped MoS2 nanosheets can be used as an efficient cocatalyst for CdS nanorods, leading to extraordinary photocatalytic H2 production under simulated solar light irradiation. The FBMC material exhibits a high rate of H2 production (196 mmol·h−1·g−1). The presence of the FBM nanosheets on the surface of the CdS nanorods effectively separates the photogenerated charge carriers and improves the surface shuttling properties for efficient H2 production due to the extraordinary number of active edge sites with superior electrical conductivity. Furthermore, the observed H2 evolution rate is much higher than those for FMC and the BBMC. Moreover, to the best of our knowledge, this is the highest H2 production rate achieved with a MoS2-based CdS photocatalyst for water splitting under solar irradiation. Considering its low cost and high efficiency, this system has great potential for the development of highly efficient photocatalysts for use in various fields. Hence, this system provides significant motivation for further research on the conversion of solar energy to chemical fuels. 7656

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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/acssuschemeng.7b00978. Preparation methods of CdS NRs, BM, and BBMC, characterization technique details and complete details of photocatalytic activity test, and PEC and Mott−Schottky analyses. XRD pattern of BM, few-layered MoS2 (FM), BBM, and FBM. Elemental analyses, FESEM and TEM images of CdS NRs, FMC, and BMC. Photocatalytic H2 production assessments of studies of CdS, BMC, FMC, BBMC, and FBMC. Mott−Schottky plots of BBM, FBM, and CdS NRs. PL spectra of all synthesized materials and XPS analysis of FMC. Comparison of photocatalytic H2 evolution rate of MoS2/CdS-based photocatalysts. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tae Kyu Kim: 0000-0002-9578-5722 Author Contributions †

D.P.K. and M.I.S. have contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Research Foundation of Korea (NRF) grants, funded by the Korean Government (MSIP) (2014R1A4A1001690 and 2016R1E1A1A01941978). This study was also financially supported by the 2017 Post-Doc. Development Program of Pusan National University.

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REFERENCES

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DOI: 10.1021/acssuschemeng.7b00978 ACS Sustainable Chem. Eng. 2017, 5, 7651−7658