Drastic Improvement of 1D-CdS Solar Driven Photocatalytic Hydrogen

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Drastic Improvement of 1D-CdS Solar Driven Photocatalytic Hydrogen Evolution Rate by Integrating with NiFe Layered Double Hydroxide Nanosheets Synthesized by Liquid Phase Pulsed Laser Ablation Hwan Lee, D. Amaranatha Reddy, Yujin Kim, So Yeon Chun, Rory Ma, D. Praveen Kumar, Jae Kyu Song, and Tae Kyu Kim ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04000 • Publication Date (Web): 28 Oct 2018 Downloaded from http://pubs.acs.org on October 28, 2018

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ACS Sustainable Chemistry & Engineering

Drastic Improvement of 1D-CdS Solar Driven Photocatalytic Hydrogen Evolution Rate by Integrating with NiFe Layered Double Hydroxide Nanosheets Synthesized by Liquid Phase Pulsed Laser Ablation Hwan Lee,†,§ D. Amaranatha Reddy,†,§ Yujin Kim,† So Yeon Chun,‡ Rory Ma,† D. Praveen Kumar,† Jae Kyu Song*,‡ and Tae Kyu Kim*,† †

Department of Chemistry and Chemistry Institute of Functional Materials, Pusan National

University, Busan 46241, South Korea ‡

Department of Chemistry, Kyung Hee University, Seoul 02447, South Korea

*Corresponding author: [email protected] (T.K.K.) and [email protected] (J.K.S.) §These

authors are contributed equally

KEYWORDS: 1D-CdS nanorods, NiFe co-catalyst, sunlight-driven photocatalyst, H2 production, reduced recombination rate.

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ABSTRACT Solar-driven semiconductor-based molecular hydrogen production is an ideal protocol for converting abundant solar energy to green fuel. However, this process suffers from costly semiconductor nanostructures, low efficiency, and poor stability. Here, we design a noble-metalfree photocatalyst, CdS-NiFe layered double hydroxide (LDH) nanocomposite, which is synthesized using the liquid-phase pulsed-laser ablation and hydrothermal method. The nanocomposite has a unique morphology of 2D-NiFe LDH nanosheets on 1D-CdS nanorods. The interfacial contact of heterostructures allows the efficient carrier transport and migration due to the appropriate potentials, which greatly reduce the recombination of carriers. It also provides a significant number of catalytically active sites for the hydrogen evolution reaction due to its thin and flexible nature and high specific surface area. The CdS/NiFe nanocomposite exhibits a hydrogen evolution rate of 72 mmol g−1 h−1, which is higher than reported nanocomposites of CdSbased co-catalyst nanostructures. We expect that the demonstrated method to form noble-metalfree CdS-based co-catalyst nanostructures and the utilization in photocatalytic hydrogen evolution reactions provide novel insights into developing cost-effective photocatalysts for hydrogen production.


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INTRODUCTION The solar-driven water splitting using noble-metal-free semiconductor catalyst is a promising technology to produce light-weight hydrogen and meet future global energy demands.1 In the converting process of solar energy to chemical energy, the semiconductor nanostructures absorb sunlight and generate electrons and holes. The resultant charge carriers diffuse to the surface of the nanostructures, where proton reduction or water oxidation occurs based on the band potentials.2 Due to its advantages, the concept of solar-driven water splitting has been validated in several semiconductor nanostructures, such as TiO2, ZnO, CdS, C3N4, CdSe, ZrO2, SrTiO3, and KTaO3.3-7 Among them, visible-light-responsive CdS with a band gap energy of 2.4 eV is one of the most prominent photocatalysts for solar-driven hydrogen evolution reactions, because of its high visible light harvesting capacity, good charge carrier mobility, and suitable band edge potentials.8 However, the hydrogen evolution rate of bare CdS nanostructures is very low due to the short electron diffusion length compared to the light penetration depth and photo-corrosive nature. Thus, the fast recombination of electron–hole pairs significantly reduces the efficiency of hydrogen evolution reactions.9 It has been found that 1D-CdS nanostructures with controlled size and thickness can improve the hydrogen evolution rate, because they provide a unidirectional path to alleviate the flow of photogenerated electrons toward the surface.10 Moreover, compared to 0D-CdS, 1D-CdS can simultaneously exhibit the quantum confinement in a radial direction and carrier transportation in the axial direction, thus reducing the carrier recombination rate and promoting the photocatalytic hydrogen evolution rate.11 However, despite the enhanced properties of 1D-CdS, the hydrogen evolution rate was still lower than the range of practical applications. To further improve the hydrogen evolution rate, co-catalysts were introduced to 1D-CdS. The co-catalysts on 3 ACS Paragon Plus Environment


semiconductor nanostructures alleviate reduction reactions by generating active sites, promoting the transfer rate, and suppressing recombination rate of charge carriers. Overall, the nanocomposite systems with co-catalysts on 1D nanostructures enhance the photocatalytic hydrogen evolution rate.12 Several co-catalyst nanostructures, including noble metals, metal oxides, metal sulfides, metal selenides, metal nitrates, metal hydroxides, and layered double hydroxide (LDH), were verified for promising candidates to improve hydrogen production reactions.13-20 Among them, LDH nanostructures have attracted much attention due to the unique physicochemical properties, such as high conductivity, large surface area, high charge density with flexible height, and compositional flexibility to create a sleek path of electron transport. Moreover, the numerous exposed reductive sites and synergetic effects between LDH and nanostructures promote charge transport to enhance photocatalytic hydrogen evolution rate.21,22 In particular, NiFe LDH holds great promise for solar-driven water splitting due to its properties, such as earth abundance, simple synthesis method, low cost, high specific surface area, significant number of catalytically active sites, suitable water reduction potentials, and ability to form close interfacial contact with various catalytically active semiconductor nanostructures.23,24 Despite of advantages of NiFe LDH nanostructures for water splitting applications, only few related nanocomposites have been fabricated to confirm the rate of hydrogen evolution. For example, Nayak et al.25 designed NiFeC3N4 nanocomposite using chemical co-precipitation combined with ultrasonication and noticed 1.488 mmol g−1 h−1 of hydrogen evolution rate under visible light conditions. Yue et al.26 reported sulfur doped NiFe LDH nanostructures/CdS-CdTe quantum dots, which shows the hydrogen evolution rate of 26.5 mmol h−1 g−1 under visible light irradiation. Zhou et al.27 synthesized NiFe LDH through reverse micro-emulsion systems and integrated on CdS nanoparticles. The NiFe/CdS 4 ACS Paragon Plus Environment

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nanostructures produces the hydrogen evolution rate of 0.469 mmol h−1 g−1 under simulated sunlight irradiation. Although some advancements such as synthesis process and construction of NiFe based nanocomposites are reported, the resultant hydrogen evolution rate is too low and synthesis methodologies are too complicated and time consuming. In addition to photocatalytic performance, the simplicity of fabrication is also important for efficient utilization. Several synthesis methods, including hydrothermal, solvothermal, electrochemical deposition, and other wet chemical and physical methods, have been adapted to prepare NiFe LDH nanostructures.28 However, normally these chemical methods require sophisticated instruments, toxic stabilizing agents, high synthesis temperatures, long duration times, and complex operations.29,30 Furthermore, low yields of synthesis have hindered the utilization of these methods. On the contrary, the pulsed-laser ablation in liquid (PLAL) is a unique fabrication technique to produce thin LDH nanostructures with high purity and little harmful surfactants. Moreover, the PLAL technique is cost-effective, eco-friendly, and easy synthetic route to produce nanostructures in dispersed liquid, which can provide the capability to integrate easily on the surface of CdS nanorods with tight binding. In addition, thin nature of LDH nanostructures from PLAL can improve light harvesting capabilities and effectively reduces the recombination rate during the catalytic reaction.31-33 In this work, thin NiFe LDH nanostructures are synthesized using PLAL for the first time, which are integrated on CdS nanorods through ultrasonication to form CdS/NiFe-LDH nanocomposites. Structural analysis demonstrates that 2D-NiFe LDH is anchored on the edges of CdS nanorods, which is investigated for solar-driven hydrogen evolution reactions using lactic acid as hole scavenger. Integrating 42 wt.% of NiFe LDH with CdS nanorods yields a hydrogen evolution rate of 72 mmol g–1 h–1, which is 30 times higher than bare CdS nanorods. The 5 ACS Paragon Plus Environment


extraordinary hydrogen evolution rate is attributed to efficient charge mobility, reduced charge recombination, and rich catalytically active sites. The dynamics of carrier transfer and migration is investigated by photoluminescence, photocurrent, and impedance analysis. The current method to form noble-metal-free co-catalysts nanostructures and the utilization in solar-driven hydrogen evolution reactions may offer novel insights to develop cost-effective photocatalysts for H2 production.

RESULTS AND DISCUSSION A schematic of the fabrication processes for NiFe LDH, CdS nanorods and CdS/NiFe-LDH nanocomposites is provided in Scheme 1. First, NiFe LDH nanosheets were synthesized using the PLAL technique. Ni foil (Sigma-Aldrich: 99.9%, 0.5 mm thickness) was cleaned with absolute ethanol and acetone by ultrasonication to remove the oxidation layers and impurities on the surface. Then, Ni foil was fixed in a glass beaker containing 10 mL FeCl3 aqueous solution, whose level was around 10 mm above the foil. The reaction cell was configured on a rotating turn-table to avoid damage from the continuous laser irradiation. Ni foil was irradiated by Nd:YAG laser with a repetition rate of 10 Hz and pulse duration of 3–6 ns. The wavelength and fluence of the pulsed laser were 1064 nm and 3.82 J/cm2, respectively. The reaction pathway for the formation of thin NiFe LDH is as follows. When Ni foil is irradiated by the high-power laser beam, the absorbed energy in Ni leads to surface vaporization, melting, and ionization. During the laser pulse, a high temperature and pressure plasma plume of Ni combusts over the laser spot. The plasma plume containing the ablated materials spreads out adiabatically at a supersonic velocity into the surrounding aqueous Fe solution, which is accompanied by the emission of a shockwave. After the pulse terminates, the plasma plume cools down, releases energy to the liquid solution, and 6 ACS Paragon Plus Environment

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forms nanosheets of NiFe.34 Second, CdS nanorods were synthesized by the hydrothermal method using cadmium acetate and thiourea as the precursor materials for Cd and S, respectively. Finally, CdS nanorods and NiFe LDH were mixed by ultrasonication to form CdS/NiFe nanocomposites. The detailed procedures for the synthesis of NiFe, CdS, and CdS/NiFe are provided in Supporting Information (SI).

Scheme 1. Schematic illustration of NiFe LDH synthesis using PLAL, CdS nanorods using the hydrothermal method, and CdS/NiFe-LDH nanocomposites using ultrasonication.

The microstructure, morphology, elemental distribution, and size of the synthesized NiFe were analyzed by FETEM with energy dispersive spectroscopy (EDS) mapping. Figure 1(a) illustrates that NiFe nanostructures have a sheet-like morphology. The HRTEM image in Figure 1(b) demonstrates that the nanostructures have clear lattice fringes with an inter-planar spacing of 0.24 nm, which agrees with (111) plane of NiFe. To characterize the elemental distribution and spatial homogeneity of the elements in NiFe LDH, the nanostructures were examined using EDS 7 ACS Paragon Plus Environment


(Figure 1(c-i)). The nanosheets were composed of uniformly distributed Ni, Fe, and O over the entire nanosheets without impurities. The quantitative elemental analysis reveals the ratio of metal element (Fe:Ni) is close to 1:1, which can be estimated to be wt.% of Fe and Ni of 50.8 : 49.2 (Figure S1). AFM topographic image (Figure S2) of the NiFe LDH nanosheets estimates the thickness of NiFe LDH nanosheets of around 12 nm. Figure S3 shows a FESEM image of the CdS nanostructures prepared by the hydrothermal method, which indicates that the nanostructures have a rod-like morphology with an average size of 15–20 nm in width and 110–200 nm in length. FETEM and EDS mapping images of CdS/NiFe nanocomposite are presented in Figure 2. It is evident in Figure 2(a) that the layered NiFe LDH is tightly bound to the edge of the CdS nanorods without damaging the morphology of the nanorods. Furthermore, the high-resolution TEM image in Figure 2(b) indicates that the lattice fringes have distances of 0.33 and 0.24 nm, which are assigned to (002) and (111) lattice planes of wurtzite CdS and NiFe, respectively. The SAED pattern of CdS and NiFe shows dotted and ring-shape patterns, respectively, demonstrating that CdS nanorods having single crystalline nature and the NiFe have polycrystalline nature (Figure 2(c, d)). Corresponding diffraction patterns are well matched to the hexagonal CdS and NiFe LDH nanostructures. The FETEM images demonstrate the close interfacial contact between CdS nanorods and NiFe LDH, which would promote the carrier transportation between the two components and improve the photocatalytic hydrogen evolution rate. To further understand the elemental distribution and spatial homogeneity in CdS/NiFe-LDH, the nanostructures were examined by EDS (Figure 2(c-i) and Figure S4), confirming uniform distributions of all elements (Cd, S, Ni, Fe and O) over the entire composites without impurities.


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Figure 1. (a) FETEM and (b) HRTEM (with lattice fringe of NiFe LDH) images, (c) HAADF image, (d-g) elemental mapping of NiFe LDH nanostructures, showing the presence of (d) Ni, (e) Fe, (f) O, and (g) all elements, and (h-i) line spectra of elements present.



Figure 2. (a) FETEM and (b) HRTEM (with the lattice fringes of CdS and NiFe LDH) images, (c-d) SAED patterns of the (c) CdS and (d) NiFe LDH, (e) HAADF image, (f-i) elemental mapping of CdS/NiFe-LDH nanocomposites, showing the presence of (f) Cd, (g) S, (h) Fe, and (i) Ni, and (j) line spectrum of all elements.

The phase, structure, and crystallinity of the synthesized CdS nanorods, NiFe LDH and NiFe co-catalyst integrated CdS nanocomposites (CdS/NiFe-LDH) were measured using powder X-ray diffraction (Figure 3(a) and Figure S5). Most of the diffraction planes of NiFe LDH in 10 ACS Paragon Plus Environment

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Figure S5 are well matched to the NiFe standard data. All diffraction peaks of the bare CdS nanorods in Figure 3(a) can be indexed to wurtzite CdS (JCPDS 89-2944) with a high crystalline nature. There are no other diffraction peaks, indicating that the temperature and time of synthesis in the hydrothermal method are appropriate for growing the CdS nanorods with high purity. The CdS/NiFe nanocomposite shows similar diffraction peaks to CdS without diffraction peaks related to NiFe. Since FETEM image shows NiFe LDH nanostructures on CdS nanorods, the diffraction results suggest that the small amount of NiFe does not contribute to form new crystal phases or alter the preferential orientations of the CdS nanorods.35 Moreover, we have noticed slight increase in the diffraction intensity of CdS/NiFe compared to the bare CdS, probably due to higher atomic scattering factor. The CdS, NiFe and CdS/NiFe composites were further investigated by X-ray photoelectron (XPS) to explore the elemental composition and valence states. The survey XPS spectrum of the CdS/NiFe nanocomposite shows Cd, S, Ni, Fe, and O in Figure 3(b). The highresolution spectrum of Cd 3d exhibits two peaks at binding energies of 404.85 and 411.73 eV in Figure 3(c), which are related to Cd 3d5/2 and 3d3/2, respectively. The difference in binding energy is approximately 6 eV, demonstrating that Cd in the nanorods is in the +2 oxidation state. However, compared with the bare CdS 3d spectrum (Figure S6(b)), the binding energy of Cd 3d of CdS/NiFe nanocomposite is shifted about 0.6 eV towards the higher binding energies. The spectrum of S 2p in Figure 3(d) shows two peaks of S 2p1/2 and 2p3/2 at the binding energies at 160.36 and 161.56 eV, respectively. The difference in binding energy is about 1.2 eV, indicating that S in the nanorods is in the -2 oxidation state.36 Compared with the S 2p spectrum of bare CdS (Figure S6(c)), the S 2p binding energy of CdS/NiFe nanocomposite is blue shifted about 0.6 eV. Because the XPS technique is generally surface-sensitive up to a depth of 10 nm, the detection of Cd and S elements 11 ACS Paragon Plus Environment


in the CdS/NiFe nanocomposite indicates that the thickness of integrated co-catalyst NiFe is below 10 nm.37 The deconvoluted spectrum of Ni 2p in the CdS/NiFe nanocomposite shows four peaks of Ni 2p3/2 (855.21 eV), Ni 2p1/2 (872.91 eV), and two satellite peaks at 861.36 and 878.94 eV in Figure 3(e).38 The spectrum of Fe 2p in Figure 3(f) shows four deconvoluted peaks, Fe 2p3/2 (710.80 eV) and Fe 2p1/2 (724.96 eV) related to Fe3+ and two satellite peaks at 861.36 and 878.94 eV.39 Overall, XPS indicates that the designed nanocomposite consists of CdS nanorod and NiFe LDH without impurities. When the binding energies of CdS/NiFe and NiFe LDH in both Ni and Fe 2p regions are compared, the binding energies of CdS/NiFe composite are shifted about 1.6 eV (Figure S7), which might be ascribed to electron transfers from CdS to NiFe. The increase or decrease of electron density can lead to the reduction or enhancement in XPS binding energies.37 Moreover, this is a direct evidence of strong electronic interfacial contacts between CdS and NiFe in the nanocomposite. We expect that this type of interaction may promote the photogenerated electron transfer rate and contribute for efficient hydrogen evolution rate.37


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Figure 3 (a) X-Ray diffraction patterns of CdS and CdS/NiFe nanocomposites. (b) XPS survey spectrum of CdS/NiFe nanocomposites. (c-f) Core-level XPS spectra of Cd 3d, S 2p, Ni 2p, and Fe 2p in CdS/NiFe nanocomposites.

To obtain an efficient photocatalytic hydrogen evolution reaction, the photo-absorption capacity in the visible region is important. UV-Vis diffuse reflectance spectra (DRS) were obtained to understand the light harvesting capacity of synthesized CdS and CdS/NiFe in Figure 4(a). The absorption edge of CdS nanorods is at approximately 505 nm, which corresponds to the band gap energy of CdS. The absorption edge of the nanocomposite was similar to that of CdS, indicating that integrating NiFe does not influence the band gap energy of CdS. However, the nanocomposites exhibit extended visible light absorption in the wavelength range of 550–800 nm compared to the bare nanorods. Since the NiFe LDH shows the absorption in 460–800 nm due to its metallic nature (Figure S8), the extended visible light absorption of the CdS/NiFe 13 ACS Paragon Plus Environment


nanocomposite can be attributed to NiFe LDH cocatalyst. The enhanced visible light absorbing nature indicates that the nanocomposites have better visible light harvesting capacity than the bare nanorods, which contributes to generation of charge carriers during the catalytic reaction.40 The efficient separation of charge carriers leads to a large number of carriers participating in hydrogen evolution reactions.41 Photoluminescence (PL) spectroscopy offers the measure of the charge transfer and separation process, because the emission intensity suggests dynamics of the photogenerated charge carriers. Figure 4(b) shows PL spectra of CdS nanorods and CdS/NiFe nanocomposites upon an excitation wavelength of 380 nm. Both spectra exhibit an emission peak at about 530 nm, which is the near band edge emission of CdS.42,43 However, the emission intensity of CdS/NiFe nanocomposites is much lower than that of CdS nanorods, which suggests the reduced recombination of photogenerated electron−hole pairs due to carrier transfer from CdS to NiFe. To further probe the carrier dynamics in CdS/NiFe nanocomposites, the lifetimes of band gap states were examined by time-resolved PL spectroscopy (Figure S9). The short lifetime of band gap states (0.19 ns) indicated the fast recombination of electron−hole pairs in the bare CdS nanorods. The lifetime of band gap states became further reduced (0.15 ns) in the CdS/NiFe nanocomposites, indicating the carrier transfer from CdS to NiFe. Despite the reduced lifetime, the transferred carriers delay the recombination of electron−hole pairs in the nanocomposites, which increases the photocatalytic activity. Moreover, the transferred electrons enhance the hydrogen production reactions in the active sites of NiFe LDH, which further improves the photocatalytic activity. Indeed, the reduced lifetime in the nanocomposites agrees with the reduced emission intensity in the PL spectra. Accordingly, integrating NiFe on CdS nanorods suppresses the recombination of electron−hole pairs during the catalytic reaction and contributes to a high hydrogen evolution rate.27 14 ACS Paragon Plus Environment

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Figure 4. (a) DRS spectra of CdS nanorods and CdS/NiFe nanocomposites. (b) Photoluminescence spectra. (c) Time dependent photocurrent responses. (d) EIS spectra.

To further understand the separation and transportation nature of charge carriers in the nanocomposites, transient photocurrent responses were measured. Figure 4(c) shows the transient photocurrent response of the bare CdS nanorods and CdS/NiFe nanocomposites during three on−off cycles. The time interval of each cycle was 60 s using a solar simulator with AM 1.5 G filter as a light source. The CdS/NiFe nanocomposites exhibit higher photocurrent intensity than the CdS nanorods, demonstrating that the nanocomposites have a higher probability of separation and transportation of charger carriers than the CdS nanorods. Accordingly, the high separation probability in the nanocomposites, which is supported by the reduced intensity and lifetime in PL 15 ACS Paragon Plus Environment


spectra, could enhance photocatalytic activity.44 To confirm the separation of the charger carriers, the electrochemical impedance spectroscopy (EIS) of the bare CdS nanorods and CdS/NiFe nanocomposites was also carried out under simulated sunlight irradiation. The EIS Nyquist plot of the CdS/NiFe nanocomposites in Figure 4(d) exhibits a smaller curve compared to that of the CdS nanorods, demonstrating that interfacial electron transfer in the CdS/NiFe nanocomposite is more efficient than that in the CdS nanorods.45 In addition to the light harvesting capability, suitable band edge potentials are critical to promote the charge transfer for efficient hydrogen evolution reaction. The levels of flat band potentials in the CdS nanorods and NiFe LDH were examined using Mott-Schottky (MS) analysis (Figure S10). Both CdS and NiFe demonstrate a positive slope and typical n-type characteristics46 with the flat band potentials of -1.05 and -0.67 V vs Ag/AgCl at pH = 7, respectively. The obtained potentials were converted to normal hydrogen electrode (NHE) scales using the formula, ENHE = EAg/AgCl + 0.197, which led to the estimated values of -0.853 and -0.473 V vs NHE at pH = 7. Clearly, the estimated value of NiFe is lower than the redox potential of H+/H2 (-0.42 V vs NHE at pH = 7), while ENHE of NiFe is higher than that of CdS. The band potentials indicate that the photogenerated electrons in CdS can be easily transferred to NiFe, which contributes to the reduction of H+ to H2 during the catalytic reaction.47 Accordingly, integrating NiFe LDH on CdS nanorods suppresses the recombination of the charger carriers in CdS and contributes to a high hydrogen evolution rate. The ability of solar-driven hydrogen production was examined in the CdS nanorods and CdS/NiFe nanocomposites in aqueous solution with lactic acid as hole scavenger (Figure 5). The hydrogen evolution rate of the CdS/NiFe nanocomposites is compared in Figure 5(a) under simulated sunlight irradiation for 5 h. The bare CdS nanorods exhibit a very low amount of H2 (2.4 16 ACS Paragon Plus Environment

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mmol g−1 h−1), which is attributed to the rapid recombination of the charger carriers and photocorrosion by the low band gap energy of CdS (2.4 eV).48 Similarly, NiFe LDH also exhibits a very low amount of hydrogen (0.09 mmol g−1 h−1) due to its limited light harvesting capacity and high recombination rate. The hydrogen evolution rate is greatly enhanced by integrating NiFe cocatalysts with CdS nanorods, reaching a maximum value of 72 mmol g−1 h−1 at 42 wt.% of NiFe. With further increase in the amount of NiFe, the hydrogen evolution rate decreased possibly due to the limited light harvesting capacity of the CdS nanorods. Evidently, the hydrogen evolution reaction of the nanocomposites is more effective than that of the bare CdS nanorods. Moreover, the hydrogen evolution rate in the CdS/NiFe nanocomposites surpasses the previous studies of cocatalyst integrated CdS nanostructures (Table S1). In particular, the PLAL-derived NiFe nanosheets are even more efficient than the noble-metal Pt nanostructures, because Pt-integrated CdS nanorods show a maximum hydrogen evolution rate of 34 mmol g−1 h−1. Under identical photocatalytic conditions, the hydrogen evolution rate was examined as a function of Fe precursor concentration used in the synthesis of NiFe LDH in Figure 5(b). The rate at FeCl3 concentration of 5, 10, 15, and 20 mM was measured to be 59, 72, 50, and 38 mmol g−1 h−1, respectively, indicating the highest rate at 10 mM. It may be due to near stiocitometric ratio and thin nature when using 10 mM FeCl3 for the synthesis of NIFe LDH. Since the concentration of hole scavenger also plays a role in hydrogen evolution reaction, the concentration effect of lactic acid was also studied by maintaining the catalyst amount and irradiation time. The hydrogen evolution rate as a function of scavenger concentration indicated that the rate increased with increasing concentration of lactic acid up to 3 mL (20% volume) in Figure 5(c), while the rate slightly decreased at higher concentrations. The decrease in the rate might be attributed to high viscosity and large number of molecules in the reaction medium, which hinder the light harvesting 17 ACS Paragon Plus Environment


capacity of the nanocomposites.49 To find out the good scavenger for high hydrogen evolution rate, several hole scavengers, such as methanol, ethanol, TEOA, and Na2S-Na2SO3, were tested in Figure 5(d), which indicates that lactic acid is one of the best scavengers for efficient hydrogen evolution reaction. Figure 5(e) shows the influence of catalyst dosage (1–4 mg) for the hydrogen evolution rate in aqueous lactic acid (20% volume) solution, which clearly indicates that 1 mg of the optimized CdS/NiFe (42 wt.%) displays the best performance. When the dosage of the CdS/NiFe nanocomposites increases, the rate gradually decreases possibly due to scattered light by excess suspended catalysts in the reaction medium.50 To assess the synthesized nanostructures photocatalytic performance, the apparent quantum yield (AQEs) of the synthesized CdS and CdS/NiFe nanocomposites were estimated under visible light using 150 W Xe lamp with 425 nm band pass filter as a light source. The measured quantum efficiencies are 0.535 and 18.2 % for CdS and CdS/NiFe, respectively. Moreover, the observed AQEs surpasses that of most preceding studies using different co-catalyst integrated CdS nanostructures (Table S1).


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Figure 5. (a) Influence of amount of NiFe LDH integrated on CdS nanorods for photocatalytic hydrogen evolution rate. (b) Effect of FeCl3 concentration on the formation of NiFe LDH and its role in the photocatalytic hydrogen evolution rate. (c) Role of lactic acid concentration in a catalytic reaction cell for photocatalytic hydrogen evolution rate over CdS/NiFe catalyst. (d) 19 ACS Paragon Plus Environment


Photocatalytic hydrogen evolution rate using CdS/NiFe nanocomposite using various hole scavengers. (e) Influence of photocatalyst dosage of CdS/NiFe on hydrogen evolution rate. (f) Stability measurements of CdS/NiFe-LDH nanocomposites over a 38 h period under simulated sunlight irradiation.

For application of the optimized catalyst, the stability of the catalyst is also crucial. To evaluate the stability of the synthesized nanostructures, the optimized catalysts were exposed to simulated sunlight for 38 h, which demonstrated a linear hydrogen production up to 25 h in Figure 5(f). The observed hydrogen evolution was approximately 280 mmol g−1, which indicated the high stability and robustness of the CdS/NiFe nanocomposites. To understand the structural stability of the synthesized CdS/NiFe nanocomposite after photocatalytic reaction, the FETEM analysis was carried out (Figure S11). The obtained results demonstrated that there were no evident structural changes after catalysis reaction, indicating that the synthesized CdS/NiFe nanocomposites are stable and have a strong interfacial contact between CdS and NiFe. The enhancement of the hydrogen evolution rate and stability of the CdS/NiFe nanocomposite over long periods of time are explained by the photocatalytic mechanism depicted in Scheme 2. When the light source illuminates the CdS/NiFe nanocomposite, the CdS nanorods in the nanocomposites absorb the light and generate electron–hole pairs. The DRS results indicate that the CdS/NiFe nanocomposites have an efficient visible light harvesting capacity. In the CdS nanorods, the photogenerated electrons in the conduction band recombine with the photogenerated holes in the valence band, which leads to low photocatalytic activity. In the presence of NiFe cocatalysts, on the other hand, an interfacial contact provides an efficient pathway to prolong the lifetime of photogenerated electrons by the electron transfer from CdS to NiFe, which efficiently 20 ACS Paragon Plus Environment

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reduces H+ for production of hydrogen molecules. Concurrently, the photogenerated holes in the valence band of CdS can be scavenged by lactic acid, which is in turn oxidized to form pyruvic acid. Efficient interfacial contact and robust separation of electrons and holes contribute to an enhanced hydrogen evolution rate in CdS/NiFe nanocomposites,20-22 as evidenced by the TEM, PL, EIS, and photocurrent results.

Scheme 2. Schematic of the proposed photocatalytic hydrogen evolution reaction mechanism in the CdS/NiFe nanocomposite

CONCLUSIONS For the first time, thin NiFe LDH nanostructures are synthesized using PLAL, which are integrated with the CdS nanorods by the ultrasonication method to form CdS/NiFe nanocomposites. Structural analyses demonstrate that NiFe LDH nanostructures are anchored onto the edges of the nanorods, which are employed for solar-driven hydrogen evolution reaction using lactic acid as hole scavenger. Introducing 42 wt.% of NiFe LDH to CdS nanorods yields a hydrogen evolution



rate (72 mmol g–1 h–1), which is 30 times higher than the bare CdS nanorods. The extraordinary hydrogen evolution rate is attributed to the efficient charge transportation, reduced charge recombination, and rich catalytically active sites. Photoluminescence, photocurrent, and impedance analysis indicate the transfer dynamics of photogenerated electrons and holes in the nanocomposites, which explain the high hydrogen evolution rates. We expect that the demonstrated methodology to form noble-metal-free co-catalysts nanostructures and the utilization for photocatalytic hydrogen evolution reactions provide novel insights to develop the cost-effective photocatalysts for efficient hydrogen production.

ASSOCIATED CONTENT Supporting Information. Experimental details; FESEM image of CdS nanorods; EDS line spectra of the CdS/NiFe nanocomposite; Mott-Schottky plots of CdS, CdS/NiFe, and NiFe LDH nanostructures; and comparison of photocatalytic H2 evolution rate reported in the literature using different LDH integrated CdS nanostructures with our present results. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] (T.K.K.) and [email protected] (J.K.S.) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS 22 ACS Paragon Plus Environment

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This work was supported by the National Research Foundation of Korea (NRF) grants, funded by the Korean Government (MSIP) (2014R1A4A1001690, 2016R1E1A1A01941978, and 2018R1A2B6001779). This work also supported by LG Yonam Foundation of Korea.

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For Table of Contents Use Only

We designed a novel highly robust, noble-metal-free photocatalytic nanohybrid CdS/NiFe for solar-driven hydrogen evolution by water splitting. The designed nanohybrid exhibits a high photocatalytic hydrogen evolution rate of 72 mmol g–1 h–1 under sunlight irradiation.


Drastic Improvement of 1D-CdS Solar Driven Photocatalytic Hydrogen

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