WEI Yn-Hu LI Gung-She WANG Jing-Ho XUE Cheng-Lin FANG Sho-Fn LI Li-Ping

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Self-assembled Nanohybrid from Opposite Charged Sheets: Alternate Stacking of CoAl LDH and MoS2①

WEI Yan-HuaaLI Guang-SheaWANG Jiang-HaobXUE Cheng-LinaFANG Shao-FanbLI Li-Pinga②

a.(130012)b.(350002)

Hybrid materials are attracting intensive attention for their applications in electronics, photoelectronics, LEDs, field-effect transistors,. Engineering new hybrid materials and further exploiting their new functions will be significant for future science and technique development. In this work, alternatively stacked self-assembled CoAl LDH/MoS2nanohybrid has been successfully synthesized by an exfoliation-flocculation method from positively charged CoAl LDH nanosheets (CoAl-NS) with negatively charged MoS2nanosheets (MoS2-NS). The CoAl LDH/MoS2hybrid material exhibits an enhanced catalytic performance for oxygen evolution reaction (OER) compared with original constituents of CoAl LDH nanosheets and MoS2nanosheets. The enhanced OER catalytic performance of CoAl LDH/MoS2is demonstrated to be due to the improved electron transfer, more exposed catalytic active sites, and accelerated oxygen evolution reaction kinetics.

CoAl LDH/MoS2nanohybrid, exfoliation-flocculation method, self-assembly, electrocatalytic oxygen evolution reaction; electrocatalytic oxygen evolution reaction;

1 INTRODUCTION

Recently, much more attention has been paid to hybrid nanomaterials because strong interaction between component materials in nanoscale could result in novel properties and functions, while the component materials maintain their unique properties. Moreover, hybrid materials have exhi- bited wide potential applications in electronics[1-3], photoelectronics[4, 5], LEDs[6-8], catalysis[9-12],. Hence it is very significant to engineer new hybrid materials and further exploit their new functions for the requirement of future science and technique development. Until now, a lot of hybrid materials have been reported. Among them, less work is devoted to fabricating the hybrids consisting of opposite charged components due to the complexity and difficulty in experimental design.

Layered double hydroxides (LDHs), known in a universal formula of [M2+1-xM3+(OH)2]+[(An-)/n]-·-mH2O, are a group of materials composed of positively charged metal hydroxide host layers and negatively charged balanced anions. Owing to the tunable structure and composition, the exchangeable interlayer anions, as well as weak interlayer interac- tion hence easy to be exfoliated, LDHs are con- sidered as an excellent basic material to engineer novel functional composites[13-15]. Moreover, the surface of exfoliated LDH in suspensions could be rich in positive charge[16], thus making its self- assembly with negatively charged nanomaterial possible.

MoS2is a typical transition metal dichalcogenide, exhibiting unique properties, such as good chemical stability, high thermal and electronic conductivity like graphite, meanwhile showing various applica- tions in electrocatalysis[17], lithium-ion batteries[18, 19], solar cells[20], supercapacitors[21], and so on. More- over, MoS2possesses a unique layered structure, in which a plane of molybdenum atoms is sandwiched by two planes of S atoms to form a monolayer of MoS2. Since stacked monolayers are held together by weak van der Waals interaction, MoS2nanosheets can be obtained by exfoliating bulk crystals into single-layered/few-layered nanosheets through various techniques including liquid-phase exfolia- tion methods in specific solvent. From the structural point of view, reducing the thickness of MoS2nanosheets to single-layer/few-layers could expose more S atoms, thus engendering MoS2nanosheets negatively charged. If CoAl-NS suspension is mixed with that of MoS2-NS, alternatively stacked CoAl LDH/MoS2hybrid from opposite charged sheets could be formed via electrostatic interaction. Moreover, in this hybrid structure, the constituents of CoAl LDH and MoS2-NS could enhance the electron transfer and accelerate the catalytic oxygen evolution reaction kinetics.

In our work, we have successfully self-assembled positively charged CoAl LDH-NS with negatively charged MoS2-NS in nanoscale into alternately stacked nanohybrid. The CoAl LDH/MoS2exhibits an enhanced catalytic performance for oxygen evolution reaction relative to the starting nanosheets. We attribute the enhanced OER catalytic perfor- mance to the improved electron transfer, large amount of exposed catalytic active sites, and accelerated catalytic oxygen evolution reaction kinetics.

2 EXPERIMENTAL

2. 1 Synthesis of CoAl LDH/MoS2 nanohybrid

All the starting materials used in the experiment were purchased from Sinopharm Chemical Reagent Co., Ltd. An exfoliation-flocculation method was adapted to prepare the CoAl LDH/MoS2nanohybrid.

2. 1. 1 Synthesis of Co-Al LDH-CO32-

Firstly, Co-Al LDH-CO32-was synthesized by a homogeneous precipitation method. In a typical procedure, Co(NO3)2·6H2O, Al(NO3)3·9H2O and urea were dissolved in 80 mL of deionized water to give the final concentrations of 10, 5, and 35 mM, respectively. The aqueous mixture was transferred into a 100 mL Teflon-lined autoclave to react at 100 ℃ for 24 h. After cooling to room temperature, the solid products were filtered, subsequently washed with water and alcohol for several times, and air-dried at ambient temperature.

2. 1. 2 Anion exchange of CoAl LDH

A two-step anion-exchange process was then used to treat the as-prepared LDH sample. 0.6 g CoAl LDH-CO32-was added to 600 mL of NaCl-HCl mixed solution (1 M NaCl and 3.3 mM HCl in deionized water) and magnetically stirred at 650 rpm for 24 h. The CoAl LDH-Cl-was separated and purified by the same procedure for CoAl LDH-CO32-as described in Part 2.1.2. Then, 0.6 g of the NaCl-HCl treated LDH sample was dispersed into 600 mL of aqueous solution containing 0.1 M NaNO3. After that, CoAl LDH-NO3-, to be used for subsequent exfoliation, was obtained by filtering, washing and drying process as that for CoAl LDH-CO32-, and -Cl-.

2. 1. 3 Exfoliation of CoAl LDH-NO3-

0.3 g CoAl LDH-NO3-was dissolved in 300 mL formamide, followed by ultrasonic treatment for 6 h. Subsequently, the solution was magnetically stirred for another 24 h. Then, the solution was treated by the centrifugation of 4000 rpm for 10 min twice to remove the remaining unexfoliated crystals and to obtain a pink suspensioncontainingCoAl-NS.

2. 1. 4 Exfoliation of MoS2

2 g bulk MoS2was dispersed in 300 mL N,N-dimethylformamide (DMF), then followed by magnetically stirring for 2 h. After 8-hour ultrasonic treatment, the mixture was magnetically stirred for another 24 h. The resulting colloidal solution was further centrifuged at 10, 000 rpm for 15 min for several times to remove the unexfoliated bulk MoS2. Finally, an olive green colloidal solution containing MoS2-NS was obtained.

2. 1. 5 Preparation of the CoAl LDH/MoS2nanohybrids

300 mL colloidal solution of MoS2-NS was slowly added into 300 mL of that of CoAl-NS at a dropping rate of 2.5 mL·min-1under continuous stirring for 24 h. The flocculated sample was obtained by centrifugation of 3000 rpm for 15 min, washed with alcohol for several times and dried at room temperature.

2. 2 Characterization

The crystal structures of the as-prepared CoAl LDH and CoAl LDH/MoS2samples were identified by powder X-ray diffraction (XRD, D/MAX 2550 and MiniFlex 600, Rigaku, respectively) with Curadiation (= 0.15418 nm).The morphologies of the as-prepared LDH samples were imaged by scanning electron microscopy (SEM, SU 8020, HITACHI). Profiles of exfoliated LDH and MoS2were obtained by a Multimode 8 DI Atomic Force Microscope instrument (AFM, Bruker, USA) in a tapping mode. TEM images of the samples were characterized by Field Emission Transmission Electron Microscopy (TEM, FEI Tecnai G2F 30) with the Accelerating voltage of 300 kV.Element distribution was measured by Energy Dispersive X-RaySpectrometer (EDX, Gnensis, EDAX, AMETEK, USA). X-ray photoelectron spectroscopy (XPS) was performed using the ESCALAB 250 (Thermo Electron Corporation, USA) and the binding energy was standardized with respect to the residual C 1peak.

2. 3 Electrochemical tests

The electrochemical measurements were per- formed using an electrochemical workstation (CHI 760 D, Shanghai) in a three-electrode tested with the prepared films on a glassy carbon electrode with an active area of 0.196 cm2as the working electrode, Hg/HgO in sat. KCl was sued as the reference electrode, and graphite as the counter electrode at a scan rate of 5 mV·s?1in 1 M KOH. To prepare the test for electrochemical performance, 4 mg of powder sample was dispersed in a solution containing 450 μL of deionized water, 500 μL of ethanol and 50 μL of Nafion. Then, the obtained suspension was treated in an ultrasonic water bath. Glassy carbon disk electrodes with a diameter of 5 mm were polished with alpha alumina powder (50 nm) suspended in deionized water on a Nylon polishing pad. After cleaning, the electrodes were thoroughly rinsed with deionized water. Before loading with catalysts, the electrodes were also cleaned in deionized water by sonication for about 1 min. 10 μL of colloid suspension was then drop-casted on the glassy carbon electrode to give a loading density of 0.2 mg·cm-2. The electrodes were dried at ambient temperature.

The potentials were calibrated with respect to the reversible hydrogen electrode (RHE) using the Equation (1),

Electrochemical impedance spectra (EIS) were measured using the electrochemical workstation at a potential of 1.6 V vs. RHE and frequency range of 100 000～0.01 Hz with amplitude of 5 mV. Cyclic voltammetry (CV) measurements at different scan rates (40, 80, 120, 160, 200 mV/s) were used to determine the electrochemical double layer capacitances (EDLC, Cdl). Chronoamperometry was tested at an applied potential of 1.63 V at pH = 14 to study the durability of the electrocatalysts.

Fig. 1. Schematic illustration of the preparation process for CoAl LDH/MoS2nanohybrid

The formation of LDH-CO32-in terms of two steps (a homogeneous hydrothermal method and a successful anion exchange to LDH-NO3-) was verified by XRD characterization. XRD patterns of CoAl LDH inserted with CO32-, Cl-, and NO3-are shown in Fig. 2(a), which are wellindexed to CoAl LDH (JCPDS, No. 51-0045). The exchange of intercalated anions leads to the XRD peak of (003) shifting towards smaller angle obviously from CO32-, Cl-, to NO3-, demonstrating an enlarged layered spacing from 7.4 to 8.6 ? according to Bragg Equation (2),

2dsin(2)

which is beneficial to the next exfoliation. The relatively strong (006) peak in CoAl LDH-NO3-shows some asymmetry, which might result from original anion residue or a minor contamination of CO32-during anion exchange. The morphologies of as-fabricated CoAl LDH-CO32-and anion exchange product LDH-NO3-were further characterized by SEM displayed in Fig. 2(b, c). CoAl LDH-CO32-nanoflakes display a typical lateral size about ten microns and thickness about tens of nanometers. After two steps of ion-exchange, no morphological changes were observed for CoAl LDH-NO3-. Both CoAl LDH-CO32-and CoAl LDH-NO3-exhibit similar morphology of hexagonal plates, demon- strating that the structure of LDH was well main- tained after two-step ion exchange.

The atomic force microscopy (AFM) profiles in Fig. 3(a, b) shows that the exfoliated CoAl LDH exhibits a typical thickness about 6～10 nanometers, while MoS2-NS of 3-6 nanometers. The presence of crack of the nanosheets may be due to the long ultrasonic time and high centrifugal rate. The ultrathin nature of the nanosheets is also confirmed by the almost transparent morphology, as shown in TEM images of Fig. 3(c, d). Besides, TEM images in Fig. 3(c, d) show that the lateral sizes of CoAl-NS and MoS2-NS are both about hundreds of nanome- ters. Furthermore, the successful exfoliation of nanoflakes to nanosheets for CoAl LDH-NO3-can also be verified by the weakening of the XRD diffraction peaks of (003), shown in Fig. 4(a). From nanoflake to nanosheet, the thickness along the direction of layer stacking is greatly reduced. Meanwhile, more octahedral edges of LDH could be exposed. It has been reported that the octahedral edges of LDH may be the OER active catalytic sites[22]. The more exposed octahedral edges, the better electrocatalytic OER performance.

Fig. 2. (a) XRD patterns of CoAl LDH before (inserted with CO32-) and after (insertedwith NO3-) ion-exchange treatment, and SEM images of (b) CoAl LDH-CO32-and (c) CoAl LDH-NO3-

Fig. 3. AFM profiles of (a) CoAl-NS, (b) MoS2-NS, and TEM images of (c) CoAl-NS, (d) MoS2-NS

The ?occulation of MoS2-NS with LDH-NS was executed by dropping MoS2-NS colloidal suspension into that of CoAl-NS. Fig. 4(a) illustrates typical XRD patterns of ?occulated product CoAl LDH/MoS2. The clear diffraction peaks at two theta of 11.5°, 23.2°, 34.6°, 38.7°, 47.3° and 61.9° are well indexed to CoAl LDH (JCPDS No. 51-0045), and the diffraction peak at 14.4° is attributed to the (002) of MoS2(JCPDS No. 87-2416). The co-existence of diffraction peaks of CoAl LDH and MoS2suggests that CoAl LDH/MoS2nanohybrid was successfully engineered. TEM and HRTEM images in Fig. 4(b～d) are shown to further illustrate the alternatively stacked structure of the hybrid. The TEM image in Fig. 4(b) demonstrates that the hybrid consists of ultrathin nanosheets. The coexistence of plane (003) of CoAl LDH and plane (103) of MoS2with simultaneously alternative stacking in HRTEM of Fig. 4(c) reveals the superlattice-like structure of CoAl LDH/MoS2. Meanwhile, HRTEM image in Fig. 4(d) illustrates that nanohybrid consisting of monolayered CoAl-NS and MoS2-NS was even formed[15, 16]. The diffraction rings of both CoAl LDH and MoS2are presented in the SAED patterns of CoAl LDH/MoS2nanohybrid in Fig. 4(e). Furthermore, the uniform distribution of Co, Al, Mo and S can be clearly observed in the HAADF- mapping images of Co–Al LDH/MoS2in Fig. 5, which further proves that MoS2and CoAl LDH were successfully integrated to form nanohybrid.

Fig. 4. (a) XRD patterns and (b) TEM, (c, d) HRTEM images and (e) SAED images of CoAl LDH/MoS2nanohybrid

Fig. 5. HAADF-mapping images of CoAl LDH/MoS2hybrid material

XPS measurement also confirms the formation of hybrid CoAl LDH/MoS2. Fig. 6(a) compares the XPS survey spectra of CoAl LDH/MoS2and CoAl-NS. Mo 3and S 2peaks can be identified in the survey spectra of CoAl LDH/MoS2, corro- borating the presence of MoS2in the hybrid material, i.e. successfully obtaining CoAl LDH/MoS2hybrid. Fig. 6(b) shows the high resolution Co 2spectra of CoAl-NS and CoAl LDH/MoS2. Both spectra can be well deconvoluted into four photoelectron peaks. For CoAl-NS, two strong peaks signals at 781.9 and 798.1 eV correspond to the core levels of Co 23/2and Co 21/2, while the other two peaks at 786.6 and 803.6 eV are the satellite peaks of Co 23/2and Co 21/2, respectively. The Co 2spectrum for CoAl-NS exhibits the following characteristics compared to that observed for cobalt oxides[23, 24]: more intense satellite peaks, higher binding energies of Co 23/2and Co 21/2, larger spin-orbital splitting and smaller energy difference between Co 23/2and its larger satellite peak. These characteristics demonstrate that cobalt in CoAl-NS is mainly Co2+ions in a high-spin state coordinated with hydroxyl[23]. When the nanohybrid was formed, Co 2peaks of CoAl LDH/MoS2shifted towards higher binding energies of 782.3 (Co 23/2) and 798.5 eV (Co 21/2). Meanwhile, two satellite bands located at 787.1 and 804.2 eV, respectively, indicate that the presence of a strong interaction between CoAl LDH and MoS2changes the electronic structure of the nanohybrid.

The CoAl LDH/MoS2nanohybrid was then tested for electrochemical oxygen evolution reaction (OER) performance in 1 M KOH solution, with CoAl-NS and MoS2-NS as the compared samples. The catalysts were electrochemically pre-conditioned to reach a stable state. Then, the water oxidation activity was measured by linear sweep voltammetry in standard three-electrode system. The CoAl LDH/MoS2nanohybrid exhibits an enhanced OER performance compared with CoAl-NS and MoS2-NS. Fig. 7 displays the comparison of OER perfor- mances of samples CoAl LDH/MoS2, CoAl-NS and MoS2-NS. The linear sweep voltammetry curves in Fig. 7(a) show that the onset potential of CoAl LDH/MoS2hybrid negatively shifted significantly compared to bare CoAl-NS and MoS2-NS. At the current density of 10 mA·cm-2, CoAl LDH/MoS2exhibited a lower overpotential of 0.376 V, superior to CoAl-NS (0.425 V) and MoS2-NS (even failing to achieve the current density of 10 mA·cm-2at an overpotential of 0.494 V). At an overpotential of 400 mV, the current density of CoAl LDH/MoS2reaches as high as 20.1 mA·cm-2, 4 times higher than that of CoAl-NS (5.41 mA·cm-2) and 8 times higher than that of MoS2-NS (0.27 mA·cm-2), depicted in Fig. 7(c). All above confirms that CoAl LDH/MoS2hybrid exhibits an enhanced electrocatalytic OER performance. Fig. 7(b) shows Tafel slopes of three samples. The lower Tafel slope of CoAl LDH/MoS2suggests its optimized oxygen evolution reaction kinetics. Moreover, a smaller Tafel slope suggests that electron transfer in CoAl LDH/MoS2is more facile compared to the other two samples, as verified by electrochemical impedance spectra (EIS). Tofurther elucidate the origin of the enhanced OER catalytic performance, EIS of CoAl LDH/MoS2and bare CoAl-NS has been obtained and is presented in Fig. 7(d). Obviously, the diameter of the semicircu- lar arc for CoAl LDH/MoS2is smaller than that for CoAl-NS, indicative of a faster charge transfer at the interface of the electrode/electrolyte after assembling CoAl-NS with MoS2, which improves the OER catalytic performance.

Fig. 6. (a) XPS survey spectra and (b) Co 2core levels of CoAl-NS and CoAl-LDH/MoS2

The electrochemical active surface area (ECSA) of samples was valued from the electrochemical double layer capacitance (EDLC, Cdl) in Fig. 7(e). The linear slope of capacitive current versus scan rates is used to evaluate the ECSA, equivalent to twice of the double-layer capacity Cdl[25, 26]. From CoAl-NS, MoS2-NS to CoAl LDH/MoS2, the ECSA increased dramatically. The higher ECSA value for CoAl LDH/MoS2strongly suggests that the obtained hybrid exposes more catalytic active sites than those electrochemically accessible for H2O or OH-. Compared to two component materials, the special structure of alternativeely stacked CoAl LDH/MoS2hybrid enables it to exhibitadvantages of faster electron transfer and more active sites responsible for its superior OER catalytic performance. The stability of CoAl LDH/MoS2was also examined by chronoamperometric measurement at an overpo- tential of 400 mV. As shown in Fig. 7(f), the current density exhibits a slight increase after small decrease, which may result from a bubble adsorption-desorp- tion process[27]. More importantly, the current density values still retained at ca. 4.67 mA·cm-2even after 12 hours of electrocatalytic reaction, suggesting that the CoAl LDH/MoS2nanohybrid is intrinsically stable under OER reaction conditions.

Fig. 7. Comparison of OER performance for samples CoAl LDH/MoS2, CoAl-NS and MoS2-NS. (a) Linear sweep voltammetry curves, (b) Tafel plots, (c) Histogram of Current density at overpotential of 400 mV (solid), and overpotential at current density of 10 mA·cm-2(hollow), (d) EIS results, (e) Charging current density differences (Δj = ja– jc) plotted against scan rates. The linear slope, equivalent to twice of the double-layer capacitance Cdl, was used to evaluate the ECSA. (f) Chronoamperometric measurement of CoAl LDH/MoS2hybrid at overpotential of 400 mV

4 CONCLUSION

In summary, we have successfully self-assembled positively charged CoAl LDH nanosheets with nega- tively charged MoS2nanosheets into alternately stacked nanohybrid. The CoAl LDH/MoS2exhibits an enhanced catalytic performance for OER when compared with pristine CoAl-NS and MoS2-NS in alkaline electrolyte of pH = 14. XPS measurement shows that Co ions in the nanohybrid are divalent and coordinate to hydroxyl. Moreover, the strong interaction between two components in CoAl LDH/MoS2nanohybrid results in a higher binding energy shift for Co 2photoelectron peaks. The results of EIS and EDLC demonstrate that the enhanced OER catalytic performance mainly attributes to the higher electron conductivity, more exposed active sites, optimized catalytic oxygen evolution reaction kinetics,. To further under- stand the mechanism of OER catalytic kinetics and interaction of the components of CoAl-NS and MoS2-NS, more works will be expected on the electron structure and physical chemistry properties. The finding about self-assembled nanohybrid from oppositely charged sheets could provide a new idea for the synthesis of new functional hybrid materials.

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26 October 2017;

9 January 2018

This work was financiallysupported by NNSFC (No. 21025104, 21271171, and 91022018)

. Li Li-Ping, professor. E-mail: lipingli@jlu.edu.cn

10.14102/j.cnki.0254-5861.2011-1869