Boosting Electrocatalytic Oxygen Evolution Performance of Ultrathin

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Boosting Electrocatalytic Oxygen Evolution Performance of Ultrathin Co/Ni-MOF Nanosheets via Plasmon-Induced Hot Carriers Minmin Wang, Ping Wang, Chuanping Li, Haijuan Li, and Yongdong Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13472 • Publication Date (Web): 26 Sep 2018 Downloaded from http://pubs.acs.org on September 27, 2018

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ACS Applied Materials & Interfaces

Boosting Electrocatalytic Oxygen Evolution Performance of Ultrathin Co/Ni-MOF Nanosheets via Plasmon-Induced Hot Carriers Minmin Wang,a,b Ping Wang,a Chuanping Li,a,b Haijuan Lia and Yongdong Jina,b* a

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, 130022, Jilin, China b

University of Chinese Academy of Sciences, Beijing 100049, China

KEYWORDS: ultrathin metal-organic frameworks, surface plasmon, hot-hole, Schottky junction, OER ABSTRACT: Ultrathin metal-organic frameworks (MOFs) nanosheets with large active sites and superior catalytic properties have attracted extensive interests and are promising for oxygen evolution reaction (OER) for water splitting. Herein, we report a novel and highly efficient hetero-nanostructured OER system based on plasmonic AuNPs and ultrathin semiconductor-like Co/Ni-MOF nanosheets. The OER performance of the hybrid system can be tuned (by varying the AuNP sizes) and the oxidation current significantly enhanced to ~ 10-fold with incorporated AuNPs of ~ 20 nm. An onset overpotential () of only 0.33 V was achieved under light illumination, which was much lower than the pure Ni/Co-MOF (0.48 V). Further analysis revealed the key role of the plasmonically induced hot holes (via electric- and combined photoexcitation) in boosting the OER performance of the resulting system. The finding and proposed concept provide a new insight for understanding the plasmon-enhancements in catalysis and may open a new avenue to design MOF hetero-nanostructures with high performance for photoelectrocatalysis.

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1. INTRODUCTION Metal-organic frameworks (MOFs) which composed of metal ions and organic ligands as a class of versatile porous materials has been used for a wide range of applications,1-5 including promising application in oxygen evolution reaction (OER) for water splitting.6-11 Apart from their inherent high surface area, MOFs with different active metals centers have been used as efficient electrocatalysts.12 The incorporation of metal nanoparticles (NPs), such as Fe3O4, Pd, Au, Ru, Cu and Pt into MOFs is an important mean to further improve their activity and stability13-17 as MOFs structures can be used as the active support to stabilize the metal or metal oxide NPs, preventing them from the agglomeration during their preparation and reaction processes. Although vast advancements have been made in designing and applying MOF-based materials for catalysis, there are still rare reports on utilizing plasmon-electrical coupling effects to boost OER performance of the semiconductor-like MOFs materials. As known that upon light illumination plasmonic metal will photogenerate a large density of carriers that can be used for photoinjection into semiconductor materials for direct surface chemical reaction.18 The effective utilization of hot plasmonic carriers from metal nanocomponents for performance-enhanced photocatalysis19-23 is now a hot research area. However, most of such photoactivity enhancements reported so far are attributed to direct hot electrons injection,24 increased carrier generation rate due to the near field electro-magnetic coupling25,26 or direct plasmon electron transferring through chemical interface damping;27,28 the role of plasmonically-generated hot holes is usually neglected because these holes are normally with low energy that cannot overcome the potential barrier. Therefore, there is little report on the roles of hot holes in plasmonic catalysis.29 Herein, by incorporating AuNPs into the ultrathin Co/Ni-MOF nanosheets


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and investigating the plasmon effect on the OER performance of the resulting heteronanostructures, we succeeded in enhancing the electrocatalytic OER performance of the system and found that the enhancement is related to the size of AuNPs. For the smaller AuNPs, higher OER activity could be obtained, which is assumed to be attributed to the AuNP size-dependent oxidation ability of the hot holes that plasmonically-generated on AuNP surfaces. To the best of our knowledge, this is the first report on synergetic utilization of the plasmonically generated hot holes for OER performance enhancement of the Co/Ni-MOF nanosheet system.

2. METHODS Synthesis of AuNPs. Uniform AuNPs seed with a diameter of 10 ± 3.6 nm was synthesized by previous report. 30 The 40.6 ± 3.6 nm AuNPs were then synthesized at room temperature by diluting 10 mL of the as-prepared 10 nm AuNPs seed solution to 100 mL using distilled water, and then adding 1 mL NH2OH·HCl (0.2 M) under vigorously stirring for 5 min. Finally, 835 μL of HAuCl4 (1 wt %) was dropwise added to the mixture with stirring for 10 min. Ligand Exchange of AuNPs with PVP. 20 ml aqueous solution of PVP (0.2 g, Mw = 55,000) was dropwise added to the as-prepared AuNP sol under vigorously stirring at room temperature, and then stirred for another 24 hours. Finally, the PVP-stabilized AuNPs were isolated by centrifugation (4000 rpm, 30 minutes), and then washed by methanol for three times, and the final AuNPs with PVP was dispersed in methanol. Synthesis of Ultrathin Co/Ni-MOF and AuNPs@Co/Ni-MOF. The synthesis was performed according to the reported procedure with minor modification.31 Typically, a mixture of Co(NO3)2·6H2O (3 mmol) and Ni(NO3)2·6H2O (1 mmol) was dissolved in 30 ml methanol to form solution A. And then 3 mmol of 2-methylimidazole was dissolved in methanol (10 mL) to


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form solution B. Then mixed the solutions A and B under stirring for 1 min. Let the mixture solution stand for 24 h at an ambient temperature. Then, the obtained yellowish precipitate (sample denoted as Co/Ni-MOF) was collected via centrifugation, washed with pure methanol and water several times and then dispersed in water. For synthesis of AuNPs@Co/Ni-MOF, a certain amount of AuNPs was added to the mixed solutions of A and B with string for 1 min, and then let the mixture solution stand for 24 h at an ambient temperature. Finally, the obtained light red precipitate was isolated by centrifugation and washed with methanol and water for three times, and then dispersed in water. Material Characterizations. UV-Vis spectrophotometer (Cary 500 Scan ,Varian, U.S.A.) was used to measure the UV-Vis absorption spectra for all the as-prepared samples. Transmission electron microscope (TEM, equipped with an energy dispersive spectrometer) was used to characterize the detailed microstructures. Chemical compositions of the samples were investigated by X-ray photoelectron spectroscopy (Escalab 250Xi, Thermo Fisher Scientific). The concentration of AuNPs in AuNPs@Co/Ni-MOF was determined by inductively coupled plasma-atomic emission spectrometer and mass spectrometer (ICP-AES/MS, icap 6000 series, Thermo Scientific). The valence state of Ni ions in AuNPs@Co/Ni-MOF was measured by ESR spectra (JES-FA200 electron spin resonance spectrometer) operating at room temperature. We use 532 nm laser with intensity of 100 mW/cm-2 in our experiments. Electrochemical Characterizations. Electrochemical measurements were performed by electrochemical workstation (CHI 660e) with a three electrode electrochemical cell and all experiments were tested at room temperature and KOH aqueous solution (pH=12) was used as the electrolyte. The detailed procedures for preparing work electrodes were shown as follows: 1 mg as-prepared catalysts were dispersed in a mixed solution of 0.2 mL ethanol, 0.6 mL water,


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0.02 mL 5 wt% Nafion solution with ultrasonication. Then dropping the as-prepared ink on the pre-polished glass-carbon electrode (GCE) uniformly. We use the Pt coil as counter electrode and Ag/AgCl as reference electrode. The current density was obtained on the geometrical area of the electrode, and the reversible hydrogen electrode (RHE) scale was converted by Nernst equation. Before electrochemical measurements, the working electrode was cycling between 0 and 1 V versus Ag/AgCl (100 mV s-1 ) for 150 cycles at room temperature. The polarization curves was obtained with a sweep rate of 5 mV s-1.

3. RESULTS AND DISCUSSION The AuNPs@Co/Ni-MOF hetero-nanostructures were prepared by in-situ encapsulating presynthesized AuNPs into the Co/Ni-MOFs. The detailed synthetic procedures are shown in experimental section. The ultrathin nanosheet-like morphology of the Co/Ni-MOF was determined by TEM and atomic force microscopy (AFM) measurements (Figure 1 and Figure S1). As shown in Figure 1a, due to ultrathin thickness of the Co/Ni-MOF, the edges of the transparent nanosheets are curled. The Tyndall light scattering effect (inset of Figure 1a) demonstrates that the as-prepared AuNPs@Co/Ni-MOF heterostructures are well-dispersed in aqueous solution, and it can be kept stable for several months. The thicknesses of the Co/NiMOF is as thin as ~ 1 nm by AFM measurement (Figure S1), which will facilitate catalytic reactions due to the existence of rich coordinative unsaturated metal sites on surfaces.32


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Figure 1. (a) TEM image of the as-synthesized AuNPs@Co/Ni-MOF. The inset shows the Tyndall light scattering of AuNPs@Co/Ni-MOF in an aqueous solution. (b-g) high angle annular dark-field scanning TEM (HAADF-STEM) and elemental mapping images of AuNPs@Co/NiMOF. The composition information was obtained by high angle annular dark-field scanning TEM (HAADF-STEM) and TEM elemental mapping measurements. As shown in Figure 1b-g, the AuNPs are sparsely decorated and elements of C, N, O, Ni and Co are uniformly distributed throughout the entire Co/Ni-MOF surfaces. In our system, AuNPs with varied sizes were prepared by a method as previous reported.30 Figure S2 shows TEM image and UV-Vis absorption spectra of the AuNPs with diameter of 40.6 ± 3.6 nm. The typical LSPR band of the AuNPs are shifted from ~ 530 nm to 534 nm after the AuNP decoration onto the Co/Ni-MOF nanosheets due to the change in dielectric environment. As shown by powder X-ray diffraction (XRD) analysis (Figure 2a), there is no apparent loss of crystallinity for the Co/Ni-MOF nanosheets after the AuNP incorporation. In addition, XRD peaks corresponding to AuNPs (JCPDS No. 04-0784) are observed, indicating successful incorporation of AuNPs. The highly porous property of the as-prepared Co/Ni-MOF nanosheets was determined by N2 sorption


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experiments. As shown in Figure 2b, both Co/Ni-MOF and AuNP@Co/Ni-MOF showed a type I isotherm which indicates that the two samples are rich in micropores.14 The as-prepared ultrathin Co/Ni-MOF nanosheets showed a higher BET surface area of 1352 m2 g-1 and a slight decrease in surface area after the incorporation of AuNPs (865 m2 g-1) due to the occupation by AuNPs. The pore size distribution (Figure 2c) for Co/Ni-MOF nanosheets and AuNP@Co/Ni-MOF was calculated to be less than 2 nm and 4 nm, respectively. The high surface area and abundant micropores of the materials will benefit for the access of electrolyte with OER active sites. XPS measurements were implemented to determine the chemical identity and detailed chemical and electronic states of the Co 2p, Ni 2p, and Au 4f species in the as-prepared AuNPs@Co/Ni-MOF heterostructures. As shown in Figure 2d, two kinds of cobalt oxidation state are detected, including the Co2+ ions at the binding energies of 780.8 eV and 796.4 eV, and Co3+ at 780.8 eV and 795.5eV. 33,34 Similarly, the Ni 2p emission spectrum (Figure 2e) was also fitted with two spin-orbit doublets. The peaks at the binding energies of 855.0 eV and 872.6 eV are attributed to Ni2+, while the peaks at 856.7 and 875.1 eV are attributed to Ni3+.35 As previously reported, the existence of high-valence state Co3+ and Ni3+ endows the as-prepared hetero-nanostructures superior catalytic activity in OER.36,37 It is known that Au is the most electronegative metal which can act as an electron sink and facilitate the oxidation of Co2+ and Ni2+. Meanwhile, the orbitals of Au 4f7/2 and Au 4f5/2 are located at 83.4 and 87.2 eV, which shift to a lower binding energy compared to free AuNPs (Figure 2f), indicating the occurrence of electron transfer from Co/Ni-MOF to AuNPs that caused by their intimate interaction. 38-40 Experimentally, we found that intimate interaction between AuNPs and Co/Ni-MOF nanosheets is vital to obtaining high electrocatalytic OER activity of the catalysts. In control experiments, the samples prepared by simple physical mixing of the two components showed


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poor dispersion and the nanosheet nature disappeared (Figure S3) due to the agglomeration of AuNPs as shown by UV-Vis spectrum change in Figure S4, which result in comparatively low electrocatalytic performance of the control system (Figure S5). As shown in Figure S6, the asprepared Co/Ni-MOF was quite stable both in alkaline KOH solutions (pH=12, which we chose as electrolyte in our system) and in acidic solutions (pH= 4). However, it can not exsit in strong acidic solutions (pH

Boosting Electrocatalytic Oxygen Evolution Performance of Ultrathin

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