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Coupling Effect Induced Acceleration of Electron Transfer for #-Ni(OH)2 with Enhanced Oxygen Evolution Reaction Activity Xinfu Zhao, Xiaotong Ding, Yuguo Xia, Xiuling Jiao, and Dairong Chen ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00360 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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ACS Applied Nano Materials

Coupling Effect Induced Acceleration of Electron Transfer for α-Ni(OH)2 with Enhanced Oxygen Evolution Reaction Activity Xinfu Zhao†‡, Xiaotong Ding†, Yuguo Xia*†, Xiuling Jiao† and Dairong Chen* † †

School of Chemistry & Chemical Engineering, National Engineering Research Center

for Colloidal Materials, Shandong University, Jinan 250100, P. R. China ‡

QiLu University of Technology (Shandong Academy of Sciences), Advanced Materials

Institute, Shandong Provincial Key Laboratory for Special Silicone-Containing Materials, Jinan 250100, P. R. China

KEYWORDS：：OER, Ni-based electrocatalyst, photo-reduction, nanocomposite, electron transfer acceleration

ACS Paragon Plus Environment

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ABSTRACT Rational design and fabrication of nanocomposites constructed by multi-components with synergistic functionalities are effective approach for materials with important energy applications such as electrocatalytic oxygen evolution reaction (OER), where there is still a challenge to obtain non-noble catalysts with high performance and practical applicability. Here, a highly active and durable OER electrocatalyst based on Ni(OH)2 nanosheets decorated by Ag nanoparticles and RGO nanosheets is designed and sequentially prepared. The Ni(OH)2-Ag-RGO nanocomposite exhibits high activity as OER electrocatalyst, achieving an overpotential of 292 mV at the current density of 10 mA cm-2 and a small Tafel slope of 58 mV dec-1 without iR-correction, and reveals good OER property comparing with commercial IrO2 electocatalyst and other previously reported Ni-based OER electrocatalysts. The acceleration of electron transferring from Ni(OH)2 nanosheets to external circuit is a result of coupling effects of Ag nanoparticle and RGO nanosheets which separately serves as to increase the energy of surface Ni3d electrons and enhance the conductivity. The above mechanism is confirmed by experimental results and computational simulation. Moreover, the generality of this strategy to the similar nanostructures with enhanced OER activity are demonstrated by fabricating Co(OH)2-Ag-RGO and FeOOH-Ag-RGO nanocomposites.


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1. INTRODUCTION Water splitting through electrochemical or photoelectrochemical processes offers a prospective strategy to solve the increasing energy demands and climate change.1-3 Among others, the efficiencies of OER and hydrogen evolution reaction (HER) which functionalize as half reactions of water splitting are crucial in the advancement of above energy technology and its practical application.4 Due to the typically higher reaction barrier in the formation of O-O bond, oxygen evolution is considered to be the most kinetically hindered step during water splitting.5 Besides, although IrOx and RuOx hold the benchmark for OER electrocatalyst, the scarcity and high-cost considerably impede their widespread utilization in commercial electrolyzes.6 In consideration of these respects, developing non-precious-metal OER catalysts to accelerate this intrinsically sluggish process by reducing the overpotential and enhancing the electron transfer between the catalysts and respective electrodes is a feasible approach. Ni-based electrocatalyst including its oxides and hydroxides has been widely studied as good earth-abundant OER electrocatalyst.7-9 However, comparing with commercial IrO2 and RuO2 electrocatalysts, Ni-based oxides and hydroxides have not been employed in practical water oxidation process due to their intrinsic low conductivity and high overpotential. Two main strategies have been widely recognized to effectively improve electrocatalytic property. The first strategy is to fabricate Ni-based oxides and hydroxides with nanoarchitectures, such as, nanosheet,10 hollow nanosponges11and nanoarrays, 12 etal, to increase surface area and exposed active sites. The other strategy is to construct hybrid nanocomposite with noble metals, metallic substrates, cocatalyst or more conductive


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carbonaceous materials, such as Au nanoparticles,13 Ni foam,14 IrO215and carbon nanotube,16 respectively. The aforementioned hybrid components can promote the conductivity between electrocatalyst and electrode to a certain extent, and prevent material shedding from the electrode. However, limited by the relative lower contact area between noble metal cluster and electrocatalyst as well as low electron transfer rate between the substrates and electrocatalyst, simply design and prepare bi-component M(S)/electrocatalyst or cocatalyst/electrocatalyst (M= metal, S=substrate) nanocomposite cannot effectively reduce the overpotential and increase the electron transfer rate simultaneously. Inspired by the efficient electron store and shuttle mechanism in TiO2-RGO-Ag nanocomposite,17 Ni(OH)2-Ag-RGO electrocatalyst is rationally designed and prepared. Herein, we initially built an ideal material model of Ni(OH)2-Ag and Ni(OH)2-RGO to illustrate the electronic structural changes of Ni(OH)2 after compositing with Ag cluster and RGO, which provides opportunities to disclose the correlation between Ag cluster, RGO and Ni(OH)2 (Detailed computational models were supplied in Figure S1). Of note, Figure 1a and Figure 1b clearly reveals that α-Ni(OH)2 exhibits a lamellar structure along c axis, and the band gap located between the valence band maximum (VBM) and conduct band minimum (CBM) indicates its semiconductor nature. The empty states localized in the Fermi level is responsible for intrinsic low conductivity of α-Ni(OH)2. Figure 1c shows the VBM relative to the Fermi level is obviously increased and new Ni3d states are generated across the Fermi level after compositing with Ag nanoparticle. The increased energy of Ni3d electrons indicates an easier conversion for Ni2+ to Ni3+/4+. Meanwhile, theoretically, the Ni3d states and Ag5s states located at the Fermi level can


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largely improve the conductivity. Besides, although the electronic structure is nearly unchanged after compositing with graphene, the whole band gap is filled up with C2p states which will further enhance the conductivity of nanocomposite. With all these advantages, we predict that coupled ternary Ni(OH)2-Ag-RGO electrocatalyst will have both lower overpotential and fast electron transfer rate. Corresponding experimental characterizations and measurements are subsequently done to demonstrate above speculation, and this strategy could pave a new approach to rationally design highly efficient OER electrocatalyst.

Figure 1. DFT calculations. (a) Schematic illustration of α-Ni(OH)2 layered crystal structure and (b) corresponding density of states, (c) density of states for Ag/α-Ni(OH)2 and (d) density of states for graphene/α-Ni(OH)2. 2. RESULTS AND DISCUSSION


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To demonstrate above speculation, Ni(OH)2-Ag-RGO nanocomposite was synthesized by a modified UV-assisted reduction method as in reference 17. Scheme 1 revealed the preparation process of Ni(OH)2-Ag-RGO nanocomposite. Note that Ag nanoparticles were firstly loaded on the Ni(OH)2 nanosheet through UV-assisted reduction instead of RGO, which ensured the Ag nanoparticles directly combined with Ni(OH)2 nanosheet.

Scheme 1. Schematic diagram for the preparation process for Ni(OH)2-Ag-RGO nanocomposite. The XRD patterns of as-prepared samples were firstly investigated as shown in Figure 2a, and the reflections labeled by rhombus could be readily indexed to hexagonal α-Ni(OH)2 (JCPDS: 38-0715), while those relative weak peaks corresponded to cubic Ag (JCPDS: 87-0597). The Raman spectra in Figure 2b showed that the value of ID/IG in Ni(OH)2-Ag-RGO nanocomposite was 1.305 and much higher than that in Ni(OH)2-RGO (ID/IG=1.187) and original GO (ID/IG=1.059), demonstrating that the proportion of the newly generated sp3 hybrid carbon atoms in RGO increased.18,19 The relative high value was considered to be caused by photoreduction process of GO occurred on the Ni(OH)2 nanosheets’ surface which destroyed the sp2 hybrid C atoms to form sp3 hybrid C atoms and the interface interaction between Ag and RGO. Therefore, strong interaction was formed between RGO and Ag particles or Ni(OH)2 nanosheets. The morphology and microstructure of the Ni(OH)2-Ag-RGO nanocomposite were further determined. As


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shown in Figure 2c, the Ag nanoparticles with a size of ca. 15 nm were uniformly deposited on Ni(OH)2 nanosheets (magnified HRTEM image was supplied in Figure S2), and the inset HRTEM image revealed the lattice fringe spacing of 2.32 Å, corresponding to (111) plane of fcc Ag. The EDS mapping of Ni(OH)2-Ag-RGO nanocomposite in Figure 2d illustrated the well-distribution of C, O, Ni elements on the whole structure, and no separated distribution of C element was observed, which indicated good composite structure was formed between RGO and Ni(OH)2 nanosheets. Meanwhile, unobvious Ag elemental signal was revealed because of its low content (the content of Ag was ca. 6.3 wt% measured by ICP-AES). To further investigate the effects of Ag nanoparticles and RGO on Ni(OH)2 nanosheets, XPS spectra were conducted to verify the valence states changes of Ni(OH)2 and Ag. As shown in Figure 2e, the Ni 2p peaks shifted successively to the higher energy direction, which illustrated Ag particles and RGO both had strong interfacial interaction with Ni(OH)2 nanosheets and indicated Ni ions in Ni(OH)2-Ag-RGO are more benefit to generate higher oxidized Ni3+/4+ active species. In addition, the Ag 3d peaks also upshifted to higher energy, which illustrated valence electrons of Ag had tendency to transfer electrons to RGO. To further confirm the electron acceptance of RGO, XPS spectra of C1s were compared between Ni(OH)2RGO and Ni(OH)2-Ag-RGO, and C1s peak was slightly shifted to low energy direction which further confirmed the electron transfer from Ag to RGO (Figure S3). Overall, interfacial interactions were formed among Ni(OH)2 nanosheets, Ag nanoparticles and RGO nanosheets.


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Figure 2. (a) XRD patterns of as-prepared samples, (b) Raman spectra of Ni(OH)2-AgRGO and GO, (c) TEM image and (d) EDX elemental mapping image of Ni(OH)2-AgRGO, and (e-f) XPS spectra of Ni 2p and Ag 3d. Inset of Figure 2c was corresponding HRTEM image. To illustrate the enhanced electrocatalytic performance produced by above nanocomposite, a three-electrode system was applied to obtain the OER activity of Ni(OH)2-Ag-RGO nanostructure in 1.0 mol dm-3 oxygen-saturated KOH electrolyte with


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Ni(OH)2-Ag, Ni(OH)2-RGO, Ni(OH)2 and commercial IrO2 as comparisons. Figure 3a showed the linear sweep voltammetry (LSV) curves of Ni-based catalysts and commercial IrO2 nanomaterials obtained at a sweep rate of 10 mV s-1 (no iR-correction), and Ni(OH)2-Ag-RGO nanocomposite exhibited lower onset potential and much higher current density than Ni(OH)2-Ag, Ni(OH)2-RGO, Ni(OH)2 and IrO2 nanomaterials after the typical oxidation peak of Ni2+ to Ni3+/4+ at 1.4-1.5 V.

13,20

Considering the effects of

coupling oxidation peak of Ni and electronic structures and surface areas changes of Ni(OH)2 due to the compositing with Ag and RGO,

overpotentials of these

electrocatalysts were obtained by a reverse scan method to eliminate aforementioned influencing factors as shown in Figure S4. The overpotentials of Ni(OH)2-Ag-RGO, Ni(OH)2-Ag, Ni(OH)2-RGO, Ni(OH)2 and commercial IrO2 were 292 mV, 349 mV, 356 mV, 387 mV and 340 mV vs RHE at the current density of 10 mA cm-2, respectively (overpotentials obtained at 20 mA cm-2 were supplemented in Table S1), suggesting the enhanced electrocatalytic activity of Ni(OH)2 after compositing with Ag nanoparticle and RGO nanosheet. Besides, the amount of loaded Ag nanoparticle was measured by LSV curves, which revealed 6.3 wt% was the optimized value for Ni(OH)2-Ag-RGO OER electrocatalyst (Figure S5). To illustrate the charge conversion efficiency during OER, Faradic efficiencies were shown in Figure S6, and all the electrocatalysts approaches to 100% in efficiencies during test, indicating the observed current was solely consumed during the process of OER. As another important parameter to evaluate the OER electrocatalytic activity, Tafel slope for Ni(OH)2-Ag-RGO nanocomposite was only 58 mV dec-1, revealing its alternated and improved kinetics for OER, which was considered to be caused by energy increase of surface Ni3d electrons and changes of electron


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transfer route. Moreover, as-prepared Ni(OH)2-Ag-RGO nanocomposite also revealed good OER property comparing with other up-to-date Ni-based electrocatalysts in the overpotential and Tafel slope as shown in Figure 3c.11,13,16,19-27 To further illustrate the enhanced electrocatalytic performance, the turnover frequency (TOF) was measured to reflect the intrinsic electrocatalytic activity. Figure 3d revealed that the TOF of Ni(OH)2Ag-RGO was similar with commercial IrO2 and higher than Ni(OH)2-Ag, Ni(OH)2-RGO, and Ni(OH)2 at the same potential. Furthermore, the mass activity and specific activity (calculation according to BET surface areas13) of Ni(OH)2, Ni(OH)2-RGO, Ni(OH)2-Ag and Ni(OH)2-Ag-RGO increased successively as shown in Figure 3e and Figure 3f. The mass activity and specific activity for Ni(OH)2-Ag-RGO were 90.6 A g-1 and 0.061 mA cm-2, which is respectively 1.34, 1.35, 1.77-fold and 2.03, 2.1, 2.26-fold as that of Ni(OH)2-Ag, Ni(OH)2-RGO and Ni(OH)2. To assess the electrochemical OER stability of Ni(OH)2-Ag-RGO nanocomposite, a long-term water oxidation was conducted at a constant overpotential of 357 mV vs. RHE without iR compensation in 1M KOH media. Figure 3g illustrated that Ni(OH)2-Ag-RGO nanocomposite retained nearly steady OER activity and no obvious current density decay was observed for longer than 18 h of oxygen release. The slightly increase of current density before 10 h might be caused by gradual electrochemical polarization or dissolution of naphthol to reach equilibrium between the electrocatalyst and electrode interface. The microstructure of Ni(OH)2-AgRGO nanocomposite was further observed as shown in Figure S7, which was nearly unchanged after I-t test, also indicating the long-term stability of the prepared Ni(OH)2Ag-RGO nanocomposite.


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Figure 3. Electrochemical performances of Ni(OH)2-Ag-RGO, Ni(OH)2-Ag, Ni(OH)2RGO, Ni(OH)2 and commercial IrO2 nanomaterials at 1.0 mol dm-3 KOH electrolyte. (a)


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The linear voltammetry curves without iR-correction, (b) Tafel slope of catalysts, (c) comparison of overpotential and Tafel slope with other Ni-based electrocatalysts, (d) TOF values, (e) mass activities, (f) specific activities of catalysts at different overpotentials, and (g) chronoamperometry curve of Ni(OH)2-Ag-RGO electrocatalyst at the overpotential of 357 mV. To reveal the electronic transfer route and coupling effects of Ag cluster and RGO nanosheet involved in Ni(OH)2-Ag-RGO electrocatalyst, the electronic structure of Ni(OH)2-Ag-RGO was measured. As shown in Figure 4a, three main emission peaks appeared at 308, 358 and 425 nm in the photoluminescence spectra (PL),28,29 and the lowest PL intensity in Ni(OH)2-Ag-RGO electrocatalyst indicated its smallest recombination rate of electrons and holes. Both the PL curve of Ni(OH)2-Ag and Ni(OH)2-RGO revealed lower intensity relative to Ni(OH)2, suggesting the electron transfer routes had been changed after compositing with Ag cluster and RGO nanosheet. The electron spin resonance (ESR) spectra were further used to obtain the direct evidence of electron transfer mechanism of as-prepared samples as shown in Figure 4b. The broad paramagenetic absorption signals with g≈10.259,4.351 and 2.247 corresponded to Ni3+ similar to that in Ni2O3,30,31 indicating the partly oxidation of as-prapared samples. The ESR signals showed little difference after loading Ag nanoparticles on Ni(OH)2 nanosheets, which illustrated that due to the formation of Ni3d impurity states in the band gap as shown in Figure 1c, the Ag nanoparticles could not directly accommodate the electrons transferred from Ni(OH)2 during ESR excitation and the composition of Ag nanoparticles just resulted in the increase of electron energy of Ni ions. On the contrary, the signals of Ni3+ significantly increased after loading RGO on Ni(OH)2 nanosheets,


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which could be attributed to the ability of C2p states of RGO to shuttle and store electrons. Therefore, the enhanced OER activity of Ni(OH)2-Ag-RGO electrocatalyst should be a synergistic effect of Ag nanoparticles and RGO nanosheet. The interfacial properties between the electrode and the electrolyte were further investigated by electrochemical impedance spectroscopy (EIS) measurements as shown in Figure 4c. The semicircle in the high frequency region of the Nyquist plots represented a charge transfer process, in which the smallest arc radius of Ni(OH)2-Ag-RGO illustrated its smallest charge transfer resistance comparing with other samples. Besides, the chargecarrier density (Na) was derived from the Mott-Schottky plots (Figure 4d). The Na for Ni(OH)2-Ag-RGO (5.10×1023) was significantly enhanced after compositing with Ag nanoparticle and RGO nanosheet comparing with original Ni(OH)2 (1.56×1022), Ni(OH)2-Ag (8.97×1022) and Ni(OH)2-RGO (1.26×1022), which further illustrated a fast electron transfer rate in Ni(OH)2-Ag-RGO. In addition, the obvious enhanced Na in Ni(OH)2-Ag-RGO comparing with Ni(OH)2-Ag and Ni(OH)2-RGO also revealed a coupling effect of Ag and RGO on Ni(OH)2 instead of simply physical superposition.


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Figure 4. (a) Photoluminescence spectra, (b) EPR spectra, (c) Nyquist plots and (d) MottSchottky plots of Ni(OH)2, Ni(OH)2-Ag, Ni(OH)2-RGO and Ni(OH)2-Ag-RGO electrocatalysts. Inset of Figure 4c was corresponding equivalent circuit. According to the computational and experimental results, we proposed the following electron transfer mechanism which was responsible for the observed enhancement for Ni(OH)2 based electrocatalysts after compositing with Ag nanoparticles and RGO nanosheet. As shown in Figure 5a, the Ag nanoparticles directly interacted with Ni(OH)2 in the form of Ag-O bonds while the interactions between Ni(OH)2 and RGO nanosheet was van der Waals force confirmed by geometries after DFT calculation, which illustrated strong interface interactions were generated between Ni(OH)2 and Ag or RGO. Different from the surface plasmon resonance of Au,13 the VBM of Ni(OH)2 relative to the Fermi level was obviously enhanced after compositing with Ag nanoparticles, which indicated an easier conversion of surface Ni atoms to high oxidized states due to higher


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energy of surface Ni3d electrons. The newly generated Ni3d and Ag5s states could largely enhance the conductivity, which was confirmed by the decreased charge transfer resistance in Nyqusit plots and increase of charge carrier density in Mott-Schottky plots, and the charge carrier medium of Ni(OH)2 was changed from Ni3d and O2p states to Ni3d states as shown in Figure 5b. Besides, although the electronic structure and charge carrier medium of Ni(OH)2 was nearly unchanged after compositing with RGO, the C2p states filled up the whole band gap of Ni(OH)2 which could enhance the nanocomposite’s conductivity further. The final OER property of Ni(OH)2-Ag-RGO was superior to Ni(OH)2 individually compositing with Ag nanoparticle or RGO. Therefore, we considered that the enhanced OER activity in Ni(OH)2-Ag-RGO electrocatalyst was the coupling effect of Ag nanoparticles and RGO nanosheet, that is, the energy of surface Ni3d electrons was increased after composting with Ag nanoparticles, and Ag nanoparticles acted as a medium for electron transfer and reduced the transmission resistance while RGO transferred the electron from surface of Ag cluster to the outside glassy carbon electrode to maintain the high activity of NiIII/IV, resulting in the enhanced OER property of Ni(OH)2-Ag-RGO electrocatalyst.


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Figure 5. (a) schematic diagram of electrode and Ni(OH)2-Ag-RGO electrocatalyst and (b) electron transfer routes for Ni(OH)2, Ni(OH)2-Ag, Ni(OH)2-RGO and Ni(OH)2-AgRGO nanocomposites. In addition, comparing with the other strategies to increase the OER property of Nibased electrocatalysts, the features involved in this paper was considered as follows. Firstly, the synthesis of Ni(OH)2-Ag-RGO electrocatalyst was template-free and secure. Toxic reagents and high temperature were not requisite compared with the synthesis methods for Ni-based sulfide 32, nitride 33 and phosphide 34. Secondly, the size and molar amount of Ag nanoparticle as well as RGO could be easily regulated to achieve the an optimized OER property. Thirdly, this strategy to enhance OER property was universal to other non-noble metal system. To confirm the generality of this strategy to enhance OER property, Co(OH)2-Ag-RGO and FeOOH-Ag-RGO catalysts were prepared with the same method as shown in Figure S8, and the two kinds of catalysts both revealed enhanced OER performance comparing with pristine Co(OH)2 and FeOOH, indicating the generality of the designed structure in improving OER catalytic activity. 3. CONCLUSION In summary, Ni(OH)2-Ag-RGO nanocomposite was designed and fabricated to improve the conductivity and efficiency of Ni(OH)2 for OER catalysis. In this nanostructure, the electrons of Ni(OH)2 were firstly transferred to Ag nanoparticles, and then shuttled by RGO nanosheets quickly to the glassy carbon electrode. The coupling effect of Ag nanoparticles and RGO nanosheets could effectively increase the population of high oxidation Ni species and improve the conductivity. The calculation results also certified the promising appropriate charge transfer route. The electrocatalytic


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measurements exhibited that the current density and Tafel slope of Ni(OH)2-Ag-RGO both was superior to that of IrO2 and other Ni-based catalysts. The generality of this kind of structure in enhancing the OER catalytic efficiency was also confirmed by fabricating the similar nanostructure of other transition-metal hydroxides. 4. EXPERIMENTAL SECTION 4.1 Materials preparation 4.1.1 Preparation of Ni(OH)2 nanosheets The synthesis of Ni(OH)2 nanosheets was processed according to Ref. 13, and the sample was stored in a desiccator for further experiment. 4.1.2 Preparation of Ni(OH)2-Ag-RGO nanocomposite In a typical process, 20 mg as-prepared Ni(OH)2 nanosheets were dispersed into 8 mL ethanol in a 20 mL bottle, which was sealed and the air was removed with nitrogen gas for 30 min. After the solution was irradiated with 254 nm UV light for another 30 min, 300 µL degassed AgNO3 solution (0.0647 mol dm-3) was added and the reaction was sustained for 12 h under dark with stirring. Then, the obtained sample was separated and washed with ethanol, dispersed in 8 mL ethanol again and degassed, reirradiated once more. After that, 1 mL GO nanosheets dispersion in ethanol (2 mg/mL) was added into the above system. After stirring for 5 h, the sample was collected by centrifugation, washed with ethanol and dried at 50 °C under vacuum. As comparisons, other nanocomposites were also fabricated. The synthesis process of Ni(OH)2-Ag nanocomposite was the same as that of Ni(OH)2-Ag-RGO, except for the addition of GO, while Ni(OH)2-RGO is synthesized without the addition of AgNO3. The


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Co(OH)2 and FeOOH nanosheets were prepared according to Ref. 25. The fabrication of Co(OH)2-Ag-RGO and FeOOH-Ag-RGO nanocomposites was similar to that of Ni(OH)2-Ag-RGO, the details were shown in the supporting information. 4.2 Structural characterization The morphology of samples was characterized by TEM (JEM-100CXII) and HRTEM (JEOL-2010) with accelerating voltage of 200 kV. The EDS mapping was obtained through FE-SEM (SU-8010，Hitachi). The phase structure of samples was measured using a Rigaku D/Max 2200-PC diffractometer with Cu Kα radiation and graphite monochromator (λ=0.15418 nm). The Raman spectra were recorded at 633 nm excitation wavelength with Horiba Jobin-Yvon double monochromator. The measurement of photoluminescence

spectra

was

conducted

on

Cary

Eclipse

Fluorescence

Spectrophotometer (Agilent Technologies) with the excited wavelength of 260 nm. EPR spectra were obtained on a JEOL-FA200 electron spin resonance spectrometer under room temperature. The Faradic efficiencies of the electrocatalytic system are measured on a gas chromatograph at 1.56V vs. RHE under solar simulated illumination. 4.3 Electrochemical tests All electrochemical experiments were measured in a three-electrode system using a CH-760e electrochemical workstation. In the three-electrode system, Pt nanoplates and Hg/HgO were used as counter and reference electrodes, respectively. The working electrode was made by dripping 7.5 µL dispersion (5.0 mg dm-3, the ratio of water to ethanol is 3:1 in solvent) onto a glassy carbon electrode (5 mm diameter), then 2 µL naphthol solution (0.5 wt%) was added and dried under an infrared lamp. The electrolyte


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was 1.0 mol dm-3 KOH aqueous solution and was purged with O2 to remove the dissolved N2. Linear sweep voltammetry curves were measured with a sweep rate of 10 mV s-1 at room temperature. EIS were measured in 1.0 mol dm-3 KOH aqueous solution over the frequency range from 100000 Hz to 0.01 Hz. 4.4 Theoretical calculations To better understanding the electronic structure changes of α-Ni(OH)2 after compositing with Ag cluster and RGO, spin-polarized calculations were performed using the Vienna ab initio Simulation Package (VASP).35, 36 The exchange-correction function was described by the Perdew-Burke-Ernzerhof generalized gradient approximation (GGA) 37

and the wavefunctions were expanded in a plane wave basis with an energy cutoff of

500 eV. The primitive cells of α-Ni(OH)2, Ag and graphite were firstly optimized, and αNi(OH)2 nanosheet, Ag cluster and RGO were represented by α-Ni(OH)2 monolayer containing 80 atoms, Ag6 cluster and graphene monolayer containing 50 atoms, respectively. To describe the strongly localized 3d electrons of Ni, DFT+U method was used with a Hubbard parameter U (U-J = 3.8 eV).38 The van der Waals (vdW) interaction was involved which was corrected by the DFT-D3 approach.39 All the structural relaxations were carried out until the residual forces were below 0.02 eV/Å. ASSOCIATED CONTENT Supporting Information. The procedure for synthesis of Co(OH)2 nanosheets, Co(OH)2-Ag-RGO composite, FeOOH nanosheets, FeOOH-Ag-RGO composite and corresponding TEM images and detailed computational models were supplied.


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AUTHOR INFORMATION Corresponding Authors *Email: [email protected] *Email: [email protected] ORCID Yuguo Xia: 0000-0002-0405-3439 Xiuling Jiao: 0000-0002-4358-7396 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors thank the Natural Science Foundation of Shandong Province (grant ZR2016BQ22), National Natural Science Foundation of China (grant 21701099) and the Taishan Scholars Climbing Program of Shandong Province (tspd20150201) for financial supports REFERENCES (1)

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Kang, D.; Kim, T. W.; Kubota, S. R.; Cardiel, A. C.; Cha, H. G.; Choi, K.-S.,

Electrochemical Synthesis of Photoelectrodes and Catalysts for Use in Solar Water Splitting. Chem. Rev. 2015, 115, 12839-12887. (4)

Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.;

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