Ar Plasma-Exfoliated Ultrathin NiCo-Layered Double Hydroxide

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Ar Plasma Exfoliated Ultrathin NiCo-Layered Double Hydroxides Nanosheets for Enhanced Oxygen Evolution Yuqian Liu, Man Zhang, Di Hu, Ruiqi Li, Kang Hu, and Kai Yan ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019

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ACS Applied Energy Materials

Ar Plasma Exfoliated Ultrathin NiCo-Layered Double Hydroxides Nanosheets for Enhanced Oxygen Evolution Yuqian Liu, Man Zhang, Di Hu, Ruiqi Li, Kang Hu and Kai Yan* Guangdong Provincial Key Laboratory of Environmental Pollution and Remediation Technology, School of Environmental Science and Engineering, Sun Yat-sen University, 135 Xingang Xi Road, Guangzhou 510275, China.

Layered double hydroxides (LDHs) materials are frequently used for oxygen evolution reaction (OER) due to their promising properties in overcoming the large energy barrier. In this work, the controllably synthesized NiCo-LDHs nanosheets are treated by Ar plasma and display the superior activity as well as high durability for OER process, where by a much lower overpotential of 299 mV at 10 mA cm-2 and a smaller Tafel slope of 45 mV dec-1 than the pristine material (347 mV and 149 mV dec-1). The characterization results reveal that numerous defects induced by Ar plasma on the surface of ultrathin NiCo-LDHs nanosheets, leading to much more exposed active sites, faster kinetics and lower resistance. This work offers an inspiration for the rational design of more active and efficient LDHs-based materials for OER.

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KEYWORDS: Ar plasma, Exfoliation, NiCo-LDHs, Nanosheets, Oxygen Evolution Reaction, Tunable

1. INTRODUCTION Water electrolysis is often used to produce hydrogen (H2) and oxygen (O2) due to its eco-friendly property and high purity, where the H2 is considered as the alternative clean fuel to the traditional fossil fuels.1 Nevertheless, the oxygen evolution reaction (OER) is a four-electron-transfer process which often requires high overpotential (η) to overcome the large energy barrier. On the other hand, the sluggish kinetics of OER is often associated with high energy consumption.2 Considerable research efforts have been devoted to developing efficient electrocatalysts to improve the electrochemical conversion efficiency and decrease the overpotential of OER process.3 The comparing benchmarks generally are precious-metals electrocatalysts such as ruthenium oxide (RuO2) and iridium oxide (IrO2) which exhibit pretty good performance for OER, however, the widespread application of them is hindered by the high cost and scarce natural resources.4-6 For this reason, it is necessary to design the cost-effective and abundant electrocatalysts with good catalytic activity and stability for OER. Many researches have studied the improvement of OER process using non-noble metals and layered double hydroxides (LDHs)-based materials in recent years.7-9 Thanks to the unique brucites-like structure of LDHs materials, the fractional trivalent metal cations could replace the divalent ones during interlayer spacing and the relatively weak bonding, the components can be easily tuned. Meanwhile, the variety of subject metal cations, compensatory anions and different molar ratios also lead to the various morphology and reactive characteristics combined with LDHs structure.10 Although such materials generally have the good electrocatalytic activity and stability,


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the large scale application is still hindered by the accumulation of layered layers and the blocked interlayer space limits the electrolyte access to the internal active sites. Hu et al. firstly reported the liquid phase exfoliation method such as the anion exchange to reduce the thickness of the bulk CoCo-, NiFe-, and NiCo-LDHs in alkaline conditions.11 These exfoliated single-layer nanosheets exhibited high OER activity, where the overpotentials were decreased by 40-54 mV at 10 mA cm2

compared with the bulk LDHs materials. It was attributed to generate single-layer nano-structure

and create more active sites. Wang et al. also reported the acid-base treatment was used to assist the exfoliation process and introduce Co, Fe and O vacancies in CoFe-LDHs simultaneously, which ultimately formed the acid etched LDHs with a low overpotential of 300 mV at 10 mA cm. However, due to the extra anions such as Cl- and NO3- needed to be induced, the synthesis of

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LDHs and exfoliation process were still complex. In consequence, Liu et al. reported the hydrothermal method to directly produce the NiMn-LDHs and generated acetate at the same time, which had the properties of the better crystallinity, increased spacing of layers and surface area as well as the atomic dispersal.13 However, this process excluded the supporting substrates benefiting the electric conductivity, the as-obtained NiMn-LDHs consequently exhibited a slightly higher overpotential of 350 mV at 10 mA cm-2. Therefore, it is essential to prepare the efficient electrocatalysts with more exposed surface active sites, good electronic conductivity, fast kinetics, smaller resistance and high durability. In this work, the NiCo-LDHs nanosheets are prepared on carbon fiber paper (abbreviated as CFP) matrix via a facile one-step hydrothermal method and the typical procedure is depicted in Figure S1, NiCo-LDHs after a short time argon gas (Ar) plasma treatment (denoted as NiCoLDHs/Ar) exhibit nice nanosheet morphology with numerous defects. The Ar plasma treatment can efficiently achieve the etching on the surface of NiCo-LDHs nanosheets, increasing the


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number of exposed actives sites and adjusting the electronic structure.14,15 Compared to the previously reported work,11-13 the synthesis process of NiCo-LDHs/Ar nanosheets is relatively simplified without long time hydrothermal treatment and the inducement of extra ions. Meanwhile, the time required for exfoliation process is significantly shrunk to 15s (more details are in Supporting Information). The as-obtained NiCo-LDHs/Ar nanosheets show the outstanding activity with a pretty low overpotential of 299 mV at 10 mA cm-2, a small Tafel slope of 45 mV dec-1, and the excellent electrocatalytic durability for OER process over 10 h.

2. EXPERIMENTAL SECTION 2.1. Materials Synthesis The NiCo-LDHs nanosheets were fabricated through the soft template and hydrothermal method. The schematic illustration of the synthesis of NiCo-LDHs nanosheets is shown in Figure S1. Ultrapure deionized water was used in all experiments. Initially, 0.2 g Hexadecyl trimethyl ammonium bromide (CTAB, VetecTM) was dissolved into 20 mL pure ethanol (Guangzhou Chemical Reagent) at 40℃ with the stirring speed of 600 r. p. m for 2 h. The 0.005 M (CH3CO2)2Co·4(H2O) (Alfa-Aesar) and 0.01 M CH3CO2)2Ni·4(H2O) (Alfa-Aesar) were then added into the CTAB solution with the temperature constant. After stirring for another 2 h, 0.03 M urea (Alfa-Aesar) was added into the mixture under continuous magnetic stirring for a further 3 h. The mixed solution was transferred into a 100 mL stainless-steel Teflon-lined hydrothermal synthesis reactor with the carbon fiber paper (Toray, denoted as CFP) placed previously for hydrothermal treatment at 160℃ for 12 h. The used CFP was cleaned with concentrated sulfuric acid for 1h and then immersed in pure ethanol for 0.5 h before reaction. After the hydrothermal reaction, the autoclave was cooled down by water to room temperature. The obtained gel deposited


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at bottom of the lining was centrifuged three times at a speed of 1,000 r. p. m for 10 min to remove the redundant CTAB, the cleaning agent used is ethanol. The CFP and washed gel were dried at 90 ℃ for 10 h under Ar atmosphere. Finally, the CFP were treated through Ar plasma at 10 s, 15 s, and 30 s respectively, and the loading of NiCo-LDHs/Ar on CFP was ~0.467 mg cm-2. 2.2. Physical Characterization X-ray diffraction (XRD) measurements were carried out on a D/max-2200vpc instrument (Rigaku, JPN) performed at 40 kV and 40 mA with Cu Kα1 radiation, the scanning rage of 2 theta was from 9° to 90° at 10° min-1. X-ray photoelectron spectroscopy (XPS) studies were operated on an EscaLab 250 X-ray photoelectron spectrometer (Thermo Scientific, USA) with a pass energy of 20 eV and a monochromatized Al Kα as the excitation source at 150 W, the energy step size was 0.05 eV step-1. Scanning electron microscopy (SEM) images were recorded using a Gemini500 instrument (Zeiss/Bruker, GER) at the accelerating voltage of 20 kV. The inductively coupled plasma optical emission spectrometerenergy (ICP-OES) was performed on 5300DV (Perkin Elmer, USA) Transmission electron microscopy (TEM) patterns were taken on a FEI Tecnai G2 F20 transmission electron microscope operated at 200 kV accelerating voltage. Atomic force microscope (AFM) was performed on a MFB-3D instrument (Asylum Research, UK) in tapping mode. 2.3. Electrochemical Analysis The cyclic polarization curves were recorded by a conventional three-electrode cell on a Multi Autolab M204 electrochemical workstation (Netherlands) in this study. The as-prepared NiCoLDHs (/Ar) supported on CFP, an Ag/AgCl electrode (3M KCl), and a platinum wire were set as the working, reference, and counter electrode, respectively. And the O2-saturated 1 M NaOH


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(Macklin) solution was used as the working electrolyte during the electrochemical OER activities analysis. The obtained potential was normalized to reversible hydrogen electrode (RHE) by: E (RHE) = E (Ag/Agcl) + 0.059 * pH + 0.197, the pH value used here was 13.24. The swept range of potential was from 0 to 0.8V (vs. REF) using a scan rate of 10 mV s-1. The catalysts were electrochemically pre-conditioned at the potential swept rate of 20 mV s-1 for 50 sans to reach the stable states. The electrochemical active surface area (ECSA) was used to compare the available active sites of as-exfoliated NiCo-LDHs/Ar nanosheets and pristine NiCo-LDH, and often approximately represented by the double-layered capacitance (Cdl) which could be calculated based on cyclic voltammograms in a non-Faradaic region. In this work, the Cdl was collected by cyclic voltammetry (CV) in a potential window of 1.17-1.27 V (vs. RHE) at different scan rates from 1 to 5 mV s-1 and obtained by plotting ΔJ (current density) = (Janodic - Jcathodic) at 1.24 V (vs. RHE) against scan rate, where the slope is twice the Cdl. 16-18

3. RESULTS AND DISCUSSION The crystalline phase of as-made NiCo-LDHs and NiCo-LDHs/Ar were firstly studied by X-ray diffraction (XRD) as shown in Figure 1a. The peaks of 11.0°, 22.3°, 33.6° and 60.0° corresponded to (003), (006), (009) and (110) crystal planes respectively according with the pattern of NiCoLDHs.19,20 In addition, as we can see, The peaks at 36.7°, 42.6°, 61.8, 74.1° and 78.0° correspond to CoO disappeared in the XRD pattern of NiCo-LDHs/Ar compared with that of pristine NiCoLDHs, which indicated that CoO was successfully cleaned out from NiCo-LDHs/Ar nanosheets after the Ar plasma treatment. Meanwhile, the framework was well maintained after Ar plasma treatment process.21 What’s more, the peak intensities of as-exfoliated NiCo-LDHs/Ar phases


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became weaker than those of pristine NiCo-LDHs material, suggesting the partial layer were successful separated on the surface of NiCo-LDHs due to the assistant of Ar plasma.22

Figure 1. (a) XRD patterns of pristine NiCo-LDHs and as-exfoliated NiCo-LDHs/Ar; XPS spectra of (b) Ni 2p ; (c) Co 2p regions of as-exfoliated NiCo-LDHs/Ar and (d) Co 2p regions of pristine NiCo-LDHs/Ar. The electronic state and element composition on the surface of NiCo-LDHs/Ar nanosheets was further investigated by X-ray photoelectron spectroscopy (XPS). The XPS spectrum of Ni 2p region (Figure 1b) display two main peaks of Ni 2p3/2 and Ni 2p1/2 at 855.0 eV and 872.6 eV, respectively, along with two shake-up satellite bands (860.9 and 878.9 eV) , which could be ascribed to Ni2+ and indicated the existence of Ni(OH)2 on the surface.23,24 Furthermore, the energy separation between Ni 2p3/2 and Ni 2p1/2 peaks was approximately ~17.6 eV, which was associated with the formation of NiOOH.25-27 Additionally, the Co 2p spectra of NiCo-LDHs/Ar nanosheets


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(Figure 1c) split into the Co 2p3/2 (780.2 eV) and Co 2p1/2 (796.1 eV) peaks accompanied by two satellite bands (785.4 and 802.0 eV), which could be assigned to Co3+.28 Besides, as shown in Figure 1c and d, the atomic ratio of Co2+/Co3+ (1.72) on the NiCo-LDHs/Ar is higher than that (0.93) of pristine NiCo-LDHs, indicating that relatively more Co2+ present in NiCo-LDH after the plasma treatment. Meanwhile, the signal in the O 1s region at 530.6 eV (Figure S2) matched with the -OH phase further indicating the hydroxide structure of Ni(OH)2.23 Besides, the inductively coupled plasma optical emission spectrometer (ICP-OES) was used to testify the atomic ratio of Ni/Co (Table S1). The atomic ratio of Ni/Co of as-exfoliated NiCo-LDHs/Ar nanosheets was well consistent with that of pristine NiCo-LDHs, indicating that the atomic ratio wasn’t influenced by Ar plasma treatment. The XRD and ICP analyses showed that the Ar plasma treatment only generated on the surface of NiCo-LDHs nanosheets.

Figure 2. SEM images of the as-synthesized samples: pristine NiCo-LDHs (a) and (b), asexfoliated NiCo-LDHs/Ar nanosheets (c) and (d).


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Figure 3. TEM and HRTEM images of the pristine NiCo-LDHs (a) and (b), as-exfoliated NiCoLDHs/Ar nanosheets (c) and (d). Scanning electron microscopy (SEM) was further utilized to understand the morphology of asprepared NiCo-LDHs and exfoliated nanosheets. As shown in Figure 2a and Figure S3, the layered structure of NiCo-LDHs was uniformly and nicely grown on the pure CFP matrix and the multiple thin layers are formed (Figure 2b). After Ar plasma treated, the layered structure was partially exfoliated (Figure 2, c and d). Transmission electron microscope (TEM) and highresolution (HR) TEM images further confirmed the transparent feature and ultrathin character of NiCo-LDHs/Ar (Figure 3a). In addition, the lattice fringe of NiCo-LDHs and NiCo-LDHs/Ar was ~0.12 nm (Figure 3, b and d), which was well matched with the (222) plane and also coincided with the XRD results. The selected area electron diffraction (SAED) patterns for NiCo-LDHs and NiCo-LDHs/Ar (Inset images in Figure 3, a and c) further revealed the lattice structure with the


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lattice fingers of ~0.27, 0.40, 0.22, 0.25, 0.15, and 0.13 nm assigned to the (009), (006), (200), (111), (220), and (311) planes, respectively. Besides, compare to NiCo-LDHs, NiCo-LDHs/Ar nanosheets possessed more defects and fine dispersed nanoparticles as shown by the darker part in Figure 3 c and d. The atomic force microscope (AFM) images further ascertained the thickness and defects of asmade NiCo-LDHs/Ar nanosheets. As shown in Figure 4 c and d, compared with NiCo-LDHs (~66.6 nm), NiCo-LDHs/Ar nanosheets showed a much promotion with extremely low thickness of ~1.1 nm. Furthermore, the dark and light spots clearly presented in AFM images, indicating the increased defects had been successfully induced after Ar plasma treatment, which also supported that the NiCo-LDHs nanosheets were partially exfoliated. These results demonstrated that the exfoliation not only led to thinner layers but also increased defects on the surface of NiCoLDHs/Ar nanosheets. In general, the special structure of NiCo-LDHs/Ar nanosheets mainly featured three aspects: (1) defects induced by Ar plasma treatment led to much more active sites on the surface; (2) ultrathin layers generated via a facile one-step hydrothermal method; (3) Ni and Co co-exist as multiple valence state, providing proper redox-oxidation environment.


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Figure 4. AFM images and the height profiles of pristine NiCo-LDHs (a and c) and as-exfoliated NiCo-LDHs/Ar nanosheets (b and d). We conveniently evaluated the OER activity of NiCo-LDHs and NiCo-LDHs/Ar nanosheets directly grown on the conductive CFP in the classical three-electrode cell in 1 M NaOH solution. All polarization curves weren’t corrected for background current and IR losses. As shown in Figure 5a, the pure CFP served as the support contributed the negligible activity. On the other hand, the high specific surface area of CFP can positively contribute to the enhanced mass transportation and increased access to the internal space.30 The onset potential for OER of NiCoLDHs/Ar was 1.44 V versus reversible hydrogen electrode (vs. RHE) which was lower than that of pristine NiCo-LDHs (1.47 V), Co(OH)2/Ar (1.51 V) and Ni(OH)2/Ar (1.46 V). Furthermore, the NiCo-LDHs/Ar nanosheets only need 299 mV to reach the current density of 10 mA cm-2, much lower than that of pristine NiCo-LDHs (347 mV), Co(OH)2/Ar (352 mV) and slightly lower than that of Ni(OH)2/Ar (302 mV), suggesting the much improved electrocatalytic performance of


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NiCo-LDHs/Ar in OER. The enhanced activity was attributed to the increased surface activity sites, improved electronic conductivity and synergy between Ni and Co in the as-exfoliated NiCoLDHs/Ar nanosheets. In addition, the influence of different processing time (10s, 15s, and 30s) by Ar plasma and the polarization curves of different original ratio of Ni/Co were shown in Figure S4. It was clear that the NiCo-LDHs nanosheets with the original ratio of Ni/Co of 2 and treated by Ar plasma for 15s showed the best activity in terms of current density and overpotential. All of them showed the much enhanced catalytic activities compared with RuO2 on the basis of our previous work.31-33 The OER activities of previously reported catalysts were also compared in the Supporting Information (Table S2), it's worth noting that the ultrathin NiCo-LDHs/Ar nanosheets present superior activity requiring a pretty low overpotential of 299 mV to achieve a current density of 10 mA cm-2. The OER catalytic kinetics was then analyzed using the Tafel plots (Figure 5b) in order to figure out the association between potential and log j, which was obtained through the Tafel equation ( 𝜂𝜂 = 𝑏𝑏 ∗ log | 𝑗𝑗 | + 𝑎𝑎 , where b denotes the Tafel slope and a is a constant).34-

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Specifically, the OER kinetics is inversely proportional to the Tafel slope value; the smaller the

value of b, the faster kinetics of OER.34 The Tafel slope value of the as-exfoliated NiCo-LDHs/Ar nanosheets was 45 mV dec-1 which was much lower than that of (149 mV dec-1), Co(OH)2/Ar (108 mV dec-1) and Ni(OH)2/Ar (83 mV dec-1), representing that NiCo-LDHs/Ar nanosheets have the fastest kinetics of OER process. Besides, to determine the ECSA, Cdl was obtained by plotting ΔJ = (Janodic - Jcathodic) at 1.24 V (vs.RHE) against scan rate, where the slope is twice the Cdl. The polarization curves were collected in a potential window of 1.17-1.27 V (vs.RHE) where is a nonFaradaic region at different scan rates from 1 to 5 mV s-1.16-18 As shown in Figure S5a and 5c, the current density of as-exfoliated NiCo-LDHs/Ar nanosheets is much higher than that of pristine NiCo-LDHs. Meanwhile, the Cdl value of NiCo-LDHs/Ar is 2.58 mF cm-2 which is more than


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twice that of pristine NiCo-LDHs (1.64 mF cm-2) indicating that the ECSA of NiCo-LDHs/Ar is also doubled. The enhancement of ECSA is associated with the increased active sites and OER performance of NiCo-LDHs/Ar nanosheets.

Figure 5. (a) Polarization curves of as-exfoliated NiCo-LDHs/Ar nanosheets, Ni(OH)2/Ar, Co(OH)2/Ar, pristine NiCo-LDHs, and the pure CFP at the scan rate of 10 mV s-1 in 1 M NaOH solution. (b) The corresponding Tafel plots. (c) Nyquist plots of impedances of as-exfoliated NiCoLDHs/Ar nanosheets and pristine NiCo-LDHs. (d) Chronoamperometric curve of as-exfoliated NiCo-LDHs/Ar nanosheets at a potential of 1.53 V vs. RHE for 10 h. The electrochemical impedance spectroscopy (EIS) analyses of the NiCo-LDHs and NiCoLDHs/Ar nanosheets were further performed to understand the electrode kinetics in OER (Figure 5c). The equivalent circuit of EIS comprises a solution resistance (RS), a constant phase element (CPE), and a charge-transfer resistance (RCT). The RCT of NiCo-LDHs/Ar nanosheets (3.86 Ω) was much smaller than that of pristine NiCo-LDHs (5.72 Ω), suggesting a faster electron transfer was


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realized through the Ar plasma exfoliating progress and was consistent with Tafel plots.37 The Ar plasma treatment could expedited the OER reaction and reduced the overpotential of NiCoLDHs/Ar nanosheets because of the numerous induced defects. Furthermore, the durability test for 10 h revealed the excellent stability of NiCo-LDHs/Ar nanosheets, and the change of current density stayed within a range of only ~3 mA cm-2 which showed the as-exfoliated NiCo-LDHs/Ar nanosheets were stable in a long-term electrochemical OER process (Figure 5d). The cyclic stability test is also important to determine the long-term cycling performance of OER catalysts.38,39 Figure S6 shows that the as-exfoliated NiCo-LDHs/Ar nanosheets exhibited good cyclic stability with slight decay of the OER activity after 350 cycles in 1 M NaOH at a sweep rate of 10 mV s−1.

4. CONCLUSIONS In summary, the Ar plasma-exfoliated NiCo-LDHs nanosheets with many defects were successfully prepared via a facile one-step hydrothermal method. The transparent electrocatalysts with extremely low thickness of ~1.1 nm displayed a pretty low overpotential of 299 mV at 10 mA cm-2, doubled ECSA of 2.58 mF cm-2, relatively low RCT value of 3.86 Ω, a quite small Tafel slope value of 45 mV dec-1 and excellent durability over 10 h and 350 cyclic CV test. The electrocatalytic activity of Ar plasma-exfoliated NiCo-LDHs nanosheets was 1.2 times that of pristine NiCo-LDHs with the decrease in overpotential of 48 mV. The Ar plasma approach provides an attractive route for the rational design of efficient LDHs-based electrocatalysts with defects for OER.

ASSOCIATED CONTENT


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxx. Low-resolution SEM images, ECSA of NiCo-LDHs (/Ar), cyclic stability test of NiCoLDHs/Ar nanosheets, comparison with previous literatures are in Supporting Information.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions Liu, Y. Q. performed the synthesis of NiCo-LDHs and studied the catalytic activities. All authors co-wrote the paper. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Key R&D Program of China (2018YFD0800700), and Science and Technology Planning Project of Guangdong Province, China (2014A050503032), National Natural Science Foundation of China (21776324), Hundred Talent Plan (201602) from Sun Yat-sen University. REFERENCES (1) Zhou, Z.-Y.; Tian, N.; Li, J.-T.; Broadwell, I.; Sun, S.-G. Nanomaterials of high surface energy with exceptional properties in catalysis and energy storage. Chem. Soc. Rev. 2011, 40, 4167-4185.


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Ar Plasma-Exfoliated Ultrathin NiCo-Layered Double Hydroxide

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