Corrosion Engineering to Synthesize Ultra-Small and Monodisperse

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Energy, Environmental, and Catalysis Applications

Corrosion Engineering to Synthesize Ultra-Small and Monodisperse Alloy Nanoparticles Stabilized in Ultrathin Cobalt (Oxy)hydroxide for Enhanced Electrocatalysis Peng Du, Yuren Wen, Fu-Kuo Chiang, Ayan Yao, Junqiang Wang, Jianli Kang, Luyang Chen, Guoqiang Xie, Xingjun Liu, and Hua-Jun Qiu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22268 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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ACS Applied Materials & Interfaces

Corrosion Engineering to Synthesize Ultra-Small and Monodisperse Alloy Nanoparticles Stabilized in Ultrathin Cobalt (Oxy)hydroxide for Enhanced Electrocatalysis Peng Du,a# Yuren Wen,b# Fu-Kuo Chiang,c# Ayan Yao,d Jun-Qiang Wang,d Jianli Kang,e Luyang Chen,f Guoqiang Xie,a Xingjun Liua,g* and Hua-Jun Qiua,* aSchool

of Materials Science and Engineering, Harbin Institute of Technology (Shenzhen), Shenzhen, 518055,

China bSchool

of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083,

China cNational dNingbo

Institute of Clean and Low Carbon Energy, Beijing 102209, China.

Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201,

China eState

Key Laboratory of Separation Membrane and Membrane Processes and School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China fSchool

of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China gState

Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Shenzhen 518055,

China

1

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Abstract 2D nanomaterials decorated with ultrasmall and well-alloyed bimetallic nanoparticles (NPs) have many important applications. Developing facile and scalable 2D materials/hybrids synthesis strategy is still a big challenge. Herein, a top-down corrosion strategy is developed to prepare ultrathin cobalt (oxy)hydroxide nanosheets decorated with ultrasmall (~1.6 nm) alloy NPs. The formation of unltrathin (oxy)hydroxides nanosheets have a retrain effect to prevent the growth of small NPs into bigger ones. Thanks to the ultrathin 2D nature and strong electronic interaction between Co(OH)2 and alloy NPs, the Pt-based binary alloy NPs are greatly stabilized by the Co(OH)2 nanosheets and the hybrids exhibit much enhanced electro-catalytic performance for water splitting. Especially, the mass activities of the PtPd and PtCu decorated samples for hydrogen evolution are ~8 times that of Pt/C. When used as both cathode and anode electrocatalysts to split water, the hybrid nanosheets outperform the commercial Pt/C-RuO2 combination. At 10 mA cm-2, the needed potential is only 1.53 V. This work provides us a highly controllable and scalable means to produce clean 2D nanomaterials decorated with a series of alloy NPs such as PtPd, PtCu, AuNi, etc. Keywords: top-down; dealloying; Co(OH)2; 2D nanosheets; electrochemical water splitting

1. Introduction Bimetallic or multi-metallic nanoparticles (NPs), stably anchored on high-surface-area porous carbon or metal oxides, have important applications in catalysis.1-14 To fully utilize metal atoms for catalysis, the size of metal NP should be ≦ 1 nm. On the other hand, enhanced catalytic performance can be expected when two or more metals are well-alloyed and they have a strong interaction with the supports.15 For example, graphene-encapsulated NiMo is very active 2


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for hydrogen evolution reaction (HER) and carbon sheet-coated NiMn alloy shows enhanced electrocatalytic performance in oxygen evolution reaction (OER).16,17 Theoretical calculations suggest that charge transfer between the carbon edges and alloy surfaces would enhance the adsorption/desorption of hydrogen.16 However, synthesis of such well-mixed and ultrasmall NPs on a high-surface-area host is a very difficult task. Usually, impregnation method is used to get supported metal/alloy NPs. In this method, the precursor solution is first filled in the void of the support and then reduced, which is very simple. However, the size of NPs prepared by this method is usually large and not uniform and for alloy NPs, they are usually not well-alloyed.1 Quite recently, by carefully controlling the adsorption of metal ions on negatively charged silica surface followed by reduction in H2 atmosphere, ultrasmall bimetallic NPs with an average size of just over 1 nm were obtained.1 However, for practical applications, scalable and more facile synthesis method is still needed. Owing to the special physiochemical properties, the interest in 2D materials continues to grow.18-24 The unusual properties make them (especially transition metal oxides/hydroxides, and dichalcogenides) as promising candidates in electronics, optics, energy storage/conversion, and catalysis.25-28 However, the reported bottom-up preparation methods usually require strict experimental condition and the addition of additive such as surfactants to guide the growth.29,30 Recently, a top-down corrosion process has been applied on Fe foil in the presence of Ni2+ to fabricate layered double hydroxides for highly stable OER.31 A top-down mechanical strategy was developed to prepare clean 2D metals by repeated cold rolling to reduce the thickness of metal foils.18 However, this method only applies to metals with good ductility and multiple steps are required. Thus, developing new facile synthesis strategy to prepare non-ligand-capped clean 2D transition-metal-based nanostructures remains a big challenge. More importantly, the decoration of these 2D materials with ultrasmall functional alloy clusters/NPs is believed to further enhance their performances and widen the application range.32,33 3



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Layered Co(OH)2 has shown great potential as electrocatalyst in alkaline solutions and anode material in lithium ion batteries with good electrochemical stability.34-36 In this work, different from these bottom-up strategies,34 we developed a simple one-step top-down dealloying strategy that can be applied to synthesize ultrathin Co(OH)2 nanosheets decorated with ultrasmall (~1.6 nm) and homogeneously alloyed NPs. For example, by one-step dealloying Al from a Al85Co14.4Pt0.3Pd0.3 precursor in an alkaline solution at 0 oC, left Co were oxidized to form ultrathin Co(OH)2 nanosheets due to its layered structure property and the decorated noble metals (Pt and Pd) which are stable under the alkaline dealloying condition would diffuse to form well-mixed ultrasmall alloy NPs trapped in the ultrathin Co(OH)2 nanosheets. The whole corrosion engineering synthesis process is shown in Scheme 1. This strategy allows the facile control on both the elemental composition and decoration amount of the ultrasmall alloy NPs by adjusting the precursor alloy composition. To demonstrate the generality of this top-down strategy, we prepared a series of alloy NP-decorated Co(OH)2 nanosheets such as PtCu, PtNi, AuNi, PdCu, PtPdAuAg, etc., by simply changing the alloy compositions of the precursor alloys. Owing to the structure advantage, this novel 2D hybrids show enhanced activities to catalyze both HER and OER. 2. Results and discussion Al which is earth-abundant, environmentally friendly and economical was used as the main component to prepare the alloy ribbons (also called as “precursor alloys”). The compositions of the alloys are basically in agreement with the designed feeding ratios (Figure S1a, Supporting Information). These precursor alloys show nearly the same X-ray diffraction (XRD) peaks and most of which could be assigned to the phases of AlxCo alloy compound and pure Al (Figure 1a). This result indicates that the addition of a small percentage of doping metals (0.6 at.%) does not cause the formation of any new intermetallic phases and they should be doped in the AlxCo 4


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phase. Due to the very low content of noble metals, obvious peak position shift of the AlxCo phase was not observed. After dealloying, EDS results show that only several percentage of residual Al can be found in the dealloyed samples (Figure S1b). XRD patterns in Figure 1b show the disappearance of original sharp XRD peaks, which demonstrates that they were fully dealloyed. Most of the newly formed weak peaks could be assigned to cobalt hydroxides. At ~43o, the weak peak can be assigned to (200) of CoO. Probably due to the decoration of alloy NPs, the (200) peak shifts obviously to the right compared with pure CoO. No peaks from the decorated metals/alloys phase are observed, suggesting that these decorated metals/alloys should be incorporated evenly in the cobalt hydroxides without the formation of any large alloy NPs, namely, these metals/alloys left in the Co(OH)2 should be in the form of ultrasmall NPs. Scanning electronic microscope (SEM) images of PtPd and NiMo incorporated hybrids are presented as representative in Figure 1c and 1d which indicate that the dealloyed samples are thin nanosheets. The zoom-in SEM image (inset in Figure 1c and 1d) verifies that these nanosheets are ultrathin with relatively large width (several hundred nanometers). Atomic force microscope (AFM) images reveal the precise thickness of the decorated Co(OH)2 nanosheets (Figure 1e-1g), which are ~3 nm in most parts and some thin parts are less than 2 nm in thickness (Figure 1g). This result indicates that these nanosheets are extremely thin (several atomic layers) and can be seen as real 2D materials. The SAED shows that the P3m1 structured single crystal nature of the hydroxide nanosheets (Figure S2a) and another series of diffraction patterns from CoO can also be observed (Figure S2b). The atomic resolution HAADF-STEM and FFT images confirm that the nanosheets mainly contain Co(OH)2 (Figure S2c). Transmission electron microscopy (TEM) images of the hybrid nanosheets are shown in Figure S3. Different alloy NPs (PtCu, PtNi, AuNi, PdCu, PtPdAuAg, etc.) with darker contrast can be clearly observed in the nanosheets since heavy elements with higher atomic number are prone to scatter more electrons. These results are further confirmed by high-resolution TEM characterization (Figure S4). 5



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To further demonstrate the alloy nature of the NPs, the PtPd, PtNi, PtPdAuAg and AuNidecorated samples were characterized by HAADF-STEM. Since heavy atoms are prone to elastically scatter electrons to high-angle annular detector, they show bright contrast in HAADFSTEM images. From Figure 2a and 2b, much brighter metal NPs ( ≦ 2 nm) can be frequently observed on the Co(OH)2 nanosheets which is very uneven based on the different color contrast. The element distributions characterized by STEM-EDS suggest that the metal NPs are comprised of well-mixed Pt and Pd, demonstrating the alloy nature of the ultrasmall NPs (Figure 2c). Ultrasmall PtNi and PtPdAuAg NPs decorated on Co(OH)2 nanosheets are also observed (Figure 2d and 2e). The relatively uniform distributions of Ni in both PtNi and AuNi-decorated samples (Figure 2e and 2f) suggest that most Ni should also be oxidized during the removal of Al due to the high chemical activity of Ni, which is confirmed later by X-ray photoelectron spectroscopy (XPS). In Figure 2f, the diameter of the Au-based NPs (~5 nm) is clearly larger than that of Ptbased NPs, which is due to the faster diffusivity of Au during dealloying. The XPS Co 2p spectrum of the PtPd sample (Figure 3a) show the binding energy peaks at 780.8 and 796.8 eV, which are ascribed to Co 2p3/2 and 2p1/2. The satellite peaks appear next to the main peaks indicating the domination of Co2+.37 In Figure 3b, the O 1s result demonstrates the Co(OH)2 nature of the formed nanosheets by showing a main metal-OH peak.38,39 In Figure 3c, the Pt 4f shows two peaks which are 4f7/2 (71.0 eV) and 4f5/2 (74.0 eV) of zero-valent platinum. The Pt binding energy peaks shift more negatively than Pt/C’s 4f7/2 (71.3 eV) and 4f5/2 (74.7 eV) peaks, indicating a clear electronic transfer or residual strain effects between the mismatched Co(OH)2 lattice and Pt.37 In Figure 3d, the two peaks of Pd 3d spectrum suggest zero-valent state of Pd. For the PtCu/Co(OH)2 case, the Co 2p and O 1s spectra which are not shown are identical to those of PtPd sample. The Pt 4f spectrum is also the same as that of the PtPd sample (Figure 3e). The weak Cu 2p peak at ~928 eV can be assigned to Cu0 (Figure 3f). 6


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These results confirm the alloy nature of these NPs. For the NiMo sample, due to their high activities, both Ni and Mo are mainly in the form of oxides (Figure 3g and 3h). Thus, it can be considered as metal ion doped Co(OH)2. The PtPd sample’s BET surface area is ~169 m2 g-1. This result demonstrates the high specific surface area of the ultrathin nanosheets compared with normal nanosheets. The detected mesopores by BET method is ~3 nm (Figure S5). According to the dealloying theory,40 metals with the chemical stability higher than Ni will exist mainly in metal states after dealloying in alkaline solutions. If their chemical stability is lower than that of nickel (for instance cobalt, manganese, titanium, etc.), oxidation of these metals will occur to form (oxy)hydroxides during the corrosion of Al in alkaline solutions.41 Therefore, in the present case, after the etching of Al, the left Co would be oxidized and reacted with OH- to form Co(OH)2/CoO. The more stable elements such as Pt, Pd, Cu, Ni, etc., would diffuse to form alloy NPs decorated on/in the Co(OH)2 nanosheets. Thus, the sizes of these alloy NPs clearly depend on the diffusion rate of these noble metals (Pt, Pd, Au, Cu, Ni, etc.), which is similar to the fact that nanoporous metal’s ligament/pore sizes are decided by the diffusion rates of more noble meals during dealloying binary alloys.42-45 Based on literature data, the diffusivities of these stable metals follow the order that Pt

Corrosion Engineering to Synthesize Ultra-Small and Monodisperse

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