Catalytic activities of Ni-decorated boron particles

Catalytic activities of Ni-decorated boron particles

Accepted Manuscript Catalytic activities of Ni-decorated boron particles Hye Jin Jung, Kyusuk Nam, Jisuk Lee, Doo-Hee Han, Hong-Gye Sung, Hyung Soo H...

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Accepted Manuscript Catalytic activities of Ni-decorated boron particles

Hye Jin Jung, Kyusuk Nam, Jisuk Lee, Doo-Hee Han, Hong-Gye Sung, Hyung Soo Hyun, Youngku Sohn, Weon Gyu Shin PII: DOI: Reference:

S0264-1275(17)30341-6 doi: 10.1016/j.matdes.2017.03.086 JMADE 2920

To appear in:

Materials & Design

Received date: Revised date: Accepted date:

10 October 2016 28 March 2017 30 March 2017

Please cite this article as: Hye Jin Jung, Kyusuk Nam, Jisuk Lee, Doo-Hee Han, HongGye Sung, Hyung Soo Hyun, Youngku Sohn, Weon Gyu Shin , Catalytic activities of Ni-decorated boron particles. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Jmade(2017), doi: 10.1016/ j.matdes.2017.03.086

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Catalytic Activities of Ni-decorated Boron Particles Hye Jin Jung,1 Kyusuk Nam,2 Jisuk Lee,2 Doo-Hee Han,3 Hong-Gye Sung,3 Hyung Soo Hyun,4 and Youngku Sohn,2,5,* Weon Gyu Shin1,* 1

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Department of Mechanical Engineering, Chungnam National University, Daejeon 34134, South Korea 2 Department of Chemistry, Yeungnam University, Gyeongsan, Gyeongbuk 38541, South Korea 3 School of Aerospace and Mechanical Engineering, Korea Aerospace University, Goyang, Gyeonggi-do 21071, South Korea 4 The Fourth R&D Institute, Agency for Defense Development, Daejeon 34188, South Korea 5 Department of Chemistry, Chungnam National University, Daejeon 34134, South Korea * Corresponding authors

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E-mail addresses: [email protected] (Y. Sohn); [email protected] (W. G. Shin)

Abstract

Ni-decorated boron (Ni/B) particles were prepared by a ball-milling method, and their

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physicochemical properties were fully characterized by scanning electron microscopy

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(SEM), X-ray diffraction (XRD) analysis, high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX) elemental mapping, and X-ray photoelectron spectroscopy (XPS). Upon decoration by Ni, the

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previously inactive boron particles showed enhanced activity in CO oxidation and photoelectrochemical hydrogen/oxygen evolution reactions (HERs/OERs). The CO

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oxidation onset and T10% were observed around 320 and 350 °C, respectively. In preliminary tests, the Ni/B (1:3 w/w) sample showed UV and visible light current responses, and current densities of 600 and 240 μA/cm2 were observed at a potential of 500 mV for HER and OER, respectively. The present study emphasizes the potential application of boron as a catalyst support material for air quality control and photoelectrochemical hydrogen/oxygen evolution.

Keywords: boron; Ni-decoration; ball-milling; CO oxidation; hydrogen evolution; oxygen evolution

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1. Introduction Boron is abundantly used in various application fields, such as borosilicate glass, neutron absorbers, highly energetic materials, and organoboron compounds [1–4]. The high melting point, stability, and unique chemical nature of boron make it a potentially

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applicable catalyst material [2, 5–10]. Mu et al. prepared nano-B4C-intercalated graphene and examined its performance in the oxygen reduction reaction (ORR) and

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fuel cells. This performance was found to be much higher than that of Pt/rGO (reduced

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graphene oxide) and commercial Pt/C catalysts [7]. Zhu et al. loaded B2O3 onto Cu/SiO2 catalysts to promote the dispersion of Cu and stabilize the particles, leading to greatly

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enhanced stability and activity toward glycerol hydrogenolysis to 1,2-propanediol [8].

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Additionally, MgO-B2O3 mixed oxides have been shown to exhibit increased dehydrogenation selectivity with increasing amounts of boron [9]. The amount of boron

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determines selectivity by affecting the ratio of acidic/basic sites on the surface. Boron

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oxide embedded in an alumina catalyst has been shown to increase the selectivity of ethane dehydrogenation to ethylene by increasing catalyst acidity [10]. Shin et al. employed dry and wet ball-milling methods to prepare CeO2 nanoparticles (NPs)

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supported on boron particles, finding a synergistic effect on CO oxidation activity

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compared with unsupported CeO2 NPs [5]. In order to efficiently enhance the catalytic activity in hydrodesulfurization of benzothiophene, Kaluza and Zdrazil prepared bimetallic CoMo and NiMo catalysts supported on γ-Al2O3 by a sequential reaction of α-boehmite with of MoO3 and Co or Ni carbonates in an aqueous paste [11]. Nickel and its oxides have been extensively used as main catalyst materials in various chemical reactions [12–24]. Moraes et al. developed a CeO2-supported Ni catalyst, tested it in low-temperature steam reforming of ethanol, and examined the enhanced role of Pt

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addition [12]. NiO was used as a CO oxidation catalyst upon coupling to the CeO2 support using a mesoporous template method [13] or a hydrothermal method [14]. The enhanced catalyst activity was attributed to the strongly attracted interfacial NiO and a reversible Ce3+ + Ni3+ = Ce4+ + Ni2+ reaction. Water splitting has attracted much

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attention as a way of solving energy and environmental issues [15–18]. Gross et al. used a Ni catalyst for the photoelectrochemical hydrogen evolution reaction (HER) in water,

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where the assembled NiO|RuP3–Zr4+–NiP catalyst showed a greatly enhanced

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efficiency [15]. Stern et al. developed a Ni2P/NiOx core-shell hydrogen evolution catalyst and reported a current density of 10 mA cm−2 at a low overpotential of 0.29 V in

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1 M KOH [17]. A transparent iron nickel oxide (NiFeOx) catalyst was produced by

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electrochemical deposition and performed well in the oxygen evolution reaction (OER), exhibiting a very high turnover frequency and a solar-to-hydrogen conversion (STH)

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efficiency of > 1.9% [18]. More specifically, for the hybrids of Ni and boron, NiB has

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been reported to exhibit catalytic activity in coal liquefaction by electrochemical reduction [25]. The NiB alloy is resistant to hydrodesulfurization conditions, showing only partial sulfidation [26]. For hydrogenation reactions involving a Ni-based catalyst,

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supporting it on boron has been shown to improve the surface area, pore diameter, and,

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notably, the sulfur-poisoning resistance [27]. In the present study, boron particles decorated with Ni particles were synthesized by wet ball milling and characterized by a variety of analysis techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectrometry (XPS). In addition, these Ni-decorated boron particles were used in CO oxidation and water splitting experiments to check if they exhibit enhanced catalytic activity compared to pure boron particles.

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2. Experimental Section 2.1. Metal-coating of boron by wet milling

Boron and nickel particles were ball-milled in wet and dry conditions using an SPEX

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SamplePrep 8000M mixer/mill. A schematic diagram of the experimental setup for the milling process is shown in Figure 1. Inside a nitrogen-filled glovebox, a milling jar

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made of tungsten carbide (WC) was filled with boron powder (H. C. Starck, average

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diameter 800 nm, purity 95%), nickel powder (Sigma-Aldrich, < 200 nm), and 40 g of tungsten carbide balls (5 mm diameter). The Ni:B weight ratios used in our experiments

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were 1:3 and 2:2. The mixture was dry-milled for 10 minutes, and 1.0 mL of oleic acid

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(Sigma-Aldrich, 95%) and 15.0 mL of hexane (Sigma-Aldrich, anhydrous, 95%) were added to the dry-milled samples, which were further wet-milled for 50 minutes. The

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nitrogen atmosphere inside the milling jar was maintained during the milling process.

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Post-processing was performed immediately after milling to remove the moisture and physisorbed residues on the boron particles, and consisted of washing, centrifugation, and drying steps. In the washing step, the boron particles were washed with methanol

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(Sigma-Aldrich, 99.9%) in an ultrasonicator and then centrifuged at 3500 rpm for 10

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minutes (MF 80, Hanil, Inc., Korea). In the drying step, the centrifuged samples were dried in an oven at 100 °C for one hour.

Fig. 1. Schematic diagram of the ball-milling process.

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2.2. Characterization of coated boron particles

Various techniques were used to analyze the physicochemical properties of Nidecorated boron particles obtained by ball milling, e.g., SEM was used to characterize their morphorogy. XPS, FT-IR, and Cs-corrected scanning transmission electron microscopy (STEM) coupled with EDX (energy-dispersive X-ray spectroscopy) were

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used to characterize the chemical bonding state and composition. X-ray diffraction (XRD) and STEM were used to analyze the crystallographic properties of non-milled

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and milled boron particles. Nickel-decorated boron samples for SEM and XPS analyses

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were prepared by attaching particles to a conductive carbon tape. SEM images were obtained on a JEOL JSM-7000F microscope operated at beam energies of 0.5 to 30 kV. XPS measurements were performed on a MultiLab 2000 instrument (Thermo Scientific,

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USA) equipped with a high-performance Al Kα X-ray source. The base pressure of the XPS system was 5 × 10–10 Torr. During data collection, the pressure was maintained at

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~5 × 10–9 Torr. High-resolution XPS spectra were collected with a pass energy of 20.0 eV and 0.1 eV/step. Data processing was performed using the Avantage 4.45 S/W software provided with the XPS system. For curve fitting of the collected XPS spectra,

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the Lorentzian to Gaussian function combination ratio and the full width at half

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maximum (FWHM) fit parameter were fixed at 30% and 2.8 ± 0.2 eV, respectively. The XPS binding energies were referenced to the C 1s peak of adventitious carbon at 284.5 eV. The XRD patterns of ball-milled boron particles were obtained in a 2θ range of 10–

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80° using a D8 Advanced diffractometer (Bruker AXS, Inc.). For STEM analysis, the particles were fully dispersed in ethyl alcohol by ultrasonication for 1 hour. A few drops

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of this dispersion were deposited onto a lacey carbon-coated Cu grid for imaging. STEM (JEM-ARM200F, JEOL Inc.) coupled with EDX (Bruker Quantax 400) was performed at 200 kV.

2.3. CO oxdiation and electrochemcial mesurements

The CO oxidation experiment was performed with 10 mg of catalyst, a temperature ramp rate of 10 K/min, and a CO (1.0%)/O2 (2.5%)/N2 gas mixture flow rate of 40 5

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mL/min. The catalyst was packed in a quartz U-tube with an inner diameter of 4 mm, and the gaseous reaction products obtained at different temperatures were examined using a SRS RGA-200 mass spectrometer. CO conversion (%) was calculated as {([CO]in – [CO]out)/[CO]in}  100%. Electrochemical measurements were performed

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using a three-electrode cell configuration, with a Ag/AgCl reference electrode, a Pt wire counter electrode, and a sample working electrode in 0.1 M Na2SO4 as an electrolyte.

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Prior to measurements, nitrogen gas was bubbled through the solution for 30 min to

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remove air. A CHI660D electrochemical workstation (CH Instruments, Austin, USA) was used for these measurements. Electrochemical impedance measurements were

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performed in the frequency range of 0.1 MHz to 0.01 Hz. To prepare the working

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electrode, a 50 mg sample was dispersed in a solution containing 5 mL of ethanol, 0.25 mL of α-terpineol, and 0.05 mL of ethyl cellulose, followed by sonication to achieve

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complete mixing. The obtained solution was placed in a vacuum oven (40 °C) to

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evaporate the solvent and produce a gel. This gel was pasted on a Si substrate and fully dried in a vacuum oven (70 °C). After drying, the electrode was finally prepared by

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establishing an electric junction with a Cu wire with the help of Ag paste.

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3. Results and discussion

3.1. Morphology analysis by BSE

Figure 2 (a) and (b) show the backscattered electron (BSE) images of the as-prepared ball-milled particles with Ni:B weight ratios of 1:3 and 2:2, respectively. These images show a clear distinction between the larger boron and smaller nickel particles with distinct brightness. Since Ni has a higher formula weight than boron, the former

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overlayer appears much brighter in the BSE image than the core boron particles. BSE imaging confirmed that boron particles were decorated by Ni particles. Comparing the Ni/B(1:3) and Ni/B(2:2) particles (Figure 2), the amount of Ni particles attached to boron is larger for the (2:2) sample than for the (3:1) sample, as expected. Ni

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caused by the high ductility of Ni during the milling process.

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agglomeration was observed for a small portion of ball-milled samples, which was

Fig. 2. BSE images of Ni-decorated boron particles with Ni:B weight ratios of (a) 1:3

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and (b) 2:2.

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3.2. XRD crystal structure

The ball-milled Ni/B particles were characterized by XRD to identify their crystal

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structure according to the weight ratio of boron and nickel. The XRD patterns of Ni/B(2:2) and Ni/B(1:3) samples are shown at the top and bottom of Figure 3, respectively. These patterns show the peaks of boron with a rhombohedral crystal structure (JCPDS 71-0157), having a much higher intensity for the Ni/B(1:3) sample than for the Ni/B(2:2) sample, as expected. Two dominant XRD peaks at 2 = 44 and 52 correspond to the metallic Ni crystal phase (cubic, JCPDS 65-2865), with their

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highest intensity observed for the Ni/B(2:2) sample. The crystalline size of metallic Ni was estimated using the well-known Scherrer’s equation to be about 34 nm showing no critical difference between the two samples. The presence of the NiO crystal phase (cubic, JCPDS 71-1179) is attributed to the oxidation of metallic Ni. Despite its weak

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intensity, we also clearly observed the presence of a NiB crystal phase (orthorhombic, JCPDS 74-1207) in both samples, which was more prominent (see inset of Fig. 3) for

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the Ni/B(1:3) sample [28]. As the boron particles are broken during the milling process,

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the internal pure boron is exposed, contributing to the increase of the amount of B-Ni bonds formed. Therefore, an increased boron content may cause the formation of

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increased amounts of the NiB crystal phase. Both samples also showed XRD peaks of

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tungsten carbide (JCPDS 72-0097), which is a component of the milling jar and balls [4,

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Fig. 3. XRD patterns of Ni-decorated boron particles with Ni:B weight ratios of 2:2 (top) and 3:1 (bottom). Inset shows amplified XRD patterns between 2=15 and 34.

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3.3. Surface composition and chemical states by XPS analysis

Figure 4(a)–(d) show the XPS spectra of Ni-decorated boron samples with weight ratios of 3:1 and 2:2. Each XPS spectrum was corrected using the reference C 1s peak at

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284.5 eV. The survey XPS spectrum (not shown here) also shows weak peaks of W and Co impurities, which were already present in the WC milling jar and balls. No W and

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Co XPS peaks were observed for original pure boron prior to ball-milling. For the high

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resolution B 1s XPS peaks in Figure 4(a) and 4(b), the peaks around 184~195 eV are attributed to three boron-containing species. The broad B 1s peak can be fitted with

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independent chemical states, such as elemental boron (B–B, 186.8 and 188.0 eV), boron

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oxide (B–O, 192.5 eV) [1], and nickel boride (NiB, 188.2 eV) [2]. The B 1s peak of NiB was included in the deconvolution because NiB phase was present in the XRD

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patterns (Figure 3) and the B 1s XPS peak of the Ni/B samples was broader compared

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with that of bare boron (Supplemental information, Fig. S1). For Figure 4(c) and 4(d), the Ni 2p3/2 spectra show a main peak around 855.5 eV and a broad satellite peak around 861 eV. The broad main peak can be resolved into three peaks, located at 852.7,

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854.5, and 856.3 eV and assigned to Ni, NiO, and Ni(OH)2, respectively (Supplemental

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information, Fig. S2) [2].

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Fig. 4. High-resolution XPS spectra: B 1s spectra of (a) Ni/B(1:3) and (b) Ni/B(2:2)

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samples; Ni 2p3/2 spectra of (c) Ni/B(1:3) and (d) Ni/B(2:2) samples.

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Tables 1 and 2 summarize the atomic ratios of Ni and B estimated from the XPS peaks, respectively. Table 1 showed the weight and atomic fractions of all Ni

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components present on the surface of Ni/B particles. The observed Ni:B weight ratios for Ni/B(1:3) and Ni/B(2:2) samples are 3.85 and 1.44, respectively, exceeding the ones used during actual milling. As shown in the SEM image above, the Ni particles on boron exist as agglomerates. For this reason, the surface of such agglomerates was only analyzed by XPS. Consequently, a smaller weight fraction of Ni was expected to be observed, as shown in Figure 4. In contrast to XRD results, the surface-sensitive XPS technique [29] led to the observation of much higher NiO and Ni(OH)2 fractions

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compared to that of metallic Ni. In addition, Ni(OH)2 was exclusively observed by XPS. Based on this result, it was concluded that the overlayer Ni particles consist of metallic Ni with a NiO shell covered by the outermost Ni hydroxide layer. The surface of metallic Ni is easily oxidized, with Ni hydroxide also readily formed by reaction with

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adsorbed water [28]. The B–O bond peak intensity in the Ni/B(1:3) sample is weaker than that in the Ni/B(2:2) sample (Supplemental Information, Fig. S1). B–O bonds are

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formed at the interface between boron and NiO-rich Ni during the milling process. Thus,

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a higher B–O bond peak intensity was observed for the Ni/B(2:2) sample, containing a larger amount of Ni than the Ni/B(1:3) sample. Additionally, the Ni–B bond peak was

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more intense for the Ni/B(1:3) sample than for the Ni/B(2:2) sample. This is explained

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by the fact that the Ni/B(1:3) sample contains a larger amount of pure elemental boron (involved in forming Ni–B bonds) than the Ni/B(2:2) sample. Considering that Ni

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particles are chemically bonded to the boron particle due to the Ni + B → NiB reaction

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occurring in the region where pure boron is exposed by milling, the reaction between the exposed elemental boron and elemental Ni or the oxygen in NiO proceeds as 3NiO + 2B → 3Ni + B2O3. Moreover, in the oxide layer of boron particles, the reaction

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between B–O and elemental Ni during milling proceeds as 3Ni + B2O3 → 3NiO + 2B.

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Thus, in the case of the Ni/B(2:2) sample, the reaction producing NiO occurs much more frequently than the one producing Ni, leading to increased atomic fractions of both B–O and Ni–O bonds (Tables 1 and 2). The Ni/B(2:2) sample showed much higher NiO and Ni(OH)2 species than the Ni/B(1:3) sample (Table 1 and Supplemental Information, Fig. S1).

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Table 1. Deconvoluted constituent fractions estimated based on Ni 2p3/2 XPS peaks. Ni (At%)

NiO (At%)

Ni(OH)2 (At%)

Total (At%)

Ni/B (1g:3g)

31.0

31.5

37.5

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Ni/B (2g:2g)

27.1

35.0

37.9

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Original Ni

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43.2

24.5

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Samples\Species

B (At%)

Boride (At%)

Ni/B (1g:3g)

86.5

7.3

Ni/B (2g:2g)

83.8

5.6

Total (At%)

6.3

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10.6

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B-O (At%)

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Samples\Species

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Table 2. Deconvoluted constituent fractions estimated from B 1s XPS peaks.

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3.4. Microstructures by TEM

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Figure 5(a) shows the bright-field scanning transmission electron microscopy (BF-

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STEM) image of a Ni-decorated boron particle (diameter ~400 nm) with a 3:1 weight ratio of B to Ni. Figure 5(a) and (b) show that small Ni particles are attached to the

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larger boron particles. Figure 5(c) and (d) show the fast Fourier transform (FFT) pattern

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of a Ni-decorated boron particle, confirming that the larger boron particle exhibits a rhombohedral crystal structure, while the attached smaller Ni particle display Ni and NiO crystal structures. These observations are consistent with the XRD results presented in Figure 3 above.

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Fig. 5. (a) BF-STEM image of a Ni/B(1:3) particle, (b) high-resolution image of the Ni

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decoration, (c) FFT pattern of the boron part, (d) FFT pattern of the Ni part.

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The Ni/B(1:3) sample was analyzed by TEM-EDX, which clearly showed the shape

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of Ni particles attached to the boron particle and its components. Figure 6 shows a darkfield (DF)-STEM image of a Ni/B particle with a diameter of ~150 nm, with the

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scanning range represented by the green line. EDX line scanning characterization

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(Figure 6) showed that the attached Ni particle had a diameter of ca. 20 nm. The elemental maps of oxygen (O), nickel (Ni), and tungsten (W) in Figure 6 show that O is present on both particles and the TEM grid, being slightly more concentrated on the surface of boron particles. Ni is present on the surface of boron in the form of studs, while W was detected in very small amounts in positions observed for nickel. Table presents the atomic and weight elemental compositions obtained at positions marked by red crosses in Figure 6. This result agrees with those of the EDX point spectrum and elemental mapping. The elemental atomic and weight compositions for the two marked 13

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positions are observed in the corresponding spectrum. At point 1, boron was detected, with almost no Ni observed. Despite being detected, carbon was not included in compositional analysis, since it was present in the TEM grid. At point 2, both boron and

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Ni were detected, since Ni was attached to the boron particle.

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Fig. 6. Dark-field (DF)-STEM image (top left) of a Ni/B(1:3) particle. EDX line scan (top right) and point spectrum analyses were performed along the green line and at the red cross marks, respectively. DF-STEM image elemental maps (bottom) for O (left), Ni (middle), and W (right).

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Table 3. Summary of atomic and weight compositions at points 1 and 2 on the Nidecorated boron sample with a Ni:B ratio of 1:3. C

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Ni

W

Total

At. %

49.6

49.4

1.0

0.1

0.0

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Point 1 Wt. %

46.7

51.7

1.4

0.3

0.0

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At. %

40.4

51.3

4.5

3.8

0.0

100

Wt.%

32.5

45.8

5.3

16.4

0.0

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Point 2

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Points\Species

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3.5. CO oxidation activity of Ni/B

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We examined the CO oxidation activities of ball-milled Ni/B samples for potential applications in air purification [5,14,30,31]. Figure 7 displays the CO conversion (%)

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profiles as a function of reaction temperature (°C) for Ni/B(1:3) and Ni/B(2:2) samples.

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Pure boron particles exhibited no CO oxidation activity below 900 °C [5]. However, upon decorating the boron surface with Ni nanoparticles, the onset of CO oxidation

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activity was dramatically shifted to 300 °C. In the first CO oxidation run of the Ni/B(1:3) sample, the oxidation onset and T10% (temperature at 10% CO conversion)

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were determined as 320 and 350 °C, respectively, with no marked changes detected in the second run. Above the onset, as the catalytic thermal activation temperature was increased the CO conversion (%) was gradually increased as expected. In the temperature range above 500 °C, the first and second runs exhibited a sight difference which is probably due to a change in the oxidation state of Ni and/or Ni-B phase formation. In the first oxidation run of the Ni/B(2:2) sample, the CO oxidation onset and T10% were observed at 360 and 405 °C, respectively. This sample showed poor CO

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oxidation activity compared to that of the Ni/B(1:3) sample. Generally, smaller particles have shown higher catalytic activity due to higher surface area and exposed active crystal facets [32, 33]. However, the crystalline sizes of the two samples were estimated to be similar, ~34 nm using the Scherrer’s equation as discussed above. This indicates

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that the difference in catalytic activity in the first run is not mainly due to size effects but different Ni species effects. In the second run, however, the CO oxidation activity

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was significantly increased, being similar to that of the Ni/B(1:3) sample. For the

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second run, the first-run sample was cooled to room temperature under the same gas flow conditions, and the CO oxidation experiment was repeated. Because the samples

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after the first run were already experienced CO oxidation at high temperature the

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samples in the second run were more thermally stable with different oxidation states. It is known that Ni and Ni(OH)2 are converted to thermally stable NiO by thermal

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oxidation reaction [29]. Therefore, all surface Ni species were commonly converted to

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the same stable NiO species after the first run. Consequently, the two samples in the second run showed similar CO oxidation activity. Based on the CO oxidation tests, we conclude that ball-milled Ni-decorated boron

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particles are potentially applicable in CO pollution control. In addition, boron particles

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can be used as a stable thermal-catalyst support for metal particles.

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Fig. 7. First and second CO oxidation runs at variable reaction temperature over Ni/B

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(1:3) and Ni/B(2:2) samples (catalyst weight ~10 mg, heating rate = 10 K/min, flow rate

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= 40 mL/min of CO (1%) and O2 (2.5%) in N2).

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3.6. Photoelectrochemical catalytic activity of Ni/B

We further briefly examined the photoelectrochemical catalytic activities of the

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above samples [15–18]. Figure 8 shows the obtained linear scan voltammogram (LSV),

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cyclic voltammogram (CV), current-time (I-t), and impedance curves for bare boron, Ni/B(1:3), and Ni/B(2:2) samples. LSV curves were recorded from –0.5 to 1.0 V (positive direction) for the above samples. Repeated measurements indicated that the current for the Ni/B(1:3) sample was much higher than that for the other two samples. For HER at an overpotential of 500 mV, the Ni/B(1:3) sample exhibited a current of 600 μA/cm2, which was much higher than the ~160 and ~60 μA/cm2 values for bare boron and Ni/B(2:2) samples, respectively. CVs were obtained at scan rates of 10, 20, 50, and

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CV curve areas were in the order of Ni/B(1:3) > bare boron > Ni/B(2:2). The positive and negative peaks were attributed to the oxidation and reduction of Ni and Ni2+,

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respectively [34]. Current-time (I-t) amperometry curves were obtained at a fixed

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voltage of 1.0 V under on-off UV and visible right irradiation to qualitatively check the photoresponse. The currents responded to both UV and visible light, indicating a

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potential for photoelectrochemical applications. Nyquist plots show the real (Z) and

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imaginary (Z) parts of sample impedance. These plots commonly feature semicircles and straight uphill lines in the high- and low-frequency regions, respectively. The

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semicircles and straight lines were attributed to the interfacial charge transfer and

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Warburg impedance diffusion resistance, respectively [29,30]. The size of the semicircles was in the order of Ni/B(1:3) < bare boron < Ni/B(2:2), which is in good

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agreement with the orders of LSV and CV data.

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Fig. 8. LSV, CV, I-t, and impedance analyses for bare boron, Ni/B(1:3), and Ni/B (2:2)

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samples.

4. Conclusion

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We demonstrated that the surface of boron particles could be decorated with Ni particles by a ball-milling method. XRD, HRTEM, and FFT analyses show that the overlayer Ni

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is mainly metallic, with some NiO present. XPS characterization of the surface composition reveals that this overlayer consists of metallic Ni with a NiO shell covered by the outermost Ni hydroxide. NiB was found to form at the interface between Ni and boron particles. As the amount of NiO increased, the B–O bonds were expected to form in greater amounts than Ni–B bonds. The Ni-decorated B particles were found to be potential HER and OER catalysts. The Ni/B(1:3) sample showed HER and OER current densities of 600 and 240 μA/cm2, respectively, at an overpotential of 500 mV. These

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currents were much higher than those observed for bare boron and Ni/B(2:2) samples. UV and visible light current responses were observed for all samples. The present study provides valuable information on metal-decorated boron particles and emphasizes that such composites can be used as catalysts for CO oxidation and photoelectrochemical

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hydrogen/oxygen evolution.

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Acknowledgements

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This work was financially supported by ADD-12-01-04-05 and by the National Research Foundation of Korea(NRF) funded by the Korean government (MSIP:

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Ministry of Science, ICT and Future Planning) (2016-912550).

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Graphical abstract

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Highlights ● Ni-decorated boron (Ni/B) particles were prepared by a ballmilling method. ● B particles showed enhanced activity in CO oxidation upon Ni-

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decoration.

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● Ni/B particles showed photoelectrochemical hydrogen/oxygen evolution reactions.

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