C Nanocatalysts from Solid

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Low-Pressure Plasma Synthesis of Ni/C Nanocatalysts from Solid Precursors: Influence of the Plasma Chemistry on the Morphology and Chemical State Emile Haye, Yan Busby, Mathieu da Silva Pires, Florian Bocchese, Nathalie Job, Laurent Houssiau, and Jean-Jacques Pireaux ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00125 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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ACS Applied Nano Materials

Low-Pressure Plasma Synthesis of Ni/C Nanocatalysts from Solid Precursors: Influence of the Plasma Chemistry on the Morphology and Chemical State

Emile Haye†, Yan Busby†*, Mathieu da Silva Pires†, Florian Bocchese†, Nathalie Job§, Laurent Houssiau† and Jean-Jacques Pireaux† †

Laboratoire Interdisciplinaire de Spectroscopie Electronique (LISE), Namur Institute of Structured Matter (NISM), University of Namur, 61 Rue de Bruxelles, 5000 Namur, Belgium §

University of Liège, Department of Chemical Engineering – Nanomaterials, Catalysis, Electrochemistry (NCE), Building B6a, Sart-Tilman, B-4000 Liège, Belgium

* Corresponding author: [email protected]

Abstract Nanocatalyst materials based on metal nanoparticles (NPs) deposited on mesoporous carbon substrates are widely used in catalysis and energy storage; however, conventional wetchemical deposition methods based on the reduction of metal salts are not always the best choice when looking for a process ensuring easy scalability and low environmental impact. Moreover, additional surface functionalization steps, such as the addition of nitrogen or oxygen containing groups are more and more explored in order to increase the activity or the chemical stability of catalysts. In this work, we investigate a new methodology for the fabrication of nickel/carbon nanocatalysts relying on a low-pressure radio-frequency plasma treatment of solid (powder) precursors. A mesoporous carbon xerogel is used as support for nickel NPs synthesized through the decomposition of an organometallic nickel precursor in a plasma discharge. Different plasma treatment conditions and chemical environments are applied by varying the plasma power and the gas mixture injected into the plasma chamber (Ar, N2, NH3 and O2). The nucleation kinetics of nickel NPs, their morphology evolution and chemical state have been fully characterized by combining analytical techniques such as in situ optical emission spectroscopy, transmission electron microscopy, X-ray diffraction and

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X-ray photoelectron spectroscopy. Results indicate that the plasma chemistry and conditions strongly influence the organometallic compound decomposition as well as the size and the oxidation state of the homogeneously-dispersed nickel NPs. We compare the organometallic precursor degradation efficiency for each plasma by defining a rational “activation power” associated

to

each

plasma

chemistry.

Moreover,

simultaneous

carbon

substrate

functionalization is obtained through plasma treatment, which demonstrates the high versatility of the plasma fabrication for developing green and efficient catalysts and energy materials.

Keywords Nickel

nanoparticles,

Nanocatalysts,

low-pressure

plasma,

organometal,

surface

functionalization, optical emission spectroscopy, photoelectron spectroscopy

Introduction Catalysts based on metal nanoparticles (NPs) supported on nanostructured materials are used in a multitude of applications, including electrochemical and energy devices. Nanocatalysts are extensively used in fuel cells, for the hydrogen transfer reaction or for water splitting.1–5 In such applications, noble metals NPs, such as platinum or gold, are known to provide the highest electrochemical mass activity; however, noble metals are rare and expensive so their use needs to be reduced for large scale applications. Low-cost green catalysts based on metalfree or non-noble materials have recently attracted intense research; within them, nickel-based nanomaterials exhibit promising properties.2,6,7 Regarding the NP substrates, high surface area carbon materials (HSAC) have been extensively used thanks to their low-cost, chemical stability, high electron conductivity and abundancy8,9. Conventional methods to deposit NPs on a carbon support are mainly based on wet (chemical) methods. The NP synthesis requires multiple oxidation/reduction/washing/filtering steps, thus involving an extensive use of solvents and energy, leading to a high environmental impact of the catalyst. Moreover, solvent and impurities may result in the metal NPs contamination with agents that may lower the electrochemical activity and lifetime of the catalyst.6,10,11 Recently, solvent-free (dry) lowpressure plasma treatments have been applied to process platinum/carbon (Pt/C) nanocatalysts for proton exchange membrane fuel cells: in particular, solid precursors (such as platinum


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acetylacetonate and carbon black) were treated in a low-pressure oxygen plasma discharge under stirring conditions. The authors suggested that the resulting Pt NPs were strongly anchored to the carbon black thanks to oxygen bridging bonds formation during the plasma.3,5 In the present work, we investigate the synthesis of Ni/C nanomaterials following a similar strategy based on low-pressure plasma treatments of solid precursors, in order to shine light on the nucleation and growth mechanisms of Ni NPs. Compared with previous works on Pt/C plasma catalysts, here we have explored the decomposition nickel acetylacetonate organometallic precursor on a new carbon xerogel support and under different plasma compositions and powers. The plasma chemistry is varied by exploring three different gas mixtures comprising an inert, an oxidizing or a reducing gas mixed with a carrier gas (argon) at different discharge power conditions. For each treatment, we compare the plasma efficiency in decomposing the Ni precursor and we compare the morphology and chemical composition of the Ni/C composite, showing that nickel nitrides are formed under ammoniabased plasma treatment. The simultaneous functionalization of the carbon xerogel surface by species contained in the plasma is also discussed. The plasma processing is demonstrated as a feasible and flexible alternative method for the NPs deposition by controlling (fairly independently) the particles morphology and their oxidation state to possibly meet the requirements for specific catalytic applications. Furthermore, the plasma decomposition kinetics of the organometallic precursor is discussed together with the possibility to simultaneously add functional groups (C-O, C=O, C-N, etc.) to the carbon substrate. The carbon functionalization is particularly relevant as it has been recently explored to enhance the catalyst efficiency and lifetime.12–15

1. Materials and methods Ni/C nanocatalysts are prepared from powder precursors containing nickel acetylacetonate (Ni(acac)2, Strem Chemicals) and a carbon xerogel with a pore size of about 100 nm, obtained following the procedure described elsewere.16 Carbon xerogel is chosen for (i) being a highpurity material,17 (ii) having a controllable porous texture consisting of mesopores and macropores,17 which limit the material transfer,18 and micropores, allowing a large anchoring surface for nanoparticles. The precursor loading in the powder mixture (containing C, O and Ni species) is fixed to reach 20 wt% of Ni. Plasma treatments are performed in an inductively coupled RF plasma reactor (13.56 MHz) described elsewhere,19 including a matching unit to



minimize the reflected power from the plasma. About 0.2 g of the powder mixture is homogeneously dispersed in a Petri box and placed into the discharge area of the plasma reactor. The reactor is pumped down by a turbo molecular pump, and mass-controlled gases (Ar, O2, N2 or NH3) are flown into the chamber before igniting the plasma. During the treatment, optical emission spectroscopy (OES) measurements were carried out using an Ocean Optics USB4000XR spectrometer by connecting an optical fibre to a quartz window. The nanocatalyst synthesis process is schematized in Figure 1. The crystal structure of the produced Ni/C nanocatalysts is analysed by X-ray diffraction (XRD, X’Pert PRO Panalytical) using the CuKα radiation (1.54056 Å). The composition, carbon surface functionalization and the chemical state of the Ni NPs have been characterized by X-ray photoelectron spectroscopy (XPS, K-Alpha Thermo Scientific) using a monochromatic Al K alpha radiation (1486.68 eV). The X-rays spot size was 250 µm; survey spectra are acquired at a pass energy of 200 eV and high-resolution spectra (C 1s, N 1s, O 1s and Ni 2p) at 30 eV. The scan number is adjusted (between 5 and 30 scans) to the specific element to get similar signal-to-noise ratios. A flood gun is used for charge compensation and spectra are calibrated by fixing the C 1s main peak at 284.8 eV. Peak-fitting is performed with the Avantage © software (Thermo Scientific). The nanocatalyst morphology is characterized by transmission electron microscopy (TEM/STEM, Tecnai OSIRIS, FEI): for the analysis, a small amount of powder is dispersed in isopropanol, ultrasonicated and finally deposited on a copper grid. Bright field (BF) and high-angle annular dark field (HAADF) TEM and STEM analyses are performed at an acceleration potential of 200 kV.

Figure 1. Sketch summarizing the plasma synthesis process of the Ni/C nanocatalysts.

2. Results and discussion ACS Paragon Plus Environment

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2.1 Study of the plasma decomposition of the organometallic precursor The RF plasma decomposition of Ni acetylacetonate and the formation of Ni NPs is investigated under three different plasma chemistries: (i) a gas mixture containing two inert gases, (Ar:N2, 5:2 sccm), (ii) a mixture comprising one reducing gas species (Ar:NH3, 5:4 sccm) and (iii) a mixture comprising one oxidizing gas species (Ar:O2, 5:2 sccm). Argon is added to promote ionization mechanisms, improve the plasma stability and help the decomposition of the organic part of the precursor. The ammonia flow rate is doubled respect to N2 to compare plasma compositions having the same amount of nitrogen species. Depending on the injected gas flow, the reactor pressure during the treatment ranges from 3 to 7 mTorr; a higher working pressure generally result in the lowering of the plasma intensity (as is clearly seen in OES signals intensity) however, within the explored pressure range the plasma intensity is not sensibly different. The RF power is varied between 90 and 200 W and the treatment is maintained until the complete decomposition of the precursor, which occurred between 45 and 60 min depending on the plasma chemistry and the discharge power. The precursor decomposition is followed by in situ OES measurements by monitoring specific emission lines associated with the precursor decomposition (as described in discussion below). Optical emission spectra are acquired in a wavelength range between 200 and 900 nm; the typical spectra obtained at the beginning of the treatment with each plasma composition are reported in Figure 2a. In the range between 680-895 nm and 395-435 nm, emission spectra are dominated by argon lines (Ar I), emission lines between 505-518 nm are ascribed to C2 Swan bands and when nitrogen species are present (N2 or NH3), additional lines appear between 310 and 390 nm, corresponding to the N2 C-B lines and CN violet system (emission lines at 386.7 and 388.2 nm).20,21 When O2 is injected into the plasma discharge, OH related emission lines are observed at 306.7, 309.2 and 313.6 nm (A-X lines20).



Figure 2. (a) OES spectra acquired in situ at the initial stage of the nitrogen-based (Ar:N2 and Ar:NH3) and oxygen-based (Ar:O2) plasma treatments. Characteristic lines from the organometallic precursor decomposition are identified and followed in situ. (b) Evolution of the selected OES signals associated with the decomposition of the organometallic precursor in the different plasma environments: for Ar:N2 and Ar:NH3 plasmas, the OES signal at 388.2 nm (CN emission lines) is selected while for the Ar:O2 plasma we follow the signal at 309.2 nm (OH emission lines).

The intensity of the Ar emission lines is fairly constant during the treatment, while other lines exhibit a clear intensity rise and fall along the plasma treatment. These lines are therefore associated with the Ni precursor decomposition: among them, we have selected the most intense lines to monitor this decomposition in situ. In nitrogen and ammonia-based plasmas, we selected the CN line at 388.2 nm, and for Ar:O2 plasma, the OH line at 309.2 nm. The rise and fall of these signal intensities in a 140 W plasma treatment are shown in Figure 2b. The direct correlation between the drop of these specific OES signals and the precursor decomposition is confirmed by acquiring ex situ XRD spectra at regular treatment time steps, showing the progressive disappearance of the XRD pattern related to the (crystalline) Ni(acac)2 precursor (Figure 3a, only the initial and final time are presented). For Ar:NH3 plasma (Figure 3b), the XRD pattern clearly indicates the formation of crystal domains attributed to nickel nitride phase (Ni3N). The average Ni3N domain size increases with the plasma treatment power as estimated by the Debye-Scherrer formula from the average full width at half maximum (FWHM) of the three most intense (110), (002) and (111) Ni3N diffraction peaks.22 For the Ar:NH3 plasma, a linear correlation is found between the Ni3N domains size (from 5.8 to 8.1 nm) and the discharge power (see the inset in Figure 3b). When the precursors are treated with Ar:O2 and Ar:N2 plasmas, only a weak NiO phase is


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observed for 200 W treatments (Figure 3a), suggesting that amorphous oxidized Ni particles are formed by these plasma treatments.

Figure 3. (a) XRD patterns of the untreated precursor and 200 W plasma-treated powder showing the disappearance of nickel acetylacetonate crystal peaks attesting its decomposition as soon as the selected OES signal drops. (b) XRD patterns obtained in Ar:NH3 treatments at different discharge power showing the formation of Ni3N domains with a size increasing with the discharge power (the average size is estimated from the Debye-Scherrer formula, inset).

The efficiency and kinetics of a given plasma chemistry in decomposing the Ni acetylacetonate is estimated from OES measurements; namely, an activation “energy” is associated to each specific plasma decomposition process. To do so, the organometallic precursor (OM) decomposition in the discharge is schematically described by the following reaction:

→ ∗

(1)

where F1* represents a generic volatile organic fragment (produced in an excited state) and F2 represents other intermediate decomposition products, containing nickel atoms. In this framework, the fast relaxation of F1* to the ground state generates the CN or OH lines observed by OES. In the second step, F1 is pumped down, while F2 fragments decompose progressively to generate Ni NPs. For a generic reaction such as → , we can define the consumption rate r of the organometallic precursor as:



=−

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= ×

(2)

where [OM] is the organometallic concentration at the reaction time t and n and k are the reaction order and reaction rate, respectively. Eq. (1) allows relating [OM] to the concentration of F1* and thus to the selected OES line intensity (IRAW, Figure 4a). In other words, by assuming that [F1*] is directly proportional to IRAW, [OM] can be followed by the expression:

= 1 −

! "

!

(3)

The Eq. (3) simply states that [OM] is proportional to the normalized integral of the OES signal intensity (IRAW), and varies from 1 (initial state, t=0) to 0 (end of the plasma treatment), )

following a decreasing sigmoid function shown in Figure 4b. The expression * I&'( +, is a constant for each treatment, called A for simplicity. Eq. (2) becomes: -./0 1

= × 21 −

! 1

3

(4)

From this, it is possible to determine n and k, by applying the natural logarithm on both sides of the previous equation:

ln67819 : = ln ln ; × ln

C Nanocatalysts from Solid

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