Advanced piezoresistance of extended metal/insulator core shell nanoparticle assemblies 
 
 

Evagelos K. Athanassiou ¹, Frank Krumeich², R. N. Grass ¹ and Wendelin J. Stark ^*1 
 
 

Supporting Information 
 
 

¹ Institute for Chemical and Bioengineering, ETH Zürich, Zurich, CH-8093 Switzerland

² Laboratory of Inorganic Chemistry, ETH Zürich, Zurich, CH-8093, Switzerland 
 
 

Submitted to Physical Review Letters 
 

* Correspondence should be addressed to Wendelin J. Stark

Email: wendelin.stark@chem.ethz.ch;

Fax: +41 44 633 10 83

Reducing flame synthesis.

Flame spray pyrolysis proceeds by the combustion of a suitable combustable metal containing organic precursor. The precursor is dispersed by an oxygen jet forming a spray, which is subsequently ignited by a premixed flame. In a conventional spray reactor, the precursor combusts to H₂O, CO₂ and metal oxide nanoparticles. In order to prepare metallic nanoparticles, the flame may be operated in a nitrogen filled glove box under oxygen limitation ([Figure S1], reducing flame synthesis). In order to ensure stable combustion conditions, the flame was encased in a double walled tube. Under oxygen limitation the combustion reaction yielded CO and H₂ instead of CO₂ and H₂O and the metal oxide nanoparticles can be reduced depending on their reduction potential.  
 

Experimental set-up for the production of NiMo/Ceramic.

A flame spray nozzle was placed in a glovebox [Figure S1] and fed with nitrogen (PanGas, 99.999%). The atmosphere was continuously recycled by a vacuum pump (Busch, Seco SV1040CV) at about 20 m³ h^-1 and the resulting combustion gases H₂O and CO₂ were removed by two adsorption columns (zeolite 4A and 13X), resulting in oxygen levels of below 100 ppm during combustion. For the synthesis of the NiMo/Ceramic nanocomposite (Ni/Mo = 7/3; CeZrO₄: 10 vol%, Table S1) an air and humidity stable liquid metal precursor was prepared from corresponding amounts of nickel 2-ethylhexanoate (6 wt % Ni), molybdenum 2-ethylhexanoate in mineral spirits (STREM Chemicals, 15 wt % Mo), zirconium 2-ethylhexanoate (13 wt % Zr) and cerium octoate (Shephard, 12.2 wt% Ce). Prior to processing the metal precursors were diluted 2:1 (w/w) with tetrahydrofurane (Fluka, tech.) and then filtered (Satorius, fluted type 288). The liquid metal precursor was delivered to the spray nozzle [1] by a micro annular gear pump (6ml min^-1, HNP Mikrosysteme, mzr-2900) and ignited by a premixed pilot flame (CH₄ 1.2 L min^-1; O₂ 2.2 L min^-1, PanGas tech.) surrounding the liquid spray. In order to allow a stable combustion the flame was encased in a porous metal tube. The produced nanocomposite was separated from the off-gas using glass fiber filters (Schleicher&Schuell, GF6, 25.7 cm diameter), mounted on a cylinder above the exit of the reducing flame synthesis reactor. During combustion the atmosphere in the glove box and the concentration of the produced gases (H₂, H₂O, CO, CO₂) was monitored by a mass spectrometer (Balzers, GAM 400).  
 
 

 Fig. S1. Experimental apparatus for the preparation of NiMo/Ceramic. 
 

Table S1. Metal composition of the liquid metal precursor.

 ^α Compositions given in wt %

Powder Analysis.

NiMo/Ceramic was analyzed by X-ray diffraction (XRD; Stoe STADI-P2, G2 monochromator, Cu K_a1, PSD detector) for crystallinity and predominant phase composition, nitrogen adsorption (BET; Tristar Micromeritics Instruments) for specific surface area and mean primary particle size, thermal gravimetric analysis (Linseis TG STA-PT1600, 25-800, 10°C min^-1) under air or H₂ in argon (7 vol% H₂), transmission electron microscopy (TEM; CM30 St-Philips, LaB₆ cathode, operated at 300 kV, point resolution ~ 4 Å), energy dispersive X-ray spectroscopy (EDXS), element microanalysis (LECO CHN-900) and scanning electron microscopy (SEM; FEG 1530 Zeiss Gemini).

Crystallinity, particle size distribution, element distribution and material homogeneity

Nickel molybdenum alloy nanoparticles (Ni/Mo = 7/3) coated with ceria/zirconia (Ce:Zr ratio = 1:1 at/at; 10 vol % overall) were produced in a single step at a production rate of about 20 g h^-1 by simultaneously combusting a spray of the corresponding metal 2-ethylhexanoates in a methane/oxygen flame [1] in a closed reactor system. The 20-50 nm as-prepared nanoparticles exhibited high air stability which is in sharp contrast to uncoated metal nanopowder’s often violent reaction with air (pyrophoricity). The absence of undesired carbon soot contamination was confirmed by quantitative carbon microanalysis (Table S2) and X-ray diffraction confirmed the formation of Ni/Mo alloy in the particle core [Fig. S2]. Energy dispersive X-ray spectroscopy followed by scanning transmission electron microscopy [Fig. S3] confirmed the presence of Ce and Zr. The primary particle size distribution [Fig. S4] was evaluated from transmission electron micrographs and confirmed that the ceramic shell did not affected the particle size distribution of the nickel/molybdenum nanoparticles [2]. The core/shell material exhibited a narrow number based particle size distribution. Assuming spherical particles, the results could be fitted to a log-normal distribution with a number based geometric mean particle diameter of 21.2 nm. This agrees well with a calculated particle diameter from specific surface area measurements through N₂ adsorption at 70 K (d_{BET, NiMo}: 16.4 nm, Table S2). The about half sized mean metal core crystallite diameter (d_XRD: 9.3 nm, Table S2) determined from the peak width using the Scherrer formula confirmed formation of polycrystalline metal particles which stays in line with earlier studies on metal nanoparticles. Transmission electron micrographs displayed nearly spherical particles with a surface coating [Fig. 1(a)] which stays in agreement with the air stability and the previous synthesis of carbon coated copper nanoparticles [Fig. 1(b)]. The preferential reduction of nickel and molybdenum to their metallic form and the simultaneous formation of cerium and zirconium oxide are in agreement with the large difference in the reduction potential of the composite constituents. Reducing flame synthesis results in the formation of metal nanoparticles, when they exhibit a positive or low negative reduction potential such as bismuth, copper [3], cobalt [4], iron, nickel (E_Red (Ni²⁺ +2e^- à Ni) = -0.25 V) and molybdenum [2] (E_Red (Mo³⁺ +3e^- à Mo) = -0.20V). On the other hand metals such as cerium (E_Red (Ce⁴⁺ +4e^- à Ce) = -1.32 V), zirconium (E_Red (Zr⁴⁺ +4e^- à Zr) = -1.45 V), aluminium and chromium have a strongly negative reduction potential and could not be reduced by the other combustion gases (hydrogen, carbon monoxide) in the present flame (O₂ below 100 ppm).

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