Encapsulation of Ni nanoparticles with oxide shell in vapor condensation 93 D O I: 1 0. 15 82 6/ ch im te ch .2 01 9. 6. 3. 02 Beketov I. V., Safronov A. P., Medvedev A. I., Murzakaev A. M., Zhidkov I. S., Cholah S. O., Maximov A. D. Chimica Techno Acta. 2019. Vol. 6, No. 3. P. 93–103. ISSN 2409–5613 I. V. Beketova*, A. P. Safronova,b, A. I. Medvedeva,b, A. M. Murzakaeva,b, I. S. Zhidkovb, S. O. Cholahb, A. D. Maximova a Institute of Electrophysics UB RAS, 106 Amundsen St., Yekaterinburg, 620016, Russian Federation b Ural Federal University, 19 Mira St., Yekaterinburg, 620002, Russian Federation *E-mail: beketov@iep.uran.ru Encapsulation of Ni nanoparticles with oxide shell in vapor condensation Controlled input of oxygen into the inert working gas flow during the produc- tion of Ni nanoparticles by the electrical explosion of wire (EEW) method leads to the formation of a crystalline oxide shell on the surface of particles during their condensation from the vapor phase. Resulting oxide shells encapsulating Ni particles weaken their agglomeration processes as well as protect the surface of the Ni nanoparticles from further oxidation. The influence of the amount of en- ergy introduced during EEW and the quantity of oxygen added to the working gas in EEW process on the properties of resulting Ni nanoparticles was studied. The obtained nickel nanopowders were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and N2 adsorption (BET) methods, which gave the specific surface area, the aver- age diameter of nanoparticles, their phase composition, the morphology of the particles and the structure of the oxide shells. It was shown that the addition of oxygen leads to a decrease in the average diameter of Ni nanoparticles and reduces the degree of their agglomeration. The encapsulation of Ni nanoparticles with 3–5 nm thick gas-tight oxide shells protects the particles from oxidation and eliminates the pyrophoricity of the powder product. Keywords: nickel nanoparticles, oxide shells, encapsulation, electrical explosion of wire. Received: 10.07.2019. Accepted: 16.09.2019. Published: 15.10.2019. © Beketov I. V., Safronov A. P., Medvedev A. I., Murzakaev A. M., Zhidkov I. S., Cholah S. O., Maximov A. D., 2019 Introduction Today, nanoparticles of transition metals are of considerable interest both from a scientific point of view due to their spe- cific physical and chemical properties, and also in connection with promising practical applications [1]. In particular, such materi- als can be used as catalysts, as components in magnetic recording systems, in bioengi- neering and biomedicine, as components in sensors, actuators, scaffolds, etc. With a  decrease in  particle size their surface activity considerably increases, promoting agglomeration and aggrega- tion of particles, which complicates their 94 further use. For instance, in the prepara- tion of metal matrix composites or com- plex ceramic composites, the uniformity of the material is achieved only at the scale of  agglomerates with sizes varying from units to tens of micrometers. Thus, the ag- glomeration of nanoparticles during their production limits the yield: with a decrease in the size of the particles, it is necessary to  reduce the productivity of  equipment to  prevent their agglomeration. In addi- tion, it is known that metal nanoparti- cles, with the exception of  noble metals, are very pyrophoric at  low temperatures and simply combust if exposed to ambient air. Usually, after the synthesis, the metal nanoparticles are passivated: a thin layer of oxide, which subsequently protects the metal from rapid oxidation, is formed on  their surface. However, some metals, such as  Cu, even after the passivation do not form a  gas-tight oxide shell that protects them from further oxidation. In particular, it is shown [2] that the process of oxidation of copper nanoparticles dur- ing their storage in air results in complete formation of CuO. To prevent agglomeration and to pro- tect the surface of  the particles, it is de- sirable to  form protective shells on  the particles during their formation. A choice of a material for a protective cover is a sep- arate issue. On the one hand, the most ac- ceptable material for  the protective shell is a material that reliably protects the sur- face of  the particle from rapid oxidation and does not interfere with, and in some cases even contributes to  the further use of such powders. On the other hand, na- noparticles with the structure of the metal core — protective shell are typically used as fillers in composite polymer materials and liquid disperse systems. Therefore, the main property of the shell material is also to  ensure the compatibility of  core / shell nanoparticles with the dispersion medium, which depends on the energy of interac- tion between the dispersion medium and the surface of such nanoparticles. In the technology of synthesis of metal nanoparticles, their surface is typically covered with pure carbon, silicon car- bide, or  polyethylene [3], which are not active at low temperatures. For instance, to create a carbon protective shell on the particles, various methods are used, such as annealing of metal particles in the at- mosphere of hydrocarbon (gasoline) [4], filling carbon tubes with metal [5], an- nealing of a mixture of metal powder with carbon [6]. All these techniques involve the processing of the finished powder. The formation of a carbon shell on the surface of  the particles in  the process of  synthe- sis of Fe, Co, Ni metal nanoparticles and their alloys using the method of  an arc discharge in  the methane atmosphere is described in Ref. [7]. In our opinion, nickel monoxide (NiO) is a  promising material of  the protective shell for Ni nanoparticles. NiO has a rel- atively high melting point, 1955 °C [8], and it forms a strong gas-tight protective film on the metal surface [9]. The objec- tive of the present study is to develop an in-situ method of  the formation of  the protective oxide shells on  the surface of Ni nanoparticles during their synthesis by the electric explosion of wire. Electric explosion of wire (EEW) is one of the most productive methods of synthesis of nano- particles of metals, alloys and their chemi- cal compounds. The productivity of EEW method ranges from 50 to 500 g / h depend- ing on  the nature of  nanoparticles. The method has low energy consumption, not exceeding 50 kW·h / kg; it is environmen- tally friendly; it provides high purity of the 95 resulting particles. Experimental setups for EEW method are compact and do not require separate facilities. This method has been described in detail elsewhere [10]. Experimental Production of nickel nanopowders was carried out in the installation of an electric explosion of  wire (EEW). The discharge circuit had an inductance of 0.4 µH. The capacitance of the capacitor bank varied from 3.2 to 4.8 µF, and the charging volt- age — from 20 to 30 kV, which provided a change in overheating ratio k (the ratio of  the energy introduced into the wire to  the sublimation energy of  the wire metal) in the range of 1–2.2. To obtain the powder, a wire of nickel (NP2 grade) with a diameter of 0.3 mm and a length of 88 mm was used. The wire was fed into an ex- plosion chamber continuously. Wire explo- sions were carried out in argon at a pres- sure of 0.12 MPa at a frequency of 0.5 Hz. The gas system of the installation con- sisted of an explosion chamber, two inertial traps with a gas flow rotation, a mechanical (cloth) filter and a fan connected in series. Inertial traps were used to separate large micron-sized particles; the rest of the pow- der was collected in  the filter. As shown by  preliminary experiments, the output of the powder in the filter depends on the amount of energy introduced into the wire and averages 85 % of the mass of all result- ing from the explosion powder. Only the powders from the filter were studied in the present work. To create shells on the sur- face of the particles in the process of the powder production, oxygen was contin- uously introduced into the working gas (Ar) flow directly before entering the ex- plosion chamber. With the help of an ad- justable flow-type throttle, the oxygen flow could vary from 0.3 to 20 cm3 / s. For the ease of comparison of powders obtained in different modes, the amount of oxygen is given below in grams per gram of nickel (g / g Ni). At low oxygen consumption, after producing the powder, it was additionally passivated with oxygen flow (0.5 cm3 / s) supplied to the EEW setup. The minimum oxygen consumption, which allows elimi- nating the pyrophoricity of  the powder, was determined experimentally. The pas- sivation process was controlled by  the oxygen partial pressure sensor. Passiva- tion stopped when the partial pressure of  oxygen reached 2 kPa. After that, the powder was freely (without the risk of its self-combustion) removed to the air. The specific surface area of the obtained powders was determined by the BET meth- od by low-temperature nitrogen adsorp- tion using Micromeritics TriStar 3000. X- ray phase analysis (XRD) was performed on the Bruker D8 DISCOVER diffractom- eter in Cu Kα radiation (λ = 1.54 Å) with a graphite monochromator on the second- ary beam. Processing of diffractograms was performed using the program TOPAS 3. High-resolution transmission electron mi- croscopy (TEM) was performed using the JEOL JEM2100 electron microscope at an accelerating voltage of 200 kV. The energy introduced into the wire was determined by  the oscillograms of  the current pulse passing through the exploding section of  the wire. Measurements of  the X-ray photoelectron spectroscopy (XPS) spectra of the main and valence levels of powders were carried out on the spectrometer PHI 5000 VersaProbe, based on  the classical scheme of X-rays with a quartz monochro- mator and a  hemispherical energy ana- lyzer operating in the energy range from 0 to 1500 eV. Electrostatic focusing with 96 magnetic shielding allows obtaining energy resolution ΔE ≤ 0.5 eV for  Al Kα radia- tion (1486.6 eV). Pumping of the analyti- cal chamber was carried out with the help of an ion pump, which provides a pressure lower than 10–7 Pa. Two-channel neutrali- zation was used to compensate the local surface charge formed during the measure- ments. The diameter of the X-ray spot was 200 mcm and X-ray power — 50 W. Cali- bration of spectra was carried out at the po- sition of the 1s line of carbon E = 285.0 eV. Processing of XPS experimental data was carried out using the complex ULVAC-PHI MultiPak Software 9.8. Results and discussion The major variable to control the size of metal particles in EEW method is the overheating ratio (k). It is a  dimension- less coefficient, which is equal to the elec- trical energy introduced into a  portion of a metal wire subjected to EEW divided by the energy of sublimation of this metal. In case of Ni, the energy of sublimation is 65.1 J / mm3. The experimental dependence of the specific surface area and the mean diameter of Ni nanoparticles on overheat- ing coefficient are shown in  Fig.  1. The mean diameter of spherical particles (D) is related to the value of their specific surface area (Ssp) by the following equation: D= 6 / ρ∙Ssp, (1) with ρ being the effective density of the ma- terial (Ni — 8.91 g / cm3, NiO — 7.45 g / cm3). It is seen that the specific surface area of Ni nanoparticles substantially increases from around 7 m2 / g up to 11 m2 / g if the overheating ratio is doubled. Consequently, with the increase in the overheating ratio, the diameter of  Ni particles diminishes from 100 down to 60 nm in average. Fur- ther on, the results for two values of the overheating ratio will be systematically compared. These values are k = 1.2, which corresponds to large Ni particles (low spe- cific surface area) and k = 2 (small parti- cles, high specific surface area). It is worth noting that overheating ratio k = 1 was not taken, because of the technical difficulties of maintaining stable EEW process at zero overheating. Using the given levels of  overheating ratio, several batches of Ni particles were synthesized with the controlled addition of  oxygen to  the EEW installation. The crystalline structure of obtained Ni nano- particles was characterized by XRD. Fig. 2, a gives examples of XRD diffractograms for two Ni batches synthesized at different amount of oxygen added to the working gas of the EEW installation. In both cases, two cubic phases were identified: the major one corresponded to  Ni with lattice pa- rameter a = 3.523(1) Å and the minor one corresponded to NiO with a = 4.174(2) Å. It means that the addition of oxygen to the working gas resulted in the in-situ oxida- tion of condensed Ni nanoparticles. Fur- Fig. 1. The dependence of the specific surface area (left) and the average diameter (right) of Ni nanoparticles synthesized by EEW method on the overheating ratio 97 ther on, the obtained particles, which con- tain both the fraction of metallic Ni and NiO are denoted as Ni@NiO. Using the diffractograms, the weight fraction of NiO in all batches was evaluat- ed. The dependence of NiO content in na- noparticles on the amount of oxygen added to the working gas in EEW installation is given in Fig. 2, b. The amount of oxygen (in weight) is divided by the weight of Ni wire consumed in the synthesis. Further on it is denoted as Q. Experimental points in Fig. 2, b relate to two values of overheat- ing ratio, k = 1.2 and k = 2. It is seen that, despite the difference in  the level of  overheating ratio, the ex- perimental points can be fitted well with the same linear dependence, %NiO = 302·Q. It means that in-situ oxidation does not depend on the average diameter of Ni particles and is governed exclusively by the amount of oxygen in the working gas. Fig. 3 presents experimental depend- ences of the specific surface area (Fig. 3, a) Fig. 2. a — Selected XRD diffraction patterns of EEW Ni nanoparticles synthesized with the addition of oxygen; b — Dependence of NiO weight fraction on the amount of oxygen added to EEW synthesis per Ni wire consumption. Closed circles correspond to overheating ratio 1.2, open circles — to overheating ratio 2 Fig. 3. Dependence of the specific surface area (a) and mean diameter (b) of Ni particles on the amount of oxygen introduced into EEW installation during the synthesis at overheating ratios 1.2 and 2 98 and the average diameter of nanoparticles (Fig. 3, b) on the amount of oxygen added to  the working gas of  the EEW installa- tion at the values of overheating ratio equal to 1.2 and 2. As  shown in  Fig.  3, a, the increase in oxygen consumption leads to an increase in the specific surface area of produced Ni nanoparticles at both levels of overheating ratio. The experimental plots are nicely fit- ted by the linear dependences: 6.9+44.2·Q (R2 =  0.981) for  k  =  1.2 and 9.2+82.4·Q (R2 = 0.998) for k = 2. In general, the in- creasing trend in  specific surface area due to the addition of oxygen is equiva- lent to the same trend due to the increase in overheating ratio (see Fig. 1). It is be- cause the oxidation of  Ni to  Ni oxide is accompanied with the evolution of heat, which adds up to the electric energy in- troduced in EEW. Thus, the total amount of energy (electrical and oxidation) in EEW process exceeds the sublimation energy of  Ni significantly more in  the presence of oxygen than in the inert atmosphere. Fig. 3, b presents the average diameter of Ni@NiO particles as a function of oxy- gen / Ni ratio. The average diameter was calculated using Eq. (1) taking into account that the density of Ni@NiO particles di- minishes with the increase of NiO content due to the difference in densities of Ni and NiO. It is seen that, at both levels of over- heating ratio, the decreasing trend in the average diameter of Ni@NiO particles is observed. The reason is the same as for the increasing trend of the specific surface ar- ea. The oxidation reaction provides extra energy to the system, and the overheating effectively increases. It is noticeable that the slopes of the lin- ear plots in Fig. 3, a are different for two levels of overheating. The slope coefficient is 44.2 if k  =  1.2, and it is 82.4 if k  =  2. However, no evident difference in  the slopes of plots can be noticed in Fig. 3, b. It is because the calculation of the average diameter of Ni@NiO particles according to Eq. (1) includes the partial correction of  increasing Ssp values by  the decreas- ing density of Ni@NiO particles with the increase in their oxidation. The plots are shifted one against the other by  the ini- tial difference of average particle diameter for  k  =  1.2 (smaller particles) and k  =  2 (larger particles). It is known [9] that in  the oxidizing atmosphere the surface of  metallic Ni is covered with dense oxide layer which ef- ficiently prevents metal from further oxida- tion. The Pilling-Batworth criterium [11] for NiO is 1.52, which means that Ni par- ticles should be covered with dense oxide shell in  the presence of  oxygen. During EEW, the formation of oxide shells on the surface of Ni particles can start only at the moment when the expanding cloud of met- al vapor after the explosion mixes up with the working gas, which contains oxygen. It does not take place at the very first mo- ments after the explosion, as the expand- ing cloud of vapors is substantially denser than the surrounding working gas. Fur- thermore, the critical temperature of the thermodynamic stability of NiO is lower than the temperature of condensation of Ni vapors into liquid phase. Therefore, oxida- tion begins when the condensation on Ni liquid droplets has been already complet- ed, and it takes place at the surface of Ni particles (liquid or solid). Fig. 4 presents high-resolution TEM image of the surface of Ni particle synthesized in the presence of oxygen. It is clearly noticeable that the surface of a particle is covered with a layer, whose crystal structure is different from that of the interior of the particle. The lattice 99 period in the surface layer evaluated from HRTEM images is 2.4±0.1 Å. This value corresponds to the lattice period for cubic or rhombohedric NiO structure. According to XPS analysis, the spectrum of the surface of Ni particles synthesized in the presence of  oxygen contained signals of  elements Ni and O.  Ni is present in  the oxidation state Ni+2, which is an indication of NiO [12], and Ni+3, which likely corresponds to Ni2O3 [13] or NiOOH [14]. The surface also contained carbon, which is the result of carbon dioxide adsorption from the air. The  thickness of  oxide layers on  Ni particles was evaluated by  the graphical analysis of  TEM images obtained with 3.7∙106 and 7.4∙106 magnification. The av- erage values of the thickness of oxide layers are given in  Table 1. According to  Table 1, the thickness of oxide layers increases with the increase in the amount of oxygen added to working gas in EEW process. At the same time, the overheating ratio does not make a noticeable influence. Table 1 Average thickness of oxide layer on Ni@NiO particles QO2, g / g Ni Overheating, k Thickness, nm 0.03 1.2 2.51 0.1 1.2 3.35 0.02 2 2.82 0.096 2 3.31 Like other chemical reactions in  the EEW process, oxidation of Ni surface takes place in non-equilibrium conditions. It re- sults in the non-uniformity of NiO distri- bution on the surface of Ni@NiO particles. It becomes more evident at large amounts of  added oxygen. Fig.  5 presents TEM image of  Ni@NiO particles obtained at QO2 = 0.1 g / g Ni. It is noticeable in  Fig.  5, a  that the spherical shape of Ni@NiO particles be- came substantially distorted due to the for- mation of oxide layers with varied thick- ness. Most likely, it is because oxidation took place at elevated temperature before the particles were cooled down to the am- bient temperature. In these conditions of  intense non-equilibrium oxidation, the initially formed oxide layer could be fractioned, and spherical shape of parti- cles would be distorted. Such distortions might as well be taken as evidence in favor of oxidation of Ni in liquid droplets, whose surface is easier to distort, rather than con- densed solid particles. At the high level of oxygen in the work- ing gas, the smallest Ni particles, which are the most chemically active, could transform completely into NiO. Fig. 5, b shows such a particle with a diameter of 15 nm, which is completely formed by  NiO. The frag- ment of crystal lattice noticeable in Fig. 5, b has a lattice period of 2.4±0.1 Å, which corresponds to NiO. Due to the presence of  separate NiO particles and thickened oxide layers, the total NiO content in Ni@ Fig. 4. High-resolution TEM image of Ni particle synthesized by EEW at k = 1.2; QO2 = 0.033 g / g Ni 100 NiO particles according to XRD increases up to 30 % if oxygen content in the working gas is raised up to 0.1 g / g Ni. As the oxidation of Ni likely takes place on the surface of liquid droplets, the for- mation of  solid oxide layer can prevent the coalescence of droplets, which causes agglomeration of particles in the product powder. Fig.  6 illustrates the influence of oxygen addition on agglomeration. In Fig. 6, a it is evident that agglom- eration of Ni particles is high. Almost all of them in the image are bonded into one cluster, which is certainly the result of coa- lescence of liquid Ni droplets in gas phase at temperatures above crystallization. Con- a b Fig. 5. a — Ni@NiO particles obtained by EEW at QO2 = 0.1 g / g Ni, k = 1.2; b — High resolution TEM image of the surface with NiO layer and small separate NiO particle (D = 15 nm) а b Fig. 6. a — Ni nanoparticles synthesized in argon; b — Ni@NiO nanoparticles synthesized in argon with the addition of oxygen, QO2 = 0.033 g / g Ni 101 trary to that, the majority of Ni@NiO na- noparticles in Fig. 6, b are separated from each other. Even if we see them on  top of  one another, we can also see distinct spherical boundaries. It means that Ni@ NiO particles are individual. Some agglom- eration can still be noticed, but it is not pre- dominant in case of Ni@NiO particles. The residual agglomeration of Ni@NiO stems from the large difference  — 1223  K  — between the temperature of vaporization of Ni, 3173 K [15], and the upper tempera- ture of oxidation, 1950 K (calculated using the software Chemical WorkBench, ver. 3, Kinetic Technologies Ltd.). This tempera- ture interval provides a time gap from the moment when liquid droplets of Ni con- dense from the vapor phase until they are covered with solid oxide, which prevents their coalescence. In case of other metals, this interval is narrower: Al — 350–400 K, Cu — 500 K. In case of these metals, the addition of oxygen prevents agglomeration completely. Nevertheless, despite the large temperature interval before oxidation, the addition of oxygen substantially decreases agglomeration of Ni nanoparticles. NiO layers on the surface of Ni nano- particles substantially enhance their sta- bility to self-combusting in the air. It was found experimentally that at  an oxygen consumption greater than 0.015 g / g Ni, the powdered product can be transferred from the EEW setup to the ambient air without the risk of combustion. Such oxygen additi- tion leads to a significant reduction in the pyrophoricity of the powder, as Ni@NiO particles are covered with an oxide shell that reliably protects them from oxidation during storage. Conclusions The  controlled oxygen injection into the inert working gas of the EEW unit dur- ing the production of Ni powder makes it possible to form a crystalline oxide shell on the particles in the process of their for- mation. It depresses the particle agglom- eration, as well as protects the surface of Ni particles from oxidation. The increase in oxygen consumption leads to an increase in the specific surface area of the powder. In the range of oxygen consumption from 0 to  0.1  g / g Ni, the specific surface area varies almost linearly and increases by 60– 80 % depending on the overheating ratio. Addition of oxygen to the working gas leads to the formation of a separate phase in the powdered product — crystalline Ni oxide, the content of which increases with increasing oxygen consumption. At low consumption of oxygen, NiO forms a dense crystalline shell on the surface of particles. The thickness of the shell is 2–5 nm and increases with increasing oxygen consump- tion. Overheating ratio has no significant effect on the shell thickness. In the range of oxygen consumption 0–0.1 g / g Ni the content of NiO linearly increases up to 30 % independently of overheating ratio. At high oxygen consumption, spherical shape of Ni@NiO nanoparticles is substantially distorted, and small separate individual NiO nanoparticles tend to appear. 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