Article pubs.acs.org/IC
Cite This: Inorg. Chem. 2018, 57, 1464−1473
One-Step Solution Combustion Synthesis of Cobalt Nanopowder in Air Atmosphere: The Fuel Effect Alexander Khort,*,† Kirill Podbolotov,‡,§ Raquel Serrano-García,∥ and Yurii Gun’ko∥,⊥ †
A.V. Luikov Heat and Mass Transfer Institute and ‡Physical-Technical InstituteNational Academy of Sciences of Belarus, Minsk 220072, Belarus § Department of Glass and Ceramic Technologies, Belarusian State Technological University, Minsk 220006, Belarus ∥ Trinity College Dublin, Dublin 2, Ireland ⊥ ITMO University, St. Petersburg 197101, Russia S Supporting Information *
ABSTRACT: In this paper, we report a new modified solution combustion synthesis technique for one-step production of metallic Co nanoparticles. The main unique feature of our approach is the use of microwave-assisted foam preparation. Also, the effect of different types of fuels (urea, citric acid, glycine, and hexamethylenetetramine) on the combustion process and characteristics of resultant solid products were investigated. It was shown that the combination of microwave-assisted foam and hexamethylenetetramine as a fuel allows us to produce metallic Co nanoparticles with the broad size distribution (∼5−40 nm), high coercivity (370 Oe), and high value of saturation magnetization (137 emu/ g) by the one-step solution combustion synthesis under normal air atmosphere without any post reduction.
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INTRODUCTION Nanomaterials of a different nature have attracted significant research interest because of their unique size-dependent properties that are not typically observed in the corresponding bulk material. Nanomaterials are characterized by a high specific surface area to volume ratio, which is the basis of their unique physical and chemical properties.1,2 In this case, a half or more atoms are located at the surface or interface. This strongly affects electronic structure, reactivity and other properties.2−5 Among the nanomaterials, metal nanoparticles (mNPs) are objects of a great research interest in modern materials chemistry and physics, as they find a wide range of applications in fields of nanoelectronics, optics, photochemistry, catalysis, etc.6−13 Cobalt nanoparticles (Co NPs) are one of the most used among mNPs. Co NPs have deserved much attention due to their application in catalysis, microwave absorption, sensors, for magnetic separations in organic synthesis, etc.14−19 Metallic Co has attracted great interest as a magnetic material because of its high saturation magnetization (Ms ≈ 168 emu/ g).20 At the same time low coercivity (Hc ≈ 10 Oe) significantly restricts its practical application in data storage devices and for microwave absorption. This problem could be overcome by using of nanoscale materials. It is known21−23 that the coercivity of superparamagnetics strongly depends on shape and magnetocrystalline anisotropies of nanoparticles. For example, Zhou et. al24 prepared Co three-dimensional cobalt microspheres with a high degree of the grains shape anisotropy and coercivity of 117.70 Oe. Other magnetic microspheres © 2018 American Chemical Society
prepared by PVP-assisted solvothermal process are characterized by higher coercivity (202 Oe) because of the combination of shape anisotropy, grain orientation, and size of nanoparticles. 25 Grass, Stark, and co-workers have investigated and published26−28 several types of carbon- and polymer-coated cobalt nanobeads with a high coercivity (∼300−500 Oe). Zhang and co-workers29 reported enhancing of Hc to greater than 1000 Oe in electrospun prepared Co nanofibers because of the strong quantum effect, but saturation magnetization in this case was less than 10 emu/g. There are many different state-of-the-art approaches for the preparation of Co NPs of different shape including: sol−gel, microfluidic and thermal reduction, electrochemical deposition, hydrothermal methods and combustion-based techniques.24,30−35 Although the latter have some advantages as it is cost- and energy efficient, very fast, and results in particles with a high specific surface area and high shape anisotropy, etc.,36,37 there are only a few papers devoted to pure metal Co NP preparations by combustion methods. The main reason is a high reactivity of metallic Co under high temperatures resulting in cobalt oxidation and a mixture of cobalt oxides and metallic Co particles, where Co is a minor phase.38 Therefore, the successful cobalt metal NPs preparation by combustion-based technique usually requires an inert atmosphere (N2, Ar) during the synthesis process to prevent metal Received: November 8, 2017 Published: January 22, 2018 1464
DOI: 10.1021/acs.inorgchem.7b02848 Inorg. Chem. 2018, 57, 1464−1473
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Figure 1. Simplified scheme of SCS of Co NPs. ⎛1 ⎞ ⎜ φ⎟C H N + 3(φ − 1)O 2 ⎝ 3 ⎠ 6 12 4 ⎛ 2φ + 3 ⎞ ⎟N + (2φ)H O = Co + (2φ)CO2 + ⎜ 2 ⎝ 2 ⎠ 2
oxidation, or an additional post reduction of metal oxides in the atmosphere of hydrogen or carbon monoxide.39 In spite of such kind of methods very attractive for Co NP production as an easier and more economically efficient, there is no information about safe one-step combustion method for pure metallic Co NPs production under normal air atmosphere or without additional post reduction. Thus, in this work, we report the synthesis of almost pure metallic Co NP by one-step modified SCS technique in normal air atmosphere without additional post reduction. The effect of the different fuels on the combustion synthesis process and structure of Co NP was also investigated. Finally, the correlation between features of phase composition, structure, and magnetic properties of Co NP was studied.
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Co(NO3)2 +
The parameter φ, the fuel to oxidizer ratio, is defined such that φ = 1 corresponds to a stoichiometric oxygen concentration, meaning that the initial mixture does not require atmospheric oxygen for complete oxidation of the fuel, while φ > 1 ( 1 the protective atmosphere formed from the oxidation products of the fuel, which provides the possibility of obtaining pure metal in normal air atmosphere. For all experiments, the ratio (φ) between the fuel and the oxidizer was kept constant and equal to 2, which allows the synthesis of pure metals.43,44 All samples were prepared by the same procedure. In a typical experiment 14.8 g cobalt nitrate hydrate was dissolved in a 5 mL of twice distilled hot water (solution 1) and a fuel (U, 12.20 g; CA, 14.24 g; G, 10.17 g; HMT, 4.75 g) was dissolved in 5 mL of aqueous solution of ammonia and additional distilled water where it was needed (solution 2). Solutions 1 and 2 were then mixed and sol was formed. Ammonia was added to the resultant solution to adjust the pH value at about 7. The obtained sol has been rapidly drying in microwave oven (800 W, 2.450 GHz), until gel and then foam has formed. The foam was ignited and burned in thermal explosion mode in preheated to 500 °C a muffle furnace in air atmosphere, leading to the formation of a fluffy powder, which was rapidly cooled in air in a quenching mode to prevent metal oxidation. Another solution in the cobalt nitrate−HMT system (HMT−furnace sample) was prepared by the same procedure, but without microwave drying by direct burning in a furnace. To carry out the time−temperature investigation, we placed precursors in a form of dried gel into a preheated 500 ± 15 °C open vertical muffle furnace in special alumina crucible with a K-type thermocouple. The output signal of the thermocouple was collected by a data acquisition system. The SCS in all cases was carried out according to the scheme shown in Figure 1. Thermal Analysis. Thermal evaluation of SCS of experimental systems was conducted by simultaneous thermal gravimetric analysis (DTA-TG) (NETZSCH STA 449 F3 Jupiter, equipped with Fouriertransform infrared spectrometer (FTIR) (Bruker Alpha). For DTATG-FTIR samples in a form of a dried gel were prepared. Twenty milligrams of powder was heated in air atmosphere, at 10 °C per minute up to 700 °C. Characterization. The phase composition and structure of the obtained combustion products was characterized by X-ray analysis using Bruker D8 ADVANCE diffractometer with a rotating copper anode CuKα radiation. The reference data was used from the PDF2 database. Rietveld refinements were conducted with the software HighScore Plus. Pseudo-Voigt function was used for the peak profile refinement. To determine the size of crystalline blocks, we used the Scherrer equation. A study by electron microscopy was performed by means of the scanning electron microscope Leo-1450 (Carl Zeiss, Germany). Transmission Electron Microscopy (TEM) images were taken on a FEI Titan Thermis 200 series operating at 300 keV and
EXPERIMENTAL SECTION
Starting Materials. All chemicals were purchased from SigmaAldrich unless stated otherwise. Cobalt nitrate hydrate (Co(NO3)2· 6H2O, Alfa Aesar, 98%) was used as a metal precursor, and urea (CH4N2O, U, 98.6%), citric acid hydrate (C6H8O7·H2O, CA, 99%), glycine (C 2 H 5 NO 2 , G, 98.5%), and hexamethylenetetramine (C6H12N4, HMT, 99.2%) were used as fuel components. Calculation. Thermodynamic calculation (TC) traditionally plays an important role in chemical technology for studying and development of many processes related to the synthesis of advanced materials.40 In the area of combustion synthesis, TC is used for estimating the adiabatic reaction temperature (Tad) and predicting the phase composition of products, which in some cases permits evaluating the interaction mechanism and modifying the synthesis conditions. TC was performed for all experimental systems over a wide range of reducer-to-oxidizer ratios φ from 0.8 to 3.0 by the same way as described in.41,42 Synthetic Procedures. Co NP samples were synthesized by modified SCS method using mixture of a metal precursor/oxidizer with a fuel. The SCS reactions take place according to the schemes suggested in eqs 1−4: Co(NO3)2 + (2φ)CH4N2O + 3(φ − 1)O2 = Co + (2φ)CO2 + (2φ + 1)N2 + (4φ)H 2O
(1)
⎛2 ⎞ ⎜ φ⎟C H O + 3(φ − 1)O 2 ⎝3 ⎠ 6 8 7 ⎛8 ⎞ = Co + (4φ)CO2 + N2 + ⎜ φ⎟H 2O ⎝3 ⎠
(2)
⎛4 ⎞ ⎜ φ⎟C C NO + 3(φ − 1)O 2 ⎝3 ⎠ 2 5 2 ⎛8 ⎞ ⎛ 2φ + 3 ⎞ ⎛ 10 ⎞ ⎟N + ⎜ = Co + ⎜ φ⎟CO2 + ⎜ φ⎟H O ⎝3 ⎠ ⎝ 3 ⎠ 2 ⎝3 ⎠ 2
(3)
(4)
Co(NO3)2 +
Co(NO3)2 +
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Inorganic Chemistry with Jeol JEM-2100, 200 kV LaB6 instruments. Samples for TEM were prepared by deposition and drying of a drop (10 μL) of the material dispersed in an appropriate solvent (water, ethanol, isopropanol) onto a carbon-coated 400 mesh cooper grid. The magnetization measurements were carried out using a homemade vibrating sample magnetometer (VSM) at room temperature with field applied up to 104 Oe. The dried sample was weighed, wrapped in paper and placed in a hexagonal PVC holder. The sample was set within a uniform magnetic field and mechanically vibrated between a series of coils. A changing magnetic flux was generated which induced a voltage in the sensing coils. The amplitude of the sinusoidal voltage is proportional to the magnetic moment of the sample. The VSM was calibrated using a nickel sample of known mass. Nickel is a ferromagnetic material with a magnetic moment of 55.4 emu/g at 104 Oe at room temperature.
Results of the DTA-TG-FTIR analysis of the experimental systems are shown in Figure 3. On the basis of DTA-TG-FTIR data analysis, we divided the process into four stages: drying, ignition, afterburning, and oxidation. At the first stage, residual solvent is evaporated. Here, only ∼3−7% weight reduction, depending on fuel used, was detected on TG curves (Figures 3a, c, e, g). During the “ignition” stage, fuels and nitrates start to decompose with gas evolution (Figure 3b, d, f, h) and initiate combustion reaction propagation. Solid phases start to form and mostly finish at this stage. At the third stage, the decomposition of residual nitrates and fuels occurs. To the end of this stage, the formation of the solid phases are finished. The next stage “oxidation” is connected with residual fuel decomposition and Co NPs reoxidation. According to TG curves at the end of the stage IV in all experimental systems there were an observation of the weight increase from 0.04 to 0.37%. It should be taken into account that at the same time, the gas evaluation at IR temperature plots (Figure 3b, d, f, h ) was detected, which compensates the real weight increase value (i.e., Co, Co3O4, and Co2O3 oxidation). Cobalt nitrate decomposition occurs during the synthesis process in all experimental systems with the formation of cobalt oxides (Co2O3 and Co3O4)45 solid phase and nitrogen oxides. The reduction of cobalt oxide particles by gaseous NH3, CO, etc., occurs. Process details in each case depend on type of fuel: its chemical composition and features of decomposition process. In the case of the cobalt nitrate−glycine system, the exothermic reaction starts at 171 °C, when the decomposition of glycine and Co(NO3)2 takes place.44,46 The main resultant products of this process are CoO, CO2 and CO. Moreover, weak peaks of NH3 and N2O are also detected (Figure 3b). Combustion reaction is promoted by exothermic reaction in nitrogen oxides-ammonia mixture. At the stage II (Figure 3a) joint cobalt nitrate and fuel decomposition gives the highest weight reduction (∼43.5%). During this stage, mixture of combustible gases accumulates in reaction volume, promote intensification of heat and mass exchange and, finally, starts to burn at the “afterburning” stage at ∼290 °C, which gives rapid increase in the DTA curve (Figure 3a). During stage III, the fuel continues to decompose, giving 27.5% weight reduction. The combustion process in the cobalt nitrate−CA system is more complex.47,48 The mixture of aconitic, itaconic, formic and acetonedicarboxylic acids forms during CA decomposition. After that, this combination decomposes with the formation of CO2 and a mixture of anhydrides. The latter then decomposes too. According to the IR−temperature plot (Figure 3d), only CO2 peaks were observed during the reaction. The weight loss during the stages II and III was almost the same: ∼29 and 33%, respectively (Figure 3c). However, the DTA signal at the stage III rapidly increases and has much higher absolute value. This can be attributed to ignition of nitrogen oxides, formed during cobalt nitrate decomposition and accumulated in the reaction volume. During the combustion, nitrogen oxides were reduced to N2, and for this reason they were not detected. It is known37 that when metal nitrate−urea gel is heated rapidly, it undergoes melting and dehydration. After that, it decomposes with the formation of such products as urea nitrate, biuret and mixture of gaseous nitrogen oxides, HNCO and NH3. This mixture is known to be hypergolic. The foam is
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RESULTS AND DISCUSSION Co NP samples were synthesized by our modified SCS method using cobalt nitrate hydrate as a metal precursor and urea, citric acid hydrate, glycine and hexamethylenetetramine as fuels. The fuel to oxidizer ratio φ = 2 was chosen on the basis of TC (Figures S1−S5) in considering of calculated phase compositions (i.e., maximum yield of Co phase and optimal yield and composition of reducing gases) and Tad, that should be high enough to finish SCS in a maximum short time, but low enough to keep Co in solid state without melting and prevent intensive grain growth. According to TC, φ value 2 allows preparing Co without cobalt oxides secondary phases in all cases. And finally, at chosen synthesis conditions amount and composition of gases released during synthesis should be sufficient to intensify mass and heat exchange and create inert or even reducing atmosphere over the reacting volume. According to data in Figure 2, the calculated adiabatic combustion temperature of the charges lies in the range from
Figure 2. Adiabatic temperatures Tad and νgas/νCo ratio as a function of a fuels type.
480 to 1280 °C: the largest one is for the HMT-sample, and the lowest one is for the U sample. However, as usual in the actual synthesis process, the combustion temperature is significantly lower because of large heat losses, which are related to the formation of a large amount of hot gases and heat transfer to environment. The calculated equilibrium compositions of the synthesized products suggest that under the selected thermodynamic conditions the formation of pure metallic Co does not present any special difficulties and is virtually complete. It can be noticed that the calculated Tad, increases with an appropriate rise in the reducing capability of fuels. 1466
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Figure 3. (a, c, e, g) TG-DTA and (b, d, f, h) IR-temperature plots of SCS in different Co(NO3)2−fuel systems Fuels: (a, b) G, (c, d) CA, (e, f) U, (g, h) HMT.
°C, the foam breaks out with a flame because of the accumulation of the hypergolic mixture of gases gives ∼68% weight reduction. As a result, the foam decomposes with the formation of the mixture of water steam, NH3, CO, CO2, and a solid product. At stage III, there is only a weak increase in DTA
forming during this process. The foam could be made up of polymers like cyanuric acid, polymeric nitrate, etc., which are combustible. The stage II starts after solvent is evaporated at 150 °C (Figure 3e). After temperature increases to specific value 228 1467
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Figure 4. (a) TG-DTA plot of SCS in Co(NO3)2−HMT system in N2 atmosphere and (b) comparison of TG curves of “oxidation” stage of Co(NO3)2−HMT system in air and N2 atmospheres.
signal and a small weight reduction (∼3.4%). Obviously, that combustion reaction is almost completed at stage II. In the cobalt nitrate−HMT system, the one-step decomposition of HMT results in the formation of nitrogen and carbon monoxides, carbon dioxide, and ammonia.49,50 The release of a high amount of gases promotes the foam formation. After the accumulation of the gas mixture the highly exothermic reaction occurs at Tig = 184 °C gives ∼64% weight reduction (Figure 3g). During stage III, decomposition of residual fuel and ignition of residual combustible gas mixture occurs, giving only ∼5.6% weight reduction. To compare and test our results, we carried out the same TG-DTA measurements in the Co(NO3)2−HMT system (as the best case, shown later) in N2 atmosphere (purge rate is 100 mL/min) (Figure 4). The nitrogen was chosen because its thermal conductivity, which is very close to thermal conductivity of air. It can be clearly seen, that there are only three stages of SCS on TG-DTA plot: drying, ignition and afterburning (Figure 4a). At the first stage, small (4.41%) mass reduction was found. The second stage of SCS begins at almost the same temperature in both cases in air and in N2 (156 and 152 °C respectively). There is the highest and close mass reduction at this stage (64.10 and 61.60% for SCS in air and in nitrogen, respectively) because of decomposition of cobalt nitrate and major part of the fuel. The main feature of the second stage for SCS in N2 is it ends at 270 °C, which is 23 °C higher than in the air. Moreover, the third stage is also much longer in the case of nitrogen than in air. Its active part ends at ∼520 °C, which is 151 °C higher than in the air, but after that, the afterburning continues in a smoldering mode. If we compare the same temperature sections of SCS of Co(NO3)2−HMT system in air (the “oxidation” stage) and in N2 (Figure 4b), it is clear that there is mass increase for curve in air, which can be attributed only to Co oxidation. In its turn, in case of N2 there is a small mass reduction during this section, which can be due to decomposition of minor part of residual fuel. Thus, we can conclude that air intensifies SCS by acceleration of redox reaction between cobalt nitrate and a fuel and promotes more complete fuel decomposition and removal at lower temperature, and that it can be achieved under inert atmosphere. From the previous data it is clear that SCS processes are complex and a sufficiently high temperature is required for completion of combustion reactions and removing of fuels’
traces, which can result in Co NPs reoxidation. Figure 5 shows normalized Gram−-Schmidt plot of SCS in different Co(NO3)2−fuel systems.51
Figure 5. Normalized Gram−Schmidt plot of SCS in different Co(NO3)2−fuel systems.
Obviously, that all combustion reactions are completed at ∼450 °C and only residual fuel decomposition occurs. Thus, we chose temperature value of 500 °C to carry out actual SCS of Co NPs with the intention of intensification of fuels and nitrate decomposition and acceleration of combustion reaction. However, according to the previous data (Figure 3b, d, f, g) at these synthesis conditions, there is a CO2 protective atmosphere over the reacting volume, which can prevent Co NP’s reoxidation. Experimental temperature−time profiles of SCS in Co(NO3)2−fuel systems are shown in Figure 6. There are the same four stages can be allocated on the curves. They are shifted and smoothed in comparison with TG-DTA curves because of intensification of the combustion reaction. The form of time−temperature profiles and specific combustion temperatures of different samples depend on the type of the fuel and features of precursor’s decomposition process. When the cobalt nitrate and fuel gel were preheated to the temperature of ∼100 °C, an intensive evaporation of water occurs in all cases. In the case of cobalt nitrate−glycine system the decomposition of glycine and Co(NO3)2 starts at Tig = 120 °C (Figure 6a) in extremely exothermic mode. The transition from stage II to stage III can be clearly seen at ∼393 °C. 1468
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Figure 6. Time−temperature curves of SCS in Co(NO3)2−fuel systems. Fuels: (a) G, (b) CA, (c) U, (d) HMT.
The actual SCS of Co NP was carried out with the microwave assistance and the foam was previously obtained (in the case of usual drying, the foam does not form) in all cases and the decomposition of precursors was more rapid. The combustion process was started in 10−30 s in a whole volume of the experimental system. Figure 7 shows the Rietveld refined XRD patterns of the synthesized Co-based materials in systems with different types of the fuel. According to the data the metallic Co phase was obtained in the cases where G, U, and HMT were used. Nevertheless, only the powder where HMT was used is characterized as a pure metallic Co in face-centered cubic (fcc) polymorph (Fm3m space group). In the cases where CA was used a mixture of CoO and Co3O4 (57 and 43%, respectively) was detected as only phases. The use of urea and glycine as fuels lead to the formation of a mixture of Co, CoO and Co3O4 crystal phases where Co is minor phase (Co content in U and G samples are 11 and 6%, respectively). For comparison of nonmodified and the modified microwave assisted SCS methods, XRD analysis and XRD profile refinement of HMT-furnace sample were performed (Figure 7). The major diffraction peaks were indexed to cubic Co phase (56%) with space group Fm3̅m. Moreover, diffraction peaks of minor CoO (38%) and Co3O4 (6%) phases were also detected for this sample. It is evident, that the using of nonmodified SCS method does not yield pure Co mNPs under normal air atmosphere without additional post reduction. In the case of an excess of fuel (φ>1), i.e., urea, glycine, and hexamethylenetetramine, there is a corresponding excess of NH3 species in the combustion wave. Moreover, ammonia aqueous solution was used for pH regulation. Ammonia reacts with the metal oxide phase formed during the decomposition of nitrate, reducing it to pure metal. As usual, metallic cobalt is not
In the case of the CA sample only a weak change in the temperature gradient at 107 °C was detected. After this point the relatively rapid temperature change was observed. However, the exothermic process did not occur and the time− temperature profile had no any strongly pronounced temperature peak. Due to a large content of carbon in CA composition the exothermic reaction switch to a smoldering mode and reaction has no expressed exothermic effect (Figure 6b). In cobalt nitrate−urea system the temperature increase on time−temperature-profile at ∼140 °C (Figure 6c) can be attributed to completion of liquid water evaporation and start of polymer’s boiling and decomposition process at higher temperature. After temperature increases to specific value 225 °C, the foam breaks out with a flame because of the accumulation of the hypergolic mixture of gases. As a result, the foam decomposes with the formation of the mixture of water steam, N2, CO, CO2, and solid product. The stage III is very weak and short. The form of time−temperature maximum in these three cases is sharp, which is connected with the rapid completion of the combustion process. In samples where HMT was used charge preheated to some specific temperature Tig = 115 °C after which, a rapid temperature increase takes place. This indicates the initiation of the combustion reaction (Figure 6d). The SCS reaction was very intensive and resulted in the higher maximum combustion temperature (Tmax ≈ 590 °C). The combustion of all precursors was associated with an intensive foaming of the gel, due to the release of a large amount of gaseous products. It can be seen that the maximum reaction temperature Tmax reaches ∼480, 366, and 590 °C for G, U, and HMT samples, respectively. In the case of CA sample Tmax (520 °C) was a bit higher than the temperature in furnace. 1469
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gaseous mixture. We suppose that the fuel with the higher reducing capability, e.g., HMT, promotes more full reduction of CoO particles that were formed during cobalt nitrate thermal decomposition. Moreover, in the case of using the microwaveassisted prepared foam, the decomposition processes during synthesis of different compounds occur in a very short period of time. Therefore, as a combustion process starts, very reactive CoO particles are reduced by hot gases (NH3, etc.) at once. In addition, there is a reducing atmosphere of a mixture of NH3 and resultant gases, such as CO, N2, and CO2, over the solid product and, because of the high porous foam, in the whole volume of the reaction mixture. Calculated average grain size values of the Co phase of the HMT sample is 20.3 nm. So, the good crystalline metallic Cu NP were produced by SCS technique in a normal air atmosphere without post reduction. Figure 8 shows TG curves of oxidation of freshly synthesized (Cof) and 7 days exposured in air (Coe) Co NPs. It can be seen
Figure 8. TG-temperature curves of freshly synthesized and 7 days exposured in air Co NPs oxidation.
Figure 7. Rietveld refined XRD patterns of SCS products.
stable at high temperature in the normal air atmosphere and metal oxidation occurs. We suppose there are two main factors enabling us to prepare metallic Co NP. First of all, the microwave-assisted rapid drying was used for foam preparation. In this case, the foam was a precursor that contains nondecomposed initial components. The latter are well-mixed in the atomic scale. According to our experience, this foam is more reactive than the foam, which is prepared by conventional drying (i.e., it starts to combust easily under the same conditions). As was noticed earlier, in the case of such foam, the combustion process started in a whole volume of the experimental system approximately at the same time and finished rapidly. There is no necessity in a long-time, high-temperature treatment for removal of residual fuel, and after the combustion process is completed, the obtained powder could be taken from the furnace and cooled rapidly. In the case of nonmodified SCS technique, it is very difficult to get foam with high specific surface in all reaction volumes simultaneously without starting the combustion process in a part of it. As the combustion process is slower, in this case, the additional time to complete solid phase formation and to remove residual fuel is needed. As a result, freshly synthesized Co NPs stay longer at hightemperature and this promotes Co reoxidation. The second factor is related to the precursor’s composition. We used fuels with different reducing capabilities and different natures, which influence the resultant composition of the
that Co NPs start to oxidize at once after heating begins. However, there is a segment up to ∼320 °C where the oxidation process is slow enough (∼3%). After this specific temperature, the gradient of mass change increases sharply and reach maximum (i.e., full Co oxidation) at ∼780 °C. On XRD patterns of the oxidized Co NPs (Figure S6) only peaks of Co3O4 were detected. According to eq 5: 3Co + 2O2 = Co3O4
(5)
The maximum theoretically achievable degree of weight increase in case of full Co oxidation is 36.16% ((MrCo3O4/ nMrCo − 1)100, where MrCo3O4 is molar mass of Co3O4, MrCo is molar mass of Co, n is molar ratio of Co atoms in oxide to metal). In the case of the Cof sample, 35.85% mass increase was detected. This means that experimental sample was almost pure Co (99.16%). In case of Coe powder, temperature of start of fast oxidation stage is shifted on ∼4° to 321 °C and maximal mass change achieves 35.54%, which means the Coe sample contains 98.29% Co and 1.71% is cobalt oxide. The room-temperature oxidation of Co NPs in air connected with its high reactivity. However, formation of thin oxidized layer on surface of the Co NPs promotes its passivation and prevent subsequent room temperature oxidation. Typical SEM and TEM images of the synthesized Co NP of HMT sample are shown in Figures 9 and 10, respectively. 1470
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Figure 9. SEM images of the Co NP in HMT sample.
Moreover, the high combustion temperature promotes powder agglomeration and crystal grains growth. As the maxima of combustion temperature in HMT-sample was high (Figure 6 d), there are medium- and large-scale agglomerates could be found in the final Co powder. Transmission electron microscopy (Figure 10) has shown a number of aggregates containing some small nanoparticles with the broad size distribution (∼5−40 nm), some nanoflakes, nanoroads, and spherical particles with a high degree of shape anisotropy. It could be noticed, that prepared Co nanoparticles are not single domain particles. The domain walls of anisotropically orientated domains of separated particles are clearly seen (Figures 10b, c). Obviously, the particles shape and size inhomogeneity is typical for combustion syntheses as well as the presence of flakelike structures. The sample of nanoparticles have been characterized using vibrating sample magnetometry (VSM) (Figure 11). Magnetization measurements revealed that these nanomaterials have a quite high saturation magnetization of 137 emu/g at 1 × 104 Oe and an anomalously high for Co NPs coercivity of 370 Oe. It should be noticed that the saturation magnetization of the sample is still lower than those of their bulk cobalt counterpart (168 emu/g). The lower Ms may be attributed to size effects and the formation of hierarchical structure, which result in a larger surface/volume ratio. The larger surface/volume ratio leads to enhanced spin disorder, and thus the total magnetic moment is significantly reduced. On the other hand, the shape and magnetostructural anisotropy prevents the grains from magnetizing in directions other than along their easy magnetic axes, leading to sample’s higher coercivity. Moreover, in the hysteresis curve a clear shift of 44 Oe can be observed. The latter could be explained by a weak exchange bias effect,52 promoting coercivity increase too.
Figure 10. TEM images of the Co NPs in HMT sample.
According to SEM (Figure 9), a great number of porous flakelike agglomerates are present in the microstructure of obtained Co powder. The results of TC (Figure 2) indicate that 7.64 mol of gases per mole of metallic Co were released for HMT sample. As is known,37 a high amount of gas evolution during combustion process affects the porosity of solid products. 1471
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of Belarusian Republican Foundation for Fundamental Research ( research project X17PM-032), Russian Foundation for Basic Research (project 17-53-04010 Bel-mol-a) and EU FP7 (FutureNanoNeeds) Marie Curie ITN grant.
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