Characterization of Bimetallic Fe-Ru Oxide Nanoparticles Prepared by Liquid-Phase Plasma Method
© The Author(s). 2016
Received: 31 May 2016
Accepted: 16 July 2016
Published: 26 July 2016
The bimetallic Fe-Ru oxide nanoparticles were synthesized in the liquid-phase plasma (LPP) method which employed iron chloride and ruthenium chloride as precursors. The active species (OH·, Hα, Hβ, and OI) and the iron and ruthenium ions were observed in the plasma field created by the LPP process. The spherical-shaped bimetallic Fe-Ru oxide nanoparticles were synthesized by the LPP reaction, and the size of the particles was growing along with the progression of the LPP reaction. The synthesized bimetallic Fe-Ru oxide nanoparticles were comprised of Fe2O3, Fe3O4, RuO, and RuO2. Ruthenium had a higher reduction potential than iron and resulted in higher ruthenium composition in the synthesized bimetallic nanoparticles. The control of the molar ratio of the precursors in the reactant solution was found to be employed as a means to control the composition of the elements in bimetallic nanoparticles.
Bimetallic catalysts have been studied by many researchers, and as a result of such studies, they have been applied to many industrial catalytic processes [1, 2]. In general, the bimetallic catalysts have higher performance than the monometallic catalysts [3, 4]. Among diverse bimetallic catalysts developed so far, the Fe-Ru catalysts are the representative alloy system successfully used in the Fischer-Tropsch synthesis . The Fe-Pt catalysts have been known since their methanol activity and selectivity would be changed by the varied proportion in the alloy phase . In addition, the increase of Fe content in zeolite-supported Pd catalysts has been known that it could increase the rate of methanol formation . In this way, the bimetallic catalysts of diverse characteristics can be produced by varying the composition of the second metal component.
On the other hand, the ruthenium oxide (RuO2) nanoparticles are added to carbonaceous materials as an electrode material to improve the performance of electrochemical capacitor [8–10]. However, owing to the price of the expensive RuO2, the cheaper iron oxide nanoparticles are occasionally added thereto instead . Therefore, if the manufacturing of Fe-Ru bimetallic nanoparticles would be enabled, then it would be presumable that they could be applicable as an electrode material of the electrochemical capacitor.
Recently, the liquid-phase plasma (LPP) process which enabled to control the size and morphology of particles has been spotlighted as a way to produce nanoparticles [12, 13]. The LPP process can produce diverse metals and metal oxide nanoparticles simply by the reduction reaction using electrons and ions which are to be generated in an aqueous solution by plasma . In our previous studies, the cases of successful synthesis of diverse metal nanoparticles including iron nanoparticles through the use of LPP process are reported [15–17]. In addition, the result of improved performance of the electrochemical capacitor electrode produced by employing the nanoparticles synthesized through the LPP process is also reported .
In this study, the synthesis of bimetallic Fe-Ru oxide nanoparticles using LPP is reported as a basic advanced study for the application of such nanoparticles to bimetallic catalyst and electrochemical capacitor electrode. The influence of LPP process parameters on the size, morphology, and chemical composition of bimetallic nanoparticles was thus also examined. In addition, the chemical and physical properties of the synthesized bimetallic nanoparticles were analyzed by using several kinds of instrumental analysis.
Materials and Experimental Equipment
Iron chloride tetrahydrate (FeCl2·4H2O; Kanto Chemical Co.) and ruthenium chloride hydrate (RuCl3∙XH2O, 45 wt% Ru; Sigma-Aldrich) were used in this study as precursors of Fe metal and Ru metal, respectively. To prevent the coagulation of the particles created by the LPP reaction in the aqueous solution, cetyltrimethylammonium bromide (CTAB; CH3(CH2)15 N(CH3)3Br; Daejung Chemicals & Metals) was used as a dispersant. Ultrapure water was employed in this study as a solvent for all applications. The LPP system exploited the power supply (Nano Technology Inc.; NTI-500 W) of high-frequency bipolar pulse which was also used in this study to produce particles from the precursors. The employed LPP system was identical to the one employed in our previous studies [15–17] from which the details of the employed LPP system can be referred to. The conditions of the voltage, frequency, and pulse width set for the creation of plasma were fixed as 250 V, 30 kHz, and 5 μ, respectively.
Preparation of Composite
The aqueous reaction solution employed to produce the Fe-Ru oxide bimetallic nanoparticles from the LPP reaction was prepared by following the ways. The aqueous solution of pH 2 was prepared by adding 0.1 N HCl to ultrapure water, and the ruthenium chloride and iron chloride were added thereto additionally with ratios of 1:9, 1:4, and 2:3, respectively, to attain a concentration 5 mM of the total metal precursor. And further, CTAB with a 40 % molar ratio with respect to the total precursor quantity (2 mM) was added and then agitated to dissolve it completely. The amount of final aqueous reaction solution used for the test conducted in this study was 300 mL. The prepared aqueous reaction solution was then put into the LPP reactor to induce LPP reaction to generate the Fe-Ru oxide nanoparticles in the solution.
Several chemical active species generated in the Fe-Ru chloride reactant solution by the LPP discharge were observed by using the optical emission spectrometer (OES; Avantes). And the pH of aqueous reaction solution varied by the LPP reaction was measured by using a pH meter (HM-30R; TOA-DKK). The morphology, size, crystal structure, and lattice of the particles generated from metal precursors by reduction were observed through the high-resolution field-emission transmission electron microscope (HR-FETEM; JEM-2100F; JEOL Ltd.). The composition of the elements comprised in the bimetallic nanoparticles was analyzed by using the energy-dispersive spectroscopy (EDS) attached to the HR-FETEM. And to look into the chemical state of iron and ruthenium which comprised the particles, the high-resolution X-ray photoelectron spectroscopy (HR-XPS; Multilab 2000 system; SSK) was used.
Results and Discussion
Optical Emission Spectra
Effect of LPP Reaction Duration
Chemical and Physical Properties of Nanoparticles
Effect of pH
Effect of Precursor Molar Ratio
In case (a, 1:9) the content of iron precursor is richer than that of ruthenium precursor in the reactant solution, the synthesis of bimetallic Fe-Ru oxide particles of which the level of iron composition is higher than that of ruthenium can be identified from the yellow dots representing an iron. In the case of the molar ratio between ruthenium precursor and iron precursor was 1:2 (c), the synthesis of bimetallic Fe-Ru oxide particles of which the level of ruthenium composition is higher than that of iron can be identified from the relative abundance as the green dots representing ruthenium. Besides, the white dots representing the oxygen element in the synthesized particles were observed from all the cases of the respective conditions; however, the amount of white dots tended to decrease in accordance with the increase of the constituent of ruthenium.
Chemical composition of the bimetallic Fe-Ru oxide nanoparticles as a different molar ratio of precursor
Thus, the higher reduction potential and lower ionization tendency of ruthenium than those of iron were identified. Owing to these properties, the ruthenium ions generated by the LPP reaction were reduced ahead of others and thereby rendered higher composition of ruthenium in the synthesized bimetallic nanoparticles. In the meantime, all the bimetallic nanoparticles synthesized under every condition showed that they were containing oxygen elements. This was estimated that it could be attributable to the oxidation of metal particles by active oxidizing species (1O2, O− 2, O•, OH•, HO2, H2O2, and O3) which were generated by the LPP reaction. In our previous study , the prevention of an oxidation of iron metal particles into iron oxide particles by ethanol employed as a solvent instead of water for the preparation of reactant solution was identified. And accordingly, the possible preparation of bimetallic Fe-Ru nanoparticles by an application of ethanol as a solvent can also be presumable. However, in this study, the application of ethanol as a solvent was not employed for the synthesis of bimetallic Fe-Ru oxide nanoparticles by taking the usage of Fe-Ru oxide as an electrode material of electrochemical capacitor into account. The preparation of bimetallic Fe-Ru nanoparticles through the use of LPP process will be considered in our future study. In addition, through results obtained from the EDX analysis, the adjustment of molar ratio between precursors contained in the reactant solution was identified that it could change the composition of the elements comprising bimetallic nanoparticles accordingly.
In this study, the bimetallic Fe-Ru oxide nanoparticles were synthesized through the LPP process. The iron chloride and ruthenium chloride were employed as precursors to produce the reactant aqueous solution of which pH became lower along with the progression of LPP reaction. And in the plasma field created by the LPP reaction, the active species (OH radical, Hα, Hβ and OI), iron ions, and ruthenium ions were observed. The spherical-shaped bimetallic Fe-Ru oxide nanoparticles of the size ranged from 5 to 80 nm were synthesized by the LPP reaction where the size was growing along with the progression of LPP reaction. The synthesized bimetallic Fe-Ru oxide nanoparticles are thereby comprised iron oxides (Fe2O3 and Fe3O4), metallic Ru (Ru0), and anhydrous ruthenium oxide (RuO2). The molar ratio between precursors contained in reactant solution would be influential on chemical composition of the constituents in bimetallic Fe-Ru oxide nanoparticles; however, the ruthenium ions were reduced ahead of iron ions owing to its comparatively higher reduction potential. And the adjustment of the molar ratio among precursors contained in the reactant solution was found that it could control the composition of the elements which are comprised in the bimetallic nanoparticles.
This work was supported by the Technology Innovation Program (10050391, development of carbon-based electrode materials with 2000 m2/g grade surface area for energy storage device) funded by the Ministry of Trade, Industry & Energy (MI, Korea).
LSJ and LH carried out the main part of experiment and drafted the manuscript. JKJ measured and analyzed the optical emission spectra. PH measured and analyzed the X-ray photoelectron spectroscopy. PYK provided assistance with the data analysis and investigated the relationship between variables and results. JSC coordinated the experimental design and contributed the manuscript writing. All the authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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