Properties and Catalytic Effects of Nanoparticles Synthesized by Levitational Gas Condensation

Abstract

One of the gas-phased methods of the levitational gas condensation (LGC) process was developed to obtain nanopowders with high purity. The instrument designed by unique concept using magnetically levitated melted droplet of metal is easily operated to synthesize nanopowder with highly defected surface. The complex compounds are also easily prepared using micron powder feeding (MPF) system in the instrument. The metals, ceramics, and carbon-coated metal nanoparticles prepared using the LGC are introduced in this chapter. Various applications such as magnetic and catalytic properties are also introduced. Nanoparticles prepared using LGC showed significantly enhanced catalytic activities during chemical reaction due to the high level of defects on their surface structure. The new heterogeneous catalysts of the solid nanoparticles were introduced in this chapter.

1. Introduction

Among the various methods for preparing nanopowders, almost all of the processes face important challenges, such as poor control of size distribution, surface contamination, the agglomeration of the particles, and so on [1, 2, 3, 4, 5]. Many attempts have been made to develop processes and techniques that can synthesize nanoparticles with specific functional properties [4, 5, 6]. Dry methods such as the levitational gas condensation (LGC) process have been developed to obtain high-purity nanopowders while suppressing the agglomeration of the produced particles [7, 8, 9, 10, 11, 12, 13, 14, 15]. The catalytic effects of nanopowders are influenced not only by the reduced size of the particles but also by their increased surface area [16, 17]. The surface of metal oxides exhibits nonstoichiometry, resulting from oxygen defect structures [17]. Particles prepared by the LGC process show enhanced catalytic activities due to the high level of defects on their surface structure. In this chapter, the synthesis processes and resulting properties of various nanoparticles prepared by LGC are introduced. The sections are focused on three aspects:

1. Levitational gas condensation (LGC): The unique instrument used for the synthesis of the nanoparticles is introduced in this section [18, 19, 20].

2. Magnetic metal and carbon-encapsulated metal nanoparticles: The produced magnetic metal (Ni and Fe) and carbon-encapsulated metal (Ni@C and Fe@C) nanoparticles showed a noncollinear magnetic structure between the core and surface layer of the particles. The morphologies and the dispersion stability kinetics in the solvents are introduced. Also, the carbon-encapsulated metal nanoparticles were successfully applied as a catalyst for the multicomponent Biginelli reaction [7, 9, 12, 18, 19, 20, 21, 22, 23].

3. Nonmagnetic metal and metal oxides: Cu oxides, Bi, and NiO nanopowders prepared by LGC are introduced. Cu oxide and NiO alloy nanopowders are widely applied as heterogeneous catalysts in oxidizing processes used in organic synthesis. Nanopowders of bismuth (Bi) with its low melting temperature were applied as a sensor electrode for detecting heavy metals in water [9, 14, 17, 24, 25].

2. Levitational gas condensation (LGC)

The LGC method is a kind of gas-phased method in which an electric current is flowed to two inductor coils, which are each wound opposite directions. The electric current following in different direction in each coil induces a magnetic field and creates a magnetic moment, which opposes gravity on the inside lower part of the coil [11, 18]. To synthesize a nanopowder, a melted metal is continuously evaporated and condensed in the levitated condition, suspended in the magnetic field. This method is shown in Figure 1(a). In this study, we modified the inductor with a downward spiral type of coil, so that the levitation region produced by the magnetic field would be more stable for the melted droplet by generating an equivalent magnetic flux density, as shown in Figure 1(b).

Figure 1.

(a) The mechanism of forming nanoparticles and the contour maps of magnetic flux density depending on shape of inductors for a cylinder type and (b) a spiral type and its strength at in-plane (a~b) and vertical (c~d) direction [11].

The total LGC system is illustrated in Figure 2. The nanoparticles formed at the surface of the liquid droplet are then flowed to a filter by the gas stream using a vacuum pump:

Figure 2.

The concept of system for LGC.

2.1. Levitational gas condensation (LGC) starting materials with a melting temperature of over 900°C

Metallic atoms were evaporated from an overheated surface and condensed by cold inert gas and then collected from the filter. To stabilize the powder surface and prohibit oxidation, the powders were passivated with thin oxide layers. The LGC apparatus consists of a high-frequency induction generator, levitation and evaporation chamber, and oxygen concentration control unit. The operating values used for the induction generator were 6, 5, 4.5, and 3 kW for the Ni, Fe, Cu, and Ag, respectively [7, 8, 9, 10, 11, 12, 13, 14, 15]. These values depended on the melting temperature as well as the magnetic permeability of the metals. The melting temperature of iron (1535°C) is higher than that of Ni (1450°C). However, the high magnetic permeability of Ni affects the magnetic force and levitation of the melted droplets. Accordingly, the input power for Ni needs to be increased up to 6 kW, the maximum power for the inductor. Preparing pure Ti nanopowders using the LGC is impossible. To do so, the temperature of the inductor must be increased up to 2000°C; however, the starting materials have to be very thin and strongly passivated by a layer of titanium oxide. The seed materials need to be fully melted before levitation, because only liquid seeds can be suspended in the inductor, due to their lower density. However, it takes too long time to melt the Ti seed to produce liquid droplet for levitation.

We also supplied the starting materials for the melted liquid droplet during synthesis. We utilized a metal wire feed system, which is very convenient for fabricating nanopowder continuously. The amount of material fed over time can be controlled. The average size of the nanopowder is increased with increasing feed speed because of the increased amount of material introduced to the liquid droplet. The optimal feeding speed is between 10 and 30 mm/s during fabrication. If the feeding speed is very slow, below 10 mm/s, the size of the liquid droplets decreased, and they disappear. In contrast, if the feeing speed is increased to over 30 mm/s, the prepared powders have a wire shape because the particles are connected with each other. By adding oxygen to the inert gas, metal oxide particles or metal particles oxidized on the surface may be obtained. Figure 3 shows metal and metal oxide particles prepared by LGC. The gas condensation (GC) method was used to obtain nanocrystalline powders of pure metal and nano-oxides with different compositions. Oxide nanopowders, such as Fe₃O₄, γ-Fe₂O₃, and Cu₂O, were produced by the LGC of metal wires at elevated pressure in an (Ar + O₂) atmosphere [8, 16].

Figure 3.

TEM images for metals and ceramic nanopowders.

High-purity Ni@C and Fe@C nanopowders were synthesized using the LGC method. The LGC apparatus consisted of a high-frequency induction generator, operating at 6 kW for Ni and 5 kW for Fe, a reaction chamber for the levitated liquid seed, and a unit to control the methane (CH₄) concentration. The starting materials were Ni and Fe wires with a diameter of 4 mm. The Ni and Fe wires were fed into a melted droplet using the wire feeding system at a feeding rate of 2 mm/min. An ingot of 85 mg, which was used as the seed material for the levitation and the evaporation reactions, was melted by using electric induction. The pressure of the mixed Ar and CH₄ gas in the chamber was 100 Torr. CH₄ was 10% of the mixture gas. The inductor was heated up to a temperature of 2000°C, and the metallic atoms were evaporated from the overheated surface of the liquid droplet and condensed by cold inert gas and then collected into the filter. At the same time, the molecular CH introduced into the chamber was converted to atomic C and H with high activity under high temperature. The highly active C atoms react with the Ni and Fe atoms, and the H atoms are converted to H molecules. The newly created H gas is vented out of the reaction chamber by continuous vacuum operation. The results indicated that all of the as-made materials were composed of nanocapsules with uniform particle size at and below 10 nm. The nanocapsules consisted of outer multi-shells of carbon [26, 27, 28].

2.2. Starting materials with a melting temperature below 900°C: Zn, Sn, Bi, and ZnO

For most metals, the optimal design of the material heating method allows a metal drop to be heated and kept in a noncontact condition in the evaporation zone by high-frequency magnetic field. However, this method does not ensure the optimal heating of light-volatile metals such as Zn, Sn, and Bi. Therefore, another material evaporation method using a refractory crucible was applied, for heating and evaporation. This method is generally suitable for evaporating materials with high vapor pressures at moderate temperature. Figure 4 shows a simple diagram of the device for obtaining light-volatile metal nanopowders. The apparatus consists of a high-frequency induction generator operating at 2.5 kW, a levitation and evaporation chamber, and an oxygen concentration control unit [11]. The wire feeding velocity (V_Zn) and mixed Ar and O₂ gas pressure in the chamber were 50 mm/min and 100 Torr, respectively. The mechanism of ZnO formation using the LGC method was analyzed. First, a liquid droplet, which is levitated against gravity by the magnetic force due to the coupled induction coils, is heated up to the temperature of 1560°C at 2.5 kW. Then Zn clusters are evaporated from the overheated surface of the liquid droplet and condensed by cold inert gas and collected into the filter. At the same time, molecular O₂ introduced into the chamber is converted to atomic O with high activity under high temperature. The highly active O atoms can diffuse into the Zn clusters and react with the Zn atoms. A large amount of the Zn phase was observed at and below an oxygen flow rate of V_O2 = 0.05 ℓ/min, whereas mixtures of ZnO and small amounts of the Zn phase were observed under O₂ flow rates in the range from V_O2 = 0.11ℓ/min to V_O2 = 0.21 ℓ/min. However, at and above 0.21 ℓ/min of O₂ flow rate, levitation was impossible. Some metals, such as Bi and Sn, have insufficient tensile strength to prepare wire. In these cases, the micron-sized powders were used as the parent materials. The powder starting materials were supplied by a powder feeding (PF) system. A detailed explanation of the PF is provided in Section 2.3.

Figure 4.

High-resolution TEM images of carbon encapsulated (a) Ni and (b) Fe.

Bi powders were prepared using metal bismuth powder as the starting material [28]. The powder was supplied by the feeding system into a graphite crucible at a rate of 20 mg/min. The crucible was heated by induction currents up to T = 700–900°C. Bi particles entering the crucible were evaporated within 1–2 sec and carried by argon flow from the hot zone. The argon flow rate was varied in the range of 80–170 ℓ/h, at pressures in the range of 70–300 torr. The dependence of the mean sizes of the bismuth particles on gas pressure at a flow rate of 80 ℓ/h was 25, 70, and 120 nm for 70, 150, and 300 torr, respectively. Thus, the optimal conditions for obtaining Bi powder were realized at an argon pressure of 70 torr and a rate of 80 ℓ/h. A simple diagram of the process for forming nanoparticles from light-volatile seed in a crucible to produce volatile nanoparticles is represented in Figure 5.

Figure 5.

(a) Elementary diagram of the process for forming nanoparticles from light-volatile seed in a crucible and (b) TEM images for the Sn, Bi, and ZnO nanoparticles prepared by LGC using light-volatile seed.

2.3. Powder starting materials: NiFe2O4 and Ti-Ni

The wire feeding (WF) system was used for synthesizing metal, ceramic, and carbon-encapsulated materials. This system easily supplies seed parent materials continuously. However, it was impossible to synthesize several complicated metal-doped materials such as ferrites, perovskite, garnet, metal-doped ZnO, Ti-Ni, and Al-Ni-Co using the wire feeder in the LGC system, because the parent materials could not be prepared as wire. A newly modified micron powder feeding (MPF) system overcomes this problem of the LGC system [15, 20]. The MPF system can be used for synthesizing brittle metals, alloys, and complex doped materials. Commercial elemental powders of Ti (99.9 at.%, ~500 μm), Ni (99.9 at.%, ~500 μm), and Fe (99.9 at.%, ~ 500 μm) were used as the starting powders for the synthesis of Ti-Ni alloy and Ni-ferrite nanopowder, using the LGC. The Ti and Ni powders were mixed by pestle and mortar to achieve the desired equi-atomic composition and were then incorporated into the micron powder feeding system, which consisted of a rotating part to supply the Ti and Ni micron powders to the melted droplet and a vibrating part for mixing the powder. The Ti and Ni micron powders were fed into the powder feeding system at a feeding rate of 38 mg/min. An 83 mg Ti-Ni alloy ingot, which was used as the seed material for the levitation and evaporation reactions, was melted by an electric induction heating with an applied power of 6 kW at an argon gas pressure of 100 torr. The evaporated powders were filtered and finally passivated by partial oxidation. The starting materials were the mixed micron powders of Ni and Fe, which has a size ranging from 100 to 500 μm. The amount of micron powder fed into the liquid seed droplet was controlled at 80 mg/min. The mixed Ar and O₂ gas pressure in the chamber was 100 torr [15, 20] (Figure 6).

Figure 6.

(a) Micron powder feeding (MPF) system and (b) wire feeding (WF) system in the LGC instrument. TEM images for (c) Ti-Ni alloys and (d) NiFe₂O₄.

3. Magnetic metal and carbon-encapsulated metal nanoparticles

3.1. Magnetic properties of Ni, Fe, Ni@C, and Fe@C

Magnetic nanoparticles have attracted much attention because of their use in nano-fluids for biomedical application, thermally conductive fluids, various catalysts, etc. However, metallic nano-fluids end to be inherently vulnerable to oxidation, dissolution, and agglomeration during synthesis. In particular, agglomeration of the particles in a solvent is a serious problem when preparing nano-fluids. To overcome these problems, encapsulating the particles in a protective shell has been recommended to improve the chemical stability of the metal nanoparticles and their dispersion stability in the solvent. It is also worth noting that after encapsulation with a carbon coating layer, these materials are not prone to agglomeration because the coating reduces their magnetic interaction. In addition, the surface diffusion processes can preserve the chemical and structural properties of the nanopowder for a long time in many chemically aggressive conditions. A graphitic carbon shell in particular is regarded as an ideal coating since it is light and shows high stability in both chemical and physical environments [29, 30, 31].

The contents of the Ni and Ni@C nanoparticles synthesized by LGC using the micron powder feeding system were confirmed by XRD pattern. The XRD results for Ni and Ni@C showed the lattice parameters and the positions of the main peaks of the Ni powders. A small amount of NiO phase and amorphous graphitic layers was found in the XRD patterns and in the TEM images as mentioned in Section 1 [18]. The diffraction peaks at 44.4°, 51.8°, and 76.3° are due to the (1 1 1), (2 0 0), and (2 2 0) planes of fcc-Ni, respectively. The Ni powders synthesized by the LGC method showed low saturation magnetization. These results were attributed to the spin-canting effect and oxide phase on the surface [32]. The magnetic properties would be weak due to the antiferromagnetic NiO phase on the powder surface. The saturation magnetization was Ms. = 42 emu/g, as shown in Figure 7(a). The slightly shifted hysteresis loop for the Ni sample can be explained by exchange bias between the ferromagnetic core of Ni and the antiferromagnetic surface of the NiO. The initial magnetization curve is not explained by the size effect. In previous studies, the virgin magnetization curve slightly spills over the limited hysteresis loop at 655 Oe. We assume that this effect is enhanced when the size of the particles is reduced, as suggested in a previous study. With decreasing particle size, the defects and the different magnetic structure on the surface of the particles are increased. The nature of this irreversibility in high magnetic fields follows a physical model and can be explained by a spin-glass or spin-canting behavior. The hysteresis loop of the as-made M@C materials in magnetic fields up to 2 T reveals their intrinsic magnetic behavior, indicated by the magnetization (M), the remanent magnetization (Mr), and the coercive force (Hc) of the M@C samples. The saturation magnetization demonstrates that the carbon-coated Ni nanocrystallites exhibited a superparamagnetic behavior at room temperature, which is related to the demagnetization effect arising from the additional energy of the magnetic fields outside the graphitic carbon encapsulation as shown in Figure 7(b). The coercive force (Hc) and magnetization (M) were 76.6 Oe and 19.6 emu/g, respectively. The ratio of remanence to the saturation magnetization (Mr/M) was 0.04. The low magnetization compared with the Ni nanoparticles without the carbon shell is due to the coexistence of nonmagnetic carbon and the large percentage of surface spin due to the disordered magnetization orientation of the nanoparticles. The magnetic properties are influenced by both the particle size and the surface properties of the particle [33, 34].

Figure 7.

M-H loops for (a) Ni and (b) carbon-encapsulated Ni measured at 20°C.

A typical hysteresis loop of the Fe nanopowder at room temperature shows a saturation magnetization of Ms = 157 emu/g and coercivity of Hc = 836 Oe as shown in Figure 8(a). An estimated single domain size of 14 nm for spherical iron particles with no shape anisotropy is reported. The size of the iron nanopowder is large enough to show very large value of coercivity. The hysteresis loops of the as-made Fe@Cs in magnetic fields up to 1 T reveal their intrinsic magnetic behavior, as shown in Figure 8(b).

Figure 8.

M-H loops for (a) α-Fe and (b) carbon-encapsulated Fe measured at 20°C.

The hysteresis loops indicate that the carbon-coated Fe nanocrystallites exhibit superparamagnetic behavior at 50 and 300 K. The magnetization was not saturated in the applied fields up to 1 T, as shown in Figure 6(b). In the nanoparticles, one can observe superparamagnetic behavior, which is related to the demagnetization effect arising from the additional energy of the magnetic fields outside the graphitic carbon encapsulation. The coercive force (Hc) and the magnetization (M) at 50 K were 130 Oe and 69.6 (emu/g), respectively. In a previous study, the Mössbauer spectrum for Fe@C nanopowder was measured at room temperature. The relative fraction of the α-Fe, Fe3C, and γ-FeC phases was determined to be about 27.6, 26.3, and 46.1%, respectively. The low magnetization compared with metal nanoparticles without a carbon shell was due to the coexistence of nonmagnetic carbon and the large percentage of surface spins due to the disordered magnetization orientation of the nanoparticles. The magnetic performance of the Ni@C and Fe@C samples was demonstrated in a liquid phase (in ethanol and polyethylene glycol) by placing a magnet bar near the glass bottle. The carbon-encapsulated magnetic metals moved under the magnetic force. This suggests that Ni@C and Fe@C materials would be ideal adsorbents and catalyst supports because they are magnetically separable [35].

3.2. Dispersion stabilities of Ni, Fe, Ni@C, and Fe@C

To evaluate the dispersion stability and agglomeration phenomena of the carbon-encapsulated Ni and Fe nanoparticles in solvents of ethanol and ethylene glycol (EG), their time-dependent sedimentation behavior was investigated using transmission profile measurements obtained with a Turbiscan Lab [36, 37, 38]. The transmission profiles were taken every 1 h for 60 h when the suspending medium was ethanol. It was found that the transmission intensity decreased at the sample top owing to clarification and increased at the sample bottom due to sedimentation. A very stable Ni@C dispersion was observed without showing any clarification or sedimentation in EG. In contrast, a progressive fall signal was observed as a function of time in the middle region of Ni nanoparticles which had an average particle size of 20 nm. This can be explained by flocculation-induced particle growth. Figure 9(a) shows the Turbiscan screen data taken every 1 h for 3 days. The time-dependent transmission rates on the top, middle, and bottom show the same tendencies. The clarification in the top region and the progressive fall in the middle region of the ΔT signal were not observed in all suspensions. These imply that flocculation due to a coalescing reaction between the nanoparticles was insignificant. A very stable Fe@C dispersion, without any clarification on the top layer or sedimentation on the bottom layer, was observed in ethanol and EG. The viscosity of the solvent affected the dispersion stability kinetics. The dispersion stability of the solvents increased in the following order: water, ethanol, and ethylene glycol (or poly ethylene glycol). Figure 9(b) shows the effect of the solvent on the dispersion stability, as measured by using Turbiscan Lab, as well as the calculated mean value of the kinetics for each transmission (ΔT) profile as a function of time. The suspensions prepared in water displayed a rapid change in the mean ΔT values. As a result, sedimentation of the Fe@C nanoparticles in suspensions of water commenced as soon as the suspension was prepared. For the suspensions prepared in ethanol and EG, the variation in the mean ΔT was much less. However, this value increased continuously. Visual inspection confirmed that the suspension was stable, but sedimentation slowly occurred. However, coalescence between the Fe@C nanoparticles rarely occurred in the suspension because the carbon shell layer prevented agglomeration of the particles. The variation in the mean ΔT for the suspension prepared in EG was the smallest. The mean value of ΔBS increased when the particles were smaller than the wavelength of the incident light (880 nm). The tendency of ΔBS was similar to those of ΔT. From these results, for three kinds of solvent, we determined EG to be the most suitable solvent [38, 39].

Figure 9.

Variations in the mean ΔT for the (a) Ni and Ni@C suspensions and (b) Fe and Fe@C suspensions prepared in various solvents (ethanol and ethylene glycol).

3.3. Catalyst for the multicomponent Biginelli reaction

In this study, we introduce the catalytic effects of the Ni and Ni@C nanopowders observed during the synthesis of S-enantiomer from 3,4-dihydropyrimidine (DHPM). The synthesis of 4-Aryl-substituted DHPM compounds by the Biginelli reaction has attracted great attention in synthetic organic chemistry due to their pharmacological and therapeutic properties such as antibacterial and antihypertensive activity as well as their behavior as calcium channel blockers. Given the versatile biological activity of DHPM, development of an alternative synthetic methodology is of paramount importance [40, 41, 42]. This has led to the development of several new synthesis strategies involving combinations of Lewis acids and transition metal salts such as mainly homogeneous catalysts, which give high yields. However, in spite of their potential utility, many of these methods involve expensive reagents, long reaction times, high temperatures, and stoichiometric amounts of catalysts and result in unsatisfactory yields. Therefore, discovering a new, inexpensive catalyst for the Biginelli-type reaction under neutral and mild conditions is of prime importance. The starting materials used in this study were ethyl acetoacetate (I) (0.25 mmol), benzaldehyde (II) (0.25 mmol), and urea (III) (0.3 mmol). First, the benzaldehyde (II) (0.25 mmol), urea (III) (0.3 mmol), 0.1 g of catalyst (Ni or Ni@C), and chiral modifier of L-proline (0.025 mmol) were mixed and react in ethanol (50 ml) at 70°C for 2 hours. In the second step, ethyl acetoacetate (I) (0.25 mmol) was added and reacted under microwave for 3h. The ratio of the s-enantiomer in the as-prepared sample was characterized by high-performance liquid chromatography (HPLC) with a chiral column (Chiralcel OD-H) [13].

Vigorous agitation appeared to be an extremely important factor influencing stereo selectivity. The results of stereo selectivity are represented in Table 1. The simultaneous use of a heterogeneous catalyst along with the chiral modifier allowed the ratio between stereoisomer in the Biginelli reaction to be changed in some experiments in favor of the S-enantiomer, with an excess of about 19.6%. The best results were obtained when using carbon-encapsulated Ni nanoparticles as the catalyst, L-proline as the chiral modifier, and methanol as the solvent. The catalytic reaction with Ni@C showed higher stereo selectivity than with Ni. The carbon shell influences the catalytic effect during synthesis [43].

Catalysts	Yield (%); Racemic	HPLC		Δ (ee. %)
		S-enantiomer	R-enantiomer
Ni	47	53.2	45.8	7.4
Ni@C	70	59.8	40.2	19.6

Table 1.

Synthesis of 3,4-dihydropyrimidine based on the Biginelli reaction using nanosized catalysts of Ni and Ni@C.

4. Nonmagnetic metal and metal oxides: Cu oxide, Bi, and NiO

4.1. Catalytic activities of Cu oxide and ZnO

Cu oxides are widely applied in various organic syntheses such as reduction and oxidation processes, various condensation processes, for the syntheses of complex compounds, etc. The surface of the nanocrystalline Cu oxide includes a defect structure, resulting in nonstoichiometry. Such materials in themselves have the advantages of both homogeneous and heterogeneous catalysts. The aim of our investigation was the development of an effective catalytic and reaction systems based on nanocrystalline Cu oxides, with high reactivity at ambient temperature. To test the catalytic reaction, both the reaction of the liquid-phase oxidation of 2,3,5-trimethyl-1,4-hydroquinone (TMHQ) and the catalase activity were chosen. The oxidation of TMHQ is an intermediate stage of the hydroxylation of 2,3,6-trimethyl phenol in the synthesis of tocopherol. The process of TMHQ oxidation was carried out in a thermostatically controlled chamber, under agitation in a mixed water and methanol solution (1:1 in volume) at 50 ± 0.2°C. The rate of air supply was 6.2 ℓ/h. The reaction was carried out using the parent material (0.66 mmol) and the nanopowders (1 mmol). To compare the catalytic properties of the Cu oxides, powders with various sizes were synthesized. The size control of the powder was carried out by altering the feeding velocity of the Cu wire from 20 to 80 mm/min. Phase control of the Cu, Cu2O, and CuO was carried out by controlling the pressure of the inert gas. In Table 2, the crystallite conditions of the copper oxides are displayed. The dehydrogenation (oxidation) of the TMHQ in solution was to practically form 2,3,5-trimethyl-1,4-quinone (TMQ) (selectivity on TMQ > 99.5%). The kinetic curves of TMHQ oxidation in the presence of the nanocrystalline Cu oxide particles (Samples 1, 2, and 3) are given in Figure 10. Samples containing mainly pure Cu (Cu 78%, Сu₂O 22%, sample a) in the structure showed rare catalytic reaction. Catalytic activity depended on both the average particle size and the oxide phase such as the concentration of Cu₂O in the nanopowders. Sample 1 with mainly Cu₂O phase and an average size of 90 nm performed as catalyst with theoretical yield. Samples 2 and 3 with an average size of 35 nm showed significantly active effect. These samples contained a small quantity of CuO, not exceeding 0.15 mol fraction. The results of the oxidation of TMHQ are represented in Table 2. The catalytic yield of Sample 1 compared with Samples 2 and 3 was relatively very low. The Samples 2 and 3 with the same range of particle sizes included different ratios of Cu and Cu oxide phase. The yield of TMHQ oxidation depended on the particle size and amount of the Сu₂O phase. Obviously, oxidation of the parent material substantially occurred under the action of the fixed oxygen, which was activated in the matrix of nanopowders [44, 45, 46].

	Average particle size (nm)	Conditions for synthesis: feeding speed, draft velocity (l/min), and pressure in chamber	Phase composition, wt. %			Oxidized yield of TMHQ	H2O2 conversion
			Cu	Cu2O	CuO
a	20	Slow feeding (20~30 mm/s); 0.0 ≤ VO2≤ 0.05, 80 torr	78	22	—	No; reaction	10.2
1	90	Fast feeding (60 mm/min); VO2 = 0.2, 120 torr	4	96	—	Initial	60.1
2	35	Slow feeding(20~30 mm/min); VO2 = 0.2, 100 torr	—	100	—	Active	99.8
3	35	Slow feeding (20~30 mm/min); 0.1 ≤ VO2≤ 0.15, 100 torr	10	85	5	Active	82.3

Table 2.

The concentration of Cu oxide nanopowders, the reaction yields for dehydrogenation of TMHQ, and hydrogen peroxide (catalase activity).

Figure 10.

Kinetic curves for the dehydrogenation of TMHQ using Cu oxide catalysts with average particle size of 35 and 90 nm.

The catalase activity is an informative parameter about the catalytic properties of the materials in the redox process. It was simulated by measuring the ability of the catalase in the decomposition of hydrogen peroxide to isolate molecular oxygen. Decomposition of the hydrogen peroxide was carried out in a thermostatically isolated chemical reactor (10 ml). A mixture of water and methanol (1:1 in volume) was agitated with a stirring rod at 50 ± 0.2°C. The reaction was carried out with hydrogen peroxide (1.7 mmol) and nanopowder (2 mg). The catalytic activity of the Co, Mn, Fe, and Cu hydroxides was also estimated by catalase activity. The aim of this work was to solve a scientific problem related to the chemical intoxication mechanisms of water phenol solutions and its derivatives during their cleaning. The data in Table 2 show the results PF the catalase activities for the same nano-Cu oxide samples. The reaction of Sample 2 with a size of 35 nm, containing mainly Cu₂O, showed much higher activity than Sample 1 with a size of 90 nm and the same oxide phase. The size of the particles was the most significant factor. The particle size affects the surface state. The specific surface areas of Samples 1, 2, and 3 were 17, 35, and 38 m²/g, respectively. The specific surface area is related to the particular manufacturing method. The LGC method produces nano-scaled powders with high specific surface area [47].

The photocatalytic activity of the ZnO was evaluated based on the photodegradation of phenol aqueous solutions under different irradiation conditions. For experiments under UV-visible light, 100 mL of 50 ppm phenol in aqueous solution with 0.5 g catalytic powders was loaded in a glass container and stirred with a magnetic stirrer a under irradiation form, a Hg-Xe lamp. Total organic carbon (TOC) values as a function of time were measured after filtration under reduced pressure. Figure 11 shows the photo mineralization of phenol with UV-visible light (solar simulator) in the presence of ZnO. Obviously, when ZnO is added to the phenol, the total organic carbon (TOC) value was reduced to 60% [48, 49, 50, 51].

Figure 11.

Photo mineralization of phenol with sunlight (TOC: total organic carbon content at times) in the presence of ZnO (Hg-Xe lamp with a wavelength of 200 ∼ 2500 nm and 1 kW of power).

4.2. Electrochemical analyses using Bi nanopowder

The anodic stripping voltammetry (ASV) method is a powerful electrochemical technique for trace metal analysis. The traditional electrodes for ASV measurements are mercury-drop electrode and a mercury-film electrode, due to high sensitivity of the mercury [52, 53, 54, 55, 56]. However, mercury is very toxic. The toxicity of the mercury has led its usage to be completely banned in some countries. In this study, we focused on searching for alternative environment-friendly electrode materials. The Bi-film electrode has been considered replacing the mercury-film electrode due to its nontoxicity. The properties of Bi materials show not only excellent resolution of neighboring peaks but also insensitivity to the dissolved oxygen in the solution. However, there are still some problems to use the electrode such as a low detection limit comparing to mercury electrode and complication of preparing electrode including processes of additional washing or polishing of the carbon surface and dissolving Bi ions into the solution for the pre-deposition of the Bi on the film electrode. In order to overcome the above weaknesses of the Bi-film electrode, a Bi nanopowder-labeled electrode with a larger electrochemical active surface area was fabricated [57, 58, 59]. In this study, the nano-Bi-fixed electrode sensor and a nanosized Bi-binding technology were developed to improve the electrochemical characteristics of Bi for detecting heavy metals. For this purpose, the Bi nanopowder was synthesized using the LGC method and was then coated on a conductive carbon layer using a Nafion solution. Figure 12 illustrates the attached Bi working electrode and the analysis system setup for measuring Zn, Cd, Pb, and Ta [14, 24, 58, 59, 60, 61, 62].

Figure 12.

Illustration for working electrode and total system for electrochemical analyses.

The working electrode was prepared using conductive carbon ink (DongYoung Chemical Co., LTD, in South Korea) painted flexible polyester film by a semiautomatic screen printing instrument. Then the prepared carbon ink with a thickness of 80 μm on painted thick film was partially covered by an insulating layer. Bi nanopowders were well dispersed into 20 ml of distilled water using an ultrasonic treatment. A Nafion solution (Fluka) was added in to the Bi-dispersed suspension for strong chemical bonding between nanopowder and the carbon paste. Finally, the Bi nanopowder-dispersed suspension was dropped onto the working area and dried in the air at room temperature. As the concentration of Nafion in suspension was increased, the value of pH was decreased due to the strong acidity of the Nafion.

When the Bi nanoparticles were dispersed in distilled water without Nafion, the zeta potential showed a positive value [14]. However, as Nafion was added in the suspension, the zeta potential changed to a negative value. The amount of Nafion should be optimized to be 200 μℓ for dispersion stability and the phase stability of Bi nanoparticles. The sensor electrodes were prepared using the screen-printed carbon surface with the Bi nanoparticles strongly attached by Nafion. A platinum wire and a saturated calomel electrode (SCE) were used as a counter electrode and a reference electrode, respectively. The supporting electrolyte was a 0.1 M NaAc and 0.025 M HCl solution of pH 5.0. The prepared nanoparticles are confirmed by XRD as shown in Figure 13(a). Also, the screen-printed Bi nanoparticles dispersed in Nafion on the electrode could be observed by TEM, as shown in Figure 13(b).

Figure 13.

XRD pattern for (a) Bi nanopowders and (b) SEM image for screen-printed Bi.

Figure 14 shows results of the anodic stripping voltammograms (ASV) using the Bi nanopowder-attached electrode for measuring various concentrations of Cd and Pb ions in solution. The ASV showed well-defined peaks at −0.85 V and −0.65 V corresponding to the oxidation of Cd and Pb, respectively. Figure 15 demonstrates the dependence of the stripping peak current density Ip on the Cd and Pb concentrations over a range of 3~30 ppb (deposition potential = −1.35 V and deposition time = 3 min). From the linearity between the metal concentration and the peak current, the values of the sensitivity of the nano-Bi-fixed electrodes were determined to be 9.01 ± 0.012 and 7.15 ± 0.007 μA/ppb·cm² for Cd and Pb, respectively. The estimated detection limits of the nano-Bi-fixed electrode were 0.31 and 0.42 ppb for Cd and Pb, respectively, on the basis of the signal-to-noise characteristics (S/N = 3) under a 10 min accumulation. These values are much lower than the domestic and the international content limits of Cd and Pb ions in drinking water, which are listed in Table 3, indicating the excellent detection of the Bi nanopowder-fixed electrode. Consequently, the low toxicity of the Bi nanopowder-fixed electrode with high sensitivity about heavy metals promises the development of an attractive sensor for monitoring toxic chemical species in environmental matrices with a clean methodology [62].

Figure 14.

Square wave anodic stripping voltammograms experimentally measured on the nano-Bi-fixed electrode for various concentrations of Zn, Cd, and Pb ions.

Figure 15.

Dependence of the stripping peak current density Ip on the Cd and Pb concentrations over the range of 3 ~ 30 ppb (deposition potential = −1.35 V; deposition time = 3 min).

Heavy metal	unit	Korea	IBWA	FDA	WHO/FAO
Cd; Pb	ppb; ppb	5; 50	5; 5	5; 5	3; 10

Table 3.

Domestic and international content limits of cd and Pb ions in drinking water.

4.3. Catalytic effect of NiO in the Biginelli and Hantzsch reaction

Nickel oxide powders were obtained by the gas condensation method in an argon-oxygen mixture flow. The argon flow rate was 130 ℓ/h, oxygen concentration 8.5 vol %, pressure equal to 90 torr, and Ni feed rate 1.8 g/h. XRD analysis showed the following phase composition: NiO 90.7%, Ni 9.3%, and mean size of nickel oxide particles 13 nm. According to the electron microscopy, the particles proved to have a nearly uniaxial shape. The nickel phase detected by the XRD method appears to result from the incomplete oxidation of some particles, and it is located in the center of the particles, while the outer layers must certainly be in an oxidized state (Figures 16 and 17).

Figure 16.

TEM image of NiO nanopowder.

Figure 17.

Chromatogram of a racemic mixture of (a) DHPM, enriched with S-enantiomer by 15.4%. The product was obtained in the presence of L-proline and NiO nanopowder mic mixture of (b) DHP, enriched with S-enantiomer by 3.4%.

The NiO nanoparticles were applied for the synthesis of both dihydropyridine (DHP) and dihydropyrimidine (DHPM) as mentioned in Section 3.3. The most plausible pathway for DHP prepared by the Hantzsch reaction has been shown to involve the interaction of benzaldehyde with one molecule of β-dicarbonyl compound 1 to give chalcone 3, while another molecule of β-dicarbonyl compound 1 is transformed into enamine 2. In route A, enamine 2 is condensed with an aldehyde and ethyl acetoacetate 1 in the reflux in a suitable solvent (methanol or ethanol) [63, 64, 65]. Route B involves the reaction of chalcone 3 with enamine 2, and it seems to give better yields of products and easier purification. In the presence of aqueous ammonia, compound 3 undergoes a partial decomposition into benzaldehyde and diketone 1, thus giving a rise to the formation of symmetrical analogues nitrendipine 16a, b. When the Hantzsch reaction is carried out at 22–25°C in the presence of L-proline and nanosized NiO, the ratio of enantiomers of nitrendipine is changed in favor of the S-enantiomer by 3.4%.

The Biginelli reaction for synthesizing DHPM was carried out in the presence of L-proline and nanosized NiO (obtained by the Institute of Metal Physics) to change the ratio of enantiomers of 3S in Section 2.3 is changed in favor of S-enantiomer by 15.4%. Our future plans involve studying the factors that affect enantiofacial discrimination for the Hantzsch and Biginelli reaction, such as the nature of nanosized metal oxides or chiral modifiers, reaction time, temperature, and solvent. We also plan to synthesize new nanosized metals and their oxides, as well as chiral modifiers.

5. Conclusion

Ceramics, such as NiO, ZnO, and Cu₂O, magnetic nanoparticles, including γ-Fe₂O₃, Fe₃O₄, and NiFe₂O_4, and metals, such as Cu, Ni, Zn, Sn, Ag, Au, Bi, and carbon-encapsulated metals (Ni and Fe), were synthesized by levitational gas condensation (LGC) method using wire feeding (WF) and micron powder feeding (MPF) systems. The magnetic properties have been characterized using a vibrating sample magnetometer (VSM). The size and shape of the nanopowders were investigated by transmission electron microscopy (TEM). The surface effect influenced the magnetic behaviors of nanopowders. Bi metals were dispersed in Nafion. The Bi particles could be applied as sensor electrode instead of mercury-based electrolyte. The particle size of carbon-coated metal with diameters in the range of up to 10 m was smaller than those of metals without a carbon shell. The dispersion stability kinetics of carbon-coated nanopowders showed good dispersion. The best results were obtained when using carbon-encapsulated Ni nanoparticles as a catalyst, L-proline as a chiral modifier, and methanol as a solvent. The catalytic reaction of Ni@C showed enhanced stereoselectivity. Also, the simultaneous use of the heterogeneous catalyst and chiral modifier may lead to an increase in the selectivity of the Biginelli reaction. Nanoparticles prepared using LGC showed significantly enhanced catalytic activities during chemical reaction due to the high level of defects on their surface structure.