The Effect of a Weak Magnetic Field (0 T to 0.4 T) on the Valence Band and Intramolecular Hydrogen of Inorganic Aerosol Metal–Nitrogen Gas Chemical Reactions in a Sparking Discharge Process

Abstract

The effects of a weak magnetic field on chemical reactions are still not well understood. In our research, we used a sparking discharge process to ionize and atomize different metal wires in ambient air under usual atmospheric conditions, with and without the presence of a magnetic field. Products were collected on a glass substrate and additionally characterized for the presence of nitrogen or nitride bonding with XPS. All samples sparked with no magnetic field provided an evidence of nitride formation. Additional characterization and comparison of samples prepared inside and outside a magnetic field was performed using FTIR and collected in deionized (DI) water to investigate the influence on conductivity and pH. When the magnetic field was present during sparking discharge, a higher concentration of nanoparticles was produced.

In 2018, we published a paper [1] on increasing the longevity of zerovalent iron nanoparticles (nZVI). In our research with a sparking discharge process, we applied a 0.4 Tesla magnetic field using a permanent magnet (Neodymium Magnet Physical Properties are graded according to information available on following website from K&J Magnetics, Inc., Pipersville, PA, USA, company, URL saved on web.archive.org https://www.kjmagnetics.com/specs.asp) to produce small and highly crystalline nano zerovalent iron (nZVI) particles. However, when we used a magnetic field of less than 0.4 T in strength, specifically in the range from 0.1 T to 0.3 T, we produced particles with different characteristics (see electronic supplementary information (ESI)) and their concentration was lower. The resulting aerosol concentrations were measured with a differential mobility analyzer (DMA) [2] at different magnetic field strengths. The results led us to investigate the reasons behind the measured influence of a weak magnetic force on chemical reactions and crystallization. The investigation focused on the abundance of free radicals and plasma ions produced during the sparking discharge process [3] and, therefore, the variety of singlet–triplet pairs which can, at certain discrete magnetic field strengths, prolong the radical pair crossing process. Furthermore, we focused on the influence of the magnetocrystalline anisotropy [4] on the lattice constant, which provided the preferred orientation, and was due to the spin-orbit coupling.

In the experimental research, we analyzed the products of a chemical reaction from a sparking discharge process, corresponding to the breaking voltage region of a current–voltage plasma discharge [5] curve (as represented and described in: Wagenaars, E. (2006). Plasma breakdown of low-pressure gas discharges. Eindhoven: Technische Universiteit Eindhoven. https://doi.org/10.6100/IR614696 and Bogaerts A, Neyts E, Gijbels R, Van der Mullen J. Gas discharge plasmas and their applications. Spectrochimica Acta Part B: Atomic Spectroscopy. 2002; 57(4), 609–58. See ESI for graphical representation). This synthesis method is also called spark ablation or spark explosion and approaches the nanoparticle synthesis via an aerosol-based chemical route [6,7,8,9,10]. Our research group approached a sparking process as a metal melt process [11,12,13,14] or as a metal vaporization with a high voltage for surface modification application. Nevertheless, both are based on the same physical mechanism, similar apparatuses (see ESI images) and the same processes or chemical reactions that take place in the sparking gap between the metal wires of sparking discharge apparatus. These processes, involving chemically active reactants, are represented in Figure 1. The anode and cathode are shown as simple metal wires that are the source of metal atoms. These wires (shown in ESI) were made from pure metal (metal wires of high purity were purchased from Advent Research Materials Ltd., Witney, UK) and were placed in electrode position of the sparking machine apparatus with a specific gap between their tips that prevented a short-circuit and to allow for the formation of breakdown voltage, a permanent discoid staking magnet was placed below the sparking discharge and maintained at the same distance (3 mm) under all experimental conditions (0 T to 0.4 T) as is represented in the figure of graphical abstract.

The magnetic field was generated from permanent discoid magnets that were stacked on top of each other in order to create different field strengths and was measured with a Phywe Tesla Meter calibrated and equipped with a hall probe; the sparking gap was distanced from the magnet equally in all conditions (0 T to 0.4 T). This is important because the gap consists of the air medium, which is a dielectric component made of atmospheric air (By using term atmospheric air we imply that it is non-dry air that contains 78% nitrogen, 21% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Air also contains on average around 1% of water vapor.). In this gap, a nitrification reaction occurs between metal ions or metallic aerosols, which are melted or ablated at a high voltage low current regime, and excite gas molecules of nitrogen [1,15] located in the dielectric medium in the gap. To summarize all reactions in the sparking gap; gasses can react between each other and/or with the vapor/aerosol of the metal wire. The latter consists of atomized metal, excited metal atoms, metal radicals, and charged aerosol particles. We characterized the surface composition, the products of the chemical reaction of sparking discharge and the nitrification with X-ray photoelectron spectroscopy (XPS). In our previous research, we have reported on iron and zinc metal wire sparking [1,2] and in the current research, investigated cobalt, nickel, titanium, vanadium, and tungsten. A full XPS report is available in ESI, and we only summarize N 1 s peak in Table 1 below. All aerosol nanoparticles produced by sparking discharge process were collected on a quartz substrate, beneath which was sometimes placed a permanent neodymium magnet, depending on the purpose of the experiment. The magnet strengths used were 0 T, 0.2 T and 0.4 T, and there was a clear influence on the crystalline structure of precipitated metal aerosols even at 0.4 T; however, we observed a metal nitride peak (400 eV to 398 eV) only when the magnetic field is less than 0.4 T.

The quality assurance and reproducible results were achieved by sparking inside air, pure nitrogen, and in the presence or absence of a magnetic field. It is clear that the lack of observable N 1 s XPS peak was due to the interference of a magnetic force. In Table 1, we present the results of sparking several types of metal wires on substrates underneath which a permanent magnet or magnetic material was not placed, performed either under pure nitrogen or ambient air atmosphere. In our XPS results, we can observe peak metal nitride values at 399 eV and peak nitrites at 407 eV.

Sparked cobalt reacts directly with nitrogen gas to produce cobalt nitride which consists of Co³⁺ and N³⁻, which implies that the breakdown voltage of the sparking gap is sufficient to break the triple bond of the nitrogen molecule. Titanium and titanium nitride are known for their noble-like properties, and in this case, the chemical reaction in the sparking discharge gap follows these steps:

Ti(s) + E(spark) → Ti(g) → Ti³⁺(g) + 3e^-

N₂(g) + E(spark) → 2N(g) + 3e^- → N^3- (g)

The chemical reactions from Formulas (1) and (2) combine in the sparking gap to precipitate titanium nitride. The process is represented in Formula 3.

Ti³⁺(g) + N³⁻(g) ⟶ TiN(g) ⟶ TiN(s)

Thermodynamically and kinetically, this reaction is possible [16,17]. The sparking process is a high energy nanomaterial generation process that allows for the vaporization/aerosolization of metals. Nitrogen molecules in the sparking gap are excited by the discharge energy flowing from the tips of the wires that are subsequently ionized. This leads to breakdown of the nitrogen triple bond. What follows is the atomization of metal wires which provides a high surface-area-to-volume ratio for the reaction to take place [18,19,20]. Moving away from the chemical reaction process, we need to prove why the described process of nitrification does not occur at a magnetic field of 0.4 Tesla and above.

We have previously published XRD and SAED data (Selected area electron diffraction) of sparked iron wire at 0 T to 0.4 T, in a pure nitrogen environment [1], as well as of sparked zinc in a pure carbon dioxide (CO₂) and nitrogen atmosphere using the same magnetic field strengths [2]. In Table 2, we summarize those findings, including the data of sparking metal wires in atmospheric conditions using an external magnetic field [11].

These Miller indices appear in XRD when sparked inside of a magnetic field with no thermal annealing or any subsequent treatment that could increase the crystallinity. A higher magnetic field, higher crystallinity, and a smaller number of peaks will appear. However, as stated previously, atmospheric metal nitride formation will only occur if the magnetic field beneath the substrate where precipitate is collected is less than 0.4 T. At any amount of magnetic field, the particles will assume a preferred orientation, which is a well-known texturing of a magnetic field [21,22]. This influence on the size and structure of a crystal can be explained by the fluctuation in energy; however, energy fluctuation cannot explain different chemical products, so a further examination of cohesive energy is required.

In terms of synthesis of new compounds and in terms of solid reaction, it is especially convenient to use cohesion energy because it signifies the energy required for solids to assemble the atoms into a structural array of reacting free atoms or reactants. The sparking discharge process disassembles solids (vaporizes) and the external magnetic field stabilizes them in a way that affects the binding of atoms

These results explain why we see nitrides at 0.2T with zinc [2] and iron [1], but do not detect them with XPS at magnetic fields equal to or higher than 0.4 T. Based on these results, we conclude with a universal chemical equation for the metal nitrides of transition elements in chemical reactions that are happening inside an external magnetic field of 0.2 T and 0.4 T, under ambient air conditions.

^Metalo + Espark + _N2 + _CO2 + _O2 ⇔0.2T Me³⁻N³⁺ + MeOx + _MeCO3 ⇔0.4T^Metalo + _N2 + _CO2 + _O2

The second important experimental observation of materials prepared in weak magnetic fields is that those prepared by sparking inside of the magnetic field are bound together strongly and have good mechanical strength. These nanoparticles are produced by a sparking discharge process [11], and if we introduce a magnetic field in the sparking gap, we will produce nanostructures with resistivity to corrosion and humidity in comparison with those which are not prepared inside a magnetic field, which are amorphous and prone to fast corrosion. Previously, we have shown influence of a uniform magnetic field on the concentration of cobalt at sparked nickel wires [23], which led us to believe that intramolecular bonding, that plays important part in the cohesiveness of material, can be characterized to evaluate for their presence inside and outside of magnetic field. FTIR experiments were conducted. Nanoparticles generated from sparking machines were collected on a quartz surface, under which was placed a 0.4 T permanent magnet. Results are shown in Figure 2, and there is an indication that the composition of thin films prepared in the magnetic field lacks metal chelate complexes and intramolecular hydrogen bonds.

The absence of FTIR spectra band positions at 2972 cm⁻¹ and 3348 cm⁻¹ in the results on the samples prepared inside of a magnetic field represent important parameters in explaining the role of cohesive forces. Namely, absence of these functional groups represents the absence of intermolecular bonding for synthetic materials prepared inside of a weak magnetic field.

It is important to note that sparking in a magnetic field also influences the valence band of sparking discharge deposited nanoparticles, as represented in Figure 3.

The valence band spectra of XPS are important for providing information about solid systems [24]. Due to the integration of different kinds of elements in the nanomaterials prepared inside and outside of the magnetic field, we can therefore synthesize materials with lower binding energies of their core electrons [25] if materials are prepared inside of a magnetic field.

To further investigate the effects of magnetic fields on spark discharge, represented in Figure 4, we placed deionized (DI) water below the sparking head and monitored differences in pH and conductivity, with and without a magnetic field present during the sparking discharge process.

The collection of nanoparticles in DI water and measuring of the parameters is represented in Table 3. Observed are the trends in effects of conductivity with increases if a magnetic field is present, which is due to the higher concentration of particles produced from a sparking head when a magnetic field is present, and the lowering of pH is due to radical production from plasma created in discharge.

Drops in pH can be attributed to the formation of acid-like rain phenomena in sparking discharge, and magnetic field does not influence this reaction; however due to increases in the number of particles produced while a magnetic field is present, there is an increase in conductivity. We investigated the influence of applying a weak magnetic field (0.1 T to 0.4 T) during a sparking discharge reaction of transition metals (Fe, Ni, V, Ti, W, Co, Zn) and dielectric or insulating gasses (N2, CO2, O2). In this paper, we provide experimental evidence of nitride formation influenced by the application of a weak external magnetic field influence on the valence band as well as the concentration of aerosol produces. This paper sheds light on an important aspect of chemical reactions in magnetic fields, which further develops the overall understanding. Chemical bonding characterization was performed using X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR). Results also showed an effect on pH, intermolecular hydrogen, and the number of particles produced from sparking discharge.

Figure 1. Schematics of the main processes involving electrons and ions which occur during spark discharge. From (Voloshko, 2015) (Image taken from: Andrey Voloshko. Nanoparticle formation by means of spark discharge at atmospheric pressure. Plasma Physics [physics.plasm-ph]. Université Jean Monnet-Saint-Etienne, 2015. English).

Figure 2. FTIR spectra of sparked discharged wires inside and outside of magnetic field.

Figure 3. XPS binding energy of nanoparticles collected with sparking discharge. Same sample as for FTIR.

Figure 4. Sparking discharge head, with wires that discharged a high voltage low current, and particles/radical collected in deionized (DI) water.

Metal Wire	Position (eV)	FWHM (eV)	Atomic Conc. (%)
Nickel–air	400.3/407.346	2.992	3.35
Nickel–pure N2	400.089/398.744	2.687	2.92
Cobalt–atmospheric air	407.304/399.806	1.421	-
Cobalt	407.191/400.211/399.155	2.370	1.02
Vanadium–air	407.228/401.873/400.032	2.875	5.08
Titanium	407.223/400.100	2.576	1.97
Vanadium	401.500	2.488	4.20
Tungsten	401.800	2.842	4.68

Metal	Conditions	Miller Indices	Crystallographic Reference
Iron (Fe)	0 T, pure N2	(002,211) and (111)	73-2102 and 83- 087726
Iron (Fe)	0.2 T, pure N2	(111,200,220) and (110)	04-016-4995; 01-080-3816
Iron (Fe)	0.4 T, pure N2	(110)	98-000-0259
Zinc (Zn)	0.2 T and 0.4 T, pure N2	(100) (212) (400)	00-005-0664; 00-023-0740; 00-035-0762
Tungsten (W)	0.4 T, ambient air	(112) (211)	00-041-1230; 03-065-6453
Nickel	0.4 T, ambient air	(111) and (200); (200)	03-065-2865 01-083-8042
Copper	0.4 T, ambient air	(002) (11-1); (111) (200)	01-080-0076. 03-065-9743
Indium (In)	0.4 T, ambient air	(101) (002) (110); (222)	03-065-9292, 01-071-2194

Time	Spark Energy	Temperature (°C)	pH	Conductivity (µS/cm) No Mag. Field	Conductivity (µS/cm) Mag. Field
0 min	0 kV 0 mA	28	7	1.35	0.68
5 min	4.75 kV 0.61 mA; 4.40 kV 0.55 mA;	28	6.38	6.17	13.08
15 min	4.78 0.57; 4.42 0.57;	28	6.18	15.93	27.70
30 min	4.76 0.58; 4.42 0.53;	28	5.77	29.68	34.84
45 min	4.77 0.58; 4.42 0.55	28	5.46	39.7	49.9
60 min	4.85 0.60; 4.48 0.54;	27	4.90	48.8	59.4
75 min	4.9 0.6; 4.48 0.55	27	4.58	68.1	79.1

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4352/10/12/1141/s1.

Author Contributions

Conceptualization, S.R. and P.S.; methodology, S.R.; software, C.B.; validation, S.R., C.B.; formal analysis, S.R. and P.S.; investigation, S.R.; data curation, P.S. and C.B.; writing-original draft preparation, S.R.; writing-review and editing, S.R.; visualization, C.B.; supervision, P.S. and S.R; project administration, S.R.; funding acquisition, S.R. and P.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received Chiang Mai University Funding.

Acknowledgments

Stefan Ručman would like to thank the postdoctoral fellowship from Chiang Mai University for help and support.

Conflicts of Interest

The authors declare no conflict of interest.

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