Tuning the structure, optical, and magnetic properties of nanostructured NiMoO4 by nitrogen plasma treatment

Herein, the nitrogen plasma treatment with different time irradiation (0, 90, 120, and 150 min) is used to tune the structure, optical, and magnetic properties of nanostructured NiMoO4 NMO NPs. The XRD patterns revealed that the crystallinity of NMO samples increases with an increase in the N2 plasma exposure time. The notable reduce in this peak’ intensity for the sample at dose of 120 min may be attributed to the energy dissipated in the defect generation. Also, the crystallite size for NMO samples was found in the range (23.9–26.7) nm. Further, EPR is used to evaluate the impact of the treatment duration on the oxygen vacancy density. The total number of spins rises as plasma irradiation duration increases, revealing that the NMO NPs can be used as a dosimeter for plasma irradiation. The optical bandgap ranged from 2.92 eV to 3.24 eV as the N2 plasma treatment duration changed. The saturation magnetization was enhanced with the rise of plasma treatment time. Furthermore, the Hc increases from 16.67 G for untreated NMO NPs to 128.41 G for N2 plasma-treated NMO NPs for 150 min. The resulted optical and magnetic properties of N2 plasma-treated NMO NPs make it candidate material for photocatalysis applications.


Introduction
Due to their complex electronic configuration and cost-effectiveness, transition metal oxides (TMOs) are numerous interesting active materials. Nevertheless, their low usage effectiveness, low electron conductivity, and limited functional site density restricted their practical applications [1][2][3]. Binary metal molybdates (MMoO 4 ; M = Zn, Bi, Cu, Ni,.) have recently been demonstrated to have exceptional characteristics that make them promising materials for significant areas such as water treatment [4], corrosion science [5], water splitting [6], antimicrobial agents [7], electrochemical electrode [8], and energy storage [9]. Between these, the NiMoO 4 NMO nanoparticles NPs have piqued the interest of scientists due to features like chemical stability, better electrochemical display, luminescence, magnetic performance, optics, and decolorization abilities at a lower cost [10][11][12][13][14]. Many investigations have been conducted to increase NMO properties, including as surface functionalization and elemental doping, with the notable results [15][16][17]. Also, NMO suffers from its high resistivity, limiting its uses in electrochemical applications. As a result, doping NMO with a suitable metal cation may boost the electrochemical characteristics. NMO NPs has mostly been used in the photoelectric area, including electrodes, supercapacitors, and photocatalytic purposes. As a photocatalyst, NMO was employed to decompose methyl orange. After 1 h of ultraviolet light irradiation, the degradation value was estimated to be 67 percent [14]. Also, due to Ni's excellent electrochemical efficiency and Mo's high electrical conductivity, NMO might be a realistic choice for pseudo-capacitive active materials [18]. Sol-gel [13,19], solid-state reaction [20], hydrothermal [21], co-precipitation [22,23], and sonochemical [24] techniques have all been described for NMO production.
When an electric field is applied to a gas-forming plasma in a reactor, plasma is generated as a confined ionized gas. Plasma can be hot or cold and have high or low pressure [25]. The plasma treatment technique, which has recently been extensively utilized in surface modification/treatment, is a successful method for defect engineering and substituents injection. The plasma-assistant reaction yields free radicals, developing crystalline vacancies on inorganic materials, modifying the surface electronic redistribution, and providing appropriate active sites [26][27][28][29][30]. Furthermore, it is required to adjust the bandgap and enhance the ion/electron conductivities. Currently, hetero element doping (N, B, F atoms) has been often used to reduce the bandgap, which may serve as an electron donor in the material's bandgap and generate a mid-gap state, for enhanced usage in energy storage and conversion. In the same context, several oxygen vacancies and species might considerably improve electrical conductivity. Plasma treatment might be an effective technique for hetero atom doping in the material under investigation. Plasma comprises highly excited atoms with enough energy to attack the surface layer and break the chemical bond. The extremely engaged atoms then react with the investigated material to introduce a unique bond, resulting in effective hetero atom doping and the existence of multiple oxygen vacancies [31][32][33][34]. Recently, Liu et al. [35] developed a unique method for synthesizing significant atomic defects on NMO-like nanosheets using N 2 plasma treatment, then filling these defects with heterocation dopants, and stabilizing them with sintering.
To our knowledge, there is no evidence of the use of nitrogen plasma to enhance the structural, optical, or magnetic characteristics of nickel molybdate NMO NPs. Herein, for the first time, nitrogen plasma treatment with different time irradiation (0, 90, 120, and 150 min) is used to tune the structure, optical, and magnetic properties of nanostructured NMO. The sol-gel technique was utilized to synthesize NMO as a facile and low-cost approach. The untreated and N 2 plasma-treated NMO NPs were characterized via numerous tools: XRD, EDX, SEM, Electron paramagnetic resonance (EPR) spectroscopy, diffuse reflectance spectroscopy (DRS), and vibrating sample magnetometer (VSM).

N 2 plasma treatment
Four disks of as-synthesized NMO NPs were exposed to nitrogen plasma at different time irradiation (0, 90, 120, and 150 min). A plasma source is used to irradiate the NMO NPs in the charged particles Lap., Radiation Physics Department, National Center for Radiation Research and Technology, Egyptian Atomic Energy Authority. The vacuum system consists of rotary and diffusion pumps that are used to evacuate the system up to 10 -4 mbar. The plasma source is placed inside the system where the backpressure inside the plasma source chamber is in the range 10 -4 mbar, while the operating pressure is 2 × 10 -3 mbar. The plasma source is DC plasma source where the cathode is cold cathode connected to the earth, and the anode is connected to positive voltage up to 4 kV, the discharge current is 2 mA. The NMO samples are placed inside the plasma source between the anode and the cathode, where the plasma is immersed in the NMO samples. Figure 1 depicts the fabrication of a cold d.c. plasma source. It has a stainless steel cylindrical anode and stainless steel disk cathode and a 5 mm inner aperture diameter. Teflon disks separate the anode from the cathode.

Characterization of NMO NPs
XRD (Shimadzu 6000) was used to illustrate the phase analyses of NMO NPs. SEM and EDX (JEOL JSM-5600 LV, Japan) were utilized to give information about surface morphology and elemental composition of NMO NPs. Fourier transform infrared FTIR spectroscopy (NICOLET iS10) is measured in range between 400 and 4000 cm −1 . The EPR signals were acquired at room temperature using a CW X-band EMX EPR spectrometer (Bruker, Germany) and an ER 4102 standard rectangular cavity. During the EPR experiment, the following operating parameters were used: microwave power, 5.053 mW; modulation amplitude, 12.00 Gauss; modulation frequency, 100 kHz; the number of x-scans, 1; resolution in x, 1024 sweep width, 10,000 Gauss; microwave frequency, 9.668 GHz; time constant, 163.84 ms; conversion time, 163.84 ms; and sweep time, 167.77 s. A Jasco UV-visible spectrophotometer (V-670 PC) had utilized to present UV diffusion reflectance spectra.   [43][44][45]. Similar results are presented by Keerthana et al. [46].

Structural analyses
Further, the figure displayed the enhancement in the peak' intensity of the preferred plane ( 2 = 29.19 • ) as the exposure time for N 2 plasma was increased. This means that the crystallinity of NMO samples increases with an increase in the N 2 plasma exposure time. In other words, this energy is dissipated to improve the ordered phases. The notable decrease in this peak' intensity at the dose of 120 min may be attributable to the energy dissipated in the defect generation, resulting in an increase in the strain in the sample as presented in Williamson-Hall plots (Fig. 4) [47]. Further, the crystallite size for untreated and N 2 plasma-treated NMO NPs was calculated and is found to be 23.9, 24.3, 26.1, and 26.7 nm with the increase in the N 2 plasma exposure time.
Also, the peak at 2 = 29.19 • with a preferred orientation at the plane (220) was shifted toward the lower diffraction angles for the treated NMO samples (see Fig. 5). The interpretation of appearing this shift can be due to the unit cell expansion of NMO NPs, which means the growth of the lattice constant of the treated NMO sample [39]. Figure 6 shows the FTIR spectra of untreated and N 2 plasma-treated NMO NPs. In the case of pristine NMO NPs, the peaks that appeared at 647 cm −1 can be attributable to the vibrations of the distorted tetrahedron (MoO 4 ) presenting in NMO NPs.    [45,48,49]. The shift in the absorption bands (828 cm −1 and 1440 cm −1 ) after exposure to N 2 plasma was confirmed and matched with the variation observed in XRD patterns.

Surface morphology of NMO NPs
The effect of N-plasma on the surface morphology of NMO NPs is illustrated in Fig. 7. SEM images of untreated and N 2 plasma-treated NMO NPs are presented in Fig. 7. Also, it is demonstrated that the untreated NMO NPs comprising agglomerated particles owned a considerable size. The figure revealed that the N-plasma possesses a marked effect on the surface morphology of NMO NPs. The remarkable discrepancy was caused by the variable surface reactivities caused by the different exposure times for N 2 plasma. Furthermore, there were intragranular pores in the treated samples, which caused the density to rise. Microstructural holes are pores trapped within grains as a consequence of increasing grain development.

Electron paramagnetic resonance (EPR) spectroscopy
Because the nitrogen plasma treatment is predicted to create a large number of crystallographic defects and vacancies on the NMO NPs surface [30,35], EPR is used to evaluate the effect of the treatment duration on the oxygen vacancy density [50]. The intensity of oxygen vacancies signal identified at g = 2.175 (Fig. 8) grows progressively as the nitrogen plasma treatment duration is extended, showing that nitrogen-based free radicals are gradually eroding the crystalline surface. We calculated the total number of spins formed in each sample (Table 1). We found that it increases as a function of plasma irradiation time, indicating the possible use of this material as a dosimeter for plasma irradiation. Figure 9 shows the typical DRS spectra of the untreated and N 2 plasma-treated NMO NPs. It is marked from the figure that the wavelength at maximum reflection (λ max ) for the untreated and N 2 plasma-treated NMO NPs was seen at ranges between 200 and 300 nm. The optical band gap can be evaluated using the reflectance results via Kubelka-Munk theory and Tauc's equation [51,52]. The optical bandgap (Eg) for untreated and N 2 plasma-treated NMO NPs decreases from 3.11 eV for pristine NMO NPs to 2.92 eV for N 2 plasma-treated NMO NPs at (150 min). The remarkable increase in optical bandgap (3.24 eV) for N 2 plasma-treated NMO NPs at (120 min) can be ascribed to the defect that occurs as presented in Fig. 10. The obtained optical behavior was matched well with XRD data. Figure 11 presents the M-H curves for the untreated and N 2 plasma-treated NMO NPs with the field sweeping from − 20,000 to + 20,000 G at room temperature. The untreated and N 2 plasma-treated NMO NPs show superparamagnetic behavior. Table 2 summarizes primary magnetic parameters, which include saturation magnetization (Ms), remanence (Mr), coercivity (Hc), and squareness (Mr/Ms) estimated out from curves. The number of magnetic molecules in a single magnetic domain is known to be proportional to the energy of a magnetic particle through an external field [53,54]. The pristine NMO NPs possess 0.0331 emu/g, 0.4370 × 10 -3 emu/g, and 16.6700 G Ms and Mr for N 2 plasma-treated NMO NPs compared to untreated NMO has been mostly due to the increased particle size and spin canting at the NMO surface [51,53]. The extraordinary drop in the value of Ms and Mr for N 2 plasmatreated NMO at 120 min matched well with XRD and optical properties. The Hc is also affected by crystallite size, as shown in Fig. 12. The aggregation of magnetic nanoparticles enhanced their interaction [53]. The Hc is increased from 16.67 G for untreated NMO NPs to 128.41 G for N 2 plasmatreated NMO NPs at (150 min). The optical and magnetic properties of N 2 plasma-treated NMO NPs make them promising candidates materials for photocatalysis applications.

Conclusion
In this study, a sol-gel technique was utilized to synthesize NiMoO 4 . The impact of nitrogen plasma treatment on the structure, optical, and magnetic properties of nanostructured NMO NPs was investigated. The crystallite size increases from 23.9 nm to 26.7 nm with the increase in the N 2 plasma exposure time. EDX spectra and elemental mapping images confirmed the existence of fundamental elements of NMO NPs without any foreign elements. The total number of spins  increases as a function of plasma irradiation time, indicating the possible use of NMO NPs as a dosimeter for plasma irradiation. Also, the values of Ms and Mr for N 2 plasmatreated NMO NPs increased than those for untreated NMO due to the increased particle size and spin canting at the NMO surface. Overall, the obtained optical and magnetic

Conflict of interest
The authors declare that they have no conflict of interest.
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