Comprehensive Investigation of Structural and Photocatalytic Properties of Cobalt and Nickel Co-Doped Magnesium Oxide Nanoparticles

Abstract

Cobalt and Nickel (Co, Ni) co-doped magnesium oxide (MgO) nanoparticles (NPs) have been synthesized using the coprecipitation method. The structural, chemical, and optical properties of the as-synthesized NPs are systematically investigated using X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and UV-visible spectroscopy. It is found that the optical bandgap of co-doped MgO NPs reduces from 2.30 to 1.98 eV (14%) with increasing Ni dopant concentrations up to 7%. The Co_0.05Ni_0.07Mg_0.88O NPs exhibit a high photocatalytic degradation efficiency of 93% for methylene blue dye (MB) under natural sunlight irradiation for 240 min. Our findings indicate that the Co_0.05Ni_xMg_0.95−xO NPs have strong potential for use as photocatalysts in industrial wastewater treatment.

1. Introduction

The textile industry, considered a backbone of global manufacturing, is currently facing significant environmental challenges, particularly its contribution to water pollution. It is well known that dyeing and finishing processes generate a massive volume of toxic wastewater, which contains various non-biodegradable organic dyes, heavy metals, complex chemicals, and other toxic pollutants that pose serious risks to the ecosystem and human health [1,2]. Different conventional methods such as adsorption, coagulation–flocculation, membrane filtration, ion exchange, and ozonation have been employed to treat wastewater [3,4]. However, these techniques are often expensive, inefficient, or generate secondary unwanted pollutants [5]. Thus, there is a high demand to develop environmentally friendly, sustainable, and cost-effective alternatives for industrial wastewater treatment.

Recently, a promising and cost-effective green technology known as photocatalysis has gained significant attention to treat wastewater by using the sunlight to degrade the pollutants [6]. This technique uses a photocatalyst that actively participates in generating reactive species within the pollutant water. These reactive species oxidize organic contaminants, converting them into non-toxic byproducts such as water and carbon dioxide. However, the selection of photocatalysts is crucial to accelerate the degradation process under sunlight exposure. Generally, semiconducting materials such as zinc oxide (ZnO), titanium dioxide (TiO₂), and a wide variety of transition metal oxide (TMO) nanoparticles (NPs) have been extensively reported as effective photocatalysts [7,8,9]. Nevertheless, the practical utilization of these photocatalysts remains limited due to their wide bandgap that restricts their activity to the ultraviolet region of the solar spectrum. Moreover, the potential toxicity of photocatalysts is another concern that should be addressed without compromising their photocatalytic activity.

Magnesium oxide (MgO), an alkaline earth metal oxide, in the form of NPs, are considered as a promising candidate to be used in a wide range of applications, from biomedicines to catalysts, due to their intrinsic characteristics such as their availability in a high natural abundance, environmental friendliness, and, importantly, chemical stability [10,11]. However, MgO nanoparticles exhibit a wide bandgap, typically in the range of 5 to 6 eV, which limits their photocatalytic activity under the exposure of sunlight, particularly within a visible range [12,13,14]. To overcome the limitation, doping of the MgO NPs with transition metals such as copper (Cu), zinc (Zn), nickel (Ni), iron (Fe), and cobalt (Co) has been proposed, as this can introduce low-energy levels within the bandgap and thereby enhance absorption of the visible sunlight [15,16,17].

For this purpose, (Co, Ni) co-doped MgO NPs were successfully synthesized using the coprecipitation method. Dopants Co²⁺ (0.65 Å) and Ni²⁺ (0.69 Å) were selected because of their ionic radii are very close to Mg²⁺ (0.72 Å), enabling substitutional doping within the MgO lattice. To the best of our knowledge, the effect of (Co, Ni) co-doping on the structural and photocatalytic activity of MgO NPs has not been investigated yet comprehensively. Our work shows that the (Co, Ni) co-doped MgO NPs exhibit a remarkable photocatalytic degradation efficiency of up to 93% for methylene blue (MB) dye under sunlight exposure for 240 min. The presented investigations highlight the potential of (Co, Ni) co-doped MgO NPs as efficient and sustainable nanomaterials for industrial wastewater treatment.

2. Results and Discussion

Figure 1 shows the XRD pattern of Co_0.05Ni_xMg_0.95−xO NPs in the 2θ range of 30° to 80°. The XRD peaks at 2θ of 42.842°, 62.167°, and 78.443° correspond to (200), (220), and (222) planes and confirm the face-centered cubic structure of Co_0.05Ni_xMg_0.95−xO NPs with the space group Fm3m, which is consistent with the JCPDS card No: No 01-075-0447.

The lattice parameter ‘a’ of Co_0.05Ni_xMg_0.95−xO NPs is determined by using the following relation:

$${\displaystyle \frac{1}{d^2}}={\displaystyle \frac{h^2+k^2+l^2}{a^2}}$$

(1)

where (hkl) denotes the Miller indices, and d is the interplanar spacing. The lattice parameter (a = b = c) of the face-centered cubic structure of Co_0.05Mg_0.95O is 4.220 Å. When increasing the concentration of the Ni dopant, the value of the lattice parameter reduces from 4.220 to 4.211 Å. The change in the lattice parameter shows a contraction or expansion in host material due to mismatching of the ionic radius of dopant with the host lattice atom. The decrease in the lattice parameters is due to the smaller ionic sizes of Co²⁺ (0.65 Å) and Ni²⁺ (0.69 Å) dopants as compared to the ionic radius of Mg²⁺ (0.72 Å) [15,18]. This gradual decrease in lattice parameters with increasing concentration of Ni dopant also suggests the possible formation of secondary phases involving Co and Ni oxides. However, no significant change is observed in our XRD data, which provide evidence of the substitutional incorporation of dopant ions into the MgO lattice. Generally, the XRD peak broadening is used to determine the crystallite size of NPs. Several factors such as instrumental broadening, dislocation density, and lattice imperfections can contribute to the broadening of XRD peaks. However, the contribution of instrumental broadening from the measured peak broadening can be reduced by considering the following relation.

$$\beta =\sqrt{{\beta }_{measured}^2-{\beta }_{instrumental}^2}$$

(2)

The average crystallite size (D) of the Co_0.05Ni_xMg_0.95−xO NPs is calculated by using the Debye–Scherrer’s expression [4]:

$$D={\displaystyle \frac{k\lambda }{\beta cos\theta }}$$

(3)

where k is the Scherer constant with a value of 0.9, λ is 0.154 nm, β is the full width at half maximum (FWHM) of the corrected peak broadening, and θ is the Bragg’s diffraction angle in radians. The calculated values of crystallite size are given in

Table 1. Determination of average crystallite size, lattice parameter, and dislocation density of Co_0.05Ni_xMg_0.95−xO NPs using Debye–Scherrer relation.

Sample ID	Average Crystallite Size (nm)	Lattice Parameter (A°)	Dislocation Density (nm−2)
Co0.05Mg0.95O	14.50	4.220	0.505
Co0.05Ni0.01Mg0.94O	13.87	4.217	0.522
Co0.05Ni0.03Mg0.92O	13.85	4.215	0.562
Co0.05Ni0.05Mg0.90O	12.76	4.213	0.614
Co0.05Ni0.07Mg0.88O	11.73	4.211	1.604

. It can be seen that when increasing the Ni dopant concentration, the average crystallite size decreases from 14.50 to 11.73 nm (19% decrease). This reduction in crystallite size also supports the successful substitution of Ni ions into the MgO crystal lattice.

In addition, the dislocation density (δ = 1/D²) provides a measure of the number of dislocation lines per unit volume of the crystal. It can be seen that dislocation density increases with the increasing concentration of Ni dopant (218% at x = 0.07). In addition, different models such as the modified Scherrer’s method, Williamson Hall method, and size–strain plot method have been employed to evaluate the crystallite size and strain [16,17,18,19,20,21,22,23,24,25,26,27,28] (see Figures S1–S5 in Section S1), and their calculated values are given in

Table 2. Determination of crystallite size and lattice strain of Co_0.05Ni_xMg_0.95−xO NPs using modified Scherrer model, Williamson Hall model, and size–strain model.

Sample ID	Modified Scherrer Model		Williamson Hall Model						SSP
			UDM		USDM		UDEDM
	D (nm)	ε (10−3)	D (nm)	ε (10−3)	D (nm)	ε (10−3)	D (nm)	ε (10−3)	D (nm)	ε (10−3)
Co0.05Mg0.95O	13.4	1.0	13.8	0.0008	13.8	0.041	13.8	0.0046	13.8	0.01
Co0.05Ni0.01Mg0.94O	12.7	0.95	12.6	0.0017	12.6	0.07	13.8	0.0078	12.6	0.002
Co0.05Ni0.03Mg0.92O	12.5	1.0	13.8	0.0008	13.8	0.046	13.8	0.0045	10.8	0.003
Co0.05Ni0.05Mg0.90O	10.8	1.0	13.8	0.0001	13.8	0.004	13.8	0.0002	9.39	0.004
Co0.05Ni0.07Mg0.88O	10.8	0.79	13.8	0.0026	10.6	0.07	9.9	0.0084	9.78	0.003

.

It can be seen that the crystallite size calculated using different XRD models are in good agreement with each other. However, the SSP model is considered as an appropriate model in our study since this model shows the best linear correlation with the experimental data.

To confirm the structural modifications in Co_0.05Ni_xMg_0.95−xO NPs, the FTIR pattern of MgO NPs is shown in Supplementary Figure S6 (Section S2). The IR peaks at 846, and 1422 cm⁻¹ are related to Mg-O-Mg and CH₂ bending vibrations [12]. The IR peak at 2355 cm⁻¹ is related to the asymmetric stretching of carbon dioxide from ambient air within the spectrometer [29]. This is due to the incomplete background subtraction of atmospheric carbon dioxide, which can cause the corresponding peak to appear either as positive or negative in the spectra, depending on baseline conditions. Similarly, the IR peaks in the wavenumber range of 1600–1630 cm⁻¹ and 3300–3600 cm⁻¹ are related to O-H stretching vibrations [30]. In contrast, the spectra of Co_0.05Ni_xMg_0.95−xO NPs show characteristic IR peaks at 629, 895, 1125, 1390, 1632, 3453, and 3753 cm⁻¹ (see Figure 1b). The FTIR peaks in the wave number range of 550–670 cm⁻¹ and 850–900 cm⁻¹ confirm the presence of Mg–O–Mg vibrations [12]. The FTIR peaks between 1026 and 1278 cm⁻¹ are related to C-O stretching vibrations [31]. A minor peak at 1390 cm⁻¹ is attributed to the adsorption of carbonate species from the environment [32]. At 1632 cm⁻¹, the IR peak appeared due to the presence of adsorbed water molecules (H-O-H bending). In addition, a wide band in the wave number range of 3300–3600 cm⁻¹ is related to the O–H stretching vibration of water molecules [33]. Similarly, the minor peak at 3753 cm⁻¹ is attributed to an adsorbed species and is most likely due to the incomplete background subtraction of the stretching vibrations of gas-phase water molecules [34]. The presence of IR peaks due to carbonate, carbonyl, and hydroxyl bonding is related to the adsorption of atmospheric CO₂ and H₂O molecules.

The optical bandgap energies of Co_0.05Ni_xMg_0.95−xO NPs are determined from the UV-visible absorption data by using the following Tauc relation for direct band transition,

$${\left(\alpha h\nu \right)}^2=A\left(h\nu -E_g\right)$$

(4)

where α is the absorption coefficient, A is the constant of proportionality, and hν is the photon energy. The optical bandgap energies are found by plotting (αhν)² versus hν and extrapolating the linear part of the curve to the intercept (hν = 0) (see Supplementary Figure S7 in Section S3). The estimated bandgap energy values are mentioned in Figure 2a. The optical bandgap of Co_0.05Mg_0.95O NPs reduces from 5.34 eV (MgO) [12] to 2.3 eV, which is consistent with the reported bandgap of co-doped MgO [35]. With the increasing concentration of the Ni dopant (at x = 0.07), the optical bandgap reduces to 1.98 eV (14%). The observed reduction in the optical bandgap is attributed to the successful incorporation of both Co²⁺ and Ni²⁺ ions into the MgO host lattice. In addition, the existence of structural defects along with the formation of additional electronic states due to the incorporation of dopant ions facilitate the transition from the valence band to the conduction band [36]. Moreover, the similar ionic radii of Co²⁺ (0.65 Å), Ni²⁺ (0.69 Å), and Mg²⁺ (0.72 Å) support the structural compatibility of these dopants within the MgO lattice [37]. Such doping not only enhances the surface area but also reduces the bandgap energy, making the synthesized NPs promising candidates for photocatalytic degradation [12,38,39].

In addition, the photocatalytic activity of Co_0.05Ni_xMg_0.95−xO NPs is investigated by monitoring the photodegradation of MB in an aqueous solution under natural sunlight exposure for 240 min. The MB solution before sunlight exposure is regarded as a reference sample (0 min) to evaluate the photocatalytic degradation efficiency (see Supplementary Figure S8 in Section S4). As the sunlight exposure time increases from 0 to 240 min, the optical absorbance gradually reduces. However, in the absence of Co_0.05Ni_xMg_0.95−xO nanoparticles, the MB showed only 15% degradation efficiency after sunlight exposure for 240 min. The maximum reduction in optical absorbance is observed when using Co_0.05Ni_0.07Mg_0.88O NPs as a catalyst. From the optical absorbance data, the ${\displaystyle \raisebox{1ex}{$A_t$}\!\left/ \!\raisebox{-1ex}{$A_0$}\right.}$ ratio is determined and plotted in Figure 2b versus the sunlight exposure time, where A_t is the optical absorbance at a specific irradiation time and A₀ relates to the optical absorbance at t = 0. It can be seen that the ${\displaystyle \raisebox{1ex}{$A_t$}\!\left/ \!\raisebox{-1ex}{$A_0$}\right.}$ ratio decreases stepwise with increasing concentrations of the Ni dopant and sunlight exposure time.

The photodegradation of Co_0.05Ni_xMg_0.95−xO NPs is determined using a pseudo-first-order kinetic model by using the following equation [40],

$$\ln {\displaystyle \frac{A_t}{A_0}}=-kt$$

(5)

where k is the rate constant. The rate constant values are determined from the slope of the linear fit to the experimental data in Figure 2c, and their numerical values are mentioned in

Table 3. The calculated values of pseudo-first-order kinetic parameters of Co_0.05Ni_xMg_0.95−xO NPs.

Sample ID	Rate Constant (min−1)	R2
Co0.05Mg0.95O	0.0021	0.998
Co0.05Ni0.01Mg0.94O	0.0025	0.973
Co0.05Ni0.03Mg0.92O	0.0032	0.991
Co0.05Ni0.05Mg0.90O	0.0041	0.994
Co0.05Ni0.07Mg0.88O	0.0046	0.992

.

Moreover, the degradation efficiency of Co_0.05Ni_xMg_0.95−xO NPs against the MB is determined by using the following relation [40]:

$$\mathit{Dye\; degratation\; efficiency}\left(\mathrm{\%}\right)={\displaystyle \frac{A_t-A_0}{A_0}}\times 100$$

(6)

The calculated degradation efficiencies of Co_0.05Ni_xMg_0.95−xO NPs with respect to exposure time are shown in Figure 2d. It can be seen that Co_0.05Ni_0.07Mg_0.88O NPs degrade the MB up to 93% with a sunlight exposure of 240 min at max. In addition, with increasing concentrations of the Ni dopant, both the rate constant and degradation efficiency increase (see Supplementary Figure S9 in Section S4). This significantly higher degradation efficiency of Co_0.05Ni_0.07Mg_0.88O NPs against MB may be attributed to the narrower optical bandgap.

The photocatalytic mechanism of Co_0.05Ni_xMg_0.95−xO NPs is shown in Figure 3. With sunlight exposure, electrons (e⁻) in the valence band (VB) are excited to the conduction band (CB). The generated electron and hole pairs are separated due to the presence of Ni and Co dopants that act as trapping centers in the bandgap and result in suppressing the electron hole recombination [12]. The excited electrons react with adsorbed oxygen molecules and reduce them to superoxide radicals. In addition, holes react with water molecules and generate hydroxyl radicals. These superoxide and hydroxyl radicals actively participate in the degradation of MB. The following chemical reactions are involved in the photocatalytic activity of Co_0.05Ni_xMg_0.95−xO NPs [41,42,43].

$${{Co}_{0.05}{Ni}_{x}Mg}_{0.95-x}O+h\nu \to e^{-}+h^{+}$$

(7)

$$e^{-}+O_2\to O_2^{·-}$$

(8)

$$h^{+}+H_{2}O\to {OH}^{.}+H^{+}$$

(9)

$$h^{+},O_2^{·-},{OH}^{.}+\mathit{MB\; dye}\to \mathit{Photodegraded\; products}+{CO}_2+H_{2}O$$

(10)

3. Experimentation

3.1. Materials

For the presented investigation, magnesium nitrate hexahydrate (Mg(NO₃)₂.6H₂O), cobalt nitrate hexahydrate (Co(NO₃)₂.6H₂O), and nickel nitrate hexahydrate (Ni(NO₃)₂.6H₂O) were used for the synthesis of Co_0.05Ni_xMg_0.95−xO NPs without further purification. Sodium hydroxide (NaOH) was used as a precipitant.

3.2. Synthesis of Co_0.05Ni_xMg_0.95−xO Nanoparticles

A stoichiometric amount of Mg(NO₃)₂.6H₂O and (Co(NO₃)₂.6H₂O) was dissolved separately in 100 mL double distilled water to prepare a homogenous solution. Both solutions were then mixed and magnetically stirred for 60 min at 70 °C. During constant stirring, a 2 M solution of NaOH was added dropwise into the homogeneous solution of salts until the desired pH value was reached at 11. This solution was vigorously stirred for 2 h and then aged for 24 h at room temperature to complete the following chemical reaction.

$$0.95\left[\mathrm{Mg}\right({\mathrm{NO}}_3{)}_2.6{\mathrm{H}}_2\mathrm{O}]+0.05[\mathrm{Co}({\mathrm{NO}}_3{)}_2.6{\mathrm{H}}_2\mathrm{O}]\stackrel{NaOH}{\to }{Co}_{0.05}{Mg}_{0.95}(OH{)}_2+by\; products$$

(11)

The precipitate of Co_0.05Mg_0.95(OH)₂ was settled at the bottom of the reaction beaker. These precipitates were filtered and washed three to four times with double distilled water to remove the unwanted impurities. The filtrate was dried at 120 °C for 24 h. After drying, the calcination of the resultant sample was performed at 500 °C for 2 h. Note that for these present investigations, the Co concentration was kept constant at 5% as a control parameter. At this concentration, co-doped MgO NPs exhibit optimized properties with a decrease in bandgap to 2.3 eV.

Similarly, for the synthesis of Co_0.05Ni_xMg_0.95−xO (x = 0, 0.01, 0.03, 0.05, and 0.07) the stoichiometric amounts of Ni(NO₃)₂.6H₂O were added to the homogeneous solution of Mg and Co nitrates to perform the following chemical reaction.

$$(0.95-\mathrm{x})\left[\mathrm{Mg}\right({\mathrm{NO}}_3{)}_2.6{\mathrm{H}}_2\mathrm{O}]+0.05[\mathrm{Co}({\mathrm{NO}}_3{)}_2.6{\mathrm{H}}_2\mathrm{O}]+\mathrm{x}\left[\mathrm{Ni}\right({\mathrm{NO}}_3{)}_2.6{\mathrm{H}}_2\mathrm{O}\left]\stackrel{2NaOH}{\to }{\mathrm{Co}}_{0.05}{\mathrm{Ni}}_{\mathrm{x}}{\mathrm{Mg}}_{0.95-\mathrm{x}}\right(\mathrm{OH}{)}_2+\mathrm{by\; products}$$

(12)

All other synthesis steps were repeated in a similar manner. The precipitate of Co_0.05Ni_xMg_0.95−x (OH)₂ was washed three to four times with double distilled water and then dried at 120 °C for 24 h. Afterwards, calcination was conducted at 500 °C to obtain Co_0.05Ni_xMg_0.95−xO NPs. However, the designations, such as Co_0.05Ni_xMg_0.95−xO, used throughout the study only indicate the nominal molar ratios of metal precursors used during synthesis. The actual composition of the nanoparticles has not been directly measured due to limitations in available instrumentation. The compositional analysis will be conducted in a future study to confirm the stoichiometry and elemental distribution of the synthesized materials.

3.3. Characterization

The structural properties of NPs were investigated by using they BRUKER D8-X-ray advanced diffractometer. The XRD patterns were obtained in the 2θ range of 30°–90° with Cu-Kα radiation. The presence of different functional groups was examined using the IRTracer-100 Fourier transform infrared spectrometer. In addition, the optical absorbance spectra were recorded with the double beam UV–Visible (UV-2800) spectrophotometer in the range of 200–900 nm. MgO NPs were used as a catalyst, and their photocatalytic activity was analyzed using the (UV-2800) spectrophotometer. The photocatalytic activity was conducted under natural sunlight exposure between 11:00 AM to 3:00 PM in May. During this period, the average solar irradiance was 734 W/m². For this purpose, a 10-ppm solution was prepared by dissolving 1 mg of MB in 100 mL distilled water and adding 30 mg of the photocatalyst. The solution was exposed to sunlight for 240 min. After each 40 min of exposure, a sample of 5 mL of the solution was collected, and the optical absorbance of the MB solution was evaluated in the wavelength range of 400–800 nm. Note that the solution was continuously stirred during the exposure period. After exposure, stirring was stopped, and the samples were kept in the dark to allow the particles to settle down at the bottom. A small quantity of the sample was collected for UV-visible spectroscopic analysis. Prior to the measurement, this sample was subjected to ultracentrifugation to remove the remaining NPs from the liquid sample.

4. Conclusions

Co and Ni co-doped Co_0.05Ni_xMg_0.95−xO NPs are successfully synthesized via the coprecipitation method. XRD analysis revealed the face-centered cubic structure of the NPs along with a systematic reduction in crystallite size that supports ionic substitution. The crystallite size and lattice strain are investigated using a modified Scherrer’s plot, Williamson Hall method, and size–strain method; the obtained results are found to be in good agreement with each other. The decrease in the optical bandgap with an increase in Ni concentration resulted in an increase in the photocatalytic degradation of methylene blue dye. Interestingly, Co_0.05Ni_0.07Mg_0.88O NPs showed up to 93% dye degradation efficiency with open sunlight exposure for 240 min. These findings suggest that Co_0.05Ni_xMg_0.95−xO NPs can be considered potential candidates for further utilization in industrial wastewater treatment.