# Synthesis and Characterization of Zinc, Iron, Copper, and Manganese Oxides Nanoparticles for Possible Application as Plant Fertilizers

Authors: Michel E. Neto; David W. Britt; Kyle A. Jackson; João H. V. Almeida Junior; Rodrigo S. Lima; Dimas A. M. Zaia; Tadeu T. Inoue; Marcelo A. Batista
Journal: Journal of Nanomaterials
 (2023)

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## Abstract

This research evaluates the synthesis and characterization of nanometric-sized metallic particles with potential application as support materials for supplying nutrients to plants. Nanoscale Zn, Mn, Fe, and Cu oxides particles were synthesized using microwave-assisted synthesis. Nanoparticles (NPs) were characterized with Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), specific surface area (SSA), and total chemical analysis. Synthesized NPs were all in oxide forms and characterized for confirmation of size, shape, surface structure, crystalline nature, and study of elemental proportion. The results indicate that synthesized NPs size was ranged between 20 and 50 nm and was all in their respective oxide forms as ZnO, Mn3O4, Fe3O4, and CuO.


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## Body

## 1. Introduction

Nanoparticles (NPs) are all those particles which present at least one dimension <100 nm, and normally show properties not found in bulk particles with the same molecular composition [1]. Industries of NPs are developing metal and metal oxides for the improvement of their services and products. The reported applications of these kind of particles have grown in all science areas like food technology, pharmacy, medicine, engineering, and even though agriculture, mainly, due its advances in chemical synthesis methodologies [2], in which, with other synthesis techniques may become easier and faster to produce nanoparticle.Because of their high energy content on surface, NPs are very reactive and thermodynamic unstable [3]. Therefore, there are several interests to synthesize, develop, and commercialize NP [4, 5]. Despite the positive benefits caused by NP, there is also a concern related to their toxicological effects to human [6, 7] and plants [8].Nanoparticles usually differ from their bulk particles exhibiting different physiochemical properties like optical, thermal, and/or electrical [9]. Normally, NPs are built by addition of reducing or precipitating agents during their synthesis [10]. Several factors are responsible for NP reactivity, including size, core composition, shape, surface properties, purity, stability, and method of manufacturing [9]. NPs can be synthesized through physical, chemical, and biological reactions. In the physical reaction, the reduction of a bulk material into smaller particles (nanometer size) occurs through milling, vaporization, and laser ablation. In the chemical reaction, molecules, atoms, or small particles join, becoming larger particles [11, 12]. In biological synthesis, different microorganisms are used to form NPs through enzymatic reactions [13]. Among manufacturing methods, microwave-assisted synthesis has been used to produce oxide NP [14, 15]. This simple method shows some advantages when compared to conventional heating. The advantages of using microwaves are due to homogeneous heating throughout the synthesis process, allowing rapid formation of NP [15]. Furthermore, it is an effective technique for the preparation of nanoparticles. And as a disadvantage, the technique has a shorter crystallization time and homogeneous nucleation due to the uniform heat of the microwave oven [16].In agriculture, NP may be used as nanofertilizer (NF). NF is proposed to enhance nutrient efficiency used by plants, delivering in an “on-demand manner” [17]. This practice promotes efficient way to avoid repeated and exceeding applications of conventional fertilizers, which may cause environmental problems due to their high solubility of conventional fertilizers [18, 19]. In addition, these losses also generate financial expenses and, consequently, negative impacts to any society inserted around these activities.The elements usually studied as NF are zinc (Zn), iron (Fe), copper (Cu), and manganese (Mn) because these elements are essential micronutrient to plants, and are often applied as soluble salts. Nevertheless, micronutrient availability to the plants applied may become low and deficiency occurs in some soils with alkaline pH, sandy texture, or those with low organic matter content [20].Presumably, NF can improve the solubility and dispersion of insoluble nutrients in the soil, reduce fixation, and increase nutrient availability [19]. In order to increase the productive capacity of agricultural areas for the coming years, one of the trends has been the development and use of new fertilizer technologies. Therefore, NF has been shown to improve the efficiency of fertilizer use by several researchers. Thus, this work aimed to quickly and cheaply synthesize and characterize Fe, Zn, Mn, and Cu NPs so that they can be later used as a source of micronutrients for cultivated plants.


## 2. Materials and Methods

### 2.1. Chemicals

The chemicals used in the experiment, iron sulfate heptahydrate (FeSO4·7H2O), zinc sulfate (ZnSO4·7H2O), manganese sulfate (MnSO4·H2O), copper sulfate (CuSO4·5H2O), and sodium hydroxide (NaOH), were purchased from Merck Chemicals Ltd. (Merck KGaA, Darmstadt, Germany) and were stored according to the supplier’s instructions and used as received.


### 2.2. Synthesis of Nanoparticles

All oxides in this research were synthesized from the coprecipitation with the 0.4 mol L−1 alkaline NaOH solution. The FeSO4·7H2O, ZnSO4·7H2O, MnSO4·H2O, and CuSO4·5H2O reagents, all in the concentration of 0.1 mol L−1, were used as precursors for the formation of Fe, Zn, Cu, and Mn nanoparticles, respectively.NaOH 0.4 mol L−1 solution of 100 mL and saline solution 0.1 mol L−1 (FeSO4·7H2O, ZnSO4·7H2O, MnSO4·H2O, CuSO4·5H2O) of 100 mL were prepared and subsequently mixed slowly dropwise (saline solutions) in 500 mL Erlenmeyer flask. Thereafter, the solution was stirred vigorously on a magnetic stirrer at 1,500 rpm in a temperature-controlled environment at 20°C for 15 uninterrupted minutes. The reaction flask was put to the microwave oven Brastemp 38 L (model BMJ38ARANA) for 2.5 min at a power of 300 W. Subsequently, the reaction flask was cooled in a temperature-controlled environment at 20°C.After these procedures, the precipitate was formed in each reaction flask. The flasks were then washed several times with ultrapure water (Milli-Q) to remove excess SO42− and Na+ remaining in solution. The washed precipitate was frozen instantaneously at −70°C in liquid nitrogen, dried by sublimation in Terroni LS 3000 lyophilizer, packed under a vacuum glass desiccator, and protected from light.


### 2.3. Characterization Techniques

The synthesized NPs were measured by X-ray diffraction (XRD),Shimadzu diffractometer using CoKα radiation where λ = 0.179 nm with a nickel filter (35 kV, 20 mA) between 20° and 75° 2θ in the transmission mode for step of 0.01° 2θ/0.6 s. The mean crystallite dimension (D) was broken through the most intense peak using the Debye–Scherrer presentation. The size and shape of the NPs were obtained through transmission electron microscopy (TEM) using a Philips EM300 transmission electron microscope operated at 80 kV with a tungsten filament. Samples were prepared by dispersing 0.05 g in 30 mL of ultrapure water and dispersing by ultrasonic vibration (Bandelin Sonoplus D-12207). The dispersions were placed in a 200 mesh polyvinyl formvar-coated grade and allowed to dry in a temperature-controlled environment.The total contents of Cu, Fe, Mn, and Zn observed in the NPs were acquired through total chemical analysis, where 100 mg of each NP sample was dissolved by adding 3 mL of concentrated H2SO4. Grades were determined using the Agilent® microwave plasma-atomic emission spectrometer (MP-AES). Expected and observed Fe, Zn, Mn, and Cu contents were calculated based on the amounts of Fe, Zn, Mn, and Cu used in the synthesis and the values obtained in the total chemical analysis, respectively, according to the synthesized mineral identified by XRD. Thermogravimetric analysis (TGA) studies were carried out using the Shimadzu DT-60H equipment, where samples were obtained from a temperature of 25–1,000°C with a heating rate of 10° min−1 under an air atmosphere. Specific surface area (SSA) was measured by Brunauer–Emmett–Teller (BET) particle absorption isotherms on Micromeritics FlowSorb II 2300 [21]. Fourier transform infrared (FTIR) spectroscopy was recorded on a 200 mg KBr disk in a Shimadzu FTIR model 8300 spectrophotometer with a spectral resolution of 4 cm−1. FTIR spectra were recorded in the range of 400–4,000 cm−1.


## 2.1. Chemicals

The chemicals used in the experiment, iron sulfate heptahydrate (FeSO4·7H2O), zinc sulfate (ZnSO4·7H2O), manganese sulfate (MnSO4·H2O), copper sulfate (CuSO4·5H2O), and sodium hydroxide (NaOH), were purchased from Merck Chemicals Ltd. (Merck KGaA, Darmstadt, Germany) and were stored according to the supplier’s instructions and used as received.


## 2.2. Synthesis of Nanoparticles

All oxides in this research were synthesized from the coprecipitation with the 0.4 mol L−1 alkaline NaOH solution. The FeSO4·7H2O, ZnSO4·7H2O, MnSO4·H2O, and CuSO4·5H2O reagents, all in the concentration of 0.1 mol L−1, were used as precursors for the formation of Fe, Zn, Cu, and Mn nanoparticles, respectively.NaOH 0.4 mol L−1 solution of 100 mL and saline solution 0.1 mol L−1 (FeSO4·7H2O, ZnSO4·7H2O, MnSO4·H2O, CuSO4·5H2O) of 100 mL were prepared and subsequently mixed slowly dropwise (saline solutions) in 500 mL Erlenmeyer flask. Thereafter, the solution was stirred vigorously on a magnetic stirrer at 1,500 rpm in a temperature-controlled environment at 20°C for 15 uninterrupted minutes. The reaction flask was put to the microwave oven Brastemp 38 L (model BMJ38ARANA) for 2.5 min at a power of 300 W. Subsequently, the reaction flask was cooled in a temperature-controlled environment at 20°C.After these procedures, the precipitate was formed in each reaction flask. The flasks were then washed several times with ultrapure water (Milli-Q) to remove excess SO42− and Na+ remaining in solution. The washed precipitate was frozen instantaneously at −70°C in liquid nitrogen, dried by sublimation in Terroni LS 3000 lyophilizer, packed under a vacuum glass desiccator, and protected from light.


## 2.3. Characterization Techniques

The synthesized NPs were measured by X-ray diffraction (XRD),Shimadzu diffractometer using CoKα radiation where λ = 0.179 nm with a nickel filter (35 kV, 20 mA) between 20° and 75° 2θ in the transmission mode for step of 0.01° 2θ/0.6 s. The mean crystallite dimension (D) was broken through the most intense peak using the Debye–Scherrer presentation. The size and shape of the NPs were obtained through transmission electron microscopy (TEM) using a Philips EM300 transmission electron microscope operated at 80 kV with a tungsten filament. Samples were prepared by dispersing 0.05 g in 30 mL of ultrapure water and dispersing by ultrasonic vibration (Bandelin Sonoplus D-12207). The dispersions were placed in a 200 mesh polyvinyl formvar-coated grade and allowed to dry in a temperature-controlled environment.The total contents of Cu, Fe, Mn, and Zn observed in the NPs were acquired through total chemical analysis, where 100 mg of each NP sample was dissolved by adding 3 mL of concentrated H2SO4. Grades were determined using the Agilent® microwave plasma-atomic emission spectrometer (MP-AES). Expected and observed Fe, Zn, Mn, and Cu contents were calculated based on the amounts of Fe, Zn, Mn, and Cu used in the synthesis and the values obtained in the total chemical analysis, respectively, according to the synthesized mineral identified by XRD. Thermogravimetric analysis (TGA) studies were carried out using the Shimadzu DT-60H equipment, where samples were obtained from a temperature of 25–1,000°C with a heating rate of 10° min−1 under an air atmosphere. Specific surface area (SSA) was measured by Brunauer–Emmett–Teller (BET) particle absorption isotherms on Micromeritics FlowSorb II 2300 [21]. Fourier transform infrared (FTIR) spectroscopy was recorded on a 200 mg KBr disk in a Shimadzu FTIR model 8300 spectrophotometer with a spectral resolution of 4 cm−1. FTIR spectra were recorded in the range of 400–4,000 cm−1.


## 3. Results and Discussion

The XRD pattern of the ZnO NP powder is shown in Figure1(a). From this analysis of XRD patterns, peak intensity, position, and width were determined. The XRD peaks were found at positions at 36.98°, 40.14°, 42.27°, 55.75°, 66.73°, and 74.43° 2θ; these peaks are characteristic and indexed as hexagonal zincite (ZnO), ICDD 050664, with lattice constants of a = b = 0.3254 and c = 0.5213 nm. The average diameter of the crystallite formed in the process was estimated from the Debye–Scherrer equation (d = K (λ/β cos θ)) using the diffraction peak corresponding to the (101) plane located at 42.27° 2θ, resulting in an average value of 26.6 nm.Figure 1
Nanoparticle (NP) X-ray diffraction: (a) Zn NP (zincite-ZnO); (b) Mn NP (hausmannite-Mn3O4); (c) Fe NP (magnetite-Fe3O4); (d) Cu NP (tenorite-CuO).
(a)(b)(c)(d)Figure1(b) shows the XRD pattern obtained from the powdered samples of Mn3O4 NP. The XRD peaks were found at positions at 33.66°, 37.76°, 42.14°, 44.61°, 51.99°, 59.82°, 69.09°, and 70° 2θ; these peaks are characteristic and indexed as hausmannite (Mn3O4), ICDD 240734, of tetragonal phase, with lattice constants of a = b = 0.5762 and c = 0.9423 nm. Debye–Scherrer emission showed that the mean crystallite size was 32.8 nm.Figure1(c) shows the XRD pattern of the powdered samples of the Fe NP. Analyzing the diffractogram, the peak intensity, position, and width were determined, total width at half maximum. The XRD peaks were found at positions at 35.26°, 41.55°, 50.69°, 63.23°, 67.60°, and 74.73° 2θ. The relative position and intensity of the peaks correspond to the characteristic peaks and indexed as cubic phase of magnetite (Fe3O4), ICDD 890691, with lattice constants of a = b = c = 0.8375 nm. The particle size according to the Scherrer equation was 14.1 nm.The diffractogram of the CuO powder material NP is shown in Figure1(d). Peak intensity, position, and width were determined. The XRD peaks were found at positions at 41.33°, 41.53°, 57.21°, 68.89°, and 72.91° 2θ. The relative position and intensity of the peaks correspond to the characteristic peaks and indexed as tenorite (CuO), ICDD 050661, of monoclinic crystals, with lattice constants of a = 0.4522, b = 0.3302, and c = 0.5157 nm. The particle size according to Scherrer’s equation was 35.27 nm.Cu, Fe, Mn, and Zn contents observed in the synthesized NP were 0.2%, 8.8%, 2.4%, and 7.7% lower than expected (Table1). The expected contents for CuO, Fe3O4, Mn3O4, and ZnO were 79.9%, 72.4%, 72.0%, and 80.4%, and the observed values were 78.7%, 66.0%, 70.3%, and 74.3%, respectively. The SSA, measured by the BET method, of the synthetic NP was 43.74, 54.62, 106.45, and 38.68 m2 g−1 for CuO, Fe3O4, Mn3O4, and ZnO, respectively. Figure 2 shows TEM images of all nanoparticles synthesized.Figure 2
Transmission electron microscopy (TEM) of the synthesized particles: (a) ZnO < 20 nm; (b) Mn3O4 < 20 nm; (c) Fe3O4 < 20 nm; (d) CuO with length >100 nm and width <50 nm.
(a)(b)(c)(d)Table 1
Total chemical analysis of synthetic nanofertilizers of Cu, Fe, Mn, and Zn.
MOExpectedObservedSDObserved/expected(mol %)(mol %)(mol %)(%)CuO79.978.70.399.8Fe3O472.466.01.591.2Mn3O472.070.31.297.6ZnO80.474.20.292.3MO, metal oxide; SD, standard deviation.Regarding FTIR results, the ZnO band at 3,163 cm−1 is due to OH stretching of hydroxyl group, probably due to adsorbed water. It should be noted that this band vanished when the samples were heated at temperatures higher than 600°C (Figure 3(a)) [22, 23]. The bands in the region from 600 to 1,700 cm−1 are due to CO2 (g) from the air. The bands at region from 629 to 745 cm−1, 845 cm−1 (sharp), and 1,187/1,303 cm−1 are attributed to in-plane deformation of CO32−, out-of-plane deformation of CO32−, and symmetric/asymmetric stretch of CO32− [24]. The absorption peak associated with Zn‒O stretching band clearly appears at 503 cm−1 confirming the formation of ZnO NP [25].Figure 3
FTIR spectrum of (a) ZnO NP, (b) Mn3O4 NP, (c) Fe3O4 NP, and (d) CuO NP.
(a)(b)(c)(d)For Mn3O4, the bands at 3,400 (broad) and 1,640 cm−1 could be attributed to OH stretching and OH bending, respectively (Figure 3) [26, 27]. The band at 656 cm−1 could be attributed to Mn–O stretching or SO42− bending [24, 27]. These bands are attributed to interaction of SO42− forming a bidentate binuclear (bridging) surface complex with Mn3O4 [28].For Fe3O4, the bands at 3,397 (broad) and 1,644 cm−1 could be attributed to OH stretching and OH bending, respectively [29]. The bands at 898, 796, and 629 cm−1 could be attributed to OH in-plane bending, OH out-of-plane bending, and Fe–O symmetric stretching, respectively [30]. The bands at 898 and 796 cm−1 are characteristic of goethite. However, X-ray diffractogram (Figure 3(c)) did not show any characteristic peak of goethite, probably because FTIR spectroscopy is a much more sensitive technique than X-ray diffractometry.For CuO, the bands at 3,402 (wide) and 1,643 cm−1 reveal the presence of OH elongation and OH bending for the alkyl group. The bands at 1,370 and 1,096 cm−1 are characteristic bands of Cu NPs due to Cu2+, –O2‒, and SO42− stretching [28, 31]. The band attributed to SO elongation of SO42− is very weak, meaning that very low concentration of SO42− adsorbed onto CuO. Furthermore, single band means that SO42− is interacting with CuO as a sphere complex [28].Figure4(a) shows TGA of ZnO NP. A high rate of mass loss was observed between 30 and 100°C. A high rate of ZnO mass loss was observed in the initial stages when the water adsorbed on the surface of the NP evaporates [32]. The second phase of water loss around 350°C occurs due to decomposition and is associated with the rupture of weak bonds and unpacking of highly strained crosslinks, resulting in the formation of straight Zn(OH)2 chains [33]. After 470°C, the aggregation of ZnO NP is expected because the exposed irradiated microwaves increase the surface energy of ZnO NP [34].Figure 4
Thermal analysis spectrum of (a) ZnO NP, (b) Mn3O4 NP, (c) Fe3O4 NP, and (d) CuO NP.
(a)(b)(c)(d)The thermal behavior of Mn NP through the TGA technique is shown in Figure4(b). There are two mass losses: the first up to 100°C and the other above 500°C. The mass loss at 100°C is due to the evaporation of water absorbed on the surface of Mn3O4. Furthermore, above 500°C, Mn3O4 will be converted into Mn2O3 [35]. The gradual weight loss above 100°C can be attributed to the superficial sintering of Mn3O4 NP, although the reason for this phenomenon is still unclear [36].The mass loss for Fe3O4 was 5.7% for the entire temperature range due to physical and chemical water removal from adsorbed water (100–130°C) (Figure 4(c)). When Fe3O4 is brought to high temperatures, products containing O2 may occur by oxidation reactions of magnetite into hematite (4Fe3O4 + O2 ⟶ 6Fe2O3) [37]. The mass loss of CuO suggests, first, up to 150°C, losses caused by evaporation of free and coordinated water in CuO NP. The second mass loss between 250 and 400°C is due to the removal of counter anions in gases [38] through dehydration and pyrolysis. Above 400°C, weight loss may be due to loss of oxygen (Figure 4(d)).The NPs synthesis process through the microwave use was efficient. The method was quick and simple. Several authors for the synthesis of oxides, hydroxides, and oxyhydroxides nanoparticles [39] have already demonstrated the efficiency of the microwave-assisted method. The benefits of this method in relation to the hydrothermal method are: (1) rapid reaction, (2) simple method, (3) short time to reach the appropriate temperature for the reaction, and (4) the morphology of the particles is in control [15]. In addition, XRD diffractogram (Figure 1) shows that the powdered particles have low impurity contents, since they did not present with any other peak that is not present in the zincite, hausmannite, tenorite, and magnetite diffractograms.The presence of other elements composing the minerals or the incomplete formation of the minerals may explain this subtle difference between the observed and expected contents of the elements (Table1). The SSA values found for ZnO in this work are larger than other ZnO synthetic nanoparticles [40, 41] and slightly lower [42]. Regarding Mn3O4, which was the highest BET value found in this work, but despite that, BET values were lower (177.6 m2 g−1) [43]. It was found that SSA varies between 7.3 and 104 m2 g−1 for Fe3O4 [44]; therefore, the results obtained by authors mention that the synthesis by different routes of precipitation and degree of oxidation alter the particle size. The values of SSA found in this work for CuO were higher [19, 45], even though within an acceptable size range.ZnO particles are well separated from each other, and it is also evident in the dark field that each particle is a single crystal of rounded shape [46]. The particle size shown by TEM was lower than 20 nm, showing a very narrow size distribution, which is characteristic of mechanically synthesized particles [47]. The estimated XRD size using the Scherrer equation was 21.3 nm and is well in agreement with the TEM images, since the sizes obtained were similar.The morphology, structure, and distribution of Mn3O4 were described by Chang et al. [48], and some synthesized particles have cubic forms. The particle size shown by TEM was around 20 nm when viewed on a single particle, having a diameter close to that estimated by the Scherrer equation in XRD (26 mm). In general, the size of the Mn3O4 particles depends on temperature during the synthesis, which means, the lower the growth temperature, the smaller the particle size [48].Regarding Fe3O4, most of the particles appear to be uniform and spherical in shape. Allied to this, the particles show high aggregation capacity. The preparation of the sample to be analyzed in the MET may be one of the reasons that induce colloid aggregation [49]. The size of the individual particles is shown to have a diameter of about 20 nm, like the diameter estimated by XRD using the Scherrer equation.CuO morphology is thin and pointed like needle or wires [50]. The aggregation is lower than the Cu particle when compared to the Fe and Mn particles, even though the Cu particles are larger when compared to the others. According to the Scherrer equation, the particle has an average size of 31.5 nm, although the TEM image showed that the particles can be up to 100 nm in length.


## 4. Conclusion

It was possible to synthesize nanoparticles of Cu, Fe, Zn, and Mn at the nanoscale. The synthesis methodology provided an increasing scale of Zn, Mn, Fe, and Cu sizes, although all particles presented at nanoscale. The characterization methods were efficient to measure the size and quality of synthesized NF.



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*Source: 1312288-2023-02-23.xml*