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Article

The Solution Combustion Synthesis of ZnO Nanoparticles Using Allium schoenoprasum (Chives) as a Green Fuel

by
Elyas Sheibani
,
Saman Soltani Alasvand
,
Neda Sami
,
Jalil Vahdati Khaki
and
Sahar Mollazadeh Beidokhti
*
Department of Materials Engineering, Faculty of Engineering, Ferdowsi University of Mashhad (FUM), Azadi Square, Mashhad 917794-8564, Iran
*
Author to whom correspondence should be addressed.
Compounds 2024, 4(3), 503-520; https://doi.org/10.3390/compounds4030030
Submission received: 2 June 2024 / Revised: 25 July 2024 / Accepted: 14 August 2024 / Published: 19 August 2024
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Abstract

Zinc oxide (ZnO) nanoparticles are widely recognized for their distinctive properties and versatile applications across diverse technological domains. However, traditional methods of synthesizing ZnO nanoparticles are characterized by environmental incompatibility, high costs, and the necessity for precise process control to attain the intended particle dimensions and morphology. The present study utilized a chives extract as a sustainable and eco-friendly fuel in the solution combustion synthesized (SCS) process to produce ZnO nanoparticles. The investigation encompassed an analysis of the impact of the fuel-to-oxidizer (F/O) ratio on the synthesized ZnO nanoparticles’ size, morphology, and crystallinity. X-ray diffraction (XRD) results showed that the particle’s crystallite size increased significantly from 12 nm to 42 nm after decreasing the F/O ratio. Furthermore, electron microscopic imagery and FTIR spectroscopy outcomes indicated that modifications in the F/O ratio significantly influenced the SCS process parameters, forming particles with diverse morphologies, including spherical, pyramid-like, hexagonal, and hexagonal plate-like shapes. This research presents a straightforward, cost-efficient, and environmentally sustainable approach for producing ZnO nanoparticles with diverse morphologies, presenting a broad potential for various applications.

1. Introduction

Nanotechnology involves the study of materials with a size between 1 and 100 nanometers. In recent decades, there has been tremendous interest in metal nanoparticles (NPs) and metal-oxide nanoparticles (MONs). Due to their small size and high volume-to-surface ratio, nanoparticles exhibit atom-like behaviors that make them unique compared to bulk materials [1,2,3]. This enables them to have various applications in medicine, drug delivery, solar cells, water purification, electronics, optics, antibacterial agents, transistors, and magnetics [2,3,4,5]. Regarding metal-oxide nanoparticles, ZnO is a promising and vital candidate [6].
ZnO exhibits several unique characteristics. These include a wide band gap of approximately 3.37 eV, a significant excitation binding energy of 60 meV, a high charge carrier mobility, a strong oxidation ability, and being one of the hardest transition metal oxides [6,7]. Due to these characteristics, ZnO can be used in various innovative applications. Biological, antibacterial, and photocatalytic agents are some of the possible applications of this compound [1,2,6]. ZnO nanoparticles (NPs) have been synthesized using conventional methods like sol-gel, co-precipitation, and chemical vapor deposition [1,2]. However, these methods are environmentally toxic due to their synthetic nature [2].
As far as synthesizing NP methods are concerned, there are two major approaches: top-down and bottom-up. By using a top-down approach, solid bulk material is broken down into smaller pieces by an external force. This force can be chemical or physical [3]. The bottom-up approach involves the combination of molecules and atoms to create nanoparticles [3]. A few of these methods are illustrated in Figure 1 [3,5]. As a result of the top-down approach, particles cannot be obtained easily, and the process is more expensive [3,4]. Alternatively, using the bottom-up approach, fine, small particles can be produced with controlled morphology [3,4].
Solution combustion synthesis (SCS) is a preferred bottom-up process for producing NPs due to its simplicity, cost-effectiveness, environmental friendliness, and ability to generate high-quality powders [8,9]. An SCS process involves highly exothermic, rapid, and self-sustaining redox reactions involving metal nitrates (mainly) as the oxidizer and one or more organic compounds serving as fuels. SCS begins with dissolving metal nitrates and organic fuel in an aqueous medium to produce a homogeneous solution. Next, heating the homogeneous solution causes dehydration, thermal decomposition, and gel formation. Finally, the gel ignites (at one point or at once in the entire volume), causing a release of heat and a large amount of gas. The combustion reaction produces nanoparticles as a porous mass that remains at the bottom of the container. The SCS process is not just about the redox reaction; it is a complex interplay of factors. Various organic materials, including glycine, urea, citric acid, etc., can be used as fuel in the SCS process. These materials provide the conditions for the redox reaction and serve as chelating agents and microstructural templates. The diverse functional groups present in these organic compounds are the key to their successful performance as fuel in SCS, enlightening the fuel’s multifaceted functions in this process. For instance, fuels containing a -NH2 group are more redox active during the combustion reaction than others with -OH or -COOH groups, leading to a more intense combustion reaction [8,9]. As a result, the selected fuel in SCS can affect the characteristics of the synthesized powder, such as purity, crystallinity, particle size, morphology, etc.
Green or biological synthesis, an environmentally friendly method that uses biological organisms or natural resources to produce NPs [3], offers a cost-effective advantage. In this method, NPs can be synthesized in a single step using microorganisms, plants, algae, yeasts, and fungi. NPs produced in this manner are more biocompatible, less dangerous, and more stable than those produced through chemical and physical synthesis methods [2,3,4,7]. Plant extracts, rich in phytochemicals and biological molecules such as amides, flavonoids, carbohydrates, phenols, carboxylic acid, etc., can serve as reducing and capping agents for NP synthesis [7,10]. The cost-effectiveness of biological synthesis is a significant advantage, as evidenced by the research on the use of extracts from various plant parts, such as aqueous extracts of fruits [7,10,11,12], plant extracts [13], and a type of algae [14] in the synthesis of ZnO NPs.
Allium schoenoprasum, commonly known as chives, is a plant that is both widely available and affordable. It is naturally widespread in Europe, Asia, and North America [15], and its cultivated variety is available year-round at a reasonable price. Chives are primarily used in cooking due to their aroma and taste. However, they also have medicinal uses, thanks to their anti-inflammatory, anti-cancer, antioxidant, anti-parasitic, and anti-blood pressure properties [16]. Fresh chives, as researched by Iosin et al. [15], can contain 268.15 μg/g of chlorophyll, which is responsible for the plant’s green color. In addition to their culinary and medicinal uses, chives contain valuable chemical compounds that can act as reducing or capping factors in the SCS process, as shown in Table 1 [16].
In this research, a chives extract is used for the first time as a green fuel to produce ZnO NPs in a solution combustion synthesis (SCS) process. Using different analyses, the role of the chives extract in the purity, crystallite size, morphology, and size of the synthesized particles is also investigated.

2. Materials and Methods

2.1. Materials

Zinc nitrate hexahydrate (Zn (NO3)2·6H2O, analytical grade-Merck, 95%) was used as the source of zinc ions and an oxidizer agent. Fresh chives were purchased from a local store (used as a reducing agent), and deionized water was from Iranshimi (used as a solvent). All materials were directly used to synthesize the ZnO nanopowders.

2.2. Preparation of Extract

This study used green, healthy chive leaves that were fully ripe and mature. These plants were procured from the local vegetable market in Mashhad, Khorasan province, Iran, which is marketed by local producers. The plants used were artificially cultivated, and the wild type of this plant was not used. The acquired chives were first carefully washed to remove any dust and dirt. After that, they were allowed to air-dry. A blender was used to extract the essence of the dried and cleaned chives leaves, resulting in a dark green solution. In order to reduce the liquid’s water content, the solution was placed in a large pan. It was allowed to evaporate naturally at room temperature for 72 h. During evaporation, green powder-like precipitates formed inside the pan. At the end of the extraction process, the chives extract was collected and stored in a sealed container for future use.

2.3. ZnO NPs Synthesis

In this research, the chives (Allium schoenoprasum) extract was used as a green fuel for ZnO NP synthesis using the SCS method. When plant extracts are used as fuel in the SCS process, their phytochemical compounds can act as reducing agents. Among the most effective ones, flavonoids, terpenoids, carbohydrates, proteins, enzymes, and organic acids (such as citric acid) can be mentioned. Phytochemical compounds’ effectiveness as reducing agents varies with their concentration in the plant extract, the plant species used, and the extraction method [17,18,19]. According to the review conducted by Zuhrotun et al. [17], 3 categories of polyphenol compounds, reducing sugars, and proteins are the main reducing agents in the green synthesis of NPs. Nevertheless, it is important to note that the exact mechanisms and specific compounds involved in the redox reaction may differ depending on the plant species used and the type of metal NPs synthesized. On the other hand, plant extracts usually contain a complex mixture of phytochemical compounds. Each compound, individually or in combination, has the potential to act as a reducing agent [17,20,21]. Consequently, determining the exact mechanism and the main reducing agent in the green synthesis of NPs is a challenging task.
Regarding chives extracts, data obtained by different analytical techniques show that this plant contains phytochemical compounds, such as phenolic compounds, flavonoids, amino acids, etc. [16]. Chives extracts also contain significant amounts of chlorophyll [15]. Each of these compounds has the potential to act as a reducing agent in ZnO synthesis through the SCS process. Among these organic compounds, chlorophylls are nature’s most essential and common pigment molecules, especially in green plants. Chlorophylls are widely distributed in green plants, algae, photosynthetic bacteria, and some animals. They play the leading role in absorbing, transporting, and transmitting light energy in photosynthesis. During photosynthesis, chlorophyll converts sunlight, carbon dioxide, and water into sugar and other substances necessary for plant growth [22,23].
Plants have different chlorophyll types, the most abundant of which is chlorophyll type (a). Chlorophyll (a) is responsible for the main task in photosynthesis [23]. Based on the searches conducted in accordance with this research, no report has been found on using chlorophyll as a fuel in the SCS process. In this research, chlorophyll (a), with the chemical formula C55H72O5N4Mg, is considered the main reducing agent in the chives extract. Equation (1) shows the reaction between zinc nitrate hexahydrate and chlorophyll (a) in the production of ZnO.
5C55H72O5N4Mg + 142Zn(NO3)2·6H2O → 142ZnO + 5MgO + 275CO2 + 1032H2O + 152N2
Five samples were prepared based on the assumption that the chives extract could contain different chlorophyll concentrations, to demonstrate the practicality of selecting chlorophyll as the primary reducing agent. Each sample contained one gram of chives extract. The chlorophyll concentration in the samples was assumed to be 100% (Sample 10X), 90% (Sample 9X), 80% (Sample 8X), 50% (Sample 5X), and 30% (Sample 3X). According to Equation(1), the required amount of zinc nitrate was initially calculated for sample 10X (assuming a chlorophyll concentration of 100%). Based on this, when using 1 g of chives extract (molecular weight for chlorophyll: 893.49 g/mol), 9.45 g of zinc nitrate hexahydrate is required (297.48 g/mol). For the other samples, zinc nitrate quantities were calculated based on assumed chlorophyll concentrations and compared to the 10X sample. For instance, the amount of zinc nitrate required for the 9X sample (assuming 90% chlorophyll concentration) was calculated by multiplying 9.45 g by 0.9. Table 2 presents the amounts of zinc nitrate used in each sample.
One of the main parameters that strongly affects the combustion reaction as well as the properties of the NPs synthesized through the SCS process is the fuel-to-oxidizer ratio, often referred to as the F/O ratio. The F/O ratio is the ratio of the total reducing capacity of the fuel to the total oxidizing capacity of the oxidizer. When the F/O ratio is equal to 1, it means that the amounts of fuel and oxidizer in the precursor solution are such that they can completely react with each other without any excess components remaining. This condition is also called the stoichiometric ratio. Specifically, the stoichiometric conditions in the 10X sample refer to the F/O = 1 ratio. A decrease in chlorophyll concentration in the chives extract is associated with a decrease in zinc nitrate. The reduction of zinc nitrate in each sample means the reduction of oxidizer agents in the precursor solution, or, in other words, increasing the F/O ratio. Various analyses will be used to evaluate the effect of changes in the F/O ratio on the properties of the synthesized NPs in each sample
The synthesis process was as follows: 1 g of the chives extract was stirred in a beaker for 10 min with 5 mL of deionized water. Afterward, the weighed quantities of zinc nitrate for each sample were added to the beaker and stirred for a further 10 min. The dark green solution obtained at this stage was transferred to a porcelain crucible and placed inside a preheated air-containing atmosphere furnace set at 500 °C. The combustion reaction was complete after four minutes, and a white powder was left at the bottom of the crucible. A mortar and pestle was used to grind the powder to be able to use it for further analyses.

2.4. Characterization of ZnO

X-ray diffraction was used for phase identification and analysis of the powder by EXPLORER GNR using Cu Kα radiation (λ = 1.541 Å). Fourier transform infrared spectroscopy (FTIR) determined the residual fuel and surface chemical bonds. The THERMO NICOLET AVATAR 370 was used for this purpose at room temperature and in the range of wave numbers 400–4000 cm−1. Differential thermal analysis and thermogravimetric analysis (DTA-TGA) were conducted to study thermal behavior during synthesis. The analysis was conducted with an STA 503 BAHR instrument made in Germany, in an air atmosphere, at a heating rate of 10 °C/min up to 700 °C. In addition, the microstructure and morphological evolution of the as-synthesized samples and their chemical composition were investigated using field emission scanning electron microscopy (FESEM) at an accelerating voltage of 15 kV and energy dispersive X-ray (EDX). The analysis was performed with the LMU TESCAN BRNO-Mira3 model. All of the samples were sputter-coated with a thin layer of gold to create a conductive surface on them. To determine the particles surface charge, the zeta potential of synthesized powders was measured by a Zeta Compact CAD FRANCE.

3. Results

3.1. XRD

Figure 2 displays X-ray diffraction (XRD) patterns of as-synthesized samples using an Allium schoenoprasum (chives) extract as fuel. According to ICDD card no. (01-079-0205), all diffraction peaks at 31.85°, 34.55°, 36.36°, 47.7°, 56.75°, and 68.17° correspond to (100), (002), (101), (102), (110), and (112) planes of hexagonal wurtzite structure, respectively. These results suggest the formation of well-crystalline ZnO in a solution combustion reaction without subsequent heat treatment. However, in samples 3X and 5X, unknown diffraction peaks are observed (marked with * symbol) that are not associated with the crystal structure of ZnO. Due to the high fuel-to-oxidizer ratio in the 3X and 5X samples, these unknown peaks are likely related to residual fuel. To further investigate the proposed peaks and measure their probability, XRD analysis was performed on the chives extract (fuel). The result is presented in Figure 3. According to ICDD card no. (01-073-0380), all indexed diffraction peaks of (200), (220), (222), (400), (420), and (422) are in good agreement with the cubic structure of potassium chloride (KCl). Paulus et al. [24] reported that chives are a rich source of potassium (K). Due to its high reactivity, the K+ ion should be found in the chives structure as KCl. According to Figure 2, the diffraction peaks related to KCl have been eliminatedfrom samples 8X, 9X, and 10X by reducing the fuel-to-oxidizer ratio.
In addition, the diffracted peaks related to KCl have some shift in the spectrum of the 3X sample compared to the spectrum of the chives extract. The change in the position of the diffraction peaks of KCl in the two samples can be attributed to the change in the parameters of the crystal lattice according to Bragg’s law. SCS, through variables such as fuel type, F/O ratio, and combustion temperature, can affect the crystal lattice characteristics of the synthesized powder [25]. Some of the crucial crystal characteristics that should be taken into consideration are the crystallite size [25], the concentration of dopants within the crystal structure [26], the lattice strain resulting from crystal defects [27], and the crystallite size. The changes created in each of these characteristics will cause a change in the parameters of the crystal lattice and then a change in the position or shift of the diffraction peaks of KCl. Further analysis of this phenomenon is beyond the goals of this study.
Another result of reducing fuel-to-oxidizer ratios is a reduction in diffraction peak broadening. In the XRD spectrum, the broadening of the diffraction peaks is directly related to the size of the crystallites. Accordingly, the crystallite size of the as-synthesized samples was calculated using the Debye–Scherrer (Equation (2)) and Williamson–Hall (Equation (3)) equations as follows:
D = k λ / β   c o s θ
β cos θ = 4 ε sin θ + k λ D
where D is the mean crystallite size, k is the crystallite-shape factor (0.9), λ is the X-ray wavelength for Cu Kα (1.5406 Å), β is the full width at half maximum of peaks (FWHM) in radian, and θ is the Bragg angle. The characteristics of the (101) planes were used to calculate the crystallite size using Equation (2). In Equation (3), using a plot of βcosθ versus 4sinθ, ε equals the slope of the line, which describes crystal strain. Furthermore, the y-intercept provides the value of kλ/D, which was used to calculate the mean crystal size.
Figure 4 shows crystallite size calculations using both methods. The results obtained from both methods indicate that crystallite size increases with decreasing F/O ratios. Among the samples, 3X and 5X have the lowest calculated crystallite size. Additionally, these samples have the highest F/O ratios. During solution combustion synthesis, changes in the fuel-to-oxidizer ratio directly affect the amount of released heat and gases. The crystallite size calculation for ZnO synthesized at different F/O ratios, previously reported by other researchers [12,28,29,30,31], is shown in Table 3. It is evident that increasing the F/O ratio has led to a reduction in crystallite size, regardless of the type of fuel used. Increasing the F/O ratio generates more gases during the combustion reaction. The increased amount of released gases, through convection, removes more combustion heat from the system [28,29,30]. Increased heat loss from the system eventually results in a reduction in crystal growth.
There is also a difference in crystallite size calculated using both methods, which is more noticeable in larger crystallite sizes. Other researchers have reported similar results [32,33]. The differences between calculated crystallite sizes can be explained by the assumptions made in the equations of these two methods. The crystallite size calculated by Scherrer’s equation mainly relies on the characteristics of one peak (the most intense peak). In contrast, the Williamson–Hall method uses the characteristics of multiple peaks to create a graph and provides a more comprehensive analysis. On the other hand, according to Scherrer’s equation, the broadening of diffraction peaks is attributed directly to crystallite size. However, in the Williamson–Hall equation, the crystal strain effect (e) is considered in addition to peak broadening to calculate the crystallite size. Despite the simplicity of calculating the crystallite size by the Scherrer method, according to the explanations given earlier in this section regarding the effects of the SCS process on the crystal lattice parameters such as the lattice strain, calculating the average crystallite size by the Williamson–Hall method is a closer choice to reality.
In this study, the highest calculated crystallite size is for the 9X and 10X samples, which have a chemical composition close to stoichiometry. In addition, in these two samples, no diffracted peaks are attributed to residual compounds. The optimal crystallinity and phase purity obtained in the nanoparticles obtained in the two mentioned samples implicitly show that the chives extract contains suitable phytochemical compounds, including chlorophyll, which can react as reducing agents in the redox reaction with oxidizing species in the precursor solution (zinc nitrate) and provide the necessary energy for forming crystalline ZnO nanoparticles. In addition, as a fuel in the SCS process, the chives extract has high efficiency. The high concentration of phytochemical compounds has led to a relatively complete reaction with the oxidizing agents present in these two samples. As a result, no detectable phase other than ZnO is observed in the XRD spectrum of these two samples. These results also show that the hypothesis considered in these two samples regarding the concentration of chlorophyll (the main reducing agent) in the chives extract is acceptable and applicable.

3.2. FTIR

FTIR spectroscopy was used to investigate chemical functional groups and surface bonds of the chives extract and synthesized zinc oxide nanoparticles. Figure 5 illustrates the FTIR spectrum of the chives extract. The mentioned chives, named Allium schoenoprasum, has several constructive compounds, including vitamin C (C6H8O6), flavonoids (C15H12O2), chlorophyll (a) (C55H72MgN4O5) and (b) (C55H70MgN4O6), and carotenoids (C40H60). Studies show a particular relation between a sample’s constructive compounds and the FTIR-related bands. Primarily, the bands at 624 and 2850 cm−1 were related to the C-H stretching symmetric of =CH2 (alkenes), and the C-H stretching asymmetric of =CH2 is at 2922 cm−1. The band at 1063 cm−1 indicates the presence of the C-O group in esters or carboxylic acids. Based on its unique structure, a double bond of carbon (C=C) could be attributed mainly to the flavonoid compounds and the allyl group in alkenes at 1404 cm−1. An N-H band could be seen in the range of 1635 cm−1, which was a specific sign of only chlorophyll (a) and (b). In addition, at 3399 cm−1, the O-H band could be related to the hydroxyl group of the -COOH group or adsorbed structural water. According to the results of FTIR analysis and the reducing property N-H and C-H related functional groups, the chives extract can be used as a fuel in redox reactions that occur in solution combustion synthesis [34,35,36,37,38,39].
Figure 6 presents the FTIR spectrum for the five synthesized samples. The absorption band in the 400–615 cm−1 range is related to the characteristic bond of Zn-O, which indicates the formation of ZnO. In the range of 1000–1226, three adsorption bands at 1037, 1108, and 1186 cm−1 are attributed to asymmetric and symmetric C-O stretching vibrations of the C-O-C linkage [38,40]. The peaks’ intensities are at their maximum in the spectra of the 3X and 5X samples. This result may be attributed to the incomplete combustion of fuel, and as a result, the presence of residual fuel in these two samples. The observed bands at 825 cm−1 and 1384 cm−1 are related to vibrational modes of nitrate ions (NO3−) caused by unreacted zinc nitrate [41,42]. The broad band observed at a range of 3311–3623 cm−1 and also a band at 1644 cm−1 represent O-H stretching and bending vibrations of water molecules, respectively [38]. These O-H bonds are likely caused by either adsorbed water molecules on the surface or compounds with hydroxyl groups (OH−) [14,38].
After the combustion reaction, a subsequent heat treatment can be performed to remove functional groups from unreacted components. However, to increase the reaction efficiency in a single-step SCS process, more advanced extraction methods can be used to extract the chives. Plant active compounds are usually located in plant matrices; as a result, it is necessary to use appropriate extraction methods to extract the active compounds [43,44]. The use of appropriate extraction methods helps to increase the reactivity between the chives extract and zinc nitrate and reduce the residual compounds after the combustion reaction.
Consequently, the reaction becomes more efficient by increasing the participation of oxidizing and reducing species in the reaction.

3.3. FESEM

A field emission scanning electron microscope (FESEM) was employed to investigate the morphology and microstructure of synthesized ZnO nanoparticles. The results show that using the chives extract as a fuel in different F/O ratios in the SCS process has resulted in producing particles with various morphologies. As per the observations in Figure 7, the decrease in the F/O ratio led to an initial agglomeration of particles, subsequent particle growth, and the manifestation of diverse morphologies. Several researchers have investigated the effects of fuel type and F/O ratio on the size and morphology of ZnO particles produced by the SCS process. The results of some of these studies are summarized in Table 3. According to the results of previous research [28,30,31,45], the three main factors affecting the morphological changes and particle size produced by the SCS method are as follows:
  • The heat of the combustion reaction;
  • The amount of gas released during the reaction;
  • The type and number of functional groups in the fuel.
Through changes in the intensity of the above factors, the type of fuel and the fuel-to-oxygen ratio affect the size and shape of the synthesized particles.
In the sample 3X, the high F/O ratio causes the release of large amounts of gases during the combustion reaction. High amounts of released gas prevent the growth and agglomeration of particles by removing more heat from the system [12,30,46]. This phenomenon causes relatively spherical-shaped particles with a relatively uniform size distribution of 20–50 nm.
In sample 5X (Figure 7b), by reducing the F/O ratio, the relatively spherical morphology of the particles is maintained. However, due to aggregation, the relatively homogeneous distribution of particles is significantly reduced, and clusters are formed. A further decrease in the F/O ratio in sample 8X (Figure 7c) causes a decreased volume of produced gas during the combustion reaction and a more favorable heat release. Favorable heat release has caused the primary clusters to merge, and as a result, increase the growth rate of particles with pyramid-shaped morphology. The increase in the particle growth caused by the reduction in the F/O ratio is in good agreement with the obtained XRD results.
The formation of hexagonal particles was another morphological change that occurred with a further reduction of F/O in samples 9X and 10X (Figure 7d,e). In contrast to samples 3X and 5X, samples 9X and 10X displayed considerably larger particle sizes, with some measuring up to 1 μm. The production of ZnO particles with hexagonal morphology indicates the good growth of particles in sample 9X. The morphological changes caused by the reduction in the F/O ratio are schematically depicted in Figure 7f.
Regarding the formation mechanism of hexagonal plate-like morphology (sample 10X), it should be mentioned that a hexagonal–wurtzite structure is the most common form of ZnO. During the crystallization of wurtzite, Zn2+ and O2− ions are arranged along the c-axis in alternating planes. Inherently, due to the higher energy of the (0001) crystal plane than other planes, ZnO crystals strongly tend to grow along the [0001] direction or the c-axis. Nevertheless, when growth along the [0001] direction is suppressed and continues along the lateral directions, hexagonal disk-like morphologies of ZnO are formed [47,48].
Significantly, the suppression of growth in the [0001] direction can be achieved through the adsorption of specific ionic species on the (0001) basal plane, and subsequently, the stabilization of the surface charge on this plane [49]. Vasei et al. [28] synthesized ZnO hexagonal nanodisks by adding a certain amount of PVP to a zinc nitrate-containing precursor solution. They explain that by adding a certain amount of PVP to the precursor, the -C=O functional groups present in PVP are adsorbed on the positive (0001) facet of the initial ZnO nuclei. Such selective adsorption suppresses the crystalline growth of the (0001) facet along the c-axis, and ultimately, the formation of hexagonal nanodisks.
Further research is needed to determine which ionic species is responsible for ZnO crystal growth orientation at 10X. However, the NO3− functional group is the most likely species to suppress the crystal growth along the [0001] direction and form the hexagonal plate-like morphology. According to Table 2, zinc nitrate concentration increased from 8.5 g in the 9X sample to 9.45 g in the 10X sample. As nitrate concentrations increase, more NO3− functional groups are absorbed on the positive (0001) facet of the primary ZnO nuclei, leading to growth suppression along the c-axis. By suppressing growth in the [0001] direction, growth continues in the lateral directions of the crystal. Consequently, hexagonal plate-like particles with an average size of 1 μm and a thickness of 50 nm are obtained at 10X. Despite the explanations provided, further investigation is required to determine the exact mechanism of hexagonal plate-like ZnO particles produced in a SCS process using the chives extract as fuel. The results show that the organic compounds in the chives extract, in addition to playing a role as a reducing agent, also acted as template agents in the SCS process and caused the production of ZnO NPs with various morphologies.
The elemental analysis of synthesized nanoparticles for two samples, 3X and 10X, is shown in Figure 8. Moreover, each graph includes an inset table showing each sample’s weight and atomic percentage of elements detected. The peaks that appeared are mainly related to Zn and O elements; however, these samples also contain small amounts of C, Cl, and K.

3.4. Zeta-Potential

Due to the importance of NPs colloidal stability, zeta potential analysis was conducted to investigate the surface charge of as-synthesized ZnO samples. Before testing, the powder was weighed and dispersed in deionized water, followed by a 30-min ultrasonic bath. Figure 9 presents the results of this analysis. Based on these results, all five samples measured a negative surface charge. Based on the graph, it can be seen that the changes in the samples’ zeta potentials have an irregular trend. However, the 8X, 9X, and 10X samples have recorded more negative zeta potential than 3X and 5X samples. Considering the similar conditions of the analysis, such as media and pH, we will examine other parameters that may affect the result. Particle size and its structure and surface functional groups are the other significant parameters that can influence the zeta potential of ZnO NPs here. In the following, particle size’s effect on the final suspension’s stability has been investigated. Smaller particles aggregate and form clumps due to their larger surface-to-volume ratio [50]. Generally, samples have changed in agreement with the above argument. This leads to increased mobility and interaction between particles. On the contrary, samples 5X and 10X show a trend unlike the others. In the mentioned samples, the particles become more significant than a critical limit and reach the limit of the opposition of the gravitational force caused by the earth and the mobility caused by the electric field [51]. Regarding surface functional groups and according to FTIR results, the role of nitrate (-NO3) and hydroxyl (-OH) groups can be investigated. According to Assad et al. [52], under neutral pH conditions, the presence of -OH groups does not normally impart a highly negative charge on the surface of ZnO nanoparticles; rather, the charge may be close to neutral or slightly positive, depending on the total surface chemistry and environmental factors.
The work of Ahmed Abdullah et al. [53] has also proved that excess zinc nitrate during the synthesis of ZnO nanoparticles could contribute to larger sizes of particles on account of increased aggregation. It can result in a less negative surface charge, which reduces the stability of the dispersion of nanoparticles. As the above results showed, it was expected that with the increase of hydroxyl and nitrate functional groups on the surface of the ZnO NPs, the surface charge would change to less negative values. However, the observed increase in the above-mentioned functional groups has shown another trend in practice and is not uniform. As a result, zeta potential analysis considers the combined effects of particle size and the surface charge (surface functional groups).

3.5. TG-DTA

TG-DTA results for samples 3X (with the highest F/O ratio) and 10X (with the lowest F/O ratio) are shown in Figure 10a,b. At first glance, it appears that both samples have experienced similar thermal behavior. There is, however, a difference in the intensity of the reactions between the two samples. The endothermic peaks at 90 and 94 °C, are attributed to the evaporation of water within the gel structure, which is associated with weight loss in samples [54,55]. There was about a 20% and 30% weight loss for samples 10X and 3X, respectively. Due to a higher proportion of fuel in a specific gel weight, sample 3X contains more structural water than sample 10X. As a result, sample 3X has shown a more severe weight loss due to structural water evaporation. As the temperature increased, an exothermic process occurred for 10X and 3X at 122 and 140 °C, respectively. The exothermic reaction is related to the initiation of the combustion reaction and the formation of zinc oxide. The difference in the weight loss of the samples, from about 140 to 300 degrees Celsius, is due to the different amounts of nitrate used in both samples. Different nitrate weight amounts cause the gaseous by-products produced during the reaction to differ in the two samples. As a result, the 10X sample experienced more weight loss with higher nitrate weight amounts [54,55,56]. The small exothermic peaks at 333 and 372 °C are related to the oxidation of organic residues in the chives extract [28,54,55,56].

4. Conclusions

Through solution combustion, this study evaluated the potential of a chives extract as a green and sustainable fuel for ZnO nanoparticle synthesis. XRD results confirm the formation of well-crystalline ZnO nanoparticles in a single-step solution combustion process. In addition, changes in the F/O ratio affect the nanoparticle’s purity, crystallinity, morphology, and size due to changes in the heat and gases released during combustion. The F/O ratio near stoichiometry produces ZnO NPs with high purity, larger size, high crystallinity, and hexagonal disk-like morphologies. These particles also exhibit good stability based on the zeta potential measurements. At the same time, the F/O ratio above stoichiometry produces small ZnO NPs with spherical morphologies along with KCl as a crystalline impurity phase. The KCl comes from unreacted fuel during the combustion reaction.
The findings demonstrate that a chives extract can function as a biocompatible fuel in the SCS process, providing the necessary reducing agents for producing crystalline ZnO NPs. Furthermore, it serves as a structural controlling agent, influencing the formation of ZnO NPs with diverse morphologies. Moreover, considering chlorophyll as a reducing agent in chives extract and then balancing its reaction with zinc nitrate has shown a favorable effect in practice..
Indeed, more research is needed to optimize using chives as a sustainable and environmentally friendly fuel in SCS, including significantly optimizing the extraction method and the selective approach regarding reducing agents. These efforts will lead to a better understanding of the combustion reaction mechanism and the formation of nanoparticles. However, the mentioned findings can enhance comprehension of the synthesis process and provide valuable insights for sustainable development.

Author Contributions

Conceptualization, J.V.K. and S.M.B.; investigation, E.S.; supervision, J.V.K. and S.M.B.; visualization, S.S.A. and N.S.; writing—original draft, S.S.A. and E.S.; writing—review and editing, S.S.A. and N.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding. The APC was funded by the corresponding author.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All relevant data generated by the authors or analyzed during the study are within this paper.

Acknowledgments

This research was supported by a financial grant from Ferdowsi University of Mashhad, Iran.

Conflicts of Interest

The authors declare no conflict of interest.

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Compounds 04 00030 g001
Figure 1. Conventional methods for synthesizing NPs.
Figure 1. Conventional methods for synthesizing NPs.
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Figure 2. XRD patterns of synthesized ZnO nanoparticles at different F/O ratios.
Figure 2. XRD patterns of synthesized ZnO nanoparticles at different F/O ratios.
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Figure 3. Comparison between XRD pattern of 3X sample and chives extract.
Figure 3. Comparison between XRD pattern of 3X sample and chives extract.
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Compounds 04 00030 g004
Figure 4. The calculated crystallite size by Scherrer and Williamson–Hall methods for samples synthesized with different F/O ratios.
Figure 4. The calculated crystallite size by Scherrer and Williamson–Hall methods for samples synthesized with different F/O ratios.
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Figure 5. FTIR spectra of the chives extract.
Figure 5. FTIR spectra of the chives extract.
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Figure 6. FTIR spectra of ZnO NPs synthesized at different F/O ratios.
Figure 6. FTIR spectra of ZnO NPs synthesized at different F/O ratios.
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Figure 7. FESEM micrographs of ZnO NPs synthesized at different F/O ratios: (a) 3X, (b) 5X, (c) 8X, (d) 9X, and (e) 10X. The morphological changes occurred due to the reduction of the F/O ratio (f).
Figure 7. FESEM micrographs of ZnO NPs synthesized at different F/O ratios: (a) 3X, (b) 5X, (c) 8X, (d) 9X, and (e) 10X. The morphological changes occurred due to the reduction of the F/O ratio (f).
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Figure 8. EDX analysis of synthesized nanoparticles for two samples, 3X and 10X.
Figure 8. EDX analysis of synthesized nanoparticles for two samples, 3X and 10X.
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Figure 9. Zeta potential results obtained from ZnO NPs samples at near neutral pH (in deionized water).
Figure 9. Zeta potential results obtained from ZnO NPs samples at near neutral pH (in deionized water).
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Figure 10. TG-DTA results obtained from the dried gel for two samples: (a) 10X and (b) 3X.
Figure 10. TG-DTA results obtained from the dried gel for two samples: (a) 10X and (b) 3X.
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Table 1. Nutritional value of Allium schoenoprasum (chives).
Table 1. Nutritional value of Allium schoenoprasum (chives).
Amino AcidsValue per gram
Tryptophan0.037 g
Threonine0.128 g
Isoleucine0.139 g
Leucine0.195 g
Lysine0.163 g
Methionine0.036 g
Phenylalanine0.105 g
Tyrosine0.095 g
Valine0.145 g
Arginine0.237 g
Histidine0.057 g
Aspartic acid0.303 g
Glutamic acid0.677 g
Glycine0.162 g
Proline0.216 g
Serine0.148 g
MineralsValue per gram
Ca (Calcium)92 mg
Fe (Iron)1.6 mg
Mg (Magnesium)42 mg
P (Phosphorus)58 mg
K (Potassium)296 mg
Na (Sodium)3 mg
Zn (Zinc)0.56 mg
Cu (Copper)1.157 mg
Mn (Manganese)0.373 mg
Se (Selenium)0.9 μg
LipidsValue per gram
Total saturated fatty acids0.146 g
Total monounsaturated fatty acids0.095 g
Total polyunsaturated fatty acids0.267 g
Phytosterols9 mg
Table 2. The exact weight of utilized reactants.
Table 2. The exact weight of utilized reactants.
Sample Code3X5X8X9X10X
Zinc nitrate (g)2.8354.727.568.509.45
Chives extract (g)11111
Table 3. A brief review of selected studies related to ZnO synthesis by a solution combustion method.
Table 3. A brief review of selected studies related to ZnO synthesis by a solution combustion method.
Fuel TypeF/O RatioCrystallite Size (nm)Calculation MethodParticle MorphologyParticle Size (nm)References
PVP (C6H9NO)n0.536Rietveld refinementPyramid shape>1000[28]
0.7522Pyramid + Cubic353
119Pyramid + Cubic353
1.518Hexagonal114
L-Valine (C5H11NO2)0.731.7Williamson–Hall - -[29]
129.5Spherical shape15–50
216.2 - -
Glutamine (C5H10N2O3)0.723.8Williamson–Hall - -[29]
121.2Spherical shape + Nano plate14–26
219.4--
Mixture of Citric acid(C6H8O7) + glycine(C2H5NO2)F/O = 1C75G25 *37SchererSemi-spherical-[31]
C50G5040Semi-spherical-
C25G7543Platelet -
C0G10063Platelet-
Urea
(CH4N2O)		0.675SchererPyramid shape (aggregated in flower-like structure)-[30]
155-
1.848-
5.4amorphousSpongy sheet-like-
C. colocynthis extractFruit portion **85SchererNanoflakes (aggregated in flower-like structure)85–100[12]
Seed portion27Hexagonal20–35
Pulp portion64Block-shaped (irregular polygons)30–80
* 75 wt.% of citric acid and 25 wt.% of glycine; ** Different parts of the C. colocynthis fruit were used as fuel in this study without regard to the F/O ratio.
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Sheibani, E.; Soltani Alasvand, S.; Sami, N.; Vahdati Khaki, J.; Mollazadeh Beidokhti, S. The Solution Combustion Synthesis of ZnO Nanoparticles Using Allium schoenoprasum (Chives) as a Green Fuel. Compounds 2024, 4, 503-520. https://doi.org/10.3390/compounds4030030

AMA Style

Sheibani E, Soltani Alasvand S, Sami N, Vahdati Khaki J, Mollazadeh Beidokhti S. The Solution Combustion Synthesis of ZnO Nanoparticles Using Allium schoenoprasum (Chives) as a Green Fuel. Compounds. 2024; 4(3):503-520. https://doi.org/10.3390/compounds4030030

Chicago/Turabian Style

Sheibani, Elyas, Saman Soltani Alasvand, Neda Sami, Jalil Vahdati Khaki, and Sahar Mollazadeh Beidokhti. 2024. "The Solution Combustion Synthesis of ZnO Nanoparticles Using Allium schoenoprasum (Chives) as a Green Fuel" Compounds 4, no. 3: 503-520. https://doi.org/10.3390/compounds4030030

APA Style

Sheibani, E., Soltani Alasvand, S., Sami, N., Vahdati Khaki, J., & Mollazadeh Beidokhti, S. (2024). The Solution Combustion Synthesis of ZnO Nanoparticles Using Allium schoenoprasum (Chives) as a Green Fuel. Compounds, 4(3), 503-520. https://doi.org/10.3390/compounds4030030

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