ZnO nanoparticles (ZnO-NP) present innovative optical, electrical, and magnetic properties that depend on specific characteristics, e.g., size, distribution, and morphology. Thus, these properties are essential to address various applications in areas such as electronics, medicine, energy, and others. In addition, the performance of this ZnO-NP depends of their preparation which can be done by chemical, physical, and biological methods. Meanwhile, nowadays, the main interest in developing ZnO-NP synthesis through biological methods bases on the decrease of use of toxic chemicals or energy applied to the procedures, making the process more cost-effective and environmentally friendly. However, the large-scale production of nanoparticles by green synthesis remains a big challenge due to the complexity of the biological extracts used in chemical reactions. That being the case, the preparation of ZnO-NP using
In recent years, the ZnO compound has been widely studied and it is usually described as a strategic, functional, versatile, and promising inorganic material with a broad range of applications [
In general, nanoparticles exhibit new or improved properties that are supported by specific characteristics such as size, distribution, and morphology [
Nanoparticles can be obtained using different techniques based on chemical (coprecipitation, sol-gel, chemical reduction, electrochemical, microemulsion, pyrolysis, photochemical, sonochemical, solvothermal, among others) [
For chemical synthesis, a common method to obtain ZnO-NP is coprecipitation. By this method, it is possible to obtain a wide range of nanoparticle diameters and morphologies. Coprecipitation is considered an easy, cost-effective, and convenient technique because it does not require complex process parameters, any expensive and sophisticated equipment, or any large space area for the set-up [
In the case of physical methods, the mechanochemical synthesis or milling is frequently used to produce nanomaterials. In this method, mechanical energy is used to grind down powders by colliding balls with the powder due to the rapid rotation of the mill. High-energy ball milling is used over bulk powders to obtain smaller and more uniform sizes, even at the nanoscale [
On the other hand, biological synthesis is considered an eco-friendly process to obtain ZnO-NP, which has arisen as a green alternative for the synthesis of nanomaterials. While chemical and physical methods may lead to high energy consumption a high temperature or pressure is required in the process [
The main interest in developing ZnO-NP synthesis through biological methods has its basis on the low (or null) use of toxic chemicals and/or energy applied to the procedures, which makes the process more cost-effective and environmentally friendly. Plants (leaves, flowers, seeds, peels, or roots) are the most important and strategic resource for the ZnO-NP biosynthesis because other bioresources as fungi or microbes require sterilization processes or controlled conditions for their cultivation. The importance of this method, by using plant-based extracts to produce metal and oxide nanoparticles has been widely reported [
For biological synthesis, the natural extracted aqueous solution contains essential phytochemical compounds for the formation of the particles at nanoscale acting as capping and reducing agents transforming metal ion to metal oxides. Depending on the part of the plant, different functional groups as flavonoids, vitamins, polysaccharides, amino acids, alkaloids, among others, appear in the aqueous extract that, being used as the reaction solvent, significantly alters the chemical dynamics of the synthesis and, consequently, the type of nanoparticles as a product. By using extract leaves, in the particular case of the
The large-scale production of nanoparticles by green synthesis remains a big challenge due to the complexity of the biological extracts used in chemical reactions. The mechanisms of formation during the nanoparticle synthesis is still be an important topic to be elucidated when they are produced at industrial quantities [
In this work, the methodology for obtaining ZnO-NP using
The leaves of
The extract used for the reduction of zinc ions (Zn2+) to ZnO-NP was prepared by placing 13 g of fine cut leaves in 500 mL glass beaker along with 400 mL of ultrapure water (18.2 MΩ·cm) at 50°C by 2 h under magnetic stirring (500 rpm), and a light-yellow aqueous solution was obtained. The extract was let to cool at room temperature and to be vacuum-assisted filtered by using a medium flow Whatman filter paper No. 1 (11 µm pore diameter) to avoid organic solid remains. The extract was stored in a refrigerator to be used for further experiments.
For the ZnO-NP synthesis (summarized in
The green synthesized ZnO-NP were confirmed and optically characterized by using a UV-Visible spectrophotometer (Libra S22, Biochrom, Ltd., Cambridge, England). Synthesized nanopowders were dispersed in 5 ml of ultrapure water (18.2 MΩ·cm) and they were sonicated for 10 min at room temperature to guarantee a powder dispersion in the solution. The absorption spectra were recorded in the wavelength range of 200–800 nm (Step 1 nm). The UV–Vis characterization gives us the insight on the actual formation of nanoparticles by a surface plasmon resonance effect.
The bandgap energy of semiconductors (including nanosized materials as films or structured particles) can be estimated by different methods and approximations. In this work we are using the Tauc relation [
where
The average size and dispersion of ZnO-NP were evaluated through dynamic light scattering (also called photon correlation spectroscopy or PCS) measured with a NICOMP Nano Z3000 System, which provides extreme versatility and high sensitivity. DLS is a widely used technique for the determination of particle size in an aqueous medium. The light scattered by nanoparticles in suspension will fluctuate with time and can be related to the particle diameter [
Average diameters were obtained with the software provided by NICOMP (ZPW 388-V2.18). The light scattering from the Brownian-motion of the ZnO nanoparticles causes photon count fluctuations on the detector. Then, a correlation function is created to determine the translation diffusion coefficient
where
Furthermore, in the same instrument, the zeta potential method was used to measure the electrostatic potential of the ZnO-NP surrounded by an aqueous medium, usually ultrapure water. Nanoparticles with a zeta potential between −10 and +10 mV are considered neutral. On the other hand, nanoparticles out of the range −30 to 30 mV are considered strongly anionic and cationic, respectively [
The synthesized ZnO-NP were structurally characterized by using a Rigaku MiniFlex/600 X-ray diffraction instrument, Japan. XRD operates at 30 kV with a current of 15 mA using Cu-Kα radiation (1.540598 Å wavelength) over a wide range of Bragg’s angles (5 to 80, 2θ degree). The average crystallite size (
where
The interplanar or reticular distance (
where
Average lattice parameters (
where
The FTIR measurement was carried out to identify the presence of characteristic bands of the ZnO products, and residues remaining before and after the thermal treatment. It was performed by using a Shimadzu IRTracer-100 spectrometer, Japan. Functional groups presence in the mid-infrared region (within the range of 400 to 4000 cm−1) were revealed in both samples, unannealed and annealed ZnO-NP. The samples were used in solid form. The Pike MIRacle single reflection horizontal ATR accessory equipped with a ZnSe ATR crystal was used for the analysis. ATR mode was used for recording the data.
Scanning electron microscopy analysis was used to determine the size and surface morphology of the synthesized ZnO-NP. FE-SEM characterization was carried out by using a FE-SEM, Hitachi Regulus 8230, Hitachi High-Tech Co., Japan. The instrument was operating at an accelerating voltage of 3.0 kV and the obtained images were taken at a constant magnification of 40.0 kX. The sample studied did not require any preparation for measurement, as is often experimentally necessary.
The UV–Vis spectrum (shown in
To determine the optical bandgap of ZnO-NP, the optical absorption edge energy must be considered since it represents the minimum energy required to excite an electron from the highest occupied molecular orbital in the valence band to the lowest unoccupied molecular orbital in the conduction band [
This Tauc relation was plotted in
In Tauc plot method, shown in
The size distribution image obtained by DLS of green synthesized ZnO-NP is shown in
As mentioned previously, the magnitude of the zeta potential indicates potential stability of ZnO-NP in an aqueous medium. For the sample annealed at temperature of 600°C, the zeta potential value was −29.73 mV (see
In a lot of cases, the ZnO-NP have been used in an aqueous medium, resulting the zeta potential characterization of a primary indicator of surface charge. For example, in nanotoxicology ZnO-NP have been commonly utilized as a biological agent to control complex processes [
The XRD patterns show the noticeable peaks of ZnO-NP indexed to (100), (002), (101), (102), (110), (103), (200), (112), and (201) planes of reflection of ZnO in agreement with Joint Committee on Powder Diffraction Standards (JCPDS) card number 01-079-0206 shown in
Annealed sample of ZnO-NP at the temperature of 600°C reveals well-defined, strong, and narrow diffraction peaks indicating a crystallite nature (see
By using the relation described in
Material | 〈 |
〈 |
---|---|---|
ZnO unannealed | 3.2491 | 5.2113 |
ZnO annealed at 600°C | 3.2471 | 5.1972 |
ZnO bulk | 3.2499 | 5.2066 |
XRD technique was used not only for the identification of crystal structure but also for different lattice parameters. Interplanar distances for each Miller indices were calculated by using Brag’s law,
φ |
φ |
|||||||
---|---|---|---|---|---|---|---|---|
(100) | 31.768 | 31.7958 | 31.8063 | 2.8145 | 2.8121 | 2.8112 | 1.8843 | 0.3861 |
(002) | 34.422 | 34.3899 | 34.4862 | 2.6033 | 2.6057 | 2.5986 | 1.8260 | 0.4239 |
(101) | 36.253 | 36.2525 | 36.2938 | 2.4759 | 2.4760 | 2.4732 | 1.6421 | 0.4431 |
(102) | 47.539 | 47.5004 | 47.6070 | 1.9111 | 1.9126 | 1.9086 | 1.8329 | 0.5461 |
(110) | 56.594 | 56.5714 | 56.6503 | 1.6250 | 1.6256 | 1.6235 | 1.4774 | 0.4587 |
(103) | 62.858 | 62.7537 | 62.9563 | 1.4773 | 1.4795 | 1.4752 | 1.2708 | 0.5671 |
FWHM is also reported in
On the other hand, analyzing the crystallographic results of the two types of samples, unannealed and annealed, there are slight reductions in parameters
ATR-FTIR spectra of ZnO-NP unannealed and annealed at 600°C in the spectral range of 400 to 4000 cm−1 are showed in
A general arrangement of the annealed ZnO-NP sample obtained by FE-SEM is presented in
Although the sample is agglomerated due to the arrangement for taking the micrograph (see
In this study, ZnO nanoparticles were successfully synthesized by using an eco-friendly green synthesis instead of conventional physical or chemical methods.
The authors are grateful to Dr. J. Quispe and Dr. H. Loro for their valuable support, to the Centro de Investigaciones Tecnológicas, Biomédicas y Medioambientales (CITBM) and finally to the Facultad de Ciencias of Universidad Nacional de Ingeniería.