Magnetic and optical properties of ZnO nanoparticles and nanorods synthesized by green chemistry

ZnO nanostructures have attracted considerable attention because of their physicochemical properties and applications as antibacterial agents, photocatalytic reactions for pollutant removal, and electronics. Hence, efficient production and knowledge of their properties under different synthesis conditions are essential. Biosynthesis has emerged as an excellent growth-directing method for synthesizing nanomaterials, representing a soft and cleaner alternative for their production. In this study, we synthesized different ZnO nanostructures using a soft chemistry method at different growth temperatures, from 200 to 800 °C every 200 °C. The crystalline structure was estudied by x-ray Diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HRTEM). The shape and size were studied by Field Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM), which revealed a ZnO hexagonal phase with two shapes: nanoparticles (NPs) with irregular shapes and nanorods of different sizes. The optical properties were studied by Raman and UV-visible spectroscopy, and optical absorption measurements showed bandgap tuning of the produced nanostructures. Finally, the magnetic characteristics of the samples demonstrated magnetic anisotropy due to the preference for crystalline formation and the size of the nanoparticles. The magnetic interaction between the two types of NPs increased the diamagnetism associated with the nanorods.

Various methods of synthesis of nanomaterials have been reported in the literature, including the colloidal method [12], co-precipitation [13], microwave irradiation [14], and the sol-gel method [15], among others, however currently green chemistry is gaining relevance, this method is one of the current procedures used to synthesize NPs of different sizes and shapes without the need for specialized equipment [16,17], as well as several substances or reagents [18], and reducing or eliminating harmful substances in the synthesis methodologies [19], in addition, to diminish synthesis times by involving fewer steps [20].
Plant species are one of the most utilized approaches for performing green synthesis, as stabilizing agents, and redox mediators [21], which influence the sizes and shapes of the resulting NPs depending on other factors such as the metal ion concentration, pH, time, and temperature [22,23].
Previous to this work, the antibacterial properties of green synthesized ZnO-NPs of different sizes and shapes were studied by the research group [24].
Considering the above, the aim of this study, was to determine the magnetic behavior of each morphology to contribute to the magnetic description of green-synthesized compounds for plausible technological applications.

Materials and methods
10 g of dried leaves of Dysphania ambrosioides were macerated in 100 ml of deionized water for 1 h at room temperature without stirring.Then, the mixture was stirred for 2 h at 40 °C and 50 rpm.The synthesis was performed by mixing 20 ml of Dysphania ambrosioides extract leaves (pH = 6.8) with 1.5 g of Zn(NO 3 ) 2 • 6H 2 O (Meyer, >99%) and stirring for 10 min at room temperature (pH = 3.6) before being placed in a muffle furnace for 1 h at different reaction temperatures: 200, 400, 600, and 800 °C, denoted as ZnO-1, ZnO-2, ZnO-3, and ZnO-4, respectively.It was then removed from the furnace and cooled to room temperature.The resulting powders from each synthesis were washed in triplicate with deionized water and dried at room temperature for 24 h [24].

Characterization
The XRD was used to examine the crystalline structure using a Bruker AXS D8 Advance x-ray diffractometer at a voltage of 40 kV and a current of 30 mA with Cu K-α radiation (1.5406 Å) between 2θ angles of 10°and 90°.
The average crystallite size was calculated by the Debye-Scherrer equation: Where: D: average crystallite size; K: shape factor usually taken as 0.9; λ: wavelength of the x-ray radiation; θ: Bragg diffraction angle; Β: Full Width at Half Maximum (FWMH) [35].
The ZnO NPs size and morphology were observed using a JEOL FESEM, model JSM7800F with a resolution of 0.8 nm equipped with an OXFORD microprobe for Energy Dispersive x-ray Spectroscopy (EDS) analysis.TEM analysis was performed in a JEM-2010 FEG electron microscope with an accelerating voltage of 200 keV, the images were analyzed using the Digital Micrograph software from GATAN.UV-visible absorption spectra of ZnO NPs were attained with a Thermo Scientific Genesys 150 spectrometer in the range of 300 to 1000 nm; the NPs were suspended in Milli-Q water and measured at room temperature.Raman spectra were acquired with a Thermo Scientific confocal microscope equipped with Micro-Raman using a λ = 532 nm laser (green), within the 50-1600 cm − 1 range at room temperature.Magnetic characterization was performed using a MPMS-3 (Quantum Design) magnetometer, the M(T) data were collected between 2-300 K with an applied field of 1 kOe, while the M(H) curves were acquired, at room temperature, with (-)30 kOe as the maximum applied magnetic field.

X-ray diffraction
Figure 1 shows the XRD pattern of the four samples of ZnO, the ZnO-4 diffractogram showed Bragg reflections indexed by comparison with the Power Diffraction File (PDF) #891397.This corresponded to the hexagonal phase of ZnO (wurtzite).The other samples had the same crystalline structure, with wider reflections indicating a reduction in the average crystallite size (table 1).The average crystalline size increased as the synthesis temperature was higher.
In addition, other crystalline phases or contaminants in the ZnO samples were not detected in the XRD patterns.

Electron microscopy characterization
SEM images of the ZnO NPs obtained with secondary electrons are shown in figure 2, showing the particle/ agglomerates of NPs size and the morphology of the ZnO NPs synthesized at different temperatures.Figures 2(A) and (B) show the ZnO-1 and ZnO-2 NPs, respectively.In both cases, agglomerates of NPs were observed but those heat-treated at 400 °C were smaller (< 50 nm).Two different NPs shapes were observed for the ZnO-3, figure 2(C), and ZnO-4, figure 2(D).The quasi-spherical NPs had an average size of 22 nm for ZnO-3 and 50 nm for ZnO-4, whereas the nanorods had an average length of 217 nm for ZnO-3 and 235 nm for ZnO-4.
The EDS analysis semi-quantified the atomic percentages of Zn and O, which were close to 50% (table 2).A 1:1 ratio of both elements was established, supporting the confirmation of ZnO NPs synthesis at different temperatures.
Figure 3 shows the results obtained by TEM of the samples under study.For the ZnO-1 and ZnO-2 samples, irregular shapes were obtained, some of them with quasi-spherical shapes.The agglomerated ZnO-1 NPs have an average particle size of 7 nm, figure 3(A), the insets show the fast Fourier transform (FFT) and an amplification of the delimited area with a dotted red line.From the FFT analysis, a wurtzite ZnO crystalline phase (hexagonal structure) with space group P63mc was determined.The measured interplanar distances were 0.254, 0.252 and 0.191 nm, corresponding to the crystallographic planes 11 01 , ( ̅ ) 101 1 ( ̅ ) and 0112 ( ̅ ) using the crystallographic reference COD: 96-101-1260, oriented along the zone axis 12 1 .
[ ̅ ] The ZnO-2 sample has an average size of 14 nm, figure 3(B), the inset shows the FFT of the delimited area with the dotted red line.The ring pattern can be indexed as the wurtzite ZnO crystalline structure too, and each diffraction ring corresponds to the 10 1 0 , ( ̅ ) 10 1 1 ( ̅ ) and 01 1 2 ( ̅ ) crystallographic planes using the same crystallographic reference.The interplanar distances were 0.287, 0.252 and 0.193 nm respectively.The average width of the nanorods is larger for ZnO-4 (90 nm), figure 3(D), than ZnO-3 (50 nm), figure 3(C).The assessed E g values are lower than those previously reported energy band gaps of 3.21 and 3.17 eV for ZnO NPs with sizes between 23-26 nm [1].Additionally, different studies suggest that the E g shift toward smaller values could be associated with vacancies, mainly on the NPs surface [30,37].Theoretical studies also suggest that variations in E g depend on the cluster size and geometry [38], inclusive of ZnO needles where the E g values drecrease for bigger clusters, and it changes with distortions in the wurtzite phase clusters [39].
From Raman spectroscopy analysis, shown in figure 5, the presence of crystalline defects, such as vacancies present in nanostructured systems, can be inferred.ZnO described by a hexagonal wurtzite-phase ZnO symmetry and point group C 6v displays the Γ = A 1 + 2B 1 + E 1 + 2E 2 vibrational modes of which only three (A 1 , E 1 , and E 2 ) are Raman active, while the remaining vibrational modes are within the IR region.A 1 and E 1 are polar modes and are split into transverse optical (TO) and longitudinal optical (LO) phonons, whereas E 2 modes (E 2Low and E 2High ) are nonpolar [34].This last Raman peak is characteristic of the hexagonal wurtzite phase, and in some studies, it has been observed that the B1 Raman mode is inactive or silent [40,41] Figure 5(A) shows the Raman spectra for the different ZnO nanostructures, from NPs (6.74 nm) to nanorods (29.53 nm), with the average sizes taken from table 1.At least 10 Raman bands related to the combination of firstand second-order vibrational modes were observed.There is a sharp band for different samples located between 80.61 and 97.96 cm −1 , corresponding to the E 2Low mode, where the narrow width of these bands is indicative of the good crystallinity of the sample [9].The bands with Raman frequencies between 1047.73 to 1078.01 cm −1 are usually related to combinations between transverse optical (TO) and longitudinal optical (LO) modes [42][43][44].E 2High mode related to the vibration of the oxygen atoms in the unit cell, this value is near to observed by M A Awad et al [40] for ZnO nanostructures and thus, this Raman band could be associated with oxygen vacancies [45].Another important feature is a small E 1 (LO) mode around 580 cm −1 related to defects and oxygen vacancies [41].Table 3 lists the Raman contributions of each band assigned after deconvolution analysis.
A detailed analysis suggested a correlation between the Raman band intensities and particle sizes in the nanostructures.It is observed for ZnO NPs between 6.74 to 16.37 nm that the Raman band maximum is the E 2Low mode while for the 29.53 nm ZnO NPs, the most intense band is the one related to the E 2High vibrational mode.Figure 6(A) shows the Raman shift position and FWHM curves plotted as a function of the nanostructure size, and the displacement position curve plotted as a function of the nanostructure size, figure 6(B).The size dependence of the Raman shift and FWHM reflects the phononic confinement effect on vibrational modes [46,47].The E 2Low vibrational mode analysis position elucidates an approximated parabolic behavior as a function of the nanoparticle size, whereas the FWHM shows a linear decrement behavior.
The analysis of the Raman intensities between E 1 (LO) ∼ 575 cm −1 and E 2High ∼ 85 cm −1 and its ratio (E 1 (LO)/E 2High ) could be modeled with an exponential decay (figure 7) when the synthesis temperature was increased, that is, the size of the nanostructures.This behavior indicates that smaller nanostructures have more intrinsic defects than nanostructures with nanorod shapes (ZnO-4).Structural defects are known to significantly affect the structural, electronic, and magnetic properties of ZnO [48].

Magnetic characterization
Figure 8 shows the magnetic loops for different samples collected up to +(−)30 kOe at room temperature.The magnetic property of the ZnO-1 sample was superparamagnetism.The superparamagnetism is related to the small magnetic monodomain; it is possible for the small NPs size (6.74 nm).The superparamagnetic curve is characterized by a fast increment in the magnetic values and magnetic saturation, it is around 0.001 emu g −1 at 8 kOe, and for greater magnetic field values, the magnetization decreases as a possible competition with a diamagnetic order.For the ZnO-2 sample, the magnetic behavior is weak ferromagnetic because the hysteresis loop is not saturated at 30 KOe, and the maximum value of the magnetic moment is comparable with the reports of Satpathy et al [3] but the coercive fields are about ten times lower than [3], around 14 Oe.The inset shows a negligible hysteresis.The formation of nanorods in the ZnO-3 sample changed their general behavior to diamagnetic.The ferromagnetic contribution predominates at low magnetic fields, where above (-)500 Oe, the magnetization of the sample saturates, and the diamagnetic contribution starts to increase.The ferromagnetic contribution practically disappears for the ZnO-4 sample.This change could be related to the decrease in the formation of quasi-spherical NPs in ZnO-4.
Figure 9 shows the zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibility (χ ρ ) curves as a function of the temperature for all the samples, with an applied magnetic field of 1 kOe.For the ZnO-1 sample, the plot showed a local maximum at 118 K, which could be related to the blocking temperature (T B ).For ZnO-2 is the T B = 125.5 K, and this higher value is in concordance with the presence of larger NPs [49].Diamagnetic behavior was not detected for these samples in the temperature range of 2-300 K in the ZFC and FC measurement modes.The M(T) plots are quite similar for the ZnO-3 and ZnO-4 samples, which have no hysteresis and a long-range diamagnetic behavior, including at room temperature.The main difference between the two samples is the interception with the x-axis; ZnO-3 NPs change to χ ρ positive values at 15 K, while ZnO-4 changes at 35 K, and the maximum negative value (2.11 × 10 −7 cm 3 g −1 ) is not too high compared to ZnO-3 (2.27 × 10 −7 cm 3 g −1 ).In addition, the ZFC and FC curves intersected at 89 K in the diamagnetic region for ZnO-3.These differences could be related to the higher amount of quasi-spherical NPs in the ZnO-3 sample compared to ZnO-4, which originates an interaction between two magnetic orders: diamagnetism and ferromagnetism.
There are sharp upturns in the ZFC and FC magnetization curves below 20 K for all samples, which may arise from an odd number of spins or isolated spins [3] and could be associated with a paramagnetic behavior [50] at low temperatures because of the small hysteresis on M(H) curves, especially for the diamagnetic samples where the ZFC an FC are very similar.Considering this for the ZnO-3 sample, it can be said that the global diamagnetic behavior is reinforced for the magnetic moment of the quasi-spherical and ferromagnetic NPs, and it requires a lower temperature, where the paramagnetic contribution arises from crossing the X-axis.
Table 4 shows the morphology and particle size related to the optical and magnetic behavior of the synthesized ZnO NPs; where the magnetic order changed from superparamagnetic to diamagnetic with an increase in the synthesis temperature owing to the formation of nanorods.

Conclusions
The change in the magnetic behavior is strongly dependent on the size and shape of the NPs.The ferromagnetic behavior was related to the quasi-spherical NPs.Furthermore, depending on the nanoparticle size,  superparamagnetism can be observed in the small NPs.On the other hand, the ZnO nanorods maintain the diamagnetic behavior of bulk ZnO.A weak contribution of small ferromagnetic NPs was detected for small magnetic fields.This ferromagnetic contribution reinforces the diamagnetism of the nanorods at low temperatures, when concentration of small NPs is enough.Also, the diamagnetic order could stay due to the ferromagnetic NPs without an external magnetic field.The bandgaps of the samples were lower than those reported in other studies; a smaller gap will activate the catalytic properties of ZnO with lower energy.Raman studies demonstrate the formation of nanorods for samples synthesized, ZnO-3 and ZnO-4, with the detection of Raman modes around 400 cm -1 associated with oxygen vibrations.

Figure 1 .
Figure 1.XRD pattern of the four samples of ZnO was taken with Cu Kα (1.5406 Å) radiation and indexed with ZnO hexagonal phase.

Figure 2 .
Figure 2. The SEM images obtained with secondary electrons show different agglomerates of NPs size: (A) ZnO-1 > (B) ZnO-2.The formation of nanorods for ZnO-3) is shown in (C), with a size range of 50-300 nm.nanorods increased in ZnO-4, as observed in (D).

Figure 3 .
Figure 3. TEM images obtained with 200 keV of accelerating voltage shows the different particle size: (A) around 5 nm and (B) around 10 nm for ZnO-1 and ZnO-2 respectively, and the insets shows the corresponding FFT transform analysis and an amplification from the delimited area by the red dotted line.The formation of nanorods at higher temperatures (600 °C) is shown in (C) for ZnO-3.The number, thickness, and length of the nanorods rise to 500 nm for ZnO-4 as shown in (D).

4. 3 .
UV-vis and Raman spectroscopy Figure4(A)shows the absorption spectra of ZnO NPs measured at room temperature.The spectra show the band edge absorption at 350, 350, and 352 nm for the ZnO-2, ZnO-3, and ZnO-4 samples, respectively, showing a 7% blue shift compared to the 376 nm plasmon surface band for bulk ZnO, due to the size effect, as reported in previous works[36].The ZnO-1 sample did not show a band-edge absorption in the wavelength range of 290-370 nm.The Tauc plots in a direct transition, figure 4(B), let the calculation of an approximated value for the E g as 2.73 eV for ZnO-1, 2.95 eV for ZnO-2, 2.96 eV for ZnO-3 and 2.92 eV for ZnO-4.

Figure 5 (
B) shows the Raman spectra of ZnO-4 (29.53 nm); after deconvolution analysis by Lorentzian functions, it is observed that the most intense band is located at 438.98 cm −1 , which corresponds to

Figure 4 .
Figure 4. UV-vis absorption spectra of the ZnO samples were recorded at room temperature and showed in (A).These spectra show a band edge absorption around 350 nm for the ZnO-2 and ZnO-3 samples, 352 nm for the ZnO-4 sample, and no band edge absorption for ZnO-1.The Tauc plots are presented in (B), and the values of the direct band gaps were estimated as the intercept of the linear fit (solid lines) along the x-axis.

Figure 5 .
Figure 5.The measured spectrum for an excitation energy of 2.33 eV (532 nm) is shown, and the Raman spectrum from 100 to 1600 cm −1 of the ZnO nanostructures is presented in (A).Deconvolution analysis by Lorentzian functions of the ZnO-4 Raman spectrum is presented in (B).

Figure 6 .
Figure 6.The E 2Low mode of the ZnO nanostructures is compared in (A).Raman shifts and FWHM of E 2Low phonon as a function of the crystallite size of the nanostructure are plotted in (B).

Figure 7 .
Figure7.Ratio E 1 (LO)/E 2High , the plot Illustrates that the ratio could be modeled by an exponential decay with increasing crystallite size.

Figure 8 .
Figure 8. Magnetization of ZnO NPs as a function of the magnetic field (H) at room temperature (300 K) at different synthesis temperatures.ZnO-1 and ZnO-2 show ferromagnetic contributions while ZnO-3 and ZnO-4 show global diamagnetic behaviour.

Figure 9 .
Figure 9. Magnetic susceptibility of ZnO NPs as a function of temperature was measured at 1 kOe in the ZFC and FC measurements modes from 2 to 300K.The diamagnetic behavior of ZnO-3 was stronger than ZnO-4 at low temperatures and could be associated with the interaction between the quasi-spherical NPs and nanorods.

Table 1 .
Average crystallite sizes of ZnO NPs.

Table 2 .
EDS analysis.Atomic percentages (% at) of Zn and O in the synthesized ZnO NPs.

Table 4 .
The different characterized parameters of the ZnO-NPs, where Eg is the energy band gap, T B is the blocking temperature, and the average size was obtained from electron microscopy analysis.