**3.1: TEM and Particle Size Analysis**

TEM images of ZnONPs were examined to study their morphology, and shapes, and to estimate their sizes (Fig. 2). The hexagonal, spherical, cubic, and cone shapes of the ZnO nanoparticles can be seen in image (A). The particles in the image (B) are nearly spherical, rod-shaped, and arranged in a star. In the image (C), ZnO nanoparticles may have spherical, cube-like, rod-like, and flower-like shapes, and some shadows can be seen surrounding the particles, possibly indicating the presence of PVP surfactant. The particles in an image (D) appear hexagonal in shape. There are a few other forms seen

in the image (E), including short rods, spheres, fibers, and hexagonal shapes. The average diameter of ZnONPs was measured for a range of PVP concentrations (5, 10, 20, 30, and 40 mL) at diameters of 47, 60, 40, 63, and 70 nm. ZnO nanoparticles increased in size with increasing PVP except at 20 mL, where they decreased.

**3.2: Analyzing X-Ray Diffraction**

Figure 3 displays the X-ray patterns of ZnONPs which were capped by different concentrations of PVP as a surfactant. The diffraction peaks are visible at \(2{\theta ^o}=\)31.69°, 34.35°, 36.17°, 47.41°, 56.40°, 62.55°, 66.11°, 67.68°, 68.78°, 72.29°, and 76.66° which correspond to the plans (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202). These results are consistent with those published in (International Center for Diffraction Data, JCDS 36-1451) [24]. Diffraction peaks at 2\(\theta ^\circ\): 31.690, 34.3, and 36.170 are consistent with the hexagonal Wuritize structure of ZnO nanoparticles [11]. Comparing the diffraction patterns of all concentrations of PVP, the increase in intensity indicates an improvement in crystallinity due to the larger particle sizes, except for the PVP concentration of 20 mL. High purity of ZnONPs results in no impurity peaks. The crystalline size was calculated for each sample using the Debye-Scherrer equation:

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

1

where k is Scherrer constant (= 0.9), λ is the X-ray wavelength = 1.54056 A\(^\circ\) of Cu\({K}_{\alpha }\) A radiation source,\(\beta\)is the peak width of half maximum (FWHM), and\(\theta ^\circ\) is the Bragg\(ʼ\)s As a result of multiplying each crystal size by its maximum intensity peak, the average crystalline size is computed by dividing each peak by the sum of the calculated crystal sizes. The average crystalline size was (22.9, 23.43, 20.55, 24.13, and 27.34 nm) at PVP (5, 10, 20, 30, and 40 mL), respectively. Based on the relationship between the intensity value and crystallinity, it can be concluded that the low crystallinity at 20 mL of PVP is due to low crystalline size. These results are in line with the results from TEM.

**3.3: FT-IR Study**

To understand PVP as a surfactant and how it binds to the surface of ZnO nanoparticles, FT-IR spectra of PVP and PVP-coated ZnONPs were recorded. The FTIR of ZnONPs prepared for different molar ratios of PVP is shown in Fig. 4. There was a strong transmittance peak at about 3460 cm− 1 in the PVP, indicating O-H stretching vibration. With increasing PVP concentrations up to 20 mL, the band of PVP-capped ZnONPs was

shifted to a lower wavenumber at 3380 cm− 1. As for PVP concentration increases, hydrogen bonds may be formed at the ZnONPs/PVP interface. A C-H stretching vibration in PVP is responsible for the strong and weak bands observed in the range (2931 cm− 1 − 2800 cm− 1). PVP has a band at 1628 cm-1 that corresponds to its carbonyl group. There is a new transmittance peak measured at 1504 cm− 1 in PVP-capped ZnONPs that may be caused by zinc carboxylate stretching vibrations. C-H, C-C, and C-O bonds exhibit bending in-plane vibration modes in PVP at 1390 cm− 1. It was determined that covalent bonds of PVP with ZnO nanoparticles caused this band to shift to 1384 cm− 1 in ZnONPs/PVP. In this band, the intensity increases with increasing PVP concentrations on ZnO surfaces, suggesting that PVP contributes to the increase of surface hydroxyl groups. C-O stretching vibrations are responsible for the band at 1044 cm-1. A small peak was observed in the range (900–1100 cm-1), which is explained by C-N bending vibrations and CH2 attachment to the pyrrole ring in PVP. There is probably a band at ~ 879 cm-1 that corresponds to the carbonate group, commonly observed in the air or indicated to C-H, C-C, C-N, or C-O out-of-plane bending vibrations of PVP. It is clear that the transmitted band at 439 cm-1 is influential. Several

characteristic stretching vibrations of Zn–O were assigned to this peak [27]. There is a summary of all the transmittance peaks and their assignments for all amounts of PVP (5mL − 40mL) in Table 1. FTIR measurements showed coordination between zinc oxide nanoparticles and PVP stabilizers through nitrogen and oxygen atoms.

Table 1

Wavenumber and assignment groups of ZnONPs prepared at different PVP concentrations

Wavenumber (cm− 1) | Assignment |

**PVP** | **PVP = 5** **mL** | **PVP = 10** **mL** | **PVP = 20** **mL** | **PVP = 30** **mL** | **PVP = 40** **mL** | |

3460 | 3380 | 3378 | 3372 | 3377 | 3391 | O-H stretching vibration |

2931 | 2931 | 2931 | 2931 | 2931 | 2931 | C-H stretching vibration |

1634 | 1628 | 1628 | 1628 | 1628 | 1628 | C = O stretching vibration |

- | 1504 | 1503 | 1501 | 1503 | 1503 | zinc carboxylate stretching vibration |

1384 | 1384 | 1384 | 1384 | 1384 | 1384 | C-H, C-C, or C-O bending vibration |

1050 | 1044 | 1044 | 1044 | 1044 | 1044 | C-O stretching vibration |

832 | 832 | 832 | 832 | 832 | 832 | C-N bending vibration |

- | 439 | 442 | 442 | 451 | 446 | Zn-O stretching vibration |

## 3.4 UV-Vis spectroscopy

UV-vis spectroscopy was performed to investigate ZnO nanoparticles. Pesika et al. demonstrated that bulk zinc oxide exhibits an optical absorption at 388 nm [28]. ZnONPs synthesized with various PVP concentrations are shown in Fig. 5, while Table 2 shows maximum absorption bands with PVP concentrations (5, 10, 20, 30, and 40 mL) at (363.56, 366.64, 362.90, and 374.90 nm). Furthermore, the quantum confinement effect of ZnONPs was demonstrated by Mun Lam et al. [29]. Compared with bulk, it was found that ZnO was synthesized at the nanoscale based on the quantum confinement effect.

Furthermore, the change in band position of ZnONPs indicates that particle size depends on the concentration of PVP. The absorption band shifted from red to blue with increasing PVP concentrations, indicating increased particle sizes except for 20 mL PVP. This band position was blue-shifted at 362.9 nm. In our study, PVP was used as a surfactant to accommodate changes in viscus condition and to reduce ion diffusivity. Therefore, it can alter nanoparticle sizes, morphology, crystallinity, and other surface properties according to their molecular structure [30, 31]. Due to the effects of diffusion and the attachment rates of PVP to the surface of ZnONPs, particle size increased with the raised of PVP (5, 10, 30, and 40 mL). When the amount of PVP is increased, the growth process is accelerated. Consequently, PVP can increase the size and enhance the crystallinity of ZnO nanoparticles. In contrast, the particle size decreased at PVP (20 mL), which can be explained by the high viscosity of PVP, which produces a more significant steric hindrance effect that prevents the tiny particles from growing into larger solids. As a result, nanoparticles of smaller sizes and newly formed crystal nuclei decelerate [13]. Eq. 2 shows how the bandgap of nanoparticle E is related to particle radius r:

$$E={E_{bulk}}+\frac{{{\hbar ^2}{\pi ^2}}}{{2e{r^2}}}\left( {\frac{1}{{{m_e}{m_o}}}+\frac{1}{{{m_h}{m_o}}}} \right) - \frac{{1.8e}}{{4\pi \varepsilon {\varepsilon _o}r}}$$

2

where \({E_{bulk}}\) is bulk bandgap, \(\hbar\) is Planck's constant divided by \(2\pi\), is elementary electric charge, \({m_e}\) is the electron effective mass, \({m_h}\) is hole effective mass, \({m_o}\) is electron mass, \(\varepsilon\) is relative permittivity, and \({\varepsilon _o}\) is the permittivity of vacuum [32–34].

When PVP concentrations were increased except at a 20 mL concentration, absorption spectra showed a redshift suggesting that the particle size of ZnO nanoparticles was decreased.

## 3.4.1 Direct, Indirect Band Gap and Urbach Energy

An electron band gap, or energy requirement, is the energy required to excite an electron from the highest occupied orbital (HOMO) in the valence band to the lowest unoccupied orbital (LUMO) in the conduction band. Figures 6 and 7 show the \({\left( {\alpha h\upsilon } \right)^2}\)and \({\left( {\alpha h\upsilon } \right)^{\tfrac{1}{2}}}\) vs. \(h\upsilon\) in the direct and indirect band transitions obtained from ZnONPs prepared with different concentrations of PVP. Within the bandgap, there are transitions from the extended valence band to extended conduction band states. The direct and indirect transitions and all other transitions relate to an electromagnetic wave interacting with an electron in the valence band that is excited in the conduction band across the fundamental band gap [35, 36]. As with direct transitions, indirect transitions are accompanied by lattice vibrations simultaneously. Figure 6 shows the weak of \({\left( {\alpha h\upsilon } \right)^2}\) at low energy (higher wavelength), representing the transition between one localized state above the valence band and another below the conduction band [35]. Tauc's proposed method uses UV-visible absorption spectra to calculate direct and indirect optical energy gaps for all prepared samples. Eq. 3 can be used to determine the direct optical energy gap [37]

$${\left( {\alpha h\upsilon } \right)^{\frac{1}{n}}}=\beta (h\upsilon - {E_g})$$

3

where, \({E_g}\) is the optical band gap, is Plank's constant, \(\alpha\) is the absorption coefficient, \(h\upsilon\) is the energy incident photon, \(\beta\) is a proportionality constant, and represents different values corresponding to allowed electronic transition [36]. When \(\left( {n=1/2} \right)\)an interband transition is

allowed directly, while \(\left( {n=2} \right)\) is an indirect interband transition. Additionally, the absorption coefficient \(\alpha\) can be calculated by following the following formula

$$\alpha =\frac{{4\pi k}}{\lambda }$$

4

From linear regression of the linear portion of \({(\alpha h\upsilon )^2}\) and \({(\alpha h\upsilon )^{1/2}}\) to zero, the band gap energies were determined using the Tauc plots in Figs. 6 and 7. The point where the line meets the incident photon energy axis represents the direct and indirect band gap energy. Table (2) summarizes the optical band gap calculated by applying the Tauc relationship. Because particle sizes and quantum size effects increased with increasing PVP concentrations, the direct and indirect band gaps decreased except for PVP 20 mL. These results appear to agree with those obtained using TEM and X-rays.

This energy represents the electron transitions from one extended valence band state to another tail state below the conduction band and/or from one extended conduction band state to another tail state above the valence band. During exciting transitions from the top of the valence band to the bottom of the conduction band, low crystalline states and disorders result from localized states caused by defects, measured by an optical parameter known as Urbach energy. This energy results from electron transitions between the extended valence band state to another tail state below the conduction. It can be seen from Fig. 8 that for low photon energies, the Urbach empirical rule describes the dependence of absorption coefficients (α) and photon energies (hν). By using the following relation (5), it can calculate Urbach energy from the absorption spectrum in this regime:

$$\alpha ={\alpha _o}\exp \left( {\frac{{h\upsilon }}{{{E_u}}}} \right)$$

5

Assume that \(\alpha\) is the absorption coefficient, \({\alpha _o}\) is constant, and \(h\upsilon\) is the incident photon energy. \(\ln \alpha\) versus photon energy \(h\upsilon\) plots are used to determine the Urbach energy \({E_u}\). The reciprocal of the slope obtained by fitting the linear portion of the curve is the Urbach energy \({E_u}\). Table (2) gives the Urbach energy, which was less than the absorption band edge. Higher \({E_u}\)indicates a low crystallinity and disorder in the nanoparticles

Table 2

Calculation of the absorption peak and direct and indirect band gaps of ZnO nanoparticles prepared at different concentrations of PVP.

Samples | PVP concentrations | Absorption peak (nm) | Direct energy gap (eV) | Indirect energy gap (eV) | Urbach energy |

S1 | 5.00 mL | 363.56 | 4.08 | 0.91 | 1.59 |

S2 | 10.0 mL | 366.64 | 4.07 | 0.50 | 1.79 |

S3 | 20.0 mL | 362.90 | 4.10 | 1.04 | 3.03 |

S4 | 30.0 mL | 373.70 | 3.87 | 0.23 | 2.33 |

S5 | 40.0 mL | 374.90 | 3.56 | 0.21 | 2.34 |