1 Introduction

Several novel syntheses have been adopted in the last few years to prepare nanoparticles with appreciable size, morphology, and desired physical properties [1]. The miniature size of nanoparticles ranging from 1 to 100 nm has made it a potential candidate for several applications in the field of medicinal chemistry, atomic physics and other technologies [2]. Based on dimensions, nanoparticles are classified into zero, one and 2-dimensional nanostructures [3]. Among them, quantum dots with zero dimensions play a major role in biological fields as nanomedicines and nanocarriers [4]. Zinc, gold, and silver nanoparticles have been considered as suitable materials for wide applications due to their superior photocatalytic property, long-term performance, and non-toxicity [5]. Food and drug administration of the US (FDA) has reported that ZnO nanoparticles are less toxic in comparison with other metal oxides [6]. ZnO nanoparticles possess a wide bandgap (3.37 eV), binding energy (60 meV), and exceptional antibacterial properties which makes them suitable for several technological applications [7, 8]. Due to its intriguing properties, ZnO nanoparticles are used for several applications in the field of photocatalysts, biosensors, optoelectronic devices, and water purification [9]. Earlier reports have shown that in comparison with chemical route techniques ZnO nanoparticles prepared by the green synthesized method showed better antibacterial effects at lower concentrations [10, 11]. The superior antibacterial properties of ZnO nanoparticles make it a potential candidate for several applications in the various fields of drug delivery, antibacterial treatments, anticancer treatments, agriculture [12,13,14,15,16]. Among the several approaches for the synthesis of nanoparticles, biosynthesis methods have been adopted due to their low cost and environmentally friendly design [17]. Greener synthesis of nanoparticles is found to be more advantageous in comparison with chemical methods due to its environmentally friendly nature, better stabilization, and control over crystal growth [18, 19].

Compared with other chemical methods of synthesis, greener synthesis of nanoparticles has better stabilization and control over crystal growth. Recently zinc oxide nanoparticles have been synthesized from many plant extracts, of these Azadirachta indica (Neem) is a traditional medicinal plant of wide application in Ayurveda, Siddha, Unani, and homeopathy [20]. Due to the presence of highly active components in this plant, both gel and leaf of A. indica have been used for pharmacological applications [21]. Jeeva Lakshmi et al. in 2012 reported the synthesis and antimicrobial effect of zinc oxide nanoparticles from hot and cold plant extracts and observed a significant change in the particle size and properties [22].

Metals are mainly used in petrochemicals, oil and gas, steel, and other industries, in all these industries interaction of metals with the environment, may damage its property and lead to corrosion [23]. The consequences due to corrosion can be minimized by the usage of corrosive inhibitors, metallic coatings, and cathodic protection [24]. Mainly organic and polymer-coated materials are used as corrosion inhibitors, but these are expensive and make adverse effects on our environment [25]. So the development of eco-friendly cost-effective corrosion inhibitors from green sources plays an important role in these industries. Recently many studies have proved that nanomaterials from different plant extracts, fruits, and seeds can act as an efficient corrosion inhibitor in the different mediums [25]. This is because plant extract acts as a capping and stabilizing agent is also adsorbed on the surface of the metal, and this will block the metal surface and thereby prevent corrosion [26]. In biogenic synthesis, plant extract can reduce the usage of harmful chemicals and also minimize the cost of production [27].

In the present work, ZnO nanoparticles were synthesized by the green route technique and their structural, optical, photocatalytic, and corrosion behavior were analyzed.

2 Materials & methods

Fresh neem plant leaves were collected and washed in distilled water, the aqueous extract is prepared by boiling 25 g of fresh leaves in 100 mL sterilized water at 65 °C for 20 min. The aqueous extract is filtered after being stirred in a magnetic stirrer until the coloration shifts from orange to brown. The plant extract is stored in a cold environment to avoid decomposition. ZnO nanoparticles are prepared by combining 25 mL of a stored extract with 25 mL of zinc acetate. The prepared solution is stirred well using a stirrer and the pH 7.0 is maintained by adding 0.5 M sodium hydroxide (NaOH) at room temperature to get a white precipitate. The collected sample is washed several times with water and ethanol to remove impurities using a centrifuge. Finally, the precipitate is dried in an oven at 60 °C and calcined at 400 °C for 1 h in a muffle furnace.

3 Characterization techniques

The crystalline structure of the prepared ZnO nanoparticles was elucidated by an X-ray diffractometer (XRD; X’Pert PRO, PANalytical operated at 40 kV and 30 mA with Cu Kαsource). The XRD pattern had been studied within the angle range from 20 to 80°.

The average particle size distribution of the prepared material is determined by using the dynamic light scattering (DLS) technique (Nanophox Sympatec, Germany) by illuminating light of a wavelength of 633 nm. The surface morphology of the prepared material is viewed by a field-emission scanning electron microscope (FESEM; JSM-6790 LS; JEOL) and the compositional analysis has been carried out using an EDX analyzer.

The optical behavior of the prepared ZnO nanoparticles was analyzed by using a UV–Visible (UV–Vis) spectrophotometer (Agilent Cary 8454, Singapore) at wavelengths from 180 to 800 nm of the electromagnetic spectral region. The bandgap energy of the prepared ZnO nanoparticles is calculated using the Tauc relation from the absorption spectra [28,29,30,31].

3.1 Sample preparation for photocatalytic activity

The photocatalytic dye degradation activity of the prepared sample was dispensed by assessing the degradation of methyl orange (MO) and Rhodamine-B (RB) dye under a constant UV light irradiation. The photocatalytic dye degradation for the prepared samples was measured by the following formula [31],

$$\eta \, = \,\frac{{C_{0} \, - \,C_{t} }}{{C_{0} }}\, \times \,100$$

Here, η is the degradation calculation, C0 is the early absorbance of dye (at 00 min); Ct is the variation in absorbance of the dye at after time intervals of the degradation.

3.2 Sample preparation for corrosion studies

The anticorrosive behavior of the prepared ZnO nanoparticles was analyzed by corrosion studies. About 15 mg of prepared ZnO nanoparticles were mixed with Polyvinylidene difluoride (PVDF) and N-methyl-2-pyrrolidone (NMP) at 80:15:5 weight proportions to create suspension. The slurry was additionally coated on the Zn metal plate surface by adopting the doctor’s blade technique [32]. The coated plate was dried within the hot-air oven at 353 K for 1 h and used for corrosion studies beneath 3.5% NaCl electrolyte.

4 Results & discussion

4.1 XRD analysis

The powder XRD pattern of the prepared ZnO nanoparticles is shown in Fig. 1a. The characteristic peaks confirmed the formation of a hexagonal wurtzite crystal structure with three most preferred orientations (1 0 0), (0 0 2), and (1 0 1), all of which are very close to the standard JCPDS Card No: 36-1451 [14]. The XRD pattern of the prepared ZnO nanoparticles shows that no characteristic impurity peaks have been obtained. This clearly shows that the prepared nanoparticles are free from contamination. The Debye–Scherrer equation was used to calculate the size of the crystallites. The calculated crystallite size of the prepared ZnO nanoparticle is 11.3 nm. Rietveld refinement of the powder XRD peaks were carried out by using FULLPROF software [33, 34]. The refinement plot of the prepared ZnO nanoparticles is shown in Fig. 1b and it can be observed that the Bragg peaks match well with the XRD data.

Fig. 1
figure 1

a Powder X-ray diffraction pattern of the ZnO nanoparticles. b Refinement plot of the ZnO nanoparticles

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4.2 UV–Vis analysis

The UV–Vis absorption spectra of ZnO nanoparticles are shown in Fig. 2. The UV absorption peak was found to be around 375 nm and the bandgap of the prepared nanoparticles was calculated from the Tauc plot. The calculated energy bandgap from the Tauc plot (Fig. 3a, b) is within the range from 3.1 to 3.3 eV, which is consistent with previous results [9, 20] The calculated energy bandgap from the Tauc plot for direct transition is 3.24 eV and the indirect transition is 3.14 eV.

Fig. 2
figure 2

UV–Vis spectrum for the prepared ZnO nanoparticles

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Fig. 3
figure 3

a, b Tauc plot of the synthesized ZnO Nanoparticles

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4.3 Morphological studies

The morphology and elemental compositional analysis of the green synthesized zinc oxide nanoparticles were ascertained by FESEM and EDX, respectively (Fig. 4a, b). SEM micrographs (Fig. 4a) revealed the presence of spherical-shaped ZnO nanoparticles with very small-sized clusters in the range between 23 and 40 nm. EDX (Fig. 4b) revealed the presence of 63.34% Zinc and 36.66% of oxygen, no traces of impurities were observed.

Fig. 4
figure 4

a SEM micrographs, b EDX studies for the prepared ZnO nanoparticles

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4.4 DLS studies

The particle size of the prepared nanoparticles was exploited by dynamic light-weight scattering. The particle size distribution of the ZnO nanoparticles is shown in Fig. 5, it signifies that the particle size ranges from 16.19 (d10) to 45.84 (d90) nm. The average particle is found to be 27.81 nm.

Fig. 5
figure 5

Particle size distribution of the prepared ZnO nanoparticles

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4.5 Photocatalytic activity

Photocatalytic activity is recorded by using prepared nanoparticles as photocatalyst in Rhodamine-B (RB) and methyl orange (MO) aqueous solution. The absorption spectra (Fig. 6a, c) of MO and RB dye were recorded by illuminating UV radiation at regular intervals (30, 60, 90, and 120 min).

Fig. 6
figure 6

a, b Photocatalytic degradation studies for ZnO nanoparticles by using methyl orange dye solution, c, d Rhodamine-B dye solution

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Irradiation of UV light on ZnO leads to the transfer of electrons from the valence band (VB) to the conduction band (CB) leaving behind positive holes in VB. The electrons transferred are termed as photogenerated electrons, the holes in VB and the photogenerated electrons in CB are accountable for the photodegradation [35, 36]. Generally, three steps are involved in a photocatalytic process (i) electron–hole pairs generation (ii) electron–hole pair separation, and (iii) generation of high active hydroxyl radicals due to the surface redox reactions [37].

The absorption spectra during the photodegradation of MO for different times (0, 30, 60, 90, 120 min) is shown in Fig. 6a. A sharp decrease in the absorption peak is observed at around 538 nm and tends to diminish after 120 min of irradiation of UV light.

The temporal variation in the UV–Vis spectra during the photodegradation of RB is shown in Fig. 6c. It can be observed that the band around 538 nm drops gradually and tends to completely vanish after 120 min of irradiation which is due to the aromatic ring opening of RB which provides a pathway for degradation [38].

The degradation of MO (Fig. 6b) for ZnO nanoparticles are around 35%, 65%, 75%, and 96% at 30, 60, 90, and 120 min, respectively. Whereas the degradation of RB (Fig. 6d) are around 50%, 70%, 85.7%, and 90% at 30, 60, 90, and 120 min, respectively. The enhancement of photocatalytic degradation may be attributed to the increase in surface area due to the decreasing particle size [39, 40, 41]. Table 1 gives a comparison of the photocatalytic responses for various catalysts using different dyes.

Table 1 Comparison on the photocatalytic responses for different dyes
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4.6 Corrosion studies

The electrochemical impedance spectra or Nyquist plot for uncoated and ZnO nanoparticles coated Zn metal surfaces is shown in Fig. 7a. At low frequencies, a semicircle was obtained within the Nyquist plot diagram, which might be linearly involving the charge transfer resistance of the reaction (associated with the corrosion process). In Fig. 7a each sample show semi-circles (an electrical phenomenon loop), however, their radius varies noticeably. The ZnO nanoparticles coated Zn metal plate had higher electrical phenomenon values than the blank Zn metal samples, indicating that the coated samples have higher barrier properties against corrosion. The high Rct values for ZnO nanoparticles coated Zn plates signify the higher corrosion resistance of the metal surface.

Fig. 7
figure 7

a Nyquist plot and b Tafel plot of uncoated and ZnO nanoparticles coated Zn metal plates in 3.5% NaCl electrolyte

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Tafel plots or Potentiodynamic Polarization studies are carried out to analyze the electrochemical corrosive characteristics of a metal surface [42,43,44]. The Tafel plot for Zn metal plates and ZnO nanoparticles coated Zn metal plates in 3.5% NaCl solution is shown in Fig. 7b.

The potential was applied from − 1.4 V to 0 V and the corrosion of the Zn plate begins from the anode side which is consistent with the Tafel plot. From the Tafel plot, the potential of the ZnO nanoparticles coated plate is shifting toward the positive region (toward the anode side) in comparison with the uncoated Zn metal plate.

The Tafel plot shows that the ZnO/Zn plate contains a lower current density than the uncoated Zn plate. The comparison of corrosion current and corrosion potential for uncoated and ZnO coated Zn plates are illustrated from the results of the Tafel plot in Table 2. The corrosion efficiency rate for the ZnO/Zn plate seems to be around 50.11% higher in comparison with the uncoated Zn metal plate. The comparative assessment on the coating of nanomaterial for various metal surfaces to improve its corrosion inhibition behavior is shown in Table 3.

Table 2 Corrosion potential (Ecorr) and corrosion current (Icorr) determined from Tafel plots of ZnO nanoparticles in 3.5% NaCl electrolyte
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Table 3 The comparative assessment on the coating of nanomaterial on different metal surfaces for improvement of corrosion inhibition behavior
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5 Conclusion

Neem plant extracts were used to prepare Zinc oxide nanoparticles by adopting green synthesis methods. X-ray diffraction studies confirmed the single-phase formation of the compound without any impurities. SEM micrographs revealed that the prepared nanoparticles were spherically shaped with sizes ranging from 23 to 40 nm and the average size was confirmed from DLS studies. The bandgap of the prepared nanoparticles from UV–Vis analysis is found to be around 3.1–3.3 eV. Photodegradation studies proved the better photocatalytic behavior for Rhodamine-B solution. The corrosion efficiency rate for the ZnO nanoparticles coated Zn plates were greater in comparison with the uncoated Zn plates.