3.1 X-Ray diffraction
The X-ray diffraction spectrum of ZnO nanostructure showed in Figure (1), the peaks of the prepared samples showed corresponding with JCPDS NO. 00-036-14511, with showed that all of ZnO Nanopowder with hexagonal wurtzite structure. The crystallite size was obtained using Scherrer's equation.
Where (d) are the average crystalline sizes (d) of the Nanopowder that observed between (33.56–30.41) nm and B represents the full width at half maximum (FWHM) in radian and θ is the Bragg diffraction angle in degree. From these results noticed that the increasing of NaOH concentration lead to reduce in the crystal, the small in crystalline size is due to super saturation and the reaction of the NaOH within the solution mixture. Subsequently, at high super saturation crystals can nucleate faster than they grow. Smaller crystals resulted as the crystal nucleation dominated crystal growth ultimately. When NaOH adding to the Zinc nitrate hexahydrate, it is suggested that the OH− attracted the Na + around the nanocrystals and a virtual capping layer was formed which inhibited the nanocrystals growth [11]. The Grain size of ZnO nanostructure has shown in Table (1).
3.2 The surface morphology studies
The surface morphology of ZnO nanostructures were obtained by SEM and AFM. The SEM images in Figure (2) shows the surface morphology of the ZnO nanostructure (showed uniform particle size distribution and their morphology was grain-like). Figures (3) shows the 2D and 3D AFM images of the ZnO Nanostructures at various NaOH concentrations. The roughness strongly depends on the precursors. When the concentration of NaOH increased, the surface roughness of ZnO Nanostructures decreased from (9.33–5.06) nm. The crystallite size decreased with increasing NaOH concentration as shown by XRD analysis. Also the surface roughness, RMS was decreased as shown in Table (3). The increasing NaOH concentration obtained homogeneous distribution of the nanoparticles.
3.3 Fourier Transform Infrared Spectroscopy measurements
The FTIR spectra of ZnO Nanopowder shows in Figure (4), the range of 400–4000 cm− 1 shown in Table (4) shows absorption peaks series. When the NaOH increase, the grain size of ZnO Nanopowder decreased and the percent of carboxylate (COO-) and hydroxyl (-H) groups in the sample increased. The FTIR peaks indicate to the successful synthesis of ZnO Nanopowder [12].
3.4 Optical properties
Figure (5), shows the optical absorption spectra of ZnO Nanopowder synthesized using different concentrations of NaOH and recorded as a function of wavelength within the range of (370–720) nm. The absorption goes up when the concentrations of NaOH increased from 1.5M to 3.5M. The absorption edges of the samples shows a sharp edge in the UV region with red shift when the molarity increased. Figures (6) shows the curve of (αhν)2 versus photon energy (hν) of ZnO prepared at different conditions where used the Tuac's equations to calculate the optical band gap (Eg):
( αhν ) n =A (hv – Eg) (2)
where hv is the photon energy, A is the constant relative to the material, and n is 2 for direct transition and 1/2 for indirect transition. As a result, by projecting the linear component of the (αhν)n – hν curve to zero, the optical band gap for absorption peak can be obtained. While Table (5) illustrated the value of optical energy gap of bulk ZnO and prepared ZnO. It observed that the band gap energy of ZnO Nanopowder increase with increased the NaOH concentrations. The increased in the optical band gap with increasing the NaOH concentrations could be due to the increase the strain value and decrease in carrier concentration.
3.5 Antibacterial activity test
The antibacterial activity of ZnO Nanopowder on gram positive (S. aurous) and gram-negative (P.aeuruginosa) Bacteria has been studied. ZnO Nanopowder was appeared higher antibacterial activity (size of inhibition zone) than control. The ZnO Nanopowder white precipitation has higher antibacterial activity on gram-positive (S. aureus) compared with antibacterial activity on gram-negative (P.aeuruginosa) bacteria in comparison with other standard antibiotics as shown in Table (6). The results summarized that ZnO Nanopowder resisted the growth of S. aurous bacteria completely because of the accumulation and adsorption of NPs in the outer membrane or cytoplasm of bacterial cells and triggered Zn2+ release, which penetrate the cell and cause disintegration of bacterial cell membrane, as a result the bacterial cells were killed, otherwise the P.aeuruginosa bacteria that may repeal the ZnO attachment onto the cell wall. Adsorption of Nanopowder leads to depolarization of cell wall, becoming more permeable as the typically negative charge of the wall is changed. The electrostatic attraction between positively charged bacterial cells and negatively charged particles is important for the activity of nanoparticles bactericidal materials. This interaction inhibits the bacterial growth and the reactive oxygen species (ROS) generation is induced, ROS are species of oxygen that are highly reactive and are produced during basic metabolism. Some ROS such as hydroxyl radicals cannot easily penetrate the negative cell membrane as they are negatively charged, meaning they which leads to cell death. The increasing in the Growth of S.aureus inhibited by ZnO Nanopowder, the increment of inhibition zone amount was due to increasing the NaOH concentration due to decrease the crystal size of ZnO Nanopowder and found the antibacterial (P.aeuruginosa and S.aureus) activity of ZnO with small crystallite sizes was stronger compared with those with large crystallite sizes [27]. The antibacterial efficiency of the ZnO Nanopowder of smaller sizes is enhanced as it can easily penetrate into bacterial membranes due to their large interfacial area [13–14]. The inhibition zone of P.aeuruginosa also increased depending on NaOH concentration.