3.1. XRD Analysis
The X-ray diffractograms of undoped ZnO and varied at.% of Yttrium doped ZnO thin films are shown in Fig. 2. The XRD patterns in all of the samples reveal phase uniformity and the same prominent peaks, indicating that the crystal structure of all coated films is hexagonal (wurtzite) with highly preferential orientation along (002) plane. The relative intensities of the peaks found in the XRD patterns appear to be in good accord with those suggested by the JCPDS for the wurtzite structure of ZnO (card No. 36-1451). There were no extra peaks of Y in any of the films studied, demonstrating the segregated Y-rich phases. This implies that the doping material (Y3+) has been successfully incorporated into the ZnO host lattice via the samples. These findings are consistent with prior findings reported by Miller et al. and Lim et al., 2011.
The lower ionisation energy of Y explains the superior crystal quality of Y doped ZnO films. In other words, Y dopants with low ionisation energy release the difference between Si's surface free energy and ZnO's c-axis preference energy. As a result, even without a catalyst, crystalline Y doped ZnO films could be easily produced on the Si substrate (Prasad Rao et al., 2009). The (002) peak has the highest intensity in pure ZnO film, showing that the film is crystalline even before doping. After raising the doping concentration beyond 4 at. %, the intensity of (002) reflection decreases, indicating a loss of crystallinity due to Y isolation at grain boundaries at high doping levels. While raising the doping concentration up to 4 at.%, the position of the (002) peak intensity is gradually relocated at the lower scattering angle (2θ) side. This tendency can be explained by the substitution of Zn2+ (0.74Å) ions by Y3+ ions (0.92 Å). For Srinivasalu et al., 2017, a similar pattern is found for metal oxide films while doping Y.
The interplanar spacing dhklvalues of undoped ZnO and Y doped ZnO thin films were calculated using the Bragg’s equation (Debye & Scherer et al., 1916). The crystallite size of the samples was determined based on the broadening of the preferential orientation (002) using the Debye-Scherer’s formula (Scherer &GöttingerNachrichten, 1918),
D = \(\frac{\text{k}\lambda }{\beta \text{c}\text{o}\text{s}\theta }\) (1)
Where k represents the shape factor (0.9), β is the full width at half maximum in radian of (002) orientation, λ is the X-ray wavelength (0.154 nm) and θ is the Bragg’s angle of the X-ray diffraction peak. The micro strain and the dislocation density were also determined for each film by using the tangent formula (Mehedi Hassan et al., 2014),
βhklcosθ =\(\frac{\text{k}\lambda }{\text{D}}\) + 4ε sinθ (2)
and
Dislocation density (δ) = \(\frac{1}{{D}^{2}}\) (3)
The calculated values of average crystallite size (D), lattice constant (c), strain (ξ) and dislocation density (δ) values for the different samples are listed in Table 1. The average crystallite size is between 4 and 15 nm. The results show that Yttrium doping at a particular amount (4 at.%) enhances the structural quality of sprayed pure ZnO thin films, lowers crystallite size (D), and thus decreases micro-strain. This finding suggests that home-made simplified spray pyrolysis may be capable of creating thin films with high crystalline quality.
Table 1
Structural parameters of undoped andZnO:Y thin films
Sample | Average Crystallite Size (D) (nm) | Lattice Constant (c) [Ȧ] | Strain (ε) | Dislocation Density (δ) (lines/m2) |
ZnO | 14.47 | 5.218 | 0.4504 | 4.775x1016 |
ZnO:Y (2 at. %) | 9.03 | 5.210 | 0.7255 | 1.226 x1016 |
ZnO:Y (4 at. %) | 4.04 | 5.188 | 0.1262 | 6.128 x1016 |
ZnO:Y (6 at. %) | 5.16 | 5.192 | 0.2242 | 3.755 x1016 |
3.2. FTIR analysis
The FTIR spectra for deposited films were recorded in the 400–4000 cm− 1 range, and the peaks confirm the numerous chemical bonds existing in thin films, as shown in Fig. 3. The spectra clearly demonstrate that the vibrational peaks range from 400 to 500 cm− 1, confirming the presence of ZnO stretching vibrations (Anandanet. al., 2013). The C-N stretching peak is represented by a narrow tiny peak at 846 cm− 1. The vibrational bonds within 1000 cm− 1 could be caused by inorganic components in the samples. At 1509 and 1601 cm− 1, C = O vibration is found (Thirumoorthi et. al., 2015). The bands at 1513 and 1417 cm− 1are attributed to carboxylic group vibration and CO2 in air, respectively (ShekDhavud et. al., 2020). Furthermore, the C-O bond vibrates at a frequency of 1384 cm− 1. The bending H-O-H vibration of water molecules adsorbed on the surface of ZnO is assigned a tiny band at 1604 cm-1.The presence of a ZnO-Y bond is responsible for the unique IR peaks below 633 cm− 1(Chen K et. al., 2009).The Y-O stretching characteristic peak is found around 505–562 cm− 1(Liu J, 2011; Raja K et al., 2014), indicating that Y3+ ions are incorporated on Zn2+ sites of the ZnO matrix. The vibrations of the Y-ZnO local bond are responsible for the band near 727 cm− 1(Repelin et. al., 1995). At 1667 cm− 1, the O-H deformation vibration is observed. The presence of hydroxyl groups in the N-O, C-H, =C-H, and C-N vibrations indicates the presence of alkane and alkene groups in the deposited films (Sato et al., 1988; Muneer et al., 2013; Mitra et al., 2013).
3.3. Optical Study
Figure 4 depicts the transmittance spectra of undoped and Yttrium doped ZnO thin films. This figure shows that all of the deposited films have good transparency in the visible and near infrared regions. The average transmittance of the undoped ZnO and ZnO:Y films in the 600 to 1200 nm wavelength range is 75%. The lack of interference fringes in the spectra shows that no reflections occur at the air/film and film/substrate interfaces (Byeongyum et al., 2006). The majority of the research demonstrated that different dopants exhibit varying optical transmittance due to optical losses in the deposited film. In general, the excellent crystalline character of the deposited films identifies a steep drop in transmission towards the basic absorption edge due to a reduction in incident electromagnetic radiation loss (Salakan et. al., 2013). As a result, the observed significant decline in the transmission spectra clearly supports our XRD findings in the current investigation.
Undoped ZnO exhibits high optical transparency in the visible region, indicating the uniformity and smoothness of the generated sample in the absence of yttrium. When yttrium is doped, the transmittance in the visible range decreases (absorption increases), and the exciton peak (365 nm) exhibits a red shift. This behavior is most noticeable in the sample doped with 4 at. % yttrium (inset of Fig. 4). This result is consistent with the discovery that higher absorbance increases visible-light photoactivity (Gotkas et al., 2013). These findings are consistent with the findings of Cheng et al. (Shan et al., 2005). As with the Y-doping concentration, 4 at.% displayed higher NIR transmittance than the other doped films, most likely due to the reduction of light scattering as carrier concentration decreased.It is important to highlight that a metal oxide with greater visible light absorption has an improved antibacterial activity. Cheng et al.'s investigation on the effect of oxide-based NPs on fish infections with visible light activated bactericidal activity (Numan Salah et al., 2013) supports this finding.
The optical energy band gap (Eg) of films can be determined by studying variation of optical absorption coefficient with wavelength of incident photons by the material (Bakin et al., 2010) using Tauc’s relation,
Here, h is plank constant, α is the absorption coefficient, Eg is optical band gap energy and B is proportionality constant. The Fig. 5 shows the different energy band gap (Eg) values (Tauc’s Plot) of the deposited films. The values that have been calculated for pure and 2 at.%, 4 at.%, and 6 at.% yttrium doped ZnO films are 3.11 eV, 3.05 eV, 3.02 eV, and 3.01 eV, respectively.
3.4 Photoluminescence Study:
Figure 6 depicts the PL spectra of ZnO:Y thin films at room temperature with respect to Y concentration (0, 2, 4, and 6 at.%). We discovered that the Y concentration influenced the PL emission from this figure. All samples exhibited two major ultraviolet (UV) emission peaks: a strong peak at 330 nm and a broad deep level emission peak between 370 and 400 nm. The integration of Y concentration (2 at.%) resulted in increased UV emission due to reduced defects such as oxygen vacancies and zinc vacancies (Ahmad Umar et al., 2015).The substitution of a higher ionic radius element (Y3+) (0.1011 nm) for a smaller ionic radius element (Zn2+) might increase the local volume of the lattice and hence eliminate defects (Kaur et al., 2016). However, increasing the Y (4 and 6 at.%) doping induces phase segregation and broad deep level emission in the region (370–400 nm). This is related to ZnO free exciton recombination. At greater Y concentrations, it is unable to replace Zn and instead occupies an interstitial position in the host lattice, resulting in phase segregation (Kumar et al., 2015). Oxygen vacancies are usually reported to be responsible for this emission band. 2 and 6at.% of Y: ZnO shows less emission in UV region, but 4 at.% of Y doped ZnO shows more emission comparing with pure and other doped materials due to enhanced incorporation of Yttrium suppresses the deep level defects and reinforces the UV emission. It is valuable to note that this band is most frequently reported band for ZnO material (Suvithet al., 2014).
3.5. Surface Morphological Study
Figure 7 shows SEM images of ZnO thin films coated with various Y doping levels: 0, 2, 4, and 6 at.%. The pure ZnO thin film features spherical-like shaped grains with good homogeneity, as shown in the picture. Pinholes and particle clusters, on the other hand, were seen. 2 at.% yttrium substituted ZnO film has a cauliflower-like structure and significant morphological changes. It is quite likely that yttrium was integrated into the ZnO lattice based on the observed sudden change in morphology (Dghoughiet al., 2010). When compared to a 2 at.% yttrium doped ZnO thin film, the film exhibits a tetra pod chain like structure with an improved surface to volume ratio at 4 at.% yttrium doping. Several continuous hollow tube growths can be seen on the substrate surface. This hollow architecture, as reported by Xu et al.,2007; (Yaoming Li et al., 2010), could be effective for trapping biomolecules and microorganisms. As a result, the 4 at.% Yttrium doped ZnO sample is expected to have improved antibacterial efficiency.Some broken tetra pod chains and background grains that are relatively spherical may be detected in the 6at.% Yttrium doped ZnO sample. This morphology-changing behavior could be produced by the interstitial entry of yttrium ions into the ZnO lattice, resulting in tension between zinc and yttrium ions (Shinde et al., 2006; Yu Q et al., 2007).
3.6. Antibacterial study:
Figure 8 depicts the zone of inhibition of pure and yttrium-doped ZnO thin films against antibiotic-resistant fish bacteria, such as Aeromonashydrophila, Salmonella enterica, Lactococcusgarvieae, and Streptococcus agalactiae. All of the deposited films (0, 2, 4, 6 at.% Y) were found to have significant antibacterial activity against the pathogens tested. The diameter of the inhibition zones surrounding each well is determined in millimetres and shown as a bar diagram (Fig. 9). This figure also shows that the produced samples have a more sensitive antibacterial nature than the standard samples.
The current findings show that the zone of inhibition was greater against gram negative bacterial strains Aeromonashydrophila and Salmonella enterica than against gram positive bacteria Lactococcusgarvieae and Streptococcus agalactiae. Maddahi et al., 2012; reported similar findings for gram negative and gram positive bacterial strains. The structural organisation of gram-positive and gram-negative bacterial cell walls differs widely. Gram positive bacteria are less susceptible to nanotoxicity of ZnO:Y nanoparticles due to the presence of a thicker peptidoglycan coating than gram negative bacteria. This could explain why the observed results show that gram-negative bacteria have a higher degree of inhibitory zone than gram positive bacteria.
ZnO is a powerful metal oxide nanoparticle that may easily limit bacterial growth (Raghupathi et al., 2011; Gunalan et al., 2012; Salini et al., 2021). According to ZnO's antibacterial mechanism, the ZnO matrix may easily release Zn2+ ions, which can directly contact bacterial cells. When Zn2+ nanoparticles come into touch with the inside and outside of cell walls, they induce damage and destruction to the cell walls and membrane. Because of the presence of carboxyl, phosphate, and amino groups, the cell wall is primarily made of peptidoglycan, which has a negative charge. The positive charge of Zn nanoparticles creates electrostatic interaction between Zn and the microorganisms' negatively charged cell membrane. To boost antibacterial activities, the surface charge of Zn NPs can be changed to produce a stronger attractive force. Zn NPs can enter microbial cells, and the released Zn2+ can interact with cellular structures and biomolecules such enzymes, lipids, proteins, and DNA. Zn NPs may release Zn2+ into and out of bacteria endlessly, and Zn ions can interact with proteins and enzymes. The Zn nanoparticles can bind to proteins found in the cell membrane that are involved in trans-membrane ATP production. The cytoplasmic damage, which occurs in varying degrees within the cell, causes the cell to lose its cell shape. Increased reactive oxygen species (ROS) cause apoptosis, lipid peroxidation, and DNA damage (Qing et al., 2018). Even though the aforementioned antibacterial process is thought to be correct, the actual antibacterial mechanism of ZnO nanoparticles is unknown.
The ability of the dopant yttrium to restrict the development of both gram-positive and gram-negative bacteria accounts for the increased efficacy of Y doped ZnO thin films. To create outstanding anti-bacterial action, Yttrium improves the inhibitory effect, generation of reactive oxygen species, and aggregation of Y: ZnO nanoparticles in the cell membrane and plasma (Tam et al., 2008; Sharma et al., 2010).
The concentration of nanoparticles determines the range of inhibition. The substitution of Y3+ions at Zn2+cation sites in YZO thin films up to 4 at. % is related to an increase in the liberation of Zn2+ and Y3+ions from the film, which supports the observed enhancement in the antibacterial efficiency of the synthesised samples. According to Maria Magdalane et al., 2021;Kaviyarasu K et al.,2017 Y3+ ions released from Y:ZnO act as a positive charge that reacts with the cell's negative charge, resulting in protein degradation. Increasing the Y concentration above 4 at. % results in more assimilation of interstitial Y atoms, resulting in grain boundary segregation. Impurity segregation at grain boundaries in nanomaterials is well recognised to affect other material properties controlled by interfaces (Manoharan et al., 2015). Grain boundary segregation can also have a significant impact on grain boundary mobility (Thongsuriwong et al., 2012) and, as a result, on the recrystallization temperature and nanocrystalline structure stabilisation (Singh et al., 2009; Maddahi et al., 2014). As a result, in the current investigation, YZO films with greater concentrations (6 at.%) were found to have lower crystallinity, resulting in decreased antibacterial activity. XRD and SEM tests show that the crystallinity of YZO films has decreased by 6 at.%. Thus, the antibacterial effectiveness of formed films against pathogenic bacteria is affected by their size, surface area, and dopant ion concentration (Mote et al., 2014). Thus, a suitable Y concentration in ZnO thin films enhances crystallinity, allowing us to modify the physical, optical, and antibacterial properties to create better and more stable nanomaterials (Azam et al., 2012).