Impact of Aluminium on ZnO Thin Films for Antimicrobial Activity

Thin lms of pure Zinc Oxide (ZnO) and Aluminium (Al) doped ZnO were deposited by two step SILAR technique. Pure and Al (1%, 3%, 5%) doped ZnO thin lm’s structural, morphology and optical properties were analyzed. Diffraction peaks of the all the samples were indexed to hexagonal wurtizite structure. The crystallite size, lattice parameters, dislocation density and microstrain were calculated for the prepared thin lms. Morphology study using FESEM shows spherical shaped structure of pure ZnO and hexagonal faced rod like structure for Al doped ZnO thin lms.T he UV-Vis absorption spectrum for the thin lms was also studied. There is decrease in bandgap as the Al doping ratio increases from 1–5%. Photoluminescence (PL) studies conrm that oxygen ion vacancy and interstitial Zn + ion were present. The maximum zone of inhibition was studied against the bacteria’s the Gram-negative (E.coli) and Gram-positive (S.aureus) by Agar diffusion method. Signicant antibacterial result was seen in pure and Al doped ZnO. Al doped ZnO shows more antibacterial activity over pure ZnO. All the samples give considerable antifungal activity which was done against Aspergillusniger.


Introduction
Zinc oxide (ZnO) is a semiconductor with possible applications in different elds because of its signi cant physical properties. It is an II-VI semiconductor which crystallizes in rock salt, wurtzite and rock salt forms. ZnO is more stable in the hexagonal wurtzite phase rather than other phases at room temperature. Studies on thin lms have improved several novelparts of analysis which depends on exclusive characteristic and structures [1]. ZnO has very good chemical stability, excellent thermal stability, high exciting binding energy (60meV) and wide bandgap (3.37eV) at room temperature. ZnO thin lms have good electrical and optical properties, very high electrical resistivity, low cost and are non-toxic [2].
The inorganic compound ZnO has very good resistance against microorganisms [3]. ZnO is an important material for preventing the in uenceof food pathogens such as Staphylococcus aureus (S.aureus) and Escherichia coli (E.coli) on the food items [4]. It is also useful in skin treating products like ointments, diaper rashes, baby powder, etc. ZnO also has good antifungal activity [5] which is used for protecting the wood from fungal activity [6] Pure ZnO lms are found to unstable because of chemisorptions and adsorption that modi es the surface conductance [7]. By adding anionic and cationic dopant such as Cu, Sn, Mn, Ag, Fe, Mg and Al, the properties of Zinc Oxide can be modi ed and the thin lms greatly conductive which could be the other options for cheaper transparent conducting layers for various applications like anti-bacterial, antifungal, gas sensing, photocatalytic properties and solar cell [8][9][10][11] etc. Among the dopants Aluminum is one of the best dopant to get thin lms of both transparent and conductive in the visible region. So, it can be used in transparent conductive composites. Currently, preparation of thin lms has number of deposition techniques like dip coating, sputtering, chemical bath deposition, spray pyrolysis, sol-gel deposition and Successive Ionic Layer Adsorption and Reaction (SILAR) [12][13][14][15][16][17]. Among these, SILAR is one of the commonly used techniques for preparing thin lms because of its ease, inexpensive, non-vacuum technique and even coating over a larger area.
The aim of the present study is to prepare pure and Al doped ZnO thin lms by SILAR technique and to investigate the structural, surface morphology and optical property for the prepared lms. Also to analyze the antibacterial and antifungal activity, since there are few studies for Al doped ZnO thin lms and merely no studies available for Al doped ZnO thin lms prepared by SILAR techniques.

Experimental Procedure
Glass slide is taken as substrate for depositing pure and Al doped ZnO thin lms by SILAR technique.
Zinc acetate dehydrates (ZAD) and aluminium acetate was used as source and dopant material for preparing the thin lms,. 0.1M of precursor (ZAD) was dissolved in 100ml of solvent (ethanol).
Monoethanolamine (MEA) is added drop by drop to obtain clear and uniform zincate solution. The zincate solutions are prepared for pure ZnO, 1%, 3% and 5% Al doped content. Film deposition was carried out by dipping the pre-cleaned microscopic glass substrate alternatively in zincate bath which is kept at room temperature and hot deionised water bath which is kept between 90ºC to 100ºC. One set of dipping involves 30 second dipping in zincate bath and in a hot deionised water bath. This process was repeated for 30 cycles to obtain the thin lm and the lm is post annealed at 250ºC. Assessment of Antibacterial Activity E.coli (Gram negative) and S.aureus (Gram positive) bacteria's were used for assessing the antibacterial activity of the prepared pure and Al doped ZnO. Agar diffusion method is used for assessing the antibacterial activity. Sterile Muller Hinton (MHA) agar was distributed within sterile petri dishes. The culture specimen was adjusted to 10 8 cfu/ml. The surface of the agar plate was swabbed over using sterile cotton swab. Prepared samples were kept at the middle of the plate and pressed softly and were incubated for 24 hours at 37 o C. The incubated plates were then tested for the interruption of growth sample. The diameter of the inhibited growth near the sample was measured to nd the zone of inhibition.

Assessment of Antifungal Activity
Agar diffusion method is used for the antifungal activity on ZnO and Al doped ZnO. The antifungal assessment used Potato dextrose agar medium and Aspergillus niger was the test organism. A petridish was dispensed with the prepared potato dextrose agar medium and the spores of fungi have been inoculated into sterile distilled water having few glass beads and shaken energetically to take the spores into suspension. The surface of the agar was diffused uniformly with 1.0 ± 0.1 ml of inoculums. The test samples were set in agar medium and they were incubated at 27°C. The zone of mycosis under and along the sides of the samples was measured after the incubation period. The incubated plates were inspected for interruption in the growth of inoculums. The size of the clear region was noted for assessment of the inhibitory impact of the assessment sample.  004) and (202) lattice plane con rms hexagonal Wurtzite structure of the prepared lms as it well matches with the JCPDS card number 36-1451. The lm pattern with the highest intensity at the (002) plane reveals the orientation of the particles are along the c-axis. This may be owing to the negligible interior stress and surface energy and also because of the high atomic density that makes the crystallites grow towards the c-axis easily [18,19]. The right shift was observed in the main peak position which is shown in g. 1(b). The doped samples illustrate that the planes (100), (002) and (101) exhibits a shift towards the higher angle side. This shift observed in the peak is owing to the attribution of ionic radii of the dopant Al 3+ ion which is small when compare to Zn 2+ ion. The Peaks also found to broaden with the increase in Al concentration. ion is small than that of Zn 2+ ion. Obtained crystallite size and the microstrain indicate that they are sensitive to Al doping. In addition, Fig. 2 shows the changes of the crystallite size along with the microstrain as a function of Al ratio, showing the likely reduction in the crystallite size which may be seen from the diffraction peak broadening as shown in g. (1b). The Dislocation density of pure ZnO thin lm is 1.1085 x 10 15 lines/m 2 and is increased to 2.9562 x 10 15 lines/m 2 after Al doping. These results highlight that there is an increase in lattice imperfection in the Al doped ZnO thin lm.  Al content. The AFM result shows that smoothness of the coating surface decreases with increase in doping concentration [22], [23]. in the ultraviolet region from 300nm -378nm. The absorption edge was shifted to the lower energy side for Aluminium doped ZnO such similar results were seen in Cd doped ZnO thin lms [24]. The material has higher optical property with the presence of Al and thereby, the absorption band edge increases from 366.67 nm for pure ZnO to 371.89 nm for 5% Al doped ZnO. The absorption spectra indicates that maximum absorption for 5% Al. This might be due to the doping of Al that affects the density of unsaturated bonds making the deviation of local states within the bandgap. Fig.7 represents the relation between (αhυ) 2 and (hυ) of the prepared ZnO lm with different Al content and the insert denotes bandgap attained by projecting the linear part of the graph and the bandgaps are 3.15eV, 3.12eV, 3.10eV and 3.06eV for pure, 1%, 3% and 5% Al doped ZnO thin lms correspondingly. The bandgap decreases with the doping of Al content and is explained in terms of stress relaxation mechanism. Such type of redshift has been reported by Mohanty et al [25]. This suggests the opening of defect states within the bandgap and it can be interpreted due to merging of impurity band into the conduction band, thus shrinking the bandgap. The photoluminescence spectra with an excitation wavelength of 325nm of Al doped ZnO is shown in g.8. It clearly indicates that the lms created a narrow ultraviolet (UV) emission peak and a green emission band. The gure clearly shows a UV peak at approximately 381 nm which is due to ZnO intrinsic emission produced through the recombination of free excitons [26] and a deep-level emission at 450-550 nm is due to zinc interstitial defects and oxygen vacancies [27]. The zone of inhibition clearly shows the method of the biocidal activity of the Aluminium doped ZnO that obliterates the external wall of the bacteria and promotes death [29]. The antibacterial effect of Al doped zinc oxide was superior on E.Coli than other strain. This is because the thickness of the cell wall is thin for gram negative bacteria.

Surface Morphology
Antifungal activity against Aspergillus niger on pure and Al doped ZnO is shown in Fig.10. The maximum zone of inhibition after four days of incubation for pure ZnO, 1%, 3% and 5% Al was found to be 18mm, 10mm, 10mm and 17mm respectively. Both doped and undoped ZnO shows signi cant antifungal activity. The ZnO particles inhibit the development of Aspergillus niger by affecting the cellular function which leads to the deformation in fungal hyphae [30]. The creation of reactive oxygen species (ROS) is responsible for the increase in the permeability of the cell membrane which enables the cell death.

Conclusion
SILAR method is utilized to prepare pure and Al doped ZnO thin lms and the lms were annealed at 250 o C. XRD result reveals that every prepared lm is having hexagonal wurtzite arrangement and the outcomes are well matched with the standard JCPDS data (card no. 36-1451). Aluminium doping in ZnO in uence the crystallite size by decreasing it sharply. FESEM images reveal spherical morphologies for pure ZnO thin lm and hexagonal faced rod like structure for Al doped ZnO thin lms. AFM study reveals that RMS values increases with the Al concentration. Incorporation of Al was con rmed from elemental analysis using EDS. In the optical study, the shift towards higher wavelength of the absorption edge was noticed. The bandgap of ZnO was found to decreases with increase in the Al doping concentration. PL spectrum shows the existence of near band emission and deep level emission. The antibacterial and antifungal results clearly shows that pure and Al doped ZnO has very good antimicrobial effect on microorganisms.

Declarations
Declarations: Not Applicable Con ict of Interest: None