Effect of Capping Agent on Structure, Composition, and Optical Properties of Low-Cost Chemically Deposited Zinc Oxide Thin Films and Their Antibacterial Activities

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Introduction
Transparent conducting oxides found extensive application in liquid crystal displays, transducers, and solar cells.Te efciency of the devices strongly depends upon materials, important key properties, high electrical conductivity with its transparency [1][2][3].Te materials such as cadmium oxide, indium oxide, and tin oxide plays a vital role in view of the fact that high transmission above wide spectral range have interesting characteristics such as moderately low cost and stability in the environment of chemistry [4][5][6].Zinc oxide (ZnO) found to exist in wurtzite structure with band gap, binding energy values 3.3 eV, 60 meV attracted researchers to fabricate efcient optoelectronic devices [7,8].ZnO is predicted as an interesting candidate for devices such as gas sensors, solar cells, and photodiodes, and varistors and its diferent structure with physical, chemical, optical, and biological properties has attracted ZnO for medical and biological applications.ZnO thin flms are mostly crystallized in wurtzite and cubic structures.Besides, it is a transparent material with diferent structures that could be synthesized in diferent forms of nanostructure [7,8].Several physical and chemical methods were employed for the preparation of ZnO thin flms with cubic and hexagonal structures [9][10][11][12][13][14]. Te method of SILAR is simply a two-step process for flm preparation in which the substrate is immersed in a precursor that consists of sources of cation with anion.Te growth of the flm deposition takes place consequently when it is dipped in precursors number of times [15].Te continuous reaction at optimized precursor concentration with solution pH values results on the formation of flms on the substrate.Te ZnO thin flm was prepared by dipping the substrate in zinc acetate complex followed by dipping in a hot water bath at the boiling temperature of 95 °C.Number of zinc complexes viz., sodium zincate, ammonium zincate prepared from zinc acetate, zinc sulphate, and zinc chloride were used as precursors for cation in flm deposition by the SILAR method.Te present work focused on the formation of ZnO thin flms from ammonium zincate obtained from zinc chloride.Te advantages and highlight of the technique is its simplicity of the equipment, ease to control the deposition rate with precursor concentration, and solution of pH values.SILAR is a simple wet chemical route for thin flm synthesis in which the formation of flm as a building block by ions instead of atoms, where the preparative parameters are controlled easily [16][17][18][19].ZnO doped with individual elements such as Ag, Al, Bi, Cd, Co, Eu, and Mn which has been reported earlier [11,12,[20][21][22][23][24].By the comparison of a number of works for ZnO thin flms with metallic dopants, few reports highlighted the growth and characterization of ZnO thin flms mediated with bio-template such as silk [25], albumen [26], orange juice [26], and pea starch [27].
In the present report, ZnO and albumen-mediated (capped) ZnO thin flms were deposited on a glass substrate made from zinc chloride, sodium hydroxide (NaOH), and liquid ammonia (NH 4 OH) in addition with albumen as the capping agent by the simple SILAR method.Te infuence of the capping agent (albumen) on structure, composition, morphology, optical properties, FTIR, and photoluminescence measurement of the deposited flms were reported.Te efect of antibacterial activity against diferent bacteria sources was reported.

Experimental Details
Te chemicals ZnCl 2 , NaOH, liquid ammonia were purchased from Sigma Aldrich with 99.99% purity.Te commercially purchased microscopic slides were cleaned using chromic acid followed by distilled water and ultrasonic cleaning with acetone and alcohol.Te commercially purchased egg white layer (albumen) was used as a capping agent.Te substrate was dipped in sodium zincite bath which was kept at temperature around 35 °C for a wellknown standardized time followed by the immersion process in boiled water for the same time of hydrogenation in which the water is maintained at temperature in the range between 90 and 95 °C.Tin flms of ZnO grown by the process of two-step chemical bath deposition method from a solution consist of 0.3 M ZnCl 2 and 0.3 M NaOH with a pH value maintained at 9.0 ± 0.1 and bath temperature 90 °C under optimized condition.In the frst dipping process, the ions in the solution get adsorbed over the substrate produced sodium zincite.Also, by following the same procedure with few number cycles by the incorporation of albumen as the capping agent with 0.3 M ZnCl 2 and 0.3 M NaOH.Te prepared flms were subjected to the process of annealing by keeping them in a mufe furnace at 300 °C for 1 hour, thereafter taking them out to determine structure, morphology, composition, and optical properties.Figure 1 shows the experimental setup used for the preparation of pure and albumen-capped ZnO thin flms.Structural feature of the prepared flms was determined by an X-ray difractometer (XPERTPRO PANalytical) with CuKα target radiation (λ �1.5406 Å).Microscopic view with flm composition was analyzed using a JEOL JSM 840 scanning electron microscope.Te optical absorbance and transmittance of the deposited flms was determined using a SHIMADZU 2000 ultra-violet-visible spectrophotometer.Photoluminescence measurement for the prepared samples was analyzed by PERKIN ELMER LS55 spectrofuorometer.Also, the appearance of bonds in the deposited flms was determined using a SHIMADZU 8400S FTIR spectrometer.Te efect of antibacterial activity for Gram (+) and Gram (−) sources was determined by disc difusion method.

Results and Discussion
3.1.Film Tickness.Te growth rate, deposition of ZnO, and albumen-capped ZnO on glass substrates were controlled by separate variables thickness and surface morphology [28][29][30].Te zinc chloride complex was prepared by dissolving 3.344 g ZnCl 2 in 250 cc demineralized water to attain a chemical solution of 0.3 M ZnCl 2 .Te solubility of zinc chloride is difcult leads to produce precipitation of the solution which is dissolved by the incorporation of few drops of acetic acid produces a transparent solution.Te transparent solution has the solution pH value of 4.5 ± 0.1 as estimated by digital pH meter (HANNA instruments).Te concentration of ZnCl 2 was found to be higher than 0.3 M in addition with 0.075 M NaOH, and particulate absorption takes place on the substrate surface that leads to produce flms with poor quality.Te molarity of ZnCl 2 was found to be 0.3 M ZnCl 2 and higher amount of 0.075 NaOH was added which leads to increase the pH value upto 9.0 ± 0.1, producing flms with higher thickness and well-defned crystallinity.Te higher value of NaOH with 0.3 M ZnCl 2 increases the pH value above 9.0 ± 0.1, and there is detachment of flm from the substrate not from the layers on the substrate.Te dependency of deposition time with flms thickness and number of dipping for the deposited flms on substrate is shown in Figure 2. Tickness of the deposited flms was estimated by the measurement of weight diference for flms deposited before and after certain intervals of time deposition on the substrate and the flm thickness was determined using the following equation: It is noted that the flm thickness increases linearly with rate of 5 more dippings for both ZnO and albumen-capped ZnO.Te observation refects a small variability in the deposition process arising probably from small experimental scattered in deposition parameters bath concentration, temperature as well as due to the nonuniformity of flm from 2 Bioinorganic Chemistry and Applications the substrate handling procedure as the deposition is carried out manually.Te process has been carried out for ZnO and albumen-capped ZnO thin flms.

Structural Properties.
XRD pattern was obtained to identify the nature, structure, and phase of the deposited ZnO and albumen-capped ZnO thin flms (Figure 3(a)).Several researchers reported that the ZnO thin flms have cubic and hexagonal structures.with its high polarity [31].Te observed peak intensity (002) along c axis is found to be a flm with higher thickness value which has been measured using weight diference method.Te intensity of ( 002) and (101) plane is found to be higher for flms have higher thickness around 1000 nm for ZnO, 960 for albumen-capped ZnO.As the time increases, the thickness of the deposited flms tends to increase and thereafter decreases for higher deposition time, due to the overgrowth of the flm [31].Te value of average crystallite size with grain sizes was obtained from X-ray line broadening analysis using the Scherrer equation assuming Gaussian-shaped peaks [15].It is evident that the capping of albumen with ZnO produces variation of structural properties which are peaks of FWHM data, the peak position of difraction angle 2θ, the crystallite size, lattice constant (c), the dislocation density (δ), and strain (ε) were determined.It is seen from the Table 1 that the lattice constant is found to increase by the addition of lesser amount of the capping agent.Te crystallite size of the deposited flms is calculated using the following equation: [32]: Te value of force acting on the deposited flms to restrict the formation of crystallites on the substrate is calculated using equation (3).Dislocation density and bond length for the ZnO and albumen-capped ZnO thin flms were determined using the following equations [15,29,31,33]: Te value lattice constants ("a" & "c"), unit cell volume, c/a ratio, atomic packing factor, and bond length were determined according to the following equations [28]: Te stress value present in ZnO and albumen-capped ZnO has been determined using the following equation: Te estimated value of lattice parameters for ZnO and albumen-capped ZnO thin flms is given in Table 1.

Morphology and Film
Composition.Morphological feature along with flm composition for ZnO and albumencapped ZnO was analyzed by EDX with SEM.Te SEM image of ZnO and albumen-capped ZnO deposited on glass substrate is shown in Figure 4.It is found that albumen added as a capping agent there is no change in surface morphology for deposited flms.A number of crystallites were grouped together to form a grain.Te sizes of the grains were found to be between 0.7 and 0.9 μm.Te average sizes of the grains were found to be 760 nm.Similar observation for albumen-mediated ZnO thin flms reported earlier [34].EDX spectra recorded for ZnO and albumen-capped ZnO thin flms are denoted in Figures 5(a   Bioinorganic Chemistry and Applications albumen-capped ZnO flms was estimated using a SHI-MADZU spectrophotometer.Te band gap value of the deposited flms was determined using the following equation: Te absorbance spectra recorded for ZnO and albumen capped ZnO thin flms are shown in Figure 6.It is noted that the deposited flms had less absorption in the visible region.Te behavior of absorption spectra which may be dependent upon flm thickness deposited and also, there is a correlation between transmittance and absorbance of the deposited flms.Te band gap value of the deposited flms was obtained by the extrapolation of linear portion of the energy axis at zero absorption coefcient (Figure 7).Te band gap value was found to be 3.28 and 3.61 eV for ZnO and albumencapped ZnO, respectively [34,35].Tere is a shift in the band gap value which can be due to the thickness value of the deposited flms.Similar behavior was exhibited by albumencapped ZnO flms that were reported earlier [35].
3.5.FTIR Spectroscopy.FTIR spectroscopy is an important analytical tool to categorize the chemical bonds, composition, and element present in the deposited ZnO and albumen-capped ZnO thin flms.Te spectroscopic observation was carried out in between the wave number 400 and 4000 cm −1 .Figure 8 shows the FTIR spectra recorded for ZnO and albumen-capped ZnO thin flms deposited on glass substrate.Te peak appearance at 848 cm −1 responds to stretching mode of Zn-O bond present in the deposited flms.Te peak appeared at 1450 cm −1 which is responsible for asymmetric, symmetric stretching modes of C�O bonds in addition with associated with present in the deposited flms after certain intervals of time as indicated in Figure 7(a) [36].Te FTIR spectra recorded for albumen-capped ZnO thin flms deposited on glass substrate is shown in Figure 7(b).Te observed peak intensity at 1585, 1018 cm −1 which may be attributed to C�C, Zn-O bands indicates capping efect of albumen with ZnO.Te transformation of OH − organic phase takes place due to the abovementioned process.Te appearance of broad absorption peak at 3400 cm −1 is probably due to superimposition of N-H stretching of amide group at 3450 cm −1 by the biotemplate and O-H stretching mode of water molecules.Besides, the appearance of some of the absorption bands in the range between 1316 and 1585 cm −1 may be due to C�O and C�C stretching modes of albumen as well as the new peak observed at 2900 cm −1 due to symmetric and asymmetric C-H bonds, respectively.Te observed results are in close agreement with the results reported earlier [37].Te observed bands at 1557 and 3452 cm −1 may be due to stretching frequency of OH group absorbed at ambient atmosphere [38].
3.6.Photoluminescence.Photoluminescence measurements were taken out to identify the observation of peak shift emission for flms of ZnO and albumen capped ZnO.Te photoluminescence spectra of for ZnO and albumencapped ZnO thin flms obtained by the SILAR technique are shown in Figure 8.It is noted that there is strong that peak emission at 440 nm corresponding to blue emission has been observed due to the process of surface defect in ZnO mainly due to Zn vacancy and weak green emission band approximately at 500 nm known as a deep level Bioinorganic Chemistry and Applications emission related to the states of defect in the deep level [16].
Te oxygen vacancy present in single ionized level which is responsible for this green emission in the ZnO [17].It may result from the process of recombination of photogenerated hole with an electron occupying the oxygen vacancies with interstitials of zinc.Te albumen capped ZnO thin flms denoted peak emission at wavelength 380 nm, and they also have a weak blue emission peak located at 420 nm (Figure 8(b)).Te ultraviolet emission is known as a near-band emission, originating from the recombination of free exciton through an exciton-exciton collision process [18].In all the samples, the emission of ultraviolet was found to be most prominent.But the albumen capped with ZnO thin flms sample a dramatic change in emission spectrum was observed.Te observation of ultraviolet emission peaks at 440-500 nm (Z 3 g and Z 6 g) (with the blue shift of single emission peak), which is considered as a signature of charge transfer reaction [19].2. We have concluded that albumen capped ZnO found to exhibit higher value antibacterial activity for E. coli and S. auereus bacterial than ZnO (see Figure 11).

Conclusions
Te aim of this research report was to explain efect of albumen as capping agent with ZnO thin flms prepared by the simple SILAR method.Structural properties reported that the prepared flms exhibited hexagonal structure with orientation along (101) and (002) for ZnO and albumen capped ZnO.Microscopic observation reported that the flms have grain such as structure with low density.EDX analysis reported that the replacement of capping agent in addition with the presence of elements as stoichiometry (Zn:O) in the deposited flms.Te shift in energy gap was determined as 3.28, 3.61 eV for the deposited ZnO and albumen-capped ZnO with diferent flm thickness values.FTIR results indicated that the appearance stretching mode at 848 cm −1 , 1018, cm −1 corresponding to Zn-O, whereas the band in present at 1585 cm −1 corresponding C�C response to efect of capping agent albumen with ZnO.Te appearance of band absorbance in between 1316 and 1585 cm −1 corresponding stretching modes of albumen also the new observed peak at 2900 cm −1 corresponding to symmetric, asymmetric modes of C-H, respectively.Te broad absorption peak at 3400 and 1325 cm −1 is probably due to superimposition of N-H stretching of amide group with water molecules.Photoluminescence measurements reported that the peaks of emission of ultraviolet blue are at 375 and 340 nm, respectively.Te flms of ZnO capping with albumen were found to possess higher value of activity than ZnO corresponding to change in surface morphology with increase in surface of the deposited flms.Due to the reason of nontoxicity and intense antibacterial activity with some other noble properties of ZnO, capping ZnO with albumen found to exhibit antibacterial activity efect against bacteria sources in nanotechnology.[40,41].

Figure 3 (
a) revealed that the prepared flms possess polycrystalline nature with a wurtzite structure.Tere are no additional secondary peaks in the XRD pattern, which denoted the flms were devoid of impurities.Te position of XRD pattern confrmed that the observation of difraction angles at 36.32, 47.56, 56.52, 62.23, 66.42, 68.06, and 69.14 corresponds to the planes (100), (002), (101), (102), (110), (103), (200), (112), and (201) with prominent refection along (101) for the flms of pure ZnO with hexagonal crystal symmetry.Te observed results were found to be in close agreement with standard fle for hexagonal ZnO[30].Te peak intensity of (101) plane is found to be higher when compared with intensities of the all other planes, respectively.Te difraction pattern of albumencapped ZnO thin flms deposited on glass substrate is shown in Figure3(b).It is evidenced that there is a slight shift in 2θ value when compared with results of pure ZnO.Te intensity of (002) plane is maximized, which denoted the ZnO thin flms with orientation along c axis when it is deposited additionally with albumen as the capping agent with pure ZnO.Te observed characteristic results reported that any peaks corresponding to secondary elements as capping agent not present in EDX pattern as shown in Figure3(b).Tere is observation of slight shift in 2θ value of ZnO thin flms in addition with albumen as capping agent which produces flms with slight diference in 2θ values as shown in Figure 3(b).Te growth of ZnO thin flms along c axis orientation is greatly favoured by the polar structure of ZnO, and it consists of Zn 2+ and anion O 2−

Figure 1 :Figure 2 :
Figure 1: Experimental setup used for the preparation of ZnO and albumen-capped ZnO thin flms.
) and 5(b).Te observation of emission lines Zn and O present in the investigated energy range indicated the formation of ZnO thin flms as EDX spectra as in Figure 4(a).Similarly, the EDX spectra of albumen-capped ZnO thin flms represent the Zn and O emission lines in the investigated energy range, but there are no impurities excluding albumen that can be acted as a capping agent in the deposited flms.Te capping agent added produces a slight change in stoichiometric ratio of Zn and O of the deposited flms on the substrate (Figure 5(b)).

Figure 10 :
Figure 10: Antibacterial activity analysis results on E coli bacteria for pure ZnO, albumen-mediated ZnO thin flms at time period of incubation at 24 hour.

Figure 11 :
Figure 11: Zone of inhibition on bacterial species for ZnO and albumen-capped ZnO thin flms.

Table 2 :
Zone of inhibition of diferent bacteria sources for ZnO and albumen-capped ZnO thin flms at 24 and 48 hours.