Characterization and Application of Nanomaterials (2019) Volume 2 Issue 2 doi:10.24294/can.v2i2.627 42  Original Research Article The variations of hydrophilic self-cleaning properties and refractive index dependence in the ZrO2 thin films by Gamma Irradiation D. Abayli, N. Baydogan* Energy Institute, Istanbul Technical University, Ayazaga Campus 34469, Istanbul, TURKEY. E-mail: dogannil@itu.edu. tr ABSTRACT ZrO2 thin film samples were produced by the sol-gel dip coating method. Four different absorbed dose levels (such as ~ 0.4, 0.7, 1.2 and 2.7 Gray-Gy) were applied to ZrO2 thin films. Hence, the absorbed dose of ZrO2 thin film was examined as physical dose quantity representing the mean energy imparted to the thin film per unit mass by gamma radiation. Modification of the grain size was performed sensitively by the application of the absorbed dose to the ZrO2 thin film. Therefore the grain size reached from ~50 nm to 87 nm at the irradiated ZrO2 thin film. The relationship of the grain size, the contact angle, and the refractive index of the irradiated ZrO2 thin film was investigated as being an important technical concern. The irradiation process was performed in a hot cell by using a certified solid gamma ray source with 0.018021 Ci as an alternative technique to minimize the utilization of extra toxicological chemical solution. Antireflection and hydrophilic properties of the irradiated ZrO2 thin film were slightly improved by the modification of the grain size. The details on the optical and structural properties of the ZrO2 thin film were examined to obtain the op- timum high refractive index, self-cleaning and anti-reflective properties. Keywords: Absorbed Dose; Irradiation; Optical Properties; Thin Film; Zirconium Oxide ARTICLE INFO Article history: Received 27 June 2019 Received in revised form 23 July 2019 Accepted 25 July 2019 Available online 7 August 2019 COPYRIGHT Copyright © 2019 D. Abayli et al. doi: 10.24294/can.v2i2.627 EnPress Publisher LLC. This work is li- censed under the Creative Commons Attribu- tion-NonCommercial 4.0 International Li- cense (CC BY-NC 4.0). http://creativecommons.org/licenses/by/4.0/ 1. Introduction Thin films of zirconium oxide (ZrO2) have attracted attention due to their advantageous properties for future thin layers technology. Zir- conium oxide (ZrO2) possesses good dielectric, optical, mechanical and chemical properties. Additionally, it exhibits high refraction index, very good transparency, great chemical stability and direct wide gap, with an optical band gap in the range of .0–5.85 eV[1,2]. Hence, ZrO2 can be used in a wide range of applications such as optical filters, laser mirrors, barrier layers, buffer layers for super-conducting ceramics, as a biomaterial for prostheses, gas sensor, optical storage elements, scin- tillators and luminescent oxygen-sensors etc.[1,3]. Pure or doped ZrO2 thin film continues to present sufficiently high thermoluminescence (TL) yield after irradiation with ionizing or ultraviolet (UV) radia- tion[3]. ZrO2 thin films with different morphologies are prepared by seve- ral methods such as chemical vapor deposition (CVD), spray pyrolysis, reactive RF sputtering, polyol route, hydrothermal method bio-temp- late method, ultra-sound assisted precipitation method, room tempera- ture precipitation method, solution combustion method and sol–gel method[4,5]. Among these techniques, the sol–gel method can be used in the production of the thin films for electronics and optics. The sol-gel 43  method is of particular intere the production of the thin films for electronics and optics. The sol-gel method is of particular interest because of several advantages such as its simplicity, low processing temperature, stoichiometry control and its ability to produce uniform, chemically homogenous films and the formation of multi and mixed layers[1,2]. The sol–gel process involves the preparation of a “sol” (mostly colloidal), the transformation of the sol into a solid “gel” phase and crystallization by heating at increased temperatures (calcination). The first and second stages determine a chemical composition and the calcination stage controls the phase evolu- tion of ZrO2 from amorphous to nanocrystalline and crystalline phase transformation (monoclinic to te- tragonal, etc.)[3]. The sol–gel process also allows the deposition of films and coatings with variable thick- ness from nm to micrometer, in an easy and fast way, by the use of either dip-, spin- or spray-coating procedures[6]. The ZrO2 thin film can be produced with refractive indices n ranging from 1.35 to 2.50 and film thicknesses d from 150 to 1500 nm. The ZrO2 thin film is hard, durable, and laser-damage- resistant thin films with high refractive index and it is widely used to produce multilayered coatings, also for the UV spectral range[7] Generation of energy by using clean and envi- ronmental-friendly technologies is one of the major aims in industrial and scientific areas. Among the various coating techniques, the coating performed with sol–gel method is one of today’s environment- protective methods. The sol-gel coating of ZrO2 att- racts attention since it is an environmental-friendly coating method[8]. As the ZrOCl2ꞏ8H2O is low toxi- city of zirconium salts low toxicity of ZrOCl2ꞏ8H2O makes the use of ZrOCl2ꞏ8H2O attractive at the pro- duction of solution in this study. Hence it was pos- sible to avoid the use of high level chemical toxici- ty by minimizing the production of hazardous che- mical solution waste. Besides, more economical and environmentally friendly technologies are research- ed to adopt more eco-friendly thin film production methods in industrial and scientific areas. The utili- zation of irradiation treatment at several production steps can be preferred as a practical and rapid pro- cess to obtain the efficient performance in the de- vices containing thin film[9]. In this study, the ion- izing radiation effect on structural and optical prop- erties was used in the ZrO2 thin film. Therefore, the improvements in the structural and optical proper- ties were obtained, avoiding the production of extra hazardous chemical solution waste. The irradiation treatment by using the gamma radiation was a key parameter to manage the optical constants and the grain sizes of the ZrO2 thin film to use them as pro- tective and biomimetic layers. The agglomeration tendency of the irradiated grains at 0.7 Gy has re- sulted with high refractive index and more hydro- philic properties. There was a relation between the refractive index and hydrophilic properties with the rise of the grain size of the irradiated ZrO2 thin film. 2. Experimental part The sol-gel dip coating method at the produc- tion of the ZrO2 thin film seems a candidate as more ecolabel coating method than other ZrO2 thin film coating methods in the advance future economic coating technology for the industry[8]. ZrOCl2ꞏ8H2O is an attractive material in sev- eral procedures due to its low toxicity[10]. ZrO- Cl2ꞏ8H2O can be used for the environmentally frien- dly methods[11]. Hence, zirconium oxychloride octahydrate (ZrOCl2ꞏ8H2O) inorganic precursor solution (Sigma Aldrich puriss grade, ≥99.0%) was used for the production of ZrO2 thin films. All the substrates were cleaned with ethyl alcohol and rinsed with deionized water and they were sterilized with water vapour in an auto clave furnace (Nuve OT 012 Ben- ch Top Steam Sterilizer) then dried at 100°C in dry- ing oven (Binder ED 53). The solvent was deter- mined as a mixture of 2-butanol and ethanol (in the ratio 1:1). Homogeneous solution including zirco- nium oxychloride octahydrate (2 wt.%) was pro- duced by mixing 1 mol of zirconium oxychloride octahydrate in 1/3 of the total volume of mixed 2-butanol and ethanol. The solution was stirred for 45 minutes by use of a magnetic stirrer (Heidolph MR 3001K). Water for hydrolysis and nitric acid for oxidation (water: HNO3: acetylacetone = 20: 0.4: 3) were added to the salt–alcohol solution. The stirring was continued for another 90 minutes to obtain 44  clear and transparent solution. The precursor solu- tion prepared at 60°C deposited on cleaned soda- lime silicate glass substrates. Dip coater (KSV dip coater LMX2) with computer controlled was used and the dip coating parameters were chosen as 10 cm/min lifting speed and 90° vertical lifting. The dip coated films were dried at room conditions and pre-fired at 150°C. This process of coating and dry- ing was repeated for 9 times in a heater. ZrO2 film samples were annealed at 500°C for 1 h in air simi- lar with the literature[12]. After the coating process, the remaining solution was stored in a dark glass b- ottle to prevent the degeneration of solution and used within 60 days at the refrigerator. A certified Co-60 radioisotope was used as a gamma ray source emitting the photons with two different energies (1.17 MeV and 1.33 MeV) to examine the rise of the absorbed dose effect on the optical properties. Hence, the provided gamma ray beam was assumed as a monochromatic beam at ~ 1.25 MeV. The used Co-60 radioisotope had an activity level of 0.018021 Ci and it was determined as an appropriate irradiation source to evaluate the changes in the structural and optical properties of the ZrO2 film samples. The absorbed dose level of the ZrO2 thin film was the important parameter as the cumulative dose for the thin film samples at the irradiation area. The properties of the used Co-60 radioisotope were presented at the irradiation pro- cess of ZrO2 thin film samples in Table 1. Table 1. The properties of the Co-60 radioisotope used in the irradiation of ZrO2 thin film Radioisotope K (R m2/Ci h) A (Ci) T1/2 (y) E (MeV) Co-60 1.32 0.018021 5.27 1.17 and 1.33 In this study, the ZrO2 thin film samples were placed around the gamma irradiation source pano- ramically (in Figure 1). The cumulative dose level of the sample was considered as the total dose re- sulting from the repeated exposures of the ionising radiation. Because radiation fields can varies with several irradiation conditions (geometry or time) for a period of time[9]. Hence, four different absorbed dose levels were obtained such as ~ 0.4, 0.7, 1.2 and 2.7 Gy to examine the details of the changes in the optical and structural properties. All of the irra- diation tests were conducted at room temperature. Figure 1. Irradiation settlement of the ZrO2 thin film samples. The thicknesses of the ZrO2 thin film samples were determined by using Veeco Dektak 6M Stylus sur- face profilometer after the optical properties of ZrO2 thin films prepared by sol-gel method were determined employing transmittance and reflec- tance spectrophotometry in UV and VIS range be- tween 190–1100 nm by using PG Instruments T80 UV-VIS spectrophotometer. Thickness measure- ment was obtained from the coated to the uncoated part of the glass. Therefore, the thickness of the thin film was determined as ~ 80 nm[13]. The use of the ionizing radiation was a key parameter to limit the chemical solution toxicity without the generation of the extra hazardous chemical solutions at the envi- ronment. 3. Results and discussion The surface morphology of the film was ex- amined using 2D Scanning Electron Microscope (SEM). There were voids between the grains in some areas of surface as depicted in Figure 2(a)-(d). The surface of the irradiated ZrO2 thin film at 0.4 Gy was composed of the equaxial grains (~50 nm) in Figure 2(a). The dimension of grains in which induced by gamma irradiation at 0.7 Gy had ~ 87.3 nm in Figure 2(b) and their dimension was the highest one with respect to the dimension of grains in other ones. The dimension of the grains was ~ 74.5 nm and decrease on the surface of the film at Irradiation Source 45  1.2 Gy in Figure 2(c) and the dimension of grains is ~ 64.8 nm of the film at 2.7 Gy in Figure 2(d). While the grains separated to more small grains due to the absorbed dose of the film, they started to gather and piled up at several places on the surface of the films at 1.2 Gy and 2.7 Gy. The changes of beta transmission of the ZrO2 thin films support- ed the variation of the grain size in our previous study[13]. The properties of the surface started to change obviously at 0.7 Gy. The absorbed dose of 0.7 Gy was important to investigate the optical constants of the thin film. It was interesting to note that when the absorbed dose of the specimen reached to 1.2 Gy, the dimension of the grains decreased again. Moreover, the gathering of the grains in the induced film took place more obviously at the certain places. on the surface of the films in Figure 2(c-d). The gathering of the grains at the surface of the film in- creased when the absorbed dose increased. But it was determined that there was the combination of grains with each other on the surfaces of irradiated specimens. The change of the valance state of im- purity atoms in the irradiated thin film can be ex- plained with the formation of new electronic order in defect centers as the result of the new configura- tion of the ions while the gathering of small grains with the rise of the absorbed dose in the thin film structure. (a) (b) (c) (d) Figure 2. SEM images of the ZrO2 thin film irradiated at (a) 0.4 (b) 0.7 (c) 1.2 2.7 Gy. The changes in transmittance (in Figure 3) and reflectance (Figure 4) of the irradiated ZrO2 thin film samples were examined with the rise of the absorbed dose. The colour of the irradiated ZrO2 thin film samples changed to darker colour tones with rise of the absorbed dose. The transmittance of the irradiated ZrO2 thin films decreased with the rise of the grain size. Besides, the transmittance shi- fted towards the higher wavelength (red shift) in UV range as the absorbed dose increased. This be- haviour is compatible with the decrease in trans- parency of the irradiated ZrO2 thin films depending on the improvement of the optical absorbance. The red shift of the irradiated ZrO2 thin film at 0.7 Gy (in Figure 3) distinguished clearly with the im- provement of the optical absorbance (in Figure 5) as the irradiated thin film (at 0.7 Gy ) reached maxi- mum grain size from ~50 nm to 87 nm. However, there is not considerable information on the details of the changes in optical constants of the irradiated ZrO2 thin film by gamma radiation in previous studies[14]. The rise of the grain size resulted with the decrease of the optical transmittance of the irra- diated thin film (at 0.7 Gy). The rise of the grain size resulted with the decrease of the reflectance of the ZrO2 thin film. The minimum reflectance of the thin film irradiated at 0.7 Gy was determined at the film with maximum grain size reached from ~ 50 nm to ~ 87 nm. The reason of this was the decrease of the grain boundaries around the course grains. The changes in refractive index of the ZrO2 thin film samples irradiated at different absorbed dose levels were determined from the transmittance ac- cording to Swanepoel’s envelope method in this study. The details on the determination of the re- fractive index were presented in the previous stud- ies[15]. The interference fringes shifted towards the higher wavelength (red shift) region. Swanepoel’s envelope method stated the minimum and maxi- mum envelops of transmittance spectrum in the weak and medium absorption regions; extrapolating the graph of the refractive index at the strong ab- sorption region in the literature[16]. The optical con- stants of ZrO2 thin films were affected by the gam- ma irradiation. It was possible to determine the ch- anges in optical constants such as the refractive in- 46  dex (n) (in Figure 6), extinction coefficient (k) (in Figure 7) and absorption coefficient () (in Figure 8) with the rise of the grain size in this study. There were the increases in n, k and  values of the irradi- ated ZrO2 thin film as the result of the increase in grain size (in Figure 2). The rises of n, k and  were related with the enhancement of optical ab- sorption as the result of the controlling of the ab- sorbed dose. The increase of the optical constants of the irradiated films was related with the gathering of the grains at the surface of the film with the con- trolling of the absorbed dose according to the SEM images (in Figure 2). Photoluminescence (PL) stu- dies provide information about the electronic band transitions, structure, defects and chemical compo- sition of the optical materials[2]. The exciton lumen- escence and intrinsic defects related luminescence are known in a number of oxides. For ZrO2, the ph- otoluminescence is of much interest for both theo- retical and experimental investigations. There are some references to make an evaluation between the results of this study and results in literature by using different irradiation sources. A broad PL emission is observed at ~ 480 nm for ZrO2 structures after the samples are induced by UV irradiation in the litera- ture[2]. The refractive index increases with the re- duction of the transmittance of the films as the ab- sorption region of the transmitance spectra rises to- wards higher wavelenght region, in the previous study of Berlin et al.[5]. In this study, it was deter- mined that there were the dramatic changes in opti- cal constants at ~ 480 nm. The photoluminescence effect at ~ 480 nm supported the rise of the optical transmittance and decrease of the optical absorb- ance slightly depending on the increase of the scat- tered photons. Besides, the decrease of the optical absorbance has led the decrease of the optical con- stants over ~ 480 nm. It was thought that the im- provement of the grain sizes in irradiated ZrO2 thin film at 0.7 Gy indicated the enhancement of the ph- otoluminescence effect dominantly at the coarse gr- ains with 87 nm. (a) (b) Figure 3. (a) The changes in transmittance of the irradiated ZrO2 thin film; (b) the details on the changes in the transmittance. (a) (b) Figure 4. (a) The changes in reflectance of the irradiated ZrO2 thin film; (b) the details on the changes in the reflectance. 47  Figure 5. The changes in optical absorbance of the ZrO2 thin film. Figure 6. The changes in the refractive index, n of the irradiat- ed ZrO2 thin films. Figure 7. The changes in extinction coefficient, k of the irradi- ated ZrO2 thin film. Figure 8. The changes in absorption coefficient, of the irradi- ated ZrO2 thin film. Figure 9. The relation between static water contact angle and refraction index of the ZrO2 films. The surface contact angle of the ZrO2 thin film was determined by using a contact angle measure- ment instrument. The contact angle values of the thin film decreased slightly and a minimum value of around 24.62° was obtained with the increase of the grain size when the absorbed dose was set to 0.7 Gy. The rise of the grain size led to increase the surface roughness and the decrease of contact angle values of the thin film resulted with the improvement of the hydrophilic properties. Hence, the contact angle was decreased by irradiation and wettability of hy- drophilic nanostructured ZrO2 surface was modified slightly with increasing surface roughness. The na- noscale roughness having coarse grains (~87 nm) has influenced the surface wettability slightly at 0.7 Gy. The ZrO2 thin film derived on soda-lime glasses has presented a relation between the refractive in- dex and hydrophilic properties as the result of the change in the size of the grains. 48  4. Conclusions The ZrO2 thin film was derived from ZrO- Cl2ꞏ8H2O (which is low toxicity of zirconium salt- s) by using sol-gel dip coating method as it is a green friendly alternative coating method. The irra- diation treatment has played a role in the enhance- ment of the optical constants (such as refractive in- dex, the extinction coefficient and absorption coef- ficient) of the ZrO2 thin film. The improvement in the irradiated grain size supported to rise the refrac- tive index by minimizing the production of ha- zardous chemical solutions. The rise of the grain size of the irradiated ZrO2 thin film improved the optical constants. 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