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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.31 No.1 pp.8-15
DOI : https://doi.org/10.3740/MRSK.2021.31.1.8

Anti-Fogging, Photocatalytic and Self-Cleaning Properties of TiO2-Transparent Coating

Shielah Mavengere, Jung-Sik Kim†
Department of Materials Science and Engineering, The University of Seoul, Seoul 02504, Republic of Korea
Corresponding author E-Mail : jskim@uos.ac.kr (J.-S. Kim, The Univ. of Seoul)
December 27, 2020 January 11, 2021 January 11, 2021

Abstract


Transparent, photocatalytic, and self-cleaning TiO2 thin film is developed by TiO2 sol-gel coating on glass and polycarbonate (PC) substrates. Acetyl acetone (AcAc) suppresses the precipitation of TiO2 by forming a yellowish (complex) transparent sol-gel. XPS analysis confirms the presence of Ti2p and O1s in the thin films on glass and PC substrates. The TiO2- sol is prepared by stabilizing titanium (IV) isopropoxide (TTIP) with diethylamine and methyl alcohol. The addition of AcAcsilane coupling solution to the TiO2-sol instantaneously turns to yellowish color owing to the complexing of titanium with AcAc. The AcAc solution substantially improves the photocatalytic property of the TiO2 coating layer in MB solutions. The coated TiO2 film exhibits super hydrophilicity without and with light irradiation. The TiO2 thin film stabilized by adding 8.7 wt% AcAc shows the highest photo-degradation for methylene blue (MB) solution under UV light irradiation. Also, the optimum photocatalytic activity is obtained for the 8.7 wt% AcAc-stabilized TiO2 coating layer calcined at 450 °C. The thin-films on glass exhibit fast self-cleaning from oleic acid contamination within 45 min of UV-light irradiation. The appropriate curing time at 140 °C improves the anti-fogging and thermal stability of the TiO2 film coated on PC substrate. The watermark-free PC substrate is particularly beneficial to combat fogging problems of transparent substrates.



초록


    © Materials Research Society of Korea. All rights reserved.

    This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    1. Introduction

    Antifogging and self-cleaning are classic properties for maintaining optical transparency and cleanliness on glass1) and polycarbonate (PC) substrates.2) The two materials are interchangeably used in construction of buildings or automobiles to create interiors with natural light for conserving energy. However, glass and PC surfaces without any protective coating are susceptible to fogging and dirt accumulation. The loss of transparency compromises safety, aesthetic beauty and rather necessitates frequent cleaning. Several researchers have reported on overcoming fogging and pollutant accumulation by coating with TiO2 sols to form protective thin-films.3-5)

    TiO2 thin-film coating layer on glass or PC imparts hydrophilicity such that in presence of moisture, thin sheets of water forms.6-8) Simultaneously, if the thin film is exposed to UV-light, TiO2 is activated to produce superoxide radicals which destroy organic and inorganic pollutants.9) The products of photocatalytic oxidation reactions are then washed away with rainwater keeping surfaces clean. This self-cleaning property eliminates glass cleaning agents which may induce more pollutants into the environment.

    Titanium dioxide (TiO2) semiconductor is merely known to decompose most volatile organic compounds (VOCs) and self-clean under ultraviolet (UV) light irradiation. Moreover, the low cost, chemical-photo stability and low toxicity characteristics are beneficial for the commercialization of TiO2 in mitigating pollution.1012) Thin film coating on glass or PC is prepared at ambient conditions followed by thermal treatment to crystallize TiO2. However, borosilicate glass is capable to withstand at most 700 °C13-15) unlike PC substrates which soften at 180 °C. Therefore, the substantial challenge in scarcely applying TiO2-sol on PC substrates is precisely the inability to withstand heat higher than 150 °C. PC substrates are interchangeably used for automobile headlamps, safety shields and lenses application instead of glass due to exceptional shutter strength. Therefore, there is need to evaluate the anti-fogging property of TiO2 thin films on PC substrates.

    In recent researches, anti-fogging tests were evaluated by visually examining the formation of fog at room temperature 25 °C after exposing coated PC or glass substrates chilling temperatures between -15 °C and -20 °C for an hour.16) The other simplified test was accomplished by exposing substrates to vapor and monitoring the presence of fog at 25 °C room temperature.17) In practical applications, the glass or PC substrates are exposed to prolonged frosty weather with humidity and temperature cyclic variations. There is need to evaluate both, cyclic anti-fogging and thermal stability of coatings on the glass or PC substrates.

    In this study, anti-fogging transparent, self-cleaning and UV photoactive monolayer TiO2 thin films were prepared by the sol-gel method. The effects of acetyl acetone (AcAc) as an additive and titanium (IV) isopropoxide (TTIP) concentration were investigated. The TiO2-sol was synthesized by sol-gel method at room temperature and uniformly coated on at least one side of glass and PC substrate.

    2. Experimental Procedure

    2.1 Experimental

    The TiO2 coating solution was synthesized by the solgel technique. An acetyl acetone ethoxysilane (AcAcsilane) of coupling solution was prepared by mixing 3- glycidoxypropyl trimethoxysilane (98.0 %), methyltriethoxysilane (98.0 %), and cetyltrimethylammonium bromide (99.0 %) in a distilled H2O in the ratio of 8.5:7.1:4.4:80 wt%, respectively. The pH of the AcAc-silane premix solution was adjusted to be 1.0 by adding an oxalic acid under magnetic stirring for 20 min at 70 °C. For comparison, the concentration of AcAc (99.0 %) was varied from 0 to 8.7 wt%. Then, a TiO2-sol was prepared by stabilizing titanium (IV) isopropoxide (TTIP, 97.0 %) with diethylamine (98.0 %) under magnetic stirring, followed by addition of methyl alcohol (99.5 %) in the ratio of 5:3.6:91.4 wt%, respectively. Finally, the AcAc-silane coupling solution was added to the TiO2-sol; the solution instantaneously turned to be yellowish owing to the complexing of titanium with AcAc.9)

    The glass substrate of 2×2 cm dimension was ultrasonically cleaned with acetone, followed by rinsing with ethanol and distilled water. Subsequently, the clean substrate was dried with O2 compressed air blow gun and flow-coated with the TiO2-sol. The coated samples were first dried in an oven at 100 °C and finally heat-treated in a muffle furnace at 350, 450 and 550 °C for 2 h; the samples were named as T-350, T-450, and T-550, respectively. The thin films for evaluating the effect of TiO2 selfcleaning property were prepared using similar experimental procedures but only TTIP concentration was varied at 0.02, 0.04 and 0.06 M with 8.7 wt% AcAc. The thin films on glass substrate were heat treated at 450 °C for 2 h. The PC substrate for evaluating anti-fogging and thermal stability was prepared as similar method. The PC substrate of 10 × 10 × 0.01 cm dimension was cleaned with ethanol, followed by rinsing with distilled water and drying with O2 compressed air blow gun. The clean PC substrate was coated with TiO2-sol and further cured in an electric oven at 140 °C for 30 min and 1 h.

    2.2 Characterization

    The morphology of the thin films containing 8.7 wt% AcAc stabilizer and 0.6 M TTIP was evaluated by scanning electron microscopy (SEM, Hitachi Field Emission S- 4300). X-ray photoelectron spectrometer (XPS) analyser (Nexsa, ThermoFisher Scientific) was used to evaluate the elemental composition of the thin films on glass and polycarbonate containing 8.7 wt% AcAc stabilizer and 0.6 M TTIP. As prepared thin films on glass and polycarbonate substrates were analyzed by XPS without sputter pre-treatment. The optical transmittance spectrum was recorded using a UV/Vis diffuse reflectance spectrophotometer (UV-1601, Shimadzu). The hydrophilicity of the thin film was evaluated using a contact angle measuring analyzer (GSTD, ST-GTD-1010, SurfaceTech. Co., Ltd).

    2.3 Self-cleaning with methylene blue pollutant

    The photocatalytic property of the TiO2 thin films was examined using methylene blue (MB) 2 ppm solution (95 %, Duksan Pure Chemical Co., Ltd) under UV irradiation. The TiO2-coated glass substrates of 2 × 2 cm dimension were immersed in a beaker with 10 mL MB solution and soaked for 30 min to achieve adsorption equilibrium. Then, two 254 nm-germicidal lamps (Sankyo Denki Co., Ltd) fixed in a protective chamber were switched on, and the solution was constantly stirred for 3 h. The 3 h-UV light irradiated MB solution was examined for absorbance variation using a UV-vis spectrophotometer (UV-3150 Shimadzu).

    2.4 Self-cleaning with oleic acid contaminant

    Self-cleaning property of TiO2-thin films on glass substrates after oleic acid contamination was determined by Surface Tech Contact Angle Measuring Analyzer (GSTD: ST-GTD-1010). Firstly, water contact angle of clean TiO2-thin film was measured before oleic acid contamination. Oleic acid contaminant solution was prepared from diluting 1 μL oleic acid in 10 mL acetone18). TiO2- thin films were contaminated by drop-coating with 0.02 μL of dilute oleic-acetone solution. Then, the glass substrates were dried in the electric oven at 70 °C. Then, the water contact angle after oleic acid contamination was determined and the thin films were exposed to UV-irradiation for TiO2 photo-activation. At every 15- minute interval, the variation of the water contact angle was measured on the Contact Angle Analyzer.

    2.5 Anti-fogging tests

    Anti-fogging tests were performed on bare or TiO2-thin film coated polycarbonate substrate by exposing to 80 °C water vapor of 15 cm height for 60 s. After exposure to vapor, the PC samples were placed on a flat tile to dry at the ambient condition. The respective photographs of the bare and TiO2-thin film coated PC substrates under fogging tests are compared. At that time, the anti-fog examined samples were dried in an electric oven at 100 °C. Photographs of the samples were visually evaluated for cleanliness or existence of watermarks.

    2.6 Thermal stability test

    Four samples per thin film coating condition were evaluated for thermal stability and cleanliness after antifogging tests. Thermal reliability test of the TiO2-coated PC substrate was evaluated by placing the samples in an electric oven at 100 °C for 8 h for 2 cycles. After one 8- h cycle, the hydrophilicity was evaluated by exposing the TiO2-coated PC substrate on the top of a beaker with 50 °C water. Then, the samples were placed in an electric oven again for further 8 h cycle and the fogging property after the 2nd cycle was evaluated. Finally, photographs of the thermally tested samples were evaluated for surface cleanliness.

    3. Results and Discussion

    Fig. 1 shows SEM micrographs for the 8.7 wt% AcAcstabilized TiO2 thin film on a glass substrate heat treated at 450 °C for 2 h. TiO2 nanoparticles were well distributed as nano-clusters along the coating layer. Fig. 2(a) shows the optical transparency of the coated films at various concentrations of AcAc added to the Ti-sol. The TiO2- coated glass sample exhibit enhanced UV-vis absorption than the bare glass substrate, which is evident from the decrease in the transmittance at 200-600 nm. In Fig. 2(b) enlarged from 200 to 500 nm wavelengths, a shift in the absorption peak is observed in the 8.7 wt% AcAc-stabilized TiO2 thin film. This shift in the absorption peak is an indication that the nanoparticle thin films absorb lower energy photons. In detail, the existence of other doping elements such as C due to AcAc additions improves light sensitization as new energy states are imparted on the thin films. The absorption of lower energy levels is equally beneficial for improving photocatalytic performances.

    Fig. 3 shows UV-vis transparency spectra of the TiO2- coated polycarbonate substrates cured at 140 °C. In Fig. 3(a) the three samples show high transparency above 90 % which is an indication that the thin-films were thin and allowing light to pass through. However, on enlarged y-axis on Fig. 3(b) the sample cured at 30 min has lower transparency than one cured for 1 h. Thus, longer curing time for coating enhances the crosslinking of TiO2 coating layer and decreases the scattering of light.

    Fig. 4(a) shows the XPS survey spectra and deconvolution peaks of (b) Ti2p (c) O1s and (d) C1s on polycarbonate (PC) substrate. Table 1 shows the elemental composition for presences of O1s, Ti2p, C1s, Si2p, N1s. Si2p of 4.5 atomic% originate from the silane stabilizer reagents. The PC substrates still contained N- from the amine groups due to low thermal treatment temperatures. However, C content was the main component due to polycarbonate substrate. Ti2p peaks at 464 and 458 eV confirm the Ti4+ state. The deconvoluted O1s peak shows broader OH non-lattice peak than the TiO2 peak. This is owing to partial crystallization of TiO2 at low temperatures like 140 °C. The deconvoluted peaks for C1s in Fig. 4(d) show three distinct maxima at 284.8, 286.3 and 288.5 eV. The highest C peak at 284.8 eV is non-oxygenated while the oxygenated C peaks are localized at 286.3 and 288.5 eV.

    Fig. 5(a) exhibit the XPS survey spectra and deconvolution peaks of (b) Ti2p (c) O1s and (d) C1s on glass substrate. Table 2 shows the elemental composition for the presence of O1s, Ti2p, Na1s C1s, Si2p. The carbon peak maybe due to instrumental impurities, whilst Na and most of Si peaks of 7.36 atomic% emanated from the glass substrate and also from the silane coupling reagents. The dominant Si2p were referenced in other research to the borosilicate substrate.15) Fig. 5(b) shows the deconvoluted Ti2p peaks at 464 and 458 eV for Ti2p3/2 and Ti2p1/2. The TiO2 peak in Fig. 5(c) is defined with higher intensity than the non-lattice OH since 450 °C temperatures completely crystallize TiO2 on the glass substrates, unlike the PC substrates in Fig. 4(c). The concentration control of reagents in a sol-preparation is critical, as in this work the optimized addition of AcAc and TiO2/TTIP precursor were 8.7 wt% and 0.06 M, respectively. At the optimized condition, the Ti-sol solution exhibited precipitate free long term stability as reported in previous studies owing to AcAc cross-linking role.9,19) TiO2 thin films on glass showed Si2p peaks in the XPS survey. Fig. 5(d) exhibit the deconvoluted peaks of C1s where the most prominent peak of non-oxygenated C is maxima at 284.8 eV followed by oxygenated C at 286.9 and 292.8 eV, respectively. The oxygenated C on glass substrates is lower than on PC substrates owing to high heat treatment procedures. Since the C detected on glass substrates is around 18 %, besides instrumental and handling contaminations, other researchers also referenced the C originated from the acetyl acetone chelating complexes. The thin films on glass with significant C impurities were reported to exhibit enhanced photocatalysis due to the low energy levels of C on TiO2 surface.20)

    Fig. 6(a) shows the photodegradation of the MB solutions with the TiO2 thin films on the glass substrate under UV-light irradiation. The photocatalytic reactivity improved with addition of AcAc in the TiO2-sol, from 10 to 74 % with respect to the increase in the concentration of AcAc from 4.5 and 8.7 wt%, respectively. The improved photocatalytic reactivity is attributed to the increase in the degree of Ti-crosslinking with AcAc. As a result the transfer of electrons to the surface of the TiO2 thin films is improved through several sites. Additionally, the high C observed in 8.7 wt% AcAc glass thin films in (Table 2 and Fig. 5) possibly caused C-doping effect which improved TiO2 photocatalytic property. Fig. 6(b) exhibit the effect of calcination temperature on the photocatalytic degradation of 8.7 wt% AcAc-stabilized TiO2 thin films. The T-450 thin films exhibited the highest photocatalytic activity followed by T-350 and T-550. Methylene blue dye pollutant was degraded efficiently by the TiO2 thin film heat treated at 450 °C. Thus, 450 °C is an optimum temperature as confirmed in other previous studies because of anatase TiO2 phase formed.3)

    Fig. 7 shows variation of water contact angles with time under UV-light germicidal lamp. The contact angle of less than 10o in the TiO2-coated glass slides is an indication of the superhydrophilic property of TiO2 thin films. After contaminating the thin films with oleic acid, the contact angle was above 40o for all the samples. However, the 0.06 M TiO2-thin films showed lowest contact angle owing to high TiO2-concentration. The TiO2- thin films exhibited self-cleaning property by degrading oleic acid contaminant within 45-min cycle of UV- irradiation. Therefore, the thin films recovered from olecic acid contamination within 45 min of UV-irradiation which is plausible for photoactive thin films. The TiO2 thin films on glass substrate exhibited self-cleaning property in the vicinity of the oleic acid pollutant by retaining the initial water contact angles after exposure to UV-light irradiation.21) Acetyl acetone as a stabilizer in TiO2-sols improved MB degradation efficiencies with increasing concentration. However, varying the amount of TiO2 (0.02, 0.04 and 0.06 M) in Ti-sol at fixed AcAc wt% of 8.7 wt%, thin films exhibited similar selfcleaning property. Therefore, crystalline TiO2 on thinfilm is a vital for facilitating self-cleaning as long as a UV light irradiates.

    Table 3 shows transparency, antifogging and thermal stability of TiO2-thin films; where symbol (O) good (Δ) fair and (X) bad. The presence of permanent water stains on the on the four samples per thin film coating was evaluated as indicators for bad, fair or good. A good thin-film is the one which when exposed to vapour; it spreads along the surface without leaving water stains. However, when a bad or fair thin film coating is exposed to vapour, droplets of water forms across the surface and on drying permanent marks appear on the substrate. This summary of coating performances elaborates that thermally stable, transparent and reusable thin films are obtained in coating solutions with more than 0.5 wt% TTIP. Whereas low TTIP in sol results in poor crosslinking in the thin-film structure hence fogging and transparency is unavoidable.

    Fig. 8 show antifogging property of (a) bare PC substrate and (b) TiO2-coated PC substrate on the top of a beaker with vapor. The bare substrate has hydrophobicity whereby water droplets accumulate and scatter light. As follows, Fig. 8(a) shows low transparency of the logo below the beaker. However, TiO2 coated PC substrate shows clearly the University of Seoul logo. Thus, TiO2 coating prevents the formation of water droplets instead it forms a thin film of water across the coated layer. Hence, high transparency is observed owing to the hydrophilic nature of TiO2-thin films. Interestingly, the coated glass and polycarbonate substrate exhibited transparency above 90 %. However, in PC substrates longer treatment time improved the transparency due to crosslinking of the thin film. Although the TiO2 thin film heat treated below 200 °C contained a low crystallinity in other reference 22), in our work above 90 % transparency at 140 °C on PC substrates is plausible for coatings. The TiO2 thin film on polycarbonate exhibited thermal resistance and good hydrophilicity or antifogging owing to the Ti-O linking confirmed by XPS analysis. In the vicinity of water vapor, the thin layer of water was formed across the TiO2-coated PC substrate, instead of water droplets. This anti-fogging property is beneficial for keeping clean surfaces.

    4. Conclusions

    Anti-fogging and photo-reactive TiO2 coating films were developed by the sol-gel method. XPS analysis confirmed the presence of Ti and O in glass and polycarbonate substrates. The addition of AcAc substantially improved the photocatalytic property of the TiO2 coating layer in MB solutions under UV light irradiation. The optimum photocatalytic activity was obtained for the 8.7 wt% AcAc-stabilized TiO2 coating layer calcined at 450 °C. The complexing property of AcAc improved the degree of Ti-Ti crosslinking in the TiO2 coating layer, promoting a broader shift in UV light absorption. This increased the generation of electron-hole pairs at the surface of the AcAc-stabilized TiO2 thin films. The TiO2 thin films on glass substrate exhibited self-cleaning property from oleic acid contamination within 45 min of UV-light irradiation. The anti-fogging and thermal stability of the TiO2 coated PC substrate was improved with extended curing time of 1 h at 140 °C. Therefore, the TiO2 monolayer coating with an appropriate addition of AcAc may protect glass or PC surfaces against dirt accumulation, pollutants and fogging problems.

    Acknowledgements

    This work was supported by the 2019 sabbatical year research grant of the University of Seoul.

    Figure

    MRSK-31-1-8_F1.gif

    SEM image of the 8.7 wt% AcAc-stabilized TiO2 thin film on glass substrate calcined at 450 °C.

    MRSK-31-1-8_F2.gif

    UV-Vis (a) transparency (red 0 wt%, blue 4.5 wt% and magneta 8.7 wt% AcAc) and (b) absorption spectra of the (black 0 wt%, red 4.5 wt% and blue 8.7 wt%) AcAc-stabilized TiO2 thin films on glass calcined at 450 °C.

    MRSK-31-1-8_F3.gif

    UV-vis optical transparency of TiO2 coated polycarbonate substrates cured at 140 °C (a) full scale and (b) enlarged at y-axis from 90 to 100%.1

    MRSK-31-1-8_F4.gif

    (a) XPS survey spectra and deconvolution peaks of (b) Ti2p (c) O1s and (d) C1s on polycarbonate (PC) substrate.

    MRSK-31-1-8_F5.gif

    (a) XPS survey spectra and deconvolution peaks of (b) Ti2p (c) O1s and (d) C1s on glass substrate.

    MRSK-31-1-8_F6.gif

    Effect of (a) AcAc concentration and (b) calcination temperature on the photocatalytic degradation of MB solutions with TiO2 thin films on glass substrates.

    MRSK-31-1-8_F7.gif

    Water contact angle variation on TiO2-thin films on glass substrates under UV-light.

    MRSK-31-1-8_F8.gif

    Antifogging property of (a) bare PC substrate and (b) TiO2- coated PC cured at 140 °C.

    Table

    Elemental composition for TiO2-thin film on polycarbonate substrate.

    Elemental composition for TiO2-thin film on glass substrates.

    Transparency, antifogging and thermal stability of TiO2-thin films on polycarbonate substrates where symbol (O) good, (Δ) fair and (X) bad.

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