1. Introduction

2. Experimental Procedure

3. Results and Discussions

3.1. Structural Properties

Figure 2 shown the X-ray diffraction spectra of ZnO, SnO2, and TiO2 films obtained by dip coating. The spectra of SnO₂ and ZnO show the presence of definite diffraction peaks, indicating the polycrystalline character of the films, as shown in Figure 2a and Figure 2b, respectively. On the other hand, the TiO₂ samples manufactured up to 450 °C do not exhibit any diffraction peaks. indicating the amorphous character of typical anatase phase. It is a matter of fact that some oxides change from amorphous to crystalline character beyond a critical temperature. This also applies to titanium dioxide, where annealing temperature up to 500 °C is the optimum condition for manufacturing quality catalyst for decoloration MB process. However, in the Photocatalisys Section it will be noted that no guarantee of better performance is assured in this annealing range. The X-ray spectrum of TiO₂ is shown in Figure 2c. In all X-ray diffraction spectra, a broad hump at 22.5 ° appears, indicating that all deposited films are thin in thickness, revealing the amorphous structure of the glass [23,24,25]. Only a wide signal appears showing the anatase phase. The diffraction peaks observed in SnO₂ and ZnO samples are consistent with those listed in the Joint Committee on Powder Diffraction Standards (JCPDS) card no. 41-1445 for rutile tetragonal type SnO2 structure [26] and 36–1451 for wurtzite type ZnO structure [27]. ZnO films present a preferential peak in the crystallographic direction (002), Figure 2 in comparison to the preferential peak in the plane (110) for SnO₂ films, Figure 3. Relatively ZnO films have a higher crystalline quality resulting in higher reflection of (002) planes. A much-detailed analysis about structural properties for SnO₂ and ZnO films as follows.

Figure 2. a. X-ray diffraction spectrum of SnO2 films with different thickness and subjected to a heat treatment at 450 °C for 1 h.

Figure 2. a. X-ray diffraction spectrum of SnO2 films with different thickness and subjected to a heat treatment at 450 °C for 1 h.

Figure 3. b. X-ray diffraction spectrum of ZnO films with different thickness and subjected to a heat treatment at 450 °C for 1 h.

Figure 3. b. X-ray diffraction spectrum of ZnO films with different thickness and subjected to a heat treatment at 450 °C for 1 h.

Figure 4. c. X-ray diffraction spectrum of TiO₂ films with different thickness and subjected to a heat treatment at 450 °C for 1 h.

Figure 4. c. X-ray diffraction spectrum of TiO₂ films with different thickness and subjected to a heat treatment at 450 °C for 1 h.

The lattice parameters were obtained using the following way. From Bragg’s law we obtain the interplanar distance d, equation 1.

$$2d\sin \theta =n\lambda$$

(1)

where λ is the wavelength of the X-rays (CuKα λ = 1.5418 Å) and n is an integer representing the diffraction order, in our case 1. The crystal size, D, is calculated using the Scherrer equation, equation 2 [28].

$$D={\displaystyle \frac{0.9 \lambda }{\beta cos\theta }}$$

(2)

where θ is the diffraction peak angle, β is the peak width at mean height (FWHM). The lattice constants c and a were obtained for the case of the SnO₂ film with tetragonal rutile phase, a = b, and (110) and (211) planes, according to the equations (3) and (4).

$${\mathrm{d}}_{\mathrm{hkl}}={\displaystyle \frac{\mathsf{\lambda }}{2{\sin \mathsf{\theta }}_{\mathrm{hkl}}}}$$

(3)

$${\displaystyle \frac{1}{d_{hkl}}}=\sqrt{{\displaystyle \frac{k^2+k^2}{a^2}}+{\displaystyle \frac{l^2}{c^2}}}$$

(4)

and in the case of the ZnO, hexagonal wurtzite phase, (002) and (101) planes were used, according to the Equations (5).

$${\displaystyle \frac{1}{d_{hkl}}}=\sqrt{{\displaystyle \frac{4\left(h^2+hk+k^2\right)}{3a^2}}+{\displaystyle \frac{l^2}{c^2}}}$$

(5)

The lattice constants a and c of bulk SnO₂ and ZnO are reported as a = 4.737 Å and c = 3.186 Å, and a = 3.25 Å and c = 5.20 Å, respectively [29]. Table 2 presents the values of the lattice constants c and a for SnO₂ and ZnO samples at different film thicknesses. TiO₂ films are amorphous, therefore their lattice constants could not be obtained. From the results obtained, it can be observed that as the film thickness increases (or as the number of cycles increases), the lattice constants of the films deviate from their bulk values, the c lattice constant decreases (the height of the basic cell decreases) and the a lattice constant increases (the basic cell widens).

The dislocation density, δ, was calculated using the inverse square of the crystal size, D, equation 6 [30].

$$\delta ={\displaystyle \frac{1}{D^2}}$$

(6)

Also the TC texture coefficient was obtained using equation 7 [31].

$$T_C={\displaystyle \frac{\frac{I\left(hkl\right)}{I_0\left(hkl\right)}}{N^{-1}{\sum }_n\frac{I\left(hkl\right)}{I_0\left(hkl\right)}}}\times 100\%$$

(7)

where I(hkl) and I₀(hkl) are the relative intensity of the plane and the standard intensity of the peak, respectively. The values of I₀(hkl) were taken from the standard, JCPDS, and N is the total number of peaks present in the X-ray spectrum. The preferred orientation of the peaks were (002) and (110) for SnO₂ and ZnO films, respectively. The density of dislocations and texture coefficients are presented in Table 3. It can be observed that as the thickness increases, the crystalline quality improves for both films, as the density of dislocations decreases and the number of crystal planes oriented towards the preferred direction increases.

3.2. Raman Analysis

As XRD analysis does not confirm positively the presence of TiO₂, then samples were studied by Raman spectroscopy in order to confirm the formation of TiO₂ in the obtained films. Irrespective of crystalline or amorphous phase, Raman spectroscopy can be utilized to observe the presence of material due to the Raman vibrational modes. Typical Raman spectra of TiO₂ anatase phase possess six different modes, where three of them are of Eg mode at 142, 200 and 654 cm^-1, one B_1g mode at 397 cm^-1 and one A_1g +B_1g mode at 522 cm^-1 [32]. Raman spectra of TiO₂ thin films obtained in this work is shown in Figure 3 (with a thickness of 227 nm) and is in good agreement with the reported Raman modes of TiO₂ anatase phase [33]. All the peaks were moderately sharp and intense which can be considered as the proof of a non-long-range order of the thin films.

Figure 5. Raman spectra for TiO₂.

Figure 5. Raman spectra for TiO₂.

3.3. Morphological Properties for SnO₂, ZnO and TiO₂ Films

Figure 4 show the surface morphology of SnO₂, ZnO, and TiO₂ films. In general, all the deposited thin films exhibit a porous surface with nanometric spherical and irregular-shaped morphology. It can be observed that the morphological properties of semiconductor oxides are not greatly influenced by the film thickness, unlike the case of ZnO films, where with greater thickness, there is a larger grain size. The average grain size mentioned was directly measured using a ruler on a group of representative grains. In the case of SnO₂ films, no notable changes in the grain size and shape were observed (Figure 4a-c, the average grain size for all films is 11 nm). For ZnO films, it can be observed that the grain size increases with the film thickness. It is also important to mention that the rate of increase in the grain size is not gradual with respect to the thickness, but rather it increases abruptly (Figure 4d-f, the grain size changes from 11 nm for films with a thickness of 100 nm to 30 nm for a film thickness of 200 and 300 nm). Although the growth of the grains occurs through the coalescence of small grains. For the TiO₂ films, it is observed that the grain size is also small (Figure 4g-i, the grain size is 10 nm for all films). It should be noted that for thin thicknesses, no grain growth was observed in some areas of the film surface (Figure 4g).

Figure 6. SEM micrographs of SnO₂ films (a-c), ZnO films (d-f), TiO₂ films (g-i).

Figure 6. SEM micrographs of SnO₂ films (a-c), ZnO films (d-f), TiO₂ films (g-i).

3.4. Measurement of Thickness for SnO₂, ZnO, and TiO₂ Films

The thickness of some SnO₂, ZnO, and TiO₂ films were obtained using cross sectional view by scanning electron microscopy technique, and the images are shown in Figure 5. The images were captured transversely to the film and in some films were not possible to observe the film/substrate interface, due to the difficulty in cutting the samples without damaging the film. In general, the measurements obtained are very close to the expected thicknesses.

Figure 7. Cross-sectional SEM images for SnO₂, with 4 cycles (a), ZnO with 2, 6 and 10 cycles (b, c and d) and TiO₂ with 10 and 16 cycles (e, and f).

Figure 7. Cross-sectional SEM images for SnO₂, with 4 cycles (a), ZnO with 2, 6 and 10 cycles (b, c and d) and TiO₂ with 10 and 16 cycles (e, and f).

Thickness measurements were also obtained from profilometry, where the thicknesses are averaged from measurements taken at three different positions on the fabricated step. Table 4 presents the results obtained from both measurement techniques, and it can be observed that the film growth is nearly linear with respect to the number of immersions. Each film has a different growth rate, therefore different numbers of immersions were performed for each film to achieve thicknesses close to 100, 200, and 300 nm, as mentioned in the experimental development section. The results of the film thickness obtained by both measurement techniques are similar, except for the films with the smallest thicknesses (100 nm), where it is more difficult to measure with profilometry. The obtaining of thicknesses by profilometry technique is more economical compared to the SEM technique, and it is also reliable. As we mentioned in the previous Section 3.3, the morphology of the films resulted in very small grains around 11 nm for SnO₂ and TiO₂ films and 30 nm for ZnO films, therefore obtaining RMS roughness values is difficult for SnO₂ and TiO₂ films. Below, we present grain size and RMS roughness values obtained by SEM and film thickness values by profilometry technique. It is observed that the RMS roughness increases as the film thickness increases, this is because the average height of the grains was taken as the film thickness, therefore, as the height of the grains increases, the surface roughness increases and consequently increases the effective surface area of the films.

The histograms of the RMS roughness values were obtained from SEM images through image processing, considering that the grain height was obtained from the film thickness. These histograms for all films are presented in Figure 6. It can be observed that for SnO₂ and ZnO films, the distribution in grain height changes from having on average shorter grains at lower and medium thicknesses (100 – 200 nm) to a distribution of taller grains for larger thicknesses (300 nm), this is better observed for ZnO films. We can say that, on average, having more grains with greater height increases the effective area of the films. For TiO₂ films, the distribution in grain height hardly changes with respect to the film thickness; that is, most of the grain height remains at half of the maximum film thickness.

Figure 8. Histogram of RMS roughness obtained from SEM and profilometry for all films.

Figure 8. Histogram of RMS roughness obtained from SEM and profilometry for all films.

3.5. Photocatalysis Study

The efficiency of decolorization or degradation (FD%) is determined by the difference in MB concentrations in darkness and under ultraviolet illumination at any given time, $C_0$ and $C_t$, respectively, according to the following equation 8.

$$FD\%={\displaystyle \frac{C_0-C_t}{C_0}}\mathrm{x} 100$$

(8)

It can be assumed that the concentrations of the dye, $C_0$, $C_t$ and the optical absorptions, $A_0$, $A_t$ are directly proportional, therefore equation 9 can be written as follows:

$$FD\%={\displaystyle \frac{A_0-A_t}{A_0}}\mathrm{x} 100$$

(9)

where $A_0$ and $A_t$ are the values of the optical absorbance of MB in darkness and under UV illumination at a given time, t, respectively. To obtain the optical absorption spectra, first a calibration curve is generated using a cell without MB content, and then the absorption spectra are obtained for each film introduced in a solution containing MB at established time intervals. The percentage of photocatalytic degradation of MB is determined from the absorption spectra by measuring the relative magnitude of the peak height of maximum absorption of MB, in our case, this occurred at a wavelength of 664 nm. We present the absorption spectra for all samples, showing the percentage of MB degradation, Figure 7.

Figure 9. AM degradation curve in the photocatalysis process, sample Z300.

Figure 9. AM degradation curve in the photocatalysis process, sample Z300.

In Figure 8, the degradation performance of MB is shown for every 30 minutes of UV light exposure for all films deposited with different materials and thicknesses. All the films exhibit similar behavior; however, they show different degrees of degradation when exposed for different times. It is clearly observed that ZnO films are the most effective in degrading MB, reaching almost 100 %, followed by SnO₂ samples with a degradation percentage of 84 - 86 %, and finally, TiO₂ films are the least effective in degrading MB, with 50 – 76 % degradation.

Figure 10. Films exhibit enhanced degradation of MB through the photocatalytic process as the UV light exposure time increases, varying material and thickness.

Figure 10. Films exhibit enhanced degradation of MB through the photocatalytic process as the UV light exposure time increases, varying material and thickness.

To determine the degradation kinetics of the dye and obtain the degradation or reaction rate constant, k (h-1), the Langmuir-Hinshelwood model was used, as the degradation rate of these organic compounds at low dye concentrations follows pseudo-first-order kinetic model [34], equation 10.

$$\ln \left({\displaystyle \frac{{\mathrm{C}}_0}{{\mathrm{C}}_{\mathrm{t}}}}\right)=\mathrm{kt}$$

(10)

Similar to the degradation percentage FD%, equation 11, the dye concentrations, $C_0$ and $C_t$, can be substituted with optical absorbance, $A_O$ and $A_t$, respectively, resulting in the following equation [35].

$$\ln \left({\displaystyle \frac{A_0}{A_t}}\right)=kt$$

(11)

Plotting ln(A_o/A_t) with irradiation time, t, the slope of this line corresponds to the reaction rate constant, k, according to equation 11. Figure 9 shows the reaction rate constants, k, for the 9 deposited samples. Plotting the values of k for each sample, it can be observed that, similar to Figure 6, the most efficient samples for degrading MB are the ZnO films, followed by the SnO₂ samples, and the least efficient samples are the TiO₂ films. The next section provides an explanation of the behavior of the reaction rates.

Figure 11. Reaction rate values, k, for the different films deposited with different film thicknesses.

Figure 11. Reaction rate values, k, for the different films deposited with different film thicknesses.

The values of photocatalytic degradation and reaction rate for all deposited films are presented in Table 5. It can be observed in general that both photocatalytic degradation and reaction rate increase as the film thickness increases. ZnO films show the highest response due to their better physical properties.

In Table 6, we are comparing our obtained results with others reported for the degradation efficiency of MB, for the three semiconductor oxides used in this work, under somewhat similar measurement conditions. It can be observed that our results presented in this work are competitive with other findings.

4. Mechanism of Photodegradation

Figure 12. Shows semiconductor with different crystalline quality: (a) a semiconductor with good crystalline quality, containing few structural defects and few energy levels within the band gap, and (b) a semiconductor with poor crystalline quality, containing more structural defects and therefore more energy levels within the band gap. The conduction band (CB), valence band (VB) and trap energy level (Et) are also labeled.

Figure 12. Shows semiconductor with different crystalline quality: (a) a semiconductor with good crystalline quality, containing few structural defects and few energy levels within the band gap, and (b) a semiconductor with poor crystalline quality, containing more structural defects and therefore more energy levels within the band gap. The conduction band (CB), valence band (VB) and trap energy level (Et) are also labeled.

Figure 13. The absorbance for SnO₂, ZnO and TiO₂ films.

Figure 13. The absorbance for SnO₂, ZnO and TiO₂ films.

5. Reusability of the Photocatalyst

Figure 14. Percentage of MB degradation for ZnO films with a thickness of 300 nm and measurement conditions that provided the maximum MB degradation for 4 cycles.

Figure 14. Percentage of MB degradation for ZnO films with a thickness of 300 nm and measurement conditions that provided the maximum MB degradation for 4 cycles.

6. Conclusions

References

1. O’Neill, C.; Hawkes, F.R.; Hawkes, D.L.; Lourenço, N.D.; Pinheiro, H.M.; Delée, W. Colour in textile effluents - Sources, measurement, discharge consents and simulation: A review. J. Chem. Technol. Biotechnol. 1999, 74, 1009–1018. [Google Scholar] [CrossRef]
2. Bae, J.S.; Freeman, H.S.; Kim, S.D. Influences of new azo dyes to the aquatic ecosystem. Fibers Polym. 2006, 1, 30–35. [Google Scholar] [CrossRef]
3. Zhu, B.; Chen, W.F.; Koshy, P.; Sorrell, C.C. Effect of film thickness on the phototcatalytic performance of TiO2 thin films deposited by spin coating. Am. Ceram. Soc. 2015. [Google Scholar]
4. Garud, R. M.; Kore, S. V.; Kore, V. S.; Kulkarni, G. S. A Short Review on Process and Applications of Reverse Osmosis. Universal Journal of Environmental Research and Technology. 2011. 1, 3 233-238.
5. Sangwoo, S.; Orest, S.; Patrick, B.; Warren, Howard, A.; Stone, Membraneless water filtration using CO₂, Nature Communications 2017. 8, 15181.
6. Gianluca, C.; Nicola, R.; The treatment and reuse of wastewater in the textile industry by means of ozonation and electroflocculation, Water Research, 2001. 35, 2, 567-572.
7. Colmenares, J.C. Colmenares, J.C. Yi-Jun, X., Heterogeneous Photocatalysis From Fundamentals to Green Applications, Springer, Green Chemistry and Sustainable Technology. [CrossRef]
8. Xueting, C.;Zhongliang, L.;Xinxin, Z.;Shibin, S.; Danxia, G.;Lihua, D.;Yansheng, Y.;Yanqiu, Z.; Efficient synthesis of sunlight-driven ZnO-based heterogeneous photocatalysts. Materials & Design 2016, 98, 324–332.
9. Alves do Nascimento, J.L.; Chantelle, L.; Garcia dos Santos, I.M.; Menezes de Oliveira, A.L. Ferreira Alves, M.C. TThe Influence of Synthesis Methods and Experimental Conditions on the Photocatalytic Properties of SnO₂: A Review. Catalysts 2022, 12, 428. [CrossRef]
10. Gbenga Peleyeju, M.; Viljoen, E.L. WO₃-based catalysts for photocatalytic and photoelectrocatalytic removal of organic pollutants from water – A review, Journal of Water Process Engineering. 2021. 40, 101930.
11. Serpone, N.; Emeline, A. V Semiconductor Photocatalysis — Past, Present, and Future Outlook, J. Phys. Chem. Lett. 2012, 5, 673–677. [Google Scholar] [CrossRef]
12. Tao, Y.; Schwartz, S.; Wu, C.Y.; Mazyck, D.W. Development of a TiO₂/AC composite photocatalyst by dry impregnation for the treatment of methanol in humid airstreams, Ind. Eng. Chem. Res. 2005. 44, 7366–7372. [CrossRef]
13. Prado, A.G.S.; Bolzon, L.B.; Pedroso, C.P.; Moura, A.O.; Costa, L.L. Nb2O5 as efficient and recyclable photocatalyst for indigo carmine degradation, Appl. Catal. B Environ. 2008. 82. 219–224.r indigo carmine degradation, Appl. Catal. B Environ. 2008.
14. Luiz, F. K.; Pedrinia, L.C.; Escaliantea, L.V.A. Deposition of TiO₂ thin Films by Dip-Coating Technique from a Two-Phase Solution Method and Application to Photocatalysis, Materials Research. 2021. 24. e20210007. [CrossRef]
15. Ferhunde, A.; Orkun, G. The effect of spinning cycle on structural, optical, surface and photocatalytic properties of sol–gel derived ZnO films, Journal of Sol-Gel Science and Technology. 2021. 100. 299–309. [CrossRef]
16. Chin-Yi.; Jui-Chung, W.; Jing-Heng, C.; Shih-Hsin, M.; Kun-Huang, C.; Tzyy-Leng, H.; Chien-Yie, T.; Chi-Jung, C.; Chung-Kwei, L.; Jerry, J. W. Photocatalyst ZnO-doped Bi2O3 powder prepared by spray pyrolysis, Powder Technology, 2015. 272, 316-321.
17. Obregón, S.; Rdz, V. Photocatalytic TiO2 thin films and coatings prepared by sol–gel processing: a brief review, Journal of Sol-Gel Science and Technology. 2018. 102. Available online: https://link.springer.com/article/10.1007%2Fs10971-021-05628-5.
18. Farsi, B. Al.; Souier, T.M.; Marzouqi, F. Al.; Maashani, M. Al.; Bououdina, M.; Widatallah, H.M.; Abri, M. Al. Structural and optical properties of visible active photocatalytic Al doped ZnO nanostructured thin films prepared by dip coating, OpticalMaterials. 2021, 113. 110868.
19. Shaham-Waldmann, N., Yaron, P. Away from TiO₂: A critical mini review on the developing of new Photocatalysts for degradation of contaminants in water, Materials Science in Semiconductor Processing 2016. 42. 72–80. [CrossRef]
20. C.J. Brinker, A.J. Hurd, “Fundamentals of sol-gel dip-coating”, J. Phys. III France 4 (1994) 1231-1242.
21. C.J. Brinker, “Dip coating”, “Chemical Solution Deposition of Functional Oxide Thin Films”, T. Schneller et al. (eds.) Springer-Verlag Wien 2013. [CrossRef]
22. Tang, X., & Yan, X. (2016). Dip-coating for fibrous materials: mechanism, methods and applications. Journal of Sol-Gel Science and Technology, 81(2), 378–404. [CrossRef]
23. Amakali, T., Daniel, L.S., Uahengo, V., Dzade, N.Y., & de Leeuw, N.H. (2020). Structural and Optical Properties of ZnO Thin Films Prepared by Molecular Precursor and Sol–Gel Methods. Crystals, 10(2), 132. [CrossRef]
24. Akgul, F.A., Gumus, C., Er, A.O., Farha, A.H., Akgul, G., Ufuktepe, Y., & Liu, Z. (2013). Structural and electronic properties of SnO2. Journal of Alloys and Compounds, 579, 50–56. 579, 50–56. [CrossRef]
25. Venkatachalam, T., Sakthivel, K., Renugadevi, R., Narayanasamy, R., Rupa, P., Predeep, P., … Varma, M.K.R. (2011). Structural and Optical Properties of TiO[sub 2] Thin Films. [CrossRef]
26. Joint Committee on Powder Diffraction Standards (JCPDS), International Centre for Diffraction Data. 1997, Card No. 41-1445.
27. McMurdie, H.F.; Morris, M.; Evans, E.; Paretzkin, B.; Wong-Ng, W.; Ettlinger, L.; Hubbard, C. Standard X-ray diffraction powder patterns from the JCPDS research associateship. Powder Diffr. 1986, 1, 64–77. [Google Scholar] [CrossRef]
28. Cullity, B.D.; Stock, S.R. Elements of X-ray Diffraction, 3rd ed.; Prentice Hall: Upper Saddle River, NJ, USA, 2001. [Google Scholar]
29. Madelung, O.; Rössler, U.; Schulz, M. (Eds.) II-VI and I-VII Compounds; Semimagnetic Compounds. Landolt-Börnstein—Group III Condensed Matter (Numerical Data and Functional Relationships in Science and Technology); Springer: Berlin/Heidelberg, Germany, 1999; Volume 41B. [Google Scholar]
30. Khan Z R, Zulfequar M and Khan M S, Optical and structural properties of thermally evaporated cadmium sulphide thin films on silicon (1 0 0) wafers, Mater. Sci. Eng. B. 2010, 174. 145–149.
31. Harris, G. Quantitative measurement of preferred orientation in rolled uranium bars, Dublin Philosophy. Mag. J. Sci.1952. 43, 336, 113–123.
32. Wiatrowski, A.; Mazur, M.; Obstarczyk, A.; Wojcieszak, D.; Kaczmarek, D.; Morgiel, J.; Gibson, D. Comparison of the Physicochemical Properties of TiO2 Thin Films Obtained by Magnetron Sputtering with Continuous and Pulsed Gas Flow, Coatings. 2018, 8, 11, 412. [CrossRef]
33. Sekiya, T.; Ohta, S.; Kamei, S.; Hanakawa, M.; Kurita, S. Raman spectroscopy and phase transition of anatase TiO2 under high pressure, Journal of Physics and Chemistry of Solids. 2001. 62. 717-721.
34. Tanaka, K.; Capule, M.F.V.; Hisanaga, T. Effect of crystallinity of TiO2 on its photocatalytic action. Chem. Phys. Lett. 1991. 187. 73–76.
35. Chang, Y.C.; Chen, C.M.; Guo, J.Y. Fabrication of novel ZnO nanoporous films for efficient photocatalytic applications. J. Photochem. Photobiol. A Chem. 2018. 356. 340–346.
36. Adeyemi, J.O., & Onwudiwe, D.C. SnS2 and SnO2 Nanoparticles Obtained from Organotin(IV) Dithiocarbamate Complex and Their Photocatalytic Activities on Methylene Blue. Materials, 2020. 13(12), 2766. doi:10.3390/ma13122766.Viet, P.V., Thi, C.M., & Hieu, L.V. (2016). The High Photocatalytic Activity of SnO₂ Nanoparticles Synthesized by Hydrothermal Method. Journal of Nanomaterials, 2016, 1–8. [CrossRef]
37. Swetha, P., Jayachamarajapura, P.S., Jayden, S., Syed, F.A., Mufsir, K., Mohammad, R.H., Baji, S., Kiran, K. Photocatalytic Degradation of Methylene Blue and Metanil Yellow Dyes Using Green Synthesized Zinc Oxide (ZnO) Nanocrystals. Crystals 2022, 12(1), 22. [CrossRef]
38. Albiss, B., & Abu-Dalo, M. Photocatalytic Degradation of Methylene Blue Using Zinc Oxide Nanorods Grown on Activated Carbon Fibers. Sustainability 2021, 13(9), 4729. [CrossRef]
39. Isai1, k. A., Shrivastava, V.S. Photocatalytic degradation of methylene blue using ZnO and 2%Fe–ZnO semiconductor nanomaterials synthesized by sol–gel method: a comparative study. N Applied Sciences. 2019. 1:1247. [CrossRef]
40. Olga, T., Anastasia, M.M., Konstantinos, M.S., Ekaterini, P.; Lakovos, Y. Highly Active under VIS Light M/TiO2 Photocatalysts Prepared by Single-Step Synthesis. Appl. Sci. 2023, 13(11), 6858. [CrossRef]
41. Mohammad Jafri, N.N., Jaafar, J., Alias, N.H., Samitsu, S., Aziz, F., Wan Salleh, W.N., … Isloor, A.M. (2021). Synthesis and Characterization of Titanium Dioxide Hollow Nanofiber for Photocatalytic Degradation of Methylene Blue Dye. Membranes, 11(8), 581. [CrossRef]
42. Maness, P.C.; Smolinski, S.; Blake, D.M.; Huang, Z.; Wolfrum, E.J.; Jacoby, W.A. Bactericidal Activity of Photocatalytic TiO2 Reaction: toward an Understanding of Its Killing Mechanism. Columbia: Applied and Environmental Microbiology. 1999. 4094–4098.
43. Landsberg, P.T. Recombination in semiconductors, Cambridge, 1991.
44. Hunge, Y.M., Yadav, A.A., Kang, S.W., Mohite, B.M. Role of Nanotechnology in Photocatalysis Application, Recent Patents on Nanotechnology.2023.17.
45. Vasiljevic, Z.Z., Dojcinovic, M. P.,Vujancevic, J.D. , Jankovic-Castvan. I., Ognjanovic, M., Tadic, N.B., Stojadinovic, S.,Brankovic, G.O., Nikolic, M.V. Photocatalytic degradation ofmethylene blue under natural sunlight using irontitanate nanoparticles prepared by a modifiedsol–gel method. R. Soc. Open Sci. 2020. 7: 200708. [CrossRef]
46. J. Zhang, P. Zhou, J. Liu, J. Yu, Phys. Chem. Chem. Phys., 2014, 16, 20382-20386.
47. J. Pascual, J. Camassel, and H. Mathieu, “Fine structure in the intrinsic absorption edge of TiO2,” Physical Review B, vol. 18, no. 10, pp. 5606–5614, 1978.
48. E. Baldini, L. Chiodo, A. Dominguez et al., “Strongly bound excitons in anatase TiO2 single crystals and nanoparticles,” Nature Communications, vol. 8, no. 1, pp. 13–13, 2017.
49. J. Pascual, J. Camassel, and H. Mathieu, “Fine structure in the intrinsic absorption edge of TiO2,” Physical Review B, vol. 18, no. 10, pp. 5606–5614, 1978.
50. W. Tian, T. Zhai, C. Zhang, S.L. Li, X. Wang, F. Liu, D. Liu, X. Cai, K. Tsukagoshi, D. Golberg, Y. Bando, Adv. Mater. 2013, 25, 4625.
51. Shaho, M. Rasul; Dlear, R. Saber; Shujahadeen, B. Aziz. In: Results in Physics, Vol 38, Iss , Pp 105688- (2022).
52. M. Zhanhong; F. Ren; X. Ming; Y. Long; A. A. Volinsky Materials, 12 (2019), p. 196.
53. Z. Nabi; A. Kellou; S. Méçabih; A. Khalfi; N. Benosman Mater. Sci. Eng. B Solid-State Mater. Adv. Technol., 98 (2003), p. 104.
54. Ishchenko O, Rogé V, Lamblin G, Lenoble D, Fechete, I. TiO 2, ZnO, and SnO 2 -based metal oxides for photocatalytic applications: principles and development. C R Chim. 2021;24(1):103–24. [CrossRef]