Synthesis and Properties of Ce3+ Doped the Multi-Functional Tio2-Sio2 nanocomposite Films

Introduction

Materials used outdoors are often damaged by ultraviolet rays and extreme weather conditions or by mechanical collisions with other materials. In recent years, the development of nano-sized coatings that both ensure aesthetics and protect surfaces in extreme conditions is of particular concern. The most prominent is the thin film synthesized from TiO₂ material, which has been developed for a long time but is still current because of their application properties in the protective film field. Titanium dioxide is a semiconductor oxide with a wide band gap, located in the ultraviolet region (3.0 – 3.2 eV) [1]. TiO₂ has attracted a lot of research scientists because it has interesting properties such as TiO₂ in Anatase phase with photocatalytic activity, self-cleaning ability, some studies show that TiO₂ materials are also capable of antibacterial ^1-3. However, Anatase phase has photocatalytic activity, under the effect of UV rays, can decompose organic components in artificial rocks, damaging the rock surface. In addition, Anatase phase is not stable to heat, when the heating temperature increases, TiO₂ materials have the ratio of Anatase and Rutile phases changes in the direction of increasing the Rutile phase ratio ⁴. Moreover, TiO₂ has poor mechanical strength and ability to adhere to the surface of other materials.

As an alternative to TiO₂, silica-titania composite material, which is an oxide system consisting of silica (SiO₂) and titania (TiO₂), can be used. This material system has been the subject of research by many scientists because of the unique properties that come from the combination of these two separate oxides.

The silica-titania composite exhibits many desirable properties such as ultraviolet light absorption, TiO₂ self-cleaning, antimicrobial material, both the stability, mechanical strength of SiO₂, and incoming properties from chemical bonding between two materials ^5-9. In addition, silica and titania are both environmentally friendly, non-toxic, and low cost materials ².

To enhance absorption and to improve some properties, materials can be doped with certain rare earth metals, transitional or nonmetallic. Cerium is a rare earth metal known for its many redox states. As an important doping material, cerium has advantages such as multiple electronic energy levels and valence states, which lead to different properties ^10,11. When doping cerium into the silica-titania composite material, there is a shift in the absorbed light from the ultraviolet to the visible light, enhancing the photocatalytic activity ¹¹.z

According to our knowledge, up to now, there has been no published study on the properties ofCe-doped TiO₂/SiO₂nanocomposite thin film and orientation for application in the field of coating for outdoor use. In this study, we have proposed the successful synthesis of Ce-doped TiO₂/SiO₂nanocomposite thin film by the sol gel method combined using the spin coating technique and their properties are discussed details

Experimental

Materials

Tetraethyl orthosilicate (TEOS, 99% Sigma-Aldrich), Titanium tetra-n-butoxide (TBT, 98%, Sigma-Aldrich), Cerium (III) nitrate hexahydrate (Xilong Scientific) were used as starting materials. Isopropanol (IPA, 99.7%, Xilong Scientific), diethylene glycol (DEG, 99.7%, Xilong Scientific) were used as a solvent. The molar ratio of TEOS and TBTwas optimized and fixed at 70:30.

Preparation

A series of TiO₂-SiO₂:xCe³⁺ nanocomposite films (x = 0 ÷ 0.08 mol) were prepared by the pechini-type sol–gel method and spin-coating technique. In the typical synthesis process, 7 mmol (1,55 mL) of TEOS was mixed with 25 mL IPA stirred for 10 – 15 minutes by a magnetic stirring hot plate at room temperature. Then HNO₃ and water were added into the solution, followed by stirring for 1 hour. Then, 3 mmol (1mL) Titanium n-butoxide (TBT) was added into the solution which was further stirred for 1 hour. Thirdly, molar ratios of ([Ce³⁺]/([Ti⁴⁺] + [Si⁴⁺]) with 2, 4, 6, and 8% of Ce(NO₃)₃.6H₂O were alternatively added into the solution, then continuously stirred for 30 minutes. Finally, DEG (1 mL) was prepended as a cross linking agent and the stirring continued for 15 minutes to get a complete homogeneous solution. To prepared films form, they were spin- coated onto a glass substrate by a spin-coater machine, following two stages: 1500 rpm for 7s and 2500 rpm for 30s. Each of five nanocomposite layers on the glass substrate was dried at several times to evaporate solvents. After that, the five-layer films were annealed in the air at 100, 200, 300 ^oC for 2 hours. The obtained nanocomposite films were employed to assess the further characterization.

Characterization

After obtaining the nanocomposite material, we carried out measurements to characterize and analyze the properties of the material. In detail, the Fourier- transform infrared spectroscopy (FT-IR, Spectrum Two, Perkin Elmer) was used to study the chemical bonding in the nanocomposite material. The crystalline structure of nanocomposite films was studied by X-ray Diffraction (D8-ADVANCE, Bruker-Germany). Thesurface morphology of films was characterized by the high-resolution field emission scanning electron microscopy (FE-SEM, JEOL JSM-7600F). The optical property of the films was investigated by using UV-vis spectrophotometer (JASCO V-750). The wettability behavior was studied by the water contact angle measurement (WCA, C017).

Results and discussion

Fig.1. shows the XRD patterns of SiO₂-TiO₂:6 %molCe³⁺ nanocomposite thin film on the glass substrate air-annealed at 100, 200 and 300 ^oC for 2 hours (a,b,c samples). The XRD patterns of (a,b) samples show that the films consist of SiO₂and TiO₂in the amorphous phase but at 300 ^oC, the sample also existed anatase phase which is located at 2θ = 23^o and TiO₂ crystals in the anatase crystal due to the characteristic peak at 2θ = 55.9^o; 31.7^o; 66.9^o (PDF#84-1286). It is noteworthy that not all the peaks of the anatase phase and no peak of Ce crystal phase are observed in the XRD patterns. Such observed can be attributed to thickness of thin-film and the greater ionic radii of Ce³⁺ (0.107 nm) than Ti⁴⁺, Si⁴⁺ (0.068, 0.040 nm, respectively). The difference in radii leads to difficulty for Ce³⁺ ions to replace Ti⁴⁺ or Si⁴⁺ ions in the SiO₂-TiO₂ matrix, hence, Ce³⁺ ions tend to bond with O^2- anion on the surface of TiO₂[12]. The trendy of bonding of Ce³⁺ with O^2- on its surface is a factor to influence the optical and wetting properties of the nanocomposite film that would be discussed in the next section.

Figure 1:  X-Ray of Ce3+ doped TiO2/SiO2 with 6% mol concentration at different temperatures for 2 hours (a:100 oC; b: 200 oC; c: 300 oC) Click here to View figure

The FT-IR spectra of SiO₂-TiO₂ and SiO₂-TiO₂:6%Ce³⁺nano composite thin film (air-annealed at 300 ^o for 2 hours) shown in Fig 2. The bands at 1070 cm^-1 and 791 cm^-1 can be assigned to the vibration of Si-O-Si bonding in SiO₂ and This trend indicated that vibration at 901 cm^-1 maybe resulted from the Ti–O–Si linkagesSi-O-Ti bonding in TiO₂-SiO₂^{13, 14}. In this case, the content of Ce³⁺ is not sufficient to influence the vibration of bonding in the SiO₂-TiO₂ matrix.

Figure 2: FT-IR spectrum TiO2/SiO2 (a); TiO2/SiO2: 6%mol Ce3+at 300 oC for 2 hours. Click here to View figure

The morphology of SiO₂-TiO₂:x%Ce³⁺nanocomposite films with a molar different ratio of Ce³⁺ (x = 0, 2, 4, 6, 8%) air-annealed at 300 ^oC for 2 hours is captured in the FE-SEM images (Fig.3). As seen in FE-SEM images, the morphology of SiO₂-TiO₂ nanocomposite films change with respect to the amount of Ce³⁺ ions doped in the film. It is clearly seen, the composite film with x = 0% shows some cracks on its surface, however, these cracks disappear when increasing Ce³⁺ ratio to 2% (Fig.3b). In addition, the surface of SiO₂-TiO₂:x%Ce³⁺ composite films display the hole-like nanoparticles that distribute in the matrix with (x = 4%, 6%) (Fig3. c, d) and the hole-like particles are distributed in the matrix having an average size of 50 – 100 nm.Further increasing the ratio of Ce³⁺ (x = 8%) in Fig.3e, the cracks re-appear in the SiO₂-TiO₂:Ce³⁺ composite film surface. The appearance of particles on the nanocomposite film surface could be attributed to high enough concentration of Ce³⁺ doped in the film to induce phrase separation and form particles on the film surface.

Figure 3: FE-SEM images of Ce3+-doped SiO2-TiO2 nanocomposite thin film at 300 oC in the air for 2 hours with a molar different ratio of Ce3+ (a) 0% (b) 2% (c) 4% (d) 6% (e) 8% (f) distribution of particles size of 6% Ce3+ sample Click here to View figure

Fig.4 shows the UV-vis transmittance spectrum of the obtained nanocomposite films. For the application of self-cleaning and protection, the film has to meet requirements of transmittance that greater than 85% in the visible region (400 – 800 nm), and absorbance in the UV-A region (300 – 400 nm) ⁸. It is clear to observe that the concentration of Ce³⁺ ion-doped in the prepared film is a factor to effect to its optical properties. As shown in Fig.4a, all of the wavelengths in the visible region (400 – 800 nm) can be transmitted through the film. By doping Ce³⁺, the transmittance (T) of the SiO₂-TiO₂ film sample is raised from 84.5% to 88.3%. Because the intrinsic bandgap of SiO₂-TiO₂ composite is 3.0 – 3.4 eV ¹⁵. In addition, the morphology of the Ce³⁺-doped film surface shows that particles are distributed relatively with at least optical defects, due to low light scattering of the films, so then the transmittance of film increased. For all of those reasons, the visible light is easily transmitted in the nanocomposite film ^{15, 16}.

Figure 4: UV-vis transmittance spectrum of SiO2-TiO2:x%Ce3+ nanocomposite films with wavelength between (a) 200 – 800 nm (b) 300 – 420 nm. Click here to View figure

On the other hand, as can be seen in Fig.4b, the absorbance is shifted towards longer wavelength from 300 – 420 nm to 355 to 390 nm, when the molar proportion of Ce³⁺ ions increases. The red-shifted of the absorbance can evidence for Ce³⁺ successfully doped in the SiO₂-TiO₂ matrix. However, with the Ce³⁺ concentration is 8%, the change is not significant in the UV-vis spectrum, also in the FE-SEM image that could be the concentration of Ce³⁺ is saturated in the SiO₂-TiO₂ matrix. Consequently, the molar ratio of Ce³⁺ doped in the film is optimized at 6%.

To investigate the effect of Ce³⁺ on the bandgap of the SiO₂-TiO₂nanocomposite, the samples prepared and annealed at 300 ^oC for 2 hours. Fig.5 illustrates the UV-vis Diffuse Reflection Spectrum (UV-vis DRS) of the SiO₂-TiO₂:x%Ce³⁺. In Fig.5a, it can be seen that the absorption edge of the Ce³⁺-doped in the samples is shifted towards the red-wavelengths when the Ce³⁺ concentration increase (from 390 to 530 nm). It is well known that the dopant can influence the structure of the host material. The configuration of Ce³⁺ is [Xe]4f¹5d⁰6s⁰ of which its f-orbital has unoccupied. When Ce³⁺ was doped in the SiO₂-TiO₂ matrix, the 4f electronic configuration of Ce which allows electron transition from 2p levels of oxygen (valance band – VB) to 4f levels of Cerium and 2p levels of Oxygen to 3d levels of Titanium (conduction band – CB) ^17-21. This leads to the generation of electron-hole pairs and the improvement of the visible light absorption, and visible light response.

According to the theory of P.Kulbelka and Munk ²² presented in 1931, the optical bandgap is calculated by applying the Kubelka-Munk function

(1(F(R_∞)hν)^1/γ = B(hν – Eg)                                                      (1)

Where R_∞ is the reflectance of an infinitely thick specimen, h is the Planck constant, ν is the photo’s frequency, Eg is the bandgap energy, B is a constant. The γ factor depends on the nature of the electron transition and is equal to ½ or 2 for the direct and indirect transition band gaps, respectively. It is known that SiO₂, TiO₂ are direct transition bandgap, therefore, Eg can be determined by plotting (F(R_∞)hν)² versus hν the optical bandgap ²³.

Figure 5: UV-vis Diffuse Reflection Spectrum of SiO2-TiO2:x% Ce3+ nanocomposite films (a) the UV-vis absorbance spectrum (b) and the calculated band gap by Tauc plot. Click here to View figure

The optical bandgap of all samples is calculated and shown in Fig.5b, for the SiO₂-TiO₂nanocomposite sample, the optical bandgap is 3.68 eV, with absorbance wavelength is 337 nm, respectively. With an increase of molar ratio of Ce³⁺ doped in the SiO₂-TiO₂ matrix, the optical bandgap of obtained sample declines from 3.4 to 2.95 eV. When Ce³⁺ doped in the SiO₂-TiO₂ matrix, the 4f levels of Cerium are occupied, leads to new electric states in the bandgap of TiO₂, which were reported by Density-functional theory (DFT) simulation articles. According to DFT simulation for Ce³⁺ doped in the TiO₂, the 4f-Ce states are near conduction bands, they are shallow impurity states. This leads to the red-shifted of the absorption edge and the bandgap of the prepared nanocomposite material is narrowed. In addition, the shallow impurity levels are a factor to influenced the generation of photo-excited electron-hole pairs because they can trap the photo-excited electrons and holes ²⁴. Therefore, when Ce³⁺ is doped in the SiO₂-TiO₂ matrix, the optical properties of the SiO₂-TiO₂ nanocomposite is influenced.

Figure 6: The water contact angle of the SiO2-TiO2:6%Ce3+nanocomposite film was exposed UV radiation 30 minutes (a: non-coated; b: coated by SiO2-TiO2:6%Ce3+) Click here to View figure

The wettability of the nanocomposite films is investigated by the contact angle method (WCA). To further investigate the surfaces of the Cedoped SiO₂-TiO₂ nanocomposite films, the wettability behavior of the films was investigated by the water contact angle (WCA) method. Fig. 6 shows the water contact angle measurements for the non-coated sample and the sample coated with a layer of 6 mol% Ce-doped SiO₂-TiO₂ film after being exposed to UV (365 nm) for 30 min. While the WCA of the noncoated sample did not change, the angle of the coated film changes from 43^o to 11^o. This result indicates that the surface of the sample has been transformed from a hydrophilic surface to asuper-hydrophilic surface when coating with 6 mol% Ce-doped SiO₂-TiO₂ film. From the literatures, this transformation process can be explained by the creation of surface oxygen vacancies due to the appearance of TiO₂nanocrystals in the coated film. In such system, the water molecules may travel into the oxygen vacancy sites, leading to the dissociative adsorption of the water molecules on the surfaces of coated film and the photoinduced reconstruction of surface hydroxyl groups. With the increase of adsorbed hydroxyl groups on the film surface, van der Waals forces and hydrogen bond interactions between water molecules and hydroxyl group will increase, leading to the super-hydrophilic properties of the composite film ²⁵.

Conclusion

In conclusion, the multi-functional SiO₂-TiO₂:x%molCe³⁺ nanocomposite film was successfully synthesized by sol-gel method. The results reported in this study illustrate that the Ce³⁺ ion is a factor to influence the morphology, optical property, and the wetting of the obtained film.  Particularly, the multi-functional SiO₂-TiO₂:6%molCe³⁺ nanocomposite film is a highly transmittance in the visible light (greater than 85%), absorbance in UV(A) light, and super-hydrophilic surface. For all of those results, the SiO₂-TiO₂:6%molCe³⁺ film is recommended to use as the protective film with some surfaces like glasses, photovoltaic devices.

Acknowledgement

This research is funded by the Hanoi University of Science and Technology (HUST) underproject number T2020-SAHEP-032. The author thankfulto R&D center of Kangaroo groups for their kind support and motivation.

Conflict of interest

No Conflict of interest.

References

1. N. Mufti, I.K.K. Laila, Hartatiek, A. Fuad, J. Phys. Conf. Ser., 853(1), (2017).
   CrossRef
2. N. Rahimi, R.A. Pax, E.M.A. Gray, Prog. Solid State Chem., 44(3), 86-105 (2016).
   CrossRef
3. Y. Hendrix, A. Lazaro, Q. Yu, J. Brouwers, World J. Nano Sci. Eng., 05(04), 161-177 (2015).
   CrossRef
4. Jaroenworaluck, A., N. Pijarn, N. Kosachan, and R. Stevens, “Nanocomposite TiO₂-SiO₂ gel for UV absorption,” Chem. Eng. J., vol. 181–182, pp. 45–55, 2012.
   CrossRef
5. Erdural, B., U. Bolukbasi, and G. Karakas, “Photocatalytic antibacterial activity of TiO₂-SiO₂ thin films: The effect of composition on cell adhesion and antibacterial activity,” J. Photochem. Photobiol.A Chem., vol. 283, pp. 29–37, 2014.
   CrossRef
6. Jesus, M. A. M. L. de, J. T. da S. Neto, G. Timò, P. R. P. Paiva, M. S. S. Dantas, and A. de M. Ferreira, “Superhydrophilic self-cleaning surfaces based on TiO₂ and TiO₂/SiO₂ composite films for photovoltaic module cover glass,” Appl. Adhes. Sci., vol. 3, no. 1, pp. 1–9, 2015.
   CrossRef
7. Sun, S., T. Deng, H. Ding, Y. Chen, and W. Chen, “Preparation of nano-TiO₂-coated SiO₂ microsphere composite material and evaluation of its self-cleaning property,” Nanomaterials, vol. 7, no. 11, 2017.
   CrossRef
8. Watanabe, F., “Composite resin.,” Nihon ShikaIshikaiZasshi, vol. 24, no. 5, p. 483, 1971.
9. Zhang, S., D. Sun, Y. Fu, and H. Du, “Recent advances of superhardnanocomposite coatings: A review,” Surf. Coatings Technol., vol. 167, no. 2–3, pp. 113–119, 2003.
   CrossRef
10. Vázquez-Velázquez, A. R., M. A. Velasco-Soto, S. A. Pérez-García, and L. Licea-Jiménez, “Functionalization effect on polymer nanocomposite coatings based on TiO₂–SiO₂ nanoparticles with superhydrophilic properties,” Nanomaterials, vol. 8, no. 6, 2018.
    CrossRef
11. Zayat, M., P. Garcia-Parejo, and D. Levy, “Preventing UV-light damage of light sensitive materials using a highly protective UV-absorbing coating,” Chem. Soc. Rev., vol. 36, no. 8, pp. 1270–1281, 2007.
    CrossRef
12. Maon, L., (2013), “Mohs’ Scale of Hardness” J. Chem. Inf. Model., vol. 53, no. 9, pp. 1689–1699, 2013.
    CrossRef
13. Latthe S, Liu S, Terashima C, Nakata K, Fujishima A (2014) Transparent, Adherent, and Photocatalytic SiO₂-TiO₂ Coatings on Polycarbonate for Self-Cleaning Applications. Coatings 4:497–507. doi: 10.3390/coatings4030497.
    CrossRef
14. Sun X, Li C, Ruan L, Peng Z, Zhang J, Zhao J, li Y (2014) Ce-doped SiO₂@TiO₂ nanocomposite as an effective visible light photocatalyst. J Alloys Compd 585:800–804. doi: 10.1016/j.jallcom.2013.10.034
    CrossRef
15. Latthe S, Liu S, Terashima C, Nakata K, Fujishima A (2014) Transparent, Adherent, and Photocatalytic SiO₂-TiO₂ Coatings on Polycarbonate for Self-Cleaning Applications. Coatings 4:497–507. doi: 10.3390/coatings4030497
    CrossRef
16. Guan K (2005) Relationship between photocatalytic activity, hydrophilicity and self-cleaning effect of TiO₂/SiO₂ films. Surf Coatings Technol 191:155–160. doi: 10.1016/j.surfcoat.2004.02.022
    CrossRef
17. Jiang Y, Jin Z, Chen C, Duan W, Liu B, Chen X, Yang F, Guo J (2017) Cerium-doped mesoporous-assembled SiO2/P25 nanocomposites with innovative visible-light sensitivity for the photocatalytic degradation of organic dyes. RSC Adv 7:12856–12870. doi: 10.1039/c7ra00191f
    CrossRef
18. Cao Y, Zhao Z, Yi J, Ma C, Zhou D, Wang R, Li C, Qiu J (2013) Luminescence properties of Sm3+-doped TiO2 nanoparticles: Synthesis, characterization, and mechanism. J Alloys Compd 554:12–20. doi: 10.1016/j.jallcom.2012.11.149
    CrossRef
19. Devi LG, Kumar SG (2012) Exploring the critical dependence of adsorption of various dyes on the degradation rate using Ln 3+ -TiO 2 surface under UV/solar light. Appl Surf Sci 261:137–146. doi: 10.1016/j.apsusc.2012.07.121
    CrossRef
20. Tsega M, Dejene FB (2016) Structural and optical properties of Ce-doped TiO2 nanoparticles using the sol-gel process. ECS J Solid State Sci Technol 5:R17–R20. doi: 10.1149/2.0341602jss
    CrossRef
21. Yao Y, Zhao N, Feng JJ, Yao MM, Li F (2013) Photocatalytic activities of Ce or Co doped nanocrystalline TiO 2-SiO2 composite films. Ceram Int 39:4735–4738. doi: 10.1016/j.ceramint.2012.11.035
    CrossRef
22. Džimbeg-Malčić V, Barbarić-Mikočević Ž, Itrić K (2012) kubelka-munk theory in describing optical properties of paper (II). Teh Vjesn 19:191–196
23. Makuła P, Pacia M, Macyk W (2018) How To Correctly Determine the Band Gap Energy of Modified Semiconductor Photocatalysts Based on UV-Vis Spectra. J Phys Chem Lett 9:6814–6817. doi: 10.1021/acs.jpclett.8b02892
    CrossRef
24. Xie K, Jia Q, Wang Y, Zhang W, Xu J (2018) The electronic structure and optical properties of Anatase TiO2 with rare earth metal dopants from first-principles calculations. Materials (Basel) 11. doi: 10.3390/ma11020179.
    CrossRef
25. Vidyadharan V, Vasudevan P, Karthika S, Joseph C, Unnikrishnan N V., Biju PR (2015) Structural, Optical and AC Electrical Properties of Ce³⁺-Doped TiO₂–SiO₂ Matrices. J Electron Mater 44:2754–2761. doi: 10.1007/s11664-015-3724-6
    CrossRef

---

This work is licensed under a Creative Commons Attribution 4.0 International License.