In recent years, water pollution caused by population growth has become one of the most health-threatening problems. To solve this problem, photocatalytic degradation of organic pollutants using solar energy is expected. Among various photocatalysts, TiO2 has been widely used and studied because of its non-toxicity, low cost, and high chemical stability. However, TiO2 has a wide band gap of 3.2 eV, so only ultraviolet light is available. Since ultraviolet rays account for only about 3~5% of sunlight, research on the effective use of visible light, which accounts for about 45% of sunlight, has been active in recent years. Known major approaches include heterojunction coupling, doping, and dye sensitization. Among them, composites of carbon nanomaterials and TiO2 have been shown to improve optical absorption in visible light. In recent years, carbon quantum dots (CQDs) have attracted attention as a new carbon nanomaterial because of their unique properties. CQDs are non-toxic, inexpensive, and easy to tune. Therefore, it has been applied as a composite material for TiO2 and has been reported to have an optical absorption edge of 419 nm and a band gap of 2.96 eV. In this study, SiO2@TiO2/CQD heterojunction structures were fabricated, and their photocatalytic activity was evaluated using methylene blue. By adjusting the particle size of SiO2@TiO2, the optical wavelength to be reflected can be selected. Among them, SiO2@TiO2 which can reflect optical wavelengths around 400 nm was adjusted to form a structure that allows more efficient optical utilization of TiO2/CQDs.SiO2@TiO2/CQD is a mixture of TiO2 and CQDs coated on the surface of a SiO2 porous structure prepared by the Stover method. This simple preparation method resulted in high surface area, efficient light utilization due to the heterostructure, and efficient charge mobility. As a result, the degradation performance against organic pollutants was shown to be improved.
Mesoporous SiO2@TiO2PhotocatalyticMethylene blueCQDPhotocatalystFile created by JATS EditorJATS Editorissue-created-year20251. Introduction
In recent years, water pollution caused by population growth has become one of the most health-threatening problems. 1 Among these, contamination by organic dyes is a serious problem. Organic dyes are a major pollutant of wastewater discharged in large quantities from the textile industry and other sources. These organic dyes cause significant damage to the land and the environment, and these residual dyes are toxic to humans and animals because they are not decomposed by microorganisms. 2 Among these, contamination by organic dyes is a serious problem. Organic dyes are a major pollutant of wastewater discharged in large quantities from the textile industry and other sources. This organic dye is very damaging to the land and environment, and the residual dye is toxic to humans and animals because it is not decomposed by microorganisms. Physical and chemical treatment methods are gaining attention to solve these problems. However, conventional physical and chemical treatment methods have problems such as secondary contamination and long treatment cycles. Among these, the removal of organic dyes by photocatalysis is attracting attention. [3; 4]
Photocatalysts have been widely used and studied because of their non-toxicity, low cost, and high chemical stability. The disadvantage of TiO2, a commonly used photocatalyst, is its wide band gap of 3.2 eV, which means that only UV light is available. [3; 5] UV light is only about 5% of sunlight. For this reason, much research has been done over the past few decades to increase the number of usable wavelength bands. Among these methods, carbon and nitrogen loading, doping, and shape-changing are known as promising methods.
As a carbon source, carbon quantum dots (CQDs) have recently attracted attention as a new carbon nanomaterial because of their unique properties. Muhammad Shafique et al. showed that the combination of TiO2 and CQDs broadens the optical absorption edge and allows degradation under visible light. 6 In addition, SiO2@TiO2 has been studied in various fields of particle shape change. One of the reasons why SiO2@TiO2 is attracting attention is that the wavelength band that can be reflected is tuned by the particle size. To reflect visible light (about 380~780nm), a particle size of about 200nm~400nm is appropriate. 7
Therefore, this study evaluated SiO2@TiO2 supported by CQD for more efficient removal of organic contaminants. Previous studies of TiO2/CQDs have shown that visible light responsivity is improved by loading CQDs onto TiO2. To utilize visible light more efficiently, TiO2 was changed to SiO2@TiO2. The SiO2@TiO2 used was adjusted to a particle size of 200 nm to 350 nm for efficient reflection and utilization of visible light. The results showed that the photocatalytic activity of SiO2@TiO2/CQDs was enhanced, suggesting that SiO2@TiO2/CQDs can remove organic pollutants that cause water pollution more efficiently compared to SiO2@TiO2.
2. Methods2.1. CQD fabrication
Put 0.720 g of urea (FUJIFILM Wako) in 20 ml of purified water and stir for 1 hour. Let this solution be Solution A. To this solution A, add 0.768 g of citric acid (FUJIFILM Wako) and stir for 1 hour. This solution is designated as Solution B. Solution B was transferred to a Teflon solution, and hydrothermal synthesis was performed at 200° for 6 hours. Impurities were then removed by filtration. The solution was stored in a refrigerator.
2.2. SiO<sub>2</sub>@TiO<sub>2</sub>/CQD
A two-step procedure was performed:
2.2.1. SiO<sub>2</sub>
Here is a detailed explanation of how to make SiO2 shown in Figure 1.
The detailed preparation of SiO2 is shown in the upper part of Figure 1 was carried out by adding 1.2 ml octadecyltrimethoxysilane (Tokyo Kasei Kogyo) and 4 ml TEOS (Fujifilm Wako) to 80 ml ethanol and stirring for 10 minutes. Next, 12 ml of ammonia was slowly added dropwise and stirred for 24 hours to form SiO2. The white precipitate was then washed several times with ethanol and dried.
2.2.2. SiO<sub>2</sub>@T iO<sub>2</sub>/CQD
This section details the preparation of the SiO2@T iO2/CQD shown in Figure 1.
In the detailed preparation of SiO2@TiO2/CQD shown in the lower part of Figure 1, 800 µl of TBOT (Fujifilm Wako) and SiO2 produced in step 1 were added to 80 ml of ethanol. The mixed solution was completely dispersed by ultrasound. In previous studies, 1.6 ml of purified water was slowly added dropwise. However, in this study, purified water was replaced by CQD solution and 1 ml, 1.6 ml and 3.2 ml of CQD solution were slowly dropped into the mixed solution and stirred for 6 hours. The yellow precipitate was then washed several times with ethanol and dried. The dried powder was annealed at 900° for 2 hours.
How to create an STC (source: by authors)
Image
Translated with DeepL.com (free version). For SiO2@TiO2, the CQD solution was changed to purified water, and the same procedure was performed.
Particle Name
Called name
Solution amount of CQD
ST
0ml
STC(1)
1ml
STC(2)
1.6ml
STC(3)
3.2ml
2.3. Photocatalytic evaluation methods
The photocatalytic activity was evaluated using model pollutants. Methylene blue (MB) (Fujifilm Wako) was used as the model contaminant. A 100 W LED floodlight was also used as the light source. UV-cut film (Azwan 2-9154-01 < 400 nm) was attached to the light source and used as a visible light source. The space from the light source to the decomposed material was fixed at 30 cm.
50 mg of the powder was mixed with 20 ml of MB solution, and the porcelain was stirred. The mixed solution was stirred for 1 hour in the dark to obtain sorption equilibrium. Then, they were irradiated with light. The mixed solution was sampled once every 30 minutes and centrifuged to remove the photocatalyst.
3. Experimental results3.1. XRD
Comparing Figure 2 (a) and (b) shows that they are similar. This is thought to be due to the low amount of CQDs held, which is why CQDs are not detected. Analysis of the diffraction peaks from Figure 2(a) and (b) shows that both are TiO2 in the anatase phase and that SiO2 is also retained. These results indicate that incorporating CQD into the ST fabrication process does not change the crystal structure of the product.
(a) The XRD data of ST and (b) the XRD data of STC. (source: by authors)
Image3.2. SEM
Figure 3 (a) is an SEM image of ST. The SEM data in (a) shows that this is a mesoporous ST and spherical. The particle size is also found to be in the range of 250 nm to 400 nm. Figure 3 (b) shows the SEM data of STC. The SEM data in (b) shows that it is also spherical. The size is also observed to be similar, ranging from 250 nm to 400 nm. However, when comparing (a) and (b), the surface of (b) is more uneven than that of (a), and it is thought that a higher surface area is acquired. This high surface area is believed to facilitate dye adsorption. In addition, the irregularity of the surface may cause incident light to enter the TiO2 surface from different angles, scattering light in various directions.
Light Absorption Rate (source: by authors)
Image3.3. Light Absorption Rate
Figure 4 shows the optical absorption coefficient. This shows that STC absorbs lighter than ST. Also, the difference in optical absorption between ST and STC increases from about 380 nm. These are thought to be influenced by CQD. The incorporation of CQD in the fabrication process improves the photo response in the ultraviolet region by increasing the surface area, and the visible light response is thought to be enhanced by the heterojunction of CQD and ST. It is also thought that making the surface uneven may lengthen the light path and allow more efficient use of light.
Light Absorption Rate(source: by authors)
Image3.4. Evaluation of photocatalytic activity
Figure 5 (a) shows a methylene blue adsorption experiment. This shows that the adsorption force varies with the amount of CQD. STC(2) is about 70% adsorbed. Compared to the conventional ST, the adsorption volume has been improved by approximately 60%. This is the same result confirmed in Figure 3. This indicates that particles with enhanced surface area can be produced by incorporating CQDs into ST. It is generally believed that the larger the surface area, the more active sites and the higher the photocatalytic activity. 8
Figure 5 (b) shows a decomposition experiment. This degradation experiment was performed under visible light. Since TiO2 generally acts only at wavelengths in the ultraviolet region, the decomposition rate of ST is 0%. In comparison, STC(1), STC(2), and STC(3) all show progressive degradation. This indicates that the synthesis of ST and CQD could create visible light-responsive photocatalysts. STC(2) is degraded by about 50%. This indicates that, as in the adsorption experiment, the photocatalytic activity varies significantly with the amount of CQD. This is thought to be due to the fact that the larger the surface area, the more active sites and the higher the photocatalytic activity, as well as the effect of heterojunction.
(a) Adsorption experiment. (b) The degradation of organic matter(source: by authors)
Image4. Discussion and conclusions
STC showed high adsorption to MB solution and high degradation under visible light. This would be attributed to the establishment of a hetero-bonded structure with the support of CQD and ST. 6 This has been shown to be useful in removing organic contaminants. In the degradation of organic matter from residual dyes, STC is expected to be a particle that combines the advantages of both the physical treatment method, which is a good adsorbent for the fast processing of organic matter, and the chemical treatment method, which is a high degradation power that prevents secondary damage. However, in terms of adsorption power, it is lower than that of smoked charcoal, etc. Therefore, it is essential to study photocatalysts with higher adsorption power.
5. Recommendation
This study has shown that the addition of CQD to the SO2@TiO2 preparation process improves the surface area, which is a drawback of SiO2@TiO2, and amplifies the available wavelength range, which is a drawback of TiO2. On this basis, future research should study and optimise the light reflectance and absorptance of SiO2@TiO2/CQDs so far. This needs to be investigated for further applications, as SIO2@TiO2 has so far been used as a scattering layer in solar cells.
Acknowledgment
The abstract of this paper was presented at the Future Smart Cities (FSC) Conference – 7th Edition which was held on the 14th - 16th of October 2024.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sector/ individuals.
Ethics Approval
Not applicable.
Conflict of Interest
The authors declare there is no conflict.
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