C₆₀- and CdS-Co-Modified Nano-Titanium Dioxide for Highly Efficient Photocatalysis and Hydrogen Production

Abstract

The inherent properties of TiO₂, including a wide band gap and restricted spectral response range, hinder its commercial application and its ability to harness only 2–3% of solar energy. To address these challenges and unlock TiO₂’s full potential in photocatalysis, C₆₀- and CdS-co-modified nano-titanium dioxide has been adopted in this work to reduce the band gap, extend the absorption wavelength, and control photogenerated carrier recombination, thereby enhancing TiO₂’s light-energy-harnessing capabilities and hydrogen evolution capacity. Using the sol-gel method, we successfully synthesized CdS-C₆₀/TiO₂ composite nanomaterials, harnessing the unique strengths of CdS and C₆₀. The results showed a remarkable average yield of 34.025 μmol/h for TiO₂ co-modified with CdS and C₆₀, representing a substantial 17-fold increase compared to pure CdS. Simultaneously, the average hydrogen generation of C₆₀-modified CdS surged to 5.648 μmol/h, a notable two-fold improvement over pure CdS. This work opens up a new avenue for the substantial improvement of both the photocatalytic degradation efficiency and hydrogen evolution capacity, offering promise of a brighter future in photocatalysis research.

1. Introduction

In recent years, photocatalytic technology has garnered significant attention from researchers owing to its cost-effectiveness, simplicity, and minimal environmental impact. This versatile technology leverages a range of photocatalysts to disintegrate a wide array of both organic and inorganic pollutants, offering the promise of converting CO₂ into valuable renewable chemicals and facilitating water decomposition [1,2,3]. Among these photocatalysts, titanium dioxide (TiO₂) stands out for its relatively high activity, rendering it highly effective in the purification of air and water, even in biomedical applications [4,5]. Consequently, its applications in the field of photocatalysis hold considerable promise [6,7].

Nevertheless, the inherent limitations of TiO₂ are its wide band gap and restricted spectral response range, which result in its ability to utilize only approximately 2–3% of solar energy and hinder its commercial application [8]. To address these challenges and unlock the full potential of TiO₂ in photocatalysis, modification techniques have emerged as a pivotal approach to enhancing its photocatalytic performance. These techniques aim to reduce the band gap, extend the absorption wavelength, and control photogenerated carrier recombination, thereby enhancing TiO₂’s light-energy-harnessing capabilities [9]. Notably, the choice of dopants significantly influences the photocatalytic activity, affecting the recombination of photogenerated electrons (e⁻) and holes (h⁺) on the material’s surface.

In the field of photoelectrochemical water decomposition, nano-TiO₂ coupling systems have been extensively researched [10,11]. Cadmium sulfide (CdS), with its narrow energy gap of 2.4 eV, has proved effective as a coupling agent for TiO₂, promoting the photocatalytic decomposition of water [12,13]. The strong coupling between TiO₂ and CdS allows for the direct combination of TiO₂ particles with dispersed CdS particles. Although CdS exhibits photoetching instability when used independently, the formation of composite nanomaterials with TiO₂, characterized by a wider band gap, effectively reduces the likelihood of photogenerated electron–hole recombination. Thus, the strategic coupling of TiO₂ with CdS mitigates the photoetching instability associated with CdS alone and expands the spectral response range [14,15,16].

Fullerene (C₆₀), renowned for its exceptional electron (e⁻) transport capabilities, has emerged as an intriguing option for mitigating charge recombination. This has fueled increasing interest in TiO₂ catalysts supported by C₆₀ [17,18]. C₆₀ boasts an extensive three-dimensional π electron system, featuring a closed core–shell structure comprising 60 delocalized π electrons and 30 bonded molecular orbitals. Even minor structural variations or changes in the solvent properties can influence the electron transfer within the system, making the incorporation of C₆₀ an attractive strategy for enhancing effective electron transfer [19,20]. With a band gap ranging from 1.6 to 1.9 eV, C₆₀ holds considerable promise for practical applications, and catalysts modified with C₆₀ are poised to achieve a remarkable photocatalytic performance.

Recognizing the complementary advantages of CdS and C₆₀, we have adopted a comprehensive approach to co-modifying TiO₂. Utilizing the sol-gel method, we have successfully synthesized CdS-C₆₀/TiO₂ composite nanomaterials with highly effective photocatalytic properties. Remarkably, the hydrogen production of CdS modified with C₆₀ exhibited a two-fold increase compared to pure CdS, reaching an average of 5.648 μmol/h. Furthermore, TiO₂ co-modified with CdS and C₆₀ demonstrated an impressive average yield of 34.025 μmol/h, representing a staggering 17-fold increase over pure CdS. This study underscores the synergistic benefits of combining CdS and C₆₀ in enhancing the photocatalytic and hydrogen production performance of TiO₂, opening up new avenues for sustainable and efficient environmental remediation processes.

2. Materials and Methods

Materials: The cadmium chloride (CdCl₂, 99.0%) and sodium sulfide (Na₂S·9H₂O) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). The Malachite Green (MG), P25 (Degussa, Hamburg, Germany), and butyl titanate (Ti(OBu)₄) were all purchased from China National Pharmaceutical Group Chemical Reagent Co., Ltd. (Beijing, China). The C₆₀ was purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). All the other reagents were analytical-grade. The pH of the solution was adjusted using nitric acid and sodium hydroxide solutions. All experiments used secondary water.

Synthesis of CdS: Dissolve 3.1148 g of CdCl₂·2.5H₂O in 20 mL of ethanol in the first bottle, and dissolve 2.4004 g of Na₂S·9H₂O in 20 mL of ethanol in the second bottle. After stirring for 10 min, the second bottle is added to the first bottle slowly, drop by drop. Gel A is stirred for two hours at room temperature before being transferred into an alumina crucible. Place the sample into a muffle furnace and set the conditions to increase from room temperature to 450 °C, with a heating rate of 2 °C/min in an air atmosphere, and maintain it there for 2 h after aging at room temperature for 24 h. The calcination conditions are the same for each sample below. Finally, create a sample of CdS powder by grinding the acquired sample in an agate mortar [21,22,23].

Synthesis of CdS-TiO₂: Make gel A as directed by the conditions listed above. In a flask with a circular bottom and labeled “gel B”, combine 20 mL of butyl titanate (TBT) with 30 mL of ethanol solution. The combination added dropwise is stirred at room temperature for 2 h. Prepare another combination using 20 mL of ethanol, 4 mL of distilled water, 2 mL of nitric acid, and 20 mL of gel A. They should be added dropwise to the room-temperature gel B solution, stirred as they are added, and left to react for 24 h. Age for 24 h in an alumina crucible. The sample is burned under the same conditions in a muffle furnace and then ground to produce a CdS-TiO₂ sample.

Synthesis of CdS-C₆₀: In order to create CdS, 30 mg of Ox-C₆₀ (also known as activated C₆₀) is added to a combination of distilled water and agitated for 2 h. An alumina crucible is used to age gel C for 24 h. Then, under the same conditions, it is calcined in a muffle furnace. The CdS-C₆₀ sample is obtained by grinding it after cooling to room temperature.

Synthesis of CdS-C₆₀-TiO₂: Synthesize gel B and gel C separately, measure 20 mL of ethanol and 4 mL of distilled water, and combine 2 mL of nitric acid and gel C to put together another mixture. The combination dropwise is added to the gel B solution at room temperature, stir whilst adding, and react for 24 h. Age it in an alumina crucible for 24 h and burn it in a muffle furnace underneath the identical conditions. Grind to achieve the CdS-C₆₀-TiO₂ sample. The experimental characterization is detailed in the Supporting Information (pages 2–4).

3. Results and Discussion

To explore the changes in the TiO₂ crystal morphology after the modification, we conducted X-ray diffraction (XRD) measurements for the pure TiO₂ and the modified TiO₂ materials in Figure 1a. As can be seen, the diffraction peaks of P25 (commercial TiO₂) appear at 2θ = 25.08°, 37.94°, 48.14°, 54.26°, 55.26°, 62.90°, 68.98°, 70.02°, and 75.24°, respectively, corresponding to the diffraction peaks of anatase-phase TiO₂ (101), (004), (200), (105), (211), (204), (116), (200), and (215), respectively, where 2θ = 25.08° was the strongest diffraction peak. The diffraction peaks of the rutile phase (110), (101), and (200) crystal faces appear at 2θ = 27.7°, 36.2°, and 42.82°, indicating that TiO₂ is a mixed crystal structure dominated by anatase phases and rutile phases [24]. Comparing P25, CdS-TiO₂, and CdS-C₆₀/TiO₂, the rutile diffraction peak weakened in turn, and the anatase-phase diffraction peak increased in turn, indicating that the introduction of CdS into the samples can inhibit the growth of their rutile phase. As we all know, rutile TiO₂, as the most stable crystalline structure form, has a good crystallization state and fewer defects, resulting in electrons and holes that are easy to compound, with almost no photocatalytic activity. Therefore, the CdS-C₆₀/TiO₂ composite with less rutile TiO₂ will have a better catalytic performance. We also tested the XPS patterns of P25, CdS-TiO₂, and CdS-C₆₀/TiO₂, as shown in Figure S1 (Supporting Information). It was proven that there is no Ti2p separation between the C₆₀, CdS, and TiO₂ phases after preparation, but the shift in the oxygen atom toward a high binding energy means that the chemical bonds between the oxygen atom and other elements become tighter and more stable, which indicates that CdS-C₆₀ has a significant effect on the modification of TiO₂, which promotes the transfer of electrons between oxygen atoms and other elements, thereby increasing the activity of the catalyst.

Not only that but most UV radiation with a wavelength of less than 400 nm is absorbed by P25. The absorption bands of CdS, CdS-C₆₀, CdS-TiO₂, and CdS-C₆₀/TiO₂ all entered the visible light region. This is mainly because the band gap of CdS is 2.4 eV, the response characteristics of visible light are better, and the crystal structure of the original substance is changed after being modified with CdS. Compared with pure CdS and pure TiO₂, the absorbance of CdS-C₆₀ and CdS-C₆₀/TiO₂ increased, showing that the band gaps of various semiconductor catalysts overlapped in the ultraviolet and visible regions, respectively, which is conducive to improving their photocatalytic performance. The C₆₀-modified sample’s absorption band edge shows some redshift, indicating that C₆₀ can narrow the catalyst’s band gap, which is due to the interaction between the components, causing surface lattice oxygen vacancy and other defects [25,26,27]. The results above indicate that the crystal structure of TiO₂ is alterable by CdS and C₆₀, leading to the creation of defects on the surface lattice. The cause of the edge absorption is the defect absorption rather than the actual absorption of the lattice [28]. Additionally, the band gap energy of TiO₂ is decreased, enabling it to be activated by visible light and to exhibit photocatalytic activity. These changes not only enhance the utilization of light energy but also improve the overall efficiency of the photocatalysis process. Meanwhile, EDX fluorescence spectroscopy analyses on each sample were performed to detect the type and content of the characteristic elements contained in each sample, as shown in Figure 1c–e.

As the FT-IR spectra show in Figure 2a,b, all the catalyst samples have strong absorption peaks at 3420–3510 cm⁻¹ and 1620–1640 cm⁻¹, corresponding to the bending vibration peaks of the H-O-H bonds, which indicates the bending vibration of the presence of numerous water molecules adsorbed onto the catalyst surface. These peaks confirm that water molecules adhere to the catalyst surface in a physical manner. Consequently, it can be concluded that these nanomaterials possess the ability to exhibit a significant photocatalytic effect, which may be the presence of the majority of hydroxyl groups on the surface, and under photoexcitation, the e⁻-h⁺ pair generated by the catalyst sample acts on the hydroxyl group to generate highly oxidizing ·OH. The abundant and diverse hydroxyl groups present on the surface also promote the creation of sites where O₂, CO₂, and CO molecules can be adsorbed [29]. The absorption peak at 1380 cm⁻¹ of the spectrum indicates that O₂ molecules are adsorbed on the surface. Whether it is the formation of highly oxidizing free radicals or the provision of adsorption sites for small molecules, this facilitates the oxidation of organic material into a variety of small molecule compounds via photocatalysis. In comparison to P25, the absorption peaks of CdS-TiO₂ and CdS-C₆₀/TiO₂ shift toward higher frequencies. The peak at 450–510 cm⁻¹ signifies the presence of titanate TiO₃²⁻, indicating that the transition of TiO₂ involves lattice distortion [30]. It can be deduced that the periodic potential field distortion generated by the modification of CdS and C₆₀ caused lattice defects in the TiO₂. The TiO₂ band gap decreases, leading to an enhanced ability to catalyze visible light. Although the sample contains a small amount of titanate TiO₃²⁻, the photocatalytic activity can be effectively improved by introducing more crystal defects [31]. The band of CdS-C₆₀-TiO₂ changes the lattice to produce some characteristic peaks at 1380 cm⁻¹, which further indicates that CdS, C₆₀, and TiO₂ are bonded, changing the original lattice structure of the TiO₂. It may be inferred from band analyses of CdS, CdS-C₆₀, CdS-TiO₂, and CdS-C₆₀/TiO₂ that the same absorption peak, which may represent a Cd-S bond vibration, is present in all of these materials.

Since the photocatalytic activity of the functionalized materials is largely dependent on their morphological structure, it can be found that P25 (Figure S2a,b, Supporting Information) is a solid microsphere. However, after being modified by CdS, the particles become less uniform, and as seen using XRD, there is an interaction between CdS and TiO₂, making its crystal lattice larger. CdS and C₆₀ uniformly cover the surface of TiO₂ in the form of lumps and microspheres, and the aggregation of small particles aggravates the surface roughness (Figure S2c–j, Supporting Information), as is consistent with the results of SEM. By comparing the TEM of the samples CdS-TiO₂ (Figure S3a, Supporting Information) and CdS-C₆₀/TiO₂ (Figure S3b, Supporting Information), we see the crystal plane of the CdS-C₆₀/TiO₂ composite is clearer and regular, the crystal form is more perfect, and the lattice fringes of TiO₂ and CdS can be seen clearly. This indicates that the introduction of C₆₀ changed the lattice structure of TiO₂, making the crystal form better. In addition, the connection between the suitable specific surface area and the variables affects the sample’s photocatalytic performance. We calculated the pore size and pore distribution of each sample using the Barrett–Joyner–Halenda technique and the Halsey equation, as shown in Table S1 (Supporting Information) [32,33,34]. From the nitrogen adsorption data of the six catalyst samples, when compared to CdS, the specific surface area of CdS-TiO₂ has increased in comparison to commercial P25 and CdS-C₆₀, but the rate of the change in the pore size is much greater than the change in the specific surface area. This may be a result of the dopant entering the lattice and causing the pore size to decrease, conducive to producing lattice defects in the crystal structure. This improves the photocatalytic activity [35]. As a product of CdS and C₆₀ co-modification, in CdS-C₆₀/TiO₂, it is hypothesized that the factors influencing the photocatalytic performance of the sample are related to an appropriate specific surface area and a certain degree of lattice defects. As the specific surface area increases, so does the number of active sites, leading to the higher adsorption performance of the catalyst. Conversely, reducing the specific surface area leads to a decrease in both the pore volume and pore size. The presence of certain lattice defects proves beneficial in enhancing the light-harnessing effect of TiO₂ [36].

By assessing the rate at which MG degrades under 120 min of visible light irradiation, the CdS- and C₆₀-co-modified TiO₂ samples can be evaluated for their visible light catalytic activity. They were put through photocatalytic degradation tests with commercial P25 under identical circumstances to assess their photocatalytic activity. The UV absorption spectra of CdS, CdS-C₆₀, CdS-TiO₂, and CdS-C₆₀/TiO₂ are displayed in Figure 3a–d. These spectra demonstrate that the absorbance values decrease with an increasing visible light irradiation period, indicating a significant and quick degradation of MG. To examine the kinetics of MG’s photocatalytic breakdown in more detail, we measured the photocatalytic degradation curves of five catalyst samples, as shown in Figure 3e–f [37]. According to the relevant formula, we calculated the apparent rate constant k_app for the catalytic reactions with different catalysts in Table S1 (Supporting information), and it was indicated that the photocatalytic efficiency of all the samples was significantly higher than that of P25 when exposed to visible light. The order of photocatalytic efficiency was CdS-C₆₀/TiO₂ > CdS-C₆₀ > CdS > CdS-TiO₂ > P25. It can be seen that the composite materials have significant catalytic activity. Compared with the pure materials, the catalytic activity of the C₆₀-modified materials is improved due to C₆₀’s ability to enhance the quantum efficiency and facilitate charge transfer: C₆₀ can enhance the efficiency of separating photogenerated e⁻-h⁺ pairs. Meanwhile, it can also enhance the adsorption efficiency during the degradation process [38,39]. This is also due to the small band gap width (2.4 eV) of CdS, which can induce photocatalysis in the visible light region. CdS can provide excited e⁻ to TiO₂, while water and oxygen are used to generate hydroxyl radicals (·OH) and superoxide anion radicals (·O₂⁻). These free radicals contribute to the photooxidation of the adsorbed organic matrix. The CdS-C₆₀/TiO₂ sample exhibits both the characteristics of C₆₀ and CdS, thus exhibiting the highest photocatalytic activity.

Moreover, we conducted adsorption mechanical analysis on the different catalysts. The pore size and pore distribution can be calculated in Table S2 (Supporting Information). The factors influencing the photocatalytic performance of the sample are connected to the combined impact of an appropriate specific surface area and a certain level of lattice defects, which is beneficial for improving the optical effect of TiO₂. The results of our adsorption-based linear fitting analysis are consistent with the desorption equilibrium data for MG using different catalysts, as shown in Figure S4 (Supporting Information). One can enhance the TiO₂ by increasing its specific surface area and modifying its crystal structure; TiO₂ co-modified with C₆₀ can be expected to improve in its photocatalytic performance; and given sufficient time, CdS-C₆₀ is comparable to CdS-C₆₀-TiO₂ in terms of its catalytic performance.

To further analyze the catalytic performance of each catalyst, we conducted a comparison of the photocatalytic hydrogen production yield over time using 10% lactic acid as a sacrificial reagent. This comparison included various catalysts under visible light conditions, as illustrated in Figure 4a. We tested the average hydrogen production of different catalysts per hour under continuous illumination for 5 h in Figure 4b. As illustrated in Figure 4a,b, the hydrogen production performance of these photocatalysts in order from best to worst is CdS-C₆₀/TiO₂ > CdS-C₆₀ > CdS > CdS-TiO₂ = P25. The visible light hydrogen production effect of pure P25 and CdS-modified TiO₂ is basically zero, and the average hydrogen production of C₆₀-modified CdS is 5.648 μmol/h. The photocatalytic activity of CdS when co-modified with TiO₂ and C₆₀ is approximately twice as high as that of pure CdS, whereas the average yield of TiO₂ co-modified with CdS and C₆₀ is 34.025 μmol/h, which is roughly 17 times higher than that of pure CdS. The narrow band gap of C₆₀ enables the photocatalyst to have a significantly broader absorption band range, thereby enhancing its light absorption capability in the visible light spectrum. C₆₀ has good conductivity and can detach photogenerated e⁻ and promote good separation of the photogenerated e⁻-h⁺, so the visible light hydrogen production performance in CdS can be enhanced through modification with C₆₀. Furthermore, both CdS and C₆₀ demonstrate significant light absorption capabilities within the visible spectrum, and C₆₀ as an electronic relay can promote the flow of CdS and TiO₂ photogenerated e⁻. C₆₀ can be used as an active site for photocatalytic hydrogen production, specifically the catalytic cleavage of water to produce H₂. This indicates that TiO₂ co-modified with CdS and C₆₀ has a high photocatalytic activity under visible light conditions.

A typical impedance equivalent plot is shown in Figure S4a (Supporting Information) to demonstrate the impact of the charge transfer on the activity of catalysts. The EIS Nyquist plots of different catalysts are shown in Figure S5b (Supporting Information). In the Nyquist plots of EIS, the semicircle’s diameter shows that CdS, CdS-C₆₀, CdS-TiO₂, and CdS-C₆₀/TiO₂ have a lower charge transfer resistance compared to pure TiO₂. The order of decreasing resistance values is R(A) < R(B) < R(C) < R(D) < R(E) < R(F). It can speed up the charge separation and promote e⁻ transfer. This helps to decrease the likelihood of recombining e⁻-h⁺ pairs and enhances the photocatalytic activity [40,41,42]. From the results of electrochemical AC impedance, we see the effective charge transfer rate of CdS is weaker than that of CdS-TiO₂, but in the photodegradation experiment, the photodegradation rate is higher than that of CdS-TiO₂. The possible reasons for this are that CdS can provide excitation of e⁻ to TiO₂; within 18 h after the modification of the empty gold sheet, the pure CdS is unstable on its own, and it may have developed photocorrosion, which, in turn, reduced the conductivity.

To illustrate the photodegradation mechanism of the catalyst CdS-C₆₀-TiO₂, we give a schematic diagram of the corresponding photodegradation mechanism, as shown in Figure 5. On the one hand, C₆₀ has a good electron transport performance, which can accelerate the photoinduced charge separation and slow down charge recombination. Under natural light irradiation, the valence electrons (e⁻) in TiO₂ are excited to the conduction band (CB), and holes (h⁺) are generated in the valence band (VB). However, these photogenerated electrons–holes can recombine quickly in the photocatalytic reaction, but since C₆₀ is a good electron acceptor and can receive electrons from the titanium dioxide, the lifetime of the photogenerated e⁻ and h⁺ will be extended during the transfer process. The photogenerated h⁺ can form hydroxyl radicals, with a strong oxidizing activity and OH⁻ adsorbed on the surface of the TiO₂. The photogenerated e⁻ form oxyanion radicals with the oxygen molecules adsorbed on the surface of the TiO₂, which can effectively photolyze organic radicals or water under the action of free radicals. The resulting active substances (•O²⁻ and •OH) have a high degradation activity for MG.

On the other hand, TiO₂ and CdS can transition under illumination simultaneously, with the latter’s conduction band position being higher than the former, so there is a misalignment of the two in the schematic diagram of the mechanism. Generally, when TiO₂ absorbs light with an energy greater than its band gap, the e⁻ in the valence band can be excited and move to the conduction band. This leads to the formation of highly active e⁻ in the conduction band and corresponding h⁺ in the valence band [43]. The photogenerated e⁻ and h⁺ are separated and moved toward the surface of the semiconductor particles due to the presence of an electric field, causing highly active photogenerated e⁻-h⁺ pairs. This indicates that the transition of the electrons from the excited state of CdS to the conduction band of TiO₂ occurs, and then these e⁻ are absorbed by O₂ molecules to form the superoxide anion radical O₂⁻ [44,45,46]. Consequently, the adsorbed organic matrix’s photooxidation characteristics are improved by this shift in vector transfer, to effectively enhance the photooxidation performance of the adsorbed organic matrix. Due to the photogenerated h⁺’s valence potential, which prevents them from oxidizing hydroxyl groups into hydroxyl radicals, CdS photocorrodes to create Cd²⁺. In short, both C₆₀ and CdS components enhance the catalytic effect of TiO₂; therefore, CdS-C₆₀/TiO₂ exhibits a high catalytic activity.

4. Conclusions

In summary, the preparation of CdS- and C₆₀-co-modified TiO₂ photocatalysts via the sol-gel method was followed by their comprehensive characterization using various techniques. Our rigorous investigation into their photocatalytic performance and hydrogen production capacity showcased the exceptional effectiveness of the ternary composite catalyst CdS-C₆₀/TiO₂ in oxidizing the targeted degradants. The results demonstrated the impressive average hydrogen production of 34.025 μmol/h for TiO₂ co-modified with CdS and C₆₀, marking a substantial 17-fold increase when compared to pure CdS. In parallel, the average hydrogen generation for C₆₀-modified CdS surged to 5.648 μmol/h, representing a notable two-fold enhancement over pure CdS. This remarkable performance can be attributed to the collaborative impact of CdS and C₆₀, which displayed synergistic effects when combined with TiO₂. The co-modifying strategy effectively minimized the likelihood of photogenerated electron–hole recombination while simultaneously reducing the material’s band gap. These findings underscore the vast potential of CdS-C₆₀/TiO₂ as a high-performance photocatalytic system, with versatile application fields such as renewable energy and environmental remediation. We have compiled a comprehensive overview of highly efficient catalysts for the degradation of MG (refer to Table S3 in the Supporting Information). Although our designed catalysts may not exhibit an optimal performance, our work offers a practical approach to enhancing the effectiveness of photocatalytic degradation by utilizing novel ternary materials.