Tailoring Electronic Structures via Ce/C Co-Doping and Oxygen Vacancy in TiO₂ Aerogels for Enhanced Solar Fuel Production

Abstract

A targeted modification approach involving the synthesis of Ce/C co-doped TiO₂ aerogels (CeCTi) via a sol–gel method combined with supercritical CO₂ drying and subsequent heat treatment is employed to enhance the photocatalytic CO₂ reduction performance of cost-effective and stable TiO₂ aerogels. The results demonstrate that the CeCTi exhibits a pearl-like porous network structure, an optical band gap of 2.90 eV, and a maximum specific surface area of 188.81 m²/g. The black aerogel sample shows an enhanced light absorption capability resulting from the Ce/C co-doping, which is attributed to the formation of oxygen vacancies. Under simulated sunlight irradiation, the production rates of CH₄ and CO reach 27.06 and 97.11 μmol g⁻¹ h⁻¹ without any co-catalysts or sacrificial agents, respectively, which are 82.0 and 5.7 times higher than those of the pristine TiO₂ aerogel. DFT reveals that C-doping facilitates the formation of oxygen vacancies, which introduces defect states within the calculational band gap of TiO₂. The proposed photocatalytic mechanism involves the light-induced excitation of electrons from the valence band to the conduction band, their trapping by oxygen vacancies to prolong the charge carrier lifetime, and their subsequent transfer to adsorbed CO₂ molecules, thereby enabling efficient CO₂ reduction, which is experimentally supported by photoluminescence measurements.

1. Introduction

In recent years, the expanding scope of industrial activities has precipitated a dual crisis of escalating carbon emissions and energy shortages, where the excessive release of CO₂ into the atmosphere not only drives climate change but also signifies a substantial waste of carbon resources [1]. Further, designing photocatalysts with CO₂ reduction performance is of great significance for promoting industrial progress and reducing carbon emissions. Researchers have developed a variety of photocatalysts with different energy band structures, such as TiO₂ [2], WO₃ [3], GaN [4], C₃N₄ [5], and BiVO₄ [6], as well as ceramic catalysts fabricated via 3D printing [7]. For a long time, TiO₂ has been very promising in the field of photocatalytic CO₂ reduction [8,9] due to its advantages such as low cost, non-toxicity, stable physicochemical properties, and abundant reserves. Lundberg et al. [10] systematically investigated the robust nanoparticle catalysts, including TiO₂, for CO₂ photoreduction, and revealed a universal volcano-plot relationship between electron transfer kinetics and catalytic performance. Xu et al. [11] constructed a 2D porphyrin MOF–TiO₂ hybrid photocatalyst through in situ growth, achieving 94.1% selective CO₂-to-CO conversion via interfacial Ti-O-N bonds that enhance charge separation and CO₂ activation. Despite its potential, TiO₂ suffers from a wide bandgap, easy recombination of photogenerated electron–hole pairs, poor light capture ability, and insufficient active sites, necessitating modifications for improved efficiency [12,13].

An ideal photocatalyst should feature an appropriately tuned bandgap for visible-light absorption, high optical activity with efficient charge separation, and a three-dimensional porous structure with a large specific surface area to maximize active site exposure [14,15]. The aerogels, which are porous materials composed of low-density, high-specific-surface-area, and high-porosity nanoparticles, have been widely used in chemical sensors, catalytic barriers, thermal insulation, and adsorbents [16,17,18,19]. The high specific surface area of aerogels provides abundant reactive sites on the photocatalyst surface, while the continuous porous 3D nano-network skeleton enhances the light-harvesting efficiency through multiple reflections [20,21]. These advantages make aerogels one of the most promising photocatalysts for CO₂ reduction. Li et al. [22] developed cyanamide-functionalized crystalline carbon nitride aerogels via a molten-salt self-assembly approach, achieving an excellent CO₂-to-CO conversion rate of 25.7 μmol g⁻¹ h⁻¹ due to the cyanamide groups acting as active sites to lower the energy barrier for *COOH formation. Saure et al. [23] utilized the high porosity of hybrid aerogel materials fabricated from hollow silica microtubes and trace amounts of reduced graphene oxide to regulate the light scattering and absorption properties, achieving an enhanced volumetric photothermal response. These structural and functional advantages make aerogel-based photocatalysts a key factor in promoting the development of solar-driven CO₂ conversion technologies. However, the fabrication process is complex, often requiring extended reaction times and post-treatment, which creates formidable barriers for large-scale manufacturing.

Concurrently, doping engineering with elements such as cerium or carbon has been widely employed to modify the wide band gap of TiO₂, effectively enabling its visible-light photocatalytic activity [24,25]. The Ce-doping boosts photocatalytic performance through their unique redox properties, oxygen vacancy (O_V) generation, and band structure modulation capabilities. Specifically, the incorporation of Ce element introduces an impurity energy level near the Fermi level, which effectively suppresses the electron–hole pair recombination and extends the optical absorption range from ultraviolet to visible light [26]. Li et al. [27] proposed S-scheme Ni₂P@ Ce-BDC-CeO₂ hollow octahedrons with hierarchical pores and O_V, where Ce-doping enhanced the charge separation and CO₂ adsorption, achieving a 61.6 μmol g⁻¹ h⁻¹ CO production rate. Zhang et al. [28] synthesized a Ce-doped TiO₂ via in situ calcination, achieving a high Li⁺ adsorption capacity of 58.42 mg/g and enhanced selectivity/stability for efficient lithium recovery from brines. Although Ce-doping effectively enhances photocatalysis through O_V generation, the limited CO yield and energy-intensive lactic acid etching process necessitate optimized material design and synthesis for performance breakthroughs. C-doping also serves as a pivotal strategy to boost photocatalytic performance through its unique capability to modify the local electronic structure, induce O_V, and regulate charge carrier dynamics [29]. Wang et al. [30] designed a dual S-scheme SrTiO₃/SrCO₃/C-doped TiO₂ heterojunction via carbon doping, demonstrating enhanced photocatalytic hydrogen production under UV–Vis light compared to individual components. C-doping often limits its efficiency in photocatalysis due to drawbacks such as increased charge recombination centers, insufficient structural stability, and weakened oxidation capability.

However, the systematic understanding of how Ce and C dopants cooperatively function within an aerogel architecture and promote CO₂ photoreduction without any co-catalysts or sacrificial agents remains limited.

Herein, this study successfully synthesized a pearl-like porous Ce/C co-doped TiO₂ aerogel (CeCTi) through a sol–gel process combined with supercritical CO₂ drying and subsequent thermal treatment. The introduction of Ce creates electron traps that prolongs the carrier lifetime and promotes the interface charge transfer. When carbon is co-doped into the lattice, it exhibits strong electronic coupling with Ce, which collectively regulates the spatial distribution and density of O_V. At elevated temperatures, carbon further participates in the partial reduction of the lattice, reacting with lattice oxygen and evolving as CO₂, thereby creating additional O_V and fine-tuning the local defect architecture. As a result, the CeCTi by heat treatment at 450 °C exhibits outstanding photocatalytic CO₂ reduction performance under simulated solar irradiation, with CH₄ and CO production rates reaching 27.06 and 97.11 μmol g⁻¹ h⁻¹, respectively. The DFT calculations further reveal that Ce/C co-doping progressively narrows the calculational band gap of TiO₂ from 1.404 eV to 0.463 eV. This work provides a solid foundation for the rational design of Ce/C co-doped TiO₂-based photocatalysts with enhanced CO₂ reduction performance in future studies.

2. Results and Discussion

2.1. Chemical Composition and Structural Analysis

Figure 1a illustrates the preparation process of the pearl-like porous CeCTi, which is successfully synthesized through a combined sol–gel method involving supercritical CO₂ drying and subsequent thermal treatment. During the sol–gel synthesis, TBOT and Ce(NO₃)₃6H₂O are used as precursors, and the introduction of R, followed by continuous stirring, promotes condensation reactions between the surface hydroxyl groups of the TiO₂ sol and the phenolic hydroxyl groups of R. After aging and solvent exchange, the wet gel is subjected to supercritical CO₂ drying and the obtained aerogel is subsequently thermally treated at 450 °C for 2 h under an Ar atmosphere. During the thermal treatment R undergoes pyrolysis accompanied by C-H bond cleavage, generating carbon that is co-doped into the TiO₂ lattice, and carbon escapes in the form of CO₂ to create O_V. Then, the samples combine synergistically with the O_V induced by Ce³⁺ substitution for Ti⁴⁺ to produce the O_V-CeCTi-450 aerogel, referred to as CeCTi. The XRD patterns of the CeTi, CTi, CeC_0.5Ti, CeC_0.75Ti, and CeC_1.0Ti samples after heat treatment at 450 °C in Figure 1b all show distinct diffraction peaks at 2θ of 25.3°, 37.8°, 47.9°, 54.2°, 55.4°, 62.6°, 70.2°, and 75.1°. The positions of these peaks are in complete agreement with the anatase TiO₂ (PDF NO. 1-562), and can be indexed to the (101), (004), (200), (105), (211), (204), (220), and (215) crystal planes, respectively [31,32,33]. The pattern of CeTi-450 is nearly identical to TiO₂, and the absence of characteristic diffraction peaks for metallic Ce or CeO_x indicates that Ce does not form other phases but is incorporated into the TiO₂ lattice as a dopant [34]. The CTi-450 exhibits a reduction in diffraction peak intensity and a weakening in crystallinity, primarily attributed to the C-doping which alters the gelation kinetics of the precursor and inhibits the formation of an ordered titanium dioxide gel network. Notably, the CeCTi-450 shows a recovered diffraction intensity relative to CTi-450, suggesting that Ce incorporation mitigates the inhibitory effect of carbon on gelation, thereby enhancing the overall structural stability. Increasing C-doping leads to an initial enhancement, followed by a reduction in diffraction peak intensity, indicating that an optimal doping level promotes orderly grain growth and prevents full width at half maximum (FWHM) broadening and intensity attenuation. The thermal treatment of CeC_0.75Ti at different temperatures revealed that the sample achieved an optimal crystalline state at 450 °C, evidenced by a maximum in diffraction intensity and a minimum in FWHM (Figure 1c). It is attributed to the moderate reducing effect of carbon on TiO₂ at 450 °C that enhances structural ordering, whereas, at 550 °C, excessive carbonization leads to lattice distortions and degraded crystal integrity. The Rietveld refinement of the XRD pattern for CeC_0.75Ti-450, showing a TiO₂ phase, achieved a high-quality fit, as reflected by the reliability indices of Rwp = 7.42% and GOF = 0.91 (Figure 1d). The refined unit cell parameters, a = b = 3.791832 Å and c = 9.516982 Å, show a close agreement with those reported for the standard anatase TiO₂ (Table S1). The absence of significant lattice expansion suggests that the actual incorporation of the Ce atom into the structure is limited, which aligns well with the Ce doping concentration. The FT-IR spectra show that the infrared bands of CeTi and TiO₂ are highly overlapping, indicating single Ce-doping did not significantly alter the framework structure of TiO₂ (Figure S1). Compared to CeTi and TiO₂, the band of CTi at 3419 cm⁻¹ is noticeably broadened with enhanced absorbance, which originates from the O-H groups enriched on the surface of carbon. The increased intensity of the band at 1630 cm⁻¹ suggests contributions from both O-H bending vibrations and C=O functional groups, with the appearance of a new C-H stretching band at 1413 cm⁻¹ [35]. The characteristic Ti-O-Ti band at 826 cm⁻¹ remains present in all samples, confirming that the main framework structure of TiO₂ is preserved, which is consistent with the XRD results. As shown in Figure S2a, the CeTi exhibit a typical porous network structure, with pores exceeding 100 nm attributed to the incomplete phase separation caused by rapid gelation. In the case of the CTi (Figure S2b), the addition of R influences the hydrolysis and condensation processes of TiO₂, leading to reduced resistance to interfacial tension, pore collapse, and overall densification. Although all samples in the CeCTi system maintain a complete three-dimensional porous nanostructure without pore channel collapse, the variation in carbon content significantly modulates their microstructure. Moderate carbon doping in CeC_0.5Ti leads to a microstructure dominated by mesoporous material, alongside a minor fraction of macropores (Figure 1e), as a result of the insufficient carbon amount to fully fill the interstices of the TiO₂ framework. In contrast, in CeC_1.0Ti, the excessively high carbon content disrupts the continuity of the TiO₂ network, leading to obvious particle agglomeration and a non-uniform pore distribution (Figure 1f). Notably, as shown in Figure 1g, the uniform “pearl-like” porous structure of CeC_0.75Ti indicates a significant effect between Ce and C elements that optimizes the mesostructured material. Ce modulates the hydrolysis-condensation kinetics of the precursor, slowing the reaction rate and suppressing excessive agglomeration, while carbon enhances the mechanical strength of the network through the formation of Ti-O-C bonds. The aerogel possesses tightly interconnected particles, an absence of agglomeration, and an ideal pore distribution for high-density active sites. Furthermore, the EDS mapping in Figure S4 shows a homogeneous distribution of Ce, Ti, O, and C elements throughout the selected area. However, the signal intensity and distribution density of Ce and C elements are lower than those of Ti and O elements, indicating that those are uniformly distributed within the TiO₂ aerogel in a low-concentration doping state.

2.2. Multiscale Structure Analysis

The TEM images of the CeC_0.75Ti-450 reveal a uniformly dispersed nanostructure composed of spherical particles (10–20 nm) that form pores of 50–70 nm, the morphological characteristics consistent with the SEM results (Figure 2a and Figure S5). As shown in Figure 2b, the distinct polycrystalline diffraction rings in the SAED pattern are assigned to the (101), (103), and (004) crystal planes of anatase TiO₂. The HRTEM reveals distinct lattice fringes with measured spacings of 0.35 and 0.23 nm, corresponding to the (101) and (103) planes of anatase TiO₂ [36], respectively, which further confirms the formation of the anatase phase (Figure 2c). As shown in Figure 2d, all adsorption isotherms exhibit mixed type II and IV characteristics, indicative of mesoporous materials, with a distinct H3-type hysteresis loop observed in the relative pressure (P/P₀) range of 0.8–1.0 [37]. The pore size distribution curves in Figure 2e further reveal a bimodal mesoporous structure for all samples, with relatively consistent distributions centered primarily around 7 and 40 nm, aligning with the mesoporous characteristics in Figure 2d. Among all samples, the CTi shows the lowest peak intensity at 7 nm and the highest around 30 nm, indicating that its pore structure is dominated by mesopores of about 30 nm. In contrast, the pore size distributions of the CeCTi overall shift towards larger pore sizes, exhibiting a weak peak near 80 nm; yet, the pores in the 40–50 nm range remain predominant. Notably, the main peak near 30 nm for CeC_0.75Ti is significantly narrower than that of other CeCTi samples, suggesting a more concentrated pore size distribution and a better uniformity of the pore structure within this size range, which is consistent with the SEM result. The increased BET surface area of CeCTi with carbon content is due to the carbon decomposition during high-temperature treatment, which releases small organic molecules that diffuse out, creating a more developed pore structure and enhanced porosity (

Table 1. The pore structures of the resulting samples.

Sample	BET Specific Surface Area (m2·g−1)	BJH Adsorption Average Pore Size (nm)	Pore Volume (cm3·g−1)
CTi	191.27	24.04	1.06
CeC0.5Ti	186.54	28.54	1.46
CeC0.75Ti	188.81	26.28	1.29
CeC1.0Ti	195.73	25.07	1.32

). Notably, the BET surface area of CeC_0.75Ti reaches 188.81 m²·g⁻¹, which is significantly higher than those of similar materials reported in the literature [38,39]. The average pore size of CeC_0.75Ti is approximately 26 nm, which indicates that the carbon decomposition process contributes to the formation of smaller pores, which is beneficial for improving the mass transfer efficiency and the accessibility of active sites, thereby promoting the efficient progression of catalytic reactions.

Figure 2f shows that all samples exhibit two distinct peaks at binding energies of approximately 464.5 and 458.8 eV, corresponding to the spin-orbit doublets of Ti 2p₁/₂ and Ti 2p₃/₂, respectively, which are in agreement with the characteristic signatures of Ti⁴⁺ [40] (Table S2). The Ti 2p peaks of the CeTi sample shift towards a lower binding energy compared to the others in the XPS spectrum, indicating an increased electron cloud density around Ti. This phenomenon can be attributed to an electronic interaction between Ce³⁺/Ce⁴⁺ and Ti⁴⁺, leading to a partial electron transfer from Ce to Ti, thereby reducing the apparent binding energy of Ti. In contrast, the binding energy of the Ti 2p core level in CeCTi samples shows no significant shift relative to CeTi, suggesting that the introduction of carbon does not substantially alter the local electron density around the Ti active centers. This observation implies that the electronic modulation induced by the Ce element incorporation likely dominates the local electronic structure of Ti. In the high-resolution C 1s XPS spectra (Figure 2g), both CeTi and TiO₂ samples show peaks at approximately 286.3 eV and 288.6 eV, which are assigned to C-O and C=O bonds, respectively [41]. In the CeCTi samples, the intensity of the C 1s peak at 286.3 eV is significantly enhanced and exhibits a slight shift towards a higher binding energy. This indicates an increased content of C-O bonds, likely resulting from the decomposition of the carbon component during treatment. In the O 1s spectra of Figure 2h, a strong characteristic peak at 530.0 eV for all samples is assigned to the Ti-O bond in the TiO₂ lattice [42], confirming that the Ce/C co-doping does not alter the fundamental crystal structure. Additionally, a shoulder peak typically appears at approximately 531.9 eV, attributed to the surface-OH resulting primarily from surface adsorption after air exposure. As depicted in Figure S6, the characteristic XPS peaks for Ce are notably weak, a phenomenon attributed principally to its low doping level relative to the major constituents of the system. The EPR spectra show an anisotropic signal at g = 2.003 for both CeTi and O_V-CeCTi (Figure 2i), which is characteristic of O_V [42,43]. The signal intensity is higher in CeCTi, indicating that the co-doping of Ce and C promotes the formation of more O_V compared to Ce-doping alone.

2.3. Photoelectrochemical Properties and Reduction Performance

The arc radius in EIS reflects the electrical conductivity of the photocatalyst, with a smaller radius typically indicating superior conductivity, lower resistance, and a higher electron transfer rate [44]. As shown in Figure 3a, under identical testing conditions, a successive decrease in the EIS arc radius is observed from TiO₂ to CeTi, CTi, and CeCTi, demonstrating a progressively lower charge transfer resistance. This trend clearly demonstrates that C-doping is more effective than Ce-doping in enhancing electrical conductivity, while their combination achieves the optimal synergistic effect, substantially improving interfacial charge migration. The photocurrent response measurements in Figure 3b show that the CeCTi sample exhibits the highest photocurrent density, significantly surpassing that of the CeTi sample. The latter, in turn, shows a markedly stronger response than both the CTi and pristine TiO₂ aerogels, which display only marginal differences between them. This trend demonstrates that Ce-doping plays a dominant role in suppressing the charge recombination and extending the carrier lifetime, while the synergistic effect of Ce/C co-doping further enhances the charge migration and separation efficiency, leading to the optimal utilization of photogenerated carriers [45]. Combined with the smaller EIS arc radius of the CeCTi, these findings indicate that Ce/C co-doping boosts the charge carrier concentration while lowering the transport resistance, collectively enhancing photocatalytic activity. Figure 3c shows that all samples exhibit a distinct PL peak at around 460 nm, which originates from the recombination of photogenerated electron–hole pairs that release energy radiatively [46]. The pristine TiO₂ aerogel shows the highest PL intensity, indicating the most severe charge recombination. Notably, the PL intensity of the CeTi sample is higher than that of CTi, suggesting that C-doping contributes more effectively to the initial charge separation. The CeCTi sample exhibits the lowest PL intensity, demonstrating the strongest suppression of recombination, which can be attributed to the effect of Ce/C co-doping. To further investigate this enhanced charge transfer dynamic, time-resolved PL decay measurements were performed on TiO₂ and CeCTi. As shown in Figure 3d, the fluorescence lifetime of TiO₂ is 0.63 ns, while that of CeCTi is prolonged to 0.83 ns. This extension directly reflects the slower carrier recombination in CeCTi, providing clear evidence of the more efficient charge separation. To further investigate the effect of Ce/C co-doping on the charge transfer efficiency, the PL decay curves of TiO₂ and CeCTi were measured and fitted using an exponential decay function. As shown in Figure 3d, the fluorescence lifetime of TiO₂ is 0.63 ns, while that of CeCTi is prolonged to 0.83 ns. The observed extension in the lifetime is a clear signature of the suppressed recombination of photogenerated charge carriers, which, in turn, leads to a significantly improved charge separation efficiency [47]. The UV–Vis spectra in Figure 3e show that CTi and CeCTi clearly exhibit characteristic absorption bands in the range of 300–400 nm, which are attributed to the intrinsic absorption of TiO₂. With an increasing C-doping content, the absorption of the CeCTi composites demonstrates an initial red shift followed by a slight blue shift, a trend driven by the C-doping optimization of the defect distribution in CeTi. Compared to CTi, the CeCTi samples demonstrate significantly enhanced absorbance in the visible light region (400–700 nm), with CeC_0.75Ti showing the highest absorption, indicating its superior visible-light-harvesting capability. The CTi aerogel retains the optical band gap of 3.20 eV in the Tauc plots, consistent with the pristine TiO₂ [48], suggesting that singular C-doping only mildly tunes the electronic structure without significantly altering the optical band gap. In contrast, the optical band gap of the CeCTi series narrows from 3.06 eV to 2.90 eV with an increasing C-doping content, then slightly widens to 3.00 eV at higher C levels. This trend demonstrates that an optimal C-doping concentration, in synergy with Ce, most effectively stabilizes the defect states and extends the visible-light absorption. To further investigate the detailed electronic properties of the CeCTi samples, Mott–Schottky measurements were carried out on its representative CeC_0.75Ti sample, as illustrated in Figure 3f. The Mott–Schottky test performed on the CeC_0.75Ti sample to further investigate the electronic band structure of CeCTi shows that the conduction band (CB) position is −0.81 V vs. SCE. Using conversion Equation (1), the CB potential is calculated to be approximately −0.57 V vs. NHE, and the 2.90 eV band gap yields a valence band (VB) potential of +2.33 V vs. NHE via Equation (2). This band alignment confirms that CeC_0.75Ti possesses sufficient thermodynamic driving force to participate in various photocatalytic redox reactions.

$$E(NHE)=E(SCE)+0.24$$

(1)

$$E_{CB}=E_{VB}-E_g$$

(2)

C-doping significantly enhances the conductivity of the material and helps stabilize the defect structure, while Ce-doping effectively prolongs the carrier lifetime, and promotes the interface charge transfer. Thus, Ce/C co-doping achieves a comprehensive improvement in the photocatalytic CO₂ reduction performance through coordinated band engineering, defect engineering and structural optimization. Based on the Mott–Schottky and UV–Vis analyses, a plausible mechanism for the photocatalytic CO₂ reduction over the CeCTi is proposed. As illustrated in Figure 3g, the CB potential of CeCTi is −0.57 V vs. NHE, which is more negative than the standard redox potentials for CO₂/CO (−0.53 eV) and CO₂/CH₄ (−0.24 eV). Meanwhile, its VB potential (+2.33 V vs. NHE) remains sufficiently positive for water oxidation. This band alignment provides the necessary thermodynamic driving force for both CO₂ reduction and H₂O oxidation. However, the photocatalytic performance is governed not only by the thermodynamic feasibility but also by the charge separation efficiency and surface reaction kinetics. Ce/C co-doping effectively introduces abundant O_V and optimizes the electronic structure of the material. First, they act as electron traps, prolonging the lifetime of photogenerated charge carriers and suppressing the electron–hole recombination. More importantly, they drive a dynamic Ti⁴⁺/Ti³⁺ redox cycle at the catalyst surface. Specifically, the electron-donating property of Ce³⁺ reduces adjacent Ti⁴⁺ ions to Ti³⁺. CO₂ molecules adsorbed on nearby O_V acquire electrons from these Ti³⁺ sites, leading to their activation and initial reduction. The resulting Ti⁴⁺ is then regenerated back to Ti³⁺ by photogenerated electrons captured by O_V, thereby sustaining an efficient electron shuttle that continuously supplies reducing equivalents to the adsorbed CO₂. Furthermore, the three-dimensional porous architecture of the CeCTi aerogel offers a high specific surface area, which exposes abundant active sites and facilitates reactant adsorption and electron transport. Figure 3h,i present the photocatalytic CO₂ reduction performance of the synthesized photocatalysts under full-spectrum light irradiation, demonstrating their promising potential [49]. It should be emphasized that the detected CO and CH₄ are exclusively derived from CO₂, as no external carbon source was introduced in the reaction system. As seen in Figure 3h, after 2 h of illumination, the CH₄ and CO production rates over the TiO₂ aerogel were 0.33 and 17.01 μmol g⁻¹ h⁻¹, respectively. The CeTi shows enhanced rates for both products, while the CTi gave a lower CH₄ yield, indicating that carbon incorporation alone does not facilitate effective light utilization. Overall, the CeC_0.75Ti-450 demonstrated the better performance, achieving CH₄ and CO production rates of 27.06 and 97.11 μmol g⁻¹ h⁻¹, respectively, which are approximately 82.0 and 5.7 times higher than those of the TiO₂ aerogel. Table S3 presents a comparison of various CO₂ photocatalytic materials (including the materials developed in this study) in terms of reaction conditions and yields. Unlike many systems that require precious metals or complex heterogeneous structures, our catalyst achieves a comparable or even superior performance through a simple Ce/C co-doping process. It demonstrates that the electronic structure optimized through Ce/C co-doping is favorable for product conversion. Furthermore, as shown in Figure 3i, the total yields of CH₄ and CO over CeC_0.75Ti-450 reach 69.81 and 376.62 μmol g⁻¹, respectively, after 5 h of simulated sunlight irradiation. Figure S7a shows that, under visible light irradiation, the CeTi and CTi samples both exhibited dramatically higher CH₄ and CO production rates than the TiO₂ aerogel, which registered only 0.41 and 0.13 μmol g⁻¹ h⁻¹, respectively. The CeC_0.75Ti-450 exhibited the best performance, with CH₄ and CO production rates reaching 5.21 and 11.23 μmol g⁻¹ h⁻¹, respectively, representing increases by factors of approximately 12.7 and 86.4 compared to the TiO₂ aerogel. This significant improvement is attributed to the effect of Ce/C co-doping, which optimizes O_V distribution, enhances visible-light absorption through defect-state introduction, and improves the charge carrier separation efficiency. Additionally, when the CeC_0.75Ti-450 sample was irradiated under visible light for 5 h, the yields of CH₄ and CO continued to increase, reaching 20.58 and 37.25 μmol g⁻¹ in Figure S7b. As shown in Figure S7c, the stability of the CeC_0.75Ti-450 catalyst is confirmed through three consecutive recycling tests (2 h per cycle), with no significant decrease in CH₄ and CO production yields observed. The results indicate that, during this test period, the yields of CH₄ and CO do not show a significant decline, suggesting that the catalyst exhibits good catalytic activity retention over this timespan. These current findings can serve as preliminary evidence of the potential stability of the catalyst and provide a basis for a further systematic investigation.

2.4. DFT Calculation

Figure 4a shows that, for TiO₂, the spin-up and spin-down electrons are symmetrically distributed around the Fermi level in the total density of states (TDOS), indicating no net spin and confirming its non-magnetic nature. The density of states (DOS) of the TiO₂ distribution reveals that the CB is primarily contributed by O 2p orbitals, while the VB originates mainly from Ti 3d orbitals. Compared to the pristine TiO₂, O_V-CeCTi introduces an impurity energy level near the Fermi level (Figure 4b). This level can act as a charge carrier trap, which is expected to facilitate more efficient charge separation and potentially enhance visible-light absorption [50]. To elucidate the individual contributions of Ce and C elements, it analyzes the enlarged view of the partial density of states (PDOS) for the Ce and C elements in CeCTi, as presented in Figure 4c. The hybrid peaks near the Fermi level mainly arise from the combined contributions of Ce 3d and C 2p orbitals, with the C 2p orbitals exhibiting a more pronounced density of states. This demonstrates the dominant role of carbon in regulating the computational band, while the Ce element enhances this effect through synergistic doping. Figure 4d shows that the DFT-calculated band gap of the pristine TiO₂ is 1.404 eV, which is significantly lower than the experimental optical band gap of 3.2 eV. This systematic underestimation is a well-known limitation of standard DFT due to its inherent self-interaction error in the exchange–correlation treatment [51]. Therefore, the DFT results presented here are used primarily for analyzing the relative trends in electronic structure modulation, rather than for predicting absolute band gap values. In comparison to the pristine TiO₂, CeTi leads to a slight increase in the calculated band gap to 1.445 eV, consistent with surface state passivation (Figure S8a). Notably, for the CeCTi, the DFT-derived electronic transition threshold drops drastically to 0.102 eV. This extremely low value corresponds not to the intrinsic optical band gap, but to defect-induced in-gap electronic excitations. The latter arise from strong C 2p–lattice hybridization, which generates near-Fermi-level impurity states and elevates the VB maximum (Figure S8). Thus, the 0.102 eV value characterizes the energy required for excitations involving these defect levels, not the fundamental band-to-band transition. Furthermore, introducing O_V into CeCTi causes the transition energy to rebound slightly to 0.463 eV, as localized electrons from the vacancies partially fill the C-doping impurity levels and shift their energy positions. This further confirms that the low-energy electronic transitions in these doped systems are governed by defect-level physics, rather than by the pristine band-to-band excitation. It is important to emphasize that the DFT-calculated transition energies for CeCTi and O_V-CeCTi are much lower than the experimental optical band gap of 2.90 eV obtained from the UV-Vis/Tauc analysis. This discrepancy underscores that the DFT values represent defect-related electronic excitations, while the UV–Vis measurement probes the intrinsic optical band gap of the material. To obtain more quantitatively accurate band gap predictions, advanced computational methods such as DFT + U would be required in future studies. To further investigate the charge transfer behavior in the O_V-CeCTi material, a differential charge density analysis was conducted. According to the results of the sectional plane shown in Figure S9b, distinct blue regions are observed around the Ce atoms, indicating that they lose electrons during the bonding process and act as electron donors (Figure 4g). Meanwhile, red regions directed towards Ti atoms are present between the Ce and Ti atoms, suggesting that electrons migrate from Ce to Ti, with Ti serving as an electron acceptor and undergoing a significant charge redistribution. This phenomenon indicates that the introduced O_V effectively modulates the charge distribution between Ce and Ti atoms, enhancing their interfacial electron interactions. The redistribution subsequently facilitates the separation and migration of photogenerated charge carriers, thereby improving the photocatalytic and electrocatalytic performance of the material. An electron localization analysis reveals that, in the pristine TiO₂, VB electrons are localized on surface O atoms while CB electrons reside on Ti atoms (Figure S9). In contrast, for O_V-CeCTi, the VB electron density concentrates around Ce/C dopants, indicating dominant contributions from C 2p and Ce 3d orbitals to the highest occupied state, whereas CB electrons accumulate around O_V (Figure 4h,i). Upon photoexcitation, electrons migrate from the VB to the CB and are effectively captured by O_V sites, prolonging the carrier lifetime. These trapped electrons are then transferred to the adsorbed CO₂, driving its reduction. This spatial separation between VB and CB suppresses the electron–hole recombination and enhances visible-light absorption, providing a clear electronic-structure basis for understanding the improved photocatalytic activity of CeCTi [52].

3. Conclusions

This study successfully synthesized CeCTi via a sol–gel method combined with supercritical CO₂ drying and subsequent heat treatment. The results show that the O_V-CeCTi-450 exhibits a pearl-like porous network structure, an optical bandgap of 2.90 eV, and a high specific surface area of 188.81 m²/g. The Ce/C co-doping enhances light absorption, with the Ce element prolonging the carrier lifetime and promoting the interface charge transfer, while carbon further contributes to the generation of additional O_V. Under simulated sunlight irradiation, the O_V-CeCTi aerogel achieves CH₄ and CO production rates of 27.06 and 97.11 μmol g⁻¹ h⁻¹ without any co-catalysts or sacrificial agents, representing 82.0- and 5.7-fold improvements over the pristine TiO₂ aerogel. DFT calculations confirm that, under Ce/C co-doping, carbon promotes O_V formation and introduces defect states within the TiO₂ band gap. Upon light irradiation, electrons are photoexcited from the VB to the CB, captured by O_V traps to prolong their lifetime, which prolongs the carrier lifetime as evidenced by the extended fluorescence lifetime from 0.63 ns (TiO₂) to 0.83 ns (CeCTi). This enhanced charge separation effectively facilitates electron delivery to the adsorbed CO₂, driving its photocatalytic reduction and offering insights for the rational design of high-performance photocatalysts.

4. Materials and Methods

4.1. Materials

Tetrabutyl titanate (TBOT, ≥99%) was purchased from Shanghai Alfa Aesar (China) Chemical Co., Ltd. (Shanghai, China). Nitric acid (HNO₃, 69%) and resorcinol (R, 99%) were bought from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Cerium (III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O, 99%) was purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). DI water and anhydrous ethanol (ETOH) were purchased from Nanjing Wanqing Chemical Glass Ware & Instrument Co., Ltd. (Nanjing, China). Ar and CO₂ were bought from Nanjing Changyuan Industrial Gases Co., Ltd. (Nanjing, China). All chemicals were used directly without further purification.

4.2. Method

4.2.1. Synthesis of the CeCTi Aerogels

First, TBOT (4.40 mL, 12.9 mmol) and HNO₃ (0.24 mL, 3.79 mmol) were mixed in EtOH (10.42 mL, 178 mmol) under constant magnetic stirring at 500 rpm for 20 min to achieve complete homogenization. Next, Ce(NO₃)₃·6H₂O (0.19 g, 0.437 mmol) was gradually introduced into a separate EtOH (5.21 mL, 89.2 mmol) and stirred at ambient temperature for an additional 20 min to guarantee thorough dissolution. At the same time, a mixture of R, EtOH, and DI water was continuously stirred for 20 min in another beaker. After mixing all prepared solutions and stirring for 15 min, the homogeneous mixture was transferred into a plastic mold and allowed to undergo wet gel by standing undisturbed at room temperature for 24 h. The gels were subsequently aged by soaking in an ethanol solution for 3 days, with the ethanol being replaced three times daily to ensure proper solvent exchange. Following aging, the wet gels were subjected to supercritical CO₂ drying to obtain CeRTi aerogels. Finally, the aerogels were heat-treated in a tube furnace under an Ar atmosphere at temperatures of 350 °C, 450 °C, and 550 °C for 2 h, corresponding to the CeCTi samples designated as O_V-CeCTi-350, O_V-CeCTi-450, and O_V-CeCTi-550, respectively. It was important to note that the molar ratios of R to TBOT were maintained at 0.5:1, 0.75:1, and 1:1, corresponding to the CeC_xTi samples designated as CeC_0.5Ti, CeC_0.75Ti, and CeC_1.0Ti, respectively. The sample without Ce/C co-doping was named TiO₂, the sample without Ce-doping was named CTi, and the sample without C-doping was named CeTi.

4.2.2. Characterizations

Scanning electron microscopy (SEM, Zeiss Gemini Sigma 300 VP SEM, Carl Zeiss AG, Oberkochen, Germany) and transmission electron microscopy (TEM, JEM 2100F, JEOL, Tokyo, Japan) were utilized to analyze the microstructure and morphology of the samples. Fourier-transform infrared spectroscopy (FT-IR, Thermo Scientific Nicolet iN10 MX, Thermo Fisher Scientific, Waltham, MA, USA) was employed to investigate the functional groups of the samples. The elemental composition of the samples was analyzed using X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, Thermo Fisher Scientific, Waltham, MA, USA) equipped with an Al Kα excitation source (1487.6 eV). The crystalline phase evolution of the samples was examined using X-ray diffraction (XRD, Mini Flex-600 X, Rigaku, Tokyo Akishima, Japan) with a Cu Kα source (λ = 0.154 nm) at a scanning rate of 10°/min. Information regarding the pores was obtained through a brunauer–emmett–teller (BET) analysis using a V-Sorb 2800P surface area and pore size analyzer (Jin Aipu Technology, Beijing, China). O_V was characterized using electron paramagnetic resonance (EPR, EMXplus-6/1, Bruker, Walzbachtal, Germany) with the following parameters: sweep time of 30.0 s, power of 2.0 mW, scan width of 100.0 G, modulation amplitude of 3.0 G, and modulation frequency of 100.0 kHz. The light response range and band structure of the samples were characterized using ultraviolet–visible absorption spectroscopy (UV–Vis, UV-2600, Shimadzu Corporation, Kyoto, Japan). Steady-state/transient fluorescence spectroscopy (FLS1000, Edinburgh, Livingston, UK) was employed to measure the photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectra, which were used to study the charge transfer kinetics and fluorescence lifetime of the samples. Electrochemical impedance spectroscopy (EIS), Mott–Schottky analysis, and photocurrent tests were conducted using a CS2350 electrochemical workstation (Wuhan Kost Instrument, Wuhan, China) to evaluate the impedance frequency and potential. A three-electrode quartz cell was used, with 0.5 M Na₂SO₄ solution as the electrolyte. Platinum (Pt) electrode, saturated calomel electrode (SCE), and fluorine-doped tin oxide (FTO) glass were employed as the counter electrode, reference electrode, and working electrode. The photocurrent (i-t) was measured via constant-potential polarization testing using a 1 cm² electrode under illumination from a 300 W xenon lamp positioned 15 cm away.

4.2.3. Photoactivity Test

Then, 5 mL H₂O was added to the bottom of a 500 mL quartz reactor, and a glass fiber membrane loaded with 3 mg of catalyst is placed above the water droplet, avoiding direct liquid–solid contact. The reactor was purged by three cycles of CO₂ evacuation and refilling to eliminate residual air, then maintained at 80 kPa to establish a reaction atmosphere comprising gaseous CO₂ and H₂O. A 300 W xenon lamp (Microsolar 300, Perfect Light, Beijing, China, 320–780 nm, 100 mW·cm⁻²) was used to simulate solar irradiation, positioned vertically 10 cm away from the catalyst during the photocatalytic reaction. Product analysis was performed using a GC-126N gas chromatograph (INESA, Shanghai, China), equipped with a flame ionization detector and a thermal conductivity detector for quantitative detection. Gas samples were collected hourly, and product concentrations were determined by converting chromatographic peak areas via external standard calibration. The final product yield was calculated based on the reaction time. Additionally, several control experiments with varying parameters were conducted to eliminate the influence of external environmental factors on the catalytic results.

4.2.4. Theoretical Calculations

DFT calculations were performed using the DMol3 module to obtain geometric structure optimization and electronic structure calculations. The generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional was employed to describe electron exchange and correlation effects. To match the experimental conditions, the (101) crystal plane of the TiO₂ phase was selected to construct the CeCTi model. A 2 × 3 × 1 supercell is built, where a Ti atom was replaced with a Ce atom to form the CeTi model, and adjacent O atom was removed to create O_V, yielding a CeTi model with O_V. The C atom was introduced into the CeTi model with O_V to construct the O_V-CeCTi catalyst model. During the geometric structure optimization, the bottom three layers of atoms were fixed, while the remaining atoms were allowed to relax freely to their ground states. A vacuum layer of 15 Å was applied along the Z-direction to eliminate interactions between periodic images. The Brillouin zone was sampled using a 2 × 3 × 1 Monkhorst–Pack k-point grid, with energy, maximum force, and maximum displacement thresholds set at 1.0 × 10⁻⁵ Hartree, 2.0 × 10⁻³ Hartree/Å, and 5.0 × 10⁻³ Å, respectively. Additionally, a 4 × 4 × 1 k-point grid was used for electronic structure calculations in this work.