CQD-Modified SrTiO₃ for Enhanced Photocatalytic CO₂ Reduction to Methane

Abstract

SrTiO₃ has attracted considerable attention owing to its favorable electronic structure and chemical stability among various semiconductor photocatalysts. However, its practical application is hindered by a wide bandgap and rapid recombination of photogenerated charge carriers. Herein, we report the fabrication of a SrTiO₃/carbon quantum dot (CQD) heterojunction via a two-step hydrothermal method for efficient CO₂-to-CH₄ photocatalysis, a strategy that circumvents the need for high-temperature treatment and noble metals. TEM images revealed well-defined lattice fringes and intimate interfacial contact between SrTiO₃ and CQDs, suggesting efficient charge transfer pathways. Optical measurements confirmed that CQD modification extends the visible-light absorption range of SrTiO₃ to 420 nm while significantly enhancing charge separation efficiency. The SrTiO₃/CQDs composite with 10 wt% CQD loading exhibited optimal activity, achieving a CH₄ evolution rate of 1.16 μmol·g⁻¹·h⁻¹—16.3 times higher than that of pristine SrTiO₃. Mechanistic investigations demonstrate that CQDs serve as efficient electron reservoirs, facilitating interfacial charge transfer and suppressing the recombination of photogenerated charge carriers. The catalyst maintained stable performance over four consecutive cycles, confirming its structural robustness and reusability. This work demonstrates that CQD modification effectively enhances the visible-light response and charge separation efficiency of SrTiO₃, offering a viable strategy for designing high-performance photocatalysts toward solar fuel production.

1. Introduction

The excessive emission of CO₂ into the atmosphere, driven by ongoing industrialization and urbanization worldwide [1,2], has not only elevated global temperatures but also accelerated ecosystem degradation [3,4]. According to the Intergovernmental Panel on Climate Change (IPCC), atmospheric CO₂ concentrations are projected to reach 590 ppm by 2100, leading to an average temperature increase of approximately 1.9 °C [5]. Achieving carbon neutrality necessitates effective strategies to mitigate this crisis. However, conventional approaches for CO₂ conversion—such as catalytic and thermochemical methods—are often economically unviable due to their reliance on high-temperature and high-pressure conditions [5]. In contrast, semiconductor-based photocatalytic CO₂ reduction has emerged as a promising technology, leveraging solar energy to convert CO₂ into value-added hydrocarbons like CH₄ [6]. This approach offers dual advantages: enhancing energy sustainability while advancing green energy utilization. As the primary component of natural gas, CH₄ exhibits high energy efficiency and clean combustion properties, making it widely applicable in the energy generation, heating, and transportation sectors [7]. Elucidating the reaction mechanisms of photocatalytic CO₂ reduction is therefore critical to optimizing catalyst design and accelerating sustainable technology development. Consequently, research on efficient modified photocatalysts has gained significant attention.

SrTiO₃ has emerged as a promising photocatalyst due to its exceptional physicochemical properties [8]. This material exhibits a suitable conduction band position for CO₂ reduction, chemical stability under irradiation, and tunable surface reactivity, though its wide bandgap (~3.2 eV) limits visible-light absorption. Notably, SrTiO₃’s adjustable band structure enables optimization of redox potentials, while its controllable surface morphology provides abundant active sites for photocatalytic reactions [9]. Recent advances demonstrate significant performance enhancements. Bi et al. [10] synthesized Cr-doped SrTiO₃ via ultrasonic-assisted co-precipitation and centrifugation, achieving a 3.7-fold increase in CH₄ yield compared with pristine SrTiO₃ during CO₂ photoreduction. Similarly, Wu et al. [11] developed Pt-modified three-dimensionally ordered macroporous SrTiO₃ using gas-foam templating, which exhibited a CO₂-to-CH₄ conversion rate 25 times higher than commercial P25 TiO₂ under identical conditions. These studies underscore the potential of engineered SrTiO₃ architectures, yet strategies to further enhance visible-light utilization—such as carbon quantum dot (CQD) hybridization—remain critically needed.

Beyond photocatalysis, the tunable surface reactivity and structural stability inherent to perovskite-type oxides (including SrTiO₃) have also been successfully exploited in high-performance gas sensing. For instance, Almaev et al. [12] reported Er-doped ZnGa₂O₄ ceramics as robust high-temperature methane sensors, in which enhanced surface adsorption and efficient interfacial electron transfer collectively contributed to sensitive and stable gas detection. Complementing this, Jiao et al. [13] summarized recent advances in nanomaterial-based conductive-type methane sensors, emphasizing that core performance metrics—including surface reaction kinetics and charge carrier transfer efficiency—are universally governed by material composition, nanostructure design, and interface engineering, regardless of whether the application is sensing or photocatalysis. This shared reliance on optimized surface activity and charge dynamics between the two fields inspires the present work: by hybridizing SrTiO₃ with CQDs, we aim to simultaneously enhance visible-light absorption and modulate surface reactivity, thereby boosting the photocatalytic CO₂-to-CH₄ performance through synergistic improvements in light harvesting, charge separation, and CO₂ activation.

CQDs, spherical or quasi-spherical carbon-based nanomaterials with diameters of 1–10 nm, exhibit exceptional optoelectronic properties critical for photocatalysis [14]. Their strong visible-light absorption capacity (extending to 600 nm) and tunable band structure enable dual functionality as electron reservoirs—accepting photogenerated electrons from semiconductors while donating holes—thereby significantly suppressing charge recombination [15]. This efficient charge separation arises from CQDs’ unique upconversion photoluminescence and rapid electron-transfer kinetics, which enhance visible-light utilization in wide-bandgap photocatalysts. Consequently, CQDs have demonstrated remarkable efficacy in biomedical imaging, optoelectronic devices, and energy conversion systems [16]. Critically for CO₂ photoreduction, their ability to extend light-harvesting ranges and facilitate interfacial charge transfer makes CQD–semiconductor hybrids ideal for overcoming the visible-light limitations of pristine SrTiO₃. Recent studies confirm that CQD-integrated photocatalysts achieve 3–5 times higher CH₄ yields than bare counterparts under solar irradiation [17], underscoring their strategic value in advancing CO₂ conversion technologies.

Based on these insights, this work systematically investigates SrTiO₃-based photocatalysts for CO₂ conversion to methane (CH₄), addressing the critical limitation of visible-light utilization in pristine SrTiO₃. Herein, we synthesize a series of SrTiO₃/CQD composites via a hydrothermal surface-modification strategy, precisely controlling CQD loadings. Comprehensive evaluation of their photocatalytic performance under simulated solar irradiation reveals that optimal 1 wt% CQD integration enhances CH₄ evolution rates by 4.2-fold compared with bare SrTiO₃, while mechanistic studies elucidate the role of CQDs in promoting interfacial electron transfer and suppressing charge recombination. This work establishes a structure–activity relationship for CQD-engineered SrTiO₃ photocatalysts and provides experimental evidence for their potential in solar-driven CO₂ valorization—offering a viable pathway toward sustainable carbon cycling.

2. Experimental and Theoretical Section

2.1. Chemicals

Strontium nitrate (Sr(NO₃)₂, AR), sodium hydroxide (NaOH, AR), urea (CH₄N₂O, AR) and ethylene glycol (C₂H₆O₂, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Tetrabutyl titanate (C₁₆H₃₆O₄Ti, AR) and citric acid (C₆H₈O₇, AR) were obtained from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Absolute ethanol (CH₃CH₂OH, AR) was purchased from Guanghua Technology Co., Ltd. (Foshan, China), and deionized water were used without further purification. Tetrabutyl titanate was stored under nitrogen atmosphere due to its moisture sensitivity before using it.

2.2. Synthesis of SrTiO₃/CQD Nanomaterials

CQDs were synthesized via hydrothermal carbonization. Citric acid (6.0 g) and urea (1.0 g) were dissolved in deionized water (20 mL, 18.2 MΩ·cm) under magnetic stirring (300 rpm, 30 min). The solution was transferred to a 50 mL polytetrafluoroethylene (PTFE)-lined autoclave and heated at 200 °C for 6 h. The resulting dark brown solution was centrifuged (10,000× g, 10 min) to remove aggregates, and the supernatant was lyophilized at 50 °C for 24 h to obtain CQD powder.

Briefly, 2.1163 g of Sr(NO₃)₂ was dissolved in deionized water (30 mL) and heated to 80 °C. Next, 3.5 mL C₁₆H₃₆O₄Ti was slowly added dropwise under vigorous stirring (500 rpm) to prevent instantaneous hydrolysis. After 1 h, 4 M NaOH (30 mL) was added to adjust pH > 12. The mixture was aged at 80 °C for 2 h, then washed 3 times with ethanol/water and dried at 60 °C for 12 h to yield SrTiO₃ nanoparticles.

CQD loadings (1, 5, 10, 15, 20 wt%) were dispersed in ethanol (10 mL) via sonication (30 min). The SrTiO₃ substrate (100 mg) was added to the dispersion and stirred (600 rpm, 30 min). The mixture was transferred to a PTFE-lined autoclave and heated at 180 °C for 24 h. Final products were washed with ethanol/deionized water (3×, 8000× g, 5 min) and dried at 60 °C for 12 h to obtain SrTiO₃/CQD_x composites (where x = CQD wt%).

2.3. Characterization

Phase composition and crystallinity were analyzed by X-ray powder diffraction (XRD; Rigaku SmartLab 9 kW, Tokyo, Japan, Cu Kα radiation λ = 1.5406 Å, 2θ = 10–80°, step size 0.02°, 2° min⁻¹). Morphology and microstructure were examined via field-emission scanning electron microscopy (FE-SEM; Hitachi Regulus 8100, Tokyo, Japan, 5 kV) and high-resolution transmission electron microscopy (HR-TEM; FEI Tecnai G₂ F30 S-TWIN, Hillsboro, OR, USA, 300 kV). Surface elemental states and chemical bonding were characterized by X-ray photoelectron spectroscopy (XPS; Thermo Fisher K-Alpha+, Waltham, MA, USA, monochromatic Al Kα hν = 1486.6 eV, 200 W, 40 eV pass energy, calibrated to C 1 s at 284.8 eV). UV-Vis diffuse reflectance spectra (DRS) were recorded using a Shimadzu UV-2600 spectrophotometer (Kyoto, Japan) with BaSO₄ as reference (wavelength range 200–800 nm, 1200 grooves mm⁻¹ grating). Photocatalytic CO₂ reduction tests were performed using a 300 W Xe lamp (CEAULIGHT CEL-HXF300, Beijing, China, AM 1.5G filter, 100 mW cm⁻² irradiance) as simulated sunlight. Charge carrier dynamics were evaluated through in situ transient photocurrent response and electrochemical impedance spectroscopy (EIS; CHI 660E, Houston, TX, USA, 10 mV AC amplitude, frequency range 0.1 Hz–100 kHz, 0.5 M Na₂SO₄ electrolyte). UV-Vis diffuse reflectance spectra (DRS) were recorded at room temperature. The measured reflectance R was converted to the Kubelka–Munk function F(R) = (1 − R)²/2R. Since SrTiO₃ is an indirect bandgap semiconductor, the optical bandgap was determined using the Tauc plot relation (αhv)^1/2 vs. hv, where α is proportional to F(R). Linear fitting was performed in the range (αhv)^1/2 = 0.1–0.5 eV^1/2 to target the fundamental absorption edge [18]. The bandgap energy Eg was obtained by extrapolating the linear region to (αhv)^1/2 = 0. Reported errors represent the standard deviation from three independent measurements.

2.4. CO₂ Photodissociation Test

Photocatalytic CO₂ reduction experiments were conducted in a Labsolar-6A closed-circulation reactor (PerfectLight, Beijing, China) under simulated solar irradiation using a 300 W Xe lamp (CEAULIGHT CEL-HXF300, Beijing, China, AM 1.5G filter, 100 mW cm⁻²). Prior to testing, CO₂ was pre-saturated in 60 mL deionized water (18.2 MΩ·cm) for 1 h to ensure complete dissolution. The reaction mixture comprised SrTiO₃/CQD_x photocatalyst (0.10 g) dispersed in this CO₂-saturated aqueous solution. The system was rigorously evacuated to ≤10 Pa and purged with ultrapure CO₂ (99.999%, 20 mL min⁻¹) for three cycles to eliminate atmospheric contaminants. Product quantification utilized gas chromatography (GC9790II, Fuli Instruments, Wenling, China) equipped with a flame ionization detector, in which CH₄ concentrations were determined through multi-point calibration using certified standards ranging from 0.5 to 100 ppm in nitrogen. Critical safety protocols included continuous hydrogen leak detection during operation of the H₂ generator (QPH-300II, Shanghai Quanpu Scientific Instrument Co., Ltd., Shanghai, China), with nitrogen carrier gas (99.999%, 30 mL min⁻¹) serving dual roles as sample transport medium and GC column coolant maintained at 35 °C, while strict H₂/air ratios (1:10) were enforced per IEC 60736 standards to ensure FID stability [19]. CH₄ evolution rates were reported in standard units of µmol g_cat⁻¹ h⁻¹ with measurements recorded at 15 min intervals over an hour period. Control experiments were carried out under two sets of conditions: using pure SrTiO₃ and without catalyst. In all control runs, the CH₄ concentration was below the detection threshold of 0.5 ppm. For stability assessment, photocatalysts underwent regeneration via ethanol washing and centrifugation (8000× g, 5 min) between consecutive cycles, with performance evaluated over four identical testing runs to confirm reproducibility. Gaseous products including CH₄, CO, and H₂ were monitored simultaneously. No other gaseous or liquid products were observed above the detection limit under the present conditions.

2.5. Computational Method and Details

All computational simulations in this study were carried out within the framework of density functional theory (DFT) using the Vienna Ab initio Simulation Package (VASP, version 6.4.3) [20]. The Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional under the generalized gradient approximation (GGA) [21] was employed, owing to its proven accuracy in describing hydrogen-bonding interactions and its reliability in modeling adsorption energetics [22]. A plane-wave kinetic energy cutoff of 450 eV was used throughout. Structural relaxations were deemed converged when the total energy change and forces fell below 1 × 10⁻⁵ eV and 1 × 10⁻⁵ eV/atom, and the residual stress was less than 0.02 GPa. Brillouin zone integration was performed using the Monkhorst–Pack scheme with a 5 × 5 × 5 k-point mesh for bulk SrTiO₃. Geometry optimization of the cubic SrTiO₃ unit cell yielded a lattice constant of a = 3.919 Å, in close agreement with the experimental value of 3.905 Å [23]. For surface calculations, a convergence test on the SrTiO₃(110) slab confirmed that 2 × 2 × 1 k-point grid yields energy variations below 0.001 eV, ensuring numerical stability. This k-point setting was thus adopted for all slab-based optimizations. Isolated CQDs and CO₂ molecules were modeled in 20 × 20 × 30 ų vacuum-separated supercells to minimize spurious inter-periodic interactions.

The CQD model for DFT calculations was constructed as a ~1 nm spherical nanoparticle with a mixed sp²/sp³ carbon framework and surface terminated by -COOH and -OH groups under charge-neutral conditions, in accordance with typical modeling strategies for carbon quantum dots [24]. This simplified model was adopted to capture the qualitative trend of interfacial electronic interactions between CQDs and SrTiO₃ rather than to reproduce absolute quantitative values. Although the experimental CQDs show a size distribution around 5 nm from TEM characterization, the key interfacial bonding and electronic coupling mechanisms remain general and representative of the CQD/SrTiO₃ composite system.

3. Results and Discussion

3.1. Phase Structure of SrTiO₃/CQD Nanomaterials

XRD patterns in Figure 1 confirm the successful synthesis of phase-pure SrTiO₃/CQD composites. All diffraction peaks align precisely with the cubic perovskite structure (space group Pm-3m) of SrTiO₃ (JCPDS No. 35-0734), exhibiting characteristic reflections at 2θ = 32.38°, 39.93°, 46.45°, 57.75°, and 67.79° corresponding to the (110), (111), (200), (211), and (220) crystallographic planes, respectively. A broad hump centered at 24.8° is distinctly observed in CQD-containing samples, attributable to the (002) plane of amorphous carbon in CQDs [25]. Notably, this feature intensifies progressively with increasing CQD loading while maintaining the SrTiO₃ peak positions within experimental error (±0.05°), confirming surface modification without lattice incorporation. The absence of peak shifting or new crystalline phases verifies that CQD integration preserves SrTiO₃’s structural integrity [26]. Crystallite size analysis of the (200) reflection using the Scherrer equation (D = Kλ/βcosθ, where K = 0.943, λ = 0.15406 nm, β = full width at half-maximum in radians) yields an average SrTiO₃ domain size of 26.1 ± 0.7 nm, consistent with the nanoparticle morphology observed in electron microscopy.

3.2. Morphology and Composition of SrTiO₃/CQD Nanomaterials

Figure 2a displays the SEM image of pristine SrTiO₃, revealing well-defined cubic nanoparticles with edge lengths predominantly in the 20–30 nm range—consistent with the crystallite size derived from XRD analysis. These particles exhibit a degree of aggregation that is typical during hydrothermal synthesis due to surface energy minimization. Figure 2b,c present the morphology of the STO/CQD nanocomposite. The CQDs, with an average size of approximately 5 nm, are uniformly dispersed on the SrTiO₃ surface. Notably, no significant morphological alteration of the SrTiO₃ cubes is observed upon CQD loading, indicating that the modification process preserves the underlying particle architecture. Figure 2d shows the corresponding elemental mapping of the region in Figure 2c, confirming the presence of Sr, Ti, O, and C throughout the composite. Sr and Ti exhibit homogeneous and concentrated distributions, consistent with the perovskite lattice, while C and O appear more diffusely—attributable to the surface-adsorbed CQDs and surface hydroxyl/adsorbed species. The spatial correlation of the C signal with the SrTiO₃ particles provides direct evidence for the successful integration of CQDs into the nanocomposite. It should be noted that the intense peak observed in the elemental intensity profile originates from exogenous elements introduced by the sample substrate during SEM/EDS analysis rather than from artifacts of the synthesis process.

TEM images shown in Figure 3a,b further resolve discrete CQDs (≈5 nm diameter) anchored on SrTiO₃ surfaces. High-resolution TEM (HRTEM) in Figure 3c measures the interplanar spacings of 0.279 nm and 0.190 nm, indexing precisely to the (110) plane of cubic SrTiO₃ and graphitic (100) domains in CQDs, respectively. This unambiguous lattice matching confirms epitaxial contact between phases without interfacial distortion. Selected-area electron diffraction (SAED) in Figure 3d exhibits polycrystalline rings from SrTiO₃ corresponding to (110) reflections, while CQDs produce diffuse rings characteristic of amorphous carbon with minor graphitic ordering—consistent with their photoluminescent properties [27,28,29,30].

3.3. Chemical Valence State of SrTiO₃/CQD Composite

XPS was employed to investigate interfacial electronic interactions in SrTiO₃/CQD₁₀ composite, with all binding energies referenced to the adventitious carbon C 1s peak at 284.8 eV. The survey spectrum shown in Figure 4a confirms the presence of Sr, Ti, O, and C elements. The Sr 3d spectrum in Figure 4b exhibits doublets at 133.73 eV (Sr 3d_5/2) and 135.47 eV (Sr 3d_3/2), consistent with Sr²⁺ in SrTiO₃ [31]. The Ti 2p spectrum in Figure 4c shows characteristic spin–orbit splitting with peaks at 458.1 eV (Ti 2p_3/2) and 463.9 eV (Ti 2p_1/2), confirming Ti⁴⁺ oxidation states [32]. Notably, the Ti 2p_3/2 binding energy is 0.3 eV lower than in pure SrTiO₃ (458.4 eV), indicating electron transfer from CQDs to SrTiO₃ that modifies surface electronic structure. The O 1s spectrum in Figure 4d resolves two components: 529.8 eV (lattice oxygen in Sr–O–Ti bonds) and 531.4 eV (surface hydroxyl groups and CQD-associated C=O moieties) [33]. Critically, the C 1s spectrum shown in Figure 4e deconvolutes into 284.8 eV (sp² C–C), 286.3 eV (C–O), and 288.8 eV (O–C=O) contributions [34,35], with the carboxyl peak intensity directly correlating with CQD loading. The absence of Ti³⁺ signatures (456.5–457.5 eV) confirms the CQDs’ function as electron mediators rather than as dopants, while the 0.3 eV Ti 2p shift provides definitive evidence of interfacial charge transfer essential for enhanced photocatalysis.

3.4. Performance Evaluation of Photocatalyst

Photocatalytic CO₂ reduction was evaluated under simulated solar irradiation (100 mW cm⁻²) at 25 °C and 70 kPa, in which CO₂ solubility in deionized water is 0.033 mol L⁻¹. Under the optimal reaction conditions, CH₄ was identified as the dominant gaseous product. The formation rates of CO, H₂ and other potential products were below the detection limit (<0.1 μmol g_cat⁻¹ h⁻¹). It should be emphasized that this high selectivity toward CH₄ is consistent with the characteristic performance of SrTiO₃-based photocatalytic systems [36]. Although trace amounts of liquid products such as methanol cannot be completely ruled out, their concentrations were below the detection range of the applied analytical methods. Therefore, the photocatalytic performance is evaluated mainly on the basis of CH₄ generation. As shown in Figure 5a, pristine SrTiO₃ exhibits a CH₄ evolution rate of 0.071 ± 0.005 μmol g_cat⁻¹ h⁻¹ after 1 h irradiation. The SrTiO₃/CQD_x composites demonstrate composition-dependent activity, with 1 wt% loading yielding 0.28 ± 0.02 μmol g_cat⁻¹ h⁻¹ (4.0× enhancement). Maximum activity occurs at 10 wt% CQD loading, achieving 1.16 ± 0.08 μmol g_cat⁻¹ h⁻¹—16.3 times higher than pristine SrTiO₃—while excessive loading (20 wt%) reduces performance to 0.41 ± 0.03 μmol g_cat⁻¹ h⁻¹ due to CQD-aggregation-induced light shielding [17]. Over 5 h (Figure 5b), the 10 wt% composite accumulates 5.11 ± 0.22 μmol g_cat⁻¹ CH₄, significantly outperforming both lower-loading composites and other SrTiO₃-based benchmarks (Cr-doped SrTiO₃ (CSTO), STO-SCO (SM), and NiO/STO) under analogous reaction conditions (see Table S2) [10,37]. This optimal loading correlates with XPS-confirmed interfacial electron transfer (Section 3.3), where CQDs function as electron reservoirs that suppress charge recombination by accepting photogenerated electrons from SrTiO₃ while donating holes to surface reaction sites. Five-cycle stability tests shown in Figure 5c reveal the 10 wt% composite retains 71% of its initial activity after 5 h total irradiation, with the 29% decay attributed to gradual CQD oxidation—a common limitation in carbon–semiconductor hybrids [38]. The retained activity (3.63 μmol g_cat⁻¹ after cycle 5) confirms a robust CO₂ conversion capability for practical applications.

3.5. Optical Properties and Electrochemical Analysis

To elucidate the enhanced photocatalytic mechanism, transient photocurrent responses and electrochemical impedance spectroscopy (EIS) were measured under simulated solar irradiation (AM 1.5G, 100 mW cm⁻²) using a standard three-electrode system (0.5 M Na₂SO₄ electrolyte, Pt counter, Ag/AgCl reference). As shown in Figure 6a, the SrTiO₃/CQD₁₀ composite exhibits a significantly higher transient photocurrent density than pristine SrTiO₃ under chopped illumination, confirming the superior separation efficiency of photogenerated electron–hole pairs. This enhancement directly correlates with the 0.3 eV negative shift in Ti 2p binding energy observed in XPS (Section 3.3), indicating CQDs act as electron reservoirs that accept photogenerated electrons from SrTiO₃ while facilitating hole transfer to surface reaction sites.

EIS was employed to evaluate the efficiency of electron transfer at the electrode interface. In general, a smaller arc radius in the Nyquist plot corresponds to lower surface resistance, which is favorable for charge transfer. As illustrated in Figure 6b, the arc radius of the SrTiO₃/CQD₁₀ composite is distinctly smaller than that of pure SrTiO₃. This observation confirms that the attachment of CQDs on the SrTiO₃ surface leads to more-efficient separation and transfer of charge carriers. The enhanced reduction performance is attributed to the dual role of CQDs: (i) extending visible-light absorption (confirmed by DRS in Section 3.6) and (ii) establishing conductive pathways that suppress charge recombination through rapid electron shuttling [38,39]. The combined electrochemical evidence—enhanced photocurrent generation and reduced charge transfer resistance—provides mechanistic validation for the 16.3 times higher CH₄ evolution rates observed in photocatalytic testing (Section 3.4), establishing CQD integration as a critical strategy for optimizing SrTiO₃-based CO₂ reduction systems.

The measured UV-Vis diffuse reflectance spectra shown in Figure 7a reveal fundamental modifications to the optical properties of SrTiO₃ upon CQD integration. Pristine SrTiO₃ exhibits a sharp absorption edge at 385 nm corresponding to a bandgap of 3.22 ± 0.03 eV, while the SrTiO₃/CQD₁₀ composite demonstrates a redshift of absorption onset to 420 nm with significantly enhanced visible-light absorption between 400 and 650 nm. Contrary to initial observations of weakened intensity, quantitative analysis shows the composite achieves 3.7-fold higher absorption at 500 nm, which is likely contributed by CQD-mediated upconversion and surface plasmon resonance effects [40]. This 3.7× enhancement at 500 nm exhibits a positive correlation with CQD concentration, further confirming the key role of CQDs in modulating the optical response. Tauc plot transformation of Kubelka–Munk functions (F(R) = (1 − R)²/2R) yields an effective bandgap of 2.95 ± 0.04 eV for the composite, in which the 0.27 eV reduction arises from CQD-induced mid-gap states rather than from bulk bandgap modification (as shown in Figure 7b). This expanded optical response directly correlates with the 2.8-fold photocurrent enhancement observed in electrochemical testing and explains the 16.3-fold increase in CH₄ evolution rates. The mechanism involves three synergistic processes: CQDs enable sub-bandgap photon utilization through upconversion photoluminescence, facilitate electron injection into SrTiO₃’s conduction band as evidenced by the 0.3 eV Ti 2p shift in XPS, and suppress charge recombination via rapid interfacial transfer. The measured effective optical gap narrowing due to mid-gap states aligns precisely with theoretical predictions for CQD–SrTiO₃ hybrids, confirming successful engineering of visible-light activity while maintaining sufficient thermodynamic potential for CO₂ reduction to methane. This discovery is consistent with the existing research results [41].

3.6. Adsorption Properties of CO₂ on the CQD-Modified SrTiO₃ Surface

Enhancing CO₂ adsorption capacity is pivotal for efficient photocatalytic CO₂ reduction, as it directly regulates reactant concentration at active sites and subsequent activation efficiency. Notably, the promotion of CO₂ adsorption by carbon quantum dot (CQD) decoration is not confined to specific semiconductor matrices but follows general regulatory principles supported by extensive research. CQDs boost CO₂ adsorption primarily through three mechanisms: (1) their ultra-small size (<10 nm) and abundant surface functional groups (-OH, C=O, -COOH) provide extra adsorption sites; (2) delocalized π-conjugated structures enable π-π stacking with CO₂, promoting chemical activation; (3) surface polarity modulation aligns with CO₂’s non-polar characteristic, enhancing dipole–dipole interactions. Similar effects have been verified in CQD/BiOIO₃/g-C₃N₄, CQD/NiAl-LDH/g-C₃N₄, and other composite systems (see Table S3) [42,43], confirming CQD decoration as a universal strategy to strengthen CO₂ adsorption and lay the foundation for improved photocatalytic activity.

TEM images clearly show that CQDs are preferentially adsorbed onto the (110) crystal plane of SrTiO₃. Guided by this experimental observation, the (110) surface was selected for modeling in this study. A (3 × 2 × 1) supercell containing 120 atoms was constructed by cleaving the geometrically optimized SrTiO₃ crystal along the (110) direction. To mitigate artificial interactions from periodic boundary conditions, a 15 Å vacuum layer was inserted perpendicular to the surface. During structural relaxation, the bottom two atomic layers—comprising an O layer and an Sr–O–Ti layer—were fixed to emulate the bulk-like stability of the underlying crystal.

To assess the impact of CQD modification on the structural stability of SrTiO₃, the formation energy ($E_f$) was employed as a key thermodynamic descriptor, defined as

E_f = E_CQDs/SrTiO3 − E_SrTiO3 − E_CQDs

where $E_{{\mathrm{CQDs}/\mathrm{SrTiO}}_3}$ is the total energy of the CQD–SrTiO₃ composite, $E_{{\mathrm{SrTiO}}_3}$ is that of pristine SrTiO₃, and $E_{\mathrm{CQDs}}$ is the energy of an isolated CQD. A negative $E_{\mathrm{f}}$ indicates a thermodynamically favorable and stable composite structure, with more-negative values corresponding to greater stability.

To evaluate CO₂ adsorption on the CQD-modified SrTiO₃(110) surface, the adsorption energy ($E_{\mathrm{ads}}$) was defined as

E_ads = E_SrTiO3+CO2 − E_CQDs/SrTiO3 − E_CO2

where $E_{{\mathrm{SrTiO}}_3+{\mathrm{CO}}_2}$ is the total energy of the system after CO₂ adsorption, and $E_{{\mathrm{CO}}_2}$ is the energy of an isolated CO₂ molecule. A negative $E_{\mathrm{ads}}$ signifies an exothermic and spontaneous adsorption process. Moreover, a larger magnitude of $-E_{\mathrm{ads}}$ reflects stronger CO₂–surface interaction and enhanced interfacial coupling [33].

To systematically compare the structural and functional differences of SrTiO₃ before and after CQD modification, geometric models of both pristine and CQD-decorated SrTiO₃(110) surfaces were constructed and fully relaxed. The optimized structures are illustrated schematically in Figure 8. To ensure reliable convergence during optimization, the initial separation between the CQD and the SrTiO₃(110) surface was set to 3 Å—a distance chosen to avoid unphysical repulsion or premature bonding. Site-screening calculations revealed that the most favorable adsorption configuration occurs when the edge carbon atoms of the CQD are positioned directly above surface Ti atoms. In this arrangement, the composite exhibits a formation energy of −9.98 eV, confirming a highly exothermic and thermodynamically stable interface.

Figure 8 compares the preferred adsorption sites of CO₂ on pristine and CQD-modified SrTiO₃. On the unmodified surface, CO₂ binds weakly with an adsorption energy of only −0.19 eV. In contrast, CQD modification induces a stronger interaction: one of the CO₂ oxygen atoms forms a coordination bond with a surface Sr atom, yielding an O–Sr distance of 2.856 Å and increasing the adsorption energy to −0.46 eV. This more-than-twofold enhancement in adsorption strength demonstrates that CQD functionalization significantly promotes CO₂ capture on SrTiO₃, thereby creating favorable interfacial conditions for subsequent photocatalytic reduction to CH₄.

3.7. Photocatalytic Schematic Illustration of SrTiO₃/CQDs

The photocatalytic CO₂ reduction schematic illustration for SrTiO₃/CQD₁₀ composite under solar irradiation is illustrated in Figure 9. Upon photon absorption, electrons in the valence band of SrTiO₃ (VB = +2.43 eV vs. RHE) are excited to the conduction band (CB = −0.82 eV vs. RHE), creating electron–hole pairs. The CB potential (−0.82 eV) thermodynamically favors CO₂ reduction to CH₄ (E° = −0.24 eV for CO₂/CH₄), while the VB potential enables water oxidation (E° = +0.82 eV for O₂/H₂O). However, rapid charge recombination in pristine SrTiO₃ limits photocatalytic efficiency, as evidenced by its low 0.071 μmol g_cat⁻¹ h⁻¹ CH₄ yield [44].

CQD integration fundamentally alters this process: Photogenerated electrons rapidly transfer from the CB of SrTiO₃ to CQDs (confirmed by the 0.3 eV negative shift in Ti 2p XPS binding energy), where they accumulate in CQD surface states acting as electron reservoirs. This interfacial electron shuttling suppresses recombination by 60.6%, increasing the electron lifetime for CO₂ reduction. The CH₄ formation pathway proceeds through an eight-electron process: CO₂ → *COOH → *CO → *CHO → *CH₂O → *CH₃O → *CH₃ → CH₄, with CQDs providing active sites for proton-coupled electron transfers [45]. Critically, the enhanced visible-light absorption (3.7× at 500 nm) and upconversion properties of CQDs enable sub-bandgap photon utilization, while the 2.95 eV effective bandgap of composite maintains sufficient driving force for CO₂ reduction. The observed 16.3× higher CH₄ yield directly correlates with this optimized charge dynamics, in which CQDs serve dual functions: (i) extending light harvesting to 420 nm, and (ii) facilitating electron transfer to adsorbed CO₂ molecules. Notably, aqueous carbonate species (HCO₃⁻/CO₃²⁻) from CO₂ dissolution—despite increasing solution alkalinity—form bicarbonate (HCO₃⁻) that competes with CO₂ for reduction sites and promotes parasitic H₂ evolution, explaining the performance decline at >10 wt% CQD loading where surface coverage impedes CO₂ adsorption [46]. The main redox reactions are shown below [47]:

SrTiO₃ + hv → SrTiO₃(e⁻) + SrTiO₃(h⁺)

(1)

SrTiO₃(e⁻) → CQDs

(2)

2H₂O + 4h⁺ → O₂ + 4H⁺

(3)

CO₂ + 8H⁺ + 8e⁻ → CH₄ + 2H₂O

(4)

CO₂(aq)/CO₃²⁻ + H₂O → OH⁻ + HCO₃⁻

(5)

CO₂(aq)/HCO₃⁻ + H₂O → OH⁻ + H₂CO₃

(6)

4. Conclusions

This work demonstrates a hydrothermal strategy for synthesizing SrTiO₃/CQD₁₀ composites that achieve a 16.3-fold enhancement in CH₄ evolution rates (5.11 ± 0.22 μmol g_cat⁻¹) compared with pristine SrTiO₃ under simulated solar irradiation. The optimal 10 wt% CQD loading maximizes visible-light harvesting (3.7× absorption increase at 500 nm) and interfacial charge transfer, as evidenced by XPS-confirmed electron injection (0.3 eV Ti 2p shift) and 60.6% reduced charge transfer resistance in EIS. Critically, CQDs function as electron reservoirs that suppress recombination while maintaining cubic morphology (26.1 ± 0.7 nm crystallites) and thermodynamic potential for CO₂-to-CH₄ conversion. The composite retains 71% of its initial activity after 5 h of cumulative irradiation, though performance decay stems from CQD oxidation and carbonate-induced site blocking—key limitations requiring mitigation in future designs. These findings establish CQD surface engineering as a viable pathway to overcome the visible-light limitations of wide-bandgap perovskites, advancing solar-driven CO₂ valorization with practical implications for sustainable fuel production. Future work should address long-term stability through CQD passivation and eliminate carbonate additives that promote parasitic hydrogen evolution, thereby enhancing the solar-to-fuel efficiency of this promising photocatalytic system.