Defect Engineering via La Doping and Hydrogenation on Bi₄Ti₃O₁₂ for Synergistically Enhancing Photocatalytic CO₂ to CH₃OH

Abstract

Developing highly efficient photocatalysts for CO₂ reduction remains a great challenge. The large band gap and poor charge carrier dynamics are the major factors limiting the performance of Bi₄Ti₃O₁₂ (BTO). Herein, a series of La-doped Bi₄Ti₃O₁₂ (BLa_xTO) nanosheets were synthesized and further modified by NaBH₄ hydrogenation to create surface defect-rich H-BLa_XTO nanosheets. Characterizations and theoretical calculations confirmed that the synergistic effect of La doping and hydrogenation significantly enhanced visible-light absorption, promoted charge separation, and improved the electron reduction capacity. When applied to photocatalytic CO₂ reduction, the H-BLa_0.2TO catalyst achieved a superior CH₃OH production rate of 7.90 μmol·g⁻¹·h⁻¹, which is 5.6 times higher than that of pristine Bi₄Ti₃O₁₂. Moreover, the H-BLa_0.2TO catalyst maintained excellent stability over four consecutive cycles. This study offers an integrated strategy for constructing high-performance bismuth-based photocatalysts through elemental doping and defect engineering.

1. Introduction

The extensive industrialization has resulted in massive consumption of fossil fuels and excessive CO₂ emissions, culminating in energy and environmental issues [1,2,3]. In this context, CO₂ conversion has emerged as a promising strategy to mitigate these issues. Among these, photocatalysis, which utilizes solar energy as a driving force, has garnered significant attention due to its green and sustainable characteristics [4,5,6]. However, the efficient and selective photocatalytic CO₂ reduction into high-value-added products such as methanol (CH₃OH) remains a considerable challenge. Accordingly, the rational design and development of highly efficient photocatalysts represents an urgent and critical need in the field of photocatalytic CO₂ reduction [7,8,9,10].

In recent years, Bi₄Ti₃O₁₂ (BTO), a typically Aurivillius-type perovskite, has attracted considerable attention in photocatalysis [11,12]. The crystal structure of single layer [Bi₂O₂]²⁺ and perovskite layer [Bi₂Ti₃O₁₀]²⁻ arranged alternately along c-axis is conducive to improving the separation rate of photogenerated electron–hole pairs. Moreover, the distinctive hybrid valence band structure, formed by Bi 6 s and O 2p orbitals, facilitates the generation of photogenerated carriers [13,14,15,16,17]. However, the pristine BTO suffers from the large band gap and poor charge carrier dynamics, which significantly inhibit its practical application in photocatalytic performance under visible light. Recent research proposed several strategies in modifying BTO to enhance its functionality. Yin et al. demonstrated that Fe doping combined with a ZnIn₂S₄ heterojunction was shown to narrow the bandgap and create a built-in electric field, significantly boosting CO₂ reduction to CO and CH₄ [18]. He et al. reported that decorating BTO with plasmonic metallic Bi and oxygen vacancies (OVs) via in situ reduction greatly improved visible-light absorption and charge separation, leading to highly efficient NO removal with suppressed toxic NO₂ by-production [19]. Geng et al. reported that Ce⁴⁺ doping was demonstrated to induce lattice distortion, enhance ferroelectric polarization, and reduce the bandgap, resulting in superior piezo-photocatalytic RhB degradation under combined light and ultrasonic excitation [20]. Collectively, these studies confirm that tailoring electronic and surface properties through elemental doping and defect creation is a powerful route to improving its catalytic performance. Inspired by these approaches, this work proposes a synergistic strategy of A-site La³⁺ doping coupled with post-synthetic hydrogenation for advanced defect engineering in BTO. While the aforementioned studies utilized Fe/Ce doping or created OVs, the combination of La doping with a controlled hydrogenation process remains less explored. This approach holds promise for simultaneously achieving narrowing of the bandgap and suppression of recombination while introducing abundant surface OV sites and Ti³⁺ sites. We hypothesize that this dual approach will not only enhance visible-light absorption but also create a synergistic effect that drastically improves charge separation and provides more active sites, thereby developing a novel hydrogenated La-doped BTO (H-BLa_xTO) catalyst for efficient photocatalytic CO₂ reduction to methanol [21,22,23]. Although defect engineering in perovskite oxides has been explored, the specific effects of combining A-site La doping with NaBH₄ hydrogenation on the photocatalytic behavior of BTO remain inadequately understood, warranting further systematic investigation.

Herein, we report La doping and NaBH₄ hydrogenation co-modified BTO (H-BLa_0.2TO) photocatalysts prepared via a glucose-assisted hydrothermal method. The performance of H-BLa_0.2TO nanosheet was evaluated based on photocatalytic CO₂ reduction into CH₃OH. The result illustrated that the deliberate defect engineering played a crucial role in enhancing photocatalytic performance. Experiments and characterizations were applied to explore the inherent nature for the verification of enhanced photocatalytic activity. Furthermore, the density functional theory (DFT) was employed to further elucidate the electronic density of states.

2. Results and Discussion

2.1. Structure and Morphology

The crystal structures of the as-synthesized BTO, BLa_XTO, and H-BLa_0.2TO nanosheets were characterized by XRD. As shown in Figure 1a, the distinct diffraction peaks observed at 2θ = 10.8°, 16.3°, 21.7°, 23.3°, 30.1°, 32.8°, and 33.1° could be assigned to the (004), (006), (008), (111), (117), (200), and (020) of Bi₄Ti₃O₁₂ (JCPDS Card No. 72-2181) [24]. No additional peaks were observed, illustrating that the introduction of La³⁺ and further NaBH₄ hydrogenation did not change the crystal structure of BTO. The crystallite sizes of samples in Table S1 were calculated using the Scherrer equation. To further investigate the effect of A-site La doping on the catalyst structure, Raman spectra of BTO, BLa_0.2TO, and H-BLa_0.2TO were measured, as shown in Figure 1b. BTO exhibited strong and sharp phonon modes at 117, 148, 189, 228, 266, 328, 356, 534, 562, 614, and 848 cm⁻¹, illustrating well-crystallized nature of the catalyst [25]. The low-frequency modes (<200 cm⁻¹) are associated with the vibrations of the [Bi₂O₂]²⁺ layers, while the high-frequency modes (>200 cm⁻¹) are generally classified as those related to the motion of cations in the [TiO₆] octahedra. Compared with BTO, BLa_0.2TO showed decreased phonon peak frequencies, peak broadening, and enhanced diffuseness. A slight red shift was observed in the high-frequency phonon modes, while the low-frequency modes remained largely unchanged. These changes suggest an altered coordination environment and modified structure of the [TiO₆] octahedra, resulting from the substitution of Bi sites in the [Bi₂Ti₃O₁₀]²⁻ layers by La [26].

The microstructures of the synthesized BTO, BLa_xTO, and H-BLa_0.2TO photocatalysts were characterized by SEM. As depicted in Figure 2a–c and Figure S2, all samples exhibited a nanosheet-like shape with varying sizes. Notably, as the concentration of the La³⁺ doping increased, the dimensions of the nanosheets gradually decreased. This increased the number of active sites and light capture sites, which was beneficial for photocatalytic reactions. Figure 2d exhibited the EDS elemental mapping of the H-BLa_0.2TO, where Bi, La, Ti, and O elements were evenly distributed throughout the nanosheets, demonstrating the successful preparation [27].

The TEM and HR-TEM images (Figure 3) provided more details of BTO, BLa_0.2TO, and H-BLa_0.2TO. All catalysts exhibited nanosheets of varying sizes, indicating that neither La incorporation nor the NaBH₄ hydrogenation process disrupted the catalyst structure. In the HRTEM image of BTO, lattice fringes with an interplanar spacing of approximately 0.404 nm were observed, corresponding to the (008) crystallographic plane of Bi₄Ti₃O₁₂. Similarly, lattice spacings of about 0.400 nm and 0.405 nm were identified in the HRTEM images of BLa_0.2TO and H-BLa_0.2TO, respectively, both of which can also be indexed to the (008) plane of Bi₄Ti₃O₁₂ according to the standard reference [28].

The chemical composition and elemental states of the BTO, BLa_0.2TO, and H-BLa_0.2TO photocatalysts were analyzed by XPS, and the results were presented in Figure 4. As shown in the Bi 4f spectra (Figure 4a), the characteristic peaks of Bi 4f_5/2 and Bi 4f_7/2 were observed at binding energies of 158.9 eV and 164.3 eV, respectively. In BLa_0.2TO, the Bi 4f_5/2 peak exhibited a slight positive shift of 0.1 eV. After hydrogenation, both the Bi 4f_5/2 and Bi 4f_7/2 peaks of H-BLa_0.2TO shifted positively by 0.3 eV and 0.2 eV, respectively. The La 3d spectra (Figure 4b) displayed characteristic doublets corresponding to La 3d_5/2 and La 3d_3/2 in both BLa_0.2TO and H-BLa_0.2TO. Peaks located at 834.0 eV and 838.0 eV were attributed to La 3d_5/2, while those at 851.4 eV and 855.0 eV were assigned to La 3d_3/2. The consistent La 3d peak positions in both samples indicated that the incorporated La³⁺ ions remained stable within the crystal lattice even after the NaBH₄ hydrogenation treatment. In the Ti 2p spectra (Figure 4c), the characteristic peaks of Ti⁴⁺ 2p_3/2 and Ti⁴⁺ 2p_1/2 were located at 457.8 eV and 463.5 eV, respectively. An additional peak observed at 466.0 eV is associated with Bi 4p_3/2. For BLa_0.2TO, the Ti⁴⁺ 2p_3/2 and 2p_1/2 peaks remained at similar positions, while a peak at 466.3 eV was assigned to Bi 4s. Notably, in H-BLa_0.2TO, a new peak appeared at 458.4 eV, which was attributed to Ti³⁺, indicating the partial reduction of Ti⁴⁺ to Ti³⁺ due to the NaBH₄ treatment. Furthermore, compared to BLa_0.2TO, the Ti⁴⁺ 2p_3/2 and 2p_1/2 peaks in H-BLa_0.2TO exhibited negative and positive shifts of 0.6 eV and 0.1 eV, respectively, likely resulting from charge compensation induced by Ti³⁺ formation. The O 1s spectra (Figure 4d) revealed two main contributions at 529.6 eV and 530.7 eV, corresponding to lattice oxygen (O_L) and oxygen vacancies (O_V), respectively. The introduction of La at the A-site did not significantly alter the concentration of surface oxygen vacancies in BLa_0.2TO. In contrast, the hydrogenation process with NaBH₄ notably increased the amount of OVs on the surface of H-BLa_0.2TO [29,30,31]. The concentration of oxygen vacancies on the catalyst surface was further determined by O₂-TPD, as shown in Figure S3. The peak observed between 200 °C and 500 °C can be corresponded to the oxidation of OVs. The introduction of La at the A-site had little influence on the oxygen vacancy concentration, whereas the high-temperature hydrogenation with NaBH₄ significantly increased it. These results are consistent with the XPS O 1s analysis [32].

2.2. Photoelectrochemical Properties

The optical absorption properties of the catalysts were analyzed using UV–vis DRS. As shown in Figure 5a, all samples exhibited strong and steep absorption edges in the visible-light region. The incorporation of La³⁺ into the Bi₄Ti₃O₁₂ lattice introduces metal impurity levels near the bottom of the conduction band (CB), reducing the bandgap and thereby enhancing light absorption. Consequently, BLa_0.2TO demonstrated improved visible-light absorption compared to BTO. The subsequent NaBH₄ hydrogenation further enhanced the visible-light absorption capability of H-BLa_0.2TO, leading to higher light utilization efficiency. Furthermore, the valence band (VB) positions of the photocatalysts were determined via XPS valence band spectra. As depicted in Figure 5b, the VB positions of BLa_0.2TO and H-BLa_0.2TO were measured to be 2.05 V and 1.75 V, respectively. The CB positions of BLa_0.2TO and H-BLa_0.2TO were estimated based on Mott–Schottky measurements. The flat-band potentials, obtained from the Mott–Schottky plots (Figure S4), were −0.95 V and −0.98 V (vs. Ag/AgCl) for BLa_0.2TO and H-BLa_0.2TO, respectively. After conversion, the corresponding CB positions were calculated to be −0.85 V for BLa_0.2TO and −0.88 V for H-BLa_0.2TO. In comparison, the conduction band of pristine BTO has been previously reported to be −0.8 V. The A-site La doping resulted in a more negative conduction band position of BLa_0.2TO than that of BTO, indicating that the electrons in the conduction band of BLa_0.2TO possess stronger reducing ability. The hydrogenation process with NaBH₄ further optimized the conduction band position of H-BLa_0.2TO, endowing it with even greater electron reduction power, which is highly beneficial for the photocatalytic CO₂ conversion to CH₃OH [33,34].

The migration and separation efficiency of photoinduced charge carriers in the samples were further investigated through I-t and EIS measurements. As shown in the Nyquist plots (Figure 6a), H-BLa_0.2TO exhibited the smallest arc radius, indicating the lowest charge transfer resistance and the fastest photoinduced electron migration rate among the samples. These results suggest that H-BLa_0.2TO possesses enhanced charge separation efficiency and reduced recombination, contributing to its superior photocatalytic activity. The I-t curve (Figure 6b) further confirmed that H-BLa_0.2TO generated the highest photocurrent density, demonstrating its excellent ability to separate photoinduced electron–hole pairs [35].

To gain further insight into the charge separation behavior, steady-state and time-resolved photoluminescence spectroscopy were conducted. The steady-state PL spectra (Figure 6c) showed that La doping at the A-site resulted in a significant quenching of the PL intensity, indicating that the introduction of La effectively suppressed the recombination of photogenerated carriers. The TRPL decay curves (Figure 6d) revealed a shorter fluorescence lifetime for the La-doped sample, suggesting an enhanced charge transfer capability due to A-site La substitution [36].

2.3. Photocatalysis Performance

The photocatalytic performance of the as-prepared samples was evaluated by the photocatalytic reduction of CO₂ to CH₃OH. Figure 7a shows the production rates of CO, CH₃OH, and CH₄ during the reaction. The CH₃OH production rates over BTO, BLa_0.1TO, BLa_0.2TO, BLa_0.3TO, BLa_0.4TO, and H-BLa_0.2TO were 1.40, 3.04, 4.70, 4.37, 3.85, and 7.90 μmol·g⁻¹·h⁻¹, respectively. Among the non-hydrogenated samples, BLa_0.2TO exhibited the highest CH₃OH yield. After hydrogenation, the CH₃OH production rate was further enhanced. This improvement can be attributed to the introduction of an optimal amount of La into the A-site, which forms impurity energy levels that facilitate the separation of photogenerated charge carriers. Additionally, the hydrogenation process promotes the formation of surface defects, modifying the band structure and thereby enhancing the photocatalytic activity. Control experiments were carried out over H-BLa_0.2TO under different conditions (Figure 7b). No CH₃OH was detected in the absence of the catalyst, light irradiation, or CO₂ flow, confirming that the CH₃OH production indeed originated from the photocatalytic reduction of CO₂. The stability of the H-BLa_0.2TO catalyst was examined through four consecutive recycling experiments (Figure 7c) and by comparing the XRD patterns before and after the reaction (Figure 7d). No significant loss of photocatalytic activity was observed after four cycles. Moreover, the XRD patterns of H-BLa_0.2TO remained virtually unchanged after the reaction, indicating that the crystal structure was well maintained and demonstrating the excellent cycling stability of the catalyst.

2.4. Photocatalytic Mechanism

Density functional theory (DFT) calculations were performed to investigate the electronic band structure, total density of states (TDOS), and projected density of states (PDOS) of H-BLa_0.2TO, providing insight into the contributions of atomic orbitals to the valence and conduction bands. As shown in Figure 8a, the calculated band gap of H-BLa0.2TO is 0.953 eV, which is significantly narrower than that of pristine BTO (1.449 eV), confirming its enhanced visible-light absorption capability. Moreover, the valence band maximum and conduction band minimum are located at different high-symmetry points in the Brillouin zone, indicating an indirect band gap nature. The TDOS of H-BLa_0.2TO (Figure 8b) reveals that the electronic states near the band edges are predominantly contributed by p-orbitals. Further analysis of the PDOS (Figure 8c–f) for Bi, La, Ti, and O atoms indicates that the valence band is mainly formed by O 2p orbitals, which exhibit higher energy states near this region. This suggests that the O 2p orbitals play a critical role in determining the electronic properties of the valence band in H-BLa_0.2TO [37,38].

Based on the above discussion, a possible reaction mechanism for the photocatalytic CO₂ reduction over the H-BLa_0.2TO catalyst is proposed (Figure 9). La doping at the A-site of Bi₄Ti₃O₁₂ via the sol–gel-assisted molten salt method introduces metal impurity energy levels, which not only narrow the bandgap of the catalyst but also effectively suppress the recombination of photogenerated charge carriers. Subsequent hydrogenation with NaBH₄ constructs abundant surface defects, further enhancing the reduction ability of the electrons in the conduction band and thereby improving the overall photocatalytic performance. Under light irradiation, H-BLa_0.2TO absorbs photons and generates electron–hole pairs. The photogenerated electrons in the conduction band then facilitate the reduction of CO₂ and H₂O to produce CH₃OH.

3. Experimental Section

3.1. Materials

Bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O, ≥99.9%), Lanthanum nitrate hexahydrate (La(NO₃)₃·6H₂O, ≥99.9%), Tetrabutyl titanate (C₁₆H₃₆O₄Ti, ≥99.9%), Potassium chloride (KCl, AR), Sodium chloride (NaCl, AR), Sodium borohydride (NaBH₄, AR), Acetic anhydride (C₄H₆O₃, AR), Ethylene glycol(C₂H₆O₂, AR), and Glacial acetic acid (CH₃COOH, AR) were obtained from Aladdin Reagent.

3.2. Fabrication of H-BLa_0.2TO Nanosheets

The hydrogenated La-doped Bi₄Ti₃O₁₂ (H-BLa_0.2TO) nanosheets were fabricated via a sequential process involving sol–gel synthesis, molten salt annealing, and a subsequent hydrogenation treatment.

Synthesis of Bi_3.8La_0.2Ti₃O₁₂ (BLa_0.2TO). Typically, 9.5 mmol of Bi(NO₃)₃·5H₂O and 5 mmol of La(NO₃)₃·6H₂O were dissolved in a mixed solvent of 7.5 mL acetic anhydride and 3 mL glacial acetic acid (solution A). Simultaneously, 7.5 mmol of C₁₆H₃₆O₄Ti was dissolved in a mixture of 12.5 mL ethylene glycol and 2 mL glacial acetic acid (solution B). Solution B was then added dropwise into solution A under vigorous stirring. The resulting mixture was continuously stirred for 30 min and aged for 10 h at room temperature. The obtained gel was dried at 110 °C for 48 h to form a xerogel. The dry xerogel was thoroughly ground with a eutectic mixture of NaCl/KCl (0.125 mol each) and subsequently calcined at 800 °C for 2 h in a muffle furnace. The product was washed repeatedly with deionized water to remove soluble salts and dried at 100 °C for 6 h, yielding Bi_3.8La_0.2Ti₃O₁₂ nanosheets, designated as BLa_0.2TO. Using the same procedure but adjusting the molar ratios of the precursors, Bi_4-xLaₓTi₃O₁₂ samples with different doping levels (x = 0.1, 0.3, 0.4) were also prepared.

Hydrogenation Treatment. The as-prepared BLa_0.2TO powder (1.0 g) was uniformly mixed with 0.2 g of sodium borohydride (NaBH₄) by grinding in an agate mortar for 30 min. The mixture was then transferred into a ceramic boat and placed in a tube furnace. Under a continuous Ar flow, the temperature was raised to 350 °C at a ramp rate of 5 °C·min⁻¹ and maintained for 1 h. After cooling down to room temperature naturally, the final product was collected and washed with deionized water until neutral, followed by drying at 80 °C for 8 h. The resulting hydrogenated catalyst was denoted as H-BLa_0.2TO. The preparation process is displayed in Figure 10.

4. Conclusions

In summary, a series of La-doped Bi₄Ti₃O₁₂ nanosheets were successfully synthesized via a sol–gel-assisted molten salt method, followed by a NaBH₄ hydrogenation treatment to obtain the hydrogenated H-BLa_0.2TO catalyst. The synergistic effect of A-site La doping and hydrogenation engineering was systematically demonstrated to significantly enhance the photocatalytic performance for CO₂ reduction to CH₃OH. The incorporation of La³⁺ into the Bi₄Ti₃O₁₂ lattice effectively narrowed the bandgap by introducing impurity energy levels, thereby extending the visible-light absorption range. More importantly, the hydrogenation process with NaBH₄ created abundant surface oxygen vacancies and partially reduced Ti⁴⁺ to Ti³⁺, which served as active sites and further optimized the electronic structure. A combination of experimental characterizations and DFT calculations confirmed that these modifications collectively promoted the separation and migration of photogenerated charge carriers while endowing the catalyst with a more negative conduction band potential, thereby enhancing its reducing power. Consequently, the optimized H-BLa_0.2TO catalyst achieved a remarkable CH₃OH production rate of 7.90 μmol·g⁻¹·h⁻¹, which was 5.6 times higher than that of pristine BTO. Furthermore, the catalyst exhibited excellent structural and catalytic stability over multiple reaction cycles. This work highlights defect engineering via elemental doping and post-synthesis treatment as a highly effective strategy for designing efficient bismuth-based photocatalysts for solar-driven CO₂ conversion to value-added fuels.