Efficient Nb₂O₅@g-C₃N₄ heterostructures for enhanced photocatalytic CO₂ reduction with highly selective conversion to CH₄

Abstract

Achieving the goal of carbon neutralization using photocatalytic CO₂ reduction has garnered widespread attention. However, rapid photogenerated charge recombination seriously impedes the further improvement of photocatalytic properties. In response, we propose a strategy to solve this limitation by way of constructing a heterojunction. A Nb₂O₅@g-C₃N₄ type II heterojunction photocatalyst was developed by a straightforward hydrothermal method. The construction of a heterojunction greatly accelerates the separation of photoinduced charges and increases the photocatalytic activity. As a result, Nb₂O₅@g-C₃N₄ (1 : 5) possesses outstanding properties for CO₂ reduction under visible light with the production of CH₄ and CO of about 19.06 and 1.93 μmol g⁻¹, respectively. The CH₄ selectivity is up to 98.79%. Furthermore, the mechanism of photocatalytic CO₂ reduction is revealed in detail based on in situ DRIFTS and photochemical characterization, thus providing guidance for the design of high-performance CO₂ photoreduction systems.

---

1. Introduction

The significantly selective catalytic reduction of CO₂ to CH₄ through the absorption of solar photon energy by photocatalytic semiconductors is a prospective way to achieve carbon neutrality.^1–6 Nevertheless, from a kinetic perspective, CH₄ formation is more challenging than the two-electron conversion of CO₂ to CO since it takes eight electrons. Now, great selectivity can be obtained using strategies like choosing an ideal semiconductor to construct a heterojunction.^7–9 Previous research has shown that the construction of heterojunctions is one of the most direct and operative ways to generate greatly selective CH₄, and it also plays a crucial role in CO₂ activation.^10–13

Among a variety of semiconductors, graphitic phase carbon nitride (g-C₃N₄) with a special structure has attracted the attention of current researchers because of its appropriate band structure and low band gap value (E_g ∼ 2.7 eV).^14,15 It is made up of two elements that are common on the Earth and have some special traits, such as good optical and electrical properties and ease of large-scale synthesis.^16,17 The well-known TiO₂ semiconductor is only functional in the UV spectrum, whereas a suitable band gap for g-C₃N₄ is 2.7 eV, which allows it to respond to visible light. Unfortunately, because photoinduced e⁻–h⁺ pairs quickly recombine, the photocatalytic capacity of pristine g-C₃N₄ is typically constrained by the inadequate availability of light.^14,18,19 In recent years, the shortcoming was solved by creating semiconductor heterojunctions with a suitable band structure, which can greatly facilitate the separation and migration of photoinduced charges.^14,20–22 To date, heterojunction photocatalysts based on g-C₃N₄ in abundance have been effectively designed to improve the photocatalytic activity, like g-C₃N₄/ZnFe₂O₄,²³ g-C₃N₄/Ag₃VO₄,²⁴ g-C₃N₄/CdS,²⁵ Bi₂WO₆/g-C₃N₄ ²⁶ and so on. However, some of the above materials have obvious drawbacks such as poor stability, high production difficulty and high cost, which ultimately limit their practical application.

Niobium oxide (Nb₂O₅), an n-type semiconductor, is a promising semiconductor material for coupling to g-C₃N₄ among other semiconductors and possesses a high specific surface area and excellent photoactivity towards CO₂ reduction and has an optical band gap of approximately 3.2 eV.^27–29 The photocatalytic performance of numerous composites, including Nb₂O₅/RGO,³⁰ Nb₂O₅/ZnO,²⁷ Nb₂O₅/iron oxide,³¹ Nb₂O₅/CuO³² and TiO₂/Nb₂O₅,³³ has significantly improved as a result of the incorporation of Nb₂O₅ powder. Long periods of synthesis and uncontrollable heterostructure formation are significant shortcomings of the systems previously discussed. These factors can result in Nb₂O₅ particles having poor dispersion on the g-C₃N₄ surface and having large particle sizes, which lower the activity of the photocatalysts. Indeed, the construction of highly selective and active heterojunction catalysts based on g-C₃N₄ is still a significant challenge.

Herein, we synthesized a novel type of Nb₂O₅@g-C₃N₄ (NB@CN) heterojunction photocatalyst by a straightforward one-step hydrothermal process. The photocatalytic conversion of CO₂ under LED light (a wavelength of 400 nm to 800 nm) simulated solar light irradiation is utilized to examine the photoreduction capacity of the as-produced photocatalysts. The synthesized NB@CN type II heterojunction exhibits remarkable photocatalytic CO₂ reduction performance with 98.79% CH₄ selectivity, greatly outperforming pristine Nb₂O₅ and g-C₃N₄ with regard to photocatalytic activity. Furthermore, the mechanism of photocatalytic CO₂ reduction is investigated in detail through photochemical characterization and in situ DRIFTS, which provides a reference and new inspiration for the development and design of efficient photocatalytic systems.

2. Experimental section

Chemicals, materials, the production of Nb₂O₅ and g-C₃N₄, characterization, the experiment of photocatalytic CO₂ conversion and in situ DRIFTS are presented in the ESI.†

2.1. Construction of the NB@CN heterojunction

In a typical hydrothermal preparation process, g-C₃N₄ (1 g) is initially added to 60 mL ultrapure water at normal temperature, stirring evenly. Then C₄H₄NNbO₉·XH₂O (200 mg) is added to the above solution. The blend is introduced into a 100 mL Teflon-lined stainless-steel autoclave after 1 hour of ultrasonication and 30 minutes of stirring. 20 hours are spent heating the autoclave to 180 °C. The white precipitates produced are centrifuged, washed five times using ultrapure water and ethanol and subsequently dried in a freezer dryer to obtain NB@CN (1 : 5). To prepare NB@CN composites with various mass ratios, the quantity of C₄H₄NNbO₉·XH₂O is regulated.

3. Results and discussion

3.1. Phase structure and morphology

XRD was conducted to look into the crystalline structure and phase purity of the produced photocatalysts. As shown in Fig. 1a, the characteristic diffraction peak of the g-C₃N₄ (002) crystal facet is visible at 27.2°.³⁴ The primary characteristic diffraction peaks for hexagonal phase Nb₂O₅ correspond to the (001), (101), (002), (110) and (102) facets at 22.5°, 28.3°, 36.6°, 46.1°, 50.8° and 55.2° (PDF# 30-0873).³⁵ Nonetheless, the diffraction strength of each peak of Nb₂O₅ falls as the g-C₃N₄ content increases, which is on account of the addition of g-C₃N₄ resulting in the reduction of Nb₂O₅ crystallinity. Furthermore, in comparison to g-C₃N₄ and Nb₂O₅, the XRD patterns of NB@CN contain all of the typical peaks of Nb₂O₅ and g-C₃N₄, and these findings can preliminarily demonstrate the successful construction of a heterojunction. The Fourier transform infrared spectrum (FT-IR) was employed to figure out the molecular structure of the catalysts. The results in Fig. 1b are in agreement with the FT-IR spectrum of the metal oxide, which exhibits no characteristic peaks beyond 1000 cm⁻¹, but a spectral band that is typically below 800 cm⁻¹.³⁶ There is an obvious peak at 803 cm⁻¹ for g-C₃N₄ and composite photocatalysts that can be attributed to the stretching vibration pattern of the triazine rings.³⁷ Besides, the characteristic stretching vibration mode of aromatic C=N and C–N in g-C₃N₄ is in the 1127–1723 cm⁻¹ region.^38,39 The wide peak at 3000–3500 cm⁻¹ is produced by the N–H of the exterior scattered amine group and the interaction of a trace of urea of partial decomposition with absorbed water.⁴⁰ All these results can confirm the ideal coupling of NB@CN heterojunctions.

Fig. 1 (a) XRD and (b) FT-IR spectrum of Nb₂O₅, g-C₃N₄ and NB@CN composites with different contents of g-C₃N₄.

SEM and TEM were employed to describe the morphology and structure of photocatalysts. Fig. 2a–c shows the NB@CN (1 : 5) heterojunction with a clumped shape, and the Nb₂O₅ flocculent is evenly dispersed on g-C₃N₄. The TEM images of the NB@CN (1 : 5) catalyst are shown in Fig. 2d–f, and g-C₃N₄ exhibits the typical amorphous characteristics, lacks lattice fringes, and exists in an amorphous form. Additionally, the catalyst has a 0.39 nm lattice spacing, which corresponds to the (001) plane of Nb₂O₅, which is consistent with the XRD findings, indicating that Nb₂O₅ is successfully attached to g-C₃N₄. Moreover, the EDX images (Fig. 2g) also prove the coexistence of Nb, C, O and N elements in the NB@CN (1 : 5) photocatalyst, which indicates that the NB@CN heterojunction is successfully constructed.

Fig. 2 (a)–(c) SEM and (d)–(f) TEM of the NB@CN (1 : 5) composite photocatalyst. (g) EDS mapping of NB@CN (1 : 5), N (green), O (blue), Nb (yellow), and C (red).

3.2. Elemental states analysis

XPS was conducted to examine the elemental states and chemical composition of the photocatalysts. Fig. 3a shows the overall spectra of Nb₂O₅, g-C₃N₄, and NB@CN (1 : 5), revealing the existence of O, Nb, C, and N elements in NB@CN (1 : 5). The spectra also show a great match with the elemental composition of g-C₃N₄ and Nb₂O₅. The C 1s XPS spectra of NB@CN (1 : 5) and g-C₃N₄ are illustrated in Fig. 3b. The three fitted peaks at binding energies of 287.6 eV, 285.8 eV, and 284.2 eV can be attributed to the N–C=N and C-NH_x (x = 1, 2) coordination peaks of the triazine ring in g-C₃N₄ and the foreign hydrocarbons (sp₂, C–C), respectively.^41,42 In Fig. 3c, the N 1s spectra exhibit three distinctive peaks located at 400.5 eV, 399.4 eV, and 397.9 eV, which correspond to the amino group (C–N–H), tertiary nitrogen N-(C)₃ groups, and sp² bonded nitrogen (C=N–C), respectively.^43,44 The O 1s spectra in Fig. 3d show two fitted peaks at 532.2 eV and 534.1 eV, which can be assigned to the lattice oxygen of Nb₂O₅ and the surface adsorbed oxygen in the –OH group, respectively.⁴⁵ The Nb 3d XPS spectra in Fig. 3e reveal two prominent peaks with spin orbits split to 2.7 eV, which are positioned at 207.9 eV and 210.6 eV and can be allocated to Nb 3d_5/2 and Nb 3d_3/2 of Nb⁵⁺.⁴⁶ The great symmetry and narrow half-peak width of the two peaks indicate that Nb⁵⁺ has a single valence state.⁴⁷ Compared with the XPS spectra of each element in pristine g-C₃N₄ and Nb₂O₅, the peaks of the NB@CN (1 : 5) composite are slightly shifted. These characteristics can be allocated to the powerful interaction between Nb₂O₅ and g-C₃N₄.

Fig. 3 (a) XPS spectra of Nb₂O₅, g-C₃N₄, and NB@CN (1 : 5). High-resolution XPS spectra of (b) C 1s, (c) N 1s, (d) O 1s, and (e) Nb 3d.

3.3. Catalytic activity

Under visible light irradiation, photocatalytic CO₂ reduction was performed for validating the impact of the constructed NB@CN (1 : 5) type II heterojunction on photo-generated carrier separation. As illustrated in Fig. 4a, the pristine Nb₂O₅ and g-C₃N₄ exhibit minimal photocatalytic CH₄ productivity, which is allocated to the rapid recombination of photo-induced charges. Fascinatingly, the CH₄ yield of the NB@CN (1 : 5) composite is significantly increased to 19.06 μmol g⁻¹ compared to those of pure Nb₂O₅ and g-C₃N₄. The reason for the phenomenon could be the building of type II heterojunctions that accelerate the separation and migration of charge carriers, promote the protonation of the photocatalyst, and boost the absorption of visible light. Among them, when the mass ratio of Nb₂O₅ to g-C₃N₄ exceeds 1 : 5, the NB@CN composite photocatalysts show decreased photocatalytic capacity. The shielding effect and unneeded recombination brought on by excess g-C₃N₄ could be the source of the occurrence. The yield changes of CO and CH₄ of the NB@CN (1 : 5) composite photocatalyst within 6 h are depicted in Fig. 4b. During the photocatalytic reaction of CO₂ reduction within 10 h, the yield of products is relatively stable, and within 6 hours, there is no overt increase or decrease, exhibiting a sustained growth trend. Interestingly, after the heterojunction construction of Nb₂O₅ and g-C₃N₄, the selectivity of CH₄ increases from 45.8% and 48% of pure Nb₂O₅ and g-C₃N₄ to 98.79%, as shown in Fig. 4c, which shows that the design of the NB@CN (1 : 5) heterojunction greatly improves the photocatalytic activity.

Fig. 4 (a) The yield of CO and CH₄ for the Nb₂O₅, g-C₃N₄ and NB@CN (1 : 5) samples with different contents of g-C₃N₄. (b) Alteration of the CO and CH₄ production by photocatalysis in 6 h for the NB@CN (1 : 5) catalyst. (c) The selectivity of CO and CH₄ for different samples. (d) The yield of CO and CH₄ in cycling tests of photocatalytic CO₂ reduction for NB@CN (1 : 5). (e) XRD spectra of NB@CN (1 : 5) before and after 5 cycles of CO₂ photoreduction. (f) CH₄ and CO mass spectra produced in ¹³CO₂ ambience. (g) The gas yield in a controlled trial. (h) Comparison of CO and CH₄ generating rate of NB@CN (1 : 5) and some Nb₂O₅, g-C₃N₄ based photocatalysts.

To demonstrate the steadiness of catalyst performance, we carried out a cycle test. As presented in Fig. 4d, the CO and CH₄ production only attenuated slightly after five cycles, indicating that the catalyst has great stability. Moreover, XRD was performed on the samples before and after the cycle, and the diffraction peaks of the XRD patterns closely match (Fig. 4e). This confirmed that the chemical states of the NB@CN (1 : 5) photocatalyst are not altered during the photocatalytic reaction process, indicating that the catalyst is extremely stable, which is also the key reason why it shows stable performance.⁴⁸ In order to clarify the sources of carbon in CH₄ and CO and eradicate the interference of polluting carbon during the process of preparation, a mass spectrometry experiment with isotope labeling was carried out under a ¹³CO₂ atmosphere, proving that every carbon-based product is transformed from CO₂,⁴⁹ as illustrated in Fig. 4f. The control experiments were conducted to demonstrate the necessary conditions of the photocatalytic process. As presented in Table S1† and Fig. 4g, the gas content in the four controlled experiments could be ignored revealing that CH₄ products were created by the photoreduction of CO₂ in NB@CN (1 : 5), and any adjustment to the reaction conditions will significantly impact the activity. In addition, we made a synthetic comparison about the catalytic reduction capability of CO₂ by some Nb₂O₅ and g-C₃N₄-based photocatalytic materials, and the results demonstrate that NB@CN (1 : 5) exhibits the best photocatalytic activity (Fig. 4h and Table S4†), further indicating that the combination of Nb₂O₅ and g-C₃N₄ is advantageous for boosting the catalytic reaction activity.

3.4. Photoelectric properties

The photoluminescence (PL), time-resolved PL decay and electrochemical impedance spectrum (EIS) Nyquist plots are obtained to further research the transport characteristics of carriers for Nb₂O₅, g-C₃N₄ and NB@CN. As displayed in Fig. 5a, at the 370 nm excitation wavelength, the peak appears at nearly 400 nm. The strength of the peak corresponds to the recombination frequency of photoinduced charges. As expected, NB@CN (1 : 5) presents the weakest peak intensity, and the photoluminescence intensity is obviously quenched, indicating that the association of g-C₃N₄ and Nb₂O₅ can successfully impede the photogenerated charge recombination. The lifetime of the photo-induced carriers was characterized by the time-resolved PL decay spectra that were captured at the corresponding steady-state emission peaks, as shown in Fig. 5b. In light of the findings of fitting, NB@CN (1 : 5) has the longest carrier average lifetime (τ_ave = 5.13 ns), which suggests that there is a more efficient photogenerated e⁻–h⁺ separation after the heterojunction construction of Nb₂O₅ and g-C₃N₄. Moreover, the EIS was utilized to further research the kinetics of charge migration for the catalysts. As illustrated in Fig. 5c, NB@CN (1 : 5) shows the smallest arc radius than that of other catalysts. In general, the resistance for charge migration is presented by the arc radius, and the decreased radius manifests a smaller resistance, that is quicker photo-induced electron–hole pair separation and transfer. Additionally, Fig. 5d illustrates that the NB@CN (1 : 5) heterojunction exhibits the strongest photocurrent response signal, compared with pure Nb₂O₅ and g-C₃N₄. Therefore, NB@CN (1 : 5) exhibits better photo-induced charge separation performance and presents better photocatalytic capacity.

Fig. 5 (a) PL spectra, (b) time-resolved PL decay spectra, (c) EIS spectra and (e) UV-vis DRS of Nb₂O₅, g-C₃N₄ and NB@CN samples with different contents of g-C₃N₄. (d) Photocurrent of the Nb₂O₅, g-C₃N₄ and NB@CN (1 : 5) samples. (f) Band gap energy spectra, (g) Mott–Schottky curves, (h) valence-band XPS spectra and (i) band structures of Nb₂O₅ and g-C₃N₄.

The optical absorption capabilities of the photocatalysts at various wavelengths are obtained through UV-Vis diffuse reflection spectroscopy. Fig. 5e depicts the UV-Vis absorption spectra in the 300–900 nm wavelength range, which demonstrates that the range of visible light response is expanded by the combination of Nb₂O₅ and g-C₃N₄. Within the wavelength range of 430–740 nm, NB@CN (1 : 5) exhibits the greatest ability to absorb visible light. As the content of g-C₃N₄ increased, samples started to gather, leading to a decrease in the utilization of visible light. According to the Tauc plot method,^50,51 the relationship curves between (αhv)² and hv obtained by UV-vis spectroscopy are shown in Fig. 5f. Nb₂O₅ and g-C₃N₄ have optical band gaps of 2.40 eV and 2.91 eV, respectively. The flat-band potentials (E_fb) of Nb₂O₅ and g-C₃N₄ are −0.66 V and −0.57 V vs. Ag/AgCl (−0.46 and −0.37 V vs. NHE), respectively (Fig. 5g). The Mott–Schottky curve has a positive slope, showing that Nb₂O₅ and g-C₃N₄ are N-type semiconductors.³⁶ As a result, the CBs of Nb₂O₅ and g-C₃N₄ are found to be −0.46 and −0.37 V, respectively. It is possible to deduce that the VBs are 1.94 and 2.54 V from the band gap value E_g. The distance between the VB and E_f may be calculated by VB-XPS (Fig. 5h), revealing that the Fermi level E_f values of Nb₂O₅ and g-C₃N₄ are 0.15 and 0.92 V, which are closer to the CB and comply with the band structure of N-type semiconductors.⁵²

The band structures of Nb₂O₅ and g-C₃N₄ are depicted in Fig. 5i according to the results discussed above. It can be seen that the CB and the VB of the semiconductor Nb₂O₅ are higher than the CB and the VB of the semiconductor g-C₃N₄. The locations of the bands indicate that a type II heterojunction between Nb₂O₅ and g-C₃N₄ has formed. Thus, when semiconductors are photoexcited, photo-induced electrons migrate from the CB of the Nb₂O₅ semiconductor to that of the g-C₃N₄ semiconductor, while photoinduced holes migrate in the opposing direction, from the VB of g-C₃N₄ to that of Nb₂O₅. Finally, the photo-generated e⁻ and h⁺ gather on the CB and VB of the semiconductors g-C₃N₄ and Nb₂O₅, respectively. Therefore, the separation of photoinduced e⁻ and h⁺ is realized, ensuring that more photoinduced electrons can be employed for photocatalytic conversion of CO₂, showing better photocatalytic performance.⁵³

3.5. Photocatalytic CO₂ reaction mechanism

The intermediates of the CO₂RR process are tracked by in situ DRIFTS under photocatalytic simulation to further research the mechanism of CO₂ conversion via photocatalysis. As illustrated in Fig. 6a–i, the critical intermediates in the CO₂ conversion process by Nb₂O₅, g-C₃N₄ and NB@CN (1 : 5) are successfully discovered after applying photocatalytic reaction conditions.^54,55 The potential mechanisms of photocatalytic conversion of CO₂ to produce CO and CH₄ utilizing water as an electron source are as follows: CO₂ → COOH* → CO* (partially, CO↑) → CHO* → CH₂O* → CH₃O* → CH₄.⁵⁶ As shown in Fig. 6a and c, the g-C₃N₄ sample exhibits signal peaks at 1761 cm⁻¹ and 1341 cm⁻¹, which correlate to the intermediate products COOH* and HCO₃* groups, respectively.⁵⁷ Generally speaking, these intermediates are regarded as the primary products in the process of converting CO₂ to CH₄ or CO. There is a small change in these groups’ positions for the Nb₂O₅ and NB@CN (1 : 5) catalysts in Fig. 6d–i. However, in general, it complies with group characteristic peak position requirements. The peaks at about 1073 and 1502 cm⁻¹ reveal the formation of CHO* and CH₂O* groups, which are typically considered to be the critical intermediates for the photocatalytic reduction of CO₂ to CH₄ and thus provide evidence that a CO₂ reduction reaction is occurring.^58,59

Fig. 6 In situ DRIFTS spectra for co-adsorption of a mixture of CO₂ and H₂O on (a) and (b) g-C₃N₄, (d) and (e) Nb₂O₅ and (g) and (h) NB@CN (1 : 5) during 30–40 min. Contour maps of in situ DRIFT of (c) g-C₃N₄, (f) Nb₂O₅ and (i) NB@CN (1 : 5).

The catalysts exhibit weaker CO* intermediates, which may be the result of rapid conversion.⁶⁰ Compared with pristine Nb₂O₅ and g-C₃N₄, the bulge peak corresponding to COOH* of NB@CN (1 : 5) at 1732 cm⁻¹ is stronger, indicating that the building of type II heterojunction between Nb₂O₅ and g-C₃N₄ can accelerate the CO₂ adsorption and activation by stabilizing the reaction intermediate COOH*. Furthermore, in the NB@CN (1 : 5) composite, the stronger CH₂O* peak corresponding to 1502 cm⁻¹ further indicates that the interaction of Nb₂O₅ and g-C₃N₄ to form a type II heterojunction is the key factor for the large amount of CH₄ formation in the CO₂ photoreduction process. These results reveal that NB@CN (1 : 5) possesses improved photocatalytic CO₂ reduction performance and increased CH₄ selectivity, which is in line with the outcomes of the performance tests.

4. Conclusions

In summary, NB@CN type II heterojunction composite photocatalysts are successfully prepared by a straightforward hydrothermal method, which combines the stable physical and chemical properties of Nb₂O₅ and g-C₃N₄ to establish a novel kind of catalytic system. The successful construction of a type II heterojunction demonstrates the effect of “killing two birds with one stone”, which not only greatly boosts the separation and migration efficiency of photo-induced charges, but also broadens the visible light response range of the composites. The above-mentioned benefits greatly facilitate the photocatalytic capacity of composites for CO₂ reduction under irradiation. The results show that the NB@CN (1 : 5) composite photocatalyst has a higher selectivity for CO₂ photoreduction to CH₄, and its CH₄ yield (19.06 μmol g⁻¹) is 15 and 20 times that of pristine Nb₂O₅ (1.23 μmol g⁻¹) and g-C₃N₄ (0.96 μmol g⁻¹) for 6 h, respectively. Moreover, the mechanism of photoreduction of CO₂ was thoroughly studied by in situ DRIFTS and photochemical characterization. This research offers a novel approach for achieving large-scale emission reduction and efficiently converting CO₂ into clean fuels.

Author contributions

Xiaofeng Wang: writing – original draft preparation and methodology. Jingwen Jiang: editing and investigation. Lilian Wang: validation and data curation. Hong Guo: research design, funding, and supervision.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

The authors acknowledge the support provided by the National Natural Science Foundation of Yunnan Province (202301AS070040), the Scientific Research Foundation of Education Department of Yunnan Province (2023Y0266), the Key Laboratory of Solid-State Ions for Green Energy of Yunnan University, the Yunnan Key Laboratory of Carbon Neutrality and Green Low-carbon Technologies, the Electron Microscope Center of Yunnan University, the MCP-WS1000 Photochemical Workstation (Beijing Perfectlight), and the GC9790Plus Gas Chromatograph (ZHEJIANG FULI ANALYTICAL INSTRUMENTS INC), and the authors would like to thank Jiao Kang from Shiyanjia Lab (https://www.shiyanjia.com) for the XPS analysis.

References

1. Q. J. Xu, J. W. Jiang, X. L. Sheng, Q. Jing, X. F. Wang, L. Y. Duan and H. Guo, Understanding the synergistic effect of piezoelectric polarization and the extra electrons contributed by oxygen vacancies on an efficient piezo-photocatalysis CO₂ reduction, Inorg. Chem. Front., 2023, 10, 2939–2950.
2. X. Wang, J. Jiang, L. Yang, Q. An, Q. Xu, Y. Yang and H. Guo, Enhanced piezoelectric polarization by subtle structure distortion to trigger efficient photocatalytic CO₂RR, Appl. Catal., B, 2024, 340, 123177.
3. L. Shi, Y. Yan, Y. Wang, T. Bo, W. Zhou, X. Ren and Y. Li, Efficient and selective photocatalytic CO₂ reduction over Ga single atom decorated quantum dots under visible light, Inorg. Chem. Front., 2023, 10, 2731–2741.
4. Q. Xu, J. Jiang, X. Wang, L. Duan and H. Guo, Understanding oxygen vacant hollow structure CeO₂@In₂O₃ heterojunction to promote CO₂ reduction, Rare Met., 2023, 42, 1888–1898.
5. F. Chen, Y. Zhang and H. Huang, Layered photocatalytic nanomaterials for environmental applications, Chin. Chem. Lett., 2023, 34, 107523.
6. S. Cao, B. Shen, T. Tong, J. Fu and J. Yu, 2D/2D heterojunction of ultrathin MXene/Bi₂WO₆ nanosheets for improved photocatalytic CO₂ reduction, Adv. Funct. Mater., 2018, 28, 1800136.
7. K. T. G. Carvalho, A. E. Nogueira, O. F. Lopes, G. Byzynski and C. Ribeiro, Synthesis of g-C₃N₄/Nb₂O₅ heterostructures and their application in the removal of organic pollutants under visible and ultraviolet irradiation, Ceram. Int., 2017, 43, 3521–3530.
8. Y. Peng, X. Guo, S. Xu, Y. N. Guo, D. Zhang, M. Wang, G. Wei, X. Yang, Z. Li, Y. Zhang and F. Tian, Surface modulation of MoS₂/O-ZnIn₂S₄ to boost photocatalytic H₂ evolution, J. Energy Chem., 2022, 75, 276–284.
9. J. Lei, Y. Chen, F. Shen, L. Wang, Y. Liu and J. Zhang, Surface modification of TiO₂ with g-C₃N₄ for enhanced UV and visible photocatalytic activity, J. Alloys Compd., 2015, 631, 328–334.
10. X. Zheng, D. Li, X. Li, J. Chen, C. Cao, J. Fang, J. Wang, Y. He and Y. Zheng, Construction of ZnO/TiO₂ photonic crystal heterostructures for enhanced photocatalytic properties, Appl. Catal., B, 2015, 168–169, 408–415.
11. J. Yang, X. Zhu, Z. Mo, J. Yi, J. Yan, J. Deng, Y. Xu, Y. She, J. Qian, H. Xu and H. Li, A multidimensional In₂S₃-CuInS₂ heterostructure for photocatalytic carbon dioxide reduction, Inorg. Chem. Front., 2018, 5, 3163–3169.
12. Z. Tang, W. He, Y. Wang, Y. Wei, X. Yu, J. Xiong, X. Wang, X. Zhang, Z. Zhao and J. Liu, Ternary heterojunction in rGO-coated Ag/Cu₂O catalysts for boosting selective photocatalytic CO₂ reduction into CH4, Appl. Catal., B, 2022, 311, 121371.
13. K. M. Kamal, R. Narayan, N. Chandran, S. Popović, M. A. Nazrulla, J. Kovač, N. Vrtovec, M. Bele, N. Hodnik, M. M. Kržmanc and B. Likozar, Synergistic enhancement of photocatalytic CO₂ reduction by plasmonic Au nanoparticles on TiO2 decorated N-graphene heterostructure catalyst for high selectivity methane production, Appl. Catal., B, 2022, 307, 121181.
14. S. Cao and J. Yu, g-C₃N₄-based photocatalysts for hydrogen generation, J. Phys. Chem. Lett., 2014, 5, 2101–2107.
15. Y. Ding, C. Wang, L. Pei, S. Maitra, Q. Mao, R. Zheng, M. Liu, Y. H. Ng, J. Zhong, L. H. Chen and B. L. Su, Emerging heterostructured C₃N₄ photocatalysts for photocatalytic environmental pollutant elimination and sterilization, Inorg. Chem. Front., 2023, 10, 3756–3780.
16. L. Xu, L. Li, Z. Hu and J. C. Yu, Boosting alkaline photocatalytic H2O2 generation by incorporating pyrophosphate on g-C₃N₄ for effective proton shuttle and oxygen activation, Appl. Catal., B, 2023, 328, 122490.
17. Q. Han, B. Wang, Y. Zhao, C. Hu and L. Qu, A graphitic-C₃N₄ “seaweed” architecture for enhanced hydrogen evolution, Angew. Chem., Int. Ed., 2015, 54, 11433–11437.
18. S. Cao, J. Low, J. Yu and M. Jaroniec, Polymeric photocatalysts based on graphitic carbon nitride, Adv. Mater., 2015, 27, 2150–2176.
19. Z. Su, J. Zhang, Z. Tan, J. Hu, F. Zhang, R. Duan, L. Yao, B. Han, Y. Zhao and Y. Yang, Facilitated photocatalytic H₂ production on Cu-coordinated mesoporous g-C₃N₄ nanotubes, Green Chem., 2023, 25, 2577–2582.
20. H. Wang, L. Zhang, Z. Chen, J. Hu, S. Li, Z. Wang, J. Liu and X. Wang, Semiconductor heterojunction photocatalysts: design, construction, and photocatalytic performances, Chem. Soc. Rev., 2014, 43, 5234–5244.
21. X. Sun, L. Li, S. Jin, W. Shao, H. Wang, X. Zhang and Y. Xie, Interface boosted highly efficient selective photooxidation in Bi₃O₄Br/Bi₂O₃ heterojunctions, eScience, 2023, 3, 100095.
22. D. W ang, Y. Zheng, H. Zhao, X. Zhu, D. Ye, Y. Yang, R. Chen and Q. Liao, Core-shell β-SiC@PPCN heterojunction for promoting photo-thermo catalytic hydrogen production, ACS Catal., 2023, 13, 10104–10114.
23. Y. Yao, F. Lu, J. Qin, F. Wei, C. Xu and S. Wang, Magnetic ZnFe₂O₄–C₃N₄ hybrid for photocatalytic degradation of aqueous organic pollutants by visible light, Ind. Eng. Chem. Res., 2014, 53, 17294–17302.
24. T. Zhu, Y. Song, H. Ji, Y. Xu, Y. Song, J. Xia, S. Yin, Y. Li, H. Xu, Q. Zhang and H. Li, Synthesis of g-C₃N₄/Ag₃VO₄ composites with enhanced photocatalytic activity under visible light irradiation, Chem. Eng. J., 2015, 271, 96–105.
25. Y. Xu and W. D. Zhang, CdS/g-C₃N₄ hybrids with improved photostability and visible light photocatalytic activity, Eur. J. Inorg. Chem., 2015, 2015, 1744–1751.
26. Y. Tian, B. Chang, J. Lu, J. Fu, F. Xi and X. Dong, Hydrothermal synthesis of graphitic carbon nitride–Bi₂WO₆ heterojunctions with enhanced visible light photocatalytic activities, ACS Appl. Mater. Interfaces, 2013, 5, 7079–7085.
27. S. M. Lam, J. C. Sin, I. Satoshi, A. Z. Abdullah and A. R. Mohamed, Enhanced sunlight photocatalytic performance over Nb₂O₅/ZnO nanorod composites and the mechanism study, Appl. Catal., A, 2014, 471, 126–135.
28. O. F. Lopes, E. C. Paris and C. Ribeiro, Synthesis of Nb₂O₅ nanoparticles through the oxidant peroxide method applied to organic pollutant photodegradation: A mechanistic study, Appl. Catal., B, 2014, 144, 800–808.
29. J. Zhang, D. Li, J. Qiu, Z. Wen, X. Luo, C. Bian, J. Chen and M. Luo, Insights into the photocatalytic degradation of triclosan over amorphous Nb₂O₅ catalysts, Mater. Res. Express, 2020, 7, 115502.
30. Z. Yue, D. Chu, H. Huang, J. Huang, P. Yang, Y. Du, M. Zhu and C. Lu, A novel heterogeneous hybrid by incorporation of Nb₂O₅ microspheres and reduced graphene oxide for photocatalytic H₂ evolution under visible light irradiation, RSC Adv., 2015, 5, 47117–47124.
31. L. C. A. Oliveira, M. Gonçalves, M. C. Guerreiro, T. C. Ramalho, J. D. Fabris, M. C. Pereira and K. Sapag, A new catalyst material based on niobia/iron oxide composite on the oxidation of organic contaminants in water via heterogeneous Fenton mechanisms, Appl. Catal., A, 2007, 316, 117–124.
32. A. E. Nogueira, O. F. Lopes, A. B. S. Neto and C. Ribeiro, Enhanced Cr(VI) photoreduction in aqueous solution using Nb₂O₅/CuO heterostructures under UV and visible irradiation, Chem. Eng. J., 2017, 312, 220–227.
33. T. A. Sedneva, E. P. Lokshin, M. L. Belikov and A. T. Belyaevskii, TiO₂- and Nb₂O₅-based photocatalytic composites, Inorg. Mater., 2013, 49, 382–389.
34. M. Liang, T. Borjigin, Y. Zhang, B. Liu, H. Liu and H. Guo, Controlled assemble of hollow heterostructured g-C₃N₄@CeO₂ with rich oxygen vacancies for enhanced photocatalytic CO₂ reduction, Appl. Catal., B, 2019, 243, 566–575.
35. Z. Xu, J. Jiang, Q. Zhang, G. Chen, L. Zhou and L. Li, 3D graphene aerogel composite of 1D-2D Nb₂O₅-g-C₃N₄ heterojunction with excellent adsorption and visible-light photocatalytic performance, J. Colloid Interface Sci., 2020, 563, 131–138.
36. J. Jiang, X. Wang, Q. Xu, Z. Mei, L. Duan and H. Guo, Understanding dual-vacancy heterojunction for boosting photocatalytic CO₂ reduction with highly selective conversion to CH4, Appl. Catal., B, 2022, 316, 121679.
37. G. Ge, X. Guo, C. Song and Z. Zhao, Reconstructing supramolecular aggregates to nitrogen-deficient g-C₃N₄ bunchy tubes with enhanced photocatalysis for H₂ production, ACS Appl. Mater. Interfaces, 2018, 10, 18746–18753.
38. L. He, M. Fei, J. Chen, Y. Tian, Y. Jiang, Y. Huang, K. Xu, J. Hu, Z. Zhao, Q. Zhang, H. Ni and L. Chen, Graphitic C₃N₄ quantum dots for next-generation QLED displays, Mater. Today, 2019, 22, 76–84.
39. W. Xing, C. Li, Y. Wang, Z. Han, Y. Hu, D. Chen, Q. Meng and G. Chen, A novel 2D/2D carbonized poly-(furfural alcohol)/g-C₃N₄ nanocomposites with enhanced charge carrier separation for photocatalytic H₂ evolution, Carbon, 2017, 115, 486–492.
40. K. C. Christoforidis, T. Montini, M. Fittipaldi, J. J. D. Jaén and P. Fornasiero, Photocatalytic hydrogen production by boron modified TiO₂/carbon nitride heterojunctions, ChemCatChem, 2019, 11, 6408–6416.
41. Y. Tan, Z. Shu, J. Zhou, T. Li, W. Wang and Z. Zhao, One-step synthesis of nanostructured g-C₃N₄/TiO₂ composite for highly enhanced visible-light photocatalytic H₂ evolution, Appl. Catal., B, 2018, 230, 260–268.
42. X. Lin, C. Liu, J. Wang, S. Yang, J. Shi and Y. Hong, Graphitic carbon nitride quantum dots and nitrogen-doped carbon quantum dots co-decorated with BiVO₄ microspheres: A ternary heterostructure photocatalyst for water purification, Sep. Purif. Technol., 2019, 226, 117–127.
43. Y. Li, S. Wang, W. Chang, L. Zhang, Z. Wu, S. Song and Y. Xing, Preparation and enhanced photocatalytic performance of sulfur doped terminal-methylated g-C₃N₄ nanosheets with extended visible-light response, J. Mater. Chem. A, 2019, 7, 20640–20648.
44. B. M. Armstrong, R. I. Sayler, B. H. Shupe, T. A. Stich, R. D. Britt and A. K. Franz, EPR evidence for the origin of nonlinear effects in an enantioselective Cu(II)-catalyzed spiroannulation, ACS Catal., 2019, 9, 1224–1230.
45. H. Jiang, C. Zang, Y. Zhang, W. Wang, C. Yang, B. Sun, Y. Shen and F. Bian, 2D MXene-derived Nb₂O₅/C/Nb₂C/g-C₃N₄ heterojunctions for efficient nitrogen photofixation, Catal. Sci. Technol., 2020, 10, 5964–5972.
46. P. Chen, W. Zhou, H. Zhang, Q. Pan, X. Zhang and B. Chu, Large thermal-electrical response and rectifying conduction behavior in asymmetrically reduced ferroelectric ceramics, ACS Appl. Electron. Mater., 2019, 1, 478–484.
47. S. Zhang, B. Zhang, D. Chen, Z. Guo, M. Ruan and Z. Liu, Promising pyro-photo-electric catalysis in NaNbO₃ via integrating solar and cold-hot alternation energy in pyroelectric-assisted photoelectrochemical system, Nano Energy, 2021, 79, 105485.
48. J. Jiang, X. Zou, Z. Mei, S. Cai, Q. An, Y. Fu, H. Wang, T. Liu and H. Guo, Understanding rich oxygen vacant hollow CeO₂@MoSe₂ heterojunction for accelerating photocatalytic CO₂ reduction, J. Colloid Interface Sci., 2022, 611, 644–653.
49. X. Wang, J. Jiang, Q. Xu, L. Duan and H. Guo, Understanding inclusive quantum dots hollow CN@CIZS heterojunction for enhanced photocatalytic CO₂ reduction, Appl. Surf. Sci., 2022, 604, 154601.
50. J. Jiang, X. Wang and H. Guo, Enhanced interfacial charge transfer/separation by LSPR-induced defective semiconductor toward high CO₂RR performance, Small, 2023, 2301280.
51. S. J. A. Moniz, S. A. Shevlin, D. J. Martin, Z. X. Guo and J. Tang, Visible-light driven heterojunction photocatalysts for water splitting—a critical review, Energy Environ. Sci., 2015, 8, 731–759.
52. J. D. Yi, R. Xie, Z. L. Xie, G. L. Chai, T. F. Liu, R. P. Chen, Y. B. Huang and R. Cao, Highly selective CO₂ electroreduction to CH₄ by in situ generated Cu₂O single-type sites on a conductive MOF: stabilizing key intermediates with hydrogen bonding, Angew. Chem., Int. Ed., 2020, 59, 23641–23648.
53. Y. Wang, K. Wang, J. Meng, C. Ban, Y. Duan, Y. Feng, S. Jing, J. Ma, D. Yu, L. Gan and X. Zhou, Constructing atomic surface concaves on Bi₅O₇Br nanotube for efficient photocatalytic CO₂ reduction, Nano Energy, 2013, 109, 108305.
54. X. Li, Y. Sun, J. Xu, Y. Shao, J. Wu, X. Xu, Y. Pan, H. Ju, J. Zhu and Y. Xie, Selective visible-light-driven photocatalytic CO₂ reduction to CH₄ mediated by atomically thin CuIn₅S₈ layers, Nat. Energy, 2019, 4, 690–699.
55. L. Liu, M. Li, F. Chen and H. Huang, Recent advances on single-atom catalysts for CO₂ reduction, Small Struct., 2023, 4, 2200188.
56. J. Wang, C. Hu, Y. Zhang and H. Huang, Engineering piezoelectricity and strain sensitivity in CdS to promote piezocatalytic hydrogen evolution, Chin. J. Catal., 2022, 43, 1277–1285.
57. Y. Liu, S. Chen, X. Quan and H. Yu, Efficient electrochemical reduction of carbon dioxide to acetate on nitrogen-doped nanodiamond, J. Am. Chem. Soc., 2015, 137, 11631–11636.
58. J. T. Yates and R. R. Cavanagh, Search for chemisorbed HCO: The interaction of formaldehyde, glyoxal, and atomic hydrogen + CO with Rh, J. Catal., 1982, 74, 97–109.
59. L. Liu and Y. Li, Understanding the reaction mechanism of photocatalytic reduction of CO₂ with H₂O on TiO₂-based photocatalysts: A review, Aerosol Air Qual. Res., 2014, 14, 453–469.
60. X. D. Zhang, T. Liu, C. Liu, D. S. Zheng, J. M. Huang, Q. W. Liu, W. W. Yuan, Y. Yin, L. R. Huang, M. Xu, Y. Li and Z. Y. Gu, Asymmetric low-frequency pulsed strategy enables ultralong CO₂ reduction stability and controllable product selectivity, J. Am. Chem. Soc., 2023, 145, 2195–2206.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qi01333b

---