Tuning the exposure of BiVO₄-{010} facets to enhance the N₂ photofixation performance

Abstract

Effective separation of photoexcited carriers and chemisorption of the N₂ molecule are two key issues to efficient nitrogen photofixation. The spatial charge separation of BiVO₄ with anisotropic exposed facets, namely the transfer of photoexcited electrons and holes to {010} and {110} facets, respectively, helps to enhance the separation ability of photogenerated carriers. Theoretical calculation results predict that a surface oxygen vacancy is easier to form on the (010) facet than on the (110) facet of BiVO₄. Accordingly, in this study, enhanced N₂ photofixation performance has been achieved for the first time by tuning the exposure of {010} facets of BiVO₄.

---

Nitrogen fixation to NH₃ is an important artificial synthesis in the chemical industry.¹ Nowadays, NH₃ is commonly produced through the Haber–Bosch process, which requires high temperature (400–500 °C) and high pressure (15–25 MPa).² This process accounts for ∼2% of the total global energy consumption and contributes ∼1.6% of the total global emissions.³ With the growing energy needs and demand for cleaner environment, a more environmentally friendly method is needed for NH₃ manufacture. Photocatalytic N₂ fixation, which employs solar energy and water to produce ammonium, is a promising sustainable and green strategy for NH₃ synthesis compared with the traditional Haber–Bosch process.^4–9 However, the photocatalytic performance of N₂ fixation is far from satisfactory due to the inefficient separation of photogenerated carriers, the high activation energy barriers and hard cleavage of the strong N≡N triple bond energy (941 kJ mol⁻¹) of the N₂ molecule.¹⁰

Crystal facet engineering of semiconductors is a significant strategy for fine-tuning the charge separation of photocatalysts.¹¹ Facet engineering of anatase TiO₂ has been given considerable research attention for photocatalytic reaction by controlling the {001} exposure ratio.^12–14 Recently, different research groups reported that the exposure of anisotropic facets of BiOX (X = Cl, Br, or I) enabled the directional transfer of photoexcited electrons and holes for spatial charge separation, accordingly improved the photocatalytic activities.^15–17 Other semiconductors including SrTiO₃,¹⁸ LaNbON₂,¹⁹ and C₃N₄ (ref. 20) also demonstrated that the predominating anisotropic facet exposure could improve the separation of photogenerated carriers. Simultaneously, the spatial charge separation of BiVO₄ with anisotropic exposed facets has been observed by direct imaging²¹ and photo-reduction or photo-oxidation reactions on {010} or {110} facets, respectively.^22–24 These results indicated that the photoexcited electrons (e⁻) and holes (h⁺) would transfer to the {010} and {110} facets, respectively. Based on the special property of BiVO₄, photocatalytic water splitting for O₂ evolution has been greatly improved.²⁵ However, a few researches on BiVO₄ with anisotropic exposed facets have been reported for photocatalytic N₂ fixation.

It is commonly understood that surface vacancies with abundant localized electrons play a critical role in N₂ photofixation by capturing and activating the inert N₂ molecule.^26,27 Efficient transfer of the photogenerated electrons to the inert N₂ molecule is also a key step for the effective photocatalytic N₂ fixation.²⁸ Considerable research results have revealed that surface oxygen vacancies could activate the N₂ molecule by chemisorption, and act as the transfer bridge of photoexcited electrons from photocatalysts to the activated N₂ molecule. For example, Pan et al. reported that the bond length of N≡N was elongated by the interaction with the surface O_VS on MoO_3−x nanobelts or W₁₈O₄₉ nanowires, and the photocatalytic activities for the N₂ fixation were directly related to the surface O_VS concentration.^29,30 The critical role of O_VS in the photofixation of N₂ was also revealed by other semiconductors including TiO₂,^10,31–33 BiOCl,³⁴ BiO quantum dots,³⁵ ultrafine Cu₂O,⁸ and amorphous CeO_x.³⁶ However, the surface O_VS on BiVO₄ for N₂ photofixation has been rarely studied.

Theoretical calculation results predicted that the formation energy (E_f) of an O_V on the surface of the representative (010) facet was lower than that on the surface of the typical (110) facet, accordingly there were more O_VS on the surface of the (010) facet. Together with the spatial charge separation property, anisotropic exposed BiVO₄ was believed to exhibit a good N₂ photofixation performance. In this study, BiVO₄ with anisotropic {010} and {110} facets were synthesized by a solid–liquid state reaction.³⁷ It was first found that the as-prepared BiVO₄ with higher percentage of exposed {010} facets exhibited better performance for N₂ photofixation without any sacrificial reagent and cocatalyst under ambient conditions. The easier formation of a surface oxygen vacancy (O_V) on the (010) facet was experimentally proved by the enhanced chemisorption of the N₂ molecule based on the temperature programmed desorption (TPD) characterization. The enhanced separation of photogenerated carriers and more surface oxygen vacancies (O_VS) made BiVO₄ with a higher exposed ratio of {010} facets to be more efficient for photocatalytic N₂ fixation.

The primitive unit cell consists of four units as shown by the side and top view of the optimized BiVO₄ (Fig. 1a and b), and the optimized lattice parameters are as follows: a = 7.33 Å, b = 11.77 Å, c = 5.18 Å, and β = 134.92°. They are in good agreement with the experimental values: a = 7.25 Å, b = 11.70 Å, c = 5.09 Å, and β = 134.225°. The formation energy of the oxygen vacancy (E_f) on the surface of (010) and (110) facets were calculated by following equation:

E_f = E_OV + E_½O2 − E_surface

where E_OV, E_½O2, and E_surface represent the total energy of surface with one oxygen vacancy, the half of total energy of oxygen and the total energy of surface without oxygen vacancy. The calculated formation energies of an oxygen vacancy were 2.91 eV and 4.93 eV on (010) and (110) surface, respectively. It means that the formation of a surface oxygen vacancy on the (010) facet was more energetically favorable than that on the (110) facet.

Fig. 1 The side view (a) and top view (b) of optimized monoclinic BiVO₄. The purple, blue and red atom indicates Bi, V and O, respectively; (c) the optimized structure of the (010) surface and (010) surface with one vacancy. (d) The optimized structure of the (110) surface and (110) surface with one vacancy.

Based on the theoretical calculation result, BiVO₄ with different exposure ratios of {010} facets was fabricated by tuning the nitric concentration in the reaction solution according to a previous report (see ESI†). The as-synthesized BiVO₄-0.50, BiVO₄-0.75, and BiVO₄-1.0 can be indexed to the monoclinic crystal structure (PDF# 14-0688) (Fig. 2a and S1†). However, the BiVO₄-0.25 sample in Fig. S1† consists of a hybrid monoclinic structure (PDF# 14-0688) and a tetragonal crystal phase (PDF# 14-0133), probably due to the low concentration of the HNO₃ solution. Furthermore, the (020) peak of BiVO₄-0.50 appears around 15° of 2θ (Fig. 2a). The as-synthesized BiVO₄ in different concentrations of HNO₃ shows different facet exposure ratios of {010} to {110}, and BiVO₄-0.50 exhibits the largest exposure ratio (Fig. 2b and S2†), which is consistent with the previous report.³⁷ The largest exposure of {010} facets must cause the appearance of the (020) peak in Fig. 2a. The typical TEM image shows the regular shape of BiVO₄-0.50 (Fig. 2c). The HRTEM interplanar spacing is 0.47 nm, corresponding to the value of (110) facet (Fig. 2d). The selected area electron diffraction (SAED) rings in the inset of Fig. 2d corresponds to the (110) and (040) crystal facets of monoclinic BiVO₄.

Fig. 2 Typical XRD pattern (a), SEM image (b), TEM image (c), and HRTEM image (d) of the as-synthesized BiVO₄-0.50. Inset in (d) SAED pattern.

Fig. 3 exhibits the temperature programmed desorption (TPD) characterization of the N₂ molecule. A single peak centering at 220 °C is observed, which is ascribed to the desorption of the chemisorbed N₂, and BiVO₄-0.50 exhibits the strongest chemisorption of the N₂ molecule. It is consistent with the theoretical calculation result shown in Fig. 1.

Fig. 3 N₂-TPD profiles of BiVO₄ synthesized in the aqueous solution with different HNO₃ concentrations.

For the general photocatalytic N₂ photofixation process in pure water, the photoexcited electrons are injected into the chemisorbed N₂ molecule via oxygen vacancy, and then the activated N₂ molecule combines with H⁺ from water to form NH₃. Simultaneously, the photogenerated holes oxidize the OH⁻ from water to produce O₂. The standard curve from the different concentrations of NH₄⁺ is shown in Fig. S4.† Photocatalytic performance test indicates that BiVO₄-0.50 exhibits the best activity for N₂ fixation among these four samples (Fig. 4a). The average NH₄⁺ evolution rate is about 15.5 μmol g⁻¹ L⁻¹ h⁻¹ for BiVO₄-0.50 during a 6 h test. The average NH₄⁺ evolution rates decrease to 13.1, 5.35, and 5.3 μmol g⁻¹ L⁻¹ h⁻¹ for BiVO₄-0.75, BiVO₄-1.00, and BiVO₄-0.25, respectively (Fig. 4a). The Brunauer–Emmett–Teller (BET) measurements indicate that the surface areas are 2.9 m² g⁻¹, 2.0 m² g⁻¹, 1.8 m² g⁻¹, and 1.5 m² g⁻¹ for BiVO₄-0.25, BiVO₄-0.50, BiVO₄-0.75, and BiVO₄-1.00, respectively. The corresponding ratios of the average NH₄⁺ evolution rate to surface area are 4.5 μmol L⁻¹ h⁻¹ m⁻², 7.8 μmol L⁻¹ h⁻¹ m⁻², 3.0 μmol L⁻¹ h⁻¹ m⁻², and 3.5 μmol L⁻¹ h⁻¹ m⁻². It is obvious that the surface area is not the determining factor of the photocatalytic activity. In order to confirm the origination of the nitrogen element in NH₄⁺, Ar gas was bubbled and the aqueous suspension of the BiVO₄-0.50 sample was irradiated by a 300 W xenon lamp during the test time. It is found that there is no detectable NH₄⁺ (Fig. 4a), and it can be concluded that the detected NH₄⁺ did not originate from environmental contamination, but from the photofixation reaction of N₂ molecule by the BiVO₄ photocatalyst. Fig. 4b shows the light-wavelength-dependent photofixation activity of BiVO₄-0.50. The photocatalytic performance decreases from 365 nm and exhibits almost no NH₄⁺ from 515 nm, and no NH₄⁺ is detected at 590 nm due to the light absorption range (Fig. S3†). The photocatalyst possessed photoexcited electrons with higher energy under the irradiation of the shorter wavelength light, and the photofixation of N₂ was more easy to occur. However, the photofixation reaction of N₂ molecule could not occur when the longer wavelength light was unable to excite the BiVO₄ photocatalyst. This result further confirmed the photofixation of N₂ in our study. The calculated value of quantum efficiency (QE) at 365 nm was about 0.003%.

Fig. 4 (a) Photocatalytic nitrogen fixation performance of BiVO₄ samples synthesized with different concentrations of nitric acid (light source: 300 W xenon lamp; photocatalyst: 0.05 g; reaction solution: 100 ml of pure water). (b) Nitrogen fixation performance of BiVO₄-0.50 illuminated by LED with different wavelengths (365 nm, 384 nm, 400 nm, 470 nm, 498 nm, 515 nm, and 590 nm).

In order to evaluate the stability of BiVO₄-0.50 for N₂ photofixation, three cycle tests were carried out, as exhibited in Fig. 5. After each cycle, the photocatalyst was washed by vacuum filtration with pure water for several times and dried in a vacuum oven. The photocatalytic activity gradually decreased probably due to the loss of photocatalyst during the wash process.

Fig. 5 Photocatalytic stability test for BiVO₄-0.50 (light source: 300 W xenon lamp; photocatalyst: 0.05 g; reaction solution: 100 ml of pure water).

In summary, BiVO₄ with exposed anisotropic {010} and {110} facets was synthesized, and studied for the first time for ammonia production by photocatalytic N₂ fixation from pure water. It was found that the sample with the high facet exposure ratio of {010} to {110} exhibited a better photocatalytic performance for N₂ fixation, which resulted from the better separation ability of the photoexcited carriers and more surface O_VS on the {010} facet. The photoexcited electrons more effectively transferred to the more surface O_VS on the typical (010) facet, where the N₂ molecule can be activated, and accordingly benefited to the enhancement of the photocatalytic performance. Our study provides a good strategy to improve the photocatalytic activity for N₂ fixation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The research was financially supported by the Chemistry and Chemical Engineering Guangdong Laboratory (Grant No. 1922018).

References

1. D. E. Canfield, A. N. Glazer and P. G. Falkowski, Science, 2010, 330, 192–196.
2. G. N. Schrauzer and T. D. Guth, J. Am. Chem. Soc., 1977, 99, 7189–7193.
3. P. Friedlingstein, M. O'Sullivan, M. W. Jones, R. M. Andrew, J. Hauck, A. Olsen, G. P. Peters, W. Peters, J. Pongratz and S. Sitch, Earth Syst. Sci. Data, 2020, 12, 3269–3340.
4. Y. Feng, Z. Zhang, K. Zhao, S. Lin, H. Li and X. Gao, J. Colloid Interface Sci., 2021, 583, 499–509.
5. W. Gao, X. Li, S. Luo, Z. Luo, X. Zhang, R. Huang and M. Luo, J. Colloid Interface Sci., 2020, 585, 20–29.
6. X. Hu, Y. Yong, Y. Xu, X. Hong, Y. Weng, X. Wang and X. Yao, Appl. Surf. Sci., 2020, 531, 147348.
7. N. Ojha, A. Bajpai and S. Kumar, J. Colloid Interface Sci., 2020, 585, 764–777.
8. S. Zhang, Y. Zhao, R. Shi, C. Zhou, G. I. Waterhouse, Z. Wang, Y. Weng and T. Zhang, Angew. Chem., Int. Ed., 2021, 60, 2554–2560.
9. Z. Zhao, C. Choi, S. Hong, H. Shen, C. Yan, J. Masa, Y. Jung, J. Qiu and Z. Sun, Nano Energy, 2020, 78, 105368.
10. H. Hirakawa, M. Hashimoto, Y. Shiraishi and T. Hirai, J. Am. Chem. Soc., 2017, 139, 10929–10936.
11. J. Bai, J. Sun, X. Zhu, J. Liu, H. Zhang, X. B. Yin and L. Liu, Small, 2020, 16, 1904783.
12. L. Ruan, X. Wang, T. Wang, Z. Ren, Y. Chen, R. Zhao, D. Zhou, G. Fu, S. Li, L. Gao, Y. Lu, Z. Wang, H. Tian, X. Kong and G. Han, ACS Appl. Mater. Interfaces, 2019, 11, 37256–37262.
13. T. Tachikawa, S. Yamashita and T. Majima, J. Am. Chem. Soc., 2011, 133, 7197–7204.
14. X. Han, Q. Kuang, M. Jin, Z. Xie and L. Zheng, J. Am. Chem. Soc., 2009, 131, 3152–3153.
15. M. Shi, G. Li, J. Li, X. Jin, X. Tao, B. Zeng, E. A. Pidko, R. Li and C. Li, Angew. Chem., Int. Ed., 2020, 59, 6590–6595.
16. T. Li, C. Wang, T. Wang and L. Zhu, Appl. Catal., B, 2020, 268, 118442.
17. M. Li, S. Yu, H. Huang, X. Li, Y. Feng, C. Wang, Y. Wang, T. Ma, L. Guo and Y. Zhang, Angew. Chem., Int. Ed., 2019, 58, 9517–9521.
18. L. Mu, Y. Zhao, A. Li, S. Wang, Z. Wang, J. Yang, Y. Wang, T. Liu, R. Chen, J. Zhu, F. Fan, R. Li and C. Li, Energy Environ. Sci., 2016, 9, 2463–2469.
19. X. Wang, T. Hisatomi, J. Liang, Z. Wang, Y. Xiang, Y. Zhao, X. Dai, T. Takata, K. Domen and K. Domen, J. Mater. Chem. A, 2020, 8, 11743–11751.
20. L. Lin, Z. Lin, J. Zhang, X. Cai, W. Lin, Z. Yu and X. Wang, Nat. Catal., 2020, 3, 649–655.
21. J. Zhu, F. Fan, R. Chen, H. An, Z. Feng and C. Li, Angew. Chem., Int. Ed., 2015, 54, 9111–9114.
22. H. Li, Y. Sun, B. Cai, S. Gan, D. Han, L. Niu and T. Wu, Appl. Catal., B, 2015, 170, 206–214.
23. R. Li, H. Han, F. Zhang, D. Wang and C. Li, Energy Environ. Sci., 2014, 7, 1369–1376.
24. J. Lu, Y. Lei, K. C. Lau, X. Luo, P. Du, J. Wen, R. S. Assary, U. Das, D. J. Miller and J. W. Elam, Nat. Commun., 2013, 4, 1432.
25. D. Wang, H. Jiang, X. Zong, Q. Xu, Y. Ma, G. Li and C. Li, Chem.–Eur. J., 2011, 17, 1275–1282.
26. S. Wang, X. Hai, X. Ding, K. Chang, Y. Xiang, X. Meng, Z. Yang, H. Chen and J. Ye, Adv. Mater., 2017, 29, 1701774.
27. T. Hou, Y. Xiao, P. Cui, Y. Huang, X. Tan, X. Zheng, Y. Zou, C. Liu, W. Zhu, S. Liang and L. Wang, Adv. Energy Mater., 2019, 9, 1902319.
28. Y. Liu, M. Cheng, Z. He, B. Gu, C. Xiao, T. Zhou, Z. Guo, J. Liu, H. He, B. Ye, B. Pan and Y. Xie, Angew. Chem., Int. Ed., 2019, 58, 731–735.
29. W. Ren, Z. Mei, S. Zheng, S. Li, Y. Zhu, J. Zheng, Y. Lin, H. Chen, M. Gu and F. Pan, Research, 2020, 3750314.
30. Y. Li, X. Chen, M. Zhang, Y. Zhu, W. Ren, Z. Mei, M. Gu and F. Pan, Catal. Sci. Technol., 2019, 9, 803–810.
31. X. Niu, Q. Zhu, S. Jiang and Q. Zhang, J. Phys. Chem. Lett., 2020, 11, 9579–9586.
32. J. Yang, Y. Guo, R. Jiang, F. Qin, H. Zhang, W. Lu, J. Wang and J. C. Yu, J. Am. Chem. Soc., 2018, 140, 8497–8508.
33. Y. Zhao, Y. Zhao, R. Shi, B. Wang, G. I. N. Waterhouse, L. Z. Wu, C. H. Tung and T. Zhang, Adv. Mater., 2019, 31, 1806482.
34. Y. Shiraishi, M. Hashimoto, K. Chishiro, K. Moriyama, S. Tanaka and T. Hirai, J. Am. Chem. Soc., 2020, 142, 7574–7583.
35. S. Sun, Q. An, W. Wang, L. Zhang, J. Liu and W. A. Goddard, J. Mater. Chem. A, 2017, 5, 201–209.
36. C. Zhang, Y. Xu, C. Lv, L. Bai, J. Liao, Y. Zhai, H. Zhang and G. Chen, Appl. Catal., B, 2020, 264, 118416.
37. H. L. Tan, X. Wen, R. Amal and Y. H. Ng, J. Phys. Chem. Lett., 2016, 7, 1400–1405.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra02739e

---