Reverse construction of dominant/secondary facets in Bi₂₄O₃₁Br₁₀ photocatalysts for boosting electronic transfer

Abstract

In this paper, it is found that the preferential growth of secondary {117} facets of Bi₂₄O₃₁Br₁₀ into dominant facets would lead to higher photocatalytic activity, although the original main {213} facet has a stronger molecular oxygen adsorption ability, which illustrates that the charge separation efficiency induced by dominant/secondary facet control plays a more important role than that of O₂ adsorptive performance in improving activity.

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With the continuous rapid development of human industrial civilization, environmental pollution has inevitably become a very prominent problem.^1–3 As a green, economically viable energy conversion technology, photocatalysis has attracted widespread attention and has made significant progress.^4–8 However, it is still suffering from poor visible-light absorption, low separation efficiency of charge carriers, and an unclear structure–performance relationship.^9–11

Bi-Based semiconductor catalysts emerged in the 1990s.^12,13 Different from the interlaced arrangement of anions and cations in most metal compounds, BiOX (X = Cl, Br, I) photocatalysts have a tetragonal crystal structure, in which halogens form an [X₂]²⁻ layer, and bismuth and oxygen form alternating [Bi₂O₂]²⁺ layers.¹⁴ BiOX photocatalysts have a large internal electric field and asymmetric polarization effect due to their unique layered structure, which improves the separation efficiency of charge.^15–18 At the same time, because of their suitable energy band positions that are able to respond to visible light,^19–22 they have aroused increasing interest in recent years. It is reported that with the increase of Bi content, the Bi-rich bismuth oxyhalide catalyst moves up the conduction band position, and the reduction ability of photogenerated electrons and photocatalytic activity can be improved. For example, compared to BiOBr, Bi₃O₄Br and Bi₂₄O₃₁Br₁₀ have a better visible light absorption capacity, and more efficient photocatalytic performance including Cr(VI) reduction and water splitting.^23,24

In photocatalytic oxidation reactions, the activation of molecular oxygen to generate reactive oxygen species (ROS) is one of the key processes determining photocatalytic activity.²⁵ Different types of ROS, such as hydroxyl radicals (˙OH), superoxide radicals (˙O₂⁻), singlet oxygen (¹O₂) and hydrogen peroxide (H₂O₂),^26–30 have different oxidizing capabilities. The main reactive oxygen species (ROS) produced by BiOX is ˙O₂⁻.^31,32 The generation of ˙O₂⁻ is determined by the concentration and transfer efficiency of free electrons generated by photocatalysis.^20,33 Adjusting the surface structure, such as facets, can promote the separation of surface electrons and holes, thereby promoting the generation of ROS.

In recent years, research on material crystal facet engineering has attracted widespread attention. In particular, research on {110}, {101}, {100}, {010} and {001} crystal facets has made considerable progress,^{16,19,20,33–35} but research on other crystal facets is still relatively limited, and there are few studies focusing on the main crystal facets of Bi-rich BiOX. Mao et al. synthesized {001}-facet exposed Bi₃O₄Br nanosheets for robust photocatalysis against phenolic pollutants.³⁶ Meng et al. synthesized Bi₁₂O₁₇Br₂ nanosheets with oxygen vacancies and {0012} as the main crystal facet.³⁷ Zeng et al. synthesized Bi₂₄O₃₁Br₁₀ nanobelts (NBs) with dominant {304} facets and nanosheets (NSs) with dominant {117} facets to degrade tetracycline hydrochloride under visible light.³⁸ However, the facet-heterojunction effects of Bi-rich BiOX are rarely paid attention.

In Bi₂₄O₃₁Br₁₀, {213} is the dominant facet and {117} is the secondary one and they can form facet heterojunctions, as shown in Fig. 1a. Herein, we synthesized Bi₂₄O₃₁Br₁₀ with different {213}/{117} crystal facet ratios in one step using ionic liquid self-combustion. The different ratios of crystal facets, and even reverse growth for {213} and {117}, are achieved by adjusting the ratio of reactants. The importance of the relationship between the O₂ adsorption of different facets and the separation efficiency of charge carriers is investigated using the degradation of a typical azo dye Rhodamine B (RhB) under visible light and theoretical analysis.

Fig. 1 (a) Schematic diagram of crystal facet positions and (b) XRD patterns of the Bi₂₄O₃₁Br₁₀ samples.

All the Bi₂₄O₃₁Br₁₀ samples are collected as powders for X-ray diffraction (XRD) characterization. The results are shown in Fig. 1b. All the diffraction peaks of Bi₂₄O₃₁Br₁₀ could be indexed to a monoclinic phase of the monoclinic structure of Bi₂₄O₃₁Br₁₀ (PDF # 01-075-0888), indicating their high purity. However, it was found that the {213} and {117} facets have different intensity ratios of 0.64, 0.74, 0.83, and 1.12 (in the following text, we use these values to represent the Bi₂₄O₃₁Br₁₀ photocatalysts with different {213}/{117} facet ratios, respectively). It implies that the preferential growth ability of the {213} and {117} facets can be controlled by means of this technique. It was also calculated that the intensity ratio of {213}/{117} in the standard Bi₂₄O₃₁Br₁₀ is 1.41. The dominant facet in the standard card is {213}, but the induction of the ionic liquid self-combustion method makes {117} the dominant one, which could be related to the adsorption ability of ions on facets in the combustion process.³⁹

As is shown in Fig. 1a, it is clear that {213} and {117} form a cross. Moreover, the morphology and structure of the as-obtained Bi₂₄O₃₁Br₁₀ samples is revealed using SEM and TEM images. As shown in Fig. S1 (ESI†), they are sheet-like in character with a thickness of about 80–100 nm. Fig. S2a (ESI†) shows the corresponding diffraction pattern, which shows the high crystallinity of the crystal. As shown in Fig. S2b and c (ESI†), it is clear that the as-obtained Bi₂₄O₃₁Br₁₀ samples are nanosheets with good crystallinity. The {213} and {117} facets can be found in the HRTEM images, as shown in Fig. S2d and e (ESI†), respectively. The {213} and {117} facet-heterojunction can be found in the HRTEM image, Fig. S2f (ESI†).

The adsorption of O₂ on the {213} and {117} facets is calculated using density functional theory (DFT), respectively, as shown in Fig. 2a and b. The calculated results are listed in Tables S2 and S3 (ESI†). O₂ can be adsorbed on the {213} and {117} facets. The adsorption energy of O₂ on the {213} facet is −0.08 eV and the adsorption energy on the {117} facet is 0.05 eV. Compared with the {117} facet, the {213} facet exhibits a more negative value, so O₂ is more easily adsorbed on the {213} facet. It can be concluded that although the {213} facet adsorbs O₂ more easily, the 1.12 sample with more preferential growth of the {213} facet does not have a higher activity, indicating that the adsorption of oxygen molecules is not as important as the separation efficiency in generating ˙O₂⁻ and improving activity.

Fig. 2 O₂ adsorption model on (a) {213} and (b) {117} facets.

Fig. S3 (ESI†) shows the nitrogen absorption–desorption isotherms and the corresponding BJH pore size distribution curves (insets) of the as-synthesized samples. According to the IUPAC classification, all isotherms are identified as type IV with an H3 hysteresis loop and exhibit mesoporous structures. The calculated BET specific surface areas (S_BET) and the pore parameters are listed in Table S4 (ESI†). The results indicate that there is only a small difference in the S_BET of the samples, implying that S_BET may not play the key role in the adsorption and degradation of RhB. Meanwhile, the effect of the defective oxygen vacancies has also been ruled out, as shown in Fig. S4 (ESI†).

The UV-Vis absorption spectra of the samples and their appearance are shown in Fig. S5a (ESI†). All samples exhibit an absorption band at approximately 470 nm. 0.74 and 0.83 Bi₂₄O₃₁Br₁₀ show the strongest photo-absorption from 200 nm to 400 nm, implying there is multi-refection of absorbed light between the {213} and {117} facets.⁴⁰ The corresponding band gap energies (E_g) are estimated to be about 2.62 eV, 2.62 eV, 2.65 eV and 2.59 eV for 0.64, 0.74, 0.83, and 1.12 Bi₂₄O₃₁Br₁₀, respectively, based on the empirical equation E_g = 1240/λg (nm).40 The Mott–Schottky curves in Fig. S5b (ESI†) show the n-type semiconducting features of the samples, and from these curves, the electron concentration of 0.74 Bi₂₄O₃₁Br₁₀ is estimated to be higher than that of the others.

In order to explore the band structure of semiconductors in depth, the conduction band position of the samples is determined using Mott–Schottky tests. As shown in Fig. S5b (ESI†), the tangent of the C⁻²–E curve of the samples is extended to the x-axis, and the intersection point is the flat band potential value. As can be seen from Fig. S5b (ESI†), the flat potential of the samples is determined to be −0.66 V, −0.68 V, −0.77 V and −0.81 V (vs. Ag/AgCl), respectively. It is then converted into the corresponding reversible hydrogen electrode (RHE) using eqn (1).1 Therefore, the flat band potential value of the samples is −0.06 V, −0.08 V, −0.17 V and −0.21 V relative to the RHE, respectively. The position of the flat band potential is very close to the position of its conduction band (CB). The position of the valence band (VB) can be obtained using eqn (2),41 thus the corresponding VB potentials of the samples are 2.56 V, 2.54 V, 2.48 V and 2.38 V. Fig. S5c (ESI†) shows the band structure diagrams.

E_RHE = E(Ag/AgCl) + 0.6 V (1)

E_CB = E_VB − E_g (2)

In order to explore the photocatalytic performance of the samples in a practical application, the photocatalytic degradation of the dye RhB using the as-synthesized samples is evaluated under visible light irradiation, as shown in Fig. 3a. Under the same test conditions, 50 mg L⁻¹ RhB does not self-degrade in the absence of the samples, indicating that the self-photolysis of RhB can be ignored. When the as-prepared photocatalysts are introduced into the RhB degradation system, after 30 min under magnetic stirring in the dark to achieve adsorption/desorption equilibrium, the C/C₀ values of 0.64, 0.74, 0.83, and 1.12 Bi₂₄O₃₁Br₁₀ are 0.99, 0.95, 0.97, 0.97, respectively. All the samples demonstrated excellent photocatalytic activity, and 0.74 Bi₂₄O₃₁Br₁₀ achieved 100% degradation and showed the highest degradation efficiency. The 1.12 Bi₂₄O₃₁Br₁₀ sample, which is closest to the standard card crystal facet ratio, has an extremely low activity. The first-order kinetic degradation curve is fitted, as shown in Fig. S6 and Table S5 (ESI†). In order to test the stability of the photocatalysts, 0.64 Bi₂₄O₃₁Br₁₀ is used for cyclic tests, as shown in Fig. 3b, after five cycles, the performance of the sample only changed slightly, this can be attributed to loss of the sample, proving that the catalyst has good stability. A series of scavenger experiments was further performed for the purpose of deeply studying the reaction mechanism in the photocatalytic process, as shown in Fig. 3c. When AO and IPA and BQ are added to the RhB degradation system, the degradation efficiency is significantly different to that without scavenger, which suggests that ˙O₂⁻, ˙OH and holes all play a key role in RhB degradation. Specifically, with the addition of BQ, the final photocatalytic degradation efficiency is significantly inhibited, illustrating that ˙O₂⁻ plays the most important role in this oxidation reaction. ˙O₂⁻ radicals can react with NBT that has a maximum absorption wavelength at 260 nm, while the product of ˙O₂⁻ and NBT does not.^42,43 As shown in Fig. 3d, the NBT degradation activity results are consistent with the order of all of the photocatalysts. This result also implies that ˙O₂⁻ plays a crucial role in oxidizing RhB.

Fig. 3 (a) Photocatalytic degradation of RhB using different samples under visible light illumination, (b) cyclic tests of 0.64 Bi₂₄O₃₁Br₁₀, (c) scavenger experiments, and (d) time-course variation of the C/C₀ of NBT under visible-light illumination.

The surface photovoltage (SPV), which is shown in Fig. 4a that arose from the transfer and/or redistribution of space-separated photogenerated charge carriers is measured.⁴⁴ It is clear that 0.74 Bi₂₄O₃₁Br₁₀ demonstrated the highest SPV response intensity, the 0.83 Bi₂₄O₃₁Br₁₀ response was preceded only by the 0.74 Bi₂₄O₃₁Br₁₀ SPV response intensity, and the 0.64 and 1.12 Bi₂₄O₃₁Br₁₀ samples showed a much weaker SPV response intensity signal, illustrating the efficient charge separation results from different {213} and {117} facet growth. To further study the separation of electron–hole pairs, time-resolved photoluminescence spectra are recorded, which are fitted and the corresponding lifetimes and percentages are listed in Table S6 (ESI†). As shown in Fig. 4b, the 0.74 Bi₂₄O₃₁Br₁₀ exhibited the longest lifetime compared to that of the others, which further explained that proper facet preferential growth is conducive to the separation of electron–hole pairs. As shown in Fig. 4c, the current intensities of the samples had not decreased significantly after six 30 s intermittent switching lamp cycle experiments, indicating their excellent reproducibility and stability. Among them 0.74 Bi₂₄O₃₁Br₁₀ showed the strongest photocurrent response, this is consistent with the results of SPV analysis (Fig. 4a). The EIS results are shown in Fig. 4d, 0.74 Bi₂₄O₃₁Br₁₀ possesses the lowest electrochemical impedance of all the samples, resulting in enhanced electron transport on the {213} and {117} facets. These results clearly indicate higher carrier concentrations and interfacial charge transport on the surface of 0.74 Bi₂₄O₃₁Br₁₀.

Fig. 4 (a) SPV spectra, (b) time-resolved fluorescence decay spectra, (c) transient photocurrent responses with light on-and-off cycles and (d) EIS measurements.

On the basis of the above results, a reasonable energy band diagram of the photocatalytic mechanism of the samples is proposed in Fig. S7 (ESI†). Under visible light irradiation, the photo-induced electrons of Bi₂₄O₃₁Br₁₀ transfer from the VB to the CB and above to form hot electrons⁴⁵ and the formed hot electrons will oxidize O₂ to form ˙O₂⁻ radicals, then the ˙O₂⁻ radicals react with RhB to form some intermediate products, and eventually generate CO₂ and H₂O.^46–48 This step is the major reaction. Meanwhile, some of the holes remaining on the VB after excitation can directly oxidize RhB, and the others react with H₂O to form ˙OH. ˙OH can also be generated from ˙O₂⁻. The ˙OH further oxidizes RhB to obtain products including CO₂ and H₂O.^39,49 This ˙OH oxidation process is the minor reaction.

In summary, Bi₂₄O₃₁Br₁₀ with adjustable crystal facets of {213} and {117} was obtained using the ionic liquid self-combustion method. Compared with the other samples, 0.74 Bi₂₄O₃₁Br₁₀ exhibits the highest RhB degradation activity under visible light. The {213} crystal facet has a stronger O₂ adsorption capacity, and the facets-heterojunction effects lead to better electron transfer abilities. The synergy of the two can enhance the activity of the catalyst more effectively. An appropriate {213} and {117} crystal facet ratio can promote the separation and transfer of surface charge and improve the ability of O₂ absorption, thereby generating more superoxide radicals (˙O₂⁻). This work provides insight into the photocatalytic mechanism with respect to the importance of the comparison between O₂ absorption ability and charge transfer, and offers a new route for facet and heterojunction construction.

This work was financially supported by the National Natural Science Foundation of China (No. 22178084, 21776059), the Foundation for Innovative Research Groups of the Natural Science Foundation of Hebei Province (No. B2021208005), and the Natural Science Foundation for Distinguished Young Scholars of Hebei Province (No. B2015208010).

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed information about materials, methods, characterization, as well as additional experimental results. See DOI: 10.1039/d1cc04003k

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