Alkanes are an important group of artificial pollutants emitted by the petrochemical industry. After their emission to the atmosphere, these pollutants will easily react with the active species in the atmospheric environment and transform to complicated pollutants of ozone and organic aerosol [1, 2]. More importantly, these secondary products display significantly enhanced negative effects on both the atmospheric environment and the human beings living around [3, 4]. Thus, efficient elimination of alkanes at the emission source is urgent for the protection of atmospheric environment and human health.

Semiconductor-based photocatalysis has been considered as an effective technique for the degradation of industrial gaseous organics [5-7]. Conventional semiconductor photocatalysts (e.g., TiO₂) are limited by their wide band gaps (e.g., 3.2 eV), which mean that they only respond to UV light [8-11]. Comparably, Bi-based semiconductors, such as BiOBr, have attracted increasing attention for the degradation of gaseous organics owing to their better optical, electrical, and photochemical properties [12-15]. For instance, Liu et al. [16] reported three times higher elimination efficiency of benzene on BiOBr than on TiO₂ under UV irradiation within 90 min, which was attributed to its better crystalline nanosheet structure. Furthermore, Feng et al. [17] obtained nearly two times higher reaction rate constant on BiOBr than on TiO₂ during the visible light photocatalytic degradation of toluene, which was attributed to the unique hierarchical structure and suitable band gap energy of BiOBr. Although BiOBr displays improved photocatalytic performance compared to that of TiO₂, its visible-light-driven efficiency in the degradation of gaseous pollutants is still not high. Meanwhile, in comparison with aromatic organics, straight-chain alkanes are well known for being more difficult to be photocatalytically degraded [18]. Hence, to more efficiently utilize the solar light and enhance the activity of BiOBr toward gaseous alkanes, several methods have been attempted, including metal deposition [19, 20], metal oxide composition [21, 22] and carbon material immobilization [23, 24]. Among these methods, the in-situ decoration of metallic Bi on BiOBr is considered as a facile operational method. Moreover, previous researchers have confirmed that metallic Bi modification can further enhance the solar light photocatalytic efficiency of BiOBr [25, 26]. Unfortunately, the available literature has focused on the application of Bi/BiOBr composite to the degradation of solution organic pollutants (e.g., ciprofloxacin [19], phenol [27], and methyl orange [28]), and attempts to utilize them in the purification of gaseous organic pollutants (e.g., alkanes) have never been made. Moreover, the relationship between the degradation performance and the synthesis parameters (e.g., solvent type and solvothermal temperature) is still unknown for the photocatalytic degradation of gaseous pollutants on Bi/BiOBr composite. In addition, the regulation of exposed facets [29, 30] or surface oxygen vacancy (SOV) [31, 32] on BiOBr caused by the solvent and solvothermal temperature was confirmed, which further enhanced the solar light photocatalytic activity.

Therefore, in this work, in-situ reduction of Bi³⁺ to immobilize metallic Bi on BiOBr with exposed (110) facets and SOV was attempted according to one-pot solvothermal method. The effects of solvent type and solvothermal temperature on the structure, composition, morphology, optical response, and band gap properties of the obtained Bi/BiOBr composite were investigated. Particularly, the alterations of the exposed facets and SOV on BiOBr due to Bi doping as well as the contribution to the enhanced degradation performance are discussed. The solar light photocatalytic performance of the Bi/BiOBr composite was evaluated by choosing n-hexane, which is one of the typical alkanes, as a model gaseous pollutant. Together with characterizations of the photoinduced current and the recombination rates of the photoexcited charge carriers and reactive species, the mechanism of enhancement of the solar light photocatalytic performance of BiOBr with exposed (110) facets and SOV caused by optimum metallic Bi decoration was tentatively revealed.

Solvothermal method was applied for the synthesis of Bi/BiOBr composite. The materials and reagents utilized are mentioned in the Supporting Information. Typically, 0.75 mmol of Bi(NO₃)₃·5H₂O and 7.5 mmol of NaBr were added to 80 mL of glycerol. The mixture was stirred for 2 h at room temperature before pouring into 100 mL of a Teflon-lined stainless autoclave. The autoclave was heated at 160 ℃ for 12 h, and the resulted precipitate was collected and thoroughly washed with ethanol. After drying at 60 ℃ for 12 h, the photocatalyst Bi/BiOBr-G-160 was obtained, where G refers to glycerol and 160 refers to the solvothermal temperature. Similarly, a series of Bi/BiOBr composites were obtained by changing the solvent type (methanol or ethylene glycol) and solvothermal temperature (180 or 200 ℃).

The crystal structures of the Bi/BiOBr composites were analyzed by X-ray diffraction (XRD; D8 ADVANCE). The functional groups of the composites were investigated by Fourier transform infrared spectrometry (FT-IR, Nicolet iS50FT-IR). X-ray photoelectron spectroscopy (XPS; Escalab 250Xi) was used to determine the chemical compositions of the composites. The SOV and reactive oxygen species were detected by electron paramagnetic resonance (EPR) spectroscopy (Bruker A300). The morphologies of the composites were characterized by scanning electron microscopy (SEM; Hitachi SU822), whereas the microstructures and elemental mappings of the composites were characterized by transmission electron microscopy (TEM; Talos F200S). UV-vis spectrophotometry (UV2550) was utilized to obtain the optical properties of the composites. The photocurrents of the composites were obtained using an electrochemical workstation (CHI650E), with a 300 W Xe lamp (PLS-SXE300C) as the light source and 0.1 mol/L Na₂SO₄ as the supporting electrolyte. A Pt foil and calomel electrode were used as the counter and reference electrodes, respectively. The photoluminescence (PL) spectra of the composites were measured by a fluorescence spectrophotometer (Fluorolog-3).

The photocatalytic activities of the Bi/BiOBr composites were estimated by measuring the degradation of n-hexane in a self-made 500 mL cylindrical polytetrafluorethylene reactor [33]. Briefly, the reactor is a hollow cylinder with a height of ca. 130 mm and an outer diameter of ca. 100 mm. Two inlets (for gas and liquid) are symmetrically fixed on the sides of the reactor. At the bottom of the reactor, 400 mg of the composite was dispersed in a glass petri dish (6 cm in diameter). A 300 W Xe lamp was used as the light source, and the distance between the light source and the catalyst surface was about 15 cm. A mixture of n-hexane and oxygen was then injected into the reactor to ensure that the initial concentration of n-hexane was 15 ppmv. After leaving for 30 min to ensure dark equilibrium, the light was turned on. Within a given time interval, about 400 μL of the gas was sampled, and the residual concentration of n-hexane was determined by a gas chromatograph (GC9800).

Fig. 1a displays the XRD patterns of the as-synthesized Bi/BiOBr composites obtained at 200 ℃ in different solvents. As can be seen, the high-temperature reaction of Bi(NO₃)₃·5H₂O with NaBr in methanol results in the production of Bi/BiOBr-M-200 with peaks at 2θ = 10.9°, 21.8°, 25.3°, 31.8°, 32.3°, 39.4°, 46.2°, and 57.2°, which correspond to the (001), (002), (101), (102), (110), (112), (200), and (104) planes, respectively. By comparing these peaks with the standard card (JCPDS no. 78-0348), they can be indexed to the tetragonal phase of BiOBr. Our result of pure BiOBr synthesis in methanol system is highly supported by those of Vadivel et al. [34] and Xiong et al. [35]. Meanwhile, these strong and sharp peaks reveal a high degree of crystallization for the BiOBr obtained in Bi/BiOBr-M-200. For Bi/BiOBr-E-200, obtained by using ethylene glycol as the reaction solvent, the characteristic peaks of metallic Bi at 2θ = 22.5°, 27.2°, 38.0°, 39.6°, 48.7°, and 56.1° (JCPDS no. 85-1329) can also be observed, together with the BiOBr peaks, which suggest the formation of Bi/BiOBr composite. Further calculation of the peak areas reveals that the relative content of Bi in this composite is approximately 29%. Significantly enhanced intensities of the peaks of metallic Bi are observed for Bi/BiOBr-G-200, synthesized using glycerol as the solvent, which lead to a further increase in the relative Bi content to 49%. Clearly, metallic Bi is only formed in systems involving ethylene glycol and glycerol, rather than methanol, which suggest that an alcohol with more functional groups benefits the generation of Bi particles. This is because a longer C chain of an alcohol is more likely to combine with metal ions [35], whereas an alcohol with more hydroxyl groups exhibits a higher reduction ability toward metal ions [36]. Therefore, an increase in the number of methyl groups in an alcohol results in a higher ability to combine with Bi³⁺, which is more easily reduced in-situ to metallic Bi by the alcohol with increased number of hydroxyl groups. Anyhow, our results of significant reduction abilities of alcohols are highly consistent with those reported by Ma et al. [27] and Wang et al [26].

Meanwhile, the exposed facets can also be deduced from the XRD results. As for Bi/BiOBr-M-200, the intensity of the (110) peak is the strongest among all the diffraction peaks, which indicates that the BiOBr in this composite contains exposed (110) facets. Furthermore, the relative intensity of the (110) peak continuously increases with the increase in the number of functional groups of the solvent. These results reveal that increased numbers of methyl and hydroxyl groups of the alcohol solvent can effectively accelerate the growth of BiOBr along the (110) plane. Li et al. [37] reported enhanced growth of the (110) facets of BiOBr by using isopropyl alcohol instead of ethanol, which was consistent with our result. The Bi/BiOBr composite with enhanced (110) facets may display improved photocatalytic activity. Peng et al. [29] reported that BiOBr with exposed (110) facets displayed better photocatalytic capability for dye degradation than BiOBr with exposed (001) facets, which supported our hypothesis.

Besides the solvent type, the effect of solvothermal temperature on the exposed facets of BiOBr and the relative content of Bi in the resulting composites was also investigated. As shown in Fig. 1b, only a peak at 32.3°, assigned to the (110) facets, is observed for all the Bi/BiOBr composites, which suggests the formation of BiOBr with mainly exposed (110) facets after glycerol efficiently inhibited the growth of BiOBr along the [102] direction. In addition, the peak intensity of metallic Bi further and further decreases upon decreasing the solvothermal temperature from 200 to 180 and 160 ℃, which leads to decreases in the relative Bi content from 49% to 40% and 22%. These results indicate that a relatively high solvothermal temperature (≥ 160 ℃) can promote the efficient reduction of Bi³⁺ by an alcohol to form metallic Bi in-situ. Ma et al. [27] also obtained Bi/BiOBr composite just under the solvothermal temperature higher than 160 ℃ by using glycerol as the solvent, which provided solid evidence for our result. Meanwhile, the results of unchanged (110) peak intensity of BiOBr and increased Bi content reveal that the formed Bi originated from Bi(NO₃)₃·5H₂O through reduction by the solvent, and not from BiOBr. The simultaneous formation of metallic Bi and BiOBr not only maintains the structure of BiOBr, but also imitates the chemical reaction between Bi and BiOBr.

To confirm the chemical bonding between Bi and BiOBr, the FT-IR spectra of the Bi/BiOBr prepared under different alcohols and solvothermal temperatures were recorded, and are displayed in Fig. 1c and 1d. As observed in all the figures, three strong peaks are noticed in the range 2000-400 cm^–1 that are assigned to the groups -OH (1618 cm^–1), C-H (1387 cm^–1), and Bi-O (510 to 533 cm^–1), which indicate the existence of both residual alcohol and BiOBr in all the prepared composites. Further comparison reveals that the wavenumber of the Bi-O group decreases from 520 cm^–1 for Bi/BiOBr-M-200 to 510 cm^–1 for Bi/BiOBr-E-200, and then increases to 533 cm^–1 for Bi/BiOBr-G-200. According to previous work [38], the FT-IR wavenumber of the Bi-O group in BiOBr is around 520 cm^–1, which is consistent with the result for Bi/BiOBr-M-200. The wavenumber shifts in the FT-IR spectra of Bi/BiOBr-E-200 and Bi/BiOBr-G-200 composites reveal that chemical bonding occurs between Bi and BiOBr during the solvothermal synthesis. Qi et al. [39] observed similar shifts during the synthesis of BaTiO₃-based composites, which confirmed the accuracy of our results. Almost the same FT-IR curves are obtained for Bi/BiOBr-G-160, Bi/BiOBr-G-180, and Bi/BiOBr-G-200, which suggest that solvothermal temperature exhibits a negligible effect on the chemical bonding reaction between Bi and BiOBr. Further, by combining the FT-IR and XRD results, it is observed that the chemical bonding mainly occurs on the (110) facets of BiOBr. This reaction between Bi and BiOBr probably results in the formation of SOV on the exposed (110) facets of BiOBr.

XPS technique was used for confirming the formation of SOV, as well as to elucidate the surface elemental compositions and oxidation states. Here, the sample Bi/BiOBr-G-160 has been taken as an example. As shown in Fig. S1, Bi, O, and Br elements are clearly detected. Fig. 2a-2c show the high-resolution Bi 4f, O 1s, and Br 3d XPS patterns, respectively. As for the Bi 4f spectrum (Fig. 2a), four peaks can be detected. The peaks with the binding energies of 164.3 and 158.9 eV are attributed to the Bi 4f_7/2 and Bi 4f_5/2 of the Bi³⁺ in BiOBr, whereas the peaks located at 162.4 and 156.9 eV belong to metallic Bi. These results suggest the formation of both Bi and BiOBr in Bi/BiOBr-G-160, which is consistent with the XRD and FT-IR results. In the case of the O 1s spectrum (Fig. 2b), three obvious peaks are discerned. According to reference [40], the peaks at 529.5, 531.4, and 532.8 eV are attributed to the lattice oxygen, surface hydroxyl oxygen, and SOV, respectively. Moreover, EPR spectroscopy was applied to confirm the presence of SOV in the composite. As shown in Fig. 2d, a remarkable signal at g = 2.003 is observed in the EPR spectrum of Bi/BiOBr-G-160, which presents solid evidence for the existence of SOV. In the case of the Br 3d spectrum, two peaks at 68.2 and 69.1 eV are observed that are ascribed to Br 3d_5/2 and Br 3d_3/2, respectively (Fig. 2c). In summary, the combination of XRD, FT-IR, XPS, and EPR data reveals the synthesis of Bi/BiOBr composite with SOV on the exposed (110) facets of BiOBr, which resulted from the chemical bonding between Bi and BiOBr.

The effects of solvent type and solvothermal temperature on the morphologies of Bi/BiOBr composites were investigated further. As shown in Fig. 3a, Bi/BiOBr-M-200 consists of bulk particles of sizes in the range several micrometers to more than ten micrometers. Further magnification of the image reveals that these bulk particles are composed of thin nanoplates (Fig. 3b). In combination with the XRD results, these nanoplates are confirmed as BiOBr with exposed (110) facets. The average length, width, and thickness of these BiOBr nanoplates are about 300, 200, and 30 nm, respectively. Xiong et al. [35] also obtained nanoplate-like BiOBr with exposed (110) facets by using methanol as the solvent, which is consistent with our result. When ethylene glycol is utilized as the reaction solvent, the resulting Bi/BiOBr-E-200 displays a micro-sized spherical morphology (Fig. 3c). Further observation reveals that these spheres show an average diameter of ca. 2 μm and are assembled by nanoplates with an average thickness of ca. 20 nm (Fig. 3d). The thinner nanoplates obtained in the case of Bi/BiOBr-E-200 than with Bi/BiOBr-M-200 suggest preferential growth of BiOBr along the (110) direction with the increase in the number of functional groups in the alcohol solvent, which is consistent with the XRD results. Notably, a few nanospheres with the average diameter of ca. 100 nm are simultaneously observed on these BiOBr microspheres. They correspond to metallic Bi, which originated from the partial reduction of Bi³⁺ by ethylene glycol under high-temperature treatment. The formation of Bi nanoparticles on BiOBr microspheres is also observed in other ethylene glycol systems [26], which is consistent with our result. Furthermore, a continuous increase in the number of these Bi nanospheres with size of ca. 100 nm is observed for Bi/BiOBr-G-200 (Fig. 3e and 3f). The result of increased content of Bi nanospheres with unchanged sizes further reveals that these newly formed Bi mainly originated from in-situ reduction of the Bi³⁺ of Bi(NO₃)₃·5H₂O by alcohol, and not from BiOBr. In addition, the thickness of the BiOBr nanoplates continuously decreases to ca. 7 nm. Together with the XRD result, it is deduced that the exposed (110) facets of the BiOBr nanoplates in Bi/BiOBr-G-200 increase with an increase in the number of functional groups in the alcohol solvent, which results in a higher percentage of these facets.

The significantly dominant (110) facets are also observed in the SEM results of the Bi/BiOBr samples obtained at different solvothermal temperatures (Fig. 4). As can be seen, both Bi/BiOBr-G-160 (Fig. 4a and 4b) and Bi/BiOBr-G-180 (Fig. 4e and 4f) consist of Bi nanospheres and BiOBr nanoplates. Meanwhile, the thickness of BiOBr nanoplates in these two composites is around 7 nm, which is close to that in Bi/BiOBr-G-200. These results indicate negligible effect of solvothermal temperature on the morphology and the exposed (110) facets of BiOBr. The exposed (110) facets of the prepared Bi/BiOBr composite were further confirmed by TEM. As shown in Fig. S2, the three elements Bi, O, and Br are uniformly dispersed on Bi/BiOBr-G-160. A low-resolution TEM image further reveals that the Bi/BiOBr-G-160 is composed of major nanoplates and a few nanospheres (Fig. 4c), which is consistent with the SEM result. Moreover, in the high-resolution TEM image (Fig. 4d), two lattice planes with the spacings of 0.277 and 0.237 nm are clearly observed. The former corresponds to the (110) lattice plane of BiOBr, whereas the latter is assigned to the (104) facets of Bi metal. These TEM results confirm the deposition of Bi nanospheres on BiOBr nanoplates with exposed (110) facets. However, increasing the solvothermal temperature from 160 and 180 to 200 ℃ leads to increased sizes of the Bi nanospheres from about 40 and 50 to 100 nm, respectively (Figs. 3 and 4). These results reveal that a higher solvothermal temperature facilitates the formation of a higher amount of metallic Bi with larger particle sizes, which may not be beneficial for the enhancement of the photocatalytic performance of Bi/BiOBr composite.

The UV-vis absorption spectra were measured to evaluate the effects of solvent type and solvothermal temperature on the optical and band gap properties of the Bi/BiOBr composites. As displayed in Fig. 5a and 5c, all the obtained Bi/BiOBr composites exhibit strong absorptions in the wavelength range 200 to 400 nm, which indicate that these composites can efficiently absorb the UV light of the solar spectrum. However, different visible light responses are observed. Bi/BiOBr-M-200 exhibits a very weak visible light absorption, whereas significant adsorptions of visible light are observed for Bi/BiOBr-E-200 and Bi/BiOBr-G-200. The corresponding photographs reveal that the colors of both Bi/BiOBr-E-200 and Bi/BiOBr-G-200 are darker than that of Bi/BiOBr-M-200 (see inset of Fig. 5a). In combination with the XRD and UV-vis results, it is believed that the increased content of metallic Bi is responsible for the color change and for the higher visible light absorption observed in Bi/BiOBr composite. Similar excellent visible light responses were obtained for Bi/BiOBr-G-160 and Bi/BiOBr-G-180 (Fig. 5c).

Furthermore, the band gaps of the Bi/BiOBr composites were calculated according to the method reported in our earlier published papers [10, 11]. As shown in Fig. 5b, the band gaps of Bi/BiOBr-M-200 (2.7 eV), Bi/BiOBr-E-200 (1.2 eV), and Bi/BiOBr-G-200 (0.6 eV) significantly decrease, and this result agrees well with the order of their visible light absorption abilities. Similarly, increased solvothermal temperature also leads to decreased band gaps of Bi/BiOBr-G-160 (2.3 eV), Bi/BiOBr-G-180 (1.4 eV), and Bi/BiOBr-G-200 (0.6 eV) (Fig. 5d). Together with all the characterization results, it is concluded that the chemical bonding between the increasingly doped Bi and BiOBr is responsible for the narrowed band gap and that the improved visible light absorption and narrowed band gap may result in more photogenerated carriers, which favors a higher photocatalytic performance of Bi/BiOBr composite.

The photocatalytic activities of the obtained Bi/BiOBr composites were evaluated using gaseous organics as the model pollutant. It should be mentioned that BiOBr-based photocatalysts show good degradation performances toward gaseous organics such as benzene [16], toluene [17], and o-dichlorobenzene [41]. In comparison with these aromatic organics, alkanes are well known for being more difficult to be photocatalytically degraded [18]. Alkanes are an important group of air pollutants in indoor and industrial environments [42]. Thus, in this work, n-hexane, a typical alkane, was selected as a model pollutant to assess the excellent degradation ability of the prepared Bi/BiOBr composite. Fig. 6 displays the photocatalytic degradation kinetics and rate constant of n-hexane on the prepared Bi/BiOBr composites under solar light irradiation. As for Bi/BiOBr-M-200 (Fig. 6a), about 62.2% of n-hexane with the initial concentration of 15 ppmv is eliminated within 120 min upon irradiation with by solar light. The degradation efficiency further increases to 76.2% on Bi/BiOBr-E-200. However, only 19.7% of n-hexane is degraded by Bi/BiOBr-G-200. The rate constant correspondingly increases from 6.3×10^–3 to 1.1×10^–2 min^–1 and then significantly decreases to 1.8×10^–3 min^–1 (Fig. 6c). It seems that a suitable decoration of metallic Bi can obviously promote the solar light photocatalytic performance of BiOBr in the degradation of n-hexane, whereas too high a content of Bi results in poor activity. A similar phenomenon is also observed in Bi/BiOBr composites synthesized under different solvothermal temperatures. As shown in Fig. 6b and 6c, upon increasing the temperature from 160 and 180 to 200 ℃, the resulting Bi/BiOBr composite shows degradation efficiencies that decrease from 97.4% and 80.1% to 19.7%, with the rate constant correspondingly decreasing from 3.0×10^–2 and 1.1×10^–2 to 1.8×10^–3 min^–1. Together with the characterization results, it is found that optimum decoration of metallic Bi (e.g., 22% in this study) efficiently narrows the band gap and enhances the visible light absorption of BiOBr with exposed (110) facets and SOV, which contributes to the excellent solar light photocatalytic performance in the degradation of n-hexane.

Besides, the abilities of the photoinduced electrons, electron-hole recombination, and the formation of reactive oxygen species on Bi/BiOBr composite can also influence its photocatalytic activity. Therefore, the photocurrent responses and the PL •OH and •O₂^– spectra of the prepared Bi/BiOBr composites were obtained and compared. Here, Bi/BiOBr-G-160 and Bi/BiOBr-G-180 were taken as examples. Fig. 7a shows the photocurrent response curves of Bi/BiOBr-G-160 and Bi/BiOBr-G-180 recorded under solar light irradiation with five on-off intermittent switchings. As observed in the figure, the photocurrent responses for both the composites are prompt, steady, and reproducible upon repeating the cycle. Furthermore, Bi/BiOBr-G-160 (ca. 0.06 mA cm^–2) shows a higher photocurrent than Bi/BiOBr-G-180 (ca. 0.04 mA cm^–2), which suggests higher positive photoinduced charge separation and transfer processes in the former. Moreover, the XRD results reveal that Bi/BiOBr-G-160 and Bi/BiOBr-G-180 contain about 22% and 40% of metallic Bi. Then, it is found that suitable Bi decoration actually facilitates the photoinduced charge separation and transfer processes on Bi/BiOBr composite. Guo et al. [43] reported that Bi/BiOBr exhibits a significantly enhanced photocurrent intensity in comparison to that of pure BiOBr, which is in agreement with our result. However, too high a Bi content leads to decreased electron generation, since metallic Bi can also act as electron-hole recombination centers.

To verify this, PL technique was used to characterize the recombination efficiency of the photoinduced electrons and holes in the Bi/BiOBr composites. As noticed in Fig. 7b, Bi/BiOBr-G-160 shows a significantly lower PL intensity than Bi/BiOBr-G-180, which indicates a lower recombination rate of the photoexcited charge carriers in the former. In addition, the PL results clearly demonstrate that the deposition of suitable metallic Bi on BiOBr greatly inhibits the recombination rate of the charge carriers, thereby promoting the accumulation of photoinduced electrons and holes in the conduction and valence bands of BiOBr to produce highly concentrated reactive oxygen species (e.g., •OH and •O₂^–).

Then, the productions of •OH and •O₂^– on Bi/BiOBr-G-160 and Bi/BiOBr-G-180 composites under solar light irradiation were compared. The •O₂^– and •OH radicals can be detected by adding 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) as a scavenger with methanol and aqueous dispersions, respectively. As revealed in Fig. 7c, a stronger DMPO-O₂^– signal is observed for Bi/BiOBr-G-160 than Bi/BiOBr-G-180, which indicates that more oxygen is reduced by the photoinduced electrons in the conduction band (CB) to generate more.O₂^– radicals. The higher concentration of •O₂^– is highly consistent with the lower recombination rate of the photoexcited charge carriers in Bi/BiOBr-G-160. As for •OH, no obvious DMPO-OH signal is observed (Fig. S3), which implies that no •OH radical is formed in the valence band (VB) of both the Bi/BiOBr composites. This is probably because the VB energy levels of both Bi/BiOBr-G-160 and Bi/BiOBr-G-180 are more negative than the OH/•OH redox potential. Liao et al. [13] observed negligible generation of •OH radicals on a BiOBr with a much more negative VB energy level, which is consistent with our result. Together with the band gap results, it seems that Bi decoration initiates CB upshifting in BiOBr, which leads to more photogenerated electrons and then •O₂^– radicals.

Based on the characterization and degradation results presented above, a mechanism scheme for enhanced photocatalytic degradation on Bi/BiOBr composite under solar light irradiation was proposed, as displayed in Fig. 8. As shown in the figure, under solar light irradiation, the BiOBr in the composite is photoexcited to generate electrons (e^–) in the CB and holes (h⁺) in the VB. The narrower band gap of BiOBr makes it much more sensitive to visible light. Subsequently, more abundant e^– and h⁺ are generated in the CB and VB of BiOBr with exposed (110) facets. Furthermore, SOV can act as electron acceptors [44]. Hence, some of electrons in the CB are trapped by the SOV to enhance the separation and transfer efficiency of the photoinduced charges, and these trapped electrons easily reduce the adsorbed O₂ to form active •O₂^– radicals, which efficiently degrade n-hexane to H₂O and CO₂. However, SOV in excess act as recombination centers for the photoinduced charge carriers, which lowers the efficiency of the photocatalytic reaction (Figs. 2d and 6b). A similar result was also reported by Wang et al. [45].

In addition to the trapping by SOV, electron transfer also occurs from the CB of BiOBr to metallic Bi, which results in significant electron accumulation on Bi. This electron transfer process can further accelerate the separation of photoinduced charges on BiOBr, which increases the lifetime of the photoinduced electrons. Similar to the reaction of BiOBr in the SOV, •O₂^– is also produced on Bi from O₂ by the photoinduced electrons. Based on the EPR results, a strong •O₂^– signal is observed for the Bi/BiOBr composites and n-hexane molecules are finally oxidized by these •O₂^– radicals to form CO₂ and H₂O. Notably, the h⁺ in the VB of BiOBr can directly degrade n-hexane molecules. In summary, the enhancement in the solar-light-driven photocatalytic degradation of gaseous n-hexane is attributed to the increases in •O₂^– radicals and h⁺ due to the synergistic effect of Bi and BiOBr.

Bi/BiOBr composites were synthesized in different alcohol solvents and at different solvothermal temperatures. The relative content of the in-situ formed metallic Bi increases with increasing number of functional groups of alcohol solvent and solvothermal temperature. Meanwhile, the chemical bonding between Bi and BiOBr with exposed (110) facets resulted in the formation of SOV on these facets. The solar light photocatalytic degradation of gaseous n-hexane revealed that the Bi/BiOBr composite synthesized in glycerol at 160 ℃ exhibited the highest removal efficiency. The synergistic effects of a high visible light response, narrow band gap, great photocurrent, low recombination rate of charge carriers, and strong •O₂^– and h⁺ formation were responsible for the high photocatalytic performance observed. The findings of this work will be helpful in the design and development of self-doped semiconductors for purification of gaseous alkanes in the petrochemical industry.

Supplementary materials related to this article can be found in the online version.