Photocatalytic technology is considered a promising solution to environmental pollution and energy shortages [1, 2]. Numerous studies have focused on the use of semiconductor photocatalysts to address these challenges. Although this photocatalytic technology has been used for hydrogen production [3] and environmental remediation [4], the development of highly active photocatalysts that can utilize solar energy efficiently is challenging. To obtain desirable photocatalytic efficiency, the photoinduced charge carriers must be effectively separated and rapidly migrate to the semiconductor surface to participate in redox reactions [5-11].

Amongst the various semiconductors, bismuth-containing materials have shown superior photocatalytic properties on visible light exposure because of their suitable band gap and layered structure. Bismuth oxyhalides (BiOX, X = Cl, Br, and I), where [Bi₂O₂]²⁺ slabs are sandwiched between double anion (X^‒) layers, have shown good photocatalytic activity. In particular, bismuth iodide (BiOI) shows strong absorption in the visible light region because of its small band gap of 1.6–1.9 eV [12, 13]. However, the high photoelectron recombination rate of BiOI limits its practical application as a visible light catalyst. To overcome this disadvantage of BiOI, various strategies, such as material design [14], decoration with noble metals [15], elemental doping [16], and heterojunctions [17, 18], have been employed.

Recently, extensive work has been conducted on the introduction of oxygen vacancies (OVs) into photocatalysts to modulate their optical absorption, charge separation and transport, and surface reactions to enhance the photocatalytic activity [19-21]. The construction of a heterostructure can increase the photocatalytic efficiency of semiconductors because the band structures of the materials are matched, which can improve three vital photocatalytic processes: electron excitation, separation and transmission, and depletion. For example, Bi/Bi₂O₃/Bi₂WO₆ [18], SrCO₃/BiOI [17], and Bi₂O₂CO₃/BiOI [22] have been reported to show highly enhanced photocatalytic activity. As a semimetal, bismuth has a very small band overlap and possesses surface plasmon resonance (SPR) properties, similar to those of the noble metals. Jiang et al. [23] prepared a Bi nanoparticle (NP)-decorated g-C₃N₄ composite and confirmed that the incorporation of the Bi NPs improved the photocatalytic activity of g-C₃N₄ via the combined heterojunction and SPR effect, both of which facilitated electron-hole (e^‒/h⁺) separation in g-C₃N₄. Thus, the simultaneous introduction of OVs and Bi NPs on BiOI effectively quenches the electron-hole recombination in BiOI and promotes photocatalytic activity.

In this paper, a ternary Bi/BiOI/(BiO)₂CO₃ heterojunction was constructed via an in situ assembly method under mild conditions. This unique heterojunction contains Bi nanoparticles, BiOI with oxygen vacancies (OV-BiOI), and (BiO)₂CO₃. The Bi₂O₂CO₃ and Bi metal deposited on BiOI can dramatically boost the NO removal efficiency via the formation of dual junctions (i.e., Bi/BiOI and BiOI/(BiO)₂CO₃), which result in increased charge carrier separation and the production of reactive radicals for visible light photocatalysis.

All chemicals were of analytical grade and were used without further purification. The Bi/BiOI/(BiO)₂CO₃ composites were prepared by a facile in situ assembly method with pure BiOI as the substrate. The BiOI was obtained by precipitation. Briefly, 0.485 g of Bi(NO₃)₃·5H₂O was added to 30 mL of deionized water and dispersed by magnetic stirring for 10 min, forming a homogeneous white suspension. Then, 0.166 g of KI was dissolved in 30 mL water and added dropwise to this mixture. Next, the mixture was stirred for 30 min at room temperature. Subsequently, the resultant brown yellow suspension was left to stand for 20 min. The product (denoted BOI) was centrifuged and washed twice with distilled water and ethanol.

The Bi/BiOI/(BiO)₂CO₃ samples were then synthesized using the produced BiOI as the precursor and NaBH₄ solution as the reducing agent. In detail, 0.352 g of BiOI powder was added to 100 mL polyvinylpyrrolidone (PVP, 0.1 g/L) solution with continuous stirring for 20 min. Then, 30 mL of NaBH₄ solution (70 mmol/L) was added dropwise to the above solution. After reaction for 1 h, the suspension was left to stand for 1 h. The resultant precipitate was centrifuged and washed twice with deionized water and absolute ethanol. Finally, the product was dried at 40 ℃ in a vacuum oven. The products are labeled based on the concentration of the NaBH₄ solution, that is, BOI-70.

The crystal phases of the samples were determined by X-ray diffractometry (XRD) with Cu K_α radiation (D/max RA, Rigaku Co., Japan). X-ray photoelectron spectroscopy (XPS) measurements were carried out using an Al K_α X-ray radiation source (Thermo Scientific ESCALAB 250, USA) to analyze the chemical composition of the samples. The morphologies and structures of the samples were observed by scanning electron microscopy (SEM, JSM-6490, Japan) and transmission electron microscopy (TEM, JEM-2010, Japan). Electron spin resonance (ESR, JES FA200) spectroscopy was used to obtain spectra of the spin-trapped    radicals  (trapping   agent: 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO)). UV-vis diffuse reflectance spectroscopy (UV-vis DRS) measurements were carried out on dry-pressed disk samples. The UV-vis spectrometer (UV2550, Shimadzu, Japan) was equipped with an integrating sphere assembly, and 100% BaSO₄ was used as the reflectance sample. Photoluminescence (PL) spectra were recorded using an Aqualog Spectrofluorometer (F-7000, Hitachi, Japan).

The visible-light-driven photocatalytic removal of NO was evaluated using a continuous-flow reactor. The rectangular reactor (30 cm × 15 cm × 10 cm) was made of polymeric glass and covered with Saint-Glass. To block UV light, a 150-W Xe commercial tungsten halogen lamp was fitted with a UV cutoff filter (420 nm), and the average light intensity was 0.16 W/cm². For the photocatalysis experiments, 0.20 g of the catalyst was coated onto two 12-mm diameter glass disks, which were placed in the reactor without irradiation when adsorption–desorption equilibrium had been reached. The initial concentration of NO was diluted to about 550 ppb by a zero air generator. The concentration of NO was continuously measured using a NO_x analyzer (Thermo Environmental Instruments Inc., 42c-TL) while analyzing the NO_x (where NO_x represents NO + NO₂) concentration. The NO removal ratio (η) was obtained using the following equation: η(%) = (1 - (C/C₀)) × 100%, where C represents the NO concentration at the outlet of the reactor after turning on the lamp, and C₀ represents the NO concentration when adsorption-desorption equilibrium is reached before turning on the lamp.

DFT calculations were carried out using the "Vienna ab initio simulation package" (VASP 5.4) [24-26] and employed a generalized gradient correlation functional [27]. The projector-augmented wave method was used with a plane-wave basis set with a cut-off energy of 500 eV [28, 29]. The Gaussian smearing width was set to 0.2 eV. The Brillouin zone was sampled with a 3 × 3 × 1 Monkhorst–Pack grid. All atoms were allowed to converge to 0.01 eV/Å. The 8 × 8 × 8 supercell of BiOI included eight oxygen atoms, eight bismuth atoms, and eight iodine atoms. The final optimization of the structural model resulted in the atomic energy converging to a minimum.

As shown in Fig. 1(a), the crystal structure of tetragonal BiOI possesses a Sillèn-like layered structure, which is composed of [Bi₂O₂]²⁺ layers with Ⅰ^‒ ions between them. Fig. 1(b) depicts the powder XRD patterns of the as-prepared BOI and BOI-70 samples. All the diffraction peaks of BOI can be well indexed to the tetragonal phase of BiOI (JCPDS No. 10-0445). No other impurity peaks are present in the XRD pattern of BOI. After the addition of a certain amount of NaBH₄, peaks corresponding to BiOI and (BiO)₂CO₃ (JCPDS No. 25-1464) were observed in the XRD pattern of BOI-70. In addition, four diffraction peaks at 27.3°, 37.9°, 39.8°, and 48.7° of the metallic Bi phase (JCPDS No. 05-0519) are also present in the XRD pattern of BOI-70. These results indicate the coexistence of Bi, (BiO)₂CO₃, and BiOI phases in the BOI-70 composites. Furthermore, the peaks of BiOI were weakened, and diffraction peaks corresponding to (BiO)₂CO₃ and Bi appear. It is possible that the weak basicity of the NaBH₄ aqueous solution results in CO₂ from the air forming CO₃^2‒, thus resulting in the conversion of (BiO)⁺ to (BiO)₂CO₃. Simultaneously, in the presence of NaBH₄, the (BiO)⁺ can be reduced to the metallic Bi phase. The signal arising from the OVs in the BOI-70 sample is obvious in the solid state EPR spectra (Fig. 1(c)) [30], showing as a characteristic signal with a g-factor of about 2.0 for BOI-70. Because of the special layered structure of BOI, the formation of oxygen vacancies on the surface during the reduction process is favorable. It can be inferred that the OVs of the as-prepared BOI-70 sample are located on the surface of BiOI (labeled OV-BiOI). These results confirm that the Bi/OV-BiOI/(BiO)₂CO₃ composites had been successfully prepared.

The surface composition and chemical states of the as-prepared samples were further analyzed by XPS measurements. Fig. 2(a) shows the elemental composition of the samples, which shows that BOI-70 contains O, C, Bi, and I. Fig. 2(b) shows the high-resolution XPS spectra of Bi. In the spectrum of BOI, the presence of Bi³⁺ is confirmed by the characteristic Bi 4f_7/2 and Bi 4f_5/2 peaks at 164.0 and 158.7 eV, respectively. In the spectrum of BOI-70, in addition to the two Bi 4f peaks of Bi³⁺, peaks corresponding to metallic Bi⁰ are detectable at 162.1 and 156.8 eV, suggesting that Bi³⁺ was partially reduced by NaBH₄ to form Bi metal in BOI-70. Fig. 2(c) shows the XPS spectra of the I 3d region, and the peaks at 618.5 and 630.1 eV can be ascribed to I 3d_5/2 and I 3d_3/2, respectively. The I 3d peaks in the spectra of the BOI-70 composites have binding energies similar to those of BOI, and the results further confirm the presence of BiOI in the as-prepared BOI-70 samples.

The morphology and structural changes of BOI and BOI-70 were investigated using SEM and high-resolution (HR)TEM measurements. The SEM images of BOI (Fig. 3(a)) and BOI-70 (Fig. 3(b)) show flower-like structures formed of nanosheets. In comparison with the smooth surfaces of the nanosheets in the BiOI sample, the surfaces of the flower-like microspheres are rough, and they are covered by some irregularly aggregated particles, as shown in Figs. 3(a) and (b). The TEM images (Figs. 3(c) and (d)) show the microstructure of the samples in detail. From the HRTEM image of BOI (Fig. 3(c)), a lattice spacing of 0.458 nm was determined, which is consistent with the (002) crystallographic plane of BiOI. As shown in Fig. 3(d), the HRTEM image clearly shows the lattice fringes with interplanar spacings corresponding to metallic Bi, tetragonal BiOI, and (BiO)₂CO₃. The interplanar spacings of 0.328, 0.458, and 0.193 nm correspond to the (012) crystallographic plane of Bi, (002) crystallographic plane of BiOI, and (020) crystallographic plane of (BiO)₂CO₃. The combined XRD, XPS, and HRTEM results show that the ternary Bi/OV-BiOI/(BiO)₂CO₃ composite can be constructed through the facile reduction of BiOI by NaBH₄. In Fig. 3(d), two borders can be seen (of Bi/OV-BiOI and OV-BiOI/(BiO)₂CO₃), which implies that an intimate interface had formed. This interface could optimize the path for charge carrier transfer in the ternary composite. The formation of Bi/BiOI/(BiO)₂CO₃ is represented by Eqs. (1)–(7).

(1)

(2)

(3)

(4)

(5)

(6)

(7)

When NaBH₄ is dissolved in water, the solution becomes weakly alkaline (Eqs. (1) and (2)). This results in ambient CO₂ becoming dissolved in the water and forming CO₃^2‒ (Eqs. (3) and (4)). During the chemical reduction process, some of the BiOI is transformed into (BiO)₂CO₃ by the presence of dissolved atmospheric CO₂ (the carbonate source) at room temperature during the long stirring process (Eqs. (5) and (6)). In addition, some of the (BiO)⁺ is reduced to Bi metal (Eq. (7)), simultaneously forming oxygen vacancies. Lastly, the Bi metal is deposited on the surface of the OV-BiOI/(BiO)₂CO₃ composite, forming a ternary heterojunction structure.

The photocatalytic removal of NO from a continuous flow of air was tested. The samples were kept in the dark until adsorption equilibrium was reached. Then, the activity was assessed under visible light (λ > 400 nm) illumination.

Fig. 4(a) shows the photocatalytic performance of BOI and BOI-70. Over the 30-min visible light photocatalytic reaction, the NO concentration decreased with increasing reaction time. The NO removal ratios (η) of BOI, P25, and BOI-70 are 1.2%, 11.5%, and 50.7% after 30 min, respectively. BOI exhibits low activity because of its high charge carrier recombination rate. Clearly, BOI-70 can remove NO more efficiently than BOI and P25, indicating that the ternary Bi/BiOI/(BiO)₂CO₃ composites can indeed improve the photocatalytic performance via the synergistic dual interface heterojunction and the introduced vacancies. On repeated testing, the photocatalytic activity of BOI-70 (Fig. 4(b)) was not reduced, implying that this catalyst is stable for NO removal and does not suffer obvious deactivation. The final products of photocatalytic NO oxidation are nitrates [17, 18], which can be removed by washing with water.

Reactive oxygen species (ROS) are considered to be involved in the photocatalytic reaction, and these species play a crucial role in the oxidative removal of NO. Figs. 4(c) and (d) show the DMPO spin-trapping ESR spectra. In Fig. 4(c), a stronger DMPO−•O₂⁻ signal intensity for BOI-70 is observed compared to that of BOI. Similarly, stronger •OH signals were detected for BOI-70 (Fig. 4(d)). The catalytic performance of the sample is closely related to its charge carrier concentration and electronic transmission properties. The formed heterojunction and presence of OVs are beneficial for charge carrier transportation, increasing the production of reactive oxygen species and significantly improving the photocatalytic activity.

To reveal the mechanism behind the enhancement in photocatalytic efficiency, UV-vis diffuse reflectance spectra (DRS) of BiOI (BOI) and (BiO)₂CO₃ (BOC), and the ternary Bi/BiOI/(BiO)₂CO₃ composite (BOI-70) were examined, and the results are presented in Fig. 5(a). Compared to BOC, BOI-70 shows enhanced photo-absorption, indicating that the number of charge carrier increased in the ternary composite. As shown in Fig. 5(b), PL quenching was observed in the spectra of the BOI-70 composites. The BiOI/(BiO)₂CO₃ heterojunction contributes to the separation and transfer of photoelectrons and increases the number of charge carriers. Metallic Bi has SPR properties in the visible light range and concentrates the incident photon energy into plasmon oscillations, thus greatly extending the photoresponsive range of the ternary composite.

The local environment and fast transmission of charge carriers by the OVs were confirmed by DFT calculations. To clarify the contribution of the OVs to the electronic transmission, DFT calculations were used to model the adsorption and activation of oxygen on the surface of the photocatalyst. The oxygen vacancies are located at the surface of the BiO⁺ layer (Figs. 6(a) and (b)). To provide more information, we analyzed the charge difference distribution of BiOI (Figs. 6(c) and (d)). On the surface of the defective BiOI (Fig. 6(d)), an electron trap is formed at the OV, and the electron density around the oxygen atoms increases. Then, the electrons are transferred from the OVs to O₂ molecules. This phenomenon does not occur in the defect-free BiOI (Fig. 6(c)), which indicates that the recombination of photogenerated charges is inhibited by the OVs because they provides sites for the transfer of electrons to the reactants. The adsorption energy (E_ads) of O₂ on the surface decreases from ‒0.29 to ‒0.76 eV from defect-free to defective BiOI, respectively, and the O–O bond length increases from 1.30 to 1.37 , respectively (Figs. 6(a) and (b)). This result implies that the OVs can promote the adsorption of O₂ molecules on the surface of the photocatalyst by reducing the oxygen adsorption energy and promote the activation of O₂ by weakening the O–O bonds. The results of the theoretical calculations are consistent with the ESR results (Figs. 4(b) and (c)), which confirm that more molecular oxygen is activated by the OVs on BOI-70 than those on BOI, generating more •O₂⁻ radicals for photocatalysis [31, 32].

The valence band (VB) and conduction band (CB) energies of BiOI and (BiO)₂CO₃ were evaluated using Eqs. (9) and (10) [33].

(9)

(10)

Here, E_g represents the band gap of the semiconductor, X is the electronegativity of the semiconductor calculated from the electronegativity of the constituent atoms, and E^e is the energy of free electrons on the hydrogen scale (approximately 4.5 eV). The X values of BiOI and (BiO)₂CO₃ are approximately 5.94 and 6.36 eV. The band gaps of BiOI and (BiO)₂CO₃ were calculated to be 1.80 and 3.40 eV, respectively. For BiOI, E_VB and E_CB were estimated to be 2.34 and 0.54 eV, respectively, and those of (BiO)₂CO₃ were estimated to be 3.56 and 0.16 eV, respectively.

BiOI is a typical p-type semiconductor where the Fermi level is located close to the valence band (VB) [34-36]. The Fermi level of n-type (BiO)₂CO₃ is close to the CB [37-39]. The coupling of an n-type semiconductor ((BiO)₂CO₃) to a p-type photosensitizer (BiOI) enables the Fermi level of (BiO)₂CO₃ and BiOI to shift to the same level, thus forming a p-n heterojunction [40-42]. Because the Fermi levels of BiOI and (BiO)₂CO₃ reach equilibrium, the CB of (BiO)₂CO₃ shifts to an energy level lower than that of BiOI. Because of the energy difference in the Fermi level of Bi NPs (–0.17 eV) [43] and the p-n heterojunction (OV-BiOI/(BiO)₂CO₃), the three components form a perfect heterojunction structure. This synergistic interaction is illustrated in Fig. 7. Because of the intimate contact between Bi, (BiO)₂CO₃ and OV-BiOI (Fig. 1(b)) and the higher Fermi level of the Bi NPs, two interfacial heterojunctions, i.e., Bi/OV-BiOI and OV-BiOI/(BiO)₂CO₃, are formed.

When the ternary composite is exposed to visible light, electron-hole pairs are excited and dissociated over the surface of the Bi NPs, as well as the BiOI semiconductor, leading to a much-increased charge carrier concentration. In addition to the injection of hot electrons from the Bi NPs into the BiOI, the concentrated resonance energy caused by the SPR effect of the Bi NPs can also be converted into a local electromagnetic field [44-46], which promotes e^‒/h⁺ separation in the ternary composite. The photoexcited electrons spontaneously migrate through the heterojunction from Bi to OV-BiOI and then to (BiO)₂CO₃ under the driving force of the energy difference between the dual heterojunction. The holes remain in the VB of BiOI, but the electrons in the CB of (BiO)₂CO₃ can react with the O₂ to produce •O₂^‒ radicals [47]. In addition, some of the electrons activate oxygen, generating •O₂^‒ radicals under the action of the OVs in the OV-BiOI. The residual holes in the VB of OV-BiOI react with OH^‒ to produce •OH radicals [48]. These reactive oxygen species are responsible for the prominent visible light photocatalytic NO removal ability of the composite [49-51]. In summary, because of the heterojunction and SPR effects of the Bi NPs and the presence of OVs, the Bi/BiOI/(BiO)₂CO₃ ternary system shows highly enhanced photocatalysis with increased charge carrier concentration and rapid electron-hole separation [52, 53].

A Bi/BiOI/(BiO)₂CO₃ ternary heterojunction photocatalyst was synthesized by an in situ assembly method using NaBH₄ as the reducing agent. During the reaction, metallic Bi and (BiO)₂CO₃ were produced from BiOI. Using both theory and experiment, we have revealed the role of each component in the ternary system in photocatalysis. This composite has a broad response range to visible light and high electron-transfer efficiency on account of the synergistic effect of the heterojunction, SPR, and OVs. The introduction of Bi, (BiO)₂CO₃, and OVs provides abundant electrons for the activation of O₂ molecules, thus promoting the production of ROS for photocatalytic NO oxidation. As a result, the Bi/BiOI/(BiO)₂CO₃ ternary composites demonstrate highly enhanced photocatalytic activity for the removal of NO under visible light irradiation. Thus, this study provides insights into the synergistic effect of the heterojunction structure, SPR effect, and OVs for enhancing visible light photocatalysis, providing a new strategy to explore other bismuth-containing heterostructured visible-light-driven photocatalysts.