English

• The over-exploited and widespread application of fossil fuels caused huge emissions of CO₂, leading a serious energy shortage and global warming [1,2]. Solar-driven CO₂ into value-added chemical fuels has been considered as a promising strategy to achieve sustainable natural carbon cycle [3,4]. However, the CO₂ conversion efficiency is still unsatisfactory due to the high thermodynamic stability of CO₂ [5,6]. Furthermore, the photocatalysts with poor light-harvesting and carrier separation remain the great challenges for CO₂ photoreduction [7–9]. Thus, it is essential to design photocatalysts for effective CO₂ reduction.

  Bismuth oxybromide (BiOBr) is one of the semiconductors with flexible and adjustable layered structure [10,11], which has been widely used in photocatalytic CO₂ reduction, nitrogen fixation and Cr(Ⅵ) removal [12–14]. However, pristine BiOBr suffer from insufficient active sites and weak visible light absorption, resulting in poor photocatalytic performance. To date, element doping, heterostructure construction, defect engineering and noble metal functionalization have been made to solve above problems [15–18]. Numerous researches revealed that noble metal nanoparticles coupling is an efficient strategy to boost photocatalytic CO₂ reduction. The noble metal nanoparticles can serve as the co-catalysts and improve the CO₂ photoreduction activity [19]. Moreover, noble metal nanoparticles can boost the charge separation [20,21]. The plasmonic noble metal can effectively expand the light-harvesting region and enhance visible light absorption capacity of photocatalysts [22]. Plasmonic nanoparticles can generate abundant high energy charges by nonradiative decay process, which is conducive to trigger a variety of chemical reactions [23–25]. Furthermore, noble metal nanoparticles can boost the CO₂ molecule adsorption/activation, accelerating CO₂ photoreduction [26,27]. Previous reports have found that Ag nanoparticles with excellent catalytic performance, which can significantly enhance the photocatalytic properties of semiconductor materials [28,29]. Inspired by above analysis, constructing Ag nanoparticles coupled BiOBr heterostructure may achieve the synchronous enhancement of light-harvesting, charge transfer and CO₂ activation.

  In this work, plasmonic Ag nanoparticles coupled BiOBr heterostructure were prepared by photodeposition strategy. The Ag nanoparticles not only significantly improve the visible light response, but also accelerate carrier transfer via the close contact interface between Ag nanoparticles and BiOBr. Besides, the heterostructure can boost the multi electron coupling process, accelerating CO₂ molecule dissociation. As a result, the optimezed Ag/BiOBr-2 composite shows excellent CO generation rate of 133.75 and 6.83 µmol/g under 5 h of 300 W Xe lamp and visible light (λ > 400 nm) irradiation, which is 1.51 and 2.81 folds compared to the BiOBr, respectively. This work offers a valuable reference to construct advanced photocatalysts for CO₂ conversion.

  The preparation process of photocatalyst is described in Fig. S1a (Supporting information). The crystalline structure of prepared BiOBr and Ag/BiOBr was confirmed by X-ray diffraction (XRD) pattern. As shown in Fig. S1b (Supporting information), the typical diffraction peaks are consistent with tetragonal BiOBr (JCPDS card No. 09–0393) [30]. Interestingly, there is no obvious characteristic peak for Ag nanoparticles, indicating a low loading amount or smaller Ag nanoparticles coupled on the surface of BiOBr [31,32]. The energy dispersive X-ray spectroscopy (EDS) indicates that the Ag/BiOBr-2 composite contains Ag (0.99 at%), Bi (32.09 at%), O (34.11 at%) and Br (32.81 at%) elements (Fig. S1c in Supporting information). Transmission electron microscopy (TEM) images show that the BiOBr possesses an ultrathin nanosheet structure, and the lattice spacing of 0.277 nm can be attributed to the (110) plane of BiOBr (Figs. S2a and b in Supporting information) [33]. Obviously, the Ag nanoparticles on the surface of BiOBr nanosheet with close contact interface and the Ag/BiOBr-2 and Ag/BiOBr-3 possess more nanoparticles than Ag/BiOBr-1 (Fig. 1a and Figs. S2c-f in Supporting information). The lattice fringe spacing of 0.23 nm corresponding to the (111) plane of Ag (Fig. 1b), indicating that the Ag nanoparticles is zero valence [34,35]. Besides, the element mapping illustrates that the Ag, Bi, O and Br are evenly dispersed in Ag/BiOBr-2 (Figs. 1c-f). The XPS spectra were used to investigate the chemical states of prepared BiOBr and Ag/BiOBr-2. As shown in Figs. S3a and b (Supporting information), the XPS survey spectra show that Bi, O and Br exists in BiOBr and Ag/BiOBr-2. The high-resolution XPS spectra for Bi 4f, O 1s and Br 3d illustrate that there is no obvious change in the valence state of elements after the Ag nanoparticles deposited on the surface of BiOBr substrate (Figs. S3c-e in Supporting information). In addition, the Ag 3d spectrum of Ag/BiOBr-2 shows two peaks at 368.0 and 374.1 eV (Fig. S3f in Supporting information), which match well with the binding energies of Ag 3d_5/2 and Ag 3d_3/2, indicating that the Ag nanoparticles in Ag/BiOBr heterostructure exists as singlet Ag [36].

  Figure 1

  

  Figure 1.  (a) TEM and (b) HR-TEM images of Ag/BiOBr-2. Elemental mapping images about (c) Bi, (d) O, (e) Br and (f) Ag of Ag/BiOBr-2.

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  UV–vis diffuse reflection spectroscopy (UV–vis DRS) was used to explore light absorption properties of obtained photocatalysts. As shown in Fig. S4a (Supporting information), the Ag/BiOBr heterostructures show an enhanced visible light absorption compared with BiOBr. Besides, strong absorption at ~500 nm could be attributed to the localized surface plasmon resonance (LSPR) effect for Ag nanoparticles [37]. The bandgap value (E_g) of BiOBr was calculated based on the plot for (αhv)^1/2 versus E_photon [38], and the E_g is determined to be 2.66 eV for BiOBr (Fig. S4b in Supporting information). As shown in Fig. S4c (Supporting information), the flat band of BiOBr was investigated by Mott-Schottky plots, which is calculated to be −0.46 V vs. NHE (pH 7). Notably, Mott-Schottky plots show a positive slope indicates that the BiOBr is one of n-type semiconductor, and the flat band potential for n-type semiconductor approximates Fermi level [39]. The valence band (VB) spectrum indicates that VB of BiOBr is 2.11 eV relative to Fermi level (Fig. S4d in Supporting information), the Fermi level of semiconductor in XPS spectrum is defined as 0 eV [8]. Combined with the above results, the VB value of BiOBr is 1.65 eV. According to E_CB = E_VB − E_g [38], the conduction band (CB) value of BiOBr is calculated to be −1.01 eV vs. NHE (pH 7). The result show that the BiOBr with suitable potential to trigger the CO₂ photoreduction reaction.

  To confirm the fast charge separation has been achieved in Ag/BiOBr heterostructure, the transient photocurrent response was measured under different single-color light. As shown in Figs. S5a-c (Supporting information), the Ag/BiOBr-2 heterostructure possesses a higher photocurrent compared with BiOBr, suggesting an enhanced carrier transfer performance after the construction of close contact interface structure [40]. Especially, the pristine BiOBr can be induced a band-gap excitation under 365 nm single-color light irradiation due to the excitation wavelength of Ag nanoparticles is hardly overlaps with ultraviolet light (Fig. S5a). Hence, the Ag nanoparticles act as 'electron sink' and accelerate carrier separation. Under 450 and 520 nm single-color light irradiation, the BiOBr only possesses weak light absorption according to UV–vis diffuses reflection spectra, the enhanced photocurrent response of Ag/BiOBr-2 heterostructure is due to the hot electron injects into the CB of BiOBr caused by LSPR of Ag nanoparticles (Figs. S5b and c). Furthermore, electrochemical impedance spectra (EIS) also indicates that the Ag/BiOBr-2 heterostructure possesses the smallest arc radius with a fast interfacial charge transfer (Fig. S6 in Supporting information) [41]. The EIS data was further fitted by an equivalent circuit the R_s, R_ct, W and CPE represents solution resistance, charge-transfer resistance, the Warburg diffusion element, and constant phase element with nonideal capacitive behavior [42], respectively. The lower R_ct value (Table S1 in Supporting information), suggesting that the Ag/BiOBr heterostructure facilitates the migration of photogenerated charge carriers [43,44].

  In order to deeply explore the charge exchange at the interface of BiOBr and Ag nanoparticles, density functional theory (DFT) calculations have been performed for photocatalysts. The surface charge density difference (Δρ) was calculated based on following formula (Eq. 1):

  (1)

  where ρ_Ag/BiOBr, ρ_BiOBr, and ρ_Ag is the total charge density for Ag/BiOBr, BiOBr and Ag, respectively. Notably, the higher charge density near the interface of Ag nanoparticles and BiOBr (Figs. 2a and b). Furthermore, the transfer electrons between BiOBr and Ag nanoparticles are decided via the Bader charge analysis (Eq. 2):

  (2)

  Figure 2

  

  Figure 2.  Electronic structural changes of (a) Ag/BiOBr and (b) Ag/BiOBr-M. Calculated charge density difference for (c) Ag/BiOBr and (d) Ag/BiOBr-M. Plane-averaged charge density difference for (e) Ag/BiOBr and (f) Ag/BiOBr-M.

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  The Bader charge results show that per supercell can transfer about 2.11 electrons to Ag layer in-situ constructed Ag/BiOBr, while the physically mixed Ag and BiOBr (denoted as Ag/BiOBr-M) only transfer 1.12 electrons, suggesting that in-situ constructed strong interfacial contact structure can achieve ultrafast charge transfer [45]. Moreover, the built-in electric field can be constructed between Ag and BiOBr, boosting the charge transfer. Figs. 2c and d show a significant charge redistribution after the construction of Ag/BiOBr interfacial, while there is less charge transfer from BiOBr to Ag layer for Ag/BiOBr-M. The plane-averaged electrostatic potential has been explored to estimate the charge transfer across the interface, which along the Z direction across the interface. The in-situ formed Ag/BiOBr with closed interface to be the electron transport channels, and the tunneling potential barriers can be calculated to ΔV ≈ 14.45 eV and ΔZ ≈ 2.63 Å (Fig. 2e), while the potential barrier for physically mixed Ag/BiOBr-M can be evaluated via ΔV ≈ 18.93 eV and ΔZ ≈ 5.58 Å (Fig. 2f). There are hardly any energy barriers at the Fermi level for Ag/BiOBr heterostructure, while the electrons at Fermi level possess a tunnel barrier of ΔV ≈ 2.24 eV and ΔZ ≈ 3.05 Å, which will hinder charge transfer over the interface. All above results show that the close contact interfacial structure between Ag nanoparticles and BiOBr can achieve excellent electrons transfer.

  The photocatalytic CO₂ reduction activity of BiOBr and Ag/BiOBr heterostructure were performed in pure water with 300 W Xe lamp and visible light (λ > 400 nm) irradiation. Under 300 W Xe lamp irradiation for 5 h, the BiOBr, Ag/BiOBr-1, Ag/BiOBr-2 and Ag/BiOBr-3 show CO production rates of 88.83, 115.31, 133.75 and 123.37 µmol/g (Fig. 3a), respectively. In addition, the Ag/BiOBr-2 achieves an excellent CO production rate (6.83 µmol/g), which is higher than that of BiOBr (2.43 µmol/g) under visible light (λ > 400 nm) irradiation for 5 h (Fig. 3b). Moreover, the Ag/BiOBr-2 also possesses outstanding CO₂ photoreduction activity compared with previous reported photocatalysts (Table S2 in Supporting information). As shown in Fig. S7 (Supporting information), the Ag/BiOBr-2 shows continuously increasing CO₂ photoreduction performance for 20 h Xe lamp irradiation. The TEM image shows that the Ag nanoparticles also coupled on the surface of BiOBr nanosheets after 20 h photocatalytic reaction (Fig. S8 in Supporting information). XRD pattern and XPS results illustrate that Ag/BiOBr-2 possesses stable crystal structure and valence state after stability test (Figs. S9 and S10 in Supporting information).

  Figure 3

  

  Figure 3.  CO yield of BiOBr and Ag/BiOBr after 5 h reaction. (a) 300 W Xe lamp and (b) visible light irradiation. In-situ FT-IR spectra of (c) BiOBr and (d) Ag/BiOBr-2. (e) Calculated free energy diagrams of the photocatalytic CO₂ reduction to CO over BiOBr and Ag/BiOBr.

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  The in-situ FT-IR spectra were used to investigate the intermediate products during photocatalytic CO₂ process. As shown in Figs. 3c and d, the characteristic peaks matched well with bicarbonate (HCO₃⁻), bidentate carbonate (b-CO₃²⁻), monodentate carbonate (m-CO₃²⁻), formic acid (HCOOH), ^*COOH and formate (HCOO⁻) [38,46,47]. The result indicates that the CO₂ photoreduction process over BiOBr is like that of Ag/BiOBr-2. Among them, ^*COOH has been considered as a crucial intermediate for CO₂ photoreduction to CO conversion [48]. According to the above results, the CO₂ reduction process can be speculated as follows (Eqs. 3–6):

  (3)

  (4)

  (5)

  (6)

  In addition, the reaction energy of possible intermediates was calculated by DFT calculations. It could be found that BiOBr shows a largest energy barrier for the generation of adsorbed ^*COOH intermediate, which indicates that the ^*COOH formation is rate-limiting step (Fig. 3e) [49]. The rate-limiting step of Ag/BiOBr changed to the formation of ^*CO from ^*COOH compared with BiOBr.

  Based on all above experimental and calculated results, the possible CO₂ photoreduction mechanism can be proposed. As shown in Fig. 4, under semiconductor excitation, the electrons in BiOBr can be excited to the CB, the excited electrons were captured by Ag nanoparticles on the surface of BiOBr. Afterwards, the adsorbed CO₂ molecule can be activated and convert to possible intermediates. Finally, the intermediates further generate the CO. While under light irradiation on the plasma resonance range, the LSPR absorption of Ag nanoparticles boost the generation of high-energy plasmonic hot electrons. Next, the hot electrons inject to the conduction band and trigger the CO₂ molecule photoreduction into CO. Hence, the CO₂ molecule can achieve an excellent photoreduction on Ag nanoparticles coupled BiOBr.

  Figure 4

  

  Figure 4.  Schematic illustration for photocatalytic mechanism of Ag/BiOBr composite.

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  In conclusion, the Ag nanoparticles coupled BiOBr heterostructure have be constructed by in-situ photoreduction strategy. The Ag nanoparticles serve as 'electron sink', achieving fast electrons transfer though the close contact interface. The BiOBr excited electrons and plasmonic Ag nanoparticles generated the high-energy hot electrons synchronous activated the C=O double bond of CO₂ molecule. Thus, the Ag/BiOBr heterostructure shows excellent CO₂ photoreduction activity with the CO production of 133.75 and 6.83 µmol/g under 5 h of 300 W Xe lamp and visible light (λ > 400 nm) irradiation, respectively. The charge transfer mechanism and CO₂ activation over Ag/BiOBr heterostructure were explored through DFT calculations and in-situ FT-IR analysis. This work not only provides some inspiration for deep understanding photocatalytic CO₂ reduction mechanism, but also developed a guide for design highly efficient photocatalysts.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was financially supported by the National Natural Science Foundation of China (Nos. 22108106, 21878134, 21576123), China Postdoctoral Science Foundation (No. 2020M680065), Hong Kong Scholar Program (No. XJ2021021), Key Laboratory of Electrochemical Energy Storage and Energy Conversion of Hainan Province (No. KFKT2021005).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.107962.

  
  
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• 

  Figure 1  (a) TEM and (b) HR-TEM images of Ag/BiOBr-2. Elemental mapping images about (c) Bi, (d) O, (e) Br and (f) Ag of Ag/BiOBr-2.

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  Figure 2  Electronic structural changes of (a) Ag/BiOBr and (b) Ag/BiOBr-M. Calculated charge density difference for (c) Ag/BiOBr and (d) Ag/BiOBr-M. Plane-averaged charge density difference for (e) Ag/BiOBr and (f) Ag/BiOBr-M.

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  Figure 3  CO yield of BiOBr and Ag/BiOBr after 5 h reaction. (a) 300 W Xe lamp and (b) visible light irradiation. In-situ FT-IR spectra of (c) BiOBr and (d) Ag/BiOBr-2. (e) Calculated free energy diagrams of the photocatalytic CO₂ reduction to CO over BiOBr and Ag/BiOBr.

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  Figure 4  Schematic illustration for photocatalytic mechanism of Ag/BiOBr composite.

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•