English

• 0.   Introduction

  Photocatalysis for the conversion of CO₂ to fuels and chemical materials holds considerable promise in reducing humanity's dependence on fossil fuels, which is fundamental to addressing energy shortages and mitigating CO₂ production^[1-3]. An efficient CO₂ reduction photocatalyst requires broad-spectrum light absorption ability, high-efficiency charge separation, and robust redox capacity. Nevertheless, developing an efficient and highly selective CO₂ catalyst remains a formidable challenge. This is mainly attributed to the high dissociation energy (750 kJ·mol^-1) required to break the C=O double bond, as well as the complexity introduced by multiple reaction intermediates^{[2, 4]}. As a result, the design of highly active photocatalytic systems for CO₂ reduction continues to be a significant challenge.

  The solar photo-driven semiconductor photocatalysts have garnered considerable attention due to their optimal redox potential, environmental compatibility, sustainability, and exceptional efficiency^[5]. However, the utilization of single-component catalysts is hindered by the rapid recombination of photogenerated carriers (h⁺/e^-), which restricts their applications owing to the minimal quantum efficiency^[6]. To overcome this challenge, researchers have devoted significant efforts to enhancing catalyst performance through techniques such as precious metal deposition^[7], atom doping^[8], or heterojunction construction^[9-11]. Recently, the integration of heterojunctions through the coupling of semiconductors has emerged as a promising strategy to enhance the performance of photocatalysts. It is important to highlight that the construction of Z-scheme heterojunctions can improve the separation of h⁺/e^- and electron transport efficiency while maintaining superior redox ability, such as Bi₂O_2.33-CdS^[12], ZnIn₂S₄@Ni(OH)₂/NiO^[13], g‑C₃N₅/BiOBr^[14]. However, it remains crucial to develop innovative Z‑scheme heterojunctions with enhanced activity for CO₂ photocatalytic reduction via a facile synthesis process.

  The bismuth-based catalysts, particularly BiOX (X=Cl, Br, and I) with a layered ternary oxide structure, have garnered significant attention has been garnered in the fields of hydrogen evolution, pollutant photodegradation, and CO₂ reduction owing to the unique structural characteristics and superior optical and electronic properties of these materials^[15]. Among them, BiOBr has been extensively investigated owing to its relatively wide bandgap energy and the appropriate alignment of its conduction and valence band (VB). Although BiOBr demonstrates advantages in visible-light absorption and structural tunability, its practical application in CO₂ photoreduction is limited by four intrinsic challenges: (ⅰ) inadequate light harvesting beyond 460 nm, (ⅱ) rapid charge recombination induced by interlayer polarization, (ⅲ) insufficient CO₂ adsorption and activation, and (ⅳ) restricted product selectivity. These challenges motivated us to design and construct a Z-type heterojunction of Co₃O₄/BiOBr by fully exploiting the synergistic advantages of both components, where the narrow-bandgap Co₃O₄ facilitates visible-light response across the entire spectrum.

  The Co₃O₄/BiOBr heterojunctions with various molar ratios of Co₃O₄ and BiOBr were synthesized using a facile chemical method at ambient temperature. The CO₂ photoreduction efficiency was systematically investigated by evaluating the influence of Co₃O₄/BiOBr molar ratios, H₂O dosage, irradiation time, and sacrificial reagent in the absence of a photosensitizer. Remarkably, the Co₃O₄/BiOBr-0.8 heterojunction exhibited an impressive CO evolution rate of 112.2 μmol·g^-1·h^-1, which was 6.3 times higher than pristine BiOBr. Experimental findings unequivocally demonstrate that the as-synthesized Co₃O₄/BiOBr heterojunction possesses a Z-scheme band structure, thereby facilitating efficient charge separation dynamics, superior redox capability, and enhanced adsorption and activation capacity towards CO₂, leading to a substantial enhancement in its photocatalytic performance for CO₂ reduction.

  1.   Experimental

  The reagents utilized in this investigation were of analytical grade and employed without any additional purification steps (Supporting information).

  The Bi(NO₃)₃·5H₂O solution was prepared by dissolving 1 mmol Bi(NO₃)₃·5H₂O in 45 mL ethylene glycol and subjected to ultrasonication for 45 min. Subsequently, solution A was formed by dissolving 1 mmol cetyltrimethylammonium bromide (CTAB) and 0.2 g polyvinylpyrrolidone (PVP) in the aforementioned Bi(NO₃)₃·5H₂O solution. Separate batches of ethylene glycol (24 mL each) were used to disperse solutions containing varying amounts (0, 0.2, 0.4, 0.6, 0.8, and 1 mmol) of Co₃O₄, respectively. The thoroughly mixed Co₃O₄ solution was then combined with the solution A under continuous stirring for 30 min to form solution B. The resulting mixture was kept in an oven at a temperature of 180 ℃ for 10 h. The resulting precipitates were cooled to room temperature before being collected through filtration, washed multiple times using deionized water, and dried at 60 ℃ for 10 h to obtain Co₃O₄/BiOBr powders exhibiting varying molar ratios of Co₃O₄ to BiOBr (0.2∶1, 0.4∶1, 0.6∶1, 0.8∶1, and 1∶1), which were referred to as Co₃O₄/BiOBr-0.2, Co₃O₄/BiOBr-0.4, Co₃O₄/BiOBr-0.6, Co₃O₄/BiOBr-0.8, and Co₃O₄/BiOBr-1.0, respectively. The pure BiOBr was prepared using the same method without the addition of Co₃O₄.

  The photocatalytic reduction of CO₂ measurement process and photoelectrochemical measurement are shown in Supporting information.

  2.   Results and discussion

  2.1   Crystal structure and morphology

  The SEM images in Fig.1a and 1b exhibit the morphology of Co₃O₄/BiOBr‑0.8, revealing a distinctive nanoflower structure with incorporated Co₃O₄ nanospheres. Furthermore, TEM image confirms the presence of a special nanoflower with nanosphere morphology in Co₃O₄/BiOBr-0.8 (Fig.1c), demonstrating evident integration between the nanospheres and nanosheets compared to pristine BiOBr (Fig.S1). The high temperature and pressure environment hinders the growth of the synthesis process and benefits the formation of heterogeneous interfaces. High-resolution transmission electron microscopy (HRTEM) analysis of Co₃O₄/ BiOBr‑0.8 revealed an intimate contact interface between the heterojunctions, as evidenced by lattice spacings of 0.28 and 0.27 nm corresponding to the Co₃O₄ (220) plane and BiOBr (003) plane^[16-17], respectively (Fig.1d). The distribution of Bi (green), O (yellow), Br (orange), and Co (purple) elements in the Co₃O₄/BiOBr-0.8 heterojunction was found to be uniform (Fig.1e and 1f). Additionally, Fig.S1 shows the SEM image of pure BiOBr, which shows the nanoflower structure combined with nanosheets.

  Figure 1

  

  Figure 1.  (a, b) SEM images, (c) TEM image, (d) HRTEM image, and (e, f) elemental mapping images of Co₃O₄/BiOBr-0.8

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  The XRD patterns presented in Fig.2a display characteristic peaks corresponding to Co₃O₄/BiOBr composites with varying molar ratios. These peaks confirm the presence of tetragonal structures, specifically BiOBr (PDF No.09‑0393) and Co₃O₄ (PDF No.42‑1467)^[17]. The sharpness of these peaks indicated a high degree of crystallinity across all samples examined. Moreover, the intensity of diffraction peaks associated with Co₃O₄ showed a gradual increase, suggesting its coexistence within the composite materials alongside BiOBr. Additionally, higher concentrations of Co₃O₄ result in enhanced intensities at the (311) plane peak position, while lower concentrations of BiOBr lead to reduced intensities at the (101) and (113) plane peaks due to interface interactions between Co₃O₄ and BiOBr.

  Figure 2

  

  Figure 2.  (a) XRD patterns of Co₃O₄/BiOBr; (b) Br3d, (c) Bi4f, and (d) O1s XPS spectra of Co₃O₄, BiOBr, and Co₃O₄/BiOBr-0.8

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  The chemical valence states and elemental composition of the synthesized samples were characterized using X-ray photoelectron spectroscopy (XPS). Fig.S2a presents the full XPS spectrum of Co₃O₄/BiOB-0.8, confirming the presence of Bi, O, Br, and Co elements. As illustrated in Fig.2b, the peaks corresponding to Br3d_5/2 and Br3d_3/2 exhibited binding energies of 68.2 and 69.2 eV, respectively, in both pure BiOBr and Co₃O₄/BiOBr-0.8, with no significant shifts observed. In Fig.2c, the characteristic peaks of pure BiOBr at 159.3 and 164.6 eV in the Bi4f spectrum correspond to Bi³⁺4f_7/2 and Bi³⁺4f_5/2, respectively. The peaks at 157.6 eV (Bi4f_7/2) and 162.9 eV (Bi4f_5/2) associate with Bi^(3-x)+ species. For Co₃O₄/BiOB-0.8, the characteristic peaks at 159.1 and 164.4 eV also correspond to Bi³⁺4f_7/2 and Bi³⁺4f_5/2, while the peaks at 157.8 eV (Bi4f_7/2) and 163.1 eV (Bi4f_5/2) reflect the presence of Bi^{(3-x)+ [18]}. In Fig.2d, the O1s spectrum of Co₃O₄ at 531.1 eV can be attributed to the Co—O bond and the adsorption of the H₂O group on the surface of the catalyst at 532.9 eV. The O1s spectrum of BiOBr can be fitted with three distinct peaks at 529.6, 530.9, and 532.4 eV, which are associated with the Bi—O bond, oxygen vacancies (OV), and the adsorption of H₂O groups, respectively. Notably, for the Co₃O₄/BiOBr‑0.8 sample, the three peaks at 529.8, 531.6, and 532.5 eV correspond to the M—O (M: metal) bond, OV, and adsorption of H₂O groups, respectively. It was observed that the intensity of the M—O peak increased, while the concentration of OV slightly decreased. The high-resolution XPS spectrum of the Co2p (Fig.S2b) exhibited two characteristic peaks, which can be associated with Co²⁺2p_1/2 (797.3 eV), Co²⁺2p_3/2 (781.8 eV), Co³⁺2p_1/2 (795.2 eV), and Co³⁺2p_3/2 (780.0 eV). This indicates the coexistence of Co²⁺ and Co³⁺ species in Co₃O₄. Notably, the Co2p peaks in the Co₃O₄/BiOBr-0.8 composite exhibited a slight shift towards lower binding energies (Co²⁺2p_1/2: 797.1 eV, Co²⁺2p_3/2: 781.6 eV; Co³⁺2p_1/2: 795.0 eV, Co³⁺2p_3/2: 779.8 eV) compared to those of pure Co₃O₄, suggesting the enhanced electron density around the cobalt sites^[19]. This shift implies the formation of a heterojunction interface that facilitates electron transfer from BiOBr to Co₃O₄. In summary, BiOBr and Co₃O₄ are primarily interconnected through Bi—O and Co—O bonds, indicating the formation of robust chemical bonds at the interface of the Co₃O₄/BiOBr-0.8 heterojunction.

  Electron paramagnetic resonance (EPR) spectroscopy was applied to BiOBr and Co₃O₄/BiOBr-0.8, as illustrated in Fig.S3. Both samples exhibited a pronounced signal at g=2.002, with the peak intensity of OV decreased with the Co₃O₄ content combined. This suggests that the Co₃O₄ content can effectively modulate the concentration of OV in the composites. Furthermore, the variations in OV and Bi^(3-x)+ species were successfully characterized. These findings indicate that during heterojunction formation, Bi^(3-x)+ ions bind to the oxygen atoms of Co₃O₄, thereby modulating the electron cloud density and facilitating electron transfer to the Co₃O₄ phase^[5]. This interaction leads to an increased Bi³⁺/Bi^(3-x)+ ratio and a decreased EPR intensity, providing robust evidence for the successful synthesis of the Co₃O₄/BiOBr-0.8 composites.

  2.2   Electronic band structure

  The UV-Vis diffuse reflectance spectra (DRS) of pure Co₃O₄, pure BiOBr, and Co₃O₄/BiOBr heterojunctions with different molar ratios are illustrated in Fig.3a. Forming a heterojunction with Co₃O₄ enhances the visible-light absorbance of BiOBr. Notably, Co₃O₄/BiOBr-0.8 exhibited a markedly improved light absorption response in the visible spectrum, which can be attributed to the synergistic effect resulting from the intimate coupling between Co₃O₄ and BiOBr. The band gap energies of pure BiOBr and Co₃O₄ were estimated by the following Eq.1.

  $ αhν = A(αhν - E_{g})^{n/2} $

  (1)

  Figure 3

  

  Figure 3.  (a) UV-Vis DRS of different catalysts; (b) Band gaps, (c) Mott-Schottky curves, and (d) VB-XPS spectra of BiOBr and Co₃O₄

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  where α, hν, A, and E_g represent the absorbance, photon energy, constant, and bandgap energy, respectively^[20]. The band gaps of pristine BiOBr and Co₃O₄ were 3.1 and 1.8 eV, respectively, which are consistent with the values reported in the literature (Fig.3b)^[21]. Furthermore, Mott-Schottky curves were used to evaluate the flat band potentials (E_fb) of pristine BiOBr and Co₃O₄. The positive slopes observed in the Mott-Schottky curves indicate that both materials exhibited n-type semiconductor characteristics (Fig.3c). The flat band potentials of pristine BiOBr and Co₃O₄ were -1.47 and -1.08 V (vs Ag/AgCl) or -0.86 and -0.47 V (vs NHE), respectively. Subsequently, the conduction band (CB) of n-type semiconductors is close to the E_fb, so that the CB potentials of BiOBr and Co₃O₄ were estimated to be -0.86 and -0.47 V vs (NHE), respectively. The VB potential of BiOBr and Co₃O₄ can be calculated using the E_g and VB energy, which were 2.24 and 1.33 V (vs NHE), respectively. The VB offset relative to the Fermi level for pure BiOBr and Co₃O₄ can be quantified through VB-XPS analysis (Fig.3d), revealing that the Fermi levels of pure BiOBr and Co₃O₄ were 0.61 and 0.17 eV, respectively.

  2.3   Photoreduction CO₂ performance

  The photocatalytic performance of CO₂ reduction was assessed on pure BiOBr, pure Co₃O₄, and Co₃O₄/BiOBr composites with different molar ratios under 5 h irradiation (Fig.4a). The results indicated that CO was the primary product of CO₂ photocatalytic reduction^[22]. Among all catalysts tested, Co₃O₄/BiOBr-0.8 exhibited the best photocatalytic performance, achieving CO evolution rates of 112.2 μmol·g^-1·h^-1 and CH₄ evolution rates of 5.5 μmol·g^-1·h^-1 (Fig.S4), which were 6.3-fold and 3.9-fold higher than BiOBr alone. Furthermore, among the various molar ratios of Co₃O₄/BiOBr heterojunctions, the evolution rates of CO and CH₄ showed a trend of rising first and then declining. Co₃O₄/BiOBr-0.8 demonstrated the highest photocatalytic activity, suggesting an optimal ratio of 0.8∶1 for maximizing the efficiency of the Co₃O₄/BiOBr heterojunction. Fig.4b illustrates that under experimental conditions such as an Ar atmosphere devoid of CO₂, or the absence of H₂O, light, or catalyst individually, trace reduction products were detected. This indicates that CO and CH₄ production produced by CO₂ molecules and confirms that all four factors—CO₂, H₂O, light, and catalyst—are indispensable for a successful photoreduction reaction. Fig.4c presents a comparative test of the photocatalytic activity for CO₂ photoreduction between the Co₃O₄/BiOBr heterojunction and a mechanical mixture of pure Co₃O₄ and BiOBr (Co₃O₄+ 0.8BiOBr). The data demonstrated that the CO productions from the Co₃O₄/BiOBr heterojunction were markedly higher compared to those from the mechanical mixing, underscoring the significant role of the heterojunction structure in enhancing CO₂ photoreduction efficiency^[23]. Furthermore, Fig.4d illustrates that the CO evolution of Co₃O₄/BiOBr‑0.8 was significantly higher than that of BiOBr, with a consistent increase observed over time. Additionally, it is confirmed that the reduction of CO follows first-order reaction kinetics, suggesting that this step is rate-limiting in the overall reaction (CO₂+2e^-+2H⁺→CO+H₂O). In contrast, the production of CH₄ was significantly more challenging due to the requirement for a greater number of electrons and protons (CO₂+8e^-+8H⁺→CH₄+2H₂O). As a result, the production of CO was markedly higher compared to CH₄, as the reduction processes leading to these products likely proceed via parallel pathways.

  Figure 4

  

  Figure 4.  (a) Productions of CO on various catalysts; (b) Productions of CO on Co₃O₄/BiOBr-0.8 with different reaction conditions; (c) Productions of CO on mechanical mixture and Co₃O₄/BiOBr-0.8; (d) Productions of CO on Co₃O₄/BiOBr-0.8 and BiOBr with different irradiation times

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  2.4   Stability of the catalyst

  Fig. 5a demonstrates that Co₃O₄/BiOBr-0.8 exhibited a minor decrease in CO production during the seven cycle reaction processes, yet it maintained overall stability over the entire duration of 28 h. The XRD pattern of the fresh Co₃O₄/BiOBr-0.8 showed significant similarity to that of the used Co₃O₄/BiOBr-0.8 after seven cycles of photocatalytic reactions (Fig. 5b). However, a slight attenuation in the peak intensity corresponding to Co₃O₄ was observed. This indicates that the catalyst possesses remarkable recyclability and stability. To elucidate the direction of electron transfer during the reaction process, XPS analyses were conducted on the catalyst both before and after the cyclic reaction. Notably, as shown in Fig. 5c, the Bi4f spectra exhibited a systematic shift toward lower binding energy, which is indicative of an electron-accumulating state. As illustrated in Fig. 5d, the Co element demonstrated a pronounced shift toward higher binding energy, reflecting electron depletion. Furthermore, the peak intensity of Co³⁺ exhibited significant attenuation, leading to a marked decline in the Co³⁺/Co²⁺ ratio. Collectively, these observations suggest that during the reaction process, electrons are transferred from the CB of Co₃O₄ to the VB of BiOBr, consistent with the electron transport pathway of the Z-scheme heterojunction.

  Figure 5

  

  Figure 5.  (a) Stability test of Co₃O₄/BiOB-0.8; (b) XRD patterns, (c) Bi4f, and (d) Co2p spectra of Co₃O₄/BiOB-0.8 before and after the stability test

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  2.5   Photocatalytic mechanism

  Fig. 6a illustrates the photoluminescence (PL) spectra of pure BiOBr and Co₃O₄/BiOBr heterojunctions with different ratios. The PL intensity decreased first and then increased with the increase of Co₃O₄ amount, suggesting that the recombination rate of electron-hole pairs in BiOBr is significantly suppressed through the incorporation of Co₃O₄^[24]. From the figure, we can see that the most suitable ratio was 0.8∶1. Fig. 6b displays the time-resolved PL decay (TRPL) curves of Co₃O₄/BiOBr-0.8, pure BiOBr, and pure Co₃O₄. The TPRL curves can be analyzed using Eq.2:

  $ {\tau }_{a}=\frac{{A}_{1}{\tau }_{1}^{2}+{A}_{2}{\tau }_{2}^{2}}{{A}_{1}{\tau }_{1}+{A}_{2}{\tau }_{2}} $

  (2)

  Figure 6

  

  Figure 6.  (a) Steady PL spectra, (b) TRPL curves, (c) transient photocurrent density curves, and (d) EIS of pure Co₃O₄, pure BiOBr, and Co₃O₄/BiOBr-0.8

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  where A₁ (the amplitude coefficient of the short-lived component) and A₂ (the amplitude coefficient of the long‑lived component) represent the corresponding amplitudes, and τ₁ and τ₂ denote the fluorescent lifetimes of fast decay and slow decay stages, respectively. For Co₃O₄/BiOBr‑0.8, τ₁=0.83 ns and τ₂=3.53 ns, which were longer than the values of pure Co₃O₄ (τ₁=0.73 ns; τ₂=2.53 ns) and BiOBr (τ₁=0.82 ns; τ₂=3.48 ns). Consequently, the average fluorescent lifetimes of Co₃O₄/BiOBr-0.8 (1.54 ns) were prolonged than pristine Co₃O₄ (1.37 ns) and BiOBr (1.30 ns) as calculated from Eq.2. The prolonged fluorescence lifetime further substantiates that the heterojunction structure can efficiently separate photogenerated electrons and holes into distinct phases, thereby significantly inhibiting the recombination of photogenerated carriers^[25]. Fig.6c illustrates the transient photocurrent responses of pure BiOBr, pure Co₃O₄, and Co₃O₄/BiOBr-0.8 under intermittent light illumination. This method is embodied by using an electrochemical method to test the current density of the photosensitive semiconductor under the switching light conversion for assessing the separation efficiency of photogenerated electron-hole pairs. The photocurrent generated by Co₃O₄/BiOBr-0.8 was notably higher than that of pure BiOBr and pure Co₃O₄, indicating a significant enhancement and stability in the separation efficiency of photoinduced electron-hole pairs for Co₃O₄/BiOBr-0.8^[26]. Fig.6d presents the electrochemical impedance spectra (EIS) of pure BiOBr, pure Co₃O₄, and Co₃O₄/BiOBr-0.8. The arc radius of Co₃O₄/BiOBr-0.8 was considerably smaller compared to pure BiOBr and Co₃O₄, suggesting superior interfacial charge transfer efficiency for Co₃O₄/BiOBr-0.8^[27]. Consequently, the Z-scheme heterojunction of Co₃O₄/BiOBr demonstrates enhanced separation of electron‑hole pairs and efficient electron transport, which aligns well with the experimental findings.

  The photocatalytic intermediates on Co₃O₄/BiOBr-0.8 were identified using in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). As illustrated in Fig.7, the DRIFTS peaks of Co₃O₄/BiOBr-0.8 exhibited enhanced intensity with prolonged reaction time, indicating that CO₂ molecules were effectively reduced under light irradiation. The characteristic peak at 1 065 cm^-1 is attributed to intermediate CHO*, resulting from the reaction of intermediate CO* with H^[28]. The peak at 1 156 cm^-1 corresponds to intermediate CH₃O*, while the peak at 1 395 cm^-1 is ascribed to bending vibrations of intermediate CH₃*^[29]. The peaks at 1 362 and 1 517 cm^-1 can be attributed to the vibrational modes associated with monodentate carbonates (m-CO₃^2-). The peaks at 1 626 and 1 651 cm^-1 are attributed to bidentate carbonate (b-CO₃^2-) species^[30]. The peaks at 1 424 and 1 457 cm^-1 are attributed to bicarbonate (HCO₃^-) produced by co-adsorption of CO₂ in H₂O vapor^[11]. Additionally, the intermediate COOH* species is confirmed by the peaks at 1 544, 1 742, and 1 772 cm^-1, which is a crucial intermediate in the photoreduction from CO₂ to CO^[31]. The characteristic peaks observed at 1 915, 1 962, and 2 081 cm^-1 indicate the formation of CO* species during the photocatalytic reduction of CO₂^[32]. These peaks can be ascribed to intermediates during the photoreduction from CO₂ to CO and CH₄ with water, the possible reaction pathway is illustrated in Fig.8.

  Figure 7

  

  Figure 7.  In-situ DRIFTS of Co₃O₄/BiOBr-0.8

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  Figure 8

  

  Figure 8.  Schematic illustration of the CO₂ photoreduction in the presence of the catalyst composites

  * denotes the adsorption state at the surface of the catalyst.

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  3.   Conclusions

  In summary, the Co₃O₄/BiOBr heterojunction was synthesized via a one-step solvothermal method. The photocatalytic reduction activity of CO₂ was comprehensively examined. It was observed that the Co₃O₄/BiOBr heterojunction exhibited substantially elevated productions of CO during the CO₂ photoreduction compared to pure BiOBr and Co₃O₄. Specifically, the evolution rate of CO for the Co₃O₄/BiOBr-0.8 heterojunction was 6.3 times higher than that of pristine BiOBr. The experimental results demonstrate that the Co₃O₄/BiOBr heterojunction exhibits a Z-scheme band structure, thereby markedly improving light utilization, improving electron transport between the two phases, and restraining the recombination of photogenerated carriers. This structure not only maintains superior redox capabilities but also significantly enhances the selectivity for CO production. This study provides valuable insights into the synthesis of highly selective Z-scheme heterojunction photocatalysts for efficient CO₂ photoreduction.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  (a, b) SEM images, (c) TEM image, (d) HRTEM image, and (e, f) elemental mapping images of Co₃O₄/BiOBr-0.8

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  Figure 2  (a) XRD patterns of Co₃O₄/BiOBr; (b) Br3d, (c) Bi4f, and (d) O1s XPS spectra of Co₃O₄, BiOBr, and Co₃O₄/BiOBr-0.8

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  Figure 3  (a) UV-Vis DRS of different catalysts; (b) Band gaps, (c) Mott-Schottky curves, and (d) VB-XPS spectra of BiOBr and Co₃O₄

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  Figure 4  (a) Productions of CO on various catalysts; (b) Productions of CO on Co₃O₄/BiOBr-0.8 with different reaction conditions; (c) Productions of CO on mechanical mixture and Co₃O₄/BiOBr-0.8; (d) Productions of CO on Co₃O₄/BiOBr-0.8 and BiOBr with different irradiation times

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  Figure 5  (a) Stability test of Co₃O₄/BiOB-0.8; (b) XRD patterns, (c) Bi4f, and (d) Co2p spectra of Co₃O₄/BiOB-0.8 before and after the stability test

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  Figure 6  (a) Steady PL spectra, (b) TRPL curves, (c) transient photocurrent density curves, and (d) EIS of pure Co₃O₄, pure BiOBr, and Co₃O₄/BiOBr-0.8

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  Figure 7  In-situ DRIFTS of Co₃O₄/BiOBr-0.8

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  Figure 8  Schematic illustration of the CO₂ photoreduction in the presence of the catalyst composites

  * denotes the adsorption state at the surface of the catalyst.

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