3.1 Structure of photocatalysts
The XRD pattern was used to reveal the crystal structure of ZnCo2O4, BiOBr, and ZCo/BB materials, as shown in Fig. 2. The (220), (311), (400), (511), and (440) crystal planes of ZnCo2O4 can be identified by the diffraction peaks at 2θ = 31.21°, 36.80°, 44.73°, 59.28°, and 65.14°, respectively. This is consistent with prior reports in the literature (JCPDS No. 23-1390). Meanwhile, the diffraction peaks with 2θ values at 10.90°, 21.93°, 25.16°, 31.69°, 32.22°, 39.38°,44.69°, 46.21°, 46.86°, 50.67°, 53.38°, 57.12°, 61.90°, 66.22°, 67.40°, 71.00°, and 76.70° correspond to (001), (002), (101), (102), (110), (112), (004), (200), (113), (104), (211), (212), (105), (204), (220), (214), and (310) planes of BiOBr (JCPDS No. 09-0393). Moreover, for ZCo/BB composites, the intensities of the characteristic peaks corresponding to BiOBr (001) in ZCo/BB gradually increase with the increase of ZnCo2O4 loading. The primary diffraction peaks that are associated with the (102) and (110) crystal planes of BiOBr in 35% ZCo/BB are significantly reduced. During the growth process in situ, ZnCo2O4 may affect the growth direction of BiOBr due to the intense interaction between ZnCo2O4 and BiOBr (Peng et al. 2023). In addition, the diffraction peaks of the complex material ZCo/BB correspond to the peaks of the standard cards of ZnCo2O4 and BiOBr, respectively. However, the diffraction peaks of ZnCo2O4 in the ZCo/BB composites are weaker than those of BiOBr, which may be because ZnCo2O4 has a low concentration and low crystallinity. The absence of other impurity peaks in the figure proves the successful composite of ZnCo2O4 and BiOBr.
To observe the microstructure and morphology of the 25% ZCo/BB heterojunction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were measured. As displayed in Fig. 3(a,d), ZnCo2O4 shows the flake-like shapes structures with different sizes. From Fig. 3(b,e), BiOBr has a three-dimensional flake-like structure. Figure 3(c,f) shows that the ZnCo2O4 particles are stacked on the three-dimensional flake-like structure of BiOBr, and it is noted that ZnCo2O4 particles are scattered around the BiOBr nanosheets probably due to the uneven distribution caused by sonication. However, the microscopic morphology of ZnCo2O4 in Fig. 3(a) is not observed in Fig. 3(f), probably due to the change in the morphology of ZnCo2O4 during the second step of the solvent thermal preparation (Wang et al. 2019). The above SEM results show that the successful composite of 25% ZCo/BB is proven.
The heterojunction of ZnCo2O4 and BiOBr is demonstrated by the TEM image of 25% ZCo/BB in Fig. 4(a), which matches the SEM results in Fig. 3(f) and illustrates the ZnCo2O4 particles are attached to the nanosheets of BiOBr. The planar crystalline spacings of 0.190 nm and 0.203 nm are assigned to the (111) crystal plane of BiOBr and the (311) crystal plane of ZnCo2O4, respectively, as shown in Fig. 4(b). The distinct lattice fringes can be displayed in the HRTEM pictures. The EDS elemental mapping is shown in Fig. 4(c). The accordant distribution of Bi, Br, O, Co, and Zn components in selected regions of the 25%ZCo/BB composite further supports the presence of ZnCo2O4 and BiOBr.
Figure 5 exhibits the XPS pattern of ZnCo2O4, BiOBr, and 25% ZCo/BB composites. As shown in Fig. 5 (a), corresponding to the EDS results, O, Zn, Co, Br, and Bi elements can be detected in the ZCo/BB sample. The C1s peak at 284.80 eV is employed as a calibration peak (Peng et al. 2022). According to the XPS spectra, the sample in Fig. 5(a) with 25% ZCo/BB contains the components O, Zn, Co, Br, and Bi elements. The individual O 1s spectra for each sample are presented in Fig. 5 (b).
In BiOBr samples, the oxygen in the lattice and adsorbed oxygen can be seen at 529.0 eV and 530.3 eV, respectively (Han et al. 2021). In addition, the peak positions of O in ZnCo2O4 are located at 530.7 eV and 529.0 eV, associated with oxygen atoms and molecular metal-oxygen bonds around oxygen defects, respectively (Uma et al. 2020). In Fig. 5(c), the peak positions of Zn 2p1/2 and Zn 2p3/2 correspond to binding energies that are situated at 1043.3 eV and 1020.2 eV, respectively (Zhu et al. 2019). The XPS high-resolution spectra of Co 2p in Fig. 5(d) were separated into six peaks, the first double peaks at 794.0 eV and 795.5 eV are assigned to Co 2p1/2; the second set of double peaks is located at 778.9 eV and 779.9 eV, belonging to Co 2p3/2. Additionally, the binding energies of two additional satellite peaks (designated "sat") are 789.2 eV and 803.8 eV (Kitchamsetti et al. 2020). It shows that in the photocatalytic MB degradation process, various cobalt ion valence states favor electron transfer (Kitchamsetti et al. 2022).
According to Fig. 5(e), the binding energy values of the Br 3d spectra are 68.0 eV and 69.2 eV, corresponding to Br 3d5/2 and Br 3d3/2, respectively, which demonstrates that Br − is present in BiOBr (Wu et al. 2021). The binding energies of BiOBr at 164.4 eV and 159.1 eV belong to Bi 4f5/2 and Bi 4f7/5, respectively, as determined by the high-resolution spectra of Bi 4f presented in Fig. 5(f). It is demonstrated that the sample embraces Bi3+ (Wu et al. 2016). Notably, the peaks of O 1s, Zn 2p, Co 2p, Br 3d, and Bi 4f in 25% ZCo/BB are shifted toward higher binding energies compared to ZnCo2O4 and BiOBr. These shifts suggest a strong interaction and electron migration between ZnCo2O4 and BiOBr in the absence of light (Zhang et al. 2020), proving the construction of a ZCO/BB heterojunction, which could improve the ability of deteriorated MB to be used as a photocatalyst.
The locations of the functional groups were identified using FT-IR spectra of ZnCo2O4, BiOBr, and 25% ZCo/BB, which are displayed in Fig. 6. The stretching of O-H and H-O-H bending vibrations of the adsorbed water molecules are responsible for the sharp spectrum peaks at roughly 3443 cm− 1 and 1620 cm− 1, respectively (Mohamed et al. 2015). The Co-O bond in ZnCo2O4 is denoted by the wavenumber of 574 cm− 1, while the stretching vibration at 664 cm− 1 is attributed to Zn-O (Vignesh et al. 2023). Furthermore, the Bi-O stretching vibration is related to the peaks at 500–1200 cm− 1. Particularly, it is suggested that the peak of the vibration of stretching Bi-O occurs around wavenumber 517 cm− 1. Additionally, at 1378 cm− 1 relates to CO2 uptake in the atmosphere (Priya et al. 2023). The presence of these peaks in the catalyst of 25%-ZCo/BB and the absence of other impurity peaks are further evidence of the successful synthesis of 25% ZCo/BB.
Specific surface area and pore size distribution were measured for BiOBr and 25% ZCo/BB, as observed in Fig. 7. Type IV isothermal curves with H3 hysteresis loops may be seen in the curves for ZnCo2O4 and BiOBr. Curves for the distribution of pore size and specific surface area of BiOBr and 25% ZCo/BB are also shown in Table 1, and it can be seen that they both have mesoporous structures. Meanwhile, compared with BiOBr, the increased specific surface area of 25% ZCo/BB provides more reactive and adsorption sites for the photocatalytically degraded MB.
Table 1
Specific surface area and pore volume of BiOBr and 25% ZCo/BB
Sample | BET (m2·g− 1) | Pore volume (cm3·g− 1) |
BiOBr | 7.13 | 0.032 |
25% ZCo/BB | 18.74 | 0.086 |
3.2 Photocatalytic performance analysis
The photocatalytic elimination of MB by ZnCo2O4, BiOBr, and various ZCo/BB composite photocatalysts is shown in Fig. 8(a). After 30 minutes of adsorption under dark reaction conditions, the concentration of MB did not change with time after the dark reaction for the 25% ZCo/BB composite photocatalyst without light, indicating that the adsorption-desorption equilibrium was reached. ZnCo2O4, BiOBr, 5% ZCo/BB, 15% ZCo/BB, 25% ZCo/BB, and 35% ZCo/BB composite photocatalysts showed 29%, 50%,51%, 70%, 92% and 74% degradation rates within 30 min, respectively. With a 63% increase in the photocatalytic efficiency over a single ZnCo2O4 and a 42% increase over a single BiOBr under identical testing conditions, 25% ZCo/BB demonstrated the highest photocatalytic degradation efficiency. Due to the band-site matching between BiOBr and ZnCo2O4, the migration efficiency of light-generated electrons and holes was increased, which prevented the carriers from compounding. Therefore, the results mentioned above demonstrated that a variety of ZCo/BB materials significantly enhanced the photocatalytic degradation of MB performance (Khan et al. 2018).
The decomposition efficiency of the binary composite photocatalyst greatly declined after the ratio exceeded 25% for ZCo/BB photocatalysts. Perhaps as a result excessive loading of ZnCo2O4 nanoparticles may reduce the number of active sites, thus decreasing the composite's capability to absorb visible light and lowering its effectiveness as a composite photocatalyst for photocatalytic degradation(Zhong et al. 2022). The maximum photocatalytic degradation activity for MB was demonstrated at 25% ZCo/BB with excellent light absorption. The photocatalytic degradation removal rate of MB also conforms to the first-order linear kinetic equation as follows,
ln(Ct/C0) = kt (1)
Where k is the rate constant, the value of k can be computed by fitting the slope of the first-order linear kinetic equation, and Ct and C0 are the concentrations at t and the initial moment, respectively. In Fig. 8(b), the calculated findings are largely compatible with the pseudo-first-order kinetics and exhibit a high linear connection. As illustrated in Fig. 8(c), all the nanocomposite samples exhibited higher values of rate constant k relative to single ZnCo2O4 and BiOBr. And the rate constants of MB photodegradation follow the following order: 25% ZCo/BB > 35% ZCo/BB > 15% ZCo/BB > 5% ZCo/BB > BiOBr > ZnCo2O4. The highest k value was determined to be 0.074 min− 1 for 25% ZCo/BB samples, which was 8.2 and 3.7 times greater than that of single ZnCo2O4 and BiOBr, respectively. It is suggested that the fast electron migration at the ZnCo2O4/BiOBr interface may promote photocatalytic efficiency. Table 2 provides a further comparison of the performance of the ZCo/BB photocatalytic heterojunction composites with the results in the literature. It shows that the 25% ZCo/BB photocatalyst has a higher degrading efficiency than other photocatalysts.
Table 2
Comparison of photocatalytic performance
Sample | Light source | Substrate | Performance | Ref |
ZnCo2O4/MoS2 | A metal halide lamp of 400 W. | MB | (120 min) DE = 95.4% | (Maksoud et al. 2022) |
BiOBr/Bi2O3 | A 300 W Xe lamp | MB | (50 min) DE = 87.1% | (Wang et al. 2015) |
BiOBr/NiFe2O4 | A 350 W Xenon lamp | MB | (60 min) DE = 90% | (Li et al. 2017) |
g-C3N4/BiOI/BiOBr | A 500 W xenon lamp | MB | (150 min) DE = 80% | (Yuan et al. 2016) |
BiOBr/BiPO4 | A 250 W halide lamp | MB | (120 min) DE = 94% | (An et al. 2015) |
25% ZCo/BB | A 300 W xenon lamp MB (λ > 420 nm) | (30 min) DE = 92% | This work |
Electron transfer effectiveness and photon utilization are assessed using photocurrent response testing, and in general, a higher density of photocurrent implies a higher separation efficiency of photogenerated carriers (Fang et al. 2023). So photocurrent tests were performed on ZnCo2O4, BiOBr, and 25% ZCo/BB. According to Fig. 9 (a), the 25% ZCo/BB composite has a higher photocurrent density than that of ZnCo2O4 and BiOBr, which is explained by the strong interaction between ZnCo2O4 and BiOBr.
The electrochemical transfer impedance (EIS) of the sample was measured in Fig. 9(b). The charge transfer efficiency at the electrode interface is reflected in the Nyquist plot's arc radius (Yang et al. 2023). A smaller arc radius indicates a faster rate of electron transfer. As seen in Fig. 9(b), the arc radii of these materials are in the following order: ZnCo2O4 > BiOBr > 25% ZCo/BB.
The complexation of 25% ZCo/BB photogenerated carriers was observed using PL spectroscopy. According to Fig. 9(c), 25% ZCo/BB has the highest photogenerated carrier separation efficiency because the PL peak is lower than that of BiOBr, which follows transient photocurrent and EIS.
The visible absorption spectrum of the MB solution on 25% ZCo/BB over time is shown in Fig. 10(a). Under the action of visible light, the intensity of the MB's distinctive peak at 664 nm gradually declines over 30 minutes, exhibiting that the substance is degrading steadily, the molecular structure of the MB is disrupted, and the photocatalyst exhibits excellent photocatalytic activity when exposed to visible light as the color of the MB changes from blue to colorless.
To shed light on the potential photocatalytic mechanism, active species trapping studies were carried out on 25% ZCo/BB to investigate active radicals. In the test, scavengers such as isopropyl alcohol (IPA), disodium (EDTA-2Na), and benzoquinone (BQ) were employed to capture ·OH, h+, and ·O2−, respectively. The addition of EDTA-2Na and IPA considerably slowed the degradation rate of MB dye, as shown in Fig. 10(b), which reduced respectively from 92–21% and 36%, and the degradation rate decreased from 92–74% with the introduction of the BQ. The outcome indicates that ·OH and h+ are essential for accelerating MB degradation, proving that h+ and ·OH are two dominant active substances in the photocatalytic reaction, while ·O2− is an auxiliary active group.
The recyclability of the 25% ZCo/BB photocatalyst was evaluated, as shown in Fig. 10 (c). 25% ZCo/BB maintained good photocatalysis after four cycles and still achieved an 80% degradation rate. XRD patterns after cycling were examined to determine whether the structure altered before and after cycling, as shown in Fig. 10(d). It is noteworthy that the crystallinity rarely changes after the photocatalytic reaction (Ma et al. 2023). The above experimental outcomes indicate that 25% ZCo/ BB has good recyclability and photostability.
The UV diffuse reflectance spectroscopy can be used as a tool to examine the intensity of visible light absorbed by the samples. According to Fig. 11 (a), the BiOBr absorption band is approximately 426 nm, and it shows the highest absorption intensity in both UV and visible regions due to the black color of ZnCo2O4. Notably, the absorption of 25% ZCo/BB photocatalyst is enhanced at 400–800 nm with the loading of ZnCo2O4 nanoparticles compared to that of a single BiOBr. It is further demonstrated that the loading of the appropriate amount of ZnCo2O4 can increase the utilization of visible light in this system and can improve photocatalytic performance. Additionally, the Tauc's equation describes the synthesized photocatalysts' band gap energies as follows,
αhν = A(hν - Eg)n/2 (2)
where the bandgap energy, Planck constant, optical frequency, and absorption coefficient are represented by Eg, h, v, and α, respectively. Here, n is determined to be 1 and 4, respectively, because ZnCo2O4 is a direct bandgap semiconductor while BiOBr is an indirect bandgap semiconductor (Chen et al. 2019, Liu et al. 2022). According to Eq. (2), the Eg of ZnCo2O4 and BiOBr were calculated to be 1.77 eV and 2.78 eV, respectively, as displayed in Fig. 11(b) and Fig. 11(c).
To further explore the shift mechanism of photogenerated charges and determine the type of semiconductor, the flat band potential (Efb) of ZnCo2O4 and BiOBr was determined using the Mott-Schottky (M-S) test. As can be seen from Fig. 11(c) and Fig. 11(d), the straight line parts of the ZnCo2O4 and BiOBr curves have negative and positive slopes, respectively. Therefore, from the slope of the above M-S curve, it can be determined that ZnCo2O4 is a p-type semiconductor while BiOBr is an n-type semiconductor. In addition, the M-S test shows that the Efb of ZnCo2O4 is around 0.93 eV and the Efb of BiOBr is -0.38 eV (vs. Ag/AgCl). The Efb is 0.1 eV lower compared to the valence band potential (EVB) for p-type semiconductors, and the Efb is 0.1 eV higher related to the conduction potential (ECB) for n-type semiconductors (Guo et al. 2021, Li et al. 2023a).
Accordingly, it is estimated that the EVB of ZnCo2O4 is around 1.03 eV (vs. Ag/AgCl) while the ECB of BiOBr is -0.48 eV (vs. Ag/AgCl). Its energy band structure can be obtained in terms of the following equation (Li et al. 2023b),
ENHE=EAg/AgCl+0.2 (3)
EVB=Eg+ECB (4)
where ENHE is the standard potential of a hydrogen electrode. According to Equ. 3, it can be speculated that the EVB of ZnCo2O4 is 1.23 eV and the ECB for BiOBr is -0.28 eV (vs. NHE), respectively. Equ. 4 is used to calculate the ECB of ZnCo2O4 and the EVB of BiOBr, which are respectively 0.54 eV and 2.50 eV based on the Eg values of 1.77 eV and 2.78 eV for ZnCo2O4 and BiOBr.
3.3 Photocatalytic mechanism
Based on calculations for energy bands and the outcomes of experiments for active species capture, the electron migration process of the 25% ZCo/BB heterojunction may follow a type II or Z mechanism. If the mechanism is consistent with type II heterogeneous junctions, the VB of ZnCo2O4 receives the holes from the valence band of BiOBr, and the electrons produced by light excitation can be transferred from the CB of ZnCo2O4 to the CB of BiOBr. Compared to the reduction potential of O2/·O2− (-0.33 eV vs. NHE), the CB potential of BiOBr is more positive (-0.28 eV), so the electrons in the CB of BiOBr are not able to reduce the O2 dissolved in the MB dye to ·O2− (Chinnathambi et al. 2021). While the holes on the VB of ZnCo2O4 are not capable of converting H2O to ·OH, this is because its VB position (1.23 eV) is lower than the potential of H2O/·OH (+ 2.40 eV vs. NHE) (Zhang et al. 2022b).
In general, the Fermi levels (Ef) of n-type BiOBr and p-type ZnCo2O4 are close to their CB and VB, respectively. When they are in close contact, the charges will be redistributed until the Fermi energy levels reach equilibrium (Wang et al. 2021). Subsequently, electron accumulation and electron depletion layers are created on the interface of BiOBr and ZnCo2O4, respectively, leading to the establishment of the internal electric field between ZnCo2O4 and BiOBr, which increases the photogenerated electrons' efficiency of transmission when the photocatalyst is illuminated. Therefore, a more plausible p/n Z-Scheme charge transfer mechanism is presented (see Fig. 12). Under the impact of the built-in electric field, strong oxidizing and reducing abilities are possessed by h+ that are retained in the BiOBr valence band and by the electrons that are retained in the ZnCo2O4 conduction band, respectively, while electrons in the VB of BiOBr can complex with holes in the VB of ZnCo2O4 via migration. Ultimately, O2 adsorbed on the surface of ZCo/BB catalysts can be rapidly converted to ·O2− by electrons on the surface of ZnCo2O4. The h+ retained in BiOBr directly oxidizes the MB dye molecule to CO2 and H2O. Because BiOBr has a greater VB potential than H2O/·OH (+ 2.40 eV vs. NHE), it can directly oxidize H2O to ·OH and finally degrade the MB dye molecule.