2.1 Crystal structure and formation mechanism of BOC samples
Bi2O2CO3/Bi2O3/Bi ternary composites were fabricated via a facile one-pot solvothermal route (Fig. 1a). Figure 2a presents the X-Ray diffraction (XRD) patterns of samples. When the molar ratio of Bi3+: OH− is 2.7, only Bi2O2CO3 peaks are observed for BOC-1 composite. As OH− dosages is increased, both diffraction peaks of Bi2O2CO3, Bi2O3 and metallic Bi could be clearly seen for BOC-2, BOC-3, BOC-4 and BOC-5, which are ascribed to orthorhombic Bi2O2CO3 (JCPDS 84-1752) , cubic Bi2O3 (JCPDS 74-1375)  and Bi (JCPDS 85-1329) , respectively. However, diffraction peaks of Bi2O3 would disappear and only signals of Bi2O2CO3 and Bi are observed with the increase of molar ratio of Bi3+: OH− to 4.7 (BOC-6). As for BOC composites, the colors of samples accompany with the increase of OH− dosages, which change from white to gray, then to dark grey (Fig. 2b). The color variations may be related with the content of metallic Bi because diffraction peaks of metallic Bi gradually increases with the increase of OH− dosages. A possible generation mechanism of Bi-based ternary composite is discussed. In this reaction system, EG is easily oxidized to glyoxal then to oxalic acid. Oxalic acid is unstable and easily decomposed into CO32− via the cleavage of C-C bond, which could be used to prepare Bi2O2CO3 (Fig. 2c). Bi2O3 is produced through ion exchange route, which utilizes Bi(NO)3 as Bi source and NaOH as OH− source. Once combining Bi3+ and OH− together, Bi(OH)3 precipitate is obtained then transforms to Bi2O3 after hydrothermal treatment owing to its instability (Fig. 2d). Bi2O2CO3 is generated by the simple combination of Bi2O3 with CO32− (Fig. 2e). In addition, EG as an excellent reductant can reduce Bi3+ to metallic Bi (Fig. 2f).
2.2 XPS analysis
X-ray photoelectron spectroscopy (XPS) is utilized to study the compositions of composites . In Fig. 3a, full survey spectrum confirms the existence of Bi, O and C elements in BOC-3. As can be seen in Fig. 3b, four peaks of Bi 4f energy level are noticed, in which two Bi signals at 164.2 and 158.9 eV are belonged to Bi 4f5/2 and Bi 4f7/2, respectively, ascribing to Bi3+ in Bi2O2CO3 and Bi2O3 [34, 35]. The other two signals locate at 162.0 and 156.8 eV, which are corresponded to metallic Bi0 . The asymmetric profile of O1s signal suggests that more than one kind of oxygen species exist. In Fig. 3c, the O1s spectrum can be split into three peaks at binding energies of 531.0, 530.2 and 529.8 eV. The peak at 531.0 eV is ascribed to the surface hydroxyl groups adsorbed on material . Besides, the signals at 530.2 and 529.8 eV are attributed to the characteristics of C-O bond in [CO3]2− layers and Bi-O bond in [Bi2O2]2+ layers [38, 39]. As shown in Fig. 3d, C 1 s signal of BOC-3 can be split into three peaks at 288.3, 285.9 and 284.6 eV, which are assigned to CO32− in Bi2O2CO3, O-bearing bonding (C-OH), and sp2 carbon, respectively .
2.3 Morphology and microstructure analysis
Morphology and microstructure of composite are studied by scanning electron microscope (SEM) and transmission electron microscope (TEM). As shown in Fig. 4a, BOC-3 sample is composed of irregular micro-plates with size of about 1–2 µm. The thickness of plates is approximately 50 nm. High resolution transmission electron microscope (HR-TEM) image of BOC-3 confirms the detailed structure. In Fig. 4b, it is clearly that three different lattice fringes with d-spacing of 0.29, 0.33 and 0.26 nm, which are attributed to (1 6 1) plane of Bi2O2CO3, (0 1 2) plane of metallic Bi and (1 2 3) plane of B2O3, respectively. SEM image and corresponding element distributions of BOC-3 are shown in Fig. 4c-f, which indicate Bi, O and C elements are evenly distributed throughout the micro-plate. These results suggest that Bi-based composites are successfully fabricated.
2.4 Photocatalytic activity of samples
Photodegradation ability of sample is studied under solar light irradiation by choosing TEC and BPA as model organic pollutants. In Fig. 5a, BOC-1 presents relatively low photodegradation efficiency owing to the large band gap of Bi2O2CO3, while photocatalytic activities of BOC-5 and BOC-6 obviously enhance because of the construction of heterojunction and modification of metallic Bi. Furthermore, after decoration of Bi2O3 and metallic Bi onto Bi2O2CO3, degradation efficiencies of samples further improve. BOC-3 presents the highest degradation efficiency and 97.8% of TEC is removed within 150 min, which is attributed to the synergy effect of heterojunction and co-catalyst. Figure 5c exhibits the kinetic constants of BOC for TEC degradation. BOC-2, BOC-3 and BOC-4 present enhanced degradation rates, which is determined to be 0.016, 0.024 and 0.014 min− 1, much higher than that of other samples. In addition, BPA is also used as target pollutant to investigate photocatalytic activity of samples. Figure 5b shows the degradation curves of BOC toward BPA under simulated solar irradiation and relevant kinetic constants are calculated in Fig. 5d. Similarly, BPA degradation rates of BOC-2, BOC-3 and BOC-4 obviously enhance in comparison with other samples. In order to further confirm the efficient photocatalytic performance of ternary composites, the degradations of various pollutants including methyl orange (MO), methylene blue (MB) and rhodamine B (RhB) are also studied. As shown in Fig. 5e, ternary samples exhibit enhanced photodegradation efficiencies. Cycling experiments of BOC-3 for TEC and BPA degradation are carried out to determine its stability. As can be seen from Fig. 5f, TEC and BPA photodegradation efficiencies of BOC-3 change little after five cycles, implying its high stability.
2.5 Photocatalytic mechanism
Photoluminescence (PL) test is an effective technology to investigate the separation and migration of photogenerated electrons-holes pairs in materials due to PL emission mainly originates from the recombination of electrons and holes . Figure 6a presents PL spectra of as-synthesized materials in the range of 320–700 nm under the excitation of 285 nm. The order of emission peaks intensity is BOC-3 < BOC-2 < BOC-4 < BOC-5 < BOC-6 < BOC-1, which is consistent with the photodegradation efficiency of pollutants. The weak peak intensity indicates the effective separation and transfer of photoinduced charge carriers. The interfacial charge separation and transfer dynamics of material during photocatalytic process are also studied by electrochemical impedance spectroscopy (EIS) . As shown in Fig. 6b, these curves are fitted with the equivalent circuit of Rs(QRf)(QRct), where Rs, Q, Rf and Rct are electrolyte resistance, constant phase element, layer resistance of materials and charge transfer resistance, respectively. Similarly, the order of semicircle arc is BOC-3 < BOC-2 < BOC-4 < BOC-5 < BOC-6 < BOC-1, which is consistent with PL analysis and photodegradation efficiency of pollutants. Typically, a smaller semicircle means smaller resistance value, which expresses faster interfacial charges migration rates. These results suggest that the construction of heterojunction and decoration of cocatalyst lead to the efficient separation of photogenerated charge carriers, further enhancing photocatalytic activity.