Facile one-step redox synthesis of Bi2O2CO3/Bi2O3/Bi ternary photocatalyst without additional carbon source

Construction of heterojunction and decoration of cocatalyst are two vital strategies to accelerate migration of charge carriers. However, the fabrication routs of multi-composites are usually complex and expensive. In this work, Bi2O2CO3/Bi2O3/Bi ternary composite was fabricated via a facile one-step redox reaction. Ethylene glycol was selected as the solvent during the whole reaction process. Moreover, ethylene glycol as an excellent reductant can reduce Bi3+ into metallic Bi0 and itself is oxidized to CO32−, which would react with Bi2O3 to generate Bi2O2CO3 without additional carbon source. Component proportions in ternary composites were optimized by the control of the ratios of raw materials. Under simulate solar light, Bi-based ternary composites exhibited enhanced photodegradation efficiencies for multifarious pollutants in comparison with single and binary samples. The enhanced photocatalytic activities were ascribed to accelerated migration rate of charge carriers owing to the construction of heterojunction and decoration of cocatalyst.


Introduction
Water contamination has become a highly concerned issue owing to its close relationship with drinking water quality and human health [1][2][3][4]. It is estimated that millions of serious illness and even death are caused by water pollution every year [5]. Therefore, developing rapid, efficient and economical technologies has become a global concern [6][7][8][9]. As we know, semiconductor-based photocatalysis, as one of the most promising strategies, has aroused widespread attention in removal of hazardous organic pollutants [10][11][12][13]. Efficient photogenerated charge carriers separation and migration is one of the main factors to improve photocatalytic activity [14][15][16][17][18]. Up to now, construction of heterojunction is considered as one of the effective strategies to inhibit charge recombination and enhance photocatalytic efficiency because of staggered band alignments between two semiconductor components [19][20][21][22]. In general, photogenerated electrons (holes) on the higher conduction (lower valence) band of semiconductor would migrate to another one with a lower conduction (higher valence) band, which could realize spatial separation of charge carriers and suppress their recombination [23][24][25]. Except heterojunction, modification of co-catalysts such as noble metals onto surface of materials is also a crucial method to accelerate separation of electron-hole pairs [26][27][28]. When the position of Fermi level of metals below the conduction band (CB) of catalysts, photogenerated electrons on the CB of catalysts can rapidly transfer to the surfaces of metals. Noble metals could act as a reservoir of electrons, preventing the recombination of charge carriers and strengthening the quantum efficiency of photocatalytic activity [29,30]. Besides, the Schottky barrier at heterojunction-metal interface can further retard electron-hole recombination owing to the internal electric field [31]. However, the scale application of noble metals is suppressed because of their high price. Some semi-metal such as metallic bismuth (Bi), as an abundant earth element with high carrier mobility and low effective electron mass, could also act as an effective co-catalyst and extensively be used in photocatalysis [32,33].
Recently, Bi-based composites have been well explored for efficient photocatalysts [34,35]. As we know, current their synthesis usually need two or multi steps, which are expensive and laborious. Herein, Bi 2 O 2 CO 3 /Bi 2 O 3 /Bi ternary composite was synthesized through a facile one-step redox reaction process, which is time-saving, labor-saving and lowcost. During synthesis, ethylene glycol (EG) is selected as solvent for the whole reaction process, which also as an excellent reducing agent can reduce Bi 3? into metallic Bi 0 and itself is oxidized to CO 3  : OHto 4.7 (BOC-6). As for BOC composites, the colors of samples accompany with the increase of OHdosages, 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 OHdosages. 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 CO 3 2via the cleavage of C-C bond, which could be used to prepare Bi 2 O 2 CO 3 (Fig. 2c). Bi 2 O 3 is produced through ion exchange route, which utilizes Bi(NO) 3 as Bi source and NaOH as OHsource. Once combining Bi 3? and OHtogether, Bi(OH) 3 precipitate is obtained then transforms to Bi 2 O 3 after hydrothermal treatment owing to its instability (Fig. 2d). Bi 2 O 2 CO 3 is generated by the simple combination of Bi 2 O 3 with CO 3 2- (Fig. 2e). In addition, EG as an excellent reductant can reduce Bi 3? to metallic Bi (Fig. 2f).

XPS analysis
X-ray photoelectron spectroscopy (XPS) is utilized to study the compositions of composites [39]. 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 4f 5/2 and Bi 4f 7/2 , respectively, ascribing to Bi 3? in   [40,41]. The other two signals locate at 162.0 and 156.8 eV, which are corresponded to metallic Bi 0 [42]. 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 [43]. Besides, the signals at 530. 2

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 two-dimensional nano-sheets with size of about 1-2 lm. The thickness of sheet 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 Bi 2 O 2 CO 3 , (0 1 2) plane of metallic Bi and (1 2 3) plane of B 2 O 3 , 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 sample. These results suggest that Bi-based composites are successfully fabricated.

Photocatalytic activity of samples
Photodegradation ability of sample is studied under solar light irradiation by choosing TEC and BPA as 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.

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 [47]. 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) [48]. As shown in Fig. 6b, these curves are fitted with the equivalent circuit of R s (QR f )(QR ct ), where R s , Q, R f and R ct 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.

Conclusion
In summary, Bi 2 O 2 CO 3 /Bi 2 O 3 /Bi ternary composite was fabricated via a facile one-step redox reaction without additional carbon source. During synthesis, EG is selected as solvent in the whole fabrication process. Besides, EG is also an effective reducing agent, which can reduce Bi 3? into metallic Bi and itself is oxidized to CO 3 2-, reacting with Bi 2 O 3 to construct Bi 2 O 2 CO 3 . In comparison with single or