Rod-like Bi4O7 decorated Bi2O2CO3 plates: Facile synthesis, promoted charge separation, and highly efficient photocatalytic degradation of organic contaminants

Accepted Manuscript Rod-like Bi4O7 decorated Bi2O2CO3 plates: Facile synthesis, promoted charge separation, and highly efficient photocatalytic degradation of organic contaminants Meng Sun, Tao Yan, Yaru Zhang, Yunhui He, Yu Shao, Qin Wei, Bin Du PII: DOI: Reference:

S0021-9797(17)31432-7 https://doi.org/10.1016/j.jcis.2017.12.042 YJCIS 23114

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

7 November 2017 12 December 2017 15 December 2017

Please cite this article as: M. Sun, T. Yan, Y. Zhang, Y. He, Y. Shao, Q. Wei, B. Du, Rod-like Bi4O7 decorated Bi2O2CO3 plates: Facile synthesis, promoted charge separation, and highly efficient photocatalytic degradation of organic contaminants, Journal of Colloid and Interface Science (2017), doi: https://doi.org/10.1016/j.jcis. 2017.12.042

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Rod-like Bi4O7 decorated Bi2O2CO3 plates: Facile synthesis, promoted charge separation, and highly efficient photocatalytic degradation of organic contaminants Meng Sun†,*, Tao Yan†, Yaru Zhang†, Yunhui He§, Yu Shao§, Qin Wei‡, Bin Du†,‡,* †

‡

School of Resources and Environment, University of Jinan, Jinan 250022, PR China.

Key Laboratory of Interfacial Reaction & Sensing Analysis in Universities of Shandong,

School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, PR China §

State Key Laboratory of Photocatalysis on Energy and Environment, Research Institute of Photocatalysis, Fuzhou University, Fuzhou 350016, PR China *

Corresponding author. Tel: +86 531-82769235; Fax: +86 531-82765969; E-mail: [email protected]; [email protected]

1

Abstract In this manuscript, rod-like Bi4O7 decorated Bi2O2CO3 plates were fabricated for the first time. Compared with pristine Bi2O2CO3, the tight decoration of Bi4O7 over Bi2O2CO3 plates not only strengthened the visible light absorption, but also enhanced the separation efficiency of photogenerated carriers. As expected, the heterostructured Bi4O7/Bi2O2CO3 composites have exhibited highly promoted photocatalytic activities in decomposing Rhodamine B (RhB) under visible light. The BOBC-2 sample displayed the best activity with a reaction rate constant of 0.0245 min-1, which was 3.8 times higher than that of pure Bi4O7. Besides RhB, the Bi4O7/Bi2O2CO3 composites also displayed superior activity toward colorless contaminants with stable chemical structures, such as phenol, p-tert-butylphenol, and o-phenylphenol. The activity enhancement should be ascribed to the proper energy levels of the materials and formation of heterojunction at their interfaces, which could facilitate the charge transfer and promote the separation efficiency. Following transient photocurrent response, electrochemical impedance spectroscopy, and photoluminescence emission tests all verified this. In addition, controlled experiments using various radical scavengers proved that •O2− and h+ played the chief role in decomposing organic pollutants. This work may provide a new method for constructing Bi-based heterostructured photocatalysts with high activity. Keywords: Photocatalyst; Bi4O7/Bi2O2CO3; Visible Light; Degradation; Heterostructure

1. Introduction 2

As we know, the rapid development of society and severe environmental pollutions usually go hand in hand. In the past many decades, the booming chemical and pharmaceutical industry has caused critical pollutions of the environment [1-4]. Till now, several methods have been attempted for environmental remediation, among which semiconductor-based photocatalysis as a promising and green technology has engendered great attentions [5-7]. In the photocatalytic system, the photocatalyst itself played the major role in the efficient removal of organic contaminants. Thus, great efforts have been devoted into exploring novel materials with superior activities. Up to now, titanium dioxide (TiO2) has been mostly studied because of its non-toxic, outstanding chemical stability, and high efficiency [8-10]. However, the large band gap (3.2 eV for anatase TiO2) determined that it was only photoactive under ultraviolet light, which just accounted for 5% of the whole solar spectrum [11,12]. In order to make full advantage of the clean and permanent solar energy, lots of visible light-responsive materials have been developed in the past many years [13-18]. Recently, many kinds of bismuth-based materials have been intensively researched and developed. For example, BiVO4 [19], BiSbO4 [20], Bi2MoO6 [21], BiOI [22], Bi12O15Cl6 [23], BiOCl [24], Bi2O2(OH)(NO3) [25], and Bi2O4 [26] have been used to photocatalytic decompose organic contaminants or split water. Huang et al. reported the hydrothermal fabrication of Bi2O2CO3 single-crystal with efficient photocatalytic activity under UV light [27]. Hu and coworkers have fabricated Bi2O4 and Bi4O7 photocatalysts, which could decompose organic pollutants under visible light [26]. However, the most of these single-phase photocatalysts commonly suffered their intrinsic faultiness. For example, the wide band gap of Bi2O2CO3 severely restricted its practical application under visible light irradiation [28]. As for the visible light-responsive Bi4O7, the rapid recombination of photo-induced carriers would severely depress the photocatalytic performances [29]. Thus, various methods have been attempted to overcome these problems, among which the semiconductor-coupling of wide- and narrow-gap materials should be a more efficient one. With proper energy band potentials, the coupling of those 3

materials could expand the light absorption range, depress the carrier recombination, and maintain their strong redox abilities at the same time [30-34]. For example, Hu et al. reported the fabrication of β-Bi2O3/Bi2O2CO3 composites with excellent activities toward o-phenylphenol [35]. Wu and co-workers reported the in-Situ synthesis of Bi2O4/BiOBr composites with efficient photocatalytic activity [36]. Li et al. reported the fabrication of core–shell fiber-shaped Ta3N5/Bi2MoO6 heterojunctions with enhanced photocatalytic performances [37]. In our previous work, we have also reported the fabrications of visible light-responsive Bi2O2CO3/Bi2O4 [38] and g-C3N4/Bi4O7 [29] composite photocatalysts. However, except for these work, Bi4O7 has rarely been reported, not to mention its coupling with other semiconductors. In this manuscript, we firstly reported the fabrication of rod-like Bi4O7 decorated Bi2O2CO3 plates, which have exhibited superior visible light photocatalytic performances in decomposing Rhodamine B (RhB), phenol, p-tert-butylphenol (PTBP), and o-phenylphenol (OPP). Because of the tight decoration of Bi4O7 over Bi2O2CO3, the heterojunctions formed on their interface could facilitate the charge transfer and separation process. The depressed charge recombination and expanded visible light absorption should be responsible for the activity enhancement.

2. Experimental section Pristine Bi4O7, Bi2O2CO3, and heterostructured Bi4O7/Bi2O2CO3 composites were fabricated using a hydrothermal method followed by a calcination process (Figure 1). The detailed preparation procedure, characterization methods, photo-electrochemical property and photocatalytic activity measurements were provided in Supporting Information.

3. Results and discussion 3.1 Structural and morphology characterization The Bi4O7/Bi2O2CO3 composites were fabricated by a two-step method. Firstly, Bi2O4/Bi2O2CO3 precursors have been synthesized by a green hydrothermal method. The XRD diffraction patterns of the obtained precursors were shown in Figure S1. As we can see, the NaBiO3·2H2O has been transformed into pure phase of Bi2O4 (JCPDS 50-0864) during the hydrothermal 4

process (Figure S1a), which was in good agreement with previous reports [38,39]. However, when certain amount of Na2CO3 was introduced, NaBiO3·2H2O would partially transform into Bi2O2CO3 after hydrothermal treatment. Accordingly, the XRD diffraction peaks ascribed to Bi2O4 and Bi2O2CO3 all have been observed for the Bi2O4/Bi2O2CO3 precursors (Figure S1b-f). It was also noticed that the diffraction peaks of Bi2O4 became weaker and weaker along with the increased addition of Na2CO3, indicating the mass ratio of Bi2O4/Bi2O2CO3 gradually changed in the precursors. In addition, an excessive introduction of Na2CO3 would result in the formation of pure Bi2O2CO3 (JCPDS 41-1488) (Figure S1g). Secondly, the precursors were further sintered at 250 ºC for 3 h in air to obtain the final Bi4O7/Bi2O2CO3 composites. Figure 2 shows the XRD patterns of the final products. As we can see, Figure 2a confirmed the complete transformation of Bi2O4 into Bi4O7 phase (JCPDS 47-1058) during the calcination process, while Bi2O2CO3 was stable enough under this calcination temperature (Figure 2g). This is in accordance with a previous report that Bi2O2CO3 was thermal stable below 300 ºC [40]. Thus, after calcination treatment, all the Bi2O4/Bi2O2CO3 precursors with different mass ratios had transformed into Bi4O7/Bi2O2CO3 composites (Figure 2b-f). Meanwhile, it was noticed that the diffraction peaks ascribed to Bi4O7 became weaker and weaker from sample BOBC-1 to BOBC-5, indicating the mass ratio of Bi4O7/Bi2O2CO3 gradually decreased in the final composites. The morphologies of the products were characterized by SEM and TEM. Figure 3a displays the typical SEM image of Bi4O7, in which numerous irregular rod-like particles with various sizes were observed. Figure 3b shows the image of pure Bi2O2CO3 after calcination at 250 ºC for 3 h, in which a large amount of plates with side length of several tens micrometers were observed. As for the obtained Bi4O7/Bi2O2CO3 composites, a typical SEM image of BOBC-2 sample was shown in Figure 3c. As we can see, numerous irregular rod-like Bi4O7 particles were decorated on the surfaces of Bi2O2CO3 plates. The tight conglutination of Bi4O7 would promote the formation of heterojunction at their interfaces, which could subsequently facilitate the charge migration and separation. Elemental mapping was further performed to delineate the 5

combination and spatial distribution of Bi4O7 and Bi2O2CO3 in the BOBC-2 composite. The element mapping images for Bi, O, and C with distinct color contrast were also shown in Figure 3. As it can be seen, the common elemental Bi and O were detected and homogeneously distributed all over the BOBC-2 composite, while the elemental C was mainly distributed in square-plate shape. A corresponding SEM-EDS image for BOBC-2 was shown in Figure S2, in which only signals for element Bi, O, and C have been observed besides the element Al form the substrate. In addition, the SEM images of the precursors (Bi2O4, Bi2O2CO3, and Bi2O4/Bi2O2CO3 composites) were also recorded and shown in Figure S3. Obviously, the particle sizes and morphologies of all the precursors had not changed before and after the calcination treatment. TEM and HR-TEM characterization could more intuitively prove the formation of heterojunction at the materials’ interfaces. Figure 4 shows the TEM images of the typical heterostructured BOBC-2 hybrid. It shows that the Bi4O7 particles were partially overlapped with Bi2O2CO3 plates. The HR-TEM image in Figure 4b displays clear lattice fringes of 0.285 and 0.372 nm, which was respectively ascribed to the (4 0 0) crystallographic plane of Bi4O7 and (0 1 1) crystallographic plane of tetragonal Bi2O2CO3. This HR-TEM image provides solid evidence to prove the formation of heterojunctions in the BOBC-2 hybrid. 3.2 DRS and XPS characterizations The optical properties of the obtained samples have been investigated by DRS. As depicted in Figure 5, the absorption band edges of all the Bi4O7/Bi2O2CO3 composites were red-shifted in comparison with Bi2O2CO3, indicating the enhanced visible light absorption efficiency. A paragraph in Figure 5 (inset) shows the color changes of Bi4O7/Bi2O2CO3 composites when varying the Bi4O7 mass contents. XPS analysis has been performed to research the surface components and chemical states of BOBC-2 sample. Figure 6a shows that the asymmetrical high-resolution XPS peaks of Bi4f5/2 and Bi4f7/2 both could be deconvoluted into two bimodal peaks. The peaks at 163.6 and 158.3 eV are ascribed to Bi(III), while that with binding energies of 164.5 and 159.2 eV are attributed to Bi(V) [39,41]. The C1s spectra of BOBC-2 sample in 6

Figure 6b had been deconvoluted into three peaks which centered at 288.6, 286.8, and 284.7 eV, respectively. According to previous reports, the peaks at 288.6 and 286.8 eV should be attributed to the carbonate ion in Bi2O2CO3 [42] and O-bearing bonding (C-OH) [43], respectively. As for the peak at 284.7 eV, it should be ascribed to adventitious carbon contaminant from XPS measurement. The O1s spectra of BOBC-2 were shown in Figure 6c. Obviously, three types of oxygen species were existed in the BOBC-2 sample. The peak at 530.1 and 531.3 eV could be assigned to the lattice oxygen in the composites and CO32- species, respectively [39,44]. The O1s peak with higher binding energy at 533.1 eV could be indexed to the adsorbed oxygen on the surfaces of composites [38,45]. Figure 6d shows the surveyed spectra of BOBC-2 with pristine Bi4O7 and Bi2O2CO3 as references. It implies that only elemental Bi, C, and O were presented in the as-prepared samples, no other element signals have been detected. So, Bi4O7/Bi2O2CO3 heterojunction photocatalysts could be easily synthesized under these current synthetic conditions. 3.3 Photocatalytic performances toward organic pollutants The visible light-responsive photocatalytic activities of heterostructured Bi4O7/Bi2O2CO3 composites were estimated by decomposing RhB and colorless organic pollutants (phenol, PTBP and OPP) in water. Figure 7a depicts the RhB degradation results over Bi4O7/Bi2O2CO3 composites using pristine Bi4O7 and Bi2O2CO3 as comparison. As we can see, the concentrations of RhB decreased with the prolonging of irradiation time for all the Bi4O7/Bi2O2CO3 composites and Bi4O7, while the degradation over Bi2O2CO3 was negligible. After 80 min of illumination, the degradation ratio of RhB over Bi4O7 was about 65%, while that for BOBC-2 was about 99%, which was the highest among all the composite photocatalysts. This activity enhancement should be attributed to the synergistic effect of Bi4O7 and Bi2O2CO3 in the composites, which facilitated the charge transfer and promoted the separation efficiency of photo-induced electron/hole pairs. Figure S4 compared the performances of BOBC-2 composite and the physical mixture of Bi4O7 and Bi2O2CO3 with the same composition. It was found that the physical mixture only exhibited a 7

poor activity compared with BOBC-2 sample. It was also clear that the mass ratio of Bi4O7 to Bi2O2CO3 could greatly affect the photocatalytic activity. A much higher or lower mass ratio was unfavorable for promoting the performances of the composites. In addition, the controlled experiment showed that RhB could hardly be decomposed over BOBC-2 without light illumination. The kinetic behaviors of the RhB degradations over different samples also have been investigated. Figure 7b shows that the photocatalytic decomposing process could be described well by the apparent first-order model: -ln(C/C0) = kt In the equation above, C0 is the RhB concentration when its adsorption and desorption over catalysts was balanced, while C and k was the temporal concentration at each irradiated time (t) and the degradation rate constant. As depicted in Figure 7c, the BOBC-2 possessed the highest k value of 0.0245 min-1. It was nearly 3.8 times higher than that of pristine Bi4O7 under identical conditions. Figure 7d exhibits the temporal concentration changes of RhB during decomposition process over BOBC-2 photocatalyst. As we can see, the main absorption peak ascribed to RhB centered at 554 nm firstly decreased drastically, followed by hypsochromic shifted to shorter wavelength, and finally disappeared after 120 min of reaction. The hypsochromic shift of absorption spectra exhibited the N-de-ethylation of RhB during degradation process. The photograph shown in Figure 7d (inset) clearly displays the gradual decolorization of RhB solution during the whole degradation process. In addition, the influences of initial RhB concentrations on the degradation rates also have been investigated. As shown in Figure S5, with a higher concentration of 2×10-5 M, the removal ratio of RhB decreased to 45% after 80 min of reaction. On the contrary, the degradation rate of RhB has been greatly accelerated when the concentration was lower (5×10-6 M). Phenol, PTBP, and OPP, three kinds of colorless toxic pollutants with stable chemical structures have also been used to further evaluate the activities of Bi4O7/Bi2O2CO3 composites. Figure 8a shows the visible light degradation results of phenol (20 mg/L) over BOBC-2. As we 8

can see, the main absorption peak of phenol centered at 269 nm became lower and lower when prolonging the irradiation time. After 200 min of reaction, the peak was almost disappeared completely. Besides phenol, PTBP and OPP also could be efficiently decomposed by BOBC-2 under visible light. Figure 8b shows that the chief absorption peak of PTBP located at 274 nm gradually decreased and finally disappeared within 160 min of reaction. Figure 8c shows the efficient degradation of OPP over BOBC-2. Obviously, the main absorption peak attributed to OPP at 281 nm decreased gradually under visible light illumination. After 100 min of irradiation, the main peak nearly completely disappeared, indicating the efficient removal of OPP by BOBC2. It has also been found that the degradation process of phenol, PTBP, and OPP all fitted well with the apparent first-order model (shown in Figure S6). Figure 8d depicts the apparent rate constants for BOBC-2 sample toward different contaminants. As we can see, the k value for phenol and PTBP decomposition was about 0.0082 and 0.0075 min-1, respectively. As for the photocatalytic decomposition of OPP over BOBC-2, a much higher apparent rate constant was observed (0.0206 min-1). The effective degradations of phenol, PTBP, and OPP indicated the strong oxidation capability of Bi4O7/Bi2O2CO3 composites toward stable environmental pollutants. In addition, the activity stabilities of BOBC-2 sample in decomposing RhB and phenol have also been tested. As depicted in Figure 9, after three runs of degradation experiments, the activity deactivation in decomposing RhB was slight, while that for phenol was a little bit more. 3.4 Charge transport and separation efficiency In general, the interfacial charge transport in heterostructured photocatalysts would promote the charge separation efficiency and enhance the photocatalytic performances. So, in order to further explicate the activity promotion of Bi4O7/Bi2O2CO3 photocatalysts, the charge transport and separation have been investigated by electrochemical impedance spectroscopy (EIS), transient photocurrent, and photoluminescence (PL) characterizations. Figure 10a displays the transient photocurrent responses of pristine Bi4O7, Bi2O2CO3, and BOBC-2 hybrid. As we can see, the photocurrent intensity over BOBC-2 sample suddenly increased to a high level when illuminated 9

by visible light, but decreased rapidly while the light was turn off. It was noticed that the photocurrent intensity over BOBC-2 was much higher than that over pristine Bi4O7, while that over Bi2O2CO3 was negligible. Since higher photocurrent implies more efficient charge migration and lower carrier recombination, the charge recombination over BOBC-2 hybrid should be severely depressed, which would result in the activity enhancement. This deduction was further supported by the EIS and PL measurements. Figure 10b depicts the EIS Nyquist plots for pristine Bi4O7, Bi2O2CO3, and BOBC-2 hybrid. Obviously, the arc radius of BOBC-2 was smaller than that of both Bi2O2CO3 and Bi4O7. Because a smaller arc radius generally represents lower resistance and higher charge transport rate, the BOBC-2 sample should possess a higher carrier transporting efficiency compared with bare Bi4O7 and Bi2O2CO3. Figure 10c displays the PL emission spectra for different samples excited at 350 nm. It was observed that the PL intensity of Bi2O2CO3 was greatly higher than that of Bi4O7, while that of BOBC-2 hybrid was very weak. That is to say, a substantial part of the total photo-generated charge carriers were recombined rapidly over pristine Bi2O2CO3 and Bi4O7, while that over BOBC-2 hybrid were efficiently separated because of the interfacial charge transfer. In a word, all the results obtained above proved the rapid and efficient separation of photo-generated carriers, resulting in the promotion of photocatalytic activities for the heterostructured catalysts. 3.5 Degradation mechanism investigation During the photocatalytic decomposition process, various active species such as •OH, •O2−, or h+ usually can be generated and responsible for the decomposition of organic contaminants. Herein, different trapping agents such as tert-butanol (TBA), ammonium oxalate (AO), and pbenzoquinone (BQ) was used to capture the generated •OH, h+, and •O2−, respectively [46,47]. Figure 10d displays the influences of different trapping agents on the RhB degradation rates under visible light illumination. When 1.00 mL of TBA was introduced, the removal ratio of RhB was decreased to 70% from 99% within 80 min of reaction. However, when AO (0.10 g) or BQ (10 mg) was introduced, the decomposition rates of RhB had been severely depressed. That is to say, 10

−

+

•O2 , •OH, and h were indeed generated in the BOBC-2 mediated degradation system, but the •O2− and h+ played a more important role in decomposing RhB. It also could be inferred that the neutral reaction system was not beneficial for the production of •OH. And then, •OH only have made less contribution to the degradation of pollutants under visible light irradiation. In the photocatalytic degradation system, the generations of active species and charge transfer were commonly determined by the band energy levels of the materials. For better understanding the interfacial charge transfer and separation mechanism, the Mott-Schottky plots of Bi4O7 and Bi2O2CO3 were recorded to measure their flat potentials (Vfb). As depicted in Figure 11, the positive slopes of the lines indicated that both Bi4O7 and Bi2O2CO3 were n-type semiconductors [48,49]. The Vfb of Bi4O7 and Bi2O2CO3 was determined to be −0.54 and −0.49 V vs. NHE (pH=7) after correction of the Hg/Hg2Cl2 potential of 0.24 V (vs. NHE), respectively. For n-type semiconductor, the conduction band potential (ECB) is usually more negative than its Vfb about 0.1 V [48,50]. From this point of view, the ECB of Bi4O7 is estimated to be −0.64 eV, while that of Bi2O2CO3 was about –0.59 eV, which was in accordance with previous literature [51]. Based on the results of trapping experiments and energy band theory, a possible degradation mechanism was presented and discussed for heterostructured Bi4O7/Bi2O2CO3 composites (Scheme 1). Under visible light illumination, Bi4O7 can be easily excited because of its narrow gap, resulting in the separation of electrons and holes. Because the ECB of Bi4O7 is more negative than that of Bi2O2CO3, the electrons stored in the conduction band of Bi4O7 would migrate to that of Bi2O2CO3 through the interfacial heterojunctions. This process could greatly depress the neutralization of photo-induced electron/hole pairs. In view of the more negative ECB of Bi2O2CO3 compared with the standard redox potential of O2/•O2− (−0.33 V vs. NHE) [52], the electrons stored there could react with dissolved O2 to produce •O2−, which would further react with H+ to generate •OH. On the other hand, the holes stored in the valance band of Bi4O7 were −

not able to react with OH to produce •OH because its valance band potential (EVB) was lower

11

−

−

than that for •OH/OH (+1.99 V vs. NHE) [53]. Alternatively, the holes, together with •O2 and •OH all could oxidize the surface absorbed organic pollutants directly.

4. Conclusions In summary, rod-like Bi4O7 decorated Bi2O2CO3 plates were successfully prepared through a two-step method. The obtained heterostructured Bi4O7/Bi2O2CO3 composites have exhibited efficient photocatalytic activities toward RhB, phenol, PTBP, and OPP under visible light −

+

irradiation. The controlled trapping experiments proved that •O2 and h played the major role in the photocatalytic decomposition of organic pollutants. The activity improvement should be attributed to the formation of heterostructure at the interfaces of Bi4O7/Bi2O2CO3 composites, which could facilitate the charge transfer and separation process. The following transient photocurrent, EIS together with PL measurements also proved this. A possible degradation mechanism for the heterostructured Bi4O7/Bi2O2CO3 photocatalyst was proposed based on the energy band theory and experimental results.

Acknowledgments This work was financially supported by the Shandong Provincial Natural Science Foundation, China (No. ZR2016BQ12, ZR2014BL017), NSFC (No. 21505051 and 21175057), the Science Foundation for Post Doctorate Research from the University of Jinan (XBH1708), Project funded by China Postdoctoral Science Foundation (No. 2017M612172), and the Open Project Program of the State Key Laboratory of Photocatalysis on Energy and Environment (Grant No. SKLPEE-KF201709).

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Figure captions 14

Figure 1. Schematic illustration of the synthetic route for Bi4O7/Bi2O2CO3 composites. Figure 2. XRD patterns of the obtained (a) Bi4O7, (b) BOBC-1, (c) BOBC-2, (d) BOBC-3, (e) BOBC-4, (f) BOBC-5, and (g) Bi2O2CO3. Figure 3. SEM images of Bi4O7 (a), Bi2O2CO3 (b), BOBC-2 (c), and the corresponding element mapping images for BOBC-2. Figure 4. TEM (a) and HR-TEM (b) images of BOBC-2 heterojunction photocatalyst. Figure 5. UV-vis DRS of Bi4O7, Bi2O2CO3, and Bi4O7/Bi2O2CO3 composites. Figure 6. The high-resolution XPS spectra of (a) Bi 4f, (b) C 1s, (c) O 1s of BOBC-2 and (d) survey XPS spectra of BOBC-2, Bi4O7, and Bi2O2CO3. Figure 7. (a) The normalized concentration changes of RhB (1×10-5 M) by using Bi4O7, Bi2O2CO3, and Bi4O7/Bi2O2CO3 composites as photocatalysts under visible light; (b) Kinetic fit curves for RhB degradation over different catalysts; (c) The corresponding degradation rate constants; (d) The temporal concentration changes of RhB mediated by BOBC-2 hybrid. Figure 8. Photocatalytic degradations of (a) phenol (4×10-4 M), (b) PTBP (60 mg L-1), (c) OPP (30 mg L-1), and (d) the corresponding degradation rate constants over BOBC-2. Figure 9. Recycling runs of the photocatalytic degradations of RhB (1×10-5 M) and phenol (4×10-4 M) over BOBC-2 hybrid. Figure 10. (a) Transient photocurrent responses, (b) EIS plot, and (c) PL emission spectra of Bi4O7, Bi2O2CO3, BOBC-2 hybrid; (d) Influences of scavengers on the photocatalytic decomposition rates of RhB over BOBC-2 hybrid. Figure 11. Mott-Schottky plots of Bi4O7 and Bi2O2CO3 electrodes in 0.05 mol L-1 Na2SO4 solutions. 15

Scheme titles Scheme 1. The proposed mechanism for organic pollutants degradation over Bi4O7/Bi2O2CO3 heterojunction under visible light.

16

Figures Figure 1.

17

Intensity (a.u.)

Figure 2.

(g) (f) (e) (d) (c) (b) (a)

10

20

30

40

50

60

2Theta (degree)

18

70

80

Figure 3.

19

Figure 4.

20

Figure 5. 100

Bi2O2CO3 BOBC-5

80

BOBC-4

R%

BOBC-3 60

BOBC-2 BOBC-1 Bi4O7

40

20

0

Bi4O7

Bi2O2CO3

-20 350

420

490

560

630

Wavelength (nm)

21

700

770

Figure 6.

284.7 Bi4f

(a)

288.6 286.9 C1s

(b)

163.6 eV 159.2 eV

162

159

156

294

291

530.1 Binding531.2 Energy533.2 (eV) O 1s

282

Bi 4f C 1s

530.1 eV

Survey Scan

Bi 4p3

531.3 eV

Bi 4p1

533.1 eV

285

(d) Counts (a.u.)

Counts (a.u.)

(c)

288

Binding Energy (eV)

Bi 5d

165

288.6 eV

O 1s

168

286.8 eV

Bi 4d3 Bi 4d5

164.5 eV

Counts (a.u.)

Counts (a.u.)

284.7 eV 158.3 eV

BOBC-2 Bi2O2CO3 Bi4O7

537

534

531

900

528

Binding Energy (eV)

750

600

450

300

Binding Energy (eV)

22

150

0

Figure 7.

23

(a) 1.0

C/C0

0.8 0.6 Bi4O7 BOBC-1 BOBC-2 BOBC-3 BOBC-4 BOBC-5 Bi2O2CO3

0.4 0.2 0.0 0

20

40

60

80

Time (min) (b) Bi4O7 BOBC-1 BOBC-2 BOBC-3 BOBC-4 BOBC-5 Bi2O2CO3

-ln(C/C0)

1.5 1.2 0.9 0.6 0.3 0.0 0

20

40

Time (min)

24

60

Figure 8.

25

(d) -1

kobs(min )

0.0206

0.0082

Phenol

0.0075

PTBP

26

OPP

Figure 9. 1.0

1

st

2

nd

3

rd

C/C0

0.8 0.6 0.4 0.2 Phenol RhB 0.0 0

80

160

240

320

400

Time (min)

27

480

560

Figure 10. 15

Photocurrent(Acm )

on 2

(a)

(b)

12

-Z'' (ohm)

off

BOBC-2

9 6 Bi2O2CO3 Bi4O7 BOBC-2

3

Bi4O7 Bi2O2CO3

0

30

60

90

120

150

20

180

30

Irradiation Time (S) Bi2O2CO3

60

0.9

C/C0

Intensity (a.u.)

(c)

(d) TBA

0.3

AO

Bi4O7

BQ No scavenger

BOBC-2

400

50

Z' (ohm)

0.6

350

40

0.0 450

500

550

Wavelength (nm)

0

20

40

Time (min)

28

60

80

Figure 11. 3.0M

Bi4O7 1250 Hz 2.5M

Bi4O7

800 Hz

2.0M

Bi2O2CO3

800 Hz

-2

-2

4

C (F cm )

Bi2O2CO3 1250 Hz

1.5M

1.0M

500.0k

0.0 -1.0

-0.9

-0.8

-0.7

-0.6

Potential (V vs. SCE)

29

-0.5

Schemes Scheme 1.

30

Rod-like Bi4O7 decorated Bi2O2CO3 square plates: Facile synthesis, promoted charge separation, and highly efficient photocatalytic degradation of organic contaminants Meng Sun†,*, Tao Yan†, Yaru Zhang†, Yunhui He§, Yu Shao§, Qin Wei‡, Bin Du†,‡,* † ‡

School of Resources and Environment, University of Jinan, Jinan 250022, PR China.

Key Laboratory of Interfacial Reaction & Sensing Analysis in Universities of Shandong,

School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, PR China §

State Key Laboratory of Photocatalysis on Energy and Environment, Research Institute of Photocatalysis, Fuzhou University, Fuzhou 350016, PR China *

Corresponding author. Tel: +86 531-82769235; Fax: +86 531-82765969; E-mail: [email protected]; [email protected]

Graphical abstract

31