Glucose-assisted hydrothermal synthesis of plasmonic Bi deposited nested Bi2O2−xCO3 photocatalysts with enhanced photocatalytic activity

Colloids and Surfaces A 583 (2019) 123946

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Glucose-assisted hydrothermal synthesis of plasmonic Bi deposited nested Bi2O2−xCO3 photocatalysts with enhanced photocatalytic activity Li Lia, Yunhui Yanc, Jinge Dua, Shuai Fua, Haiping Liua, Fengying Zhaoa, Jianguo Zhoua,b,

T

⁎

a

School of Environment, Key Laboratory of Yellow River and Huai River Water Environment and Pollution Control (Ministry of Education), Henan Engineering Laboratory of Environmental Functional Materials and Pollution Control, Henan Normal University, Xinxiang, 453007, Henan, PR China b Key Laboratory of Green Chemical Media & Reactions (Ministry of Education), Xinxiang, 453007, Henan, PR China c Department of Chemistry, Xinxiang Medical University, Xinxiang, Henan, 453003, PR China

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: Bi plasmon Oxygen vacancies Synergistic effects Nested Bi2O2CO3

Crystal defects play an important role on the physical and chemical properties of semiconductors. In this work, a series of Bi/Bi2O2−xCO3 photocatalysts have been fabricated by an one-pot hydrothermal method, where glucose is used as reducing and morphological control agent. The phase structure, morphology, optical and photoelectrochemical properties of the photocatalysts are characterized by various techniques. These oxygen vacancies (OVs) have been confirmed and characterized by ESR spectroscopy and XPS spectra. The Bi/Bi2O2−xCO3 photocatalysts exhibits highest removal efficiency of Lanasol Red 5B and ciprofloxacin. The corresponding degradation rate constant for Lanasol Red 5B is about 2.28 times higher than that of Bi2O2CO3 under simulated solar irradiation, and 10.9 times under visible light irradiation, respectively. Radical scavenger experiments futher indicated that holes (h+) and hydroxyl radical (%OH) are the main active species for Lanasol Red 5B degradation. This enhanced photocatalytic performance can be rationalized by the synergistic effects of Bi plasmon and OVs, resulting in, enhanced light-harvesting and electron- hole separation ability of Bi/Bi2O2−xCO3 photocatalysts.

Corresponding author at: School of Environment, Key Laboratory of Yellow River and Huai River Water Environment and Pollution Control (Ministry of Education), Henan Engineering Laboratory of Environmental Functional Materials and Pollution Control, Henan Normal University, Xinxiang, 453007, Henan, PR China. E-mail address: [email protected] (J. Zhou). ⁎

https://doi.org/10.1016/j.colsurfa.2019.123946 Received 25 July 2019; Received in revised form 7 September 2019; Accepted 7 September 2019 Available online 12 September 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.

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1. Introduction

promising replacement for the usage of precious metals. In addition, metal nanoparticles can promote surface charge separation and act as catalytically active site for hydrogen evolution. Several Bi particle deposited semiconductors have been developed, such as Bi/Bi2O2CO3 [28], Bi/BiOX (BiOCl, BiOBr, BiOI) [29–31], Bi/Bi2MoO6 [32], Bi/ BiVO4 [33] and Bi/Bi2S3 [34]. These metal-semiconductor hybrids exhibited the higher photocatalytic activities compared to that of pure semiconductors. Herein, we successfully synthesized a Bi/Bi2O2−xCO3 photocatalysts via a facile one-pot hydrothermal process. Glucose has not only been used as the reducing agent, to in-situ reduce Bi3+ to metallic Bi and induce the OVs formation. In addition, a nested morphology has been found. Bi/Bi2O2−xCO3 showed slightly improved photocatalytic activity for degradation of Lanasol Red 5B and ciprofloxacin than that of pure Bi2O2CO3 under both simulated solar irradiation, but greatly higher activity than pure Bi2O2CO3 under visible light irradiation. To understand the enhanced photocatalytic performance of the Bi/ Bi2O2−xCO3 system, the optical and electrochemical properties, photogenerated electron hole dynamics, and the reactive species of during photocatalytic process were investigated. A possible photocatalytic mechanism has been proposed based on the synergistic effects of Bi plasmon and OVs.

Semiconductor-driven photocatalysis is one of the most promising technologies to solve the global environmental issues [1]. Over decades, thoundants types of semiconductor photocatalysts have been developed, such as metal oxides [2–5], sulfides [6,7], multi-anion compounds [8–11]. Among them, Bi-based photocatalysts have been widely studied due to their layered crystal structure and flexible composition, such as “sillén” phase compounds [Bi2O2][Xm] (X = anion group). Bi2O2CO3 is a typical sillén phase compound and it has a band gap (3.1–3.5 eV) [12]. To improve its photocatalytic activity, many strategies have been reported including defect and crystal engineering and heterojunction fabrication etc [13–18]. Defects engineering has been adopted to enhance photocatalytic performance [19]. These defects can tailor the electronic structure and create in-gap states to improve photocatalytic activity. For instance, significant amount of defects have been reported in Bi-based photocatalyst and the enhanced photocatalytic performance were found. Bi metal and phosphate defects in BiPO4 has been prepared with assistance of NaBH4 as reductant [20]. Bismuth vacancy mediated single unit cell Bi2WO6 nanosheets was obtained via adopting BiOBr as template-directed strategy [21]. Huang et.al reported Bi2O3/Bi2O2CO3 heterojunctions with high content of OVs via heat treatment [22]. Dong et al. obtained the defective Bi2O2CO3 microspheres via vacuum heat treatment with different temperature [23]. These methods are complex, rigorous or risky conditions like high temperature and hydrogen atmosphere. Hence, it is of great significance to regulate the concentration of defect under mild condition. Metal-semiconductor hybrid is another strategy to improve the photocatalytic activity [24–27]. Low-cost Bi has exhibited a similar plasmonic effect to enhance visible light adsorption, resulting in an

2. Experimental section 2.1. Synthesis of nested Bi/Bi2O2−xCO3 All of the reagents were analytical grade without further purification. Firstly, 4 mmol Bi(NO3)3·5H2O was first dissolved in 1 M HNO3 (40 mL) and stirred for 10 min, and 1 g urea was subsequently added in the above solution and stirred for another 10 min to dissolve it. Then

Fig. 1. (a) XRD patterns of as-synthesized samples; (b) ESR spectra of as-prepared samples; (c) Schematic illustration of Bi0 and oxygen vacancies for Bi-BOC-2. 2

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Fig. 2. XPS spectra of BOC and Bi-BOC-2: (a) full spectra; (b) C 1s; (c) Bi 4f; (d) O1s; (e) VB-XPS of BOC. (f) contact angle of BOC and Bi-BOC-2.

certain amount of glucose was added into the solvent and continuously stirring until the solution became clear. The pH value of the mixture was adjusted to 9 by using the 2 M NaOH aqueous solution. The obtained solution was transferred to a 100 mL Teflon-lined stainless steel autoclave, annealing at 180 ℃ for 1 h and then naturally cooled to room temperature. The as-synthesized products were filtered, washed with distilled water and absolute ethanol for three times, and dried at 60 ℃ overnight. Depending on the different amount of glucose (0.2 g, 0.4 g, 0.8 g), the samples were correspondingly labeled as Bi-BOC-1, Bi-BOC2, Bi-BOC-3. The pure Bi2O2CO3 was also synthesized and denoted as BOC.

2.3. Measurement of electrochemical performances All photoelectrochemical experiment results were performed in an electrochemical system (CHI660E). Photocurrent response and electrochemical impedance spectroscopy (EIS) spectra were performed with a standard three-electrode system in 0.1 M Na2SO4 solution. Homemade catalyst-coated ITO glass with a fixed exposure surface area (1.5 cm2) was implemented as the working electrode, and platinum wire electrode and saturated Ag/AgCl electrode were used as the counter and reference electrode, respectively. 2.4. Evaluation of photocatalytic activity

2.2. Characterization

The photocatalytic activity of the prepared samples was evaluated by removal of Lanasol Red 5B (LR5B) and Ciprofloxacin (CIP) in an aqueous solution under simulated solar (500 W Xe lamp) and visible light (500 W Xe lamp with 420 nm cutoff filter) irradiation, respectively. The photocatalysts (40 mg) was dispersed in LR5B (40 mL, 40 mg/L) and CIP (40 mL, 10 mg/L) respectively. The aqueous suspensions were stirred magnetically for 30 min in the dark to ensure adsorption-desorption equilibrium before irradiation. During the photocatalytic degradation process, about 4 mL of the suspension was extracted at specific time intervals, then centrifuged at 8000 rpm for 10 min to remove the suspended photocatalysts, and examined using UV–vis spectrophotometer (UV-2450) at a maximum absorption wavelength of 529 nm for LR5B and 272 nm for CIP. For the cyclic testing, the photocatalyst was recollected by centrifugation and re-dispersed in the fresh pollutant solution, and the photocatalytic activity was recorded as the process mentioned above. The experiments on trapping active species were performed by using disodium ethylenediaminetetraacetic acid (EDTA-2Na), 1,4-benzoquinone (BQ) and isopropanol (IPA) acted as the scavengers of the holes (h+), the superoxide radicals (·O2−) and the hydroxyl radicals (·OH), respectively.

The crystal and phase structure of the samples were characterized by X-ray diffraction (XRD, X’ Pert 3 Powder) with Cu-Kα radiation (λ =0.15418 nm). The morphology and microstructure were studied using a field-emission scanning electron microscope (FESEM, SU8010) equipped with an Energy Dispersive X-ray Spectroscopy (EDX) detector and transmission electron microscope (TEM, JEM-2100). The X-ray photoelectron spectroscopy (XPS) spectra were characterized by AXISUltra instrument (Kratos) with Al Kα radiation. The UV–vis diffuse reflectance spectra (DRS) were detected on Lambda 950 Spectrophotometer (Perkin Elmer) using BaSO4 as reference. The photoluminescence (PL) spectra were obtained using a Fluorescence Spectrophotometer (FP-6500, Japan) with an excitation wavelength of 340 nm. The transient fluorescence spectra was measured on a FLS980 spectrophotometer with a 450 W xenon lamp (Edinburgh Instruments). Electron spin resonance (ESR) spectra of paramagnetic species spintrapped with 5,5-dimethyl-l-pyrroline-N-oxide (DMPO) were recorded with (ER300-SRC, Bruker) by mixing 0.05 g of the photocatalyst in 25 mM 5,5′-dimethyl-1-pirroline-N-oxide (DMPO) solution with a 50 mL aqueous dispersion for DMPO-·OH under simulated solar irradiation. 3

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Fig. 3. SEM image of BOC (a); Bi-BOC-1 (b); Bi-BOC-2 (c); Bi-BOC-3 (d); TEM image (e) and HRTEM image (f) of Bi-BOC-2; EDS results of Bi-BOC-2 (g); N2 adsorption–desorption (h); pore size distribution (i) of BOC and Bi-BOC-2.

Fig. 4. (a) UV–vis diffuse reflectance spectra of the different samples; (b) Plots of (αhν)1/2 versus energy (hν) for the band gap energy determination.

3. Results and discussion

detected. In addition, the XRD results shows that the amount of glucose affected the crystallinity of the product. With the increasing amount of glucose, the peak intensity of samples gradually decreased, reflecting that the glucose may affect the Bi2O2CO3 crystallization processes [32]. Compared to pure BOC, a diffraction peak at around 27.1° can be attributed to the (012) plane of the metallic Bi (JCPDS No. 44-1246). The formation of Bi metal phase can be assigned to the reduction of Bi3+ to

3.1. Crystal structure and morphology The XRD patterns of the as-synthesized samples are shown in Fig. 1a. The diffraction peaks of BOC can be assigned to a tetragonal Bi2O2CO3 phase (JCPDS No. 41-1488) and no impurities can be 4

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Fig. 5. (a) XRD patterns of Bi-BOC-2 after different growth periods; SEM images of Bi-BOC-2 after different growth periods: (b) 0 min; (c) 20 min; (d) 40 min and (e) 60 min.

Bi0 by glucose at a higher hydrothermal temperature. In addition, the partial reduction of Bi3+ introduce schottky defect pairs, which further result in significant amount of OVs. The presence of oxygen vacancies is examined by ESR spectroscopy. Fig. 1b shows that there is no signal for BOC, while a characteristic signal at a g value of 2.001 can be observed for the Bi-BOC composites, reflecting the presence of OVs [19,35]. It is worthy to note that the intensity of ESR signal gradually increases from Bi-BOC-1 to Bi-BOC-3, which is consistent to the higher concentration of OVs. The chemical state and surface composition of BOC and Bi-BOC-2 have been investigated by XPS analysis. All binding energies have been calibrated by using the C 1s peak at 284.6 eV. In Fig. 2a, the survey

spectra clearly show the existence of Bi, O and C elements in both BOC and Bi-BOC-2. There are three splitted peaks at 284.6, 286.2 and 288.8 eV in the C 1s spectra for both BOC and Bi-BOC-2 (Fig. 2b). The former two peaks may correspond to the external hydrocarbon contamination, and the latter peak is assigned to the carbonate ions of BOC [36]. Fig. 2c showed two well resolved peaks of Bi 4f in BOC at 164.3 and 159.0 eV, which are consistent with the characteristic peaks of Bi3+ [37]. For the Bi-BOC-2, two new peaks at 157.9 and 163.2 eV can be detected, and they are the feature of the Bi0 [29]. Furthermore, the ratio of Bi0/Bi3+ in Bi-BOC-2 is determined to be 1:10. It is worthy to note that the peaks of Bi3+ in Bi-BOC-2 slightly shifted to lower binding energy compared to that of pure BOC, reflecting the the preservation of electrostatic 5

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BOC samples, for instance, Bi-BOC-1 and Bi-BOC-2 (Fig. 3b and c) showed similar nested structure assembled by nanosheets. The thickness of nanosheets in Bi-BOC-1 and Bi-BOC-2 is around 5 and 10 nm, respectively. Notably, the thickness of nanosheets gradually increases with the increasing amount of glucose during the synthesis process, is consistent with the previous report [43]. Meanwhile, several Bi particles with a diameter around 100 nm are found on the surface of BOC nanosheets. With further increase amount of glucose, the nanosheets of Bi-BOC-3 become thicker and larger Bi particles ∼500 nm are observed (Fig. 3d). TEM and HRTEM are implemented to further investigate the composition Bi-BOC-2. The TEM image of Bi-BOC-2 (Fig. 3e) clearly shows the Bi metal particles on the surface of BOC nanosheets. As shown in Fig. 3f, the lattice spacing of 0.295 and 0.328 nm could be well matched to the (013) plane of BOC and (012) plane of Bi, respectively. These lattice parameters are in agreement with the XRD result. The EDS results, as shown in Fig. 3g, reveals that Bi-BOC-2 is comprised of Bi, O, and C elements, and no other purities are found. The content of Bi, O, C elements in Bi-BOC-2 was estimated to be 85%, 12%, 3%, respectively. The nitrogen adsorption−desorption isotherms are displayed in Fig. 3h. The specific surface areas of BOC and Bi-BOC-2 are 2.8275 and 14.7026 m2/g, respectively. The enhanced specific surface area of BiBOC-2 can be rationalized by the hierarchical nanosheets morphology. The corresponded pore size distribution has been summarized in Fig. 3i. For BOC, the pore size distribution shows mesopores with peak pore diameters of about 2.1 and 12.5 nm, and large macropores with peak pore diameters of about 90 nm. In contrast, Bi-BOC-2 is bimodal in pore size distribution and shows mesopores (peak pore diameters of about 13 nm) and macropores (peak pore diameters of about 90 nm), but its pore volume is higher than that of BOC.

Scheme 1. Schematic illustration of the formation process of the Bi-BOC-2 composite.

balance during the above reduction processes [38]. The binding energies of O 1s of BOC and Bi-BOC-2 are 530.3, 531.5 and 532.9 eV, and they can be assigned to lattice oxygen, oxygen defects, and absorbed oxygen species on the surface [39,40], respectively (Fig. 2d). In previous study, the peak intensity of adsorbed oxygen could be used to estimate the concentration of OVs [41]. Notably, the peak intensities of adsorbed oxygen (532.9 eV) and oxygen defects (531.5 eV) of Bi-BOC-2 are higher than those of BOC, suggesting the higher concentration of OVs in the Bi-BOC-2. This result clearly shows that glucose can be used as effective reduction agent to induce OVs formation in BOC samples. The contact angle test has been used to evaluate the hydrophilicity of BOC and Bi-BOC-2. The higher contact angle indicated the lower of hydrophilicity [42]. As shown in Fig. 2f, the contact angle of BOC (43°) is higher than that of Bi-BOC-2 (21°), reflecting the hydrophilicity of BOC is lower than Bi-BOC-2. The morphologies of the as-synthesized BOC and Bi-BOC composites are characterized by SEM. For BOC, Fig. 3a shows nanosheets feature and the thickness of nanosheet is around 1–3 nm. For the series of Bi-

3.2. Optical properties UV-vis diffuse reflectance spectra (DRS) is used to investigate the optical absorption properties. In Fig. 4a, the absorption edge of BOC is at 381 nm, suggesting that BOC is wide band gap semiconductor and it

Fig. 6. (a) Removal of LR5B profiles with different samples under simulated solar irradiation; (b) the kinetic fit for the degradation of LR5B with different samples under simulated solar irradiation; (c) removal of LR5B profiles with different samples under visible light irradiation; (d) the kinetic fit for the degradation of LR5B with different samples under visible light irradiation; (e) removal of LR5B with different amount of Bi-BOC-2; (f) removal of different concentration of LR5B. 6

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Fig. 7. (a) Removal of CIP profiles with different samples under simulated solar irradiation; (b) kinetic fit for the degradation of CIP with different samples under simulated solar irradiation; (c) removal of CIP profiles with different samples under visible light irradiation; (d) kinetic fit for the degradation of CIP with different samples under visible light irradiation.

cannot harvest visible light [44]. The absorption band edge of nanohybrids shows a notable red shift toward the visible region, suggesting that the Bi nanoparticle can significant enhance the visible-light response of the photocatalysts. This results are also consistent with the color change (from white to brown) (inset of Fig. 4a). The optical band gap can be calculated from the classic Tauc approach through the equation αhν=A(hν-Eg) n/2, where α, h, ν, A and Eg are the absorption coefficient, planck constant, light frequency, a constant and band gap, respectively. According to the fitted optical bandgap (Fig. 4b), the band gap of pristine BOC is determined to be 3.25 eV, while the band gap of Bi-BOC-1, Bi-BOC-2 and Bi-BOC-3 are calculated to be 2.92, 2.75 and 2.50 eV, respectively. This result confirmes that the existence of Bi particle can significant narrow the bandgap and enhance the adsorption of the visible light range.

initially irregular nanosheets gradually assemble to nested structure (Fig. 5d). After 60 min, the intensity of diffraction peak at 27.1° further increases, suggesting the formation of Bi particles (Fig. 5e). The formation mechanism of nested Bi/Bi2O2−xCO3 is proposed as five reactions, as illustrated in Scheme 1. Firstly, Bi(NO3)3 dissolved to form Bi3+ in nitric acid solution, which strongly hydrolyzed by forming BiONO3. Meanwhile, urea decomposes and generates CO2, which will dissolve in water and result in CO32− formation. The glucose can carbonize into the hydrophilic carbon colloid, which can dehydrate and condense by forming catenarian C60H102O51 during alkaline hydrothermal treatment (reactions (1)–(3)). It has been reported that glucose would not form carbon spheres due to the short reaction time. Then the produced BiONO3 reacts with CO32− to form Bi2O2CO3. The carboxyl and hydroxyl on the surface of hydrophilic carbon colloid could attract Bi3+ and serve as a soft template to form a nested structure. Finally, Bi3+ is reduced to Bi0 by the glucose.

3.3. Formation mechanism of the composites

Bi(NO3 )3 + H2 O

To understand the formation process of the nested Bi/Bi2O2−xCO3, time dependent morphology and crystal phase evolutions have been investigated. The nested Bi/Bi2O2−xCO3 start crystallization within 1 h under hydrothermal conditions. Fig. 5a shows the XRD patterns of the samples, which are obtained at different reaction times from 0 to 60 min. At the initial stage, Bi(NO3)3 was hydrolyzed to form amorphous BiONO3. The corresponding SEM image shows irregular nanoplates (Fig. 5b). After 20 min, the nanosheets start to form (Fig. 5c) and their corresponding XRD peaks can be assigned to Bi2O2CO3 (JCPDS No. 41-1488). Upon increasing the reaction time (40 min), the new peak at 27.1° appeared, indicating the Bi particle formation. The

BiONO3 + 2H+ + 2NO3

NH2 CONH2 + 2H2 O

10C6 H12 O6

C60 H102O51

+

2NH+4

C60 H102 O51+9H2 O

2BiONO3 + CO32

Bi3+

CO32

Bi2O2 CO3 + 2NO3

Bi0

(1) (2) (3) (4) (5)

3.4. Photocatalytic activity The photocatalytic performance of as-prepared samples is assessed 7

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Fig. 8. (a) Cycle runs of Bi-BOC-2 for the degradation of LR5B under simulated solar irradiation; (b) XRD patterns of the Bi-BOC-2 sample before and after five-cycle experiments; SEM image of Bi-BOC-2 (c) before degradation of LR5B; (d) after degradation of LR5B.

by degrading LR5B under simulated solar light and visible light irradiation. A background experiment has been conducted. The results shows that a trace amount of LR5B is degraded in the absence of photocatalyst under simulated solar light, indicating that photo bleaching of LR5B could be ignored. As shown in Fig. 6a, Bi-BOC-1 and Bi-BOC-2 85% showed improved photocatalytic activity (69% and 85%, respectively) compared to that of BOC (58%), under simulated solar light irradiation. However, Bi-BOC-3 showed lower photocatalytic activity (54%) than that of BOC. For a clearer presentation of their photocatalytic performance, the data was fitted by the following pseudofirst-order kinetic model as shown in Eq. (6): –ln(C/C0)=kt

higher photocatalytic activity than that of BOC, because BOC shows no absorption in visible light range, the introduction of Bi and OVs can create the visible light adsorption, thus enhancing the visible photocatalytic activity. The apparent rate constants of the samples are followed the order of Bi-BOC-2 > Bi-BOC-1 > Bi-BOC-3 > BOC (Fig. 6d). It is worthy to note that the apparent rate constant of Bi-BOC-2 is 10.9 times higher than that of BOC under visible light irradiation. Moreover, the amount of photocatalyst (Bi-BOC-2) is optimized from to 1 g L−1, as shown in Fig. 6e. The pollution concentration effect on degradation efficiency has been carried out by varying the LR5B concentration from 10 to 80 mg L−1 and the amount of photocatalyst are fixed to be 1 g L−1. As shown in Fig. 6f, the degradation efficiency had no significant change when the pollution concentration is below 40 mg L−1 of LR5B. However, when the concentration of LR5B is higher than 0.04 g L−1, the degradation efficiency significantly decreased. This phenomenon may attribute to the limited adsorption kinetics of LR5B molecules. In addition, the fixed amount of photocatalyst could generate limited number of active radicals, which may slow down the degradation kinetics [45]. To exclude the photosensitization effect on the LR5B decomposition, we evaluate the photocatalytic activity by removing CIP (Fig. 7). The photodegradation of CIP shows almost negligible without the presence of photocatalyst. Again, Bi-BOC-2 showed the highest removal efficiency (88%) after 2 h under simulated solar light irradiation (Fig. 7a), the CIP degradation kinetics follow an pseudo-first-order reaction (Fig. 7b). The k values are 0.00017 min−1, 0.00875 min−1, 0.00962 min−1, 0.00632 min−1 and 0.00358 min−1 for blank, BOC, BiBOC-1, Bi-BOC-2 and Bi-BOC-3, respectively. Meanwhile, the

(6)

where C0 is the initial absorbance of concentration (after stirring for 30 min in the dark) and C is the concentration at time t. Parameters k and t are the rate constant and time, respectively. The reaction rate constant will be used to quantitatively evaluate the photodegradation efficiency. The fitting results are displayed in Fig. 6b. The catalytic activity and apparent rate constants of the samples followed the order of Bi-BOC-2 > Bi-BOC-1 > BOC > Bi-BOC-3. The photocatalytic activity shows an volcano shape as a function of the glucose content, and Bi-BOC-2 shows optimum concentration for Bi metal and OVs to achieve highest activity. We speculate that the photocatalytic activity is highly related to the content of Bi and OVs. The excessive of Bi or OVs would become the recombination centers of electrons and holes, thus resulting in the decreased degradation rate [28,36]. The photocatalytic activity has also been assessed under visible light irradiation, as shown in Fig. 6c. All Bi/Bi2O2−xCO3 composites showed 8

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Fig. 9. (a) PL spectra of BOC and Bi-BOC composites; (b) nanosecond time-resolved emission decay of BOC and Bi-BOC-2; (c) transient photocurrent response of BOC and Bi-BOC composites; (d) EIS Nyquist plots of BOC and Bi-BOC composites.

Fig. 10. (a) Effect of different scavengers on LR5B degradation under simulated solar irradiation of Bi-BOC-2; (b) ESR spectra of DMPO-%OH adducts on Bi-BOC-2 under simulated solar irradiation.

degradation kinetics of CIP are further studied under visible light irradiation (Fig. 7c). The degradation efficient and apparent rate constants show the similar trend as that of LR5B, which are followed the order of Bi-BOC-2 > Bi-BOC-1 > Bi-BOC-3 > BOC (Fig. 7d).

stability of Bi-BOC-2, and the results are shown in Fig. 8. It is obvious that the photocatalytic activity of LR5B can be well preserved after five consecutive cycles of degradation tests (Fig. 8a). Moreover, the Bi-BOC2 samples before and after the 5th run cycle were characterized by XRD and SEM analysis (Fig. 8b–d). It could be found that the phase and morphologies of the recycled Bi-BOC-2 samples had almost no obvious discrepancy compared with the unirradiated one. The results reveals its excellent stability and great potential value in environmental purification.

3.5. Stability test The stability of photocatalysts is essential for their practical applications. Recycle experiments have been performed to evaluate the 9

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degradation process. In addition, there is negligible change on photocatalytic activity observed by adding BQ, suggesting that %O2− has limited contribution in the LR5B photocatalytic degradation. ESR analysis of Bi-BOC-2 has been performed to detect %OH during the simulated solar irradiation driven photocatalytic processes (Fig. 10b). Four characteristic peaks with an intensity ratio of 1:2:2:1 has been observed for Bi-BOC-2 in aqueous dispersion systems, which demonstrates the existence of %OH radicals. As such, h+ and %OH are most likely the main active radicals in the photocatalytic process. By summarizing the above results, a photocatalytic mechanism is proposed (Scheme 2). Under the simulated solar irradiation, both Bi and BOC generate photoinduced electrons, and the OVs can create ingap states. Electrons from in-gap state can hop to the conduction band of BOC. Meanwhile, the electrons on the Bi surface can transfer to the CB (0.05 eV) of BOC and the OVs induced defect states because the Fermi level of metallic Bi (-0.17 eV) is more negative [28]. After the electrons transfer, the fermi level of Bi would shift to more positive potentials to produce positive charges and then accept the electrons from VB of BOC to return to its primal state. The potential of CB of BOC could not reduce O2 to %O2− because the redox potential of O2/%O2− is −0.33 eV, which is more negative than that of the CB (0.05 eV) of BOC. In contrast, the photogenerated electrons in BOC could be excited to OV-induced defect states, and the holes can accumulate in the VB of BOC. In addition, the potential of VB of BOC (3.3 eV) is more positive than the redox potential of %OH/H2O (1.99 eV). The holes can also oxidize OH− into %OH, in consistence to the results from active radical trapping experiments and ESR analysis. Overall, both Bi and OVs play an important role on photocatalytic activity. Firstly, the existence of Bi can efficiently increase visible light absorption and the SPR effect of Bi can promote separation of electron−hole pairs. Secondly, OVs can extend the light absorption range due to the existence of defect states and create more active sites to improve photocatalytic activity. Moreover, OVs can act as electron scavenger to improve the separation efficiency of the photogenerated charge carriers. Therefore, we conclude that the synergistic effect of Bi plasmon and OVs can significant improve photocatalytic activity of the Bi-BOC-2.

Scheme 2. Proposed mechanism of the Bi-BOC-2 photocatalyst under simulated solar irradiation.

3.6. Photocatalytic mechanism Photoluminescence spectroscopy (PL) is used to investigate the separation capacity of the photoinduced electron-hole pairs. It has been reported that the recombination of excited electrons and holes can lead to high intensity of the PL signals [46]. As shown in Fig. 9a, the peaks intensity of Bi-BOC-1 and Bi-BOC-2 significantly lower than that of other samples. This may attribute to metallic Bi and OVs can serve as electron traps and inhibit the electron−hole recombination [47]. Whereas, Bi-BOC-3 showed higher intensity than BOC, reflecting excessive Bi and OVs could act as recombination centers and promote recombination rate of electron-hole pairs [23]. To further investigate dynamics of photogenerated electron and hole, their lifetime in BOC and Bi-BOC-2 has been measured by the time-resolved fluorescence emission decay spectra. As shown in Fig. 9b, the nanosecond time-resolved emission of Bi-BOC-2 displayed faster decay than that of BOC [48,49]. By using the triexponential fitting process, the average lifetimes for BOC and Bi-BOC-2 are calculated to be 3.473 and 2.577 ns, respectively. The short lifetime suggest that the new non-radiative processes for charge carriers may occur in Bi-BOC-2 [48]. The transient photocurrent response of BOC and Bi-BOC composites has been performed to study the electron formation and diffusion. As shown in Fig. 9c, the current density increases as light-on and decreases as light-off. Bi-BOC-2 exhibits the highest photocurrent density. The high photocurrent suggests that the certain amount of Bi and OVs can facilitate photogenerated charge carriers separation, thus improving photocatalytic activity. Electrochemical impedance spectroscopy (EIS) analysis has been applied to study the charge separation processes of BOC and Bi-BOC composites. The radius of the arc represents the charge-transfer resistance and the smaller radius indicates the lower resistance and higher separation efficiency of photogenerated electrons and holes. Bi-BOC-2 shows smallest radius among all the samples, revealing that the certain amount Bi and OVs accelerate the separation rate of photoinduced charge carriers (Fig. 9d). The active species trapping experiments has been employed to verify the main active species for LR5B photodegradation. As shown in Fig. 10a, the photocatalytic activity is inhibited by adding EDTA-2Na as hole capture and the LR5B degradation rate declined from 85% to 26%. This indicates that h+ is the dominant species in the photodegradation LR5B processes. Meanwhile, the degradation efficiency of LR5B reaches 42% after adding IPA, reflecting that %OH played secondary role in the

4. Conclusions In summary, we have successfully synthesized a nested Bi/ Bi2O2−xCO3 with OVs by a one-pot hydrothermal process by using glucose as both morphological control agent and reducing agent. The synergistic effects of Bi plasmon and OVs result in outstanding photocatalytic activity of the Bi-BOC-2 for LR5B and CIP degradation. The removal efficiency of LR5B and CIP are 85% and 88%, which are higher than that of BOC under simulated solar irradiation. More importantly, cyclic testing demonstrated that the Bi-BOC-2 possesses excellent structure stability. The trapping experiments and ESR analysis clearly show that the active radicals of h+ and %OH are the main active species in the degradation process. The present work could not only develop a novel strategy for construction of nested photocatalyst with abundant oxygen vacancies, but also introduce low-cost metallic Bi as a substitute for noble metals to promote the utilization of solar energy. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work is supported by the Science and Technology Innovation Talents Program of Xinxiang City (CXRC17001, CXGG17004), Major Science and Technology Program for Water Pollution Control and Treatment of China (2015ZX07204-002), Natural Science Foundation 10

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of Henan Province (162300410212), Foundation of Henan Normal University (20180446).

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