Two-Dimensional CoreâShell-Structured Bi2O4

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1D/2D Core-Shell Structured Bi2O4/BiO2-x Heterojunction for Highly Efficient Broad Spectrum Light Driven Photocatalysis: Faster Interfacial Charge Transfer and Enhanced Molecular Oxygen Activation Mechanism Jun Li, Yuan Li, Gaoke Zhang, Hongxia Huang, and Xiaoyong Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21693 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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ACS Applied Materials & Interfaces

1D/2D

Core-Shell

Structured

Bi2O4/BiO2-x

Heterojunction for Highly Efficient Broad Spectrum Light Driven Photocatalysis: Faster Interfacial Charge Transfer and Enhanced Molecular Oxygen Activation Mechanism Jun Li,a,1 Yuan Li,a,1 Gaoke Zhang,*,a Hongxia Huang,a Xiaoyong Wu a a

Hubei Key Laboratory of Mineral Resources Processing and Environment, State Key

Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China

1

These authors contributed equally.

KEYWORDS:

core-shell; Bi2O4/BiO2-x composite; anti-photocorrosion; near-infrared light;

molecular oxygen activation; DFT study

1 ACS Paragon Plus Environment

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ABSTRACT

The deliberate tuning of nanoparticles encapsulated with nanosheet shells can bring about fascinating photocatalytic properties due to the fast charge transfer characteristics of a nanosized core-shell structure. Herein, a novel core-shell structured Bi2O4/BiO2-x composite was fabricated through one-step hydrothermal method. The core-shell Bi2O4/BiO2-x composite presented distinct optical absorption property, including the UV, visible and near-infrared (NIR) lights regions. Compared to Bi2O4 and BiO2-x, the Bi2O4/BiO2-x composite revealed improved broad spectrum light responsive molecular oxygen activation into •O2-, especially, achieving •O2- generation under NIR light irradiation. The achievement that enhanced broad spectrum light activated molecular oxygen activation could be ascribed to the faster electron transfer confirmed by the electron spin resonance (ESR) spectra, photoluminescence (PL) spectra, photoelectrochemical test and quantitative analysis of •O2-. The strong interface effect of Bi2O4/BiO2-x composite was confirmed by X-ray photoelectron spectroscopy (XPS) analysis. Density functional theory (DFT) calculated results suggested Bi2O4/BiO2-x composite revealed increased density of states near the Femi level, suggesting it possessed higher carrier mobility as compared to Bi2O4 and BiO2-x, contributing to the faster separation of photoinduced carriers and the generation of •O2-. Benefiting to the heterojunction, Bi2O4/BiO2-x composite showed improved photocatalytic activity and anti-photocorrosion activity during rhodamine B (RhB) and ciprofloxacin (CIP) degradation with the irradiation of UV, visible and NIR lights. Besides, the possible photocatalytic mechanism and transformation pathway of RhB and CIP degradation by Bi2O4/BiO2-x composite were proposed by the analyses of the liquid chromatography-mass spectrometry. This study furnishes a new strategy for fabricating high-efficient and broad spectrum light driven heterojunction photocatalysts for environment purification.


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1. INTRODUCTION Fabricating core-shell structured semiconductor photocatalysts, integrating distinct ingredients with different functionalities in one system, have triggered the extensively exploration.1-10 Among the strategies for assembling core-shell structured heterostructures, hydrothermal strategy is regarded as an effective way for construction of novel core-shell structured heterostructures with promising applications in thermos-electronics, photocatalysis and so on.1112

Considering the distinct physicochemical characteristics of bismuth oxides, fabricating core-

shell structured Bi-based photocatalysts together with their appropriate energy level match are promising photocatalysts for pollutants removal. To date, several core-shell structured semiconductors constructed by bismuth oxides has been developed, such as Bi@Bi2O3, Fe3O4@Bi2O3 and [email protected] These photocatalysts with abilities to facilitate electronhole separation reveal commendable photocatalytic performances for pollutants removal under visible light irradiation. However, the main works focus on the studies of UV or UV-visible light responsive photocatalysts, the utilization of NIR light accounting for about ca. 50% of sunlight still desired to be further explored. The bismuth-based photocatalysts with NIR light harvest ability have recently gained great concern in photocatalysis owing to its facile hole mobility, narrow band gap and fantabulous optical property.16-20 Especially, when the broad spectrum light driven photocatalysts were achieved, the NIR-driven photocatalytic mechanism of the bismuth-based photocatalysts has been further acquainted. For instance, Bi2WO6 nanosheets revealed excellent photocatalytic performance because of introducing rich oxygen vacancies, which could increase the Fermi level and decrease the band edge.21 The dual VBi-O’’ defects induced defect states in monolayer BiO2-x sheets and doping Ni2+ induced doped states in BiO2-x nanosheets could effectively improve the



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separation efficiency of photoinduced carriers and contribute to realize better photocatalytic performance for degradation RhB and phenol.22-23 Although the Bi-based photocatalysts exhibited satisfactory photocatalytic activity, exploring relative convenient and effective way to strengthen NIR light responsive Bi-based photocatalytic performance and further comprehending the mechanism are still indispensable. The fabrication of heterojunction photocatalysts presents admirable advantages in charge separation.24-30 Herein, the core-shell structured Bi2O4/BiO2-x binary heterojunctions were fabricated via one-step hydrothermal strategy. The core-shell structured Bi2O4/BiO2-x composite was observed by microscopy technology. Moreover, the electron transfer mechanism of Bi2O4/BiO2-x heterojunction has been confirmed by chemical trapping experiments and ESR analysis. DFT calculations also revealed that the Bi2O4/BiO2-x heterojunction favored to the separation of photoinduced carriers. The experiments and theorical calculations demonstrated the formation of Bi2O4/BiO2-x heterojunction enhanced the generation of •O2- under visible and NIR lights irradiation, achieving •O2- generation under NIR light irradiation. In accordance with expectation, the Bi2O4/BiO2-x composite exhibited raised photocatalytic activity and antiphotocorrosion performance for RhB and CIP degradation. Furthermore, the possible photocatalytic mechanisms of RhB and CIP degradation were proposed. This work provides a novel high-efficient broad spectrum light driven heterojunction photocatalyst with enhanced molecular oxygen activation for environment remediation. 2. EXPERIMENTAL 2.1. Preparation of Photocatalysts.


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Fabrication of Core-Shell Bi2O4/BiO2-x Heterojunction Photocatalyst: In a typical experiment, 7.2 g NaOH was dissolved into the 60 mL deionized water with violently stirring. Therewith, 2.8 g NaBiO3 was dispersed to the NaOH solution. After being continuously stirred for 0.5 h, the suspensions were transferred into a 90 mL PPL-lined stainless steel autoclave, and sealed and heated at 180 oC for 6 h. When the reaction finished, the autoclave was cooled to room temperature naturally. The precipitate was collected by centrifugation and washed with deionized water for five times, and finally dried at 80 ◦C for 4 h in drying chamber. The Bi2O4 nanorods and BiO2-x nanosheets were prepared by a facile hydrothermal strategy. The synthesized processes of Bi2O4 and BiO2-x were presented in the supplementary materials. 2.2. Characterization. The phase composition of the as-prepared products was detected by X-ray diffraction on a X’Pert PRO diffractometer. The optical properties of the sample were observed on a spectrophotometer (Lambda 750 S). X-ray photoelectron spectra (XPS) was employed on the PHI Quantera II system with an exciting source of Mg Kα. Transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) images were gained from JEM-2100F machine. The electron paramagnetic resonance (EPR) (A300-10/12, Bruker) was performed to observe the main radical signals at room temperature. Photoluminescence (PL) emission spectra were carried out by a spectrofluorometer (RF-5301PC, Shimadzu, Japan). Agilent 6410 liquid chromatography mass spectrometer (LC-MS, USA) was performed to determine the main intermediate products after photocatalytic degradation of RhB and CIP. The photoelectrochemical characterizations were implemented using the electrochemical workstation (CHI660E) containing a typical three-electrode system. The 0.5 M Na2SO4 aqueous was selected



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as the electrolyte. Nitroblue tetrazolium (NBT) is a common method to quantify the amount of superoxide radical (•O2-) produced in the photocatalytic process. 100 mg samples were dispersed in a 500 beaker with 0.1 L NBT solution. After the dark adsorption for 1 h, the suspensions were irradiated with visible light for 2 h. At given time intervals, 6 ml suspensions were collected and centrifuged. The NBT concentration after irradiation was analyzed by employing a UV-vis spectrophotometer (UV751GD, China) at 260 nm. The •O2- concentration can be calculated based on that 1 mmol NBT would react with 4 mmol •O2-. 2.3. Theoretical Calculations. For observing the electronic structure of Bi2O4, BiO2-x and the charge distribution characteristic of Bi2O4/BiO2-x heterojunctions, the calculations were implemented by using the Vienna ab initio simulation package and projector-augmented wave (PAW) pseudopotentials were selected.31 The exchange-correlation functional was achieved based on generalized gradient approximation (GGA) in the parametrization of Perdew, Burke and Enzerhof (PBE).32 The optimizations of atomic geometry were employed until the components of residual force and iterative energy difference were less than 0.02 eV•Å-1 and 10-4 eV, respectively. The lattice parameters of monoclinic Bi2O4 were a = 12.367 Å, b = 5.118 Å, c = 5.299 Å. The lattice parameter of cubic BiO2-x is a = 5.475 Å. In addition, the k-point meshes of Bi2O4 and BiO2-x were selected to be 1 × 2 × 2 and 9 × 9 × 9, respectively. 2.4. Photocatalytic Activity. The broad spectrum driven photocatalytic performance of Bi2O4/BiO2-x heterojunction was performed by degradation of RhB and CIP. 100 mg Bi2O4/BiO2-x composite were put into 100 mL of RhB or CIP solution. After the dark adsorption for 1 h, the suspensions were illuminated


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by 100 W LED lamp with different wavelengths. At set time intervals, 6 ml suspensions were taken out and then centrifuged. The absorbance of residual RhB or CIP was analyzed by employing a UV-vis spectrophotometer (UV751GD, China). In this work, 100 W LED lamps with the wavelengths of 365, 420 and 770-860 were selected as UV, visible and NIR light sources. The light intensities of the UV, visible and NIR light sources were measured to be 73, 55 and 25 mW•cm-2, separately. 3. RESULTS AND DISCUSSION 3.1. Texture Properties. BiO2-x nanosheets, Bi2O4 nanorods and core-shell structured Bi2O4/BiO2-x composite were all fabricated by a hydrothermal method and presented in Scheme 1. The phase structures of the samples were characterized by XRD technique (Figure 1). The diffraction peaks at 2θ = 28.21o, 32.69o, 46.92o, 55.64o, 58.33o were indexed to the (111), (200), (220), (311) and (222) planes of the cubic structure BiO2-x (JCPDS No. 47-1057), while the peak at 2θ = 26.83o, 29.48o and 30.33o were attributed the (111), (31-1) and (400) planes of the monoclinic phase Bi2O4 (JCPDS NO. 50-0864) and no other impurity peaks were identified. According to the above analysis, it can be deduced that indicated that the Bi2O4/BiO2-x composite was fabricated successfully.



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Scheme 1. Schematic depiction of the prepared route of BiO2-x nanosheets, Bi2O4 nanorods and core-shell structured Bi2O4/BiO2-x composite.

Figure 1. XRD patterns of BiO2-x, Bi2O4 and Bi2O4/BiO2-x composite. 3.2. Morphologic Structure. Figure 2 revealed the TEM images of BiO2-x, Bi2O4 and Bi2O4/BiO2-x composite photocatalysts. Figure 2 (a-b) reveal that BiO2-x was hexagon structured nanosheet. By measuring its lattice fringes, the fringe spacing was 0.317 nm, which belongs to the (111) plane of BiO2-x. As shown in Figure 2 (c-d), pure Bi2O4 is the nanorod morphology, and its lattice distance of 0.332 nm corresponds to the (111) plane. Furthermore, the Bi2O4/BiO2-x composite was revealed through the TEM images. It was very clearly that Bi2O4 cores were coated by the BiO2-x shells (Figure 2e-f), indicating the existence of core-shell structured Bi2O4/BiO2-x. The Bi2O4/BiO2-x composite reserved the morphological features of BiO2-x nanosheets and Bi2O4 nanorods. In the growth process, we could conclude that the concentration of NaOH solution played a dominant role for


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the formation of Bi2O4/BiO2-x composite with core-shell structure. Compared with the synthesis condition of pure BiO2-x nanosheets and pure Bi2O4 nanorods, we could know that the high concentration of NaOH solution was in favor of the growth of BiO2-x nanosheets, while the low concentration of NaOH solution was in favor of the growth of Bi2O4 nanorod. Thus, the growth process can be summarized as follows: In the first stage, the concentration of NaOH was 0.5 mmol/L, the relative high concentration of NaOH solution promoted the growth of BiO2-x nanosheets. Secondly, when the reaction process went to the intermediate stage, the concentration of NaOH solution decreased, the residual of NaBiO3 distributing on the surface of BiO2-x nanosheets could further react forming Bi2O4 nanorod. Thirdly, in the growth process of Bi2O4 nanorods, the growing BiO2-x nanosheets was wrapping the Bi2O4 nanorods and then the Bi2O4/BiO2-x composite with core-shell structure was obtained.

Figure 2. TEM and HRTEM images of the samples. (a-b) BiO2-x; (c-d) Bi2O4 and (e-f) Bi2O4/BiO2-x composite.



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3.3. Composition and Surface Structure Analysis. For investigating the surface chemical state and interface interaction in Bi2O4/BiO2-x composite structure, XPS analysis was performed. The survey XPS spectrum in Figure 3a indicated that composition of the Bi2O4/BiO2-x composite included Bi and O elements. The peaks at 158.3 and 163.6 eV correspond to the Bi 4f7/2 and Bi 4f5/2, respectively, which belongs to the mixed valence states Bi (Ⅲ, Ⅴ)-O band in the samples (Figure 3b).33-35 The peaks of Bi2O4/BiO2x

composite shifted to higher binding energies compared to BiO2-x. The enhanced binding

energies of Bi 4f spectrum observed in Bi2O4/BiO2-x composite compared to BiO2-x can be ascribed to the construction of strong interfacial interaction between Bi2O4 and BiO2-x, revealing the coupling state of Bi2O4/BiO2-x heterojunction is chemical hybridization not the simple physical attachment. 36-37 The C 1 s spectrum in Figure 3c represented mainly one peaks with the binding energy at 284.6 eV, which resulted from the adventitious carbon species for measurement. The O 1s spectra shown in Figure 3d was fitted by three peaks at 529.2, 530.7 and 531.7 eV corresponded to lattice oxygen, surface hydroxyl group and oxygen vacancies in the asprepared samples, respectively.38-39 The above results demonstrated that the presence of the strong interaction between BiO2-x and Bi2O4 and revealed the formation of a Bi2O4/BiO2-x heterojunction.


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Figure 3. XPS spectra of the Bi2O4/BiO2-x composite: (a) survey, (b) Bi 4f, (c) C 1s and (d) O 1s spectra. 3.4. Optical Property. To further research the optical properties of Bi2O4, BiO2-x and Bi2O4/BiO2-x composite, UVVis-NIR DRS measurement was performed. The absorption edge of the Bi2O4/BiO2-x composite revealed an evident red-shift and was located at ~850 nm (Figure 4a). The absorption intensity of Bi2O4/BiO2-x composite was stronger than that of the Bi2O4 nanorods, indicating the improvement of absorption capacity due to the presence of BiO2-x on the surface of Bi2O4. In addition, it is worth noting that it presents an obvious positive correlation with the presence of BiO2-x for the Bi2O4/BiO2-x composite in the UV-vis-NIR spectra. The BiO2-x nanosheets



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possibly induce modifications of the formation scheme of photogenerated carriers under solar light irradiation.40 The band gap (Eg) of Bi2O4 and BiO2-x can be determined using the KubelkaMunk equation:33, 41-42 αhν = A (hν − Eg)n/2 In this equation, ν and h represent the absorption coefficient and Planck constant, separately. For Bi2O4, n is equal to 4 resulting from the indirect transition semiconductor features. The Eg is calculated to be ~1.63 eV (Figure 4b). Based on our reported work, the band gap of BiO2-x was ~1.46 eV. Moreover, the Mott-Schottky (MS) plots of BiO2-x and Bi2O4 were revealed in Figure 4c, the flat potentials of Bi2O4 and BiO2-x were calculated to be -0.51 and -0.55 V versus the Ag/AgCl electrode, respectively, and they were equivalent to -0.27 and -0.31 versus the normal hydrogen electrode (NHE). The conduction band minimum potential (ECB) of n-type semiconductor is usually more negative by about 0.1 V than the flat band potential. Therefore, ECB potentials of them were -0.37 and -0.41 V, respectively. So, combined with the results of bad gaps and CB potentials, the valence band (VB) minimum of them was calculated to be 1.26, 1.05 V, separately. Both the samples possess the abilities to generate •O2- with the irradiation of UV, visible and NIR lights.


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Figure 4. (a) UV-vis-NIR DRS spectra of BiO2-x, Bi2O4 and Bi2O4/BiO2-x. (b) Plots of (αhν)1/2 versus the photon energy of Bi2O4. (c) Mott-Schottky plots of pure BiO2-x and Bi2O4. 3.5. Photocatalytic Performance Evaluation. In order to assess the photocatalytic performance of the products, RhB in aqueous solution was selected as the representative pollutants. The photolysis of CIP in aqueous solution is negligible, showing the stability of CIP molecular (Figure S1a). The degradation ratio of RhB showed no appreciable change at the presence of photocatalysts in the dark, indicating the happen of photocatalytic reaction (Figure 5 (a-c)). Under UV, visible and NIR lights irradiation, the core-shell structured Bi2O4/BiO2-x composite exhibited satisfactory photocatalytic activity compared to pure BiO2-x and Bi2O4, and achieved 84 %, 96 % and 80 % removal efficiency of RhB, respectively. In addition, the corresponding photocatalytic degradation rates of RhB by the



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products was presented in Figure 5 (d-f). With the irradiation of UV or visible light irradiation, the calculated degradation rate k values of Bi2O4/BiO2-x composite, BiO2-x and Bi2O4 were estimated to be 2.64571 (2.63950), 1.02607 (1.66867) and 1.25298 h-1 (1.94085 h-1), respectively, and followed the order BiO2-x < Bi2O4 < Bi2O4/BiO2-x composite. While under the NIR light irradiation, the calculated k values of Bi2O4/BiO2-x composite, BiO2-x and Bi2O4 were about 0.23770 h-1, 0.18278 h-1, and 0.04918 h-1, following the order Bi2O4 < BiO2-x < Bi2O4/BiO2-x composite. Considering the utilization of broad spectrum light and the facile synthesis of the Bi2O4/BiO2-x composite, the Bi2O4/BiO2-x composite could be considered as a promising and highly efficient photocatalysts for pollutants removal. Furthermore, to get an insight into the intermediates of RhB degradation by Bi2O4/BiO2-x composite, LC coupled with MS was employed to observe the intermediate products. According to the experimental results and reported works, the possible evolution process of RhB degradation under NIR light irradiation as illustrated in Scheme 2.43-44 The corresponding mass spectra and chemical structures of the possible intermediates were listed in Table S1. Firstly, abundant N-de-ethylated intermediate products could be investigated in the N-de-ethylation process. The active radicals generated in the aqueous solution attacked the N-deethylation intermediate products resulting in the generation of some primary oxidation products. Furthermore, the mass peaks at m/z 415.3 and 387.2 were identified as N, N-diethylrhodamine, N-ethyl-N-ethylrhodamine. Thirdly, the generated products were further degraded into smaller intermediates by photocatalysis. Finally, the smaller products were decomposed into CO2 and H2O.


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Figure 5. The photocatalytic performances of the products for degradation of RhB under (a) UV (RhB; 10 mg/L), (b) visible (RhB; 10 mg/L) and (c) NIR lights (RhB; 5 mg/L) irradiation, respectively. Linear plots of ln(C0/C) vs. degradation time under (d) UV, (e) visible and (f) NIR lights irradiation, separately.



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Scheme 2. Schematic diagram of possible intermediates of photocatalytic RhB degradation. The cycling experiments was performed to evaluate the anti-photocorrosion performance and stability of the Bi2O4/BiO2-x composite (Figure 6). The photocatalytic performance of the Bi2O4/BiO2-x composite remained almost negligible decrease after 4 cycles under the same conditions under UV, visible and NIR lights irradiation, and RhB removal efficiencies varies from 93% to 87% under UV light irradiation, 100% to 96.5% under visible light irradiation and 80.0% to 70.0 % under NIR light irradiation (Figure 6(a-c)). Figure 6d showed the XRD patterns of fresh and recycled Bi2O4/BiO2-x composite, and there were no significant changes, confirming the excellent stability and anti-photocorrosion performance of the Bi2O4/BiO2-x composite photocatalyst. In addition, the recycled Bi2O4/BiO2-x composite can still maintain its core-shell nanostructure, suggesting the core-shell structure of Bi2O4/BiO2-x composite is stability (Figure S2).


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Figure 6. The cycling experiments of Bi2O4/BiO2-x composite for photocatalytic degradation of RhB under (a) UV, (b) visible and (c) NIR lights irradiation. (d) XRD patterns of Bi2O4/BiO2-x composite of fresh and used Bi2O4/BiO2-x composite. To eliminate the potential dye sensitization effect, the photocatalytic performance of Bi2O4/BiO2-x composite for CIP degradation was also performed under UV-NIR light irradiation. The photolysis of RhB in aqueous solution is ignorable, showing the stability of RhB molecular (Figure S1b). After adding the samples, the CIP in aqueous solution was gradually degraded by the photocatalysts (Figure 7(a-c)). The core-shell structured Bi2O4/BiO2-x composite exhibited the better photocatalytic performance that by pure BiO2-x nanosheets and Bi2O4 nanorods, implying that the Bi2O4 nanorods encapsulated by BiO2-x nanosheets could accelerate the carriers’ separation and further improve the photocatalytic activity (Figure 7d). 17 ACS Paragon Plus Environment


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Furthermore, their decomposition kinetic constants (k) of Bi2O4, BiO2-x and Bi2O4/BiO2-x composite were about 0.383, 0.673 and 0.787 h-1, respectively. Particularly, the kinetic constant of the Bi2O4/BiO2-x composite was about 2.01 and 1.17 times higher than that of Bi2O4 and BiO2-x, separately (Figure 7e). The Bi2O4/BiO2-x composite still possesses high photocatalytic activity after 4 cycles under UV-NIR light irradiation (Figure 7f). The intermediates of CIP degradation were identified by LC-MS. The corresponding mass spectra, chemical structures of the possible intermediates were presented in Table S2. Moreover, two possible photocatalytic degradation pathways of CIP were proposed based on relevant literatures and identifying intermediates (Scheme 3).45-46 In the pathway (1), it is the broken process of the piperazinyl moiety. The piperazine was destroyed and transferred into the products at m/z=362 derived from the oxidation of •O2- and h+. And then, the products at m/z=362 lost two -CO groups and turn into the products at m/z=306, which further were oxidized into the products at m/z=291. Finally, the products underwent the process of decarboxylation and decarbonylation, changing to the products at m/z=219. Finally, the piperazinyl substituent of CIP was broken. The pathway (2) exhibited a defluorination and hydroxylation process. CIP molecular goes through varying degrees of hydrolysis and produced the products at m/z=334. These intermediates could be further oxidized, such as the cleavage of quinolone, hydroxylation and decarboxylation, and degraded into CO2, H2O, NO3- and F- in the end. In addition, the total organic carbon (TOC) tests was performed to measure the mineralization rate of the organic pollutants. As shown in Figure S3, the removal efficiencies of RhB and CIP were 36 % and 43 %, respectively, which were positive correlation to their degradation rate.


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Figure 7. UV-vis absorption spectral variations of CIP in aqueous solution over (a) Bi2O4, (b) BiO2-x and (c) Bi2O4/BiO2-x composite with the irradiation of UV-NIR light, respectively. (d) Photocatalytic performances of the samples for degradation of CIP, and (e) the corresponding kinetic analyses. (f) Cycling photocatalytic test of Bi2O4/BiO2-x composite.



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Scheme 3. The possible photocatalytic degradation pathways of CIP in aqueous solution. 3.6. Broad Spectrum Light Driven Photocatalytic Mechanism. To explore the photocatalytic mechanism of Bi2O4/BiO2-x composite for degradation of RhB under UV, visible and NIR lights irradiation, the trapping experiments were performed.47-49 When sodium oxalate (Na2C2O4) or benzoquinone (BQ) was added in the system, the photocatalytic performance of RhB was inhibited obviously, confirming that h+ and •O2- were the dominant active species under UV, visible and NIR lights irradiation (Figure 8(a-c)). While isopropyl alcohol (IPA) was selected as •OH scavenger, the photocatalytic activity slightly decreased, indicating the •OH wasn’t the active species with the irradiation of UV, visible and NIR lights. The above results reveal that •O2- and h+ were the dominant active radicals, while •OH performs a negligible influence. For confirming and providing in-depth understanding to the enhanced molecular oxygen activation into •O2-, the nitroblue tetrazolium (NBT) was used to


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quantify the concentration of •O2- produced by Bi2O4, BiO2-x and Bi2O4/BiO2-x composite. In general, 1 mmol NBT would react with 4 mmol •O2-, so the •O2- concentration can be determined (Figure 8d). The •O2- generation of Bi2O4, BiO2-x and Bi2O4/BiO2-x composite was calculated to be 0.45, 3.2 and 4 μmol•L-1, respectively. As expected, the Bi2O4/BiO2-x composite reveals higher •O2- generation activity than that of Bi2O4 and BiO2-x under UV-NIR light irradiation, resulting from the faster separation of photogenerated carriers induced by the heterojunction.

Figure 8. Trapping experiment of active species during the photocatalytic degradation of RhB under LED lamp irradiation with the wavelength of (a) 365, (b) 420, (c) 770-860 nm. (d) Quantitative determination of •O2-’s generation for Bi2O4, BiO2-x and Bi2O4/BiO2-x composite under UV-NIR light irradiation.



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To further identify the active species in the broad spectrum light responsive photocatalytic process, ESR spectra were measured.50-55 As shown in Figure 9a, the signals of the DMPO-•O2could be distinctly detected under visible and NIR lights irradiation in the presence of the samples. The ESR analyses are in keeping with the chemical trapping experiment results. It is worth mentioning that the pure BiO2-x could hardly generate •O2- under NIR light irradiation in our previous works. The generation of •O2- in Bi2O4/BiO2-x composite system derived from the faster carriers’ separation efficiency. Moreover, the characteristic signals of the DMPO-•OH couldn’t be observed with the irradiation of visible and NIR lights, confirming that •OH played the minor role in the photocatalytic process with the irradiation of visible and NIR lights (Figure 9b). The enhanced generation of •O2- in Bi2O4/BiO2-x composite contributes to its higher photocatalytic activity.

Figure 9. ESR signals of the (a) DMPO-•O2- and (b) DMPO-•OH for Bi2O4/BiO2-x for 5 min under UV, visible and NIR lights irradiation.


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The photoelectrochemical properties of the products were measured to present the photoresponse and electrochemical interfacial reactions.56-59 The photocurrent response of Bi2O4/BiO2x

composite is about 3 and 1.5 times higher than that of Bi2O4 and BiO2-x, separately (Figure 10a).

The electrochemical impedance spectra in Figure 10b showed that the Bi2O4/BiO2-x composite displayed a reduced radius of Nyquist plot compared to that of pure Bi2O4 and BiO2-x, confirming combining BiO2-x and Bi2O4 can increase charge transport and lower carriers’ recombination rate. The above analyses demonstrated the Bi2O4/BiO2-x composite can generate more photogenerated electrons with extended carriers’ lifetime that could contribute to the generation of •O2- for photocatalytic degradation of pollutants. Additionally, photoluminescence (PL) spectra were also observed to value the charge separation efficiency of the products. Figure 10(c-e) presented the PL spectra of Bi2O4, BiO2-x and Bi2O4/BiO2-x composite. The photoluminescence (PL) spectra revealed three emission peaks at about 370, 470 and 730 nm, which corresponded to the UV, visible and NIR light region, separately, confirming the presence of oxygen vacancies in their structure,23,60 which was in accordance with the above XPS analysis.



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Figure 10. (a) The transient photocurrent response and (b) electrochemical impedance spectra of Bi2O4, BiO2-x and Bi2O4/BiO2-x composite. (c-e) The PL spectra of BiO2-x, Bi2O4 and Bi2O4/BiO2-x composite. 3.7. Electronic Property Calculation. For investigating the effect of the introduced Bi2O4 on the electronic structure of the BiO2-x and the boosted photocatalytic activity of the Bi2O4/BiO2-x composite, the DFT calculations were employed. Figure 11a showed the total density of states (TDOS) and partial density of states (PDOS) of BiO2-x, Bi2O4 and Bi2O4/BiO2-x composite. The VB and CB of BiO2-x and Bi2O4 were mainly composed by Bi 4p and O 2p orbits showing their half-metallic property. It was worthy to mentioned that the DOS at the Fermi level in the Bi2O4/BiO2-x composite was significantly larger than that of BiO2-x and Bi2O4, suggesting that the Bi2O4/BiO2-x composite possesses higher


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carrier mobility.61 The high conductivity of the Bi2O4/BiO2-x composite could favor carriers’ separation and further improve the reactions between pollutants and the active species.

Figure 11. (a) TDOS and PDOS of Bi2O4, BiO2-x and Bi2O4/BiO2-x composite. (b-c) Calculated electrostatic potentials for Bi2O4 nanorods and BiO2-x nanosheets, respectively. (d) Charge difference distribution between BiO2-x and Bi2O4: charge accumulation is in cyan and depletion in yellow. (e) ELF of Bi2O4/BiO2-x composite. To better study the heterojunction formed between Bi2O4 nanorods with (111) planes and BiO2-x nanosheets with (111) planes, their work functions were calculated. The work functions of Bi2O4 with (111) plane and BiO2-x with (111) plane were evaluated to be 6.46 and 4.34 eV, separately, showing that the electrons in BiO2-x can flow to Bi2O4 through the heterojunction (Figure 11b).62-64 These results were described by the charge density difference and the result was present in Figure 11c. The cyan and yellow regions are on behalf of electron depletion and accumulation, separately, demonstrating the photogenerated electrons could transfer from BiO2-x



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to the Bi2O4 in heterojunction between the BiO2-x and Bi2O4. Figure 11d showed the charge difference distribution of Bi2O4 on BiO2-x. In addition, the Bi-O layer of Bi2O4 obtained rich electrons from Bi elements of BiO2-x and became the photoactive sites. The electronic location function (ELF) (Figure 11e) also confirmed the presence of a strong interface interaction between the Bi elements of BiO2-x and O elements of Bi2O4, providing a transfer channel of photoinduced electrons. Combined to the analyses of electronic structures and ELF, the electron transfer from Bi elements of BiO2-x and O elements of Bi2O4, indirectly demonstrates the type-Ⅱ electron transfer pathway. It’s well known that the potentials of the photocatalysts can affect the redox characteristics of photocatalysts. Further to verify the trapping experiments and explore the possible separation process of the photon-generated carriers, the potentials are described in Figure 12. According to the above analyses of DRS and XPS valence band spectra, we obtain the potentials of BiO2-x and Bi2O4, respectively. The CB and VB potentials of BiO2-x were -0.41 and 1.05 V (vs. NHE). The ones of Bi2O4 were -0.37 and 1.39 V (vs. NHE). Generally, the standard redox potential of •O2/O2 is evaluated to be -0.33 V (vs. NHE).65 The CB potentials of the BiO2-x and Bi2O4 were located at about -0.41 and -0.37 V, which were more negative than the reduction potential of •O2/O2. Therefore, the photogenerated electron on the CB of Bi2O4 is thermodynamically able to react with O2 for producing •O2-. The VB potentials of BiO2-x and Bi2O4 are 1.05 and 1.39 V (vs. NHE), respectively. The VB potentials of them were more positive than the standard redox potential of •OH/H2O (+2.27 V vs. NHE),66 indicating •OH can’t be formed on the VBs of the BiO2-x and Bi2O4. Therefore, a possible photocatalytic mechanism of Bi2O4/BiO2-x composite for degradation of RhB and CIP was described in Figure 12. Under UV, visible and NIR lights irradiation, the photogenerated electrons on the CB of BiO2-x transferred to the CB of Bi2O4,


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accelerating the fast separation of photoinduced carriers. The e- on the CB of Bi2O4 could react with O2 to producing •O2-. In the trapping experiments, the •OH free radical has been confirmed to play the minor roles in the photocatalytic activity. The generation of •OH free radical could be attributed to the decompose of H2O2 generated by the reaction of resolved O2 and H+. Meanwhile, the photoinduced holes of Bi2O4 migrated to the VB of BiO2-x. Finally, the •O2-, •OH and h+ would attack the RhB and CIP molecular in aqueous solution, resulting in their decomposition into CO2, H2O and other intermediates. The highly efficient Bi2O4/BiO2-x heterojunction is a promising UV, visible and NIR responsive photocatalyst for environmental modification.

Figure 12. The possible photocatalytic mechanism of Bi2O4/BiO2-x composite for photocatalytic degradation of pollutants under UV, visible and NIR lights irradiation. 4. CONCLUSION The novel core-shell structured Bi2O4/BiO2-x composite was successfully fabricated via a hydrothermal strategy. The UV-Vis-NIR DRS spectra revealed that Bi2O4/BiO2-x composite



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possessed broad spectra absorption from 200-2500 nm. The chemical trapping experiments and ESR measurement demonstrated •O2- and h+ were the dominant active species for RhB and CIP degradation. The much more •O2- was generated in Bi2O4/BiO2-x heterojunction under visible and NIR lights irradiation. The Bi2O4/BiO2-x heterojunction displays satisfactory photocatalytic activity and anti-photocorrosion property for degradation of RhB and CIP as compared to that of Bi2O4 nanorods under UV, visible and NIR lights irradiation. The highly efficient photocatalytic activity of Bi2O4/BiO2-x composite could be ascribed to the fast charge transfer via the heterojunction interface inducing the generation of •O2-. Our work not only provides a promising routine for synthesizing high-efficient broad spectrum light driven photocatalysts, but also a new idea to enhance the anti-photocorrosion activity of photocatalysts. ASSOCIATED CONTENT Supporting Information The synthesized processes of Bi2O4 and BiO2-x; the photolysis of CIP and RhB in aqueous solution under visible-NIR light irradiation; TEM images of recycled Bi2O4/BiO2-x composite; TOC removal of CIP and RhB in aqueous solution under the photocatalytic process in the presence of Bi2O4/BiO2-x composite and the detailed photocatalytic intermediates of RhB and CIP degraded by Bi2O4/BiO2-x composite. AUTHOR INFORMATION Corresponding Author *E-mails: [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This work was supported by National Science Foundation of China (No.51472194), the National Science Foundation of Hubei Province (2016CFA078) and National Program on Key Basic Research Project of China (973 Program) 2013CB632402.



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Two-Dimensional CoreâShell-Structured Bi2O4

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Two-Dimensional CoreâShell-Structured Bi2O4