g-C3N4 heterojunction for efficient photocatalytic degradation of 2,3-dihydroxynaphthalene

Journal Pre-proofs Visible-light responsive Z-scheme Bi@β-Bi2O3/g-C3N4 heterojunction for efficient photocatalytic degradation of 2,3-dihydroxynaphthalene Yunlong Lan, Zesheng Li, Dehao Li, Wenyu Xie, Guangxu Yan, Shaohui Guo PII: DOI: Reference:

S1385-8947(19)33101-8 https://doi.org/10.1016/j.cej.2019.123686 CEJ 123686

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

2 September 2019 16 November 2019 1 December 2019

Please cite this article as: Y. Lan, Z. Li, D. Li, W. Xie, G. Yan, S. Guo, Visible-light responsive Z-scheme Bi@βBi2O3/g-C3N4 heterojunction for efficient photocatalytic degradation of 2,3-dihydroxynaphthalene, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123686

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Visible-light responsive Z-scheme Bi@β-Bi2O3/g-C3N4 heterojunction for efficient photocatalytic degradation of 2,3-dihydroxynaphthalene Yunlong Lan a, b, Zesheng Li a, Dehao Li a,*, Wenyu Xie a, Guangxu Yan b, Shaohui Guo b,** a

Guangdong Provincial Key Laboratory of Petrochemical Pollution Process and

Control, Guangdong University of Petrochemical Technology, Maoming, Guangdong 525000, China b State

Key Laboratory of Heavy Oil Processing, Beijing Key Laboratory of Oil and

Gas Pollution Control, China University of Petroleum, Beijing 102249, China

* Corresponding author. email: [email protected] (D. Li) ** Corresponding author. email: [email protected] (S. Guo)

Abstract In this study, all-solid-state Z-scheme Bi@β-Bi2O3/g-C3N4 heterojunction was successfully constructed by in situ deposition and oxidation, where the mediator bismuth acted as a bridge to shuttle electrons between β-Bi2O3 and g-C3N4. The micromorphology, crystal structure, electronic environment and optical property of Z-scheme catalysts were systematically studied by XRD, XPS, SEM, TEM, UV-vis DRS, etc. techniques. The characterizations confirmed Bi@β-Bi2O3/g-C3N4 with the core-shell structure was successfully fabricated at 230°C. The Z-scheme heterojunction exhibited the superior visible-light degradation capacity for 2,3-dihydroxynaphthalene (2,3-DHN) with a removal ratio of 87.0% after 100 min irradiation. The significantly enhanced photoactivity was attributed to Z-scheme heterojunction facilitating the spatial and temporal separation of photoinduced carriers and maintaining the original strong oxidation and reduction reaction center of correlative component. The possible photodegradation pathway of 2,3-DHN was proposed by the determined degradation byproducts. This study could inspire new ideas for building efficient metal-bridge Z-scheme heterojunctions, and also provided novel insights into the elimination mechanism of polycyclic aromatic hydrocarbons and their derivatives in photocatalysis.

Keywords: Bismuth metal; Bismuth trioxide; Graphitic carbon nitride; Z-scheme heterojunction; Visible-light photocatalysis.

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CONTENTS Abstract ...................................................................................................................1 1. Introduction .........................................................................................................3 2. Experimental .......................................................................................................5 2.1. Synthesis of photocatalysts .......................................................................5 2.2. Characterization methods..........................................................................6 2.3. Photocatalytic degradation measurement..................................................7 3. Results and discussion.........................................................................................8 3.1. Crystalline and chemical composition ......................................................8 3.2. Morphological structure ..........................................................................11 3.3. Optical absorption and photocatalytic performance ...............................12 3.4. Formation and photodegradation mechanism .........................................16 4. Conclusions .......................................................................................................21 Acknowledgments.................................................................................................22 References .............................................................................................................22

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1. Introduction Industrial development and accelerated urbanization have caused global pollution of toxic chemicals in the environment. Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous contaminants in nature and have potential teratogenicity, mutagenicity and carcinogenicity to humans [1,2]. PAHs can come from the burning of gas fuel, coke production, waste incineration, oil refinery process and forest fires [3–5]. 2,3-Dihydroxynaphthalene (2,3-DHN), as a derivative of PAHs, can accumulate in the food chain due to its high stability and hydrophobicity, threatening environmental safety and human health. Hence, it is urgent to effectively remove 2,3-DHN from the environment. Nevertheless, traditional degradation techniques for PAHs, such as physical adsorption and biological treatment, are limited by processing efficiency, operating costs and secondary pollution [6,7]. Photodegradation has recently become the most attractive water remediation strategy, with a bright future for PAHs degradation [8–10]. However, photocatalytic efficiency is still a constraint on practical applications. Therefore, semiconductors with superior photocatalytic activity are required for the degradation of 2,3-DHN. Graphitic carbon nitride (g-C3N4) as a layered polymer possesses high physicochemical stability and unique electronic structures [11–13]. g-C3N4 has been widely used for photodegradation, which is an ideal support due to the suitable band structure [14,15]. Bi2O3 is a single bismuth-based oxide semiconductor that can be excited by visible light, and wide attention has been attracted due to its matchless properties [16,17], such as non-toxicity, stability and low cost. Bi2O3 has five kinds of crystal forms, which are monoclinic α-Bi2O3, tetragonal β-Bi2O3, body-centered cubic γ-Bi2O3, cubic δ-Bi2O3 and triclinic ω-Bi2O3 [18,19]. Compared with other polymorphs, β-Bi2O3 performs the best for the visible-light harvesting due to its 3

narrowest bandgap and shows the prospect for removing contaminants from water [20]. However, the quantum efficiency of β-Bi2O3 is unsatisfactory because of the easily recombined photogenerated carriers. And the large crystal size also limits its interfacial reaction rate [21,22]. Therefore, it is still essential to improve the photocatalytic activity of bulk β-Bi2O3 through a rational nanomaterial configuration system. Z-scheme heterojunction system has received much attention among various strategies due to the remarkable advantages in enhancing catalytic activity [23–25]. Z-scheme heterostructures mainly include direct “photocatalyst I - photocatalyst II” and all-solid-state “photocatalyst I - conductor - photocatalyst II” [26]. The artificial all-solid-state Z-scheme is generally made up of two semiconductors with suitable band structures and an electron conductor [27–29]. Due to the fine conductivity of metal, the electron mediator can be Au, Ag, Pt and Bi as a transfer bridge for photogenerated

electrons

between

two

semiconductors

[28,30].

These

multi-semiconductor Z-scheme solutions enable spatial and temporal separation of photogenerated electrons and holes, and the two reaction centers for oxidation and reduction remain separately at the original site of corresponding component [31,32]. Z-scheme heterojunctions essentially preserve the conduction band minimum of one component and the valence band maximum of the other, overcoming the shortcomings of individual catalysts [33]. Hence, Z-scheme is highly advantageous in photodegradation reactions due to the excellent catalytic activity and stability. Deng et al. [34] prepared Z-scheme black BiOCl/Bi2O3 with Bi as the mediator for visible-light degradation of 2-nitrophenol. Gong et al. [35] fabricated Z-scheme Ag2CrO4/Ag/g-C3N4 for 2,4-dichlorophenol degradation. Therefore, it is feasible to enhance the photocatalytic activity of β-Bi2O3 by constructing a Z-scheme

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heterojunction system. So far, reports on photodegradation of 2,3-DHN by Z-scheme Bi@β-Bi2O3/g-C3N4 heterojunction are quietly scarce. Herein, all-solid-state Z-scheme Bi@β-Bi2O3/g-C3N4 heterojunction was successfully constructed by in situ deposition and oxidation, where the mediator metal Bi acted as an electron transport channel between β-Bi2O3 and g-C3N4. The micromorphology, crystal structure, electronic environment and optical property of Z-scheme catalysts were systematically studied. Bi@β-Bi2O3/g-C3N4 heterojunction exhibited outstanding photocatalytic degradation capacity for 2,3-DHN under visible light. The structural evolution of multi-component heterojunction was explored and the Z-scheme mechanism was elucidated for the enhancement of photocatalytic activity. The distinctive Z-scheme system considerably overcame the defects of single-component β-Bi2O3 to achieve efficient separation of photogenerated carriers and strong redox capability. Moreover, a possible degradation mechanism for 2,3-DHN was proposed by identifying the photodegradation intermediates.

2. Experimental 2.1. Synthesis of photocatalysts In the typical synthesis, 0.485 g Bi(NO3)3•5H2O was added to 7 mL of deionized water. 3 mL of 4 mol/L HNO3 solution was added dropwise and stirred until a clear solution was formed. After that, the clear solution of 55 mL ethylene glycol (EG) containing 0.6 g of polyvinylpyrrolidone (PVP, MW 24000) was put in the above solution. 0.60 g of g-C3N4 was then added and ultrasonically dispersed for 30 min. The mixture was transferred to a 100 mL autoclave and hydrothermally treated at 160°C for 12 h. The collected product was washed several times with distilled water and dried at 60°C under vacuum for 12 h to obtain the precursor Bi/g-C3N4 (designated as Bi/CN). Then 0.1 g of Bi/CN was placed in a tube furnace and heated 5

respectively at 200°C, 230°C, 260°C and 290°C for 1 h in air with a heating rate of 10 °C/min. After natural cooling, the obtained products were named as FU200, FU230, FU260 and FU290, respectively. According to the following characterizations, FU230 was identified as Bi@β-Bi2O3/g-C3N4. β-Bi2O3 was prepared by a facile calcination method for comparison. 1.940 g of Bi(NO3)3•5H2O was dissolved in 20 mL of 1 mol/L HNO3 solution and stirred until clear. Then 0.804 g of sodium oxalate was added and stirred for 1 h. The filtered product was washed with distilled water and ethanol and then dried at 60°C under vacuum for 12 h. Then, 0.1 g of the sample was placed in a tube furnace, heated to 270°C at 10 °C/min, and kept for 4 h in air. After natural cooling, β-Bi2O3 was obtained and labeled as BiO. g-C3N4 was prepared and denoted as CN [36] (see the supporting information). 2.2. Characterization methods The phase composition of the photocatalysts was characterized by an X-ray diffractometer (XRD, Rigaku Ultima IV-285E, Cu Kα, λ = 0.15406 nm, 2θ = 10° 70°). X-ray photoelectron spectroscopy (XPS) analysis was carried out to investigate the chemical compositions (Thermo Fisher Escalab 250Xi). Fourier transform infrared spectroscopy (FT-IR) was conducted on a Thermo Fisher Nicolet 460 to determine the chemical functional groups. The morphological structure was studied using a scanning electron microscope (SEM, Zeiss SUPRA 40) and transmission electron microscope (TEM, FEI Tecnai G20). The Brunauer-Emmett-Teller (BET) specific surface areas of the catalysts were measured using Micromeritics ASAP 2020 with all samples degassed at 180°C before the analysis. The ultraviolet-visible diffuse reflectance spectra (UV-vis DRS) were obtained by a scan UV-vis spectrophotometer (Shimadzu UV-2550). The total organic carbon (TOC) concentration was measured 6

on

an

automated

analyzer

(Shimadzu

TOC-V

CPH).

Electrochemical

characterizations were conducted on a CHI660D workstation with a three-electrode cell, including a saturated calomel reference electrode, a Pt counter electrode and a working electrode. The electron spin resonance (ESR) signals of radicals spin-trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were detected on a Bruker A300 spectrometer. 2.3. Photocatalytic degradation measurement The photocatalytic activity of the prepared samples was evaluated by photodegradation of 2,3-DHN under visible light. The degradation experiments were performed under a 300 W Xe lamp combined with a 420 nm cutoff filter which provided the visible light. All experiments were performed under the same initial conditions. 0.0400 g of the photocatalyst was mixed with 200 mL of 20 mg/L 2,3-DHN solution under constant magnetic stirring. Before the irradiation, the suspension was stirred in dark for 30 min to reach adsorption equilibrium of the system. 5 mL of the suspension was taken at predetermined intervals during the photocatalytic process. After the suspension was filtered, the concentration of 2,3-DHN was analyzed at 227 nm by a UV-vis spectrophotometer (Shimadzu UV-2550). The

degradation

intermediates

of

2,3-DHN

were

analyzed

by

gas

chromatography-mass spectrometer (GC-MS) system (Shimadzu GCMS-QP2010) equipped with a SH-Rxi-5Sil column (30.0 m × 0.25 mm × 0.25 μm). When the degradation reaction of 2,3-DHN by FU230 reached 20 min and 80 min, 50 mL of the sample was taken out and transferred to a separatory funnel. Then 15 mL of trichloromethane was put in the mixture. The thoroughly shaken mixture was separated after standing. Anhydrous sodium sulfate was added to the extracted phase 7

to remove the water contained. The approach for GC-MS was as follows: GC was on a scan mode and injected in a split mode. The inlet temperature was set at 250.0°C. The oven temperature was initially 60.0°C and held for 2 min, and then it raised up to 290.0°C at a rate of 20.00 °C/min and held for 2 min.

3. Results and discussion 3.1. Crystalline and chemical composition The composition and phase structure of the catalysts were investigated by XRD (Fig. 1). There were two characteristic peaks of 27.4° and 13.1° in CN, which represented the typical diffraction peaks of g-C3N4 (PDF #87-1526). The strong peak at 27.4° was indexed as (002) plane of interplanar stacked graphitic sheets and the weak peak at 13.1° corresponded to (100) plane of the in-plane repeating motif. The peaks of the precursor Bi/CN at 27.2°, 38.0°, 39.6° and 48.7° were put down to the (012), (104), (110) and (202) facets of Bi-metal (PDF #85-1329), indicating hydrothermally reduced Bi-metal was anchored on the g-C3N4 support under the effect of the reducing agent EG and the capping reagent PVP [37]. The XRD pattern of FU200 was identical to Bi/CN, implying the phase structure of the precursor hardly changed after calcination at 200°C. As the calcination temperature rose above 230°C, the diffraction peaks of β-Bi2O3 in the patterns of FU230, FU260 and FU290 were enhanced increasingly, where the peaks of 27.9°, 31.7°, 32.7°, 46.2° and 46.9° were indexed to (201), (002), (220), (222) and (400) facets of β-Bi2O3 (PDF #78-1793). Meanwhile, the peak intensities of Bi at 27.2°, 38.0° and 39.6° decreased with increasing temperature. It indicated when the temperature was above 230°C, the outer layer of Bi-metal in Bi/CN was gradually oxidized by O2 in the air to β-Bi2O3, forming heterogeneous catalysts. The peak of g-C3N4 at 27.4° was obscured in the patterns of FU230, FU260 and FU290 due to the shadowing effect of the strong (201) 8

peak of β-Bi2O3. There were distinct peaks of Bi and β-Bi2O3 in FU230, revealing FU230 was composed of Bi, β-Bi2O3 and g-C3N4. And no obvious Bi peak in FU290 suggested that bismuth in the precursor Bi/CN was fully converted into β-Bi2O3 and the binary heterojunction of β-Bi2O3/g-C3N4 was fabricated at 290°C. Therefore, the calcination temperature was a critical factor for the composition of prepared catalysts. In addition, the peaks in BiO were consistent with β-Bi2O3 (PDF #78-1793), suggesting pure β-Bi2O3 was favorably synthesized by a facile calcination method. XPS analysis was performed for elemental compositions of catalysts (Fig. 2). The survey spectra demonstrated evident peaks of C, N, O and Bi in FU200, FU230, FU260 and FU290, and revealed the presence of C and N in g-C3N4. The low-intensity O peak in CN corresponded to adsorbed H2O or O2. The peak around 287.8 eV in the spectra of C 1s was assigned to sp2 C in the N-C=N groups of the aromatic ring, and the other peak near 284.7 eV corresponded to the adventitious carbon. The binding energy of sp2 C in the composites was shifted to a lower value compared with the sp2 C of g-C3N4 at 288.1 eV, implying the increased electron density of C atoms for g-C3N4 component. In N 1s spectra, the peaks around 398.3 eV, 399.7 eV and 400.8 eV were ascribed to sp2 N in triazine rings (C=N-C), tertiary nitrogen in N-(C)3 groups and sp2 N in amino groups (C-N-H) [38], respectively. Similar to the results of C 1s, the lower binding energy of sp2 N in the composites compared to g-C3N4 indicated electrons was acquired by the sp2-hybridized N, revealing electrons in the composites migrated to the heterocycles of g-C3N4 [39]. The peaks near 164.0 eV and 158.7 eV in Bi 4f spectra were attributed to Bi 4f5/2 and Bi 4f7/2 of β-Bi2O3, while two small peaks around 161.9 eV and 156.6 eV represented Bi 4f5/2 and Bi 4f7/2 of Bi-metal. As the temperature increased from 200°C to 290°C, the peak intensity of Bi was weakened by degrees until it was not detected in FU290. It

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revealed Bi in the precursor Bi/CN was etched by O2 from the surface layer to inner core with the increased calcination temperature. For Bi 4f in FU200, the peak of β-Bi2O3 was more pronounced than Bi-metal because a thin Bi2O3 amorphous film was coated on the Bi surface in FU200 [40,41]. Two peaks of O 1s around 529.3 eV and 531.3 eV were originated from the crystal lattice oxygen (Bi-O) of β-Bi2O3 and O2 (or H2O) adsorbed on the surface. Further comparison in the Bi 4f and O 1s spectra indicated that the Bi and O binding energies of β-Bi2O3 in FU230 were reduced compared with FU260 and FU290, revealing the increased electron density of β-Bi2O3 in FU230. And the binding energies of Bi-metal in FU230 were shifted to higher values relative to FU200 that consisted mainly of Bi/g-C3N4, confirming bismuth acted as an electron donor in the composite and electrons migrated from Bi-metal to β-Bi2O3. Clearly, FU230 with a high bismuth content was markedly affected by Bi-metal, suggesting metallic bismuth played a critical role in the chemical microenvironment of composite components. XPS analysis demonstrated calcination temperature greatly affected the conversion of Bi to β-Bi2O3 and the microenvironment for the composites, and it also confirmed FU230 was constructed with three components of Bi, β-Bi2O3 and g-C3N4. The chemical structures of catalysts were further explored by FT-IR (Fig. S1). For CN, the absorption peaks between 3500 cm-1 and 3000 cm-1 were ascribed to the N-H stretching vibration modes, and the peaks in the range of 1700 cm-1 - 1180 cm-1 belonged to the stretching modes of C-N and C=N. The band at 1636 cm-1 was attributed to the C=N stretching vibration, and the peaks at 1571 cm-1, 1544 cm-1 and 1463 cm-1 were due to stretching modes of tri-s-triazine heterocycles. The absorption peak at 1406 cm-1 was assigned to the C-N stretching of tertiary bridging nitrogen in the tri-s-triazine, whereas those at 1322 cm-1, 1240 cm-1 and 1204 cm-1 corresponded

10

to the C-N stretching of secondary bridging nitrogen [42]. And the absorption bands at 892 cm-1 and 813 cm-1 were respectively assigned to the deformation of cross-linked heptazine and the bending modes of tri-s-triazine heterocycles. For BiO, the peaks at 531 cm-1 and 592 cm-1 were associated with the Bi-O vibration of BiO6 units, while those at 642 cm-1 and 846 cm-1 belonged to the Bi-O modes of BiO3 units. And the band at 1391 cm-1 was assigned to the Bi-O stretching modes. As observed from FT-IR spectra, there were distinct absorption peaks of g-C3N4 in FU230, but no obvious peaks of β-Bi2O3. This could be due to that the weak vibration peaks of β-Bi2O3 in the composite were masked by the intense infrared response of g-C3N4. 3.2. Morphological structure The morphology and surface structure of the samples were investigated by SEM (Fig. S2). g-C3N4 presented a lamellar structure and β-Bi2O3 was assembled from nanosheets with a thickness of less than 100 nm. The Bi-metal spheres, having a diameter of about 150 nm, were tightly anchored on the surface of g-C3N4 in FU200. Relative to the smooth Bi-spheres in FU200, small sheets began to cover the surface of Bi in FU230, indicating the surface layer of Bi-metal was transformed into flaky β-Bi2O3 when the temperature came to 230°C. The nano-sheets on the outer layer of Bi spheres in FU260 became distinct and stereoscopic, suggesting Bi-metal spheres on g-C3N4 were deeply carved towards the core by O2 at 260°C. The spherical β-Bi2O3 in FU290 was maintained at a diameter of around 160 nm. Therefore, for the composites, calcination temperature significantly influenced the surface morphology of spheres on the g-C3N4 support. Besides, the BET specific surface areas of CN, FU200, FU230, FU260 and FU290 were respectively 12.0 m2/g, 26.8 m2/g, 27.3 m2/g, 28.9 m2/g and 29.6 m2/g (Fig. S3). The largely increased specific surface areas of composites benefited from the microspheres loaded on g-C3N4, which favored the 11

employment of active sites and the mass transfer of pollutants during the photodegradation. TEM images of the samples were further investigated for the microstructure (Fig. 3). CN was provided with a typical irregular layered structure and Bi-spheres in FU200 were tightly bonded to g-C3N4. It was clearly observed that the composite FU230 was a typical Bi@β-Bi2O3 (core@shell) structure supported on g-C3N4, which confirmed the surface layer of Bi in the precursor Bi/g-C3N4 was oxidized at 230°C to bismuth trioxide as the outer shell of Bi core. The β-Bi2O3 shell in FU260 was obviously thicker than FU230, while spheres in FU290 was monolithic without core-shell structures. It was confirmed Bi-metal was continuously oxidized from outside to inside at increasing temperature, consistent with the SEM analysis. There was a clear interface between bismuth core and bismuth trioxide shell in the HRTEM image of FU230. The lattice spacing of 0.328 nm and 0.319 nm on both sides of the interface corresponded to (012) facet of Bi (PDF #85-1329) and (201) facet of β-Bi2O3 (PDF #78-1793), respectively. Bi-metal in FU230 acted as a “linker” to intimately bind the two semiconductor components of β-Bi2O3 and g-C3N4, contributing to the rapid transfer of photoinduced electrons and the sufficient separation of electron-hole pairs. Combined with XRD and TEM results, FU230 was identified as a Bi@β-Bi2O3/g-C3N4 heterojunction with a core-shell structure on the support. 3.3. Optical absorption and photocatalytic performance The band structure of catalysts determined the photocatalytic activity, and UV-vis DRS of the composites were shown in Fig. 4. FU230 owned the best absorption capability in the visible light region (λ ≥ 420 nm) among the calcined composites. The broad absorption peak of FU200 in the visible region was attributed 12

to the surface plasmon resonance (SPR) effect of Bi-metal [22,41]. The similar absorption peaks between FU260 and FU290 implied the comparable heterojunction compositions of these two. And it could also be speculated the degradation capability of FU260 was close to the binary heterojunction FU290 which was made up of β-Bi2O3/g-C3N4. The analogous adsorption structure of FU260 and FU290 in the range of ≥ 420 nm could be ascribed to that the small amount of Bi-metal remaining in FU260 exerted a limited impact on its optical property, where most of the bismuth in FU260 was converted into β-Bi2O3 based on XRD analysis. Thus, the optical performance of FU260 resembled the binary β-Bi2O3/g-C3N4. Metallic bismuth in the composites influenced not only the chemical environment of components but also the optical property, which contributed to the marvelous photo-degradability of Z-scheme Bi@β-Bi2O3/g-C3N4 that contained abundant Bi-metal. The bandgap (Eg) of a semiconductor was evaluated using the equation αhν = k(hν - Eg)n/2, where α, h, ν and k were the absorption coefficient, Planck constant, light frequency and a constant, respectively. The value of n was determined by the optical transition type, where n of g-C3N4 was 4 for the indirect absorption and n of Bi2O3 was 1 for the direct absorption [18,36]. The experimental bandgap of the sample was estimated by plotting (αhν)2/n versus hν. The bandgap values of CN and BiO were respectively 2.55 eV and 2.40 eV (Fig. S4). The band positions of CN and BiO were calculated by the empirical equation [38]: EVB = χ - Ee + 0.5Eg, where EVB was the valence band (VB) potential, χ was the geometric mean of the electronegativity of constituent atoms, and Ee was the energy of free electrons on hydrogen scale (about 4.5 eV). The χ values of CN and BiO were 4.73 eV and 5.95 eV, and the EVB values were respectively calculated to be 1.51 eV and 2.65 eV. The conduction band (CB) potentials of CN and BiO were -1.04 eV and 0.25 eV based on the formula ECB = EVB - Eg. The results 13

demonstrated CN was endowed with a fairly negative CB potential while BiO possessed a considerably positive VB potential, implying each of CN and BiO was correspondingly with sufficient reduction or oxidation ability. The photocatalytic performance of composites was evaluated by the degradation efficiency of 2,3-DHN under visible light (Fig. 5a). Relative to CN, the poorer degradation performance of BiO was due to the defect of the easily recombined photogenerated carriers in β-Bi2O3 [21,22]. FU200 was composed of Bi/g-C3N4 and exhibited better degradation activity compared to CN as a result of the SPR effect of Bi in the composite [43–46]. FU230 performed the best for 2,3-DHN degradation among all catalysts, revealing Bi@β-Bi2O3/g-C3N4 heterojunction with the core-shell structure owned unparalleled advantages in photodegradation reactions. The degradation efficiency of FU260 was close to but better than FU290, confirming bismuth residue in FU260, as the metal bridge connecting g-C3N4 and β-Bi2O3, was significant for enhancing the photocatalytic activity of the composite. The degradation efficiency (E) was calculated by the formula E = (1 - Ct/C0) × 100%, where Ct and C0 were the concentration of the solution at the illumination time t min and 0 min. The degradation efficiencies of BiO, CN, FU200, FU230, FU260 and FU290 for 2,3-DHN were respectively 45.3%, 58.7%, 65.9%, 87.0%, 76.7% and 72.8% after 100 min photoreaction. The removal efficiency of FU230 was increased by 92.0% and 48.3% compared with BiO and CN. Thus, as the Bi@β-Bi2O3/g-C3N4 heterojunction, FU230 had the best photocatalytic activity for 2,3-DHN under visible light. In order to quantitatively investigate the photodegradation reactivity of catalysts, the removal rates were obtained by the following first-order kinetic equation: ln(C0/Ct) = k1 t, where C0, Ct and k1 denoted respectively the initial 2,3-DHN concentration (mg/L) after adsorption in the dark, the concentration of 2,3-DHN

14

solution at the irradiation time t h, and the first-order reaction rate constant (h-1). The fitting results demonstrated the photodegradation of 2,3-DHN well conformed to the first-order kinetic model (Fig. 5b). And the rate constants (k1) of BiO, CN, FU200, FU230, FU260 and FU290 were 0.348 h-1, 0.503 h-1, 0.614 h-1, 1.190 h-1, 0.823 h-1 and 0.728 h-1, respectively. The k1 constant of FU230 was 3.42 and 2.37 times that of BiO and CN, revealing the remarkable photodegradation rate of Bi@β-Bi2O3/g-C3N4 heterojunction. Total organic carbon (TOC) could be the mineralization index for the degradation system. And the TOC removal efficiencies of BiO, CN and FU230 were respectively 19.0%, 37.9% and 75.6% after 100 min of visible-light irradiation (Fig. S5). It indicated that FU230 achieved the best mineralization effect on 2,3-DHN compared to individual catalysts, which was consistent with the degradation efficiency results. TOC measurement confirmed that Bi@β-Bi2O3/g-C3N4 basically avoided secondary pollution and effectively mineralized organic pollutants, including 2,3-DHN and its degradation intermediates, which particularly mattered for the actual practice. As shown in Table S1, the catalyst loading of Bi@β-Bi2O3/g-C3N4 was much lower compared with previously reported photocatalysts for degrading DHN, and the composite in this work still exhibited an excellent removal ratio during the reaction; the maximum kinetic constant of Z-scheme catalyst revealed the advantageous reaction efficiency. Moreover, relative to these reported catalysts, the facile preparation of Bi@β-Bi2O3/g-C3N4 and no additional oxidants (such as H2O2) needed for degradation demonstrated its great potential for practical applications and the exceptional performance advantage. The stability of Z-scheme Bi@β-Bi2O3/g-C3N4 was crucial for actual applications, so the recyclability of FU230 was evaluated in four successive cycling-experiments under visible light. The conditions of recycle-experiments were

15

the same as those of photodegradation, except that FU230 was reused after each cycle. As shown in Fig. S6a, the 2,3-DHN oxidation efficiency of FU230 decreased gently with the increased reuse-cycles, and it stayed at 81.9% after 4 times recycling. This could be due to that FU230 with the Bi2O3 shell prevented its Bi-metal core from being corroded during the reaction, thereby exhibiting high degradation stability. To further determine the reusability and stability, XRD of the used FU230 was measured after the fourth cycle. No significant changes were detected compared to the original FU230 from the XRD pattern (Fig. S6b), suggesting its attractive structural stability. Hence, Bi@β-Bi2O3/g-C3N4 was provided with high-quality stability for efficient elimination of 2,3-DHN. 3.4. Formation and photodegradation mechanism The structure evolution of Bi@β-Bi2O3/g-C3N4 heterojunction with the core-shell structure was proposed in Fig. 6 based on above characterizations. g-C3N4 and PVP were put in the ethylene glycol (EG) solution containing Bi3+. The mixture was transferred to an autoclave and hydrothermally treated at 160°C for 12 h. Ethylene glycol with a high boiling point could act as the complexing agent for metal ions so that the Bi3+-EG complexing polymer was formed at the initial stage of the reaction to resist the rapid hydrolysis of Bi3+ [47,48]. During the hydrothermal process, Bi-metal was reduced in situ to the g-C3N4 support under the joint action of EG (as a metal-ion reducing agent) and PVP (as a capping agent) [37], which contributed to the intimate interfacial contact between Bi-metal and g-C3N4. The resulting precursor Bi/g-C3N4 was then placed in a tube furnace and calcined in air at a suitable temperature. The Bi nanospheres supported on g-C3N4, as the self-sacrificial templates, were oxidized in situ to β-Bi2O3 by reacting with O2. The composition and structure of the composites depended on the calcination temperature. At last, Bi@β-Bi2O3/g-C3N4 heterojunction 16

with outstanding photocatalytic activity was successfully constructed at a temperature of 230°C. The separation efficiency of photogenerated electron-hole pairs was investigated by electrochemical experiments (Fig. 7). The photocurrent density of FU230 was about 1.5 times and 2.0 times that of CN and BiO when visible light was irradiated, suggesting more charge carriers were photogenerated by FU230. As the Bi-bridge Z-scheme heterojunction, Bi@β-Bi2O3/g-C3N4 produced abundant charge carriers and efficiently inhibited their recombination, which was particularly advantageous for enhancing the reaction activity of photodegradation. Electrochemical impedance spectroscopy (EIS) was employed to further determine the transfer efficiency of photoinduced charge carriers. As shown in the EIS Nyquist plots (Fig. 7b), the lowest arc radius of FU230 represented the smallest charge transfer resistance and the most efficient separation of electron-hole pairs. Thus, Z-scheme Bi@β-Bi2O3/g-C3N4 heterojunction was highly favorable for effective separation of charge carriers to enhance the photocatalytic activity. To clarify the radical species responsible for the photodegradation of 2,3-DHN, ESR spin-trapping over BiO, CN and FU230 for DMPO-O2•- and DMPO-•OH were conducted under visible light (Fig. 8) [31,49]. With the visible light illumination, both FU230 and CN exhibited distinct DMPO-O2•- characteristic peaks, while strong peaks of FU230 and BiO were observed in the DMPO-•OH spectrum. This result indicated that for individual catalysts, O2•- was the main reactive species of g-C3N4 in the photodegradation reaction, while β-Bi2O3 degraded contaminants mainly through •OH radicals, consistent with the calculated band structures. Having an adequate negative CB potential, g-C3N4 reduced dissolved oxygen to O2•- radicals in solution. The fairly positive VB potential of β-Bi2O3 led to the generation of •OH radicals by oxidation.

17

Relative to BiO and CN, the more prominent peaks of radical species in FU230 suggested that Bi@β-Bi2O3/g-C3N4 thoroughly exploited the advantages of each component, promoted the separation of photoexcited carriers and enhanced the oxidative degradation ability. Reactive species capture experiments were performed to further ascertain the role of radicals during the photodegradation of 2,3-DHN by FU230 (Fig. S7). Sodium oxalate (Na2C2O4) and tert-butyl alcohol (TBA) were applied as corresponding scavengers for the quenching of photoinduced holes and hydroxyl radicals, while the effect of superoxide radicals on photodegradation was examined by degassing with N2 to prevent the formation of O2•- from oxygen. Obviously, the removal efficiency of 2,3-DHN was considerably inhibited under N2 atmosphere, and the addition of TBA also noticeably reduced the degradation efficiency. This indicated both O2•- and •OH radicals were of great significance for the photocatalytic system. Moreover, the slight decrease in the degradation ratio after adding Na2C2O4 suggested that h+ made a limited effect on the removal of 2,3-DHN. Therefore, Bi@β-Bi2O3/g-C3N4 heterojunction produced sufficient O2•- and •OH radicals during photodegradation for the strong photooxidation capacity. The possible pathway for photodegradation of 2,3-DHN by FU230 was illustrated in Fig. 9, and the photodegradation intermediates of 2,3-DHN were analyzed by GC-MS (Fig. S8). As the reaction time prolonged, 2,3-DHN was continuously photodegradated with the decrease in the intensity of its characteristic peak (peak * in Fig. S8). The intermediates at 20 min of the degradation reaction were listed in Fig. S9 and most of the byproducts were not detected at 80 min. The •OH addition reaction was carried out at position 1 of 2,3-DHN for the photodegradation (Step 1 in Fig. 9), and further oxidation led to the formation of naphthalene-1,2-dione (Step 2) [50]. 2H-chromen-2-one formed by the ketonization reaction could generate

18

phenyl 3-methylbut-2-enoate through the aromatic ring cleavage under the action of photoactivated

alkyl

radicals

(Steps

3

and

4)

[51].

1,4-Dihydronaphthalene-1,2,3,4-tetraol was produced by the addition of •OH radicals at the 1st and 4th positions of 2,3-DHN, and then naphthalene-1,4-dione was generated by the dehydration reaction (Steps 5 and 6) [52]. After the •OH attack on the benzoquinone ring, the resulted phthalic acid continued the alkylation reaction, producing dibutyl phthalate under photo-activated alkanes (Steps 7 and 8) [6,50]. As a result of the continuous attack of free radicals, the aromatic rings of intermediates were broken to form ester compounds, such as pentadecan-4-yl octanoate, (Z)-hex-3-en-1-yl 3-methylbutanoate, tridecan-4-yl 2-methoxyacetate and allyl decanoate (Step 9). Chain-alkanes and ethers were then obtained by further oxidation and cleavage (Step 10) [36]. In the later degradation stage, the intermediates of 2,3-DHN could be mineralized into small organic molecules or CO2 by redox reactions [50,51]. The photodegradation mechanism of Z-scheme heterojunction of Bi-bridge Bi@β-Bi2O3/g-C3N4 was presented in Fig. 10 based on the characterizations and analyses. All-solid-state Bi@β-Bi2O3/g-C3N4 was constructed of g-C3N4 support and β-Bi2O3, wherein the Bi-metal core acted as a linker to bond together the two components closely. Under visible light irradiation, both β-Bi2O3 and g-C3N4 components were photo-activated due to the appropriate bandgaps, and electrons (e-) were then excited from the valence band (VB) to the respective conduction band (CB) while leaving an equal amount of holes (h+) on their VB (Eqs. 1 and 2) [53–55]. The photogenerated electrons on the CB of β-Bi2O3 could transfer to the VB of g-C3N4 through the tightly coupled Bi-metal bridge, that was, the electrons of β-Bi2O3 annihilated with the holes of g-C3N4 [56,57]. As a result, Bi@β-Bi2O3/g-C3N4

19

exhibited

extraordinary

photodegradation

capability

compared

with

binary

β-Bi2O3/g-C3N4 and single catalysts. This greatly enhanced activity was thanks to its unique Z-scheme structure that efficiently separated charge carriers and sufficiently inhibited

their

recombination.

Especially,

Z-scheme

Bi@β-Bi2O3/g-C3N4

heterojunction facilitated the spatial and temporal separation of respective electron-hole pairs of the two components and maintained the original oxidation and reduction center of correlative component [58,59]. Strong reductive electrons on the CB of g-C3N4 in Bi@β-Bi2O3/g-C3N4 reduced dissolved O2 to O2•- radicals due to the more negative potential than E(O2/O2•-) (Eqs. 3). O2•- radicals could be further evolved into H2O2, •HO2 and •OH through chain reactions of radicals (Eqs. 4 - 9) [36,53]. Meantime, strong oxidizing holes on the VB of β-Bi2O3 generated •OH radicals by oxidizing H2O and OH- (Eqs. 10 and 11) [29,32]. Eventually, 2,3-DHN was effectively removed through radicals and oxidants produced by visible-light responsive Z-scheme Bi@β-Bi2O3/g-C3N4 (Eqs. 12). β-Bi2O3 + hν → h+ + e-

(1)

g-C3N4 + hν → h+ + e-

(2)

O2 + e- → O2•-

(3)

O2•- + e- + 2H+ → H2O2

(4)

O2 + 2H+ + 2e-→ H2O2

(5)

O2•- + H+ → HO2•

(6)

2HO2• → H2O2 + O2

(7)

O2•- + H2O2 → •OH + OH- + O2

(8)

H2O2 + e- → •OH + OH-

(9)

h+ + OH- → •OH

(10)

H2O + h+ → •OH + H+

(11)

20

2,3-DHN + O2•- (or •OH, •HO2, H2O2, h+) → products

(12)

4. Conclusions In summary, all-solid-state Z-scheme Bi@β-Bi2O3/g-C3N4 heterojunction was successfully fabricated by in situ deposition and oxidation, where Bi-metal acted as a bridge to shuttle electrons between β-Bi2O3 and g-C3N4. XRD, XPS and TEM analyses confirmed Bi@β-Bi2O3/g-C3N4 with the core-shell structure was successfully constructed at 230°C, where the calcination temperature was a determinant factor affecting the compositions and morphologies of the composites. The Z-scheme heterojunction exhibited the remarkable visible-light degradation activity for 2,3-DHN with a removal ratio of 87.0% after 100 min photoreaction. The significantly enhanced photoactivity of Bi@β-Bi2O3/g-C3N4 was attributed to Z-scheme facilitating the spatial and temporal separation of photogenerated carriers and maintaining the original strong oxidation and reduction capability. Sufficient O2•and •OH radicals were produced by Bi@β-Bi2O3/g-C3N4 for the outstanding photooxidation capacity, owing to the preserved CB minimum and VB maximum of the correlated component. The possible degradation pathway of 2,3-DHN was proposed through the determined photodegradation intermediates. This study presented an inspiring method for building efficient metal-bridge Z-scheme heterojunctions and revealed novel insights for PAHs degradation mechanism in photocatalysis.

Acknowledgments This work was supported by the National Natural Science Foundation of China

21

(No. 21777034).

Declaration of interest statement 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.

References [1]

V. Bansal, K.-H. Kim, Review of PAH contamination in food products and their health hazards, Environ. Int. 84 (2015) 26–38. doi:https://doi.org/10.1016/j.envint.2015.06.016.

[2]

N. An, S. Liu, Y. Yin, F. Cheng, S. Dong, X. Wu, Spatial distribution and sources of polycyclic aromatic hydrocarbons (PAHs) in the reservoir sediments after impoundment of Manwan Dam in the middle of Lancang River, China, Ecotoxicology. 25 (2016) 1072–1081. doi:10.1007/s10646-016-1663-5.

[3]

W. Qi, X. An, F. Zhang, H. Liu, J. Qu, Facet-dependent intermediate formation and reaction mechanism of photocatalytic removing hydrophobic anthracene under simulated solar irradiation, Appl. Catal. B Environ. 206 (2017) 194–202. doi:https://doi.org/10.1016/j.apcatb.2017.01.018.

[4]

J. Han, Y. Liang, B. Zhao, Y. Wang, F. Xing, L. Qin, Polycyclic aromatic hydrocarbon (PAHs) geographical distribution in China and their source, risk

22

assessment analysis, Environ. Pollut. 251 (2019) 312–327. doi:https://doi.org/10.1016/j.envpol.2019.05.022. [5]

C. Zhan, D. Wan, Y. Han, J. Zhang, Historical variation of black carbon and PAHs over the last ~200 years in central North China: Evidence from lake sediment records, Sci. Total Environ. 690 (2019) 891–899. doi:https://doi.org/10.1016/j.scitotenv.2019.07.008.

[6]

M. Wasiewicz, A.G. Chmielewski, N. Getoff, Radiation-induced degradation of aqueous 2,3-dihydroxynaphthalene, Radiat. Phys. Chem. 75 (2006) 201–209. doi:10.1016/j.radphyschem.2005.08.015.

[7]

R.E. Parales, J.D. Haddock, Biocatalytic degradation of pollutants, Curr. Opin. Biotechnol. 15 (2004) 374–379. doi:https://doi.org/10.1016/j.copbio.2004.06.003.

[8]

D. Chen, B. Li, Q. Pu, X. Chen, G. Wen, Z. Li, Preparation of Ag-AgVO3/g-C3N4 composite photo-catalyst and degradation characteristics of antibiotics, J. Hazard. Mater. 373 (2019) 303–312. doi:https://doi.org/10.1016/j.jhazmat.2019.03.090.

[9]

Y. Sang, X. Cao, G. Dai, L. Wang, Y. Peng, B. Geng, Facile one-pot synthesis of novel hierarchical Bi2O3/Bi2S3 nanoflower photocatalyst with intrinsic p-n junction for efficient photocatalytic removals of RhB and Cr(VI), J. Hazard. Mater. 381 (2020) 120942. doi:https://doi.org/10.1016/j.jhazmat.2019.120942.

[10]

Z. Xiong, Z. Wang, M. Muthu, Y. Zhang, Construction of an in-situ Fenton-like system based on a g-C3N4 composite photocatalyst, J. Hazard.

23

Mater. 373 (2019) 565–571. doi:https://doi.org/10.1016/j.jhazmat.2019.03.114. [11]

H. Zhao, G. Li, F. Tian, Q. Jia, Y. Liu, R. Chen, g-C3N4 surface-decorated Bi2O2CO3 for improved photocatalytic performance: Theoretical calculation and photodegradation of antibiotics in actual water matrix, Chem. Eng. J. 366 (2019) 468–479. doi:https://doi.org/10.1016/j.cej.2019.02.088.

[12]

M. Xie, J. Tang, L. Kong, W. Lu, V. Natarajan, F. Zhu, J. Zhan, Cobalt doped g-C3N4 activation of peroxymonosulfate for monochlorophenols degradation, Chem. Eng. J. 360 (2019) 1213–1222. doi:https://doi.org/10.1016/j.cej.2018.10.130.

[13]

N. Ullah, S. Chen, R. Zhang, Mechanism of the charge separation improvement in carbon-nanodot sensitized g-C3N4, Appl. Surf. Sci. 487 (2019) 151–158. doi:https://doi.org/10.1016/j.apsusc.2019.05.032.

[14]

J. Li, Y. Yin, E. Liu, Y. Ma, J. Wan, J. Fan, X. Hu, In situ growing Bi2MoO6 on g-C3N4 nanosheets with enhanced photocatalytic hydrogen evolution and disinfection of bacteria under visible light irradiation, J. Hazard. Mater. 321 (2017) 183–192. doi:https://doi.org/10.1016/j.jhazmat.2016.09.008.

[15]

Y. Xu, L. Zhang, M. Yin, D. Xie, J. Chen, J. Yin, Y. Fu, P. Zhao, H. Zhong, Y. Zhao, X. Wang, Ultrathin g-C3N4 films supported on Attapulgite nanofibers with enhanced photocatalytic performance, Appl. Surf. Sci. 440 (2018) 170– 176. doi:https://doi.org/10.1016/j.apsusc.2018.01.127.

[16]

J. Hou, C. Yang, Z. Wang, W. Zhou, S. Jiao, H. Zhu, In situ synthesis of α–β phase heterojunction on Bi2O3 nanowires with exceptional visible-light

24

photocatalytic performance, Appl. Catal. B Environ. 142–143 (2013) 504–511. doi:https://doi.org/10.1016/j.apcatb.2013.05.050. [17]

K.K. Bera, R. Majumdar, M. Chakraborty, S.K. Bhattacharya, Phase control synthesis of α, β and α/β Bi2O3 hetero-junction with enhanced and synergistic photocatalytic activity on degradation of toxic dye, Rhodamine-B under natural sunlight, J. Hazard. Mater. 352 (2018) 182–191. doi:https://doi.org/10.1016/j.jhazmat.2018.03.029.

[18]

X. Xiao, R. Hu, C. Liu, C. Xing, C. Qian, X. Zuo, J. Nan, L. Wang, Facile large-scale synthesis of β-Bi2O3 nanospheres as a highly efficient photocatalyst for the degradation of acetaminophen under visible light irradiation, Appl. Catal. B Environ. 140–141 (2013) 433–443. doi:https://doi.org/10.1016/j.apcatb.2013.04.037.

[19]

Y. Huang, J. Qin, C. Hu, X. Liu, D. Wei, H.J. Seo, Cs-doped α-Bi2O3 microplates: Hydrothermal synthesis and improved photochemical activities, Appl. Surf. Sci. 473 (2019) 401–408. doi:https://doi.org/10.1016/j.apsusc.2018.12.165.

[20]

C. Yu, P. Yang, L. Tie, S. Yang, S. Dong, J. Sun, J. Sun, One-pot fabrication of β-Bi2O3@Bi2S3 hierarchical hollow spheres with advanced sunlight photocatalytic RhB oxidation and Cr(VI) reduction activities, Appl. Surf. Sci. 455 (2018) 8–17. doi:https://doi.org/10.1016/j.apsusc.2018.04.201.

[21]

T. Chen, Q. Hao, W. Yang, C. Xie, D. Chen, C. Ma, W. Yao, Y. Zhu, A honeycomb multilevel structure Bi2O3 with highly efficient catalytic activity

25

driven by bias voltage and oxygen defect, Appl. Catal. B Environ. 237 (2018) 442–448. doi:https://doi.org/10.1016/j.apcatb.2018.05.044. [22]

Q. Hao, R. Wang, H. Lu, C. Xie, W. Ao, D. Chen, C. Ma, W. Yao, Y. Zhu, One-pot synthesis of C/Bi/Bi2O3 composite with enhanced photocatalytic activity, Appl. Catal. B Environ. 219 (2017) 63–72. doi:https://doi.org/10.1016/j.apcatb.2017.07.030.

[23]

X. Ruan, H. Hu, G. Che, P. Zhou, C. Liu, H. Dong, Fabrication of Z-scheme γ-Bi2MoO6/Bi12GeO20 heterostructure for visible-light-driven photocatalytic degradation of organic pollutants, Appl. Surf. Sci. (2019) 143668. doi:https://doi.org/10.1016/j.apsusc.2019.143668.

[24]

Y.-Y. Li, Z.-H. Wei, J.-B. Fan, Z.-J. Li, H.-C. Yao, Photocatalytic CO2 reduction activity of Z-scheme CdS/CdWO4 catalysts constructed by surface charge directed selective deposition of CdS, Appl. Surf. Sci. 483 (2019) 442– 452. doi:https://doi.org/10.1016/j.apsusc.2019.03.333.

[25]

R. Shen, C. Jiang, Q. Xiang, J. Xie, X. Li, Surface and interface engineering of hierarchical photocatalysts, Appl. Surf. Sci. 471 (2019) 43–87. doi:https://doi.org/10.1016/j.apsusc.2018.11.205.

[26]

H. Li, T. Hu, R. Zhang, J. Liu, W. Hou, Preparation of solid-state Z-scheme Bi2MoO6/MO (M=Cu, Co3/4, or Ni) heterojunctions with internal electric field-improved performance in photocatalysis, Appl. Catal. B Environ. 188 (2016) 313–323. doi:https://doi.org/10.1016/j.apcatb.2016.02.015.

[27]

F. Guo, W. Shi, H. Wang, M. Han, W. Guan, H. Huang, Y. Liu, Z. Kang, Study

26

on highly enhanced photocatalytic tetracycline degradation of type Ⅱ AgI/CuBi2O4 and Z-scheme AgBr/CuBi2O4 heterojunction photocatalysts, J. Hazard. Mater. 349 (2018) 111–118. doi:https://doi.org/10.1016/j.jhazmat.2018.01.042. [28]

B. Shao, X. Liu, Z. Liu, G. Zeng, Q. Liang, C. Liang, Y. Cheng, W. Zhang, Y. Liu, S. Gong, A novel double Z-scheme photocatalyst Ag3PO4/Bi2S3/Bi2O3 with enhanced visible-light photocatalytic performance for antibiotic degradation, Chem. Eng. J. 368 (2019) 730–745. doi:https://doi.org/10.1016/j.cej.2019.03.013.

[29]

D. Huang, J. Li, G. Zeng, W. Xue, S. Chen, Z. Li, R. Deng, Y. Yang, M. Cheng, Facile construction of hierarchical flower-like Z-scheme AgBr/Bi2WO6 photocatalysts for effective removal of tetracycline: Degradation pathways and mechanism, Chem. Eng. J. 375 (2019) 121991. doi:https://doi.org/10.1016/j.cej.2019.121991.

[30]

J. Zhang, Y. Hu, X. Jiang, S. Chen, S. Meng, X. Fu, Design of a direct Z-scheme photocatalyst: Preparation and characterization of Bi2O3/g-C3N4 with high visible light activity, J. Hazard. Mater. 280 (2014) 713–722. doi:https://doi.org/10.1016/j.jhazmat.2014.08.055.

[31]

Y. Jia, S. Li, J. Gao, G. Zhu, F. Zhang, X. Shi, Y. Huang, C. Liu, Highly efficient (BiO)2CO3-BiO2-x-graphene photocatalysts: Z-Scheme photocatalytic mechanism for their enhanced photocatalytic removal of NO, Appl. Catal. B Environ. 240 (2019) 241–252.

27

doi:https://doi.org/10.1016/j.apcatb.2018.09.005. [32]

X. Ma, G. Wang, C. Wang, S. Li, J. Wang, Y. Song, NiS2 as trapezoid conductive channel modified ternary Z-scheme photocatalyst system, NiGa2O4/NiS2/WO3, for highly photocatalytic simultaneous conversions of NO2− and SO32−, Chem. Eng. J. 350 (2018) 364–377. doi:https://doi.org/10.1016/j.cej.2018.05.149.

[33]

J. Jin, J. Yu, D. Guo, C. Cui, W. Ho, A Hierarchical Z-Scheme CdS–WO3 Photocatalyst with Enhanced CO2 Reduction Activity, Small. 11 (2015) 5262– 5271. doi:10.1002/smll.201500926.

[34]

F. Deng, Q. Zhang, L. Yang, X. Luo, A. Wang, S. Luo, D.D. Dionysiou, Visible-light-responsive graphene-functionalized Bi-bridge Z-scheme black BiOCl/Bi2O3 heterojunction with oxygen vacancy and multiple charge transfer channels for efficient photocatalytic degradation of 2-nitrophenol and industrial wastewater treatment, Appl. Catal. B Environ. 238 (2018) 61–69. doi:https://doi.org/10.1016/j.apcatb.2018.05.004.

[35]

Y. Gong, X. Quan, H. Yu, S. Chen, Synthesis of Z-scheme Ag2CrO4/Ag/g-C3N4 composite with enhanced visible-light photocatalytic activity for 2,4-dichlorophenol degradation, Appl. Catal. B Environ. 219 (2017) 439–449. doi:https://doi.org/10.1016/j.apcatb.2017.07.076.

[36]

Y. Lan, Z. Li, D. Li, G. Yan, Z. Yang, S. Guo, Graphitic carbon nitride synthesized at different temperatures for enhanced visible-light photodegradation of 2-naphthol, Appl. Surf. Sci. 467–468 (2019) 411–422.

28

doi:https://doi.org/10.1016/j.apsusc.2018.10.152. [37]

Y. Sun, Y. Xia, Shape-Controlled Synthesis of Gold and Silver Nanoparticles, Science (80-. ). 298 (2002) 2176. doi:10.1126/science.1077229.

[38]

W. Shan, Y. Hu, Z. Bai, M. Zheng, C. Wei, In situ preparation of g-C3N4/bismuth-based oxide nanocomposites with enhanced photocatalytic activity, Appl. Catal. B Environ. 188 (2016) 1–12. doi:https://doi.org/10.1016/j.apcatb.2016.01.058.

[39]

R. He, J. Zhou, H. Fu, S. Zhang, C. Jiang, Room-temperature in situ fabrication of Bi2O3/g-C3N4 direct Z-scheme photocatalyst with enhanced photocatalytic activity, Appl. Surf. Sci. 430 (2018) 273–282. doi:https://doi.org/10.1016/j.apsusc.2017.07.191.

[40]

F. Dong, Q. Li, Y. Sun, W.-K. Ho, Noble Metal-Like Behavior of Plasmonic Bi Particles as a Cocatalyst Deposited on (BiO)2CO3 Microspheres for Efficient Visible Light Photocatalysis, ACS Catal. 4 (2014) 4341–4350. doi:10.1021/cs501038q.

[41]

F. Dong, Z. Zhao, Y. Sun, Y. Zhang, S. Yan, Z. Wu, An Advanced Semimetal– Organic Bi Spheres–g-C3N4 Nanohybrid with SPR-Enhanced Visible-Light Photocatalytic Performance for NO Purification, Environ. Sci. Technol. 49 (2015) 12432–12440. doi:10.1021/acs.est.5b03758.

[42]

R.C. Dante, P. Martín-Ramos, F.M. Sánchez-Arévalo, L. Huerta, M. Bizarro, L.M. Navas-Gracia, J. Martín-Gil, Synthesis of crumpled nanosheets of polymeric carbon nitride from melamine cyanurate, J. Solid State Chem. 201

29

(2013) 153–163. doi:https://doi.org/10.1016/j.jssc.2013.02.016. [43]

X. Xu, L. Meng, Y. Dai, M. Zhang, C. Sun, S. Yang, H. He, S. Wang, H. Li, Bi spheres SPR-coupled Cu2O/Bi2MoO6 with hollow spheres forming Z-scheme Cu2O/Bi/Bi2MoO6 heterostructure for simultaneous photocatalytic decontamination of sulfadiazine and Ni(II), J. Hazard. Mater. 381 (2020) 120953. doi:https://doi.org/10.1016/j.jhazmat.2019.120953.

[44]

J. Li, W. Zhang, M. Ran, Y. Sun, H. Huang, F. Dong, Synergistic integration of Bi metal and phosphate defects on hexagonal and monoclinic BiPO4: Enhanced photocatalysis and reaction mechanism, Appl. Catal. B Environ. 243 (2019) 313–321. doi:https://doi.org/10.1016/j.apcatb.2018.10.055.

[45]

Q. Wang, J. He, Y. Shi, S. Zhang, T. Niu, H. She, Y. Bi, Designing non-noble/semiconductor Bi/BiVO4 photoelectrode for the enhanced photoelectrochemical performance, Chem. Eng. J. 326 (2017) 411–418. doi:https://doi.org/10.1016/j.cej.2017.05.171.

[46]

X. Li, W. Zhang, W. Cui, J. Li, Y. Sun, G. Jiang, H. Huang, Y. Zhang, F. Dong, Reactant activation and photocatalysis mechanisms on Bi-metal@Bi2GeO5 with oxygen vacancies: A combined experimental and theoretical investigation, Chem. Eng. J. 370 (2019) 1366–1375. doi:https://doi.org/10.1016/j.cej.2019.04.003.

[47]

X. Xiao, W.-D. Zhang, Hierarchical Bi7O9I3 micro/nano-architecture: facile synthesis, growth mechanism, and high visible light photocatalytic performance, RSC Adv. 1 (2011) 1099–1105. doi:10.1039/C1RA00323B.

30

[48]

M.-S. Gui, W.-D. Zhang, A facile preparation strategy for hollow-structured Bi2O3/Bi2WO6 heterojunction with high visible light photocatalytic activity, J. Phys. Chem. Solids. 73 (2012) 1342–1349. doi:https://doi.org/10.1016/j.jpcs.2012.06.009.

[49]

W. He, Y. Sun, G. Jiang, Y. Li, X. Zhang, Y. Zhang, Y. Zhou, F. Dong, Defective Bi4MoO9/Bi metal core/shell heterostructure: Enhanced visible light photocatalysis and reaction mechanism, Appl. Catal. B Environ. 239 (2018) 619–627. doi:https://doi.org/10.1016/j.apcatb.2018.08.064.

[50]

H. Jia, J. Zhao, X. Fan, K. Dilimulati, C. Wang, Photodegradation of phenanthrene on cation-modified clays under visible light, Appl. Catal. B Environ. 123–124 (2012) 43–51. doi:https://doi.org/10.1016/j.apcatb.2012.04.017.

[51]

X. Zhao, Z. Cai, T. Wang, S.E. O’Reilly, W. Liu, D. Zhao, A new type of cobalt-deposited titanate nanotubes for enhanced photocatalytic degradation of phenanthrene, Appl. Catal. B Environ. 187 (2016) 134–143. doi:https://doi.org/10.1016/j.apcatb.2016.01.010.

[52]

X. Wang, C. Chen, J. Li, X. Wang, Ozone degradation of 1-naphthol on multiwalled carbon nanotubes/iron oxides and recycling of the adsorbent, Chem. Eng. J. 262 (2015) 1303–1310. doi:https://doi.org/10.1016/j.cej.2014.10.107.

[53]

P. Li, T. He, Common-cation based Z-scheme ZnS@ZnO core-shell nanostructure for efficient solar-fuel production, Appl. Catal. B Environ. 238

31

(2018) 518–524. doi:https://doi.org/10.1016/j.apcatb.2018.07.057. [54]

L. Ye, J. Liu, C. Gong, L. Tian, T. Peng, L. Zan, Two Different Roles of Metallic Ag on Ag/AgX/BiOX (X = Cl, Br) Visible Light Photocatalysts: Surface Plasmon Resonance and Z-Scheme Bridge, ACS Catal. 2 (2012) 1677– 1683. doi:10.1021/cs300213m.

[55]

H. Wang, L. Zhang, Z. Chen, J. Hu, S. Li, Z. Wang, J. Liu, X. Wang, Semiconductor heterojunction photocatalysts: design, construction, and photocatalytic performances, Chem. Soc. Rev. 43 (2014) 5234–5244. doi:10.1039/C4CS00126E.

[56]

M. Ou, S. Wan, Q. Zhong, S. Zhang, Y. Song, L. Guo, W. Cai, Y. Xu, Hierarchical Z-scheme photocatalyst of g-C3N4@Ag/BiVO4 (040) with enhanced visible-light-induced photocatalytic oxidation performance, Appl. Catal. B Environ. 221 (2018) 97–107. doi:https://doi.org/10.1016/j.apcatb.2017.09.005.

[57]

H. Li, Y. Sun, B. Cai, S. Gan, D. Han, L. Niu, T. Wu, Hierarchically Z-scheme photocatalyst of Ag@AgCl decorated on BiVO4 (040) with enhancing photoelectrochemical and photocatalytic performance, Appl. Catal. B Environ. 170–171 (2015) 206–214. doi:https://doi.org/10.1016/j.apcatb.2015.01.043.

[58]

Y. Deng, L. Tang, C. Feng, G. Zeng, J. Wang, Y. Zhou, Y. Liu, B. Peng, H. Feng, Construction of plasmonic Ag modified phosphorous-doped ultrathin g-C3N4 nanosheets/BiVO4 photocatalyst with enhanced visible-near-infrared response ability for ciprofloxacin degradation, J. Hazard. Mater. 344 (2018)

32

758–769. doi:https://doi.org/10.1016/j.jhazmat.2017.11.027. [59]

M. Ren, Y. Ao, P. Wang, C. Wang, Construction of silver/graphitic-C3N4/bismuth tantalate Z-scheme photocatalyst with enhanced visible-light-driven performance for sulfamethoxazole degradation, Chem. Eng. J. 378 (2019) 122122. doi:https://doi.org/10.1016/j.cej.2019.122122.

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Fig. 1. XRD patterns of CN, Bi/CN, FU200, FU230, FU260, FU290 and BiO. Fig. 2. XPS spectra of the samples: (a) survey and high-resolution spectra of (b) C 1s, (c) N 1s, (d) Bi 4f and (e) O 1s. Fig. 3. TEM images of (a) CN, (b) FU200, (c) FU230, (d) FU260, (e) FU290 and the HRTEM image of (f) FU230. Fig. 4. UV-vis diffuse reflectance spectra of the samples. Fig. 5. (a) Photocatalytic degradation of 2,3-DHN and (b) linear first-order models fitting for the catalysts. Fig. 6. Schematic illustration of the Bi@β-Bi2O3/g-C3N4 construction by in situ deposition and oxidation. Fig. 7. (a) Transient photocurrent responses and (b) EIS Nyquist plots of BiO, CN and FU230. Fig. 8. ESR spin-trapping of BiO, CN and FU230 for (a) DMPO-O2•- and (b) DMPO-•OH under visible light for 8 min. Fig. 9. Scheme of the possible pathway for photodegradation of 2,3-DHN by FU230. Fig. 10. Illustration of 2,3-DHN photodegradation by Z-scheme Bi@β-Bi2O3/g-C3N4 under visible light.

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Fig. 1. XRD patterns of CN, Bi/CN, FU200, FU230, FU260, FU290 and BiO.

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Fig. 2. XPS spectra of the samples: (a) survey and high-resolution spectra of (b) C 1s, (c) N 1s, (d) Bi 4f and (e) O 1s.

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Fig. 3. TEM images of (a) CN, (b) FU200, (c) FU230, (d) FU260, (e) FU290 and the HRTEM image of (f) FU230.

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Fig. 4. UV-vis diffuse reflectance spectra of the samples.

Fig. 5. (a) Photocatalytic degradation of 2,3-DHN and (b) linear first-order models fitting for the catalysts.

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Fig. 6. Schematic illustration of the Bi@β-Bi2O3/g-C3N4 construction by in situ deposition and oxidation.

Fig. 7. (a) Transient photocurrent responses and (b) EIS Nyquist plots of BiO, CN and FU230.

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Fig. 8. ESR spin-trapping of BiO, CN and FU230 for (a) DMPO-O2•- and (b) DMPO-•OH under visible light for 8 min.

Fig. 9. Scheme of the possible pathway for photodegradation of 2,3-DHN by FU230.

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Fig. 10. Illustration of 2,3-DHN photodegradation by Z-scheme Bi@β-Bi2O3/g-C3N4 under visible light.

Highlights

(1) Bi@β-Bi2O3/g-C3N4 was successfully constructed by in situ deposition and oxidation. (2) 2,3-DHN was effectively degraded by Z-scheme catalysts under visible light. (3) Z-scheme favored the separation of carriers and kept the original reaction centers. (4) The elimination pathway of 2,3-DHN was proposed based on GC-MS results.

Declaration of interest statement

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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.

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