Hollow β-Bi2O3@CeO2 heterostructure microsphere with controllable crystal phase for efficient photocatalysis

Journal Pre-proofs Hollow β-Bi2O3@CeO2 Heterostructure Microsphere with Controllable Crystal Phase for Efficient Photocatalysis Xiaoyan Yang, Yiming Zhang, Yonglin Wang, Chenglong Xin, Peng Zhang, Dan Liu, Bhekie B. Mamba, Kebede K. Kefeni, Alex T. Kuvarega, Jianzhou Gui PII: DOI: Reference:

S1385-8947(20)30091-7 https://doi.org/10.1016/j.cej.2020.124100 CEJ 124100

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

3 October 2019 22 December 2019 10 January 2020

Please cite this article as: X. Yang, Y. Zhang, Y. Wang, C. Xin, P. Zhang, D. Liu, B.B. Mamba, K.K. Kefeni, A.T. Kuvarega, J. Gui, Hollow β-Bi2O3@CeO2 Heterostructure Microsphere with Controllable Crystal Phase for Efficient Photocatalysis, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124100

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Hollow β-Bi2O3@CeO2 Heterostructure Microsphere with Controllable Crystal Phase for Efficient Photocatalysis Xiaoyan Yang,a,b† Yiming Zhang,a† Yonglin Wang,c Chenglong Xin,d Peng Zhang,c Dan Liu,c* Bhekie B. Mamba,e Kebede K. Kefeni,e Alex T. Kuvarega,e Jianzhou Guia,c* a State

Key Laboratory of Separation Membranes and Membrane Processes, School of

Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China b

School of Chemistry and Chemical Engineering, Shangqiu Normal University,

Shangqiu 476000, China c

Tianjin Key Laboratory of Green Chemical Technology and Process Engineering,

School of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, China d Shandong e

Center for Disease Control and Prevention, Jinan 250014, China

University of South Africa, College of Science, Engineering and Technology,

Nanotechnology and Water Sustainability Research Unit, Florida Science Campus 1710, South Africa 

Corresponding Authors

Tel: +86-022-83955221, E-mail: [email protected] (Dan Liu); Tel: +86-022-83955221, E-mail: [email protected] (Jianzhou Gui) † The two authors contributed equally to this work. 1

Abstract Herein, a two-step hydrothermal synthesis is used to successfully prepare a Bi2O3 microsphere coated with CeO2 nanosheets on the surface. The resultant core/shell Bi2O3@CeO2 microsphere is calcined at different temperatures, yielding a series of Bi2O3@CeO2-t hollow microspheres. It is found that, with the increasing calcination temperature, the Kirkendall effect occurred can impel the interior Bi2O3 to gradually diffuse outward the Bi2O3@CeO2 microsphere, meanwhile the crystal structure of Bi2O3 transforms from the metastable tetragonal phase () to the stable monoclinic phase () at 350 °C. Phase-controllable preparation of the Bi2O3@CeO2-350 composite with the highly active -Bi2O3 is pursued to obtain the high electron-transfer efficiency between -Bi2O3 and CeO2, thus exhibiting superior photocatalytic performance in the photocatalytic degradation of tetracycline (TC) under visible light irradiation. Therefore, Kirkendall effect can be exploited as a new methodology to effectively improve the photocatalytic performance of heterojunction photocatalysts.

Keywords: -Bi2O3; CeO2; Kirkendall effect; photocatalysis; hollow structure

2

1. Introduction Nowadays, the exploitation of solar energy is being regarded as an important strategy to solve the global energy crisis and environmental pollution.[1-4] As an advanced photochemical technology, photocatalysis recently acquires extensive developments and successfully drives many reactions, including water splitting,[5-8] CO2 reduction,[9-11] and removal of pollutants.[12-16] Compared with some photocatalysts with a large bandgap (such as TiO2[17-20] and ZnO[21, 22]), the semiconductor materials with narrow bandgap can be excited by visible light, thus achieving higher solar energy utilization efficiency. Particularly, Bi2O3 is believed to be an important visible-light-responsive photocatalyst due to its non-toxicity, abundant earth reserves, and good chemical stability, and its direct bandgap is found to depend heavily on its crystal phase, varying in a wide range of 2-3.9 eV.[23] Generally, Bi2O3 has four main polymorphs, namely monoclinic , tetragonal , body-centred cubic , and face-centred cubic  with bandgaps of 2.8, 2.58, 2.8, and 3.01 eV, respectively.[24, 25] In previous studies, several researchers have reported that β-Bi2O3 exhibits a superior photocatalytic activity compared with others, resulting from its narrower bandgap and higher optical absorption.[26, 27] However, β-Bi2O3 is thermodynamically metastable and easily transforms into stable α-Bi2O3 under high-temperature treatment.[28] Thus, maintaining the β phase is vital to enhance the photocatalytic performance of Bi2O3. However, the photocatalytic activity of pure β-Bi2O3 is still not good enough to meet the industrial standard, due to its low quantum yield and fast electron/hole combination rate. It is preferable to fabricate the heterojunction composites containing β-Bi2O3,[29] 3

in which the hybridized semiconductor will efficiently separate the photogenerated electron/hole pairs in Bi2O3 to improve its photocatalytic activity.[30-34] Because electron transfer mainly occurs at the interface of two semiconductors, their contact areas significantly affect the overall photochemical property of the heterojunction composite.[22, 35] To maximally enhance the photocatalytic performance of Bi2O3, it is imperative to increase the interfacial area between Bi2O3 and another hybridized material. As a classical thermodynamic phenomenon, the Kirkendall effect normally refers to the non-equilibrium diffusion through the interface of coupled materials,[36] which has been widely adopted for preparing multifunctional nanocomposites with hollow structures.[37, 38] Recently, In and coworkers[39] have reported depositing amorphous TiO2 on the surface of a Cu3N nanocube, and then thermally treating the resultant product to fabricate the cube-like CuO-TiO2-xNx hollow composites via the Kirkendall effect. This CuO-TiO2-xNx hollow nanocube was found to exhibited a good photocatalytic performance for the conversion of CO2 into methane. Based on the microscale Kirkendall effect, Huang and coworkers[40] have developed an anion exchange strategy to synthesize the Bi2WO6 hierarchical hollow architecture using a BiOBr solid microsphere as the template. They also found that these Bi2WO6 hollow microspheres had high CO2 adsorption capacity and photocatalytic efficiency for the conversion of CO2 into methanol. Furthermore, Chen and coworkers[41] adopted Bi2O3 nanospheres as the template to successfully obtain the hollow Bi2S3 nanospheres via the Kirkendall effect, displaying good photocatalytic reduction of Cr(VI) in electroplating industry wastewater. Those impressive works indicated that the 4

Kirkendall effect can be utilized to tune the contact areas between two heterojunction semiconductors, further affecting the overall photocatalytic activity of heterojunction photocatalysts. Herein, a Bi2O3 microsphere is hydrothermally obtained and used as the precursor to grow CeO2 nanosheets via another hydrothermal method, finally fabricating the core/shell Bi2O3@CeO2 microsphere. During the subsequent calcination at different temperatures, the crystal phase of Bi2O3 core transforms from metastable β-Bi2O3 to stable α-Bi2O3. Simultaneously, the Kirkendall effect impels interior Bi2O3 to gradually diffuse outside the Bi2O3@CeO2 microsphere, thus increasing the interfacial area and electron-transfer efficiency between Bi2O3 and CeO2. A calcination temperature of 350 °C is found to be an appropriate temperature to obtain the heterojunction Bi2O3@CeO2 photocatalysts, which exhibit the highest photocatalytic activity for the degradation of tetracycline (TC) under visible light illumination.

2. Experimental 2.1 Preparation of photocatalysts Bi2O3 MS: Bi2O3 microspheres were used as the matrix and synthesized by the hydrothermal method reported in a previous work.[23] In a typical experimental procedure, 1.455 g Bi(NO3)3·5H2O (3 mmol) was dissolved in 30 mL of a solution of glycerol and absolute ethanol (v:v = 1:1). After ultrasonically dispersing for 20 min and magnetically stirring for another 10 min, the resulting solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave and maintained at 160 °C for 3 h. When the reaction was completed, the as-obtained precipitates were collected by centrifugation, 5

and were then washed for several times with distilled water and ethanol. After having been completely dried at 60 °C, the Bi2O3 matrix was finally prepared and denoted as Bi2O3 MS in this work. Bi2O3@CeO2-t photocatalysts: 0.233 g Bi2O3 MS (0.5 mmol) was ultrasonically dispersed in 10 mL of distilled water to form solution a. Solution b was obtained by dissolving 0.434 g of Ce(NO3)3·6H2O (1 mmol) in 28 mL of distilled water. Afterward, solution b was added drop-wise to solution a, yielding a mixture. After magnetically stirring for 10 min, the suspension was transferred to a 50 mL Teflon-lined stainlesssteel autoclave and heated at 200 °C for 24 h. Finally, the hydrothermal product was collected by centrifugation, thoroughly washed with water and ethanol, and then dried in the oven at 60 oC for 12 h. In this work, the hydrothermal product was denoted as Bi2O3@CeO2. Subsequently, the Bi2O3@CeO2 sample was loaded into a tube furnace and calcined at the specified temperature for 3 h under N2 atmosphere. In terms of different calcination temperatures (t = 250 °C, 350 °C, 450 °C, 550 °C), four Bi2O3@CeO2-t products were separately obtained and used as the photocatalysts. Contrast samples: In the absence of Bi2O3 MS, pure CeO2 was hydrothermally synthesized under the same conditions with the Bi2O3@CeO2. Both the Bi2O3 MS and pure CeO2 samples were then calcined at 350 °C under N2 atmosphere, and the resultant products were denoted as the Bi2O3-350 and CeO2-350, respectively. 2.2 Characterization of photocatalysts The morphology and structure of various samples were determined by field-emission scanning electron microscopy (FESEM, Hitachi S4800), transmission electron 6

microscopy (JEOL JEM-2100), and high resolution TEM (JEOL JEM2100). Crystal phases of materials were characterized by X-ray diffraction (XRD, Bruker D8 Advance A25, Cu-Kα radiation, 40 kV, 40 mA). The surface chemical compositions of various Bi2O3@CeO2-t samples were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Aepra). The UV-Vis diffuse reflectance spectra (DRS) were recorded over the spectral range of 200-800 nm on a Shimadzu UV 2700 with an integrating sphere attachment. Corresponding plots of (Ahv)2 vs. (hv) were calculated by (hν)n = m(hν - Eg), where , ν, Eg, m and h are the absorption coefficient, light frequency, bandgap energy, a constant and Planck’s constant, respectively. Absorption coefficient () is proportional to absorbance (A), and n depends on semiconductor species (2 for -Bi2O3 and CeO2). N2 adsorption-desorption isotherms were measured at -196 °C with a Micromeritics ASAP 2020 instrument, of which samples were firstly degassed at 220 °C for 12 h. Photoelectrochemical measurements were taken on an electrochemical work station (CHI 760E, Chenhua Instrument Co. Ltd., Shanghai, China) in 0.1 M Na2SO4 aqueous solution using the standard three-electrode system. A platinum wire and a saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. Working electrodes were prepared as follows: 4 mg of product was ultrasonically dispersed in 2 mL of ethanol for 10 min to yield slurry, which was subsequently carefully dipped into a groove (1 cm × 1 cm) on an indium-tin oxide (ITO) glass. After having been totally dried at 60 °C, the sample/ITO electrodes were heated at 250 °C for 6 h under N2 atmosphere. A 500 W Xe lamp was used as the excitation 7

light source for the visible light irradiation. Photocurrent response was measured at 0.3 mV under ambient conditions. Electrochemical impedance spectroscopy (EIS) was performed at 0.0 V by applying an AC voltage with 50 mV amplitude in a frequency range from 10000 Hz to 0.1 MHz. Valence band (VB) and conduction band (CB) potentials of -Bi2O3 and CeO2 were calculated using the following empirical equations: ECB = χ − Ee − 1/2Eg EVB = Eg + ECB where χ is the absolute electronegativity of a semiconductor; Ee is the energy of free electrons on the hydrogen scale (ca. 4.5 eV); and Eg is the bandgap energy of the photocatalyst. The χ values of -Bi2O3 and CeO2 are approximately 5.938 and 5.53 eV, while their bandgap energies are 2.45 and 3.03 eV, respectively. As a result, the CB and VB potentials of -Bi2O3 and CeO2 are obtained as 0.213 eV, 2.663 eV and -0.47 eV, 2.56 eV, respectively. 2.3 Photocatalytic experiments Tetracycline (TC) was selected as the probe organic to evaluate the photocatalytic activities of various products under visible light irradiation. The visible light was supplied by a 500 W Xe lamp, where the UV light ( < 420 nm) has been removed by the optical filter. For the catalytic reaction, 20 mg of catalysts and 40 mL of TC aqueous solution (10 mg/L) were placed in a 50 mL quartz reactor, and magnetically stirred in the dark for 30 min to establish the adsorption/desorption equilibrium. When the Xe lamp was turned on, about 4 mL of suspension was sampled from the reaction solution 8

at intervals of 30 min, and centrifuged to remove the powder catalyst. The filtrate absorbance was measured by a UV-Vis spectrophotometer (Thermo Scientific EvolutionTM 300) at 356 nm which is the wavelength for maximum absorption of TC. The TC degradation process was monitored by the C/C0 ratio, where C and C0 refer to absorbance of filtrates at different reaction times and absorbance of the initial TC aqueous solution, respectively. The intermediate products of TC were analyzed by high performance liquid chromatographytandem mass spectrometry (HPLC-MS, Waters Xevo TQ-S). Repeatability and stability of the Bi2O3@CeO2-350 were investigated using the standard reaction procedure, whereas only measuring the solution absorbance after 180 min. When the reaction was complete, the reused catalysts were recovered by centrifugation, washed three times using ethanol, and dried at 80 °C for the next cycle. A series of active species-trapped experiments were also performed to analyze the photocatalytic mechanism of the Bi2O3@CeO2 photocatalyst. Under the standard TC photodegradation conditions, 0.5 mmol of scavengers were introduced into the reaction solution, and then monitored the C/C0 variation over the Bi2O3@CeO2-350, where ethyl-enediaminetetraacetic acid disodium salt (Na2C2O4), tert-butylalcohol (t-BuOH), and silver nitrate (AgNO3) were used to trap holes (h+), hydroxyl radicals (·OH), and electrons (e-), respectively.

3. Results and discussions The chemical compositions of various products could be confirmed by the XRD patterns, as shown in Figure 1. Series of featured peaks assigned to the CeO2 (JCPDS 9

no. 43-1002) are clearly observed in five XRD patterns, indicating that the CeO2 phase in hydrothermal product has been maintained during the thermal treatment process. Four diffraction peaks at ca. 32, 46, 55, and 59 ° are observed in the presence of the Bi2O3@CeO2, Bi2O3@CeO2-250, and Bi2O3@CeO2-350, suggesting the existence of -Bi2O3 (JCPDS no. 27-0050) in their structure. When the calcination temperature reaches 450 °C, the metastable -Bi2O3 phase could be transformed into the stable Bi2O3 phase in the Bi2O3@CeO2-450 and Bi2O3@CeO2-550, due to the emergence of diffraction peaks of -Bi2O3 (JCPDS no. 27-0052) and the disappearance of peaks from -Bi2O3. This thermal transition process of the Bi2O3 crystal phase has also been reported in previous studies.[23, 27] Besides the featured peaks of -Bi2O3, it is noted that the XRD pattern of the Bi2O3@CeO2-450 also include a borderline peak at ca. 32 ° from -Bi2O3. This result demonstrates that in this case, -Bi2O3 cannot completely become -Bi2O3 at 450 °C, thus the Bi2O3@CeO2-450 contains slight -Bi2O3. As for the Bi2O3@CeO2-550, no -Bi2O3 phase could be found in its XRD pattern and diffraction peaks of -Bi2O3 is stronger than that of the Bi2O3@CeO2-450, resulting from that the high-temperature treatment impels the increase of crystallite size of Bi2O3. In terms of the Scherrer equation, the crystallite sizes of CeO2 and Bi2O3 in five Bi2O3@CeO2 composites can be estimated and summarized in Table S1. Moreover, the XRD results reveal that the two contrast samples are pure Bi2O3 and CeO2, as shown in Figure S1.

10

Figure 1. XRD patterns of Bi2O3@CeO2 and Bi2O3@CeO2-t samples.

From the SEM image (Figure 2a), it is clearly seen that the original hydrothermal product has a well-dispersed spherical morphology and uniform size. After the second hydrothermal reaction, the final product Bi2O3@CeO2 generally maintains the spherical appearance of the Bi2O3 MS, as shown in Figure 2b. However, a hierarchical structure was evident in the Bi2O3@CeO2: a large number of nanosheet subunits are located on the surface of the microsphere. Figure 3a-d are representative SEM images of various calcined products, showing a similar hierarchical morphology as the Bi2O3@CeO2. However, in terms of statistical data (Figure 3e), the size of various Bi2O3@CeO2-t samples is found to be gradually decreasing with the increasing calcination temperature. The average diameters of the Bi2O3@CeO2, Bi2O3@CeO2-250, Bi2O3@CeO2-350, Bi2O3@CeO2-450, and Bi2O3@CeO2-550 are estimated to be 1.87, 1.84, 1.82, 1.77, and 11

1.63 m, respectively. Consequently, although the high-temperature treatment does not significantly destroy the physical structure of Bi2O3@CeO2 products, a shrinkage in size could be confirmed during the heat treatment. In comparison, the SEM image show a collapsed sphere structure for the Bi2O3-350 (Figure S2a), while the CeO2-350 is composed of well-defined octahedral crystals (Figure S2b).

Figure 2. Typical SEM images of (a) Bi2O3 MS and (b) Bi2O3@CeO2.

Figure 3. Typical SEM images of (a) Bi2O3@CeO2-250, (b) Bi2O3@CeO2-350, (c) Bi2O3@CeO2-450, (d) Bi2O3@CeO2-550 and (e) their corresponding size distributions. 12

To explore the detailed structure, a broken Bi2O3@CeO2-350 sample has been picked out and analyzed by its SEM image (Figure 4a), where a cavity is clearly observed inside the microsphere. Meanwhile, the microsphere shell of the Bi2O3@CeO2-350 is measured to be ca. 600 nm (Figure 4a) and wrapped by many CeO2 nanosheets on the surface, as shown in its TEM image (Figure 4b). It is reported that the formation of this hollow structure mainly results from the thermodynamic removal of Bi(III)-glycerol complex micelles self-assembled in the synthesis solution.[23] The HRTEM image (Figure 4c) exhibits two distinct lattice spacing with the distance of 0.196 and 0.312 nm, which are attributed to Bi2O3 (222) and CeO2 (111), respectively. Therefore, the Bi2O3@CeO2-350 product obtained in this study has an unique hierarchical structure: a large number of CeO2 nanosheet-subunits cover on the surface of the hollow Bi2O3 microsphere.

Figure 4. (a) SEM, (b) TEM, and (c) HRTEM images of Bi2O3@CeO2-350.

The pore structure and specific surface area of Bi2O3@CeO2-t products have been confirmed by their N2 adsorption-desorption isotherms, as shown in Figure 5. In the 13

relative pressure range of 0.4-0.9, the type IV hysteresis loop could be clearly found in four samples to verify abundant mesopores in their structures, particularly the Bi2O3@CeO2-350 with the largest loop area. Compared with others, the Bi2O3@CeO2550 exhibits an obvious decreasing N2-adsorbed amount, indicating that the hightemperature will destory the hierarchical porous structure of the Bi2O3@CeO2-t microsphere. According to the N2 adsorption-desorption isotherms, the BrunauerEmmett-Teller (BET) specific surface areas of the Bi2O3@CeO2-250, Bi2O3@CeO2350, Bi2O3@CeO2-450, and Bi2O3@CeO2-550 have been respectively estimated to be 27.0, 29.2, 24.8, and 7.7 m2/g, as summarized in Table 1.

Figure 5. N2 adsorption-desorption isotherms of various Bi2O3@CeO2-t samples.

In the present work, XPS is utilized to further analyze the variation of surface chemical composition of Bi2O3@CeO2-t composites during the high-temperature calcination process. The XPS survey spectra of various Bi2O3@CeO2-t photocatalysts (Figure S3) show a series of strong peaks from Bi 4f, C 1s, O 1s, and Ce 3d, respectively. 14

In the high-resolution XPS of Bi 4f (Figure 6a), the energy gap between two peaks is measured to be 5.4 eV, confirming the existence of Bi2O3 in all samples. Particularly, two peaks in the Bi2O3@CeO2, Bi2O3@CeO2-250, and Bi2O3@CeO2-350 composites are observed at 158.7 and 164.1 eV, which are attributed to -Bi2O3.[26] For the Bi2O3@CeO2-450 and Bi2O3@CeO2-550, the two peaks of Bi 4f are observed to shift to 158.8 and 164.2 eV, resulting from -Bi2O3.[31] It is indicated that phase transformation from the original -Bi2O3 to -Bi2O3 has tanken place in Bi2O3@CeO2-t products due to the thermal treatment, which is in accordance with the XRD results. As shown in Figure 6b, the Ce 3d XPS of five Bi2O3@CeO2-t composites is found to have a rather complex structure, being deconvoluted into eight peaks. Those fitting peaks could be divided into two multiplets of 3d3/2 and 3d5/2, noted as v and u in this work. The v1/u1, v3/u3, and v4/u4 doublets are ascribed to the photoemissions from the Ce(IV), while the spin-orbit doublets of v2/u2 mainly result from the photoemissions of Ce(III). Obviously, Ce species in all Bi2O3@CeO2-t products exist as a mixed chemical state of Ce3+ and Ce4+. Previous works have reported that the as-formed Ce3+ will yield oxygen vacancies around Ce4+ to satisfy the charge equilibrium, further accelerating the reduction reaction.[42] Based on the XPS peak area, Ce3+/Ce4+ atomic ratios in five samples have been separately calculated and are listed in Table 1. In summary, more Ce3+ species could be observed in the Bi2O3@CeO2-250 and Bi2O3@CeO2-350, thus both of these should be expected to show superior photocatalytic performance.

15

Table 1. BET specific surface area, atomic ratios of Ce3+/Ce4+ and Bi/Ce obtained from XPS data, and reaction constants of various Bi2O3@CeO2-t samples Sample

BET specific surface area (m2/g)

Ce3+/Ce4+

Bi/Ce

Reaction constant (min-1)

Bi2O3@CeO2

-

0.06

3.75

0.0032

Bi2O3@CeO2-250

27.0

0.26

3.46

0.0044

Bi2O3@CeO2-350

29.2

0.23

3.76

0.0111

Bi2O3@CeO2-450

24.8

0.09

6.57

0.0063

Bi2O3@CeO2-550

7.7

0.16

5.37

0.0031

Furthermore, Table 1 also lists the atomic ratios of the Bi/Ce on the surface of various Bi2O3@CeO2-t photocatalysts. In general, an escalating trend could be discerned in the data from Bi2O3@CeO2 to Bi2O3@CeO2-550, suggesting that the surface area of Bi species increase gradually with the calcination temperature. This result demonstrates that, besides the crystal-phase transformation of Bi2O3, the Kirkendall effect has also occurred under the high-temperature calcination, which will impel interior Bi2O3 in Bi2O3@CeO2-t microspheres continually migrates to the surface. During this process, the individual size of various Bi2O3@CeO2-t samples will gradually decrease with the thermal temperature, as indicated by their corresponding size distribution data given above (Figure 3e).

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Figure 6. High-resolution XPS spectra of (a) Bi 4f and (b) Ce 3d states of Bi2O3@CeO2 and Bi2O3@CeO2-t samples.

The heterojunction structure of the Bi2O3@CeO2-t hollow microspheres significantly affects their optical properties, which are measured by the UV-Vis DRS. As shown in Figure 7a, the Bi2O3-350 has good absorption of visible light, while the CeO2-350 is found to slightly absorb visible light. As for the four Bi2O3@CeO2-t composites, the overall intensity of light absorbed is further enhanced compared with the Bi2O3-350 and CeO2-350, although an obvious distinction could be barely witnessed in their UVVis DRS. It is verified that the hybridization between Bi2O3 and CeO2 apparently reduces their respective bandgap to strengthen the visible light absorption efficiency. As shown in Figure 7b, from plots of (Ahv)2 vs. (hv), the integral bandgap of the Bi2O3@CeO2-350 is estimated to be ca. 2.38 eV, which is smaller than those of the 17

Bi2O3-350 (2.47 eV) and CeO2-350 (3.05 eV).

Figure 7. (a) UV-Vis DRS of Bi2O3@CeO2, Bi2O3@CeO2-t samples, Bi2O3-350, and CeO2-350; (b) Plots of (Ahv)2 vs. (hv) for Bi2O3@CeO2-350, Bi2O3-350, and CeO2350.

Photocurrent responses of a series of products have also been carried out to investigate their photochemical properties, of which the detailed results are shown in Figure 8. All involved current curves are observed to rise instantaneously when the light source is switched on, indicating good photosensitivity. As shown in Figure 8a, the current density of Bi2O3@CeO2-t samples increases with the calcination temperature until it reaches 350 °C, and inversely declines from 350 °C to 550 °C. It is widely reported that the electron-transfer efficiency will strongly depend on the contact area of two semiconductor materials.[22, 35] In consideration of the outward diffusion of interior Bi2O3 at elevated temperatures, it is conjectured that the calcination temperature below 350 °C would impel interior Bi2O3 to hybridize the surface CeO2 nanosheets, thus increasing their interfacial area. Subsequently, at 450 °C or above, excess Bi2O3 18

migrates outward the Bi2O3@CeO2 hollow microsphere to inversely decreasing the electron-transfer efficiency of the Bi2O3@CeO2-450 and Bi2O3@CeO2-550. As shown in Figure 8b, the current density of the Bi2O3@CeO2-350 is found to be significantly higher than that of the Bi2O3-350 and CeO2-350. In their Nyquist plots (Figure 8c), the Bi2O3@CeO2-350 shows a smaller circular arc radius, suggesting a rapider intrinsic electron-diffused rate in comparison to the pure CeO2 and Bi2O3 samples. Therefore, both the photocurrent and EIS results demonstrated that the heterojunction interface in the Bi2O3@CeO2-350 will greatly facilitate the electron transfer between Bi2O3 and CeO2, thus the Bi2O3@CeO2-350 is probably endowed with a superior photocatalytic activity.

Figure 8. (a) Photocurrent responses of various Bi2O3@CeO2-t electrodes, (b) photocurrent responses and (c) Nyquist plots of Bi2O3@CeO2-350, Bi2O3-350, and CeO2-350 electrodes in 0.1 M Na2SO4 solution under visible light irradiation.

As a common antibiotic, tetracycline (TC) is adopted to evaluate the photocatalytic performance

of various

Bi2O3@CeO2-t

photocatalysts.

After

reaching

the

adsorption/desorption equilibrium in dark, the photocatalytic activity of various catalysts was subsequently investigated under the visible light irradiation. From the 19

time-dependent absorbance of TC solution over the Bi2O3@CeO2-350 (Figure 9a), the intrinsic peak of TC molecules at 356 nm gradually decreases with the reaction time, while the self-degradation of TC could be proved to be negligible (Figure S4). It is indicated that the Bi2O3@CeO2-350 has a good visible-light-driven catalytic activity. The TC-removed processes of the Bi2O3@CeO2 and four Bi2O3@CeO2-t products have been shown in Figure 9b. The Bi2O3@CeO2-350 is found to have the largest saturated adsorption amount for TC in dark, which can adsorb 47 % TC within 30 min. Meanwhile, the adsorption capacity of the Bi2O3@CeO2 products exhibits the same trend with their BET specific surface areas, thus the Bi2O3@CeO2-250, Bi2O3@CeO2450, and Bi2O3@CeO2-550 adsorb 37 %, 28 %, and 17 % TC under the same conditions, as shown in Figure 9b. Besides, under visible light irradiation, the Bi2O3@CeO2-350 also possesses the highest TC removal efficiency and attains almost complete removel of TC within 180 min. To quantitatively compare the photocatalytic activities of various catalysts, their photodecomposition reaction could be fitted by the expression -ln(C/C0) = kt, where k is the rate constant (min-1). As shown in Figure 9c, a good linear relationship between ln(C/C0) and time is clearly observed in five photocatalysts, indicating the first-order reaction process. An increasing trend in photocatalytic performance is observed from Bi2O3@CeO2 to Bi2O3@CeO2-350, with rate constants of 0.0032, 0.0044, and 0.011 min-1, respectively. On the contrary, when the calcination temperature is higher than 350 °C, the catalytic efficiency of Bi2O3@CeO2-t samples is decreased, of which the Bi2O3@CeO2-450 and Bi2O3@CeO2-550 have the rate constants of 0.0063 and 0.0031 min-1. From the aforementioned analysis, it is concluded 20

that the Bi2O3@CeO2-350 possesses the highest photon-to-electron conversion efficiency among all samples, and is composed of the highly active -Bi2O3. Consequently, both the Kirkendall effect and the crystal-phase transformation of Bi2O3 simultaneously confirm that 350 °C is an appropriate thermal-treatment temperature to maximally enhance the photocatalytic performance of the Bi2O3@CeO2-t hollow microspheres.

Figure 9. (a) Time-dependent absorbance of TC over Bi2O3@CeO2-350 under visible light irradiation; photocatalytic degradation of TC over (b) various Bi2O3@CeO2-t samples and (d) Bi2O3-350 and CeO2-350; (c, e) their corresponding kinetic plots; and (f) recycle photocatalytic performance of Bi2O3@CeO2-350 for TC degradation under visible light irradiation within 180 min.

The Bi2O3-350 and CeO2-350 are used as two contrast samples to evaluate the photocatalytic performance of the Bi2O3@CeO2-350. Their photodegradation processes for TC and corresponding kinetic plots are shown in Figure 9d and e. From Figure 9d, 21

three fast degradation plots could be clearly seen and indicate that the three products have good photocatalytic activities under the visible-light irradiation. Particularly, the photocatalytic activity of the Bi2O3-350 is found to be higher than that of the CeO2-350. After forming the composite structure, the Bi2O3@CeO2-350 exhibits a greatly improved photocatalytic performance due to its heterojunction structure. In terms of the fitting reaction constant (Figure 9e), it is estimated that the photocatalytic activity of the Bi2O3@CeO2-350 is about 2.3 and 6.5 times as high as the Bi2O3-350 and CeO2350, respectively. Besides the superior photocatalytic activity, the as-obtained Bi2O3@CeO2-350 hollow microsphere is also found to have great stability under the visible light irradiation. As shown in Figure 9f, during the five repeated reactions, the Bi2O3@CeO2350 composite consistently maintains the high TC removal efficiency, confirming high repeatability of photocatalytic activity and stability. Therefore, the high catalytic activity and stability endow the Bi2O3@CeO2-t photocatalysts a good potential practicability in the remediation of effluents. Photodegraded intermediates of the Bi2O3@CeO2 hollow microsphere could be confirmed by HPLC-MS, and corresponding spectra have been shown in Figure S5. In the initial TC solution, a strong peak at m/z of 445 has been only found, which is attributed to the TC molecule (Figure 10). Besides the TC, many other peaks at 427, 410, 362, 318, 302, and 274 are detected in the photoreaction solution. According to this, a possible degradation pathway of TC is proposed and shown in Figure 10. During the photocatalytic process, the TC molecules can undergo series of reactions, including 22

the dehydroxylation, deamination and ring opening, thus degraded to various lowmolecular-weight organic intermediates.[1, 43] Finally, those organic by-products are thoroughly mineralized into CO2 and H2O, and the toxic TC is removed from the waste water.

Figure 10. Possible degradation pathways of TC over Bi2O3@CeO2-350.

In the present case, the trapping experiment has also been employed to detect the photoreactive species during the catalytic process of the Bi2O3@CeO2 microsphere, including h+, ·OH, and e−. As shown in Figure 11, it is clearly observed that the photocatalytic performance of the Bi2O3@CeO2-350 has been significantly impeded in the presence of Na2C2O4, while exhibiting a slight reduction when AgNO3 and t-BuOH are introduced. Therefore, h+ plays as the main active species in the TC photodegradation over the Bi2O3@CeO2 photocatalyst, not e- and ·OH.

23

Figure 11. Photocatalytic degradation of TC over Bi2O3@CeO2-350 in presence of no scavenger and various scavengers under the visible light irradiation.

Scheme 1. Photocatalytic mechanism proposed for Bi2O3@CeO2-350 hollow microsphere for TC degradation under visible light irradiation.

The possible photocatalytic mechanism of the Bi2O3@CeO2-350 has also been proposed and shown in Scheme 1. Exposed to the visible light ( > 420 nm), -Bi2O3 and CeO2 can absorb the photons to separately yield conduction band electrons (e-) and 24

valence band holes (h+), although they have different bandgaps. However, most of the photogenerated h+ will recombine with e- within an extremely short time, thus the pure -Bi2O3 and CeO2 exhibit a low quantum efficiency and photocatalytic activity. After forming the heterojunction structure, in the interface of -Bi2O3 and CeO2, h+ in the VB-Bi2O3 (2.663 eV) can rapidly transfer to VBCeO2 (2.56 eV), while e- migrates from CBCeO2 (-0.47 eV) to CB-Bi2O3 (0.213 eV). As a result, the interfacial contact successfully improves the separation efficiency of photogenerated electron/hole pairs and prolongs their lifetime. As shown in Scheme 1, h+ has been proved to play as reactive species to degrade organic molecules in the irradiation process. Obviously, the integral photocatalytic activity of various Bi2O3@CeO2-t microspheres strongly depends on the electron-transfer efficiency between -Bi2O3 and CeO2. As discussed in the photocurrent experiments above, the highest photocurrent density recorded for the Bi2O3@CeO2-350 indicates the largest contact area of -Bi2O3 and CeO2 among all the photocatalysts involved. Therefore, the Bi2O3@CeO2-350 is found to possess the superior photocatalytic activity in the photodegradation of TC. In addition, it is reported that surface Ce4+ can capture photogenerated e- to become Ce3+, which is simultaneously re-oxidized to Ce4+ by h+.[44, 45] The interconvertible Ce3+/Ce4+ pairs also efficiently suppress the recombination of photoinduced charge carriers, thus further enhancing the photocatalytic performance of the Bi2O3@CeO2-350 hollow microsphere.[42]

4. Conclusions In summary, the core/shell Bi2O3@CeO2 microsphere is successfully prepared via a 25

two-step hydrothermal synthesis, and then calcined at different temperatures to obtain a series of Bi2O3@CeO2-t hollow microspheres. It is found that the high temperature facilitates the gradual outward diffusion of the interior -Bi2O3, leading to the more effective heterojunction structure. However, the phase transtion from the active -Bi2O3 to the stable -Bi2O3 occurs at 450 °C or above. As a result, the Bi2O3@CeO2-350 exhibits the highest photodegradation efficiency for TC under visible light irradiation, which almost completes removal of TC within 180 min. The superior photocatalytic performance can be ascribed to the high electron-transfer efficiency between Bi2O3 and CeO2, as well as the intrinsic highly active -Bi2O3. Consequently, the fabrication of Bi2O3@CeO2 hollow microspheres as described in this study shows that the Kirkendall effect can be exploited to design and synthesize heterojunction photocatalysts with enhanced photocatalytic performance.

Acknowledgements This work was partly supported by the National Natural Science Foundation of China (No. 21576211 and No. 21706190), Natural Science Foundation of Tianjin City (No. 18JCQNJC76300), and Key Scientific and Innovative Research Team in the University of Tianjin (No. TD13-5031).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:xxx 26

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Declaration of interests ☒ 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.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Bi2O3@CeO2 Hollow Microsphere with Controllable Crystal Phase and Heterojunction Structure for Efficient Photocatalytic Performance

Xiaoyan Yang, Yiming Zhang, Yonglin Wang, Chenglong Xin, Peng Zhang, Dan Liu,* Bhekie B. Mamba, Kebede K. Kefeni, Alex T. Kuvarega, Jianzhou Gui*



Series of Bi2O3@CeO2 hollow microspheres have been fabricated.



Kirkendall effect impels interior Bi2O3 to diffuse outside microspheres.



Crystal phase of Bi2O3 is transformed from  to .



Bi2O3@CeO2-350 shows the superior visible-light photocatalytic performance.

36