BiVO4 Chain-like Hollow Microstructures: Synthesis

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Bi/BiVO4 Chain-like Hollow Microstructures: Synthesis, Characterization and Application as Visible-Light-Active Photocatalysts Qifeng Jing, Xinyan Feng, Xiaojun Zhao, Zeiyu Duan, Jiangling Pan, Limiao Chen, and You-Nian Liu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00330 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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ACS Applied Nano Materials

Bi/BiVO4 Chain-like Hollow Microstructures: Synthesis, Characterization and Application as Visible-Light-Active Photocatalysts Qifeng Jing1, Xinyan Feng1, Xiaojun Zhao1, Zeiyu Duan1, Jiangling Pan,2 Limiao Chen*1, Younian Liu1 1

School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, PR China; 2School of Physics and Electronics, Central South University, Changsha 410083, PR China; e-mail: [email protected] Tel.: +86 73188879616 Fax: +86 73188879616

Abstract Bi/BiVO4 microstructures with novel hollow chain-like morphology have been successfully fabricated for the first time and its application as photocatalyst is explored. Composites composed of metallic Bi and amorphous BiVO4 (denoted as Bi/a-BiVO4) are firstly prepared through a simple solvothermal route using ethylene glycol (EG) as solvent and then hydrothermally treated in a basic solution to crystallize BiVO4. The Bi content in the Bi/BiVO4 microstructures can be facilely adjusted by changing the molar ratio of Bi3+ to V5+ in the reaction system. The photocatalytic

activity

of

the

Bi/BiVO4

microstructures

is

assessed

by

photodegradation of Rhodamine B (RhB) under visible-light illumination. The Bi/BiVO4 microstructures exhibit an obviously improved photocatalytic performance compared to the BiVO4 and Bi. The synergistic effects of BiVO4 and Bi may contribute to the improved photocatalytic activity. The present research may provide a new approach to design and fabricate BiVO4-based photocatalyst with high photocatalytic activity. Keywords: Bismuth vanadate, Bismuth, Composites, Photocatalytic activity, Dye degradation

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Introduction Recently, materials with a hollow structure have received extensive concern owing to their low density, tunable refractive index, big surface area, and their promising applications in many fields including optical devices, catalysis, sensing and drug delivery.[1-7] Compared to the solid counterparts, materials with hollow interior usually display enhanced physical and chemical properties, thus providing a new strategy to adjust the properties of the materials. Up to now, many methods have been explored to fabricate hollow micro- and nanostructures including hollow spheres, olives, tubes, plates, and polyhedrals.[8-12] These synthesis methods usually include conventional hard and soft templating method, template-free method, spray synthesis route, Ostwald ripening process, and Kirkendal effect.[13-15] Even if great successes have been achieved, the preparation of tunable novel metal-semiconductors with well-defined hollow interiors still remains of great interest and challenge. Monoclinic BiVO4 has been considered as a very promising photocatalyst due to its small band-gap energy (2.45 eV) and excellent visible-light photocatalytic activity.[16, 17] Nevertheless, the rapid recombination of photogenerated charge pairs and poor electrical conductivity of BiVO4 limit its wide application in photodecomposition of organic pollutant and photocatalytic evolution of O2.[18] To overcome aforementioned limitation, various strategies have been developed, which includes morphological control, loading co-catalyst, impurity doping, and heterojunction construction.[18-27] Among them, coupling BiVO4 with metal or other matched semiconductor into composites has been regarded as a very effective approach and various composites based on BiVO4 have been fabricated, such as BiVO4/Bi2S3, BiVO4/BiOCl, BiVO4/Ag, BiVO4/Bi2O3, BiVO4/AgI, BiVO4/WO3, and BiVO4/Ag/Ag3PO4.[20-27] The enhancement of photocatalytic activity of these composites is mainly ascribed to the improved visible-light adsorption and reduced recombination rate of photoinduced electron-hole pairs. However, as far as we know, there are few reports on coupling BiVO4 with Bi to construct composite photocatalyst and its photocatalytic activity.[28-29] 2


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Bismuth (Bi), a typical semimetal material, has aroused extensive attention owing to its especial properties including narrow band-gap, high carrier motility, and low effective mass.[30] Recently, Bi was also found to show a direct plasmonic photocatalytic activity, which can act as a potential candidate to replace noble metals.[29, 31-32] Herein, in this investigation, we report on a novel chain-like Bi/BiVO4 hollow microstructure synthesized by a two-step chemical method. Firstly, chain-like hollow composites composed of Bi nanocrystals and amorphous BiVO4 (denoted as Bi/a-BiVO4) were prepared through a solvothermal route using ethylene glycol (EG) as solvent. Then, as-synthesized Bi/a-BiVO4 composites were hydrothermally treated in a basic solution to crystallize BiVO4. The amount of Bi metal in the Bi/BiVO4 composites can be facilely adjusted by tuning the molar rate of Bi(NO3)3·5H2O to NH4VO3 (marked as Bi3+/V5+) in the reaction solution. The photocatalytic testing of as-obtained Bi/BiVO4 composites were performed using RhB molecules as pollutant probes under visible-light irradiation. It was revealed that the Bi/BiVO4 microstructures show much improved photocatalytic performance compared to the pure BiVO4 and Bi.

2. Experimental Section 2.1. Preparation of Bi/a-BiVO4 and Bi/c-BiVO4 composites Bi/a-BiVO4 composites were synthesized through a solvothermal route using EG as solvent. In a typical synthesis, 1mmol Bi(NO3)3·5H2O and 1 mmol sodium dodecyl sulfate (SDS) were dissolved in 15 mL EG with stirring for 30 min to obtain transparent solution. At the same time, a suitable amount of NH4VO3 powders was dissolved in 15 mL EG with heating and stirring for 30 min to obtain milk solution. Afterwards, the two solutions were blended with continuous stirring to generate a yellow suspension. The molar ratios of Bi3+/V5+ in several different suspensions were 1, 1.2, 1.4, 1.6, 1.8 and 2 respectively. After 30 min of additional stirring, the suspension was moved into a stainless steel autoclave (40 mL in volume) and maintained at 180 °C for 8 h. Then the autoclave was cooled down to normal temperature and the precipitates were separated by filtration, washed with alcohol and 3



deionized water for five times, and then placed in a vacuum oven at 60 °C for 8 h. In order to describe the samples conveniently, the samples obtained under various molar ratios of Bi3+/V5+ are denoted as B/a-BVO-x, where x is the molar ratio of Bi3+/V5+. For comparison, Bi/a-BiVO4 composites were also prepared in the presence of ethylenediaminetetraacetic acid (EDTA) and poly(vinylpyrrolidone) (PVP) or without any organic additives to study the effect of organic additives on the shape and size of the products. Composites based on metallic Bi and crystalline BiVO4 (denoted as Bi/c-BiVO4) were prepared by hydrothermally treating the B/a-BVO-x composites in a basic solution. Typically, a suitable amount of Bi/a-BiVO4 composites were dispersed in 30 mL basic solution (pH=8) with ultrasonic treatment for 30 min and then moved into a 40 mL stainless steel autoclave. Subsequently, the hydrothermal reaction was conducted at 150 °C for 5 h. As the autoclave was cooled down to normal temperature, the solid substances were separated by filtration and washed with distilled water for several times. They were then dried in a vacuum oven. As-obtained composites were denoted as B/c-BVO-x, where x is the molar ratio of Bi3+/V5+ in the reaction solution. The synthesis process is presented in scheme 1.

Scheme 1 The synthesis process of Bi/c-BiVO4 hollow chain-like structures. 2.2. Characterization The crystal structures of all the samples were determined by X-ray diffraction (XRD) analysis (Bruker/AXS D8 Advance). The XRD patterns were recorded in the 2θ range from 10 to 70° using Cu Kα radiation (λ=0.15406 nm) with a step scan of 8 °/min. The shape and microstructure of the as-synthesized samples were observed with a scanning electron microscope (SEM, Philips XL 30 FEG, operated at 10 kV). The chemical compositions of the samples were investigated by the energy dispersive 4


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X-ray (EDX) spectroscopy. The morphology and microstructure of the as-prepared samples were characterized with transmission electron microscope (TEM, Philips CM200, operated at 200 kV) and high-resolution transmission electron microscope (HRTEM). X-ray photoelectron spectroscopy (XPS) test was carried out with an ESCALB MK-II VG XPS with an excitation source of a monochromatic Mg KR light. Raman measurement was carried out by using an In Via-reflex spectrometer (Renishaw) equipped with a microscope under 532 nm laser radiation. Photoluminescence (PL) emission spectra were obtained from a Hitachi F-4500 fluorescence spectrophotometer. UV-visible (UV-vis) absorption spectra were obtained from a UV-vis spectrophotometer (Shimadzu, Model UV-2700), in which BaSO4 was used as a reflectance standard. The Brunauer-Emmett-Teller (BET) surface area and pore diameter distribution were determined by N2 adsorption isotherms at 77 K with the use of a Micromeritics ASAP2020 analyzer after the products had been placed in vacuum at 120 °C for 8 h. 2.3. Measurements of Photocatalytic Activity The photocatalytic performances of Bi, BiVO4 and Bi/c-BiVO-x series were determined by the visible-light photodegradation of RhB in aqueous solution by using a 500 W Xe lamp equipped with a cut-off filter (λ > 420 nm). In a typical process, 30 mg photocatalysts were added to 30 mL of RhB aqueous solution (10 mg L-1) under vigorous stirring. Before light irradiation, the mixed suspension was kept in darkness for 30 min with continuous stirring in order to establish an adsorption-desorption balance of RhB molecules on the surface of catalysts. During the visible light irradiation, about 3 mL of suspension was sampled at 1 h intervals and centrifuged to remove the photocatalysts completely. Then the light absorption of the centrifuged solution was measured. In order to identify the radical species involved in the photodegredation of RhB molecules, EDTA, methyl alcohol (MeOH), and isopropanol (IPA) molecules were used as quenchers and added in the photocatalytic reaction system. The reaction condition was similar to that in the above photodegredation experiments except that additional scavengers were added. 5



3. Results and Discussions

Fig.1 (A) XRD patterns of (a) pure BiVO4, (b) pure Bi and the samples obtained with: no organic additives (c), PVP (d), SDS (e), and EDTA (f). (B) XRD patterns of different samples synthesized with SDS: (a) B/a-BVO-1, (b) B/a-BVO-1.2, (c) B/a-BVO-1.4, (d) B/a-BVO-1.6, (e) B/a-BVO-1.8, and (f) B/a-BVO-2. The crystal structure and phase purity of the sample synthesized with a 1:1.6 molar ratio of Bi3+/V5+ in the presence of different organic additives are determined by XRD analysis, as shown in Fig. 1A. For comparison, the XRD patterns of pure Bi and BiVO4 are also included. The curve c in Fig. 1A displays the XRD pattern of the products prepared without any organic additives. The characteristic diffraction peaks could be well indexed on the basis of monoclinic BiVO4 and rhombohedral Bi, indicating that as-prepared products consist of BiVO4 and Bi phase. The curves d-f in Fig. 1A display the XRD patterns of the samples synthesized with PVP, SDS and EDTA, respectively. Obviously, all the diffraction peaks in these patterns can be assigned to Bi phase, and no characteristic peaks due to the BiVO4 phase can be observed though there are a large number of NH4VO3 in the reaction solution. These results suggest that the PVP, SDS and EDTA molecules can prevent crystallization of BiVO4. Fig.1B presents the XRD patterns of the products synthesized with various molar ratios of Bi3+/V5+ in the presence of SDS. It can be found that all reflection peaks can be assigned to Bi phase, and no characteristic peaks due to the Bi2O3 phase can be detected though there are excess Bi(NO3)3·5H2O in the reaction solution. Moreover, the peaks intensities increase gradually with increase in the molar ratio of Bi3+/V5+, suggesting more metallic Bi was formed. 6


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Fig.2 (a, b) SEM, (c) TEM and (d) HRTEM images of the as-prepared B/a-BVO-1.6 sample. (e) The EDS spectrum of B/a-BVO-1.6. (f-h) EDX elemental mapping images of B/a-BVO-1.6 sample. The morphology and crystal structure of the B/a-BVO-1.6 sample are investigated with SEM and TEM. The SEM image in Fig. 2a indicates that as-obtained products consist of numerous chain-like microstructures, which are composed of many sphere-like particles. Fig.2b shows the high-magnification SEM image, indicating that the surface of the spheres is not very smooth due to some nanoparticles sparsely adhered to the surface. The SEM image in Fig.2b also shows a typical broken region (as indicated by arrow), suggesting the hollow interior structure of the chain-like microstructures. The TEM image in Fig. 2c also indicates that the chain-like microstructures are interconnecting hollow microspheres. The wall thickness of the hollow microspheres is about 70 nm. Fig. 2d presents a HRTEM image obtained from the edge of the wall. Both crystalline and amorphous phases are clearly observed. The apparent lattice fringes with d = 0.395, 0.328, and 0.237 nm correspond to the distances of (003), (012) and (104) planes of rhombohedral Bi.[30] The corresponding 7



Fourier transform pattern (the inset in Fig. 2d) further identifies the single-crystal nature of the Bi. It is worth noting that no lattice interplanar spacing corresponding to BiVO4 can be observed, which might be due to the formation of amorphous BiVO4. The chemical composition of the chain-like microstructures is determined by the EDX analysis. The EDX spectrum in Fig. 2e displays the presence of the Bi, V and O elements (other elements originated from the TEM copper grid). The atomic ratio of Bi/V is approximately 1.56, which is very close to the molar ratio (1.6) of Bi3+/V5+ in the reaction system. EDX elemental mapping patterns indicate that both Bi and V element are uniformly dispersed in the whole chain-like microstructures (Fig. 2f-h). These results suggest that the chain-like microstructures may contain vanadium based oxides such as amorphous BiVO4 and V2O5. Raman characterization is performed to further investigate the structure of the chain-like microstructures. As shown in Fig. S2a, the weak and broad peaks at 119, 202 and 818 cm-1 can be assigned to the typical vibrations of BiVO4,[33] confirming the formation of amorphous BiVO4 in the products. All of these results manifest that as-obtained products consist of crystalline Bi and amorphous BiVO4. SEM and TEM are also utilized to analyze the shape of the products prepared with no or other organic additives (EDTA and PVP). It is found that sphere-like nanoparticles with the size of 100~225 nm were formed in the absence of any organic additives, as shown in Fig. S3a. Moreover, some nanospheres are interconnected to form dimmers. The corresponding TEM image (Fig. S3b) reveals the solid texture of as-obtained nanospheres. When PVP was used, the morphology of the products is similar with that of the products prepared without any surfactants. However, in the presence of EDTA, solid microspheres with uniform diameter and smooth surface were obtained (Fig. S3e-f). These results demonstrate that SDS plays a crucial role in the fabrication of hollow microstructures. Fig. S4 presents the SEM images of the samples synthesized with various quantities of SDS. It can be found that the average particle size is increased and surface becomes smoother gradually with increasing the amount of SDS. When the amount of SDS was 1.0 mmol, well-defined chainlike microstructures with smooth surface were formed. The effect of the molar ratio of 8


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Bi3+/V5+ in the reaction solution on the shape of the particles is also studied. It is observed that the products synthesized with various molar ratios of Bi3+/V5+ have a similar morphology (Fig. S5), indicating that the molar ratio of Bi3+/V5+ has little influence in the shape of the particles. To discover the formation process of chain-like Bi/a-BiVO4 hollow microstructures, a series of contrast experiments were conducted and the products obtained at different growth stages are analyzed by SEM, TEM and XRD. Fig. S6 displays the SEM and TEM images of the products obtained at different hydrothermal reaction stages (0.5, 1, 1.5, 2, 4, and 6 h). It can be observed from Fig. S6a, after hydrothermal reaction for 0.5 h, the products are composed of sphere-like and irregular nanoparticles with the size of 10~100 nm. The corresponding TEM image (the inset in Fig. S6a) indicates that most of these nanoparticles have hollow interior structure. As the hydrothermal reaction proceeds for 1 h, most of the irregular nanoparticles disappeared and a large number of spherical particles with big size (~ 1µm) are formed (Fig. S6b). Moreover, these sphere-like particles are interconnected to form chain-like structures. The TEM image (the inset in Fig. S6b) indicates the solid texture of these chain-like structures. As the reaction time is increased to 1.5 h, the chain-like structures become more mature and the irregular nanoparticles are disappeared completely (Fig. S6c). The inset in Fig. S6c displays the corresponding TEM observations, where the dark periphery and slightly greyish center of the chain-like structures can be observed, suggesting the chain-like structures start to become hollow. As the reaction time is further increased, the chain-like morphology of the products is nearly unchanged, while the average length of the chains is increased and the area of the greyish center becomes larger, as shown in Fig. S6d-f. The phase and purity of the products formed at different reaction stages is examined by XRD. It is found that the products exhibit an amorphous structure as the reaction time is shorter than 2 h (Fig. S7a-c). As the hydrothermal reaction proceeds for 2 h, weak diffraction peaks centered at 27.2, 37.9, 39.6, and 48.8° appear in the XRD pattern (Fig. S7d) and they are characteristics of the Bi phase, revealing metallic Bi phase start to form. Furthermore, with prolonging the reaction time, the intensities of the diffraction peaks assigned to Bi phase 9



strengthened gradually (Fig. S7d-g), implying that the Bi content in the products is increased.

Fig. 3 Schematic illustration of the formation process for the Bi/a-BiVO4 chain-like hollow microstructures. Based on the study on the formation of chain-like Bi/a-BiVO4 hollow microstructures, a possible growth mechanism is proposed and illustrated in Fig. 3. Firstly, the SDS molecules formed sphere-like vesicles under vigorous stirring at room temperature and the Bi3+ ions are adsorbed on the surface of the SDS vesicles owing to the electrostatic attractions. At the early stage of hydrothermal process, the Bi3+ ions on the surface of SDS vesicles provide the nucleation area for the reaction between VO3- and Bi3+ ions, and the subsequent mineralization leads to the formation BiVO4 hollow nanoparticles. Due to the high surface energy, as-obtained hollow nanoparticles tend to aggregate and further grow into chain-like structures with relatively big size through a dissolution-recrystallization process.[34] As compared to the outer shells, the inner core of the chain-like structures has higher surface energy because the inner core resulted from the aggregation of small hollow nanoparticles at an early stage. To reduce the energy, the inner cores are dissolved and recrystallized gradually on the surface of the shell to form hollow interior structure.[13] At the same time, the Bi3+ ions (the excess Bi3+ ions or the Bi3+ ions dissociated from BiVO4) start to react with EG to generate metallic Bi nanoparticles on the surface of chain-like structures. Finally, the Bi/BiVO4 hollow chain-like structure is formed successfully.

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Fig. 4 The XRD spectra of the samples: (a) B/c-BVO-1, (b) B/c-BVO-1.2, (c) B/c-BVO-1.4, (d) B/c-BVO-1.6, (e) B/c-BVO-1.8 and (f) B/c-BVO-2. The crystal structure and morphology of the Bi/a-BiVO-x composites after hydrothermal treatment in basic solution are characterized with XRD, Raman and SEM. The XRD patterns of these samples are displayed in Fig. 4. Obviously, two groups of reflection peaks can be observed in each XRD pattern. The peaks with 2θ values of 18.8, 28.8, 30.5, 35.1, 42.5, 47.2, 53.3 and 59.3° can be perfectly indexed as the BiVO4 phase, indicating the amorphous BiVO4 in Bi/a-BiVO-x has been transformed into crystalline BiVO4 after hydrothermal treatment. The composites based on Bi nanocrystals and crystalline BiVO4 were denoted as Bi/c-BiVO4. After comparison one can found that the peak intensities of BiVO4 in B/c-BVO-2 sample are much lower than those in other samples, indicating the lower content or crystallinity of BiVO4 in this sample. The Raman spectrum of the B/c-BVO-1.6 is presented in Fig. S2b. The major Raman peaks at around 817, 367, 328, 203 and 140 cm-1 can be attributed to the typical vibrational bands of BiVO4. Moreover, the intensities of Raman peaks of BiVO4 in B/c-BVO-1.6 are much stronger than those in B/a-BVO-1.6, indicating the improved crystallinity of BiVO4 in B/c-BVO-1.6. It should be noted that the pH value of the reaction solution has effect on the phase of the final products. Bi2O3 and BiVO4 phases would be obtained when the B/a-BVO-x composites are hydrothermally treated in deionized water (Fig. S8). Fig. S9 shows the 11



typical SEM images of the B/c-BVO-x samples. As can be seen, after hydrothermal crystallization, the chain-like morphology and particle size remain nearly unchanged. The crystal structure of the representative B/c-BVO-1.6 sample is examined by TEM, as shown in Fig. 5b. The clear contrast between the shell and the interior indicates hollow structure is retained after hydrothermal crystallization. The HRTEM image (Fig. 5c) obtained from the edge of the shell suggests that the shell is composed of metallic Bi and crystalline BiVO4. The obvious lattice interplanar spacing of about 0.395 and 0.260 nm corresponds to the (003) planes of Bi and the (200) planes of BiVO4,[31, 33] respectively. All of these results manifest that the amorphous BiVO4 in B/a-BVO-1.6 is transformed into crystalline BiVO4 after hydrothermal crystallization. The BiVO4 content in B/c-BVO-1.6 is about 60.6 wt% ascertained by EDX analysis (Table S1). The mapping images of B/c-BVO-1.6 sample (Fig. 5d-f) are similar with those of B/a-BVO-1.6 (shown in Fig. 2f-h), indicating that the hydrothermal crystallization didn’t change the distribution of compositions.

Fig. 5 (a) SEM, (b) TEM，and (c) HRTEM images of B/c-BVO-1.6 sample; (d-f) EDX elemental mapping images of B/c-BVO-1.6 sample. The special surface area and porosity of the B/c-BVO-1.6 sample are investigated using nitrogen adsorption-desorption isotherm. As can be observed clearly from Fig.S10, the B/c-BVO-1.6 sample shows a typical type III isotherm (according to the IUPAC classification) with a hysteresis loop in the range of P/P0 ≥ 0.8, indicating the 12


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presence of mesoporous and macroporous microstructures. The corresponding pore-size distribution curve (the inset in Fig.S10) further confirms that B/c-BVO-1.6 composites contain both mesopores and macropores up to more than 30 nm in size. The BET surface area of the B/c-BVO-1.6 calculated form the linear part of BET plot is about 10.3m2/g, which is around four times higher than that of pure BiVO4 polyhedrals (2.6m2/g). The BET surface areas of other B/c-BVO-x samples were also measured. As shown in Table S1, the surface area increases slightly with the increase in the Bi content in the B/c-BVO-x composites.

Fig. 6 Typical XPS spectra of pure BiVO4 (a) and B/c-BVO-1.6 (b) samples: (A) survey scan, (B) B i4f, (C) V 2p, and (D) O 1s. The surface elemental composition and metal oxidation states of B/c-BVO-1.6 and pure BiVO4 samples were determined by XPS analysis. The wide-scan XPS spectra (shown in Fig. 6A) of the B/c-BVO-1.6 and pure BiVO4 clearly display the existence of C 1s, Bi 4f, V 2p, and O 1s peaks. The C 1S peaks may be ascribed to the signal from the hydrocarbon of the XPS instruments. Fig. 6B displays the high resolution XPS spectra of Bi 4f. For pure BiVO4, two wide and strong peaks located at about 164.3 and 159.0 eV can be indexed to the binding energies of Bi 4f7/2 and Bi 4f5/2 of Bi3+ species, respectively. However, for B/c-BVO-1.6 sample, the binding energies of Bi 4f peaks were shifted negatively by 0.3 eV. Besides, there are two weak peaks centered at 157.2 and 162.6 eV, corresponding to the Bi0 in metal Bi. Fig. 8C 13



shows the V 2p regions of XPS spectra of B/c-BVO-1.6 and pure BiVO4 samples. It is found that the binding energies of the V 2p peaks are also shifted negatively by ca. 0.3 eV for B/c-BVO-1.6 compared with the pure BiVO4. The O 1S peaks of pure BiVO4 (Fig. 6D) located at 529.8 and 531.6 eV can be attributed to the Bi-O bonds in BiVO4 and surface chemisorb oxygen, respectively. As for B/c-BVO-1.6, the peak locations of O 1S peaks also shift toward lower binding energy. The shifts of the binding energies for Bi 4f, V 2p and O 1s may be caused by the interaction of Bi metal with BiVO4 and the Bi vacancy in the Bi/BiVO4 composites.[35-36]

Fig. 7 UV–vis absorption spectra (A) and plots of (ahv)2 versus (hv) (B) of pure BiVO4 (a), B/a-BVO-1(b), B/c-BVO-x (c-h): (c) B/c-BVO-1, (d) B/c-BVO-1.2, (e) B/c-BVO-1.4, (f) B/c-BVO-1.6, (g) B/c-BVO-1.8, (h) B/c-BVO-2, and pure Bi (i). UV–vis absorption spectrum (DRS) is utilized to illustrate the light absorption ability of Bi, BiVO4 and B/c-BVO-x composites. It can be found from Fig. 7, the metallic Bi demonstrates an obvious absorption band in the region of 200~350 nm, while BiVO4 has an intense absorption in visible-light region besides that in the UV region. All the B/c-BVO-x composites show a wide absorption from the UV region to the visible light region. Moreover, the intensity of visible light absorption of the B/c-BVO-x composites decreased gradually with the increase in the Bi content owing to the shielding effect of metal Bi. The band gaps (Eg) of these samples were determined by using the equation: αhν = A(hν - Eg)n/2, where α, hν, A and Eg are the absorption coefficient, the discrete photo energy, proportionality constant, and the optical band gap energy, respectively. The Eg values of all the products are calculated according to the intercept of the plots of (αhν) 2 versus hν (Fig. 7B) and tabulated in Table S1. It was found that the Eg value firstly decreased from 2.68 to 2.32 eV with increasing the BiVO4 content from 30.32 to 39.40%, and thereafter it increased to 14


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2.44 eV as the BiVO4 content increased to 77.50%. The B/c-BVO-1.6 sample has the lowest Eg energy among these samples, suggesting that B/c-BVO-1.6 sample may generate the more charge pairs in comparison with other samples under the identical conditions. The photocatalytic performances of B/c-BVO-x composites are evaluated in terms of visible-light degradation of RhB in aqueous solution. For the sake of comparison, a blank experiment without any catalyst and the photodecomposition of RhB over P25, pure Bi and bare BiVO4 particles were also conducted under the same visible-light conditions and presented in Fig. 8A. As can be seen, the P25 and BiVO4 have only very low degradation rates to RhB in 240 min illumination. As for pure Bi, it also exhibits low degradation efficiency (46.1%). However, high degradation efficiency (91.8%) can be achieved by the B/c-BVO-1.6 composites. The synergetic effects of metal Bi and BiVO4 may give rise to the activity enhancement of B/c-BVO-1.6 composites: the BiVO4 widens the range of visible-light absorption and the Bi reduces the recombination efficiency of photoinduced charge pairs. In addition, the relative high BET surface area of B/c-BVO-1.6 sample might also have contribution to the enhanced photocatalytic performance. Fig. 8B shows the photodegradation of RhB over B/c-BVO-x composites. It is revealed that the content of metal Bi in the B/c-BVO-x composites can affect their photocatalytic activity. The degradation efficiency in 240 min illumination firstly increases from 12.7% to 91.8% with increasing the metallic Bi content from 22.50 to 60.60%, and thereafter it decreases to 53.7% as the content of metal Bi is further increased to 69.68%. The B/c-BVO-1.6 sample exhibits the highest degradation efficiency among the B/c-BVO-x series. Regarding the BET surface area, the B/c-BVO-x composites have the similar surface areas (shown in Table S1). Therefore, the discrepancy in the photocatalytic activity may be caused by the difference in the metal Bi content in the Bi/BiVO4 composites, instead of the BET surface area. With increasing the Bi content (22.5-60.60%), more Bi nanoparticles are formed in the surfaces of Bi/BiVO4 composites, effectively inhibiting the recombination of photoinduced electron-hole pairs. As a result, the improved photocatalytic performance is achieved. However, when the content of 15



Fig. 8 (A, B) Photodecomposition efficiencies of RhB under different photocatalytic conditions. (C) Repeated photocatalytic activity of B/c-BVO-1.6 under visible-light illumination. (D) The influence of different scavengers in the photodegradation of RhB over B/c-BVO-1.6 photocatalysts. metal Bi is further increased, excess Bi would act as mediators for recombination of photoinduced charge pairs, giving rise to a decreased photocatalytic performance.[37] The photocatalytic activity of the BiVO4 and B/c-BVO-x series was also characterized by the photodecomposition of phenol under the same conditions. It can be observed from Fig. S11, the photodecomposition of phenol over BiVO4 and B/c-BVO-x composites exhibits the similar trend with RhB, and the B/c-BVO-1.6 sample exhibits the highest photocatalytic activity. The reusability and stability of B/c-BVO-1.6 sample are further investigated, as presented in Fig. 8C. It is clear that the photocatalytic performance of B/c-BVO-1.6 is slightly improved after several photocatalytic reactions. The B/c-BVO-1.6 photocatalysts after photocatalytic measurement are characterized with XRD and SEM (Fig. S12). It was found that the XRD pattern of B/c-BVO-1.6 after repeated irradiation is similar with that in Fig. 4d, 16


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which suggests its phase stability. However, SEM image in Fig. S13 indicates that the chain-like structures become partially broken after several photocatalytic reactions. Therefore, the surface area of the B/c-BVO-1.6 photocatalyst could be increased. As a result, more active sites become available and there is enhanced photocatalytic activity. PL spectroscopy is a valid technique to analyze the recombination of photogenerated electron-hole pairs in semiconductors. Fig. S14 displays the PL emission spectra of the pure BiVO4 and B/c-BVO-1.6 sample under the excitation of 300 nm. The PL emission spectrum of bare BiVO4 displays an intense band located at around 537 nm, corresponding to the band-gap recombination of electron-hole pairs.[38] In comparison with that of the pure BiVO4, the PL intensity of B/c-BVO-1.6 sample is obviously decreased. It has been demonstrated that a high PL intensity usually implies a low separation efficiency of photoinduced electrons and holes.[39] Therefore, the recombination rate of photogenerated electron hole pairs in the B/c-BVO-1.6 sample can be effectively restrained. This result is in well accordance with the result of photodecomposition of RhB. To analyze the predominant active radicals generated in the photodegradation process, a series of trapping experiments were performed using B/c-BVO-1.6 sample as representative photocatalysts. In these experiments, EDTA, MeOH and IPA molecules were added in the photocatalytic system of RhB degradation as quenchers of h+, ·O2- and ·OH, respectively. It can be observed from Fig. 8D that the photodecomposition rate of RhB is slightly reduced with the addition of MeOH, suggesting that ·O2- radicals only made small contribution to the photodecomposition of RhB. On the contrary, with the addition of EDTA and IPA, the photodecomposition rates of RhB are only about 31.1 and 55.1% after 240 min irradiation, respectively, which are less than 93.5% conversion rate of RhB in the absence of any scavengers. Thus, it can be inferred that the h+ and ·OH radicals may play major roles for the photodecomposition of RhB over Bi/BiVO4 composites. Based on the previous reports and above experimental results, a possible 17



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photocatalytic reaction mechanism upon the Bi/BiVO4 microstructures for RhB photodegradation is proposed, as presented in Fig. 9. Under visible-light illumination, photoinduced electron-hole pairs are formed on the surface of metallic Bi. Owing to the Fermi level of metal Bi (about -0.17 eV) is more negative than the conduction band (CB) of BiVO4 (about 0.07eV),[36, 40] the photoinduced electrons on the surface of metal Bi could migrate to the CB of BiVO4. As a result, the potential of metal Bi would become more positive due to the photoinduced holes still remain in metal Bi. After trapping electrons from the valence band (VB) of BiVO4, the metal Bi could recover to its initial state, which is in accordance with the previous results.[37, 41] The electron migration process at the interface of Bi and BiVO4 would be beneficial to reducing the recombination efficiency of the photoinduced charge pairs. The photogenerated electrons would be trapped by the O2 molecules adsorbed on the surface of Bi/BiVO4 to generate H2O2 owing to the relatively positive redox potential of O2/H2O2 (0.70 eV).[42-43] Then, the H2O2 could be further changed into ·OH after accepting an electron. At the same time, the OH- can be oxidized by the holes to form ·OH because the standard redox potential of OH-/·OH (1.99 eV) is more negative in comparison with the VB of BiVO4 (2.47 eV).[44] Based on the trapping experiment results and the potential analysis, it is deduced that the holes and ·OH may be the predominant active radicals. Finally, the holes and ·OH would degrade the RhB molecules in the photocatalysis process. The photocatalytic reaction may occur as follows: Bi/BiVO4 + hν → e- + h+

(1)

O2 + 2H+ + 2e- → H2O2

(2)

H2O2 + e- →·OH + OH−

(3)

h+ + OH− →·OH

(4)

h+ + RhB → Degradation

(5)

·OH + RhB → Degradation

(6)

18


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Fig. 9 A proposed photocatalytic mechanism for decomposition of RhB over the Bi/BiVO4 microstructures under visible-light illumination.

4. Conclusion In summary, chain-like Bi/BiVO4 hollow microstructures have been successfully prepared for the first time through a two-step chemical method, in which the Bi content in Bi/BiVO4 microstructures can be facilely adjusted by tuning the molar ratio of Bi3+/V5+ in the reaction solution. The evaluation of the photocatalytic activity indicates that the Bi/BiVO4 composites obviously exhibited an improved photoactivity in comparison with the bare BiVO4. This enhancement in the photocatalytic performance can be ascribed to the synergistic effects of BiVO4 and Bi. Among the B/BVO-x composites, the B/c-BVO-1.6 possesses the strongest visible-light photoactivity and excellent photochemical stability under repeated illumination, which is especially promising for its practical application. The present study may provide a new approach to fabricate BiVO4-based photocatalysts with hollow structure and may be of great importance to solve the water pollution problem in the future.

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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Synthesis procedure of BiVO4 and Bi, SEM images, TEM images, XRD patterns, Raman spectra, N2 adsorption-desorption isotherms, Photocatalytic activities of Bi/ BiVO4 composites for the degradation of phenol under visible light irradiation, PL spectra of pure BiVO4 and Bi/ BiVO4 composites.

Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 21776317 and 21636010).

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Table of Contents

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BiVO4 Chain-like Hollow Microstructures: Synthesis

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