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Materials and Interfaces

Bi quantum dots decorated Bi4V2O11 hollow nanocakes: Synthesis, characterization and application as photocatalysts for CO2 reduction Xiaojun Zhao, Zeiyu Duan, and Limiao Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01737 • Publication Date (Web): 28 May 2019 Downloaded from http://pubs.acs.org on June 4, 2019

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Industrial & Engineering Chemistry Research

Bi quantum dots-decorated Bi4 V2O11 hollow nanocakes: Synthesis, characterization and application as photocatalysts for CO2 reduction Xiaojun Zhao, Zeyu Duan, Limiao Chen* College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, P.R. China e-mail: [email protected] Tel.: +86 73188879616 Fax: +86 73188879616

Abstract Bi-quantum-dots-modified-Bi4 V2O11 (BQDs/Bi4 V2O11 ) hollow nanostructures were prepared via a two-step hydrothermal process and used as photocatalysts for CO2 photoreduction under simulated sunlight irradiation. Characterization techniques including XRD, FE-SEM, TEM, UV-vis DRS, XPS, PL spectroscopy, N2 adsorption-desorption and photoelectrochemical measurements were utilized to investigate

the

crystal

structure,

morphology,

optical

property,

and

photoelectrochemical property of as-prepared BQDs/Bi4 V2O11 composites. The characterization results indicated that the loading of BQDs on Bi4 V2O 11 remarkably increase the photoabsorption performance, CO2 absorption capacity, and separation efficiency of photoinduced charge pairs in Bi4 V2O11 . Most importantly, a significantly enhanced photocatalytic activity toward CO2 photoreduction was obtained over the BQDs/Bi4 V2O 11 composites under simulated sunlight irradiation. A possible enhancement mechanism of CO2 photoreduction over BQDs/Bi4 V2O11 composites was proposed that the BQDs as co-catalysts can improve the photocatalytic activity by borrowing the redox conversion between CO2 and BQDs.

Keywords: Bi4V2O11, Bi quantum dots, hollow structure, CO2 photoreduction

1

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. Introduction Construction of novel semiconductor photocatalysts with excellent photocatalytic activity and high stability has been regarded as one of the best strategies to solve the environmental and energy issues. Over the past several decades, numerous semiconductor-based photocatalysts have been fabricated and applied in many fields, such as organic pollutant degradation, CO2 reduction, and hydrogen production.[1-5] It was demonstrated that, owing to their large band gap, the conventional photocatalysts including TiO2, ZnS, and ZnO respond to only ultraviolet light, which only takes up about 4% of the whole solar spectrum, leading to poor visible-light photocatalytic performance.[6] Therefore, developing photocatalysts with desirable wide spectrum optical response is an attractive and promising strategy to achieve high efficiency of photocatalytic reaction. Up to now, considerable numbers of novel semiconductor photocatalysts such as Bi-contained, W-contained and Mo-contained photocatalysts have been designed and fabricated to solve the problems of the traditional photocatalysts.[7-9] Especially, Bi-contained photocatalysts have been widely investigated in photocatalytic application because of their outstanding electrical and optical properties.[10-16] As one of important Bi-containing compounds, Bi4 V2 O11 has aroused considerable interest owing to its especial lamellar structure and good visible-light response.[17] To date, numerous efforts have been payed to improve the photocatalytic activity of Bi4 V2O11 through fabrication of Bi4 V2O 11 structures with special morphology and particle size since the morphology and particle size have a great influence in the photocatalytic performance of photocatalysts.[18-20] For instance, Chen et al.[17] synthesized novel Bi4 V2O11 hollow microspheres via a surfactant-free solvothermal process and found that they possessed good visible-light photocatalytic performance for the photodecomposition of rhodamine B (RhB). Pan et al.

[18]

fabricated two-dimensional

Bi4 V2O11 nanosheets by a hydrothermal route and demonstrated that as-obtained Bi4 V2O11 nanosheets exhibited a better ability of light-harvesting compared to monoclinic BiVO4 . Despite all these great efforts, the practical application of 2


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Bi4 V2O11 in photopcatalyst is still suppressed because of the high recombination rate of photoinduced charge pairs in pristine Bi4 V2 O11. Therefore, a low recombination rate of the photoexcited electron-hole pairs is very necessary for improving the photocatalytic efficiency of Bi4 V2O11. Loading of noble metal such as Ag, Au, Pt and Pd on the surface of semiconductor photocatalysts has been considered to be an effective approach to enhance their photocatalytic activities because the noble metals can serves as an electron trap and substantially enhance the separation efficiency of photoinduced electrons and holes through the Schottky barriers at the interface of semiconductor and noble metal.[21-25] Nevertheless, the relatively high price of the noble metals seriously limits their industrial applications. Recently, it has been demonstrated that metallic Bi nanoparticles also exhibits the surface plasmon resonance (SPR) effects similar to the noble metals and can be used as a probable candidate to substitute the noble metals.[26] Several cases of photocatalysts based on Bi metal and semiconductor, such as Bi/BiVO4, Bi/BiOCl, Bi/Bi2 MoO6, Bi/Bi2WO 6, and Bi/C3N4 have been reported.[27-31] These photocatalysts exhibit improved photocatalytic activity compared to their individual semiconductors owing to the presence of metal Bi nanoparticles. However, to date, utilizing the SPR effects of metallic Bi nanoparticle to improve the photocatalytic performance of Bi4 V2O 11 has not been investigted. Here, Bi-nanoparticle-modified-Bi4 V2O11 (Bi/Bi4 V2O11) nanohybrid has been fabricated by deposition of Bi nanoparticles on pre-synthesized Bi4 V2O 11 hollow nanocakes through a hydrothermal method. The particle size and number density of Bi nanoparticles on the surface of Bi4 V2 O11 nanocakes can be tuned by adjusting the N2H4·H2O concentration. The performance for photocatalytic CO2 reduction with H2O in gas-phase under simulated sunlight irradiation was investigated. It was revealed that the Bi-quantum-dots-modified-Bi4 V2O 11 (BQDs/Bi4 V2O11) exhibit a higher photoactivity toward photoreduction CO2 compared to the bare Bi4 V2O11 . The present work may inspire ongoing interest in utilizing Bi quantum dots as co-catalyst to boost the photocatalytic activity of photocatalysts for photocatalytic CO 2 conversion to valuable fuels using solar energy. 3



2. Experimental Section 2.1 Fabrication of Bi/Bi4 V2O11 composites Bi/Bi4 V2O11 composites were fabricated via a two-step route. Firstly, Bi4V2O11 nanostructures were synthesized using a solvothermal approach. The detailed synthesis process is as follows: 1mmol NH4 VO 3 and 2 mmol Bi(NO3)3∙5H2O were added to 15 and 20 mL ethylene glycol under continuous stirring, respectively. Afterward, the NH4 VO3 solutions were slowly added into the Bi(NO3)3∙5H2O solutions with constant magnetic stirring to form yellow suspensions. Subsequently, the resulting yellow suspensions were poured into a 50 mL Teflon-lined stainless steel autoclave, which was then sealed and maintained at 180 °C. After hydrothermal reaction for 8 h and cooling down to normal temperature, the formed brown precipitates were separated by centrifugation, washed with deionized water and alcohol several times, and vacuum-dried at 50 °C. As-obtained products were denoted as S0. For the preparation of Bi/Bi4 V2O11 composites, 500 mg as-prepared Bi4V2O11 powders were added into a 60 ml of N2H 4·H2O solution with vigorous stirring. The concentrations of N2H4·H2O in the reaction solution were about 80, 160, 240, 320, and 480 mmol/L, respectively. Subsequently, the mixture suspensions were poured into an autoclave of 80 mL capacity, which was then sealed and kept at 110 °C. After hydrothermal reaction for 4 h and cooling to normal temperature, the brown precipitates were separated by centrifugation, washed with deionized water several times, and finally vacuum-dried at 50 °C. The Bi/Bi4 V2O11 composites fabricated with 80, 160, 240, 320, and 480 mmol/L N2H 4·H2O were defined as S1, S2, S3, S4, and S5. 2.2. Catalyst Characterization Powder X-ray diffraction (XRD) analysis of as-fabricated pure Bi4 V2O 11 and Bi/Bi4 V2O11 composites were performed on a Bruker/AXS D8 Advance X-ray diffractometer equipped with Cu Kα radiation (λ=0.15406 nm). The morphology and microstructure of all samples were determined through field-emission scanning electron microscopy (FE-SEM, JEOL model JSM-6490), transmission electron 4


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microscopy (TEM, JEOL model JEM-2100F), and high resolution transmission electron microscopy (HRTEM, JEM-2100F). Energy dispersive X-ray spectroscopy (EDS) spectra and EDS mapping images of Bi4 V2O 11 were obtained on focused ion beam scanning electron microscopy (FIB-SEM, Helios NanoLab 600i). The metal Bi content in Bi/Bi4 V2O11 samples was determined with an inductively coupled plasma atomic emission spectrometry (ICP-AES, 725ES). X-ray photoelectron spectroscopy (XPS) measurements were carried out using an ESCALB MK-II VG X-ray photoelectron spectrometer with a monochromatic Al Kα irradiation (hν = 1486.6 eV). Photoluminescence (PL) analysis of the products was accomplished on a fluorescence spectrophotometer (Hitachi F-2500). The UV−vis diffuse reflective spectrum (DRS) was recorded on a spectrophotometer (U˗4100, Hitachi) using BaSO4 as a reflectance sample. The specific surface area and CO2 adsorption ability of the products were measured on a Micromeritics ASAP 2020 adsorption apparatus. Photocurrent, electrochemical impedance, and flat-band potential tests were conducted using a CHI660E Electrochemical workstation (Chenhua Instrument Company, China) according to the previously reported method.[32] The working electrode, counter electrode, and reference electrode used in the tests are fluorine-doped tin oxide (FTO) glass coated with samples, Pt wire, and Ag/AgCl (or Hg/Hg2Cl2 ) electrode, respectively. 2.3. Photocatalytic Activity The photocatalytic CO2 conversion experiments were performed in 200 mL homemade reactor under the conditions of room temperature and 1 atm pressure. A 300 W Xe-illuminator was employed as simulated sun-light source to start the photoreaction. The detailed process is as follows: 0.10 g of catalysts and 20 mL of deionized water were added into the reactor. After ultrasonic dispersion for 1 h, the reactor was kept at 80 °C for several hours in order to make the sample particles deposit on the bottom of the reactor and form a fine film. Subsequently, 1.0 mmol NaHCO3 was introduced into the homemade reactor. Before the reaction, the reactor was sealed and blown with pure N2 for 0.5 h to ensure the air was completely eliminated. After elimination of the air, 0.2 mL H2 SO4 aqueous solution with the 5



concentration of 2 mmol/L was added into the reactor through injection. CO 2 and H2O vapor were obtained through the reaction between NaHCO 3 and H2SO 4 aqueous solution in the reactor. After irradiation for 1 h, 1 mL of gas product in the reactor was sampled and analyzed by a gas chromatograph (GC-SP7890, Ruihong, China) with a flame ionized detector (FID).

3. Results and discussion

Figure 1 XRD patterns of Bi4 V2O11 (a) and Bi/Bi4 V2O11 (b-f) samples: (b) S1, (c) S2, (d) S3, (c) S4, (f) S5. The crystal structures of as-fabricated Bi4 V2O11 and Bi/Bi4 V2O 11 samples were determined using XRD. Figure 1a displays the XRD pattern of pure Bi4 V2O 11. All the noticeable characteristic peaks can be perfectly indexed to the orthorhombic Bi4 V2O11 phase (JCPDS No. 42-0135), suggesting a pure Bi4 V2O11 phase of as-obtained products. Figure 1b-f show the XRD patterns of Bi/Bi4 V2O11 composites obtained at different N2H4·H 2O concentrations. The XRD pattern in Figure 1b reveals that all diffraction peaks of S1 sample can be assigned to the Bi4 V2O11 phase and no peaks of metallic Bi can be observed. This may be due to the low amount of metallic Bi in S1 sample. From Figure 1c-f one can clearly find two sets of XRD diffractions in each XRD pattern. These peaks located at 2θ = 27.28, 37.89, 39.62, 48.66, 55.96, 62.22 and 64.48°are readily assigned to (012), (104), (110), (202), (204), (116) and (122) 6


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reflections of Bi phase, while the remaining diffraction peaks are perfectly ascribed to the Bi4 V2O11 phase, confirming that partial Bi4 V2O11 molecules were reduced to form metal Bi at hydrothermal conditions. Meanwhile, the relative peak intensity of metallic Bi increased gradually with the increase in the N2H4·H 2O concentration. It was also found that the 2θ positions of the characteristic peaks of Bi4 V2 O11 in the Bi/Bi4 V2O11 composites remained unchanged, implying that metallic Bi was on the surface of Bi4 V2O 11 rather than entering the crystal lattice.

Figure 2 SEM (a, b) and TEM (c) images of S0 sample . (d) HRTEM image obtained at the edge of a nanocake. The morphology and detailed structure of pure Bi4 V2O11 sample were investigated with SEM, TEM, and HRTEM. Figure 2a reveals that the morphology of pure Bi4 V2O11 sample is nanocakes constructed by numerous nanosheets. The average diameter and thickness of the nanokcakes are around 900 and 150 nm, respectively. Figure 2b displays a SEM image of a partially broken nanocake, which gives strong evidence that the nanocakes have a hollow interior structure. A typical TEM image of a single Bi4 V2O11 nanocake is presented in Figure 2c, which also reveals that the nanocake is built up by lots of nanoplates. Moreover, the periphery of the nanocake is darker than their center, further confirming the hollow structure of as-prepared 7



nanocakes. The representative HRTEM image obtained at the edge of a nanoplate was shown in Figure 2d. The interplanar spacing of about 0.312 nm corresponds to the (113) plane of Bi4 V2O11 .[15] The chemical components of the nanocakes were examined by the EDS. The EDS spectrum in Figure S1 (Supporting Information (SI)) shows that there are sharp peaks from V, Bi, and O. The atomic ratio of Bi : V : O is estimated to be 4 : 2 : 11, giving a normal composition of Bi4 V2O11. EDS mapping of Bi4 V2O11 was performed to examine the elemental distribution. From Figure S2 (SI) one can see that the colored dots representing the Bi, V, and O elements follow the shape of the Bi4 V2 O11 nanocake, indicating the uniform distribution of these elements. Figure S3 (SI) demonstrates the time-dependent morphological evolution of hollow Bi4 V2O11 nanocakes, which suggests that they may form via a dissolutionrecrystallization process.[33]

Figure 3 (a-e) Typical TEM images of different samples: (a) S1, (b) S2, (c, d) S3, and (d) S4. (f) HRTEM image of Bi nanoparticles on the surface of Bi4 V2O11 nanocakes (S3 sample). TEM measurements were also performed to analyze the morphology and detailed microstructure of the Bi/Bi4 V2 O11 composites. Figure 3 presents the TEM images of Bi/Bi4 V2O11 composites prepared with various concentration of N2H4·H 2O. When the concentration of is 80 mmol/L, no apparent Bi nanoparticles can be found in the TEM 8


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image (Figure 3a). This may be due to the low content of metal Bi in the composites and its very small particle size. As the concentration is increased to 160 mmol/L, some Bi nanoparticles with very small size (1~5 nm) are sparsely distributed on the surface of nanocakes. With raising the N2H4·H2O concentration to 240 mmol/L, the number density and size of Bi nanoparticles are obviously increased (Figure 3d).The particle size of Bi nanoparticles is in the range of 5~15 nm. Moreover, the surface of the nanocakes become relatively rougher compared to that of unreduced Bi4 V2O11 nanocakes. In addition, a few big spherical particles with diameter of ~100 nm can be found (as shown by the arrow in Figure 3c), which may be Bi nanoparticles. When the N2H4·H2O concentration is increased to 320 mmol/L, a layer of Bi nanoparticles with size of 10~30 nm are loaded on the surface of the nanocakes. The HRTEM image of Bi nanoparticles on the surface of nanocakes is given in Figure 3f. The interplanar spacing is determined to be 0.329 nm, corresponding to the (012) plane of metallic Bi.[34] These results indicate that Bi nanoparticles were successfully loaded on the surface of the Bi4 V2O11 nanocakes. The Bi nanoparticles may originate from the partial reduction of Bi4 V2O11 molecules via the reaction presented in equation 1.[18] The metal Bi content in the Bi/Bi4 V2O11 sample is determined by ICP-AES and presented in table 1. It is found that the metal Bi content increases with increase in the N2H4·H2O concentration. The surface areas of pure Bi4 V2O11 and Bi/Bi4 V2O11 composites were determined according to the N2 adsorption−desorption isotherms and presented in table 1. The surface areas of the Bi/Bi4 V2O11 composites first increased with the increased metal Bi content and then significantly decreased. The change of surface area may be connected with the particle size and amount of metallic Bi nanoparticles on the surface of Bi4 V2O11 nanocakes. Bi4 V2O11 + 3N2H4 + 2OH- → 4Bi + 3N2 + 2VO3- + 7H2O

(1)

To discover the chemical compositions and states of the elements in the Bi4 V2O11 and Bi/Bi4 V2O11 samples, XPS characterization was used. The high-resolution spectra of Bi 4f obtained from S0 and S2 samples were presented in Figure 4a. Two characteristic peaks of Bi 4f centered at about 158.8 and 164.1 eV are observed in the Bi fine spectrum of Bi4 V2O 11, which corresponds to the Bi 4f7/2 and Bi 4f5/2 binding 9



Figure 4 The XPS spectra for S0 (i) and S2 (ii) samples: (a) Bi 4f, (b) V 2p, and (c) O1s high resolution spectra. energies of Bi3+,[35-36] respectively. Nevertheless, in the Bi 4f XPS spectrum of Bi/Bi4 V2O11, the Bi 4f peaks are shifted to 159.2 and 164.5 eV, respectively. The vacancy of Bi in Bi4 V2O 11 and the interaction between Bi4 V2O11 and metallic Bi may result in such blue shift.[37] In addition, two very weak peaks at around 163.2 and 158.0 eV are attributed to the metal Bi in the Bi/Bi4 V2O11 composites,[23] which is good agreement with the results of XRD and HRTEM. Figure 4b displays the V 2p XPS spectra obtained from Bi4 V2O11 and Bi/Bi4 V2O11 samples. It can be seen that the V 2p binding energies of the two samples also exhibit an apparent discrepancy. This difference may be attributed to the change of V5+ coordination environment in the two samples.[29,

37]

The O 1s XPS spectrum of the Bi4 V2O11 (Figure 4c) can be

deconvoluted into two peaks. The peaks centered at 529.5 and 531.0 eV correspond to the crystal lattice oxygen and adsorbed oxygen species on the surface, [35-36] respectively. Because of the influence of metallic Bi and the vacancy of Bi in the Bi/Bi4 V2O11, the O 1s peaks of Bi/Bi4 V2O11 also slightly shift in comparison with that of Bi4 V2O11.

Figure 5 UV−vis DRS (a) and the plots of (αhv)2 vs. (hv) (b) for different samples. 10


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The light absorption properties of pristine Bi4 V2O11 and Bi/Bi4 V2O11 composites were examined using the UV–vis DRS, as presented in Figure 5a. Clearly, pure Bi4 V2O11 has a relatively weak absorption in the 550-800 nm light region, and exhibits an absorption edge at 560 nm, which is smaller than that of Bi4 V2 O11 hollow microspheres.[17] However, for the Bi/Bi4 V2O11 composites, the visible-light absorption in the 550-800 nm light region is gradually enhanced by the growing Bi plasmon band.[26] This observation is in accordance with the previous reports that Bi can display SPR absorptions in the visible-light region.[26-27,

37-38]

The band-gap

energies (Eg) of Bi4 V2O11 and Bi/Bi4 V2O11 were determined by the Kubelka–Munk equation: (αhν)2 = A(hν−Eg) n,[30] where α, h, ν, and A are the absorption coefficient, Plank constant, light frequency, and a constant, respectively. The n value for Bi4 V2O11 is 1 because it is a direct transition semiconductor.[17] Plots of (αhν)2 vs. (hν) of pure Bi4 V2O11 and Bi/Bi4 V2O11 composites are presented in Figure 5b. The Eg values are approximately 2.25, 2.04, 1.91, 1.93, and 1.90 eV for S0, S1, S2, S3, and S4, respectively, indicating that the loading Bi nanoparticles on Bi4 V2O11 can broaden the absorption band and reduce the band-gap of Bi4 V2O11 . Similar phenomenon was also observed for Bi/Bi2 WO6 and Bi/BiOIO3 .[39-40] Table 1 The Content of metallic Bi in the sample, Eg value, surface area, and CO2 chemisorption of different samples. Sample code

S0

S1

S2

S3

Content of metal Bi

0

4.17

9.45

17.33

25.68

BET (m /g)

20.43

23.77

27.62

31.86

29.25

Eg (eV)

2.25

2.04

1.91

1.93

1.90

CO2 chemisorption(cm3/g)

2.01

2.76

3.69

4.88

4.30

2

S4

PL spectroscopy was utilized to analyze the separation efficiency of photogenerated electrons and holes. Figure 6 illustrates the PL emission spectra of pristine Bi4 V2O11 and Bi/Bi4 V2O11 composites at an excitation wavelength of 300 nm. Obviously, the pure Bi4 V2O11 shows a stronger emission intensity in the 400~525 nm range compared to the Bi/Bi4 V2O11 composites, which is well accordance with the previously reported results.[41] After loading Bi nanoparticles, the PL intensity of 11



Bi/Bi4 V2O11 composites decreases greatly, among which S2 sample shows the lowest PL intensity. It is generally accepted that a stronger emission intensity is indicative of more recombination of photoinduced charge pairs.[40] Hence, the radiative recombination of photoexcited electron-hole pairs would be effectively inhibited at the surface of the Bi/Bi4 V2O11 composites.

Figure 6 PL emission spectra of different samples: (a) S0, (b) S1, (c) S2, (d) S3, and S4. Transient photocurrent response measurement was employed to inspect the effect of metallic Bi content on the migration and separation of photoexcited electrons and holes. Figure 7a shows the photocurrent density of Bi4 V2O 11 and Bi/Bi4 V2O 11 samples as a function of the illumination time under simulated sunlight irradiation. Generally, the photocurrent densities of all the samples are well responsive to the switching on/off of the xenon light. Nevertheless, the photocurrent density of Bi/Bi4V2O11 is much higher than that of pure Bi4 V2O11, indicating that the Bi/Bi4 V2O11 has a great advantage over bare Bi4 V2O11 in charge pair migration and separation. In addition, the photocurrent density on the Bi/Bi4 V2O11 electrode first increases with increasing of metallic Bi content and then significantly decreases with a further increase in the metal Bi content. S2 sample shows the highest photocurrent density among these samples. The Bi-content-dependent photocurrent density may be explained as follows. Insufficient amount of Bi nanoparticles in the Bi/Bi4 V2O 11 composites would not effectively separate photoinduced charge carriers, leading to a low photocurrent 12


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density. On the other hand, over-loading of metal Bi nanoparticles on the surface of Bi4 V2O11 nanocakes would decrease the photocurrent owing to the fact that excess amount of Bi nanoparticles serve as a recombination center of photoinduced electron-hole pairs and promotes their recombination.

Figure 7 (a) Photocurrent response, (b) CO2 adsorption isotherm curves, and (c) CO evolution rates of different samples. (d) Stability test of CO2 photoreduction activity of S2 sample. It is well known that the CO2 adsorption capacity of photocatalyst has a great influence of in its photocatalytic activity in CO2 photoreduction. The CO 2 adsorption capacity of pristine Bi4 V2O 11 and Bi/Bi4 V2O11 composites was measured. Figure 7b shows the CO2 adsorption isotherm curves of pristine Bi4 V2O11 and Bi/Bi4 V2O11 composites. As can be seen, the CO2 adsorption ability firstly enhances with the increase in the metal Bi content and then decreases significantly. The CO2 adsorption capacity of S0, S1, S2, S3, and S4 at P/P0 = 1.0 is about 2.01, 2.76, 3.69, 4.88, and 4.30 cm-3·g-1, respectively. The decreased adsorption ability of S4 sample may be ascribed to its decreased surface area. These results suggest that the metal Bi content can improve the CO2 adsorption capacity of Bi/Bi4 V2 O11, and the sample with larger surface area exhibits a higher CO2 adsorption capacity, which may promote the surface reaction kinetics.[42] The photocatalytic performance of pristine Bi4 V2O11 and Bi/Bi4 V2O11 composites was evaluated in terms of CO2 photoreduction under simulated sunlight illumination. 13



For comparison, several controlled experiments were also carried out. It was demonstrated that only CO2 gas was detected in the reaction system when the photocatalytic reaction was performed in the absence of catalysts or in the dark. While, in the presence of both photocatalysts and light irradiation, CO as the only reaction product is detected, confirming that the CO2 photoreduction surely occurred through a photocatalytic reaction. Figure 7c shows the evolution rate of CO for Bi4 V2O11 and Bi/Bi4 V2O11 catalysts after 4h of light illumination. As can be seen, the amount of CO evolution first increases with increasing the metallic Bi content and then decreases greatly. Among these samples, S2 sample exhibits the highest CO generation rate of 3.24 μmol/g·h, which is about 3 times as high as that of pure Bi4 V2O11 under the identical conditions. The present CO evolution rate is close to that of previously reported photocatalysts used for CO2 conversion into CO.[43] It is well acknowledged that the surface area, recombination rate of photoinduced charge pairs, particle size, and CO2 adsorption ability of photocatalysts have a great influence in the photocatalytic activity of photocatalysts.[44-46] Although S3 and S4 samples have bigger surface areas and higher CO2 adsorption capacities, they possess lower CO evolution rate compared to S2 sample. This implies that particle size of metal Bi and recombination rate of photoinduced charge pairs in Bi/Bi4 V2O11 are the major factors affecting its photocatalytic activity. In order to examine the photostability of S2 sample, the cyclic CO2 photoreduction test was performed and the results were presented in Figure 7d. It is clear that S2 sample maintain its high photocatalytic CO2 reduction activity in three continuous runs. The XRD pattern, SEM and TEM images of S2 sample after three continuous photocatalytic reactions were presented in Figure S4 (SI). It is found that the crystal structure and morphology of Bi/Bi4 V2O11 are well kept after the photocatalytic reactions. The HRTEM image of S2 sample after three continuous photocatalytic reactions is also shown in Figure S4. It displays the clear interplanar spacing of 0.328 and 0.249 nm, which belongs to the (012) planes of Bi4 V2O11 and (102) planes of Bi2O3 , respectively. The Bi2O3 nanoparticles may derive from the partial oxidation of Bi nanoparticles during the process of photocatalytic reactions. 14


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To discover the photocatalytic mechanism of Bi/Bi4 V2O11 composites, the band structure of Bi4 V2O11 was analyzed on the basis of the combination of the Eg value and flat-band potential. Figure S5 (SI) shows the Mott-Schottky plot of pure Bi4 V2 O11 (S0 sample) using 0.2 M Na2SO 4 solution as an electrolyte. Clearly, the slope of the plot is positive, indicating that the Bi4 V2O11 belongs to an n-type semiconductor. The measured flat-band potential of Bi4 V2O11 is about -0.462 V vs. Hg/Hg2Cl2 at pH =7. The conduction band (CB) edge potential of Bi4 V2O 11 can be calculated by the conversion formula: E(NHE) = E(Hg/Hg2 Cl2) ˗ Eɵ + 0.059pH, [47]

where E(NHE) is

the normal hydrogen electrode potential (NHE), E(Hg/Hg2Cl2) is the Hg/Hg2Cl2 electrode potential, and Eɵ (Hg/Hg2 Cl2, at pH = 7) is 0.2415 V. The estimated CB value of Bi4 V2O11 is -0.291 V (vs NHE, pH=7), which is more positive than that of amorphous Bi4 V2O11 (-0.544 V (vs NHE, pH=7)).[48] Combining with the band-gap of Bi4 V2O11 (2.25 eV), the valance band (VB) edge potential of Bi4 V2O 11 can be calculated according to the formula: ECB = EVB − Eg. The calculated VB edge potential of Bi4 V2O11 is 1.96 eV (vs NHE, pH=7), which is similar to the previous reported value.[41]

Figure 8 Migration and separation process of photoexcited charge pairs and possible photocatalytic mechanism of BQDs/Bi4 V2O11 for CO2 reduction. Based on the above results and the previous reports, the possible migration and separation process of photoexcited electrons and holes in BQDs/Bi4 V2O11 composites is elucidated schematically in Figure 8. Under simulated sunlight irradiation, Bi4 V2O11 is excited to produce charge pairs. Due to the Fermi level of metal Bi (-0.17eV vs NHE) is more positive than the CB of Bi4 V2O11 (-0.224 eV),[49] the photogenerated electrons in Bi4 V2O11 would transfer to metallic Bi, while the holes 15



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accumulate in the VB of Bi4 V2O11. The H2O molecules can be directly oxidized by holes to form O2 and protons. Since the redox potential of CO2/CO (-0.53 V) is higher than the Fermi level of Bi,[50] the photogenerated electrons accumulated in Bi could not reduce the CO 2 to CO. Very recently, it has been demonstrated that the Bi quantum dots (BQDs) as co-catalyst can enhance the photocatalytic activity of catalysts in CO2 reduction by borrowing redox conversion between Bi2O3 and Bi.[51] In our case, the Bi nanoparticles in S1 and S2 samples may also directly react with CO2 to form Bi2O3 and CO due to the very small particle size distribution (1~5 nm). Subsequently, as-formed Bi2O3 quantum dots can be reduced to BQDs by the photoinduced electrons from Bi4 V2O11 because of the quantum confinement effect of Bi2O3 quantum dots. [51] As for S3 and S4 samples, though they have larger CO 2 adsorption capacities compared to S1 and S2 samples, the Bi nanopartciels in these sample could not reduce CO2 to CO due to their big particle size (over 5 nm), resulting in a very low evolution rate of CO2. Therefore, the particle size of Bi nanoparticle may be a critical factor for the photocatalytic CO2 reduction. The proposed reaction pathway may be illustrated by the equations (2)-(6) In addition, the SPR effect of Bi BQDs can propel Bi4 V2O11 to absorb more visible light and enhance charge migration and separation,[52] thereby greatly boosting the photocatalytic activity of S1 and S2 samples. Bi/Bi4 V2O11 + hν → e- + h+ + Bi/Bi4 V2O11

(2)

4h+ + 2H2O → O2 + 4H+

(3)

2Bi + 3CO2 → Bi2O3 + 3CO

(4)

4Bi + 3O2 → 2Bi2O3

(5)

Bi2O3 + 6H+ + 6e- → 2Bi + 3H2O

(6)

4. Conclusions In summary, a novel hollow cake-like BQDs/Bi4 V2O11 nanostructure was fabricated via a two-step hydrothermal process for the first time and used as photocatalyst for CO2 photoreduction under simulated sunlight irradiation. The 16


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optimized BQDs/Bi4 V2O 11 composites exhibited a lower recombination rate of photoexcited charge pairs and a smaller particle size of metallic Bi, and thus possess a significantly enhanced photocatalytic performance toward photoreduction CO2 under simulated sunlight irradiation. The optimal BQDs/Bi4 V2O11 composites showed a CO2 conversion rate of 3.24 μmol/g·h, which is about 3 times as high as that of pure Bi4 V2O11. It is expected that as-fabricated BQDs/Bi4 V2O11 heterostructures can find a wide range of application in environmental pollution and energy production.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: EDX spectrum and elemental mapping images of Bi4 V2O14; TEM images of Bi4 V2O14 obtained at different reaction time; XRD pattern, SEM, and TEM image of S2 sample after photocatalytic reaction; Mott–Schottky plots of Bi4 V2O14 .

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

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