Oxygen-Induced Bi5+-Self-Doped Bi4V2O11 with a p–n Homojunction

Jun 27, 2017 - Benefiting from these favorable properties, Bi5+-BVO exhibits a ... x Solid Solution toward Promoting Visible-Light Driven Photocatalyt...
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Oxygen-Induced Bi5+-Self-Doped Bi4V2O11 with a p−n Homojunction Toward Promoting the Photocatalytic Performance Chade Lv, Gang Chen,* Xin Zhou,* Congmin Zhang, Zukun Wang, Boran Zhao, and Danying Li MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China S Supporting Information *

ABSTRACT: Bi5+-self-doped Bi4V2O11 (Bi5+-BVO) nanotubes with p−n homojunctions are fabricated via an oxygeninduced strategy. Calcinating the as-spun fibers with abundant oxygen plays a pivotal role in achieving Bi5+ self-doping. Density functional theory calculations and experimental results indicate that Bi5+ self-doping can narrow the band gap of Bi4V2O11, which contributes to enhancing light harvesting. Moreover, Bi5+ self-doping endows Bi4V2O11 with n- and ptype semiconductor characteristics simultaneously, resulting in the construction of p−n homojunctions for retarding rapid electron−hole recombination. Benefiting from these favorable properties, Bi5+-BVO exhibits a superior photocatalytic performance in contrast to that of pristine Bi4V2O11. Furthermore, this is the first report describing the achievement of p−n homojunctions through self-doping, which gives full play to the advantages of self-doping. KEYWORDS: self-doping, p−n homojunction, photocatalysis, Bi4V2O11, density functional theory

1. INTRODUCTION Thus far, unrelenting efforts have been devoted to the optimization of photocatalysts by various kinds of methods, such as doping,1 constructing heterojunctions,2 and nanostructuring.3 Self-doping, as an exceptional element doping, is emerging as a promising strategy to improve the photocatalytic performance.4−7 Promoted transport of charge carriers can be achieved by self-doping, while the latent adverse impact stemmed from doping with adventitious species, for instance, undesirable thermal instability, can be avoided.8 In addition, it is conductive to strengthening the light-harvesting efficacy due to the narrowed band gap or formation of intermediate levels between the conduction band (CB) and valence band (VB).9−11 Therefore, self-doping holds the potential to exploit semiconductor photocatalysts to the utmost. Motivated by these excellent properties, self-doping has been introduced into a variety of photocatalysts by elaborate approaches, especially through reduction treatment, which can result in low-valence self-doping with little hindrance.12,13 Although great progresses have been achieved in introducing self-doping by the reduction treatment, there still remain limitations, which are as follows: (a) the risk of forming metal rather than low-valence self-doping in some frequently investigated catalysts, such as Bi-based semiconductors, which are considered as one category of the most active photocatalysts;14 (b) induced by the reduction treatment, oxygen vacancies are generated accompanied with low-valence selfdoping, which might produce deep, localized, and filled states near the valence band maximum (VBM).15 States close to the © 2017 American Chemical Society

VBM are fully occupied and therefore do not benefit electron transport.15 Relative to the reduction treatment, the oxidation treatment will not give rise to the generation of oxygen vacancies, while the oxygen atom can be introduced into the lattice.16 This feature provides an alternative approach to realizing highvalence self-doping through an oxidation method. Very recently, Bi4V2O11 with intrinsic oxygen vacancies and a layered structure has been regarded as an excellent photocatalyst for oxygen evolution and water purification.17−20 Inspired by its peculiar crystalline structure, it is conjectured that Bi4V2O11 possesses adequate space for introducing oxygen atoms to realizing high-valence self-doping. Furthermore, an n−p-type transformation in Bi4V2O11 might occur because of the implanted oxygen, leading to the coexistence of n- and p-type domains.21 Thereby, a p−n-type photochemical homojunction could be constructed, where the Fermi-level (Ef) alignment of the p- and n-type domains emerges, creating a built-in field.22 The photogenerated electrons and holes would separate effectively by means of migrating in opposite directions at the homojunction interface,23 ulteriorly promoting the photocatalysis.24 Thus far, aiming at constructing p−n homojunctions, surface modification,22 element doping,23 and electrodeposition25 are employed. Nevertheless, the construction of p−n homojunctions Received: April 15, 2017 Accepted: June 27, 2017 Published: June 27, 2017 23748

DOI: 10.1021/acsami.7b05302 ACS Appl. Mater. Interfaces 2017, 9, 23748−23755

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2. EXPERIMENTAL SECTION

achieved via self-doping is overlooked. Thus, in view of the merits introduced by self-doping, we hypothesize that Bi5+-selfdoped Bi4V2O11 (Bi5+-BVO) could serve as a standout photocatalyst. In this work, oxygen-induced Bi5+-BVO is fabricated by tuning the oxygen ambience during the heat treatment of asspun fibers (Scheme 1). The electronic band structures,

2.1. Preparation and Characterizations. The fabrication of photocatalysts is similar to that in our previous work (details are presented in the Supporting Information).20 To fabricate Bi4V2O11 without Bi5+ doping (denoted BVO), 0.5 g of as-spun fibers was annealed in a covered crucible to furnish insufficient oxygen ambience, as shown in Scheme 1. For Bi5+-BVO, 0.5 g of as-spun fibers was calcinated in an uncovered crucible with sufficient oxygen. The crystalline structures of Bi5+-BVO and BVO were confirmed by X-ray diffraction (XRD) on a Rigaku D/max-2000 diffractometer with Cu Kα radiation (λ = 0.15406 nm). Diffraction patterns were collected from 10 to 90° at a speed of 4 °C min−1 with a scan width of 0.02°. The morphology characterizations were observed by a HELIOS NanoLab 600i field emission scanning electron microscope. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed on FEI Tecnai G2 S-Twin operating at 300 kV. Brunauer−Emmett−Teller (BET) data were obtained with a Micromeritics accelerated surface area and porosimetry system 2020 (ASAP 2020). Ultraviolet−visible (UV−vis) diffuse reflectance spectra were acquired by HITACHI UH-4150. X-ray photoelectron spectroscopy (XPS) was carried out using a Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectrometer with a pass energy of 20.00 eV and an Al Kα excitation source (1486.6 eV). Electron paramagnetic resonance (EPR) spectra were recorded on a Bruker A200 EPR spectrometer operating at room temperature with micro frequency of 9.86 GHz. Photoluminescence (PL) spectra and time-resolved fluorescence decay spectra were recorded on HORIBA FluoroMax-4. 2.2. Photocatalytic and Photoelectrochemical Measurements. The photocatalytic reduction of Cr(VI) is consistent with that in our previous work.20 The photocatalytic reduction of Cr(VI) was irradiated under visible light (>400 nm) in the presence of citric acid. In a typical procedure, 0.05 g of the photocatalyst was added into 100 mL of the Cr(VI) solution (10 mg L−1, which was based on Cr in a dilute K2Cr2O7 solution). The photocatalyst was dispersed in the solution by ultrasonic treatment for 5 min. Then, the solution was stirred for 40 min in the dark to attain adsorption equilibrium. Prior to irradiation, 0.05 g of citric acid was added into the solution. The photocatalyst was removed by centrifugation at given time intervals,

Scheme 1. Schematic Illustration of Fabricating Bi5+-BVO by Tuning the Oxygen Ambience

photoabsorption, and photoinduced charge carrier transport properties of Bi5+-BVO are investigated by density functional theory (DFT) calculations and systematic measurements. Enhanced photoabsorption and boosted photoinduced charge carrier transport are realized in Bi5+-BVO. Furthermore, Bi5+ self-doping leads to the construction of p−n homojunctions, which could facilitate the separation of photoinduced electrons and holes stemmed from the as-created built-in field. Accordingly, the above factors determine the superior photocatalytic performance of Bi5+-BVO for reduction of Cr(VI).

Figure 1. XRD patterns of the Bi5+-BVO and BVO samples at different ranges: (a) 10−60° and (b) 27−30°; (c) crystal structure of Bi4V2O11; XPS spectra of the Bi5+-BVO and BVO samples: (d) Bi 4f spectra, (e) V 2p spectra, and (f) O 1s spectra. 23749

DOI: 10.1021/acsami.7b05302 ACS Appl. Mater. Interfaces 2017, 9, 23748−23755

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Figure 2. SEM and TEM characterizations of Bi5+-BVO: (a) SEM image; the insert is the magnified cross section, (b) TEM image, (c) HAADF image, (d) line scan profile, (e) HRTEM image, and the (f) SAED pattern. and the Cr(VI) concentration was determined at 352 nm using a HITACHI UH-5300 UV−vis spectrometer. The Mott−Schottky plots of Bi5+-BVO and BVO were measured by applying potentials in the range of 2.6−0.6 V versus an Ag/AgCl reference electrode. A 300 W Xe light source was employed. An Autolab electrochemical working station with a standard threecompartment cell was employed for the transient photocurrent response and electrochemical impedance spectroscopy (EIS) measurements of the samples. A fluorine-doped tin oxide glass with a photocatalyst coating was used as the working electrode, and a piece of Pt sheet, a Ag/AgCl electrode, and 0.5 M sodium sulfate served as the counter electrode, reference electrode, and electrolyte, respectively. 2.3. Computational Details. All DFT calculations were performed using the Vienna ab initio simulation package within the projector-augmented wave (PAW) formalism.26,27 PAW28 potentials for treating electron−ion interactions and Perdew−Burke−Ernzerhof functionals were employed. The cutoff energy of 400 eV and 6 × 4 × 6 k-point mesh centered at the G point was used for the plane-wave basis set. The convergence tolerance for self-consistent energy is set at 10−5 eV, and all atomic structures were fully relaxed until the residual forces on all atoms were smaller than 0.01 eV/Å.

crystalline structure of Bi4V2O11 (Figure 1c), the layered structure might furnish adequate space for introducing oxygen atoms to oxide Bi3+ into Bi5+. To shed light on the defective structure, XPS is carried out. In Bi 4f XPS spectra (Figure 1d), only two photoelectron peaks located at 159.2 and 164.5 eV are detected in BVO, which are corresponding to Bi3+ in Bi4V2O11.20,30 However, for Bi5+-BVO, except for Bi3+ peaks, two new peaks arise at 160.6 and 165.9 eV, which could be assigned to the Bi5+ defects.31,32 As for V 2p XPS spectra shown in Figure 1e, V peaks located at 516.7 and 524.3 eV belong to V5+, whereas no V4+ emerges, indicating that only Bi5+ defects serve as self-doping elements.20,30 From the observation in O 1s spectra (Figure 1f), two O 1s peaks could be clearly distinguished in BVO, which are located at 529.7 and 530.8 eV. The former corresponds to the lattice oxygen (M−O), whereas the latter could originate from the intrinsic oxygen vacancy (Vo) in Bi4V2O11.33−35 Nevertheless, upon calcinating with abundant oxygen, the Vo peak disappears in Bi5+-BVO. Instead, two new peaks emerge, located at 531.5 and 533.0 eV, which could be assigned to M−OH and H2O(l).36 EPR analysis (Figure S1) can also suggest the absence of oxygen vacancy in Bi5+-BVO, whereas BVO possesses a typical signal of oxygen vacancies at g = 2.001.37 On the basis of the above results, we can safely draw the conclusion that Bi5+ self-doping is successfully induced by abundant oxygen, which accomplishes the achievement of selfdoping relying on the presence of intrinsic oxygen vacancies. Moreover, the failure in introducing Bi5+ in other Bi-based photocatalysts (such as Bi2WO6 and Bi2MoO6) without intrinsic oxygen vacancies can further affirm the above opinions.38,39 Therefore, the doping equation could be described as follows

3. RESULTS AND DISCUSSION In a typical synthesis, the first step is to electrospin precursor fibers (Scheme 1). Bi5+-BVO is obtained by calcinating the asspun fibers in an uncovered crucible with sufficient oxygen. When the crucible is covered to isolate the as-spun fibers from a sufficient oxygen ambience, Bi4V2O11 without Bi5+ doping (BVO) is fabricated. The XRD analysis (Figure 1a) suggests that Bi4V2O11 (BVO) and Bi5+-BVO consist of pure Bi4V2O11 (JCPDS No. 420135).18,20 Tuning the oxygen ambience might have impacts on the crystalline structure of the samples. In magnified XRD patterns (Figure 1b), the peak shifts of Bi5+-BVO toward lower 2θ values can be observed. Because the ionic radius of Bi5+ (76 pm) is relatively small than that of Bi3+ (103 pm), the peak shift could be due to the presence of defective Bi5+ in Bi4V2O11.29 Moreover, as displayed in the schematic presentation of the

2BiBi + O2 → 2Bi•• Bi + 2O″ i 5+

(1) 5+

In addition to the Bi defect, Bi -BVO also possesses a tubular structure, which is subjected to detailed morphological 23750

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Figure 3. (a) UV−vis DRS of Bi5+-BVO and BVO and (b) plots of (αhν)n/2 vs hν (n = 4).

Figure 4. Crystal structures of BVO (a) and Bi5+-BVO (b), electronic band structures of BVO (c) and Bi5+-BVO (d), and DOS plots of BVO (e) and Bi5+-BVO (f). The insert in (f) is the corresponding magnified DOS plot.

pattern also suggests that Bi5+-BVO is assigned to the orthorhombic phase. As shown above, with abundant oxygen ambience, we obtain Bi5+-BVO with a tubular structure, which is derived from solid as-spun fibers (Figure S3). In a sharp contrast, in the absence of abundant oxygen ambience, Bi4V2O11 nanofibers are fabricated with a solid interior (Figure S4). These results imply that tuning the oxygen ambience not only induces Bi5+ self-doping but also results in the formation of a tubular structure. To further affirm the above viewpoint, the as-spun fibers are calcinated in O2 flow and H2/Ar atmosphere (denoted Bi5+BVO-O2 and H-BVO, respectively). When calcinated in O2 flow, the Bi5+ concentration in Bi5+-BVO-O2 does not show an

characterizations as below. The diameter of the nanotubes is approximately 60 nm (Figure 2a), and the insert image in Figure 2a is the magnified cross section of the Bi5+-BVO nanotubes. Careful TEM (Figures 2b and S2) and high-angle annular dark-field (HAADF) (Figure 2c) observations can also confirm the tubular structure. The energy-dispersive X-ray line profile (Figure 2d) indicates a typical curve shape of a tubular structure. In addition, Bi, V, and O are well-distributed throughout the nanotube with good homogeneity. As shown in the HRTEM image (Figure 2e), the interplanar spacing of 0.28 nm corresponds to the (200) plane of orthorhombic Bi4V2O11. The selected-area electron diffraction (SAED) 23751

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Figure 5. Mott−Schottky plots of Bi5+-BVO (a) and BVO (b) and the illustration of charge transfer in the p−n homojunction of Bi5+-BVO (c).

obvious increase compared to that in Bi5+-BVO, indicating that self-doping of Bi5+ in Bi5+-BVO has already reached the upper limit (Figure S5). The absence of the tubular structure and Bi4V2O11 phase proves that a sufficient oxygen atmosphere is the determinant factor for constructing Bi4V2O11 with a tubular structure (Figures S6 and S7). Therefore, the oxygen-induced mechanism of Bi5+ self-doping and formation of the tubular structure is proposed (see Figure S8 and the corresponding explanation in the Supporting Information). Moreover, as demonstrated by BET measurements (Figure S9), the tubular structure endows Bi5+-BVO with a higher surface area (7.09 m2 g−1) relative to that of BVO possessing a solid interior (3.52 m2 g−1). For the purpose of illuminating the impact of Bi5+ selfdoping, UV−vis diffuse reflection spectra (DRS) are acquired to analyze the optical properties of the samples. As shown in Figure 3a, BVO exhibits favorable visible-light absorption and the absorption edge is located at approximately 550 nm. In contrast with BVO, the as-fabricated Bi5+-BVO has an extended absorption edge up to ca. 580 nm, indicating that the lightharvesting ability is promoted after the introduction of Bi5+ selfdoping. Such a red shift for Bi5+-BVO results in a distinct color change from yellow (pristine Bi4V2O11) to orange red (Bi5+BVO) (the inset in Figure 3a). The band gaps of Bi5+-BVO and BVO are calculated to be 2.15 and 2.25 eV, respectively (Figure 3b). Aiming at exploring the impact of Bi5+ self-doping on the crystal structure and electronic structure of Bi5+-BVO, a full DFT calculation is performed. On the basis of the above proposed doping equation and experimental results, one oxygen atom is introduced to Bi4V2O11 and the model of Bi5+-BVO is constructed with six oxygen atoms surrounding the central Bi5+ ion to form a distorted octahedron BiO6 (Figure 4a,b). The calculated electronic band structure along the Brillouin zone path of the orthorhombic lattice and the density of state (DOS) plot of pristine Bi4V2O11 and Bi5+-BVO are shown in Figure 4c−f, respectively. Figure 4c shows that the CB minimum and VBM are located at the Γ point with a direct band gap of 2.17 eV, which is consistent with the experimental and previous theoretical results.18 The DOS (Figure 4e) shows that the CB is mainly contributed by the O 2p and Bi3+ 6s,p states, wheras the VB is composed primarily of the V 3d and O 2p states, indicating a strong ionic bond character of the Bi−O and V−O bonds. With respect to Bi5+-BVO, Bi5+ and O-add states appearing in the CB and VB near the Fermi level (Figure 4d,f) result in the lower of the CB and the increase of the VB, which leads to narrower band gap. The results are in good agreement with our experimental observation. Interestingly, for

pristine Bi4V2O11, the Fermi level occurs at the VBM, whereas Bi5+ self-doping causes the VBM to go upward and exceed the Fermi level, consequently leading to p-type Bi4V2O11. In contrast with the intrinsic n-type conductivities of Bi4V2O11, the introduction of Bi5+ self-doping might endow Bi4V2O11 with both n- and p-type conductivities, hence constructing p−n homojunctions. To corroborate the above argument experimentally, the Mott−Schottky curves of Bi5+-BVO and BVO are performed (Figure 5a,b). As observed in Figure 5b, BVO exhibits an intrinsic n-type characteristic due to the simple presence of a positive slope. Interestingly, in a sharp contrast with Bi5+-BVO, the straight lines with both positive and negative slopes can be observed in Bi5+-BVO simultaneously (Figure 5a), suggesting that Bi5+ self-doping endows Bi4V2O11 with n- and p-type characteristics. Hence, a p−n-type homojunction is constructed, ascribed to the coexistence of the n- and p-type domains in Bi5+-BVO, which meets the above expectations. The construction of a p−n-type homojunction can introduce a builtin field at the interface stemmed from the Fermi-level (EF) alignment of the p- and n-type domains (Figure 5c).22 Attributed to the as-constructed p−n homojunction, the photoinduced charge carriers can separate and transport effectively to participate in the photocatalysis reaction.22,24,25,40 For all we know, this should be the first report on the construction of p−n homojunctions induced by self-doping. The Bi5+ self-doping-induced band structure modulation leads to the construction of p−n homojunctions as above. It is anticipated that this p−n homojunction is conductive to facilitating the separation and transport of photoinduced carriers during the photocatalysis process. Furthermore, the charge separation and transport dynamics of photoelectrons are examined by means of photocurrent responses.41 Figure 6a exhibits the current responses of Bi5+-BVO and BVO from 0.4 to 0.8 eV (vs Ag/AgCl) under illumination of visible light. By contrast, the enhanced photocurrent intensity could be discerned in Bi5+-BVO, whereas BVO shows a faintish photocurrent response. This reveals that the rapid transport of photoinduced electron−hole pairs is realized in p−n homojunctions of Bi5+-BVO.42 EIS experiments are utilized to monitor the electron generation and charge transport characteristics of Bi5+-BVO and BVO.43,44 From the observation in Figure 6b, the arc radius on the EIS Nyquist plot of Bi5+-BVO is much smaller compared to that of BVO, implying promoted separation and transport efficiency of photoinduced charge carriers due to the construction of p−n homojunctions in Bi5+-BVO. 23752

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Figure 6. (a) Photocurrent response of Bi5+-BVO and BVO electrodes under visible-light illumination, (b) electrochemical impedance spectra of Bi5+-BVO and BVO, (c) PL spectra and (d) time-resolved fluorescence decay spectra of Bi5+-BVO and BVO, and photocatalysis measurements of BVO and Bi5+-BVO: (e) Cr(VI) reduction and (f) cycling performance.

revealed by the above analysis, Bi5+ self-doping leads to the construction of p−n homojunctions; therefore, the transport of photoinduced electron−hole pairs is effectively boosted. Stimulated by Bi5+ self-doping, which leads to enhanced photoabsorption and fantastic transport efficiency of photogenerated charge carriers, the as-fabricated Bi5+-BVO could be conductive to being used as a promising catalyst for photocatalysis applications. Figure 6e shows the photocatalytic reduction of Cr(VI) on Bi5+-BVO and BVO under visible-light illumination. Compared with BVO, Bi5+-BVO exhibits a better photocatalytic activity, reaching approximately 87% of Cr(VI)

To further confirm the enhanced separation efficiency of photogenerated charge carriers in Bi5+-BVO, PL and timeresolved fluorescence decay spectra are recorded.45 In contrast to the strong emission peaks of BVO (Figure 6c), Bi5+-BVO displays rather weak PL peaks, indicating that the p−n homojunction can effectively suppress the electron−hole recombination.46 From the observation of time-resolved fluorescence decay spectra (Figure 6d), the decay kinetics of Bi5+-BVO is relatively slow.47 The increased lifetimes of carriers in Bi5+-BVO could be ascribed to the promotion of the photoinduced charge carrier separation and transfer. As 23753

DOI: 10.1021/acsami.7b05302 ACS Appl. Mater. Interfaces 2017, 9, 23748−23755

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ACS Applied Materials & Interfaces Notes

reduction after 100 min. However, only 47% of Cr(VI) can be reduced under the same conditions. The comparable performance of both photocatalysts observed after 100 min discloses that introduction of Bi5+ self-doping can dramatically enhance the photoreduction activity of Bi4V2O11. In addition, as the morphology of Bi5+-BVO is basically maintained after the photocatalysis reaction (Figure S10), good cycling performance is achieved, as displayed in Figure 6f. On the basis of the above analysis, there are three main reasons for the promoted visible-light photocatalytic activity of Bi5+-BVO. Remarkably, the narrow band gap stemmed from Bi5+ doping expands light harvesting, which can excite more photogenerated electrons and holes for participating in photocatalysis reactions (Figure S11). Second, an enlarged surface area originated from the tubular structure contributes to more active sites for photocatalysis. Last but not the least, attributed to Bi5+ self-doping, the construction of a p−n homojunction is achieved, which can efficiently boost the separation and transfer of photogenerated electrons and holes. Consequently, the above-mentioned merits determine the superior photocatalytic performance of Bi5+-BVO.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21471040 and 21303030).

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4. CONCLUSIONS In summary, oxygen-induced Bi5+-BVO with promoted photocatalytic activity is fabricated by tuning the oxygen ambience during the heat treatment procedure. The DFT calculation results reveal that Bi5+ self-doping can narrow the band gap realized by lowering the CB position and increasing the VB position simultaneously. The enhanced light-harvesting ability is verified by UV−vis DRS. In addition, a p−n homojunction is constructed in Bi4V2O11 due to Bi5+ self-doping; therefore, facilitated photoinduced charge carrier transport and retarded electron−hole recombination are realized, as confirmed by the photocurrent response, EIS, PL, and time-resolved fluorescence decay spectra. Hence, Bi5+-BVO shows enhanced photocatalytic properties for Cr(VI) relative to those of pristine Bi4V2O11. The current findings might open up new ways for the design and fabrication of self-doped photocatalysts with outstanding activities.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b05302. EPR spectra of Bi5+-BVO and BVO (Figure S1), TEM images of Bi5+-BVO (Figure S2), SEM images of BVO (Figure S3), SEM images of as-spun fibers (Figure S4), XPS spectra of Bi5+-BVO and Bi5+-BVO-O2 (Figure S5), SEM images of H-BVO (Figure S6), XRD pattern of HBVO (Figure S7), formation mechanism of nanotubes and pristine Bi4V2O11 solid nanofibers (Figure S8), BET measurements of Bi5+-BVO and BVO (Figure S9), SEM images of the recycled Bi5+-BVO catalyst (Figure S10), and photocatalysis Cr(VI) reduction plots of BVO and Bi5+-BVO (>550 nm) (Figure S11) (PDF)

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REFERENCES

(1) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. VisibleLight Photocatalysis in Nitrogen-Doped Titanium Oxides. Science 2001, 293, 269−271. (2) Li, H. J.; Zhou, Y.; Tu, W. G.; Ye, J. H.; Zou, Z. G. State-of-theArt Progress in Diverse Heterostructured Photocatalysts toward Promoting Photocatalytic Performance. Adv. Funct. Mater. 2015, 25, 998−1013. (3) Osterloh, F. E. Inorganic Nanostructures for Photoelectrochemical and Photocatalytic Water Splitting. Chem. Soc. Rev. 2013, 42, 2294−2320. (4) Zuo, F.; Wang, L.; Wu, T.; Zhang, Z. Y.; Borchardt, D.; Feng, P. Y. Self-Doped Ti3+ Enhanced Photocatalyst for Hydrogen Production under Visible Light. J. Am. Chem. Soc. 2010, 132, 11856−11857. (5) Zuo, F.; Bozhilov, K.; Dillon, R. J.; Wang, L.; Smith, P.; Zhao, X. Active Facets on Titanium(III)-Doped TiO2: An Effective Strategy to Improve the Visible-Light Photocatalytic Activity. Angew. Chem., Int. Ed. 2012, 124, 6327−6330. (6) Zhang, X.; Zhang, L. Z. Electronic and Band Structure Tuning of Ternary Semiconductor Photocatalysts by Self Doping: The Case of BiOI. J. Phys. Chem. C 2010, 114, 18198−18206. (7) Dong, G. H.; Zhao, K.; Zhang, L. Z. Carbon Self-Doping Induced High Electronic Conductivity and Photoreactivity of g-C3N4. Chem. Commun. 2012, 48, 6178−6180. (8) Huang, H. W.; Li, X. W.; Wang, J. J.; Dong, F.; Chu, P. K.; Zhang, T. R.; Zhang, Y. H. Anionic Group Self-Doping as a Promising Strategy: Band-Gap Engineering and Multi-Functional Applications of High-Performance CO32−-Doped Bi2O2CO3. ACS Catal. 2015, 5, 4094−4103. (9) Wang, J. Q.; Su, S. Y.; Liu, B.; Cao, M. H.; Hu, C. W. One-Pot, Low-Temperature Synthesis of Self-Doped NaTaO3 Nanoclusters for Visible-Light-Driven Photocatalysis. Chem. Commun. 2013, 49, 7830− 7832. (10) Ding, X.; Ho, W. K.; Shang, J.; Zhang, L. Z. Self Doping Promoted Photocatalytic Removal of NO under Visible Light with Bi2MoO6: Indispensable Role of Superoxide Ions. Appl. Catal., B 2016, 182, 316−325. (11) Yuan, X. T.; Wang, X.; Liu, X. Y.; Ge, H. X.; Yin, G. H.; Dong, C. L.; Huang, F. Q. Ti3+-Promoted High Oxygen-Reduction Activity of Pd Nanodots Supported by Black Titania Nanobelts. ACS Appl. Mater. Interfaces 2016, 8, 27654−27660. (12) Zhang, Y.; Xing, Z. P.; Liu, X. F.; Li, Z. Z.; Wu, X. Y.; Jiang, J. J.; Li, M.; Zhu, Q.; Zhou, W. Ti3+ Self-Doped Blue TiO2(B) SingleCrystalline Nanorods for Efficient Solar-Driven Photocatalytic Performance. ACS Appl. Mater. Interfaces 2016, 8, 26851−26859. (13) Zhou, C.; Zhao, Y. F.; Shang, L.; Cao, Y. H.; Wu, L. Z.; Tung, C. H.; Zhang, T. R. Facile Preparation of Black Nb4+ Self-Doped K4Nb6O17 Microspheres with High Solar Absorption and Enhanced Photocatalytic Activity. Chem. Commun. 2014, 50, 9554−9556. (14) He, R. A.; Cao, S. W.; Zhou, P.; Yu, J. G. Recent Advances in Vsible Light Bi-Based Photocatalysts. Chin. J. Catal. 2014, 35, 989− 1007. (15) Socratous, J.; Banger, K. K.; Vaynzof, Y.; Sadhanala, A.; Brown, A. D.; Sepe, A.; Steine, U.; Sirringhaus, H. Electronic Structure of LowTemperature Solution-Processed Amorphous Metal Oxide Semiconductors for Thin-Film Transistor Applications. Adv. Funct. Mater. 2015, 25, 1873−1885. (16) Etacheri, V.; Seery, M. K.; Hinder, S. J.; Pillai, S. C. Oxygen Rich Titania: A Dopant Free, High Temperature Stable, and Visible-Light Active Anatase Photocatalyst. Adv. Funct. Mater. 2011, 21, 3744−3752.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: (+86)-451-86413753 (G.C.). *E-mail: [email protected]. Fax: (+86)-451-86413753 (X.Z.). ORCID

Gang Chen: 0000-0003-1502-0330 23754

DOI: 10.1021/acsami.7b05302 ACS Appl. Mater. Interfaces 2017, 9, 23748−23755

Research Article

ACS Applied Materials & Interfaces (17) Chen, X. F.; Liu, J. B.; Wang, H.; Ding, Y. L.; Sun, Y. X.; Yan, H. One-Step Approach to Novel Bi4V2O11 Hierarchical Hollow Microspheres with High Visible-Light-Driven Photocatalytic Activities. J. Mater. Chem. A 2013, 1, 877−883. (18) Jiang, Z. Y.; Liu, Y. Y.; Li, M. M.; Jing, T.; Huang, B. B.; Zhang, X. Y.; Qin, X. Y.; Dai, Y. One-Pot Solvothermal Synthesis of Bi4V2O11 as A New Solar Water Oxidation Photocatalyst. Sci. Rep. 2016, 6, No. 22727. (19) Lv, C. D.; Chen, G.; Sun, J. X.; Zhou, Y. S.; Fan, S.; Zhang, C. M. Realizing Nanosized Interfacial Contact via Constructing BiVO4/ Bi4V2O11 Element-Copied Heterojunction Nanofibres for Superior Photocatalytic Properties. Appl. Catal., B 2015, 179, 54−60. (20) Lv, C. D.; Chen, G.; Sun, J. X.; Zhou, Y. S. Construction of α−β Phase Junction on Bi4V2O11 via Electrospinning Retardation Effect and Its Promoted Photocatalytic Performance. Inorg. Chem. 2016, 55, 4782−4789. (21) Xu, H.; Ye, H. T.; Coathup, D.; Mitrovic, I. Z.; Weerakkody, A. D.; Hu, X. J. An Insight of p-Type to n-Type Conductivity Conversion in Oxygen Ion-Implanted Ultrananocrystalline Diamond Films by Impedance Spectroscopy. Appl. Phys. Lett. 2017, 110, No. 033102. (22) Liu, G. G.; Zhao, G. X.; Zhou, W.; Liu, Y. Y.; Pang, H.; Zhang, H. B.; Hao, D.; Meng, X. G.; Li, P.; Kako, T.; Ye, J. H. In Situ Bond Modulation of Graphitic Carbon Nitride to Construct p−n Homojunctions for Enhanced Photocatalytic Hydrogen Production. Adv. Funct. Mater. 2016, 26, 6822−6829. (23) Liu, Y. D.; Cai, Y. Q.; Zhang, G.; Zhang, Y. W.; Ang, K. W. AlDoped Black Phosphorus p−n Homojunction Diode for High Performance Photovoltaic. Adv. Funct. Mater. 2017, 27, No. 1604638. (24) Pan, L.; Wang, S. B.; Xie, J. W.; Wang, L.; Zhang, X. W.; Zou, J. J. Constructing TiO2 p-n Homojunction for Photoelectrochemical and Photocatalytic Hydrogen Generation. Nano Energy 2016, 28, 296− 303. (25) Jiang, T. F.; Xie, T. F.; Yang, W. S.; Chen, L. P.; Fan, H. M.; Wang, D. J. Photoelectrochemical and Photovoltaic Properties of p-n Cu2O Homojunction Films and Their Photocatalytic Performance. J. Phys. Chem. C 2013, 117, 4619−4624. (26) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (27) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (28) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (29) Ding, X.; Zhao, K.; Zhang, L. Z. Enhanced Photocatalytic Removal of Sodium Pentachlorophenate with Self-Doped Bi2WO6 under Visible Light by Generating More Superoxide Ions. Environ. Sci. Technol. 2014, 48, 5823−5831. (30) Liu, Z. S.; Niu, J. N.; Feng, P. Z.; Sui, Y. W.; Zhu, Y. B. One-Pot Synthesis of Bi24O31Br10/Bi4V2O11 Heterostructures and Their Photocatalytic Properties. RSC Adv. 2014, 4, 43399−43405. (31) Ding, Y. B.; Yang, F.; Zhu, L. H.; Wang, N.; Tang, H. Q. Bi3+ Self Doped NaBiO3 Nanosheets: Facile Controlled Synthesis and Enhanced Visible Light Photocatalytic Activity. Appl. Catal., B 2015, 164, 151−158. (32) Wang, W. J.; Chen, X. Q.; Liu, G.; Shen, Z. R.; Xia, D. H.; Wong, P. K.; Yu, J. C. Monoclinic Dibismuth Tetraoxide: a New Visible-Light-Driven Photocatalyst for Environmental Remediation. Appl. Catal., B 2015, 176−177, 444−453. (33) Banger, K. K.; Yamashita, Y.; Mori, K.; Peterson, R. L.; Leedham, T.; Rickard, J.; Sirringhaus, H. Low-Temperature, HighPerformance Solution-Processed Metal Oxide Thin-Film Transistors Formed by a ‘Sol−Gel on Chip’ Process. Nat. Mater. 2011, 10, 45−50. (34) Hsieh, P. T.; Chen, Y. C.; Kao, K. S.; Wang, C. M. Luminescence Mechanism of ZnO Thin Film Investigated by XPS Measurement. Appl. Phys. A 2008, 90, 317−321. (35) Chen, M.; Wang, X.; Yu, Y. H.; Pei, Z. L.; Bai, X. D.; Sun, C.; Huang, R. F.; Wen, L. S. X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy Studies of Al-doped ZnO Films. Appl. Surf. Sci. 2000, 158, 134−140.

(36) Ali-Löytty, H.; Louie, M. W.; Singh, M. R.; Li, L.; Casalongue, H. G. S.; Ogasawara, H.; Crumlin, E. J.; Liu, Z.; Bell, A. T.; Nilsson, A.; Friebel, D. Ambient-Pressure XPS Study of a Ni−Fe Electrocatalyst for the Oxygen Evolution Reaction. J. Phys. Chem. C 2016, 120, 2247− 2253. (37) Li, H.; Shang, J.; Ai, Z. H.; Zhang, L. Z. Efficient Visible Light Nitrogen Fixation with BiOBr Nanosheets of Oxygen Vacancies on the Exposed {001} Facets. J. Am. Chem. Soc. 2015, 137, 6393−6399. (38) Liu, X. N.; Lu, Q. F.; Wei, M. Z.; Wang, C. Q.; Liu, S. W. Facile Electrospinning of CeO2/Bi2WO6 Heterostructured Nanofibers with Excellent Visible-Light-Driven Photocatalytic Performance. Chem. Asian J. 2015, 10, 1710−1716. (39) Zhao, J.; Lu, Q. F.; Wei, M. Z.; Wang, C. Q. Synthesis of OneDimensional a-Fe2O3/Bi2MoO6 Heterostructures by Electrospinning Process with Enhanced Photocatalytic Activity. J. Alloys Compd. 2015, 646, 417−424. (40) He, Z. Q.; Shi, Y. Q.; Gao, C.; Wen, L. N.; Chen, J. M.; Song, S. BiOCl/BiVO4 p-n Heterojunction with Enhanced Photocatalytic Activity under Visible-Light Irradiation. J. Phys. Chem. C 2014, 118, 389−398. (41) Yan, J. Q.; Wang, T.; Wu, G. J.; Dai, W. L.; Guan, N. J.; Li, L. D.; Gong, J. L. Tungsten Oxide Single Crystal Nanosheets for Enhanced Multichannel Solar Light Harvesting. Adv. Mater. 2015, 27, 1580−1586. (42) Zhou, Y. S.; Chen, G.; Yu, Y. G.; Zhao, L. C.; Sun, J. X.; He, F.; Dong, H. J. A New Oxynitride-Based Solid State Z-Scheme Photocatalytic System for Efficient Cr(VI) Reduction and Water Oxidation. Appl. Catal., B 2016, 183, 176−184. (43) Lv, C. D.; Chen, G.; Sun, J. X.; Yan, C. S.; Dong, H. J.; Li, C. M. One-Dimensional Bi2O3 QD-Decorated BiVO4 Nanofibers: Electrospinning Synthesis, Phase Separation Mechanism and Enhanced Photocatalytic Performance. RSC Adv. 2015, 5, 3767−3773. (44) Yan, M.; Hua, Y. Q.; Zhu, F. F.; Gu, W.; Jiang, J. H.; Shen, H. Q.; Shi, W. D. Appl. Catal., B 2017, 202, 518−527. (45) He, F.; Chen, G.; Zhou, Y. S.; Yu, Y. G.; Li, L. Q.; Hao, S. E.; Liu, B. ZIF-8 Derived Carbon (C-ZIF) as a Bifunctional Electron Acceptor and HER Cocatalyst for g-C3N4: Construction of a MetalFree, All Carbon-Based Photocatalytic System for Efficient Hydrogen Evolution. J. Mater. Chem. A 2016, 4, 3822−3827. (46) Wu, Q. F.; Bao, S. Y.; Tian, B. Z.; Xiao, Y. F.; Zhang, J. L. Double-Diffusion-Based Synthesis of BiVO4 Mesoporous Single Crystals with Enhanced Photocatalytic Activity for Oxygen Evolution. Chem. Commun. 2016, 52, 7478−7481. (47) He, F.; Chen, G.; Miao, J. W.; Wang, Z. X.; Su, D. M.; Liu, S.; Cai, W. Z.; Zhang, L. P.; Hao, S. E.; Liu, B. Sulfur-Mediated SelfTemplating Synthesis of Tapered C-PAN/g-C3N4 Composite Nanotubes toward Efficient Photocatalytic H2 Evolution. ACS Energy Lett. 2016, 1, 969−975.

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DOI: 10.1021/acsami.7b05302 ACS Appl. Mater. Interfaces 2017, 9, 23748−23755