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• 0.   Introduction

  Over the past decades, visible-light-active Bi based compounds are recommended as a category of the hottest photocatalysts, which is because Bi³⁺ ion in the crystal lattice exhibits unique d¹⁰ configuration and 6s² lone pair electrons. Such an electronic structure contributes to promoting the transfer of photogenerated carriers and the regulation of band gap by the orbital hybridization of Bi³⁺ with other ions^[1-2]. Among various Bi based photocatalysts, Bi₂O₃ is the simplest and foremost one, thanks to its innocuity, adjustable band gap and high oxidation power of valence band (VB) hole^[3-5]. Generally, Bi₂O₃ has five different polymorphs, i.e., monoclinic α, tetragonal β, body-centered cubic γ, face-centered cubic δ and triclinic ω-phases^[6-9]. Among them, β-Bi₂O₃ usually exhibits the best visible-light activity for its smaller band gap and unique tunnel structure^[6]. However, β-Bi₂O₃ still doesn't have sufficient efficiency for the photocatalytic application due to its fast recombination of light-induced hole and electron. So, many researchers have tried to adopt various modification methods to improve the photocatalytic performance of β-Bi₂O₃ under visible light irradiation, such as doping transition metal ions^[9-11] and constructing heterojunction semiconductor^[12-14]. Impurity doping in β-Bi₂O₃ has been confirmed to be an effective strategy to improve its visible light activity. Luo et al. found that Gd-doped β-Bi₂O₃ showed much higher photocatalytic activity for the photodegradation of phenol than pure β-Bi₂O₃, since the doped Gd³⁺ ion serves as an efficient scavenger to entrap photo-generated electrons thereby effectively separating photoinduced electron-hole pairs^[9]. Liang et al. reported that Fe-doped β-Bi₂O₃ photocatalyst with 4% molar fraction of Fe displayed desired photocatalytic activity, because Fe doping provides the photocatalyst with the ability to promote the migration of electron-hole pairs, thereby adding to photocatalytic activity^[10]. Besides doped ions, oxygen vacancy (OV) often was introduced into semiconductor to improve the light absorption and charge separation ability of catalyst^[15-17]. Lu et al. pointed out that the construction of oxygen vacancies on the surfaces of photocatalyst can improve the visible light absorption, target air pollutants enrichment and electron mobility^[16]. However, most of the OV-modified photocatalysts are synthesized under a harsh condition such as in a hydrogen or nitrogen atmosphere or under vacuum deoxidation condition^{[16, 19-20]}. The OV produced by this method is unstable and easy to be filled by oxygen in air atmosphere. Therefore, it is very essential for the development of a facial and controllable method to generate stable oxygen defect photocatalysts.

  In this work, we prepared Zn-doped OV-β-Bi₂O₃ nanoparticles (OV-Zn: Bi₂O₃) with two crystal defects, OV and doped Zn²⁺, via a sol-gel method followed by in-situ carbon thermal reduction treatment. The OV in OV-Zn: Bi₂O₃ samples is stable and controllable by regulating the content of doped Zn²⁺. As a reference, traditional β-Bi₂O₃ without OV and OV-β-Bi₂O₃ having OV but without doped Zn²⁺ were also synthesized. Various characterization methods were adopted to investigate the effect of OV and doped Zn²⁺ on the improvement of photocatalytic activity of OV-Zn: Bi₂O₃ series.

  1.   Experimental

  1.1   Preparation of the catalysts

  OV-Zn: Bi₂O₃ catalyst was synthesized via a simple sol-gel method followed by in-situ carbon thermal reduction treatment. Typically, 0.072 mol of citric acid monohydrate and 0.015 mol of Bi(NO₃)₃·5H₂O were dispersed in 30 mL of distilled water and ultrasonicated for 10 min to form a transparent sol. The as-obtained sol was stirred vigorously for 30 min while the ammonia solution of disodium ethylenediaminetetra acetate (EDTA) was slowly added to afford a new sol denoted as sol A. 1.5 mmol of Zn(CH₃COO)₂·2H₂O was dissolved in 5 mL of ammonia and stirred to form sol B. The as-obtained sol B was dropped into sol A under stirring with a water-bath (80 ℃) for 1 h to afford sol C. After being cooled to room temperature, 2 mL of sol C was carbonized in an electric furnace to obtain a viscous black product at 300 ℃. The viscous black product was further calcined at 400 ℃ for 2 h to yield a yellow powder, OV-Zn: Bi₂O₃-0.1 with a theoretical molar ratio (n_Zn/n_Bi) of 0.1. A series of Zn-doped OV-β-Bi₂O₃ phtocatalysts with theoretical n_Zn/n_Bi of 0.3, 0.5 and 1.0 were prepared in the same manners while the amount of Zn(CH₃COO)₂·2H₂O was adjusted to be 4.5, 7.5 and 15.0 mmol. Corresponding products were labeled as OV-Zn: Bi₂O₃-0.3, OV-Zn: Bi₂O₃-0.5 and OV-Zn: Bi₂O₃- 1.0, respectively. Pure OV-β-Bi₂O₃ powder with abundant oxygen vacancies but without doped Zn²⁺ was also prepared with the same procedure in the absence of Zn(CH₃COO)₂·2H₂O.

  For comparison, traditional β-Bi₂O₃ powder without OV was prepared by the precipitation method^[21-22]. Briefly, 2 g of Bi(NO₃)₃·5H₂O and 0.1 g of CTAB (cetyltrimethyl ammonium bromide) were both dissolved in 20 mL of nitric acid (1 mol·L^-1) solution with stirring for 20 min. Then, 0.4 g of oxalic acid was added in above solution and reacted under stirring for 30 min. The white precipitate was filtered, washed and calcined at 270 ℃ to obtain the yellow powder, traditional β-Bi₂O₃.

  1.2   Characterization

  A JSM-7610F scanning electron microscope (SEM) was performed with an accelerating voltage of 15 kV to observe the morphology and microstructure of as-prepared traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series. The crystal structure of the catalysts was characterized using a Bruker D8 X-ray powder diffractometer equipped with Cu Kα radiation source (λ = 0.154 8 nm); the tube current was 40 mA, operating voltage was 40 kV, and the scan range was 5°≤2θ≤70°. A Thermo Fisher Scientific Escalab 250Xi X-ray photoelectron spectrometer (XPS, monochromatic Al Kα excitation source) was deployed to analyze the chemical states of as-prepared photocatalysts. The ultravioletvisible light diffuse reflectance spectra (UV-Vis DRS) of as-prepared photocatalysts were recorded with a PE Lambda 950 spectrometer equipped with an integrating sphere attachment. Electron spin resonance (ESR) spectra were obtained with a Bruker E500 apparatus. The measurement was conducted at room temperature in ambient air, without vacuum-pumping. The g-tensors of the ESR signals were obtained by setting g of diphenyl picryl hydrazyl (DPPH, g=2.003 6) as the reference. The photoluminescence (PL) spectra of the products were obtained with a PuXi TU-1900 fluorescence spectrophotometer under the excitation at 380 nm. The photocurrent response and voltammogram (I-V) curve were measured at CHI 650E electrochemical workstation (Chenhua Instrument Company) with a three -electrode system including a modified ITO glass as working electrode, a platinum wire as counter electrode and an Ag/AgCl electrode as reference. A 300 W xenon lamp with a 420 nm cutoff filter was used as a visible light source.

  1.3   Evaluation of visible light photocatalytic activity

  The photocatalytic activity of as-prepared photocatalysts were evaluated by the degradation of methylene blue (MB) and 2, 4, 6-trichlorophenol (2, 4, 6-TCP) in a glass cylindrical reactor under visible light irradiation (300 W xenon lamp equipped with a 420 nm cutoff filter). The reactor was surrounded by a water-cooling system so as to ensure that the photocatalytic reaction occurs at room temperature. In each experiment, 100 mg of the to-be-tested photocatalysts was suspended in 150 mL MB or 2, 4, 6-TCP solution, which concentrations were all 10 mg·L^-1. Differently, the light intensity for the degradation of MB was 23 mW·cm^-2 and that for 2, 4, 6-TCP was 45 mW·cm^-2. Prior to illumination, the suspension was fiercely stirred in dark for 1 h until the absorption-desorption balance was achieved. During the photocatalytic reaction, 5 mL of the suspension was taken at certain time intervals to determine the concentration of MB and 2, 4, 6-TCP by UV-Vis spectro- photometry (TU-1810, Puxi Instrument Company, China) at the maximum absorption wavelength of 665 nm and 310 nm, respectively. The degradation rate of pollutant was calculated as (c₀-c)/c₀×100%, where c₀ refers to the initial concentration of pollutant after the adsorption-desorption equilibrium was reached, and c is the concentration of pollutant measured at a certain time interval during the photocatalytic reaction process.

  2.   Results and discussion

  2.1   Crystal structure

  Scheme 1 displays the schematic diagram for preparation process of traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series. For OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃, the critical step marked by red circle in diagram is the carbonization of EDTA-Bi complex on an electric stove. The reductive carbon produced in the carbonization process can reduce Bi³⁺ in the subsequent calcination process, resulting in the escaping of some oxygen atoms in β-Bi₂O₃ or Zn-doped Bi₂O₃ and leading to the formation of OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃.

  Scheme 1

  

  Scheme 1.  Schematic illustration of preparation for traditional β-Bi₂O₃ (a), OV-β-Bi₂O₃ (b) and OV-Zn: Bi₂O₃ series (c)

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  Fig. 1a depicts the XRD patterns of traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series with different n_Zn/n_Bi. The diffraction peaks of β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series at 2θ =27.94°, 31.76°, 32.69°, 46.21°, 46.90°, 54.26°, 55.48° and 57.75° are indexed to the (201), (002), (220), (222), (400), (203), (421) and (402) crystal planes of tetragonal β-Bi₂O₃ (PDF No. 27-0050)^[10-11], indicating that all as-prepared catalysts possess the same crystal structure. OV-Zn: Bi₂O₃ catalysts with a n_Zn/n_Bi of 0.1~1.0, i. e., OV-Zn: Bi₂O₃-0.1, OV-Zn: Bi₂O₃-0.3, OV-Zn: Bi₂O₃-0.5 and OV-Zn: Bi₂O₃-1.0, showed the same XRD patterns as traditional β-Bi₂O₃ and OV-β-Bi₂O₃ did. And all the OV-Zn: Bi₂O₃ photocatalysts did not show diffraction peaks of zinc oxide. Moreover, the enlarged XRD patterns shown in Fig. 1b demonstrate that the (201) peak of the OV-Zn: Bi₂O₃ photocatalysts tended to slightly shift towards lower 2θ value with the increase of zinc concentration. This means that as -prepared OV-Zn: Bi₂O₃ photocatalysts do not consist of the mixtures of Bi₂O₃ and ZnO phases but Zn-doped tetragonal Bi₂O₃. It is generally recognized that there are two modes of ion doping, substitutional doping and interstitial doping. The ionic radii of Bi³⁺ ion and Zn²⁺ ion are 0.103 and 0.074 nm, respectively^[23]. On one hand, if the lattice sites of Bi³⁺ in Bi₂O₃ crystal are partly substituted by smaller Zn²⁺ ion, the lattice parameters of Bi₂O₃ would decrease in association with the slight shift of the XRD peaks towards higher 2θ values. This supposition, however, is not supported by the above-mentioned XRD data. On the other hand, if some Zn²⁺ ions are doped in Bi₂O₃ crystal by way of interstitial doping, the volume of Bi₂O₃ crystal cell would be expanded and the distance of the crystal plane (d spacing) would be increased. This supposition, indeed, is supported by relevant XRD data. Namely, according to Bragg's law, 2dsinθ= nλ, the decrease of θ refers to the increase of d; and the volume expansion of the OV-β-Bi₂O₃ crystal cell after Zn doping proves that Zn²⁺ ion is doped into OV-β-Bi₂O₃ crystal via interstitial doping to afford the OV-Zn: Bi₂O₃ photocatalysts in this work.

  Figure 1

  

  Figure 1.  (a) XRD patterns of as-prepared traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series with different n_Zn/n_Bi; (b) Enlarged (201) peak in the 2θ range of 26°~30° for OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series

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  2.2   Morphology observation

  Fig. 2 shows the SEM images of traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series with different Zn content. It can be seen that both OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series exhibited similar spherical shape and had an average size of 100 nm (Fig. 2a~2e). This indicates that the doping of zinc has little effect on the morphology of OV-β-Bi₂O₃. Compared to OV-β-Bi₂O₃, traditional β-Bi₂O₃ prepared by the precipitation method consisted of many nano-sheets with thickness of about 10 nm (Fig. 2f), which was consistent with that reported in the literature^[21] and might be the reason for much larger specific surface area of 10 m²·g^-1 than that of OV-β-Bi₂O₃ with a value of 2 m²·g^-1.

  Figure 2

  

  Figure 2.  SEM images of pure OV-β-Bi₂O₃ (a), OV-Zn: Bi₂O₃-0.1 (b), OV-Zn: Bi₂O₃-0.3 (c), OV-Zn: Bi₂O₃-0.5 (d), OV-Zn: Bi₂O ₃-1.0 (e) and traditional β-Bi₂O₃ (f)

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  2.3   XPS analysis

  The chemical states of related elements in as-prepared samples were analyzed by XPS. Fig. 3a shows the high-resolution XPS spectra of Bi4f in traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃-0.3. Compared to traditional β-Bi₂O₃, the Bi4f peaks of OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃-0.3 shifted to the lower binding energies, indicating that the Bi atom in both samples with OV has higher electron density (or lower chemical valence) than that in β-Bi₂O₃ due to the charge compensation. The absence of partial oxygen atoms can reduce the coordination number of Bismuth ion leading to the decrease of binding energy of Bi4f. The Zn2p_3/2 and Zn2p_1/2 peaks of OV-Zn: Bi₂O₃-0.3 were centered at 1 022.9 and 1 046.1 eV (Fig. 3b), which means that the doped Zn is present in the form of Zn²⁺ in Zn-doped β-Bi₂O₃^[24]. Besides, as shown in Fig. 3c, the O1s spectrum of traditional β-Bi₂O₃ was split into two peaks, Ⅰ and Ⅱ: peak Ⅰ at 530.3 eV is assigned to the lattice oxygen in β-Bi₂O₃ crystal and peak Ⅱ at 531.4 eV is as- cribed to adsorbed oxygen in the form of hydroxyl group^{[7, 15, 25-26]}. Different with traditional β-Bi₂O₃, a new peak Ⅲ appeared for OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃-0.3, which was situated at about 532.6 eV corresponding to the OV defect^{[7, 27]}.

  Figure 3

  

  Figure 3.  High-resolution XPS spectra of Bi4f (a), Zn2p (b) and O1s (c) for traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃-0.3 samples

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  2.4   ESR analysis

  ESR test was adopted to confirm the existence of oxygen vacancies in OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series as a direct and sensitive way. As shown in Fig. 4, no ESR signal could be observed for traditional β-Bi₂O₃. While there was a strong symmetric ESR peak for OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series both having a center at g=2.004, which is attributed to single-electron-trapped OV (V_O^·) with a positive charge^[28-29]. For OV-Zn: Bi₂O₃ series, XRD results suggested that Zn²⁺ ion is doped into OV-β-Bi₂O₃ crystal via interstitial doping not substitution, which leads to more positive charge in semiconductor. So, with the increase of the doped Zn²⁺ content, the concentration of V_O^· decreased and the intensity of ESR signal decreased gradually. This demon- strats that the concentration of OV may be modulated by the amount of doped Zn²⁺ in OV-Zn: Bi₂O₃ series. Many studies reported that, OV can not only broaden the photoabsorption range of the catalyst but also serve as trapping center for excited electrons to improve the separation efficiency of photogenerated carriers^{[16, 30]}. To study the trapping role of OV, many ESR spectra under laser irradiation of different catalysts will be further compared.

  Figure 4

  

  Figure 4.  ESR spectra of traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series at room temperature in air

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  2.5   Optical properties of as-prepared samples

  It is well known that both OV and doped ions can change the optical absorption and electronic energy structure of semiconductor, which can be determined by UV-Vis DRS. Fig. 5a shows the UV-Vis DRS spectra of traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ samples with different n_Zn/n_Bi. The absorption edge of traditional β-Bi₂O₃ was approximately 460 nm. The absorption range of OV-β-Bi₂O₃ was clearly broadened, with the absorption edge extending to approximately 500 nm. For OV-β-Bi₂O₃, which has the wider light absorption range, more electron-hole pairs can be generated to participate in photocatalytic reaction under visible light irradiation. Compared with OV-β-Bi₂O₃, the absorption edge of OV-Zn: Bi₂O₃ samples gradually blue-shifted to shorter wavelength with the increase of Zn content. However, the absorption range of nearly all OV-Zn: Bi₂O₃ samples, except OV-Zn: Bi₂O₃-1.0, was still wider than that of traditional β-Bi₂O₃. The band gap can be estimated using the following equation (Eq.1):

  Figure 5

  

  Figure 5.  UV-Vis DRS spectra (a) and plots of (αhν)² vs photon energy (b) of traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series

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  $(\alpha h \nu)^{2}=A\left(h \nu-E_{\mathrm{g}}\right)^{n}$

  (1)

  Where α, ν, A and E_g are absorption coefficient, light frequency, a constant and band gap energy, respectively. The value of n is 1 for direct transition and 4 for indirect transition of semiconductor. According to the literature^[9], the value of n is 1 for β-Bi₂O₃. The band gap energies of traditional β-Bi₂O₃, OV-β-Bi₂O₃, OV-Zn : Bi₂O₃-0.1, OV-Zn: Bi₂O₃ -0.3, OV-Zn: Bi₂O₃-0.5 and OV-Zn: Bi₂O₃-1.0 were estimated from a plot of (αhν)² vs hν to be 2.72, 2.40, 2.46, 2.50, 2.53 and 2.80 eV, respectively (Fig. 5b).

  To further investigate the effect of Zn doping on the VB position of OV-Zn: Bi₂O₃, we adopted Eq.2 to determine the VB edge position of pure OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series^[31-32].

  $E_{\mathrm{VB}}=X-E_{\mathrm{e}}+0.5 E_{\mathrm{g}}$

  (2)

  Where E_e is about 4.5 eV on the hydrogen scale; E_g is the band gap of OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃; E_VB is the valence band edge potential; and X is the absolute electronegativity of the solid (the X value for Bi₂O₃ was calculated to be 6.24, Supporting information). Simultaneously, the conduction band (CB) position of the samples can be obtained from E_CB=E_VB-E_g. As shown in Table 1, compared to OV-β-Bi₂O₃, the VB position of OV-Zn: Bi₂O₃ samples gradually moved down with the increase of n_Zn/n_Bi, which suggests that the oxidizing ca-Bipacity of the photoexcited holes tends to rise with the increase of Zn content. Furthermore, the VB spectra of traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃-0.3 as the representative of OV-Zn: Bi₂O₃ series were mea-Bisured by XPS. As shown in Fig. 6, the VB potentials of OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃-0.3 were lower than that of traditional β-Bi₂O₃ for the narrow band gap due to the existence of OV, and the VB potential of OV-Zn: Bi₂O₃-0.3 was higher than that of OV-β-Bi₂O₃, which may imply that the photo-induced holes of OV-Zn: Bi₂O₃- 0.3 possesses stronger oxidizing ability than that of OV-β-Bi₂O₃. These results are well consistent with above DRS analysis.

  Table 1

  

  Table 1.  Values of E_g, E_VB and E_CB for traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series

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  Catalyst	Eg/eV	EVB/eV	ECB/eV
  β-Bi2O3	2.72	3.10	0.38
  OV-β-Bi2O3	2.40	2.94	0.54
  OV-Zn: Bi2O3-0.1	2.46	2.97	0.51
  OV-Zn: Bi2O3-0.3	2.50	2.99	0.49
  OV-Zn: Bi2O3-0.5	2.53	3.01	0.48
  OV-Zn: Bi2O3-1.0	2.80	3.14	0.34

  Figure 6

  

  Figure 6.  VB XPS spectra of traditional β-Bi₂O₃ (a), OV-β-Bi₂O₃ (b) and OV-Zn: Bi₂O₃-0.3 (c) as the representative of OV-Zn: Bi₂O₃ series

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  2.6   Visible light photocatalytic activity

  Different pollutants including MB dye and colorless phenol compound, 2, 4, 6-TCP, were employed as the substrate to evaluate the photocatalytic activitiy of traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ samples under visible light irradiation. Fig. 7a and 7b shows the photocatalytic performance of different catalysts toward MB dye degradation with the light intensity of 23 mW·cm^-2 (λ=420 nm). It can be seen that, in the absence of any photocatalyst, nearly 80% of MB was retained after 3 h of visible light irradiation. In the presence of traditional β-Bi₂O₃ and OV-β-Bi₂O₃, about 40% and 70% of MB was degraded after 4 h of visible light irradiation, respectively, which indicates that, although the specific surface area of OV-β-Bi₂O₃ was smaller than that of traditional β-Bi₂O₃, the former promotes much higher degradation rate of MB than the latter for the wider light absorption range resulted from OV. Furthermore, OV-Zn: Bi₂O₃ catalysts with different n_Zn/n_Bi can more efficiently catalyze the degradation of MB under visible light irradiation as compared with OV-β-Bi₂O₃; and in particular, OV-Zn: Bi₂O₃-0.3, with aphotocatalytic efficiency similar to that of OV-Zn: Bi₂O₃-0.5, can catalyze the photodegradation of up to 91% of MB within 4 h of visible light irradiation. The linear relationship between ln(c₀/c) and irradiation time for MB degradation is shown in Fig. 7b. The reaction rate constant (k) and linear correlation coefficient (R²) for different catalysts are summarized in Table 2. It can be seen that all as-prepared catalysts had an R² value of above 0.97, which means that the degradation of MB catalyzed by β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series is of the first order kinetic nature. Moreover, OV-Zn: Bi₂O₃-0.3 had the highest rate constant among all the tested catalysts, which demonstrates that the optimal n_Zn/n_Bi of OV-Zn: Bi₂O₃ catalysts should be 0.3.

  Figure 7

  

  Figure 7.  Photocatalytic degradation of MB (a) and linear fitting for first order kinetics (b) over different catalysts under visible light irradiation; Photocatalytic degradation of 2, 4, 6-TCP (c) and linear fitting for first order kinetics (d) over different catalysts under visible light irradiation

  λ> 420 nm, the light intensities were 23 mW·cm^-2 for MB and 45 mW·cm^-2 for 2, 4, 6-TCP

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  Table 2

  

  Table 2.  Reaction rate constant (k) and correlation coefficient (R²) of traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series for photocatalytic degradation of MB under visible light irradiation

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  Catalyst	R2	k/min-1
  β-Bi2O3	0.972	0.001 91
  OV-β-Bi2O3	0.995	0.004 98
  OV-Zn: Bi2O3-0.1	0.979	0.005 75
  OV-Zn: Bi2O3-0.3	0.985	0.009 71
  OV-Zn: Bi2O3-0.5	0.995	0.009 13
  OV-Zn: Bi2O3-1.0	0.998	0.006 21

  2, 4, 6-TCP, as a well-known organic pollutant in wastewater of pharmaceutical, paint and pesticide industries, can cause severe nervous system and respiratory disease. Fig. 7c shows the photocatalytic activity of traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ samples toward the degradation of 2, 4, 6-TCP under visible light with the intensity of 45 mW·cm^-2 (λ=420 nm). It can be seen that, after irradiation for 100 min, about 51% and 71% of 2, 4, 6-TCP was degraded by OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃- 0.3, respectively, while only 37% of 2, 4, 6-TCP was removed over traditional β-Bi₂O₃ within 100 min. Mover, as illustrated in Fig. 7d, the order of the reaction rate constant in the degradation of 2, 4, 6-TCP was k_{OV-Zn: Bi2O3-0.3}>k_{OV-Zn: Bi2O3-0.5}>k_{OV-Zn: Bi2O3-1.0}>k_{OV-Zn: Bi2O3-0.1}>k_OV-β-Bi2O3>k_β-Bi2O3, which was the same with that of the degradation of MB dye, confirming the optimal n_Zn/n_Bi of OV-Zn: Bi₂O₃ catalysts was 0.3.

  The above systematic photocatalytic experiments revealed that, compared to traditional β-Bi₂O₃, both OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃exhibited superior activity due to the introduction of OV. Moreover, the synergistic effect of OV and doped zinc ions can promote the photocatalytic activity of OV-Zn: Bi₂O₃ in the degradation of MB and 2, 4, 6-TCP under visible light. In terms of the practical applications of catalysts, the reusability and stability are two vital factors worth special emphasis. Therefore, cycling experiments of OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃-0.3 as the representative samples were carried out to evaluate their reusability for the degradation of MB under visible light irradiation. As shown in Fig. 8, after 4 cycles running, the degradation rate of MB in the presence of OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃-0.3 decreased slightly. The XRD patterns of OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃-0.3 before and after four cycles of the photocatalytic degradation of MB are compared in Fig. 9. It can be seen that, after four-cycle experiments, there was not any change in the crystal structure for both samples, which demonstrates that both OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ possess strong structural stability during the photocatalytic degradation of MB.

  Figure 8

  

  Figure 8.  Plots of c/c₀ versus irradiation time over OV-β-Bi₂O₃ (a) and OV-Zn: Bi₂O₃-0.3 (c) as well as photodegradation efficiency of MB over OV-β-Bi₂O₃ (b) and OV-Zn: Bi₂O₃-0.3 (d) in different recycling runs

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  Figure 9

  

  Figure 9.  XRD patterns of OV-Zn: Bi₂O₃-0.3 (a) and OV-β-Bi₂O₃ (b) before and after four-cycling degradation of MB under visible light irradiation

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  2.7   Photocatalytic mechanism

  To investigate the active species involved in the MB degradation over OV-Zn: Bi₂O₃ catalysts, we adopted triethanolamine (TEOA) and tert- butanol (TBA) as the radical scavengers for holes and hydroxyl radicals to conduct trapping experiments^[33-35]. And N₂ was also added to remove oxygen and prevent the formation of superoxide radicals^[36]. Fig. 10 displays the photocatalytic activity of OV-Zn: Bi₂O₃-0.3 sample for the photocatalytic degradation of MB in the presence of TEOA and TBA (their concentration in the photocatalytic system was 6 mmol·L^-1). It can be seen that TEOA could strongly inhibit the photocatalytic activity of OV-Zn: Bi₂O₃-0.3, which indicates that h⁺ is the main active species for the degradation of MB. And after the introduction of TBA, the photodegradation of MB was obviously suppressed, which confirms that ·OH also plays an important role in the photocatalytic degradation of MB. This is consistent with DRS results. As shown in Fig. 11, although the electrons in the VB of OV-Zn: Bi₂O₃-0.3 can be excited to the CB by visible light, the active ·O₂^- radical cannot be generated, due to the lower potential of O₂/·O₂^- (-0.33 eV (vs NHE)) than that of CB potential of OV-Zn: Bi₂O₃-0.3 (0.49 eV (vs NHE), Table 1) ^[37-38]. However, the holes in VB can oxidize OH^- (or H₂O) to generate ·OH or directly oxidize MB, due to the higher VB potential of OV-Zn: Bi₂O₃-0.3 (2.99 eV (vs NHE), Table 1) than that of OH^-/·OH (2.40 eV (vs NHE)) and H₂O/·OH (2.72 eV (vs NHE))^[39-40]. These results confirm that h⁺ and ·OH are the main active species in the photodegradation process of MB over OV-Zn: Bi₂O₃-0.3 photocatalyst.

  Figure 10

  

  Figure 10.  Effect of different scavengers on MB degradation process over OV-Zn: Bi₂O₃-0.3 catalyst

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  Figure 11

  

  Figure 11.  Photocatalytic degradation mechanism of OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ under visible light irradiation

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  In order to investigate the effect of OV and doped Zn²⁺ on the improvement of photocatalytic activity, photoelectrochemical experiments were carried out to evaluate the charge carrier density and the photogenerated carrier separation efficiency of different catalysts. The density of charge carriers, which is an important parameter affecting the photocatalytic activity of catalyst, can be obtained by the following Nernst equation:

  $E_{\mathrm{f} 1}-E_{\mathrm{f} 2}=k T \ln \left(N_{\mathrm{fl}} / N_{\mathrm{f} 2}\right) / e$

  (3)

  Herein, E_f1 and E_f2 is the quasi-Fermi level of sample 1 and sample 2, respectively. N_f1 and N_f2 is the carrier density of two samples. In addition, k, T and e are the Boltzmann constant (1.381×10^-23 J·K^-1), temperature and elementary charge (1.602×10^-19 C). With the addition of methylviologen dichloride (MVCl₂) serving as the fast electron acceptor, the photocurrent onset potential in a voltammogram can be considered as the quasiFermi level of majority carriers^{[15, 41-43]}. As shown in Fig. 12a, the photocurrent onset potentials of OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃-0.3 were-0.37 and -0.41 V, both of which were higher than that of traditional β-Bi₂O₃ with a value of -0.45 V. According to above Nernst equation, the carrier densities of OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃-0.3 were determined as 22.5 and 4.7 times larger than that of traditional β-Bi₂O₃, respectively. This suggests that the existence of OV can largely increase the charge carrier density of semiconductor by expanding the light absorption range of catalyst. Compared to OV-β-Bi₂O₃, although the carrier density of OV-Zn: Bi₂O₃-0.3 was reduced for the wider band gap, the oxidation ability of photogenerated holes was enhanced and the separation efficiency of photo-generated carriers might be increased for the electron trapping role of the doped Zn²⁺. The transient photocurrent responses of different catalysts under visible light are shown in Fig. 12b. It can be seen that, the photocurrent densities of both OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃-0.3 were much larger than that of traditional β-Bi₂O₃. It demonstrates that the efficient generation and separation of photoinduced electron-hole pairs of both OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ arise from the OV. Moreover, the photocurrent density of OV-Zn: Bi₂O₃-0.3 was larger than that of OV-β-Bi₂O₃, suggesting that the doped Zn²⁺ may also play an important role in promoting the separation efficiency for OV-Zn: Bi₂O₃ series. Fig. 12c shows the Zn2p XPS spectra of OV-Zn: Bi₂O₃-0.3 before and after MB degradation reaction. It can be seen that, after reaction, the binding energies of Zn2p_1/2 and Zn2p_3/2 shifted to lower energy, indicating that parts of doped Zn²⁺ was reduced by trapped electron to a new chemical state with lower valence. This further confirms that moderate doped Zn²⁺ can also serve as an electron trapping center to promote the separation of photogenerated carriers. The PL spectra of catalysts can also reflect the separation efficiency of photogenerated electron-hole pairs in semiconductor. It is well known that a stronger PL intensity represents lower separation efficiency and higher recombination rate of the photogenerated charge carriers. Fig. 12d shows the PL spectra of different samples at the excitation wavelength of 380 nm. It can be seen that I_β-Bi2O3 > I_OV-β-Bi2O3 > I_{OV-Zn: Bi2O3-1.0} > I_{OV-Zn: Bi2O3-0.3}, indicating that E_{OV-Zn: Bi2O3-0.3} (separation efficiency of OV-Zn: Bi2O3 -0.3) > E_{OV-Zn: Bi2O3-1.0} > E_OV-β-Bi2O3 > E_β-Bi2O3, which was consistent with transient photocurrent results of catalysts. All the above data suggest that OV and appropriate doped Zn²⁺ can promote the separation efficiency of photogenerated carriers, however, excessive zinc may also become the recombination center, reducing the catalytic activity of OV-Zn: Bi₂O₃ series.

  Figure 12

  

  Figure 12.  (a) I-V curves of traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃-0.3; (b) Photocurrent response of traditional β- Bi₂O₃, OV- β-Bi₂O₃, OV-Zn: Bi₂O₃-0.3 and OV- Zn: Bi₂O₃ -1.0 under visible light (λ>420 nm); (c) Comparison of Zn2p XPS spectra of OV-Zn: Bi₂O₃-0.3 before and after reaction; (d) PL spectra of β-Bi₂O₃, OV-β-Bi₂O₃, OV-Zn: Bi₂O ₃-0.3 and OV-Zn: Bi₂O₃-1.0

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  To further study the role of OV in promoting the separation efficiency of photogenerated carriers, the ESR spectra were recorded for OV-Zn: Bi₂O₃ series under laser irradiation (λ =420 nm) in air. Fig. S1 (Supporting information) are the ESR spectra of OV-Zn: Bi₂O₃ series, where the dashed and solid lines correspond to the signals before and after laser irradiation, respectively. It can be seen that, the ESR signal intensity of V_O^· in all catalysts having OV could be increased under visible light irradiation, indicating that the OVs in OV-Zn: Bi₂O₃ series prepared in this work are very stable and there are more V_o^· produced under visible light. It is well known that, besides V_O^·, there are two other kinds of OV, two-electron-trapped OV (effective charge=0, denoted as V_O^×) and no-electron-trapped OV (effective charge=+2, denoted as V_O^··)^{[27, 29]}, both of which do not contribute to ESR signals because of the absence of unpaired electrons. Under visible light irradiation, the electrons can be photoexcited to conduction band, leaving the holes in VB. If there are trapping electron centers, i.e., V_O^·· or trapping hole centers V_O^× on the surface of catalyst, the electron of the conduction band may be trapped by surface V_O^·· and the hole of the VB may be captured by surface V_O^×. Both kinds of OVs transformed into V_O^· and the signal intensity became stronger accordingly (Fig. 11). The separation efficiency of photogenerated carriers will be also improved. In other words, Δh (Δh=h_light-h_dark, h_light and h_dark refer to the intensities of ESR signal at g= 2.004 under visible light and in dark, respectively) is closely related to the separation efficiency of photogenerated carriers caused by surface OV. The dependences of Δh and the reaction constant (k) for the degradation of MB by OV-Zn: Bi₂O₃ series on the intensity of ESR signal for V_O^· (h_{ESR, OV}) are shown in Fig. 13a. It can be seen that with the increase of h_{ESR, OV}, both of Δh and kfirst increased and then decreased, suggesting that excessive OV may be detrimental to the activity of catalyst. Fig. 13b shows the dependences of h_{ESR, OV} and k for the degradation of MB by OV-Zn: Bi₂O₃ series on n_Zn/n_Bi. It can be seen that, with the increase of doped Zn²⁺, h_{ESR, OV} decreased and there was a peak of reaction con- stant situated at n_Zn/n_Bi=0.3 for OV-Zn: Bi₂O₃-0.3. This suggests that the concentration of OV can be modulated by the doped zinc and excessive zinc doping may also reduce the separation efficiency of photogenerated carriers.

  Figure 13

  

  Figure 13.  (a) Dependence of Δh and reaction rate constant (k) for the degradation of MB by OV-Zn: Bi₂O₃ series on intensity of ESR signal for V_o^· (h_{ESR, OV}); (b) Dependence of h_{ESR, OV} and k on n_Zn/n_Bi

  Δh=h_light-h_dark; h_light and h_dark refer to the intensities of ESR signal at g=2.004 under visible light and in dark, respectively

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  Two kinds of OV on the surface of catalysts, V_o^·· and V_O^×, can serve as trapping centers of electrons and holes, respectively. And moderate doped Zn²⁺ can also act as electron capture sites to promote the separation efficiency of photogenerated carriers. Thus, the visible light activity of both OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series can be improved. Of course, compared to OV-β-Bi₂O₃, moderate doped Zn²⁺ can result in the optimal energy level structure for OV-Zn: Bi₂O₃, and OV-Zn: Bi₂O₃-0.3 exhibited the highest activity for the degradation of MB and 2, 4, 6-TCP.

  3.   Conclusions

  OV-β-Bi₂O₃ and Zn-doped OV-β-Bi₂O₃ series (OV-Zn: Bi₂O₃) were prepared by a simple sol-gel method followed by in-situ carbon thermal reduction treatment. The concentration of oxygen vacancy in OV-Zn: Bi₂O₃ can be modulated by the content of doped zinc. The more the content of doped Zn²⁺ is, the less the concentration of oxygen vacancy will be. The introduction of oxygen vacancy and moderate zinc can not only regulate the energy structure of OV-Zn: Bi₂O₃, but also promote the separation efficiency of photogenerated carriers. So, the visible light activity of OV-Zn: Bi₂O₃ series was superior to that of OV-β-Bi₂O ₃, and the activity of OV-β-Bi₂O₃ was better than that of traditional β-Bi₂O₃. This study offers a new method to prepare the novel visible-light-responsive catalyst with stable controllable oxygen vacancy.

  
  Acknowledgments: The authors acknowledge the financial support provided by the Science & Technology Research Project of Henan Province (Grant No.19A150020), the Program for Science & Technology Innovation Team in Universities of Henan Province (Grant No.19IRTSTHN029), Innovation and Entrepreneurship Training Program of Henan Province (Grant No. S202010475087) and Science & Technology Development Plan Project of Kaifeng (Grant No.1808001). Supporting information is available at http://www.wjhxxb.cn
  
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• 

  Scheme 1  Schematic illustration of preparation for traditional β-Bi₂O₃ (a), OV-β-Bi₂O₃ (b) and OV-Zn: Bi₂O₃ series (c)

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  Figure 1  (a) XRD patterns of as-prepared traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series with different n_Zn/n_Bi; (b) Enlarged (201) peak in the 2θ range of 26°~30° for OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series

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  Figure 2  SEM images of pure OV-β-Bi₂O₃ (a), OV-Zn: Bi₂O₃-0.1 (b), OV-Zn: Bi₂O₃-0.3 (c), OV-Zn: Bi₂O₃-0.5 (d), OV-Zn: Bi₂O ₃-1.0 (e) and traditional β-Bi₂O₃ (f)

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  Figure 3  High-resolution XPS spectra of Bi4f (a), Zn2p (b) and O1s (c) for traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃-0.3 samples

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  Figure 4  ESR spectra of traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series at room temperature in air

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  Figure 5  UV-Vis DRS spectra (a) and plots of (αhν)² vs photon energy (b) of traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series

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  Figure 6  VB XPS spectra of traditional β-Bi₂O₃ (a), OV-β-Bi₂O₃ (b) and OV-Zn: Bi₂O₃-0.3 (c) as the representative of OV-Zn: Bi₂O₃ series

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  Figure 7  Photocatalytic degradation of MB (a) and linear fitting for first order kinetics (b) over different catalysts under visible light irradiation; Photocatalytic degradation of 2, 4, 6-TCP (c) and linear fitting for first order kinetics (d) over different catalysts under visible light irradiation

  λ> 420 nm, the light intensities were 23 mW·cm^-2 for MB and 45 mW·cm^-2 for 2, 4, 6-TCP

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  Figure 8  Plots of c/c₀ versus irradiation time over OV-β-Bi₂O₃ (a) and OV-Zn: Bi₂O₃-0.3 (c) as well as photodegradation efficiency of MB over OV-β-Bi₂O₃ (b) and OV-Zn: Bi₂O₃-0.3 (d) in different recycling runs

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  Figure 9  XRD patterns of OV-Zn: Bi₂O₃-0.3 (a) and OV-β-Bi₂O₃ (b) before and after four-cycling degradation of MB under visible light irradiation

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  Figure 10  Effect of different scavengers on MB degradation process over OV-Zn: Bi₂O₃-0.3 catalyst

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  Figure 11  Photocatalytic degradation mechanism of OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ under visible light irradiation

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  Figure 12  (a) I-V curves of traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃-0.3; (b) Photocurrent response of traditional β- Bi₂O₃, OV- β-Bi₂O₃, OV-Zn: Bi₂O₃-0.3 and OV- Zn: Bi₂O₃ -1.0 under visible light (λ>420 nm); (c) Comparison of Zn2p XPS spectra of OV-Zn: Bi₂O₃-0.3 before and after reaction; (d) PL spectra of β-Bi₂O₃, OV-β-Bi₂O₃, OV-Zn: Bi₂O ₃-0.3 and OV-Zn: Bi₂O₃-1.0

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  Figure 13  (a) Dependence of Δh and reaction rate constant (k) for the degradation of MB by OV-Zn: Bi₂O₃ series on intensity of ESR signal for V_o^· (h_{ESR, OV}); (b) Dependence of h_{ESR, OV} and k on n_Zn/n_Bi

  Δh=h_light-h_dark; h_light and h_dark refer to the intensities of ESR signal at g=2.004 under visible light and in dark, respectively

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  Table 1.  Values of E_g, E_VB and E_CB for traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series

  Catalyst	Eg/eV	EVB/eV	ECB/eV
  β-Bi2O3	2.72	3.10	0.38
  OV-β-Bi2O3	2.40	2.94	0.54
  OV-Zn: Bi2O3-0.1	2.46	2.97	0.51
  OV-Zn: Bi2O3-0.3	2.50	2.99	0.49
  OV-Zn: Bi2O3-0.5	2.53	3.01	0.48
  OV-Zn: Bi2O3-1.0	2.80	3.14	0.34

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  Table 2.  Reaction rate constant (k) and correlation coefficient (R²) of traditional β-Bi₂O₃, OV-β-Bi₂O₃ and OV-Zn: Bi₂O₃ series for photocatalytic degradation of MB under visible light irradiation

  Catalyst	R2	k/min-1
  β-Bi2O3	0.972	0.001 91
  OV-β-Bi2O3	0.995	0.004 98
  OV-Zn: Bi2O3-0.1	0.979	0.005 75
  OV-Zn: Bi2O3-0.3	0.985	0.009 71
  OV-Zn: Bi2O3-0.5	0.995	0.009 13
  OV-Zn: Bi2O3-1.0	0.998	0.006 21

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•