Heterostructured Bi₂O₃/Bi₂MoO₆ nanocomposites: simple construction and enhanced visible-light photocatalytic performance

Abstract

Heterostructured Bi₂O₃/Bi₂MoO₆ nanocomposites with enhanced visible-light photocatalytic activity are successfully constructed by a simple two-step route, employing bismuth nitrate and sodium molybdate as the main raw materials in the presence of polyvinyl pyrrolidone (PVP). The phase and the morphology of the as-prepared composites are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), (high-resolution) transmission electron microscopy (TEM/HRTEM), and energy dispersive spectrometry (EDS). It is found that heterostructured Bi₂O₃/Bi₂MoO₆ nanocomposites exhibit stronger catalytic activities for the degradation of Rhodamine B (RhB) and 2,4-dinitrophenol (2,4-DNP) under the irradiation of visible light than the single component (Bi₂O₃ or Bi₂MoO₆). The above enhanced photocatalytic performance should be attributed to the formation of the p–n junction between the p-type Bi₂O₃ and n-type Bi₂MoO₆, which effectively halts the fast recombination of photogenerated electrons and holes. Moreover, the as-obtained Bi₂O₃/Bi₂MoO₆ nano-heterostructures also display improved photoelectochemistry performance and high photochemical stability, which is important for conquering the photo-corrosion of photocatalysts. Also, the present work is a useful attempt for the design of new visible-light photocatalytic materials with heterojunction structures.

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1. Introduction

Photocatalysis based on semiconductor materials, as a low-cost and sustainable green chemical technology for converting solar energy to chemical energy and completely eliminating organic pollutants, is drawing increasing research interest because of the global energy crisis and environmental pollution.^1,2 Over the past decade, various semiconductor-based photocatalysts have been widely investigated, including Bi-based, Ag-based, In-based, TiO₂-based and Cu-based ones.^3–7 However, some traditional semiconductor photocatalysts such as TiO₂ and ZnO can only utilize the UV light below 400 nm due to their large band gaps, which leads to poor photocatalytic efficiency under the irradiation of visible light.^8,9 Cu₂O is also unsatisfied due to its wide size distribution and the high recombination rate of electron–hole pairs.¹⁰ As typical visible-light photocatalysts, Bi-based semiconductor materials, such as Bi₂WO₆, BiVO₄, Bi₂MoO₆, and BiOX (X = Br, Cl, I),^11–14 have been paid much attention in recent years because of their layer structures and high catalytic properties. Among them, Bi₂MoO₆ is an n-type semiconductor with the layered Aurivillius structure, which consists of [Bi₂O₂]²⁺ layers sandwiched between MoO₄²⁻ slabs.¹⁵ Its band gap locates at 2.5–2.8 eV, which largely depends on its structure, shape and components.¹⁶ Undoubtedly, Bi₂MoO₆ is a promising photocatalyst in organic pollutant degradation and water splitting. However, the photocatalytic efficiency of individual Bi₂MoO₆ is still low due to the rapid recombination of photogenerated electrons and holes. Therefore, it is still necessary to further optimize its photocatalytic performance.¹⁷

In order to overcome the intrinsic defects of single component and to improve the photocatalytic performance, different types of Bi₂MoO₆-based heterostructures with appropriate band positions have been attempted by several groups. For example, heterostructured Bi₂MoO₆/TiO₂ nanocomposites, in which Bi₂MoO₆ nanosheets grew on the surface of TiO₂ nanobelts, were synthesized by Tian and coworkers.¹⁸ Against single Bi₂MoO₆ and TiO₂, the improved photocatalytic activity of heterostructures was attributed to the larger specific surface areas and the formation of heterojunction. In particular, the formation of the p–n heterojunction can generate the internal electric field in a semiconductor photocatalyst, which greatly promotes the migration rates of the photogenerated electrons and holes.¹⁹ At present, forming a p–n junction in semiconductor photocatalysts has been considered to be the most effective strategy to significantly optimize the photocatalytic activity of photocatalysts. As a p-type semiconductor, Bi₂O₃ is a fascinating material for the photooxidation of pollutants and the water splitting due to its high refractive index, dielectric permittivity, thermal stability and direct band gap of 2.8 eV.^20,21 Obviously, it is probable that the formation of a p–n junction between Bi₂MoO₆ and Bi₂O₃ can largely promote the photocatalytic efficiency of the photocatalyst.

To date, few reports are found in the literature on heterostructured Bi₂O₃/Bi₂MoO₆ nanocomposites.^22,23 For instance, Xu et al. successfully prepared hierarchical β-Bi₂O₃/Bi₂MoO₆ heterostructured flower-like microspheres assembled from nanoplates by a facile one-step template-free solvothermal route.²² Hao and coworkers designed an alkali-etching route to synthesize α-Bi₂O₃/Bi₂MoO₆ composite nanoflakes using α-Bi₂O₃ nanoflakes as the original materials.²³ The as-obtained heterostructures both exhibited better photocatalytic activities for the degradation of RhB than pure Bi₂O₃ or Bi₂MoO₆ samples. In the current work, we successfully construct a novel heterostructured Bi₂O₃/Bi₂MoO₆ nanocomposite through a two-step solution route. α-Bi₂O₃ nanorods were first obtained via the hydrothermal treatment of Bi(NO₃)₃ in diluted HNO₃ solution with PVP. Then, heterostructured Bi₂O₃/Bi₂MoO₆ nanocomposites were solvothermally prepared through treating a glycol–water mixed system containing α-Bi₂O₃ nanorods, Bi(NO₃)₃ and Na₂MoO₄. Interestingly, Bi₂O₃/Bi₂MoO₆ nanocomposites exhibited obviously different morphology from pure Bi₂O₃ and Bi₂MoO₆, and increasing BET surface area. TEM observations uncovered that spherical Bi₂MoO₆ was distributed on the surface of the thin Bi₂O₃ nanoflakes to form Bi₂O₃/Bi₂MoO₆ heterostructures, which differed from the previous reports.^22,23 Furthermore, the photocatalytic experiments showed the as-obtained Bi₂O₃/Bi₂MoO₆ nanocomposites presented significantly enhanced visible-light-driven photocatalytic activity for the degradation of RhB and 2,4-dinitrophenol. Compared with some reported works, the as-obtained Bi₂O₃/Bi₂MoO₆ nanocomposites also presented better photocatalytic activity.

2. Experimental

2.1 Materials

Bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O), sodium molybdate dihydrate (Na₂MoO₄·2H₂O), polyvinylpyrrolidone (PVP; MW 29000), nitric acid (HNO₃), sodium hydroxide (NaOH), absolute ethanol, ethylene glycol (EG), rhodamine B (RhB), 2,4-dinitrophenol (2,4-DNP) were used in this work. All chemicals were analytically pure and used without further purification. Deionized water was used throughout experiments.

2.2 Synthesis of Bi₂O₃ nanorod bundles

In a typical experiment, 1.45 g of Bi(NO₃)₃·5H₂O was firstly dissolved in 30 mL 1.5 mol L⁻¹ of HNO₃ under magnetic stirring. Then, 0.1 g of PVP was added under continuously stirring. After the mixed system was continuously stirred for 15 min, the pH of the system was adjusted to 11 with a NaOH solution of 5 mol L⁻¹. Finally, the above mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave and kept at 100 °C for 4 h. The white product was centrifuged, washed with deionized water and ethanol for several times, and dried in a vacuum oven at 60 °C.

2.3 Preparation of heterostructured Bi₂O₃/Bi₂MoO₆ nanocomposites

To obtain Bi₂O₃/Bi₂MoO₆ nanocomposites, 0.1 g of Bi₂O₃ nanorods were added into 15 mL EG with 0.485 g Bi(NO₃)₃·5H₂O under magnetic stirring. Subsequently, a 15 mL solution containing 0.121 g of Na₂MoO₄·2H₂O was introduced. After continuously stirring for another 30 min, the system was sealed in a Teflon-lined stainless steel autoclave and heated at 160 °C for 20 h. Subsequently, the system was cooled down to room temperature naturally. The final product was collected and washed several times with deionized water and absolute ethanol, and dried at 60 °C for 4 h.

As a control, pure Bi₂MoO₆ was also prepared through the above process in the absence of Bi₂O₃. Simultaneously, a mixture of Bi₂O₃ and Bi₂MoO₆, labeled as PM-Bi₂O₃/Bi₂MoO₆, was prepared by the simple physical mixing method.

2.4 Visible-light-driven photocatalytic performance

To investigate the photocatalytic activities of as-prepared samples, RhB and 2,4-DNP were used as the pollutants and a 500 W xenon lamp with a cutoff filter at 420 nm as the visible light source. In a typical experiment, 30 mg of catalyst was added to 50 mL of RhB (or 2,4-DNP) solution with a concentration of 10 mg L⁻¹. Then, the system was continuously stirred in the dark for 60 min to establish an adsorption–desorption equilibrium between the catalyst and the pollutant. Finally, the system was irradiated under a high-pressure xenon lamp equipped with a UV filter for desired durations (during irradiation, the system was ceaselessly stirred). The concentration change of the pollutant was monitored with a Metash 6100 UV-vis absorption spectrophotometer (Shanghai).

2.5 Photoelectrochemical measurements

Photocurrent density was measured in a standard three-electrode cell. The product deposited on the ITO glass was used as the working electrode, a platinum coil (0.5 mm × 4 cm) and an Ag/AgCl electrode as the counter and the reference electrodes, respectively. A 0.5 M of Na₂SO₄ solution that was purged with nitrogen for 10 min prior to the measurement was selected as the electrolyte. The working electrode was prepared as follows: 2 mg of the as-prepared samples were firstly dispersed into 1 mL of twice distilled water under ultrasonication. Next, 100 μL of the sample suspension was uniformly dropped onto the surface of the ITO glass. Afterwards, the ITO glasses were heated at 60 °C for 2 h to evaporate the solvent. The photoelectrochemical properties were monitored with a computer-controlled CHI660D electrochemical workstation under ambient conditions through illumination of a 500 W Xe lamp with a cut-off filter (λ = 420 nm).

2.6 Characterization

The X-ray powder diffraction patterns of the products were carried out on a Bruker D8 Advance X-ray diffractometer equipped with Cu Kα radiation (λ = 0.154060 nm), employing a scanning rate of 0.2 °s⁻¹ and 2θ ranges from 10° to 80°. Transmission electron microscopy (TEM) images were carried out on a Hitachi HT7700 transmission electron microscope, employing an accelerating voltage of 100 kV. High resolution transmission electron microscopy (HRTEM) images were obtained on an FEI Tacnai G² transmission electron microscope, employing an accelerating voltage of 200 kV. Field emission scanning electron microscopy (FESEM) images and energy dispersive spectrometry (EDS) analysis were obtained on a Hitachi S-4800 field emission scanning electron microscope, employing an accelerating voltage of 5 kV and 15 kV, respectively. UV-vis absorption spectra were recorded on a Metash 6100 UV-vis absorption spectrophotometer (Shanghai). The solid UV-vis diffuse reflection was performed on Shimadzu UV-2450 spectrophotometer. X-ray photoelectron spectroscopy (XPS) of the product was obtained on a Thermo ESCALAB 250 instrument, employing monochromic Al Kα (hν = 1486.6 eV) at a power of 150 W. Photoluminescence (PL) spectra were recorded on a FLSP 920 with a Xe lamp at room temperature, employing the excitation wavelength of 335 nm from a He–Cd laser. The Brunauer–Emmett–Teller (BET) surface areas of the product were measured on a Micromeritics ASAP 2460 analyzer.

3. Results and discussion

3.1 Structure and morphology characterization

Fig. 1a shows the XRD patterns of as-prepared Bi₂O₃, Bi₂MoO₆, and Bi₂O₃/Bi₂MoO₆. According to the diffraction patterns, pure Bi₂O₃ and Bi₂MoO₆ prepared in the present work can be separately determined as the monoclinic α-Bi₂O₃ phase (PDF no. 76-1730) and the orthorhombic Bi₂MoO₆ one (PDF no. 72-1524). The diffraction peaks of the as-prepared Bi₂O₃/Bi₂MoO₆ nanocomposites are very similar to those of pure Bi₂MoO₆, but they markedly widen against those of pure Bi₂MoO₆. This implies that the final product possibly owns smaller particle size. Furthermore, the main diffraction peak situations of Bi₂O₃ and Bi₂MoO₆ are very close. Their overlapping each other is maybe another reason causing the wideness of diffraction peaks. Fig. 1b gives the EDX analysis the final nanocomposites. Only O, Bi and Mo elements are detected, and the atomic ratio of Bi/Mo is calculated to be ∼3.25 : 1, which is bigger than 2 : 1 in Bi₂MoO₆. This fact implies the presence of Bi₂O₃ in the final product.

Fig. 1 (a) XRD patterns of Bi₂O₃, Bi₂MoO₆ and Bi₂O₃/Bi₂MoO₆ nanocomposites and (b) the EDS analysis of Bi₂O₃/Bi₂MoO₆ nanocomposites.

Further evidences came from the XPS analysis of the product. As shown in Fig. 2a, the sample contains O, C, Mo and Bi elements. The C peak should be ascribed to the XPS instrument itself. The Bi 4f spectrum is displayed in Fig. 2b. The peaks centered at 164.5 and 159.2 eV are assigned to Bi 4f_5/2 and Bi 4f_7/2 of Bi³⁺, respectively.¹⁶ However, two Bi 4f peaks in pure Bi₂O₃ locate at 164.2 and 158.6 eV and the values in pure Bi₂MoO₄ 164.6 and 159.3 eV.²³ The above facts confirm the formation of Bi₂O₃/Bi₂MoO₆ composites. In Mo 3d spectrum shown in Fig. 2c, the peaks at 235.5 and 232.5 eV are ascribed to Mo 3d_3/2 and Mo 3d_5/2 of Bi(VI), respectively.¹⁶ The O 1s spectrum contains three peaks (see Fig. 2d). The peaks at 529.9 and 530.7 eV are assigned to the crystal lattice oxygen of Bi₂MoO₄ and Bi₂O₃; and the one at 531.5 eV should be ascribed to the adsorbed oxygen.²⁴

Fig. 2 XPS analyses of Bi₂O₃/Bi₂MoO₆ nanocomposites: (a) survey spectrum, (b) Bi 4f, (c) Mo 3d and (d) O 1s.

Fig. 3 depicts the typical FESEM images of as-prepared Bi₂O₃, Bi₂MoO₆ and Bi₂O₃/Bi₂MoO₆ composite. Pure Bi₂O₃ samples prepared by the hydrothermal route are composed of abundant nanorod bundles (see Fig. 3a and its inset). Pure Bi₂MoO₆ samples obtained by the solvothermal approach comprise of plentiful irregular nanoplates with the length of ∼50 nm (Fig. 3b). However, the final Bi₂O₃/Bi₂MoO₆ composites present flake-like nanostructures and the arrangement of the nanoflakes are unordered (see Fig. 3c). Obviously, the morphology of the final composite is different from the nanorod bundles of Bi₂O₃ and the nanoplates of Bi₂MoO₆. Furthermore, FESEM observations showed that only micro-rods were obtained when no extra Bi(NO₃)₃ was added into the system with Bi₂O₃ and Na₂MoO₄ (see Fig. 3d). TEM observations further proved the above difference of the morphology. As shown in Fig. 4a, the nanorod bundles of Bi₂O₃ can be readily distinguished. The lengths of nanorods are polydispersed and the diameter of single nanorod is ∼20 nm. Similarly, the plate-like nanostructures of Bi₂MoO₆ are also clearly visible (see Fig. 4b), which is in good agreement with the result of FESEM. Fig. 4c displays a representative TEM image of the final composite. Abundant nanoparticles with the mean size of ∼10 nm are distributed on the surfaces of the nanoflakes. Comparing with the morphologies of Bi₂O₃ and Bi₂MoO₆, one can find that the final composite has obviously different morphology. Combining the results of Fig. 3d and b, one can find that Bi₂O₃ nanorods and Bi(NO₃)₃ are necessary in the formation of the final composites. The shape change of Bi₂O₃ nanorod bundles implies than nanorods take part in the reaction of forming Bi₂MoO₆ and change the nucleation environment of Bi₂MoO₆. Thus, the final composite owns markedly different morphology from single component. Fig. 4d shows a HRTEM image of the composite. Owing to different contrasts, one can find that several near-spherical nanoparticles strew the surface of a thin nanoflake. The d-spacing of the thin nanoflake is measured to 0.32 nm, which is very close to 0.32434 nm of the (120) plane of α-Bi₂O₃. The distance between planes of a spherical nanoparticle is ∼0.316 nm, which is close to 0.31562 nm of the (131) plane of orthorhombic Bi₂MoO₆. The above facts confirm the formation of Bi₂O₃/Bi₂MoO₆ composites. Furthermore, the selected area electron diffraction (SAED) pattern shown in the inset of Fig. 4d presents concentric rings, implying the polycrystalline nature of the composite.

Fig. 3 SEM images of as-prepared Bi₂O₃ nanorod bundles (a), Bi₂MoO₆ nanoplates (b), Bi₂O₃/Bi₂MoO₆ nanocomposites (c) and the product from the system with Bi₂O₃ nanorods and Na₂MoO₄ (d).

Fig. 4 TEM images of (a) Bi₂O₃ nanorod bundles, (b) Bi₂MoO₆ nanoplates, and (c) Bi₂O₃/Bi₂MoO₆ nanocomposites; (d) HRTEM image and SEAD pattern of Bi₂O₃/Bi₂MoO₆ nanocomposites.

The above shape change strongly affected the surface areas and the pore volumes of the products. Fig. 5 depicts the N₂ adsorption–desorption isotherms and the pore size distributions of the as-obtained products. The BET surface areas of Bi₂O₃, Bi₂MoO₆ and Bi₂O₃/Bi₂MoO₆ composites are 0.4288 m² g⁻¹, 18.97 m² g⁻¹ and 54.32 m² g⁻¹ in turn. The pore volumes of Bi₂MoO₆ and Bi₂O₃/Bi₂MoO₆ composites are calculated to be 0.22 cm³ g⁻¹ and 0.49 cm³ g⁻¹, respectively. However, no pore volume of Bi₂O₃ was obtained in BET measurement since its surface area was too small to meet BJH criteria. Obviously, the Bi₂O₃/Bi₂MoO₆ composites bear the larger BET surface area and pore volume than Bi₂MoO₆ nanoplates. Furthermore, the Bi₂O₃/Bi₂MoO₆ composites also presented different optical properties from pure Bi₂O₃ and Bi₂MoO₆. Fig. 6a exhibits UV-vis diffuse-reflectance spectra (DRS) of Bi₂O₃, Bi₂MoO₆ and Bi₂O₃/Bi₂MoO₆ composites. The absorption edges of three samples are in turn at 443, 477 and 461 nm. Namely, the absorption edge of the composite locates between those of two components. The PL spectra of three samples are displayed in Fig. 6b, from which one can easily find that the Bi₂O₃/Bi₂MoO₆ composite bears the weakest luminescent emission under the excitation of 310 nm UV light. The above facts show that the fast recombination of photo-induced electron–hole pairs in pure Bi₂O₃ and Bi₂MoO₆ has been efficiently restrained after forming the Bi₂O₃/Bi₂MoO₆ composite. Obviously, this is favorable for the enhancement of the photocatalytic performance.

Fig. 5 N₂ adsorption–desorption isotherms and corresponding pore size distributions of the as-obtained products.

Fig. 6 (a) UV-vis diffuse reflectance and (b) PL spectra of Bi₂O₃, Bi₂MoO₆ and Bi₂O₃/Bi₂MoO₆ nanocomposites.

3.2 Photocatalytic performance

To investigate the photocatalytic activity of the as-prepared Bi₂O₃/Bi₂MoO₆ composites, toxic organic compounds such as 2,4-dinitrophenol (2,4-DNP) and RhB were employed as the pollutants. For comparison, Bi₂O₃ nanorod bundles, Bi₂MoO₆ nanoplates and their physical mixture (PM-Bi₂O₃/Bi₂MoO₆) were used as photocatalysts under the same experimental conditions, too. Fig. 7a depicts the UV-vis absorption spectra of 2,4-DNP solution irradiated by the visible light for various durations in the presence of Bi₂O₃/Bi₂MoO₆ composites. Markedly, the peak intensity of 2,4-DNP gradually decreases with the prolonging of the irradiation time, indicating good photodegradation activity of the as-obtained composites. Fig. 7b depicts the correlation curves between the concentration changes of 2,4-DNP and the irradiation durations in the presences of various photocatalysts. After irradiating for 80 min, the degradation efficiencies of four catalysts for 2,4-DNP are in turn 27.6% for Bi₂O₃, 4.3% for Bi₂MoO₆, 63.9% for PM-Bi₂O₃/Bi₂MoO₆ and 88.9% for Bi₂O₃/Bi₂MoO₆ composites. Although the PM-Bi₂O₃/Bi₂MoO₆ presents the appreciably higher photocatalytic activity than pure Bi₂O₃ and Bi₂MoO₆, its catalytic ability is still much lower than that of Bi₂O₃/Bi₂MoO₆ composites. Obviously, the highly efficient separation of photogenerated carriers in the composite is main reason to promote the photocatalytic activity of the catalyst. Additionally, the high surface area and the big pore volume of the composite are also favorable for the adsorption of organic molecules and the acceptance of the incident light. Hence, the Bi₂O₃/Bi₂MoO₆ composites present the enhanced photocatalytic performance.

Fig. 7 (a) The absorption spectra of 2,4-DNP solution solution after irradiated by the visible light for various durations under the presence of 30 mg Bi₂O₃/Bi₂MoO₆ nanocomposites; (b) the concentration–time curves of 2,4-DNP irradiated by the visible light in the presence of 30 mg various catalysts.

Similar experimental phenomena were also found on the degradation of RhB dye. As seen from Fig. 8a, under the presence of 30 mg Bi₂O₃/Bi₂MoO₆ nanocomposites, the maximal absorbance (λ_max) gradually shifts from the initial 554 nm towards shorter wavelength and finally to 496 nm with the expansion of the irradiation time. This is in good agreement with the degradation of RhB catalyzed by BiVO₄.²⁵ The blue-shift of absorption band should be mainly attributed to the stepwise removal of the N-ethyl group during the degradation of RhB. After the de-ethylated process is fully completed, the produced rhodamine is further decomposed due to the gradual destruction of the conjugated structure.²⁵ Finally, the color of the solution is faded (see the inset in 8a). Fig. 8b shows the concentration–time curves of RhB dye in the presences of various catalysts. When Bi₂O₃/Bi₂MoO₆ composites were used as the photocatalyst, RhB was fully degraded after irradiation for 60 min under the visible light. The degradation efficiency is far higher than 37.7% of Bi₂O₃, 45.9% of Bi₂MoO₆ and 70.8% of PM-Bi₂O₃/Bi₂MoO₆. Moreover, compared with some previous reports (Table 1), the present Bi₂O₃/Bi₂MoO₆ composites also presented better photocatalytic activity for the photodegradation of RhB.

Fig. 8 (a) The absorption spectra of RhB solution irradiated by the visible light for various durations under the presence of Bi₂O₃/Bi₂MoO₆ composites; (b) the concentration–time curves of RhB solution irradiated by the visible light in the presences of various catalysts. The inset in (a) is the photographs of RhB solution irradiated by the visible light for various durations under the presence of Bi₂O₃/Bi₂MoO₆ composites.

Table 1 Photocatalytic efficiency of RhB over various Bi-based ternary oxides photocatalysts under visible light irradiation

Photocatalyst	CRhB	Light source	Photocatalytic efficiency	Ref.
Bi2O3/Bi2MoO6 (30 mg)	10 mg L^−1	500 W Xe lamp	100% 60 min	This work
Bi2MoO6/ZnTiO3 (50 mg)	10 mg L^−1	150 W Xe lamp	82% 240 min	26
Fe3O4/SiO2/Bi2MoO6 (100 mg)	10 mg L^−1	300 W Xe lamp	100% 120 min	27
BiIO4/Bi2MoO6 (50 mg)	0.01 mM	1000 W Xe lamp	55% 300 min	28
Bi2MoO6/BiOCl (50 mg)	10 mg L^−1	500 W Xe lamp	100% 300 min	29
Bi2WO6/ZnWO4 (20 mg)	10 mg L^−1	500 W mercury lamp	80%, 80 min	30
Ag–graphene–Bi2WO6 (50 mg)	0.01 mM	350 W Xe lamp	100% 90 min	31
C3N4@Ag–Bi2WO6 (50 mg)	10 mg L^−1	350 W Xe lamp	100% 90 min	32
CuO/BiVO4 (200 mg)	10 mg L^−1	300 W Xe lamp	100% 150 min	33
BiVO4/BiOI (30 mg)	0.02 mM	500 W Xe lamp	100% 300 min	19

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To ascertain the mineralization degree of organic species in the photodegradation of RhB by Bi₂O₃/Bi₂MoO₆ composites, the total organic carbon (TOC) value was measured. As shown in Fig. S1,† the TOC value slowly decreases with the increase of irradiation time from 0–100 min; and then rapidly decreases, indicating that RhB is continuously mineralized. After irradiating for 160 min, the TOC value changes from 14.951 to 6.157 mg L⁻¹. The mineralization ratio of ∼59% is reached.

Furthermore, the stability and reusability of the present photocatalyst were also investigated. As shown in Fig. 9, the photocatalytic efficiency of Bi₂O₃/Bi₂MoO₆ composites after 5 cycles under the same conditions still reaches ∼91%, implying the good stability of the present photocatalyst. The above facts clearly indicate that the as-obtained Bi₂O₃/Bi₂MoO₆ composites are outstanding photocatalyst and have potential applications in the field of environmental treatment and protection.

Fig. 9 The photocatalytic efficiency of Bi₂O₃/Bi₂MoO₆ composites to RhB dye after five cycles.

To faithfully demonstrate the stability of the present catalyst, we suppose that the photodegradation process of RhB by Bi₂O₃/Bi₂MoO₆ composites in each cycle is the pseudo-first-order model. Thus, the rate constant, k, could be obtained according to the below equation:

ln(C/C₀) = kt (1)

where, C₀ and C are the concentrations of the dye solution at time 0 and t, respectively. Fig. S2a† exhibits the plots of the rate constant vs. the irradiation time for five cycles. The k value is calculated from the slope of the plots of ln(C/C₀) vs. t. The k value has marked decrease after 4 cycles. Fig. S2b† gives the histogram of the kinetic constant vs. the cycle number, which faithfully shows the above change of the k value.

In the PL spectra of the samples shown in Fig. 6b, Bi₂O₃/Bi₂MoO₆ composites own the weakest emission intensity, implying the efficient charge separation of the photocatalyst. To further evaluate the charge separation efficiencies of different photocatalysts, the photo-current densities of various catalysts were measured before and after visible light illumination. As shown in Fig. 10, all of the catalysts exhibit small current densities in the dark. However, under visible light irradiation the photocurrent densities of all catalysts markedly increase. At 1.2 V, for example, the photocurrent density changes from 237 to 388 μA cm⁻² for Bi₂O₃, from 194 to 400 μA cm⁻² for Bi₂MoO₆, from 243 to 366 μA cm⁻² for PM-Bi₂O₃/Bi₂MoO₆ and from 200 to 500 μA cm⁻² for Bi₂O₃/Bi₂MoO₆. Among them, the Bi₂O₃/Bi₂MoO₆ composites present the highest photocurrent density and the biggest increase. Mentioned by the previous text, Bi₂O₃/Bi₂MoO₆ composites have the biggest surface area, which is beneficial for light harvesting.³⁴ Thus, more electron–hole pairs are generated in Bi₂O₃/Bi₂MoO₆ composites. Simultaneously, the recombination of the produced electron–hole pairs can be effectively suppressed, resulting in the marked increase of the photocurrent density of Bi₂O₃/Bi₂MoO₆ composites. Moreover, it is worthy pointing out that PM-Bi₂O₃/Bi₂MoO₆ has smaller photocurrent density than Bi₂O₃ and Bi₂MoO₆, but it presents stronger photocatalytic activity. To ascertain the reason causing the above contradiction, the BET surface area of PM-Bi₂O₃/Bi₂MoO₆ was investigated. Fig. S3† depicts N₂ adsorption–desorption isotherm and corresponding pore size distributions of PM-Bi₂O₃/Bi₂MoO₆. The BET surface area of PM-Bi₂O₃/Bi₂MoO₆ was 10.45 m² g⁻¹, lower than 18.97 m² g⁻¹ of Bi₂MoO₆. Thus, PM-Bi₂O₃/Bi₂MoO₆ should absorb fewer incident photons than Bi₂MoO₆, which resulted in the lower photocurrent density. In the photocatalytic degradation, however, PM-Bi₂O₃/Bi₂MoO₆ owned stronger ability to restrain the fast recombination between the photoelectrons and poles than single component. As a result, PM-Bi₂O₃/Bi₂MoO₆ exhibited better photocatalytic activity.³⁵

Fig. 10 Photocurrent densities of Bi₂O₃, Bi₂MoO₆, PM-Bi₂O₃/Bi₂MoO₆ and Bi₂O₃/Bi₂MoO₆ composites measured under a 500 W Xe lamp with a cut-off filter (λ > 420 nm) in a 0.5 mol L⁻¹ Na₂SO₄ solution.

3.3 Photocatalytic mechanism

In the present Bi₂O₃/Bi₂MoO₆–RhB system, to ascertain the definite oxidative species during the photodegradation process, some scavengers including iso-propanol (IPA), ethylene diamine tetraacetic acid disodium salt (EDTA-2Na), and benzoquinone (BQ) were employed, respectively. As seen from Fig. 11, when IPA was used as the scavenger, the photocatalytic degradation of RhB dye was only slightly affected, implying that ˙OH group hardly had contribution to the RhB oxidation. When BQ was selected as the scavenger, the photodegradation of RhB dye was markedly inhibited, indicating that ˙O₂⁻ played the important role in the photodegradation of RhB. While EDTA-2Na was employed as the scavenger, the photocatalytic degradation of RhB dye was strongly restrained, showing that h⁺ played the crucial role in the photodegradation of RhB.

Fig. 11 Photocatalytic degradation efficiency of RhB solutions with various scavengers in the presence of 30 mg Bi₂O₃/Bi₂MoO₆ composites.

Usually, when p-type and n-type semiconductors contact each other, carriers will diffuse due to the concentration gradient of carries between p–n junctions. Holes transfer from the p-type semiconductor to the n-type semiconductor side, and electrons enter the p-type side from the n-type one. Hence, the region in the n-type semiconductor that loses electrons will have a net positive charge; and the region in the p-type semiconductor that accepts the electrons will have a net negative charge. At the interface of the semiconductors, the charge redistribution creates an internal electric field in the space charge region from the n-type side to the p-type side.¹⁷ The direction of the internal electric field is opposite to the direction of the diffusion current, so when the redistribution of mobile electrons or holes will reach a balance, which is called as the Fermi energy level (E_F) equilibrium state between p–n junctions. In the present work, the optical band gap of pure Bi₂O₃ and Bi₂MoO₆ could be calculated from the absorption spectra shown in Fig. 6a through the equation of αhν = A(hν − E_g) ^n/2. Where α, h, ν, A and E_g are in turn the absorption coefficient, Planck constant, light frequency, constant and band gap. And n depends on the characteristics of transition in a semiconductor (n = 4 for indirect band gap and = 1 for direct band gap). Here, n = 4 for Bi₂O₃ and n = 1 for Bi₂MoO₆.³⁶ Fig. 12 depicts the plots of (αhν)^2/n ∼ hν of Bi₂O₃ and Bi₂MoO₆. The band gaps of Bi₂O₃ and Bi₂MoO₆ are 2.67 and 2.80 eV, respectively.

Fig. 12 The plots of (αhν)^2/n vs. hν of Bi₂O₃ and Bi₂MoO₆.

Thus, the positions of conduction and valence band can be determined by the following equation:

E_CB = X − E_e − 0.5E_g (2)

E_CB = E_VB − E_g (3)

where, E_CB is the CB edge potential. X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms. The X values for Bi₂O₃ and Bi₂MoO₆ are ∼5.99 and 5.55 eV,^1,24 respectively. The E_e value is the energy of free electrons on the hydrogen scale (∼4.5 eV). According to the eqn (2) and (3), E_CB and E_VB values of Bi₂MoO₆ are determined to be −0.35 eV and 2.45 eV; and those of pure Bi₂O₃ are 0.16 eV and 2.83 eV. The energy band structures of Bi₂O₃ and Bi₂MoO₆ are shown in the left of Fig. 13. The difference of CB and VB positions between Bi₂O₃ and Bi₂MoO₆ will facilitate the move of the photogenerated electrons and holes in opposite directions, which can effectively inhibit their recombination. Moreover, as a p-type semiconductor, the E_F of Bi₂O₃ is on the verge of the valence band, whereas the E_F of n-type Bi₂MoO₆ is close to the conduction band. Before the hybrid of p-type Bi₂O₃ with n-type Bi₂MoO₆, the Fermi level of Bi₂O₃ lower than that of the Bi₂MoO₆ (see the left in Fig. 13). After the hybrid of Bi₂MoO₆ and Bi₂O₃, the E_F of Bi₂O₃ shifts upward, whereas the E_F of Bi₂MoO₆ shifts downward until an equilibrium state is formed. Eventually, the conduction band bottom of Bi₂MoO₆ becomes lower in energy than that of Bi₂O₃, resulting in the formation of a p–n junction (see the right in Fig. 13). Under the visible light irradiation, Bi₂O₃ with a narrow band gap is excited easily, and generates photoelectrons and holes. The electrons in the CB of Bi₂O₃ can easily migrate to that of Bi₂MoO₆ and meanwhile, the produced holes remain in the VB of Bi₂O₃. Since the CB potential (−0.35 eV) of Bi₂MoO₆ is more negative than that of E⁰ (O₂/˙O₂⁻) (−0.046 eV vs. NHE),³⁷ the active species ˙O₂⁻ can be produced due to the reduction of adsorbed O₂ by photogenerated electrons. Subsequently, the h⁺ and ˙O₂⁻ take part in photoredox reactions to degrade organic pollutants directly or indirectly. Furthermore, the internal electric field in the Bi₂O₃/Bi₂MoO₆ p–n junction increases the migration of photogenerated electrons and holes,³⁸ which also promotes the photocatalysis of organic pollutants.

Fig. 13 Schematic diagrams for the energy band of Bi₂O₃ and Bi₂MoO₆ (the left) and the possible charge separation in Bi₂O₃/Bi₂MoO₆ composites (the right).

4. Conclusion

In summary, the Bi₂O₃/Bi₂MoO₆ composites have been successfully prepared via a two-step solvothermal route. Electron micrographs showed that the as-synthesized Bi₂O₃/Bi₂MoO₆ composites owned the obviously different morphology from pure Bi₂O₃ and Bi₂MoO₆. N₂ sorption–desorption experiments and photoelectrochemical measurements showed that Bi₂O₃/Bi₂MoO₆ composites had the biggest surface area and the highest photocurrent density. It was found that as-obtained Bi₂O₃/Bi₂MoO₆ composites presented stronger catalytic activities to the degradation of RhB and 2,4-DNP under the irradiation of the visible light than single component (Bi₂O₃ or Bi₂MoO₆) and their physical mixture. The above enhanced photocatalytic performance originated from the p–n junction between p-type Bi₂O₃ and n-type Bi₂MoO₆, in which the fast recombination of photogenerated electrons and holes was efficiently restrained. Simultaneously, experiments uncovered that the hole (h⁺) was the main oxidative species and ˙O₂⁻ radicals also acted as the important role in the photodegradation of organic pollutants. The present photocatalyst also presented good cycling stability. After 5 cycles, the catalytic efficiency still retained ∼91%, indicating that the as-obtained photocatalyst has potential application in environmental protection and wastewater treatment.

Acknowledgements

The authors thank the National Natural Science Foundation of China (21571005), High School Leading talent incubation programme of Anhui province (gxbjZD2016010) and The Recruitment Program for Leading Talent Team of Anhui Province for the fund support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra02760e

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