Novel BiIO₄/BiVO₄ composite photocatalyst with highly improved visible-light-induced photocatalytic performance for rhodamine B degradation and photocurrent generation

Abstract

A novel BiIO₄/BiVO₄ heterojunction photocatalyst has been successfully developed by a facile hydrothermal route for the first time. X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-vis diffuse reflectance spectra (DRS) were utilized to characterize the crystal structures, morphologies and optical properties of the as-prepared products. Under visible light irradiation (λ > 420 nm), the BiIO₄/BiVO₄ composite exhibits much better photoelectrochemical performance for rhodamine B (RhB) degradation and photocurrent (PC) generation compared to pure BiIO₄ and BiVO₄. This significant enhancement on visible-light-driven photocatalytic activity should be ascribed to the formation of the BiIO₄/BiVO₄ heterojunction, which can result in the high separation and transfer efficiency of photogenerated charge carriers. It was verified by electrochemical impedance spectra (EIS). The active species trapping experiment demonstrated that h⁺ play a critical role during the photocatalytic process, which is consistent with the supposed photocatalytic mechanism.

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1. Introduction

Semiconductor photocatalysts show great potential for environment remediation and energy conversion.^1–8 Generally, the high performance of a photocatalyst requires that it must have a very negative conduction band (CB) and a positive valence band (VB), thus providing enough reduction power for the electrons and high oxidation power for the holes.^9–11 These requirements can be met by the wide-band-gap (WBG) semiconductors. Nevertheless, a photocatalyst with wide band gap always suffers from insufficient absorption of visible light. Therefore, fabrication of a heterojunction photocatalyst by coupling with a narrow-band-gap (NBG) semiconductor is a very effective approach to modifying the WBG semiconductor and acquiring high visible-light-driven photocatalytic activity.^12–14

Bismuth compounds as photocatalysis active materials have attracted intensive attention for the advantages of high activity, high stability and non-toxity.^15–21 Lately, bismuth iodate (BiIO₄) has been found possessing highly efficient photocatalytic activity for the degradation of organic pollutants.^22,23 The layered crystal structure and polarity are considered to be very favorable for the separation of photogenerated charge carriers. Nevertheless, BiIO₄ almost could not absorb visible light, which can extremely limit its practical application from the point of view of utilization of solar energy. Thus, coupling with other semiconductor with a relatively larger band gap as visible light sensibiliser was an effective route to enhance the photocatalytic activity of the system. As a visible-light-induced photocatalyst with a narrow band gap of 2.3–2.5 eV, BiVO₄ has been recognized as a potentially suitable photocatalyst for water splitting.^24,25 However, it has been reported that the photocatalytic activity of BiVO₄ is usually not satisfied because the photogenerated electrons and holes tend to rapidly decay through recombination, which significantly restricted the practical applications of BiVO₄ in photocatalytic degradation of pollutants. Therefore, it is also necessary to develop effective strategies to improve the charge separation efficiency and improve visible-light photocatalytic performance of BiVO₄ photocatalyst.

Herein, we for the first time report a novel BiIO₄/BiVO₄ composite with a heterojunction structure, which was successfully synthesized by a facile hydrothermal process. The photoelectrochemical performance of BiIO₄/BiVO₄ heterostructure was evaluated by photodecomposition of rhodamine B (RhB) and photocurrent generation (PC) under visible light irradiation (λ > 420 nm). The results indicated that BiIO₄/BiVO₄ heterojunction exhibits highly enhanced photoelectrochemical properties than the two single samples. Furthermore, the photocatalytic mechanism of the BiIO₄/BiVO₄ composite was discussed in details.

2. Experimental section

2.1 Materials and synthesis procedure

The raw materials were all in analytical grade and utilized as received without further purification. The samples of BiIO₄, BiVO₄ and BiIO₄/BiVO₄ composites were all obtained by a hydrothermal method. In a typical synthesis of 15% BiIO₄/BiVO₄ composite, 0.15 mmol of I₂O₅ and 2 mmol of NH₄VO₃ were dissolved in 30 mL deionized water under ultrasound. Meanwhile, a stoichiometric amount of Bi(NO₃)₃·5H₂O was also added in 30 mL deionized water to obtain a transparent solution. Then, the bismuth salt solution was added into the above solution drop by drop under magnetic stirring. Afterwards, the resulting suspensions were transferred into a 100 mL Teflon-lined stainless autoclave and kept at 180 °C for 24 h. Finally, the as-prepared products were filtrated and washed with deionized water and ethanol for several times, and then dried at 60 °C for 5 h. The other BiIO₄/BiVO₄ composites and pure BiIO₄ and BiVO₄ were obtained using the same method.

2.2 Catalyst characterization

X-ray powder diffraction (XRD) was performed to characterize the crystal structures of the samples on a Bruker D8 with Cu Kα radiation. The microstructure and morphology of the products were investigated by a S-4800 scanning electron microscopy (SEM). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analyses were obtained on a JEM-2100 electron microscope (JEOL, Japan) operating at 200 kV. A PerkinElmer Lambda 35 UV-vis spectrometer was utilized to record the UV-vis diffuse reflectance spectra (DRS). The photoluminescence (PL) spectra were recorded on a JOBIN 10 YVON FluoroMax-3 fluorescence spectrophotometer, and a 150 W xenon lamp was used as the excitation lamp.

2.3 Photocatalytic activity experiment

The photocatalytic performance of BiIO₄, BiVO₄ and BiIO₄/BiVO₄ composites were evaluated by degradation of RhB and bisphenol A (BPA) under visible light (λ > 420 nm, 500 W xenon lamp). 50 mg of as-prepared photocatalyst was dispersed in an aqueous solution of RhB (50 mL, 0.01 mM) or BPA (50 mL, 10 mg L⁻¹). Before photoreaction, the photocatalyst and dye solution were vigorously stirred in the dark for 1 h to complete an adsorption–desorption equilibrium. Then, the light was turned on. About 3 mL of the liquid was taken for the time interval of 1 h, and separated to remove the solid by centrifugation. The concentrations of RhB and BPA were obtained by using a Cary 5000 UV-vis spectrophotometer.

2.4 Photoelectrochemical measurements

The photocurrent (PC) generation and electrochemical impedance spectra (EIS) were obtained in a three-electrode system with an electrochemical analyzer (CHI-660B, China). The electrolyte solution was 0.1 M Na₂SO₄ solution. Platinum wires and saturated calomel electrodes (SCE) were utilized as the counter electrode and reference electrode, respectively. The working electrodes were the photocatalyst films of BiIO₄, BiVO₄ and BiIO₄/BiVO₄ composite coated on indium-tin oxide (ITO) glass. The detailed procedure of coating on ITO was as follows: 5 mg of photocatalyst was added to 0.5 mL deionized water. After sonicating for 15 min, the suspension was deposited onto the ITO conductive glasses surface (the conductive side) with 5 mL plastic sucker. The electrode was dried and then calcined at 373 K for 10 h. The electrochemical tests were all measured at 0.0 V with light intensity 1 mW cm⁻².

2.5 Active species trapping experiments

In older to detect the active species generated in the photocatalytic process, the scavengers, including disodium ethylenediaminetetraacetate (EDTA-2Na), 1,4-benzoquinone (BQ) and tert-butyl alcohol (IPA) were added as quenchers of holes (h⁺), superoxide radical (˙O₂⁻) and hydroxyl radicals (˙OH), respectively.^26,27 The trapping experiment was similar to the above photodegradation tests except that RhB was replaced.

3. Results and discussion

3.1 Characterization

BiIO₄ and BiVO₄ crystallize in orthorhombic space group Pca2 and monoclinic space group C2/c, respectively. Fig. 1a shows the crystal structure of BiIO₄, which is composed of (Bi₂O₂)²⁺ and (IO₃)⁻ nonbonding layers. BiVO₄ also exhibits the obvious layered structure consisting of alternated connected VO₄ tetrahedra and BiO_x polyhedra as seen in Fig. 1b. These layered structures are supposed to be favorable for the separation of photogenerated electrons and holes. Fig. 2 presents the X-ray powder diffraction (XRD) patterns of the as-prepared BiIO₄, BiVO₄ and 15% BiIO₄/BiVO₄ composite. It can be found that all the samples were well crystallized. The sharp diffraction peaks of BiIO₄ and BiVO₄ can all well be indexed into orthorhombic phase BiIO₄ (ICSD #262019) and monoclinic phase BiVO₄ (JCPDS File no. 14-0688). There are no impure peaks found, suggesting the high purity and crystallinity of the samples. The BiIO₄/BiVO₄ product exhibit a mixed phase of both BiIO₄ and BiVO₄. It indicated the co-existence of BiIO₄ and BiVO₄ in the as-prepared composite.

Fig. 1 Crystal structures of BiIO₄ and BiVO₄.

Fig. 2 XRD patterns of BiIO₄, BiVO₄ and 15% BiIO₄/BiVO₄ composite.

The microstructure and surface morphology of BiIO₄, BiVO₄ and 15% BiIO₄/BiVO₄ composite have been investigated by scanning electron microscopy (SEM) as shown in Fig. 3. It can be seen from Fig. 3a that BiIO₄ exhibits nanocube and nanorod structure with crystal size of 200–500 nm. In contrast, the BiVO₄ products were composed of dendritic particles, and the length range from hundreds of nanometers to several microns (Fig. 3b). The SEM images of BiIO₄/BiVO₄ composite were shown in Fig. 3c and d. It is clear to see that in the composite the above two phases all can be found, and the BiIO₄ nanorods were firmly assembled on the surface of the BiVO₄ dendrites. Moreover, BiIO₄, BiVO₄ and BiIO₄/BiVO₄ composite were further investigated by transmission electron microscopy (TEM). Fig. 4a and b present the TEM images of BiIO₄ and BiVO₄, respectively, which confirmed the nanorods of BiIO₄ and dendritic microparticles of BiVO₄. With respect to BiIO₄/BiVO₄ composite, the HRTEM image (Fig. 4c and d) indicates two sets of lattice fringes with interplanar spacing of 0.309 and 0.236 nm, which correspond to the (121) planes of BiIO₄ and BiVO₄, respectively. This result further confirmed the co-existence of BiIO₄ and BiVO₄ in the composite.

Fig. 3 SEM images of (a) BiIO₄, (b) BiVO₄ and (c and d) 15% BiIO₄/BiVO₄ composite.

Fig. 4 TEM images of (a) BiVO₄, (b) BiIO₄ and (c and d) HRTEM of 15% BiIO₄/BiVO₄ composite.

Fig. 5a displayed the UV-vis DRS spectra of BiIO₄, BiVO₄ and 15% BiIO₄/BiVO₄ heterostructure. The absorption edge of BiIO₄ and BiVO₄ are located at about 400 nm and 540 nm in the visible region, respectively. Comparatively, the BiIO₄/BiVO₄ heterostructure exhibit a similar spectrum with BiVO₄, which assures the light absorption on visible light. Thus, the BiVO₄ can serve as an effective visible-light sensibilizer for BiIO₄.

Fig. 5 (a) UV-vis diffuse reflectance spectra of the BiIO₄, BiVO₄ and 15% BiIO₄/BiVO₄ heterojunction. (b) Band gaps of BiIO₄ and BiVO₄.

The band gap (E_g) can be determined by the Kubelka–Munk function:^28,29

αhv = A(hν − E_g)^n/2 (1)

where α, E_g, hν and A are the absorption coefficient, band gap, the photonic energy and a constant, respectively. In this equation, n is determined by the type of optical transition of a semiconductor (n = 1 for direct transition and n = 4 for indirect transition). As BiIO₄ is an indirect transition semiconductor, the n value is 4. For BiVO₄, the n value is 1 for its direct optical transition property. Fig. 5b presented the plots of absorption^1/2 versus energy and absorption² versus energy. The band gaps of BiIO₄ and BiVO₄ are estimated to be 3.15 and 2.37 eV, respectively.

3.2 Photocatalytic performance

The photocatalytic activity of BiIO₄, BiVO₄ and BiIO₄/BiVO₄ composites were assessed by photodecomposition of RhB under visible light irradiation (λ > 420 nm). As shown in Fig. 6a, the adsorption of RhB in dark and degradation catalysed by BiIO₄ could be neglected, and pure BiVO₄ also exhibits a poor photocatalytic activity. Compared to the two pure samples, the BiIO₄/BiVO₄ composites with molar ratios of 5%, 10%, 15% and 20% all display much better photocatalytic performance. Among which, the most significantly improved activity was observed in 15% BiIO₄/BiVO₄ composite. To quantitatively understand the reaction kinetics of the photocatalytic degradation process of RhB, the apparent pseudo-first-order model according to the Langmuir–Hinshelwood (L–H) kinetics model was applied:³⁰

ln (C₀/C) = k_appt (2)

where C₀ is initial RhB concentration (mg L⁻¹), C is RhB concentration in aqueous solution at time t (mg L⁻¹), k_app is the apparent pseudo-first-order rate constant (h⁻¹). The photodegradation apparent rate constants were shown in Fig. 6b. The corresponding k_app values are calculated to be 0.00396 h⁻¹, 0.027 h⁻¹, 0.108 h⁻¹, 0.129 h⁻¹, 0.28 h⁻¹ and 0.173 h⁻¹, for BiIO₄, BiVO₄, and BiIO₄/BiVO₄ composites with molar ratios of 5%, 10%, 15% and 20%, respectively. Thus, 15% BiIO₄/BiVO₄ composite exhibits the highest photocatalytic efficiency, which is almost 70.7 and 10.4 times higher than those of pure BiIO₄ and BiVO₄, respectively. Moreover, in order to exclude the photosensitization of RhB, the photodegradation of colorless organic pollutant bisphenol A (BPA) over BiIO₄, BiVO₄ and BiIO₄/BiVO₄ composite photocatalysts under visible light was performed. As shown in Fig. 6c, the photodegradation efficiency of BPA over the BiIO₄/BiVO₄ composite photocatalysts is still much higher than those of the single BiIO₄ and BiVO₄. This result further confirms the more efficient photocatalytic reactivity of BiIO₄/BiVO₄ composite.

Fig. 6 (a) Photocatalytic degradation curves and (b) apparent rate constants of RhB over the BiIO₄, BiVO₄ and BiIO₄/BiVO₄ composites under visible light (λ > 420 nm). (c) Photodegradation curves of BPA over BiIO₄, BiVO₄ and 15% BiIO₄/BiVO₄ under visible light (λ > 420 nm).

The photocatalytic activity is closely related with the generation of the photoinduced charge carrier in the photocatalytic process, which can be directly monitored by the photocurrent.^16,31 Fig. 7 presents the photocurrent densities of BiIO₄, BiVO₄ and 15% BiIO₄/BiVO₄ composite under visible light irradiation. It can be obviously seen that BiIO₄/BiVO₄ possess a highly enhanced photocurrent respond, and the intensity was about 3 and 5 times higher than those of pure BiIO₄ and BiVO₄. It is in good agreement with the order of their photocatalytic activities. The significant enhancement on degradation rate and photocurrent generation should be attributed to fabrication of the BiIO₄/BiVO₄ heterojunction, thus resulting in highly efficient separation of photogenerated charge carriers. It will be discussed in details in the following photocatalytic mechanism Section.

Fig. 7 Photocurrent generation of BiIO₄, BiVO₄ and 15% BiIO₄/BiVO₄ heterojunction photocatalysts under visible-light irradiation (λ > 420 nm, [Na₂SO₄] = 0.1 M).

3.3 Photocatalytic mechanism of BiIO₄/BiVO₄ composite

In view of the significantly enhanced photocatalytic activity in BiIO₄/BiVO₄, some synergistic effects may exist in the composite. The conduction band (CB) and valence band (VB) positions of BiIO₄ and BiVO₄ can be obtained by the following equations:³²

E_VB = X − E_e + 0.5E_g (3)

E_CB = E_VB − E_g (4)

where E_CB is the CB potential, E_VB is the VB potential, E_e is the energy of free electrons on the hydrogen scale (E_e = 4.5 eV), E_g is the band gap energy of the semiconductor, X is the electronegativity of the semiconductor (geometric average of the absolute electronegativity of the constituent atoms). Accordingly, the E_CB and E_VB of BiIO₄ were separately estimated to be 0.77 and 3.92 eV. The E_CB and E_VB of BiVO₄ were calculated to be 0.48and 2.85 eV, respectively.

In order to survey the active species in the photodegradation process of RhB over BiIO₄/BiVO₄ composite, the trapping experiment was performed. Various scavengers, including benzoquinone (BQ), ethylene diamine tetraacetic acid disodium salt (EDTA-2Na) and iso-propanol (IPA) were added to as the quenchers of superoxide radicals (˙O₂⁻), holes (h⁺) and hydroxyl radicals (˙OH), respectively.^26,27 From Fig. 8, it can be found that degradation efficiency of RhB was only slightly affected by the addition of BQ and IPA. Nevertheless, a significant inhibition was observed after with the EDTA-2Na was added into the photocatalytic system. The results indicated that holes (h⁺) are the main active species and can dominate the degradation process.

Fig. 8 Photocatalytic degradation of RhB over the 15% BiIO₄/BiVO₄ photocatalyst with the addition of scavengers BQ, EDTA and IPA.

Based on the above band energy level analysis and results from active species trapping experiments, the possible photocatalytic mechanism was supposed. As illustrated in Fig. 9, the BiIO₄ and BiVO₄ photocatalysts possess matchable band energy levels. Then, an effective heterojunction structure between BiIO₄ and BiVO₄ can be fabricated. Under visible light irradiation, the electrons in BiVO₄ can be excited to transfer to the CB of BiVO₄, and holes remained in its VB. As the CB potential of BiVO₄ is more negative than that of BiIO₄, the photogenerated electrons in the CB of BiVO₄ are easier to migrate to the CB of BiIO₄ due to the interfacial interaction between two photocatalysts. Thus, the photogenerated charge carriers in the composite can be effectively separated. As the CB of BiIO₄ (0.77 eV) is much more positive than the level of O₂/˙O₂⁻ (−0.28 eV vs. NHE),³³ the ˙O₂⁻ radicals could almost not be generated by reducing the O₂ with an electron. Therefore, the holes in the VB of BiVO₄ with powerful oxidation ability serving as the main active species play important roles in the photocatalytic process.

Fig. 9 Schematic diagram of BiIO₄/BiVO₄ heterojunction photocatalyst under visible light irradiation.

Electrochemical impedance spectra (EIS) can be used to investigate the migration and transfer processes of photogenerated electrons and holes in a semiconductor. The radius of the arc in the EIS spectra demonstrates the interface layer resistance occurred on the surface of electrode.³⁴ The smaller radius of the arc implied the higher efficiency of charge transfer. The Nyquist plots of BiIO₄, BiVO₄ and BiIO₄/BiVO₄ heterojunction with and without visible light irradiation were shown in Fig. 10. Obviously, the arc radius of BiIO₄/BiVO₄ is much smaller than those of pure BiIO₄ and BiVO₄ in the cases of both light and in dark. Thus, BiIO₄/BiVO₄ heterojunction holds a stronger ability in migration and transfer of photogenerated electrons and holes. The above PL and EIS results revealed that fabrication of a heterojunction can truly promote the transfer of electrons and holes, and hinder its recombination.

Fig. 10 Electrochemical impedance spectra of BiIO₄, BiVO₄ and BiIO₄/BiVO₄ heterojunction photocatalysts under visible-light irradiation (λ > 420 nm, [Na₂SO₄] = 0.1 M).

4. Conclusions

In summary, a novel BiIO₄/BiVO₄ heterojunction has been successfully developed by a one-step hydrothermal method for the first time. The results demonstrated that the BiIO₄/BiVO₄ heterojunction exhibits the much better photoelectrochemical properties for rhodamine B (RhB) degradation and photocurrent (PC) generation than the two individuals under visible light irradiation (λ > 420 nm). The significantly enhanced photocatalytic activity should be ascribed to fabrication of a BiIO₄/BiVO₄ heterojunction, which can result in an efficient interfacial charge transfer. It was proved by PL and EIS. The supposed photocatalytic mechanism dominated by holes (h⁺) was confirmed by active species trapping experiment.

Acknowledgements

This work was supported by the National Natural Science Foundations of China (Grant no. 51302251, 51172245), the Fundamental Research Funds for the Central Universities (2652013052), and the special coconstruction project of Beijing city education committee, Key Project of Chinese Ministry of Education (no. 107023).

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