Novel I-BiOBr/BiPO₄ heterostructure: synergetic effects of I⁻ ion doping and the electron trapping role of wide-band-gap BiPO₄ nanorods

Abstract

A novel I⁻-doped BiOBr/BiPO₄ (I-BiOBr/BiPO₄) heterostructure was constructed by a facile deposition-precipitation method using wide-band-gap BiPO₄ nanorods as the substrate. The as-prepared I-BiOBr/BiPO₄ composite exhibited a notably enhanced photocatalytic performance in the decomposing of methyl orange and phenol under visible light (λ > 420 nm) irradiation in comparison with BiOBr, I-BiOBr, BiPO₄ and BiOBr/BiPO₄ reference samples. The photocatalytic activity improvement of I-BiOBr/BiPO₄ mainly resulted from the synergetic effects of doped I⁻ ions in I-BiOBr and the heterojunction interface between I-BiOBr and BiPO₄. I⁻ ions doping narrowed the band-gap energy of BiOBr to generate more photocharges under visible light irradiation. Moreover, BiPO₄ quickly trapped and transferred the electrons originating from I-BiOBr. This study affords a facile strategy to intensively improve the visible light photocatalytic activity of the BiOX system for water contaminants removal.

---

1. Introduction

Semiconductor photocatalysis has attracted great interest because it provides a promising pathway for solving energy supply and environmental pollution problems.^1–3 To date, various pure semiconductor materials, including elementary substances,⁴ metal oxides,^5,6 metal sulfides,⁷ metal nitrides⁸ and solid solutions,^9,10 have been employed as photocatalysts. However, weak photocatalytic activities are still the biggest obstacle considering the practical applications and therefore it needs efficient strategy to largely strengthen the separation efficiency of photocharges such as modulating morphology,^11,12 construction of the heterostructure,^13–15 employment of doping ions^16,17 and depositing of metals^18–21.

As for the novel BiOX (X = Cl, Br and I) ternary system, it has been extensively applied in contaminants removal.^22,23 With increasing the atomic number of X⁻, the visible light activity of BiOX increases due to intensively strengthened visible light absorption. Thus, to improve the activity of BiOBr and BiOCl, the primary step is to broaden the visible light absorption range. As the most direct way, inner ion doping^24–31 was successfully employed in a BiOX system through implanting the impurity level. However, the recombination of photocharges will also seriously exist along with the positive effects of ion doping on the activity enhancement of BiOX. On this condition, it is still a big challenge to further improve the photocharge separation efficiency of ion-doped BiOX.

Inspired by the above mentioned methods, a potential way to couple a suitable component for constructing an ion-doped BiOX based heterostructure is possible. Our previous report concerning the couple of I⁻ ions doped BiOBr with BiOI (BiOBr_0.9I_0.1/BiOI)³² displayed excellent activity for organic contaminants degradation. The activity enhancement of BiOBr_0.9I_0.1/BiOI not only resulted from the heterostructure interface, but also the intensive visible light utilization of the BiOI component. The challenging task is whether a wide-band-gap semiconductor can also be used as an appropriate component to couple with ion-doped BiOX. BiPO₄ is a newly founded photocatalyst with excellent UV performance.³³ Enhanced visible light activity was discovered in many BiPO₄-based composites,^34–36 which suggests that BiPO₄ could act as an efficient surface electron trapper, although it cannot be excited under visible light. Therefore, another potential method is to introduce BiPO₄ in the ion-doped BiOX system. To the best of our knowledge, no result has been reported for the fabrication, activity evaluation and electron separation discussion of the ion-doped BiOX/BiPO₄ heterostructure.

In this study, for the first time, we constructed a novel I⁻ ions doped BiOBr/BiPO₄ (I-BiOBr/BiPO₄) heterostructure through loading I-BiOBr nanoparticles on the surface of BiPO₄ nanorods. Methyl orange and phenol were employed as probes to evaluate the activity of I-BiOBr/BiPO₄ under visible light (λ > 420 nm) irradiation. The enhanced activity of I-BiOBr/BiPO₄ was discussed in detail depending on the doped I⁻ ions, as well as the formed I-BiOBr/BiPO₄ interface. The significant findings of this study provide an efficient strategy to largely improve the activity of BiOX via synergistic effects of ion-doping and the electron trapping role of a wide-band-gap semiconductor.

2. Experimental

2.1. Sample preparation

All analytical grade chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and were used without further purification. Deionized water was used throughout this study.

I-BiOBr/BiPO₄ was synthesized by a facile deposition-precipitation method. The process included the following steps: first, the BiPO₄ substrate was prepared via a hydrothermal method. 0.033 mol Bi(NO₃)₃·5H₂O was dissolved in 90.3 mL dilute nitric acid solution (11.0 vol%). Then, 0.033 mol Na₂HPO₄·12H₂O dissolved in 60 mL deionized water was added dropwise to the abovementioned Bi-based solution with stirring. The pH value was adjusted to 3.0 by adding ammonia water. After stirring for 1 h, the resulting precursor suspension was transferred into a 50 mL Teflon-lined stainless steel autoclave and maintained at 180 °C for 24 h. Subsequently, the white BiPO₄ precipitate was filtered, washed with deionized water and dried at 80 °C for 12 h. Second, 0.50 g BiPO₄ was dispersed in a 30 mL acidic Bi(NO₃)₃ solution (pH = 1.96) adjusted by glacial acetic acid. Then, a mixed solution containing 0.032 g NaBr and 0.003 g KI (the molar percentage of I/Br was 5%) was slowly added into the abovementioned solution with stirring. After stirring for 5 h at room temperature, the I-BiOBr/BiPO₄ precipitate was filtered, washed with deionized water and dried at 80 °C for 12 h. The molar percentage of I-BiOBr to BiPO₄ was 10%.

As references, BiOBr/BiPO₄, BiOBr and I-BiOBr were also prepared. BiOBr/BiPO₄ was fabricated like I-BiOBr/BiPO₄ by only adding NaBr solution in the absence of KI. BiOBr was synthesized similar to BiOBr/BiPO₄ without adding the BiPO₄ substrate. I-BiOBr was also obtained like I-BiOBr/BiPO₄ without adding the BiPO₄ substrate.

2.2. Characterization

XRD was performed on a Bruker D8 Advance X-ray diffractometer with a scanning speed of 10° min⁻¹ and Cu Kα radiation (λ = 1.5406 Å). SEM was conducted on an FEI Sirion 200 field emission scanning electron microscope with 5.00 kV scanning voltages. TEM and HRTEM were performed with a JEOL-2011 transmission electron microscope with an accelerating voltage of 200 kV. DRS were recorded on a TU-1901 UV-vis spectrophotometer in the wavelength range of 200 to 800 nm. XPS was carried out on a Thermo Escalab 250 with Al Kα (1486.6 eV) line at 150 W. BET surface area measurements were calculated using a Micromeritics ASAP2020 apparatus at liquid nitrogen temperature (77.3 K). Photocurrent Mott–Schottky curves were measured on a CHI660E electrochemical system in a standard three-electrode experimental system using 0.1 M Na₂HPO₄ and 0.1 M NaH₂PO₄ aqueous solution as supporting electrolyte. Surface photovoltage (SPV) spectroscopy was measured on a TLS-SPV530 spectrometer (Zolix instruments Co., Beijing, China).

2.3. Photocatalytic activity measurements

The photocatalytic activities of the as-prepared samples were evaluated using azo dye MO and non-biodegradable organic phenol as model compounds. The initial concentration of MO (phenol) solution was 10 mg L⁻¹. In experiments, 0.10 g of catalyst was dispersed into 50 mL dye MO (phenol) solution and was then stirred for 30 min in the dark to establish an adsorption–desorption equilibrium. Subsequently, the suspension was exposed to visible light irradiation with a 500 W Xe lamp equipped with a 420 nm cut-off glass filter as the light source. At given time intervals, about 2.7 mL of the suspension was taken out and filtered through a 0.2 μm filter. Finally, the MO solution was analyzed with a 722 s spectrophotometer at 464 nm, whereas the phenol solution was analyzed by a PGeneral TU-1901 UV-vis spectrophotometer (China).

3. Results and discussion

3.1. Characterization

3.1.1. XRD analysis. XRD patterns (Fig. 1) were obtained to investigate the phase structures of the as-prepared samples. It was observed that pure BiPO₄ substrate was monoclinic (JCPDS file no. 15-0767), whereas BiOBr was of tetragonal phase (JCPDS file no. 09-0393). All the diffraction peaks of I-BiOBr coincided with those of pure BiOBr except that (102) and (110) peaks were slightly shifted to lower angles due to the doped I⁻ ions.³¹ As for the I-BiOBr/BiPO₄ heterostructure, both the peaks of I-BiOBr and BiPO₄ coexisted. Owing to the little content, the peak intensity of loaded I-BiOBr was very weak in I-BiOBr/BiPO₄. As a reference, the BiOBr/BiPO₄ composite was also measured for successful preparation. In addition, no impurity peak was found in any of the samples.

Fig. 1 XRD patterns of BiOBr, I-BiOBr, BiOBr/BiPO₄, I-BiOBr/BiPO₄ and BiPO₄.

3.1.2. SEM and EDS analysis. Fig. 2 shows the SEM images of the I-BiOBr/BiPO₄ composite and reference samples. Fig. 2a exhibits that the pure BiOBr sample was of hierarchical microflower morphology gathered by a lot of nanosheets. Through doping I⁻ ions, I-BiOBr changed to a close-gathered microsphere with a diameter about 5 μm (Fig. 2b). However, BiPO₄ (Fig. 2c) substrate was of nanorod structure with diameters of 100–220 nm. As for the BiOBr/BiPO₄ (Fig. 2d) and I-BiOBr/BiPO₄ (Fig. 2e) composites, BiOBr and I-BiOBr nanoparticles were, respectively, adhered on the surface of the BiPO₄ nanorods. This indicates that the BiOBr/BiPO₄ and I-BiOBr/BiPO₄ heterostructures were successfully constructed via the facile deposition-precipitation method.

Fig. 2 SEM images of (a) BiOBr, (b) I-BiOBr, (c) BiPO₄, (d) BiOBr/BiPO₄, (e) I-BiOBr/BiPO₄ and (f) EDS spectra of the various samples.

Furthermore, elemental compositions of BiOBr, I-BiOBr, BiPO₄, BiOBr/BiPO₄ and I-BiOBr/BiPO₄ were examined by EDS analysis (Fig. 2f). It can be observed that BiOBr was only composed of Bi, O and Br elements, whereas BiPO₄ contained Bi, O and P elements. It can be observed that only Bi, O, I and Br elements existed in the I-BiOBr sample. The weak peak intensity of I element suggests its content was low in I-BiOBr. As for BiOBr/BiPO₄, four signals for Bi, O, Br and P elements were well detected. Moreover, the Bi, O, Br, I and P elements were present in I-BiOBr/BiPO₄, demonstrating that the I-BiOBr/BiPO₄ composite was composed of both I-BiOBr and BiPO₄. The peak changes of the I-BiOBr/BiPO₄ composite indicate the interaction between I-BiOBr and BiPO₄.

Moreover, to visually exhibit the elemental composition and dispersion in the I-BiOBr/BiPO₄ sample, EDS mapping was measured and the results are shown in Fig. 3. Clearly, Br, Bi, I, O and P elements were homogeneously distributed in I-BiOBr/BiPO₄. This result indicates that I-BiOBr nanoparticles were uniformly dispersed on the surface of the BiPO₄ substrate, which facilitates the formation of the efficient I-BiOBr/BiPO₄ heterostructure.

Fig. 3 EDS mapping of the I-BiOBr/BiPO₄ sample (a). SEM image of (b) Br, (c) Bi, (d) I, (e) O and (f) P.

The BET surface areas of the BiOBr, I-BiOBr, BiPO₄, BiOBr/BiPO₄ and I-BiOBr/BiPO₄ samples were tested to be 12.71, 9.50, 3.26, 4.48 and 3.31 m² g⁻¹, respectively. In comparison with BiOBr and I-BiOBr, the other three samples had small BET surface areas. This indicates that the BET surface area was not the major influencing factor for the photocatalytic performance of different samples.

3.1.3. TEM and HRTEM analysis. To ensure the formation of a heterojunction interface between I-BiOBr and BiPO₄ in the I-BiOBr/BiPO₄ composite, TEM and HRTEM investigations were carried out. The TEM image (Fig. 4a) reveals that I-BiOBr/BiPO₄ was composed of BiPO₄ nanorods decorating many I-BiOBr nanoparticles, which is consistent with the SEM observation (Fig. 2e). In addition, two types of lattice fringes with spacing of 0.163 and 0.212 nm were distinctly found in the HRTEM image of I-BiOBr/BiPO₄ (Fig. 4b), which can be indexed to the (114) plane of I-BiOBr and (211) plane of BiPO₄, respectively. This suggests that the I-BiOBr/BiPO₄ heterostructure was successfully constructed so that it is expected to efficiently improve the separation of photocharges in the photocatalytic process.

Fig. 4 (a) TEM and (b) HRTEM images of the I-BiOBr/BiPO₄ composite.

3.1.4. XPS analysis. XPS spectra were further characterized to investigate the surface compositions and chemical states of the as-prepared photocatalysts. All the peak positions were calibrated by adventitious C 1s (284.7 eV) as a reference. Fig. 5a shows the XPS survey spectra of BiOBr, I-BiOBr/BiPO₄ and BiPO₄. The Bi, O, P, Br and I elements were clearly displayed in I-BiOBr/BiPO₄. Based on this, the corresponding high-resolution XPS spectra of Bi, O, P, Br and I elements were presented in Fig. 5b–f, respectively. Two strong peaks with binding energies of 164.8 and 159.5 eV (Fig. 5b), corresponding to Bi 4f_5/2 and Bi 4f_7/2, indicate that the chemical state of the Bi element was +3 valence.³⁷ As shown in Fig. 5c, the O 1s spectrum was fitted to two peaks. The one peak located around 530.2 eV belonged to the Bi–O bond originating from the photocatalyst, whereas the other peak with a higher binding energy of 532.0 eV could be assigned to the surface adhered hydroxyl groups on the photocatalyst.³⁷ The spectrum of P 2p (Fig. 5d) was located at 132.8 eV,³⁸ confirming the presence of PO₄³⁻ in the composite. The Br 3d peaks (Fig. 5e) were of binding energy of 70.1 and 69.1 eV, associated with Br 3d_3/2 and Br 3d_5/2, which is the characteristic of Br⁻ ions in the composites.³⁹ Moreover, the I 3d peaks (Fig. 5f) at about 630.7 and 619.2 eV corresponded to I 3d_3/2 and I 3d_5/2, affirming the existence of I⁻ ions in the I-BiOBr/BiPO₄ composite.⁴⁰ It can be noted that the peak characteristics of Bi, O, P, Br elements had a slight difference from those in BiOBr and BiPO₄, indicating that the surrounding chemical conditions of the abovementioned elements have been disturbed owing to the formation of the I-BiOBr/BiPO₄ interface.

Fig. 5 XPS spectra of BiPO₄, BiOBr and I-BiOBr/BiPO₄: (a) survey, (b) Bi 4f, (c) O 1s, (d) P 2p, (e) Br 3d and (f) I 3d.

3.1.5. DRS analysis. Fig. 6a presents the UV-vis diffuse reflectance spectra of the as-prepared samples. Clearly, BiPO₄ had no visible light response with an absorption band edge at about 290 nm. The absorption band edge of I-BiOBr was broadened to 515 nm due to the significant doping effect of I⁻ ions³¹ in comparison with pure BiOBr. The light absorption of BiOBr/BiPO₄ was located between those of BiOBr and BiPO₄ owing to the combined effect of both materials. It can be noted that the existence of I-BiOBr greatly expanded the visible light absorption of BiPO₄ from 290 to 500 nm, i.e., I-BiOBr acted as an efficient sensitizer for BiPO₄ in the I-BiOBr/BiPO₄ system.

Fig. 6 (a) UV-vis diffuse reflectance spectra and (b) corresponding band-gap energy of BiPO₄, BiOBr, I-BiOBr, BiOBr/BiPO₄ and I-BiOBr/BiPO₄.

On the basis of the abovementioned absorption spectra, the band-gap energy of the samples were calculated according to eqn (1):⁴¹

ahν = A(hν − E_g)^n/2 (1)

where a, h, n, A and E_g stand for the absorption coefficient, Plank constant, light frequency, a constant and the band-gap energy, respectively. Among them, n was determined by the type of optical transition of a semiconductor. The n values of BiPO₄ and BiOBr were taken as 4 in view of their indirect transitions.^37,42 As a result, the E_g values of BiPO₄, BiOBr, BiOBr/BiPO₄, I-BiOBr and I-BiOBr/BiPO₄ were estimated to be 4.12, 2.77, 2.85, 2.15 and 2.25 eV (Fig. 6b), respectively.

3.1.6. Photoelectrochemical analysis. In general, the photocatalytic activity of a composite photocatalyst is determined by the separation of photocharges, which is largely dependent on its photocatalytic heterojunction interface role. As a useful tool for detecting the separation efficiency of photocharges, the transient photocurrent response spectra was applied to measure the photoelectrochemical behavior of the I-BiOBr/BiPO₄ composite under visible light (λ > 420 nm) irradiation. Fig. 7 shows positive photocurrent signals with different intensities were displayed, which suggests that different samples had discrete separation efficiency of photocharges under visible light irradiation. Undoubtedly, BiPO₄ exhibited the lowest photocurrent intensity owing to its inability to absorb visible light. BiOBr had a slightly higher photocurrent response than BiPO₄ due to its weak visible light absorption. The BiOBr/BiPO₄ heterojunction exhibited a much higher photocurrent response than the single BiPO₄ and BiOBr because of the BiOBr/BiPO₄ interface effect on photocharge separation. In addition, after doping with I⁻ ions, more visible light was utilized and this resulted in the excellent photocurrent intensity of I-BiOBr. Most important was the coupling of I-BiOBr and BiPO₄, as it led to an intense enhancement of the photocurrent response. This may have resulted from the synergistic effects of doped I⁻ ions and the formed I-BiOBr/BiPO₄ heterojunction interface. In consideration of the relationship between the photocurrent response and photocatalytic activity, it is expected that the I-BiOBr/BiPO₄ system will demonstrate the best activity among all the samples.

Fig. 7 Transient photocurrent responses of different samples in phosphate buffered saline aqueous solution under visible light (λ > 420 nm) irradiation.

Moreover, surface photovoltage (SPV) spectra can also be employed for detecting the transfer direction and illustrating the separation efficiency of photocharges. It is considered a powerful instrument to understand the photocatalytic mechanism of semiconductors.⁴³ The distinct positive SPV response (Fig. 8) suggests that the photoinduced positive holes migrated to the surface while the electrons moved oppositely to the interior of the photocatalysts. In this regard, a very weak SPV signal for BiPO₄ in the UV region indicates that little excess photocharges were generated on account of their fast recombination rate. Pure BiOBr presents a strong SPV intensity in the region of 300–400 nm, originating from the band-to-band transition. The reason is that a great many holes aggregated on the surface of BiOBr and the electrons quickly transferred to the inside of BiOBr. However, except for the similar SPV peak as that of BiOBr, a new SPV peak for I-BiOBr around 430 to 500 nm arose from the sub-band transition of the doped I⁻ ion impurity level. It should be noted that I-BiOBr/BiPO₄ composite demonstrated a further decreased SPV intensity, which implies that the enhanced separation efficiency of photocharges prompted the assembling of electrons on the surface of I-BiOBr/BiPO₄. In such a way, the net amount of positive holes on I-BiOBr/BiPO₄ was ultimately reduced.

Fig. 8 SPV responses of various as-prepared samples.

3.2. Photocatalytic activity

The photocatalytic activities of I-BiOBr/BiPO₄ for MO and phenol degradation are presented in Fig. 9. Corresponding to the transient photocurrent responses, all the samples displayed different photocatalytic performances for MO and phenol removal. For example, the activity of MO degradation decreased as follows (Fig. 9a): I-BiOBr/BiPO₄ > I-BiOBr > BiOBr/BiPO₄ > BiOBr > BiPO₄. The changes in activity mainly depended on the separation efficiency of photocharges under visible light (λ > 420 nm) irradiation. I-BiOBr/BiPO₄ had the best photocharge separation efficiency so it possessed the highest activity for MO elimination. As mentioned above, the doped I⁻ ions and I-BiOBr/BiPO₄ interface synergistically controlled the separation of photocharges, as well as the activity of the I-BiOBr/BiPO₄ composite. In view of the apparent pseudo-first-order constant (k_app) calculated from eqn (2), the activity of I-BiOBr/BiPO₄ was enhanced 8.1 and 2.2 times relative to those of BiOBr/BiPO₄ and I-BiOBr, not to mention those of pure BiOBr and BiPO₄ under the identical conditions (Fig. 9b).where k_app is the pseudo-first-order rate constant (h⁻¹), C₀ is the original concentration of phenol and MO (10 mg L⁻¹), C is the concentration of phenol and MO at reaction time t (mg L⁻¹). The excellent activity of I-BiOBr/BiPO₄ was also well discovered in the degradation process of colourless phenol under visible light (Fig. 9c and d) irradiation.

Fig. 9 Photocatalytic degradation curve of contaminants and corresponding k_app values over different samples under visible light (λ > 420 nm) irradiation for (a) and (b) MO and (c) and (d) phenol.

3.3. Photocatalytic mechanism

3.3.1. Reactive species detection. To clearly conform the major reactive species involved the photocatalytic process, tert-butyl alcohol (TBA),⁴⁴ benzoquinone (BQ)⁴⁵ and ammonium oxalate (AO)⁴⁶ with the same final concentrations (0.1 mmol L⁻¹) were used to judge the reactive hydroxyl radicals (˙OH), superoxide radical (˙O₂⁻) and holes (h⁺), respectively. The k_app values of I-BiOBr/BiPO₄ for MO degradation were shown in Fig. 10 after adding the different scavengers. It shows that the k_app values of MO degradation were significantly reduced to 0.01, 0.24, and 0.49 h⁻¹ in the presence of BQ, AO and TBA, respectively. This result suggests that reactive ˙O₂⁻, h⁺ and ˙OH all determined MO degradation. Comparatively, ˙O₂⁻ and h⁺ played more important roles than ˙OH. The judgment of reactive species provided fundamental understanding for discussing the transfer and separation pathway of photocharges in the photocatalytic process of contaminants removal over I-BiOBr/BiPO₄ under visible light (λ > 420 nm) irradiation.

Fig. 10 k_app values of reactive species in the photodegradation process of MO over I-BiOBr/BiPO₄ under visible light (λ > 420 nm) irradiation.

3.3.2. Energy band structure of I-BiOBr/BiPO₄. For a heterojunction photocatalyst, the matching energy band structure of two components is the premise for acquiring excellent separation efficiency of photocharges. Thus, it was necessary to ascertain the energy band structure of I-BiOBr/BiPO₄ in detail. First, the flat band potentials (V_fb) of BiOBr, I-BiOBr and BiPO₄ were calculated using a Mott–Schottky plot (Fig. 11). The positive slope of the C⁻²–E plot indicated that BiOBr, I-BiOBr and BiPO₄ belonged to the n-type semiconductors category. Utilizing the commonly used extrapolation method, the V_fb values of BiOBr, I-BiOBr and BiPO₄ approximately equalled −0.91, −0.91 and −0.84 V vs. Ag/AgCl at pH 6.8, equivalent to −0.71, −0.71 and −0.64 V vs. NHE at pH 6.8,⁴⁷ respectively. For an n-type semiconductor, the E_CB is very close to the V_fb,⁴⁷ so the E_CB values were regarded as −0.71, −0.71 and −0.64 V for BiOBr, I-BiOBr and BiPO₄, respectively. On the basis of the abovementioned E_CB values of the samples, the VB potentials (E_VB) were further calculated according to eqn (3):

E_CB = E_VB − E_g (3)

Fig. 11 Mott–Schottky plot of the as-prepared samples in phosphate buffered saline aqueous solution (pH = 6.8).

As a result, the corresponding E_VB values were estimated to be 2.01, 1.44 and 3.48 V for the BiOBr, I-BiOBr and BiPO₄ samples, respectively. It was observed that both BiOBr/BiPO₄ and I-BiOBr/BiPO₄ composites had matching type II energy band structures. The most important factor was that the E_CB values of BiOBr (−0.71 V) and I-BiOBr (−0.71 V) were more negative than that of BiPO₄ (−0.64 V), which facilitates separation of photocharges. Furthermore, the more negative E_VB value of I-BiOBr (1.44 V) resulted in more visible light absorption than single BiOBr (2.01 V).

3.3.3. Photocharge separation mechanism. As a whole, the excellent photocatalytic activity of I-BiOBr/BiPO₄ primarily depended on the enhanced separation efficiency of photocharges, as shown in Fig. 12. Under visible light irradiation (λ > 420 nm), only the I-BiOBr component could be activated for generating necessary photocharges because BiPO₄ nanorods had a wide band-gap energy (4.12 eV). Subsequently, the subsequent transfer processes mainly relied on the relative position of the VB and CB of I-BiOBr and BiPO₄. The electrons located on the CB of I-BiOBr quickly moved to the empty CB of BiPO₄ and then formed reactive species ˙O₂⁻ (E^o(O₂/˙O₂⁻) = −0.33 V/NHE)^48,49 that degraded MO and phenol efficiently. Moreover, partial ˙O₂⁻ could be transformed to ˙OH that also degraded MO and phenol. Nevertheless, the remaining holes on the VB of I-BiOBr could not transfer to the VB of BiPO₄ due to the negative potential but gathered on the surface of I-BiOBr. Because the VB potential (1.44 V) was not enough to oxidize H₂O to ˙OH (E^o(˙OH, H⁺/H₂O) = 2.72 V/NHE),⁵⁰ the holes oxidized the contaminants directly.

Fig. 12 Schematic of activity enhancement of I-BiOBr/BiPO₄ under visible light (λ > 420 nm) irradiation.

Comparatively speaking, I-BiOBr/BiPO₄ exhibited a much higher activity than pure BiOBr, BiPO₄, ion-doped I-BiOBr and BiOBr/BiPO₄ heterojunctions, i.e., I-BiOBr/BiPO₄ was an outstanding assembly integrated with doped-ions and heterojunction interfaces. The doped I⁻ ions prompted I-BiOBr to absorb more visible light and generated more photocharges than pure BiOBr. Simultaneously, the I-BiOBr/BiPO₄ heterojunction interface quickly separated the photocharges via the electron trapper role of wide-band-gap BiPO₄. In such a way, I-BiOBr/BiPO₄ displayed superior photocatalytic activity for contaminants removal under visible light irradiation. This study provided a potential way to largely boost the activity of the BiOX system via integrating inner ion doping and modifying the surface heterostructure.

4. Conclusions

A novel I-BiOBr/BiPO₄ heterojunction was fabricated through loading a little amount of I-BiOBr on a monoclinic BiPO₄ nanorod. The largely enhanced photocatalytic activity ensured that I-BiOBr/BiPO₄ was an excellent visible light induced composite photocatalyst for methyl orange and phenol degradation. The wide-band-gap BiPO₄ acted as a highly efficient electron capturer to quickly transfer electrons originating from the I-BiOBr component after good visible light absorption. We supplied a simple and useful strategy to intensively enhance the photocatalytic activity of BiOX through inner I⁻ ions doping and surface modification with wide-band-gap BiPO₄.

Acknowledgements

This study was financially supported by the Natural Science Foundation of China (51472005, 51272081), the Natural Science Foundation of Educational Committee of Anhui Province (KJ2014A221, KJ2015A027, gxyqZD2016413, gxyqZD2016414) and State Key Laboratory of Structural Chemistry (20160014).

References

1. K. Honda and A. Fujishima, Nature, 1972, 238, 37–38.
2. M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemann, Chem. Rev., 1995, 95, 69–96.
3. N. Serpone and R. F. Khairutdinov, Stud. Surf. Sci. Catal., 1997, 103, 417–444.
4. G. Liu, P. Niu, L. C. Yin and H. M. Cheng, J. Am. Chem. Soc., 2012, 134, 9070–9073.
5. V. Kandavelu, H. Kastien and K. R. Thampi, Appl. Catal., B, 2004, 48, 101–111.
6. A. Mclaren, T. Valdes-Solis, G. Q. Li and S. C. Tsang, J. Am. Chem. Soc., 2009, 131, 12540–12541.
7. Y. C. Weng, Y. D. Chou, C. J. Chang, C. C. Chan, K. Y. Chen and Y. F. Su, Electrochim. Acta, 2014, 125, 354–361.
8. P. Niu, L. L. Zhang, G. Liu and H. M. Cheng, Adv. Funct. Mater., 2012, 22, 4763–4770.
9. J. Gaudet, A. C. Tavares, S. Trasatti and D. Guay, Chem. Mater., 2005, 17, 1570–1579.
10. S. W. Tao and J. T. S. Irvine, J. Solid State Chem., 2002, 165, 12–18.
11. D. H. Wang, G. Q. Gao, Y. W. Zhang, L. S. Zhou, A. W. Xu and W. Chen, Nanoscale, 2012, 4, 7780–7785.
12. H. Q. He, J. Yin, Y. X. Li, Y. Zhang, H. S. Qiu, J. B. Xu, T. Xu and C. Y. Wang, Appl. Catal., B, 2014, 156–157, 35–43.
13. W. J. Zhou, H. Liu, J. Y. Wang, D. Liu, G. J. Du, S. J. Han, J. J. Lin and R. J. Wang, Phys. Chem. Chem. Phys., 2010, 12, 15119–15123.
14. M. Y. Wang, L. Sun, Z. Q. Lin, J. H. Cai, K. P. Xie and C. J. Lin, Energy Environ. Sci., 2013, 6, 1211–1220.
15. F. F. Duo, Y. W. Wang, X. M. Mao, X. C. Zhang, Y. F. Wang and C. M. Fan, Appl. Surf. Sci., 2015, 340, 35–42.
16. W. K. Ho, J. C. Yu and S. C. Lee, Chem. Commun., 2006, 10, 1115–1117.
17. H. Gnayem and Y. Sasson, ACS Catal., 2013, 3, 186–191.
18. H. X. Lin, L. Y. Ding, Z. X. Pei, Y. G. Zhou, J. L. Long, W. H. Deng and X. X. Wang, Appl. Catal., B, 2014, 160–161, 98–105.
19. Y. P. Gao, L. B. Wang, Z. Y. Li, C. Z. Li, X. X. Cao, A. G. Zhou and Q. K. Hu, Mater. Lett., 2014, 136, 295–297.
20. Z. K. Zheng, B. B. Huang, X. Y. Qing, X. Y. Zhang, Y. Dai and M. H. Whangbo, J. Mater. Chem., 2011, 21, 9079–9087.
21. C. L. Ren, B. F. Yang, M. Wu, J. Xu, Z. P. Fu, Y. lv, T. Guo, Y. X. Zhao and C. Q. Zhu, J. Hazard. Mater., 2012, 182, 123–129.
22. X. Zhang, A. Z. Ai, F. L. Jia and L. Z. Zhang, J. Phys. Chem. C, 2008, 112, 747–753.
23. M. Shang, W. Z. Wang and L. Zhang, J. Hazard. Mater., 2009, 167, 803–809.
24. Z. S. Liu, B. T. Wu, Y. L. Zhao, J. N. Niu and Y. B. Zhu, Ceram. Int., 2014, 40, 5597–5603.
25. G. H. Jiang, X. H. Wang, Z. Wei, X. Li, X. G. Xi, R. B. Hu, B. L. Tang, R. J. Wang, S. Wang, T. Wang and W. X. Chen, J. Mater. Chem. A, 2013, 1, 2406–2410.
26. R. J. Wang, G. H. Jiang, X. H. Wang, R. B. Hu, X. G. Xi, S. Y. Bao, Y. Zhou, T. Tong, S. Wang, T. Wang and W. X. Chen, Powder Technol., 2012, 228, 258–263.
27. Z. Y. Zhao and W. W. Dai, Inorg. Chem., 2014, 53, 13001–13011.
28. M. Q. He, W. B. Li, J. X. Xia, L. Xu, J. Di, H. Xu, S. Yin, H. M. Li and M. N. Li, Appl. Surf. Sci., 2015, 331, 170–178.
29. G. H. Jiang, X. Li, Z. Wei, X. H. Wang, T. T. Jiang, X. X. Du and W. X. Chen, Powder Technol., 2014, 261, 170–175.
30. X. Li, G. H. Jiang, Z. Wei, X. H. Wang, W. X. Chen and L. Shen, MRS Commun., 2013, 3, 219–224.
31. H. L. Lin, X. Li, J. Cao, S. F. Chen and Y. Chen, Catal. Commun., 2014, 49, 87–91.
32. X. M. Jia, J. Cao, H. L. Lin, Y. Chen, W. F. Fu and S. F. Chen, J. Mol. Catal. A: Chem., 2015, 409, 94–101.
33. C. S. Pan and Y. F. Zhu, J. Mater. Chem., 2011, 21, 4235–4241.
34. H. F. Ye, H. L. Lin, J. Cao, S. F. Chen and Y. Chen, J. Mol. Catal. A: Chem., 2015, 397, 85–92.
35. T. Lv, L. K. Pan, X. J. Liu and Z. Sun, RSC Adv., 2012, 2, 12706–12709.
36. X. Lin, D. Liu, X. Y. Guo, N. Sun, S. Zhao, L. M. Chang, H. J. Zhai and Q. W. Wang, J. Phys. Chem. Solids, 2015, 76, 170–177.
37. Y. N. Huo, J. Zhang, M. Miao and Y. Jin, Appl. Catal., B, 2012, 111–112, 334–341.
38. C. S. Pan, J. Xu, Y. Y. Wang, D. Li and Y. F. Zhu, Adv. Funct. Mater., 2012, 22, 1518–1524.
39. H. Liu, Y. Su, Z. Chen, Z. T. Jin and Y. Wang, J. Hazard. Mater., 2014, 266, 75–83.
40. L. F. Zhu, C. He, Y. L. Huang, Z. H. Chen, D. H. Xia, M. H. Su, Y. Xiong, S. Y. Li and D. Shu, Sep. Purif. Technol., 2012, 91, 59–66.
41. S. M. Sun, W. Z. Wang, L. Zhang, L. Zhou, W. Z. Yin and M. Shang, Environ. Sci. Technol., 2009, 43, 2005–2010.
42. J. Cao, B. Y. Xu, H. L. Lin and S. F. Chen, Chem. Eng. J., 2013, 228, 482–488.
43. H. G. Kim, P. H. Borse, W. Y. Choi and J. S. Lee, Angew. Chem., Int. Ed., 2005, 44, 4585–4589.
44. J. Kim, C. W. Lee and W. Choi, Environ. Sci. Technol., 2010, 44, 6849–6854.
45. M. C. Yin, Z. S. Li, J. H. Kou and Z. G. Zou, Environ. Sci. Technol., 2009, 43, 8361–8366.
46. F. T. Li, Q. Wang, X. J. Wang, B. Li, Y. J. Hao, R. H. Liu and D. S. Zhao, Appl. Catal., B, 2014, 150–151, 574–584.
47. S. X. Weng, B. B. Chen, L. Y. Xie, Z. Y. Zheng and P. Liu, J. Mater. Chem. A, 2013, 1, 3068–3075.
48. J. Kim, C. W. Lee and W. Choi, Environ. Sci. Technol., 2010, 44, 6849–6854.
49. D. T. Sawyer and J. S. Valentine, Acc. Chem. Res., 1981, 14, 393–400.
50. G. T. Li, K. H. Wong, X. W. Zhang, C. Hu, J. C. Yu, R. C. Y. Chan and P. K. Wong, Chemosphere, 2009, 76, 1185–1191.

---