Enhancing visible light photocatalytic activity of BiOBr/rod-like BiPO₄ through a heterojunction by a two-step method

Abstract

Visible light driven BiOBr/rod-like BiPO₄ composites with different BiOBr to BiPO₄ molar ratios were fabricated via a facile deposition-precipitation method. X-ray powder diffraction, field emission scanning electron microscopy, transmission electron microscopy, UV-Vis diffuse reflectance spectroscopy and Brunauer–Emmett–Teller surface area were used to characterize the as-synthesized samples. Results showed that the BiOBr/rod-like BiPO₄ composite with equal mole ratio exhibited the best photocatalytic performance for rhodamine B (RhB) degradation under visible light irradiation. The enhanced photocatalytic performance could be mainly attributed to the effective separation of the photogenerated electrons and holes at the heterojunction interface of p-BiOBr and n-BiPO₄.

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Introduction

The photocatalytic splitting of water on TiO₂ electrodes demonstrated by Fujishima and Honda heralded a new era for heterogeneous photocatalysis.¹ After that, TiO₂-based photocatalysts have been widely investigated for environmental remediation and alternative clean energy supplies for the sustainable development of the human society. In general, a semiconductor photocatalytic cycle comprises three critical steps: firstly, illumination induces a transition of electrons from the valance band (VB) to the conduction band (CB) of the semiconductor, leaving an equal number of holes and electrons; secondly, the excited electrons and holes migrate to the surface; thirdly, they react with absorbed electron donors and electron acceptors, respectively.² However, the high electron and hole recombination rate hinders the charge transfer to the surface of the semiconductor in the second step, decreasing the efficiency of the photocatalyst. Research has demonstrated that improving the generation and separation of photoinduced electron–hole pairs by semiconductor heterostructures can effectively enhance the activity of a photocatalyst.^3–5

Recently, BiPO₄ with a novel nonmetal oxy-acid salt structure has been revealed as an excellent photocatalyst. The photocatalytic activity of BiPO₄ is twice that of TiO₂ (P25, Degussa) for the degradation of organic dye under UV light.⁶ It is well known that the energy of UV light accounts for only smaller part (5%) of the total solar energy than that of visible light (45%).⁷ Nevertheless, the large band gap energy of BiPO₄ (3.85 eV) prevented it from utilizing the visible light to meet the requirement of practical application.⁶ For achieving the desirable visible light photocatalytic activity of BiPO₄, many semiconductors such as CdS,⁸ AgBr,⁹ BiVO₄,¹⁰ BiOI,^11,12 AgI,¹³ Ag₃PO₄,¹⁴ g-C₃N₄,^15–17 have been coupled with BiPO₄ to construct the heterojunctions to improve visible light response of BiPO₄.

Bismuth oxy halides (BiOX, X = Cl, Br, I) belong to the family of main group V–VI–VII multicomponent metal oxyhalides. They have been widely applied in the fields of catalysis, ionic conductors, photochromic devices, ferroelectric materials, pigments and solar cells due to the good optical, electrical and magnetic properties.^18,19 Among these, BiOBr is of great research interest since it is an active and stable visible light photocatalyst.²⁰ However, the photocatalytic activity of BiOBr has been limited by the high recombination of electron and hole.²¹ Constructing p–n junction in composites such as g-C₃N₄/BiOBr,²² TiO₂/BiOBr²³ and ZnFe₂O₄/BiOBr²⁴ further facilitated the separation of electron and hole and enhanced their photocatalytic activity.

Very recently, small amount (5–20 mol%) of nanosized BiPO₄ particle (50 nm) has been loaded in the hierarchical microsphere of BiOBr (1–2 μm) by a one-pot solvothermal method.²⁵ Results showed that the enhanced separation of the photoinduced electron and hole drastically improve the degradation efficiency of RhB. HRTEM revealed that the BiPO₄ nanoparticle is aggregation of quantum dot (about 5 nm). Nanoparticles of small size tend to agglomerate and could induce recombination of electron–hole pair that limited the photocatalytic activity.²⁶ In fact, monoclinic BiPO₄ synthesized by hydrothermal method often forms the rod-like morphology.²⁷ It is reported that graphene/rod-shaped TiO₂ showed higher photocatalytic activity than graphene/spherical TiO₂ since the photogenerated electrons could be delocalized effectively and free to move throughout the length of the crystal, which reduces the recombination of the electrons and holes.²⁸

For keeping the rod-like morphology of BiPO₄, BiPO₄ has been prepared in advance in this work. After that, composites between BiOBr and rod-like BiPO₄ have been prepared by a deposition-precipitation method for the first time. The photocatalytic activity has been systematically investigated by degradation of RhB under the irradiation of visible light.

Experimental

Synthesis

All the reagents were of analytical purity and were used as received. BiPO₄ was synthesized via a hydrothermal method in advance. 2.91 g of Bi(NO₃)₃·5H₂O was dissolved in 55 mL deionized water and 2.28 g Na₃PO₄·12H₂O and 0.2 g sodium dodecylsulphate was added. The pH of the mixed suspensions was adjusted to 1 with HNO₃ and was stirring for 2 h. The mixed suspensions were transferred into a Teflon-lined stainless steel autoclave with a volume of 100 mL for the hydrothermal reaction and sustained at 180 °C for 24 h. Then the reactor was cooled to room temperature naturally. The precipitation was filtered out and washed with distilled water and absolute ethanol several times and dried at 80 °C for 24 h.

BiOBr/rod-like BiPO₄ photocatalyst was fabricated via a facile deposition-precipitation method. Bi(NO₃)₃·5H₂O and cetyltriethylammonium bromide (CTAB) was dispersed in ethylene glycol, respectively. Subsequently, BiPO₄ was added into Bi(NO₃)₃·5H₂O suspension with constant stirring for 1 h. Then, CTAB suspension was slowly added into above the mixed solution, the reaction was continued stirring at 120 °C in oil bath for 8 h. Products were separated by centrifugation, washed with deionized water and alcohol for several times, and then dried at 80 °C for 24 h. According to this method, different mole ratios of BiOBr/BiPO₄ samples at 1 : 2, 1 : 1, and 3 : 1 were obtained and denoted as 33.3% BiOBr/BiPO₄, 50% BiOBr/BiPO₄, and 75% BiOBr/BiPO₄, respectively. In addition, BiOBr was prepared by the simple solvothermal method under the same conditions for comparison.

Characterization

Crystallinity and phase composition of samples were identified by X-ray powder diffraction (XRD) with Cu Kα radiation (D8 Advance, Bruker, Germany), using a voltage of 40 kV, a current of 40 mA and scanning rate of 3° min⁻¹, in 2θ ranges from 10 to 60°. The morphology of as-prepared powder has been observed by field emission scanning electron microscopy (FESEM, S-4800, Hitachi, Japan), transmission electron microscope (TEM) and high resolution transmission electron microscopy (HRTEM) (TEI, Tecnai, F20). The chemical composition of the samples was determined by X-ray energy dispersion spectrum (EDS) on Oxford instruments INCA X-ray energy-dispersive spectroscopy. The specific surface area of samples were evaluated through Brunauer–Emmett–Teller (BET) (ASAP 2010, Micromeritics, USA). The band gap was measured by the UV-Vis diffused reflectance spectra (UV 3150, Shimadzu, Japan). The photoluminescence (PL) spectra were measured by fluorescence spectrophotometer (Cary Eclipse, Varian, USA).

Photocatalysis measurement

The photocatalytic activity of the as-prepared catalysts were evaluated by degradation of 10 mg L⁻¹ RhB solution under visible light (500 W xenon lamp, λ ≥ 420 nm). Prior to illumination, 0.3 g of samples were dispersed in 300 mL RhB solution and magnetically stirred in darkness for 30 min to achieve adsorption/desorption equilibrium. During the visible light irradiation, approximately 5 mL suspension was taken at 30 min intervals. Then filtered through 0.45 μm Millipore filter and analysed using a UV-Vis spectrometer (UVT6, Purkinje General, China) at the absorption wavelength of 554 nm.

Results and discussion

The crystal structure and phase composition of samples were examined by the powder X-ray diffraction (XRD). Fig. 1 shows that the XRD patterns of BiPO₄, BiOBr and the BiOBr/BiPO₄ composites with different molar ratios. The diffraction peaks of pure BiPO₄ are in good accordance with the standard card of monoclinic phase of BiPO₄ (JCPDS Card no. 80-0209). It confirms the purity of the as-prepared BiPO₄. Compared with pure BiPO₄, the characteristic peaks of BiOBr (JCPDS Card no. 09-0393) appears and their relative intensity increases gradually with increasing the BiOBr contents in BiOBr/BiPO₄ composite. These results reveal the coexistence of both BiOBr and BiPO₄ phases in the BiOBr/BiPO₄ composites.

Fig. 1 XRD patterns of as-prepared BiOBr, BiPO₄ and BiOBr/BiPO₄ composite samples.

Fig. 2 shows morphologies of BiPO₄, BiOBr and BiOBr/BiPO₄ composites observed by FESEM. As shown in Fig. 2a, the pure BiPO₄ is composed of rod-like crystals with diameters of 130–560 nm, lengths of 0.4–3.5 μm, which is consistent with previous reports.^29,30 By contrast, pure BiOBr in Fig. 2e consists of hierarchical microsphere. Close inspection reveal that the microspheres compose of massive BiOBr nanosheets (30 nm in thickness). Similar morphology has been observed in literature.³¹ When small amount of BiOBr (33.3 mol%) is introduced, several nanoplates have been attached on the surface of BiPO₄ rod (Fig. 2b). The contact area between BiPO₄ crystal with BiOBr nanoplates increase with increasing of BiOBr in composite (50% BiOBr/BiPO₄ Fig. 2c). However, when the content of BiOBr is increased to 75%, some microspheres with average diameters about 2–4 μm appear. On the other hand, many BiOBr microspheres have been penetrated by the rod of BiPO₄. It indicates the closely contact of BiOBr and BiPO₄ particles. EDS and elemental mapping analysis (Fig. S1†) of 50% BiOBr/BiPO₄ (Fig. 2e) results imply coexistence of O, Br, P and Bi elements. Moreover, the atomic percentage ratio of Br/P is 48.5%, which is close to the theoretical value 50% within experimental error. It further confirms formation of BiOBr on surface of BiPO₄. Compared with the BiOBr/nanoparticle BiPO₄ (50 nm), effectively delocalized electrons of rod-like BiPO₄ could further suppress the recombination of photoinduced electron and hole.²⁸ On the other hand, the larger size of the BiPO₄ rod (Fig. 2a) benefits the separation and reuse of as-prepared photocatalyst.

Fig. 2 FESEM images of samples: (a) BiPO₄, (b) 33.3% BiOBr/BiPO₄, (c) 50% BiOBr/BiPO₄, (d) 75% BiOBr/BiPO₄, (e) BiOBr.

The 50% BiOBr/BiPO₄ heterostructure is further characterized by TEM and HRTEM. Fig. 3a shows TEM image of the 50% BiOBr/BiPO₄ sample. It can be seen that there are many small plate attaching on the large rod. Comparing with FESEM results in Fig. 2, rod shape crystal and small sheets are determined as BiPO₄ and BiOBr, respectively. Fig. 3b presents the HRTEM image of interface between two phases in 50% BiOBr/BiPO₄ composite. The interfaces between BiPO₄ and BiOBr are clearly visible. Clear fringe with an interval of 0.410 nm could be indexed to (−111) lattice plane of monoclinic phase of BiPO₄ and 0.159 nm is in accordance with (212) lattice plane of tetragonal BiOBr. In addition, the characteristic layer structure of BiOBr sheets has also been observed. These results not only verify the coexistence but also demonstrate the formation of the heterojunction between BiOBr and BiPO₄.

Fig. 3 (a) TEM and (b) HRTEM images of 50% BiOBr/BiPO₄.

The specific surface area and porosity of the as-prepared samples were investigated by nitrogen adsorption and desorption isotherms, as shown in Table 1. It can be seen that BiOBr has higher BET surface areas about 17.06 m² g⁻¹ and larger pore volume about 0.083 cm³ g⁻¹, which could be well explained by the agglomerated structure of nanosized plates (Fig. 2e inset). Compared with BiOBr, the BET surface areas and pore volumes of the pure BiPO₄ nanorods are smaller (3.648 m² g⁻¹ and 0.0087 cm³ g⁻¹), due to the dense and smooth morphology (Fig. 2a inset). During in situ growth of BiOBr in the present of BiPO₄, the BET surface areas and pore volumes of BiOBr/BiPO₄ samples firstly slightly increase, reaching the maximum at 50% BiOBr/BiPO₄, and then decrease with further increasing BiOBr content. However, the BET surface areas of the BiOBr/BiPO₄ composites as well as pore volumes do not show an obvious difference, which implies that the BET surface area is not the main influencing factor for the photocatalytic performance of different BiOBr/BiPO₄ composites. This phenomenon is consistent with other reports.^32,33

Table 1 BET surface areas, pore volumes and band gap energies of different samples

Sample	BET surface areas (m^2 g^−1)	Pore volumes (cm^3 g^−1)	Band gap energies (eV)
BiPO4	3.648	0.0087	4.11
33.3% BiOBr/BiPO4	8.150	0.059	2.94
50% BiOBr/BiPO4	9.612	0.071	2.56
75% BiOBr/BiPO4	8.168	0.058	2.48
BiOBr	17.057	0.083	2.63

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The UV-Vis diffuse reflectance spectra (DRS) of the as-prepared samples are displayed in Fig. 4a. BiPO₄ could merely absorb the UV light with wavelength less than 300 nm, which is in agreement with the results previous reported.³⁴ After the deposition of BiOBr, the absorption of BiOBr/BiPO₄ is obviously extended to the visible light range (λ ≥ 420 nm) and the relative absorption intensity is enhanced in the wavelength range of 300–400 nm, which indicates that BiOBr is a good visible light sensitizer to BiPO₄.²⁵ The band gap energies (E_g) of samples can be calculated by the following formula:³⁵

αhν = A(hν − E_g)^n/2 (1)

where α, h, and ν, E_g and A are absorption coefficient, Planck constant, light frequency, band gap energy, and a constant, respectively. Among them, n depends on the type of optical transition of a semiconductor (n = 1 for direct transition and n = 4 for indirect transition). As reported previously, the n values of BiPO₄ and BiOBr were both 4.^34,36 According to eqn (1), the band-gap energy (E_g) of sample can be estimated from a plot of (αhν)^1/2 vs. energy (hν) in Fig. 4b. Thus, the band gaps of BiPO₄ and BiOBr are estimated to be 4.11 and 2.63 eV, respectively, which are very close to the results previously reported.^34,36 As seen in Table 1 E_g of BiOBr/rod-like BiPO₄ heterostructures are estimated to be 2.94, 2.56 and 2.48 eV for 33.3%, 50% and 75% BiOBr/BiPO₄, respectively. This notable variation of the absorption edge could be well explained by the large interval of composition employed in this work. This result is obviously different with previous report.²⁵

Fig. 4 (a) UV-Vis diffuse reflectance spectra and (b) plot of the (αhν)^1/2 vs. photon energy (hν) of as-prepared samples.

The photocatalytic activity of different catalysts is measured by the decomposition of pollutant RhB in aqueous solution under visible light irradiation (λ ≥ 420 nm), as shown in Fig. 5a. It can be seen that in absence of any photocatalyst, RhB self-degradation is almost negligible. The degradation efficiency of RhB by pure BiPO₄ is only about 2.34% under visible light for 120 min, which may be ascribed to the weak dye photosensitization role of RhB. As for BiOBr, it degrades 93.7% of RhB at the same irradiation. Significantly, the 50% BiOBr/BiPO₄ heterojunction degrades 97.5% RhB within 90 min, demonstrating the higher photocatalytic activity than either individual BiPO₄ or BiOBr. Composite with other molar ratios also show notable visible photocatalytic activity (Fig. S2†). The photocatalytic activity of composite first sharply increases, and then slightly decreases. The rate constant of RhB degradation process were calculated according to the following apparent pseudo-first-order kinetics equation:³⁷

ln C₀/C = k_appt (2)

where k_app is the apparent first-order rate constant (min⁻¹). The determined k_app for as-synthesized photocatalyst are calculated (Fig. 5b). It was indicated that the photocatalytic reactivity order is 50% BiOBr/BiPO₄ > 75% BiOBr/BiPO₄ > BiOBr > 33.3% BiOBr/BiPO₄ > BiPO₄, which is consistent with the variation trend of RhB degradation. Meanwhile, BiPO₄, BiOBr and 50% BiOBr/BiPO₄ also show excellent photocatalytic activity under UV light irradiation (Fig. S3†).

Fig. 5 (a) Photocatalytic activity of samples for RhB degradation, (b) effect of different BiOBr/BiPO₄ molar ratio on the rate of RhB degradation rate under visible light irradiation.

Photoluminescence (PL) spectrum has been carried out to examine the separation efficiency of the photogenerated electron–hole pairs in a semiconductor. Fig. S4† presents the PL spectra of pure BiPO₄, BiOBr and 50% BiOBr/BiPO₄ with an excitation wavelength of 250 nm. Emission intensity of main PL peak (around 490 nm) of 50% BiOBr/BiPO₄ is much weaker than those of BiPO₄ and BiOBr samples, which indicates that the recombination of photogenerated charge carriers is retrained. This result is similar with other reports.^38,39 It further confirms that the higher photocatalytic activity than those of pure BiPO₄ and BiOBr is the result of enhanced separation of the photoinduced electron and hole.

In addition, the stability and photocatalytic efficiency of photocatalysts are also very significant for practical application. To demonstrate the stability and reusability of catalysts, the catalytic activity of 50% BiOBr/BiPO₄ composite photocatalyst has been performed forth runs under the same conditions. Results in Fig. 6 reveal great stability and reusability of 50% BiOBr/BiPO₄ composite for RhB degradation.

Fig. 6 Cycling runs photocatalytic degradation of RhB by 50% BiOBr/BiPO₄ under light irradiation.

It is well known that the larger specific surface areas can absorb more active species and reactants on its surface, which seems to benefit the photodegradation of dyestuff in the solution. However, 50% BiOBr/BiPO₄ exhibits the higher degradation rate than BiOBr with the largest BET surface area (Table 1). It can be concluded that BET is not the main influence factor for the photocatalytic activity of the BiOBr/BiPO₄ composites.²⁵ On the other hand, photocatalytic activity is closely related with the generation, separation, migration efficiency of photogenerated carriers.⁴⁰ When n-BiPO₄ is coupled with p-BiOX (X = Br, I), the Fermi level of BiOBr will shift upward to reach new balance with that of BiPO₄, resulting the formation of the p–n junction. The internal electric field will drive the electron on the CB of BiOBr to that of BiPO₄, while keep the hole stay on the VB of BiOBr.^25,41 The schematic diagram is shown in Fig. 7. This process suppresses the electron–hole recombination of BiOBr and increases the probability of hole to surface to oxidize the RhB. Naturally, the separation efficiency is greatly affected by the contacting area between BiOBr and rod-like BiPO₄ (density of p–n junctions). When 33.3% BiOBr is loaded, the low concentration of BiOBr make highly dispersed nanosheets of BiOBr randomly distribute on the surface of BiPO₄ rods. The intimate contact effectively forms the p–n junction. As the content of BiOBr increases to 50%, more nanosheets evenly attached on the surface of BiPO₄ cause the more effective contact area and further increase degradation rate of RhB. However, there are so many of BiOBr that most of them have agglomerated to form the hierarchy microsphere along the BiPO₄ rod (Fig. 2d) as the content of BiOBr approaches 75%. The self-assembly of BiOBr nanosheets cannot effectively increase the contact area with BiPO₄ and from more p–n junctions but agglomerated as BiOBr particle with high electron and hole recombination rate. Therefore, the degradation rate of RhB is decreased. The most effective formation of p–n junctions in the composition with 50% BiOBr makes it show the highest photoactivity.

Fig. 7 Schematic diagram: (a) energy band of BiOBr and BiPO₄, (b) the formation of p–n junction and the possible charge separation process.

Conclusions

The BiOBr/rod-like BiPO₄ heterojunction is prepared by a two-step method. Results show that formation of heterojunction enhances the degradation of RhB under visible light irradiation due to the effective separation of photogenerated electrons and holes. The distribution state of BiOBr nanoplates on BiPO₄ rods greatly affects the photoactivity of composite catalyst. The relatively high content and even distribution of BiOBr nanoplates make 50% BiOBr/BiPO₄ exhibit much higher degradation rate of RhB than the pure BiOBr and BiPO₄.

Acknowledgements

This study was supported by the Program for the National Natural Science Foundation of China (Grant No. NSFC 51102130), the Program for Key Laboratory of Inorganic Function Material and Device, Chinese Academy of Sciences (Grant No. KLIFMD-2011-01) and the Program for Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Chinese Academy of Sciences (Grant No. PCOM 201518).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra08421k

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