Synthesis and photocatalytic activity of g-C₃N₄/BiOI/BiOBr ternary composites

Abstract

A novel ternary composite photocatalyst (g-C₃N₄/BiOI/BiOBr) was prepared via a facile solvothermal method. The samples were characterized by powder X-ray diffraction, transmission electron microscopy, UV-visible diffuse reflection spectrometry, X-ray photoelectron spectrometry and photoluminescence measurements. Under irradiation with visible light, the g-C₃N₄/BiOI/BiOBr photocatalyst showed a higher photocatalytic activity than pure g-C₃N₄ and BiOI/BiOBr for the degradation of methylene blue. Among the hybrid photocatalysts, 3% g-C₃N₄/BiOI/BiOBr showed the highest photocatalytic activity for the degradation of MB. These results suggest that the heterostructure combination of g-C₃N₄, BiOI and BiOBr provides a synergistic effect through an efficient charge transfer process.

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1. Introduction

Contamination by organic pollutants has become a major environmental concern with industrial development and increases in the human population.¹ Photocatalytic techniques are often used to eliminate hazardous pollutants from the atmosphere and the aqueous environment.² TiO₂ has attracted much attention as a photocatalyst for the decomposition of organic pollutants as a result of its high oxidative power, photostability and non-toxicity.³ However, because of its wide band gap (3.20 eV) and rapid recombination of photoinduced electrons–holes, TiO₂ has a low efficiency for the utilization of solar energy, which limits its applications. Therefore there is a need to develop new photocatalysts with high activity under irradiation with visible light.⁴

Polymeric graphitic carbon nitride (g-C₃N₄) is a novel metal-free visible light induced semiconductor with a narrow band gap of 2.7 eV.⁵ It has been used in many applications, such as environmental purification,⁶ H₂ production⁷ and CO₂ reduction.⁸ However, its low quantum efficiency and fast charge recombination limits its applications.⁹ As composite photocatalysts have been shown to have a better photocatalytic performance than individual photocatalysts,^10,11 g-C₃N₄ has been used to make composite photocatalysts with narrow or wide band gap semiconductors, such as ZnO,¹² Co₃O₄,¹³ WO₃,¹⁴ In₂O₃,¹⁵ AgX (X = Br, I),¹⁶ Ag₃PO₄,¹⁷ SmVO₄,¹⁸ CdS,¹⁹ Pt/ZnO,²⁰ Au/Pt,²¹ Ag₃VO₄,²² BiOCl,²³ BiOI²⁴ and Ag₂O.²⁵ Some ternary photocatalysts have also been synthesized, such as TiO₂–In₂O₃@g-C₃N₄ (ref. 26) and g-C₃N₄/Bi₂O₃/TiO₂.²⁷ By combining semiconductors with g-C₃N₄, the ternary composites obtained, such as TiO₂–In₂O₃@g-C₃N₄, showed a much higher photocatalytic performance than individual photocatalysts for the degradation of Rhodamine B under irradiation with visible light and in the production of hydrogen. The g-C₃N₄/Bi₂O₃/TiO₂ composite showed enhanced activity under visible light irradiation and improved photoelectrochemical activity.

The V–VI–VII ternary bismuth compounds (BiOX; X = Cl, Br, I) have attracted considerable attention as a result of their excellent electrical and optical properties resulting from the internal static electric fields between the [Bi₂O₂]²⁺ slabs and interleaved halogen ion layers.²⁸ Coupling BiOX with other types of semiconductors to construct heterostructures is considered to be an effective strategy to improve their photocatalytic reactivity.²⁹ BiOX heterojunctions, including BiOBr/Bi₂O₃,³⁰ BiOBr/BiOI,³¹ BiOCl/Bi₂S₃ (ref. 32) and g-C₃N₄/BiOCl_xBr_1−x (ref. 33) have been reported. The coupling of BiOX with appropriate semiconductors has resulted in enhanced photocatalytic activities because the other semiconductor helps to increase the photocatalytic response of BiOX by providing a suitable band gap, efficient electron transfer and a low recombination rate for the photogenerated charge carriers.

Based on above investigations, binary BiOX composites have been synthesized for the enhanced photocatalytic activity, whereas there have been few reports on ternary composites based on BiOX binary composites. In this work, g-C₃N₄ was introduced to a BiOI/BiOBr binary composite to form the ternary composite g-C₃N₄/BiOI/BiOBr. The photocatalysts were characterized by XRD, SEM, EDS, HRTEM and diffuse reflectance spectrometry. The photocatalytic performance of the photocatalysts in the degradation of methylene blue (MB) was tested under irradiation with visible light. A possible photocatalytic mechanism is proposed based on the relative band positions in the three semiconductors.

2. Experimental section

2.1. Synthesis of photocatalysts

All the starting reagents (analytical-reagent grade purity) were purchased from Sinopharm and used without further purification.

2.1.1. Synthesis of g-C₃N₄. The g-C₃N₄ powder was prepared by heating dicyandiamide powder to 520 °C for 4 h at a heating rate of 20 °C min⁻¹ in a muffle furnace according to a previously reported method.¹⁴ The yellow product was collected and ground into a powder.

2.1.2. Synthesis of BiOI/BiOBr composite. In a typical process, 1 mmol of Bi(NO₃)₃·5H₂O was dissolved in 10 mL of ethylene glycol with magnetic stirring for about 30 min (solution A). Then 0.6 mmol of cetyltriethylammonium bromide (CTAB) and 0.4 mmol of KI were dissolved in 10 mL of water with ultrasonic treatment for 30 min (solution B). Solution A was then added into solution B with magnetic stirring and a yellow precipitate formed immediately (solution C). Solution C was stirred for 10 min and then transferred into a 25 mL Teflon-lined autoclave to 80% of the volume, which was then sealed and heated at 140 °C for 24 h.³⁴ After cooling to room temperature naturally, the products was collected by filtration and washed five times with distilled water before drying at 80 °C for 12 h.

2.1.3. Synthesis of g-C₃N₄/BiOI/BiOBr composites. In a typical synthesis of 1% g-C₃N₄/BiOI/BiOBr composite, 7.7 mg of g-C₃N₄ were added to solution C (total mass of Bi(NO₃)₃·5H₂O + CTAB + KI = 770 mg) and the suspension was stirred for 10 min and then transferred into a 25 mL Teflon-lined autoclave up to 80% of the volume, which was then sealed and heated at 140 °C for 24 h. After cooling to room temperature naturally, the products were collected by filtration and washed five times with distilled water and then dried at 80 °C for 12 h before being used in the photocatalytic reactions and for further characterization.

In suspension C, taking the mass of g-C₃N₄ as m and the total mass of Bi(NO₃)₃·5H₂O + CTAB + KI as M, based on the result of (m/M) × 100% = 1%, the obtained product was named as the 1% g-C₃N₄/BiOI/BiOBr composite. According to this method, different mass ratios of the g-C₃N₄/BiOI/BiOBr composites were synthesized and labeled as 3% g-C₃N₄/BiOI/BiOBr, 5% g-C₃N₄/BiOI/BiOBr and 7% g-C₃N₄/BiOI/BiOBr.

2.2. Characterization

The g-C₃N₄/BiOI/BiOBr nanocomposites were analyzed by XRD using a Bruker D8 diffractometer with Cu Kα radiation (λ = 1.5418 Å) in the 2θ range 10–80°. The morphology and structure of the obtained samples were examined with transmission electron microscopy (TEM; Hitachi H-600-II). Ultraviolet-visible (UV-visible) diffuse reflectance spectra were measured using a UV-visible spectrophotometer (Shimadzu UV-2450) in the range 200–800 nm. BaSO₄ was used as the standard reflectance material. Fourier-transform infrared (FTIR) spectra of the samples were recorded on a Nicolet Avatar-370 spectrometer at room temperature. Photoluminescence (PL) spectra of the catalysts were measured on a QuantaMaster 40 spectrofluorometer (Photon Technology International) with an excitation wavelength of 420 nm. X-ray photoemission spectrometry (XPS) was carried out on a PHI5300 instrument with a monochromatic Mg Kα source.

2.3. Photocurrent measurements

Photocurrent tests and electrochemical impedance spectroscopy (EIS) measurements were conducted using an electrochemical analyzer with a standard three-electrode configuration. A 500 W xenon lamp was used as the photo source. The working electrodes were ITO glass (0.5 × 1.5 cm²) coated with the as-prepared samples (0.1 mg) and the counter electrode was a platinum wire. The reference electrode was a saturated Ag–AgCl electrode and an aqueous solution of 0.1 M phosphate-buffered saline (pH 7.0) was used as the electrolyte. EIS was performed in the dark using a 0.1 M KCl solution containing 5 mM Fe(CN)₆³⁻/Fe(CN)₆⁴⁻.

2.4. Photocatalytic activity

The photocatalytic activities of the as-prepared samples were evaluated by the degradation of MB (20 mg L⁻¹) under visible light irradiation at room temperature. A 75 mg mass of the photocatalyst was suspended in 75 mL of aqueous MB. Before irradiation, the suspension was magnetically stirred in the dark for about 30 min to establish an adsorption–desorption equilibrium between the photocatalysts and the MB dye and the solution was also magnetically stirred during the reaction. Under visible light illumination, 4 mL aqueous sample was withdrawn at certain time intervals and centrifuged at 10 000 rpm before analysis. The MB concentration was determined by spectrometry by recording the intensity of the maximum band at 280 nm in the UV-visible absorption spectrum.

3. Results and discussion

3.1. XRD analysis

Fig. 1 shows the XRD patterns of the g-C₃N₄, BiOI/BiOBr and g-C₃N₄/BiOI/BiOBr composites. Two broad peaks were observed in the XRD pattern of g-C₃N₄. The strongest XRD peak at 27.4° originates from the (002) interlayer reflection of a graphite-like structure and the other pronounced XRD peak indexed as (100) at about 13.1° arises from the in-plane ordering of tris-triazine units.³⁵ For BiOI/BiOBr, the diffraction peaks can be readily indexed to the tetragonal phases of BiOBr (JCPDS file no.73-2061)³⁶ and BiOI (JCPDS file no. 10-0445).³⁷ In the case of the g-C₃N₄/BiOI/BiOBr composites, the diffraction peaks can be indexed to the tetragonal phases of BiOBr and BiOI. However, no typical diffraction peak of g-C₃N₄ appeared in the g-C₃N₄/BiOI/BiOBr composite. This is a result of the low contents of g-C₃N₄ in the g-C₃N₄/BiOI/BiOBr composites and the fact that g-C₃N₄ was fully mixed with BiOI/BiOBr. This result is similar to that reported previously.³⁸ The existence of g-C₃N₄ was confirmed by FTIR spectrometry and XPS.

Fig. 1 XRD patterns of g-C₃N₄, BiOI/BiOBr, 1% g-C₃N₄/BiOI/BiOBr, 3% g-C₃N₄/BiOI/BiOBr, 5% g-C₃N₄/BiOI/BiOBr and 7% g-C₃N₄/BiOI/BiOBr.

3.2. SEM and TEM analyses

The dispersion state and the structure of the g-C₃N₄, BiOI, BiOBr, BiOI/BiOBr, 3% g-C₃N₄/BiOI/BiOBr and 7% g-C₃N₄/BiOI/BiOBr composites were obtained by TEM (Fig. 2). The TEM image of the prepared g-C₃N₄ sample showed a lamellar structure (Fig. 2A), which is in agreement with previous work.³⁹ TEM images of the prepared BiOI and BiOBr samples are shown in Fig. 2B and C. BiOI formed rounded sheets with a size of about 50–150 nm and BiOBr formed quadrate sheets of about 200–500 nm. For BiOI/BiOBr, it could be clearly seen that the quadrate BiOBr substrates overlapped with rounded thin pieces of the BiOI nanosheets (Fig. 2D), which confirmed the presence of BiOI/BiOBr.²⁹ In the TEM images of the 3% g-C₃N₄/BiOI/BiOBr (Fig. 2E) and 7% g-C₃N₄/BiOI/BiOBr (Fig. 2F) composites, the composites showed a much darker color than BiOI/BiOBr, which is a result of the introduction of g-C₃N₄. Although having undergone ultrasonic treatment before the TEM observations, 7% g-C₃N₄/BiOI/BiOBr showed a firm connection between g-C₃N₄, BiOI and BiOBr (Fig. 2F), in which it can be clearly seen that curved g-C₃N₄ was attached to the surface of the composite, which is favorable for the formation of heterojunctions. The morphologies of the 3% g-C₃N₄/BiOI/BiOBr composite were examined by SEM (Fig. 2G); the as-prepared 3% g-C₃N₄/BiOI/BiOBr showed a sheet-like structure. The chemical composition of the 3% g-C₃N₄/BiOI/BiOBr composite was further confirmed by EDS analysis. Fig. 2H shows that Bi, O, C, N, Br and I peaks were present in the spectrum; the Au peak is attributed to the gold plating.

Fig. 2 TEM images of (A) g-C₃N₄, (B) BiOI, (C) BiOBr, (D) BiOI/BiOBr, (E) 3% g-C₃N₄/BiOI/BiOBr and (F) 7% g-C₃N₄/BiOI/BiOBr. (G) SEM image and (H) EDS spectrum of 3% g-C₃N₄/BiOI/BiOBr.

3.3. XPS analysis

XPS was carried out to further analyze the elemental composition and valence bond structure of the g-C₃N₄, BiOI/BiOBr and g-C₃N₄/BiOI/BiOBr composites (Fig. 3). The survey XPS spectrum (Fig. 3A) clearly showed that the 3% g-C₃N₄/BiOI/BiOBr composite was composed of C, N, Bi, O, I and Br. High-resolution spectra of Bi4f, Br3d, I3d, O1s, C1s and N1s are shown in Fig. 3B–G. The Bi4f spectra of BiOI/BiOBr and 3% g-C₃N₄/BiOI/BiOBr (Fig. 3B) could be resolved into two spin orbit components at binding energies of 158.7 and 163.9 eV, which were assigned to the Bi4f_7/2 and Bi4f_5/2 of Bi³⁺ in BiOBr, respectively.⁴⁰ In the BiOI/BiOBr composite, the typical peaks at binding energies of about 67.6 and 68.4 eV (Fig. 3C) belonged to Br3d_5/2 and Br3d_3/2.^41,42 The I3d peaks of BiOI/BiOBr were located at 618.4 and 629.6 eV (Fig. 3D), which correspond to the I3d_5/2 and I3d_3/2 binding energies.⁴³ However, in the case of the 3% g-C₃N₄/BiOI/BiOBr composite, the binding energy of Br3d_3/2 (68.5 eV) and I3d_3/2 (629.8 eV) showed a slight shift compared with that of the BiOI/BiOBr composite. The shift of Br3d_3/2 and I3d₅ confirmed the interaction between g-C₃N₄ and BiOI/BiOBr.

Fig. 3 (A) XPS survey spectrum and high-resolution XPS spectra of (B) Bi4f, (C) Br3d, (D) I3d, (E) O1s, (F) C1s and (G) N1s regions for 3% g-C₃N₄/BiOI/BiOBr.

The XPS spectra of O1s for BiOI/BiOBr and 3% g-C₃N₄/BiOI/BiOBr are shown in Fig. 3E. For BiOI/BiOBr, the O1s profile is asymmetrical and can be fitted to three symmetrical peaks located at 529.5, 530.8 and 532.5 eV, respectively, indicating three different O species in the sample. The O1s peak at 529.5 eV was attributed to crystal lattice O atoms (Bi–O). The peak at 530.8 eV was attributed to the Bi–O bonds in the [Bi₂O₂] slabs of BiOI.⁴⁴ The peak at 532.5 eV was associated with the presence of the –OH group or water molecules on the surface of the BiOI/BiOBr and 3% g-C₃N₄/BiOI/BiOBr composites.⁴⁵ For the 3% g-C₃N₄/BiOI/BiOBr composite, the O1s peak at 530.8 eV for BiOI/BiOBr shifted to 531.0 eV after hybridization, indicating a slight change in the oxygen environment after the introduction of g-C₃N₄ into BiOI/BiOBr.

The XPS spectra of C1s for g-C₃N₄ and the 3% g-C₃N₄/BiOI/BiOBr composite are shown in Fig. 3F. For g-C₃N₄, the peak at 284.8 eV is regarded as the carbon species from g-C₃N₄.⁴⁶ The peak at 286.2 eV was attributed to the C–NH₂ species on g-C₃N₄.⁴⁷ The other peak at 288.3 eV was assigned to the sp²-hybridized carbon in the N=C–N₂ coordination,⁴⁸ which is ascribed to carbon atoms that have one double and two single bonds with three N neighbors, respectively.⁴⁹ After hybridization, the peak at 288.3 eV for pure g-C₃N₄ shifted to 287.8 eV, indicating that there were interactions among g-C₃N₄, BiOI and BiOBr. Compared with pure g-C₃N₄, the peak intensity of C1s (287.8 eV) in the 3% g-C₃N₄/BiOI/BiOBr composite decreased significantly, which is a result of the lower levels of g-C₃N₄.

The XPS results for N1s in g-C₃N₄ and the 3% g-C₃N₄/BiOI/BiOBr composite are shown in Fig. 3G. The N1s binding energy of g-C₃N₄ at 398.8 eV can be assigned to sp²-hybridized nitrogen (C=N–C),^50,51 confirming the presence of sp²-bonded graphitic carbon nitride. The binding energies at 400.1 and 401.3 eV can be assigned to tertiary nitrogen bonded to carbon atoms by C–NH and N–(C)₃ bonds,⁴⁹ respectively. Compared with g-C₃N₄, the binding energy of the N1s (398.4 eV) of the 3% g-C₃N₄/BiOI/BiOBr composite had a negative shift and the intensity decreased significantly. This shift further indicates that there are interactions among g-C₃N₄, BiOI and BiOBr. As a result of the low content of g-C₃N₄ in the 3% g-C₃N₄/BiOI/BiOBr composite, the peak intensity of N1s is very weak. This has also been found in similar systems.³⁸

Based on the results of the XRD, TEM, EDS and XPS analyses, the interaction among g-C₃N₄, BiOI and BiOBr was confirmed, which may be favorable for charge transfer and the separation of photogenerated electron–hole pairs, which could promote the photocatalytic performance.

3.4. FTIR analysis

Fig. 4 shows the FTIR spectra for g-C₃N₄ and the BiOI/BiOBr and g-C₃N₄/BiOI/BiOBr composites, with different weight ratios of g-C₃N₄ in the g-C₃N₄/BiOI/BiOBr composites. For pure g-C₃N₄, the band at 808 cm⁻¹ is related to the breathing mode of the heptazine arrangement, whereas those at 1244–1639 cm⁻¹ can be assigned to aromatic C–N breathing modes.^52,53 For the BiOI/BiOBr sample, the valent symmetrical A_2u-type vibrations of the Bi–O bond were observed at 508 cm⁻¹ in BiOI/BiOBr.⁵⁴ The absorption at 767 cm⁻¹ was assigned to the asymmetrical stretching vibration of the Bi–O bond.^55,56 The sharp and strong absorption at 1626 cm⁻¹ was assigned to the bending vibrations of O–H resulting from the adsorption of free water molecules on the photocatalyst surface. The characteristic peaks of g-C₃N₄ and BiOI/BiOBr are present in all the g-C₃N₄/BiOI/BiOBr composites.

Fig. 4 FTIR spectra of g-C₃N₄, BiOI/BiOBr, 1% g-C₃N₄/BiOI/BiOBr, 3% g-C₃N₄/BiOI/BiOBr, 5% g-C₃N₄/BiOI/BiOBr and 7% g-C₃N₄/BiOI/BiOBr.

3.5. UV-visible diffuse reflectance spectrometry

The optical properties of the g-C₃N₄, BiOI/BiOBr and g-C₃N₄/BiOI/BiOBr composites were investigated by UV-visible diffuse reflectance spectrometry. As shown in Fig. 5, the g-C₃N₄ and BiOI/BiOBr samples absorb UV to visible light, which signifies their visible-light-driven photocatalytic activities. g-C₃N₄ can absorb light with a wavelength <450 nm,⁵⁷ and the BiOI/BiOBr sample showed strong absorption over the whole UV-visible range of 200–600 nm.⁵⁸ The absorption of the g-C₃N₄/BiOI/BiOBr composites decreased with the increasing g-C₃N₄ mass ratios, whereas they still showed an absorption edge in the visible region. The result implies that the fabrication of the composites provides the potential in the following visible-light photocatalytic test.

Fig. 5 UV-visible diffuse reflectance spectra of g-C₃N₄, BiOI/BiOBr, 1% g-C₃N₄/BiOI/BiOBr, 3% g-C₃N₄/BiOI/BiOBr, 5% g-C₃N₄/BiOI/BiOBr and 7% g-C₃N₄/BiOI/BiOBr.

3.6. Photocatalytic activity and kinetics

The photocatalytic capability of the g-C₃N₄, BiOI/BiOBr and g-C₃N₄/BiOI/BiOBr composites was evaluated by decomposing MB under visible light. Fig. 6A shows that MB self-degradation was almost negligible in the absence of a photocatalyst. g-C₃N₄ and BiOI/BiOBr could degrade MB up to 50% and 40%, respectively, within 2.5 h and all the g-C₃N₄/BiOI/BiOBr composites showed a higher photocatalytic activity than both g-C₃N₄ and BiOI/BiOBr under visible light irradiation. Significantly, the 3% g-C₃N₄/BiOI/BiOBr composite had the best activity for the decomposition of MB, resulting in 80% degradation within 2.5 h. However, on further increasing the proportion of g-C₃N₄, the degradation rate showed a slight decrease, although it remained higher than that of g-C₃N₄ and BiOI/BiOBr. The decrease in the activity of the samples with a heavy loading of g-C₃N₄ may be a result of the shading effect,⁵⁹ which can block the absorption of the incident light by BiOI/BiOBr.

Fig. 6 (A) Photodegradation of MB on g-C₃N₄/BiOI/BiOBr composites. (B) Kinetic fit for the degradation of MB with g-C₃N₄, BiOI/BiOBr and g-C₃N₄/BiOI/BiOBr composites.

To quantitatively investigate the reaction kinetics of the degradation of MB, the experimental data were fitted by a first-order model as expressed by the formula:

ln(C₀/C) = kt

where C₀ and C are the dye concentrations in solution at times 0 and t, respectively, and k is a first-order rate constant.

Fig. 6B shows that the pseudo-first-order rate constants (k) for the degradation of MB with the g-C₃N₄, BiOI/BiOBr and g-C₃N₄/BiOI/BiOBr composites (1, 3, 5 and 7%) were about 0.27, 0.21, 0.46, 0.68, 0.54 and 0.35 h⁻¹, respectively. The rate constant of the 3% g-C₃N₄/BiOI/BiOBr composite was 2.5 times as high as that of g-C₃N₄ and 3.2 times as high as that of BiOI/BiOBr. As a result, the 3% g-C₃N₄/BiOI/BiOBr composite had the best performance and was selected for the subsequent recycling experiment.

3.7. PL analysis

To investigate the effect of the separation efficiency of the generated electrons and holes in the semiconductors, the PL spectra for the BiOI/BiOBr and 3% g-C₃N₄/BiOI/BiOBr composites were determined at an excitation wavelength of 420 nm (Fig. 7); the results were similar to previous reports.⁴¹ The BiOI/BiOBr and 3% g-C₃N₄/BiOI/BiOBr composites both showed a similar peak shape and peak position. Compared with the BiOI/BiOBr composite, the intensity of the peaks for the 3% g-C₃N₄/BiOI/BiOBr composite was much lower, suggesting that the composite had a much lower recombination rate for the photogenerated charge carriers.

Fig. 7 Photoluminescence spectra of BiOI/BiOBr and 3% g-C₃N₄/BiOI/BiOBr.

3.8. Photocurrent measurements

Photocurrent measurements were performed to further investigate the photoelectric performance because a stronger photocurrent intensity often suggests a higher separation efficiency of holes and electrons.⁶⁰ Hence the higher the photocurrent, the better the electron and hole separation efficiency and therefore the higher the photocatalytic activity. Fig. 8 shows the transient photocurrent response curves of the g-C₃N₄, BiOI/BiOBr and 3% g-C₃N₄/BiOI/BiOBr composites. Compared with g-C₃N₄ and BiOI/BiOBr, the 3% g-C₃N₄/BiOI/BiOBr composite showed an enhanced photocurrent response, which was 5 and 1.7 times the intensity of the g-C₃N₄ and BiOI/BiOBr composites, respectively. This result indicates that the formation of heterojunctions is favorable for charge separation.⁶¹

Fig. 8 Transient photocurrent response for pure g-C₃N₄, BiOI/BiOBr and 3% g-C₃N₄/BiOI/BiOBr.

3.9. EIS analysis

The charge transfer efficiency at the interface of semiconductor photoelectrodes can be indicated by EIS. The EIS Nyquist plots of the g-C₃N₄, BiOI/BiOBr and 3% g-C₃N₄/BiOI/BiOBr composites were measured (Fig. 9). The results show that the BiOI/BiOBr and 3% g-C₃N₄/BiOI/BiOBr composites both had smaller diameter Nyquist circles than g-C₃N₄. In addition, the 3% g-C₃N₄/BiOI/BiOBr composite had a smaller diameter than BiOI/BiOBr. Compared with pure g-C₃N₄ and BiOI/BiOBr, 3% g-C₃N₄/BiOI/BiOBr composite had the lowest resistance, which is favorable for the interfacial charge transfer process. The results of the EIS analysis suggested that the combination of g-C₃N₄ and BiOI/BiOBr can improve the separation and charge transfer efficiency of photogenerated electron–hole pairs and enhance the photocatalytic activity.⁶²

Fig. 9 EIS profiles for pure g-C₃N₄, BiOI/BiOBr and 3% g-C₃N₄/BiOI/BiOBr.

3.10. Stability evaluation

The stability and recyclability of photocatalysts are important in practical applications. Additional experiments were carried out to degrade MB under visible light irradiation over three cycles with the 3% g-C₃N₄/BiOI/BiOBr sample. Fig. 10A shows the relationship between the removal ratio for MB and the cycle time. After three repeated runs, the MB removal ratio decreased to 55%, which may be a result of the loss of the photocatalysts during the cycling experiment. The sample was collected after the cycling runs and analyzed by XRD (Fig. 10B). The XRD peaks of the used 3% g-C₃N₄/BiOI/BiOBr sample were identical to those of the fresh sample, which implied that the sample was stable during degradation.

Fig. 10 (A) Cycling runs of 3% g-C₃N₄/BiOI/BiOBr composite under visible light irradiation for 2.5 h. (B) XRD patterns of 3% g-C₃N₄/BiOI/BiOBr before and after the cycling photocatalytic experiments.

3.11. Detection of reactive species

Photoinduced holes, superoxide radicals and hydroxyl radicals are the main active species in the degradation of organic pollutants.⁶³ The roles of active species in the photodegradation process were tested by adding individual scavengers to the photodegradation suspensions containing 3% g-C₃N₄/BiOI/BiOBr. The scavengers used were isopropanol for hydroxyl radicals (˙OH), N₂ for superoxide radicals (˙O₂⁻) and triethanolamine for holes (h⁺). Fig. 11 shows that the addition of isopropanol, N₂ and triethanolamine caused a decrease in the photodegradation efficiency of 3% g-C₃N₄/BiOI/BiOBr, indicating that ˙OH, ˙O₂⁻ and h⁺ are the active species in this photocatalytic process. A significant suppression of photocatalytic performance was observed when triethanolamine was added, which indicates that h⁺ is the main active species. These experiments suggest that ˙OH, ˙O₂⁻ and h⁺ affect the degradation of MB, but that h⁺ is the main reactive species in the degradation of MB.^64,65

Fig. 11 Reactive species trapping experiments over 3% g-C₃N₄/BiOI/BiOBr.

3.12. Possible photocatalytic mechanism of the ternary hybrid composites

Two possible photocatalytic mechanisms based on the band structures and microstructures of the catalysts are proposed for g-C₃N₄/BiOI/BiOBr (Fig. 12). The valence band (VB) and conduction (CB) levels for g-C₃N₄ were 1.57 and −1.12 eV,^66,67 respectively, and those for BiOI were 2.31 and −0.63 eV;⁶⁸ the VB and CB edge potentials of BiOBr were 0.24 and 3.05 eV.³⁴ Based on the band gap structures of g-C₃N₄, BiOBr and BiOBr, the separation processes for the photoexcited electrons–holes are shown in Fig. 12A and B. If the charge carriers of the g-C₃N₄/BiOI/BiOBr composite transfer as shown in Fig. 12A, then the generic electron–hole separation process seen in a great number of composite photocatalysts takes place. g-C₃N₄, BiOI and BiOBr all absorbed visible light and became excited. Because the CB edge of g-C₃N₄ is more negative than that of both BiOI and BiOBr, electrons from g-C₃N₄ were injected into the less negative CB of the surrounding BiOI. Similarly, the electrons of BiOI jumped toward BiOBr and the holes in the VB of BiOBr migrated to the VB of BiOI and g-C₃N₄. As a result, the accumulated electrons in the CB of BiOBr cannot reduce O₂ to ˙O₂⁻ and the holes in the VB of g-C₃N₄ cannot oxidize OH⁻ to ˙OH. Therefore when the charge carriers of the photocatalyst transfer in accordance with the traditional model, the formation of the active species ˙O₂⁻and ˙OH is not favorable. Therefore the mechanism in Fig. 12A does not match the results of the reactive species experiment.

Fig. 12 Proposed mechanisms for the photodegradation of MB on g-C₃N₄/BiOI/BiOBr composites (see text for discussion).

However, if the charge carriers of the g-C₃N₄/BiOI/BiOBr photocatalyst transfer according to Fig. 12B, which involves a direct Z-scheme,^69,70 then the photoexcited electrons in the CB of BiOBr and the photoexcited holes in the VB of BiOI are rapidly combined. At the same time, the electrons in the CB of BiOI, which have a more negative potential, reduce molecular oxygen to yield ˙O₂⁻. The holes in the VB of BiOBr, which have a more positive potential, generate abundant active ˙OH radicals. The holes in the VB of BiOBr and g-C₃N₄ are then available to degrade organic pollutants. Thus a model based on the Z-scheme between BiOBr and BiOI is favorable for the production of the ˙O₂⁻, ˙OH and h⁺ reactive species.

4. Conclusion

Novel g-C₃N₄/BiOI/BiOBr composite photocatalysts were successfully prepared via a facile solvothermal method. The structure, surface morphology and chemical states of the elements were investigated and the interaction between g-C₃N₄ and BiOI/BiOBr was determined. Compared with the g-C₃N₄ and BiOI/BiOBr composites, the g-C₃N₄/BiOI/BiOBr composites showed a remarkable improvement in photocatalytic activity under irradiation with visible light. The degradation rate constant of the optimized 3% g-C₃N₄/BiOI/BiOBr composite was 0.68 h⁻¹, 3.2 times that of BiOI/BiOBr. A possible photocatalytic mechanism is proposed based on the experimental results. The matched band positions of g-C₃N₄, BiOI and BiOBr allowed the development of a model containing the Z-scheme, which is favorable for the separation and transfer of photogenerated charge carriers and the production of ˙O₂⁻, ˙OH and h⁺ reactive species.

Acknowledgements

The authors genuinely appreciate the financial support of this work from the National Nature Science Foundation of China (21406094, 21476097 and 21416078), Postdoctoral Foundation of China (2015M571693) and the Foundation of Jiangsu University (14JDG184).

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