Synthesis of BiOI nanosheet/coarsened TiO₂ nanobelt heterostructures for enhancing visible light photocatalytic activity

Abstract

For high photocatalytic activity, BiOI nanosheets were deposited on acid-corroded TiO₂ nanobelts, where a large number of BiOI nanosheet/TiO₂ nanobelt heterojunctions could be built. The structure, morphology and optical properties of the BiOI nanosheet/coarsened TiO₂ nanobelt heterostructure composites (BiOI/TiO₂ CNHs) were characterized by XRD, XPS, SEM, HRTEM, DRS, PL, TOC and nitrogen sorption. The XRD results show that only two phases of TiO₂ and BiOI were found in the composites. The HRTEM image shows clear lattice fringes, which prove the formation of heterojunctions at the interface between BiOI and coarsened TiO₂. The photocatalytic activity of the BiOI/TiO₂ CNHs for the degradation of methyl orange under visible-light irradiation was evaluated. The results reveal that the BiOI/TiO₂ CNHs exhibited much higher photocatalytic activity compared with coarsened TiO₂ nanobelts and pure BiOI nanosheets due to the introduction of BiOI onto the surface of the coarsened TiO₂ nanobelts and the formation of heterojunctions. BiOI acts as a visible light photosensitizer in the BiOI/TiO₂ CNHs. In addition, the large surface area and matched energy bands of the BiOI/TiO₂ CNHs both contribute to enhancing the photocatalytic activity. In addition, the possible mechanism of promotion of the photocatalytic performance of the new heterostructure system is explained.

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Introduction

Nanometer TiO₂ with various morphologies, including nanobelts, nanowires and nanotubes, has been at the core of current nanoscience and nanotechnology and has experienced increasing interest in environmental protection and green energy fields.^1–4 For instance, TiO₂, as well as Ga₂O₃ obtained by calcination of GaOOH, are both able to photocatalyse the overall splitting of water by using solar energy.^5–9 Because of its favorable electronic band structure, biochemical compatibility, strong oxidizing power, nontoxicity and long-term stability against photochemical corrosion,¹⁰ TiO₂ has been considered to be the most promising material and has been applied in photocatalysts,^11,12 solar cells,^13,14 gas sensors,^15,16 and lithium batteries.^17,18 Nano-TiO₂ is considered to be a very important photocatalyst for removal of organic pollutants in dye wastewater. However, one of the critical drawbacks of the TiO₂ photocatalyst is its wide band gap of 3.2 eV, which restricts the excitation to UV light irradiation and greatly decreases the utilization of sunlight due to the small ultraviolet fraction (4–5%) of the solar spectrum. Therefore, it is very meaningful for practical application to develop effective means to expand the light absorption area.^19–22

Many studies on shifting the optical response of TiO₂ to the visible light range and then promoting the photocatalytic efficiency have been reported, including those on nonmetallic doping, deposition of noble metals and sensitization.^23–25 In addition, the formation of heterostructures by coupling nano-TiO₂ with an effective visible light semiconductor photocatalyst is also an effective way to enhance the visible light photocatalytic activity. The heterostructure can not only expand the spectral response range to visible light but also improve the efficiency of the charge carrier separation of the photocatalyst. For the past few years, forming a heterojunction between TiO₂ and a narrow-gap semiconductor has been frequently reported. For instance, Chai et al. prepared TiO₂/BiOI photocatalysts consisting of flower-like BiOI decorated with TiO₂ nanoparticles by a facile one-pot solvothermal route.²⁶ Tian et al. reported a broad spectrum photocatalyst with 3D Bi₂WO₆/TiO₂ nanobelt heterostructures by introducing Bi₂WO₆ nanosheets on surface-coarsened TiO₂ nanobelts.²⁰ Zhou et al. prepared Ag₂O nanoparticles/TiO₂ nanobelts by an acid-assisted hydrothermal method which exhibited enhanced photocatalytic activity.²⁷ Liu et al. fabricated bicomponent SnO₂/TiO₂ nanofibers with controllable heterojunctions by the electrospinning process, improving the photocatalytic activity of TiO₂.²⁸ Zhang et al. reported a visible-light-activated photocatalyst based on BiOI/TiO₂ heterostructured powders.²⁹ Among these heterostructures, BiOI/TiO₂ heterostructures show good activity for the photocatalytic degradation of organic pollutants under visible-light irradiation, for the band gap of BiOI of about 1.77 eV can be stimulated by visible light irradiation. Many BiOI-based composites have been investigated, such as BiOBr/BiOI,³⁰ AgI/BiOI,³¹ and BiOCl/BiOI.³²

In this work, we concentrate on working on the modified BiOI/TiO₂ system. BiOI/TiO₂ CNHs were prepared by using BiOI and surface-coarsened TiO₂ nanobelts through a hydrothermal method at certain temperature. The surface-coarsened TiO₂ nanobelts are expected to offer a large amount of active sites for the growth of BiOI nanosheets while facilitating a uniform distribution of BiOI nanosheets on the surface of the coarsened TiO₂ nanobelts. The photocatalytic activity and recycling ability of the BiOI/TiO₂ CNHs were investigated by the degradation of MO. Furthermore, in the process of degradation of MO, the BiOI/TiO₂ CNHs show better photocatalysis efficiency than the pure coarsened TiO₂ nanobelts and BiOI nanosheets. To the best of our knowledge, this is the first report on heterojunctions of BiOI/TiO₂ CNHs with a unique BiOI flake-coarsened TiO₂ belt structure and excellent visible light photocatalytic activity and stability.

Experimental

Materials

All the reagents used were of analytical grade. Titania P-25 (commercial TiO₂, ca. 80% anatase and 20% rutile) was bought from China National Medicines Co., Ltd. Hydrochloric acid and sulfuric acid were obtained from Sino-pharm Chemical Reagent Co., Ltd. NaOH, Bi(NO₃)₃·5H₂O, KI and ethylene glycol (EG) were purchased from Tianjin Reagent Co., Ltd.

Preparation of TiO₂ nanobelts and surface-coarsened TiO₂ nanobelts

TiO₂ nanobelts were prepared via the hydrothermal process using commercial TiO₂ powder. First, 0.3 g of P-25 powder was dispersed in 40 mL of 10 M NaOH aqueous solution, followed by magnetic stirring for 30 min. After that, the solution was transferred into a 40 mL Teflon-lined stainless steel autoclave, heated at 180 °C for 72 h, and cooled down to room temperature under ambient conditions. The treated product was washed thoroughly with deionized water several times, followed by a filtration process. Subsequently, the wet product was dispersed in a 0.1 M HCl aqueous solution for 48 h, and then washed thoroughly with deionized water. The resultant hydrogen titanate nanobelts were carefully dried at 80 °C for 8 h in an oven and further heat-treated in the muffle furnace at 600 °C for 2 h, obtaining TiO₂ nanobelts.

The hydrogen titanate nanobelts were dispersed in 20 mL of a 0.02 M H₂SO₄ aqueous solution under magnetic stirring for 30 min, and then heated in a Teflon-lined stainless steel autoclave at 100 °C for 12 h. After washing and drying, the sample was annealed at 600 °C for 2 h, and finally the surface-coarsened TiO₂ nanobelts were successfully synthesized.

Preparation of BiOI/TiO₂ CNHs and BiOI/TiO₂ nanobelt heterostructure composites (BiOI/TiO₂ NHs)

BiOI/TiO₂ CNHs with different molar ratios from 1 : 3 to 3 : 1 were synthesized using a three-step hydrothermal method. As an example, the preparation process of 1 : 1 BiOI/TiO₂ CNHs is presented as follows: 0.485 g Bi(NO₃)₃·5H₂O and 0.166 g KI were each separately dissolved in 40 mL of EG. After stirring magnetically for 30 min, the KI solution was added to the Bi(NO₃)₃·5H₂O solution drop by drop. Next, the calculated amount of coarsened TiO₂ nanobelts was added to the mixed solution, which was stirred for 30 min and then ultrasonically treated for 10 min. Then, the above mixed solution was transferred into a 40 mL Teflon-lined stainless steel autoclave, and heated at 160 °C for 6 h. Finally, BiOI/TiO₂ CNHs were obtained after washing with deionized water and ethanol several times and drying at 80 °C. For comparison, individual BiOI was also synthesized by the same process.

BiOI nanosheet/TiO₂ nanobelt heterostructure composites (BiOI/TiO₂ NHs) were synthesized using the hydrothermal method. In a typical procedure, 0.485 g Bi(NO₃)₃·5H₂O and 0.166 g KI were each separately dissolved in 20 mL of EG. After stirring magnetically for 30 min, the KI solution was added to the Bi(NO₃)₃·5H₂O solution drop by drop. Next, the calculated amount of TiO₂ nanobelts was added to the mixed solution, which was stirred for 30 min and then ultrasonically treated for 10 min. Then, the above mixed solution was transferred into a 40 mL Teflon-lined stainless steel autoclave, and heated at 160 °C for 6 h. Finally, BiOI/TiO₂ NHs were obtained after washing with deionized water and ethanol several times and drying at 80 °C.

Characterization

X-ray powder diffraction (XRD) patterns of the catalysts were recorded on a Bruker D8 Advance powder X-ray diffractometer with Cu-Ka (λ = 0.15406 nm) radiation. X-ray photoelectron spectroscopy (XPS), which was carried out on an ESCALAB 250 photoelectron spectrometer, was used to analyze the surface chemical composition and valence state. The morphologies of the samples were characterized by scanning electron microscopy (SEM, Hitachi, Japan), transmission electron microscopy (TEM, TecnaiF20) and high-resolution TEM (HRTEM, Jeol, Japan). UV-Vis diffuse reflectance spectra (DRS) of the dry-pressed disk samples in the wavelength range of 200–800 nm were obtained using a UV-Vis spectrometer (Shimadzu U-3010) and BaSO₄ as a reflectance standard. Photoluminescence (PL) spectra were recorded on a FLS920 fluorescence spectrometer with an excitation wavelength of 380 nm. A Micromeritics Tristar 3000 and TOC analyzer (TOC-V, Shimadzu, Japan) were employed to measure nitrogen adsorption–desorption isotherms and total organic carbon (TOC) concentration, respectively. The Brunauer–Emmett–Teller (BET) surface area was estimated using the adsorption data. The Mott–Schottky measurement was conducted on an electrochemical system (CHI 660E, Shanghai). A three electrode single compartment immersed in 0.5 M Na₂SO₄ solution was used for capacitance analysis. The as-prepared film coated on FTO was used as a working electrode while Ag/AgCl and platinum were used as the reference and counter electrodes, respectively.

Photocatalytic activity test

The photocatalytic activity of the BiOI/TiO₂ heterostructures was examined by detecting the photodegradation of MO at ambient temperature under simulated visible light. MO was selected as the model chemical to evaluate BiOI/TiO₂ photocatalytic activity. The reactivity was tested with 50 mL of 20 mg L⁻¹ MO solution illuminated by a 350 W Xe arc lamp (wavelength > 410 nm by pass filter), which was used as the light source of a homemade photoreactor. The BiOI/TiO₂ concentration was 1.0 g L⁻¹. Before irradiation, the MO suspension was placed in the dark for 60 min with magnetic stirring to achieve adsorption/desorption equilibrium. At given irradiation time intervals of 15 minutes, 4 mL MO suspension was taken out. The residual MO concentration was calculated from absorbance at 464 nm on a UV-Vis spectrophotometer.

Results and discussion

Structure and morphology analysis

The crystallinity and crystal structures of the different samples were characterized by XRD. In Fig. 1a, there are four relatively wide diffraction peaks at around 29.6°, 31.7°, 45.5° and 55.2°, which correspond to (012), (110), (004) and (212), and all of the diffraction peaks can be indexed to the tetragonal phase of BiOI (space group P4/nmm, JCPDS 10-0445). As seen from Fig. 1b, the diffraction peaks located at 25.2°, 37.9°, 48.1°, 53.9° and 62.8° in the XRD pattern of the TiO₂ nanobelts correspond to the (101), (004), (200), (105) and (201) planes of anatase TiO₂ (JCPDS file no. 21-1272), respectively. In addition, several peaks at 28.8°, 33.3° and 43.4° are also observed and are indexed to TiO₂-B (JCPDS no. 46-1237). The diffraction peaks of the coarsened TiO₂ nanobelts in Fig. 1c remain almost consistent with those in Fig. 1b, which illustrates that the surface modification by H₂SO₄ solution does not change the crystalline phase of the TiO₂ nanobelts. However, the TiO₂-B peaks become sharp after acid corrosion, probably due to the formation of heterostructures composed of nanoparticles and nanobelts, which may promote the migration of the crystalline phase to be more complete.³³ Fig. 1d and e show that all the diffraction peaks can be attributed to the BiOI and TiO₂ phases, which indicates that BiOI and anatase TiO₂ coexist and there remains good chemical compatibility between them.

Fig. 1 XRD patterns of (a) BiOI, (b) TiO₂ nanobelts, (c) coarsened TiO₂ nanobelts, (d) BiOI/TiO₂ NHs and (e) BiOI/TiO₂ CNHs.

In order to further determine the surface chemical composition of the BiOI/TiO₂ CNHs, XPS studies were conducted and the spectra are shown in Fig. 2. Fig. 2a shows the existence of Ti, O, Bi, I and C elements. The XPS peak for C 1s (284.6 eV) is ascribed to the adventitious carbon from the XPS instrument. In the high resolution XPS of Bi 4f (Fig. 2b), the two peaks of Bi 4f at 159.1 eV and 164.5 eV belong to Bi 4f_7/2 and Bi 4f_5/2, respectively, corresponding to the characteristics of Bi³⁺ in BiOI.^34,35 The satellite peaks, with a distance of about 1.8 eV away from the Bi 4f_7/2 and Bi 4f_5/2 main peaks, are consistent with the reported values.³⁶ Fig. 2c is the survey XPS spectrum of I 3d, and the two peaks at binding energies of 630.4 eV and 619.1 eV are attributed to I 3d_3/2 and I 3d_5/2, respectively, which can be ascribed to I⁻ in pure BiOI.^34,35 The XPS results demonstrate the coexistence of BiOI and TiO₂ in the BiOI/TiO₂ CNHs and are consistent with the XRD results.

Fig. 2 (a) XPS full spectrum, (b) Bi 4f XPS spectrum and (c) I 3d XPS spectrum of BiOI/TiO₂ CNHs.

SEM and EDX were performed to describe the morphologies and element compositions of the samples. As shown in Fig. 3a, the surfaces of the TiO₂ nanobelts are very smooth and clean, and the TiO₂ nanobelts are typically several micrometers in length, 50–300 nm in width and 25–40 nm in thickness. Compared with the TiO₂ nanobelts in Fig. 3a, the surface-coarsened TiO₂ nanobelts treated by the acid process (see Fig. 3b) show rough surfaces and high specific surface area, which can provide abundant nucleation sites for the assembly of BiOI nanosheets. Fig. 3c shows that the synthesized BiOI has a flower-like spherical morphology with a diameter of 2–4 μm and is composed of flakes. The typical SEM images of the BiOI/TiO₂ NHs and BiOI/TiO₂ CNHs are shown in Fig. 3d and e. It can be noted that low density BiOI nanosheets with non-uniform size are anchored on the surface of the untreated TiO₂ nanobelts (Fig. 3d). In contrast, homogeneous BiOI nanosheets are thickly arranged on the surface-coarsened TiO₂ nanobelts, just like branches covered with snow, which indicates a large number of heterojunctions formed on the surface of the coarsened TiO₂ nanobelts. The morphologies of the two BiOI/TiO₂ heterostructures are different, which originates from the diversity of surface roughness. The EDX spectrum of the BiOI/TiO₂ CNHs given in Fig. 3f reveals that the BiOI/TiO₂ CNHs only contain Bi, O, I, and Ti, without impurity elements. To further confirm the formation, EDX mapping of the BiOI/TiO₂ CNHs was conducted. As shown in Fig. 4a–e, the four constituent elements O, Ti, I, and Bi can all be detected, and they are homogeneously distributed in the composite. These results imply that uniformly structured BiOI/TiO₂ CNHs are obtained.

Fig. 3 SEM images of (a) TiO₂ nanobelts, (b) coarsened TiO₂ nanobelts, (c) BiOI nanoflowers, (d) BiOI/TiO₂ NHs and (e) BiOI/TiO₂ CNHs, and (f) EDX analysis of BiOI/TiO₂ CNHs. Insets in (a, c and e) are high-magnification SEM images of TiO₂ nanobelts, BiOI nanoflowers and BiOI/TiO₂ CNHs, respectively.

Fig. 4 SEM image (a) and EDX mappings of BiOI/TiO₂ CNHs, showing the distributions of O (b), Ti (c), I (d) and Bi (e).

The typical transmission electron microscopy (TEM) and HRTEM images of TiO₂ nanobelts and BiOI/TiO₂ CNHs are presented in Fig. 5. The TiO₂ nanobelts in Fig. 5a with a width of about 100 nm are consistent with Fig. 3a. It can be clearly observed from Fig. 5b that the BiOI nanosheets adhere on the surface of the coarsened TiO₂ nanobelts and the BiOI/TiO₂ CNHs still keep a one-dimensional morphology. The high-resolution TEM images of the BiOI/TiO₂ CNHs are shown in Fig. 5c and d at different magnifications. The lattice spacings of TiO₂ and BiOI are 0.35 nm and 0.30 nm, respectively, corresponding to the interplanar spacing of the (101) plane of TiO₂ and (012) plane of BiOI, which indicates the formation of heterojunctions in the BiOI/TiO₂ CNHs.

Fig. 5 TEM images of (a) TiO₂ nanobelts and (b) BiOI/TiO₂ CNHs; (c and d) HRTEM images of BiOI/TiO₂ CNHs.

The N₂ adsorption–desorption isotherms were measured to determine the pore structure and size distribution of the as-prepared coarsened TiO₂ nanobelts, BiOI/TiO₂ NHs, and BiOI/TiO₂ CNHs, as shown in Fig. 6. In Fig. 6b, d and f, all the isotherms are consistent with type IV, exhibit H1 hysteresis loops observed in the range of 0.5–1.0 P/P₀ and illustrate the mesoporous characteristics of the samples.³⁷ From Fig. 6a and c, it can be seen that the main pore diameters of the coarsened TiO₂ nanobelts and BiOI/TiO₂ NHs are in the ranges of 25–45 nm and 1–30 nm, respectively. Meanwhile, the pore diameters of the BiOI/TiO₂ CNHs are mostly in the range of 2–13 nm, as shown in Fig. 6e. The difference in pore sizes may come from the crystal growth process.

Fig. 6 Pore volume curves of (a) coarsened TiO₂ nanobelts, (c) BiOI/TiO₂ NHs, and (e) BiOI/TiO₂ CNHs. Nitrogen adsorption–desorption isotherms of (b) coarsened TiO₂ nanobelts, (d) BiOI/TiO₂ NHs and (f) BiOI/TiO₂ CNHs.

By using the BET method, the surface area values of the coarsened TiO₂ nanobelts, BiOI/TiO₂ NHs and BiOI/TiO₂ CNHs are calculated to be 26.9 m² g⁻¹, 29.6 m² g⁻¹ and 38.6 m² g⁻¹, respectively. Moreover, the pore volumes of the coarsened TiO₂ nanobelts, BiOI/TiO₂ NHs and BiOI/TiO₂ CNHs are calculated to be 0.06 cm³ g⁻¹, 0.13 cm³ g⁻¹ and 0.18 cm³ g⁻¹, respectively. These results indicate that the BiOI/TiO₂ CNHs have a larger surface area and pore volume after coating of BiOI on the coarsened TiO₂ nanobelts. The large surface area and pore volume can provide more surface active sites and facilitate the transfer of reactants, leading to an enhanced photocatalytic activity.³⁸

Optical properties analysis

The UV-Vis diffuse reflectance spectra (DRS) of the coarsened TiO₂, BiOI, BiOI/TiO₂ NHs and BiOI/TiO₂ CNHs are shown in Fig. 7a and b. In Fig. 7a, the coarsened TiO₂ nanobelts exhibit an absorption edge at about 400 nm, indicating that the light absorption of TiO₂ is limited to the UV range.³⁹ BiOI shows strong capability of light absorption in both the UV and visible light regions of about 400–650 nm, in addition to the intrinsic absorption from the band gap transition, which will lead to a good visible light response. The UV-Vis spectra of the BiOI/TiO₂ NHs and BiOI/TiO₂ CNHs also display almost the same absorption band as BiOI. Compared with the coarsened TiO₂ nanobelts, there is an obvious wavelength shift to about 650 nm for the BiOI/TiO₂ NHs and BiOI/TiO₂ CNHs due to the presence of BiOI. It has been reported that the inhibited recombination between photoelectrons and holes could result in a strong response in the visible region.⁴⁰

Fig. 7 (a) UV-Vis diffuse absorption spectra and (b) the plot of (αhν)^1/2 vs. photo energy of coarsened TiO₂, BiOI, BiOI/TiO₂ NHs and BiOI/TiO₂ CNHs, and (c) PL spectra of coarsened TiO₂ nanobelts and BiOI/TiO₂ CNHs with a mole ratio of 1 : 1, λ_ex = 380 nm.

The band gaps can be evaluated from the DRS using the following equation (eqn (1)):⁴¹

where α, n, E_g and A are the absorption coefficient, light frequency, indirect band gap and constant, respectively. Among them, n depends on the transition characteristics of the semiconductor. The n value of BiOI and coarsened TiO₂ is 4.⁴² The E_g is the energy intercept, as shown in Fig. 7b. The band gap of the coarsened TiO₂ nanobelt is estimated to be 3.0 eV, which is smaller than that of P-25 (about 3.2 eV). The band gap energies of the BiOI/TiO₂ CNHs and BiOI/TiO₂ NHs can be evaluated to be about 1.83 eV and 1.88 eV, respectively, while that of BiOI is 1.80 eV. The E_g of BiOI/TiO₂ CNHs is between that of pure coarsened TiO₂ and BiOI, which indicates the formation of the heterostructures.

In order to explore the fate of photoinduced electron–hole pairs in the semiconductors, photoluminescence (PL) spectra of the coarsened TiO₂ nanobelts and BiOI/TiO₂ CNHs were studied, as shown in Fig. 7c. The PL emission peak appears when the photogenerated free carriers recombine again. The higher peaks indicate a higher recombination of carriers, while the lower peaks suggest a lower recombination.⁴³ Fig. 7c shows the PL of the coarsened TiO₂ nanobelts and BiOI/TiO₂ CNHs. The two strong emission peaks of 470 nm and 525 nm might belong to the band gap transition of the pure coarsened TiO₂ nanobelts.⁴⁴ It is obvious that the PL intensities of the BiOI/TiO₂ CNHs have decreased compared with the coarsened TiO₂ nanobelts. This kind of situation should be caused by fewer photons reaching the TiO₂ because of the BiOI on the surface of the coarsened TiO₂ nanobelts. The BiOI could be easily stimulated, with induction of carriers after absorbing photons, and the heterojunctions formed between the BiOI and coarsened TiO₂ are appropriate for the transfer of the photogenerated carriers to the TiO₂, as in the subsequent discussion of the mechanism. The transfer of electrons from BiOI to coarsened TiO₂ is beneficial for the separation of photoinduced electrons and holes in coarsened TiO₂ and thus inhibits the recombination of the photogenerated charge carriers; hence, the photocatalytic activity of the BiOI/TiO₂ CNHs is enhanced.

Photocatalytic activities analysis

The photocatalytic degradation capabilities of the coarsened TiO₂ nanobelts, BiOI, BiOI/TiO₂ NHs, and BiOI/TiO₂ CNHs were measured by decomposition of aqueous MO solution under visible light irradiation, as shown in Fig. 8. All measurements were carried out under the same conditions for accurate comparison. Before the light irradiation, the photocatalysts and MO were stirred for 60 min in the dark to achieve the balance of the adsorption/desorption equilibrium. Fig. 8a shows a temporal evolution of the spectral changes during the photodegradation of the MO solution in the presence of BiOI/TiO₂ (mole ratio 1 : 1) CNHs during the photocatalytic process. It can be seen that the typical absorption peak of MO decreases along with the increase in photocatalytic time, and almost disappears after 120 min, illustrating that the BiOI/TiO₂ (mole ratio 1 : 1) CNHs possess good photocatalytic activity for MO decomposition under visible light. As shown in Fig. 8b, it is clear that the concentration of the MO solution without catalyst is undiminished after 60 min in the dark, while the decrease with coarsened TiO₂ is only 3%, and that with BiOI is 5%, which results from the higher adsorption ability of BiOI than coarsened TiO₂. Furthermore, the BiOI/TiO₂ CNHs exhibit a slightly stronger adsorption ability toward the MO with an adsorption value of 11% in the dark, while that of BiOI/TiO₂ NHs is about 8%. This may be due to the large specific surface area of the BiOI/TiO₂ CNHs, with more BiOI nanosheets in the BiOI/TiO₂ CNHs, as mentioned in Fig. 3 and 6. This kind of attractive nanomaterial with an adsorption capacity will have important applications in water treatment to remove organic pollutants and even some toxic heavy metal ions.⁴⁵ After visible light irradiation, it can be seen from Fig. 8b that the coarsened TiO₂ nanobelts exhibit a low 15% photodegradation rate of MO because of their low absorption of visible light. On the contrary, BiOI shows a little higher photodegradation rate of 50% of MO after 120 min of visible light irradiation, due to its low band gap (1.80 eV), which can be stimulated by visible light (less than 2.95 eV). With the same illumination time, the efficiency for MO degradation of the BiOI/TiO₂ NHs reaches 61%, which is higher than that of BiOI, possibly due to the visible light response of BiOI and the formation of heterostructures, which inhibited the recombination of photogenerated electron–hole pairs in the interface between the BiOI nanosheets and TiO₂ nanobelts. It is noteworthy that almost 98.2% of MO is degraded by the BiOI/TiO₂ CNHs, which is much higher than that of BiOI/TiO₂ NHs. In general, the photocatalytic activity of a composite photocatalyst is connected to its content and components, surface area, absorbance ability, heterojunction interface, band structure matching, etc. However, no remarkable difference in the absorbance ability is found between the BiOI/TiO₂ NHs and BiOI/TiO₂ CNHs (Fig. 6), which indicates that the major influences for the enhanced photocatalytic activity of the BiOI/TiO₂ CNHs are the large specific surface area and the greater formation of a heterostructure interface with matched energy bands between the surface-coarsened TiO₂ nanobelts and BiOI nanosheets.

Fig. 8 (a) Time-variant optical absorption spectra for the degradation of MO using the BiOI/TiO₂ (mole ratio 1 : 1) CNHs under visible-light irradiation, (b) comparison of photodegradation rates of MO using different photocatalysts, (c) ln(c/c₀) versus irradiation time under visible light, and (d) recycling properties of the BiOI/TiO₂ (mole ratio 1 : 1) CNHs.

Hence, it is necessary to analyze the degradation data using the pseudo-first-order reaction kinetics model as expressed in eqn (2), which is generally used to express the photocatalytic degradation process:⁴⁶

ln(c₀/c) = kt (2)

where c₀ and c are the initial absorption value before irradiation and timed absorption value after irradiation at 464 nm of the MO solution, respectively. The rate constants of the samples were calculated according to Fig. 8c and are summarized in Fig. 8d. The rate constant of the BiOI/TiO₂ CNHs was the highest, which was almost two, three and sixteen times higher than those of the BiOI/TiO₂ NHs, BiOI and coarsened TiO₂, respectively. All the measurements suggest that the BiOI/TiO₂ CNHs exhibit much more effective photocatalytic activity.

Further studies were necessary to probe the influence of the mole ratio of BiOI to TiO₂ on the photocatalytic ability of the BiOI/TiO₂ CNHs. It can be seen from Fig. 9a that with an increasing mole ratio of BiOI/TiO₂, the photocatalytic ability of the BiOI/TiO₂ CNHs first strengthens and then decreases. The maximum value of the BiOI/TiO₂ CNHs is reached when BiOI/TiO₂ is 1 : 1. Here are the reasons for this phenomenon. Firstly, the visible light photocatalytic activity of the BiOI/TiO₂ CNHs increases with increasing fraction of BiOI due to the good absorption of visible light by BiOI and the high photocatalytic activity under visible light. Secondly, when the molar ratio is lower than 1 : 1, the formed heterojunctions between the BiOI and surface-coarsened TiO₂ nanobelts can promote the photocatalytic activity by facilitating the separation of photogenerated electron–hole pairs, which results in higher visible photocatalytic activity of the BiOI/TiO₂ CNHs. However, for the sample with a mole ratio beyond 1 : 1, excessive BiOI nanosheets cover the surface of the coarsened TiO₂ nanobelts, which is disadvantageous to the formation of heterojunctions and impedes the transmission of the photoinduced electrons on the heterostructure interface of the BiOI/TiO₂ CNHs and hence results in a lower photocatalytic activity.

Fig. 9 (a) Photocatalytic decomposition of MO over BiOI/TiO₂ CNHs with different mole ratios under visible light, (b) recycling properties of the BiOI/TiO₂ CNHs (1 : 1) and (c) removal ratio of TOC with BiOI/TiO₂ (1 : 1) CNHs at different times.

Although the as-synthesized BiOI/TiO₂ CNHs show good photocatalytic activity, they will has no practical value if the photocatalyst is not stable under repeated operations. The reusability and photocatalytic stability of the BiOI/TiO₂ CNHs were investigated by reuse of the BiOI/TiO₂ CNHs in the decomposition of MO under visible-light. As shown in Fig. 9b, after four cycles the degradation rate only decreased by about 8.1%, which indicates an excellent photocatalytic stability of the BiOI/TiO₂ CNHs under visible light irradiation.

The efficient mineralization of organic dyes and pollutants is very important for preventing secondary pollution. TOC analysis was used to determine the degree of mineralization of the MO. The original TOC concentration of the MO solution with the BiOI/TiO₂ CNHs is 20 mg L⁻¹. After visible light irradiation for 100 min, the TOC concentration of the MO solution decreases by about 62% (Fig. 9c), which indicates that the BiOI/TiO₂ CNHs exhibit an excellent performance in the mineralization of MO.

Mechanism of photocatalytic activity enhancement

It is known that the photocatalytic activity is determined by the energy band structure and the separation efficiency of photogenerated electron–hole pairs. The photogenerated electrons and holes could react with the adsorbed reactants on the surface of the catalyst or recombine again.

In particular, BiOI is a p-type semiconductor,^26,47,48 and TiO₂ is an n-type semiconductor,^26,48,49 which was further confirmed by the Mott–Schottky plots. According to the Mott–Schottky equation, a linear relationship of 1/C² versus applied potential can be obtained, with the negative and positive slopes corresponding to p- and n-type conductivities,^50,51 respectively. As shown in Fig. 10, the TiO₂ and BiOI/TiO₂ CNHs show positive slopes, as expected for an n-type semiconductor, while the BiOI shows a negative slope, indicating that BiOI is a p-type semiconductor.

Fig. 10 Variation of capacitance (C) with the applied potential in 0.5 M Na₂SO₄ presented as the Mott–Schottky relationship for (a) TiO₂ and BiOI/TiO₂ CNHs, and (b) pure BiOI. The capacitance was determined by electrochemical impedance spectroscopy.

The band edge positions of BiOI and TiO₂ were predicted theoretically from the absolute electronegativity,⁵² and the conduction and valence band positions were evaluated using the following equation (eqn (3)):⁵³

where X is the absolute electronegativity of the semiconductor, E_e is the energy of free electrons on the hydrogen scale (4.5 eV), and E_g is the band gap of the semiconductor. The X values for BiOI and TiO₂ are calculated to be about 5.99 eV and 5.84 eV, respectively. The E_g values of BiOI and TiO₂ are 1.80 eV and 3.0 eV, respectively. On the basis of the above eqn (3), the CB and VB of BiOI are calculated to be 0.59 and 2.39 eV, respectively, while the CB and VB values of the TiO₂ are −0.16 and 2.84 eV, respectively.

As reported earlier, the flat band potential represents the apparent Fermi level (E_F) of a semiconductor in equilibrium with a redox couple,^54,55 and the flat band position of semiconductors can be determined by means of the extrapolation of the Mott–Schottky plot. Therefore, we can propose the change in energy band structure of the two semiconductors before and after the contact, as shown in Fig. 11a and b. The E_F values of TiO₂ and BiOI are 0.03 and 2.18 V (vs. AgCl), respectively. As an n-type semiconductor, the Fermi level of TiO₂ lies close to the CB, while the p-type BiOI has an E_F close to its VB.^56,57 The above analysis indicates that the results of the energy band positions obtained through the band gap values combined with either the conduction or valence band positions are reasonable.

Fig. 11 (a) Mechanism diagrams for the energy bands of p-BiOI and n-TiO₂ before contact; (b) the formation of a p–n junction and its energy band diagram at equilibrium and transfer of photoinduced electrons from p-BiOI and n-TiO₂ under visible-light irradiation.

In Fig. 11a, it can be seen that the CB of TiO₂ is higher than that of BiOI before contact, and the Fermi level of TiO₂ is also higher than that of BiOI. In addition, BiOI could be easily stimulated by visible-light (energy less than 2.95 eV) due to its narrow band gap of 1.80 eV, which induces the photogenerated electron–hole pairs. However, the photocatalytic activity of BiOI is low (Fig. 8b) due to recombination of the photogenerated electrons and holes, which is caused by its narrow band gap. On the contrary, TiO₂ nanobelts cannot be stimulated by visible-light because of the wide band gap of 3.0 eV in this work.

After contact (Fig. 11b), the Fermi level of TiO₂ is moved down and BiOI is moved up until reaching an equilibrium.⁵⁸ Meanwhile, the energy bands of TiO₂ shift downward along with the Fermi energy level, whereas those of BiOI shift upward in the process, and the CB edge of TiO₂ is lower than that of BiOI. Meanwhile, an electric field is formed in the interface under the equilibrium state, thus TiO₂ has a positive charge, while a negative charge exists in the BiOI region. The flat band potential of the BiOI/TiO₂ CNHs was determined to verify the above deduction. As can be seen from Fig. 11a, the Fermi level position of the BiOI/TiO₂ CNHs was 0.15 V (vs. AgCl), which indicates that the Fermi level position of TiO₂ shifts down and that of BiOI shifts up after the formation of the p–n junction. In the visible light region, the photogenerated electrons in the VB of BiOI could be excited up to the CB of BiOI, and photogenerated holes remain in the VB of BiOI. The photogenerated electrons transfer to the CB of TiO₂, crossing the interface field which will promote the transmission of the photo-generated electron–hole pairs. On the contrary, the holes in the VB of TiO₂ transfer to the VB of BiOI. Therefore, the photogenerated electron–hole pairs could be effectively separated by the heterojunction at the p-BiOI/n-TiO₂ interface. As a result, more effective photoinduced electrons and holes participate in the photodegradation process, with enhanced photocatalytic activity. Therefore, the separation of photoinduced electron–hole pairs is the key factor enhancing the photocatalytic activity of the BiOI/TiO₂ CNHs. According to the band gap structure of BiOI/TiO₂, the possible path for the photocatalytic degradation of MO or other dye molecules by the BiOI/TiO₂ CNHs is proposed in the following equation (eqn (4)):

Conclusions

In summary, novel BiOI/TiO₂ CNHs were successfully prepared via an efficient hydrothermal method. A dense mass of thick BiOI nanosheets was attached on the surface of coarsened TiO₂ nanobelts. Importantly, the BiOI/TiO₂ CNHs exhibited excellent decomposition efficiency in the photocatalytic degradation of MO compared to BiOI/TiO₂ NHs and pure coarsened TiO₂ nanobelts under visible-light irradiation. The photocatalytic activity of the BiOI/TiO₂ CNHs varies with the BiOI to TiO₂ mole ratios and reaches a maximum value at 1 : 1. In addition, the recycling testing of the BiOI/TiO₂ CNHs for MO degradation suggests good stability of the BiOI/TiO₂ CNHs. The enhanced photocatalytic activity can be attributed to the well-matched energy bands of the BiOI/TiO₂ heterostructure, high specific surface area and suppressed photoelectron–hole recombination. Furthermore, the novel BiOI/TiO₂ CNHs will provide a promising platform for potential applications in degradation of organic pollutants and nanotechnology.

Acknowledgements

The authors thank the Key Laboratory of Pulp and Paper Science & Technology of Ministry of Education of China (Grant no. 0308031378) for financial support. They also thank Shandong Provincial Key Laboratory of Processing and Testing Technology of Glass and Functional Ceramics for technological support.

Notes and references

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