Construction of heterostructured TiO₂/InVO₄/RGO microspheres with dual-channels for photo-generated charge separation

Abstract

A novel triple-component TiO₂/InVO₄/RGO photocatalyst with dual channels for photogenerated charge separation has been successfully synthesized for the first time to improve photocatalytic activity under visible light. The synthesis involved loading of RGO particles on the surface of InVO₄ microspheres to form RGO/InVO₄, and then depositing TiO₂ nanocrystals on the surface of InVO₄ by hydrolysis of Ti(SO₄)₂ at low-temperature hydrothermal conditions. The TiO₂/InVO₄/RGO exhibited superior photocatalytic performance to bare InVO₄, TiO₂, TiO₂/InVO₄, RGO/TiO₂, and RGO/InVO₄ in degradation of Rhodamine B (Rh B) under visible light. It is suggested that the photogenerated electrons in the conduction band (CB) of InVO₄ can quickly migrate to RGO, while the electrons also can be transferred to the CB of TiO₂. The dual transfer channels at the interfaces of TiO₂/InVO₄/RGO result in effective charge separation, leading to enhanced photocatalytic activity. The concept of establishing dual channels for charge separation in a triple-component heterostructure provides a promising way to develop photocatalysts with high efficiency.

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1. Introduction

Semiconductor photocatalysts, especially those with high photocatalytic activity and strong structure stability under visible light irradiation, have been regarded as promising materials for application in solar energy conversion and water pollutant treatment.^1–4 In the past few decades, various researchers have been focused on exploiting novel and efficient photocatalysts for degradation of organic pollutants.^5–7 Among them, InVO₄ was found to be an important fundamental semiconductor photocatalyst,⁸ and has received considerable attention in photocatalysis field due to its narrow band gap.^9–11 However, similar to other semiconductor photocatalysts, poor quantum yield and poor visible light absorption efficiency are still great challenges to enhance the photocatalytic performance of pure InVO₄ to meet the practical application requirements.^12–14 Thus, it would be beneficial if a more effective photocatalytic system was developed to promote the charge separation of InVO₄.

Recently, many studies have revealed that InVO₄-based photocatalytic processes could enhance the photocatalytic degradation of organic contaminants under visible-light illumination. The TiO₂/InVO₄,¹⁵ RGO/InVO₄,¹⁶ CeO₂/InVO₄,¹⁷ and g-C₃N₄/InVO₄ (ref. 18) composites have demonstrated enhanced photocatalytic efficiency due to the fast electron–hole separation in these heterostructures. However, only one charge transfer channel is usually established in these binary-component photocatalysts, either an electron transfer channel or a hole transfer channel. It is therefore interesting to study whether a triple-component photocatalyst with two transfer channels could further enhance the photo-generated charges separation and result in better photocatalytic performance than the binary-component photocatalysts. In our previous work, we presented the synthesis of ternary heterostructured Ag–BiVO₄/InVO₄ and Ag₃PO₄/BiVO₄/InVO₄ composites and tested their photocatalytic properties.^19,20 The as-fabricated ternary heterostructured composites showed superior photocatalytic activities than that of pure InVO₄, or binary heterostructured BiVO₄/InVO₄ composite. Motivated by the above efforts, we further study the preparation of TiO₂/InVO₄/RGO composite for improving photocatalytic efficiency. To the best of our knowledge, there are no reports on the synthesis and application of TiO₂/InVO₄/RGO heterostructure.

Herein, a novel ternary heterostructured TiO₂/InVO₄/RGO photocatalyst has been synthesized. The as-prepared TiO₂/InVO₄/RGO composite exhibited enhanced photocatalytic activity in degradation of organic Rh B in aqueous solution under visible light. The microstructure of TiO₂/InVO₄/RGO and the underlying mechanism for the enhanced photocatalytic performance have been investigated.

2. Experimental section

2.1. Preparation of photocatalysts

2.1.1 Preparation of RGO nanoparticles. All reagents for synthesis and analysis were commercially available and used without further treatments. In the typical procedure, GO sheets from Hummers' method (50 mg)²¹ were sonicated in a mixed solution of concentrated H₂SO₄ (75 mL) and HNO₃ (25 mL). After refluxed at 100 °C for 24 h, the mixed solution was filtered through a 0.22 μm microporous membrane to remove the acids. The brown filtrate was then transferred to a Teflon-lined autoclave (20 mL in capacity) and heated at 200 °C for 24 h. After cooling to room temperature, the resultant hydrothermally treated brown suspension was further purified in a dialysis bag for 3 days, producing highly stable RGO suspension.

2.1.2 Preparation of InVO₄. InVO₄ was synthesized by hydrothermal method. In a typical procedure, In(NO₃)₃·4.5H₂O (1 mmol) was dissolved in 3 mL of HNO₃ (2 mol L⁻¹) followed by vigorous stirring 3 h for a uniform suspension. At the same time, NH₄VO₃ solid (2 mmol) was dissolved in 12 mL of deionized water to obtain a clear solution. The solution was added rapidly to the suspension and soon afterwards stirred for additional 3 h at room temperature. After carefully adjusting the pH value of 4 using 25 wt% NH₃·H₂O solution, the mixed solution was transferred into a 20 mL Teflon-lined steel autoclave, which was heated in an oven at 150 °C for 24 h. At last, the obtained InVO₄ was collected and washed with ethanol and distilled water several times, and dried at 100 °C for 2 h.

2.1.3 Preparation of RGO/InVO₄. The coupling of RGO with InVO₄ crystals was achieved by using chitosan as the linker molecule.²² First, a certain amount (5 mg) RGO was dispersed in the chitosan solution (0.50 wt%, 50.0 mL) and the pH was adjusted to 4.5 using 0.1 M HCl. This process would result in the formation of RGO–chitosan composite. Subsequently, a given concentration of InVO₄ crystal suspension (100 mg, 20.0 mL deionized water) was added to the RGO–chitosan solution under ultrasonication, which allowed for the chemical adsorption of RGO onto the InVO₄ surface. The products (RGO/InVO₄) were collected by centrifugation and dried in vacuum for later use. The theoretical wrapping amount of RGO was 5 wt%.

2.1.4 Preparation of TiO₂/InVO₄/RGO. The deposition of TiO₂ nanoparticles on the surface of InVO₄ crystals was carried out by low-temperature hydrothermal method with RGO/InVO₄ powders and Ti(SO₄)₂ as starting materials in aqueous solution. In a typical preparation process: 0.10 g of RGO/InVO₄ and 0.01 g of Ti(SO₄)₂ were added into 15 mL of deionized water. After stirring for 1 h, the obtained solution was transferred into a 20 mL Teflon-sealed autoclave and maintained at 120 °C for 9 h. Then, the resulting gray precipitate of TiO₂/InVO₄/RGO sample was isolated by centrifugation at 6000 rpm for 10 min and then the precipitate was dried in a vacuum oven at 60 °C for 2 h, and followed by heat-treated at 450 °C for 30 min. The products (TiO₂/InVO₄/RGO) were washed with ethanol and distilled water several times, and dried in vacuum. For comparison, the TiO₂/InVO₄ and TiO₂/RGO samples were prepared under the same conditions.

2.2. Characterization of photocatalysts

The crystal structures of the samples were characterized by X-ray diffraction (XRD) on a Rigaku (Japan) D/max 2500 X-ray diffractometer (Cu K_α radiation, λ = 0.154 18 nm). The morphologies and structure details of the as-synthesized samples were detected using field emission scanning microscopy (SEM, JSM-6510) and transmission electron microscopy (TEM, JEM-2100F). X-ray photoelectron spectroscopy (XPS) analysis was performed with an ESCALa-b220i-XL electron spectrometer (VGScientific, England) using 300 W Al K_α radiation. The specific surface areas of the samples were measured through nitrogen adsorption BET method (BET/BJH Surface Area, 3 H-2000PS1). The photoluminescence (PL) spectra of the photocatalysts were obtained by a F4500 (Hitachi, Japan) photoluminescence detector with an excitation wavelength of 325 nm. The UV-vis diffuse reflectance spectra (DRS) were recorded using a scan UV-vis spectrophotometer (UV-2550).

2.3. Photocatalytic activities studies

The photocatalytic properties of the as-prepared samples were evaluated using Rh B as a model compound. The Rh B is a very stable compound, which has been used widely as a representative reaction for examining the performance of numerous visible light active catalysts. In experiments, the Rh B solution (0.01 mmol L⁻¹, 100 mL) containing 0.02 g of photocatalyst was mixed in a pyrex reaction glass. A 300 W Xe lamp (λ > 420 nm) was employed to provide visible light irradiation. A 420 nm cut-off filter was inserted between the lamp and the sample to filter out UV light (λ < 420 nm). Prior to visible light illumination, the suspension was strongly stirred in the dark for 40 min. Then the solution was exposed to visible light irradiation under magnetic stirring. At given time intervals, 4 mL of the suspension was periodically collected and analyzed after centrifugation. The Rh B concentration was analyzed by a UV-2550 spectrometer to record intensity of the maximum band at 552 nm in the UV-vis absorption spectra.

2.4. Active species trapping experiments

For detecting the active species during photocatalytic reactivity, some sacrificial agents, such as 2-propanol (IPA), disodium ethylenediamine tetraacetic acid (EDTA-2Na) and 1,4-benzoquinone (BQ) were used as the hydroxyl radical (˙OH) scavenger, hole (h⁺) scavenger and superoxide radical (O₂˙⁻) scavenger, respectively. The method was similar to the former photocatalytic activity test with the addition of 1 mmol of quencher in the presence of Rh B.

3. Results and discussion

Fig. 1 shows the XRD patterns of TiO₂, InVO₄, RGO/TiO₂, TiO₂/InVO₄, RGO/InVO₄ and TiO₂/InVO₄/RGO. All the diffraction peaks in the pattern of InVO₄ can be indexed to the specific crystal planes of InVO₄ phase (JCPDS no. 48-0898),¹⁸ and all the diffraction peaks of TiO₂ can be indexed to the anatase phase (JCPDS no. 21-1272).¹⁵ Besides the diffraction peaks of InVO₄, a peak at 25.3° in the pattern of TiO₂/InVO₄ matches well with the (101) plane of anatase TiO₂,¹⁵ indicating the coexistence of TiO₂ and InVO₄ in the composite. RGO/InVO₄ exhibits a similar pattern as pure InVO₄ and TiO₂/InVO₄/RGO shows a similar pattern as TiO₂/InVO₄, suggesting that the incorporation of RGO has little influence on the phase structure of TiO₂ and InVO₄, which may be resulted from small crystal size or low percentage of RGO.

Fig. 1 XRD patterns of the as-synthesized samples: (a) TiO₂, (b) RGO/TiO₂, (c) InVO₄, (d) TiO₂/InVO₄, (e) RGO/InVO₄, (f) TiO₂/InVO₄/RGO.

Herein, the successful loading of RGO was verified by XPS and FTIR spectra. Fig. 2 represents the XPS survey spectra of the TiO₂/InVO₄/RGO. C 1s XPS spectrum (Fig. 2a) clearly shows three characteristic peaks, corresponding to nonoxygenated ring C bond (284.8 eV, including C–C, C=C, and C–H), the C–O in C–O–C or C–OH groups (286.1 eV), and the carbonyl C in C=O (287.0 eV), respectively.²² The In 3d peaks are detected at 453.31 eV and 445.79 (Fig. 2b), corresponding to the signals of In³⁺ species.²³ The peak around 453.37 eV is assigned to Ti 2p_3/2 of TiO₂, corresponding to Ti⁴⁺ (Fig. 2c).²⁴ The V 2p peak is centered at about 515.38 eV (Fig. 2d), corresponding to V⁵⁺.²³ The structural information of the as-prepared RGO and TiO₂/InVO₄/RGO samples was also confirmed by the FTIR analysis (Fig. 3). In the spectrum of RGO, the peaks at 1740, 1634, 1381, and 1041 cm⁻¹ are assigned to the vibrations of oxygen-containing groups carboxyl C=O, C=C, carboxyl C–O, and C–O–C,¹⁶ respectively. Furthermore, all characteristic absorption bands of RGO are observed in the spectrum of TiO₂/InVO₄/RGO, indicating the successful loading of RGO.

Fig. 2 XPS spectra of the as-obtained TiO₂/InVO₄/RGO sample: (a) C 1s spectrum, (b) In 3d spectrum, (c) Ti 2p spectrum, (d) V 2p spectrum.

Fig. 3 FTIR spectra of the as-prepared RGO and TiO₂/InVO₄/RGO samples.

The morphology and microstructure of the as-prepared photocatalysts were further studied by SEM and TEM. The bare InVO₄ crystals are of regular spherical shape with an average diameter of several micrometers (Fig. 4a and c). The morphology of TiO₂/InVO₄/RGO sample is microspheres with size of 200 nm–10 μm (Fig. 4b). TiO₂/InVO₄, RGO/InVO₄, and TiO₂/InVO₄/RGO samples show a similar morphology to pure InVO₄ sample, except that the surfaces of these composites are rough (Fig. 4d–f). The formation of the RGO/BiVO₄/InVO₄ heterostructure was confirmed by the elemental mapping of the TiO₂/InVO₄/RGO sample (Fig. 5a–d). Maps of In–L, V–K, Ti–K, and C–K have the same shape and location, which demonstrates the existence of TiO₂, InVO₄, and RGO in the as-fabricated composite. This gives solid evidence of the formation of TiO₂/InVO₄/RGO heterostructure. EDX elemental microanalysis confirms Ti, In, V, C, and O as major elements in the ternary heterostructured TiO₂/InVO₄/RGO composite (Fig. 5e). In order to further ascertain the heterostructure among TiO₂, InVO₄ and RGO, TiO₂/InVO₄/RGO sample was investigated by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM), as displayed in Fig. 6. Fig. 6a shows the spherical structure of the as-synthesized TiO₂/InVO₄/RGO composite. HRTEM image of the TiO₂/InVO₄/RGO sample further confirms the formation of a novel ternary heterostructure (Fig. 6b). An interconnected fine particulate morphology observed indicates the existence of the ternary heterostructure. By measuring the lattice fringes, the resolved interplanar distances are about 0.249 nm, 0.389 nm, and 0.338 nm, corresponding to the (101) plane of TiO₂, the (111) plane and the (−220) plane of InVO₄, respectively.

Fig. 4 SEM images of as-synthesized InVO₄ (a and c), TiO₂/InVO₄/RGO (b and f), TiO₂/InVO₄ (d), RGO/InVO₄ (e).

Fig. 5 The corresponding EDS elemental mapping images (a–d) and EDX spectrum (e) of TiO₂/InVO₄/RGO.

Fig. 6 The TEM (a) and HRTEM (b) images of as-synthesized TiO₂/InVO₄/RGO sample.

The optical absorption properties of the photocatalysts were studied by UV-vis DRS spectra. As shown in Fig. 7a, bare TiO₂ exhibits strong absorbance in wavelengths shorter than 400 nm, and bare InVO₄ shows strong absorbance in wavelengths shorter than 500 nm. The TiO₂/InVO₄ shows a similar absorption property to bare InVO₄, while the RGO/InVO₄ shows much stronger absorption than InVO₄ and TiO₂/InVO₄ in the visible light range due to the existence of RGO.¹⁶ The TiO₂/InVO₄/RGO exhibits a similar absorption curve to that of RGO/InVO₄ in the visible light region, indicating that the improved visible light absorption of InVO₄ mainly arises from the hybrid with RGO rather than with TiO₂.²¹ It also can be seen that all the absorbance edges of RGO based composites show significant red shift, and the absorption intensities also increase in the visible-light region, which therefore improve the utilization of the solar spectrum. These results show that the as-prepared TiO₂/InVO₄/RGO composite can be potentially used in solar energy application. The band gap energies of the pure TiO₂ and InVO₄ can be obtained through the following formula:

αhν = A(hν − E_g)^n/2 (1)

where α, ν, E_g and A are the absorption coefficient, the light frequency, the band-gap energy, and a constant, respectively. Among these parameters, n is determined by the type of optical transition of a semiconductor (n = 1 for a direct transition and n = 4 for an indirect transition). For TiO₂ and InVO₄, the value of n is 1 for the direct transition.^15,24 According to eqn (1), the band-gap energies (E_g) of TiO₂ and InVO₄ can be estimated from a plot of (ahν)² versus energy (hν), as illustrated in Fig. 7b. Thus, the band gaps of the as-prepared TiO₂ and InVO₄ are calculated to be 3.11 eV and 2.40 eV, respectively.

Fig. 7 (a) UV-vis DRS of as-synthesized samples. (b) Plots of (αhν)² versus photon energy (hν) for the band gap energies of TiO₂ and InVO₄.

In our experiment, heterostructure formed in the TiO₂/InVO₄/RGO composite played an important role in the efficient separation of photoinduced electron–hole pairs. The band positions of TiO₂ and InVO₄ were calculated by the following empirical formulas:

E_VB = X − E^e + 0.5E_g (2)

E_CB = E_VB − E_g (3)

where E_VB is the valence band edge potential, E_CB is the conduction band edge potential, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, E^e is the energy of free electrons on the hydrogen scale (about 4.5 eV), E_g is the band gap energy of the semiconductor. Using the eqn (2) and (3), the CB and VB edge potentials of TiO₂ are calculated to be −0.01 eV and 3.10 eV, respectively. And the CB and VB edge potentials of InVO₄ are determined to be −0.70 eV and 1.70 eV, respectively. The energy band structure diagram of TiO₂, InVO₄, and RGO is thus schematically illustrated, as shown in Scheme 1. Under visible light irradiation, InVO₄ can be excited to produced h⁺ and e. Under normal case, most of electrons–holes pairs recombine rapidly, thus pure InVO₄ has a respectively low photocatalytic activity. Due to the well-matched overlapping band-structures and intimate interfaces of TiO₂/InVO₄/RGO, photogenerated electrons on the CB of InVO₄ can easily be transferred to the CB of TiO₂ (electron transfer I: InVO₄ CB → TiO₂ CB). In addition, RGO may serve as an effective electron acceptor for InVO₄, due to its high electrical conductivity. Thus, photogenerated electrons on the CB of InVO₄ also can effectively migrate to RGO (electron transfer II: InVO₄ CB → RGO). The ternary heterostructure formed among InVO₄, TiO₂, and RGO prevents electrons and holes from recombination, and thus the photocatalytic activity is enhanced greatly.

Scheme 1 Schematic diagram of the separation and transfer of photogenerated charges in the heterostructure under visible light irradiation.

The photocatalytic performance of TiO₂/InVO₄/RGO was evaluated by degradation of Rh B under visible light. The TiO₂, InVO₄, RGO/TiO₂, TiO₂/InVO₄ and RGO/InVO₄ samples were also studied under the same conditions for comparison. Fig. 8a shows the photocatalytic degradation curves of Rh B as a function of time. Before light irradiation, the solution of Rh B and photocatalyst was magnetically stirred in dark for 40 min to establish an adsorption desorption equilibrium. The decrease of the Rh B concentration due to the adsorption of RGO/TiO₂ (5%), RGO/InVO₄ (9%) or TiO₂/InVO₄/RGO (around 10%) is much higher than that of InVO₄ (2.2%) and TiO₂ (2%) samples, indicating that RGO in the composites can enhance the absorption of organic molecules on the surface of photocatalysts. The enhanced adsorptivity is mainly attributed to the π–π stacking and electrostatic attraction between Rh B and aromatic regions of the high-surface-area RGO.^25,26

Fig. 8 (a) Photodegradation efficiencies of Rh B as a function of irradiation time for different photocatalysts. (b) Cycling runs for the photocatalytic degradation of Rh B over TiO₂/InVO₄/RGO sample under visible light irradiation.

N₂ adsorption desorption isotherms (Fig. S1†) of the as-fabricated InVO₄, TiO₂/InVO₄, RGO/InVO₄, and TiO₂/InVO₄/RGO samples were performed to determine the surface areas of the samples. The BET surface areas of the as-prepared InVO₄, TiO₂/InVO₄, RGO/InVO₄, TiO₂/InVO₄/RGO samples are 6.32, 8.29, 28.24 and 33.60 m² g⁻¹, respectively. The larger BET surface area can facilitate more efficient contact of TiO₂/InVO₄/RGO composite with organic contaminants, which is beneficial to the improvement of photocatalytic efficiency.

From the catalytic experiments, TiO₂/InVO₄/RGO sample was detected to be more photoactive towards Rh B solution than the pure TiO₂, InVO₄, TiO₂/InVO₄, RGO/TiO₂, and RGO/InVO₄ sample. In addition, the as-prepared TiO₂/InVO₄/RGO sample showed a higher photocatalytic activity compared with TiO₂/InVO₄ or RGO/InVO₄ reported in the literatures,^15,16 revealing that the ternary heterostructured photocatalysts with two transfer channels can further enhance the photo-generated charges separation and result in better photocatalytic performance than the binary heterostructured composites. The RGO coating can not only improve the visible light absorption efficiency (Fig. 7a) but also enhance the adsorptivity of the photocatalyst (Fig. S1†), which are both beneficial for the photocatalyst to photolyze Rh B, thus enhancing the photocatalytic performance of the composite. Besides, efficient heterostructure interface between two (or three) components can restrain the recombination of photoinduced charges effectively.^27–31 The comparison of PL spectra of the pure InVO₄, TiO₂/InVO₄, RGO/InVO₄, and TiO₂/InVO₄/RGO samples under the excitation wavelength of 325 nm is shown in Fig. S2.† It can be observed that the PL peak intensity of TiO₂/InVO₄/RGO decrease obviously. This result shows that the heterostructure effect of TiO₂, InVO₄, and RGO contributes to the effective electron–hole pair separation, which may be a reason for the TiO₂/InVO₄/RGO sample showing enhanced photocatalytic performance under visible light irradiation.

In addition to photocatalytic activity, the stability of photocatalyst is another important issue. In order to investigate the stability of the photocatalyst, five runs of cycling photodegradation experiments have been carried out for TiO₂/InVO₄/RGO (Fig. 8b). The photocatalyst still maintained a high level of degradation activity in the repeated experiments, indicating that the photocatalyst showed good stability during the photocatalytic reaction.

It is important to detect the main oxidative species in the photocatalytic process for revealing the photocatalytic mechanism. The main oxidative species in the Rh B photocatalytic degradation process were detected through the trapping experiments of radicals and holes in the presence of various scavengers including 2-propanol (IPA, hydroxyl radical scavenger), 1,4-benzoquinone (BQ, superoxide radical scavenger) and EDTA-2Na (hole scavenger).³² As revealed in Fig. 9, under the visible-light irradiation of the as-prepared TiO₂/InVO₄/RGO composite, the photodegradation rate of Rh B decreased after the addition of hydroxyl radical scavenger IPA as well as hole scavenger EDTA-2Na. The photodegradation rate of Rh B was decelerated significantly after the addition of superoxide radical scavenger BQ. The above results show that the active species including O₂˙⁻, h_VB⁺, and ˙OH are responsible for the degradation of Rh B over the TiO₂/InVO₄/RGO composite under visible light illumination.

Fig. 9 Trapping experiment of active species during the photocatalytic degradation of Rh B over TiO₂/InVO₄/RGO sample under visible light irradiation.

On the basis of aforementioned experimental data and analysis, the potential electrons transfer route and photocatalytic mechanism for Rh B degradation over TiO₂/InVO₄/RGO heterostructure is presented as shown in Scheme 2. Under visible light irradiation, InVO₄ are excited to produced h⁺ and e⁻. Due to the well-matched overlapping band-structures and intimate interfaces of TiO₂/InVO₄/RGO, the electrons on the CB of InVO₄ can migrate to TiO₂ and then react with adsorbed O₂ to produce superoxide radical ˙O₂⁻, which enhance the separation efficiency of photogenerated electrons and holes of InVO₄. Meanwhile, electrons on the CB of InVO₄ also can be transferred to the RGO, and then ˙O₂⁻ obtained. These ˙O₂⁻ can react with H⁺ to produce hydroxyl radical ˙OH. Both these radicals are strong oxidants that can completely oxidize organic molecules to water and carbon dioxide.³² The efficient charges separation leads to the enhancement of photocatalytic activity.³² In addition to the advantage of the improved charges separation, other properties of the composite, such as the strong adsorption ability for organic molecule due to the loading of RGO, the narrow band-gap and high light absorption in visible-light-region also contribute to the improved photocatalytic performance of TiO₂/InVO₄/RGO.

Scheme 2 Schematic illustration of the charge separation and photocatalytic mechanism of TiO₂/InVO₄/RGO heterostructure under visible light irradiation.

4. Conclusions

In summary, a novel TiO₂/InVO₄/RGO heterostructure has been successfully synthesized. The TiO₂/InVO₄/RGO exhibited superior photocatalytic activity to bare InVO₄, TiO₂, TiO₂/InVO₄, RGO/TiO₂, and RGO/InVO₄ samples in degradation of Rh B under visible light irradiation. It is suggested that, due to the alignments of the electronic band structures of TiO₂, InVO₄ and RGO, the photogenerated electrons in the CB of InVO₄ can be quickly transferred to RGO through an electron transfer channel of InVO₄ → RGO, while the photogenerated electrons can migrate to the CB potentials of TiO₂ through another electron transfer channel of InVO₄ → TiO₂. The dual-channel for photogenerated charges separation at the interfaces of TiO₂/InVO₄/RGO finally suppresses the recombination of photo-generated electron–hole pairs and leads to an enhanced photocatalytic performance of TiO₂/InVO₄/RGO. The concept of constructing multifunctional InVO₄-based photocatalyst by establishing dual transfer channels for photogenerated charges separation can also be extended to develop other high efficient photocatalysts.

Acknowledgements

This work was supported by National Natural Science Foundation of China (21407059, 21576112), China Postdoctoral Science Foundation (2013M531286), the Science Development Project of Jilin Province (20130522071JH), and the Science and Technology Research Project of the Department of Education of Jilin Province (2015220).

References

1. M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemannt, Chem. Rev., 1995, 95, 69.
2. Y. Mao and S. S. Wong, J. Am. Chem. Soc., 2006, 128, 8217.
3. R. Mohan, K. Krishnamoorthy and S. J. Kim, Solid State Commun., 2012, 152, 375.
4. L. Zhang, W. Z. Wang, S. M. Sun, Y. Y. Sun, E. P. Gao and Z. J. Zhang, Appl. Catal., B, 2014, 148–149, 164.
5. Z. G. Yi, J. H. Ye, N. Kikugawa, T. Kako, S. X. Ouyang, H. Stuart-Williams, H. Yang, J. Y. Cao, W. J. Luo, Z. S. Li, Y. Liu and R. L. Withers, Nat. Mater., 2010, 9, 559.
6. S. Fukuzumi, T. Kobayashi and T. Suenobu, Angew. Chem., Int. Ed., 2011, 50, 728.
7. H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri and J. Ye, Adv. Mater., 2012, 24, 229.
8. Y. Li, M. H. Cao and L. Y. Feng, Langmuir, 2009, 25, 1705.
9. L. Ge, M. X. Xu and H. B. Fang, Mater. Lett., 2007, 61, 63.
10. A. Martirosyan, A. Bazes and Y. J. Schneider, Nanotoxicology, 2014, 8, 573.
11. J. Mao, T. Y. Peng, X. H. Zhang, K. Li and L. Zan, Catal. Commun., 2012, 28, 38.
12. J. Ye, Z. Zou, M. Oshikiri, A. Matsushita, M. Shimoda, M. Imai and T. Shishido, J. Photochem. Photobiol., A, 2002, 148, 79.
13. Y. Li, S. H. Jiang, J. R. Xiao and Y. D. Li, Int. J. Hydrogen Energy, 2014, 39, 731.
14. Y. X. Qin, G. T. Fan, K. X. Liu and M. Hu, Sens. Actuators, B, 2014, 190, 141.
15. H. Chang, E. H. Jo, H. D. Jang and T. O. Kim, Mater. Lett., 2013, 92, 202.
16. J. F. Shen, X. F. Li, W. S. Huang, N. Li and M. X. Ye, Mater. Res. Bull., 2013, 48, 3112.
17. J. Chaisorn, K. Wetchakun, S. Phanichphant and N. Wetchakun, Mater. Lett., 2015, 160, 75.
18. W. L. Shi, F. Guo, J. B. Chen, G. B. Che and X. Lin, J. Alloys Compd., 2014, 612, 143.
19. X. Lin, X. Y. Guo, W. L. Shi, L. N. Zhao, Y. S. Yan and Q. W. Wang, J. Alloys Compd., 2015, 635, 256.
20. X. Lin, X. Y. Guo, W. L. Shi, F. Guo, G. B. Che, H. J. Zhai, Y. S. Yan and Q. W. Wang, Catal. Commun., 2015, 71, 21.
21. B. Q. Lu, N. Ma, Y. P. Wang, Y. W. Qiu, H. H. Hu, J. H. Zhao, D. Y. Liang, S. Xu, X. Y. Li, Z. Y. Zhu and C. Cui, J. Alloys Compd., 2015, 630, 163.
22. W. Zhao, J. H. Li, Z. B. Wei, S. M. Wang, H. He, C. Sun and S. G. Yang, Appl. Catal., B, 2015, 179, 9.
23. F. Guo, W. L. Shi, X. Lin, X. Yan, Y. Guo and G. B. Che, Sep. Purif. Technol., 2015, 141, 246.
24. Y. L. Min, K. Zhang, Y. C. Chen and Y. G. Zhang, Ultrason. Sonochem., 2012, 19, 883.
25. J. P. Zou, J. Ma, Q. Huang, S. L. Luo, J. Yu, X. B. Luoa, W. L. Dai, J. Sun, G. C. Guo, C. T. Au and S. L. Suib, Appl. Catal., B, 2014, 156–157, 447.
26. P. F. Wang, Y. H. Ao, C. Wang, J. Hou and J. Qian, Carbon, 2012, 50, 5256.
27. S. M. Wang, D. L. Li, C. Sun, S. G. Yang, Y. Guan and H. He, Appl. Catal., B, 2014, 144, 885.
28. S. X. Yang, Q. Yue, F. M. Wu, N. J. Huo, Z. H. Chen, J. H. Yang and J. B. Li, J. Alloys Compd., 2014, 597, 91.
29. N. J. Huo, Q. Yue, J. H. Yang, S. X. Yang and J. B. Li, ChemPhysChem, 2013, 14, 4069.
30. S. X. Yang, X. J. Cui, J. Gong and Y. L. Deng, Chem. Commun., 2013, 49, 4676.
31. X. Yang, H. Y. Yang, H. Y. Ma, S. Guo, F. Cao, J. Gong and Y. L. Deng, Chem. Commun., 2011, 47, 2619.
32. D. L. Jiang, L. L. Chen, J. J. Zhu, M. Chen, W. D. Shi and J. M. Xie, Dalton Trans., 2013, 42, 15726.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17676j

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