The p–n heterojunction with porous BiVO₄ framework and well-distributed Co₃O₄ as a super visible-light-driven photocatalyst

Abstract

The p–n heterojunction with mesoporous BiVO₄ framework well-distributed Co₃O₄ is fabricated. The mesoporous structure is prepared by nanocasting technique using KIT-6 as template, while the incorporation of Co₃O₄ particles is completed by the impregnation technique. Transmission electron microscopy (TEM) image shows that Co₃O₄ particles have been successfully loaded in the framework of the mesoporous BiVO₄. Energy dispersive spectrum (EDS) mapping reveals that Co₃O₄ particles distribute uniformly in the sample, which is the result of the template effect of the mesoporous structure. The photocatalytic removal rate of RhB with BiVO₄ is enhanced by the construction of the p–n heterojunction of Co₃O₄/mesoporous BiVO₄. This increase can be attributed to the enhanced separation efficiency of the photogenerated charge carriers produced by the p–n heterojunction. Mesoporous BiVO₄ enables the guest material of Co₃O₄ to be well distributed in the matrix of the mesoporous BiVO₄, thereby providing a large contact area of Co₃O₄ and BiVO₄. This promotes the charge transferring across the interface, thereby increasing the separation of electron–hole pairs, and leading to the enhancement of photocatalytic ability. Furthermore, the mesoporous structure itself contributes to the enhanced photocatalytic performance.

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1. Introduction

Semiconductor photocatalysis has attracted considerable attention because of its promising performance in environmental pollution control.^1–3 TiO₂ has been proved to be an excellent photocatalyst due to its powerful photocatalytic activity and outstanding stability.^4–6 However, the wide band gap (3.2 eV) of TiO₂ limits the utilization of visible light, which accounts for a large part of the solar energy. Recently, BiVO₄, with a narrow band gap of 2.4 eV, has been the focus of the field of photocatalysis. It satisfies the basic requirements as a photocatalyst for pollution control, including being responsive to visible light,⁷ and possessing good stability.^8,9 However, the low light absorption efficiency and fast recombination of photogenerated electron–hole pairs still limit the photocatalytic activity of BiVO₄. Therefore, a few attempts have been devoted for improving the photocatalytic performance of BiVO₄, such as porous structure construction,^10,11 noble metal modification,^12,13 and heterojunction composite fabrication.^14,15

Coupling BiVO₄ with another semiconductor to form a heterojunction (especially a p–n heterojunction) can restrain the recombination of the photogenerated electron–hole pairs, and then enhance the photocatalytic activity of BiVO₄. The large contact area of the two semiconductors can promote the charge transfer across the interface, thereby increasing the separation of electron–hole pairs, leading to the enhancement of photocatalytic ability. Thus, it is an ideal method to increase the contact area of semiconductors in the heterojunction to bring about enhanced photocatalytic ability.¹⁶

Mesoporous structure construction can also enhance the photocatalytic performance.^17,18 The construction of mesoporous structures in the bulk BiVO₄ can elevate the surface area of the photocatalyst to offer a large number of reactive sites.^19,20 It can also enhance the light absorption efficiency because of more photons being distributed onto the surface of the photocatalyst, using the pores as light transfer paths. In addition, mesoporous materials themselves can be used as a support of guest materials.²¹ Mesopores in the structure enable the guest materials to get highly distributed and restrain them to be small particles.^22,23 High distribution and the small particle nature of the guest materials in the framework of the host can enhance their contact area. Inspired by this idea, Co₃O₄ was chosen as the guest material to be loaded in the mesoporous BiVO₄ to form the p–n heterojunction. It is a p-type semiconductor and a visible-light-driven photocatalyst with a band gap of 2.07 eV.²⁴ The conduction band level (E_CB) of Co₃O₄ is +0.37 eV vs. NHE, which is more positive compared to that of BiVO₄ (E_CB = 0 eV vs. NHE) and the valence band (E_VB) level of Co₃O₄ is +2.44 eV vs. NHE, which is also more positive than that of BiVO₄ (E_VB = 2.40 eV vs. NHE). The matching of band levels and the types between Co₃O₄ and BiVO₄ makes Co₃O₄ a suitable material for constructing a p–n heterojunction with BiVO₄.

Herein, Co₃O₄/mesoporous BiVO₄ p–n heterojunction with a large contact area is first fabricated to get enhanced photocatalytic ability under visible light. The distribution role of mesoporous structure in Co₃O₄/mesoporous BiVO₄ as a visible-light-driven photocatalyst is discussed. RhB, a common pollutant in industry wastewater, is chosen as a test substance to evaluate the photocatalytic performance of the as-prepared samples under visible light.

2. Experimental section

2.1. Sample preparation

2.1.1. Preparation of mesoporous silica KIT-6. The mesoporous silica KIT-6 with cubic Ia3̄d symmetry was prepared as follows:²⁵ 6 g of P123 was dissolved in 217 mL of distilled water to which 10 mL of concentrated HCl (35%) and 7.41 mL of butanol (99.4%) were added under stirring for 1 h at 35 °C. 13.8 mL of TEOS (98%) was added at 35 °C (TEOS–P123–HCl–H₂O–BuOH = 1 : 0.017 : 1.83 : 195 : 1.31 in mole ratio). After stirring at 35 °C for 24 h, the mixture was transferred to an enclosed polypropylene bottle, for hydrothermal strategy at 100 °C for 6 h. The product was filtered and washed three times with deionized water and absolute ethanol, and dried at 100 °C for 5 h. Finally, the solids were calcined at 550 °C (2 °C min⁻¹) for 5 h.

2.1.2. Preparation of mesoporous BiVO₄. 2 mmol of Bi(NO₃)₃·5H₂O was dissolved in 10 mL of 2 M HNO₃ under ultrasound, and a transparent colorless solution was obtained. The same molar mass of NH₄VO₃ was added to the solution under ultrasound for 30 min to form a bright yellow solution. Then, 0.3 g of prepared KIT-6 was added. After the complete immersion of the KIT-6, the mixture was placed into the electrothermal constant-temperature dry box at 80 °C for 12 h to completely fill the BiVO₄ precursor into the void of KIT-6. Then, the dried sample was converted by calcination at 400 °C for 12 h. Finally, the mesoporous BiVO₄ was obtained with a 2 M NaOH aqueous solution to eliminate KIT-6. The final product was filtered and washed with distilled water and absolute ethanol. Reference BiVO₄ was prepared by the same procedure without the KIT-6.

2.1.3. Preparation of Co₃O₄/BiVO₄ composites. The loading of Co₃O₄ on BiVO₄ was completed by the impregnation method from an aqueous solution of Co(NO₃)₂. The preparation of Co₃O₄/mesoporous BiVO₄ was carried out by the following typical procedure: mesoporous BiVO₄ powder (1.0 g) was added to 10 mL solution of Co(NO₃)₂ containing an appropriate amount of Co(NO₃)₂ as 2% Co (recorded by S₁), 4% Co (recorded by S₂), and 6% Co (recorded by S₃) to BiVO₄ in a beaker. The suspension was stirred during the evaporation of water under the irradiation of an infrared light. The resulting powder was collected and calcined in air at 300 °C for 2 h. Co₃O₄/BiVO₄ was prepared by the same procedure with mesoporous BiVO₄ replaced by BiVO₄, and the mass percentage of Co to BiVO₄ was 4%.

2.2. Sample characterization

X-ray diffraction (XRD) spectra were recorded by Rigaku D/MAX-2400 with Cu Kα radiation, accelerating voltage of 40 kV, current of 30 mA. The scanning rate was 8° (2θ) min⁻¹, and the scanning range was 10°–80°. The light absorption intensities were measured using a UV-vis spectrophotometer (Shimadzu, UV-2450) with a wavelength range of 200–800 nm. The micro-morphologies of the samples were obtained using a transmission electron microscopy (TEM; JEM-2100(UHR) JEOL). Energy dispersive spectrum (EDS) measurements were performed using a scanning electron microscope (SEM, JSM-6460LV) with an energy spectrometer (X-Max50). X-ray Photoelectron Spectroscopy (XPS) of the samples was obtained by X-ray photoelectron spectrometer (AMICUS). Fluorescence (FL) spectra were recorded by FL spectrometer (LS-55, PE). Tristar 3000 was used to examine the N₂ adsorption and desorption properties of the as-prepared samples at 77 K. The surface photovoltage spectra (SPS) were recorded by surface photovoltage measurement system, which consists of a monochromator (model Omni-λ3005) and a lock-in amplifier (model SR830-DSP) with an optical chopper (model SR540) running at a frequency of 20 Hz.

2.3. Measurement of photocatalytic activity

The photocatalytic activities of the as-obtained samples were monitored through the photodegradation of RhB under visible light irradiation. Photocatalytic reactions were conducted in a 100 mL cubic quartz reactor. A 300 W Xe lamp was employed as the visible light source. The light was passed through a filter, which shielded all wavelengths below 420 nm. In all the experiments, photocatalysts (0.10 g) were added to 100 mL RhB aqueous solution (5 mg L⁻¹). During each photocatalytic experiment, 5 mL of the suspension was collected at predetermined time intervals. The suspension was centrifuged at 9500 rpm for 10 min, and the concentration of RhB in the supernatant was determined by measuring the absorbance at λ = 553 nm with a Shimadzu UV2000 spectrophotometer. The adsorption ability of photocatalysts was measured under the same condition without the light illumination.

3. Results and discussion

3.1. Crystal structure

The XRD patterns of BiVO₄, Co₃O₄/BiVO₄, mesoporous BiVO₄, S₁, S₂, and S₃ are shown in Fig. 1(a). Clear characteristic peaks with 2θ at 18.8°, 28.6°, 30.5°, 35.2°, 39.7° and 53.1° are observed. This indicates that BiVO₄ in the samples are monoclinic scheelite structure, which could be indexed to the standard cards (JCPDS no. 14-0688). It is clear that the processes of constructing the mesoporous structure and loading Co₃O₄ do not influence the crystal form of BiVO₄. No diffraction peaks of Co species are observed over the composite samples, which is because of the small crystallite size or low concentration of Co species. To determine the existence and species of Co, XPS was recorded and that of S₂ is shown in Fig. 2(b). The peaks ascribed to Bi, V, O and Co are found, certifying the existence of Co species. The high-resolution spectrum for Co_2p (Fig. 3(b)) shows two major peaks with binding energies at 779.3 and 195.2 eV, corresponding to Co 2p_2/3 and 2p_1/3, respectively, which is the characteristic of a Co₃O₄.²⁶

Fig. 1 (a) XRD patterns of BiVO₄, Co₃O₄/BiVO₄, mesoporous BiVO₄, S₁, S₂ and S₃ and XPS spectra of (b) S₂ and (c) Co_2p (high resolution).

Fig. 2 TEM images of (a) BiVO₄, (b) mesoporous BiVO₄, (c) S₂ and (d) the rectangular area of (c), inset is the HRTEM image of the rectangular area of (d).

Fig. 3 EDS mapping of Co element of Co₃O₄/BiVO₄ (a) and S₂ (b).

3.2. Morphologies

The particle size of BiVO₄ based on TEM image (Fig. 2(a)) is about 500 nm. The big particle size and nonporous structure lead to a low surface area of BiVO₄ (1.54 m²). From Fig. 2(b), the replica of KIT-6 with several mesopores in BiVO₄ is determined. The determined surface area of mesoporous BiVO₄ is 41.2 m² g⁻¹, which is highly larger than that of BiVO₄. This enhancement can be attributed to the mesoporous structure and the small size of the BiVO₄ particles. The big surface area of mesoporous BiVO₄ can provide more sites for the loading of foreign materials. Fig. 2(c) and (d) show typical TEM images of S₂. Most pores in mesoporous BiVO₄ are filled with Co₃O₄ particles. It can be measured from HRTEM image (inset of Fig. 2(d)) that d-spacings are 0.308 and 0.243 nm, which are in agreement with the (121) plane of BiVO₄ and the (311) plane of Co₃O₄, respectively. It is apparent that the Co₃O₄ particles have been successfully loaded in the mesoporous BiVO₄. To investigate the distribution of Co₃O₄ in mesoporous BiVO₄, the Co element distribution of Co₃O₄/BiVO₄ and Co₃O₄/mesoporous BiVO₄ is studied using EDS mapping. The EDS mapping image indicates that the Co element is well dispersed in Co₃O₄/mesoporous BiVO₄ (Fig. 3(b)). This can be attributed to the template action of the mesoporous structure of the composite, which can highly distribute the guest materials and modulate the guest materials to be small particles. The good dispersion of Co element in the framework of mesoporous BiVO₄ can enhance the contact area of Co₃O₄ and BiVO₄, and thus elevate the photocatalytic ability of Co₃O₄/mesoporous BiVO₄. From Fig. 3(a), compared with that of Co₃O₄/mesoporous BiVO₄, a few accumulation of Co element on BiVO₄ particles is determined, suggesting that the Co element in Co₃O₄/BiVO₄ is not well distributed. The proposed loading mechanism of Co₃O₄ on the surface of BiVO₄ and mesoporous BiVO₄ is illustrated in Scheme 1.

Scheme 1 The proposed loading mechanism of Co₃O₄ on the surface of BiVO₄ and mesoporous BiVO₄.

3.3. Separation ability of photogenerated charge carriers

FL analysis is used to reveal the separation efficiency of the photogenerated electrons²⁷ and holes in semiconductors, and the results are shown in Fig. 4(a). Considering that the FL emission results from the free charge carrier recombination, the lower peak indicates lower the rate of recombination of photogenerated charge carriers. As can be seen from this figure, the FL intensity of BiVO₄ is the highest, suggesting that BiVO₄ has the highest recombination rate of photogenerated charge carriers. The results confirm that the separation of photogenerated electrons and holes can be improved by the construction of mesoporous BiVO₄ or/and the formation of p–n heterojunctions between Co₃O₄ and BiVO₄, leading to the enhancement of photocatalytic activity. The FL intensity of Co₃O₄/BiVO₄ is higher than those of Co₃O₄/mesoporous BiVO₄. Moreover, the mesoporous structure, the larger contact area of Co₃O₄/mesoporous BiVO₄ caused by the good distribution of Co₃O₄ in the composite contributes to the efficient separation of photogenerated of electrons and holes of Co₃O₄/mesoporous BiVO₄. The FL intensities of S₁, S₂ and S₃ are different, and S₂ gets the lowest. The reason why the FL intensity of S₁ is higher than S₂ is the lower content of Co, which affects the formation of the p–n heterojunctions structure. However, upon the introduction of considerably high Co₃O₄ into mesoporous BiVO₄, the pores and channels of samples may get crammed by Co₃O₄. On the basis of the low recombination rate of S₂, it could be expected that S₂ should have high photocatalytic ability. SPS is one of indexes evaluating the separation efficiency of the photogenerated holes and electrons.²⁸ In general, the larger surface photovoltage the photocatalyst has, the better the separation ability of the photogenerated carriers. The SPS are recorded, and the results are shown in Fig. 4(b). The result obtained by the analyses of FL is further confirmed by SPS result.

Fig. 4 (a) FL spectra and (b) SPS of BiVO₄, Co₃O₄/BiVO₄, mesoporous BiVO₄, S₁, S₂ and S₃.

3.4. Optical absorption

The color of the Co₃O₄/mesoporous BiVO₄ composite powder is dark green and becomes darker and darker with increasing Co content. The UV-vis diffuse reflectance spectra of the BiVO₄, Co₃O₄/BiVO₄, mesoporous BiVO₄, S₁, S₂ and S₃ are shown in Fig. 5(a). All the samples with mesoporous structure (mesoporous BiVO₄, S₁, S₂ and S₃) show enhanced absorption intensity over BiVO₄, indicating the benefits of the mesoporous structure for the absorption of the photons. This result has also been obtained in other published work.²⁹ Moreover, obvious red shifts of the band gap edge are observed in the spectra of Co₃O₄/BiVO₄, S₁, S₂ and S₃. The calculated band gaps of Co₃O₄/BiVO₄, S₁, S₂ and S₃ are 2.34, 2.20, 2.09 and 2.31 eV (shown in Fig. 5(b)), which are smaller than that of BiVO₄ (2.43 eV). This should be attributed to the small band gap of Co₃O₄ (2.07 eV). Among Co₃O₄/mesoporous BiVO₄ composites, S₂ has the strongest absorption intensity and the narrowest band gap. Thus, under the same light intensity, S₂ can absorb more photons, and the enhanced photocatalytic ability can be anticipated.

Fig. 5 (a) UV-vis absorption spectra of BiVO₄, Co₃O₄/BiVO₄, mesoporous BiVO₄, S₁, S₂ and S₃ and (b) calculation of the band gap by Kubelka–Munk function of BiVO₄, Co₃O₄/BiVO₄, S₁, S₂ and S₃.

3.5. Adsorption ability

Adsorption ability of the target substance is very important for the photocatalytic reaction. After 70 min, the removal rate of MB in the solution with mesoporous BiVO₄ is 20.0%, whereas that with BiVO₄ is 4.15% (Fig. 6). It is obvious that the adsorption ability of BiVO₄ is enhanced by the mesoporous structure, which can be attributed to the increase of the surface area. The adsorption ability of mesoporous BiVO₄ slightly decreases when Co₃O₄ is loaded on it. The decreased surface area of S₂ (29.3 m² g⁻¹) results in this decline.

Fig. 6 RhB concentration C_t/C₀ vs. time for RhB adsorption.

3.6. Photocatalytic activity and stability

In this experiment, RhB was adopted as a typical pollutant to evaluate the photocatalytic activity of photocatalysts under visible light irradiation. The variation of RhB concentration (C_t/C₀) vs. irradiation time with photocatalysts is shown in Fig. 7(a). In 70 min, only 15.7% of RhB is removed with BiVO₄, while the RhB removal rate with mesoporous BiVO₄ is 53.8%, indicating that the mesoporous structure dramatically enhances the photocatalytic ability of BiVO₄. The kinetic analysis shows that the photocatalytic reaction with BiVO₄ can be considered as a first order reaction (see Fig. 7(b)), and calculated k value of mesoporous BiVO₄ is 0.0106 min⁻¹, which is higher than that of BiVO₄ (0.00228 min⁻¹). The enhanced light absorption ability, adsorption ability and separation efficiency of the photogenerated charge carriers are responsible for this increase. In 70 min, the RhB removal rate with S₂ is 90.3%. The k value of the S₂ (0.0323 min⁻¹) is nearly three times as high as that of mesoporous BiVO₄. The loading of Co₃O₄ on the mesoporous BiVO₄ to form the p–n heterojunction can enhance the photocatalytic performance of mesoporous BiVO₄. To confirm the heterojunction effect, the same amounts of mesoporous BiVO₄ (95 mg) and Co₃O₄ (5 mg) to those of S₂ are used as the photocatalyst to degrade RhB. Under the same condition, the RhB removal rate is largely lower than S₂ (Fig. 7(c)), confirming that the enhanced photocatalytic ability of S₂ is attributed to the heterojunction effect. This enhancement can be attributed to the increased light absorption ability and separation efficiency of photogenerated holes and electrons. The loading quantity of Co₃O₄ affects the photocatalytic degradation ability of Co₃O₄/mesoporous BiVO₄. Among the Co₃O₄/mesoporous BiVO₄ composites, the removal rate of S₂ is the highest one. However, the determined RhB removal rate of S₃ is even lower compared to that of mesoporous BiVO₄, indicating that considerably high Co₃O₄ loaded on the surface of mesoporous BiVO₄ can decrease its photocatalytic ability. This result is beyond the explanation based on the light absorption ability and the separation efficiency of photogenerated charge carriers. Thus, it can be reasonably speculated that there are other factors that affect the photocatalytic ability of Co₃O₄/mesoporous BiVO₄. It is reported that the pores in the photocatalysts can accelerate the diffusion of the reactants, and then promote the surface reaction of the photocatalysis.³⁰ The determined pore volumes of mesoporous BiVO₄ and S₃ are 0.0820 and 0.0615 cm³ g⁻¹, respectively, indicating that the loading of Co₃O₄ can decrease the pore volume and that some of pores of photocatalysts may be crammed. Thus, the surface reaction should be restrained, and the photocatalytic ability will decrease. To further analyze the role of mesoporous structure, the photocatalytic ability of Co₃O₄/BiVO₄ is compared with that of S₂. On one hand, the enhanced photocatalytic ability of S₂ can be attributed to the increase in the light absorption ability, separation efficiency of photogenerated charge carriers and adsorption ability caused by the construction of the mesoporous structure. On the other hand, Co₃O₄/mesoporous BiVO₄ has a large contact area of Co₃O₄ and BiVO₄, which can promote the charge transfer across the interface, thereby increasing the separation of electron–hole pairs, and leading to the enhancement of photocatalytic ability.

Fig. 7 RhB concentration C_t/C₀ (a) and ln(C₀/C_t) (b) vs. time for the photocatalytic degradation of RhB with BiVO₄, Co₃O₄/BiVO₄, mesoporous BiVO₄, S₁, S₂ and S₃, RhB concentration C_t/C₀ vs. time for photocatalytic degradation of RhB with S₂ and the mixture of mesoporous BiVO₄ and Co₃O₄ (c), UV-vis absorbance spectra of RhB solution after photocatalytic degradation with S₂ (d) and cycling runs in the photocatalytic degradation of RhB with S₂ (e) under visible light irradiation.

It is well-reported that the RhB photodegradation occurs via two competitive processes: N-deethylation and the destruction of the conjugated structure.^31–33 The intermediates produced in RhB degradation process include DER, EER, DR and ER, which result from losing one and/or two ethyl groups from the xanthene ring in the parent RhB structure. If RhB is degraded by N-deethylation, these intermediates would be generated and the color of the oxidized RhB would gradually change from pink to green. On the other hand, if RhB is decomposed by the destruction of the conjugated structure (cleavage of the chromophore structure), the maximum absorption wavelength of the solution during degradation will not obviously change. With the photocatalysis of S₂, the absorption maximum of the solution does not obviously shift (Fig. 7(d)), indicating RhB is decomposed by the destruction of the conjugated structure.

The stability of the Co₃O₄/mesoporous BiVO₄ composite (S₂) was also evaluated and the result is shown in Fig. 7(e). After recycling five times for the photocatalytic degradation of RhB, no obvious loss in activity is observed, indicating that Co₃O₄/mesoporous BiVO₄ composite is very stable and recyclable.

4. Conclusions

Co₃O₄/mesoporous BiVO₄ with mesoporous BiVO₄ networks containing well-dispersed Co₃O₄ particles is successfully fabricated. The construction of p–n heterojunction with mesoporous structure as the framework can enhance the photocatalytic ability of BiVO₄. Mesoporous BiVO₄ provides well-distributed space to host Co₃O₄ particles. The high distribution of Co₃O₄ particles in the matrix of the mesoporous BiVO₄ provides a large contact area of Co₃O₄ and BiVO₄, which promotes the charge transfer across the interface, thereby increasing the separation of photogenerated electron–hole pairs, and leading to the enhancement of photocatalytic ability. Furthermore, mesoporous structure in BiVO₄ can enhance the photo absorption ability, adsorption ability and separation ability of photogenerated charge carriers, which contributes to the enhanced photocatalytic ability. On the basis of the results obtained here, it is confirmed that using mesoporous structure as the network to construct p–n heterojunction is an efficient way to enhance the photocatalytic ability.

Acknowledgements

This work was supported by the National Science Fund China (project no. 21107007) and Cultivation Program for Excellent Talents of Science and Technology Department of Liaoning Province (no. 2014026009).

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