One-dimensional Ag₃PO₄/TiO₂ heterostructure with enhanced photocatalytic activity for the degradation of 4-nitrophenol

Abstract

One-dimensional TiO₂ nanofibers were synthesized using an electrospinning method. Subsequently, Ag₃PO₄/TiO₂ heterostructures were successfully fabricated through a simple deposition–precipitation reaction. The Ag₃PO₄/TiO₂ heterostructures exhibited enhanced photocatalytic performance under visible light. The improved photocatalytic activities are attributed to the visible light absorption enhanced by Ag₃PO₄ and the formation of a heterojunction between TiO₂ and Ag₃PO₄, which can effectively accelerate the charge separation and transfer. The heterostructures can be reclaimed easily by sedimentation without a decrease in the photocatalytic activity due to the large length to diameter ratio of the nanofiber framework.

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1. Introduction

The applications of semiconductor photocatalysts in solar energy conversion and degradation of pollution have received extensive attention in modern society.^1–3 To utilize solar energy more effectively, the development of efficient visible-light-driven (VLD) photocatalysts has attracted worldwide attention.^4–18 Recently, a new type of VLD photocatalyst, Ag₃PO₄, demonstrated by Ye et al.¹⁹ exhibited high photooxidative capabilities for O₂ evolution from water as well as organic dye decomposition under visible light irradiation. To further enhance its photochemical reactivity and stability, many studies have focused on constructing the Ag₃PO₄–base composite photocatalyst from different materials.^20–24 Up to now, Ag₃PO₄–base composite materials have been mainly divided into two kinds, zero dimensional nanoparticles and two dimensional films.^25–28 Among these materials, zero-dimensional Ag₃PO₄–base composite nanoparticles exhibited a high photocatalytic activity due to their high surface area. However, the suspended particulate catalysts are easily lost in the process of the photocatalytic reaction and separation, which may pollute the treated water again. In contrast, the two dimensional Ag₃PO₄–base nanofilms can be fixed and reclaimed easily, but the immobilization of nanofilms dramatically reduces the interfacial contact between the photocatalysts and pollutants, resulting in lower photocatalytic efficiency. Therefore, it is of great interest to design efficient and practical Ag₃PO₄ photocatalysts with excellent photocatalytic characteristics and favorable recycling capabilities.

With a great potential to overcome these drawbacks, one-dimensional electrospun nanofibers might be a promising support for immobilization of nanostructured photocatalysts. Compared with the corresponding nanoparticles and thin films of Ag₃PO₄–base, the effects of the one-dimensional composite nanomaterials on the photocatalytic properties of Ag₃PO₄ have been rarely studied.^29,30 More specifically, the use of one-dimensional electrospun nanofibers as co-photocatalysts is attracting increasing attention in the fields of photochemistry and photocatalysis. For example, our group reported that Bi₄Ti₃O₁₂ composites showed rapid sedimentation over a time scale of minutes, which can also be useful for the gravity separation of these particles in photocatalytic applications.⁹ Furthermore, in contrast to an individual semiconductor photocatalyst, hybrid one-dimensional photocatalysts integrate the synergistic effects of the individual species, which can endow the composite systems with prolonged carrier lifetime, enhanced catalytic performance and higher chemical stability.

Based on the above considerations, in this work we report a successful attempt for the fabrication of the Ag₃PO₄/TiO₂ heterostructures via a simple electrospinning technique and deposition–precipitation method. The as-prepared Ag₃PO₄/TiO₂ heterostructures have an interesting structure with uniform size, consisting of a TiO₂ nanofiber core and a nanocube-based Ag₃PO₄ shell. The special hierarchical structure, high light-harvesting capacity and nanoscale heterostructure make it be an excellent candidate for the degradation of pollution with enhanced photocatalytic efficiency.

2. Experimental

2.1. Preparation of TiO₂ nanofibers

Firstly, 2 g poly(vinyl pyrrolidone) powder (PVP, M_w = 1 300 000) was added to a mixture of 9 mL absolute ethanol and 5 mL acetic acid in a capped bottle. The obtained solution was stirred for 1 h to generate a homogeneous solution. Then, 2.0 g Ti(OC₄H₉)₄ was added to the solution, and the mixture was continuously stirred for another 1 h to make the precursor solution. 3 mL of the precursor solution was placed in a 5 mL syringe equipped with a blunt metal needle of 0.8 mm outer diameter and 0.6 mm inner diameter. A stainless steel plate covered with a sheet of aluminum foil was employed as the collector. The distance between the needle tip and collector was 15 cm, and the voltage was set at 9 kV. The as-collected nanofibers were calcined at 550 °C for 2 h to form anatase TiO₂ nanofibers.

2.2. Fabrication of Ag₃PO₄/TiO₂ hierarchical nanostructures

Subsequently, Ag₃PO₄/TiO₂ composites were prepared by deposition–precipitation and photoreduction processes. Briefly, 50 mg TiO₂ nanofibers was added to 100 mL of distilled water, and the suspension was stirred magnetically for 30 min. Then, 50 mL aqueous AgNO₃ solution (15 mM) was added into the suspension and stirred for another 30 min. After that, a small amount of NH₃·H₂O (2 wt% NH₃) was added to make sure that the Ag⁺ reacted with the NH₃. Finally, 50 mL of aqueous Na₂HPO₄·12H₂O solution (5 mM) was added dropwise into the suspension, and stirred for 120 min. All the above processes were carried out at ambient temperature. The products were filtered, washed with deionized water, and then dried at 70 °C for 2 h. For comparison, Ag₃PO₄ samples were also prepared using a similar process in the absence of TiO₂ nanofibers.

2.3. Characterization

The morphology of the samples was observed using a field emission scanning electron microscope (FE-SEM; SU-70, Hitachi, Japan) equipped with an energy dispersive X-ray (EDX) spectrometer. The crystalline structures of the samples were characterized by X-ray powder diffraction (XRD, D/max2600, Rigaku, Japan) and transmission electron microscopy (TEM; FEI, Tecnai TF20). UV-vis diffuse reflectance spectra (DRS) of the samples were obtained by using a UV-vis spectrometer (PerkinElmer, Lambda 850).

The photocurrent responses were determined by using an electrochemical analyzer (CHI660E, Shanghai, China) with a standard three-electrode configuration, which employed a Pt wire as a counter electrode, a saturated calomel electrode as a reference electrode and fluorine-doped tin oxide (FTO) as a working electrode. 10 mg of sample powder was dispersed in 2 mL of N,N-dimethylformamide under ultrasonication for 10 min to obtain slurries. The as-prepared slurries were spread onto the surface of the FTO glasses to obtain sample films with a size of 1 × 1 cm. These as-prepared FTO glass electrodes were dried at 100 °C for 60 minutes under ambient conditions to improve adhesion. 50 mL 0.5 M Na₂SO₄ (pH = 6.8) was used as the electrolyte solution. A 300 W xenon lamp with a 420 nm cut-off filter was employed as a visible light photosource.

2.4. Photocatalytic test

The degradation of 4-nitrophenol (4-NP) was carried out in a 200 mL beaker containing 100 mL 4-NP at a concentration of 1 × 10⁻⁵ mol L⁻¹ and 50 mg of the as-prepared photocatalysts, with vigorous magnetic stirring at room temperature. A 300 W xenon lamp with a 420 nm cut-off filter was employed as a visible light photosource. The solution was stirred in the dark for 30 min to obtain good dispersion and reach adsorption–desorption equilibrium between the 4-NP molecules and the catalyst surface. The concentration of 4-NP was measured using a UV-visible spectrophotometer at given intervals during the degradation process of 4-NP.

3. Results and discussion

The morphology, crystallinity and composition of the products were characterized by SEM, TEM, XRD and EDX. Fig. 1a shows a SEM image of the pure TiO₂ nanofibers that were fabricated by electrospinning followed by calcination at 550 °C for 2 h. It can be clearly seen that the TiO₂ nanofibers have a relatively smooth surface without secondary structures and their diameters are 200 nm. However, after being subjected to the solution of Ag⁺ and PO₄³⁻, Ag₃PO₄ nanostructures grew on the surface of the TiO₂ nanofibers after the deposition–precipitation process. Fig. 1b shows the SEM images of the Ag₃PO₄/TiO₂ heterostructures. As observed in the inset of Fig. 1b, Ag₃PO₄ nanocubes grow on the surface of the TiO₂ nanofibers after the deposition–precipitation reaction. It can be seen from the high magnification SEM image of the Ag₃PO₄/TiO₂ heterostructures that the diameters of the Ag₃PO₄ nanocubes are about 50–150 nm. Close inspection of the junction of the TiO₂ nanofibers and Ag₃PO₄ nanocubes shows that the Ag₃PO₄ secondary structures have their roots inside the TiO₂ nanofibers, suggesting that the Ag₃PO₄ nanocubes are not just loosely attached to the surface of the TiO₂ nanofibers. Fig. 1c shows a typical TEM image of the Ag₃PO₄/TiO₂ heterostructures. The TEM image reveals that the Ag₃PO₄ secondary structures are coated around the primary TiO₂ nanofiber substrates, coinciding with the results from the above SEM observations. The HRTEM image of the Ag₃PO₄/TiO₂ heterostructures displays two types of clear lattice fringes as shown in Fig. 1d. The interplanar distance between one set of adjacent lattice fringes is 0.35 nm, which agrees well with the (101) plane of the anatase TiO₂. The other set of fringes, with an interplanar distance of 0.26 nm, corresponds to the (210) lattice plane of Ag₃PO₄. The HRTEM image further exhibits the formation of the heterostructures.

Fig. 1 (a) SEM images of TiO₂ nanofibers. (b) SEM images of Ag₃PO₄/TiO₂ heterostructures. (c) TEM images of Ag₃PO₄/TiO₂ heterostructures. (d) HRTEM images of Ag₃PO₄/TiO₂ heterostructures. (e) XRD patterns of the samples. (f) EDX patterns of the Ag₃PO₄/TiO₂ heterostructures.

The XRD pattern (Fig. 1e) shows the crystallinity and phase purity of the Ag₃PO₄, TiO₂ nanofibers and Ag₃PO₄/TiO₂ heterostructures. The diffraction peaks of the TiO₂ nanofibers are indexed to pure anatase TiO₂ (JCPDS no. 21-1272) and those of the Ag₃PO₄ powders are coincident with the cubic structure of Ag₃PO₄ (JCPDS no. 06-0505). No peaks of impurities can be observed, demonstrating the high phase purity of the as-prepared Ag₃PO₄ powders and TiO₂ nanofibers. In comparison with the Ag₃PO₄ powders and TiO₂ nanofibers, the Ag₃PO₄/TiO₂ heterostructures are well crystallized and all the diffraction peaks can be indexed to the anatase TiO₂ and cubic structure of Ag₃PO₄. No characteristic peaks for impurities, such as Ag₂O, are observed, indicating that the Ag₃PO₄/TiO₂ heterostructures are successfully achieved using the deposition–precipitation reaction. Additionally, the EDX spectrum of the Ag₃PO₄/TiO₂ heterostructures was measured to determine the chemical composition of the heterostructures. The analysis of the results shows that the atomic ratio of Ag to Ti is about 3 : 10 (Fig. 1f).

The UV-vis diffuse reflectance spectra of the pure TiO₂ nanofibers, Ag₃PO₄ powders and Ag₃PO₄/TiO₂ heterostructures are shown in Fig. 2. As observed in Fig. 2, the diffuse reflectance spectrum of the TiO₂ nanofibers only exhibits the fundamental absorption band in the UV region, and there are no more absorption bands in the visible wavelengths. The UV-vis spectrum of the Ag₃PO₄ sample indicates that it absorbs sunlight with a wavelength less than 510 nm, corresponding to a band gap energy of 2.45 eV. However, for the Ag₃PO₄/TiO₂ heterostructures, aside from the absorption band edge (370 nm) in the UV light range, a characteristic band edge of Ag₃PO₄ appears in the visible light range based on its UV-vis spectrum. These features of the visible light absorption properties of the prepared Ag₃PO₄/TiO₂ photocatalyst can be attributed to the small band gap and large absorption coefficient of Ag₃PO₄. Taking into account the efficient use of visible light in a large part of the solar spectrum, we believe that the Ag₃PO₄/TiO₂ heterostructures, with their long wavelength absorption band, provide an attractive photocatalyst for pollutant degradation.

Fig. 2 UV-vis diffuse reflectance spectra of samples.

To investigate the photocatalytic properties of the as-prepared photocatalyst, we chose 4-nitrophenol (4-NP) to evaluate the photocatalytic activity as it is more difficult to degrade than other dyes in aqueous media. It is well-known that a 4-NP solution exhibits a strong absorption peak at 317 nm under neutral or acidic conditions. Temporal changes in the concentration of 4-NP, as monitored by the maximal absorption of 4-NP in the UV-vis spectra when mixed with the as-prepared photocatalysts, are shown in Fig. 3a. The degradation efficiency of all the samples is defined as C/C₀, where C and C₀ represent the residual and initial concentration of 4-NP, respectively. TiO₂ nanofibers, Ag₃PO₄ powder and Degussa-P25 were used as the photocatalytic references. As shown in Fig. 3a, after visible light irradiation for 50 min, the TiO₂ nanofibers and Degussa-P25 displayed no photocatalytic activity under visible light, except for the adsorption of 4-NP. However, the degradation efficiencies for 4-NP are about 70 and 99% for the Ag₃PO₄ powder and Ag₃PO₄/TiO₂ heterostructures, respectively. Clearly, the Ag₃PO₄/TiO₂ heterostructures exhibit enhanced photocatalytic activity for the degradation compared to the TiO₂ nanofibers and Ag₃PO₄ powder. The enhanced photocatalytic activity can be attributed to the epitaxial growth of Ag₃PO₄ nanocubes on the surfaces of the TiO₂ nanofibers and the strong visible light absorptivity.

Fig. 3 (a) Comparison of the photocatalytic activities among the samples. (b) Degradation curves of 4-NP over the Ag₃PO₄/TiO₂ heterostructures when reusing them 3 times (inset: photographs of 4-NP solutions undergoing visible light photo-degradation for 50 min with P25 and Ag₃PO₄/TiO₂ heterostructures, after sedimentation for 30 min).

The enhanced photocatalytic performance of Ag₃PO₄/TiO₂ is due to the following factors: on the one hand, according to DRS analysis, the Ag₃PO₄/TiO₂ heterostructures exhibit enhanced absorption in the visible light region. It is evident from the results that the Ag₃PO₄/TiO₂ heterostructures absorbed more visible light than pure TiO₂ and thus displayed better photocatalytic activity. On the other hand, the formed junction between Ag₃PO₄ and TiO₂ in the heterostructured photocatalysts can also prevent the recombination between photoelectrons and holes. In this work, the as-adopted fabrication route is successful in realizing a close contact between Ag₃PO₄ with TiO₂ nanoparticles in the Ag₃PO₄/TiO₂ heterostructures, as evidenced by SEM and TEM observations. Such close contact is more effective in the suppression of electron–hole recombination. The migration of photogenerated carriers was promoted because less of a barrier exists between the Ag₃PO₄/TiO₂ heterostructures.

To confirm the stability of the high photocatalytic performance of the Ag₃PO₄/TiO₂ photocatalyst, the sequential runs of the photodegradation of 4-NP under visible light (λ > 420 nm) were checked. As shown in Fig. 3b, each experiment was carried out under identical conditions and after three cycles the photocatalytic activity of the Ag₃PO₄/TiO₂ heterostructures remained almost unchanged, clearly indicating their stability. Fig. S1† shows the XRD pattern and SEM image of the Ag₃PO₄/TiO₂ catalyst after the catalytic reaction. After three catalytic runs, the positions and the ratios of the peaks are nearly the same as those of the fresh photocatalyst. In addition, the secondary Ag₃PO₄ nanostructures are still very complete, clearly indicating their stability. In addition, the photograph in Fig. 3b shows that the samples can be easily separated from the solution by sedimentation for 30 min, probably due to the large length to diameter ratio of the one-dimensional nanofibrous Ag₃PO₄/TiO₂ heterostructures. In comparison, the Degussa-P25 nanoparticles are still suspended after 30 minutes of precipitation in aqueous solution. This indicates that the Ag₃PO₄/TiO₂ nanostructures display efficient photoactivity for the degradation of organic pollutants under visible light irradiation and can easily be separated for reuse.

Scheme 1 illustrates a plausible mechanism for the photodegradation of 4-NP over the Ag₃PO₄/TiO₂ heterostructures. As shown in Scheme 1, Ag₃PO₄, with a narrow band gap energy (2.45 eV in this work), can be easily excited by visible light (λ > 420 nm, energy less than 2.95 eV) and induce the generation of photoelectrons and holes. In the case of TiO₂, it can not be excited by visible-light irradiation with energy less than 2.95 eV due to its wide energy gap of about 3.11 eV in this work. The potentials of both the conduction band and valence band of TiO₂ are more negative than those of Ag₃PO₄, and photon-generated holes in an Ag₃PO₄ particle quickly transfer to a TiO₂ particle, whereas photon-generated electrons migrate to the surface of an Ag₃PO₄ particle. In such a way, the photoinduced electron–hole pairs can be effectively separated. The separation of electrons (in Ag₃PO₄) and holes (trapped in TiO₂) prevents the charge recombination, leading to the higher photocatalytic activity of Ag₃PO₄. During the photocatalytic process, Ag⁰ nanoparticles are produced by the partial reduction of Ag₃PO₄ by the photogenerated electrons (Fig. S2†). The resultant Ag metal can trap the photo-generated electrons and thus inhibit the further decomposition of Ag₃PO₄.^31,32 Therefore, the formed Ag/Ag₃PO₄/TiO₂ interfaces can effectively promote charge separation and enhance the photocatalytic activity of the catalyst. The better separation of electrons and holes in the Ag₃PO₄/TiO₂ heterostructures is confirmed by the transient photocurrent responses (Fig. 4). In comparison with pure Ag₃PO₄, the Ag₃PO₄/TiO₂ heterostructures exhibit an increased current density, about 3.5 times than that of the bare Ag₃PO₄. The increased current density indicates enhanced separation efficiency of photoinduced electrons and holes, which could be attributed to the heterojunctions between Ag₃PO₄ and TiO₂ and the electron trapping role of the Ag nanoparticles. Moreover, the photoluminescence emission spectra of the prepared TiO₂ nanofibers and Ag₃PO₄/TiO₂ heterostructures are also consistent with the above results (Fig. S3†).

Scheme 1 Schematic of the band structures of the Ag₃PO₄/TiO₂ heterostructures and possible electron–hole separation.

Fig. 4 Transient photocurrent response of Ag₃PO₄ and the Ag₃PO₄/TiO₂ heterostructures under visible light irradiation.

4. Conclusion

In summary, by using a deposition–precipitation reaction and electrospinning technology, Ag₃PO₄/TiO₂ heterostructures were successfully fabricated. In comparison to TiO₂ and Ag₃PO₄ samples, the Ag₃PO₄/TiO₂ heterostructures exhibit a high photocatalytic behavior for the decomposition of 4-NP, benefiting from the heterojunction reducing the recombination of photogenerated electrons and holes due to the photoinduced potential difference generated at the Ag₃PO₄/TiO₂ heterojunction interface. These results indicated that the Ag₃PO₄/TiO₂ heterostructures are a promising candidate material for wastewater treatment.

Acknowledgements

This work was partially supported by the Natural Science Foundation of China (no. 51172058, 51472066 and 51402076), the Natural Science Foundation of Heilongjiang Province (ZD201112 and QC2014C056), Institution of Higher Education Doctoral Fund Jointly Funded Project (20112329110001) and Graduate Students’ Scientific Research Innovation Project of Harbin Normal University (HSDSSCX2014-06).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra15946b

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