Mingyi Zhang,
Lu Li and
Xitian Zhang*
Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, P. R. China. E-mail: xtzhangzhang@hotmail.com
First published on 17th March 2015
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.
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.
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.
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.
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.
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†).
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Scheme 1 Schematic of the band structures of the Ag3PO4/TiO2 heterostructures and possible electron–hole separation. |
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Fig. 4 Transient photocurrent response of Ag3PO4 and the Ag3PO4/TiO2 heterostructures under visible light irradiation. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra15946b |
This journal is © The Royal Society of Chemistry 2015 |