Synthesis and Photocatalytic Application of BiVO 4 Nanocrystalline Functionalized by porphyrin with Two Different Crystalline Phases as an Efficient Photocatalyst

A series of different Nanocrystalline phases, Bismuth vanadate (BiVO4) (photocatalysts monoclinic/tetragonal heterophase) and BiVO4 Nanocrystalline functionalized by porphyrin, have been prepared. The prepared sample was characterized by powder X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy (FT-IR). Photocatalytic activities samples were evaluated by the methyl orange decomposition under visible and UV light irradiation. The degradation was observed by the decrease in the absorbance peak studied with UV-Vis spectrophotometer.


Introduction
Porphyrins as a supermolecular compound and primary chromophores in photosynthesis, are nature's light harvesting system [1].Porphyrins capture a broad range of the solar spectrum due to their strong absorption band in both visible and near infrared regions [2].Heterogeneous photocatalytic oxidation, using semiconductors, has attracted great interest as a promising technology for the purification of contaminated water [3][4][5][6].Among the various semiconductor photocatalysts, TiO2 has been used widely, but some disadvantage of this photocatalyst limits its applications [7].Thus, investigations have been concentrated on the design and development of other metal oxides properties of photoinduced charge separation under excitation light illumination.Among several studies of oxides with activity, under visible-light irradiation, BiVO4 has received special attention [8,9].There are three crystalline phases reported for BiVO4: tetragonal zircon (z-t), monoclinic scheelite (s-m) and tetragonal scheelite (s-t) [10].Bismuth vanadate (BiVO4) has been recognized as a strong photocatalyst (band gap: 2.4 eV) for pollutant decomposing, due to its photocatalytic response to the visible-light region and relatively strong oxidation properties for water purification [11].In this work, we synthesized mixed monoclinic and tetragonal structure of BiVO4 and BiVO4-TCPP via a simple chemical method.Photocatalytic activities of samples were evaluated by the methyl orange decomposition under visible and UV light irradiation.The present study has focused on the preparation of catalyst with high photocatalytic activity.The results show that the presence of interface in monoclinic-tetragonal heterophase provides a special condition for charge transfer, increase the separation of photoinduced electron-hole pairs, and changes the migration direction of photoinduced carriers.

1. Synthesis of Bismuth Vanadate
In the present work, for the synthesis of bismuth vanadate (BiVO4) with monoclinic-tetragonal phases, NH4VO3 and Bi (NO3)3.5H2O in equimolar aqueous solution was mixed.PEG was added to the solution including Bi (NO3)3.5H2O,then the mixture was stirred for 1h and sonicated by ultrasonic for 30 min at room temperature.The yellow precipitate of BiVO4 was separated from the solution by centrifuging and was washed with distilled water and ethanol.The final production was dried in the oven at 60˚C.The corresponding product was calcined at 450˚C for 120 min.

Synthesis of porphyrin and BiVO4 functionalized by porphyrin
Tetrakis (4-carboxyphenyl) porphyrin (TCPP) was prepared by refluxing 4-Carboxybenzaldehyde and pyrrole in propionic acid.In order to sensitize the BiVO4 with porphyrin compounds, BiVO4 and TCPP with 10:1 ratio was refluxed in the DMF solvent.After washing with DMF, the synthesized product obtained.

Characterization
Figure . 1 shows the XRD patterns of BiVO4 and BiVO4-TCPP.Fig. 1a shows the BiVO4 sample is a heterophase including tetragonal Zircon (z-t) and monoclinic scheelite (s-m) with the presence of peaks 2θ = 24.4˚,32.7˚ (JCPDS no.14-0133) for tetragonal zircon (z-t) structure and peaks of 2θ = 28.8˚(JCPDS no.14-0688) for monoclinic scheelite (s-m) structure.As Fig. 1b shows, the significant change have not seen in the XRD pattern of BiVO4-TCPP compounds, therefor, in the BiVO4 functionalized with porphyrin compounds, the distance between the plates of crystalline BiVO4 and its structure was not changed.The representative SEM images of the prepared BiVO4 and BiVO4-TCPP composites are shown in Fig. 2. According to the image 2b, the presence of TCPP did not change morphology of BiVO4.The diffuse reflectance spectra (DRS) of the synthesized samples are shown in Fig. 3.These results have been indicated that the porphyrin sensitization had great effect on the photocatalytic activity of BiVO4 in visible light.DRS analysis is an important method for investigation of the functionalization of metal oxide with the porphyrin compounds.As Fig. 3 shows, the sensitization of BiVO4 by porphyrin change the absorption edge of BiVO4.
Fourier transform infrared spectroscopy (FT-IR) were shown in Fig. 4.Moreover, the bands at 700-800 cm −1 for both samples, pure BiVO4 and BiVO4 composite, were associated with the bending vibration of the VO4 3− .

Photocatalytic Activity of Prepared Samples
The photocatalytic activity of BiVO4 sample was evaluated by the degradation of methyl orange in the aqueous solution (10 ppm) under the visible light and UV light irradiation.As shown in Fig. 5, under the irradiation of UV light (λ > 400 nm), 25% of the methyl orange dye was destructed in the presence of the BiVO4 sample after 240 min, which was higher than the irradiation of visible light.The result showed that under the irradiation of visible light, 47% of the methyl orange dye was destructed in the presence of the BiVO4-TCPP sample after 240 min.Fig. 5. Photodegradation of MO using pure BiVO4 and BiVO4-TCPP.

Conclusions
In tetragonal/monoclinic heterophase BiVO4, a tight interface junction has been formed between BiVO4 (s-m) and BiVO4 (z-t) in a nanosize level.Monoclinic scheelite BiVO4 (s-m) was mainly formed at the surface of heterophase BiVO4 nanoparticles after calcination.Tetragonal/monoclinic heterophase BiVO4 has higher photocatalytic activity under the irradiation of UV light.The new BiVO4-TCPP photocatalyst was successfully prepared through a simple method.As expected, the new BiVO4-TCPP not only has high photodegradation efficiency than the pure BiVO4, but also excellent photostability for the photodegradation of MO under visible light irradiation.This is because the energy level of the excited TCPP and the conduction band of BiVO4 was properly overlapped which leads to decrease the recombination of the electron-hole pair and thus promotes the photodegradation efficiency.