Synthesis of ZnO nanorods modified with TPP, TPPS and Cu-TPPS for photodegradation of MB

In this study, ZnO nanorods have been synthesized by a simple precipitation technique with zinc nitrate as starting material in the presence of ammonium hydroxide followed by calcination at 450  ̊C. Then meso-tetrakis(4-phenyl) porphyrin (TPP), meso-tetrakis(4-sulfonatophenyl) porphyrin (TPPS), and Copper meso-tetrakis(4-sulfonatophenyl) porphyrin (Cu-TPPS) have been immobilized on the ZnO nanorods and the potential of the obtained photocatalysts in degradation of methylene blue dye was studied under UV and visible irradiation. The photocatalysts were characterized by various techniques such as XRD, SEM and FT-IR. The influence of experimental parameters on the dye photodecolorization process was studied. The role of porphyrins as sentisizers was also investigated on decolorization rate. Under these optimum conditions, the obtained decolorization efficiency for MB dye was more than 90% under visible light. The reusability of the intended catalyst was also investigated. ZnO nanorod


Introduction
Semiconductors such as ZnO and TiO 2 have a filled valence band and an empty conduction band and therefore can act as sensitizers for light-reduced redox processes [1]. Among semiconductors, ZnO is a very common material with excellent chemical and thermal stability [2]. It can provide high mobility [3], high transparency [4], nontoxicity and biocompatibility [5]. According to great function of zinc oxide, several techniques have been proposed for zinc oxide synthesis such as thermal decomposition method [5,6], hydrothermal synthesis [4,7], sol-gel synthesis [8], flame 2 spray pyrolysis [9] and precipitation method [10]. Since environmental problems such as water pollution, semiconductor based photocatalytic reactions have attracted interest as an effective candidate for purifying contaminants [11]. The most efficient materials for photocatalytic applications are semiconductor oxides on nanoscale. Furthermore, shape of accumulation of crystals, have an important role in its applications [8], because the electrical and optical properties of nanomaterials depend on both size and shape of the particles. Therefore, it is desired to synthesize nanomaterials in a simple method which both shape and size factors can be controllable. For zinc oxide particles, various shapes including nanorods [11,12], whiskers [13,14] and nanowires [15] have been successfully prepared.
Many studies on the performance of ZnO particles as Photocatalyst have been performed to remove contaminants [14][15][16]. It is known that the solar spectra usually contain about 4% UV light. Therefore, pure ZnO, with large direct band gap energy of 3.37 eV at room temperature, cannot make use of the solar spectra efficiently [4]. To overcome this problem, several approaches have been proposed in the literature in which the response of the semiconductor was extended towards the visible region. Among them, dye sensitization has been widely used in order to extend the absorption range to the visible region [17].

Materials and Methods
All of the Chemicals used in this work were analytical grade reagents and used without further purification. Zinc nitrate (Zn(NO 3 ) 2 .6H 2 O) , ammonia (25%), pyrrole, propionic acid, benzaldehyde and sulfuric acid were purchased from Merck company. Deionized water was used to prepare all solutions.
The samples were characterized by X-ray powder diffraction (XRD) using JEOL X-ray diffractometer with Cu Kα radiation. The particle morphologies of the ZnO powder were 3 observed by an AIS2100 (Seron Technology) scanning electron microscopy (SEM). The FT-IR analyses were carried out on a Shimadzu FTIR-8400S spectrophotometer using a KBr pellet for sample preparation. DRS spectra were prepared via a Shimadzu (MPC-2200) spectrophotometer.

Preparation of copper meso-tetrakis(4-sulfonatophenyl) porphyrin (TPPS)
10 mmol of freshly distilled pyrrole (0.7 ml), 10 mmol of benzaldehyde, 100 ml of propionic acid and 15 ml of nitrobenzene were added to a 250 mL flask. The mixture was allowed to reflux under stirring at 120 ˚C for 60 min. After that, the resulting mixture was cooled overnight in room temperature and filtrated under reduced pressure. The crude product was purified by column chromatography (Silica gel, chloroform/ethyl acetate = 20:1 as an eluent) and a desired purple solid of meso-tetrakis(phenyl) porphyrin (TPP) was obtained (30%).
For synthesis of meso-tetrakis(4-sulfonatophenyl) porphyrin (TPPS), 0.  The resulting solution was cooled in room temperature for 24 h. Consequently, the product was washed with ethanol until no porphyrin could be detected in the supernatant by UV-visible spectrophotometer.

Photocatalysis procedure
In a typical process, the catalytic reaction was carried out in a 100 ml photoreactor (Scheme 2), which contain 10 ml of MB dye (20 mg/l) solution and 6 mg of catalyst. Before the irradiation, the solution was stirred in the dark (15 min) to allow equilibrium of the system. Irradiation was carried out using 400 W tungsten and 400 W high pressure mercury lamps as the light sources.
All photocatalytic experiments were carried out at the same conditions. The distance between photoreactor and light sources was 20 cm. Samples (3 ml) were collected during the irradiation and MB solution were separated from the photocatalyst by centrifugation. The degradation was monitored by measuring the absorbance amount using a double beam UV-vis spectrophotometer (Shimadzu UV-1700) at 664 nm wavelength.

Morphological characterizations
The morphology of the ZnO particle was examined from SEM images, as shown in Fig. 1. It was found that all synthesized ZnO nanorods were quite uniform in size. Fig. 1A,B show SEM images of the pure ZnO and Fig. 1C,D are related to TPPS/ZnO nanorods.   The synthesized ZnO nanorods diameter was calculated using Debye-Scherrer formula.

The X-ray powder diffraction
Where 0.89 is Scherrer's constant, λ is the wavelength of X-rays, θ is the Bragg diffraction angle, and β is the full width at half-maximum (FWHM) of the diffraction peak corresponding to plane (101). The average particle size of the sample was found to be 21.05 nm which is derived from the FWHM of more intense peak corresponding to (1 0 1) plane located at 36.30• using this formula.
8 Fig. 2. The XRD pattern of the synthesized ZnO nanorods.

Optical properties
The UV-vis diffuse reflectance spectra (DRS) of the synthesized ZnO nanorods and TPPS/ZnO and the UV-vis spectrum of pure TPPS (in methanol) at room temperature are shown in Fig. 3A.
The spectrum reveals a characteristic absorption peak of ZnO at wavelength of 368, which can be assigned to the intrinsic band-gap absorption of ZnO due to the electron transitions from the valence band to the conduction band (O 2p → Zn 3d ) [20].
In semiconductor, the absorption coefficient is a function of the incident photon energy. Near the absorption edge, the absorption coefficient for direct transition is given by [21]: where Eg (eV) is the energy gap, hν (eV) is the energy of incident photon and constant A varies for different transitions, and is related to effective mass of carriers, refractive index, oscillator strength and so on. The n is an index which equals 1/2 corresponding to allowed direct transitions [22].
The extrapolation method is usually used to determine the band gap of semiconductors providing that the transition in the semiconductor at certain wavelength scope is direct. In this work, when the index n is 1/2 in above Eq., by using the relationship of reflectivity and transmission with wavelength, we calculated the square of the absorption coefficient as a function of the incident photon energy (as shown in Fig. 3B). The energy band gap Eg measurement is 3.23 eV.
The   For the FT-IR of TPPS/ZnO, the appearance of peaks corresponded to TPPS confirm that porphyrin have immobilized on ZnO nanorods but these peaks are difficult to give precise values because of very low intensities due to a low loading of the macrocycle into the inorganic matrix.

Photocatalytic degradation of methylene blue
The photocatalytic activities of as-synthesized two kinds of catalysts were evaluated by the degradation of organic dyes methylene blue in aqueous solution under light irradiation. The TPPS/ZnO nanorods and ZnO nanorods, with a high specific surface area, were used as 11 photocatalysts for the decomposition of methylene blue by the superoxides and/or hydroxyl radicals formed at their interface. The characteristic absorption of MB at 664 nm was chosen to monitor the photocatalytic degradation process. Concerning the initial MB concentration, it can be concluded that there was a decrease in the photodegradation of MB with increasing initial MB concentration. Fig. 5 shows a typical photocatalytic degradation process of MB (initial concentration:10 mg/l, 20 ml) using TPPS/ZnO (0.01 g) under visible light irradiation. The absorption peaks corresponding to MB diminished gradually as the exposure time was extended.

Conclusion
The photocatalytic degradation of MB in water was studied using ZnO and visible and UV light irradiation. The obtained results indicate that the photodegradation of MB was affected by the initial MB concentration and type of light irradiaton.