PHOTOCATALYTIC DEGRADATION OF ORGANIC DYES BY INFINITE ONE DIMENSIONAL COORDINATION POLYMER BASED ON Zn(II) IN WATER

A new dbased Zn(II) coordination polymer (CP), formulated as [Zn(SO4)3(DMF)3]n (1), has been synthesized and structurally characterized. The single crystal X-ray diffraction results indicates that CP1 forms 1D inorganic –[Zn–SO4–Zn]n– chain along the ac plane. The thermal stability, UV/Vis diffuse-reflection spectrum and photocatalytic behavior of 1against organic dyes have been investigated. The photocatalysis results indicate that1displays better photocatalytic activities against RhBin comparison toMV. The probable mechanism associated with the photocatalysis of 1 against organic dyes has been addressed using density of states (DOS) calculations.


INTRODUCTION
Currently much attention has focused on numerous harmful industrial organic pollutants that have been released into the environment and as a consequence the biosphere [1][2][3][4]. To cope-up with this problem, in recent years, many efforts of various groups worldwide have been dedicated to develop new and efficient photocatalytic materials based on coordination polymer and semiconductors with fascinating structural motifs that can convert these organic pollutants into environmentally safe species [5][6].
It had now been established that the photocatalytic decomposition of dyes had been successfully in the presence of MIL-53 [7]; Fe 3 O 4 /MIL-100(Fe) core-shell microspheres [8], ZIF-8 [9], etc. Hence, application of these MOFs in the photocatalytic decompositions of dyes revealed their great potential as photocatalysts [10]. It is well known that the electronic properties of MOFs depend on their chemical compositions and structures [11][12], viz. the nature of the organic ligands (linkers) metal ions, and metal-oxide clusters. The explanation of electronic properties of MOFs were constructed by the SBUs of metal-oxide cluster and conjugated organic linkers, which represent the discrete quantum dotanalogue the photon antenna, respectively [13].
Based on these aspects and in an attempt to investigate photo-current-generating properties of MOFs, herein, in the presented work, a new Zn-based coordination polymer, formulated as [Zn(SO 4 )(DMF)] n (1) has been synthesized and structurally characterized by single-crystal X- ray diffraction. The UV-Vis absorption, diffuse reflectance spectra and photocatalytic behaviours have been investigated.

General considerations
All the chemicals were obtained from commercial sources and used without further purification. The powder X-ray diffraction (PXRD) data was collected on Bruker D8 Advance X-ray diffractometer with Cu-Kα radiation (λ=1.5418 Å) at 50 kV, 20 mA with a scanning rate of 6°/min and a step size of 0.02°. The simulated powder patterns for 1 was obtained using Mercury 2.0 program. The purity and homogeneity of the bulk products were assessed by comparing the simulated and experimental X-ray powder diffraction patterns. Fourier transform infrared (FT-IR) spectra as KBr pellet were recorded using Nicolet Impact 750 FTIR spectrometer in the range of 400-4000 cm -1 . Thermogravimetric analysis (TGA) was performed under nitrogen atmosphere from room temperature to 800 °C at a heating rate of 10 °C min -1 , using SDT Q600 thermogravimetric analyzer.

X-Ray crystallography
Single crystal X-ray diffraction data collection was carried out on Bruker Smart Apex diffractometer which was equipped with graphite monochromated Mo radiation (λ = 0.71073 Å) by using an -scan technique. The intensities of the absorption effects were corrected by using SADABS. The structures were solved by direct methods (SHLEXS-2014) and refined by a full-matrix least-squares procedure based on F 2 (Shelxl-2014) [14]. All the hydrogen atoms were generated geometrically and refined isotropically using the riding model. Crystallographic details and selected bond dimensions for 1 are listed in Tables 1 and 2 (CCDC: 1816723).

Photocatalytic method
The photocatalytic reactions were performed as follows: 50 mg of 1 were dispersed in 50 mL aqueous solution of RhB/MV (10 mg/L) under stirring in the dark for 30 min to ensure the establishment of an adsorption-desorption equilibrium. Then the mixed solution was exposed to UV irradiation from an Hg lamp (250 W) and kept under continuous stirring during irradiation for 100 min. Samples of 5 mL were taken out every 10 min and collected by centrifugation for analysis by UV-Vis spectrometer. By contrast, the simple control experiment was also performed under the same condition without adding any catalysts. The photocatalytic activity studies were carried out in a Shimadzu UV-Vis 2501PC recording spectrophotometer.

Computational details
The probable mechanism associated with the photocatalytic property of 1 against organic dyes has been ascertained using density of states (DOS) and partial DOS calculations. For that the optimized molecular geometry of 1 have been computed using the B3LYP exchange-correlation functional. 24 The 6-31G** basis set for all the atoms were used for geometry optimization All the calculations were performed using Gaussian 09 programme [15]. GaussSum 3.1 was used to obtain density of state (DOS) plots [16].

[Zn(SO 4 ) 3 (DMF) 3 ] n (1)
Single crystal X-ray diffraction analysis shows that the asymmetric unit of 1 consists of three crystallographically independent Zn(II) ions, three coordinated sulfate ions and three coordinated DMF molecules. The coordination environment of Zn(II) ions are shown in Figure  1a

Diffuse-reflectance UV/Vis spectroscopy
The UV-Vis absorption spectrum of 1 in the solid state at room temperature was measured. As shown in Figure 2a, the CP1 displays one absorption band at 315 nm, which may be ascribed to the intra-ligand charge-transfer transition (ILCT) [21]. In the diffuse reflectance spectroscopy (DRS), scattered radiation is collected and the reflected light is matching closely with the Kubelka-Munk function given by F(R) = (1-R) 2 /2R, where R is the reflectance at a given energy. The plot of [F(R)hv] 2 vs energy is presented in Figure 2b.The energy band gap (E g ) obtained by extrapolating the linear portion of the absorption edge was estimated to be 2.74 eV. The band gap of 1 indicates that it may show absorption responses to UV light and in turn may have the potential capability for catalytic photodegradation [22].

Photocatalysis
The photocatalytic activity of 1 was evaluated by photo-degradation of MV/Rh B dyes in aqueous medium. The degradation ratios of MV/Rh B dyes in water were monitored by observing the intensity of the characteristic absorption band of these dyes. No new absorption band corresponding to dyes arises in the UV-Vis absorption spectra, indicating the complete decomposition of MV/Rh B in water. In the presence of 1, 47% of MV and 76% of Rh B dyes decomposed in 80 min (Figure 3). For sake of comparison, the total catalytic degradation efficiency of the control experiment (without the use of catalyst) had also been carried out. The degradation ratio of MV and RhB were merely 25.2% and 18.6%, respectively, in 80 min under the same condition in the absence of CP 1. These results demonstrate that the presence of 1 is necessary for the degradation of MV and Rh B. Also, 1 displayed better photocatalytic activity against Rh B than that of MV under similar conditions [23]. In addition, the photostability of 1 was monitored by PXRD analysis during the course of the photocatalytic reactions. The PXRD experiments confirmed that 1can be recovered from the catalytic system and reused without obvious loss of crystallinity (as revealed by PXRD analysis) ( Figure 4).   To investigate the kinetics of MV/Rh B photocatalytic degradation by 1, experimental data can be described by the Langmuir-Hinshelwood model as expressed by ln (C/C 0 ) = −kt (k = apparent reaction rate constant). WhereC 0 is the initial concentration of MV/Rh B, t is the reaction time, and C is the concentration of Rh B/MV at the reaction time t. The plot of ln (C 0 /C) and irradiation time (t) is approximately linear and approximated the first-order kinetic equation ( Figure 5). The calculated apparent rate constant k values of Rh B and MV are 0.761×10 -2 and 0.378×10 -2 min -1 for 1, respectively. Thus, Rh B is much higher than that of MV. Thus, 1 could be chosen as photocatalysts to degrade Rh B [24].
The probable mechanism associated with the photocatalytic property of 1 against organic dyes has been suggested on the basis of band structure calculations on 1 which is based on density functional theory (vide supra). The density of states (DOS) and partial DOS for 1 is presented in Figure 6 which indicates that the valence band lying just below the Fermi level is mainly contributed by the oxygen and nitrogen centers with lesser contribution by the Zn(II) centers. The conduction band lying just above the Fermi level in the range of -1.93 to 0.05 eV have been derived from carbon and nitrogen, with admixture of Zn(II) centers. Therefore, the electronic transition in 1 mainly takes place from the Zn(II) center and oxygen and nitrogen region. In the typical photocatalytic process, the sample 1 may be excited to produce electronhole pairs under visible light irradiation and as band structure calculations reveal that the hole moves to Zn(II) centers and the electron migrates to aromatic entity. The generation of holes on the d 10 Zn(II) centers will correspond to its oxidation which is now capable to oxidize the dye to re-reduce back to Zn(II) again. The mechanistic scheme suggested to operate during the photocatalytic decomposition of RhB/MV in the presence of 1 can be summarised as follows [25,26] O 2 ·¯/ HO 2 · + S  oxidation products (5) 1 when irradiated gets excited to generate electron-hole pair (equation 1). The reaction of the photo-excited CP 1 with water molecule (i.e. oxidative hole trapping) then generates bound hydroxyl (OH · ) radical (equation 2). This hydroxyl radical reacts subsequently with the organic dyes viz. Rh B/MV by following different ways to produce oxidised products. The obtained oxidised products are different in Rh B (Scheme 1) [27] and MV (Scheme 2) [28]. The dioxygen behaves as an efficient oxidant for the reduced 1(e¯) and its key action is to regenerate the reduced photocatalyst 1. Scheme 2. The possible oxidation products obtained during the photodegradation of MV in presence of 1.

CONCLUSION
Our results indicated that the coordination polymer 1 reported herein possesses better photocatalytic activity against Rh B. The presented investigation indicated that 1 acted as an excellent and selective adsorbent material for organic dye Rh B and can be utilized repeatedly for effective adsorption of Rh B from waste-water without huge alleviation in the adsorption capacity.