English

• 1. INTRODUCTION

  The pollutants in wastewater have significant threat regarding humanity's daily life because they contain many kinds of refractory complex pollutant constituents, such as dye molecules, drugs, aromatic compounds, etc^[1-3]. Especially, dye effluents are difficult to degrade. Even with quite low concentrations, they can still reduce the transmissivity of water and eventually destroy the ecosystems of water^[4]. Thus, controlling the organic dye pollution in the water environment in time is urgent.

  There are many conventional treatment methods to solve this problem, such as adsorption, microbial degradation, electrolysis, and so forth^[5-7]. Among these ways, photocataly- tic process has been recognized as one of the current technologies with energy-saving and high efficiency for the removal of pollutants from water^[8]. Up to now, multitudinous inorganic materials have been adopted as photocatalysts for photocatalytic process^[9-14]. Coordination polymers developed in recent years have been a new type of photocatalytic materials for their outstanding advantages of both organic and inorganic materials^[15-17]. Coordination polymers are excellent composites combining high dimensional stability, rigidity of inorganic components as well as molecular decoration and clipping property of organic fractions, with more optimized functions^[18-26]. In general, in order to obtain coordination polymers with desirable framework structures and func- tionalities, many efforts have been devoted to the rational selection of metal entities and organic linkers. In this regard, Fe-based polymers show great potential as photocatalysts, and have attracted extensive attention^[27-29]. Meanwhile, owing to the rapid redox cycling of iron and faster production of ·OH, the Fe-based polymers were used as the catalyst and showed good catalytic effects. In addition, 1, 10-phenanthroline (phen) and its derivatives with rigid planar and electron-poor heteroaromatic system are the ideal organic links to construct coordination polymers^[30-36]. Many studies have shown that phen and its derivatives are good electron-transporting materials, and have a bright future, which will help the electrons and holes to separate, so the catalytic effects may be improved^{[37, 38]}.

  Herein, we report a new Fe-based coordination polymer, [Fe(C₂O₄)_0.5(4-NCP)]_n·2nH₂O (1), with 1, 10-phenanthroline derivative (2-(4-carboxyphenyl)-1H-imidazo(4, 5-f)-(1, 10)- phenanthroline, 4-HNCP) and oxalic acid (H₂C₂O₄) as the organic ligands and Fe(II) as the metal center. 1 was characterized by infrared spectrum, elemental analysis, single-crystal X-ray diffraction, power X-ray diffraction (XRD), and ultraviolet-visible (UV-vis) absorption spectrum. The photocatalytic activity toward the degradation of rhodamine B (RhB) dye was investigated from aqueous solution in the presence of H₂O₂ under visible light irradiation.

  2. EXPERIMENTAL

  2.1 Chemicals

  FeSO₄·7H₂O and oxalic acid (H₂C₂O₄·2H₂O) were purchased from Sinopharm, China. 4-HNCP was purchased from Jinan Henghua Sci. & Tec. Co. Ltd, China. The other chemicals used in this study were of analytical grade without further purification.

  2.2 Preparation of 1

  1 was prepared by hydrothermal process. FeSO₄·7H₂O (0.0278 g, 0.1 mmol), H₂C₂O₄·2H₂O (0.0126 g, 0.1 mmol) and 4-HNCP (0.017 g, 0.05 mmol) were added into 15 mL of aqueous solution. The mixture was stirred for 30 min at room temperature, and then transferred into a 25 mL Teflon-lined stainless-steel autoclave and heated at 443 K for 72 h. Upon cooling and opening the bomb, brown block crystals of 1 were collected with a yield of 45% (based on Fe) by filtration and washed with distilled water. Elemental anal. Calcd. (%) for C₂₁H₁₂N₄O₆Fe (1, M_r = 472.20): C, 25.22; H, 12.10; N, 56.03. Found (%): C, 25.35; H, 12.21; N, 55.96. IR (cm⁻¹): 3414s, 1642s, 1588s, 1554s, 1449m, 1391s, 1311s, 1079m, 1014s, 967m, 922w, 794w, 730w, 532w, 492w.

  2.3 Characterization

  Elemental analysis for C, H and N was carried out on a Perkin-Elmer 240C elemental analyzer. The infrared (IR) spectrum was recorded as KBr pellets on a Perkin-Elmer 2400LSII spectrometer. Powder X-ray diffraction (PXRD) patterns were characterized by D/MAX-3C diffractometer with CuKα radiation (λ = 1.5406 Å). The morphology of 1 was observed by a JSM-6510 scanning electron microscope (SEM). The surface composition and chemical environment were analyzed by X-ray photoelectron spectroscopy (XPS, VG Scientific). Thermogravimetric analysis (TGA) of 1 was performed on a NETZSCH STA 449C analyzer heated from 40 to 1000 ℃ under N₂ atmosphere. The UV-vis spectra were recorded by Shimadzu UV-2500 spectrometer with BaSO₄ as reference.

  2.4 Single-crystal structure determination

  Single-crystal X-ray diffraction data for polymer 1 were measured at 293(2) K on a Bruker SMART APEX CCD area detector diffractometer by graphite-monochromated MoKα radiation (λ = 0.71073 Å). The structure was solved by direct methods of SHELXS-97 and refined by full-matrix least- squares techniques using the SHELXL-97 program^{[39, 40]}. The non-hydrogen atoms were anisotropically refined against F² by the full-matrix least-squares technique. 1 crystallizes in triclinic system, space group P$ \overline 1 $ with a = 8.5721(9), b = 9.3063(11), c = 12.3923(14) Å, α = 98.181(2)°, β = 92.118(2)°, γ = 110.166(2)°, V = 914.60(18) ų, Z = 2, C₂₁H₁₂N₄O₆Fe, M_r = 472.20, D_c = 1.715 g⋅cm^–3, μ = 0.877 mm^–1, F(000) = 480, GOOF = 1.032, the final R = 0.0468 and wR = 0.1017 for 2758 observed reflections with I > 2σ(I). The selected bond distances of the title complex are given in Table 1.

  Table 1

  

  Table 1.  Selected Bond Length (Å) and Bond Angle (°) for 1

  DownLoad: CSV

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Fe(1)–N(1)	2.185(3)		Fe(1)–N(2)	2.176(3)		Fe(1)–O(2)	2.131(2)
  Fe(1)–O(1)	2.161(2)		Fe(1)–O(4)	2.026(2)		Fe(1)–O(3)	2.196(2)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(4)–Fe(1)–O(2)	92.97(10)		O(4)–Fe(1)–O(1)	97.99(10)		O(2)–Fe(1)–O(1)	90.84(9)
  O(4)–Fe(1)–N(2)	94.75(10)		O(2)–Fe(1)–N(2)	171.67(10)		O(1)–Fe(1)–N(2)	91.17(9)
  O(4)–Fe(1)–N(1)	167.71(11)		O(2)–Fe(1)–N(1)	96.44(10)		O(1)–Fe(1)–N(1)	89.77(10)
  N(2)–Fe(1)–N(1)	75.49(10)		O(4)–Fe(1)–O(3)	88.90(10)		O(2)–Fe(1)–O(3)	76.77(9)
  O(1)–Fe(1)–O(3)	166.16(9)		N(2)–Fe(1)–O(3)	100.24(9)		N(1)–Fe(1)–O(3)	85.59(10)

  2.5 Photocatalytic test

  In a typical photocatalytic degradation process, 20 mg of 1 was mixed with an aqueous solution of RhB (100 mL, 10 mg/L) in a glass reaction flask. After the dark adsorption in 30 min, H₂O₂ (2 mmol) was added to the mixture solution. Afterwards, the suspension was exposed to visible-light illumination using a 250W Xe lamp with a cut off filter (420 nm). At every 20 min interval, 4 mL of the suspension was extracted and RhB solution was measured using a UV-vis spectrophotometer after centrifugation.

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure of 1

  The single-crystal X-ray crystallographic study reveals that 1 crystallizes in the triclinic crystal system with P$ \overline 1 $ space group. The crystal structure of 1 possesses a 2D network with one Fe(II) center, one 4-NCP ligand, a half C₂O₄ anion and two free water molecules in the asymmetric unit. Each Fe(II) center is six-coordinated [FeO₄N₂] in an octahedral coor- dination environment (Fig. 1). The basal plane and apical positions are occupied by N(1), N(2), O(2), O(4) and O(1), O(3), respectively. The Fe–O bond lengths are in the range of 2.026(2)~2.196(2) Å and the Fe–N bond lengths are 2.185(3) and 2.176(3) Å (Table 1), which are in accord with those reported Fe(II) complexes^[41]. It is more interesting that adjacent four Fe(II) centers are connected by 4-NCP ligands in (κ¹-κ¹)-(κ¹-κ¹)-μ₃ modes, leading to a novel 1-D double chain structure, with the Fe···Fe distances of 4.7134 and 15.3265 Å (Fig. 2). There exist π-π interactions between the neighboring 4-NCP ligands in the 1D chains, as indicated by a center-to-center distance of 3.523(2) Å, a dihedral angle of 6.85(17)^o. In addition, these 1D chains are connected with each other through C₂O₄ ligands in (κ¹-κ¹)-(κ¹-κ¹)-μ₂ manners, resulting in a 2D network, with the Fe···Fe distances of 5.5935 and 15.3265 Å. The overall network topology of 1 can be described as a (4, 4)-connected network. In addition, hydrogen bonds of O–H···O interactions are also found in the 2D network between the coordinated water molecules and carboxylic oxygen atoms, which further stabilize the 2D network (Table 2).

  Figure 1

  

  Figure 1.  Coordination environment of Fe atom in 1. Symmetry codes: #1: 1–x, 1–y, 1–z; #2: 2–x, –y, –z; #3: 2–x, 1–y, –z

  DownLoad: Full-Size Img PowerPoint

  Figure 2

  

  Figure 2.  View of the 2D structure of 1 (left) and (4, 4) net topology of the 2D layer (right)

  DownLoad: Full-Size Img PowerPoint

  Table 2

  

  Table 2.  Hydrogen-bonding Geometry (Å, °) for 1

  DownLoad: CSV

  D–H···A	d(D–H)	d(H···A)	d(D···A)	∠DHA
  O(1W)–H(1WA)···O(3)#3	0.76	2.03	2.776(4)	167
  O(1W)–H(1WB)···O(2W)	0.87	1.94	2.798(5)	166
  Symmetry code for 1: #3: x+1, y, z

  3.2 Characterization of 1

  The morphology and particle size of 1 were examined by SEM (Fig. 3). The as-prepared 1 sample is composed of block-like crystal with a length of tens of micrometers. The experimental and computer-simulated powder X-ray diffraction (XRD) patterns of 1 are presented in Fig. 4. They coincided well between the as-synthesized pattern and the simulated one, which indicates the phase purities of the sample. Fig. 5 shows XPS spectra of Fe orbitals before and after photocatalytic process. The consistency of XPS spectra before and after the catalysis indicated that the structure of 1 is intact and not collapse.

  Figure 3

  

  Figure 3.  SEM image of 1

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  Figure 4

  

  Figure 4.  PXRD patterns of the simulated (black), as-synthesized 1 (red), and after photocatalysis RhB (green)

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  Figure 5

  

  Figure 5.  XPS spectra of Fe orbital of 1 before and after catalysis

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  The UV-Vis diffuse reflectance spectrum is recorded to estimate the UV-vis absorption spectrum of 1. As shown in Fig. 6, 1 displays strong absorption in the range of 200~750 nm, which can be attributed to absorption induced by ligand-to-metal charge transfer (LMCT). The absorption onset of 1 is located at approximately 486 nm; thus, based on the relation E_g = 1240/λ^[42], the calculated bandgap of 1 is 2.55 eV (inset in Fig. 6).

  Figure 6

  

  Figure 6.  UV-Vis diffuse reflectance spectrum of 1

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  The thermal behavior of 1 was studied by TGA. As shown in Fig. 7, the TG curve of 1 shows two main steps of weight loss in the temperature range of 40~1000 ℃. The weight loss of 7.45% from 60 to 133 ℃ results from the release of water molecules (calcd. 7.62%), and that from 231 ℃ corresponds to the decomposition and collapse of the structure.

  Figure 7

  

  Figure 7.  TGA curve of polymer 1

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  3.3 Catalytic activity of 1

  The photodegradation of RhB was carried out under irradiation by visible light to evaluate the catalytic performance of 1. Fig. 8a displays the variations of RhB concentration (C/C₀) as a function of reaction time under different conditions. After visible light irradiation for 80 min, no observation of the photolysis of RhB in the absence of the catalyst 1, reflecting that RhB was quite stable toward incident light. As shown in Fig. 8a, only 4.68% of RhB was degraded in 80 min with the addition of H₂O₂ under visible light irradiation. And when only catalyst 1 was added under visible light irradiation, 4.93% of RhB was removed within 80 min. On the other hand, 25.16% of RhB was degraded in the presence of 1 and H₂O₂ under dark condition. Interestingly, 99.50% of RhB was decomposed after 80 min, when using 1 with H₂O₂ irradiated by visible light. As shown in Fig. 8b, the visible band of the dye decreased gradually as the reaction progressed. The photocatalytic activity of bare 1 was not satisfactory due to the rapid recombination of photogenerated electron-hole pairs according to the literature^[27-29]. Intri- guingly, the introduction of external H₂O₂ could hinder the recombination of photogenerated carriers and improve the photocatalytic activity of 1. The results demonstrate 1 exhibits good photocatalytic activity for RhB degradation in the presence of H₂O₂ under visible irradiation.

  Figure 8

  

  Figure 8.  (a) Degradation of RhB under different reaction conditions. (b) UV-vis spectral changes during RhB decolorization by 1

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  To determine the catalytic mechanism of 1, trapping experiments of active species were performed. The isopro- panol (IPA), ammonium oxalate (AO) and benzoquinone (BQ) were applied to the reaction system as ‧OH, h⁺ and ‧O₂^– scavengers, respectively. As shown in Fig. 9, in the presence of IPA, AO and BQ, the degradation efficiencies of RhB were 13.08%, 20.79% and 61.77%, respectively. The inhibitory effects of IPA and AO were much higher under visible light irradiation, contrast to the BQ scavenger. The free radical trapping experiments indicated that ‧OH and h⁺ are the major active species during the photocatalytic process. Fig. 10 shows the measured photocatalytic stability of 1. The photocatalytic efficiency decreased slightly through each cycle, indicating good reusability of 1 for RhB degradation under visible-light irradiation.

  Figure 9

  

  Figure 9.  Trapping experiments of active species degradation of RhB under visible-light irradiation

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  Figure 10

  

  Figure 10.  Reusability of 1 for the photocatalytic during the photocatalytic reaction

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  4. CONCLUSION

  A novel Fe-based coordination polymer was synthesized based on 4-HNCP and H₂C₂O₄ ligands under hydrothermal conditions. Polymer 1 possesses a (4, 4)-connected 2D network and further stabilizes the 2D framework through hydrogen bonding O–H···O interactions. Besides, polymer 1 exhibits good photocatalytic activity for RhB degradation in the presence of H₂O₂ under visible irradiation. 99.50% of RhB was degraded after 80 min in the presence of 1 and H₂O₂, which reveals 1 has promise as a photocatalyst for the treatment of dye wastewater.

  
  
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  Figure 1  Coordination environment of Fe atom in 1. Symmetry codes: #1: 1–x, 1–y, 1–z; #2: 2–x, –y, –z; #3: 2–x, 1–y, –z

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  Figure 2  View of the 2D structure of 1 (left) and (4, 4) net topology of the 2D layer (right)

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  Figure 3  SEM image of 1

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  Figure 4  PXRD patterns of the simulated (black), as-synthesized 1 (red), and after photocatalysis RhB (green)

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  Figure 5  XPS spectra of Fe orbital of 1 before and after catalysis

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  Figure 6  UV-Vis diffuse reflectance spectrum of 1

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  Figure 7  TGA curve of polymer 1

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  Figure 8  (a) Degradation of RhB under different reaction conditions. (b) UV-vis spectral changes during RhB decolorization by 1

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  Figure 9  Trapping experiments of active species degradation of RhB under visible-light irradiation

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  Figure 10  Reusability of 1 for the photocatalytic during the photocatalytic reaction

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  Table 1.  Selected Bond Length (Å) and Bond Angle (°) for 1

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Fe(1)–N(1)	2.185(3)		Fe(1)–N(2)	2.176(3)		Fe(1)–O(2)	2.131(2)
  Fe(1)–O(1)	2.161(2)		Fe(1)–O(4)	2.026(2)		Fe(1)–O(3)	2.196(2)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(4)–Fe(1)–O(2)	92.97(10)		O(4)–Fe(1)–O(1)	97.99(10)		O(2)–Fe(1)–O(1)	90.84(9)
  O(4)–Fe(1)–N(2)	94.75(10)		O(2)–Fe(1)–N(2)	171.67(10)		O(1)–Fe(1)–N(2)	91.17(9)
  O(4)–Fe(1)–N(1)	167.71(11)		O(2)–Fe(1)–N(1)	96.44(10)		O(1)–Fe(1)–N(1)	89.77(10)
  N(2)–Fe(1)–N(1)	75.49(10)		O(4)–Fe(1)–O(3)	88.90(10)		O(2)–Fe(1)–O(3)	76.77(9)
  O(1)–Fe(1)–O(3)	166.16(9)		N(2)–Fe(1)–O(3)	100.24(9)		N(1)–Fe(1)–O(3)	85.59(10)

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  Table 2.  Hydrogen-bonding Geometry (Å, °) for 1

  D–H···A	d(D–H)	d(H···A)	d(D···A)	∠DHA
  O(1W)–H(1WA)···O(3)#3	0.76	2.03	2.776(4)	167
  O(1W)–H(1WB)···O(2W)	0.87	1.94	2.798(5)	166
  Symmetry code for 1: #3: x+1, y, z

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