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Ultrasound irradiation effect on morphological properties of a 3D nano zinc(II) supramolecular coordination polymer
Ultrasonics - Sonochemistry 41 (2018) 67–74
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Ultrasound irradiation effect on morphological properties of a 3D nano zinc (II) supramolecular coordination polymer Hai Ning Chang, Suo Xia Hou, Zeng Chuan Hao, Guang Hua Cui
MARK
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College of Chemical Engineering, Hebei Key Laboratory for Environment Photocatalytic and Electrocatalytic Materials, North China University of Science and Technology, No. 21 Bohai Road, Caofeidian New-city, Tangshan, Hebei 063210, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Coordination polymer Nano Photocatalytic Sonochemical Supramolecule
Nano-structures of [Zn(L)(atpt)]n (1) (L = 1,2-bis(2-methylbenzimidazol-1-ylmethyl)benzene and H2atpt = 2aminoterephthalic acid) were obtained by hydrothermal and sonochemical approaches, characterized by scanning electron microscopy (SEM), IR, powder X-ray diffraction (PXRD), and elemental analysis. CP 1 features a 2D (4,4) network with the point symbol {44.62}, the 3D supramolecular architecture in CP 1 is controlled through π⋯π stacking interactions. The influence of various concentrations of initial reagents, power of ultrasound irradiation, and ultrasound time on the morphology and size of nano-structured CP 1 were studied in detail. In addition, the luminescence and photocatalytic properties of the nanoparticles of CP 1 for the degradation of methyl blue (MB) have also been investigated.
1. Introduction Crystal engineering of coordination polymers (CPs) is of great current interest to develop new crystalline materials for possible use in a variety of applications, such as sensing, catalysis, magnetism, gas storage and separation [1–7]. These functional materials are finely tuned by judicious combination of metal centers with predesigned bridging ligands under suitable conditions [8,9]. Zinc(II) is really a well-known metal with a variety of applications in numerous areas, they can adopt different coordination modes when they react with organic ligands, their d10 configuration is assigned to a flexible coordination environment to ensure that different geometries can be generated to tailormade materials [10–13]. Organic ligands can help control the framework and topology as one-/two-/three-dimensional architectures [14,15]. Nowadays, organic dyes from the textile, dyeing and other industries are discharged into the local environment without adequate treatment [16,17]. Conventional methods have been established for wastewater treatment only make the pollutants transfer from the liquid phase to the solid phase. Hence, it is essential to search green and economical treatment to purify the pollutants, photocatalysis technology has been used to remove organic contaminants from water and air [18,19], while coordination polymer is regarded as one of the most potent photocatalyst for its low band gap, which can accelerate separation rate of photogenerated electron-hole pairs [20,21]. Further, we select 2-aminoterephthalic acid as connectors, because aminated
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linkers are known to enhance the absorption of light and afford photocatalysis with CPs under visible-light irradiation [22]. Assembly of CPs with diverse shape and sizes are very significant for the applications such as heterogeneous photocatalyst. In nano-sized particles, the ratio of surface area to volume is more increased than the particles with bulk sizes. The size effect plays an important role on physical and chemical properties such as catalytic behavior [23]. To prepare nano-structured CPs, utilization of ultrasound has been attracted much attention recently [24–27]. Sonochemistry is a research area of chemical reactions that are influenced by the application of powerful ultrasound radiation (20 kHz–10 MHz). Ultrasound induces chemical or physical changes during cavitation, a phenomenon involving the formation, growth, and instantaneously implosive collapse of bubbles in a liquid. This can generate local hot spots with temperatures up to 5000 °C, 500 atm pressures and a lifetime of a few microseconds. These extreme conditions can drive chemical reactions and can promote the formation of nano-structures, mostly via an increase of crystallization nuclei [28–30]. Hence, we utilize sonochemical method to synthesize nano-sized ZnII coordination polymer [Zn(L)(atpt)]n (1) (L = 1,2-bis(2-methylbenzimidazol-1-ylmethyl)benzene and H2atpt = 2-aminoterephthalic acid). Scanning electron microscopy (SEM) revealed that the concentration of initial reagents, ultrasound irradiation time and power exhibit significant effects on the size and shape of CP 1. Further, the luminescence and photocatalytic properties of the CP 1 were also presented.
Corresponding author. E-mail address: [email protected] (G.H. Cui).
http://dx.doi.org/10.1016/j.ultsonch.2017.09.024 Received 24 July 2017; Received in revised form 14 September 2017; Accepted 15 September 2017 Available online 18 September 2017 1350-4177/ © 2017 Elsevier B.V. All rights reserved.
Ultrasonics - Sonochemistry 41 (2018) 67–74
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2. Experimental 2.1. Materials and methods 1,2-bis(2-methylbenzimidazol-1-ylmethyl)benzene was synthesized according to the literature procedure [31]. Other reagents and solvents were purchased from Jinan Henghua Sci. and used without further purification. Powder X-ray diffraction (PXRD) patterns were measured on a Rigaku D/Max-2500 diffractometer. Ultrasound was generated by a multi-wave KQ2200DE at a frequency of 40 kHz. Elemental analyses of C, H and N were determined using a PerkinElmer 240 C elemental analyzer. IR spectra using KBr pellets were recorded on an Avatar 360 (Nicolet) spectrophotometer in the region of 4000–400 cm−1. The TGA measurement was performed on a NETZSCH TG 209 thermal analyzer from room temperature to 800 °C with a heating rate of 10 °C·min−1 under N2 atmosphere. The luminescence spectra were obtained by using FS5 fluorescence spectrophotometer equipped with a continuous xenon lamp at room temperature. Solid-state UV/Vis diffuse reflectance spectra were measured using a UV–Vis Puxi T9 UV–visible spectrophotometer with BaSO4 as a reference. The crystallite sizes of selected samples were estimated using the Scherrer equation. The morphology of the suitable samples was investigated using a JSM-IT100 scanning electron microscope after gold coating. 2.2. Synthesis of [Zn(L)(atpt)]n as single crystal A mixture of Zn(OAc)2·2H2O (0.2 mmol, 43.9 mg), L ligand (0.2 mmol, 73.2 mg), H2atpt (0.2 mmol, 36.2 mg), and H2O (10 mL) was sealed in a 25 mL Teflon-lined autoclave and heated to 140 °C for 3 days under autogenous pressure. Afterwards, the autoclave was cooled to room temperature at a rate of 5 °C·h−1. Colorless block-like crystals of CP 1 were obtained. Yield: 42.4% based on Zn. Calc. for C32H27ZnN5O4 (610.98): C, 62.91; H, 4.45; N, 11.46%. Found: C, 62.45; H, 4.16; N, 11.75%. IR (KBr, cm−1): 3331 w, 3142 w, 2914 w, 1570 s, 1370 s, 1260 m, 1132 m, 1067 m, 840 w, 537 w. 2.3. Synthesis of [Zn(L)(atpt)]n via a sonochemical process Ultrasonic syntheses of CP 1 were carried out in an ultrasonic bath at room temperature and atmospheric pressure. To prepare the nanoparticle, a solution of zinc(II) acetate two hydrate with certain concentration in water was placed in an ultrasonic bath. Into this solution, we added a mix-ligand solution of L ligand and H2atpt in water dropwise. For further studies, these series of experiments were performed in the concentration of all reagents of 0.005 M, times of 60 min, and power of 70 W. After that, the resulting precipitates were isolated by centrifugation, washed with some quantities of distilled cold water and drying in air. 64.1% of sample 1 was obtained referred to Zn. Calc. for C32H27ZnN5O4 (610.98): C, 62.91; H, 4.45; N, 11.46%. Found: C, 62.57; H, 4.21; N, 11.45%. IR (KBr, cm−1): 3331 w, 3134 w, 2925 w, 1551 s, 1370 s, 1251 m, 1128 m, 1069 m, 830 w, 529 w. These processes were also done with other two various concentrations of all reagents (0.005, 0.01 and 0.05 M), different times (30 and 90 min), and different powers (40 and 100 W) to study its effects on the size and morphology of nanostructured CP 1. 2.4. X-ray crystallography Diffraction data for CP 1 was collected at 296(2) K with a Bruker Smart 1000 CCD diffractometer with Mo-Kα radiation (λ = 0.71073 Å) and ω scan mode. Absorption corrections were applied using the SADABS program [32]. The structure was solved by direct methods and refined with full-matrix least-squares technique based on F2 using the SHELXL-2016 program [33]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms bonded to carbon atoms were placed in geometrically calculated positions and refined with isotropic thermal
(caption on next page)
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H.N. Chang et al. Fig. 1. (a) The coordination environment for Zn(II) center in CP 1, showing thermal ellipsoids at 30% probability. Symmetry codes: A: x, −y + 1/2, z−1/2, B: −x + 2, y + 1/ 2, −z + 3/2; (b) View of the 2D wave-like network; (c) The schematic view of a 2D (4,4) net topology; (d) The 3D supramolecular framework is extended via π⋯π stacking interactions (purple dashed lines, centroid-to-centroid distance of 3.572 Å). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
higher thermal stability than nano CP 1, due to the much higher surface to volume ratio of the nanoparticles, as more heat is needed to annihilate the lattices of the single crystals [36]. The IR spectra of the nanoparticles and the bulk materials of CP 1 show similar absorption bands (Fig. S3). The band around 3331 cm−1 is attributed to the absorption of the N–H group of the atpt2− ligands. The characteristic asymmetric and symmetric vibration bands of the carboxylate groups can be viewed at about 1570 and 1370 cm-1.
parameters riding on the corresponding parent atoms. Crystallographic data and structure determination summaries for CP 1 are listed in Table S1. Selected bond lengths and angles are listed in Table S2.
3.3. Morphology Ultrasonic synthesis is a simple and quickly approach utilized in the synthesis of nano materials. The morphology and size of the nanostructures of CP 1 prepared by the ultrasonic method were characterized using Scanning Electron Microscopy (SEM) at room time by changing three parameters, such as: concentration of initial reagents, sonication time, and sonication power. For CP 1, concentration of initial reagents as a parameter was changed at a constant sonication time and sonication power of 60 min and 70 W, respectively. Nanoplates of CP 1 were obtained for all three different concentrations (0.005, 0.01 and 0.05 M, marked as samples 1, 2, and 3, respectively). Comparison between the products with different concentration of 0.005 and 0.01 M displays that higher concentration of initial reagents led to more regular nanostructures and narrower morphology (Fig. 2a and b). When the concentration of initial reagents is up to 0.05 M (Fig. 2c), nonuniform distribution of particle size occurs and morphology changes to an agglomerated structure, which is due to the fact that a higher concentration of starting reagents could lead to a decrease in the nucleation rate and the number of nucleation centers, leading to larger particle sizes [37,38]. In addition, to investigate the role of sonication time on the morphology and size of products, reactions were performed with different times (30, 60 and 90 min, labelled as samples 4, 2, and 5, respectively, Figs. 3a, 2b, and 3b) at a constant concentration of initial reagents and sonication power of 0.01 M and 70 W, separately. There are uniform nanoparticles with good size distribution in all cases, while the thickness of nano-sized coordination polymer plates increases and the morphology of these nanostructures becomes more regular over the sonication time. We still discussed the effect of ultrasonic power on morphology and size of nano-structured CP 1. The reactions were carried out with different ultrasonic powers (40, 70 and 100 W, labelled as samples 6, 4, and 7, respectively, Figs. 4a, 3a, and 4b) at a constant concentration of initial reagents and sonication time of 0.01 M and 30 min, separately. In contrast with sample f, the image of sample d showed that the size was smaller than that of sample f, and size of sample g is the smallest one, indicating that high power ultrasound irradiation decreased agglomeration, and thus led to decrease particles size. Table 1 shows an overview of the comparison of the concentration of initial reagents, ultrasound times and powers effect of ultrasonic irradiation on the morphologies of nanoparticles of CP 1. In addition, SEM images of CP 1 show in Fig. 5, indicating that the morphologies of CP 1 synthesized from sonochemistry method is similar to that obtained by hydrothermal means.
3. Results and discussion 3.1. Crystal structure of [Zn(L)(atpt)]n (1) CP 1 crystallizes in the monoclinic space group P21/c. The asymmetrical unit contains one crystallographically independent Zn(II) center, one L ligand and one atpt2- ligand. As depicted in Fig. 1a, each Zn(II) center is four-coordinated by two nitrogen atoms (Zn1–N1 = 2.024(2) Å and Zn1–N4A = 2.096(3) Å, symmetry code: A = x, −y + 1/2, z − 1/2) of two L ligands and two carboxylate oxygen atoms (Zn1–O1 = 1.912(2) Å and Zn1–O3B = 1.965(2) Å, symmetry code: B = −x + 2, y + 1/2, −z + 3/2) of two atpt2- ligands, giving a distorted tetrahedron geometry with a τ4 value of 0.86 [34]. The coordination angles range from 97.06(10) to 127.44(11)°, which are comparable to values observed for similar zinc(II) CPs [35]. Each L ligand adopts the trans-conformation to bridge the neighboring Zn(II) centers to give an infinite 1D [Zn(L)]n wave-like chain with the non-bonding distance of Zn⋯Zn is 9.176(8) Å. The Zn(II) centers are bridged by the monodentate carboxylic groups to generate 1D [Zn(atpt)]n zigzag chains, spanning the non-bonding distance of Zn···Zn is 10.877(6) Å. The two type chains are further connected each other to form a 2D wave-like network (Fig. 1b), which can be simplified as a 4-connected sql net with the point symbol of {44.62}. As shown in Fig. 1c, the 2D later contains two distinct parallelograms, such as quadrangle Zn1-Zn1A-Zn1C-Zn1B (Zn1–Zn1A = 9.176(8) Å, Zn1–Zn1B = 10.877(6) Å, Zn1A–Zn1–Zn1B = 86.01(4)°, and Zn1–Zn1A–Zn1C = 93.99(4)° symmetry code: C = −x + 2, −y + 1, −z + 1), the adjacent quadrilateral, sharing an edge with quadrangle Zn1Zn1AZn1CZn1 B, has the dimension of 9.176(8) Å × 10.877(6) Å, and the angles of 77.26(4)° and 102.74(4)°. Finally, the adjacent two layers are further interdigitated into each other, fabricating a 3D supramolecular architecture through π⋯π stacking interactions between the imidazole and benzene rings of distinct L ligands (Cg1: N1–C11–N2–C17–C12, Cg2: C12–C13–C14–C15–C16–C17, centroid-tocentroid distance of 3.572 Å) (Fig. 1d), and the interplanar angle α is 0.57(17)°, slipping angles β and γ are 21.09° and 21.31°, respectively. 3.2. PXRD, TGA, and IR To check the phase purities of CP 1 synthesized by hydrothermal method and prepared by the ultrasonic irradiation, the powder X-ray diffraction patterns were carried out at room temperature. As shown in Fig. S1, the slight differences in 2θ were found between the experimental and simulated patterns, demonstrating the good phase purity of samples. CP 1 produced by ultrasonic irradiation as nanostructures was well identical to that single crystal diffraction. To estimate the thermal stability of bulk and nano-structured CP 1, TG analyses were carried out. As shown in Fig. S2, the TG curves of single crystal and nanostructured CP 1 both show a sharp weight loss stage from 360 to 700 °C, corresponding to the decomposition of the L and atpt2− ligands, the initial collapse temperature value of 360 °C indicates CP 1 have high stability. The residue weights (13.9% for bulk CP 1, 12.5% for nano-sized CP 1) are in correspondence with the calculated value (13.3%) of ZnO. The result shows that bulk CP 1 has
3.4. Photophysical properties The d10 metal centers based coordination polymers have received remarkable attention in view of various potential applications, such as in chemical sensors, photochemistry, electroluminescent display, and so on [39–41]. The solid-state luminescent emission properties of free L ligand and nanoparticles of CP 1 were investigated at room temperature (Fig. 6). The main emission peaks of free L ligand is at 375 nm (λex = 269 nm), which can be assigned to the π∗ → π or π∗ → n transitions [42]. Upon photoexcitation, nano-structured CP 1 displays the maximum emission peaks at 422 nm (λex = 330 nm). Comparing with 69
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Fig. 3. SEM images of nanostructured CP 1 with concentration of initial reagents of 0.01 M, power of 70 W, and different times (30 and 90 min, respectively) corresponding to pictures a and b.
property of CP 1, according to the formula: Eg = 1240/λg (which λg is absorption threshold and Eg is the energy gap), it can be estimated that the Eg of CP 1 was 2.85 eV. The results suggest that the CP 1 photocatalyst may be possesses the high photocatalytic activity, which corresponded to the process of photodegradation. Organic dyes are common compounds that can be used in many fields. Hence, large amounts of dyes are released into effluent streams [44,45]. Methylene blue (MB) as typical pollutants in aqueous media owing to their toxicity and degradation-resistant, have become one of the most serious environmental issues. Herein, we selected MB as a target pollutant to evaluate the photocatalytic performance of nanosized CP 1. The photodegradation experiment under UV irradiation was carried out after the dark adsorption–desorption equilibrium achieved, the detailed photocatalytic reaction procedures were listed in the ESI. As shown in Figs. 8, 9 and S4a, after 60 min, during the dark adsorption–desorption equilibrium, conversion ratio percent of absorbed MB by nano CP 1 and TiO2 are 12.7% and 23.3%, respectively, while conversion ratio percent of the existence of only MB is 1.6%. When CP 1 was dispersed in MB solution under UV irradiation for 90 min, the absorption peak of MB distinctly decrease along with increasing reaction time, the degradation efficiency of MB reached 90.7%, this value is higher than many other Zn-based CPs [46,47]. Moreover, we also tested the photocatalytic behaviors of the P25 TiO2 catalysts under the same conditions as contrast experiment, and we find the photodegradation efficiency of TiO2 is up to 84.6% (Figs. 9 and S4b), which is lower than the nano-structured CP 1. The efficiency of the control experiment for the photodegradation of MB without any catalysts is 15.8%, in which
Fig. 2. SEM images of nanostructured CP 1 with sonochemical time of 60 min, power of 70 W, and different concentration of initial reagents (0.005, 0.01 and 0.05 M, respectively) corresponding to pictures a, b, and c.
the emission of free L ligand, the emission band of nano-sized CP 1 indicate significant red shifts (by 47 nm), which can be attributed to the charge transfer from benzimidazole-based ligands to Zn(II) centers (LMCT) [43]. In order to explore the conductivities of CP 1, their diffuse reflectance UV/Vis spectrum was measured for nano-sized CP 1 in order to obtain the band gap Eg. As shown in Fig. 7, the intense absorption peaks at 330 and 390 nm for CP 1, which can be attributed to π∗ → π or π∗ → n transitions of the ligand or LMCT. To obtain the semiconductor 70
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Fig. 5. SEM images of CP 1 prepared by hydrothermal method.
Fig. 4. SEM images of nanostructured CP 1 with concentration of initial reagents of 0.01 M, time of 30 min, and different powers (40 and 100 W, respectively) corresponding to pictures a and b. Fig. 6. Solid-state emission spectra of free L ligand and nanoparticles of CP 1. Table 1 The influence of concentration of initial reagents, reaction time and sonication power on the size of CP 1.
Sample Sample Sample Sample Sample Sample Sample a b c d
1 2 3 4 5 6 7
T(°C)a
C(M)b
t(min)c
P(W)d
Average diameter (nm)
Yield/%
25 25 25 25 25 25 25
0.005 0.01 0.05 0.01 0.01 0.01 0.01
60 60 60 30 90 30 30
70 70 70 70 70 40 100
315 192 – 156 702 234 67
84.1 71.3 67.4 79.5 65.3 74.2 86.2
Represents reaction temperature. Represents concentration of initial reagents. Represents reaction time. Represents sonication power.
the concentration of MB almost does not change. To further confirm the catalytic activity of the as-synthesized materials, UV kinetics and UV/catalyst degradation kinetics are described by Langmuir–Hinshelwood kinetic equation (1),
ln(C0/ C ) = kt
Fig. 7. UV/Vis absorption spectra of nano-sized CP 1 with BaSO4 as background, and diffuse reflectance spectra of Kubelka-Munk function vs. energy of nano-sized CP 1.
(1) and 0.01784 min−1 for TiO2, respectively. All these indicate that the asprepared nano-sized CP 1 possesses higher photocatalytic activity. The second kinetic equation of photo-degradation can be indicated the Eq. (2). And the linear relation of 1/C and t was fitted by Fig. 11.
where k is pseudo first order dynamic rate constant. C0 and C are the initial and the remaining concentrations of MB at regular time intervals, respectively. According to Eq. (1), the linear relation of ln(C0/C) and t was followed Fig. 10, and the k values were calculated to be 0.02593 min−1 for nano CP 1, 0.00187 min−1 for control experiment, 71
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Fig. 11. The second-kinetic plots for the photodegradation of MB in the presence of nano CP 1 or P25 TiO2, and in the absence of catalyst under UV irradiation.
Fig. 8. Absorption spectra of the MB solutions during the photodegradation reaction under UV light with the presence of nano-sized CP 1.
Table 2 Kinetics parameters for the photodegradation of different targets. First kinetic equation 2
nano CP 1 without catalyst TiO2
Second kinetic equation −1
R
k/min
0.99809 0.97970 0.67646
0.02593 0.00187 0.01784
R2
k/L·mol−1·min−1
0.89409 0.98631 0.86115
0.44146 0.00786 0.22336
can be seen that the first kinetic linear regression coefficients (R2) are higher than the second kinetic linear regression coefficients for CP 1 nanoparticles. It suggests that the degradation process of nano CP 1 should be better fitted the first kinetic equation. On the contrary, the photodegradation course of control experiment and TiO2 are better fitted the second kinetic equation, and the k values of the second kinetic equation were calculated to be 0.44146 L·mol−1·min−1 for nano CP 1, 0.00786 L·mol−1·min−1 for control experiment, and 0.22336 L·mol−1·min−1 for TiO2, respectively. The results are still displayed nano-sized CP 1 shows high photocatalytic behavior. To make clear what active species are involved in the photocatalytic process occurring on CP 1, the formation of %OH, h+, and %O2− on the surface of UV-illuminated CP 1 was respectively detected by using tbutyl alcohol (TBA), ammonium oxalate (AO), and benzoquinone (BQ) as scavengers. As shown in Figs. 12 and S5, we can observe that the
Fig. 9. The experiment results of the photodegradation of MB.
Fig. 10. The first-kinetic plots for the photodegradation of MB in the presence of nano CP 1 or P25 TiO2, and in the absence of catalyst under UV irradiation.
1 1 − = kt C C0
(2)
According to the Langmuir–Hinshelwood kinetic equation, kinetics parameters for photocatalytic degradation of first and second kinetic equation of different targets were listed in Table 2. From the results, it
Fig. 12. Trapping experiment of active species during the photocatalytic reaction for nano-sized CP 1.
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size of nanoparticles were also studied. The results demonstrate that the concentrations of initial reagents of 0.01 M, the sonication time of 30 min, and the ultrasonic power of 100 W may obtain optimal nanoparticles, the size of nanoparticles can up to 67 nm. Further, the thermostability and photocatalytic behaviors of the nano-sized CP 1 may make it an excellent candidate as a catalysis material. Acknowledgments The project was supported by the National Natural Science Foundation of China (51474086) and Natural Science Foundation – Steel and Iron Foundation of Hebei Province (B2015209299). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ultsonch.2017.09.024.
Fig. 13. Schematic illustration of the photocatalytic mechanism of the nano-sized CP 1.
presence of TBA greatly suppressed the degradation activity of nanoparticles of CP 1 toward MB solution. The %OH quenching experiment indicates that the photodegradation process of MB with nano-sized CP 1 as catalyst predominantly involves attack by %OH radicals, implying that the formation rate of %OH radicals is an important factor influencing photocatalytic activity. Moreover, it can be found that the photodegradation of MB was also moderately affected by the addition of BQ and AO, indicating that the existence of the photogenerated %O2−, h+ and %OH, among them, %OH radical is the main active species of CP 1 for the photodegradation of MB under UV light irradiation. Consequently, a simplified model of photocatalytic reaction mechanism was proposed as depicted in Fig. 13, the HOMO is mainly contributed by oxygen and (or) nitrogen 2p bonding orbitals (valence band, VB) and the LUMO by empty Zn orbitals (conduction band, CB). Under UV light irradiation, electrons (e−) in the HOMO (VB) of CP 1 were excited to its LUMO (CB), with same amount of holes (h+) left in VB. The HOMO strongly demands one electron to return to its stable state. Therefore, one electron was captured from water molecules, which was oxygenated into %OH active species. Meanwhile, the electrons (e-) in LUMO could be combined with the oxygen adsorbed on the surfaces of CP 1 to form %O2−, then they might transform to the hydroxyl radicals (%OH). Then the formed •OH radicals could cleave MB effectively to complete the photocatalytic process. Further, the stability and recyclability of the CP 1 is a very significant element for practical applications, especially for commercial and industrial applications. The reusability of nano-sized CP 1 was tested in the degradation of MB under the irradiation of UV light. After the completion of the reaction, 3.0 mL of the reaction mixture was taken out, separated through centrifugation, washed with water and ethanol, dried and reused for five successive cycles of reaction without evident change of its degradation efficiency, indicating the stable and high recycling efficiency of nanoparticles of CP 1 (Fig. S6). Further, PXRD was also performed to confirm the chemical stability of nanosized CP 1. As shown in Fig. S7, the results of the sample after the fifth run were almost the same to that of the simulated pattern, which indicates that nanoparticles of CP 1 were stable during the photocatalytic process.
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4. Conclusions A nanostructured Zn(II) coordination polymer based on 2-aminoterephthalic acid (H2atpt) and 1,2-bis(2-methylbenzimidazol-1-ylmethyl)benzene (L) was successfully synthesized by conventional hydrothermal synthesis and sonochemical irradiation. The crystal structure of CP 1 is a 2D 4-connected sql net with the point symbol of {44.62}, which is further extended into a 3D supramolecular framework by π⋯π stacking interactions. Effects of the concentrations of initial reagents, sonication time and ultrasonic power on the morphology and 73
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Ultrasound irradiation effect on morphological properties of a 3D nano zinc (II) supramolecular coordination polymer Hai Ning Chang, Suo Xia Hou, Zeng Chuan Hao, Guang Hua Cui
MARK
⁎
College of Chemical Engineering, Hebei Key Laboratory for Environment Photocatalytic and Electrocatalytic Materials, North China University of Science and Technology, No. 21 Bohai Road, Caofeidian New-city, Tangshan, Hebei 063210, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Coordination polymer Nano Photocatalytic Sonochemical Supramolecule
Nano-structures of [Zn(L)(atpt)]n (1) (L = 1,2-bis(2-methylbenzimidazol-1-ylmethyl)benzene and H2atpt = 2aminoterephthalic acid) were obtained by hydrothermal and sonochemical approaches, characterized by scanning electron microscopy (SEM), IR, powder X-ray diffraction (PXRD), and elemental analysis. CP 1 features a 2D (4,4) network with the point symbol {44.62}, the 3D supramolecular architecture in CP 1 is controlled through π⋯π stacking interactions. The influence of various concentrations of initial reagents, power of ultrasound irradiation, and ultrasound time on the morphology and size of nano-structured CP 1 were studied in detail. In addition, the luminescence and photocatalytic properties of the nanoparticles of CP 1 for the degradation of methyl blue (MB) have also been investigated.
1. Introduction Crystal engineering of coordination polymers (CPs) is of great current interest to develop new crystalline materials for possible use in a variety of applications, such as sensing, catalysis, magnetism, gas storage and separation [1–7]. These functional materials are finely tuned by judicious combination of metal centers with predesigned bridging ligands under suitable conditions [8,9]. Zinc(II) is really a well-known metal with a variety of applications in numerous areas, they can adopt different coordination modes when they react with organic ligands, their d10 configuration is assigned to a flexible coordination environment to ensure that different geometries can be generated to tailormade materials [10–13]. Organic ligands can help control the framework and topology as one-/two-/three-dimensional architectures [14,15]. Nowadays, organic dyes from the textile, dyeing and other industries are discharged into the local environment without adequate treatment [16,17]. Conventional methods have been established for wastewater treatment only make the pollutants transfer from the liquid phase to the solid phase. Hence, it is essential to search green and economical treatment to purify the pollutants, photocatalysis technology has been used to remove organic contaminants from water and air [18,19], while coordination polymer is regarded as one of the most potent photocatalyst for its low band gap, which can accelerate separation rate of photogenerated electron-hole pairs [20,21]. Further, we select 2-aminoterephthalic acid as connectors, because aminated
⁎
linkers are known to enhance the absorption of light and afford photocatalysis with CPs under visible-light irradiation [22]. Assembly of CPs with diverse shape and sizes are very significant for the applications such as heterogeneous photocatalyst. In nano-sized particles, the ratio of surface area to volume is more increased than the particles with bulk sizes. The size effect plays an important role on physical and chemical properties such as catalytic behavior [23]. To prepare nano-structured CPs, utilization of ultrasound has been attracted much attention recently [24–27]. Sonochemistry is a research area of chemical reactions that are influenced by the application of powerful ultrasound radiation (20 kHz–10 MHz). Ultrasound induces chemical or physical changes during cavitation, a phenomenon involving the formation, growth, and instantaneously implosive collapse of bubbles in a liquid. This can generate local hot spots with temperatures up to 5000 °C, 500 atm pressures and a lifetime of a few microseconds. These extreme conditions can drive chemical reactions and can promote the formation of nano-structures, mostly via an increase of crystallization nuclei [28–30]. Hence, we utilize sonochemical method to synthesize nano-sized ZnII coordination polymer [Zn(L)(atpt)]n (1) (L = 1,2-bis(2-methylbenzimidazol-1-ylmethyl)benzene and H2atpt = 2-aminoterephthalic acid). Scanning electron microscopy (SEM) revealed that the concentration of initial reagents, ultrasound irradiation time and power exhibit significant effects on the size and shape of CP 1. Further, the luminescence and photocatalytic properties of the CP 1 were also presented.
Corresponding author. E-mail address: [email protected] (G.H. Cui).
http://dx.doi.org/10.1016/j.ultsonch.2017.09.024 Received 24 July 2017; Received in revised form 14 September 2017; Accepted 15 September 2017 Available online 18 September 2017 1350-4177/ © 2017 Elsevier B.V. All rights reserved.
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2. Experimental 2.1. Materials and methods 1,2-bis(2-methylbenzimidazol-1-ylmethyl)benzene was synthesized according to the literature procedure [31]. Other reagents and solvents were purchased from Jinan Henghua Sci. and used without further purification. Powder X-ray diffraction (PXRD) patterns were measured on a Rigaku D/Max-2500 diffractometer. Ultrasound was generated by a multi-wave KQ2200DE at a frequency of 40 kHz. Elemental analyses of C, H and N were determined using a PerkinElmer 240 C elemental analyzer. IR spectra using KBr pellets were recorded on an Avatar 360 (Nicolet) spectrophotometer in the region of 4000–400 cm−1. The TGA measurement was performed on a NETZSCH TG 209 thermal analyzer from room temperature to 800 °C with a heating rate of 10 °C·min−1 under N2 atmosphere. The luminescence spectra were obtained by using FS5 fluorescence spectrophotometer equipped with a continuous xenon lamp at room temperature. Solid-state UV/Vis diffuse reflectance spectra were measured using a UV–Vis Puxi T9 UV–visible spectrophotometer with BaSO4 as a reference. The crystallite sizes of selected samples were estimated using the Scherrer equation. The morphology of the suitable samples was investigated using a JSM-IT100 scanning electron microscope after gold coating. 2.2. Synthesis of [Zn(L)(atpt)]n as single crystal A mixture of Zn(OAc)2·2H2O (0.2 mmol, 43.9 mg), L ligand (0.2 mmol, 73.2 mg), H2atpt (0.2 mmol, 36.2 mg), and H2O (10 mL) was sealed in a 25 mL Teflon-lined autoclave and heated to 140 °C for 3 days under autogenous pressure. Afterwards, the autoclave was cooled to room temperature at a rate of 5 °C·h−1. Colorless block-like crystals of CP 1 were obtained. Yield: 42.4% based on Zn. Calc. for C32H27ZnN5O4 (610.98): C, 62.91; H, 4.45; N, 11.46%. Found: C, 62.45; H, 4.16; N, 11.75%. IR (KBr, cm−1): 3331 w, 3142 w, 2914 w, 1570 s, 1370 s, 1260 m, 1132 m, 1067 m, 840 w, 537 w. 2.3. Synthesis of [Zn(L)(atpt)]n via a sonochemical process Ultrasonic syntheses of CP 1 were carried out in an ultrasonic bath at room temperature and atmospheric pressure. To prepare the nanoparticle, a solution of zinc(II) acetate two hydrate with certain concentration in water was placed in an ultrasonic bath. Into this solution, we added a mix-ligand solution of L ligand and H2atpt in water dropwise. For further studies, these series of experiments were performed in the concentration of all reagents of 0.005 M, times of 60 min, and power of 70 W. After that, the resulting precipitates were isolated by centrifugation, washed with some quantities of distilled cold water and drying in air. 64.1% of sample 1 was obtained referred to Zn. Calc. for C32H27ZnN5O4 (610.98): C, 62.91; H, 4.45; N, 11.46%. Found: C, 62.57; H, 4.21; N, 11.45%. IR (KBr, cm−1): 3331 w, 3134 w, 2925 w, 1551 s, 1370 s, 1251 m, 1128 m, 1069 m, 830 w, 529 w. These processes were also done with other two various concentrations of all reagents (0.005, 0.01 and 0.05 M), different times (30 and 90 min), and different powers (40 and 100 W) to study its effects on the size and morphology of nanostructured CP 1. 2.4. X-ray crystallography Diffraction data for CP 1 was collected at 296(2) K with a Bruker Smart 1000 CCD diffractometer with Mo-Kα radiation (λ = 0.71073 Å) and ω scan mode. Absorption corrections were applied using the SADABS program [32]. The structure was solved by direct methods and refined with full-matrix least-squares technique based on F2 using the SHELXL-2016 program [33]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms bonded to carbon atoms were placed in geometrically calculated positions and refined with isotropic thermal
(caption on next page)
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H.N. Chang et al. Fig. 1. (a) The coordination environment for Zn(II) center in CP 1, showing thermal ellipsoids at 30% probability. Symmetry codes: A: x, −y + 1/2, z−1/2, B: −x + 2, y + 1/ 2, −z + 3/2; (b) View of the 2D wave-like network; (c) The schematic view of a 2D (4,4) net topology; (d) The 3D supramolecular framework is extended via π⋯π stacking interactions (purple dashed lines, centroid-to-centroid distance of 3.572 Å). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
higher thermal stability than nano CP 1, due to the much higher surface to volume ratio of the nanoparticles, as more heat is needed to annihilate the lattices of the single crystals [36]. The IR spectra of the nanoparticles and the bulk materials of CP 1 show similar absorption bands (Fig. S3). The band around 3331 cm−1 is attributed to the absorption of the N–H group of the atpt2− ligands. The characteristic asymmetric and symmetric vibration bands of the carboxylate groups can be viewed at about 1570 and 1370 cm-1.
parameters riding on the corresponding parent atoms. Crystallographic data and structure determination summaries for CP 1 are listed in Table S1. Selected bond lengths and angles are listed in Table S2.
3.3. Morphology Ultrasonic synthesis is a simple and quickly approach utilized in the synthesis of nano materials. The morphology and size of the nanostructures of CP 1 prepared by the ultrasonic method were characterized using Scanning Electron Microscopy (SEM) at room time by changing three parameters, such as: concentration of initial reagents, sonication time, and sonication power. For CP 1, concentration of initial reagents as a parameter was changed at a constant sonication time and sonication power of 60 min and 70 W, respectively. Nanoplates of CP 1 were obtained for all three different concentrations (0.005, 0.01 and 0.05 M, marked as samples 1, 2, and 3, respectively). Comparison between the products with different concentration of 0.005 and 0.01 M displays that higher concentration of initial reagents led to more regular nanostructures and narrower morphology (Fig. 2a and b). When the concentration of initial reagents is up to 0.05 M (Fig. 2c), nonuniform distribution of particle size occurs and morphology changes to an agglomerated structure, which is due to the fact that a higher concentration of starting reagents could lead to a decrease in the nucleation rate and the number of nucleation centers, leading to larger particle sizes [37,38]. In addition, to investigate the role of sonication time on the morphology and size of products, reactions were performed with different times (30, 60 and 90 min, labelled as samples 4, 2, and 5, respectively, Figs. 3a, 2b, and 3b) at a constant concentration of initial reagents and sonication power of 0.01 M and 70 W, separately. There are uniform nanoparticles with good size distribution in all cases, while the thickness of nano-sized coordination polymer plates increases and the morphology of these nanostructures becomes more regular over the sonication time. We still discussed the effect of ultrasonic power on morphology and size of nano-structured CP 1. The reactions were carried out with different ultrasonic powers (40, 70 and 100 W, labelled as samples 6, 4, and 7, respectively, Figs. 4a, 3a, and 4b) at a constant concentration of initial reagents and sonication time of 0.01 M and 30 min, separately. In contrast with sample f, the image of sample d showed that the size was smaller than that of sample f, and size of sample g is the smallest one, indicating that high power ultrasound irradiation decreased agglomeration, and thus led to decrease particles size. Table 1 shows an overview of the comparison of the concentration of initial reagents, ultrasound times and powers effect of ultrasonic irradiation on the morphologies of nanoparticles of CP 1. In addition, SEM images of CP 1 show in Fig. 5, indicating that the morphologies of CP 1 synthesized from sonochemistry method is similar to that obtained by hydrothermal means.
3. Results and discussion 3.1. Crystal structure of [Zn(L)(atpt)]n (1) CP 1 crystallizes in the monoclinic space group P21/c. The asymmetrical unit contains one crystallographically independent Zn(II) center, one L ligand and one atpt2- ligand. As depicted in Fig. 1a, each Zn(II) center is four-coordinated by two nitrogen atoms (Zn1–N1 = 2.024(2) Å and Zn1–N4A = 2.096(3) Å, symmetry code: A = x, −y + 1/2, z − 1/2) of two L ligands and two carboxylate oxygen atoms (Zn1–O1 = 1.912(2) Å and Zn1–O3B = 1.965(2) Å, symmetry code: B = −x + 2, y + 1/2, −z + 3/2) of two atpt2- ligands, giving a distorted tetrahedron geometry with a τ4 value of 0.86 [34]. The coordination angles range from 97.06(10) to 127.44(11)°, which are comparable to values observed for similar zinc(II) CPs [35]. Each L ligand adopts the trans-conformation to bridge the neighboring Zn(II) centers to give an infinite 1D [Zn(L)]n wave-like chain with the non-bonding distance of Zn⋯Zn is 9.176(8) Å. The Zn(II) centers are bridged by the monodentate carboxylic groups to generate 1D [Zn(atpt)]n zigzag chains, spanning the non-bonding distance of Zn···Zn is 10.877(6) Å. The two type chains are further connected each other to form a 2D wave-like network (Fig. 1b), which can be simplified as a 4-connected sql net with the point symbol of {44.62}. As shown in Fig. 1c, the 2D later contains two distinct parallelograms, such as quadrangle Zn1-Zn1A-Zn1C-Zn1B (Zn1–Zn1A = 9.176(8) Å, Zn1–Zn1B = 10.877(6) Å, Zn1A–Zn1–Zn1B = 86.01(4)°, and Zn1–Zn1A–Zn1C = 93.99(4)° symmetry code: C = −x + 2, −y + 1, −z + 1), the adjacent quadrilateral, sharing an edge with quadrangle Zn1Zn1AZn1CZn1 B, has the dimension of 9.176(8) Å × 10.877(6) Å, and the angles of 77.26(4)° and 102.74(4)°. Finally, the adjacent two layers are further interdigitated into each other, fabricating a 3D supramolecular architecture through π⋯π stacking interactions between the imidazole and benzene rings of distinct L ligands (Cg1: N1–C11–N2–C17–C12, Cg2: C12–C13–C14–C15–C16–C17, centroid-tocentroid distance of 3.572 Å) (Fig. 1d), and the interplanar angle α is 0.57(17)°, slipping angles β and γ are 21.09° and 21.31°, respectively. 3.2. PXRD, TGA, and IR To check the phase purities of CP 1 synthesized by hydrothermal method and prepared by the ultrasonic irradiation, the powder X-ray diffraction patterns were carried out at room temperature. As shown in Fig. S1, the slight differences in 2θ were found between the experimental and simulated patterns, demonstrating the good phase purity of samples. CP 1 produced by ultrasonic irradiation as nanostructures was well identical to that single crystal diffraction. To estimate the thermal stability of bulk and nano-structured CP 1, TG analyses were carried out. As shown in Fig. S2, the TG curves of single crystal and nanostructured CP 1 both show a sharp weight loss stage from 360 to 700 °C, corresponding to the decomposition of the L and atpt2− ligands, the initial collapse temperature value of 360 °C indicates CP 1 have high stability. The residue weights (13.9% for bulk CP 1, 12.5% for nano-sized CP 1) are in correspondence with the calculated value (13.3%) of ZnO. The result shows that bulk CP 1 has
3.4. Photophysical properties The d10 metal centers based coordination polymers have received remarkable attention in view of various potential applications, such as in chemical sensors, photochemistry, electroluminescent display, and so on [39–41]. The solid-state luminescent emission properties of free L ligand and nanoparticles of CP 1 were investigated at room temperature (Fig. 6). The main emission peaks of free L ligand is at 375 nm (λex = 269 nm), which can be assigned to the π∗ → π or π∗ → n transitions [42]. Upon photoexcitation, nano-structured CP 1 displays the maximum emission peaks at 422 nm (λex = 330 nm). Comparing with 69
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Fig. 3. SEM images of nanostructured CP 1 with concentration of initial reagents of 0.01 M, power of 70 W, and different times (30 and 90 min, respectively) corresponding to pictures a and b.
property of CP 1, according to the formula: Eg = 1240/λg (which λg is absorption threshold and Eg is the energy gap), it can be estimated that the Eg of CP 1 was 2.85 eV. The results suggest that the CP 1 photocatalyst may be possesses the high photocatalytic activity, which corresponded to the process of photodegradation. Organic dyes are common compounds that can be used in many fields. Hence, large amounts of dyes are released into effluent streams [44,45]. Methylene blue (MB) as typical pollutants in aqueous media owing to their toxicity and degradation-resistant, have become one of the most serious environmental issues. Herein, we selected MB as a target pollutant to evaluate the photocatalytic performance of nanosized CP 1. The photodegradation experiment under UV irradiation was carried out after the dark adsorption–desorption equilibrium achieved, the detailed photocatalytic reaction procedures were listed in the ESI. As shown in Figs. 8, 9 and S4a, after 60 min, during the dark adsorption–desorption equilibrium, conversion ratio percent of absorbed MB by nano CP 1 and TiO2 are 12.7% and 23.3%, respectively, while conversion ratio percent of the existence of only MB is 1.6%. When CP 1 was dispersed in MB solution under UV irradiation for 90 min, the absorption peak of MB distinctly decrease along with increasing reaction time, the degradation efficiency of MB reached 90.7%, this value is higher than many other Zn-based CPs [46,47]. Moreover, we also tested the photocatalytic behaviors of the P25 TiO2 catalysts under the same conditions as contrast experiment, and we find the photodegradation efficiency of TiO2 is up to 84.6% (Figs. 9 and S4b), which is lower than the nano-structured CP 1. The efficiency of the control experiment for the photodegradation of MB without any catalysts is 15.8%, in which
Fig. 2. SEM images of nanostructured CP 1 with sonochemical time of 60 min, power of 70 W, and different concentration of initial reagents (0.005, 0.01 and 0.05 M, respectively) corresponding to pictures a, b, and c.
the emission of free L ligand, the emission band of nano-sized CP 1 indicate significant red shifts (by 47 nm), which can be attributed to the charge transfer from benzimidazole-based ligands to Zn(II) centers (LMCT) [43]. In order to explore the conductivities of CP 1, their diffuse reflectance UV/Vis spectrum was measured for nano-sized CP 1 in order to obtain the band gap Eg. As shown in Fig. 7, the intense absorption peaks at 330 and 390 nm for CP 1, which can be attributed to π∗ → π or π∗ → n transitions of the ligand or LMCT. To obtain the semiconductor 70
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Fig. 5. SEM images of CP 1 prepared by hydrothermal method.
Fig. 4. SEM images of nanostructured CP 1 with concentration of initial reagents of 0.01 M, time of 30 min, and different powers (40 and 100 W, respectively) corresponding to pictures a and b. Fig. 6. Solid-state emission spectra of free L ligand and nanoparticles of CP 1. Table 1 The influence of concentration of initial reagents, reaction time and sonication power on the size of CP 1.
Sample Sample Sample Sample Sample Sample Sample a b c d
1 2 3 4 5 6 7
T(°C)a
C(M)b
t(min)c
P(W)d
Average diameter (nm)
Yield/%
25 25 25 25 25 25 25
0.005 0.01 0.05 0.01 0.01 0.01 0.01
60 60 60 30 90 30 30
70 70 70 70 70 40 100
315 192 – 156 702 234 67
84.1 71.3 67.4 79.5 65.3 74.2 86.2
Represents reaction temperature. Represents concentration of initial reagents. Represents reaction time. Represents sonication power.
the concentration of MB almost does not change. To further confirm the catalytic activity of the as-synthesized materials, UV kinetics and UV/catalyst degradation kinetics are described by Langmuir–Hinshelwood kinetic equation (1),
ln(C0/ C ) = kt
Fig. 7. UV/Vis absorption spectra of nano-sized CP 1 with BaSO4 as background, and diffuse reflectance spectra of Kubelka-Munk function vs. energy of nano-sized CP 1.
(1) and 0.01784 min−1 for TiO2, respectively. All these indicate that the asprepared nano-sized CP 1 possesses higher photocatalytic activity. The second kinetic equation of photo-degradation can be indicated the Eq. (2). And the linear relation of 1/C and t was fitted by Fig. 11.
where k is pseudo first order dynamic rate constant. C0 and C are the initial and the remaining concentrations of MB at regular time intervals, respectively. According to Eq. (1), the linear relation of ln(C0/C) and t was followed Fig. 10, and the k values were calculated to be 0.02593 min−1 for nano CP 1, 0.00187 min−1 for control experiment, 71
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Fig. 11. The second-kinetic plots for the photodegradation of MB in the presence of nano CP 1 or P25 TiO2, and in the absence of catalyst under UV irradiation.
Fig. 8. Absorption spectra of the MB solutions during the photodegradation reaction under UV light with the presence of nano-sized CP 1.
Table 2 Kinetics parameters for the photodegradation of different targets. First kinetic equation 2
nano CP 1 without catalyst TiO2
Second kinetic equation −1
R
k/min
0.99809 0.97970 0.67646
0.02593 0.00187 0.01784
R2
k/L·mol−1·min−1
0.89409 0.98631 0.86115
0.44146 0.00786 0.22336
can be seen that the first kinetic linear regression coefficients (R2) are higher than the second kinetic linear regression coefficients for CP 1 nanoparticles. It suggests that the degradation process of nano CP 1 should be better fitted the first kinetic equation. On the contrary, the photodegradation course of control experiment and TiO2 are better fitted the second kinetic equation, and the k values of the second kinetic equation were calculated to be 0.44146 L·mol−1·min−1 for nano CP 1, 0.00786 L·mol−1·min−1 for control experiment, and 0.22336 L·mol−1·min−1 for TiO2, respectively. The results are still displayed nano-sized CP 1 shows high photocatalytic behavior. To make clear what active species are involved in the photocatalytic process occurring on CP 1, the formation of %OH, h+, and %O2− on the surface of UV-illuminated CP 1 was respectively detected by using tbutyl alcohol (TBA), ammonium oxalate (AO), and benzoquinone (BQ) as scavengers. As shown in Figs. 12 and S5, we can observe that the
Fig. 9. The experiment results of the photodegradation of MB.
Fig. 10. The first-kinetic plots for the photodegradation of MB in the presence of nano CP 1 or P25 TiO2, and in the absence of catalyst under UV irradiation.
1 1 − = kt C C0
(2)
According to the Langmuir–Hinshelwood kinetic equation, kinetics parameters for photocatalytic degradation of first and second kinetic equation of different targets were listed in Table 2. From the results, it
Fig. 12. Trapping experiment of active species during the photocatalytic reaction for nano-sized CP 1.
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size of nanoparticles were also studied. The results demonstrate that the concentrations of initial reagents of 0.01 M, the sonication time of 30 min, and the ultrasonic power of 100 W may obtain optimal nanoparticles, the size of nanoparticles can up to 67 nm. Further, the thermostability and photocatalytic behaviors of the nano-sized CP 1 may make it an excellent candidate as a catalysis material. Acknowledgments The project was supported by the National Natural Science Foundation of China (51474086) and Natural Science Foundation – Steel and Iron Foundation of Hebei Province (B2015209299). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ultsonch.2017.09.024.
Fig. 13. Schematic illustration of the photocatalytic mechanism of the nano-sized CP 1.
presence of TBA greatly suppressed the degradation activity of nanoparticles of CP 1 toward MB solution. The %OH quenching experiment indicates that the photodegradation process of MB with nano-sized CP 1 as catalyst predominantly involves attack by %OH radicals, implying that the formation rate of %OH radicals is an important factor influencing photocatalytic activity. Moreover, it can be found that the photodegradation of MB was also moderately affected by the addition of BQ and AO, indicating that the existence of the photogenerated %O2−, h+ and %OH, among them, %OH radical is the main active species of CP 1 for the photodegradation of MB under UV light irradiation. Consequently, a simplified model of photocatalytic reaction mechanism was proposed as depicted in Fig. 13, the HOMO is mainly contributed by oxygen and (or) nitrogen 2p bonding orbitals (valence band, VB) and the LUMO by empty Zn orbitals (conduction band, CB). Under UV light irradiation, electrons (e−) in the HOMO (VB) of CP 1 were excited to its LUMO (CB), with same amount of holes (h+) left in VB. The HOMO strongly demands one electron to return to its stable state. Therefore, one electron was captured from water molecules, which was oxygenated into %OH active species. Meanwhile, the electrons (e-) in LUMO could be combined with the oxygen adsorbed on the surfaces of CP 1 to form %O2−, then they might transform to the hydroxyl radicals (%OH). Then the formed •OH radicals could cleave MB effectively to complete the photocatalytic process. Further, the stability and recyclability of the CP 1 is a very significant element for practical applications, especially for commercial and industrial applications. The reusability of nano-sized CP 1 was tested in the degradation of MB under the irradiation of UV light. After the completion of the reaction, 3.0 mL of the reaction mixture was taken out, separated through centrifugation, washed with water and ethanol, dried and reused for five successive cycles of reaction without evident change of its degradation efficiency, indicating the stable and high recycling efficiency of nanoparticles of CP 1 (Fig. S6). Further, PXRD was also performed to confirm the chemical stability of nanosized CP 1. As shown in Fig. S7, the results of the sample after the fifth run were almost the same to that of the simulated pattern, which indicates that nanoparticles of CP 1 were stable during the photocatalytic process.
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