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Theoretical study on adsorption characteristics and environmental effects of dimetridazole on TiO2 surface
Computational and Theoretical Chemistry 1150 (2019) 10–17
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Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc
Theoretical study on adsorption characteristics and environmental effects of dimetridazole on TiO2 surface
T
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Hai-Chuan Qin, Qiao-Qiao Qin, Hui Luo, Wei Wei, Liu-Xie Liu, Lai-Cai Li College of Chemistry and Material Science, Sichuan Normal University, Chengdu 610068, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Density functional theory TiO2 photocatalyst Dimetridazole Explanation
In this paper, the adsorption characteristics of dimetridazole on anatase TiO2(1 0 1) and (0 0 1) crystal surfaces has been studied by using density functional theory. Adsorption structures of dimetridazole on anatase TiO2(1 0 1) and (0 0 1) crystal surfaces have been optimized under vacuum, water, acidic and alkaline conditions, respectively. The optimum adsorption site, adsorption energy and the electronic structure of the stable adsorption model were calculated. By analyzing the optimal adsorption site, we found that the possibility of degradation of dimetridazole on the surface of TiO2 and reaction site of degradation were the opening ring of CeN bond on the imidazole ring. By comparing the adsorption characteristics of dimetridazole on two different crystal planes of TiO2 under acidic and alkaline conditions, we found that the adsorption wavelengths of electron transition between conduction bands and valence bands of dimetridazole on anatase TiO2(1 0 1) crystal plane are within the range of visible wavelength. The results show that TiO2(1 0 1) can effectively utilize visible light and catalyze the adsorption and degradation reaction of dimetridazole on TiO2(1 0 1) surface. Our results show that TiO2(1 0 1) crystal surface can effectively use visible light under acidic and alkaline conditions. Our conclusion can explain the experimental result that the use of visible light on TiO2(0 0 1) face is greatly affected by the environment.
1. Introduction Dimetridazole (DMZ), 1,2-dimethyl-5-nitroimidazole, a class of nitroimidazole antibiotics is widely used in livestock farming [1]. It is mainly used to intervene and treat poultry and pigs infected by bacteria and protozoa [2]. Since DMZ and its metabolites contain heterocycle which will lead to the side effects of genetic toxicity, carcinogenicity and mutation [3], many countries including China, have banned it as a feed additive or remain in food [4]. Because DMZ is a refractory organic substance, the traditional biodegradation method has little degradation effect on it [5,6], so it will finally entry into water body. Leung [7] has carried out detection and risk assessment of drug concentration for 133 samples from waterworks in 13 cities in China. The results show that there are 89% samples exsit DMZ and the concentration is 6.9–17.7 ng L−1, posing a great threat to the health of infants. Activated carbon adsorption is the main method to treat DMZ at present, but it only separates DMZ from the environment rather than degrade it [8]. TiO2 photocatalysis is a promising advanced oxidation technology with mild reaction conditions, simple operation and no secondary pollution. It has great application potential in refractory organic compounds treatment [9]. A series of studies have shown that the
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photocatalytic technology of TiO2 has a good effect on the treatment of nitrogen-containing heterocyclic antibiotics. TiO2 can effectively degrade moxifloxacin [10]. Gao et al. [11] used TiO2 to remove sulfamethoxazole from water, and the oxidation of hydroxyl radicals dominated the process. Sánchezd-Polo et al. [12]. used Gamma rays to degrade DMZ, in which electrons played an important role, and the removal rate was about 70%. Chang Rui [13] combined photocatalytic and biological methods to degrade imidazole ionic liquids. HPLC-MS was used to analyze the oxidized fragments. It was found that the degradation of N-containing heterocyclic compounds originated from the oxidation of carbon adjacent to nitrogen atom. With the increase of the length of the upper side chain of nitrogen atom, the imidazole ring is more difficult to be attacked because of the influence of steric hindrance [14]. Moreover, Guanine also contains imidazole ring structure. HouRuobing [15] has studied the mechanism of 8-hydroxyguanine opening ring reaction. In this paper, adsorptions of DMZ on anatase TiO2(1 0 1) and (0 0 1) crystal planes in vacuum and aqueous solution are studied. Effects of acidic and alkaline condition on adsorption are also discussed. It is hoped that this study can provide theoretical information for the study of antibiotic degradation.
Corresponding author. E-mail address: [email protected] (L.-C. Li).
https://doi.org/10.1016/j.comptc.2019.01.002 Received 16 November 2018; Received in revised form 4 January 2019; Accepted 4 January 2019 Available online 05 January 2019 2210-271X/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. The model of crystal TiO2(1 0 1) and (0 0 1) surfaces. (a) and (c) are side views of (1 0 1) and (0 0 1) faces; (b) and (d) are overhead views of (1 0 1) and (0 0 1) faces along the Z axis.
2. Calculation method
molecular dynamics simulation is carried out by LAMMPS [20] program under NVE ensemble. The length of relaxation is 0.1 fs and the duration is 10 ps. Based on the results of LAMMPS relaxation, structures corresponding to local minimum are selected for further structural optimization of DFT [21–23]. All structural optimization of DFT in this work are performed on the basis of spin-polarized plane waves. The energy of electronic exchange can be calculated by the PBE method of generalized gradient approximation (GGA) [24] in the VASP [25] program, and the K-point is set to 1 × 1 × 1 [26]. We have used the double numerical plus polarized (DNP) basis set [27]. The truncation energy of plane waves is 380 eV, the energy convergence criterion is less than 0.001 eV, and the force convergence criterion is less than 0.05 eV/Å. The dispersion force plays a crucial role in the non-covalently combined system, while the traditional DFT is not sufficient to describe the dispersion interaction. In recent years, the development of some correction schemes has been used to make up for the shortcomings. Many studies [28–30] have proved that the DFT-D3 method shows good calculation results in noncovalent interacting systems. DFT-D3 dispersion correction is also used in this paper. DFT calculation will underestimate the energy band value [31] by using some correction methods can get good results, such as DFT + U [32]. In this study, the (1 0 1) surface adopts U = 7.0 eV [33], and the (0 0 1) surface adopts U = 8.5 eV [34]. The adsorption energy is defined as the change of material energy before and after adsorption.
There are two kinds of Ti atoms in anatase TiO2(1 0 1) crystal plane, which are five-coordinated Ti(5) and six-coordinated Ti(6), respectively. There are also two kinds of oxygen atoms, which are two-coordinated O(2) and three-coordinated O(3), respectively. As shown in Fig. 1(a), (b). Based on a preliminary study of the effect of plate thickness on surface energy [16], (1 0 1) crystal plane can be balanced between the calculation time and accuracy by using a three-layer model. The periodic boundary condition is set in XY plane and a vacuum layer of 15 Å is added to the Z direction. The super cell consists of 108 atoms of 1×3×3 units and the size is 10.89 Å × 11.33 Å × 25.00 Å. In the environment of neutral solution, about 48 H2O molecules are added according to the density of 1 g/cm3 by using the Universal force field. Anatase titanium dioxide (0 0 1) surface has five-coordinated Ti(5) and six-coordinated Ti(6) located in the inner layer, and the surface has two-coordinated O(2) and threecoordinated O(3). As shown in Fig. 1(c), (d), the crystal surface adopts one-layer model [17]. The periodic boundary condition is set in XY plane, and the vacuum layer of 15 Å is added to the Z direction. The super cell is composed of 108 atoms of 3 × 3 × 1 units and the size is 11.33 Å×11.33 Å×27.65 Å. In neutral solution, about 69 H2O molecules are added according to the density of 1 g/cm3 using the Universal force field. Under acidic and alkaline conditions, we replace one water molecule with one hydrogen chloride molecule and one sodium hydroxide molecule, respectively [18]. DMZ molecules are put on TiO2(1 0 1) and (0 0 1) crystal planes. In order to avoid the strong interaction between molecule and surface, the distance between them is set to be larger than 3.8 Å, and then the molecular dynamics is calculated in the ReaxFF force field [19]. The
Eads = (E + Esur ) − Eadsorption Eadsorption is the total energy of the system after adsorption, E is the energy of the adsorbed material, and Esur is the energy of the adsorption surface. 11
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Fig. 2. Adsorption configurations of DMZ on TiO2 (1 0 1) and TiO2(0 0 1) surfaces in vacuum(Å).
3. Calculation results
Li et al. [37] found that hydrogen bond between collagen, PVP and TiO2 enhances the stability of nanocomposite scaffolds. It can be seen that the formation of hydrogen bond can increase the stability of adsorption configurations. In five stable adsorption structures on the surface of TiO2(1 0 1), no hydrogen bond was found between DMZ molecular and the 3-coordinated O(3) atom on the surface of TiO2(1 0 1) indicating that the O(2) atoms on the surface of TiO2(1 0 1) were more active than O(3) atoms. The characteristics of adsorption on the TiO2(0 0 1) surface is similar to that on TiO2(1 0 1). The oxygen atoms of nitro group on the imidazole ring can also be adsorbed on the five-coordinated Ti(5) atoms on the surface of TiO2(0 0 1). Hydrogen atoms of methyl bonded to N (1), of methyl bonded to C(2) and hydrogen atom bonded to C(4) form hydrogen bond with O(2) atom and O(3) atom of TiO2(0 0 1) crystal plane and the length of hydrogen bond between DMZ and O(2) atom on TiO2(0 0 1) crystal plane is shorter, which may enhance the interaction between DMZ molecule and TiO2(0 0 1) crystal plane. A1, A2, A3, A4 and A5 are adsorption configurations in vacuum. Table 1 shows that A3 adsorption configuration has the largest adsorption energy of 1.67 eV, which is the best adsorption configuration. In the A3 configuration, the N(3) atom of DMZ is adsorbed at the Ti(5)
3.1. Adsorption of DMZ on the surface of TiO2 Molecular structures of dimetridazole and TiO2(1 0 1) and (0 0 1) stable crystal planes were optimized. The molecular dynamics behavior of DMZ on TiO2(1 0 1) and (0 0 1) crystal planes was simulated by LAMMPS program. The most stable adsorption configuration [35] was selected and the exact structure was optimized by VASP program. Fig. 2 show five stable adsorption configurations of DMZ on TiO2(1 0 1) and (0 0 1) crystal planes. The adsorption energy and the adsorption energy gap are listed in Table 1. The adsorption configuration of DMZ on TiO2 was multiple-sited adsorption. Oxygen atom of nitro bounded to N(3) and C(5) in imidazole ring can be absorbed on five-coordinated Ti(5) atom on the TiO2(1 0 1) surface. Hydrogen bonds are formed between hydrogen atoms in methyls bonded to C(2) and N(1) and hydrogen atoms on C(4) and O(2) atom of 2-coordination on the TiO2(1 0 1) crystal plane. Zhang et al. [36] studied the interface adsorption of dye/ TiO2 and found that the effect of hydrogen bond increases the stability of dye aggregating on the surface of TiO2 during the adsorption process.
Table 1 Adsorption energies of DMZ adsorbed on TiO2 surface. TiO2(1 0 1) surface
TiO2(0 0 1) surface
Condition
Structures
Eads (eV)
Energy gap (eV)
Structures
Eads (eV)
Energy gap (eV)
Vacuum
A1 A2 A3 A4 A5
−0.42 −0.50 −1.67 −0.93 −1.40
2.30 2.18 1.57 1.92 1.68
a1 a2 a3 a4 a5
−0.60 −1.35 −0.41 −0.44 −1.36
2.65 1.63 2.65 2.52 1.53
Neutral solution
B1 B2 B3 B4 B5
−1.72 −2.10 −2.28 −1.68 −1.98
2.27 2.30 2.06 2.22 2.39
b1 b2 b3 b4 b5
−1.85 −0.44 −0.20 −1.59 −2.17
0.77 1.13 0.88 1.18 1.19
Acidic solution
C1 C2 C3 C4 C5
−1.84 −0.64 −1.11 −2.03 −2.32
1.88 1.85 2.43 2.27 1.82
c1 c2 c3 c4 c5
−1.13 −1.86 −2.47 −1.31 −1.15
1.27 1.77 1.75 2.01 1.29
Basic solution
D1 D2 D3 D4 D5
−1.77 −1.82 −2.15 −1.30 −1.02
0.95 1.68 1.87 1.66 2.08
d1 d2 d3 d4 d5
−1.20 −1.15 −1.06 −1.75 −2.19
0.93 1.36 1.93 1.15 1.64
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TiO2, water molecules and the surface of TiO2, some bond lengths on the surface of TiO2 change. In solution, oxygen atoms of water molecules are easily adsorbed on Ti(5) atoms on TiO2 surface. Because O(2) atoms on TiO2 surface protrude on TiO2 surface, hydrogen bonds will be formed between hydrogen atom and O(2) atom on TiO2 surface. Because the position of O(3) atom on TiO2 surface is lower, no hydrogen atoms in water molecules are found to be adsorbed on O(3) atoms on the surface of TiO2. In addition, the oxygen atoms of nitro and H atoms on the DMZ molecule form hydrogen bonds with the hydrogen and oxygen atoms on the water molecules, respectively, and the exposed N(3) atoms on the imidazole ring can also form hydrogen bonds with the hydrogen atoms on the water molecules. In aqueous solution, the adsorption characteristics of DMZ on TiO2(1 0 1) and (0 0 1) crystal planes are similar to each other, and hydrogen bond plays an important role in the adsorption process. B1, B2, B3, B4 and B5 are adsorption configurations in neutral solution. It can be seen from Table 1 that the B3 mode has the largest adsorption energy of 2.28 eV, which is the optimal adsorption configuration. The same adsorption characteristics as the A3 adsorption structure is in gas phase. The b5 adsorption structure is the most stable adsorption site of DMZ on the surface of TiO2(0 0 1), which is similar to the most stable adsorption of a5 under gas phase condition. We have compared the adsorption characteristics of DMZ on TiO2(0 0 1) and (1 0 1) crystal planes under the conditions of gas phase and solvent, and have found that the most stable absorption structures are similar. The most stable adsorption structures of DMZ on TiO2(1 0 1) and (0 0 1) planes under gas phase condition are A3 and a5 respectively. In the neutral water solvent, the most stable adsorption configurations of DMZ on the TiO2(1 0 1) and (0 0 1) planes are B3 and b5, respectively. The solvent effect makes the interaction of hydrogen bonding between the molecules enhanced and absorption energy of system enhanced.
site on the crystal plane of TiO2(1 0 1), and the adsorption distance is 2.279 Å. the hydrogen atoms of two methyls on DMZ and the hydrogen atom bonded to C(4) on DMZ form hydrogen bond with O(2) atom on TiO2(1 0 1) crystal plane. The bond lengths are 2.389 Å, 2.213 Å and 1.916 Å, respectively. It’s beneficial to enhance the stability of adsorption. Compared with the structure of DMZ before adsorption, in A3 configuration, the bond lengths of N(3)-C(2) and N(3)-C(4) of DMZ increase from 1.343 Å and 1.356 Å to 1.358 Å and 1.365 Å, respectively, and bond length of N(1)-C(2) decreased from 1.370 Å to 1.358 Å due to the adsorption effect of DMZ on the crystal plane of TiO2(1 0 1). The degradation of nitrogen-containing heterocyclic compounds generally starts from the oxidation of carbon adjacent to nitrogen atoms, and the N(3)-C(2) bond of DMZ is weakened in favor of the attack of hydroxyl radicals. A5 is the sub-stable adsorption configuration of DMZ on the TiO2(1 0 1) plane and the adsorption energy is 1.40 eV. In the A5 configuration, oxygen atom of nitro in DMZ nitro is adsorbed on the surface of Ti(5) atom, and the adsorption distance is 2.294 Å. The hydrogen atoms of methyl bonded to C(2) atom and the hydrogen atom bonded to C(4) atom form weak hydrogen bonds with O(2) atoms on the TiO2(1 0 1) crystal plane, and the bond lengths are 2.530 Å, 2.506 Å and 2.412 Å, respectively. The bond lengths of C(2)-N(3) and C(4)-C(5) double bonds both have been increased. The reason for the lengthening of C(2)-N(3) bond may be the hydrogen bond formed between hydrogen atom of methyl bonded to C(2) in DMZ and O(2) atom. However, The lengthen of C(4)-C(5) bond may be due to the interaction between the oxygen atom of nitro, hydrogen atom bonded to C(4) in DMZ and the TiO2 crystal plane. In general, the hydrogen atom on DMZ is easy to form hydrogen bond with oxygen atom on TiO2 surface, which makes DMZ molecule adsorbed to TiO2 surface. N(1) atom of DMZ is bonded to methyl group with short branching chain and small steric hindrance, which makes more atoms on the molecule being adsorbed to TiO2 surface. Table 1 shows that among the five stable adsorption structures of DMZ on the TiO2(0 0 1) crystal plane, adsorption energy of A5 configuration is the largest of 1.36 eV, which is the best adsorption configuration. In a5 configuration, N(3) atom of DMZ is adsorbed on the Ti(5) atom on the TiO2(0 0 1) crystal plane, and the adsorption distance was 2.312 Å. Because of the interaction between N(3) atom of DMZ and Ti (5) atom, the bond lengths of N(3)-C(2) and N(3)-C(4) in DMZ increased from 1.343 Å and 1.356 Å to 1.358 Å and 1.364 Å, respectively. The bond length of N(1)-C(2) is shortened from 1.371 Å to 1.364 Å, and the N(3)-C(2) bond of DMZ is weak, which is beneficial to the attack of hydroxyl radical and the ring opening degradation. The sub-stable configuration of DMZ adsorbed on the surface of TiO2(0 0 1) is a2 and the adsorption energy is 1.35 eV. The oxygen atom of nitro on DMZ is adsorbed on Ti(5) atom on TiO2(0 0 1) crystal plane in a distance of 2.313 Å. The hydrogen atom of the methyl group bonded to C(2) and the H atom bonded to C(4) in DMZ both form hydrogen bonds with O (2) atoms on the surface of TiO2(0 0 1), and the bond lengths are 2.285 Å, 2.261 Å and 2.419 Å respectively, which is beneficial to increase the adsorption stability. Similar to the adsorption results of TiO2(1 0 1) surface, the N(3)-C(2) bond of DMZ also is weak in a2 configuration, which is favorable to the attack of hydroxyl radical and ring opening degradation. In order to investigate adsorption characteristics of DMZ under solvent conditions, we optimized five stable adsorption configurations of DMZ on (1 0 1) and (0 0 1) crystal planes of TiO2 in aqueous solution by using the same method. The molecular dynamics behavior of DMZ on TiO2(1 0 1) and (0 0 1) crystal planes was simulated by LAMMPS program. The most stable adsorption configuration was selected and the exact structure was optimized by VASP program. As shown in Fig. 3, in which the adsorption configurations and distances of DMZ on TiO2 surface are shown. In aqueous solution, the interaction between DMZ molecule and atom of TiO2 surface is weaker than that under gas phase condition, and the adsorption distance is longer because of the action of water molecule. Due to the interaction between DMZ and the surface of
3.2. Adsorption of DMZ on TiO2 surface under acidic and alkaline conditions Under acidic condition, we used the Lammps program to simulate molecular dynamics of DMZ on the TiO2(1 0 1) and (0 0 1) crystal faces. We selected five relatively stable absorption configurations of DMZ on the TiO2(1 0 1) and (0 0 1) crystal faces in acidic solution, and optimized these structures by VASP program. Fig. 4 shows the adsorption configurations and adsorption distances of DMZ on TiO2(1 0 1) and (0 0 1) crystal planes in acidic solution, respectively. Because of the interaction of DMZ molecules, water molecules, protons and chloride ions with atoms of TiO2 surface, some bond lengths on the surface of TiO2 change. In the acidic solution, free protons (H+) are relatively easily adsorbed onto the N(3) atom of imidazole ring. It can be seen from Table 1 that the C5 mode has the largest adsorption energy of 2.32 eV, which is the optimal adsorption configuration. In C5 configuration, DMZ molecule was adsorbed on the surface of TiO2 in a nearly parallel manner, and the chloride ions were also adsorbed on the Ti(5) atom of TiO2(1 0 1) surface with a adsorption distance of 2.458 Å. The bond length of C(2)-N(3) of DMZ increased from 1.343 Å before adsorption to 1.359 Å, and the bond length of N(1)C(2) decreased from 1.370 Å before adsorption to 1.361 Å. The weakening of C(2)-N(3) bond of DMZ is beneficial to the attack of hydroxyl radical and degradation. The sub-stable adsorption configuration of DMZ on the surface of TiO2(1 0 1) is C4 adsorption configuration, and the adsorption energy is 2.03 eV. In C4 configuration, the DMZ molecule was adsorbed on the surface of TiO2 in a nearly vertical manner. This adsorption also made the C(2)-N(3) bond of DMZ weaker and favors the attack of hydroxyl radicals. This is consistent with the situation in C5, which is similar to the case of DMZ in aqueous solution. In Table 1, we also list the adsorption energy of three relatively stable adsorption configurations of DMZ on TiO2(0 0 1) crystal plane in the acidic solution. The c3 mode has the largest adsorption energy of 2.47 eV, which is the optimal adsorption configuration. The bond 13
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Fig. 3. Adsorption configurations of DMZ on TiO2(1 0 1) and TiO2(0 0 1) surfaces in water solution (Å).
length of C(2)-N(3) of DMZ increased from 1.343 Å before adsorption to 1.360 Å, and this adsorption also weakened the C(2)-N(3) bond of DMZ. Under alkaline condition, Fig. 5 shows the adsorption configurations and adsorption distances of DMZ on TiO2(1 0 1) and (0 0 1) surfaces, respectively. The OH− is easily adsorbed on the surface of TiO2, which makes the adsorption of DMZ molecule on surface of TiO2 weakened. It can be seen from Table 1 that the D3 mode has the largest adsorption energy of 2.15 eV, which is the optimal adsorption configuration. In D3 configuration, the N(3) atom of DMZ is adsorbed on the Ti(5) atom of the TiO2(1 0 1) crystal plane, and the adsorption distance is 2.369 Å. During the adsorption process, hydrogen bonds are formed between two hydrogen atoms of the DMZ C(4) and C(2) and the O(2) atom of the TiO2(1 0 1) crystal plane, and the bond lengths are 2.174 Å, 2.285 Å, and 2.472 Å, respectively. It’s beneficial to increase the stability of adsorption. Compared with the structure of DMZ before adsorption, in D3 configuration, the bond length of C(2)-N(3) on the imidazole ring increased from 1.343 Å before adsorption to 1.357 Å, which makes C(2)-N(3) bond on the imidazole ring weaken and easy to be attacked by hydroxyl radicals. Under alkaline condition, the most stable adsorption configuration of DMZ on TiO2(0 0 1) crystal plane is
d5 with adsorption energy of 2.19 eV and this adsorption are similar to those of DMZ on TiO2(1 0 1) crystal plane, also weakened the C(2)-N(3) bond of DMZ. We calculated the energy bands of TiO2(1 0 1) and (0 0 1) crystal faces under gas phase condition, respectively. After adsorption, the valence band and conduction band energy levels of TiO2 decreased, and the energy gap became narrow. The energy gap value of TiO2 is calculated to be 3.20 eV before adsorption, which is consistent with the experimental value of 3.20 eV [38]. After adsorption, the energy gap values of A1 to A5 are reduced to 2.30 eV, 2.18 eV, 1.57 eV, 1.92 eV, and 1.68 eV, respectively; and a1 to a5 reduced to 2.65 eV, 1.63 eV, 2.65 eV, 2.52 eV, and 1.53 eV, respectively. The adsorption of DMZ on TiO2(1 0 1) and (0 0 1) crystal planes, of which the electron transition wavelength between the valence band and the conduction band is within the visible range, can effectively utilize the visible light to drive the degradation of DMZ on the surface of TiO2. 3.3. Density of state analysis of adsorption configurations It can be seen from Fig. S3 (supporting information) in neutral solution, the energy band structure of TiO2 changes due to the adsorption
Fig. 4. Adsorption configurations of DMZ on TiO2(1 0 1) and TiO2(0 0 1) surfaces in acidic solution (Å). 14
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Fig. 5. Adsorption configurations and adsorption distances (Å) of DMZ on TiO2(1 0 1) and TiO2(0 0 1) surfaces in basic solution.
composed of the d-orbital of Ti before adsorption. The s-orbital component of energy level of valence band increases, and the range of porbital energy is widened from −5 eV to 0 eV before adsorption to −9 eV to 0 eV, and two sets of peaks appear, whose height and area increase. After adsorption, the energy gap of TiO2 is narrowed, and the b1 ∼ b5 energy gap is reduced to 0.77 eV, 1.13 eV, 0.88 eV, 1.18 eV and 1.19 eV, respectively. The wavelengths of electronic transition between valence band and conduction band are: 1612 nm, 1098 nm, 1411 nm, 1052 nm and 1043 nm, respectively, none of those within the range of visible length, indicating that TiO2(0 0 1) crystal plane in neutral solution cannot effectively utilize visible light. As can be seen from Fig. S7, the band structure of TiO2(0 0 1) crystal face in acidic solution is similar to that in neutral solution. Cl− ions are not easily adsorbed by TiO2, while H+ ions are easily adsorbed on the exposed N(3) atom of the imidazole ring and the O(2) atom on the surface of TiO2. When H+ ions are adsorbed on O(2), the component of s-orbital in valence band is slightly increased. After adsorption, the energy gap of TiO2 is narrowed, and the energy gaps of c1 ∼ c5 are reduced to 1.27 eV, 1.77 eV, 1.75 eV, 2.01 eV and 1.80 eV. Except for c1, the wavelengths of electronic transition between valence band and conduction band are: 977 nm, 701 nm, 709 nm, 689 nm, and 618 nm, respectively, within the range of visible wavelengths. It can be seen from Fig. S8 that under alkaline condition, structure of energy band of the TiO2(0 0 1) crystal plane is similar to that under neutral condition. Na+ ions are not easily adsorbed by TiO2, while OH− ions are easily adsorbed on Ti(5) atoms on the surface of TiO2. A few porbital components appears in the range of −22 eV ∼ −15 eV, and the energy gap of TiO2 becomes narrow after adsorption. The energy gaps of d1 ∼ d5 are reduced to 0.93 eV, 1.36 eV, 1.93 eV, 1.15 eV and 1.64 eV, respectively. The wavelengths of electronic transition between valence band and conduction band are: 1335 nm, 913 nm, 643 nm, 1079 nm and 757 nm, except for d3 and d5, none of those in the range of visible light, which indicates that TiO2(0 0 1) crystal plane does not make good use of visible light under alkaline condition. Fig. 6 shows the electron density of DMZ on the TiO2 surface under vacuum condition. We observed an overlap between the charge density of DMZ and the TiO2 surface, indicating that there is electron transfer between DMZ and the TiO2 surface and a new chemical bond is formed. In this process, chemical adsorption occurs between DMZ molecule and TiO2 surface.
of water molecules on the TiO2(1 0 1) surface. Two s-orbital peaks appear between −20 eV and −15 eV. The energy level of conduction band is still composed of the d orbital of Ti before adsorption. The sorbital component increases in the energy level of valence band, and the energy range of the p-orbital is widened from −5 eV to 0 eV before adsorption to −8 eV to 0 eV. Two peaks appeared, whose height and area increase. After adsorption, the energy gap of TiO2 is narrowed, and the energy gaps of B1 ∼ B5 are reduced to 2.27 eV, 2.30 eV, 2.06 eV, 2.22 eV and 2.39 eV, respectively. The wavelengths of electronic transition between valence band and conduction band are 547 nm, 540 nm, 603 nm, 559 nm, 519 nm, respectively, and the frequency of absorption to light is reduced, all are in the range of visible wavelength, which improves the efficiency of absorption to visible light. As can be seen from Fig. S4, in acidic solution, the band structure of the TiO2 (1 0 1) crystal face is similar to that in neutral solution. Cl− ions are not easily adsorbed by TiO2, while H+ ions are more easily adsorbed on the exposed N(3) atoms of the imidazole ring and on the O (2) atom on the surface of TiO2. When H+ ions are adsorbed on O(2), the component of s-orbital of valence band increases and the energy gap of TiO2 becomes narrower. The energy gaps of C1 to C5 are reduced to 1.88 eV, 1.85 eV, 2.43 eV, 2.27 eV and 1.82 eV, respectively. The wavelengths of electronic transition between valence band and conduction band are 660 nm, 671 nm, 511 nm, 547 nm, and 682 nm, respectively, and the frequency of absorption to light is reduced in the range of visible wavelength, which improves the efficiency of absorption to visible light. As can be seen from Fig. S5, in alkaline solution, the band structure of the TiO2(1 0 1) crystal plane is similar to that in neutral solution. Na+ ions are not easily adsorbed by TiO2, while OH− ions are easily adsorbed on Ti(5) atoms on the surface of TiO2. A little p-orbital component appears in the range of –22 eV ∼ −15 eV, and the energy gap of TiO2 becomes narrow after adsorption. The energy gaps of D1 ∼ D5 are reduced to 0.95 eV, 1.68 eV, 1.87 eV, 1.66 eV, and 2.08 eV, respectively. Except for D1, the wavelengths of electronic transition between valence band and conduction band are: 1307 nm, 739 nm, 664 nm, 748 nm, and 597 nm, respectively, in the range of visible length, which improves the efficiency of adsorption to visible light. It can be seen from Fig. S6 that the TiO2(0 0 1) crystal plane in the neutral solution is similar to the TiO2(1 0 1) crystal plane, and the TiO2(0 0 1) crystal plane adsorbs water molecules, which changes the structure of energy band of TiO2. Two s-orbital peaks appear between −22 eV and −15 eV. The energy level of conduction band is still
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Fig. 6. The electron density of DMZ on the TiO2 crystal surface under vacuum condition.
4. Conclusions
References
In this paper, the density functional theory is used to study the adsorption of DMZ on anatase TiO2(1 0 1) and (0 0 1) crystal planes. The most stable adsorption structure was optimized and characteristics of adsorption structure were discussed. DMZ can be effectively adsorbed on TiO2 surface both in vacuum and in neutral, acidic and alkaline solution. The results of adsorption lengthen the CeN bond length of imidazole ring on DMZ, which is beneficial to the attack of hydroxyl radicals and the degradation reaction. With the change of molecular after adsorption, we found the reaction site of ring-opening degradation of DMZ on the surface of TiO2. At the same time, study found that the effect of hydrogen bond is obvious in the adsorption process of DMZ on TiO2 surface. Under neutral, acid or alkaline condition, the effect of hydrogen bond on the adsorption characteristics and properties in the adsorption process is very significant. For different acid or base condition, the adsorption wavelengths of electron transition between conduction bands and valence bands are both within the range of visible wavelength, indicating that TiO2(1 0 1) crystal plane can effectively utilize visible light and can photocatalyze the adsorption and degradation of DMZ on TiO2(1 0 1) crystal plane. However, the use of visible light in TiO2(0 0 1) crystal piane is greatly affected by the condition, and DMZ is more susceptible to TiO2(0 0 1) surface in acidic condition. We compared the adsorption characteristics of DMZ on two different crystal planes of TiO2 under acidic and alkaline conditions and we found that the TiO2(1 0 1) crystal plane can effectively utilize visible light under acidic and alkaline conditions, which can explain experimental results that DMZ can effectively be degraded by TiO2(1 0 1) crystal plane, whether in acidic,neutral or alkaline condition. The crystal plane of TiO2(1 0 1) accounts for 94% of anatase TiO2. Our results indicate that anatase TiO2 can play a good photocatalytic role in the degradation of nitroimidazole antibiotics.
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Acknowledgments We are grateful for the financial support of this work by the Sichuan province department of education (13ZA0150) and Sichuan province (2014JY0099).
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.comptc.2019.01.002. 16
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[22] M. Higuchi, K. Higuchi, Pair density functional theory, Comput. Theor. Chem. 1003 (2013) 91–96. [23] F. Buendia, J.A. Vargas, R.L. Johnston, M.R. Beltran, Study of the stability of small AuRh clusters found by a genetic algorithm methodology, Comput. Theor. Chem. 1119 (2017) 51–58. [24] J. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865–3868. [25] G. Kresse, J. Furthmüller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Comp. Mater. Sci. 6 (1996) 15–50. [26] L.X. Liu, K. Li, X. Chen, X.Q. Liang, Y. Zheng, L.C. Li, Amino acid adsorption on anatase (101) surface at vacuum and aqueous solution: a density functional study, J. Mol. Model. 24 (2018) 107–115. [27] B. Delley, An all-electron numerical method for solving the local density functional for polyatomic molecules, J. Chem. Phys. 92 (1990) 508–517. [28] J. Moellmann, S. Grimme, DFT-D3 study of some molecular crystals, J. Phys. Chem. 118 (2014) 7615–7621. [29] T. Risthaus, S. Grimme, Benchmarking of London dispersion-accounting density functional theory methods on very large molecular complexes, J. Chem. Theory Comput. 9 (2013) 1580–1591. [30] J. Rezáč, K.E. Riley, P. Hobza, S66: A well-balanced database of benchmark interaction energies relevant to biomolecular structures, J. Chem. Theory Comput. 7
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Theoretical study on adsorption characteristics and environmental effects of dimetridazole on TiO2 surface
T
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Hai-Chuan Qin, Qiao-Qiao Qin, Hui Luo, Wei Wei, Liu-Xie Liu, Lai-Cai Li College of Chemistry and Material Science, Sichuan Normal University, Chengdu 610068, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Density functional theory TiO2 photocatalyst Dimetridazole Explanation
In this paper, the adsorption characteristics of dimetridazole on anatase TiO2(1 0 1) and (0 0 1) crystal surfaces has been studied by using density functional theory. Adsorption structures of dimetridazole on anatase TiO2(1 0 1) and (0 0 1) crystal surfaces have been optimized under vacuum, water, acidic and alkaline conditions, respectively. The optimum adsorption site, adsorption energy and the electronic structure of the stable adsorption model were calculated. By analyzing the optimal adsorption site, we found that the possibility of degradation of dimetridazole on the surface of TiO2 and reaction site of degradation were the opening ring of CeN bond on the imidazole ring. By comparing the adsorption characteristics of dimetridazole on two different crystal planes of TiO2 under acidic and alkaline conditions, we found that the adsorption wavelengths of electron transition between conduction bands and valence bands of dimetridazole on anatase TiO2(1 0 1) crystal plane are within the range of visible wavelength. The results show that TiO2(1 0 1) can effectively utilize visible light and catalyze the adsorption and degradation reaction of dimetridazole on TiO2(1 0 1) surface. Our results show that TiO2(1 0 1) crystal surface can effectively use visible light under acidic and alkaline conditions. Our conclusion can explain the experimental result that the use of visible light on TiO2(0 0 1) face is greatly affected by the environment.
1. Introduction Dimetridazole (DMZ), 1,2-dimethyl-5-nitroimidazole, a class of nitroimidazole antibiotics is widely used in livestock farming [1]. It is mainly used to intervene and treat poultry and pigs infected by bacteria and protozoa [2]. Since DMZ and its metabolites contain heterocycle which will lead to the side effects of genetic toxicity, carcinogenicity and mutation [3], many countries including China, have banned it as a feed additive or remain in food [4]. Because DMZ is a refractory organic substance, the traditional biodegradation method has little degradation effect on it [5,6], so it will finally entry into water body. Leung [7] has carried out detection and risk assessment of drug concentration for 133 samples from waterworks in 13 cities in China. The results show that there are 89% samples exsit DMZ and the concentration is 6.9–17.7 ng L−1, posing a great threat to the health of infants. Activated carbon adsorption is the main method to treat DMZ at present, but it only separates DMZ from the environment rather than degrade it [8]. TiO2 photocatalysis is a promising advanced oxidation technology with mild reaction conditions, simple operation and no secondary pollution. It has great application potential in refractory organic compounds treatment [9]. A series of studies have shown that the
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photocatalytic technology of TiO2 has a good effect on the treatment of nitrogen-containing heterocyclic antibiotics. TiO2 can effectively degrade moxifloxacin [10]. Gao et al. [11] used TiO2 to remove sulfamethoxazole from water, and the oxidation of hydroxyl radicals dominated the process. Sánchezd-Polo et al. [12]. used Gamma rays to degrade DMZ, in which electrons played an important role, and the removal rate was about 70%. Chang Rui [13] combined photocatalytic and biological methods to degrade imidazole ionic liquids. HPLC-MS was used to analyze the oxidized fragments. It was found that the degradation of N-containing heterocyclic compounds originated from the oxidation of carbon adjacent to nitrogen atom. With the increase of the length of the upper side chain of nitrogen atom, the imidazole ring is more difficult to be attacked because of the influence of steric hindrance [14]. Moreover, Guanine also contains imidazole ring structure. HouRuobing [15] has studied the mechanism of 8-hydroxyguanine opening ring reaction. In this paper, adsorptions of DMZ on anatase TiO2(1 0 1) and (0 0 1) crystal planes in vacuum and aqueous solution are studied. Effects of acidic and alkaline condition on adsorption are also discussed. It is hoped that this study can provide theoretical information for the study of antibiotic degradation.
Corresponding author. E-mail address: [email protected] (L.-C. Li).
https://doi.org/10.1016/j.comptc.2019.01.002 Received 16 November 2018; Received in revised form 4 January 2019; Accepted 4 January 2019 Available online 05 January 2019 2210-271X/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. The model of crystal TiO2(1 0 1) and (0 0 1) surfaces. (a) and (c) are side views of (1 0 1) and (0 0 1) faces; (b) and (d) are overhead views of (1 0 1) and (0 0 1) faces along the Z axis.
2. Calculation method
molecular dynamics simulation is carried out by LAMMPS [20] program under NVE ensemble. The length of relaxation is 0.1 fs and the duration is 10 ps. Based on the results of LAMMPS relaxation, structures corresponding to local minimum are selected for further structural optimization of DFT [21–23]. All structural optimization of DFT in this work are performed on the basis of spin-polarized plane waves. The energy of electronic exchange can be calculated by the PBE method of generalized gradient approximation (GGA) [24] in the VASP [25] program, and the K-point is set to 1 × 1 × 1 [26]. We have used the double numerical plus polarized (DNP) basis set [27]. The truncation energy of plane waves is 380 eV, the energy convergence criterion is less than 0.001 eV, and the force convergence criterion is less than 0.05 eV/Å. The dispersion force plays a crucial role in the non-covalently combined system, while the traditional DFT is not sufficient to describe the dispersion interaction. In recent years, the development of some correction schemes has been used to make up for the shortcomings. Many studies [28–30] have proved that the DFT-D3 method shows good calculation results in noncovalent interacting systems. DFT-D3 dispersion correction is also used in this paper. DFT calculation will underestimate the energy band value [31] by using some correction methods can get good results, such as DFT + U [32]. In this study, the (1 0 1) surface adopts U = 7.0 eV [33], and the (0 0 1) surface adopts U = 8.5 eV [34]. The adsorption energy is defined as the change of material energy before and after adsorption.
There are two kinds of Ti atoms in anatase TiO2(1 0 1) crystal plane, which are five-coordinated Ti(5) and six-coordinated Ti(6), respectively. There are also two kinds of oxygen atoms, which are two-coordinated O(2) and three-coordinated O(3), respectively. As shown in Fig. 1(a), (b). Based on a preliminary study of the effect of plate thickness on surface energy [16], (1 0 1) crystal plane can be balanced between the calculation time and accuracy by using a three-layer model. The periodic boundary condition is set in XY plane and a vacuum layer of 15 Å is added to the Z direction. The super cell consists of 108 atoms of 1×3×3 units and the size is 10.89 Å × 11.33 Å × 25.00 Å. In the environment of neutral solution, about 48 H2O molecules are added according to the density of 1 g/cm3 by using the Universal force field. Anatase titanium dioxide (0 0 1) surface has five-coordinated Ti(5) and six-coordinated Ti(6) located in the inner layer, and the surface has two-coordinated O(2) and threecoordinated O(3). As shown in Fig. 1(c), (d), the crystal surface adopts one-layer model [17]. The periodic boundary condition is set in XY plane, and the vacuum layer of 15 Å is added to the Z direction. The super cell is composed of 108 atoms of 3 × 3 × 1 units and the size is 11.33 Å×11.33 Å×27.65 Å. In neutral solution, about 69 H2O molecules are added according to the density of 1 g/cm3 using the Universal force field. Under acidic and alkaline conditions, we replace one water molecule with one hydrogen chloride molecule and one sodium hydroxide molecule, respectively [18]. DMZ molecules are put on TiO2(1 0 1) and (0 0 1) crystal planes. In order to avoid the strong interaction between molecule and surface, the distance between them is set to be larger than 3.8 Å, and then the molecular dynamics is calculated in the ReaxFF force field [19]. The
Eads = (E + Esur ) − Eadsorption Eadsorption is the total energy of the system after adsorption, E is the energy of the adsorbed material, and Esur is the energy of the adsorption surface. 11
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Fig. 2. Adsorption configurations of DMZ on TiO2 (1 0 1) and TiO2(0 0 1) surfaces in vacuum(Å).
3. Calculation results
Li et al. [37] found that hydrogen bond between collagen, PVP and TiO2 enhances the stability of nanocomposite scaffolds. It can be seen that the formation of hydrogen bond can increase the stability of adsorption configurations. In five stable adsorption structures on the surface of TiO2(1 0 1), no hydrogen bond was found between DMZ molecular and the 3-coordinated O(3) atom on the surface of TiO2(1 0 1) indicating that the O(2) atoms on the surface of TiO2(1 0 1) were more active than O(3) atoms. The characteristics of adsorption on the TiO2(0 0 1) surface is similar to that on TiO2(1 0 1). The oxygen atoms of nitro group on the imidazole ring can also be adsorbed on the five-coordinated Ti(5) atoms on the surface of TiO2(0 0 1). Hydrogen atoms of methyl bonded to N (1), of methyl bonded to C(2) and hydrogen atom bonded to C(4) form hydrogen bond with O(2) atom and O(3) atom of TiO2(0 0 1) crystal plane and the length of hydrogen bond between DMZ and O(2) atom on TiO2(0 0 1) crystal plane is shorter, which may enhance the interaction between DMZ molecule and TiO2(0 0 1) crystal plane. A1, A2, A3, A4 and A5 are adsorption configurations in vacuum. Table 1 shows that A3 adsorption configuration has the largest adsorption energy of 1.67 eV, which is the best adsorption configuration. In the A3 configuration, the N(3) atom of DMZ is adsorbed at the Ti(5)
3.1. Adsorption of DMZ on the surface of TiO2 Molecular structures of dimetridazole and TiO2(1 0 1) and (0 0 1) stable crystal planes were optimized. The molecular dynamics behavior of DMZ on TiO2(1 0 1) and (0 0 1) crystal planes was simulated by LAMMPS program. The most stable adsorption configuration [35] was selected and the exact structure was optimized by VASP program. Fig. 2 show five stable adsorption configurations of DMZ on TiO2(1 0 1) and (0 0 1) crystal planes. The adsorption energy and the adsorption energy gap are listed in Table 1. The adsorption configuration of DMZ on TiO2 was multiple-sited adsorption. Oxygen atom of nitro bounded to N(3) and C(5) in imidazole ring can be absorbed on five-coordinated Ti(5) atom on the TiO2(1 0 1) surface. Hydrogen bonds are formed between hydrogen atoms in methyls bonded to C(2) and N(1) and hydrogen atoms on C(4) and O(2) atom of 2-coordination on the TiO2(1 0 1) crystal plane. Zhang et al. [36] studied the interface adsorption of dye/ TiO2 and found that the effect of hydrogen bond increases the stability of dye aggregating on the surface of TiO2 during the adsorption process.
Table 1 Adsorption energies of DMZ adsorbed on TiO2 surface. TiO2(1 0 1) surface
TiO2(0 0 1) surface
Condition
Structures
Eads (eV)
Energy gap (eV)
Structures
Eads (eV)
Energy gap (eV)
Vacuum
A1 A2 A3 A4 A5
−0.42 −0.50 −1.67 −0.93 −1.40
2.30 2.18 1.57 1.92 1.68
a1 a2 a3 a4 a5
−0.60 −1.35 −0.41 −0.44 −1.36
2.65 1.63 2.65 2.52 1.53
Neutral solution
B1 B2 B3 B4 B5
−1.72 −2.10 −2.28 −1.68 −1.98
2.27 2.30 2.06 2.22 2.39
b1 b2 b3 b4 b5
−1.85 −0.44 −0.20 −1.59 −2.17
0.77 1.13 0.88 1.18 1.19
Acidic solution
C1 C2 C3 C4 C5
−1.84 −0.64 −1.11 −2.03 −2.32
1.88 1.85 2.43 2.27 1.82
c1 c2 c3 c4 c5
−1.13 −1.86 −2.47 −1.31 −1.15
1.27 1.77 1.75 2.01 1.29
Basic solution
D1 D2 D3 D4 D5
−1.77 −1.82 −2.15 −1.30 −1.02
0.95 1.68 1.87 1.66 2.08
d1 d2 d3 d4 d5
−1.20 −1.15 −1.06 −1.75 −2.19
0.93 1.36 1.93 1.15 1.64
12
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TiO2, water molecules and the surface of TiO2, some bond lengths on the surface of TiO2 change. In solution, oxygen atoms of water molecules are easily adsorbed on Ti(5) atoms on TiO2 surface. Because O(2) atoms on TiO2 surface protrude on TiO2 surface, hydrogen bonds will be formed between hydrogen atom and O(2) atom on TiO2 surface. Because the position of O(3) atom on TiO2 surface is lower, no hydrogen atoms in water molecules are found to be adsorbed on O(3) atoms on the surface of TiO2. In addition, the oxygen atoms of nitro and H atoms on the DMZ molecule form hydrogen bonds with the hydrogen and oxygen atoms on the water molecules, respectively, and the exposed N(3) atoms on the imidazole ring can also form hydrogen bonds with the hydrogen atoms on the water molecules. In aqueous solution, the adsorption characteristics of DMZ on TiO2(1 0 1) and (0 0 1) crystal planes are similar to each other, and hydrogen bond plays an important role in the adsorption process. B1, B2, B3, B4 and B5 are adsorption configurations in neutral solution. It can be seen from Table 1 that the B3 mode has the largest adsorption energy of 2.28 eV, which is the optimal adsorption configuration. The same adsorption characteristics as the A3 adsorption structure is in gas phase. The b5 adsorption structure is the most stable adsorption site of DMZ on the surface of TiO2(0 0 1), which is similar to the most stable adsorption of a5 under gas phase condition. We have compared the adsorption characteristics of DMZ on TiO2(0 0 1) and (1 0 1) crystal planes under the conditions of gas phase and solvent, and have found that the most stable absorption structures are similar. The most stable adsorption structures of DMZ on TiO2(1 0 1) and (0 0 1) planes under gas phase condition are A3 and a5 respectively. In the neutral water solvent, the most stable adsorption configurations of DMZ on the TiO2(1 0 1) and (0 0 1) planes are B3 and b5, respectively. The solvent effect makes the interaction of hydrogen bonding between the molecules enhanced and absorption energy of system enhanced.
site on the crystal plane of TiO2(1 0 1), and the adsorption distance is 2.279 Å. the hydrogen atoms of two methyls on DMZ and the hydrogen atom bonded to C(4) on DMZ form hydrogen bond with O(2) atom on TiO2(1 0 1) crystal plane. The bond lengths are 2.389 Å, 2.213 Å and 1.916 Å, respectively. It’s beneficial to enhance the stability of adsorption. Compared with the structure of DMZ before adsorption, in A3 configuration, the bond lengths of N(3)-C(2) and N(3)-C(4) of DMZ increase from 1.343 Å and 1.356 Å to 1.358 Å and 1.365 Å, respectively, and bond length of N(1)-C(2) decreased from 1.370 Å to 1.358 Å due to the adsorption effect of DMZ on the crystal plane of TiO2(1 0 1). The degradation of nitrogen-containing heterocyclic compounds generally starts from the oxidation of carbon adjacent to nitrogen atoms, and the N(3)-C(2) bond of DMZ is weakened in favor of the attack of hydroxyl radicals. A5 is the sub-stable adsorption configuration of DMZ on the TiO2(1 0 1) plane and the adsorption energy is 1.40 eV. In the A5 configuration, oxygen atom of nitro in DMZ nitro is adsorbed on the surface of Ti(5) atom, and the adsorption distance is 2.294 Å. The hydrogen atoms of methyl bonded to C(2) atom and the hydrogen atom bonded to C(4) atom form weak hydrogen bonds with O(2) atoms on the TiO2(1 0 1) crystal plane, and the bond lengths are 2.530 Å, 2.506 Å and 2.412 Å, respectively. The bond lengths of C(2)-N(3) and C(4)-C(5) double bonds both have been increased. The reason for the lengthening of C(2)-N(3) bond may be the hydrogen bond formed between hydrogen atom of methyl bonded to C(2) in DMZ and O(2) atom. However, The lengthen of C(4)-C(5) bond may be due to the interaction between the oxygen atom of nitro, hydrogen atom bonded to C(4) in DMZ and the TiO2 crystal plane. In general, the hydrogen atom on DMZ is easy to form hydrogen bond with oxygen atom on TiO2 surface, which makes DMZ molecule adsorbed to TiO2 surface. N(1) atom of DMZ is bonded to methyl group with short branching chain and small steric hindrance, which makes more atoms on the molecule being adsorbed to TiO2 surface. Table 1 shows that among the five stable adsorption structures of DMZ on the TiO2(0 0 1) crystal plane, adsorption energy of A5 configuration is the largest of 1.36 eV, which is the best adsorption configuration. In a5 configuration, N(3) atom of DMZ is adsorbed on the Ti(5) atom on the TiO2(0 0 1) crystal plane, and the adsorption distance was 2.312 Å. Because of the interaction between N(3) atom of DMZ and Ti (5) atom, the bond lengths of N(3)-C(2) and N(3)-C(4) in DMZ increased from 1.343 Å and 1.356 Å to 1.358 Å and 1.364 Å, respectively. The bond length of N(1)-C(2) is shortened from 1.371 Å to 1.364 Å, and the N(3)-C(2) bond of DMZ is weak, which is beneficial to the attack of hydroxyl radical and the ring opening degradation. The sub-stable configuration of DMZ adsorbed on the surface of TiO2(0 0 1) is a2 and the adsorption energy is 1.35 eV. The oxygen atom of nitro on DMZ is adsorbed on Ti(5) atom on TiO2(0 0 1) crystal plane in a distance of 2.313 Å. The hydrogen atom of the methyl group bonded to C(2) and the H atom bonded to C(4) in DMZ both form hydrogen bonds with O (2) atoms on the surface of TiO2(0 0 1), and the bond lengths are 2.285 Å, 2.261 Å and 2.419 Å respectively, which is beneficial to increase the adsorption stability. Similar to the adsorption results of TiO2(1 0 1) surface, the N(3)-C(2) bond of DMZ also is weak in a2 configuration, which is favorable to the attack of hydroxyl radical and ring opening degradation. In order to investigate adsorption characteristics of DMZ under solvent conditions, we optimized five stable adsorption configurations of DMZ on (1 0 1) and (0 0 1) crystal planes of TiO2 in aqueous solution by using the same method. The molecular dynamics behavior of DMZ on TiO2(1 0 1) and (0 0 1) crystal planes was simulated by LAMMPS program. The most stable adsorption configuration was selected and the exact structure was optimized by VASP program. As shown in Fig. 3, in which the adsorption configurations and distances of DMZ on TiO2 surface are shown. In aqueous solution, the interaction between DMZ molecule and atom of TiO2 surface is weaker than that under gas phase condition, and the adsorption distance is longer because of the action of water molecule. Due to the interaction between DMZ and the surface of
3.2. Adsorption of DMZ on TiO2 surface under acidic and alkaline conditions Under acidic condition, we used the Lammps program to simulate molecular dynamics of DMZ on the TiO2(1 0 1) and (0 0 1) crystal faces. We selected five relatively stable absorption configurations of DMZ on the TiO2(1 0 1) and (0 0 1) crystal faces in acidic solution, and optimized these structures by VASP program. Fig. 4 shows the adsorption configurations and adsorption distances of DMZ on TiO2(1 0 1) and (0 0 1) crystal planes in acidic solution, respectively. Because of the interaction of DMZ molecules, water molecules, protons and chloride ions with atoms of TiO2 surface, some bond lengths on the surface of TiO2 change. In the acidic solution, free protons (H+) are relatively easily adsorbed onto the N(3) atom of imidazole ring. It can be seen from Table 1 that the C5 mode has the largest adsorption energy of 2.32 eV, which is the optimal adsorption configuration. In C5 configuration, DMZ molecule was adsorbed on the surface of TiO2 in a nearly parallel manner, and the chloride ions were also adsorbed on the Ti(5) atom of TiO2(1 0 1) surface with a adsorption distance of 2.458 Å. The bond length of C(2)-N(3) of DMZ increased from 1.343 Å before adsorption to 1.359 Å, and the bond length of N(1)C(2) decreased from 1.370 Å before adsorption to 1.361 Å. The weakening of C(2)-N(3) bond of DMZ is beneficial to the attack of hydroxyl radical and degradation. The sub-stable adsorption configuration of DMZ on the surface of TiO2(1 0 1) is C4 adsorption configuration, and the adsorption energy is 2.03 eV. In C4 configuration, the DMZ molecule was adsorbed on the surface of TiO2 in a nearly vertical manner. This adsorption also made the C(2)-N(3) bond of DMZ weaker and favors the attack of hydroxyl radicals. This is consistent with the situation in C5, which is similar to the case of DMZ in aqueous solution. In Table 1, we also list the adsorption energy of three relatively stable adsorption configurations of DMZ on TiO2(0 0 1) crystal plane in the acidic solution. The c3 mode has the largest adsorption energy of 2.47 eV, which is the optimal adsorption configuration. The bond 13
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Fig. 3. Adsorption configurations of DMZ on TiO2(1 0 1) and TiO2(0 0 1) surfaces in water solution (Å).
length of C(2)-N(3) of DMZ increased from 1.343 Å before adsorption to 1.360 Å, and this adsorption also weakened the C(2)-N(3) bond of DMZ. Under alkaline condition, Fig. 5 shows the adsorption configurations and adsorption distances of DMZ on TiO2(1 0 1) and (0 0 1) surfaces, respectively. The OH− is easily adsorbed on the surface of TiO2, which makes the adsorption of DMZ molecule on surface of TiO2 weakened. It can be seen from Table 1 that the D3 mode has the largest adsorption energy of 2.15 eV, which is the optimal adsorption configuration. In D3 configuration, the N(3) atom of DMZ is adsorbed on the Ti(5) atom of the TiO2(1 0 1) crystal plane, and the adsorption distance is 2.369 Å. During the adsorption process, hydrogen bonds are formed between two hydrogen atoms of the DMZ C(4) and C(2) and the O(2) atom of the TiO2(1 0 1) crystal plane, and the bond lengths are 2.174 Å, 2.285 Å, and 2.472 Å, respectively. It’s beneficial to increase the stability of adsorption. Compared with the structure of DMZ before adsorption, in D3 configuration, the bond length of C(2)-N(3) on the imidazole ring increased from 1.343 Å before adsorption to 1.357 Å, which makes C(2)-N(3) bond on the imidazole ring weaken and easy to be attacked by hydroxyl radicals. Under alkaline condition, the most stable adsorption configuration of DMZ on TiO2(0 0 1) crystal plane is
d5 with adsorption energy of 2.19 eV and this adsorption are similar to those of DMZ on TiO2(1 0 1) crystal plane, also weakened the C(2)-N(3) bond of DMZ. We calculated the energy bands of TiO2(1 0 1) and (0 0 1) crystal faces under gas phase condition, respectively. After adsorption, the valence band and conduction band energy levels of TiO2 decreased, and the energy gap became narrow. The energy gap value of TiO2 is calculated to be 3.20 eV before adsorption, which is consistent with the experimental value of 3.20 eV [38]. After adsorption, the energy gap values of A1 to A5 are reduced to 2.30 eV, 2.18 eV, 1.57 eV, 1.92 eV, and 1.68 eV, respectively; and a1 to a5 reduced to 2.65 eV, 1.63 eV, 2.65 eV, 2.52 eV, and 1.53 eV, respectively. The adsorption of DMZ on TiO2(1 0 1) and (0 0 1) crystal planes, of which the electron transition wavelength between the valence band and the conduction band is within the visible range, can effectively utilize the visible light to drive the degradation of DMZ on the surface of TiO2. 3.3. Density of state analysis of adsorption configurations It can be seen from Fig. S3 (supporting information) in neutral solution, the energy band structure of TiO2 changes due to the adsorption
Fig. 4. Adsorption configurations of DMZ on TiO2(1 0 1) and TiO2(0 0 1) surfaces in acidic solution (Å). 14
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Fig. 5. Adsorption configurations and adsorption distances (Å) of DMZ on TiO2(1 0 1) and TiO2(0 0 1) surfaces in basic solution.
composed of the d-orbital of Ti before adsorption. The s-orbital component of energy level of valence band increases, and the range of porbital energy is widened from −5 eV to 0 eV before adsorption to −9 eV to 0 eV, and two sets of peaks appear, whose height and area increase. After adsorption, the energy gap of TiO2 is narrowed, and the b1 ∼ b5 energy gap is reduced to 0.77 eV, 1.13 eV, 0.88 eV, 1.18 eV and 1.19 eV, respectively. The wavelengths of electronic transition between valence band and conduction band are: 1612 nm, 1098 nm, 1411 nm, 1052 nm and 1043 nm, respectively, none of those within the range of visible length, indicating that TiO2(0 0 1) crystal plane in neutral solution cannot effectively utilize visible light. As can be seen from Fig. S7, the band structure of TiO2(0 0 1) crystal face in acidic solution is similar to that in neutral solution. Cl− ions are not easily adsorbed by TiO2, while H+ ions are easily adsorbed on the exposed N(3) atom of the imidazole ring and the O(2) atom on the surface of TiO2. When H+ ions are adsorbed on O(2), the component of s-orbital in valence band is slightly increased. After adsorption, the energy gap of TiO2 is narrowed, and the energy gaps of c1 ∼ c5 are reduced to 1.27 eV, 1.77 eV, 1.75 eV, 2.01 eV and 1.80 eV. Except for c1, the wavelengths of electronic transition between valence band and conduction band are: 977 nm, 701 nm, 709 nm, 689 nm, and 618 nm, respectively, within the range of visible wavelengths. It can be seen from Fig. S8 that under alkaline condition, structure of energy band of the TiO2(0 0 1) crystal plane is similar to that under neutral condition. Na+ ions are not easily adsorbed by TiO2, while OH− ions are easily adsorbed on Ti(5) atoms on the surface of TiO2. A few porbital components appears in the range of −22 eV ∼ −15 eV, and the energy gap of TiO2 becomes narrow after adsorption. The energy gaps of d1 ∼ d5 are reduced to 0.93 eV, 1.36 eV, 1.93 eV, 1.15 eV and 1.64 eV, respectively. The wavelengths of electronic transition between valence band and conduction band are: 1335 nm, 913 nm, 643 nm, 1079 nm and 757 nm, except for d3 and d5, none of those in the range of visible light, which indicates that TiO2(0 0 1) crystal plane does not make good use of visible light under alkaline condition. Fig. 6 shows the electron density of DMZ on the TiO2 surface under vacuum condition. We observed an overlap between the charge density of DMZ and the TiO2 surface, indicating that there is electron transfer between DMZ and the TiO2 surface and a new chemical bond is formed. In this process, chemical adsorption occurs between DMZ molecule and TiO2 surface.
of water molecules on the TiO2(1 0 1) surface. Two s-orbital peaks appear between −20 eV and −15 eV. The energy level of conduction band is still composed of the d orbital of Ti before adsorption. The sorbital component increases in the energy level of valence band, and the energy range of the p-orbital is widened from −5 eV to 0 eV before adsorption to −8 eV to 0 eV. Two peaks appeared, whose height and area increase. After adsorption, the energy gap of TiO2 is narrowed, and the energy gaps of B1 ∼ B5 are reduced to 2.27 eV, 2.30 eV, 2.06 eV, 2.22 eV and 2.39 eV, respectively. The wavelengths of electronic transition between valence band and conduction band are 547 nm, 540 nm, 603 nm, 559 nm, 519 nm, respectively, and the frequency of absorption to light is reduced, all are in the range of visible wavelength, which improves the efficiency of absorption to visible light. As can be seen from Fig. S4, in acidic solution, the band structure of the TiO2 (1 0 1) crystal face is similar to that in neutral solution. Cl− ions are not easily adsorbed by TiO2, while H+ ions are more easily adsorbed on the exposed N(3) atoms of the imidazole ring and on the O (2) atom on the surface of TiO2. When H+ ions are adsorbed on O(2), the component of s-orbital of valence band increases and the energy gap of TiO2 becomes narrower. The energy gaps of C1 to C5 are reduced to 1.88 eV, 1.85 eV, 2.43 eV, 2.27 eV and 1.82 eV, respectively. The wavelengths of electronic transition between valence band and conduction band are 660 nm, 671 nm, 511 nm, 547 nm, and 682 nm, respectively, and the frequency of absorption to light is reduced in the range of visible wavelength, which improves the efficiency of absorption to visible light. As can be seen from Fig. S5, in alkaline solution, the band structure of the TiO2(1 0 1) crystal plane is similar to that in neutral solution. Na+ ions are not easily adsorbed by TiO2, while OH− ions are easily adsorbed on Ti(5) atoms on the surface of TiO2. A little p-orbital component appears in the range of –22 eV ∼ −15 eV, and the energy gap of TiO2 becomes narrow after adsorption. The energy gaps of D1 ∼ D5 are reduced to 0.95 eV, 1.68 eV, 1.87 eV, 1.66 eV, and 2.08 eV, respectively. Except for D1, the wavelengths of electronic transition between valence band and conduction band are: 1307 nm, 739 nm, 664 nm, 748 nm, and 597 nm, respectively, in the range of visible length, which improves the efficiency of adsorption to visible light. It can be seen from Fig. S6 that the TiO2(0 0 1) crystal plane in the neutral solution is similar to the TiO2(1 0 1) crystal plane, and the TiO2(0 0 1) crystal plane adsorbs water molecules, which changes the structure of energy band of TiO2. Two s-orbital peaks appear between −22 eV and −15 eV. The energy level of conduction band is still
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Fig. 6. The electron density of DMZ on the TiO2 crystal surface under vacuum condition.
4. Conclusions
References
In this paper, the density functional theory is used to study the adsorption of DMZ on anatase TiO2(1 0 1) and (0 0 1) crystal planes. The most stable adsorption structure was optimized and characteristics of adsorption structure were discussed. DMZ can be effectively adsorbed on TiO2 surface both in vacuum and in neutral, acidic and alkaline solution. The results of adsorption lengthen the CeN bond length of imidazole ring on DMZ, which is beneficial to the attack of hydroxyl radicals and the degradation reaction. With the change of molecular after adsorption, we found the reaction site of ring-opening degradation of DMZ on the surface of TiO2. At the same time, study found that the effect of hydrogen bond is obvious in the adsorption process of DMZ on TiO2 surface. Under neutral, acid or alkaline condition, the effect of hydrogen bond on the adsorption characteristics and properties in the adsorption process is very significant. For different acid or base condition, the adsorption wavelengths of electron transition between conduction bands and valence bands are both within the range of visible wavelength, indicating that TiO2(1 0 1) crystal plane can effectively utilize visible light and can photocatalyze the adsorption and degradation of DMZ on TiO2(1 0 1) crystal plane. However, the use of visible light in TiO2(0 0 1) crystal piane is greatly affected by the condition, and DMZ is more susceptible to TiO2(0 0 1) surface in acidic condition. We compared the adsorption characteristics of DMZ on two different crystal planes of TiO2 under acidic and alkaline conditions and we found that the TiO2(1 0 1) crystal plane can effectively utilize visible light under acidic and alkaline conditions, which can explain experimental results that DMZ can effectively be degraded by TiO2(1 0 1) crystal plane, whether in acidic,neutral or alkaline condition. The crystal plane of TiO2(1 0 1) accounts for 94% of anatase TiO2. Our results indicate that anatase TiO2 can play a good photocatalytic role in the degradation of nitroimidazole antibiotics.
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Acknowledgments We are grateful for the financial support of this work by the Sichuan province department of education (13ZA0150) and Sichuan province (2014JY0099).
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.comptc.2019.01.002. 16
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