Theoretical study of the adsorption characteristics and the environmental influence of ornidazole on the surface of photocatalyst TiO2

In this paper, density functional theory (DFT) was performed to study the adsorption properties of ornidazole on anatase TiO2(101) and (001) crystal facets under vacuum, neutral and acid-base conditions. We calculated the adsorption structure of ornidaozle on the anatase TiO2 surface, optimal adsorption sites, adsorption energy, density of states, electronic density and Milliken atomic charge under different conditions. The results show that when the N(3) atom on the imidazole ring is adsorbed on the Ti(5) atom, the largest adsorption energy and the most stable adsorption configuration could be achieved. According to the analysis of the adsorption configuration, we found that the stability of C(2)-N(3) bond showed a weakening trend. The adsorption wavelengths of the electronic transition between the valence band and conduction band of ornidazole on the TiO2 surface were in the visible light wavelengths range, showing that the TiO2 crystal plane can effectively make use of visible light under different conditions. We speculate the possibility of ornidazole degradation on the surface of TiO2 and found that the reactive site is the C-N bond on the imidazole ring. These discoveries explain the photocatalytic degradation of ornidazole by TiO2 and reveal the microscopic nature of catalytic degradation.

as a photocatalyst to degrade two types of reactive azo dyes. By continuously changing the test parameters, the two dyes could be completely degraded. Combining quantum chemical theory calculations with experiments, Liu 12 employed the yttrium-doped TiO 2 (TiO 2 /Ce) hydrosol as a photocatalyst to analyze the degradation effect of the pesticide residue dimethoate, and conducted meritorious studies on the subsequent pesticide residues as well. Marothu 13 found that the heterogeneous photocatalytic degradation technology is very effective towards the anti-Parkinson-like entacapone with anatase TiO 2 , and they studied the effects of the parameters of degradation, such as the catalyst loading, acidity and alkalinity of the solution, and initial concentration. In the present study, anatase TiO 2 was utilized as a catalyst to study the adsorption properties of ornidazole on the TiO 2 (101) and (001) facets under different conditions. We hope to supply some theoretical information for the research of ornidazole.

Results and Discussion
The molecular structure of ornidazole (seen in Fig. S1) and the stable crystal planes of TiO 2 (101) and (001) were optimized. The molecular dynamics of ornidazole on the TiO 2 (101) and (001) crystal facets was simulated by the LAMMPS program. Based on the relaxation results, we selected the relatively stable adsorption configurations to further optimize the molecular structures by the Materials Studio program. The adsorption energies for the adsorption configurations are shown in Table 1. Thus, it is shown that the Ti(5) and O(2) atoms are more active than the Ti(6) and O(3) atoms. Some bond lengths on the surface of TiO 2 are slightly deformed owing to the interaction of the ornidazole molecule with the TiO 2 surface. Zhang 14 founded that the hydrogen bond can enhance the stability of the multilayer dye aggregates on the TiO 2 surface. The study by Chang showed that the hydrogen bond between HNO 3 and TiO 2 can enhance the adsorption energy and the stability of the adsorption configuration 15 . It is observed that the adsorption configuration can be stabilized by the formation of hydrogen bonds.
From Table 1, can be observed that under the vacuum conditions, A1 is the most stable adsorption configuration on the TiO 2 (101) surface. For A1 mode, the N(3) atom adsorbed on the Ti(5) atom, and the H atom of the methyl moiety and the C(2) atomic branch form hydrogen bonds with the O(2) atom on the TiO 2 (101) plane. The adsorption distances are 2.341, 2.073 and 2.422 Å, respectively. Due to the interaction of the ornidazole molecule with the surface of TiO 2 , some bond lengths are changed. C(2)-N(3) and N(3)-C(4) in the imidazole ring change from 1.335 and 1.354 Å to 1.352 and 1.361 Å, respectively, and the C(2)-N(3)-C(4) bond angle changes from 106.1°to 107.3°. The bond length of C(2)-N(3) increases even more than the N(3)-C(4), which means that the process of adsorption make C(2)-N(3) more unstable and favors the attack of the hydroxyl radicals.
Similarly, Table 1 shows that under vacuum conditions, a1 is the most favorable configuration with the highest adsorption energy about 2.83 eV. In a1 configuration (Fig. 1), the N(3) atom adsorbed on the Ti(5) atom and the H atom on the C(2) atomic branch of ornidazole adsorbed on the O(2) atom. The adsorption distances are 2.267 and 2.444 Å, respectively. The C(2)-N(3) bond length in the imidazole ring has been greatly changed due to the adsorption, increasing from 1.335 Å to 1.346 Å. This result indicates that the stability of C(2)-N(3) bond weakens, which favors the attack of the hydroxyl radicals and ring opening degradation.
Adsorption under solvent conditions. To take into account the adsorption characteristics of the ornidazole molecule on the TiO 2 crystal surface under solvent conditions, we used the same method to optimize the stable adsorption structures of ornidazole on the TiO 2 (101) and (001) crystal facets under solvent conditions. Adsorption under neutral conditions. The modes of B1~B5 and b1~b5 are shown in Fig. 2 (101) and (001) surfaces, respectively. The degree of deformation of the crystal facets is greater than that under vacuum conditions to maintain the stability of the adsorption configuration. In terms of the adsorption energies (Table 1), the B5 and b1 configurations are the most stable structures of ornidazole on the anatase TiO 2 (101) and (001) crystal facets, which are 2.45 and 2.64 eV, respectively. The adsorption characteristics of B5 and b1 are similar to those of the A1 and a1 configurations, and the stability of C(2)-N(3) bond tends to be weak and is susceptible to attack by hydroxyl radicals. After adsorption the N(3)-C(2) bond length become longer compare to vacuum conditions. In the solvent conditions, H 2 O molecules are revolved around ornidazole, there may be strong interactions between H 2 O molecules and the ornidazole molecule. Meanwhile, the adsorption energy increase relative to the vacuum conditions. Zhang 16  Adsorption under acidic conditions. Figure 3 shows the adsorption distances and sites of C1~C5 and c1~c5 configurations under acidic conditions. Due to the interactions of the ornidazole molecule, water molecules, proton and chloride ion with TiO 2 , a subtle deformation of the TiO 2 crystal plane occurs. As shown in Table 1, in terms of the adsorption energy, C1 mode is the most stable configuration under acidic conditions.  www.nature.com/scientificreports www.nature.com/scientificreports/ The adsorption properties of C1 are similar to those of A1 except that more hydrogen bonds have formed. The c2 configuration has the largest adsorption energy, 2.89 eV, and is the most stable adsorption configuration of ornidazole on the TiO 2 (001) surface under acidic conditions. In the c2 configuration, the N(3) atom of ornidazole is not adsorbed on the Ti(5) atom. However, by investigating the bond lengths of the c2 configuration, we found that the bond lengths of C(2)-N(3) and C(4)-C(5) increase after adsorption from 1.335 and 1.384 Å to 1.354 and 1.398 Å, respectively. The length of C(2)-N(3) is longer than that of C(4)-C(5). At the same time, we also analyzed the other four configurations and found that the bond lengths of the ornidazole molecule change differently and that the C(2)-N(3) bond length obviously increases, disclosing adsorption makes the stability of C(2)-N(3) weaker and more susceptible to attack by hydroxyl radicals.
Adsorption under alkaline conditions. As demonstrated in Fig. 4, D1~D5 and d1~d5 are five adsorption configurations of ornidazole on the TiO 2 (101) and (001) facets under alkaline conditions, respectively. From Table 1 we observe that D5 (adsorption energy of 2.52 eV) is the most stable adsorption configuration of ornidazole on the TiO 2 (101) surface. Similarly, in terms of the adsorption energy (shown in Table 1), d5 (2.89 eV) is the most stable adsorption configuration of ornidazole on the TiO 2 (001) surface. The adsorption properties of the D5 and d5 configurations are also similar to those of A1 and a1 under vacuum conditions, except that in the d5 structure, more hydrogen bonds form and the O atom in the hydroxyl moiety interacts with Ti(5). Adsorption  www.nature.com/scientificreports www.nature.com/scientificreports/ also makes the stability of C(2)-(N3) weaker and thus susceptible to attack by hydroxyl radicals and ring-opening degradation.
By analyzing the adsorption energy and adsorption configuration of the ornidazole molecule on the TiO 2 crystal surface under vacuum and aqueous conditions, we found that the adsorption configuration is more stable under aqueous conditions. The adsorption of ornidazole on the TiO 2 surface is affected by the intermolecular surface tension and hydrogen bonding. After adsorption, the crystal surface is slightly deformed due to the influence of the ornidazole molecule, water molecules, proton and ion on the crystal plane of TiO 2 . We also found that when the ornidazole molecule adsorbed on the TiO 2 (001) crystal plane, the degree of deformation of the crystal plane is much greater than that of the (101) plane under vacuum or aqueous solution conditions, which may be related to the fact that the anatase TiO 2 (001) crystal plane has more unsaturated titanium ions and a higher surface activity [18][19][20] . Therefore, the (001) surface may be more favorable for photocatalysis. Additionally, the overall adsorption energy of ornidazole is found to be the highest under acidic conditions. The isoelectric point of TiO 2 is 6.3 21 , which indicates that the positive charge on the surface of TiO 2 is beneficial to the adsorption of the ornidazole molecule when the pH is less than 6.3. In contrast, the negative charge on the surface of TiO 2 is not conducive to the adsorption of the ornidazole molecule. The results may provide a certain reference for the degradation conditions of ornidazole.
On the basis of the characteristics of the molecular adsorption structure, we found that the most stable adsorption configuration of ornidazole on two surfaces of TiO 2 is the N(3) atom adsorbed on the Ti(5) atom. After the ornidazole adsorbed on TiO 2 surface, the bond length of C-N much longer. Thus, according to the adsorption results, it is reasonable to speculate that the ring-opening reaction site of ornidazole is the C-N bond 22 . From these figures we found that the s-orbital composition of the TiO 2 (001) plane is even more than that of the TiO 2 (101) plane. For semiconductor photocatalytic materials, the electronic transition between the CB and VB is affected by the energy gap. If the energy gap is in the visible light range, visible light can be effectively utilized. Therefore, the energy gaps of different crystal plane adsorption configurations, which are the differences between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies, can be used to judge the utilization of visible light. By calculating the energy gap, their values are the difference. The calculated energy gap of bulk TiO 2 is 2.87 eV, which is close to the experimental value of 3.20 eV 25 26,27 , demonstrating that using visible light to drive the degradation of ornidazole on TiO 2 surface is effective.
The DOS and PDOS of adsorption configuration under neutral aqueous conditions are plotted in Figs S4 and S5. The band structure of TiO 2 changes due to the action of H 2 O molecules on TiO 2 surface, in the water solvent conditions. After adsorption, the Ti 3d states still govern the CB edge, and the s-orbital component of the VB energy level is increased. Two peaks of s orbital form between −21 eV and −15 eV, and the energy range of the p orbital is broadened from −5~0 eV to −8~0 eV. From −8~0 eV, two sets of peaks appear, in which the peak height and peak area are increased compared to that of pure TiO 2 . The increase in the s-orbital and p-orbital components elucidates that the 1 s orbital of the H atom and the 2p orbital of the O atom in the H 2 O molecules participate in hybridization. The s orbital appears near the Fermi level, which corresponds to the HOMO orbital of the system. When the number of electrons in the HOMO or LUMO orbital increase, the electron donating ability of the system also increase, showing that the chemical activity of TiO 2 is improved. As well, the TiO 2 (001) surface has more p-orbital components, indicating that there may be more 2p orbitals of O in the H 2 O molecules involving in the hybridization and that the chemical activity of the TiO 2 (001) surface is greater than that of the (001) plane. The TiO 2 energy gap is narrowed after adsorption. The energy gaps of B1~B5 are reduced to 2.313, 2.153, 2.121, 2.331, and 2.251 eV. The b1~b5 energy gaps are reduced to 2.127, 1.704, 2.005, 2.029, and 2.077 eV, respectively. The above energy gaps all in the visible light range, showing that the TiO 2 surface can effectively utilize visible light under water solvent conditions. The DOS and PDOS for the adsorption of ornidazole on anatase TiO 2 (001) and (001) facets under acidic conditions are shown in Figs S6 and S7, respectively. Clearly, the TiO 2 band structure is similar to that of the neutral solution. The energy gap is narrower after adsorption than that of pure anatase TiO 2 . The C1~C5 energy gaps are reduced to 2.301, 2.306, 1.900, 2.052, and 1.162 eV, respectively. The light absorption frequency is reduced, except for the C5 configuration, and they are all in the visible wavelength range. The energy gaps of c1~c5 are 1.789, 1.824, 1.722, 2.131, and 1.958 eV, respectively, which are in the visible light range. These results show that, under acidic conditions, the TiO 2 crystal surface can effectively use visible light.
The DOS and PDOS of adsorbed-TiO 2 under alkaline conditions are shown in Figs S8 and S9, respectively. The band structure is similar to that of the neutral conditions. Adsorption results in the narrowing of the TiO 2 energy gap. Specifically, the D1~D5 energy gaps are reduced to 2.325, 1.978, 1.640, 2.327, and 2.336 eV, respectively, and the d1~d5 energy gaps are 1.990, 2.043, 1.700, 1.932 and 1.930 eV, respectively. All of the energy gaps are in the visible range, and thus, the TiO 2 crystal plane can effectively use visible light, under basic conditions. Figure 5 shows the electron density of ornidazole on the TiO 2 crystal surface under vacuum conditions. We observed an overlap between the charge density of the imidazole ring and the TiO 2 surface. From Table S1, we www.nature.com/scientificreports www.nature.com/scientificreports/ can see that under different conditions, the number of electrons on the imidazole ring increases after adsorption, which shows that the electrons of the crystal surface have shifted to the imidazole ring during the adsorption process. These results demonstrate that electron transfer occurs between the imidazole ring and TiO 2 surface and that a new chemical bond is formed. During the process of adsorption, the ornidazole molecule interacts with the surface of TiO 2 and undergoes chemical adsorption.

Conclusion
In this work, DFT was used to study the adsorption characteristics of ornidazole on the anatase TiO 2 (101) and the (001) facets under different conditions. The result showed that ornidazole can absorb on the TiO 2 surface in vacuum or aqueous solution conditions, especially acidic conditions. After adsorption the bond length of C-N in the imidazole ring becomes longer, which is conducive to the attack and ring-opening degradation of the hydroxyl radicals. Through the molecular adsorption structure change characteristics, we found that the reaction site of degradation is the ring-opening of the C-N bond on the imidazole ring. At the same time, the hydrogen bonding played a role in the process of ornidazole adsorbed on the surface of TiO 2 . Compared with vacuum conditions, the hydrogen bonding effect in the adsorption process under aqueous conditions is more significant for the change in the adsorption characteristics. For different conditions, we found that the adsorption wavelengths of the electronic transition between the VB and CB of each adsorption configuration on the TiO 2 (101) and (001) crystal facets correspond to visible light. Our results reveal that the TiO 2 can effectively use visible light and can be used as a photodegradation catalyst for ornidazole.

Methods
The anatase TiO 2 (101) and (001) crystal facets were investigated in this paper. From Fig. 6, it is can be observed that the surface of TiO 2 show 5-fold and 6-fold coordinated Ti atoms (Ti(5) and Ti (6)), as well as 2-fold and 3-fold coordinated oxygen atoms (O(2) and O(3)). Notably, Ti(6) site does not exist in TiO 2 (001) surface layer. Based on a preliminary study of the effect of the plate thickness on the surface energy, when the (101) surface adopts a three-layer model 28 31 .
Placing the ornidazole molecule on the TiO 2 (101) and (001) crystal facets, the distance between them was set to approximately 3.8 Å to avoid a strong interaction, and then we introduced the reactive force field (ReaxFF) and NVE ensemble under the LAMMPS program to perform a molecular dynamic calculation 32,33 . Based on the LAMMPS relaxation results, the local minimum structure was selected for further optimization by DFT.
The DFT calculation was performed using the DMol3 code of the MS package 34 . DMol3 applied the dual digital base group and polarization function to extend the electronic wave function and all structural optimization was performed on the basis of spin-polarized plane waves. The Kohn-Sham one-electron equations were solved in the generalized gradient approximation (GGA) by using the Perdew-Burke-Ernzerhof (PBE) functional 35 , and the effective core potentials (ECP) was used to describe the core electrons. The polarized DNP 36 basis set was used to describe the atomic orbitals and the cutoff radius was set to 4.5 Å. For the calculation of the adsorption results, the www.nature.com/scientificreports www.nature.com/scientificreports/ convergence criterion is set to the following criteria: the energy was smaller than 2 × 10 −5 Ha, the force was below 4 × 10 −3 Ha/Å, and the max displacement was 5 × 10 −3 Å. In addition, the self-consistent field (SCF) iterative energy tolerance was set to 1 × 10 −5 Ha, and the multipole expansion was performed by the octupole moment.
The adsorption energy (E ads ) is defined as

ads s urf m ol total
where E total is the free energy for the ornidazole molecule absorbed on TiO 2 surface, E surf is the energy of the TiO 2 surface, and E mol is the energy for the ornidazole molecule. The lattice parameters of bulk anatase TiO 2 optimized by the above method are a = b = 3.776 Å, and c = 9.486 Å, which are consistent with the experimental values of a = b = 3.785 Å and c = 9.514 Å 37,38 . The agreement shows that our calculation method and results are reliable.