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A computational study on the effect of Ni impurity and O-vacancy on the adsorption and dissociation of water molecules on the surface of anatase (101)
Journal of Physics and Chemistry of Solids 136 (2020) 109176
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A computational study on the effect of Ni impurity and O-vacancy on the adsorption and dissociation of water molecules on the surface of anatase (101) Mohammadreza Elahifard a, b, *, Hajar Heydari c, Reza Behjatmanesh-Ardakani c, d, Bijan Peik b, Seyedsaeid Ahmadvand b a
Department of Chemical Engineering, Faculty of Engineering, Ardakan University, Ardakan, Iran Department of Metallurgical Engineering, University of Nevada, Reno, Reno, NV, 89557, USA c Department of Chemistry, Payame Noor University, PO Box, 19395-3697, Tehran, Iran d Research Center of Environmental Chemistry, Payame Noor University, Ardakan, Yazd, Iran b
A R T I C L E I N F O
A B S T R A C T
Keywords: Anatase (101) surface Ni impurity Oxygen vacancy Density of states Water adsorption
Ni impurity has been broadly used in the structure of TiO2 to extend the excitation response to the visible region and thus increase the photocatalytic activity. In this study, a full potential density functional theory has been used to study the effect of Ni impurity, substituted on the surface of anatase (101), on the electronic structure, Ovacancy formation energy, charge transfer, and adsorption and dissociation energies of water molecule. To this end, anatase TiO2 (101) surface is simulated and analyzed in its pure, Ni-doped, defective, and defective Nidoped forms. According to the results, oxygen vacancies and nickel impurities shift the occupied Ti 3d-orbitals and unoccupied Ni 3d-orbitals below the conduction band and inside the band gap, respectively. In addition, Ni impurity reduces the O-vacancy formation energy and increases the water adsorption energy significantly. While the molecular adsorption is preferred on the surface of plain and Ni-doped anatase (101), the adsorption of the dissociated form is more favorable upon O-vacancy development. Simultaneous presence of O-vacancy and Ni impurity creates an occupied defect state inside the band gap, which mainly corresponds to the Ni 3d-orbitals, as well as a positive synergic effect on the surface reactivity of the anatase (101) by increasing the adsorption energy of water especially in dissociated form. This provides OH groups on the surface as the main reactive specious to trigger the photocatalytic process.
1. Introduction
Ni, as a cationic substitutional impurity, narrows TiO2 band gap by creation of new states within the band gap region, which in turn shifts the optical absorption to the visible region [21–27]. On the other hand, well-studied oxygen defects have been shown to be stable and essential for catalytic activity of TiO2 in decomposition of water molecules and generation of hydroxyl groups [28–31]. Formation of hydroxyl radical plays an important role in oxidation of organic compounds and removal of microorganisms [32]. Therefore, a thorough knowledge of water adsorption and dissociation energetics is critical to increase the photo catalytic efficiency. Rutile and anatase are the main polymorphs of TiO2, with anatase having higher photocatalytic activity. The tetragonal anatase structure consists of four TiO2 units in its unit cell, and the most stable TiO2 anatase surface is anatase (101). However, fewer studies are focused on
Due to its abundance, chemical stability, and low toxicity, TiO2 is of a great importance in many industrial applications, such as hydrogen production, elimination of organic contaminants, and water disinfection [1–10]. With a wide band gap of 3.0–3.2 eV, TiO2 is mainly active under the ultraviolet (UV) light, lowering its efficiency in solar devices. However, this problem could be tackled by generating cationic/anionic substitutional/interstitial impurities and defects in TiO2 structure, i.e., band gap engineering [11–20]. These defects and impurities create some electronic states inside the TiO2 band gap, extending its excitation response to a larger range of electromagnetic wave. Among transition metals, Ni has been employed more frequently as a dopant to broaden the photocatalytic activity of TiO2. Experimental studies indicate that
* Corresponding author. Ayatollah Khatami Boulevard, P.O. Box:184, Ardakan University, Ardakan, Iran. E-mail addresses: [email protected], [email protected] (M. Elahifard). https://doi.org/10.1016/j.jpcs.2019.109176 Received 3 August 2019; Received in revised form 29 August 2019; Accepted 2 September 2019 Available online 5 September 2019 0022-3697/© 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Slab (a) and top (b) views of the pure anatase (101) surface. Red and blue spheres represent O and Ti atoms, respectively.
Fig. 2. Total and partial density of states of the pure, O-vacancy@-, Ni@-, and O-vacancy@Ni@anatase (101) surfaces.
the surface of anatase. Experimental and theoretical results indicate that similar to the most stable surface of rutile, rutile (110), water molecular adsorption is preferable on the plain anatase (101) surface. However, in the presence of oxygen vacancy, adsorption of dissociated water (OH þ
H) is thermodynamically more favorable compared to molecular adsorption [33,34]. Anatase is known as the most active photocatalytic phase of TiO2, and yet, its uneven surface adds more complication to computational 2
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Fig. 3. The optimized structures for molecular and dissociated water adsorption on the pure anatase (101) surface. Red and blue spheres represent O and Ti atoms, respectively.
the vacuum was set to 15 Å. This structure contains four layers of O–Ti–O and thus 12 atomic layers consisting of 16 titanium atoms and 32 oxygen atoms. Fig. 1 shows a perfect structure of anatase (101) surface including 6-fold (Ti6c) and 5-fold (Ti5c) coordinated Ti atoms, and 2- (Ob or O2c) and 3- (O3c) coordinated oxygen atoms. Ni-doped anatase (101) surface was made by replacement of either Ti6c or Ti5cTi5c Ti5c atoms Ti6c Ti6c with a Ni atom. The most stable form of Ovacancy was made by removing a two-fold coordinated bridge oxygen atom (Ob). Yet, all possible positions of O-vacancy were considered with respect to the substituted nickel atom; Ni6c with O2c (vacancy) and Ni5c with O2c (vacancy), along with different distances defined as near and far structures. The formation energy of O-vacancy (Eformation ) in pure and doped anatase surface was calculated by, Eformation ¼ Eðsystem without defectÞ
Eðsystem with defectÞ þ μO
(1)
where, Eðsystem without defectÞ and Eðsystem with defectÞ are the total energy of pure anatase (101) surface and that with Ni dopant for cases with and without oxygen vacancy, respectively. In relative conditions, chemical potential of oxygen atom (μO ) can be taken as half of the ground state energy of the oxygen molecule [35,36]. Adsorption energies were obtained by, � � ads Eads Ecomplex ðEsur þ EH2 O Þ (2) mol ; E dis ¼
Fig. 4. The difference between charge densities of the complex (water adsorbed on anatase (101) surface) and separate species (water þ anatase (101)). The blue and yellow lobes represent the negative and positive levels of isosurfaces, respectively. Red and blue spheres represent O and Ti atoms, respectively.
studies. Recently, we have shown that Ni-impurity increases the adsorption energy of water on rutile (110) surface noticeably [11]. Herein, we extend our study to investigate the effects of the Ni impurity, O-vacancy, and simultaneous O-vacancy and Ni-impurity on the surface of anatase (101). The structural and electronic properties of anatase (101) with or without impurities/defects are inquired. Both 5- and 6-fold Ti atoms are replaced by Ni atom, and their properties are compared. In addition, the role of Ni impurities and oxygen defects is investigated in adsorption and dissociation of water. Finally, partial density of states (pDOS) analysis is used to discuss the effects of nickel doping as well as oxygen vacancy on the electronic structure of the anatase surface.
where, Ecomplex is the energy of anatase (101) surface, with/without defects/impurities with one water molecule, Esur is the energy of the surface without the water molecule, and EH2 O is the energy of one water ads molecule. Eads mol and Edis denote the adsorption energies of water in mo lecular and dissociated forms, respectively. All calculations were carried out using Fritz Haber Institute ab initio Molecular Simulations (FHI-aims) computational package; an allelectron and full potential code [37,38]. Also, correlation exchange function of generalized gradient approximation (GGA) and the revised form of Perdew, Burke, and Ernzerhof (rpbe) [39] functional of DFT [40–43] were employed. However, band gap energies were refined using hybrid method of HSE06 [44]. The Monkhorst-pack grids of 4 � 6 � 1 and 6 � 6 � 2 were used for the geometry optimization of the surface and bulk, respectively. Half of the layers were frozen while the remaining atoms were relaxed. The convergence criteria were set to
2. Computational details In our modeling, periodic slab model of 2 � 2 surface of anatase (101) unit cells with single adsorbed water molecule were used, while 3
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Table 1 The Hirshfeld charge difference for all surficial atoms before and after molecular and dissociated adsorption of water.
1 2
Ovacancy@Ni-doped (dis.)
Ovacancy@Ni-doped (mol.)
Ni-doped (dis.)
Ni-doped (mol.)
O-vacancy (dis.)
O-vacancy (mol.)
Pure (dis.)
Pure (mol.)
Configuration
0.02 0.001 0.0005 0.002 0.001 0.001 0.4 0.4 … 0.002 0.002 0.006 0.01 0.002 0.001 0.02 0.01
0.03 0.006 0.01 0.004 0.02 0.02 0.02 0.37 … 0.05 0.003 0.05 0.03 0.42 0.13 0.02 0.03
0.11 0.005 0.005 0.002 0.01 0.006 0.26 0.2 0.06 0.011 0.01 0.01 0.002 0.2 0.005 0.013 0.012
0.007 0.02 0.02 0.0003 0.01 0.01 0.015 .02 0.001 0.06 0.002 0.04 0.06 0.05 0.11 0.030 0.032
0.05 0.002 0.002 0.001 0.002 0.002 0.004 0.04 … 0.004 0.003 0.004 0.006 0.04 0.013 0.01 0.01
0.03 0.012 0.05 0.00002 0.012 0.012 0.03 0.06 … 0.005 0.005 0.005 0.002 0.01 0.15 0.06 0.06
0.053 0.004 0.005 0.003 0.01 0.006 0.01 0.07 0.007 0.006 0.0005 0.026 0.004 0.02 0.004 0.016 0.005
0.002 0.017 0.017 0.0005 0.006 0.006 0.01 0.07 0.005 0.0008 0.003 0.017 0.008 0.002 0.11 0.002 0.024
O2C O2C O2C O2C Ti5C Ti5C 1 Ti5C/Ni5C 2 Ti5C/Ni5C O2C/vacancy O3C O3C O3C O3C Ti6C/Ni6C OW HW1 HW2
water adsorption on Ni replaced by Ti5C. water adsorption on Ti5C (more stable state) and Ni replaced by Ti5C.
Fig. 5. The optimized structures of molecular and dissociated water adsorption on the anatase (101) surface with an O-vacancy. Red and blue spheres represent O and Ti atoms, respectively.
SC_accuracy_rho 1 E 5, SC_accuracy_eev 2 E 3 and SC_accuracy_etot 1 E 6 for the self-consistency cycle, sum of eigen values and the total energy, respectively [45].
surface simulations. The Hirshfeld charge analysis of anatase (101) re sults in a similar charge for Ti6c atoms of the surface and bulk. Also, the Ti5c atoms are about 0.05 e more positive compared to the Ti6c atoms. Furthermore, the surficial Ni─O bond has a weaker polarization, where nickel is about 0.26 e less positive than titanium. This results in a higher bond length for Ni─O (1.86 Å) compared to that of Ti─O (1.83 Å) despite of the smaller radius of Ni ion with respect to Ti ion. However, in rutile (110), the less flexible structure restricts atomic displacements and results in a lower bond length for Ni─O compared to that of Ti─O [11]. The calculated formation energies of O-vacancy in the pure and Nidoped anatase (101) surface are 6.73 eV and 4.13 eV, respectively. The presence of Ni.NiNi has lowered this formation energy significantly (2.6 eV). This increment of the photocatalytic activity via elevation of Ovacancy concentration might be the result of Ti4þ substitution by Ni2þ and advent of the Burstein-Moss effect [48,49].
3. Result and discussion 3.1. Surfaces analysis Anatase TiO2 has a tetragonal structure with the space group of C19 4h I41 =amd comprised of four titanium atoms and eight oxygen atoms. a ¼ b ¼ 3.785 Å and c ¼ 9.51 Å are the optimized lattice constants with a less than 1% error compared to the experimental values [46,47]. Fig. 1 represents the simulated anatase surface made of Ti5cTi5c Ti5c Ti5c , Ti6c, O2c and O3c atoms. Similar to rutile, replacing Ti5cTi5c Ti5c Ti5c Ti5c with Ni NiNiresults in a more stable structure [11]. This implies that Ni im purity prefers surficial positions and emphasizes the importance of 4
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contribution from Ni 3d orbitals. The spin-polarized calculation results in a nonzero magnetization for O-vacancy@anatase (101) due to the difference between the population of spin-up and spin-down states. This magnetization is zero for Ovacancy@Ni@anatase (101) with a symmetric DOS. This could be due to the fact that the resultant electrons from O-vacancy compensate the replacement of Ti by Ni, paring all the electrons in the occupied orbitals. However, this trend is not followed by O-vacancy@Ni@rutile (110) based on our previous study, which could be a subject of further investigations. 3.2. Water adsorption 3.2.1. Anatase (101) surface Similar to rutile, Ti5c is the most stable site for adsorbing water molecule with an adsorption energy of 0.5 eV. The experimental values for this adsorption energy lays between 0.5 and 0.7 eV, introducing
Fig. 6. The difference between charge densities of the complex (water adsorbed on the O-vacancy@anatase (101) surface) and separate species. The blue and yellow lobes represent the negative and positive levels of isosurfaces, respec tively. Red and blue spheres represent O and Ti atoms, respectively.
Consistent with previous studies, titanium 3d and oxygen 2p orbitals have the major contributions to the bottom edge of the conduction band and the top edge of the valence band, respectively, for both surface and bulk [24,27]. The band gap of pure anatase surface is 3.03 eV, which is comparable with the experimental value of 3.23 eV [24]. While, plain DFT methods underestimate the band gap values, the hybrid method used in this study obtains a better agreement with experiment compared to previous theoretical reports [24,50]. Two extra electrons, released by O-vacancy in the structure, occupy the 3d states of the near Ti atoms, which arise below the conduction band (Fig. 2). This gives rise to an n-type semiconductor and increases the band gap by 0.3 eV in this case that is consistent with the bulk [22,25,26]. On the other hand, Ni in duces unoccupied defect states inside the anatase band gap which stem from hybridization of Ni 3d and unoccupied oxygen 2p orbitals. The position of these defect states slightly differs between the bulk and surface [22,25]. Once Ni doping is accompanied by O-vacancy, an occupied band inside the band gap emerges. Hybridization of O 2p, and Ti and Ni 3d orbitals give rise to this occupied band with a larger
Fig. 8. The difference between charge densities of the complex (water adsorbed on Ni@anatase (101) surface) and separate species. The blue and yellow lobes represent the negative and positive levels of isosurfaces, respectively. Red, blue, and yellow spheres represent O, Ti and Ni atoms, respectively.
Fig. 7. The optimized structures of molecular and dissociated water adsorption on the Ni@anatase (101) surface. Red, blue, and yellow spheres represent O, Ti, and Ni atoms, respectively. 5
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Fig. 9. The optimized structures of molecular and dissociated water adsorption on the O-vacany@Ni@anatase (101) surface. Red, blue, and yellow spheres represent O, Ti and Ni atoms, respectively.
molecular and dissociated forms of water is 0.8 eV and 1.45 eV respec tively, i.e., dissociated from is more favorable. This is again consistent with our findings for O-vacancy@rutile (110) and previous studies [13]. Similar to defectless surface, charge transfer occurs from water molecule to the surface (Fig. 6). However, this transferred charge in this case is slightly lower for the defectless surface (Table 1). 3.2.3. Ni@anatase (101) surface Lower coordination number of surficial atoms is in favor of Ni atoms with lower activity compared to Ti atoms. Similar to Ni@rutile (110) surface, Ti5C and Ni5C atoms are the most stable sites for water adsorption, with 0.52 eV adsorption energy related to the nearest Ti (5fold) atom to the Ni impurity (Fig. 7). Supported by experiment, Ni doping increases the hydrophilicity, and thus photocatalytic and selfcleaning properties of TiO2 [53–55]. This is consistent with the ligand effect [56–59]; Ni doping increases the activity of the near-Ti atoms. Accordingly, the energies of water adsorption on Ti5C near and far from Ni5C impurity are 0.52 and 0.47 eV, respectively. The adsorption energy of dissociated water in the most stable form, as shown in Fig. 7, is 0.29 eV. This is unlike Ni@rutile (110) surface, where molecular adsorption of water is less thermodynamically favorable. These results indicate that Ti5C near to the Ni impurity is the active site of the Ni@anatase (101) surface. This is unlike Ni@rutile (110) where Ni atom is the active site of the surface. Also, compared to the pure surface, the molecular and the dissociated adsorption is slightly higher for Ni@a natase (101) surface. According to our previous report, Ni impurity has more positive ef fect on water dissociation and charge transfer from water the surface of Ni@rutile (110). Also, the charge transfer (Fig. 8) and Hirshfeld analyses (Table 1) both confirm an overall positive effect of Ni impurity on mo lecular water adsorption, by facilitating the charge transfer from the water molecule to the surface of Ni@anatase (101).
Fig. 10. The difference between charge densities of the complex (water adsorbed on O-vacancy@Ni@anatase (101) surface) and separate species. The blue and yellow lobes represent the negative and positive levels of isosurfaces, respectively. Red, blue, and yellow spheres represent O, Ti and Ni atoms, respectively.
anatase as a self-cleaning material [51,52]. Fig. 3 shows the adsorption of water on anatase (101) in the molecular and dissociated forms. The charge transfer on the local water-surface interaction is visualized by the corresponding difference in electron densities. Fig. 4 exhibits the charge transfer from water molecule to anatase (101) upon surface adsorption. The trend of charge transfers for different atoms in different structures can be seen in Table 1. The adsorption energy of the dissociated water is 0.4 eV that is 0.1 eV less than that of molecular one. This is comparable with a previous theoretical study with values of 0.72 and 0.44 eV for the adsorption energies of molecular and dissociated water, respectively [52], con firming the more favorable adsorption of the molecular form of water compared to the dissociated form.
3.2.4. O-vacancy@Ni@anatase (101) surface Similar to O-vacancy@Ni@rutile (110) surface, the O-vacancy is the most active site of O-vacancy@Ni@anatase (101) surface for water adsorption (Fig. 9). The most stable structure, made by removing the Ob near the Ni impurity, has the adsorption energy of 0.84 eV for molecular water. The dissociated adsorption energy is 1.5 eV resulting in an
3.2.2. O-vacancy@anatase (101) For the O-vacancy@anatase (101) surface, the vacant site is preferred for water adsorption (Fig. 5), wherein the adsorption energy of 6
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enthalpy of 0.66 eV for the whole dissociation process. According to Hireshfild analysis, the charge transfer from water molecule to the surface increases and Ni atom becomes 0.37 e more negative (Table 1). Fig. 10 shows the difference of charge densities in local water-surface interaction confirming the Hireshfild results. Overall, Ni impurity facilitates not only water adsorption and dissociation but also the formation of O-vacancy. This leads to further formation of OH radicals on the surface, which in turn increases the photocatalytic of O-vacancy@Ni@anatase (101) in aqueous solutions.
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4. Conclusion In this study, DFT-based full potential methods are employed to investigate the effect of O-vacancy and Ni impurity on the electronic structure and thermodynamics of water adsorption and dissociation on the surface of anatase (101). Also, the results of this study are compared with our previous investigation on rutile (110) surface. According to pDOS analysis, O 2p and Ti 3d orbitals have the major contribution to the upper edge of valence and the lower edge of conduction bands, respectively, for the pure anatase (101) surface. Occupied defect states, with major contribution from Ti 3d orbitals, are created near the con duction band via formation of O-vacancy. Ni impurity creates unoccu pied defect states inside the band gap, whereas simultaneous presence of Ni impurity and O-vacancy create an occupied impurity state inside the band gap. As far as electronic configuration is concerned, adding an Ni impurity is expected to counteract the elimination of an oxygen atom resulting in a total zero magnetization. Surprisingly, this is only followed by O-vacancy@Ni@anatase (101) but not O-vacancy@Ni@rutile (110). In terms of water dissociation process, adsorption of molecular water is more thermodynamically favorable on both anatase (101) and rutile (110) pure surfaces. This is in favor of dissociated water for both Ovacancy@anatase (101) and O-vacancy@rutile (110). The presence of solo Ni impurity changes the active site of the Ni@anatase (101) compared to Ni@rutile (110) surface, as well as the order of molecular versus dissociated adsorption of water. The most significant effect arises from the simultaneous presence of Ni impurity and O-vacancy, where Ni impurity facilitates the formation of O-vacancy, and they both facilitate the overall dissociation of water on both O-vacancy@Ni@anatase (101) and O-vacancy@Ni@rutile (110) surfaces, especially the former one. This increases the production of OH radicals in aqueous solutions, and thus the photocatalytic activity of TiO2. Taken together, this study shows that Ni and O-vacancy could synergistically activate the surface of anatase (101) by facilitating the formation of active species such as OH radical. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpcs.2019.109176. References [1] K. Bourikas, C. Kordulis, A. Lycourghiotis, Titanium dioxide (anatase and rutile): surface chemistry, liquid–solid interface chemistry, and scientific synthesis of supported catalysts, Chem. Rev. 114 (2014) 9754–9823. https://doi.org/10.1021/ cr300230q. [2] A.L. Linsebigler, G. Lu, J.T. Yates Jr., Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758. https://doi.org/ 10.1021/cr00035a013. [3] W. Choi, A. Termin, M.R. Hoffmann, The role of metal ion dopants in quantumsized TiO2: correlation between photoreactivity and charge carrier recombination dynamics, J. Phys. Chem. 98 (1994) 13669–13679. https://doi.org/10.1021/j1001 02a038. [4] C. Ng, J.H. Yun, H.L. Tan, H. Wu, R. Amal, Y.H. Ng, A dual-electrolyte system for photoelectrochemical hydrogen generation using CuInS2-In2O3-TiO2 nanotube array thin film, Mater. Sci. China 61 (2018) 895–904. https://doi.org/10.1007/s40 843-017-9237-2. [5] M.R. Elahifard, S. Rahimnejad, S. Haghighi, M.R. Gholami, Apatite-coated Ag/ AgBr/TiO2 visible-light photocatalyst for destruction of bacteria, J. Am. Chem. Soc. 129 (2007) 9552–9553. https://doi.org/10.1021/ja072492m.
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[31] R. Schaub, P. Thostrup, N. Lopez, E. Lægsgaard, I. Stensgaard, J.K. Nørskov, F. Besenbacher, Oxygen vacancies as active sites for water dissociation on rutile TiO2 (110), Phys. Rev. Lett. 87 (2001) 266104. https://doi.org/10.1103/Ph ysRevLett.87.266104. [32] C.S. Turchi, D.F. Ollis, Photocatalytic degradation of organic water contaminants: mechanisms involving hydroxyl radical attack, J. Catal. 122 (1990) 178–192. https://doi.org/10.1016/0021-9517(90)90269-. [33] A. Vittadini, A. Selloni, F. Rotzinger, M. Gr€ atzel, Formic acid adsorption on dry and hydrated TiO2 anatase (101) surfaces by DFT calculations, J. Phys. Chem. B 104 (2000) 1300–1306. https://doi.org/10.1021/jp993583b. [34] A. Vittadini, A. Selloni, F. Rotzinger, M. Gr€ atzel, Structure and energetics of water adsorbed at TiO2 anatase (101) and (001) surfaces, Phys. Rev. Lett. 81 (1998) 2954. https://doi.org/10.1103/PhysRevLett.81.2954. [35] Y.F. Zhao, C. Li, S. Lu, L.J. Yan, Y.Y. Gong, L.Y. Niu, X.J. Liu, Effects of oxygen vacancy on 3d transition-metal doped anatase TiO2: first principles calculations, Chem. Phys. Lett. 647 (2016) 36–41. https://doi.org/10.1016/j.cplett.2016.01.0 40. [36] C.G. Van de Walle, J. Neugebauer, First-principles calculations for defects and impurities: applications to III-nitrides, J. Appl. Phys. 95 (2004) 3851–3879. http s://doi.org/10.1063/1.1682673. [37] V. Blum, R. Gehrke, F. Hanke, P. Havu, V. Havu, X. Ren, K. Reuter, M. Scheffler, Ab initio molecular simulations with numeric atom-centered orbitals, Comput. Phys. Commun. 180 (2009) 2175–2196. https://doi.org/10.1016/j.cpc.2009.06.022. [38] S. Ahmadvand, M.R. Elahifard, B. Peik, R. Behjatmanesh-Ardakani, B. Abbasi, B. Abbasi, Predictive modeling of corrosion in Al/Mg dissimilar joint, ChemEngineering 3 (2019) 70. https://doi.org/10.3390/chemengineeri ng3030070. [39] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865. https://doi.org/10.1103/PhysRevLett .77.3865. [40] V. Mohammadrezaei, M. Ebrahimi, S.A. Beyramabadi, Study of the effect of molecular cluster size alanine concentrations in water by using the activity coefficient method and density functional theory, Bulg. Chem. Commun 49 (2017), 147-15. [41] O Lykhin Aleksandr, Seyedsaeid Ahmadvand, Sergey A. Varganov, Electronic transitions responsible for C60þ diffuse interstellar bands, J. Phys. Chem. Lett. 10 (2018) 115–120. https://doi.org/10.1021/acs.jpclett.8b03534. [42] V. Mohammadrezaei, M. Ebrahimi, S.A. Beyramabadi, Calculation of concentration of alanine in water using the activity coefficient model and ab initio model, Bulg. Chem. Commun. 49 (2017) 106–108. [43] M.R. Elahifard, M.P. Jigato, J.W. Niemantsverdriet, Direct versus hydrogenassisted CO dissociation on the Fe (100) Surface: a DFT study, ChemPhysChem 13 (2012) 89–91. https://doi.org/10.1002/cphc.201100759. [44] M.R. Elahifard, R. Behjatmanesh-Ardakani, S. Ahmadvand, B. Abbasi, A mechanistic study of photo-oxidation of phenol and AB92 by AgBr/TiO2, Res. Chem. Intermed. 1–12 (2019). https://doi.org/10.1007/s11164-019-03867-4. [45] E. Weinan, W. Ren, E. Vanden-Eijnden, Simplified and improved string method for computing the minimum energy paths in barrier-crossing events, J. Chem. Phys. 126 (2007) 164103. https://doi.org/10.1063/1.2720838.
[46] C.E. Ekuma, D. Bagayoko, Ab-initio electronic and structural properties of rutile titanium dioxide, Jpn. J. Appl. Phys. 50 (2011) 101103. https://doi.org/10. 1143/JJAP.50.101103. [47] C. Arrouvel, M. Digne, M. Breysse, H. Toulhoat, P. Raybaud, Effects of morphology on surface hydroxyl concentration: a DFT comparison of anatase–TiO2 and γ-alumina catalytic supports, J. Catal. 222 (2004) 152–166. https://doi.org/10.10 16/j.jcat.2003.10.016. [48] B. Bharti, S. Kumar, H.-N. Lee, R. Kumar, Formation of oxygen vacancies and Ti3þ state in TiO2 thin film and enhanced optical properties by air plasma treatment, Sci. Rep. 6 (2016) 32355. https://doi.org/10.1038/srep32355. [49] L. Kong, J. Kang, Y. Wang, L. Sun, L. Liu, X. Liu, X. Zhang, R. Han, Oxygenvacancies-related room-temperature ferromagnetism in polycrystalline bulk Codoped TiO2, Electrochem. Solid State Lett. 9 (2006) G1–G3. https://doi.org/ 10.1149/1.2130263. [50] S.M. Esfandfard, M.R. Elahifard, R. Behjatmanesh-Ardakani, H. Kargar, DFT study on oxygen-vacancy stability in rutile/anatase TiO2: effect of cationic substitutions, Phys. Chem. Res. 6 (2018) 547–563. https://doi.org/10.22036/pcr.2018.12 8713.1481. [51] U. Aschauer, Y. He, H. Cheng, S.-C. Li, U. Diebold, A. Selloni, Influence of subsurface defects on the surface reactivity of TiO2: water on anatase (101), J. Phys. Chem. C 114 (2009) 1278–1284. https://doi.org/10.1021/jp910492b. [52] D. Selli, G. Fazio, G. Seifert, C. Di Valentin, Water multilayers on TiO2 (101) anatase surface: assessment of a DFTB-based method, J. Chem. Theory Comput. 13 (2017) 3862–3873. https://doi.org/10.1021/acs.jctc.7b00479. [53] S. Banerjee, D.D. Dionysiou, S.C. Pillai, Self-cleaning applications of TiO2 by photoinduced hydrophilicity and photocatalysis, Appl. Catal. B Environ. 176 (2015) 396–428. https://doi.org/10.1016/j.apcatb.2015.03.058. [54] I. Ganesh, A. Gupta, P. Kumar, P. Sekhar, K. Radha, G. Padmanabham, G. Sundararajan, Preparation and characterization of Ni-doped TiO2 materials for photocurrent and photocatalytic applications, Sci. World J. (2012) (2012). https:// doi.org/10.1100/2012/127326. [55] I. Ganesh, A. Gupta, P. Kumar, P.C. Sekhar, K. Radha, G. Padmanabham, G. Sundararajan, Preparation and characterization of Co-doped TiO2 materials for solar light induced current and photocatalytic applications, Mater. Chem. Phys. 135 (2012) 220–234. https://doi.org/10.1100/2012/127326. [56] A. Groß, Tailoring the reactivity of bimetallic overlayer and surface alloy systems, J. Phys. Condens. Matter 21 (2009), 084205. https://doi.org/10.1088/0953-8984/ 21/8/084205. [57] M.R. Elahifard, E. Fazeli, A. Joshani, M.R. Gholami, Ab-Initio calculations of the CO adsorption and dissociation on substitutional Fe–Cu surface alloys relevant to Fischer–Tropsch Synthesis: bcc-(Cu) Fe (100) and fcc-(Fe) Cu (100), Surf. Interface Anal. 45 (2013) 1081–1087. https://doi.org/10.1002/sia.5228. [58] T.R. Esch, T. Bredow, Band positions of Rutile surfaces and the possibility of water splitting, Surf. Sci. 665 (2017) 20–27. https://doi.org/10.1016/j.susc.2017.08.006 . [59] M. Farsad, M.R. Elahifard, R. Behjatmanesh-Ardakani, Full-potential DFT study of CO dissociation on Fe-Cu cluster, Theor. Chem. Acc. 137 (2018) 142. https://doi. org/10.1007/s00214-018-2346-5.
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A computational study on the effect of Ni impurity and O-vacancy on the adsorption and dissociation of water molecules on the surface of anatase (101) Mohammadreza Elahifard a, b, *, Hajar Heydari c, Reza Behjatmanesh-Ardakani c, d, Bijan Peik b, Seyedsaeid Ahmadvand b a
Department of Chemical Engineering, Faculty of Engineering, Ardakan University, Ardakan, Iran Department of Metallurgical Engineering, University of Nevada, Reno, Reno, NV, 89557, USA c Department of Chemistry, Payame Noor University, PO Box, 19395-3697, Tehran, Iran d Research Center of Environmental Chemistry, Payame Noor University, Ardakan, Yazd, Iran b
A R T I C L E I N F O
A B S T R A C T
Keywords: Anatase (101) surface Ni impurity Oxygen vacancy Density of states Water adsorption
Ni impurity has been broadly used in the structure of TiO2 to extend the excitation response to the visible region and thus increase the photocatalytic activity. In this study, a full potential density functional theory has been used to study the effect of Ni impurity, substituted on the surface of anatase (101), on the electronic structure, Ovacancy formation energy, charge transfer, and adsorption and dissociation energies of water molecule. To this end, anatase TiO2 (101) surface is simulated and analyzed in its pure, Ni-doped, defective, and defective Nidoped forms. According to the results, oxygen vacancies and nickel impurities shift the occupied Ti 3d-orbitals and unoccupied Ni 3d-orbitals below the conduction band and inside the band gap, respectively. In addition, Ni impurity reduces the O-vacancy formation energy and increases the water adsorption energy significantly. While the molecular adsorption is preferred on the surface of plain and Ni-doped anatase (101), the adsorption of the dissociated form is more favorable upon O-vacancy development. Simultaneous presence of O-vacancy and Ni impurity creates an occupied defect state inside the band gap, which mainly corresponds to the Ni 3d-orbitals, as well as a positive synergic effect on the surface reactivity of the anatase (101) by increasing the adsorption energy of water especially in dissociated form. This provides OH groups on the surface as the main reactive specious to trigger the photocatalytic process.
1. Introduction
Ni, as a cationic substitutional impurity, narrows TiO2 band gap by creation of new states within the band gap region, which in turn shifts the optical absorption to the visible region [21–27]. On the other hand, well-studied oxygen defects have been shown to be stable and essential for catalytic activity of TiO2 in decomposition of water molecules and generation of hydroxyl groups [28–31]. Formation of hydroxyl radical plays an important role in oxidation of organic compounds and removal of microorganisms [32]. Therefore, a thorough knowledge of water adsorption and dissociation energetics is critical to increase the photo catalytic efficiency. Rutile and anatase are the main polymorphs of TiO2, with anatase having higher photocatalytic activity. The tetragonal anatase structure consists of four TiO2 units in its unit cell, and the most stable TiO2 anatase surface is anatase (101). However, fewer studies are focused on
Due to its abundance, chemical stability, and low toxicity, TiO2 is of a great importance in many industrial applications, such as hydrogen production, elimination of organic contaminants, and water disinfection [1–10]. With a wide band gap of 3.0–3.2 eV, TiO2 is mainly active under the ultraviolet (UV) light, lowering its efficiency in solar devices. However, this problem could be tackled by generating cationic/anionic substitutional/interstitial impurities and defects in TiO2 structure, i.e., band gap engineering [11–20]. These defects and impurities create some electronic states inside the TiO2 band gap, extending its excitation response to a larger range of electromagnetic wave. Among transition metals, Ni has been employed more frequently as a dopant to broaden the photocatalytic activity of TiO2. Experimental studies indicate that
* Corresponding author. Ayatollah Khatami Boulevard, P.O. Box:184, Ardakan University, Ardakan, Iran. E-mail addresses: [email protected], [email protected] (M. Elahifard). https://doi.org/10.1016/j.jpcs.2019.109176 Received 3 August 2019; Received in revised form 29 August 2019; Accepted 2 September 2019 Available online 5 September 2019 0022-3697/© 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Slab (a) and top (b) views of the pure anatase (101) surface. Red and blue spheres represent O and Ti atoms, respectively.
Fig. 2. Total and partial density of states of the pure, O-vacancy@-, Ni@-, and O-vacancy@Ni@anatase (101) surfaces.
the surface of anatase. Experimental and theoretical results indicate that similar to the most stable surface of rutile, rutile (110), water molecular adsorption is preferable on the plain anatase (101) surface. However, in the presence of oxygen vacancy, adsorption of dissociated water (OH þ
H) is thermodynamically more favorable compared to molecular adsorption [33,34]. Anatase is known as the most active photocatalytic phase of TiO2, and yet, its uneven surface adds more complication to computational 2
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Fig. 3. The optimized structures for molecular and dissociated water adsorption on the pure anatase (101) surface. Red and blue spheres represent O and Ti atoms, respectively.
the vacuum was set to 15 Å. This structure contains four layers of O–Ti–O and thus 12 atomic layers consisting of 16 titanium atoms and 32 oxygen atoms. Fig. 1 shows a perfect structure of anatase (101) surface including 6-fold (Ti6c) and 5-fold (Ti5c) coordinated Ti atoms, and 2- (Ob or O2c) and 3- (O3c) coordinated oxygen atoms. Ni-doped anatase (101) surface was made by replacement of either Ti6c or Ti5cTi5c Ti5c atoms Ti6c Ti6c with a Ni atom. The most stable form of Ovacancy was made by removing a two-fold coordinated bridge oxygen atom (Ob). Yet, all possible positions of O-vacancy were considered with respect to the substituted nickel atom; Ni6c with O2c (vacancy) and Ni5c with O2c (vacancy), along with different distances defined as near and far structures. The formation energy of O-vacancy (Eformation ) in pure and doped anatase surface was calculated by, Eformation ¼ Eðsystem without defectÞ
Eðsystem with defectÞ þ μO
(1)
where, Eðsystem without defectÞ and Eðsystem with defectÞ are the total energy of pure anatase (101) surface and that with Ni dopant for cases with and without oxygen vacancy, respectively. In relative conditions, chemical potential of oxygen atom (μO ) can be taken as half of the ground state energy of the oxygen molecule [35,36]. Adsorption energies were obtained by, � � ads Eads Ecomplex ðEsur þ EH2 O Þ (2) mol ; E dis ¼
Fig. 4. The difference between charge densities of the complex (water adsorbed on anatase (101) surface) and separate species (water þ anatase (101)). The blue and yellow lobes represent the negative and positive levels of isosurfaces, respectively. Red and blue spheres represent O and Ti atoms, respectively.
studies. Recently, we have shown that Ni-impurity increases the adsorption energy of water on rutile (110) surface noticeably [11]. Herein, we extend our study to investigate the effects of the Ni impurity, O-vacancy, and simultaneous O-vacancy and Ni-impurity on the surface of anatase (101). The structural and electronic properties of anatase (101) with or without impurities/defects are inquired. Both 5- and 6-fold Ti atoms are replaced by Ni atom, and their properties are compared. In addition, the role of Ni impurities and oxygen defects is investigated in adsorption and dissociation of water. Finally, partial density of states (pDOS) analysis is used to discuss the effects of nickel doping as well as oxygen vacancy on the electronic structure of the anatase surface.
where, Ecomplex is the energy of anatase (101) surface, with/without defects/impurities with one water molecule, Esur is the energy of the surface without the water molecule, and EH2 O is the energy of one water ads molecule. Eads mol and Edis denote the adsorption energies of water in mo lecular and dissociated forms, respectively. All calculations were carried out using Fritz Haber Institute ab initio Molecular Simulations (FHI-aims) computational package; an allelectron and full potential code [37,38]. Also, correlation exchange function of generalized gradient approximation (GGA) and the revised form of Perdew, Burke, and Ernzerhof (rpbe) [39] functional of DFT [40–43] were employed. However, band gap energies were refined using hybrid method of HSE06 [44]. The Monkhorst-pack grids of 4 � 6 � 1 and 6 � 6 � 2 were used for the geometry optimization of the surface and bulk, respectively. Half of the layers were frozen while the remaining atoms were relaxed. The convergence criteria were set to
2. Computational details In our modeling, periodic slab model of 2 � 2 surface of anatase (101) unit cells with single adsorbed water molecule were used, while 3
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Table 1 The Hirshfeld charge difference for all surficial atoms before and after molecular and dissociated adsorption of water.
1 2
Ovacancy@Ni-doped (dis.)
Ovacancy@Ni-doped (mol.)
Ni-doped (dis.)
Ni-doped (mol.)
O-vacancy (dis.)
O-vacancy (mol.)
Pure (dis.)
Pure (mol.)
Configuration
0.02 0.001 0.0005 0.002 0.001 0.001 0.4 0.4 … 0.002 0.002 0.006 0.01 0.002 0.001 0.02 0.01
0.03 0.006 0.01 0.004 0.02 0.02 0.02 0.37 … 0.05 0.003 0.05 0.03 0.42 0.13 0.02 0.03
0.11 0.005 0.005 0.002 0.01 0.006 0.26 0.2 0.06 0.011 0.01 0.01 0.002 0.2 0.005 0.013 0.012
0.007 0.02 0.02 0.0003 0.01 0.01 0.015 .02 0.001 0.06 0.002 0.04 0.06 0.05 0.11 0.030 0.032
0.05 0.002 0.002 0.001 0.002 0.002 0.004 0.04 … 0.004 0.003 0.004 0.006 0.04 0.013 0.01 0.01
0.03 0.012 0.05 0.00002 0.012 0.012 0.03 0.06 … 0.005 0.005 0.005 0.002 0.01 0.15 0.06 0.06
0.053 0.004 0.005 0.003 0.01 0.006 0.01 0.07 0.007 0.006 0.0005 0.026 0.004 0.02 0.004 0.016 0.005
0.002 0.017 0.017 0.0005 0.006 0.006 0.01 0.07 0.005 0.0008 0.003 0.017 0.008 0.002 0.11 0.002 0.024
O2C O2C O2C O2C Ti5C Ti5C 1 Ti5C/Ni5C 2 Ti5C/Ni5C O2C/vacancy O3C O3C O3C O3C Ti6C/Ni6C OW HW1 HW2
water adsorption on Ni replaced by Ti5C. water adsorption on Ti5C (more stable state) and Ni replaced by Ti5C.
Fig. 5. The optimized structures of molecular and dissociated water adsorption on the anatase (101) surface with an O-vacancy. Red and blue spheres represent O and Ti atoms, respectively.
SC_accuracy_rho 1 E 5, SC_accuracy_eev 2 E 3 and SC_accuracy_etot 1 E 6 for the self-consistency cycle, sum of eigen values and the total energy, respectively [45].
surface simulations. The Hirshfeld charge analysis of anatase (101) re sults in a similar charge for Ti6c atoms of the surface and bulk. Also, the Ti5c atoms are about 0.05 e more positive compared to the Ti6c atoms. Furthermore, the surficial Ni─O bond has a weaker polarization, where nickel is about 0.26 e less positive than titanium. This results in a higher bond length for Ni─O (1.86 Å) compared to that of Ti─O (1.83 Å) despite of the smaller radius of Ni ion with respect to Ti ion. However, in rutile (110), the less flexible structure restricts atomic displacements and results in a lower bond length for Ni─O compared to that of Ti─O [11]. The calculated formation energies of O-vacancy in the pure and Nidoped anatase (101) surface are 6.73 eV and 4.13 eV, respectively. The presence of Ni.NiNi has lowered this formation energy significantly (2.6 eV). This increment of the photocatalytic activity via elevation of Ovacancy concentration might be the result of Ti4þ substitution by Ni2þ and advent of the Burstein-Moss effect [48,49].
3. Result and discussion 3.1. Surfaces analysis Anatase TiO2 has a tetragonal structure with the space group of C19 4h I41 =amd comprised of four titanium atoms and eight oxygen atoms. a ¼ b ¼ 3.785 Å and c ¼ 9.51 Å are the optimized lattice constants with a less than 1% error compared to the experimental values [46,47]. Fig. 1 represents the simulated anatase surface made of Ti5cTi5c Ti5c Ti5c , Ti6c, O2c and O3c atoms. Similar to rutile, replacing Ti5cTi5c Ti5c Ti5c Ti5c with Ni NiNiresults in a more stable structure [11]. This implies that Ni im purity prefers surficial positions and emphasizes the importance of 4
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contribution from Ni 3d orbitals. The spin-polarized calculation results in a nonzero magnetization for O-vacancy@anatase (101) due to the difference between the population of spin-up and spin-down states. This magnetization is zero for Ovacancy@Ni@anatase (101) with a symmetric DOS. This could be due to the fact that the resultant electrons from O-vacancy compensate the replacement of Ti by Ni, paring all the electrons in the occupied orbitals. However, this trend is not followed by O-vacancy@Ni@rutile (110) based on our previous study, which could be a subject of further investigations. 3.2. Water adsorption 3.2.1. Anatase (101) surface Similar to rutile, Ti5c is the most stable site for adsorbing water molecule with an adsorption energy of 0.5 eV. The experimental values for this adsorption energy lays between 0.5 and 0.7 eV, introducing
Fig. 6. The difference between charge densities of the complex (water adsorbed on the O-vacancy@anatase (101) surface) and separate species. The blue and yellow lobes represent the negative and positive levels of isosurfaces, respec tively. Red and blue spheres represent O and Ti atoms, respectively.
Consistent with previous studies, titanium 3d and oxygen 2p orbitals have the major contributions to the bottom edge of the conduction band and the top edge of the valence band, respectively, for both surface and bulk [24,27]. The band gap of pure anatase surface is 3.03 eV, which is comparable with the experimental value of 3.23 eV [24]. While, plain DFT methods underestimate the band gap values, the hybrid method used in this study obtains a better agreement with experiment compared to previous theoretical reports [24,50]. Two extra electrons, released by O-vacancy in the structure, occupy the 3d states of the near Ti atoms, which arise below the conduction band (Fig. 2). This gives rise to an n-type semiconductor and increases the band gap by 0.3 eV in this case that is consistent with the bulk [22,25,26]. On the other hand, Ni in duces unoccupied defect states inside the anatase band gap which stem from hybridization of Ni 3d and unoccupied oxygen 2p orbitals. The position of these defect states slightly differs between the bulk and surface [22,25]. Once Ni doping is accompanied by O-vacancy, an occupied band inside the band gap emerges. Hybridization of O 2p, and Ti and Ni 3d orbitals give rise to this occupied band with a larger
Fig. 8. The difference between charge densities of the complex (water adsorbed on Ni@anatase (101) surface) and separate species. The blue and yellow lobes represent the negative and positive levels of isosurfaces, respectively. Red, blue, and yellow spheres represent O, Ti and Ni atoms, respectively.
Fig. 7. The optimized structures of molecular and dissociated water adsorption on the Ni@anatase (101) surface. Red, blue, and yellow spheres represent O, Ti, and Ni atoms, respectively. 5
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Fig. 9. The optimized structures of molecular and dissociated water adsorption on the O-vacany@Ni@anatase (101) surface. Red, blue, and yellow spheres represent O, Ti and Ni atoms, respectively.
molecular and dissociated forms of water is 0.8 eV and 1.45 eV respec tively, i.e., dissociated from is more favorable. This is again consistent with our findings for O-vacancy@rutile (110) and previous studies [13]. Similar to defectless surface, charge transfer occurs from water molecule to the surface (Fig. 6). However, this transferred charge in this case is slightly lower for the defectless surface (Table 1). 3.2.3. Ni@anatase (101) surface Lower coordination number of surficial atoms is in favor of Ni atoms with lower activity compared to Ti atoms. Similar to Ni@rutile (110) surface, Ti5C and Ni5C atoms are the most stable sites for water adsorption, with 0.52 eV adsorption energy related to the nearest Ti (5fold) atom to the Ni impurity (Fig. 7). Supported by experiment, Ni doping increases the hydrophilicity, and thus photocatalytic and selfcleaning properties of TiO2 [53–55]. This is consistent with the ligand effect [56–59]; Ni doping increases the activity of the near-Ti atoms. Accordingly, the energies of water adsorption on Ti5C near and far from Ni5C impurity are 0.52 and 0.47 eV, respectively. The adsorption energy of dissociated water in the most stable form, as shown in Fig. 7, is 0.29 eV. This is unlike Ni@rutile (110) surface, where molecular adsorption of water is less thermodynamically favorable. These results indicate that Ti5C near to the Ni impurity is the active site of the Ni@anatase (101) surface. This is unlike Ni@rutile (110) where Ni atom is the active site of the surface. Also, compared to the pure surface, the molecular and the dissociated adsorption is slightly higher for Ni@a natase (101) surface. According to our previous report, Ni impurity has more positive ef fect on water dissociation and charge transfer from water the surface of Ni@rutile (110). Also, the charge transfer (Fig. 8) and Hirshfeld analyses (Table 1) both confirm an overall positive effect of Ni impurity on mo lecular water adsorption, by facilitating the charge transfer from the water molecule to the surface of Ni@anatase (101).
Fig. 10. The difference between charge densities of the complex (water adsorbed on O-vacancy@Ni@anatase (101) surface) and separate species. The blue and yellow lobes represent the negative and positive levels of isosurfaces, respectively. Red, blue, and yellow spheres represent O, Ti and Ni atoms, respectively.
anatase as a self-cleaning material [51,52]. Fig. 3 shows the adsorption of water on anatase (101) in the molecular and dissociated forms. The charge transfer on the local water-surface interaction is visualized by the corresponding difference in electron densities. Fig. 4 exhibits the charge transfer from water molecule to anatase (101) upon surface adsorption. The trend of charge transfers for different atoms in different structures can be seen in Table 1. The adsorption energy of the dissociated water is 0.4 eV that is 0.1 eV less than that of molecular one. This is comparable with a previous theoretical study with values of 0.72 and 0.44 eV for the adsorption energies of molecular and dissociated water, respectively [52], con firming the more favorable adsorption of the molecular form of water compared to the dissociated form.
3.2.4. O-vacancy@Ni@anatase (101) surface Similar to O-vacancy@Ni@rutile (110) surface, the O-vacancy is the most active site of O-vacancy@Ni@anatase (101) surface for water adsorption (Fig. 9). The most stable structure, made by removing the Ob near the Ni impurity, has the adsorption energy of 0.84 eV for molecular water. The dissociated adsorption energy is 1.5 eV resulting in an
3.2.2. O-vacancy@anatase (101) For the O-vacancy@anatase (101) surface, the vacant site is preferred for water adsorption (Fig. 5), wherein the adsorption energy of 6
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enthalpy of 0.66 eV for the whole dissociation process. According to Hireshfild analysis, the charge transfer from water molecule to the surface increases and Ni atom becomes 0.37 e more negative (Table 1). Fig. 10 shows the difference of charge densities in local water-surface interaction confirming the Hireshfild results. Overall, Ni impurity facilitates not only water adsorption and dissociation but also the formation of O-vacancy. This leads to further formation of OH radicals on the surface, which in turn increases the photocatalytic of O-vacancy@Ni@anatase (101) in aqueous solutions.
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4. Conclusion In this study, DFT-based full potential methods are employed to investigate the effect of O-vacancy and Ni impurity on the electronic structure and thermodynamics of water adsorption and dissociation on the surface of anatase (101). Also, the results of this study are compared with our previous investigation on rutile (110) surface. According to pDOS analysis, O 2p and Ti 3d orbitals have the major contribution to the upper edge of valence and the lower edge of conduction bands, respectively, for the pure anatase (101) surface. Occupied defect states, with major contribution from Ti 3d orbitals, are created near the con duction band via formation of O-vacancy. Ni impurity creates unoccu pied defect states inside the band gap, whereas simultaneous presence of Ni impurity and O-vacancy create an occupied impurity state inside the band gap. As far as electronic configuration is concerned, adding an Ni impurity is expected to counteract the elimination of an oxygen atom resulting in a total zero magnetization. Surprisingly, this is only followed by O-vacancy@Ni@anatase (101) but not O-vacancy@Ni@rutile (110). In terms of water dissociation process, adsorption of molecular water is more thermodynamically favorable on both anatase (101) and rutile (110) pure surfaces. This is in favor of dissociated water for both Ovacancy@anatase (101) and O-vacancy@rutile (110). The presence of solo Ni impurity changes the active site of the Ni@anatase (101) compared to Ni@rutile (110) surface, as well as the order of molecular versus dissociated adsorption of water. The most significant effect arises from the simultaneous presence of Ni impurity and O-vacancy, where Ni impurity facilitates the formation of O-vacancy, and they both facilitate the overall dissociation of water on both O-vacancy@Ni@anatase (101) and O-vacancy@Ni@rutile (110) surfaces, especially the former one. This increases the production of OH radicals in aqueous solutions, and thus the photocatalytic activity of TiO2. Taken together, this study shows that Ni and O-vacancy could synergistically activate the surface of anatase (101) by facilitating the formation of active species such as OH radical. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpcs.2019.109176. References [1] K. Bourikas, C. Kordulis, A. Lycourghiotis, Titanium dioxide (anatase and rutile): surface chemistry, liquid–solid interface chemistry, and scientific synthesis of supported catalysts, Chem. Rev. 114 (2014) 9754–9823. https://doi.org/10.1021/ cr300230q. [2] A.L. Linsebigler, G. Lu, J.T. Yates Jr., Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758. https://doi.org/ 10.1021/cr00035a013. [3] W. Choi, A. Termin, M.R. Hoffmann, The role of metal ion dopants in quantumsized TiO2: correlation between photoreactivity and charge carrier recombination dynamics, J. Phys. Chem. 98 (1994) 13669–13679. https://doi.org/10.1021/j1001 02a038. [4] C. Ng, J.H. Yun, H.L. Tan, H. Wu, R. Amal, Y.H. Ng, A dual-electrolyte system for photoelectrochemical hydrogen generation using CuInS2-In2O3-TiO2 nanotube array thin film, Mater. Sci. China 61 (2018) 895–904. https://doi.org/10.1007/s40 843-017-9237-2. [5] M.R. Elahifard, S. Rahimnejad, S. Haghighi, M.R. Gholami, Apatite-coated Ag/ AgBr/TiO2 visible-light photocatalyst for destruction of bacteria, J. Am. Chem. Soc. 129 (2007) 9552–9553. https://doi.org/10.1021/ja072492m.
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