Article pubs.acs.org/JPCC
Self-Trapped Charge Carriers in Defected Amorphous TiO2 Kulbir K. Ghuman† and Chandra V. Singh*,†,‡ †
Department of Materials Science and Engineering, University of Toronto, 184 College Street, Suite 140, Toronto, ON M5S 3E4, Canada ‡ Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, ON M5S 3G8, Canada ABSTRACT: Self-trapped polarons and excitons associated with defects can localize energy in ways that can modify the material properties. The doped amorphous titanium dioxide (aTiO2) that possesses self-trapped polarons and excitons due to its inherited disorder and extrinsic doping has recently drawn increased attention as potential cheap visible light photocatalyst. However, the synergetic role that intrinsic and extrinsic defects play in enhancing the photoactivity of doped aTiO2 is still a mystery. To gain insights into the photocatalytic behavior of defected aTiO2, in this study, we analyzed three Fe-doped aTiO2 models having Fe in Fe(II), Fe(III), and Fe(IV) oxidation states. The density functional theory with Hubbard energy correction (DFT+U) calculations conducted in this study clearly indicate that the visible light absorption of aTiO2 will be improved by doping it with Fe. Furthermore, our investigation reveals that even though all the doped aTiO2 models showed highly localized states at mid gap and band edges, doping with Fe(II) led to maximum visible light absorption. This distinct behavior of Fe(II)-doped aTiO2 is attributed to the unique position of its mid gap states, high self-trap energy, low mobility, and weak chemical bonds.
1. INTRODUCTION The self-trapping of holes, excess electrons, and excitons along with any corresponding structural relaxations play important roles in the radiation induced processes.1−3 Understanding details of charge trapping is thus of fundamental importance in the fabrication of radiation resistant metal-oxide semiconductor devices, optical fibers, and photocatalysts. The localization of polarons or excitons in crystals generally requires an initial structural perturbation in order to break the symmetry and trigger the polaronic trapping.4−6 The existence of weak polaron is reported by various theoretical and experimental investigations in several oxides, including anatase TiO2, doped BaTiO3, KNbO3. and SiO2.7−12 Although there are many studies on electronic structures, defect levels, and polaron formation13−17 of crystalline titanium dioxide (cTiO2), there is a lack of such investigations on amorphous TiO2 (aTiO2) despite their many possible applications such as active photocatalyst, as a substrate, or as a protection layer.18−20 In cTiO2 structural perturbation leads to deep trapping of the formed charge carriers, usually located on single Ti or O atom, subsequently forming small polarons.13,21,22 However, aTiO2 despite having qualitatively similar electronic structure as of cTiO223−25 forms large localized bound state on several O and Ti atoms due to the strong electron interaction with the lattice distortion.23,25,26 To increase the photoactivity of aTiO2, doping with suitable dopants has been suggested by many studies.27−33 However, the synergetic role of amorphousness and defects created due to doping on the photoactivity of aTiO2 is still a mystery. It is not known how the localization of charge in aTiO2 can be utilized to create trapping centers instead of recombination centers. Since doping induces incremental structural distortion © XXXX American Chemical Society
in aTiO2, there could be a tendency for even large polarons and excitons to form in the doped aTiO2 than pristine aTiO2. Thus, in this study we conducted ab initio analysis to understand the dependence of charge trapping, recombination, mobility, and localization on the intrinsic as well as extrinsic disorder. For this we chose to study Fe-doped aTiO2 since Fe ions are expected to act as electron traps and to cause a red shift in the absorption band.34,35 As previous studies36 showed the excessive amount of Fe can cause an adverse effect on photocatalytic activity, we have also studied the concentration effect of Fe doping in aTiO2. The preparation of aTiO2 models used for doping was previously reported.25 These aTiO2 models showed electronic properties in good agreement with experimental and theoretical data available. In the present study, these aTiO 2 models are doped with different concentrations of Fe leading to different Fe oxidation states (Fe(II), Fe(III), and Fe(IV)), and their electronic structures, optical properties, carrier dynamics and defect properties are calculated.
2. MODELS AND METHODS The melt-quenching method used to prepare aTiO2 models is demonstrated in our previous work.25 This size sensitive study showed that the properties of the three aTiO2 models prepared in this study are in good agreement with the experimental and theoretical data available. The structural analysis conducted therein suggested that the local structural features of bulk cTiO2 are retained in aTiO2. Electronic properties and Received: July 21, 2016 Revised: November 14, 2016 Published: November 23, 2016 A
DOI: 10.1021/acs.jpcc.6b07326 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C structural origin of tail and defect states of these models were also discussed earlier.32 In the present study, out of these three models the 2 × 2 × 4 supercell, i.e., 96-atom model of aTiO2 has been considered37 for Fe doping. We have considered three models of Fe-doped aTiO2: (1) substitutional doping of a single Ti atom by one Fe atom resulting into an uncommon Fe(IV) oxidation state with 1.04 mol % dopant concentration, (2) substitutional doping of a single Ti atom by one Fe atom and one O vacancy resulting into Fe(II) oxidation state with 1.05 mol % dopant concentration, and (3) substitutional doping of two Ti atoms by two Fe atoms and one O vacancy resulting into a Fe(III) oxidation state with 2.11 mol % dopant concentration. The aTiO2 model indicating the positions of Fe atoms and O vacancy is shown in Figure 1. This doping level
The electronic characteristics of aTiO2 sample have been investigated using ab initio method. The plane-wavepseudopotential approach, together with the Perdew−Burke− Ernzerhof (PBE)40 exchange-correlation functional, and Vanderbilt ultrasoft pseudopotentials41 were utilized throughout. The kinetic energy cutoffs of 544 and 5440 eV were used for the smooth part of the electronic wave functions and augmented electron density, respectively. The QuantumESPRESSO code, PWSCF package,42 was used to perform the calculations. Brillouin zone integrations were performed using a Monkhorst−Pack,43 grid of 4 × 4 × 6 k points. All calculations are spin polarized. The structures were relaxed by density functional theory (DFT) using a conjugate gradient minimization algorithm until the magnitude of residual Hellman−Feynman force on each atom was less than 10−3 Ry/Bohr. In all electronic density of states (DOS) and projected density of states (PDOS) plots a conventional Gaussian smearing of 0.1 eV was utilized. Appreciable underestimation of band gap and delocalization of d and f electrons are well-known limitations of DFT. Therefore, we used DFT with Hubbard energy correction (DFT+U) formalism with U = 4.2 eV applied to Ti 3d electrons and U = 6.00 eV applied to Fe 3d electrons for analyzing electronic properties of pristine and doped aTiO2. However, the optical absorption properties are calculated by the usual DFT method. The U value convergence test has been done for quantum espresso software, and the U parameter was chosen based on the position of the bandgap states of crystalline rutile, details of which are given in our previous publication.45 The value of U for Ti has been chosen not solely on the basis of band gap but also based on the property of interest,44 which, in the current study, is the photocatalytic behavior of TiO2 that in-turn depends upon the position of band gap states and their effect on the electronic structure.45 This value of U for Ti is consistent with theoretical investigations by Morgan et al.,46 who calculated it by fitting the peak positions for surface oxygen vacancies to experimental XPS data. Further the value of U for Fe is taken from ref 47, which showed that the density of states (DOS) of FeO shows
Figure 1. Supercell (2 × 2 × 4) of aTiO2. Ti, O, Fe, and O vacancy (O-vac) atoms are highlighted in gray, red, green, and blue, respectively.
can be readily achieved for 3d transition metals in rutile TiO2.38,39 It is reported that the optimum doping amount of Fe ions in cTiO2 is 0.005 mol %, and this can enhance the photocatalytic activity, while too great an amount (5 mol %) makes the Fe ions become recombination centers for the electron−hole pairs and reduce the photocatalytic activity.38
Figure 2. (a) Optical absorption curves and (b) imaginary parts of the dielectric function, ε2(ω), of undoped aTiO2, Fe(IV)-aTiO2, Fe(II)-aTiO2, and Fe(III)-aTiO2 models. B
DOI: 10.1021/acs.jpcc.6b07326 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C characteristics similar to experimental observations48 with U = 6.0 eV.
3. RESULTS AND DISCUSSION 3.1. Optical Absorption Properties. We first determined the optical properties of doped and undoped aTiO2 models by the frequency-dependent dielectric function ε(x) = ε1(x) + iε2(x), which is mainly a function of electronic structure. The imaginary part of the dielectric function ε2(x) can be calculated from the momentum matrix elements between the occupied and unoccupied wave functions. The real part ε1(x) can be evaluated from the imaginary part ε2(x) by the famous Kramer−Kronig relationship. The corresponding absorption spectrum was estimated using the following equation: I(ω) = 21/2ω{[ε12(ω) + ε2 2(ω)]1/2 − ε1(ω)}1/2
The optical absorption spectra of the pure and doped aTiO2 systems are shown in Figure 2a. The results show clearly that aTiO2 will absorb mainly UV light; however, after the introduction of dopants into aTiO2, the absorption edges are shifted to the visible-light region, which is in good agreement with the literature.27 The Fe(II)-doped aTiO2 showed the maximum visible light absorption with overall optical absorption in the range of 400−1000 nm. Fe(III)-doped aTiO2, however, presents least visible light absorption. Further, the imaginary part of dielectric function presented in Figure 2b shows that the visible light absorption observed for the Fedoped models in the current work (Figure 2a) and in the literature27 is due to visible optical transition (E1). The E1 transition is observed at 1.5 and 2.2 eV for Fe(II) and Fe(IV)doped aTiO2, respectively, with Fe(IV)-doped aTiO2 having lower intensity than Fe(II)-doped aTiO2. After defect incorporation, the optical transition of the peak representing band gap (Eg) remains almost the same for all the models (∼4.80 eV), which means that the band gaps of the doped systems are not changed after doping. 3.2. Electronic Density of States. In order to understand the reason for different spectra for three models, we studied the electronic density of states (DOS) and partial DOS (PDOS) for the d electrons of Fe and Ti atoms and p electrons of O atoms for doped and undoped aTiO2 models (Figure 3). In all DOS and PDOS figures the Fermi level is set to 0 eV on the energy axis. It can be seen from Figure 3 that the magnitude of the Γ point electronic gap, defined as the difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is 2.5, 2.5, 2.2, and 2.2 eV for undoped, Fe(IV)-doped, Fe(II)-doped, and Fe(III)-doped aTiO2, respectively. Further, the PDOS analysis shows that the highest valence band of pristine aTiO2 mainly consists of the O 2p states and that the lowest conduction band is dominated by Ti 3d states (Figure 3a). These results for pristine aTiO2 model are comparable to anatase and agree well with the previous investigations on aTiO2.28 The DOS and PDOS for Fe(IV)-doped, Fe(II)-doped, and Fe(III)-doped bulk aTiO2 models shown in Figure 3b−d, respectively, illustrate that the band structure of aTiO2 changes when Fe and/or O vacancy defects are introduced in pristine aTiO2. Fe(IV)-doping results into a spin up mid gap state of 0.4 eV width at 1.1 eV below the CB edge, whereas Fe(II)-doping resulted into an impure state of width 0.3 eV just above the valence band maxima (VBM). It should be noted that both Fe(IV)- and Fe(II)-doped aTiO2 models showed no states at
Figure 3. Total density of states (TDOS) and the projected density of states on the p and d orbitals of (a) undoped aTiO2, (b) Fe(IV)aTiO2, (c) Fe(II)-aTiO2, and (d) Fe(III)-aTiO2 models. The zero energy value is set at the Fermi energy represented by the vertical dashed line.
the conduction band minima (CBM) and left CB unaltered. Fe(III) doping, however, not only resulted into mid gap states of width 0.6 at 0.8 eV below the CBM but also shows energy states at both VBM and CBM. These mid gap states in Fe(IV)doped, Fe(II)-doped, and Fe(III)-doped aTiO2 models are mainly due to Fe 3d and O 2p orbitals. Further, PDOS analysis also shows that along with mid gap states the Fe 3d orbitals also contribute to CBM of Fe(III)-doped and Fe(IV)-doped aTiO2 and VBM of Fe(II)-doped aTiO2. The electronic structure calculations (Figure 3) clearly shows that the E1 for Fe(II)-doped aTiO2 is due to transition from occupied Fe 3d states above the VB maxima to the unoccupied Ti 3d states in the CB minima, whereas E1 for Fe(IV)-doped aTiO2 is due to optical transition from O 2p states in the highest valence band to unoccupied O 2p and Fe 3d states in the mid gap. It should be noted that although Fe(III) doping also led to the mid gap states in the band gap, it showed the least visible light absorption (Figure 2) probably due to the forbidden optical transition between O 2p and mid gap Fe 3d states. These results clearly indicate that the position of mid gap states play an important role in increasing the visible light absorption and that maximum visible light adsorption of Fe(II)doped aTiO2 should be prepared experimentally. Further, the PDOS results indicate that the optical transition at 4.80 eV (Eg) C
DOI: 10.1021/acs.jpcc.6b07326 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C can be ascribed to the intrinsic transition between the O 2p states in the highest valence band and the Ti 3d states in the lowest conduction band. These impurity states due to Fe and/or O vacancy not only can change visible light absorption properties but also can act as traps for photoinduced carriers. The trapping of the carriers could further exert two opposite effects on the separation of the photogenerated carriers. On one hand, the trapping of the charges can hinder the charge recombination because the trapped electron and hole have to conquer an energy barrier from the localized trapping states to encounter each other. However, the lower mobility of the trapped charge carriers depresses their migration from these trap-sites to surface reaction sites of photocatalyst and thereby increases the recombination probability. Furthermore, since we have introduced defects in an already distorted structure, i.e., amorphous phase of TiO2, the effects of doping on trapping and recombination process can be different in doped aTiO2 as compared to doped cTiO2, which is explored by analyzing charge localization in the next section. 3.3. Charge Localization and Self-Trapping. To investigate the localization of the tail states and to obtain the mobility band gap of doped and undoped aTiO2 systems, inverse participation ratio (IPR) analysis was performed following the procedure suggested by Justo et al.49 The IPR of an orbital ψn(rl⃗ ),I(ψn), is accordingly defined by
Figure 4. Total density of states (TDOS) (black line, right scale) and the corresponding values of the inverse participation ratio (IPR) (blue dots, left scale) on the left side and the spin density (ρ↑ − ρ↓) on the right side for (a) undoped aTiO2, (b) Fe(IV)-aTiO2, (c) Fe(II)-aTiO2, and (d) Fe(III)-aTiO2 models. The zero energy value is set at the Fermi energy represented by the vertical dashed line. The isovalue used for spin density plots is 0.01 eÅ−3.
N
I(ψn) = N
∑i = 1 |ψn( ri⃗)|4 N
[∑i = 1 |ψn( ri⃗)|2 ]2
where N is the number of volume elements in the cell, and i is the index of the volume element. The IPR is large for highly localized states and small for delocalized states. Ideally, a localized orbital means I(ψ) = N, whereas a delocalized orbital means I(ψ) = 1. The IPR can identify a level as belonging to the delocalized band, to the partially localized band tail, or to the highly localized band gap. Thus, IPR analysis provides a reasonable method to understand the localization of energy states. Further, IPR analysis can also be used for band gap prediction by analyzing the change in degree of localization of electronic states near the valence band (VB) and conduction band (CB) edges. The bottom of the CB and the top of the VB were determined as the energy levels where the IPR becomes larger than the average IPR of levels at few eV above the CB bottom and below the VB top, respectively. The IPR analysis for undoped aTiO2 (Figure 4a) shows that the band tail electronic states for aTiO2 are strongly localized, which is in agreement with the literature.28 This is also consistent with other disordered systems such as amorphous carbon.50 This large IPR for the states in the bottom of CB and the top of VB is a result of the localized character of the Ti 3d and O 2p states, respectively. Further, the mobility band gap value for the undoped aTiO2 model is about 3.0 eV (approx.), which is in close agreement with the experimental value of 3.4 eV.51 It should be noted that, due to a large number of levels with different IPRs in the top of the VB and bottom of the CB, there is some uncertainty in defining the gap. The IPR plot for doped aTiO2, depicted in Figure 4, suggests that the localization of VB and CB tails increases for all the doped aTiO2 systems as compared to undoped aTiO2. The localization at VBM of Fe(II)-doped aTiO2 is mainly due to O 2p and Fe 3d states, whereas VBM of Fe(III)-doped and Fe(IV)-doped aTiO2 are localized only due to O 2p states. The
localization at CBM of Fe(II)-doped aTiO2 is mainly due to Ti 3d states, whereas CBM of Fe(III)-doped and Fe(IV)-doped aTiO2 are localized due to both Ti 3d and Fe 3d states (see Figure 3). Further, due to doping, the charge is also localized at the mid gap due to O 2p and Fe 3d energy states. The localization at mid gap state is very high in comparison to localization at CBM and VBM for all three doped systems. The spin density plot for doped systems (Figure 4) also shows that the spin densities are strongly localized on the Fe atom and O1− sites in the nearest-neighbor position relative to the Fe dopant. It is interesting to note that the valence tail states are more localized as compared to the conduction tail states in undoped aTiO2 (Figure 4a), as also reported by Prasai et al.28 This localization of VB tail becomes even more stronger than CB tail for Fe(II)-doped aTiO2 (Figure 4c). However, for Fe(IV)- and Fe(III)-doped systems the CB tail is more localized as compared to VB tail. As also reported for the case of GaN,52 since the VB tail states are more localized as compared to the CB tail states for undoped and Fe(II)-doped aTiO2, it would be more difficult to move its Fermi level toward the valence edge, leading to poor mobility. However, for Fe(III)- and Fe(IV)doped aTiO2 the Fermi level is shifted toward VB (Figure 4b,d), indicating that the charge carriers in these systems will be more mobile as compared to Fe(II)-doped aTiO2 system. Further, since the optical band gap of all three doped aTiO2 models remains the same as the optical band gap of undoped aTiO2, the localization observed at the band edges of these models do not contribute to the reduction in band gap and the visible light absorption observed (Figure 2). Previous reports have shown that the distortion of the cTiO2 structure influences the coordination number of TiO6 octahedra and, in turn, the nature of polarons and excitons.53−56,26 In rutile, the octahedra D
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It should be noted that smaller Ef values result in easier incorporation of impurities into the TiO2 supercell. The formation energy of Fe-doped aTiO2 is reduced to a greater degree under O-rich conditions than under the Ti-rich conditions, indicating that the incorporation of Fe into aTiO2 at the site of the Ti atom is favorable under O-rich conditions than Ti-rich conditions. In addition, the formation energy calculation shows that Fe(III)-doped aTiO2 is the most stable system under O-rich conditions. This suggests that the higher concentrations of Fe facilitate the synthesis of Fe-doped aTiO2 under O-rich condition, which is also observed for Fe-doped cTiO2.58,59 The experiments conducted in this work also showed that the Fe(III)-doped aTiO2 samples are very stable; however, it is very difficult to stabilize Fe(II)- and Fe(IV)doped aTiO2. Furthermore, in order to understand the nature of interatomic charge distributions quantitatively, Bader analysis based on the atom-in-molecule (AIM) theory was carried out.60 The charge states of atomic species in doped aTiO2 were calculated using a grid based decomposition algorithm. The oxidation states of Fe (electronic configration 3s23p64s23d6) in Fe(IV)-, Fe(II)-, Fe(III)-doped aTiO2 models (Table 1) reveal that the Fe(IV) valence shell loses 1.65e charge, Fe(II) valence shell loses 1.26e charge, and Fe(III) valence shell loses 1.44e and 1.79e charges to form bonds with adjacent oxygen atoms. As a comparison, Ti in pure aTiO2 loses ∼2.30e to form bonds with adjacent oxygen atoms. This is due to substituting Ti in aTiO2 with comparatively smaller atomic radii Fe atom. This smaller charge accumulation around the Fe center as compared to Ti shows that there is weaker coupling between O and Fe atoms in all the three models. Among these three models Fe(IV)-doped aTiO2 shows the strong interaction with neighboring atoms; however, Fe(II)-doped aTiO2 and one of the Fe(III)-doped aTiO2 atoms show the weakest chemical bonds. These analyses suggests that the highly localized states in Fe(II)- and Fe(III)-doped aTiO2 as compared to Fe(IV)doped aTiO2 is due to the formation of weak chemical bonds near the VB minima of these structures. The relative imperfect and hence loose chemical bonds resulted in more pronounced atomic relaxations, therefore deepening the trapping energy.
are almost undistorted and the polarons are considered free; in anatase and aTiO2, instead, the distortion is much higher, and charge carrier pairs are localized in the octahedral sites, originating the so-called self-trapped polarons. Since doping further enhances the structural distortion, the self-trapped polarons will be present in our doped aTiO2 samples. In order to quantify the effect of doping on localization, the self-trapping energies (EST) of polarons were calculated in doped aTiO2 (Table 1). EST is usually defined as a difference between the Table 1. Calculated Formation Energy, Ef (in eV), Bader Charges (e), and Self-Trapping Energy, EST (in eV), for Undoped and Doped aTiO2 Models formation energy, Ef (eV) atomic charge (e)
self-trapping energy, EST (eV)
models
O-rich
Tirich
Fe(IV)aTiO2 Fe(II)-aTiO2 Fe(III)aTiO2
−6.49
3.58
1.65
0.33
−4.62 −10.03
0.41 5.08
1.26 1.44, 1.79
1.75 1.85
energy of the delocalized state at zero distortion (perfect lattice), EUD, and the energy of the localized (trapped) state at its equilibrium distortion, ED.57 Here, EST values, calculated as the difference between the initial atomic structure energy (with the free/delocalized polarons) and the energy of atomic relaxed system with the localized polarons, were found to be ∼0.33, 1.75, and 1.85 eV for Fe(IV)-, Fe(II)-, and Fe(III)-doped aTiO2, respectively (Table 1). The positive value obtained suggests that the localization of the polarons could be one of local minima instead of the global one. Our calculations therefore have suggested that EST (or localization) of polarons in the Fe(III)- and Fe(II)-doped aTiO2 is very high as compared to the EST of Fe(IV)-doped aTiO2. These results agree with the localization results obtained from the IPR analysis as shown in Figure 4. 3.4. Structural Stability and Charge Analysis. To examine the relative stability and understand the variation in charge localization of aTiO2-doped with various concentrations of Fe, we further calculated the defect formation energies by using following formula:
4. CONCLUSIONS Overall, in this work we analyzed Fe(IV)-, Fe(II)-, and Fe(III)doped aTiO2 models in order to understand the effect of doping on photocatalytic properties of aTiO2 through computational methods. The analysis of electronic band structure, optical absorption spectra, and localized states showed that all the doped aTiO2 models possess increased charge localization at band edges aided by the original localization of band edges in the undoped amorphous phase. Along with the localization of band edges, Fe-doping also leads to very highly localized polarons at mid gap states. However, despite having the mid gap states in all the doped models, all of them showed different visible light absorption due to the different nature and distribution of these defect states. In Fe(III)-doped aTiO2 system these mid gap states are due to localization of holes, and they lie approximately 0.8 eV away from localized electrons of VBM. In Fe(IV)-doped aTiO2 system the energy difference between the electron and the holes of an exciton increase to 1.1 eV (approx.). In contrast, in Fe(II)-doped aTiO2 system the mid gap states are due to localization of electrons, and they are approximately 1.9 eV away from localized holes of CBM. Thus, the probability of recombination of electrons and holes is much lower for Fe(II)-
Ef = E(Ti32 − n1O64 − n2 Fen3) − [E(Ti32O64 ) − n1μTi − n2μO + n3μFe ]
where Ef(Ti32−n1O64−n2Fen3) and E(Ti32O64) are the total energies of the doped aTiO2 and pure aTiO2, respectively. μTi, μO, and μFe are the chemical potentials of the Ti, O, and Fe elements, respectively; and n1, n2, and n3 denote the numbers of Ti atoms substituted, O vacancies, and Fe-dopant atoms, respectively. The formation energy depends on the growth conditions, which can be Ti-rich or O-rich. For TiO2, μTi and μO satisfy the relationship μTi + 2μO = μTiO2. Under the O-rich growth condition, μO is determined by the total energy of an O2 molecule (μO = μO2/2) and μTi is determined by the formula μTi = μTiO2 − 2μO. Under the Ti-rich growth condition, μTi is the energy of one Ti atom in bulk Ti and μO is determined by μO = (μTiO2 − μTi)/2. Table 1 summarizes the calculated formation energies for TiO2 doped with various concentrations of Fe. E
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doped system. This is also reflected by our absorption spectra analysis that showed that Fe(II)-doped aTiO2 possesses the maximum visible light absorption among all the models studied. Further, it was also found that Fe-doping does not reduce the optical band gap and that an increase in photoactivity of Fedoped aTiO2 is purely due to highly localized states near VBM. These states create self-trapped charge carriers with reduced mobility leading to enhanced activity. Hence, if the preparation methods can be controlled appropriately, Fe(II)-doped aTiO2 can prove to be a cheaper and more abundant alternative photocatalyst to crystalline forms of TiO2. Overall, this study proves that the localized band edges of natural aTiO2 can accelerate the generation of visible light-active, highly localized charge carriers by appropriate doping.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +1 416 946 5211. Fax: +1 416 978 4155. ORCID
Chandra V. Singh: 0000-0002-6644-0178 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Computations were performed on the HPC supercomputer at the SciNet HPC Consortium and Calcul Quebec/Compute Canada. SciNet is funded by the Canada Foundation for Innovation under the auspices of Compute Canada; the Government of Ontario; Ontario Research Fund−Research Excellence; and the University of Toronto. The authors gratefully acknowledge the continued support of above organizations.
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