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I, N-Codoping Modification of TiO for Enhanced Photoelectrochemical HO Splitting in Visible-Light Region 2

Mang Niu, Jun Zhang, and Dapeng Cao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08782 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017

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The Journal of Physical Chemistry

I, N-Codoping Modification of TiO2 for Enhanced Photoelectrochemical H2O Splitting in Visible-Light Region

Mang Niu,1 Jun Zhang,1* and Dapeng Cao2*

1

College of Science, China University of Petroleum (East China), Qingdao 266580, Shandong Province, P. R. China 2

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, P. R. China

*

Authors to whom correspondence should be addressed. Electronic addresses: [email protected] Fax: +860532-86983366; [email protected] Fax: +86010-64443254. 1

ACS Paragon Plus Environment


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ABSTRA ACT

The

donor-acceptor codoping is an effective approach to tune the

photoelectrochemical properties of TiO2. Here, we systematically investigate the effects of (I+N) codoping on the electronic structures and H2O splitting reactions of anatase TiO2 by using density functional theory. It is found that the codoping of stable charge-compensated (I+N) donor-acceptor pair in anatase TiO2 can not only prevent the recombination of photo-generated electron-hole pairs, but also can effectively reduce the band gap to 2.251 eV by forming an intermediate band within the band gap. The band edge alignment of (I+N) codoped TiO2 is desirable for H2O splitting, and the calculated optical absorption curve of (I+N) codoped TiO2 verifies that (I+N) codoping can significantly improve visible-light absorption. Moreover, we also calculate the chemical reaction pathways of H2 generation via H2O splitting on (I+N) codoped TiO2 surfaces by using climbing nudged elastic band (cNEB) method, and find that the (I+N) codoping in TiO2 can efficiently reduce the energy barrier of H2 production by about 1.0 eV. These findings imply that the (I+N) codoped anatase TiO2 is a promising visible-light photocatalyst for H2O splitting.

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INTRODUCTION TiO2 as one of the most important semiconductor materials has attracted much attention due to its promising applications in the dye-sensitized solar cells and photocatalysis.1-4 For example, the photoelectronchemical (PEC) H2O splitting reaction on TiO2 photocatalyst can produce H2 through solar light irradiation.5-8 However, the photoreaction efficiency of TiO2 is severely limited by its wide band gap (3.0 eV for rutile and 3.2 eV for anatase), which leads to that TiO2 can only absorb ultraviolet portion of the solar spectrum. Therefore, the desirable photocatalyst should have a band gap around 2.0 eV and its band edges must straddle the water redox potentials to achieve high efficiency of PEC water splitting under visible-light irradiation.5-8 In the last few decades, great efforts have been devoted to improving the visible-light photocatalytic activity of TiO2. It is suggested that the incorporation of high level of non-metals,9-10 transition metals,11-12 and rare earths13-14 in TiO2 would be a promising way to reduce its band gap by inducing impurity levels within the band gap. Although the doping method can improve the visible-light light absorption of TiO2 to some extent, the photocatalytic performance of these monodoped TiO2 is still limited by the relatively high recombination rate of photo-generated carriers.15-17 Gai et.al proposed that the coincorporation of donor-acceptor pairs in TiO2 is an effective method to overcome the shortcomings of monodoped TiO2.6 It is believed that the charge compensation effect in donor-acceptor codoped TiO2 can not only significantly reduce the band gap of TiO2, but also can prevent the recombination of

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photon-generated carriers.18-19 The transition metals (such as W, Cr, Mn)20-21 and non-metal (such as N, C)22-23 dopants are often chosen as the traditional donor-acceptor pairs, which could induce impurity levels below the conduction band minimum (CBM) and above the valence band maximum (VBM) of TiO2, respectively.6 However, the donor impurity levels induced by the transition metals could lower the reduction potential of the conduction band edge, resulting in poor H2O splitting. To design a visible-light photocatalyst for H2O splitting by using the codoping method, the ideal impurities should be soluble in TiO2 to reduce the band gap and enhance the optical absorption, and do not lower the reduction potential of the conduction band edge of TiO2. It is reported that the incorporated I atom would occupy the Ti site in TiO2 lattice, which can act as the donor dopant. Actually, I-doping in anatase TiO2 can lead to a signiﬁcant uphill shift of CBM position, thus the reduction ability of TiO2 can be enhanced.6,

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Moreover, the I-doping can

effectively reduce the band gap of anatase TiO2 and thus improves the visible-light absorption of TiO2.25-26 For the acceptor dopants, N is the most typical one and it has been intensively investigated so far.17 On one hand, N dopant prefers to filling the oxygen deficiency of TiO2, so N-doped TiO2 can be easily prepared by various physical or chemical methods.22, 27 On the other hand, because N has higher atomic p-orbital energy than O does, it can forms an acceptor level above the VBM of TiO2, and thus reduced the phototransition energy. Therefore, I and N are predicted to be the best dopants as donor and acceptor, respectively.

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Up to now, there is no corresponding theoretical investigations on PEC properties of compensated (I+N) codoped anatase TiO2. Thus, we constructed the compensated

(I+N)

codoped

anatase

TiO2

model

to

investigate

its

photoelectrochemical properties by using first-principles density functional theory (DFT) calculations. We firstly calculate the electronic properties of (I+N) codoped anatase TiO2. Then, we investigate the optical absorption properties and the band edge shifts of the codoped system. Finally, the chemical reaction pathways of H2 generation via H2O splitting on different (I+N) codoped TiO2(101) surfaces are discussed. Our calculation results indicate that the (I+N) codoped anatase TiO2 is a promising visible-light photocatalyst for H2O splitting.

COMPUTATIONAL DETAILS The plane-wave based density functional theory (DFT) were performed by using the frozen-core all-electron projector-augmented-wave (PAW)28-29 method as implemented in the Vienna Ab-initio Simulation Package (VASP).30-31 The exchange and correlation potential were described by the Perdew-Burke-Ernzerhof (PBE)32 parameterization of generalized gradient approximation (GGA). The cutoff energy for the plane-wave basis set was set to 500 eV. To ensure the convergence with respect to the number of k-point mesh33 for all calculations, we have performed extensive tests. The geometry optimizations were carried out until the forces on each atom was reduced below 0.01 eV/Å. Then, the resulting structures were used for the electronic structures calculations.

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Because the CBM of anatase TiO2 is about 0.1 eV higher than that of the rutile phase, which facilitates the hydrogen production, we use anatase TiO2 in our calculations. The pure anatase TiO2 was mpdeled by a 2 × 2 × 1 anatase TiO2 supercell. The codoped TiO2 systems were constructed by inducing the dopants to the inner part of anatase supercell: one O site in TiO2 was substituted by one N atom, and one Ti site in TiO2 was substituted by one I atom. The supercell models of pure, I-doped, N-doped, and (I+N) codoped anatase TiO2 are displayed in Figure S1(a-d) in Supporting Information, respectively. These settings are consistent with the corresponding

experimental

results.26,

34-35

Here,

we

used

the

Heyd-Scuseria-Ernzerhof (HSE06)27, 36 hybrid functional for the electronic structures and the optical properties calculations of the pure and doped TiO2 systems. In the HSE06 functional, the exchange contribution was divided into short- and long-ranged part. The short-ranged part of PBE exchange was mixed with 25% Hartree-Fock (HF) exchange: HSE EXC =

1 HF,SR 3 EX （µ）+ EXPBE,SR（µ） +EXPBE,LR（µ）+ECPBE , 4 4

(1)

where SR and LR represent the short- and long-ranged parts of the exchange interaction, respectively. µ ( µ = 0.2 Å-1) is the parameter that defines the range-separation of Coulomb kerne. The climbing nudged elastic band (cNEB)37-39 method was used to investigate the reaction pathways of H2 generation via H2O splitting on the pure and the (I+N) codped TiO2(101) surfaces. The cNEB method is a small modification to the NEB method in which the exact saddle point (the highest energy image) can be obtained

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during the calculation. Here, the PBE functional was used for cNEB calculations. Considering the convergence of saddle point calculations, only the surface dopants and H2O molecule are allowed to relax in H2O/TiO2 systems.

RESULTS AND DISCUSSION Electronic Structures of (I+N) Codoped TiO2. The calculated band structure plots for pure anatase TiO2 were displayed in Figure 1(a). It is found that the pure anatase TiO2 is an indirect semiconductor, and the HSE06 calculated band gap is 3.26 eV, which is consistent with the experimental result (3.2 eV).40 The calculated band structures of I doped, N doped, and (I+N) codoped antase TiO2 are shown in Figure 1(b-d), respectively. As shown in Figure 1(b), the I doping in anatase TiO2induces a fully occupied midgap energy level in its band gap. The HSE06 calculated band gap of I doped TiO2 is 1.922 eV. Moreover, the I-doped anatase TiO2 showing a typical n-type metallic characteristic: the Fermi level locates a few tenth of an electronvolt above its CBM. For the N-doped anatase TiO2, there is a partially occupied impurity level locates above the its VBM [see Figure 1(c)], showing a typical p-type doping. Because N has one less electron than O, the N doping on O site in TiO2 would generate a hole in the N-doped anatase TiO2. As shown in Figure 1(d), the Fermi level of (I+N) codoped anatase TiO2 shifts at the top of VBM, corresponding to a semiconductor

behavior

of

this

system.

Accordingly,

the

photo-generated

electron-hole recombination rate of (I+N) codoped anatase TiO2 could be much lower than that of I-doped or N-doped anatase TiO2.6 The HSE06 calculated band gap of

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(I+N) codoped TiO2 is 2.251 eV, which is about 1.0 eV narrower than that of pure TiO2. The possible reason is that an electron on I donor level passivates a hole on N acceptor level. Therefore, the compensated (I+N) codoping in anatase TiO2 can prevent the recombination of photo-generated carries, and thus improved the photocatalytic efficiency.36

Figure 1. HSE06 calculated band structures of (a) pure, (b) I-doped, (c) N-doped, and (d) (I+N) codoped anatase TiO2. The Fermi levels are displayed with purple dashed lines.

To further understand the passivated effect of coincorporated I and N in anatase TiO2, the density of states (DOSs) of pure, I-doped, N-doped, and (I+N) codoped anatase TiO2 were calculated and shown in Figure 2. The highest occupied state is chosen as the Fermi energy and is set to zero. As shown in Figure 2(a), the valance band edge of pure anatse TiO2 consists mainly of O-2p state, whereas the conduction band edge is dominated by Ti-3d state. Figure 2(b) shows that I-doping in TiO2 introduces a fully occupied band gap state about 1.3 eV above the VBM. The partial

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DOS plots indicate that the band gap state consists of I-5s states and the O-2p states.41 As a result, the I dopant in anatse TiO2 exists as an I5+ cation, as found experimentally.26, 35 Although the band gap of I-doped TiO2 (1.922 eV) is suitable for visible-light absorption, the free electrons induced by I-doping can act as recombination sites and thus reduce the photocatalytic efficiency. Figure 2(c) shows that the incorporation of N dopants in anatase TiO2 induces partially occupied N-2p impurity states above the VBM. Therefore, N-doping in TiO2 can improve the visible-light absorption of TiO2 but shows a minor effect on the band gap reduction.38, 42

The DOS plot of (I+N) codoped anatase TiO2 is displayed in Figure 2(d). It is found

that Fermi level of (I+N) codoped anatase TiO2 is located at the top of VBM, and the VBM mainly consists of fully occupied I-5s and N-2p states. Because the substitution of I and N atoms on Ti and O sites in TiO2 act as single donor and acceptor, respectively. The electrons induced by I-donor can be passivated by the same amount of holes induced by N-acceptor. As a result, the (I+N) codoped anatase TiO2 still keeps the intrinsic semiconductor characters. Moreover, due to the higher neutral orbital energy of I-5s than that of N-2p, the energy position of I-5s state is higher than that of N-2p state in (I+N) codoped anatase TiO2. It is suggested that the (I+N) is an ideal codoping combination to improve the visible-light photocatalytic activity of anatase TiO2. On one hand, the fully occupied impurity energy levels induced by (I+N) codoping lead to an effective band gap reduction in anatase TiO2 (2.251 eV), which can enhance the visible-light absorption of anatase TiO2. On the other hand, the compensate effect of the I-N donor-acceptor makes the (I+N) codoped anatase TiO2

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still keeps the intrinsic semiconductor characters, and thus can prevent the photo-excited carriers from recombination.6

Figure 2. The HSE06 calculated DOSs for (a) pure, (b) I-doped, (c) N-doped, and (d) (I+N) codoped anatase TiO2. The Fermi levels are displayed with black dashed lines.

Optical Absorption Properties and Band Edge Shifts of (I+N) Codoped TiO2. It is well-known that the optical absorption property of a material is determined by its imaginary part of the dielectric tensor ε 2 (ω ) .43 To obtain accurate optical absorption property of (I+N) codoped anatase TiO2, we also adopt the HSE06 functional for 10


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ε 2 (ω ) calculations. Figure 3 show the ε 2 (ω ) curve of (I+N) codoped anatase TiO2, and the ε 2 (ω ) curve of pure anatase TiO2 was also displayed for comparison. For the pure anatase TiO2, because of its large band gap of 3.2 eV, the main optical absorption was confined in the UV light region, with a wavelength less than 400 nm. However, the (I+N) codoped anatase TiO2 can absorb a wider range light covering 300-600 nm, indicating that the optical absorption is extended to the visible region. It is noticed that the enhanced visible-light absorption of (I+N) codoped anatase TiO2 is in good agreement with the band gap prediction.

Figure 3. The HSE06 calculated imaginary part of the dielectric function ( ε 2 ) for pure anatase TiO2 and (I+N) codoped anatase TiO2. The imaginary part of the dielectric functions were averaged over the three (x, y, and z) polarization vectors.

To verify the PEC activity of (I+N) codoped anatase TiO2, the shifts of band

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edges for this system were calculated with respect to pure anatase TiO2, as shown in Figure 4. It is found that the increment of VBM and CBM for (I+N) codoped anatase TiO2 is 1.40 and 0.39 eV, respectively. The calculated results indicate that the (I+N) codoping in anatase TiO2 can not only enhance the reducing power, but also keep the oxidizing power of pure anatase TiO2. As a result, the (I+N) codoped anatase TiO2 is a suitable and efficient photocatalyst for PEC water splitting in visible-light region.

Figure 4. Comparison of the HSE06 calculated VBM and CBM positions of pure and I +N codoped anatase TiO2. A positive number indicates an uphill shift in energy with respect to pure anatase TiO2. The energy positions of the hydrogen production level (up) and the water oxidation level (down) are indicated by two horizontal blue dashed lines.

To verify the stability of (I+N) codoped anatase TiO2, the defect pair binding energy Eb is given by 12


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Eb = E (I) + E (N) − E (I+N) − E (TiO 2 ),

(2)

where E is the total energy of the system calculated with the same supercell. The calculated binding energy Eb for I-N pair is 1.918 eV. The positive Eb indicates that the I-N defect pairs tend to bind with each other when both of them are doped in anatase TiO2. It may be due to the charge transfer form I donor to N acceptor and the Coulomb interaction between positively charged I donor and negatively charged N acceptor. Accordingly, the binding of I and N could stabilize the N dopant to enhance the stability of (I+N) codoped anatase TiO2.

Reaction Pathways of H2 Generation via H2O Splitting on (I+N) Codoped TiO2(101) Surfaces. As mentioned above, the (I+N) codoped anatase TiO2 has a band gap of 2.251 eV and its band edges straddled the water redox potentials. Therefore, the (I+N) codoped anatase TiO2 could be a high-activity photocatalyst to produce hydrogen (H2) by water (H2O) splitting reaction under visible-light radiation. To explore the chemical reaction pathways on (I+N) codoped anatase TiO2, we constructed the supercell models of the pure anatase TiO2, the bulk, the I-surface and the N-surface of (I+N) codoped TiO2, which are displayed in Figure 5 (a-d), respectively. The pure anatase TiO2 was simulated by a two-layer 1 × 2 anatase TiO2(101) slab along [101̅] and [010] directions. To ensure the decoupling between neighboring systems, a vacuum region of 15 Å above anatase TiO2(101) slabs was used.44 Moreover, to obtain better surface relaxation energies, the atoms in the bottom layer of TiO2(101) slab were ﬁxed to their bulk positions and these in the top layer

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were allowed to relax.45 As shown in Figure 5 (b), the bulk of (I+N) codoped anatase TiO2 was simulated by replacing one bulk Ti and one bulk O atoms in pure anatase TiO2 with one I and one N atoms, respectively. For the I-surface of (I+N) codoped anatase TiO2, one surface Ti atom in pure anatase TiO2 was substituted by one I atom, whereas one bulk O atom in pure anatase TiO2 was substituted by one N atom [see Figure 5 (c)]. For the N-surface of (I+N) codoped anatase TiO2, one surface O atom in pure anatase TiO2 was substituted by one N atom, whereas one bulk Ti atom in pure anatase TiO2 was substituted by one I atom [see Figure 5(d)]. It is noticed that there are six kinds of surface atoms in these models: the six-coordinated Ti atom (Ti-6c), the five-coordinated Ti atom (Ti-5c), the three-coordinated O atom (O-3c), the two-coordinated O atom (O-2c), the five-coordinated I atom (I-5c), and the two-coordinated N atom (N-2c).

Figure 5. Supercell models of the pure and (I+N) codoped anatase TiO2. (a) the pure anatase TiO2, (b) the bulk of (I+N) codoped TiO2, (c) the I-surface of (I+N) codoped TiO2, and (d) the N-surface of (I+N) codoped TiO2. The light blue, red, 14


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purple, and blue balls represent Ti, O, I, and N atoms, respectively.

Then, we optimized the H2O molecule in various adsorption configurations on pure anatase TiO2. The adsorption energy, Eads , is calculated using the expression Eads = Eslab + EH 2O − EH 2O / slab

(3)

where Eslab is the energy of the clean TiO2(101) slab, EH 2O represents the energy of the H2O molecule, and EH 2O / slab represents the total energy of the adsorbed H2O/TiO2 system. A positive value of Eads > 0 indicates stable adsorption. As displayed in Figure 6(a), it is found that H2O on the top site of Ti-5c atom on pure anatase TiO2(101) surface was the global minimum.46. Where the O atom of H2O has an interaction with Ti, and the adsorption energy of H2O on pure anatase TiO2(101) surface is 0.647 eV (see Supporting Information). It is noticed that one of the H-O bond dangles with the H atom pointing toward the vacuum; The other H-O bonds is almost parallel to pure anatase TiO2(101) surface, and the H atom that points to the nearest O-2c atom is 2.63 Å from the nearest O-2c atom. Then, we investigated a possible chemical reaction pathway by using the global minimum structures as reactants.47 The details on searching the global minimum structures are discussed in Supporting Information (Figure S2-S6). The H2O splitting reaction and its possible reaction pathway on pure anatase TiO2 are shown in Figure 6(a). The first splitting of a H atom from H2O is an endothermic reaction, with an energy barrier of 0.73 eV. This step can be considered as a catalytic process: The Ti-5c atom has an electronic interaction with H2O, which can facilitate H dissociation. After the first H dissociation,

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the dissociated H atom forms O-H species with the O atom on pure anatase TiO2. During the second splitting reaction, the H atom of OH that adsorbs on Ti-5c atom can bond to a O-H species on pure anatase TiO2. H2 can then form directly if the energy barrier of 6.13 eV is surmounted. The high energy barrier of H2 formation indicates that H2 generation reaction is very hard on pure anatase TiO2. Globally, H2 generation is endothermic by 6.09 eV Finally, the distance between H2 molecule and the O-2c atom on pure anatase TiO2 increases to 2.62 Å, which indicates that it can easily desorb to the pure anatase TiO2(101) surface. The H2O splitting reaction and its possible reaction pathway on the bulk of (I+N) codoped anatase TiO2 surface are shown in Figure 6(b). In this case, the reaction pathway of H2 formation is similar to that on pure anatase TiO2. The energy barrier of the first splitting of a H atom from H2O is 0.80 eV. For the second splitting reaction, the energy barrier of H2 generation is 5.16 eV, which is 0.97 eV lower than that on pure anatase TiO2. Moreover, the global H2 formation is endothermic by 4.28 eV, which is 1.81 eV lower than that on pure anatase TiO2. These results indicate that the H2 generation reaction on the bulk of (I+N) codoped anatase TiO2 surface is more energetic favorable than that on pure anatase TiO2 surface.

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Figure 6. Calculated relative energies and most favorable reaction pathways for H2 generation via H2O splitting on (a) the pure anatase TiO2, (b) the bulk, (c) the I-surface and (d) the N-surface of (I+N) codoped anatase TiO2. The transition states (TSs) with reaction barrier energies (eV) are given in blue numbers.

For the I-surface of (I+N) codoped anatase TiO2, the global minimum adsorption geometry of the H2O is shown in Figure 6(c), where the O atom of H2O molecule was bonded with the doped I-5c atom, and one of H atom was bonded with the O-2c atom. The lengths of O-I and H-O bonds are 3.00 Å and 1.89 Å, respectively. The firstly splitted H atom from H2O was bonded to the nearest O-2c atom. Then the second splitted H atom of OH adsorbed on I-5c atom of the I-surface of (I+N) codoped anatase TiO2 approaches the firstly splitted H atom, which was diffused to the surface O-3c atom, and therefore forms H2 molecule. The energy barriers of the first splitting of H2O and H2 generation on the I-surface of (I+N) codoped anatase TiO2 are 1.07 eV

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and 5.21 eV, respectively. The global H2 formation is endothermic by 3.45 eV. It is found that the reaction pathway of H2 generation on the N-surface of (I+N) codoped anatase TiO2 can be divided to three steps, as shown in Figure 6(d). In the first step, one of the H atom of H2O splitted to the N-2c atom and formed N-H bond on the N-surface of (I+N) codoped anatase TiO2. Then, the H atom of OH adsorbed on Ti-5c atom also splitted to the N-2c atom, distributed with H-N-H species on the N-surface of (I+N) codoped anatase TiO2. Finally, the H2 molecule can be produced from H-N-H species through desorption. The first and second splitting of H2O on the N-surface of (I+N) codoped anatase TiO2 are exothermic, with the energy barriers of 0.71 eV and 1.30 eV, respectively. In addition, the energy barrier of H2 generation on the N-surface of (I+N) codoped anatase TiO2 is 5.23 eV, and the global reaction is endothermic by 3.26 eV. Actually, the H2 generation via H2O splitting reaction on (I+N) codoped anatase TiO2(101) surfaces is more energetic favorable than that on pure anatase TiO2(101) surface. The low energy barriers of H2 generation and the low relative energies of final H2 adsorption geometries for (I+N) codoped anatase TiO2 can be attributed to its stronger reducing power than the pure anatase TiO2.

CONCLUSIONS In summary, the electronic structures and the reaction pathways of H2 generation via H2O splitting of (I+N) codoped anatase TiO2 have been investigated systematically by using density functional theory calculations. The results indicate that the charge compensation effects between I-donor and N-acceptor can not only

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prevent the recombination of photo-generated electron-hole pairs, but also effectively reduce the band gap of anatase TiO2 (to 2.251 eV). The calculated band edges of (I+N) codoped anatase TiO2 straddle the water redox potentials, and the (I+N) codoped anatase TiO2 has stronger reducing power than pure anatase TiO2, which would facilitate the H2 generation. The calculated optical absorption curve of (I+N) codoped anatase TiO2 verifies that I+N codoping can significantly improve visible-light absorption. The calculated binding energy for I-N pair is 1.918 eV, indicating that the I+N defect pair is stable in (I+N) codoped anatase TiO2. The chemical reaction pathways of H2 generation via H2O splitting on pure and (I+N) codoped anatase TiO2 surfaces have also been explored comprehensively by using climbing nudged elastic band method. The results indicate that the H2 generation via H2O splitting reaction on (I+N) codoped anatase TiO2(101) surfaces is more energetic favorable than on pure anatase TiO2(101) surface. The (I+N) codoping in TiO2 can reduce the energy barrier of H2 production by about 1.0 eV. The enhanced reducing power of (I+N) codoped anatase TiO2 leads to the low energy barriers of H2 generation and the low relative energies of final H2 adsorption geometries on (I+N) codoped anatase TiO2(101) surfaces. These findings imply that the (I+N) codoped anatase TiO2 is a promising visible-light photocatalyst for H2O splitting.

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ASSOCIATED CONTENT Supporting Information The supercell models of pure, I-doped, N-doped, and (I+N) codoped anatase TiO2; The global minimum structures during the H2 generation reactions for the pure anatase TiO2, the bulk, the I-surface and the N-surface of (I+N) codoped anatase TiO2.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]. Phone: +86-0532-86983366.

*

E-mail: [email protected]. Phone: +86-010-64443254.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This research was financially supported by the National Natural Science Fund of China (No. 21603275), the Shandong Provincial Natural Science Foundation of China (ZR2016BB12),

and

the

Fundamental

Research

Funds

for

the

Central

Universities(15CX02118A, 15CX08003A). D. Cao is greatly thankful for the support of National Science Fund for Distinguished Young Scholars (No. 21625601).

REFERENCES 1.

Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode.

Nature 1972, 238, 37-38. 2.

Khan, S. U. M.; Al-Shahry, M.; Ingler Jr, W. B. Efficient Photochemical Water Splitting by a

20


Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Chemically Modified N-TiO2. Science 2002, 297, 2243-2245. 3.

Chen, X.; Mao, S. S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and

Applications. Chem. Rev. 2007, 107, 2891-2959. 4.

Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of

Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69-96. 5.

Khaselev, O.; Turner, J. A. A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen

Production Via Water Splitting. Science 1998, 280, 425-427. 6.

Gai, Y.; Li, J.; Li, S. S.; Xia, J. B.; Wei, S. H. Design of Narrow-Gap TiO2: A Passivated Codoping

Approach for Enhanced Photoelectrochemical Activity. Phys. Rev. Lett. 2009, 102, 36402. 7.

Yin, W. J.; Tang, H.; Wei, S. H.; Al-Jassim, M. M.; Turner, J.; Yan, Y. Band Structure Engineering

of Semiconductors for Enhanced Photoelectrochemical Water Splitting: The Case of TiO2. Phys. Rev. B 2010, 82, 045106. 8.

Liu, G.; Yin, L. C.; Wang, J.; Niu, P.; Zhen, C.; Xie, Y.; Cheng, H. M. A Red Anatase TiO2

Photocatalyst for Solar Energy Conversion. Energy Environ. Sci. 2012, 5, 9603-9610. 9.

Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visible-Light Photocatalysis in

Nitrogen-Doped Titanium Oxides. Science 2001, 293, 269-271. 10. Irie, H.; Watanabe, Y.; Hashimoto, K. Nitrogen-Concentration Dependence on Photocatalytic Activity of TiO2-X Nx Powders. J. Phys. Chem. B 2003, 107, 5483-5486. 11. Choi, W.; Termin, A.; Hoffmann, M. R. The Role of Metal Ion Dopants in Quantum-Sized TiO2: Correlation between Photoreactivity and Charge Carrier Recombination Dynamics. J. Phys. Chem. 1994, 98, 13669-13679. 12. Litter, M. I. Heterogeneous Photocatalysis: Transition Metal Ions in Photocatalytic Systems. Appl. Catal. B: Environ. 1999, 23, 89-114. 13. Yin, J. B.; Zhao, X. P. Preparation and Electrorheological Activity of Mesoporous Rare-Earth-Doped TiO2. Chem. Mater. 2002, 14, 4633-4640. 14. Xu, A. W.; Gao, Y.; Liu, H. Q. The Preparation, Characterization, and Their Photocatalytic Activities of Rare-Earth-Doped TiO2 Nanoparticles. J. Catal. 2002, 207, 151-157. 15. Mu, W.; Herrmann, J. M.; Pichat, P. Room Temperature Thotocatalytic Oxidation of Liquid Cyclohexane into Cyclohexanone over Neat and Modified TiO2. Catal. Lett. 1989, 3, 73-84. 16. Tang, J.; Durrant, J. R.; Klug, D. R. Mechanism of Photocatalytic Water Splitting in TiO2. Reaction of Water with Photoholes, Importance of Charge Carrier Dynamics, and Evidence for Four-Hole Chemistry. J. Am. Chem. Soc. 2008, 130, 13885-13891. 17. Zhang, J.; Wu, Y.; Xing, M.; Leghari, S. A. K.; Sajjad, S. Development of Modified N Doped TiO2 Photocatalyst with Metals, Nonmetals and Metal Oxides. Energy Environ. Sci. 2010, 3, 715-726. 18. Niu, M.; Xu, W.; Shao, X.; Cheng, D. Enhanced Photoelectrochemical Performance of Rutile TiO2 by Sb-N Donor-Acceptor Coincorporation from First Principles Calculations. Appl. Phys. Lett. 2011, 99, 203111. 19. Ma, X.; Wu, Y.; Lu, Y.; Xu, J.; Wang, Y.; Zhu, Y. Effect of Compensated Codoping on the Photoelectrochemical Properties of Anatase TiO2 Photocatalyst. J. Phys. Chem. C 2011, 115, 16963-16969. 20. Choi, H.; Shin, D.; Yeo, B. C.; Song, T.; Han, S. S.; Park, N.; Kim, S. Simultaneously Controllable Doping Sites and the Activity of a W–N Codoped TiO2 Photocatalyst. ACS Catalysis 2016, 6, 2745-2753. 21. Phattalung, S. N.; Limpijumnong, S.; Yu, J. Passivated Co-Doping Approach to Bandgap

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 24

Narrowing of Titanium Dioxide with Enhanced Photocatalytic Activity. Appl. Catal. B: Environ. 2017, 200, 1-9. 22. Xie, J.; Xie, H.; Su, B.; Cheng, Y.; Du, X.; Zeng, H.; Wang, M.; Wang, W.; Wang, H.; Fu, Z. Mussel-Directed Synthesis of Nitrogen-Doped Anatase TiO2. Angew. Chem. Int. Ed. 2016, 55, 3031-3035. 23. Muhich, C. L.; Westcott IV, J. Y.; Fuerst, T.; Weimer, A. W.; Musgrave, C. B. Increasing the Photocatalytic Activity of Anatase TiO2 through B, C, and N Doping. J. Phys. Chem. C 2014, 118, 27415-27427. 24. Niu, M.; Cui, R.; Wu, H.; Cheng, D.; Cao, D. Enhancement Mechanism of the Conversion Effficiency of Dye-Sensitized Solar Cells Based on Nitrogen-, Fluorine-, and Iodine-Doped TiO2 Photoanodes. J. Phys. Chem. C 2015, 119, 13425-13432. 25. Hou, Q.; Zheng, Y.; Chen, J.-F.; Zhou, W.; Deng, J.; Tao, X. Visible-Light-Response Iodine-Doped Titanium Dioxide Nanocrystals for Dye-Sensitized Solar Cells. J. Mater. Chem. 2011, 21, 3877-3883. 26. Hong, X.; Wang, Z.; Cai, W.; Lu, F.; Zhang, J.; Yang, Y.; Ma, N.; Liu, Y. Visible-Light-Activated Nanoparticle Photocatalyst of Iodine-Doped Titanium Dioxide. Chem. Mater. 2005, 17, 1548-1552. 27. Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb Potential. J. Chem. Phys. 2003, 118, 8207-8215. 28. Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953-17979. 29. Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758-1775. 30. Kresse,

G.;

Hafner,

J.

Ab-Initio

Molecular-Dynamics

Simulation

of

the

Liquid-Metal-Amorphous-Semiconductor Transition in Germanium. Phys. Rev. B 1994, 49, 14251-14269. 31. Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab-Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. 32. Hammer, B.; Hansen, L. B.; Nørskov, J. K. Improved Adsorption Energetics within Density-Functional Theory Using Revised Perdew-Burke-Ernzerhof Functionals. Phys. Rev. B 1999, 59, 7413-7421. 33. Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188-5192. 34. Ma, T.; Akiyama, M.; Abe, E.; Imai, I. High-Efficiency Dye-Sensitized Solar Cell Based on a Nitrogen-Doped Nanostructured Titania Electrode. Nano Lett. 2005, 5, 2543-2547. 35. Tojo, S.; Tachikawa, T.; Fujitsuka, M.; Majima, T. Iodine-Doped TiO2 Photocatalysts: Correlation between Band Structure and Mechanism. J. Phys. Chem. C 2008, 112, 14948-14954. 36. Paier, J.; Marsman, M.; Hummer, K.; Kresse, G.; Gerber, I. C.; Ángyán, J. G. Screened Hybrid Density Functionals Applied to Solids. J. Chem. Phys. 2006, 124, 154709. 37. Sheppard, D.; Terrell, R.; Henkelman, G. Optimization Methods for Finding Minimum Energy Paths. J. Chem. Phys. 2008, 128, 134106. 38. Henkelman, G.; Uberuaga, B. P.; Jónsson, H. A Cimbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901-9904. 39. Henkelman, G.; Jónsson, H. Improved Tangent Estimate in the Nudged Elastic Band Method for Finding Minimum Energy Paths and Saddle Points. J. Chem. Phys. 2000, 113, 9978-9985. 40. Burdett, J. K.; Hughbanks, T.; Miller, G. J.; Richardson Jr, J. W.; Smith, J. V. Structural-Electronic

22


Page 23 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Relationships in Inorganic Solids: Powder Neutron Diffraction Studies of the Rutile and Anatase Polymorphs of Titanium Dioxide at 15 and 295 K. J. Am. Chem. Soc. 1987, 109, 3639-3646. 41. Yang, K.; Dai, Y.; Huang, B.; Whangbo, M. H. Density Functional Characterization of the Band Edges, the Band Gap States, and the Preferred Doping Sites of Halogen-Doped TiO2. Chem. Mater. 2008, 20, 6528-6534. 42. Niu, M.; Cheng, D.; Cao, D. Understanding Photoelectrochemical Properties of B-N Codoped Anatase TiO2 for Solar Energy Conversion. J. Phys. Chem. C 2013, 117, 15911-15917. 43. Sun, J.; Wang, H. T.; He, J.; Tian, Y. Ab-Initio Investigations of Optical Properties of the High-Pressure Phases of ZnO. Phys. Rev. B 2005, 71, 125132. 44. Long, R. Electronic Structure of Semiconducting and Metallic Tubes in TiO2/Carbon Nanotube Heterojunctions: Density Functional Theory Calculations. J. Phys. Chem. Lett. 2013, 4, 1340-1346. 45. Meng, S.; Kaxiras, E. Electron and Hole Dynamics in Dye-Sensitized Solar Cells: Influencing Factors and Systematic Trends. Nano Lett. 2010, 10, 1238-1247. 46. Walle, L.; Borg, A.; Johansson, E.; Plogmaker, S.; Rensmo, H.; Uvdal, P.; Sandell, A. Mixed Dissociative and Molecular Water Adsorption on Anatase TiO2 (101). J. Phys. Chem. C 2011, 115, 9545-9550. 47. Chen, P. T.; Sun, C. L.; Hayashi, M. First-Principles Calculations of Hydrogen Generation Due to Water Splitting on Polar GaN Surfaces. J. Phys. Chem. C 2010, 114, 18228-18232.

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I, N-Codoping Modification of TiO2 for Enhanced ... - ACS Publications

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