Formation of New Phases to Improve the Visible-Light Photocatalytic

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Formation of New Phases to Improve the Visible-Light Photocatalytic Activity of TiO2 (B) via Introducing Alien Elements Baihai Li, Yun Lin, Jianlin Wang, Xue Zhang, Yun-Rui Wang, Yu Jiang, Ting Shuai Li, Li-Min Liu, Liang Chen, Wanli Zhang, and Yanrong Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07509 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 8, 2016

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The Journal of Physical Chemistry

Formation of New Phases to Improve the Visible-Light Photocatalytic Activity of Tio2 (B) Via Introducing Alien Elements Bai-Hai Li†, ‡, Yun Lin†, Jian-Lin Wang†, Xue Zhang†, Yun-Rui Wang†, Yu Jiang†, Ting-Shuai Li†, Li-Min Liu*, ǁ, Liang Chen*, §, Wan-Li Zhang‡, Yan-Rong Li‡ †

School of Energy Science and Engineering, University of Electronic Science and Technology of China, Chengdu,

611731, P. R. China ‡

State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and

Technology of China, Chengdu 611731, P. R. China ǁ §

Beijing Computational Science Research Center, Beijing, 100193, P. R. China Ningbo Institute of Industrial Technology, Chinese Academy of Sciences, Ningbo, 315201, P.R. China

E-mail: [email protected] (+86)0574-86685160, [email protected] (+86)010-82687086

Abstract Designing highly efficient photocatalysts using first principles calculations is an urgent challenge. In this work, the potential structures of doped bronze TiO2 were systematically designed and screened using cluster expansion (CE) and first principles calculations. The ordered TiPtO4 phase, which can be fabricated by substituting the Ti cations in bronze TiO2 with Pt, is predicted to be a promising visible light-responsive photocatalyst with excellent thermodynamic stability. Further calculations suggest that TiPtO4 is a semiconductor with a suitable band gap (1.96 eV) for the absorption of visible light and appropriate band edge positions relative to the redox potential of water splitting. This work not only reports a potential highly efficient material for photocatalysis but also sheds light on the design and rationalization of this new photocatalyst using first principles calculations.

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1. Introduction The use of photocatalytic technologies for harvesting and converting sunlight into chemical energy carriers that can be stored (e.g., hydrogen) is a promising and rapidly developing research topic. In principle, a suitable band gap and appropriate band edge positions are prerequisites for a photocatalyst with a sufficiently high redox capability to catalyze water-splitting. Moreover, high transfer coefficient and fast charge carrier separation are important factors. Differences in orbital characteristics and carrier mobility at the conduction band minimum (CBM) and valance band maximum (VBM) are highly beneficial to prevent electron-hole pair recombination, thus elongating the carrier lifetimes. Since Fujisima and Honda1 first reported their water-splitting system, hundreds of new materials have been demonstrated to exhibit photocatalytic activity for water splitting.2-3 However, the ability of these materials to absorb and convert sunlight is still below the required 10% photoelectric transformation efficiency because of a wide band gap and/or misaligned band edge positions. Although TiO2 is considered the most promising candidate for commercial photocatalysts, its wide band gap is still a serious problem leading to low quantum efficiency under visible-light, hindering commercial and industrial applications. There have been numerous attempts in the literature to improve the performance by substituting or doping a foreign species into TiO2 to narrow the band gap and adjust the band edge position.4-10 However, increases in efficiency have proven to be rather limited, largely because precisely controlling the doping concentration and atom positions in a host material is extremely difficult. Some of the primary issues are whether the dopants are substitutional, interstitial, or incorporated into grain boundaries and whether they form complexes such as dopant pairs or complexes involving cation interstitials and/or anion vacancies.11 However, introducing trace foreign atoms usually results in structural instability, flat and highly localized in-gap impurity levels in the band structures that decrease carrier mobility and strong carrier recombination or trapping at the defect site, all of which eventually lower the photocatalytic activity. The experimental trial-and-error method alone has been


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unable to offer an optimal solution to improve the photoelectrochemical conversion efficiency of photocatalysts. Fortunately, the rapid developments in first principles have made it possible to screen a large number of possible structures under given conditions and obtain a detailed picture of the interactions between the dopants and the host material, thus providing ideal options for the dopant elements, positions and concentrations needed to develop the best-performing photocatalyst. Bronze TiO2 [TiO2 (B)],12-13 a meta-stable monoclinic polymorph in the TiO2 family, was chosen for our investigation. Due to the lack of commercial TiO2 (B) products, limited studies have been conducted on TiO2 (B) compared with anatase and rutile TiO2. Recently, however, TiO2 (B) has received significant attention because it can be prepared as nanofibers,14 nanotubes15 or heterostructures with anatase16 for photocatalysis. Its unique open structure with edge- and corner-linked octahedral qualities facilitates the transport and extraction of hydrogen or other small cations (e.g., Li). Nevertheless, the wide band gap (3.0~3.2 eV)17-19 and weak electron conductivity impedes wide application. Recent reports that anatase TiO2 doped with dilute metallic atoms (e.g., Ru, Zr, Pt, Sn) has a narrower band gap and thus more efficient solar energy absorption and conversion20-24 suggest that partially replacing the Ti ions of TiO2 (B) with appropriate foreign elements might also improve photocatalytic performance to generate better visible-light responsive catalysts. In this study, a computational screen approach based on combining first principles calculations with the cluster expansion (CE) formalism25-28 was employed to determine which element(s) among the selected RMs can be doped into TiO2 (B), to gauge the extent to which the dopant(s) can replace Ti atoms, and to investigate the possibility of generating new ordered structures. Monte Carlo (MC) simulations were used to study the thermodynamic stability of the promising phase(s) within the bronze prototype. The electronic structures and optical absorption of the stable phase(s) were calculated using an accurate HSE06 method to evaluate the photoelectrochemical activity. Ten representative metals (RMs) were systematically explored via first principles calculations and CE prediction, including precious metals (Pt and Pd), normal transition metals (V, Mn, Mo, and Ru), d0 elements (Nb and Zr), and d10



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elements (Ge and Sn) with +4 valences and similar ionic radii to Ti4+. Interestingly, our calculations reveal that Pt and Ge can be doped into TiO2 (B) to generate ordered phases, whereas the other eight metals likely cannot be doped into TiO2 (B). Moreover, the new predicted thermodynamically stable TiPtO4 exhibits a favorable band gap and reasonable band edge positions for water splitting, thereby showing great potential as an outstanding photocatalyst for solar energy conversion.

2. Computational Methodology 2.1. Cluster Expansion For the Ti-RM-O system, the order over the cation sublattice determines the configurational entropy and thus the grand canonical free energy, which dictate the thermodynamic properties of the system. The atomic arrangement on the cationic sublattice sites of the system can be assigned with spin-like occupation variables for each chemical species, e.g., σi = +1 and -1 for RM and Ti in the cation sites,

ur

respectively. Therefore, the occupation variable collection

σ = {σ 1 ,..., σ 2 ,...σ M }

uniquely constructs the arrangement of Ti and RM in the system. Within the cluster expansion, the energy can be written in terms of polynomials of the basic function for each site

ur E σ = V0 + ∑ Viσ i + ∑ Vijσ iσ j + ∑ Vijkσ iσ jσ k + ...

( )

i

i, j

(1)

i , j ,k

in which the coefficients Vi, Vij, Vijk, …, are the effective cluster interactions (ECIs) for all pairs, triplets, quadruplets, etc. The ECI values were determined by fitting Equation 1 to the first-principles energies of selected configurations using a root mean square method. However, in practice, the polynomials in Equation 1 for clusters above the maximum size can be omitted, and the optimal ECIs can be obtained according to the weighted cross-validation (CV) score, which is a criterion for the quality of the cluster expansion in predicting the energy of structures not included in the fitting.25, 29-30 The effective Hamiltonian was determined by a genetic algorithm from a set of ECIs.31 Using optimal ECIs and the effective Hamiltonian, the phase


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equilibrium of the system at finite temperatures can be predicted by performing grand canonical Monte Carlo simulations.

2.2. First-Principles Calculation All spin-polarized first-principles calculations were performed using the Vienna Ab initio Simulation Package (VASP).32 The projector augmented-wave (PAW) method33 was used to treat core electrons. The electron exchange and correlation were treated within

a

generalized

gradient

approximation

(GGA)

using

the

Perdew-Burke-Ernzerhof (PBE) functional.34 The cutoff energy was set to 420 eV for all first-principles calculations. To perform a first-principles optimization on the atomic positions and lattice constants of the selected configurations within the cluster expansion, appropriate K-point meshes were sampled based on a Monkhorst-Pack scheme,35 ensuring that the force on each atom converged to within 0.02 eV/Å. To overcome the well-known deficiency of standard DFT in describing the exchange-correlation energy, the screened Heyd−Scuseria−Ernzerhof (HSE06) hybrid functional36-38 was employed in the present work to perform secondary geometry optimization and electronic structures calculations of the thermodynamically stable phases. The exact exchange contribution was 22% HF and 78% PBE contributions, which has produced accurate lattice constants and band gaps for rutile and anatase TiO2 compared to the experimental data and other calculation values.39-40 The HSE06 calculated lattice parameters of TiO2 (B) a=12.182 Å, b=6.532 Å, c=3.753 Å are in excellent agreement with the experimental values.12 Furthermore, the HSE06 method reproduces a reasonable band gap of TiO2 (B) to be 3.58 eV, which is slightly larger than the experimental band gap of 3.0~3.2 eV,

17-19

while seems better than the

screened exchange (sX) functional yielding the value of 3.71 eV.41

3. Results and Discussion To investigate the dissolution of RMs within TiO2 (B), the cluster expansion calculations were performed to describe the dependence of the energies of Ti1-xRMxO2 (B) on the arrangement of Ti and RMs over the cationic sites of TiO2 (B). At 0 K, the



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feasibility of forming a doped Ti1-xRMxO2 (B) structure was determined by the formation energy with respect to the total energies of TiO2 (B) and RMO2 (B) using the following equation: tot tot  E f = ETito1−t x RM x O2 − (1 − x ) ETiO + xERMO 2 2 

(2)

in which the Etot terms after the equal sign are the DFT-calculated total energies of the doped structures Ti1-xRMxO2 (B), pure TiO2 (B) and RMO2 (B), from left to right, and x is the RM doping concentration. Next, first-principles calculations were performed on a total of 755 different structures (with over 50 different Ti-RM configurations for each RM element in the Ti-sites of TiO2 (B) initially generated by CE) to obtain the formation energies according to Equation 2. Note that the structures with ultra-low doping concentrations were not considered in this work. The calculated formation energies of the doped structures at various concentrations of each RM element ranging from 0.125 to 0.875 in the TiO2 (B) unit cell are compared in Figure 1. The values for the CE-generated RM’ candidates (a subset of the transition metals V, Mn, Ru, Mo, Nb, Zr, and Sn) and Pd-doped TiO2 (B) are largely positive, indicating that these structures are unstable. Importantly, many configurations of the doped TiO2 (B) with Pt and Ge at various concentrations yield negative formation energies, indicating that these two elements can be introduced into TiO2 (B) and may generate stable and ordered structures. The results can be understood by comparing the relative strength of the RM-O binding interactions. As implied by the formation energy defined in equation 2, we suggest that the configurations of Ti1-xRMxO2 (B) are actually generated during the mixing process of TiO2 and RMO2. Thus, in principle, moderate interactions between RM and O are required to form stable Ti1-xRMxO2 (B) phases. The energies of RM binding to O within the RMO2 compounds are compared in Table S1 (Part A, Supporting Information). The Pt and Ge binding to O (binding energies of -4.28 and -4.98 eV/atom, respectively) are more appropriate than that of RM’-O in RM’O2 oxides (binding energies in the range of -5.13 ~ -7.01 eV/atom), which may be the reason


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that only Pt and Ge can be introduced into TiO2 (B) to form stable Ti1-xRMxO2 (B) with negative formation energies. The excessively strong binding interactions of RM’ with O atoms should presumably induce those RM’ atoms to precipitate from Ti1-xRM’xO2 (B) and form RM’O2 oxides, thereby causing the positive formation energies of Ti1-xRM’xO2 (B) structures. We also found that some doped TiO2 (B) was induced to have a magnetic structure by the transition metals, including V, Mn, Ru, Mo and Nb, as shown in Table S2 (Part A, Supporting Information). Therefore, these Ti1-xRMxO2 (B) configurations have been reconfirmed as unstable structures because none of the reported compounds have a non-zero magnetic moment, as previously reported. In addition, the much larger sizes of the alien atoms (Mo, Ru, Nb, Sn, and Zr) compared to Ti constitute another adverse factor, and as a result, the doped structures exhibit a remarkable volume expansion, as listed in Table S2 (Part A, Supporting Information), degrading the geometric stabilities of the Ti1-xRMxO2 (B) configurations. Pd has the weakest binding interactions with the O atom and would prefer to escape from the bronze lattice and exist as a pure metal, which accounts for the positive formation energies of the Ti1-xPdxO2 (B) configurations. We believe that even if the process for doping RM’ and Pd atoms into TiO2 (B) is assisted by high external energy, the solubility of the alien atoms will be rather limited. Our calculations also suggest that it is difficult to dope RM atoms at the interstitial sites of TiO2 (B) to form ordered crystal structures. Even though the RM diluted TiO2 (B) could be obtained under the external conditions, the mobility of the carriers on the doping levels might be rather low, making negative contributions to the photocatalytic efficiency, as demonstrated in Part B of Supporting Information. We next focus on identifying the ground states of Pt-doped TiO2 (B). To this end, the formation energies of the Ti1-xPtxO2 (B) configurations were used in the CE fitting to calculate the coefficients of the CE, and a least-squares fit was used to calculate the ECI values. The obtained cluster expansion includes the empty cluster and 2 points, 19 pairs, 5 triplets and 3 quadruplets clusters. The cross-validation score for the cluster expansion is 6.7 meV/f.u. (f.u.=formula unit) with a root mean square



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(rms) error of 5.5 meV/f.u.. TiO2 (B) and PtO2 (B) were set as the references to plot the formation energy diagrams, as shown in Figure 2 (a). The consistent formation energies obtained by DFT calculations (red points) and CE prediction (green points) confirm that Pt can replace Ti in the bronze lattice at various concentrations. Clearly, CE is able to describe the first principles energies with high accuracy. Furthermore, the convex hulls of the formation energies obtained by cluster expansion (green line) and first-principles (red line) are converged well. Hence, the point at xPt=0.5 representing the newly discovered compound TiPtO4 was identified as a unique global minimum phase by both cluster expansion and first-principles calculations. The calculated formation energy of the most stable phase is -101.87 meV/f.u. Regarding Ti1-xGexO2 (B), the ground states are found to be not suitable for visible light-responsive photocatalysis due to their too large band gaps, thus we present the results of the CE of this system in Part C of Supporting Information. In contrast to the formation of the ground phase TiPtO4, our previous calculations suggested that Pt can not be doped into Anatase and Brookite TiO2 at high concentrations to form ordered crystal structures, since the formation energies are largely positive.42 Grand canonical Monte Carlo simulations on the Ti1-xPtxO2 (B) system were performed

following

the

cluster

expansion

prediction

to

investigate

the

thermodynamic properties at finite temperatures. At each site within a 12×12×12 supercell (6912 Ti/Pt sites), 8000 Monte Carlo moves were performed at each temperature and chemical potential. The first 3000 moves were used for equilibration, and the following 5000 moves were used for the final analysis. The difference in chemical potential µPt-µTi as a function of the average Pt concentration xPt is shown in Figure 2 (b). The appearance of kinks and plateaus in the curves for 373 and 673 K indicates that TiPtO4 is a long-range ordered crystal structure at finite temperatures. The solubility of Pt in TiO2 (B) and TiPtO4 increases gradually with the temperature, which results in the wider and smoother kink. Finally, the two phases become indistinguishable from Pt, as illustrated by the curves at 1073 K. A pseudo-binary phase diagram of the TiO2 – PtO2 (B) system constructed from


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the output of the Monte Carlo simulations enabled us to investigate the stability of the bronze phases. The boundaries of the phase fields and the two-phase regions can be determined by comparing the grand canonical free energy integration of µPt in both directions with respect to the Pt fraction. As shown in Figure 3 (a), three phases – TiO2 (B), TiPtO4 and PtO2 (B) – are separated by two miscibility gaps, one at the Ti-rich end at ~850 K and the other one at the Pt-rich end at ~1100 K. However, as demonstrated experimentally by Punnoose and Seriani, the most stable hexagonal α-PtO2 phase decomposes at temperatures above 870 K.38,

43

PtO2 (B) should be

unstable even at lower temperatures because the formation enthalpy of PtO2 (B) is calculated to be 0.83 eV/f.u. higher than that of α-PtO2. Thus, the miscibility gap between TiPtO4 and PtO2 (B) might not exist at higher temperatures. The thermodynamic phase diagram indicates that TiPtO4 is stable at lower temperatures, whereas it undergoes a second-order phase transition and becomes homogeneous at temperatures higher than 1350 K, which is determined by the transition temperature and compositions with an order parameter characterizing when the ordered phases become indistinguishable from each other.44 TiPtO4 has a similar monoclinic structure to TiO2 (B) and is in the symmetry group (B2/m, 12). The HSE06 calculations showed that the unit cell volume of TiPtO4 is 6.59% larger than that of TiO2 (B), with lattice parameters of a=12.059 Å, b=6.704 Å, c=3.874 Å, and γ=75.632° (the geometry structure in .cif format can be found in Supporting Information). Consequently, the [TiO6] and [PtO6] octahedra retain their symmetry and only suffer subtle distortions. The geometric structure of TiPtO4 is displayed in Figure 3 (b), in which the Ti and Pt atoms as well as four types of O atoms are denoted by different balls and arrows. Two- and three-coordinated O1 and O2 formed bonds with one Pt and one Ti atom and with two Pt and one Ti atom in the ab plane, respectively. Furthermore, three-coordinated O3 and four-coordinated O4 atoms formed O3-Ti-O3-Ti… and O4-Pt-O4-Pt…. chains along the c direction and simultaneously bound to another lateral Pt or Ti atom, respectively. The bonding interactions and charge transfer intimately refer to the coordination environment. A



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Bader charge analysis45 showed that O1, O2, O3, and O4 atoms obtain 1.19, 1.18, 1.29 and 1.15 electrons from cations, respectively, whereas each Ti and Pt atom donated 2.76 and 2.05 electrons, respectively. Clearly, the values deviated remarkably from the formal ionic charges based on the classical model. To explore the potential application of the new TiPtO4 material, the electronic structures were investigated. The density of states (DOS) of TiPtO4 calculated by HSE06 and PBE methods were displayed in Figures 4 (a) and S6 (part D, Supporting Information), respectively. It is clearly shown that the top of the valence band (VB) is mainly dominated by the states of Pt5d hybridized with O2p. This characteristic is in significant contrast to that of pristine TiO2 and other conventional oxides, in which the VBM is occupied by O2p orbitals.18, 46 The electrons at the VB edge are therefore expected to be more active because hybridized Pt5d-O2p states are less restricted than O2p orbitals due to the introduction of Pt. Moreover, the bottom of the conduction band (CB) is primarily occupied by the Pt5d and is accompanied by some of the Ti-O hybridized states. The l-projected DOS of Pt aotms shown in Figure S7 (part D, Supporting Information) clearly presents that the VBM and CBM are mainly attributed to Pt5d

zx

and Pt5 d 2

x -y 2

, respectively. Consistently, as visualized by the

single-state charge density contour maps in Figure 4 (b) and (c), the electron density at the VBM mainly accumulated on the Pt-O bond along the c axis, characteristic of Pt5 d zx and O 2p z , whereas that of the CBM was occupied by the Pt5 d 2

x − y2

orbitals with

little contribution from the Ti and O atoms. The CBM and VBM distributions on the different metal ions may effectively reduce

the

recombination

rates of

photo-generated electron hole pairs. The calculated band structures clearly showed that TiPtO4 is an intrinsic semiconductor with a band gap of 1.96 eV from A to M in the Brillouin zone [Figure 4 (d)]. In addition, there is a direct band gap of 2.47 eV at the M point. The narrow band gaps suggest that TiPtO4 is capable of responding to the wide range of visible-light as a photocatalyst. Furthermore, the energy bands at the edges are quite


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dispersive, implying that photo-generated carriers can rapidly transfer to the surface. Therefore, we decided to estimate the effective mass of the electron and hole by fitting the energy-wave vector dependence onto the states at the VBM and CBM. The effective mass should be anisotropic along different directions in the Brillouin zone because the energy isosurface of TiPtO4 is non-spherical, as demonstrated by several summits or valleys at the VB and CB edges, respectively. Indeed, the electron effective mass at the CBM (me*) is calculated to be 0.15 me along LM or 0.27 me along AM, which suggests that the excited electron has excellent mobility in the conduction bands of TiPtO4, even better than in anatase TiO2 (me*=1 me).47 Nevertheless, the diffusion of photo-generated holes is relatively weak because the hole effective mass at a VBM (mh*) of 1.29 me along MA or 0.77 me along ΓA is much larger than me*. These results suggest that the rate of electron hole recombination should be low in this system, considering the large difference in the effective masses of me* and mh*. The position of the CBM and VBM with respect to the free energy of the relevant redox reactions was another key factor to determine the efficiency of a photocatalytic reaction. The CBM and VBM of a water-splitting catalyst should span both the reduction and oxidation potentials of water. Specifically, the valence band must be lower than the level of O2/H2O (-5.67 eV at pH=0), and the conduction band must be higher than the level of H+/H2 (-4.44 eV at pH=0).48-49 Herein, we used a periodic slab model containing a semi-infinite surface and a sufficiently thick vacuum region (30 Å) to calculate the work functions for determining the positions of the CBM and VBM referring to the vacuum level, as suggested by Toroker et al..50 Five low-index stoichiometric surfaces of TiPtO4 with identical terminations for their respective slabs were chosen for the calculations such that there were no dipole-dipole interactions for each of the surface models, as shown in Figure S8 (Part D, Supporting Information). The surface energies and Bader charge analyses of the optimized slabs displayed in Table S3 (Part D, Supporting Information) suggest that TiPtO-(001), TiPtO-(010) and PtO-(100) surfaces are more stable than the two O-(010) and TiO-(100) surfaces. Therefore, these three stable surfaces were selected to



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calculate the work functions and the relative positions of the band edges. The work functions of the TiPtO-(001), TiPtO-(010) and PtO-(100) surfaces are 6.15, 5.94 and 6.35 eV, respectively, which places the CBMs above the reduction potential of water in the order of TiPtO-(010) > TiPtO-(001) > PtO-(100), whereas the VBMs are below the oxidation potential of water in the order of TiPtO-(010) < TiPtO-(001) < PtO-(100). The redox potentials of water compared to the vacuum level are displayed as red dashed lines in Figure 4(e). The calculated results indicated that the surfaces of TiPtO4 were indeed suitable to catalyze water-splitting. However, it is worth noting that redox reactions or the adsorption of materials (e.g., water or pollutants) might occur on the surfaces at ambient conditions, which would shift the electrostatic potential and therefore the band edges of surfaces. The optical absorption spectrum of TiPtO4 calculated by the HSE06 method is shown in Figure 5. The indirect interband transition from point A at the VBM to point M at the CBM leads to the initial absorption at approximately 2.0 eV. The very steep optical absorption onset with the linear portion (red dash line) extrapolating to 2.3 eV is a remarkable feature of the spectrum. Furthermore, TiPtO4 exhibits strong optical absorption of visible-light with a coefficient rapidly reaching the order of 105~106 cm-1 within the energy range of approximately 0.5 eV, which is mainly attributed to the collective contributions from interband transitions at high-symmetry points and/or axes in Brillouin zone. The steep onset of absorption and the high absorption coefficient are favorable factors for TiPtO4 as a photocatalyst.

4. Conclusion In summary, first principles calculations have been performed to screen a large group of doped TiO2 (B) structures with different transition metals at various concentrations. Our systematic theoretical studies based on combining first principles calculations with CE predictions and MC simulations show that TiPtO4, which is generated by introducing Pt into the Ti-site of TiO2 (B), is a thermodynamically stable phase at temperatures lower than 850 K. Ge can also be doped into TiO2 (B); however,


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the resulting stable structures are not suitable for photocatalysis because of the excessively large band gaps. The other RM elements cannot be doped into TiO2 (B) at 0 K because of their large positive formation energies. Furthermore, HSE06 calculations showed that TiPtO4 is an intrinsic semiconductor with a narrow band gap within the visible light spectrum, indicating that TiPtO4 is a potentially promising as a visible light-responsive photocatalyst. The VBM and CBM were mainly composed of the Pt5d-O2p hybridized states and Pt5d orbitals, respectively. In addition, the effective masses of the carriers at the VBM and CBM were significantly different. The unique electronic structures would substantially promote the separation of photo-generated electron-hole pairs. The positions of the VBM and CBM of the low index surfaces spans both redox potentials of water, suggesting that TiPtO4 can catalyze water-splitting to produce hydrogen and oxygen. It might be difficult to dope high concentration of Pt into TiPtO4, however, we believe that there would be other way to synthesize the thermodynamically stable phase TiPtO4. Finally, we expect that the theoretical design methodology will shed light on the development of high-efficiency and lower-cost photocatalysts.

Supporting Information Additional information and figures. This information is available free of charge via the Internet at http://pubs.acs.org/.

Acknowledgements We gratefully acknowledge the financial support by the National Key Basic Research Program of China (Grant No. 2013CB934800), National High Technology Research and Development Program of China (863 Program) (Grant No. 2015AA034202), National Science Foundation of China (Grant No. 51472254), NSF of Zhejiang Province (LR14E020004), the program for Ningbo municipal science and technology innovative research team (2015B11002) and Fundamental Research Funds for the Central Universities (Grant No. ZYGX2014J086).



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Captions

Figure 1. The formation energies obtained via first principles calculations of transition metal (RMs=V, Pt, Mn, Ru, Mo, Nb, Zr, Sn, Ge, and Pd)-doped TiO2 (B) at various concentrations with respect to TiO2 (B) and RMO2 (B). Figure 2. (a) The formation energy of TixPt1-xO2 (B) in the bronze prototype obtained by first-principles calculations (red) and cluster expansion prediction (green). The red solid line


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represents the first-principles convex hull, which overlaps the green line representing the convex hull of the cluster expansion. (b) The chemical potential difference as a function of average Pt concentration in TixPt1-xO2 (B) at 373, 673 and 1073 K, calculated by grand canonical Monte Carlo simulations. Figure 3. (a) The phase diagram of TiO2 (B)-PtO2 (B) obtained by Monte Carlo simulations. (b) The geometric structure of TiPtO4 obtained from the cluster expansion. Figure 4. The electronic structures of TiPtO4: (a) the total and partial density of states. (b) and (c) The single states of VBM and CBM at 0.003 e/Å, respectively. (d) The band structure. (e) The positions of valance and conduction bands compared to the vacuum level. (001), (010) and (100) represent TiPtO-(001), TiPtO-(010) and PtO-(100) surfaces, respectively. Figure 5. The optical absorption spectrum of TiPtO4 calculated via the HSE06 method. The red dashed line denotes the energy range in which absorption increases steeply.

Figures



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Figure 1

Figure 2

Figure 3


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Figure 4



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Figure 5


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TOC figure

The new ordered structure TiPtO4 predicted via first principles calculations and Monte Carlo simulations is a semiconductor. The suitable band gap (1.96 eV) and appropriate band edge positions relative to the redox potential of water splitting suggest the material is a potential photocatalyst.


Formation of New Phases to Improve the Visible-Light Photocatalytic

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