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Rutile TiO2 (011) 2×1 Reconstructed Surfaces with Optical Absorption over Visible Light Spectrum Rulong Zhou, Dongdong Li, Bingyan Qu, Xiaorui Sun, Bo Zhang, and Xiao Cheng Zeng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10718 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 25, 2016
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ACS Applied Materials & Interfaces
Rutile TiO2 (011) 2×1 Reconstructed Surfaces with Optical Absorption over Visible Light Spectrum
Rulong Zhou1*, Dongdong Li1, Bingyan Qu1, Xiaorui Sun1, Bo Zhang1*, and Xiao Cheng Zeng2,3* 1
Laboratory of Amorphous Materials, School of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui 230009, P. R. China 2 Department of Chemistry and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, Lincoln, Nebraska 68588 3 Hefei National Laboratory for Physical Sciences at Microscale and Collaborative Innovation Center of Chemistry for Energy Materials, University of Science and Technology of China, Hefei, Anhui 230026, China *
[email protected], *
[email protected], *
[email protected] Abstract The stable structures of the reconstructed rutile TiO2 (011) surface are explored on the
basis
of
an
evolutionary
method.
In
addition
to
the
well-known
“brookite(001)-like” 2×1 reconstruction model, three 2×1 reconstruction structures are revealed for the first time, all being more stable in the high Ti-richness condition. Importantly, the predicted Ti4O4-2×1 surface model not only is in excellent agreement with the reconstructed metastable surface detected by Tao et al [Nat. Chem. 3, 296 (2011)] from their STM experiment, but also gives consistent formation mechanism and electronic structures with the measured surface. The computed imaginary part of the dielectric function suggests that the newly predicted reconstructed surfaces are capable of optical absorption over entire visible-light spectrum, thereby offering high potential for photocatalytic applications.
Keywords: Surface reconstruction, Rutile TiO2, First-principle Calculation, Electronic Structure, Photocatalysis
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Introduction Titanium dioxide has attracted extensive research interests because many of its special functionalities can be exploited in broad areas of applications such as photocatalysis1-7, hydrogen production from water8 or organic chemicals9, air and water purification,10 self-cleaning surface,11 oxygen gas sensor,12 dye-sensitized solar cells13, rechargeable batteries/supercapacitors,14 and biomedical devices15. In photocatalytic applications, for example, the ground-state electrons of TiO2 are excited via absorbing photon energy, followed by subsequent electron transfer and/or energy transfer to adsorbed molecules.1
Hence, the optical absorption efficiency is
critical to the use of TiO2 in photocatalysis. However, most TiO2 polymorphs with relatively wide bandgap (3.0 eV) can only absorb light in the ultraviolet region with the energy being only ~5% of the sunlight spectrum. Over the past few decades, many efforts have been made for reducing the bandgap of TiO2 in order to harvest a wider spectrum of sunlight. One common strategy is to dope TiO2 with impurities.16-18 It is known that the dopants not only can introduce donor or accept states in the band gap to reduce the band gap of TiO2, but also can serve as the charge carrier trapping or recombination sites. Although the strategy of doping TiO2 has led to success in achieving the visible-light activity, the photocatalytic efficiency of TiO2 is still downgraded. In recent years, the self-structural modification of surfaces has received increasing attentions because the dangling bonds on the surfaces can introduce different states within the band gap while having little influence on the charge carriers in the bulk part. Ariga et al reported photo-oxidation, within the visible-light spectrum, on pure rutile TiO2(001) surface in 2009.19 Two years later, a metastable rutile TiO2 (011) 2×1 reconstructed surface was reported by Tao et al, which has an optical band gap of ~ 2.1 eV.20 More recently, Dette et al reported a new anatase TiO2 (101) surface with a reduced band gap of ~2 eV and high chemical reactivity.21 These experimental achievements imply that some metastable reconstructed surfaces of TiO2 may possess even better photocatalytic properties than the bulk. 2
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To dates, although extensive studies have been devoted on the clean surfaces of rutile and anatase TiO2, only a few stably reconstructed surfaces have been observed in experiments. For anatase, the most studied reconstructed surface is the (001) surface which exhibits the 1×4 reconstruction.22 For rutile, reconstruction of the (110) surface has been extensively studied both experimentally and theoretically.23 The reconstructions of (1×1),24-25 single-linked (1×2),23,
26
cross-linked (1×2),27-28 and
pseudohexagonal rosette structures29 have been observed, and several structural models, e.g. Ti2O3-1×2 model, 26 Ti2O-1×2 model, 24,25 Ti3O6-1×2 model, 28 have been proposed theoretically. However, consistent conclusion has yet to be achieved. Until recently, Wang et al confirmed that the previously proposed Ti2O3-1×2 and Ti2O-1×2 models, as well as two newly predicted Ti3O2-1×2 and Ti3O3-2×1 reconstructions, are stable based on extensive search using the evolutionary method.30 More recently, the studies of reconstruction of the rutile (011) surface become more active due to suggested higher photocatalytic activity. The most favorable reconstruction of the rutile (011) surface is the (2×1) reconstruction whose STM image shows zigzag bright spots.31-34 Several models, e.g. titanyl model,31 microfaceting missing-row structural model,33 and “brookite(001)-like” model,34-35 are proposed for the rutile (011) (2×1) reconstruction. The “brookite(001)-like” model proposed by Torrelles et al Gong et al
34
35
and
are proven to be the most energetically stable, consistent with the
experimental results. Besides the most stable “brookite(001)-like” rutile (011) (2×1) reconstruction surface, the metastable reconstruction reported by Tao et al is of particular interest because its band gap matches the visible light and thus possibly gives high reactivity.20 The obtained STM image shows a (distorted) hexagonal arrangement of bright spots, not predicted by any structural models proposed previously. Very recently, Wang et al suggested that the titanyl model is possibly the structure obtained by Tao et al, based on the simulated STM image and calculated band gap.36 However, their results may be not sufficiently accurate due to the use of asymmetric models with only four TiO2 layers while the calculated relative surface energies and electronic structures seemed strongly dependent on the model symmetry, the thickness of the slabs, the existence of the fixed atoms and the dangling bonds on 3
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the bottom atoms.33 The inconsistency in the orders of surface energies between those computed by Wang et al using the asymmetric models and those computed by Gong et al using the symmetric models indicates that the using of asymmetric models for the TiO2 surfaces is not the best choice.34 So, the structure of the metastable reconstructed surface obtained by Tao et al is still unresolved.
Clearly, knowledge of
the atomic structure of this new metastable reconstructed surface is of importance both for fundamental understanding of its photocatalytic reactivity and for future applications. In this work, we performed an extensive search for the stable and metastable reconstructions of rutile (011) surface using evolutionary method. From our search, besides the “brookite(001)-like” model, three
new 2×1 reconstruction structures are
revealed, which are more stable in the high Ti-richness condition. The predicted Ti4O4-2×1 model is in good agreement with the metastable surface revealed by Tao et al in their experiment,20 particularly in the arrangement of surface atoms, simulated STM image, and computed electronic structure, indicating it is most likely be the metastable surface found in experiment. The computed imaginary part of the dielectric function shows that the Ti4O4-2×1 model can absorb sunlight over nearly all the wavelengths, thereby offering high potential for photocatalytic applications.
Computational method The search for the stable and metastable structures of the rutile (011) surface is performed using the evolutionary algorithm implemented in the USPEX package.37-40 The algorithm has been shown by many previous researchers to be very effective in predicting the most stable structures of bulk crystals,39, 41-42 nanostructures,37 as well as solid surfaces30, 40. For the surface structure search, each candidate surface structure is divided into the vacuum, surface region, and substrate, and only the surface region is optimized. Here we performed two distinct searches, separately, so that we can confirm the most stable and metastable rutile TiO2 (011) reconstruction surfaces observed experimentally, while predict some new stable or metastable surface 4
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structures. The first one is fixed-cell search and the second one is variable-cell search, for both the compositions are allowed to vary. For the fixed-cell search, only the (2×1) reconstruction is considered as only the (2×1) reconstruction is reported from experiments. Each supercell contains a vacuum layer of 10-12 Å, a surface region with maximally four Ti atoms and eight O atoms, and a substrate layer of three TiO2 layers. The atoms in the surface region and those in the top region of 2.5 Å of the substrate are fully relaxed. 100 populations are generated randomly in the initial generation, and then 30 populations are generated according to the evolution operations in the following generations. Mostly, 30 generations and more than 1000 structures are explored in the entire fixed-cell search. For the variable-cell search, the 1×1, 1×2 and 2×1 supercells are considered, and mostly two Ti and four O atoms are added in the surface region per surface unit cell. Different from the fixed-cell search, 200 populations and 40 populations are generated for the initial generation and the following generations, respectively, and mostly 40 generations are explored. Two different substrate structures are employed in our search to assure non-missing of any low-energy reconstructed surfaces. One is the cleaved slab with the top surface terminated with O atoms while the bottom surface terminated with Ti atoms. Another one is the unreconstructed rutile (011) surface with the same top and bottom surfaces. Each structure is relaxed using the first-principles method at medium accuracy level, and the low-energy structures are collected for ensuing high-level optimization and detailed analysis. To obtain accurate relative formation energies, a symmetric slab containing 11 TiO2 layers for the lowest-energy structure of each stoichiometry is made for structural relaxation without any constraints, followed by electronic structural calculations. The spin-polarized density-functional theory (DFT) computations for structural relaxation and electronic structural calculation are performed using the VASP package43-44. The Perdew-Burke-Enzerhof (PBE)45 exchange-correlation functional within the generalized gradient approximation is adopted. An all-electron plane-wave basis set with an energy cutoff of 450 eV is used. The projector augment wave (PAW) pseudopotentials are chosen to describe the interactions of elements, where the 5
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valence configurations of O and Ti are 2s22p4 and 3p63d24s2, respectively. A dense K-point sampling with the grid spacing less than 2π × 0.04 Å-1 in the Brillouin zone is selected for the structural relaxation. The ionic positions are fully relaxed until the residual force acting on each ion is less than 0.01 eV/Å. In the electronic computation, a more dense K-point sampling with the grid spacing less than 2π × 0.02 Å-1 is selected together with the GGA+U (U = 4.1 eV)30 method.
Results and Discussion The relative stabilities of different surface structures can be evaluated according to the formation energies given by the following formula:
Eformation = where Etot
1 [ Etot − nO µO − nTi µTi ] N
(1)
is the total energies of the surface under consideration ; nO and nTi are
the number of O and Ti atoms in the supercell; µO and µTi are the chemical potential of O and Ti, respectively; N is the number of surface unit cells in the supercell.
Chemical
potentials
(
µO
and
µTi
)
have
the
1 µO , µTi ≤ µTibulk and µTi + 2 µO = ETiO2 , in which ETiO2 is the 2 2 internal energy of the bulk rutile TiO2 unit cell. Eq. (1) can be converted to 1 Eformation = Etot − nTi ETiO2 − µO ( nO − 2nTi ) (2) N
constraints: µO ≤
According to the constraints of the chemical potentials, the permitted range of µO is
(
)
1 1 ETiO2 − µ Tibulk ≤ µO ≤ µO2 . 2 2
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Fig. 1 Computed formation energies of stoichiometric surfaces vs. chemical potentials of O.
The computed formations energies of different stoichiometric surfaces with respect to the chemical potential of O are shown in Fig. 1. Clearly, within most of the permitted range of µO ( µO > -8.94 eV), the previously proposed “brookite(001)-like” (2×1) reconstruction model possesses the lowest formation energies, suggesting its highest stability and likelihood to be formed. The unreconstructed surface is less stable with 0.76 eV higher in formation energy, compared to the “brookite(001)-like” model. The titanyl model proposed by Beck et al
31
is less stable than the
unreconstructed surface and the “brookite(001)-like” model, with 0.30 eV and 1.06 eV higher in formation energy, respectively. The energetic order of these three stoichiometric TiO2 surfaces from our calculation is the same as that calculated by Gong et al 34 but different from that obtained by Wang et al, which indicates that the slab model adopted in the calculation can have significant influence on the results for TiO2 surfaces. Besides the “brookite(001)-like” model, four additional stable structures close to the low limit of µO , denoted as Ti4O4-2×1, Ti2O2-2×1, Ti4O7-2×1 and MF(111)-TiO, respectively, also exist. The MF(111)-TiO structure is obtained by Wang et al from their extensive search (also found in our search),36 which is the most energetically stable for µO < ~ -9.0 eV based on our calculation. Ti2O2-2×1 has the same surface stoichiometry as MF(111)-TiO model but with different structure. Its formation energy is slightly higher (0.16 eV higher) than that of the MF(111)-TiO model. The Ti4O7-2×1 is also more stable than the “brookite(001)-like” model for µO 7
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< ~ -9.0 eV,
but a little less stable than MF(111)-TiO. It is the lowest-energy
metastable structure within the range of (-9.32 eV, -9.00 eV) of µO . The Ti4O4-2×1 structural model is energetically more stable than the “brookite(001)-like” model when µO is < -9.28 eV, and becomes the lowest-energy metastable surface for µO < -9.44 eV.
Within the range of (-9.32, -9.44) of µO , Ti2O2-2×1 is energetically more
stable than Ti4O4-2×1 and Ti4O7-2×1 models. Although the Ti4O4-2×1, Ti2O2-2×1 and Ti4O3-2×1 models are energetically less stable than the MF(111)-TiO model, the energy differences among them are quite small ( ∆Eformation
