Combining High Photocatalytic Activity and Stability via Subsurface

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Combining High Photocatalytic Activity and Stability via Subsurface Defect in TiO

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Yan Liu, Qing Zhu, Xiyu Li, Guozhen Zhang, Yudan Liu, Shaobin Tang, Edward Sharman, Jun Jiang, and Yi Luo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04037 • Publication Date (Web): 08 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018

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The Journal of Physical Chemistry

Combining High Photocatalytic Activity and Stability via Subsurface Defect in TiO2 ‖

Yan Liu†§, Qing Zhu†§, Xiyu Li†, Guozhen Zhang†, Yudan Liu†, Shaobin Tang‡, Edward Sharman , Jun Jiang*†, Yi Luo† †

Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation

Center of Chemistry for Energy Materials), CAS Center for Excellence in Nanoscience, Department of Chemistry and Materials Science, University of Science and Technology of China (USTC), Hefei, Anhui 230026, China ‡

Key Laboratory of Organo-Pharmaceutical Chemistry of Jiangxi Province, Gannan Normal University,

Ganzhou 341000, China ‖

Department of Neurology, University of California, Irvine, California 92697, USA

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ABSTRACT: Surface defects consisting of oxygen vacancy in TiO2 provide sites with high photocatalytic activity, while their photocatalytic stability is often undermined because of the trapping of oxygen species. Here we conducted a theoretic study to propose the use of subsurface oxygen vacancies in anatase TiO2 (101) for photocatalytic water-splitting, aiming to combine both high activity and stability. This study demonstrates that subsurface defects expand light harvesting ability of TiO2 to visible light region, and facilitate photo-generated charge separation, these results were then verified by experimental. These vacancies also bestow high catalytic activity on sites of water adsorption and oxidation above them on the defect-free surface. Importantly, subsurface defects are inert toward reactive oxygen species, ensuring high stability for the nearby surface catalytic sites. These results demonstrate the role of subsurface oxygen vacancies in catalyzing oxygen evolution reaction, leading to new design strategy for photocatalytic or catalytic oxide materials.


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1. Introduction Sunlight-driven chemical transformation by metal oxide holds great promise for clean energy and sustainable technology development. Especially, Titanium dioxide (TiO2) based photocatalytic watersplitting have attracted great attentions.1-3 The practical utilization of TiO2 is hindered by low energy conversion efficiency, mainly due to wide energy band gaps (3.05 eV for rutile and 3.18 eV for anatase phase). Such wide gaps result in poor harvesting efficiencies because a large proportion of the energetic electron-hole pairs produced by solar light illumination are wasted due to their high charge recombination probability (~90%).2 Introduction of defects (especially, the oxygen vacancy) on oxide catalyst has been used to modify electronic structures, so as to obtain high photocatalytic activity of visible-light harvesting4, charge separation5, and chemical interactions with adsorbates.6-9 The impact of defects occurring in metal-oxide photocatalysis or catalysis is normally restricted to studies with oxygen vacancies on surface (O ), which serve as chemically active sites for photocharge collection and chemical adsorption/reaction.5-7However, the unsaturated bonding nature of O often causes poor stability during catalysis functioning. In water-splitting applications which involve the oxygen evolution reaction (OER), surface vacancies easily trap oxygen intermediates (hydroxyl and oxygen) and O2 products with high binding energies, so that these species then become hard to release them for further reactions. This not only reduces the generation of O2 products, but also destroys the oxygen defects, thus directly eliminate the surface active sites.10-12 Especially for nanoparticles of anatase TiO2—which holds higher photocatalytic activity than the rutile phase particles—surface defects quickly become clogged with oxygen species or evolve into bulk defect.10,13,14 However, anatase TiO2 also always contains many subsurface oxygen vacancy defects (O ) right below regions of defect-free surface, which may promote chemical activity of the nearby surface.14-17 For instance, it was found that O could steer photo-excited charges to near-surface region.10 Scanning tunneling microscopy (STM) measurements have found that O2 molecule exhibits good adsorption and ACS Paragon Plus Environment


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dissociation characteristics at surface sites near O in negatively-charged TiO2.15 Theoretical predictions show that O facilitates water molecule dissociation both directly and indirectly.16,18 Photoelectron spectra measurements have determined that O favors water adsorption on anatase TiO2 (101) surface.17 Importantly, since O defects are insensitive to reactive oxygen species during reactions, they are incapable of trapping oxygen species and thereby the active surface sites near O are preserved and the useful catalytic lifetime is substantially extended. In this work, we conducted a theoretical study on TiO2 with a subsurface oxygen defect O , to identify systems which combine the high photocatalytic activity with stability of surface sites near for sunlight-driven water-splitting. Our results show that surface oxygen vacancy defects are vulnerable to environmental oxygen, while subsurface defects hold high structural stability. The O induce impurity electronic states inside the energy band gap of pristine TiO2, extending the light harvesting ability of TiO2 into the visible light region. In addition, O collect photo-generated hole charges, a property which not only facilitates charge separation, but also promotes water-splitting reaction by tuning electronic structures around defective sites. These theoretical predictions were then verified by the results of experimental measurements. With extra charges in TiO2 to simulate photo-excited states, we found that water molecules are adsorbed strongly at defect-free surface sites near O , and that the subsequent oxidation reaction holds a low reaction energy barrier. The subsurface oxygen vacancies in TiO2 are stable active sites that exhibit high photocatalytic activity. 2. Computational methods First-principles calculations were carried out at the spin-polarized density functional theory (DFT) level using the VASP package.19 The Perdew, Burke, and Ernzerhof (PBE)20 exchange-correlation functional and the projector augmented-wave (PAW)21 potential were employed. The PBE+U method22 was applied to describe partially filled d-orbitals by considering coulomb and exchange corrections, with 3.5 eV U-correction to the Ti d-orbitals.23 An energy cutoff of 500 eV was used for the plane-wave ACS Paragon Plus Environment

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expansion of the electronic wave function. The Monkhorst-Pack24 sampling scheme of the Brillouin zone was adopted to generate 4×4×1 k-point grids. All geometric structures were optimized until the maximum force on each atom was reduced to less than 0.02eV/Å. The empirical correction method (DFT-D2) was used to describe the long-range van der Waals (vdW) interactions.25 TiO2/molecules interface was built using a slab model with periodic boundary condition (PBC) and a 25 Å vacuum region above a supercell surface. In addition, the frequency dependent dielectric functions were calculated to reflect the optical absorption properties of pristine TiO2 and TiO2− O (details in Supplementary Information). To calculate the stability of oxygen vacancies in TiO2, we defined the formation energies as:

( ) = + − ， where is the total energy of an O2 molecule, and and are the total energies of the TiO2 surface with and without oxygen vacancies, respectively. The adsorption energies of H2O and O2 on the anatase surface were calculated using () = + − @ , where Ex represents the total energies of H2O or O2, represents the total energies of the TiO2 surface, @ represents the total energies of the adsorbate/TiO2 system. Since the calculated adsorption energies of intermediate species (−O, −OH, −OOH) in OER are different from those of H2O and O2, here we list their definitions as:

() = + − @ ，

() = + − − @ ，

() = + − − @ where @ , @ , @ denote the total energies of the intermediates (−O, −OH, −OOH) adsorbing on the TiO2 surface. denotes the total energies of the TiO2 surface. , , and denote the total energies of the H2O, O2, H2 and H2O2 molecules, respectively. ACS Paragon Plus Environment


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3. Results and discussion 3.1 Structure and Stability of TiO2 with Different Oxygen Vacancies

Figure 1. Atomic structures of bulk anatase TiO2 (a), the TiO2 (101) surface (b), the TiO2 (101) surface with one surface oxygen vacancy TiO2−O (c), and the TiO2 (101) surface with one sub-surface oxygen vacancy TiO2−O (d), showing defect formation energy of 4.24 and 4.05 eV, respectively. The grey and red beads represent Ti and O atoms, and the green circles represent oxygen vacancies. ACS Paragon Plus Environment

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The optimized structure cell of anatase TiO2 consists of 4 Ti atoms and 8 O atoms with lattice parameters of a=b=3.894 Å and c=9.674 Å (Fig. 1a), agreeing well with reported experimental measurements.26 The (101) surface was found to be the most stable surface,27 and we have built a 3 3 1 supercell of a (101) surface with 36 Ti atoms and 72 O atoms (Fig. 1b). An oxygen vacancy (O ) on surface or subsurface was introduced by removing an oxygen atom from the supercell, resulting in TiO2−O and TiO2−O systems with a defect concentration of ~1.39% (Fig. 1c and d). The computed formation energies of various surface defects are 4.24~4.94 eV, while that of a subsurface defect is 4.05 eV, implying the fact that subsurface defects may be a bit more stable than surface defects (Fig. S1). Meanwhile, there is an energy barrier of ~3.46 eV for the evolution of one O to O (i.e. an oxygen atom at the defect-free surface migrates to fill the oxygen vacancy at the subsurface).

Figure 2. Top view of the geometrical structures for the surface of (a) pristine TiO2, (b) TiO2−O

,

and (c) TiO2−O . The surfaces show undercoordinated Ti5c atoms and O2c atoms. In the pristine TiO2,

all Ti5c sites are the same. The surface oxygen vacancy O

adsorption and reaction in TiO2−O

is the catalytic site for molecular

. There are four different Ti5c sites (1-4) in the surface of

TiO2−O . The grey and red beads represent Ti and O atoms, and the green circles represent oxygen vacancies.



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Table 1. The computed adsorption energies (Ea) of reactant H2O, intermediate species (−O, −OH, −OOH), and product O2 during the water-splitting OER process catalyzed by defect-free TiO2 (101), TiO2−O , and TiO2−O with surface stie 1-4 defined in Fig. 2. Structure TiO2

Site -

TiO2−O!"#$ !"%

TiO2−O

Site 1 Site 2 Site 3 Site 4

Ea(H2O) 1.10 1.30

Ea(O) -1.65 4.02

Ea(OH) -1.86 1.66

Ea(OOH) -0.80 1.69

Ea(O2) 0.85 3.73

1.10 1.09 1.15 0.92

1.45 1.36 1.34 1.80

0.38 0.34 0.35 0.48

0.60 0.51 0.51 0.66

1.60 1.22 1.20 1.24

All Ea values are in eV. We firstly examined in theory the stability of O and O . During water-splitting reaction, the TiO2 surface interacts cyclically with H2O reactants, intermediate oxygen species (−O, −OH, −OOH), and O2 products. The (101) surface is terminated with Ti-5c (five-coordinated Ti), Ti-6c, O-2c and O-3c atoms, among which Ti-5c atoms are found to be the most stable adsorption sites for reactant molecules. The surface of pristine TiO2, the O site in TiO2−O , and four different surface sites near O in TiO2−O (defined as site 1-4 in Fig. 2), readily adsorb H2O molecules with adsorption energy of 1.10, 1.30, and 0.92~1.15 eV, respectively (Table 1). In contrast, the adsorption energies of the intermediate oxygen species and O2 products (Fig. 3a) on the O site are high: 4.02 eV (−O), 1.66 eV (−OH), 1.69 eV (−OOH), 3.73 eV (O2). In these cases, either extra energy is needed to remove oxygen species from the O site, severely reducing the energy conversion efficiency, or the surface defect will be irreversibly occupied, resulting in loss of catalytic site. Fortunately, all normal surface sites near O in TiO2−O interact weakly to oxygen species (Fig. 3b and Fig. S2), with adsorption energies (Table 1) of 1.34~1.80 eV (−O), 0.34~0.48 eV (−OH), 0.51~0.66 eV (−OOH), 1.20~1.60 (O2). The 1.0~2.5 eV adsorption energies (low compared to those of surface defects), enable the TiO2− O system to maintain high catalytic stability during reaction. Meanwhile, the optimized atomic structures of O2 ACS Paragon Plus Environment

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adsorbed on TiO2−O (Right columns in Fig. 3b and Fig. S2 b, c, d) suggest that the high stability of subsurface oxygen vacancy defects prevents their migration to the surface.

Figure 3. The optimized atomic structures of those molecular groups involved in water splitting (H2O, −O, −OH, −OOH, O2) adsorbed onto the surface of TiO2−O (a) and TiO2−O (b) via site 1 as defined in Fig. 2, together with corresponding adsorption energies. The grey and red beads represent Ti and O atoms, and the green circles represent oxygen vacancies.

3.2 Photo-response of TiO2−O!"%



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Figure 4. The computed total density of states (TDOS) (a) and potential surfaces (b) of the anatase (101) facets of the pristine TiO2, TiO2−O , and TiO2−O structures, from which workfunction values are found to be 6.86, 5.35, 5.47 eV, respectively. (c) The simulated charge distribution associated with one extra/photogenerated hole (left) and electron (right) charges in the TiO2−O structure. The grey and red beads represent Ti and O atoms, the green circles


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represent oxygen vacancies, and the yellow and cyan bubbles delineate electron and hole charge densities, respectively. The photo-response of TiO2−O was examined next. As expected, an oxygen defect changes the electronic structure of TiO2. The computed total density of states (TDOS) in Fig. 4a shows that both O and O defects induce impurity electronic states inside the energy band gap of pristine TiO2. These substantially reduce the band gap values, extending the light harvesting ability of pristine TiO2 into the visible light region.28,29 The electronic potential surfaces were plotted in Fig. 4b; these surfaces determine the workfunction values of pristine TiO2, TiO2−O , and TiO2−O as 6.86, 5.35, and 5.47 eV, respectively. Both types of defects substantially lower the workfunction of pristine TiO2 surface by 1.4~1.5 eV. Given that electrons tend to flow from a material with a lower workfunction to that with a higher one, it is to be expected that charge separation would occur, so that charge due to photo-generated electrons accumulates at perfect surface sites, while hole charges accumulate separately at defect sites. Upon light irradiation, the spatial separation of photo-generated electron and hole charges holds the key to efficient photocatalysis.30 To understand how O may affect charge distribution, we examined charged systems with either extra electrons or with extra holes. We calculated the charge density differences between either a positively charged or negatively charged system and a neutral one, as shown separately in Fig. 4c. It was confirmed that the dominant charges around O are positive hole charges, while electrons tend to go everywhere except nearing O . Meanwhile, based on the computed adsorption energies of oxygen species on surface (Table S1), it is apparent that TiO2−O with hole charges in O would maintain stability during the reaction cycle.


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3.3 Catalytic Ability of TiO2−O!"%

Figure 5. (a) The energy potential maps on the top layer (surface) of TiO2−O at the neutral state and positively charged state (0.3 and 0.5 h+). Here the dotted red circles represent the position of O below the surface. (b) The energy band alignment of CBM (conduction band


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maximum) and VBM (valance band minimum) states for TiO2−O with 0 h+, 0.3 h+, and 0.5 h+ charges. (c) and (d) The schematic show of the Gibbs free energy changes for the four elementary OER steps catalyzed by the pristine TiO2(101) surface (c) and TiO2−O surface site 1 (d) as defined in Fig. 2, based on situations of 0 h+, 0.3 h+, and 0.5 h+ charges. (e) The computed overpotential (η) values for OER catalyzed by the pristine TiO2 (101) surface and TiO2−O surface sites. We then moved forward to investigate the catalytic function of the charged TiO2 after photoexcitation. Using 0.3, 0.5, 0.7 and 1.0 h+ to model the hole charging effect near O , we have simulated the electric potential map of the top surface layer of TiO2−O (Fig. 5a and Fig. S3a). The electric potential near O decreases substantially with increasing hole charges, confirming the accumulation of holes at oxygen vacancy. By calculating the TDOS and potential surfaces of the charged systems (Fig. S4), we have determined the energy band alignment (Fig. 5b and Fig. S3b). The conduction band minimum (CBM) and valence band maximum (VBM) were substantially lowered if hole charges were collected by O (shift of 0.3~1 h+ → 0.23 ~1.39 eV). Therefore, the combination of a defect-containing region and a defect-free region of TiO2 would form a z-scheme photocatalysis architecture in which the un-shifted CBM of defectfree TiO2 ensures H2 production while the very low VBM of TiO2−O promotes OER. As for the chemical reactions involved in water-splitting, Norskov and co-workers31,32 have proposed that the OER process may be decomposed into four steps [A to D]： *+ H2O → *OH + (H+ + e-)

(A)

*OH → *O + (H+ + e-)

(B)

*O + H2O → *OOH + (H+ + e-)

(C)


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*OOH → O2 + (H+ + e-)

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(D)

where * represents a surface site, and *x (x=O, OH, OOH) represents a species x adsorbed on the surface. The Gibbs free energy change (∆) ) of each of these reaction step was computed; based on these we deduced the overpotential (η) for OER (Details in Supplementary Information). As shown in Fig. 5c, the rate-limiting step of OER catalyzed by the pristine TiO2 surface is the first, *OH-generating step. The overpotential needed for neutral TiO2 is 1.26 eV (1.23 eV with 0.3 h+ and 0.5 h+), which is close to the potential (1.23 eV) required by water-splitting. We then studied the catalytic properties of the site 1 on the TiO2−O surface (Fig. 5d). For the neutral state, the first step of forming *OH needs only ~0.25 eV, which is significantly lower than the watersplitting potential (1.23 eV). On the other hand, formation of *OOH became the rate-limiting step. This resulted in the overpotential of 0.93 eV, which is more conducive to OER than in the case of pristine TiO2. Moreover, with 0.3 h+ and 0.5 h+ charges in the TiO2− O , the overpotentials are lowered to 0.67 and 0.71 eV, respectively. Similar results are found for OER via the other 3 surface sites near O (Fig. S5). Based on the free energies and overpotential values for all systems under investigation (Table S3), the subsurface defect can substantially lower the OER overpotential of pristine TiO2, and thereby promotes the chemical activity of nearby surface sites (Fig. 5e). 3.4 Experimental Results to Verify Photo-response of TiO2−O!"%


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Figure 6. The simulated (a) and experimental (b) UV-Vis absorption spectra of pristine TiO2 and TiO2−O . (c) The experimentally measured photocurrent response vs. time for pristine TiO2 and TiO2−O under chopped irradiation at a bias potential of 0.4 V vs. Ag/AgCl (λ ≥ 400 nm). To further verify the photo-response of TiO2−O , we prepared pristine TiO2 and TiO2 containing subsurface oxygen vacancies for comparison (Fig. S6 and details in Supplementary Information). Here we started with commercially available anatase samples of pristine TiO2， and used NaBH4 as reducing agent to introduce defects into the TiO2, as first reported by Tan. et. al33 to remove O atoms from the crystal lattice. The sample of TiO2−(O + O ) and TiO2−O remain stable in the presence of H2O and O2 when exposed to air. After the removal of surface defects by H2O2 treatment, the electron spin resonance (ESR in Fig. S6b) of TiO2−O sample shows only the strong signal of Ti3+ center (reflecting O ), indicating the removal of O and the presence of O (see details in the Supplementary Information Fig. S6). This suggests that molecules in air (particularly water and oxygen) cause no migration of O to O . The optical absorption spectra of both samples were characterized by UV–Vis absorption. In the measured photo-absorption spectra, the white, pristine TiO2 sample only responds to high-energy UV light, while the dark-blue sample of TiO2− O shows a considerably large absorption in the visible and NIR regions (Fig. 6a). As expected, the


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simulated absorption of TiO2−O also confirmed the improvement in harvesting visible light (Fig. 6b). Here our calculations underestimated the energy band gap of these materials due to a known limitation of DFT method, resulting in ~100 nm red-shifts in the simulated absorption spectra for both pristine and defect-containing TiO2 as compared to experimental values. The strong visible absorption was ascribed to electron transitions of impurity states from O . Subsequently, experimental photoelectrochemical measurements (Fig. 6c) revealed that TiO2−O generates an appreciably larger photocurrent than pristine TiO2. Therefore, these experimental results verify that O site are much stable than surface oxygen vacancies, which can easily be removed by oxygen species. Moreover, TiO2−O possesses improved visible light absorption, lower recombination of photogenerated charges and more efficient migration of energetic carriers. These properties can readily be explained by the simulated charge distributions where photo-generated electrons and holes in pristine TiO2 are uniformly distributed and can easily recombine (Fig. S7), while holes in TiO2− O are trapped by subsurface oxygen vacancies (O ) but electrons are distributed everywhere except near O . 4. Conclusion In summary, we have studied the photocatalytic water dissociation reaction on the defect-free surface near a subsurface oxygen vacancy defect O in TiO2 (101). It was demonstrated that the O defect holds similar high photocatalytic activity as the surface defect, and possesses benefits of efficient harvesting at visible-light wavelengths, promoting photo-generated charge separation, collecting hole charges, and a lowering of the OER reaction barrier. Importantly, the O are inert toward all molecular groups during each reaction step, so nearby defect-free surface sites maintain the same structural stability as the pristine TiO2 (101) surface when interacting with oxygen species. Hopefully, this new concept of combining the photocatalytic


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activity of subsurface defects with the structural stability of a defect-free oxide surface, can expand the utility of the subsurface oxygen vacancies in designing new systems of metal oxides used for photocatalytic/catalytic application. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Formation energies, Optimized structure of various relevant species for water splitting, Adsorption energy, Free energy, Energy potential, including Table S1−S4 and Figures S1−S8.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Jun Jiang: 0000-0002-6116-5605 §Author Contributions Yan, Liu and Qing, Zhu contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This work was financially supported by the MOST 973 Program (No. 2014CB848900), the National Natural Science Foundation of China (NSFC) (No. 21473166), Hefei Science Center CAS (2016HSC-IU012), CAS Key Research Program of Frontier Sciences (QYZDB-SSWSLH018), the Steady High Magnetic Field Facilities, High Magnetic Field Laboratory, CAS, and the Fundamental Research Funds for the Central Universities. REFERENCES (1) Fujishima, A.; Honda, K.Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38. (2) Ma, Y.; Wang, X.; Jia, Y.; Chen, X.; Han, H.; Li, C. Titanium Dioxide-Based Nanomaterials for Photocatalytic Fuel Generations. Chem. Rev. 2014, 114, 9987–10043. (3) Chen, X.; Liu, L.; Yu, P.; Mao, S. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746-750. (4) Wang, J.; Wang, Z.; Huang, B.; Ma, Y.; Liu, Y.; Qin. X.; Zhang, X.; Dai, Y. Oxygen Vacancy Induced Band-Gap Narrowing and Enhanced Visible Light Photocatalytic Activity of ZnO. ACS Appl. Mater. Interfaces 2012, 4, 4024-4030. (5) Zhang, N.; Li, X.; Ye, H.; Chen, S.; Ju, H.; Liu, D.; Lin, Y.; Ye, W.; Wang, C.; Xu, Q.; et al. Oxide Defect Engineering Enables to Couple Solar Energy into Oxygen Activation. J. Am. Chem. Soc. 2016, 138, 8928-8935. (6) Hussain, H.; Tocci, G.; Woolcot, T.; Torrelles, X.; Pang, C.; Humphrey, D.; Yim, C.; Grinter, D.; Cabailh, G.; Bikondoa, O.; et al. Structure of a Model TiO2 Photocatalytic Interface. Nat. Mater. 2017, 16, 461-466.


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Combining High Photocatalytic Activity and Stability via Subsurface

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