Strong Influence of Oxygen Vacancy Location on Charge Carrier

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Cite This: J. Phys. Chem. Lett. 2019, 10, 2676−2683

Strong Influence of Oxygen Vacancy Location on Charge Carrier Losses in Reduced TiO2 Nanoparticles Yeonsig Nam,†,‡ Linqiu Li,‡ Jin Yong Lee,*,† and Oleg V. Prezhdo*,‡ †

Department of Chemistry, Sungkyunkwan University, Suwon 16419, Korea Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States

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S Supporting Information *

ABSTRACT: Oxygen vacancies in TiO2 nanoparticles are important for charge carrier dynamics, with recent studies reporting contradictory results on TiO2 nanoparticle photocatalytic activity. We demonstrate that ground state multiplicity, defect levels, and formation energies depend strongly on vacancy location. Quantum dynamics simulations show that charges are trapped within several picoseconds and recombine over a broad range of time scales from tens of picoseconds to nanoseconds. Specifically, nanoparticles with missing partially coordinated surface oxygens showed fast recombination, while nanoparticles with missing highly coordinated subsurface oxygens or singly coordinated oxygens at tips showed slow recombination, even slower than in the pristine system. The results are rationalized by energy gaps and electron−hole localization, the latter determining nonadiabatic coupling and quantum coherence time. The diverse charge recombination scenarios revealed by the nonadiabatic dynamics simulations rationalize the contradictory experimental results for photocatalytic activity and provide guidelines for rational design of nanoscale metal oxides for solar energy harvesting and utilization.

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light.12 The formation of Ti3+ by reduction of TiO2 was observed in many experimental studies employing photoelectron spectroscopy,12 electron paramagnetic resonance,13 and shifts in the core-level binding energies.14 Theoretical investigations15,16 confirmed the experimental findings. It has been known that reduction of the band gap does not necessarily lead to enhancement of the photocatalytic activity.17 This is because the generated e−h pairs can recombine in the middle of a photocatalytic process, undermining the photocatalytic activity. In particular, it has been reported that defects can act as charge recombination sites, accelerating electron and hole losses. These findings emphasize the importance of studying charge carrier recombination dynamics and band gap engineering. Recent experimental papers investigated the dynamics of photogenerated electrons and holes using femtosecond time-resolved diffuse reflectance spectroscopy18,19 and time-resolved photoluminescence.20 However, the results reported for the electron−hole recombination dynamics of reduced TiO2 NPs are controversial. Some studies21,22 demonstrated that an Ov induced deep trap states around 1.2 eV below the CBM and that these states were efficient charge recombination centers due to strong interaction with both conduction and valence bands. In contrast, other studies20,23 reported that surface trap states resulting from Ov could localize electrons and holes,

itanium dioxide (TiO2) has been an important semiconductor in numerous studies aimed at various applications, such as solar cells,1,2 environmental cleanup,3 and photocatalysis.4,5 In particular, the use of TiO2 in photocatalytic water splitting and degradation of organic pollutants has attracted significant interest. The multiple advantages of TiO2 over other materials include its low cost, nontoxicity, and stability. A photocatalytic process consists of three steps. (1) Upon absorption of light with energy equal to or greater than the TiO2 band gap, an electron is excited from the valence band (VB) to the conduction band (CB), generating an electron−hole (e−h) pair. (2) The e−h pair dissociates into free charge carriers, which diffuse to the surface. (3) An electron or a hole is transferred to an adsorbed molecule, e.g., water or organic species, initiating a chemical reaction. The photocatalytic activity of TiO2 is considerably limited by its intrinsic large band gap6 (>3.0 eV), which restricts light absorption to the ultraviolet region, hindering exciton generation. To enhance absorption of visible light, many strategies, such as native defects,7 chemical doping,8 noble metal deposition,9 and nanostructuring,10 have been suggested. Oxygen vacancies (Ov) in particular, being native defects, can play an important role in visible light absorption.11 One neutral oxygen vacancy provides two excess electrons, resulting in the reduction of Ti4+ to Ti3+. The defect states appear around 0.8− 1.0 eV below the conduction band minimum (CBM). The large band gap of TiO2 nanoparticles (NPs) is reduced to an optimal gap of 2.2−2.4 eV, enhancing utilization of visible © 2019 American Chemical Society

Received: April 8, 2019 Accepted: May 8, 2019 Published: May 8, 2019 2676

DOI: 10.1021/acs.jpclett.9b00987 J. Phys. Chem. Lett. 2019, 10, 2676−2683

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The Journal of Physical Chemistry Letters

bipyramidal anatase NPs constructed on the basis of the Wulff method.28 This bipyramidal atomic structures were suggested by Barnard et al.,29 who considered relative stabilities of (TiO2)n anatase particles exhibiting the lowest-energy surfaces, eight (101) facets, with n up to 455 units, using a selfconsistent tight-binding approach. The choice was further supported by density functional theory (DFT) calculations16,30 using various functionals such as PBE, PBE0, and B3LYP with all-electron basis sets. We used the most stable structure among the two isomers30 of (TiO2)35. The pristine and defective structures remained stable during our MD simulation at 300 K. Thus, these structures should provide a faithful representation of the geometric and electronic properties of experimentally synthesized NPs used in photochemical reactions catalyzed by TiO2. The properties of Ov in (TiO2)35 were investigated by removing one neutral oxygen atom from the total of 70 oxygen atoms. Among them, there are 25 distinct oxygen atoms that cannot be superimped by symmetry operations. Figure 1 shows

leading to electron−hole separation and long-lived excited states. These results indicated that all defects may not be equally detrimental to photocatalytic performance. Rather, certain defect types could be photocatalytically beneficial. Synthesized nanoscale TiO2 contains mixed phases, strained lattices, and interfaces and is abound with Ov defects that are distributed over various sites and exhibit site-dependent properties. Therefore, it is especially important to investigate charge carrier dynamics in reduced TiO2 depending on the Ov location. Recent studies of TiO2 NPs provide clear evidence that various types of oxygen vacancies exist in differing locations and coordination numbers.16,21 It is difficult to investigate the effects of Ov with experimental approaches, because it is impossible to achieve full synthetic control over NP size and shape,23 as well as the location and concentration of Ov. Also, it is difficult to disentangle the contribution of Ov from that of dopants, impurities, etc. Many theoretical studies have been dedicated to investigation of structural and electronic properties of reduced TiO2 in bulk,24 surface, and subsurface25 structures, providing important information. At the same time, bulk or surface models cannot reflect the properties of Ov in synthesized NPs. Isolated (TiO2)n clusters (n < 17) have been investigated,26 and their geometric and electronic properties have been compared with those of bulk phases. However, the sizes of such clusters are too small to explain the experimental observations, because such small clusters cannot maintain (101) facets observed in the nanosized anatase particles. Recently, Kim et al.16 reported that the bipyramidal (TiO2)35 NP is a proper model for studying the physical and electronic properties of oxygen vacancies, because it retains (101) facets of anatase after geometry relaxation and because its size (∼2 nm) is appropriate for rationalizing the experimental results for TiO2 NPs. They reported the existence of various kinds of oxygen defects depending on the domain and the coordination number of the oxygen atom. As an example, they found onecoordinated oxygen atoms that were not observed in the previous bulk and surface models. The results showed that the ground state multiplicity, vacancy formation energy, and defect energy levels were significantly affected by the location of the defect sites. Thus, charge carrier dynamics studies of the reduced bipyramidal (TiO2)35 model can reveal important properties that have not been observed in other bulk and surface models and can rationalize the contradictory experimental results. In this Letter, we investigate the bipyramidal (TiO2)35 cluster model and classify various oxygen vacancy sites by considering the vacancy domain, coordination number, and distance from the NP center. We show that nonradiative charge recombination in systems with defects inside the NP or at the NP tip is significantly slowed due to strong localization of electrons and holes. In comparison, defects located at NP facets increase the recombination rate due to greater nonadiabatic coupling and additional charge recombination channels. We show that the variations in the charge recombination mechanism originate from differences in the intrinsic properties of the oxygen defects in various NP regions. The quantum dynamics simulation results are rationalized by analyzing transition energies, nonadiabatic coupling, and quantum coherence times. To investigate the influence of Ov on the electron−hole recombination dynamics in TiO2 NPs, we used the (TiO2)35 anatase cluster, known as the minimum size model16,27 among

Figure 1. Schematic of different oxygen vacancy domains (left) and four representative structures for the reduced TiO2 NPs with a single oxygen vacancy at various sites (right). Yellow shows the charge density of defect states with an isosurface value of 0.001 e au−3, demonstrating the strong influence of the location of the oxygen vacancy on charge localization. Blue and red spheres denote titanium and oxygen atoms, respectively.

the four vacancy domains [left: tip (T), facet (F), edge (E), and inside (I)] and the four representative structures (right) for the reduced TiO2 NPs (Ti35O69). We named each defect site as Xm-n, where X designates the type of domain, m reflects the coordination number (CN) of the removed oxygen, and n is sequentially numbered by the distance of the removed oxygen from the center of the NP. For example, F2-3 represents the oxygen vacancy site at the facet domain (F) coordinated with two Ti atoms (CN = 2) and third nearest to the center of mass. We examined a total of 25 oxygen vacancy sites to investigate their defect state energy levels, Ov formation energy (EOf ), and ground state multiplicity, as shown in Figure S1 and Table S1. For triplet ground state (TGS) systems, two defect energy levels appeared around 0.1−0.8 eV below the LUMO. We denote these defect levels as D1 and D2, where D1 denotes the shallower trap closer to LUMO and D2 the deeper trap closer to HOMO of the pristine NP. For singlet ground state (SGS) systems, only one defect energy level appeared, denoted as D1. Most of the reduced TiO2 NPs showed a 2677

DOI: 10.1021/acs.jpclett.9b00987 J. Phys. Chem. Lett. 2019, 10, 2676−2683

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The Journal of Physical Chemistry Letters

Figure 2. Density of states (DOS) for the reduced and pristine TiO2 NPs. Black, blue, and red lines represent the total DOS, partial DOS of Ti, and partial DOS of O, respectively. The black dotted rectangles highlight the states originating from the oxygen vacancies, and the green rectangles indicate states originating from the singly coordinated oxygen atoms.

Figure 3. Charge densities of HOMO to LUMO (+1) orbitals in the reduced (TiO2)35 nanoparticles. The isosurface value is 0.001 e au−3.

site (3.08 eV) because it showed the lowest EOf . This is in good accordance with the experimental observation for well-defined surfaces of anatase31 that the Ov site is favorable in the threecoordinated domain located at the subsurface region. The all-

preference for the spin-unrestricted triplet ground state over the closed-shell singlet state. The positive value of EOf of all reduced TiO2 NPs indicates that additional energy is required to produce Ov. Among them, I3-2 was the most favorable Ov 2678

DOI: 10.1021/acs.jpclett.9b00987 J. Phys. Chem. Lett. 2019, 10, 2676−2683

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Figure 4. Electronic energy levels involved in the charge carrier trapping and recombination dynamics for (a) triplet ground state (TGS) NPs and (b) singlet ground state (SGS) NPs. HT, ET, ER, and RC refer to hole trapping, electron trapping, electron relaxation, and recombination, respectively.

both electron and hole traps significantly affects the charge carrier dynamics. The electronic structure of the reduced TiO2 NPs was validated further by PBE+U calculations. The energies of the key orbitals were similar for PBE and PBE+U (Table S2). The energy gaps, such as ΔELUMO−HOMO, ΔELUMO−D2, and ΔELUMO−D1, changed little. The band gap in the pristine system increased from 2.654 to 2.932 eV, while the band gaps for the reduced TiO2 NPs remained almost the same. The charge densities of the key states calculated with PBE+U also demonstrate that the results are qualitatively the same (Figure S2). We also validated the size of the vacuum layer. The electronic structure calculated with the larger layers of 15, 20, and 30 Å showed similar trends. As an example, the energies of the key orbitals for I3-1 decreased uniformly with increasing vacuum (Table S3). However, the energy differences between the orbitals showed only slight changes: 2.532 (10 Å), 2.528 (15 Å), 2.522 (20 Å), and 2.541 (30 Å) for ΔELUMO−HOMO. These results and the charge densities of the key states at 30 Å of vacuum (Figure S3) demonstrate that spurious interactions between periodic images are negligible already with the 10 Å vacuum. Next, we investigated nonradiative electron−hole recombination by performing nonadiabatic molecular dynamics (NAMD) simulations within the framework of real-time time-dependent density functional theory (TDDFT) in the Kohn−Sham formulation.35 The light electrons were treated quantum mechanically, using ab initio DFT, while the heavier and slower nuclei were treated (semi)classically. Decoherence induced surface hopping36 (DISH) was used. Decoherence is induced by coupling of electrons to quantum phonons and is essential for modeling the slow charge recombination that involves quantum transitions across large energy gaps. The methodology has been applied successfully to the excited state dynamics in a broad range of materials, such as perovskites,37−39 semiconductor quantum dots,40 graphene,41 carbon nanotubes,42 two-dimensional materials,34,43−45 metallic nanoclusters,46 and other systems.47,48 We considered excitation of an electron from HOMO to LUMO (LUMO+1 for SGS). For TGS, the hole could relax from HOMO to the hole trap states (S1 and S2), and the electron could relax from LUMO to the electron trap states

electron DFT calculations performed with a hybrid functional lead to the same conclusion.16 However, the modest energy difference between the smallest (I3-2, 3.08 eV) and the largest (F3-8, 5.31 eV) EOf values clearly shows that various types of Ov can coexist in synthesized TiO2 NPs, exhibiting a range of electron−hole recombination scenarios. Thus, we classified the 25 Ov models into seven groups considering ground state multiplicity and defect energy levels. We selected seven models that showed the lowest EOf value for each group: T1-1, I3-1, I32, I3-4, F3-10, F2-1, and F2-3. Among them, T1-1, I3-1, I3-2, and I3-4 are TGS systems, while F3-10, F2-1, and F2-3 are SGS systems. Overall, there are 18 TGS and 7 SGS among the total of 25 distinct Ov structures. The density of states (DOS) shown in Figure 2 demonstrates that the HOMO arises mainly from atomic orbitals of O atoms and the LUMO from Ti atoms, which is generally known.32 The defect energy levels are marked with black dotted rectangles. The higher defect level D1 is very close to the LUMO (14 meV). However, the direct recombination between a hole in D1 and an electron in the LUMO could occur competitively with HT3, providing an additional recombination channel. The NAC for RC2 was comparable to that for HT3. For example, the NACs for HT3 in F3-10, F2-1, and F2-3 were 4.980, 2.862, and 2.272 meV, respectively, while those of RC2 were 1.761, 2.767, and 2.974 meV, respectively. The strong dependence of the charge recombination mechanisms on the defect site is an entirely new finding that has not been reported in the previous studies of TiO2 NPs. The energy gap between D1 and LUMO is small in SGS, such that the e−h pair can even be generated thermally at room temperature. Because of the small gap, SGS showed decreased carrier lifetimes of tens of picoseconds.20,32 Specifically, F3-10, F2-1, and F2-3 showed values of 29.83, 33.24, and 43.55 ps, respectively (Table 1). Interestingly, F2-1 showed a lifetime that was longer than that of F3-10 despite a smaller transition energy and a shorter pure-dephasing time for the e−h recombination between D1 and the LUMO. This is because F2-1 showed a smaller NAC for HT3 (2.862 meV) than for F3-10 (4.98 meV) and a larger average transition energy for the HT3 transition, which had a much longer time (23.71 ps vs 9.68 ps). The results showed that charge recombination in both TGS and SGS is governed by coupling between S2 and defect states, which correspond to RC for TGS and HT3 for SGS. TGS showed an average NAC (0.667 meV) much smaller than that

(D1 and D2) (Figure 4a). For SGS, the hole could relax from HOMO to D1 through S1 and S2, and the electron could relax from LUMO+1 to LUMO (Figure 4b). Note that D1 in SGS acts as a hole trap because it is located below the Fermi energy. Ultimately, the electron and hole recombined, and the ground state population increased with time. The charge carrier dynamics processes denoted as HT, ET, ER, and RC refer to hole trapping, electron trapping, electron relaxation, and recombination, respectively. For example, HT1 refers to hole trapping from HOMO to S1, HT2 refers to hole moving from S1 to S2, and HT3 denotes hole transition from S2 to D1. We present 10 ps of population evolution of the key states involved in the charge trapping and recombination dynamics (Figure S4 and Table S4). The time scales of the trapping processes were obtained by fitting with the single-exponential expression P(t) = A exp(−x/t) + C, separately for rise and decay. The fitting is motivated by the exponential behavior observed in experiments.18,20,21 The reported trapping times are the sums of the rise and decay times. The growth of the ground state population was fitted with the equation P(t) = 1 − exp(t/τ) ≈ t/τ to obtain the charge recombination time τ, as shown separately in Figure S5. The key properties governing the excited state dynamics for each state are listed in Table S5, including the average transition energy, nonadiabatic coupling (NAC), and pure-dephasing time. Generally, a larger NAC and a smaller transition energy favor faster nonradiative relaxation based on Fermi’s golden rule and the energy gap law. Loss of quantum coherence, represented by the pure-dephasing time, slows quantum dynamics. We estimated the pure-dephasing time by fitting the pure-dephasing functions (Figure S6) with a Gaussian. The charge carrier trapping and recombination dynamics of TGS and SGS showed different mechanisms. Charge recombination occurred in TGS between the electron trapped in D2 and the hole in S2. Specifically, the photogenerated hole was rapidly, within 1 ps, trapped from HOMO to S1 and then S1 to S2. This can be explained by the larger NAC for HT1 and HT2 compared to the other transitions (Table S6). For example, the NAC for HT1 in I3-1 (22.64 meV) was much larger than for the direct electron−hole recombination between HOMO and LUMO (1.593 meV), HOMO and D1 (1.785 meV), and HOMO and D2 (1.560 meV) as well as direct hole trapping from HOMO to S2 (3.261 meV). Similar arguments can be applied to hole trapping in other systems and electron trapping in all TGS (Table S6). The electron was trapped from LUMO to D1, and then D2 on a longer, 7 ps time scale. The shallow trap states are strongly hybridized with the HOMO and LUMO, resulting in greater wave function overlap and a larger NAC compared to those of the deep trap states. The fast trapping of both electrons and holes within 2680

DOI: 10.1021/acs.jpclett.9b00987 J. Phys. Chem. Lett. 2019, 10, 2676−2683

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The Journal of Physical Chemistry Letters of SGS (3.371 meV). Valentin et al.49 and Carey et al.50 reported that electrons and holes can be more easily localized at Ti and O atoms with a lower coordination number. Our system contained one six-coordinated Ti atom (Ti6c) in the subsurface region and several Ti5c and Ti4c atoms at edges, facets, and tips on the surface. Oxygen atoms in the subsurface region were all triply coordinated (O3c). Facets contained several O2c and O3c atoms; edges contained O2c, and tips had O1c. Figure 3 demonstrates that hole densities are highly localized at the tip O1c. Therefore, it would be easier for the hole to recombine with electrons localized at nearby facets or edges rather than inside the particle. Comparing localizations of electrons in TGS and SGS, we observe that SGS have electrons at the surface, while electrons are localized more inside the bulk in TGS. Because the distance between the trapped electron and hole is larger in TGS, they exhibit smaller NACs and slower charge recombination. Interestingly, T1-1 shows the smallest NAC even though the electron density is localized on Ti3c. Removal of the oxygen atom at the NP tip produced the longest distance between the trapped electron and hole. Recently, Sinhamahapatra et al.19 and Yamanaka et al.18 reported separately that an optimal concentration of Ov exists for achieving the highest photocatalytic activity and that above the optimal concentration the activity decreases. It may be possible to rationalize these findings by the proximity of Ov defects at high concentrations. An excess of Ov induces many defect levels. The energy gaps between defect states and the HOMO/LUMO as well as between different defect states become small, resulting in a large overlap of corresponding charge densities, increased NACs, and faster charge recombination. Therefore, our findings suggest that control over Ov defect sites is needed to enhance the photocatalytic activity of TiO2 NPs. In conclusion, investigated charge carrier dynamics of reduced TiO2 NPs using nonadiabatic molecular dynamics and real-time time-dependent density functional theory. We examined 25 oxygen vacancy sites and classified them into seven groups, based on ground state multiplicity, formation energy, and defect energy level. We demonstrated that the level of charge recombination in systems with defect sites inside the NP and at the NP tips decreased compared to that of the pristine system due to small nonadiabatic coupling, resulting from highly localized electrons and holes. In contrast, charge recombination in systems with defect sites at NP facets increased due to enhanced nonadiabatic coupling, as a result of the short electron−hole distance, small energy gaps, and multiple charge recombination channels. Our simulations explain the contradictory experimental results on the charge carrier dynamics of reduced TiO2 NPs and suggest that control over oxygen vacancy sites can greatly enhance the photocatalytic activity. The detailed insights into the energy relaxation and charge recombination processes are valuable for investigating the photocatalytic activity of nanoscale metal oxides for solar energy harvesting and utilization.

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Simulation details; orbital energy levels; formation energies; ground state multiplicities; charge carrier trapping and recombination dynamics; fitting parameters; average transition energies, nonadiabatic couplings, pure-dephasing times, and nonradiative trapping and recombination times; phonon influence spectra; puredephasing functions; and evolution of ground state populations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +82-31-299-4560. Fax: +82-31-290-7075. E-mail: [email protected]. *Phone: +1-213-821-3116. Fax: +1-213-740-2701. E-mail: [email protected]. ORCID

Jin Yong Lee: 0000-0003-0360-5059 Oleg V. Prezhdo: 0000-0002-5140-7500 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.N. and J.Y.L. acknowledge support by the POSCO Science Fellowship of the POSCO TJ Park Foundation and the National Research Foundation (NRF) grant funded by the Korean government (2016R1A2B4012337). The authors are grateful to the KISTI supercomputing center for support through the strategic program for the supercomputing application research (KSC-2018-CHA-0025). L.L. and O.V.P. acknowledge funding of the U.S. Department of Energy (Grant DE-SC0014429).

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REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b00987. 2681

DOI: 10.1021/acs.jpclett.9b00987 J. Phys. Chem. Lett. 2019, 10, 2676−2683

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DOI: 10.1021/acs.jpclett.9b00987 J. Phys. Chem. Lett. 2019, 10, 2676−2683

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DOI: 10.1021/acs.jpclett.9b00987 J. Phys. Chem. Lett. 2019, 10, 2676−2683