Increasing the Photocatalytic Activity of Anatase TiO2 through B, C

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Increasing the Photocatalytic Activity of Anatase TiO2 through B, C and N Doping Christopher L Muhich, Jay Y Westcott, Thomas F Fuerst, Alan W. Weimer, and Charles B. Musgrave J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 03 Nov 2014 Downloaded from http://pubs.acs.org on November 4, 2014

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The Journal of Physical Chemistry

Increasing the Photocatalytic Activity of Anatase TiO2 through B, C and N Doping Christopher L. Muhicha, Jay Y. Westcott IVa, Thomas Fuersta, Alan W. Weimera, and Charles B. Musgravea,b* a

Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80303, USA b

Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, USA *

Corresponding Author: Email address: [email protected]; Phone number: (303) 735-0411; Fax: (303) 492-8425; Address: 596 UCB, University of Colorado, Boulder; Boulder, CO 80309

ACS Paragon Plus Environment

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ABSTRACT We utilized density functional theory (DFT) to systematically investigate the ability of B, C and N interstitial and O substitutional surface and near-surface dopants in TiO2 to facilitate O2 reduction and adsorption. Periodic boundary condition calculations based on the PBE+U DFT functional show that dopants that create filled band gap states with energies higher than that of the near surface O2 πz* molecular orbital enable O2 adsorption and reduction. Sites that create unoccupied band gap states with energies below that of the O2 πz* orbital reduce TiO2’s reduction ability as these states result in photoexcited electrons with insufficient reduction potential to reduce O2. B dopants in interstitial and relaxed substitutional sites, whose gap states lie > 1.5 eV above the valence band maximum (VBM) and hence above O2’s πz* level, facilitate the reduction of O2 to the peroxide state with adsorption energies on TiO2 of -1.22 to -2.77 eV. However, N dopants, whose gap states lie less than ~1 eV above the VBM impede O2 adsorption and reduction; O2 on N doped (101) anatase relaxes away from the surface. Interstitial and substitutional N dopants require two photoexcited electrons to enable O2 adsorption. C doping, which introduces gap states between those introduced by N and B, aids O2 adsorption as a peroxide for interstitial doping, although substitutional C does not facilitate O2 adsorption. Dopants for enhancing the photocatalytic reduction of O2 in order of predicted effectiveness are interstitial B, relaxed substitutional B, and interstitial C. In contrast, substitutional C, and interstitial and substitutional N hinder O2 reduction despite increasing visible light absorption. Dopants within the surface layer likely deactivate quickly due to the high exothermicity of O 2 reacting with them to form BO2, CO2, and NO2. KEYWORDS: Organic pollutant degradation, defect engineering, density functional theory, oxygen reduction reaction, visible light photocatalysis


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1. Introduction TiO2 is a widely-used and studied photocatalyst due to its remarkable ability to harness the energy of photons to drive oxidation/reduction reactions.1 In addition to being photochemically active, TiO2 is inexpensive, generally stable in solution, and its band edges straddle the reduction-oxidation potentials of many valuable chemical processes. These reactions include water2,3 and carbon dioxide splitting,4-6 NOx oxidation and reduction,7 and organic pollutant (OP) degradation, which is currently the most common application.8-10 However, TiO2’s effectiveness as a photocatalyst is constrained both by its inefficient use of solar energy that results from the small fraction of incident solar radiation that possess energies exceeding its 3.2 eV band gap, and the slow reaction kinetics of the photochemical processes that occur on TiO2’s surface. Despite these limitations, TiO2 is nevertheless widely used as a photocatalyst where efficiency and rapid reactions are not critical, such as in self-cleaning and antibacterial surfaces.11,12 In addition to being the most common commercial application of TiO2 as a photocatalyst, organic pollutant degradation (OPD) also serves as a convenient model reaction for TiO2 photocatalytic activity because it suffers from slow reaction rates and low efficiencies. Furthermore, the reduction and oxidation potentials of O2 and H2O, which are the primary redox reactants in OPD, are similar to those of other redox reactants of interest. The two key steps of OPD are the oxygen reduction reaction (ORR) and water oxidation reaction, both of which are critical reactions in a variety of important redox processes. In photocatalytic OPD, photoexcited electrons reduce adsorbed O2 to O22-, which subsequently accepts two protons to form H2O2 (hydrogen peroxide) that readily oxidizes the OP. The photoexcited holes either oxidize the OP directly or oxidize H2O to produce H2O2 or O2, depending on the operating conditions. Similar to


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H2O2 produced by O2 reduction, H2O2 produced by H2O oxidation also oxidizes the OP. In the case of OPD, the reduction of O2 is the rate-limiting step.13,14 The slow kinetics of oxygen reduction stem from O2’s inability to adsorb to the perfect anatase (101) surface, which is the most stable surface of TiO2’s most photocatalytically active phase.15-17 The rate of electron transfer to O2 that is in solution and not adsorbed to the TiO2 surface is insignificant, and thus the rate of oxygen reduction by photoexcited electrons on the perfect (101) anatase surface is slow. Additionally, because O2 scavenges excited electrons slowly, significant electron-hole recombination likely occurs within photoexcited TiO2. Two previous approaches to increase the rate and efficiency of OP photodegradation include deposition of Pt clusters onto the TiO2 surface to increase UV activity18-27 and doping TiO2 in an attempt to induce visible light photoactivity26,28-37 and thereby increase the fraction of absorbed solar photons. Although neither of these approaches were intended to increase the adsorption of oxygen and the rate of the ORR, they both resulted in improved photocatalytic activity towards OPD. Recently, we performed density functional theory (DFT) calculations based on the PBE+U Hubbard corrected exchange-correlation functional to show that Pt particles deposited on TiO2 increase photocatalytic activity by enhancing O2 adsorption, and consequently increasing the rate of O2 reduction upon illumination with UV light.38 In the absence of Pt clusters or dopants, O2 adsorbs and accepts excited electrons to undergo reduction at significant rates only near O sub-surface vacancy or Ti interstitial sites due to the inability of O2 to adsorb to the defect free anatase (101) surface.17,38 The presence of O vacancies, Ti interstitial atoms, and Pt clusters provide electron density that occupies band gap states with sufficient energy to reduce O2 by electron transfer to unfilled O2 π* states and form O2-Ti bonds between O2 π* orbitals and d-orbitals of surface Ti atoms or O2-Pt bonds between O2 π* orbitals and d-orbitals of Pt atoms


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on the surface of TiO2 supported Pt clusters. Subsequently, the now unoccupied gap states created by anatase defects or the Pt cluster accept electron density upon photoexcitation, either a) directly: VB → O vacancy states, Ti interstitial states, or Pt cluster states or b) indirectly: VB → CB → O vacancy states, Ti interstitial states, or Pt clusters states, where photoexcited electrons relax electronically to the induced levels in the band gap; thus, upon photoexcitation of anatase, these states are again occupied by electrons capable of reducing O2.38 Furthermore, O2 adsorbs more favorably to (101) anatase at O vacancy sites and on Pt clusters than H2O, which enables O2 reduction even in aqueous environments.17,38,39 Although O vacancies and Ti interstitials in TiO2, and Pt clusters on the (101) anatase surface promote O2 reduction, new approaches to enhance ORR and OPD could overcome major challenges in TiO2 photocatalysis. The high formation energies of O vacancies and Ti interstitials result in equilibrium defect densities that are too low to be effective at enhancing the rate of ORR.39,40 Furthermore, although Pt clusters supported on (101) anatase act as effective oxygen reduction catalysts because of Pt’s high work function,38 Pt is expensive, prone to deactivation via agglomeration,41-43 and Pt clusters act as electron-hole recombination centers independent of Pt cluster loading.38 Therefore, new and preferably inexpensive TiO2 modification schemes that produce permanent and abundant surface O2 adsorption/reduction sites are needed. For this new approach to be viable, the modification must create sites that provide localized, near-surface electron density that possesses sufficient energy to reduce O2 and that gives rise to states near the conduction band that do not bridge the band gap to avoid significant acceleration of electron-hole recombination. One of the most straightforward ways to modify TiO2 involves inducing defects by doping TiO2 with either non-metals, such as B, C, N, S and P44-55 or other transition metals such


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as V, Fe, Cr, Nb, and Mn.35,56-59 Although modified TiO2 materials are often created in an attempt to enable visible light photoactivity, B doped TiO2 outperforms undoped TiO2 even under UV illumination and specific reaction and preparation conditions;60,61 in contrast, N doping was shown to have a detrimental effect on UV light photoactivity.28,60,62 In addition to altering the OPD rates, doping TiO2 may also alter electron-hole recombination, which is likely a non-radiative process, by decreasing the bandgap or by altering the phonon modes. However, the specific effect of doping on recombination is not yet determined and likely depends on the particular dopants and preparation methods used.2,63-65 While many theoretical studies have examined the effect of doping on the band structure of TiO2, from which O2 reduction and photocatalytic behavior has been hypothesized (where high energy states and band gap lowering dopants are associated with increasing the catalytic rates), to our knowledge no study has attempted to prove or quantify these effects. Our objective in this investigation is to use quantum chemical calculations of the adsorption and reduction of O2 on B, C and N doped (101) anatase to: a) develop a fundamental description of the effect of these dopants on O2 reduction on TiO2 and b) provide governing principles for developing effective and practical reduction photocatalysts for ORR, and hence OPD. 2. Computational Methods Calculations of TiO2 surfaces were performed using the PBE+U method, which combines the Perdew-Burke-Ernzerhof (PBE) DFT generalized gradient approximation (GGA) exchange correlation functional66 with a Hubbard onsite Coulomb repulsion term.67 Slab supercell models of the (101) anatase surface were described using plane wave periodic boundary condition calculations within the Vienna Ab initio Simulation Program (VASP).43,44 Projector augmented wave (PAW) pseudopotentials68 that explicitly describe the 2s and 2p electrons of boron, carbon,


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nitrogen and oxygen atoms, as well as the 3p, 4s and 3d electrons of titanium atoms were used to reduce the computational cost of the calculations. All calculations utilized a 400 eV cut off energy as determined by a convergence analysis performed over a range of 200 to 500 eV and were conducted at the Γ-point rather than using a k-point expansion because extensive Brillouin zone folding for larger supercells results in the Γ-point spanning a large fraction of k-space without the added computational expense of an explicit k-point expansion. All calculations explicitly treated electron spin and the system energy was minimized by relaxing the total spin of the system. DFT calculations of first row transition metal oxides such as TiO2 require careful treatment of onsite correlation effects, which often lead to partial occupancies of degenerate 3d states, and thus significant self-interaction errors when treated by various DFT functionals. To avoid these errors we employed the GGA+U method with an onsite Hubbard correction U to treat correlation of the 3d electrons at Ti sites. A Hubbard term67 of Ueff = 3.5 eV was used to incorporate the onsite correlation effects of Coulomb repulsion between Ti d-electrons occupying the same d-orbitals. This value for Ueff for the rotationally invariant formulation of the onsite Hubbard-U term was chosen as a compromise between accurate reaction energies and band gaps,69 and has previously been used to describe O2 interactions with the anatase (101) surface.17 The DFT+U method was chosen over more demanding methods such as HSE06 due to the impractically high computational costs of full HSE06 electronic and geometric optimizations of the 192 atom supercells with 15 Å of vacuum space utilized in this work. To validate the use of the DFT+U functional, the adsorption energy of O2 on TiO2 doped with a B atom occupying interstitial site 7 was calculated with the HSE06 level of theory at the PBE+U geometry and


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found to be -1.70 eV. The PBE+U method predicts an adsorption energy of -1.76 eV, suggesting that PBE+U results in similar O2 adsorption energies at significantly lower computational costs. The (101) anatase TiO2 surface was modeled using a four double-layer thick (101) slab consisting of a 4 × 2 primitive cell containing 192 atoms, as shown in Figure 1. All four double

Figure 1: a) The 4×2×4 anatase TiO2 slab which was used to represent the (101) surface. Small red spheres are O atoms and large gray spheres are Ti atoms. b) Density of states plot of the slab showing the oxygen dominated valence band (VB) and Ti dominated conduction band (CB). The solid gray line shows the total density of states, the red short dashed line shows the O states and the blue long dashed line shows the Ti states. The “~” denotes that a large fraction of the vacuum space above the slab in the top part of the supercell is not shown. layers were allowed to relax. The four double layer slab representation of the (101) surface enables a reliable description of bulk properties (for example, a 3.878 Å inter layer Ti-Ti distance for the second and third double layer of the slab compared to a 3.836 Å inter layer Ti-Ti distance in the bulk), while simultaneously correcting the energy of dangling bonds of frozen surface atoms by allowing both slab surfaces to relax. For instance, a single N dopant in an interstitial bulk position in an equivalent three layer thick slab with a frozen backside double layer exhibits O shoulder states associated with the frozen surface atoms that extend into the band gap, as shown in Supplemental Information (SI), Figure S1. All supercells employ a 15 Å


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vacuum gap between the top surface and the backside of the periodic image of the slab. Increasing the vacuum gap to 20 Å changed the O2 adsorption energy to the B doped interstitial 7 site by less than 0.003 eV, and therefore the less computationally demanding 15 Å vacuum gap was utilized. Geometric relaxations were carried out using the Quasi-Newton-Raphson method and were converged to below 1×10-3 eV. All electron density changes were determined using a Bader charge analysis using software from the Henkelman group.70,71 The density of states plots presented in this work were shifted to align the Ti 2p states, which are the lowest energy states in the system. Dopant formation energies are calculated by Equations 1 and 2 for interstitial and substitutional dopants, respectively: 𝐸𝑑𝑜𝑝𝑖𝑛𝑔_𝑖𝑛𝑡 = 𝐸𝑑𝑜𝑝𝑒𝑑_𝑖𝑛𝑡 − (𝐸𝑏𝑢𝑙𝑘 + 𝜇𝑑𝑜𝑝𝑎𝑛𝑡 ) 𝐸𝑑𝑜𝑝𝑖𝑛𝑔_𝑠𝑢𝑏 = 𝐸𝑑𝑜𝑝𝑒𝑑_𝑠𝑢𝑏 + 𝜇𝑂 − (𝐸𝑏𝑢𝑙𝑘 + 𝜇𝑑𝑜𝑝𝑎𝑛𝑡 )

(1) (2)

where Edoping_int and Edoping_sub are the enthalpies of doping, Edoped_int and Edoped_sub are the calculated energies of the doped cell for interstitial and substitutional dopants, Ebulk is the energy of the undoped surface, μdopant is the chemical potential of the dopant. μdopant is taken to be a fixed value, which is calculated to be a) half of the PBE energy of an N2 molecule (μN = ½ EN2), the b) energy of a C atom in graphite (μC = EC in graphite) or c) the calculated energy of a B atom in B2H6 (μB = ½ EB2H6 – 3/2 EH2) for N, C and B dopants, respectively. μO is given by μO = ½ EO2 + μO’, where EO2 is the calculated energy of an isolated O2 molecule and -2 eV < μO’ < 0 eV. The relationship between reference μO’ temperature and pressure values and actual equivalents was derived by Reuter et al.72


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O2 adsorption energies were calculated using Equation 3: 𝐸𝑎𝑑𝑠 = 𝐸𝑠𝑢𝑟𝑓•𝑂2 − (𝐸𝑠𝑢𝑟𝑓 + 𝐸𝑂2 )

(3)

where Eads is the energy of adsorption, Esurf•O2 is energy of the surface of interest with an adsorbed O2, and EO2 is the energy of the isolated ground state triplet O2 molecule. A negative energy indicates that the process is exothermic, although more favorable adsorptions are often described as having “larger” adsorption energies. In order to avoid expensive excited state calculations, we have modeled photoexcited electrons by adding additional electrons (at most two) to the slab model. This results in the “excited” electron occupying the lowest previously unoccupied band. Modeling a photoexicted electron simply by the addition of an extra electrons is valid if the photoexcited electron and the photoexcited hole do not strongly interact. In the case of TiO2, Di Valentin et al. demonstrated that photoexcited electrons and holes do not strongly interact and behave similarly regardless of whether they are modeled as individual quasi-particles, or by adding or removing an electron from the TiO2 supercell or as an exciton.73 3. Results and Discussion In this work, we considered both interstitial and O-site substitutional B, C and N dopants on the surface and in the first subsurface layer, as shown in Figure 2. As expected, due to their low coordination with other atoms, all dopant atoms initially located at interstitial positions 2, 3 and 6 on the surface are unstable and relax away from the surface and into the vacuum upon geometry optimization. In contrast, dopants initially placed at interstitial sites 1, 4, 5 and 7 and all substitutional sites are stable upon relaxation and are examined further. The formation energies for these defects at 227 °C and an O2 partial pressure of 0.1 atm are reported in Table 1;


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for the doping energies at various O2 partial pressures see supplemental Figure SF18 and for the

Figure 2: Surface and near surface dopant sites of (101) anatase TiO2. Blue spheres in a) represent substitutional sites and the blue squares in b) and c) represent interstitial sites, numbers denote dopant site name. Large gray spheres represent Ti atoms and small red spheres represent O atoms. The “~” denotes that a large fraction of the vacuum space above the slab in the top part of the supercell is not shown. O2 pressure at which a substitutional dopant is more favorable than an interstitial dopant see supplemental Table ST2. It is worth noting that for C and B dopants, the most favorable substitutional dopant is not favored over the least favorable interstitial dopant at all temperatures under 727°C and O2 partial pressures above 1×10-18 atm. In the case of N doping, the most favorable substitutional doping location (substitutional site 5) only becomes favorable over the least favorable interstitial location (non-NO3 form of interstitial site 7) at 227 °C and 0.1 atm. Before discussing the effect of TiO2 doping of B, C and N on its O2 reduction activity, we first briefly summarize the adsorption and reduction of O2 on undoped TiO2 to establish the basic mechanism of O2 adsorption and reduction. TiO2 structures that exhibit strong O2 adsorption and reduction indicate that those sites will be active ORR sites and therefore facilitate OP degradation reactions.


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Table 1: Formation energies of B, C and N dopants at various stable or metastable doping sites at 227 °C and 0.1 atm of O2. Dopant site

B (eV)a

Interstitial 1

-0.37

1.15

N (eV) 2.68

Interstitial 4

1.01

1.75

4.35

Interstitial 5

3.57

2.0

4.34

Interstitial 7

0.66

1.91

3.90

Substitutional 1

5.67 (7.72)

4.62

4.88

Substitutional 2

5.74(7.85)

4.52

4.94

Substitutional 3

5.31 (8.27)

4.75

4.79

Substitutional 4

5.48 (7.85)

4.46

4.77

4.49

4.66

Substitutional 5

5.78 (7.73)

C (eV)

a

The substitutional dopants are relaxed substitutional site (RSS) dopants while meta-stable substitutional site (MSS) dopants are shown in parenthesis. See Section 3.2 for a description of these dopant structures. 3.1 Adsorption and Reduction of O2 on Undoped (101) Anatase Here, we briefly summarize the adsorption and reduction of O2 on the undoped (101) TiO2 anatase surface to establish a point of comparison for the reactivity of doped TiO2 surfaces. A more extensive description of the adsorption of O2 on TiO2 can be found elsewhere17,38,74,75 and in SI, Section 1. As has been previously described,15-17,38 O2 does not adsorb to the defect free TiO2 surface unless sufficient electron density is present in the conduction band to enable the formation of Ti-O bonds between the adsorbing O2 molecule and a Ti atom on the surface while concomitantly reducing O2. An O2 molecule near the non-photoexcited TiO2 surface does not adsorb, but rather relaxes into the vacuum space. The two half-occupied degenerate O2 π* discrete molecular orbitals are, however, broadened and lower in energy when O2 is near (interacts with) a surface Ti atom than when it is far from the surface; additionally these π*


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levels split, such that the π*z orbital lies lower in energy than the π*y orbital, as shown in Figure 3a and SI, Figure S2. Nearsurface, but unadsorbed ground state triplet O2 was modeled using an O2 molecule 2.07 Å from a surface Ti atom oriented parallel to the [010] row of surface Ti atoms. At this position, we calculate that O2 has an O-O bond length of 1.24 Å, compared to a calculated bond length of 1.23 Å in the gas phase. Splitting of the O2 π* orbitals to break their degeneracy is caused by interactions with the Ti surface atom’s unfilled 3d orbitals; the lower energy O2 π*z orbital relative to the O2 π*y orbital results from larger overlap between the O2 π*z and Ti dyz orbital than between the π*y and the Ti dxy orbital. When an electron is in the conduction

band,

for

example

after

Figure 3: Density of states plot for undoped TiO2. a) an O2 molecule near the TiO2 surface where the O2 is free to relax along the O-O bond but constrained from relaxing either towards or away from the surface. For an O 2 molecule far above the TiO2 surface see SI figure S.2. b) O2 adsorbed to the (101) anatase TiO2 surface when one excited electron is present in the slab. The insert shows the electron density isosurface for the highest occupied band and the characteristic ~1.33 Å O-O bond length of a super oxide species. c) O2 adsorbed to a (101) TiO2 anatase surface with two excited electrons present in the slab. The inserts show the isosurface for the two highest occupied bands and the characteristic ~1.44 Å O-O bond length of a peroxide species. d) (101) anatase slab with a sub-surface oxygen vacancy. For all DOS plots, the solid gray line indicates the total density of states, the red short dashed line indicates O2 states and the blue long dashed line indicates Ti states. In d) the red short dashed line shows the O atoms’ of the TiO2 slab. The total density of states has been reduced 15 fold in a and c, and 20 fold in b in order to highlight the details in the PDOS lines.


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promotion from the valence band by photoexcitation, it provides electron density to O2 π*z-Ti dyz bonding orbitals, thus allowing O2 to adsorb (Eads=-1.10 eV), as shown in the insert of Figure 3b and Table 2. O2 adsorption to the (101) anatase surface with one photoexcited electron results in a superoxide state, O2- with one unpaired electron, as indicated by the lengthening of the O2 O-O bond to 1.33 Å and the Bader charge of -0.53 e localized on adsorbed O2, which corresponds to an O2 with a formal charge of -1. The difference between the Bader and formal charges of ½ for adsorbed O2 stems from electron sharing between the O2 and the surface Ti atom;38 therefore the formal charge of the adsorbed O2 molecule can be determined by doubling its Bader charge. When a second excited electron is present near the adsorbing O2, the O2 is further reduced to the peroxide state, O22-, by the formation of a second bond, in this case an O πy*-Ti dxy d-p π bond, as shown in the insert of Figure 3c and evidenced by the Bader charge of -1.02 and the formal charge of -2. Electron density provided by occupied defects states, such as those accompanying subsurface O vacancies, facilitates O2 adsorption to undoped TiO2 when these states have sufficient reducing potential to reduce adsorbing O2. Oxygen vacancies facilitate O2 adsorption because they provide high energy electron density which is localized on the Ti atoms neighboring the O vacancy. This electron density corresponding to two unpaired electrons occupies gap states with energies of 1.68 and 2.08 eV above the VBM, as shown in Figure 3d, which are 0.28 and 0.68 eV above the near surface O2 molecule’s πy* and πz* molecular orbitals, respectively. Consequently, the excess electron density provided by vacancies a few layers below the surface lies sufficiently close to the surface and high in energy to transfer electron density to an adsorbing O2, resulting in an Eads of -2.59 eV, and a surface peroxide O22- species, as corroborated by its calculated Bader and formal charges of -0.99 and -2, respectively. The


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overall spin of the supercell in this configuration is zero. Although near-surface vacancies are predicted to act as O2 reduction sites, their low equilibrium concentration in TiO2 and the relatively low kinetic barriers to fill these vacancies with O atoms in the presence of O240,76 render an approach using O vacancies to enhance O2 adsorption impractical. This study aims to determine if near surface dopants that possess occupied gap states with energies greater than that of the O2 πz*-Ti dyz LUMO indeed facilitate O2 adsorption and reduction, the rate-limiting step of photocatalytic organic pollutant degradation, and therefore provide a predictive method and design principles for surface modifications to increase photoactivity. Table 2: O2 adsorption characteristics on undoped (101) anatase

a

Characteristic

Singly photoexcited electron

Doubly photoexcited electron

Subsurface O vacancy

Adsorption energy (eV)

-1.10

-1.74

-2.59

O-O bond length (Å)

1.33

1.45

1.45

O2 charge gain (e)a

0.53

1.02

0.99

Formal O2 charge state

-1

-2

-2

The reported charges are negative (O2 is reduced on adsorption) and referenced to triplet (ground state) O2 in the vacuum.

3.2 Adsorption and Reduction of O2 on Boron Doped (101) Anatase Introducing B atoms into the surface and near subsurface layers of TiO2 results in B dopants nominally located in either interstitial or oxygen substitutional positions. However, B atoms positioned at O substitutional sites on the O sublattice are only metastable and upon relaxation move into interstitial BO2-like sites and in the process create an O vacancy. This is


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demonstrated by the relaxation of a B atom initially placed at O substitutional site 3, as shown in Figures 4a and b. We refer to the O sublattice substitutional site as a meta-stable substitutional site (MSS) and the relaxed structure as a relaxed substitutional site (RSS). We report the calculated relative energies of B dopants in Table 1. An O atom from

Figure 4: Example geometries for the B doped TiO2 slab showing the a) RSS 3 and b) interstitial 7 sites. Large gray spheres represent Ti atoms, medium pink spheres represent B atoms, and small red spheres represent O atoms.

either an H2O or O2 molecule, upon encountering a B RSS dopant, readily fills the O vacancy created by the RSS B atom, to create a shallow interstitial B defect and no O vacancy. Because B migration from a MSS to a RSS is exothermic by at least 1.89 eV, and filling the RSS O vacancy with an oxygen atom from O2 is exothermic by at least 6.29 eV, we focus on interstitial B dopants and how they affect TiO2 photocatalytic activity; RSS and MSS sites are addressed in the SI.


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


Interstitial and substitutional B defects in bulk TiO2 decrease its effective band gap by inducing gap states

near

minimum

the

conduction

(CBM),

as

band shown

previously.77,78 Figure 5 illustrates the projected density of states (PDOS) for the representative interstitial site 7 and substitutional site 3; the DOS plots for all B dopant sites can be found in SI, Figure S.3-5 and the relaxed positions of all dopants can be seen in SI, Figure S.6-8. Doping TiO2 with a B atom results in a supercell with an odd number of electrons. B doped TiO2

Figure 5: Density of states plots for B doped TiO2 for a) RSS 3 and b) interstitial 7 sites. The solid gray line indicates the total density of states, the red short dashed line the O states, the blue long dashed line the Ti states and the pink dotted line represents B states. The total density of states has been reduced 15 fold in order to highlight the details in the PDOS lines.

supercells were found to have one unpaired electron, except for B in interstitial site 4 and substitutional sites 1 and 4, which have three unpaired electrons. The differences in the character of the dopant induced band gap states between the various dopant sites arise from the nature of the interactions between the B dopant atom and the surrounding TiO2 material. When a B atom resides in the BO3-like interstitial position the neighboring O atoms relax away from the Ti atoms to which they are bound (the TiO bonds elongate from 2.11 Å to 2.14-2.25 Å) and towards the interstitial B atom forming B-O bonds with an ~1.40 Å bond length. A Bader charge analysis indicates that in the BO3 interstitial


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structure B’s three valence electrons are transferred to its three O nearest-neighbors leaving the B with a +3 formal oxidation state. Because the B interstitial atom donates electron density towards filling the valence orbitals of its nearest-neighbor O atoms, the O atoms draw less electron density from their neighboring Ti atoms. The Ti atoms, therefore, retain more of their valence electron density, giving rise to occupied Ti states in the band gap near the conduction band. An examination of these gap states shows that in fact, they are not solely localized on the Ti atoms nearest to the B dopant, but are instead distributed over the Ti atoms of the slab. This stems from the large number of Ti atoms within the supercell model that are second nearest-neighbors to the B dopant, and whose initially unoccupied 3d states at the CBM are stabilized by delocalization of the electron density provided by the B dopant. For a similar analysis of MSS and RSS dopants, refer to SI section SI.2. The high energy occupied states introduced by B doping in TiO2, which are predicted to have similar energies to the band gap states created by O vacancies, suggest that B doping provides electron density with sufficient reducing potential to reduce adsorbing O2 and thus that B doping enhances the photocatalytic activity of TiO2. We calculate that O2 adsorbs exothermically to B doped (101)

Figure 6: O2 adsorption to B doped TiO2 anatase (101) surface at a) interstitial 7 and b) RSS 3 sites. The O-O bond length is indicated in the right hand panels. Large gray spheres represent Ti atoms, medium pink spheres represent B atoms, and small red spheres represent O atoms.

anatase TiO2 with adsorption occurring at Ti sites, as discussed in detail below. However, for B doping on the surface at oxygen substitutional sites 1 and 2 and at interstitial site 5, O2 adsorbs


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directly to the B dopant atom to form a BO3 species, as shown in SI, Figure S5a. Although adsorption of O2 to B at surface substitutional sites 1 and 2 and at interstitial site 5 is highly favorable, these surface sites are directly exposed to the aqueous solvent; the high solubility of B2O3 in water suggests that exposed BO3 structures will dissolve in aqueous conditions and that their dissolution will lead to the deactivation of these dopant structures. Consequently, we do not discuss these structures further except to mention that they will contribute to the catalytic activity until they dissolve. Interstitial B dopants give rise to high energy occupied gap states that enable strong O 2TiO2 bonding with adsorption energies of -1.56, -2.77 and -1.76 eV for interstitial sites 1, 4 and 7, respectively. The strong adsorption of O2 on interstitial B doped (101) anatase corresponds to the formation of surface peroxide species by reduction of O2 to O22- and a supercell with a single unpaired electron. This is evident from both the characteristic ~1.45 Å peroxide O-O bond length and the ~-0.95 e charge of the adsorbed O2 molecule, exact Bader charges and bond lengths are shown in Table 3 and Figure 6a for interstitial site 7. O2 gains only -0.95 e of electron density, as predicted by a Bader charge analysis, in stark contrast to the nominal two electrons O2 gained on forming the peroxide species because the adsorbed O2 molecule bonds to a Ti surface atom with which it shares electron density through an O2 π*z-Ti dyz and an O2 π*y-Ti dxy bond, analogous to the O2-Ti bond formed upon O2 adsorption on the undoped (101) anatase surface as illustrated by the inset of Figure 3c. The highly favorable adsorption of O2 to interstitial site 4 (Easd=-2.77 eV) arises from interactions between O2 and the partial vacancy created by a surface O atom that relaxes into the bulk to associate with the B atom at interstitial site 4, as seen in SI, Figure S.4i. When the O2 molecule adsorbs on (101) anatase, it rotates away from being parallel to the row of Ti atoms to point towards the descended surface O atom (see SI, Figure S.9b).


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Table 3: O2 adsorption characteristics on B doped TiO2

Site Inter. 1 Inter. 4 Inter. 7

O2 O2 B charge B charge O2 Formal adsorption bond before O2 after O2 charge O2 energy length adsorption adsorption gain charge (eV) (Å) (e) (e) (e)b state -1.56

1.45

0.0

0.0

0.93

-2

-2.77

1.45

0.0

0.0

1.02

-2

-1.76

1.45

0.0

0.0

0.96

-2

a

O2 forms BO2 at interstitial site 5 The Bader predicted charge gain, from which the formal charge of O2 can be determined by multiplying by two When O2 adsorbs to the B doped (101) anatase TiO2 surface electron density from the b

highest energy occupied gap states, which are localized over the Ti atoms of the TiO2 slab, transfers to the adsorbing O2. It is important to mention here that the highest occupied band after O2 adsorption to the B doped (101) anatase surface is not comprised of Ti-O2 bonds (SI, Figure S.3d); instead, it is composed of the same Ti 3d states distributed throughout the slab that provide the electron density to the adsorbing O2. Because each B dopant donates three electrons to the unit cell of our supercell model of the TiO2 surface and only two electrons are required to fully reduce O2, one electron is expected to remain in the high energy Ti states. This indicates that each interstitial B atom provides electron density capable of reducing more than one adsorbing O2; indeed, when a second O2 molecule is near the interstitial B dopant in the (101) anatase surface it does in fact adsorb exothermically with an adsorption energy of -1.01 eV relative to one adsorbed O2 near the interstitially doped B (101) anatase surface (SI, Figure S.9c). When two O2 are adsorbed, each O2 has been reduced by ~-0.7 e, resulting in a state that is intermediate between the superoxide and peroxide states whereas one O2 adsorbed near the interstitial B dopant is reduced by ~-0.95 e to the peroxide state.


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Not only does B doping facilitate O2 absorption to the TiO2 surface regardless of doping structure, but excited electrons also increase the O2 adsorption strength and reduce the B dopant to facilitate additional O2 reduction. When an excited electron encounters an O2 adsorbed to the anatase surface doped with an interstitial B the excited electron backfills the hole created in the previously occupied gap states oxidized upon reducing O2. For the interstitial B case the photoexcited electron slightly stabilizes the Ti-O2 adsorbate, which increases the adsorption energy from -1.76 to -1.88 eV; additionally, rather than further reducing the adsorbed O2, which remains a peroxide with a -2 charge state, the excited electron backfills the Ti 3d state that provided electron density to the adsorbing O2 molecule. Overall, doping the (101) anatase TiO2 surface with B facilitates O2 adsorption and consequently, oxygen reduction regardless of the doping site. Therefore, our results predict that B doping is an effective approach for increasing the photoactivity of TiO2. However, it is worth noting that because B dopants create gap states, which at the surface or near surface enhances O2 adsorption, bulk B dopants also act as electron/hole recombination centers that lower the number of excited electrons and thus hinder electron transfer to the surface and lower the overall photochemical efficiency. Further study is required to quantify these effects. Therefore, to the degree that it is practical, B dopants should be introduced in the near surface region where they act as both O2 adsorption sites and as reducing agents. 3.3 Adsorption and Reduction of O2 on Carbon Doped (101) Anatase Both near surface substitutional and interstitial C atoms introduce states into the band gap of TiO2 where interstitial C doping generally introduces states that lie higher in the band gap than substitutional C, as previously predicted for bulk structures79 and shown for near surface C dopants in Figures 7 and 8, respectively. Density of states plots and geometries of all structures


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considered can be found in SI, Figures S.1011 and S.12-13, respectively. Substitutional dopants result in spin neutral cells, while interstitial dopants result in cells with two unpaired electrons. Interstitial

C

atoms

introduce

occupied gap states with energies that lie between 1.16 and 1.59 eV above the VBM, which we predict are sufficiently high in energy to reduce O2 and thus enable its adsorption

on

(101)

anatase.

In

the

interstitial position the C atom takes on a +1 or +2 oxidation state, where the further from the surface the C atom resides the more likely it is to take on a +2 oxidation state, as Figure 7: Density of states plots for interstitial site 7 C doped TiO2 (101) anatase. a) before O2 adsorption and b) after O2 adsorption. The solid gray line indicates the total density of states, the blue long dashed line the Ti states, the brown dotted line represents C states, and the red short dashed line the a) near surface O2 states before O2 adsorption and b) O2 states after adsorption. O2 states before adsorption and shown in a) are taken from Fig. 3a. The O atoms of the near surface O2 are 2.07 Å from a surface Ti atom of an undoped TiO2 slab and the O2 has an O-O bond length of 1.23 Å. The total density of states has been reduced 30 and 20 fold in a and b respectively in order to highlight the details in the PDOS lines. c) Geometry of the O2 adsorption to interstitial site 7. Large gray spheres represent Ti atoms, medium brown spheres represent B atoms, and small red spheres represent O atoms.

shown in Table 4. In contrast, substitutional C atoms introduce both occupied and unoccupied gap states and are reduced to the -1 oxidation state, as electron density transfers to the C atom from the Ti atoms to which it is bonded. The occupied states possess energies between 0.29 eV and 1.02 eV above the VBM, which we predict lie


22

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below the O2 π* LUMO energy, and thus do not have sufficient reducing potential to reduce O2 and consequently are unable to facilitate O2 adsorption. The unoccupied gap states induced by interstitial C lie higher in the gap at approximately 2 eV above the VBM. The predicted formation energies of C dopants are reported in Table 1. For a more in depth discussion of the C based gap states see SI, section SI.3. The relative energies of interstitial C’s occupied gap states compared to O2’s π* orbitals indicate that O2 adsorption to the (101) anatase surface is only facilitated by C dopants positioned at interstitial sites 1, 4 and 7 because only these C interstitial defects induce occupied gap states with adequate reducing potential to reduce O2. When an O2 molecule is initially placed on the

(101)

surface

near

all

other

substitutional or interstitial C defects it relaxes away from the surface and into the

Figure 8: Density of states plots for substitutional site 4 C doped (101) anatase TiO2. a) before O2 adsorption and b) after O2 adsorption to the photoexcited surface. The solid gray line indicates the total density of states, the blue long dashed line shows Ti states, the brown dotted line represents B states, and the red short dashed line the near O2 states a) before O2 adsorption and b) after O2 adsorption to the surface with an excited electron. O2 states in a) before adsorption are taken from Fig. 3a.The total density of states has been reduced 15 and 20 fold in a and b respectively in order to highlight the details in the PDOS lines. c) Geometry of the thermoneutral O2 adsorption to (101) anatase doped with C in substitutional site 4 and in the presence of a photoexcited electron. Large gray spheres represent Ti atoms, medium brown spheres represent B atoms, and small red spheres represent O atoms.

vacuum, or, as in the case of C in substitutional sites 1 and 2 and interstitial sites 1 and 5, it


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forms CO2, thus eliminating these defects as viable ORR and OPD reaction sites. Our geometry optimizations predict that O2 does not adsorb to surfaces with dopants that do not induce occupied states with energies above the states corresponding to O2’s π* orbitals, that is that lack sufficient reducing potential to transfer electrons to O2, and corroborate our predictions based on the calculated relative energies of the filled electronic energy levels. O2 adsorbed at interstitial sites 4 and 7 adsorbs favorably by -1.92 and -1.84 eV in peroxide O22- states and has calculated O-O bond lengths of 1.45 and 1.44 Å, 1.88 Å Ti-O bond lengths and accepts -0.95 and -0.97 e of electron density, respectively, as reported in Table 4. O2 adsorbed on C doped (101) anatase at sites 4 and 7 results in a supercell with two and zero unpaired electrons, respectively. The electron density transferred to O2 is partially provided by the interstitial C dopant. A C atom in interstitial site 4 donates -0.87 e to the adsorbing O2 molecule whereas a C atom in interstitial site 7 only loses -0.22 e and the remaining electron density donated to the adsorbing O2 molecule is provided by Ti 3d states. For both dopant sites the C atom has a formal oxidation state of +2 after O2 adsorption. Similar to O2 adsorption on surfaces containing interstitial and RSS B atoms, O2 adsorbs as a peroxide on interstitially C doped TiO2 and forms Ti dyz-O2 π*z and Ti dxy-O2 π*y d-p π bonds to surface Ti atoms. Our results predict that C interstitials give rise to occupied states sufficiently high in the band gap to reduce O2, and thus facilitate O2 adsorption and reduction, whereas substitutional C atoms do not. The ab initio description of the electronic structure of C doped anatase clearly establishes how the calculated density of states induced by dopants predicts whether a specific dopant enables O2 adsorption; the calculated energy of occupied gap states of dopants that provide electron density with energy in excess of the O2 π* LUMO indicates favorable O2 adsorption and reduction, while occupied gap states calculated to lie below O2’s LUMO predict that O2 will not adsorb, as shown in Figures 7 and 8.


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Table 4: O2 adsorption characteristics on interstitially C doped (101) anatasea C charge after O2 ads(e)

O2 charge gain (e)b

Formal O2 charge state

Site

O2 adsorption energy (eV)

O2 bond length (Å)

C charge before O2 ads (e)

Inter. 4

-1.92

1.45

3.17

2.30

0.95

-2

Inter. 7

-1.84

1.44

2.16

1.94

0.97

-2

a

O2 does not adsorb to the substitutionally C doped (101) anatase TiO2 surface and forms CO2 at interstitial site 5. b The Bader predicted charge gain, from which the formal charge of O2 can be determined by multiplying by two. Photoexcited electrons are necessarily present under photocatalytic conditions and

because of the presence of unoccupied gap states associated with substitutional C doping, they are predicted to have an effect on the O2 adsorption behavior on C doped TiO2. Therefore, we proceed to discuss these interactions in some detail. Generally, C atoms in interstitial sites induce occupied gap states sufficiently high in the gap to reduce O2. However, substitutional C provides both occupied gap states that are too low in energy to reduce O2 and unoccupied gap states that are predicted to be capable of reducing O2 when occupied by photoexcited electrons. Indeed, the presence of a photoexcited electron strengthens the O2-Ti bonds of interstitially C doped TiO2, where the O2 adsorption energy changes from -1.75 eV to -1.85 eV. Although substitutional C atoms enable O2 adsorption when coupled with photoexcited electrons, O2 adsorption is only thermoneutral. In the case of C occupying substitutional site 4, both one and two photoexcited electrons result in thermoneutral O2 adsorption in the O2- peroxide state (1.33 Å O-O bond length and a Bader predicted ~-0.6 e of electron density transferred to O2, resulting in -1 formal charge state). The thermoneutral nature of O2 adsorption on photoexcited substitutionally C doped TiO2 suggests that the increase in energy of the electron in the half occupied O2 π* orbital due to Ti-O bond formation involves an energy penalty similar to the energy decrease of the photoexcited


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Page 26 of 36

electron due to relaxation of the electron from the C induced gap state to the O2–Ti π*z-dzy bond. The transfer of less than one electron to O2 adsorbing on the doubly photoexcited TiO2 surface with a substitutional C atom (-0.6 e of electron density is transferred to O2 resulting in a -1 charge state) suggests that the photoexcited electron occupying the normally vacant second induced gap state above the Fermi energy does not have sufficient reducing power to transfer electron density to O2-; i.e. one photoexcited electron lies higher in energy than the O2 πz*-Ti dzy bond, but the second photoexcited electron occupies a level lower in energy than the O2 πy*-Ti dxy bond, as shown in Figures 8b and c. However, the formation of interstitial C defects, which facilitate O2 reduction, is energetically favored over the formation of substitutional C dopants. Similar to the B doped (101) anatase surface, C doping likely increases O2 reduction most effectively for dopants located in the near surface region where electron trapping promotes O2 reduction rather than electron hole recombination, but further study is required to quantify electron hole recombination kinetics. 3.4 Adsorption and Reduction of O2 on Nitrogen Doped (101) anatase Substitutional and interstitial N doping of bulk and surface TiO2 was previously shown80,81 to decrease the band gap of TiO2 by introducing states that create a shoulder in the DOS at the VBM or that induce states in the band gap near the VBM, as shown in Figure 9. DOS plots and N geometries for all dopant sites are shown in SI, Figures S.14/15 and S.16/17, respectively. All interstitial and substitutional N dopants result in supercells with 1 unpaired electron because N atoms have an odd number of electrons. The N atom is not oxidized when occupying either an interstitial or substitutional site; rather the N gains some electron density in interstitial sites, -0.33 e to -0.46 e according to a Bader analysis: or significant electron density


26

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when occupying substitutional sites, gaining -0.97 e to -1.18 e. This corresponds to formal oxidation states for N of 0 and -1 for interstitial and substitutional sites, respectively. The smaller band gap explains the visible light photocatalytic activity of N doped TiO 2.32 The position of the highest lying occupied states introduced by both the interstitial and substitutional N dopants are found to lie within 1 eV of the VBM where interstitial sites create distinct, occupied band gap states while substitutional sites create shoulder states. Additionally, substitutional N dopants induce empty levels that also lie only ~1 eV above the VBM. Interstitial and substitutional N atoms induce states that are significantly lower in the band gap of TiO2 than the other dopants investigated, and because they lie below O2’s lowest lying empty π* orbitals they have insufficient reducing potential to reduce O2, although they do significantly redshift TiO2’s excitation energy. Thus, N doping that forms substitutional and interstitial dopant structures are not predicted to enhance O2 adsorption and thus, do not facilitate the ORR more than merely enabling visible light absorption. For a more detailed description of surface N doping see the SI, section SI.4.


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In addition to substitutional and interstitial structures, N dopants also form a structure in the near-surface region we call the No/Ov structure where O atoms near N interstitials relax into the TiO2 pore (see Figure 9). This relaxation reduces steric repulsion within the No/Ov structure to stabilize it by 0.60 eV relative to N in interstitial site 7. As with the other N dopant structures, the additional electron associated with the No/Ov dopant structure results in one unpaired electron in

the system.

This

relaxation involves significant displacement of surface Ti and O atoms into the vacuum, which is precluded for bulk relaxation of the No/Ov dopant structure. This structure induces occupied gap states that lie 1.3 eV above the VBM, significantly higher than the states induced by other N dopant structures. Nevertheless, our results predict that these energy levels lie too low in the band gap to facilitate O2 adsorption.

Figure 9: Density of states plots for N doped TiO2 in a) substitutional site 4, b) interstitial site 7 and c) the No/Ov structure. The solid gray line indicates the total density of states, the dark blue long dashed line Ti states, the bright blue dotted line N states, the light red short dashed line near O2 states before O2 adsorption and the dark red dashed line in c) indicates the O atom in the pore. The O 2 states before adsorption are taken from Figure 3a. The total density of states has been reduced 15 (a and b) and 2 (c) fold in order to highlight the details in the PDOS lines.


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The relaxation of O2 into the vacuum in geometry optimizations where O2 is initially placed on the interstitial and substitutional N doped (101) anatase surface corroborates the prediction that this doping induces gap states that lie too low in the band gap to facilitate O 2 adsorption. This occurs for N doping at all substitutional and interstitial sites with the exceptions of N occupying interstitial sites 1 and 5, where instead NO2 is formed, and interstitial site 7, where O2 forms an O2- metastable adsorbate with an endothermic adsorption energy of 0.10 eV, as referenced to the No/Ov structure. Upon O2 adsorption, the overall number of unpaired electrons in the supercell remains one, meaning that the two unpaired electrons of the adsorbing triplet O2 singlet pair upon adsorption. The formation of a peroxide state corresponds to a Bader charge analysis predicted -0.50 e of electron density transferring to the O2, -0.39 e of which is provided by the N atom. This results in a formal charge state on the adsorbed O2 of -1 and oxidation state of the N atom of 0. Upon O2 adsorption at interstitial sites 1 and 5 NO2 is formed, which makes this site inactive for further reaction. The inability of O2 to favorably bond to the TiO2 surface in the presence of near surface N dopants results from the high energy of O2’s π* orbitals relative to the energies of the highest occupied gap states induced by N doping; thus, occupied gap states induced by N doping have insufficient reducing potential to reduce O2 and enable O2 adsorption and ORR. Although N dopants increase photoexcited electron density in TiO2 by decreasing its band gap to therefore allow absorption of longer wavelength solar photons, even singly excited TiO2 near a N dopant does not aid O2 adsorption because when a photoexcited electron encounters a N dopant it loses reduction potential by relaxation to the lower energy N induced empty gap states. Instead of adsorbing, O2 relaxes into the vacuum when initially placed on singly photoexcited (101) anatase near substitutional N dopants. To enable O2 adsorption on


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substitutionally N doped TiO2, two photoexcited electrons must be added to the slab: one that fills the lower energy gap state, giving N an oxidation state of -1.5, and a second that occupies the second lowest normally unoccupied state that lies at the CBM and which is not associated with the N atom and possesses sufficient reducing potential to reduce adsorbing O2. Because O2 reduction in the presence of N dopants requires a second photoexcited electron, N doping hinders O2 reduction and, in the case of UV light photoexcitation, is worse than no doping at all, which only requires a single photoexcited electron to enable the adsorption of O2. Only N dopants that form the No/Ov structure enable a single photoexcited electron to reduce O2 and facilitate its adsorption, which is exothermic by -0.48 eV. O2 adsorption at No/Ov sites occurs because the initially unfilled N-O π* states lie above the O2 LUMO, as shown in Figure 9. These results suggest that although N doping lowers TiO2’s band gap, N’s propensity to act as a low energy electron trap hinders the ability of TiO2 to reduce oxygen such that overall the effect of N on TiO2’s UV photocatalytic activity is detrimental, as is observed experimentally. For further results and discussion of O2 adsorption to photoexcited N doped TiO2 see the SI, section SI.5. 4. Conclusions In this work we have calculated electronic structures of doped TiO2 surfaces to elucidate how doping affects O2 adsorption and reduction and to predict that doping the TiO2 surface with elements that create filled gap states with energy greater than the near surface O2 πz* molecular orbital (O2 LUMO) increases O2 adsorption and reduction, the rate-limiting step in photocatalytic organic pollution degradation and other photocatalytic processes. However, sites that create empty states lower than the O2 πz* orbitals handicap TiO2’s reduction abilities, as these states decrease the photoexcited electron’s reduction potential. We have shown that the most effective dopants for photocatalytic ORR in order of predicted effectiveness are interstitial B,


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substitutional (relaxed) B, interstitial C, and substitutional C. Our calculations predict that interstitial N and substitutional N are in fact deleterious to O2 adsorption and reduction, despite their effect of lowering the band gap of TiO2. In contrast, B dopants increase O2 adsorption regardless of whether the dopant is in a substitutional or interstitial site, although B atoms in substitutional sites will likely rapidly relax to the more stable interstitial sites. C dopants enhance O2 reduction depending on the dopant location with the more stable interstitial sites enhancing adsorption with or without photoexcited electrons. Although N doping has previously been used in attempts to enhance the ORR by increasing the visible light photoactivity of TiO2, we predict that it generally hinders O2 adsorption and reduction and therefore limits UV photocatalytic ORR, although as intended, they do enable visible light photoactivity by lowering the band gap. To increase UV activity, it is important that B and C dopants reside near the surface where they act as O2 reduction sites rather than in the bulk where they act as electron traps or worse, electron-hole recombination centers. We have also demonstrated that the likelihood of photocatalytic oxygen reduction can be predicted by calculating the relative energies of the highest occupied band and the O2 πz* molecular orbital. This enables the rapid evaluation and screening of dopants and catalysts for oxygen reduction reactions and is applicable to TiO 2 OPD and to ORR on other substrates and reactions. 5. Acknowledgement The authors gratefully acknowledge the support of the National Science Foundation for support under grant CHE-1038015 and the Department of Education for support through a Renewable and Sustainable Energy Graduate Assistance in Areas of National Need (GAANN) Fellowship. This work utilized the Janus supercomputer, which is supported by the NSF (award number CNS-0821794) and the University of Colorado Boulder. Additionally, we would like to


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acknowledge that the images were created using the VESTA 3D visualization software.82 The authors also thank Brian Ehrhart for help in figure and manuscript preparation. CBM is a Fellow of the Materials Science Program and the Renewable and Sustainable Energy Institute of the University of Colorado Boulder. 6. Supplemental Information Further descriptions of O2 interaction with the undoped TiO2 surface, details of B dopants in the MSS, and further discussion of the nature of C and N doping as well as supplemental figures including all DOS’s and slab geometries are available in the Supplemental Information. This information is available free of charge via the Internet at http://pubs.acs.org 7. References (1) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Photocatalysis on Tio2 Surfaces - Principles, Mechanisms, and Selected Results. Chemical Reviews. 1995, 95, 735-758. (2) Ni, M.; Leung, M. K. H.; Leung, D. Y. C.; Sumathy, K. A Review and Recent Developments in Photocatalytic Water-Splitting Using for Hydrogen Production. Renewable and Sustainable Energy Reviews. 2007, 11, 401-425. (3) Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science. 2011, 331, 746-750. (4) Liu, L.; Zhao, H.; Andino, J. M.; Li, Y. Photocatalytic Co2 Reduction with H2o on Tio2 Nanocrystals: Comparison of Anatase, Rutile, and Brookite Polymorphs and Exploration of Surface Chemistry. ACS Catalysis. 2012, 2, 1817-1828. (5) Wang, W.-N.; An, W.-J.; Ramalingam, B.; Mukherjee, S.; Niedzwiedzki, D. M.; Gangopadhyay, S.; Biswas, P. Size and Structure Matter: Enhanced Co2 Photoreduction Efficiency by Size-Resolved Ultrafine Pt Nanoparticles on Tio2 Single Crystals. J. Am. Chem. Soc. 2012, 134, 11276-11281. (6) Dhakshinamoorthy, A.; Navalon, S.; Corma, A.; Garcia, H. Photocatalytic Co2 Reduction by Tio2 and Related Titanium Containing Solids. Energy & Environmental Science. 2012, 5, 9217-9233. (7) Wu, Q.; van de Krol, R. Selective Photoreduction of Nitric Oxide to Nitrogen by Nanostructured Tio2 Photocatalysts: Role of Oxygen Vacancies and Iron Dopant. J. Am. Chem. Soc. 2012, 134, 9369-9375. (8) Lachheb, H.; Puzenat, E.; Houas, A.; Ksibi, M.; Elaloui, E.; Guillard, C.; Herrmann, J.-M. Photocatalytic Degradation of Various Types of Dyes (Alizarin S, Crocein Orange G, Methyl Red, Congo Red, Methylene Blue) in Water by Uv-Irradiated Titania. Applied Catalysis B: Environmental. 2002, 39, 75-90. (9) Peral, J.; Ollis, D. F. Heterogeneous Photocatalytic Oxidation of Gas-Phase Organics for Air Purification: Acetone, 1-Butanol, Butyraldehyde, Formaldehyde, and M-Xylene Oxidation. J. Catal. 1992, 136, 554-565. (10) Chatterjee, D.; Dasgupta, S. Visible Light Induced Photocatalytic Degradation of Organic Pollutants. J. Photochem. Photobiol. C-Photochem. Rev. 2005, 6, 186205. (11) Parkin, I. P.; Palgrave, R. G. Self-Cleaning Coatings. J. Mater. Chem. 2005, 15, 1689-1695. (12) Ireland, J. C.; Klostermann, P.; Rice, E. W.; Clark, R. M. Inactivation of Escherichia-Coli by Titanium-Dioxide Photocatalytic Oxidation. Appl. Environ. Microbiol. 1993, 59, 1668-1670. (13) Gerischer, H.; Heller, A. The Role of Oxygen in Photooxidation of Organic-Molecules on Semiconductor Particles. J. Phys. Chem. 1991, 95, 5261-5267. (14) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chemical Reviews. 1995, 95, 69-96. (15) Mattioli, G.; Filippone, F.; Amore Bonapasta, A. Reaction Intermediates in the Photoreduction of Oxygen Molecules at the (101) Tio2 (Anatase) Surface. J. Am. Chem. Soc. 2006, 128, 13772-13780. (16) Filippone, F.; Mattioli, G.; Bonapasta, A. A. Reaction Intermediates and Pathways in the Photoreduction of Oxygen Molecules at the (1 0 1) Tio2 (Anatase) Surface. Catal Today. 2007, 129, 169-176. (17) Aschauer, U.; Chen, J.; Selloni, A. Peroxide and Superoxide States of Adsorbed O-2 on Anatase Tio2 (101) with Subsurface Defects. Phys. Chem. Chem. Phys. 2010, 12, 12956-12960. (18) Zhou, Y.; King, D. M.; Liang, X. H.; Li, J. H.; Weimer, A. W. Optimal Preparation of Pt/Tio2 Photocatalysts Using Atomic Layer Deposition. Appl. Catal. B-Environ. 2010, 101, 54-60. (19) Cho, K. C.; Hwang, K. C.; Sano, T.; Takeuchi, K.; Matsuzawa, S. Photocatalytic Performance of Pt-Loaded Tio2 in the Decomposition of Gaseous Ozone. Journal of Photochemistry and Photobiology a-Chemistry. 2004, 161, 155-161. (20) Kennedy, J. a. D., AK Photothermal Heterogeneous Oxidation of Ethanol over Pt/Tio2. J. Catal. 1998, 179, 375-389. (21) Lam, S., et al The Effect of Platinum and Silver Deposits in the Photocatalytic Oxidation of Resorcinol. Appl. Catal. B-Environ. 2007, 72, 363-372.


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Increasing the Photocatalytic Activity of Anatase TiO2 through B, C

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