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Two-Dimensional TiO Nanosheets for Photo and Electro -Chemical Oxidation of Water: Predictions of Optimal Dopant Species From First Principles Namhoon Kim, Emily M. Turner, Yoonyoung Kim, Shintaro Ida, Hidehisa Hagiwara, Tatsumi Ishihara, and Elif Ertekin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04725 • Publication Date (Web): 15 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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The Journal of Physical Chemistry

Two-dimensional TiO2 Nanosheets for Photo and Electro -Chemical Oxidation of Water: Predictions of Optimal Dopant Species from First Principles Namhoon Kim,† Emily M. Turner,‡ Yoonyoung Kim,¶,§ Shintaro Ida,k Hidehisa Hagiwara,¶,§ Tatsumi Ishihara,¶,§ and Elif Ertekin∗,†,§ †Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 1206 W Green St, Urbana, IL 61801, United States ‡Department of Materials Science and Engineering, University of Florida, 1724 Gale Lemerand Dr, Gainesville, FL 32603, United States ¶Department of Applied Chemistry, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan §International Institute for Carbon Neutral Energy Research (WPI-I2 CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan kGraduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo, Kumamoto 860-8555, Japan E-mail: [email protected] Phone: +1 (510) 847-7073

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Abstract Due to a high surface area to volume ratio, two-dimensional nanosheets have gained interest for photo and/or electro -catalytic water splitting. In particular, experimental Rh doping of lepidocrocite TiO2 nanosheets has significantly increased catalytic activity. We use first-principles density functional theory to consider the oxygen evolution reaction (OER) on both pristine and transition metal doped systems. While the undoped TiO2 nanosheets exhibit several limitations and require high overpotentials during the water splitting reaction, selected dopants modify the binding strength of reaction intermediates and can reduce rate limiting thermodynamic barriers and theoretical required overpotentials. We present an activity volcano for these nanosheets, with the full spectrum of 3d, 4d, and 5d transition metals as candidate dopants. Subsequent photocatalytic measurements of OER activity with selected dopants are carried out to validate the predictions, and the trends are found to be consistent. These results help describe how surface dopants affect reaction mechanisms and provide general design principles for high performance catalysts during the water splitting reaction.

Introduction A longstanding goal of renewable energy research is the conversion of water to hydrogen and oxygen gas in a green and sustainable manner. Water splitting typically occurs through two half-reactions, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), both of which require energy input and suffer from slow kinetics. 1–3 The required energy can be provided by electrocatalysis, photocatalysis, or a combination of the two. Electrocatalysis uses an electric potential to provide energy to carriers in a semiconductor, while photocatalysis utilizes sunlight to excite carriers. Compared to better-performing metal oxides such as IrO2 and RuO2 , TiO2 requires large overpotentials and exhibits particularly sluggish kinetics in electrocatalysis. 4 However, TiO2 can be used as a photocatalyst for water splitting due to a large band gap and high energy holes which enable oxygen evolution. 5–7 2


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In addition to proper band alignment relative to the water redox potentials, it also provides stability under irradiation and is relatively inexpensive. 5–7 For these reasons, TiO2 is the most frequently studied water splitting photocatalyst. Despite these advantages, reaction rates are still too slow and the performance of TiO2 still insufficient for wide scale adoption of TiO2 based water splitting technology. The introduction of alternative species and dopants is one possible approach to accelerate the water splitting reactions. Previous studies show that doping can improve the catalytic activity of host semiconductors, and impurity introduction may be a promising technique for improving the activity of TiO2 photoelectrolysis systems. For instance, Pt is often incorporated as a co-catalyst to increase the rate of hydrogen evolution. 8–10 Transition metal dopants have also been demonstrated to enhance the OER on various anatase and rutile titania surfaces. 11–13 Doped titania materials therefore have the potential to offer a cheap alternative to electro or photocatalytic water splitting. The precise role and mechanism of dopant species, however, is not always clear. Exploring dopant-enhanced catalysis requires a clear picture of composition-structureproperty relationships. Recently, two-dimensional (2D) nanosheets have captured interest for catalytic applications. The nanosheets are typically obtained by exfoliating a layered oxide material although other synthesis routes are also possible. Various 2D TiO2 systems have been demonstrated to be active, including pure TiO2 nanosheets 14–17 and metal-doped TiO2 nanosheets. 18–21 Thanks to the high surface area of the nanosheets, which can have thickness down to ∼1 nm or less, a large portion of the atoms exist at the surface. Therefore, they serve as a unique platform to explore the possibility of chemical modifications to the surface via incorporation of dopants. In our recent work, we demonstrated that isolated Rh dopants incorporated as substitutional defects at the Ti sites of TiO2 nanosheets can increase the photocatalytic hydrogen production rate by a factor of ten. 22 The atomic-scale mechanism for improved hydrogen evolution is not yet well-understood. It is also of interest to establish how dopants at the

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surface can affect the full water splitting reaction, including oxygen as well as hydrogen evolution. To fill these gaps, in this work we report the full OER profile on doped and undoped TiO2 nanosheets, as obtained from first-principles density functional theory (DFT). DFT has been extensively used to investigate surface reactivity of catalysts for oxygen evolution, by identifying correlations between activity and adsorption strength of reaction intermediates. 23–25 These studies typically assume a particular reaction mechanism, in which the OER occurs via intermediate steps involving -HO∗ , -O∗ , and -HOO∗ adsorbates. Photocatalytic or electrocatalytic activity is governed by the binding strength of these intermediates to the surface, and scaling relationships between binding strengths give rise to so-called “activity volcanos" which establish relationships between binding energies and activity. 13,26 Here we carry out this analysis for the TiO2 nanosheets, both undoped and transition metal doped. We compare the atomic-scale mechanism of the OER on the nanosheets to that of the more common polymorphs anatase 24 and rutile 25 titania. The rate limiting step on undoped 2D TiO2 surfaces is identified as the first proton-coupled electron transfer (PCET) that involves dissociative adsorption of water on the surface. Through modifications to the electronic structure that allow the formation of different reaction intermediates, several dopants are found to reduce both the thermodynamic energy barrier of key rate limiting steps and the theoretical required overpotential. Based on these insights, we present an activity volcano for transition metal doped TiO2 nanosheets. Subsequent measurements of the photocatalytic activity of selected candidates are carried out using a KIO3 aqueous solution. The measured trends are consistent with the first-principles predictions.

Methods Density Functional Theory and Structure Our study is based on spin-polarized DFT 27,28 calculations invoking the generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) 29 as implemented in the Vienna Ab-initio 4


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(a)

(b)

c a b

c a b

Ti

O

H

Dopant

(c) c b Step A

Step B

Step C

Step D

Figure 1: The atomic geometries of the TiO2 with lepidocrocite structure (a) unit cell of the lepidocrocite TiO2 slab model, (b) (4 × 3) slab model of transition metal doped TiO2 nanosheet, and (c) Four oxygen evolution reaction steps : step A (TiO2 ), step B (TiO2 + OH ∗ ), step C (TiO2 + O∗ ), and step D (TiO2 + OOH ∗ ). Simulation Package. 30,31 Pseudopotentials generated according to the projector augmented wave (PAW) scheme 32,33 are used to replace the core electrons, and the Kohn-Sham orbitals are expanded in a plane wave basis set with energy cutoff sufficient to converge total energies to the number of significant figures shown. As we are considering transition metal dopants, it may also be reasonable to incorporate DFT+U functionals. However it is not obvious what value of U is most applicable for each species and how to compare results obtained from different functionals. We therefore adopt standard DFT for this initial analysis. The TiO2 nanosheets crystallize in the lepidocrocite structure. 34–37 The structure can be obtained by considering two monolayers of anatase (001), and shifting one in-plane relative to the other. The shifted configuration is stable, because the surface Ti atoms become fully six-fold coordinated, in contrast to the five-fold (under-coordinated) Ti atoms at the anatase (001) surface. Our model TiO2 lepidocrocite nanosheet comprises of a (4 × 3) surface cell. The lepidocrocite nanosheet unit cell, and a supercell with a transition metal dopant atom present, are both shown in Figure 1. We use a 2 × 2 × 1 k-point sampling of the Brillouin zone. Consecutive nanosheet images in the periodic supercells are separated by an ≈15 Å vacuum with a dipole correction 38 included. All atomic geometries are relaxed until the forces acting on each atom are smaller than 0.01 eV/Å. With these simulation parameters, we find lattice constants of a = 3.77 Å and b = 3.03 Å, which are in good agreement with

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the experimental values (a = 3.8 Å, b = 3.0 Å), 39 and a band gap of 2.74 eV (in comparison to the experimental value of 3.84 eV 40 ).

Oxygen Evolution Reaction Model The OER activity is calculated using a straightforward scheme 23 that has previously been applied on anatase 24 and rutile 4,25 titania surfaces. In the OER, two water molecules are split to create two protons and an oxygen molecule via several intermediate steps. The OER is believed to occur through four proton coupled electron transfers (PCETs), and the overall activity is governed by the binding strengths of reaction intermediates to the surface. Four PCETs and their Gibbs free energy changes ∆G are: Step A → B:

∆GH2 O/OH ∗ ,

Step B → C:

∆GOH ∗ /O∗ ,

Step C → D:

∆GO∗ /OOH ∗ ,

Step D → E:

∆GOOH ∗ /O2 .

(1)

These intermediate steps, step A (TiO2 ), step B (TiO2 + OH ∗ ), step C (TiO2 + O∗ ), and step D (TiO2 + OOH ∗ ), are shown in Figure 1. Due to the fully coordinated nature of the atoms in the lepidocrocite nanosheet, it is expected to be non-reactive in an aqueous environment and exhibit limited interactions with water. Compared to other common anatase and rutile surfaces, the lepidocrocite nanosheet is probably most similar to anatase (101), which also exhibits little reactivity with water molecules. 24 Lepidocrocite nanosheets are different from both anatase (001) 24,41 and rutile (110) 42,43 which are believed to exist in a hydroxylated state in aqueous environments. In common with anatase (101) surfaces, we expect adsorption at the surface to be facilitated by delivery of a hole to the surface. This enables OH adsorption and proton release, followed by an electron transfer to the proton. 6


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For each intermediate step, the Gibbs free energy change is 23

∆G = ∆E + ∆ZP E + ∆H − T ∆S − |e|U .

(2)

The change in the internal energy ∆E is determined through DFT total energy calculations, and zero-point vibrational contributions (∆ZP E) are included from DFT vibrational frequency calculations. For each PCET step, we considered several possible starting geometries and choose the most energetically favorable relaxed configuration. Enthalpic ∆H and entropic T ∆S contributions are obtained using standard thermodynamic data for the molecules in the gas and liquid phases. 44 The term −e|U | refers to the applied overpotential in electrocatalysis, or the energy of a photoexcited electron-hole pair in photocatalysis. Note that since reaction barriers between the intermediates are neglected, this framework provides an initial but not complete picture of the reaction. We have not included the aqueous water environment nor do we consider the possibility of active site mediated processes (from other defects that may be present), which are all approximations but reasonable ones when comparing trends and patterns in structurally similar materials. 26 We have used the standard hydrogen electrode (SHE) as the reference potential, 23 which amounts to setting the hydrogen chemical potential for the reaction (H + + e− ) to that of 1/2H2 as 1 µH = G[H2 ] = G[H + + e− ] . 2

(3)

The reference energies for H2 and H2 O are directly calculated within DFT, and the free energy change ∆G of the total reaction 2H2 O → O2 + 2H2 is fixed at the experimental value of 4.92 eV for standard conditions when e|U | = 0. In this way we avoid DFT calculations of O2 , which are problematic in PBE. 45 Lastly, the entropy of gas-phase water is calculated at 0.035 bar, the equilibrium pressure at room temperature, so that the free energy of gas and liquid -phase water are equal. 25 Further details on the protocol can be found in Refs. [ 23–25 ]. 7



Results and Discussion Dopant Screening and Selection (a) 3.0 Ta

2.5 4GOH*/O* (eV)

Period 4 Period 5 Period 6

W Nb

2.0

Re Mo

V

1.5 1.0 0.5 0.0 0.0

Tc Os

Cu

0.5

1.0

Cr Ir Ru Co Rh Mn Pt Ni Fe Hf Ag Ti Zr Au Pd

1.5 2.0 4GH2O/OH* (eV)

2.5

3.0

(b) 3.0 Cr

2.5 V

4GO*/OOH* (eV)

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2.0

Mo Cu

1.5 1.0

Ta

W Nb

Fe Pt Pd Zr Mn Hf Ni Au Ru Co Ag Ir Rh Tc Os

Re


0.5 0.0 0.0

Ti

0.5

1.0

1.5 2.0 4GH2O/OH* (eV)

2.5

3.0

Figure 2: Scaling relationships between free energy changes of four proton coupled electron transfers (PCET) for oxygen evolution reaction at the transition metal doped TiO2 nanosheets. (a) relationship between the energies of first and the second PCETs, (b) relationship between the energies first and the third PCETs. To test the possibility that transition metal dopants may be effective for OER activity, we calculated the OER landscape for all transition metal species in the 3d, 4d, and 5d series to observe the trends. We are concerned with a general picture of composition-structureproperty relationships so we considered all transition metals, but we note that in practice it may be difficult to achieve doping with some of these species. 8


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0.0

> OverpotenŸal (eV)

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>0.5

Cu Os Tc Ag Co Ir Re Ru Ni Rh V


Mo Nb

>1.0

W

>1.5

Cr

Ta

Au Fe Mn Pt Zr Hf Pd Ti

>2.0 0.0

0.5

1.0

1.5 2.0 4GH2O/OH* (eV)

2.5

3.0

Figure 3: Activity volcano of the transition metal doped TiO2 nanosheets showing overpotential vs. free energy change of the first PCET. The OER activity is governed by the four energies in Eq. (1). The theoretical overpotential required (eV) is given by

max[

∆GH2 O/OH ∗ , ∆GOH ∗ /O∗ , ∆GO∗ /OOH ∗ , ∆GOOH ∗ /O2 ] − 1.23

(4)

For other titania polymorphs, empirical scaling relationships, such as ∆GH2 O/OH ∗ vs. ∆GH2 O/OOH ∗ (the latter is ∆GH2 O/OH ∗ + ∆GOH ∗ /O∗ + ∆GO∗ /OOH ∗ ), have been identified that simplify the identification of the overpotential. 4,13,26,46 Accordingly, we attempted to identify several possible scaling relationships for lepidocrocite TiO2 as well, and illustrate two in Figure 2. Although correlations are present to some extent in all cases, they appear less well-established for this system than others. 4,13,26,46 The point marked as Ti in Figure 2 represents the pristine TiO2 nanosheet, and other points are marked as a dopant which is incorporated into the nanosheet. First, our analysis suggests an inverse relationship between the first and second PCETs, ∆GH2 O/OH ∗ vs. ∆GOH ∗ /O∗ (Figure 2a). The gray dotted line is the least-squares constant offset, ∆GOH ∗ /O∗ = −∆GH2 O/OH ∗ + 2.90. Most atoms are located below and to the right

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side of the graph, indicating that the free energy change of the second PCET is typically less than the first. It is observed that an undoped TiO2 nanosheet requires the highest energy for the first PCET, since the lepidocrocite nanosheet is stable and largely non-reactive. Second, our analysis reveals a correspondence between the first and the third PCET ∆GH2 O/OH ∗ vs. ∆GO∗ /OOH ∗ , both of which involve dissociation of water molecules (Figure 2b). The leastsquares constant offset is given by ∆GO∗ /OOH ∗ = ∆GH2 O/OH ∗ + 0.09, illustrating that the free energy change for these two steps are quite similar. Here also most atoms are located above and to the right of the panel. These two observations indicate that for most dopants either the first or the third PCET reactions are rate limiting ; we also provide the OER profiles for all transition metal dopants in the supporting information to directly show this. The presence of a dopant species is largely to reduce the cost of the first or third PCET, at the cost of increasing the cost of the second and fourth. Since the free energy change for the first and third PCET is quite similar for a given dopant, we propose that the most suitable activity descriptor is ∆GH2 O/OH ∗ , the energy of the first PCET. We show in Figure 3 the corresponding activity volcano. An ideal catalyst, in terms of required overpotential, is one for which water oxidation can occur just above the equilibrium potential. This requires that all four charge transfer steps in Eq. (1) have the same reaction free energy change of 1.23 eV, which corresponds to the peak in the activity volcano. 47 The solid black line of the volcano is formed from the scaling relationship between ∆GH2 O/OH ∗ and ∆GOH ∗ /O∗ , while the dashed line of the volcano is formed from the scaling relationship between ∆GH2 O/OH ∗ and ∆GO∗ /OOH ∗ . The actual overpotentials for each dopant, obtained directly from Eq. (4), are also shown on the diagram (if the scaling relationships were exact, all points would appear on the volcano). To understand the trends in Figure 3, we note that for the species appearing to the right of the peak, the first or third PCET is rate limiting. For the species on the left, the second is rate limiting. Amongst all systems considered, the undoped sheet is most rightwards, with a very large rate limiting ∆GH2 O/OH ∗ . Dopants have the desired effect of reducing ∆GH2 O/OH ∗ 10


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at the cost of ∆GOH ∗ /O∗ , and overall bringing the system closer to ideal. For most dopants, the reduction of ∆GH2 O/OH ∗ is not sufficient to bring the species to the peak and the dopant remains on the right of the peak. The species that appear on the left are the ones for which the dopant sways the balance between ∆GH2 O/OH ∗ and ∆GOH ∗ /O∗ enough that the second PCET, rather than the first, becomes rate limiting.

Oxygen Evolution Reaction Profiles 6 Integrated UG (eV)

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5 4 3 TiO2 Rh-TiO2 Nb-TiO2 Pd-TiO2

2 1 0

A

B C KÆÇP v À}oµŸ}v Œ

D Ÿ}v •š ‰•

A

Figure 4: Gibbs free energy changes according to the oxygen evolution reaction steps shown in Figure 1 (Black: TiO2 , red: Rh-TiO2 , blue: Pd-TiO2 , and green: Nb-TiO2 ). To better understand the trends exhibited in the activity volcano, we compare the full OER free energy profiles for −e|U | = 0 eV vs. SHE for undoped sheets, and Rh, Nb, and Pd dopants in Figure 4. Rh was chosen amongst the species to the right of the volcano peak, because enhanced activity of Rh-TiO2 was demonstrated in our previous study. 22 Nb was chosen from the group of dopant species to the left of the peak. The dopant Pd is also considered, as it a dopant that was able to be experimentally characterized (discussed later). In Figure 4, we observe that, interestingly, the free energy change associated with A → C appears to be relatively constant for all systems considered. The difference between the dopants appears instead to arise from differences in the individual steps A → B and B → C. The profile for the undoped TiO2 (Figure 4 black lines) indicates large thermodynamic barriers at the first and third PCETs, 2.95 eV for steps A → B and 2.68 eV for steps C → D. 11



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These steps both involve dissociative water adsorption, and the large barriers arise from the over-coordinated state of the Ti atoms after adsorption. The large barriers are consistent with previous experiments, which suggested minimal water interaction with lepidocrocite. 42 Undoped TiO2 nanosheets are far from the ideal (minimum overpotential) scenario, as the ∆G with −e|U | = 0 for the four steps are respectively 2.95 eV, 0.07 eV, 2.68 eV, and -0.78 eV rather than 1.23 eV each. Similar to the anatase (101) 24 and rutile (110) surface, 25 the rate limiting step is the first PCET, ∆GH2 O/OH ∗ , which requires an overpotential of 1.72 V (−|e|U = −2.95 eV) to make ∆GH2 O/OH ∗ = 0. When transition metal dopants are introduced, several profile features are modified (red lines for Rh, green lines for Nb in Figure 4). Broadly, the dopants “even out” the energy barriers associated with A → B and B → C, bringing the profiles closer to ideal. For Rh, ∆GH2 O/OH ∗ (A → B) is easier but ∆GOH ∗ /O∗ (B → C) is more difficult. The ∆G values for the four PCETs are 2.12 eV, 0.92 eV, 2.13 eV, and -0.25 eV. Although the ∆GH2 O/OH ∗ (A → B) is still rate limiting, the theoretical overpotential is reduced to 0.90 V (−|e|U = −2.13 eV vs. SHE). For Nb, with −e|U | = 0, the corresponding ∆G values are 0.86 eV, 2.25 eV, 1.17 eV, and 0.64 eV. The reduction in energy for the first PCET is large enough to shift the balance so that the second PCET (B → C) is rate limiting.

Bader Charges and Possible Descriptors of Activity – Dopant Oxidation States To understand the trends in the profiles, Figure 5 indicates how the atomic geometries and the number of electrons surrounding each atomic species (Bader charges 48 ) change during A → B of Rh-doped, Nb-doped, and undoped systems, associated with ∆GH2 O/OH ∗ . The geometries show that it is preferable for the adsorbed OH ∗ in step B to attach not to a dopant, but on a neighboring Ti atom. 49 There are differences in the nature of the adsorbed OH ∗ with and without dopant, suggesting different reaction intermediates. Without a dopant, the OH ∗ stays between the bridging O atom and the neighboring Ti atom. However, with Rh the 12


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Step A 7.02

7.23 1.41

7.20

7.01 8.39

7.11 8.38

Step B 2.07 7.03

1.52 7.01 6.58

7.22 7.43

1.39

TiO2

1.47 6.99 6.53 8.68

Rh-TiO2

2.62 7.08

7.61

8.34

Pd-TiO2

Nb-TiO2

Figure 5: The atomic geometries and bader charges of (left to right) undoped, Rh-doped, Pd-doped, and Nb-doped TiO2 nanosheet during the oxygen evolution steps A to B, the first proton coupled electron transfer. OH ∗ appears to interact with a bridging O atom, forming an O − O − H bond. The O − O distance is only 1.52 Å, compared to 2.07 Å in the undoped sheet. The geometries of the Pd case are similar to those of Rh. The Nb dopant shows a further elongated O − O distance of 2.62 Å between the oxygen atoms, revealing that the OH ∗ avoids the bridging O atom near the dopant. 50 The different reaction intermediates in Figure 5 and the differences in ∆GH2 O/OH ∗ can be largely explained by considering the Bader charges of species present and favorable oxidation states of the dopants. While the most common oxidation state of Ti is +4, Rh is most commonly observed in a +3 oxidation state but is multi-valent and can often also be +4. Meanwhile Nb is most commonly observed in a +5 oxidation state. We suggest that in step A → B, in the undoped system Ti remains in a +4 state throughout and Nb in a +5 state throughout, whereas A → B is associated with a change in the oxidation state of Rh from +4 to +3. It is important to note that this analysis is speculative, but represents our effort to understand the physical mechanisms underlying the trends. In configuration A, Figure 5 indicates that the effect of both dopants on the pristine nanosheet is to reduce the number of electrons (e) on the bridging O atoms near the dopant. For undoped sheets the Bader charge on each O atom in the nanosheet is 7.23 e, which is reduced to 7.02 e with Rh and to 7.11 e with Nb. (The absolute values of e are not

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important, but changes to e can qualitatively suggest trends.) The dopants have higher electronegativity than Ti, and tend to pull electrons more tightly around themselves and away from the neighboring O. In the case of Nb dopant which adopts a +5 oxidation state, the additional e donated to the nanosheet is found from Bader charges to be distributed fairly evenly amongst all other atoms, rather than localized at a particular site. In any case, for both dopants the depletion of e around the bridging O atoms in step A creates different local environments that, in combination with changes to dopant oxidation states, modify the reaction intermediates formed when the OH species is adsorbed in step B. In the undoped sheet, the OH ∗ adopts a position in between the bridging O atom and the neighboring Ti atom. When OH ∗ is adsorbed the charge on the bridging O drops from 7.23 e to 7.03 e (Figure 5). The charge on the OH ∗ is 7.22 e, compared to its value of 7.00 e for an isolated molecular fragment. Therefore e from the bridging O are almost completely transferred to the OH ∗ during the first PCET. The Bader charge of the Ti atoms remain largely unchanged during the adsorption, with Ti retaining its +4 oxidation state. This change in charge distribution is associated with a large ∆GH2 O/OH ∗ = 2.95 eV. With Rh present, the charge on the bridging O drops more significantly from 7.02 e to 6.58 e from A → B, while the charge on the OH ∗ is now only 7.00 e. Meanwhile the charge on the Rh increases from 7.20 e to 7.42 e. This rearrangement is consistent with a change in the oxidation state of Rh from +4 to +3. The accommodation of the additional e at the Rh site enables the formation of the O − O − H intermediate, and reduces ∆GH2 O/OH ∗ to 2.12 eV. The O − O distance is 1.52 Å, suggesting the formation of an O2−2 peroxo species, and a weakly attached H. Based on its favorable oxidation states, the mechanism for Pd (assessed later experimentally) is likely similar to that of Rh. During A → B, for the Pd doped systems the geometries are similar to Rh and the electronic charge on the Pd increases from 8.39 e to 8.68 e. This may be associated with a change in the oxidation state from +4 to something more similar to +2 or +3. When Nb atoms are present, recall that the +5 oxidation state of Nb results in a dis-

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tribution of an extra electron throughout the nanosheet in configuration A. The adsorption of OH causes little change to the charge on the bridging O (7.11 e vs. 7.08 e), but the charge on the OH ∗ is now 7.61 e, much larger than the other cases. The oxidation state of Nb remains +5 throughout the process, and electrons instead are pulled from the other atomic species in the nanosheet, which had been accommodating extra charge in state A. This greatly reduces ∆GH2 O/OH ∗ to ∆G = 0.86 eV. The OH ∗ charge of 7.61 e indicates the formation of a OH − hydroxide that is only weakly attached to the nanosheet. In fact, when considering all transition metal dopants we found that as the charge of OH ∗ approached 8 e, its binding strength to Ti was reduced, thus lowering the required energy for A → B (not explicitly shown here). Based on these trends, we suggest possible descriptors of dopant activity and mechanisms in these nanosheets. When low oxidation state dopants are used (Rh), the formation of O − O − H bonds with stable O2−2 peroxo species lowers the thermodynamic barrier of the rate limiting step. With higher oxidation state dopants (Nb), the barrier is also reduced, now instead due to the ease of formation of OH − weakly attached to the nanosheet.

Experiments 5 O2 ' v Œ Ÿ}v ~…u}o•

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


4 3 2 d]K2 ZZrd]K2 W rd]K2

1 0

0

30

60 90 120 150 /ŒŒ ] Ÿ}v d]u ~u]v•

180

Figure 6: Oxygen evolution as a function of time for undoped (black circle), Rh-doped (red triangle), and Pd-doped (blue square) TiO2 nanosheets. To assess the model presented above, undoped and M -doped TiO2 nanosheets (M : Rh, Pd) were prepared from M -doped cesium titanium oxide, Cs0.7 Ti1.82−x Mx O4 (x = 0.018). 15



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Ideally it would be interest to test dopant species located on both the left and right side of the volcano plot (Figure 3). However, in the experiments, we are unfortunately limited by the dopants for which our synthesis procedure currently works, and unfortunately this limits us to Rh and Pd. We present the results for these species as an initial indication that the model results are consistent with experiment, although more validation will be necessary in the future. In our experiments, first Cs0.7 Ti1.82−x Mx O4 (x = 0.018) was prepared from CsCO3 , TiO2 , and RhCl3 · 3H2 O or PdCl2 . The mixtures were calcined in air at 673 K for 1 h, followed by grinding and calcination at 1073 K for 10 h. Cs0.7 Ti1.82−x Mx O4 (0.2 g) was converted into the protonated form by acid-exchange processing in a 0.1 M HCl solution (50 mL) for 1 day. Following the proton exchange reaction, the powder was washed in several changes of water by centrifugation. Nanosheet suspensions were obtained by stirring the paste of protonated powder (0.0236 g) in a 0.025 M tetrabutylammonium hydroxide (TBAOH) aqueous solution (50 mL) for 3 days. After the exfoliation reaction, the nanosheets were washed in 0.1 M HCl and water to remove TBAOH, and nanosheet powders were obtained by a freeze drying method. In our previous work, 22 high-resolution scanning tunneling microscopy revealed that the Rh dopants are incorporated into the nanosheets as substitutional dopants on the Ti sites. Note that we assume here that the dopant concentrations achieved in the TiO2 nanosheet powders have similar dopant concentration as in the M -doped cesium titanium oxide. Also note that the Pd-doped titania nanosheet has not been characterized in detail, and we again assume that Pd dopants are also incorporated in the lattice as other transition metal dopants such as Co, Ni, Zn, Mn, and Fe have been shown to be. 51–55 All of these factors necessarily introduce some degree of uncertainty in our experimental validation. Photocatalytic measurements were carried out using 50 mL of 0.01 M KIO3 aqueous solution (amount of nanosheet powder: 5 mg) under UV-light irradiation (500 W Xe-lamp).

16


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The photocatalytic reactions are

IO3− + 3H2 O + 6e− → I − + 6OH − (reduction)

(5)

H2 O → O2 + 4H + + 4e− (oxidation)

(6)

The electrons and holes used in the reaction are supplied by photo-excitation of the nanosheet. The photocatalytic reaction was performed using a conventional closed circulation system. A quartz reaction cell was irradiated by a 500 W Xe lamp. The amount of O2 formed was measured by gas chromatography with a thermal conductivity detector, which was connected to a conventional volumetric circulating line. In Figure 6, the oxygen evolution as a function of time is shown for undoped, Rh-doped, and Pd-doped TiO2 nanosheets. The results are reasonably linear, showing a steady rate of O2 production for around three hours. The activity is greatest for the Rh-doped sheet, followed by the Pd-doped sheet, and then the undoped sheet. This activity ordering is consistent with the volcano plot in Figure 3, for which Rh is closest to the vertex and Ti is farthest. Establishing direct correspondence between the experimental results and our attributed mechanism is always challenging, however, the agreement between the measurements and the straightforward computational predictions is encouraging and suggests that the analysis can be useful to estimate trends.

Conclusions We have carried out a detailed first-principles assessment of the oxygen evolution reaction (OER) on doped and undoped two-dimensional lepidocrocite TiO2 nanosheets. We assessed the full spectrum of 3d, 4d, and 5d transition metals as candidate dopants. There are several performance limiting features to the OER on undoped TiO2 nanosheet, necessitating the application of large overpotentials. However, we find that the incorporation of transition metal as a dopant modifies the reaction intermediates in a favorable manner, reducing the 17



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energy of the rate limiting step and the theoretical required overpotential. We established a connection between the degree to which the dopant extracts electrons from neighboring O atoms, linked to the dopant electronegativity and oxidation states, to its effectiveness for enhanced OER. Photocatalytic measurements of the oxygen generation for undoped, Rh-doped, and Pd-doped TiO2 nanosheets are consistent with the modeling.

Supporting Information Available Figure illustrating OER profiles for all transition metal dopants in titania nanosheets.

Acknowledgement The authors gratefully acknowledge the support of the International Institute for Carbon Neutral Energy Research (WPI-I2 CNER), sponsored by the Japanese Ministry of Education, Culture, Sports, Science and Technology. We also acknowledge the support of the National Science Foundation under Grant No. 1545907. E.M.T. acknowledges the support of the National Center for Supercomputing Applications SPIN (Students Pushing Innovation) undergraduate research internship program. Computational resources were provided by (i) the Extreme Science and Engineering Discovery Environment (XSEDE) allocation DMR-140007, which is supported by National Science Foundation grant number ACI-1053575, and (ii) the Illinois Campus Computing Cluster.

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(49) To be certain, we considered a configuration with the adsorbed OH ∗ attached to the dopant itself. This resulted in an internal energy increase of around 0.7 eV. (50) We tested the robustness of these geometries by taking the undoped geometry of step B and replacing a Ti atom with a Rh or Nb atom and re-relaxing, as well as replacing the dopant with Ti in step B relaxed doped geometries. In all cases, the geometries returned to those indicated in Figure 5. (51) Osada, M.;

Ebina, Y.;

Fukuda, K.;

Ono, K.;

Takada, K.;

Yamaura, K.;

Takayama-Muromachi, E.; Sasaki, T. Ferromagnetism in Two-Dimensional Ti0.8 Co0.2 O2 Nanosheets. Phys. Rev. B 2006, 73, 153301. (52) Gao, T.; Norby, P.; Okamoto, H.; Fjellvåg, H. Syntheses, Structures, and Magnetic Properties of Nickel-Doped Lepidocrocite Titanates. Inorg. Chem. 2009, 48, 9409– 9418. (53) Gao, T.; Fjellvaåg, H.; Norby, P. Defect Chemistry of a Zinc-Doped Lepidocrocite Titanate Csx Ti2−x/2 Znx/2 O4 (x = 0.7) and Its Protonic Form. Chem. Mater. 2009, 21, 3503–3513. (54) Dong, X.; Osada, M.; Ueda, H.; Ebina, Y.; Kotani, Y.; Ono, K.; Ueda, S.; Kobayashi, K.; Takada, K.; Sasaki, T. Synthesis of Mn-Substituted Titania Nanosheets and Ferromagnetic Thin Films with Controlled Doping. Chem. Mater. 2009, 21, 4366–4373. (55) Costa, A. M. L. M.; Marinkovic, B. A.; Suguihiro, N. M.; Smith, D. J.; da Costa, M. E. H. M.; Paciornik, S. Fe-Doped Nanostructured Titanates Synthesized in a Single Step Route. Mater. Char. 2015, 99, 150–159.

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Graphical TOC Entry Integrated energy change (eV)

4 3 2 1 0

A

B

C D KÆÇP v À}oµŸ}v Œ Ÿ}v •š ‰•

25


TiO2 Rh-TiO2 A

Two-Dimensional TiO2 Nanosheets for Photo and ... - ACS Publications

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