Oxygen evolution reaction on doped and undoped lepidocrocite TiO2 nanosheets:design rules for the identification of optimal dopant species from first principles

Nam-Hoon Kim1, Emily M. Turner2, Shintaro Ida3,4,5, Tatsumi Ishihara3,4, and Elif Ertekin1,4

1Department of Mechanical Science & Engineering, 1206 W Green St.,University of Illinois at Urbana-Champaign, Urbana IL 61801, United States

2Department of Industrial and Enterprise Systems & Engineering,104 S. Matthews Ave., University of Illinois at Urbana-Champaign, Urbana IL 61801

3Department of Applied Chemistry, Graduate School of Engineering,Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

4International Institute for Carbon Neutral Energy Research (WPI-I2CNER),Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan and

5Department of Chemical Engineering, Graduate School of Engineering,Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan∗

(Dated: January 1, 2016)

The environmentally friendly production of H2 from water using only sunlight as energy inputhas long been a goal of sustainable energy research. Despite several advances, there remain se-vere limitations to overcome before photocatalytic water splitting is suitable for deployment atglobal scales. Designing photocatalysts is challenging, due to stringent requirements: good opticalabsorption of the solar spectrum, stabilty under exposure to ultraviolet light, efficient delivery ofphotoexcited carriers to the surface, and surfaces that are amenable to the desired reaction. Inspiredby the performance of recently synthesized Rh-doped lepidocrocite two-dimensional nanosheets, weused first-principles density functional theory to map the oxygen evolution reaction (OER) on thesenanosheets, both pristine and those containing transition metals. We assessed the key steps of theOER, including water molecule adsorption at the surface and the subsequent deprotonation steps.While undoped lepidocrocite nanosheets exhibit several limitations to effective performance, remark-ably we find that Rh dopants accelerate the process by eliminating the need for overpotentials andby reducing the thermodynamic barrier to dissociative water molecule adsorption. Based on theseinsights, we assessed the full spectrum of 3d, 4d, and 5d transition metals as candidate dopants. Weidentified general design principles to the selection of optimal dopants to enhance the OER. Theseresults present a set of predictions on dopant performance that can be tested in the laboratorynot only to understand in detail how surface dopants affect reaction mechanisms, but also towardsachieving high performance photocatalysts.

I. INTRODUCTION

A longstanding goal of renewable energy research is theconversion of water to hydrogen and oxygen using onlysolar energy as input. One approach is photocatalyticwater splitting1–4, in which sunlight is used to photoex-cite carriers in a semiconductor and provide the energyfor the reaction to take place. Titanium dioxide (TiO2)is the most frequently studied photocatalyst for solarenergy conversion, due to its many favorable propertiessuch as stability under irradiation and proper band align-ment relative to the water redox potentials2–4. The watersplitting reaction on TiO2 however suffers from slow ki-netics and requires large overpotentials, particularly forhydrogen production, thanks to the relative positions ofthe conduction band edge and the reference potential forH2 production5. To overcome this barrier, co-catalystssuch as Pt are often incorporated to increase the rate ofproduction6,7, although the precise role and mechanismof the co-catalysts are not always clear.

Recently, two-dimensional nanosheets have capturedinterest for photocatalytic applications. Such nanosheetsare typically obtained by exfoliating a layered ox-ide material although other synthesis routes are alsopossible. Various systems have been realized and

demonstrated to be active for photocatalysis, includingTiO2 nanosheets8, Pt/TiO2 nanosheets9, and CaNbO3

nanosheets10. Thanks to the high surface area of thenanosheets, which can have thickness down to ∼ 1 nmor less, a large portion of the atoms present exist at thesurface. Therefore, they serve as a unique platform to ex-plore the possibility of chemical modifications to the sur-face via the incorporation of dopant species. In our recentwork, we demonstrated that isolated Rh dopant speciesincorporated into 2D TiO2 lepidocrocite nanosheets cansubstantially increase (by a factor of ten in some cases)the hydrogen production rate11. The full mechanismfor the improved photocatalytic activity, however, andatomic-scale understanding of how the dopants affect thephotocatalytic process, is not yet well-understood. To fillthis gap, in this work we report the full oxygen evolutionreaction (OER) profile on these doped and undoped lep-idocrocite nanosheets, as obtained from first-principlestotal energy electronic structure methods based on den-sity functional theory (DFT).

We compare the atomic-scale mechanism of the OERto that of anatase and rutile TiO2. On anatase (001)12

and rutile (110)13, the first deprotonation is consideredto be the rate limiting step, requiring high overpoten-tials of ≈ 0.7 V (1.93 V vs. SHE, standard hydrogen

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Figure 1. Unit cell of lepidocrocite-TiO2 slab model (upper),slab model for Rh doped lepidocrocite-TiO2 nanosheet (lower)

electrode). Our analysis identifies the corresponding ratelimiting steps of the OER on lepidocrocite surfaces andfurther elucidates the role of surface dopant species inenabling the water splitting reaction. The presence ofRh dopant species at the surface is found to reduce theenergy barrier of several key rate limiting steps, includingthe initial adsorption of water molecules and some of thedeprotonation steps. The reduction of the energy bar-rier is associated with the elimination of the need for anoverpotential and a reduced thermodynamic barrier forwater molecule adsorption, which points to design rulesfor selection of optimal dopant species. With these designrules as a guide, we have screened through all transitionmetal dopant species, and also suggest other promisingcandidates. This set of predictions offers a unique test-bed for assessing how isolated dopant atoms can affectthe mechanism and pathway of the oxygen evolution re-action.

II. METHODS

Our study is based on spin-polarized DFT14,15 calcula-tions invoking the generalized gradient approximation ofPerdew-Burke-Ernzerhof (PBE)16 as implemented in theVienna Ab-initio Simulation Package17–20. Pseudopo-tentials generated according to the PAW scheme21,22 areused to replace the core electrons, and the Kohn-Shamorbitals are expanded in a plane wave basis set with en-ergy cutoff sufficient to converge total energies to thenumber of significant figures shown. We consider TiO2

lepidocrocite model nanosheet generated by a (4×3) sur-face cell, with 2×2×1 sampling of the Brillouin zone. Thelepidocrocite nanosheet unit cell, and a supercell witha Rh dopant atom present, are both shown in Figure1. Consecutive nanosheet images in the periodic super-cells are separated by an ≈ 15 A vacuum with a dipolecorrection23 included. All atomic geometries are relaxed

until the forces acting on each atom are smaller than 0.01eV/A. With these simulation parameters, we find latticeconstants of a = 3.77 A and b = 3.03 A, which are in goodagreement with the experimental values (a = 3.8 A, b =3.0 A)24, and a band gap of 2.74 eV (in comparison tothe experimental value of 3.84 eV)25.

The free energy profile of the OER is calculated us-ing a straightforward scheme26 that has previously beenapplied on anatase12 and rutile5,13 TiO2. In the reac-tion, two water molecules are split to create two hydrogenmolecules and an oxygen molecule via several intermedi-ate steps. We have not included the surrounding aque-ous water environment, which is an approximation but areasonable one when comparing trends and patterns inotherwise structurally similar materials27. The interme-diate steps are listed in Table I. For each intermediatestep, the Gibbs free energy change is26

∆G = ∆E + ∆ZPE + ∆H − T∆S − |e|U . (1)

The change in the internal energy ∆E is determinedthrough DFT total energy calculations, and zero-pointvibrational contributions (∆ZPE) are included fromDFT vibrational frequency calculations. For each OERstep, we considered several possible starting geometriesfor the atomic configurations and choose the one thatoffers the energetically most favorable relaxed configu-ration. Enthalpic ∆H and entropic T∆S contributionsare obtained using standard thermodynamic data for themolecules in the gas and liquid phases28.

We have used the standard hydrogen electrode (SHE)as the reference potential26, which amounts to setting thechemical potential (free energy per H) for the reaction(H+ + e−) to that of 1/2H2) as

µH =1

2G[H2] = G[H+ + e−] . (2)

For all reaction steps in Table I that involve an electrontransfer (the four deprotonation steps), an overpotentialof 0.78 V (U = 2.01 eV vs. SHE) is applied. This valuecorresponds to the energy of a photoexcited electron/holepair, and is chosen to make the free energy change offirst deprotonation, which is found here to be the biggest,equal to zero for the undoped system. The referenceenergies for H2 and H2O are directly calculated withinDFT, and the free energy change ∆G of the total reaction2H2O → O2 + 2H2 is fixed at the experimental value of4.915 eV for standard conditions. In this way we avoidDFT calculations of O2, which are problematic in PBE29.Lastly, the entropy of gas-phase water is calculated at0.035 bar, the equilibrium pressure at room temperature,so that the free energy of gas-phase water is equal tothe free energy of liquid water.13 Further details on theprotocol adopted here can be found in Refs. [12, 13, and26].

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Table I. Calculated free energies (eV) of elementary steps in oxygen evolution reaction at the surface of lepidocrocite TiO2and Rh-doped TiO2. The legend for the symbols is: A (TiO2), B (TiO2 + HO*+ H*), C (TiO2 + HO*), D (TiO2 + O*), E(TiO2 + HO* + HO*), F (TiO2 + HO* + O*), G (TiO2 + O* + O*).

TiO2 Rh-TiO2

Steps ∆E + ∆ZPE ∆H − T∆S −|e|U ∆G ∆E + ∆ZPE ∆H − T∆S −|e|U ∆GA + H2O → B 0.74 0.57 0.00 1.30 0.35 0.57 0.00 0.91

B + h+ → C + H+ 2.17 -0.16 -2.01 0.00 1.36 -0.16 -2.01 -0.81C + h+ → D + H+ -0.14 -0.16 -2.01 -2.31 1.26 -0.16 -2.01 -0.91

D + H2O → E 1.26 0.57 0.00 1.83 0.34 0.57 0.00 0.91E + h+ → F + H+ 1.38 -0.16 -2.01 -0.79 1.37 -0.16 -2.01 -0.80F + h+ → G + H+ 0.19 -0.16 -2.01 -1.98 1.03 -0.16 -2.01 -1.14

G → A + O2 -0.64 -0.54 0.00 -1.18 -0.75 -0.54 0.00 -1.30

Figure 2. Atomic configurations of oxygen evolution reaction at the surface of lepidocrocite TiO2 and Rh-doped TiO2 shownas unit cells (upper), Gibbs free energy changes according to the oxygen evolution reaction pathway (lower)

III. RESULTS AND DISCUSSION

A. Atomic structure

The TiO2 nanosheets crystallize in the lepidocrocitestructure30–32 as illustrated in Figure 1. This structurecan be obtained by considering two monolayers of anatase(001), and shifting one relative to the other. The config-uration is stable for the nanosheets, because when shiftedthe surface Ti atoms become fully six-fold coordinated,in contrast to the under-coordinated (five-fold) Ti atomsthat appear at the anatase (001) surface. Given the fullcoordination of the surface Ti atoms in lepidocrocite, theadsorption of water molecules is expected to be unlikely.Accordingly, in experiments no strong evidence for waterinteraction with lepidocrocite is observed33.

B. Free energy change along OER pathway

The computed OER free energy profiles for undopedand Rh-doped TiO2 nanosheets and the correspondingatomic geometries for each step are shown in Fig. 2.The contributions to ∆G for each step are broken downin Table I.

The profile for the undoped system (black line in Fig-ure 2) exhibits both similarities and differences comparedto the OER on anatase (001) and rutile (110) surfaces.Similar to anatase and rutile, the first deprotonationB → C is the most endothermic, necessitating the ap-plication of a large overpotential. The deprotonationstep that determines the overpotential is the one withthe highest energy change ∆E + ∆ZPE + ∆H − T∆S,since it will be the last step to become downhill as the

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Figure 3. Bader charge of each atom for lepidocrocite TiO2

(upper) and Rh-doped TiO2 (lower) for OER step A to B,dissociative water molecule adsorption.

Figure 4. Bader charge of each atom for lepidocrocite TiO2

(upper) and Rh-doped TiO2 (lower) for OER step B to C,removal of hydrogen atom.

overpotential is increased. For anatase and rutile, thefirst deprotonation is the most difficult, necessitatinglarge overpotentials ≈ 0.7 eV12,13,34 and limiting perfor-mance. We find a similarly large overpotential of 0.78 V(−|e|U = −2.01 eV) for lepidocrocite TiO2, also arisingfrom the first deprotonation (B → C), see the left sideof Table I. On the other hand, a notable difference be-tween undoped lepidocrocite vs. anatase or rutile is thehighly endothermic nature of the steps involving disso-ciative adsorption of H2O. Unlike the case of anatase12,35

and rutile33,36 for which dissociative H2O adsorption isfavorable, there are large thermodynamic barriers (stepA → B, 1.30 eV and step D → E, 1.83 eV for un-doped lepidocrocite). This is consistent with experimentssuggesting that water interaction with lepidocrocite isminimal33, and occurs because the lepidocrocite Ti atoms

are fully coordinated (as opposed to undercoordinated).After H2O is adsorbed (step B, undoped case), the at-tachment of the OH∗ fragment at a Ti site leaves the Tiover-coordinated.

When Rh dopants are introduced (red line in Fig. 2and right side of Table I), several differences are observedin the OER. Remarkably, the presence of Rh reduces theoverpotential from 0.78 V to ≈ 0 V (−|e|U = −2.01 eVto −|e|U = −1.20 eV vs. SHE). In other words, with Rhpresent, no overpotential is required for the deprotona-tion steps to satisfy ∆G ≤ 0; the energies of the photo-excited electrons and holes are already sufficient. (Notethat in Fig. 2 we have drawn both reaction profiles for−e|U | = −2.01 eV which makes B → C go downhill withRh present). Evidently the first deprotonation B → C iseasier in the presence of Rh: based on the ∆E + ∆ZPEcontribution in Table I, the uphill climb is reduced from2.17 eV to 1.36 eV. This is consistent with other indica-tions in the literature that the introduction of transitionmetal dopants can reduce overpotentials37. It is also con-sistent with the large photocatalytic (not electrocatalytic)activity observed in the doped nanosheets32. The atomicgeometries in Fig. 2 reveal differences in the nature of theadsorbed OH∗ in step C with and without Rh. WithoutRh the OH∗ avoids the bridging O atoms, while with Rhthe OH∗ appears to interact with a bridging O atom.The O − O distance is only 1.52 A, compared to 2.07A without Rh. We tested the robustness of these ge-ometries by taking the undoped geometry of step C andreplacing a Ti atom with an Rh atom and re-relaxing, aswell as by taking the doped geometry and replacing theRh atom with a Ti atom and re-relaxing. In all cases,the geometries returned to those indicated in Fig. 2.

A second notable difference that arises when Rhdopants are present is related to water molecule adsorp-tion steps, which are rate limiting according to the pro-files. For these two steps, ∆G changes from 1.30 eV to0.91 eV for A → B and 1.83 eV to 0.91 eV for D →E without and with Rh dopants, respectively. Althoughit remains endothermic, the thermodynamic barriers arestrongly reduced in the presence of the dopant. The re-duction of the barrier of the rate limiting step is also con-sistent with the observed increase in water splitting activ-ity on the nanosheets in the presence of the dopants11.Interestingly, as the geometry in Figure 2 shows, it ispreferable for the adsorbed OH∗ to be attached not tothe Rh dopant itself, but on the neighboring Ti atom.To be certain, we considered a configuration with theadsorbed OH∗ attached to the Rh itself, but found theinternal energy to be 0.75 eV higher than the configura-tion pictured in Fig. 2

While the presence of Rh reduces the barriers for watermolecule adsorption and for the first deprotonation, thetotal ∆G for the full reaction must be the same whetherdopants are present or not since the starting and end-ing points are the same. Correspondingly, there must besteps in the OER profile which offset the reduction of thethermodynamic barriers for water molecule adsorption

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steps and the first deprotonation. From Table I, thesesteps are the second C → D and fourth F → G deproto-nations. In the second deprotonation, ∆G = −2.31 eVwithout Rh dopants, but ∆G = −0.91 eV when dopantsare present. In the fourth deprotonation, ∆G = −1.98eV without Rh dopants, but ∆G = −1.14 eV whendopants are present.

C. Bader Charge Analysis

To better understand the differences observed in theOER profiles with and without Rh, we used Bader chargeanalysis38 which can roughly and qualitatively illustratethe nature of the electron rearrangement that takes placeduring the OER steps. Fig. 3 indicates how the numberof electrons surrounding each atomic species changes forthe first H2O adsorption A→ B for doped and undopedsystems. Before the water molecule is adsorbed, the effectof the Rh dopant is to reduce the number of electrons onthe neighboring O from 7.23e to 7.02e. This is probablydue to the higher electronegativity of the Rh atom, com-pared to the Ti atom which it has replaced, which allowsthe Rh to hold its electrons more tightly away from thesurrounding O neighbors. Consequently, the surface Oatoms around the dopant are somewhat depleted of elec-trons in step A. This creates a local environment thatis more amenable to the dissociative adsorption of H2O,which requires extracting 0.4e (qualitative) to break themolecular bonds. As Fig. 3 shows, the 0.4 electronsare nearly completely transferred to the bridging O dur-ing dissociative adsorption. Since this O is depleted ofelectrons when Rh is present, this makes the electronredistribution more favorable, reducing the energy costof the dissociative adsorption from ∆G = 1.30 eV to∆G = 0.91 eV.

Figure 4 also shows how the electrons are redistributedwith and without Rh atoms present for the first depro-tonation B → C. The geometries and Bader chargesfor B and C represent the charge redistribution associ-ated with the removal of the H atom, which occurs as aproton-coupled electron transfer (PCET) H+ + e−. Forundoped B → C, removal of H causes the number ofelectrons on the bridging O to drop from 7.65e to 7.00eand on the OH∗ to drop from 7.60e to 7.27e. The netloss of e is ≈ 1, corresponding to the electron that is lostduring the PCET; stripping the electrons from the elec-tronegative O atoms makes this step energetically costly.With Rh present, in step C it appears that the formationof an O − O bond is possible, and the resulting electronsharing between the O atoms makes it easier to strip theO atoms of electrons during the PCET. Now the removalof the H causes the number of electrons on the bridgingO to drop more substantially from 7.51e to 6.58e and onthe OH∗ to drop from 7.60e to 7.01e. The net loss of enow is ≈ 1.5, only 1e of which is lost during the PCET.In other words, the formation of the O−O bond actuallycauses the O atoms to lose more electrons than necessary

Figure 5. Adsorption energy vs. charge of surface oxygenatom (top), adsorption energy vs. electronegativity (middle),charge of surface oxygen atom vs. electronegativity (bottom)

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for the removal of the H atom. The remaining 0.5e thatare lost need to be accomodated as well, and they redis-tributed to the neighboring atoms, particularly the Rhitself. We surmise that once again, the electronegativ-ity of the Rh atom allows it to accommodate the excesselectrons to enable the formation of the O-O bond. TheO−O distance is 1.52 A, suggesting the formation of anO−2

2 peroxo species and a weakly attached H.

D. Dopant Screening and Selection

For the lepidocrocite nanosheets, the rate limiting stepof the OER is dissociative H2O adsorption as Figure 2suggests. If the dopant species is able to deplete thecharge around the surface O atoms, then a local environ-ment more suitable for H2O adsorption is created and thethermodynamic barrier can be reduced. Since the degreeto which the dopant species can deplete charge aroundneighboring O atoms depends on its electronegativity,it is possible that the more electronegative a particularatomic species is, the more effectively it will facilitateH2O adsorption. We considered this possibility in detail,by calculating the thermodynamic barrier for dissociativeH2O adsorption for all transition metal species in the 3d,4d, and 5d series. In Fig. 5, we compare our results us-ing scatter plots that compare a) the adsorption energyvs. the charge of the neighboring O atoms according toBader analysis, b) the adsorption energy vs. the Allenelectronegativity39 of the dopant, and c) the charge ofthe surface atom vs. the Allen electronegativity.

The top panel indicates a good correlation between theadsorption energy of the water molecule and the Badercharge on the surrounding oxygen atoms. This reaf-firms the possibility that depleting the neighboring oxy-gen atoms of charge makes water molecule adsorption lessunfavorable. In fact, several dopant species, such as Fe,Co, Ni, and Cu, are found to have very small (≈ 0 eV) ad-sorption energies and may also be effective for increasingthe water splitting photocatalytic activity. Some species(Zn, Cd, and Hg) even have slightly negative adsorptionenergies so that the adsorption is thermodynamically fa-vorable, although in some cases concerns about toxicitymay preclude their usage.

The correlation between the adsorption energy and theAllen electronegativity and between the charge of thesurface oxygen and the Allen electronegativity is not asgood, but still visible in the middle and lower panelsof Fig. 5 respectively. We also carried out the sameanalysis using the more common Pauling electronegativ-ity scale40, but find that the correlation is not as good.These results are available in the Supplementary Infor-mation for comparison purposes. The Allen electronega-tivity scale was established with the intent of improving

the description of transition metal electronegativity sincetransition metals tend to form ionic bonds with oxygen(while the Pauling scale is based on the strength of thecovalent TM-H bond)

IV. CONCLUSIONS

We have carried out a detailed first-principles assess-ment of the oxygen evolution reaction on doped and un-doped two-dimensional lepidocrocite nanosheets. Thereare several performance limiting features to the OER onundoped lepidocrocite, including the need for large over-potentials comparable in magnitude to anatase and rutileTiO2 and thermodynamic barriers to dissociative watermolecule adsorption. However, we find that the incorpo-ration of Rh as a dopant atom at the surface enhancesperformance in several critical ways: it eliminates theneed for overpotentials and reduces the thermodynamicbarrier to dissociative water adsorption.

Based on these insights, we have assessed the fullspectrum of 3d, 4d, and 5d transition metals as can-didate dopant species for incorporation into these two-dimensional photocatalytic systems. We suggest a con-nection between the degree to which the dopant extractselectrons from neighboring O atoms, somewhat linkedto the dopant electronegativity, to its effectiveness as adopant. We offer a set of predictions for test in the labo-ratory to understand in detail how surface dopants affectreaction mechanisms, and towards the realization of high-performance systems for photocatalytic water splitting.

V. ASSOCIATED CONTENT

A. Supporting Information Available

Assessment of water molecule adsorption energy usingPauling electronegativity scale.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the support of theInternational Institute for Carbon Neutral Energy Re-search (WPI-I2CNER), sponsored by the World PremierInternational Research Center Initiative (WPI), MEXT,Japan. E.M.T. acknowledges the support of the NationalCenter for Supercomputing Applications SPIN (StudentsPushing Innovation) undergraduate research internshipprogram. Computational resources were provided by (i)the Extreme Science and Engineering Discovery Environ-ment (XSEDE) allocation DMR-140007, which is sup-ported by National Science Foundation grant numberACI-1053575, and (ii) the Illinois Campus ComputingCluster.

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Figure 6. Adsorption energy vs. charge of surface oxygenatom (top), adsorption energy vs. electronegativity (middle),charge of surface oxygen atom vs. electronegativity (bottom)

Supporting Information