A Density Functional Theory Study of Water Photo-Oxidation at

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

A Density Functional Theory Study of Water Photo-Oxidation at Copper Oxide Nanostructures on the Anatase (101) Surface Victor Meng'wa, Nicholas Makau, George Amolo, Sandro Scandolo, and Nicola Seriani J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03671 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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The Journal of Physical Chemistry

A Density Functional Theory Study of Water Photo-Oxidation at Copper Oxide Nanostructures on the Anatase (101) Surface †,‡

Victor Meng'wa,

Nicholas Makau,

†

George Amolo,

¶

§

Sandro Scandolo,

and

∗,§

Nicola Seriani

†Department

of Physics, University of Eldoret, P.O Box 1125-30100, Eldoret, Kenya

‡Department

of Physics and Computer Science, Alupe University College, P.O Box 845-50400, Busia, Kenya

¶Department

of Physics and Space Science, The Technical University of Kenya, P.O BOX 52428-00200, Nairobi, Kenya

§The

Abdus Salam International Centre for Theoretical Physics, Condensed Matter and Statistical Physics Section, Strada Costiera 11, 34151 Trieste, Italy

E-mail: [email protected]

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

Abstract Copper-modied titania photocatalysts are known to display a higher activity towards water splitting than pure titania, but the mechanisms of this enhancement are unclear. We investigated oxidation of water on nanostructured Cu2 O/TiO2 and CuO/TiO2 systems by density functional theory. Under photoelectrochemical conditions, the most stable structure has a CuO stoichiometry, and yields an overpotential of 0.66 V for water oxidation, with the active site being at the interface between copper oxide and titanium dioxide. A lower overpotential of 0.48 V is found on a metastable Cu2 O nanostructure. These values are lower than the values for pure titania and for copper oxide. Moreover, in the case of the most stable structure, the active site is directly at the interface between CuO and TiO2 . We therefore argue that a cooperative catalytic eect is at play in this system, beside the known increased photoabsorption coming from the lower band gap. The nanostructures display a stronger adsorption of OH with respect to pristine titania, which is essential in lowering the overpotential, and switching the overpotential-determining step from hydroxyl formation to dehydrogenation of the adsorbed hydroxyl. This insight into the water oxidation reaction might provide some guiding principles for the design of improved photocatalysts for solar water splitting.

Introduction Solar energy provides a renewable and clean energy source that could satisfy the current global energy demand. 1 To store the energy harvested from the Sun, it would be convenient to eciently convert it into chemical energy and possibly into a storable, high-energy-density fuel, such as hydrogen. To achieve this, a possible route is photoelectrochemical (PEC) water splitting, which directly converts water to hydrogen and oxygen. 2 Hydrogen could then be used directly in fuel cells, e.g. in vehicles, 3 or could be used to produce hydrocarbons from carbon dioxide. The water splitting reaction (2H2 O → O2 + 2H2 ) is endothermic; reactants and prod2


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ucts are at equilibrium at an applied bias of 1.23 V with respect to the reversible hydrogen electrode (RHE). 1 In a photoelectrochemical cell, two half-reactions take place at dierent electrodes, the oxygen evolution reaction (OER) at the photoanode and the hydrogen evolution reaction (HER) at the cathode. The two reactions can be written as 2H2 O → O2 +4H+ + 4e and 4H+ + 4e → 2H2 , respectively. The OER is a major obstacle as it generally requires a large overpotential, 4 and this is the motivation driving current research. TiO2 is one of the most important photoanode materials for the OER due to its low cost, chemical and biological inertness and long-term stability against photo-chemical corrosion. 58 However, water splitting on TiO2 still displays a large overpotential for the oxygen evolution reaction. Nozik 9 determined an overpotential of about 1 V at a TiO2 rutile anode. Firstprinciples calculations based on Nørskov's approach conrmed this, with an overpotential of 0.78 V on the (110) surface of rutile, 10 and of 1.39 V on the (101) surface of anatase. 11 Intense research eorts have been devoted to reducing the OER overpotential on TiO2 and to shift the TiO2 absorption range from ultraviolet to visible range, for instance by doping with various elements. 12,13 Liu et al. 13 established in their experiments that TiO2 doped with dierent transition metals (TM) considerably suppressed the overpotential for oxygen evolution reaction (OER). Nevertheless, doping can lead to defects that can act as recombination centres, thus reducing the eciency of the photocatalyst regardless of the visible light absorption. 14 It is still unclear how to design a photocatalyst with high OER activity, largely because the chemical processes and kinetics of the OER are not fully understood. Besides doping, another eective strategy to enhance TiO2 photocatalytic activity is to couple with Cu2 O and CuO nanoparticles/clusters. Gawande et al. 15 reported that Cu2 O and CuO nanoparticles/clusters coupled to TiO2 signicantly enhanced HER. Wu and Lee 16 reported that deposited Cu particles on TiO2 , which are in the oxidized state during reaction, cause up to 10-fold enhancement in H2 production. Recent work on photocatalytic water splitting in a compact PEC cell with CuO-modied TiO2 photoanode, established enhanced 3



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photocurrent density, hydrogen production and solar to hydrogen eciency of about 20 % and 50 % in open spectrum and AM 1.5 G ltered light, respectively, from solar light. 17 It is however unclear what is the main mechanism leading to an improved activity in copper-modied titania. First, copper oxides are active on their own. Cu2 O has been reported as photocatalyst for water splitting under visible light irradiation. 18 CuO is an active catalyst for OER. In an experimental study of dierent morphologies of CuO as water oxidation catalysts, CuO nanowire exhibited the lowest overpotential for water oxidation with an onset potential of ∼ 0.90 V at pH 9.2. 19 Further reduction of the onset potential to

∼ 0.8 V (vs. RHE) at pH 13.6 towards OER was also established in annealed CuO on uorine doped tin oxide. 20 Moreover, copper oxides enhance absorption of visible light with respect to pure titania. Finally, the presence of titania and copper oxides might be favourable for separation of electron-hole pairs. In order to clarify the eect of copper modication on the functional behaviour of titania towards water splitting, it is necessary to investigate the processes contributing to the reaction. It is often assumed that the reaction proceeds on a single active site, with four proton-coupled electron transfers, going through a nucleophilic attack with formation of adsorbed OOH intermediates. 21 Indeed, Nakamura et al. identied OOH and OO species under reaction conditions, 22,23 and proposed that they formed through nucleophilic attack of a water molecule on a surface-trapped hole at surface lattice oxygen. Density functional theory (DFT) calculations of the OER energetics have also showed that the rst proton coupled electron transfer (PCET) process, i.e. the hydroxyl formation, is the rate determining step on both rutile 10 and anatase 11 TiO2 . Recent work has however suggested that the reaction is initiated by proton transfer (PT) followed by electron transfer (ET). 24 An atomic-level understanding of the OER chemical processes on Cu2 O/TiO2 and CuO/TiO2 not only would be of great scientic interest, but could also be helpful for the design of photoelectrochemical water splitting cells with improved eciencies. The nanostructures of copper oxides considered here were determined in our previous work, 25 which found Cu2 O and CuO 4


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nanowires to be more stable than clusters on TiO2 (101). Herein, we show by DFT calculations that the overpotential is reduced from 1.39 V for pure TiO2 (101) to 0.48 V and 0.66 V for the two main structures of CuOx -modied titania, with adsorbants at the CuOx -TiO2 boundary playing a crucial role. These values are lower than the values of 1 V for pure titania 9 and of 0.9 V for pure CuO. 19 We therefore suggest that a synergy between the two oxides is at play to increase the photocatalytic activity of the system.

Computational Methodology We performed spin-polarized DFT calculations using the Quantum-Espresso code, 26 which uses a plane wave basis set. Ultrasoft pseudopotentials were used to model electron-ion interactions. 27 GGA-PBE 28 exchange-correlation functional was used with eective Coulomb interaction (U) 29,30 to correct the self interaction error of the electrons in 3d states of titanium and copper. The value of U terms, drawn from literature are 4.2 eV 31 and 5.2 eV 32 for Ti 3d and Cu 3d states, respectively. We used a kinetic energy cuto of 40 Ry for wave functions and 480 Ry for charge density. A (1 × 2 × 1) k-point mesh 33 was chosen to sample the Brillouin zone. The TiO2 (101) surface was modeled with a rectangular (1 × 2) surface cell of size (10.27 Å × 7.57 Å), as in our previous work. 25 We loaded one side of anatase-TiO2 (101) surface slab with the copper oxide nanowires. The vacuum width between the TiO2 slab and its periodic image was ∼14 Å. In geometry optimizations, all atomic positions were relaxed using Broyden-Fletcher-Goldfarb-Shanno (BFGS) quasi-Newton algorithm gradient method until the force on each atom was smaller than 0.03 eV/Å. Isolated H2 and H2 O molecular calculations were performed by placing each one of them in a cubic box of (∼10 Å). A single water molecule was adsorbed either molecularly or dissociatively (split into OH and H) on possible adsorption sites on Cu2 O/TiO2 and CuO/TiO2 nanostructures. We calculated adsorption energy to determine the most favorable adsorption congurations as

5



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follows:

Eads = ET iO2 /Cux O + EH2 O − ET iO2 /Cux O/H2 O .

(1)

where Eads is the adsorption energy, ET iO2 /Cux O/H2 O , ET iO2 /Cux O and EH2 O are DFT total energies of the respective species. In this case, a higher positive value of Eads corresponds to stronger adsorption. We established the dierence in adsorption energy between single water molecule adsorbed on one side and on both sides of the TiO2 slab to be ∼ 5.1 meV. Moreover, from Lowdin charge analysis, no net charge transfer is observed on the clean TiO2 slab.

Results and Discussions Water Adsorption

Molecular and dissociative adsorption have been considered on the nanostructures from Ref. 25 We tested ten congurations for each system, which reduced to symmetries reported in Figs. 1 and 2, that are based on stabilities and preferential adsorption sites. For the system with Cu2 O, the most stable congurations are shown in decreasing order of stability in Fig. 1. The corresponding adsorption energies are reported in Table 1. (a)

(b)

(c)

(d)

(e)

(f)

Figure 1: Optimized congurations of adsorbed H2 O on Cu2 O/TiO2 anatase (101) nanowire surface arranged in decreasing order of stability. Blue balls: titanium; red balls: oxygen; red-brown balls: copper and white balls: hydrogen

6


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Table 1: Calculated adsorption energies of water molecule on Cu2 O/TiO2 nanowire surface.

Structure a b c d e f

Eads (eV) 0.78 0.67 0.50 0.43 0.32 0.30

Water prefers to adsorb molecularly on Cu2 O/TiO2 with the oxygen atom from water molecule being bonded to ve-fold coordinated Ti (Ti5C ) on the TiO2 surface with Eads of 0.78 eV (Fig. 1a). The most stable dissociate conguration has OH on Ti5C and H on O2C site (Fig. 1c). On the contrary, on CuO/TiO2 dissociated water is favoured, with OH species located at a bridge site between Ti and Cu atoms, and H bound to an O atom on CuO (Fig. 2 and Table 2). The adsorption energy is 1.12 eV, larger than on Cu2 O/TiO2 . This behaviour should be compared with those on pure oxides. On TiO2 (101), water is known to adsorb molecularly at low coverage, 3437 with co-existence of molecular and dissociated water at high (monolayer) coverage. 38,39 The trend we nd, with molecular adsorption in presence of Cu2 O and dissociative adsorption in presence of CuO, is in line with the behaviour of the pure copper oxides. DFT indeed predicts molecular adsorption on (111) 40 and (110) 41 surface of Cu2 O, with an adsorption energy of 0.82 eV on the former. Dissociative adsorption is favoured on CuO(111), with an adsorption energy of 0.9 eV. 42 We can therefore conclude that CuO contributes to water dissociation, while Cu2 O does not play a role at this stage, since water still prefers to adsorb at sites of the TiO2 surface. Also metallic copper is unlikely to be a good site for water adsorption or dissociation, since it is known to have low adsorption energies, around 0.4 eV, 43 with partial dissociation appearing above 150 K. 44 It is important to note that most stable water terminations on Cu2 O/TiO2 and CuO/TiO2 are consistent with empty states at the bottom of the conduction band found in the projected density of states of these

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systems in our previous work. 25 Water preferentially adsorbes on copper when copper states are at the bottom of the conduciton band, and vice cersa. (a)

(b)

(c)

(e)

(f)

(g)

(d)

(h)

Figure 2: Optimized geometries of adsorbed H2 O on CuO/TiO2 (101) nanowire surface arranged in decreasing order of stability. Table 2: Calculated adsorption energies of water molecule on CuO/TiO2 nanowire surface.

Structure a b c d e f g h

Eads (eV) 1.12 1.07 1.03 0.90 0.81 0.53 0.51 0.32

Thermodynamic Stability

Water splitting leads to the formation of surface hydroxyls, hydrogens, oxygens, and other adsorbed species. The thermodynamic stability of the surface in presence of these adsorbants has been determined, as a function of the applied electric bias. In the following, we denote a given surface termination as Sm k (k=1-3 and m=1-2) where k=1: pristine TiO2 anatase (101) surface; k=2: Cu2 O/TiO2 terminated surface; k=3: CuO/TiO2 terminated surface. We considered two terminations for each of Cu2 O/TiO2 and CuO/TiO2 , and we have used m=1-2 to distinguish them. Moreover, Sm k 8


(αβγ)

has α O*,

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β HO*, and γ HOO* adsorbed species. The free energy is given as:

GS m (αβγ) = ES m (αβγ) + (ZP E − T S)S m (αβγ) , k

k

k

(2)

where ES m (αβγ) is the total energy of the respective terminations calculated using DFT+U. k

ZPE and TS are zero point energy and entropy contributions, respectively, from the adsorbed species. These contributions to the free energies of the reaction intermediates are calculated at room temperature (298 K). The ZPE and TS values of intermediates on dierent metal oxides do not change considerably, 45 hence the ZPE and TS values are taken from Ref. 10 (see Table S1 of SI). To determine the stability of the surface adsorbed species under PCET conditions, we used the method developed by Nørskov and co-workers 46 to calculate the surface free energy:

γ(Ub ) =

X 1 [G − ni µi (Ub )], A i

(3)

where G is the free energy of pristine TiO2 anatase (101) surface, Cu2 O/TiO2 or CuO/TiO2 covered with intermediate species (O, OH, OOH), ni and µi are the number of atoms and chemical potential of type i, respectively, Ub is the eective bias and A is the area of each surface on which the species are adsorbed. In the case of pristine TiO2 anatase (101) surface, it will be 2A, where the factor of 2 is included since both surface orientations at the top and bottom are equivalent. The redox potential for the photogenerated holes, according to the photoelectrochemical studies 47 is about 3.25 V versus the normal hydrogen electrode (NHE) for TiO2 at pH=0. Thus, the stability of oxidation species at values of Ub ≥ 3.25 V are of great interest. The chemical potentials of dierent atomic types (O, H, Ti, Cu) are modied by the bias as indicated in supporting information (see SI for details). The chemical potentials of the bulk TiO2 , (µT iO2 ) is approximated to be equal to DFT energy per formula unit (εT iO2 ). For Cu, it's approximated to be equal to free energy of Cu metal (GCu ) that is given by the

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DFT energy per copper atom. Thus the surface free energy of Cu2 O/TiO2 and CuO/TiO2 in reference to Cu metal atom is:

1 γS m (αβγ) = {G − [nT i (εT iO2 − 2GH2 O + 2GH2 ) k A + nCu (GCu ) 1 + nO (GH2 O − GH2 ) + nH GH2 2 + (2nO − 4nT i − nH )eUb ]}.

(4)

where nT i , nCu , nO and nH denote number of Ti, Cu, O and H atoms, respectively. We calculated the reaction free energy for each reaction step using the approach developed by Nørskov et al. 46 More details can be found in the supporting information, as well as in Ref. 4851 In Fig. 3, we show plots of surface free energy against potential for various adsorbed species on Cu2 O/TiO2 and on CuO/TiO2 . The vertical dashed line indicates a Ub value of 3.25 V versus the normal hydrogen electrode (NHE) at which photogenerated holes are produced. For each termination, the most stable conguration is that with an adsorbed OOH. At an eective bias of Ub = 3.25 V, the CuO/TiO2 systems are more stable than the Cu2 O/TiO2 . This is an eect of the photoelectrochemical conditions. Indeed, as already reported before, 25 at zero applied bias the order of stability is reversed. This is consistent with some observation that, under reaction conditions, Cu2 O experiences surcial oxidation reaction to CuO. 52 1(001)

The most stable species, OOH on CuO/TiO2 (S3

), is located at the bridge site be-

tween Ti and Cu atoms, testifying of a possible cooperative behaviour of the two elements. The hydroperoxo is in fact the most stable on all considered terminations. The same was found to apply to Ga2 O3 -covered hematite. 53 On the contrary, the hydroxyl is the least stable species on most terminations. 10


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0

2

γS (eV/Å )

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


1(010)

S2

2(100)

S2

2(001)

2(010)

S3

S2

1(001)

S3

1(001)

S2

-0.4 0

0.5

1

1.5

2

2.5 3 Ub (V)

2(001)

S2

3.5

4

4.5

5

Figure 3: Surface free energies, γs (eV/Å2 ) of dierent water oxidation species as a function (αβγ) (αβγ) of applied bias on Cu2 O/TiO2 (Sm ) and CuO/TiO2 (Sm )(101) surface with bulk 2 3 1(100) (blue copper as reference. Other surface terminations not indicated are shown as: S3 2(100) 1(010) 2(010) solid line), S3 (orange solid line), S3 (black solid line), S3 (violet solid line) and 1(100) 2(100) 1(010) 2(010) S2 (brown solid line), S2 (indigo solid line). S2 /S2 overlap. Photo-Oxidation of Water

The process proceeds through four sequential proton-coupled electron transfer steps (PCET) giving rise to the intermediate species hydroxyl (HO*), atomic oxygen (O*) and hydroperoxo (HOO*). The water oxidation reaction is given by: A

2H2 O + (∗ ) − → HO∗ + H2 O + H + + e− B

− → O∗ + H2 O + 2(H + + e− ) C

− → HOO∗ + 3(H + + e− ) D

− → O2 + (∗) + 4(H + + e− ).

(5)

where (*) is the active site on the photoanode. The reaction step labelled A denotes hydroxylation of an active site, step B is the dehydrogenation of an adsorbed hydroxyl group, step C is the formation of hydroperoxo species and step D creates the release of an O2 molecule from the surface. We shall follow this reaction path in our calculations. The reaction free energies (4E) at zero bias for each PCET step are summarised in Table S3 of

11



SI. As a preliminary check, we calculated the reaction cycle also on the pristine (101) surface of anatase. There, the overpotential-determining step is the hydroxyl formation from the water molecule, with an overpotential of 1.39 V (see the SI for more details). This is in agreement with previous calculations. 10,11 The overpotential is calculated as follows: overpotential=max(GA ,GB ,GC ,GD )/e - 1.23 V. The calculated overpotential is a thermodynamic quantity, determined from the dierences between equilibrium states, as opposed to the kinetic barriers that exists between the states. Nevertheless, the computed overpotential compares well with measured overpotential, which takes into consideration the concentration of active sites and the current density. 54 Nanostructured Cu2 O/TiO2 Anatase (101) Surface

We considered two terminations of Cu2 O/TiO2 for the water oxidation reactions; one at the Ti5C atom (see Fig. 1a) and the other at a Cu atom on Cu2 O-supported TiO2 substrate (see Fig. 1b). The reaction pathways are the same as those on clean TiO2 anatase (101) surface. Our calculations of water photooxidation established overpotentials of 0.77 V and 0.48 V for the two terminations, whose water oxidation cycles are shown in Figs. 4a and 4b, respectively. The overpotential-determining steps are the dehydrogenation of the adsorbed hydroxyl group and formation of the hydroperoxo (OOH) species, respectively. This is different from the case of the pristine anatase surface, where the overpotential-determining step is the hydroxylation. There are however many systems where these steps determine the overpotential: dehydrogenation of the hydroxyl group is responsible for overpotentials of ∼ 0.8 V on bare and gallium oxide-covered hematite (0001) surfaces; 53 hydroperoxo formation determines overpotentials of 0.37 V and 0.56 V on RuO2 and IrO2 (110) surfaces, respectively. 55 The calculated overpotentials of 0.77 V and 0.48 V represent a substantial decrease from the value of 1.39 V calculated for clean TiO2 , and the value of 0.9 V measured for copper oxide. 19 For the Ti5C site, we attribute the decrease in overpotential to the presence of 12


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(a)

(b)

Figure 4: Water oxidation cycle on Cu2 O/TiO2 anatase (101) surface (a) Water-oxidation reaction occurring at Ti5C site (S21 ), (b) Water oxidation reaction on Cu2 O covered TiO2 (S22 ). The free energy of 1.71 eV at step C in (b) is much lower compared to energetics of the other overpotential determining steps of Cu oxides/TiO2 and responsible for the lowest overpotential of 0.48 V. the copper oxide, which alters the adsorption energies of the reaction intermediates. In particular, comparing the reaction energies and congurations on pristine anatase and on this system, it is possible to observe that the key contribution of the copper oxide is to stabilize the adsorbed OH. For the adsorption at the Ti5C site, this is possible because 13



copper modies the geometry of this site. In fact, in the presence of Cu, the range of O2C bond to two Ti atoms increased (∼ 0.1 Å, and ∼ 0.3 Å, respectively), accordingly, the Ti5C OH bond decreased by 0.06 Å, compared to similar bond on the pristine TiO2 surface. This indicates that OH adsorbs well at Ti5C site on Cu2 O/TiO2 surface, which is consistent with our thermodynamic studies. This assertion is further conrmed by adsorption energies of OH at Ti5C site on pristine TiO2 surface and Cu2 O/TiO2 surface, which are 1.42 eV and 2.56 eV, respectively. In the case of the cycle on the Cu atom, the stronger OH adsorption is due to the low O-coordination of the copper atom, bound to only one oxygen, i.e. to the nanostructured nature of the copper oxide. In fact, the low coordination of that copper atom is also related to the appearance of states in the band gap of TiO2 . 25 As an additional eect, Cu 3d and O 2p states extending in the gap could contribute to a better photo-response, 25,56 and to an enhanced electron transfer to adsorbed OH. 11 We notice that the calculated overpotential of 0.48 V is comparable to that of Mn-doped TiO2 nanowires, 13 and only higher than that of RuO2 (0.37 V 55 ), which is however a costly material. Nanostructured CuO/TiO2 Anatase (101) Surface

On this system, we investigated the catalytic cycle including the most favourable conguration with dissociative adsorbed water (Fig. 2a), and the cycle taking place completely on CuO (Fig. 2d). In the former, OH adsorbs at a bridge site between a Ti and a Cu atom, while hydrogen binds to an oxygen atom of the CuO nanostructure. The two water oxidation cycles are shown in Figs. 5a and 5b, respectively. They yield overpotentials of 0.66 V and 0.96 V, again considerably lower than the 1.39 V of clean TiO2 . The value of 0.96 V for the cycle taking place on the CuO nanostructure is in line with the experimental value for CuO. 19 At both sites, the overpotential-determining step is the dehydrogenation of the adsorbed hydroxyl, again at variance from what happens at the 14


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(a)

(b)

Figure 5: Water oxidation cycle on CuO/TiO2 anatase (101) surface. In both cases, the energetics at step B, which correspond to dehydrogenation of OH species are unfavorable. (a) Water-oxidation reaction at the interface of CuO and TiO2 anatase surface (S31 ), (b) Water-oxidation reaction on CuO supported by TiO2 anatase surface (S32 ). pristine anatase surface.

15



Conclusion We investigated the energetics of photoelectrocatalytic oxidation of water on the TiO2 anatase (101) surface modied by copper oxides, by means of density functional theory (DFT). The overpotentials for this reaction amount to 0.48 V for Cu2 O/TiO2 (S22 ), and 0.66 V for CuO/TiO2 (S31 ), a sharp decrease with respect to pristine titania (1.39 V). The main reason for this improvement is the stronger adsorption of hydroxyl (OH) on the coppermodied systems. This also reects in the nature of the overpotential-determining step: on the pristine surface, it is the hydroxyl formation, while on the systems considered here it is the dehydrogenation of adsorbed hydroxyl group in most cases. There is however a dierence between the two oxides at zero electric bias, with molecular water adsorption prevailing in presence of Cu2 O and dissociative adsorption being favoured over the CuO system. Finally, we show that, under photoelectrochemical conditions, the CuO system becomes more stable than the Cu2 O, contrary to what happens in absence of an electric bias. Therefore, CuO will most likely be present under photoelectrochemical reaction conditions. This work shows that there is a synergetic eect arising from the addition of nanostructured copper oxide to titania, leading to an improvement of the energetics of the reaction and to a lowering of the overpotential. The calculated overpotential is among the lowest overpotentials for this reaction, and is obtained with cheap and safe materials.

Acknowledgements Victor Meng'wa acknowledges the nancial support from OFID Postgraduate Fellowship Programme at ICTP provided under the ICTP/IAEA Sandwich Training Educational Programme (STEP). Computational facilities were provided by the Abdus Salam ICTP and Centre for High Performance Computing (CHPC). The authors are most grateful.

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Supporting Information Available It contains thermodynamic model of water oxidation, free-energy changes and overpotential calculations, thermodynamic stability, optimized geometries of water oxidation species, photo-oxidation on pristine TiO2 surface, free energy diagrams, and copper oxide-titania atomic models. This material is available free of charge via the Internet at http://pubs.

acs.org/.

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