Theoretical Study of Structure, Stability, and the Hydrolysis Reactions

Article pubs.acs.org/JPCA

Theoretical Study of Structure, Stability, and the Hydrolysis Reactions of Small Iridium Oxide Nanoclusters Xin Zhou,*,† Jingxiu Yang,†,‡ and Can Li† †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, Dalian 116023, China ‡ Graduate School of the Chinese Academy of Sciences, Dalian 116023, China S Supporting Information *

ABSTRACT: The geometric structures and relative stabilities of small iridium oxide nanoclusters, IrmOn (m = 1−5 and n = 1−2m), have been systematically investigated using density functional theory (DFT) calculations at the B3LYP level. Our results show that the lowest-energy structures of these clusters can be obtained by the sequential oxidation of small “core” iridium clusters. The iridium-monoxide-like clusters have relatively higher stability because of their relatively high binding energy and second difference in energies. On the basis of the optimized lowest-energy structures of neutral and cationic (IrO2)n (n = 1−5), DFT has been used to study the hydrolysis reaction of these clusters with water molecules. The calculated results show that the addition of water molecules to the cationic species is much easier than the neutral ones. The overall hydrolysis reaction energies are more exothermic for the cationic clusters than for the neutral clusters. Our calculations indicate that H2O can be more easily split on the cationic iridium oxide clusters than on the neutral clusters.

1. INTRODUCTION

process using solar energy, most semiconductor-based photocatalysts require suitable cocatalysts for both reduction and oxidation reactions. It is believed that the cocatalysts provide reaction sites, promote the separation of photoexcited charges and decrease the activation energy for H2/O2 evolution.5 Transition-metal oxides such as IrOx and RuOx are usually applied as effective cocatalysts for O2 generation.6−9 In fact, these oxides have been known to be good catalysts for water oxidation for many years.10−13 Harriman and co-workers investigated a number of such oxides, including RuO2, IrO2, Co3O4, and Mn2O3,and found that IrO2 was both stable and very active in the water oxidation with sacrificial reagents under photochemical conditions.14 Several groups have now found iridium oxide as a favorable cocatalyst that promotes the rate of oxygen evolution in photocatalytic and photoelectrochemical water splitting.8,9,15−18 Kudo et al. used a simple photodeposition method to load IrOx on La-doped NaTaO3 and found that the activity of NaTaO3:La for O2 evolution was increased by 1.5 times by the promoter IrOx. Their experimental results indicated that the iridium oxide plays important roles in O2 evolution and the suppression of the charge recombination.8 Domen et al. investigated the effect of loading IrOx by impregnation method on La−In-based oxysulfide, and

Photocatalytic water splitting to generate H2 and O2 has attracted enormous attention in recent years as a potential means for the large scale production of H2 from water using abundant solar light.1−4 Oxygen evolution reaction is currently considered a major bottleneck for photocatalytic water splitting, because it is an uphill reaction involved a four electron transfer process. So the research on the oxygen evolution reaction is important to develop effective photocatalytic systems for overall water splitting. As illustrated in Scheme 1, for the water-splitting Scheme 1. Schematic Energy Diagram of Photocatalytic Water Splitting

Received: June 29, 2012 Revised: September 14, 2012 Published: September 17, 2012 © 2012 American Chemical Society

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dx.doi.org/10.1021/jp3064068 | J. Phys. Chem. A 2012, 116, 9985−9995

The Journal of Physical Chemistry A

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Figure 1. Lower-energy isomers of IrOn and Ir2On clusters. The blue and red balls represent the iridium and oxygen atoms, respectively. The spin multiplicity, relative energies (in eV) with respect to that of the corresponding lowest-energy isomer and the calculated expectation values of the S2 operator are provided below the structures, respectively.

performed a systematic study on IrmOn clusters with m ranging from 1 to 5 and n from 1 to 2m. Our purposes are 4-fold: (i) to compare the structures of IrmOn clusters at different O/Ir ratios, and investigate the oxidation process of small iridium oxides clusters with continuous addition of O atoms, (ii) to study the structures of ionic IrmOn clusters and compare with the neutral ones, because in experiments charged clusters are usually involved, (iii) to estimate the relative stability of IrmOn clusters according to binding energies and second difference in energies, and (iv) to investigate the mechanism of hydrolysis reaction of (IrO2)n (n = 1−5) in the calculated lowest-energy states.

the oxidation of water to O2 was promoted by 3-fold under visible light.9 Li and co-workers explored that the effect of the loading amount of IrOx on the photocatalytic O2 production on IrOx− ZnO/Zn2−xGeO4−x−3yN2y.16 Their results showed that the photocatatlytic activity of O2 evolution decreases when the amount of IrOx is increased.15 Domen and co-workers examined SrNbO2N as a photoelectrode for water splitting under visible light, and they found that loading colloidal IrOx enhanced the rate of O2 evolution by an order of magnitude.18 These cocatalysts are typically present as nanoparticles on the semiconductor surface by impregnation or in situ photodeposition.19 However, experimental evidences for structure and the size evolution for the iridium oxide nanoclusters are scarce. Nahor and co-workers radiolytically prepared iridium oxide clusters as catalysts in the photochemical oxidation of water. They found that the clusters contained four or five Ir atoms in a mixture of IrIII and IrIV states.20 Recently, Banerjee and co-workers have reported that annular dark field scanning transmission electron microscopy images provide evidence revealing the structure of IrOx nanocrystals as a core shell structure, where the inner structure is rich in Ir and the outer area is rich in O.21 Despite some efforts, the structures and properties of iridium oxide nanoparticles are still far from complete understanding. A number of researchers have indicated that one can gain insight into the physics and chemistry of nanostructures from the atomic clusters, which play an important role as a bridge the molecules and the condensed-phase materials.22−26 Understanding geometrical and electronic structures of small iridium oxide clusters are important for further investigation of catalytic reactions on iridium oxide clusters and construction of nanostructures with larger size. Therefore, in this work, we

2. COMPUTATIONAL METHODS All calculations are performed with the GAUSSIAN09 package.27 The Becke three-parameter hybrid functional with the Lee− Yang−Parr correlation functional (B3LYP)28 is chosen for taking into account the exchange and correlation effects of electrons, which has been successfully applied to study oxide clusters.29−33 Geometries were optimized at the unrestricted (U)B3LYP level of theory and without any symmetry constraint. (U)B3LYP vibrational frequencies were calculated to characterize the stationary points or transition states located on the potential energy surface and to obtain the zero-point energy corrections as well as the thermal and free energy corrections at 298 K. An allelectron double-ζ basis set 6-31++G(d,p) was used for oxygen and hydrogen including polarization and diffuse functions.34−36 The iridium atom was represented by the LANL2DZ pseudopotential and its corresponding basis set.37−39 This scheme is a good compromise between accuracy and computational effort. Several low-lying spin states of a given model have been considered in the calculations. Solvent effects have been 9986



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Figure 2. Lower-energy isomers of Ir3On clusters. The blue and red balls represent the iridium and oxygen atoms, respectively. The spin multiplicity, relative energies (in eV) with respect to that of the corresponding lowest-energy isomer and the calculated expectation values of the S2 operator are provided below the structures, respectively.

considered in the framework of the self-consistent reaction field polarizable continuum model (PCM).40

Doublet through octet spin states are considered for these two clusters. Our calculations show that IrO has a quartet ground state. The optimized bond length of IrO is 1.721 Å. For IrO2, the lowest energy structure is a doublet. Our calculations predict that an obtuse isosceles triangle structure (Figure 1(a2)) is much more stable than acute triangular structure (Figure 1(b2)). The linear OIrO and IrOO structures as those found in other metal oxides have not been located as stable structures.30,42 Singlet through septet spin states for Ir2On clusters are examined in the calculations. As shown in Figure 1, all the lowest energy structures of clusters tend to have lower spin states: triplet states for Ir2O, Ir2O2, and Ir2O3 and singlet state for Ir2O4. The ground state of the Ir2O cluster has an acute isosceles triangle structure with the apex angle of 81.6°. When two O atoms are connected to two Ir atoms (Ir2O2), the lowest-energy structure is a rhombus as shown in Figure 1 (a4). For Ir2O3, the structure with two O atoms bonded to the each Ir atom in Ir2O-triangle is energetically more favorable than the monocyclic structure (Figure 1 (d5)), suggesting that the O−O bond is not favorable in iridium oxide clusters. The Ir2O4 cluster, which has an O/Ir ration of 2:1, prefers a structure with an Ir2O2 rhombus and two IrO bonds on each side of the rhombus (Figure 1 (a6)). The structure in such a configuration is much lower in energy than the structure with two O atoms bonded to the same Ir atom of the Ir2O2 rhombus (Figure 1 (b6)). Ir3On (n = 1−6). The low-energy structures of Ir3On clusters are shown in Figure 2. We have considered doublet through octet

3. RESULTS AND DISCUSSION 3.1. Geometrical Structures. An extensive search for the ground state of IrOx clusters was performed, which included several two- and three-dimensional configurations. The choice of the initial geometries was mostly dependent upon the previous studies of clusters of silicon oxide,41,42 titanium oxide,43,44 vanadium oxide,45 and lead oxide.30 Some lower-energy structures of the clusters obtained from our present calculations are plotted in Figures 1−4, and other higher-energy structures are shown in Figure S1−S3 (Supporting Information). Several isomers are plotted for each cluster and relative energies with respect to the corresponding lowest-energy isomers are also shown in the figures. Because all calculations were performed within the unrestricted formalism UB3LYP, the problem of spin contamination and its effect on the calculated energetics arises. As shown in Figures 1−4, the calculated expectation values of S2 of the high-spin states are as follows: 0.75−0.82 for doublets, 2− 2.14 for triplets, 3.75−4.05 for quartets, 6−6.48 for quintets, 8.75−8.89 for sextets, 12−12.05 for septets, and 15.75−15.86 for octets. Some of the triplet, quartet, and quintet species were found to be spin-contaminated, but the calculated expectation values of the S2 operator are still within reasonable limits. IrOn (n = 1−2) and Ir2On (n = 1−4). Optimized lowest energy structures for IrO and IrO2 clusters are shown in Figure 1. 9987



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oxygen atoms and additional terminal oxygen atoms symmetrically attached to iridium atoms. The lowest energy isomer of Ir3O6 has a hexagonal arrangement of iridium−oxygen atoms, with three oxygen atoms attached to iridium atoms. As shown in Figure 2, our calculations show the preference of Ir for 3-fold coordinated configurations. Ir4On (n = 1−8). Figure 3 illustrates the low-energy structures of Ir4On. Singlet through quintet spin states are considered for calculations for Ir4O7 and Ir4O8, and singlet through septet spin states are considered for all other Ir4On (n = 1−6) clusters. Spin states of the lowest energy structures vary between singlet and quintet. The recent calculations performed by Jiang and coworkers indicate that a planar D4h structure has been obtained as

spin states for all the clusters in the calculations. All the lowest energy structures of clusters tend to have lower spin states: doublet states for Ir3O3, Ir3O4, and Ir3O5, and quartet states for Ir3O, Ir3O2, and Ir3O6. For Ir3O, the most stable structure is one with an O atom bonded directly to the lowest-energy structure of the Ir3 cluster (Figure 2 (a1)).46 The most favorable Ir3O2 structure has a four-membered ring, with an oxygen atom attached to iridium atom adjacent to oxygen atom in the ring. In the case of Ir3O3, the most stable structure is an extension of the corresponding structure in Ir3O2, with additional oxygen attached to one of the iridium atoms leading to Cs symmetry for the isomer. For Ir3O4 and Ir3O5, the energetically favorable structure in each case is a five-membered ring with two bridging 9988



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the ground state of the Ir4 cluster.46 Our results show that the most stable structure of the Ir4On (n = 1−3) cluster can be obtained by a sequential oxidation process of the planar Ir4 cluster by adding oxygen atoms one by one into the one of the terminal sites of the Ir4 cluster. For Ir4O4, the 4-fold structure

with bridging oxygen atoms is more favorable in energy than the one with terminal oxygen atoms. Furthermore, the square Ir4O4 ring is lengthened by attaching the terminal oxygen atoms to iridium atoms in Ir4O5 and Ir4O6 clusters. The lowest-energy isomer of Ir4O7 can be constructed from the Ir4O6 cluster by 9989



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Figure 5. Most stable structures of cationic and anionic Ir4On (n = 1−8). The blue and red balls represent the iridium and oxygen atoms, respectively. The spin multiplicity and the calculated expectation values of the S2 operator are provided below the structures, respectively.

Figure 6. Most stable structures of cationic and anionic Ir5On (n = 1−10). The blue and red balls represent the iridium and oxygen atoms, respectively. The spin multiplicity and the calculated expectation values of the S2 operator are provided below the structures, respectively.

Ir5On (n = 1−10). Figure 4 shows the optimized lower energy isomers for Ir5On (n = 1−10) clusters. Doublet through octet spin states are considered for calculations of the Ir5On (n = 1−4) clusters; doublet through sextet spin states are considered for the Ir5On (n = 5−10) clusters. Our calculations indicate that lower spin states (doublet or quartet) for most isomers are energetically accessible. Jiang’s calculations predict that the square pyramid

adding a bridging oxygen atom between two Ir atoms opposite to each other. In the case of Ir4O8, our calculations predict the monocyclic structure (Figure 3 (a8)) is more stable than the multiring structure. As summarized in Figure 3, the structures with more 3-fold coordinated Ir atoms are energetically more favorable. 9990



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structure is the most favored in energy in the three isomers of Ir5 cluster.46 As shown in Figure 4, the lowest-energy structures of Ir5On (n = 1−4) clusters can be obtained by bonding oxygen atoms directly to the ground state of Ir5 cluster one by one. When the O/Ir ratio goes to 1, our calculated results show that Ir5O5 cluster favors the planar butterfly structure. From Ir5O6 to Ir5O9, the most favorable isomers are constructed by bonding oxygen atoms directly to the planar Ir5O5 cluster one by one. When the O/Ir ratio equals to 2, the most stable structure is composed of two connected sixe-membered rings with dangling IrO bonds, which is different from those of other oxygen rich clusters. Ionic Clusters. On the basis of the structures of the neutral Ir4On and Ir5On clusters, we have performed further calculations to study the structures of cationic and anionic clusters Ir4On± (n = 1−8) and Ir5On± (n = 1−10). To search for the energetically favorable structures of the ionic clusters, several lower-energy isomers of each size of neutral Ir4On and Ir5On clusters were selected as initial structures and then optimized with the addition of one positive or one negative charge. Several spin states (doublet through sextet for Ir4On± and singlet through quintet for Ir5On±) are again considered in the calculations. The lowestenergy isomers for each ionic cluster are presented in Figures 5 and 6. The most stable structures of all Ir4On+ clusters and Ir4On− (n = 1−6) clusters are calculated to be similar to those of neutral ones. The lowest-energy structures Ir4O7− and Ir4O8− clusters correspond to the lower-energy (about 0.3 eV higher than the most favorable structure) neutral isomers c7 and b8 in Figure 3, respectively. Except Ir5O4+ and Ir5O8+, all the cationic clusters are predicted to have the lowest-energy structures similar to the corresponding neutral clusters. The most stable structures of the Ir5O4+ and Ir5O8+ clusters are the lower-energy isomers of neutral species depicted in Figure 4. For the anionic Ir5On clusters, the energetically favorable structure of Ir5O5− is distorted square pyramid structure, rather than the planar butterfly structure of the neutral one; the second-lowest-energy isomer of neutral Ir5O8 cluster is calculated to be the most stable structure of the anionic species; other anionic Ir5On clusters are computed to have the lowest-energy structures similar to the corresponding neutral clusters. Therefore, our results indicate that most of the small iridium oxide clusters Ir4On and Ir5On are stable upon losing one electron or accepting one electron. Oxidation Motif of Iridium Oxide Clusters. The survey of Ir4On0,± (n = 1−8) and Ir5On0,± (n = 1−10) lowest-energy structures displays a feature of oxidation pattern in the small iridium oxide clusters. Primarily, ground states of all the clusters can be obtained by the sequential oxidation of small “core” iridium clusters. At the beginning, the oxygen atom favors the terminal site of iridium atoms. During this process, the lowestenergy structures of iridium clusters are maintained. When the oxygen-to-iridium ratio equals 1, a planar symmetrical structure with bridging oxygen atoms can be obtained. When more oxygen atoms are added, they locate around the central iridium clusters in either bridging or terminal types. Our calculations are in good agreement with the recent experimental observation of iridium oxide nanocrystals as a core shell structure.21 3.2. Stability. The relative stability of Ir4On0,± (n = 1−8) and Ir5On0,± (n = 1−10) clusters as a function of the O/Ir ratio has been investigated via the analysis of binding energies per oxygen atom Eb(O), and second difference in energies Δ2E, which are defined by

Δ2 E(IrmOn 0, ±) = E(IrmOn − 10, ±) + E(IrmOn + 10, ±) − 2E(IrmOn 0, ±)

(2)

Figures 7 and 8 display the Eb(O) and Δ E curves with respect to the number of O atoms. The calculated results show that the 2

Figure 7. Binding energies of neutral and ionic Ir4On (a) and Ir5On (b) clusters.

binding energy decreases with cluster size. The binding energy values for ionic clusters follow a trend similar to the neutral cluster exhibiting the expected behavior: anionic ones being more stable and cationic ones less stable, compared to the respective neutral counterparts. As shown in Figure 7, when the number of oxygen atoms equals the number of iridium atoms in neutral and cationic clusters, the binding energy is higher than that of their neighbors. The relative stability of iridium monoxide Ir4O40,+ and Ir5O50,+ can be also observed from the curve of second difference in energies of Figure 8 which shows peaks at these species. 3.3. Hydrolysis Potential Energy Surfaces. Nahor and coworkers’ investigation found that after the initial IrOx cluster is oxidized to Ir4+, further oxidation of the cluster leads to oxidation of water to O2.20 On the basis of the experiments, we investigated the hydrolysis reaction of (IrO2)n (n = 1−5) nanoclusters. Reactions with two H2O molecules have been studied because one O2 molecule is produced by oxidizing two H2O molecules. The calculated lowest-energy states of (IrO2)n (n = 1−5) are

E b 0, ±(O) = [E(Irm 0, ±) + n/2E(O2 ) − E(IrmOn 0, ±)]/n (1) 9991



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from Ir2O3(OH)2·H2O to Ir2O2(OH)4 is −9.4 kcal/mol, and the reaction barrier is predicted to be 9.6 kcal/mol. Ir3O6. The overall hydrolysis reaction of Ir3O6 with two H2O molecules is predicted to be exothermic. The calculated reaction energy is −1.1 kcal/mol for the first hydrolysis step and −2.9 kcal/mol for the second hydrolysis step. The reaction barriers are computed to be 16.2 kcal/mol for the first hydrolysis step and 7.7 kcal/mol for the second hydrolysis step. The addition of water molecule in every step is predicted to be energetically favorable. Ir4O8. As depicted in Figure 9, the addition of the first and second H2O molecules is calculated to be endothermic by 3.5 and 2 kcal/mol, respectively. The reaction energies are −4.0 kcal/mol for the first hydrolysis step and −6.5 kcal/mol for the second hydrolysis step. And the reaction barriers are computed to be 15.4 kcal/mol for the first hydrolysis step and 19.0 kcal/mol for the second hydrolysis step. In this case, the terminal IrO bonds are more easily hydrolyzed than the bridging Ir−O bonds. Ir5O10. The addition of the first water molecule is predicted to be exothermic by 1.5 kcal/mol. The potential energy surface of the first hydrolysis step is quite flat. The reaction energy is −1.8 kcal/mol with a low barrier of 2.3 kcal/mol. The addition of the second water molecule is endothermic by 2.2 kcal/mol. The calculated reaction energy of the second hydrolysis step is −5.0 kcal/mol and the reaction barrier is 15.6 kcal/mol. Summary of Potential Energy Profiles. Our calculated results show that the overall hydrolysis reaction of the (IrO2)n are endothermic processes for n = 1 and 2, and exothermic processes for n = 3, 4, and 5. The addition of the first water molecule is computed to energetically favorable for n = 3 and 5 and unfavorable for n = 1, 2, and 4. The addition of the second water molecule is predicted to endothermic for every cluster except n = 3. The dissociative adsorptions of H2O molecules on (IrO2)n nanoclusters are calculated to be slightly more favorable than the molecular adsorptions. Recently, Dixon and co-workers theoretically studied the hydrolysis reaction of (MO2)n (M = Ti, Zr, and Hf, n = 1−4).47,48 They found that the hydrolysis of all the nanoclusters for both the singlet ground states and the lowest-energy triplet excited states are exothermic processes with relatively low reaction barriers. Their calculations show the H2O readily reacts with (MO2)n nanoclusters to form the hydroxides, which is quite different from our results. Usually in the photocatalytic systems, TiO2, ZrO2, and HfO2 are considered as photocatalysts to absorb the input light, produce photoinduced electron/hole, react with water and generate O2 and H2. As mentioned in the Introduction, IrOx are usually applied as oxidation cocatalysts for O2 generation. Thermodynamically, the electron-transfer process in the oxidation reaction on the surface of semiconductor is described as follows. By light irradiation as shown in Scheme 1, the electrons in the top of valence band of the photocatalyst (such as TiO2, ZrO2, and HfO2) are excited to the bottom of the conduction band, while the holes are left in the top of the valence band. And then the electrons of cocatalysts (such as IrO2) nanoclusters transfer to the top of valence band of the photocatalyst. After that, the electrons in H2O transfer to the IrO2, and O2 is continuously generated as electrons sequentially transfer. Meanwhile, the electrons excited to the bottom of the conduction band of the photocatalyst participant in the H2 generation to make the cycle continuously run. Namely, after losing electrons, IrO2 nanoclusters react with water to generate O2. Therefore, we have further investigated the hydrolysis reactions of cationic (IrO2)n (n = 1−5) clusters. Reactions with

Figure 8. Second differences in energy of neutral and ionic Ir4On (a) and Ir5On (b) clusters.

considered as the reactants. The water molecule can dissociate so that the hydroxyl group bonds with an iridium atom and the proton goes either to a terminal oxygen or to a bridging oxygen atom. So in this work, several possible pathways have been studied and the most energetically favorable pathway for every (IrO2)n is shown in Figure 9. IrO2. As shown in Figure 9, the hydrolysis reaction begins with an endothermic addition of H2O to IrO2 to form a complex, IrO2·H2O. A H atom migrates from the H2O to a terminal O atom of the cluster, leading to the formation of two Ir−OH bonds. The reaction energy barrier from IrO2·H2O is computed to be 15.1 kcal/mol. The reaction energy from IrO2·H2O to IrO(OH)2 is exothermic by 17.9 kcal/mol. The addition of a second H2O molecule to form IrO2(OH)2·H2O is calculated to be endothermic by 10.9 kcal/mol. A H atom migrates from the H2O again to form two additional Ir−OH bonds. The reaction barrier for the second hydrogen transfer step from IrO2(OH)2·H2O is computed to be endothermic by 19.3 kcal/ mol. The reaction energy from IrO2(OH)2·H2O to Ir(OH)4 is endothermic by 5.9 kcal/mol. Ir2O4. The hydrolysis reaction for the addition of the first water molecule is endothermic by 6.9 kcal/mol. After the formation of the Ir2O4·H2O complex, a H atom transfer from H2O to a terminal O atom, which occurs with a barrier from the complex of 9 kcal/mol. The addition of the second water molecule to Ir2O4 is endothermic by 6.3 kcal/mol. The computed reaction energy 9992



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Figure 9. Potential energy profiles (ΔG298K) for (IrO2)n + 2H2O → IrnO2n−2(OH)4 (O in red, Ir in blue, and H in white). Relative energies are calculated at the B3LYP/6-31++G(d,p) level in kcal/mol.

nanocluster to form a stable complex. For the first hydrolysis step, the reaction energy barriers from (IrO2)n·H2O+ are calculated to be ca. 11−50 kcal/mol. The energy differences between the molecular and the dissociative adsorptions of first water on (IrO2)n+ clusters are predicted to be 0.8−5.2 kcal/mol. The addition of the second water molecule is computed to be exothermic by 11.0−23.4 kcal/mol. The second hydrolysis reaction steps are endothermic by 1.1−7.0 kcal/mol and the reaction barriers are predicted to be 10.2−31.3 kcal/mol. Comparing the overall potential energy surfaces for the hydrolysis of (IrO2)n with (IrO2)n+, one can observe that the addition of water molecules to the cationic species is much easier

two H2O molecules have been studied. The calculated lowestenergy states of (IrO2)n+ (n = 1−5) are considered as the reactants. Like the hydrolysis of the neutral species, H transfer steps to both a terminal O atom and a bridge O atom have been studied and the most energetically favorable pathway for every (IrO2)n+ is shown in Figure 10. Hydrolysis Potential Energy Surfaces of Cationic Species. As shown in Figure 10, the overall reaction energies for the addition of two H2O molecules are exothermic for all the (IrO2)n+ (n = 1− 5) nanoclusters, which are energetically more favorable than the neutral clusters. The hydrolysis reaction of all of cationic species begins with the exothermic molecular adsorption of H2O to the 9993



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Figure 10. Potential energy profiles (ΔG298K) for (IrO2)n+ + 2H2O → IrnO2n−2(OH)4+ (O in red, Ir in blue, and H in white). Relative energies are calculated at the B3LYP/6-31++G(d,p) level in kcal/mol.

than the neutral ones. The overall hydrolysis reaction energies are more exothermic for the cationic clusters than for the neutral clusters. Experimentally, the hydrolysis reactions usually occur in solutions. So we took neutral and cationic Ir4O8 clusters as examples to investigate potential energy surfaces of the overall

hydrolysis reactions in solution (water). The solvent effects were estimated at the PCM level using gas-phase-optimized geometries. The calculated potential energy surfaces are shown in Figure S4 (Supporting Information). The results show that hydrolysis reactions are more difficult to occur in solution than in 9994



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gas phase. In term of the overall potential energy surface, H2O can be more easily split on the cationic clusters than on the neutral clusters in solution and gas phase.

4. CONCLUSIONS In this work, we have performed a systematic study on neutral IrmOn (m = 1−5 and n = 1−2m) and ionic Ir4On and Ir5On clusters. It is found that the lowest-energy structures of all these clusters can be obtained by the sequential oxidation of small “core” iridium clusters. The analysis of binding energy and second difference in energies indicates that the iridiummonoxide-like clusters have great stability. Our calculations on the hydrolysis reaction of (IrO2)n0,+ (n = 1−5) show that the overall hydrolysis reactions are energetically more favorable for the cationic nanoclusters than for the neutral species. And the adsorptions of water molecules are predicted to be easier to the cationic clusters than to the corresponding neutral ones. The molecular and dissociative adsorptions of H2O molecules on both neutral and cationic clusters are comparative. We hope that the results reported in this article will offer valuable assistance to future experimental and theoretical studies on iridium oxide nanostructures.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Three figures showing additional structures, the potential energy surfaces of neutral and cationic Ir4O8 clusters, and full author list of ref 27 are provided. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Tel: 86-411-84379302. Fax: 86-41184694447. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Basic Research Program of China (2009CB220010), National Science Foundation of China under Grant 21003115, 21090342, and 21061140361, the Solar Energy Initiative of the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant. No. KGCX2-EW-311), and DICP SKLC Program No. R201003.

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Theoretical Study of Structure, Stability, and the Hydrolysis Reactions

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