Unraveling the Mechanism of Photocatalytic Water Splitting in Î±

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

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Unraveling the Mechanism of Photocatalytic Water Splitting in #-GaO Loaded with Nickel Oxide Cocatalyst: A First-Principles Investigation Yidan Wang, Ping Zhuang, Haiying Yue, Hao Dong, and Xin Zhou

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00047 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 12, 2019

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The Journal of Physical Chemistry

Unraveling the Mechanism of Photocatalytic Water Splitting in Ga2O3 Loaded with Nickel Oxide Cocatalyst: A First-Principles Investigation Yidan Wanga, Ping Zhuanga, Haiying Yuea, Hao Donga,* , Xin Zhoub,* a

School of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian

116029, Liaoning, China b

College of Environment and Chemical Engineering, Dalian University, Dalian 116622,

Liaoning, China

* Corresponding authors. E-mail addresses: [email protected] (H. Dong), [email protected] (X. Zhou).

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ABSTRACT: In the heterogeneous photocatalytic overall water splitting reaction, introducing suitable cocatalysts has an important effect on improving the photocatalytic performance. However, the working mechanism of cocatalysts is still not fully understood. In this work, we have performed density functional theory calculations on -Ga2O3 loaded with nickel oxide cocatalyst, which is a highly active photocatalyst of decomposing water in UV region. We have investigated the adsorption behavior of NinOn (n=1, 2, 4, 6) clusters on (001) and (012) surfaces of -Ga2O3. It is found that all NinOn clusters adsorb strongly on both (001) and (012) surfaces, with adsorption energies ranging from -3.9 eV to -6.6 eV. The more stable adsorption configurations have a larger number of new interfacial bonds formed between the surface and the cluster. The calculated density of states demonstrate that the adsorption of NinOn clusters on (012) surface is more favorable to the separation and transfer of photogenerated electrons and holes. Our results reveal that on the -Ga2O3 surfaces covered with nickel oxide cocatalyst, the active site of hydrogen evolution reaction is the oxygen atom on the surface, and the interfacial dissociation of water is the most stable due to more bonds formed. The favorable site of oxygen evolution reaction is on the highly unsaturated Ni atoms in NinOn clusters. The two half reactions of decomposing water occur at the different sites in the photocatalytic system, which benefits the separation of electrons and holes. Our calculations have reasonably explained the experimental observation on the significant enhancement of activity for photocatalytic water-splitting reaction after loading nickel oxide cocatalyst in -Ga2O3.

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1. INTRODUCTION Gallium oxide, Ga2O3, is a large bandgap semiconductor known for its applications for electronic and optoelectronic devices.1-5 Recently, Ga2O3 as a photo-absorber has been widely used in photocatalytic fields, such as organics degradation,6-8 overall water-splitting,

9-12

and

CO2 reduction.13-15 Ga2O3 possesses two advantages for exhibiting superior photocatalytic activities: one is its appropriate valence and conduction band edge with respect to oxidation and reduction electrode potentials, respectively,16 the other is its conduction band formed by hybridized sp orbitals with large dispersion able to generate photoexcited electrons with large mobility.17,18 Ga2O3 has five commonly identified polymorphs, designated as rhombohedral α, monoclinic β, defective spinel , cubic δ, and orthorhombic ε.19-21 These distinct crystal structures essentially influence the surface properties of Ga2O3, leading to the variation in catalytic activity among the polymorphs.22,23 In these phases, -Ga2O3 is the most stable crystal phase under normal conditions and has attracted most of the recent attention. The rhombohedral corundum α phase is metastable but can exist under ambient conditions. The  phase can be transformed into the α phase under hydrostatic pressure at higher temperatures. While recent investigations indicated that -Ga2O3 can be easily obtained by nanosizing Ga2O3 at relatively low temperatures.24,25 Muruganandham et al. found that in the degradation of organic compounds, self-assembly -Ga2O3 microspheres showed higher photocatalytic performance than the well-known photocatalyst TiO2.7 Wang et al. revealed that the original -Ga2O3 undergoes gradual  to  phase transformation upon increasing the calcination temperature. The photocatalytic activity of splitting water in pure  phase is higher than that in pure  phase.11 Li et al. reported that photocatalytic H2 evolution rate in pure water for -Ga2O3 is almost one order higher than that for -Ga2O3.26 A realistic photocatalytic system is usually comprised of semiconductors and cocatalysts. In such photocatalytic hybrid structures, semiconductors are the light-harvesting components for generating photo-induced charge carriers. Cocatalysts are loaded on the surface of semiconductor with a quite small amount and have seldom or no activity in photocatalytic reactions. They are considered to be responsible for promoting electron-hole separation by forming an interface with the semiconductor and serving as active redox reaction sites on semiconductor surfaces.27-29 Cocatalysts can be classified into two functions: reduction 3



cocatalysts trapping electrons for reduction half reactions, and oxidation cocatalysts trapping holes for oxidation half reaction. Commonly, noble metals (e.g. Pt, Ru, Rh, Au and Ag),30-32 metal sulfides (e.g. MoS2, WS2 and NiS) are acted as reduction cocatalysts,33-36 and transition metal oxides (e.g. IrO2, RuO2, and CoOx) are considered as oxidation cocatalysts.18,37-39 Recently, plenty of experiments have found that nickel oxide is one of the most efficient cocatalysts for the high activity in photocatalytic overall water splitting systems such as Ga2O3, NaTiO3, SrTiO3, and layered perovskite structures.9,

40-43

However, there are some critical

questions unsolved experimentally, which limit the further optimization of photocatalytic systems. For instance, what are the components and structure of cocatalysts? Where is the favorable adsorption site of cocatalyst on the semiconductor surface? What is the reaction mechanism of photocatalytic semiconductors loaded with cocatalysts? First-principles calculation based on density functional theory (DFT) has been proven to be a useful tool for complementing the experimental findings and investigating geometrical structure, electronic property and microscopic mechanism of semiconductor-based systems on the atomic level.4448

In this work, we have theoretically modeled a well-known photocatalyst α-Ga2O3 loaded

with nickel oxide cocatalyst, which is successfully applied to overall water-splitting reaction. Our aim is to find the stable adsorption structures of nickel oxide clusters with different sizes on low-index surfaces of α-Ga2O3, to study the effect of loading cocatalyst on electronic structures, to investigate the interaction between water and the semiconductor-cocatalyst hybrid structure, and to explore the active sites of hydrogen evolution reactions (HER) and oxygen evolution reactions (OER). 2. COMPUTATIONAL METHODS We have performed spin-polarized DFT calculations employing the generalized gradient approximation (GGA) and the plane wave implementation in the Vienna ab initio simulation package.49,50 The exchange-correlation interactions are considered using the Pedrew-BurkeErnzerhof (PBE) functional.51 The interaction between ionic cores and valence electrons is described by the projector-augmented wave method.52,53 The electronic wave functions are expanded in a plane-wave basis with a cut-off energy of 400 eV. Atomic positions are relaxed until the maximal forces on each atom are less than 0.01 eV/Å. The GGA+U method with onsite Coulomb correction is used on Ni 3d orbital electrons.54 We only consider Ueff, which is the difference between the on-site Coulomb energy U and exchange parameter J. The value of Ueff for Ni 3d orbital electrons is equal to 5.3 eV based on previous theoretical findings.55-57 4


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Bulk α-Ga2O3 with corundum structure is rhombohedral with a R c space group. The unit cell includes 30 atoms with six-coordinated Ga atoms and four-coordinated O atoms, which is fully optimized using a 993 k-point sampling. The relaxed lattice parameters, a=b=5.032 Å, c=13.504 Å, and =120, agrees well with experimental values.58 Our previous findings showed that (001) and (012) surfaces of α-Ga2O3 have the highest and almost the same stabilities in the most investigated low-index surfaces of semiconductors with corundum structure.59 As displayed in Figure 1a and 1b, (001) and (012) surfaces are represented by rhombic and rectangular supercells with 120 atoms, respectively. A vacuum layer of 15 Å is added in the slab and a 3  2  1 Monkhorst-Pack k-point sampling is applied. In this work, we mainly focus on the structural and electronic properties of NinOn (n=1, 2, 4 and 6) clusters adsorbed on (001) and (012) surfaces as indicated in Fig. 1c, 1d, 1e and 1f. Due to the unsaturated coordination of atoms, Ni-O bond lengths in free clusters are shorter than in the bulk (2.084 Å), e.g. 1.661 Å in NiO, 1.808 Å in Ni2O2, 1.930 Å in Ni4O4, 1.912 Å and 2.108 Å in Ni6O6. During the relaxation of surfaces covered with adsorbates, the top half of the slab and adsorbates can move, while the bottom half of the slab is fixed at positions in its optimized bulk structure. Dipolar corrections are included along the axis normal to the surface. Based on the relaxed structures obtained using the GGA-PBE method, the screened Coulomb hybrid functional HSE06 with 33% HF exchange was applied to calculate the electronic structures.60-62 The calculate band gap of bulk -Ga2O3 is 4.36 eV, which is good agreement with the experimental value of  4.5 eV.6 To study the interaction between an adsorbant and a semiconductor surface, and estimate the stability of hybrid systems, we define the adsorption energy Eads as Eads = Etotal – Esurf – Emolecule

(1)

where Etotal is the total energy of a surface with an adsorbed molecule, Esurf is the energy of a surface without an adsorbed molecule, Emolecule is the energy of a free molecule in a vacuum. A negative value of Eads means the thermodynamic stability of the cluster adsorption. In our calculation, surfaces with and without an adsorbed molecule and the free molecule are optimized in the same periodic supercell with the same computational parameters. The initial (NiO)n clusters were cut from the bulk structure of nickel oxide. 3. RESULTS AND DISCUSSION 5



3.1. Adsorption configurations of NinOn clusters on α-Ga2O3 surfaces Different adsorption configurations are relaxed to find the stable structure of NinOn (n=1, 2, 4, 6) clusters supported on α-Ga2O3(001) surface. Three stable configurations for each size of cluster are as shown in Figure 2, denoted as (a), (b), and (c). The adsorption energies are also given below each structure. It is found that the adsorption site of clusters and the number of interfacial bonds are related to the adsorption energies. For each cluster, we will focus on the most stable adsorption structure in the remainder of the paper, and some structural parameters for these configurations are listed in Figure 3. The oxygen atoms in NinOn cluster and α-Ga2O3 surfaces are denoted as Oc and Os, respectively. For the most stable structure of NiO indicated in Figure 3a, the cluster binds to the surface with three interfacial Ni-Os bonds with distances of 1.952, 2.022 and 2.138 Å, and with one interfacial Ga-Oc bond with a bond length of 1.870 Å. The Ni-Oc bond in cluster is lengthened by 0.159 Å compared to that in the free cluster. The adsorption energy is calculated to be -4.46 eV. When a square Ni2O2 cluster is deposited on the surface (Figure 3b), six interfacial bonds are formed: four Ni-Os bonds with distances of 2.006, 2.024, 2.082 and 2.117 Å, and two Ga-Oc bonds with distances of 1.881 and 1.913 Å. From the top view of Ni2O2(a) in Figure 2, one can find that one of Ni-Oc bonds is mostly parallel with the binding Ga-Os bond in surface. The adsorption energy is -5.41 eV, which is lower than that of NiO(a) in Figure 2. This is possibly due to more new bonds between the cluster and the surface in Ni2O2(a). As to the structure named Ni4O4(a) with a cubic cluster supported on (001) surface in Figure 2, only one Ni atom binds to the surface and the structure of Ni4O4 cluster near to the surface is distorted obviously. The computed adsorption energy is -4.98 eV. Three Ni-Os bonds and two Ga-Oc bonds form with the distances of 1.972, 2.026, 2.093, 1.896 and 1.934 Å, as listed in Figure 3c. From the top view of Ni6O6(a) in Figure 2, one can find that some Ni-Oc bonds adjacent to the surface are broken. The structure has an adsorption energy of -6.55 eV. As shown in Figure 3d, there are seven interfacial bonds. The Ni-Os bond lengths are 2.037, 2.047, 2.081, and 2.179 Å, respectively. The Ga-Oc bond distances are 1.883, 1.884 and 1.928 Å, respectively. Figure 4 displays three energetically favorable structures of NinOn adsorbed on α-Ga2O3(012) surface for each series. As listed in Table 1, the resulting adsorption energy of the most stable structure are -3.94 eV for NiO, -6.34 eV for Ni2O2, -5.11 eV for Ni4O4, and -5.76 eV for Ni6O6, which shows a reasonable dependence on the number of interfacial bonds, similar to the results obtained in (001) surface. In other words, the more the interfacial bonds form, the more stable 6


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the structure is. For the most stable adsorption of NiO shown in Figure 3e, the Ni atom binds to two Os atoms with distances of 1.919 and 1.985 Å, and the Oc atom binds to a Ga atom with a distance of 1.846 Å. The bond length of Ni-Oc is 1.758 Å, which is shorter than that in the case of (001) surface (1.820 Å). In the most stable adsorption configuration of Ni2O2, Figure 3f, two Ni atoms are coordinated to three Os atoms, with Ni-Os bond lengths of 1.914, 1.956 and 2.226 Å. Two Oc atoms are coordinated to two Ga atoms, with Ga-Oc bond distances of 1.889 and 1.902 Å. When a Ni4O4 cluster is deposited on the surface, two Ni-Os bonds and two Ga-Oc bonds generate as displayed in Figure 3g. The bond lengths are 1.956, 1.981, 1.908 and 1.910 Å, respectively. As shown in Figure 3h, Ni6O6 binds to the surface through three Ni-Os bonds with distances of 1.931, 1.941 and 1.943 Å, and three Ga-Oc bonds with distances of 1.943, 1.990 and 2.006 Å. It is found that all NinOn (n=1, 2, 4 and 6) clusters adsorb strongly on both (001) and (012) surfaces, with adsorption energies ranging from -3.9 eV to -6.6 eV. The more stable adsorption configurations have a larger number of new interfacial bonds formed between the surface and the cluster. For some cases, Ni atoms create interfacial bonds to Ga atoms. Comparing the geometries of clusters in the most stable configurations in Figure 2 and Figure 4, we observe that the NinOn cluster with the same size undergo less distortion on (012) surface than on (001) surface. For example, the average bond lengths of Ni-Oc bonds are 1.820 Å in NiO and 1.976 Å in Ni2O2 adsorbed on (001) surface, and 1.758 Å in NiO and 1.926 Å in Ni2O2 adsorbed on (012) surface. Bader atomic charges in Table 1 demonstrate that, in NinOn cluster supported on the surface, Ni atoms donate electrons to the surface and carry positive charge, while O atoms withdraw electrons from the surface and carry negative charge.63 3.2. Electronic structure of NinOn clusters on α-Ga2O3 surfaces For the most favorable configuration of each supported NinOn cluster, HSE06 method is applied to calculate the total density of states (TDOS), the projected density of states (PDOS) onto Ga2O3 surface, Ni 3d and Oc 2p states, which are summarized in Figure 5 for (001) surface and Figure 6 for (012) surface. In Figure 5a and Figure 6a, PDOS of Os 2p and Ga states in Ga2O3 surfaces are depicted. As reported previously, both the valence band maximum (VBM) and the conduction band minimum (CBM) of Ga2O3 surfaces are mostly composed of the mixture of Os 2p and Ga states, which indicates the character of covalent bonding in this semiconductor.59 Most of Oc 2p states 7



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are in the VBM, while Ni 3d states have important contributions in both the VBM and the CBM. In the case of (001) surface displayed in Figure 5a, 5b, 5c and 5d, there are some Ga2O3 surface states right above the Fermi level, which is higher in energy than NinOn cluster states. Upon photoexcitation, electrons in the VBM of Ga2O3 are excited to the CBM, and the generated holes in the VBM hardly transfer to the NinOn states due to the unfavorable energy position. The holes may recombine with excited electrons, which is disadvantage to the photocatalytic process. By contrast, Figure 6a, 6b, 6c and 6d reveal the NinOn states in the VBM is higher than Ga2O3 states in (012) surface. The electrons in NinOn clusters tend to transport to the VBM of Ga2O3 and neutralize the holes induced upon light irradiation. The holes left in NinOn clusters are able to take part in OER. As unoccupied Ga2O3 states overlap with empty NinOn states for both (001) and (012) surfaces, electrons have the opportunity of transferring from the Ga2O3 surface to the cluster. As a result, the electrons accumulated on both Ga2O3 surface and metal oxide cluster can participant in HER. The calculated electronic structures show that the adsorption of nickel oxide clusters benefits the transfer of photogenerated electrons and holes from Ga2O3 surface to cocatalyst and decreases the possibility of charge recombination, especially on (012) surface, which may explain the improved photocatalytic activity by loading nickel oxide cocatalyst.9,11 3.3. The Mechanism of Hydrogen Evolution Reaction As we mentioned in the part of Introduction, experimental observations show that Ga2O3 loaded by nickel oxide as cocatalyst exhibits a high activity in photocatalytic decomposition of water into H2 and O2. During the process, HER and OER occur simultaneously and are closely interrelated to photo-induced electrons and holes, respectively. Generally, HER involves adsorbing hydrogen intermediates to the surface of a catalyst and recombination desorption of molecular hydrogen, which plays an important role in hydrogen fuel cells, hydrogen energy storage, corrosion of metals and so on.64-66 Previous studies reveal that the HER activity is directly related to the Gibbs free energy of adsorbing a single hydrogen atom on a catalyst surface, namely an optimal catalyst for HER should exhibit a free energy of the hydrogen adsorption close to zero (ΔGH 0).67-69 Here we explore the mechanism of HER by calculating the Gibbs free energy of a hydrogen atom binding to NinOn supported on -Ga2O3 surfaces and studying the most favorable reaction site. The Gibbs free energy of a hydrogen atom is calculated by ΔGH = ΔEH + ΔZPE TΔSH 8


(2)

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where ΔEH is the calculated reaction energy, and ΔZPE is the difference in zero point energy, which is computed by using DFT calculations of vibrational frequencies and standard tables for gas-phase molecules.70 We assume that S equals to zero for the adsorbants on the surface. So ΔSH is approximately equal to the negative value of half the entropy of H2 in the gas phase at standard conditions. As for the surface covered with NinOn cluster, we initially examined all the possible active sites for HER, including Os, Ga, Oc and Ni sites. Four representative relaxed configurations and the corresponding Gibbs free energies are displayed in Figure 7 for (001) surface and Figure 8 for (012) surface. It is found that the adsorption of hydrogen atom leads to the structural distortion of hybrid systems to some extent. Since most of the studied systems show the similar trend, we take Ni2O2 adsorbed on surfaces as examples to analyze the results. As shown in Figure 7, the calculated ΔGH is 0.16 eV on Os (Ni2O2(a)), -1.12 eV on Oc (Ni2O2(b)), 1.43 eV on Ni (Ni2O2(c)), 1.34 eV on Ga (Ni2O2(d)). Figure 8 indicates the computed ΔGH is 0.06 eV on Os (Ni2O2(a)), -0.27 eV on Oc (Ni2O2(b)), 1.56 eV on Ni (Ni2O2(c)), 1.38 eV on Ga (Ni2O2(d)). The results demonstrate that, when a hydrogen atom is attached to an oxygen atom on the surface near to the NinOn cluster, the absolute value of ΔGH is closer to zero. The adsorption of a hydrogen atom on a Oc site is an exothermic reaction, namely a relative strong adsorption and a weak desorption. While the hydrogen adsorption on a Ni atom or a Ga atom is an endothermic reaction. That means it is quite difficult to make the adsorption of hydrogen happen. Therefore, our calculations reveal that the most favorable site of hydrogen adsorption is the oxygen atom of the α-Ga2O3 surface. 3.4. Water adsorption on surfaces In heterogeneous photocatalytic applications, most of reactions are performed in aqueous solution, or at least, need the participation of water. It is vital to get insight into the interaction between water and semiconductor surfaces. The calculated electronic structures in the session 3.2. indicate that decomposing water to generate O2 tends to occur on α-Ga2O3 (012) surface loaded with cocatalysts. So we focus on searching the stable adsorption configurations of water on the (012) surface covered with nickel oxide clusters. Our aim is to obtain the active site of decomposing water. Initially we add a water molecule to the different site (Ga and Ni) on the surface of hybrid system. Figure 9 summarizes optimized structures of the dissociative water adsorption. 9



For NiO and Ni2O2, two stable structures are displayed, respectively. From NiO(a) and Ni2O2(a), we can find that the hydroxyl group on a Ni site binds to the nearest Ga atom on the surface, and the dissociated H atom attaches to the oxygen atom in cluster, which shows the interaction between water and the interface. The dissociative adsorption of water on Ga site far away from nickel oxide cluster (structure (b)) is less stable than that on the interface (structure (a)) by 1.00 eV for NiO and 0.50 eV for Ni2O2, respectively. As for Ni4O4 and Ni6O6, we show three energetically favorable structures of water decomposition, named (a), (b) and (c). In the structure (a), water dissociates at the Ni atom in the atomic layer of cluster near to the α-Ga2O3 surface, and one of H atoms transfers to a closest Oc atom. In the structure (b), the decomposition of water occurs on the upper layer of nickel oxide cluster. In the configuration (c), water forms a bond with a surface Ga atom and one of H atoms moves to the adjacent Os atom. The adsorption energy is computed to be -1.21 eV for Ni4O4(a), -1.05 eV for Ni4O4(b), -0.79 eV for Ni4O4(c), -2.06 eV for Ni6O6(a), -1.14 eV for Ni6O6(b), and -0.41 eV for Ni6O6(c), respectively. Our results reveal that it is energetically favorable to decompose water on the α-Ga2O3 surface loaded with nickel oxide cocatalysts. Among the different sites, the interfacial site between the cluster and the α-Ga2O3 surface is the most propitious to the dissociation of water, since the hydroxyl group binds to a Ni atom and a Ga atom simultaneously to stabilize the whole system. 3.5. The Mechanism of Oxygen Evolution Reaction In the overall water splitting reaction, OER involves a process of four-electron transfer and is more complicated than HER. Therefore, OER is considered to be the rate-determining step in the whole reaction, and it draws more and more attention in the photocatalytic field.71,72 In this work, photo-excitation and charge migration are considered to generate the electrochemical potentials for elecrocatalytic H2O decomposition. We only investigate the thermodynamic process of OER by using the method developed by Nørskov et al.,73-75 in which the molecular oxygen is generated via a surface HOO* intermediate and the reaction happens at the coordinatively unsaturated atoms. In this electrochemical computation, OER is considered to include four elementary reaction steps, each involving the electron transfer coupled with proton removal: 10


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H2O + *→HO* + H+ + e– HO* → O* + H+ + e– O* + H2O → HOO* + H+ + e– HOO* → * + O2 + H+ + e–

(A) (B) (C) (D)

where * represents a surface site and X* denotes an attached X intermediate on the surface. We get the energy of H+ + e– implicitly by referencing it to the energy of H2 using the standard hydrogen electrode. This means that at standard conditions (pH=0, p=1 bar and T=298 K) the free energy of H+ + e– can be taken equal to be half the formation energy of H2. The reaction free energy, ΔG=ΔE+ΔZPE-TΔS, is computed as follows: The reaction energy ΔE is obtained from standard DFT calculations. The zero point energy ΔZPE and entropic contributions TΔS are calculated using computed vibrational frequencies and standard tables for the reactants and products in the gas phase.70 The entropies for the atoms and molecules adsorbed to the surface active site are assumed to be zero. The temperature dependence of the enthalpy is neglected in these calculations. Applying an external bias U on each proton-coupled electron transfer step is accounted by including a eU term in the reaction free energy. For simplicity, the effect of pH is not considered here and we restrict the calculations to pH=0. Therefore, the reaction free energies are expressed as follows: ΔGA= E(HO*)EH2O + 1/2 EH2+(ΔZPETΔS)AeU

(3)

ΔGB= E(O*) E(HO*) + 1/2 EH2 + (ΔZPETΔS)BeU

(4)

ΔGC= E(HOO*) E(O*) EH2O+ 1/2 EH2 + (ΔZPETΔS)CeU ΔGD= E(*) E(HOO*) +EO2+1/2EH2 + (ΔZPETΔS)DeU

(5) (6)

where E(*), E(HO*), E(O*) and E(HOO*) are the calculated DFT energies of the clean surface and surfaces covered with adsorbed groups HO, O and HOO, respectively. EH2O, EH2 and EO2 are the energies for the isolated gaseous molecules H2O, H2 and O2, respectively. The freeenergy change of the total reaction H2O  1/2 O2 + H2 is fixed at the experimental value of 2.46 eV per water molecule. This implies that in the reaction step involving the formation of O2, we consider that ΔG(2H2OO2+2H2) = 4.92 eV =EO2+ 2EH2 2EH2O +(ΔZPE-TΔS)(2H2OO2+2H2) .

The reaction overpotential can be calculated from the difference between the voltage at which

all free-energy steps become downhill and the minimum voltage required for the OER. 11



Based on the dissociative adsorption structures of water shown in the top left of every panel in Figure 10, the intermediates including OH*, O* and OOH* are relaxed and the calculated free energy diagrams for each case at pH = 0, T = 298 K and different overpotentials are depicted. The calculated results show that, for no applied bias, U = 0 V, all steps in all the surfaces are uphill. Even at standard equilibrium potential for OER at U = 1.23 V, some of the steps become downhill but the rest still remain uphill. Therefore, it is necessary to add a overpotential (or bias) on all the surfaces to make every step downhill. During the four-step process of OER, the one with the largest change in ΔG is the rate-limiting step, which determines the reaction overpotential. As shown in Figure 10, the rate-limiting step is the second step for all the cases, in which a proton is transferred from the adsorbed OH* species to the electrolyte. As for the surface adsorbed with NiO seen in Figure 10a and 10b, the overpotential is calculated to be 0.81 V (= 2.04 -1.23 V) at the interfacial site and 1.05 V at the Ga site. Considering the system loaded with Ni2O2 presented in Figure 10c and 10d, the overpotential at the interfacial site of 0.97 V is smaller than that at the Ga site by 0.44 V. The computed bias is equal to be 1.01 V for Ni4O4 in Figure 10e and 1.14 V for Ni6O6 in Figure 10h at the interfacial site, 0.78 V for Ni4O4 in Figure 10f and 0.84 V for Ni6O6 in Figure 10i at the Ni site, and 1.28 V for Ni4O4 in Figure 10g and 1.46 V for Ni6O6 in Figure 10j at the Ga site, respectively. The results reveal that the required overpotential of OER is the smallest at the Ni site and the largest at the Ga site, which mainly results from the distinct structures of OH*, O* and OOH* at different sites. This means that the occurence of OER on NinOn cluster is more favorable than at the Ga site. The smallest overpotential of OER is 0.81 V for NiO, 0.97 V for Ni2O2, 0.78 V for Ni4O4, and 0.84 V for Ni6O6, which does not change much with the size of nickel oxide clusters. Conclusions We have performed a detailed investigation on the adsorption behavior of NinOn (n=1, 2, 4 and 6) clusters on α-Ga2O3 (001) and (012) surfaces, and explored their respective HER and OER performances by means of spin-polarized DFT+U calculations. Our results indicate that small NinOn clusters have strong interactions with α-Ga2O3 surfaces based on the computed adsorption energies. The stability of the adsorption configurations is related to the number of new interfacial bond between the cluster and the surface. NinOn clusters undergo a smaller distortion on (012) surface than (001) surface. The electronic structure shows that the adsorption of NinOn cluster on (012) surface is more favorable to the transfer of photogenerated 12


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electrons and holes between the semiconductor and the cocatalyst. Water dissociation tends to happen at the interfacial site. Our calculations demonstrate that the most favorable sites of HER and OER are the O atom on the semiconductor surface and the Ni atom in the cocatalyst, respectively, which benefits the separation of photoinduced carriers. These results provide an in-depth understanding on the reaction mechanism of α-Ga2O3 loaded with nickel oxide cocatalysts, and can be utilized to rationalize the experimental observations. Supporting Information Supporting Information Available: Optimized structures with higher energy for (NiO)n/Ga2O3(001) and (NiO)n/-Ga2O3(012) (n =1,2,4,6). Acknowledgements This work is financially supported by National Natural Science Foundation of China under Grant 21473183 and the open fund of Key Laboratory of Computational Physical Sciences (Fudan University), Ministry of Education. References (1) Vanithakumari, S. C.; Nanda, K. K. A One-Step Method for the Growth of Ga2O3-Nanorod-Based WhiteLight-Emitting Phosphors. Adv. Mater. 2009, 12, 3581-3584. (2) Chandiran, A. K.; Tetreault, N.; Humphry-Baker, R.; Kessler, F.; Baranoff, E.; Yi, C.; Nazeeruddin, M. K.; Grätzel, M. Subnanometer Ga2O3 Tunnelling Layer by Atomic Layer Deposition to Achieve 1.1 V OpenCircuit Potential in Dye-Sensitized Solar Cells. Nano Lett. 2012, 12, 3941-3947. (3) Kumar, S.; Singh, R. Nanofunctional Gallium Oxide (Ga2O3) Nanowires/Nanostructures and Their Applications in Nanodevices. Physica status solidi (RRL) – Rapid Research Letters 2013, 7, 781-792. (4) Pearton, S. J.; Yang, J.; Cary, P. H.; Ren, F.; Kim, J.; Tadjer, M. J.; Mastro, M. A. A Review of Ga2O3 Materials, Processing, and Devices. Appl. Phys. Rev. 2018, 5, 011301-1-011301-16. (5) Xue, H.; He, Q.; Jian, G.; Long, S.; Pang, T.; Liu, M. An Overview of the Ultrawide Bandgap Ga2O3 Semiconductor-Based Schottky Barrier Diode for Power Electronics Application. Nanoscale Res. Lett. 2018, 13, 290. (6) Hou, Y.; Wu, L.; Wang, X.; Ding, Z.; Li, Z.; Fu, X. Photocatalytic Performance of α-, β-, γ-Ga2O3 for the Destruction of Volatile Aromatic Pollutants in Air. J. Catal. 2007, 250, 12-18.

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(7) Muruganandham, M.; Amutha, R.; Abdel Wahed, M. S. M.; Ahmmad, B.; Kuroda, Y.; Suri, R. P. S.; Wu, J. J.; Sillanpää, M. E. T. Controlled Fabrication of α-GaOOH and α-Ga2O3 Self-Assembly and Its Superior Photocatalytic Activity. J. Phys. Chem. C 2012, 116, 44-53. (8) Li, D.; Duan, X.; Qin, Q.; Fan, H.; Zheng, W. Ionic Liquid-Assisted Synthesis of Mesoporous α-Ga2O3 Hierarchical Structures with Enhanced Photocatalytic Activity. J. Mater. Chem. A 2013, 1, 12417-12421. (9) Yanagida, T.; Sakata, Y.; Imamura, H. Photocatalytic Decomposition of H2O into H2 and O2 over Ga2O3 Loaded with NiO. Chem. Lett. 2004, 33, 726-727. (10) Sakata, Y.; Matsuda, Y.; Yanagida, T.; Hirata, K.; Imamura, H.; Teramura, K. Effect of Metal Ion Addition in a Ni Supported Ga2O3 Photocatalyst on the Photocatalytic Overall Splitting of H2O. Catal. Lett. 2008, 125, 22-26. (11) Wang, X.; Xu, Q.; Li, M.; Shen, S.; Wang, X.; Wang, Y.; Feng, Z.; Shi, J.; Han, H.; Li, C. Photocatalytic Overall Water Splitting Promoted by an α-β Phase Junction on Ga2O3, Angew. Chem. Int. Ed. 2012, 51, 13089-13092. (12) Busser, G. W.; Mei, B.; Weide, P.; Vesborg, P. C. K.; Stührenberg, K.; Bauer, M.; Huang, X.; Willinger, M.-G.; Chorkendorff, I.; Schlögl, R.; Muhler, M. Cocatalyst Designing: A Regenerable MolybdenumContaining Ternary Cocatalyst System for Efficient Photocatalytic Water Splitting. ACS Catal. 2015, 5, 5530-5539. (13) Tsuneoka, H.; Teramura, K.; Shishido, T.; Tanaka, T. Adsorbed Species of CO2 and H2 on Ga2O3 for the Photocatalytic Reduction of CO2. J. Phys. Chem. C 2010, 114, 8892-8898. (14) Yamamoto, M.; Yoshida, T.; Yamamoto, N.; Nomoto, T.; Yamamoto, Y.; Yagi, S.; Yoshida, H. Photocatalytic Reduction of CO2 with Water Promoted by Ag Clusters in Ag/Ga2O3 Photocatalysts. J. Mater. Chem. A 2015, 3, 16810-16816. (15) Huang, Z.; Teramura, K.; Asakura, H.; Hosokawa, S.; Tanaka, T. Recent Progress in Photocatalytic Conversion of Carbon Dioxide over Gallium Oxide and Its Nanocomposites. Curr. Opin. Chem. Eng. 2018, 2, 114-121. (16) Xu, Y.; Schoonen, M. A. A. The Absolute Energy Positions of Conduction and Valence Bands of Selected Semiconducting Minerals. Am. Mineral. 2000, 85, 543-556. (17) He, H.; Orlando, R.; Blanco, M. A.; Pandey, R. First-Principles Study of the Structural, Electronic, and Optical Properties of Ga2O3 in Its Monoclinic and Hexagonal Phases. Phys. Rev. B 2006, 74, 195123. (18) Inoue, Y. Photocatalytic Water Splitting by RuO2-Loaded Metal Oxides and Nitrides with d0- and d10Related Electronic Configurations. Energy Environ. Sci. 2009, 2, 364-386.

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Table 1. The adsorption energy Eads (eV), the number of interfacial bonds (Nbonds) and the averaged bader charge on Ni and O atoms in NinOn clusters (a.u.) in the most stable configuration for each composition. (001) NiO Ni2O2 Ni4O4 Ni6O6

Eads Nbonds -4.46 5 -5.41 6 -4.98 5 -6.55 7

QNi +0.73 +0.83 +0.98 +0.90

QO (012) -0.53 NiO -0.76 Ni2O2 -0.93 Ni4O4 -0.89 Ni6O6

Eads Nbonds -3.94 3 -6.34 6 -5.11 5 -5.76 6

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QNi +0.74 +0.64 +0.81 +0.93

QO -0.77 -0.69 -0.81 -0.94

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Figure 1. The structures of -Ga2O3 (a) (001) surface, (b) (012) surface, (c) NiO, (d) Ni2O2, (e) Ni4O4, and (f) Ni6O6 with optimized bond lengths. The red, brown, yellow and spheres represent oxygen atoms in surfaces, gallium atoms, oxygen atoms in clusters and nickel atoms, respectively.

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Figure 2. Stable configurations of (NiO)n/-Ga2O3(001) (n =1,2,4,6) clusters (side and top views). The adsorption energies are also given below each structure. Coloring scheme: red (surface O), brown (Ga), blue (Ni) and yellow (cluster O). 22


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Figure 3. Geometric parameters for the most stable structures of (NiO)n/-Ga2O3(001): (a) NiO, (b) Ni2O2, (c) Ni4O4, and (d) Ni6O6; and for the most stable strucures of (NiO)n/Ga2O3(012): (e) NiO, (f) Ni2O2, (g) Ni4O4, and (h) Ni6O6.

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Figure 4. Stable configurations of (NiO)n/-Ga2O3(012) (n =1,2,4,6) clusters (side and top views). The adsorption energies are also given below each structure. Coloring scheme: red (surface O), brown (Ga), blue (Ni) and yellow (cluster O).

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Figure 5. Density of states for the most stable configuration of (a) NiO, (b) Ni2O2, (c) Ni4O4 and (d) Ni6O6 adsorbed on (001) surfaces of -Ga2O3. The Fermi level is shown by the vertical dashed line.

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Figure 6. Density of states for the most stable configuration of (a) NiO, (b) Ni2O2, (c) Ni4O4 and (d) Ni6O6 adsorbed on (012) surfaces of -Ga2O3. The Fermi level is shown by the vertical dashed line.

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Figure 7. The relaxed structures of H adsorbed NinOn/-Ga2O3(001) surface at different sites, and the corresponding Gibbs free energy is listed below each structure. Coloring scheme: red (surface O), brown (Ga), blue (Ni), yellow (cluster O) and white (H).

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Figure 8. The relaxed structures of H adsorbed NinOn/-Ga2O3(012) surface at different sites, and the corresponding Gibbs free energy is listed below each structure. Coloring scheme: red (surface O), brown (Ga), blue (Ni), yellow (cluster O) and white (H). 28


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Figure 9. The relaxed structures of dissociative water adsorbed NinOn/-Ga2O3(012) surface at different sites, and the corresponding adsorption energy is listed below each structure. Coloring scheme: red (surface O), brown (Ga), blue (Ni), yellow (cluster O), green (water O) and white (H).

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Figure 10. Free-energy diagram at pH=0 and T = 298 K for OER at the different applied potentials for the -Ga2O3(012) surface (a) and (b) covered with NiO, (c) and (d) covered with Ni2O2, (e), (f) and (g) covered with Ni4O4, and (h), (i) and (j) covered with Ni6O6.

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TOC Graphic

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Unraveling the Mechanism of Photocatalytic Water Splitting in Î±

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