Influence of Ag Clusters on the Electronic Structures of β-Ga2O3 Photocatalyst Surfaces.

In order to understand the photocatalytic carbon dioxide reduction over Ag-loaded β-Ga2O3 photocatalysts, first principles calculations based on density functional theory were performed on the surface model of a Ag cluster-adsorbed β-Ga2O3 system. The stable adsorption structures of Ag n (n = 1 to 4) clusters on the β-Ga2O3 (100) surface were determined. In the electronic structure analysis, the valence states of all Ag clusters mixed with the top of the O 2p valence band of Ga2O3, leading the Fermi level of Ag n /β-Ga2O3 to shift to the bottom of the conduction band. It was also revealed that the unoccupied states of Ag n clusters overlapped with the Ga unoccupied states, and occupied electronic states of Ag clusters were formed in the band gap. These calculation results corresponded to the experimental ones obtained in our previous study, i.e., small Ag clusters had strong interaction with the Ga2O3 surface, enhancing the electron transfer between the Ag clusters and the Ga2O3 surface. That is, excited electrons toward Ag n clusters or the perimeter of Ag-Ga2O3 should be the important key to promote photocatalytic CO2 reduction.

Introduction

Photocatalytic reduction of carbon dioxide by semiconductor photocatalysts is drawing much attention as one of the solutions to environmental and energy problems, because carbon dioxide as a causative substance of global warming can be recycled by solar energy.[1−6] From the viewpoint of realizing artificial photosynthesis, it is important to develop a material that can accelerate the carbon dioxide reduction reaction in water using water as the electron source.

Since the reduction of carbon dioxide in water competes with the reduction of protons in water, which is thermodynamically favorable, it is necessary to promote the carbon dioxide reduction by loading an appropriate cocatalyst on the surface of a semiconductor photocatalyst. In general, cocatalysts are considered to have the function of providing reaction sites and reducing the activation energy of the reaction and also promoting the charge separation of the electrons and holes generated in the semiconductor by light.[5,6] Previous studies have reported that the activity and the selectivity of the carbon dioxide reduction reaction is improved when silver is loaded as a cocatalyst on semiconductor photocatalysts.[7−15]

Among semiconductor photocatalysts, β-gallium oxide (β-Ga2O3) is known to be a highly active semiconductor photocatalyst for overall water splitting.[16] It has been also reported that β-Ga2O3 exhibits activity for reduction of carbon dioxide in water with a silver cocatalyst.[10,11] In our previous research, the adsorption behavior of carbon dioxide on silver-loaded β-Ga2O3 (Ag/β-Ga2O3) was observed by in situ FT-IR measurement, and we have found that silver clusters promote the formation of the intermediates in the carbon dioxide reduction reaction.[11] However, it is not clear how silver clusters on β-Ga2O3 work and promote the reduction of carbon dioxide. It is experimentally difficult to investigate the local structure and electronic states of minute reaction sites created by extremely small metal clusters and metal oxide surfaces.

In order to clarify the physical and chemical properties of such surface reaction sites and to understand the photocatalytic reaction mechanism, theoretical research using first principles calculations will be a powerful technique. For example, the first principles electronic structure calculations were performed on metallic nickel and nickel oxide-loaded β-Ga2O3 (Ni–NiO/β-Ga2O3) photocatalytic systems, and it was suggested that metallic nickel contributes to the hydrogen evolution reaction in the overall water splitting and nickel oxide would take part in the oxygen evolution reaction.[17] However, as far as we know, first principles calculations were not performed on the surface model of the Ag cluster/β-Ga2O3 system.

In this study, in order to understand the photocatalytic carbon dioxide reduction over the Ag/β-Ga2O3 photocatalyst, first principles calculations were performed on the surface model of the Ag cluster/β-Ga2O3 system for the first time. It has been reported that the (100) plane is the most stable in β-Ga2O3,[17,18] thus we chose the β-Ga2O3 (100) surface. In this study, the surface structure and surface energy were calculated for two different types of termination of β-Ga2O3 (100) surfaces. Ag (n = 1 to 4) clusters were adsorbed to the stable surface, and the most stable adsorption structure was determined. Furthermore, we examined the density of states (DOS) and the spatial distribution of wave functions for the Ag/β-Ga2O3 system. DOS is a powerful tool to analyze the energy levels of the system. The region between the top of the valence band and the bottom of the conduction band are similar to the HOMO and LUMO, which are related to the photocatalytic reactivity of Ag/β-Ga2O3.[19,20]

Results and Discussion

First, structural optimization was performed for the unit cell of β-Ga2O3 (the left of Figure ). As a result, lattice constants a, b, c and lattice angle β were calculated to be 12.42, 3.09, 5.88 Å and 103.74°, respectively. These values are almost the same as previously reported calculation results and in good agreement with the experimental result.[17,21]

Figure 1

Unit cell of β-Ga2O3 and (100) surface models with different types of termination.

Structural optimization and surface energy calculations of a β-Ga2O3 (100) surface were carried out for two different terminations of the (100) plane. One of the termination referred to as (100)A is shown in the upper right of Figure and the other denoted as (100)B is represented in the lower right of Figure . The surface energy γ was calculated by the following equation:where Eslab is the total energy of the surface slab model, Eperf is the total energy of the unit cell, n is the number of Ga2O3 units included in the surface slab model, and S is the surface area of the surface slab model.

The surface energies are summarized in Table . The (100)B surface was more stable than the (100)A surface. The values of the surface energy were almost the same between 8 and 12 atomic layers with the difference within 0.01 J/m2. Therefore, we assumed that the surface energy sufficiently converged at 12 layers.

Table 1

Surface Energies of Different Numbers of Atomic Layers and Termination Types of the β-Ga2O3 (100) Surface

slab	surface energy [J/m2]
(100)A–8 layers	0.783
(100)A–12 layers	0.779
(100)B–8 layers	0.457
(100)B–12 layers	0.449

Subsequently, the (100)B surface with 12 atomic layers was expanded to (3 × 2) to make the surface model where the Ag (n = 1 to 4) cluster was adsorbed. The adsorption energy of the Ag cluster Ead(n) was calculated by the following equation:where Eslab + Ag(n) is the total energy of the Ag cluster-adsorbed β-Ga2O3 (100) surface slab model, Eslab is the total energy of the bare β-Ga2O3 (100) surface slab model, and EAg(n) is the is the total energy of the optimized Ag cluster in a vacuum.

First, the Ag1 cluster was arranged at six different adsorption sites on the β-Ga2O3 (100) surface as initial conditions as shown in Figure . Figure S1 shows the structures of the Ag1/β-Ga2O3 (100) surface after optimization for each initial conditions. Bonds are drawn between atoms within a distance of 3.5 Å. The relative energies with respect to the most stable surface and the adsorption energies of the Ag1 cluster are summarized in Table S1.

Figure 2

Side (upper) and top (lower) views of structures of the Ag1/β-Ga2O3 (100) surface at different sites as initial conditions.

For the case of n ≥ 2, initial structures of Ag/β-Ga2O3 (100) were constructed by adding a Ag atom on one of the possible adsorption sites near the Ag cluster of the most stable configuration of the Ag/β-Ga2O3 (100) models. Results of optimized structures and adsorption energies of the Ag cluster (n = 2 to 4) on the β-Ga2O3 (100) surface are shown in Figures S2–S4 and Tables S2–S4, respectively.

Figure shows the most stable structures of the Ag/β-Ga2O3 (100) surface (n = 1 to 4). Ag atoms and the O atoms that are the nearest neighbor to the Ag atoms are labeled. By comparing with the adsorption energies of the most stable structures of the Ag/β-Ga2O3 (100) surface (n = 1 to 4) in Tables S1–S4, we found that the adsorption energy tends to become less negative with increasing of Ag atoms while odd number clusters (Ag1 or Ag3) were more stable, suggesting the Ag cluster size dependence of the adsorption energy.[22]

Figure 3

Most stable structures of the Ag/β-Ga2O3 (100) surface. (a) n = 1, (b) n = 2, (c) n = 3, and (d) n = 4.

Tables 345 summarize the bond lengths in the Ag/β-Ga2O3 (100) surfaces. The most stable adsorption site for an isolated Ag atom (Figure a) is a hollow site in the middle of four nearest O atoms with an adsorption energy of −0.694 eV (Ag1(a) in Table S1), resulting in four Ag–O bonds with a bond length of 2.681–2.753 Å. When a Ag2 dimer is adsorbed on the Ga2O3 (100) surface, the most energetically favorable structure (Figure b) consists of a Ag–Ag bond with a length of 2.644 Å and eight Ag–O bonds of 2.875–3.353 Å. These Ag–O bonds are longer than the Ag–O bond length in the Ag1/β-Ga2O3 (100) surface, suggesting that the formation of the Ag–Ag bond results in the increase of the Ag–O bond length. The adsorption energy of the Ag dimer structure is calculated to be −0.452 eV (Ag2(a) in Table S2), less stable than the adsorption energy of the isolated Ag atom by 0.242 eV. It can be supposed that the weaker adsorption of the Ag dimer on the β-Ga2O3 surface results in longer Ag–O bond distances.[23]

Table 2

Bond Length between Silver–Silver or Silver–Neighboring Oxygen of the Ag1/β-Ga2O3 (100) Surface

bond	length [Å]
Ag(I)–O(I)	2.682
Ag(I)–O(II)	2.751
Ag(I)–O(III)	2.681
Ag(I)–O(IV)	2.753

Table 3

Bond Length between Silver–Silver or Silver–Neighboring Oxygen of the Ag2/β-Ga2O3 (100) Surface

bond	length [Å]
Ag(I)–Ag(II)	2.644
Ag(I)–O(I)	3.353
Ag(I)–O(II)	3.300
Ag(I)–O(IV)	3.361
Ag(I)–O(V)	3.295
Ag(II)–O(II)	2.880
Ag(II)–O(III)	3.029
Ag(II)–O(V)	2.875
Ag(II)–O(VI)	3.017

Table 4

Bond Length between Silver–Silver or Silver–Neighboring Oxygen of the Ag3/β-Ga2O3 (100) Surface

bond	length [Å]
Ag(I)–Ag(II)	2.711
Ag(I)–Ag(III)	2.668
Ag(II)–Ag(III)	2.660
Ag(I)–O(I)	2.277
Ag(I)–O(II)	3.453
Ag(I)–O(III)	3.341
Ag(II)–O(III)	3.339
Ag(II)–O(IV)	2.269

Table 5

Bond Length between Silver–Silver or Silver–Neighboring Oxygen of the Ag4/β-Ga2O3 (100) Surface

bond	length [Å]
Ag(I)–Ag(II)	2.695
Ag(II)–Ag(III)	2.813
Ag(II)–Ag(IV)	2.681
Ag(III)–Ag(IV)	2.629
Ag(I)–O(I)	3.193
Ag(I)–O(II)	3.362
Ag(I)–O(III)	3.275
Ag(I)–O(IV)	3.445
Ag(II)–O(IV)	2.478
Ag(III)–O(V)	3.324
Ag(III)–O(VI)	3.227
Ag(III)–O(VII)	2.328

As for the lowest energy structure of the Ag3/Ga2O3 (100) surface (Figure c), the adsorption energy is calculated to be −1.654 eV (Ag3(a) in Table S3), suggesting the strong adsorption of the Ag trimer on Ga2O3. Three Ag–Ag bond lengths are evaluated as 2.660–2.711 Å, which are almost equivalent to those in the Ag2/β-Ga2O3 (100) surface. On the other hand, two shorter and three longer Ag–O bonds are found and the corresponding lengths are 2.269–2.277 Å and 3.339–3.453 Å, respectively. The short Ag–O bonds would be involved in the strong adsorption of the Ag trimer on Ga2O3.

The introduction of the extra Ag atom to the Ag trimer on Ga2O3 suppresses the adsorption energy to −1.089 eV (Ag4(a) in Table S4) compared to the adsorbed state of the Ag trimer. Investigating the lowest energy structures shown in Figure c,d , the difference in Ag–Ag bond lengths is small. On the other hand, the number ratio of two short Ag–O bonds of 2.328–2.478 Å to six long Ag–O bonds of 3.193–3.445 Å in Figure d is 1/3, which is less than 2/5, calculated with the number of short and long Ag–O bonds in Figure c. The lower ratio of the short Ag–O bond would be related to the less stable adsorption of the Ag4 cluster compared to the Ag3 cluster.

Total DOS (TDOS) and projected partial DOS (PDOS) of Ag/β-Ga2O3 (100) systems are shown in Figure . In the graph, the energy at the upper end of the valence band of the β-Ga2O3 slab is set to 0 eV. The valence states of all Ag clusters mixed with the top of the O 2p valence band of Ga2O3, leading the Fermi level of Ag/β-Ga2O3 to shift to the bottom of the conduction band.[23] Furthermore, the obvious occupied states of Ag clusters are generated between the band gap of Ga2O3, and they increase with the number of Ag atoms. On the other hand, the unoccupied states of Ag clusters overlapped with the Ga unoccupied 4s and 4p states. Under photoexcitation, electrons from the O occupied states move to the Ga unoccupied state. Since the Ga unoccupied states overlap with the unoccupied Ag states, the excited electrons generated in the Ga2O3 photocatalyst can transfer to Ag.[17]Table summarizes some essential values for each surface model.

Figure 4

DOS of the bare and Ag-adsorbed β-Ga2O3 (100) surface models. (a) β-Ga2O3, (b) Ag1/β-Ga2O3, (c) Ag2/β-Ga2O3, (d) Ag3/β-Ga2O3, and (e) Ag4/β-Ga2O3. Fermi levels are represented by dashed lines.

Table 6

Summary of the Calculation Results for Each Surface Modela

surface	adsorption energy of the Ag cluster [eV]	average length of the Ag–Ag bond [Å]	average length of the Ag–O bond [Å]	Fermi level [eV]	energy levels of occupied states of the Ag cluster [eV]
Ag1/β-Ga2O3	–0.694		2.717	2.227	2.12
Ag2/β-Ga2O3	–0.452	2.644	3.139	1.581	1.48
Ag3/β-Ga2O3	–1.654	2.680	2.936	2.262	0.22
Ag4/β-Ga2O3	–1.089	2.705	3.079	1.858	0.14	0.72	1.82

Fermi levels and energy levels of occupied states of Ag clusters are relative to the valence band maximum level of β-Ga2O3.

In Figure , we visualize spatial distribution of the electronic states by the isosurfaces at 0.005 Å–3 of the norm of the wave functions with the energy range from the Fermi level to the energy state 0.5 eV below the Fermi level. As shown in the figure, the highest valence bands are relatively localized on Ag1, Ag2, and Ag4 clusters while they extend to the Ga2O3 surface in the Ag3/β-Ga2O3 (100) system. Since the electrons are localized on Ag1, Ag2, and Ag4 clusters, these Ag clusters would be effective CO2 adsorption sites.[24]

Figure 5

Side (upper) and top (lower) views of the charge density isosurface for the energy level between the Fermi energy and the energy −0.5 eV below from the Fermi energy of the Ag/β-Ga2O3 (100) system.

Figure shows isosurfaces of electron probability densities for the localized bands having higher energy states than the valence states, namely, O 2p orbitals, of the β-Ga2O3 slab in the Ag4/β-Ga2O3 (100) system. Figure shows the schematic diagram of their band structure of the Ag4/β-Ga2O3 (100) system. In this system, the highest occupied states are localized on the Ag4 cluster, and the occupied states derived from the Ag4 cluster would contribute to the local excitation from Ag clusters to the interface between Ag clusters and Ga2O3.[25] It should be noted that our calculations based on the density functional theory underestimate the band gap compared to the experiment. The quantitative values of the energy positions of the gap states shown in Figure will be changed if more accurate calculations such as the hybrid density functional ones are performed. However, the characteristics of the chemical bonding between Ga2O3 and Ag clusters will remain the same regardless of the formalism of the first principles calculations.

Figure 6

Side (upper) and top (lower) views of the charge density isosurface for the internal band between the band gap of the Ag4/β-Ga2O3 (100) system. Relative energies from the valence band maximum level of β-Ga2O3 are written at the bottom of each graph.

Figure 7

Schematic diagram of the band structure of the Ag4/β-Ga2O3 (100) system.

These calculation results correspond to the experimental ones obtained in our previous study,[10,11] i.e., Ag/β-Ga2O3 photocatalysts were examined for photocatalytic reduction of CO2 with water, and the effects of structural and chemical states of the Ag cocatalyst on the photocatalytic reactivity was investigated by studying TEM images, DR UV–vis, and XANES spectra. In particular, Ag L3-edge XANES and O K-edge XANES spectra suggested a strong interaction between 1 nm sized Ag clusters and the Ga2O3 surface, which supports the electron transfer between them. In addition, our in situ FT-IR measurements also revealed that the effective electron transfer can promote the reduction of CO2 adsorbates to the reaction intermediate species on the perimeter of Ag clusters.

In this study, the electronic structure analyses suggest that the loading of Ag clusters enhances the photocatalytic performance by transferring the excited electrons generated in the Ga2O3 photocatalyst to Ag clusters due to the overlap of unoccupied states of Ag clusters and Ga as well as the shift of the Fermi level to the conduction band of Ga2O3. It was also revealed that occupied electronic states of Ag clusters formed in the band gap may enable local excitation from Ag clusters to the interface between Ag clusters and Ga2O3. Excited electrons toward Ag clusters or the perimeter of the Ag-Ga2O3 interface would be used to reduce CO2 adsorbates to the reaction intermediate species and which should enhance the photocatalytic performance.

Conclusions

We performed first principles calculations to study the atomic and electronic structures of the Ag/β-Ga2O3 surface in order to clarify the role of adsorbed Ag clusters in the catalysis of carbon dioxide reduction. Surface models of Ag/β-Ga2O3 (n = 1 to 4) systems were built for first principles electronic structure calculations. The adsorption energy tended to increase with increasing of Ag atoms, while odd number clusters (Ag1 or Ag3) were more stable.

In the electronic structure analysis, the valence states of all Ag clusters mixed with the top of the O 2p valence band of Ga2O3, leading the Fermi level of Ag/β-Ga2O3 to shift to the bottom of the conduction band. It was also revealed that the unoccupied states of Ag clusters overlapped with the Ga unoccupied states. Therefore, the excited electrons generated in the Ga2O3 photocatalyst can transfer to Ag efficiently. Regarding Ag4/β-Ga2O3, occupied states derived from Ag4 clusters were formed in the band gap and right above the valence band of β-Ga2O3, which may enable local excitation from Ag clusters to the interface between Ag clusters and Ga2O3. These calculation results suggest the loading of Ag clusters would enhance the photocatalytic performance, corresponding to the experimental fact that Ga2O3 with small Ag clusters showed high activity for CO2 reduction.

Calculation Methods

We obtain total energies and energy-minimized configurations using first principles calculations based on the plane-wave basis sets and the projector augmented wave method[26] implemented in the VASP code.[27] The Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) is used for the exchange-correlation functional.[28] For all calculations, the cutoff energy of the plane waves was set to be at 500 eV. Structure optimization calculations were performed until all forces acting on atoms of calculated structure models were less than 0.02 eV/Å. The valence electron configurations are 3d10 4s2 4p1 for gallium, 2s2 2p4 for oxygen, and 4d10 5s1 for silver.

The calculation of the β-Ga2O3 unit cell was performed with 2 × 6 × 4 k-point meshes of the Brillouin zone sampling based on the Monkhorst-Pack scheme.[29] A surface slab model was used for calculations of the β-Ga2O3 (100) surface. A (1 × 1) slab model containing 8 or 12 atomic layers in the (100) direction with a 20 Å vacuum spacing was constructed from the optimized β-Ga2O3 unit cell. For the calculation of the surface where the Ag cluster (n = 1 to 4) was adsorbed, (3 × 2) supercells of the surface slab model were constructed. The mesh sizes of the Brillouin zone sampling were 6 × 4 × 1 and 2 × 2 × 1 in the clean surface model and the Ag-absorbed surface model, respectively. The two of the central atomic layers of Ga2O3 were fixed for all surface slab model calculations. The energy of isolated Ag clusters in vacuum was used to calculate the adsorption energy of Ag clusters. In this study, all crystal and surface models are visualized by VESTA.[30]