Catalytic Mechanism of Pd Adsorption on S-Terminated GaAs(001)-(2 × 6) Surface

Structural and electronic properties of Pd adsorption on clean and S-terminated GaAs(001)-(2 × 6) surfaces are studied using first-principle simulations. Our calculations show that the Pd atom prefers to occupy the HH3 site. The Pd atom is lower than the S atom with 0.15 Å. The density of states analysis confirms that S–Ga bond plays an important role in Heck reaction. We also find that the Pd catalysis activity for Pd adsorption on clean GaAs(001)-(2× 6) surface is weak while it is enhanced when the Pd atom is adsorbed on the S-terminated GaAs(001)-(2 × 6) surface, which is in good agreement with the experiments.


Introduction
Transition metals such as palladium (Pd) and platinum (Pt) are very important catalysts in the field of synthetic chemistry since they have favorable catalysis properties in Heck reaction [1][2][3].Heck reaction plays an important role in drug discovery and pharmaceutical industries.However, the homogeneous Pd catalysis has some disadvantages, such as low reusability, difficulty of separating the catalyst from the products or solvents after reaction.
Recently, heterogeneous catalyst has been extensively investigated in order to reduce the waste of noble metal and develop the high yield of Heck reaction.For Pd adsorption on the sulfur-terminated GaAs(001) surface, experimental results show that the Pd catalyst is reusable [4][5][6][7][8].Arisawa et al. prepare it with Pd(PPh 3 ) 4 [5].They find that Pd is adsorbed on the S-terminated GaAs surface by X-ray photoelectron spectroscopy (XPS) and Pd can be reused at least 3 times.Takamiya et al. find that Ga-S bond fixes Pd on S-terminated GaAs(001)-(2 × 6) surface [6,7], which plays an important role in Heck reaction.It effectively enhances the activity of the Heck reaction more than homogeneous Pd catalyst and could be recycled at least 10 times.In their further study [8], using a new Pd source of Pd(OAc) 2 and another technique of heated washing, they obtain a higher catalysis activity and stability for Heck reaction than the sophisticated method of deposition organometallic Pd(PPh 3 ) 4 on S-terminated GaAs(001)-(2 × 6) surface.Recently, Konishi et al. report that S-GaAs(2 × 6) surface structure is preserved after deposition Pd(CH 3 COO) 2 on its surface using reflection highenergy electron diffraction (RHEED) and scanning tunneling microscopy (STM) [9].However, the detailed structure and micromechanism are unknown.Earlier theoretical study is focused on the system of Pd adsorption on S-terminated GaAs(001)-(2 × 1) surface [10], which is different from the experiments [7,9,11].
In this paper, in order to explain the intrinsic mechanism of Pd catalysis, we investigate the surface structure and electronic property of Pd adsorption on S-GaAs(2×6) surface, based on the density functional theory (DFT).We confirm that the S-Ga band is essential for catalytic reaction.

Calculation Method
The ion-electron interaction was described by the projector augment wave method (PAW).Local density approximation (LDA) approach was used to deal with the energy of exchange correlation.The d electrons are included for Ga atom.Integration over an irreducible Brillouin zone was carried out using the Monkhorst-Pack grid of 5 × 5 × 1.The unit supercell of S adsorption on GaAs(001)-(2 × 6) surface consists of 5 atomic layers and plus a vacuum layer equivalent to 5 atomic layers in thickness.The position of bottom layer As atoms was fixed.Hydrogenlike pseudoatoms with 0.75 charges were used to saturate the surface As dangling bonds at the bottom of slab.The optimized lattice constant was 5.610 Å, consistent with the experimental value of 5.653 Å [12].The detailed structures of Pd adsorption on GaAs(001)-(2 × 6) and S-terminated GaAs(001)-(2 × 6) surfaces are presented.We find that the position of Pd is lower than the surface As or S atom for both model cases, which is similar to the result of Pd adsorption on S-terminated GaAs(001)-(2 × 1) surface [10].The height between Pd and underlying S layer is 0.15 Å and Pd-S bond length is 2.53 Å on S-terminated GaAs surface.Pd-As bond length is calculated to be 2.51 Å on clean GaAs surface, which is similar to the Au-As bond length of 2.50 Å for Au adsorption on GaAs(001) surface [13].For Pd adsorption on S-terminated GaAs (001)-(2 × 6) surface, the S-S dimer remains on the substrate and the bond length is calculated to be 3.568 Å, which is consistent with the experimental findings that (2 × 6) reconstruction is preserved when Pd immobilized on the S-terminated GaAs from the RHEED and STM investigations [9].as Pd adsorption on TiO 2 and Au adsorption on MgO [14][15][16].It is derived from the fact of charge polarization by metal adsorption and an increase of the potential in the adsorption layer.For Pd adsorption on the clean GaAs(001)-(2 × 6) surface, the catalyst activity of Pd is weak due to a few electronic states at the Fermi energy.

Electronic Properties.
In Figure 3, the TDOS and LDOS of Pd adsorption on S-terminated GaAs(001)-(2 × 6) surface at HH3 site are presented.Compared with the TDOS of S-terminated GaAs(001)-(2 × 6) surface, we find that the Pd adsorption makes the DOS shift to lower energy, and the Fermi level exists above the conduction band, which is attributed to the strong interaction between Pd adatom and substrate.In addition, the band gap for Pd adsorption on S-terminated GaAs(001) surface is almost disappeared.It is because that the metal induced gap states (MIGS) are found in the energy gap, which is composed of Ga 4p and S 3p states.MIGS can lead to increasing the activity of chemical reaction due to the increase of the probability of getting electrons.It is consistent with the experimental result of important role for S-Ga bond in Heck reaction [7,9].On the other hand, comparing the DOS of Pd adsorption on clean GaAs(001) surface, we can see that the electronic state at the Fermi level is remarkably increased for Pd adsorption on S-terminated GaAs(001) surface.It indicates that the Pd catalysis activity is enhanced.Meanwhile, DOS at the Fermi level is mainly arisen from the contribution of Pd 4d and S 3p states, and the hybridization between Pd 4d and S 3p states both in valence band and conduction band is also found.Therefore, a strong Pd-S bond is formed on the surface.

Conclusion
We have investigated the structural and electronic properties of Pd adsorption on clean and S-terminated GaAs(001)-(2 × 6) surfaces using first-principle simulations.The calculated adsorption energy shows that Pd atom prefers to occupy the HH3 site.The detailed structure is presented and the Pd atom is lower than the S atom with 0.15 Å.The density of states demonstrates that S-Ga bond plays an important role in Heck reaction.We also find that the Pd catalysis activity for Pd adsorption on clean GaAs(001)-(2 × 6) surface is weak while it is enhanced when Pd atom is adsorbed on the S-terminated GaAs(001)-(2 × 6) surface, which is in good agreement with the experiments.

Figure 2 :Figure 3 :
Figure 2: The total and local density of states (DOS) of Pd adsorption on clean GaAs(001)-(2×6) surface at HH3 site.The dashed line denotes DOS for the clean GaAs(001)-(2 × 6) surface without Pd adsorption.The dashed vertical line at zero represents the Fermi level.