Substrate dependence of Pt₄ electronic properties

The excellent catalytic behavior and exceptional chemical reaction properties of Pt clusters have stimulated significant research interest in the use of these nanostructures in important industrial applications such as catalytic converters for vehicles to reduce toxic pollutants such as CO, NO_x, and hydrocarbons, and in hydrogen fuel cells [1–3]. An interesting characteristic of Pt clusters is that the supporting substrate can greatly affect their chemical reactivity and selectivity, which could originate from the modification of the electronic structure of Pt by the supporting substrate. The mechanism behind such a substrate-induced electronic modification of Pt clusters, however, is so far not well established. In this communication, we show that the electronic properties (whether metallic or insulator) of Pt₄ clusters can be tuned by adsorption on substrates (γ-Al₂O₃ and CaZrO₃) with different electronic valence characters. γ-Al₂O₃ and CaZrO₃ substrates are industrially important catalysts for conventional automobile and fuel cell applications [4–6]. Both have insulating properties and wide band gaps, but the former is a p-block metal oxide while the latter has a strong valence d character.

The tetrahedral structure of the Pt₄ cluster was chosen because of its relative stability over a planar rhombus isomer [7–9]. Also, this structure was shown to be more stably adsorbed on γ-Al₂O₃(111) than the planar structure [10]. The calculated gas phase isolated Pt₄ cluster with a tetrahedral structure has an average Pt–Pt distance of 2.62 Å, in agreement with related studies [7–11]. For the γ-Al₂O₃ substrate, the Pt₄ cluster was adsorbed on the (111) facet of a slab modeled as a 2 × 2 supercell with 32 Al and 48 O atoms and a vacuum space of ∼25 Å. This model is based on the proposed structure of γ-Al₂O₃(111) in similar density functional theory (DFT) studies [10, 12]. The adsorption of the Pt₄ cluster on the (001) facet of the CaZrO₃ substrate was modeled with an 84-atom (16 Ca, 16 Zr, 48 O and 4 Pt atoms) supercell with 0.25 ML adsorbate coverage and a vacuum layer of ∼16 Å. The CaZrO₃ orthorhombic perovskite structure is based on the related literature [13, 14]. The CaZrO₃(001) and γ-Al₂O₃(111) facets were chosen because of their stability [12]. The Pt₄ cluster and the top 40 atoms of the slab were fully relaxed to obtain a stable adsorption structure of the cluster on the surface sites shown in figure 1. For the γ-Al₂O₃(111) surface, the a, b, and c sites correspond to O hollow sites, Al hollow sites, and Al atop sites, respectively. These sites are expected to be the most favorable adsorption sites since these configurations can promote a strong Pt–O bonding interaction. For CaZrO₃(001), sites a, b and c correspond to the positions above Ca, O, and Zr, respectively. The adsorption energy was computed by taking the difference between the total energy of the Pt₄ cluster–slab system in the lowest energy adsorption site and the summed energies of the relaxed clean surface and the gas phase Pt₄ cluster.

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Spin-polarized density functional theory (DFT) calculations were implemented via the Vienna ab initio simulation package (VASP) [15–18]. The interactions between ions and electrons were described using the projector augmented wave (PAW) method [19, 20]. Plane-wave basis sets were employed with an energy cut-off of 400 eV. The exchange–correlation term was described using the generalized gradient approximation (GGA) based on the Perdew–Burke–Ernzerhof (PBE) [21, 22] functional. The surface Brillouin zone integrations were performed on a grid of 3 × 3 × 1 Monkhorst–Pack k-points [23]. The conjugate gradient algorithm was used to relax the ions into their ground state. An electric dipole correction layer in the vacuum area was used to cut the dipole interactions between the repeated image layer systems. Convergence of the numerical results with respect to the slab thickness, the kinetic energy cut-off and the k-point was established.

We first discuss the adsorption of Pt₄ clusters on γ-Al₂O₃(111) and CaZrO₃(001) surfaces. The Pt₄ cluster adsorbs strongly on site b of γ-Al₂O₃(111) and site c of CaZrO₃(001), with adsorption energies of −4.09 eV and −4.30 eV respectively, in excellent agreement with a previous study [10]. The bottom plane of the tetrahedral Pt₄ cluster is parallel to the surface of γ-Al₂O₃(111). For CaZrO₃(001), one of the surface Pt atoms goes deeper into the surface by ∼0.3 Å as compared to the other two atoms, making the bottom plane of the tetrahedral structure slightly inclined on the surface. This is because of corrugation of the CaZrO₃(001) surface, as shown in figure 1. The average Pt–Pt distances of the Pt atoms directly bonded to the surface are 2.60 Å and 2.81 Å for γ-Al₂O₃(111) and CaZrO₃(001) surfaces, respectively. The greater surface area of the bottom plane of the tetrahedral cluster on CaZrO₃(001) than on γ-Al₂O₃(111) results in stronger binding of the cluster on the CaZrO₃(001) surface.

We then report the difference in the electronic properties of the Pt₄ cluster upon its adsorption on γ-Al₂O₃(111) and CaZrO₃(001) surfaces. The density of states (DOS) projected on the Pt atom (denoted as 'Pt' in figure 3 for adsorbed systems) is shown in figure 2. The gas phase tetrahedral Pt₄ cluster has metallic characteristics and exhibits a magnetic character, as shown by the spin polarization of the DOS (figure 2(a)) and the magnetization of 0.44 μ_B/atom. Upon adsorption on the surfaces, these states are broadened and shifted, due to the interaction of the adsorbate with the surface. For the Pt₄ cluster adsorbed on the γ-Al₂O₃(111) surface, the presence of states in the Fermi level (figure 2(b)) shows that Pt has metallic characteristics. It is noted that the occupied regions from ∼ − 2.0 eV to the Fermi level are essentially unchanged. The same metallic characteristics are also noted for the other three Pt atoms (their DOS plots are not shown). Interestingly, Pt exhibits insulator properties on the CaZrO₃(001) surface, with an energy band gap of 2.2 eV. Since the energy band gap in PtO₂ has been found and calculated in both experimental [24, 25] and theoretical [26] studies, Pt exhibits oxide-like properties on CaZrO₃. The same properties were noted for the other three Pt atoms (DOS plots are not shown). For both of these substrates, the spin polarization of the Pt d state is retained. Such a spin-polarized Pt d state was also observed for a Pt overlayer on a ferromagnetic substrate [27].

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Next, we discuss the changes in the electronic properties of the substrate atoms which directly interact with the Pt₄ cluster. Figure 3 shows the charge density difference plot projected on a $+0.0 1 e/{a}_{0}^{3}$ isosurface value. Strong interaction between atoms is shown by the accumulation of electron density (yellow). For the γ-Al₂O₃(111) surface, the Pt atoms near the surface interact mainly with the Al atoms which protrude out of the surface (which we now refer to as Al_Pt). This was also noted in a related study [10]. While for CaZrO₃(001) surface, Pt atoms interact strongly with the nearest Zr (which we also now refer to as Zr_Pt). As shown in figure 4, the valence charge is largely associated with the O p state, suggesting the polar character of the Al–O bond. Such a property has been reported in the literature [12]. The adsorption of the Pt₄ cluster on γ-Al₂O₃(111) dramatically decreases the energy band gap of Al_Pt from 4.1 to 0.6 eV. For the CaZrO₃(001) surface, the adsorption of the Pt₄ cluster results in downward shifting of the occupied and unoccupied states of Zr_Pt, but the energy band gap is essentially unchanged (figure 5). To understand these differences, we first note that for Zr_Pt, the d state participates mainly in the interaction with the Pt₄ cluster. The DOS for s and p states is negligible, and too small to significantly alter the Pt d states. Therefore, the Pt d state has to adapt to the insulator characteristics of Zr_Pt to efficiently hybridize with the Zr_Pt d state. Thus, the strong hybridization between Pt d and Zr d states is promoted by the electronic switching of the Pt d state from metallic to insulator. For Al_Pt, however, the weak hybridization of the Pt d state with Al states allows the Pt d state to maintain its metallic electronic properties upon its adsorption on the surface. We note that the occupied and unoccupied Al_Pt states have shifted closer to the Fermi level, narrowing the energy band gap.

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In conclusion, we have shown that the electronic properties of Pt₄ clusters can be tuned by adsorption on substrates with different electronic structures. Pt₄ clusters exhibit metallic characteristics on γ-Al₂O₃(111) and insulator properties on CaZrO₃(001). The noted difference indicates the role of electronic valence states of the substrate atoms that directly bond with Pt. The strong hybridization between Pt d and Zr d states for Pt₄/CaZrO₃(001) is promoted by the electronic switching of the Pt d state from metallic to insulator. On the other hand, weak hybridization between Pt d and Al states allows the Pt cluster to retain metallic characteristics on γ-Al₂O₃(111).

Acknowledgments