Adsorption model for atoms and molecules on doped semiconducting oxides

Fundamental understanding of the interaction between atoms and molecules with the surfaces of oxides including semiconducting oxides is crucial for the development of several thermo-, photo-, and electrocatalytic reactions as well as any application where surfaces are exposed to an environment beyond vacuum. While previous studies have postulated material features (descriptors) that to some extent suggest the adsorption energy trends on semiconducting oxides, a physics based model to describe the interaction of atoms and molecules with the surfaces of these materials is still lacking. In this study, we perform a series of controlled in silico experiments involving doping of quintessential semiconducting oxides (${\mathrm{SrTiO}}_3, {\mathrm{SrZrO}}_3$, and ${\mathrm{TiO}}_2$) to identify the perturbation by the dopant to the electronic structure of the host oxide and its resultant effect on the adsorption energies of simple atoms and molecules. We identify that a combination of three surface features: unique surface resonance states of the host-metal and lattice oxygen atoms of the terminating surface oxide layer as well as the gap states dominated by the introduced dopants contribute to the adsorption energy in a concerted fashion. We find that this intricate interplay between on the one hand host-metal and on the other hand oxygen surface resonance states with the adsorbate, respectively, results in a deviation from the well-established adsorbate scaling relations seen for ${\mathrm{NH}}_x$ ($x=0$–2) and ${\mathrm{CH}}_x$ ($x=0$–3) but not ${\mathrm{OH}}_x$ and ${\mathrm{SH}}_x$. Through this lens, we develop a physics based adsorption model hitherto referred as the generalized concerted coupling model (GCC model). The introduced model provides a physical understanding with an associated electronic structure descriptor rooted in the unique surfaces resonances that accurately captures the adsorption energy trends on doped semiconducting oxides. This paves the way for the atomistic design of doped semiconducting oxides for different catalytic applications, including sustainable energy applications such as electrochemical water splitting.

Figure 1

Side view of the atomic structure of the (001)-${\mathrm{BO}}_2$ doped STO surface (STMO) used in the calculations indicating the location of the subsurface dopant layer. Top view of atomic structure of the (001)-${\mathrm{BO}}_2$ doped STO (STMO) surface. The highlighted region in the periodic table shows the different $3d, 4d$, and $5d$ metals $M$ considered as dopants in this study.

Figure 2

Adsorbate scaling relations between the central atom ($A$) and the different $A{\mathrm{H}}_x$ species for adsorption on-top the surface Ti atom of the (001)-${\mathrm{BO}}_2$ surface of STMO for (a) ${\mathrm{OH}}_x(x=0$–1), (b) ${\mathrm{NH}}_x(x=0$–2), (c) ${\mathrm{CH}}_x(x=0$–3), and (d) ${\mathrm{SH}}_x(x=0$–1). $M$ corresponds to $3d, 4d$, and $5d$ transition metals doped in the subsurface of STO.

Figure 3

The projected density of states (PDOS) of the different adsorbates in their most favored geometry optimized position on the (001)-${\mathrm{BO}}_2$ surface of undoped STO. From top to bottom: H-$1\mathit{s}$ (adsorbed on-top of O); C-$2p$, N-$2p$, S-$3p$, and O-$2p$ (adsorbed on-top of Ti) states. The corresponding adsorbed configurations on the preferred surface sites are shown in the insets, and the vertical dashed line corresponds to the Fermi level.

Figure 4

$\mathrm{\Delta }\mathrm{PDOS}$ of the Ti-$3d$ and O-$2p$ states of the surface atoms of the (001)-${\mathrm{BO}}_2$ surface of STO induced by the presence of different adsorbates. The corresponding adsorbate states are also shown: (a) O-$2p$, (b) N-$2p$, (c) C-$2p$, and (d) S-$3p$.

Figure 5

$\mathrm{\Delta }\mathrm{PDOS}$ of the Ti-$3d$ and O-$2p$ states of the surface atoms of the (001)-${\mathrm{BO}}_2$ Mo-doped STMO surface induced by the presence of the adsorbate (O) at different distances above the surface Ti atom adsorption site. The corresponding adsorbate states, as well as the $\mathrm{\Delta }\mathrm{PDOS}$ of the dopant states (subsurface Mo and O) are also shown.

Figure 6

$\mathrm{\Delta }\mathrm{PDOS}$ of the Ti-$3d$ and O-$2p$ states of the surface atoms of the (001)-${\mathrm{BO}}_2$ surface of STO doped with Ru induced by the presence of the adsorbate (O), for the geometry optimized case (top) and with the constrained surface (bottom). The corresponding adsorbate states, as well as the $\mathrm{\Delta }\mathrm{PDOS}$ of the dopant states (subsurface Ru and O) are also shown.

Figure 7

$\mathrm{\Delta }\mathrm{PDOS}$ of the Ti-$3d$ and O-$2p$ states of the surface atoms of the (001)-${\mathrm{BO}}_2$ surface of Ru-doped STMO induced by the presence of the adsorbates (a) O and (b) H, when moved across the surface at a fixed distance above the surface (from top to bottom: Ti-top, in-between Ti and O, and O-top). The corresponding adsorbate states, as well as the $\mathrm{\Delta }\mathrm{PDOS}$ of the dopant states (subsurface Ru and O) are also shown. Inset shows the top view of the adsorption configurations. Color codes: Ti-grey,O surface-red, Sr-green and adsorbate (O or H): blue.

Figure 8

Calculated hybridization energy ($\mathrm{\Delta }{E}_{\mathrm{hyb}}$) using the Newns-Anderson model and two semiellipse fits for the PDOS; one for the surface Ti and the other for the surface O atoms of STO. The 2D heat map of $\mathrm{\Delta }{E}_{\mathrm{hyb}}$ as a function of varying positions of the O-$2p$ valence band (VB) and Ti-$3d$ conduction band (CB) for a given adsorbate state (${\varepsilon }_a$) and different relative coupling strengths between the adsorbate and surface O and Ti atoms. (a) Strong Ti coupling (top), comparable concerted coupling (middle) and strong O coupling(bottom). (b) $\mathrm{\Delta }{E}_{\mathrm{hyb}}$ for the different $4d$ transition metal doped STMO as well as the undoped STO computed using the Newns-Anderson model for the three different relative coupling strengths. (c) DFT computed O adsorption energy with adsorption on-top of the surface Ti atom of $4d$ transition metal doped STMO.

Figure 9

$\mathrm{\Delta }\mathrm{PDOS}$ of the surface Ti-$3d$ and O-$2p$ states, as well as the dopant states in the gap, computed by taking the difference between the PDOS of the oxide with the dopant (STMO) and without the dopant (STO). The Mo-doped STMO system is shown here as an example. The surface resonance states are indicated as OSR and HMSR. The vertical dashed line corresponds to the Fermi level.

Figure 10

Schematic illustration of the generalized concerted coupling model (GCC model). The interaction between the adatom (adsorbate) state and the oxygen surface resonance (OSR) and host-metal surface resonance (HMSR) results in the bonding and antibonding adsorbate states, captured by the quenching of the surface resonance peaks. The quenching of the dopant states in the gap shows the charge transfer from the dopant layer (Fermi level) and the newly formed bonding state. Mo-doped STMO is used as an example.

Figure 11

Correlation between the adsorption energy of (a) ${\mathrm{OH}}_x(x=0$–1) and (b) ${\mathrm{NH}}_x(x=0$–2) adsorbed on-top of the Ti surface atom of the (001)-${\mathrm{BO}}_2$ surface of STMO and the identified descriptor $|{\varepsilon }_{\mathrm{OSR}}/{\varepsilon }_{\mathrm{HMSR}}|$ in the GCC model. $M$ corresponds to $3d, 4d$, and $5d$ transition metals doped in the subsurface of STMO. The intercept corresponds to the adsorption energy on undoped STO. The corresponding correlation plots for SZMO and ${\mathrm{TiMO}}_2$ are shown in Figs. S50 and S51 in Ref. [36], respectively.

Figure 12

The changes in the $\mathrm{\Delta }\mathrm{PDOS}$, with increasing coupling strength (top to bottom) between the H adsorbate and surface O atom on Ru-doped STMO. The $\mathrm{\Delta }\mathrm{PDOS}$ of the surface atoms, dopant layer atoms and the adsorbate are shown.

Figure 13

Correlation between the DFT calculated adsorption energy of H adsorbed on-top of the O atom of the (001)-${\mathrm{BO}}_2$ surface (constrained) of STMO and the inverse of the identified descriptor in GCC model. M corresponds to $3d, 4d,$ and $5d$ transition metals doped in the subsurface of STO. The intercept corresponds to the adsorption energy on undoped STO.