Dopamine Adsorption on Rutile TiO2(110): Geometry, Thermodynamics, and Core-Level Shifts from First Principles

The modification of the rutile TiO2(110) surface with dopamine represents the best example of the functionalization of TiO2-based nanoparticles with catecholamines, which is of great interest for sunlight harvesting and drug delivery. However, there is little information on the dopamine–TiO2(110) adsorption complex in terms of thermodynamic properties and structural parameters such as bond coordination and orientation of the terminal ethyl–amino group. Here, we report a density functional theory (DFT) investigation of dopamine adsorption on the TiO2(110) surface using the optB86b-vdW functional with a Hubbard-type correction to the Ti 3d orbitals, where Ueff = 3 eV. Guided by available X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) data, our simulations identify enolate species with bidentate coordination at a submonolayer coverage, which are bonded to two neighboring 5-fold-coordinated Ti atoms at the TiO2(110) surface through both deprotonated oxygen atoms of the dopamine, i.e., in a bridging fashion. The process is highly exothermic, involving an adsorption energy of −2.90 eV. Calculated structural parameters suggest that the molecule sits approximately upright on the surface with the amino group interacting with the π-like orbitals of the aromatic ring, leading to a gauche-like configuration. The resulting NH···π hydrogen bond in this configuration can be broken by overcoming an energy barrier of 0.22 eV; in this way, the amino group rotation leads to an anti-like conformation, making this terminal group able to bind to other biomolecules. This mechanism is endothermic by 0.07 eV. Comparison of existing spectroscopic data with DFT modeling shows that our computational setup can reproduce most experimentally determined parameters such as tilt angles from NEXAFS and chemical shifts in XPS, which allows us to identify the preferred mode of adsorption of dopamine on the TiO2(110) surface.


S2
This Supporting Information contains: • Geometries of gas-phase dopamine obtained with optB86b-vdW (Table S1) • Adsorption energies for the dopamine-TiO2(110) adsorption complex calculated within the optB88-vdW and PBE approaches ( Table S2) • Analysis of the presence of co-adsorbed hydrogen on the surface • Core-level shift calculations within the initial state approximation for the most stable configuration • Bader charge analysis for the "Bridging G" configuration Table S1. Computed structural parameters for gas-phase dopamine predicted by the optB86b-vdW functional. For numbering, refer to Figure 1 in the main section.

Analysis of the Presence of Co-Adsorbed Hydrogen on the Surface
To assess the effect of both dissociated hydrogen atoms on the dopamine-TiO2 (110) adsorption complex, test calculations for the most stable configuration were undertaken for two different locations relative to the adsorbed dopamine, which are labeled (1) and (2) in Figure   S1. In location (1) the resulting surface hydroxyls were intentionally placed far away from the adsorption site to isolate the remaining dopamine, whereas in location (2) hydrogen interactions between both hydroxyl groups and enolate dopamine were promoted. It was found that a configuration resulting from placing both protons at location (2) is energetically more favorable than its counterpart by 0.29 eV. In this configuration the calculated tilt angles are α=68° and β=23° (see Figure 4 in the main section). However, this adsorption geometry is not compatible with the values determined by angle-resolved NEXAFS spectra (α=78±5° and β=11±5°). Therefore, the dissociated hydrogen atoms from dopamine were placed as far as possible from the adsorption site in all configurations.
The above calculations lead us to two conceptions. First, identification of adsorption geometry adopted by the dopamine onto TiO2(110) surface cannot be done on the basis of adsorption energies alone; consequently, it is necessary to resort to a direct comparison with experiment. That was also the case for benzoate adsorption onto this surface, where a S4 combination of scanned-energy mode PhD and DFT calculations identified an upright/coplanar structure, even though theoretical modelling suggested that more tilted/twisted adsorption geometries were also possible. 1 In this system, however, there is not a direct effect of coadsorbed hydrogens in the tilted angles, which prefer to be adjacent to the adsorbed benzoate.
And second, surface hydroxyls at location (2)     Within this approximation, relaxation effects are completely neglected, i.e., only the state of the system before the photoionization process is considered.

Configuration
According to Table S3, the calculated shifts in the C 1s BEs indicate that initial state effects dominate the relative chemical shifts in the benzene ring. On the other hand, such chemical shifts in the ethyl chain are directed by the so-called final state effects.

S7
We have calculated the effective atomic charges using the Bader methodology, 5 where an atom is defined as a region of real space delimited by surfaces through which the flux of the density gradient is zero. For our Bader analysis we employed the implementation by Henkelman et al. [6][7][8] Bader charge analysis shows that a net charge of 0.45 e is transferred to the TiO2 (110) surface. The amount of transferred charge is practically accounted by the benzene ring and the oxygens in the adsorbate which causes a charge redistribution among these atoms. Such a redistribution is not seen in C7 and C8.