Adsorption studies of p-aminobenzoic acid on the anatase TiO₂(101) surface.

Andrew G Thomas,Michael Wagstaffe,Johan Adell,Natalia Martsinovich,Karen Syres,Mark J Jackman,Hanna Radtke,Anna Lévy

doi:10.1021/la5032619

Abstract

The adsorption of p-aminobenzoic acid (pABA) on the anatase TiO2(101) surface has been investigated using synchrotron radiation photoelectron spectroscopy, near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT). Photoelectron spectroscopy indicates that the molecule is adsorbed in a bidentate mode through the carboxyl group following deprotonation. NEXAFS spectroscopy and DFT calculations of the adsorption structures indicate the ordering of a monolayer of the amino acid on the surface with the plane of the ring in an almost upright orientation. The adsorption of pABA on nanoparticulate TiO2 leads to a red shift of the optical absorption relative to bare TiO2 nanoparticles. DFT and valence band photoelectron spectroscopy suggest that the shift is attributed to the presence of the highest occupied molecular orbitals in the TiO2 band gap region and the presence of new molecularly derived states near the foot of the TiO2 conduction band.

Highlights

The adsorption of organic molecules for the functionalization of titania is of interest for a number of technological applications including photovoltaics[1], biosensors[2] and targeted biomaterials[3] to name but a few
Photoelectron spectroscopy indicates that the molecule is adsorbed in a bidentate mode through the carboxyl group following deprotonation
Density Functional Theory (DFT) and valence band photoelectron spectroscopy suggest the shift is attributed to the presence of the highest occupied molecular orbitals in the TiO2 band gap region and the presence of new molecularly derived states near the foot of the TiO2 conduction band

Summary

Introduction

The adsorption of organic molecules for the functionalization of titania is of interest for a number of technological applications including photovoltaics[1], biosensors[2] and targeted biomaterials[3] to name but a few. The aromatic ring offers resonance structures for electron or hole trapping This trapping, it has been suggested, can lead to an increase in the rate and efficiency of charge transfer when using cysteine as a linker molecule for CdSe quantum dots on nanostructured TiO2 surfaces[13]. In order to calculate NEXAFS spectra of the gas phase pABA molecule GaussView and Gaussian 0331 were used to produce energy minimized geometry optimized structures in order to obtain the atomic co-ordinates. These calculations were carried out using DFT B3LYP theory and the 631G(d,p) basis set. Summation of the individual energy calibrated spectra gives the theoretical angle-integrated NEXAFS spectrum for the molecule

Results and discussion

Stobe pABA C atom spectra

Isolated pABA DOS pABA HOMO pABA LUMO