Monoxide carbon frequency shift as a tool for the characterization of TiO2 surfaces: Insights from first principles spectroscopy

PABLO G. LUSTEMBERG; DAMIAN A SCHERLIS

JOURNAL OF CHEMICAL PHYSICS

AMER INST PHYSICS

Lugar: New York; Año: 2013 vol. 138 p. 124702 - 124710

0021-9606

The adsorption and vibrational frequency of CO on defective and undefective titanium dioxide surfacesis examined applying first-principles molecular dynamics simulations. In particular, the vibrationalfrequencies are obtained beyond the harmonic approximation, through the time correlationfunctions of the atomic trajectories. In agreement with experiments, at low CO coverageswe find an upshift in the vibration frequency with respect to the free CO molecule, of 45 and 35cm−1 on the stoichiometric rutile (110) and anatase (101) faces, respectively. A band falling 8 cm−1below the frequency corresponding to the perfect face is observed for the reduced rutile (110) surfacein the low vacancy concentration limit, where the adsorption is favored on Ti4 + sites. At a higherdensity of defects, adsorption on Ti3 + sites becomes more stable, accompanied by a downshift in thestretching band. In the case of anatase (101), we analyze the effect of subsurface oxygen vacancies,which have been shown to be predominant in this material. Interestingly, we find that the adsorptionof CO on five coordinate Ti atoms placed over subsurface vacancies is favored with respect toother Ti4 + sites (7.25 against 6.95 kcal/mol), exhibiting a vibrational redshift of 20 cm−1. Theseresults provide the basis to quantitatively assess the degree of reduction of rutile and anatase surfacesvia IR spectroscopy, and at the same time allow for the assignment of characteristic bands in theCO spectra on TiO2 whose origin has remained ambiguous.