Titanium dioxide is one of the most important metal oxides because of its applications as white pigment, as important component in solar cells and as photocatalyst. In the last two applications the relevant phenomena are occurring at the surface of anatase nanoparticles, which are generally considered to be more active than rutile ones. Therefore it is relevant for both technological and fundamental motivations to study the structure of the different surfaces terminating the anatase nanocrystals. In our work we performed periodic DFT calculations of the structure of (101), (100), (001) and (112) anatase faces and of the vibrational properties of CO adsorbed on them at two coverages. Comparison of these results with HRTEM images of the adsorbing nanocrystals of P25 (a highly crystalline TiO2 characterized by outstanding photocatalytic properties and well shaped crystals) and with the experimental FTIR spectra of adsorbed CO on them at 60 K allowed to assign with unprecedented precision the complex spectrum of CO. The relevance of this achievement can be briefly described as follows. CO is a weak Lewis base and its stretching frequency changes when the molecule interact with the surface Ti4+ sites since the greater is the electrophilicity of the metal cation, the higher is the blue shift with respect to the value in gas phase. As the electrophylic character of Ti4+ sites depends upon the face, the FTIR spectrum of adsorbed CO is composed by several peaks, each one corresponding to a specific facelet. On this basis it is evident that the IR spectrum give information about nanocrystals morphology and about the reactivity of different faces. To test the reactivity aspect, the water and acetylene adsorption has been studied. It is shown that the different faces are characterized by different reactivity. Acetylene is a special probe of the surface properties. In fact, unlike CO, it has acidic character and hence is able to probe the basic O2- ions present on the different faces. It is shown that acetylene is irreversibly interacting with basic sites with formation of colored oligomeric species. This reaction seems to be particularly interesting since these in situ grown acetylene polymers can act as potential sensitizers to improve the visible light photoactivity of TiO2.
Surface properties and reactivity of TiO2 nanocrystals: a combined experimental and ab initio study
MINO, LORENZO;FERRARI, Anna Maria;SPOTO, Giuseppe;BORDIGA, Silvia;ZECCHINA, Adriano
2012-01-01
Abstract
Titanium dioxide is one of the most important metal oxides because of its applications as white pigment, as important component in solar cells and as photocatalyst. In the last two applications the relevant phenomena are occurring at the surface of anatase nanoparticles, which are generally considered to be more active than rutile ones. Therefore it is relevant for both technological and fundamental motivations to study the structure of the different surfaces terminating the anatase nanocrystals. In our work we performed periodic DFT calculations of the structure of (101), (100), (001) and (112) anatase faces and of the vibrational properties of CO adsorbed on them at two coverages. Comparison of these results with HRTEM images of the adsorbing nanocrystals of P25 (a highly crystalline TiO2 characterized by outstanding photocatalytic properties and well shaped crystals) and with the experimental FTIR spectra of adsorbed CO on them at 60 K allowed to assign with unprecedented precision the complex spectrum of CO. The relevance of this achievement can be briefly described as follows. CO is a weak Lewis base and its stretching frequency changes when the molecule interact with the surface Ti4+ sites since the greater is the electrophilicity of the metal cation, the higher is the blue shift with respect to the value in gas phase. As the electrophylic character of Ti4+ sites depends upon the face, the FTIR spectrum of adsorbed CO is composed by several peaks, each one corresponding to a specific facelet. On this basis it is evident that the IR spectrum give information about nanocrystals morphology and about the reactivity of different faces. To test the reactivity aspect, the water and acetylene adsorption has been studied. It is shown that the different faces are characterized by different reactivity. Acetylene is a special probe of the surface properties. In fact, unlike CO, it has acidic character and hence is able to probe the basic O2- ions present on the different faces. It is shown that acetylene is irreversibly interacting with basic sites with formation of colored oligomeric species. This reaction seems to be particularly interesting since these in situ grown acetylene polymers can act as potential sensitizers to improve the visible light photoactivity of TiO2.
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