An investigation on the bulk and surface properties of Ta-doped TiO2 nanomaterials

INTRODUCTION Titanium dioxide is a widely adopted compound in the field of nanomaterials for manifold applications, such as production of green fuels, water splitting, solar cells, drug delivery systems, surface functionalization and photocatalysis [1-2]. The doping with metal species was variously proposed as a possible technique able to vary the structural, electronic and surface properties and also to increase the performance of the TiO2 nanopowders [3]. Recently, we studied tantalum doping of TiO2 as a possible procedure for enhancing TiO2 activity under UV irradiation [4]. In fact, theoretical studies expected tantalum to improve the photocatalytic performance of TiO2 by providing additional free electrons (thanks to the higher oxidation state of Ta with respect to Ti) and energy states available for excitation in the conduction band [4]. Nonetheless, to the best of our knowledge, an in-depth study able to correlate the modification of the lattice provided by Ta dopant with the modification of the TiO2 surface properties, for differently synthesized TiO2 nanopowders, is not present in the literature yet. In the present work, different sol-gel procedures permitted to synthesize titania samples with a different phase composition (pure anatase and anatase / brookite mixtures). Corresponding Ta-doped samples were prepared. The influence of the synthetic procedure and of the effect of Ta addition were investigated by means of a wide variety of characterizations. The synthesized nanomaterials were then adopted as photocatalysts in the degradation of the active principle of the bestseller on the- count (OTC) drug in Italy, that is acetaminophen (paracetamol), which in the last few years has become an important emerging pollutant of water bodies due to its extensive use. EXPERIMENTAL Material Synthesis. The TiO2 samples were synthesized by sol-gel syntheses. The doping with Ta was provided by an impregnation method, in which the amorphous TiO2 powder is suspended in a TaCl5 ethanol/water solution. Finally, a thermal treatment at 400 °C was performed. Material Characterization. The nanopowders were characterized by XRPD and BET specific surface area analyses in order to investigate the structural and the morphological properties (phase composition, crystallite dimension, porosity). Further analyses allowed to get information about the active site properties in terms of surface acidity and hydroxylation by adsorption / desorption of a probe molecule (phenylethylamine). HRTEM was used to discuss the presence of different titania phases and tantalum moieties. Granulometry analyses and suspension stability tests were also performed. Photocatalytic tests. The undoped and Ta-doped samples were tested in the photodegradation of acetaminophen, under UV irradiation, both concerning the disappearance and the final mineralization of the molecule. The photocatalytic tests were performed also in the presence of OḢ• and h+ scavenger compounds (i-propanol and EDTA), in order to evaluate the role of these oxidizing species in the reaction route. RESULTS AND DISCUSSION Structural characterization by XRPD of the differently synthesized samples revealed different phase compositions. The addition of acetic acid in the reaction environment promoted anatase growth, avoiding brookite formation. Ta inclusion slightly modified the phase composition, while the morphology of the nanomaterials was deeply affected by synthesis and doping procedures. CONCLUSION Ta-doped TiO2 nanomaterials were synthesized according to different synthetic routes. The effect of the dopant on anatase and anatase / brookite systems was investigated. Bulk, surface and photocatalytic properties (even in the presence of scavengers) were studied in order to provide a comprehensive analysis on these compounds, thus enlightening their perspective use for different applications where a superior activity under UV irradiation is needed. REFERENCES 1. X. Chen et al., Chem. Rev. 107, 2891 (2007) 2. G. Soliveri et al., J. Phys. Chem. C 116, 26405 (2012) 3. O. Carp et al., Prog. Solid State Chem. 32, 33 (2004) 4. L. Rimoldi et al., J. Phys. Chem. C 19, 24104 (2015)