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TUNABLE FEATURES OF TIO2 NANOPARTICLES FOR PHOTOCATALYTIC AND PHOTOVOLTAIC APPLICATIONS: A FUNDAMENTAL STUDY OF ELECTRONIC AND STRUCTURAL ASPECTS.
Get PDFNanosized titanium dioxide has been widely explored as both pristine and doped semiconductor. TiO2 nanocrystals were successfully synthesized by three different procedures. Two of them are based on a controlled sol–gel reaction, implemented either by doping or by a hydrothermal growth step with the use of surfactants, exploiting three-dimensional micelles. The latter one is a microemulsion-mediate route, to accurately control the growth of Ti precursor in nanoreactors by the confinement in aqueous droplets. As for the doping, both metal and non-metal elements have been chosen, namely praseodymium, silver and nitrogen. In this latter case, both organic and inorganic sources, in turn, were used to modulate the N content of doped-titania.
All samples were characterized from the morphological, structural, optical, and electronic points of view. Moreover, for selected N-TiO2 samples, the role of the aging time after the calcinations of the powders (from “freshly prepared” to “old” samples) both on the intensity of the optical features and on the bulk paramagnetic nitrogen concentration was also considered.
Two main fields of applications for titania powders and films were investigated: photocatalysis and photovoltaics. Thus, the photocatalytic activity of several samples was tested using either UV or visible light irradiation with regard to different pollutants, ranging from ethanol (both in aqueous and gaseous media) to the more complex methylene blue molecule (deposited onto the oxide film). Besides, advanced oxidation processes were successfully applied to the degradation and final mineralization of bisphenol A and 4-cumylphenol. The use of TiO2 immobilized in thin films is of paramount importance for the plant-scale applicability of the process, especially if a low intensity irradiation source, such as solar light, is to be exploited. Then, photocatalytic reduction of hexavalent chromium was conducted with the use of electrodeposited TiO2 layers.
Even though (doped) titanium dioxide is one of the most commonly adopted semiconductors in photocatalysis, it suffers from a relatively high bandgap – hence the doping to increase visible light absorption – and from recombination of photogenerated charge carriers. Furthermore, univocal conclusions on the Fermi energy levels, chemical nature and location of the doping centers, and related charge transfer processes versus dopant concentration are absent in the literature. Therefore, electronic effects induced by the presence of the heteroatom in the TiO2 nanocrystals were investigated by a synergistic combination of electrochemical experiments (Mott-Schottky plots, photovoltage and photocurrent measurements) and theoretical DFT calculations. Results on the doped materials point towards a reduced tendency to charge carriers’ recombination and different effects on the (quasi-)Fermi energy location of the final material.
Recombination is a central issue also in solar cell devices, resulting in possible low efficiencies. The ability of spin-coated and spray-pyrolyzed TiO2 blocking layers in preventing or reducing losses arising from electron transfer via the transparent conductive substrate has been examined in bilayer hybrid dye-sensitized solar cell (DSC) devices. Then, different types of both solid-state dye-sensitized solar cells and DSCs with a liquid electrolyte were tested making use of the more effective spray-pyrolyzed TiO2 as “blocking layer” and optimizing home-made nanostructured titania pastes
Tailore TiO2 layers for the photocatalytic ozonation of cumylphenol, a refractory pollutant exerting hormonal activity
No full textPhotocatalytic ozonation by Al-supported TiO2 films is successfully applied to the mineralization of 4-cumylphenol, a recalcitrant endocrine disrupting compound. A possible reaction mechanism implying the intermediate formation of bisphenols is proposed on the grounds of HPLC-MS and FTIR analyses
Doped and undoped titania nanocrystals : theoretical bottom-up approach vs. experimental flatband potential studies
No full textA synergistic theoretical and experimental approach has been developed in order to determinate the electronic band structure in pure and nitrogen doped titania nanocrystals. Theoretical ab initio Density Functional Theory (DFT) calculations were employed using the state of the art DFT functionals available within the VASP suite of codes. On one hand, these studies allowed us to calculate the density of the electronic states (DOS), including the Fermi Energy location at the bandgap and the eventual presence of intra-gap states. On the other hand, electrochemical characterizations were carried out on samples of titania nanocrystals with doping concentrations comparable to the theoretical models. These brought us to the determination of fundamental parameters, such as the effective donor density (ND), the flat-band potential (FB), the position of the conduction and valence band edges at the interface. The FB potentials of pure and nitrogen-doped nanotitania thin films, prepared in our laboratory, have been determined by the capacitance method, based on Mott-Schottky plots.
Merging these results with the theoretical ones provided us with a unison physical picture. This allowed us to better understand the N-doping effects on titania nanocrystals, such as the formations of intraband gap states, and to elucidate a long standing diatriba about the possible bandgap narrowing induced by N doping of titania
Pure and N-doped TiO2 nanoparticles : Flat-band potential evaluation by different approaches
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Tailored TiO2 nanoparticles by means of template and microemulsion routes
No full textTiO2 materials with tailored pore size and particle dimensions, high specific surface area, and well-defined crystalline frameworks have potential applications in several fields such as solar energy conversion, photocatalysis, catalytic production of gases, and Li-ion batteries. Therefore, a great deal of efforts has been devoted in recent years to the development of new synthetic routes that allow the relevant morphological properties of titania powders to be controlled.1,2
In the present work, two synthetic methods have been optimized in order to obtain titania particles with controlled structure, morphology and surface state: a template route3,4 and a microemulsion mediated method,5,6 both starting from alkoxide precursor in hydroalcoholic medium.
In the template route, a block copolymer employed as a structure-directing agent is added in the pre-synthesized titania nanoparticulate sol. Alternatively, the titania nanoparticles are synthesized using a reverse microemulsion system containing surfactant and co-surfactant, water and non-polar solvents.7
All samples are characterized under the structural (XRPD), morphological (BET, HRTEM), optical (DRS) and spectroscopic (XPS, Raman) points of view. Selected samples are tested for their photocatalytic activity.
The effects of several synthetic parameters, such as the polymer content, the role of the surfactant, the evaporation method and the annealing conditions, on sample morphology and surface/bulk properties are discussed
N, Ag codoped nano-TiO2: the effect of synthetic parameters on gas phase photocatalytic activity
No full textThe use of traditional, simple, quick sol-gel synthesis for the obtainment of well-performing N,Ag codoped TiO2 photocatalysts has revealed fortunate. Thanks to the concomitant presence of both metal and non-metal dopants, the nanopowders show high surface areas, abundant mesopores and a wide absorption in the visible region, leading to a good photocatalysis under solar irradiation
Electrochemical capabilities in both photoreduction and detection of toxic Cr(VI) pollutant
No full textChromium is widely used in several industrial processes such as metal plating,
leather tanning, paint making and others. Due to its acute toxicity,
carcinogenic action and high mobility in water, Cr(VI) is in the list of priority
pollutants of most countries. In aquatic environments, chromium is present mostly
as hexavalent Cr(VI) and trivalent Cr(III). Cr(III) is less noxious and usually
immobile through precipitation or adsorption onto solid phase. Therefore, the rapid
and accurate monitoring of both chromium species and the efficient reduction of
Cr(VI) to Cr(III) in contaminated waters are highly desirable.
Specifically, Cr(VI) ions in water can be completely photoreduced to Cr(III) at the
surface of UV-excited TiO2 photocatalysts. The transfer of photocatalytic
results from slurry to immobilized particles on substrates is a general concern,
primarily due to the elimination of filtration steps. Thus, the first part of this
presentation is focused on the electrophoretic deposition (EPD) of TiO2 as an
alternative procedure to deposit the oxide in dense layers, avoiding slurry filtration
steps. The second part of the presentation will be devoted to the study of the
photocatalytic performances of titania layers by comparing traditional
polarographic techniques with a new environmentally friendly electroanalytical
approach based on the use of bismuth screen printed electrodes
Time effects on the stability of the induced defects in TiO2 nanoparticles doped by different nitrogen sources
No full textN-doped TiO2 samples are claimed to be the most promising among the so-called secondgeneration photocatalysts, but their success in photocatalysis is still under debate. In this study, N-doped TiO2 nanocrystals are obtained by a simple, quick, and effortless procedure, starting from titanium alkoxide as the precursor for the sol-gel route, with the N source being either inorganic (NH3) or organic (triethylamine, urea). Structural, morphological, and optical characterizations are compared with electron paramagnetic resonance (EPR) data to give an integrated picture of such materials. No literature data on the "aging" features in the dry state of the fresh calcined samples on the EPR and diffuse reflectance spectra (DRS) measurements are reported. Our N-doped TiO2 powders show different stabilities of paramagnetic and optical signals. The photocatalytic activity is tested, toward the degradation of ethanol in aqueous media, under both visible and UV irradiation, in this latter case resembling the same trend of the paramagnetic species decay
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