| Resumo: |
The proponents of the present project synthesized very recently a new highly efficient nanostructured composite photocatalyst. This photocatalyst is made of TiO2 nanoparticles chemically bonded to graphene platelets. The new photocatalyst proved to have a much higher photocatalytic activity compared to the standard P25 from Evonik, or to the state-of-the-art photocatalysts such as vlp7000 from Kronos or PC500 from Millenium.
When a TiO2 nanoparticle is illuminated with photons with energy equal or larger than the band gap, electrons (e-) are injected from the valence band into the conduction band leaving a positive hole (h+). The injected electrons can move to the semiconductor surface and originate a reduction reaction or they can recombine with holes originating heat only. On the other hand, holes that reach the surface of the semi-conductor, driven by the so-called Helmholtz layer, can originate oxidation reactions. Due to the strong oxidation potential of the TiO2 (anatase), ca. 3 V, compared to the relatively small reduction one, ca. -0.2 V, titania is used mostly in photooxidation reactions. The large band gap of titania, 3.2 V, allows that only photons with a wavelength smaller than 386 nm can be absorbed; this corresponds to around 5 % of the solar spectrum energy. To increase the activity of a photocatalyst various strategies can be used: a) increasing its surface area, b) decreasing the recombination rate of the photo generated electron-hole pairs, c) extending the light absorption range to longer wavelengths and d) providing better access to the reactants and preventing the adsorption of the reaction products. Activated carbon has a large surface area and is a great adsorbent for many chemical species. It was observed that when used together with titania it has the ability to improve the photooxidation of certain chemical species. The photooxidation reaction is well described by the Langmuir-Hinshelwood mechanism. This means that the pollutants  |
Resumo
The proponents of the present project synthesized very recently a new highly efficient nanostructured composite photocatalyst. This photocatalyst is made of TiO2 nanoparticles chemically bonded to graphene platelets. The new photocatalyst proved to have a much higher photocatalytic activity compared to the standard P25 from Evonik, or to the state-of-the-art photocatalysts such as vlp7000 from Kronos or PC500 from Millenium.
When a TiO2 nanoparticle is illuminated with photons with energy equal or larger than the band gap, electrons (e-) are injected from the valence band into the conduction band leaving a positive hole (h+). The injected electrons can move to the semiconductor surface and originate a reduction reaction or they can recombine with holes originating heat only. On the other hand, holes that reach the surface of the semi-conductor, driven by the so-called Helmholtz layer, can originate oxidation reactions. Due to the strong oxidation potential of the TiO2 (anatase), ca. 3 V, compared to the relatively small reduction one, ca. -0.2 V, titania is used mostly in photooxidation reactions. The large band gap of titania, 3.2 V, allows that only photons with a wavelength smaller than 386 nm can be absorbed; this corresponds to around 5 % of the solar spectrum energy. To increase the activity of a photocatalyst various strategies can be used: a) increasing its surface area, b) decreasing the recombination rate of the photo generated electron-hole pairs, c) extending the light absorption range to longer wavelengths and d) providing better access to the reactants and preventing the adsorption of the reaction products. Activated carbon has a large surface area and is a great adsorbent for many chemical species. It was observed that when used together with titania it has the ability to improve the photooxidation of certain chemical species. The photooxidation reaction is well described by the Langmuir-Hinshelwood mechanism. This means that the pollutants have to be adsorbed before being degraded. The enhancement originated by the activated carbon was then assigned to the adsorption of these pollutants in the activated carbon, close to the photocatalyst. It was suggested that in composite titania/carbon nanotubes, the electron-hole recombination is minimized because the migration of excited electrons from the semiconductor to the carbon nanotube (CNT), decreases the direct recombination with the holes and provides an adsorption site for the species to be oxidized. Also, it could be developed a sensitized mechanism, similar to the one that occurs in the so called dye-sensitized solar cells, where an excited electron from the CNT is injected into the conduction band of the catalyst. Graphene have an ultrathin geometry and properties such as high charge carrier mobility, excellent thermal conductivity and high mechanical strength. The graphene/TiO2 platelet composite photocatalyst benefits potentiality of many, if not all, activity improving mechanisms described above; in addition, graphene is a one atom thick sheet with two faces, which allows TiO2 to attach both faces, and should also be safe for living animals. The present project aims to characterize the newly synthesized graphene-TiO2 platelet composite photocatalyst and modify it for optimized use as ambient NOx photoabatement and for water splitting for the production of H2. The research team has been involved in the photoabatement of ambient NOx, where the photocatalyst is immobilized in paints, and on the water splitting process, using mostly hematite and tungsten oxide. The driving force of this project relies on the possibility of decreasing the band gap, minimize the electron-hole pair recombination, increase the reduction edge potential of this newly synthesized photocatalyst and, with it, obtain a much safer and active photocatalyst for NOx abatement as well as the direct photocleavage of water. |