Claire Costa-Coquelard

Photocatalysis with Polyoxometalates Associated to Porphyrins under Visible Light: An Application of Charge Transfer in Electrostatic Complexes

Absorption spectrum of derived Dawson sandwich polyoxometalates (POM) [M(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](n-) with n = 16 for M = Zn(II), Ni(II), and n = 12 for M = Fe(III) have been extended in the visible range forming electrostatic complexes with a chromophore, the zinc tetracationic porphyrin [ZnTMePyP](4+). Formation of such complexes was followed by steady-state absorption and luminescence spectroscopies. The electrostatic complexation gives in all cases a strong, neutral, and nonluminescent complex. A charge transfer between the two units was shown by transient absorption spectroscopy. Upon a visible excitation of the porphyrin subunit, an electron transfer from the porphyrin to the POM occurs and imparts it a catalytic activity. This has been demonstrated studying a model reaction such as the reduction of silver cations leading to nanoparticles. In all cases, the reduction of the silver cations takes place. We showed that the catalytic activity depends of the nature of the metal of the tetraoxometallic central cluster of the Dawson sandwich POM.

Role of substrate wettability in the "bubble deposition method" applied to the CeVO4 nanowire films.

Homogeneous two-dimensional structures of CeVO(4) nanowires (NWs) deposited on silicon substrates are obtained by means of the bubble deposition method (BDM). Surface wettability (i.e., surface energy) and film ripening (i.e., film thickness) are two major parameters in nanoparticle confinement and deposition. As the presence of surfactant could be detrimental to applications, a washing treatment is developed without CeVO(4) chemical changes or NW film modifications. Careful investigations of the film topography are carried out by atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) is used to check the chemical composition of the film at different stages. Finally, samples made by BDM are compared to those made by dip-coating method, demonstrating the higher efficiency of the BDM in providing large areas of well-organized and dense CeVO(4) monolayer.