Abdelkrim Chemseddine

PbSe nanocrystal shape development: oriented attachment at mild conditions and microwave assisted growth of nanocubes

PbSe nanocrystals have been synthesized by conventional hot injection process and also using microwave technique. For PbSe nanocrystals obtained via hot injection process at 160 °C, the quality of the interface between the isolated and purified nanocrystals, after removal of capping ligands, strongly depends on the structure and stoichiometry of the surface. High resolution transmission electron microscopy has been used to investigate the interface between size selected PbSe nanocrystals. Oriented attachment of nanocrystals is achieved by protonation of oleate capping ligands using wet methanol and subsequent solvent evaporation. The removal of oleate from the surface is monitored using infrared spectroscopy. The attachment between preformed PbSe NCs has taken place exclusively through the {100} facets of the rock-salt lattice. In contrast to other related observations, it is found that nanocrystals can also attach through adjacent {100} facets. We have also studied the shape evolution of cubic PbSe NCs during microwave synthesis using transmission electron microscopy. The formation of PbSe nanocubes is attributed to the higher growth rate in the {111} direction compared to that in the {100} direction.

Selective photo-deposition of Cu onto the surface of monodisperse oleic acid capped TiO2 nanorods probed by FT-IR CO-adsorption studies

A novel, non-aqueous, organometallic route to nanocomposite Cu@TiO2 materials is presented. TiO2 nanorods stabilized with oleic acid (OLA) were used as support for the photo-assisted deposition of Cu using the organometallic Cu(II) precursor [Cu(OCH(CH3)CH2N(CH3)2)2] (1). The copper precursor penetrates through the shell of OLA and is photo reduced to deposit Cu0 directly at the surface of the TiO2 rods. The obtained Cu decorated nanorods were still soluble in nonpolar organic solvents without change of the morphology of nanorods. The Cu@TiO2 colloid was characterized by means of UV-VIS, XRD, AAS, and HRTEM. FTIR CO adsorption studies provide evidence for Cu0 anchored at the titania surface by a characteristic absorption at 2084 cm-1. Comparative studies of Cu-deposition were performed using CuCl2 as simple Cu source which proved that the concept of organometallic disguise of the metal centre results in a higher reaction rate and the circumvention of non-selective reduction, parasitic side reactions and undesired agglomeration of the OLA stabilized titania nanorods.

Solution-processed multilayered BiVO4 photoanodes: influence of intermediate heat treatments on the photoactivity

Spin coating of successive layers is a convenient method for fabricating metal oxide photoelectrodes with tunable thickness from a precursor solution. In this study, the crystallization behavior and photoelectrochemical properties of spin-coated BiVO4 films are investigated as a function of the brief heat treatments applied after depositing each individual layer. We find that full crystallization of the final films can only be obtained when the films are subjected to 10 min intermediate heat treatments to at least 350 °C. Heat treatments at lower temperatures result in films which remain partially amorphous, even after final treatment at 460 °C. For intermediate heat treatments above 350 °C the layers fully crystallize to the desired monoclinic scheelite phase, showing improved carrier separation efficiencies but lower efficiencies for charge injection into the electrolyte. These findings suggest that choosing the right processing conditions is an essential first step towards improving the performance of solution-processed BiVO4 photoelectrodes.

Spray pyrolysis of CuBi2O4 photocathodes: improved solution chemistry for highly homogeneous thin films

Dense, homogeneous CuBi2O4 thin films are prepared, for the first time, by spray pyrolysis. Major challenges related to the chemical stability of the precursor solution and spreading behavior of the sprayed droplets are revealed and addressed. Triethyl orthoformate (TEOF) is added as a water scavenger to avoid fast hydrolysis and polycondensation of bismuth ions in the precursor solution, thereby reducing powder formation during the spray deposition process. Polyethylene glycol (PEG) is used to improve the spreading behavior of sprayed droplets over the entire CuBi2O4 film surface, which prevents powder formation completely and allows for the deposition of dense, homogeneous films with thicknesses over 420 nm. These highly uniform CuBi2O4 thin films are well-suited for fundamental studies on the optical and photoelectrochemical properties. Additionally, they produce record photocurrent densities for CuBi2O4 up to 2.0 mA cm−2 under AM1.5 simulated sunlight along with incident photon-to-current efficiency (IPCE) and absorbed photon-to-current efficiency (APCE) values up to 14% and 23%, respectively (for 550 nm light at 0.6 VRHE with H2O2 as an electron scavenger).

Gradient Self-Doped CuBi2O4 with Highly Improved Charge Separation Efficiency

A new strategy of using forward gradient self-doping to improve the charge separation efficiency in metal oxide photoelectrodes is proposed. Gradient self-doped CuBi2O4 photocathodes are prepared with forward and reverse gradients in copper vacancies using a two-step, diffusion-assisted spray pyrolysis process. Decreasing the Cu/Bi ratio of the CuBi2O4 photocathodes introduces Cu vacancies that increase the carrier (hole) concentration and lowers the Fermi level, as evidenced by a shift in the flat band toward more positive potentials. Thus, a gradient in Cu vacancies leads to an internal electric field within CuBi2O4, which can facilitate charge separation. Compared to homogeneous CuBi2O4 photocathodes, CuBi2O4 photocathodes with a forward gradient show highly improved charge separation efficiency and enhanced photoelectrochemical performance for reduction reactions, while CuBi2O4 photocathodes with a reverse gradient show significantly reduced charge separation efficiency and photoelectrochemical performance. The CuBi2O4 photocathodes with a forward gradient produce record AM 1.5 photocurrent densities for CuBi2O4 up to -2.5 mA/cm2 at 0.6 V vs RHE with H2O2 as an electron scavenger, and they show a charge separation efficiency of 34% for 550 nm light. The gradient self-doping accomplishes this without the introduction of external dopants, and therefore the tetragonal crystal structure and carrier mobility of CuBi2O4 are maintained. Lastly, forward gradient self-doped CuBi2O4 photocathodes are protected with a CdS/TiO2 heterojunction and coated with Pt as an electrocatalyst. These photocathodes demonstrate photocurrent densities on the order of -1.0 mA/cm2 at 0.0 V vs RHE and evolve hydrogen with a faradaic efficiency of ∼91%.

Interlayer tuning of electronic and magnetic properties in honeycomb ordered Ag3LiRu2O6

We report a switching of electronic, magnetic and lattice properties in honeycomb ruthenates by interlayer cation exchange. The new material Ag3LiRu2O6 was made by ion-exchange of the ordered Li/Ru honeycomb material Li2RuO3 in an AgNO3 melt at 200 °C. Neutron powder diffraction and electron microscopy show that the Li/Ru order is preserved in the honeycomb layers, however, significant stacking disorder is found between layers. In contrast to Li2RuO3, which is insulating, dimerised and diamagnetic, Ag3LiRu2O6 has low electrical resistivity (0.01 ohm cm−1) and a large magnetic susceptibility at room temperature. This is attributed to the electronic influence of the highly polarisable interlayer Ag+ cation. The combination of two dimensionality, good conductivity and stacking disorder means this family of materials have potential for thermoelectric applications.

Enhanced chemical reactivity of the tris-(2,2′-bipyridyl)ruthenium(II) complex due to electrostatic binding to colloidal particles as evidenced on the basis of the reaction with azidyl radicals

Ion binding of Ru(bpy)32+ ions to negatively charged colloidal particles (silica or titania) or poly(vinyl sulfate) ions causes a pronounced reactivity increase. Respective evidence comes from the fact that the otherwise unreactive Ru(II) ions react with azidyl radicals (N3•) provided they are firmly attached to the negatively charged particles. The increase in reactivity is explained in terms of strong electronic coupling between the Ru(II) complex and the host particle. As a consequence the oxidation potential of the system Ru(bpy)33+/Ru(bpy)32+ is significantly reduced.