Céline Olivier

Nanostructured SnO2–ZnO Heterojunction Photocatalysts Showing Enhanced Photocatalytic Activity for the Degradation of Organic Dyes

Nanoporous SnO(2)-ZnO heterojunction nanocatalyst was prepared by a straightforward two-step procedure involving, first, the synthesis of nanosized SnO(2) particles by homogeneous precipitation combined with a hydrothermal treatment and, second, the reaction of the as-prepared SnO(2) particles with zinc acetate followed by calcination at 500 °C. The resulting nanocatalysts were characterized by X-ray diffraction (XRD), FTIR, Raman, X-ray photoelectron spectroscopy (XPS), nitrogen adsorption-desorption analyses, transmission electron microscopy (TEM), and UV-vis diffuse reflectance spectroscopy. The SnO(2)-ZnO photocatalyst was made of a mesoporous network of aggregated wurtzite ZnO and cassiterite SnO(2) nanocrystallites, the size of which was estimated to be 27 and 4.5 nm, respectively, after calcination. According to UV-visible diffuse reflectance spectroscopy, the evident energy band gap value of the SnO(2)-ZnO photocatalyst was estimated to be 3.23 eV to be compared with those of pure SnO(2), that is, 3.7 eV, and ZnO, that is, 3.2 eV, analogues. The energy band diagram of the SnO(2)-ZnO heterostructure was directly determined by combining XPS and the energy band gap values. The valence band and conduction band offsets were calculated to be 0.70 ± 0.05 eV and 0.20 ± 0.05 eV, respectively, which revealed a type-II band alignment. Moreover, the heterostructure SnO(2)-ZnO photocatalyst showed much higher photocatalytic activities for the degradation of methylene blue than those of individual SnO(2) and ZnO nanomaterials. This behavior was rationalized in terms of better charge separation and the suppression of charge recombination in the SnO(2)-ZnO photocatalyst because of the energy difference between the conduction band edges of SnO(2) and ZnO as evidenced by the band alignment determination. Finally, this mesoporous SnO(2)-ZnO heterojunction nanocatalyst was stable and could be easily recycled several times opening new avenues for potential industrial applications.

Low-Temperature UV Processing of Nanoporous SnO2 Layers for Dye-Sensitized Solar Cells

Connection of SnO₂ particles by simple UV irradiation in air yielded cassiterite SnO₂ porous films at low temperature. XPS, FTIR, and TGA-MS data revealed that the UV treatment has actually removed most of the organics present in the precursor SnO₂ colloid and gave more hydroxylated materials than calcination at high temperature. As electrodes for dye-sensitized solar cells (DSCs), the N3-modified 1-5 μm thick SnO₂ films showed excellent photovoltaic responses with overall power conversion efficiency reaching 2.27% under AM1.5G illumination (100 mW cm⁻²). These performances outperformed those of similar layers calcined at 450 °C mostly due to higher V(oc) and FF. These findings were rationalized in terms of slower recombination rates for the UV-processed films on the basis of dark current analysis, photovoltage decay, and electrical impedance spectroscopy studies.

Size and shape fine-tuning of SnO2 nanoparticles for highly efficient and stable dye-sensitized solar cells

Innovative solution routes led to two types of tin dioxide nanocrystals, i.e. 10–15 nm spheroid cassiterite nanoparticles and 50–150 nm anisotropic cassiterite particles showing octahedral facets. Nanoporous SnO2 electrodes of various architectures (mono- or bilayered) were then processed by the screen-printing method using suitable combinations of these SnO2 particles; the final texture, composition and morphology of the photoanodes obtained depending upon the nature of the post-treatment (with or without TiCl4). After sensitization by the ruthenium dye N719, ATR-FTIR studies revealed that chemisorption of the dye onto porous cassiterite SnO2 layers took place through a bridging coordination mode. As-prepared dye-sensitized photoanodes, when embedded in DSC devices containing a liquid electrolyte, led to a record overall power conversion efficiency (PCE) of 3.2% for pure SnO2 composed of both kinds of particles and to very promising PCE above 4% for photoanodes post-treated with TiCl4. The remarkable photovoltaic performances of the photoanodes including both kinds of particles, associated or not with a TiCl4 post-treatment, were due to improved Voc and FF, and were related to: (i) lower charge transfer resistance at the SnO2–N719–electrolyte interface; (ii) onset of dark current occurring at higher potential; (iii) enhanced electron lifetimes as determined by transient Voc decay measurements. Finally, the most striking feature of this study concerns the improvement of the power conversion efficiency upon aging under ambient conditions and the amazing long-term stability of DSCs fabricated from different SnO2-based photoanodes since standard devices built from N719 dye and I3−/I− electrolytes usually show fast decrease of efficiency.

Functionalization of a ruthenium-diacetylide organometallic complex as a next-generation push-pull chromophore.

The design and preparation of an asymmetric ruthenium-diacetylide organometallic complex was successfully achieved to provide an original donor-π-[M]-π-acceptor architecture, in which [M] corresponds to the [Ru(dppe)2] (dppe: bisdiphenylphosphinoethane) metal fragment. The charge-transfer processes occurring upon photoexcitation of the push-pull metal-dialkynyl σ complex were investigated by combining experimental and theoretical data. The novel push-pull complex, appropriately end capped with an anchoring carboxylic acid function, was further adsorbed onto a semiconducting metal oxide porous thin film to serve as a photosensitizer in hybrid solar cells. The resulting photoactive material, when embedded in dye-sensitized solar cell devices, showed a good spectral response with a broad incident photon-to-current conversion efficiency profile and a power conversion efficiency that reached 7.3 %. Thus, this material paves the way to a new generation of organometallic chromophores for photovoltaic applications.

Push–pull ruthenium diacetylide complexes: new dyes for p-type dye-sensitized solar cells

A couple of novel donor–π–acceptor dyes based on organometallic ruthenium diacetylide complexes (SL1 and SL2) have been designed and synthesized for use in NiO-based p-type dye-sensitized solar cells (p-DSCs). The optical and electrochemical properties of the dyes were assessed and theoretical calculations were performed to rationalize the experimental data. The best performing dye in NiO-based p-DSC devices is the red dye SL1, which gives a photocurrent density of 2.25 mA cm−2 and maximum IPCE of 18%. This represents a promising result, paving the way for future finely tuned organometallic efficient dyes for such application.

Breaking the symmetry: assembly of cylindrical nanostructures with a C3-symmetrical ligand.

The reaction of a tripodal ligand containing terminal 2,3-dihydroxypyridine groups with (arene)ruthenium(II) complexes resulted in the formation of cylindrical nanostructures.

Tuning visible-light absorption properties of Ru–diacetylide complexes: simple access to colorful efficient dyes for DSSCs

A series of σ-dialkynyl ruthenium complexes showing a D–π–[M]–π–A structure (where [M] = [Ru(dppe)2], dppe = bisdiphenylphosphinoethane) were designed and synthesized for dye-sensitized solar cell (DSSC) applications. The molecular structure of these highly modular organometallic complexes was fine-tuned through the introduction of a bithiophene, rhodanine or benzothiadiazole unit. This original molecular engineering approach combined with convergent synthetic pathways thus afforded efficient photosensitizers with tunable colors across the visible spectrum, ranging from red to purple, blue and blue-green dyes. The optoelectronic properties of the new complexes were fully assessed and the dyes were tested in standard single-dye devices as well as in co-sensitized DSSCs, yielding 7.5% power conversion efficiency in the presence of an iodine-based liquid electrolyte.

Oligocarbazole‐Based Chromophores for Efficient Thin‐Film Dye‐Sensitized Solar Cells

Carb your enthusiasm: Carbazole-based sensitizers with high extinction coefficients are synthesized for application in dye-sensitized solar cells (DSCs). The dyes perform efficiently with both iodine and cobalt electrolytes, showing power conversion efficiencies of up to 5.8% on TiO₂ films of 15 μm thickness, and retaining 90% of their efficiency in devices with thinner films.

Supercritical CO2-assisted deposition of NiO on (101)-anatase-TiO2 for efficient facet engineered photocatalysts

NiO/(101)-anatase-TiO2 heterostructure nanoparticles were synthesized by depositing NiO onto the (101) facet of anatase crystals via the supercritical fluid chemical deposition (SFCD) route. Thorough characterization experiments performed by various techniques (XRD, UV-Vis DRS, N2 sorption, HR-TEM, EDX, and XPS) indicate that the SFCD method allowed a good dispersion of NiO onto the TiO2 nanoparticles for NiO amounts below 2 wt%. Compared to pure TiO2, the 0.1–1 wt% NiO–TiO2 nanocomposites showed enhanced photocatalytic properties for methylene blue (MB) and methyl orange (MO) decomposition under UV light irradiation, the 0.25 wt% NiO–TiO2 system leading to the highest efficiencies. The photocatalytic properties were then rationalized in terms of the acidic properties and electronic structures of the NiO–TiO2 nanocomposites. This higher photocatalytic activity was mainly related to the heterocontact at the interface of the NiO–TiO2 crystallites and to the enhanced reaction rates at the NiOx surface.