Thierry Toupance

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.

Near- and Supercritical Alcohols as Solvents and Surface Modifiers for the Continuous Synthesis of Cerium Oxide Nanoparticles

Supercritical fluids offer fast and facile routes toward well-crystallized tailor-made cerium oxide nanoparticles. However, the use of surfactants to control morphology and surface properties remains essential. Therefore, although water, near-critical (nc) or supercritical (sc), is a solvent of choice, the poor water solubility of some surfactants could require other solvent systems such as alcohols, which could themselves behave as surface modifiers. In here, the influence of seven different alcohols, MeOH, EtOH, PrOH, iPrOH, ButOH, PentOH, and HexOH, in alcothermal conditions (300 °C, 24.5 MPa) over CeO(2) nanocrystals (NCs) size, morphology, and surface properties was investigated. The crystallite size of the CeO(2) nanocrystals can be tuned in the range 3-7 nm depending on the considered alcohol, and their surface has been modified by these solvents without the use of surfactants. Mechanisms are proposed for the interaction of primary and secondary alcohols with CeO(2) surface and its functionalization during the synthesis based on FTIR and TGA-MS studies. This study allows apprehending the role of alcohols during the synthesis and may lead to an informed choice of solvent as a function of the required size and surface properties of CeO(2) NCs. It also opens new route to CeO(2) functionalization using supercritical alcohol derivatives.

Tin dioxide thin films prepared from a new alkoxyfluorotin complex including a covalent SnF bond

Abstract Fluorine-doped tin dioxide (FTO) films were elaborated from a new Sn(IV) molecular precursor, the tert amyloxyfluorodipentan-2,4-dionatotin(IV) complex, to be used via the sol–gel route. Upon controlled hydrolysis in acetonitrile, a fluorinated polystannoxane was formed as a stable xerosol, which was characterized by multinuclear magnetic resonance and Mossbauer spectroscopies, elemental analysis, and thermogravimetry coupled to mass spectrometry. This xerosol was further dissolved in acetonitrile for depositing highly transparent films by the spin-coating and spray pyrolysis techniques. Film composition was determined by X-ray photoelectron spectroscopy, electron probe microanalysis and energy dispersive X-ray analysis. The F/Sn atomic ratio was 3% for spin-coated films and 10% for the sprayed films. A noticeable amount of carbon (C/Sn #10 at.%) was detected in the films whatever the deposition technique. In every case, the film resistivity was found to be larger than the one obtained from separate tin and fluorine precursors. This result was attributed to the excess of carbon and fluorine content in the oxide layers relieved from the xerosol.

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.

CeO2 Nanocrystals from Supercritical Alcohols: New Opportunities for Versatile Functionalizations?

The fast and controlled synthesis of surface-modified cerium oxide nanoparticles was carried out in supercritical {ethanol + alcohol derivative} mixtures. The newly found ability of supercritical alcohols to graft onto cerium oxide nanocrystals (CeO2 NCs) during their synthesis was exploited to control their surface chemistry via the addition of three aminoalcohols: ethanolamine, 3-amino-1-propanol and 6-amino-1-hexanol. Although the ethanol to aminoalcohol ratio was consistent (285:1), the successful grafting of these alcohol derivatives onto CeO2 NCs was identified based on Fourier transform infrared (FTIR) and thermogravimetric analysis-mass spectrometry (TGA-MS) measurements. Smaller crystallite size of CeO2 NCs synthesized in the presence of aminoalcohols, compared to those synthesized in supercritical ethanol alone, were also noticed and attributed to a possible intervention of amine groups helping the grafting of the alcohols, allowing one to stop the growth of the CeO2 NCs faster. The use of supercritical alcohol mixture-ethanol with hexanol, dodecanol, or octadecanol, with a 285:1 ratio-was also investigated. Such mixtures allow accessing a finer control in CeO2 NCs crystallite size compared to pure alcohols, according to calculation made from X-ray diffraction measurements. Finally, fluorescent molecules (fluorescein isothiocyanate) were grafted onto amine-modified CeO2 NCs. The powders displayed a fluorescent behavior under UV light, confirming the suitability and interest of CeO2 NCs surface modification by such technique.

A new single molecular precursor route to fluorine-doped nanocrystalline tin oxide anodes for lithium batteries

Abstract A fluorine-doped nanocrystalline tin oxide has been prepared by thermolysis of xerosol derived from a single molecular precursor. A fluorine to tin atomic ratio, as high as 0.14, was estimated from the results of elemental analysis. The oxidation state of Sn and conducting properties were determined by Mossbauer spectroscopy and resistivity measurements. According to the electrochemical experiments, a reversible capacity up to ∼800 mAh/g has been achieved.

Semiconductivity and gas-sensing properties of crown-ether-substituted lutetium bisphthalocyanines

Abstract The conductivity of thin films of crown ether (15-crown-5, 18-crown-6) substituted lutetium bisphthalocyanines has been studied in the presence of various gases. Under vacuum the films show intrinsic semiconducting properties (σ≈10 −6 Ω −1 cm − ). When exposed to an oxidant (O 2 , I 2 , Br 2 ), the molecular semiconductors are progressively transformed into doped insulators (σ≈10 −9 Ω − cm −1 ). Exposure to wet nitrogen yields fully reversible conductivity changes: a two orders of magnitude difference in conductivity is measured on oxidized films when the relative humidity varies from 10% to 90%.

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.

A TIPS-TPDO-tetraCN-Based n-Type Organic Field-Effect Transistor with a Cross-linked PMMA Polymer Gate Dielectric

Recent improvement in the performance of the n-type organic semiconductors as well as thin gate dielectrics based on cross-linked polymers offers new opportunities to develop high-performance low-voltage n-type OFETs suitable for organic complementary circuits. Using TIPS-tetracyanotriphenodioxazine (TIPS-TPDO-tetraCN) and cross-linked poly(methyl methacrylate) (c-PMMA), respectively as n-type organic semiconductor and gate dielectric, linear regime field-effect mobility (1.8 ± 0.2) × 10(-2) cm(2) V(-1)s(-1), small spatial standard deviation of threshold voltage (∼0.1 V), and operating voltage less than 3 V are attainable with the same device structure and contact materials used commonly for p-type OFETs. Through comparative static and dynamic characterizations of c-PMMA and PMMA gate dielectrics, it is shown that both smaller thickness and larger relative permittivity of c-PMMA contributes to reduced operating voltage. Furthermore, negligible hysteresis brings evidence to small trap states in the semiconductor near gate dielectric of the n-type OFETs with c-PMMA. The use of TIPS-TPDO-tetraCN and c-PMMA is fully compatible with polyethylene terephthalate substrate, giving promise to various flexible applications.