Revisiting the Potential of Group VI Inorganic Precatalysts for the Ethenolysis of Fatty Acids through a Mechanochemical Approach.

The utilization of biobased feedstocks to prepare useful compounds is a pivotal trend in current chemical research. Among a varied portfolio of naturally available starting materials, fatty acids are abundant, versatile substrates with multiple applications. In this context, the ethenolysis of unsaturated fatty acid esters such as methyl oleate is an atom-economical way to prepare functional C10 olefins with a biobased footprint. Despite the existence of a variety of metathesis catalysts for the latter process, there is a lack of readily available, efficient, and inexpensive catalytic systems based on earth-abundant metals (Mo, W) whose preparation does not require sophisticated syntheses and manipulations. Here, a systematic exploration of homogeneous and heterogeneous inorganic Mo, W (oxy)halides shows that MoOCl4, while inactive as a homogeneous species, forms active and selective silica-supported ethenolysis precatalysts able to reach equilibrium conversion of methyl oleate within a few minutes upon activation with SnMe4. Such heterogeneous MoOCl4-based precatalysts were easily accessed through mechanochemical solvent-free procedures and found to contain, upon characterization by elemental analysis and Raman spectroscopy, isolated (≡SiO)Mo(=O)Cl3 units or polymeric silica-supported [-O(≡SiO)nMoCl4-nO-]m (n = 1, 2) complexes depending on the molybdenum loading. The former isolated species exhibited a higher catalytic performance. The developed heterogeneous precatalysts could be applied to the ethenolysis of various substrates, including polyunsaturated fatty acid esters and industrial fatty acid methyl ester (FAME) mixtures from palm oil transesterification.

Catalysts Prepared from Atomically Dispersed Ce(III) on MgO Rival Bulk Ceria for CO Oxidation.

Atomically dispersed cerium catalysts on an inert, crystalline MgO powder support were prepared by using both Ce(III) and Ce(IV) precursors. The materials were used as catalysts for CO oxidation in a once-through flow reactor and characterized by atomic-resolution scanning transmission electron microscopy, X-ray absorption near-edge structure spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction, among other techniques, before and after catalysis. The most active catalysts, formed from the precursor incorporating Ce(III), displayed performance similar to that reported for bulk ceria under comparable conditions. The catalyst provided stable time-on-stream performance for as long as it was kept on-stream, 2 days, increasing slightly in activity as the atomically dispersed cerium ions were transformed into ceria nanodomains represented as CeOx and having increased reducibility on the MgO support. The results suggest how highly dispersed supported ceria catalysts with low cerium loadings can be prepared and may pave the way for improved efficiencies of cerium utilization in oxidation catalysis.

Simple Halogen-Free, Biobased Organic Salts Convert Glycidol to Glycerol Carbonate under Atmospheric CO2 Pressure.

Glycerol carbonate (GC) has emerged as an attractive synthetic target due to various promising technological applications. Among several viable strategies to produce GC from CO2 and glycerol and its derivatives, the cycloaddition of CO2 to glycidol represents an atom-economic an efficient strategy that can proceed via a halide-free manifold through a proton-shuttling mechanism. Here, it was shown that the synthesis of GC can be promoted by bio-based and readily available organic salts leading to quantitative GC formation under atmospheric CO2 pressure and moderate temperatures. Comparative and mechanistic experiments using sodium citrate as the most efficient catalyst highlighted the role of both hydrogen bond donor and weakly basic sites in the organic salt towards GC formation. The citrate salt was also used as a catalyst for the conversion of other epoxy alcohols. Importantly, the discovery that homogeneous organic salts catalyze the target reaction inspired us to use metal alginates as heterogeneous and recoverable bio-based catalysts for the same process.

Microwave-Assisted Non-aqueous and Low-Temperature Synthesis of Titania and Niobium-Doped Titania Nanocrystals and Their Application in Halide Perovskite Solar Cells as Electron Transport Layers.

Undoped and Nb-doped TiO2 nanocrystals are prepared by a microwave-assisted non-aqueous sol-gel method based on a slow alkyl chloride elimination reaction between metal chlorides and benzyl alcohol. Sub-4 nm nanoparticles are grown under microwave irradiation at 80 °C in only 3 h with precise control of growth parameters and yield. The obtained nanocrystals could be conveniently used to cast compact TiO2 or Nb-doped TiO2 electron transport layers for application in formamidinium lead iodide-based photovoltaic devices. Niobium doping is found to improve the cell performance by increasing the conductivity and mobility of the electron transport layer. At the same time, a measurable decrease in parasitic light absorption in the low wavelength portion of the spectrum was observed.

Physico-chemical investigation of ZnS thin-film deposited from ligand-free nanocrystals synthesized by non-hydrolytic thio-sol-gel.

Ultra-small and monodispersed zinc sulfide nanocrystals (NCs) (d ≤ 3 nm) have been prepared without the use of any surfactants by a synthetic route using benzyl mercaptan as a source of sulfur. The prepared NCs are dispersible in highly polar solvents and display the capability to closely pack-up in a bulky film. The NCs were characterized by TEM, XRD and UV-vis optical absorption as well as by steady-state and time-resolved photoluminescence (PL) spectroscopies. Uniform films of ZnS were spin-coated on glass and ITO-glass substrates using a NCs dispersion in N,N-dimethylformamide. The NCs and the resulting films were characterized by morphological and optoelectronic probing techniques such as AFM, SEM, diffuse reflectance, PL and photoelectron spectroscopy in air. These physical investigations confirmed that the chalcogenide NCs grown by this method have the potential to be utilized directly as photocatalysts and are potentially useful building-blocks/starting materials for the fabrication of semiconductor thin films for optoelectronic applications such as LED, luminescent screens, field effect transistor and solar cells. Insights on the chemistry involved in the NCs growth have been provided revealing that their formation proceeds through a mechanism involving a thioether elimination reaction.

Amorphous Tin Oxide as a Low-Temperature-Processed Electron-Transport Layer for Organic and Hybrid Perovskite Solar Cells.

Chemical bath deposition (CBD) of tin oxide (SnO2) thin films as an electron-transport layer (ETL) in a planar-heterojunction n-i-p organohalide lead perovskite and organic bulk-heterojunction (BHJ) solar cells is reported. The amorphous SnO2 (a-SnO2) films are grown from a nontoxic aqueous bath of tin chloride at a very low temperature (55 °C) and do not require postannealing treatment to work very effectively as an ETL in a planar-heterojunction n-i-p organohalide lead perovskite or organic BHJ solar cells, in lieu of the commonly used ETL materials titanium oxide (TiO2) and zinc oxide (ZnO), respectively. Ultraviolet photoelectron spectroscopy measurements on the glass/indium-tin oxide (ITO)/SnO2/methylammonium lead iodide (MAPbI3)/2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene device stack indicate that extraction of photogenerated electrons is facilitated by a perfect alignment of the conduction bands at the SnO2/MAPbI3 interface, while the deep valence band of SnO2 ensures strong hole-blocking properties. Despite exhibiting very low electron mobility, the excellent interfacial energetics combined with high transparency (Egap,optical > 4 eV) and uniform substrate coverage make the a-SnO2 ETL prepared by CBD an excellent candidate for the potentially low-cost and large-scale fabrication of organohalide lead perovskite and organic photovoltaics.

The Silicon:Colloidal Quantum Dot Heterojunction.

A heterojunction between crystalline silicon and colloidal quantum dots (CQDs) is realized. A special interface modification is developed to overcome an inherent energetic band mismatch between the two semiconductors, and realize the efficient collection of infrared photocarriers generated in the CQD film. This junction is used to produce a sensitive near infrared photodetector.

Ag29(BDT)12(TPP)4: A Tetravalent Nanocluster.

The bottom-up assembly of nanoparticles into diverse ordered solids is a challenge because it requires nanoparticles, which are often quasi-spherical, to have interaction anisotropy akin to atoms and molecules. Typically, anisotropy has been introduced by changing the shape of the inorganic nanoparticle core. Here, we present the design, self-assembly, optical properties, and total structural determination of Ag29(BDT)12(TPP)4, an atomically precise tetravalent nanocluster (NC) (BDT, 1,3-benzenedithiol; TPP, triphenylphosphine). It features four unique tetrahedrally symmetrical binding surface sites facilitated by the supramolecular assembly of 12 BDT (wide footprint bidentate thiols) in the ligand shell. When each of these sites was selectively functionalized by a single phosphine ligand, particle stability, synthetic yield, and the propensity to self-assemble into macroscopic crystals increased. The solid crystallized NCs have a substantially narrowed optical band gap compared to that of the solution state, suggesting strong interparticle electronic coupling occurs in the solid state.

Nano-design of quantum dot-based photocatalysts for hydrogen generation using advanced surface molecular chemistry.

Efficient photocatalytic hydrogen generation in a suspension system requires a sophisticated nano-device that combines a photon absorber with effective redox catalysts. This study demonstrates an innovative molecular linking strategy for fabricating photocatalytic materials that allow effective charge separation of excited carriers, followed by efficient hydrogen evolution. The method for the sequential replacement of ligands with appropriate molecules developed in this study tethers both quantum dots (QDs), as photosensitizers, and metal nanoparticles, as hydrogen evolution catalysts, to TiO2 surfaces in a controlled manner at the nano-level. Combining hydrophobic and hydrophilic interactions on the surface, CdSe-ZnS core-shell QDs and an Au-Pt alloy were attached to TiO2 without overlapping during the synthesis. The resultant nano-photocatalysts achieved substantially high-performance visible-light-driven photocatalysis for hydrogen evolution. All syntheses were conducted at room temperature and in ambient air, providing a promising route for fabricating visible-light-responsive photocatalysts.

Tethering metal ions to photocatalyst particulate surfaces by bifunctional molecular linkers for efficient hydrogen evolution.

A simple and versatile method for the preparation of photocatalyst particulates modified with effective cocatalysts is presented; the method involves the sequential soaking of photocatalyst particulates in solutions containing bifunctional organic linkers and metal ions. The modification of the particulate surfaces is a universal and reproducible method because the molecular linkers utilize strong covalent bonds, which in turn result in modified monolayer with a small but controlled quantity of metals. The photocatalysis results indicated that the CdS with likely photochemically reduced Pd and Ni, which were initially immobilized via ethanedithiol (EDT) as a linker, were highly efficient for photocatalytic hydrogen evolution from Na2S-Na2SO3-containing aqueous solutions. The method developed in this study opens a new synthesis route for the preparation of effective photocatalysts with various combinations of bifunctional linkers, metals, and photocatalyst particulate materials.

Light harvesting with multiwall carbon nanotube/silicon heterojunctions.

We report on a significant photocurrent generation from a planar device obtained by coating a bare n doped silicon substrate with a random network of multiwall carbon nanotubes (MWCNTs). This MWCNT/n-Si hybrid device exhibits an incident photon to current efficiency reaching up to 34% at 670 nm. We also show that MWCNTs covering a quartz substrate still exhibit photocurrent, though well below than that of the MWCNTs coating the silicon substrate. These results suggest that MWCNTs are able to generate photocurrent and that the silicon substrate plays a fundamental role in our planar device. The former effect is particularly interesting because MWCNTs are generally known to mimic the electronic properties of graphite, which does not present any photocurrent generation. On the basis of theoretical calculations revealing a weak metallic character for MWCNTs, we suggest that both metallic and semiconducting nanotubes are able to generate e-h pairs upon illumination. This can be ascribed to the presence of van Hove singularities in the density of states of each single wall carbon nanotube constituting the MWCNT and to the low density of electrons at the Fermi level. Finally, we suggest that though both MWCNTs and Si substrate are involved in the photocurrent generation process, MWCNT film mainly acts as a semitransparent electrode in our silicon-based device.

Photocurrent generation in Ge nanocrystal/Si systems.

We report on the generation of photocurrent in the visible and ultraviolet range from planar devices built from the Ge nanocrystals grown on a heavy n-doped Si(001) substrate covered with 5 nm thick thermally grown SiO2. These Ge nanostructures/SiO2/n(+)-Si devices are shown to generate photocurrent with an Incident-Photon-to-electron Conversion Efficiency (IPCE) spectral range depending on the Ge nanocrystals size. The increase of the IPCE value of our devices in the 350-600 nm range correlates well with the absorbance of Ge.