Molecular Precursors Research Articles

Halide-free synthesis of metastable graphitic BC3.

Layered BC3, a metastable phase within the binary boron-carbon system that is composed of graphite-like sheets with hexagonally symmetric C6B6 units, has never been successfully crystallized. Instead, poorly-crystalline BC3-like materials with significant stacking disorder have been isolated, based on the co-pyrolysis of a boron trihalide precursor with benzene at around 800 °C. The halide leaving group (-X) is a significant driving force of these reactions, but the subsequent evolution of gaseous HX species at such high temperatures hampers their scaling up and also prohibits their further use in the presence of hard-casting templates such as ordered silicates. Herein, we report a novel halide-free synthesis route to turbostratic BC3 with long-range in-plane ordering, as evidenced by multi-wavelength Raman spectroscopy. Judicious pairing of the two molecular precursors is crucial to achieving B-C bond formation and preventing phase-segregation into the thermodynamically favored products. A simple computational method used herein to evaluate the compatibility of bottom-up molecular precursors can be generalized to guide the future synthesis of other metastable materials beyond the boron-carbon system.

Controllable and facile one-pot synthesis of high surface area amorphous, crystalline, and triphasic TiO2: catalytic and photocatalytic applications

A simple and controllable synthesis led to both amorphous and crystalline porous TiO2 with remarkably high surface area and pore volume. Their distinctive characteristics influenced their effectiveness in photocatalytic and catalytic processes.

The dual nature of biomimetic melanin.

Melanin-inspired nanomaterials offer unique photophysical, electronic and radical scavenging properties that are widely explored for health and environmental preservation, or energy conversion and storage. The incorporation of functional melanin building blocks in more complex nanostructures or surfaces is typically achieved via a bottom-up approach starting from a molecular precursor, in most cases dopamine. Here we demonstrate that indeed, the oxidative polymerization of dopamine, for the synthesis of melanin-like polydopamine (PDA), leads to the simultaneous formation of more than one nanosized species with different compositions, morphologies and properties. In particular, a low-density polymeric structure and dense nanoparticles (NP) are simultaneously formed. The two populations could be separated and analyzed in real time using a chromatographic technique free of any stationary phase (flow field fractionation, FFF). The results following the synthesis of melanin-like PDA showed that the NP are formed only during the first 6 hours as a result of a supramolecular self-assembly-driven polymerization, while the formation of the polymer continues for about 36 hours. The two populations were also separated and characterized using TEM, UV-vis absorption spectroscopy, fluorescence and light scattering spectroscopy, DLS, FTIR, ζ-potential measurements, gel electrophoresis and pH titrations. Interestingly, very different properties between the two populations were observed: in particular the polymer contains a higher number of catechol units (8 mmol g-1 -OH) with respect to the NP (1 mmol g-1 -OH) and presents a much higher antioxidant activity. The attenuation of light by NP is more efficient than that by the polymer especially in the Vis-NIR region. Moreover, while the NP scatter light with an efficiency up to 27% they are not fluorescent, and the polymer does not scatter light but shows an excitation wavelength-dependent fluorescence typical of multi-fluorophoric uncoupled systems.

A soft molecular single-source precursor approach to synthesize a nanostructured Co9S8 (pre)catalyst for efficient water oxidation and biomass valorization

A nanocrystalline Co9S8 (pre)catalyst derived from a novel {CoIIS4} complex is reported for the oxygen evolution reaction (OER) and selective biomass valorization. During OER, Co9S8 reconstructs completely into a CoOOH active phase via S-leaching.

Synthesis and structures of cobalt-expanded zirconium- and cerium-oxo clusters as precursors for mixed-metal oxide thin films.

Transforming current complementary metal-oxide-semiconductor (CMOS) technology to fabricate memory chips and microprocessors into environmentally friendlier electronics requires the development of new approaches to resource- and energy-efficient electron transport and switching materials. Metal and multi-metal oxide layers play a key role in high-end technical applications. However, these layers are commonly produced through high-energy and high-temperature procedures. Herein, we demonstrate our first attempts to obtain stimuli-responsive mixed-metal oxide thin films from solution-processed molecular precursors under milder conditions. The molecular compounds of interest were prepared by one-pot reactions of a CoII carboxylate complex, triethylamine (Et3N), N-butyldiethanolamine (H2bda), and a hexanuclear complex [Ce6O4(OH)4(piv)12] (Hpiv = pivalic acid) or [Zr6O4(OH)4(ib)12(H2O)]·3Hib (Hib = isobutyric acid) in acetonitrile solution. The resulting charge-neutral, heterometallic coordination compounds display a ligand-supported pentanuclear {CeIV3CoIII2} core (in 1) and a dodecanuclear {ZrIV6CoII6} core (in 2), exhibiting thermal stability up to ca. 100 °C in air. Compound 2 was deposited and analyzed on Au(111) and SiO2/Si(100) surfaces to explore its potential as a single-molecule precursor for the preparation of atomically precise, complex mixed-metal oxide thin films. The adsorption characteristics of it demonstrate the ability to form stable agglomerates on the investigated surfaces.

Novel synthesis of semiconductor chalcohalide anti-perovskites by low-temperature molecular precursor ink deposition methodologies

Ag3SX (X = I, Br) possess a unique anti-perovskite structure (similar to perovskites but switching anions by cations and vice-versa). Here, we propose a new low-cost low-temperature synthesis methodology based on thiol-amine molecular ink deposition.

Electronic and magnetic properties of porphyrin nanoribbons with chelated metals.

Advances in surface-assisted synthesis routes now allow for precise control in the preparation and modification of low-dimensional structures. The choice of molecular precursors plays a fundamental role in these processes since the structural details and properties of the resulting nanostructures directly depend on the molecular block used. From this perspective, units based on porphyrins have proven to be promising candidates for the construction of nanosystems with nontrivial geometry. In particular, efforts have been made to synthesize different arrangements of π-conjugated porphyrins. With this motivation, we use computational simulations to investigate the electronic and magnetic properties of nanoribbons constructed from the concatenation of π-extended porphyrins hosting transition metal atoms. We show that the binding energy of these systems and the specific way the electrons populate the d-shells are strongly influenced by the type of the transition metal. Furthermore, it was observed that most systems with chelated metals (except Ni and Zn) feature magnetic properties. The systems considered in this work have analogs in finite structures recently synthesized in the laboratory so the nanomaterials proposed here have a high potential to be produced in the near future.

Transition mechanism of the coverage-dependent polymorphism of self-assembled melamine nanostructures on Au(111).

Molecular self-assembled films have recently attracted increasing attention within the field of nanotechnology as they offer a route to obtain new materials. However, careful selection of the molecular precursors and substrates, as well as exhaustive control of the system evolution is required to obtain the best possible outcome. The three-fold rotational symmetry of melamine molecules and their capability to form hydrogen bonds make them suitable candidates to synthesize this type of self-assembled network. In this work, we have studied the polymorphism of melamine nanostructures on Au(111) at room temperature. We find two coverage-dependent phases: a honeycomb structure (α-phase) for submonolayer coverage and a close-packed structure (β-phase) for full monolayer coverage. A combined scanning tunnel microscopy and density functional theory based-calculations study of the transition regime where both phases coexist allows describing the mechanism underlying this coverage driven phase transition in terms of the changes in the molecular lateral tension.

M-Ge-Si thermolytic molecular precursors and models for germanium-doped transition metal sites on silica.

The synthesis, thermolysis, and surface organometallic chemistry of thermolytic molecular precursors based on a new germanosilicate ligand platform, -OGe[OSi(OtBu)3]3, is described. Use of this ligand is demonstrated with preparation of complexes containing the first-row transition metals Cr, Mn, and Fe. The thermolysis and grafting behavior of the synthesized complexes, Fe{OGe[OSi(OtBu)3]3}2 (FeGe), Mn{OGe[OSi(OtBu)3]3}2(THF)2 (MnGe) and Cr{OGe[OSi(OtBu)3]3}2(THF)2 (CrGe), was evaluated using a combination of thermogravimetric analysis; nuclear magnetic resonance (NMR), ultraviolet-visible (UV-Vis), and electron paramagnetic resonance (EPR) spectroscopies; and single-crystal X-ray diffraction (XRD). Grafting of the precursors onto SBA-15 mesoporous silica and subsequent calcination in air led to substantial changes in transition metal coordination environments and oxidation states, the implications of which are discussed in the context of low-coordinate and low oxidation state thermolytic molecular precursors.

Theoretical prediction and experimental synthesis of a Ba0.5Pb0.5S alloy via the molecular precursor route

We synthesized Ba–Pb–S chalcogenide compounds with tunable bandgap properties using a discrete molecular precursor strategy. Ba0.5Pb0.5S has a 1.77 eV optical bandgap, p-type characteristics, and shows great potential for optoelectronic applications.

The origin and evolution of molecular precursors for quantum dot synthesis

In this review article, we reflect on the origins of some of the molecular precursor chemistry behind modern quantum dot synthesis and how it developed into the modern discipline that is routinely used world-wide.

Enhanced photoluminescence of InP@ZnS quantum dots induced by increasing the indium myristate-to-tris(trimethylsilyl)phosphine molecular precursors ratio

Enhanced photoluminescence of InP@ZnS quantum dots induced by increasing the indium myristate-to-tris(trimethylsilyl)phosphine molecular precursors ratio

Green synthesis and structural and cytotoxic properties of a new silver(I) complex: Solid-state, single-source precursor of Ag and Ag2O NPs

A new silver complex, [Ag(2-pymc)(NO3)] (Ag-1) [2-pymc = methyl-2-(pyridine-2yl-methylene) hydrazinecarboxylate], was prepared and subjected to elemental analysis including Fourier transform infrared (FT-IR) and ultraviolet–visible (UV–vis) spectroscopic methods. Single-crystal X-ray diffraction (SC-XRD) study demonstrated [Ag(2-pymc)(NO3)] monomers in the crystal structure. The silver ion is triple coordinated by an N, N-chelating 2-pymc and one oxygen atom of a monodendate nitrate anion. Interestingly, Ag-1 acted as a single-source, solid-state molecular precursor (SSMP) for synthesizing both Ag and Ag2O nanoparticles (NPs) at different reaction conditions. The crystallinity and purity of the as-synthesized Ag and Ag2O NPs were investigated by powder XRD, whereas the size and surface morphology were determined by TEM analysis. The results showed that the formed Ag NPs were well dispersed and spherically shaped with an average diameter of 30–32 nm. The Ag2O NPs were not homogeneous but showed irregular shapes with an average diameter of 40–42 nm. Ag-1 showed antibacterial activity against selected Gram-positive (Bacillus megaterium) and Gram-negative (Pseudomonas Stutzeri) bacteria. The cytotoxicity of Ag-1 against MCF-7 cancer cells was also tested.

Kinetic formation of iron oxide nanoparticles using un- and γ-irradiated singular molecular precursor of Tris(pentanedionato)iron(III) complex

Iron oxide nanoparticles were synthesized from tris(pentanedionato)iron(III) singular molecular precursor through pyrolysis in static air. FT-IR, XRD, TGA, BET and SEM/EDS techniques were used to follow the reactions and identify the products. Single crystalline iron oxide sizes were found to be 18.23 ± 0.23, 16.42 ± 0.21, and 13.964 ± 0.11 nm using Scherrer equation, and the BET surface area were measured to be 110.909, 121.352, and 153.9911 m2g−1 for the corresponding iron oxides obtained through pyrolysis of un-irradiated and γ irradiated tris(2,4 pentanedionato)iron(III) with 100 and 300 kGy, respectively. Nonisothermal kinetics formation of iron oxide nanoparticles over the thermal decomposition of un-irradiated and γ-irradiated tris(pentanedionato)iron(III) molecular precursor with 100 and 300 kGy total γ-ray doses were investigated under air flow with a heating rate of 5, 10, 15, 20, and 25 °C/min, from 25 °C to 500 °C. Kinetic parameters were attained through model-fitting and model-free approaches, and artificial isokinetic relationship (IKR) for multi-step processes. The decomposition for both un-irradiated and γ-irradiated tris(pentanedionato)iron(III) molecular precursor with 100 and 300 kGy total γ-ray doses proceed over one major step with two-dimensional diffusion (bi-dimensional particle shape) Valensi equation (D2).

Molecular Precursor Approach to Sulfur-Free CuInSe2: Replacing Thiol Coordination in Soluble Metal Complexes.

Solution-processed CuInSe2 films have generally relied on sulfide or sulfoselenide precursor films that, during the grain growth process, hamper the growth of thicker films and lead to the formation of a fine-grain layer. However, recent research has indicated that sulfur reduction in the precursor film modifies the grain growth mechanism and may enable the fabrication of thicker absorbers that are free of any fine-grain layer. In this work, we pursue direct solution deposition of sulfur-free CuInSe2 films from the molecular precursor approach. To this end, we tune the amine-thiol reactive solvent system and study the changes to the resulting soluble complexes through a combination of analytical techniques. We show that by reactively dissolving indium(III) selenide and selenium in solutions of n-butylamine and 1,2-ethanedithiol, a metal thiolate species is formed, and that this metal thiolate can be modified by isolation from the thiol-containing solvent via precipitation. As the quantity of selenium in the ink increases, the thiol content in the complex decreases, eventually producing soluble [InSex]- species. Extending this method to be used with copper selenide as a copper source, molecular precursor inks can be made for solution-processed, sulfur-free CuInSe2 films. We then show that these CuInSe2 precursor films can be fully coarsened without a fine-grain layer formation, even at the desired thicknesses of 2 μm and greater.

Copper Phosphinate Complexes as Molecular Precursors for Ethanol Dehydrogenation Catalysts.

Nowadays, the production of acetaldehyde heavily relies on the petroleum industry. Developing new catalysts for the ethanol dehydrogenation process that could sustainably substitute current acetaldehyde production methods is highly desired. Among the ethanol dehydrogenation catalysts, copper-based materials have been intensively studied. Unfortunately, the Cu-based catalysts suffer from sintering and coking, which lead to rapid deactivation with time-on-stream. Phosphorus doping has been demonstrated to diminish coking in methanol dehydrogenation, fluid catalytic cracking, and ethanol-to-olefin reactions. This work reports a pioneering application of the well-characterized copper phosphinate complexes as molecular precursors for copper-based ethanol dehydrogenation catalysts enriched with phosphate groups (Cu-phosphate/SiO2). Three new catalysts (CuP-1, CuP-2, and CuP-3), prepared by the deposition of complexes {Cu(SAAP)}n (1), [Cu6(BSAAP)6] (2), and [Cu3(NAAP)3] (3) on the surface of commercial SiO2, calcination at 500 °C, and reduction in the stream of the forming gas 5% H2/N2 at 400 °C, exhibited unusual properties. First, the catalysts showed a rapid increase in catalytic activity. After reaching the maximum conversion, the catalyst started to deactivate. The unusual behavior could be explained by the presence of the phosphate phase, which made Cu2+ reduction more difficult. The phosphorus content gradually decreased during time-on-stream, copper was reduced, and the activity increased. The deactivation of the catalyst could be related to the copper diffusion processes. The most active CuP-1 catalyst reaches a maximum of 73% ethanol conversion and over 98% acetaldehyde selectivity at 325 °C and WHSV = 2.37 h-1.

Visualizing the Hierarchical Evolution of Aryl-Metal Bonding in Organometallic Nanostructures on Ag(111).

On-surface dehalogenative coupling reactions are promising for constructing nanostructures with diverse properties and functionalities. Extensive efforts have been devoted to single aryl-halogen (C-X) substituents and substitutions at various functionalization sites (typically including meta- and para-substitutions) to generate aryl-aryl single bonds. Moreover, multiple C-X substituents at the ortho-site and the peri- and bay-regions have been applied to create a variety of ring scaffolds. However, for multiple C-X substituents, the hierarchy of aryl-metal bond formation and dissociation remains elusive. Herein, by combining scanning tunneling microscopy imaging and density functional theory calculations, we have visualized and demonstrated the hierarchical evolution of aryl-metal bonding in organometallic intermediates involved in a dehalogenative coupling reaction on Ag(111), using a molecular precursor with both para-substitution and potential bay-region substitution. Our results elucidate how metal atoms are progressively embedded into and removed from organometallic intermediates, enhancing the understanding of on-surface dehalogenative coupling reactions for the controlled construction of the desired nanostructures.

Tuning Mesopore Accessibility of Ce0.18Zr0.64Y0.15La0.03O2-δ by Hydrothermal Post-treatment─A Case Study for Ceria-Based Oxidation Storage Materials.

The connectivity and thermal stability of pores in heterogeneous, mesoporous metal oxide catalysts are key properties controlling their (long-term) efficacy. In this study, we investigate the influence of pH and temperature during a common hydrothermal aftertreatment step in the synthesis of mesoporous CexZr1-x-y-zYyLazO2-δ oxides obtained from molecular precursors via hydrothermal synthesis. This study has a strong focus on the methodological approach, elucidating whether and how even the smallest changes in morphology and connectivity may be unraveled and related to the underlying chemical processes to uncover key parameters for the ongoing improvement of material properties. Deep insights into the mesopore space were obtained by state-of-the-art physisorption (including hysteresis scanning), electron tomography, and small-angle X-ray scattering (SAXS) analysis. We also provide a simple tool to simulate SAXS curves from electron tomography data that allow direct comparison to experimentally obtained SAXS curves. Furthermore, the impact on surface-bound nitrate groups and the development during calcination were studied in detail by thermogravimetric analysis coupled with mass spectrometry. The key observations indicate a significant increase in thermal stability at temperatures as high as 1050 °C and improved mesopore accessibility with an increase in pH of the aftertreatment solution. The combined observations from the employed methods suggest a pH-dependent removal of surface-bound nitrate groups as well as a dissolution and reprecipitation-based fusing of the primary particles that constitute the mesopore skeleton. This transformation yields a mechanically and thermally stronger mesopore space with the capability to endure high temperatures.

Taphonomic experiments on caridean shrimp elucidate the timing and influence of organic matter on diagenetic pyritization and sulfurization

Sulfur is an element particularly valuable for preserving organisms in the fossil record. On one hand, sulfurization (i.e. incorporation of sulfur into organic matter) forms macromolecules more likely to be preserved over geological timescales than their low-weight molecular precursors. On the other hand, pyrite (FeS2) is one of the main minerals replicating organisms and contributes to the exceptional preservation of soft tissues. However, the incomplete understanding of the processes, controls, timing and relation (competition, succession …) of these reactions may conceal critical biases within the fossil record, used to reconstruct past biodiversity and environment. In particular, organic matter (OM) is critical in these fossilization modes, but the roles of its composition and abundance remain to be better assessed, in particular for pyritization. Therefore, laboratory experiments were conducted on crustaceans' carcasses in mesocosms containing natural water and sediment. Tanks were treated with variable conditions to test the relative roles on preservation potential of oxic perturbation, thermal degradation of OM, and internal soft tissues, or lack thereof. Various bio-physico-chemical parameters and sulfur speciation were then monitored. The precipitation of a macroscopic amorphous iron monosulfide was observed after 100 days, both in the sediment and within carcasses; sulfurization of carcasses was detected within tens of days. A periodic aeration of the sediment where the carcasses were decayed impeded neither pyritization nor sulfurization while thermal degradation of OM did. Further, absence of internal soft tissues led solely to sulfurization, which suggests (i) that labile OM may be the factor driving the balance between pyritization and sulfurization and (ii) that internal soft tissues may promote pyritization through an organomineralization process, with sulfur-bearing organic ligands acting as nucleation sites. These results invite further experiments to test additional factors, in order to help refine environmental and taxonomical controls, and further investigation of fossil material, to evaluate the significance of sulfurization in the fossil record by searching sulfurized organisms.

XPS study of NiO thin films obtained by chemical vapor deposition

Nickel oxide (NiO) thin films are of great importance for a variety of technological applications, especially in (photo)electrocatalysis for clean energy production and pollutant degradation. In this field, various research efforts are devoted to the preparation of thin films with controllable chemicophysical properties. In the framework of our research activities, we have recently fabricated NiO thin films by means of chemical vapor deposition (CVD) using a series of closely related Ni(II) β-diketonate-diamine molecular precursors. In the present work, the attention is focused on the x-ray photoelectron spectroscopy (XPS) analysis of a representative NiO film grown at 400 °C in an O2 + H2O reaction atmosphere. Besides the wide scan spectrum, high resolution spectra for C 1s, O 1s, and, in particular, Ni 2p are reported and discussed in detail.