Fe-Ni porphyrin/mesoporous titania thin film electrodes: a bioinspired nanoarchitecture for photoelectrocatalysis.

Porphyrin and porphyrinoid derivatives have been extensively studied in the assembly of catalysts and sensors, seeking biomimetic and bioinspired activity. In particular, Fe and Ni porphyrins can be used for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) by immobilization of these molecular catalysts on semiconductor materials. In this study, we designed a hybrid material containing a crystalline mesoporous TiO2 thin film in which the catalytic centres are Ni-porphyrin (NiP), Fe-porphyrin (FeP), and a NiP/FeP bimetallic system to assess whether the coexistence of both metalloporphyrins improves the OER activity. The obtained photoelectrodes were physicochemically and morphologically characterized through high-resolution FE-SEM images, UV-vis and Raman spectroscopies, cyclic voltammetry, and impedance measurements. The results show a differential behavior of the mono- and bimetallic porphyrin systems, where the Fe(iii) centre in FeP may increase the acidity and lower the reduction potential of the Ni2+/3+ couple when co-deposited with NiP leading to an improved photoelectrochemical water-oxidation performance. We have validated the cooperative effect of both metal complexes within this novel system, where the µ-peroxo-bridged interaction between Fe and Ni is integrated into a supramolecular heterometallic structure of porphyrins.

Sunlight-Driven Photocatalysis for a Set of 3D Metal-Porphyrin Frameworks Based on a Planar Tetracarboxylic Ligand and Lanthanide Ions.

Metal-porphyrin frameworks (MPFs) with trivalent lanthanide ions are the most sought-after materials in the past decade. Their porosities are usually complemented by optical properties imparted by the metal nodes, making them attractive multifunctional materials. Here, we report a novel family of 3D MPFs obtained through solvothermal reactions between tetrakis(4-carboxyphenyl) porphyrin (H4TCPP) and different lanthanide sources, yielding an isostructural family of compounds along the lanthanide series: [Ln2(DMF)(TCPP)1.5] for Ln = La, Ce, Nd, Pr, Er, Y, Tb, Dy, Sm, Eu, Gd, and Tm. Photoluminescent properties of selected phases were explored at room temperature. Also, the photocatalytic performance exhibited by these compounds under sunlight exposure is promising for its implementation in organic pollutant degradation. In order to study the photocatalytic activity of Ln-TCPPs in an aqueous medium, methylene blue (MB) was used as a contaminant model. The efficiency for MB degradation was Sm > Y > Yb > Gd > Er > Eu > either no catalyst or no light, obtaining more than 70% degradation at 120 min with Sm-TCPP. These results open the possibility of using these compounds in optical and optoelectronic devices for water remediation and sensing.

Elucidating the Role of the Metal Catalyst and Oxide Support in the Ru/CeO2-Catalyzed CO2 Methanation Mechanism.

This study addresses the yet unresolved CO2 methanation mechanism on a Ru/CeO2 catalyst by means of near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) complemented with periodic density functional theory (DFT) calculations. NAP-XPS results show that the switch from H2 to CO2 + H2 mixture oxidizes both the Ru and CeO2 phases at low temperatures, which is explained by the CO2 adsorption modes assessed by means of DFT on each representative surface. CO2 adsorption on Ru is dissociative and moderately endergonic, leading to polybonded Ru-carbonyl groups whose hydrogenation is the rate-determining step in the overall process. Unlike on Ru metal, CO2 can be strongly adsorbed as carbonates on ceria surface oxygen sites or on the reduced ceria at oxygen vacancies as carboxylates (CO2 -δ), resulting in the reoxidation of ceria. Carboxylates can then evolve as CO, which is released either via direct splitting at relatively low temperatures or through stable formate species at higher temperatures. DRIFTS confirm the great stability of formates, whose depletion relates with CO2 conversion in the reaction cell, while carbonates remain on the surface up to higher temperatures. CO generation on ceria serves as an additional reservoir of Ru-carbonyls, cooperating to the overall CO2 methanation process. Altogether, this study highlights the noninnocent role of the ceria support in the performance of Ru/CeO2 toward CO2 methanation.

Thermocatalytic CO2 Conversion over a Nickel-Loaded Ceria Nanostructured Catalyst: A NAP-XPS Study.

Despite the increasing economic incentives and environmental advantages associated to their substitution, carbon-rich fossil fuels are expected to remain as the dominant worldwide source of energy through at least the next two decades and perhaps later. Therefore, both the control and reduction of CO2 emissions have become environmental issues of major concern and big challenges for the international scientific community. Among the proposed strategies to achieve these goals, conversion of CO2 by its reduction into high added value products, such as methane or syngas, has been widely agreed to be the most attractive from the environmental and economic points of view. In the present work, thermocatalytic reduction of CO2 with H2 was studied over a nanostructured ceria-supported nickel catalyst. Ceria nanocubes were employed as support, while the nickel phase was supported by means a surfactant-free controlled chemical precipitation method. The resulting nanocatalyst was characterized in terms of its physicochemical properties, with special attention paid to both surface basicity and reducibility. The nanocatalyst was studied during CO2 reduction by means of Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS). Two different catalytic behaviors were observed depending on the reaction temperature. At low temperature, with both Ce and Ni in an oxidized state, CH4 formation was observed, whereas at high temperature above 500 °C, the reverse water gas shift reaction became dominant, with CO and H2O being the main products. NAP-XPS was revealed as a powerful tool to study the behavior of this nanostructured catalyst under reaction conditions.

Zinc porphyrin/mesoporous titania thin film electrodes: a hybrid material nanoarchitecture for photocatalytic reduction.

In this work, photocatalytic reduction of methyl viologen is achieved using zinc tetra(4-N-methylpyridyl)porphine (ZnP) functionalized mesoporous titania thin films (MTTF). Metalloporphyrins are the core of natural systems that harvest energy from the sun. Thus, a bioinspired approach is used, taking advantage of ZnP sensitizing capabilities and MTTF organized structure.

Unexpected Optical Blue Shift in Large Colloidal Quantum Dots by Anionic Migration and Exchange.

Compositional changes taking place during the synthesis of alloyed CdSeZnS nanocrystals (NCs) allow shifting of the optical features to higher energy as the NCs grow. Under certain synthetic conditions, the effect of those changes on the surface/interface chemistry competes with and dominates over the conventional quantum confinement effect in growing NCs. These changes, identified by means of complementary advanced spectroscopic techniques such as XPS (X-ray photoelectron spectroscopy) and XAS (X-ray absorption spectroscopy), are understood in the frame of an ion migration and exchange mechanism taking place during the synthesis. Control over the synthetic routes during NC growth represents an alternative tool to tune the optical properties of colloidal quantum dots, broadening the versatility of the wet chemical methods.

Understanding and Reducing Photothermal Forces for the Fabrication of Au Nanoparticle Dimers by Optical Printing.

Optical printing holds great potential to enable the use of the vast variety of colloidal nanoparticles (NPs) in nano- and microdevices and circuits. By means of optical forces, it enables the direct assembly of NPs, one by one, onto specific positions of solid surfaces with great flexibility of pattern design and no need of previous surface patterning. However, for unclear causes it was not possible to print identical NPs closer to each other than 300 nm. Here, we show that the repulsion restricting the optical printing of close by NPs arises from light absorption by the printed NPs and subsequent local heating. By optimizing heat dissipation, it is possible to reduce the minimum separation between NPs. Using a reduced graphene oxide layer on a sapphire substrate, we demonstrate for the first time the optical printing of Au-Au NP dimers. Modeling the experiments considering optical, thermophoretic, and thermo-osmotic forces we obtain a detailed understanding and a clear pathway for the optical printing fabrication of complex nano structures and circuits based on connected colloidal NPs.

Inorganically coated colloidal quantum dots in polar solvents using a microemulsion-assisted method.

The dielectric nature of organic ligands capping semiconductor colloidal nanocrystals (NCs) makes them incompatible with optoelectronic applications. For this reason, these ligands are regularly substituted through ligand-exchange processes by shorter (even atomic) or inorganic ones. In this work, an alternative path is proposed to obtain inorganically coated NCs. Differently to regular ligand exchange processes, the method reported here produces core-shell NCs and the removal of the original organic shell in a single step. This procedure leads to the formation of connected NCs resembling 1D worm-like networks with improved optical properties and polar solubility, in comparison with the initial CdSe NCs. The nature of the inorganic shell has been elucidated by X-ray Absorption Near Edge Structure (XANES), Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Photoelectron Spectroscopy (XPS). The 1D morphology along with the lack of long insulating organic ligands and the higher solubility in polar media turns these structures very attractive for their further integration into optoelectronic devices.