Boosting Oxygen Reduction Reaction Selectivity in Metal Nanoparticles with Polyoxometalates

The lack of selectivity toward the oxygen reduction reaction (ORR) in metal nanoparticles can be linked to the generation of intermediates. This constitutes a crucial constraint on the performance of specific electrochemical devices, such as fuel cells and metal-air batteries. To boost selectivity of metal nanoparticles, a novel methodology that harnesses the unique electrocatalytic properties of polyoxometalates (POM) to scavenge undesired intermediates of the ORR (such as HO2 -) promoting selectivity is proposed. It involves the covalent functionalization of metal nanoparticle's surface with an electrochemically active capping layer containing a new sulfur-functionalized vanadium-based POM (AuNP@POM). To demonstrate this approach, preformed thiolate Au(111) nanoparticles with a relatively poor ORR selectivity are chosen. The dispersion of AuNP@POM on the surface of carbon nanofibers (CNF) enhances oxygen diffusion, and therefore the ORR activity. The resulting electrocatalyst (AuNP@POM/CNF) exhibits superior stability against impurities like methanol and a higher pH tolerance range compared to the standard commercial Pt/C. The work demonstrates for the first time, the use of a POM-based electrochemically active capping layer to switch on the selectivity of poorly selective gold nanoparticles, offering a promising avenue for the preparation of electrocatalyst materials with improved selectivity, performance, and stability for ORR-based devices.

Electrochemically Versatile Graphite Nanoplatelets Prepared by a Straightforward, Highly Efficient, and Scalable Route

Nanostructured carbon materials with tailor-made structures (e.g., morphology, topological defect, dopant, and surface area) are of significant interest for a variety of applications. However, the preparation method selected for obtaining these tailor-made structures determines the area of application, precluding their use in other technological areas of interest. Currently, there is a lack of simple and low-cost methodologies versatile enough for obtaining freestanding carbon nanostructures that can be used in either energy storage or chemical detection. Here, a novel methodology for the development of a versatile electrochemically active platform based on freestanding graphite nanoplatelets (GNP) has been developed by exploiting the interiors of hollow carbon nanofibers (CNF) comprising nanographene stacks using dry ball-milling. Even though ball-milling could be considered as a universal method for any carbonaceous material, often, it is not as simple (one step, no purification, and no solvents), efficient (just GNP without tubular structures), and quick (just 20 min) as the sustainable method developed in this work, free of surfactants and stabilizer agents. We demonstrate that the freestanding GNP developed in this work (with an average thickness of 3.2 nm), due to the selective edge functionalization with the minimal disruption of the basal plane, can act either as a supercapacitor or as a chemical sensor, showing both a dramatic improvement in the charge storage ability of more than 30 times and an enhanced detection of electrochemically active molecules such as ascorbic acid with a 236 mV potential shift with respect to CNF in both cases. As shown here, GNP stand as an excellent versatile alternative compared to the standard commercially available carbon-based materials. Overall, our approach paves the way for the discovery of new nanocarbon-based electrochemical active platforms with a wide electrochemical applicability.

High-yield halide-assisted synthesis of metal-organic framework UiO-based nanocarriers

The synthesis of nanosized metal-organic frameworks (NMOFs) is requisite for their application as injectable drug delivery systems (DDSs) and other biorelevant purposes. Herein, we have critically examined the role of different synthetic parameters leading to the production of UiO-66 crystals smaller than 100 nm. Of note, we demonstrate the co-modulator role conferred by halide ions, not only to produce NMOFs with precise morphology and size, but also to significantly improve the reaction yield. The resulting NMOFs are highly crystalline and exhibit sustained colloidal stability in different biologically relevant media. As a proof of concept, these NMOFs were loaded with Rhodamine 6G (R6G), which remained trapped in most common biologically relevant media. When incubated with living mammalian cells, the R6G-loaded NMOFs were efficiently internalized and did not impair cell viability even at relatively high doses.

Crystal engineering and ferroelectricity at the nanoscale in epitaxial 1D manganese oxide on silicon

Ferroelectric oxides have attracted much attention due to their wide range of applications, particularly in electronic devices such as nonvolatile memories and tunnel junctions. As a result, the monolithic integration of these materials into silicon technology and their nanostructuration to develop alternative cost-effective processes are among the central points in the current technology. In this work, we used a chemical route to obtain nanowire thin films of a novel Sr1+δMn8O16 (SMO) hollandite-type manganese oxide on silicon. Scanning transmission electron microscopy combined with crystallographic computing reveals a crystal structure comprising hollandite and pyrolusite units sharing the edges of their MnO6 octahedra, resulting in three types of tunnels arranged along the c axis, where the ordering of the Sr atoms produces natural symmetry breaking. The novel structure gives rise to ferroelectricity and piezoelectricity, as revealed by local direct piezoelectric force microscopy measurements, which confirmed the ferroelectric nature of the SMO nanowire thin films at room temperature and showed a piezoelectric coefficient d33 value of 22 ± 6 pC N-1. Moreover, we proved that flexible vertical SMO nanowires can be harvested providing an electrical output energy through the piezoelectric effect, showing excellent deformability and high interface recombination. This work indicates the possibility of engineering the integration of 1D manganese oxides on silicon, a step which precedes the production of microelectronic devices.

Direct and converse piezoelectric responses at the nanoscale from epitaxial BiFeO3 thin films grown by polymer assisted deposition

We use an original water-based chemical method to grow pure epitaxial BiFeO3 (BFO) ultra-thin films with excellent piezoelectric properties. Particularly, we show that this novel chemical route produces higher natural ferroelectric domain size distribution and coercive field compared to similar BFO films grown by physical methods. Moreover, we measured the d33 piezoelectric coefficient of 60 nm thick BFO films by a direct approach, using Direct Piezoelectric Force Microscopy (DPFM). As a result, first piezo-generated charge maps of a very thin BFO layer were obtained applying this novel technology. We also performed a comparative study of the d33 coefficients between standard PFM analysis and DPFM microscopy showing similar values i.e. 17 pm V-1 and 22 pC N-1, respectively. Finally, we proved that the directionality of the piezoelectric effect in BFO ferroelectric thin films is preserved at low thickness dimensions demonstrating the potential of chemical processes for the development of low cost functional ferroelectric and piezoelectric devices.

Electric and Mechanical Switching of Ferroelectric and Resistive States in Semiconducting BaTiO3-δ Films on Silicon

Materials that can couple electrical and mechanical properties constitute a key element of smart actuators, energy harvesters, or many sensing devices. Within this class, functional oxides display specific mesoscale responses which often result in great sensitivity to small external stimuli. Here, a novel combination of molecular beam epitaxy and a water-based chemical-solution method is used for the design of mechanically controlled multilevel device integrated on silicon. In particular, the possibility of adding extra functionalities to a ferroelectric oxide heterostructure by n-doping and nanostructuring a BaTiO₃ thin film on Si(001) is explored. It is found that the ferroelectric polarization can be reversed, and resistive switching can be measured, upon a mechanical load in epitaxial BaTiO_3-δ /La_0.7 Sr_0.3 MnO₃ /SrTiO₃ /Si columnar nanostructures. A flexoelectric effect is found, stemming from substantial strain gradients that can be created with moderate loads. Simultaneously, mechanical effects on the local conductivity can be used to modulate a nonvolatile resistive state of the BaTiO_3-δ heterostructure. As a result, three different configurations of the system become accessible on top of the usual voltage reversal of polarization and resistive states.

High quality thin films of thermoelectric misfit cobalt oxides prepared by a chemical solution method

Misfit cobaltates ([Bi/Ba/Sr/Ca/CoO]_n^RS[CoO2]_q) constitute the most promising family of thermoelectric oxides for high temperature energy harvesting. However, their complex structure and chemical composition makes extremely challenging their deposition by high-vacuum physical techniques. Therefore, many of them have not been prepared as thin films until now. Here we report the synthesis of high-quality epitaxial thin films of the most representative members of this family of compounds by a water-based chemical solution deposition method. The films show an exceptional crystalline quality, with an electrical conductivity and thermopower comparable to single crystals. These properties are linked to the epitaxial matching of the rock-salt layers of the structure to the substrate, producing clean interfaces free of amorphous phases. This is an important step forward for the integration of these materials with complementary n-type thermoelectric oxides in multilayer nanostructures.

Room-temperature ferromagnetism in thin films of LaMnO3 deposited by a chemical method over large areas

Hole-doping into the Mott insulator LaMnO3 results in a very rich magneto-electric phase diagram, including colossal magnetoresistance and different types of charge and orbital ordering. On the other hand, LaMnO3 presents an important catalytic activity for oxygen reduction, which is fundamental for increasing the efficiency of solid-oxide fuel cells and other energy-conversion devices. In this work, we report the chemical solution (water-based) synthesis of high-quality epitaxial thin films of LaMnO3, free of defects at square-centimeter scales, and compatible with standard microfabrication techniques. The films show a robust ferromagnetic moment and large magnetoresistance at room temperature. Through a comparison with films grown by pulsed laser deposition, we show that the quasi-equilibrium growth conditions characteristic of this chemical process can be exploited to tune new functionalities of the material.

Tunnel conduction in epitaxial bilayers of ferromagnetic LaCoO₃/La₂/₃Sr₁/₃MnO₃ deposited by a chemical solution method

We report magnetic and electronic transport measurements across epitaxial bilayers of ferromagnetic insulator LaCoO3 and half-metallic ferromagnet La2/3Sr1/3MnO3 (LCO/LSMO: 3.5 nm/20 nm) fabricated by a chemical solution method. The I-V curves at room temperature and 4K measured with conducting atomic force microscopy (CAFM) on well-defined patterned areas exhibit the typical features of a tunneling process. The curves have been fitted to the Simmons model to determine the height (φ) and width (s) of the insulating LCO barrier. The results yield φ = 0.40 ± 0.05 eV (0.50 ± 0.01 eV) at room temperature (4K) and s = 3 nm, in good agreement with the structural analysis. Our results demonstrate that this chemical method is able to produce epitaxial heterostructures with the quality required for this type of fundamental studies and applications.