Erika M. A. Fuentes-Fernandez

Understanding and controlling water stability of MOF-74

Metal organic framework (MOF) materials in general, and MOF-74 in particular, have promising properties for many technologically important processes. However, their instability under humid conditions severely restricts practical use. We show that this instability and the accompanying reduction of the CO2 uptake capacity of MOF-74 under humid conditions originate in the water dissociation reaction H2O → OH + H at the metal centers. After this dissociation, the OH groups coordinate to the metal centers, explaining the reduction in the MOFs CO2 uptake capacity. This reduction thus strongly depends on the catalytic activity of MOF-74 towards the water dissociation reaction. We further show that—while the water molecules themselves only have a negligible effect on the crystal structure of MOF-74—the OH and H products of the dissociation reaction significantly weaken the MOF framework and lead to the observed crystal structure breakdown. With this knowledge, we propose a way to suppress this particular reaction by modifying the MOF-74 structure to increase the water dissociation energy barrier and thus control the stability of the system under humid conditions.

The effect of poling conditions on the performance of piezoelectric energy harvesters fabricated by wet chemistry

The effect of poling conditions on the power output of piezoelectric energy harvesters using sol–gel based Pb(Zr0.53,Ti0.47)O3–Pb(Zn1/3,Nb2/3)O3 piezoelectric thin-films has been investigated. A strong correlation was established between the poling efficiency and harvester output. A method based on simple capacitance–voltage measurements is shown to be an effective approach to estimate the power output of harvesters poled under different conditions. The poling process was found to be thermally activated with an activation energy of 0.12 eV, and the optimum poling conditions were identified (200 kV cm−1, 250 °C for 50 min). The voltage output and power density obtained under optimum poling conditions were measured to be 558 V cm−2 and 325 μW cm−2, respectively.

Hetero-epitaxial BiFeO3/SrTiO3 nanolaminates with higher piezoresponse performance over stoichiometric BiFeO3 films

In this research, BiFeO3 (BFO) films are integrated into BFO/SrTiO3 (STO)/BFO nanolaminates (BSB-NLs) featuring nanometer-scale thickness of BFO and STO layers. By introducing the STO layer in between two BFO layers, the leakage current density is reduced by two orders of magnitude with respect to relatively high leakage currents of current single BFO layers, i.e., from 10−5 A/cm2 to 10−7 A/cm2. The BSB-NL also shows very high piezoelectric response, which is ∼5 times higher than that of the pure BFO with the same thickness. The highly strained state of the BFO layers concurrently with the chemical/crystallographic state of the interfaces between the BFO and STO layers contribute to the very high values of piezoresponse and very low leakage current observed in the BSB-NLs.

Cluster assisted water dissociation mechanism in MOF-74 and controlling it using helium

We show that the water dissociation reaction H2O → OH + H in the confined environment of MOF-74 channels can be precisely controlled by the addition of noble gas He. Elucidating the entire reaction process using ab initio methods and infrared (IR) spectroscopy, we prove that the interaction between water molecules is critical to the formation of water clusters, which reduce the dissociation barrier by up to 37% and thus influence the reaction significantly. Our time-resolved IR measurements confirm that the formation of these clusters can be suppressed by introducing He gas, providing unprecedented control over water dissociation rates. Since the water dissociation reaction is the cause of the structural instability of MOF-74 in the presence of water, our finding of the reaction mechanism lays the groundwork for designing water stable versions of MOF-74 as well as understanding water related phenomena in MOFs in general.

Study on the Microstructure and Electrical Properties of Pb(Zr0.53 Ti0.47)O3 Thin-Films

In the present study a complete analysis of the morphological and electrical properties of PZT layers with composition 53Zr-47Ti is presented. Three different samples composed of 3, 6, and 9 PZT layers were analyzed on a substrate consisting of ZrO2-SiO2-Si structures. The PZT and ZrO2 layers were deposited via Sol-Gel, whereas the SiO2 layer, on every sample, was deposited via PECVD. SEM results showed morphology of very small granules on the 3 layered thin-film samples (12 nm), on the 6 layered thin-film samples a mixture of small and large size (100-300 nm) granule formation was observed, with the 9 layered thin-film samples exhibiting very large granule sizes (bigger than 300 nm). XRD results showed that increasing the number of deposited layers caused an incremental increase on the detected peak intensities, aided in the promotion of the perovskite phase, and diminished the presence of the pyrochlore phase. It was also observed, during electrical measurements, that increasing the number deposited layers directly increased the overall capacitance of the thin-film structure. This effect was attributed primarily to the large amount of perovskite and large size of grains presented on thick samples.

Chemistry in confined spaces: reactivity of the Zn-MOF-74 channels

Using infrared spectroscopy combined with ab initio methods we study reactions of H2O and CO inside the confined spaces of Zn-MOF-74 channels. Our results show that, once the water dissociation reaction H2O → OH + H takes place at the metal centers, the addition of 40 Torr of CO at 200 °C starts the production of formic acid via OH + H + CO → HCO2H. Our detailed analysis shows that the overall reaction H2O + CO → HCO2H takes place in the confinement of MOF-74 without an external catalyst, unlike the same reaction on flat surfaces. This discovery has several important consequences: it opens the door to a new set of catalytic reactions inside the channels of the MOF-74 system, it suggests that a recovery of the MOFs adsorption capacity is possible after it has been exposed to water (which in turn stabilizes its crystal structure), and it produces the important industrial feedstock formic acid.

Synthesis and Characterization of Pb(Zr., Ti.)O-Pb(Nb/, Zn/)O Thin Film Cantilevers for Energy Harvesting Applications

A complete analysis of the morphology, crystallographic orientation, and resulting electrical properties of Pb(Zr0.53,Ti0.47)O3− Pb(Nb1/3, Zn2/3)O3 (PZT-PZN) thin films, as well as the electrical behavior when integrated in a cantilever for energy harvesting applications, is presented. The PZT-PZN films were deposited using sol-gel methods. We report that using 20% excess Pb, a nucleation layer of PbTiO3 (PT), and a fast ramp rate provides large grains, as well as denser films. The PZT-PZN is deposited on a stack of TiO2/PECVD SiO2/Si3N4/thermal SiO2/Poly-Si/Si. This stack is designed to allow wet-etching the poly-Si layer to release the cantilever structures. It was also found that the introduction of the poly-Si layer results in larger grains in the PZT-PZN film. PZT-PZN films with a dielectric constant of 3200 and maximum polarization of 30 μC/cm2 were obtained. The fabricated cantilever devices produced ~300–400 mV peak-to-peak depending on the cantilever design. Experimental results are compared with simulations.