Brendan Bulfin

Perovskite oxides for application in thermochemical air separation and oxygen storage

Perovskites AMO3−δ are ideal for thermochemical air separation due to their oxygen nonstoichiometry δ, which can be varied by changing the temperature and oxygen partial pressure. We show in this work how materials can be selected for chemical looping air separation from thermodynamic considerations and present thermogravimetric experiments carried out on (Ca,Sr) ferrites and manganites, and doped variants, all synthesized via a citric acid auto-combustion method. SrFe0.95Cu0.05O3−δ and Ca0.8Sr0.2MnO3−δ show the best gravimetric oxygen storage capacity of all tested materials at T < 1200 °C. The redox reactions are completed in <1 min in air and are highly reversible. A significant re-oxidation reaction of reduced samples was observed at temperatures as low as 250 °C at an oxygen partial pressure of 0.16 bar. We studied phase formation via XRD and lattice expansion during reduction via in situ XRD experiments. The objective is to validate the potential and boundary conditions of such materials to pave the way for competitive air separation based on thermochemical cycling.

Redox chemistry of CaMnO3 and Ca0.8Sr0.2MnO3 oxygen storage perovskites

Perovskite oxides CaMnO3 and Ca0.8Sr0.2MnO3 show continuous non-stoichiometry over a range of temperatures and oxygen partial pressures. In this work a thermobalance equipped with an oxygen pump was used to measure the equilibrium non-stoichiometry of both materials for temperatures in the range 400–1200 °C and oxygen partial pressures in the range 1–10−5 bar. Analysis of the data showed that Ca0.8Sr0.2MnO3 has a lower enthalpy of reduction and thus can be more easily reduced. The strontium added sample was also robust against a phase transition that was seen in CaMnO3 at high temperatures. A statistical thermodynamic model of the system suggests that the defects form clusters of the form . The oxidation kinetics were also investigated with Ca0.8Sr0.2MnO3 showing faster kinetics and maintaining activity at lower temperatures. Overall, Ca0.8Sr0.2MnO3 shows very promising properties for redox applications, including gravimetric oxygen storage up to 4% by mass, high stability and rapid reversibility, with re-oxidation in less than 1 min at 400 °C. Finally, the redox chemistry of Ca0.8Sr0.2MnO3 was also investigated using in situ X-ray photoelectron spectroscopy and near-edge X-ray absorption measurements at near ambient pressure in oxygen atmospheres.

Manipulating and probing the growth of plasmonic nanoparticle arrays using light

Highly ordered self-assembled silver nanoparticle (NP) arrays have been produced by glancing angle deposition on faceted c-plane Al2O3 templates. The NP shape can be tuned by changing the substrate temperature during deposition. Reflectance anisotropy spectroscopy has been used to monitor the plasmonic evolution of the sample during the growth. The structures showed a strong dichroic response related to NP anisotropy and dipolar coupling. Furthermore, multipolar resonances due to sharp edge effects between NP and substrate were observed. Analytical and numerical methods have been used to explain the results and extract semi-quantitative information on the morphology of the NPs. The results provide insights on the growth mechanisms by the glancing angle deposition. Finally, it has been shown that the NP morphology can be manipulated by a simple illumination of the surface with an intense light source, inducing changes in the optical response. This opens up new possibilities for engineering plasmonic structure over large active areas.

Writing with atoms: Oxygen adatoms on the MoO2/Mo(110) surface

AbstractWriting at the nanoscale using the desorption of oxygen adatoms from the oxygen-rich MoO2+x/Mo(110) surface is demonstrated by scanning tunnelling microscopy (STM). High-temperature oxidation of the Mo(110) surface results in a strained, bulk-like MoO2(010) ultra-thin film with an O-Mo-O trilayer structure. Due to the lattice mismatch between the Mo(110) and the MoO2(010), the latter consists of well-ordered molybdenum oxide nanorows separated by 2.5 nm. The MoO2(010)/Mo(110) structure is confirmed by STM data and density functional theory calculations. Further oxidation results in the oxygen-rich MoO2+x/Mo(110) surface, which exhibits perfectly aligned double rows of oxygen adatoms, imaged by STM as bright protrusions. These adatoms can be removed from the surface by scanning (or pulsing) at positive sample biases greater than 1.5 V. Tip movement along the surface can be used for controlled lithography (or writing) at the nanoscale, with a minimum feature size of just 3 nm. By moving the STM tip in a predetermined fashion, information can be written and read by applying specific biases between the surface and the tip.

Applications and limitations of two step metal oxide thermochemical redox cycles; a review

Two step metal oxide thermochemical redox cycles have seen growing interest in the research community over the last two decades. In particular, they have often been studied as a means of converting heat from concentrated solar power to chemical energy, which can subsequently be used for a number of applications. In this work, we offer critical perspective and valuable insight on these research fields, from authors with a combined experience of 50+ years in metal oxide redox cycles. The current fundamental understanding of thermochemical redox materials, and the implications and limitations this has on the redox thermodynamics are discussed. The underlying fundamentals of the redox materials are then used to give insight into the theoretical limitations imposed on a number of applications including; solar thermochemical fuel production, solar energy storage for off sun power generation, thermochemical air separation, oxygen pumping, and thermochemical heat pumping. A number of recent novel process developments are also presented, which offer valuable motivation and direction for the respective fields. The analysis shows that oxides which undergo a stoichiometric phase change during reduction, such as ZnO or Co3O4, have much larger specific energy storage than materials undergoing partial reduction such as ceria and perovskites. The partial reduction materials generally have faster kinetics and better activity at low temperature, and the selection of materials for the various applications is often a compromise between the importance of high specific energy storage vs. fast kinetics and low temperature operation.

Decoupling the refractive index from the electrical properties of transparent conducting oxides via periodic superlattices

We demonstrate an alternative approach to tuning the refractive index of materials. Current methodologies for tuning the refractive index of a material often result in undesirable changes to the structural or optoelectronic properties. By artificially layering a transparent conducting oxide with a lower refractive index material the overall film retains a desirable conductivity and mobility while acting optically as an effective medium with a modified refractive index. Calculations indicate that, with our refractive index change of 0.2, a significant reduction of reflective losses could be obtained by the utilisation of these structures in optoelectronic devices. Beyond this, periodic superlattice structures present a solution to decouple physical properties where the underlying electronic interaction is governed by different length scales.