Hybrid magnetic materials based on layered double hydroxidesfrom the chemistry towards the applications

Abstract

Layered double hydroxides (LDHs) are the leitmotiv of this dissertation. Contradicting the assertion that “any past was better”, LDHs have been continuously revisited from the middle of the twentieth century, and represent an excellent example of the never-ending beauty of Chemistry. New synthetic perspectives are giving a new impetus to LDH chemistry, which among hybrid materials, are finding their heyday. This is resulting in novel materials and also paving the way for new fundamental and practical insights. This dissertation is focused on magnetic LDHs, and for the sake of clarity it is organized in three main parts: we will move from basic synthetic and physical aspects of LDHs, through the preparation of multifunctional hybrids that respond to external stimuli, and towards some of the most innovative applications in energy storage and spintronics. Due to the wide number of techniques employed, and to facilitate the reading, we have included the experimental procedures and the specific instrumentation at the end of each Chapter. The first part deals with the chemical design, synthesis, exfoliation and physical characterization of new magnetic layered double hydroxides. Chapter 2 provides an in-depth insight into the synthesis of highly crystalline, hexagonal-shaped NiFe–LDH with tunable composition, a kind of LDH that up to now had been elusive. In addition, exfoliation of this magnetic LDH into unilamellar nanosheets has been achieved for the very first time. Chapter 3 is devoted to the physical properties of these new layered materials, unveiling their controllable magnetic behaviour, and shedding light on the cation order within the layers, elucidating the origin of the spin-glass behaviour in this sort of materials. Concluding this part, Chapter 4 addresses the profound effect exerted by extrinsic impurities in the magnetic behaviour of LDHs, highlighting the enormous possibilities offered by these magnetic materials. The second part explores the preparation of new hybrid magnetic multilayers based on LDHs, with special emphasis on the control of the magnetic properties of LDHs by means of an external stimulus. In this sense, Chapter 5 and 6 focuses on the physical effect induced by the intercalation of a macrocyclic phthalocyanine and porphyrins in the interlamellar space provided by magnetic LDHs, respectively. Moreover, the synthesis and characterization of novel stimuli responsive hybrid materials has been surveyed in this section, showing the very first examples of the reversible switching of the magnetic properties in LDH-based materials. Along this front, Chapter 7 deals with the photo-switching in a hybrid material made of azobenzene intercalated LDHs, whilst Chapter 8 examines the reversible thermo-responsive switching of a thermochromic-LDH hybrid. Finally, the third part is devoted to the applications of several LDH-based materials in different fields of utmost importance, such as energy storage and conversion or spintronics. On the one hand, in Chapter 9 we have studied the intrinsic electrocatalytic properties towards water oxidation of NiFe–LDHs. We have also investigated the effect of their hybridization with graphene showing an excellent behaviour as supercapacitors and water oxidation electrocatalysts. On the other hand, in Chapter 10 we have reported the use of hybrid NiFe–LDHs as catalytic nanoreactors for the in-situ synthesis of novel magnetic nanocomposites formed by FeNi3 nanoparticles embedded in carbon nanoforms. These hierarchical nanocomposites can be used as precursors for the low-temperature isolation of nanocarbons, ranging from carbon nanoonions to graphene. Furthermore, we have tested these nanocomposites as electrode materials for supercapacitors, obtaining high values of specific capacitance and excellent rate capabilities. In the end, we have explored the application of the previously synthesized graphene–FeNi3 nanocomposites in the field of spintronics. Concretely, Chapter 11 describes the magnetotransport properties of these hybrids, showing striking crossover behaviour from negative to positive magnetoresistance, arising from the correlation of the physical properties of its initial constituents in a synergistic way.