Nano-Structured Spinel Ferrites and Their Applications as Antimicrobial Agents, Gas Sensors, and Dye Adsorbents

Introduction

Nanomaterials are particles with one or more external and internal structure dimensions or surface dimensions falling in the range of 1 nm–100 nm (Rana et al., 2021). When the particle size of materials is reduced to the nanoscale, they show very different properties compared to what they show on the macroscale (Jacob & Khadar, 2010; McHenry & Laughlin, 2000). The nano-sized particles possess a high surface area hence, the effectiveness also gets high. There are two basic types of fabrication methods for nanomaterials: top down and bottom up. Nanomaterials can be constructed by a top-down approach, producing very small structures from larger pieces of the material. For example, laser ablation, ball-milling, arc-discharge methods, etc. They can also be fabricated by bottom-up techniques, producing from atom by atom or molecule by molecule. i.e., chemical vapor deposition, sol-gel method, co-precipitation method, etc. Nanomaterials are important in various areas from basic research to various applications in electronics, medical, catalysis, cosmetics, and energy.

The history of ferrites began with the discovery of stones that would attract iron. A large amount of these stones was found in the district of Magnesia in Asia Minor and hence this mineral is named magnetite (Fe₃O₄) (Shaikh et al., 2016). The first application of magnetite was the “Lodestones”, which were used by early navigators to detect magnetic poles (Mills, 2004). Lodestone is a naturally available mineral that is rich in iron oxide and hence behaves like a permanent magnet. The etymology of ferrites is from the Latin word ‘Ferrum’ which meaning is iron (Garcia-Munoz et al., 2020). Ferrites are a large class of oxides containing Fe³⁺ and other metal cations that have been investigated and applied in many fields for more than 50 years. They are classified according to their crystal structure and the arrangement of oxygen anions around the metal cations, such as spinel, garnet, magnetoplumbite, and orthoferrite. Among the various types of ferrites, spinel ferrites are widely used in electrical, magnetic, and medical applications. The systematic study on the relationship between chemical composition and magnetic properties of various ferrites was firstly reported by Hilpert in 1909 (Hilpert, 1909). At that time, Hilpert had successfully prepared spinel ferrites, such as magnesium, cobalt, manganese, copper, and zinc. The commendable aspects of his work were that he already had attempted to prepare barium, lead, and calcium ferrites and for that, he had applied for a German patent (Hilpert, 1909) on his forehanded idea of the ferrite core.

An important ferrite material is magnetite, Fe²⁺Fe³⁺₂O₄ (typically known as Fe₃O₄), a natural spinel oxide and probably the oldest magnetic material which is a very active research field at present, with the practical applications due to its interesting properties associated with the coexistence of ferrous and ferric cations. Magnetite is naturally available in large quantities, chemically stable, and environmentally robust. It is a weak hard ferrite material. Another important material by its structure and applications is maghemite or γ-Fe₂O₃, which can be considered a defective spinel (Valenzuela, 2012).