Introductory Chapter: Cerium Oxide - Applications and Attributes

1. Introduction

Cerium belongs to lanthanide series and available most abundantly in the crust of the earth with an average concentration of 50 ppm as a rare earth element. Elemental cerium is a flexible and malleable lustrous metal. Cerium metal is iron-gray in color and is highly reactive. It is also known as a strong oxidizing agent and exists as cerium oxide in association with oxygen atoms. It exists as either cerous (Ce³⁺, trivalent state) or ceric (Ce⁴⁺, tetravalent state) in the form of compounds [1].

It is clear from the title that this book is related to cerium oxide (CeO₂) which is one of the important transition metal oxides acting as n-type semiconductor materials. It possesses several features resulted from the combination of high amount of oxygen in its structure and the facile change between the reduced and oxidized states (Ce³⁺ and Ce⁴⁺) [2]. The CeO₂ has cubic fluorite structure, in which each cerium atom is surrounded by eight equivalent oxygen atoms and each oxygen atom is surrounded by a tetrahedron of four cerium atoms. Ideally, CeO₂ should have a formal charge of −2 and distance between oxygen–oxygen atoms should be 2.705 Å, in which the formal charge of cerium ions is +4 [3].

The main unique characteristics of cerium oxide involve a band gap of 3–3.6 eV, high value of dielectric constant up to κ = 23–26, high refractive index of n: 2.2–2.8, and high dielectric strength reached to 2.6 MV cm⁻¹ [4]. Such properties qualify cerium oxide-based materials to be employed in various applications, especially when they are in nanosized particles. The cerium oxide is a famous member of nanostructured materials having a wide range of applications. Cerium oxide materials/nanomaterials have been utilized in numerous fields including adsorption, catalysis, photocatalysis, sensing, fuel cells, hydrogen production, semiconductor devices as well as biomedical uses [5, 6, 7, 8, 9, 10].

Commercial uses of CeO₂ could be utilized in the pure form or in a concentrated dose as a polishing powder for glasses as well as ophthalmic lenses or precision optics. Cerium oxide is also employed as a glass constituent for preventing solarization and discoloration, particularly in television screens. The CeO₂ contributes in heat-resistant alloy and ceramic coatings. Cerium oxide is also used in petroleum refining and emission controlling system in gasoline engines as well as a diesel fuel-borne catalyst to reduce particulate matter emissions. In recent years, CeO₂ nanoparticles have gained more consideration in biomedical research community since they could be used as inhibiting cellular agent along with their antimicrobial and antioxidant activity [1, 5].

Owing to the dramatical and widespread industrial uses of cerium oxide materials, the National Institute of Environmental Health Sciences is suggested and nominated CeO₂ for toxicological characterization because of its limited toxicity data, and a lack of toxicological studies for nanoscale CeO₂. CeO₂, which is one of important transition metal oxides, acts as n-type semiconductor materials that have diverse applications such as adsorption, catalysis, photocatalysis, sensing, fuel cells, hydrogen production, semiconductor devices as well as biomedical uses.