**1. Introduction**

Metal Oxides (MO) are a class of compounds that are as abundant in the nature than in the library of synthetic inorganic compounds. While the use of MO as bulk materials is widely applied, developing new class of materials based on MO, understanding their chemistry, tuning their characteristics, and developing novel potential engineering are still under investigation. MO nanoparticles (MO-NP) are MO with a diameter ranging between 10 and 100 nm with a high surface-to-volume ratio, which explains their advantageous features compared to similar materials of micro- or macro-size [1].

MO-NPs can contain either a single metallic specie (nanoparticle, NP) or two or more different metallic species (nanocomposite, NCP). When the metal species are separated and independent at the molecular level, the nanocomposites could refer to a simple mixture of oxides with several component phases. In some systems, nanocomposites consist in stoichiometric (or non-stoichiometric) replacement of a metal ion by another one in the first oxide crystallographic structure. Such substitution systems are referred as mixed MOs e.g. spinels [2] or perovskites [3, 4].

MO-NPs also include core-shell architectures where each particle has an inner core made of one type of MO, and the outer shell consisting in another MO. Just as single nanoparticles, composite nanoparticles exhibit size confinement properties (optical and electronic). Their composition and the atomic order of aggregates are pivotal factors for their specific features, which are tailored during their synthesis. MO-NPs have a high surface-to-volume ratio, which increases the reactivity. Dispersed in base fluid (BF), MO-NP enhances the native properties of the solution compared to the case in which no MO-NPs were added.

This chapter does not intend to list, in an exhaustive manner, the features of nanofluid. Rather, the authors aim to discuss, from both the chemistry and the engineering point of view, the key features of MO-NFs transferable to the carbon capture utilization and storage (CCUS). The chapter will cover the different synthesis methods of the NP and NCP, the formulation of NF as well as the application in respect of CCUS.
