**1. Introduction**

Advanced nanostructured materials may be defined as materials having one dimension in the 1 to 100 nm range. The massive academic and industrial research efforts concerning these materials over the past decade arose from the remarkable variations in their physical and chemical properties when their dimension shrinks to the nanometric scale. In this category of materials, the interest for hexagonal-boron nitride (*h*-BN, but expressed here as BN) grew during the past few decades in relation to their unique combination of key properties.

BN is a synthetic chemical compound containing boron (B) and nitrogen (N) atoms in a one-to-one ratio. The in-plane atoms are linked through covalent bonds, while the out-of-plane layers are bonded by weak interactions (van der Waals forces) between B and N atoms, alternatively, providing anisotropic properties. BN displays a large band gap (~5.5 eV) and offers the lowest density (*d* = 2.26 g·cm<sup>í</sup><sup>3</sup> ) among non-oxide ceramics. It proposes relatively good thermal stability in air and vacuum, high thermal conductivity, good thermal shock resistance, high electrical resistance, a low dielectric constant and loss tangent, microwave transparency, non-toxicity and easy machinability. Furthermore, it is non-abrasive, lubricating and non-reactive towards molten metals [1–6].

BN was obtained for the first time by Balmain [7] in 1842 through the reaction between boric oxide and potassium cyanide. It is nowadays produced by conventional powder technology, requiring nitridation or carbothermal reaction of boric acid/boric oxide with melamine or urea and the use of additives during the further sintering process [8]. It is used in various fields of chemistry, metallurgy, high temperature technology, electronics and in thermal management applications. However, beside the fact that the use of boric oxide inherently induces the presence of oxygen-containing phases, BN is only produced as powders with a plate-like morphology and workpieces. This inherently limits the development of BN.

Recently, interest at the academic level has arisen in both the synthesis of nanostructured BN and their applications for energy and the environment [9–13]. The important industrial challenges in line with nanostructured BN production requires the development of materials in which topologies, shapes and morphologies are tuned on demand. Inherent difficulties of traditional techniques to manufacture such materials can be addressed by the development of synthetic pathways where molecular/inorganic chemistry, processing and material chemistry/science are combined rationally to process BN with tailor-made properties [14]. The key step in nanostructured BN preparation is the selection of the BN precursors. Precursors with a good B:N ratio where/for which hydrogen (H) is the only element added to B and N are preferred. Borazine and derived polyborazylene are the most appropriate candidates [15,16]. Within this context, in this review article, we discuss the use of borazine (BZ) as a single-source molecular precursor used for the design of BN nanoparticles (NPs), hollow-cored BN-NPs that are strongly facetted and hollow core-mesoporous shell NPs. The latter have been used as host materials to encapsulate and store ammonia borane (AB).
