During past few years, rapid advancements in modern technology and growing interest in sustainable living have invoked immense interest in the development of multifunctional nanomaterials. Among the various categories of advanced nanomaterials that are currently being developed, layered materials have emerged out as notable crystalline solids that have gained substantial attention specifically in the field of materials science. Basically, Layered solids belongs to the class of two-dimensional (2-D) solids consisting of stacked layers owing to the bonds among the intraplanar atoms that are much stronger than the interaction among the adjacent planes [1]. The intraplanar atoms are held together by covalent bonds while VanderWaal attraction forces hold the adjacent layers in place, thereby contributing to the formation of the layered structure. The compositional layers may vary in thickness i.e. the layers may be one layer thick, or may be made of several atoms in thickness. Layered solids are classified based on the thickness of the individual layers as well as on the presence of fixed charges in the planar macromolecule [2]. Based on the parameter of layer thickness, layered materials can be categorized into Class I, Class II and Class III type. Class I layered solids are formed from atomic monolayers; typical examples for this class are graphite, that is homopolar and boron nitride, a binary layered material [3]. Layered dichalcogenides and FeOCl belongs to Class II layered solids that are formed from layers which are a few atoms thick. While class III consists of the constituent layers which are many atoms thick, for example, silicate clays and metal (IV) phosphates. Furthermore, depending on the presence of fixed charges in the planar macromolecule, these materials may be (i) cationic - that have negatively charged layers (eg. Metal phosphates and phospohonates, phyllosilicate-group containing clays), (ii) neutral that have no charge on layers (eg. Graphite, magnesium hydroxide) or (iii) anionic that have positive charge on layers (eg. Layered double hydroxides (LDHs), layered hydroxy salts (LHSs)) that is offset by the presence of interlayer anions to restore charge neutrality [4]. The creation of charges on the layers may be a consequence of chemical reduction processes, or the substitution of cationic and/or anionic species in the lattice. A variety of network solids such as clays, LDHs, metal hydroxides (M(OH)2) and hydroxysalt, zirconium phosphate, transition metal chalcogenides, layered perovskites, graphite and metal phosphonate belong to this class of materials that have been widely explored for diverse applications [5]. For instance, graphite is a neutral layered solid with a 2-D array of hexagonally packed carbon atoms stacking together to form 3-D structures that find utility in electrode fabrication, energy conversion, foundries, and lubricants [[6], [7], [8]]. Similarly, smectite clays are examples of cationic layered solids that are widely used in paint and insecticide, nuclear, foundries, and civil engineering industries [9,10]. While anionic clays such as, α-hydroxides, hydroxy double salts (HDS), LDHs, and LHSs holds immense role in different applications, i.e., ceramics, catalyst, water purification, biomedical, sensing, supercapacitors, etc. [11].

The study of anionic layered solids in particular is of much interest due to their exemplary properties that are a result of the anisotropy in bonding. These structural features lend a multitude of properties such as anion exchange, intercalation, ability to swell in solvents, and formation of monolayer colloidal dispersions. Furthermore, the structural arrangement of layered materials open up the potential for storage of unstable biomolecules by intercalation between layers with novel strategies for their use in supercapacitors and batteries, water remediation, catalysis, sensing, and biomedical applications [8,[12], [13], [14], [15]].

The structure of a typical layered solid have been shown schematically in Fig. 1. Herein, an individual 2-D unit is referred to as ‘lamella’ or layer. The bary-centres of two layers present next to each other are separated by some degree, and this gap is known as interlayer or interlamellar distance or basal spacing and is denoted as ‘d’. The free distance between the adjacent layers, called the gallery height, can be obtained by the subtraction of thickness of the layer from interlayer distance [16]. Due to these unique structural attributes of anionic clays, they have become the focal points of extensive research in the last few years. Majority of the review focus on LDHS, which have been extensively studied, but literature on other types of anionic layered materials (ALMs), i.e., LHSs, HDSs, and α-hydroxides remain scarce. Additionally, a detailed study on the wide range of advanced synthetic methodologies, structural features of ALMs, and their interplay with functional properties has not been widely discussed. However, even though the traditional methods that are generally utilized for ALM synthesis have been widely reported and discussed in several reviews, emerging methodologies—such as template-assisted synthesis, mechanochemical routes, and other innovative techniques needs to be explained, thus creating significant gap in the current literature [17].

To address existing knowledge gaps, this review systemically summarizes the diverse aspects of ALMs, with the primary focus on their assembly chemistry, structures, and key properties. A brief discussion distinguishing layered salts from double salts has also been incorporated to outline the fundamental differences between these two groups of materials. The review further examines the various strategies adopted for the synthesis of ALMs, highlighting advanced methodologies employed to engineer precise morphological and compositional features for enhanced multifunctional applications. Functional properties that govern the applicability of ALMs in various fields have also been analyzed in detail. The advanced characterization techniques employed to examine the nuanced structural and compositional attributes of ALMs with advanced application in several critical domains has also been discussed. Through an integrative and critical evaluation of contemporary research, this review aims to provide a robust conceptual framework and guidance for future research innovation in design and applications of ALMs.

Double salts are crystalline materials that consist of multiple cations/anions following the mixing of two different metal salts in fixed proportions. Due to this, the crystal structure of resultant salt is different than those of both the constituent salts, and they exhibit the properties of their individual constituent salts in solid state. The most important property of double salts is they generally contain water of hydration, and double salts break down completely into their compositional ions on their dissolution in water. Several types of double salts have been reported in literature,that include alums having molecular formula MIMIII[SO4]2•6H2O, MI = K+, NH4+and MIII = Al3+, Cr3+) or Tutton salts having molecular formula [MI]2MII[SO4]2•6H2O, MI = K+, Rb+, Cs+, NH4+ and MII = Zn2+, Cr2+, Ni2+, Co2+) [18]. While in case of ALMs, they are anionic clay-type materials wherein the layered arrangement consists of uniformly distributed cations with charge compensating interlayer anions and water molecules in the galleries. They are 2-D nanomaterials that share structural similarities with brucite mineral. Layered double salts encompass a wide variety of nanostructures, including LDHs (Layered Double Hydroxides), LMHs (Layered Metal Hydroxides), HDSs (Hydroxy Double Salts), etc. [19,20]. They possess unique physiochemical properties i.e. low toxicity, biocompatibility, high specific surface area with unique heat and chemical resistance. Additionally, LDHs, a subclass of layered double salts, also possess the unique memory effect that enable them to regain their original structure on being calcined and subsequently being dispersed in solution of desired anion. Due to the multitude of desirable properties and advantageous features, layered double salts find applications in various fields such as water remediation, biomedical imaging, catalysis, tissue engineering, drug delivery systems, flame retardants, sensing, catalysis, etc.