# Designing Dual-Function Nanostructures for Water Purification in Sunlight
## 1 Introduction  
It is known that only about 1% of water is available for human consumption. However, water is one of the most abundant natural resources in the world. The main problem of freshwater resources is a continuous contamination by inorganic and organic pollutants. The presence of heavy metals, cations, anions and dyes in water creates serious human health and environmental issues due to potentially rise toxicity. The existing technologies for treating the wastewater have many drawbacks such as generation of toxic sludge and incomplete pollutant removal in addition to high energy requirement. The most familiar processes for water purification are adsorption method and photocatalytic degradation technique. Although some researchers focused on removal of organic pollutants using adsorption techniques, the majority of adsorbents do not have optical activity. In the other side, many researchers concentrated on photocatalytic degradation of organic pollutants. However, the majority of the optical materials are effective for photocatalytic reactions in UV light which makes high running costs. In order to increase the efficiency of water purification from industrial pollutants and avoid the drawbacks of the existing technologies, it can be achieved by combining building blocks together into a well-designed nanostructure to be effective as photocatalyst in sunlight and adsorbent. The effective photocatalysts in sunlight can reduce the high-energy requirement for water purification and completely convert theorganic pollutants to carbon dioxide and water. In addition, the active adsorbents can easily remove the inorganic and organic pollutants and accelerate the photocatalytic reactions. Amazing and unexpected properties can be produced by the combination of various building blocks. Among several building blocks, layered double hydroxides (LDHs) have been widely studied. Recently, in our laboratory, LDHs have used as building blocks for producing nanofibers structures [1]. LDHs have nanolayered structures basing on brucite-like sheets. These sheets possess positive charges that were produced from presence of tri- and di-valent metals in the same layer. By intercalation of anions to neutralize the positive charges, nanolayered structures of LDHs are produced through two-dimensional (2D) lamellar nanostructure. The general formula of LDHs can be represented as [M\({}^{2+}\)1-xM\({}^{3+}\)x(OH)2](A\({}^{\text{n}}\)-)\({}_{\text{v/n}}\cdot\)mH\({}_{2}\)O, where M\({}^{2+}\) and M\({}^{3+}\) are di- and tri-valent cations; respectively. A\({}^{\text{n}-}\) is an interlayer anion. Among the most studied advanced functional structures, LDHs have been used in the fields of environmental protection, catalysis, materials science, energy and biology in recent years [2; 3; 4]. In addition, graphene considers an ideal kind of building block because it is well known as one of the thinnest materials in the world [5]. It possesses 2D carbon nanostructure. Graphene has many applications as polymer composites [6], biosensors [7], liquid crystal devices [8], supercapacitors [9], nanoelectronics [10], drug delivery systems [11] and energy storage materials [12]. According to the above considerations, graphene and LDHs are attractive building blocks for producing effective materials for water purification through the dual process of both adsorption and photocatalytic degradation. However, the efficiency of LDHs is widely restricted because of their low electrical conductivity and aggregation behavior. In addition, there is a big problem that limits the applications of graphene. In addition to the chemical inertness of graphene [13], its sheets tend to agglomerate because of van der Waals interactions. It means that restacking process occurs for graphene sheets to form graphite if they are not well separated from each other [11]. By combining the sheets of both graphene and LDHs, the chemical reactivity of LDHs can integrate with the chemical inertness of graphene [14].