Synthesis of graphene oxide-intercalated α-hydroxides by metathesis and their decomposition to graphene/metal oxide composites

Abstract

Graphene oxide-intercalated α-metal hydroxides were prepared using layers from the delaminated colloidal dispersions of cetyltrimethylammonium-intercalated graphene oxide and dodecylsulfate-intercalated α-hydroxide of nickel/cobalt as precursors. The reaction of the two dispersions leads to de-intercalation of the interlayer ions from both the layered solids and the intercalation of the negatively charged graphene oxide sheets between the positively charged layers of the α-hydroxide. Thermal decomposition of the intercalated solids yields graphene/nanocrystalline metal oxide composites. Electron microscopy analysis of the composites indicates that the nanoparticles are intercalated between graphene layers.

Introduction

Layered composites in which the layers from two different layered solids are stacked alternatively are important as they would exhibit unique physiochemical properties such as 2-D magnetism and conductivity due to the combined effect of the properties of the structural unit layers. Metastable mixed layered composites have been prepared through soft chemical routes. Layered composites, such as clay/metal hydroxide [1], [2], [3], MoS₂/Ni(OH)₂ or Co(OH)₂ [4], graphite oxide/birnessite [5], have been prepared by intercalation of M²⁺ ions in the interlayer of the host followed by titrating the product with alkali. Birnessite/NiOH₂ [6], birnessite/LiAl₂(OH)₆ [7], clay/LiAl₂(OH)₆ [8] and clay/SnO₂ [9] have been prepared by intercalation followed by hydrothermal treatment. Graphite/NiOH₂ has been prepared through intercalation of Ni²⁺ ions in graphite and subjecting the product to electrochemical cycling in the presence of alkali [10]. The synthesis approaches to such composites are restricted to a few select systems and are not general in nature.

Delamination of layered solids yields solvated monolayers that could be used as synthons in the preparation of layered composites. We have shown that interstratified composites of similarly charged layers of layered solids having related structures [11], [12], [13] and of layered solids having different structures [14] can be prepared starting from the monolayer colloidal dispersions of layered solids. We reported the preparation of 1-D solid solutions of α-hydroxides of Ni and Co [11], two different layered double hydroxides (LDHs), namely Mg–Al and Co–Al LDHs [12], two different cationic clays – a dioctahedral clay and a trioctahedral clay [13] and graphite oxide (GO)/smectite [14] through the delamination–costacking approach.

It is also possible to make layer-by-layer composites of two oppositely charged layered solids through delamination [15], [16]. We have recently shown that positively charged GO layers could be introduced into the interlayer of exfoliated anionic clay by ion-exchange [17]. In this work, we describe a metathesis route to layer-by-layer composites. If we denote the anionic and cationic layered solids as L⁺–A⁻ and L⁻–C⁺ where L⁺ and L⁻ are the layers of the anionic and cationic layered solids and A⁻ and C⁺ are the interlayer anions and cations then we can visualize a reaction of the type

$${\mathrm{L}}^{+}-{\mathrm{A}}^{-}+{\mathrm{L}}^{-}-{\mathrm{C}}^{+}\to {\mathrm{L}}^{+}-{\mathrm{L}}^{-}+\text{AC}$$

if A⁻ and C⁺ are surfactant ions then the reaction may be carried out in an organic solvent in which both the layered solids would delaminate. The product AC being highly soluble in the organic solvent, the reaction will be driven forward to yield L⁺–L⁻, the layer-by-layer composite.

Graphene (G) based composites are important as they are potent materials for applications in varied fields such as nanoelectronics, sensors, batteries, supercapacitors, hydrogen storage, catalysts, catalyst supports, adsorbents and magnetic materials [18]. Graphite/polymer [19], graphite/nanoparticle composites [20], supercapacitor and battery electrode materials [21], [22] have been fabricated starting from GO as it is dispersible in water and alkaline medium [23] and in organic solvents after modification with amines [24], [25], [26].

α-Hydroxides of nickel and cobalt are important electrode materials in alkaline secondary batteries [27], [28]. In addition, these solids decompose at relatively low temperatures (<300 °C) to give the corresponding oxides. The hydroxides of nickel and cobalt crystallize in different polymorphic modifications, mainly the β- and the α-form [29], [30]. The β-form is of the formula M(OH)₂ and it is an ordered stacking of neutral layers of the composition [M(OH)₂] with an interlayer spacing of 4.6 Å. The α-form is a hydroxyl-deficient compound and consists of a stacking of positively charged layers of composition $[\text{M}(\text{OH}{)}_{2-\text{x}}({\text{H}}_2\text{O}{)}_{\text{x}}{]}^{\text{x}+}$, which intercalate anions such as ${\text{NO}}_3^{-}$, Cl⁻, OAc⁻, ${\text{SO}}_4^{2-}$, etc. along with water molecules in the interlayer region to restore charge neutrality. Consequently, the α-hydroxides have a higher interlayer spacing, which varies with the size of the interlayer anion. On intercalation of surfactant anions, dodecylsulfate (DS) and dodecylbenzene sulfonate (DBS), these solids delaminate to give a colloidal dispersion of layers in organic solvents such as 1-butanol [31]. It would be interesting to prepare layer-by-layer composites of α-hydroxides and GO. Thermal decomposition of these composites would reduce GO to G and convert hydroxides to oxides resulting in G-metal oxide composites. In this paper, we report the metathesis between alkylammonium ion-intercalated GO and DS-intercalated α-hydroxide of nickel/cobalt that yields GO/α-hydroxide layer-by-layer composites (GO sheets intercalated α-hydroxides). These composites on heating in different atmospheres yield interesting G/inorganic material composites.