Morphological evolution of prussian yellow Fe[Fe(CN)₆] colloidal nanospheres

Abstract

A simple hydrothermal system was developed for controllable morphologies of the Prussian yellow Fe[Fe(CN)₆] nanostructures in the presence of organic additives. Hollow and solid nanospheres of the Prussian yellow materials were successfully synthesized with suitable experimental conditions. It is found that the amounts of organic additives CTAB could result in the formation of the spherical nanocrystals and the hydrolysis of phosphate in the solution could play a role in the final morphology of the products. A possible formation mechanism of the Prussian yellow nanostructures is proposed.

Introduction

Recently, many scientists have explored the correlation between physical or chemical properties and morphology of inorganic nanomaterials driven by the excitement of understanding new science and of potential hope for applications in daily life devices [1], [2], [3], [4]. Precise control of the morphologies of inorganic materials is often manipulated with solution self-assembly using electrodeposition method [5], sol-gel method [6], hydrothermal synthesis [7] and so on. Among these methods, hydrothermal process bears the advantage that it can initiate the nucleation in the growth of micro- and nano-crystals, and promote the formation of crystalline products to completion under non-equilibrium conditions [8], which might assist the growth of inorganic materials with special morphologies. In recent years, inorganic salt-assisted hydrothermal method that is convenient and effective, could be helpful for controlling the shape, surface nature, and structure of nanostructures [9]. In general, it is still a great challenge to develop solution self-assembly route for the controllable synthesis of various inorganic nanomaterials with tailored morphology.

Prussian blue (PB), the first synthetic coordination compound, is a mixed-valence iron-(III) hexacyanoferrate(II) compound of composition Fe₄[Fe(CN)₆]₃·XH₂O with a face-centered-cubic structure, in which Fe³⁺ in the N-coordinated sites is in the high-spin state and Fe²⁺ in the C-coordinated sites is in the low-spin state [10]. Prussian blue and its analogs with different morphologies such as spheres [11], cubes [12], [13], polyhedrons [14], wires [15] and tubes [16] have attracted intensive research attention due to their unique properties when applied in molecular magnets [17], [18], electrochemistry [19], optics [20], hydrogen storage [21], [22] and biosensors [23]. Prussian yellow FeIII[FeIII(CN)₆] (PY), as the chemical oxidation of PB, displays wonderful properties in electrocatalysis and electrochromism [24]. The synthesis of PY materials needs two steps [25]: PB materials are firstly prepared and then obtained PB materials are oxidized to form PY materials. However, this synthesizing method is too complicated or unfit for the achievement of the PY nanomaterials, which is crucial for opening up possibilities for particular functional devices. Inorganic colloidal nanospheres, especially hollow nanospheres, have attracted growing attention [26] because of their potential applications in the controlled release, catalysis and drug delivery [27]. Therefore, it is of great scientific interest and technological significance for the synthesis of the PY colloidal nanospheres that may extend the application in construction of functional device.

Herein, we report the controllable synthesis of Prussian yellow Fe[Fe(CN)₆] nanostructures with the hydrothermal method in the presence of organic additives. The colloidal nanospheres (hollow and solid nanospheres) of Prussian yellow nanomaterials were successfully synthesized. The spherical morphology of the PY nanocrystals is related to the amounts of organic additives CTAB. The effects of phosphate on the morphology of the PY nanomaterials have been realized. Finally, we proposed a possible formation mechanism for the Prussian yellow nanomaterials. These results should give a useful enlightenment for the development of other novel inorganic nanomaterials.