Hematite nanostructuring using electrohydrodynamic lithography

Highlights

• Electrohydrodynamic destabilization was achieved on a polymer/iron salt thin film.

• Two kinds of structures were obtained: submicrometric domes and nanometric droplets.

• The nanometric droplets mimic the structures of a master electrode.

• Hematite is obtained upon pyrolysis and the structure is conserved (≈35% shrinkage).

• Partial polymer protonation and micelle formation explain this low shrinkage.

Abstract

Tailoring hematite thin film nanostructure is particularly interesting since this oxide's function is closely related to its structure, for example when implemented as a photoanode in water splitting solar cells. In this study, electrohydrodynamic destabilization was designed to grow hematite nanodroplets with morphologies controlled by a master electrode. A polymer/iron salt film was destabilized by electrohydrodynamic destabilization and the resulting structures were pyrolysed to achieve crystalline α-Fe₂O₃ nanodroplets of 30 nm height and 70 nm radius. NEXAFS spectroscopy proved that the structures contain ferrihydrite, which is converted into hematite during pyrolysis, while the polymer was decomposed. Homogeneous nanoparticle precipitation in the bulk of the polymer, due to encapsulation of the iron precursor in the polymer matrix, is accounted for the good preservation of the structures. This study represents the first step towards the use of electrohydrodynamic destabilization for nanostructuring of hematite thin films, with a control over the feature size.

Introduction

Metal oxide thin films based on nanopillar array have been suggested as efficient photoanode systems for solar water splitting and hydrogen fuel generation in photo-electrochemical cells [1]. Iron oxides such as α-Fe₂O₃ (hematite) are low-cost, environmentally benign and abundant semiconductors for use as such photoanodes. But hematite has the major disadvantage that its optical thickness is large (118 nm at λ = 550 nm) [2] compared to its charge carrier recombination length (2–4 nm) [3]. To overcome this drawback, various approaches for micro- or nano-structuring of iron oxide into nanopillar, nanorod and nanotube arrays, for example by aqueous chemical growth [4] or anodization [5], [6] have been investigated.

Here we present a novel and simple method of obtaining iron oxide structures, taking advantage of the self-organization occurring in molten polymers subjected to an external electric field. This phenomenon is known as electrohydrodynamic destabilization (EHD) and has been previously used to tailor micrometric droplets or pillar arrays [7], [8]. The polymer structures obtained by Heier et al. [9], with spatially modulated electric fields, show that the sizes and aspect ratios obtained can be controlled using parameters such as the electric field strength and the lateral field modulation. The control provided by such heterogeneous electric fields has been exploited by Schaffer et al. [10] to achieve features with lateral dimensions down to 140 nm using a structured electrode. Moreover this electrolithographic phenomenon is a determining factor in the upscaling of the destabilized region [11]. The use of this technique to oxide films has been initiated by Voicu et al. [12]. They destabilized titanium alkoxide-alcohol solutions and obtained micrometric TiO₂ patterns. To the best of our knowledge no other attempt has been made to use EHD to structure inorganic materials. Our study focuses on the development of electrohydrodynamic destabilization for the high-fidelity conversion of polymer/iron salt structures obtained by EHD destabilization into hematite (α-Fe₂O₃) structures. The strategy adopted is to encapsulate an iron salt in a polymer thin film and then tailor the polymer structure using EHD. After structuring, the film is pyrolysed at 500 °C in an air vented furnace to decompose the polymer matrix, whilst at the same time converting the iron to hematite.