Hematite nanostructuring using electrohydrodynamic lithography
Graphical abstract
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.
Section snippets
Materials and methods
The thin film for destabilization was spin-coated from an aqueous solution on a silicon wafer. This solution was prepared by mixing 0.5 M in pyridyl groups of poly(4-vinylpyridine) (P4VP) (Mw ≈ 60,000, Sigma–Aldrich) and 0.5 M of FeCl3 (reagent grade, 97% anhydrous, Sigma–Aldrich) in double deionized water, leading to a 1:1 molar ratio between pyridyl groups and aqueous iron chloride. The resulting aqueous solution is acidic with a pH of 1.5. The spin coating was realized using a Primus STT15
Results
The destabilization of the P4VP/FeCl3 thin film results in the formation of two different kinds of features: oblate droplets with sub-micrometer size, and droplets with nanometer size (Fig. 2a). Using complementary Matlab and Fiji procedures (see Supporting Information/Matlab-Fiji code for image processing) the nanometric distribution has been mathematically separated from the submicrometric distribution by using a high pass filter (Fig. 2, Fig. 3). The size distribution of the nanometric
Conclusions
This study shows that it is possible to destabilize thin polymer films loaded with an iron containing precursor. Self-organized nanostructures may be grown by means of a master nanostructured master electrode. After the electric field assisted self-organization, the film can be pyrolised, allowing the simultaneous combustion of the polymer and conversion of the iron into hematite. This chemical conversion is achieved with conservation of the nanostructure. These results are promising and
Acknowledgements
We acknowledge the financial support of the Swiss National Science Foundation for the project SNF 200021-137868 – Reaction–diffusion processes for the growth of patterned structures and architectures: a bottom-up approach for photoelectrochemical electrodes. R.T. is grateful for financial support from the Marie Heim-Vögtlin Program under project number PMPDP2-139698.
The Helmholtz Zentrum Berlin is acknowledged for granting synchrotron beamtime at BESSY II under application number 2012-1-111500.
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