Deterministic Surface Growth of Single-Crystalline Iron Oxide Nanostructures in Nonequilibrium Plasma Urosˇ Cvelbar*,† and Kostya (Ken) Ostrikov†,‡,⊥ Jozˇef Stefan Institute, JamoVa cesta 39, Ljubljana SI1000, SloVenia, EU, and CSIRO Materials Science and Engineering, P.O. Box 218, Lindfield NSW2070, Australia
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 12 4347–4349
ReceiVed July 29, 2008; ReVised Manuscript ReceiVed October 8, 2008
ABSTRACT: An innovative approach to precise tailoring of surface density, shapes, and sizes of single-crystalline R-Fe2O3 nanowires and nanobelts by controlling interactions of reactive oxygen plasma-generated species with the Fe surface is proposed. This strongly nonequilibrium, rapid, almost incubation-free, high-rate growth directly from the solid-solid interface can also be applied to other oxide materials and is based on deterministic control of the density of oxygen species and the surface conditions, which determine the nanostructure nucleation and growth. Presently, the search for the most effective chemical synthesis routes of multipurpose, low-dimensional nanomaterials such as high-aspect-ratio nanowires is rapidly gaining momentum.1-4 These efforts aim at finding the most appropriate process environments and controls to effectively achieve the desired density, shape, structure, morphology, and other attributes of nanowires required for specific applications.5,6 Iron oxide (e.g., R-Fe2O3) nanowires are particularly interesting owing to their widespread applications in catalysis, water splitting, detection sensors, dye solar cells, magnetic storage media, controlled drug delivery, detection of elusive infectious agents such as prions, and several others.7-9 The synthesis of these nanowires can be successfully implemented via various chemical routes such as hydrothermal,10-12 sol-gel mediated,13-15 solvothermal,16 gas decomposition,17,18 thermal oxidation,19-21 chemical vapor deposition (CVD)22 or plasmaassisted synthesis.23,24 However, most of the processes feature complex synthesis conditions that are very difficult to control in order to obtain tailored nanowires. In vapor-based routes such as thermal CVD, it is particularly difficult to enable localized oxidation of iron and control nucleation sites on the surface, which often leads to bulk films without any nanoscale features. Here we show that these common problems can be overcome through direct oxidation of Fe atoms on the surface of a thin iron foil exposed to high-density fluxes of O atoms created in reactive O2 plasmas. In this process, no metal vapor is supplied from the gas phase and oxygen atoms serve both as building units (BUs) to form metal oxide nanostructures and as reactive radicals that provide localized surface heating upon recombination. It is shown that the nanowire synthesis can be effectively controlled by the interaction of plasma-generated oxygen species with the metal surface. Moreover, the effect of the O2 plasma parameters on the synthesis of R-Fe2O3 nanowires is quantified for the first time. In addition to specifying the macroscopic synthesis control parameters, which is a common practice nowadays, here we precisely measure and calculate a range of microscopic plasma parameters such as the exact numbers of oxygen atoms supplied for chemical reactions on the surface, as well as the densities and fluxes of electrons or ions, temperature of electrons, kinetic energy of ions, and the neutral gas temperature. Knowledge of these parameters is indispensable to understand and control the nanowire growth kinetics, which strongly depends on the demand and actual supply of plasmagenerated species to the surface, surface conditions, and the oxidation rates of the surface material. Under conditions of our * To whom correspondence should be addressed. E-mail:
[email protected]. † Jozˇef Stefan Institute. ‡ CSIRO Materials Science and Engineering. ⊥ Also affiliated with University of Sydney, Sydney NSW 2006, Australia.
experiments, these reactions enable highly unusual epitaxial growth at the metal-oxide interface. Figure 1 depicts three scanning electron microscopy (SEM) images of single-crystalline R-Fe2O3 nanowires (NWs) and nanobelts (NBs) synthesized in low-pressure inductively coupled radiofrequency oxygen plasmas. These images represent three typical stages of growth and are related to the balance of BUs: (i) poor NW growth with insufficient supply of O atoms (Figure 1a); (ii) optimum NW growth with a balanced supply and demand of O atoms (Figure 1b); and (iii) oversupply of BUs where NWs grow into NBs (Figure 1c). All the nanostructures are made predominantly of single-crystalline R-Fe2O3 and grow along the [110] crystallographic direction. Moreover, oxygen vacancy ordering appears in all nanostructures in every fourth plane of (11j2) or on every 10th of (33j0) plane with an ordering distance of 1.45 nm.25 The ordering planes are also aligned with the growth direction. The probability of the nanowires forming a bundle-like morphology is rather rare; this occurs under certain process parameters and in particular when the nanowires nucleate close enough (approximately
