Low-temperature, carbon-free reduction of iron oxide

Article abstract of DOI:10.1016/S0039-6028(02)01374-2

We report here that iron oxide can be reduced completely to elemental iron, through a carbon-free and low-temperature reaction. Nanoparticles of Fe₂O₃ are completely reduced, resulting in the formation of nanoparticles of Fe, on the surfaces of Si or Ge, at ～740 and ～440 °C respectively. We show that this phenomenon is due to the oxygen atoms changing the bonding partner from Fe to Si or Ge, followed by the desorption of the respective monoxides. Therefore, the reduction temperature is dictated by the desorption temperatures of SiO or GeO molecules. The nanoparticles thus formed are magnetic and are of uniform size and shape. On graphite surfaces, however, Fe₂O₃ retains the original stoichiometry even after annealing at higher temperatures.

Full text of DOI:10.1016/S0039-6028(02)01374-2

Surface Science 506 (2002) L250–L254
www.elsevier.com/locate/susc
Surface Science Letters
Low-temperature, carbon-free reduction of iron oxide
a,
K. Prabhakaran , K.V.P.M. Shafi , A. Ulman , P.M. Ajayan , Y. Homma,
b
b
c
*
a
T. Ogino
a
NTT Basic Research Laboratories, Device Physics Research Laboratory, 3-1 Morinosato Wakamiya Atsugi-shi,
2
43-0198 Kanagawa, Japan
Department of Chemical Engineering, Chemistry, and Materials Science, Polytechnic University, and The NSF MRSEC for Polymers
at Engineered Interfaces, Brooklyn NY 11201, USA
b
c
Materials Science and Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
Received 20 December 2001; accepted for publication 22 January 2002
Abstract
We report here that iron oxide can be reduced completely to elemental iron, through a carbon-free and low-tem-
perature reaction. Nanoparticles of Fe are completely reduced, resulting in the formation of nanoparticles of Fe, on
2 3
O
the surfaces of Si or Ge, at ꢀ740 and ꢀ440 °C respectively. We show that this phenomenon is due to the oxygen atoms
changing the bonding partner from Fe to Si or Ge, followed by the desorption of the respective monoxides. Therefore,
the reduction temperature is dictated by the desorption temperatures of SiO or GeO molecules. The nanoparticles thus
2 3
formed are magnetic and are of uniform size and shape. On graphite surfaces, however, Fe O retains the original
stoichiometry even after annealing at higher temperatures. Ó 2002 Elsevier Science B.V. All rights reserved.
Keywords: X-ray photoelectron spectroscopy; Surface chemical reaction; Oxidation; Germanium; Silicon; Iron oxide; Silicon oxides;
Clusters
The carbothermal reduction of iron ore (Fe
2
O
3
)
to the environment and cause global warming [1–
3]. It is therefore highly desired to have an alter-
native process for the production of iron, which is
green and less harmful to nature. We find that the
reduction temperatures of oxides can be down-
shifted significantly by employing surface reac-
into iron and the manufacturing of steel is one of
the most important industrial manufacturing
processes to date. Large amounts of power get
consumed in this process since the reduction of
iron oxide occurs at very high temperatures
(
ꢀ1600 °C). In addition, the process creates
tions. In particular, we have found that Fe O3
2
greenhouse gas (CO ), which could cause damage
2
nanoparticles can be reduced completely to nano-
particles of pure Fe, through a bonding partner
change reaction, which we call chemical bond
manipulation. This can be directed to occur by
proximal thermal treatment of the oxide particles
with Si and Ge surfaces. We believe that this re-
action, which has attractive features such as low
*
Corresponding author. Tel.: +81-462403474; fax: +81-
62404718.
E-mail address: prab@will.brl.ntt.co.jp (K. Prabhakaran).
4
0
039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S0039-6028(02)01374-2

K. Prabhakaran et al. / Surface Science 506 (2002) L250–L254
L251
thermal budget and carbon-free, can lead to the
formation of high purity Fe.
The Fe nanoparticles are synthesized [4] by
2 3
the initial samples constitute stoichiometric Fe O
species, but as the sample temperature is raised,
the Fe 2p signal shifts towards lower binding en-
ergies, indicating reduction of the particles through
a bonding partner change reaction. Oxygen chan-
ges the bonding partner from Fe to Si and subse-
quently desorb from the surface. Similar reaction
takes place on Ge wafer surface as well (see Fig. 1).
Insets to the Fig. 1 plot the changes in the Si 2p
and Ge 2p regions and clearly show the disap-
pearance of the oxide signals. Independent exper-
iments on the behavior of ultrathin oxides on Si
and Ge clearly indicated that the desorption of the
respective monoxides takes place at ꢀ740 and
ꢀ440 ° C. In other words, the complete reduction
of iron oxide occurs at these respective temp-
eratures on Si and Ge surfaces. This was unam-
biguously shown by photoelectron spectroscopy
employing synchrotron radiation [5]. Therefore,
2
O
3
the decomposition of the precursor FeðCOÞ by
5
2
highly intense sound waves (20 kHz, 100 W/cm ).
This method yields particles of nearly uniform size
and spherical shape. The particles are attached to
the Si wafer, by dispersion from an ethanol sus-
pension, using ultrasonic bath. The sample is loa-
ded into the ultrahigh vacuum (UHV) chamber and
annealed to various temperatures. The nature of
surface species was characterized in situ by pho-
toelectron spectroscopy. The samples were then
examined, out side the chamber, by a variety of
techniques. Furthermore, the reaction pathway was
examined in detail by employing synchrotron ra-
diation photoelectron spectroscopy (will be pub-
lished elsewhere).
In Fig. 1 we plot the XPS spectra in the Fe 2p
region, from the Fe
2
O
3
particles as a function of
2 3
the reduction temperature of Fe O particles is
annealing temperature. As clearly seen by the peak
position of Fe 2p as well as the spin–orbit splitting,
dictated by the desorption temperature of the cor-
responding monoxides. The oxygen atoms change
the bonding partner from Fe to Si or Ge to form
stronger bonds during the early stages of the an-
nealing (bond strengths of Si–O > Ge–O > Fe–O
are 799, 659 and 390 kJ/mol respectively). The
chemical identity of the nanoparticles formed on
the surface as a result of the reduction process was
further ascertained by mapping the Auger signals
(Fig. 2). Reactions occurring on the surface are
summarized as follows:
Fe O þ Si ! 2Fe þ 3SiO
2
3
Fe
2
O
3
þ Ge ! 2Fe þ 3GeO
Fig. 1. XPS measurements of various core levels (Si 2p, Ge 2p,
Fe 2p) from Si/Fe
2
O
3
and Ge/Fe
2
O
3
showing complete reduc-
Fig. 2. SEM micrograph of the Fe nanoparticles on dot pattern
and the corresponding mapping of Auger signals from (b) Fe
and (c) Si. Bright and dark contrasts indicate positive and
negative intensities.
tion at temperatures 760 and 440 °C respectively. The Si 2p and
Ge 2p levels show the absence of any oxygen related features at
the reduction temperature.

L252
K. Prabhakaran et al. / Surface Science 506 (2002) L250–L254
The Auger maps of the particles clearly show
that they consist almost entirely of elemental Fe,
suggesting that the Fe gets completely trans-
2
O
3
formed to Fe as shown by the white contrast in
the Fe AES map in Fig. 2. The corresponding
Si Auger displays negative intensity (dark con-
trast) indicating that mixing or silicide formation
is negligible. Similar observations are made on Ge
surfaces as well.
In order to verify the validity of such a reaction
occurring in bulk, we tested the reaction in the
solid state. Bulk Si or Ge in powder form was
2 3
mixed with Fe O powder and annealed in a fur-
nace at 760 and 550 °C respectively for 5–10 h and
then examined by XPS. The spectra show that the
reduction of the oxide takes place, resulting in the
formation of elemental Fe (Fig. 3). Since the re-
duced samples were exposed to air before mea-
surements were carried out, we can see a mixture
of the oxides and the reduced Fe species. X-ray
diffraction measurements substantiate these con-
clusions. These results imply that the reactivity is
enhanced considerably when the material is in the
nanostructured form. We therefore believe that the
findings reported here may have implications in
regard to the commercial synthesis of iron. It is to
be pointed out that a recent calculation [6] predicts
that a carbon-free process for the formation of Fe
2 3
Fig. 3. The observation of reduction of Fe O in the solid state.
(a) and (b) are Fe 2p XPS recorded from a mixture of powder
samples of Fe
2
O
3
/Si, before and after annealing (750 °C) in a
furnace. The measurements were performed several days after
the samples were annealed and therefore a mixture of oxides
and the reduced Fe species are observed.
Fig. 4. AFM image of (a) Ge wafer which was treated with Fe
2
O
3
particles and subsequently annealed in UHV and (b) after etching
the sample with HF, results in the retention of uniform sized Fe nanoparticles.

K. Prabhakaran et al. / Surface Science 506 (2002) L250–L254
L253
2 3
Fig. 5. (a) AFM image of a graphite sample which was treated with Fe O particles and annealed in UHV, showing the self-assembly
process and (b) shows that the Fe 2p core level does not exhibit any noticeable shift on annealing, indicating the absence of any
reduction reaction.
from iron ore can save power consumption by
about 30%.
on a graphite surface. The AFM and the XPS re-
sults are shown in Fig. 5. In the case of graphite
surfaces the particles retain their original stoichi-
ometry even after annealing, as the Fe 2p level
doesn’t exhibit any noticeable shift (see Fig. 5(c)).
The AFM images indicate that the particles
nucleate on step edges on the surface forming a
type of one-dimensional chains through the self-
assembly of the oxide particles (see Fig. 5 (a)). The
zoom up of the AFM image shown in Fig. 5(b)
show the self-assembly of the oxide particles.
To summarise, we describe an environment
friendly, low-temperature method for reducing
iron oxide through surface reactions. We have to
address issue of cost-effectiveness arising primarily
In Fig. 4(a) we show the AFM image of the Ge
sample which was treated with the particles
and subsequently annealed in UHV chamber. This
sample was etched by HF and shows the presence
of uniform sized Fe particles Fig. 4(b). These
magnetic semiconductors have been gaining tre-
mendous importance in the area of spintronics.
Controlled introduction of nanomagnetic particles
into semiconductors is the key point in this field,
for possible applications.
In order to compare the behavior of the nano-
particles described above with that on a less reac-
tive substrate, we performed similar experiments

L254
K. Prabhakaran et al. / Surface Science 506 (2002) L250–L254
[
2] J.T. Houghton, et al. (Eds.), Climate Change 1995: The
Science of Climate Change, Cambridge University Press,
Cambridge, 1996.
due to the low natural occurrence of Ge and
therefore the recovery of Si or Ge used up in the
reaction has to be considered.
[
[
[
3] P.D. Jones et al., Rev. Geophys. 37 (1993) 173.
4] K.V.P.M. Shafi et al., J. Phys. Chem. B 101 (1997) 6409.
5] K. Prabhakaran et al., Appl. Phys. Lett. 76 (2000)
2
244.
References
[6] Scenarios of US Carbon Reductions: Potential Impacts of
Energy Technologies by 2010 and Beyond (ORNL/CON-
444), Oak Ridge National Laboratory, Oak Ridge, TN,
1997.
[
1] A.H. Rosenfeld, T.M. Kaarsberg, J. Romm, Physics Today
November (2000) 29.