Photo-Chemical Synthesis of Iron Oxide Nanowires Induced by Pulsed
Laser Ablation of Iron Powder in Liquid Media
Abstract
Iron oxide nanowires are synthesized by photochemical method through nanosecond pulsed laser ablation of iron
powder in liquid media. The synthesis is tried using various wavelengths (248 and 532 nm) of pulsed laser at different
liquids (water, ethanol, methanol, isopropanol and glycol). The solution of iron oxide nanowires is obtained only in
methanol when the iron powder (size < 60 μm) is irradiated at 248 nm. Ablation with 532 nm in methanol resulted in
solutions of iron oxide nanoparticles. It is very interesting to note that irradiating solutions of the nanoparticles,
obtained via 532 nm ablation, with laser pulses at 248 nm laser gives rise to the solution of iron oxide nanowires
again. It is to be recorded that only nanoparticles are obtained in water, ethanol, isopropanol and glycol when ablated
with any of the said two lasers. Chemical and structural characterization suggest that the as-synthesized products are
mainly amorphous goethite [FeO(OH)]. Annealing above 400°C, converts it into crystalline hematite (α-Fe2O3). The
formation mechanism of the nanowires from extended 2D nanosheets is discussed.
I. Introduction
Nanosized materials exhibit different chemical and physical properties compared to the bulk form [1]. Particle surface
plays an important role in the magnetic performance of the material in the nanometric scale. It has become evident
that the dependence of magnetic properties on size and dimensionality is among the most magnificent size effects [1].
Iron oxides are widely used in the fields of semiconductor, recording materials, photo-catalyst etc [2–4]. Goethite [αFeO(OH)] is a common iron oxide and ingredient of iron rust. It is generally yellowish in colour and well known for its
use as a pigment. It has an open channel structure which is accountable for its extraordinary capability to incorporate
and fix ions from migrating solutions [5, 6]. Hematite (α-Fe2O3) is one of the most accepted phases for research due
to its novel chemical and physical properties. It has been demonstrated as photoanode for photoassisted electrolysis of
water, active component of gas sensors, photocatalyst as well as an ordinary catalyst [7–10]. Conversion of goethite
to hematite has many technological applications [11].
Hematite nanowires are synthesized by different techniques like surface oxidation, thermal oxidation, electrochemical
deposition etc [12–16]. Most of these are complicated processes taking a long time (1.5–120 h), needing different gas
flow and high temperature (400–800°C). Therefore, there is a necessity for fabrication of nanowires of these
important oxides in the least complicated procedure with minimum time. In this direction, we have prepared the
goethite/hematite nanowires with a very simple technique by laser ablation with the shortest time reported so far. A
comparison of different preparation techniques is reported earlier [17]. The present article testifies the photo-chemical
synthesis of iron oxide nanowires induced by pulsed laser ablation of iron powder in liquid media and their chemical as
well as structural characterization.
II. Experimental Techniques
Iron oxide nanowires are synthesized by ultraviolet (UV) photochemical technique using the laser pulses from a
Lambda-Physik LPX 210i excimer laser operating at 248 nm with pulse duration of 25 ns and Nd-YAG laser operating
at wavelength 532 nm with pulse duration 15 ns [17, 18]. Iron (Fe) powder from Goodfellow Cambridge Ltd. with
purity more that 99% and maximum particle size of 60 μm is used as the starting material. It is placed in an open
glass vessel mounted on an X-Y translation motorised stage. Sufficient powder is taken to make a thin layer ( 5 mm)
on the bottom of the vessel. Then it is filled with different liquids (water, ethanol, methanol, isoproponal and glycol)
up to a height of
2 cm. 120 mJ laser pulses are focused to produce a rectangular spot and fluence of 11 J/cm2. The
laser spot is raster scanned over the whole bottom area of the vessel once every 15 s. A laser shot repetition rate of
20 Hz is used throughout the experiments. Iron oxide nanowires are obtained only with methanol (AR grade, Fisher
Scientific UK Ltd) and laser of wavelength 248 nm. However, the other liquids result in solution of nanoparticles with
both the lasers. The possible reactions of iron with methanol are suggested below.
and finally
The starting methanol is clear and colourless. At the beginning, the iron oxide nanobelts are formed when the laser of
248 nm is scanned for 5 minutes and the colour becomes cloudy yellowish. If it is collected and annealed with this
laser for another 10 minutes, it becomes clear yellow. It should be mentioned that if the irradiation is continued to 15
minutes instead of 5 minutes, we get the same yellow colour solution. The yellow solution is dried on glass substrate
as well as silicon wafer and annealed at a temperature of 100–800°C. The structure of the iron oxide
nanowires/nanobelts and nanoparticles is investigated by FEI Quanta 200 environmental scanning electron microscope
(ESEM) and Philips CM200 high-resolution transmission electron microscope (HRTEM). The HRTEM is operated at 200
kV with a LaB6 filament and has a line resolution of 0.14 nm. The type of oxide is confirmed from the Raman
spectroscopy of the samples taken by Reinshaw Ramanscope. It is further verified by the x-ray photoemission
spectroscopy (XPS) data taken by an Omicron Multiprobe ultra high vacuum (UHV) system equipped with an Omicron
EA125 hemispherical analyzer. XPS spectra are acquired with a pass energy of 50 eV for survey and 20 eV for higher
resolution respectively, using Mg Kα radiation (hν = 1256.4 eV) from a VG XR3E2 twin anode source. The base
pressure is maintained at
10−9 mbar for the entire set of experiments. To compensate for possible charging effects,
binding energies are normalized with respect to the position of Au substrate 4f 7/2 line at 84 eV. Typically an energy
step of 0.1eV is used for a dwell time of 1s and 50 scans are accumulated for each spectrum. Fitting and
deconvolution of the spectra are performed by CASA XPS software using a Gaussian-Lorenzian line-shape and a
Shirley type background.
III. Results and Discussion
Electronic Microscopic (SEM and TEM) Studies
The starting iron powder demonstrates granules of size less than 60 μm [17]. When these are rusted in ethanol,
methanol, water, isopronol and glycol with pulsed laser of wavelength 532 nm, we obtain iron oxide nanoparticles.
These are also attained in all liquids except methanol when ablated with 248 nm laser. Scanning electron microscopic
(SEM) image of the nanoparticles obtained in water with 248 nm laser with frequency 20 Hz and ablation of 10 min
are displayed in Fig. 1. The particle sizes are less than 100 nm. Though the laser ablation of iron powder in water,
ethanol, isopropanol and glycol by 248 nm laser give the iron oxide nanoparticles, it results in iron oxide nanoribbons
when ablated in methanol for 5 minutes (Fig. 2(a)). When the crushing time is increased to 7.5 min, we obtain badly
formed nanowires along with the ribbons as shown in Fig. 2(b). However, if the rusting is made for more than 20 min,
we get all circular shaped nanowires as displayed in Fig. 2(c). SEM image of post ablated iron powder which was
collected from the bottom of the vessel after removing the methanol is shown in Fig. 3. It exhibits that the granules
are not of spherical size.
Figure 1 SEM image of the iron oxide nanoparticles of size
less than 100 nm synthesized by 248 nm laser rusting the iron
powder in water.
View larger
version(228K)
Figure 2 SEM image of (a) iron oxide nanoribbons
synthesized by 5 min rusting of iron powder with 248 nm
laser through methanol, (b) badly formed nanowires when the
rusting is increased to 7.5 min and (c) nanowires when the
rusting is increased to 10 min or more.
View larger
version(141K)
Figure 3 SEM image of post ablated iron powder showing
granular size.
View larger
version(99K)
If we use 532 nm laser with same frequency and power, we get naoparticles even with methanol. SEM image of 532
nm laser irradiated iron oxide nanopartices achieved in methanol is demonstrated in Fig. 4(a). It is to be noted that
when that solution is irradiated with 248 nm laser for 5 min, we obtain the mixture of nanowires with fragmented
edges and naopartices (Fig. 4(b)). However, if the solution is annealed with 248 nm laser for more than 15 min, we
get the circular shaped iron oxide nanowires as shown in Fig. 4(c). A possible mechanism of the formation of
nanowires from nanoribbons is schematically shown in Ref. [18]. Initially, the nanoribbons are formed with smooth
edges when laser (248 nm) shot is run for 5 min [18]. When the laser annealing is increased to
10 min, the edges of
the ribbons start fragmented. Then the wider ribbons are disintegrated into much smaller width ribbons. Ultimately,
these turn into round shaped nanowires when laser annealed for
20 min [18]. It should be mentioned that the iron
oxide nanowires are found to be stable at room temperature and air for several months, which is very important for
possible device applications. The
20 min laser annealed solution is coated on glass slides/silicon substrates and
annealed at different temperatures of 100–800°C for 2–4 h in air using a tube furnace. Their transmission electron
microscopic (TEM) images demonstrate that the as-produced wires are of circular shape and amorphous in nature
[17,18]. When it is annealed at 400°C, it starts crystallizing [17, 18]. Nevertheless, annealing at 800°C crystallizes
the nanowires fully [17, 18]. Selected area electron diffraction (SAED) pattern of as-produced iron oxide nanowires
shows its amorphous nature with broad and diffuse rings [17]. However, the SAED pattern of 600°C annealed samples
clearly exhibits the crystallization with sharp and thin rings [17]. Therefore, from TEM and SAED images, it may be
concluded that the annealing in air above 400°C, crystallizes the samples.
Optical Spectroscopic (UV, Raman and XPS) Studies
As mentioned earlier, the hazy yellow product become clear yellow with further laser annealing for another 10
min.Figure 5 displays the ultra violet (UV) absorption spectra of laser annealed and non-annealed iron oxide
nanowires/nanoribbons. Figures 5d, c, b, a demonstrate, respectively, the UV absorption spectra of as produced iron
oxide nanowires obtained by laser ablation for 7.5 min, collected product laser annealed for another 10 min, laser
ablation of 10 min and collected product laser annealed for another 10 min. It has been observed that the product
absorb the UV strongly at
248 nm and there is little absorption at 532 nm. The absorption is greater when the
collected product is laser annealed for another 10 min (Fig. 5a and c) compared to that of laser irradiated as-produced
hazy product (Fig. 5b and b). This is due to the formation of more iron oxide nanowires with further laser annealing.
Figure 4 SEM image of (a) iron oxide nanoparticles
synthesized by 532 nm laser in methanol, (b) laser annealed
for 10 minutes with 248 nm laser showing the mixture of
nanowires and nanoparticles and (c) laser annealed for more
than 20 minutes with 248 nm laser exhibiting complete
formation of nanowires.
View larger
version(121K)
Figure 5 Ultra-violet (UV) absorption spectra of 7.5 min
laser (248 nm) irradiated as-synthesized product (d), laser
annealed for another 10 min (c), 10 min laser irradiated assynthesized product (b) and laser annealed for another 10 min
(a).
View larger
version(48K)
To determine the type of iron oxide, we have taken the Raman spectra of the 10 min laser irradiated as-produced
product (unannealed) and the temperature annealed samples. Raman spectra of unannealed and annealed (at 400,
600 and 800°C) samples illustrate the peaks corresponding to α-FeO(OH), α-Fe2O3, and silicon substrate [5, 19]. It
demonstrates that the unannealed sample has a weak peak corresponding to α-FeO(OH) which disappears at higher
temperatures. Nevertheless, with the increase of annealing temperature, the peak height corresponding to αFe2O3increases consistent with the literature [20, 21]. Some new peaks due to it also appear at 600 and 800°C
annealed samples [17]. As the annealing temperature is increased, the sample colour changes from yellowish to
reddish and it becomes totally red at 800°C. Therefore, with the increase of annealing temperature, more α-Fe2O3 are
produced and α-FeO(OH) is reduced which is also supported by XPS data as discussed below.
The Fe 2p3/2 and Fe 2p1/2 XPS spectra [17] of as-produced and 600°C annealed samples demonstrate that Fe2p3/2and
Fe 2p1/2 peaks appear at 710.7, 711 eV and 724, 724.5 eV for as-produced and 600°C annealed samples which are
close to the values [22] observed for both α-Fe2O3 and α-FeO(OH). Equally the position and the separation of these
peaks are consistent with the presence of Fe3+ species in the samples and indicate the formation of the α-FeO(OH)
phase in as-produced material and α-Fe2O3 phase in 600°C annealed product [17–19] which are in good agreement
with other published data [23]. Nevertheless, it has been experimentally observed that the Fe 2p core levels regions
present very little variation between different iron compounds or phases due to their broad and asymmetric features.
This does not allow the dissimilar oxidized species to be reasonably well distinguished and their complex satellite
structures complicate things further [23]. Values for Fe 2p binding energy in both α-Fe2O3 and α-FeO(OH) range from
710.4 to 711.5 eV, presumably due to the effect of the background subtraction methods and the choice of the lineshapes used for spectra analysis, as well as the effects stated above [24, 25]. Both spectra [17] display the presence
of relatively broad shake-up satellite peaks, which represent a well known characteristic of oxide spectra, occurring at
the high binding energy side of both of the Fe2p peaks, indicating again the presence of Fe3+species in these samples
[25].
For more information on the samples, the O 1s region is investigated where O1s peak can invariably be deconvoluted
into two components for both the samples [17]. In O1s XPS spectra of as-produced and 600°C products exhibit that
the peaks at 530.6 eV in as-produced material and at 530.2 eV in 600°C annealed one are because of the oxygen
within the α-Fe2O3 [17]. However, those at 532.6 eV in as-produced sample and at 532.1 eV in 600°C annealed one
are by the reason of oxygen within α-FeO(OH) which are in good agreement with the literature [26]. These indicate
the presence of α-Fe2O3 and α-FeO(OH) in both the samples at different ratio. The relative ratio between the two
peaks corresponding to α-Fe2O3 and α-FeO(OH) is 22 and 78 for as-produced sample, and 75 and 25 for 600°C
annealed one. Therefore, the majority of the as-synthesized product is α-FeO(OH) whereas for the annealed one, αFe2O3 dominates which may be due to the chemical reaction
Heating in ultra high vacuum (UHV) at 70°C of as-produced sample shows only a slight change in the ratio of O 1s
peaks originating from α-Fe2O3 and α-FeO(OH) with their position remaining the same [17]. Hence, we can definitely
exclude the presence of OH group derived from water in the sample. However, the OH peak reduces a lot in 600°C
annealed sample compared to that of 70°C annealed one in UHV. The positions of Fe 2p peaks before and after
heating in UHV remained the same but the amount of C in the sample reduced upon heating as expected. The high
resolution scan of the C1s region and its deconvolution presents two peaks [17], one being attributed to aliphatic
C&bond;C bonding and the other to the more electronegative C&bond;O bonding of the residual organic species from
the solvent (methanol). It is found that the peak due to C&bond;O bonding reduces enormously by heat treatment at
70°C [17].
IV. Conclusion
In conclusion, we have shown that UV pulsed laser (248 nm) irradiation of iron powder in methanol produces solutions
of iron oxide nanowires. Nanoparticles are obtained with this laser in other media like water, ethanol, isopropanol, and
glycol. Nanoparticles are also achieved when a visible laser is used in methanol, but can be transformed into
nanowires when annealed with a 248 nm laser. Though five minutes laser ablation produce nanobelts, increasing the
ablation time results in more nanowires. This is due to the fragmentation of the nanobelts leading to nanowires. More
iron oxide nanowire product is obtained with the increase of laser ablation time as evident from UV absorption of the
samples. Raman and XPS data confirm that formation of α-Fe2O3 increases with the enhancement of annealing
temperature. This approach is very simple and has many advantages over traditional surface oxidation methods since
enormous amount of iron oxide nanowires may be synthesized at room temperature in methanol with the least time.
The segregation of the solution of α-FeO(OH)/α-Fe2O3 nanowires is very easy as the evaporation of methanol is
incredibly simple. The nanowires are extremely stable up to a temperature of 100°C and may have constant use for
suitable application for a wide range of temperature.
Acknowledgment
One of the authors (S.M.) is grateful to Commonwealth Scholarship Commission in the United Kingdom for providing
him a fellowship to work in U.K. The authors are thankful to EPSRC for funding in the form of a Portfolio grant. 532 nm
pulsed laser facility provided by Prof. M. N. R. Ashfold, School of Chemistry, University of Bristol, U.K. is appreciatively
acknowledged.
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