# Iron Oxide Doped Alumina-Zirconia Nanoparticle Synthesis  by Liquid Flame Spray from Metal Organic Precursors

Authors: Juha-Pekka Nikkanen; Helmi Keskinen; Mikko Aromaa; Mikael Järn; Tomi Kanerva; Erkki Levänen; Jyrki M. Mäkelä; Tapio Mäntylä
Journal: Research Letters in Nanotechnology
 (2008)

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## Abstract

The liquid flame spray (LFS) method was used to make iron oxide doped alumina-zirconia nanoparticles. Nanoparticles were generated using a turbulent, high-temperature (Tmax∼3000 K) H2-O2 flame. The precursors were aluminium-isopropoxide, zirconium-n-propoxide, and ferrocene in xylene solution. The solution was atomized into micron-sized droplets by high velocity H2 flow and introduced into the flame where nanoparticles were formed. The particle morphology, size, phase, and chemical composition were determined by TEM, XRD, XPS, and N2-adsorption measurements. The collected particulate material consists of micron-sized aggregates with nanosized primary particles. In both doped and undoped samples, tetragonal phase of zirconia was detected in room temperature while alumina was found to be noncrystalline. In the doped powder, Fe was oxidized to Fe2O3. The primary particle size of collected sample was approximately from 6 nm to 40 nm. Doping was observed to increase the specific surface area of the powder from 39 m2/g to 47 m2/g.


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## Body

## 1. Introduction

Liquid flame spray (LFS) technique can be used to synthesize the broad spectrum of inorganic
oxide nanoparticles like Al2O3, ZrO2, Mn2O3,
[1, 2], TiO2 [3], Fe2O3,
Fe3O4 [4], and
nanoscale metallic particles like Ag and Pd [4]. Liquid feeding-based flame methods also offer
a controlled way to  synthesize mixed oxides like MgAl2O4, CeO2/ZrO2, Y2O3-ZrO2 [1], and SiO2/TiO2 [5].
In Liquid flame spray (LFS), a high-velocity high-temperature flame is used to
produce nanoparticles. Liquid precursor is axially injected into the  flame
where it evaporates, decomposes, reacts, and finally nucleates to the nanosized particles. Particles can be deposited directly on the substrate as a layer [6], or
collected from gas stream by filters [7] or by electrostatic
precipitators [3].The unique properties of nanoscale materials
offer several interesting applications in different fields of industry. The
remarkable optical, electronic, magnetic, and mechan-ical properties of
nanoparticles and materials made of na-noparticles open up a possibility to modify the functionality of electrochemical, photochemical and structural
materials, and coatings [8, 9]. A fractional size and controlledshape of metal and
functionalized metal nanoparticles, as well as properties of doped mixed oxides
like Al2O3-ZrO2, could provide a unique
possibility for highly efficient catalysis [9, 10]. The
catalysis provides numerous applications like alkylation and isomerization in
the chemical industry [10, 11], catalytic reduction of NO and NH3 [12], or
preparation of carbon nanotube-Al2O3 composites with Fe
catalysts [13].
Several applications of the catalytic agents together with increasing demand of
high-quality structural materials call for new and controllable methods to
produce nanoscale building blocks. LFS technique opens up a possibility to
produce multicomponent oxide nanomaterials together with additional catalyst
for further designing and processing of functional or structural nanomaterials.The aim of this work was to demonstrate the
synthesis of iron oxide doped Al2O3-ZrO2 mixed
oxide by an LFS. The collected particles were analyzed in order to determine
resulting particle size, morphology, phases, and chemical composition. The
characteristics of iron oxide doped sample were compared to the undoped sample.


## 2. Experimental

Aluminium isopropoxide (C9H21AlO3), zirconium (IV) n-  propoxide (Zr[O(CH2)2CH3]4),
and ferrocene ((C5H5)2Fe) were
dissolved in p-xylene. Molar ratio of (C9H21AlO3): (Zr[O(CH2)2CH3]4):((C5H5)2Fe)
in precursor solution was 52 : 22 : 1. Also in undoped solution, the molar ratio of (C9H21AlO3):
(Zr[O(CH2)2 CH3]4)
was 52 : 22. The precursor solution was fed into turbulent, high-temperature H2-O2 flame. The feed rate was 5.2 mL/min. The gas flows were 20 L/min for H2 and 10 L/min for O2.
Liquid precursor was atomized in the high-velocity H2 flow
into micron-sized droplets and introduced into oxygen stream and the upwards
directed flame where particles were formed. Particles were collected by a
cylindrical electrostatic precipitator with a diameter of 100 mm and length of
1000 mm, using 6.0 kV voltage on the central rod, and 0.3 mA current from rod
to grounded cylinder. Total amount of collected material was in the order of
one gram. LFS-process is schematically
illustrated in Figure 1.Figure 1
Schematic description of the LFS-process [3].The crystal
structure of the sample was determined by X-ray diffractometer (Kristalloflex D-500, Siemens) using monochromatized CuKα radiation. The particle morphology was determined by a
transmission electron microscope (TEM) (JEOL, JEM 2010). Then the specific surface area of sample was measured by nitrogen adsorption using the Brunauer-Emmett-Teller (BET) method (Coulter Omnisorp 100 cx). The X-ray photoemission (XPS) spectra were recorded using a Physical Electronics Quantum 2000 ESCA instrument. The XPS measurements were performed at a base pressure of
1×10−9 torr using the AlKα X-ray source. Operating power was 12.5
kW and the spot diameter 50  μm. The low
and high resolutions of pass energy inspectral acquisition were 117.4 and 23.5 eV, respectively. Surfaceconcentrations were determined with Multipak 6.1 software using peak areas to give surface concentrations in atomic percent.


## 3. Results and Discussion

The XRD diffraction patterns (Figure2) of LFS particulate
materials contained only the peaks of tetragonal ZrO2. Since no
other than t-ZrO2 was detected, Al and Fe are probably in amorphous
phase.Figure 2
XRD-diffraction pattern of Al2O3-ZrO2 (a) and iron oxide doped
Al2O3-ZrO2 mixed oxide (b); t: tetragonal-ZrO2.The XPS spectra
of the collected samples are shown in Figure3. In XPS measurements, iron was
detected and the Fe 2p3/2 peak position was around 711 eV,
indicating that most of the iron is Fe2O3. The Al 2p peak
position was around 73.8 eV and position of Zr 3d3/2 peak around 186
eV which proves that both Al and Zr are oxidized to Al2O3 and
ZrO2. The signal at around 285 eV corresponds to adventitious
carbon.Figure 3
XPS survey spectra of Al2O3-ZrO2 and Fe2O3 doped Al2O3-ZrO2 mixed oxide. The insert in the figure represents the Fe 2p high-resolution spectra of Fe2O3 doped Al2O3-ZrO2.The surface
concentrations of oxygen, aluminum, zirconium, and iron, in atomic percent, are
given in Table1. From the high-resolution XPS spectra, the calculated
concentrations are compared to the theoretical acquisition of named atoms. From
XPS spectra, calculated concentrations of oxygen and iron are higher whereas
the concentrations of aluminum and zirconium are lower than the theoretical
acquisition. It is supposed that there is adsorbed oxygen and meanwhile more Fe2O3 particles on the surface of the sample than inside of it.Table 1
Calculated surface concentrations in atomic percentage from XPS (1) and theoretical acquisition (2) of oxygen, aluminum, zirconium,
and iron of Fe2O3 + Al2O3-ZrO2 and
Al2O3-ZrO2.
SampleOxygen at -%Aluminum at -%Zirconium at -%Iron at -%(1)Fe2O3 + Al2O3-ZrO271.0122.355.870.77(2)Fe2O3 + Al2O3-ZrO261.9126.4911.090.51(1)Al2O3-ZrO271.3722.536.10—(2)Al2O3-ZrO262.2326.6311.15—The specific surface area of the doped sample was 46.7 m2/g whereas it was 38.8 m2/g
for undoped sample. Higher surface area of doped sample refers to small size of
Fe2O3 particles. Another explanation for the increase of specific surface area may be the decreased particle size. Anyway, the
difference is so small that it cannot be observed in the peak width in the XRD
patterns (Figure 2), but the phenomenon has been observed in earlier studies [3, 14]. The TEM images of the collected samples are shown in Figure 4. The primary particle size as estimated from TEM graphs is approximately from 6 to 40 nm in both,
doped (Figure 4(a)) and undoped (Figure 4(b)) samples. The slightly higher specific
surface area of doped sample is probably caused by less dense packing or increased
surface irregularities
caused by small Fe2O3 particles. The shape of the
particles is mainly spherical in doped and undoped samples.TEM micrograph of Fe2O3 doped Al2O3-ZrO2 (a) and Al2O3-ZrO2 mixed oxide (b).
(a)(b)The crystallite size oft-ZrO2 was
estimated from XRD pattern by using Scherrer’s formula t = (0.9λ/B cos θ) [15], where t is in nm, λ the wavelength of X-ray in Å (1.5418 Å), B is
the full width half maxima in radians, and θ is the Bragg angle. This calculation gave
the size of 7.4 nm for both samples. Zirconia has a monoclinic phase structure
at low temperature and at normal pressure. The martensitic transformation upon
heating to tetragonal phase occurs approximately at about 1150°C,
whereas reverse transformation upon cooling occurs at about 900°C
because of the hysteresis effect [2, 16]. The presence of tetragonal ZrO2 at low
temperature can be explained by the small size of the crystals. Phase
transformation from tetragonal to monoclinic occurs at a critical crystal size
of 8–18 nm. Tetragonal
phase is thermodynamically stable if the size of the crystals is below that [2, 17].


## 4. Conclusions

Iron oxide doped nanoscale Al2O3-ZrO2 mixed oxide was prepared
by liquid flame spray. The crystal structure,particle and crystal size,
specific surface area, and surface concentrations of the elements were
analyzed. Analysis using XRD showed that the structure of alumina is not
crystalline, whereas zirconia exists in tetragonal phase. According to XPS
study, Fe exists mostly in oxide (Fe2O3). On the grounds
of the TEM dissection, the particle size of the sample is in nanometer scale.
The calculated crystallite size of zirconia was found to be 7.4 nm. In small
particles, the tetragonal structure is energetically favorable. This gives the
explanation to the existence of the tetragonal form of zirconia at the room
temperature.



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*Source: 516478-2008-05-18.xml*