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

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 (𝑇_max∼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.

1. Introduction

Liquid flame spray (LFS) technique can be used to synthesize the broad spectrum of inorganic oxide nanoparticles like Al₂ O₃, ZrO₂, Mn₂ O₃, [1, 2], TiO₂ [3], Fe₂ O₃, Fe₃ O₄ [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 MgAl₂ O₄, CeO₂/ZrO₂, Y₂ O₃-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 Al₂ O₃-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-Al₂ O₃ 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 Al₂ O₃-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 (C₉ H₂₁AlO₃), zirconium (IV) n- propoxide (Zr[O(CH₂ )₂CH₃ ]₄), and ferrocene ((C₅ H₅ )₂Fe) were dissolved in p-xylene. Molar ratio of (C₉ H₂₁AlO₃): (Zr[O(CH₂ )₂CH₃ ]₄):((C₅ H₅ )₂Fe) in precursor solution was 52 : 22 : 1. Also in undoped solution, the molar ratio of (C₉ H₂₁AlO₃): (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 CuK_𝛼 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 AlK_𝛼 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 (Figure 2) 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 Al2 O3-ZrO2 (a) and iron oxide doped Al2 O3-ZrO2 mixed oxide (b); t: tetragonal-ZrO2.

The XPS spectra of the collected samples are shown in Figure 3. In XPS measurements, iron was detected and the Fe 2p_3/2 peak position was around 711 eV, indicating that most of the iron is Fe₂ O₃. The Al 2p peak position was around 73.8 eV and position of Zr 3d_3/2 peak around 186 eV which proves that both Al and Zr are oxidized to Al₂ O₃ and ZrO₂. The signal at around 285 eV corresponds to adventitious carbon.

Figure 3 

XPS survey spectra of Al2 O3-ZrO2 and Fe2 O3 doped Al2 O3-ZrO2 mixed oxide. The insert in the figure represents the Fe 2p high-resolution spectra of Fe2 O3 doped Al2 O3-ZrO2.

The surface concentrations of oxygen, aluminum, zirconium, and iron, in atomic percent, are given in Table 1. 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 Fe₂ O₃ 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 Fe₂ O₃ + Al₂ O₃-ZrO₂ and Al₂ O₃-ZrO₂.

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 Fe₂ O₃ 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 Fe₂ O₃ particles. The shape of the particles is mainly spherical in doped and undoped samples.

(a)

(b)

Figure 4 

TEM micrograph of Fe2 O3 doped Al2 O3-ZrO2 (a) and Al2 O3-ZrO2 mixed oxide (b).

The crystallite size of t-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 Al₂ O₃-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 (Fe₂ O₃). 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.

Acknowledgment

This work is partially supported by EU research project IP NANOKER (FP6-515784-2).