# Photochemistry Aspects of the Laser Pyrolysis Addressing the Preparation of Oxide Semiconductor Photocatalysts

Authors: R. Alexandrescu; I. Morjan; F. Dumitrache; M. Scarisoreanu; I. Soare; C. Fleaca; R. Birjega; E. Popovici; L. Gavrila; G. Prodan; V. Ciupina; G. Filoti; V. Kuncser; L. Vekas
Journal: International Journal of Photoenergy
 (2008)

---

## Abstract

The laser pyrolysis is a powerful and a versatile tool for the gas-phase synthesis of nanoparticles. In this paper, some fundamental and applicative characteristics of this technique are outlined and recent results obtained in the preparation of gamma iron oxide (γ-Fe2O3) and titania (TiO2) semiconductor nanostructures are illustrated.
Nanosized iron oxide particles (4 to 9 nm diameter values)
have been directly synthesized by the laser-induced pyrolysis of a
mixture containing iron pentacarbonyl/air (as oxidizer)/ethylene
(as sensitizer). Temperature-dependent Mossbauer spectroscopy
shows that mainly maghemite is present in the sample obtained at
higher laser power. The use of selected Fe2O3 samples for the preparation of water-dispersed magnetic nanofluids is also discussed. TiO2 nanoparticles comprising a mixture of anatase and rutile phases were synthesized via the laser pyrolysis of TiCl4- (vapors) based gas-phase mixtures. High precursor concentration of the oxidizer was found to favor the prevalent anatase phase (about 90%) in the titania nanopowders.


---

## Body

## 1. Introduction

In recent years,  research
interests for the preparation and application of oxide semiconductor photocatalysts
have continuously grown [1]. Particular attention is paid to the use of
semiconductor materials as photocatalysts for the removal of organic and
inorganic species from aqueous or gas phase. This method has been suggested in
environmental protection due to its ability to oxidize the organic and
inorganic substrates [2]. Several steps should characterize the heterogeneous
photocatalysis reaction among which most important should be the adsorption and
the reaction onto the surface [3].Studies were made on compositions consisting
mainly of various oxide semiconductors. For instance, commonly used Fe2O3 semiconductor oxide is known for its
catalytic activity [4, 5]. TiO2 nanoparticles offer additional advantages (low cost, chemical inertness,
highly photo-activity, and suitable band gap (Eg = 3.2 eV)) [6–8].The photocatalytic
activity of a particulate semiconductor system depends on the crystal
structure, crystallinity, particle size and shape,  size distribution, surface
area, and surface hydroxyls. Considerable enhancement of the absorption can be
observed in small nanocrystals, where the surface-to-volume ratio is very high and the share of the surface
atoms is sufficiently large. Consequently, the synthesis method plays a fundamental
role.A
wide variety of gas-phase techniques has been developed for the fabrication of
nanoparticulate structures [9] which could allow for homogeneous
nucleation and may provide to some extent the control of particle size,
particle shape, size distribution, particle composition, and degree of particle
agglomeration. Among these synthesis procedures, the laser pyrolysis technique
has been developed recently as a powerful tool for the synthesis of
technological important nanomaterials such as metallic (Fe, Ti) oxides and
composites [10–13], silicon crystals [14], nanocarbon [15], and iron-siloxane
polymer nanoparticles [16].This paper represents an extension of our earlier analysis on the
principles of the laser pyrolysis and its applications [17]. Here, we shortly
discuss the background of the method and recent development in the field, with
special emphasis on the preparation of two types of semiconductor
photocatalysts: maghemite iron oxide and titanium dioxide. In Section 2, the pyrolysis
of Fe(CO)5-based gas mixtures and the preparation of magnetic
nanofluids are presented  in a sequence
relevant for the phenomenology of the process and the nanopowder structure. In Section 4, main
characteristics of the laser-sensitized pyrolysis of TiCl4-based
mixtures for producing titania and anatase titania in connection with their
specific structural characteristics are discussed.


## 2. Laser Pyrolysis Synthesis

### 2.1. Photochemical Aspects of the Laser Pyrolysis

The specific features of the CO2 laser pyrolysis rely on the IR vibrational photochemistry. The process is based on the
overlapping of the emission line of the laser (the strong 10P (20) line that
peaks at 10.591 μm) with an absorption line of one or more gas
precursors [17]. The molecular mechanism involves the increase of the
internal energy of the ground electronic state, the evolution of the
vibrational states through collisional relaxation processes, and finally the
thermochemical reactions. Flynn et al. [18] show that in any environment where multiple collisions occur on the
time scale of a chemical reaction, the vibrational modes of a polyatomic
molecule will equilibrate, and reaction occurs without energy localization in a
single mode. An additional substance, the so-called sensitizer is used in case
of nonabsorbing gas precursors. It will absorb the energy and transmit it to
the precursors by collisions. This energy transfer agent plays a high practical
role (detailed for ethylene in what follows) because it assures the coupling
between the laser radiation and the absorbing system. As a final effect
of the coupling between the laser radiation and the absorbing system, the laser
acts as a localized heat source. As compared to heating the gases in a furnace,
the laser heating is a localized one occurring only in a small volume
delimitated by the radiation cross section and the emergent gas flows. Short millisecond-scale residence times,
spatial uniformity of the reaction zone, and continuous source of activation
energy are the key attributes of the process.Fe(CO)5 and TiCl4 are greatly used as precursors in conventional CVD or in photolytic or
thermally laser-induced processes [19]. Fe(CO)5 and TiCl4 molecules
meet also other prerequisites for CVD, like stability and relevant vapor
pressure at room temperature (around 25 torr and 8 torr at 300 K,
resp.).  The IR spectra of both of
Fe(CO)5 and TiCl4 present no absorption bands near the emission frequencies of the CO2 laser.In the sensitized CO2 laser pyrolysis of Fe(CO)5/TiCl4, the IR
radiation is used to resonantly excite C2H4 sensitizer.
The absorption coefficient of C2H4 at the laser radiation
wavelength of the usual CO2 lasers is 1.7×10−3 torr-1 cm-1. Ethylene is excited in a superposition of two fundamentals: ν7 (949.3 cm-1) and ν8 (940.6 cm-1). The
dissociation energy H2C-CH-H (for one hydrogen stripping) is rather
high (103 Kcal/mol [20]).Ideally, it is supposed that
ethylene only transfers its energy by collisions to the other precursors, but
do not participate in the reaction.
However, the study of the evolution of the exhaust gases often shows
that at higher temperatures ethylene could also decompose. Indeed, as a first
product of decomposition, acetylene is released and appears as traces in the IR
spectrum of the exhaust gases. One should note that the observed C2H4 dehydrogenation could be significantly promoted by the active Fe sites
generated by Fe(CO)5 dissociation. As
shown in [21], although the thermodynamically favored process is the iron pentacarbonyl
dissociation, the hydrocarbon decomposition could be speeded up by the
catalytic surface of freshly formed iron nanoparticles.


### 2.2. Advantages and Drawbacks

The
laser pyrolysis processes are characterized by high temperatures, fast heating
rates, and short reaction times. These features make the laser pyrolysis
attractive method for the manufacture of technological useful materials at
lower costs compared to conventional gas-phase processes (such as the flame
spray processes). Some other advantages are (i) the relative simple equipment, (ii)
the versatility—the formation of
virtually any nanoproducts, provided a suitable choice of precursors, and (iii)
formation of high purity products.The drawbacks of the laser pyrolysis technique are sometimes found in the
constraints imposed on the precursors [6]: the desired elements must be present
in vapor phase and at least one reactant should contain an infrared absorption
band for the coupling of the energy with the laser beam.


### 2.3. Experimental Set-Up

The flow reactor is the core of the laser pyrolysis technique, ensuring
the practical application of the characteristic interplay between resonant and
nonresonant processes. It was presented in details elsewhere [17]. A schematic
view of the setup is presented in Figure 1. Briefly, the system is based on a
cross-flow configuration. The reactant flow emerges in the reactor through a nozzle system where it
is orthogonally intersected by the focused IR radiation beam. Pyrolysis in the gas phase
occurs in the small volume defined by the radiation-gases crossing where a “flame” usually appears.
It is believed that the visible emission is mainly resulting from the hot
freshly nucleated particles. The confinement
of gas precursors and of the particles toward
the flow axis is achieved by a coaxial argon flow. Nucleated particles are kept
entrained by the gas stream and collected in a removable tank at the exit of
the reaction cell. Main process parameters are the nature of the gas (vapor)
precursors, gas flow rates, pressure, as well as laser wavelength and laser
power. For liquid precursors, the bubbled vapors are entrained into the
reaction zone by a carrier gas.Figure 1
Schematic
view of the laser pyrolysis setup for the production of iron oxide and titania
nanoparticles, respectively.Fe(CO)5/C2H4 and TiCl4/C2H4—were alternatively used as basic precursor
mixtures for the preparation of iron and titanium oxide nanoparticles,
respectively. In Figure 1, both these experimental arrangements are suggested.
Additives such as air were employed for obtaining specific oxide nanostructures.
In order to prevent the NaCl windows from being coated with powder, they were
continuously flushed with Ar. Usually, either ethylene or argon was carrying
through a bubbler the vapors of the liquid metal precursor into the reaction
zone. In each of these systems, the installation for laser pyrolysis was
adapted to the demands imposed by the nature of the precursors and the final
aim of the synthesis process. In case of titania nanoparticles, a main variable
trend was the change of a double nozzle system with a triple one (not shown in Figure 1)
thus trying to avoid the direct reaction of TiCl4 with air.


### 2.4. Material Characterization

After synthesis, the
morphology and composition of the iron oxides nanopowders were characterized by
transmission electron microscopy (TEM), selected area electron diffraction
(SAED), X-ray diffraction (XRD), and Mössbauer
spectroscopy. The XRD pattern was recorded on
a DRON DART UM-2 diffractometer equipped with a Cu Kα radiation and
a graphite monochromator in the diffracted beam. The crystallite sizes were
calculated from the corrected full width and half maximum (FWHM) applying the
Debye-Scherrer formula. The powdery deposits were imaged in a transmission electron
microscope Philips CM120ST (Customized
Microscope 120 Super Twin, 120 kV max. acceleration voltage, about 2 A
resolution, Cs = ~1.2 mm). The samples were analyzed by the different electron
microscopy techniques such as Bright
Field Transmission Electron Microscopy (BF-TEM), Selected Area Electron
Diffraction (SAED) and High Resolution Transmission Electron Microscopy
(HRTEM). 57Fe Mössbauer spectroscopy was used for analyzing
the samples at temperatures ranging from 80 K to 230 K. Mössbauer spectra were
collected in transmission geometry by inserting the sample in a bath
liquid-nitrogen cryostat. A Mössbauer drive system operating in constant
acceleration mode combined with conventional electronics and a 57Co
(Rh matrix) source of about 25 mCi activity were employed.


## 2.1. Photochemical Aspects of the Laser Pyrolysis

The specific features of the CO2 laser pyrolysis rely on the IR vibrational photochemistry. The process is based on the
overlapping of the emission line of the laser (the strong 10P (20) line that
peaks at 10.591 μm) with an absorption line of one or more gas
precursors [17]. The molecular mechanism involves the increase of the
internal energy of the ground electronic state, the evolution of the
vibrational states through collisional relaxation processes, and finally the
thermochemical reactions. Flynn et al. [18] show that in any environment where multiple collisions occur on the
time scale of a chemical reaction, the vibrational modes of a polyatomic
molecule will equilibrate, and reaction occurs without energy localization in a
single mode. An additional substance, the so-called sensitizer is used in case
of nonabsorbing gas precursors. It will absorb the energy and transmit it to
the precursors by collisions. This energy transfer agent plays a high practical
role (detailed for ethylene in what follows) because it assures the coupling
between the laser radiation and the absorbing system. As a final effect
of the coupling between the laser radiation and the absorbing system, the laser
acts as a localized heat source. As compared to heating the gases in a furnace,
the laser heating is a localized one occurring only in a small volume
delimitated by the radiation cross section and the emergent gas flows. Short millisecond-scale residence times,
spatial uniformity of the reaction zone, and continuous source of activation
energy are the key attributes of the process.Fe(CO)5 and TiCl4 are greatly used as precursors in conventional CVD or in photolytic or
thermally laser-induced processes [19]. Fe(CO)5 and TiCl4 molecules
meet also other prerequisites for CVD, like stability and relevant vapor
pressure at room temperature (around 25 torr and 8 torr at 300 K,
resp.).  The IR spectra of both of
Fe(CO)5 and TiCl4 present no absorption bands near the emission frequencies of the CO2 laser.In the sensitized CO2 laser pyrolysis of Fe(CO)5/TiCl4, the IR
radiation is used to resonantly excite C2H4 sensitizer.
The absorption coefficient of C2H4 at the laser radiation
wavelength of the usual CO2 lasers is 1.7×10−3 torr-1 cm-1. Ethylene is excited in a superposition of two fundamentals: ν7 (949.3 cm-1) and ν8 (940.6 cm-1). The
dissociation energy H2C-CH-H (for one hydrogen stripping) is rather
high (103 Kcal/mol [20]).Ideally, it is supposed that
ethylene only transfers its energy by collisions to the other precursors, but
do not participate in the reaction.
However, the study of the evolution of the exhaust gases often shows
that at higher temperatures ethylene could also decompose. Indeed, as a first
product of decomposition, acetylene is released and appears as traces in the IR
spectrum of the exhaust gases. One should note that the observed C2H4 dehydrogenation could be significantly promoted by the active Fe sites
generated by Fe(CO)5 dissociation. As
shown in [21], although the thermodynamically favored process is the iron pentacarbonyl
dissociation, the hydrocarbon decomposition could be speeded up by the
catalytic surface of freshly formed iron nanoparticles.


## 2.2. Advantages and Drawbacks

The
laser pyrolysis processes are characterized by high temperatures, fast heating
rates, and short reaction times. These features make the laser pyrolysis
attractive method for the manufacture of technological useful materials at
lower costs compared to conventional gas-phase processes (such as the flame
spray processes). Some other advantages are (i) the relative simple equipment, (ii)
the versatility—the formation of
virtually any nanoproducts, provided a suitable choice of precursors, and (iii)
formation of high purity products.The drawbacks of the laser pyrolysis technique are sometimes found in the
constraints imposed on the precursors [6]: the desired elements must be present
in vapor phase and at least one reactant should contain an infrared absorption
band for the coupling of the energy with the laser beam.


## 2.3. Experimental Set-Up

The flow reactor is the core of the laser pyrolysis technique, ensuring
the practical application of the characteristic interplay between resonant and
nonresonant processes. It was presented in details elsewhere [17]. A schematic
view of the setup is presented in Figure 1. Briefly, the system is based on a
cross-flow configuration. The reactant flow emerges in the reactor through a nozzle system where it
is orthogonally intersected by the focused IR radiation beam. Pyrolysis in the gas phase
occurs in the small volume defined by the radiation-gases crossing where a “flame” usually appears.
It is believed that the visible emission is mainly resulting from the hot
freshly nucleated particles. The confinement
of gas precursors and of the particles toward
the flow axis is achieved by a coaxial argon flow. Nucleated particles are kept
entrained by the gas stream and collected in a removable tank at the exit of
the reaction cell. Main process parameters are the nature of the gas (vapor)
precursors, gas flow rates, pressure, as well as laser wavelength and laser
power. For liquid precursors, the bubbled vapors are entrained into the
reaction zone by a carrier gas.Figure 1
Schematic
view of the laser pyrolysis setup for the production of iron oxide and titania
nanoparticles, respectively.Fe(CO)5/C2H4 and TiCl4/C2H4—were alternatively used as basic precursor
mixtures for the preparation of iron and titanium oxide nanoparticles,
respectively. In Figure 1, both these experimental arrangements are suggested.
Additives such as air were employed for obtaining specific oxide nanostructures.
In order to prevent the NaCl windows from being coated with powder, they were
continuously flushed with Ar. Usually, either ethylene or argon was carrying
through a bubbler the vapors of the liquid metal precursor into the reaction
zone. In each of these systems, the installation for laser pyrolysis was
adapted to the demands imposed by the nature of the precursors and the final
aim of the synthesis process. In case of titania nanoparticles, a main variable
trend was the change of a double nozzle system with a triple one (not shown in Figure 1)
thus trying to avoid the direct reaction of TiCl4 with air.


## 2.4. Material Characterization

After synthesis, the
morphology and composition of the iron oxides nanopowders were characterized by
transmission electron microscopy (TEM), selected area electron diffraction
(SAED), X-ray diffraction (XRD), and Mössbauer
spectroscopy. The XRD pattern was recorded on
a DRON DART UM-2 diffractometer equipped with a Cu Kα radiation and
a graphite monochromator in the diffracted beam. The crystallite sizes were
calculated from the corrected full width and half maximum (FWHM) applying the
Debye-Scherrer formula. The powdery deposits were imaged in a transmission electron
microscope Philips CM120ST (Customized
Microscope 120 Super Twin, 120 kV max. acceleration voltage, about 2 A
resolution, Cs = ~1.2 mm). The samples were analyzed by the different electron
microscopy techniques such as Bright
Field Transmission Electron Microscopy (BF-TEM), Selected Area Electron
Diffraction (SAED) and High Resolution Transmission Electron Microscopy
(HRTEM). 57Fe Mössbauer spectroscopy was used for analyzing
the samples at temperatures ranging from 80 K to 230 K. Mössbauer spectra were
collected in transmission geometry by inserting the sample in a bath
liquid-nitrogen cryostat. A Mössbauer drive system operating in constant
acceleration mode combined with conventional electronics and a 57Co
(Rh matrix) source of about 25 mCi activity were employed.


## 3. Recent Examples of and Advances in the Preparation by Laser Pyrolysis of Oxide Semiconductor Particles

### 3.1. Spinel-Like Iron (III) Oxide (γ-Fe2O3)

#### 3.1.1. The Preparation of the Nanoparticles and of Their Magnetic
Fluids Counterparts

Nanoparticle Preparation
For producing nanosized
iron oxide particles, the focused continuous-wave CO2 laser
radiation (80 W maximum output power, λ = 10.6 μm) orthogonally crossed the gas flows
emerging through two concentric nozzles (Figure 1). The gas mixture which
contained air and Fe(CO)5 vapors (about 25 Torr vapor pressure at 20°C), entrained by C2H4, was admitted through
the central inner tube. Iron
oxide samples (labeled as 01 and 02) were obtained by varying the laser
power between 35 W and 55 W as presented in Table 1. The other experimental
conditions, namely, the reactor pressure, the relative flow of the oxidizing
agent (air), and the flow of ethylene (as Fe(CO)5 carrier) were maintained constant
(their values are displayed in Table 1).Table 1
Experimental
parameters and main crystallographic characteristics (estimated from XRD
analysis) for the as-synthesized nano iron oxide samples.
SampleΦC2H4* (through Fe(CO)5 [sccm]Φair* [sccm]P** [mbar]PL* [watt]XRD analysisEDAX-elemental analysis, at%a (unit cell parameter) [′]D (mean crystallite dimension) [nm]FeOC0114570300558.3644.536.2952.9910.720214570300358.3773.035.0955.129.79*ΦC2H4 and Φair, are the flows of the ethylene (carrier of Fe(CO)5 vapors) and of air oxidizer,
respectively (in standard cubic centimeter per minute);
**P is the pressure in the reaction chamber (in mbar);
***PL is the laser power (in Watt).Magnetic Fluid Preparation
The as-synthesized iron oxide nanopowders were used for the preparation of magnetic
nanofluids. In what follows, the obtained magnetic fluid samples will be
labeled MF01 and MF02 and are derived by dispersing in water samples 01 and 02,
respectively. Water-based magnetic nanofluids are a special category of polar
magnetic nanofluids showing particularities as concerning particle interactions
and agglomerates formation.  The
synthesis of magnetic fluids has two main stages (1) preparation of nanosized
magnetic particles, in particular by laser pyrolysis, and (2) subsequent
stabilization/dispersion of the nanoparticles in water carrier. Electrostatic
and steric stabilization procedures are both applicable for water-based
magnetic nanofluids. Double layers steric + electrostatic (combined) stabilization
of magnetic nanoparticles in water carrier was experimented recently using
various chain length of carboxylic acid surfactants, as well as dodecyl
benzensulphonic acid (DBS) [22].  In case
of magnetic nanoparticles prepared by laser pyrolysis, a complementary
difficulty appears due to the large agglomerates existing already in the nanopowder
state, as a consequence of the magnetic dipolar interactions between uncoated
nanoparticles. The sequence of steps of the preparation procedure for water-based
magnetic nanofluids may be summarized as follows: (a) addition of NaOH (6N) to
the water/magnetic nanopowder mixture in order to ensure a pH value of about
8.5–9, followed by
heating up the reaction medium to about 75–80°C;
(b) chemisorption of the surfactant (in the present case, lauric acid (LA)); (c)
magnetic organosol; (d) magnetic decantation (elimination of residual salts); (e)
correction of pH (using NaOH); (f) dispersion of surface-coated magnetic
nanoparticles; (g) primary magnetic nanofluid; (h) magnetic
decantation/filtration; (i) water-based magnetic nanofluid (diluted, with solid
volume fraction of about 5%).


#### 3.1.2. Nanoparticle Characterization

The
XRD analysis of the as-synthesized nanopowders points to the presence of the
spinel-like iron (III) oxide [10], namely, maghemite/magnetite iron phase
(according to JCPDS 19-0629 for standard maghemite and JCPDS 39–1346 for
maghemite). In Figure 2, a representative diffractogram obtained for the as-synthesized
oxides is displayed. Although it refers to sample 01, its characteristic phase
identification features are similar to that of sample 02 (not presented here). The
unit-cell parameter (Table 1) was estimated from the 311 and 440 reflections.
It presents relatively lower values to the standard magnetite value (Fe3O4 with
a = 8.396 Å), suggesting the formation of a more likely maghemite phase (γ-Fe2O3).
The broadness of the diffraction lines is associated with the formation of very
small particles as well as with a high degree of structural/crystallographic
disorder. The estimated mean crystallite dimension D (nm) is presented in Table 1. A higher crystallinity and slightly larger mean crystallite size may be
deduced for the sample obtained at the higher laser power.Figure 2
XRD diffraction patterns for a laser-synthesized
iron oxide (sample 01): peak identification points to a chemical composition
containing maghemite/magnetite iron oxide phases.The
identification of elements present in powders was performed using the energy
dispersive analysis by X-rays (EDAX). The results are presented in the last
column of Table1. The stoichiometry, suggested by the relative quantities of
oxygen and iron present in the synthesized powders, corresponds to a Fe3+ iron oxide, most probably maghemite. A rather low carbon contamination (about
10%), mainly due to the unwanted ethylene dissociation may be noticed. Although
revealed by EDAX, the XRD analysis seems not able to reveal traces of carbon. It
is worth to note that the sensitivity of this analysis is down to a few atomic
percent and that low Z elements are detectable with limited sensitivity.TEM
analysis of samples 01 and 02 reveals an almost polycrystalline morphology (Figures3(a), 3(b)). The nanoparticles show coalescent features. Cross-linked chains may be
sometimes observed. An estimation of the particle mean sizes gives about 4.5
and 6 nm for samples 01 and 02, respectively.TEM analysis for iron oxides obtained from (a) sample 01 and (b) sample 02.
(a)(b)The
reflections revealed by the SAED
analysis (Figure4) may be attributed to a cubic structure of the iron oxide,
namely, maghemite/magnetite. The strong diffraction ring at 0.25 nm may be
noticed. Although difficult to distinguish between
these phases from electron diffraction patterns alone, it is probable that
maghemite prevails by referring to XRD results (see, e.g., the value of the
estimated lattice parameter a in
Table 1). Increased crystallinity seems to characterize the nanomaterial
obtained at higher laser power. This can be considered as a consequence of both the higher temperature
gradients in the reactions zone and the fast quenching of the condensed nanoparticles
in the condensation zone. As revealed by the characterization methods employed,
this process greatly favors maghemite phase in powders.Figure 4
SAED analysis of sample 02, revealing main diffraction rings of maghemite/magnetite.HRTEM analysis was performed on many different domains of the nanopowders
and is presented in Figure5.
Nanocrystals with different orientation and mean diameters of about 5-6 nm are
displayed in Figure 5(a). From the indicated squarish areas, the Fourier
transform images are presented as insets. The left-side and the right-side
groups of nanocrystals in Figure 5(a) display characteristics of the cubic
structure of a maghemite crystal. The lattice fringes, corresponding to
the (113)  (d113 = 0.253 nm) and to the (220)  (d220 = 0.313 nm) planes, are evidenced. A small distribution of d-values is caused by
the imperfect crystallographic structure, probably due to the quenching of the
material when leaving the reaction zone. The HRTEM image of Figure 5(b)
presents agglomerated nanocrystals. They seem to be surrounded by an amorphous
layer.HRTEM
images for samples: (a) 01 and (b) 02.  Space representations of Fourier transform
(displayed as insets on figure) seem to evidence the diffraction planes of
maghemite/magnetite.
(a)(b)Figure 6
Mössbauer
spectra (at 80 K) of the two as-synthesized nanopowder samples; at the right side,
the probability distributions of the hyperfine field are also displayed.


#### 3.1.3. Magnetic Properties for the As-Synthesized Particles and for the Processed Magnetic Nanofluids

The
magnetic properties of the samples were studied by temperature-dependent
Mössbauer spectroscopy. The 80 K Mössbauer spectra of the analyzed samples are
shown in Figure5. The derived probability distributions of the hyperfine field
are presented at the right side. The narrow peak of the distribution, centered
at about 52 T, evidences the presence of mainly maghemite in sample 01. The
larger distribution peak centered at about 48 T in sample 02 might have two
different reasons: (i) a starting magnetic relaxation process already present
at 80 K and (ii) a much defected structure shifting structurally from maghemite
to magnetite (the average magnetic hyperfine field of magnetite being lower
than the one of maghemite). The magnetic relaxation process was analyzed [23]
in terms of the collapsing behavior of the Mössbauer spectra at higher temperatures.
From the Mossbauer spectra obtained at different temperatures, blocking
temperatures of about 400 K and 200 K were derived for samples 01 and 02,
respectively. Particle average sizes of about 9 nm for sample 01 and,
respectively, 7 nm for samples 02 were estimated, starting from the above-mentioned
blocking temperatures and considering noninteracting nanoparticles.The ferrofluids obtained by dispersing the
nanopowders in water showed blocking temperatures of 230 K and 200 K for
samples MF01 and MF02, respectively. It is worth mentioning that the higher
blocking temperature observed for 01 as compared to MF01 is most probably
generated by nonnegligible interparticle interactions in the powder sample (e.g., the substantial presence of maghemite with its ferri-magnetic behavior).
In the defected maghemite-magnetite mixtures (sample 02), a spin-glass-like
structure with reduced (almost negligible) net magnetic moment per particle
seems to be present, affecting/reducing drastically the magnetic dipolar
interactions, even in the powder form. The best magnetic performances (a
maximum amount of magnetic nanoparticles at room temperature and related
highest magnetization) correspond to the ferrofluid obtained by dispersing the
well-formed maghemite-based nanoparticles synthesized under a laser power of 55 W.The reduced magnetization curves of the as-synthesized
nanopowders are presented in Figure7. It is worth mentioning that none of the
samples is saturated at the maximum available field of  1.1 T. This behavior can be considered an
additional evidence for the disordered spin configuration, connected with the
defected structure inside nanoparticles, in agreement with the Mössbauer data.   Therefore,
the reduced magnetization curves for the two samples were obtained with respect
to the specific magnetization at the maximum field (denoted as Ms).  The
magnetization Ms of the 01 sample, 50 emu/g, is much greater
than for sample 02 (15 emu/g) supporting a three times larger magnetic moment
per particle in sample 01 as compared with sample 02 in the assumption that the
particle mean sizes of the two samples are almost similar.  On the other hand, the reduced magnetization
of sample 01 at very low field is clearly larger than that of sample 02, providing
evidence for a larger amount of magnetic state at room temperature in this
sample in agreement with the larger blocking temperature evidenced by Mössbauer
spectroscopy.Figure 7
Reduced magnetization curves for 01
and 02 nanopowders.Following
the procedures described in a
previous section, the iron oxide nanopowders were subsequently
synthesized as water-based magnetic fluids (MFs). TEM analysis of the
ferrofluids showed that depending on the nanopowder synthesis parameters,
different degrees of agglomeration are still present.  The morphology of samples MF01 and MF02 may
be examined in Figures8(a) and 8(b). The nanoparticles exhibit a spherical shape,
confirmed by the 0.86 value of the form factor. From SAED analysis (no
presented here), an iron oxide cubic structure is deduced, with lattice
constants of 0.835 nm and 0.837 nm for MF01 and MF02, respectively. This
estimation may indicate that the maghemite-like structure is favored in the ferrofluid-derived
from the sample synthesized at high laser power. On the other hand, one may
observe that the MF prepared from 02 nanoparticles (Figure 8(b)) presents a
slightly lower degree of agglomeration as compared with MF02 (Figure 8(a)). The
suppression of interparticle interaction in sample MF01 could be less
effective, in spite of the fact that insignificant magnetic relaxation differences
separate  the MF samples.TEM
images showing the morphology of the magnetic nanofluids prepared from samples MF01
and MF02, respectively.
(a)(b)The
reduced magnetization curves of the ferrofluid samples MF01 and MF02,
synthesized from the powders 01 and 02, respectively, are shown in Figure9.
The dependencies of the reduced magnetizations versus the applied magnetic
field are quite similar to the ones of the powder samples. A major difference
concerns the values of the magnetization, Ms, at the maximum field
of 1.1 T. This time these are expressed in emu/cm3 (or A/m) in order
to take into account the inhomogeneous character of the ferrofluid, seen as a
liquid-solid composite (Ms given in emu/cm3 derives from
the specific magnetization in emu/g times the ferrofluid density, in g/cm3).
The Ms value in sample MF01 is almost 7 times larger than the value
in sample MF02, proving clearly the better magnetic performances of the
ferrofluid derived from the 01 powder sample. In addition, one observes that
the low field part of the reduced magnetization in the magnetic fluid sample
MF01 is lower than for the 01 sample and that a certain amount of
suparparamagnetic phase is to be found in this sample even at 1.1 T. Therefore,
due to the cancellation of interparticle interactions, the magnetometry data
infer that the magnetic relaxation at room temperature has to be more advanced
in the MF01 ferrofluid sample as compared with the 01 powder sample (in
agreement with the Mossbauer data showing a lower blocking temperature in the
ferrofluid). Hence, the magnetic relaxation may hide the information about the
static spin disorder. The specific magnetizations are not strictly related to
the different degree of spin disorder in the samples but, at least
qualitatively, the magnetometry results, in agreement with the Mossbauer ones,
support very well the conclusion that the lowest spin disorder is present in
the 01 sample and in the corresponding MF01 ferrofluid.Figure 9
Reduced
magnetization curves of water magnetic fluid samples prepared from 01 and 02
nanopowders.


### 3.2. Titania, with Major TiO2 Anatase Phase

#### 3.2.1. Particle  Preparation and Characterization

The synthesis of TiO2
nanopowder was performed by employing a modified version of the pyrolysis setup
[24] in which TiCl4 and air,
as gas-phase precursors, are simultaneously allowed to emerge into the flow
reactor (see also Figure 1). Ethylene served as a carrier for the TiCl4 vapors and as an energy transfer agent. A special irradiation geometry
employing a central triple nozzle system was used. The focused IR radiation
(400 W nominal power) was intersecting at right angles the reactant flows. During
all experiments, the total pressure in the reactor was held at 550 mbar.The samples (labeled SN,
SA, and S0) are studied relatively to the input variation of their
oxygen precursors. Sample SN was obtained by replacing air with
nitrous oxide. A much lower air flow was used to obtain sample SA as
compared to sample S0. The main experimental parameters are listed in Table 2.
In the last columns of the table are gathered the mean crystallite sizes [25]
as well as the lattice parameters (a,c) as extracted from the XRD patterns
(see below).Table 2
Synthesis parameters and crystallographic parameters of nanoTiO2 particles.
SampleΦair* [sccm]ΦC2H4* (TiCl4) [sccm]TiO2-phase proportion**Lattice parameters**D Mean crystallite dimension [nm]A (%)R (%)A (JCPDS 21-1272)R (JCPDS 21-1272)ARa = 3.785 Åa = 4.593 Åc = 9.513 Åc = 2.959 Åa (Å)c (Å)a (Å)c (Å)SN10 (N2O)10064.135.93.8059.564.702.9214.44.2SA5010074.625.43.819.574.642.96517.815.0S015010092.37.73.819.554.632.9518.921.4*ΦC2H4 and
Φair, are the flows of the
ethylene (carrier of Fe(CO)5 vapors) and of the air oxidizer,
respectively (in standard cubic centimeter per minute);
**A and R stand for anatase and rutile, respectively; a and c are the values of the
lattice parameters of titania, according to standard JCPDS 21-1272 data.The obtained titanium oxide
nanostructures were characterized by XRD, TEM, and SAED.XRD phase analysis (Figure10
and the last columns of Table 2) reveals an overwhelming dominancy of the
anatase phase (more than about 92%) in the sample S0, obtained at the highest
air precursor concentration. It was already observed [26] that for titania
produced by RF thermal plasma, rutile formation is favored by oxygen-deficient
gas-phase TiO2 clusters while anatase tends to condense from
oxygen-rich clusters. We should mention that for both air and nitrous oxide, at
lower air flows (e.g., 10 sccm for sample SN), the powder was gray
with slight blue parts. At high air input flows (150 sccm for sample S0), the
color of the powder becomes light yellow to white. Some salient features
characterize the SN, SA, and S0 series: (i) the
significant decrease of the mean particle sizes for SN and SA relatively to S0, particularly concerning the rutile phase; and (ii) the
decrease of the total amount of the TiO2 phases relatively to the S0
sample, especially in case of the anatase phase. For the TiO2 SA sample, anatase percentage is approximately 74%, which is quite close to the
well-known Degussa P25 commercial sample.Figure 10
XRD
diffraction patterns for the nanotitania samples SN, SA,
and S0, with anatase (A) and rutile (R) peak identification.Low-magnification TEM micrographsfor TiO2 samples SN and SA (Figure 11) show groups of
loosely-bound aggregated nanoparticles.General TEM view of the samples SN (a) and SA (b).
(a)(b)The SAED patterns for samples SA                               and S0 are presented in Figure 12. The identification of the interplanar
distances is indicated by arrows and these qualitative results can be directly
compared with those obtained by means of XRD. In general, SAED analysis
confirms that the titania nanoparticles are a mixture of anatase and rutile.
However, increased crystallinity and dominant anatase phase are found for the
reference sample (S0). We should note that the diffusivity observed in the SAED
pattern of sample SA could be due not only to an amorphous material
but also to the increased number of crystalline nanometer-sized particles [27]. The most intense diffraction rings can be assigned to the (101) anatase (at
3.52 Å) and (110) rutile (at 3.25 Å). Some rings may appear by a superposition
of anatase and rutile reflections, such as the rings at 2.48 Å  and 1.68 Å. In
case of the better crystallinity of sample S0, the complementary ring at 1.4 Å  (corresponding to rutile (007) and anatase (204)) is identified.SAED patterns for samples SA and S0.
(a)(b)In Figure13, a comparative TEM
analysis for medium- (a, c, and e) and high-resolution (b, d, and f) images of SN,
SA and S0 titania samples is performed.  At a higher magnification, the TEM
micrographs of samples discussed in this work exhibit both round-shaped and
elongated or facetted particles with irregular shapes and sizes (Figures 13(a)–13(f)). Primary
nanocrystals seem to be often present. In Figure 13(b), an isolated SN primary nanocrystal is presented, showing an elongated (about 15 nm) shape.  Particles surrounded by an amorphous layer (Figures
13(a) and 13(c)) appear in sample SN and SA, respectively. The
rather well-defined microcrystallites of sample S0 exhibit irregular, mostly
polyhedral shapes (Figure 13(f)).Comparative TEM analysis for medium- (a, c,
and e) and high-resolution (b, d, and f) images of titania samples, showing the
different morphologies of SN, SA, and S0.
(a)(b)(c)(d)(e)(f)


## 3.1. Spinel-Like Iron (III) Oxide (γ-Fe2O3)

### 3.1.1. The Preparation of the Nanoparticles and of Their Magnetic
Fluids Counterparts

Nanoparticle Preparation
For producing nanosized
iron oxide particles, the focused continuous-wave CO2 laser
radiation (80 W maximum output power, λ = 10.6 μm) orthogonally crossed the gas flows
emerging through two concentric nozzles (Figure 1). The gas mixture which
contained air and Fe(CO)5 vapors (about 25 Torr vapor pressure at 20°C), entrained by C2H4, was admitted through
the central inner tube. Iron
oxide samples (labeled as 01 and 02) were obtained by varying the laser
power between 35 W and 55 W as presented in Table 1. The other experimental
conditions, namely, the reactor pressure, the relative flow of the oxidizing
agent (air), and the flow of ethylene (as Fe(CO)5 carrier) were maintained constant
(their values are displayed in Table 1).Table 1
Experimental
parameters and main crystallographic characteristics (estimated from XRD
analysis) for the as-synthesized nano iron oxide samples.
SampleΦC2H4* (through Fe(CO)5 [sccm]Φair* [sccm]P** [mbar]PL* [watt]XRD analysisEDAX-elemental analysis, at%a (unit cell parameter) [′]D (mean crystallite dimension) [nm]FeOC0114570300558.3644.536.2952.9910.720214570300358.3773.035.0955.129.79*ΦC2H4 and Φair, are the flows of the ethylene (carrier of Fe(CO)5 vapors) and of air oxidizer,
respectively (in standard cubic centimeter per minute);
**P is the pressure in the reaction chamber (in mbar);
***PL is the laser power (in Watt).Magnetic Fluid Preparation
The as-synthesized iron oxide nanopowders were used for the preparation of magnetic
nanofluids. In what follows, the obtained magnetic fluid samples will be
labeled MF01 and MF02 and are derived by dispersing in water samples 01 and 02,
respectively. Water-based magnetic nanofluids are a special category of polar
magnetic nanofluids showing particularities as concerning particle interactions
and agglomerates formation.  The
synthesis of magnetic fluids has two main stages (1) preparation of nanosized
magnetic particles, in particular by laser pyrolysis, and (2) subsequent
stabilization/dispersion of the nanoparticles in water carrier. Electrostatic
and steric stabilization procedures are both applicable for water-based
magnetic nanofluids. Double layers steric + electrostatic (combined) stabilization
of magnetic nanoparticles in water carrier was experimented recently using
various chain length of carboxylic acid surfactants, as well as dodecyl
benzensulphonic acid (DBS) [22].  In case
of magnetic nanoparticles prepared by laser pyrolysis, a complementary
difficulty appears due to the large agglomerates existing already in the nanopowder
state, as a consequence of the magnetic dipolar interactions between uncoated
nanoparticles. The sequence of steps of the preparation procedure for water-based
magnetic nanofluids may be summarized as follows: (a) addition of NaOH (6N) to
the water/magnetic nanopowder mixture in order to ensure a pH value of about
8.5–9, followed by
heating up the reaction medium to about 75–80°C;
(b) chemisorption of the surfactant (in the present case, lauric acid (LA)); (c)
magnetic organosol; (d) magnetic decantation (elimination of residual salts); (e)
correction of pH (using NaOH); (f) dispersion of surface-coated magnetic
nanoparticles; (g) primary magnetic nanofluid; (h) magnetic
decantation/filtration; (i) water-based magnetic nanofluid (diluted, with solid
volume fraction of about 5%).


### 3.1.2. Nanoparticle Characterization

The
XRD analysis of the as-synthesized nanopowders points to the presence of the
spinel-like iron (III) oxide [10], namely, maghemite/magnetite iron phase
(according to JCPDS 19-0629 for standard maghemite and JCPDS 39–1346 for
maghemite). In Figure 2, a representative diffractogram obtained for the as-synthesized
oxides is displayed. Although it refers to sample 01, its characteristic phase
identification features are similar to that of sample 02 (not presented here). The
unit-cell parameter (Table 1) was estimated from the 311 and 440 reflections.
It presents relatively lower values to the standard magnetite value (Fe3O4 with
a = 8.396 Å), suggesting the formation of a more likely maghemite phase (γ-Fe2O3).
The broadness of the diffraction lines is associated with the formation of very
small particles as well as with a high degree of structural/crystallographic
disorder. The estimated mean crystallite dimension D (nm) is presented in Table 1. A higher crystallinity and slightly larger mean crystallite size may be
deduced for the sample obtained at the higher laser power.Figure 2
XRD diffraction patterns for a laser-synthesized
iron oxide (sample 01): peak identification points to a chemical composition
containing maghemite/magnetite iron oxide phases.The
identification of elements present in powders was performed using the energy
dispersive analysis by X-rays (EDAX). The results are presented in the last
column of Table1. The stoichiometry, suggested by the relative quantities of
oxygen and iron present in the synthesized powders, corresponds to a Fe3+ iron oxide, most probably maghemite. A rather low carbon contamination (about
10%), mainly due to the unwanted ethylene dissociation may be noticed. Although
revealed by EDAX, the XRD analysis seems not able to reveal traces of carbon. It
is worth to note that the sensitivity of this analysis is down to a few atomic
percent and that low Z elements are detectable with limited sensitivity.TEM
analysis of samples 01 and 02 reveals an almost polycrystalline morphology (Figures3(a), 3(b)). The nanoparticles show coalescent features. Cross-linked chains may be
sometimes observed. An estimation of the particle mean sizes gives about 4.5
and 6 nm for samples 01 and 02, respectively.TEM analysis for iron oxides obtained from (a) sample 01 and (b) sample 02.
(a)(b)The
reflections revealed by the SAED
analysis (Figure4) may be attributed to a cubic structure of the iron oxide,
namely, maghemite/magnetite. The strong diffraction ring at 0.25 nm may be
noticed. Although difficult to distinguish between
these phases from electron diffraction patterns alone, it is probable that
maghemite prevails by referring to XRD results (see, e.g., the value of the
estimated lattice parameter a in
Table 1). Increased crystallinity seems to characterize the nanomaterial
obtained at higher laser power. This can be considered as a consequence of both the higher temperature
gradients in the reactions zone and the fast quenching of the condensed nanoparticles
in the condensation zone. As revealed by the characterization methods employed,
this process greatly favors maghemite phase in powders.Figure 4
SAED analysis of sample 02, revealing main diffraction rings of maghemite/magnetite.HRTEM analysis was performed on many different domains of the nanopowders
and is presented in Figure5.
Nanocrystals with different orientation and mean diameters of about 5-6 nm are
displayed in Figure 5(a). From the indicated squarish areas, the Fourier
transform images are presented as insets. The left-side and the right-side
groups of nanocrystals in Figure 5(a) display characteristics of the cubic
structure of a maghemite crystal. The lattice fringes, corresponding to
the (113)  (d113 = 0.253 nm) and to the (220)  (d220 = 0.313 nm) planes, are evidenced. A small distribution of d-values is caused by
the imperfect crystallographic structure, probably due to the quenching of the
material when leaving the reaction zone. The HRTEM image of Figure 5(b)
presents agglomerated nanocrystals. They seem to be surrounded by an amorphous
layer.HRTEM
images for samples: (a) 01 and (b) 02.  Space representations of Fourier transform
(displayed as insets on figure) seem to evidence the diffraction planes of
maghemite/magnetite.
(a)(b)Figure 6
Mössbauer
spectra (at 80 K) of the two as-synthesized nanopowder samples; at the right side,
the probability distributions of the hyperfine field are also displayed.


### 3.1.3. Magnetic Properties for the As-Synthesized Particles and for the Processed Magnetic Nanofluids

The
magnetic properties of the samples were studied by temperature-dependent
Mössbauer spectroscopy. The 80 K Mössbauer spectra of the analyzed samples are
shown in Figure5. The derived probability distributions of the hyperfine field
are presented at the right side. The narrow peak of the distribution, centered
at about 52 T, evidences the presence of mainly maghemite in sample 01. The
larger distribution peak centered at about 48 T in sample 02 might have two
different reasons: (i) a starting magnetic relaxation process already present
at 80 K and (ii) a much defected structure shifting structurally from maghemite
to magnetite (the average magnetic hyperfine field of magnetite being lower
than the one of maghemite). The magnetic relaxation process was analyzed [23]
in terms of the collapsing behavior of the Mössbauer spectra at higher temperatures.
From the Mossbauer spectra obtained at different temperatures, blocking
temperatures of about 400 K and 200 K were derived for samples 01 and 02,
respectively. Particle average sizes of about 9 nm for sample 01 and,
respectively, 7 nm for samples 02 were estimated, starting from the above-mentioned
blocking temperatures and considering noninteracting nanoparticles.The ferrofluids obtained by dispersing the
nanopowders in water showed blocking temperatures of 230 K and 200 K for
samples MF01 and MF02, respectively. It is worth mentioning that the higher
blocking temperature observed for 01 as compared to MF01 is most probably
generated by nonnegligible interparticle interactions in the powder sample (e.g., the substantial presence of maghemite with its ferri-magnetic behavior).
In the defected maghemite-magnetite mixtures (sample 02), a spin-glass-like
structure with reduced (almost negligible) net magnetic moment per particle
seems to be present, affecting/reducing drastically the magnetic dipolar
interactions, even in the powder form. The best magnetic performances (a
maximum amount of magnetic nanoparticles at room temperature and related
highest magnetization) correspond to the ferrofluid obtained by dispersing the
well-formed maghemite-based nanoparticles synthesized under a laser power of 55 W.The reduced magnetization curves of the as-synthesized
nanopowders are presented in Figure7. It is worth mentioning that none of the
samples is saturated at the maximum available field of  1.1 T. This behavior can be considered an
additional evidence for the disordered spin configuration, connected with the
defected structure inside nanoparticles, in agreement with the Mössbauer data.   Therefore,
the reduced magnetization curves for the two samples were obtained with respect
to the specific magnetization at the maximum field (denoted as Ms).  The
magnetization Ms of the 01 sample, 50 emu/g, is much greater
than for sample 02 (15 emu/g) supporting a three times larger magnetic moment
per particle in sample 01 as compared with sample 02 in the assumption that the
particle mean sizes of the two samples are almost similar.  On the other hand, the reduced magnetization
of sample 01 at very low field is clearly larger than that of sample 02, providing
evidence for a larger amount of magnetic state at room temperature in this
sample in agreement with the larger blocking temperature evidenced by Mössbauer
spectroscopy.Figure 7
Reduced magnetization curves for 01
and 02 nanopowders.Following
the procedures described in a
previous section, the iron oxide nanopowders were subsequently
synthesized as water-based magnetic fluids (MFs). TEM analysis of the
ferrofluids showed that depending on the nanopowder synthesis parameters,
different degrees of agglomeration are still present.  The morphology of samples MF01 and MF02 may
be examined in Figures8(a) and 8(b). The nanoparticles exhibit a spherical shape,
confirmed by the 0.86 value of the form factor. From SAED analysis (no
presented here), an iron oxide cubic structure is deduced, with lattice
constants of 0.835 nm and 0.837 nm for MF01 and MF02, respectively. This
estimation may indicate that the maghemite-like structure is favored in the ferrofluid-derived
from the sample synthesized at high laser power. On the other hand, one may
observe that the MF prepared from 02 nanoparticles (Figure 8(b)) presents a
slightly lower degree of agglomeration as compared with MF02 (Figure 8(a)). The
suppression of interparticle interaction in sample MF01 could be less
effective, in spite of the fact that insignificant magnetic relaxation differences
separate  the MF samples.TEM
images showing the morphology of the magnetic nanofluids prepared from samples MF01
and MF02, respectively.
(a)(b)The
reduced magnetization curves of the ferrofluid samples MF01 and MF02,
synthesized from the powders 01 and 02, respectively, are shown in Figure9.
The dependencies of the reduced magnetizations versus the applied magnetic
field are quite similar to the ones of the powder samples. A major difference
concerns the values of the magnetization, Ms, at the maximum field
of 1.1 T. This time these are expressed in emu/cm3 (or A/m) in order
to take into account the inhomogeneous character of the ferrofluid, seen as a
liquid-solid composite (Ms given in emu/cm3 derives from
the specific magnetization in emu/g times the ferrofluid density, in g/cm3).
The Ms value in sample MF01 is almost 7 times larger than the value
in sample MF02, proving clearly the better magnetic performances of the
ferrofluid derived from the 01 powder sample. In addition, one observes that
the low field part of the reduced magnetization in the magnetic fluid sample
MF01 is lower than for the 01 sample and that a certain amount of
suparparamagnetic phase is to be found in this sample even at 1.1 T. Therefore,
due to the cancellation of interparticle interactions, the magnetometry data
infer that the magnetic relaxation at room temperature has to be more advanced
in the MF01 ferrofluid sample as compared with the 01 powder sample (in
agreement with the Mossbauer data showing a lower blocking temperature in the
ferrofluid). Hence, the magnetic relaxation may hide the information about the
static spin disorder. The specific magnetizations are not strictly related to
the different degree of spin disorder in the samples but, at least
qualitatively, the magnetometry results, in agreement with the Mossbauer ones,
support very well the conclusion that the lowest spin disorder is present in
the 01 sample and in the corresponding MF01 ferrofluid.Figure 9
Reduced
magnetization curves of water magnetic fluid samples prepared from 01 and 02
nanopowders.


## 3.1.1. The Preparation of the Nanoparticles and of Their Magnetic
Fluids Counterparts

Nanoparticle Preparation
For producing nanosized
iron oxide particles, the focused continuous-wave CO2 laser
radiation (80 W maximum output power, λ = 10.6 μm) orthogonally crossed the gas flows
emerging through two concentric nozzles (Figure 1). The gas mixture which
contained air and Fe(CO)5 vapors (about 25 Torr vapor pressure at 20°C), entrained by C2H4, was admitted through
the central inner tube. Iron
oxide samples (labeled as 01 and 02) were obtained by varying the laser
power between 35 W and 55 W as presented in Table 1. The other experimental
conditions, namely, the reactor pressure, the relative flow of the oxidizing
agent (air), and the flow of ethylene (as Fe(CO)5 carrier) were maintained constant
(their values are displayed in Table 1).Table 1
Experimental
parameters and main crystallographic characteristics (estimated from XRD
analysis) for the as-synthesized nano iron oxide samples.
SampleΦC2H4* (through Fe(CO)5 [sccm]Φair* [sccm]P** [mbar]PL* [watt]XRD analysisEDAX-elemental analysis, at%a (unit cell parameter) [′]D (mean crystallite dimension) [nm]FeOC0114570300558.3644.536.2952.9910.720214570300358.3773.035.0955.129.79*ΦC2H4 and Φair, are the flows of the ethylene (carrier of Fe(CO)5 vapors) and of air oxidizer,
respectively (in standard cubic centimeter per minute);
**P is the pressure in the reaction chamber (in mbar);
***PL is the laser power (in Watt).Magnetic Fluid Preparation
The as-synthesized iron oxide nanopowders were used for the preparation of magnetic
nanofluids. In what follows, the obtained magnetic fluid samples will be
labeled MF01 and MF02 and are derived by dispersing in water samples 01 and 02,
respectively. Water-based magnetic nanofluids are a special category of polar
magnetic nanofluids showing particularities as concerning particle interactions
and agglomerates formation.  The
synthesis of magnetic fluids has two main stages (1) preparation of nanosized
magnetic particles, in particular by laser pyrolysis, and (2) subsequent
stabilization/dispersion of the nanoparticles in water carrier. Electrostatic
and steric stabilization procedures are both applicable for water-based
magnetic nanofluids. Double layers steric + electrostatic (combined) stabilization
of magnetic nanoparticles in water carrier was experimented recently using
various chain length of carboxylic acid surfactants, as well as dodecyl
benzensulphonic acid (DBS) [22].  In case
of magnetic nanoparticles prepared by laser pyrolysis, a complementary
difficulty appears due to the large agglomerates existing already in the nanopowder
state, as a consequence of the magnetic dipolar interactions between uncoated
nanoparticles. The sequence of steps of the preparation procedure for water-based
magnetic nanofluids may be summarized as follows: (a) addition of NaOH (6N) to
the water/magnetic nanopowder mixture in order to ensure a pH value of about
8.5–9, followed by
heating up the reaction medium to about 75–80°C;
(b) chemisorption of the surfactant (in the present case, lauric acid (LA)); (c)
magnetic organosol; (d) magnetic decantation (elimination of residual salts); (e)
correction of pH (using NaOH); (f) dispersion of surface-coated magnetic
nanoparticles; (g) primary magnetic nanofluid; (h) magnetic
decantation/filtration; (i) water-based magnetic nanofluid (diluted, with solid
volume fraction of about 5%).


## 3.1.2. Nanoparticle Characterization

The
XRD analysis of the as-synthesized nanopowders points to the presence of the
spinel-like iron (III) oxide [10], namely, maghemite/magnetite iron phase
(according to JCPDS 19-0629 for standard maghemite and JCPDS 39–1346 for
maghemite). In Figure 2, a representative diffractogram obtained for the as-synthesized
oxides is displayed. Although it refers to sample 01, its characteristic phase
identification features are similar to that of sample 02 (not presented here). The
unit-cell parameter (Table 1) was estimated from the 311 and 440 reflections.
It presents relatively lower values to the standard magnetite value (Fe3O4 with
a = 8.396 Å), suggesting the formation of a more likely maghemite phase (γ-Fe2O3).
The broadness of the diffraction lines is associated with the formation of very
small particles as well as with a high degree of structural/crystallographic
disorder. The estimated mean crystallite dimension D (nm) is presented in Table 1. A higher crystallinity and slightly larger mean crystallite size may be
deduced for the sample obtained at the higher laser power.Figure 2
XRD diffraction patterns for a laser-synthesized
iron oxide (sample 01): peak identification points to a chemical composition
containing maghemite/magnetite iron oxide phases.The
identification of elements present in powders was performed using the energy
dispersive analysis by X-rays (EDAX). The results are presented in the last
column of Table1. The stoichiometry, suggested by the relative quantities of
oxygen and iron present in the synthesized powders, corresponds to a Fe3+ iron oxide, most probably maghemite. A rather low carbon contamination (about
10%), mainly due to the unwanted ethylene dissociation may be noticed. Although
revealed by EDAX, the XRD analysis seems not able to reveal traces of carbon. It
is worth to note that the sensitivity of this analysis is down to a few atomic
percent and that low Z elements are detectable with limited sensitivity.TEM
analysis of samples 01 and 02 reveals an almost polycrystalline morphology (Figures3(a), 3(b)). The nanoparticles show coalescent features. Cross-linked chains may be
sometimes observed. An estimation of the particle mean sizes gives about 4.5
and 6 nm for samples 01 and 02, respectively.TEM analysis for iron oxides obtained from (a) sample 01 and (b) sample 02.
(a)(b)The
reflections revealed by the SAED
analysis (Figure4) may be attributed to a cubic structure of the iron oxide,
namely, maghemite/magnetite. The strong diffraction ring at 0.25 nm may be
noticed. Although difficult to distinguish between
these phases from electron diffraction patterns alone, it is probable that
maghemite prevails by referring to XRD results (see, e.g., the value of the
estimated lattice parameter a in
Table 1). Increased crystallinity seems to characterize the nanomaterial
obtained at higher laser power. This can be considered as a consequence of both the higher temperature
gradients in the reactions zone and the fast quenching of the condensed nanoparticles
in the condensation zone. As revealed by the characterization methods employed,
this process greatly favors maghemite phase in powders.Figure 4
SAED analysis of sample 02, revealing main diffraction rings of maghemite/magnetite.HRTEM analysis was performed on many different domains of the nanopowders
and is presented in Figure5.
Nanocrystals with different orientation and mean diameters of about 5-6 nm are
displayed in Figure 5(a). From the indicated squarish areas, the Fourier
transform images are presented as insets. The left-side and the right-side
groups of nanocrystals in Figure 5(a) display characteristics of the cubic
structure of a maghemite crystal. The lattice fringes, corresponding to
the (113)  (d113 = 0.253 nm) and to the (220)  (d220 = 0.313 nm) planes, are evidenced. A small distribution of d-values is caused by
the imperfect crystallographic structure, probably due to the quenching of the
material when leaving the reaction zone. The HRTEM image of Figure 5(b)
presents agglomerated nanocrystals. They seem to be surrounded by an amorphous
layer.HRTEM
images for samples: (a) 01 and (b) 02.  Space representations of Fourier transform
(displayed as insets on figure) seem to evidence the diffraction planes of
maghemite/magnetite.
(a)(b)Figure 6
Mössbauer
spectra (at 80 K) of the two as-synthesized nanopowder samples; at the right side,
the probability distributions of the hyperfine field are also displayed.


## 3.1.3. Magnetic Properties for the As-Synthesized Particles and for the Processed Magnetic Nanofluids

The
magnetic properties of the samples were studied by temperature-dependent
Mössbauer spectroscopy. The 80 K Mössbauer spectra of the analyzed samples are
shown in Figure5. The derived probability distributions of the hyperfine field
are presented at the right side. The narrow peak of the distribution, centered
at about 52 T, evidences the presence of mainly maghemite in sample 01. The
larger distribution peak centered at about 48 T in sample 02 might have two
different reasons: (i) a starting magnetic relaxation process already present
at 80 K and (ii) a much defected structure shifting structurally from maghemite
to magnetite (the average magnetic hyperfine field of magnetite being lower
than the one of maghemite). The magnetic relaxation process was analyzed [23]
in terms of the collapsing behavior of the Mössbauer spectra at higher temperatures.
From the Mossbauer spectra obtained at different temperatures, blocking
temperatures of about 400 K and 200 K were derived for samples 01 and 02,
respectively. Particle average sizes of about 9 nm for sample 01 and,
respectively, 7 nm for samples 02 were estimated, starting from the above-mentioned
blocking temperatures and considering noninteracting nanoparticles.The ferrofluids obtained by dispersing the
nanopowders in water showed blocking temperatures of 230 K and 200 K for
samples MF01 and MF02, respectively. It is worth mentioning that the higher
blocking temperature observed for 01 as compared to MF01 is most probably
generated by nonnegligible interparticle interactions in the powder sample (e.g., the substantial presence of maghemite with its ferri-magnetic behavior).
In the defected maghemite-magnetite mixtures (sample 02), a spin-glass-like
structure with reduced (almost negligible) net magnetic moment per particle
seems to be present, affecting/reducing drastically the magnetic dipolar
interactions, even in the powder form. The best magnetic performances (a
maximum amount of magnetic nanoparticles at room temperature and related
highest magnetization) correspond to the ferrofluid obtained by dispersing the
well-formed maghemite-based nanoparticles synthesized under a laser power of 55 W.The reduced magnetization curves of the as-synthesized
nanopowders are presented in Figure7. It is worth mentioning that none of the
samples is saturated at the maximum available field of  1.1 T. This behavior can be considered an
additional evidence for the disordered spin configuration, connected with the
defected structure inside nanoparticles, in agreement with the Mössbauer data.   Therefore,
the reduced magnetization curves for the two samples were obtained with respect
to the specific magnetization at the maximum field (denoted as Ms).  The
magnetization Ms of the 01 sample, 50 emu/g, is much greater
than for sample 02 (15 emu/g) supporting a three times larger magnetic moment
per particle in sample 01 as compared with sample 02 in the assumption that the
particle mean sizes of the two samples are almost similar.  On the other hand, the reduced magnetization
of sample 01 at very low field is clearly larger than that of sample 02, providing
evidence for a larger amount of magnetic state at room temperature in this
sample in agreement with the larger blocking temperature evidenced by Mössbauer
spectroscopy.Figure 7
Reduced magnetization curves for 01
and 02 nanopowders.Following
the procedures described in a
previous section, the iron oxide nanopowders were subsequently
synthesized as water-based magnetic fluids (MFs). TEM analysis of the
ferrofluids showed that depending on the nanopowder synthesis parameters,
different degrees of agglomeration are still present.  The morphology of samples MF01 and MF02 may
be examined in Figures8(a) and 8(b). The nanoparticles exhibit a spherical shape,
confirmed by the 0.86 value of the form factor. From SAED analysis (no
presented here), an iron oxide cubic structure is deduced, with lattice
constants of 0.835 nm and 0.837 nm for MF01 and MF02, respectively. This
estimation may indicate that the maghemite-like structure is favored in the ferrofluid-derived
from the sample synthesized at high laser power. On the other hand, one may
observe that the MF prepared from 02 nanoparticles (Figure 8(b)) presents a
slightly lower degree of agglomeration as compared with MF02 (Figure 8(a)). The
suppression of interparticle interaction in sample MF01 could be less
effective, in spite of the fact that insignificant magnetic relaxation differences
separate  the MF samples.TEM
images showing the morphology of the magnetic nanofluids prepared from samples MF01
and MF02, respectively.
(a)(b)The
reduced magnetization curves of the ferrofluid samples MF01 and MF02,
synthesized from the powders 01 and 02, respectively, are shown in Figure9.
The dependencies of the reduced magnetizations versus the applied magnetic
field are quite similar to the ones of the powder samples. A major difference
concerns the values of the magnetization, Ms, at the maximum field
of 1.1 T. This time these are expressed in emu/cm3 (or A/m) in order
to take into account the inhomogeneous character of the ferrofluid, seen as a
liquid-solid composite (Ms given in emu/cm3 derives from
the specific magnetization in emu/g times the ferrofluid density, in g/cm3).
The Ms value in sample MF01 is almost 7 times larger than the value
in sample MF02, proving clearly the better magnetic performances of the
ferrofluid derived from the 01 powder sample. In addition, one observes that
the low field part of the reduced magnetization in the magnetic fluid sample
MF01 is lower than for the 01 sample and that a certain amount of
suparparamagnetic phase is to be found in this sample even at 1.1 T. Therefore,
due to the cancellation of interparticle interactions, the magnetometry data
infer that the magnetic relaxation at room temperature has to be more advanced
in the MF01 ferrofluid sample as compared with the 01 powder sample (in
agreement with the Mossbauer data showing a lower blocking temperature in the
ferrofluid). Hence, the magnetic relaxation may hide the information about the
static spin disorder. The specific magnetizations are not strictly related to
the different degree of spin disorder in the samples but, at least
qualitatively, the magnetometry results, in agreement with the Mossbauer ones,
support very well the conclusion that the lowest spin disorder is present in
the 01 sample and in the corresponding MF01 ferrofluid.Figure 9
Reduced
magnetization curves of water magnetic fluid samples prepared from 01 and 02
nanopowders.


## 3.2. Titania, with Major TiO2 Anatase Phase

### 3.2.1. Particle  Preparation and Characterization

The synthesis of TiO2
nanopowder was performed by employing a modified version of the pyrolysis setup
[24] in which TiCl4 and air,
as gas-phase precursors, are simultaneously allowed to emerge into the flow
reactor (see also Figure 1). Ethylene served as a carrier for the TiCl4 vapors and as an energy transfer agent. A special irradiation geometry
employing a central triple nozzle system was used. The focused IR radiation
(400 W nominal power) was intersecting at right angles the reactant flows. During
all experiments, the total pressure in the reactor was held at 550 mbar.The samples (labeled SN,
SA, and S0) are studied relatively to the input variation of their
oxygen precursors. Sample SN was obtained by replacing air with
nitrous oxide. A much lower air flow was used to obtain sample SA as
compared to sample S0. The main experimental parameters are listed in Table 2.
In the last columns of the table are gathered the mean crystallite sizes [25]
as well as the lattice parameters (a,c) as extracted from the XRD patterns
(see below).Table 2
Synthesis parameters and crystallographic parameters of nanoTiO2 particles.
SampleΦair* [sccm]ΦC2H4* (TiCl4) [sccm]TiO2-phase proportion**Lattice parameters**D Mean crystallite dimension [nm]A (%)R (%)A (JCPDS 21-1272)R (JCPDS 21-1272)ARa = 3.785 Åa = 4.593 Åc = 9.513 Åc = 2.959 Åa (Å)c (Å)a (Å)c (Å)SN10 (N2O)10064.135.93.8059.564.702.9214.44.2SA5010074.625.43.819.574.642.96517.815.0S015010092.37.73.819.554.632.9518.921.4*ΦC2H4 and
Φair, are the flows of the
ethylene (carrier of Fe(CO)5 vapors) and of the air oxidizer,
respectively (in standard cubic centimeter per minute);
**A and R stand for anatase and rutile, respectively; a and c are the values of the
lattice parameters of titania, according to standard JCPDS 21-1272 data.The obtained titanium oxide
nanostructures were characterized by XRD, TEM, and SAED.XRD phase analysis (Figure10
and the last columns of Table 2) reveals an overwhelming dominancy of the
anatase phase (more than about 92%) in the sample S0, obtained at the highest
air precursor concentration. It was already observed [26] that for titania
produced by RF thermal plasma, rutile formation is favored by oxygen-deficient
gas-phase TiO2 clusters while anatase tends to condense from
oxygen-rich clusters. We should mention that for both air and nitrous oxide, at
lower air flows (e.g., 10 sccm for sample SN), the powder was gray
with slight blue parts. At high air input flows (150 sccm for sample S0), the
color of the powder becomes light yellow to white. Some salient features
characterize the SN, SA, and S0 series: (i) the
significant decrease of the mean particle sizes for SN and SA relatively to S0, particularly concerning the rutile phase; and (ii) the
decrease of the total amount of the TiO2 phases relatively to the S0
sample, especially in case of the anatase phase. For the TiO2 SA sample, anatase percentage is approximately 74%, which is quite close to the
well-known Degussa P25 commercial sample.Figure 10
XRD
diffraction patterns for the nanotitania samples SN, SA,
and S0, with anatase (A) and rutile (R) peak identification.Low-magnification TEM micrographsfor TiO2 samples SN and SA (Figure 11) show groups of
loosely-bound aggregated nanoparticles.General TEM view of the samples SN (a) and SA (b).
(a)(b)The SAED patterns for samples SA                               and S0 are presented in Figure 12. The identification of the interplanar
distances is indicated by arrows and these qualitative results can be directly
compared with those obtained by means of XRD. In general, SAED analysis
confirms that the titania nanoparticles are a mixture of anatase and rutile.
However, increased crystallinity and dominant anatase phase are found for the
reference sample (S0). We should note that the diffusivity observed in the SAED
pattern of sample SA could be due not only to an amorphous material
but also to the increased number of crystalline nanometer-sized particles [27]. The most intense diffraction rings can be assigned to the (101) anatase (at
3.52 Å) and (110) rutile (at 3.25 Å). Some rings may appear by a superposition
of anatase and rutile reflections, such as the rings at 2.48 Å  and 1.68 Å. In
case of the better crystallinity of sample S0, the complementary ring at 1.4 Å  (corresponding to rutile (007) and anatase (204)) is identified.SAED patterns for samples SA and S0.
(a)(b)In Figure13, a comparative TEM
analysis for medium- (a, c, and e) and high-resolution (b, d, and f) images of SN,
SA and S0 titania samples is performed.  At a higher magnification, the TEM
micrographs of samples discussed in this work exhibit both round-shaped and
elongated or facetted particles with irregular shapes and sizes (Figures 13(a)–13(f)). Primary
nanocrystals seem to be often present. In Figure 13(b), an isolated SN primary nanocrystal is presented, showing an elongated (about 15 nm) shape.  Particles surrounded by an amorphous layer (Figures
13(a) and 13(c)) appear in sample SN and SA, respectively. The
rather well-defined microcrystallites of sample S0 exhibit irregular, mostly
polyhedral shapes (Figure 13(f)).Comparative TEM analysis for medium- (a, c,
and e) and high-resolution (b, d, and f) images of titania samples, showing the
different morphologies of SN, SA, and S0.
(a)(b)(c)(d)(e)(f)


## 3.2.1. Particle  Preparation and Characterization

The synthesis of TiO2
nanopowder was performed by employing a modified version of the pyrolysis setup
[24] in which TiCl4 and air,
as gas-phase precursors, are simultaneously allowed to emerge into the flow
reactor (see also Figure 1). Ethylene served as a carrier for the TiCl4 vapors and as an energy transfer agent. A special irradiation geometry
employing a central triple nozzle system was used. The focused IR radiation
(400 W nominal power) was intersecting at right angles the reactant flows. During
all experiments, the total pressure in the reactor was held at 550 mbar.The samples (labeled SN,
SA, and S0) are studied relatively to the input variation of their
oxygen precursors. Sample SN was obtained by replacing air with
nitrous oxide. A much lower air flow was used to obtain sample SA as
compared to sample S0. The main experimental parameters are listed in Table 2.
In the last columns of the table are gathered the mean crystallite sizes [25]
as well as the lattice parameters (a,c) as extracted from the XRD patterns
(see below).Table 2
Synthesis parameters and crystallographic parameters of nanoTiO2 particles.
SampleΦair* [sccm]ΦC2H4* (TiCl4) [sccm]TiO2-phase proportion**Lattice parameters**D Mean crystallite dimension [nm]A (%)R (%)A (JCPDS 21-1272)R (JCPDS 21-1272)ARa = 3.785 Åa = 4.593 Åc = 9.513 Åc = 2.959 Åa (Å)c (Å)a (Å)c (Å)SN10 (N2O)10064.135.93.8059.564.702.9214.44.2SA5010074.625.43.819.574.642.96517.815.0S015010092.37.73.819.554.632.9518.921.4*ΦC2H4 and
Φair, are the flows of the
ethylene (carrier of Fe(CO)5 vapors) and of the air oxidizer,
respectively (in standard cubic centimeter per minute);
**A and R stand for anatase and rutile, respectively; a and c are the values of the
lattice parameters of titania, according to standard JCPDS 21-1272 data.The obtained titanium oxide
nanostructures were characterized by XRD, TEM, and SAED.XRD phase analysis (Figure10
and the last columns of Table 2) reveals an overwhelming dominancy of the
anatase phase (more than about 92%) in the sample S0, obtained at the highest
air precursor concentration. It was already observed [26] that for titania
produced by RF thermal plasma, rutile formation is favored by oxygen-deficient
gas-phase TiO2 clusters while anatase tends to condense from
oxygen-rich clusters. We should mention that for both air and nitrous oxide, at
lower air flows (e.g., 10 sccm for sample SN), the powder was gray
with slight blue parts. At high air input flows (150 sccm for sample S0), the
color of the powder becomes light yellow to white. Some salient features
characterize the SN, SA, and S0 series: (i) the
significant decrease of the mean particle sizes for SN and SA relatively to S0, particularly concerning the rutile phase; and (ii) the
decrease of the total amount of the TiO2 phases relatively to the S0
sample, especially in case of the anatase phase. For the TiO2 SA sample, anatase percentage is approximately 74%, which is quite close to the
well-known Degussa P25 commercial sample.Figure 10
XRD
diffraction patterns for the nanotitania samples SN, SA,
and S0, with anatase (A) and rutile (R) peak identification.Low-magnification TEM micrographsfor TiO2 samples SN and SA (Figure 11) show groups of
loosely-bound aggregated nanoparticles.General TEM view of the samples SN (a) and SA (b).
(a)(b)The SAED patterns for samples SA                               and S0 are presented in Figure 12. The identification of the interplanar
distances is indicated by arrows and these qualitative results can be directly
compared with those obtained by means of XRD. In general, SAED analysis
confirms that the titania nanoparticles are a mixture of anatase and rutile.
However, increased crystallinity and dominant anatase phase are found for the
reference sample (S0). We should note that the diffusivity observed in the SAED
pattern of sample SA could be due not only to an amorphous material
but also to the increased number of crystalline nanometer-sized particles [27]. The most intense diffraction rings can be assigned to the (101) anatase (at
3.52 Å) and (110) rutile (at 3.25 Å). Some rings may appear by a superposition
of anatase and rutile reflections, such as the rings at 2.48 Å  and 1.68 Å. In
case of the better crystallinity of sample S0, the complementary ring at 1.4 Å  (corresponding to rutile (007) and anatase (204)) is identified.SAED patterns for samples SA and S0.
(a)(b)In Figure13, a comparative TEM
analysis for medium- (a, c, and e) and high-resolution (b, d, and f) images of SN,
SA and S0 titania samples is performed.  At a higher magnification, the TEM
micrographs of samples discussed in this work exhibit both round-shaped and
elongated or facetted particles with irregular shapes and sizes (Figures 13(a)–13(f)). Primary
nanocrystals seem to be often present. In Figure 13(b), an isolated SN primary nanocrystal is presented, showing an elongated (about 15 nm) shape.  Particles surrounded by an amorphous layer (Figures
13(a) and 13(c)) appear in sample SN and SA, respectively. The
rather well-defined microcrystallites of sample S0 exhibit irregular, mostly
polyhedral shapes (Figure 13(f)).Comparative TEM analysis for medium- (a, c,
and e) and high-resolution (b, d, and f) images of titania samples, showing the
different morphologies of SN, SA, and S0.
(a)(b)(c)(d)(e)(f)


## 4. Conclusions

Recent results obtained in the
preparation of gamma iron oxide (γ-Fe2O3) and titania (TiO2)
semiconductor nanostructures using the laser pyrolysis method are outlined.
The properties of different nanometric iron oxides prepared by laser pyrolysis
were analyzed from the point of view of their morphology and magnetic behavior.
The nanoparticles are characterized by a cubic maghemite/magnetite structure.
Maghemite phase seems to prevail in the nanopowders obtained at the highest
laser power. These nanoparticles also present a better crystallinity and slightly
larger mean crystallite dimensions. The analysis of samples as water-based
nanofluids shows that the best magnetic performances (a maximum amount of
magnetic nanoparticles at room temperature and related highest magnetization)
correspond to the ferrofluid derived from maghemite-based nanoparticles synthesized
under a higher laser power.TiO2 nanoparticles comprising a mixture of anatase and rutile phases were
synthesized via the sensitized laser pyrolysis of TiCl4- (vapors)
based gas-phase mixtures. TiO2 samples were obtained by varying the
amount of oxidizing agent (air). Enhanced crystallinity and larger crystallite mean
sizes are observed at increased amount of the oxidizing agent. Samples with a
major concentration of anatase phase (about 90%) were obtained by the same technique
by using as precursors very high air flows.



---

*Source: 604181-2008-08-20.xml*