Criteria for flame-spray synthesis of hollow, shell-like, or inhomogeneous oxides

Article abstract of DOI:10.1111/j.1551-2916.2005.00249.x

The influence of metal precursor and solvent composition on the morphology of SiO₂, Bi₂O₃, and other oxide particles made by flame spray pyrolysis (FSP) was investigated. Silica precursors with boiling points T_bp = 299-548 K dissolved in xylene were used as well as different solvents (T_bp = 308-557 K) with tetra-ethyl-orthosilicate (TEOS) as the silica precursor. For Bi₂O₃, nonvolatile bismuth nitrate pentahydrate was dissolved in solvents with T_bp = 338-468 K. Product powders were characterized by nitrogen adsorption, X-ray diffraction, and scanning and transmission electron microscopy. From these data as well as from the literature of FSP synthesis of Bi₂O₃, CeO₂, MgO, ZnO, Fe₂O₃, Y₂O ₃, Al₂O₃, and Mg-Al spinel, it is inferred that hollow/inhomogeneous particles are formed at low combustion enthalpy densities and when the solvent boiling point is comparable or smaller than the precursor melting or decomposition point.

Full text of DOI:10.1111/j.1551-2916.2005.00249.x

J. Am. Ceram. Soc., 88 [6] 1388–1393 (2005)
DOI: 10.1111/j.1551-2916.2005.00249.x
ournal
J
Criteria for Flame-Spray Synthesis of Hollow, Shell-Like, or
Inhomogeneous Oxides
R. Jossen, S. E. Pratsinis,w W. J. Stark, and L. Madler
¨
Particle Technology Laboratory, Department of Mechanical and Process Engineering, ETH Zurich, CH-8092 Zurich,
Switzerland
The influence of metal precursor and solvent composition on the
morphology of SiO2, Bi2O3, and other oxide particles made by
flame spray pyrolysis (FSP) was investigated. Silica precursors
with boiling points Tbp 5 299–548 K dissolved in xylene were
used as well as different solvents (Tbp 5 308–557 K) with tetra-
ethyl-orthosilicate (TEOS) as the silica precursor. For Bi2O3,
nonvolatile bismuth nitrate pentahydrate was dissolved in sol-
vents with Tbp 5 338–468 K. Product powders were character-
ized by nitrogen adsorption, X-ray diffraction, and scanning and
transmission electron microscopy. From these data as well as
from the literature of FSP synthesis of Bi2O3, CeO2, MgO,
ZnO, Fe2O3, Y2O3, Al2O3, and Mg–Al spinel, it is inferred that
hollow/inhomogeneous particles are formed at low combustion
enthalpy densities and when the solvent boiling point is compa-
rable or smaller than the precursor melting or decomposition
point.
spinel by spray pyrolysis using magnesium and aluminum ni-
¨
trate codissolved in ethanol. Madler and Pratsinis13 obtained
hollow bismuth oxide particles by FSP of bismuth nitrate dis-
solved in nitric acid/ethanol. By replacing ethanol with acetic
acid, they produced homogeneous, non-hollow, solid Bi2O3 par-
ticles. Similarly, solid but inhomogeneous ceria particles were
made by FSP using cerium acetate dissolved in acetic acid.8
Adding an iso-octane/2-butanol mixture to the precursor solu-
tion resulted in rather homogeneous powders of 4–8 nm primary
particle diameter. Tani et al.14 produced thinly shelled hollow
(eggshell-like) or solid g-Al2O3 particles by FSP of aqueous
emulsion of aluminum nitrate when using air or oxygen, respec-
tively, as dispersion/oxidant gas. They also produced hollow
TiO2, ZrO2, Y2O3 or solid ZnO, Fe2O3, CeO2, and MgO par-
ticles by FSP of aqueous emulsion in air of the corresponding
nitrate or chloride precursors.14 Some of the solid product pow-
ders (CeO2, ZnO, and MgO) were quite inhomogeneous with a
broad, nearly bimodal size distribution.
Even though hollow or porous particles can be useful for in-
sulation, catalyst supports, or encapsulation matrices,15 they are
generally an undesired by-product. However, systematic studies
of the effect of process variables on FSP-made particle mor-
phology are still missing. It is therefore of interest to identify
FSP conditions and criteria that prevent synthesis of inhomo-
geneous powders. In this study the effect of liquid precursor on
product morphology is investigated during FSP synthesis of sil-
ica and bismuth oxide. Design and operation criteria for syn-
thesis of homogeneous particles are proposed and compared
with the present data and pertinent ones in the literature.
I. Introduction
ILICA and titania nanoparticles are produced in megaton
quantities annually by flame technology, a manufacturing
S
route that is investigated currently for synthesis of other com-
modity and novel functional metal and ceramic nanoparticles.1,2
While volatile metal-chlorides are fed as gases in a flame result-
ing in their metal oxides, liquids are sprayed during flame spray
pyrolysis (FSP).3 Liquid precursors considerably broaden the
range of accessible products from simple and mixed oxides for
lasing materials4 to alumina-supported platinum catalysts5 for
enantioselective hydrogenation of ethyl pyruvate in synthesis of
chiral pharmaceuticals, to name just a couple of examples.
Recent research activities have improved the understanding
of particle formation and growth during FSP for control of
product particle size, crystallinity, and even scale-up.4–8 For ex-
II. Experimental Procedure
(1) Powder Synthesis
Silica and bismuth oxide powders were produced by FSP7,8 us-
ing air or oxygen (499.95% purity, Pan Gas, Luzern, Switzer-
land) as oxidant/dispersion gas. A constant 1.5 bar gas pressure
drop at the burner nozzle tip was maintained by adjusting the
nozzle-gap width. The gas flow rates were controlled by mass
flow controllers (Bronkhorst, Ruurlo, the Netherlands). The
liquid precursor solution was fed through the nozzle by a syringe
pump (IER-232, Inotech, Oberwinterthur, Switzerland). The re-
sulting spray was ignited by a ring of 18 supporting premixed
flamelets sustained by 0.5 L/min CH4 and 2 L/min O2.8 An ad-
ditional oxygen sheath gas flow (5 L/min) was supplied through
a sintered metal plate ring 8 mm wide with an inner diameter of
9 mm surrounding the supporting flamelets.8 The particles were
collected on a glassfiber filter (diameter of 150 mm; Whatman
GF/A, Springfield Mill, Maidstone, Kent, U.K.) using a vacu-
um pump (Vaccubrand, Type RZ 16, Wertheim, Germany).
The effect of precursor composition on flame-made silica
characteristics was investigated by spraying 1.5 mL/min of
various silicon precursors (Table Ia, 0.88M Si in xylene
(498%, Sigma-Aldrich, Fluka Chemie GmbH, Buchs, Switzer-
land)) to the flame. The combustion enthalpy density was de-
fined as the ratio of the fed liquid precursor combustion
enthalpy (kJ/min) over the total gas flow (ggas/min) within the
ample, Madler et al.7 found that the specific surface area (SSA)
¨
of silica made by FSP of hexamethyldisilane (HMDSO) at con-
stant liquid volumetric flow rate was decreased using methanol,
ethanol, or iso-octane as solvents in this order. As the combus-
tion enthalpy per-unit volume of these solvents was increased,
higher flame temperatures enhanced sintering, resulting in larger
particles. As result, this process has been scaled up for synthesis
up to 1 kg/h of silica9 and ZrO2 nanoparticles. Limaye and
10
Helble11 made zirconia particles by FSP of zirconium(IV)
n-butoxide in butanol, and they postulated liquid- and vapor-
phase pathways of the final particle formation depending on the
applied flame temperature.
In some cases, however, FSP has produced inhomogeneous
powders. Suyama and Kato12 made mostly hollow Mg–Al
L. C. Klein—contributing editor
Manuscript No. 10623. Received October 24, 2003; approved November 9, 2004.
Supported by TH Gesuch No. 34/02-3 and the Swiss Commission for Technology and
Innovation, TopNano21 No. 5978.2.
wAuthor to whom correspondence should be addressed. e-mail: pratsinis@ptl.mavt.
ethz.ch
1388

June 2005
Criteria for Flame-Spray Synthesis of Hollow, Shell-like, or Inhomogeneous Oxides
1389
Table I. Process Conditions and Properties of Solvents and Precursors Employed in the FSP Synthesis of SiO2 and Bi2O3
Solvent
Boiling point (K)
Feed rate (mL/min)
Concentration (mol/L)
Sourcew, purity (%)
(a) Si-Precursor properties for silica production dissolved in xylene (Tbp 5 410–416 K)
TMS, Si(CH3)4
299–300
374
393–347
436–440
548
1.5
1.5
1.5
1.5
1.5
0.88
0.88
0.88
0.88
0.88
F, 499.5
F, 499
F, 499
A, 498
F, 497
HMDSO, C6H18OSi2
TEMOS, Si(CH3O)4
TEOS, Si(CH3CH2O)4
TEPOS, Si(CH3CH2CH2O)4
(b) Solvents for TEOS (Tbp 5 436–440 K)
Pentane, C5H12
Hexane, C6H14
308–309
342
372
397–401
441–451
486–489
523–526
556–559
2.00
1.89
1.81
1.80
1.73
1.70
1.67
1.65
0.64
0.68
0.71
0.72
0.75
0.77
0.78
0.79
F, 499
F, 499.5
F, 499.5
F
Iso-Octane, C8H18
Octane fraction
Decane fraction
Dodecane, C12H26
Tetradecane, C14H30
Hexadecane, C16H34
F
F, 90–95
F, 497
F, 498
(c) Solvent properties employed in the FSP synthesis of Bi2O3
Methanol (MeOH)
Ethanol (EtOH)
Methoxy-2-propanol
Ethoxy-ethanol
338
351
391
1.31
1.00
1.03
0.80
0.89
0.90
0.31
0.41
0.39
0.51
0.46
0.45
F, 499.8
F, 499.9
F, 498
408–410
413–423
500
F, 499
Propylene glycol propylether
Diethylene glycol-monoethylether
wSigma-Aldrich, Chemie GmbH, Buchs, Switzerland. HMDSO, hexamethyldisilane; FSP, flame spray pyrolysis; TEOS, tertaethyl-orthosilcate; TMS, tetramethylsilane;
F, 498.8
F, 498
TEPOS, tetrapropyl-orthosilicate; TEMOS, tetramethyl-orthosilicate.
spray. The effect of solvent composition (Table Ib) on silica
characteristics was studied using tetraethyl-orthosilicate (TEOS)
as Si-precursor. Since the combustion enthalpy per mass of
alkane solvents is similar but their densities vary from 0.62
(pentane) to 0.77 g/cm3 (hexadecane), the silica precursor con-
centration was adjusted between 0.64M and 0.79M. All solvent
experiments were performed at combustion enthalpy density of
6.470.1 kJ/ggas and silica production rate of 4.65 g/h, with 3 L/
min of air as dispersion gas using liquid feed rates from 1.65
(hexadecane) to 2 mL/min (pentane).
cles.13 Further analysis was performed by transmission electron
microscopy (TEM, Zeiss 912 Omega; Jena, Germany operated
at 100 kV using a slow CCD camera and the Proscan software).
TEM pictures were prepared by dipping the TEM grid into the
product powder.
III. Results and Discussion
(1) SiO2 Particle Size and Morphology
Bismuth trinitrate pentahydrate (Bi(NO3)3 Á 5H2O, 98%; Ald-
rich) was dissolved in HNO3/alcohol mixtures (15 vol% of nitric
acid (65 %, Aldrich) and 85 vol% of the corresponding alcohol,
Table I(c)). Since the density and combustion enthalpy per unit
mass of these alcohols vary, the feed rate and bismuth concen-
tration were adjusted (Table I(c)) to keep production rate at
11.5 g/h Bi2O3 and total combustion enthalpy density of
2.270.03 kJ/ggas. The total combustion enthalpy density was
increased to 4.070.09 and 4.770.2 kJ/ggas by increasing the
precursor feed rates by a factor of 2 and 2.65, respectively. For
Bi2O3 3 L/min oxygen was used as dispersion/oxidant gas and
1.58 L/min CH4 and 1.38 L/min O2 for the supporting flamelets.
The effect of silica precursor/solvent composition on product
primary particle size, dBET, was investigated at constant metal
concentration, gas flow rate and combustion enthalpy density.
Using different silica precursors and solvents, the relative vola-
tility of the sprayed liquid was varied systematically. The relative
volatility, TR, is the ratio of the boiling point of the precursor
over that of the solvent. Figure 1 shows the dBET of silica as a
function of TR for various precursors (Table I(a)) dissolved in
xylene (triangles) and for TEOS (circles) dissolved in various
alkane solvents (Table I(b)). Error bars represent twice the
standard deviation of reproduced experiments. The total com-
bustion enthalpy density was kept constant at 6.2 kJ/ggas. In-
creasing Si-precursor Tbp from 299 to 548 K (increases TR from
0.75 to 1.37) had no effect on the dBET (13.871 nm). Likewise,
increasing the solvent boiling point (Tbp) from 308 to 559 K
(decreasing TR from 1.42 to 0.79) also had no significant influ-
ence on dBET, indicating that the employed solvent/precursor
compositions are of minor importance in flame spray synthesis
of silica at these conditions. Figure 2 shows TEM images of
homogeneous silica agglomerates made using TEOS/pentane
(a, TR 5 1.42), tetramethylsilane (TMS)/xylene (b, TR 5 0.75),
TEOS/hexadecane (c, TR 5 0.79), and tetrapropyl-orthosilicate
(TEPOS)/xylene (d, TR 5 1.37). No distinct size differences or
hollow particles were observed in any silica products. This mor-
phology is similar to fumed silica made by feeding gaseous Si-
precursor (e.g., SiCl4 or HMDSO) to flames resulting in fumed
silica.17
(2) Powder Characterization
The powder SSA was measured by nitrogen adsorption at 77 K
(BET-method, Micrometrics Inc., Gemini, The Netherlands) af-
ter degassing samples for at least 1 h at 1501C under nitrogen.
The average BET-equivalent primary particle diameter is
dBET 5 6/(SSA Â rp), where rp is the density of SiO2 (2.2 g/
cm3) or b-Bi2O3 (8.9 g/cm3). The crystallinity of bismuth oxide
was measured by X-ray diffraction (XRD; 40 kV, 40 mA) using
a step size of 0.051 and a scan speed of 0.251/min (Burker Ad-
vance D8, Karlsruhe, Germany). The XRD spectra were ana-
lyzed using the Topas 2.0 software (Bruker AXS, 2000). Meas-
ured XRD patterns were regressed with the crystalline data of
b-Bi2O3 (PDF #78-1793).16 The morphology of product pow-
ders was analyzed by scanning electron microscopy (SEM) op-
erated at 30 kV (Hitachi Model S900, Tokyo, Japan). Both
secondary electron (SE) and back-scattered electron (BS) SEM
images were taken to distinguish from hollow and dense parti-
Apparently, silica particle formation takes place in the gas
phase as the precursor fully evaporates and forms product par-
ticles by chemical reaction, coagulation, and sintering.7 If par-
ticle formation would have taken place at or within the droplet,

1390
Journal of the American Ceramic Society—Jossen et al.
Vol. 88, No. 6
icant effect on particle formation (size and morphology), so
major process parameters are combustion enthalpy and metal
concentration in the flame. This is in agreement with Briesen
et al.18 who found that the combustion enthalpy or adiabatic
flame temperature determines the product silica primary particle
size made in a vapor-fed premixed flame reactor.
(2) Bi2O3 Particle Size and Morphology
Since typically nonvolatile metal precursors are employed in
FSP, the effect of solvent composition was investigated for syn-
thesis of Bi2O3 from nonvolatile bismuth nitrate dissolved in
different alcohols (Table I(c)). Experiments were carried out at
three combustion enthalpy densities but constant metal concen-
tration and gas flow rates.
Figure 3 shows the dBET of Bi2O3 as a function of the em-
ployed alcohol (solvent) boiling point, Tbp, at combustion enthal-
py density of 2.2 (diamonds), 4.0 (circles), and 4.7 kJ/ggas (trian-
gles). The primary particle size is not affected by the alcohol
boiling point (Tbp) as with silica and remains constant at 1471,
1771, and 2371 nm for combustion enthalpy densities of 2.2,
4.0, and 4.7 kJ/ggas, respectively. The increase in particle size from
14 to 23 nm by increasing the combustion enthalpy density is
consistent with the current understanding of the effect of adia-
batic flame temperature and product particle size.18 With increas-
ing combustion enthalpy density from 2.2 to 4.7 kJ/ggas, the flame
height increased from 55 to 75 mm, similar to vapor-fed flame
synthesis of silica.18 The increased residence time at high temper-
ature in the flame leads to longer residence time for particle sin-
tering, resulting in larger primary particles and lower SSA.1,7 For
the lower combustion enthalpy densities, 2.2 and 4.0 kJ/ggas, and
for the lowest boiling point solvent (MeOH), a slightly larger
Fig. 1. Primary particle BET diameter of silica using tetraethyl-ortho-
silicate (TEOS)/alkanes (circles) and xylene/Si-precursors (triangles) as a
function of the relative boiling point (Tbp(Si-precursor)/Tbp(solvent)) at
constant enthalpy density of 6.2 kJ/ggas. The volatility of either precursor
or droplet has no effect on particle size and morphology as homogene-
ous particles were formed at all conditions.
mass transfer to the gas phase would be the rate-limiting step,
and spherical or broken shells may be formed.15 This was not
observed and therefore dBET was not affected by the precursor
and solvent composition (Fig. 1). Even though the difference in
vapor pressure of TEPOS (Tbp 5 548 K) and xylene (Tbp 5 410–
416 K) is about five orders of magnitude at room temperature,
no significant difference in dBET was observed as the total
combustion enthalpy density was rather constant.7 The droplet
evaporation history for volatile silica precursors has no signif-
d
BET, was measured at each combustion enthalpy. The latter
Bi2O3 powders, however, are inhomogeneous and contain larger
particles that decrease the SSA and increase the dBET, as it will be
shown shortly by microscopy and XRD.
Figure 4 shows TEM images of bismuth oxide particles made
with (a) EtOH as solvent at a feed rate of 3.5 mL/min resulting
in combustion enthalpy density of 4.7 kJ/ggas; (b) methoxy-2-
propanol as solvent at a feed rate of either 1.80 mL/min result-
Fig. 3. Primary particle BET diameter of Bi2O3 made by flame spray
pyrolysis from bismuth nitrate pentahydrate/HNO3/alcohol solutions as
a function of the boiling point of the employed alcohol solvent for three
combustion enthalpy densities. At constant combustion enthalpy densi-
ty, the solvent composition has little influence on product primary par-
ticle size. Higher production rate corresponding to higher combustion
enthalpy density and longer flames prolong the particle residence time at
high temperature resulting larger particles.
Fig. 2. Transmission electron microscope images of flame spray pyro-
lysis-made silica at constant combustion enthalpy density and produc-
tion rate from (a) tetraethyl-orthosilicate (TEOS)/pentane, (b) TMS/
xylene, (c) TEOS/hexadecane, and (d) TEPOS/xylene. At all conditions
homogenous particles were formed.

June 2005
Criteria for Flame-Spray Synthesis of Hollow, Shell-like, or Inhomogeneous Oxides
1391
Fig. 6. X-ray diffraction spectra of Bi2O3 powder made by flame spray
pyrolysis from bismuth nitrate dissolved in EtOH for a combustion
enthalpy density of 2.0 (solid line) and of 4.7 (broken line) kJ/ggas. There
is a distinct difference (hump) between inhomogeneous (solid line) and
homogeneous (broken line) Bi2O3.
Fig. 4. Transmission electron microscope images of solid, nanostruc-
tured Bi2O3 particles made by flame spray pyrolysis from a bismuth ni-
trate pentahydrate/HNO3 solution in (a) EtOH (at a combustion en-
thalpy density of 4.7 kJ/ggas), (b) methoxy-2-propanol at 4.0 or (c) at
2.0 kJ/ggas, and (d) ethoxy-ethanol at 4.0 kJ/ggas
.
ing in combustion enthalpy density of 4.0 kJ/ggas or (c) 0.9 mL/
min resulting in combustion enthalpy density of 2.0 kJ/ggas, and
(d) ethoxy-ethanol as solvent at a feed rate of 1.6 mL/min re-
sulting in combustion enthalpy density of 4.0 kJ/ggas. In all cases
(Figs. 4(a)–(d)), only homogeneous particles were formed. Hol-
low or large particles were not found at high combustion enthal-
py density ( ꢀ 4.7 kJ/ggas) or with solvents having boiling points
larger than 391 K (Table I(c)).
In contrast, Fig. 5 shows images of inhomogeneous Bi2O3
powder made using EtOH as solvent at a feed rate of 1 mL/min
resulting in a combustion enthalpy density of 2.2 kJ/ggas. Figure
5(a) shows a TEM picture of a shell-like particle together with
fine particles (B10 nm). Figure 5(b) shows a SEM picture of the
morphology of a hollow particle with diameter of about 2–4 mm.
In more detail, Fig. 5(c) shows a SE-SEM image and Fig. 5(d)
the corresponding BS-SEM image of larger, fine and shell-like
particles. Dense, large particles can be observed (white struc-
tures in Fig. 5(d)) with particle diameters varying from 100
to 500 nm whereas the hollow ones appear gray and the fine
particles are barely visible. Similar results were obtained with
MeOH as solvent.
XRD was used to further investigate particle homogeneity.
Figure 6 shows XRD spectra of the Bi2O3 powders in Fig. 4(a)
(broken line) and Fig. 5 (solid line). This figure shows a clear
distinction between a smooth (broken line) and a hump-con-
taining (solid line) XRD corresponding to homogeneous and
inhomogeneous, respectively, powders made at 4.7 and 2.2 kJ/
ggas combustion enthalpy density. For the inhomogeneous pow-
der (Fig. 5), a bimodal crystal size distribution can be derived
from XRD,16 resulting in average crystal sizes of 108 nm (15%
by mass) and 3 nm (85% by mass) for each size mode. This gives
a quantitative measure of crystalline powder homogeneity and is
used throughout this study.
The above results can be placed in the FSP parameter space
of combustion enthalpy density and solvent boiling point, Tbp.
Figure 7 shows that hollow, shell-like, or inhomogeneous pow-
ders were produced at low combustion enthalpy densities
(o4.7 kJ/ggas) and with solvents having low boiling points (open
symbols). In all other cases, solid homogeneous, nanostructured
Fig. 5. Images of hollow, large and fine solid Bi2O3 made by flame
spray pyrolysis from a bismuth nitrate pentahydrate/HNO3/EtOH so-
lution at a combustion enthalpy density of 2.0 kJ/ggas: (a) transmission
electron microscopy of a hollow Bi2O3 particle surrounded by very fine
ones, (b) scanning electron microscopy of inhomogeneous Bi2O3 pow-
ders with hollow particles of 2–4 mm, (c) secondary-electron image and
corresponding (d) backscattered-electron images. The latter shows solid
particles (white contrast) of about 100–500 nm and a hollow broken shell
of 2 mm in diameter (gray).
Bi O powders were observed (filled symbols). Similarly, Madler
¨
2
3
and Pratsinis13 used acetic acid (AcOH, Tbp 5 391 K) as solvent
and produced homogeneous powders. The particle morphology
was independent of the combustion enthalpy density since they
chose a high boiling point solvent (Fig. 7, filled triangles). When
using, however, more than 20% EtOH with the balance of
AcOH as solvent, inhomogeneous particles were formed.13 By
calculating the boiling point of AcOH/EtOH mixture with the

1392
Journal of the American Ceramic Society—Jossen et al.
Vol. 88, No. 6
Fig. 8. Morphology mapping of various solid (filled symbols) and hol-
low/shell-like (open symbols) ceramic oxide particles in the flame spray
pyrolysis parameter space of the ratio of the solvent boiling point over
the precursor decomposition or melting point (Tbp/Td/mp) and combus-
tion enthalpy density. Homogeneous particles were made for Tbp/Td/mp
41.05 and for combustion enthalpy densities 44.7 kJ/ggas
.
Fig. 7. Morphology mapping of solid (filled symbols) and hollow/shell-
like (open symbols) particles as a function of solvent boiling point and
combustion enthalpy density during flame spray pyrolysis synthesis of
Bi2O3 from bismuth nitrate pentahydrate.
was formed (open diamonds) as the combustion enthalpy den-
sity was decreased to 4.2 kJ/ggas. Homogeneous iron oxide pow-
der (inverse triangle crosshair) was produced even though air
was used as dispersion gas (Tbp/Td/mp 5 1.16, 4.5 kJ/ggas).
Inhomogeneous powders were observed when making ceria
(Tbp/Td/mp 5 0.88, 4.6 kJ/ggas, hexagon), zinc oxide (Tbp/Td/
mp 5 0.92, 4.6 kJ/ggas, circle containing cross), yttria (Tbp/Td/
mp 5 1.00, 4.5 kJ/ggas, triangle containing cross), and magnesium
oxide (Tbp/Td/mp 5 1.01, 4.6 kJ/ggas, square containing cross,
Fig. 8). Suyama and Kato12 (diamond containing cross) pre-
pared Mg–Al spinel by spray pyrolysis of Mg(NO3)3 Á 6H2O and
Al(NO3)3 Á 9H2O dissolved in EtOH. The melting point of the
aluminum nitrate is 347 K, while the magnesium nitrate precur-
Antoine equation, the latter data are in excellent agreement with
the present study (Fig. 7, open triangles).
(3) Morphology Mapping of FSP-Made Oxides
The limit between the region of hollow and homogeneous par-
ticles can be found close to the melting point of the bismuth
nitrate (Tmp 5 349 K), thus corroborating the outlined mecha-
nism. The decomposition/melting point of bismuth nitrate
(Td/mp 5 349 K) can be used to relate the present results to other
FSP-studies of particle morphology. Figure 8 maps the powder
morphology in the FSP parameter space of combustion enthal-
py density as a function of the ratio Tbp/Td/mp. Filled symbols
represent homogeneous powders while open represent hollow or
inhomogeneous powders.
sor decomposes at 371 K. They made hollow particles at Tbp
Td/mp 5 1.01 (Fig. 8) and combustion enthalpy density of 1.2 kJ/
gas. Hollow particles were formed if a concentration gradient is
/
g
formed during solvent evaporation19 and if the molten oxide
precursor decomposes within this layer. If the salt shells are im-
permeable to the solvent, the pressure within the particle in-
creases substantially and particles fragment or explode,20
resulting in broken shells. From Figs. 7 and 8 it may be inferred
that the precursor precipitates on the droplet surface, forming a
shell that can trap remaining solvents (Tbp/Td/mpo1) that evap-
orate within that shell. The resulting high pressure inside that
hollow sphere disrupts it forming shell-like fragments. For high
boiling solvent (Tbp/Td/mp41), no shells were formed. The pre-
cursor decomposed in the hot solvent and solid nuclei are
formed throughout the droplet. Upon solvent evaporation,
these nuclei form extremely fine oxide particles. Figure 8 shows
that homogeneous powders were only found at high combustion
enthalpy densities (44.7 kJ/ggas) and at Tbp/Td/mp41.05 for all
these oxides.
Circles represent the Bi2O3 data from this study (Fig. 7). Hol-
low particles and inhomogeneous Bi2O3 powders were only
found at Tbp/Td/mpo1 and at combustion enthalpy densities
o4.7 kJ/ggas. The triangles refer to Madler and Pratsinis,13 who
¨
prepared Bi2O3 by FSP as discussed before (Fig. 7). Inhomoge-
neous silica powders (filled squares) were not made here, since
the employed Tbp/Td/mp was 41.4 and the combustion enthalpy
densities were about 6.2 kJ/ggas. The inverse triangles show data
of ceria8 made by FSP from cerium acetate (Td/mp 5 573 K)
dissolved in acetic acid (Tbp 5 391 K) resulting in Tbp/Td/mp
5 0.68. By adding a mixture of iso-octane/2-butanol, the com-
bustion enthalpy density was increased from 1.2 to 1.8 kJ/ggas at
a liquid precursor feed rate of 1 mL/min. A mixture of large and
small particles (open inverse triangles) was produced at com-
bustion enthalpy densities of 1.2, 1.8, and 3.2 kJ/ggas
(2 mL/min feed rate). When increasing, however, the liquid
feed rate to 4 mL/min resulting in a combustion enthalpy den-
sity of 5.9 kJ/ggas, solid, and homogeneous ceria particles were
formed (filled inverse triangles).
IV. Conclusions
Formation of hollow/inhomogeneous ceramic oxide particles by
FSP was examined, since FSP is one of the promising techniques
for synthesis of a wide spectrum of nanostructured oxides.
By analyzing Bi2O3 and silica powders made with various
precursors and solvents, low process temperatures and low
boiling point solvents favor formation of hollow or inhomo-
geneous powders. These criteria were found to match data
with flame-spray-made Bi2O3, SiO2, CeO2, MgO, ZnO, Fe2O3,
Y2O3, Al2O3, or Mg–Al spinel. In particular, inhomogeneous
particles were formed at low combustion enthalpy densities
(o4.7 kJ/ggas) and when the solvent boiling point is smaller
Tani et al.14 made nanoparticles by FSP of aqueous emul-
sions of precursor metal nitrates mixed with kerosene and a
surfactant that was sprayed in a flame reactor. Here, the Tbp/Td/mp
was calculated using the boiling point of water (Tbp 5 373 K)
and the melting point of the nitrates. The diamonds represent
Al2O3 powders with Tbp/Td/mp 5 0.92. Using oxygen as oxidant/
dispersion gas, homogeneous alumina powder (filled diamonds)
was formed as the combustion enthalpy density was 5.4 kJ/ggas.
Using air as oxidant/dispersion gas, inhomogeneous alumina

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Criteria for Flame-Spray Synthesis of Hollow, Shell-like, or Inhomogeneous Oxides
1393
9R. Mueller, L. Madler, and S. E. Pratsinis, ‘‘Nanoparticle Synthesis at High
¨
than the melting or decomposition point of the metal precursor
(Tbp/Td/mpo1.05).
Production Rates by Flame Spray Pyrolysis,’’ Chem. Eng. Sci., 58 [10] 1969–76
(2003).
10R. Mueller, R. Jossen, S. E. Pratsinis, M. Watson, and M. K. Akhtar, ‘‘Zir-
conia Nanoparticles Made in Spray Flames at High Production Rates,’’ J. Am.
Ceram. Soc., 87 [2] 197–202 (2004).
Acknowledgments
The authors thank M. Muller for technical support for the EM facilities, O.
¨
Wilhelm and F. Krumeich for the SEM images and H. K. Kammler for the TEM
images.
11A. U. Limaye and J. J. Helble, ‘‘Morphological Control of Zirconia Nano-
particles Through Combustion Aerosol Synthesis,’’ J. Am. Ceram. Soc., 85 [5]
1127–32 (2002).
12Y. Suyama and A. Kato, ‘‘Characterization and Sintering of Mg–Al Spinel
Prepared by Spray-Pyrolysis Technique,’’ Ceram. Int., 8 [1] 17–21 (1982).
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