Sensing of Organic Vapors by Flame-Made TiO
2
Nanoparticles
A. Teleki
*
, S.E. Pratsinis
*
, K. Kalyanasundaram
**
and P.I. Gouma
**
*
Swiss Federal Institute of Technology, PTL
ML2 F13, Zurich 8092, Switzerland, pratsinis@ptl.mavt.ethz.ch
**
State University of New York at Stony Brook, Stony Brook, NY, USA, pgouma@notes.cc.sunysb.edu
ABSTRACT
Anatase TiO
2
nanoparticles were produced by flame
spray pyrolysis (FSP) and characterized by
transmission/scanning electron microscopy, X-ray
diffraction and nitrogen adsorption. Thick films (30 –
50 µm) of these powders were prepared by drop-coating
technique and tested for sensing of acetone, isoprene and
ethanol at 500 °C in dry N
2
/O
2
. A high n-type sensor signal
was recorded at ppm levels of these organic vapors with
fast response and recovery times. Heat-treatment at 900 °C
caused a nearly complete anatase to rutile transformation
and a transition to p-type sensing behavior. The rutile
sensor had a poor signal to all hydrocarbons tested and
considerably longer recovery times.
Keywords: titanium dioxide, anatase, gas sensors, isoprene,
acetone, ethanol
1 INTRODUCTION
Titanium dioxide (TiO
2
) has been applied in sensors for
measuring many gases including oxygen [1], carbon
monoxide [2], hydrogen [3], nitrous/nitric oxide [4], water
vapor [5] and hydrocarbon gases [6]. TiO
2
sensor material
synthesis routes must meet the demands of close size
control, large and easily accessible surface area, high
crystallinity and the ability to include noble metal doping.
Larger surface area materials provide high sensitivity at low
gas concentrations, e.g. Gao et al. [7] found that nano-scale
titania films exhibited better oxygen sensing performance
than micron-sized ones. Thermal pre-treatment of sensing
devices is often required to ensure sensor stability. This
causes grain growth of the material, resulting in a lower
surface area and poor sensor response. Moreover, in the
case of TiO
2
, temperatures over 600 °C lead to a
crystallographic phase transition from anatase to rutile [8].
Dopants are typically added to titania either to increase its
thermal stability (Si: [9]; Nb: [10]; Ta: [11]; La: [12]) or
sensor sensitivity and selectivity (CuO: [13]). Nano- and
micrometer TiO
2
particles for gas-sensing have been
produced by sol–gel [5], oxidation of metallic titanium foil
[14], laser pyrolysis [10], magnetron sputtering [3],
supersonic cluster beam deposition [15] and ball milling of
commercial powders [16].
Flame technology is used largely for manufacture of
about 2 million tons/year pigmentary titania [17]. Size,
crystallinity and morphology of flame-made nanostructured
TiO
2
can be controlled by changing the high temperature
residence time of the particles in the flame [18]. Doped-
TiO
2
can be readily made by co-oxidation of precursors in
the flame [19, 20]. Further, flame spray pyrolysis (FSP)
processes allow for the addition even of low-volatility
dopant precursors (e.g. for platinum [21]).
The high and fast response of FSP-made SnO
2
[22] and
Pt/SnO
2
[21] nanoparticles towards propanal, NO
2
and CO
has already been demonstrated. In this study, the sensing of
volatile organic compounds is explored using flame-made
TiO
2
anatase nanoparticles [23]. The sensor signal to
ethanol, isoprene (2-methyl-1,3-butadiene) and acetone was
investigated. Acetone is a common airborne contaminant
[24], isoprene can be found in human breath [25] and over
forested areas [26]. Ethanol detection is required for
applications such as breath analyzers, monitoring devices
for food-quality [27].
2 EXPERIMENTAL
A flame spray pyrolysis (FSP) reactor [28] was used for
the synthesis of TiO
2
nanoparticles. Solutions of 0.5 or
0.67 M titanium-tetra-isopropoxide (TTIP, Aldrich, purity >
97 %) in a xylene (Fluka, > 98.5 %)/acetonitrile (Fluka,
> 99.5 %) mixture (11/5 by volume) were fed at 5 ml/min
through the inner reactor capillary. Oxygen (Pan Gas,
purity > 99%) was supplied at 5 l/min through the
surrounding annulus, dispersing the precursor solution into
a combustible spray. The pressure drop at the nozzle tip
was held constant at 1.5 bar. The spray was ignited by a
premixed methane/oxygen flame (1.5/3.2 l/min). The spray-
flame could be sheathed with 40 l/min of oxygen gas and
enclosed by a 40 cm long glass tube resulting in higher
temperatures. Suspensions of the product powders in 1-
heptanol (Acros Organics) were prepared and drop-coated
onto alumina substrates with interdigitated Au electrodes
(10×10 mm; Electronics Design Center, MicroFabrication
Lab, Case Western Reserve University). The substrates
were dried at 100 °C, at least, for 1 hour in an oven.
X-ray diffraction (XRD) patterns of product powders
and sensing films were obtained with a Bruker AXS D8
Advance diffractometer (40 kV, 40 mA, Karlsruhe,
Germany) operating with Cu Kα radiation. Anatase and
rutile crystallite sizes, and phase composition were
determined by the fundamental parameter approach and the
Rietveld method. The BET powder-specific surface area
(SSA), was measured by nitrogen adsorption at 77K
423NSTI-Nanotech 2006, www.nsti.org, ISBN 0-9767985-8-1 Vol. 3, 2006
(Micromeritics Gemini 2375) after degassing the sample, at
least, for 1 h at 150 °C in nitrogen. The BET equivalent
average diameter (d
BET
) was calculated as d
BET
=
6/(SSA*ρ
p
), where ρ
p
is the weighted density of TiO
2
(4260
or 3840 kg/m
3
for rutile or anatase, respectively). The
product powder morphology was analyzed by transmission
electron microscopy (TEM; CM30ST microscope, FEI
(Eindhoven), LaB6 cathode, operated at 300 kV,
SuperTwin lens, point resolution ~2Å). Scanning electron
microscope (SEM, LEO 1530 Gemini microscope) images
were prepared of the sensing films.
DC electrical measurements (sensor tests) were
performed to monitor the response to acetone (1 – 7.5
ppm), isoprene (2-methyl-1,3-butadiene; 1 – 9 ppm), and
ethanol (10 – 75 ppm) in a dry N
2
/O
2
atmosphere (all gases
Specgas, Inc.). The pulse time of the gases at each
concentration was usually 180 s. For the sensor tests the
substrate was placed in the center of a quartz tube (2.5 cm
diameter and 60 cm length), which in turn was introduced
into a tubular furnace (Lindberg/Blue). Gold wires were
melted onto the sensor electrodes and externally connected
to a digital multimeter (Agilent 34401) recording the sensor
resistance. The furnace was heated to 500 °C in 1 hour and
kept at this temperature during the sensor tests. A total gas
flow rate of 1 L/min was passed through the quartz tube and
controlled by mass flow controllers (1479 MKS). The
measurements were done in 10 % accompanying O
2
with
the balance N
2
. The sensors were allowed to stabilize for, at
least, 1 hour at the sensing temperature and N
2
/O
2
flow.
Sensors could also be heat-treated in ambient atmosphere at
900 °C for 6 hours prior to sensing at 500 °C. The sensor
signal is given in the following as the resistance ratio R
0
-
R
gas
/R
gas
, where R
0
and R
gas
denote the sensors’ resistances
in the absence and presence of the gas to be sensed,
respectively. The sensor response is defined as the time
required until 90 % of the response signal is reached. The
recovery time denotes the time needed until 90 % of the
original baseline signal is recovered.
3 RESULTS AND DISCUSSION
3.1 Particle and Sensing Film Properties
Figure 1 shows TEM images of as-prepared TiO
2
nanoparticles. Sample P1 (Fig. 1a) was produced from a
0.5 M TTIP solution, while sample P2 (Fig. 1b) was
produced from a 0.67 M solution with the glass tube
enclosing the flame. The particles in sample P1 (Fig. 1a)
are spherical and non-agglomerated of 15 nm in BET
diameter consistent with Schulz et al. [28] for FSP-made
TiO
2
at similar conditions. The particles in sample P2 (Fig.
1b) are larger, 43 nm BET-diameter, and polyhedral as they
were made at higher temperature (in the enclosed flame)
and higher TTIP concentration than those of sample P1.
The phase composition in both samples is about 85 wt%
anatase and the balance rutile, typical for TiO
2
formed in
oxygen-rich vapor-fed [20] or spray [28] flames.
Figure 1: TEM images of FSP made TiO
2
nanoparticles.
Spherical, non-agglomerated particles are visible in sample
P1 (a), while particles in sample P2 (b) are polyhedral since
they experienced higher temperature residence times than
P1 during their flame synthesis.
The anatase crystallite size is larger in sample P2 than
P1, 60 and 20 nm, respectively. In both cases the crystallite
sizes are larger than the BET diameter indicating non-
spherical particles, as seen in TEM images of sample P2
(Fig. 1b). The longer high temperature residence time of P2
particles than P1 enables the growth of these large, non-
spherical crystals.
Sensing films S1 and S2 were prepared from samples
P1 and P2, respectively. The S2 sensor was tested as-
prepared and after heat-treatment. Figure 2 shows a cross-
sectional SEM image of heat-treated film S2 after sensor
test. The alumina substrate is also visible. The film is dense
with a thickness is about 20 – 30 µm, irregularities stem
from the drop-coating technique. Prior to heat-treatment
more porous structures were observed and the films were
40 – 50 µm thick.
Figure 2: SEM image of TiO
2
sensing layer S2 on an
alumina substrate after heat-treatment at 900 °C and
sensing at 500 °C.
424 NSTI-Nanotech 2006, www.nsti.org, ISBN 0-9767985-8-1 Vol. 3, 2006
The phase composition of film S1 after sensor test was
87 wt% anatase consistent with the as-prepared powder P1.
Also the anatase and rutile sizes were unchanged compared
to as-prepared particles. Thus no anatase to rutile phase
transformation or grain growth had taken place during the
sensor tests indicating the high stability of flame-made
particles at these conditions. A nearly complete phase
transformation to rutile, however, had taken place in the
heat-treated film S2 as expected at these temperatures [29].
The rutile crystallite size increased significantly to 159 nm.
3.2 Gas Sensing Properties
Sensor S1 was tested for 1 – 9 ppm of isoprene during a
forward and backward cycle. The resistance decreased
during the gas exposure, a typical behavior for anatase as an
n-type semiconductor [6]. The sensor signal consistently
increased with increasing isoprene concentration. The
forward and the backward cycles nearly coincided though
the signal during the backward cycle was slightly higher
than the forward.
A similar signal was obtained with sensor S2 tested for
acetone (triangles), isoprene (diamonds) and ethanol
(rectangles) as shown in Figure 3. At 1 ppm for acetone and
isoprene, the sensor first self-recovered before stabilizing at
the sensor signal. The sensor signal increases rather linearly
with increasing gas concentration consistent with sensor S1
and in agreement with Zhu et al. [30] for ZnO-TiO
2
thick
film acetone sensors. The response curves of acetone and
isoprene coincide (Fig. 3), only at 7.5 ppm acetone gives a
higher signal than isoprene. The interaction of both acetone
and isoprene with hydroxyl groups on the surface of TiO
2
might explain the sensor signal similarity of the two gases.
Also for ethanol at 10 – 75 ppm the sensor signal increases
rather linearly with increasing concentration, in agreement
with ethanol sensing at higher gas concentration [27]. The
signal is higher than for acetone and isoprene due to the
higher ethanol vapor concentration.
The response times were within 2-3 seconds for all
gases at the tested concentrations. The recovery time for
acetone increased nearly linearly from 144s at 1 ppm to
302s at 7.5 ppm. The same dependence of recovery time
with concentration was observed of isoprene but the sensor
recovered faster than for acetone. The slower recovery after
acetone exposure might stem from molecular adsorption of
acetone on the surface [24]. In contrast, the recovery time
decreased with increasing ethanol concentration, indicating
a different sensing mechanism for ethanol. The sensing
behavior of titania might not only rely on interaction of
ethanol with adsorbed oxygen species, but rather on the
direct adsorption at semiconductor surface sites, as was also
suggested by Ferroni et al. [27].
Acetone or isoprene concentration, ppm
02468
Sensor signal, -
0
10
20
30
Ethanol concentration, ppm
020406080
Acetone
Isoprene
Ethanol
Figure 3. Sensor signal of S2 under exposure to acetone
(triangles), isoprene (diamonds) and ethanol (squares) in a
dry N
2
/O
2
atmosphere at 500 °C.
After heat-treatment sensor S2 showed a p-type
behavior towards sensing of acetone and isoprene, as an
anatase to rutile transformation had taken place. However,
exposing the sensor to ethanol, the response changed back
to n-type. For all gases response and recovery times were
slower than before the heat-treatment. This indicated that
the time for heat treatment might not have been long
enough to equilibrate the rutile lattice with oxygen.
4 CONCLUSIONS
TiO
2
nanoparticles were produced in a flame spray
pyrolysis reactor by combustion of titanium-tetra-
isopropoxide. Particles consisting mainly of anatase phase
(85 wt%) with BET-equivalent particle diameters of 15 or
43 nm were prepared. These particles were drop-coated
from heptanol suspensions onto alumina substrates
interdigitated with gold electrodes resulting in porous films
40 – 50 µm thick. The gas sensing properties of these films
were investigated for ppm levels of ethanol, acetone and
isoprene vapors at 500 °C. The sensors had n-type response
to these vapors with response and recovery times within a
few seconds or minutes, respectively. Denser films about
30 µm thick were formed after heat-treatment at 900 °C and
a complete phase transformation to rutile took place. This
resulted in a n- to p-type transition and poor sensor signals.
425NSTI-Nanotech 2006, www.nsti.org, ISBN 0-9767985-8-1 Vol. 3, 2006
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