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Pyroelectrically Driven •OH Generation by Barium Titanate and
Palladium Nanoparticles
Annegret Benke,*,∥,† Erik Mehner,∥,‡ Marco Rosenkranz,§ Evgenia Dmitrieva,§ Tilmann Leisegang,‡
Hartmut Stöcker,‡ Wolfgang Pompe,† and Dirk C. Meyer‡
†
Institute of Materials Science and Max Bergmann Center of Biomaterials, TU Dresden, D-01062 Dresden, Germany
Institute of Experimental Physics, TU Bergakademie Freiberg, Leipziger Straße 23, D-09596 Freiberg, Germany
§
Leibniz Institute for Solid State and Materials Research Dresden (IFW Dresden), Helmholtzstraße 20, D-01069 Dresden, Germany
‡
ABSTRACT: The disinfection of bacteria by thermally excited pyroelectric
materials in aqueous environments provides opportunities for the development
of new means of sanitization. However, little is known about the formation of
reactive oxygen species (ROS) at the surface of the thermally excited pyroelectric
materials. To investigate the pyroelectrically driven ROS generation we
performed OH radical specific measurements of thermally stimulated barium
titanate nanoparticles in contact with palladium nanoparticles. Through electron
spin resonance measurements with the spin trap BMPO (5-tert-butoxycarbonyl
5-methyl-1-pyrroline n-oxide) and fluorescence spectroscopy of 7-hydroxycoumarin, OH radical generation was detected, which confirms the hypothesis of
pyroelectric ROS production. Since pyroelectric potential changes are insufficient
for direct electrochemical OH radical generation, we propose a two-step chargetransfer model facilitated by intermittent contact between the palladium and the pyroelectric nanoparticles and the pyroelectric
effect as the driving force for charge transfer.
■
INTRODUCTION
Commercial water disinfection currently relies on chemical
methods using chlorine- or ozone-based chemicals, whereas
physical methods like thermal disinfection or ultraviolet
radiation are less often employed. Due to their high oxidative
potential, reactive oxygen species (ROS) are well suited as a
physical means of disinfection. A completely new approach for
creating ROS is the utilization of the pyroelectric effect,1 which
seems favorable when naturally occurring temperature changes
can be employed for the excitation of the pyroelectric materials
and, thus, offer an environmentally friendly method of water
disinfection.
In an aqueous solution the spontaneous polarization at the
surface of a ferroelectric is screened, for example, by dissolved
ions or dissociated water molecules. Changes in temperature
trigger the pyroelectric effect. The imbalance of polarization
and screening charges changes the effective surface potential. It
was shown that these potential changes whether they stem from
changes in temperature or strain can be used to drive
electrochemistry between physisorbed molecular species.1,2
For example Hong et al. demonstrated water splitting on
mechanically excited surfaces of BaTiO3 and ZnO. Gutmann et
al. proposed that the observed water disinfection with thermally
stimulated LiNbO3 and LiTaO3 is facilitated by production of
ROS at the surface of the pyroelectric materials.
Free radicals have high oxidation potentials, especially the
OH radical whose oxidation potential is twice that of chlorine
which is commonly used for disinfection. It is known that OH
radicals can pull H atoms from C−H and S−H bonds and split
© 2015 American Chemical Society
aromatic rings. Living cells are damaged by radicals reacting
with amino acids and DNA molecules.3 Photocatalytic E. coli
inactivation with TiO2 showed cell damage caused by various
ROS, such as OH radicals, hyperoxide radicals, and H2O2.4
Basically ROS react immediately at the place of their origin.
Their reaction rate with biomolecules is very high being 107 to
1010 mol−1 s−1 in the diffusion-limited regime.5 As they are
short-lived on the time scale of 70 ns,6 only short diffusion
lengths of 3−20 nm result. Consequently, all methods for ROS
detection function indirectly, for example, degradation of dyes
or other organics in aqueous solutions,7,8 fluorescence spectroscopy of marker molecules,9 like 2′,7′-dichlorodihydrofluorescin (DCFH)10,11 or 7-hydroxycoumarin,12 or oxidation of Jtriiodide to J 3 -triiodide 13 or para-chlorbenzoic acid
(pCBA).14,15 ROS detection by oxidation of DCFH appears
to be nonspecific for ROS because it was shown that not only
ROS contribute to the reaction.16 The reaction mechanism
itself proceeds over several stages and is not understood
entirely. Several substances have been identified which oxidize
DCFH directly, whereas others catalyze the reaction.17 The
established ROS detection methods are mainly applied for
biochemically and photocatalytically generated radicals. To the
best of our knowledge their viability for detection in the vicinity
of thermally stimulated pyroelectric materials has not been
examined. In this article we report detection of pyroelectrically
Received: May 13, 2015
Revised: July 10, 2015
Published: July 17, 2015
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generated ROS with two independent methods: fluorescence
spectroscopy of 7-hydroxycoumarin and an electron spin
resonance (ESR) based approach.
We investigated the pyroelectric generation of ROS using
pyroelectric, thermally excited barium titanate powder in
combination with palladium nanoparticles. Noble metal
nanoparticles are of great importance as catalysts and are
often used in photocatalytic water disinfection. Catalytic activity
depends on particle size and interactions between carrier and
metal nanoparticles. 18 These parameters influence the
electronic structure of the nanoparticles and can enhance the
reactivity of the carrier. Examples of reactions that are typically
analyzed with metal cocatalysts are water splitting19,20 and the
oxidation of carbon monoxide (CO).21−23 Inoue et al.
investigated differently polarized surfaces of lithium niobate
decorated with small palladium particles. An increase of CO
oxidation on the positive polarized surface was observed. The
electron transfer from palladium into the pyroelectric material
results in electron depletion of the metal and weakening of
chemisorption between metal and CO.
It is the intention of this study to reveal the basic mechanism
of ROS production in the BaTiO3−Pd nanoparticle system.
Besides the evidence of pyroelectrically generated ROS, we
propose a model for the ROS generation reaction. It explains
how the pyroeletric effect acts as the driving force for the
exchange of charge carriers between pyroelectric and metal
nanoparticles. Due to the impact of particle size on the crystal
structure of barium titanate and the fact that pyroelectricity
requires the tetragonal phase of barium titanate we put special
emphasis on the structural analysis of the barium titanate
employed.
Transmission Electron Microscopy (TEM). The palladium nanoparticles were investigated by TEM using a LIBRA
200 transmission electron microscope (Zeiss) in order to
visualize their shape and to measure the size distribution.
Samples were prepared on a carbon-coated copper grid by
mounting a 10 μL drop of palladium nanoparticle solution, with
a settling time of this drop of 10 min, and a final rinsing with
ultrapure water.
X-ray Diffraction and Fluorescence (XRD/XRF). The asreceived and poled crystalline BaTiO3 powder materials were
characterized using XRD. Diffraction patterns were recorded in
reflective Bragg−Brentano geometry with Cu-Kα radiation on a
θ−θ goniometer (Bruker D8 Advance). The diffractometer
employs primary and secondary 2.3° axial Soller collimators
and a Johansson-type secondary graphite monochromator. The
equatorial beam divergence was limited to 2°, whereas the focal
point was constrained to 0.025°. Samples were continuously
rotated during the measurement at 60 rpm. The instrumental
broadening and shapes of reflection profiles were calibrated and
fitted with program TOPAS26 and a fundamental parameter
approach27 using the diffraction pattern of NIST SRM 640d
silicon standard powder. Accordingly, crystallite sizes can be
extracted from a cos θ convolution using Scherrer’s formula. Xray fluorescence spectra were recorded with a wavelengthdispersive spectrometer (Bruker S8 Tiger) and evaluated with
the programs SpectraPlus and QuantExpress (Bruker).
Fluorescence Spectroscopy of Coumarin/7-Hydroxycoumarin. Coumarin, a well-known probe molecule for
specific detection of photocatalytically generated OH radicals,12
was used first for the detection of such radicals in the context of
thermally stimulated pyroelectric materials. Reacting with OH
radicals coumarin forms the highly fluorescent 7-hydroxycoumarin with a specific fluorescence emission maximum at
wavelength of 455 nm. Coumarin works as a qualitative specific
test method for OH radicals. By measuring the fluorescence
intensity the amount of radicals can be quantified.
For preparing the samples, 30 mg of as-received or poled
barium titanate powder was weighed in a reaction cap and
mixed with 50 μL of palladium nanoparticle solution. Palladium
particles were not immobilized on the barium titanate surface;
instead all particles are free in the solution and can form a
temporary contact. Then, 150 μL of a solution with 1 mmol of
coumarin (Sigma-Aldrich) in ultrapure water was added. The
samples were heated from 20 to 70 °C (temperature stability of
the coumarin solution was verified up to 80 °C) and then
cooled to 20 °C in a thermoshaker (Thermomixer comfort,
Eppendorf) at a heating and cooling rate of 5 K/min. In
intervals of 3 min, the samples were mixed at 600 rpm for a
period of 9 s. This procedure was carried out 5 times in total.
Finally, the samples were centrifuged (14 000 min−1, 20 min),
and the specific fluorescence intensities of the supernatants
were measured with a fluorescence spectrometer (Nanodrop
ND 3300, ThermoScientific) at wavelengths of 360 and 455 nm
for excitation and emission, respectively. Control samples
without thermal excitation, without palladium nanoparticles,
and containing only coumarin or only palladium nanoparticles
were measured for all samples. A calibration curve for the
concentration of 7-hydroxycoumarin/•OH was captured by
measuring the fluorescence intensity of 0.1, 0.25, 0.5, and 1
μmol of 7-hydroxycoumarin (Sigma-Aldrich) solution in 1
mmol of coumarin (Figure 5b). All experiments were protected
from light to exclude photo effects.
■
MATERIALS AND METHODS
Pyroelectric BaTiO3 Powder. BaTiO3 was purchased from
IoLiTec as a nanopowder material (nominal particle size 100
nm) with a purity of 99.9% and a relative permittivity of 2500−
2800. The pyroelectric coefficient of bulk barium titanate is
approximately 200 μC/m2 K.24 It was used as received and after
poling in an electric field, respectively. Powders were poled with
a constant high voltage (6 MV/m) for 1 h as a dielectric in a
parallel plate capacitor placed in a high vacuum chamber at 1 ×
10−5 mbar pressure. By poling an alignment of domains is
expected and therefore an enhancement of polarization
uniformity.
Preparation of Palladium Nanoparticles. Palladium
nanoparticles were synthesized by the route of Bigall,25 albeit
with slight modifications. An amount of 4.4 mg of palladium
chloride K2PdCl4 (Sigma-Aldrich) was dissolved in 1 μL of
concentrated hydrochloric acid and then injected in 50 mL of
ultrapure boiling water through a filtering syringe with a pore
size of 0.22 μm resulting in a final concentration of the metal
precursor of 0.27 mmol. After 1 min 1.1 mL of the mild
reducing agent containing 1% sodium citrate and 0.05% citric
acid was injected. After another half minute 0.55 mL of a freshly
prepared strong reducing agent with 0.08% sodium borohydrate, 1% sodium citrate, and 0.05% citric acid were added, and
the solution was left to boil for 10 min before cooling.
Scanning Electron Microscopy (SEM). The powder
sample morphology of BaTiO3 was investigated by SEM
using a DSM 982 Gemini electron microscope (Zeiss). The
powder samples were mixed in a drop of water, mounted on
carbon pads, dried, and carbon-coated before insertion into the
microscope.
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Figure 1. SEM micrograph of barium titanate powder. Individual particles (a) are aggregated forming spheres of different sizes (b).
Figure 2. Powder diffraction pattern of the commercial barium titanate powder. Measured (Yobs) and calculated intensities (Ycalc) are given on a
logarithmic scale, whereas the difference (Ydiff = Yobs − Ycalc) is given on a linear scale. The solid line (red) shows the best Rietveld fit (see results in
Table 1). Insets show composition of 111 and 002/020 reflections with respect to cubic and tetragonal fractions on a linear scale (au: arbitrary
units).
A first long thermal excitation step (4 h, seven cycles
between 20 and 70 °C) of the pyroelectric barium titanate in
combination with the palladium nanoclusters has been applied
in water to remove adsorbed gases in the Stern bilayer and
equilibrate screening charges on the barium titanate powder
surface. Then, 20 μL of aqueous solution of the spin trap
molecule was added to each sample so that a final
concentration of 50 mmol is achieved followed immediately
by the thermal excitation for trapping the free radicals by the
spin trap. Thermal excitation took 15 min (seven cycles)
between 5 °C (ice water) and 30 or 40 °C (thermoshaker)
while mixing (protected from light to exclude photo effects).
BMPO (5-tert-butoxycarbonyl 5-methyl-1-pyrroline N-oxide,
high purity, Enzo Life Science) was employed as a spin trap to
detect specifically short-lived O-, C-, S-, and N-centered free
radicals by formation of stable radical adducts. The
concentration of BMPO was 50 mM. BMPO radical adducts
are much more stable compared to other spin traps like DMPO
(5,5-dimethyl-1-pyrroline N-oxide).30 The half-life time is 23
min (Enzo Life Science) at room temperature; therefore,
thermal excitation was only 15 min. Finally, the samples were
centrifuged (14 000 min−1, 5 min) to separate all barium
Electron Spin Resonance (ESR) Spectroscopy. ESR
(also known as electron paramagnetic resonance spectroscopy)
is a method for studying chemical species that have at least one
unpaired electron leading to absorption of microwave radiation
in an external magnetic field. Beyond its many applications,
ESR has already been used for detection of ROS and especially
free radicals.28,29 Spin trap molecules reacting with free radicals
in solution generate stable products, which in turn are directly
observable by ESR. Spin trapping is therefore a valuable tool for
studying the very short-lived free radicals.
To measure electron spin resonance, a sample volume of
about 1300 μL was used. This volume has been split into six
parts for sample preparation in order to get virtually the same
sample volume and powder mass for thermal excitation in the
thermoshaker like in the fluorescence spectroscopy experiments. For each sample fraction, 32.5 mg of as-received or
poled barium titanate powder was weighed in a reaction cap
and mixed with 54.2 μL of palladium nanoparticle solution.
Palladium particles were not immobilized on the barium
titanate surface, but all particles are free in the solution and can
form temporary contacts. Then, 142.5 μL of ultrapure water
was added.
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Table 1. Summary of Rietveld Refinementa
refinement model
single cubic
cubic BaTiO3 lattice
constant acubic (Å)
tetragonal BaTiO3 lattice
constants atetra, ctetra (Å)
4.012(3)
−
−
single tetragonal
−
4.006(1)
4.025(3)
“simple” linear combination
cubic + tetragonal
4.006(6)
“best fit” linear combination
cubic + tetragonal
4.007(4)
4.006(9)
4.025(4)
4.006(7)
4.024(9)
crystallite
size (nm)
mass fraction
(wt %)
residual weighted
profile RWP (%)
goodness
of fit
188 ± 12
−
57 ± 27
−
155 ± 5
48 ± 16
51 ± 6
465 ± 91
46 ± 12
55 ± 6
500 ± 102
46 ± 12
98.4 ± 0.6
−
1.7 ± 0.3
−
98.5 ± 0.4
1.5 ± 0.2
20.5 ± 1.7
77.7 ± 1.8
1.7 ± 0.2
22.9 ± 1.6
75.4 ± 1.7
1.7 ± 0.2
10.9
3.25
7.3
2.19
6.5
1.96
5.7
1.73
a
Crystallite size and mass fraction refer to cubic BaTiO3, tetragonal BaTiO3, and BaCO3 impurity, respectively. Given errors are at a 3σ confidence
level, without respect to serial error correlation.
The diffraction pattern (Figure 2) shows no clear splitting of
the 020 and 002 reflections of BaTiO3 hinting to the cubic
phase. However, the 111 reflection is considerably sharper than
the 020 and 002 reflections, which is a distinct indication of the
tetragonal phase. Hence, refinements using both cubic and
tetragonal BaTiO3 structures with space groups Pm3̅m32,33 and
P4mm33 were attempted. Structure models34,35 as well as
structures for orthorhombic and rhombohedral BaTiO335,36
were not pursued since initial tests yielded low R-factors. For all
refinements a fourth-order polynomial background was
employed. Barium occupancy factors were adopted from XRF.
The higher indexed reflections are significantly broader than
lower ones, which is typically caused by microstrain. Although
isotropic strain models are usually an oversimplified ansatz, we
used a tan θ convolution to model the reflection broadening at
higher angles because elastic properties of BaTiO3 are not of
interest here.
The refinements for a single cubic or tetragonal phase agree
considerably better with the tetragonal phase since the different
widths of the 111, 020, and 002 reflections are better
reproduced by the tetragonal structure model. However,
intensities and shapes of the reflections are not satisfactorily
matched and require further improvement (RBragg). Following
the surface reconstruction model by Hoshina et al. a refinement
with a linear combination of cubic and tetragonal phases was
attempted and yields a better refinement.37 On the basis of the
average particle size found by SEM the Hoshina model predicts
radii for its cubic surface layer and gradient lattice strain layer of
approximately half the crystallite size of the refined cubic phase
(see Table 1). Comparing the obtained crystallite size for the
tetragonal phase with the SEM micrographs suggests at least
partially oriented lattice intergrowth between the small particles
(Figure 1a). The best conformity with the measured pattern
was realized by incorporating platy textures along [111] and
[011] directions which may stem from compaction of
intergrown particles (Figure 1) during sample preparation.
Since powder diffraction is unable to spatially localize the cubic
fraction, investigations with TEM were conducted. The TEM
micrographs exhibited strong strain-related contrast, although
no indication for a separated core−shell structure was found.
Consequently, we assume the cubic phase as a modification of
the tetragonal phase that is likely to be located at the surface of
the particles. Defining the characteristics of this cubic phase
more precisely by the Rietveld refinement is hindered by the
titanate and palladium particles. Supernatants of all samples
were collected for injection into the ESR flat cell. Control
samples without palladium nanoparticles were also investigated.
The ESR spectra were recorded by an EMX plus X-band CW
spectrometer (Bruker) using an optical cavity (ER 4104OR,
Bruker) and the Xenon software package (Bruker). A special
ESR flat cell (Quarzglastechnik Ltd.) was used for the
measurements in aqueous solution. The ESR spectra were
measured at a modulation amplitude of 2 G (at 100 kHz) and
microwave power of 5 mW.
To determine the number of spins, the ER213ASC alanine
spin concentration sample provided by Bruker BioSpin GmbH
was used. The determined spin concentration of this standard
was 2.00 × 1017 spins. Using this recalibration system and a
special polynomial-sensitivity pattern for the used resonator,
the absolute number of spins of an unknown sample can be
determined with an accuracy of ∼20%.
■
RESULTS
Morphological and Structural Characterization of
BaTiO3 Powder. The SEM micrograph shows single nanoparticles with a size of approximately 150 nm similar to the
supplier’s declaration (Figure 1a), and particles are aggregated
in random spheres of micrometer size (Figure 1b). In aqueous
solution these aggregated spheres are difficult to deagglomerate,
even by ultrasonic treatment. Therefore, it has to be assumed
that nanoparticles are aggregated at least partially during the
radical generation experiments.
X-ray fluorescence analysis confirms the commercial BaTiO3
powder to be nearly stoichiometric
Ba0.988±0.002Ti1.000±0.005O3.2±0.2, notwithstanding elements lighter
than oxygen. Expected impurities like calcium or strontium
were found to be below (<100 ppm), whereas traces of sodium
(0.6 wt %), chlorine (0.1 wt %), and phosphorus (200 ppm)
were detected. A powder diffraction pattern was recorded in the
2θ range of 15−125° and demonstrated that the powder is
almost phase-pure BaTiO3 (Figure 2). The long exposure time
of 80 s per point allowed the identification of barium carbonate
as an impurity phase (1.7 wt %), which accounts for all the low
intensity reflections not belonging to barium titanate.31 Thus,
we expect the main phase to be slightly barium deficient (pdoped) related to a sample composition of Ba1−δTiO3 +
δBaCO3.
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low crystallite size. Refinement models where the barium
deficiency is confined to the cubic phase versus a homogeneous
distribution over all phases yield almost indistinguishable Rfactors. The same holds for the disambiguation of the polarcubic configuration (Yashima’s model33 ICSD Coll. Code
164385) and the cubic-nonpolar configuration (Buttner’s
model32). We attribute the remaining difference between
modeled average structure and measured structure to the
particle size distribution and intergrowth.
Poling of the powder had little impact on the diffraction
pattern. If any, a small fraction (within the margins of error) of
the cubic phase is converted to tetragonal. The summarized
results for all refinements (Table 1) show that the model
impact on the results outweighs statistical errors. However, all
but the “simple cubic” models show a significant prevalence of
the tetragonal phase and indicate the powder as ferroelectric
and pyroelectric.
Visualization of Palladium Nanoparticles. With transmission electron microscopy palladium clusters could be
specified as round-shaped particles with a size of (40.3 ±
7.2) nm (Figure 3).
Figure 4. OH radical-specific evidence by fluorescence detection of 7hydroxycoumarin. Fluorescence spectrum of 7-hydroxycoumarin: OHspecific emission maximum at 455 nm is shown for both pyroelectric
materials combined with palladium nanoparticles after thermal
treatment.
excitation or with coumarin solution or palladium nanoparticles
show no or only very small intensities. By means of the
calibration curve (Figure 5b) the concentration of 7hydroxycoumarin /•OH is estimated to approximately 0.74
μmol for as-received and 0.8 μmol for poled barium titanate
with palladium nanoparticles.
ESR Spectroscopy for Investigation of Radicals. Results
for pyroelectrically generated radicals are shown in Figure 6.
Although the BMPO spin trap captures a variety of radicals, the
recorded ESR spectra indicate that only OH radicals, the
radicals with the highest oxidation potential, were produced in
the experiments. All ESR measurements were conducted after
the samples have been thermocycled seven times between 20
and 70 °C without the spin trap. In this way, impacts of gas
desorption and other one-time effects were minimized in the
ESR measurements. Figure 6a shows the signal pattern for the
measured adducts BMPO/•OH (mixture of two diastereomers) from samples with poled barium titanate powder
with palladium nanoparticles and temperature excitation of
seven cycles with ΔT = 35 K between 5 and 40 °C compared to
the simulated spectra for the same adducts BMPO/•OH.
When the signal pattern is matched, the OH radical is detected
as the main reaction product. A simulated spectrum shown by a
magenta line in Figure 6a consists of a mixture of two BMPO/
•OH diastereomers using the following hyperfine coupling
constants: aN = 14.3 G, aβH = 14.1 G, aγH = 1.4 G for
diastereomer 1 (80%); aN = 14.0 G, aβH = 12.6 G, aγH = 0.7 G
for diastereomer 2 (20%). The values of these parameters
resemble closely those found in the literature.30
In Figure 6b the numbers of spins are shown for each sample,
representing the amount of spin-trap adducts. These values can
be directly related to the number of radicals generated in the
system. Both ESR spectroscopy and fluorescence spectroscopy
of 7-hydroxycoumarin confirm a significant difference in free
radical production with respect to the presence or absence of
palladium nanoparticles in the sample. Only for samples with
palladium nanoparticles can OH radicals clearly be detected.
Spin numbers from poled powder samples are slightly increased
compared to samples with as-received powder. This is in good
agreement with the results from the fluorescence test with
coumarin. Apparently, an increased thermal excitation leads to
higher ESR intensity (Figure 6b).
Figure 3. TEM micrograph of palladium nanoparticles, consisting of
several grains each, with an average size of 40 nm.
Fluorescence Spectroscopy of 7-Hydroxycoumarin.
Fluorescence intensity of 7-hydroxycoumarin at 455 nm
wavelength indicating OH radicals was recorded for both
pyroelectric materials, as received and poled barium titanate,
combined with palladium nanoparticles and necessary control
samples (see Figure 4). The highest fluorescence intensities
stem from samples with thermally excited barium titanate
combined with palladium nanoparticles, whereas poling the
barium titanate yielded only marginally higher values (Figure
5a). Significant generation of OH radicals is only achieved by
the combination of pyroelectric material and palladium
nanoparticles under thermal excitation. In contrast, barium
titanate alone does not lead to a substantial amount of radicals;
i.e., the fluorescence spectra do not show the fluorescence peak
of 7-hydroxycoumarin. Control samples without thermal
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Figure 5. Specific evidence of OH radicals by fluorescence detection of 7-hydroxycoumarin. Peak fluorescence intensities at 455 nm for control
samples, samples without thermal treatment, and samples with nanoparticles and thermal treatment (a). The calibration curve is obtained by plotting
the fluorescence intensity at 455 nm of samples with prepared concentrations of 7-hydroxycoumarin in 1 mmol coumarin solution (b).
Figure 6. Accumulated ESR spectra of spin adducts of BMPO: Comparison of the simulated spectrum of a mixture of two diastereomers to the
measured spectrum reveals only a signal of BMPO/•OH as a trapped radical (a). The absolute number of measured spins in the sample volume
strongly depends on the composition of the sample. Only the combination of barium titanate with palladium nanoparticles and thermal treatment
leads to significant generation of OH radicals (b).
■
about 50 nmol •OH L−1 min−1 K−1 for fluorescence
spectroscopy experiments and 0.1 nmol •OH L−1 min−1 K−1
for ESR experiments, respectively. Assuming that both
experiments yield the same radical-generation rate, we find
the conversion yield for the ESR experiment to be 500 times
lower than that of the fluorescence experiment. Whether an
inactivation of bacteria can be achieved with our realized OH
radical generation rates is debatable since quantitative data on
this subject are scarce. Gao et al. described disinfection by OH
radical generation via ultrasonication reaching a disinfection of
Bacillus subtilis of 2.5 log levels at a maximum with an OH
radical concentration of about 2.1 × 104 nmol min−1.41 Cho et
al. inactivated 2 log levels of Escherichia coli bacteria with
photocatalytically generated OH radicals at a concentration of
about 4.4 × 10−5 nmol min−1.42 The difference of 9 orders of
magnitude for a similar disinfection level underlines the
sensitivity of such experiments to changes in experimental
conditions like type of microorganism, number of cells, specific
experimental setup, combination with ROS besides OH
radicals, and ultrasonication or UV light. Still, with OH radicals
being an important oxidant species for bacteria inactivation43
and previous work1 we are optimistic that pyroelectrically
driven radical generation is suitable for disinfection purposes.
Impact of Poling of Barium Titanate on Radical
Generation. Results of both methods for OH radical detection
DISCUSSION
OH Radical Detection. OH radicals have been clearly
detected by both spectroscopic methods. Moreover, measurements without thermal excitation, palladium, or barium titanate
nanoparticles, respectively, reveal the pyroelectric nature of the
radical generation. Furthermore, the palladium nanoparticles
enhance the radical generation. Gauging the amount of radicals
created requires knowledge of the respective conversion yield
for each marker (coumarin and BMPO) and OH radicals under
the chosen experimental conditions. Literature data on trapping
efficiency and conversion yields are scanty, although it is known
that the trapping efficiency of BMPO depends on the
temperature during •O2− capture.38 The decay time of
BMPO depends on the reaction conditions, namely, the
presence of metal ions, pH, temperature, and solvent.39 In our
experiments, such influences may stem from the barium
carbonate impurity, especially since carbonate ions are known
to trap hydroxyl radicals.40 No information has been found in
the literature on the temperature dependence of the coumarin/
7-hydroxycoumarin reaction.
It is nonetheless possible to estimate the lower limit of the
radical generation rate by assuming the conversion yield as
unity. Scaling the fluorescence intensity and number of spins
linearly by sample volume, detection-agent concentration,
undergone temperature change, and time per sample yields
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the mid-band gap in Figure 7 and assume that the pyroelectric
effect generates an internal potential, which tilts the conduction
and valence band.
(fluorescence spectroscopy of 7-hydroxycoumarin and ESR
spectroscopy with BMPO spin trap) show only a slight increase
of OH radical generation with poled powder in comparison to
the as-received one; differences are within the margin of error
for each method. Changes, occurring in the powder during
poling, result in contrary effects. For instance, the z-faces of a
single domain, single crystal particle will exhibit stronger
surface potential changes under a given thermal excitation than
a random polydomain configuration of that particle. However,
the latter configuration may have a larger z-surface area due to
the possibility of 90° domains in barium titanate. Since the
required minimum potential change for the OH radical
generation is not deducible from our experiments, this question
will be subject to future investigation.
The spatial arrangement of tetragonal and cubic phases in or
between particles and thus related screening effects are not
entirely clear yet.
Charge Transfer: The Role of Palladium Nanoparticles
and Thermal Excitation. The generation of OH radicals in
our experiments is fascinating for two reasons: (a) noble-metal
nanoparticles showing promising results in catalysis experiments are often much smaller than the 40 nm palladium
nanoparticles employed here.44,45 (b) Given the applied
thermal excitation (ΔT ≤ 50 K), permittivity, and pyroelectric
constant of barium titanate (see Materials and Methods), the
electrical surface potential change during a temperature half
cycle of the barium titanate powder is expected to be less than
200 mV, depending on the actual domain size. This potential
change is only a fraction of the minimal redox potential
required for OH radical generation: h+ + H2O → •OH + H+
(2.18 V in neutral solution46). We therefore assume that the
OH radical-generating reaction is, although pyroelectrically
driven, strongly depending on additional electronic parameters
at the site of charge transfer. Without immobilization the
palladium particles are in random intermittent contact with the
barium titanate. Subsequently, we consider the electronic
contact situation during which the particles are immobile and
adsorbed. Although the work function of palladium (depending
on size and orientation47,48) and the band structure of barium
titanate49 are known, the type of contact is difficult to classify
due to the ill-defined electron affinity of barium titanate.50 The
Fermi level of barium titanate is often placed close to the
conduction band edge (e.g., Burbure et al.51) near an oxygen
vacancy induced donor state assuming (or specifically
preparing) barium titanate as an n-type semiconductor.
However, the used barium titanate was found to be slightly
barium deficient and not heat treated under reducing
atmosphere. Consequently, we assume the material as p-type
if not an intrinsic semiconductor. Defects may pin the Fermi
level at a different energy. In this case, defects are expected due
to barium vacancies and microstrain resulting in Fermi level
pinning (see Results and Neubrand et al.52). Furthermore, both
electron affinity and band bending of a ferroelectric depend on
the polarization state and the compensating adsorbates
constituting the Gouy−Chapman layer.53 Finally, the pyroelectric nature of the experiment will result in a time-periodic
band tilting in the barium titanate and thus bias the contact in a
forward or reverse direction. Depending on the polarization
state of the domain (c+ or c−, the a± and b± are nonpolar) and
the temperature at the time of contact, electrons or holes will
be driven across the interface. Owing to this complex situation
and the limited amount of available information, we employ a
simplified model placing the Fermi level of barium titanate at
Figure 7. Schematic energy level diagrams and ROS-generating charge
transfer resulting from thermal excitation of the pyroelectric barium
titanate (CB: conduction band, VB: valence band, Φ: work function,
EVAC: vacuum energy, EF: Fermi energy, E0NHE: standard electrode
potential): Energy bands of palladium and barium titanate prior to
contact. The bands are bent at barium titanate c±-surfaces due to the
ferroelectric polarization (a). Effect of the pyroelectric band tilting
(exaggerated) and subsequent internal compensation by intrinsic
charge carriers (b). Redox potentials between palladium nanoparticles,
barium titanate, and neutral aqueous surrounding relative to standard
hydrogen electrode are compared for selected species (c).
Figure 7c shows the redox potentials of a few ROS species
that may be created by charge carrier injection from either the
conduction band and valence band of barium titanate or the
Fermi level of palladium. However, the expected concentration
of charge carriers available for ROS generation from barium
titanate alone is very low owing to the assumption of an
intrinsic semiconductor. This is consistent with the experimental results shown in Figures 5a and 6b. Since these
experiments also showed no significant ROS generation for
barium titanate combined with palladium without thermal
excitation we have to conclude that pyroelectric-band tilting is
the primary process which supplies charge carriers for the
chemical reactions: Electrons are transferred into the palladium
nanoparticles leaving holes in the valence band of barium
titanate. These holes are likely to be the source of the detected
OH radicals because they can readily be injected from the
barium titanate valence band into the OH−/H2O redox system.
This reaction path can be further promoted by an increased
surface hydroxylation, if the surface is Ti−O terminated and
oxygen vacancies in the TiO2 layer are available.54 The barium
deficiency of the used powder in combination with the reported
barium loss in aqueous media are cases in point.52
The expected reduction reactions 2H+ + •O2− + e− → H2O2
and H+ + •HO2− + e− → H2O2 (with Eredox = 1.71 eV and
Eredox = 1.42 eV, respectively,55 Figure 7) resulting from
electron injection from the palladium into the O2 redox system
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The Journal of Physical Chemistry C
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have not been detected. This is not surprising for the
fluorescence experiments since coumarin is a marker molecule
specific to OH radical detection. However, BMPO is capable of
trapping •HO2. Monroe et al. found the complex of DMPO
and •HO2 to be short-lived on the time scale of minutes,
quickly changing into DMPO−/•OH.56 Thus, our ex-situ ESR
experiment may have been too slowly paced to catch •HO2.
The resulting H2O2 can also contribute to an increased
hydroxylation of the barium titanate surface.57 It is obvious that
more ROS may have gone undetected in our experiments due
to their OH radical specificity. Many OH radical production
and decay reactions go through hydrogen peroxide which in
turn may be produced at lower redox potential cost than OH
radicals.
■
CONCLUSION
We report the pyroelectrically driven OH radical generation by
barium titanate in combination with palladium nanoparticles.
The OH radicals were detected with two independent
methods: fluorescence spectroscopy of 7-hydroxycoumarin
and ESR spectroscopy of BMPO adducts. To the best of our
knowledge this is the first evidence of OH radicals created by
thermally excited pyroelectric materials. Due to the contact
between the pyroelectric and metal nanoparticles, it can be
assumed that the palladium nanoparticles facilitate charge
transfer between palladium, barium titanate, and the adsorbed
species: Charge carriers are pyroelectrically driven toward the
interface to water. Hole injection from the valence band of
barium titanate is supposed to be the primary source of the OH
radicals in the experiments reported here. The generation of
other ROS species through the proposed mechanism is also
expected, though their detection is beyond the scope of this
study.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
Author Contributions
∥
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors owe many thanks to Dr. Mathias Lakatos and
Elisabeth Preuße for their support in the preparation of
palladium nanoparticles, to Dr. Anja Blüher and Axel Mensch
for providing assistance with TEM investigations, to Juliane and
Florian Hanzig for the fruitful discussions of the radical
mechanism, and furthermore to the colleagues from the
Institute of Genetics of TU Dresden for the use of the
fluorescence spectrometer. We thank Quirina Roode-Gutzmer
for proof-reading the manuscript. Financial support by the
Deutsche Forschungsgemeinschaft (DFG, BE 4857/1-1), the
BMBF (VIP0364), the European Union (European Regional
Development Fund), and the Ministry of Science and Art of
Saxony (SMWK) within the PyroConvert junior research group
(100109976) is gratefully acknowledged.
■
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