Pyrocatalytic oxidation-strong size-dependent poling effect on catalytic activity of pyroelectric BaTiO3nano- And microparticles

Article abstract of DOI:10.1039/d0cp03158e

Pyrocatalysis is an emerging advanced oxidation process for wastewater remediation with the potential for thermal energy harvesting and utilization. Although several studies explored the potential of new pyrocatalyst materials to degrade harmful organic water pollutants, the role of important material properties and electric poling procedures on the pyrocatalytic activity is still unclear. In this work, we investigate the interdependence between particle size, electric poling and pyrocatalytic activity of BaTiO3 powders with nominal particle sizes of 100, 200 and 500 nm by using the dichlorofluorescein redox assay. Depending on the particle size, the influence of surface area or phase composition on the pyrocatalytic activity predominates. Moreover, we demonstrate that poling of pyrocatalysts leads to a strong size-dependent increase of pyrocatalytic activity. This poling effect increases with particle size up to +247% and can be explained with size-dependent changes in phase composition and domain structure. Combining all results, the progression of the pyrocatalytic activity as a function of particle size was derived and a future strategy for maximizing the catalytic performance of pyrocatalysts was developed. This study greatly improves the understanding about the role of important material properties and electric poling on pyrocatalytic activity, thus enabling an effective catalyst design. With the help of highly active catalysts, the pyrocatalytic process can take the next step in its development into a new and energy-efficient advanced oxidation process for water remediation.

Full text of DOI:10.1039/d0cp03158e

Volume 22
Number 41
7 November 2020
Pages 23413–24136
PCCP
Physical Chemistry Chemical Physics
rsc.li/pccp
ISSN 1463-9076
PAPER
Patrick Braeutigam et al.
Pyrocatalytic oxidation – strong size-dependent
poling effect on catalytic activity of pyroelectric
BaTiO3 nano- and microparticles

PCCP
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PAPER
Pyrocatalytic oxidation – strong size-dependent
poling effect on catalytic activity of pyroelectric
BaTiO3 nano- and microparticles†
ab
Cite this: Phys.Chem.Chem.Phys.,
2020, 22, 23464
Sascha Raufeisen,
Peter Neumeister,c Johannes R. Buchheim,ab
Michael Stelterabd and Patrick Braeutigam
*
ab
Pyrocatalysis is an emerging advanced oxidation process for wastewater remediation with the potential
for thermal energy harvesting and utilization. Although several studies explored the potential of new
pyrocatalyst materials to degrade harmful organic water pollutants, the role of important material
properties and electric poling procedures on the pyrocatalytic activity is still unclear. In this work, we
investigate the interdependence between particle size, electric poling and pyrocatalytic activity of
BaTiO3 powders with nominal particle sizes of 100, 200 and 500 nm by using the dichlorofluorescein
redox assay. Depending on the particle size, the influence of surface area or phase composition on the
pyrocatalytic activity predominates. Moreover, we demonstrate that poling of pyrocatalysts leads to a
strong size-dependent increase of pyrocatalytic activity. This poling effect increases with particle size up
to +247% and can be explained with size-dependent changes in phase composition and domain
structure. Combining all results, the progression of the pyrocatalytic activity as a function of particle size
was derived and a future strategy for maximizing the catalytic performance of pyrocatalysts was
developed. This study greatly improves the understanding about the role of important material
properties and electric poling on pyrocatalytic activity, thus enabling an effective catalyst design. With
the help of highly active catalysts, the pyrocatalytic process can take the next step in its development
into a new and energy-efficient advanced oxidation process for water remediation.
Received 12th June 2020,
Accepted 27th August 2020
DOI: 10.1039/d0cp03158e
rsc.li/pccp
source of such potentially toxic and ecotoxic micropollutants,
the introduction of advanced oxidation processes (AOPs) is
1 Introduction
Ongoing progress in medical research in combination with an discussed as part of an additional water treatment stage.6–8
ageing society led to the widespread release of a large number
The purpose of AOPs is the oxidative degradation of (micro)-
of pharmaceuticals into the water cycle.1,2 Enabled by advance- pollutants to nontoxic transformation products with the help
ments in analytical techniques, trace amounts of these phar- of in situ generated free radical species like hydroxyl radicals
maceuticals as well as their metabolites and transformation ꢀOH.9,10 Well-established AOP techniques are ozonation (O3),
products were detected in almost any fresh water resource.3–5 Fenton (Fe2+/H2O2), sonolysis, UV oxidation, electrocatalysis
Since urban wastewater treatment plants are the main emission and photocatalysis (e.g. UV/TiO2).11–16 Photocatalytic techni-
ques in particular were greatly improved over the last years due
a Institute of Technical Chemistry and Environmental Chemistry, Friedrich Schiller
to the development of innovative materials and material
combinations.17 One driving force for material innovations is
the reduction of energy demand and costs for the photoexcita-
tion. This reduction can be achieved by adjusting the absorp-
tion of the photocatalyst toward the solar spectrum, thus
enabling the harvesting of solar energy.18,19
The harvesting of energy from the environment and its
conversion into chemical energy for wastewater remediation
can also be reached with an emerging AOP technique, the
pyrocatalysis.20 This technique utilizes the pyroelectric effect
of crystals with a spontaneous polarization, which changes in
response to temperature fluctuations. Under thermal cycling,
University Jena, Philosophenweg 7a, 07743 Jena, Germany.
E-mail: patrick.braeutigam@uni-jena.de
b Center for Energy and Environmental Chemistry (CEEC Jena), Friedrich Schiller
University Jena, Philosophenweg 7a, 07743 Jena, Germany
c Fraunhofer IKTS, Fraunhofer Institute for Ceramic Technologies and Systems,
Winterbergstraße 28, 01277 Dresden, Germany
d Fraunhofer IKTS, Fraunhofer Institute for Ceramic Technologies and Systems,
Michael-Faraday-Straße 1, 07629 Hermsdorf, Germany
† Electronic supplementary information (ESI) available: Contains reaction
scheme of DCF assay, DCF calibration data, sample allocation during experiment,
detailed temperature profiles, X-ray diffractograms and results for Rietveld
refinements for BT samples, and BET specific surface area for BT samples. See
DOI: 10.1039/d0cp03158e
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transient variations of the polarization magnitude occur and correlates mainly with their BET surface area. However, a
lead to a build-up of polarization charges on the pyroelectric comparison with literature data led to the overall picture that
crystal surface. Although these polarization charges are the importance of size-dependent phase composition and
screened by compensation charges at equilibrium, transient tetragonality increases strongly for smaller particles. Moreover,
net charges can trigger redox reactions. Thus, pyroelectrically the catalytic activity of BT catalysts was greatly increased
generated reactive oxygen species (ROS) such as ꢀOH can through poling up to +247%. It was shown under directly
convert organic micropollutants into nontoxic degradation comparable conditions that the relative pyrocatalytic activity
products.
enhancement by poling is size-dependent and increases with
The application of pyrocatalysis for wastewater remediation particle size. This finding was explained with the increased
has the potential for a great reduction of energy demand and fraction of tetragonal phase, the transition from ferroelectric
costs by harvesting thermal energy from natural temperature single-domain to multidomain crystals, as well as the better
gradients as well as waste heat from industrial processes.21 movability of domain walls of larger particles. These size-
However, there is a great gap in knowledge about the exact dependent aspects led to an improved alignment of randomly
catalytic mechanism as well as the influence of essential distributed ferroelectric dipoles in larger, multidomain parti-
process/reaction/material parameters on the pyrocatalytic activity. cles during poling, which in turn led to a higher relative
In recent years, various experimental and some theoretical increase of pyrocatalytic activity. All results were compared
studies investigated the catalytic potential of pyroelectric mate- with literature data and a future strategy for maximizing the
rials for water remediation (disinfection,20 micropollutant catalytic performance of pyrocatalysts was developed. With the
degradation22,23) but also for hydrogen generation through help of this strategy, the pyrocatalytic process can take the next
water splitting.24–26 Several studies introduced new pyrocata- step in its development into a new AOP technique for water
lysts (LiNbO3, LiTaO3, BaTiO3, BiFeO3, etc.) with different remediation, accompanied with a great potential for thermal
particle shapes (e.g. nanoparticles, nanofibers) and explored energy harvesting.
their potential to degrade model substances like rhodamine
B.27,28 In addition, Zhang et al. developed a number of concepts
to increase the activity of pyrocatalysts such as the combination
with co-catalysts or cycling the temperature around the Curie
2 Experimental
2.1 Reagents and chemicals
temperature TC of materials with low TC (high pyroelectric
coefficient).28
BaTiO3 powders with different nominal particle sizes in the range
from 100 to 500 nm were supplied by US Research Nanomater-
The experimental studies were important in order to show
ials, Inc. (BT100: 100 nm; BT200: 200 nm, BT500: 500 nm; all
that the concept of a pyrocatalytic process can be transferred to
99.9%). 20,70-Dichlorodihydrofluorescein diacetate (DCHF-DA;
a variety of different pyroelectric materials and applications.
497%), 20,70-dichlorofluorescein (DCF; reagent grade) and
Unfortunately, they are not comparable and therefore not
suited for a detailed investigation of the influence of important
material parameters such as the particle size/shape, the pyro-
electric coefficient or the relative permittivity on the pyrocata-
lytic activity. Three main reasons for this are (i) the broad
variety of process parameters, especially the temperature curves
used for thermal excitation, (ii) the lack of information about
important material properties such as crystallographic struc-
NaH2PO4ꢁ2H2O (498%) were purchased from Sigma Aldrich,
Alfa Aesar and Merck. Methanol (499.8%) was purchased from
VWR Chemicals. Ultrapure water (0.055 mS cmꢂ1, GenPure UV,
Fisher Scientific GmbH) was used for sample preparation and
analysis. All Stock solutions were stored in a refrigerator at 9 1C
and were protected from light. DCHF-DA was stored in a refrig-
erator at ꢂ10 1C.
ture, particle size/shape distribution as well as specific surface
area, and (iii) the comparison of materials from different
2.2 Recycling and poling procedures
synthesis routes.29 Only studies that allow a direct comparison In order to investigate the catalytic durability of the pyrocata-
of pyroelectric catalyst materials with identical process/reaction lysts, the unpoled BT100 was recycled four times. The recycling
parameters in combination with an adequate material charac- was done directly after centrifugation within the previous
terization are suited for a maximization of catalytic activity. pyrocatalytic activity experiments. The precipitated powders
Furthermore, the potential benefits of an electric poling proce- from several experiments were combined in a centrifugation
dure on the catalytic activity of pyrocatalysts were ignored with tube (PE; 15 mL) and subsequently suspended and washed with
few exceptions.23,30,31 This is remarkable, since several studies ultrapure water. After centrifugation (3858 rcf; 10 min), the
of the related piezocatalysis have shown a strong increase of supernatant was replaced with fresh ultrapure water and this
piezocatalytic activity through such a remanent alignment of procedure was repeated eight times in order to remove all
ferroelectric dipoles inside ferroelectric particles.32–34
residual DCF. The washed and precipitated powder was then
In this work, the influence of the particle size on the dried and carefully de-agglomerated by hand into a fine,
pyrocatalytic activity of BaTiO3 powder (BT) catalysts containing homogeneous powder.
nanoparticles with nominal particle sizes of 100, 200 and
For poling, the BT samples were pressed (4.9 kN) without
500 nm was investigated. It was shown under directly comparable additives into circular discs with a diameter of 10 mm and
conditions that the pyrocatalytic activity of BT catalysts 2 mm thickness. Then, these unsintered powder compounds
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were placed between two copper plates used as electrodes and and simultaneous shaking at 1200 rpm. The internal temperature
poled under a nominal electric field of 2 kV mmꢂ1 for 5 s in air.
Y
in was measured with a type K thermocouple (NiCr-Ni, ꢃ1.5 1C)
In order to be usable as suspended pyrocatalysts, the poled BT directly placed into the reaction tube. Prior to the temperature
discs were carefully de-agglomerated by hand into fine, homo- program, the reaction tubes were equilibrated for 5 min in the
geneous powders.
pre-heated aluminum block (32.5 1C). The temperature program
consisted of four uniform cycles with a length of 40 min
each (Fig. 1(a)) followed by a 5 min cooling phase back to
2.3 Pyrocatalyst characterization
The crystal structures of BT samples were characterized by 20 1C (Fig. S2, ESI†). The preset temperature YP and the
using X-ray diffraction (XRD, Rigaku MiniFlex600) with Cu-Ka
radiation (l = 1.54059 Å) over the range of 2y from 20 to 901
with a scanning rate of 0.0081 sꢂ1. The unit cell parameters and
weight fractions were obtained via Rietveld refinement of the
XRD data. The instrumental broadening and shapes of reflec-
tion profiles were calibrated and fitted with program MAUD16
using the diffraction pattern of LaB6 standard powder. Scan-
ning electron microscopy (SEM) was used to investigate the BT
powder morphology with a JSM-7001F (JEOL). The BT samples
were dried, mounted on carbon pads, coated with carbon and
then inserted into the microscope. At the cathode, an accelera-
tion voltage for the electrons of 15 kV was set. Nitrogen
adsorption/desorption isotherms of the BT samples were mea-
sured by physical adsorption of nitrogen (N2) at about 77 K on a
Quantachrome Autosorb IQ instrument. Prior to the measure-
ments, the powders were degassed at 30 1C for 3 h under
vacuum. The specific surface area SBET was calculated by the
Brunauer–Emmett–Teller (BET) method.
2.4 Pyrocatalytic activity experiments
To compare the ROS-induced oxidation capability of pyroelec-
tric BT in aqueous solution, the DCF redox assay was used.35
The DCHF-based reaction protocol was introduced by Carthcart
et al. and further modified in a previous study (Scheme S1,
ESI†).36,37 This method was proven pH- and cation-
independent over a wide range; it offers a high precision and
enables the determination and consideration of autoxidation
processes. A DCHF reaction solution was prepared by dissol-
ving DCHF-DA in methanol (140 mL per mg DCHF-DA) and
adding 0.01 M NaOH (570 mL per mg DCHF-DA) to trigger a
complete deesterification of DCHF-DA to DCHF within 30 min.
Finally, 25 mM NaH2PO4 (1.6 mL per mg DCHF-DA) was used to
neutralize the base and the DCHF solution was diluted with
ultrapure water to a concentration of 1 mM. In order to get
reliable results, all solvents were degassed prior to use and all
stock, reaction and sample solutions were protected from light
and air during all procedures.
In a typical pyrocatalytic experiment, 60 mg of BT was placed
within a micro tube (PP, amber, Vmax = 1.85 mL) and suspended
in 1750 mL DCHF reaction solution. The micro tube was
sealed with Parafilm and shaken thoroughly. The thermal excita-
tion of the suspended pyroelectric powders was realized with the
Fig. 1 (a) Measured temperature inside the reaction vessel Yin over time t
and corresponding heating/cooling rate DYin/Dt for one thermal cycle
used in the DCHF-oxidation experiments. (b) XRD patterns (c) and
enlarged XRD patterns of the unpoled BaTiO3 powders with nominal
particle sizes of 100, 200 and 500 nm (BT100, BT200, BT500). (d) Weight
fraction distribution of ferroelectric tetragonal (wftetr) and paraelectric
cubic (wfcub) BaTiO3 phases for poled and unpoled samples.
help of a thermomixer (heating/cooling thermomixer MKR13,
Hettich Benelux) equipped with an aluminum block for 24 micro
tubes. Up to 12 micro tubes were placed in every second slot of
the aluminum block while the remaining slots were left free
(Fig. S1, ESI†). The oxidation experiments were carried out with
thermal cycles between 32.5 and 69.9 1C inside the micro tubes
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measured temperature of the aluminum block YAl (internal percentage of the latter.38,39 Rietveld refinement method was
thermocouple) are displayed in Fig. S3, ESI.† The heating used to further investigate the phase compositions of the BT
and cooling rates were kept constant at around +2 K minꢂ1 samples. The complete results for the unpoled as well as the
and ꢂ2 K minꢂ1 (Fig. 1(a)). The samples were then centri- poled BT are listed in Tables S2 and S3 (ESI†).
fuged twice at 20 1C (17 300 rcf, 5 and 15 min) to remove all BT
In addition, the weight fractions of ferroelectric tetragonal
particles. Immediately afterwards, the fluorescence intensity (wftetr) and paraelectric cubic (wfcub) BaTiO3 phases are com-
of the supernatant was measured. Every experiment was pared in Fig. 1(d). As indicated in the ongoing splitting of the
performed twice and the relative error of cDCF corresponds peak at 2y of 45.21, wftetr rises with the increasing nominal
to the standard deviation.
particle diameter from 61.8 ꢃ 2.4% up to 91.3 ꢃ 3.3% for the
unpoled BT batches (Fig. 1(c)). This result can be confirmed by
the results of Benke et al.30 The refinement for BT with a size of
about 150 nm resulted in a wftetr of 75.4 ꢃ 1.7%. Furthermore,
Lan et al. found a wftetr of about 0% for BT with a size o100 nm
and wftetr of about 60 and 80% for BT with sizes between 100
and 300 nm although the exact particle sizes are difficult to
determine from the relevant SEM micrographs.40 Additionally,
Benke et al. found out that poling of their BT had no significant
impact on the diffraction pattern and that only an insignificant
amount of the cubic phase was converted to tetragonal. We can
confirm these findings for BT samples with different nominal
particle sizes when comparing the results for unpoled and
poled BT (Tables S2, S3, ESI† and Fig. 1(d)). However, the
results show that the tetragonal is the dominant phase for all
BT, which means that all of them are ferroelectric and thus
pyroelectric.
SEM was used to analyze the morphological and structural
properties of the unpoled BT samples (Fig. 2). The particles of
the powder sample with a nominal particle diameter of
100 nm (BT100) have an irregular round shape and a particle
size between 100 and 200 nm (Fig. 2(a)). Powder samples of
BT200 consist of particles with a size between 180 and 350 nm
and show a transition from a round to an angular shape
(Fig. 2(b)). For BT500, the particles show a mixture of differ-
ent angular particle shapes and most particles have a size
2.5 Analysis methods
Fluorescence measurements of DCF were conducted with a
fluorescence detector (FP-4025, Jasco) which was equipped with
a square cell holder for 10 ꢄ 10 mm square cells (Type 3/G/10,
Starna Scientific). The wavelengths for excitation and emission
were 480 and 525 nm and a detector gain of 1 was used. In a
typical measurement 400 mL of a sample were diluted with
2900 mL degassed ultrapure water. The DCF stock solution (50 mM)
was prepared by dissolving DCF in methanol (234 mL per mg DCF)
and subsequent dilution of this methanolic solution with
25 mM NaH2PO4ꢁ2H2O (2.33 mL per mg DCF). In order to get
reliable results, it was crucial to protect the samples from light
during the whole measurement process to prevent further
DCHF oxidation. The fluorescence intensities correlated with
the DCF concentration cDCF in a linear fashion (Fig. S4, ESI†).
The quantitative analysis was done by external calibration
with standards in three concentration ranges and threefold
measurements. In Table S1 (ESI†) calibration parameters for
the linear regressions are displayed.
3 Results and discussion
3.1 Pyrocatalyst characterization
XRD was used to analyze the phase compositions of BT samples between 0.5 and 1.0 mm (Fig. 2(c)). The particles in BT100 and
(unpoled powders in Fig. S5–S7, ESI†). In Fig. 1(b) and (c), the BT200 are agglomerated in random, micrometer-sized
XRD patterns for the unpoled BT samples are compared in spheres. The poling process had no influence on the particle
detail. For the fine-grained BT100, the peak at 2y of 45.21 is shape or size.
broad, whereas it shows an increasing but not complete split-
In addition to the particle size and shape, the BET
ting for BT samples containing larger particles (BT200 and specific surface area SBET of all poled and unpoled powders
BT500; Fig. 1(c)). This development indicates a mixture of was measured and is shown in Fig. S8 (ESI†). SBET increases
paraelectric cubic (COD 1507757, Pm3m) and ferroelectric with decreasing particle size from 2.3 m2 gꢂ1 for BT500 up to
%
tetragonal phase (COD 1507756, P4mm) with an increasing 9.4 m2 gꢂ1 for BT100. The poling process resulted only for
Fig. 2 SEM micrographs of unpoled BaTiO3 powders with nominal particle sizes of 100, 200 and 500 nm (BT100, BT200, BT500).
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BT100 in a slight increase of SBET (+11%), while no signifi- 66.7 mg mLꢂ1. As shown in Fig. 3(c), there is a linear correlation
cant change was observed for BT200 and BT500.
between cDCF and bBT100 over the whole range (7.7 nM DCF per
mg mLꢂ1 BT100). This result corresponds to results of our
previous study with BT200 (1.2 nM DCF per mg mLꢂ1
BT200).37 Wu et al. and Qian et al. on the other hand found
that the pyrocatalytic degradation of rhodamine B with BiFeO3
3.2 Pyrocatalytic activity – BT100
The pyrocatalytic activity of BT100 nanoparticles was character-
ized by their ability to oxidize DCHF into DCF under a cyclic
thermal excitation between 32.5 and 69.9 1C with 2 K minꢂ1
(Scheme S1, ESI†). Fig. 3(a) demonstrates the increase of the
characteristic fluorescence spectra of the oxidized DCF after 0
to 8 thermal cycles. Additionally, the DCF concentration cDCF
from experiments with an increasing number of thermal cycles
was plotted in Fig. 3(b). cDCF increased nearly linearly with the
thermal cycle count up to 492 nM DCF for 8 cycles. This
corresponds to a conversion of 49% based on the initial DCHF
concentration c0,DCHF. In a reference experiment without pyr-
ocatalyst, less than 14 nM DCF was generated through thermal
cycling alone. These findings demonstrate that the pyrocatalyst
BT100 is responsible for the oxidation of DCHF into DCF under
thermal cycling.
and ZnO powders reached a plateau for a low b of 1 mg mLꢂ1 41,42
.
For the piezocatalytic degradation of RhB and Acid Orange 7 with
sonicated suspensions of K0.5Na0.5NbO3 and Pb(Zr0.52Ti0.48)O3, an
optimum and a plateau was reached at higher b values of
4 mg mLꢂ1 and 12.5 mg Lꢂ1 43,44
In these studies, the reduction
.
or saturation of the pyro- or piezocatalytic degradation efficiencies
at higher values of b were explained by the increased collision
probability between suspended particles. Each collision increases
the recombination probability of pyro- or piezoelectrically induced
opposite surface charges, which are necessary for pollutant
degradation. This explanation is comprehensible, but as
can be seen from the broad range for b found in literature
between 1 and 12.5 mg mLꢂ1, there have to be additional
influencing factors like the material (e.g. pyro-/piezoelectric
coefficient, permittivity, morphology, size), reaction (e.g. type/
concentration of pollutant, pH, conductivity) and process
In a next step, the dependence of cDCF from the powder
amount bBT100 for the standard temperature program with four
thermal cycles was determined in the range between 1.7 and
Fig. 3 Pyrocatalytic activity of the BaTiO3 catalyst powder BT100 (100 nm nominal particle size) after thermal treatment: (a) fluorescence excitation and
emission spectra (lem = 525 nm; lex = 480 nm) and (b) DCF concentration cDCF with (‘‘DT + BT100’’) and without pyrocatalyst (‘‘DT only’’) after 0 to 8
thermal cycles. Inset: Color change of the DCHF solution after 0 to 8 thermal cycles. (c) DCF concentration cDCF versus powder amount bBT100
.
(d) DCF concentration cDCF and residual catalytic activity cr/cr0 after recycling. DCF: 20,70-dichlorofluorescein; DCHF: 20,70-dichlorodihydrofluorescein;
DT: thermal cycling.
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parameters (e.g. temperature, fluid mechanics). Given that
many parameters in this study differ greatly from previous
studies, a much wider linear range for the linear correlation
between cDCF and bBT100 is plausible.
Next, the stability and reusability of BT100 was investigated.
As shown in Fig. 3(d), the residual catalytic activity cr/cr0
decreases discontinuously. After twofold recycling, the amount
of oxidized DCHF decreased only slightly (87.5%) and dropped
down to 68 ꢃ 6% after four recycling steps. XRD measurements
revealed no notable changes in crystal structure and phase
composition. In addition, SBET decreased only slightly by less
than ꢂ5%. A possible explanation for the decrease of pyro-
catalytic activity can be a consecutive adsorption of DCHF and
DCF on active sites of the BaTiO3 particles in combination with
a slight aggregation of smaller particles. As the reusability of
powder catalysts is crucial for a future application, strategies
for preserving the residual catalytic activity after several recy-
cling steps should be developed. Possible approaches are the
washing of the catalyst with organic solvents or the disaggrega-
tion of particles with the help of ultrasound.
3.3 Pyrocatalytic activity – size dependency
After studying the correlation between cDCF and bBT100 as well as
the catalyst reusability, experiments with BT of different nom-
inal particle diameters dnom of 100, 200 and 500 nm (BT100,
BT200, BT500) were performed. Fig. 4(a) shows that cDCF is
decreasing with increasing dnom of unpoled BT. In order to
reveal how much of the increased reactivity of the fine-grained
BT100 can be explained by its larger surface area, cDCF was
plotted as a function of SBET (Fig. 4(b)). Although the number of
data points is small, cDCF is found to be proportional to SBET in
good approximation. Accordingly, the increased reactivity of
the fine-grained BT100 in this study can be mainly be explained
by its larger surface area.
Fig. 4 Pyrocatalytic activity of poled and unpoled BaTiO3 catalysts with
nominal particle sizes of 100, 200 and 500 nm (BT100, BT200, BT500).
(a) 20,70-Dichlorofluorescein (DCF) concentration cDCF after thermal treat-
ment. Inset plot: Relative increase of cDCF through poling of BT. (b) Plot of
cDCF as a function of BET surface area SBET. BET: Brunauer–Emmett–Teller.
While SBET correlates to the adsorption capability, wftetr defines
Nevertheless, the authors would like to point out that for BT the fraction of BT that is actually pyroelectric, i.e. catalytically
containing smaller particles, further parameters exist which active, and (c/a ꢂ 1) correlates with the pyroelectric coefficient
can outweigh the influence of SBET (Table 1). For instance, in a and how much charge is generated during temperature
direct comparison Wu et al. found that a BT with low particle changes. A hypothetical progression of the size-dependent
sizes and a XRD pattern with cubic symmetry features tetra- pyrocatalytic activity cDCF = f (dnom) of unpoled BT was derived
gonality had a lower pyrocatalytic performance for RhB degra- from the assumption that the pyrocatalytic activity is in a
dation than a BT with larger particles and a XRD pattern rough estimation proportional to the product of SBET, wftetr
showing tetragonal symmetry features.29 These findings are and (c/a ꢂ 1) and is shown in Fig. 5. Based on this schematic
supported by another study of Wu et al. regarding the role of plot, BT pyrocatalysts can be categorized into three classes
the ferroelectric polarization on the piezocatalytic activity of depending on their particle size. BT catalysts with large particle
annealed BT.34 They determined the highest piezocatalytic sizes (41 mm) belong to the first class C-I and should have a
performance for RhB degradation for an annealed BT with low pyrocatalytic activity as a consequence of a very low SBET
medium particle size, tetragonality c/a and SBET. The perfor- ({1 m2 gꢂ1). BT catalysts with particle sizes between 1 mm and
mance of annealed BT containing smaller and larger particles 100 nm belong to the second class C-II. In this class, SBET
declined (Table 1).
increases drastically with decreasing particle size, outweighing
An explanatory approach, which combines and mostly the moderate decline of wftetr and (c/a ꢂ 1). As a result, cDCF
explains the previously mentioned results, is that the size- increases drastically when the particle size is decreased and
dependency of the pyrocatalytic activity has its origin in a SBET is increased, as shown in this study for BT100, BT200 and
combination of opposing, size-dependent changes in SBET BT500 (Fig. 4(b)). The third class C-III covers BT catalysts
and the crystallographic composition. As SBET increases with with further decreased particle sizes down to around 30 nm.
a decreasing particle size (SBET B 1/d), wftetr and c/a are The ferroelectric, tetragonal phase is suppressed due to bound-
decreasing (Fig. S8, ESI† and Fig. 1(d), Table S2, ESI†).45,46 ary effects below this critical particle size dcrit (C-0).47 In this
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Table 1 Comparison of material parameters with the pyro- or piezocatalytic performance of differently sized BaTiO3 powders
Tetragonal phase
weight fraction wftetr
BET specific
surface area SBET
Particle size
Catalytic activity
Ref(s).
Increase cDCF (pyro):
276 nM
89 nM
100–200 nm (BT100)
180–350 nm (BT200)
0.5–1.0 mm (BT500)
62%
82%
91%
9.4 m2 gꢂ1
3.3 m2 gꢂ1
2.3 m2 gꢂ1
This work
50 nM
Degradation degree
RhB (pyro):
26%a
b
r100 nma
Lowa
—
Wu et al.29
b
150–300 nma
Higha
—
48%a
Rate constant MO
degradation (piezo):
0.014 minꢂ1 a
0.019 minꢂ1
0.005 minꢂ1
o100 nma
100–350 nma
c1 mma
Lowa; c/a = 1.001a
21.1 m2 gꢂ1
4.7 m2 gꢂ1
0.23 m2 gꢂ1
Mediuma; c/a = 1.005a
High (100%a); c/a 4 1.011
Wu et al.34
a
b
Estimate based on the experimental data reported. Not applicable; wftetr: tetragonal phase weight fraction; SBET: Brunauer–Emmett–Teller (BET)
specific surface area; BTX00: BaTiO3 powder with X00 nm nominal particle size; cDCF: concentration of 20,70-dichlorofluorescein (DCF); RhB:
rhodamine B; MO: methyl orange.
third class, wftetr and (c/a ꢂ 1) drop down to zero, whereas SBET to confirm this explanatory approach in order to be able to
only triples with decreasing particle size. These opposing predict optimal particle sizes for piezo- and pyrocatalysts.
tendencies lead to a weakening of the increase of cDCF until a
3.4 Pyrocatalytic activity – poling effect
maximum in pyrocatalytic activity is reached at the optimal
particle size. Below this optimal particle size, the decrease of
wftetr and (c/a ꢂ 1) outweighs the increase of SBET, causing cDCF
to drop down rapidly towards zero at dcrit. It should be noted,
however, that the exact development of the catalytic activity as a
function of particle size as well as the optimal particle size
depend on additional factors such as the excitation mecha-
nism, the model reaction or the particle shape. Nevertheless,
the results of both studies of Wu et al. are in good agreement
with this classification.29,34 The BT compared in the studies of
Wu et al. (Table 1) belong to different classes. In both studies,
the fine-grained BT belongs to the third and the medium-sized
BT to the second class. Considering that the catalytic activity for
BT catalysts of the third class is sensitive to slight changes of
wftetr and (c/a ꢂ 1), the lower piezocatalytic activity of the fine-
grained BT compared to the medium-sized BT despite its much
higher SBET is plausible. Further studies should be performed
A strong increase of the catalytic activity of pyrocatalysts has
been achieved by implementing a poling treatment. The influ-
ence of this on the pyrocatalytic activity of BT batches with
increasing particle size was investigated and is shown in
Fig. 4(a). Poling led to a strong and size-dependent increase
of cDCF whereby the strongest relative increase of +247% was
found for BT500 with the largest particle. For the finest powder,
BT100, the increase turned out to be only +91%. Changes in
the particle shape/size, SBET or wftetr cannot explain these
results, as they were not significantly affected by the poling.
The literature does not describe changes in these parameters
for poled BT either.30,31,34 A summary of relevant parameters
and results of previous studies on the influence of poling on the
piezo- and pyrocatalytic activity of BT is given in Table 2 for
comparison. These studies are difficult to compare as different
excitation mechanisms (thermal vs. mechanical), poling proce-
dures and model reactions were used. In addition, the available
material characterization data differ. However, the combi-
nation with the fully comparable data of this study results in
a largely consistent picture: (i) poling of BT with a particle size
larger than ca. 200 nm (BT200, BT500; Chen et al.) up to several
micrometers (Wu et al.) and with a high wftetr leads to strong
improvements of piezo- and pyrocatalytic activity.31,34 (ii) For
fine-grained BT with a considerably reduced wftetr of {80%
and a particle size near 100 nm, only marginal (Benke et al.) or
reduced (BT100) positive effects can be achieved by poling.30
A possible explanation for similar observations in piezo-
catalysis was provided by Wu et al. and links the ferroelectric
polarization of individual BaTiO3 particles with their
size, crystal and domain structure (schematically shown in
Fig. 6).34 Small BaTiO3 particles below the critical size dcrit of
about 30 nm have a cubic phase and are therefore paraelectric
and non-pyroelectric (Fig. 6(a1) and (b1)). Particles with a
higher particle size up to about 100 nm are ferroelectric single
Fig. 5 Hypothetical progression of cDCF, the product of the tetragonal phase
weight fraction wftetr times the tetragonality (c/a ꢂ 1) and SBET as a function of
the nominal particle diameter dnom. BET: Brunauer–Emmett–Teller.
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Table 2 Comparison of material and poling parameters with the effect of poling on the pyro- or piezocatalytic performance of BaTiO3 powders
Tetragonal phase
weight fraction wftetr
BET specific
surface area SBET
Poling effect on
catalytic activity
Particle size
Poling parameters
Ref(s).
100–200 nm
(BT100)
180–350 nm (BT200)
61.8 ꢃ 2.4% (unpol.)
66.7 ꢃ 2.9% (poled)
82.0 ꢃ 2.2% (unpol.)
81.9 ꢃ 2.0% (poled)
91.3 ꢃ 3.3% (unpol.)
92.0 ꢃ 3.2% (poled)
75.4%
9.4 m2 gꢂ1 (unpol.)
10.5 m2 gꢂ1 (poled)
3.3 m2 gꢂ1 (unpol.)
3.8 m2 gꢂ1 (poled)
2.3 m2 gꢂ1 (unpol.)
2.1 m2 gꢂ1 (poled)
Increase cDCF (pyro):
+91%
+155%
2 kV mmꢂ1; 5 s;
powder pellet
This work
0.5–1.0 mm (BT500)
ca. 150 nm
+247%
b
—
3 kV mmꢂ1; 1 h;
powder pellet
Coumarin dosimetry
(pyro):
Benke et al.30
Chen et al.31
Wu et al.34
Marginal effect
Decomposition ratio
RhB (pyro):
+100/300/560%
Rate constant MO
degradation (piezo):
+35a/80%
b
ca. 350 nm
c1 mma
Tetragonal phase
—
1.5/3/5 kV mmꢂ1; 30 min;
110 1C; Au-coated wafer
100%a
0.23 m2 gꢂ1
2/4 kV mmꢂ1; 30 min;
powder pellet
c/a 4 1011a
a
b
Estimate based on the experimental data reported. Not applicable; wftetr: tetragonal phase weight fraction; SBET: Brunauer–Emmett–Teller
(BET) specific surface area; BTX00: BaTiO3 powder with X00 nm nominal particle size; RT: room temperature; cDCF: concentration of
20,70-dichlorofluorescein (DCF); RhB: rhodamine B; MO: methyl orange.
domains in a multidomain single crystal as well as individual
crystallites in a polycrystalline particle should be randomly
oriented, resulting in a macroscopic polarization of zero.50 Since
a reduction in polarization has a negative effect on the pyro-
electric coefficient and thus on the pyrocatalytic activity, coarse-
grained BaTiO3 pyrocatalysts can benefit from poling.34,51 During
the poling procedure, the ferroelectric domains in individual
polycrystalline particles and in multidomain single crystals align
along the direction of a sufficiently strong electric field (Fig. 6(e2)
and (f2)). This alignment results in an increased remanent
polarization PR of these particles after the field is reduced back
to zero (ferroelectric hysteresis; Fig. 6(C)).52 The increase of PR is
beneficial for the pyroelectric coefficient and thus increases the
pyrocatalytic activity of these particles.34,51 On the contrary, the
pyrocatalytic activity of small ferroelectric particles with only a
single domain should be independent of the poling treatment
(Fig. 6(c2) and (d2)). For these particles, the polarization rises and
falls linearly with the external electric field, so that there should
be no further significant increase in PR (Fig. 6(B)).53 Conse-
quently, the positive effect of poling reduces with decreasing
particle size. The further the particle size distribution of a BaTiO3
powder is shifted towards smaller particle sizes, the higher the
Fig. 6 Schematic representation of the relationships between particle
proportion of paraelectric and single-domain ferroelectric BaTiO3
particles that do not benefit from poling.
Considering all previously discussed findings, theoretical
explanations and comparisons with literature, a hypothetical
size, polarization, crystal and domain structure (a1)–(f1) before poling
and (a2)–(f2) after poling with a sufficiently strong external electric
field. Typical progressions of polarization versus electric field (P–E) for
(A) paraelectric, (B) ferroelectric single-domain, and (C) ferroelectric
multidomain crystals (demonstration only; not to scale).
progression of the size-dependent pyrocatalytic activity cDCF
=
f (dnom) for poled BT was derived based on assumptions
explained in Section 3.3 (Fig. 5). Additionally, the influence of
crystals with a single ferroelectric domain (Fig. 6(c1) and (d1)) the size-dependent crystal and domain structure on the poling
Larger particles develop a multidomain structure with a behavior and the resulting increase in pyrocatalytic activity was
domain size of tens to hundreds of nanometers depending estimated. While the relative positive effect on pyrocatalytic
on the particle size (Fig. 6(e1) and (f1)). With continuous activity should increase with increasing dnom, the absolute
growth, for example during an annealing procedure, BaTiO3 effect should have an optimum for medium-sized BT. Again,
particles can additionally develop a polycrystalline structure it should be noted that the exact development of the catalytic
through partial sintering.47–49 In their virgin state, individual activity as a function of particle size for poled BT depend on
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additional factors such as the excitation mechanism, the model
reaction or the particle shape. Therefore, further studies should be
performed to confirm this explanatory approach in order to be able
to predict optimal particle sizes for poled piezo- and pyrocatalysts.
However, considering all previously discussed findings, a basic
future strategy for maximizing the pyrocatalytic performance
of BaTiO3 nanoparticles for DCHF oxidation can be developed:
(I) SBET should be maximized through a further decrease of the
particle size while maintaining sufficiently high values for wftetr
and c/a. (II) All ferroelectric BT catalysts should be poled in order to
increase the alignment of the intrinsic ferroelectric dipoles of
multidomain or polycrystalline BaTiO3 particles.
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C. Faggio, Sci. Total Environ, 2018, 631–632, 789–794, DOI:
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4 Conclusions
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T. A. Ternes and A. Joss, Water Res., 2016, 95, 240–249,
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2018.04.103.
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J. Phys. Chem. Ref. Data, 1988, 17, 513–886, DOI: 10.1063/
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10 D. B. Miklos, C. Remy, M. Jekel, K. G. Linden, J. E. Drewes
and U. Hubner, Water Res., 2018, 139, 118–131, DOI:
10.1016/j.watres.2018.03.042.
11 S. Yan, A. Jia, S. Merel, S. A. Snyder, K. E. O’Shea,
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In conclusion, the interdependence between the particle size,
the poling procedure and the pyrocatalytic activity of BaTiO3
powders with nominal particle sizes of 100, 200 and 500 nm
was revealed by using the DCF redox assay under directly
comparable conditions. Based on our results, we were able to
estimate and confirm the basic progression of the pyrocatalytic
activity of unpoled BaTiO3 powders as a function of nominal
particle size. Moreover, we demonstrated that poling of pyr-
ocatalysts leads to a strong size-dependent increase of pyro-
catalytic activity. This poling effect increased with particle size
up to +247% and was explained with size-dependent changes in
phase composition and domain structure. Based on the find-
ings of the experimental investigations, a future strategy for
maximizing the catalytic performance of BaTiO3 pyrocatalysts
was developed. Further research topics are the long term and
cycling stability of poled BT, the influence of poling conditions
on the pyrocatalytic activity and the influence of heating/cooling
rates as well as temperature range. With the help of highly active
pyrocatalysts, the pyrocatalytic process can take the next step in
its development into a new AOP technique for water remediation,
accompanied with a great potential for thermal energy harvest-
ing. We believe that this study provides a deeper understanding
of the influence of important material parameters on the catalytic
performance of pyrocatalysts.
15 N. Singh and B. R. Goldsmith, ACS Catal., 2020, 10,
3365–3371, DOI: 10.1021/acscatal.9b04167.
16 M. Abinaya, R. Rajakumaran, S.-M. Chen, R. Karthik and
V. Muthuraj, ACS Appl. Mater. Interfaces, 2019, 11, 38321–38335.
17 C. Xu, P. Ravi Anusuyadevi, C. Aymonier, R. Luque and
S. Marre, Chem. Soc. Rev., 2019, 48, 3868–3902.
18 J. You, Y. Guo, R. Guo and X. Liu, Chem. Eng. J., 2019, 373,
624–641.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
19 R. Katal, M. Salehi, M. H. Davood Abadi Farahani,
S. Masudy-Panah, S. L. Ong and J. Hu, ACS Appl. Mater.
Interfaces, 2018, 10, 35316–35326.
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The authors thank the group of Lothar Wondraczek for their
help with the material characterization.
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