Enhanced photocatalytic activity on polarized ferroelectric KNbO3

Article abstract of DOI:10.1039/c6ra23344a

In this paper, we demonstrate the enhanced photodegradation of rhodamine B on polarized ferroelectric KNbO₃ (KNO) particles. High-quality KNO samples were prepared using a solid-state reaction and polarized under different electric fields. The variation of XRD and piezoelectric coefficient (d₃₃) suggest an increased intensity of the ferroelectric polarization in the polarized samples. In contrast to the slow photodegradation rate of unpolarized KNO, the normalized photodegradation reaction rate constant was remarkably increased to 0.317 min^-1 by the polarized KNO. The longer photoluminescence lifetime indicates that the enhancement of photocatalytic activity in polarized ferroelectric KNO powder is mainly attributed to the enhanced internal field. We believe this work may open up new avenues in photocatalysis using polarized ferroelectric materials.

Full text of DOI:10.1039/c6ra23344a

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Enhanced photocatalytic activity on polarized
ferroelectric KNbO3†
Cite this: RSC Adv., 2016, 6, 108883
Qiang Fu,ab Xianjie Wang, Changyu Li,c Yu Sui,a Yaping Han,b Zhe Lv,a Bo Songad
a
*
e
*
and Ping Xu
In this paper, we demonstrate the enhanced photodegradation of rhodamine B on polarized ferroelectric
KNbO3 (KNO) particles. High-quality KNO samples were prepared using a solid-state reaction and
polarized under different electric fields. The variation of XRD and piezoelectric coefficient (d33) suggest
an increased intensity of the ferroelectric polarization in the polarized samples. In contrast to the slow
photodegradation rate of unpolarized KNO, the normalized photodegradation reaction rate constant was
remarkably increased to 0.317 minꢀ1 by the polarized KNO. The longer photoluminescence lifetime
indicates that the enhancement of photocatalytic activity in polarized ferroelectric KNO powder is mainly
attributed to the enhanced internal field. We believe this work may open up new avenues in
photocatalysis using polarized ferroelectric materials.
Received 20th September 2016
Accepted 4th November 2016
DOI: 10.1039/c6ra23344a
www.rsc.org/advances
Furthermore, ferroelectric polarization helps to inhibit the
recombination of electrons and holes, just like p–n junctions of
1. Introduction
a typical photovoltaic or other diode structure. With a similar
mechanism as that of p–n junctions, ferroelectric materials can
also serve as new candidates for photocatalysis, and these have
a signicant inuence on the surface photochemistry. For
example, acceleration of the separation of electrons and holes
by polarization in BaTiO3 (BTO) bulk pellets has been demon-
strated via the spatial selectivity of photo reduction and oxidation
reactions.14 Cui et al. observed an enhanced photocatalytic
activity in Ag-loaded BTO as a result of the inuence of the
ferroelectricity on the carrier separation.15 Recently, Su et al.
prepared different-sized BTO nanoparticles (7.5 nm) with high
monodispersity to investigate the effects of a ferroelectricity-
enhanced photocatalytic reaction. These results clearly indicate
that ferroelectricity can directly affect photocatalytic activity and
that photocatalytic performance can be signicantly increased
by attaching Ag to the BTO surface.16 However, the relationship
between polarization and photocatalytic reactions is not clear.
Recently, Density Functional Theory (DFT) and electronic
structure analysis suggested that polarized ferroelectric oxide is
a useful way to design an efficient photocatalyst.17,18 It is well
known that the polarization of ferroelectrics can be reoriented by
an applied electric eld. KNbO3 (KNO) is one of the most
promising candidates for ferroelectric-based photovoltaic appli-
cations due to a higher Curie temperature (about 440 ꢁC).19 Much
attention has been paid to morphological control and noble
metal doping to improve the photocatalytic activity of KNO. Ag–
KNO nanocomposites enhance the photocatalytic activity
because the deposition of Ag nanoparticles results in continuous
band gap states and increases electron density adjacent to the
Fermi level.20 Lan et al. observed an enhanced photocatalytic
The discharge of industrial effluent containing organic pollut-
ants is becoming a serious challenge for society. Signicant
efforts, including ltration, sorption processes, biological
treatment, and catalytic oxidation, are being made to remove
these pollutants.1–6 Among the techniques that are being
investigated, semiconductor photocatalysis is regarded as an
effective way to degrade and remove hazardous pollutants from
water. Electron–hole pairs can be generated in a semiconductor
under light irradiation, and these are able to participate in
redox reactions with pollutants. Several semiconductor systems
have been investigated, but no satisfactory cost-effective
approach has yet been found because some precious metals,
such as Au and Ag, or their oxides must be added to enhance the
photocatalytic activity.7–10
Ferroelectric materials have spontaneous polarization
arising from the displacement of the center of the positive and
negative charges in a unit cell, and this could provide new
possibilities for designing photovoltaic devices by promoting
the separation of photo-excited carriers to a desirable extent.11–13
aDepartment of Physics, Harbin Institute of Technology, Harbin 150001, China.
E-mail: wangxianjie@hit.edu.cn
bDepartment of Physics, Northeast Forestry University, Harbin 150040, China
cMaterial Science and Engineering College, Northeast Forestry University, Harbin
150040, China
dAcademy of Fundamental and Interdisciplinary Sciences, Harbin Institute of
Technology, Harbin 150001, China
eSchool of Chemistry and Chemical Engineering, Harbin Institute of Technology,
Harbin 150001, China. E-mail: pxu@hit.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra23344a
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activity in one-dimensional KNO nanowires with Au nano- absorption spectra were collected using ultraviolet-visible
particles due to surface plasmon resonance as well as interband (UV-vis) spectroscopy (TU-1901, Beijing Purkinje General). The
transitions on Au nanoparticles.21 Zhang et al. found that photoluminescence (PL) lifetime was measured using a laser
orthorhombic KNO nanowires displayed RhB photodegradation system with a wavelength of 375 nm and a frequency of 20 MHz
about two-fold as large as their monoclinic counterparts.22 (Becker and Hickl Gmbh).
However, their photocatalytic efficiency was still very low.
In this paper, we investigated the inuence of polarization
3. Results and discussion
on the photodegradation of rhodamine B (RhB) by KNO parti-
cles. High quality KNO samples were polarized under different
electric elds, and variations of XRD and the piezoelectric
coefficient (d33) clearly suggested increased intensity of ferro-
Fig. 1a shows the XRD patterns of KNO samples at room
temperature (RT), which can be indexed to a perovskite struc-
ture of pure KNO of an orthorhombic crystal system with the
electric polarization. The photodegradation rate and photo-
Amm2 space group (JCPDS 71-0946).23 The intensity of the (220)
luminescence lifetime were strongly increased aer
peak was higher than that of the (002) peak (Fig. 1b). The XPS
polarization, and this was mainly attributed to the enhanced
spectra of K and Nb of the KNO samples are shown in Fig. S1.†
internal eld aer polarization.
The binding energy was determined by reference to the
C 1s line. The binding energies of Nb 3d5/2 and Nb 3d3/2 are
2. Experimental
2.1 Sample preparation
206.7 eV and 209.4 eV, respectively, similar to the reported
values of Nb5+
.
The binding energies of K 2p3/2 and K 2p1/2
20,24
are 291.3 eV and 294.1 eV, respectively, which conrms the
existence of K+. Temperature-dependent dielectric constants at
various frequencies (Fig. S2†) clearly show four phase transi-
tions of KNO, and TC is about 436 ꢁC. The above results conrm
the high quality of the as-obtained KNO samples.25,26
Fig. 2 shows the P–E hysteresis loops of the polarized KNO
samples at RT. It is clear that Pr of the KNO samples increases
monotonically with the increase in the applied electric eld,
which is attributed to the fact that the ferroelectric domains can
be switched easily at a higher applied electric eld. The result
veries the existence of an internal electric eld in KNO due to
an intrinsic electric polarization.27,28
Stoichiometric powder with the composition of KNbO3 (KNO)
was prepared via the solid-state reaction technique, where the
dried K2CO3 (99.5%) and Nb2O5 (99.9%) powders were used as
raw materials. The raw materials were mixed and well-ground in
absolute ethyl alcohol until it dried. The powder was then
ꢁ
pressed into pellet disks and calcined at 900 C for 24 h. The
calcined mixture was then ground using an agate mortar and
pressed into pellet disks 13 mm in diameter, and these were
ꢁ
sintered at 1000 C for 24 h surrounded by sacricial powder
of the same composition to inhibit volatilization of potassium.
The prepared samples were polarized using a lab-mad polar-
izing device. The system consists of two conductive needles,
and the KNO pellet disk coated with palladium–silver paste on
both sides was put in the middle for polarization. Three
different polarized elds (5 kV cmꢀ1, 10 kV cmꢀ1, and 15 kV
cmꢀ1) were used to polarize the KNO samples for 30 min. It is
noted that the KNO pellet disk is destroyed if the polarizing eld
The KNO samples were polarized using a lab-mad polarizing
device with three polarization elds (5 kV cmꢀ1, 10 kV cmꢀ1
,
and 15 kV cmꢀ1). The polarization intensity can be character-
ized from the ratio of I(220)/I(002) from Fig. 1b. The relative
intensities of (220)/(002) are 2.81, 2.74, and 3.03 aer polarizing
under 5, 10, and 15 kV cmꢀ1 (1.40 for the non-polarized),
is higher than 20 kV cmꢀ1
.
2.2 Characterization
The crystal structure of the KNO particles was characterized by
X-ray diffraction (XRD, using the X'Pert XRD spectrometer with
˚
Ni-ltered Cu Ka radiation, l ¼ 1.5418 A) and X-ray photoelec-
tron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scien-
tic). The dielectric properties of the samples were measured
using the Agilent E4980A LCR meter. Ferroelectric data were
collected with the Precision Premier II Ferroelectric Tester
(Radiant Technology).
2.3 Photocatalysis
The photodegradation of rhodamine B (RhB) was assessed
under UV light irradiation to evaluate the photocatalytic activity
of the KNO samples. The concentration of the RhB solution was
10 mg Lꢀ1, and 0.2 g of the KNO samples was used to degrade
200 ml of the RhB solution. Before light illumination, the
suspension was magnetically stirred for 30 min in the dark to
Fig. 1 (a) XRD patterns of non-polarized KNO and 5 kV cmꢀ1, 10 kV
cmꢀ1, and 15 kV cmꢀ1 polarized KNO samples. (b) The enlarged view of
establish complete adsorption–desorption equilibrium. The the (220) and (002) crystal planes.
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Fig. 2 P–E hysteresis loops of KNO polarized with different electric
fields.
respectively, indicating more electric domains along the (220)
crystal plane with increasing polarized elds.29,30 Furthermore,
the alignment of electric domains enhances the separation of
photogenerated electrons and holes, and thus the photo-
catalytic efficiency. As shown in Table S1,† the variation of
lattice parameters calculated from XRD also suggest that the
degree of polarization increases with stronger polarized electric
eld because the ferroelectric polarization of KNO arises from
the displacement of the center of the positive and negative
charges in a unit cell. The piezoelectric coefficient (d33) is
a useful parameter to conrm the ferroelectric polarization. The
increased value of d33 also conrms the enhanced intensity of
the ferroelectric polarization aer being polarized (Table S1†).
It is well known that ferroelectric polarization helps to
inhibit the recombination of holes and electrons that partici-
pate in redox reactions. Here, we nd that the photodegradation
rate can be strongly enhanced aer polarization (Fig. 3). Only
40% RhB was degraded aer 70 min of illumination using the
non-polarized KNO. However, RhB will be completely degraded
within 15 min using the polarized KNO samples. It has been
generally assumed that the kinetics of photocatalytic decolor-
ization of most organic compounds follows the Langmuir–
Hinshelwood model:5,31,32
Fig. 3 (a) Photodegradation profiles of RhB under UV light irradiation,
the inset shows the cycle stability of the 15 kV cmꢀ1 polarized KNO
sample; (b) decay profiles of photoluminescence for non-polarized
KNO and the polarized samples.
and the reaction rate, kobs, can be obtained from the tted linear
plot of ln(C0/C) vs. t, which is shown in Table S2.† It is clear that
the photodegradation rate of RhB increases from 0.010 minꢀ1 to
0.317 minꢀ1 with the increase of the polarized voltage.
Fig. S4† shows the UV-vis spectral changes of the RhB solu-
tions in the presence of polarized KNO with a polarized voltage
of 15 kV cmꢀ1 and its corresponding successive colour change.
The polarized KNO is very stable for the photodegradation of
RhB (inset in Fig. 3a). It is well known that most of the ferro-
electric photocatalysts used to degrade organic pollutants are
made into nanostructures, and loaded with noble metal such as
Ag, Au, and Pt, and thus synthetic processes are very compli-
cated.7–10,16,28 Here, our samples are synthesized using a simple
solid state reaction, and moreover, the degradation efficiency of
the polarized KNO catalyst is markedly enhanced and the
normalized photodegradation reaction rate constant was
remarkably increased to 0.317 minꢀ1 by the polarized KNO,
which is much more effective than that of previous catalysts,
such as Ag2O–BTO hybrid nanocubes (about 2 h),33 Ag–KNO
nanocomposites (90 min),20 and Ag–BTO hybrid nano-
composites (40 min).9 Therefore, our results clearly demon-
strate that ferroelectric polarization is a useful way to enhance
photocatalytic activity. In order to demonstrate the advantage of
dCi
dt
kKCi
ri ¼ ꢀ
¼
(1)
1 þ kKCi
where Ci is the molar concentration of the dye solution, k is the
reaction rate constant, and K is the adsorption coefficient of the
dye to the catalyst. When Ci is small (Ci < 10ꢀ3 M)3, kKCi ꢂ 1,
and eqn (1) can be simplied to a pseudo-rst-order equation:20
dCi
ri ¼ ꢀ
¼ kKCi;
(2)
dt
and eqn (2) can be given in the following relationship:
C0
ln
¼ kobs
t
(3)
C
where C0 is absorbance related to the initial concentration of
the dye, and kobs ¼ kK is the observed pseudo-rst-order reac-
tion rate constant. The tted liner curves are shown in Fig. S3,†
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polarization more directly, time-resolved photoluminescence the formation of $OH and $O2ꢀ, respectively. As shown in Fig. 4a,
(PL) lifetime was measured. As shown in Fig. 3b, the electrons $OH was generated through the oxidation of OHꢀ and H2O on
ꢀ
excited from the polarized samples have a longer lifetime than the surface (ꢀ), and $O2 was formed from O2 molecules
that in the non-polarized one. This result clearly demonstrates through accepting electrons on the surface (+). Both $OH and
ꢀ
that the internal eld can inhibit the charge recombination, $O2 can oxidize RhB and nally degrade it into H2O, CO2, and
promoting the separation of the electrons and holes to enhance other degradation products. Therefore, our results demonstrated
the photocatalytic activity.11,34,35
clearly that the enhancement of photocatalysis activity in polar-
The above results imply that the enhanced photocatalytic ized ferroelectric powders can be mainly attributed to the
activity of polarized KNO samples may be interpreted from the enhanced internal eld. Remanent polarization could assist in
enhanced internal electric eld. Though there exists sponta- the increase of the photocatalytic activity in KNO.28
neous polarization in non-polarized KNO, the intensity of the
polarization is very limited because of the unordered ferroelec-
tric domain. However, when an electric eld was applied to
4. Conclusion
polarize KNO, the ferroelectric domain in KNO tended to become In summary, this study shows that the photodegradation rate of
ordered, and the polarization direction tended to point to the RhB can be strongly increased through the polarization of
same direction. As shown in Fig. 4a, the spontaneous polariza- ferroelectric KNO particles. Variations of the I(220)/I(002) ratio
tion with polarization vector P can be screened by free electrons in XRD and d33 conrm the increased intensity of ferroelectric
and holes, respectively, and/or by ions or molecules adsorbed polarization aer polarizing the KNO samples. The longer PL
onto the surface from the solution forming a stern layer. The lifetime reveals that the increased photocatalytic activity in
accumulation of free electrons on the surface (+) and holes on polarized KNO particles can be mainly attributed to the
the surface (ꢀ) leads to downward and upward band bending, enhanced internal eld. Our results provide clear evidence that
respectively. Aer the polarizing process, a permanent internal ferroelectric materials can act as promising photocatalysts in
electric eld was generated in KNO by the remanent polarization, dye degradation and that polarization can greatly enhance the
as shown in Fig. 4b and c. When the polarized powder was photocatalytic activity.
irradiated with UV light, the electrons and holes were photo-
generated and then separated by the internal electric eld
because of ferroelectric polarization, which led to the spatial
Acknowledgements
separation of the oxidation and reduction reactions on opposite This work is supported by National Natural Science Foundation of
surfaces.15 Zhang et al. carried out control experiments with China (No. 51472064, 51672057, 21671047), Science and Tech-
a number of scavengers to identify the major active species nology Innovation Talent Foundation of Harbin (2012RFXXG110),
responsible for RhB degradation, and suggested that the $OH and Program for Innovation Research of Science in Harbin
plays a more important role under UV than under visible- Institute of Technology (PIRS of HIT 201616). Fundamental
light.20,22 The photogenerated holes and electrons participated in Research Funds for the Central University (Grant No. HIT.-
BRETIII.201220, HIT.NSRIF.2012045, HIT.ICRST.2010008), Inter-
national Science & Technology Cooperation Program of China
(2012DFR50020) and the Program for New Century Excellent
Talents in University (NCET-13-0174).
Notes and references
1 H. J. Yun, D. M. Lee, S. Yu, J. Yoon, H.-J. Park and J. Yi, J. Mol.
Catal. A: Chem., 2013, 378, 221–226.
2 Y. Kang, Y. Yang, L. C. Yin, X. Kang, G. Liu and H. M. Cheng,
Adv. Mater., 2015, 27, 4572–4577.
3 S. T. Ong, P. S. Keng, W. N. Lee, S. T. Ha and Y. T. Hung,
Water, 2011, 3, 157–176.
4 H. Zhang, C. Wei, Y. Huang and J. Wang, Ultrason.
Sonochem., 2016, 30, 61–69.
5 I. K. Konstantinou and T. A. Albanis, Appl. Catal., B, 2004, 49,
1–14.
6 G. Liu, S. You, M. Ma, H. Huang and N. Ren, Environ. Sci.
Technol., 2016, 50, 11218–11225.
7 T. Som, G. V. Troppenz, R. R. Wendt, M. Wollgarten,
J. Rappich, F. Emmerling and K. Rademann, ChemSusChem,
2014, 7, 854–865.
Fig. 4 (a) Schematic of photocatalysis with ferroelectric materials. The
8 J. J. Chen, J. C. S. Wu, P. C. Wu and D. P. Tsai, J. Phys. Chem.
C, 2012, 116, 26535–26542.
schematic of remanent polarization (Pr) of (b) non-polarized KNO and
(c) polarized KNO samples.
108886 | RSC Adv., 2016, 6, 108883–108887
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Paper
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9 N. T. Khoa, S. W. Kim, D. H. Yoo, S. Cho, E. J. Kim and 23 H. Du, W. Zhou, F. Luo, D. Zhu, S. Qu and Z. Pei, Appl. Phys.
S. H. Hahn, ACS Appl. Mater. Interfaces, 2015, 7, 3524–3531. Lett., 2007, 91, 182909.
10 S. Liu, C. Li, J. Yu and Q. Xiang, CrystEngComm, 2011, 13, 24 R. Wang, Y. Zhu, Y. Qiu, C.-F. Leung, J. He, G. Liu and
2533–2541. T.-C. Lau, Chem. Eng. J., 2013, 226, 123–130.
11 A. Kakekhani, S. Ismail-Beigi and E. I. Altman, Surf. Sci., 25 B. Sundarakannan, K. Kakimoto and H. Ohsato, J. Appl.
2016, 650, 302–316. Phys., 2003, 94, 5182–5187.
12 L. Li, P. A. Salvador and G. S. Rohrer, Nanoscale, 2014, 6, 24– 26 G. Shirane, R. Newnham and R. Pepinsky, Phys. Rev., 1954,
42. 96, 581–588.
13 L. Liang, X. Kang, Y. Sang and H. Liu, Adv. Sci., 2016, 3, 27 P. A. Markovin, V. V. Lemanov, M. E. Guzhva, P. P. Syrnikov
1500358. and T. A. Shaplygina, Phys. Solid State, 2014, 56, 989–995.
14 J. L. Giocondi and G. S. Rohrer, J. Phys. Chem. B, 2001, 105, 28 S. Park, C. W. Lee, M.-G. Kang, S. Kim, H. J. Kim, J. E. Kwon,
8275–8277.
S. Y. Park, C.-Y. Kang, K. S. Hong and K. T. Nam, Phys. Chem.
Chem. Phys., 2014, 16, 10408–10413.
29 S. Li, A. S. Bhalla, R. E. Newnham, L. E. Cross and
C. Y. Huang, J. Mater. Sci., 1994, 29, 1290–1294.
30 T. Takaaki, K. Yutaka, O. Naoki, T. Tadashi and F. Osamu,
Jpn. J. Appl. Phys., Part 1, 1997, 36, 5970.
15 Y. Cui, J. Briscoe and S. Dunn, Chem. Mater., 2013, 25, 4215–
4223.
16 R. Su, Y. Shen, L. Li, D. Zhang, G. Yang, C. Gao and Y. Yang,
Small, 2014, 11, 202–207.
17 A. Kakekhani and S. Ismail-Beigi, Phys. Chem. Chem. Phys.,
2016, 18, 19676–19695.
18 B. Modak and S. K. Ghosh, RSC Adv., 2016, 6, 9958–9966.
31 W. Baran, E. Adamek and A. Makowski, Chem. Eng. J., 2008,
145, 242–248.
19 N. Yawata, H. Nagata and T. Takenaka, Ferroelectrics, 2014, 32 J.-M. Herrmann, Catal. Today, 1999, 53, 115–129.
458, 158–162.
33 H. Li, Y. Sang, S. Chang, X. Huang, Y. Zhang, R. Yang,
H. Jiang, H. Liu and Z. L. Wang, Nano Lett., 2015, 15,
2372–2379.
20 T. Zhang, W. Lei, P. Liu, J. A. Rodriguez, J. Yu, Y. Qi, G. Liu
and M. Liu, J. Phys. Chem. C, 2016, 120, 2777–2786.
21 J. Lan, X. Zhou, G. Liu, J. Yu, J. Zhang, L. Zhi and G. Nie, 34 H. Yu, X. Wang, W. Hao and L. Li, RSC Adv., 2015, 5, 72410–
Nanoscale, 2011, 3, 5161–5167. 72415.
22 T. Zhang, W. Lei, P. Liu, J. A. Rodriguez, J. Yu, Y. Qi, G. Liu 35 H. Lee, K. Heo, J. Park, Y. Park, S. Noh, K. S. Kim, C. Lee,
and M. Liu, Chem. Sci., 2015, 6, 4118–4123.
B. H. Hong, J. Jian and S. Hong, J. Mater. Chem., 2012, 22,
8372–8376.
This journal is © The Royal Society of Chemistry 2016
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