Efficient separation of photogenerated electron-hole pairs by the combination of a heterolayered structure and internal polar field in pyroelectric BiOIO3 nanoplates

Article abstract of DOI:10.1002/chem.201302884

Electrify your chemistry! Pyroelectric heterolayered BiOIO₃ nanoplates efficiently separate photogenerated electron-hole pairs due to the combined effect of their heterolayered structure and internal polar field (see scheme). Pyroelectric BiOIO₃ nanoplates, synthesized by a simple hydrothermal method, were found to possess a superior photocatalytic activity under UV irradiation. Copyright

Full text of DOI:10.1002/chem.201302884

COMMUNICATION
DOI: 10.1002/chem.201302884
Efficient Separation of Photogenerated Electron-Hole Pairs by the
Combination of a Heterolayered Structure and Internal Polar Field
in Pyroelectric BiOIO Nanoplates
3
[
a]
[a]
[b]
[a]
[a]
Wenjun Wang, Baibiao Huang,* Xiangchao Ma, Zeyan Wang, Xiaoyan Qin,
[
a]
[b]
[c]
Xiaoyang Zhang, Ying Dai, and Myung-Hwan Whangbo
Semiconductor photocatalysts are important for new
energy exploitation and decontamination of hazardous or-
ganic pollutants because they can photoconvert solar energy
into chemical energy, photodecompose organic materials,
[1]
and photoreduce carbon dioxide. However, their practical
applications still remain beyond our grasp due to their poor
quantum yield, which arises from the rapid recombination
[2]
of photogenerated electron-hole (e-h) pairs. In principle,
heterostructures can separate photogenerated electrons and
holes efficiently because the different components possess
different valence band (VB) and conduction band (CB)
[3]
Figure 1. a) A perspective view of the Bi
2
O
4
layer in BiOIO
3
, in which
edges. However, the electron-hole separation becomes in-
efficient if there is no good contact between the different
components. It is challenging to find a simple method that
efficiently separates photogenerated electrons and holes.
Bismuth-based layered semiconductors have long been an
important subject for photocatalysis. Nanostructures of
the large cyan and small white circles represent the Bi and O atoms, re-
spectively. b) A perspective view of the BiOIO slab, in which a Bi
layer shares O corners with IO units. The arrows along the c-direction
indicate the direction of the pyroelectric polarization, which results from
the dipoles of the individual IO units.
3
2 4
O
3
3
[
4]
[5]
[6]
[7]
Bi WO ,
Bi MoO ,
BiOX (X=Cl, Br, I),
Bi S ,
2 3
2
6
2
6
[
8]
Bi O CO
have been prepared by various methods and
the BiO pyramids, but from the IO pyramids. The local
6 3
2
2
3
used as the photocatalysts for decomposing organic pollu-
tants. However, bismuth-based layered materials with high
photocatalytic activity have not been found yet. The hetero-
dipole moments of the BiO pyramids are practically can-
6
celed out, but those of the IO pyramids are not, thereby
3
leading to a pyroelectric polarization along the c-axis direc-
[9]
[9]
layered bismuth oxide iodate, BiOIO , is pyroelectric, and
tion (Figure 1b). Under an electric field, electrons and
3
3
+
5+
[10]
its stereoactive lone-pairs on both Bi and I cations are
located at the BiO hexagonal and IO trigonal pyramidal
holes move in opposite directions. Bastard et al. reported
that the CB and VB electrons in GaAs quantum wells can
be spatially separated by applying an external electric field.
Thus, it is reasonable to suppose that photogenerated e-h
pairs of a pyroelectric compound can be effectively separat-
ed by the internal polar field and hence exhibit a good pho-
tocatalytic activity. In this Communication we verify this hy-
pothesis by studying the heterolayered pyroelectric com-
6
3
sites, respectively. The BiO pyramids edge-share to form
6
the Bi O layers (Figure 1a), the two surfaces of which are
2
4
corner-shared with IO pyramids to form BiOIO slabs (Fig-
3
3
ure 1b). The pyroelectricity of BiOIO does not arise from
3
[
a] W. Wang, Prof. Dr. B. Huang, Z. Wang, X. Qin, X. Zhang
State Key Laboratory of Crystal Materials
Shandong University, Jinan 250100 (P. R. China)
Fax : (+86)0531-88365969
pound BiOIO to find that it is an efficient photocatalyst
3
under ultraviolet (UV) light irradiation, far superior to that
of TiO (P25).
2
E-mail: bbhuang@sdu.edu.cn
We synthesize BiOIO nanoplates by using a simple hy-
3
[
b] X. Ma, Prof. Dr. Y. Dai
drothermal method. Bi ACHTUNGTRENNG(U NO ) ·5H O and KIO were used as
3 3 2 3
School of Physics
Shandong University, Jinan 250100 (P. R. China)
the sources of Bi and IO , which are environmentally friend-
3
ly and inexpensive. Figure 2 shows the typical XRD pattern
[
c] Prof. Dr. M.-H. Whangbo
Department of Chemistry
North Carolina State University
Raleigh, North Carolina, 27695-8204 (USA)
of the as-prepared BiOIO product. All the XRD peaks are
3
indexed to orthorhombic BiOIO (space group: Pca21; a=
3
[9]
5
.6584(4), b=11.0386(8), c=5.7476(4) ꢀ), which indicates
a high purity of the obtained product. The intense and sharp
diffraction peaks demonstrate that the product is well crys-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302884.
Chem. Eur. J. 2013, 19, 14777 – 14780
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
14777

lected 2-propanol, which has no absorption in the UV/Vis
region of our experiment, to further reveal the photocatalyt-
ic properties of the as-prepared BiOIO (see Figure S6 in
3
the Supporting Information).
To gain insight into a probable reason for this excellent
photocatalytic property of BiOIO , we examine its electron-
3
ic band structure determined from density functional calcu-
lations, which is presented in terms of the density of states
(
DOS) plots in Figure 4. The VB has strong O 2p contribu-
Figure 2. XRD patterns of the as-prepared BiOIO
after (b) the photocatalytic reaction.
3
sample before (a) and
tallized. The diffraction peak of the (121) plane is very
strong, which indicates the preferential growth along the
[121] direction. After recycling four consecutive runs of the
photodecomposition of methyl orange (MO), the crystal
structure of BiOIO is still well maintained, which indicates
3
a good stability of the product. The EDX spectrum, XPS
spectra, TEM images, and DRS spectra are shown in Figur-
es S1–S4 in the Supporting Information.
The photocatalytic activities of the products were evaluat-
ed by photodegradation of MO under UV light irradiation
at room temperature. The absorption peak at 464 nm de-
creases gradually as the irradiation time increases and com-
pletely disappears after 10 min, which indicates the destruc-
tion of the conjugated structure. Figure 3b shows the photo-
Figure 4. The total and projected DOS plots calculated for BiOIO
energies in the horizontal axes are given in eV. a) c: O 2p; c: I
p+Bi 6p; c: TDOS. b) c: O1 2p; c: O2 2p; c: O3+O4 2p;
3
. The
5
c) c: I 5s; c: I 5p. d) c: Bi 6s; c: Bi 6p.
tions, and the CB the Bi 6p and I 5p contributions (Fig-
ure 4a). There are three kinds of oxygen atoms in BiOIO3,
namely, the O1 atoms forming the OBi tetrahedra within
4
each Bi O layer, the O2 atoms bridging the BiO and IO
3
2
4
6
pyramids, and the O3/O4 atoms forming the terminal atoms
Figure 3. a) The temporal evolution of the absorption spectra of the MO
of IO pyramids (Figure 1). The VB top has O 2p contribu-
3
solution with as-prepared BiOIO
time. b) The comparison of the photocatalytic activities of different pho-
tocatalysts.
: Blank; *: dark; ~: P25; !: BiOIO
3
sample as a function of irradiation
tions from all these O atoms (Figure 4b), so that the photo-
excitations from the VB top to the CB generate holes at the
O 2p states in all O atoms, and electrons at the p-states of
&
3
.
3
+
5+
the Bi and I sites (Figure 4a,b).
catalytic activities of different catalysts. The degradation ef-
In principle, photogenerated carriers at BiO6 pyramids
ficiencies are defined as C/C , in which C and C represent
can migrate through the O2 atoms to adjacent IO pyramids,
0
0
3
the remnant and initial concentration of MO, respectively.
MO is quite stable and its self-photolysis is negligible. The
and vice versa. It is noted that the CB bottom has no O 2p
contribution from O2, which suggests that the electrons that
3
+
superior activity was obtained over the BiOIO sample, on
have arrived there will not easily migrate between the Bi
3
5
+
which more than 92% of MO was degraded within 12 min.
The nitrogen adsorption–desorption isotherms of the sample
and P25 are shown in Figure S5 in the Supporting Informa-
tion. The BET surface areas were calculated to be 12.253
and I sites. Consider an e-h pair generated at each IO3
pyramid. If the hole goes to the terminal O sites of IO , it
3
5
+
gets trapped there (Figure 5). The electron at I can move
out from the IO pyramid through the bridging O2 atom to
3
2
À1
3+
and 60.017 m g , respectively. Thus the surface area is not
the main factor that affects the photocatalytic properties of
the sample. In addition to the decoloration of MO, we se-
the neighboring Bi sites if it gets into the upper part of
the CB in which the O2 contribution is present. This e-h
separation at each IO pyramid will be enhanced by the in-
3
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Chem. Eur. J. 2013, 19, 14777 – 14780

Separation of Photogenerated Electron-Hole Pairs
COMMUNICATION
rating photogenerated electron-hole pairs at the BiO and
6
IO pyramids.
3
Experimental Section
Preparation: All the chemicals used in the experiment were analytical
grade reagents and were used without further purification. In a typical
procedure, bismuth nitrate pentahydrate (0.485 g, 1 mmol) was added to
deionized water (80 mL) under constant stirring, and then potassium
iodate (0.214 g, 1 mmol) was dissolved in the above suspension. After
stirring for 10 min, the mixture was transferred into a 100 mL Teflon-
lined stainless autoclave. The autoclave was sealed and heated at 1508C
for 5 h. After naturally cooling down to room temperature, the product
was obtained by filtering and washing with deionized water and ethanol
four times and then dried at 608C for 12 h.
Figure 5. A schematic diagram showing how the photogenerated e-h
pairs generated at BiO and IO pyramids are separated under the action
6 3
of the internal polar field and how the separated electrons and holes are
consumed. The black and blue arrows indicate the directions of the hole
and electron movements.
Characterization: The crystal structure and phase purity of the products
were characterized by X-ray powder diffraction recorded on a Bruker
AXS D8 diffractometer by using CuKa radiation. The energy-dispersive
X-ray spectrum was examined by an energy dispersive X-ray spectrome-
ter equipped with scanning electron microscopy (Hitachi S-4800). The
morphology and microstructure of the products were observed by a trans-
mission electron microscopy (JEOL JEM-2100F). X-ray photoelectron
spectroscopy (XPS) measurements were carried out on a Thermo Fisher
Scientific Escalab 250 spectrometer with monochromatized AlKa excita-
tion, and C 1s (284.6 eV) was used to calibrate the peak positions of the
elements. The UV/Vis diffuse reflectance spectrum of the product was
determined on a Shimadzu UV 2550 spectrophotometer equipped with
an integrating sphere. The Brunauer–Emmett–Teller (BET) surface area
and the pore-size distribution of the products were identified by a Micro-
meritics ASAP 2020 apparatus.
ternal polar field. In a similar manner, the hole generated at
each BiO pyramid can move through the bridging O2 to
6
the IO pyramid and eventually to the terminal O atoms of
3
IO . The electrons generated at each BiO pyramid and at
3
6
each IO pyramid might end up in the Bi 6p CB bottom
3
(
Figure 5). In this picture, the photo-oxidation would occur
on the surface IO3 layer, and the photoreduction at one
edge of the nanoplate in which the Bi O layer is exposed.
2
4
The photogenerated holes on the terminal O 2p VB top
on the surface of the BiOIO nanoplate can directly oxidize
3
MO to the final products (see Figure S7 in the Supporting
Information). Because of the strong oxidizing ability of the
holes (4.08 eV), MO could be decomposed fast and effi-
ciently. The electrons that move to the Bi atoms are finally
Photocatalytic evaluation: The photocatalytic activities of the products
were evaluated by the photodecomposing MO at room temperature. A
3
00 W Xe arc lamp (PLS-SXE300) was used as the light source. In a typi-
cal experiment, 0.1 g photocatalyst was dispersed into 100 mL of
À1
captured by O dissolved in water. It should be pointed out
20 mgL MO solution. Before the irradiation process, the suspension
2
was stirred for 1 h in the dark to reach the adsorption/desorption equilib-
rium. At given time intervals, 3 mL of the suspension was continually col-
lected from the beaker. The solution and the photocatalyst were separat-
ed by centrifuging and the concentration of the remaining MO was deter-
mined at 464 nm by a UV/Vis spectrophotometer (Xinmao UV-7502PC).
that BiFeO is ferroelectric and has a three-dimensional per-
3
ovskite structure. BiFeO3 was found to be photocatalytic
[11]
under both UV and visible light,
but is not an efficient
photocatalyst (e.g., ~6 h of UV/Vis irradiation is needed to
decompose 90% of MO). The superior photocatalytic activi-
ty of BiOIO relative to that of BiFeO indicates the impor-
Density functional calculations: Our DFT calculations employed the pro-
jector augmented wave method coded in the Vienna ab initio simulation
3
3
[
12]
package, and the generalized gradient approximation of Perdew, Burke
tance of its heterolayered structure in separating e-h pairs.
In addition, we investigated the photocatalytic properties of
[
13]
and Ernzerhof for the exchange and correlation corrections with the
plane wave cutoff energies of 400 eV, a set of 8ꢂ4ꢂ8 k-points and the
À4
nonpolar compounds BiOX (X=Cl, Br, I) and Bi O CO
2
2
3
threshold of self-consistent-field energy convergence of 10 eV.
(
see Figure S8 in the Supporting Information) to find them
to show much less photocatalytic activity than the as-pre-
pared BiOIO . BiOX and Bi O CO are constructed by
3
2
2
3
2
+
[
Bi O ]
layers interleaved by slabs comprising anions,
2
2
Acknowledgements
which is similar to BiOIO . However, BiOIO is polar with
a dipole moment of 63.32 D of the IO pyramid. The band
gaps of Bi O CO and BiOCl are similar to that of BiOIO ,
but those of BiOBr and BiOI are smaller. This further sup-
ports our conclusion that the internal polar field enhances
the photocatalytic properties.
3
3
[9]
3
This work was financially supported by the National Basic Research Pro-
gram of China (973 program, No. 2013CB632401) and the National Natu-
ral Science Foundation of China (Nos. 21333006, 11374190, 51002091
and 21007031 ).
2
2
3
3
In summary, we have found a simple hydrothermal
method to prepare pyroelectric BiOIO3 nanoplates and
shown that they possess a superior photocatalytic activity
under UV irradiation. This enhanced photocatalytic proper-
Keywords: BiOIO3
heterolayered structure · photocatalysts · polar field ·
semiconducters
·
electron-hole
separation
·
ty of BiOIO nanoplates is attributed to two factors; one is
3
the heterolayered structure and the other is the internal
polar field. Both features are necessary in effectively sepa-
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Received: July 23, 2013
Published online: October 2, 2013
14780
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ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 14777 – 14780