Fabrication of highly crystalline SnNb2O6 shell with a visible-light response on a NaNbO3 nanowire core

Article abstract of DOI:10.1021/ic4002175

A visible-light-absorbing SnNb₂O₆ shell with high crystallinity was successfully fabricated on a NaNbO₃ nanowire through a molten salt treatment of the NaNbO₃ nanowire of the starting material with SnCl₂, whereas the fabrication was not successful on the TT phase of a niobia nanowire. The difference will come from the formation processes of SnNb₂O₆ crystals (ion-exchange reaction vs thermally induced crystallization reaction). The core/shell nanowire obtained from NaNbO₃ showed photocatalytic activity comparable to that of H₂ evolution in the presence of an electron donor under visible-light irradiation (λ > 420 nm), compared with the corresponding bulky counterpart.

Full text of DOI:10.1021/ic4002175

Communication
pubs.acs.org/IC
Fabrication of Highly Crystalline SnNb O Shell with a Visible-Light
2
6
Response on a NaNbO Nanowire Core
3
,†,‡,§
†,⊥
Kenji Saito*
and Akihiko Kudo
†Department of Applied Chemistry, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjyuku-Ku, Tokyo 162-8601,
Japan
‡
PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan
⊥
Division of Photocatalyst for Energy and Environment, Research Institute of Science and Technology, Tokyo University of Science,
1
-3 Kagurazaka, Shinjuku-Ku, Tokyo 162-8601, Japan
*
S Supporting Information
11
Nb O nanowire (TT-Nb O -NW) as a reference. These
2
5
2
5
ABSTRACT: A visible-light-absorbing SnNb O6 shell
with high crystallinity was successfully fabricated on a
2
treated nanowires were employed as photocatalysts for H and
2
O evolution reactions in the presence of sacrificial reagents
2
NaNbO nanowire through a molten salt treatment of the
3
under visible-light irradiation (λ > 420 nm).
NaNbO nanowire of the starting material with SnCl ,
3
2
Figure 1 shows XRD patterns of powders obtained after the
molten SnCl treatment of TT-Nb O -NW and NaNbO -NW.
whereas the fabrication was not successful on the TT
phase of a niobia nanowire. The difference will come from
the formation processes of SnNb O crystals (ion-
2
2
5
3
2
6
exchange reaction vs thermally induced crystallization
reaction). The core/shell nanowire obtained from
NaNbO showed photocatalytic activity comparable to
3
that of H evolution in the presence of an electron donor
2
under visible-light irradiation (λ > 420 nm), compared
with the corresponding bulky counterpart.
xtensive efforts have been made to devise a strategy leading
to structural changes in functional semiconductor materials
E
from 3D networks to lower dimensional forms such as
nanoparticle, nanowire, and nanoplate. It is important to apply
these nanomaterials as potential building blocks in a variety of
1
−4
fields.
SnNb O possesses a foordite structure in which two
2 6
corner-sharing NbO sheets linked together at their edges and a
6
distorted SnO sheet due to the existence of a lone-pair electron
8
Figure 1. XRD patterns of (a) SnNb O /TT-Nb O -NW, (b)
5
II
2
6
2
5
are alternating. This material absorbs visible light because Sn
SnNb O /NaNbO -NW, (c) SnNb O -B, (d) TT-Nb O (PDF 28-
2
6
3
2
6
2
5
2
with a 5s electronic configuration contributes to the formation
0
317), and (e) NaNbO (PDF 33-1270). Miller indices due to TT-
3
of a hybrid orbital with O 2p as a valence band. So far, SnNb O6
Nb O and NaNbO are highlighted in blue and red, respectively, for
clarity.
2
2 5 3
has been reported to function as a H - and O -evolving
2
2
photocatalyst in the presence of sacrificial reagents and
6
,7
appropriate cocatalysts under visible-light irradiation. The
photocatalytic performance was enhanced by structural trans-
formation from bulk to nanoplate, which was prepared by a
molten SnCl treatment of layered perovskite Sr Nb O as a
The XRD patterns indicated that obtained powders were a
mixture of monoclinic SnNb O [lattice constants are a =
2
6
1
7.063(3) Å, b = 4.8838(7) Å, and c = 5.5775(11) Å] with
2
2
2
7
8
pseudohexagonal TT-Nb O [lattice constants are c = 3.951(13)
starting material. Further structural changes of this material are
of interest. However, a nanowire or nanoparticle structure of
SnNb O has yet to be reported. The molten salt treatment has
2
5
Å and a = 3.6158(9) Å] or monoclinic SnNb O [lattice
2
6
constants are a = 17.065(3) Å, b = 4.8755(10) Å, and c =
.5781(12) Å] with orthorhombic NaNbO [lattice constants
2
6
5
been found to be effective in converting the surface of a NaNbO3
3
are a = 5.5588(3) Å, b = 15.608(3) Å, and c = 5.4853(3) Å].
NaNbO -NW was converted to SnNb O more efficiently than
nanowire (NaNbO -NW), which was obtained by the thermal
3
reaction of a mixture of (NH ) [NbO(C O ) ]·H O, trioctyl-
3
2
6
4
3
2
4 3
2
9
TT-Nb O -NW. (600)/(002) of SnNb O /TT-Nb O -NW was
amine, and NaOH to visible-light-absorbing AgNbO . In this
2 5 2 6 2 5
3
stronger than that of SnNb O -B. The crystal structure of
context, fabrication of a SnNb O shell on the NaNbO nanowire
2
6
2
6
3
core has merited attention.
In the present study, the molten SnCl treatment of a NaNbO
nanowire was examined, in addition to using the TT phase of a
Received: November 7, 2012
Published: April 30, 2013
2
3
1
0
©
2013 American Chemical Society
5621
dx.doi.org/10.1021/ic4002175 | Inorg. Chem. 2013, 52, 5621−5623

Inorganic Chemistry
Communication
SnNb O with inputting crystal plane (Figure S1) suggests that
NW (Table S1 in the SI). Because the thickness of the SnNb O
2
6
2
6
(
600) has possibly causes the preferred orientation if anisotropic
layer was ca. 20 nm, the broadening of the (−111) diffraction
grain growth occurs along with bc plane. Half-widths of (−111)
reflections of SnNb O were determined in order to roughly
peak compared with that of SnNb O -B will be due to the small
2
6
crystallite size. Using the Scherrer equation, the crystallite size of
2
6
estimate their crystallinities. The half-widths were in the order
SnNb O in SnNb O /NaNbO -NW was determined as 17.7 ±
2
6
2
6
3
SnNb O -B (0.20°) < SnNb O /TT-Nb O -NW (0.26°) =
0.6 nm. This was consistent with the TEM result (Figure 2b).
SnNb O /TT-Nb O -NW also had a core/shell structure
2
6
2
6
2
5
SnNb O /NaNbO -NW (0.27°). To confirm which of the peak
2
6
3
2
6
2
5
broadenings of SnNb O /TT-Nb O -NW and SnNb O /
judging from the contrast (Figure 2c). However, lattice fringes
were not observed in this case, suggesting less crystallinity of the
SnNb O shell than that of SnNb O /NaNbO -NW. TEM-EDX
2
6
2
5
2
6
NaNbO -NW compared with that of SnNb O -B was due to
3
2
6
poor crystallinity or small crystallite size, microscopic analyses by
SEM and high-resolution TEM (HR-TEM) were carried out.
SEM images (Figure S2) indicated that SnNb O /TT-Nb O -
2
6
2
6
3
results suggested that only 8.1% of SnNb O existed in the
2
6
2
6
2
5
SnNb O /TT-Nb O -NW sample. An excess amount of Nb was
2
6
2
5
11
NW obtained from TT-Nb O -NW possessed a structure in
detected for SnNb O /TT-Nb O -NW by XPS compared with
2
5
2 6 2 5
which grains with ca. 100 nm size were connected with each
other, forming a continuous network. Short nanowires and
particles coexisted near the surface of the network. SnNb O /
SnNb O -B. The results would originate from one of the
2 6
following possibilities: (1) this sample was a mixture of
2
6
SnNb O /TT-Nb O -NW with unreacted TT-Nb O -NW; (2)
2
6
2
5
2
5
TT-Nb O -NWs prepared by using 5% excess SnCl under heat
the unreacted TT-Nb O region was partially exposed on the
2
5
2
2 5
treatment at 623 K for 30 h and a 2-fold excess of SnCl under
SnNb O /TT-Nb O -NW surface. These results precluded a
2 6 2 5
2
heat treatment at 673 K for 12 h possessed nanoparticles and ca.
clear estimation of the reason for XRD peak broadening. The
SnNb O crystallite size in SnNb O /TT-Nb O -NW was 41.1
1
00-nm-thick plate morphologies, respectively. Thus, the
2
6
2
6
2
5
continuous network seen in SnNb O /TT-Nb O -NW was
± 2.7 nm using the Scherrer equation and thereby larger than
2
6
2
5
created by severely limited conditions. On the other hand,
that in SnNb O /NaNbO -NW. The differences in the
2
6
3
1
0
nanowire morphology of NaNbO3 was maintained for
SnNb O /NaNbO -NW at this SEM resolution. TEM of
formation and crystallization of the SnNb O shells NaNbO -
2 6 3
2
6
3
and TT-Nb O -NW used herein will be due to the reaction
2
5
SnNb O /NaNbO -NW showed evidence of a core/shell
processes. When TT-Nb O -NW is used, thermally induced
2
6
3
2 5
structure that appeared as areas of contrast density (Figure
S2). Clear lattice fringes on the core and the widths of the fringes
were estimated as 0.82 ± 0.01 nm, the value of which agreed well
with (200) due to SnNb O (Figure 2a and 2b). STEM-EDX
crystallization of SnNb O like a solid-state reaction on the
2 6
nanowires takes place. On the other hand, NaNbO can function
3
+
as a reaction substrate for an ion-exchange reaction of Na with
2+
2
6
Sn . The latter process will be favorable to maintaining the
nanowire structure. BET surface areas of SnNb O /TT-Nb O -
2
6
2
5
2
−1
NW and SnNb O /NaNbO -NW were 17 and 14 m g ,
2
6
3
respectively. These values were more than 10 times larger than
2
−1
the SnNb O bulk (0.9 m g ) but lower than the pristine
2
6
2
−1
nanowires (63 and 38 m g for TT-Nb O -NW and NaNbO -
2
5
3
NW, respectively). As established by the microscopic and
spectroscopic results, highly crystalline SnNb O was homoge-
2
6
neously fabricated on NaNbO -NW. Such an increase in the
3
crystallinity generally lowers the surface area. In contrast, the
molten SnCl treatment lowered the structural homogeneity of
2
TT-Nb O -NW, affording a mixture of grains as a major species
2
5
with short nanowires and particles. This would be the major
reason for the low surface area of SnNb O /TT-Nb O -NW.
2
6
2
5
DRS spectra of SnNb O /TT-Nb O -NW and SnNb O /
2
6
2
5
2
6
NaNbO -NW are shown in Figure S2 in the SI, in addition to
3
nontreated TT-Nb O -NW, NaNbO -NW, and SnNb O -B as
2
5
3
2
6
the references. Onset wavelengths in absorptions were red-
shifted significantly by the treatment of TT-Nb O -NW and
2
5
NaNbO -NW with molten SnCl . The spectral shapes indicated
3
2
that SnNb O /TT-Nb O -NW contained a small amount of the
2
6
2
5
Figure 2. HR-TEM images of (a) SnNb O /NaNbO -NW, (b) the
2
6
3
SnNb O component, in good agreement with the XRD and
2
6
magnified view of part a, and (c) SnNb O /TT-Nb O -NW.
2
6
2
5
TEM-EDX results. SnNb O /NaNbO -NW contained a larger
2
6
3
amount of SnNb O than SnNb O /TT-Nb O -NW. The red
2
6
2
6
2
5
elemental mapping images further supported this result (Figure
S3). It is difficult to distinguish the thin shell from the core
because of the small sample volume of the shell. However, Nb
and Na components were clearly overlapped with the blurred Sn
component, suggesting a NaNbO /SnNb O core/shell struc-
shift in the absorptions was attributable to the contribution of the
Sn 5s orbital to the formation of a hybrid orbital with O 2p as the
2
valence band at a more negative potential than the O 2p orbital.
The band gap estimated by absorption edges was 2.3 eV, identical
with that of SnNb O -B.
3
2
6
2
6
ture. Consistent with the TEM results, XPS indicated that
SnNb O existed at the surface of the nanowire because the
Photocatalytic reactions for H and O evolution (Table 1)
2 2
were carried out in the presence of methanol or AgNO , each of
2
6
3
atomic ratio of Sn/Nb was almost the same between SnNb O /
which acts as an electron donor or an acceptor, under visible-light
2
6
NaNbO -NW and SnNb O -B (Table S1 in the SI). The
irradiation (λ > 420 nm). Both nanowires showed almost no
3
2
6
composition of a single nanowire was further analyzed by TEM-
EDX. NaNbO existed at a rate of 30.9% in SnNb O /NaNbO -
photoresponse regarding an O evolution reaction, although
2
highly crystalline SnNb O was formed on the NaNbO
3
2
6
3
2
6
3
5
622
dx.doi.org/10.1021/ic4002175 | Inorg. Chem. 2013, 52, 5621−5623

Inorganic Chemistry
Communication
Table 1. H and O Evolution Reactions over SnNb O -Based
induced crystallization versus ion exchange. SnNb O /NaNbO -
2
6
3
2
2
2
6
Photocatalysts in the Presence of Methanol and AgNO under
NW showed an activity for the H evolution reaction comparable
2
3
a
Visible-Light Irradiation
with that of the corresponding bulk in the presence of an electron
donor under visible-light irradiation.
rate of gas
evolution/
μmol h−1
ASSOCIATED CONTENT
Supporting Information
Experimental section, crystal structure of Sn Nb O with
■
b
c
d
,e
*
S
photocatalyst
BG/eV
Pt cocatalyst/wt % H2
O2
2
2
6
SnNb O /TT-Nb O -NW
2.3
2.3
2.3
3.3
3.3
2.3
2.3
2.3
2.3
2.3
3.4
3.4
2.3
2.3
2.3
2.3
0
0
0
2
6
2
5
inputting (600), SEM images, STEM-EDX mapping results,
summary of elemental analyses, and DRS. This material is
available free of charge via the Internet at http://pubs.acs.org.
SnNb O /TT-Nb O -NW
0.3
1.0
1.0
0
0.7
1.0
0
2
6
2
5
SnNb O /TT-Nb O -NW
2
6
2
5
TT-Nb O -NW
2
5
TT-Nb O -NW
0
t
2
5
AUTHOR INFORMATION
Corresponding Author
E-mail: ksaito@eng.niigata-u.ac.jp.
■
SnNb O /NaNbO -NW
0
2
6
3
SnNb O /NaNbO -NW
0.3
0.5
1.0
1.5
1.0
0
5.5
5.6
10
2
6
3
*
SnNb O /NaNbO -NW
2
6
3
SnNb O /NaNbO -NW
Present Address
2
6
3
§
SnNb O /NaNbO -NW
5.4
1.1
K.S.: Office for Development of Young Researchers, Research
2
6
3
NaNbO -NW
Planning and Promotion Division, Niigata University, 8050
Ikarashi 2-no-cho, Nishi-ku, Niigata 950-2181, Japan.
Author Contributions
The manuscript was written through the contributions of all
authors. All authors have given approval to the final version of the
manuscript.
3
NaNbO -NW
0
3
SnNb O -B
0
0
14
12
0
0.9
2
6
SnNb O -B
0.3
0.5
1.0
2
6
SnNb O -B
−
−
2
6
SnNb O -B
2
6
a
Catalyst, 0.1 g; reactant solution, 150 mL; light source, 300 W xenon-
Notes
b
arc lamp (λ > 420 nm); cell, top-irradiation Pyrex cell. BG represents
the band gap. 10 vol % of an aqueous methanol solution. 0.02 mol
L
The authors declare no competing financial interest.
c
d
−1
e
of an aqueous silver nitrate solution. Initial rate of O evolution.
2
ACKNOWLEDGMENTS
■
This work was financially supported by the PRESTO
(Precursory Research for Embryonic Science and Technology)
project.
nanowire surface. However, this is in agreement with the
6,7
previous reports using SnNb O -B. SnNb O /TT-Nb O -NW
2
6
2
6
2
5
showed low activity for the H evolution reaction. In contrast, a
2
SnNb O /NaNbO composite nanowire showed activity com-
2
6
3
REFERENCES
■
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(
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(
(
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band gap and of NaNbO -NW with a 3.4 eV band gap will lie at
3
−
0.3 and −0.4 eV vs NHE, respectively, because the valence
bands of these nanowires consist of O 2p orbitals, of which the
potential is ca. 3 eV vs NHE. Taking band gaps of SrNb O (3.85
2
6
1
2
eV) and SnNb O (2.3 eV) into account, the conduction and
2
6
valence band levels of SnNb O will lie at around −0.85 and 1.55
2
6
eV, respectively. Hence, photogenerated electron transfer from
the SnNb O shell to the TT-Nb O - or NaNbO -NW core is
(
2
6
2
5
3
8
exothermic. However, because the edge and sidewall of the TT-
Nb O - or NaNbO -NW core will be covered with SnNb O , the
6
86.
2
5
3
2
6
(
(
(
10) Saito, K.; Kudo, A. Inorg. Chem. 2010, 49, 2017.
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conduction band electron in the TT-Nb O or NaNbO core is
2
5
3
considered to make no contribution to the water reduction
reaction. Thus far, efforts devoted to enhance the photocatalytic
performance of the bulk under visible light by semiconductor
13,14
nanowires have been hampered.
Herein, this was overcome
by transcription of the SnNb O crystal onto NaNbO -NW while
(14) Yu, J.; Kudo, A. Adv. Funct. Mater. 2006, 16, 2163.
2
6
3
+
maintaining high crystallinity, through ion exchange of Na with
2+
Sn ions. This work will offer a new strategy to developing a
visible-light-responsive semiconductor nanowire possessing a
superior photocatalytic performance.
In conclusion, a molten salt treatment of simple and complex
niobate (TT-Nb O and NaNbO ) nanowires was examined in
2
5
3
order to fabricate visible-light-responsive SnNb O layers on the
2
6
niobate nanowires. Structural characterizations using XRD, HR-
TEM, and XPS indicated that the SnNb O shell on the NaNbO
3
2
6
nanowire core possessed high crystallinity, in contrast to that on
TT-Nb O -NW. The nanowire-dependent crystallization of
2
5
SnNb O originated from different reaction processes: thermally
2
6
5
623
dx.doi.org/10.1021/ic4002175 | Inorg. Chem. 2013, 52, 5621−5623