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Angewandte
Chemie
Deutsche Ausgabe:
DOI: 10.1002/ange.201708645
Internationale Ausgabe: DOI: 10.1002/anie.201708645
Water Oxidation Hot Paper
Enhanced Photoactivity from Single-Crystalline SrTaO2N Nanoplates
Synthesized by Topotactic Nitridation
Jie Fu and Sara E. Skrabalak*
Abstract: There are few methods yielding oxynitride crystals
with defined shape, yet shape-controlled crystals often give
enhanced photoactivity. Herein, single-crystalline SrTaO2N
nanoplates and polyhedra are achieved selectively. Central to
these synthetic advances is the crystallization pathways used, in
which single-crystalline SrTaO2N nanoplates form by topotactic nitridation of aerosol-prepared Sr2Ta2O7 nanoplates
and SrTaO2N polyhedra form by flux-assisted nitridation of
the nanoplates. Evaluation of these materials for the oxygen
evolution reaction (OER) and hydrogen evolution reaction
(HER) showed improved performance for the SrTaO2N
nanoplates, with a record apparent quantum efficiency
(AQE) of 6.1 % for OER compared to the polyhedra (AQE:
1.6 %) and SrTaO2N polycrystals (AQE: 0.6 %). The enhanced
performance from the nanoplates arises from their morphology and lower defect density. These results highlight the
importance of developing new synthetic routes to high quality
oxynitrides.
Photocatalytic water splitting offers a solution to clean and
renewable energy technologies of which the critical challenges lie in identification and synthesis of suitable photocatalysts.[1] Metal oxynitrides are attractive photocatalysts
because they absorb visible light and have appropriate band
positions for water splitting.[2] They are normally prepared by
nitridation, in which ammonia is continuously passed though
the precursor at high temperature (900–1100 8C) and for
many hours (10 or more). Oxynitride polycrystals with illdefined structures are typically produced. Synthesis of oxynitride materials with better crystallinity, defined nanostructural features, and higher surface areas is critical to realizing
the potential of these materials and accessing high photoactivity.[3] However, the synthesis of oxynitride samples with
desired features is difficult because of the harsh nitridation
conditions typically applied, prohibiting use of crystal growth
modifiers. Oxide precursors with controlled features are
easier to achieve than oxynitrides, but both the mismatch in
unit-cell parameters and cation stoichiometry between the
oxide precursor(s) and oxynitride product restrict crystallinity
and morphology control during nitridation.[4] Herein, we
report the use of aerosol-assisted molten salt synthesis
[*] J. Fu, Prof. S. E. Skrabalak
Department of Chemistry
Indiana University, Bloomington
800 E. Kirkwood Ave., Bloomington, IN, 47405 (USA)
E-mail: sskrabal@indiana.edu
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201708645.
Angew. Chem. 2017, 129, 14357 –14361
(AMSS) to prepare single-crystalline oxide nanoplates for
topotactic nitridation to oxynitride nanocrystals with shape
preservation and high photoactivity.
A template approach to shape-defined oxynitride single
crystals was recently demonstrated by Domen and co-workers, where single-crystalline La2Ti2O7 plates was prepared and
nitridated to LaTiO2N with shape preservation.[3b] With CoOx
co-catalyst deposited, the LaTiO2N plates produced twice as
much O2 when illuminated compared to polycrystalline
LaTiO2N without shape control. This enhancement was
attributed to the single-crystallinity of the plates and expression of more active sites. The formation of LaTiO2N plates is
interesting given its perovskite structure. While not discussed,
the conservation of the plate morphology during nitridation
likely occurred through a topotactic transformation enabled
by the related lattice orientations of La2Ti2O7 and LaTiO2N.
A potentially useful outcome of topotactic chemistry is that
the crystal faceting and crystallinity of the product are
determined by the precursor phase. Moreover, this chemistry
can yield single crystals of non-equilibrium shape which are
potentially more active for catalysis.[5]
Motivated by the idea that oxynitride single crystals with
controlled morphology could be synthesized through topotactic transition, we identified SrTaO2N, a promising material for solar water splitting, as an exciting synthetic target.
The [010] direction of orthorhombic Sr2Ta2O7 and the [100]
direction of tetragonal SrTaO2N are crystallographically
related and suitable for topotactic conversion. Here, singlecrystalline Sr2Ta2O7 nanoplates were prepared by AMSS,
followed by nitridation to SrTaO2N with shape and singlecrystallinity preserved. Sr2Ta2O7 belongs to orthorhombic
crystal system with a space group of Cmcm (left panels of
Figure 1) and has a layered perovskite structure. It consists of
perovskite-type slabs of corner-shared TaO6 octahedra and
two Sr sites, one within the slabs and the other between the
slabs. SrTaO2N has a tetragonally distorted perovskite
structure with a space group of I4/mcm, comprising of
corner-linked TaO6 octahedra and Sr in between (right
panels of Figure 1). The similar crystal orientation along the
[010] direction of Sr2Ta2O7 and the [100] direction of SrTaO2N
should allow for topotatctic transition in which part of the
oxygen sites are replaced with nitrogen and each 4-layered
perovskite slab of Sr2Ta2O7 contracts to be corner-shared,
forming the structure of SrTaO2N. However, large crystals of
Sr2Ta2O7 can disrupt the conferral of single-crystallinity
because of built-up strain (crystal parameters relationships
during transition: anitride = 0.21 boxide, bnitride = coxide, cnitride =
2 aoxide), as demonstrated by Teshima and co-workers.[6] Our
work reported herein highlights the value of nanoscale
templates for topotactic chemistry.
T 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 1. A crystal structure model for the topotactic conversion of
Sr2Ta2O7 (left panels) to Sr2TaO2N (right panels). Crystal structures
were visualized using the VESTA suite of programs.[7] A) Side views of
[010] direction of Sr2Ta2O7 and [100] direction of SrTaO2N. B) Views of
the Sr2Ta2O7 (010) facet and SrTaO2N (100) facet. Pink spheres Sr, gray
spheres Ta atoms, blue polyhedra represent the TaO6 or TaO4N2
octahedral units. Unit cells are indicated by the black boxes.
Previously, we showed that nanocrystals with unconventional shapes can form by AMSS, which couples molten salt
chemistry with the spatial and temporal confinement of
aerosol droplets.[3a, 5a, 8] This method was used to prepare the
nanoscale Sr2Ta2O7 for topotactic nitridation. Specifically, an
aqueous solution of TaClx(OCH3)5@x and SrCl2 in a mole ratio
of 1:12 was ultrasonically nebulized into micron-sized droplets, which were then transported into a furnace (800 8C)
where product formation occurred (Figure S1 in the Supporting Information). SrCl2 was in excess as it served as both the
Sr-source and major flux component for crystal growth of
Sr2Ta2O7 (see Supporting Information for full discussion of
SrCl2 as a flux). As shown in Figure 2, powder composed of
single-crystalline nanoplates was produced. X-ray diffraction
confirmed a single phase of Sr2Ta2O7 (Figure 2 B). The
relative intensities of 0k0 diffraction peaks compared to
other reflections were higher for our measured sample than
those from the reference ICDD PDF, indicating that Sr2Ta2O7
nanoplates have a preferred orientation of [010]. This assignment is supported by electron diffraction of an individual
plate (which also confirms their single-crystallinity) and the
coherence and d-spacing of the lattice fringes in the high
resolution TEM (HRTEM) image. The morphology of the
product is mainly determined by its crystal structure in
solution-phase crystallization processes without any organic
additives or templates. As mentioned earlier, the structure of
Sr2Ta2O7 consists of covalently bonded TaO6 octahedral layers
(a- and c-axis), which are attracted to each other through
weak van der Waals force along the b-axis. The higher free
energy of the covalent bonds compared to the van der Waals
interactions lead to faster growth rates along the c- and a-
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Chemie
Figure 2. A) SEM image and B) XRD pattern (ICDD #01-072-0921) of
AMSS-derived Sr2Ta2O7 nanoplates, C) TEM image (inset: electron
diffraction pattern), and D) HRTEM image of a portion of the nanoplate marked in (C).
axes, which account for [010] facets being expressed. The
thickness of the plates was measured to be 30 : 1 nm and the
dimension of the basal plane was (300 : 53 nm) X (198 :
45 nm) from SEM images. The nanoscale dimension along
the [010] direction should help transfer the single-crystallinity
of the Sr2Ta2O7 nanoplates to SrTaO2N, along with shape
preservation.
Indeed, single-crystalline SrTaO2N nanoplates were achieved by direct nitridation of the Sr2Ta2O7 nanoplates with
ammonia at 950 8C for 15 hours, as examined with SEM,
TEM, and XRD (Figure 3). SrTaO2N was converted directly
from Sr2Ta2O7 by replacing two oxygen planes with one
nitrogen plane and shrinking along the [010] direction
Figure 3. A) SEM image and B) XRD pattern (ICDD #01-079-1311) of
SrTaO2N nanoplates, C) TEM image (inset: electron diffraction pattern
of nanoplate outlined by dashed line) and D) HRTEM image nanoplate
portion indicated by a black box in (C).
T 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. 2017, 129, 14357 –14361
Zuschriften
(Figure 1). The reduced unit-cell volume accounts for the
surface roughness, porosity, and irregular plate-like shapes of
the SrTaO2N particles, which are visible in the SEM and TEM
images. The thickness of the SrTaO2N nanoplates from SEM
images is 29.5 : 0.9 nm, which is slightly thinner than that of
Sr2Ta2O7. Sometimes, two plates were found to sinter edge to
edge, but extensive compaction was not observed (Figure 3 C). Electron diffraction taken from a single nanoplate
confirmed their single-crystalline nature, with (100) facets
exposed. This finding is also supported by the continuity and
d-spacing in the HRTEM image. The topotactic nitridation
process is represented in Scheme 1, in which an AMSSderived Sr2Ta2O7 nanoplate with (010) facet expression
directly transforms to a SrTaO2N nanoplate with (100) facet
expression.
Angewandte
Chemie
common on account of either incomplete or over nitridation;
such deviations could give differences in electronic structure.
Thermal gravimetric analysis (TGA) was employed to
determine the nitrogen content of the SrTaO2N samples, in
which they were oxidized under air by slow heating back to
Sr2Ta2O7 (Figure S5). The compositions of the SrTaO2N
nanoplates, polyhedra, and polycrystals were similar, estimated to be SrTaO1.87N1.09, SrTaO1.88N1.08, and SrTaO1.93N1.05,
respectively. Diffuse reflectance spectroscopy (DRS) of all
SrTaO2N samples were measured, and their direct band gaps
were calculated using Tauc plots (Figure 4 A and Table S1).
Scheme 1. Top: A Sr2Ta2O7 nanoplate topotactically transforms to
a SrTaO2N nanoplate. Bottom: The Sr2Ta2O7 nanoplate undergoes
dissolution then crystallization to polyhedral SrTaO2N in molten SrCl2.
The AMSS-derived Sr2Ta2O7 nanoplates were also nitridated with molten SrCl2 under the same conditions with the
anticipation that the flux may prevent any SrTaO2N nanoplates from sintering and minimize surface roughness. Interestingly, single-crystalline SrTaO2N polyhedra were generated instead of nanoplates (Figure S3). This morphology is
consistent with dissolution of the Sr2Ta2O7 nanoplates in the
molten media, followed by nucleation and growth of SrTaO2N
(Scheme 1). Unlike products produced by gas–solid nitridation of oxide precursor particles,[9] the SrTaO2N crystallized
from the flux do not show porosity. The polyhedral crystal
habit is consistent with the perovskite structure. The morphological contrast of the resulting SrTaO2N particles highlights the sensitivity of conditions for direct topotactic
nitridation and the importance of developing novel synthetic
strategies that provide control of crystal growth pathways. For
catalytic comparison, conventional SrTaO2N polycrystals
without shape control were prepared by nitridation of
a mixture of SrCO3 and Ta2O5 (Figure S4).[10]
High surface area is often associated with more active
sites, which can enhance the photoactivity.[11] The BET
surface areas of the SrTaO2N nanoplates, polyhedra, and
polycrystals were measured to be 11.7 m2 g@1, 5.4 m2 g@1, and
6.6 m2 g@1. The higher surface area of the SrTaO2N nanoplates
might stem from their 2D nanostructure and porosity. Oxynitrides with stoichiometries that deviate from the ideal are
Angew. Chem. 2017, 129, 14357 –14361
Figure 4. A) Diffuse reflectance spectroscopy (DRS) spectra of
SrTaO2N samples. SrTaO2N polycrystals (dashes), SrTaO2N polyhedra (dots), and SrTaO2N nanoplates (solid line). Amount of B) O2
and C) H2 evolved in the first hour of irradiation from SrTaO2N
samples. (Irradiation of 400 nm , l , 500 nm. For OER, 20 mg photocatalyst and La2O3 as a pH buffer were dispersed in 12 mL AgNO3
(0.05 m) electron scavenger solution. For HER, 20 mg photocatalyst
was dispersed in 12 mL aqueous solution containing 10 vol % methanol.)
T 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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As anticipated from their similar compositions, these samples
have similar band-gap energy (ca. 2.2 eV). However, the
background absorption beyond the absorption edge
(> 600 nm) is much more significant for the SrTaO2N polycrystals than nanoplates and polyhedra, indicative of defect
sites.[3e] This observation is expected as more defect sites
should be present in polycrystalline SrTaO2N compared with
single-crystalline samples.
Oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) from the SrTaO2N samples were
performed to investigate the roles of morphology and
crystallinity to photoactivity. Visible light irradiation with
wavelengths between 400 nm and 500 nm was selected based
on the band edge positions. Bare SrTaO2N nanoplates
generated the largest amount of oxygen and hydrogen
within the first hour of illumination, followed by SrTaO2N
polyhedra, and SrTaO2N polycrystals (Figure 4). CoOx or Pt is
normally loaded on oxynitride photocatalysts independently
to facilitate OER and HER, respectively. 2 wt % CoOx and
3 wt % Pt was deposited separately on each of the SrTaO2N
samples as this amount is reported as optimal for most
oxynitrides.[3b,e, 12] The same trend for the OER and HER was
observed (Figure 4 and Figure S6). SrTaO2N nanoplates
evolved the most product (4.47 mmol oxygen and 0.295 mmol
hydrogen) within the first hour of illumination, which are
roughly three and ten times of that generated from SrTaO2N
polyhedra (1.22 mmol oxygen and 0.109 hydrogen) and
SrTaO2N polycrystals (0.478 mmol oxygen and 0.014), respectively (Figure 4). The ratio of H2 to O2 deviates from ideal
(2:1), which might be due to photo-excited electron trapping
states that could perform Ag+ reduction but not HER.[3b]
AQE measurements for the SrTaO2N samples within the
first hour are summarized in Table S1. To our knowledge, an
AQE of 6.1 % (400 nm , l , 500 nm) for OER, achieved with
the SrTaO2N nanoplates, is among the highest recorded for
this material. A decreased oxygen evolution rate was
measured with increased illumination time for samples with
and without CoOx ; this trend is commonly observed in
powdered photocatalytic OER as a result of active site
blocking from gradual Ag deposition and a decreased Ag+
concentration (Figure S6).[3b] We note that powder XRD of
the SrTaO2N nanoplates after OER reveals Ag deposition but
also preservation of the oxynitride phase, indicating that the
material is relatively stable during use (Figure S7). Ag
deposition precludes tests of material reuse.
Loading of co-catalysts on the SrTaO2N samples did
promote the surface reaction of OER and HER compared to
the bare ones. The similarity in enhancements of OER (ca. 4fold) for all SrTaO2N samples implies that factors other than
surface reaction give rise to the high AQE of the SrTaO2N
nanoplates compared to other samples. Unlike OER, deposition of Pt boosted HER more dramatically for SrTaO2N
nanoplates than polyhedra and polycrystals. The band gaps of
the SrTaO2N samples are almost the same, so the quantity of
photons absorbed should not vary substantially from sample
to sample. Also, when the AQE is normalized by surface area,
the SrTaO2N nanoplates still display enhanced performance
(OER: 0.52 % m@2 vs. 0.30 % m@2 for the polyhedra and
0.090 % m@2 for the polycrystals, and HER: 0.017 % m@2 vs.
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Chemie
0.014 % for the polyhedra and 0.0029 % m@2 for the polycrystals). Thus, the higher surface area of the nanoplates
alone is insufficient to fully explain the greater performance.
Rather, the enhanced photoactivity of the single-crystalline
SrTaO2N nanoplate sample can be attributed to its lower
defect density compared to the SrTaO2N polycrystals and
shorter charge diffusion distance compared to both the
SrTaO2N polyhedra and polycrystals. Taken together, low
defect density and the 2D nanostructure of the nanoplates
greatly enhance SrTaO2N for the OER and HER, with a high
AQE of 6.1 % for OER under irradiation of 400–500 nm.
In summary, the synthesis of single-crystalline oxynitride
particles with controlled size and shape is a great challenge.
Yet, as we showed, control over these structural features
greatly enhances the performance of materials for photocatalytic water splitting. To achieve such structural control,
innovations in material synthesis are required. Herein, we
demonstrated that the preparation of single-crystalline
SrTaO2N nanoplates are possible by topotactic nitridation
of appropriate oxide precursor. This approach could be
a general way to other functional materials with defined yet
unconventional shapes when suitable crystallographic relationships are identified. This study should motivate exploration of new synthetic strategies toward oxynitrides that
integrate methods for controlled crystal growth.
Acknowledgements
Support comes from Indiana University, NSF DMR-1608711,
and the Camille Dreyfus Teacher-Scholar Program. Access to
the powder X-ray powder diffractometer was provided by
NSF CRIF CHE-1048613. We thank the Nanoscale Characterization Facility and Electron Microscopy Center of Indiana
University for access to SEM, TEM, and STEM-EDS.
Conflict of interest
The authors declare no conflict of interest.
Keywords: aerosol synthesis · nanoplates · photocatalysis ·
topochemistry · water oxidation
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Accepted manuscript online: September 18, 2017
Version of record online: October 4, 2017
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