Synthesis of single-crystalline, porous TaON microspheres toward visible-light photocatalytic conversion of CO2 into liquid hydrocarbon fuels

Article abstract of DOI:10.1039/c6ra19368d

Single-crystalline, porous TaON microspheres were prepared for photocatalysis via facile nitridation of uniform amorphous Ta₂O₅ sphere formed by hydrothermal treatment. The amorphous property facilitates shrinkage of Ta₂O₅ into the porous TaON nanostructure during the ammonification process. The porous spherical architecture of TaON plays a significant role in deciding the CO₂ photocatalytic conversion efficiency into ethanal and ethanol under visible light irradiation, relative to its counterpart from commercial Ta₂O₅, including availability of more reaction sites, easy trapping of incident illumination, and shortening of charge transfer distance from the interior to the outer surface to expedite charge separation. Loading Pt as an electron sinker over the porous TaON, boosting the separation of the photogenerated electron-hole pairs, not only improves the photoconversion efficiency, but also alters the product species.

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Synthesis of single-crystalline, porous TaON microspheres
toward visible-light photocatalytic conversion of CO2 into
liquid hydrocarbon fuels
Received 00th January 20xx,
Accepted 00th January 20xx
Qiutong Han,a, b, c, d Yong Zhou,*, a, b, c, d Lanqin Tang,a, b, c, d, e Ping Li,a, b, c, d Wenguang Tu,a, b, c, d
Liang Li,a, b, c, d Haijin Li, a, b, c, d and Zhigang Zou*,a, b,c
DOI: 10.1039/x0xx00000x
www.rsc.org/
Single-crystalline, porous TaON microspheres were prepared for photocatalysis via facile nitridation of
uniform amorphous Ta2O5 sphere formed by hydrothermal treatment. The amorphous property facilitates
shrinkage of Ta2O5 into the porous TaON nanostructure during the ammonification process. The porous
spherical architecture of TaON plays a significant role in deciding the CO2 photocatalytic conversion efficiency
into ethanal and ethanol under visible light irradiation, relative to its counterpart from commercial Ta2O5,
including availability of more reaction sites, easy trapping of incident illumination, and shortening of charge
transfer distance from interior to outer surface to expedite charge separation. Loading Pt as an electron
sinker over the porous TaON, boosting the separations of the photogenerated electron-hole pairs, not only
improves photoconversion efficiency, but also alters sorts of the product species.
Keywords: Photocatalysis, Photoconversion of CO2, Hydrocarbon Fuel, Porous Spherical Architecture
as (oxy)nitrides have attracted considerable attention due to their
proper band gap and the right position. Tantalum oxynitride (TaON)
is one of the most towardly oxynitrides that displays remarkable
Introduction
The development of photocatalysts for reduction of CO2 to
hydrocarbon fuels over an artificial photosynthesis is a very
important means to solve energy storage and global warming
problems.1-3 The artificial photosynthesis is based on the
advantages for the rational range of visible light absorption (λ ＞
420 nm). The valence band top of TaON consists of hybridization of
N 2p and O 2p that is clearly different from Ta2O5 which is made up
of total O 2p. According to the semiconductor energy band theory,
the nitrogen 2p orbital is more potential negative compared to the
oxygen 2p orbital and result in a red-shift to the visible light range.11
Photoreduction of H+ into H2 and photooxidation of H2O into O2
were early tested to assess the photocatalytic capabilities of TaON.
Fabrication of an efficient TaON photoanode with cocatalyst is the
5
photoexcitation of semiconductors such as TiO2,4 WO3 , CdS,6
Zn2GeO4,7 Fe2V4O13,8 and so on. A series of hydrocarbon products
(e.g., C1, C2, and C3 compounds) could be obtained via different
photocatalysts, which depends on the number of electrons and
protons (e−/H+) involved in multiple reactions.9 Most of currently
developed photocatalysts can only show the photochemical activity
in the ultraviolet (UV) light with energy accounts for less than 5% in
the total solar spectrum owing to the wide band gaps.10 Therefore,
how to narrow the band gap of semiconductor nanostructures and
enhance the utilization of visible light is of technical and commercial
first
demonstration
of
oxynitride
photoanode
for
photoelectrochemical water splitting into a considerable production
of O2.12 TaON-based photocatalysts for oxygen evolution in a two-
step water splitting system has also been successfully exhibited
particularly high photocatalytic activity.13-14 Furthermore, several
noble metals (Pt, Ag and Ru) were found to be potential H2-
evolution O2-evolution promoters.13,15-16 While arbitrary TaON
shows reasonable property in splitting water into H2 and O2, and
degrading various organic contaminations,17 morphology-controlled
nanostructures including hollow urchin-like sphere18 and hollow
sphere19 allow further enhancement of the photocatalytic
performance.
interests. In strict looking for
a
suitable catalyst for
maximum use of solar energy, certain nitrogen oxide materials such
a.Jiangsu Key Laboratory for Nano Technology, Nanjing University, Nanjing
210093, P. R. China. E-mail: zhouyong1999@nju.edu.cn
b.Key Laboratory of Modern Acoustics, MOE, Institute of Acoustics, Department of
Physics, Nanjing University, Nanjing 210093, P. R. China. E-mail:
zgzou@nju.edu.cn
c. National Laboratory of Solid State Microstructures, Department of Physics, and
Collaborative Innovation Center of Advanced Microstructures, Nanjing University,
Nanjing 210093, P.R. China
In this paper, single-crystalline, porous TaON microspheres were
successfully synthesized via high-temperature nitridation of uniform
amorphous Ta2O5 sphere formed in isopropyl alcohol solvent. The
participation of nitrogen narrows the bandgap, which availably
enhances the sensitivity to visible light. As the porous
d.Eco-materials and Renewable Energy Research Center (ERERC), Nanjing
University, Nanjing 210093, P. R. China
e. College of Chemistry and Chemical Engineering, Yangcheng Institute of
Technology, Yancheng 22401, P. R. China
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spherical architecture could easily capture incident illumination and counted by the Brunauer−Emmeꢁ−Teller (BET) method. The
shorten of charge transfer distance from interior to outer surface to adsorbing capacity of CO2 was assessed at 273 K on an automatic
expedite charge separation, the TaON microsphere is confirmed to volumetric adsorption equipment (Micromertics ASAP 2010). The
significantly improve the photocatalytic efficiency toward chemical state was researched by X-ray photoelectron spectroscopy
conversion of CO2 into ethanol and ethanal, under visible light (XPS; K-Alpha, Thermo Fisher Scientific).
irradiation, superior to its counterpart from commercial Ta2O5. By Measurement of Photocatalytic Activity
loading Pt as an electron sinker to boost the separations of the
Photocatalytic reactions for carbon dioxide reduction were carried
photogenerated electron-hole pairs, the photoconversion efficiency
out in the Pyrex reaction vessel has a top irradiation area of 50.27
cm2 at room temperature. Typically, 0.1 g of the as-prepared
was further increased, and the product species can be altered.
catalyst powder was dispersed in 100 ml NaHCO3 water solution (1
mol L-1) under magnetic stirring. The high purity of CO2 gas was
Experimental Section
continuously bubbled into mixed solution for 30 minutes in order to
Preparation of porous TaON microspheres
eliminate air and provide the necessary reaction gas. The samples
were irradiated by a solar simulator (Microsolar 300 xenon lamp,
nominal power 50 W, diameter 2.5 cm, intensity 1.02 kW m-2) fitted
with a filter to select only light at visible wavelengths (λ > 420 nm).
The temperature of the reaction solution was invariably kept at
room temperature by flowing of cooling water during the reaction
process. Starting from the Xe lamp was lighted, about 2 ml of the
reactant solution was constantly taken from the reaction vessel
every hour and injected into a vial for succeeding reaction products
Total chemical reagents from commercial sources were of analytical
grade and were not purified during use. In a emblematic synthesis,
30 ml of isopropyl alcohol solution (99.8%; Kanto Chemicals)
containing the 0.1 g of TaCl5 (99.9%; High Purity Chemicals) and
additionally 0.1 g of tetrabutylammonium hydroxide (10% in Water)
were stirred for 30 min. A 50 ml inner volume of Teflon-lined
stainless steel autoclave was prepared to take in the resulting
solution. Then the synthesis was put in an electric oven to
continuously heat at 200 for 24 h before starting to cool down
naturally to room temperature. The bottom sediment was washed
with deionized water and centrifugated, and dried in the freeze
dryer. The white spherical structure of Ta2O5 was obtained. A
suitable amount of Ta2O5 powder was then nitrogenized in a quartz
tube at 1123 K for 10 h in ammonia atmosphere with flowing rate
of 100 ml min-1. From the start heating to down to ambient
temperature, the ammonia gas always kept flowing to avoid being
oxidized gain. Finally, the yellow-green porous TaON microspheres
were obtained. Meanwhile, commercial Ta2O5 powder as
references is named C-Ta2O5. C-TaON was synthesized with the
same method described above to nitride the commercial Ta2O5.
analysis by using
a liquid chromatograph (LC-2014, Agilent
Technologies, USA). The irradiation time is 5 hours.
Results and discussion
Pt-loaded TaON was prepared by a precipitation method. 20 ml of
methanol solution containing the 0.1 g of TaON powder and
additionally 0.05 ml of H2PtCl6 (0.5 wt%) aqueous solution were
poured into a glass beaker with 80 ml deionized water. The mixture
was stirred for 5 h under the light irradiation of a 300 W Xe lamp
before being washed and dried.
Fig.1 Optical images of (a) as-synthesized Ta2O5 microspheres, and
(b) porous TaON microspheres.
Characterization of porous TaON microspheres
The crystallographic phases and purity situation of the prepared
powder samples were researched by X-ray diffractometry (XRD;
Rigaku Ultima III, Japan) at room temperature. The XRD curve was
described with a sweep rate of 10° min-1 using Cu-Ka radiation (λ =
0.154178
nm)
at
40
kV
and
40
mA.
The morphology and core structure of samples were characterized
with the field emission scanning electron microscopy (FE-SEM) on
an XL30 ESEM FEG scanning electron microscopy conducting at 20
kV. The transmission electron microscopy (TEM) and fine resolution
transmission electron microscope (HRTEM) images were taken on a
JEM 200CX TEM apparatus. The diffuse reflectance graph of the
powder was depicted with a UV−vis spectrophotometer (UV-2550,
Shimadzu) and switched to the absorption spectrum on the basis of
the Kubelka−Munk connecꢀon. The size of specific surface area for
the sample was detected by nitrogen sorption on a surface area at
77 K and porosity analyzer (Micromeritics TriStar 3000, USA) and
Fig.2 Low and high magnification SEM images of (a1) and (a2) Ta2O5
microspheres, (b1) and (b2) single-crystalline porous TaON
microspheres.
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The color of the samples obviously change from white for as- The phase transformation process of samples were characterized by
obtained Ta2O5 to yellow green for the TaON (Fig. 1), illustrating X-ray diffraction XRD patterns. Compared with the XRD peaks of C-
that the oxygen atoms gradually replaced with the nitrogen atoms Ta2O5 (Fig. 4a), the large valley packages at about 26° and 55°
over increasing the nitriding time. The FE-SEM images show that indicates the amorphous or poor crystallinity of the Ta2O5
the Ta2O5 of 400−500 nm in diameter displays relatively uniform,
microsphere (Fig. 4b). After nitridation with heating treatment,
monodisperse, spherical microstructure with smooth surface (Fig. the strong diffraction peaks can well index to TaON according to
2a1), in contrast with irregular C-Ta2O5 (See Supporting Information). JCPDS card no. 71-0178 (Fig. 4c). It was found that the nitridation
With high-temperature nitridation, the formed TaON microspheres time greatly affects the crystal formation of the nitride samples.
exhibit surface roughness and contain abundant pores (Fig. 2b2).
Extension of the nitridation time to 15 hours led to the formation of
Ta3N5 (JCPDS No. 79-1533) (Fig. 4d).
Fig.5 (a) UV-visible absorption spectrum of Ta2O5 microsphere
and porous TaON microsphere. The band edge of (b) Ta2O5
microsphere and porous TaON microsphere.
UV–visible absorption edge of the Ta2O5 is approximately 300 nm so
that it only shows the response ability in the ultraviolet (UV) light
(Fig. 5). The TaON sphere displays obviously strong absorption in
visible light because the nitrogen 2p orbital is more potential
negative than the oxygen 2p orbital, which is advantageous to the
spectral position of the band edge red-shift to around 520 nm (Fig.
5a). The band gap of the Ta2O5 and TaON microsphere were
calculated to be 3.9 and 2.4 eV (Fig. 5b), respectively, according to
Fig.3 (a1) and (a2) TEM images of Ta2O5 microsphere; (b1) and (b2)
TEM image of porous TaON microsphere with the inset
corresponding to the SAED.
The TEM images further reveal that the Ta2O5 microsphere consists
of numerous extremely tiny particles (Fig. 3a1 and 3a2). The
amorphous property of those particles (See below XRD analysis)
facilitates shrinkage of Ta2O5 into the porous TaON nanostructure
during the nitridation process at high temperature. The light
contrast of the TaON microsphere indicates the formation of
nanopore (Fig. 3b1). The corresponding diffraction dot diagram of
the selected-area electron diffraction (SAED) manifests the single-
crystal property of TaON microsphere. The interplanar distance
between the well-defined lattice fringes is 0.42 nm, assigned to the
(110) crystal plane of the TaON crystal (Fig. 3b2).
the equation (1)20-21
:
n
（α
hν
）= A h
ν
− Eg
(1)
(
)
Here A is a constant, hν is the light energy, Eg is optical band gap
energy, α is absorption coefficient, and n is a number that changes
over the transition process which equals to 1/2 for indirect
transition and for 2 for direct transition.
Fig.6 Nitrogen adsorption–desorption isotherms of porous TaON
microsphere. Inset shows the corresponding pore diameter
distribution.
Fig.4 XRD pattems of (a) C-Ta2O5; (b) Ta2O5 microsphere; (c) porous
TaON microsphere; (d) Ta3N5 microsphere, and (e) standard JCPDS
card (no. 71-0178) for TaON.
The isotherms adsorption–desorption behavior of porous TaON
microsphere was detected by the BET measurement. As
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the pressure increases, the porous TaON microsphere received the originates from input CO2 gas rather than the decomposition of any
relatively higher adsorbed volume of the nitrogen (Fig. 6). A constituent.Fig. 8 shows that CO2 can be photoreduced to C2H5OH
hysteresis loop of nitrogen adsorption-desorption isotherms and CH3CHO by using TaON samples as photocatalysts with noble-
represents the characteristic of porous structure and adsorption metal co-catalysts such as Pt. The C2H5OH rate of generation of
without limits at high relative pressure.22 The inset of pore diameter TaON and C-TaON are measured to be 2.03 μmol h-1 g-1 and 0.84
distribution curve further reveals the average pore size diameter of μmol h-1 g-1, respectively. Also, the TaON (CH3CHO: 0.52 μmol h-1 g-1)
about 32 nm for the porous TaON sphere. The specific surface area
of porous TaON microsphere is about 11.12 m2 g-1, 1.5-time larger
than the counterpart from C-Ta2O5 (7.41 m2 g-1).
Fig.8 (a) Comparation of the average product formation rates of
samples. (b) Photocatalytic C2H5OH and CH3CHO evolution amounts
for C-TaON, TaON microspheres and Pt-loaded TaON microspheres.
shows about a 3.25 times improvement in conversion rate than the
referred C-TaON (0.16 μmol h-1 g-1) (Fig. 8a), which is ascribed to
following reasons: (1) The mesoporous structure enhances gas
Fig.7 XPS spectra analysis of the TaON microsphere: (a) wide scan
capture/adsorption of the reactants and provides more reaction
for sample ; narrow scan for (b) Ta element, (c) O element and (d)
sites; (2) The porous spheriform structure potentially takes on a
Ta and N element; (e) location of the VB.
photon capture trap well to allow the multi-scattering of incidence
light for enhancement of light absorption, as demonstrated in the
precedent TiO2 hierarchical nanostructure;24-26 (3) The porous
structure shortens charge transfer distance from interior to outer
surface for expediting separation of photogenerated charge. The
output in the C2H5OH of the porous TaON increased with the
photocatalytic time, and a ultimate average output of C2H5OH
received in the experiment after 5 h of continuous irradiation is
about 11.70 μmol h-1 g-1, which is far better than a final yield of the
C-TaON (4.21 μmol h-1 g-1) (Fig. 8b). The generation rate of C2H5OH
over the porous TaON microspheres could be further increased to
2.34 μmol h-1 g-1 through loading Pt (0.5 wt%) as an electron sinker
to boost the separations of the photogenerated electron-hole
pairs.27 Interestingly, the CH3CHO generation rate was oppositely
decreased after loading Pt. It is possible that Pt nanoparticles
accelerate the electron transfer to allow the part of the forming
CH3CHO to further continuously reduce into C2H5OH.
Fig. 7a displays the presence of Ta, O, and N in the full-scale XPS
spectrum of TaON microsphere. The strong peaks of binding
energies at 25.6 eV and 27.5 eV correspond to the spin orbit
separation of the Ta 4 f5/2 and Ta 4 f7/2 ingredients, respectively (Fig.
7b), indicating the formation of the Ta5+.23 The two different binding
energies of O element can be assigned to the features of Ta—O
bond (530.2 eV) and oxygen in carbonate species or hydroxyl
groups (531.4 eV) (Fig. 7c).20 The strong peaks for Ta 4 P2/3 and N 1s
in Fig. 7d reveals that the N 1s region concentrated at 396.3 eV
linked to the binding energy of approximately 403.5 eV for Ta 4 P2/3,
further confirming the formation of numerous Ta–N bonds.23 Based
on the bandgap of the TaON of 2.40 eV, the corresponding EVB and
ECB were detected located at about 2.0 eV (Fig. 7e) and -0.4 eV,
respectively, which satisfies the qualification for CO2
photoreduction potential.
The photocatalytic CO2 conversion was carried out with visible light
(λ>420 nm) under CO2-saturaed NaHCO3 electrolytes. The identical
CO2 reduction experiment carried out in the dark or in the presence
of highly pure nitrogen/argon exhibits no appearance of the
hydrocarbons, indicating that the formation of ethanal and ethanol
The solar-to-chemical energy conversion efficiency over the present
system was calculated using the following formula. Here the energy
stored in the products is acquired from the heat energy released
from complete combustion of per mole pure substance,
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∇HC H OH =1366 KJ / mol ∇HC
ARTICLE
,
[The principle of
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=1168 KJ / mol
O
2H4
2
5
general chemistry (fourth edition), 85], multiplied by the amount of
products (See the detail in Experimental Section).
Energy stored in products
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E =
（2）
Irradiation energy
The average conversion rate of the solar-to-chemical energy was
listed in Table 1. The irradiation energy is the solar intensity
multiplied by the irradiation area and irradiation time.
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3057.
solar-to-
CH3CHO
solar-to-
C2H5OH
Total solar to
hydrocarbon
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E (%)
E (%)
E (%)
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C-TaON
TaON
0.001
0.003
0.002
0.006
0.015
0.017
0.007
0.018
0.019
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The single-crystalline, porous spherical architecture of TaON was
successfully obtained through nitridation of the Ta2O5 microsphere
precursor. This nanostructure exhibit high photocatalytic activity for
reduction of CO2 to liquid hydrocarbon fuels in the presence of
NaHCO3 electrolytes under visible light (λ > 420 nm) illumination,
relatively to C-TaON. The porous structure allows for effectively
capturing incident illumination and providing more reaction sites.
Loading of Pt cocatalyst enables to further improve the
photocatalytic efficiency and tune the conversion products.
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Acknowledgements
This work was supported by 973 Programs (No. 2014CB239302 and
2013CB632404), National Natural Science Foundation of China (No.
21473091, 51272101, and 51202005), and National Science
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Table of Contents
Synthesis of single-crystalline, porous TaON microspheres
toward visible-light photocatalytic conversion of CO2 into
liquid hydrocarbon fuels
Single-crystalline, porous TaON microspheres were prepared for
photocatalysis via facile nitridation of uniform amorphous Ta2O5
sphere. The porous spherical architecture of TaON plays
a
significant role in deciding the CO2 photocatalytic conversion
efficiency into ethanal and ethanol under visible light irradiation,
relative to its counterpart from commercial Ta2O5, including
availability of more reaction sites, easy trapping of incident
illumination, and shortening of charge transfer distance from
interior to outer surface to expedite charge separation.
6 | J. Name., 2012, 00, 1-3
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