Self-Hybrid Transition Metal Oxide Nanosheets Synthesized by a Facile Programmable and Scalable Carbonate-Template Method

Article abstract of DOI:10.1002/smll.202103176

2D transition metal oxides (TMO) nanosheets have attracted considerable attention in both fundamental research and practical applications. Herein, a convenient programmable and scalable carbonate crystals templating synthesis is developed to produce high-quality self-hybrid TMO nanosheets (Si-WO_3?x, Ta_xO_y, Mn_xO_y) and their respective polymetallic oxide hybrid nanosheets with tunable composition, low-cost and high-yield. Taking tungsten oxide nanosheets as example, silicotungstic acid precursor is in situ converted into tungsten oxide nanosheets like scales on the surface of calcium carbonate crystals through the simple soaking-drying-calcination process, and after selectively dissolving calcium carbonate by etching, the dispersive tungsten oxide nanosheets with unique self-hybrid Si-doped h-WO₃/ε-WO₃/WO₂ compositions are obtained, which show excellent acetone gas-sensing performances at low temperatures. This carbonate-template method opens up the possibility to economically produce various functional TMO nanosheets with specific compositions for diverse applications.

Full text of DOI:10.1002/smll.202103176

ReseaRch aRticle
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Self-Hybrid Transition Metal Oxide Nanosheets
Synthesized by a Facile Programmable and Scalable
Carbonate-Template Method
Wenhe Xie, Yuan Ren, Bingjie Yu, Xuanyu Yang, Meiqi Gao, Junhao Ma, Yidong Zou,
Pengcheng Xu, Xinxin Li, and Yonghui Deng*
been developed for preparation of TMO
nanosheets, such as exfoliation,[10] wet-
chemical synthesis,[9,11,12] and vacuum-
based epitaxial growth.[13] Mechanical
exfoliation technique and liquid exfolia-
tion by sonication were first used to pro-
duce ultrathin 2D nanomaterials, but the
low yield restricted their practical appli-
cations. Besides, these methods, as well
as the alternative ion-intercalation and
exfoliation method, are only applicable to
those materials with bulk-layered struc-
tures.[14] Alternatively, a variety of wet
chemical synthesis methods, including
hydro/solvothermal,[15] self-assembly of
nanocrystals,[16] 2D-templated synthesis,[17]
and soft colloidal templated methods,[9,18]
were developed toward non-layered struc-
tured nanosheets. However, the structure-
directing agents and surfactants were
usually used to produce and stabilize the
2D transition metal oxides (TMO) nanosheets have attracted considerable
attention in both fundamental research and practical applications. Herein,
a convenient programmable and scalable carbonate crystals templating
synthesis is developed to produce high-quality self-hybrid TMO nanosheets
(Si-WO3−x, TaxOy, MnxOy) and their respective polymetallic oxide hybrid
nanosheets with tunable composition, low-cost and high-yield. Taking tung-
sten oxide nanosheets as example, silicotungstic acid precursor is in situ con-
verted into tungsten oxide nanosheets like scales on the surface of calcium
carbonate crystals through the simple soaking-drying-calcination process,
and after selectively dissolving calcium carbonate by etching, the dispersive
tungsten oxide nanosheets with unique self-hybrid Si-doped h-WO3/ε-WO3/
WO2 compositions are obtained, which show excellent acetone gas-sensing
performances at low temperatures. This carbonate-template method
opens up the possibility to economically produce various functional TMO
nanosheets with specific compositions for diverse applications.
2D nanostructure in wet chemical synthesis and they could
contaminate the surface and compromise the performances
owing to possible blockage of active sites and change of band
gaps.[19]
1. Introduction
Since the discovery of graphene in 2004,[1] 2D nanomaterials
have always been a research focus due to their unique physical
and chemical performances.[2–4] As a major class of 2D nano-
materials, transition metal oxides (TMO) nanosheets have
shown great potential in various applications such as catal-
ysis,[5] energy storage and conversion,[6,7] sensing, and optical
nanodevices.[8,9] To date, varieties of synthetic strategies have
Vacuum-based epitaxial growth, such as chemical vapor
deposition (CVD), is a promising route to synthesize high-
quality and surface-clean nanosheets on solid substrates.
Conventionally used 2D substrates (e.g., Au foils, SiO2/Si,
mica, sapphire)[20–22] possessing a limited surface area are
incompatible for scalable production. Thus, powder-like sub-
strates containing SiO microparticles[23] and diatomite[24] were
used for practical CVD routes. The CVD method requires
harsh reaction preparation conditions and special instru-
ments which may limit its yield. Moreover, conventional
methods are usually not suitable for convenient synthesis of
2D nanomaterials with hybrid structures and well-organized
textures. Hybrid metal oxide materials usually have a better
performance than either pure phase alone. A typical example
is P25 Mixed-Phase TiO2, in which charges produced on
rutile are stabilized through electron transfer to lower energy
anatase lattice trapping sites, making P25 exhibit high photo-
catalytic activity.[25] Generally, the hybrid materials can be
synthesized with multiple precursors or strictly control-
ling the critical temperature of phase transition with single
precursor.[26,27]
W. Xie, Y. Ren, B. Yu, X. Yang, M. Gao, J. Ma, Y. Zou, Y. Deng
Department of Chemistry
State Key Laboratory of Molecular Engineering of Polymers
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials
iChEM
Fudan University
Shanghai 200433, China
E-mail: yhdeng@fudan.edu.cn
P. Xu, X. Li, Y. Deng
State Key Laboratory of Transducer Technology
Shanghai Institute of Microsystem and Information Technology
Chinese Academy of Sciences
Shanghai 200050, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.202103176.
DOI: 10.1002/smll.202103176
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Recently, salt-template methods combining the advan-
tages of wet-chemical synthesis and powder surface epitaxial
growth were proposed.[19,28–31] Typical synthesis process was
promoted by lattice matching between target materials and
salt substrates on the flat surface of salt crystals.[28] Till now,
diverse 2D nanomaterials, including TMO,[32] transition metal
dichalcogenides,[33] transition metal nitrides,[34] transition metal
phosphide,[32] carbon-based hybrid composites,[36] and single-
atom catalysts anchored on graphene-like carbon,[37] have been
reported using salt-assisted synthesis methods. Although the
above approaches can be used for mass production of high-
quality TMO nanosheets, they still suffer several shortcomings
which hamper their wide applicability. First, lattice matching
mechanism restricts the synthesis of TMO species using chlo-
ride and sulfate templates. Second, it is difficult to synthesize
hybrid TMO in a programmable mean via existing salt-template
methods. Third, the precursor solvent must be organic agents
because the templates are water-dissolvable.[30]
Herein, we report a carbonate-template method to synthe-
size various self-hybrid TMO nanosheets (e.g., WO3−x, TaxOy,
MnxOy) via a simple and programmable soaking-drying-
calcination-etching process. The self-hybrid nanosheets are
synthesized with single precursor in a wide temperature
range, while the hybrid character is induced by the tem-
plates. Taking the synthesis of Si-doped h-WO3/ε-WO3/WO2
(i.e., Si–WO3−x) nanosheets as an example, silicotungstic acid
(H4SiW12O40·H2O, abbreviated as H4SiW) aqueous solution
could form a liquid film on the flat surface of polycrystalline
CaCO3 during the drying process. Guided by the confinement
of the liquid film and partial lattice matching between tungsten
oxide and carbonate crystals, the H4SiW molecules can grow
into Si–WO3−x nanosheets with a mean dimension of about
100 nm and thickness of about 10 nm on the surface of CaCO3
like scales during the calcination process. Interestingly, the
obtained Si–WO3−x nanosheets are self-hybrid nanocomposites
consisting of WO2, h-WO3, and ε-WO3. The self-hybrid nano-
composite nanosheets were found to exhibit superior acetone
sensing performance with a good response (Rair/Rgas = 119
for 50 ppm of acetone), high selectivity, and fast response/
recovery dynamics (21 s/44 s) under low temperature (175 °C)
due to the ultrathin structure, narrow hand gap (≈2.37 eV),
rich oxygen defects and phase boundaries. Besides, this car-
bonate-templated method can be extended readily to synthesize
tion). Field emission scanning electron microscopy (FESEM)
and transmission electron microscopy (TEM) images reveal
that this color change is owing to the growth of Si–WO3−x
nanosheets on the surface of CaCO3 like scales (Figure 1a
and Figure S3b, Supporting Information). Highly-dispersed
Si–WO3−x nanosheets can be obtained after the selectively
etching of CaCO3 by hydrochloric acid solutions.
FESEM and TEM observations reveal that the obtained
ultrathin Si–WO3−x nanosheets have a mean dimension of
about 100 nm and a thickness of about 10 nm (Figure 1b,c).
Due to the small size and thickness, these nanosheets tend to
stack together to form flower-like nanostructures after drying
(Figure 1b), exposing a large available surface area and active
interface sites (Figure S4, Supporting Information). High-
resolution TEM (HRTEM) and selected-area electron diffrac-
tion pattern show a polycrystalline feature (Figure 1d,e). In a
typical image of Figure 1d, the WO2 is dominant component
while small grains of ε-WO3 and h-WO3 with domains of
several nanometers are embedded in the WO2, indicative of
self-hybrid tungsten oxides with abundant phase boundaries
(yellow circle). The blurred lattice fringes are mainly attrib-
uted to the existence of amorphous silica produced by the
doping of silicon.[38] Energy-dispersive X-ray mapping images
show the uniform distribution of W, O, and Si throughout the
nanosheets, indicative of a homogenous growth of Si–WO3−x
nanosheets (Figure 1f).
The crystal composition of the purified nanosheets was
determined by wide-angle X-ray diffraction (XRD) and charac-
teristic peaks correspond to hexagonal h-WO3, unusual mono-
clinic ε-WO3, and monoclinic WO2 (Figure 2b), while h-WO3,
as well as ε-WO3, have high surface polarity and WO2 has good
electron transfer properties.[39,40] Such self-hybrid tungsten
oxides derived from single precursor integrate the feature of
three crystal phases. For comparison, H4SiW was directly cal-
cinated in N2 without carbonate templates, and pure ε-WO3
crystals with bulk morphology were obtained (Figure S3a, Sup-
porting Information).[41] Thus, it is deduced that both of 2D
nanostructure of Si–WO3−x and the formation of h-WO3 and
WO2 are attributed to the induction of carbonate templates.
Thermogravimetry (TG) tests reveal that the complete thermal
decomposition of H4SiW in N2 occurs below 500 °C, which
is consistent with the XRD results, meanwhile, the CaCO3
templates are thermally stable in this temperature range.
(Figure S4, Supporting Information). Besides, UV measure-
ment (Figure S5, Supporting Information) reveals Si–WO3−x
nanosheets exhibit a bandgap of 2.37 eV, much lower than that
(2.58 eV) of bulk ε-WO3, which is mainly due to the presence of
narrow bandgap WO2.
various polymetallic oxide hybrid nanosheets (e.g., Al/WO3−x
,
Cu/WO3−x, Mo/WO3−x) for various applications.
2. Results and Discussion
Furthermore, to investigate the formation mechanism of
unusual ε-WO3, ammonium metatungstate (AMT) was used
as precursor and was directly calcined in N2. Unlike the met-
astable ε-WO3 obtained with H4SiW precursor, the obtained
WO3 crystals possessing bulk morphology and stable mono-
clinic γ-phase (Figure 2a and Figure S3c, Supporting Informa-
tion) agree well with Raman spectroscopy measurement results
(Figure 2c). Such a difference is attributed to the unique H4SiW
precursor, in which the Si–O tetrahedron is surrounded by
twelve W–O octahedrons while AMT doesn’t have heteroatoms
in its W–O framework. The in situ doping of silicon induces
Micron-sized calcite-type polycrystalline CaCO3 crystals were
used as templates for the growth of tungsten oxide nanosheets
(Figures S1a and S2, Supporting Information). The program-
mable soaking-drying-calcination-etching process is illustrated
in Scheme 1. For the Si–WO3−x nanosheets synthesis, polyoxo-
metalate precursor (H4SiW) was dissolved in water and mixed
with CaCO3 crystals. The dried mixture was annealed at 500 °C
in N2 atmosphere to decompose H4SiW and form Si–WO3−x
.
After annealing, the CaCO3 powders showed a visible color
change from white to gray (Figure S1c, Supporting Informa-
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Scheme 1. Schematic illustration of the synthesis process of TMO nanosheets. Metal oxide precursor solution is mixed with carbonate template
(Step-I). As solvent evaporates, the precursor evenly spread over the surface of carbonate with the aid of interface affinity (Step-II). The as-prepared
carbonate@precursor mixture is calcinated at decomposition temperature of precursor to form TMO carbonate@nanosheets (Step-III). After selec-
tively etching of carbonate by hydrochloric acid solution, the dispersive nanosheets are obtained (Step-IV).
the lattice distortion of the WO3 and stabilize the metastable
structure at high temperature (>350 °C).[38,42]
worth noting that, besides the H4SiW, by using other tungsten
source (e.g., WCl6, Na2WO4, H3PW12O40), self-hybrid WO3−x
nanosheets with similar compositions and morphology can be
readily obtained (Figures S6 and S7, Supporting Information).
It suggests that the carbonate-template method is versatile to
synthesize self-hybrid tungsten oxide nanosheets.
X-ray photoelectron spectroscopy (XPS) was employed
to gain further information about the valance states of the
Si–WO3−x composite nanosheets. The W 4f, O 1s, and Si 2p
core-level peaks are shown in Figure 2d–f. The W 4f spec-
trum indicates the existence of W4+ 4f7/2 at 35.4 and W4+ 4f5/2
at 37.6 eV, respectively, revealing an approximate content of
33.9% for WO2. The peaks at 37.1 and 39.3 eV correspond to
W6+ 4f7/2 and W6+ 4f5/2, respectively. The peaks at 36.3 and
38.5 eV are assigned to W5+ 4f7/2 and W5+ 4f5/2, corresponding
to a content of 45.5%, much higher than that 20.6% for W6+,
implying abundant local distortion and variation of the coor-
dination mode for the W–O bond exist in the structure. The
defects on the surface of metal oxides are beneficial for adsorp-
In order to gain insight about the growth of Si-WO3−x
nanosheets, H4SiW precursor solutions of different concen-
trations were used for the synthesis via the soaking-drying-
calcination-etching process, while the volume of precursor
solution and weight of CaCO3 template was fixed to 2 mL
and 0.5 g, respectively. A series of samples were obtained and
fully characterized by FESEM. When the concentration is low
(0.005 g mL−1), Si–WO3−x preferentially nucleate and grow on
the edges, corners, and curved surfaces of the micron-sized
cubic CaCO3 crystals as nanoparticles (Figure S8, Supporting
Information). With the increase of the H4SiW concentration
from 0.005 to 0.05 g mL−1, the adjacent nanoparticles experi-
ence a fusion growth to form nanosheets (Figure S9, Sup-
porting Information). With an appropriate H4SiW concentration
(0.05 g mL−1), Si–WO3−x nanoparticles grow into nanosheets
which further closely arrange on the whole surface of CaCO3,
forming a thin plate analog to fish scales (Figure 1a). However,
too high H4SiW concentration (0.1 g mL−1) can lead to the over-
growth of Si–WO3−x coating, resulting in bulk-phase Si-WO3–x
−
tion of oxygen in air. Both O− and O2 can be regarded as the
adsorbed oxygen species, whose content reflects the abundance
of oxygen vacancies in the tungsten oxide crystal lattice. The
spectra of O 1s, disclose content of dissociative O− (27.5%) and
−
molecular-type adsorbate O2 (15.8%).[43] These results clearly
suggest the presence of rich oxygen vacancies in the obtained
Si–WO3−x composite nanosheets. In Si 2p spectra, the peaks at
103.8 and 101.8 eV can be assigned to aggregated Si–O–Si and
uniformly doped Si–O–W.[38] It suggests that Si atoms derived
from H4SiW are in situ doped in the WO3−x nanosheets. It is
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Figure 1. Structural characterization of Si–WO3−x nanosheets. FESEM images of Si–WO3−x nanosheets a) before and b) after removing the CaCO3
templates. c) TEM and d) HRTEM of Si–WO3−x nanosheets. e) The selected-area electron diffraction pattern revealing the polycrystalline feature.
f) Elemental mapping (W, O, Si) of the obtained nanosheets.
around the CaCO3 crystals, possibly due to lack of effective sur-
face confinement to retard the interface growth of Si–WO3−x
(Figure S10c,d, Supporting Information). Therefore, with an
optimized concentration, dispersed Si–WO3−x nanosheets with
the highest yield and regular morphology can be obtained.
Based on the above results, it is speculated that such a
unique formation of Si–WO3−x nanosheets on the surface
of CaCO3 crystals is guided by the interface affinity and pro-
moted by the lattice matching.[19,28] During the soaking process,
the evaporation of solvent can leave a liquid film confined on
the flat surface of CaCO3 crystals due to the good affinity of the
precursor solution towards metal carbonate templates. Thus,
the precursor solution spreads uniformly over the template
surfaces by virtue of the confinement of the liquid film. Under
the effect of geometry distribution of precursors and the lattice
matching between metal oxides and the underlying carbonate
templates, metal oxides nanosheets were finally generated via
this effective and low-cost soaking-drying-calcination-etching
process. The lattice mismatch between CaCO3 and h-WO3
along x and y directions are 1.4% and 0.2% (Figure S11, Sup-
porting Information), and the lattice mismatch between CaCO3
and WO2 in the two directions are 0.05% and 1.1%, respectively
(Figure S12, Supporting Information). Meanwhile, the lattice
mismatch between CaCO3 and ε-WO3 in the two directions is
4.3% and 2.5%.[28,20,32,35] Thus, it is speculated that the formation
of h-WO3 and WO2 on the surface of CaCO3 is favored and has
close probability, while the growth of ε-WO3 on the surface of
CaCO3 is restrained. Direct thermal treatment of the H4SiW
precursor only leads to ε-WO3, due to the lack of structure-
inducing effect exerted by carbonate crystals’ interface. Besides,
other salt templates with different crystal structures can also
influence the crystal phases and dominant exposed planes of
tungsten oxides (Figure S13, Supporting Information).
It is noteworthy that WO2 is usually obtained at high tem-
perature (>700 °C) under inert atmosphere.[44,45] However, the
above results indicate the WO2 can be readily obtained through
our carbonate template method at much lower temperature
of 500 °C even in air (Figure S13a, Supporting Information).
The ε-WO3, h-WO3, and WO2 can remain in the self-hybrid
Si–WO3−x products at 500 °C or higher reaction temperature
no matter N2 or air was chosen as the calcination atmosphere.
The WO2 is dominant component in the products obtained at
500 °C in N2, while its content declines to form ε-WO3 with
temperature from 500 to 600 °C and the change of calcination
atmosphere from N2 to air (Figure S14, Supporting Informa-
tion). XRD, XPS, and HRTEM results reveal that the self-hybrid
products obtained at 500 °C in N2 can be converted into pure-
phase ε-WO3 nanosheets with single-crystal structure and fewer
defects through thermal treatment at temperatures higher than
300 °C (Figures S15 and S16, Supporting Information). These
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Figure 2. Elemental and structural analysis of the Si–WO3−x nanosheets. a) XRD patterns of H4SiW and AMT together with corresponding products
after thermal treatment at 500 °C in N2 without templates. b) XRD patterns of Si–WO3−x nanosheets and corresponding products without templates.
c) Raman spectra of Si–WO3−x nanosheets and WO3−x nanosheets (the control sample) obtained using AMT as the inorganic precursor. d–f), XPS
showing the W 4f (d), O 1s (e), and Si 2p (f) core level peak regions of Si–WO3−x nanosheets.
results suggest that the relative content of each component in
self-hybrid Si–WO3−x can be regulated through the choice of
calcination temperature and atmosphere.
reaches its maximum at 175 °C (Figure 3d). It is worth men-
tioning that previously reported WO3-based materials usually
exhibit optimized sensing performance at temperature higher
than 250 °C.[46] Such a unique low-temperature sensing perfor-
mance is attributed to the existence of WO2, whose narrow gap
(≈0.6 eV) facilitates the carrier migration on the surface and
interface.[44]
Tungsten oxide is a typical n-type semiconductor with elec-
−
trons as carriers. The active oxygen species (e.g., O2 , O−)
absorbed on the surface of tungsten oxide extract electrons
from the conduction band, causing the increase of depletion
layer (EDL). When reducing gases such as acetone react with
The Si–WO3−x nanosheets gas sensors show a rapid response
and high sensitivity over a wide range of concentrations. The
sensitivity gradually rises from 6.2 to 119 within a range of
1–50 ppm and the resistance can always recover to the base-
line (Figure 3e–g), indicative of good cycling stability. The sen-
sors have obvious response to a low concentration of gas down
to 100 ppb (Ra/Rg = 2.2), showing great potential for detec-
tion of trace acetone (Figure S16, Supporting Information).
For comparison, the gas sensing performances of the pure
Si–WO3 nanosheets with pure ε-WO3 phase obtained by treating
Si–WO3−x nanosheets at 300 °C in air for 2 h (Figures S15
and S16, Supporting Information) were investigated (blue
curves in Figure 3e), which reveals much lower response value
than the Si–WO3−x nanosheets at 175 °C. In 50 ppm acetone, the
Si–WO3−x nanosheets sensors displayed a fast response of 21 s
and recovery of 44 s (Figure 3g). Long-term tests indicate these
gas sensors can maintain their performance over two weeks
(Figure S18, Supporting Information). Such long-term stability
for these tungsten oxides is attributed to the integrated WO2
and WO3 phases with rich stable surface/interface.[47] In order
to study the selectivity of sensors, six kinds of representative
−
adsorbed active oxygen species (C3H6O + Ox → H2O + CO2),
the extracted electrons return back to the conduction band of
tungsten oxide, causing the shrinkage of layer in the surface
and the decrease of resistance (Figure 3b,c). For the 2D nano-
materials such as nanosheets, the electron depletion layer can
cover almost all of the areas, making it ideal candidates for
development of high-performance gas sensors.
Owing to the 2D morphology, rich oxygen vacancies, and
phase boundaries, the Si–WO3−x nanosheets obtained at 500 °C
in N2 were chosen to test the gas-sensing performances, which
was performed on a side-heated type gas sensor platform
(Figure 3a) and the sensitivity of reducing gases is defined as
the ratio of the sensor resistance in air (Ra) to sensor resistance
in tested gas (Rg), namely S = Ra/Rg.
In general, the sensitivity of gas sensor is strongly affected by
working temperature which is relevant with carrier concentra-
tion of the gas-sensitive materials and the activation energy of
the surface reactions. To explore the optimal working tempera-
ture for the self-hybrid Si–WO3−x nanosheets, a series of tests
was carried out toward 50 ppm of acetone and the sensitivity
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Figure 3. Gas-sensing performances of the Si–WO3−x nanosheets. a) Sketch of the structure of a side-heated gas sensor on Si-WO3−x nanosheets.
b) Schematic diagram of the interaction between Si–WO3−x nanosheet and acetone. c) Diagram of the electron depletion layer model for acetone-gas
sensing. d) Responses of the Si–WO3−x nanosheets sensor to 50 ppm acetone at different operating temperatures (160–200 °C). e) Response-recovery
curves of self-hybrid Si–WO3−x nanosheets (red line) and Si–WO3 nanosheets with pure ε-WO3 phase (blue line) at 175 °C at different gas concentra-
tions. f) Response (S = Ra/Rg) of the Si–WO3−x nanosheets sensor versus acetone concentrations at 175 °C. g) Response-recovery curve of the Si–WO3−x
nanosheets sensor to 50 ppm of acetone at 175 °C. h) Repeating response-recovery curve of the Si–WO3−x nanosheets sensor to 50 ppm acetone at
175 °C. i) Responses of the Si–WO3−x nanosheets sensor to different gases of 50 ppm at 175 °C.
gases, including benzene, ammonia, methanol, formalde-
hyde, carbon monoxide, and hydrogen gas, were chosen
to compare with acetone under the same conditions. The
response value of acetone at 50 ppm was 119, much higher
than the rest six kinds of gases (Figure 3i). The good selec-
tivity is attributed to the interstitial doping of silicon, which
induces the formation of ferroelectric ε-WO3 with higher
surface polarity.[38,48] Compared with previous reported WO3-
based sensing materials and other acetone sensitive metal
oxides materials, our sensors show good comprehensive
sensing performances (Table S1, Supporting Information),
which is due to the easy electron transport, large available
surface area, abundant active phase components and rich
oxygen vacancies.[49–52]
The carbonate-templated method can be extended to syn-
thesize other TMO nanosheets, including TaxOy and MnxOy
(Figure 4 and Figures S19 and S20, Supporting Information),
while BaCO3 and Na2CO3 were optimal templates to syn-
thesize TaxOy and MnxOy nanosheets, respectively. FESEM
images show the nanosheets grow on the surface of BaCO3
and Na2CO3 like scales (Figure S21, Supporting Information).
TG tests confirm that the templates maintain chemical sta-
bility during the calcination process (Figure S22, Supporting
Information). The obtained nanosheets have high crystallinity
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Figure 4. TaxOy and MnxOy nanosheets by using other carbonate as templates. a) FESEM, b) TEM, and c) HRTEM images of TaxOy nanosheets.
d) FESEM, e) TEM, and f) HRTEM images of MnxOy nanosheets. The insets in c,f) are corresponding selected-area electron diffraction pattern revealing
the crystalline nature.
and polycrystalline feature. TaxOy nanosheets have larger
size (200–800 nm) and good dispersity, while the MnxOy
3. Conclusion
nanosheets are smaller (100–200 nm) and tend to aggregate
together (Figure 4). The too high resistance of TaxOy and too
low resistance of MnxOy make them non-ideal materials for
gas-sensing. Nonetheless, TaxOy and MnxOy nanosheets may
find applications in other areas, such as catalysis and energy
storage.
In summary, a facile and programmable carbonate-template
method was developed to synthesize high-quality self-hybrid
TMO nanosheets (Si–WO3−x, TaxOy, MnxOy) with single pre-
cursor via simple soaking-drying-calcination-etching process.
As a representative system, the tungsten oxide nanosheets pro-
duced by CaCO3 template and H4SiW precursor have unique Si-
doped h-WO3/ε-WO3/WO2 compositions, which show excellent
gas-sensing performances toward acetone at low temperature
with a high sensitivity and good selectivity owing to its ultrathin
structure, large available surface area, abundant oxygen vacan-
cies, and well cooperation among the active components. This
carbonate-template method is beneficial for green industrial
production of various nanosheets with high-yield and low-cost.
Furthermore, the carbonate templates can be extended to syn-
thesize ultrathin transition metal dichalcogenides, transition
metal nitrides, and other 2D materials for various applications
in sensing, catalysis, energy storage, and conversion.
Except for self-hybrid TMO nanosheets, the carbonate-tem-
plate method can also be used to produce polymetallic oxide
hybrid nanosheets, such as Al/WO3−x, Cu/WO3−x, and Mo/
WO3−x. Compared with pure WO3−x, the addition of small
amounts of other metal sources can cause obvious morpho-
logical change and the decrease of specific surface area of the
nanosheets (Figure S23, Supporting Information). Compared
with Si–WO3−x nanosheets, the gas-sensing performances of
polymetallic oxide hybrid nanosheets change obviously, which
confirms that the element doping can influence the gas-sensing
performances in many aspects, such as the optimum oper-
ating temperature, sensitivity, and selectivity (Figure S25, Sup-
porting Information). The ratio of different components can
be adjusted within a certain range (Figures S26 and S27, Sup-
porting Information). All these results confirm the universality
of the carbonate-template method for the scalable production of
TMO nanosheets.
4. Experimental Section
Chemicals and Materials: Silicotungstic acid hydrate (H4SiW12O40·xH2O,
analytical reagent (AR)), phosphotungstic acid (H3PW12O40 ·xH2O,
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analytical reagent (AR)), AMT hydrate ((NH4)6H2W12O40 · xH2O, 99.5%
metals basis), sodium tungstate dihydrate (Na2WO4 ·2H2O, 99.5%),
ammonium molybdate tetrahydrate (H24Mo7N6O24 ·4H2O, 99.9% metals
basis), aluminum chloride hexahydrate (AlCl3 ·6H2O, 97%) and tantalum
chloride anhydrous (TaCl5, >95%) were purchased from Aladdin.
Calcium carbonate (CaCO3, AR), sodium carbonate (Na2CO3, AR),
barium carbonate (BaCO3, AR), manganese (II) chloride tetrahydrate
(MnCl2 ·4H2O), copper (II) chloride dihydrate (CuCl2 ·2H2O),
hydrochloric acid (HCl, 36.0–38.0%) were purchased from Sino-Pharm
Chemical Reagent Co. Ltd.
Synthesis of Si–WO3−x nanosheets: In a typical synthesis, H4SiW
(0.10 g) was dissolved in H2O or ethanol (2.0 mL) to obtain tungsten
oxide precursor solution (namely H4SiW solution). Then, CaCO3 crystals
(0.5 g) were dispersed in the precursor solution (2.0 mL) in a beaker
under gentle magnetic stirring. The mixture was placed in a hood to
allow the evaporation of solvent. After 12 h, the mixture was further
dried at 100 °C for 2 h, and the obtained white powder was annealed at
500 °C for 1 h under N2 atmosphere (heating rate, 1 °C min−1 below 350
and 5 °C min−1 above 350 °C). Finally, the obtained gray powder sample
was treated in diluted hydrochloric acid to remove the CaCO3 and dark
green Si–WO3−x powders were collected and washed with water through
centrifugation.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This work was financially supported by National Key R&D Program of China
(No. 2020YFB2008600), NSF of China (21875044, 22005058, 22005057),
Key Basic Research Program of Science and Technology Commission of
Shanghai Municipality (20JC1415300), Program of Shanghai Academic
Research Leader (19XD1420300), China Postdoctoral Science Foundation
(2020M670973, BX20200085, 2021TQ0066, 2021M690660), and the state
key laboratory of Transducer Technology of China (Grant no. SKT1904).
Conflict of Interest
The authors declare no conflict of interest.
Synthesis of TaxOy Nanosheets: TaxOy nanosheets were synthesized by
following a similar method with Si–WO3−x except for tantalum chloride
anhydrous (0.1 g) dissolved in ethanol (2.0 mL) was used as the
precursor solution and BaCO3 powder (1 g) was used as template. The
calcination conditions were set as 900 °C for 0.5 h under air atmosphere
(5 °C min−1). The final TaxOy products were white powders.
Data Availability Statement
The data that support the findings of this study are available from the
corresponding author upon reasonable request.
Synthesis of MnxOy Nanosheets: MnxOy nanosheets were synthesized
by following a similar method with Si–WO3−x except for manganese (II)
chloride tetrahydrate (0.1 g) dissolved in ethanol (2.0 mL) was used as
the precursor solution and Na2CO3 (1.5 g) was used as template. The
calcination conditions were set as 400 °C for 1 h under air atmosphere
(5 °C min−1). The final MnxOy products were black powders.
Keywords
carbonate-template, gas-sensing, self-hybrid, transition metal oxide
nanosheets
Synthesis of WO3-Based Polymetallic Oxide Hybrid Nanosheets:
M/WO3−x (M = Mo/Al/Cu) nanosheets were synthesized by following
a similar method with Si–WO3−x except for AMT hydrate (0.1 g)
together with ammonium molybdate tetrahydrate/aluminum chloride
hexahydrate/copper (II) chloride dihydrate (0.01–0.1 g) dissolved in H2O
(2.0 mL) were used as precursor solution.
Received: June 1, 2021
Revised: July 8, 2021
Published online:
Measurement and Characterization: FESEM images were collected
with a GM500 FESEM (Zeiss, Germany) performed at 0.5–3 kV. TEM
images were recorded with an HT7800 high contrast TEM (Hitachi,
Japan) at 100 kV, and HRTEM images were obtained at a Tecnai G2 F20
S-Twin field emission TEM (FEI, America) at 200 kV. N2 adsorption-
desorption isotherms were obtained at 77 K with an ASAP 2420 analyzer.
Before measurements, the samples were degassed in vacuum at 180 °C
for 6 h to remove impurities. The specific surface area was recorded
with Brunauer–Emmett–Teller method and the size distributions were
dealt with Barrett–Joyner–Halenda model. Powder XRD patterns were
obtained at a D4 XRD (Bruker, Germany) equipped with Ni-filtered Cu Kα
radiation (40 kV, 40 mA). XPS data were collected with an AXIS ULTRA
DLD XPS System with MONO Al source (Shimadzu Corp., Japan).
Gas Sensing Testing: The gas sensing performances of all the
samples were fabricated into side-heated type of gas sensor and
evaluated on MA1.0 gas sensing measuring system (Narui Corp.
Ltd., China). The nanosheet powders were ground and dispersed
into ethanol, and the obtained dispersion was uniformly deposited
on alumina tubes. The distance between the two Au electrodes on
alumina tubes was 2.0 mm on average. The as-fabricated tubes with
deposited samples were then subjected to annealing at 100 °C for 2 h
and then at 175 °C for 48 h to ensure the stability of the devices.
The target liquid analytes were injected onto an evaporation plate to
generate target gases. The response value is defined as S = Ra/Rg,
while the Ra is the sensor resistance in air and Rg is the sensor
resistance in tested gases. Besides, the response time and recovery
time are the times required to achieve a 90% saturation value after the
import and/or release of the target gas within the step function.
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