Synthesis of orthogonally assembled 3D cross-stacked metal oxide semiconducting nanowires

Article abstract of DOI:10.1038/s41563-019-0542-x

Assemblies of metal oxide nanowires in 3D stacks can enable the realization of nanodevices with tailored conductivity, porous structure and a high surface area. Current fabrication methods require complicated multistep procedures that involve the initial preparation of nanowires followed by manual assembly or transfer printing, and thus lack synthesis flexibility and controllability. Here we report a general synthetic orthogonal assembly approach to controllably construct 3D multilayer-crossed metal oxide nanowire arrays. Taking tungsten oxide semiconducting nanowires as an example, we show the spontaneous orthogonal packing of composite nanorods of poly(ethylene oxide)-block-polystyrene and silicotungstic acid; the following calcination gives rise to 3D cross-stacked nanowire arrays of Si-doped metastable ε-phase WO₃. This nanowire stack framework was also tested as a gas detector for the selective sensing of acetone. By using other polyoxometallates, this fabrication method for woodpile-like 3D nanostructures can also be generalized to different doped metal oxide nanowires, which provides a way to manipulate their physical properties for various applications.

Full text of DOI:10.1038/s41563-019-0542-x

Articles
https://doi.org/10.1038/s41563-019-0542-x
Synthesis of orthogonally assembled 3D cross-
stacked metal oxide semiconducting nanowires
1
,5
1,5
2
1
1
1
Yuan Ren , Yidong Zouꢀ ꢀ , Yang Liu , Xinran Zhou , Junhao Ma , Dongyuan Zhaoꢀ ꢀ ,
3
2
4
1
Guangfeng Weiꢀ ꢀ , Yuejie Ai , Shibo Xi and Yonghui Dengꢀ ꢀ *
Assemblies of metal oxide nanowires in 3D stacks can enable the realization of nanodevices with tailored conductivity, porous
structure and a high surface area. Current fabrication methods require complicated multistep procedures that involve the initial
preparation of nanowires followed by manual assembly or transfer printing, and thus lack synthesis flexibility and controllability.
Here we report a general synthetic orthogonal assembly approach to controllably construct 3D multilayer-crossed metal oxide
nanowire arrays. Taking tungsten oxide semiconducting nanowires as an example, we show the spontaneous orthogonal pack-
ing of composite nanorods of poly(ethylene oxide)-block-polystyrene and silicotungstic acid; the following calcination gives
rise to 3D cross-stacked nanowire arrays of Si-doped metastable ε-phase WO . This nanowire stack framework was also tested
3
as a gas detector for the selective sensing of acetone. By using other polyoxometallates, this fabrication method for woodpile-
like 3D nanostructures can also be generalized to different doped metal oxide nanowires, which provides a way to manipulate
their physical properties for various applications.
ature is abundant with fascinating materials, such as natural with inorganic precursors. Within these structures, the interfaces
diatomite, shell and nacre. They are constructed via self- between the wires are distinct, and multiple components can
N
assembly with ordinary building units into multifunctional be integrated in each layer for applications other than those of the
nanostructures and can exhibit superior physiochemical proper- single component-based structures.
1
ties . Recently, self-assembly nanomaterials based on organic–inor-
The co-assembly of amphiphilic copolymers and inorganic
ganic assembled nanostructures, such as biological mineralization precursors was developed to be a more effective approach for
and bionic materials, have resulted in extensive and interdisciplin- various ordered nanostructures by virtue of the tunable and
ary applications as sensor devices, drug delivery vehicles and energy reproducible interfacial interactions between the amphiphi-
2,3
22–26
storage and electrochromic devices . The self-assembly strategy has lic structure-directing molecules and inorganic precursors
.
emerged as a flexible and powerful approach to design various func- However, little work has been done yet to controllably construct
tional nanostructures via the spontaneous organization of host mol- metal oxide semiconductor nanowire arrays through the coopera-
ecules and inorganic and/or organic guests through non-covalent tive assembly of organic amphiphilic copolymers and inorganic
interactions, which include hydrogen bonding, π–π stacking, hydro- precursors that are suitable for the reliable, and flexible mass
philic–hydrophobic interactions and even electrostatic interac- production of nanowire-based nanostructures and nanodevices
4
,5
27–29
tions . Among various nanostructures, nanowire-based structures, for various applications
.
particularly semiconducting nanowires, have attracted particular Here we successfully explored an unusual BCP-directed orthogo-
attention because of their excellent properties, such as direct elec- nal assembly to conveniently construct 3D multilayer-crossed metal
6,7
tron transport and good mechanical stability . Traditional tech- oxide semiconducting nanowire arrays through the co-assembly of
niques require special equipment and multistep procedures, which amphiphilic diblock copolymers and polyoxometallates (POMs)
include the preparation of nanowires and further manual assembly and the subsequent calcination-induced structure transformation.
8–10
or costly transfer printing, and thus lack fabrication flexibility
.
Taking the fabrication of 3D tungsten oxide nanowire arrays as an
The efficient and convenient synthesis of nanowire-based struc- example (Fig. 1 and Supplementary Fig. 1), poly(ethylene oxide)-
tures with highly controllable morphology, size and functionalities block-polystyrene (PEO-b-PS) and hydrated silicotungstic acid
11–14
still remains a major challenge
.
(H SiW O ∙15H O, abbreviated as H SiW) can co-assemble to
4
12 40
2
4
Block copolymers (BCPs) exhibit rich phase separation behav- form multilayer-crossed tungsten oxide nanowire arrays (MC-WO -
3
15
iours and can self-assemble into various nanostructures . Based on NWAs). Interestingly, the obtained tungsten oxide nanowires were
their flexible soft chemistry behaviour, BCPs have been employed found to have a unique metastable ε-WO phase due to the intro-
3
as structure-directing agents to construct organic or inorganic duction of silicon atoms in situ generated by H SiW. The obtained
4
16–21
nanowire patterns . These inorganic structures usually consist semiconducting MC-WO -NWAs were used as acetone sensors
3
of inorganic nanowires faithfully derived from the self-assembled and showed sensitivity down to 10.0 ppb, fast response-recovery
structure of the BCPs via either conversion of the silicon-contain- dynamics and good selectivity due to the simultaneous effect of
ing segments or post-decoration of the assembled nanostructures high surface areas with numerous active sites for gas–solid catalytic
1
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, State Key Laboratory of Molecular Engineering
2
of Polymers, and iChEM, Fudan University, Shanghai, China. College of Environmental Science and Engineering, North China Electric Power University,
Beijing, China. School of Chemical Science and Engineering, Shanghai Key Laboratory of Chemical Assessment and Sustainability, Tongji University,
Shanghai, China. Institute of Chemical and Engineering Sciences, Agency for Science, Technology and Research in Singapore (A*STAR), Jurong Island,
Singapore. These authors contributed equally: Yuan Ren, Yidong Zou. *e-mail: yhdeng@fudan.edu.cn
3
4
5
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Articles
NATure MATeriAlS
a
b
c
1
00 nm
H SiW
4
d
e
f
100 nm
100 nm
200 nm
g
h
i
26.8 nm
0
.378 nm
020)
(
100 nm
50 nm
10 nm
Fig. 1 | Co-assembly of PꢃO-b-PS and H SiW O . a, Optical photographs of the transparent colloidal PEO-b-PS/H SiW/THF solution showing a pale
4
12 40
4
blue due to the light scattering of the micelle solution (left) and corresponding Tyndall effect under a 650ꢀnm red laser illumination (right). b, Cryo-TEM
image of the colloidal micelles. c, Structural models of composite micelles. Yellow line, PS block; blue line, PEO block; red dot, O atom; blue polyhedron,
W–O octahedron; orange dot, Si atom). d, TEM image of the spherical and cylindrical micelles. e,f, Field-emission scanning electron microscopy (FESEM)
images of the as-cast PEO-b-PS/H SiW hybrid film composed of multilayer-crossed cylindrical micelles. g–i, FESEM (g) and TEM (h and i) images with a
4
selected-area electron diffraction pattern (h) of MC-WO -NWAs obtained after calcination.
3
4
–
32
reactions, the high electron transport along the nanowires and the of SiW O is about 1.5nm (ref. ), the THF-solvation shell thick-
12
40
4–
high electric dipole moment of ε-WO .
ness around SiW O is about 0.5nm. Strikingly, when the colloi-
3
12 40
Remarkably, this BCP-directed co-assembly method can be dal solution was dropped directly onto carbon-coated copper grids
extended readily to synthesize various multilayer-crossed metal for normal TEM characterization, numerous cylindrical micelles
oxide nanowires with different compositions. We have demon- and some spherical micelles could be observed (Fig. 1d). The nano-
strated the 3D cross-stacked nanowire structures of Si-doped MoO , spheres have the same size as that in the cryo-TEM image, whereas
3
P-doped WO and P-doped MoO by using H SiMo O , H PW O
the cylindrical micelles have different lengths that range from 50nm
3
3
4
12 40
3
12 40
and H PMo O , respectively, which have promising applications in toseveralmicrometres,butthesamediameterofabout25nm(Fig.1d
3
12 40
catalysis, sensing, energy storage and conversion, and so on.
and Supplementary Fig. 4). Moreover, the cylindrical micelles
exhibit a bumpy morphology with close spherical tips, which sug-
gests that most of the spherical micelles gradually fused into cylin-
ꢄnterfacial behaviour of micelles
Through mixing the two homogeneous colourless tetrahydrofuran drical micelles in line with the THF evaporation (Supplementary
(
THF) solutions, one that contained dissolved PEO-b-PS copoly- Fig. 4d). When the whole colloidal solution was cast on Petri dishes,
mers and the H SiW, a stable, transparent and pale blue colloidal an organic–inorganic composite film that consists of multilayer-
4
–
1
solution with a solid (copolymer and H SiW) content of 0.08gml
crossed cylindrical micelles can be obtained due to the slow assem-
4
(
Supplementary Fig. 2), which exhibits a typical Tyndall effect bly of micelles under a higher concentration with sufficient building
Fig. 1a). The formation of such a colloidal solution can be explained blocks (Fig. 1e,f and Supplementary Fig. 5).
(
as follows. The PEO segments of PEO-b-PS can be quickly proton-
+
ated by H ions released from H SiW that further interact with Structural transformation induced by calcination
4
4–
30,31
SiW O anions by electrostatic attractions , which significantly The as-formed H SiW/PEO-b-PS film was calcined at 500°C in
1
2
40
4
increases the repulsion force among the copolymers and induces nitrogen and then at 400°C in air, which resulted in an unusual kind
their spontaneous micellization to reduce interface energy. Thus, of 3D MC-WO -NWAs (Fig. 1g). The nanowires are ordered in one
3
uniform spherical micelles with a THF-swelled PS core and H SiW- direction within each layer but crossed between adjacent layers, and
4
associated PEO shell formed, as confirmed by cryo-transmission most of the cross-angles are close to 90° (Supplementary Fig. 6).
electron microscopy (TEM) (Fig. 1b). Dynamic light scattering mea- The spacing between two neighbouring nanowires is ~20.0nm, and
surements show bimodal narrow size distributions (Supplementary the diameter of the WO nanowires is ~15.0nm. By increasing the
3
Fig. 3). The first distribution peak, centred at 28.5nm, corresponds mass ratio of H SiW/PEO-b-PS from 2.0 to 3.0, 4.0 and 5.0, similar
4
to PEO-b-PS/H SiW spherical micelles, and the other peak at 2.5nm nanostructures of nanowire arrays can be obtained, except that the
4
4–
is attributed to free SiW O anions. Given that the molecule size diameter of the nanowires increases from ~10.5 to ~18.5nm because
12
40
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NATure MATeriAlS
Articles
a
b
c
Experimental data
Sum
Background
Si–O–Si
Si–O–W
Experimental data
Sum
Experimental data
Sum
Background
Background
6
6
5
5
+
+
+
+
–
2
–
W
W
W
W
4f7/2
4f5/2
4f7/2
4f5/2
O
O
I
n
t
e
n
s
i
t
y
(
a
.
u
.
)
44
42
40
38
36
34
32
108
106
104
102
100
98
96
540
538
536
534
532
530
528
526
Binding energy (eV)
Binding energy (eV)
Binding energy (eV)
d
e
f
*
ε-WO3
*
ε-WO3
*
*
#
γ-WO3
# γ-WO3
*
4
5
00 °C, air
00 °C, N2
*
MC-WO -NWAs
3
MC-WO -NWAs
*
#
*
3
*
PEO-b-PS/H SiW-100 °C
*
4
#
#
#
##
#
Mesoporous WO3
#
#
#
Mesoporous WO3
#
H SiW12O ·15H O
4
40
2
JCPDS NO.20-1324
20
30
40
50
60
70
20
30
40
50
60
70
100 200 300 400 500 600 700 800 900 1,000
–
1
2θ (°)
2θ (°)
Raman shift (cm )
Fig. 2 | ꢃlemental and structural analysis of the MC-WO -NWAs. a–c, X-ray photoelectron spectroscopy showing the W 4f (a), O 1s (b) and Si 2p (c) core
3
level peak regions of MC-WO -NWAs. d, XRD patterns of completely Si-doped ε-WO after thermal treatment at 500ꢀ°C in N . After calcination in air at
3
3
2
4
00ꢀ°C to remove the residual carbon derived from the organic templates, the Si-doped ε-WO materials had crystalline features. e,f, XRD patterns (e) and
3
Raman spectra (f) of MC-WO -NWAs and the mesoporous WO (the control sample) obtained using WCl as the inorganic precursor. The MC-WO -NWAs
3
3
6
3
−1
sample displays peaks at 363, 682 and 804ꢀcm , which correspond to the acentric ε-phase (space group Pc). In contrast, mesoporous WO displays peaks at
3
−1
324, 710 and 805ꢀcm , assigned to the γ-WO phase (space group P21/n). a.u., arbitrary units.
3
more H SiW species are involved in interacting with the ethylene arrays at 340 °C, due to the accelerating effect under vacuum that
4
3
3–35
oxide segments of PEO-b-PS copolymers (Supplementary Fig. 7). promotes the decomposition of the composite . X-ray photo-
Excess addition of H SiW can result in degenerated MC-WO - electron spectroscopy characterization of the MC-WO -NWAs
4
3
3
NWAs that consist of non-uniform and even broken nanowires. reveals the core level peaks of W 4f, O 1s and Si 2p (Fig. 2a–c and
TEM images (Fig. 1h,i) show a clearly porous structure formed by Supplementary Fig. 14), which confirms the presence of W and
the interweaving crystalline semiconducting WO nanowires, and Si and species. The peak-differentiation-imitating analysis indi-
3
5
+
the nanowires are ‘welded’ at the crossing points. The selected-area cates the presence of small amount of W (24.3%), and the O 1s
electron diffraction pattern reveals spotty diffraction arrays, in peak also reveals the oxygen vacancy in the WO crystal lattice of
3
which the diffraction spots are slightly elongated. A close observa- MC-WO -NWAs. Energy dispersive spectra show the uniform dis-
3
3
6
tion on a single nanowire reveals the presence of a distorted crystal tribution of W, O and Si throughout the MC-WO -NWAs sample
3
lattice and some dislocations (Supplementary Fig. 8a,b) or lattice (Supplementary Fig. 15).
mismatches at crystal grain boundaries (Supplementary Fig. 8c,d).
The obtained MC-WO -NWAs show well-resolved X-ray dif-
3
Therefore, the nanowires in the MC-WO -NWAs are not perfect fraction (XRD) peaks attributed to the unusual ε-WO crystallites
3
3
single crystals. The pore structure was evaluated by nitrogen adsorp- (JCPDS No. 20-1324) (Fig. 2d,e). Compounds with the ε-phase
tion–desorption isotherms, which indicate a mean pore size of WO have been extensively studied as ferroelectric materials for
3
2
–1
about 29.0nm and specific surface area of 55m g (Supplementary their spontaneous polarization behaviour and this phase usually
3
7–39
Fig. 9 and Supplementary Note 1).
exists only at a low temperature (<350 °C) (refs
). Following
The unique structural transformation from cylindrical compos- a similar solvent-evaporation-induced co-assembly, WCl was
6
ite micelles into nanowires induced by calcination can occur even used as the tungsten source to assemble with PEO-b-PS via the
if the concentration of the PEO-b-PS/H SiW solution changes complexation between the PEO segments and the tungsten spe-
4
4
0
(
Supplementary Figs. 10–13 and Supplementary Note 2). In situ cies , and an ordered mesoporous WO was obtained via sol–gel
3
TEM was employed to further study the formation by heating the chemistry (Supplementary Fig. 16). The obtained mesoporous
as-casted PEO-b-PS/H SiW under TEM and by real-time record- WO has 3D interconnected spherical mesopores and the polycrys-
4
3
ing images of the sample (Supplementary Video 1). The well- talline γ-phase WO . Such a dramatic difference in crystal phase
3
aligned cylindrical micelles can be maintained up to 300 °C during is also evidenced by the Raman spectroscopy measurements
TEM observation, and are quickly transformed into nanowire (Fig. 2f). These results clearly indicate that the silicon-containing
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Articles
NATure MATeriAlS
Si-MoO3
P-WO3
P-MoO3
a
b
c
100 nm
100 nm
200 nm
d
e
f
100 nm
100 nm
200 nm
g
h
i
1
00 nm
100 nm
100 nm
Fig. 3 | Different 3D cross-stacked nanowire structures by using other POMs as inorganic precursors. a–c, TEM images of as-cast PEO-b-PS/POM
cylindrical micelles of SiMO (a), PWO (b) and PMoO (c). d–i, FESEM images of hybrid films before (d–f) and after (g–i) calcination. The corresponding
3
3
3
MC-metal oxide-NWAs were obtained using silicomolybdic acid (a, d and g), phosphotungstic acid (b, e and h) and phosphomolybdic acid (c, f and i) as
the inorganic precursors.
species play an important role in stabilizing the metastable ε-WO , (Supplementary Video 2, Supplementary Figs. 29 and 30 and
3
especially when they are used at temperatures higher than 350°C. Supplementary Note 4). In the second stage, the cylindrical micelles
The one-step in situ Si-doping strategy using a Si-containing tung- with a high aspect ratio in the solution usually tend to self-assemble
sten precursor shows great advantages in synthesizing ε-WO
into a 2D hexagonal structure (P6mm) to reach a closest packing
3
41,42
materials, with controllable nanostructures and compositions as the solvent evaporation proceeds . The distance between the
pffiffiffiffi
(
Supplementary Note 3).
neighbouring two layers is 3r, where r is the radius of the micelles
It is noteworthy that, following a similar process, other Keggin- (Fig. 4b, right). However, the PEO-b-PS/H SiW cylindrical micelles
4
type POMs, such as H SiMo O , H PW O and H PMo O , can were closely stacked parallel within every layer, and nearly orthogo-
4
12 40
3
12 40
3
12 40
be employed to co-assemble with PEO-b-PS copolymers to control- nally between adjacent layers. In this case, the interlamellar spac-
lably synthesize Si-doped MoO , P-doped WO and P-doped MoO
ing increases to 2r (bottom of Fig. 4b), which favours formation of
3
3
3
NWAs, respectively (Fig. 3). The co-assembly, structural transforma- a stable stacked structure due to the weaker electrostatic repulsion
tionandthemorphologyandstructureoftheobtainedP-dopedmate- between the cylindrical micelles.
rials are similar to those of MC-WO -NWAs, as confirmed by various
To illustrate the formation mechanism for the orthogonally
3
characterization techniques, which include electron microscopy and assembled structure, a simplified simulation was carried out. As
element mapping (Supplementary Table 1), XRD (Supplementary mentioned in the above dynamic light scattering results, the diam-
Fig. 17), thermogravimetric analysis (Supplementary Fig. 18) and eter of the cylindrical micelles is about 25nm, and the thickness of
X-ray photoelectron spectroscopy (Supplementary Figs. 19–21). the THF solvated region around a single H SiW O is about 0.5nm.
4
12 40
Moreover, binary hybrid nanowire arrays can also be obtained These cylindrical micelles further precipitate and co-assemble at the
through similar methods, and other BCPs with features similar to solid (silicon substrate)–liquid (precursor solution)–gas (air) inter-
those of PEO-b-PS can be used for such syntheses (Supplementary face where a meniscus exists (Fig. 4a). This allows for a rapid evapo-
4
0
Figs. 22–28 and Supplementary Note 3) .
ration of solvent, which induces the cylindrical micelles to assemble
parallel and orthogonally such that neighbouring layers minimize
the repulsion force among them. The dynamic distance between the
Synthesis mechanism
Based on the above results, it was speculated that the formation of neighbouring cylindrical micelles within the same layer is 1.0nm,
the 3D interlaced metal oxide nanowires (for example, MC-WO - which corresponds to twice the thickness of the THF-solvated
3
NWAs) experiences three stages, (1) the co-assembly of PEO-b-PS region. Based on these considerations, the electrostatic repulsive
2
2
and silicotungstic acid into spherical composite micelles, (2) the force (defined as F=q /4πεr , where q is the quantity of electric
fusion of the spherical micelles into cylindrical micelles that subse- charge of the micelles and ε is the dielectric constant of the solution)
quently self-assemble into a multilayer-crossed nanostructure and between a single free cylindrical micelle and the preformed arrays
(
3) the thermal-treatment-induced structural transformation into of cylindrical micelles varies as the crossing angle changes from 0
MC-WO -NWAs, as vividly illustrated in Fig. 4a and the animation to 180° (top of Fig. 4c). The electrostatic repulsive force (F) reaches
3
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NATure MATeriAlS
Articles
a
Precursor solution
Cylindrical
composite micelle
Gas
=
=
THF
PEO-b-PS
Liquid
precursor
=
=
= H SiW12O40
4
Solid substrate
Si-WO3
Calcination-induced structural transformation
Pyrolysis of PEO-b-PS, migration and
decomposition of H SiW12O40, and growth of
4
Orthogonally packed
composite micelles
Si-doped WO3 nanowires
MC-WO -NWAs
3
b
c
r
√
3r
0
°
45°
90°
Hexagonal
r
2
r
r
Lamellar
0
20 40 60 80 100 120 140 160 180
Cross-angle (°)
Fig. 4 | Schematic illustration and theoretical simulation of the formation process of the MC-WO -NWAs. a, First, PEO-b-PS and H SiW O in a
3
4
12 40
THF solution co-assemble into the spherical composite micelles. Second, solvent evaporation induces the fusion of the composite spherical micelles
into cylindrical micelles that subsequently self-assemble into a multilayer-crossed nanostructure. Finally, the removal of the organic templates and the
transformation of H SiW O in the organic–inorganic composites into Si-doped WO nanowires. The inset in the centre of the scheme depicts the
4
12 40
3
assembly of cylindrical composite micelles into orthogonally packed arrays at the solid–liquid–gas interface with the continuous evaporation of THF.
b, Comparison of the hexagonal and lamellar structure constructed by PEO-b-PS/H SiW cylindrical composite micelles. c, A stacking model of the
4
negatively charged PEO-b-PS/H SiW cylindrical micelles (top) and simulation results of the electrostatic repulsive force between a single free cylindrical
4
micelle and the preformed arrays of cylindrical micelles as the cross-angle changes from 0 to 180° bottom).
its minimum at 90° (bottom of Fig. 4c and Supplementary Table 2). Fig. 31b) >W6 substituted (−171.55 eV, Supplementary Fig. 31c).
+
4
+
Clearly, the cylindrical micelles between the layers prefer to cross This indicates that Si embedded in the crystal lattice of γ-WO
3
vertically to reach the most stable structure, and the theoretical is an optimal and stable doping type, which can induce partial
simulation results agree well with the scanning electron microscopy lattice distortion and lead to the ε-WO phase. In addition, the
3
and TEM characterizations.
isosurface of the electron density difference for Si-doped WO
3
represents rigorously vertical transitions, and is most amenable
to depict optically populated Franck–Condon states (Fig. 5b), and
DFꢁ calculations and ꢃXAFS analyses
6
+
4+
To understand the contribution of Si in stabilizing the unusual a partial charge transfer occurred from W to Si to form stable
metastable ε-WO phase of MC-WO -NWAs, synchrotron-radia- Si–O–W bonds. The density of states (DOS) for pure WO and
3
3
3
tion-based X-ray absorption fine structure (XAFS) spectroscopy Si-doped WO indicate that the calculated bandgap of Si-doped
3
and density functional theory (DFT) were applied to directly WO (0.99 eV) is lower than that for pure WO (1.29 eV). This
3
3
4
+
resolve the crystal structure and atomic energy of MC-WO - implies that the introduction of Si can reduce the bandgap and
3
NWAs. It was found that the change of formation energy caused adjust the physiochemical properties (Fig. 5c,d). The doping
4
+
4+
by Si doping followed the order of Si embedded (Type I, 23.53 eV, of Si can induce a partial lattice distortion and an electronic
4
+
Fig.5a) >0 eV >Si embedded(TypeII,−10.36 eV,Supplementary effect that results in strain and atomic rearrangement, which can
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Articles
NATure MATeriAlS
a
b
c
d
1
1
1
40
20
00
120
00
Total
W
Total
W
1
O
O
Si
8
6
4
2
0
0
0
0
0
8
6
4
2
0
0
0
0
0
0.99 eV
1.29 eV
–8
–6
–4
–2
0
2
4
6
–10
–8
–6
–4
–2
0
2
4
E – EF (eV)
E – EF (eV)
e
f
8
6
4
2
0
6
4
2
0
2
4
6
Mesoporous WO3
MC-WO -NWAs
3
Mesoporous WO
3
–
–
–
3
FIT (mesoporous WO )
MC-WO
3
-NWAs
A1
3
4
5
6
7
k (Å
8
1
9
10 11 12
–
)
A2
Mesoporous WO
3
3
FIT (MC-WO -NWAs)
MC-WO -NWAs
3
A3
0
2
4
6
8
10,190
10,200
10,210
10,220
10,230
R (Å)
Photon energy (eV)
Fig. 5 | DFꢁ calculation and ꢃXAFS analysis of MC-WO -NWAs. a, The optimized structure of Si-doped WO . Red, oxygen atom; light blue, tungsten
3
3
atom; yellow, silicon atom. b, Isosurface of a 3D electron density difference for the optimized Si-doped WO . c,d, Total and partial DOS of pure WO3
3
(
c) and Si-doped WO (d). The dashed circle in d highlights the overlapping of the W, O and Si curves, implying strong hybridization. e, Fourier transform
3
2
W L -edge EXAFS spectra of the samples. Inset: W L -edge EXAFS oscillation function k χ(k)). f, W L -edge XANES spectra of Si-doped WO . The blue
and brown isosurfaces represent charge accumulation and depletion, respectively, in the space. E , Fermi energy.
3
3
3
3
F
increase the conduction and electron transport capability. More to W 5d unoccupied states with multiple excitations for hybridized
computational details are shown in Supplementary Table 3 and W 5d–O 2p conduction band states. The W L -edge spectra are
3
Supplementary Note 5.
consistent with the ε-WO electronic structure based on crystal-
3
The local electronic and atomic structure of Si-doped ε-WO3 field 10 D of the O symmetries. It exhibits two typical areas, A
q
h
1
were investigated by X-ray absorption near-edge spectroscopy and A which correspond to the splitting of the W 5d orbital into
2
,
(
XANES) and extended X-ray absorption fine structure (EXAFS). the t and e , respectively, degenerate states according to the crys-
2
g
g
43
W L -edge EXAFS spectra (Fig. 5e) were obtained via a Fourier tal field effect . By contrast, the decomposed A (t ) and A (e )
3
1
2g
2
g
transform treatment of the raw data (Supplementary Fig. 32). feature for Si-doped ε-WO obtained from subtracting the arctan-
3
They show that Si-doped ε-WO exhibits shifted peaks with gent (step function A ) curve from the best fitted Gaussian curves
3
3
higher intensities than those of pure WO , which demonstrates (dashed green line in Fig. 5f) is different for pure WO . According
3
3
4
+
different local atomic arrangements (Supplementary Table 4 and to O crystal field theory, the introduction of Si changes the t –e
h
2g
g
Supplementary Note 6). The prominent resonance of XANES splitting energy levels and DOS t and e orbitals in the W 5d,
2
g
g
near the absorption edge in the energy region of 10,195–10,225 eV which demonstrates a distortion of O symmetry due to the trans-
h
(
Fig. 5f) is attributed to the excited electron transfer from W 2p
formation from the γ-WO to the ε-WO phase by Si doping.
3
/2
3
3
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NATure MATeriAlS
Articles
–
Ox
Potential barrier
a
b
c
e
–
E
c
+
e
–
MC-WO -NWA coating
3
In air
WO3
Au electrode
Heating wire
E
F
e–
2
H O + CO
2
Ev
Potential barrier
e–
Electronic
depletion layer
e–
–
e
Acetone
–
E
c
e
WO3
E
F
Pt wire
In acetone
E
v
d
e
f
2
2
1
1
50
500
400
5
4
3
2
1
0
00
50
00
3
2
1
00
00
00
0
50
200
250
300
350
0
200 400 600 800 1,000 1,200 1,400
Time (s)
0
100
200
300
400
Working temperature (°C)
Acetone concentration (ppm)
g
h
i
2
2
1
1
50
00
50
00
4
3
2
1
0
4
3
2
1
0
50 ppm
CH COCH
3
3
)
g
R
/
a
R
5 s
12 s
R e s p o n s e
(
5
0
0
C H OH
H S
2
2
5
NH3
CH OH
CO
3
HCHO
8
00
900
1,000
200 400 600 800 1,000 1,200 1,400
Time (s)
Gases (50 ppm)
Time (s)
Fig. 6 | Gas-sensing performances of the MC-WO -NWAs. a, Sketch of the structure of a side-heated gas sensor on MC-WO -NWAs. b, Diagram of the
3
3
interaction between acetone molecules and Si-doped ε-WO nanowires. c, Schematic diagram of the mechanism for acetone-gas sensing. d, Response
3
of the sensor to 50ꢀppm acetone at different temperatures. E , conduction band edge. E , valence band edge. e, Response–recovery curve of the sensor to
c
v
acetone at different concentrations (1.0–400ꢀppm) at 300ꢀ°C. f, Response (Sꢀ=ꢀR /R ) of the sensor versus acetone concentrations. g, Response–recovery
a
g
curve of the sensor to 50 ppm of acetone at 300ꢀ°C. h, Responses of the sensor to different 50ꢀppm gases at 300ꢀ°C (for example, Sꢀ=ꢀ16 for 50ꢀppm
ethanol and Sꢀ=ꢀ21 for 50ꢀppm H S). i, Repeating response–recovery curve of the sensor to 50ꢀppm acetone.
2
Gas-sensing performances
a sensing signal of decreased resistance (Fig. 6b,c). In general, the
The sensing performance of the MC-WO -NWAs was tested on a activity of the sensing materials is strongly affected by working tem-
3
platform based on a side-heated type of gas sensor (Fig. 6a). The peratures and even light illumination (Supplementary Figs. 33–35
44,45
sensitivity is defined as S=R /R for reducing gases (where R and and Supplementary Note 7) . To optimize the working tempera-
a
g
a
R are the resistance of the sensor in air and tested gas, respectively) ture, the WO -based sensor was tested towards 50ppm of acetone
g
3
or S=R /R for oxidizing gases, and the response of the sensor to at 200–350°C (Fig. 6d). The sensing response first rises and then
g
a
the target gas can be calculated by detecting the changes in resis- decreases with the increase of the testing temperature, and the sen-
tance (Supplementary Fig. 33). WO is a typical n-type semicon- sitivity reaches its maximum at 300°C. Thus, the working tempera-
3
ductor, in which electrons behave as carriers. The oxygen molecules ture was selected as 300°C for subsequent tests.
–
−
adsorbed on WO could be activated as O species, such as O ,
The gas sensors show a fast response to various concentrations
3
x
2
−
2−
O and O , by extracting electrons from the conduction band of acetone from 1.0 to 400ppm (Fig. 6e) due to the highly open
of WO , which causes an increased resistance due to the forma- structure, abundant active sites and enhanced electron transport,
3
tion of an electron depletion layer in the surface region of WO . and the resistance of the sensors can almost recover to their initial
3
When the sensor is exposed to reducing gases such as acetone, value, which indicates a good reversibility. Moreover, the response
acetone molecules can react with the adsorbed oxygen species values (defined as S=R /R ) continuously increase from 4.25 to 474
a
g
–
–
(
acetone+O =CO +H O+e ), which releases the trapped elec- (Fig. 6f). The sensor in 50ppm acetone displayed a response and
x
2
2
trons back to the conduction band. As a result, the sensor outputs recovery time of 5 and 12seconds, respectively (Fig. 6g), indicative of
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Articles
NATure MATeriAlS
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WO nanomaterials, our sensor shows an aggregate good sensing
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performance (Supplementary Table 5). Several works describe min-
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2
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Articles
NATure MATeriAlS
–1
tolerance of force on each atom during the structure relaxation was set at 0.02eVÅ .
The self-consistent field iterations were considered converged when the change of
Methods
3
Synthesis of multilayer-crossed nanowire arrays of Si-doped WO . A typical
−5
total energy was smaller than 10 eV. The room-temperature monoclinic structure
synthesis was carried out as follows: 0.10g of the amphiphilic diblock copolymer
–
1
for the pristine γ-WO model with a typical space group P21/n was constructed
PEO114-b-PS156 (M
n
=21,500gmol , polydispersity index=1.06) designed through
3
4
0
based on the structure of an octahedron and the optimized unit cell parameters
a classical atom-transfer radical polymerization strategy was dissolved in 4.0ml
of THF with a high-speed mixer to form a homogeneous solution. Almost
were a=7.294Å, b=7.418Å, c=7.591Å and β=91.5°, and the optimized unit
cell consisted of 8W and 24O atoms. The formation energy (E ) of the doping
simultaneously, 0.30g of H SiW (analytical reagent grade (AR), Aladdin) was also
form
4
system was calculated as Eform = EWO – EWO –Si +μSi, where EWO –Si is the total
dissolved in 1.0ml of THF to form the precursor solution. Aꢃer all the solid-phase
components had been dissolved, they were mixed to produce a light-blue colloidal
solution (the mass ratio of PEO-b-PS/POMs was 1:3). Aꢃer 1.0h, this solution
was cast onto glass Petri dishes to evaporate THF slowly at room temperature for
3
3
3
energy of the WO –Si system, EWO is the total energy of pure WO and μSi is the
3 3
3
chemical potential of Si. The results are listed in Supplementary Table 4.
XAFS measurements and analysis. To explore the local structural and compositional
1
2h (air humidity 20–60%), followed by a sequential thermal treatment at 50°C
and 100°C for 24h each. ꢁe target Si-doped WO ꢀlm was generated through a
gradient calcination treatment at 500°C for 1h in N
environment of the absorbing atom, the W LIII-edge XAFS spectra for pure WO
3
3
–
1
3
and Si-doped ε-WO were obtained from the Shanghai Synchrotron Radiation
2
(heating rate, 1°Cmin below
–1
50°C and 5°Cmin– above 350°C) and then at 400°C for 20min in air (5°Cmin ).
1
Facility. During the data-collection process, all the data were recorded with a typical
fluorescence mode based on a high-purity Ge solid-state detector, and all the signals
3
given are average values of triplicates. To obtain structural information, all the spectra
Synthesis of multilayer-crossed nanowire arrays of P-doped WO
3
. MC-P-
55,56
were collected through a classical fitting via the common IFEFFIT software
.
WO -NWAs were synthesized by following the same method as that for the
3
In addition, the theoretical EXAFS amplitudes, including phase functions for W–O
multilayer-crossed nanowire arrays of Si-doped WO
3
except that commercial
43,57
single-scattering paths, were generated based on FEFF 7.0 . The main fitted
phosphotungstic acid hydrate (H
used as the inorganic precursor.
3
PW12O40·21H O, AR, Sigma-Aldrich) was
2
parameters, such as interatomic distance, coordination number and Debye–Waller
43
factor were also confirmed with reasonable guesses and were fitted in R-space .
Synthesis of multilayer-crossed nanowire arrays of Si-doped MoO and P-doped
3
Data availability
MoO
by following a similar method except that commercial silicomolybdic acid
hydrate (H O, AR, Aladdin) and phosphomolybdic acid hydrate
O, AR, Aladdin) were used as inorganic precursors, respectively.
The mass ratio of PEO-b-PS/POMs was 1:2. The thermal treatment procedure was
3 3 3
. The MC-Si-MoO -NWAs and MC-P-MoO -NWAs were synthesized
The datasets generated and/or analysed during the current study are available from
the corresponding author on reasonable request.
4
O
SiMo12
O40·30H
2
(
H
3
PMo12
40·30H
2
References
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calcination at 350°C for 2h in a N
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obtained (Supplementary Fig. 22).
1
Characterizations and measurements. The scanning electron microscopy images
were collected with a Zeiss Ultra 55 FESEM recorded at 3kV. The dried samples
were directly used for observation without any treatment. The TEM images were
2
recorded with a JEM-2100 F (JEOL) microscope. The N -adsorption isotherms were
obtained at 77K with a Micromeritics Tristar 3020 analyser. To remove impurities
(
such as adsorbed H O), all the target materials were degassed in vacuum conditions
2
(
180°C, 6h) and, in addition, the specific surface area was recorded with a Brunauer–
Emmett–Teller technique and the pore size distribution was measured via a Barrett–
Joyner–Halenda strategy. Fourier transform infrared spectra of all the samples
were obtained with classical KBr pellets with a Nicolet Fourier spectrophotometer.
Thermogravimetric analysis data were collected from 25 to 800°C in air (SDTQ600
–1
analyser, heating rate 10°Cmin ). The in situ TEM tests were carried out in a
differentially pumped Hitachi H-9500 ETEM operating at 300keV with a home-
46
made microelectromechanical-system-based ultrastable heating holder . The ETEM
−4
57. Rehr, J. J., Albers, R. C. & Zabinsky, S. I. High-order multiple-scattering
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−4
–1
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3
397–3400 (1992).
Gas-sensing tests. The gas sensing performances of all the samples were evaluated
on a typical side-heated type of gas sensor by a sensing system (HW-30A, Hanwei
Electronics Co. Ltd). To quantitatively evaluate the material properties, the
response value is considered as S was used. Moreover, the response time and recovery
time are the times to achieve a 90% saturation value after the import and/or release
Acknowledgements
This work was financially supported by the National Natural Science Foundation of
China (Grant nos 51422202, 21673048 and 21875044), Key Basic Research Program
of Science and Technology Commission of Shanghai Municipality (17JC1400100) and
Youth Top-notch Talent Support Program of China. The authors thank G. Zhou and
Z. Shan for assistance in TEM characterization.
3
of the target gas within the step function. For all the tests, the MC-WO -NWA
powder was dispersed into terpineol to produce a uniform paste. The as-prepared
slurry samples were deposited on an alumina tube for 2h of annealing at 100°C
and further solidifying at 300°C for 2h. The coating on each single ceramic tube
Author contributions
contained 5.0± 0.5mg WO
3
nanowire materials. The distance between the two
Y.R., Y.Z. and Y.D. conceived the project and designed the experiments. Y.R., Y.Z., X.Z.,
J.M., D.Z., G.W. and Y.D. were primarily responsible for the data collection and analysis.
Y.L. and Y.A. analysed the structures with DFT calculations. S.X. and Y.Z. analysed the
EXAFS data. Y.R., Y.Z. and Y.D. prepared the figures and wrote the main manuscript text.
All the authors contributed to the discussions and manuscript preparation.
Au electrodes was 2.0mm on average. The thickness of the coating was 500μm on
average. The electric resistance of the sensor device under ambient atmosphere was
about 11± 3MΩ at the optimum working temperature (300°C) and an illustration
of the electric circuit of a sample is shown in Supplementary Fig. 33.
DFT theoretical calculations. DFT calculations were used to evaluate the
Competing interests
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structural relaxation and electronic properties of MC-WO -NWAs, and our
The authors declare no competing interests.
calculations on the WO
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complex were performed using the plane-wave basis
47,48
Vienna Ab-initio Simulation Package code and the projector augmented
Additional information
4
9–51
wave method used to describe the electron–ion interaction . The exchange-
Supplementary information is available for this paper at https://doi.org/10.1038/
s41563-019-0542-x.
5
2,53
correlation functional of the generalized gradient approximation was applied
to describe the exchange correlation potential. For geometry optimization, the
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4
Correspondence and requests for materials should be addressed to Y.D.
Brillouin zone was sampled with a Monkhorst–Pack mesh of 5×5×2 k-points.
The cutoff energies for the plane waves were set to be 400eV, and the convergence
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