Dynamic Coassembly of Amphiphilic Block Copolymer and Polyoxometalates in Dual Solvent Systems: An Efficient Approach to Heteroatom-Doped Semiconductor Metal Oxides with Controllable Nanostructures.

Dynamic coassembly of block copolymers (BCPs) with Keggin-type polyoxometalates (POMs) is developed to synthesize heteroatom-doped tungsten oxide with controllable nanostructures, including hollow hemispheres, nanoparticles, and nanowires. The versatile coassembly in dual n-hexane/THF solvent solution enables the fomation of poly(ethylene oxide)-b-polystyrene (PEO-b-PS)/POMs (e.g., silicotungstic acid, H4SiW12O40) nanocomposites with different morphologies such as spherical vesicles, inverse spherical micelles, and inverse cylindrical micelles, which can be readily converted into diverse nanostructured metal oxides with high surface area and unique properties via in situ thermal-induced structural evolution. For example, uniform silicon-doped WO3 (Si-WO3) hollow hemispheres derived from coassembly of PEO-b-PS with H4SiW12O40 were utilized to fabricate gas sensing devices which exhibit superior gas sensing performance toward acetone, thanks to the selective gas-solid interface catalytic reaction that induces resistance changes of the devices due to the high specific surface areas, abundant oxygen vacancies, and the Si-doping induced metastable ε-phase of WO3. Furthermore, density functional theory (DFT) calculation reveals the mechanism about the high sensitivity and selectivity of the gas sensors. On the basis of the as-fabricated devices, an integrated gas sensor module was constructed, which is capable of real-time monitoring the environmental acetone concentration and displaying relevant sensing results on a smart phone via Bluetooth communication.

Introduction

Amphiphilic block copolymers (BCPs) can self-assemble into a range of ordered mesostructures driven by the opposite long-range repulsive and short-range attractive forces because the constituent blocks are chemically incompatible but connected by covalent bonds.[1−6] The self-assembled BCPs with specific compositions can be converted into ordered mesoporous materials (polymer, carbon, silica, etc.) by selectively removing some blocks via calcination or solvent extraction.[7−12] This process can be viewed as self-templating synthesis. The coassembly of amphiphilic block copolymers (also serving as sacrificial templates) and inorganic precursors (e.g., metal alkoxides or salts, inorganic nanocrystals) with organic surfactants as sacrificial templates has proved to be an efficient route to produce ordered mesoporous materials with different compositions, high specific surface area, interconnected uniform pores, and rich active sites in the pore wall.[13−26] Such a process is the well-known soft-templating synthesis. To endow ordered mesoporous materials with improved performance, great efforts have been made to introduce functional heteroatoms, nanoclusters, or even nanoparticles into the pore wall mostly via postmodification approaches; however, the post modification method usually leads to uncontrolled distribution of the guest species and pore blocking. To solve these problems, nanosized metal–oxygen clusters with intrinsic multiple compositions such as polyoxometalates (POMs) have recently been employed as precursor to synthesize mesostructures with different morphologies and heteroatom-doped frameworks without using other inorganic sources.[27−31] For example, nanostructured inverse hexagonal polyoxometalate composite films were cast directly from solution using diblock copolymers poly(butadiene-block-2-(dimethylamino)ethyl methacrylate) (PB-b-PDMAEMA) as structure directing agents. Phosphomolybdic acid (H3[PMo12O40], H3PMo) as an inorganic source was selectively incorporated into the PDMAEMA domains because of electrostatic interactions between PMo3– anions and protonated PDMAEMA.[27] It is worth noting that polyoxometalates consist of transition metal elements (e.g., V, Mo, W) and nonmetallic element (e.g., Si, P); therefore, adopting POMs as the inorganic metal source to coassemble with amphiphilic block copolymers is particularly beneficial to the construction of multicomponent nanostructured metal oxides, carbides, and even nitrides.[32,33] However, until now, little work has been done to the designed synthesis of nanostructured metal oxides with controllable morphology and chemical composition through coassembly of amphiphilic block copolymer with polyoxometalates. Our group recently succeeded in constructing 3D orthogonally cross-stacked metal oxide semiconducting nanowire arrays through the interfacial electrostatic coassembly of BCPs and POMs and micelle fusion and packing during solvent evaporation.[34,35] However, previous studies were limited to the electrostatic interactions between block copolymers and polyoxometalates and rarely studied the phase transition behavior of block copolymers under the influence of solvents, so the structures obtained were monotonous (e.g., nanowires). The dynamic assembly of amphiphilic BCPs and POM clusters and corresponding derived controllable nanostructures have been rarely explored.

Herein, a novel dynamic interfacial coassembly method was developed to construct amphiphilic block copolymers/polyoxometalates nanocomposites in a mixed-solvent system consisting of tetrahydrofuran (THF) and n-hexane. Through tuning the volume ratio of THF/n-hexane, poly(ethylene oxide)-b-polystyrene (PEO-b-PS)/silicotungstic acid (H4SiW12O40) nanocomposites with diverse morphologies including spherical vesicles, inverse spherical micelles, and inverse cylindrical micelles could be obtained. Correspondingly, after thermal treatments, Si-doped WO3 nanostructures with different morphologies including nanobowls, nanoparticles, and nanowires were obtained. This general and flexible method could be extended to synthesize other nanostructured metal oxides (P-WO3, Si-MoO3, and P-MoO3) by coassembly of PEO-b-PS with other polyoxometalates (e.g., H3PW12O40, H4SiMo12O40, and H3PMo12O40) following a similar process. As an example, the Si-WO3 hollow hemispheres were used as semiconducting sensing materials to fabricate a gas sensor and showed excellent sensing performance to acetone, including high sensitivity (Rair/Rgas = 37 vs 50 ppm), low limit of detection (<0.1 ppm), fast response speed (7 s), and good selectivity (Sacetone/Sgas > 5), due to its high specific surface areas, highly crystalline framework, and abundant oxygen deficiency caused by homogeneous in situ Si-doping. Moreover, the gas sensing mechanism was systematically studied by in situ FTIR spectroscopy and theoretical DFT calculations. Furthermore, an integrated gas sensor module was fabricated, and real-time monitoring of acetone concentrations on a smart phone via Bluetooth communication was realized, which shows great potential in noninvasive early diagnosis of diabetes via detecting acetone in human exhaled breath.

Results and Discussion

Synthesis and Characterization of the Si-WO3 Nanobowls

In this study, a mixed solvent consisting of miscible THF and n-hexane was employed to enable the coassembly of PEO-b-PS copolymers and polyoxometalates into various nanostructures by simply tuning the volume ratio of n-hexane/THF. Taking the assembly of PEO-b-PS copolymers and silicotungstic acid (H4SiW12O40, H4SiW) as a sample (Scheme ), in THF solution without n-hexane, amphiphilic PEO-b-PS molecules interact with SiW12O404– to form core–shell spherical micelles with PS as the core and PEO/SiW12O404– as the shell, due to strong electrostatic attractions between the protonated PEO blocks (PEO-H+) and the SiW12O404– anions (Scheme a). When a certain amount of n-hexane, a poor solvent for SiW12O404–, was added, the spherical micelles gradually transformed into spherical vesicles, inverse spherical micelles, and inverse cylindrical micelles, in sequence upon the increase of n-hexane due to the decreased solubility of POMs in the mixed solvent, and when these colloidal solutions containing PEO-b-PS/H4SiW12O40 micelles were cast onto the glass substrate, an organic–inorganic nanocomposite film could be obtained. The subsequent annealing treatments result in nanobowls, nanoparticles, and nanowires of Si-WO3, respectively, due to decomposition of PEO-b-PS and pyrolysis of H4SiW12O40 into Si-doped WO3 (Scheme ).

Scheme 1

Coassembly of PEO-b-PS with H4SiW12O40 and Formation of PEO-b-PS/H4SiW12O40 Spherical Micelles, Spherical Vesicles, Inverse Spherical Micelles, and Inverse Cylindrical Micelles under Different Volume Ratio of n-Hexane/THF (V= V/VTHF), and the Corresponding Derived Si-WO3 Nanostructures after Thermal Treatments

Upon mixing PEO-b-PS and H4SiW12O40 hydrate, both predissolved in THF, a transparent light blue colloidal solution with a distinct Tyndall effect was formed immediately (Figure S2), indicating that PEO-b-PS/H4SiW12O40 hybrid micelles were formed directly. A cryogenic transmission electron microscopy (Cryo-TEM) image of the colloidal solution reveals the formation of core–shell spherical micelles with a gray PS chain as the core and PEO/SiW12O404– as the shell due to the dramatic mass contrast between the tungsten species and PS segments, and the average size of the spherical micelles was estimated to be about 35 nm (Figure a). Interestingly, when adding n-hexane into the colloid solution and the volume ratio of n-hexane/THF reaches 0.75, spherical vesicles with a diameter of about 80 ± 20 nm were obtained, with gray PS as the inner layer, dark PEO/H4SiW12O40 as the intermediate layer, and gray PS as the shell, as revealed by the TEM images (Figure b). With further increase of the volume ratio of n-hexane/THF from 0.75 to 1.0 and 1.25, the spherical vesicles gradually transformed into inverse spherical micelles (Figure c) and inverse cylindrical micelles (Figure d) respectively.

Figure 1

(a) Cryo-TEM and (b–d) TEM images and the corresponding structural models (insets) of the PEO-b-PS/H4SiW12O40 nanocomposites. (a) Spherical micelles (V/VTHF = 0), (b) spherical vesicles (V/VTHF = 0.75), (c) inverse spherical micelles (V/VTHF = 1.0), and (d) inverse cylindrical micelles (V/VTHF = 1.25).

Such a morphology and structure evolution are mainly due to the decreasing solubility of the mixed solvents for PEO/SiW12O404–. First, in the THF solution, driven by the microphase separation, spherical micelles with PS as the core and PEO/H4SiW12O40 as the shell can be formed to reduce the interface energy, due to the strong electrostatic Coulomb force (S+I–) between the protonated PEO block (PEO-H+) of PEO-b-PS surfactants and the inorganic POM anion (SiW12O404–). With the addition of n-hexane, PS phase was swelled by n-hexane, while the PEO-H4SiW12O40 phase tends to shrink inward because n-hexane is a precipitating agent for PEO-H4SiW12O40. As a result, spherical vesicles with PS as the inner layer, PEO/H4SiW12O40 as the intermediate layer, and PS as the outer layer were formed. As more n-hexane was introduced in the assembly solution, the hydrophilic PEO-H4SiW12O40 phase shrinks further inward, and reverse spherical (or cylindrical) micelles with PEO/H4SiW12O40 as the core and PS as the shell were formed (Scheme ).

Scheme 2

Illustration of the Transformation of the PEO-b-PS/H4SiW12O40 Hybrid Micelles/Vesicles under Different n-Hexane/THF Ratio

The controllable formation of reverse spherical and cylindrical micelles was realized by tuning the BCPs/POMs mass ratio (Figure S1). The structures of self-assembled amphiphilic block copolymers are determined by the packing parameter: p = v/a0l,[36−39] where v and l represent the volume and length of the hydrophilic segment, respectively, while a0 denotes the contact area of the hydrophobic block. Generally, in the case of p < 1/3, spherical micelles or spheres with body-centered cubic (bcc) and face-centered cubic (fcc) packing modes can be formed. In the case of 1/3 < p < 1/2, cylindrical micelles or hexagonally packed cylinders (Hex) can be formed. In this study, when the weight ratio of BCPs/POMs reaches 1/3, the p value is less than 1/3, and spherical micelles were formed; and when the weight ratio of BCPs/POMs is 1/4, p value ranges between 1/3 and 1/2, namely, 1/3 < p < 1/2, and cylindrical micelles were formed. The structures of the above four kinds of micelles and vesicles were also be confirmed by dynamic light scattering (DLS) characterizations (Figure S2, S3).

An in situ X-ray photoelectron spectroscopy (XPS) technique was employed to confirm the three-layered core-interlayer-shell structures of PEO-b-PS/H4SiW12O40 spherical vesicles. The PEO-b-PS/H4SiW12O40 nanocomposite film composed of spherical vesicles was fabricated on a silicon wafer (2 cm × 2 cm), and the element (Si, W, O, C) content was measured by XPS before and after the surface was sputtered using the Ar-GCIS beam, during which about 5–10 nm of surface coating would be stripped off (Figure ). For the composite film composed of PEO-b-PS/H4SiW12O40 spherical vesicles, before Ar+ sputtering, the element content measured by XPS was Si = 0.83 at %, W = 4.70 at %, O = 15.80 at %, and C = 78.67 at %, respectively, indicating that the surface of hybrid film was mainly composed of organic copolymers. After Ar+ sputtering treatment for 180 s, the Si, W, and O content was increased to 1.97 at %, 17.17 at %, and 61.63 at %, respectively (Figure e). Correspondingly, the C content decreased to 19.55 at %. Extended delocalized electrons in aromatic rings can always result in satellite peaks, and their binding energy is several eV higher than the main sp3 C 1s peak. The π–π* satellite peak could be seen at ∼291.6 eV before etching but vanished after etching; it demonstrates that the PS phase dominates in the composite film surface. The above results indicate that the PEO/SiW12O404– layer was exposed to the surface after Ar+ sputtering, during which the PS domain in the surface of the composite film was etched away (Figure f). The combined results of TEM characterization and XPS analysis unambiguously confirmed the spherical vesicle structure with PEO/SiW12O404– acting as the wall and PS chains spreading outward and inward, respectively. In contrast, no π–π* satellite peak in the C 1s peak was observed for the cylindrical micelles (transformed from spherical micelles) before Ar+ sputtering (Figure S5a), and the W4+ species were found after Ar+ sputtering (Figure S4), indicating a different core–shell structure (PS as the core and PEO/SiW12O404– as the shell).

Figure 2

X-ray photoelectron spectroscopy (XPS) of PEO-b-PS/H4SiW12O40 spherical vesicles (V/VTHF = 0.75) supported on silicon wafer before and after Ar+ sputtering: (a) Si 2p, (b) W 4f, (c) O 1s, and (d) C 1s. (e) Element contents measured by XPS and (f) the corresponding structural models of the PEO-b-PS/H4SiW12O40 spherical vesicles before and after Ar+ sputtering.

Interestingly, PEO-b-PS/H4SiW12O40 spherical vesicles nanocomposite film deposited on the substrate underwent an unconventional thermal-induced structural transformation into Si-WO3 hollow hemispheres (nanobowls). In other words, the H4SiW12O40 species on the top of the spherical vesicles exposed larger surface areas and possess higher energy, which is relatively unstable. During the annealing process to decompose PEO-b-PS copolymers and pyrolyze H4SiW12O40 into Si-WO3, the H4SiW12O40 species would migrate to the contacting regions of neighboring vesicles and spaces between the vesicles and the substrate, thus forming Si-WO3 nanostructures of hollow hemispheres (Figure a). After casting the colloidal solution containing PEO-b-PS/H4SiW12O40 spherical vesicles onto the glass substrate, an inorganic–organic nanocomposite film was obtained after the solvent evaporates completely (Figure b, c). The subsequent thermal treatment at 500 °C in the N2 atmosphere and 400 °C in air can remove the organic template and decompose H4SiW12O40, forming unique nanobowl-like Si-doped WO3 nanoparticles. FESEM images reveal the Si-doped WO3 nanobowl-like particles are hollow hemispheres (HHSs) structures of Si-WO3 with an inner diameter of about 40 nm, external diameter of about 62 nm, and thickness of about 11 nm (Figure d, e). TEM characterizations at low magnification and from different directions confirmed unusual “nanobowls” morphology of Si-WO3, and the selected area electron diffraction (SAED) patterns with spotty rings indicate a polycrystalline property of the framework (Figure f, S6). A high-resolution TEM (HRTEM) image showed the lattice spacing of 0.383 nm, corresponding to the (001) plane of orthorhombic WO3 (JCPDS. 20-1324) (Figure g). Energy dispersive spectra (EDS) show a homogeneous distribution of the W, O, Si element throughout the Si-WO3 HHSs sample (Figure S7). The abnormally high Si content is due to the overlap of the peaks of W and Si elements. In this case, it is difficult to evidence Si at low concentrations by EDS when the W is present at high concentration because all energies of the SiK and WM are very close.[44−47]

Figure 3

(a) Schematic illustration of the thermal-induced structural transformation of PEO-b-PS/H4SiW12O40 spherical vesicles into Si-WO3 nanobowls, (b, c) FESEM images of PEO-b-PS/H4SiW12O40 vesicles (V/VTHF = 0.75) supported on silicon wafer, (d, e) FESEM, (f) TEM and SAED pattern (inset), and (g) HRTEM images of the Si-WO3 nanobowls. The insets in (c) and (e) are the corresponding structural models. The inset in (d) is diameter distribution of the Si-WO3 nanobowls.

In order to investigate the structural transformation process, the nanocomposite film composed of PEO-b-PS/POMs spherical vesicles was, respectively, treated at 500 °C for 0.5, 1.0, and 2.0 h (Figure S8). Before annealing, they possess spherical morphology (Figure a, b). After annealing at 500 °C for 0.5 h, due to the gradual decomposition of block copolymers, H4SiW12O40 species anchored at the micelles’ surface can gradually migrate from top of the vesicles to the contacting region between vesicles and substrate because they exposed a higher surface area and possess a high surface energy. At the same time, they were decomposed into Si-doped WO3, and nanobowls with thin walls and small openings were obtained. After annealing at 500 °C for 1 h, the tungsten species further migrate from the top to the contact region between the nanobowls and the substrate, resulting in nanobowls with thicker walls and larger openings; prolonging the annealing time to 2 h caused the nanobowls to become more flat and thicker due to the further migration of the tungsten species. The migration of tungsten species and the morphology evolution indicate that it is a thermal-induced structural transformation process.

It is worth mentioning that the morphology of the Si-WO3 nanobowls could be well controlled by fine-tuning the volume ratio of n-hexane/THF (V/VTHF) (Figure S9). The size of inner and outer diameters and wall thickness of Si-WO3 nanobowls are all controllable by tailoring V/V from 0.25 to 0.90. As increasing the V/VTHF value, the volume of the inner cavity becomes smaller and the wall becomes thicker. The morphological changes of Si-WO3 nanobowls originate from the inward contraction of the PEO/H4SiW12O40 domain of the PEO-b-PS/H4SiW12O40 spherical vesicles because of the decrease of solubility for H4SiW12O40 in n-hexane/THF. Finally, when V/VTHF reaches 1.0, the inner PS chains of the PEO-b-PS/H4SiW12O40 spherical vesicles completely disappeared, and the spherical vesicles evolved into inverse spherical micelles completely with PEO/H4SiW12O40 as the core and PS chain as the shell. Correspondingly, uniform solid Si-WO3 nanoparticles were obtained (Figures S12, S13).

The X-ray diffraction (XRD) patterns of the PEO-b-PS/H4SiW12O40 nanocomposites and Si-WO3 hollow hemispheres are shown in Figure a; after pyrolysis in 500 °C-N2 and 400 °C-air, the H4SiW12O40 are completely transformed into Si-doped tungsten oxides, and diffraction peaks ranging from 0 to 70° matched well with the orthorhombic ε-WO3 phase (JCPDS no. 20-1324). The sharp diffraction peaks indicate a highly crystalline framework of Si-WO3 HHSs, and the average crystallite size is calculated to be 10.4 nm according to the Scherrer equation (D = Kλ/B cos θ). The Fourier transform infrared (FTIR) characterizations also revealed the conversion of H4SiW12O40 into Si-WO3 (Figure S10). Before calcination, typical absorption peaks at 2922 and 3026 cm–1 can be clearly visible in the as-made PEO-b-PS/H4SiW12O40 composite, which was attributed to the —C–H and =C–H groups, respectively, and the absorption peaks at 788, 926, and 980 cm–1 were atrributed to Keggin polyoxometallates. While after calcination at 500 °C in N2 and 400 °C in air, all these peaks disappear, and a new absorption peak at 808 cm–1 appears, implying the formation of WO3 and a complete removal of the PEO-b-PS copolymers. The nitrogen adsorption–desorption isotherms of the Si-WO3 HHSs show a typical IV-type hysteresis loop, indicating a well-defined mesostructure with large accessible pores, and the specific surface area and pore volume are calculated to be 42.2 m2/g and 0.077 cm3/g, respectively. The pore size distributions calculated by the BJH method centered at 30.5 nm (Figure S11). The large surface areas of Si-WO3 HHSs could provide a large number of active sites for guest molecules (e.g., gas molecules) and would be favorable for promoting chemical reaction rates in heterogeneous catalysis and gas sensing.

Figure 4

(a) XRD patterns of the commercial H4SiW12O40·15H2O, PEO-b-PS/H4SiW12O40 nanocomposites treated at 100 °C, and Si-WO3 hollow hemispheres obtained after thermal treatment at 500 °C in N2 and 400 °C in air. (b) Solid-state 29Si NMR spectrum of Si-WO3 hollow hemispheres; (c) UV–vis absorption spectra and the extracted band gap profile of the mesoporous WO3 obtained using WCl6 as the inorganic precursor and the Si-WO3 hollow hemispheres. X-ray photoelectron spectroscopy showing the (d) O 1s, (e) W 4f, and (f) Si 2p core level peak regions of Si-WO3 hollow hemispheres.

X-ray photoelectron spectroscopy (XPS) characterizations of the Si-WO3 HHSs revealed core level peaks of W 4f, O 1s, and Si 2p (Figure d–f), confirming the presence of W, Si, and O species. The peak-differentiating and imitating analysis indicated the presence of a small amount of W5+ (22.5%), and the O 1s peak also revealed abundant (27.5%) surface-adsorbed oxygen species (O–, O2–), indicating rich oxygen vacancies in the crystal lattice of WO3. The above results confirmed that the Si atoms are successfully doped into a crystal lattice of tungsten oxide, thus causing the enhancement of surface-adsorbed oxygen species according to the charge compensation mechanism. Furthermore, the Si-doping enables the W atoms to deviate from the center of the W–O octahedron, resulting in an ε-phase WO3 with a high dipole moment. The Si content calculated from XPS (6.2 at %) is much higher than the theoretical content from the stoichiometric ratio of H4SiW12O40 (2.0 at %) (Figure f), indicating that little silicon-containing species (SiO) were located on the surface of WO3 except for atomic doping into the crystal lattice.

Solid state 29Si MAS NMR spectra of Si-WO3 hollow hemispheres (Figure b) showed Q1, Q2, Q3, and Q4 peaks of the Si element. It indicates that except for the formation of amorphous SiO2 (Q4), there are abundant Si species embedded into the lattice of tungsten oxide with the form of Si4+ (Q1, Q2, Q3). Such a result coincides well with the XPS characterizations. The UV–vis absorption spectra and the WCl6-derived mesoporous WO3 and the H4SiW-derived Si-WO3 HHSs also showed different absorption curves within the range of 250–800 nm (Figure c), and the corresponding extracted band gap (Eg) of Si-WO3 (2.76 eV) is significantly higher than that of WO3 (2.50 eV). The relationship between carrier concentration (n) and band gap (Eg) follows the equation: n = (NcNv)∧(1/2) exp[−Eg/(2kT)]. Therefore, the higher band gap can result in lower carrier concentration. As a typical n-type semiconductor, the carrier concentration of Si-WO3 is dominated by number of electrons. In semiconductor gas sensing tests, according to the definition of sensitivity (Ra/Rg), it could be predicted that the higher carrier concentration formed in air would lead to a smaller variation in the density of the charge carrier by the gas sensing reaction, and lower carrier concentration would be more advantageous for achieving a high gas sensing response. As a result, when exposed to reducing gases, the sensor based on Si-WO3 may show higher response because of the higher variation of the charge carrier concentrations.

Synthesis and Characterization of the Si-WO3 Nanoparticles and Nanowires

Following a similar casting process, nanocomposite films consist of PEO-b-PS/H4SiW12O40 inverse spherical micelles and inverse cylindrical micelles could also be constructed on the substrate (Figure a, b). After annealing at 500 °C in N2 and 400 °C in air, a film composed of Si-WO3 nanoparticles and nanowires were obtained (Figure S13–S16). As shown in Figure c, e, the Si-WO3 nanoparticles possessed uniform spherical morphology with a narrow size distribution of about 27 nm, and the Si-WO3 nanowires exhibited irregular-curved nanowires with a high aspect ratio (about 15 nm in diameter and several microns in length) (Figure d, f). In accordance with electron diffraction patterns from TEM, both Si-WO3 nanoparticles and nanowires are highly crystalline. Furthermore, the film composed of Si-WO3 NPs and NWs are highly porous due to the stacking of the NPs and NWs (Figure S17), endowing them potential for applications such as heterogeneous catalysis, gas sensing, and photoelectric conversions by virtue of their large amount of adsorption sites and semiconducting properties.

Figure 5

FESEM images of the PEO-b-PS/H4SiW12O40: (a) inverse spherical micelles, (b) inverse cylindrical micelles; FESEM and TEM images of the Si-WO3 (c, e) nanoparticles and (d, f) nanowires. The upper right insets in (a–d) are the corresponding structural models, and the insets in (e, f) are the corresponding selected area electron diffraction (SAED) patterns. The insets at the bottom of panels (c, d) are diameter distributions of the Si-WO3 (c) nanoparticles and (d) nanowires.

Generality of the Coassembly Strategy

It is found that this dynamic coassembly in miscible n-hexane and THF mixed solvent can be used as a general and flexible approach to synthesize other nonmetal elements (including Si and P) doped transition metal oxides (WO3/MoO3) nanomaterials (Figure S18–S20). For example, following similar coassembly and the annealing process, P-WO3 nanobowls and nanoparticles, P-MoO3 nanobowls, and Si-MoO3 nanoparticles could be readily synthesized from the coassembly of PEO-b-PS with phosphotungstic acid (H3PW12O40), silicomolybdic acid (H4SiMo12O40), and phosphomolybdic acid (H3PMo12O40), respectively, in THF/n-hexane. Similar to synthesis of Si-WO3 nanomaterials, the hollow hemispheres and nanoparticles could be generated from constructing PEO-b-PS/POMs spherical vesicles and inverse spherical micelles, respectively. The Si-/P-doped metal oxides possessed uniform well-defined nanostructures, high specific surface areas, highly crystalline frameworks, and semiconducting properties (Figure S19, 20), and these doped metal oxide nanostructures hold great potential for applications in many applications such as heterogeneous catalysis, gas sensing and energy storage, and conversions.

Gas Sensing Performances of the Si-WO3 Nanobowls

The gas sensing performances of the Si-WO3 HHSs materials were tested on a side-heated type gas sensor by a dynamic gas distribution test system (Figure , S21). First, the sensor was tested for 50 ppm acetone at different temperatures (100–350 °C) to obtain the optimum working temperature (Figure b, S22). With an increase of the working temperature, the sensing response (S = Ra/Rg) first rose and then went down, and the response time kept decreasing. The sensor reached its maximum response value (S = 37) at 300 °C, with a short response time (t = 7 s). Therefore, in this study, the optimal working temperature of the sensor was found to be around 300 °C. As shown in Figure a, the resistance signal drops rapidly when acetone gas was injected into the test chamber, and it can be completely restored to its initial value when the sensor was exposed to air. The response of the sensor rose from 2.0 to 132.3 as the acetone concentration increases gradually from 1 to 500 ppm (Figure b). Moreover, the sensor was tested to the subppm level acetone, and the sensitivity was 1.09, 1.13, and 1.19 to 0.1, 0.2, and 0.5 ppm acetone, respectively. Obviously, the limit of detection (LOD) is lower than 0.1 ppm. Furthermore, to evaluate its stability, the sensor was tested to 0.1–50 ppm acetone for ten cycles (Figure d–i), and all the response–recovery curves were repeated well, indicating a good long-term stability. Thanks to the porous nanostructures and crystalline framework that facilitates diffusion and transport of gases and electron conduction, respectively, the gas sensing device showed fast response-recovery dynamics. For example, in 50 ppm acetone atmosphere, the response time was as short as 7 s (Figure S23). The selectivity is also an important parameter for a gas sensor; in this study, nine volatile interference gases, including ethanol, hydrogen sulfide, benzene, formaldehyde, carbon monoxide, ammonia, methanol, nitrogen dioxide, and methane were introduced, respectively, to study the sensing response of the sensor. As shown in Figure c, the sensitivity of the sensor to 50 ppm acetone reaches 37, while all the interference gases show sensitivity of less than 7.2, at least five times lower than that in acetone. The specific selectivity toward acetone could be explained as follows. The Si-doping in lattice of WO3 formed a large amount of oxygen defects, and created abundant oxygen species adsorbed on the surface of WO3 in air, which facilitates the catalytic oxidation of reducing gas molecules at the working temperature. On the other hand, the Si-doping leads to lattice distortion of WO3. It can cause the W atoms to deviate from the center of the W–O octahedrons, leading to a spontaneous polarization effect and enhancing its adsorption capacity for gas molecules with a high dipole moment (e.g., acetone: 2.88 D). The increased gas–solid interface adsorption favors the catalytic oxidation of acetone molecules, resulting in a high sensing response.[40−43] The high sensitivity toward acetone of the Si-WO3 sensor is very meaningful for applications of early diagnosis of diseases, as the acetone content in exhaled gas of type-I diabetes mellitus patients was significantly higher (>1.8 ppm) than that of healthy people.[41]

Figure 6

Gas sensing performances of Si-WO3 nanobowls-based sensor. (a) Response-recovery curves of the sensor to acetone of different concentrations (1–500 ppm) at 300 °C, (b) sensing responses of the Si-WO3 hollow hemispheres-based sensor to 1–500 ppm acetone at different temperatures, and (c) responses of the sensor to different gases of 50 ppm at 300 °C. Cycling tests to acetone with different concentrations: (d, g) 0.1–0.5 ppm, (e, h) 1–5 ppm, and (f, i) 10–50 ppm.

In order to clarify the effect of silicon doping on the gas sensing performance, the nondoped WO3 nanostructure was synthesized by using WCl6 as the tungsten source instead of silicotungstic acid, to coassemble with PEO-b-PS templates (Figure S24), and mesoporous nondoped WO3 was obtained and used to fabricate sensing devices toward acetone. As shown in Figure S25, the sensing response was 1.4, 2.3, 3.9, 7, 11.6, and 17.1 to 1, 2, 5, 10, 20, and 50 ppm acetone, respectively, at 300 °C. Compared to the Si-doped WO3 nanobowls (Ra/Rg = 37.0 @ 50 ppm), the mesoporous WO3 based sensor showed much lower sensitivity (Ra/Rg = 17.1) to acetone. These results further confirm that the interface interaction between guest gas molecules and host sensing materials is important to the sensitivity of gas sensors.

For comparison, we also fabricated semiconductor gas sensors based on Si-WO3, P-WO3, Si-MoO3, and P-MoO3 nanoparticles and investigated their gas sensing performances. As shown in Figures S26–29, the Si-WO3 sensor showed better acetone sensing performances than P-WO3 nanoparticles. The sensing response (S = Rair/Rgas) of the Si-WO3 nanoparticles to 50 ppm of acetone is S = 9.5, much higher than that of P-WO3 (S = 4.2). The in situ doping of Si atoms in the lattice of WO3 leads to more oxygen vacancies and surface-adsorbed oxygen species (O–, O2–), meanwhile it can increase the dipole moment of WO3. Therefore, the sensitivity of the gas sensor to acetone with a large dipole moment (D = 2.88) is dramatically improved. For P-WO3, P species are unstable and P2O5 would sublimate at high temperatures (>360 °C), thus resulting in less lattice defects and lower dipole moment, and the acetone sensing performance is not as good as Si-doped WO3, and its gas sensing behavior (e.g., selectivity) is close to the undoped WO3. Similarly, the Si-MoO3 showed higher sensitivity than P-MoO3 to reducing gases such as acetone (SSi-MoO3 = 2.8 and SP-MoO3 = 1.3) and ethanol (SSi-MoO3 = 3.8 and SP-MoO3 = 1.6).

Study on Gas Sensing Mechanism

To investigate the gas sensing mechanism, in situ Fourier transform infrared spectroscopy (in situ FTIR) was conducted to reveal the catalytic reaction pathways of acetone on the surface of Si-WO3 (Figure S30). The Si-WO3 sample and KBr were pressed together, heated to 300 °C from room temperature, then acetone gas was injected into the chamber, and the infrared spectrum was monitored in real time. From the results we can observe that with an increase of the working temperature, the absorption peaks around 1737 cm–1 disappeared gradually, which was attributed to the carbonyl (C=O) in acetone, indicating that that acetone decomposes gradually. Meanwhile, new absorption peaks at 2306 and 2383 cm–1 appeared gradually, which could be referred to as carbon dioxide (CO2). No other peaks were observed during the testing process. From the above results, it could be concluded that the reaction pathways during the gas sensing process could be described as follows:That is, acetone absorbed on the sensing materials was oxidized to carbon dioxide and water directly, and electrons flow from acetone to Si-WO3 materials, resulting in a resistance decrease of the sensing layer.

Theoretical DFT Calculations

In order to further study the gas sensing mechanism in depth, a theoretical calculation based on density functional theory (DFT) was conducted to explain the high sensitivity and selectivity toward acetone of the Si-WO3 based sensor (Figure S31). The Si-WO3 structure was initially built and optimized, serving as a dominant interface region interacting with acetone molecules. The adsorption configurations of the (020) plane on the Si-WO3 is optimized, and the adsorption energies (Eads) for acetone was calculated to be −2.16 eV, indicating a strong adsorption capacity to acetone molecules of Si-WO3 (Figure S31a). In comparison, the optimized structures for the other nine interfering gases absorbed on Si-WO3 were also built, and the corresponding adsorption energies were calculated (Figure S32). Among all the investigated gases, Eads for acetone was the most negative one (−2.16 eV). While for other gases, the adsorption energies are all higher than −1.0 eV (Figure S31i). It indicates that the (020) facets of Si-WO3 greatly benefit adsorption and subsequent catalysis of acetone compared with other gases. In addition, no obvious changes in molecular structures were observed after the adsorption of gas molecules onto the Si-WO3, and therefore, it can be proven that all the gases on the Si-WO3 are physically adsorbed. Meanwhile, the Eads for acetone absorbed on the undoped WO3 was calculated to be −1.09 eV (Figure S31c), much higher than that of Si-WO3, indicating that Si doping is beneficial to the adsorption of acetone. The above results are consistent with the experimental results, further confirming the ultrahigh selectivity to acetone of the Si-WO3. Further calculations reveal that the band gap is narrowed after acetone absorption (Figure S31g, h), and the density of states (DOS) of the Si-WO3 + acetone displays a new energy level in the conduction band (Figure S31e, f), due to the strong bonding adsorption of acetone on the Si-WO3 and electron transfer from acetone to Si-WO3, which resulted in a decrease in resistance. These results consistently demonstrate that the acetone adsorption changes electronic structure and surface energy level of the Si-WO3.

Charge density difference was further calculated to elucidate the accurate electronic transfer during the acetone sensing process. As shown in Figure S31b, the blue and yellow lobes represent the charge depletion and accumulation, respectively, due to adsorption and oxidation of acetone molecules. The Bader charge analysis presents that there is 0.74 e of charge transfer (Δq) from acetone to Si-WO3, while the Δq is 0.56 e for acetone absorbed on undoped WO3 (Figure S31d), obviously lower than that of Si-WO3. This result is in accordance with the DOS results and charge distribution in the 2D plane of Si-WO3 (Figure S31k). In conclusion, the gas sensor based on the Si-WO3 nanobowls shows ultrahigh selectivity and sensitivity toward acetone because of the Si-WO3 that provides abundant active sites for acetone adsorption and accelerates electronic transfer during the gas sensing process.

Construction of Gas Sensor Module

To further study the application possibility of the sensing devices, an advanced integrated gas sensor module based on the Si-WO3 sensing device was constructed for efficient real-time monitoring acetone concentration on a smart phone via Bluetooth communication. The module was composed of a Si-WO3 gas senor, a battery, a microcontroller unit, and a wireless data communication. As shown in Figure S33, the integrated sensor module was tested to 5, 10, 20, and 30 ppm acetone, respectively, and the sensor outputs a concentration signal of 4.5, 10.7, 20.5, and 28.8 ppm, respectively, with an error less than 10%. All the tests were repeated for three times, and the reproducibility was well maintained (Figure S34). The above results indicated that the sensor module is promising for application in real-time monitoring concentration of target gases.

Conclusions

In summary, a dynamic coassembly of PEO-b-PS and H4SiW12O40 in THF/n-hexane dual solvent system has been developed to construct a variety of nanostructured Si-WO3 (nanobowls, nanoparticles, and nanowires). The THF/n-hexane volume ratio of the solution was found to dominate the morphologies of the assembled hybrid composites due to the changes of solubility of block copolymers and their assembly behaviors. The as-formed spherical vesicles, inverse spherical micelles, and inverse cylindrical micelles of PEO-b-PS/H4SiW12O40 were thermally converted into the above nanostructured Si-WO3 with distinct features. This synthetic strategy can be extended to the coassembly of PEO-b-PS with other Keggin-type POMs (H3PW12O40, H4SiMo12O40, and H3PMo12O40) to synthesize different kinds of heteroatom-doped transition metal oxides (P-WO3, Si-MoO3, and P-MoO3) with diverse nanostructures. The Si-WO3 hollow hemispheres-based semiconductor gas sensor exhibits excellent acetone sensing performances with high sensitivity and selectivity, due to its high specific surface areas and abundant surface adsorbed oxygen species originated from in situ Si doping. The excellent gas sensing performance makes it promising to develop miniaturized and wearable semiconductor gas sensors. Furthermore, this universal and flexible BCPs-POMs coassembly strategy offers great opportunity to design of various inorganic nanomaterials (metal carbide and metal nitride, etc.) with novel topological structures, abundant chemical compositions, and unique physicochemical properties, which show huge potentials in applications of catalysis, sensing, and energy storage and conversion.

Experimental Section

Chemicals

H4SiW12O40·15H2O, AR, Aladdin; H3PW12O40·21H2O, AR, Sigma-Aldrich; H4SiMo12O40·30H2O, AR, Aladdin; and H3PMo12O40·30H2O, AR, Aladdin. All chemicals were used without further purification.

Synthesis of the Si-WO3 and P-WO3 Hollow Hemispheres, Nanoparticles, and Nanowires

In a typical synthesis, 0.05 g of the lab-made amphiphilic block copolymer PEO114-b-PS156 (Mn = 21500 g/mol, PDI = 1.06) synthesized according to our previous report[48] was dissolved in 3 mL THF, to form the homogeneous solution A. 0.15 g of H4SiW12O40·15H2O (or H3PW12O40·21H2O) was dissolved in 1 mL THF to form the precursor solution B. The solution A and B were mixed to form a light blue transparent colloidal solution with further stirring for 0.5 h, then 3 mL n-hexane was added dropwise to form a white colloidal solution, which was then poured into a Petri dish to evaporate the solvent at room temperature for 12 h, followed by sequential heating at 100 °C for 24 h. Finally, the as-cast film was calcined at 500 °C for 1 h in N2 atmosphere (heating rate: 1 °C/min from room temperature to 350 and 5 °C/min from 350 to 500 °C) and 400 °C for 30 min in air (heating rate: 5 °C/min), and the sample of Si-WO3 (or P-WO3) hollow hemispheres was obtained.

The synthetic process for Si-WO3 (or P-WO3) nanoparticles and nanowires was the same with that of the Si-WO3 (or P-WO3) hollow hemispheres, except that the volume of n-hexane was 4 and 5 mL, respectively, and for nanowires the mass ratio of BCPs/POMs is 1/4.

Synthesis of the Si-MoO3 and P-MoO3 Hollow Hemispheres and Nanoparticles

In a typical synthesis, 0.05 g of the lab-made amphiphilic block copolymer PEO114-b-PS156 (Mn = 21500 g/mol, PDI = 1.06) was dissolved in 3 mL THF to form the homogeneous solution A. 0.10 g of H4SiMo12O40·30H2O (or H3PMo12O40·30H2O) was dissolved in 1 mL THF to form the precursor solution B. The solution A, B and n-hexane (3 mL for hollow hemispheres, 4 mL for nanoparticles) were mixed to form a transparent colloidal solution with further stirring for 0.5 h, which was then poured into a Petri dish to evaporate the solvent at room temperature for 12 h, followed by sequential heating at 100 °C for 24 h. Finally, the as-cast film was calcined at 350 °C for 3 h in N2 atmosphere (heating rate: 1 °C/min) and at 350 °C for 2 h in air (5 °C/min), and the sample of Si-MoO3 (or P-MoO3) hollow hemispheres (or nanoparticles) was obtained.

Synthesis of mesoporous WO3

0.1 g of PEO114-b-PS156 was dissolved in 5 mL THF, forming homogeneous solution A. 0.3 g of WCl6 was dissolved in 1 mL EtOH and 0.5 mL acetylacetone to form the precursor solution B. The solution A, B was mixed to form a green transparent solution with further stirring for 0.5 h, which was then poured into a Petri dish to evaporate the solvent at room temperature for 12 h, followed by sequential heating at 100 °C for 24 h. Finally, the as-cast film was calcined at 500 °C for 1 h in N2 atmosphere (heating rate: 1 °C/min below 350 and 5 °C/min above 350 °C) and 400 °C for 1 h in air (heating rate: 5 °C/min), and mesoporous WO3 was obtained.

In Situ X-ray Photoelectron Spectroscopy Measurements

X-ray photoelectron spectroscopy measurements were carried out with an AXIS Supra by Kratos Analytical Inc. using monochromatized Al Ka radiation (hv = 1486.6 eV, 225 W) as an X-ray source with a base pressure of 10–9 Torr. Survey scan spectra were acquired using a pass energy of 160 eV and a 1 eV step size. Narrow region scans were acquired using a pass energy of 40 eV and a 0.1 eV step size. The hybrid lens mode was used in both cases. The analyzed area of all XPS spectra was 300 × 700 μm2. A charge neutralizer was used throughout as the samples were mounted such that they were electrically isolated from the sample bar. All spectra were calibrated by C 1s (284.8 eV). This surface was sputtered using the Ar-GCIS beam (n = 1000, 10 keV beam energy); the sputtering area is 3 × 3 mm2. The etch rate was ∼30 nm/min for poly(lactic-co-glycolic acid (PLGA, standard).