General Synthesis of Mixed Semiconducting Metal Oxide Hollow

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Functional Inorganic Materials and Devices

General Synthesis of Mixed Semiconducting Metal Oxide Hollow Spheres with Tunable Compositions for Low-Temperature Chemiresistive Sensing Gen Wang, Xinran Zhou, Jing Qin, Yan Liang, Bingxi Feng, Yonghui Deng, Yongxi Zhao, and jing wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08694 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

General Synthesis of Mixed Semiconducting Metal Oxide Hollow Spheres with Tunable Compositions for LowTemperature Chemiresistive Sensing Gen Wang,†# Xinran Zhou,‡# Jing Qin,† Yan Liang,§ Bingxi Feng† Yonghui Deng,*‡ Yongxi Zhao,† and Jing Wei*† †The

Key Laboratory of Biomedical Information Engineering of Ministry of Education, School of Life Science and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, P. R. China ‡Department

of Chemistry, State Key Laboratory of Molecular Engineering of Polymers, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, P. R. China §Department

of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia

KEYWORDS: mesoporous materials, hollow spheres, self-template synthesis, polyphenol, gas sensor ABSTRACT: Metal oxide hollow spheres (MOHSs) with multicomponent metal elements exhibit intriguing properties due to the synergistic effects of different components. However, it remains a great challenge to develop a general method to synthesize multicomponent MOHSs due to the different hydrolysis and condensation rates of precursors for different metal oxides. Herein, we demonstrate a general strategy for the controllable synthesis of MOHSs with up to five metal elements by decomposition of metal-phenolic coordination polymers (MPCPs), which are prepared by chelation of tannic acid with various metal ions. After calcination to burn out the organic component and induce heterogeneous contraction of MPCPs, a series of MOHSs with multishell structure, high specific surface area (55-171 m2/g) and crystalline mesoporous framework are synthesized, including binary (Fe-Co, Ni-Zn and Ni-Co oxides), ternary (Ni-Co-Mn and NiCo-Zn oxides) and quinary (Ni-Co-Fe-Cu-Zn oxides) MOHSs. The gas sensing nanodevices based on quinary MOHSs show much higher response (10.91) than those based on single component toward 50 ppm of ethanol at 80 °C with the response/recovery time of 85/160 s. The quinary oxides sensor also displays high selectivity to ethanol against other interfering gases (e.g., methanol, formadehyde, toluene, methane and hydrogen) and long-term stability (~94.0% after 4 weeks), which are extremely favorable for practical applications.

1. Introduction Metal oxide hollow spheres (MOHSs) with controllable compositions and inner architectures have attracted increasing interests in diverse areas, such as catalysis, sensing, and sustainable energy conversion and storage.1-8 Integration of different metal oxides into single hollow sphere holds a great promise to create well-defined interfaces among oxide domains, and this can offer intriguing properties that exceed unitary component.9-15 For example, semiconducting metal oxides with hybrid nanostructures and compositions can be used for the fabrication of room-temperature gas sensor devices with high sensitivity and selectivity.16-18

to synthesize MOHSs with diverse compositions and nanostructures.19 Despite great successes for synthesis of MOHSs via template-synthesis strategy, this method usually required external template (e.g., carbon spheres) and tedious multistep synthesis. Based on the selftemplate strategy, coordination polymers (CPs) or metalorganic frameworks (MOFs) have recently been extensively used as a precursor for the synthesis of MOHSs with well-reserved morphology and tunable compositions.20-22 In these syntheses, CPs and MOFs serve as both metal precursors and templates and no additional sacrificial templates are required. The resultant MOHSs can be well controlled by tailoring the compositions of CPs (or MOFs) and calcination conditions. MOHSs with ternary or multiple metal elements might exhibit special properties. However, such materials have been rarely

The synthesis of MOHSs usually relies on sacrificial templating strategy. For example, Wang and coworkers have developed a general sequential templating approach

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and tunable metal elements can be easily obtained. After calcination, a series of binary (Fe-Co, Ni-Zn and Ni-Co oxides), ternary (Ni-Co-Mn and Ni-Co-Zn oxides) and quinary (Ni-Co-Fe-Cu-Zn oxides) MOHSs with high specific surface area and crystalline mesoporous framework were obtained. As a proof-of-concept application, the quinary MOHSs (i.e., Ni-Co-Fe-Cu-Zn oxides) were used as gas sensing materials. The results revealed the quinary MOHSs sensor showed much higher response to gaseous ethanol than metal oxide with unitary component (e.g., ZnO, NiO, Co3O4, CuO and Fe2O3) due to the synergistic effect between different constituent metal oxides. It is also found that the quinary MOHSs can detect ethanol with concentrations from 0.5 to 100 ppm at a low operating temperature (25-80 °C), which could effectively reduce energy consumed and risk of gas explosion as well as improve the long-term stability of the sensor.

reported due to the limited availability of CPs or MOFs with multicomponent metal species. Thus, it is extremely desired to develop a general effective strategy to synthesize new CPs (or MOFs) containing multiple metal species by exploring proper precursors and a reliable method for controllable conversion of CPs (or MOFs) to MOHSs. Amorphous coordination polymers have flexible compositions and morphologies, which are suitable for the synthesis of metal oxides with complicated compositions and structures. For example, Guan et al. reported a general synthesis of multishell mixed metal oxide particles by using amorphous CPs (metalisophthalic acid) as a precursor.23 Because isophthalic acid can generally coordinate with different metal species, a series of binary and ternary metal oxide (Ni-Co, Mn-Co, Mn-Ni, Zn-Mn and Mn-Co-Ni oxides) particles with multishell structure were synthesized based on the selftemplate strategy. Similar to other methods like microemulsion synthesis,24 hydrothermal synthesis,25,26 coprecipitation27 and hard-templating synthesis28, the composition of mixed metal oxides can also be well controlled via self-template strategy. Tannic acid (abbreviated as TA) is one kind of widely available plant polyphenols in nature. Each TA molecule has five catechol groups and five galloyl groups. It can chelate 18 different metal ions to form amorphous metal-phenolic coordination polymers (MPCPs) via a metal-catechol coordination bond.29-38 It is reasonable to hypothesize that MPCPs with multiple metal ions in the framework would be an excellent precursor for the rational design and synthesis of multi-component metal oxide materials with unique structure and fascinating properties. In our recent work, we have successfully synthesized mesoporous metal oxide spheres with unitary metal element (e.g., Al, Zn, Cu, Co and Fe) using MPCPs as a metal precursor.37 The internal nanostructure of metal oxide spheres depends on the types of metal species because different metal oxide precursors have different thermal decomposition behaviors and crystallization dynamics. Therefore, if MPCPs containing two or more metal species can be synthesized and used as a precursor, the nanostructure evolution of the mixed metal oxide composites could be more complicated. Moreover, the multicomponent metal oxides could form rich oxide interfaces and thus exhibit fascinating physical and chemical properties, but all these are still unexplored yet. To the best of our knowledge, the synthesis of binary, ternary or quinary MOHSs have not been reported yet using polyphenolic coordination polymers as a precursor.

2. Results and Discussion Binary MOHSs were firstly synthesized using bi-metalTA CPs as a metal oxide precursor. The precursors and the corresponding metal oxides were denoted M1-M2-TA (for CPs) and M1-M2-TA-x (M and x refer metal and calcination temperature, respectively). Scanning electron microscopy (SEM) image for Fe-Co-TA revealed a spherical morphology (Figure S1a). The average diameter of Fe-Co-TA was around 335 nm from dynamic light scattering (DLS) measurement (Figure S1g). After calcination, the obtained Fe-Co-TA-400 showed wellreserved morphology (~188 nm) as indicated by transmission electron microscopy (TEM) image (Figure 1b). The average diameter of Fe-Co-TA-400 (~210 nm) from DLS results was smaller than that of the parent CPs (~335 nm) due to the shrinkage of framework during calcination process (Figure S1h). TEM image further revealed hollow structure with multiple layers (Figure 1b, c). Element mapping showed that Fe and Co elements were distributed in the MOHSs uniformly (Figure 1d). The atomic ratio of Co to Fe was 0.73:1 from energydispersive X-ray spectroscopy (EDX), which was slightly different from the atomic ratios (e.g., 0.42:1) in the metal precursor (Figure S2a). Co(II) ions may have stronger chelate ability with TA than Fe(II) ions, leading to a higher content in the resultant MOHSs. The selected area electron diffraction (SAED) pattern of the obtained MOHSs revealed a polycrystalline feature (Figure 1b, inset). The powder X-ray diffraction (XRD) patterns showed similar diffractions with CoFe2O4 (JCPDS No. 221086) (Figure S3a). X-ray photoelectron spectroscopy (XPS) measurement for Fe-Co-TA-400 revealed the existence of C, O, Fe and Co elements in the samples (Figure S3b, Table S1). The carbon from the sample may be ascribed to the incomplete decomposition of coordination polymers and adventitious contaminates (Figure S3e). The XPS spectra for Fe 2p exhibited nine peaks, which were assigned to Fe (II), Fe (III) and satellite

Herein, we demonstrate a general strategy to synthesize metal oxide hollow spheres consisting up to five metal elements and multishell structure by thermal decomposition of the rationally designed metal-phenolic coordination polymers (Figure 1a). Because each TA molecule can simultaneously coordinate with different kinds of metal ions, MPCPs with amorphous framework


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ACS Applied Materials & Interfaces isotherms (Figure 1e-g, Figure S1-S12). All the MOHSs showed multishell structure and mesoporous crystalline framework. The constituent metal elements were homegenously distributed in the entire sphere. The atomic ratios of metal elements in the obtained MOHSs were generally proportional to corresponding synthesis recipes. The MOHSs had high specific surface area of 57171 m2/g and large pore volume of 0.26-0.63 cm3/g (Table S2).

peaks respectively (Figure S3c).39 XPS spectra for Co 2p also showed the co-existence of Co(II) and Co(III) (Figure S3d). In order to prove the universality of this strategy, spherical Ni-Zn-TA, Ni-Co-TA, Ni-Co-Mn-TA and Ni-CoZn-TA CPs and their derived binary (Ni-Zn-TA-400 and Ni-Co-TA-400) and ternary (Ni-Co-Mn-TA-400 and NiCo-Zn-TA-400) MOHSs were also successfully synthesized and confirmed by TEM, XRD and N2 sorption

Figure 1. (a) Schematic synthesis of multicomponent MOHSs. (b, c) TEM images and SAED patterns (inset), (d) STEM image and element mapping for Fe-Co-TA-400. (e) SEM, (f) TEM, (g) STEM image and element mapping for Ni-Co-MnTA-400. MOHSs, we further extended this strategy to the synthesis of quinary MOHSs. In this study, spherical NiCo-Fe-Cu-Zn-TA CPs with a uniform diameter of ~325 nm (from DLS results) were synthesized via the simultaneous coordination of TA with five metal ions and used as the precursor (Figure s1f, i). After calcination, the obtained metal oxides (denoted Ni-Co-Fe-Cu-Zn-TA-400) exhibited spherical morphology with mean diameter of ~220 nm and crystalline framework (Figure 2a, b, Figure S13a, b). Despite the complicated composition of synthesis system, five metal elements were still distributed uniformly in the obtained Ni-Co-Fe-Cu-Zn-

MOHSs with five or more metal elements may show unpredictable physical and chemical properties. Typically, crystalline high-entropy oxides contain five or more metal elements, and show unique physical properties and potential applications.40-42 Sarkar et al. found that the high-entropy oxides (Co0.2Cu0.2Mg0.2Ni0.2Zn0.2)O had high cycling stability and storage capacity retention for Li ion batteries due to the entropy stabilization effect.42 The synthesis of MOHSs with five or more metal elements has been rarely reported using MOFs or CPs as a precursor. Inspired by the successful synthesis of binary and ternary



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face centered cubic (fcc) structure (Figure 3a). Three diffraction peaks are similar to (111), (200) and (220) planes of fcc Co3O4 (JCPDS No. 42-1467), implying a pure crystalline phase of the quinary MOHSs. N2 sorption isotherms showed a mesoporous structure (Figure 3b). The specific surface area, pore volume and pore size were 92 m2/g, 0.21 cm3/g and 6.6 nm, respectively (Table S2).

TA-400 with retained spherical morphology (Figure 2c-h). The relative atomic percentage for Fe, Co, Ni, Cu and Zn were 17.6±1.0%, 18.2±0.8%, 21.9±1.3%, 21.5±2.4% and 20.8±2.2% respectively calculated by the inductively coupled plasma mass spectrometry (ICP-MS) (Figure 3d). XRD patterns exhibited three diffraction peaks at 36.08, 43.22 and 62.72°, indicating a crystalline framework with

Figure 2. (a) TEM, (b) HRTEM, (c-h) STEM images and element mapping for Ni-Co-Fe-Cu-Zn-TA-400. TEM images for (i) Ni-Co-Fe-Cu-Zn-TA, (j, k) Ni-Co-Fe-Cu-Zn-TA-300 and (l) Ni-Co-Fe-Cu-Zn-TA-350. (m) Schematic illustration of the possible formation process of multishell hollow structure. 2p3/2 indicated the presence of Ni (III) and Ni (II) (Figure 3g).49 The XPS spectra for Cu 2p revealed the presence of CuO and Cu2O. Cu (I) had a single peak at 932.0 eV, while Cu (II) had one main peak at 934.2 eV and shakeup satellites at 939.6 and 942.1 eV.50-53 The presence of Cu(I) may be due to the reduction of Cu(II) under X-ray irradiation in UHV.53 The Zn 2p spectrum showed two peaks at 1020.0 and 1043.0 eV, which can be ascribed to Zn 2p3/2 and Zn 2p1/2 (Figure 3i), respectively.54 Calculations based on both XPS and ICP-MS data indicated that the relative atomic percentage of each metal element in the quinary MOHSs was larger than 10%

XPS spectrum was employed to further investigate the composition and valence states of Ni-Co-Fe-Cu-Zn-TA400 (Figure 3c). XPS results revealed the existence of C, O, Ni, Co, Fe, Cu and Zn. Their relative contents were shown in Table S1. The XPS spectra for Fe 2p exhibited nine peaks, which were assigned to Fe (II), Fe (III) and satellite peaks respectively (Figure 3e).43,44 The Co 2p spectrum could be fitted to two spin-orbit doublets, which were ascribed to Co(II) and Co(III), and two satellite peaks.45-49 Ni 2p XPS spectrum displayed five peaks at 853.6, 855.5, 859.9, 871.6 and 878.0 eV. The peak-fitting curves for Ni


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ACS Applied Materials & Interfaces (Figure 2l). During the calcination process, the out surface of MPCPs was firstly decomposed by air, generating a dense layer of metal oxide on the surface of particles (Figure 2m). This phenomenon was also observed when Cu-TA was used as a precursor.37 The interface layer between the dense shell and inner CPs received two different forces from opposite directions, i.e. the inward cohesive force (σco) from inner CPs and outward adhesive force (σad) from dense layer.55 When two or more different metal species are involved in the hybrid CPs networks, they may accelerate the decomposition of polymer via a catalytic effect. And the quick shrinkage of the inner CPs may lead to a situation that σco is larger than σad. The inner CPs contract inward and can be separated from the dense layer on the surface, thus multishell metal oxide spheres with hollow structure and mesoporosity are formed after calcination. When the calcination temperature increased to 450 °C, the hollow structure was partially collapsed due to the overgrowth of nanocrystals (Figure S13c).

(Figure 3d). The quinary MOHSs consisting of multiple transition elements exhibited pure crystalline phase with fcc structure, thus they could be considered as new family of unusual high-entropy metal oxide spheres with hollow structure and mesoporosity. To the best of our knowledge, this is the first example to demonstrate the successful synthesis of high-entropy metal oxides with hollow structure and mesoporous framework via a self-template strategy. The multishell structure was formed due to the heterogeneous contraction of MPCPs.55 MPCPs (i.e. NiCo-Fe-Cu-Zn-TA) showed solid internal structure (Figure 2i). After calcination at 300 °C, a yolk-shell structure was observed in some nanospheres (Figure 2j). Small nanocrystals were observed on the surface of particles simultaneously (Figure 2k). Such results proved MPCPs experienced a severe shrinkage of framework and crystalline process for metal species. When the calcination temperature increased to 350 °C, almost all the nanospheres showed multishell hollow structure

Figure 3. (a) XRD patterns, (b) N2 sorption isotherms and (inset) pore size distributions, (c) XPS survey spectra, (d) metal contents calculated from XPS and ICP-MS, (e) Fe 2p, (f) Co 2p, (g) Ni 2p, (h) Cu 2p and (i) Zn 2p spectra for Ni-Co-Fe-CuZn-TA-400.



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MPCPs. The internal structure is determined by the crystallization temperature of metal oxides and the thermal stability of polymer networks. In this work, we found that by carefully choosing binary, ternary or quinary MPCPs as a precursor, multishell mixed MOHSs were synthesized for the first time. Such results proved that the different combinations of metal ions also played a pivotal role in determining the internal structure of metal oxide spheres. Fast and reliable detection of volatile organic compounds (VOCs) with high selectivity and sensitivity is highly required for non-invasive medical evaluation and environmental monitoring.54,56 MOHSs are composed of semiconducting transition metal oxides, which possess chemo-resistive response to active gaseous molecules. The MOHSs also showed high specific surface area and crystalline mesoporous framework. Therefore, we further explored the potential application of the quinary MOHSs (Ni-Co-Fe-Cu-Zn-TA-400) as sensing materials for VOCs detection. The gas sensing system of AM1.0 was used. The photograph of the device was shown in Figure S14. The electric circuit of gas sensing measurements was shown in Figure 4a. The output resistance increased when gaseous ethanol was injected into the chamber, indicating a behavior of p-type semiconductor gas sensor (Figure 4b, Figure S15). The optimized operating temperature was 80 °C (Figure 4c). When the concentration of ethanol was 50 ppm, the response (Rg/Ra) was 10.91, and the response/recovery time was 85 and 160 s respectively (Figure 4c, Figure S16a). The response time did not change significantly under different concentrations of ethanol (Figure S16b). However, the recovery time increased with the increase of ethanol concentration.

Figure 4. (a) Schematic diagrams of the semiconducting metal oxides (SMOs) sensor and the test circuit. (b) Simplified gas sensing mechanism of p-type oxide semiconductors. (c) Responses of quinary MOHSs-based sensor toward 50 ppm of ethanol at different operating temperatures (25-200 °C). (d) Comparison of responses at 25 and 80 °C using different metal oxide nanoparticles as a sensing material. (e) Response curve of quinary metal oxide sensor toward ethanol gas with different concentrations (0.5-500 ppm). Inset shows the corresponding calibration curve (0.5-25 ppm). (f) Responses of semiconducting metal oxide based ethanol gas sensors at different operating temperatures (○: reference, ★: this work).

The quinary MOHSs showed higher response than unitary metal oxides synthesized via a similar method (Figure 4d). The response increased from 1.2 to 11.9 when the concentration of ethanol increased from 0.5 to 100 ppm (Figure 4e, Figure S16c). Specifically, a linear relationship between responses and the concentrations of ethanol was calculated with the concentrations ranging from 0.5 to 25 ppm (Figure 4e, inset). The responses to ethanol, methanol, formaldehyde, toluene, methane and hydrogen with concentration of 50 ppm at 80 °C were 10.91, 4.20, 2.35, 1.27, 1.1 and 1.05 respectively, indicating a high selectivity to ethanol gas (Figure S16d, e). Five cycles of a dynamic response-recovery curves toward 50 ppm of ethanol at 80 °C showed a good reproducibility of the sensor (Figure S16f). After testing for 4 weeks, the response to ethanol gas kept almost constant (~94.0%), indicating a good repeatability and long-term stability for quinary MOHS based gas sensor (Figure S17). The response toward ethanol under different humid conditions (40%, 65% and 90%) did not show obvious change, indicating that the quinary MOHSs sensor had good resistance to the humidity (Figure S18). The different valance states of metal oxides in quinary MOHSs

Based on the above results, we propose that this selftemplating formation of the multishell metal oxide hollow spheres is due to the multiple inhomogeneous structure shrinkage of metal-phenolic coordination polymers during calcination process. TA molecule can simultaneously chelate different metal ions to form amorphous MPCPs, making it possible to synthesize MOHSs with complicated compositions and structures. During the calcination process in air, the organic polymer network was gradually decomposed into CO2 and H2O, inducing a dramatic structural shrinkage of the framework and the formation of nanopores. Simultaneously, the metal species in the polymer network can be in situ converted into oxides via the selftemplating effect of the spherical MPCPs. And the crystalline metal oxide particles can retain the morphology of parent CPs. However, the metal oxide particles derived from MPCPs showed various internal structures due to the diversity of metal species in the


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ACS Applied Materials & Interfaces species such as Ni, Co, Fe and Cu in the quinary MOHSs had variable valence states, which may affect the adsorption and desorption of molecules on the surface of materials. As a result, the sensing performance such as recovery time and humidity-independent sensing performance can be enhanced. Such results indicate that MOHSs with multiple metal elements may exhibit unexpected properties and find many applications in catalysis, energy conversion, etc.

may play an important role by reversing reaction of water poisoning.57,58 The operating temperature is one of key factors to evaluate the gas sensing performance. The operating temperature for traditional semiconducting metal oxide gas sensors is usually ranging from 300 to 500 °C.59,60 The high operating temperature will bring safety issues when operating in flammable and explosive environments. Lowtemperature gas sensor can reduce energy consumed and risk of gas explosion as well as improve the long-term stability. It is found that the quinary MOHSs-based sensor can even detect ethanol at room temperature (i.e. 25 °C) (Figure 4d). The response increased from 1.71 to 4.86 as the concentration of ethanol increased from 10 to 500 ppm (Figure 4e). Such quinary MOHSs sensor showed advantage of low operating temperature in comparison with other semiconducting metal oxide-based gas sensors (Figure 4f, Table S3).

3. Conclusion In summary, a series of binary (Fe-Co, Ni-Zn and Ni-Co oxides), ternary (Ni-Co-Mn and Ni-Co-Zn oxides) and quinary (Ni-Co-Fe-Cu-Zn oxides) metal oxide hollow spheres with multi-shell structure, high specific surface area and crystalline mesoporous framework were synthesized by thermal decomposition of amorphous metal-phenolic coordination polymers. Polyphenol can chelate different kinds of metal species simultaneously to form metal-phenolic coordination polymers, which were proved to be an ideal precursor to tailor the nanostructures and compositions of the derived metal oxides. The formation of multishell structure is based on the heterogeneous contraction of coordination polymers during calcination process. Due to their high specific surface area (55-171 m2/g), multishell hollow structure and crystalline mesoporous framework, the gas sensing nanodevices based on quinary MOHSs show excellent comprehensive sensing performance at low operating temperature (25-80 °C) toward low concentration of ethanol. This work opens a new avenue for the development of room-temperature chemiresistive gas sensing materials by integration of multicomponent semiconducting metal oxides into single nanoparticles. Considering the simplicity of preparing amorphous metal-phenolic coordination polymers and conversion into various multicomponent metal oxides via thermal decomposition, such MOHSs with multishell structure and crystalline mesoporous framework have potential applications in various fields such as lithium ion batteries, catalysis and gas sensing, etc.

The interactions between semiconducting metal oxide and gas (oxygen, ethanol) generally change the conductivity of the metal oxide, which can be used for detection of target gas. The quinary MOHSs-based gas sensor showed a typical p-type semiconductor behavior. When p-type semiconductor based sensor was in air, the oxygen molecules were adsorbed on the surface of semiconductor (Figure 4b). Oxygen molecule could capture free electrons from the surface of semiconductor, resulting into an increase of concentrations of holes on the surface. As a result, the resistance of semiconductor decreased. When gaseous ethanol was introduced, the reducing gas may react with the oxygen anions adsorbed on the surface of semiconductor and decreased the concentrations of holes in the depletion layer of semiconductor, resulting into an increased resistance. However, the gas sensing mechanism for low-temperature (< 100 °C) detection of ethanol is still unclear and little work has been done about it.16,61 The metal oxides such as NiO, CuO and Co3O4 are p-type semiconductors, while ZnO and Fe2O3 are n-type semiconductors. The quinary MOHSs contain five metal elements with different valence states. Their sensing mechanism is more complicated than that of unitary metal oxide. Though it is difficult to elaborate clearly the sensing mechanism here, the excellent sensing performance at a low operating temperature for quinary MOHSs can be mainly ascribed to the unique structure and compositions. Firstly, quinary MOHSs showed multishell hollow structure with high surface area (92 m2/g) and large pore size (6.6 nm), which could afford large amounts of active sites to adsorb oxygen and ethanol species and promote the rapid and effective gas diffusion toward the entire sensing surfaces via the mesoporous structures.61,62 Secondly, the components in the quinary MOHSs would show a synergic effect to enhance the sensing performance.63 For example, some metal oxides (i.e., Co3O4 and CuO) have been proven to be a good catalyst to promote the oxidation of VOCs at low temperature.63 The metal

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the internet at http://pubs.acs.org. Experimental section; SEM images of metal-phenolic coordination polymers; EDX spectra, XRD patterns, N2 sorption isotherms and pore size distributions for Fe-Co-TA400, Ni-Co-TA-400, Ni-Zn-TA-400, Ni-Co-Mn-TA-400 and Ni-Co-Zn-TA-400; TEM images of Ni-Co-TA-400, Ni-Zn-TA400 and Ni-Co-Zn-TA-400; XPS spectra of Fe-Co-TA-400 and Ni-Co-Mn-TA-400; SEM image of Ni-Co-Fe-Cu-Zn-TA-400 and Ni-Co-Fe-Cu-Zn-TA-450; Particle size distribution for Ni-Co-Fe-Cu-Zn-TA-400; Dynamic response-recovery curves of quinary MOHSs sensor; Selectivity and long-term stability for quinary MOHSs sensor; Textual properties of mixed



(11) Zhang, G.; Lou, X. W. General Synthesis of Multi-Shelled Mixed Metal Oxide Hollow Spheres with Superior Lithium Storage Properties. Angew. Chem. Int. Ed. 2014, 53, 9041-9044. (12) Wang, X.; Xiao, H.; Li, A.; Li, Z.; Liu, S.; Zhang, Q.; Gong, Y.; Zheng, L.; Zhu, Y.; Chen, C.; Wang, D.; Peng, Q.; Gu, L.; Han, X.; Li, J.; Li, Y. Constructing NiCo/Fe3O4 Heteroparticles within MOF-74 for Efficient Oxygen Evolution Reactions. J. Am. Chem. Soc. 2018, 140, 15336-15341. (13) Joshi, N.; da Silva, L. F.; Jadhav, H. S.; Shimizu, F. M.; Suman, P. H.; M'Peko, J. C.; Orlandi, M. O.; Seo, J. G.; Mastelaro, V. R.; Jr., O. N. Oliveira. Yolk-shelled ZnCo2O4 Microspheres: Surface Properties and Gas Sensing Application. Sens. Actuator. B 2018, 257, 906-915. (14) Kaur, N.; Zappa, D.; Ferroni, M.; Poli, N.; Campanini, M.; Negrea, R.; Comini, E. Branch-like NiO/ZnO Heterostructures for VOC Sensing. Sens. Actuator. B 2018, 262, 477-485. (15) Hong, Y. J.; Son, M. Y.; Park, B. K.; Kang, Y. C. One-Pot Synthesis of Yolk-Shell Materials with Single, Binary, Ternary, Quaternary, and Quinary Systems. Small 2013, 9, 2224-2227. (16) Zhang, J.; Liu, X.; Neri, G.; Pinna, N. Nanostructured Materials for Room-Temperature Gas Sensors. Adv. Mater. 2016, 28, 795-831. (17) Joshi, N.; Hayasaka, T.; Liu, Y.; Liu, H.; Jr., O. N. Oliveira; Lin, L. A Review on Chemiresistive Room Temperature Gas Sensors Based on Metal Oxide Nanostructures, Graphene and 2D Transition Metal Dichalcogenides. Microchim. Acta. 2018, 185, 213. (18) Liu, X.; Ma, T.; Pinna, N.; Zhang, J. Two-Dimensional Nanostructured Materials for Gas Sensing. Adv. Funct. Mater. 2017, 27, 1702168. (19) Mao, D.; Wan, J.; Wang, J.; Wang, D. Sequential Templating Approach: A Groundbreaking Strategy to Create Hollow Multishelled Structures. Adv. Mater. 2018, 30, 1802874. (20) Chen, Y. Z.; Zhang, R.; Jiao, L.; Jiang, H. L. Metal-Organic Framework-Derived Porous Materials for Catalysis. Coordin. Chem. Rev. 2018, 362, 1-23. (21) Dang, S.; Zhu, Q. L.; Xu, Q. Nanomaterials Derived from Metal-Organic Frameworks. Nat. Rev. Mater. 2018, 3, 17075. (22) Liu, D.; Wan, J.; Pang, G.; Tang, Z. Hollow MetalOrganic-Framework Micro/Nanostructures and their Derivatives: Emerging Multifunctional Materials. Adv. Mater. 2018, e1803291. (23) Guan, B. Y.; Kushima, A.; Yu, L.; Li, S.; Li, J.; Lou, X. W. Formation of Onion-Like NiCo2S4 Particles via Sequential Ion-Exchange for Hybrid Supercapacitors. Adv. Mater. 2017, 29, 1605051. (24) Niu, X.; Du, W. P; Du, W. M. Preparation and Gas Sensing Properties of ZnM2O4 (M=Fe, Co, Cr). Sens. Actuator. B 2004, 99, 405-409. (25) Vadivel, S.; Balaji, G.; Rathinavel, S. High Performance Ethanol and Acetone Gas Sensor Based Nanocrystalline MnCo2O4 Using Clad-modified Fiber Optic Gas Sensor. Opt. Mater. 2018, 85, 267-274. (26) Joshi, N.; da Silva, L. F.; Jadhav, H.; M'Peko, J. C.; Torres, B. B. M.; Aguir, K.; Mastelaro, V. R.; Jr., O. N. Oliveira. One-step Approach for Preparing Ozone Gas Sensors Based on Hierarchical NiCo2O4 Structures. RSC Adv. 2016, 6, 92655-92662. (27) Zhang, B.; Qu, F.; Zhou, X.; Zhang, S.; Thomas, T.; Yang, M. Porous Coral-like NiCo2O4 Nanospheres With Promising Xylene Gas Sensing Properties. Sens. Actuator. B 2018, 261, 203-209. (28) Zhao, R.; Li, Q.; Wang, C.; Yin, L. Highly Ordered Mesoporous Spinel ZnCo2O4 as a High-performance Anode Material for Lithium-ion Batteries. Electrochim. Acta. 2016, 197, 58-67.

metal oxide; Comparison of sensing performance for different metal oxides.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (D. Y. H.). *E-mail: [email protected] (W. J.). Author Contributions #These

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authors contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Science Foundation of China (No. 21701130, 21673048 and 21875044), the Fundamental Research Funds for the Central Universities, “Young Talent Support Plan” of Xi’an Jiaotong University, Key Basic Research Program of Science and Technology Commission of Shanghai Municipality (17JC1400100), Program of Shanghai Academic Research Leader (19XD1420300), and Youth Top-notch Talent Support Program of China. We thank Miss Jiao Li, Mr Zijun Ren at Instrument Analysis Center of Xi'an Jiaotong University for their assistance with TEM and SEM analysis.

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General Synthesis of Mixed Semiconducting Metal Oxide Hollow

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