Metal Organic Framework-Templated Chemiresistor: Sensing Type

Aug 4, 2017 - Facile synthesis of porous nanobuilding blocks with high surface area and uniform catalyst functionalization has always been regarded as...
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Metal Organic Framework-Templated Chemiresistor: Sensing Type Transition from P‑to‑N Using Hollow Metal Oxide Polyhedron via Galvanic Replacement Ji-Soo Jang,† Won-Tae Koo,† Seon-Jin Choi,†,§ and Il-Doo Kim*,† †

Department of Materials Science and Engineering and §Applied Science Research Institute, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea S Supporting Information *

ABSTRACT: Facile synthesis of porous nanobuilding blocks with high surface area and uniform catalyst functionalization has always been regarded as an essential requirement for the development of highly sensitive and selective chemical sensors. Metal−organic frameworks (MOFs) are considered as one of the most ideal templates due to their ability to encapsulate ultrasmall catalytic nanoparticles (NPs) in microporous MOF structures in addition to easy removal of the sacrificial MOF scaffold by calcination. Here, we introduce a MOFs derived n-type SnO2 (n-SnO2) sensing layer with hollow polyhedron structures, obtained from p−n transition of MOF-templated p-type Co3O4 (p-Co3O4) hollow cubes during galvanic replacement reaction (GRR). In addition, the Pd NPs encapsulated in MOF and residual Co3O4 clusters partially remained after GRR led to uniform functionalization of efficient cocatalysts (PdO NPs and p-Co3O4 islands) on the porous and hollow polyhedron SnO2 structures. Due to high gas accessibility through the meso- and macrosized pores in MOF-templated oxides and effective modulation of electron depletion layer assisted by the creation of numerous p−n junctions, the GRR-treated SnO2 structures exhibited 21.9-fold higher acetone response (Rair/Rgas = 22.8 @ 5 ppm acetone, 90%RH) compared to MOF-templated p-Co3O4 hollow structures. To the best of our knowledge, the selectivity and response amplitudes reported here for the detection of acetone are superior to those MOF derived metal oxide sensing layers reported so far. Our results demonstrate that highly active MOF-derived sensing layers can be achieved via p−n semiconducting phase transition, driven by a simple and versatile GRR process combined with MOF templating route.

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INTRODUCTION Recent advances in rationally designed metal−organic frameworks (MOFs), which are easily synthesized by controlled coupling of metal ions and a variety of organic ligands, have attracted tremendous attention owing to their giant surface area (2000−7000 m2/g), high porosity, and unique physicochemical properties.1−3 MOF-based materials have been extensively applied to diverse fields, including gas adsorption/storage, catalysts, separation, chemical sensors, and energy storage.4−8 Furthermore, the facile immobilization of various metal nanocatalysts inside the cavities of microporous MOFs offers new intriguing materials properties.9−13 However, the poor thermal and chemical stability of the MOFs often limits broad ranges of applications, particularly for devices that operate at high temperature (>200 °C) such as semiconducting metaloxides (SMOs) based chemiresistors.14−16 Thus, thermally stable MOFs-derived materials are needed for practical applications in chemiresistive gas sensors. To address these critical issues, a high temperature calcination in air atmosphere can be used to produce thermally and chemically stable mesoporous SMOs structures by simultaneous thermal decomposition of organic linkers and oxidation of metal © 2017 American Chemical Society

nodes. The MOFs-templated SMOs also possess high surface area and high gas accessible structures due to their inherent open porosity. In particular, high yield of desired nanoarchitectures without any severe aggregation or structural collapse would be of tremendous value in terms of reproducibility and scalable production. For example, Lü et al. simply developed Co based MOF, i.e., zeolitic imidazolate framework-67 (ZIF-67), -templated porous Co3O4 concave nanocubes as high temperature operating ethanol sensing layers.14 Furthermore, Koo and co-workers successfully developed PdO nanoparticles (NPs) functionalized Co3O4 hollow nanocages by using direct calcination of Pd-loaded ZIF-67.49 However, due to the inherent limitation of p-type Co3O4 materials as high response sensing layers, their gas responses (Rair/Rgas) are mostly in the ranges of 2−5 at gas concentration levels of several hundred ppm. To overcome the poor sensing characteristics of MOF templated p-type SMOs, Li and co-workers fabricated VOCs sensors by using the ZIF-8 templated n-type ZnO nanocages.17 Gao et al. also reported Received: May 22, 2017 Published: August 4, 2017 11868

DOI: 10.1021/jacs.7b05246 J. Am. Chem. Soc. 2017, 139, 11868−11876

Article

Journal of the American Chemical Society

Figure 1. (a) Schematic illustration of synthetic process for the n-SnO2 HNCs functionalized with Co3O4 and PdO, (b) SEM images of Pd NPs encapsulated ZIF-67, (c) magnified SEM images of Pd encapsulated ZIF-67, (d,e) SEM images of PdO loaded p-Co3O4 HNCs, and (f) and (g) SEM images of Co3O4−PdO loaded n-SnO2 HNCs.

for the creation of desired SMO architectures with tunable phase and composition. So far, SMO based chemiresistors prepared by combining GRR with MOF templating route have never been studied. In this work, we first developed a process for preparing nSnO2 hollow polyhedron nanocubes (HNCs), that are functionalized with discrete p-type Co3O4 (p-Co3O4) islands by combining MOF-templating routes with GRR. Our strategy provides reliable p−n transition in MOF templated SMOs while maintaining highly porous and hollow nanostructure. Interestingly, the replacement process of p-Co3O4 to n-SnO2 creates the macrosized pores on the surfaces of polyhedron HNCs, resulting in bimodal porous structure with meso- and macrosized pores. Furthermore, tiny PdO cocatalysts, which were initially embedded in MOFs, were simultaneously decorated on calcined n-SnO2 HNCs. With these functional factors such as p−n transition, catalytic effect, and porous structures, the remarkably enhanced gas responses and outstanding selectivity to other interfering gases can be achieved.

MIL-88A templated n-type Fe2O3 nanorods for detection of acetone molecules.18 Nonetheless, sufficient sensing performances with great selectivity have not been achieved using MOFdriven oxide materials. Therefore, the development of various MOF-derived SMO sensors with enhanced gas response and selectivity, particularly for the detection of gases at sub-ppm level, is highly required. So far, a number of metal nodes including Zn, Co, Al, Ti, Fe, In, Ni, Mg, and Cu elements have been coupled with organic linkers for formation of a variety of MOFs. However, only a few papers in the literature have reported on Sn metal node based MOFs, which possessed insufficient MOF characteristics; small surface area (14.24 m2/ g), and low porosity.19,20 In this sense, the synthesis of reliable SnO2 building blocks through the direct calcination of Sn metal node-based MOFs is still very challenging. In fact, limited solubility of certain organic linkers, poor chemical stability, and different formation inherency of metal clusters have limited the direct synthesis of Sn-based MOFs with desired surface areas, porosity, and morphology. Thus, the novel strategy for the synthesis of MOF-driven SnO2 phase should be developed since n-type SnO2 (n-SnO2) materials are able to provide highest gas sensing properties.1,21,22 Meanwhile, the galvanic replacement reaction (GRR), which is driven by the difference in electrochemical potentials of two metallic species, is known as an effective method for controlling composition and porosity of materials.23−25 Furthermore, in 2013, Oh et al. first showed that GRR can occur in crystalline SMOs, thereby leading to artificial switching of materials phase and composition.26 Thus, combining GRR with MOF templating can be considered as a new processing platform

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RESULTS AND DISCUSSION Figure 1a indicates the synthetic process of n-SnO2 hollow nanocubes (HNCs) functionalized with cocatalysts, i.e., partially decorated Co3O4 islands and PdO nanoparticles (NPs) (hereafter, Co3O4−PdO loaded n-SnO2 HNCs). Single crystalline ZIF-67, which have polyhedral structures, is formed by binding of Co ions and 2-methylimidazole (HMIM) organic linkers in methanol solvent. Then, Pd ions dispersed in aqueous solution are infiltrated into the cavities of as-synthesized ZIF-67 11869

DOI: 10.1021/jacs.7b05246 J. Am. Chem. Soc. 2017, 139, 11868−11876

Article

Journal of the American Chemical Society and the subsequent reduction process induced the precipitation of Pd NPs in the pore-sites of ZIF-67.27 Figure 1a(i) indicates a magnified scheme of ZIF-67 comprising Pd NPs encapsulated Co ions−organic linker composite. The networks of Pd NPs in ZIF-67 were clearly observed by TEM analysis (Figure S1 of the Supporting Information). The uniformly sized (2−3 nm) Pd NPs loaded ZIF-67 showed clear polyhedral structures as shown in Figure 1b,c. Due to the fact that ordered organic linkers and Co ions coexisted in ZIF-67, selective thermal decomposition of organic linkers and oxidation of Co ions stemming from high temperature calcination leads to formation of Co3O4 HNCs. During the heat treatment, the faster diffusion rate of inner Co cations relative to atmospheric oxygen causes the formation of hollow structures in a process that is known as the Kirkendall effect.28−30 Note that the ramping rate during heat treatment was set up to 10 °C min−1, which is a 2-fold higher ramping rate compared to conventional ramping rate (under 5 °C min−1). Fast ramping rates induce the fast gas diffusion of CO2 or H2O, which originates from thermal decomposition of organic materials, in the outward direction in ZIF-67. During this process, the migration of Co cations or Pd NPs to the outer side of HNCs also occurred through the fast gas diffusion.31 Due to this physical migration effect, Pd NPs encapsulated in ZIF-67 were oxidized to PdO and tightly immobilized on both the interior and exterior sides of hollow Co3O4 polyhedral structures simultaneously (hereafter, PdO loaded p-Co3O4 HNCs, see Figure 1a(ii)). The SEM images of PdO loaded p-Co 3 O 4 HNCs showed obvious hollow polyhedral structures with threefold edges (Figure 1d and e). In order to further transform the phase of p-Co3O4 to nSnO2, we conducted a galvanic replacement reaction (GRR) in a metal oxide system.26 For the GRR process, the surface of hollow Co3O4 was dissolved into solution and Sn2+ ions were precipitated to SnO2. Mesoporous PdO loaded Co3O4 HNCs can induce the great accessibility of Sn(II) ions into the structures, resulting in the formation of Co3O4−PdO loaded nSnO2 HNCs (Figure 1f,g). In addition, dissolution of Co3O4 leads to formation of more mesosized pores on the surface of Co3O4−PdO loaded n-SnO2 HNCs (see the yellow dotted elliptical circles in Figure 1g). The detailed microstructures of porous SnO2 regions functionalized by cocatalysts (Co3O4 and PdO) were indicated in Figure 1a(iii). In terms of the microstructure such as pore distribution and surface area, Co3O4−PdO loaded n- SnO2 HNCs showed higher BET surface area (31.76 m2/g) compared to that (26.98 m2/g) of PdO loaded Co3O4 HNCs (Figure S3a,b). In addition, the pore volume of mesosized pores, especially ranging 2−5 nm, of Co3O4−PdO loaded n-SnO2 HNCs are dramatically increased compared with that of PdO loaded Co3O4 HNCs (Figure 2). Figure 3a shows a schematic illustration for formation mechanism of Co3O4−PdO loaded n-SnO2 HNCs. In detail, the higher standard electrode potential of Co3+/Co2+ (1.87 V) than Sn4+/Sn2+ (−0.09 V) leads to redox process in PdO loaded p-Co3O4 HNCs, thereby forming the precipitation of SnO2 and dissolution of Co3O4, simultaneously. Due to the higher potential difference of Co3+/Co2+ (1.87 V) − Sn4+/Sn2+ (−0.09 V) compared with that of Pd2+/Pd (0.951 V)-Sn4+/Sn2+ (−0.09 V), the GRR preferentially occurs in Co/Sn, thereby maintaining the PdO catalysts in phase-transformed structures. The overall redox reaction is expected, as expressed in chemical reaction 1.26

Figure 2. Pore size distribution of PdO loaded Co3O4 HNCs and Co3O4−PdO loaded n-SnO2 HNCs.

Co3O4 → Co3O4 /SnO2 (Co3 +reduced to Co2 +and Sn 2 +oxidized to SnO2)

(1)

When the dissolution of Co3O4 on the surface of PdO loaded p-Co3O4 HNCs has occurred, mesosized pores are formed, allowing the permeation of Sn solutions onto both inner and exterior sides of PdO loaded p-Co3O4 HNCs. The phase transition from p-Co3O4 to n-SnO2 can occur when relative amounts of SnO2 are larger than Co3O4. In the p−n transition reaction of Co3O4 during GRR, the exposure time to Sn precursor solution and concentration of Sn solution are key factors. According to previous research, the concentration of Sn precursor in GRR solution systems is the most important parameter for achieving perfect substitution of SnO2 replacing Co3O4.26,32,33 In this work, we used a 20 vol % of Sn precursorcontained aqueous (2 M) solution for all GRR experiments. When the Sn(II) chloride solution is added to a suspension of MOF driven PdO loaded p-Co3O4 HNCs and continuously reacted for over 1.5 h, p−n transition from PdO loaded pCo3O4 HNCs to Co3O4−PdO loaded n-SnO2 HNCs occurs. The TEM image of Co3O4−PdO loaded n-SnO2 HNCs showed porous hollow polyhedral structures (Figure 3b). However, this transformation reaction preferentially occurred around the edge sites of the Co3O4 HNCs, thereby showing blunt edges in Co3O4−PdO loaded n-SnO2 HNCs compared with that of PdO loaded p-Co3O4 HNCs.34 The high-resolution transmission electron microscopy (HRTEM) image and selected area electron diffraction (SAED) pattern revealed that their major crystalline phase is tetragonal n-SnO2 with lattice distance of rutile SnO2 structure (i.e., 0.335 nm corresponding to SnO2 (110), Figure 3c,d). Despite the solution based synthetic process with low temperature (@ 90 °C), microstructure of Co3O4−PdO loaded n-SnO2 HNCs showed highly crystalline SnO2 structures. Furthermore, PdO NPs with diameter ranging 2−3 nm were uniformly functionalized on the wall of Co3O4−PdO loaded n-SnO2 hollow nanocubes (yellow arrows in Figure 3c). On the other hand, insufficient GRR time (0.5 h) in a PdO loaded p-Co3O4 HNCs system did not lead to a successful p−n transition reaction. After 0.5 h GRR, discontinuous and small particles of n-SnO2 were decorated on PdO loaded Co3O4 HNCs, whereas their major semiconducting properties were governed by p-type Co3O4 (Figure 3e,f). The corresponding SAED analysis results also indicated that the major crystal structure of insufficient GRR driven product was spinel Co3O4 (Figure 3g). These 11870

DOI: 10.1021/jacs.7b05246 J. Am. Chem. Soc. 2017, 139, 11868−11876

Article

Journal of the American Chemical Society

Figure 3. (a) Schematic illustration on the mechanism of p−n transition from p-Co3O4 to n-SnO2 HNCs, (b) TEM image of porous Co3O4−PdO loaded n-SnO2 HNCs, (c) HRTEM lattice spacing image of Co3O4−PdO loaded n-SnO2 HNCs, (d) SAED pattern of Co3O4−PdO loaded n-SnO2 HNCs, (e) TEM image of SnO2−PdO loaded p-Co3O4 HNCs, (f) HRTEM lattice spacing image of SnO2−PdO loaded p-Co3O4 HNCs, (g) SAED pattern of SnO2−PdO loaded p-Co3O4 HNCs.

grain growth and the formation of second phase compound oxides between Co3O4 and SnO2, thereby maintaining separate crystallinity (SnO2 or Co3O4) with well-preserved hollow nanocube structures. In order to investigate the chemical state of each element for Co3O4−PdO loaded n-SnO2 HNCs and SnO2−PdO loaded p-Co3O4 HNCs, X-ray photoelectron spectroscopy (XPS) analysis was carried out. XPS results revealed that the Co3O4 and SnO2 coexisted in hollow polyhedral structures with obvious different peak intensity areas in each sample. As shown in Figure 4b,c, two specific peaks of 2p3/2 at the binding energies of 780 and 781.3 eV, that correspond to Co3+ and Co2+, were clearly shown in both ntype and p-type hollow polyhedral materials.35 Note that the energy difference between Co 2p3/2 and 2p1/2 was approximately 15.4 eV.36 However, peaks of Co2+ in Co3O4−PdO loaded n-SnO2 HNCs, which were mainly attributed to CoO (satellite structure), were much stronger than peaks of Co2+ in SnO2−PdO loaded p-Co3O4 HNCs. This can be explained by the reduction of Co3O4 during the GRR. The phase transition from Co3+ to Co2+ occurred by oxidization of Sn2+ to Sn4+, leading to significant formation of satellite peaks in XPS data. Sn related peaks exhibited Sn4+ state at the binding energies of 487.2 (3d5/2) and 495.6 eV (3d3/2) (Figure 4d,e), also indicating the formation of SnO2 phase in the hollow polyhedral structures.15 However, a much larger surface area under Sn peaks of Co3O4−PdO loaded n-SnO2 HNCs

results indicate that electrical properties, morphology, and porosity of phase transformed materials can be manipulated by controlling the GRR time. The EDS elemental mapping images and line scan profile of both Co3O4−PdO loaded n-SnO2 HNCs and SnO2−PdO loaded p-Co3O4 HNCs further support the these TEM analysis results (Figure S4). In the case of longer time (>2 h) GRR in PdO loaded p-Co3O4 HNCs, collapsed and disordered nanostructures were observed (Figure S5). Since the excessive transformation reaction induces the immoderate removal of edge sites, it leads to collapsed structures. Hereafter, we denote insufficient time (