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MOF-derived hierarchical hollow ZnO nanocages with enhanced low-concentration VOCs gas-sensing performance
Sensors and Actuators B 225 (2016) 158–166
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
MOF-derived hierarchical hollow ZnO nanocages with enhanced low-concentration VOCs gas-sensing performance Wenhui Li a,b , Xiaofeng Wu a,∗ , Ning Han a , Jiayuan Chen a,b , Xihui Qian a,b , Yuzhou Deng a,b, Wenxiang Tang a,c , Yunfa Chen a,∗ a
State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China c Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136, USA b
a r t i c l e
i n f o
Article history: Received 21 July 2015 Received in revised form 8 October 2015 Accepted 6 November 2015 Available online 10 November 2015 Keywords: MOF-derived ZnO nanocages Hierarchical hollow structure ppb or sub-ppm level VOCs gas-sensors
a b s t r a c t The design and synthesis of nanostructured ZnO with high chemical sensing properties, especially towards ppb or sub-ppm level VOC gases is still highly desired and challengeable. Herein, the hierarchical hollow ZnO nanocages were synthesized by a facile strategy through the simple and direct pyrolysis of Zn-based metal-organic framework. The as-synthesized hollow ZnO products present the typical hierarchical structures with hollow interiors enveloped by interpenetrated ZnO nanoparticles as porous shell, providing structurally combined meso-/macro-porous channels for facilitating the diffusion and surface reaction of gas molecules. The gas-sensing experiments demonstrate that, in contrast with singular ZnO nanoparticles, the ZnO nanocages show significantly enhanced chemical sensing sensitivity and selectivity towards low-concentration volatile organic compounds, typically, acetone and benzene. Furthermore, the ZnO hollow nanocages perform sub-ppm level sensitivity with 2.3 ppm−1 towards 0.1 ppm benzene, and ppb level sensitivity with 15.3 ppm−1 towards 50 ppb acetone, respectively. The enhanced sensing performance of the MOF-derived ZnO nanocages is ascribed to the unique hierarchical structure with high specific surface area and abundant exposed active sites with surface-adsorbed oxygen. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Hierarchical hollow metal oxides have always been an extremely hot topic over the past decades in nanotechnology [1–5]. The unique structure with large specific surface area and abundant exposed active sites has significantly enhanced physicochemical properties compared with their singular nanoscale building blocks. High-sensitive sensing materials were highly demanded for low-concentration volatile organic compounds (VOCs) detection in the views of environmental protection and human health. As a wide bandgap metal oxide semiconductor (MOS), zinc oxides have attracted extensive interests applied to chemical gas-sensing materials with high performance, low cost and environmental friendliness [6–8]. However, due to the easy recombination of electron–hole pairs, low exposure of reactive crystal planes and few pathways for gas-solid contact, traditional ZnO-based sensing materials still suffer from poor sensitive and selectivity towards
∗ Corresponding authors. E-mail addresses: [email protected] (X. Wu), [email protected] (Y. Chen). http://dx.doi.org/10.1016/j.snb.2015.11.034 0925-4005/© 2015 Elsevier B.V. All rights reserved.
low-concentration VOCs, resulting in the limitation of practical application [9–11]. Continuous efforts were made to overcome these drawbacks for improving the ZnO sensing properties, such as ion-doping [12], noble metal–MOS [13] or MOS–MOS heterostructure [14] to increase the mobility and separation efficiency of electron–hole pairs, and structure-controlling to form hollow or porous nanostructure for increasing exposure active sites [15,16]. Nevertheless, these improved materials still have undesirable sensing performances towards low-concentration VOC gas, especially undetectable ppb or sub-ppm level sensing responses, severe signal drifts, and much less selectivity [17–19]. Therefore, it is highly desired to synthesize the materials with high chemical sensing properties, especially towards ppb or sub-ppm level VOC gases. Metal–organic frameworks (MOFs) as a new class of organic–inorganic hybrid materials have received considerable attention because of the distinctive characteristics and widely potential technical application, e.g. gas adsorption and separation, catalysis, chemical sensors and drug delivery [20–28]. Most recently, MOFs have been explored as promising self-sacrificial templates or precursors to construct porous oxide nanostructures
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with intriguing properties. For example, Fe2 O3 @TiO2 composite nanoparticles were synthesized by using Fe-containing nanoscale MOFs coated with amorphous titania for visible light-driven hydrogen production from water [29]. High symmetric porous Co3 O4 hollow dodecahedra were fabricated from ZIF-67 with highly enhanced lithium storage capability [30]. After thermal treatment of core–shell ZIF-8@ZIF-67 crystals, selectively functionalized nanoporous hybrid carbon was constructed with a distinguished electrochemical performance as an electrode in supercapacitors [31]. These great progresses were achieved because of the remarkable properties with selectable metal ion centers, multifunctional organic ligands, well-developed porosity and diverse morphologies of metal-organic frameworks. It is challengeable to recognize and detect low-concentration VOCs, especially ppb to sub-ppm level from indoor or outdoor emission sources at residential or work place. In these places, the exposure to low-concentration VOCs, especially BTX (benzene–toluene–xylene), is dangerous and carcinogenic. Accordingly, our emphasis is mostly placed on improved sensitive ZnO-based nanostructures for detecting low-concentration VOC at ppb–ppm range. Indifferent with the mostly reported literature, the MOF-derived hollow ZnO perform improved VOC-sensing properties towards BTX, ethyl acetate, acetone, etc., of which are common VOC pollutants in indoor or outdoor environments, and good sensitivity at relative low concentration (ppb–sub-ppm level). Also, our work presented a facile and cost-effective way to synthesize high-sensitive hollow ZnO gas sensors, when compared with the previous reports [32–35]. Herein, the unique hierarchical ZnO nanocages with hollow interiors enveloped by porous shells was synthesized by simply and directly temperature-programmed decomposition of spherical Zn-based metal–organic frameworks (MOF-5) precursor. More importantly and interestingly, the as-synthesized ZnO nanocages exhibited enhanced chemical sensing performances towards two typical low-concentration VOCs, acetone and obstinate benzene at ppb and sub-ppm levels, exhibiting the good promise of this MOF pyrolysis strategy in developing highly sensitive and selective gas sensing materials. 2. Experimental 2.1. Materials Zn(NO3 )·6H2 O was obtained from Beijing Chemical Works. 1,4-dicarboxybenzene (H2 DBC), Polyvinyl pyrrolidone (PVP, M.W. 30,000) and N,N-dimethylformamide (DMF) were purchased from Sinopharm. All chemicals were used directly without further purification. The water was made from Millipore Milli-Q water (15 M cm). 2.2. Synthesis of hierarchical hollow ZnO nanocages The MOF-5 precursors were firstly synthesized referenced by the previous literature [36], but modified. Typically, 5.436 g PVP was dissolved into the 60 ml DMF/ethanol (5/3 in v/v) mixed solvent. Then, 0.1088 g Zn(NO3 )·6H2 O, 0.225 g H2 DBC were added under vigorous stirring to form a clear solution. The solution was transferred into a 100 ml Teflon autoclave, and placed into an oven with a constant temperature of 140 ◦ C for 3 h, then cooled to the room temperature. The white precipitates were isolated from the supernatant by centrifugation and washed with DMF and CHCl2 at least 3 times, and then dried at 80 ◦ C overnight. Finally, the collected white powders were used as precursors to prepared ZnO samples by carefully temperature-programmed thermal decomposition process with a slow rate of 1 ◦ C min−1 from room
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temperature. When the temperature lifted to 450 ◦ C, the samples were kept continuously for 1 h and gradually cooled to the room temperature. 2.3. Characterization The crystallinity of the products was characterized by powder X-ray diffraction (XRD) on a Panalytical X’Pert PRO system using Cu-K␣ radiation. The morphologies and microstructure were investigated using a field-emission scanning electron microscope (FE-SEM, JEOL JSM-6700F, Japan) operating at 15 kV and a transmission electron microscopy (TEM, JEOL JEM-2010F) with an accelerating voltage of 200 kV. Thermogravimetric analysis (TGA) was performed on a NETZSCH STA449F3 apparatus with a heating rate of 10 ◦ C min−1 from 35 ◦ C to 900 ◦ C under air atmosphere. The N2 adsorption–desorption isotherms were measured on an automatic surface analyzer (SSA-7300, China). Before the measurement, the samples were outgassed at 150 ◦ C for 10 h. The surface analysis of the products was obtained by using X-ray photoelectron spectroscopy (XPS) on an XlESCALAB 250Xi electron spectrometer from VG Scientific with a monochromatic Al K␣ radiation. Photoluminescence (PL) spectra were measured at room temperature on an Edinburgh FLS-5 spectrometer with excitation wavelength of 325 nm. 2.4. Sensor fabrication and measurements The gas-sensing experiments were performed on a homemade tube-furnace sensor test system, similar to our previous report [37]. Briefly, two Pt wires were fixed on an Al2 O3 substrate by Ag paste (Wuhan Youle Optoelectronics Technology Co., Ltd., China), and then calcined at 550 ◦ C for 30 min in a tube furnace to fix Pt wires and substrate. After that, a proper amount of the assynthesized ZnO ultrasonically dispersed into ethanol and then drop-coated onto the Al2 O3 substrate, and placed into the tube furnace at 400 ◦ C for 20 h to ensure good ohmic contacts. The ohmic contact was confirmed by a rectangular sweep voltage (continuously set at −5 V, −2.5 V, 0 V, 2.5 V, and 5 V) before gas sensing test using Keithley 2601 Sourcemeter (Keither Instrument Inc., USA). The targeted gases with different concentrations of 0.1–5 ppm for benzene, and 50–1000 ppb for acetone were mixed by a mixer connected with synthetic air (20.9 vol.% O2 , 79.1 vol.% N2 ) and standard gas (49.8 ppm for benzene, 51.8 ppm for acetone in synthetic air, respectively. Beijing Hua Yuan Gas Chemical Industry Co., Ltd., China) controlled by two digital mass flow controllers. The target gas of certain concentrations was introduced into the furnace with a 600 ml min−1 flow rate, and tested at different operating temperature. The bias was fixed at 5 V and the current was recorded using Keithley 2601 Sourcemeter. The sensor response is defined as the ratio Ra /Rg − 1, where Ra and Rg are the sensor resistance in air and in the test gas, respectively. The sensitivity is defined as the ratio of response towards gas-concentration (response/concentration, ppm−1 ). The response time is defined as the time required for sensor resistance reaching to 90% of the final value after exposed to the target gas. The recovery time is defined as the time required for reducing to 10% of the saturation value after exposed to the clean air.
3. Results and discussion 3.1. Morphological and structural characteristics The typical morphologies and microstructures of MOF-5 precursors and the derived ZnO products were shown in Fig. 1. In contrast,
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Fig. 1. (a, c) SEM and TEM images of MOF-5 precursors; (b, d) SEM and TEM images of the as-synthesized hierarchical hollow ZnO nanocages; (e) HRTEM image of ZnO nanocages obtained at 450 ◦ C (insets are lattice distance and SAED pattern). (For interpretation of the references to color in the citation of this figure, the reader is referred to the web version of the article.)
the derived ZnO nanostructures preserve the well-defined spherical morphology after thermal decomposition of spherical MOF-5 particles, as observed by SEM in Fig. 1a and b. The average particle size is ∼100 nm for MOF-5, and ∼100 nm for MOF-derived ZnO products. No significant size shrinkage was observed when MOF precursor was thermally decomposed into ZnO nanostructures. Despite the similar morphology, the discernable difference in microstructures is presented in TEM images in Fig. 1c for MOF particles and Fig. 1d and e for MOF-derived ZnO. In Fig. 1c, the MOF-5 particles appear as solid spheres with ∼10 nm nanoparticles as building blocks, which are different from traditional micro-size MOF-5 cubes because of the steric hindrance effects of PVP capping molecules, inhibiting the crystal grains growing up [38]. It clearly shows in Fig. S1, the crystal grains without PVPencapsulation will be form to micro-size cubes. Interestingly, the distinct microstructure evolution occurs after pyrolysis in 450 ◦ C for 1 h. The resultant ZnO products show the typical hollow nanostructures with ∼60 nm cavity in diameter and ∼25 nm porous shell in thickness, clearly seen by TEM observation in Fig. 1d and e. A few broken particles are also found out with hollow interiors as indicated by the yellow circles in Fig. 1b, in agreement with the above TEM results. The shell of the as-prepared ZnO hollow nanostructures was seemingly constructed by ultrafine interpenetrated nanoparticles with an average diameter of ∼20 nm. Furthermore, high-resolution TEM observation in right-upper inset of Fig. 1e reveals the lattice interplane distance of ZnO is 0.259 nm, assigned to the (002) interplane spacing of hexagonal wurtzite phase of ZnO, and the selected area electron diffraction (SAED) reveals the quasimonocrystalline structure in porous ZnO hollow nanocages, the discrete diffraction cycles are assigned to the (100), (101), (103) and (203) crystal planes of wurtzite ZnO. All these results suggest a progressive inward-hollowing process occurs during the thermal
decomposition of MOF-5 spherical particles into ZnO, indicative of the ZnO species outward diffusion and structural reconstruction. This interesting phenomenon based on Kirkendall effect is also found in the previous report [39]. The corresponding powder X-ray diffraction (XRD) patterns of MOF precursor and the derived hollow ZnO nanostructures are shown in Fig. 2a. Obviously, the diffraction peaks of MOF precursors are in good agreement with the stimulated MOF-5. The diffraction peaks of the as-synthesized ZnO can be perfectly indexed to the hexagonal wurtzite structure of ZnO (JCPDS card No. 361451) without any impurities, consistent with the above SAED cycle assignment. The average diameter of ZnO nanoparticles is ∼25 nm by Scherrer’s equation based on the (101) lattice plane, in accordance with the TEM results. Thermogravimetric (TG) analysis confirmed that 40% weight loss occurs before 500 ◦ C (Fig. 2b). Such a weight loss suggests that large amounts of H2 O, CO2 will be released during the MOF pyrolysis process. After 500 ◦ C, no weight loss was observed and pure ZnO was obtained. Combining XRD, SEM and TEM results, 450 ◦ C is a suitable calcination temperature to maintain the morphology of MOF-5 precursors and transform completely into ZnO hollow porous nanostructures. The Brunauer–Emmett–Teller (BET) surface area of MOF-5 precursors and ZnO nanocages are 154 m2 /g and 30 m2 /g (Fig. 2c and d), respectively. Typically, such high specific surface area and unique hollow porous structures of ZnO are due to the three-dimensional network with large internal space of porous selfsacrificial MOF-5 frameworks. Therefore, this may offer sufficiently exposed interfaces, facilitating the adsorption–desorption process and interfacial charge transfer between analyte gas and ZnO nanoparticles. It can be anticipated that the as-prepared hollow ZnO nanostructures perform the enhanced chemical gas-sensing properties.
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Fig. 2. (a) XRD patterns of MOF-5 and as-synthesized hierarchical hollow ZnO nanocages; (b) TG curve of MOF-5 precursors; (c, d) N2 adsorption–desorption isotherm of the MOF-5 precursors and hollow ZnO nanocages, respectively (insets are the corresponding BJH pore size distribution).
3.2. ppb or sub-ppm level VOCs gas-sensing performance Fig. 3a and c displays the electrical resistance response against operating temperature when exposed to 500 ppb acetone and 1 ppm benzene, respectively. Obviously, the sensing responses experience a climbing increase firstly, and the highest response is reached at 300 ◦ C with a response value of 6.36 for acetone, and 450 ◦ C with a value of 0.92 for benzene. After these peaks, the response value sharply decreased. The influence of operating temperature on sensing responses was ascribed to the surface chemical activation and dynamic adsorption–desorption balance between analyte gas and ZnO nanocages [40]. For lower operating temperature, the low chemical activation of ZnO nanocages and slow diffusion of analyte molecules lead to a poor gaseous acetone response. When increasing the operating temperature, the adsorbed gas molecules with high activation may easily escape from sensor surface, resulting to a decreasing response. Furthermore, the combustion of the target gas before interfacial chemical reaction taking place may be another factor for the decreased gas response in high operating temperature. Fig. 3b and d depicts the comparative sensing performance of the as-prepared ZnO nanocages and singular ZnO nanoparticles as reference towards low-concentration of acetone and benzene, respectively. In contrast with the singular ZnO nanoparticles as reference, the as-prepared hierarchical ZnO nanocages perform the significantly enhanced sensing responses towards acetone at a wide concentration range of 50–1000 ppb as shown in Fig. 3b, and the sensitivity is one order of magnitude higher than those of the ZnO nanoparticles as reference, as listed in Table 1. The important parameters of response and recovery times are also investigated, as shown in Table 1. For example, the response/recovery time of hierarchical hollow ZnO nanocages is 187/253 s for 100 ppb acetone, longer than 176/156 s of singular ZnO nanoparticles and other types of
ZnO gas sensors towards acetone (Table S1). This may be explained that the low-concentration VOCs detection leads to the relatively slow surface adsorption–desorption kinetics of analytes, thus producing relatively longer response/recovery time. Nevertheless, when compared with the reported literatures [15,19,41], our MOF-derived hollow ZnO nanocages perform higher sensitivity towards acetone at comparable low-concentration level as summarized in Table 2. More importantly, for carcinogenic and obstinate benzene molecules, the developed MOF-derived ZnO hollow cages behave the enhanced sub-ppm level sensing responses and faster response/recovery time as shown in Fig. 3d and Table 1 when compared with the referenced ZnO nanoparticles, also perform the improved sensing properties than other reported ZnO nanomaterials, as summarized in Table 2, which is very valuable in trace detecting the indoor toxic aromatic compounds. The response stability of ZnO nanocages was characterized at 300 ◦ C operating temperature with 100 ppb gaseous acetone and 400 ◦ C operating temperature with 0.1 ppm gaseous benzene. Fig. 4a and b shows the ZnO nanocages have good response stability of 2.2 ± 7.2% for acetone and 0.28 ± 14% for benzene, and reproducibility with high sensing performance after six on–off cycles. In addition, the ZnO nanocages also perform good longterm stability (Fig. 4c). In order to evaluate the selectivity of the ZnO hollow nanocages as gas-sensing materials, the selectivity of hierarchical hollow ZnO nanocages was tested at 300 ◦ C operating temperature towards low-concentrations of acetone, ethanol, ethyl acetate, toluene and benzene range from 50 ppb to 1000 ppb (Fig. 4d). Obviously, the sample had much lower response towards the difficult-to-decompose toluene and benzene compared with acetone, ethanol and ethyl acetate under the 300 ◦ C operation temperature. This operating temperature is the optimal one for detecting acetone, as shown in Fig. 3a, but poor for detecting
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Fig. 3. (a) Acetone gas response of hierarchical hollow ZnO nanocages to 500 ppb at different operating temperatures; (b) acetone gas response comparison between hierarchical hollow ZnO nanocages (ZnO-H) and singular 0D ZnO nanoparticles (ZnO-S) with different low-concentrations at 300 ◦ C operating temperature; (c) benzene gas response of ZnO-H to 1 ppm at different operating temperature; (d) Benzene gas response comparison between ZnO-H and ZnO-S with concentrations range from 0.1 to 5 ppm at 400 ◦ C operating temperature.
Table 1 Response/recovery time and sensitivity of hierarchical hollow ZnO nanocages (ZnO-H) and singular ZnO nanoparticles (ZnO-S) to different acetone and benzene gas concentrations at 300 ◦ C/400 ◦ C operating temperature. Sample
Gas
Concentration (ppb/ppm)
Response timea (s)
Recovery timeb (s)
Sensitivityc (ppm−1 )
ZnO-H
Acetone
ZnO-S
Acetone
50 100 500 1000 50 100 500 1000
107 187 322 441 236 176 158 158
284 253 268 384 107 156 152 165
15.3 19.0 12.8 13.8 1.5 1.3 0.4 0.3
ZnO-H
Benzene
ZnO-S
Benzene
39 78 84 64 134 246 189 134
118 139 129 144 177 194 172 197
a b c
0.1 0.5 1 5 0.1 0.5 1 5
Response time: the time required for sensor resistance reaching to 90% of the final value after exposed to the target gas. Recovery time: the time required for reducing to 10% of the saturation value after exposed to the clean air. Sensitivity: the ration of response and gas-concentration (response/concentration).
2.3 1.0 0.8 0.3 0.9 0.2 0.1 0.05
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Table 2 Comparison of the gaseous acetone and benzene sensing performance with different ZnO-based sensors. Materials
Gas
Temperature (◦ C)
Concentration (ppm)
Sensitivity (ppm−1 )
Reference
ZnO microsphere ZnO microbelts ZnO nanosheets ZnO nanocages
Acetone Acetone Acetone
400 300 400 300
5 100 100 1 0.05 100 10 50 5 0.1
0.40 0.41 0.32 13.8 15.3 0.075 0.11 0.20 0.35 2.30
[41] [15] [19] This work This work [15] [42] [43] This work This work
ZnO microbelts ZnO microspheres ZnO nanorods ZnO nanocages
Acetone Benzene Benzene Benzene Benzene
300 300 370 400
benzene, as shown in Fig. 3c. Evidently, the ZnO nanocages had the much greater sensing response to acetone than other five reductive VOCs gases. This implied that hierarchical hollow ZnO nanocages, although the sub-ppm level gas-sensing performance towards obstinate benzene, had better gas-sensing selectivity towards acetone at specified 300 ◦ C operating temperature. This trend may be ascribed to the bond dissociation energy (BDE) of different gases. The BDE of H CH2 COCH3 (393 kJ/mol) in acetone is less than
that of H OCH2 CH3 (436 kJ/mol) in ethanol, H CH2 COOCH2 CH3 (410 kJ/mol) in ethyl acetate and H C6 H5 (431 kJ/mol) in benzene, but similar with CH3 C6 H5 (389 kJ/mol) in toluene [44]. Another factor may be associated with the number of electrons released during the redox reaction (Eq. (2)) at 300 ◦ C operating temperature. Therefore, the hollow ZnO nanocages show good selectivity towards acetone with the relatively lower bond dissociation energy and more released electrons.
Fig. 4. Response reproducibility of hierarchical hollow ZnO nanocages at (a) 300 ◦ C operating temperature with 100 ppb gaseous acetone and (b) 400 ◦ C operating temperature with 0.1 ppm gaseous benzene; (c) long-term response stability of 100 ppb acetone and 0.1 ppm benzene; (d) selectivity tests of ZnO nanocages with low-concentrations gaseous acetone, ethanol, ethyl acetate, toluene and benzene at 300 ◦ C operating temperature.
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3.3. Gas-sensing mechanism of ZnO nanocages It is well known that the sensing mechanism of MOS oxides is based on the thickness changes of surface electron depletion layers in grain boundary of MOS interparticles, or D–L model [45–47]. For n-type ZnO, the surface electron depletion layers of ZnO grain boundary are thickened when exposed to air, because the conduction band (CB) electron is captured by surface-adsorbed oxygen, then thinned by election injection originated from the surface redox reaction when shifted from oxidative air to the reducing atmosphere of the analytes, producing larger electron-conducting cross-section within grain boundaries and bigger conductivity. From the viewpoint of surface reaction, the pre-adsorbed oxygen molecules are ionized and turned into the more active oxygen species (like O2− , O− or O2 − ) through capturing the free electrons in CB of ZnO. When exposed to the reducing analytes, like acetone or benzene, the active oxygen species oxidize the reducing molecules and simultaneously inject electrons back to the CB, increasing the conductivity of ZnO sensor [48]. Therefore, the related surface reaction of ZnO hollow nanocages is written as Eqs. (1)–(3) when exposed to air and reducing analytes, like acetone and benzene.
(1) CH3 COCH3 + 8O(ads) 2− → 3CO2 + 3H2 O + 16e−
(2)
C6 H6 + 15O(ads) 2− → 6CO2 + 3H2 O + 30e−
(3)
Furthermore, the chemical sensing response is closely related to the surface oxygen vacancies [49]. Actually, as the electron acceptor, oxygen vacancies with positive charge easily capture the electron generated by redox reaction between reducing gas and oxygen species, resulting in effective separation the electron–hole pairs and enhancing gas-sensing performance [50,51]. X-ray photoelectron spectroscopy (XPS) results in Fig. 5a and b show the contents of oxygen vacancies are 27.4% in hierarchical hollow ZnO nanocages, which are much higher than singular ZnO nanoparticles (17.1%), suggesting highly capacity for electron–hole pairs separation and surface oxidizability towards VOCs. The results of photoluminescence (PL) spectra further support the above-mentioned facts (Fig. 5c). Typically, the UV near-bandedge emission around 380 nm is considered as the characteristic emission of ZnO, the green emission around 510 nm is generally accepted as the emission from ZnO structural defects with oxygen vacancies [52]. Moreover, the relative intensity ratio of the UV near-band-edge emission to the deep-level green emission is used to estimate the concentration of oxygen vacancies [53]. A stronger green emission derived from abundant oxygen vacancies will lead to a smaller intensity ratio. The calculation result shows that such intensity ratios in hierarchical hollow ZnO nanocages and singular ZnO nanoparticles are 0.28 and 2.19, respectively, indicating a much higher concentration of oxygen vacancies in hierarchical hollow ZnO nanocages. In addition, it is anticipated that the hierarchical hollow ZnO nanocages with interpenetrated nanoparticles within porous shells offer a relative lower potential barrier than agglomeration singular ZnO nanoparticles due to larger grain boundary cross-section of melt ZnO interparticles within shells (observed in Fig. 1e and schematically depicted in Fig. 6), and the high specific surface area facilitating surface adsorption–diffusion and reaction kinetics for oxygen and analyte molecules [54]. In conclusion, the hierarchical MOF-derived ZnO nanocages provides
Fig. 5. O 1s XPS spectra of (a) hierarchical hollow ZnO nanocages (ZnO-H) and (b) singular 0D ZnO nanoparticles (ZnO-S) (OI : lattice oxygen; OII : electron-donor defects with oxygen vacancies; OIII : adsorbed molecular water); (c) PL spectra of hierarchical hollow ZnO nanocages and singular 0D ZnO nanoparticles.
more surface oxygen vacancies due to their more exposed active sites and larger specific surface area, which act as the reactive locations for the ionization of the pre-adsorbed oxygen molecules and oxidation process of reducing analytes, producing the enhanced chemical sensing properties towards acetone and benzene, respectively.
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Fig. 6. Schematic comparison of potential barrier between (a) melt ZnO nanoparticles within shells of hierarchical hollow ZnO nanocages and (b) singular ZnO nanoparticles.
4. Conclusions In conclusion, we have successfully synthesized the unique hierarchical hollow ZnO nanocages by a facile strategy through the direct thermolysis of spherical Zn-based metal–organic framework. The as-synthesized hollow ZnO products present the typical hierarchical structures with hollow interiors enveloped by interpenetrated ZnO nanoparticles as porous shell, providing structurally combined meso-/macro-porous channels for facilitating the diffusion and surface reaction of gas molecules. The gas-sensing experiments demonstrate that, in contrast with singular ZnO nanoparticles, the ZnO nanocages show significantly enhanced chemical sensing sensitivity and selectivity towards lowconcentration volatile organic compounds, typically, acetone and benzene. The hierarchical MOF-derived ZnO nanocages provides more surface oxygen vacancies due to their more exposed active sites and larger specific surface area, which act as the reactive locations for the ionization of the pre-adsorbed oxygen molecules and oxidation process of reducing analytes, producing the enhanced chemical sensing properties towards acetone and benzene, respectively. This work clearly elucidates the architecture-dependent performance in gas sensing, and also provides a facile strategy to synthesize high performance ZnO sensors. Acknowledgements This work was financially supported by National Natural Science Foundation of China (No. 51272253), the 863 Hi-tech Research and Development Program of China (Grant No. 2013AA031801), and the Strategic Project of Science and Technology of Chinese Academy of Sciences (No. XDB05050000). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2015.11.034. References [1] J.H. Pan, G. Han, R. Zhou, X.S. Zhao, Hierarchical N-doped TiO2 hollow microspheres consisting of nanothorns with exposed anatase {101} facets, Chem. Commun. 47 (2011) 6942–6944. [2] H. Zhang, Q. Zhu, Y. Zhang, Y. Wang, L. Zhao, B. Yu, One-pot synthesis and hierarchical assembly of hollow Cu2 O microspheres with nanocrystals-composed porous multishell and their gas-sensing properties, Adv. Funct. Mater. 17 (2007) 2766–2771. [3] H. Su, Y.F. Xu, S.C. Feng, Z.G. Wu, X.P. Sun, C.H. Shen, J.Q. Wang, J.T. Li, L. Huang, S.G. Sun, Hierarchical Mn2 O3 hollow microspheres as anode material of lithium ion battery and its conversion reaction mechanism investigated by XANES, ACS Appl. Mater. Interfaces 7 (2015) 8488–8494. [4] M. Samadpour, S. Gimenez, A.I. Zad, N. Taghavinia, I. Mora-Sero, Easily manufactured TiO2 hollow fibers for quantum dot sensitized solar cells, Phys. Chem. Chem. Phys. 14 (2012) 522–528. [5] J.-H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sens. Actuators B 140 (2009) 319–336.
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Biographies Wenhui Li is now a PhD student at Institute of Process Engineering, Chinese Academy of Sciences. His researches focus on sensing materials and environmental catalytic materials. Xiaofeng Wu received his PhD at Institute of Process Engineering, Chinese Academy of Sciences, in 2007. After post-doctoral research at Chonbuk National University in South Korea, he joined in the Institute of Process Engineering, in 2009, and now he is a professor of the Institute of Process Engineering. His research interests are preparation of nanostructural inorganic materials (metal@oxide semiconductor core–shell, hollow particles etc.) and their application in sensors, photocatalysis, thermal insulator, energy storage, etc. Ning Han received his PhD at Institute of Process Engineering, Chinese Academy of Sciences, in 2010. After post-doctoral research at City University of Hong Kong, he joined in the Institute of Process Engineering, in 2014, and now he is a professor of the Institute of Process Engineering. His research interests are preparation and application of gas sensing materials and technologies to fabricate gas sensors and the gas sensing mechanism. And he is also interested in III–V compound semiconductors preparation and applications. Jiayuan Chen is now a PhD student at Institute of Process Engineering, Chinese Academy of Sciences. His researches focus on graphene and graphene composite materials. Xihui Qian is now a postgraduate student at Institute of Process Engineering, Chinese Academy of Sciences. Her researches focus on hollow structural oxides and organic–inorganic composite materials. Yuzhou Deng is now a PhD student at Institute of Process Engineering, Chinese Academy of Sciences. His researches focus on metal-oxides materials and environmental catalytic materials. Wenxiang Tang is now a postdoctoral research assistant in University of Connecticut, USA. His researches focus on the synthesis of porous metal-oxides and the application in VOCs’ catalytic combustion. Yunfa Chen received his PhD in Material Science at Université Louis Pasteur Strasbourg (ULP), France in 1993. He is a professor of the University of Chinese Academy of Sciences, and Research Professor and Vice Director of Institute of Process Engineering. His current research interests are preparation and assembly of nanoparticles, functional materials, organic–inorganic composite materials and layered materials. And he is also interested in industrial application of nanomaterials.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
MOF-derived hierarchical hollow ZnO nanocages with enhanced low-concentration VOCs gas-sensing performance Wenhui Li a,b , Xiaofeng Wu a,∗ , Ning Han a , Jiayuan Chen a,b , Xihui Qian a,b , Yuzhou Deng a,b, Wenxiang Tang a,c , Yunfa Chen a,∗ a
State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China c Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136, USA b
a r t i c l e
i n f o
Article history: Received 21 July 2015 Received in revised form 8 October 2015 Accepted 6 November 2015 Available online 10 November 2015 Keywords: MOF-derived ZnO nanocages Hierarchical hollow structure ppb or sub-ppm level VOCs gas-sensors
a b s t r a c t The design and synthesis of nanostructured ZnO with high chemical sensing properties, especially towards ppb or sub-ppm level VOC gases is still highly desired and challengeable. Herein, the hierarchical hollow ZnO nanocages were synthesized by a facile strategy through the simple and direct pyrolysis of Zn-based metal-organic framework. The as-synthesized hollow ZnO products present the typical hierarchical structures with hollow interiors enveloped by interpenetrated ZnO nanoparticles as porous shell, providing structurally combined meso-/macro-porous channels for facilitating the diffusion and surface reaction of gas molecules. The gas-sensing experiments demonstrate that, in contrast with singular ZnO nanoparticles, the ZnO nanocages show significantly enhanced chemical sensing sensitivity and selectivity towards low-concentration volatile organic compounds, typically, acetone and benzene. Furthermore, the ZnO hollow nanocages perform sub-ppm level sensitivity with 2.3 ppm−1 towards 0.1 ppm benzene, and ppb level sensitivity with 15.3 ppm−1 towards 50 ppb acetone, respectively. The enhanced sensing performance of the MOF-derived ZnO nanocages is ascribed to the unique hierarchical structure with high specific surface area and abundant exposed active sites with surface-adsorbed oxygen. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Hierarchical hollow metal oxides have always been an extremely hot topic over the past decades in nanotechnology [1–5]. The unique structure with large specific surface area and abundant exposed active sites has significantly enhanced physicochemical properties compared with their singular nanoscale building blocks. High-sensitive sensing materials were highly demanded for low-concentration volatile organic compounds (VOCs) detection in the views of environmental protection and human health. As a wide bandgap metal oxide semiconductor (MOS), zinc oxides have attracted extensive interests applied to chemical gas-sensing materials with high performance, low cost and environmental friendliness [6–8]. However, due to the easy recombination of electron–hole pairs, low exposure of reactive crystal planes and few pathways for gas-solid contact, traditional ZnO-based sensing materials still suffer from poor sensitive and selectivity towards
∗ Corresponding authors. E-mail addresses: [email protected] (X. Wu), [email protected] (Y. Chen). http://dx.doi.org/10.1016/j.snb.2015.11.034 0925-4005/© 2015 Elsevier B.V. All rights reserved.
low-concentration VOCs, resulting in the limitation of practical application [9–11]. Continuous efforts were made to overcome these drawbacks for improving the ZnO sensing properties, such as ion-doping [12], noble metal–MOS [13] or MOS–MOS heterostructure [14] to increase the mobility and separation efficiency of electron–hole pairs, and structure-controlling to form hollow or porous nanostructure for increasing exposure active sites [15,16]. Nevertheless, these improved materials still have undesirable sensing performances towards low-concentration VOC gas, especially undetectable ppb or sub-ppm level sensing responses, severe signal drifts, and much less selectivity [17–19]. Therefore, it is highly desired to synthesize the materials with high chemical sensing properties, especially towards ppb or sub-ppm level VOC gases. Metal–organic frameworks (MOFs) as a new class of organic–inorganic hybrid materials have received considerable attention because of the distinctive characteristics and widely potential technical application, e.g. gas adsorption and separation, catalysis, chemical sensors and drug delivery [20–28]. Most recently, MOFs have been explored as promising self-sacrificial templates or precursors to construct porous oxide nanostructures
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with intriguing properties. For example, Fe2 O3 @TiO2 composite nanoparticles were synthesized by using Fe-containing nanoscale MOFs coated with amorphous titania for visible light-driven hydrogen production from water [29]. High symmetric porous Co3 O4 hollow dodecahedra were fabricated from ZIF-67 with highly enhanced lithium storage capability [30]. After thermal treatment of core–shell ZIF-8@ZIF-67 crystals, selectively functionalized nanoporous hybrid carbon was constructed with a distinguished electrochemical performance as an electrode in supercapacitors [31]. These great progresses were achieved because of the remarkable properties with selectable metal ion centers, multifunctional organic ligands, well-developed porosity and diverse morphologies of metal-organic frameworks. It is challengeable to recognize and detect low-concentration VOCs, especially ppb to sub-ppm level from indoor or outdoor emission sources at residential or work place. In these places, the exposure to low-concentration VOCs, especially BTX (benzene–toluene–xylene), is dangerous and carcinogenic. Accordingly, our emphasis is mostly placed on improved sensitive ZnO-based nanostructures for detecting low-concentration VOC at ppb–ppm range. Indifferent with the mostly reported literature, the MOF-derived hollow ZnO perform improved VOC-sensing properties towards BTX, ethyl acetate, acetone, etc., of which are common VOC pollutants in indoor or outdoor environments, and good sensitivity at relative low concentration (ppb–sub-ppm level). Also, our work presented a facile and cost-effective way to synthesize high-sensitive hollow ZnO gas sensors, when compared with the previous reports [32–35]. Herein, the unique hierarchical ZnO nanocages with hollow interiors enveloped by porous shells was synthesized by simply and directly temperature-programmed decomposition of spherical Zn-based metal–organic frameworks (MOF-5) precursor. More importantly and interestingly, the as-synthesized ZnO nanocages exhibited enhanced chemical sensing performances towards two typical low-concentration VOCs, acetone and obstinate benzene at ppb and sub-ppm levels, exhibiting the good promise of this MOF pyrolysis strategy in developing highly sensitive and selective gas sensing materials. 2. Experimental 2.1. Materials Zn(NO3 )·6H2 O was obtained from Beijing Chemical Works. 1,4-dicarboxybenzene (H2 DBC), Polyvinyl pyrrolidone (PVP, M.W. 30,000) and N,N-dimethylformamide (DMF) were purchased from Sinopharm. All chemicals were used directly without further purification. The water was made from Millipore Milli-Q water (15 M cm). 2.2. Synthesis of hierarchical hollow ZnO nanocages The MOF-5 precursors were firstly synthesized referenced by the previous literature [36], but modified. Typically, 5.436 g PVP was dissolved into the 60 ml DMF/ethanol (5/3 in v/v) mixed solvent. Then, 0.1088 g Zn(NO3 )·6H2 O, 0.225 g H2 DBC were added under vigorous stirring to form a clear solution. The solution was transferred into a 100 ml Teflon autoclave, and placed into an oven with a constant temperature of 140 ◦ C for 3 h, then cooled to the room temperature. The white precipitates were isolated from the supernatant by centrifugation and washed with DMF and CHCl2 at least 3 times, and then dried at 80 ◦ C overnight. Finally, the collected white powders were used as precursors to prepared ZnO samples by carefully temperature-programmed thermal decomposition process with a slow rate of 1 ◦ C min−1 from room
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temperature. When the temperature lifted to 450 ◦ C, the samples were kept continuously for 1 h and gradually cooled to the room temperature. 2.3. Characterization The crystallinity of the products was characterized by powder X-ray diffraction (XRD) on a Panalytical X’Pert PRO system using Cu-K␣ radiation. The morphologies and microstructure were investigated using a field-emission scanning electron microscope (FE-SEM, JEOL JSM-6700F, Japan) operating at 15 kV and a transmission electron microscopy (TEM, JEOL JEM-2010F) with an accelerating voltage of 200 kV. Thermogravimetric analysis (TGA) was performed on a NETZSCH STA449F3 apparatus with a heating rate of 10 ◦ C min−1 from 35 ◦ C to 900 ◦ C under air atmosphere. The N2 adsorption–desorption isotherms were measured on an automatic surface analyzer (SSA-7300, China). Before the measurement, the samples were outgassed at 150 ◦ C for 10 h. The surface analysis of the products was obtained by using X-ray photoelectron spectroscopy (XPS) on an XlESCALAB 250Xi electron spectrometer from VG Scientific with a monochromatic Al K␣ radiation. Photoluminescence (PL) spectra were measured at room temperature on an Edinburgh FLS-5 spectrometer with excitation wavelength of 325 nm. 2.4. Sensor fabrication and measurements The gas-sensing experiments were performed on a homemade tube-furnace sensor test system, similar to our previous report [37]. Briefly, two Pt wires were fixed on an Al2 O3 substrate by Ag paste (Wuhan Youle Optoelectronics Technology Co., Ltd., China), and then calcined at 550 ◦ C for 30 min in a tube furnace to fix Pt wires and substrate. After that, a proper amount of the assynthesized ZnO ultrasonically dispersed into ethanol and then drop-coated onto the Al2 O3 substrate, and placed into the tube furnace at 400 ◦ C for 20 h to ensure good ohmic contacts. The ohmic contact was confirmed by a rectangular sweep voltage (continuously set at −5 V, −2.5 V, 0 V, 2.5 V, and 5 V) before gas sensing test using Keithley 2601 Sourcemeter (Keither Instrument Inc., USA). The targeted gases with different concentrations of 0.1–5 ppm for benzene, and 50–1000 ppb for acetone were mixed by a mixer connected with synthetic air (20.9 vol.% O2 , 79.1 vol.% N2 ) and standard gas (49.8 ppm for benzene, 51.8 ppm for acetone in synthetic air, respectively. Beijing Hua Yuan Gas Chemical Industry Co., Ltd., China) controlled by two digital mass flow controllers. The target gas of certain concentrations was introduced into the furnace with a 600 ml min−1 flow rate, and tested at different operating temperature. The bias was fixed at 5 V and the current was recorded using Keithley 2601 Sourcemeter. The sensor response is defined as the ratio Ra /Rg − 1, where Ra and Rg are the sensor resistance in air and in the test gas, respectively. The sensitivity is defined as the ratio of response towards gas-concentration (response/concentration, ppm−1 ). The response time is defined as the time required for sensor resistance reaching to 90% of the final value after exposed to the target gas. The recovery time is defined as the time required for reducing to 10% of the saturation value after exposed to the clean air.
3. Results and discussion 3.1. Morphological and structural characteristics The typical morphologies and microstructures of MOF-5 precursors and the derived ZnO products were shown in Fig. 1. In contrast,
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Fig. 1. (a, c) SEM and TEM images of MOF-5 precursors; (b, d) SEM and TEM images of the as-synthesized hierarchical hollow ZnO nanocages; (e) HRTEM image of ZnO nanocages obtained at 450 ◦ C (insets are lattice distance and SAED pattern). (For interpretation of the references to color in the citation of this figure, the reader is referred to the web version of the article.)
the derived ZnO nanostructures preserve the well-defined spherical morphology after thermal decomposition of spherical MOF-5 particles, as observed by SEM in Fig. 1a and b. The average particle size is ∼100 nm for MOF-5, and ∼100 nm for MOF-derived ZnO products. No significant size shrinkage was observed when MOF precursor was thermally decomposed into ZnO nanostructures. Despite the similar morphology, the discernable difference in microstructures is presented in TEM images in Fig. 1c for MOF particles and Fig. 1d and e for MOF-derived ZnO. In Fig. 1c, the MOF-5 particles appear as solid spheres with ∼10 nm nanoparticles as building blocks, which are different from traditional micro-size MOF-5 cubes because of the steric hindrance effects of PVP capping molecules, inhibiting the crystal grains growing up [38]. It clearly shows in Fig. S1, the crystal grains without PVPencapsulation will be form to micro-size cubes. Interestingly, the distinct microstructure evolution occurs after pyrolysis in 450 ◦ C for 1 h. The resultant ZnO products show the typical hollow nanostructures with ∼60 nm cavity in diameter and ∼25 nm porous shell in thickness, clearly seen by TEM observation in Fig. 1d and e. A few broken particles are also found out with hollow interiors as indicated by the yellow circles in Fig. 1b, in agreement with the above TEM results. The shell of the as-prepared ZnO hollow nanostructures was seemingly constructed by ultrafine interpenetrated nanoparticles with an average diameter of ∼20 nm. Furthermore, high-resolution TEM observation in right-upper inset of Fig. 1e reveals the lattice interplane distance of ZnO is 0.259 nm, assigned to the (002) interplane spacing of hexagonal wurtzite phase of ZnO, and the selected area electron diffraction (SAED) reveals the quasimonocrystalline structure in porous ZnO hollow nanocages, the discrete diffraction cycles are assigned to the (100), (101), (103) and (203) crystal planes of wurtzite ZnO. All these results suggest a progressive inward-hollowing process occurs during the thermal
decomposition of MOF-5 spherical particles into ZnO, indicative of the ZnO species outward diffusion and structural reconstruction. This interesting phenomenon based on Kirkendall effect is also found in the previous report [39]. The corresponding powder X-ray diffraction (XRD) patterns of MOF precursor and the derived hollow ZnO nanostructures are shown in Fig. 2a. Obviously, the diffraction peaks of MOF precursors are in good agreement with the stimulated MOF-5. The diffraction peaks of the as-synthesized ZnO can be perfectly indexed to the hexagonal wurtzite structure of ZnO (JCPDS card No. 361451) without any impurities, consistent with the above SAED cycle assignment. The average diameter of ZnO nanoparticles is ∼25 nm by Scherrer’s equation based on the (101) lattice plane, in accordance with the TEM results. Thermogravimetric (TG) analysis confirmed that 40% weight loss occurs before 500 ◦ C (Fig. 2b). Such a weight loss suggests that large amounts of H2 O, CO2 will be released during the MOF pyrolysis process. After 500 ◦ C, no weight loss was observed and pure ZnO was obtained. Combining XRD, SEM and TEM results, 450 ◦ C is a suitable calcination temperature to maintain the morphology of MOF-5 precursors and transform completely into ZnO hollow porous nanostructures. The Brunauer–Emmett–Teller (BET) surface area of MOF-5 precursors and ZnO nanocages are 154 m2 /g and 30 m2 /g (Fig. 2c and d), respectively. Typically, such high specific surface area and unique hollow porous structures of ZnO are due to the three-dimensional network with large internal space of porous selfsacrificial MOF-5 frameworks. Therefore, this may offer sufficiently exposed interfaces, facilitating the adsorption–desorption process and interfacial charge transfer between analyte gas and ZnO nanoparticles. It can be anticipated that the as-prepared hollow ZnO nanostructures perform the enhanced chemical gas-sensing properties.
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Fig. 2. (a) XRD patterns of MOF-5 and as-synthesized hierarchical hollow ZnO nanocages; (b) TG curve of MOF-5 precursors; (c, d) N2 adsorption–desorption isotherm of the MOF-5 precursors and hollow ZnO nanocages, respectively (insets are the corresponding BJH pore size distribution).
3.2. ppb or sub-ppm level VOCs gas-sensing performance Fig. 3a and c displays the electrical resistance response against operating temperature when exposed to 500 ppb acetone and 1 ppm benzene, respectively. Obviously, the sensing responses experience a climbing increase firstly, and the highest response is reached at 300 ◦ C with a response value of 6.36 for acetone, and 450 ◦ C with a value of 0.92 for benzene. After these peaks, the response value sharply decreased. The influence of operating temperature on sensing responses was ascribed to the surface chemical activation and dynamic adsorption–desorption balance between analyte gas and ZnO nanocages [40]. For lower operating temperature, the low chemical activation of ZnO nanocages and slow diffusion of analyte molecules lead to a poor gaseous acetone response. When increasing the operating temperature, the adsorbed gas molecules with high activation may easily escape from sensor surface, resulting to a decreasing response. Furthermore, the combustion of the target gas before interfacial chemical reaction taking place may be another factor for the decreased gas response in high operating temperature. Fig. 3b and d depicts the comparative sensing performance of the as-prepared ZnO nanocages and singular ZnO nanoparticles as reference towards low-concentration of acetone and benzene, respectively. In contrast with the singular ZnO nanoparticles as reference, the as-prepared hierarchical ZnO nanocages perform the significantly enhanced sensing responses towards acetone at a wide concentration range of 50–1000 ppb as shown in Fig. 3b, and the sensitivity is one order of magnitude higher than those of the ZnO nanoparticles as reference, as listed in Table 1. The important parameters of response and recovery times are also investigated, as shown in Table 1. For example, the response/recovery time of hierarchical hollow ZnO nanocages is 187/253 s for 100 ppb acetone, longer than 176/156 s of singular ZnO nanoparticles and other types of
ZnO gas sensors towards acetone (Table S1). This may be explained that the low-concentration VOCs detection leads to the relatively slow surface adsorption–desorption kinetics of analytes, thus producing relatively longer response/recovery time. Nevertheless, when compared with the reported literatures [15,19,41], our MOF-derived hollow ZnO nanocages perform higher sensitivity towards acetone at comparable low-concentration level as summarized in Table 2. More importantly, for carcinogenic and obstinate benzene molecules, the developed MOF-derived ZnO hollow cages behave the enhanced sub-ppm level sensing responses and faster response/recovery time as shown in Fig. 3d and Table 1 when compared with the referenced ZnO nanoparticles, also perform the improved sensing properties than other reported ZnO nanomaterials, as summarized in Table 2, which is very valuable in trace detecting the indoor toxic aromatic compounds. The response stability of ZnO nanocages was characterized at 300 ◦ C operating temperature with 100 ppb gaseous acetone and 400 ◦ C operating temperature with 0.1 ppm gaseous benzene. Fig. 4a and b shows the ZnO nanocages have good response stability of 2.2 ± 7.2% for acetone and 0.28 ± 14% for benzene, and reproducibility with high sensing performance after six on–off cycles. In addition, the ZnO nanocages also perform good longterm stability (Fig. 4c). In order to evaluate the selectivity of the ZnO hollow nanocages as gas-sensing materials, the selectivity of hierarchical hollow ZnO nanocages was tested at 300 ◦ C operating temperature towards low-concentrations of acetone, ethanol, ethyl acetate, toluene and benzene range from 50 ppb to 1000 ppb (Fig. 4d). Obviously, the sample had much lower response towards the difficult-to-decompose toluene and benzene compared with acetone, ethanol and ethyl acetate under the 300 ◦ C operation temperature. This operating temperature is the optimal one for detecting acetone, as shown in Fig. 3a, but poor for detecting
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Fig. 3. (a) Acetone gas response of hierarchical hollow ZnO nanocages to 500 ppb at different operating temperatures; (b) acetone gas response comparison between hierarchical hollow ZnO nanocages (ZnO-H) and singular 0D ZnO nanoparticles (ZnO-S) with different low-concentrations at 300 ◦ C operating temperature; (c) benzene gas response of ZnO-H to 1 ppm at different operating temperature; (d) Benzene gas response comparison between ZnO-H and ZnO-S with concentrations range from 0.1 to 5 ppm at 400 ◦ C operating temperature.
Table 1 Response/recovery time and sensitivity of hierarchical hollow ZnO nanocages (ZnO-H) and singular ZnO nanoparticles (ZnO-S) to different acetone and benzene gas concentrations at 300 ◦ C/400 ◦ C operating temperature. Sample
Gas
Concentration (ppb/ppm)
Response timea (s)
Recovery timeb (s)
Sensitivityc (ppm−1 )
ZnO-H
Acetone
ZnO-S
Acetone
50 100 500 1000 50 100 500 1000
107 187 322 441 236 176 158 158
284 253 268 384 107 156 152 165
15.3 19.0 12.8 13.8 1.5 1.3 0.4 0.3
ZnO-H
Benzene
ZnO-S
Benzene
39 78 84 64 134 246 189 134
118 139 129 144 177 194 172 197
a b c
0.1 0.5 1 5 0.1 0.5 1 5
Response time: the time required for sensor resistance reaching to 90% of the final value after exposed to the target gas. Recovery time: the time required for reducing to 10% of the saturation value after exposed to the clean air. Sensitivity: the ration of response and gas-concentration (response/concentration).
2.3 1.0 0.8 0.3 0.9 0.2 0.1 0.05
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Table 2 Comparison of the gaseous acetone and benzene sensing performance with different ZnO-based sensors. Materials
Gas
Temperature (◦ C)
Concentration (ppm)
Sensitivity (ppm−1 )
Reference
ZnO microsphere ZnO microbelts ZnO nanosheets ZnO nanocages
Acetone Acetone Acetone
400 300 400 300
5 100 100 1 0.05 100 10 50 5 0.1
0.40 0.41 0.32 13.8 15.3 0.075 0.11 0.20 0.35 2.30
[41] [15] [19] This work This work [15] [42] [43] This work This work
ZnO microbelts ZnO microspheres ZnO nanorods ZnO nanocages
Acetone Benzene Benzene Benzene Benzene
300 300 370 400
benzene, as shown in Fig. 3c. Evidently, the ZnO nanocages had the much greater sensing response to acetone than other five reductive VOCs gases. This implied that hierarchical hollow ZnO nanocages, although the sub-ppm level gas-sensing performance towards obstinate benzene, had better gas-sensing selectivity towards acetone at specified 300 ◦ C operating temperature. This trend may be ascribed to the bond dissociation energy (BDE) of different gases. The BDE of H CH2 COCH3 (393 kJ/mol) in acetone is less than
that of H OCH2 CH3 (436 kJ/mol) in ethanol, H CH2 COOCH2 CH3 (410 kJ/mol) in ethyl acetate and H C6 H5 (431 kJ/mol) in benzene, but similar with CH3 C6 H5 (389 kJ/mol) in toluene [44]. Another factor may be associated with the number of electrons released during the redox reaction (Eq. (2)) at 300 ◦ C operating temperature. Therefore, the hollow ZnO nanocages show good selectivity towards acetone with the relatively lower bond dissociation energy and more released electrons.
Fig. 4. Response reproducibility of hierarchical hollow ZnO nanocages at (a) 300 ◦ C operating temperature with 100 ppb gaseous acetone and (b) 400 ◦ C operating temperature with 0.1 ppm gaseous benzene; (c) long-term response stability of 100 ppb acetone and 0.1 ppm benzene; (d) selectivity tests of ZnO nanocages with low-concentrations gaseous acetone, ethanol, ethyl acetate, toluene and benzene at 300 ◦ C operating temperature.
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3.3. Gas-sensing mechanism of ZnO nanocages It is well known that the sensing mechanism of MOS oxides is based on the thickness changes of surface electron depletion layers in grain boundary of MOS interparticles, or D–L model [45–47]. For n-type ZnO, the surface electron depletion layers of ZnO grain boundary are thickened when exposed to air, because the conduction band (CB) electron is captured by surface-adsorbed oxygen, then thinned by election injection originated from the surface redox reaction when shifted from oxidative air to the reducing atmosphere of the analytes, producing larger electron-conducting cross-section within grain boundaries and bigger conductivity. From the viewpoint of surface reaction, the pre-adsorbed oxygen molecules are ionized and turned into the more active oxygen species (like O2− , O− or O2 − ) through capturing the free electrons in CB of ZnO. When exposed to the reducing analytes, like acetone or benzene, the active oxygen species oxidize the reducing molecules and simultaneously inject electrons back to the CB, increasing the conductivity of ZnO sensor [48]. Therefore, the related surface reaction of ZnO hollow nanocages is written as Eqs. (1)–(3) when exposed to air and reducing analytes, like acetone and benzene.
(1) CH3 COCH3 + 8O(ads) 2− → 3CO2 + 3H2 O + 16e−
(2)
C6 H6 + 15O(ads) 2− → 6CO2 + 3H2 O + 30e−
(3)
Furthermore, the chemical sensing response is closely related to the surface oxygen vacancies [49]. Actually, as the electron acceptor, oxygen vacancies with positive charge easily capture the electron generated by redox reaction between reducing gas and oxygen species, resulting in effective separation the electron–hole pairs and enhancing gas-sensing performance [50,51]. X-ray photoelectron spectroscopy (XPS) results in Fig. 5a and b show the contents of oxygen vacancies are 27.4% in hierarchical hollow ZnO nanocages, which are much higher than singular ZnO nanoparticles (17.1%), suggesting highly capacity for electron–hole pairs separation and surface oxidizability towards VOCs. The results of photoluminescence (PL) spectra further support the above-mentioned facts (Fig. 5c). Typically, the UV near-bandedge emission around 380 nm is considered as the characteristic emission of ZnO, the green emission around 510 nm is generally accepted as the emission from ZnO structural defects with oxygen vacancies [52]. Moreover, the relative intensity ratio of the UV near-band-edge emission to the deep-level green emission is used to estimate the concentration of oxygen vacancies [53]. A stronger green emission derived from abundant oxygen vacancies will lead to a smaller intensity ratio. The calculation result shows that such intensity ratios in hierarchical hollow ZnO nanocages and singular ZnO nanoparticles are 0.28 and 2.19, respectively, indicating a much higher concentration of oxygen vacancies in hierarchical hollow ZnO nanocages. In addition, it is anticipated that the hierarchical hollow ZnO nanocages with interpenetrated nanoparticles within porous shells offer a relative lower potential barrier than agglomeration singular ZnO nanoparticles due to larger grain boundary cross-section of melt ZnO interparticles within shells (observed in Fig. 1e and schematically depicted in Fig. 6), and the high specific surface area facilitating surface adsorption–diffusion and reaction kinetics for oxygen and analyte molecules [54]. In conclusion, the hierarchical MOF-derived ZnO nanocages provides
Fig. 5. O 1s XPS spectra of (a) hierarchical hollow ZnO nanocages (ZnO-H) and (b) singular 0D ZnO nanoparticles (ZnO-S) (OI : lattice oxygen; OII : electron-donor defects with oxygen vacancies; OIII : adsorbed molecular water); (c) PL spectra of hierarchical hollow ZnO nanocages and singular 0D ZnO nanoparticles.
more surface oxygen vacancies due to their more exposed active sites and larger specific surface area, which act as the reactive locations for the ionization of the pre-adsorbed oxygen molecules and oxidation process of reducing analytes, producing the enhanced chemical sensing properties towards acetone and benzene, respectively.
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Fig. 6. Schematic comparison of potential barrier between (a) melt ZnO nanoparticles within shells of hierarchical hollow ZnO nanocages and (b) singular ZnO nanoparticles.
4. Conclusions In conclusion, we have successfully synthesized the unique hierarchical hollow ZnO nanocages by a facile strategy through the direct thermolysis of spherical Zn-based metal–organic framework. The as-synthesized hollow ZnO products present the typical hierarchical structures with hollow interiors enveloped by interpenetrated ZnO nanoparticles as porous shell, providing structurally combined meso-/macro-porous channels for facilitating the diffusion and surface reaction of gas molecules. The gas-sensing experiments demonstrate that, in contrast with singular ZnO nanoparticles, the ZnO nanocages show significantly enhanced chemical sensing sensitivity and selectivity towards lowconcentration volatile organic compounds, typically, acetone and benzene. The hierarchical MOF-derived ZnO nanocages provides more surface oxygen vacancies due to their more exposed active sites and larger specific surface area, which act as the reactive locations for the ionization of the pre-adsorbed oxygen molecules and oxidation process of reducing analytes, producing the enhanced chemical sensing properties towards acetone and benzene, respectively. This work clearly elucidates the architecture-dependent performance in gas sensing, and also provides a facile strategy to synthesize high performance ZnO sensors. Acknowledgements This work was financially supported by National Natural Science Foundation of China (No. 51272253), the 863 Hi-tech Research and Development Program of China (Grant No. 2013AA031801), and the Strategic Project of Science and Technology of Chinese Academy of Sciences (No. XDB05050000). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2015.11.034. References [1] J.H. Pan, G. Han, R. Zhou, X.S. Zhao, Hierarchical N-doped TiO2 hollow microspheres consisting of nanothorns with exposed anatase {101} facets, Chem. Commun. 47 (2011) 6942–6944. [2] H. Zhang, Q. Zhu, Y. Zhang, Y. Wang, L. Zhao, B. Yu, One-pot synthesis and hierarchical assembly of hollow Cu2 O microspheres with nanocrystals-composed porous multishell and their gas-sensing properties, Adv. Funct. Mater. 17 (2007) 2766–2771. [3] H. Su, Y.F. Xu, S.C. Feng, Z.G. Wu, X.P. Sun, C.H. Shen, J.Q. Wang, J.T. Li, L. Huang, S.G. Sun, Hierarchical Mn2 O3 hollow microspheres as anode material of lithium ion battery and its conversion reaction mechanism investigated by XANES, ACS Appl. Mater. Interfaces 7 (2015) 8488–8494. [4] M. Samadpour, S. Gimenez, A.I. Zad, N. Taghavinia, I. Mora-Sero, Easily manufactured TiO2 hollow fibers for quantum dot sensitized solar cells, Phys. Chem. Chem. Phys. 14 (2012) 522–528. [5] J.-H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sens. Actuators B 140 (2009) 319–336.
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Biographies Wenhui Li is now a PhD student at Institute of Process Engineering, Chinese Academy of Sciences. His researches focus on sensing materials and environmental catalytic materials. Xiaofeng Wu received his PhD at Institute of Process Engineering, Chinese Academy of Sciences, in 2007. After post-doctoral research at Chonbuk National University in South Korea, he joined in the Institute of Process Engineering, in 2009, and now he is a professor of the Institute of Process Engineering. His research interests are preparation of nanostructural inorganic materials (metal@oxide semiconductor core–shell, hollow particles etc.) and their application in sensors, photocatalysis, thermal insulator, energy storage, etc. Ning Han received his PhD at Institute of Process Engineering, Chinese Academy of Sciences, in 2010. After post-doctoral research at City University of Hong Kong, he joined in the Institute of Process Engineering, in 2014, and now he is a professor of the Institute of Process Engineering. His research interests are preparation and application of gas sensing materials and technologies to fabricate gas sensors and the gas sensing mechanism. And he is also interested in III–V compound semiconductors preparation and applications. Jiayuan Chen is now a PhD student at Institute of Process Engineering, Chinese Academy of Sciences. His researches focus on graphene and graphene composite materials. Xihui Qian is now a postgraduate student at Institute of Process Engineering, Chinese Academy of Sciences. Her researches focus on hollow structural oxides and organic–inorganic composite materials. Yuzhou Deng is now a PhD student at Institute of Process Engineering, Chinese Academy of Sciences. His researches focus on metal-oxides materials and environmental catalytic materials. Wenxiang Tang is now a postdoctoral research assistant in University of Connecticut, USA. His researches focus on the synthesis of porous metal-oxides and the application in VOCs’ catalytic combustion. Yunfa Chen received his PhD in Material Science at Université Louis Pasteur Strasbourg (ULP), France in 1993. He is a professor of the University of Chinese Academy of Sciences, and Research Professor and Vice Director of Institute of Process Engineering. His current research interests are preparation and assembly of nanoparticles, functional materials, organic–inorganic composite materials and layered materials. And he is also interested in industrial application of nanomaterials.