R Y s N P V a b c d e a A R R A A K Z Z C Y O I 1 t h c N b r ( i c e l G C ( h 0 Sensors and Actuators B 257 (2018) 906–915 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical jo u r nal homep age: www.elsev ier .com/ locate /snb esearch Paper olk-shelled ZnCo2O4 microspheres: Surface properties and gas ensing application irav Joshia,b,∗, Luís F. da Silvac, Harsharaj S. Jadhavd, Flavio M. Shimizua, edro H. Sumane, Jean-Claude M’Pekoa, Marcelo Ornaghi Orlandie, Jeong Gil Seod, almor R. Mastelaroa, Osvaldo N. Oliveira Jr a São Carlos Institute of Physics, University of São Paulo, CP 369, São Carlos 13560-970, São Paulo, Brazil Department of Mechanical Engineering, University of California, Berkeley, USA Department of Physics, Federal University of São Carlos, Rodovia Washington Luis km 235, 13565-905 São Carlos, SP, Brazil Department of Energy Science and Technology, Myongji University, Cheoin-gu, Yongin-si, South Korea Department of Physical-Chemistry, Institute of Chemistry, São Paulo State University, P.O. Box 355, 14800-900 Araraquara, SP, Brazil r t i c l e i n f o rticle history: eceived 13 July 2017 eceived in revised form 6 November 2017 ccepted 9 November 2017 vailable online 11 November 2017 eywords: a b s t r a c t The need to improve the sensitivity, selectivity and stability of ozone gas sensors capable of monitoring the environment to prevent hazard to humans has sparked research on binary metal oxides. Here we report on a novel ozone gas sensor made with ca. 0.5 �m yolk-shelled ZnCo2O4 microstructures synthesized via an eco-friendly, co-precipitation method and subsequent annealing. With these ZnCo2O4 microspheres, ozone concentrations down to 80 parts per billion (ppb) could be detected with a.c. and d.c. electrical measurements. The sensor worked within a wide range of ozone concentrations, from 80 to 890 ppb, being n-Co glycolate nCo2O4 o-precipitation olk-shelled structures zone gas sensing mpedance spectroscopy also selective to ozone compared to CO, NH3 and NO2. The high performance could be attributed to the large surface area to volume ratio inherent in yolk-shell structures. Indeed, ozone molecules adsorbed on the ZnCo2O4 surface create a layer of holes that affect the conductivity, as in a p-type semiconductor. Since this mechanism of detection is generic, ZnCo2O4 microspheres can be further used in other environment monitoring devices. © 2017 Elsevier B.V. All rights reserved. . Introduction Monitoring toxic and harmful gases is now considered essen- ial all over the world [1–5], owing to the increasing release of armful gases, liquids and chemicals from industrial effluents, agri- ultural chemicals and fertilizers [6]. Toxic air pollutants such as CO, O2 and NH3 have been detected with chemiresistor gas sensors ased on metal oxides [7,8], which still require improvements to each the selectivity needed for practical applications [9]. Ozone O3) is a potentially harmful gas that has received less attention, n spite of its effects on the human respiratory system that may ause loss of consciousness [6]. Continuous monitoring ozone in the nvironment is therefore important [10,11], as exposure to ozone evels above 120 ppb should be avoided, according to European uidelines (2002/3/EG) [12,13]. Sensors made with metal semicon- ∗ Corresponding author at: São Carlos Institute of Physics, University of São Paulo, P 369, São Carlos 13560-970, São Paulo, Brazil. E-mail addresses: nirav.joshi1986@gmail.com, nirav.joshi@berkeley.edu N. Joshi). ttps://doi.org/10.1016/j.snb.2017.11.041 925-4005/© 2017 Elsevier B.V. All rights reserved. ducting oxides are normally uncapable to meet the requirement of detecting ozone at the ppb level [14]. There are exceptions though, including some n-type semiconductors, viz. ZnO [15], SnO2 [16], WO3 [17], �-AgWO4 [18], NiCo2O4 [19] and In2O3 [20], especially the nanocrystalline In2O3 films able to operate at room tempera- ture and detect ozone levels down to 15 ppb [21]. Despite the high sensitivity of the latter n-type semiconductor sensors, issues have to be addressed to improve stability and decrease response and recovery times [22]. Even more challenging is to achieve such high sensitivity if sensors based on p-type semiconductors are used [22]. Another possible avenue to enhance sensitivity and stability is to prepare gas sensors with binary metal oxides, which can be produced with a variety of methods, including hydrothermal [23–26], co-precipitation/digestion [27,28], microemulsion [29], template-assisted synthesis [30,31], pyrolysis [32,33], solvother- mal [34–37] and thermal decomposition [38,39] methods. The resulting binary metal oxides (AB2O4) may adopt shapes such as nano/microflowers, nanowires, nanoarrays, nanorods, and hollow microspheres [40]. Gas sensors with these binary oxides have been used for detection of carbon monoxide, Liquefied Petroleum Gas https://doi.org/10.1016/j.snb.2017.11.041 http://www.sciencedirect.com/science/journal/09254005 http://www.elsevier.com/locate/snb http://crossmark.crossref.org/dialog/?doi=10.1016/j.snb.2017.11.041&domain=pdf mailto:nirav.joshi1986@gmail.com mailto:nirav.joshi@berkeley.edu https://doi.org/10.1016/j.snb.2017.11.041 Actuators B 257 (2018) 906–915 907 ( t o a t u o t ( e s c s h 2 ( l C 2 p p a 3 c b w T a r d i 2 X R a o n e p H f P ( E a t g c b s s u r N. Joshi et al. / Sensors and LPG), chlorine, ethanol, formaldehyde [27,29,30,33,41] in addition o ozone with p-type hexagonal platelets [19]. In this study, we build upon the successful use of binary metal xides for sensing, now employing binary oxides made of cobalt nd zinc. This choice was motivated by the high electronic conduc- ivity and electrochemical activity of zinc cobalt oxide (ZnCo2O4) sed in Li-ion batteries [42] and as a hole transport layer in rganic photovoltaics (PVs) [43]. Nanostructured ZnCo2O4 in dis- inct shapes has been used to detect volatile organic compounds VOCs) [29,30], such as methanol, formaldehyde, acetone and thanol [22,34,44]. Here, we report on the use of ZnCo2O4 yolk- hell microspheres, synthesized in a cost-effective, eco-friendly o-precipitation method, for detecting ozone with electrical mea- urements. Significantly, a high sensitivity was reached with the igh surface area to volume ratio inherent in yolk-shell structures. . Experimental section The chemical reagents zinc (II) acetate (Zn (CH3COO)2·2H2O 99%)), cobalt (II) acetate (Co (CH3COO)2·4H2O (99%)), and ethy- ene glycol (HOCH2CH2OH, A.R.) were supplied by Sigma-Aldrich o. LLC. and used without any further purification. .1. Synthesis of ZnCo2O4 yolk-shell microspheres The ZnCo2O4 compound was synthesized using the co- recipitation method [45], in which ZnCo2O4 microspheres were repared by dissolving 2 mM of zinc acetate and 4 mM of cobalt cetate in 50 mL ethylene glycol (EG), left under vigorous stirring for 0 min. This homogeneous solution was then transferred to a glass ontainer, which was kept in an oil bath for 2 h at 170 ◦C, followed y cooling down to room temperature. The precipitate powder was ashed several times with deionized water, ethanol and acetone. his powder was then dried in a vacuum oven overnight at 80 ◦C, fter which it was heated in an electric furnace to 350 ◦C at a heating ate of 1 ◦C min−1 during 5 h, leading to crystalline ZnCo2O4 pow- er. The synthesis procedure of pristine ZnCo2O4 is schematically llustrated in Fig. S1 in the electronic Supplementary information. .2. Material characterization The crystalline phase of ZnCo2O4 samples was identified by -ray diffraction (XRD), using CuK� radiation (Rigaku, Rotaflex U-200B) in the 2� range from 10◦ to 80◦ with a step of 0.02◦ nd step scanning of 2◦ min−1. The morphological characteristics f ZnCo2O4 microspheres were analyzed by field emission scan- ing electron microscopy (FE-SEM, Zeiss Sigma) operating at 5 kV, quipped with X-ray energy dispersive spectroscopy (EDS). Sam- le cross-sections were obtained in a dual-beam microscope (FEI elios NanoLab 600i). The phase and crystallinity of ZnCo2O4 were urther investigated using transmission electron microscopy (TEM; hilips, CM200) operated at 200 kV. Thermo-gravimetric analysis TGA) was carried out using a thermogravimetric analyzer (Perkin lmer TGA7) at a scan rate of 10 ◦C min−1 in air. The specific surface rea and pore size distribution of the material were estimated using he Brunauer-Emmett-Teller (BET) method based on the nitro- en adsorption-desorption isotherms (BELSORP-mini II, Japan). The omposition and chemical state of the final products were analyzed y X-ray photoelectron spectroscopy (XPS) using an ESCALAB-MKII pectrometer (UK) with Al K� radiation (1486.6 eV) as the X-ray ource for excitation. The binding energies (BEs) were evaluated sing C1s spectrum (BE = 284.6 eV) as the reference with an accu- acy of ±0.1 eV. Fig. 1. XRD pattern of ZnCo2O4 annealed at 350 ◦C for 5 h in ambient atmosphere. 2.3. Fabrication of ZnCo2O4 sensing film and gas-sensing measurements Gas sensing devices were fabricated by ultrasonically dispersing the as-prepared ZnCo2O4 powders (10 mg) in 1 mL isopropyl alco- hol and the suspension was then drop-cast onto a SiO2/Si substrate containing 100 nm thick Pt electrodes separated by a distance of 50 �m. Details of preparation of the interdigitated electrodes (IDEs) are given in [19]. After dropping the ZnCo2O4 solution, the sub- strates were heated to 100 ◦C for 10 min to evaporate the solvent, followed by calcination at 350 ◦C for 2 h in an electric furnace in air to stabilize the sample before the gas sensing measurements. Pictures of the system to measure the chemiresistive gas sensing performance are shown in Fig. S2. The sensor was inserted into a chamber with temperature control from 100 to 300 ◦C under dif- ferent ozone concentrations. Dry air was used as both the reference and the carrier gas for all measurements, with a constant total flow of 500 SCCM kept via mass flow controllers. Ozone gas was formed by oxidation of oxygen molecules of dry air with a pen-ray UV lamp (UVP, model P/N 90-0004-01), which was calibrated using a toxic gas detector (ATI, model F12) that provided ozone level concentra- tion in the range of 80–890 ppb. The applied DC voltage was 1 V and the electrical resistance was measured using an electrometer Keithley (model 6514). The ozone-containing dry air was blown directly onto the sample, which was placed in a heated holder sys- tem. For gas sensing characterization, the sensors were exposed to 1 ppm (parts-per-million) of various gases, namely nitrogen diox- ide (NO2), carbon monoxide (CO) and ammonia (NH3) controlled by mass flowmeters. Details of the gas-sensing experiments are avail- able in [46]. The sensor response was calculated from the response curves using Eq. (1): Sensor Response (%)= |�R Ra | × 100 (1) where �R = Ra − Rg for oxidizing gases like ozone, and Rg and Ra are the electrical resistances of the sensor film with target gases and dry air, respectively. The response and recovery times were defined as the time needed for reaching 90% of total change in resis- tance upon exposure to the target gas and fresh air, respectively. During the measurements, the relative humidity was kept within the range 30–50% RH (Termo-Higrometro model HT-700). The AC Impedance spectroscopy data were obtained with the ZnCo2O4 film using an impedance/gain-phase analyzer (Solartron SI 1260) in the frequency range from 1 Hz to 1 MHz at an operating temperature of 200 ◦C. 908 N. Joshi et al. / Sensors and Actuators B 257 (2018) 906–915 F at 35 y 3 3 e s g a fi i m t 3 o p s 5 c N p t t o m s T m m 9 e i ig. 2. (a and b) FE-SEM images with different magnifications of ZnCo2O4 annealed olk and the shell parts. . Results and discussion .1. Structural and microstructural characterizations ZnCo2O4 yolk-shelled microspheres were synthesized via an co-friendly precipitation route without any surfactant. Fig. S3 hows the thermogravimetric analysis (TGA) curve of ZnCo- lycolate precursor which indicates a total weight loss of 45.5% bove 300 ◦C. Two major weight loss steps were identified: the rst loss of ∼12% up to 210 ◦C is attributed to the release of chem- cally and physically adsorbed ethylene glycol and other organic olecules, while the other major weight loss of 33.5% is attributed o thermal decomposition of ZnCo-glycolate into ZnCo2O4. Above 00 ◦C no weight loss was observed, which confirms the formation f pure ZnCo2O4 phase. To ensure complete decomposition of the recursor, we chose 350 ◦C as the calcination temperature for the ynthesis of ZnCo2O4. Fig. 1 displays the XRD pattern of ZnCO2O4 powder annealed for h at 350 ◦C, where all reflections were indexed to a face-centered- ubic (fcc) arrangement with Fd3m (227) space group (JCPDS file o. 23-1390) and spinel structure, by comparing with the standard attern illustrated by the vertical red lines. The hysteresis on the N2 adsorption–desorption isotherms for he yolk-shelled ZnCo2O4 in Fig. S4 is classified as type III, with a ype H4 hysteresis loop, according to IUPAC (International Union f Pure and Applied Chemistry). These are typical of mesoporous aterials formed by agglomerated particles [47]. The BET specific urface area was 52.73 m2 g−1, with a pore volume of 0.10 m3 g−1. he pore size distribution, derived from desorption data and esti- ated from the isotherm using the Barrett–Joyner–Halenda (BJH) odel (inset in Fig. S4), indicates a pore diameter ranging from 5 to 1 nm, with an average of 10 nm. This mesoporous structure is ben- ficial to sensing because the target gas molecule easily penetrates nto the pores, thus leading to a larger analyte/sample contact area 0 ◦C for 5 h. (c) As deposited and (d) FIB-sectioned ZnCo2O4 sample displaying the and providing interconnected paths that facilitate electron trans- port and accelerate species diffusion [48,49]. Fig. 2(a) shows the spherical-like morphological features of ZnCo2O4 in the FE-SEM images, which are preserved upon cal- cination. A typical broken yolk-shelled microsphere is shown in Fig. 2(b). The core of the material is highly porous and composed of polycrystalline nanosized particles, in good agreement with BET results. The formation process of yolk-shelled microspheres is due to Ostwald ripening by minimization of surface energy [45]. In the synthesis, ZnCo-glycolate is formed due to interlinking of ethylene glycol at 170 ◦C; owing to a high surface energy and aggregation it turns into spheres with reduced free energy [45,50]. The yolk- shelled structure arises from the heat treatment during thermal decomposition of ZnCo-glycolate under non-equilibrium condi- tions. As the temperature reaches 350 ◦C during calcination, the ZnCo-glycolate core starts to decompose into ZnCo2O4, thus even- tually leading to a ZnCo2O4 yolk-shelled microsphere [45]. Fig. 2(c) illustrates the typical yolk-shell spheres while Fig. 2(d) shows the cross-section of the larger sphere made by FIB (Focused Ion Beam), confirming the small yolk surrounded by a thin shell (white arrows in Fig. 2(d)). The purity of the as-obtained ZnCo2O4 particles is ensured by the EDS mapping images in Fig. S5, where there are only Zn, Co and O elements uniformly distributed throughout the structure, in addition to Si from the substrate. The gap between the outer shell and the inner yolk in the micro- spheres is illustrated in the low and high magnification TEM images in Fig. 3(a) and (b). Some broken spheres do not possess a shell due to the ultrasound bath used for TEM sample preparation, as observed in Fig. 3(a). Details of a yolk-shell sphere are marked with a white box in Fig. 3(b), featuring a yolk diameter of 0.5 �m and wall thickness of ca. 50 nm. The selected area electron diffraction (SAED) pattern for spheres annealed at 350 ◦C in Fig. 3(c) displays concen- tric rings, characteristic of polycrystalline materials. The diffraction rings are assigned as (111), (220), (311), (400), (422), (511) and (440) planes of the cubic structure of the ZnCo2O4 phase, consis- N. Joshi et al. / Sensors and Actuators B 257 (2018) 906–915 909 o2O4 t i i i m X e p l a 1 Z s t a X l b s g m Fig. 3. (a and b) Low and high magnification TEM images of the yolk-shelled ZnC ent with the XRD analysis. Fig. 3(d) shows the high resolution TEM mage of a particle at the shell edge (white square region), where the nterplanar distance is 0.47 nm, corresponding to the (111) Miller ndices of spinel-type ZnCo2O4 phase. The chemical elements comprising the ZnCo2O4 yolk-shelled icrospheres are inferred from the XPS spectra in Fig. 4. The survey PS spectrum in Fig. 4(a) reveals the presence of C, Zn, Co, and O lements, with the absence of impurities. The appearance of the C eak is derived from adventitious carbon species. The high reso- ution XPS spectra of Zn 2p, Co 2p, and O 1s after Gaussian fitting re shown in Fig. 4(b–d). Two main peaks appear at 1020.8 and 043.9 eV for Zn2p in Fig. 4(b), typical of Zn2+ with the orbits of n 2p3/2 and Zn 2p1/2, respectively [51]. The well resolved Co 2p pectrum in Fig. 4(c) shows peaks at 779.4 and 794.2 eV assigned o Co 2p3/2 and Co 2p1/2, respectively, with spin-orbit splitting of round 15 eV owing to mixed Co2+ and Co3+ ions [52–54]. The O 1s PS spectrum in Fig. 4(d) can be deconvoluted into three peaks. The ower energy peak at 529.2 eV can be attributed to metal-oxygen onding (oxygen bonding with Zn and Co) in ZnCo2O4 [54,55]. The econd peak at 530.9 eV is assigned to oxygen of surface hydroxyl roups [56–58], while the third peak at 532.3 eV is due to oxygen olecules chemisorbed onto the semiconductor surface [59]. microspheres. (c) SAED pattern (d) HRTEM image at the edge of a microsphere. 3.2. Gas sensing properties The gas sensing properties of ZnCo2O4 yolk-shelled micro- spheres were investigated by measuring the response of a sensor exposed to 560 ppb of ozone at an operating temperature of 200 ◦C, with different exposure times (15, 30, and 60 s). Fig. 5(a) reveals that ZnCo2O4 microspheres were sensitive to ozone even for the shortest exposure time (15 s), with no evidence of saturation upon increasing the time. Fig. S6 in the Supplementary material indicates that even after 3 min of exposure to ozone gas, the sample response was not fully saturated and the recovery time became longer than 30 min. The lack of saturation is probably due to the high surface area to volume ratio of the microspheres [60–62] in addition to the insufficient time for the adsorption process to reach equilib- rium. The optimum operating temperature was 200 ◦C, according to Fig. 5(b), similarly to traditional oxide gas sensors [15,63–65]. The reason why a peak in performance appears is as follows: the reac- tion kinetics of gases with oxygen species chemisorbed on an oxide surface becomes faster with increasing temperatures, but above 200 ◦C the rate of desorption is higher than for adsorption, thus decreasing the response. Therefore, there is an optimum operat- ing temperature, where adsorption and desorption processes reach equilibrium. We do not expect any structural change during the sensing measurements because the material had been annealed at a higher temperature than the sensing operating temperature 910 N. Joshi et al. / Sensors and Actuators B 257 (2018) 906–915 Fig. 4. XPS spectra of the ZnCo2O4 structure. (a) Survey scan and hig Table 1 Ozone gas sensing parameters for ZnCo2O4 microspheres at 200 ◦C. O3 level (ppb) Sensor response (%) Response time (s) Recovery time (min) 80 23.3 8.4 9.7 290 35.5 12 17 [ s t r t 3 t g d r e u s c i s d t s ( s C , whose average value was 0.912 × 10−10 F, close to the nF order 560 55 21 18 890 71 37.4 21 55,66]. In subsidiary experiments we found that ZnCo2O4 yolk- helled microspheres could detect ozone even at lower operating emperatures, but the recovery time was too long or the original esistance could not be reached back. With regard to the concen- ration dependence, the response time varied from 8.4 s (80 ppb) to 7 s (890 ppb), while the recovery time varied from 9.7 min (80 ppb) o 21 min (890 ppb) as shown in Fig. 5(c). It is worth noting that an ozone level above 120 ppb is dan- erous to human health, possibly causing problems such as lung amage. The ZnCo2O4 sensor displays total reversibility and good eproducibility in Fig. 6(a) when exposed to various ozone lev- ls in three measurement cycles. The sample resistance decreased pon exposure to the oxidizing gas, which is indicative of p-type emiconductor behavior. The sensor response increases with ozone oncentration and may even not reach saturation for 890 ppb, as ndicated in Fig. 6(b). Due to the limitations of our gas sensing ystem, 80 ppb is the lowest concentration that can be reliably elivered to the sensor device. Table 1 shows the sensing parame- ers with respect to different ozone levels. Fig. 7 shows the sensor response of ZnCo O yolk-shell micro- 2 4 pheres exposed to 1 ppm of reducing (CO and NH3) and oxidizing O3 and NO2) gases at an operating temperature of 200 ◦C. One hould note that for reducing gases the change in electrical resis- h resolution scan of (b) Zn 2p, (c) Co 2p, and (d) O 1s regions. tance in Eq. (1) is given as �R = Rg − Ra. The magnitude of the electrical resistance change was considerably higher for O3 (∼71%) than for NO2 (6.5%), NH3 (3.8%) and CO (4.8%). Therefore, the ZnCo2O4 yolk-shell microspheres are very selective for ozone, which is promising for sensor devices. The response of this sen- sor film is also very stable, as indicated by the long-term stability measurements for over 30 days in Fig. S7 (a) and (b) in the electronic Supplementary information. 3.3. Impedance spectroscopy measurements of ZnCo2O4 yolk-shell microspheres The ZnCo2O4 sensors were also tested for ozone detection through electrical impedance experiments, in concentrations rang- ing from 80 ppb to 890 ppb at a fixed operating temperature of 200 ◦C. Fig. 8(a) shows that the impedance data consists of a sin- gle semicircle in the Nyquist plot, whose diameter decreases with ozone concentration, consistent with the decreasing resistance upon ozone exposure discussed in the last subsection. In order to obtain further information about the contribution from each region of the sensor, the impedance data were successfully fitted with the equivalent electrical circuit shown in Fig. 9, which also depicts a schematic representation of the sensing sample under ozone flow. Table 2 brings the parameters extracted from data fit- ting. Rb refers to the bulk contribution, which manifested here as a non-zero intercept of the impedance data with the real axis towards the highest frequencies; in passing, this parameter did not change under ozone flow. A similar behavior was found for the capacitance s often observed for surface effects [67,68]. We recall that the sys- tem under study contains a yolk-shell-modified particle-to-particle contact surface, the consequence of which is the observation of a N. Joshi et al. / Sensors and Actuators B 257 (2018) 906–915 911 Fig. 5. (a) Dynamic electrical resistance of ZnCo2O4 sample exposed for different periods of time to 560 ppb of O3 at 200 ◦C. (b) Sensor response of the ZnCo2O4 sample exposed to 560 ppb O3 at different operating temperatures. (c) Response and recovery time as a function of ozone concentration from 80 to 890 ppb at 200 ◦C. F one co f c e ( c T s i t ig. 6. (a) Dynamic electrical response of ZnCo2O4 sample of three cycles for each oz or ZnCo2O4 microspheres at 200 ◦C. apacitance value somewhat lower than expected in typical surface ffects. The resistance attributed to this surface-like contribution Rs), whose values are shown in Fig. 8(b), decreased with ozone oncentration according to an exponential behavior. Note from able 2 that Rs >> Rb, meaning that these contact surfaces act as emi-blocking resistive regions. This is a charge storage effect (i.e., nterface polarization of Maxwell-Wagner type [67]), giving rise to he capacitance component observed (impedance semicircle inci- ncentration (b) Sensor response vs ozone concentration in the range of 80–890 ppb dence) in Fig. 8(a). The change in impedance calculated from the Nyquist plot by subtracting the spectra before and after ozone expo- sure is depicted in Fig. 8(c). At high frequencies, the impedance did not vary with ozone concentration, but it did so between 1 Hz and 10 kHz as the electrical response was dominated by surface contri- butions [69]. It is clear that ozone concentrations well below 80 ppb could be detected, but this was not pursued owing to limitations of the sensor setup. The ozone sensing activity in this material, just 912 N. Joshi et al. / Sensors and Actuators B 257 (2018) 906–915 Fig. 7. Selectivity histogram of the ZnCo2O4 yolk-shell microspheres exposed to common gases present in the atmosphere. F b) Rb v ( o 1 MH a e t e t t a m o t ig. 8. (a) Nyquist plot of ZnCo2O4 film in air and after ozone exposure at 200 ◦C. ( c) Relative magnitude impedance spectra of ZnCo2O4 sample at 200 ◦C from 1 Hz t s found in parent compounds, is shown to be a surface dependent ffect, with resistance being the appropriate parameter to moni- or. Back to the impedance data in Fig. 8(a) and (c), we found no vidence of contribution from the material-electrode interface to he total impedance at the lowest frequencies. In fact, such a con- ribution would have involved higher capacitance values, normally pproaching �F order of magnitude, instead of nF [67,68]. The finding above is consistent with an electronic transport echanism with a non-blocking electrode effect. The adsorbed zone molecules are expected to act as acceptors while electron rapping on the surface causes band bending, leading to an increase alues from fitting with an equivalent circuit as a function of ozone concentration. z for different ozone concentrations. in free holes near the interface and therefore a decrease in the resis- tance. The yolk-shell microstructure could be thought of as a filter, where a molecule penetrates the shell and is “trapped” to interact with the inner surface of the shell and the outer surface of the core. Significantly, these yolk-shell microspheres possess high surface area to promote gas adsorption-desorption processes that enhance gas sensing performance. The whole spectra of normalized magnitude impedance data for all the samples were analyzed with the multidimensional projection technique Interactive Document Map (IDMAP) using the PEx-Sensors software [70–72]. In IDMAP data dimensions are N. Joshi et al. / Sensors and Actuators B 257 (2018) 906–915 913 Fig. 9. Schematic representation of the gas sensing material and electrical equivalent circuit to fit the impedance data. Fig. 10. IDMAP plot of normalized magnitude impedance spectra for different ozon Table 2 Parameters from fitting the impedance spectra using an RC equivalent circuit. Rb is the bulk resistance (volume effect), while Rs and Cs are the yolk-shell-modified surface resistance and capacitance contributions to the total impedance. O3 (ppb) Rb (�) Rs (k�) Cs (pF) 0 458.4 ± 37.1 488.7 ± 3.0 90.7 ± 0.7 80 427.5 ± 62.2 258.4 ± 2.6 90.7 ± 1.2 290 442.9 ± 30.2 139.6 ± 0.6 91.0 ± 0.6 r p d I s c a b 4 b p 560 447.2 ± 26.5 94.7 ± 0.4 91.5 ± 0.6 890 455.7 ± 24.6 71.9 ± 0.3 92.0 ± 0.6 educed with the Fastmap technique, and then each spectrum was rojected as a data point on the projected space. Dissimilarity was efined in terms of Euclidian distances between the spectra. The DMAP plot in Fig. 10 confirms the efficiency of ZnCo2O4 for ozone ensing, since the distance between the points with no gas to the oncentration 80 ppb is large. Quantitatively, this distinguishing bility was represented by a silhouette coefficient of 0.91 [70], to e compared with the maximum value of 1.0. . Conclusion We have shown that ZnCo2O4 microspheres can be synthesized y co-precipitation and subsequent annealing. The structure, mor- hology and composition of the ZnCo2O4 were confirmed with e gas concentrations. The scale bar represents the Euclidean distance of 0.1. several methods, namely XRD, FE-SEM, TEM and XPS. The yolk- shelled ZnCo2O4 sensor was highly sensitive to detect ozone gas down to 80 ppb with both a.c. and d.c. electrical measurements, with fast response and recovery, and good selectivity. The mech- anism of detection was found to be based on adsorption of ozone molecules on the ZnCo2O4 surface, as a layer of holes is created which affects the conductivity, as in a typical p-type semiconductor. Finally, the enhanced performance owing to the large surface area to volume ratio of ZnCo2O4 yolk-shelled microspheres is promising for developing further gas sensor devices to monitor the environ- ment. Acknowledgements This work had financial support from CNPq and FAPESP (2012/15543-7, 2013/14262-7, 2013/07296-2, 2014/23546-1, 2016/23474-6). The authors are also grateful to Angelo L. Gobbi and Maria H.O. Piazzetta for the use of the Microfabrication Laboratory (LMF-20509) facilities to manufacture electrodes (LMF/LNNano-LNLS, Campinas, Brazil). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.snb.2017.11.041. http://dx.doi.org/10.1016/j.snb.2017.11.041 http://dx.doi.org/10.1016/j.snb.2017.11.041 http://dx.doi.org/10.1016/j.snb.2017.11.041 http://dx.doi.org/10.1016/j.snb.2017.11.041 http://dx.doi.org/10.1016/j.snb.2017.11.041 http://dx.doi.org/10.1016/j.snb.2017.11.041 http://dx.doi.org/10.1016/j.snb.2017.11.041 http://dx.doi.org/10.1016/j.snb.2017.11.041 http://dx.doi.org/10.1016/j.snb.2017.11.041 http://dx.doi.org/10.1016/j.snb.2017.11.041 9 Actua R [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ 14 N. Joshi et al. / Sensors and eferences [1] J.-H. Lee, Gas sensors using hierarchical and hollow oxide nanostructures: overview, Sens. Actuators B: Chem. 140 (2009) 319–336. [2] N. Barsan, D. Koziej, U. Weimar, Metal oxide-based gas sensor research: how to? Sens. Actuators B: Chem. 121 (2007) 18–35. 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