RSC Advances PAPER Pu bl is he d on 2 8 Se pt em be r 20 16 . D ow nl oa de d by U N IV E R SI D A D S A O P A U L O o n 18 /1 1/ 20 16 1 2: 59 :4 3. View Article Online View Journal | View Issue One-step approa aSão Carlos Institute of Physics, University of 13560-970, Brazil. E-mail: nirav.joshi1986@ bLIEC, Institute of Chemistry, São Paulo St Araraquara, SP, Brazil cDepartment of Energy Science and Tech Yongin-si, South Korea dAix Marseille Université, CNRS IM2NP (UM 13397, France † Electronic supplementary informa 10.1039/c6ra18384k Cite this: RSC Adv., 2016, 6, 92655 Received 19th July 2016 Accepted 22nd September 2016 DOI: 10.1039/c6ra18384k www.rsc.org/advances This journal is © The Royal Society of C ch for preparing ozone gas sensors based on hierarchical NiCo2O4 structures† Nirav Joshi,*a Lúıs F. da Silva,b Harsharaj Jadhav,c Jean-Claude M'Peko,a Bruno Bassi Millan Torres,a Khalifa Aguir,d Valmor R. Mastelaroa and Osvaldo N. Oliveira Jra Nanostructured semiconducting oxides have been used as resistive gas sensors of toxic and non-toxic gases, but little emphasis has been placed on ozone sensing. Here we present a new ozone gas sensor based on hierarchical NiCo2O4 cubic structures synthesized via a facile urea-assisted co-precipitation method and annealed at 450 �C, which showed a low detection level. Ozone detection was carried out through electrical measurements with an optimized performance at 200 �C, with fast response (�32 s) and recovery (�60 s) time with suitable concentration range (from 28 to 165 ppb) for technological applications. Furthermore, NiCO2O4 platelets are selective to ozone compared to other oxidizing and reducing gases. The low detection level can be attributed to the coexistence of 3D structures based on hexagonal platelet-like and porous flower-like shape, which were revealed by field emission scanning electron microscopy (FE-SEM). In summary, NiCo2O4 is promising for detection of sub-ppb levels of ozone gas. 1. Introduction Metal semiconducting oxides (MSO) have been used in various applications such as gas sensors, UV light emitting diodes, dye- sensitized solar cells, and industrial nanodevices.1,2 MSO are advantageous as sensing materials mainly because they are inexpensive, chemically stable, easily processible, yield mass production, and possess a Debye length corresponding to the target molecules.3,4 Indeed, various researchers have demon- strated the technological potential of MSO as gas sensing materials, including ZnO,5,6 CuO,7 WO3,8,9 Fe2O3,10 SnO2,11 In2O3 (ref. 12) and Co3O4.13 Among the many gases to be investigated is ozone (O3), which is an oxidizing gas used in drinking-water treatment and microelectronic cleaning processes.6,14–16 The monitoring of ozone levels is important owing to its potential risk to public health and environment, as it may damage the respiratory system with a decrease in lung function, inam- mation of airways and pain.17 Exposure to ozone levels above 120 ppb for long times is not recommendable for humans;18 São Paulo, CP 369, São Paulo, São Carlos gmail.com ate University, P.O. Box 355, 14800-900, nology, Myongji University, Cheoin-gu, R 7334), FS St Jérôme S152, Marseille, tion (ESI) available. See DOI: hemistry 2016 therefore, the continuous monitoring of ozone levels is essen- tial.19 Ozone gas sensors have been based on ZnO,6 In2O3,20 SnO2 (ref. 21) and WO3,22 a-Ag2WO4,23 SrTi1�xFexO3 (ref. 24) which are suitable in terms of their operating temperature, selectivity, response, and stability.6 Ternary metal oxides can be even more useful owing to synergy in properties achieved by combining distinct compounds. Nickel cobalt oxide (NiCo2O4) has such outstanding properties as high electrical conductivity, good structural stability, easy electrolyte penetration and high elec- trochemical activity.25 The preparation of NiCo2O4 has been accomplished using different routes, including solvothermal,26 co-precipitation27 and hydrothermal.28 These methods allow NiCo2O4 to be obtained in a variety of shapes, including owers, nanowires, nanoarrays, nanorods, and hollow microspheres.29 Furthermore, this compound has been used in supercapacitors, electrocatalysts, and Li-ion batteries.30–32 NiCo2O4 has also been used to detect reducing gases, e.g. ethanol, chlorine, carbon monoxide.25,33,34 In this study, we report an approach for obtaining NiCo2O4 hexagonal platelets via a simple and cost-effective urea-assisted co-precipitation method, which are then applied as ozone gas sensors. The co-precipitation method is advantageous for its low processing temperature, high purity, low cost, and controlled morphology compared with most conventional methods. X-ray diffraction and eld emission scanning electron microscopy were used to investigate the structural and micro- structural properties of NiCo2O4 hexagonal platelets. Gas sensing experiments were performed at different working RSC Adv., 2016, 6, 92655–92662 | 92655 http://crossmark.crossref.org/dialog/?doi=10.1039/c6ra18384k&domain=pdf&date_stamp=2016-09-27 http://dx.doi.org/10.1039/c6ra18384k http://pubs.rsc.org/en/journals/journal/RA http://pubs.rsc.org/en/journals/journal/RA?issueid=RA006095 Fig. 1 (a) Schematic illustration of NiCo2O4-sensor device. (b) Pt- interdigitated Si/SiO2 substrate and NiCo2O4 deposited Si/SiO2 substrate. RSC Advances Paper Pu bl is he d on 2 8 Se pt em be r 20 16 . D ow nl oa de d by U N IV E R SI D A D S A O P A U L O o n 18 /1 1/ 20 16 1 2: 59 :4 3. View Article Online temperatures towards ozone, nitrogen dioxide, and ammonia gas. Impedance spectroscopy measurements were also per- formed for supporting the ozone gas sensing mechanism of NiCo2O4 hexagonal platelets. 2. Experimental 2.1 Synthesis of NiCo2O4 hexagonal platelets The synthesis of NiCo2O4 was accomplished using the urea- assisted co-precipitation method.35 To prepare NiCo2O4 hexag- onal platelets, 10 mM of NiCl2$6H2O (99.5% Aldrich), 20 mM of CoCl2$6H2O (99.95% Aldrich) and 100 mM urea (98% Aldrich) were dissolved in 100 mL deionized (DI) water, and le under vigorous stirring for 30 min to form a clear pink color solution. This homogeneous solution was transferred to a glass container, which was kept in an oil bath and maintained for 10 h at 90 �C, followed by cooling to room temperature. The precipitate powder was washed several times with DI water, ethanol and collected by centrifugation and then dried in a vacuum oven overnight. The powder obtained was annealed for 2 h at 450 �C under atmospheric air using a heating rate of 2 �C min�1 for obtaining crystalline NiCo2O4 powders. The synthesis of pristine NiCo2O4 is schematically displayed in Fig. S1 in the ESI.† 2.2 Materials characterization The crystalline phase of NiCo2O4 samples was identied by X- ray diffraction (XRD), CuKa radiation (Rigaku Rotaex RU- 200B) in the 2q range from 10 to 90� with a step of 0.02� and step scanning at 2� min�1. The morphology of the as-obtained sample was studied by eld emission scanning electron microscopy (FE-SEM, Zeiss Sigma) operating at 5 kV and high- resolution transmission electron microscopy (HR-TEM, Phi- lips Tecnai F20 at 200 kV). Thermo-gravimetric analysis (TGA) analysis was carried out using a thermogravimetric analyzer (Perkin Elmer TGA7) at a scan rate of 10 �C min�1 in an air atmosphere. 2.3 Fabrication of NiCo2O4 sensing lm and gas-sensing measurements The NiCo2O4 powders (10 mg) were dispersed in 1 mL isopropyl alcohol by an ultrasonic cleaner for 30 min and the suspension was then dropped onto a SiO2/Si substrate containing 100 nm thick Pt electrodes separated by a distance of 50 mm. The patterning of Pt substrates with photolithographic and li-off processes reported in our previous reports.19,36,37 The substrates were then heated to 80 �C for 10 min to evaporate the solvent, followed by calcinations at 350 �C for 2 h in an electric furnace in air to stabilize the sample before the gas sensing measurements were performed. The electrical contact and sensor lm illustration mentioned in Fig. 1. The sensor sample was inserted into a test chamber for temperature control under different ozone concentrations. The sensor operating tempera- ture (Topt.), which varied from 150 �C to 300 �C, was maintained by external heating. The dc voltage applied was 1 V while the electrical resistance was measured using a Keithley (model 92656 | RSC Adv., 2016, 6, 92655–92662 6514) electrometer. Dry air was used as both the reference and the carrier gas for all gases, maintaining a constant total ow of 500 sccm via mass ow controllers. The ozone gas was formed by oxidation of oxygen molecules of dry air (500 sccm) with a pen-ray UV lamp (UVP, model P/N 90-0004-01), which was calibrated using a gas toxic detector (ATI, model F12) for an O3 level between 28 and 560 ppb (parts-per-billion). The dry air containing ozone was blown directly onto the sensor placed on a heated holder. Nitrogen dioxide (NO2) and ammonia (NH3) gas sensing measurements were performed under concentrations varying from 1 to 1000 ppm (parts-per- million). A photograph of the chamber used for gas-sensing experiments is shown in ref. 24. The sensor response (%) was calculated using eqn (1) Responseð%Þ ¼ � � � � Rg � Ra Ra � � � � � 100 (1) where, Rg and Ra are the electrical resistances of the sensor exposed to target gases and dry air, respectively. The response time was dened as the time required for a change in the sample's electrical resistance to reach 90% of the initial value when exposed to the target gas. Similarly, the recovery time was dened as the time required for the electrical resistance of the sensor to return to 90% of the initial value aer the target gas has been turned off. During the measurements, the humidity was kept within the range 30–50% RH (Termo-Higrometro mod.HT-700). Impedance spectroscopy data were obtained with the NiCo2O4 lm 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. 3. Results and discussion 3.1 Structural and microstructural characterizations Fig. 2 displays the XRD pattern of NiCo2O4 powder annealed for 2 h at 450 �C, with the XRD peaks indexed as a cubic structure of NiCo2O4 with F*3 (202) space group (Joint Committee on Powder Diffraction Standards, JCPDS, le no. 20-0781). In order to conrm the crystal structure and composition, Fig. S2† shows the XRD pattern of before and aer annealing of NiCo2O4 powder which clearly indicates that before annealing samples is composed of double mixed metal hydroxides and its crystalline This journal is © The Royal Society of Chemistry 2016 http://dx.doi.org/10.1039/c6ra18384k Fig. 2 XRD pattern of NiCo2O4 annealed at 450 �C for 2 h under an ambient atmosphere. Paper RSC Advances Pu bl is he d on 2 8 Se pt em be r 20 16 . D ow nl oa de d by U N IV E R SI D A D S A O P A U L O o n 18 /1 1/ 20 16 1 2: 59 :4 3. View Article Online nature which is quite similar to those of Ni or Co hydroxide (JCPDS no. 46-0605 or 22-0752), which demonstrates the complete decomposition of the precursors to highly pure NiCo2O4 aer annealing.38,39 The morphological features of the NiCo2O4 sample are illustrated in Fig. 3(a and b), where two types of shapes can be seen, viz. 7–8 mm hexagonal platelets and nanoowers attached on these platelets. Fig. S3† show the transmission electron microscope (TEM) morphology of the as- transformed NiCo2O4 hexagonal platelets. As can be seen, NiCo2O4 sample exhibits uniform hexagonal plate morphology. Fig. 3 (a and b) FESEM images of NiCo2O4 platelets annealed at 450 �C for 2 h under an ambient atmosphere. This journal is © The Royal Society of Chemistry 2016 The selected area electron diffraction (SAED) patterns taken from NiCo2O4 platelets display hexagonally arranged diffraction spots, suggesting that NiCo2O4 platelets exist as a single crystal. The measured interplanar distance is 0.47 nm, which matches well to the (111) plane of cubic NiCo2O4. The N2 adsorption–desorption isotherms for the NiCo2O4 sample in Fig. 4 shows a typical hysteresis loop, indicating mesoporous features. The Brunauer–Emmett–Teller (BET) specic surface area was 57.3 m2 g�1 and the pore volume was 0.228 m3 g�1. The pore size distribution, derived from desorption data and calculated from the isotherm using the Barrett–Joyner–Halenda (BJH) model (inset in Fig. 4), indicates a pore diameter ranging from 5 to 81 nm, with an average of 15 nm.12,40 This hierarchical structure is benecial to the sensing material because the target molecule gas easily penetrate into the pores, thus leading to a larger analyte/sample contact area and providing interconnected paths that facilitate electron transport and accelerate species diffusion.40–42 In order to understand the weight loss with the temperature during the annealing process, and to determine the annealing temperature of the sample, TGA curve in air was obtained in Fig. S4† The total weight loss of 31.5% upto 430 �C, mainly attributed to loss of residual water and burnout of organic species involved in precursor. Above 430 �C, no obvious weight loss is observed indicating that there is no additional phase or structural change in NiCo2O4, which is consistent with previous reports.35,43,44 Finally, as synthesizedmaterial annealed at 450 �C to get high purity of NiCo2O4 material. 3.2 Gas sensing properties The electrical response of the NiCo2O4 sample was investigated at a xed operating temperature under the exposure of 560 ppb of ozone gas for 1 min exposure time. Fig. 5 shows that the best sensor response was obtained at 200 �C, which is close to that for traditional gas sensors.7,13,45,46 The electrical response of the NiCo2O4 sample was investigated at a xed operating Fig. 4 N2 adsorption–desorption isotherm and pore size distribution (inset) of the NiCo2O4 platelets. RSC Adv., 2016, 6, 92655–92662 | 92657 http://dx.doi.org/10.1039/c6ra18384k Fig. 5 Gas sensing response of NiCo2O4 exposed to 560 ppb O3 at different operating temperatures. Fig. 7 (a) Sensor response versus gas concentration in the range of 28 to 165 ppb O3. (b) Gas sensing response and recovery time as a function of ozone concentration in the range of 28 to 165 ppb O3 at 200 �C. RSC Advances Paper Pu bl is he d on 2 8 Se pt em be r 20 16 . D ow nl oa de d by U N IV E R SI D A D S A O P A U L O o n 18 /1 1/ 20 16 1 2: 59 :4 3. View Article Online temperature of 200 �C under the exposure of 80 ppb of ozone gas at different times (0.5, 1.0 and 5.0 min). As displayed in Fig. S5,† the sample was sensitive to ozone gas even for the shortest exposure time, with no evidence of saturation upon increasing the time. Additionally, we emphasize that with NiCo2O4 platelets we could detect the same amount of ozone gas, i.e. 80 ppb, at lower temperatures, even at room tempera- ture. However, the recovery time was too long or could even not be reached, as displayed in Fig. S6–S12 in the ESI.† For the remaining experiments, we used the optimized temperature of 200 �C. Fig. 6 shows that the NiCo2O4 sensor displayed good sensor response in the 28–165 ppb range, with total reversibility and good reproducibility for different ozone levels. As displayed in Fig. S13,† for each measurement cycle, the sample was exposed for 1 min with different ozone concentration which indicating a stable and reproducible response. The resistance of the sample decreased upon expo- sure to the oxidizing gas, indicating a p-type semiconductor behavior. In addition, the response time varied from 32 s (28 Fig. 6 Ozone gas sensing response for the NiCo2O4 as a function of the gas level at an operating temperature of 200 �C. 92658 | RSC Adv., 2016, 6, 92655–92662 ppb) to 48 s (165 ppb), while the recovery time varied from 1min (28 ppb) to 6.5 min (165 ppb). Fig. 7(a) shows the sensor response for NiCo2O4 platelets as a function of ozone concentrations. The response and recovery time is one of the important parameters for toxic gas sensor applications. The response and recovery time as a function of ozone concentrations shows in Fig. 7(b), it can be seen that the response time increases with increasing ozone concentration. The response time increases from 32 to 48 s while recovery time also increases from 1 to 6.5 min as the concentration increases from 28 to 165 ppb. However at low concentration, more ozone molecules easily interact with adsorbed oxygen ions providing a fast response compared to higher concentration.47 In order to verify the selectivity of the sensing materials, the NiCo2O4 sample was also exposed at 200 �C to NH3 and NO2, with the results being shown in Fig. S14 and S15† respectively, while Fig. 8 compares the result for these gases with those of higher ozone concentrations. For the reducing NH3, the resistance increased slightly whereas for the oxidizing NO2 the resistance decreased, as expected. The magnitude of these changes in resistance is much smaller than for ozone, thus conrming that ozone can be detected in a selective manner using NiCo2O4 platelets. Table 1 shows a comparison of parameters for ozone sensing with NiCo2O4 with traditional ozone gas sensors, including ZnO, WO3, SnO2, and In2O3. To our knowledge, the sensor This journal is © The Royal Society of Chemistry 2016 http://dx.doi.org/10.1039/c6ra18384k Fig. 8 Selectivity histogram for NiCo2O4 hexagonal platelets upon exposure to different NO2, NH3 and ozone concentrations at an operating temperature of 200 �C. Fig. 9 Impedance spectra of the fresh and ozone exposed NiCo2O4 Paper RSC Advances Pu bl is he d on 2 8 Se pt em be r 20 16 . D ow nl oa de d by U N IV E R SI D A D S A O P A U L O o n 18 /1 1/ 20 16 1 2: 59 :4 3. View Article Online reported here could detect the lowest level of ozone for the temperature with optimized performance. film measured at 200 �C. The blue and red colour lines represent the fitting of the spectra using an equivalent circuit. Table 2 Impedance parameters for a NiCo2O4 film in the absence, and presence of 80 ppb ozone, from fitting the experimental curves with the equivalent circuit Sample R0 (U) R1 (U) C1 (nF) Fresh NiCo2O4 200 6413 12.73 Ozone exposed-NiCo2O4 200 5451 14.11 3.3 Impedance spectroscopy of NiCo2O4 hexagonal platelets The interaction between NiCo2O4 sensing material and ozone gas was further investigated using impedance spectroscopy measurements, which may provide information about the different contributions to the sensor response. Fig. 9 shows the Nyquist plots for the NiCo2O4 lm exposed to an air atmo- sphere, and then to 80 ppb ozone. In both cases, perfect single semicircles are observed from the low to the high-frequency region. These curves were tted with an equivalent circuit comprising a resistance (R0) connected in series with a parallel resistor (R1)–capacitor (C1) block, as shown in the gure inset. There was good agreement between experimental data and simulated curves using the electrical parameters given in Table 2. As for the physical signicance of these components, R0 remained unchanged with exposure to either air or ozone atmosphere, and should be attributed to the low resistance effect coming from the conductive-like bulk material. R1 and C1 are the respective interfacial resistance and capacitance between NiCo2O4 grains, to be understood here as the particle- to-particle contacts, according to the criteria of magnitude order of capacitance.52,53 Upon exposure to ozone, R1 decreased while C1 increased. The interaction of ozone mole- cules with the sensing material generates lone-pair electrons trapped on the NiCo2O4 surface, which leads to a hole- accumulating layer. This behavior implied in a reduction of the electrical resistance, and an increase in capacitance of the Table 1 Comparison of ozone gas sensor parameters for a NiCo2O4 co Sensor Synthesis route Ozone detecti limit (ppb) ZnO Hydrothermal method 60–1000 WO3 RF-sputtering 30–800 SnO2 Spray pyrolysis deposition 1000 In2O3 Sol–gel method and RGTO technique 30–900 SrTi1�xFexO3 Polymeric precursor method 30–1000 NiCo2O4 Co-precipitation 28–165 This journal is © The Royal Society of Chemistry 2016 NiCo2O4 compound.54–56 These ndings will support the gas sensing mechanism proposed for the NiCo2O4 compound below. 3.4 Gas sensing mechanism of NiCo2O4 hexagonal platelets The results above can be rationalized according to the following gas sensing mechanism for NiCo2O4. Under ambient condi- tions, oxygen from air is adsorbed on NiCo2O4, with surface states thus being created.54,57 These surface states allow elec- trons to be excited from the valence band, and therefore addi- tional holes are induced in the p-type NiCo2O4 grains.58 Detection of oxidizing gases on a p-type sensor material is known to arise from surface adsorption of gas molecules, which leads to a hole accumulation layer (HAL) near the surface because of electrostatic interaction between oppositely charged mpound with other metal oxides on Operating temperature (�C) Response time Recovery time Reference 250 15 s 60 s 48 250 1 s 60 s 49 280 2 s 900 s 16 300 25 s 1200 s 50 and 51 260 28 s 161 s 19 200 32 s 60 s Present study RSC Adv., 2016, 6, 92655–92662 | 92659 http://dx.doi.org/10.1039/c6ra18384k Fig. 10 Schematic illustration representing (a) interaction between oxygen and NiCo2O4 grain and corresponding energy band diagram; (b) interaction between O3 and NiCo2O4 grain and corresponding energy band diagram. RSC Advances Paper Pu bl is he d on 2 8 Se pt em be r 20 16 . D ow nl oa de d by U N IV E R SI D A D S A O P A U L O o n 18 /1 1/ 20 16 1 2: 59 :4 3. View Article Online species. The latter again establishes the electronic core–shell conguration and induces an increase in the current density.59,60 Fig. 10(a and b) depicts such a mechanism where upon exposing NiCo2O4 to air, atmospheric oxygen is adsorbed in an ionic form at the surface as O2 � (<100 �C), O� (100–300 �C) and O2� (>300 �C), which is a function of the operating temperature, as described elsewhere.61 For our measurements at 200 �C, adsorbed oxygen is mainly present in the O� form. Fig. 10(a) shows a schematic diagram for the interaction of O2 and O3 gas with NiCo2O4 grains, along with energy level diagrams. With adsorbed O� species, an accumulation layer is formed at the surface of NiCo2O4 grains. This is represented in eqn (2)–(5), where adsorption of negatively charged oxygen can generate holes. O2(gas) / O2(ads.) (2) O2(ads.) + e� / O2(ads.) � (3) O2(ads.) � + e� / 2O(ads.) � (4) O(ads.) � + e� / O(ads.) 2� (5) When exposed to oxidizing ozone (O3), the NiCo2O4 surface is covered with adsorbed ozone molecules acting as acceptors, as depicted in Fig. 10(b). This surface trapping of lone-pair of electrons causes band bending, with free holes and increase in hole concentration near the interface. It is the hole accumulated layer that decreases the resistance of NiCo2O4. The gas sensor response of NiCo2O4 lm for oxidizing gas (O3) can be explained by eqn (6).60 The reactions between reductive molecules and pre- adsorbed O� release free electrons and neutralize the holes, which are the majority carriers in p-type NiCo2O4, as shown in eqn (7). 92660 | RSC Adv., 2016, 6, 92655–92662 O3(gas) + e� / O2(des.) + O(ads) � (6) e� + h / null (7) Then, for p-type semiconductors, oxygen adsorption produces an increase in the number of holes in the valence band, which increases conductivity. Note that thermal energy is needed for the reaction between ozone gas molecules and surface adsorbed oxygen species. With increasing temperature, the surface reaction is thermally activated, causing a large decrease in the resistance, which means enhanced response, until saturation occurs and increasing temperature no longer brings increased performance. 4. Conclusion NiCo2O4 synthesized in one-step approach with a urea-assisted co-precipitation method has been proven excellent for ozone detection with both dc and ac electrical measurements. A low detection level of 28 ppb was achieved at the temperature with optimized performance, below the values in the recent litera- ture.34,62,63 The range for detection was from 28–165 ppb, with good sensitivity, selectivity and fast response (�32 s) and recovery time (�60 s). Themechanism of detection was found to be based on adsorption of ozone molecules on the NiCo2O4 surface, which creates a layer of holes that affect the conduc- tivity, as in a p-type semiconductor. The results presented here already demonstrate that NiCo2O4 is promising for monitoring the environment, and further optimization in ozone sensing may be achieved by analyzing their magnetic and electrical properties, and long-term stability, in addition to the possible use for other reducing and oxidizing gases. In particular, sensitivity can be further enhanced, even reaching sub ppb levels, by varying crystal shapes with tuning by varying param- eters such as reaction time and temperature, pH value, solvent type and concentration, in addition to the use of a catalyst. Acknowledgements The authors are grateful for the nancial support from FAPESP (2014/23546-1). Acknowledgments are also due to Angelo L. Gobbi and Maria H. O. 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See DOI: 10.1039/c6ra18384k One-step approach for preparing ozone gas sensors based on hierarchical NiCo2O4 structuresElectronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18384k One-step approach for preparing ozone gas sensors based on hierarchical NiCo2O4 structuresElectronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18384k One-step approach for preparing ozone gas sensors based on hierarchical NiCo2O4 structuresElectronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18384k One-step approach for preparing ozone gas sensors based on hierarchical NiCo2O4 structuresElectronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18384k One-step approach for preparing ozone gas sensors based on hierarchical NiCo2O4 structuresElectronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18384k One-step approach for preparing ozone gas sensors based on hierarchical NiCo2O4 structuresElectronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18384k One-step approach for preparing ozone gas sensors based on hierarchical NiCo2O4 structuresElectronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18384k One-step approach for preparing ozone gas sensors based on hierarchical NiCo2O4 structuresElectronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18384k One-step approach for preparing ozone gas sensors based on hierarchical NiCo2O4 structuresElectronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18384k One-step approach for preparing ozone gas sensors based on hierarchical NiCo2O4 structuresElectronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18384k One-step approach for preparing ozone gas sensors based on hierarchical NiCo2O4 structuresElectronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18384k