Nanowire-Assembled Hierarchical ZnCo2O4 Microstructure Integrated

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Nanowire-Assembled Hierarchical ZnCoO Microstructure Integrated with Low Power Microheater for Highly Sensitive Formaldehyde Detection Hu Long, Anna Harley-Trochimczyk, Siyi Cheng, Hao Hu, Won Seok Chi, Carlo Carraro, Tielin Shi, Zirong Tang, and Roya Maboudian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11054 • Publication Date (Web): 27 Oct 2016 Downloaded from http://pubs.acs.org on November 1, 2016

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ACS Applied Materials & Interfaces

Nanowire-Assembled Hierarchical ZnCo2O4 Microstructure Integrated with Low Power Microheater for Highly Sensitive Formaldehyde Detection Hu Long1-3, Anna Harley-Trochimczyk1,2, Siyi Cheng3, Hao Hu3, Won Seok Chi1,2, Carlo Carraro2, Tielin Shi3, Zirong Tang3, Roya Maboudian1,2,* 1 Berkeley Sensor & Actuator Center, University of California at Berkeley, Berkeley, CA 94720, USA. 2 Department of Chemical and Biomolecular Engineering, University of California at Berkeley, Berkeley, CA 94720, USA. 3 State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan 430074, China. KEYWORDS. Hierarchical structure, ZnCo2O4, Microheater, Formaldehyde, Sensing.

Abstract. Nanowire assembled 3D hierarchical ZnCo2O4 microstructure is synthesized by a facile hydrothermal route and a subsequent annealing process. In comparison to simple nanowires, the resulting dandelion-like structure yields more open spaces between nanowires, which allow for

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better gas diffusion and provides more active sites for gas adsorption while maintaining good electrical conductivity. The hierarchical ZnCo2O4 microstructure is integrated on a low-power microheater platform without using binders or conductive additives. The hierarchical structure of the ZnCo2O4 sensing material provides reliable electrical connection across the sensing electrodes. The resulting sensor exhibits an ultralow detection limit of 3 ppb towards formaldehyde with fast response and recovery as well as good selectivity to CO, H2 and hydrocarbons such as n-pentane, propane and CH4. The sensor only consumes ~5.7 mW for continuous operation at 300 oC with good long-term stability. The excellent sensing performance of this hierarchical structure based sensor suggests the advantages of combining such structures with microfabricated heaters for practical low-power sensing applications.


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Introduction Solid-state gas sensors have attracted considerable attention since the 1960’s due to the wide range of applications, such as detecting hazardous gases in industrial areas, analyzing breath, and monitoring air quality.1-8 However, commercially available gas sensors are still bulky, expensive and consume high power (~500 mW) due to the need for a heater element to promote gas detection, which limits their use in battery-powered portable devices.9 Thus, researchers have focused on the development of miniaturized gas sensors with small size, low power consumption and robust sensing performance.10 Such sensors can be achieved by advances on two fronts: smaller sensor elements and highly sensitive sensing materials. Gas sensor miniaturization can be accomplished through microfabrication methodologies, leveraging the well-developed industrial processes to produce reproducible and low cost devices.11-19 Decreasing the size of the heated sensing area gives a sensor with lower power consumption and fast thermal response time, which can allow for low duty cycle operation to further decrease the power consumption.1519

Solid-state gas sensors have long relied on metal oxides as the sensitive material. The composition and structure of the metal oxide material are both important factors in the gas sensing performance. To reach appreciable conductivity and sensitivity, the metal oxide materials are usually heated. Using a microfabricated platform with integrated heating elements lowers the power consumption but it also decreases the active area for the sensing material.17 Nanostructured materials such as nanoparticles and nanowires display high sensitivity due to their small grain size and high surface area,20 but integration of nanomaterials onto a microfabricated platform is difficult to control. Nanomaterial aggregation during sensing film formation and sensor operation leads to a loss of active surface area and slows the diffusion of


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the target gas to the active sites. Assembling nanostructures into an interconnected hierarchical microstructure can maintain the high surface area and porous structure, while also ensuring simple integration and excellent electrical properties.18 In addition to the sensing material structure, the composition is critical to the sensing performance. The choice of metal oxide is usually driven by the target gases of interest for sensing. Formaldehyde is a common air pollutant that is the primary cause of sick building syndrome21 and as such would benefit from indoor air quality monitoring that could detect it sensitively and selectively. The US Environmental Protection Agency (EPA) has set 0.04 ppm formaldehyde as the emission standard because of its high toxicity.21 For formaldehyde sensing, various metal oxides have been investigated, such as SnO2, NiO, Fe2O3, ZnO, and Co3O4,23-34 but most of them cannot reach the low detection limit required. Cobalt oxide (Co3O4) has shown promise for the sensitive detection of other volatile organic compounds due to its catalytic activity.33,34 Partially replacing the cobalt with another metal such as Ni, Zn, Sn, or Cu to form a mixed metal oxide35-41 is reported to increase the conductivity and electrochemical activity while retaining the catalytic properties.35-38 ZnCo2O4, which shares a similar crystal structure with Co3O4, is one of these mixed metal oxides that has been demonstrated as a high performance material in a number of applications including as electrodes for Li-ion batteries and supercapacitors.37-40 There have been a few reports using ZnCo2O4 nanospheres and nanoparticles as gas sensing materials,41-46 but not as a hierarchical microstructure. Here we present the fabrication of a highly sensitive and selective formaldehyde sensor based on hierarchical ZnCo2O4 microstructures integrated onto a low power microheater. The hierarchical ZnCo2O4 microstructure is synthesized using a hydrothermal process and subsequent annealing. By assembling the ZnCo2O4 nanowires into a dandelion-like microstructure, the


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material exhibits a high specific surface area and allows for fast gas diffusion to the active sites. The micrometer size of the hierarchical structure also allows for a simple and reliable method to integrate the ZnCo2O4 onto a microheater platform, yielding excellent electrical connection. The formaldehyde gas sensing performance of the hierarchical ZnCo2O4 sensor demonstrates a low detection limit, fast response and recovery, and good selectivity.

Experimental section Synthesis and materials characterization. All reagents are of analytical grade and were used without further purification. The hierarchical ZnCo2O4 microstructures were synthesized by a hydrothermal reaction, similar to our previous report.37 In a typical synthesis procedure, 2 mmol of zinc nitrate (Zn(NO3)2*6H2O) and 4 mmol of cobalt nitrate (Co(NO3)2*6H2O) were dissolved in 70 ml of deionized (DI) water to form a clear pink solution. Then, 4 mmol of ammonium fluoride (NH4F) and 10 mmol of urea (CO(NH2)2) were added with constant stirring to form a homogeneous solution. The solution was then transferred into a Teflon-lined stainless autoclave and kept at 120 °C for 12 h. After reaction, the autoclave was cooled down to room temperature naturally and the resulting pink precipitate was rinsed with ethanol and distilled water several times before being dried at 60 ºC in an oven overnight. Following this step, the samples were placed in a quartz tube and calcined at 400 ºC for 2 h in air with a heating and cooling rates of 2 ºC min-1. The morphology of the products was characterized using field-emission scanning electron microscopy (FESEM, JEOL JSM-6700F), transmission electron microscopy (TEM, FEI, Tecnai G2 20) and field-emission transmission electron microscopy (FETEM, FEI, Tecnai G2 F30). The crystal structure was characterized using X-ray diffraction (XRD; X’Pert PRO, PANalytical


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B.V., the Netherlands) employing radiation from a Cu target (Kα, λ= 0.15406 nm). Chemical characterization using X-ray photoelectron spectroscopy (XPS) was carried out using a VG MultiLab 2000 system with a monochromatic Al Kα X-ray source (Thermo VG Scientific). Microheater-based sensor fabrication and testing. Microheaters were fabricated using a 4mask surface micromachining process to create a polycrystalline silicon (poly-Si) microheater embedded in a thin silicon nitride membrane. The fabrication details can be found in our previous reports.15-18 Briefly, 100 nm in-situ doped poly-Si was patterned and encapsulated in 200 nm low-stress silicon nitride (LSN) film. The sensing electrodes and microheater contacts were made using 10 nm of titanium and 90 nm of platinum. Finally, the wafers were etched with KOH from the backside to remove the silicon under the microheaters, leaving only the thin silicon nitride membrane. The wafer was diced into 3.5 × 3.5 mm2 chips, which contain four individual microheaters per chip where one trace has a width of 6 µm and approximate length of 100 µm. Once individualized, the microheater chips were wire-bonded into a 14-pin ceramic dual in-line package for electrical and gas sensing characterizations. The as-synthesized ZnCo2O4 was sonicated into suspension and deposited from a solution of DI water and isopropyl alcohol (IPA). A 0.25 µl drop of 1 mg mL-1 solution was placed on the microheater chip while the microheater was powered to 2 mW (100 °C), with the voltage controlled by a Keithley 2602A sourcemeter. Heating the microheater promotes solvent evaporation and leads to localized material deposition at the center of the heated area. The packaged microheater was placed within a gas flow chamber with a volume of 1 cm3. The sensor was exposed to different gases using a computer-controlled gas delivery system. Mass flow controllers (Bronkhorst) controlled by LabView were used to dilute the gas with clean air and deliver these gases to the sensor chamber. For gas sensing characterization, the sensors were


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exposed to various gases, namely formaldehyde (Praxair, 20 ppm in N2), CO (Praxair, 5000 ppm in N2), propane (Praxair, 5%), n-pentane (Praxair, 5000 ppm), methane (Praxair, 5%), hydrogen (Praxair, 5% in N2), O2 (Praxair, 100%), and N2 (Praxair, 100%) using the same gas delivery system. Relative humidity was adjusted by evaporating deionized water and mixing it with the gases. All sensor testing was performed at a constant flow rate of 300 sccm and at room temperature (25 °C). Stream balance and purge were made up of house air passed through pressure swing adsorption dryers to remove humidity and an activated carbon scrubber to remove other contaminants. The measurement of the microheater sensor was done with a Keithley 2602 source-meter controlled by Zephyr, an open-source Java-based instrument and control and measurement software suite. Zephyr was also used to acquire data from the gas delivery system, including gas flow rates and concentrations. The sensor measurement was taken by continuously applying a bias voltage and recording the current, which was used to calculate the resistance, R. The sensor response is defined as (Rgas-Rair)/Rair, where Rgas is the resistance during exposure to a given gas concentration and Rair is the average resistance in clean air before any gas exposure.

Results and discussion Structural and morphological characteristics. Representative scanning electron micrographs of the as-synthesized ZnCo2O4 (after thermal annealing at 400 °C) are shown in Figures 1(a) and (b). The low-magnification FESEM reveals that the product is an array of dandelion-like microstructures with diameters around 10 µm. Higher resolution FESEM images show that the microstructures are made up of nanowires grown radially from the center of the dandelion-like structures. The well-assembled nanowires


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yield more V-type channels between nanowires which are expected to facilitate gas diffusion and provide large surface area for gas reaction. Figure 1 (c) exhibits a FESEM image of a broken dandelion-like structure, showing that the microstructure is a double-shelled structure composed of nanosheets assembled microsphere at the core and nanowires as an outer shell. Further information about the ZnCo2O4 structure is obtained by transmission electron microscopy (TEM). Figure S1 (Supporting Information) shows a low-magnification TEM image of the edge of a microstructure, where the ZnCo2O4 nanowires can be seen. A higher magnification TEM image shown in Figure 1(d) shows a typical needle-shape ZnCo2O4 with a diameter from 10 to 80 nm. Figure 1(e) shows a high-resolution TEM image of the nanowire where the nanowire is seen to be polycrystalline. The lattice fringes shown in Figure 1(f) have a lattice spacing of 0.247 nm, which corresponds to (311) plane of ZnCo2O4.37-39 As shown in Figure S2 (Supporting Information), the relevant SAED pattern shows distinct diffraction rings which could be readily indexed to the (111), (220), (311), (400), (422) and (400) crystal planes of spinel ZnCo2O4. Furthermore, the Brunauer–Emmett–Teller (BET) analysis from our previous work shows that similar types of hierarchical structure using the same synthesis process yield highly porous, high surface area (about 155 m2/g) nanomaterials.


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Figure 1. (a-b) FESEM images of the hierarchical ZnCo2O4 structure with different magnifications. (c) FESEM image of a broken dandelion-like structure showing the double shelled structure. (d) Low-magnification TEM image of a typical needle-shaped ZnCo2O4 nanowire. (e) HRTEM image of single ZnCo2O4 nanowire. (f) Enlarged HRTEM image of the ZnCo2O4 nanowires. A possible synthesis mechanism for the hierarchical microstructure is proposed, as schematically shown in Figure 2. At the initial stage of the hydrothermal reaction, Zn2+ and Co3+ ions nucleate and form the seeds (nanoparticles) upon heating (Figure 2 (a)). The newly formed seeds with high surface energies tend to aggregate into nanosheets in the presence of NH4F.47 The NH4F enhances the adhesion between the nanosheets so that they further aggregate into microspheres in order to minimize the surface energy of the system.47 As shown in the SEM image in Figure 2(c), the microspheres are obtained after 5 hours of hydrothermal reaction and thermal annealing at 400 °C. The addition of urea to the solution promotes the growth of


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nanowires on the surface of the microsphere by acting as a surfactant.37, 39, 48 With additional ripening time, the structure continues to grow into dandelion-like features in the solution, shown in the SEM image in Figure 2(d), after 12 hours of hydrothermal reaction and thermal annealing at 400 °C. Such a mechanism is commonly used to explain crystal growth and is observed for similar material systems.47-48 Owing to the release of CO2 and H2O during the thermal annealing process, a porous nanostructure is effectively formed.

Figure 2. Schematic of the formation process of hierarchical ZnCo2O4 structure. (a) Nucleation; (b) Aggregation; (c) Further aggregation; (d) Nanowire growth. SEM images correspond to the hierarchical ZnCO2O4 structures with (c) 5 hours of hydrothermal reaction (after thermal annealing) and (d) 12 hours of hydrothermal reaction (after thermal annealing). X-ray diffraction is conducted to identify the composition and crystalline phases of the ZnCo2O4 material (Figure 3). All the diffraction peaks match well with the spinel structure of


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ZnCo2O4 (JCPDS Card No. 23-1390). No other peaks corresponding to other materials are observed, indicating the high purity of the ZnCo2O4 product.

Figure 3. XRD pattern of as-prepared hierarchical ZnCo2O4 structure. X-ray photoelectron spectroscopy is used to further characterize the surface composition of the hierarchical ZnCo2O4 structure. Figure 4(a) shows a wide-scan spectrum, where the characteristic peaks of Zn, Co, O and C are clearly identified. No other peaks are observed. The C 1s peak at 284.6 eV is used for calibration. Figures 4 (b-d) show the high resolution XPS scan of Zn 2p, Co 2p and O 1s regions, respectively. The spectrum from Zn 2p region shows two peaks with binding energies at 1020.3 and 1043.6 eV, which can be assigned to Zn 2p3/2 and Zn 2p1/2, respectively, of Zn2+ oxidation state, as expected for ZnCo2O4. In the Co 2p region, two peaks at 779.4 eV and 794.8 eV are observed, which can be assigned to Co 2p3/2 and Co 2p1/2, respectively, of Co3+ oxidation state of ZnCo2O4. In Figure 4d, the O 1s peak can be separated into two peaks. The peak at 529.2 eV corresponds to the oxygen species in the spinel ZnCo2O4 phase. The peak observed close to 531.2 eV indicates the presence of –OH (hydroxyl) species adsorbed on the surface which is commonly reported in ex situ analyzed samples.18 The XPS


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results, in conjunction with other aforementioned studies, confirm the formation of ZnCo2O4 with normal spinel structure.

Figure 4. XPS spectra for the ZnCo2O4 structure: (a) survey spectrum and high-resolution spectra from (b) Zn 2p, (c) Co 2p and (d) O 1s regions. Gas sensing performance. Figure 5(a) shows a schematic of the microheater sensor, consisting of suspended polysilicon heater embedded in a silicon nitride membrane with Pt/Ti metal contacts for the microheater and the sensing materials. The suspended microheater design enhances the thermal efficiency and minimizes power loss through conduction to the substrate. The efficient design of the microheater results in a low power consumption of 5.7 mW at an operation temperature of 300 oC.15-18 The hierarchical ZnCo2O4 microstructure is integrated,


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using drop-casting methods, on the microheater platform to form the sensor. The ZnCo2O4 microstructures easily bridge the gap between the sensing electrodes which is 2 µm in size. Typically, inkjet printing is employed for depositing well-prepared inks (mixing nanomaterials with polymer and solvent) to form a thin film on the microheater. However, a uniform and stable ink with nanomaterials is hard to achieve and nanomaterials aggregation during film formation and sensor operation leads to a loss of effective surface area and slows the diffusion of target gas to the active sites. Consisting of a self-assembled network of nanowires, the ZnCo2O4 hierarchical microstructure maintains the high surface area with good inter-nanowire contact and controllable morphology. The large size of each dandelion further allows a strong connection between the sensing electrodes. Thus, the hierarchical ZnCo2O4 microstructure provides high surface area, which facilitates the sensor fabrication by easily connecting the sensing electrodes while maintaining the advantage of one-dimensional materials as well as high surface in an accessible porous network.

Figure 5. (a) Schematic of the hierarchical dandelion-like ZnCo2O4 structures integrated microheater sensor. (b) Optical image of the as-fabricated microheater containing 4 four heaters. Inset: optical image of one microheater showing Pt/Ti sensing electrodes above the polysilicon heater.


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A typical current-voltage (IV) curve of a hierarchical ZnCo2O4 based sensor is shown in Figure S3 (Supporting Information). In the range of -2 V to 2V, the sensor shows a linear IV behavior indicating the formation of Ohmic contact with Pt/Ti electrodes. For gas sensing measurements, a bias voltage of 1 V is applied to the sensor channel to measure the resistance across the sensing material. Operation temperature is an important parameter which has a major impact on the sensing performance, such as the sensitivity and the response and recovery times. To determine the optimum operating temperature for the sensor, the sensor is exposed to formaldehyde gas while the microheater is set to different temperatures from 25 to 400 oC. Figure 6(a) shows that the response to 5 ppm formaldehyde rapidly increases and reaches a maximum at 250 °C, and then decreases gradually with a further increase in temperature.


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Figure 6. (a) Sensor response to 5 ppm formaldehyde at different microheater temperatures. (b) Response and recovery times (t90) to 5 ppm formaldehyde at different temperatures. (c) Relationship between microheater power and temperature for the microheater. (d) Real time resistance change of the sensor to different formaldehyde concentrations at 300 °C. (e) Calculated response versus formaldehyde concentrations. Figure 6(b) shows that the response and recovery times of the sensor decrease with increasing temperature when exposed to 5 ppm formaldehyde at various temperatures. The reported times are taken as average times to reach 90% of the final sensing and baseline signals (so-called t90). The response and recovery times at 300 °C are found to be significantly shorter than at 250 °C, but with further increase in temperature, these times gradually decrease. Balancing between sensitivity, response and recovery times, and power consumption, the sensing temperature of 300 °C is chosen as the optimum operating temperature. At 300 °C, the power consumption of the sensor is only 5.7 mW (Figure 6(c)), which is nearly two orders of magnitude lower than many commercial metal oxide sensors. Figure 6(d) shows the real-time sensor response to different formaldehyde concentrations with the microheater set to 300 °C. The sensor displays an increase in resistance upon exposure to formaldehyde. The mechanism of ZnCo2O4 gas sensing in air is likely based on a surface controlled reaction and charge transfer, resulting in a change in electrical resistance of the materials. ZnCo2O4 is a well-known p-type material and its resistance is highly influenced by surface states, which are affected by the adsorption of oxygen species. In ambient air and depending on the temperature, chemisorbed oxygen molecules, O2ads, capture electrons from ZnCo2O4 and are subsequently converted to (O2 ads)-, (Oads)- and (Olattice)2- species on the surface, e.g., via reaction 1:


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1/2O2(g) + e- → (O ads) -

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(1)

The withdrawal of electrons from metal oxides results in an electron-depleted surface region, which, for a p-type material, leads to a decrease in the resistance of the sensor in air. To elucidate the effect of oxygen on the sensor performance, Figures S4(a-c) (Supporting Information) show the sensor baseline resistance, the sensor response to 1 ppm formaldehyde and response and recovery times vs. O2 concentration, respectively, with the microheater at 300 °C. In agreement with the proposed mechanism (reaction 1), the sensor baseline resistance shows a large decrease when exposed to 5% O2 in N2 compared to pure N2. The effect plateaus at higher O2 concentrations as the coverage of oxygen species on the surface reaches a saturation point (Supporting Information Figure S4 (a)). In pure N2 ambient, the presence of formaldehyde causes an increase in the sensor resistance, which is also consistent with a p-type sensing behavior given the electron-donating characteristics of formaldehyde. 42-44 In 5% O2 environment (Supporting Information Figure S4 (b-c)), the sensor shows a smaller response to formaldehyde but with faster response and recovery compared to in pure N2, which may indicate that the dominating sensing mechanism changes from the direct adsorption in pure N2 to a surface reaction controlled mechanism. In presence of O2, formaldehyde is oxidized by the chemisorbed oxygen species forming CO2 and H2O, via reaction 2: HCHO + 2O-(ads) → CO2 + H2O + 2e-

(2)

This reaction causes the release of electrons trapped by oxygen species, leading to an increase in the resistance of the sensor. In an O2 environment, a balance is reached between removing preadsorbed oxygen species by the above reaction and continuous oxygen adsorption; thus, increasing the O2 concentration enhances the oxygen adsorption relative to the oxygen removal


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by reaction 2, which decreases the effects of formaldehyde adsorption and reaction on the electron depletion layer thickness, leading to a slightly lower sensor response. In practical applications for air quality monitoring, the sensor is normally exposed to atmospheric condition (21% oxygen). Under such conditions, the working temperature impacts the sensing reaction as shown in Figure 6(a). To understand the effect of working temperatures, there are two main factors controlling the reaction, namely the thermal energy required for the activation of reaction 2, and the amount of oxygen adsorbed (reaction 1). At low temperatures, the reaction rate is slow; thus, the response of the sensor is relatively small. When the temperature is increased, the thermal energy provided is high enough to overcome the activation energy barrier for surface reaction; thus the reaction rate is enhanced and the sensor shows increasing response to formaldehyde. However, when the temperature increases above 300 °C, the desorption of the chemisorbed oxygen is enhanced more than the reaction rate, which leads to a decrease in response. To find the limit of detection and detection range, the sensor was exposed to different formaldehyde concentrations with the microheater set to 300 °C (Figure 6(d)). The hierarchical ZnCo2O4 sensor shows a rapid increase in resistance upon exposure to formaldehyde. As discussed previously, the observed increase in resistance upon exposure to formaldehyde is consistent with p-type sensing behavior.42-44 When the sensor is exposed to clean air again, the resistance quickly returns to the baseline, indicating the fast recovery of the sensor. The average t90 response and recovery times of the sensor are around 18 and 24 s, respectively. The sensor response versus formaldehyde concentration is displayed in Figure 6(e). The sensor response increases with increasing formaldehyde concentration and the lack of significant plateau suggests that the sensor can detect higher formaldehyde concentrations as well. At lower concentrations,


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the sensor shows a linear response. Due to the limitations of the gas delivery system, 50 ppb is the lowest concentration that can be reliably delivered to the sensor. By using a signal-to-noise threshold value of 3, the calculated detection limit of the sensor to formaldehyde is 3 ppb, which is well below the threshold value of 40 ppb set by the EPA. In order to better assess the advantage of combining the hierarchical ZnCo2O4 microstructure and the low-power microheater, the comparison between this work and other metal oxide based formaldehyde sensors is listed in Table S1 (supporting information). The sensor presents superior sensing performance than other metal oxide based sensors, such as a low detection limit, fast response and recovery times and low power consumption. The highly sensitive feature of the sensor can be ascribed to the high surface area and small grain size of the nanowire assembled hierarchical ZnCo2O4 structure. It is well known that the decrease in grain size to less than twice the Debye length leads to electron depletion of the whole grain, which maximizes the effect of changes in electron depletion layer thickness, resulting in high sensitivity. The well assembled nanowires provide a large number of the surface sites participating in the interactions which also enhance the sensitivity. The highly porous structure and the V-channels formed between the nanowires provide high gas permeability to active sites, allowing fast gas diffusion and reaction, resulting in fast response and recovery. The device also exhibits excellent stability, as shown in Figure S5 (Supporting Information). For 6 hours of continuous heating at working temperature of 300 ºC, the sensor shows less than 5% baseline drift with low noise. In addition, the time taken by the sensor to reach a stable baseline is less than 3 min (Inset in Figure S4), which further demonstrates the potential practical application of this hierarchical ZnCo2O4 based low power sensor. The effect of relative humidity on the gas sensing properties is also investigated. Figure S6 (Supporting Information) shows that


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the baseline resistance increases with increasing relative humidity. This is due to adsorbed water acting as an electron donor upon forming hydroxyl groups (-OH-) on the sensor surface and thus increasing the sensor resistance.22 The sensor maintains good formaldehyde response when the relative humidity is increased to 30%. However, the sensor response decreases with further increase in relative humidity level. It is known that there is a competitive adsorption between O2 and H2O related surface species. At high humidity levels, the higher percentage of OH- groups may negatively affect the surface reaction between formaldehyde and surface oxygen species, resulting in lower sensor response.22,43 But when the relative humidity level returns to 0% again, the sensor response is restored, indicating the temporary effect of humidity rather a permanent “poisoning” effect. One of the main drawbacks of metal oxide sensing technologies has been their poor selectivity. The composition and structure of the metal oxide sensing material affect the way it interacts with various gases. The selectivity of the hierarchical ZnCo2O4 sensors is shown in the bar graph in Figure 7. The sensor is exposed to a variety of common reducing gases at 300 °C, which have similar physical and chemical properties to formaldehyde and are also common indoor air pollutants. The sensor shows high response to low concentrations of formaldehyde (10 ppm), but considerably lower response to 10 ppm carbon monoxide (CO), 500 ppm n-pentane, 5000 ppm hydrogen (H2), 5000 ppm propane and 5000 ppm methane (CH4). Metal oxide semiconductors with distinctive surface reactivity and gas adsorption properties may enhance gas selectivity as well as improve recovery rates.49 As shown in Figure S4, the hierarchical ZnCo2O4 structure shows good formaldehyde sensing properties in pure N2, which might result in a high selectivity for formaldehyde against other gases. Further study is needed to fully understand the selectivity mechanism.


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Figure 7. Selectivity of the hierarchical ZnCo2O4 structure based low power sensor at 300 °C

Conclusion In summary, we have synthesized a 3D hierarchical ZnCo2O4 microstructure composed of 1D nanowires with a facile hydrothermal route and subsequent annealing process. The structure, morphology and composition of the material are confirmed with FESEM, TEM, XRD and XPS. The ZnCo2O4 nanowires assembled into a microstructure yields high surface area per device footprint and make it easy to electrically connect the nanowires without using a binder or conductive additive. The hierarchical ZnCo2O4 is easily integrated on a low power microheater platform by drop casting and gives reliable electrical connections across the microfabricated sensing electrodes due to the microscopic size. The hierarchical ZnCo2O4 sensor exhibits excellent sensing performance towards formaldehyde, including an estimated low detection limit of 3 ppb, fast response and recovery (less than 20 s), good selectivity, low power consumption (5.7 mW for 300 ºC operation), and a highly stable baseline. The excellent sensing performance is attributed to the unique structure of ZnCo2O4, which forms a 3D conductive network consisting of well-aligned porous nanowires with high surface area. The combination of


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hierarchical structure integrated with low power microheater has great potential to work in battery powered portable devices for indoor air quality monitoring or breath analysis.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Low resolution TEM image, SAED pattern, IV curve of the sensor, impact of O2 concentrations, long term dynamic resistance change of the sensor, impact of relative humidity (PDF) AUTHOR INFORMATION Corresponding Author *R. Maboudian. Email: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank Ameya Rao for many useful discussions. This work was partially supported by the National Science Foundation of China (No. 51275195 and No. 91323106) and Program for Changjiang Scholars and Innovative Research Team in University (Grant no. IRT13017) which provided for materials synthesis and characterization. This work was also supported by Berkeley Sensor and Actuator Center (BSAC) Industrial Members and National Science Foundation (NSF grant # IIP 1444950) which provided for the design of experiments, student support (H.L., A.H.T.), and sensor fabrication and performance characterization. A.H.-T. acknowledge additional support though the NSF Graduate Research Fellowship (DGE 1106400).


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Figure 1. (a-b) FESEM images of the hierarchical ZnCo2O4 structure with different magnifications. (c) FESEM image of a broken dandelion-like structure showing the double shelled structure. (d) Low-magnification TEM image of a typical needle-shaped ZnCo2O4 nanowire. (e) HRTEM image of single ZnCo2O4 nanowire. (f) Enlarged HRTEM image of the ZnCo2O4 nanowires. 180x94mm (300 x 300 DPI)



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Figure 2. Schematic of the formation process of hierarchical ZnCo2O4 structure. The SEM images correspond to the hierarchical ZnCO2O4 structures with (c) 5 hours of hydrothermal reaction and (d) 12 hours of hydrothermal reaction. 150x88mm (300 x 300 DPI)


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Figure 4. XPS spectra for the ZnCo2O4 structure: (a) survey spectrum and high-resolution spectra from (b) Zn 2p, (c) Co 2p and (d) O 1s regions. 140x119mm (300 x 300 DPI)



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Figure 3. XRD pattern of as-prepared hierarchical ZnCo2O4 structure. 85x70mm (300 x 300 DPI)


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Figure 5. (a) Schematic of the hierarchical dandelion-like ZnCo2O4 structures integrated microheater sensor. (b) Optical image of the as-fabricated microheater containing 4 four heaters. Inset: optical image of one microheater showing Pt/Ti sensing electrodes above the polysilicon heater. 116x54mm (300 x 300 DPI)



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Figure 6. (a) Sensor response to 5 ppm formaldehyde at different microheater temperatures. (b) Response and recovery times to 5 ppm formaldehyde at different temperatures. (c) Relationship between power consumption and temperature for the microheater. (d) Real time resistance change of the sensor to different formaldehyde concentrations at 300 °C. (e) Calculated response versus formaldehyde concentrations. 171x125mm (300 x 300 DPI)


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Figure 7. Selectivity of the hierarchical ZnCo2O4 structure based low power sensor 85x59mm (300 x 300 DPI)



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80x35mm (300 x 300 DPI)


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Nanowire-Assembled Hierarchical ZnCo2O4 Microstructure Integrated

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