Single and Networked ZnOâCNT Hybrid Tetrapods for Selective Room

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Single and Networked ZnO−CNT Hybrid Tetrapods for Selective Room-Temperature High-Performance Ammonia Sensors Fabian Schütt,† Vasile Postica,‡ Rainer Adelung,*,† and Oleg Lupan*,†,‡ †

Institute for Materials Science, Kiel University, Kaiser str. 2, D-24143 Kiel, Germany Department of Microelectronics and Biomedical Engineering, Technical University of Moldova, 168 Stefan cel Mare Avenue, MD-2004 Chisinau, Republic of Moldova

‡

S Supporting Information *

ABSTRACT: Highly porous hybrid materials with unique high-performance properties have attracted great interest from the scientific community, especially in the field of gas-sensing applications. In this work, tetrapodalZnO (ZnO-T) networks were functionalized with carbon nanotubes (CNTs) to form a highly efficient hybrid sensing material (ZnO-T− CNT) for ultrasensitive, selective, and rapid detection of ammonia (NH3) vapor at room temperature. By functionalizing the ZnO-T networks with 2.0 wt % of CNTs by a simple dripping procedure, an increase of 1 order of magnitude in response (from about 37 to 330) was obtained. Additionally, the response and recovery times were improved (by decreasing them from 58 and 61 s to 18 and 35 s, respectively). The calculated lowest detection limit of 200 ppb shows the excellent potential of the ZnO-T−CNT networks as NH3 vapor sensors. Room temperature operation of such networked ZnO−CNT hybrid tetrapods shows an excellent long-time stability of the fabricated sensors. Additionally, the gas-sensing mechanism was identified and elaborated based on the high porosity of the used three-dimensional networks and the excellent conductivity of the CNTs. On top of that, several single hybrid microtetrapod-based devices were fabricated (from samples with 2.0 wt % CNTs) with the help of the local metal deposition function of a focused ion beam/scanning electron microscopy instrument. The single microdevices are based on tetrapods with arms having a diameter of around 0.35 μm and show excellent NH3 sensing performance with a gas response (Igas/Iair) of 6.4. Thus, the fabricated functional networked ZnO−CNT hybrid tetrapods will allow to detect ammonia and to quantify its concentration in automotive, environmental monitoring, chemical industry, and medical diagnostics. KEYWORDS: CNT, ZnO tetrapod, hybrid, networks, microsensor, ammonia sensor

1. INTRODUCTION The ammonia in Earth’s atmosphere is emitted by human activities, and the annual emission/deposition rate is increasing each year worldwide, especially in Central and Western Europe.1,2 Different problems are associated with this, including serious health threats, formation of nanometer airborne particles, acidity in rainwater, soil acidification, eutrophication, change in vegetation, corrosions, and others.1−4 To detect ammonia and quantify its concentration on extensive areas and Earth’s atmosphere levels (0−15 km), new types of lightweight nano- and microsensors are required, particularly for environmental monitoring, automotive, chemical industry, and medical diagnostics. Functional nano- and micromaterials, like carbon nanotubes (CNTs), zinc oxide (ZnO), and other semiconducting oxides, are promising candidates as main components in such sensors and microdevices. Integration of low-dimensional and quasi-one-dimensional carbon nanomaterials, such as CNTs with their unique properties, into functional nano- and microdevices is still very challenging;5,6 thus, their application is still rather limited to laboratory environment. Nevertheless, CNTs are well known © XXXX American Chemical Society

for their exceptional electrical, thermal, and mechanical properties and are thus one of the most studied nanomaterials within the last decades. Sensors based on carbon nanomaterials are known to be very sensitive to gaseous species at room temperature with “low noise” nature and high stability, which give the possibility to operate at low applied bias voltages.7,8 For example, Esser et al. demonstrated the possibility to detect sub-ppm concentrations of ethylene using single-walled CNTs (SWCNTs).9 Schnorr et al. fabricated sensor arrays of covalent functionalized SWCNTs with a promising long-term stability for the detection of explosive gases such as cyclohexanone and nitromethane.10 Saetia et al. fabricated flexible sensors based on functionalized multiwalled CNT (MWCNT) films coated on a porous electrospun fiber mat for real-time detection of a nerve agent stimulant.11 Furthermore, Rigoni et al. demonstrated the possibility to detect ammonia at room temperature and very low concentrations down to 20 ppb using SWCNTs.12 Wang et Received: March 15, 2017 Accepted: June 14, 2017

A

DOI: 10.1021/acsami.7b03702 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

capable of detecting low concentrations under UV illumination.31 Cui et al. reported on a possibility to improve NH3 gassensing properties of MWCNTs by hybridization with SnO2 and Ag nanoparticles. The proposed sensing mechanism was explained on the basis of the electronic sensitization and attraction of NH3 molecules to hollow sites on the oxidized Ag surface, with H atoms pointing toward Ag atoms and electron donation from H to the hybrid sensor.32 However, some issues as low gas sensitivity, selectivity, and especially rapidity still need to be improved. Therefore, the investigation of the interaction between carbon-based nanomaterials and nano- and microstructures of metal oxides for enhanced gas-sensing properties is of major interest.33 By combining the advantages of both material classes and thereby eliminating the single disadvantages, more advanced sensing materials can be obtained. Thus, investigating different combinations of materials (type of metal oxide and morphology) with carbonbased nanomaterials and finding the most efficient one is a necessity, especially in the field of gas-sensing applications, which is the aim of this research. In this work, we combine the capability of CNTs to detect gaseous species at room temperature,34 with the very short response and recovery times of the tetrapodal-ZnO (ZnO-T) 3D networks based sensors due to their specific morphology and detection mechanism.21,22,35 The hybrid ZnO-T−CNT networks were functionalized with different contents of CNTs to improve NH3 sensing properties at room temperature. The influence of humidity and long-time stability was also investigated. ZnO-T−CNTs demonstrated excellent selectivity to NH3 with the lowest detection limit (LDL) for NH3 vapor beyond 0.2 ppm or 200 ppb (in the case of functionalization with 2.0 wt % CNTs). The gas-sensing mechanism was proposed and described in detail. Several microdevices fabricated using individual hybrid ZnO-T−CNT show excellent NH3 sensing performances at room temperature. On top of that, the influence of the diameter of the arms of the zinc oxide tetrapods was also investigated, showing that a decrease in the diameter leads to an increase in the rapidity and gas response.

al. fabricated NH3 gas sensors based on SWCNT/CuPcTIP and SWCNT/CuPcTTMP hybrid materials with improved performances at room temperature compared to pristine SWCNTs.13 Another interesting work was presented by Some et al., in which one-headed polymer optical fiber sensor arrays with hydrophilic graphene oxide and hydrophobic reduced graphene oxide were investigated for their selectivity to volatile organic compounds in highly unfavorable environments.14 Han et al. implemented SWCNTs-based sensors on a cotton yarn for the detection of NH3, resulting in good mechanical robustness against bending.15 Chow et al. reported on the transport of energetic electrons through single, wellaligned MWCNTs, suggesting that it has the potential to realize new classes of collimators and beam optics for energetic particles, ions as well as electrons, or to enable the placement of dopant atoms with nanometer precision.16 Doping also has been widely used for improving the gas-sensing properties of carbon-based nanomaterials.17 Recently, our group demonstrated low-powered, tunable, and ultralight gas sensors for climate monitoring8 based on aerographite,18 which can operate even at 1 mV applied bias voltage at room temperature. The gas-sensing mechanism for chemiresistive sensors based on carbon micro- and nanostructures is widely explained on the basis of the charge transfer with gas molecules adsorbed on the surface.7 However, the disadvantages such as long recovery times and strong influence of ambient factors (e.g., humidity) often limit the potential of carbon-based nanostructures for real applications. On the other hand, semiconducting oxide based sensors are known to be highly sensitive, low-cost, and widely used in industry.19,20 In this context, ZnO is an extensively studied material due to the ease of nanostructure fabrication. It has already been demonstrated that three-dimensional (3D) networks made of interconnected tetrapodal-shaped ZnO microparticles show excellent prospects for the development of ultra-fast UV and gas sensors due to their interconnected arms and anti-agglomeration properties leading to a specific sensing mechanism based on their enormous porosity and efficient diffusion of gases even at the bottom layers of the networks.21,22,59 However, some issues remain, such as the necessity for high operating temperatures for the efficient detection of gases together with a sufficient long-term stability, which increase the complexity of the sensor structures and power consumption of the heater elements.23−27 In this context, the hybridization of ZnO (and other metal oxides) nanostructures with carbon-based materials (which are ideal building nanoblocks for the construction of hybrid structures) is known to be a powerful tool to essentially improve the physical and chemical properties for gas-sensing applications. Van Hieu et al. investigated the room temperature NH3 gas-sensing properties of SnO2/MWCNT composites and demonstrated a much better response and rapidity compared to pristine SnO2 or CNT material.23 Singh et al. demonstrated the detection of 1 ppm of CO, NH3, and NO2 gases at room temperature by ZnO-decorated graphene.28 Yi et al. fabricated flexible gas sensors based on vertically aligned ZnO nanorods with graphene-based top electrode capable of detecting ethanol vapor at room temperature.29 Lee et al. reported gas sensors based on carbon nanoflake/SnO2 composites (with 10% of SnO2) for the detection of NH3 showing 3 times higher sensor response and better repeatability than the gas sensors based on pristine SnO2.30 Ding et al. demonstrated acetone vapor sensors based on SWNT-TiO2 core/shell hybrid nanostructures

2. EXPERIMENTAL SECTION For the fabrication of ZnO tetrapodal networks, a flame transport synthesis was used.21,22 Thereby, zinc powder, having a grain size of 1−5 μm, was mixed with polyvinyl butyral (1:2 by mass) and the mixture was heated in a muffle furnace for 30 min at 900 °C (heating rate of 60 °C·min−1). The obtained loose powder was pressed into cylindrical pellets (h = 3 mm, d = 6 mm), with a density of 0.3 g/cm3 and reheated to 1150 °C for 4 h. For the CNT infiltration of the templates, a commercially available aqueous CNT dispersion (CarboByk 9810) was used, which was diluted with distilled water to contain 0.1 wt % of CNTs. The dispersion was treated in an ultrasonification bath for 20 min to reduce the amount of agglomerates before each infiltration. After that, the dispersion was placed in a computer-controlled syringe and slowly dropped (130 μL) on the templates (complete infiltration, no more dispersion could be taken up by the template). Subsequently, the templates were dried for at least 1 h. To increase the amount of CNTs, the process could be repeated several times. In this study, the amount of CNTs varied between 0.4 and 4.0 wt % in the ceramic networks. Further details on the fabrication as well as on mechanical and electrical properties regarding the ZnO-T-CNT composite materials can be found in a previous work.36 The morphological properties of ZnO-T−CNT hybrid networks were investigated using scanning electron microscopy (Zeiss Ultraplus SEM at 7 kV). The detailed characterization of the ZnO-T material was reported in another works.21,35 B


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Figure 1. SEM images of ZnO-tetrapod−CNT networks at different magnifications (from lower to higher, from left to right) with different contents of CNTs: (a−c) 0.0 wt %, (d−f) 0.8 wt %, (g−i) 2.0 wt %, and (j−l) 4.0 wt %. In this work, we mainly focused on practical applications of developed ZnO-T−CNT hybrid materials, namely, on the gas sensing with respect to NH3. The gas-sensing measurements were performed as reported previously by our research group at room temperature (RT ∼26 °C) and two values of relative humidity (RH), namely, 30% RH and 75% RH.24,26,37,38 Gas response (S) was defined as the ratio of electrical current values in tested gas (Igas) and under exposure to air (Iair), that is, S = Igas/Iair. Response and recovery times were defined as the time required to reach and recover 90% of the response signal, respectively. The sensor structures were fabricated accordingly to the procedure described in our previous work for ZnO-T networks, without additional annealing at high temperatures but with annealing at 400 °C for 30 min before each measurement to provide a better connection of the ZnO-T with the CNTs.21,56,57,59 The microsensor structures were fabricated and investigated accordingly to the procedure described in previous work for a single ZnO-T-based sensor by Lupan et al.35,56−58

3. RESULTS AND DISCUSSION 3.1. Morphological Properties. Figure 1 shows the SEM images of ZnO-T−CNT networks at different magnifications (from lower to higher, from left to right) with different contents of CNTs, (a−c) 0.0 wt %, (d−f) 0.8 wt %, (g−i) 2.0 wt %, and (j−l) 4.0 wt %. The diameter of the tetrapod arms is in the range of 0.8−8 μm, whereas the length of the arms varies in the range of 5−60 μm (see Figure 1). The diameter of the CNTs is in the range of 15−50 nm. The amount of CNTs added to the 3D template can be easily adjusted by simply changing the concentration of the used aqueous CNT dispersion or by repeated infiltrations, leading to a gradual increase in CNTs on its surface (see Figure 1f,i,l).36 The inset in Figure 2a shows the ceramic template before and after CNT infiltration. Due to the hydrophilic character and high porosity of the 3D ZnO network, the aqueous dispersion is taken up by the network upon infiltration and forms a homogeneous layer upon drying. At high CNT concentrations, networks consisting C


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Figure 2. (a) Gas response to 100 ppm of NH3 vs the content of CNTs (inset showing the highly porous 3D ZnO network (left) and the CNTinfiltrated network (right)). (b) Gas response of the ZnO-T−CNT-based samples vs the content of CNTs (concentration of gases and vapors: 100 ppm of NH3 vapors, 1000 ppm of ethanol (EtOH) vapor, 10 000 ppm of H2, 10 000 ppm of CH4, 10 000 ppm of CO2, and 1000 ppm of acetone vapor). (c) Dynamic NH3 response (100 ppm) for ZnO-T samples with different contents of CNTs: 0.0, 0.8, 2.0 wt %, and (j−l) 4.0 wt %. (d) Dynamic NH3 response for ZnO-T−CNT2.0 (2.0 wt % CNTs) samples to different concentrations of NH3 vapor.

∼111, which is still higher than that for pristine ZnO-T networks (∼38). Even though ZnO-T−CNT hybrid sensors show a higher response to NH3 vapors, the selectivity is a major challenge for metal oxide-based gas sensors. Therefore, we investigated the selectivity of the hybrid networks by individual gas exposure to other gases (1000 ppm of ethanol (EtOH) vapor, 10 000 ppm of H2, 10 000 ppm of CH4, 10 000 ppm of CO2, and 1000 ppm of acetone vapor). The results are presented in Figure 2b. No considerable response (S > 3) to other tested gases was detected, demonstrating an excellent selectivity to NH3 vapor of the ZnO-T−CNT samples at room temperature. The selectivity of sensors is usually investigated by alternating exposure of various gases on the tested devices and the selectivity factor is calculated as the sensitivity ratio determined for individual gases.39 In this case, it was evaluated as the ratio of NH3 response to other tested gases. The calculated results for devices based on ZnO-T−CNTs with 2.0 wt % CNTs are SNH3/SEtOH ∼ 200, SNH3/SH2 ∼ 290, SNH3/SCH4 ∼ 300, SNH3/SCO2 ∼ 300, and SNH3/Sacetone ∼ 180. Dynamic NH3 responses (100 ppm) at room temperature for ZnO-T and ZnO-T−CNT samples are presented in Figure 2c. Two consecutive pulses with the same concentration were applied to check the repeatability of the sensor. No considerable deviation in the gas response ( 1.2 (i.e., the sensor signal is considered to be true if the gas response is changed with a value higher than 1.2.41), the LDL for NH3 vapor can be extrapolated from the linear curve (from Figure 3a) when the gas response is equal to 1.2. The estimated value is ∼200 ppb. An illustration of the sensor structure used in gas-sensing investigations with respective electrical connections is presented in the inset of Figure 3a. Information on the rapidity of the sensors and other calculated parameters are generalized in Table S1 and Figure S2. A considerable increase in the rapidity E


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Figure 4. Structural illustration of cross-sectional view of the ZnO-T and ZnO-T−CNT networks (two arms touching each other) along its length for the gas-sensing mechanism, with the respective energy band diagram in the inset. (a) Exposure of ZnO-T to air. Oxygen molecules adsorb onto the surface of ZnO-T-arms and form an EDR (blue). The potential barrier qVs1 between the two T-arms is also presented in the inset (down left). (b) Exposure of ZnO-T to NH3 vapor; due to donated electrons, the width of the EDR is decreased, leading to a decrease in the height of the potential barrier (qVs2) between arms and increased conduction channel. (c) Exposure of ZnO-T−CNT2.0 (2.0 wt %) to air. (d) On exposure to NH3 vapor, a higher change in the height of the potential barrier between the ZnO-T-arms can be obtained due to adsorbed NH3 molecules on the CNT leading to an electron transfer to ZnO through the CNT. (e, f) At a higher content of carbon nanomaterial (4.0 wt %), percolating networks of CNTs can be obtained, leading to a current flow (red arrow) at the surface, diminishing the influence of the potential barrier between ZnO-T-arms.

other vapors,43 as well as higher electron-donating ability of ammonia; and (ii) the contribution of NH4+ cations, which can lead to a crucial increase in conductance (see ref 44 for more details). As the gas-sensing mechanism of the metal oxides/carbonbased nanomaterial is still under consideration, we can speculate that the enhancement of the NH3 response and rapidity of the ZnO-T−CNT samples are in our case based on two main principles: first, the high porosity (∼93%) of the networks, which facilitates the adsorption as well as the desorption of gas molecules,21,25 and second, the high conductivity of the CNTs, which enhances the transfer of electrons. As a result, the adsorption/desorption of oxygen molecules and oxidation of NH3 molecules are increased, which in return improve the overall performance of the networks.34,45 In the following, the sensing mechanism is identified and explained using energy band diagrams in the radial direction and structural models of the conduction mechanism through the single ZnO arm.42 At room temperature, mainly adsorbed oxygen molecules (O2−) can be found on the surface of ZnO, leading to the formation of an electron-depleted region (EDR) at the surface of the tetrapod arms (Figure 4a)35,37,38,42

NH3 were performed. The corresponding results are presented in Table S1 and Figure 3c, demonstrating no significant change in the stability to high RH values of the samples after the addition of carbon nanomaterial on ZnO-T networks. However, a decrease in the gas response with an increase in the RH value is observed for all samples (about ≈55−60%). This can be explained by the low stability of the carbon nanomaterial in humid environment.34 Furthermore, a decrease in the rapidity of the samples was also observed (Figure 3d and Table S1) due to the degradation of the sensing properties in humid environment.24 The calculated response and recovery times for both values of RH (30 and 75%) are presented in Table S1. This behavior is also typical for sensors based on metal oxides24,26 and can be explained on the basis of the competitive reactions between NH3 molecules and H2O molecules on the surface of ZnO tetrapods.24,26 After the exposure of ZnO-T and ZnO-T−CNT samples to higher RH values (75%), an increase in the current is observed (not shown here). This is the result of the H2O molecule’s donor effect.24 In short, the initial value of Iair and active sites decreases, leading to a lower gas response.26 3.3. Gas-Sensing Mechanism of Hybrid ZnO-T−CNT Networks. The room temperature ammonia sensing of ZnO can be explained as follows: (i) low ionization energy (10.18 eV) and kinetic diameter (0.36 nm) of ammonia compared to

O2 (g) ↔ O2 (ad) + e− → O−2 (ad) F

(1)


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Figure 5. SEM images of the fabricated microdevices based on a single ZnO-tetrapod-CNT (from samples with 2.0 wt %) with different dimensions: (a) tetrapod arm length (L) = 57 μm and tetrapod arm diameter at the end (D) = 4 μm; (b) L = 12 μm and D = 1.8 μm; (c) L = 22 μm and D = 1.5 μm; (d) L = 20 μm and D = 1 μm; (e) L = 27 μm and D = 0.5 μm. (f) Gas response of the microdevices (1−5) measured at room temperature (concentration of gases and vapors: 100 ppm of NH3 vapors, 10 000 ppm of H2 gas, and 1000 ppm of ethanol vapor).

temperature (which is an electron-donating gas species), that is, NH3 molecules will donate electrons to CNTs (Figure S1a), the electrons are easily transferred to the underlying ZnO-T, thereby decreasing the height of the potential barrier qVs2 more efficiently. In conclusion, a higher modulation of qΔVs was obtained by increasing the CNT content (Figure 4c,d) (from 0.8 to about 2.0 wt %).27,38,50 In the case of a higher CNT content (about 4.0 wt %), a high number of conductive pathways is formed through the 3D network. This enables a current flow between the arms of different tetrapods, thus excluding the influence of the barrier height modulation (see Figure 4e,f, wherein only the situation with percolating path is illustrated showing that the effect of the potential barrier is excluded from gas sensing).51 Figure S3 shows a considerable increase in the electrical current for ZnO-T−CNT4.0 samples, which confirms the formation of a percolating carbon nanomaterial network.23,34,52−54 Also, a higher content of carbon nanomaterial lowers the adsorption sites for oxygen molecules.38,55 Thus, in this case, the gas response is lowered,

Thus, between different tetrapodal arms (T-arms), potential barriers (qVs1) are formed, see Figure 4a. When exposed to NH3 vapor, the ammonia molecules interact with the adsorbed oxygen and release electrons to the ZnO-T by the following reaction46 2NH3(ad) + (3/2)O−2 (ads) ↔ N2 + 3H 2O + 3e−

(2)

As a result, the width of the EDR is reduced, leading to a decrease in the height of the potential barriers (qVs2) between the tetrapod arms and an increase in the current flow through the network (Figure 4b). In this case, the gas response depends mainly on the variation in the potential barrier qΔVs = qVs1 − qVs2.35,56−58 In contrast, by adding CNTs on the surface of ZnO-T, an Ohmic contact is formed at the interface due to the higher work function of n-ZnO (5.1−5.4 eV47) compared to the work function of CNTs (4.7−4.9 eV48), leading to an easy electron transfer process from ZnO to CNTs and vice versa depending on the gaseous environment.49 Considering that the CNTs can efficiently adsorb NH3 molecules at room G


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Figure 6. Dynamic gas responses at room temperature for devices based on a single ZnO-tetrapod-CNT (made from samples with 2.0 wt % CNT): (a) device #2; (b) device #3; (c) device #4; and (d) device #5. (Concentration of gases and vapors: 100 ppm of NH3 vapors, 10 000 ppm of H2, and 1000 ppm of ethanol (EtOH) vapor.)

content, see Figure S4); therefore, the method used for the integration of single ZnO structures27,37,38,56 is also compatible for the hybrid material presented here. The corresponding current−voltage characteristics (in −1 to +1 V bias voltage range) of the different devices are shown in Figure S5, whereas the results of the gas-sensing investigations at room temperature are presented in Figure 5f and Table S2. For all gassensing measurements, the applied bias voltage was 1 V to exclude a self-heating effect at higher voltages. The biggest ZnO-T−CNT, namely, device #1 with L ≈ 57 μm and D ≈ 4 μm, showed poor gas-sensing performances, as no response to 1000 ppm of EtOH, 10 000 ppm of H2, 10 000 ppm of CH4, 10 000 ppm of CO2, and 1000 ppm of acetone was detected. Only a slight variation in current under exposure to 100 ppm of NH3 was observed, showing a gas response of Igas/Iair ≈ 1.02. Therefore, due to the high diameter of the T-arm (D ≈ 4 μm) no considerable gas response could be obtained by integrating an individual large tetrapod in a single device. Thus, it can be concluded that in the case of single structures, the gas response value mainly originates from the size variation of the electron depletion region at the surface of a semiconducting oxide by adsorption/desorption of gaseous species. Therefore, the structure diameter plays a crucial role, which has already been demonstrated experimentally elsewhere.35,56,57,60 In the case of device #2 with L ≈ 12 μm and D ≈ 1.8 μm, a slight improvement in the gas response was observed. The gas response to NH3, H2, and EtOH increased to ∼1.32, 1.06, and 1.02, respectively (see Figure 5f). On the basis of pure geometrical factors and coaxial geometry and excluding the

which is in agreement with our experimental results (Figure 2a). 3.4. Gas-Sensing Properties of Microdevices Based on a Single Tetrapod ZnO-T−CNT. In the following, we describe the fabrication technology of microdevices by using a single ZnO tetrapod hybridized with CNTs (ZnO-T−CNT), which was released from the previous mentioned hybrid networks with 2.0 wt %. The release of single microstructures, followed by the transfer to a SiO2-coated Si (SiO2/Si) substrate and the subsequent separation of a lower density of tetrapods on the surface, was performed as reported by Lupan et al.25,35,42,56−58 A single microtetrapod (micro-T) ZnO-T−CNT was contacted to pre-patterned Au/Cr contacts on a SiO2/Si substrate with a Pt complex by using the maskless deposition functions in a focused ion beam/scanning electron microscopy (SEM) instrument system.56−58 More details on the fabrication of such nanodevices and microdevices are provided by Lupan et al. in previous works.8,25,35,38,42,56,57,59 In this study, five different single ZnO-T−CNT devices were fabricated with different diameters and lengths of the arms, hereafter designated as devices #1, #2, #3, #4, and #5. In Table S2, all of the geometrical parameters of different single ZnOT−CNT devices are listed, namely, length (L) and diameter (D) at the end of the arms. In the following sections, the influence of geometrical parameters on the gas response is investigated. The SEM images of all of the devices are presented in Figure 5a−e. After transferring a single structure of ZnO-T−CNT for further device fabrication, the initial density of CNTs on its surface did not change (no loss of CNT H


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Table 1. NH3 Vapor Sensors Based on Micro- and Nanostructures of Different Metal Oxides, Polymers, and Carbon-Based Materials morphology and properties of sensing material broken In2O3 nanotubes CuO nanocrystalline powder SnO2 NC-functionalized CuO nanowires (NWs) TeO2 NWs polypyrrole-coated SnO2 hollow spheres SnO2/MWCNT composites ZnO−CNT nanosphere heterostructures ZnO-T−CNT networks with 2.0 wt % CNT single ZnO NW graphene nanomesh single polypyrrole NW single polyaniline NW individual SWCNT single ZnO-T−CNT a

NH3 conc. (ppm)

gas response (Rg/Ra)a or (Ra/Rg)b

operating temperature (°C)

20 100 10 000

∼2400 ∼10c ∼5c

RT RT RT

10 min ∼100 sc ∼9 s

20 s 500 s ∼6 s

63 64 51

100 20 200 20 100

∼1.05c ∼3 ∼28c ∼1000 330

RT RT RT RT RT

10 min 9s ∼5 min >10 min 18.4 s

30 min >3 minc ∼5 min 30 s 35 s

65 62 23 45 this work

∼1.1 ∼1.14 ∼1.2c ∼55c ∼100 6.4

RT RT RT RT RT RT

420 s

66 67 68 69 70 this work

100 100 300 100 10 000 100

response time

10 min ∼15 min ∼1−2 min 20 s

recovery time

ref

Gas response for p-type semiconducting oxides. bGas response for n-type semiconducting oxides. cValue approximated from a graphical plot.

increases with decrease in the diameter of the tetrapod arms. For example, devices #2 and #3 showed relatively high response time of about 200 and 110 s, respectively, whereas the recovery time for both devices was higher than 5 min. By decreasing the diameter of the tetrapod arm at the end to about 0.35 μm (device set #5), the rapidity was essentially improved by decreasing the response and recovery times to beyond 20 and 420 s, respectively (see Table S2). Anyway, environmental monitoring of ambient NH3 levels does not require extremely fast sensors.1 Thus, the devices presented here are of acceptable rapidity for such important applications. Table 1 summarizes the most relevant gas-sensing results of NH3 sensors, which are based on individual micro- and nanostructures together with a comparison of our results. All sensors demonstrated good and promising results at room temperature. Even though sensors with a higher gas response have been reported, our sensors can detect NH3 vapors more rapidly, which is a major criterion for real applications.

influence of junctions at the tetrapod core, the gas response of the devices presented here can be estimated as follows61 Igas Iair

≈

⎛ ⎞2 D ⎜ ⎟ ⎝ D − 2ΔW ⎠

(3)

where D is the diameter of the tetrapod arm and ΔW the change in the EDR on the surface of the ZnO-T−CNT. Thus, the gas-sensing mechanism of single tetrapod structures is mainly based on the modulation of conduction channels by surface reactions. Equation 3 reveals that a lower diameter of the structures leads to a higher variation in the current under exposure to gaseous species, as observed for the structures presented here. By further decreasing the diameter of the Tarm to D ≈ 1.5, 1.0, and 0.35 μm, the NH3 gas response is increased to 2.1, 2.65, and 6.4, respectively (see Figure 5f). The gas response to H2 and EtOH also increases slightly. A gas response to other gases (CH4, CO2, and acetone) was not detected with such structures. Even though the pristine single ZnO-T (as presented in a previous work) showed a higher selectivity to hydrogen gas,35 in our case, the higher selectivity to NH3 can be explained on the basis of the presence of CNTs on the surface of the ZnO-T, which are known to be excellent NH3 sensing materials having a low hydrogen response at room temperature (demonstrated experimentally, see Figure S1).8,53,62 Also, as discussed above, CNTs effectively adsorb NH3 molecules at room temperature and transfer donated electrons to the underlying ZnO-T, leading to a widening of the conduction channel.34,45,49 Thus, by the addition of carbonbased nanomaterials (namely CNTs in our case) on the surface of ZnO, the improvement in the room temperature sensing properties can be effectively enhanced for microstructure devices. The dynamic gas response at room temperature of the fabricated devices (devices #2, #3, #4, and #5) are presented in Figure 6. The dynamic response for device #1 is not presented due to its low gas-sensing performances and was excluded from further experiments. The calculated response and recovery times are presented in Table S2. All of the devices showed slow recovery, especially in the case of devices based on tetrapods with thicker arms. However, it can be seen that the rapidity

4. CONCLUSIONS Ultrasensitive and rapid NH3 vapor sensors at room temperature based on individual and networked ZnO tetrapods functionalized with CNTs were fabricated. The optimal CNT concentration in the highly porous ceramic ZnO-T networks was found to be 2.0 wt %, showing a high gas response of Igas/ Iair ≈ 330 at room temperature to 100 ppm of NH3. Besides, an increase in the response time and a considerable increase in the rapidity were induced. Thus, the calculated time constants of the response and recovery curves for 3D networks are τr ≈ 18.4 s and τd ≈ 35 s, respectively, demonstrating a rapid sensing performance of NH3 vapor at room temperature. ZnO-T− CNTs also demonstrated excellent selectivity, being promising materials for selective NH3 gas sensors, which is most important for environmental monitoring wherein different gases are present. Due to room temperature operation of the developed ZnO-T−CNT sensing material, a long-time stability was obtained, which is very important for the lifetime of gas sensors in various applications. The high sensing performances were explained based on the porosity of the networks, which facilitate adsorption/desorption of gas molecules, as well as on the high conductivity of CNTs, which can enhance the transfer I



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of the electrons as the result of adsorption/desorption processes of oxygen molecules and oxidation of NH3 molecules. Due to the open porous structure and the large pore diameter (several micrometers) of the template, an easy access of the gaseous species to a high surface is ensured. In the case of a single hybrid ZnO-T−CNT with an arm diameter of about 0.35 μm, the NH3 response was found to be Igas/Iair ≈ 6.4, with a response time beyond 20 s. NH3 sensors based on individual ZnO-T−CNT hybrid tetrapod microdevices (from samples with 2.0 wt % CNTs) were developed in premiere. We consider that the presented results are of high scientific interest for the readers and the community and can be an essential step in the field of new hybrid materials with high performances for practical applications in automotive, environmental monitoring, chemical industry, and medical diagnostics.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03702. Dynamic gas response of CNT networks at room temperature and calculated response and recovery times versus content of carbon in ZnO-T−CNT samples; current−voltage characteristics of the sensor structures based on ZnO-T and ZnO-T−CNT networked samples and of the microdevices based on a single ZnO-T−CNT; SEM images of the ZnO-T−CNT tetrapod arm integrated in devices labeled as #2, #3, #4, and #5; two tables with calculated parameters of the ZnO-T- and ZnO-T−CNT-based sensors at room temperature and different RH values, as well as geometrical parameters and gas-sensing parameters of single ZnO tetrapods hybridized with CNTs (from samples with 2.0 wt %) used for fabrications of devices (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.A.). *E-mail: [email protected], [email protected] (O.L.). ORCID

Oleg Lupan: 0000-0002-7913-9712 Notes

The authors declare no competing financial interest.

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REFERENCES

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S Supporting Information *

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Research Article

ACKNOWLEDGMENTS

Dr. Lupan acknowledges the Alexander von Humboldt Foundation for the research fellowship for experienced researchers number 3-3MOL/1148833 STP at the Institute for Materials Science, Kiel University, Germany. The authors acknowledge the support from German Research Foundation (Deutsche Forschungsgemeinschaft (DFG)) under SFB 1261 A5. Prof. Adelung gratefully acknowledges partial project funding by the DFG contract AD183-17/1 and support from the EU in the framework of the Graphene Flagship. This research was in part supported by the Project Institutional (Project No. 45inst-15.817.02.29A) funded by the Government of the Republic of Moldova and the STCU within the Grant 6229. J


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