Enhanced Ammonia Adsorption on Directly Deposited Nanofibrous Carbon Films

Research Article Enhanced Ammonia Adsorption on Directly Deposited Nanofibrous Carbon Films Alexander G. Bannov , 1,2 Ondřej Jašek, 3 Jan Prášek , 4,5 Jiří Buršík , 6 and Lenka Zajíčková 1,3 1 Central European Institute of Technology, Masaryk University, Kamenice 5, CZ-62500 Brno, Czech Republic 2 Department of Chemistry and Chemical Technology, K. Marx 20, Novosibirsk State Technical University, 630073 Novosibirsk, Russia 3 Department of Physical Electronics, Faculty of Science, Masaryk University, Kotlářská 2, CZ-61137 Brno, Czech Republic 4 Central European Institute of Technology, Brno University of Technology, Technická 3058/10, CZ-61600 Brno, Czech Republic 5 Centre of Sensors, Information and Communication Systems, Faculty of Electrical Engineering and Communication, Technická 3058/10, CZ-61600 Brno, Czech Republic 6 Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Žižkova 22, CZ-61662 Brno, Czech Republic Correspondence should be addressed to Alexander G. Bannov; [email protected] Received 12 April 2018; Revised 9 July 2018; Accepted 29 July 2018; Published 17 September 2018 Academic Editor: Michele Penza Copyright © 2018 Alexander G. Bannov et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The ammonia adsorption on the nanostructured carbon thin film was significantly influenced by the choice of deposition temperature and deposition time of thin film synthesis. The thin films were prepared on Si/SiO 2 substrates by chemical vapour deposition in Ar/C 2 H 2 gas mixture using iron catalytic nanoparticles. The analysis of the grown layer by the scanning and transmission electron microscopy showed the transition from long multiwalled nanotubes (MWCNTs) to bamboo-like hollow carbon nanofiber structure with the decrease of the deposition temperature from 700 to 600 ° C. Further, the material was analyzed by energy-dispersive X-ray spectroscopy and Raman spectroscopy confirmed the transition from graphitic sp 2 structure to highly defective structure at lower deposition temperature. The resistance of the prepared layer strongly depends on deposition temperature (Td) and deposition time (td). High resistance layer, 38.6 kΩ, was formed at Td600 ° C and td10 min, while at Td700 ° C and td60 min, the resistance decreased to 860 ohms. Such behaviour is consistent with MWCNTs being responsible for the formation of the conductive network. Such system was studied using chemiresistor ammonia gas sensor configuration. The sensor resistance increased when exposed to ammonia in all the cases, but their response varied considerably. A decrease in deposition time, from 60 to 10 min, and the deposition temperature, from 700 to 600 ° C, led to the 10-fold increase in the sensor response. The measurements carried out at room temperature showed the higher sensor response than the measurements carried out at 200 ° C. This behaviour can be explained by the change in adsorption-desorption equilibrium at different temperatures. Analysis of dependence of the sensor response on the ammonia concentration proved that the underlying resistance change mechanism is chemisorption of ammonia molecules on the carbon network corresponding to the Langmuir isotherm. 1. Introduction A safety and sustainable development require creation of new highly sensitive devices for the detection of toxic and flammable gases along with the control of environmental pollution. Such devices require new materials which extend the number of detectable gases and lower their detection limits. One of the most dangerous gases that have a negative influence on human health and environment is ammonia (NH 3 ). According to US OHSA (Occupational Safety and Health Administration) and CDC (Centers for Disease Control and Prevention) regulations, the long-term exposure limits are 25–35 ppm for Hindawi Journal of Sensors Volume 2018, Article ID 7497619, 14 pages https://doi.org/10.1155/2018/7497619 workers. The smell threshold of ammonia ranges from 5 to 15 ppm. The concentrations above 50 ppm induce irritation to the mouth, nose, wheezing, etc. The concentrations from 300 to 500 ppm are dangerous for life. In industry, ammonia can be detected at higher concentrations (from 1000 ppm to 40000 ppm, depending on rooms, facilities, etc.). Therefore, study of ammonia adsorption on nanostructured surfaces and development of new materials for ammonia gas sensors attracted a lot of attention of scientific community. Single-wall carbon nanotubes (SWCNTs) and multiwall carbon nanotubes (MWCNTs) belong to one of the most promising candidates in the gas sensing field [1–9] because of their ability to change the electrical properties (e.g., resistivity) under gas adsorption. This fact is complemented by the enhanced adsorption of gases by carbon nanomaterials [8]. There are a wide range of gases on which the carbon nanotube- (CNT-) based sensors can be used, such as NH 3 [4, 10–16], NO 2 [17, 18], CH 4 [19, 20], H 2 [21], H 2 S[22–25], CO 2 [26], ethanol [25], methanol [25], hydrocarbons [7], and other gases. Commercially available sensors require a high temperature for their operation. For wider use of such devices, the room temperature operation of ammonia gas sensors is an important requirement and was recently studied by several authors. In [27], the authors created HCl-doped MWCNT/ polyaniline composite sensors with good sensing response and high reproducibility. In [28], the authors created SWCNT- (40% metallic and 60% semiconducting) based ammonia sensors using inkjet-printed electrodes which possessed the maximal response of 27.3% for 500 ppm at room temperature. Cui et al. [29] developed the room temperature ammonia sensor based on Ag nanocrystal-functionalized MWCNTs that exhibited enhanced response of 9% and fast response at room temperature with the full recovery within several minutes in air. In [12], the authors created the room temperature sensor based on SWCNT for ammonia sensing with the extremely low detection limit (3 ppb). The enhancement of the response was realized by two techniques: drop casting and sonication. In this paper, a promising potential of managing the NH 3 gas sensor performance by the control of nanofibrous carbon (NFC) chemical vapour deposition (CVD) synthesis conditions is investigated in detail. Up to now, the CNT-based NH 3 gas sensors were closely investigated from the performance point of view but the influence of the synthesis conditions of these materials on the sensing properties has not yet been studied. Previous studies were based on the creation of the sensors by the direct deposition of CNTs using only certain conditions [30–33] without detailed investigation of their synthesis role in the formation of sensor response. The influence of NFC deposition parameters (growth temperature and time) on the materials’properties and NH 3 sensing characteristics is determined. The interconnection between the sensor resistance and response has been found. 2. Materials and Methods 2.1. Nanofibrous Carbon Film Preparation. NFC layers were grown by atmospheric pressure chemical vapour deposition using an iron catalyst. Polished single-crystal Si (c-Si) pieces (8 mm ×8 mm) coated by a thermal SiO 2 film, 92 nm in thickness, were used as substrates. Nanoparticles (NPs) of iron catalyst were deposited by a microwave (MW) plasma torch from the iron pentacarbonyl, Fe(CO) 5 , vapours mixed with argon. The experimental set-up is described by Synek et al. [34, 35] in detail. The c-Si/SiO 2 substrates were put in a special holder for 4 samples. The flow rate of argon through the central part of the nozzle was 700 sccm. The outer concentric part of the nozzle was used to deliver Fe(CO) 5 vapours (0.1 sccm) carried by Ar flow of 28 sccm. The MW torch was ignited with the power of 210 W. The deposition time of the nanoparticles was 15 s. Direct deposition of nanoparticles was used as preferred form of the catalyst because it enabled us to form a sparse network of fibrous carbon. In case of the thin film catalyst, dense structure of the nanofibrous carbon network with low resistance and negligible response was formed. NFC growth was carried out in a quartz tubular furnace. The processes started by heating the furnace from the room temperature to deposition temperature (Td): 600 ° C, 650 ° C, and 700 ° C, under the argon flow of 1400 sccm. The heating speed was 25 ° C/min. Then, the catalytic nanoparticles were reduced in Ar/H 2 flow (1400 sccm of Ar and 500 sccm of H 2 ) for 10 min. After reduction, the hydrogen flow was switched offand NFC growth was carried out in Ar/C 2 H 2 mixture with the flow rates of 1400 and 25 sccm, respectively. The growth lasted for 10 min, 40 min, and 60 min as summarized in Table 1. The reactor was cooled down to the room temperature under argon flow. The samples were taken out and placed in a vacuum evaporator for the deposition of 6.65 mm ×2.33 mm gold contacting pads overlapping by 1 mm with the field of NFC (Figure 1). Thickness of the Au layer was 350 nm, and a 15 nm thick Ni/Cr layer was used to improve Au adhesion to the substrate. Such prepared substrates were used as sensors for measurement of resistance change under various gas atmospheres. 2.2. Investigation Methods of NFC Films. The as-prepared catalytic NPs and NFCs on c-Si/SiO 2 substrates were investigated by scanning electron microscopy (SEM) with MIRA II Table 1: Summary of conditions used for the growth of NFC by CVD. Sample Reduction and deposition temperature (Td), ° C Reduction time, min Deposition time (td), min CNF600-10 600 10 10 CNF600-40 600 10 40 CNF600-60 600 10 60 CNF650-10 650 10 10 CNF650-40 650 10 40 CNF650-60 650 10 60 CNF700-10 700 10 10 CNF700-40 700 10 40 CNF700-60 700 10 60 2 Journal of Sensors (TESCAN, Brno, Czech Republic) equipped with the EDX detector (Oxford Instruments, UK). Raman spectra of NFCs were obtained using the Renishaw inVia (Renishaw, Gloucestershire, UK) spectrometer in the range 100–3200 cm −1 (λ= 514 nm). Raman spectra were treated using Lorentzian fitting. Structure and morphology of carbon nanomaterials and catalytic nanoparticles were additionally investigated using the CM12 STEM transmission electron microscope (Philips, Eindhoven, Netherlands). Gas sensing characteristics of the sensors were determined by measuring the changes of sensor resistance during ammonia exposure in a custom-built system equipped with two gas channels and a measurement chamber (Figure 1). Synthetic air (80% N 2 , 20% O 2 , Linde, Brno, Czech Republic) was used as a gas carrier in one gas channel. Along with it, the second gas line was flowed by ammonia diluted in nitrogen (calibration gas 5000 ppm of NH 3 in N 2 , Linde, Brno, Czech Republic). Total volume flow rate of gases was set constant at 500 sccm for all stabilizations and measurements, but the concentration of NH 3 in the mixture with synthetic air and N 2 was changed. The additional channel for oxygen was used in the setup, and oxygen was admixed when increasing ammonia concentration in order to keep the concentration of air and nitrogen mixture the same as in synthetic air. Gas distribution system was made of stainless steel (connections, tubes). The volume of the chamber where the sensors were examined was 160 cm 3 (length: 10 cm, width: 8 cm, height: 2 cm). The sensor was placed on a heater inside the measurement chamber, and the measurements were carried out at two temperatures: room temperature (25 ±2 ° C) and 200 ° C (±2 ° C). The heating temperature was controlled by the DC power supply Agilent U3606A (Agilent, Santa Clara, California, USA) using temperature calibration curves. The resistance was measured by two electrodes that were placed to Au pads. Electrodes were plugged to a high voltage source meter Keithley 2410 (Keithley, Cleveland, OH, USA) using 1 V bias voltage. In the present work, no efforts were made to dope or treat NFCs for response enhancement. Solely, the correlations between the NFC deposition conditions, influencing the structure and composition of the deposit, and the gas sensor properties were investigated. Relative humidity (RH) in the chamber was controlled by the SHT25 sensor (Sensirion, Staefa, Switzerland). Also, the measurements of humidity influence on sensing properties were carried out. The level of RH during measuring the response in dry gases from cylinders was 2.5–3%. The measurements in different RH conditions were carried out by feeding the wet air to the chamber additionally with dry air and analyte. Before each response measurement, the sensor was conditioned and recovered for 30 min at 200 ° C in the synthetic air flow of 1000sccm. The baseline of the sensor resistance was stabilized at desired temperature in 500 sccm synthetic air flow for 60 min before the first measurement of each sample. After the baseline stabilization, the sensor response to NH 3 was measured using alternating cycles, 10 min in synthetic air and 10 min in the mixture of the NH 3 with synthetic air. This cycle was repeated for several tested concentrations, 100 ppm, 250 ppm, and 500 ppm of NH 3 . Air Active material NiCr/ Au contacts Valve Flow controller Measuring cell Pressure controller Exhaust Si/SiO2 substrate NH3 Figure 1: The sensor layout and experimental set-up for measurement of sensor response to ammonia. 3Journal of Sensors The baseline resistance, R0, was linearly extrapolated along the measurement time, and the sensor response was determined by subtracting the baseline resistance signal from the sample resistance under ammonia exposure, R. The measurement of baseline and extrapolation has been done for each measurement of all samples. The sensor response was defined as follows: ΔR R0 =R−R0 R0 ⋅100% 1 The instrumental detection limit of the sensor response was 10 ppm, the value was limited by the flow controller characteristics (low accuracy of flow rate control at the value up to 1 sccm) for the sensor measurement. To estimate the sensor selectivity, they were also tested for detection of H 2 and iC 4 H 10 . These gases were also fed diluted in N 2 (5000 ppm of NH 3 ). The resolution of scanning of sensor resistance was 0.5 s (one experimental point per 0.5 s). 3. Results and Discussion 3.1. Characterization of Catalytic Nanoparticles and Carbon Nanostructures. The typical SEM and TEM micrographs of catalytic nanoparticles are shown in Figure 2. The catalyst formed aggregates consisting of nanoparticles with the size of 5–25 nm. It was difficult to determine the phase composition of the catalyst by X-ray diffraction or Raman spectroscopy because of the low amount of nanoparticles on the substrate. Therefore, the phase composition was studied by electron diffraction in TEM. The analyses revealed that the catalyst consisted of iron oxides, predominantly maghemite (γ-Fe 2 O 3 ). Maghemite (γ-Fe 2 O 3 ) phase was detected (Figure S1 Supplementary Materials) by selected area electron diffraction (SAED) by TEM. It is in agreement with the data obtained by Synek et al. [34]. The SEM and TEM micrographs of the NFCs grown for 10 min at 600 ° C (Figure 3) show both the typical hollow carbon structures observed also in other samples, long MWCNTs (2–5μm) with walls consisting of graphene layers parallel to the tube axis, and strongly curved short bamboolike hollow structures. Besides, the samples contained also carbon-encapsulated Fe and Fe 3 C nanoparticles. The diameter of MWCNTs, 10–30 nm, suggests that the growth was initiated from small, 5–25 nm, catalytic NPs. The bamboo-like structures, 50–120 nm in diameter, are expected to grow from larger NPs (>45 nm in diameter). The difference in their growth mechanism, as compared to MWCNTs, induces a strong curvature with chain-like morphology that has some similarities to CNFs [36]. Initially, a higher amount of small NPs, a lower activity of large NPs, and limited growth time were the reasons for much higher amount of MWCNTs than bamboo-like CNFs on the substrates (Figure 3(a)) [37]. SEM micrographs of the NFC samples prepared at different temperatures and times are presented in Figure S2 (Supplementary Materials). At 600 ° C, the increased deposition times, 40 and 60 min, led to the growth of longer nanotubes and a formation of more dense nanotube aggregates compared to 10 min. According to EDX analysis of the CNF600-10 sample, the C/Fe weight ratio was 26.5 and it increased to 85 and 107 with increasing deposition time for CNF600-40 and CNF600-60 samples, respectively. At higher temperatures (650, 700 ° C), the yield of CNT increased and a denser MWCNT network was formed. Raman spectroscopy of all the NFC samples was carried out for assessing the overall structure of the deposits. Raman spectra for the CNF600-10, CNF650-10, and CNF700-10 samples are shown in Figure 4(a). The spectra were dominated by two peaks corresponding to disordered Dand graphitic G bands [38]. The positions of the Dand Gpeaks ranged from 1349 to 1352 cm −1 and from 1581 to 1594 cm −1 , respectively (Table 2). The second-order peaks were presented above 2500 cm −1 . The peaks of silicon at 520 cm −1 and 960 cm −1 were observed in case of sparsely coated substrates (low growth of temperature and time). The ratio of Dand Gpeak intensities, ID/I G , provided information about material’s disorder (a) (b) Figure 2: SEM (a) and TEM (b) micrographs of catalytic nanoparticles synthesized by the MW torch. 4 Journal of Sensors degree [38]. The dependencies of the I D /I G on the NFC growth time are shown in Figure 4(b). The disorder degree increases with increasing growth time. It can be attributed to the loss of nanoparticle catalytic activity. Less defective samples, synthesized for only 10 min at 600, 650, and 700 ° C, had I D /I G = 0.88, 0.7, and 0.44, respectively. It reveals that higher temperature had a positive effect on the NFC graphitization degree, but the role of the temperature in the suppression of defects is lower for a longer growth time. 3.2. Ammonia Gas Response Measurements of NFC Films. The sensor resistances varied in a wide range from 0.87 kΩ to 38.60 kΩ(Table 3). The highest resistance for each deposition time was obtained at 600 ° C. The resistance dropped from 38.60 to 2.72 kΩ(for 10 min deposition time) when the deposition temperature increased from 600 to 700 ° C. The resistance variations can be explained with the help of high resolution SEM images (Figure 5) that divided the sensors into three groups. The first group is represented by only one sample CNF600-10 which possesses the highest resistance. The sensing material is composed of short carbon nanofibers connected with each other by rarely dispersed MWCNTs. The second group consists of CNF650-10, CNF600-40, and CNF600-60 samples. The length of the CNFs and CNTs increases, and it creates additional connections and forms the network with a lower sensor resistance (R≈6–9kΩ). The third group is represented by CNF650-60, CNF700-10, CNF700-40, and CNF700-60 samples. Increasing the amount of MWCNTs enhances the formation of the conductive network between nanotubes whereas the role of carbon nanofibers becomes negligible (sensor resistance R<3 kΩ). The conductive network formed during longer deposition time and with the increase of the deposition temperature, and it has a certain similarity with the formation of percolating networks. The sample in the first group can be understood as a quasi-insulating state of the network (in terms of percolation theory); the second group of samples is situated in the transition region, and the third group represents a conductive network in which resistance is weakly influenced by the further increase in deposition time and temperature. The changes of the sensor resistance could be partially explained also by a changed defectiveness of carbon structures. Indeed, a higher nanotube graphitization degree was proved by Raman spectroscopy, i.e., lower ID/I G ratio, (a) (b) (c) (d) Figure 3: SEM (a) and TEM (b, c, d) micrographs of the CNT600-10 sample. 5Journal of Sensors when the temperature during the growth was increased from 600 to 650 and 700 ° C (Figure 4(b)). However, the sensor resistances did not increase with increasing growth time although ID/I G was higher. The increased time resulted in the deposition of higher amount of interconnecting long MWCNTs, and it had much stronger influence on the sensor resistance that dropped significantly for 40 min of the growth, especially for 600 ° C. It is worth noting that according to chemical equipment and plasma equipment in industry, it is possible to carry out growing of CNTs on large wafers. Moreover, a CVD process is a very simple way for CNT growth, and it is appropriate for practical application. The processes with a floating catalyst for the deposition of iron nanoparticles are also successively used for plasma-enhanced CVD of nanoparticles for subsequent growth of CNTs. The possibility to control the growth time and deposition temperature is an advantage of the process used in this paper, because it is possible to obtain the defined resistance of the sensing layer that makes the process flexible and favorable for industry. 3.3. Ammonia Adsorption on Carbon Nanostructured Films. All the sensors exhibited increased resistance upon ammonia exposure. The increase of the NFC sensor resistance during an ammonia adsorption has been explained on the basis of the hole depletion in p-type MWCNTs [39, 40]. Adsorbed ammonia molecules donate electrons to CNTs inducing a decrease of charge carriers (holes). The same effect was observed for p-type SWCNTs [40]. The resistance of CNF600-10, 38.60 kΩ, was quite high for the MWCNTbased sensors [40, 41], but this sample exhibited the highest response, 2.8% to 500 ppm at room temperature (Figure 6). In addition, the sample CNF600-40 exhibited the response comparable to CNF600-10, e.g., 2.5% at 500 ppm (room temperature). Comparing results of Raman spectroscopy 0 5000 10000 15000 CNT600-10 CNT650-10 Intensity, arb. un. CNT700-10 500 1000 1500 2000 2500 3000 Raman shift, cm−1 (a) 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 I(D)/I(G) 600 ºC 650 ºC 700 ºC 10 20 30 40 50 60 Time, min (b) Figure 4: (a) Raman spectra of CNF600-10, CNF650-10, and CNF700-10 samples. (b) Intensity ratio I D /I G vs. NFC growth time. Table 2: Raman spectroscopy data of the samples. Sample Dpeak position, cm −1 Dpeak FWHM, cm −1 Gpeak position, cm −1 Gpeak FWHM, cm −1 I D /I G CNF600-10 1352 102 1590 70 0.88 CNF600-40 1350 150 1593 73 0.92 CNF600-60 1350 125 1594 71 1.03 CNF650-10 1350 80 1586 55 0.70 CNF650-40 1349 84 1587 64 0.78 CNF650-60 1351 158 1592 72 0.92 CNF700-10 1349 71 1581 44 0.44 CNF700-40 1352 79 1589 62 0.76 CNF700-60 1350 82 1586 66 0.84 6 Journal of Sensors (Figure 4(b)), SEM images (Figure 5), and sensor resistance (Table 3), it can be concluded that the sensor response is mostly linked to the microstructure and high sensor resistance. The high resistance sensors possessed low charge carrier concentration, and thus, they were more sensitive to any change of NH 3 adsorption. Mishra et al. [32] supposed Table 3: Summary of sensor properties. Sensor resistances at room temperature and 200 ° C are denoted RRT and R200°C, respectively. The sensor responses, S 100 ppm ,S 250 ppm , and S 500 ppm to 100, 250, and 500 ppm of ammonia, respectively, at room temperature (RT) and 200 ° C were calculated as ΔR/R0. Sample RRT,ΩR200°C,ΩS 100 ppm ,% S 250 ppm ,% S 500 ppm ,% RT 200 ° C RT 200 ° C RT 200 ° C CNF600-10 38600 24100 1.8 1.1 2.4 1.4 2.8 2.1 CNF600-40 8940 2670 1.8 0.7 2.3 1.4 2.5 1.9 CNF600-60 8610 2580 0.6 1.1 0.8 n/a 1 1.1 n/a 1 CNF650-10 6860 4380 1.2 0.5 1.7 0.7 2.0 1.1 CNF650-40 2660 800 0.4 0.1 0.7 0.2 1.0 0.2 CNF650-60 1680 670 0.6 0.1 0.8 0.2 0.9 0.3 CNF700-10 2720 1340 0.4 0.4 0.6 0.7 0.7 1.1 CNF700-40 1420 530 0.3 0.2 0.4 0.4 0.5 0.5 CNF700-60 870 440 0.1 n/a 1 0.2 n/a 1 0.2 n/a 1 1 The sensor response was comparable with noise. (a) (b) (c) (d) Figure 5: SEM images of NFCs: (a) CNF600-10 (sample with the highest resistance), (b) CNF600-40 and (c) CNF650-10 (samples from the group with medium resistance), (d) CNF700-10 (sample from the group with the lowest resistance). 7Journal of Sensors that the increase of the disorder degree improved the sensor response because a higher defect density increased the number of active sites in which ammonia can be adsorbed. The ID/I G of the five sensors from the third group (the resistance below 3 kΩ) was quite different, 0.44–0.92, but their response did not prove to be dependent on it. From structural studies and ammonia adsorption characterization, one can conclude that not only properties of individual components of thin film (CNFs and CNTs) but also the amount and structural arrangement of nanostructures are important for thin film response towards ammonia. The above discussed results suggest that the response of the sensors can be increased by the decrease of the deposition temperature below 600 ° C. However, the preliminary experiments carried out for the NFC active layers synthesized at 500 ° C and 550 ° C, showed that these sensors possessed extremely high resistance and were almost without response to NH 3 . So much reduced temperatures had a negative effect on the reduction of catalytic NPs prior to the CNT and CNF growth. It resulted in a very low yield of NFCs, and iron oxide and Fe 3 C NPs were covered by carbon. It is worth noting that no iron was detected on the surface of our samples by XPS analysis even for the deposition temperature of 600 ° C. In this connection, the iron nanoparticles have no direct contact with ammonia and have no influence on the response from the point of adsorption. The catalytic nanoparticles can change the conductivity of carbon nanomaterials but only for carbon nanomaterials synthesized using low deposition time, e.g., 10 min, or low temperature (600 ° C). Their presence in the active material increases the resistance of the active layer but only for limited samples, and we supposed that it has a little effect on the sensor response, because conductivity in such materials is mainly determined by the number of contacts formed between the nanotube and nanofiber network (this effect can be treated as percolation). The response of the sensors at 100 ppm, 250 ppm, and 500 ppm NH 3 was in the ranges of 0.1–1.8%, 0.2–2.4%, and 0.2–2.8%, respectively. In comparison with previously published results, such as Cui et al. [29] obtained the response of 2.8% for bare MWCNTs under exposure to 1% of NH 3 (i.e., 10000 ppm). Hoa et al. [42] achieved approximately 8% response to 6% of NH 3 (60000 ppm) using CNTs on anodized alumina template; our NFC thin films exhibited the 2.8% response under much lower exposure of 0.05% (500 ppm) of NH 3 . This is comparable to a more expensive SWCNT-/cellulose-based sensor [16] or chemically modified CNTs reported by Randeniya et al. [4], which showed 1–2% resistance growth of an acidtreated CNT yarn sensor under 550 ppm of ammonia. The sensors discussed in this work were not surface treated, and it is expected that some surface treatment will improve further their sensing properties. We recently reported that plasma treatment of PECVD-grown CNTs could enhance their response [43]. The adsorption on ammonia on the surface can be divided into two categories: physisorption and chemisorption. Bannov et al. [43] discussed that the response curve is a combination of both categories; the first steep part is related to physisorption of ammonia on the structure surface and later, an asymptotic part to chemisorption. In case of the recovery process, the response can quickly recover by the 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Room temperature 500 ppm 250 ppm ΔR/R0, % Time, min 100 ppm 200 ºC 0 102030405060 (a) 600 620 640 660 680 700 1 2 3 Deposition temperature, ºC 100 ppm (room temperature) 100 ppm (200 ºC) 250 ppm (room temperature) 250 ppm (200 ºC) 500 ppm (room temperature) 500 ppm (200 ºC) ΔR/R0, % (b) Figure 6: (a) Response curves of the CNF600-10 sample to ammonia at room temperature and 200 ° C. Ammonia exposure period and its concentration are depicted in the graph. (b) CNF600-10 sensor response at the room temperature (solid symbols) and 200 ° C (open symbols) as function of the NFC deposition temperature for the deposition time of 10 min. 8 Journal of Sensors desorption of physisorped molecules but the recovery of chemisorbed molecules is slow or irrecoverable. For our films, we propose that the main response mechanism is physisorption and can be modelled by Langmuir isotherm. It is worth noting that it is not possible to separate chemisorption and physisorption. Nevertheless, we cannot exclude the partial chemisorption, since there is an incomplete recovery of sensor response without using heating. The use of thermal recovery makes it possible to fully carry out the desorption of ammonia molecules. The Langmuir isotherm is usually used for the description of NH 3 adsorption on carbon materials taking into account few assumptions: each site holds each molecule, all sites are equivalent, and there is no interaction between NH 3 molecules on sites [44]. The adsorption equilibrium constant can be estimated by the following equation: θ=θ∞ K⋅p 1+ K⋅p, 2 where θis the fraction of active sites covered by NH 3 (with the assumption that the value is proportional to the change of resistance), θ∞is the total number of active sites, Kis the adsorption equilibrium constant, and pis the partial pressure of NH 3 . Results of the sample fitting by adsorption isotherm are shown in Figure 7(b). The fitting was presented taking into account that the concentration of active sites is proportional to the sensor resistance. According to the fitting, the adsorption constant of K=005 Pa −1 was different compared with the reported one for graphene-like materials [45] (K=016 Pa −1 ) and the graphene-based NH 3 sensor reported in [46]. The sensors studied can be used as industrial sensors for the determination of ammonia concentration in chemical engineering (apparatuses), oil refining, etc. The data on fitting the experimental data using Langmuir isotherm makes it possible to find the response for low NH 3 concentrations that makes it possible to predict the sensor response for 14 ppm (according to OSHA regulations) and below that is appropriate for environmental control sensors. It is worth noting that there is a necessity to control also the higher concentrations (1000–20000 ppm) for turning on the emergency ventilation in compressor rooms at the facilities used anhydrous ammonia and the sensors studied can be also used for this purpose. 3.4. Influence of the Operating Temperature on NFC Film Response and Its Recovery. NFC-based sensors possessed good response at room temperature that is an advantage as compared with semiconductor-based conventional sensors [47]. The sensors working at room temperature were also prepared using graphene-based materials. For example, Katkov et al. [45] created a fluorine-functionalized graphene sensor and reached 10.2% response to 10000 ppm of ammonia. The response 3–4% for 0.1% (1000 ppm) NH 3 in Ar was obtained on reduced graphene oxide-silver nanowires [48]. The sensor responses at room temperature and 200 ° C were quite different. The dependence of ΔR/R0on the concentration had almost linear behaviour at 200 ° C, whereas it became nonlinear at room temperature (Figure 6(b)). The maximum response at 200 ° C was 2.5% in comparison with 2.8% at room temperature. The enhanced sensor response at the room temperature can be explained by the thermodynamics of adsorption. The adsorption is an exothermic process, and therefore, an increase of temperature enhances the desorption and higher temperatures (compared to room temperature) are favourable for this process. Additionally, it can be caused by weaker desorption at room temperature that lead to accumulation of small amount of ammonia on NFC surface. For example, the recovery percentage (measured after 10 min recovery in synthetic air) of the sensor CNF600-10 for 100 ppm at room temperature is only 23% in comparison with 62% at 200 ° C. The full adsorption-desorption cycle for wide range of concentrations of NH 3 (50–500 ppm) can be seen in Figure 7(a). We can see that although the recovery at 200 ° C was significantly better than at room temperature, we can observe at higher concentration saturation of the NFC thin film response an increase of background resistance. 0 2 4 6 500 ppm 375 ppm 300 ppm 250 ppm 175 ppm 100 ppm ΔR/R0, % Time, min 50 ppm 0 20 40 60 80 100 120 140 160 (a) 0 1020304050 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 Partial pressure, Pa ΔR/R0 = 3.54⁎0.05⁎p/(1+0.05⁎p) ΔR/R0, % (b) Figure 7: CNF600-10 response curve to NH 3 in a wide concentration range at 200 ° C and data fitting by Langmuir adsorption isotherm (R2=0984). 9Journal of Sensors