Nitrogen Dioxide Sensing Using Multilayer Structure of Reduced Graphene Oxide and α-Fe2O3

Multilayers consisting of graphene oxide (GO) and α-Fe2O3 thin layers were deposited on the ceramic substrates by the spray LbL (layer by layer) coating technique. Graphene oxide was prepared from graphite using the modified Hummers method. Obtained GO flakes reached up to 6 nanometers in thickness and 10 micrometers in lateral size. Iron oxide Fe2O3 was obtained by the wet chemical method from FeCl3 and NH4OH solution. Manufactured samples were deposited as 3 LbL (GO and Fe2O3 layers deposited sequentially) and 6 LbL structures with GO as a bottom layer. Electrical measurements show the decrease of multilayer resistance after the introduction of the oxidizing NO2 gas to the ambient air atmosphere. The concentration of NO2 was changed from 1 ppm to 20 ppm. The samples changed their resistance even at temperatures close to room temperature, however, the sensitivity increased with temperature. Fe2O3 is known as an n-type semiconductor, but the rGO/Fe2O3 hybrid structure behaved similarly to rGO, which is p-type. Both chemisorbed O2 and NO2 act as electron traps decreasing the concentration of electrons and increasing the effective multilayer conductivity. An explanation of the observed variations of multilayer structure resistance also the possibility of heterojunctions formation was taken into account.


Introduction
Nitrogen dioxide is a typical air pollutant, being a component of motor vehicle exhaust gas and is also generated in the house and industrial combustion processes. Hightemperature combustion leads to an increase in NOx emissions. The early applications of NOx detectors for diesel engine exhausts utilized dense zirconia-based electrolytes. It is shown in more recent investigations of nitrogen oxide sensors that good results can be obtained by integrating the sensor with NOx trap materials [1] or by the use of LaSrMnO 3 type perovskite electrodes [2]. However, the sensors connected with vehicles exhaust work at elevated temperatures, frequently exceeding 300 • C. Exposures to NO 2 are also dangerous for the respiratory system; the presence of nitrogen dioxide in the atmosphere is a cause of photochemical smog and acid rains. Manufacturing of inexpensive NO 2 sensors working at low temperatures is therefore of high importance.
One of the materials recently investigated in gas sensors technology is graphene and structures based on graphene. There are many attempts to increase the sensitivity of graphene-based sensors in view of interaction with the ambient gas atmosphere, enabling practical applications. Pure monolayer graphene with a large surface area and a high conductivity is a good candidate for high-performance gas sensors. However, its production methods as epitaxial growth or chemical vapor deposition, are known as cost-prohibitive with small-scale yield. Modified graphene materials, i.e., graphene oxide (GO) or its reduced form (rGO), can be much easier manufactured, and their specific properties caused by lattice defects connected with, e.g., oxygen functional groups enhance absorption of ambient gas molecules. Increased sensitivity of rGO can be obtained by hybridization of this material with typical in gas sensor technology metal oxides (MOX), as SnO 2 [3][4][5], WO 3 [6,7], TiO 2 [8,9], In 2 O 3 [10], and ZnO [11,12]. One observes also hybridization of rGO with metal disulfides, as MoS 2 [13][14][15] and WS 2 [16], conducting polymers (graphene/PANI [17]) or with metal nanoparticles like Cu [18], Pt [19], Pd [20], and Pd-Pt [21]. It was also discovered that the working temperature of graphene/metal oxide sensors was lower than that of MOX sensors and sometimes can even reach room temperature (RT) [22][23][24][25]. Good sensitivity of GO to acetone at room temperature was obtained recently by using a microwave-based gas sensor realized as a coupled-line section covered with a graphene oxide film [26].
Another gas-sensitive oxide, α-Fe 2 O 3 (hematite), is recently investigated as a NO 2 sensor [27] but more frequently in the form of a nanocomposite with rGO [28][29][30]. The nanocomposites of α-Fe 2 O 3 /rGO are also used in detection of acetone [31,32], ethanol [33,34], and ethylene [35]. Among all crystal phases of iron oxide, essentially α-Fe 2 O 3 is used in gas sensors technology. This phase is the most stable, environmentally friendly, sensitive, and easy in preparation. Mesoporous α-Fe 2 O 3 samples with a large specific surface area were obtained with the use of templates [36,37]. Comparison of sensitivities for α-Fe 2 O 3 and γ-Fe 2 O 3 phases and their mixture is analyzed in [38].
Sensors of NO 2 based on hematite, as can be inferred from the literature (e.g., see discussion in [27]), work at rather elevated temperatures of order 200 • C. In comparison to pure hematite, improved sensing properties vs. NO 2 were obtained in hybrid nanocomposites rGO/Fe 2 O 3 obtained by a facile hydrothermal method [28]. The enhanced response of the nanocomposite at RT was 3.86, compared to 1.38 for pure rGO. In [29] grapheneencapsulated α-Fe 2 O 3 hybrids were investigated. The samples revealed good NO 2 sensing properties at RT with the response time of order a few minutes. In this case, the hybrid was obtained by the hydrolysis of Fe 3+ ions in the colloidal solution of GO and following hydrothermal procedure. The hydrothermal synthesis at 120 • C was applied by the authors of [30] to manufacture α α-Fe 2 O 3 /rGO nanocomposites. In effect, nanosphere-like α-Fe 2 O 3 of 40-50 nm diameters and single intercalated sheets of rGO were obtained.
In the present work, α-Fe 2 O 3 and rGO were obtained separately and deposited on the substrate in the form of a multilayer by the spray method. Such a deposition method allowed for easy formation of slits and pores between the GO sheets, making diffusion of the ambient gas easier. Good sensitivity to NO 2 was also obtained at temperatures close to RT but with quite high response and recovery times.

Sample Preparatiom
Graphene oxide (GO) was prepared using a wet chemical method from graphite (modified Hummers method). Obtained flakes consisted of 6-7 graphene layers with a total thickness of ca. 6 nm and reached the lateral size up to a few micrometers (manufacturing procedure of GO and rGO is described in detail in [39]). Ferric oxide α-Fe 2 O 3 was also obtained by the wet chemical method as follows: to 100 mL of 0.1 M FeCl 3 , 300 mL of 0.1 M NH 4 OH solution was added. An obtained reddish brown suspension was heated for 60 min at 100 • C. The precipitate was washed with warm water on a filter with pore size of 0.2 µm. Both GO and Fe 2 O 3 were deposited on the substrate by spraying, as is schematically illustrated in Figure 1. pore size of 0.2 µm. Both GO and Fe2O3 were deposited on the substrate by spraying, as is schematically illustrated in Figure 1. The GO/Fe2O3 multilayer structure was deposited by LbL (layer by layer) coating technique using alumina substrate with interdigitated Au electrodes manufactured in thick film technology, Separation between electrodes was 0.3 mm. The geometry of electrodes with adequate masking enabled the deposition of two different structures with, e.g., different thicknesses. For investigations, two kinds of samples were prepared: 3 LbL structure consisting of 3 layers (Fe2O3-GO-Fe2O3) and 6 LbL structure (3 Fe2O3 and 3 GO layers) with Fe2O3 layer always on top.

Electrical Measurements
Measurements of sensor resistance were performed in conditions of selected temperature, humidity, and ambient atmosphere composition. The high-quality equipment (electrometer, current and voltage sources, scanners, mass flow controllers) of Agilent (Agilent Technologies, Santa Clara, CA, USA), Keithley (Keithley Instruments Inc., Cleveland, OH, USA) and MKS (MKS Instruments, Andover, MA, USA) manufacturers was used. The measurement chamber contained the sample, Pt 100 temperature probe and digital humidity sensor. The current was measured by an electrometer working in a constant voltage mode. Measurements of resistance varying in a range over nine orders of magnitude were possible. The temperature inside the chamber was changed by applying the voltage from an additional power source. All devices were controlled from the LabView custom application of Agilent Technologies interface card. The required NO2 gas concentration was obtained by controlling the ratio of gas to airflow rates. The humidity of the gas atmosphere was always kept constant with RH = 50% (detailed description of the measurement setup is given in [40]). The GO/Fe 2 O 3 multilayer structure was deposited by LbL (layer by layer) coating technique using alumina substrate with interdigitated Au electrodes manufactured in thick film technology, Separation between electrodes was 0.3 mm. The geometry of electrodes with adequate masking enabled the deposition of two different structures with, e.g., different thicknesses. For investigations, two kinds of samples were prepared: 3 LbL structure consisting of 3 layers (Fe 2 O 3 -GO-Fe 2 O 3 ) and 6 LbL structure (3 Fe 2 O 3 and 3 GO layers) with Fe 2 O 3 layer always on top.

Electrical Measurements
Measurements of sensor resistance were performed in conditions of selected temperature, humidity, and ambient atmosphere composition. The high-quality equipment (electrometer, current and voltage sources, scanners, mass flow controllers) of Agilent (Agilent Technologies, Santa Clara, CA, USA), Keithley (Keithley Instruments Inc., Cleveland, OH, USA) and MKS (MKS Instruments, Andover, MA, USA) manufacturers was used. The measurement chamber contained the sample, Pt 100 temperature probe and digital humidity sensor. The current was measured by an electrometer working in a constant voltage mode. Measurements of resistance varying in a range over nine orders of magnitude were possible. The temperature inside the chamber was changed by applying the voltage from an additional power source. All devices were controlled from the LabView custom application of Agilent Technologies interface card. The required NO 2 gas concentration was obtained by controlling the ratio of gas to airflow rates. The humidity of the gas atmosphere was always kept constant with RH = 50% (detailed description of the measurement setup is given in [40]).

Structural and Morphological Characteristics
TEM measurements were performed with the help of high resolution scanning electron microscope HITACHI S-5500 (HITACHI Ltd., Tokyo, Japan) in a transmission mode. An example of a TEM image for the GO sample is shown in Figure 2a. Different transparencies for attenuated electron beams were caused by the stacking nanostructure of the investigated layers. Dark areas indicate the overlap of several layers. SEM pictures were obtained with the help of FEI Versa 3D Dual Beam microscope (Thermo Fisher Scientific, Waltham, MA, USA). In Figure 2b, one can see the flakes of GO sample deposited on Si/SiO 2 substrate.
Sensors 2021, 21, x FOR PEER REVIEW

Structural and Morphological Characteristics
TEM measurements were performed with the help of high resolution scann tron microscope HITACHI S-5500 (HITACHI Ltd., Tokyo, Japan) in a transmissio An example of a TEM image for the GO sample is shown in Figure 2a. Different t encies for attenuated electron beams were caused by the stacking nanostructu investigated layers. Dark areas indicate the overlap of several layers. SEM pictu obtained with the help of FEI Versa 3D Dual Beam microscope (Thermo Fisher S Waltham, MA, USA). In Figure 2b, one can see the flakes of GO sample depo Si/SiO2 substrate.

Chemical Composition
The surface chemistry of investigated samples was analyzed using X-ray photoelectron spectroscopy. A Microlab 350 (Thermo Electron, VG Scientific) spectrometer with nonmonochromatic Al Kα radiation (hν = 1486.6 eV, power 300 W, voltage 15 kV) was used for this purpose. The analyzed area was 2 × 5 mm. The hemispherical analyzer was used for collecting the high-resolution (HR) XPS spectra with the following parameters: pass energy 40 eV, energy step size 0.1 eV. The collected XPS spectra were fitted using the Avantage software (version 5.9911, Thermo Fisher Scientific), where a Smart function of background subtraction was used to obtain XPS signal intensity and an asymmetric Gaussian/Lorentzian mixed-function was applied. The position of the carbon C1s peak was assumed to be at 284.4 eV and used as an internal standard to determine the binding energy of other photoelectron peaks. XPS spectra for GO/Fe 2 O 3 structure are shown in Figure 4 and for rGO/Fe 2 O 3 structure in Figure 5. energy of other photoelectron peaks. XPS spectra for GO/Fe2O3 structure are shown in Figure 4 and for rGO/Fe2O3 structure in Figure 5.    Table 1. In particular, the decrease in C-O bonds is clearly seen after thermal reduction (decrease from 34.2% to 20.5%). Sim-  Table 1. In particular, the decrease in C-O bonds is clearly seen after thermal reduction (decrease from 34.2% to 20.5%). Similar observations are reported by other investigators of rGO/α-Fe 2 O 3 composites, [28,32]. The recorded XR-XPS spectra of Fe2p revealed the chemical state of iron. Two wide peaks for both structures, located at the binding energies of ca. 711.0 eV and ca. 725.0 eV for Fe2p 3/2 and Fe2p 1/2 , respectively, are characteristic for Fe 3+ species in α-Fe 2 O 3 , which is in good agreement with other reports, e.g., [29,31].

Gas Sensing Properties
Measurements of resistance for GO/Fe 2 O 3 multilayers in response to oxidizing NO 2 atmosphere indicate that the structures behave as p-type semiconductors (under oxidizing atmosphere resistance of the sample decreases). Calculated gas response defined as S = (R a − R g )/R a where R a and R g are the sensor resistances in air and NO 2 , respectively, indicate that the structures are sensitive even at temperatures as low as 30 • C (Figure 6a,b). With increasing working temperature, the response also increased, as is shown for the structure GO/Fe 2 O 3 6 LbL with an increased number of layers, Figure 6c. As can be noticed, an increase in the number of layers in the structure does not influence the response remarkably. Variation of the baseline resistance with time at a constant NO 2 concentration is shown in Figure 6d, and the comparison of sensitivities for the 6LbL multilayer and the single rGO layer is shown in Figure 6e.
The selectivity of GO/Fe 2 O 3 6LbL structure was tested by exposing the sensor to different interference gases, both oxidizing and reducing, as shown in Figure 7. Comparing the response to acetone (1.2% at 25 ppm) and hydrogen (0.9% at 50 ppm) with that to NO 2 , one can summarize that the influence of the tested interference gases is negligible. However, the influence of humidity on the sensor response is remarkable and is clearly seen at elevated temperatures. The example measurements of that response at 90 • C in a wide range of RH is shown in Figure 7b. The results indicate that both the increase and decrease of RH in comparison to 50% give variations of the response not higher than ±5%.
The response and recovery times of the structures are high, especially in the low operation temperature range. With increasing working temperature, these times significantly decreased, as shown in Figure 8 and Table 2. The selectivity of GO/Fe2O3 6LbL structure was tested by exposing the sensor to different interference gases, both oxidizing and reducing, as shown in Figure 7. Comparing

Gas Sensing Mechanism
Fe 2 O 3 is known as an n-type semiconductor, but the rGO/Fe 2 O 3 hybrid structure behaves similarly to p-type rGO. Both chemisorbed O 2 and NO 2 act as electron traps decreasing the concentration of electrons. This is clearly seen in rGO in the decrease of resistance (hole density increases). Pure Fe 2 O 3 is nearly insensitive to NO 2  As a result, the unbalance of charge on the surface of Fe 2 O 3 is compensated by transferring additional electrons from rGO to Fe 2 O 3 surface, which results in additional holes in rGO, Figure 9, and then the increase in conductivity.

Gas Sensing Mechanism
Fe2O3 is known as an n-type semiconductor, but the rGO/Fe2O3 hybrid stru haves similarly to p-type rGO. Both chemisorbed O2 and NO2 act as electron traps ing the concentration of electrons. This is clearly seen in rGO in the decrease of r (hole density increases). Pure Fe2O3 is nearly insensitive to NO2, but in rGO/Fe2O sition, NO2 reacts with O2 -adsorbed on Fe2O3 surface, forming an intermediate N plex what was suggested in [30] 2NO2 (gas) + O2 -(ads) + e -→ 2 NO3 -(ads) As a result, the unbalance of charge on the surface of Fe2O3 is compensated ferring additional electrons from rGO to Fe2O3 surface, which results in additio in rGO, Figure 9, and then the increase in conductivity. In Table 2, O3 on the increased sensitivity to NO2 can be understood from F At the interface between rGO flakes and Fe2O3 grains, the p-n heterojunction formed. The numerical value of work function for rGO was taken from [41], an ergy gap was evaluated from [42] for oxygen content in rGO equal O/C = 10%, ously determined in elemental composition experiments [40]. The work function was taken from Guo et al. [31]. The difference in work functions of both materia the transfer of electrons from rGO to Fe2O3 and holes in the opposite direction. the hole accumulation region is formed on the p-rGO side. The concentration o the accumulation layer on the p-side increases additionally after interaction w leading to the increased conductivity of GO flakes in the presence of NO2 gas, as ically shown in Figure 11. In Table 2, O 3 on the increased sensitivity to NO 2 can be understood from Figure 10. At the interface between rGO flakes and Fe 2 O 3 grains, the p-n heterojunctions can be formed. The numerical value of work function for rGO was taken from [41], and the energy gap was evaluated from [42] for oxygen content in rGO equal O/C = 10%, as previously determined in elemental composition experiments [40]. The work function for Fe 2 O 3 was taken from Guo et al. [31]. The difference in work functions of both materials causes the transfer of electrons from rGO to Fe 2 O 3 and holes in the opposite direction. In effect, the hole accumulation region is formed on the p-rGO side. The concentration of holes in the accumulation layer on the p-side increases additionally after interaction with NO 2 , leading to the increased conductivity of GO flakes in the presence of NO 2 gas, as schematically shown in Figure 11.  Potential barriers between Fe2O3 grains were not taken into account as the multilayer structure in response to NO2 behaves as a p-type semiconductor, then rGO layers play the essential role in conductivity.
As it was mentioned, the spray deposition technology allowed for easy formation of slits between the GO sheets, and additionally, Fe2O3 prevented these sheets from restacking. The slits can act as channels for gas diffusion, providing a higher number of active sites for the interaction with nitrogen dioxide, thus influencing the electrical transport.

Conclusions
Among air pollutants, NO2 is one of the most harmful gases, and measurements of its concentration with the help of inexpensive chemoresistive structures with very small power consumption are of great importance. In contrary to the pure Fe2O3 with small sensitivity to NO2 in the low-temperature range, rGO/Fe2O3 multilayer structures indicated good sensitivity to low concentrations of NO2 in the ambient air, going down to 1 ppm. Both materials of the structure were easily obtained by the wet chemical method and were sprayed on the substrate to form the needed composition. The sensitivity of the multilayer increased with temperature. Response time of order 1 min was possible to obtain at temperatures below 150 °C, but the recovery time was still high, of order tens of minutes. An  Potential barriers between Fe2O3 grains were not taken into account as the multilaye structure in response to NO2 behaves as a p-type semiconductor, then rGO layers play th essential role in conductivity.
As it was mentioned, the spray deposition technology allowed for easy formation o slits between the GO sheets, and additionally, Fe2O3 prevented these sheets from restack ing. The slits can act as channels for gas diffusion, providing a higher number of activ sites for the interaction with nitrogen dioxide, thus influencing the electrical transport.

Conclusions
Among air pollutants, NO2 is one of the most harmful gases, and measurements o its concentration with the help of inexpensive chemoresistive structures with very sma power consumption are of great importance. In contrary to the pure Fe2O3 with small sen sitivity to NO2 in the low-temperature range, rGO/Fe2O3 multilayer structures indicate good sensitivity to low concentrations of NO2 in the ambient air, going down to 1 ppm Both materials of the structure were easily obtained by the wet chemical method and wer sprayed on the substrate to form the needed composition. The sensitivity of the multilaye increased with temperature. Response time of order 1 min was possible to obtain at tem peratures below 150 °C, but the recovery time was still high, of order tens of minutes. A Potential barriers between Fe 2 O 3 grains were not taken into account as the multilayer structure in response to NO 2 behaves as a p-type semiconductor, then rGO layers play the essential role in conductivity.
As it was mentioned, the spray deposition technology allowed for easy formation of slits between the GO sheets, and additionally, Fe 2 O 3 prevented these sheets from restacking. The slits can act as channels for gas diffusion, providing a higher number of active sites for the interaction with nitrogen dioxide, thus influencing the electrical transport.

Conclusions
Among air pollutants, NO 2 is one of the most harmful gases, and measurements of its concentration with the help of inexpensive chemoresistive structures with very small power consumption are of great importance. In contrary to the pure Fe 2 O 3 with small sensitivity to NO 2 in the low-temperature range, rGO/Fe 2 O 3 multilayer structures indicated good sensitivity to low concentrations of NO 2 in the ambient air, going down to 1 ppm. Both materials of the structure were easily obtained by the wet chemical method and were sprayed on the substrate to form the needed composition. The sensitivity of the multilayer increased with temperature. Response time of order 1 min was possible to obtain at temperatures below 150 • C, but the recovery time was still high, of order tens of minutes. An increase in the number of layers of the structure did not influence the response remarkably. The sensing behavior of the multilayer structure was explained by both individual interactions of constituting materials with the ambient oxidizing gas and by the formation of heterojunctions at the contact regions of GO and Fe 2 O 3 .