2. N-Doped Graphene and Its Derivatives as Resistive Gas Sensors: Theoretical Studies
With the progressive advancement of computational power and the availability of better supercomputers, theoretical studies of materials have become more feasible. Since gas sensor reactions include chemical reactions in which bonds are formed or broken and often result in a transfer of charge, the first principle (ab initio) or a quantum approach is preferred over a classical mechanics method. Quantum approach methods are useful for studying gas–surface interactions owing to their ability to model the adsorption mechanisms, the adsorption energy, charge transfer, electronic modification after gas adsorption, the structural changes, and the electronic properties involved. In this regard, density functional theory (DFT) calculations are considered a powerful technique to carry out many simulations, including the gas-sensing capability of different materials, without any side effects. DFT calculations are required for understanding experimental results at the molecular level, or as a predictive tool for the design of new sensing materials 
The adsorption of some gases, namely CO, NO, O2
, and NO2
, on defective graphene and N-doped graphene was investigated through DFT calculations. It was theoretically revealed that all CO, NO, and O2
gases were chemisorbed on the defective graphene with large charge transfers. This can lead to the poor selectivity of CO in the presence of other gases. In contrast, only CO molecule with a large charge transfer was chemisorbed on the N-doped graphene, while other gases were only weakly adsorbed on it, reflecting good selectivity for CO gas sensing. Hence, through DFT calculations, it was shown that with a simple doping of “N” in a graphene lattice, it is possible to improve the selectivity to CO gas 
Based on DFT studies, the adsorption of CO molecules on pristine, N-doped, as well as Al-doped graphene was studied. The results showed that the most stable adsorption configuration was on top of a carbon (N or Al) atom of graphene sheets. The adsorption energies of CO molecules on pristine, N-doped, and Al-doped graphene were −0.01 eV, −0.03 eV, and −2.69 eV, respectively. In fact, the adsorption of CO molecules on pristine and N-doped graphene was physical adsorption, while it was chemisorbed on the Al-doped graphene. Moreover, calculations on band structure, density of states (DOS), and differential charge density showed that the adsorption of CO on Al-doped graphene was remarkably different from other materials in their study 
. In another work, the adsorption of HCHO on Si-, Al-, Cr-, B-, N-, Au-, and Mn-doped graphene was studied using DFT calculations. It was reported that the HCHO molecule was weakly adsorbed on B- and N-doped graphene, while HCHO chemisorption was predicted on Si-, Al-, Cr-, Mn-, and Au-doped graphene. Additionally, based on adsorption geometries, charge transfers, adsorption energies, and density of states, it was found that Al and Mn dopants are the best ones for sensing enhancement towards HCHO gas 
. The next work was related to an adsorption study of CO2
on the P-, N-, B-, and Al- doped monovacancy graphene through DFT calculations. The band gap was changed from 0.004 to 0.478 eV and the electrical conductivity was changed from metallic to semiconducting when a monovacancy was created in graphene. Furthermore, the doped graphene had a larger band gap relative to its undoped counterpart. The authors reported that Al-doped monovacancy graphene was better than other gas sensors for CO2
gas sensing due to the suitable adsorption strength and band gap. Before the adsorption of CO2
gas, the band gaps of N-doped monovacancy graphene and P-doped monovacancy graphene had no change, but those of B-doped monovacancy graphene and Al-doped monovacancy graphene changed. The band gap of Al-doped monovacancy graphene changed from almost 0 to 0.229 eV 
. Hence, the electrical features were significantly changed, showing the high potential of Al-doped monovacancy graphene for CO2
are colorless gases and useful reagents in organic synthesis 
. The energetic and electronic properties of pristine and N-doped graphene upon the adsorption of BC13
, and BF3
gases were studied using DFT calculations. The computed dipole moment revealed that the amount of change in dielectric of the sensing material depends on the type of molecule. Furthermore, it was reported that N-doped graphene had much higher adsorption energy and higher net charge transfer relative to pristine graphene thanks to Lewis acid–base interactions between these gases and the sensing material. Additionally, the Lewis acidity increased as follows: BF3
with adsorption energies of −8.7, −18.3, and −26.5 kJ/mol, respectively, revealing better adsorption of the B(OCH3
on the N-doped graphene. Furthermore, very low adsorption energies were calculated on pristine graphene, suggesting a poor sensing response of pristine graphene to these gases 
Based on the DFT calculation results, it can be concluded that, overall, N doping can enhance the adsorption of gas molecules on graphene or rGO, relative to pristine (undoped) graphene or rGO. However, it should be also mentioned that, in comparison to metallic dopants such as Al, Ti, or even B dopant, generally, N-doped graphene shows weaker adsorption of the target gas molecules. However, the adsorption behavior of codoped graphene such as Ti/N, and codoping in graphene in general, need to be further explored using DFT calculations.
3. N-Doped Graphene and Its Derivatives as Resistive Gas Sensors: Experimental Studies
Graphene quantum dots (GQDs) have lateral dimensions smaller than 10 nm and the combination of QDs and graphene characteristics in these materials leads to revealing enhanced features such as low toxicity, chemical stability, high strength, tunable energy band gaps, outstanding optical properties, and excellent microelectronic properties 
. In this regard, Masemola et al. 
prepared a N-doped GQDs/PANI composite for ethanol-sensing studies.
The NGQDs/PANI sensor revealed a better response compared to that of pristine PANI along with a faster response time and recovery time of 85 s and 62 s, respectively, to 100 ppm ethanol vapor at 25 °C. The enhanced response of the composite sensor was related to the incorporation of N in the GQDs. It was related to the enhanced conductive pathways between the PANI and N-doped GQDs, which induced conductive pathways created by strong synergetic effects through interchain π interactions between the NGQDs and PANI. Furthermore, the surface area of the composite sensor (41.0 m2 g−1) was higher than that of bare PANI (31.2 m2 g−1), leading to higher adsorption of ethanol molecules on the composite sensor.
CPs are used in gas sensors due their tunable properties, low cost, and room-temperature operation 
. In another study, Gavgani et al. 
realized VOC gas sensors based on pristine N-doped GQDs, poly (3,4 ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS), and N-doped GQDs/PEDOT-PSS nanocomposites. The fabricated sensors at room temperature were exposed to 100 ppm acetone, toluene, propanol, chlorobenzene, methanol, ethanol, and water. It was reported that the N-doped GQDs/PEDOT-PSS nanocomposite gas sensor showed a good response to methanol, whereas it revealed almost no response to other gases. The response of the optimal sensor to 100 ppm methanol was 154.4%, which was 12 times as high as that of the PEDOT-PSS sensor. The sensing mechanism of the optimal sensor was described using the following reasons: (i) The swelling of CPs due to the diffusion of methanol into the CPs. In this case, the interchain distance of CPs increases and the electron hopping process becomes more difficult. (ii) N-doped GQDs integrated into the CP matrix acted as conductive paths, favoring the electron hopping. Therefore, a combination of the above two processes leads to the modulation of the electrical resistance in the presence of methanol gas.
Nitrogen dioxide (NO2
), with a highly toxic nature, emits from fossil fuels and can damage the human respiratory system 
. In this regard, a series of N-doped GQD 3-dimensional ordered macroporous (3DOM) In2
composites were synthesized for NO2
gas-sensing studies. The results showed that the sensor exhibited a better response to NO2
gas with fast and recovery times and a low working temperature (100 °C), relative to pristine In2
and rGO/3DOM In2
sensors. Furthermore, it showed good selectivity and high long-term stability. The conductivity enhancement due to the presence of N-doped GQDs and the generation of heterojunctions between In2
and N-GQDs, as well as the presence of macrospores, accounted for the enhanced response to NO2
P-type rGO and N-doped rGO were explored for NO gas detection at 25 °C. The response of the N-rGO sensor to 1000 ppb NO (1.7) was higher than that of the rGO sensor (0.012) and it was even able to detect 0.4 ppm NO gas. The increase in the NO gas-sensing performance was related to N doping. First, the N doping produced polarization in the sp2
carbon network and further affected the physical and the electrical properties of the sensor. Second, the improved gas response was related to the improved active sites of pyridinic N and/or pyrrolic N. In particular, pyridinic N, where the N atom is bonded to two C atoms and donates one p-electron to the aromatic π system, modified the band structure of carbon and lowered the work function. Additionally, the electron density on the neighboring C atom decreased, which resulted in the transfer of electrons from the C to N atoms, and N back-donate electrons to the neighboring C pz orbitals. These processes facilitated the dissociation of oxygen molecules on the C atoms and helped to form a strong bond between O and C atoms 
is an n-type semiconductor which offers high chemical stability and excellent electronic properties. Yan et al. 
investigated the sensing properties of C- and N-doped rGO/TiO2
composites with special exposed facets synthesized via a hydrothermal method at 180 °C using HF. The gas-sensing performance of the N-doped rGO/TiO2
sensor was better than that of the C-doped rGO/TiO2
. The N-doped rGO/TiO2
sensor exhibited the highest response to isopropanol, ethanol, and acetone gases at 210, 240, and 270 °C, respectively. The N-doped rGO/TiO2
sensor exhibited the highest response to 100 ppm acetone with a response of 14.12, while its responses to ethanol and isopropanol were 5.85 and 3.47, respectively.
Lin et al. 
fabricated mesoporous p-type Co3
/N-doped rGO nanocomposites via a metal–organic framework (MOF) template. Elemental mapping revealed the uniform distribution of Co, O, C, and N elements in the sample. The effect of initial rGO concentration on the gas-sensing characteristics of the composite sensor was studied. It was found the Co3
/N-doped rGO-0.5 (rGO mass = 0.5 mg) nanocomposite sensor had a better response to ethanol than other sensors, and a response of 44.5 to 200 ppm ethanol at 200 °C was obtained. The enhanced sensing response was related to the presence of rGO with high electron mobility, which facilitated the migration of carriers in the sensing process, leading to improved sensing performance. Furthermore, the coupling effect between Co3
and N-doped rGO enhanced the oxygen reduction ability in the gas-sensing process and increased the transfer rate of charge carriers. In addition, the sensor had a high specific surface area, and so provided more effective active sites during ethanol adsorption than the other samples. The N atoms were mainly bonded with rGO as pyridine-N. They were bonded with the “C” atoms of the rGO sheets, which provided lone pair electrons to increase the electronic partial density of states near the Fermi level. The higher density of states facilitated the electron flow during the sensing reactions.
Modak et al. 
gas sensors using a SnO2
-N-doped rGO composite for NO2
-rGO composites with three amounts of SnO2
(low, medium, and high) were synthesized via a hydrothermal reaction. The samples were coded as SR1, SR2, and SR3. For N-doped rGO, urea was used as a source of N atoms. They were coded as SRN1, SRN2, and SRN3 based on 0.09, 0.18, and 0.27 mg of urea, respectively.
N doping led to the generation of more active sites which were beneficial for NO2 adsorption. Additionally, the higher surface area and higher pore volume of the SRN2 sensor facilitated a higher response to NO2 gas. Furthermore, the large number of defects and oxygen vacancies in SRN2 enhanced the gas response. Indeed, N-doped rGO had a greater number of defects in the rGO sheets, which acted as adsorption sites for the NO2 gas. Additionally, the SRN2 sensor had higher conductivity compared to SR sensors, owing to the presence of a lone pair of electrons on the N atom.
Nie et al. 
synthesized N- and Si-codoped (N/Si-doped) graphene. The N atoms acted as the active sites for the NO2
adsorption, whereas the Si atoms remarkably modified the electronic features of the graphene. The N/Si-doped graphene exhibited p-type conductivity, indicating that the hole doping from Si atoms overcame the electron doping from N atoms. The sensor was able to detect NO2
gas at 25 °C.
, as a toxic gas, is an irritant to the eyes, skin, and respiratory system 
. Among different CPs, PANI is popular thanks to merits such as the ease of synthesis, low cost, stability, and good response to NH3
. Nevertheless, in pristine form, it has a response 
. Therefore, it is often used in composite form. In this regard, Gavgani and coworkers 
introduced a flexible NH3
sensor using a S/N-codoped GQDs/PANI composite. The response of the S/N-codoped GQDs/PANI sensor to 100 ppm NH3
was 5-fold higher than that of the PANI sensor, with a faster response time and recovery time. Additionally, the S/N-doped GQDs/PANI hybrid gas sensor showed better selectivity to NH3
gas relative to the PANI sensor.
The increased NH3
-sensing features were attributed to the swelling of the PANI in the presence of NH3
, as well as the reversible acid–base doping/dedoping of the PANI in the presence of NH3
gas, which changed it from a conductive state to an insulating state, leading to an increase in the resistance and formation of heterojunctions between the PANI and the S/N-codoped GQDs. In another study, an NH3
gas sensor based on a combination of PANI and different amounts of N-doped GQDs was fabricated and Ag or Al was used for the electrodes. The sensor with Ag and Al contact showed Ohmic and Schottky behaviors, respectively. The sensor with a Ag electrode had a much higher response to NH3
than that of the sensor with an Al electrode. The (50 wt%) N-GQDs/PANI sensor with a Ag electrode displayed a high response of 110.92 to 1500 ppm NH3
at room temperature, which was around 30% higher than that of the Al electrode 
. In another research, the sensing response of a PEDOT-PSS sensor to 1500 ppm NH3
at room temperature increased from 30.13% to 212.32% with 50 wt% N-doped GQDs. Furthermore, the sensor with N-GQD showed faster dynamics and higher stability 
. Purbia et al. reported N-doped GQDs/SnO2
nanocomposites for the detection of NO2
gas at 150 °C. The response to 100 ppb NO2
gas was 292 with a response time of ~3 min. Furthermore, it revealed good selectivity to NO2
gas. The formation of SnO2
-N-doped GQDs, the presence of defects, and the high surface area of the gas sensor led to a better response in the composite sensor relative to the bare SnO2
gas sensor 
There are various methods to fabricate N-doped graphene or rGO. N doping in graphene and rGO can not only induce defects, but can also increase the electrical conductivity and induce n-type conductivity in graphene. Through N doping, it is feasible to tune the band gap of graphene and rGO. Furthermore, N dopants are cheaper than noble metals such as Au, Pt, and Pd.