Electrochemically chlorinated graphene for ultrafast NO₂ detection at room temperature

Abstract

Functionalization of two-dimensional (2D) materials is a key approach to enhancing the performance of gas sensors since it effectively modulates the intrinsic chemical properties. Various atoms have been exploited to change the molecular interaction between sensing materials and target species. In particular, chlorine has been widely studied due to its extremely high surface reactivity and high electronegativity. However, traditional chlorination methods have been conducted by hazardous processes. Herein, we report a non-toxic electrochemical chlorination of graphene that enables superior nitrogen dioxide (NO₂) gas sensing properties at room temperature. Chlorinated graphene (Cl-Gr) was synthesized by an electrochemical reaction using an aqueous sodium chloride (NaCl) solution under an applied voltage. The Cl-Gr gas sensors exhibited enhanced sensitivity and improved reversibility upon exposure to NO₂ at room temperature. The response and recovery times were dramatically decreased by 75.8% and 86.4%, respectively. The role of chlorine in the sensing performance was investigated by first-principles density functional theory (DFT) calculations, which were in agreement with experimental results. This work extends the potential use of functionalized 2D material-based gas sensors and deepens the understanding of their gas sensing mechanism.

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Introduction

2D materials have been attracting growing attention for next-generation sensing applications due to their unique properties such as adjustable electronic characteristics, ease of surface chemistry modification and flexibility.^1,2 Although semiconductor metal-oxide-based gas sensors have been commercialized in various fields, the high power consumption and low thermal stability still remain as challenges. A key advantage of 2D materials in chemical sensors is their high sensitivity at room temperature. However, 2D materials show slow response and recovery steps, which hinder their practical use. Pristine graphene-based gas sensors have exhibited difficulties in achieving a rapid reaction time and full recovery to baseline resistance after gas exposure at room temperature.³ The defects on the graphene surface, such as vacancies and dangling bonds, result in higher binding energy with target gas molecules, leading to slower chemical sensing reactions.^4–6 To solve this problem, various approaches have been explored such as surface modification, self-activation, heterostructures, noble-metal decoration, and chemical functionalizations.^7–10 Among these methods, functionalization is one of the most promising strategies to control the intrinsic sensing properties, as it can be applied to a wide range of elements with almost no limitations, and various methods have been developed for its implementation.

Halogenation is widely studied for carbon-based materials to modify their intrinsic electronic and chemical properties. One of the key advantages of halogenation is its inherent high reactivity, which minimizes the effort required to introduce functional groups onto target materials. Once halogen groups are introduced to the pristine materials, their electronic and chemical properties vary and significantly influence molecular interactions between the sensing materials and the target species.^11,12 The novel properties resulting from the halogenation of graphene-based materials have demonstrated their potential for various applications such as energy storage,¹³ catalysis,¹⁴ and gas sensing.^10,15 Despite this, studies focusing on their application in gas sensors remain limited. For instance, fluorination of graphene oxide exhibited selective, reversible and rapid NH₃ sensing behaviour as the fluorine functional group changes the overall charge distribution of graphene oxide.¹⁰ Additionally, Weis et al. reported that non-covalent incorporation of chlorine-, bromine-, and iodine-containing functional groups on carbon nanotubes can selectively enhance the responses to target species while also greatly reducing detection limits to sub-ppm levels.¹⁵ However, to the best of our knowledge, chlorination of pristine graphene for gas sensing applications has not been reported, and consequently, the sensing mechanisms remain unexplained.

Chlorine with high electronegativity promotes electron transfer, leading to significant changes in the surface chemistry of intrinsic materials.¹⁶ As a result, chlorinated graphene has shown superior characteristics in lithium-ion batteries,¹⁷ field-effect transistors¹⁸ and photodetectors.¹⁹ However, despite its exceptional properties, efficient chlorination of graphene has been achieved through toxic and complex processes, such as plasma treatment, photochemical reactions and thermal exfoliation methods.^20–22 To address this problem, non-toxic methods such as laser-induced processes²³ and electrochemical methods²⁴ have been explored.

Herein, we report the electrochemical chlorination of monolayer graphene, which enables fast response and recovery for NO₂ sensing at room temperature. Cl-Gr sensors were fabricated via a facile electrochemical chlorination process, which involved drop-casting a sodium chloride solution onto a patterned graphene monolayer and applying a bias voltage. In contrast to the conventional chlorination approaches, this electrochemical method is a much simpler and less toxic process, not requiring complex equipment. Chemical vapor deposited (CVD) monolayer graphene was chlorinated using NaCl solutions with different concentrations. Gas sensing property changes of CVD grown graphene induced by chlorination were investigated. Although a slight improvement in NO₂ response was observed with increasing chlorination levels, the predominant effect was a significant reduction in response and recovery times, by 75.8% and 86.4%, respectively. These results highlight the crucial role of chlorination in enabling ultrafast sensing dynamics under ambient conditions. The sensor also exhibited good linearity in response as a function of NO₂ concentration. Chlorination was verified by X-ray photoelectron spectroscopy (XPS), Raman analysis and electron dispersive spectroscopy (EDS). The role of chlorination of graphene in sensing performance was investigated by first-principles DFT calculations. The influence of chlorination of graphene on sensing performance was demonstrated through both experiments and theoretical calculations. These findings suggest that chlorination can overcome the inherent limitation of slow response and recovery in carbon-based sensing applications, offering valuable insights for further research.

Experimental

Materials

Sodium chloride (NaCl), potassium dihydrogen phosphate (KH₂PO₄), phosphoric acid, and CVD grown monolayer graphene.

Fabrication of patterned graphene gas sensors

Monolayer graphene was synthesized on a piece of Cu foil (purity: 99.99%) using a thermal chemical vapor deposition method at 990 °C with a hydrocarbon source (CH₄, 50 sccm) and hydrogen (H₂, 10 sccm). Monolayer graphene on copper foil was wet-transferred onto the SiO₂/Si (285 nm/525 μm) substrate, following the removal process of PMMA using acetone for 30 minutes. Subsequently, the sample was patterned by photolithography and oxygen (O₂) plasma treatment (17 s) with 50 W plasma power, followed by thermal annealing at 350 °C for 7 hours to perfectly remove the PMMA residue. After that, graphene samples were contacted at one end using conductive silver paste (ELCOAT P-100).

Electrochemical chlorination process

NaCl solutions with dissolved potassium phosphate buffer were prepared with concentrations of 3.0 M and 5.0 M. A NaCl drop is carefully placed on a graphene pattern without touching the silver paste region at the end of the graphene pattern. An AgCl/Ag electrode was inserted into a NaCl drop and a gold tungsten probe tip (M5BG, MS Tech) was placed in contact with silver paste. The voltage of 1.6 V was applied using pulse mode (using a Keithley 2400 device), which consisted of total 24 cycles of 10 s and 5 s. The sample was rinsed with deionized water and naturally dried in air.

Gas sensor measurements

Gas sensing properties were measured at room temperature under a constant flow of 1000 sccm in a quartz tube. A Keithley 2400 recorded sensor's resistance at 1 s intervals under a DC bias voltage of 0.1 V, while the flow gas was alternately changed from dry air (99.999%) to calibrated target gases (balanced with dry air) using a mass flow controller. The response was calculated as (R_gas − R_air)/R_air × 100, where R_gas and R_air are the measured resistances of the sensors exposed to the target gases and dry air, respectively.

Characterization

The chlorinated graphene was characterized with a Field Emission Scanning Electron Microscope (FE-SEM, Hitachi S-4800, Japan) and an Energy Dispersive Spectroscopy system (EDS, Bruker, USA) using a 10 kV beam. Raman scattering was carried out using a Raman spectrometer and 532 nm laser light for excitation. X-ray Photoelectron Spectroscopy (XPS) measurements were conducted using a PHI5000 VersaProbe II (ULVAC-PHI, Japan), equipped with a monochromatic Al Kα source (1486.6 eV) at the Korea Research Institute of Standards and Science (KRISS). The X-ray beam spot size and power were set to 200 μm and 50 W, respectively, with the base pressure in the XPS chamber maintained at less than 3.5 × 10⁻⁸ Pa. The X-ray beam was directed perpendicularly onto the sample surface, and the detector collected photoelectrons at 45 degrees relative to the surface normal. XPS spectra were fitted using CasaXPS software, employing Shirley background subtraction and a Voigt function for peak deconvolution. Fitting parameters were optimized to minimize the residual standard deviation (STD) between the measured and fitted XPS spectra.

DFT calculations

First principles DFT calculations were performed using the projector augmented wave (PAW) method and the local density approximation (LDA) for the exchange correlation potential, implemented in the Vienna Ab initio Simulation Package (VASP) code.^25–27 Monkhorst–Pack k-point sampling with a grid of 4 × 5 × 1 was used for Brillouin zone integration.²⁸ An energy cutoff of 520 eV was used for the plane wave representation of the wavefunctions, and atomic structures were relaxed until all the Hellmann–Feynman forces were less than 0.01 eV Å⁻¹. The graphene structure was constructed from 24 carbon atoms (a 3 × 4 rectangular supercell). More than 15 Å vacuum space was employed along the perpendicular direction to prevent artificial interaction between the periodic images. The detailed experimental procedures are provided in the SI.

Results and discussion

Monolayer graphene grown on Cu foil by the CVD method was wet transferred onto a SiO₂/Si substrate, followed by photolithography to make a H-shaped pattern with 10 μm channel width (Fig. S1). After that, thermal annealing was conducted to eliminate the PMMA residue, which interferes with the surface functionalization of graphene or reduces the detection characteristics of gas sensors, in order to enhance surface reactivity. As shown in Fig. 1a, a NaCl solution was drop-cast on the as-prepared graphene pattern without touching silver paste at the other end of the graphene pattern, and the voltage was applied by inserting an AgCl/Ag electrode into a NaCl drop and making contact with silver paste at the end of the graphene pattern. A voltage of 1.6 V was applied for 24 cycles, followed by washing and drying processes. Afterward, silver paste was placed at both ends of the graphene pattern to prepare for gas measurement. The samples showed a slight increase in resistance upon chlorination (Fig. S2).

Fig. 1 Fabrication of H-shape patterned graphene and the chlorination process. (a) Schematic illustration of wet transfer of monolayer graphene on a SiO₂/Si (285 nm/525 μm) substrate and H-shape patterning via photolithography. After that, monolayer graphene was chlorinated with a NaCl aqueous solution. (b) OM image of the graphene channel of 10 μm width. (c) EDS mapping images of Cl-Gr.

As can be seen in the current graph in Fig. S3, the voltage of 1.6 V was applied using pulse mode, which consists of 24 cycles of consecutive 10 s and 5 s. Although oxidation of the graphene surface can also occur under these electrochemical conditions, this process was effectively suppressed in the presence of high chloride concentration due to the preferential formation of chlorine radicals (Cl˙), as demonstrated by previous studies. When a voltage was applied, chloride ions (Cl⁻) in the NaCl solution migrated toward the graphene surface and were electrochemically oxidized to chlorine radicals (Cl˙), which subsequently reacted with the graphene basal plane to achieve chlorination.²⁴ However, under continuous voltage application, these ions were continuously consumed at the electrode interface, reducing their local concentration and leading to the formation of a depletion layer, which can diminish the efficiency of the electrochemical reactions. To avoid these inefficiencies in electrochemical reactions, we proceeded in pulse mode while applying the voltage.²⁹

When a voltage was applied in the NaCl solution, we also observed an instantaneous increase in current, followed by a gradual decrease over time (Fig. S3a). In contrast, with the buffer solution, no initial spike occurred upon applying voltage, and the current remained constant (Fig. S3b). These initial graph jumps can be explained by a temporarily rapid increase in current as ions move quickly toward the electrode under the influence of the electric field.³⁰ This phenomenon may also be attributed to the rapid formation of an electric double layer (EDL) between the electrode and the solvent when the voltage was first applied, causing the current to increase momentarily before stabilizing.³¹ However, as time passed and the EDL stabilized, the ion movement near the electrode decreased, and the current value gradually decreased accordingly. This is because the ions have accumulated enough on the electrode surface to no longer actively move.³² Besides, there are several reasons for the current decrease trend. First, ions participate in electrode reactions, altering the electric potential and reducing the current, indicating that electrochemical reactions slow the ion movement in the electrolyte and potentially cause further current reduction.³³ Second, the accumulation of oxides or other reaction products formed on the electrode surface can also affect the current decrease. The substances formed on the electrode surface reduce the active area of the electrode, resulting in a decrease ofthe current.³⁴

Fig. 1b presents an optical microscope (OM) image of the chlorinated graphene channel along with the expected molecular structure, and Fig. 1c exhibits the surface morphology of graphene after the chlorination process, as observed by scanning electron microscopy (SEM). The sample was chlorinated using a 5.0 M NaCl solution, which was chosen as the upper concentration limit due to the observed salt precipitation beyond this point. Energy dispersive X-ray spectroscopy (EDS) mapping images in Fig. 1c also illustrate the green images of Cl atoms, which indicate the presence of Cl atoms on the graphene pattern and their uniform distribution. The signals of Cl atoms are detected not only in the NaCl crystal but also in its surroundings.

As shown in Fig. 2a, b and S4, X-ray photoelectron spectroscopy (XPS) analysis confirms the formation of covalent bonds between C and Cl atoms after chlorination, leading to an increase from sp³-hybridized carbon in graphene. Specifically, the relative area of the sp³ component in the C 1s core-level spectrum increased from 16.77% to 19.48% after chlorination, suggesting surface functionalization of graphene (Table S1). An increase in the ratio of sp³ to sp² orbitals and a decrease in the composition ratio of oxygen-related peaks were observed due to chlorination. Thus, we can infer that the oxygen bonded to the graphene surface was replaced by chlorine, indicating a reduction process, while the existing sp² orbitals of graphene were rehybridized into sp³ orbitals due to the introduction of covalent bonds between carbon and chlorine atoms. Miao et al. reported that the structural transformation of C–C bonds from sp² to sp³ configuration occurred through chlorination of graphene.²³ The Cl 2p XPS spectrum exhibited distinct peaks corresponding to the presence of chlorine atoms bonded to the graphene lattice, with characteristic binding energies of about 199.6 eV for Cl 2p_3/2 and 201.2 eV for Cl 2p_1/2, confirming the formation of C–Cl bonds (Fig. 2b and S5). To clarify the chemical state of chlorine in Cl-Gr, the Cl 2p spectrum was deconvoluted into two components: C–Cl and NaCl. As shown in Table S2, for the 5.0 M NaCl chlorinated Cl-Gr sample, 84.2% of the Cl signal was attributed to covalent C–Cl bonding, while 15.8% originated from residual NaCl, indicating the successful chemical incorporation of chlorine into the graphene lattice. In addition, Raman spectroscopy complemented XPS results by providing information on the vibrational modes of chlorinated graphene. The introduction of chlorine into the graphene structure affected the Raman peaks, especially the G and 2D bands. Changes in peak positions were observed, indicating modifications in the electronic structure and bonding properties due to chlorination (Fig. 2c–e and S6). The blue shift of G and 2D bands was observed, consistent with previous reported studies about chlorination of graphene.^18,35,36 This blue shift was caused by electron doping, which occurred as the electron density in graphene increased. Also, the I_D/I_G value was calculated by averaging the Raman measurements taken at five different points on a sample. The increase in the I_D/I_G ratio from 0.30 to 0.51 also indicates the transition from sp² to sp³ hybridization (Fig. 2e), suggesting an increase in defect density.³⁷

Fig. 2 High-resolution XPS for the (a) C 1s region and (b) Cl 2p region of Cl-Gr. (c) Raman spectra and (d) G band of pristine and Cl-Gr chlorinated by 5.0 M NaCl solution. (e) I_D/I_G and I_G/I_2D ratios of pristine and Cl-Gr.

The NO₂ gas sensor was fabricated using this Cl-Gr as a sensing material. Fig. 3a and b show the sensing behaviour of Cl-Gr samples upon exposure to 5 ppm NO₂ at room temperature under an applied voltage of 0.1 V. After chlorination, response and recovery times significantly improved when measured at room temperature, unlike the samples treated with other Cl sources (Fig. 3a and S7). The responses of pristine, 3.0 M and 5.0 M chlorinated samples were 0.81, 2.01 and 2.04%, respectively, demonstrating a slight increase in signal with a higher chlorination level, along with a progressive improvement in response and recovery speeds. In particular, the pristine sample, before chlorination, exhibited an unstable and excessively slow response with significant noise, despite having a clean graphene surface. Moreover, even at 3.0 M NaCl solution, saturation was not effectively achieved when exposed to NO₂ gas and subsequent air. The response time (t_res,50) and recovery time (t_rec,50) are defined as the time taken to reach 50% of response value and the time taken to return to 50% of the initial resistance value, respectively. Since the pristine and 3.0 M NaCl-chlorinated samples did not fully recover to the baseline, we adopted the t₅₀ value to enable a comparison of response and recovery times within a measurable time range.^38–41 When chlorinated by a higher level of NaCl solution of 5.0 M concentration than 3.0 M, the response time and recovery times were significantly shortened (Fig. 3b). When comparing the pristine graphene-based sensor and Cl-Gr sensor, the response time decreased from 157 s to 38 s, and the recovery time decreased from 1485 s to 202 s, corresponding 75.8% and 86.4% of decline rates, respectively, indicating remarkable resilience of the Cl-Gr gas sensor. Further details, including the t₉₀ values, are provided in Fig. S8 and S9 and Table S3. Moreover, this trend of enhancement of response and recovery times was sustained even after 1000 seconds of exposure time, indicating the sensor's consistent performance under extended exposure durations to 5 ppm NO₂ and 50 ppm NH₃ gases at room temperature (Fig. S10). In this regard, the gas sensor exhibited the best performance when chlorinated in 5.0 M NaCl, which corresponded to a higher doping level compared to lower concentrations.

Fig. 3 NO₂ sensing performance of the Cl-Gr sensor under a bias voltage of 0.1 V at room temperature (25 °C). (a) Response curves of pristine and Cl-Gr sensors upon exposure to 5 ppm NO₂. (b) Response time (t_50,res) and recovery time (t_50,rec) of the Cl-Gr sensors as a function of different levels of chlorination. (c) Normalized response curves with fits to the exponential decay formula for the sensors and (d) decay time, τ, as a function of doping level.

Exponential decay fitting was performed for accurate comparison since pristine and 3.0 M chlorinated samples did not saturate (Fig. 3c). The exponential fitting is widely used to measure how quickly a gas sensor responds to changes in gas concentration, or to contribute to understanding the behaviour of a gas sensor.^42–44 The exponential decay formula refers to ΔR/R₀(t) = exp(−t/τ) + R_∞, where τ is the time constant and R_∞ is the steady state resistance. The time constant, τ value, indicates how quickly the sensor's response decays; a smaller τ means the sensor responds more quickly to changes in gas concentration and returns to its original state more quickly. The τ value for pristine and 3.0 M and 5.0 M chlorinated samples were calculated as 2465, 218 and 46 s, respectively. After chlorination, the τ value decreased further, and this trend became stronger as the level of chlorination increased (Fig. 3d), suggesting that the sensitivity and response time of the sensor have improved.

The performance of the Cl-Gr gas sensor was retained even after three consecutive exposures to 5 ppm NO₂ at room temperature in terms of response, response time and recovery time (Fig. 4a). The plateau region observed in the response curves corresponds to a saturation state similar to that reported in previous study.⁴⁵ This behaviour is attributed to the redistribution of adsorbed gas molecules among different adsorption sites on the sensor surface, leading to a slow change in resistivity despite continuous exposure. However, the long-term stability of the sensor was limited, as prolonged ambient exposure led to performance degradation (Fig. S11 and S12). The sensor response was also evaluated for various concentrations of NO₂ from 1 ppm to 10 ppm at room temperature, and linear fitting was performed by using these five values (Fig. 4b and c). The responses of the sensor were −0.39, −1.04, −1.79, −2.74 and −4.32% to 1, 3, 5, 7 and 10 ppm NO₂ gas, respectively, and the value of r², the measure of goodness-of-fit of the linear regression, was calculated to be 0.9893 (Fig. 4c). Low-concentration measurement results below 1 ppm also are presented in Fig. S13. Based on this linear fitting, the theoretical limit of detection (LOD) was calculated to be approximately 0.013 ppm,^46,47 and the value of r² was calculated to be 0.9996. Considering that the occupational safety and health administration (OSHA) sets the permissible NO₂ exposure limit at 5 ppm (5000 ppb) for an 8 hour workday, this LOD value indicates that it is highly effective in providing early detection and warning of hazardous gases at safe levels. Also, maintaining good linearity ensures the sensor's output signal remains predictable and stable, facilitating accurate real-time detection of gas concentrations and enabling dependable operation in safety and monitoring systems. The sensor was also tested in various humidity environments of 0%, 50% and 100% relative humidity (RH) using a bubbler system to create humid conditions (Fig. 4d, e and S14). The RH values were calculated from the flow rate ratio of saturated air (from the bubbler column) to dry air.⁴⁸ All previous measurements were conducted under conditions of 0% humidity. In humid environments, the performance of gas sensors may deteriorate owing to the competitive behaviour of water molecules and target gas molecules.^49–51 However, stable operation even in harsh environments of more than 90% is essential for some applications such as human breathing analysis.^52,53 Under humid conditions, the Cl-Gr sensor exhibited improved trends consistent with previous reports on graphene-based gas sensors, which demonstrate enhanced response and recovery behaviour in the presence of moisture.^54–57 Specifically, the NO₂ response increased by approximately −1.85, −2.94 and −2.92% under RH 0%, 50% and 100% for the pristine, 3.0 M NaCl-, and 5.0 M NaCl-chlorinated samples, respectively. Furthermore, the humid atmosphere shows negligible effect on the response time (38 s at RH 0%, 30 s at RH 50%, and 35 s at RH 100%), but the recovery time enhanced from 202 s at RH 0% to 113 s and 124 s at RH 50% and RH 100%, respectively. We further analyzed this humidity-accelerated recovery mechanism by DFT calculations and identified that O₂ adsorption energies become more negative with increasing the number of adsorbed H₂O molecules (−1.09 eV on Cl-Gr, −1.18 eV on Cl-Gr + H₂O, and −1.22 eV on Cl-Gr + 2H₂O). Therefore, the NO₂ recovery on our Cl-Gr sensor can be accelerated under humid conditions, similar to previous studies on graphene-based gas sensors. The calculation details for the humidity-accelerated recovery are provided in the SI (Fig. S15). The gas sensor was also exposed to various gases such as 50 ppm CH₃COCH₃, 50 ppm C₂H₅OH, 50 ppm H₂, 50 ppm NH₃, 5 ppm NO₂ and 100 ppm C₇H₈ at room temperature (Fig. 4f). After chlorination in the 5.0 M NaCl solution, the response exhibited changes of 8.12% to 10.90% for 50 ppm NH₃ at room temperature. When exposed to NH₃ and NO₂ gases, increased sensitivity was notable. This can be ascribed to the change in adsorption energy between the sensing material and target gas molecules.

Fig. 4 NO₂ sensing properties of the Cl-Gr sensor under a bias voltage of 0.1 V at room temperature (25 °C). (a) Response curves of the Cl-Gr sensor upon exposure to three consecutive pulses of 5 ppm NO₂. (b) Response curves to different NO₂ concentrations and (c) its linear fit of the responses as a function of NO₂ concentration. (d) Response curves upon exposure to 5 ppm NO₂ in 0%, 50% and 100% of relative humidity atmosphere. (e) Response time (t_50,res) and recovery time (t_50,rec) of the sensor in various humidity environments (RH 0%, 50% and 100%), with error bars representing three repeated measurements. (f) Responses of the Cl-Gr sensor to 50 ppm NH₃, 5 ppm NO₂, 50 ppm C₂H₅OH, 50 ppm CH₃COCH₃, 100 ppm C₇H₈ and 50 ppm H₂.

We performed DFT calculations on both pristine graphene (pristine) and chlorinated graphene (Cl-Gr) to investigate the effect of chlorination on NO₂ gas sensing. As the interaction between graphene and gas molecules plays a crucial role in gas sensing, our study focused on how chlorination alters the adsorption energies of gas molecules. In our NO₂ gas sensing measurement (Fig. 3a), the gas sensor was first exposed to NO₂ gas for the response, followed by exposure to pure air (NO₂ free) for recovery. Accordingly, we considered NO₂ response as the adsorption of NO₂ molecules and NO₂ recovery as the adsorption of O₂ molecules from air, as O₂ exhibited stronger interactions with pristine and Cl-Gr than N₂ (Fig. S16).

We first identified the surface configuration of Cl-Gr, wherein graphene was processed by electrochemical chlorination. Based on our XPS analysis, Cl-Gr showed clear sp³ hybridization between chlorine and graphene (Fig. 2a and b). To investigate this sp³ hybridization of Cl-Gr, a chlorine atom was adsorbed on graphene by considering various sites, but the sp³ hybridization was not clearly observed in all possible configurations (Fig. S17). Meanwhile, our Raman analysis showed the I_D/I_G increase after the electrochemical chlorination, indicating the increase of defect formation (Fig. 2e).³⁷ Therefore, we further investigated the possibility that chlorine adsorption helps to form carbon vacancies, which lead to the sp³ hybridization.

To investigate the effect of chlorine, we calculated the defect formation energies of carbon vacancy without/with chlorine adsorption. Without chlorine adsorption, the carbon vacancy formed three carbon dangling bonds (Fig. 5a), resulting in a significantly high formation energy of 8.09 eV (Fig. 5c, left). However, with chlorine adsorption, the chlorine passivated one of the dangling bonds and pulled the bonding carbon atom up to 0.81 Å, indicating a clear sp³ hybridization (Fig. 5b). Owing to chlorine passivation, the formation energy was effectively reduced to 4.13 eV, indicating relatively easier formation of carbon vacancies (Fig. 5c, right). Therefore, we suggest that chlorine adsorption facilitates the formation of carbon vacancies on graphene during the electrochemical chlorination. For subsequent calculations, we considered this defected graphene as Cl-Gr.

Fig. 5 Optimized vacancy defect configurations (a) without and (b) with chlorine adsorption. (c) Calculated defect formation energies without/with chlorine adsorption.

We then tried to understand how the Cl-Gr gas sensor rapidly detected the NO₂ gas molecules. To validate the rapid NO₂ sensing of Cl-Gr, we investigated the adsorption behaviour of NO₂ and O₂ gas molecules on both pristine and Cl-Gr surfaces. For this purpose, various molecular orientations were examined over symmetrically inequivalent sites on pristine and Cl-Gr. Fig. 6a shows the most energetically favourable configurations of NO₂ and O₂ gas molecules on pristine (left) and Cl-Gr (right). On pristine graphene, NO₂ is positioned above the center of a double carbon ring and O₂ lies above the center of a single carbon ring. On Cl-Gr, however, NO₂ directly binds to a single carbon dangling bond and O₂ bridges between two adjacent carbon dangling bonds near the chlorine atom. These distinct adsorption geometries result in significantly shorter bonding distances, as presented in Fig. 6b: the C–NO₂ distance decreases from 3.14 Å (pristine) to 1.57 Å (Cl-Gr), and the C–O₂ distance decreases from 3.05 Å to 1.44 Å. These shortened bonding distances indicate stronger surface interactions on Cl-Gr rather than pristine. Correspondingly, the calculated adsorption energies further support this observation, as shown in Fig. 6c. The NO₂ adsorption is enhanced from −0.29 eV (pristine) to −0.61 eV (Cl-Gr), and O₂ adsorption is similarly enhanced from −0.16 eV to −1.09 eV. Therefore, we confirm that the electrochemical chlorination significantly improves the NO₂ and O₂ adsorption capability of the graphene surface, suggesting superior sensing speed for NO₂ detection. Our DFT calculation results are also in agreement with the rapid NO₂ response and recovery time of our experimental Cl-Gr gas sensor (Fig. 3b).

Fig. 6 (a) Adsorption configurations of NO₂ and O₂ gas molecules on pristine and Cl-Gr. (b) Average bonding distances between the adsorbed gas molecules and the nearest carbon atoms in pristine and Cl-Gr. (c) Calculated adsorption energies of NO₂ and O₂ on pristine and Cl-Gr surfaces.

To further understand how the electrochemical chlorination can enhance the NO₂ and O₂ gas adsorption, we analysed the charge density differences of pristine (left) and Cl-Gr (right) upon NO₂ and O₂ adsorption (Fig. 7a). On pristine graphene, both NO₂ and O₂ molecules exhibit only minimal charge redistribution, indicating weak electronic interaction with graphene. In contrast, Cl-Gr shows significant charge redistribution between gas molecules and graphene, indicating a strong electronic interaction. Bader charge analysis supports this observation, revealing that the charge transfer from graphene to NO₂ increases from +0.20e (pristine) to +0.39e (Cl-Gr) and that to O₂ increases from +0.11e to +1.16e. The relatively larger charge transfer to O₂ is attributed to its binding with two adjacent dangling bonds, while NO₂ binds with only one. These results confirm that the electrochemical chlorination promotes significant charge transfer from graphene to gas molecules, thereby enhancing their adsorption on the Cl-Gr surface.

Fig. 7 (a) Charge density difference for NO₂ and O₂ adsorption on pristine (left) and Cl-Gr (right). Blue and red isosurfaces represent regions of electron accumulation and depletion, respectively. The isosurfaces are plotted at ±0.05 e Å⁻³. Charge transfer from graphene to gas molecules are quantified by Bader charge analysis. (b) Electronic density of states (DOS) of Cl-Gr before and after NO₂ (left) and O₂ (right) adsorption. The red dashed lines indicate the Fermi level of Cl-Gr. All DOS curves are aligned to the Fermi level of Cl-Gr.

The enhanced NO₂ and O₂ gas adsorption can also be understood by the electronic density of states (DOS) of Cl-Gr, as shown in Fig. 7b. Before NO₂ adsorption, the NO₂ states appear as sharp localized peaks between −4 eV and −3 eV below the Fermi level. After NO₂ adsorption, these NO₂ states become hybridized with the valence states of Cl-Gr, forming a wide band. A similar change is observed for O₂ adsorption, where the localized O₂ states near −2 eV are hybridized with the Cl-Gr states overall. Note that such hybridization of NO₂ and O₂ gas molecules is not observed on pristine, which is discussed in the SI (Fig. S18). Therefore, the hybridization between gas molecules and Cl-Gr also confirmed that NO₂ and O₂ can strongly interact with Cl-Gr, enhancing NO₂ response and recovery.

Conclusion

In this work, CVD graphene was functionalized into Cl-Gr by a facile electrochemical method, enabling the fabrication of high-performance NO₂ gas sensors operating at room temperature. The method is non-toxic and simple, in contrast to conventional chlorination processes that rely on toxic chlorine gas or require additional reactors. The incorporation of chlorine atoms into the graphene structure dramatically improved the sensing characteristics of the gas sensor at room temperature, including high sensitivity and rapid response and recovery steps. Furthermore, the excellent linearity and humidity stability of Cl-Gr demonstrate its potential for diverse applications, including environmental monitoring, breath analysis, and industrial use. DFT calculations were employed to reveal the gas sensing mechanism of Cl-Gr, showing good agreement with experimental data. These findings highlight the potential of chlorination as an effective surface engineering strategy to enhance the sensing performances of graphene and other 2D materials.

Author contributions

J. Oh and H. Kim contributed equally to this work. Y. Kim, D. Lee, B. H. Hong, and S. An conceived and supervised the project. J. Oh fabricated and analysed the Cl-Gr gas sensors. H. Kim and D. Lee conducted the DFT calculations. S. Cho assisted with data investigation. S. Choi and A. Kim carried out XPS analysis. J. Sim and B. H. Hong provided and analysed CVD-grown graphene. W. Lee contributed to the electrochemical chlorination process. The manuscript was mainly written by J. Oh and H. Kim. All authors discussed the results and commented on the manuscript at all stages.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data generated or analyzed during this study are included in this article and its SI.

Supplementary information is available. See DOI: https://doi.org/10.1039/d5ta05009j.

Acknowledgements

This research was supported by the Young Researcher Project and Development of Measurement Technology for High-Tech Strategic Industries funded by the Korea Research Institute of Standards and Science (KRISS – 2025 – GP2025 – 0013). This research was also supported by the National Research Council of Science & Technology (NST) grant by the Korea government (MSIT) (No. CAP25031-000).

Notes and references

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Footnote

† Co-first authors.

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