A multifunctional covalently linked graphene – MOF hybrid as an e ﬀ ective chemiresistive gas sensor †

A hybrid of GA@UiO-66-NH 2 was synthesized based on the covalent assembly of graphene acid (GA) and the amine functionalized UiO-66 metal – organic framework through amide bonds. This strategy endows the material with unique properties, such as hierarchical pores, a porous conductive network decorated with functional groups, a high speci ﬁ c surface area, and a good chemical and thermal stability. The resultant hybrid has an electrical resistance of (cid:1) 10 4 U , whereas the pristine GA and UiO-66-NH 2 possess an electrical resistance of (cid:1) 10 2 U and (cid:1) 10 9 U , respectively. The hybrid GA@UiO-66-NH 2 was demonstrated for CO 2 chemiresistive sensing and displayed a very fast response and quick recovery time of (cid:1) 18 s for 100% CO 2 , at 200 (cid:3) C. While the pristine GA exhibits negligible response under the same conditions, GA@UiO-66-NH 2 exhibited a response of 10 (cid:4) 0.6%. Further, in situ temperature dependent Raman studies during CO 2 exposure con ﬁ rm the presence of strong hydrogen bonding interaction between CO 2 and the amide functionality present on GA@UiO-66-NH 2 . The resulting gas sensing characteristics of GA@UiO-66-NH 2 are majorly attributed to the better interaction of CO 2 at the amide/ amine functional groups and the readily accessible hierarchical pores. This design strategy opens new horizons in the development of covalently linked hybrids with hierarchical porous conductive networks which can help to improve the gas sensing properties of MOF-based materials.


Introduction
2][3] From the CO 2 concentration in the atmosphere of closed rooms and buildings, the occupancy by people can be determined. 4This can be used to predict and prevent events related to overcrowding of areas, which ultimately can lead to disasters.Also, CO 2 detection can be used by rescuers aer earthquakes or similar incidents to detect survivors that are trapped under the rubble of collapsed structures. 5Current technologies detect CO 2 based on non-dispersive IR optical sensors. 6Even though these sensors are well established, they still have some drawbacks, including their complexity, cost, scalability, and energy consumption.[9][10][11][12][13][14][15] Upon an outer stimulus, these sensors rely on changes in electrical resistance, that allow continuous monitoring of

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View Journal | View Issue various gas concentrations.One of the most critical challenges in the development of CO 2 gas sensors is the inclusion of active sites for adsorption/desorption, where CO 2 gas can cause changes in electrical parameters.4][25][26][27][28] However, these amine-based composite sensors display sharply decreasing sensitivity with reduced interactions over time.Hence, the need for the modications of the active species over graphene-based materials to improve the signal-transducer interaction is essential for CO 2 sensing.
0][31][32][33] In particular, conjugated layered two-dimensional (2D) MOFs have recently been demonstrated as attractive CO 2 gas sensors by combining the desired properties of conductivity, nano porosity and the presence of functional groups for analyte adsorption. 11,12,34However, their response and recovery times are still limiting their potential application.Although many pristine MOFs have conductivity limitations (mostly near insulators), there are some examples of conductive 2D layered MOF structures. 356][47][48] In particular, the covalent linking of graphene with appropriately functionalized MOFs has shown promising electronic properties for energy applications.Additionally, the strong bonding between each of the pristine components, the hierarchical pore architecture, and the enhanced thermal and chemical stabilities are all features which are sought aer for the development of robust chemiresistive sensors.
Herein, we report the use of a covalently assembled hybrid constructed from graphene acid (GA) and amine functionalized UiO-66-NH 2 (Zr 6 O 4 (OH) 4 (NH 2 -bdc) 6 with NH 2 -bdc 2À ¼ 2-amino-1,4-benzenedicarboxylate) [49][50][51][52][53] for chemiresistive CO 2 sensing.In the material, UiO-66-NH 2 is covalently bound with GA via amide bonds, creating the hybrid GA@UiO-66-NH 2 (Fig. 1).The material shows hierarchical porosity, wherein the micropores are located at the octahedral UiO-66-NH 2 nanocrystals and the mesopores are built up through the stacking of GA through the MOF nanocrystals.The hierarchical architecture facilitates gas diffusion through the material, hence providing faster access to the interaction sites for the analytes.On the other hand, the interconnected conductive network ensures quick readout of the signal.In comparison, graphene has a low electrical resistance ($10 À6 U) and high carrier electron mobility, but the lack of specic and strong interaction sites for the adsorption of gaseous analytes, renders it unusable for chemiresistive sensing in its pristine form.Here, the resultant hybrid GA@UiO-66-NH 2 provides interaction sites at the amide linkages present at the interface of graphene and the MOF for detectable change in resistance upon CO 2 interaction.These results are claried by temperature dependent in situ Raman spectroscopy during exposure of the material to CO 2 .In this study, a good correlation has been achieved between sensing characteristics and the novel functional properties obtained aer integrating the MOF with graphene.

Synthesis of graphene acid
Graphene acid was prepared according to the literature. 31To prepare graphene acid, the commercial uorinated graphite (120 mg) was sonicated in presence of 15 mL of N,N-dimethylformamide (DMF) for 4 h under nitrogen atmosphere in a 25 mL round-bottom ask.To this mixture, 800 mg sodium cyanide (NaCN) was added and the mixture was heated to 130 C with a condenser under stirring (500 rpm) for 24 h.During the reaction procedure, the intermediate and nal product were cooled to room temperature, aer which an equal amount of acetone was added.The materials were then separated by centrifugation and further puried by successive washing steps using DMF, dichloromethane, acetone, ethanol and water (each step was repeated four times).Additionally, hot (80 C) DMF and water were also used.During the nal centrifugation step with water, it was necessary to apply centrifugal force of up to 25 000 rpm to isolate the product.The resultant cyanographene was mixed with HNO 3 (65%) under stirring at room temperature.The mixture was then heated at 100 C under reux with stirring (350 rpm) for 24 h.Further, the product was thoroughly washed with water to remove soluble impurities to obtain graphene acid as the nal material.It should be noted that, the stable aqueous suspensions of GA were prepared by adjusting the pH of the puried suspension to $8.

Synthetic procedure for UiO-66-NH 2
Octahedral nano crystals of UiO66-NH 2 were prepared according to a slightly modied literature procedure.ZrCl 4 (0.233 g) and 2-aminoterepthalic acid (0.181 g)and 4-aminobenzoic acid (0.244 g) were mixed in 10 mL dimethylformamide (DMF) solvent in glass vial and dissolved by addition of 0.16 mL of hydrochloric acid (HCl).Later, the glass vial was tightly closed and placed in an oven at 120 C for 2 days.Aer cooling to room temperature, the supernatant was decanted, and the pale-yellow precipitate was washed three times with DMF and three times with methanol.Methanol was decanted and replaced once per day during the course of three days and later it was removed under vacuum.The product was heated under vacuum to 150 C for 5 h.The sample was cooled to room temperature and stored under ambient conditions.

Preparation of hybrid GA@UiO-66-NH 2
A sample of 100 mg of GA was exfoliated in 10 mL of DMF solution for 30 minutes under sonication.To this GA suspension, ZrCl 4 (0.233 g), 2-aminoterepthalic acid (0.181 g), 4-aminobenzoic acid (0.244 g) and 0.16 mL of HCl was added into a 25 mL glass vial.The resultant mixture was sonicated for 30 minutes.The resultant mixture was transferred to a 50 mL glass vial and placed at 120 C for 2 days.The obtained black gel was washed several times with methanol and dried under nitrogen atmosphere at 100 C.

Chemiresistive sensing measurement
All the materials tested for chemiresistive CO 2 sensing were weighed and pressed into pellets of uniform thickness.150 mg of dried powdered sample were weighed and pressed with a pressure of 75 kg cm À2 using a hydraulic pellet press.Pellets with uniform thickness of 1 mm were obtained.Using a toothpick, small silver dots (diameter of 1 mm) were placed on the upper surface of the pellet.This silver conductive adhesive paste has a sheet resistance of <0.025 ohm square À1 @ 0.001 mm thick.A Keithley source meter (Model no.2450) was used to measure the resistance change in the presence and absence of CO 2 gas.The argon (reference/balance gas) and CO 2 gas ow into the gas reactor chamber were controlled by using Alicat Mass Flow Controllers (MFCs).A computer-controlled data acquisition system was used to monitor the sensor responses through the resistance-based studies.In order to carry out the sensing studies, the pellet was placed on a hollow graphite heating block containing the halogen lamp to provide the thermal energy.Argon was chosen both as reference and balancing gas.Initially, up to a desired temperature of sensing (controlled by temperature control unit), argon gas was introduced through the inlet in the gas reactor chamber.A calculated amount of CO 2 gas was injected on obtaining a stable resistance value under argon ow.The mixing chamber was used to mix the CO 2 and argon gas uniformly to obtain the desired quantity of gas ow.The resistance of the sensor/pellet was measured in presence and absence of CO 2 gas.

Results and discussions
The coupling of the carboxylate functional groups of GA with pristine UiO-66-NH 2 under solvothermal conditions was done in analogy to our previously reported study. 54The powder X-ray diffraction of the resulting hybrid (GA@UiO-66-NH 2 ) material conrms the structural integrity of the metal-organic framework (Fig. 2a).
Hybridization of GA with UiO-66-NH 2 through covalent bonds yielded a hybrid system featuring electrical conductivity, hierarchical pores, amine/amide functional groups and a good CO 2 capacity, which is ideally suited for CO 2 gas sensing applications.The gas sensing measurements were carried out using a home-built two-probe gas sensing station.The twoprobe method is suitable for measuring the electrical resistance of materials that have a much higher resistance than the contacts and wires used during the measurement.The probes are connected to a Keithley source meter.The gas sensing/ testing chamber is equipped with a heater and gas-supplying channel that were controlled precisely by a temperaturecontrol device and mass ow controllers (MFCs), respectively (Fig. S3 †).The sensing characteristics of the material in the form of pressed pellets tested in this work include response%, response time and recovery time towards different amounts of CO 2 (100% to 5%) by recording the change in resistance versus time (Fig. S4 †).Using the above-described setup, the resistance of pellets of GA, UiO-66-NH 2 and GA@UiO-66-NH 2 were determined, obtaining values of around 10 2 U, 10 9 U and 10 4 U, respectively (Fig. 5a).The low resistance of pristine GA results from the delocalized p-electrons.Nevertheless, GA do not possess any favourable interaction sites for CO 2 .UiO-66-NH 2 features suitable interaction sites but has a high resistance due to the very poor orbital overlap between the Zr(IV) d-orbitals and the linker frontier orbitals.Covalent assembly of these two components leads to an intermediate resistance in the range of $10 4 U which is in the range of typical metal-oxide semiconductor based sensors like SnO 2 . 56 Based on these properties we explored GA@UiO-66-NH 2 as a CO 2 chemiresistive sensor.][59][60][61][62][63][64] As expected, due to the lack of strong interaction sites, GA shows negligible response towards CO 2 (100% to 5%) at 200 C (Fig. S5 †).Remarkably, GA@UiO-66-NH 2 shows signicant response of 10 AE 0.6% at the same temperature for 100% CO 2 .For 50, 30, 10 and 5% CO 2 , GA@UiO-66-NH 2 showed responses of 8 AE 0.8%, 6 AE 0.5%, 4 AE 0.2% and 4 AE 0.3% respectively, at 200 C (Fig. 5b).The response is found to also decrease almost linearly with decrease in CO 2 concentration (Fig. S6 †).The recyclability was demonstrated with three cycles of responserecovery curves (Fig. S7 and S8 †).The effect of temperature on the sensing characteristics of the material is shown in Fig. 5d  and e.The CO 2 response increased with increase in temperature from 2 AE 0.5% at 100 C to 8 AE 0.8% at 200 C for 50% CO 2 .Further increase in temperature to 250 C resulted in a slight drop in the response to 7 AE 0.4%.A very fast response and quick recovery time ($18 s) are observed at 200 C.,56,[65][66][67][68][69][70][71] The gas sensing performance of GA@UiO-66-NH 2 in presence of synthetic air is shown in Fig. S9.† The measured response% for 50% CO 2 in presence of synthetic air balance was 7%.This response% value is like the one measured in presence of argon balance gas, that is 8 AE 0.8%.The response and recovery times were in the range of 15-18 s.Hence, the effect of synthetic air was negligible on the gas sensing performance of GA@UiO-66-NH 2 .For pristine UiO-66 MOF, a stable resistance of 0.5 Â 10 À11 U was measured at 200 C under argon ow.Upon exposure to CO 2 , this material did not show any detectable changes in resistance (Fig. S10 †).
This could be due to the lack of strong interactive sites in the MOF.Nevertheless, the amide bridge at GA@UiO-66-NH 2 connecting the graphene basal plane and -NH 2 functionality on the pristine UiO-66-NH 2 are known to have affinity towards CO 2 .The positive sensing curves (Fig. 5b) suggest that donor (amide/ amine)-acceptor (CO 2 ) interactions led to a decrease in the conductivity of GA in the hybrid material.We rule out the possibility of sole contribution of UiO-66-NH 2 to the sensing characteristics due to its several orders of magnitude higher resistance compared to GA.It is reasonable to predict that the CO 2 interactions with the amide functionality at the interface of GA and the MOF are majorly responsible for the observed gas sensing properties (Fig. 6).The MOF has two roles in this hybrid, on one hand it controls the distance between the conductive GA layers constructing the mesoporous diffusion channels and on the other hand it provides the interaction sites at the amide functionalities at the GA-MOF interface.
Raman spectroscopy is a very useful technique to understand the interaction of gas molecules with MOFs.In order to verify the interactions proposed in Fig. 6, temperature dependent in situ Raman spectroscopy studies were conducted on GA@UiO-66-NH 2 in the presence of CO 2 .Two new broad features appear around 1230 cm À1 and 1383 cm À1 which are assigned to adsorbed CO 2 Fermi resonance modes (Fig. S11 and enlarged gures of gure of S11 shown in Fig. S12-S18 †).Dosing of Ar instead of CO 2 , did not yield any new signals in this region (Fig. S19 †).Variable temperature Raman spectroscopy studies were conducted in CO 2 environment from room temperature (RT) to 300 C (Fig. 7).Fig. 7a shows the temperature dependent Raman shi of the vibrational modes corresponding to the C]O stretching, as well as the D and G bands of the GA@UiO-66-NH 2 .The temperature dependent mode behaviour in Fig. 7a exhibits the expected monotonous soening (decrease in the frequency) with increase in temperature suggesting that the hybrid is stable in the temperature range of RT to 300 C in the CO 2 environment and there is no effect of CO 2 adsorption on these modes.The Raman modes corresponding to the amide CO-NH linkage of GA@UiO-66-NH 2 and the Fermi resonance mode of  CO 2 show the expected soening up to 125 C.This suggests that there is a gradual uptake of CO 2 into the pore space without strong interactions with the walls of the pores of the hybrid with rise in temperature.Between 125 C to 200 C, we observe that the change in frequency shi remains nearly constant until 200 C suggesting that further uptake of CO 2 is assisted by a strong chemical interaction between CO 2 and the CO-NH bond.We observe that above 200 C there is a sudden increase in the frequency of both the CO-NH linkages and the two Fermi resonance modes of CO 2 and they shi back close to the values observed at room temperature.This suggests that CO 2 interacts only weakly with CO-NH above 200 C, possibly due to the high kinetic energy associated with CO 2 .This is consistent with the chemiresistive gas sensing results (Fig. 5d) where 200 C is found to be the optimum temperature for achieving the highest CO 2 response.Further, the relative intensity ratio of CO 2 with respect to the D band (I CO 2 /D ) as shown in Fig. 7c and we observe that the CO 2 intensity decreases up to 125 C and then increases to a maximum intensity at 200 C and then falls steeply beyond.

Conclusions
In summary, we have reported a uniquely simple, convenient, and scalable covalent assembly of graphene acid with amine functionalized UiO-66-NH 2 under solvothermal conditions.The GA@UiO-66-NH 2 hybrid acts as a chemiresistive CO 2 sensor with signicant efficiency and remarkable stability.The novel CO 2 sensing characteristics might be attributed to the synergistic effect between GA and UiO-66-NH 2 in terms of good conductivity, hierarchical porous nature facilitating gas diffusion and amide/amine based interaction sites.Temperaturedependent Raman studies during CO 2 adsorption conrm interaction of CO 2 molecules with the amide bonds of GA@UiO-66-NH 2 .The present data suggests a proof-of-concept for the use of this hybrid material for gas-sensing applications and there is a lot of room for the development of other conductive graphene-MOF hybrids through covalent assembly for various gas and bio-sensing applications.

Fig. 1
Fig. 1 (a) Schematic representation of the multifunctional (hierarchical porosity, conductive network, amine/amide functional groups) graphene-MOF hybrid (b where carboxyl groups of graphene acid are covalently linked via amide bonds to the amine groups of UiO-66-NH 2 MOF.

Fig. 5
Fig.5(a) Comparison between the resistance of UiO-66-NH 2 , GA and GA@UiO-66-NH 2 , (b) gas sensing performance of GA@UiO-66-NH 2 to 5% to 100% CO 2 concentration at an operating temperature of 200 C, (c) comparison of gas sensing characteristics (response%, response time and recovery time) from 100% to 5% CO 2 at an operating temperature of 200 C. Effect of temperature on the sensing characteristics of GA@UiO-66-NH 2 (d) response% and (e) response and recovery times.
Fig. 7b shows the temperature dependent Raman mode behaviour of the amide (CO-NH) linkage connecting the MOF and GA, and the Fermi doublet of the CO 2 modes.Interestingly, the observed behaviour can be divided into three distinct regions (i) RT to 125 C, (ii) 125 C to 200 C and (iii) above 200 C.

Fig. 6
Fig. 6 Schematic illustration of possible sensing mechanism: (a) GA@UiO-66-NH 2 shows porous network with amide linkage (b) the CO 2 interactions with amide linkage present at the basal plane of the GA could bring about the positive change in resistance of GA@UiO-66-NH 2 chemiresistive sensor.(c) Upon removal of CO 2 , the steady state baseline has been retrieved suggesting that CO 2 interaction is reversible.

Fig. 7
Fig. 7 Temperature dependent Raman study of GA@UiO-66-NH 2 under CO 2 atmosphere-(a) C]O stretching, D and G band, (b) CO-NH and Fermi resonance mode of CO 2 .(c) Intensity ratio vs. temperature plot showing maximum adsorption of CO 2 around 200 C.