Reduced Graphene Oxide and Graphene Composite Materials for Improved Gas Sensing at Low Temperature

Supplementary Information Reduced Graphene Oxide and Graphene Composite Materials for Improved Gas Sensing at Low Temperature Alexander Zöpfla, Michael-Maximilian Lembergera, Matthias Königb, Guenther Ruhlb, FrankMichael Matysika, and Thomas Hirscha a Institute of Analytical Chemistry, Chemoand Biosensors, University of Regensburg, Universitaetsstrasse 31, 93053 Regensburg, Germany; E-Mail: alexander.zoepfl@ur.de michael.lemberger@ur.de; frank-michael.matysik@ur.de; thomas.hirsch@ur.de b Infineon Technologies AG, 93049 Regensburg, Germany E-Mail: Guenther.Ruhl@infineon.com; Matthias.Koenig@infineon.com


Introduction
The simple and reliable monitoring of gas concentrations is important in everyday-life.In industrial processes hazardous gases need to be controlled to guarantee safety.Controlling air quality can save energy in automated air conditioning.Also the detection of environmental pollution (like NO x ) is of great interest. 1Up to now, solid-state gas sensors based on metal oxide chemiresistors are well established and widely used in detecting gases. 2,3They are operated at high temperatures, 4 which consumes excessive energy and limits their long-term stability, thus leading to the development of new gas sensor concepts which overcome these drawbacks.In the last decade carbon nanomaterials, like carbon nanotubes 5,6 or graphene, [7][8][9][10] were shown to be capable of gas detection due to their high sensitivity to various gases even at low operating temperatures.Especially graphene has recently attracted intense attention because of its high charge carrier mobility (15 000 cm 2 V À1 s À1 under ambient conditions) and large surface area as a consequence of having a thickness of only one atom and the absence of any bulk phase. 11,12any different preparation methods for graphene are known so far, but only a few of them are applicable in terms of sensor preparation.Chemically derived graphene obtained by the reduction of graphene oxide (GO) 13,14 is inexpensive and the synthesis is easily scalable.Due to a more defective structure its electrical properties are not as outstanding as those of pristine graphene, 15,16 but are still suitable for sensitive gas detection. 17It has also been described that graphene with a more defective structure shows an improved adsorption of gas molecules. 18urthermore, reduced graphene oxide (rGO) can be dispersed in solutions, simplifying the transfer to a sensor set-up by e.g.spraying, printing and casting methods. 19,20imilar to solid state gas sensors, gas adsorption on graphene leads to a change in its electrical resistance (Fig. 1).Therefore, any kind of interaction between graphene sheets and adsorbates inuences the electronic structure of the graphene, leading to an altered charge carrier concentration or electrical conductance of the material, respectively. 17,21Nevertheless, up to now most of such sensor concepts have been demonstrated only in inert gas mixtures or at elevated temperatures.For any practical application it is necessary to overcome the limitations of poor selectivity. 7Many works address this issue by chemically modifying the carbon material with functional groups or doping with metals and metal oxides.][24] Here we report on rGO as a sensitive material for gas detection and the possibilities of functionalization to introduce selectivity.Reduced graphene oxide was prepared by the chemical reduction of GO, which was obtained from the oxidation of graphite.The resulting product can be dispersed in water, enabling an easy transfer to pre-structured microelectrodes comprising an interdigital structure.Application of rGO via spin coating was optimized and resulted in consistent layers of reproducible quality in terms of the electrical properties.The conductance of such modied electrodes was measured in the presence of various gases diluted in synthetic air (NO 2 , CH 4 , and H 2 ) at moderate temperatures (85 C).Chemical modications were applied by the insertion of functional groups and by doping with metals and metal oxides.The resulting materials were characterized and tested for their gas sensing behaviour.It was demonstrated that a combination of simple modications of rGO can lead to sensors with different characteristics, allowing a pattern detection of various gases.

Materials and instrumentation
Unless otherwise stated, all chemicals were purchased from Merck (Darmstadt, Germany) or Sigma-Aldrich (Steinheim, Germany) and used without further purication.Ultra-pure water (0.055 mS cm À1 ) was used in all experiments.All gases and mixtures were purchased from Linde AG.Raman spectra were recorded on a Thermo Scientic DXR Raman microscope with a 532 nm excitation laser (10 mW).Scanning electron microscopy (SEM) was performed with a Zeiss Ultra 55, EDX on a JOEL JSM-6700F and TGA-FTIR with a Netzsch Iris TG209 connected to a Bruker Equinox 55, all at Inneon Technologies AG Regensburg.Transmission electron microscopy (TEM) was performed using a 120 kV Philips CM12 microscope.

Synthesis of reduced graphene oxide and modications
GO was synthesized using a slightly modied Hummers method. 13Briey, 100 mg graphite (China ake graphite, K. W. Thielmann & Cie KG) was mixed with 75 mg NaNO 3 , 7.5 mL conc.H 2 SO 4 and 450 mg KMnO 4 .The mixture was sonicated for 3 h and stirred for 3 days at room temperature.Subsequently, 7.5 mL of 5% H 2 SO 4 was added and the mixture was stirred under reux for 2 h at 100 C, followed by the addition of 1.5 mL of 30% H 2 O 2 under constant stirring for 1 h at room temperature.For purication the obtained GO was washed with the following solutions: four times in 3% H 2 SO 4 with 0.5% H 2 O 2 ; two times in 3% HCl; and three times in water.The product was dialyzed against water (14 kDa cut-off) for 10 days.
To obtain rGO, 14 7 mL of a GO suspension (0.5 mg mL À1 ) was mixed with 31 mL NH 3 (32% in water).Aer adding 5 mL of 98% hydrazine hydrate the reaction mixture was reuxed for 1 h at 100 C. Finally the resulting black suspension was washed with water.

Modication with octadecylamine
To modify rGO with octadecylamine (ODA), the synthesis of surface functionalized graphene by Wang et al. was adapted. 255 mg GO was suspended in 5 mL dichlorobenzene.Aer adding 50 mg ODA, the reaction mixture was sonicated for 24 h at 80 C. To get rid of excess ODA and dichlorobenzene, the reaction mixture was treated with 40 mL ethanol resulting in a black precipitate.The mixture was centrifuged and the resulting solid was washed twice with ethanol and toluene.This washing step was repeated once more and the product (rGO-ODA) was dried in air.

Modication with MnO 2
A synthesis for GO-MnO 2 nano composites was adapted. 265.1 mg GO and 21 mg MnCl 2 $4H 2 O were dispersed in 5 mL isopropanol and sonicated for 30 min.The reaction mixture was reuxed at 83 C under vigorous stirring.Aerwards a solution of 12 mg KMnO 4 in 0.4 mL water was added and the slurry was reuxed at 83 C for another 30 min before it was cooled to room temperature.The nano composite was then centrifuged and washed twice with water.The resulting brown product (rGO-MnO 2 ) was dried at 60 C overnight.

Modication with TiO 2
For the functionalization of carbon nanomaterials Li et al. have reported the modication of single-walled carbon nanotubes with TiO 2 . 27In an analogous way rGO was modied with TiO 2 (rGO-TiO 2 ): 1 mg of TiO 2 with an average particle size of 1 mm was ground thoroughly.Subsequently the powder was dispersed in 2 mL water, sonicated for 1 h and 1 mL of an aqueous rGO suspension (0.5 mg mL À1 ) was added.Aer another sonication for 1 h a grey suspension was formed where the modied rGO akes tended to aggregate.

Modication with Pd and Pt nanoparticles
Both approaches were adapted from the preparation of graphene-metal nanocomposites by Xu et al. and modied slightly. 285 mL of a GO suspension (1 mg mL À1 ) was mixed with 10 mL ethylene glycol.Aerwards 0.5 mL of a 0.01 M solution of a metal precursor (K 2 PtCl 4 or PdCl 2 ) in water was added and the mixture was rst stirred for 30 min at room temperature and then for 6 h at 100 C.During the heating in both cases the colour became black indicating that the GO had been reduced by ethylene glycol.The resulting suspensions containing the metal doped carbon nanomaterials (rGO-Pd and rGO-Pt) were centrifuged for 5 min at 3000 rpm and washed three times with water.

Electrode preparation
Suspensions of rGO and the modications were deposited on gold microelectrodes (Fig. 2) via spin coating.The respective suspensions in a 1 : 1-mixture of water and isopropanol (0.25 mg mL À1 ) were sonicated for 10 min before 2 mL of each was spin coated onto the interdigital electrode structure with a Laurell WS-400BZ-6NPP/LITE spin coater (5 s at 500 rpm and 20 s at 3000 rpm).Aerwards the electrodes were heated to approximately 230 C for 30 s on a hot plate.

Gas measurements
Gas measurements were performed on a homemade gas mixing device, consisting of mass ow controllers (UFC-8160A and UFC-1660 from Unit Instruments Inc.), a ow cell with temperature control by two heating elements, and a Pt-100 temperature sensor (Fig. SI 1 †).
Measurement of the electrical resistance was accomplished by contacting the two gold conducting paths in the mid-position of the interdigital electrode structure (Fig. 2) using a Keithley 2400 sourcemeter at a constant bias voltage of 50 mV.Synthetic air (N 2 : 80%, O 2 : 20%) was used as the carrier gas and was mixed with 300 ppm NO 2 , 1% H 2 or 1% CH 4 .The test gases were all diluted by the synthetic air.Furthermore the gas adsorption tests were accomplished at a constant gas ow of 100 sccm (300 sccm for the concentration dependency of NO 2 ) and a constant temperature of 85 C. It was also necessary to heat the electrodes for 20 s at 230 C before each measurement, ensuring that no analyte gas had been previously adsorbed by reaching the initial resistance.For measuring the inuence of air humidity, the gas ow was piped through the head space of a ask containing water.

Choice of materials and sensor fabrication
The transfer of graphene materials is still the most critical step in sensor preparation using this type of carbon nanomaterial.Graphene prepared by e.g. the CVD technique 29 requires a sophisticated transfer method of the carbon nanomaterial from the metallic substrate to the substrate of choice.This can be done e.g. by a transfer with stamping methods, 30 which leads to contamination of the graphene and therefore such processes lack reproducibility.In contrast, rGO suspensions can be easily processed.Reduced graphene is usually transferred to electrodes by drop casting, which results in layers of inhomogeneous thickness and low reproducibility.The sensitivity of chemoresistors strongly depends on the thickness of the conductive layer.Therefore we used an interdigital electrode structure which was covered with a thin layer of rGO via spin coating.Parameters affecting the spin coating, such as the concentration of rGO, the rotation speed and time, as well as the usage of certain additives like isopropanol for improving the spreading of the suspension were optimized to obtain consistent layers with reproducible total resistance.

Characterization of the microelectrodes modied with reduced graphene oxide and composite materials
Graphene oxide was prepared by a modied Hummers method and reduced with hydrazine as reported previously. 14The size of the graphene akes ranged from 100 nm to about 1 mm as indicated by SEM studies (Fig. SI 2a †).The Raman spectrum of rGO (Fig. 3a) exhibits the typical broad D and G bands at 1342 cm À1 and 1602 cm À1 . 31The D/G ratio and position of the G band indicates crystalline domains of several nanometers in size. 32egarding the thermogravimetric analysis (TGA) of GO and rGO (Fig. 3b), a loss of mass can be observed at temperatures of about 200 C.In FTIR-TGA (Fig. SI 3 †)  this effect can be ascribed to a thermal reduction, where CO and CO 2 are released. 33Graphene oxide loses about 25% of its mass, while rGO loses around 15%, which shows that the reduction by hydrazine is incomplete.
Further, the rGO modied electrodes were heated to 230 C to guarantee a thermal reduction.During this step the electrical resistance dropped from about 8.0 AE 3.8 MU to 180 AE 78 kU.The oxidation process of graphite introduces many defects into the sp 2 structure which act like a barrier for the electron ux, resulting in better conductance at higher levels of reduction. 34unctionalization of rGO was performed by wet chemical methods before the composite material was also transferred to the microelectrodes.Generally all dopants primarily bind/coordinate with the oxygen functionalities of rGO.Whereas ODA reacts most probably with the epoxy groups of GO, 25 doping with metals and metal oxides usually includes the interaction of a precursor metal ion with the carboxyl groups of rGO. 28,35The successful modication of rGO was revealed by Raman studies.Peaks at 1125 and 1489 cm À1 (ODA), 140, 392, 510 and 644 cm À1 (TiO 2 ), and 579 and 633 cm À1 (MnO 2 ) were found (Fig.  Covalent attachment of functional groups 36 or doping with metals and metal oxides 37 may change the electronic structure and level of n-/p-doping within the material, but may also increase the sensor surface or have a catalytic effect on gas adsorption.

Gas sensor response
To assess the effect of gas adsorption on the conductance of rGO, the electrical resistance of the coated microelectrodes was measured for different gases of The reversibility and concentration dependence of the sensor was evaluated by continuous adsorption and desorption of the different gases.The results for NO 2 are shown in Fig. 4, and the changes in conductivity for other gases are listed in Table 1.A linear behaviour within the applied concentration range (25-150 ppm), a sensitivity of 0.56 ppm À1 and a detection limit of 0.3 ppm (S/N ¼ 3) were observed for NO 2 .In contrast to the signal drop for NO 2 , the adsorption of H 2 and CH 4 led to an increase in electrical conductance.The electron withdrawing effect of the adsorbed NO 2 leads to more positive charge carriers since rGO is described to show a p-type semiconducting behaviour. 16,38Gases such as H 2 or CH 4 cannot act as electron donors or acceptors.It can be assumed that previously physisorbed molecules from synthetic air (most probably oxygen) at the surface of the graphene are replaced by H 2 or CH 4 , leading to an increase in electrical resistance. 39,40hese results clearly demonstrate that rGO on the one hand is an excellent sensor material for detecting gases.In contrast to metal oxide sensors, which are the most common gas sensors so far, sensors based on graphene can be operated at low temperatures with fast response times.On the other hand it is demonstrated that graphene based sensors lack selectivity.2][43][44][45][46][47][48] To overcome this drawback graphene itself may be doped by nitrogen, as was theoretically calculated by Ma et al. 49 Another possibility is to functionalize graphene or to create graphene nanocomposites. 50,51hemical insertion of ODA led to slightly increased signal changes towards H 2 and CH 4 .For NO 2 detection almost no inuence of the modication could be found.The composite material with TiO 2 showed unaffected behaviour towards H 2 and CH 4 , but a lower response towards NO 2 .This indicates that the adsorption sites for NO 2 may be blocked.Electrodes coated with rGO which was doped with MnO 2 showed almost no signal change for all of the utilized gases.Therefore, this modication could only be used as a reference signal to compensate for small uctuations in temperature.The most signicant change in response was achieved by chemical doping with Pd-and Pt-nanoparticles.The resulting composite materials showed an increased sensitivity towards H 2 (Fig. SI 9b †).The same observations with comparable results have been made with carbon nanotubes decorated with Pt and multilayered graphene nanoribbons doped with Pd. 24,52 Though the composite material with rGO is easier to produce and process.
In summary, all of the composites of rGO presented here are simple to prepare and show different behaviour in their electrical conductance in the presence of various gases.The sensor responses of different functionalized rGO materials towards NO 2 , H 2 and CH 4 (Fig. 5) are summarized in Table 1.

Principal component analysis for pattern recognition of different gases
The altered sensitivity of every sensor towards different analyte gases enables pattern recognition using multivariate analysis based on principal component analysis (PCA).With this chemometric technique it is possible to simplify multidimensional datasets without crucial loss of information.Here the data matrix (Table 1) contains the normalized sensor response of each sensor material to a certain gas and concentration.This multidimensional matrix can be reduced to two principle components (PC1, PC2).The variance of PC1 (95.75%) and PC2 (2.94%) is above 98% and therefore these components already contain signicant information to represent the data in two dimensions (Fig. 6).Clear separation between the clusters representing individual gases with no overlap and a recognizable trend of concentration allows the identication of all analytes with a set of six different electrodes.Regeneration of the sensor material still has to be further investigated.In this study the regeneration was mostly performed by a short term heat treatment at  230 C between measurements.Furthermore this step ensures the complete desorption of any physisorbed impurities as demonstrated by the TGA measurements.In an application this could be realized by different operation temperatures with short regeneration cycles.

Conclusion
Reduced graphene oxide is an ideal sensor material for chemiresistive gas sensors due to its simple preparation and functionalization enabling altered sensitivities.Compared to commercial solid state gas sensors, rGO sensors have the advantage of operating at the low temperature of 85 C to exclude the strong inuences of humidity.It was demonstrated that the spin coating of rGO composites results in reproducible sensor behaviour suitable for well established fabrication technologies.In a chemiresistor setup, unmodied rGO showed high sensitivity towards NO 2 on the ppm level at ambient conditions.The sensor was rather unselective and also showed responses towards H 2 and CH 4 .Upon different functionalizations it was possible to achieve different sensor behaviors for different gases.This can be used to apply PCA to discriminate between each individual gas.It is expected that this approach could be extended to build up sensor arrays for detecting the concentration of many individual gases in a complex matrix at low temperatures.

Fig. 1
Fig. 1 Scheme of the principle of chemiresistor gas sensors based on graphene.

Fig. 2
Fig.2Microelectrodes with an interdigital structure consisting of gold conducting paths on a Si/SiO 2 wafer substrate. SI 4 †).Graphene composites with conductive Pt-and Pd-nanoparticles could be observed on distinct regions of rGO in SEM and TEM images.The average particle size was below 20 nm (Fig. SI 5 and SI 6 †).The elemental composition of the different modied materials was conrmed using energy-dispersive X-ray spectroscopy (EDS) (Fig. SI 7 †).

Fig. 3
Fig. 3 (a) Raman spectra and (b) thermogravimetric analysis of GO and rGO.

Fig. 4
Fig. 4 Changes in the electrical resistance of a rGO layer at 85 C in the presence of different concentrations of NO 2 .

Table 1
Comparison of the response of unmodified rGO and modifications of rGO in the presence of different concentrations of NO 2 , H 2 and CH 4 at 85 C Open Access Article.Published on 27 June 2014.Downloaded on 20/04/2015 08:23:08.This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.